Electric field fingerprint sensor apparatus and related methods

0001The present invention relates to the field of personal identification and verification, and, in particularly, to the field of fingerprint sensing and processing.

0002Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. In particular, a common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in storage to determine proper identification of a person, for verification purposes.

0003A typical electronic fingerprint sensor is based upon illuminating the finger surface using visible light, infrared light, or ultrasonic radiation. The reflected energy is captured with some form of camera, for example, and the resulting image is framed, digitized and stored as a static digital image. The specification of U.S. Patent No. 4,210,899 discloses an optical scanning fingerprint reader cooperating with a central processing station for a secure access application, such as admitting a person to a location or providing access to a computer terminal. The specification of U.S. Patent No. 4,525,859 discloses a video camera for capturing a fingerprint image and uses the minutiae of the fingerprints, that is, the branches and endings of the fingerprint ridges, to determine a match with a database of reference fingerprints.

0004Unfortunately, optical sensing may be affected by stained fingers or an optical sensor may be deceived by presentation of a photograph or printed image of a fingerprint rather than a true live fingerprint. Accordingly, an optical fingerprint sensor may be unreliable in service in addition to being bulky and relatively expensive due to optics and moving parts.

0005In the event of a failure to form an acceptable image of a fingerprint, the specification of U.S. Patent No. 4,947,443 discloses a series of indicator lights which give the user a simple go or no-go indication of the acceptability of the fingerprint scanning.

0006The specification of U.S. Patent No. 4,353,056 discloses another approach to sensing a live fingerprint.

0007The specification of U.S. Patent No. 5,325,442 discloses a fingerprint sensor including a plurality of sensing electrodes. Active addressing of the sensing electrodes is made possible by the provision of a switching device associated with each sensing electrode.

0008An object in fingerprint sensing and matching for identification and verification are desirable and may prevent unauthorized use of computer workstations, appliances, vehicles, and confidential data.

0009An object of the present invention is to provide a fingerprint sensor and related methods for accurately sensing a fingerprint, and which sensor is rugged, compact, reliable and relatively inexpensive.

0010Advantageously, a fingerprint sensor comprising an array of electric field sensing electrodes, a dielectric layer on the electric field sensing electrodes with the dielectric layer for receiving a finger adjacent thereto, and drive means for applying an electric field drive signal to the electric field sensing electrodes and adjacent portions of the finger so that the electric field sensing electrodes produce a fingerprint image output signal. Accordingly, the many shortcomings and disadvantages of prior art optical sensors are thus overcome, as the sensor in accordance with the present invention may be readily made to be rugged, compact, relatively low cost, and accurate.

0011In one embodiment the drive means preferably comprises coherent drive means for driving the array with a coherent signal. More particularly, the coherent drive means may include a drive electrode adjacent the electric field sensing electrodes, a second dielectric layer between the drive electrode and the electric field sensing electrodes, and a drive circuit for powering the drive electrode to generate the coherent electric field drive signal with a predetermined frequency. The sensor also preferably includes a finger electrode positioned adjacent the dielectric layer for contact with the finger.

0012Another important embodiment and aspect of the invention includes a respective shield electrode associated with each of the electric field sensing electrodes for shielding each electric field sensing electrode from adjacent sensing electrodes. Each shield electrode may be provided by an electrically conductive layer surrounding a respective sensing electrode with a dielectric layer there between.

0013The present invention advantageously includes dynamic contrast enhancing means operatively connected to the electric field sensing electrodes and within the integrated circuit of the sensor for dynamically enhancing contrast and uniformity of the fingerprint image output signal.

0014The dynamic contrast enhancing means may be provided by a capacitor matrix operatively connected to the electric field sensing electrodes, and an alternating current (AC) capacitor matrix drive means for driving the capacitor matrix. In addition, the AC capacitor matrix drive means may be provided, in part, by the synchronous demodulator described above. The dynamic contrast enhancing means may also comprise a resistor array or matrix operatively connected to the electric field sensing electrodes.

0015The fingerprint sensor may desirably be implemented using semiconductor processing techniques and wherein the upper dielectric layer, upon which the finger is placed, is an upper exposed portion of the semiconductor chip' containing the sensing and drive electrodes, as well as associated active electronic circuitry. In particular, an amplifier may be operatively connected to each electric field sensing electrode, and multiplexing means provided for selectively reading each of the electric field sensing electrodes. The sensor may also preferably include package means in one embodiment for enclosing the substrate, the active semiconductor layer, the electric field sensing electrodes, and the dielectric layer. The package means preferably has an opening therethrough in registry with the dielectric layer.

0016The present invention includes a fingerprint sensor comprising: <ul id="ul0001" list-style="none" compact="compact"><li>an array of electric field sensing electrodes;</li><li>a dielectric layer on said electric field sensing electrodes, said dielectric layer for receiving a finger adjacent thereto; and</li><li>coherent drive means for applying a coherent electric field drive signal to said electric field sensing electrodes and adjacent portions of the finger so that said electric field sensing electrodes produce a fingerprint image output signal.</li></ul>

0017The invention also includes a method for sensing a fingerprint and generating a fingerprint image output signal, the method comprising the steps of: <ul id="ul0002" list-style="none" compact="compact"><li>providing an array of electric field sensing electrodes with a dielectric layer on said electric field sensing electrodes for receiving a finger adjacent thereto; and</li><li>applying a coherent electric field drive signal to said electric field sensing electrodes and adjacent portions of the finger so that said electric field sensing electrodes produce a fingerprint image output signal, including the step of shielding said electric field sensing electrodes by positioning a respective shield electrode surrounding each of said electric field sensing electrodes to shield each electric field sensing electrode from adjacent electric field sensing electrodes.</li></ul>

0018Conveniently, the fingerprint sensor may interface with the computer processor so that the electronics associated with the sensor may be simplified. Thus, the computer processor preferably comprises access control means for permitting operation of the computer workstation only upon determining a match between a fingerprint sensed by the protectively mounted fingerprint sensor and an authorized reference fingerprint. The fingerprint sensor may be the electric field fingerprint sensor described herein or other sensors may be used.

0019The invention will now be described, by way of example, with reference to the accompanying drawings in which: <ul id="ul0003" list-style="none" compact="compact"><li>FIG. 1 is a schematic diagram of the fingerprint sensor in combination with a notebook computer;</li><li>FIG. 2 is a schematic diagram of the fingerprint sensor in combination with a computer workstation and associated information processing computer and local area network (LAN);</li><li>FIG. 3 is a schematic perspective view of an embodiment of a fingerprint sensor in;</li><li>FIG. 4 is a schematic plan view of a portion of the sensor and an overlying fingerprint pattern with a portion thereof greatly enlarged for clarity of illustration;</li><li>FIG. 5 is a greatly enlarged plan view of a portion of the fingerprint sensor with the upper dielectric layer removed;</li><li>FIG. 6 is a schematic perspective view of a portion of the fingerprint sensor;</li><li>FIG. 7 is a schematic fragmentary view of a portion of the fingerprint sensor;</li><li>FIG. 8 is a schematic side view, partially in section, illustrating the electric fields;</li><li>FIG. 9 is a schematic circuit diagram of a portion of the fingerprint sensor;</li><li>FIG. 10 is an enlarged schematic side view, partially in section, further illustrating the electric fields;</li><li>FIG. 11 is a schematic block diagram of the fingerprint sensor and associated circuitry in one embodiment;</li><li>FIG. 12 is a schematic block diagram of the fingerprint sensor and associated circuitry in another embodiment;</li><li>FIG. 13 is a schematic block diagram of an embodiment of a sensor circuit;</li><li>FIG. 14 is a schematic block diagram of another embodiment of a sensor circuit;</li><li>FIG. 15 is a schematic block diagram illustrating a plurality of sensor units;</li><li>FIG. 16 is a schematic block diagram of an embodiment of a portion of the signal processing for the fingerprint sensor;</li><li>FIG. 17 is a schematic block diagram of another embodiment of a portion of the signal processing for the fingerprint sensor;</li><li>FIG. 18 is a schematic block diagram of yet another embodiment of signal processing circuitry for the fingerprint sensor;</li><li>FIG. 19 is a schematic circuit diagram of yet another embodiment of a portion of the signal processing for the fingerprint sensor;</li><li>FIG. 20 is a schematic circuit diagram of yet another embodiment of a portion of the signal processing for the fingerprint sensor illustrating a resistor matrix for dynamic contrast enhancement;</li><li>FIG. 21 is a schematic circuit diagram of yet another embodiment of a portion of the signal processing for the fingerprint sensor illustrating a capacitor matrix implementation for dynamic contrast enhancement;</li><li>FIG. 22 is a schematic block diagram of an embodiment of the fingerprint sensor package;</li><li>FIG. 23 is a schematic diagram of another embodiment of the fingerprint sensor package;</li><li>FIG. 24 is a schematic block diagram of another aspect of the sensor for illustrating near real-time positioning feedback of finger placement;</li><li>FIG. 25 is a schematic perspective diagram of a computer illustrating near real-time positioning feedback of finger placement;</li><li>FIG. 26 is a schematic perspective diagram of a fingerprint sensor including indicators for illustrating near real-time positioning feedback of finger placement.</li></ul>

0020Referring to FIGS. 1-3, the fingerprint sensor <b>30</b> includes a housing or package <b>51,</b> a dielectric layer <b>52</b> exposed on an upper surface of the package which provides a placement surface for the finger, and a plurality of signal conductors <b>53</b>. A conductive strip or electrode <b>54</b> around the periphery of the dielectric layer <b>52</b> also provides a contact electrode for the finger as described in greater detail below. The sensor <b>30</b> may provide output signals in a range of sophistication levels depending on the level of processing.

0021The fingerprint sensor <b>30</b> is used for personal identification or verification purposes. For example, the sensor <b>30</b> may be used to permit access to a computer workstation, such as a notebook computer <b>35</b> including a keyboard <b>36</b> and associated folding display screen <b>37</b> (FIG. 1). In other words, user access to the information and programs of the notebook computer <b>35</b> may only be granted if the desired fingerprint is first sensed.

0022The sensor <b>30</b> may be used to grant or deny access to a fixed workstation <b>41</b> for a computer information system <b>40</b>. The system may include a plurality of such workstations <b>41</b> linked by a local area network (LAN) <b>43</b>, which in turn, is linked to a fingerprint identification server <b>43</b>, and an overall central computer <b>44</b>.

0023Referring to FIGS. 4-10, the sensor <b>30</b> is described in greater detail. The sensor <b>30</b> includes a plurality of individual pixels or sensing elements <b>30a</b> arranged in array pattern as shown perhaps best in FIGS. 4 and 5. These sensing elements are relatively small so as to be capable of sensing the ridges <b>59</b> and intervening valleys <b>60</b> of a typical fingerprint (FIG. 4). Live fingerprint readings as from the electric field sensor <b>30</b> may be more reliable than optical sensing, because the conduction of the skin of a finger in a pattern of ridges and valleys is extremely difficult to simulate. In contrast, an optical sensor may be deceived by a readily prepared photograph or other similar image of a fingerprint.

0024The sensor <b>30</b> includes a substrate <b>65</b>, and one or more active semiconductive layers <b>66</b> thereon. A ground plane electrode layer <b>68</b> is above the active layer <b>66</b> and separated therefrom by an insulating layer <b>67</b>. A drive electrode layer <b>71</b> is positioned over another dielectric layer <b>70</b> and is connected to an excitation drive amplifier <b>74</b>. The excitation drive signal may be typically in the range of about 1 KHz to 1 MHz and is coherently delivered across all of the array. Accordingly, the drive or excitation electronics are thus relatively uncomplicated and the overall cost of the sensor <b>30</b> may be reduced, while the reliability is increased.

0025Another insulating layer <b>76</b> is on the drive electrode layer <b>71</b>, and an illustratively circularly shaped sensing electrode <b>78</b> is on the insulating layer <b>76</b>. The sensing electrode <b>78</b> may be connected to sensing electronics <b>73</b> formed in the active layer <b>66</b> as schematically illustrated.

0026An annularly shaped shield electrode <b>80</b> surrounds the sensing electrode <b>78</b> in spaced relation therefrom. As would be readily appreciated by those skilled in the art the sensing electrode <b>78</b> and its surrounding shield electrode <b>80</b> may have other shapes, such as hexagonal, for example, to facilitate a close packed arrangement or array of pixels or sensing elements <b>30a</b>. The shield electrode <b>80</b> is an active shield which is driven by a portion of the output of the amplifier circuit <b>73</b> to help focus the electric field energy and, moreover, to thereby reduce the need to drive adjacent electrodes. Accordingly, the sensor <b>30</b> permits all of the sensing elements to be driven by a coherent drive signal in sharp contrast to prior art sensors which required that each sensing electrode be individually driven.

0027Referring to FIGS. 8-10, the excitation electrode <b>71</b> generates a first electric field to the sensing electrode <b>78</b> and a second electric field between the sensing electrode <b>78</b> and the surface of the finger <b>79</b>, over the distances <b>d1</b> and <b>d2</b>, respectively. In other terms, a first capacitor <b>83</b> (FIG. 9) is defined between the excitation electrode <b>71</b> and the sensing electrode <b>78</b>, and a second capacitor <b>85</b> is defined between the finger skin <b>79</b> and ground. The capacitance of the second capacitor <b>85</b> varies depending on whether the sensing electrode <b>78</b> is adjacent a ridge or valley. Accordingly, the sensor <b>30</b> can be modeled as a capacitive voltage divider. The voltage sensed by the unity gain voltage follower or amplifier <b>73</b> will change as the distance <b>d2</b> changes.

0028The sensing elements <b>30a</b> operate at very low currents and at very high impedances. For example, the output signal from each sensing electrode <b>78</b> is desirably about 5 to 10 millivolts to reduce the effects of noise and permit further processing of the signals. The approximate diameter of each sensing element <b>30a</b>, as defined by the outer dimensions of the shield electrode <b>80</b>, may be about 0.002 to 0.005 inches in diameter. The excitation dielectric layer <b>76</b> and surface dielectric layer <b>54</b> may desirably have a thickness in the range of about 1 µm. The ground plane electrode <b>68</b> shields the active electronic devices from the excitation electrode <b>71</b>. A relatively thick dielectric layer <b>67</b> will reduce the capacitance between these two structures and thereby reduce the current needed to drive the excitation electrode. The various signal feedthrough conductors for the electrodes <b>78</b>, <b>80</b> to the active electronic circuitry may be readily formed as would be understood by those skilled in the art. The illustrated signal polarities may be readily reversed.

0029The overall contact or sensing surface for the sensor <b>30</b> may desirably be about 0.5 by 0.5 inches -- a size which may be readily manufactured and still provide a sufficiently large surface for accurate fingerprint sensing and identification. The sensor <b>30</b> in accordance with the invention is also fairly tolerant of dead pixels or sensing elements <b>30a</b>. A typical sensor <b>30</b> includes an array of about 256 by 256 pixels or sensor elements, although other array sizes are also contemplated by the present invention. The sensor <b>30</b> may also be fabricated at one time using primarily conventional semiconductor manufacturing techniques to thereby significantly reduce the manufacturing costs.

0030FIG. 11 refers to functional partitioning of an apparatus <b>90</b> including the fingerprint sensor <b>30</b> is described. The fingerprint sensor apparatus <b>90</b> may be configured to provide one or more of displacement sensing of the fingerprint, provide an image present trigger, perform analog-to-digital conversion, provide full image capture and image integrity determination, provide contrast enhancement and normalization, and provide image binarization. In the illustrated embodiment, the sensor <b>30</b> is connected to a parallel processor and memory array <b>92</b>, and control processor <b>93</b> via the illustrated interface <b>91</b>. The parallel processor <b>92</b> may provide image quality and bad block determinations; provide edge enhancement and smoothing and thinning; generate ridge flow vectors, smooth the vectors and generate ridge flow characteristics as may be desired for fingerprint matching; identify the center of the fingerprint; generate, smooth and clean curves; and provide minutiae identification. The illustrated control processor <b>93</b> may provide minutiae registration and matching, minutiae storage, generate authorization codes, and communicate with the host via the illustrated interface <b>94</b>. The illustrated local non-volatile memory <b>95</b> may also be included in the apparatus <b>90</b>.

0031A variation of the apparatus <b>90</b> of FIG. 11 is illustrated by the apparatus <b>100</b> of FIG. 12. This embodiment includes a two chip version of the sensor and processing electronics. The apparatus <b>100</b> includes a sensor chip <b>96</b> and an authenticator chip <b>97</b> connected via a local memory bus interface <b>99</b>. A scan control processor <b>98</b> is also included in the illustrated embodiment of FIG. 12, the remaining functional components are the same as in FIG. 11.

0032Demodulation and preliminary processing of the detected signals from the sensor <b>30</b> are further understood with reference to FIGS. 13 and 14. Both of the illustrated circuits <b>110</b>, <b>120</b> desirably use an alternating current excitation. In addition, the amplitude of the voltage on the sensor is proportional to the displacement of the local ground plane, hence, the signal has to be demodulated before further use. FIG. 13 illustrates a local comparator <b>112</b> to allow the control to manage the A/D conversion process in parallel. The processor can present a sequence of a reference voltages to an entire row or column of pixels or sensor elements <b>30a</b> and monitor the transitions on the <b>Sig0</b> lines. A successive approximation conversion could be implemented, first stepping large steps, and then stepping in progressively finer steps over a smaller range, as would be readily understood by those skilled in the art. The <b>Sig0</b> output can be a binary bus connection while the <b>SigA</b> output is a demodulated analog signal that can be used as part of analog reference voltage generating circuit.

0033The circuit <b>120</b> illustrated in FIG. 14 has storage to do localized contrast enhancement for all sensor units or pixels simultaneously. The computation can use the analog comparator <b>112</b> for a decision element. The binarized output image can be shifted out of the binary shift registers provided by the illustrated latches <b>113</b>. Alternately, the output image could by read out as with conventional memory array addressing as would be readily understood by those skilled in the art. Since the circuit <b>120</b> has its own local memory, it does not need a separate set of buffers to store the pixel data.

0034Variations in skin conductivity and contamination may cause phase shift of the electric field signal. Accordingly, the processing electronic circuits <b>110</b>, <b>120</b> of FIGS. 13 and 14 preferably include a synchronous demodulator or detector <b>111</b> so that the overall circuit has less sensitivity to any such variations in conductivity.

0035Interconnections of the sensor units or pixels <b>30a</b> in a portion of an array are schematically illustrated in FIG. 15. Column data transfer lines <b>121</b>, row data transfer lines <b>122</b>, and comparator reference lines <b>123</b> are shown connected to the array of sensor units <b>30a</b>. The interconnections may be desirably made in an 8-by-8 block of sensor units.

0036The circuit <b>130</b> of FIG. 16 includes a charge coupled device (CCD) shift register <b>131</b> which, in turn, includes a plurality of individual shift registers <b>135</b>. The shift registers <b>131</b> function as a tapped delay line to facilitate image signal processing. The registers <b>135</b> feed respective A/D converters <b>132</b> operated under control of the illustrated block processor <b>134</b>. The sensing amplifier outputs are connected to the CCD analog shift registers <b>135</b>, with one shift register per row of pixels. A row of data is then shifted out of the register either to an A/D converter <b>132</b> which serves as the active conversion device. Each pixel is converted to an 8 bit digital word as it arrives at the converter. The conversion process and the A-to-D reference voltage are under control of block processors, where each block processor may control one or more rows, such as, for example, 16 rows per each processor. A limited degree of dynamic contrast compensation can be achieved using data from the previous pixel conversion to scale the reference voltage; however, significant downstream digital image processing may still be required.

0037The circuit <b>140</b> of FIG. 17 is similar to that of FIG. 16. In FIG. 17, a comparator <b>141</b> operates under control of the illustrated block processor <b>134</b> to provide the image output signals.

0038FIG. 18 depicts another aspect of the signal processing configurations. This circuit embodiment <b>150</b> is similar to that embodiment illustrated in FIG. 11 and described above. The circuit <b>150</b> of FIG. 18 illustratively includes a 16-by-16 array of sensor units or image cells <b>30b</b> selectively addressed and read by the illustrated row select data input multiplexor <b>151</b>, column select bus drivers <b>153</b>, and comparator reference voltage dividers <b>152</b>. Once an image has been captured from the electric field sensing electrodes and digitized, fingerprint features can be extracted from the image. FIG. 18 illustrates a high level view of a sensor connected to a bank of digital signal processors <b>92</b>. A 128 x 128 pixel array, in this instance, has been partitioned into a 16 x 16 array of image cells <b>30b</b>, wherein each image cell is formed of an 8 x 8 pixel array.

0039Each image cell <b>30b</b> has a single comparator reference line that services the entire cell. When a cell <b>30b</b> is being scanned, one of the parallel processors manages the reference voltage for that cell <b>30b</b> and records the digitized signals for all of the sensors in that cell. During the process of scanning the sensors in the cell <b>30b</b>, the processor can simultaneously correlate the data from the cell to generate a preliminary estimate of the ridge flow direction in that cell. In the illustrated embodiment, a control processor <b>93</b> manages the sensor signal scanning and digitization, and supervises a bank of parallel processors <b>92</b> that perform feature extraction and matching functions.

0040Turning to FIG. 19, a 4 x 4 processor matrix circuit <b>180</b>, such as might be used for a pipeline style implementation of the fingerprint minutiae processing, is illustrated. The circuit <b>180</b> includes an array of processors <b>184</b>, a sensor array input/output portion <b>181</b>, a non-volatile memory interface <b>182</b>, and the illustrated multi-processor array clock and control unit <b>182</b>. The illustrated circuit <b>180</b> may be used to identify and locate the fingerprint's unique minutiae to determine a match between a sensed fingerprint and one of a plurality of reference fingerprints. The processors <b>184</b> may match the minutiae against a set of previously stored reference minutia, to complete the identification process. When a positive identification has been made, for example, the circuit <b>180</b> may notify an external processor by sending an appropriately encrypted message over a host procossor interface.

0041There is a general need to ensure sufficient contrast between the ridges and valleys of the fingerprint over the entire area of the fingerprint. The circuit <b>160</b> of FIG. 20 schematically illustrates a resistive network or matrix <b>161</b> including a plurality of interconnected resistors <b>162</b> for providing dynamic contrast enhancement for the array of pixels <b>30a</b>. The effect of adjacent pixels is used to normalize the output of each pixel and while providing sufficient contrast. The circuit includes a pair of amplifiers <b>163</b>, <b>164</b> for providing the enhanced contrast output signals.

0042Each pixel's value is determined by comparing the sensor signal to a reference signal that sums the block reference signal with a weighted average of the signals from all of the sensors in the immediate area. The square resistive grid or matrix provides the necessary weighted average to each of the pixel comparators simultaneously. The global block reference line <b>165</b> is preferably driven with a staircase waveform while the comparator outputs are monitored for change of state. Each pixel's gray-scale value may be determined by noting which step of the staircase causes that pixel's comparator to change state.

0043A variation for dynamic contrast enhancement is understood with reference to the circuit <b>170</b> of FIG. 21. Dynamic contrast enhancement can also be implemented by an array <b>172</b> of capacitors <b>171</b> interconnecting the pixel nodes <b>174</b>. In this embodiment, the array <b>172</b> receives an alternating current signal derived from the synchronous demodulator <b>175</b> described in greater detail above. The capacitors <b>171</b> serve as an AC impedance network distributing and averaging the AC signals in a fashion analogous to the behavior of the resistive network <b>161</b> (FIG. 20) for DC signals. In the AC contrast enhancing circuit <b>170</b>, the lowpass filtering that in other embodiments may be part of the demodulator circuit, is moved to the comparator <b>177</b> circuit portion. The capacitor array <b>172</b> is readily implemented using conventional semiconductor processing techniques and may offer an advantage of relatively small size as compared to the resistor array implementation described above.

0044The resistive matrix circuit <b>160</b> and capacitor matrix circuit <b>170</b> may provide weighting for image contrast enhancement. An alternative is to conduct such enhancement via downstream software which may take a relatively long time to fully process. Accordingly, the resistor matrix and capacitor matrix arrangement may provide greater overall processing speed. In addition, such preliminary processing at the sensor <b>30</b> may allow relaxation of A/D conversion from an 8 bit AD converter to a 1 bit converter in some embodiments, while still providing high speed and at a relatively low cost. For example, processing of the fingerprint image and determination of a match may desirably take only several seconds for certain applications to avoid user frustration.

0045Referring to FIG. 22, the sensor <b>30</b> may be contained within a secure sensor package <b>190</b>. The sensor <b>30</b> is desirably mounted to prevent flexing or shifting which may stress the chip or its electrical connections. More particularly, the overall package may include a tamper resistant housing <b>191</b> as would be readily understood by those skilled in the art. For example, the housing <b>191</b> may be formed of a hard plastic material or metal that is strong and resistant to cutting, abrading or sawing. Alternately, the housing <b>191</b> may be a material which crumbles and destroys its internal circuit components if cutting, dissolution, or other forms of entry are attempted.

0046The sensor package <b>190</b> also includes the illustrated substrate <b>195</b>, processor <b>192</b>, destructible memory <b>195</b>, and encrypted output circuit <b>194</b>. More particularly, the encrypted output circuit <b>194</b> provides an output signal that can only be decrypted by the intended downstream device. Such encryption techniques will be readily understood by those skilled in the art and may include the use of various keys, passwords, codes, etc. The specifications of U.S. Patent Nos. 4,140,272; 5,337,357; 4,993,068 and 5,436,972 each disclose various approaches to encryption.

0047The output of the sensor package <b>190</b> may be communicated to associated downstream decryption equipment via electrically conductive leads or pins, or may be inductively or optically coupled to associated equipment. Electrical or other types of protection may be provided on the encrypted output portion to ensure that data, such as a database of fingerprints stored on the memory <b>193</b>, is not readily readable by external connections and/or signal manipulations.

0048The sensor <b>30</b> and processor <b>192</b> may be configured to provide any of a range of integral sensor processing features. For example, the encrypted output may be a raw image, a processed image, fingerprint minutiae data, a yes/no match indication, or personal identification and digital signature keys.

0049The illustrated sensor package <b>190</b> also includes a bead <b>196</b> of sealing material at the interface between the upper dielectric layer <b>52</b> of the sensor <b>30</b> and the adjacent portions of the housing <b>191</b>. Other sealing arrangements are also contemplated by the present invention, for desirably providing a fluid tight seal at the interface between the exposed upper dielectric layer and the adjacent housing portions. In addition, a cleaning liquid may be used to routinely clean the window and reduce the contamination thereof. Since various alcohols, such as isopropyl alcohol are likely to be used as cleaning solutions, the housing <b>191</b> and sealing bead <b>196</b> are desirably resistant to such chemicals.

0050Turning to FIG. 23 another sensor package <b>220</b> is illustrated, and the problems and solutions with respect to an integrated circuit package are discussed. A fingerprint sensor integrated circuit presents a special packaging difficulty since it has to be touched by the finger being scanned. The main contaminants of concern are sodium and the other alkaline metals. These contaminants may cause mobile ions in the SiO₂ layers that are typically used to passivate the integrated circuit. The resulting oxide charge degrades device characteristics especially in MOS technology.

0051One conventional approach to controlling mobile ionic contamination uses hermetic packaging with a phosphorus-doped passivation layer over the integrated circuit. The phosphorus doping reduces contaminant mobility by trapping mechanisms. Plastic packaging has now become more widespread, and a silicon nitride passivation layer may be used with the plastic packaging. Silicon nitride may greatly reduce the permeability to contaminants to permit direct contact between the finger of the user and the integrated circuit. Accordingly, silicon nitride may preferably be used as a passivation layer of the fingerprint sensor.

0052A fingerprint sensor as in the present invention also raises several unique packaging requirements including: the package needs to be open to enable finger-to-sensor die contact; the package should be physically strong in order to withstand rough use; the package and die should be able to withstand repeated cleaning with detergent and/or disinfectant solutions, and including scrubbing; the die should be able to withstand contact with a wide variety of organic and inorganic contaminants, and should be able to withstand abrasion; and finally the package should be relatively inexpensive.

0053The illustrated package <b>220</b> of FIG. 23 addresses these packaging issues. The package <b>220</b> includes an integrated circuit die <b>221</b> mounted on a metal paddle <b>222</b> that is connected to the leadframe <b>223</b> during injection molding of the surrounding plastic material <b>191</b> of the package. Connections are made by bond wires <b>227</b> and the lead frame <b>223</b> to the outwardly extending leads <b>228</b> as would be readily understood by those skilled in the art. The upper surface of the plastic housing <b>191</b> includes an integrally molded opening <b>52</b> which permits contact to the die <b>221</b>. The adhesion between the plastic molding compound and the adjacent upper surface portions of the die creates a seal in this illustrated embodiment. Accordingly, no separate sealing compound or manufacturing step may be needed.

0054The integrated circuit die <b>221</b> may also include a passivation layer <b>224</b> of silicon nitride for reasons highlighted above. In addition, as shown in the illustrated sensor package <b>220</b>, the die <b>221</b> may be provided with a second protective coating <b>225</b>. Each of the coatings <b>224</b>, <b>225</b> are desirably relatively thin, such as on the order of about a micrometer, in order to retain sensor sensitivity. The outer coating <b>225</b> may be an organic material, such as polyimide or PTFE (Teflon™) which yields advantages in wear resistance and physical protection. Inorganic coatings, such as silicon carbide or amorphous diamond, may also be used for the outer layer <b>225</b> and may greatly enhance wear resistance, especially to abrasive particles In addition, the material of the protective die coating <b>225</b> is preferably compatible with standard IC pattern definition methods in order to enable bond pad etching, for example.

0055The bond pads on the integrated circuit die <b>221</b> may be provided by aluminum. Another perhaps more preferable approach seals the pads with a gold plug, as may be applied by electroplating. In order to reduce the height created by the looped bond wires <b>227</b>, the die <b>221</b> may be directly flip-chip bonded in another embodiment of the invention, not shown. The sensor package <b>220</b> in other embodiments may be manufactured using tape automated bonding techniques.

0056Returning to FIG. 22, yet another aspect of the sensor package <b>190</b> is that the memory <b>198</b> and/or other integrated circuit components may be made to destruct or be rendered secure upon breach of the housing <b>191</b>, for example. A coating <b>193</b> of material may be applied to the integrated circuit die(s) that causes destruction of the die if the coating is dissolved away as would be readily understood by those skilled in the art. The memory <b>193</b> may also self-destruct or empty its contents upon exposure to light or upon removal of a sustaining electrical current. Those of skill in the art will readily appreciate other approaches to ensuring the integrity of the data and processing capabilities of the sensor package <b>190</b>. Accordingly, the present invention provides that sensitive data, such as a database of authorized fingerprints, encryption keys, or authorization codes, are not readily stolen from the sensor package <b>190</b>. In addition, although the sensor package <b>190</b> may desirably incorporate the electrical field sensor <b>30</b>.

0057The various embodiments of the sensor <b>30</b> and its associated processing circuitry may implement any of a number of conventional fingerprint matching algorithms. Fingerprint minutiae, that is, the branches or bifurcations and end points of the fingerprint ridges, are often used to determine a match between a sample print and a reference print database. Such minutiae matching may be readily implemented by the processing circuitry of the present invention as would be readily understood by those skilled in the art. The specifications in U.S. Patent Nos. 3,859,633 and 3,893,080 are directed to fingerprint identification based upon fingerprint minutiae matching. The specification of U.S. Patent No. 4,151,512 describes a fingerprint classification method using extracted ridge contour data. The specification of U.S. Patent No. 4,185,270 discloses a process for encoding and verification also based upon minutiae. The specification of U.S. Patent No. 5,040,224 discloses an approach to preprocessing fingerprints to correctly determine a position of the core of each fingerprint image for later matching by minutiae patterns.

0058Because of the relatively fast and efficient processing of a fingerprint image provided by above identified sensor <b>30</b> and associated circuitry of the invention, the user may be provided with nearly real-time feedback regarding positioning of his finger on a fingerprint sensor, such as the illustrated electric field sensor <b>30</b>. Accordingly, the user may quickly and accurately reposition his finger, have his identification accurately determined, and promptly move forward with the intended task. In the past only a simple go or no-go indication has been described for a user as in the specification of U.S. Patent No. 4,947,443 and with such an indication most likely taking a relatively long time. Unless such an indication can be given within several seconds, user frustration is likely to rise dramatically with any further passage of time. Moreover, a simple go/no-go indication may only prompt the user to try again without any useful guidance on what may be causing the no-go indication.

0059The apparatus <b>200</b> (FIG. 24) illustratively includes a fingerprint sensor <b>30</b> operatively connected to an image processor <b>201</b>. Along the lines as discussed above, the image processor <b>201</b> may include the tapped delay line or other functional center point calculator <b>202</b> for determining a center point from the sensed fingerprint as will be readily appreciated by those skilled in the art. The location of the center point relative to a predetermined reference center point may be determined and an indication given the user via a position indicator <b>203</b>. The image may also be further analyzed, and if the applied finger pressure is too great or too little, such an indication may also be given to the user. Accordingly, potential user frustration may be significantly reduced. A need to clean the sensor may also be effectively communicated to the user if repositioning and/or pressure changes are ineffective, such as after a predetermined number of attempts.

0060Turning to FIG. 25, a practical implementation of the position feedback sensing and indication is further described as applied in a computer workstation, such as the illustrated notebook computer <b>35</b> of the type including a keyboard <b>36</b> and display <b>37</b>.

0061The fingerprint sensor <b>30</b> receives the finger of the user. The processor of the computer in cooperation with the fingerprint sensor <b>30</b> generates a display of the fingerprint image <b>206</b> along with its center point <b>205</b> on an image of a window <b>207</b> on the display <b>37</b>. The display also includes a target center point <b>208</b> to assist the user is repositioning his finger for an accurate reading.

0062In addition to the visual image indication, a further indication may be given by display of the words "move upward" and "move left" along with the illustrated associated directional arrows. An indication may also be given concerning a desired pressure, such as the illustrated words "increase pressure".

0063Yet another variation of the feedback and pressure indications may be in the form of synthetically generated speech messages issued from a speaker <b>39</b> mounted within the housing of the computer. For example, the generated voice messages illustratively include an annunciation to "move finger up and to the left" and "increase finger pressure".

0064Still another embodiment of finger position feedback sensing and indication is understood with further reference to the apparatus <b>210</b> of FIG. 26. In this embodiment, the sensor <b>30</b> is used to operate an access controller <b>211</b> which, in turn, may operate a door, for example, to permit a properly identified user to enter. Simple visual indications in the form of LEDs <b>212</b>, <b>213</b> for up and down motion, and left and right motion, respectively, may be provided to indicate to the user the proper positioning or repositioning of his finger. The illustrated embodiment also includes a plurality of LEDs <b>214</b> for indication of pressure.

0065A fingerprint sensor includes an array of electric field sensing electrodes, a dielectric layer on the sensing electrodes with the dielectric layer for receiving a finger adjacent thereto, a driver for applying an electric field drive signal to the sensing electrodes and adjacent portions of the finger so that the sensing electrodes produce a fingerprint image output signal, the driver provides a coherent drive signal for the array. A respective shield electrode is associated with each of the electric field sensing electrodes for shielding each electric field sensing electrode from adjacent sensing electrodes. Each shield electrode is actively driven for further shielding. The fingerprint sensor includes a synchronous demodulator and contrast enhancer for more accurate output image signals.