Fingerprint detection

This invention is related to fingerprint pattern recognition, and more particularly, to enhanced fingerprint pattern sensing circuit and method.

Capacitive sensors to detect a fingerprint pattern are currently well known. There are a number of patents which describe capacitive sensors for determining the pattern of ridges and valleys in a person's fingerprint. <patcit id="pcit0001" dnum="US4353056A"><text>U.S. Patent No. 4,353,056</text></patcit> describes a capacitive fingerprint sensor which has a sensing surface for receiving a fingerprint and sensing the ridges and valleys of the skin using an array of small capacitors. <patcit id="pcit0002" dnum="US5325442A"><text>U.S. Patent No. 5,325,442</text></patcit> also describes a fingerprint sensing and recognition circuit using a plurality of capacitors arranged in rows and columns.

Some capacitive sensors operate using two or more plates in silicon with a negative feedback as shown in <figref idref="f0001">Figures 1 and 2</figref>, taken from <patcit id="pcit0003" dnum="US5973623A"><text>U.S. Patent No. 5,973,623</text></patcit>. In the multiple plate system, the finger of the user changes the capacitance that exists between the two plates in silicon. A negative feedback circuit is used in the detection of whether a ridge or a valley is present in the fingerprint.

Some capacitive sensors operate using a single capacitor plate in silicon per sensor cell. This type of sensor is shown in <patcit id="pcit0004" dnum="US5325442A"><text>U.S. Patent No. 5,325,442</text></patcit>. <figref idref="f0002">Figures 3 and 4</figref> are prior art. Figures from this patent illustrate a single capacitive plate 14 and a switching device 16 to access the sensor cell. The finger of the user provides the other plate of the capacitor. The value of the capacitor is then sensed to determine the presence of a ridge or a valley.

One of the problems with the prior art capacitive fingerprint sensors is their level of sensitivity to distinguish a ridge from a valley. The difficulty of obtaining a good fingerprint pattern through capacitive sensing can vary from individual to individual. With some persons, depending upon their skin characteristics the pattern can be much more difficult to obtain than from other persons. It is therefore preferred to have an enhanced method of capacitive sensing which provides more uniform sensitivity from one individual to another.

According to principles of the present invention, a capacitive fingerprint sensing device is provided which places a variable voltage on the body of the individual whose fingerprint pattern is being sensed. The finger is thus performing the dual function of providing the variable charge transfer during input while also providing the variable capacitance to be sensed. The effect of performing both functions effectively multiplies the measurable effects of having a ridge or a valley at each sensor cell. The sensing is significantly enhanced in recognizing and distinguishing the difference between a ridge and a valley.

The electrical connection to the person's body can be made by any number of acceptable techniques. According to one preferred embodiment, an electrical contact is provided to the person's finger while it is placed on the fingerprint sensor pad. A logic control circuit connected to the electrical conductor places a step voltage on the electrical conductor according to a preset timing sequence in conjunction with the timing of the sensing. In this embodiment, there is a direct connection between the variable input voltage and the finger of the user for which the fingerprint is being sensed.

In a further embodiment, the voltage change is placed on the person's body by a capacitive transfer. A large plate capacitor in the silicon provides one plate to transfer charge to the finger to provide the change in voltage while the sensing occurs. Alternatively, the voltage change can be coupled to the body of the user either via another finger or some other appendage. For example, the connector can be in contact with the other hand of the person, their arm or any other acceptable portion of the body since all that is required is to place a change in charge on the person's body in order to obtain the enhanced capacitive sensing capability.

The invention can be used with sensor cells of the type having one capacitive plate in silicon and the finger providing the other plate, or with the type using multiple capacitor plates in silicon and the finger being a conductor that modifies their relative capacitance.

In one embodiment, the fingerprint or sensor circuit includes a negative feedback amplifier having the two plates of a feedback capacitor in silicon with a field therebetween. The field between the two plates is varied by the user's finger, whether ridge or valley. The effect of having a valley is that the sense capacitance will be greater than in the case of having a ridge. When a ridge is present, the feedback capacitance is reduced, as compared to a valley. In other words, the further the skin is from the plates of the capacitor, the less effect it has on reducing the effective capacitance between them and the sensed capacitance will be higher with a valley than with a ridge. On the other hand, the input capacitance will decrease with the greater distance of the skin from the sensor, such as is present at a valley. Since it is a ratio of the input capacitance to the sense (feedback) capacitance which determines the sensitivity of the circuit, a circuit which causes them to change in opposite directions results in a greatly multiplied effect of increasing the sensitivity of the measurement. Therefore, significant enhancement is provided by using the body as both the input capacitor and the variable plate to a capacitor whose value is being sensed.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic representation of a fingerprint sensor according to the prior art.</li><li><figref idref="f0001">Figure 2</figref> is an electrical schematic representation of an individual cell of the fingerprint sensor of <figref idref="f0001">Figure 1</figref> according to the prior art.</li><li><figref idref="f0002">Figure 3</figref> is a schematic representation of a fingerprint sensor circuit according to different prior art embodiment.</li><li><figref idref="f0002">Figure 4</figref> is an electrical schematic of a fingerprint sensor cell as used in the prior art of <figref idref="f0002">Figure 3</figref>.</li><li><figref idref="f0003">Figure 5</figref> is an electrical schematic of a fingerprint sensor according to principles of the present invention.</li><li><figref idref="f0003">Figure 6</figref> is a schematic representation of a fingerprint sensor according to principles of the present invention.</li><li><figref idref="f0004">Figure 7</figref> is an electrical schematic of an alternative embodiment of a fingerprint sensor according to principles of the present invention.</li><li><figref idref="f0005">Figure 8A</figref> is an electrical schematic of fingerprint sensor cells for aid in understanding the present invention.</li><li><figref idref="f0005">Figure 8B</figref> is an electrical schematic of fingerprint sensor cells illustrating one embodiment of the invention.</li><li><figref idref="f0006">Figure 9</figref> is an electrical schematic of a plurality of cells while sensing a fingerprint ridge and valley pattern according to an embodiment of the present invention.</li><li><figref idref="f0006">Figure 10</figref> is an electrical equivalent circuit showing the electrical interaction between various equivalent electrical components according to principles of the present invention.</li><li><figref idref="f0007">Figure 11</figref> is a top view of a housing containing a sensor according to the present invention.</li><li><figref idref="f0007">Figure 12</figref> is a partial cross-sectional view of the substrate of <figref idref="f0007">Figure 11</figref> taken along the lines 12-12.</li><li><figref idref="f0008">Figure 13</figref> is a partial cross-sectional view of a substrate and housing containing the invention.</li><li><figref idref="f0008">Figure 14</figref> is a partial cross-sectional view of an alternative substrate and housing of the present invention.</li><li><figref idref="f0009">Figures 15A and 15B</figref> are side views of a further alternative embodiments to place a voltage change on the finger of the user.</li><li><figref idref="f0010">Figures 16A and 16B</figref> illustrate alternative interference mechanisms which may increase the difficulty of properly reading a fingerprint of a user.</li><li><figref idref="f0011">Figure 17</figref> is an electrical schematic of the circuit according to principles of the present invention.</li></ul>

<figref idref="f0001">Figure 1</figref> illustrates a substrate I having a plurality of sensor cells 2 thereon. The sensor cells 2 are in the form of an array 3 and organized to sense a fingerprint pattern of a finger which is placed on top of the substrate. The array 3 is driven by horizontal scan electronics 5 and also by vertical scan electronics 6 so that each individual cell 2 is selected on a known timing sequence under control of the controller 7.

In particular, each sensor cell 2 includes electronics for sensing the presence or absence of a ridge or a valley of a fingerprint pattern for a finger which is placed on top of the substrate 1. The controller 7 includes a power supply circuit 120 which receives power from a power source 95. Each sensing cell 2 provides an output on line 17 of each individual column. The output is provided on the chip bus 9 which is provided to an output amplifier 8 and provided to the output of the sensor array 3 at terminal 10. The output of the sensor array 3 at terminal 10 is provided to fingerprint pattern recognition electronics which organizes the signal into a fingerprint pattern and compares it to other fingerprint patterns to perform recognition of the fingerprint pattern which is placed thereon. The specific details of the array 3 and interaction between the controller 7 to provide fingerprint pattern recognition is described in <patcit id="pcit0005" dnum="US5973623A"><text>U.S. Patent No. 5,973,623</text></patcit>, which is incorporated herein by reference. The fingerprint pattern recognition may also be obtained and performed by any acceptable technique as described in many other patents and publications which are available to those of skill in the art today.

<figref idref="f0001">Figure 2</figref> illustrates the electronics of a single sensor cell 2 which is acceptable for use in sensing the presence or absence of a ridge or valley according to the prior art. The cell shown in <figref idref="f0001">Figure 2</figref> is of the type that has two plates in the semiconductor substrate for each cell and the finger provides a third plate to modify the capacitance value of the existing capacitor. It can be considered a three plate cell, two plates in the substrate and one above. The specific electronics for the cell 2 are provided as only one example by which the cell can be constructed. Other acceptable techniques may also be used since the electronics of the individual cell are provided as an example only.

The cell 2 includes a dielectric layer 11 onto which a person's finger is placed. The dielectric layer 11 has a top surface 125 which comes in direct contact with the person's finger 18. The finger 18 will have a ridge or a valley at the position in which it comes in contact with the cell 2, the example shown for <figref idref="f0001">Figure 2</figref> including having a ridge thereon so that it is pressed against the substrate 11 and in contact with surface 125. The dielectric layer I 1 has a thickness D as shown in <figref idref="f0001">Figure 2</figref>.

A capacitor having plates 24 and 23 is against the back surface of the dielectric layer 11. The input plate of the cell 24 is connected to the input terminal of an inverting amplifier 13 and the output plate 23 is connected to the output of the inverting amplifier 13. A reset switch 19 is closed when sensing is not being performed to reset the inverting amplifier 13 to a known state. A capacitor 20 provides a step voltage N as applied through V_ref at terminal 21. The capacitor C_in is of a known value to provide a known charge to the input node 16 of inverting amplifier 13.

<figref idref="f0002">Figures 3 and 4</figref> illustrates another acceptable sensor array 3 and cell 2 which may be modified to use the present invention. The particular sensor cell 2 is known from prior art <patcit id="pcit0006" dnum="US5325442A"><text>U.S. Patent No. 5,325,442, to Knapp</text></patcit>, which is hereby incorporated herein by reference. In particular, the sensor cell of the 442 patent to Knapp to include a row driver 100 and a sensor circuit 102.

The cell 2 of <figref idref="f0002">Figures 3 and 4</figref> is of the type having one plate in the semiconductor substrate for each cell and the finger provides the second plate to form the capacitor with two plates. The row driver circuit couples to a row conductor 104 to the gate of an access transistor 106. One capacitor plate 108 in the substrate acts as a sense electrode. A finger 18 provides the other plate of the capacitor λ and the presence of a ridge or valley is determined based on the distance between the plate 108 and plate 18 that form the capacitor. The capacitance of the individual capacitors varies as a function of the spacing between the finger and the sense plate 108. The larger capacitance occurs where the finger ridge is adjacent and very close to the plate 108 and the smaller capacitances occurring where a valley is adjacent the plate 108, but spaced further from the plate because it is a valley.

The capacitive fingerprint sensor, whether the three plate cell of <figref idref="f0001">Figure 2</figref> or the two plate cell of <figref idref="f0002">Figure 4</figref>, operates on the principle that the amount of charge stored on a capacitor is proportional to the voltage multiplied by the capacitance. <maths id="math0001" num="(1)"><math display="block"><mi>q</mi><mo>=</mo><mi mathvariant="italic">vc</mi></math><img file="EP1990759A2_D0001.tif" /></maths> where q equals the amount of charge, ν equals the voltage on the capacitor, and C equals the capacitance value. As is known, the capacitance value of a capacitor varies with the distance of the plates from each other. In a fingerprint pattern sensor, the fingerprint, whether a ridge or a valley, acts as a plate of the capacitor. The distance varies in the fingerprint, small for a ridge and large for a valley, the capacitance is also different. In a three plate sensor, such as <figref idref="f0001">Figure 2</figref> shows, the capacitance value also varies depending upon the fringing effect between the two capacitor plates in the substrate as described by the later equations herein.

The value of the charge stored or the rate of charge stored can be measured and sensed in one embodiment, if desired. This is done in some devices by placing a current or voltage on the capacitor and obtaining a measurement corresponding to the stored charge.

A variation on this method is to measure a change in output voltage based on a change of charge on the capacitor. The basic equation for output voltage sensing can be obtained from equation 1 as follows: <maths id="math0002" num="(2)"><math display="block"><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mi>q</mi><mi>c</mi></mfrac></math><img file="EP1990759A2_D0002.tif" /></maths> where <i>V_out</i> is the output voltage from the cell 2 and q is the charge, and C is the capacitive value.

It is often desirable to measure a change in <i>V_out</i> from a known reset of reference value rather than measure <i>V_out</i> as an absolute value. For cells in which a change in voltage is measured to sense a ridge or a valley, the following equation applies: <maths id="math0003" num="(3)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>q</mi></mrow><msub><mi>C</mi><mi>s</mi></msub></mfrac></math><img file="EP1990759A2_D0003.tif" /></maths> in which <i>ΔV_out</i> equals the change in voltage on terminal 17, Δ<i>q</i> equals the change in charge on the capacitance, and <i>C_s</i> is the capacitance being sensed, namely, the capacitance of the capacitor plates in the sensor with the finger 18 adjacent thereto, whether a ridge or a valley. The presence of a ridge and a valley will cause a variation in the changes in the capacitance. In particular, as described later herein, the presence of a ridge or a valley near sensor cell 2 will cause the capacitance being sensed to be reduced or increased, respectively, so that a ridge and a valley can be distinguished.

<figref idref="f0003">Figures 5, 6</figref> and <figref idref="f0004">7</figref> illustrate, respectively, alternate embodiments according to the present invention. In each of the <figref idref="f0003 f0004">Figures 5-7</figref>, a dielectric layer 11 is positioned over a sensor cell 2. Directly on top of the dielectric layer 11, a finger 18 of a user is positioned for sensing the fingerprint pattern. Of course, the term fingerprint as used herein includes the variations in skin texture on any body part of a person, such as a thumb, a toe, a foot, a forehead, or any other surface whose variations are being sensed to identify a person. In the examples shown in <figref idref="f0003 f0004">Figures 5-7</figref>, the finger 18 has a ridge directly over the sensor cell 2. Accordingly, the finger appears as a single flat member against the substrate 11. Since, in most cases, a sensor cell 2 will be smaller than the width of a ridge it will appear, from the view of the cell 2, that the finger 18 is abutting against the top surface of the dielectric layer 11. In other figures, such as <figref idref="f0005">Figures 8A, 8B</figref>, <figref idref="f0006">9</figref>, <figref idref="f0009">15A and 15B</figref>, a plurality of different sensor cells 2 are shown to illustrate the response of the array to different parts of the fingerprint, whether a ridge or a valley.

The operation of the circuit of <figref idref="f0003">Figure 5</figref> is illustrated schematically in <figref idref="f0005">Figures 8A and 8B</figref>, as will now be explained. The circuit of <figref idref="f0003">Figures 6</figref> and <figref idref="f0004">7</figref> is illustrated schematically in <figref idref="f0006">Figures 9 and 10</figref> and they will be discussed in more detail with respect to these two figures.

<figref idref="f0005">Figures 8A and 8B</figref> illustrate the sensing principles to illustrate the present invention using a sensor cell of the type generally shown in <figref idref="f0003">Figure 5</figref>. <figref idref="f0005">Figure 8A</figref> shows a two plate capacitor system for each cell, the finger 18 being one plate and the plate 14 being the sensor electrode and other plate. Two cells are shown, one at a ridge and the other at a valley.

While in the ideal case, <i>C_s</i> is the only capacitance which effects the equation such that equation (3) is a direct characterization solely of the finger being sensed output, in reality, there are various parasitic capacitance which occur in different parts of the circuit. If the effect of all the parasitic capacitance at the various parts of the circuit are combined, they are summed with the capacitance being sensed as shown by equation (4) below: <maths id="math0004" num="(4)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>q</mi></mrow><mrow><msub><mi>C</mi><mi>s</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0004.tif" /></maths> where <i>C_p</i> is the parasitic capacitance affecting the cell output. Unfortunately, the parasitic capacitance <i>C_p</i> can sometimes be quite large. Depending on the particular circuit design <i>C_p</i> may be much larger than <i>C_s.</i> In some cases it may be over ten times larger. If <i>C_p</i> is too large, then it will dominate the equation such that changes in <i>C_s</i> cause only a very small change in the overall capacitance being sensed as can be seen by viewing equations (2) and (4) together. Accordingly, if the parasitic capacitance is much, much larger than the capacitance being sensed, then the sensitivity of cell 2 is reduced. In most applications, the capacitance being sensed is extremely small since it is based on detecting merely the difference in capacitance value between a ridge and a valley in a single fingerprint.

The sensitivity of the sensor can be defined based on the ability to distinguish between the maximum signal output and the minimum signal output. As an equation, sensitivity can be expressed as <maths id="math0005" num="(5)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out max</mi></msub><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0005.tif" /></maths> where δ is the sensitivity, <i>ΔV_outmax</i> is the highest possible output voltage and <i>ΔV_outmin</i> is the lowest possible output voltage during a measurement. The sensitivity thus increases with a larger difference between the maximum and minimum changes in output voltages and by reducing the minimum change in output voltage to the lowest possible value.

In the circuit of <figref idref="f0005">Figure 8A</figref>, the output voltage will be different from cell 1 to cell 2 based on the difference between <i>C_{plate ridge}</i> and <i>C_{plate valley}.</i> All other aspects of the cells will be the same. Thus, both cells will have the same <i>C_in</i> value and the same <i>C_p</i> value.

Viewing now <figref idref="f0005">Figure 8A</figref>, the change in output voltage can be defined by: <maths id="math0006" num="(6)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate body</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0006.tif" /></maths> where <i>ΔV_in</i> is the change in input voltage during sensing, <i>C_in</i> is the input capacitor, <i>C_p</i> is the parasitic capacitor and <i>C_plate body</i> is the capacitance that is composed of the plate 14 and the body 18.

It is, of course, desired to distinguish for the body, <i>C_{plate body},</i> the difference between a ridge and a valley, defined as <i>C_{plate ridge}</i> and <i>C_{plate valley},</i> respectively.

The plate capacitor value is at its maximum for a ridge and its minimum for a valley, so equation (6) becomes: <maths id="math0007" num="(7)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>max</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate valey</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0007.tif" /></maths> for a valley over the cell, as shown for cell 2. For a ridge, the results are: <maths id="math0008" num="(8)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0008.tif" /></maths>

Since <i>C_{plate ridge}</i> is larger than C_{plate valley} by some factor A, these equations can be simplified by stating that: <maths id="math0009" num="(9)"><math display="block"><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>=</mo><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></math><img file="EP1990759A2_D0009.tif" /></maths> where <i>ΔC_{plate ridge}</i> is the difference in capacitance between a valley and a ridge. Substituting this into equation (9) provides: <maths id="math0010" num="(10)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0010.tif" /></maths> It is desired to understand the overall sensitivity of the system, which was stated in equation (5) and is provided again below. <maths id="math0011" num="(11)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out max</mi></msub><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0011.tif" /></maths> Substituting equation (7) and (10) for the above values, equation (12) is obtained: <maths id="math0012" num="(12)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">palte ridge</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0012.tif" /></maths> It can be seen that the effect of both parasitic capacitance <i>C</i>_p and input capacitance <i>C</i>_in is to reduce effective sensitivity. We can define idealized sensitivity for the plate, <i>δ_plate,</i> (in the absence of parasitics) to be: <maths id="math0013" num="(13)"><math display="block"><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mn>.</mn></math><img file="EP1990759A2_D0013.tif" /></maths> For simplicity from an algebra standpoint, a value a will be set equal to <maths id="math0014" num="(14)"><math display="block"><mi>α</mi><mo>=</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mn>.</mn></math><img file="EP1990759A2_D0014.tif" /></maths> If equations (13) and (14) are substituted into equation (12). The result is: <maths id="math0015" num="(15)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mfrac><mo>⋅</mo><mfrac><mn>1</mn><mrow><mn>1</mn><mo>+</mo><mi>α</mi></mrow></mfrac><mo>=</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>⋅</mo><mfrac><mn>1</mn><mrow><mn>1</mn><mo>+</mo><mi>α</mi></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0015.tif" /></maths> In a semiconductor substrate, <i>C_in</i> can be carefully controlled and can be reduced to a very low value so as to be negligible, approaching zero. On the other hand, <i>C_p</i> will be over 10 times greater than <i>C_{plate valley},</i> may be up to 20 to 100 times greater. <i>Cp</i> is difficult to reduce to zero, so we assume <i>C_p</i> equals 10 x <i>C_{plate valley.}</i>

This provides <maths id="math0016" num="(16)"><math display="block"><mi>α</mi><mo>=</mo><mfrac><mrow><mn>0</mn><mo>+</mo><mn>10</mn></mrow><mn>1</mn></mfrac><mo>=</mo><mn>10</mn></math><img file="EP1990759A2_D0016.tif" /></maths> and so resulting effective sensitivity is: <maths id="math0017" num="(17)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mn mathvariant="italic">11</mn></mfrac><mn>.</mn></math><img file="EP1990759A2_D0017.tif" /></maths>

Thus, for this type of sensing, the sensitivity is reduced to the plate sensitivity <i>δ_plate,</i> divided by 11. The effect of <i>C_p</i> thus causes a significant reduction in sensitivity. As <i>C_p</i> becomes higher, the sensitivity will reduce even further.

<figref idref="f0005">Figure 8B</figref> illustrates one embodiment of the present invention showing how applying <i>V_in</i> to the body can increase the sensitivity and reduce the effects of <i>C_p.</i> In <figref idref="f0005">Figure 8B</figref>, the step voltage <i>ΔV_in</i> is applied to the finger 18 and the body acts as capacitor <i>C_in.</i> In this case, the value of <i>C_in</i> will be different for each cell, depending on whether a valley or a ridge is present over the cell. For cell 1, a ridge, the value <i>C_in</i> will be higher for a ridge and lower for a valley. The capacitance change caused by ridge or valley being present can be effectively considered as multiplying the change in output since it affects both the input signal and the sense signal. The results can be seen from the following equations: <maths id="math0018" num="(18)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>•</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">plate body</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0018.tif" /></maths>

Recalling that <i>C_{plate ridge}</i> is larger than <i>C_{plate valley}</i> by a value <i>ΔC_{plate ridge}</i> and that <i>δ_plate</i> is defined as <maths id="math0019" num=""><math display="inline"><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mo>,</mo></math><img file="EP1990759A2_D0019.tif" /></maths> the following equations are obtained: <maths id="math0020" num="(19)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>max</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⁢</mo><mfenced separators=""><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mfenced></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0020.tif" /></maths><maths id="math0021" num="(20)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mfenced><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfenced></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0021.tif" /></maths><maths id="math0022" num="(21)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out max</mi></msub><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow></mfrac><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mo>•</mo><mfrac><msub><mi>C</mi><mi>p</mi></msub><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0022.tif" /></maths>

Equation (21) shows that <i>C_p</i> is in both the numerator and denominator. Thus the effect of <i>C_p</i> will be to cancel itself out, towards a value equal to 1 in the equation. Even if <i>C_p</i> becomes large, the result will be to cancel out its effect on the sensitivity since it is in both top and bottom in the ratio. Defining <maths id="math0023" num="(22)"><math display="block"><mi mathvariant="italic">β</mi><mo>=</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi>p</mi></msub></mfrac></math><img file="EP1990759A2_D0023.tif" /></maths><maths id="math0024" num="(23)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mo>⁢</mo><mfenced><mfrac><msub><mi>C</mi><mi>p</mi></msub><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></mfenced><mo>=</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>•</mo><mfrac><mn>1</mn><mrow><mn>1</mn><mo>+</mo><mi mathvariant="italic">β</mi></mrow></mfrac></math><img file="EP1990759A2_D0024.tif" /></maths>

Thus, if we assume again that <i>C_p=</i>10<i>C_{plate valley},</i> along with the further assumption that C_p≈ΔC_{plate ridge}, the results obtained from the above equations show that <maths id="math0025" num="(24)"><math display="block"><mfrac><mn>1</mn><mrow><mn>1</mn><mo>+</mo><mi mathvariant="normal">β</mi></mrow></mfrac><mo>≈</mo><mfrac><mn>1</mn><mn>2</mn></mfrac></math><img file="EP1990759A2_D0025.tif" /></maths> so that: <maths id="math0026" num="(25)"><math display="block"><mi mathvariant="normal">δ</mi><mo>=</mo><mfrac><msub><mi mathvariant="normal">δ</mi><mi mathvariant="italic">plate</mi></msub><mn>2</mn></mfrac><mn>.</mn></math><img file="EP1990759A2_D0026.tif" /></maths>

Equation (25) shows that the invention results in a 5-times improvement in sensitivity for the invention applied to the simple case of <figref idref="f0005">Figure 8B</figref>.

<figref idref="f0003">Figure 5</figref> shows a circuit of a single cell that takes advantage of the principles of the invention that have just been shown in the example of <figref idref="f0005">Figure 8B</figref>. A select transistor 112, coupled to a plate 14 having a finger 18 thereon provides an output <i>V_out. A</i>. voltage terminal 21 having a step voltage input signal <i>V_in</i> is connected to the finger 18.

<figref idref="f0003">Figure 6</figref> illustrates another example of fingerprint sensing according to principles of the present invention. The sensor cell as shown in <figref idref="f0003">Figure 6</figref> is a further example that provides even more remarkable results than the single plate sensor cell of <figref idref="f0003">Figure 5</figref> as explained herein as will be clear from the technical discussion.

According to principles of the present invention, during the sensing operation, an electrical conductor 21 is connected to the body of the person whose fingerprint is being sensed. It may be coupled at any convenient location, such as the finger itself whose print is being sensed, the hand of the user, the unused hand of the user, the foot, via a conductive foot pad, or other location. Electrical conductor 21 has a terminal 99 connected to a variable voltage supply Δ<i>V_in</i>. The Δ<i>V_in</i> is directly controlled in a timing sequence with electronic controller 7 to place a known change of voltage on the body and thus on the finger 18 in conjunction with the timing of sensing in each individual cell 2. In particular, at the beginning of sensing, the <i>ΔV_in</i> will normally be a low value and, once sensing has begun will undergo step voltage to high value as shown by the signal wave 114 applied to the terminal Δ<i>V_in</i>. After the sensing has occurred while the voltage <i>ΔV_in</i> is at a high value then the cell 2 will be reset and the electrical conductor 21 will be returned to a low value so as to place the both the cell and the body in a reset mode in preparation for the next sensing cycle at which time <i>ΔV_in</i> undergoes another step value upwards as shown by signal wave 114 in <figref idref="f0002">Figure 4</figref>.

The step on Δ<i>V_in</i> is timed in conjunction with the timing sequence of a clock signal ϕ_r which controls the opening and closing of switch 19. Namely, switch 19 is opened and after which the <i>ΔV_in</i> undergoes the step function upwards so that the change in voltage Δ<i>V_in</i> occurs when switch 19 is open. This creates a change on input node 25 to inverting amplifier 13 which creates a change in voltage on <i>V_out</i> corresponding to <i>ΔV_out</i> as discussed in the prior equations.

The three plate capacitor system of <figref idref="f0001">Figures 2</figref>, <figref idref="f0003">6</figref>, <figref idref="f0004">7</figref>, <figref idref="f0006">9, and 10</figref> causes a change in the output voltage by two effects, the plate effect, already discussed, and the fringing effect between the two plates 23 and 24. For the fringing effect, the presence of a ridge will greatly reduce the fringe capacitive between plates 23 and 24, whereas a valley will cause little or no reduction. Having a conductor adjacent the plates of the capacitor will ground some of the field lines between the capacitance plates. On the other hand, if a valley is present over sensor cell 2, then the sensed capacitance Cs will be greater than that of a ridge since a valley has a conductor spaced farther from the cell 2. In the ideal sense, the reduction of capacitance may be zero though, in a real sense, even a valley may cause some slight reduction in the capacitance value. Nevertheless, the general principle applies for a multiple plate capacitive sensor that Cs will be larger with a greater distance of the finger 18 from the cell such as occurs at a valley and Cs will decrease with the finger closer to the cell such as occurs at a ridge. Namely, the sensed plate capacitance Cs for a valley is greater than for a ridge.

Comparing the plate effect and the fringe effect provides: <maths id="math0027" num="(26)"><math display="block"><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>&gt;</mo><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></math><img file="EP1990759A2_D0027.tif" /></maths><maths id="math0028" num="(27)"><math display="block"><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>&lt;</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub><mn>.</mn></math><img file="EP1990759A2_D0028.tif" /></maths>

As previously stated, <maths id="math0029" num="(9)"><math display="block"><msub><mi mathvariant="normal">C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>=</mo><msub><mi mathvariant="normal">C</mi><mi>plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi mathvariant="normal">C</mi><mi mathvariant="italic">plate ridge</mi></msub></math><img file="EP1990759A2_D0029.tif" /></maths> and <maths id="math0030" num="(13)"><math display="block"><msub><mi mathvariant="normal">δ</mi><mi mathvariant="italic">plate</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mn>.</mn></math><img file="EP1990759A2_D0030.tif" /></maths>

For the fringe capacitance, between plates 23 and 24, it can also be stated that: <maths id="math0031" num="(28)"><math display="block"><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub><mo>=</mo><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub></math><img file="EP1990759A2_D0031.tif" /></maths> and that: <maths id="math0032" num="(29)"><math display="block"><msub><mi mathvariant="normal">δ</mi><mi mathvariant="italic">fringe</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mfrac></math><img file="EP1990759A2_D0032.tif" /></maths> where <i>δ_fringe</i> is the idealized sensitivity between plates 23 and 24 for the fringe capacitor.

Viewing <figref idref="f0001">Figure 2</figref>, the prior art results of the sensing with <i>V_in</i> applied to the capacitor <i>C_in</i> can be seen in the equations: <maths id="math0033" num="(30)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe body</mi></msub></mfrac></math><img file="EP1990759A2_D0033.tif" /></maths><maths id="math0034" num="(31)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>max</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mfrac></math><img file="EP1990759A2_D0034.tif" /></maths><maths id="math0035" num="(32)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">in</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0035.tif" /></maths>

When these equations are put into the sensitivity equation, (11), the result is: <maths id="math0036" num="(33)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>max</mi></mrow></msub><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub></mrow></mfrac><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">fringe valley</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mfrac><mn>.</mn></math><img file="EP1990759A2_D0036.tif" /></maths>

Turning now to <figref idref="f0003">Figure 6</figref>, in which the invention is used to apply the input voltage to the body. The fringe capacitor, <i>C_fringe</i> is represented by field lines 39 and <i>C_plate</i> by field lines 37, the results obtained will be: <maths id="math0037" num="(34)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">platebody</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringebody</mi></msub></mfrac></math><img file="EP1990759A2_D0037.tif" /></maths> recalling the equations for <i>C_{fringe ridge}</i> and <i>C_{fringe valley},</i> the following is obtained: <maths id="math0038" num="(35)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>max</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⁢</mo><mfenced separators=""><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mfenced></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mfrac></math><img file="EP1990759A2_D0038.tif" /></maths><maths id="math0039" num="(36)"><math display="block"><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mrow><mi mathvariant="italic">out</mi><mspace width="1em" /><mi>min</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mfenced><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfenced></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0039.tif" /></maths><maths id="math0040" num="(37)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out max</mi></msub><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out min</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0040.tif" /></maths><maths id="math0041" num="(38)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mfrac><mo>+</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mfrac><mo>+</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe valley</mi></msub><mo>⋅</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><mrow><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0041.tif" /></maths><maths id="math0042" num="(39)"><math display="block"><mi>δ</mi><mo>=</mo><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub><mo>+</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>+</mo><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub><mo>⋅</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mn>.</mn></math><img file="EP1990759A2_D0042.tif" /></maths>

As can be seen, the idealized sensitivity increases greatly over the prior art.

Turning now to <figref idref="f0004">Figures 7</figref>, <figref idref="f0006">9 and 10</figref>. A further embodiment of a circuit is shown in <figref idref="f0004">Figure 7</figref>, and theoretical schematics are shown in <figref idref="f0006">Figures 9 and 10</figref>. In this embodiment, an additional compensation voltage Δ<i>V_c</i> is applied to a filter capacitor <i>C_f</i> with the voltage <i>ΔV_c</i> being opposite the wave form <i>ΔV_in.</i>

In the non-ideal case, the grain G of amplifier 13 is finite and, as shown in <figref idref="f0006">Figure 10</figref>, there are parasite capacitors <i>C_p</i> on the input and output nodes to the amplifier 13.

Since the body is being used as the input capacitor, the value of the <i>C_plate</i> contribute to the total input node capacitance. <maths id="math0043" num="(40)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>⋅</mo><msub><mi>C</mi><mi mathvariant="italic">plate body</mi></msub></mrow><mrow><mfrac><mrow><mi>G</mi><mo>+</mo><mn>1</mn></mrow><mi>G</mi></mfrac><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe body</mi></msub><mo>+</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">plate body</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow><mi>G</mi></mfrac></mrow></mfrac></math><img file="EP1990759A2_D0043.tif" /></maths> for a very, very large G, one of the factors goes to 1 and the other factor goes to zero so: <maths id="math0044" num="(41)"><math display="block"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">out</mi></msub><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mo>•</mo><msub><mi>C</mi><mi mathvariant="italic">plate body</mi></msub></mrow><msub><mi>C</mi><mi mathvariant="italic">fringe body</mi></msub></mfrac><mn>.</mn></math><img file="EP1990759A2_D0044.tif" /></maths>

Substituting from prior equations provides <maths id="math0045" num="(42)"><math display="block"><mi>δ</mi><mo>=</mo><mfrac><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mrow><mn>1</mn><mo>+</mo><mi>σ</mi></mrow></mfrac><mo>+</mo><mfrac><mrow><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>•</mo><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub></mrow><mrow><mn>1</mn><mo>+</mo><mi>ε</mi></mrow></mfrac><mo>+</mo><mfrac><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub><mrow><mn>1</mn><mo>+</mo><mi>ε</mi></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0045.tif" /></maths>

As G becomes large then σ goes toward zero and ε goes toward zero.

The invention thus has the affect, in the non-ideal case of canceling out completely the parasitic capacitance as well as increasing the sensitivity as the gain G of amplifier 13 goes higher. In the above equation (42), <maths id="math0046" num="(43)"><math display="block"><mi>σ</mi><mo>=</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub></mrow><mrow><mfenced separators=""><mi>G</mi><mo>+</mo><mn>1</mn></mfenced><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow></mfrac></math><img file="EP1990759A2_D0046.tif" /></maths> and <maths id="math0047" num="(44)"><math display="block"><mi>ε</mi><mo>=</mo><mfrac><mrow><msub><mi>C</mi><mi mathvariant="italic">plate valley</mi></msub><mo>+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">plate ridge</mi></msub><mo>+</mo><msub><mi>C</mi><mi>p</mi></msub></mrow><mrow><mfenced separators=""><mi>G</mi><mo>+</mo><mn>1</mn></mfenced><mo>⁢</mo><msub><mi>C</mi><mi mathvariant="italic">fringe ridge</mi></msub></mrow></mfrac><mn>.</mn></math><img file="EP1990759A2_D0047.tif" /></maths>

The result obtained, even in the non-ideal case with a large, but finite gain G, thus approaches: <maths id="math0048" num="(45)"><math display="block"><mi>δ</mi><mo>=</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>+</mo><mo>⋅</mo><msub><mi>δ</mi><mi mathvariant="italic">plate</mi></msub><mo>⋅</mo><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub><mo>+</mo><msub><mi>δ</mi><mi mathvariant="italic">fringe</mi></msub><mn>.</mn></math><img file="EP1990759A2_D0048.tif" /></maths>

<figref idref="f0004">Figure 7</figref> illustrates an alternative embodiment for a circuit according to principles of the present invention. Now that the mathematical calculations have been shown, the circuit of <figref idref="f0004">Figure 7</figref> will be described in more detail. Namely, the circuit of <figref idref="f0004">Figure 7</figref> is different from <figref idref="f0003">Figure 6</figref> in that a separate capacitor <i>C_f</i> is present on the input 25. This separate capacitor <i>C_f</i> acts as a compensation capacitor or filter to the input node 25 to enhance and further increase the sensitivity. One plate of the capacitor <i>C_f</i> is connected to the sense node 25 whereas the other plate of the capacitor is connected to a voltage source <i>V_c</i> representing a compensation voltage. The voltage source <i>V_c</i> also undergoes a change in voltage according to a known timing sequence and thus is shown as a <i>ΔV_c.</i> In a preferred embodiment, the compensation voltage <i>ΔV_c</i> undergoes a change in voltage which is exactly opposite that of Δ<i>V_in</i> which is placed on the body. Namely, the signals <i>ΔV_in</i> and <i>ΔV_c</i> will normally be the inverse of each other such that when Δ<i>V_in</i> undergoes a step function up, Δ<i>V_c</i> will undergo step down. According to principles of the present invention, the magnitude and absolute direction of Δ<i>V_in</i> and <i>ΔV_c</i> is not critical; a change in voltage of some magnitude is desired. Namely, the <i>ΔV_in</i> shown in <figref idref="f0004">Figure 7</figref> as undergoing a step function from a low signal to a high signal could, of course, undergo a change from a low signal to a negative voltage, or from a high signal to a low signal to place the change in voltage on the finger 18. Similarly, <i>ΔV_in</i> can transition from low to high and from high to low, respectively, as shown in <figref idref="f0004">Figure 7</figref> or can transition from high to low and from low to high, respectively, in order to provide the enhanced sensing according to principles of the present invention.

<figref idref="f0006">Figures 9 and 10</figref> will be used to illustrate the theory of the operation. As will be appreciated, the mathematical equations which characterize the human body, and the electrical interaction between the human body and the sensor cell are very complicated if all factors and parasitics are included therein. Accordingly, not all possible factors and parasitics are shown and a simplified electrical explanation is provided that includes the major factors so that the basic operation of the invention can be understood.

As shown in <figref idref="f0006">Figure 9</figref>, the terminal 99 has applied thereto a change in voltage <i>ΔV_in</i> and the terminal 29 also as applied thereto, a change in voltage Δ<i>V_c</i> as previously described with respect to <figref idref="f0004">Figure 7</figref>. Viewing now the connection between the <i>ΔV_in</i> at terminal 99 and the actual finger 18, it can be seen that the human body has a certain impedance shown as Z BODY 31. The impedance of the body 31 is characterized according to a number of different electrical equations, and is sufficient for purposes of the invention to view the overall impedance of the body whether composed of various capacitors, resistors or combinations thereof as the overall body impedance 31. The body also has a resistance to ground 33 which varies greatly from one person to another and also varies based on the position of connection to the body. As can be appreciated, in some embodiments the resistance to ground 33 characterized by resistor R<i>_g</i> can be extremely large.

For example, if the person is electrically insulated from ground, such as having thick rubber soles, then the resistance R_g will approach infinity so that in fact, the person is not grounded and, from an electrical voltage potential, the voltage on the body is floating. In other embodiments, the resistance R<i>_g</i> will be extremely small, because the body is connected directly to ground. For example, if the human skin is in direct electrical contact with the grounding electrode, then resistance to ground 33 will be very small. Accordingly, the value for resistor Rg may vary from one circumstance to another.

<figref idref="f0006">Figure 9</figref> shows two sensor cells 2 which are spaced from each other in different parts of array 3. For ease of illustration, the first cell 2 is shown with a ridge directly over the cell 2, whereas the second cell is shown with a valley over the cell 2 so that the differences in operation can be easily explained and understood. Concerning first the situation in which a ridge of the finger 18 is directly over the cell 2. In this situation, the distance between the skin is D₁ since the skin is in direct contact with the upper surface 125 of the dielectric layer 11. In this instance, the effect of the ridge 18 is to reduce the value of the fringe capacitance between plates 24 and 23 of the negative feedback amplifier 13. In particular, the capacitance between plates 24 and 23 is shown by the fringe field lines 35 with fewer field lines illustrated in a reduction in the capacitance value. Namely, since the ridge 18 is close to the plates 24 and 23, some of the field lines are interfered with by the finger 18 so that the overall capacitance value between 24 and 23 is reduced due to the fringing effect of an adjacent conductor. This is represented by some of the field lines 35 going into the finger 18 rather than extending between capacitor plate 24 and 23.

The input capacitance, which is an active part of the sensing circuit, also has effect on the sensor cell 2. Namely, the input capacitor <i>C_in</i> is also variable as coming from the body will go high at a ridge, as compared to a valley. Namely, <i>C_in</i> has its maximum value at a ridge. <i>C_s,</i> on the other hand, goes down at a ridge, namely <i>C_s</i> will have its minimum value at a ridge. The recognition that a ridge has an opposite effect on the two capacitance values, namely that it causes a decrease in the fringe capacitance <i>C_s,</i> which is sensed and an increase in the input capacitance is a principle of the present invention, which is used to provide enhanced sensitivity for fingerprint pattern recognition.

According to the mathematical equations for sensing the fingerprint pattern, C<i>_in</i> and C<i>_s</i> are a ratio which is used for changing the output value V<i>_out</i>. Thus, if C<i>_s</i> and C<i>_in</i> change in opposite directions, this will have the effect of greatly changing the ratio, particularly, over the change that would occur if only one of the values C<i>_s</i> or C<i>_in</i> were to change. In particular, according to principles of the present invention, the body itself is used as the input capacitor as well as for the sense capacitor so that enhanced sensitivity is obtained, as will be explained with respect to the following equations. For understanding the charge change which takes place on the body when the voltage step <i>ΔV_in</i> is applied thereto, it can be appreciated that: <maths id="math0049" num="(46)"><math display="block"><msub><mi mathvariant="normal">Q</mi><mi mathvariant="italic">in</mi></msub><mo>=</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msub><mi>V</mi><mi mathvariant="italic">in</mi></msub><mfenced><msub><mi mathvariant="normal">C</mi><mi mathvariant="italic">in</mi></msub></mfenced></math><img file="EP1990759A2_D0049.tif" /></maths> where <i>Q_in</i> is the amount of charge which is placed on the body, and thus corresponds to <i>Δ_q</i> when the body undergoes a change in voltage. <i>ΔV_in</i> is the step function change in voltage as applied to terminal 99 and <i>C_in</i> is the value of the input capacitance.

According to the prior art, the value <i>C_in</i> was fixed so that Δ<i>V_in</i> x <i>C_in</i> created a step function change of the amount of charge applied at node 16. Namely, the value <i>Q_in</i> was always a known value for each cell and did not vary. On the other hand, if the body is used for <i>C_in,</i> when the body undergoes a step function <i>ΔV_in</i> the amount of charge provided at each individual cell will be different from that provided to other cells since the value locally at the individual cell will be different for each capacitor <i>C_in.</i> Accordingly, the amount of charge transferred to the input node 25 will also vary for each individual cell. As will be appreciated, the actual local capacitance of each individual cell will not be precisely known, and will vary over a wide range. For principles of the present invention, we can now consider two extremes, namely, the input capacitance of the valley <i>C_valley</i> and the capacitance of a ridge at <i>C_ridge.</i> The capacitor <i>C_in</i> will be at a maximum value while at a ridge with the skin in direct contact with the dielectric layer 11. On the other hand, <i>C_in</i> will go down, and be at a minimum value while at a valley. This can be understood according to the equations C<i>_in</i> = <i>min</i> at valley and C<i>_in</i> = <i>max</i> at a ridge. Since the fingerprint pattern of an individual varies between ridge and valleys, it will be appreciated that the actual input capacitance will also vary somewhat from the two extremes of the minimum and maximum value.

Turning back now to <figref idref="f0006">Figure 9</figref>, over the second cell 2b a valley is present. In this circumstance, the finger 18 is separated by a distance d₂ from the upper surface 125 of dielectric layer 11. Since a valley is curved, the actual shape may vary from a normal distance d₂ to a maximum distance and an intermediate distance. Taking the case in which the valley is quite large, there will be no interference between the fringe capacitive field lines 35 that extend between the plates 24 and 23 of the feedback capacitor to the negative amplifier 13. In particular, all of the fringe field lines 35 which existed between plates 24 and 23 still exist when the valley is adjacent the cell such that it does not interfere with the capacitance value in any way. The capacitance value C<i>_s</i> is therefore at a maximum value for the presence of a valley. On the other hand, the input capacitance is represented by field lines 37 is at a minimum value. Namely, with the valley present and the finger 18 spaced a far distance from the plate 24, the input capacitance is very small, and for a large valley is at a minimum value. This is to be compared with the ridge of cell 2a at which the input capacitance was large and there were a large number of field lines 37, as shown.

<figref idref="f0006">Figure 10</figref> illustrates an electrical schematic showing equivalent circuit of some components which will exist under normal operating conditions. In particular, from an electrical equivalent standpoint, the input node 25 will have a parasitic capacitor 41 coupled thereto. In addition, output node 17 will have a parasitic capacitor 43 coupled thereto. These parasitic capacitors <i>C_p</i> will have an effect on the electrical operation of the circuit. Further, the electrical equivalent of the negative feedback amplifier 13 is a simple inverter 13, as shown in <figref idref="f0004">Figure 7</figref>, assuming, the situation of an ideal negative amplifier with infinite gain. In this situation, a number of field lines 35 extend between plates 24 and 23 of the capacitor, which provides the negative feedback from the output 17 to the input 25. The field lines 37 represent the value and strength of the input capacitance provided from the body being used as the input capacitor. As previously described, there is applied to terminal 99 a step voltage <i>ΔV_in</i> and to terminal 21 a step voltage Δ<i>V_c</i> of opposite polarity type. The actual values of Δ<i>V_in</i> and Δ<i>V_c</i> are not of particular importance and in many circumstances, they will be a different value. In alternative embodiments, they are inverse signals of each other, such that they vary exactly the opposite in timing and in magnitude.

The plate 43 for the capacitance of the body as represented by the finger 18 in contact with the substrate 1. The step voltage Δ<i>V_in</i> is connected to capacitance of the body via Z BODY, as previously shown with respect to <figref idref="f0003">Figure 6</figref>. When the body has a conductor, such as a portion of the skin which is very closely adjacent the plates 24 and 23 of the feedback capacitor, then the plate 43 of the capacitance of the body has a different effect than when the conductor of the body is spaced farther from the plates 24 and 23. In particular, when a ridge is present, then the input field lines 37 are significantly increased so that the capacitance 43 of the body and the overall body conductor has the effect of causing an increase in the input capacitance at node 25. At the same time, since the same body portion is adjacent the capacitor plates 24 and 23 for the same cell for which sensing is occurring, the same effect will cause a significant decrease in the field lines 35 passing from plates 24 and 23 as illustrated with respect to <figref idref="f0003">Figure 6</figref>. Namely, the effect of using the body for both the input capacitance and sense capacitance will be opposite, in one case causing an increase in the input capacitance and in the other case causing a decrease in the sense capacitance. Since these two portions of the equations are a ratio with respect to each other, the overall sensitivity of the measurement is significantly enhanced. Similarly, if the body capacitance 43 has a valley adjacent the cell 2, it will have a reduced input capacitance represented by reduced field lines 37 and an increased capacitance between plates 24 and 23, since the valley does not interfere with the field lines 35, again shown in <figref idref="f0003">Figure 6</figref>. The parasitic capacitance 41 and 43 thus have an overall reduced effect on the circuit.

<figref idref="f0007">Figures 11 and 12</figref> illustrate one acceptable technique for placing the change in voltage <i>ΔV_in</i> on the human body while the fingerprint sensing is occurring. The substrate 1 is included within a housing 60. Positioned on the face of the substrate 1 are a plurality of electrical conductors 30, as shown in <figref idref="f0005">Figures 8</figref> and <figref idref="f0006">9</figref>. The conductors 30 and 31 are electrically conductive strips formed in the upper surface of the substrate 1. In particular, these electrical conductors will be in direct electrical contact with the finger 18 of the user that is placed on the substrate 1.

According to principles of the present invention, the electrical conductors 30 and 31 have applied thereto the step voltage <i>ΔV_in</i> or, a grounding voltage, depending on the timing of the sense operation and the location of the cell being sensed relative to the electrical conductors 30, 31. One set of conductors 30 has the step voltage Δ<i>V_in</i> applied thereto so as to place Δ<i>V_in</i> on the body, according to the timing sequences previously described. The other conductors are grounded, so as to ground the portion of the finger 18 which is not adjacent the cell being sensed. In this case, the resistance to ground 33 as shown in <figref idref="f0003">Figure 6</figref> is reduced to 0.

The operation of the circuit according to <figref idref="f0007">Figures 11 and 12</figref> will be as follows. When a user places their finger on the substrate 1, a portion of the finger will be in contact with the array of wires 30, whereas another portion of the finger will be in contact with the array of wires 31. Underneath each of the grid of wires is the array 3 composed of a plurality of sensor cells 2. When sensing is being performed at a particular cell 2a as shown in <figref idref="f0007">Figure 12</figref>, one of the array of wires, either 30 or 31 is coupled to ground and the other array is coupled to <i>ΔV_in.</i> In a first embodiment, the grid closest to the cell, 30, will be coupled to ground, and the grid spaced further from the fringes, grid 31, will be coupled to <i>ΔV_in.</i> In a second, alternative embodiment, the grid of wires which is adjacent that particular cell will be connected to Δ<i>V_in</i> by switch 51.

In the first embodiment, as <i>ΔV_in</i> varies from high to low, the electrical conductor 31 which is spaced from sensor cell 2 is in direct contact with the finger, so the finger will also track <i>ΔV_in.</i> The other grid 30 will be coupled to ground so that the adjacent portion of the finger 18 is held at ground. Sensing will progress using circuits of any acceptable type, one of which uses the vertical and horizontal scan patterns shown in <figref idref="f0001">Figure 1</figref>, but others may also be used. As the sensing progresses through the array, in each instance the localized grid of wires, whether 30 or 31 which is spaced from the cell 2 being sensed will be directly connected to <i>ΔV_in</i> so that the body voltage and that portion of the finger tracks the voltage <i>ΔV_in.</i> The other portion of the grid will be connected to ground. Namely, when those cells which are underneath the grid 31 are being sensed, the grid 31 will be connected to Δ<i>V_in.</i> Thereafter, when the cells which are under grid 31 are being sensed, grid 31 will be connected to ground, and grid 30 will be connected to <i>ΔV_in.</i>

In the second embodiment, the grid adjacent the cell being sensed is coupled to <i>ΔV_in</i> and the grid spaced from the finger, or area around the periphery of the device is coupled to ground. See, for example, <figref idref="f0008">Figures 13, 14</figref> and accompanying text in which there is a grounded conductor at the peripheral edge of the entire device.

In the example shown in <figref idref="f0007">Figure 11</figref>, there is a space distance between grids 30 and 31 so that they do not electrically interfere with each other. Since the difference between the first and second embodiments relates to whether the grid adjacent the fringe is connected to ground or to Δ<i>V_in</i>, respectively, also applies to the description just given the second embodiment, with the locations reversed and thus it need not be repeated. As an alternative, there could be a third electrical grid around the edges or positioned between the two grids, as shown in <figref idref="f0008">Figures 13 and 14</figref> or by the middle electrode 53 of <figref idref="f0007">Figure 12</figref>, respectively. This third electrode can be placed at ground, coupled to Δ<i>V_in</i>, or switched from <i>ΔV_in</i> to ground during the operation. Alternatively, it may be sufficient to have merely two voltage grids on or adjacent the substrate 11.

<figref idref="f0008">Figure 13</figref> illustrates a further alternative embodiment according to principles of the present invention. According to this embodiment, the housing 60 has contained therein a substrate 1 which includes the sensor array 3, as previously described. The substrate 1 has a sealing material 37 so as to provide a weatherproof and rugged package 60 for housing the substrate 1 with a sensor array 3 thereon. On an upper surface at an edge portion is provided conductors 32 and 34. The conductors 32 and 34 are directly adjacent, or in some instances overlapped by the sealing material 37. The conductors 32 and 34 are preferably not directly over portion of the sensor array. Instead, they are spaced sufficiently far from the array that they do not have an electrical impact on the capacitance value of each individual pixel within cell 2. In one embodiment, the conductor 32 is connected to <i>ΔV_in</i> during the entire sensing operation, and the conductor 34 is connected to ground. In this instance, the electrical conductor 32 provides the change in voltage <i>ΔV_in</i> for all of the cells 2 during sensing. Similarly, conductor 34 is always ground so as to ground a portion of the body such that R_g is at or near 0. The output of each individual sensor cell 2 is provided so that the fingerprint sensing occurs as previously described.

In an alternative embodiment, the electrical conductors 32 and 34 are connected to each other so as to provide a conductive strip which is a continuous single strip all the way around the peripheral edge of the substrate 1 within the package 60. As can be appreciated, for preferred operation, the finger of the user is in contact at all times with either conductor 32 or conductor 34. Unfortunately, if the user moves their finger, they may come out of contact with one or two of the conductors and press into the other conductor. Since this would have a detrimental effect on the sensing capability, it may be preferred that the entire conductor surrounding the sensor array 3 be at the same voltage at all times so that if the person's finger moves, they can always be assured of being in contact with a known electrical voltage. In this instance, the conductive ring 32 and 34 are electrically connected to each other so that both are always held at the same voltage. According to principles of the present invention, the voltage will be <i>ΔV_in</i> so that the voltage on the human body is directly controlled during the sensing operation. In this instance, the resistance to ground is not controlled. Namely, resistor R<i>_g</i> as shown in <figref idref="f0002">Figure 3</figref> becomes an unknown value.

<figref idref="f0008">Figure 14</figref> illustrates a conductive housing seal 34 for providing electrical connection to the finger. A conductive particle, such as carbon, iron, graphite, or the like is embedded into the sealing member 34 so that the voltage step can be applied to the housing 60 and it will be provided to the person's finger. The conductive epoxy is connected so as to not short out the lead lines, wire bonding, pad connectors, etc.

<figref idref="f0009">Figure 15A</figref> illustrates an additional structure for placing a voltage change on the person's finger 18. According to this embodiment, a large conductive metal line 92 is connected in the upper portion of the substrate so as to be in an electrical connection with the user's finger 18. (The individual electrical connections are not shown for simplicity.) A metal line 92 may be an array of lines, interspaced throughout the array and positioned between the individual cells 2 or positioned on the edge of the array, as shown in <figref idref="f0008">Figures 13 and 14</figref>. Thus, when a person places their finger on the finger print sensor, it automatically directly contacts the conductor 93. Charge can be injected into, or removed from, the person's finger by varying the voltage on the electrical conductor 92. With a large number of lines 92 spaced throughout the array, the user can be assured of having good electrical contact with a sufficient number of lines that the desired step voltage is placed on the person's finger during the sensing operation. Accordingly, while a single line is shown for purposes of illustration, it is understood that other lines in various patterns, such as a number of parallel lines, a grid or other pattern, may be positioned within the semiconductor substrate and having an exposed upper surface with a variable voltage source connected thereto.

<figref idref="f0009">Figure 15B</figref> illustrates a further circuit for placing a voltage change on the person's finger 18. A large plate of a capacitor is formed in the substrate. This is separate than and spaced away from the array 3 that is composed of sensor cells 2. (The individual electrical connections are not shown for simplicity.) The plate 93 of the charge transfer capacitor is connected to the input voltage, <i>ΔV_in.</i> The finger of the user acts as the other plate of the input capacitor. In this case, the plate 93 of the input capacitor is very large, so that it covers many ridges and valleys. Its primary purpose is charge transfer, not sensing. When a voltage change <i>ΔV_in</i> is placed on this plate 93, the body, acting as the other plate of the capacitor, sees a step input voltage. The effect of this is sensed at the individual cells 2 in the manner described herein. An array of small capacitors interspersed throughout the array could also be used in place of the large one.

<figref idref="f0010">Figures 16A and 16B</figref> illustrate some practical applications of the present invention based on potential problems encountered in daily use. As can be appreciated, fingerprint sensors can be advantageously used in a daily routine. For example, a fingerprint sensor may be placed on a car door so that a person may push their thumb, or some other member having a fingerprint pattern against the car door, and upon being recognized be permitted to open the car door and operate the car. Similarly, a fingerprint sensor may be part of a remote car lock and unlock mechanism of the type shown and described in Application No. 09/475,686, incorporated herein by reference. There are numerous other situations when sensing a fingerprint may be important, such as identification of individuals at various times during prison incarceration, a safety feature for a handgun to prohibit firing until the correct palm print or fingerprint has been placed thereon, to lock or unlock certain rooms to which restricted access is provided to only selected individuals, or other potential uses. Each of these uses, particularly those for an automobile and a safety device for a handgun, operate in relatively dirty environments. For example, a car may be parked outside, in adverse weather conditions for extended periods of time. In addition, the person wishing to drive the car may have dirt, grease, or other debris on their fingerprint which, under standard conditions, may interfere with the proper sensing of the fingerprint pattern. Further, the person may be wearing a glove, such as a thin glove composed of latex, neoprene, rubber, or other non-permeable, elastic membrane to provide a sterile working environment for the hand, as may often be the case in hospitals or other medical procedures. It is desirable to be able to sense a fingerprint pattern of a user even though their hands may be covered with dirt, debris, a thin glove or other membrane. The present invention has been found to be particularly useful in those environments which represent actual operating conditions for users, and daily work. <figref idref="f0010">Figures 16A and 16B</figref> illustrate two potential situations which may occur in practical use of the fingerprint sensor for which the present invention provides a solution.

<figref idref="f0010">Figure 16A</figref> shows the dielectric substrate 11 having individual sensor cells 2 formed in the array 3 therein. A finger 11 of a user is positioned for placing on the dielectric layer 11. A layer 95 of foreign material is interposed between the dielectric layer 11 and the finger 18 of the user. The layer 95 may be any foreign material whose properties and even presence cannot reliably be predicted. It may include, for example, a drops, bits of ice which has frozen from a thin sheet of water, frost, snow, dirt or grime, or other foreign material which may build up on the dielectric 1 1 since the fingerprint sensor is exposed to outdoor weather conditions. Of course, it is desired that a user remove as much foreign debris and only layers of water, snow, ice and dirt before sensing. A large amount of foreign material, particularly if it is highly conductive, will make proper operations of the sensor difficult. The debris may also be local and positioned only over some sensor cells and not others, such as a small drop of rain, etc. It is desired to minimize or remove as much unplanned debris as possible, even so, some may be present. Thus, in one embodiment, the layer 95 represents a buildup of foreign debris based on the fingerprint sensor being in an outdoor environment for an extended period of time. In addition, the layer 95 may be a layer worn by the user. In some situations, the layer 95 may be a latex glove, a rubber glove, or some other protective layer which a user may be wearing. A glove with appear to the sensor to be a smooth, flat layer 95 on top of the dielectric surface. In one embodiment it may be an additional protective layer, such as wax paper, a thin plastic wrap coating, or other material which is positioned on top of the entire sensor array 3 in order to protect it from damage due to adverse weather and harsh environmental conditions. In such a situation, the layer 95 is a removable layer which may be taken off and replaced on a regular basis. Thus, the substrate 11 remains clean and free of debris, and the protective cover 95 is merely removed, discarded, and a new, clean coating 95 placed thereon to maintain the entire sensor in good operating condition.

For example, in one embodiment, the top of package 60, or potentially the entire package 60, including the substrate and sensor array 3, may be completely enclosed in a watertight, thin plastic layer. This will provide the advantage that the sensor array is sealed from a high-moisture or other harsh operating environment. The protective layer 95 is then replaced as needed. Accordingly, the layer 95 represents any layer of material which is positioned between the dielectric layer and the fingerprint on the finger 18 which is being sensed whether intentionally or not. As can be appreciated, the physical shape, electrical properties, and effect on capacitive sensing of the layer 95 cannot always be completely nor accurately predicted. In addition, it may change over time, particularly in those cases where the layer 95 represents a buildup of debris, or other temporary material which may be partially cleaned from the surface 11. Thus, for a period of time the sensor may operate having the finger 18 in direct contact with the substrate 11 and, in different situations the very same sensor may operate having some foreign material in the form of a layer 95 between the finger 18 and the dielectric layer 11.

<figref idref="f0010">Figure 16B</figref> illustrates a layer 96 and debris 98 which has a different configuration than the layer 95 of <figref idref="f0010">Figure 16A</figref>. In particular, the layer 96 is more conformal to the fingerprint pattern of the user, whereas the layer 95 is flat, and more conformal to the dielectric layer 11. Further, some material 98 may be intermittently positioned on the finger of a user either in a valley, on top or a ridge, or some other location. The material that adheres to the finger, in the form of layer 96 may be intermittent, or may have different thickness at different locations and may have air pockets under the material, such as the air pocket under debris 98. Examples of material 96 include such items as a thin layer of oil on a person's thumb which may build up slowly during the day due to normal bodily functions. It may, alternatively, be a drop of water, which may be perspiration, wet hands from a raindrop, or some other thin coating 96 which is generally conformal to the person's finger, but in fact may only partially follow the ridges and valleys of the thumbprint, based, of course, on the properties of the layer 96. Debris 98 is of the type which may occur when the user has been in a harsh working environment, such as working in the field and having dirt, grime, grease, or some other material which has adhered to their finger. In the event the sensor is used on a handgun to sense the identity of the palm print of a user, it may very well be the residue from handling the gun and loading it, such as the oil from the gun, gunpowder, small flakes of lead, or other debris which often adheres to a person's finger during such activities. Thus, the material 98 may appear in various pockets on different parts of the person's hand, and may or may not be conformal to the hand depending on the type of debris.

The present invention is particularly applicable to use of the invention in the embodiments of <figref idref="f0010">Figures 16A and 16B</figref>, in which additional material, whose properties may not be totally predictable, are positioned over the dielectric layer 11 between the finger of the user. The terminal 99 is coupled to the body of the user through any acceptable technique, whether capacitive coupling is shown in <figref idref="f0009">Figure 15B</figref>, coupling to the other hand of the user, some other body part, or to the finger itself. The present invention, by increasing the sensitivity of the distinction between the ridges and valleys as described in the equations briefly herein, is able to more distinctly and accurately sense the difference between a ridge and a valley. The circuit of the present invention is therefore useful for outputting the relative differences between the effect of a ridge or a valley on the fingerprint sensor rather than the absolute values of each. The circuit of the present invention is particularly useful for producing a signal which distinguishes a ridge from a valley within a fingerprint, since the relative effect of each is amplified using the circuit of the present invention as has been previously explained herein.

<figref idref="f0011">Figure 17</figref> illustrates the electronics 101 which may be used in controlling the present invention. The electronics 101 may be positioned within a package 60 directly under the sensor array 3 as shown in <figref idref="f0008">Figure 13</figref>. Alternatively, the electronics 101 may be positioned in the same semiconductor substrate as the sensor array 3 and positioned in a peripheral region such that the control electronics are included in the same integrated circuit as the entire sensor array, as shown in <figref idref="f0009">Figure 15</figref>. The electronics 101 include the sensor array 3, and the appropriate clocks, electronic controllers, and interface, together with the memory and recognition logic for proper operation of the fingerprint sensor. For example, coupled as an output to the sensor array 3 may be a A/D converter 48 whose output is provided to the controller, recognition and logic 7, also shown in <figref idref="f0001">Figure 1</figref>. The integrated circuit interface circuit 40 receives signals from the controller 7 and provides output to the appropriate circuits, such as a D/A converter 42 and a timing generator 44. A bias generator 50, as well as an oscillator 46 are coupled to the appropriate circuits for providing the correct bias voltages as well as clock circuits as needed. The integrated circuit interface circuit 40 also includes the appropriate I/O circuits so as to output a signal indicating that a valid match has been found, or an invalid match has been found. After the integrated circuit interface 40 has output the results of the match, then the appropriate action can be enabled based on the results of the match. For example, if a valid match, it may enable starting the car, opening the door, access to the bank account, the firing of a handgun or some other activity. On the other hand, if a valid match is not found then the interface circuit may put out the appropriate signal to block any further activity, send an alert signal to a security system indicating that an unauthorized user has attempted access, or perform other functions as desired.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.