Raised microstructures

TECHNICAL FIELD

0001The present invention relates to raised microstructures for silicon based devices.

BACKGROUND OF THE INVENTION

0002The use of silicon based capacitive transducers as microphones is well known in the art. Typically, such microphones consist of four elements: a fixed backplate; a highly compliant, moveable diaphragm (which together form the two plates of a variable air-gap capacitor); a voltage bias source and a buffer.

0003The batch fabrication of acoustic transducers using similar processes as those known from the integrated circuit technology offers interesting features with regard to production cost, repeatability and size reduction. Futhermore, the technology offers the unique possibility of constructing a single transducer having a wide bandwidth of operation with a uniform high sensitivity. This provides for a transducer that, with or no modification, can be used in such diverse applications as communications, audio, and ultrasonic ranging, imaging and motion detection systems.

0004The key to achieve wide bandwidth and high sensitivity lies in creating a structure having a small and extremely sensitive diaphragm. Designs have previously been suggested in <patcit id="pcit0001" dnum="US5146435A"><text>U.S. Patent No. 5,146,435 to Bernstein</text></patcit>, and in <patcit id="pcit0002" dnum="US5452268A"><text>U.S. Patent No. 5,452,268 to Bernstein</text></patcit>. In these structures, the diaphragm is suspended on a number of very flexible movable springs. However, the implementation of the springs leads to an inherent problem of controlling the acoustic leakage in the structure, which in turn affects the low frequency roll-off of the transducer. Another approach is to suspend the diaphragm in a single point, which also provides an extremely sensitive structure. See <patcit id="pcit0003" dnum="US5490220A"><text>U.S. Patent No. 5,490,220 to Loeppert</text></patcit>. Unfortunately, in this case the properties of the diaphragm material become critical, especially the intrinsic stress gradient which causes a free film to curl. Eventually, this leads to a similar problem for this structure concerning the reproducibility of the low frequency roll-off of the transducer.

0005The two mechanical elements, the backplate and diaphragm, are typically formed on a single silicon substrate using a combination of surface and bulk micromachining well known in the art.

0006One of these two elements is generally formed to be planar with the surface of the supporting silicon wafer. The other element, while itself generally planar, is supported several microns above the first element by posts or sidewalls, hence the term "raised microstructure."

0007In general, the positioning of the two elements with respect to each other affects the performance of the entire device. Intrinsic stresses in the thin films comprising the raised microstructure cause the structure to deflect out of the design position. In a microphone in particular, variations in the gap between the diaphragm and backplate affect the microphone sensitivity, noise, and over pressure response.

0008Many other factors also affect the manufacture, structure, composition and overall design of the microphone. Such problems are more fully discussed and addressed in <patcit id="pcit0004" dnum="US5408731A"><text>U.S. Patent No. 5,408,731 to Berggvist</text></patcit>; <patcit id="pcit0005" dnum="US5490A"><text>U.S. Patent No. 5,490</text></patcit>, <patcit id="pcit0006" dnum="US220A"><text>220 to Loeppert</text></patcit>, and <patcit id="pcit0007" dnum="US5870482A"><text>U.S. Patent No. 5,870,482 to Loeppert</text></patcit>.

0009In the specific example of the design of a microphone backplate as a raised microstructure, the goal is to create a stiff element at a precise position relative to the diaphragm. One method to achieve this is to form the backplate using a silicon nitride thin film deposited over a shaped silicon oxide sacrificial layer which serves to establish the desired separation. This sacrificial layer is later removed through well known etch processes, leaving the raised backplate. Intrinsic tensile stress in the silicon nitride backplate will cause it to deflect out of position. Compressive stress is always avoided as it causes the structure to buckle.

0010<figref idref="f0008">FIG. 12</figref> depicts one such raised microstructure <b>110</b> of the prior art. After the oxide is removed leaving the raised microstructure <b>110,</b> an intrinsic tension will be present within the plate <b>112.</b> This tension <b>T</b> results from the manufacturing process as well as from the difference between the coefficient of expansion of the material of the raised microstructure <b>110</b> and the supporting wafer <b>116.</b> As shown, the tension <b>T</b> is directed radially outwards. The tension <b>T</b> intrinsic in the plate <b>112</b> will result in a moment as shown by arrow <b>M</b> about the base <b>118</b> of sidewall <b>114.</b> This moment <b>M</b> results in a tendency of the plate <b>112</b> to deflect towards the wafer <b>116</b> in the direction of arrow <b>D.</b> This deflection of plate <b>112</b> results in a negative effect on the sensitivity and performance of the microphone.

0011A number of undesirable means to negate the effects of this intrinsic tension within a thin-film raised microstructure are known in the prior art. Among them are that the composition of the thin film can be adjusted by making it silicon rich to reduce its intrinsic stress levels. However, this Technique has its disadvantages. It results in making the thin film less etch resistant to HF acid, increasing the difficulty and expense of manufacture. An additional solution known in the prior art would be to increase the thickness of the sidewall supporting the raised backplate thereby increasing the sidewall's ability to resist the intrinsic tendency of the thin film to deflect. While this sounds acceptable from a geometry point of view, manufacture of a thick sidewall when the raised microstructure is made using thin film deposition is impractical.

0012The object of the present invention is to solve these and other problems.

SUMMARY OF THE INVENTION

0013One aspect of the present invention results from a realization that a diaphragm has the highest mechanical sensitivity if it is free to move in its own plane. Furthermore, if the diaphragm is resting on a support ring attached to the perforated member, a tight acoustical seal can be achieved leading to a well controlled frequency roll-off of the transducer. Additionally, if a suspension method is chosen such that the suspension only allows the diaphragm to move in its own plane and does not take part in the deflection of the diaphragm to an incident sound pressure wave, complete decoupling from the perforated member can be achieved which reduces the sensitivity to external stresses on the transducer.

0014The present invention consists in a raised microstructure for use in a silicon based device, the raised microstructure comprising; a generally planar thin-film plate having a periphery; a ribbed sidewall, the ribbed sidewall including a plurality of ridges and grooves, the ridges and grooves extending about substantially the entire periphery and further being arranged substantially perpendicular to an edge of the thin-film plate defined by the periphery, the ribbed sidewall arranged to support the generally planar thin-film plate along the periphery; wherein the plurality of ridges and grooves of the ribbed sidewall form at least one rib, and wherein at least one rib stiffens the ribbed sidewall. The rib may be of generally arcuate, triangular or rectangular cross section.

BRIEF DESCRIPTION OF THE DRAWINGS:

0015<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is an enlarged schematic cross-sectional view taken along the line 1-1 in <figref idref="f0001">FIG. 2</figref> of an acoustic transducer with clamped suspension in accordance with the present invention;</li><li><figref idref="f0001">FIG. 2</figref> is a top plan view, partially in phantom, of the acoustic transducer of <figref idref="f0001">FIG. 1</figref>;</li><li><figref idref="f0002">FIG. 3</figref> is a cross-sectional perspective view of the acoustic transducer of <figref idref="f0001">FIG. 2</figref> taken along line 3-3 of <figref idref="f0001">FIG. 2</figref>;</li><li><figref idref="f0002">FIG. 4</figref> is an enlarged partial top view, partially in phantom, of an acoustic transducer similar to <figref idref="f0001">FIG. 2</figref> wherein the perforated member includes an optionally shaped attach perimeter;</li><li><figref idref="f0003">FIG. 5</figref> is an enlarged schematic cross-sectional view taking along the plane 5-5 in <figref idref="f0003">FIG. 6</figref> of an acoustic transducer with high compliance spring suspension in accordance with the present invention;</li><li><figref idref="f0003">FIG. 6</figref> is a top plan view, partially in phantom, of the acoustic transducer of <figref idref="f0003">FIG. 5</figref>;</li><li><figref idref="f0004">FIG. 7</figref> is a cross-sectional perspective view of the acoustic transducer of <figref idref="f0003">FIG. 6</figref> taken along plane 7-7;</li><li><figref idref="f0004">FIG. 8</figref> is a greatly enlarged partial top view, partially in phantom, of an acoustic transducer similar to <figref idref="f0003">FIG. 5</figref> wherein the perforated member includes an optionally shaped attach perimeter;</li><li><figref idref="f0005">FIG. 9</figref> is an electrical circuit for the detection of the change of the microphone capacitance whilst maintaining a constant electrical charge on the microphone;</li><li><figref idref="f0006">FIG. 10</figref> is an electrical circuit for the detection of the change of the microphone capacitance while maintaining a constant electrical potential on the microphone;</li><li><figref idref="f0007">FIG. 11</figref> is a cross-sectional perspective view of the acoustic transducer of <figref idref="f0002">FIG. 4</figref>;</li><li><figref idref="f0008">FIG. 12</figref> is a cross sectional schematic of a raised microstructure known in the prior art;</li><li><figref idref="f0009">FIG. 13</figref> is a cross sectional perspective view of a raised microstructure embodying the present invention;</li><li><figref idref="f0009">FIG. 14</figref> is a cross section of the raised microstructure of <figref idref="f0009">FIG. 13</figref>; and</li><li><figref idref="f0010">FIG. 15</figref> is a plan view of <figref idref="f0009">FIG. 13</figref>.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION:

0016Referring now to the drawings, and particularly to <figref idref="f0001 f0002">FIGS. 1-3</figref>, an acoustic transducer in accordance with the present invention is disclosed. The acoustic transducer <b>10</b> includes a conductive diaphragm <b>12</b> and a perforated member <b>40</b> supported by a substrate <b>30</b> and separated by an air gap <b>20.</b> A very narrow air gap or width <b>22</b> exists between the diaphragm <b>12</b> and substrate <b>30</b> allowing the diaphragm to move freely in its plane, thereby relieving any intrinsic stress in the diaphragm material and decoupling the diaphragm from the substrate. A number of small indentations <b>13</b> are made in the diaphragm to prevent stiction in the narrow gap between the diaphragm and substrate. The lateral motion of the diaphragm <b>12</b> is restricted by a support structure <b>41</b> in the perforated member <b>40,</b> which also serves to maintain the proper initial spacing between diaphragm and perforated member. The support structure <b>41</b> may either be a continuous ring or a plurality of bumps. If the support structure <b>41</b> is a continuous ring, then diaphragm <b>12</b> resting on the support structure <b>41</b> forms tight acoustical seal, leading to a well controlled low frequency roll-off of the transducer. If the support structure <b>41</b> is a plurality of bumps, then the acoustical seal can be formed either by limiting the spacing between the bumps, by the narrow air gap <b>22,</b> or a combination thereof .

0017The conducting diaphragm <b>12</b> is electrically insulated from the substrate <b>30</b> by a dielectric layer <b>31.</b> A conducting electrode <b>42</b> is attached to the non-conductive perforated member <b>40.</b> The perforated member contains a number of openings 21 through which a sacrificial layer (not shown) between the diaphragm and perforated member is etched during fabrication to form the air gap <b>20</b> and which later serve to reduce the acoustic damping of the air in the air gap to provide sufficient bandwidth of the transducer. A number of openings are also made in the diaphragm <b>12</b> and the perforated member <b>40</b> to form a leakage path <b>14</b> which together with the compliance of the back chamber (not shown), on which the transducer will be mounted, forms a high-pass filter resulting in a roll-off frequency low enough not to impede the acoustic function of the transducer and high enough to remove the influence of barometric pressure variations. The openings <b>14</b> are defined by photo lithographic methods and can therefore be tightly controlled, leading to a well defined low frequency behavior of the transducer. The attachment of the perforated member <b>40</b> along the perimeter <b>43</b> can be varied to reduce the curvature of the perforated member due to intrinsic internal bending moments. The perimeter can be a continuous curved surface (<figref idref="f0001 f0002">FIGS. 1-3</figref>) or discontinuous, such as corrugated (<figref idref="f0002">FIG. 4</figref>). A discontinuous perimeter <b>43</b> provides additional rigidity of the perforated member <b>40</b> thereby reducing the curvature due to intrinsic bending moments in the perforated member <b>40</b>.

0018Turning to <figref idref="f0003 f0004">FIGS. 5-7</figref>, an alternative embodiment of an acoustic transducer in accordance with the present invention is depicted. The transducer <b>50</b> includes a conductive diaphragm <b>12</b> and a perforated member <b>40</b> supported by a substrate <b>30</b> and separated by an air gap <b>20</b>. The diaphragm <b>12</b> is attached to the substrate through a number of springs <b>11</b>, which serve to mechanically decouple the diaphragm from the substrate, thereby relieving any intrinsic stress in the diaphragm. Moreover, the diaphragm is released for stress in the substrate and device package.

0019The lateral motion of the diaphragm <b>12</b> is restricted by a support structure <b>41</b> in the perforated member <b>40</b>, which also serves to maintain the proper initial spacing between diaphragm and perforated member <b>40</b>. The support structure <b>41</b> may either be a continuous ring or a plurality of bumps. If the support structure <b>41</b> is a continuous ring, then diaphragm <b>12</b> resting on the support structure <b>41</b> forms tight acoustical seal, leading to a well controlled low frequency roll-off of the transducer. If the support structure <b>41</b> is a plurality of bumps, then the acoustical seal can be formed by limiting the spacing between the bumps, or by providing a sufficiently long path around the diaphragm and through the perforations <b>21.</b>

0020The conducting diaphragm <b>12</b> is electrically insulated from the substrate <b>30</b> by a dielectric layer <b>31</b>. A conducting electrode <b>42</b> is attached to the non-conductive perforated member <b>40</b>. The perforated member contains a number of openings <b>21</b> through which a sacrificial layer (not shown) between the diaphragm <b>12</b> and the perforated member is etched during fabrication to form the air gap <b>20</b> and which later serves to reduce the acoustic damping of the air in the air gap to provide sufficient bandwidth of the transducer. A number of openings are made in the support structure <b>41</b> to form a leakage path <b>14</b> (<figref idref="f0003">FIG. 6</figref>) which together with the compliance of the back chamber (not shown) on which the transducer can be mounted forms a high-pass filter resulting in a roll-off frequency low enough not to impede the acoustic function of the transducer and high enough to remove the influence of barometric pressure variations. The openings <b>14</b> are preferably defined by photo lithographic methods and can therefore be tightly controlled, leading to a well defined low frequency behavior of the transducer. The attachment of the perforated member along the perimeter <b>43</b> can be varied to reduce the curvature of the perforated member due to intrinsic internal bending moments. The perimeter <b>43</b> can be smooth (<figref idref="f0003 f0004">FIGS. 5-7</figref>) or corrugated (<figref idref="f0004">FIGS. 8</figref> and <figref idref="f0007">11</figref>). A corrugated perimeter provides additional rigidity of the perforated member thereby reducing the curvature due to intrinsic bending moments in the perforated member.

0021In operation, an electrical potential is applied between the conductive diaphragm <b>12</b> and the electrode <b>42</b> on the perforated member. The electrical potential and associated charging of the conductors produces an electrostatic attraction force between the diaphragm and the perforated member. As a result, the free diaphragm <b>12</b> moves toward the perforated member <b>40</b> until it rests upon the support structure <b>41</b>, which sets the initial operating point of the transducer with a well defined air gap <b>20</b> and acoustic leakage through path <b>14</b>. When subjected to acoustical energy, a pressure difference appears across the diaphragm <b>12</b> causing it to deflect towards or away from the perforated member <b>40</b>. The deflection of the diaphragm <b>12</b> causes a change of the electrical field, and consequently capacitance, between the diaphragm <b>12</b> and the perforated member <b>40</b>. As a result the electrical capacitance of the transducer is modulated by the acoustical energy.

0022A method to detect the modulation of capacitance is shown in <figref idref="f0005">FIG. 9</figref>. In the detection circuit <b>100</b>, the transducer <b>102</b> is connected to a DC voltage source <b>101</b> and a unity-gain amplifier <b>104</b> with very high input impedance. A bias resistor <b>103</b> ties the DC potential of the amplifier input to ground whereby the DC potential "Vbias" is applied across the transducer. Assuming in this circuit a constant electrical charge on the transducer, a change of transducer capacitance results in a change of electrical potential across the transducer, which is measured by the unity-gain amplifier.

0023Another method to detect the modulation of capacitance is shown in <figref idref="f0006">FIG. 10</figref>. In the detection circuit <b>200</b>, the transducer <b>202</b> is connected to a DC voltage source <b>201</b> and a charge amplifier configuration <b>205</b> with a feedback resistor <b>203</b> and capacitor <b>204</b>. The feedback resistor ensures DC stability of the circuit and maintains the DC level of the input of the amplifier, whereby the DC potential "Vbias-Vb" is applied across the transducer. Assuming in this circuit a constant potential across the transducer, due to the virtual ground principle of the amplifier, a change of capacitance causes a change of charge on the transducer and consequently on the input side of the feedback capacitor leading to an offset between the negative and positive input on the amplifier. The amplifier supplies a mirror charge on output side of the feedback capacitor to remove the offset, resulting in a change of output voltage "Vout." The charge gain in this circuit is set by the ratio between the initial transducer capacitance and the capacitance of the feedback capacitor. An advantage of this detection circuit is that the virtual ground principle of the amplifier eliminates any parasitic capacitance to electrical ground in the transducer, which otherwise attenuate the effect of the dynamic change of the microphone capacitance. However, care should be taken to reduce parasitic capacitances to minimize the of gain of any noise on the signal "Vb" and the inherent amplifier noise.

0024An embodiment of the raised microstructure <b>110</b> of the present invention is shown in <figref idref="f0009">FIGS. 13 and 14</figref>. The raised microstructure <b>110</b> comprises a generally circular thin-film plate or backplate <b>112</b> supported by a sidewall <b>114</b>.

0025The raised microstructure <b>110</b> is comprised of a thin film plate <b>112</b> of silicon nitride deposited on top of a sacrificial silicon oxide layer on a silicon wafer <b>116</b> using deposition and etching techniques readily and commonly known to those of ordinary skill in the relevant arts. The sacrificial silicon oxide layer has already been removed from the figure for clarity. The sidewall <b>114</b> of the raised microstructure <b>110</b> is attached at its base <b>118</b> to the silicon wafer <b>116</b> and attached at its opposite end to the plate <b>112</b>. The sidewall <b>114</b> is generally perpendicular to plate <b>112</b>, but it is noted other angles may be utilized between the sidewall <b>114</b> and the plate <b>112</b>.

0026<figref idref="f0010">FIG. 15</figref> shows a plan view of the assembly of <figref idref="f0009">FIG. 13</figref> with a surface of the sidewall <b>114</b> of the present invention shown in phantom. It can be seen that the sidewall <b>114</b> of the present invention as shown in <figref idref="f0009 f0010">FIGS. 13-15</figref> is ribbed, forming a plurality of periodic ridges <b>120</b> and grooves <b>122.</b> In the preferred embodiment, the ridges <b>120</b> and grooves <b>122</b> are parallel and equally spaced, forming a corrugated structure. Furthermore, the preferred embodiment utilizes ridges <b>120</b> and grooves <b>122</b> of a squared cross section. The effect of corrugating the side wall in this manner is to create segments <b>124</b> of the sidewall <b>114</b> that are radial, as is the intrinsic tension <b>T</b> of the plate <b>112.</b> By making portions of the sidewall <b>114</b> radial, as is the tension <b>T</b>, the sidewall <b>114</b> is stiffened. It has been found that the sidewall <b>114</b> of the prior art, which is tangential to plate <b>112</b>, is easily bent as compared to the radial segments <b>124</b> of the present invention.

0027Other geometries than that shown in <figref idref="f0009 f0010">FIGS. 13-15</figref> of the corrugations or ridges <b>120</b> and grooves <b>122</b> can be imagined and used effectively to increase the sidewall's <b>114</b> ability to resist moment <b>M</b> and the geometry depicted in the <figref idref="f0009 f0010">FIGS. 13-15</figref> is not intended to limit the scope of the present invention.

0028For example, a generally annular geometry, generally triangular geometry or any combination or variation of these geometries or others could be utilized for the ridges <b>122</b> and grooves <b>124</b>.

0029In the preferred embodiment, the corrugations are radial and hence the sidewalls <b>114</b> are parallel to the tension in the backplate <b>112</b>. Furthermore, the sacrificial material is etched in such a way that the sidewalls <b>114</b> are sloped with respect to the substrate to allow good step coverage as the thin film backplate <b>112</b> is deposited.