Abstract:
A method of making an integrated texture sensor for sensing a texture is described. In one embodiment, the method is directed to a sensor that that is protected from external contaminating particulates and will self-equalize using air from outside the sensor. Further combinations of such protection among various membrane switches, in combination with various types of membranes, is described. In another embodiment, a method of making a skin-texture sensor for sensing a skin texture having a plurality of ridges and a plurality of valleys is described, such that when completed, applying a ridge of the texture to a membrane switch will cause flexure of the membrane resulting in a contact between the lower electrode and the upper electrode, the contact establishing an electrical communication between said one of the row lines and said one of the column lines, whereas disposing a valley of the texture over said each membrane switch will not result in the contact between the lower electrode and the upper electrode.

Description:
RELATED APPLICATION DATA 
     This application is a divisional of U.S. patent application Ser. No. 10/038,505 filed Dec. 20, 2001, now U.S. Pat. No. 6,889,565 which is a continuation-in-part of U.S. patent application Ser. No. 09/571,765, filed May 16, 2000, now U.S. Pat. No. 6,578,436, which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to biometric identification systems and methods, and in particular to fingerprint or skin texture identification systems and methods using switch arrays. 
     BACKGROUND OF THE INVENTION 
     The fingerprint sensing industry uses several different conventional technologies to capture images of an individual&#39;s fingerprints. Two prominent technologies are optical-based sensors and capacitance-based sensors. In a typical optical sensor, a light source, lenses and a prism are used to image the ridges and valleys on a fingerprint, based on differences in the reflected light from the features. Conventional capacitance sensors include two-dimensional array of capacitors defined on a silicon chip, and fabricated by semiconductor CMOS processing. The individual sensors on the chip form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate for the various localized sensors. Upon contact with the array of sensors, the individual distance from each sensor to the corresponding point on the skin above the sensor is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint identifies the fingerprint. 
     Capacitive and optical sensors can be sensitive to oils or grease on the finger and to the presence or absence of moisture on the finger. In addition, the ambient temperature can affect these sensors at the time of sensing. Under hot or cold conditions, capacitive sensors can provide erroneous readings. Finally, most sensors have abrasion resistant coatings. The thickness of the protective coating can affect the measurements. The combined effect of these variables can result in distorted fingerprint images. Finally, in the case of silicon chip based fingerprint sensors, the placement of the finger directly onto the silicon increases the risk of electrostatic discharge and damage to the sensor. 
     Accordingly, there remains a need for a device suitable for use as a texture image capture sensor that has high sensitivity, yet can provide high lateral resolution. Moreover, there further remains a need for a sensor that is suitable for use in fingerprint image capture that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease. 
     SUMMARY OF THE INVENTION 
     A texture sensor for sensing a texture having a plurality of protrusions and a plurality of valleys, such as a fingerprint or other skin texture, includes an array of membrane switches disposed on a base. Each membrane switch comprises a fixed electrode rigidly coupled to the base, and a flexible upper membrane structures disposed over the base such that a cavity separates a central region of the membrane structure and the base. The membrane structure comprises a movable electrode disposed facing the fixed electrode. Disposing a protrusion of the texture over the membrane switch causes a flexure of the membrane resulting in a change in contact state between the fixed electrode and the movable electrode. Disposing a valley of the texture over the membrane switch does not result in the change in contact state between the fixed electrode and the movable electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where: 
         FIGS. 1-A  and  1 -B are schematic diagrams of a switch array forming part of a sensing circuit according to the preferred embodiment of the present invention. 
         FIG. 2  shows an isometric view of four adjacent membrane switches according to the preferred embodiment of the present invention. 
         FIGS. 3-A  and  3 -B show isometric and side sectional views, respectively, of a membrane switch according to the preferred embodiment of the present invention. 
         FIGS. 4-A ,  4 -B, and  4 -C show isometric, top, and side sectional views, respectively, of the structure resulting after a lower electrode is formed on a sensor substrate, according to the preferred embodiment of the present invention. 
         FIGS. 5-A ,  5 -B, and  5 -C show isometric, top, and side sectional views, respectively, of the structure resulting after the formation of a pair of sacrificial layers above the lower electrode, according to the preferred embodiment of the present invention. 
         FIGS. 6-A ,  6 -B, and  6 -C show isometric, top, and side sectional views, respectively, of the structure resulting after an upper electrode membrane is formed above the sacrificial layers, according to the preferred embodiment of the present invention. 
         FIGS. 7-A  and  7 -B illustrate, in top and side sectional views, respectively, the formation of a protective field oxide over the structure of  FIGS. 6-A  through  6 -C, according to the preferred embodiment of the present invention. 
         FIGS. 8-A  and  8 -B illustrate, in top and side sectional views, respectively, the formation of a polymer diaphragm and vent seals on the structure of  FIGS. 7-A ,  7 -B, according to the preferred embodiment of the present invention. 
         FIGS. 9-A  through  9 -C show side sectional views of three membrane switches according to alternative embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, it is understood that each recited element or structure can be formed by or be part of a monolithic structure, or be formed from multiple distinct structures. Unless otherwise specifically stated, the statement that a first layer or structure is disposed or deposited on a second layer or structure is understood to allow for the presence of parts of the first or second layer or structure that are not so disposed or deposited, and further allow for the presence of intermediate layers or structures between the first and second layers or structures. The terms “chip base” and “chip substrate” are understood to encompass monolithic substrates as well as structures containing multiple layers or parts. The terms “upper” and “lower” are used to describe relative positions, and do not necessarily refer to the direction of gravity during operation of the sensor. A set of elements is understood to include one or more elements. A plurality of elements is understood to include two or more elements. Any recitation of an element is understood to refer to at least one element. 
     The following description illustrates embodiments of the invention by way of example and not necessarily by way of limitation. 
       FIGS. 1-A  and  1 -B are schematic diagrams illustrating the electrical connections of a fingerprint or texture sensor  20  according to the preferred embodiment of the present invention. Sensor  20  comprises an array of switches (cells) spaced from each other by an appropriate distance that is a fraction of the inter-ridge spacing of the fingerprint or skin texture to be sensed. The preferable range of this fraction is ½ to 1/20. Typical spacings between fingerprint ridges are 400-500 μm. Each switch corresponds to an intersection of a row line and a column line illustrated in  FIGS. 1-A  and  1 -B. Closed switches are denoted by their associated resistances R 5 . The 8×8 switch array illustrated in  FIGS. 1-A  and  1 -B is part of a larger m×n array. 
     A plurality of metallic leads arranged in an m x n orthogonal grid and electrically isolated from each other serve as the electrical input and output lines for each switch. The orthogonal grid of metallic leads comprises conductive rows  22  and columns  24 , which lead into and out of each switch. Each row/column combination of conducting leads corresponds uniquely to a specific switch in the array. Each switch is addressed by applying a voltage and sensing a current between the row and column leads corresponding to the switch. 
     Each column lead  24  is connected to an input of a column multiplexer  30 , while each row lead  22  is connected to an input of a row (downstream) multiplexer  32 . Column multiplexer  30  and row multiplexer  32  are used to individually address each of the switches in the matrix, and thereby determine the state (open or closed) of each switch. Such multiplexers and the manner of causing the addressing of each switch are known, and thus a detailed description thereof is not necessary for an understanding of the present invention. 
     Each switch includes a lower conductive electrode, and a membrane structure including an upper electrode disposed over the lower electrode. The lower electrode of each switch is electrically connected to a column (or row) lead, and the upper conducting electrode is electrically connected to a row (or column) lead. Since the row and column conductor leads are electrically isolated from each other, the switch is electrically open in the quiescent state, and no current passes between the row and the column corresponding to the switch. 
     When a fingerprint is placed on the sensor, the upper electrodes of certain switches are pressed downward by the ridges of the fingerprint, while the upper electrodes of the other switches are not sufficiently deflected to close the switches. With appropriate design and fabrication of the switches, the upper electrodes deflect downward and establish contact with the corresponding lower electrodes when a ridge of a fingerprint is applied to the switches. The upper electrodes then revert to their original positions when the ridge of the fingerprint is removed. If a switch in the array resides directly beneath a ridge of the fingerprint, it is deflected and the switch is closed. If a switch resides beneath a valley of the fingerprint, then it is not deflected and it remains open. When closed, a switch establishes electrical contact between the row and column corresponding to that switch. In this way, one can generate a map of the contact points or the ridges on the fingerprint, and get an accurate reproduction of the fingerprint. 
     An electrical circuit such as a conventional counter, shift register and operational amplifier attached to a multiplexer, combined with voltage sources and current/voltage detectors can be used to detect the output current or voltage from the row/column being addressed. The magnitude of the output current indicates whether a particular switch is closed or open. Each switch can be polled individually using the multiplexer and simple controlling electronics and software to acquire a map of the closed switches. The switch map represents a map of the fingerprint image, since the image is a reproduction of the ridges on the fingerprint. 
     A large resistive device is preferably placed in series with each switch in the array so that one can easily distinguish between the open and closed states of the switch when the switch is polled. The resistive device is preferably a passive resistor with a resistance of 5,000Ω to 500,000Ω. Other suitable resistive devices include active devices such as p-n diodes with low forward resistance and low leakage current characteristics, field effect transistors, thin film transistors, and other types of transistors with similar characteristics. A pull-down resistor R p  is connected between ground and each input of downstream multiplexer  32 . Pull-down resistor R p  is preferably a passive resistor with a resistance of approximately 50-250Ω. Pull-down resistor R p  and the resistor placed in series with each switch facilitate the reliable detection of the state (open/closed) of each switch. Ascertaining the state of the switch relies on detecting a significant difference in the voltages (or currents) measured for the closed and open states of the switch. 
     Consider a source of constant current, sourcing from the column multiplexer  30  and sinking through the row or downstream multiplexer  32 .  FIG. 1-A  illustrates the current paths through sensor  20  as column E and row  4  are addressed. Addressing column E and row  4  is used to determine the state of the switch E 4 . The current that passes down the addressed column E encounters a parallel network of closed and open switches. Each of rows  2 ,  4 ,  6 , and  7  contains a closed switch along column E. Only the closed switches provide current paths through the sensor, since the open switches provide essentially infinite impedance. The pull-down resistors R p  at the inputs of the row multiplexer  32  are selected such that each closed switch on the addressed column, including the switch on the addressed row, presents approximately the same resistance to the current. 
     The current passing through column E is split evenly between all the closed switches R s  along column E, in this case the closed switches R s  on rows  2 ,  4 ,  6 , and  7 . If there are no closed switches on the addressing column, then the current that flows through the switch being addressed can be approximated by I/n, where I is the total source current. Some of the current that makes its way to the addressed row is lost when it shunts back out through the closed switches on the addressed row. However, by making the switch resistance adequately high, this shunting can be reduced to a small fraction of the current that sinks through row multiplexer  32 . Additionally, some of the current distributed throughout the fingerprint array does not sink through the various row pull-downs and makes its way back to the addressed row, increasing the current detected by the circuit. 
       FIG. 1-B  illustrates the current paths through sensor  20  as column E and row  5  are addressed. Row  5  does not share any of the initial current that is split between the closed switches R s  on rows  2 ,  4 ,  6 , and  7  on the addressed column E. The only current in the addressed row  5  is a portion of the current that does not sink through the various pull-down resistors R p  on the rows  2 ,  4 ,  6 , and  7  with closed switches R s  on the addressed column E. 
     The ratio of currents through the row multiplexer  32  for a closed versus an open switch can be considered to be the quality factor for detecting the state of any switch in the array. An equation that approximates this ratio is:
 
 I   closed   /I   open =1 +NX/n (1 −X )  [1]
 
wherein X=(R sw /n+R sw /n 2 +R pd /n 2 )/(R pd +R sw /n+R sw /n 2 +R pd /n 2 ),
     R sw =Resistance in series with each switch,   R pd =Pull-down resistance at inputs to the row multiplexer,   N=Total number of switches in any given row or column,   n=Average number of closed switches on any given row or column.   

     Some typical values for the parameters in the above equations are:
     R sw =100,000Ω   R pd =100Ω   N=256 Switches   n=128 Switches   X=0.887
 
which yields a current ratio or quality factor of I closed /I open =16.7, a ratio that is large enough to provide resolution in distinguishing between a closed and an open switch.
   

     In accordance with an aspect of a method of fingerprint identification/verification in accordance with the invention, before fingerprint measurements are performed, the quiescent impedance of each switch is measured in order to determine whether there is a stress on a particular switch. Measuring the quiescent impedance of each switch with no finger on the sensor provides a baseline measurement value for the switch. The baseline can be established either immediately prior to or immediately following the imaging of the fingerprint. The impedance measurement for each switch is repeated with the finger on the sensor, and the switches that have changed state from electrically open to electrically closed are recorded. The state-change information is mapped for the entire switch array to obtain an image of the fingerprint. 
     Comparing fingerprint measurements to the baseline allows a reduction of the effect of ambient temperature, humidity, and stress on the measurements. The baseline comparison also reduces the effect of individual bad sensors that are electrically closed prior to the application of the fingertip. Some switches may be closed or may appear to be closed in the absence of applied pressure, due to processing errors or undue deflection of some membranes. 
       FIG. 2  shows an isometric view of four adjacent switches  40  forming part of sensor  20 , according to the preferred embodiment of the present invention. As illustrated, switches  40  are disposed in a cartesian array along a planar surface, and are electrically interconnected by row leads  44  and column leads  46 . The interiors of the switches  40  disposed along each row are interconnected through tunnels  48 , such that the interior chambers of the switches  40  disposed along each row define a common space. Tunnels  48  provide pathways for equalizing the pressures within the chambers of different switches  40 . Such pressure differences can affect the force required to close different switches  40 . 
       FIGS. 3-A  and  3 -B show isometric and side sectional views, respectively, of a membrane switch  40  according to the preferred embodiment of the present invention. Switch  40  is shown in its quiescent, unflexed state.  FIG. 3-B  corresponds to the section AA′ illustrated in  FIG. 3-A . Referring to  FIG. 3-B , switch  40  is formed on a base comprising a substrate  62  and an insulating layer  64  extending over substrate  62 . A conductive, planar lower electrode  50  is disposed over insulating layer  64 . A flexible membrane structure  52  is disposed over lower electrode  50  and is separated from lower electrode  50  by a chamber or gap  54 . Preferably, gap  54  is filled with air and is capable of pressure equalization with the outside atmosphere. The edges of membrane structure  52  are anchored to the stationary part of switch  40  and remain fixed, while the center of membrane structure  52  is capable of downward deflection in response to the application of downward pressure by a fingerprint ridge. 
     Membrane structure  52  includes a conductive, creased upper electrode  66  disposed facing lower electrode  50 , and a diaphragm or button  68  stacked above upper electrode  66 . Diaphragm  68  forms the top, external boundary of switch  40 . The fingerprint or texture of interest is pressed directly on diaphragm  68 . Diaphragm  68  provides added thickness, mechanical stability, and impact resistance to membrane structure  52 . Preferably, the height of the top surface of diaphragm  68  is within less than +4 μm, in particular within −0.5 μm or less, of the height of the rigid area surrounding diaphragm  68 . Excessively increasing the height of membrane structure  52  can make switch  40  vulnerable to external shocks. Excessively lowering the height of membrane structure  52  relative to its fixed surroundings can impede the protrusion of texture ridges to positions needed for establishing effective contact with the upper surface of membrane structure  52 . The height of the top surface of diaphragm  68  can exceed the height of the surrounding rigid area by a few microns if improved sensitivity is desired. Preferably, the height of the top surface of diaphragm  68  is not below the height of the surrounding rigid area by more than 0.25 to 0.5 μm, if at all. 
     Upper electrode  66  is disposed along the bottom of membrane structure  52 . Upper electrode  66  includes a planar contact surface facing lower electrode  50 , such that downward flexing of membrane structure  52  establishes electrical contact between the contact surfaces of upper and lower electrodes  50 ,  66 . For clarity, upper electrode  66  and lower electrode  50  are hatched in  FIG. 3-B . 
     Lower electrode  50  is electrically connected to row lead  44  through a passive resistor  70  disposed over insulating layer  64 . In general, lower electrode  50  may also form part of row lead  44 . Upper electrode  66  is electrically connected to column lead  46  (shown in  FIG. 3-A ) through a conductor which forms part of the cover of tunnel  48  extending between chamber  54  and column lead  46 , as will be further illustrated below. 
     Row and column leads  44 ,  46  are made of a highly conductive material that can be easily patterned using known photolithography and etching techniques. Leads  44 ,  46  are electrically isolated from each other and from all other structures by films of insulating material. Lower and upper electrodes  50 ,  66  are preferably made of a material that is resistant to corrosion and oxidation, has a relatively high conductivity or forms an oxide having high conductivity, and is amenable to patterning by existing processes. Gold, copper, chromium, molybdenum, ruthenium, and indium tin oxide (ITO) are examples of preferred materials for lower and upper electrodes  50 ,  66 . 
     Diaphragm  68  is made of a material which is resistant to corrosion and oxidation, which can be made to adhere well to upper electrode  66 , and which has a desired stiffness. Diaphragm  68  is preferably made of a plastic or polymer. In a present implementation, diaphragm  68  is made of Shin-Etsu SINR 3180, a silicone-based polymer having a Young&#39;s modulus of about 40 MPa. Other suitable materials for diaphragm  68  include aluminum oxide, silicon dioxide, silicon nitride, metallic films, and polymer films such as elastomers, polyacrylates, etc. The choice of materials for diaphragm  68  and/or upper electrode  66  affects the flexibility and reliability of membrane structure  52 . The materials and dimensions of diaphragm  68  and upper electrode  66  are preferably chosen such that the performance of membrane structure  52  does not degrade due to repeated deflections over the lifetimes of switch  40 . Moreover, the materials and dimensions of membrane structure are chosen such that membrane structure fully deflects under typical pressures applied by fingerprints, but does not substantially deflect in the absence of applied fingerprint pressures. 
     Typically, the load applied by an individual&#39;s finger on a sensor is in the range of 100-500 grams. The fingerprint is approximately 15 mm×15 mm in general diameter. Thus, an array of switches with total dimensions of 15 mm×15 mm is generally appropriate for sensing fingerprints. The spacing between typical fingerprint ridges is on the order of 400 μm. If the switches are assumed to be placed 50 μm apart on a two dimensional x-y grid, an array of on the order of 300×300 switches would be suitable for covering a sensor surface area of 15 mm×15 mm. There are a total of 90,000 sensors in such an array, and the applied load from the fingertip can be assumed to be distributed over these 90,000 sensors. As a first order approximation, one can assume that the area of the ridges is equal to that of the valleys. Thus, approximately 45,000 sensors bear the applied load. If one conservatively assumes an applied load of 90 grams from the fingerprint, then each cell bears an approximate load of about 2 mg. 
     Membrane structure  52  is preferably designed such that it deflects adequately under the application of 2 mg of load to establish contact between upper electrode  66  and lower electrode  50 , and then reverts to its original, quiescent position when the load is removed. The geometry and material properties of membrane structure  52  can be empirically tailored so membrane structure  52  causes closure of its corresponding switch if membrane structure  52  is positioned under a ridge or protrusion of the texture of interest, and does not cause such closure if membrane structure  52  is positioned under a valley or depression of the texture of interest. 
     An order-of-magnitude estimate of the dependence of the central deflection of a circular membrane on the properties of the membrane can be calculated by considering an ideal, flat disk-shaped monolithic membrane anchored around its circular edge. The central deflection of such a membrane is on the order of 
     
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       Pha 
                       4 
                     
                     
                       
                         EA 
                         p 
                       
                       ⁢ 
                       
                         h 
                         4 
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
         
         where P is applied pressure, 
         h is the membrane thickness, 
         a is the membrane radius, 
         E is Young&#39;s Modulus for the membrane material, 
         and A p  is a dimensionless stiffness coefficient.
 
Equation [2] applies to both flat and corrugated diaphragms. For a flat diaphragm and a Poisson ratio μ=0.30, the value of A p  is 5.86. The value of A p  is higher for corrugated membranes.
 
       
    
     Consider some approximate values for the variables in Eq. [2]:
     P=3×10 4  Pa;   E=3×10 11  Pa;   a=1.6×10 −5  m (16 μm);   h=1.5×10 −7  m (0.15 μm);   A p =6.
 
The value of P above corresponds approximately to 5 psi, or a load of about 2 mg applied over a circle having a radius of about 16 μm. The value of E above is on the order of the Young&#39;s moduli of metals such as Cr (2.8×10 11  Pa) and Mo (3.24×10 11  Pa). The values above yield a central deflection of about 3×10 −7  m, or about 0.3 μm. Actual deflection values will depend on the particular materials, dimensions, and geometries (e.g. corrugation) employed in a given switch. Furthermore, the deflection of membranes comprising multiple stacked layers will depend on the properties of those layers. Eq. [2] nevertheless provides a useful indication of the effect of several variables on the deflection of the upper membrane in response to applied force.
   

     If the quiescent state separation of the upper and lower electrode is made to be slightly smaller than the typical deflection of the membrane in response to an applied fingerprint ridge, the upper electrode makes contact with the lower electrode under applied pressure, allowing detection of the fingerprint ridge. The separation between the upper electrode and the lower electrode is preferably larger than any height or thickness variability that might arise in the membrane due to processing induced stresses, such that such stresses do not result in closure of the switch in the absence of applied pressure. 
     The fabrication of switches  40  will be described with reference to  FIGS. 4-A  through  8 -B. The description below will focus on a single switch  40 . As is apparent to the skilled artisan, structures corresponding to multiple switches  40  are formed in each step described below. The various deposition/patterning steps can be performed using known processes such as dry etching, wet chemical etching, and deposition using photolithographic liftoff stencils. 
       FIGS. 4-A ,  4 -B, and  4 -C show isometric, top, and side sectional views, respectively, of the structure resulting after lower electrode  50  is formed along the base of a switch  40 , according to the preferred embodiment of the present invention. The view of  FIG. 4-C  is taken along the line AA′ shown in  FIG. 3-A . The fabrication process starts with the base formed by substrate  62  and insulating layer  64 . Preferred materials for substrate  62  include silicon, aluminum oxide, and glass. Preferred materials for insulating layer  64  include silicon dioxide, silicon oxinitride, and aluminum oxide. Alternative materials for substrate  62  include insulators such as plastics. Insulating layer  64  can be formed on substrate  62  by sputtering or other known methods. In a present implementation, substrate  62  is made of silicon or glass while insulating layer  64  is made of silicon dioxide. 
     Referring back to  FIGS. 4-A ,  4 -B, and  4 -C, a passive resistor strip  70  is deposited onto insulating layer  64  and patterned using dry etching, wet etching, or using a photolithographic liftoff stencil during deposition. Alternatively, resistor strip  70  can be electroplated in the required pattern. Resistor strip  70  is preferably made of a high-resistivity material such as tantalum oxide, titanium dioxide, doped silicon, or another oxidized metal or doped semiconductor. The thickness of resistor strip  70  is preferably between 200 Å and 5000 Å. The thickness, length, width, and composition of resistor strip  70  are chosen so as to yield a desired resistance value. In a present implementation, resistor strip  70  consists of a 2500 Å-thick layer of tantalum oxide, having a resistance of approximately 100,000Ω. 
     A straight conductive row lead  44  is formed on insulating layer  64 . Row lead  44  is preferably made of highly-conductive material(s) such as Cu, Cr, Au, Mo, and ITO. The thickness of row lead  44  is preferably between 1000 and 5000 Å, in particular between 2000 and 4000 Å. In a present implementation, row lead  44  is formed by a 300 Å Cr/2500 Å cu/1000 ÅCr stack. 
     Lower electrode  50  is deposited onto insulating layer  64 , for example using a photolithographic liftoff stencil. Lower electrode  50  has a substantially planar upper contact surface for establishing electrical contact with the upper electrode of the switch. The top surface of lower electrode  50  is preferably formed of metal(s) or alloy(s) that are resistant to corrosion, abrasion, frictional forces, and etchant materials used in subsequent processing steps, as described below. Preferred materials for lower electrode  50  include Cr, Au, Ru, Mo, and ITO. The thickness of lower electrode  50  is preferably between 500 and 5000 Å, in particular between 1000 and 2000 Å. In a present implementation, lower electrode  50  is formed by a 1000 Å Cr/500 Å Au/300 Å Ru stack. 
       FIGS. 5-A ,  5 -B, and  5 -C show isometric, top, and side sectional views, respectively, of the structure resulting after several additional fabrication steps performed according to the preferred embodiment of the present invention. An insulator layer  80  is deposited along the entire surface of the structure shown in  FIGS. 4-A ,  4 -B, and  4 -C, except for the middle part of lower electrode  50 . The uncovered middle part of lower electrode  50  is to form the contact surface of the switch. Insulator layer  80  serves to insulate row lead  44 , resistor  70 , and lower electrode  50  from conductive structures which are to be subsequently deposited. Insulator layer  80  extends over the edge of lower electrode  50 , and forms an annular protrusion  81  around the edge of lower electrode  50 . Annular protrusion  81  serves to generate an annular crease in subsequently deposited layers, including the upper electrode of switch  40 . Preferred materials for insulator layer  80  include alumina, silicon oxide, and silicon oxynitride. In alternative implementations, insulator layer  80  can be formed by a plastic or polyimide. The thickness of insulator layer  80  is preferably between 500 Å to 1 μm. In a present implementation, insulator layer  80  is formed by a 5000 Å-thick alumina layer. 
     A straight column lead  46  is deposited over insulator layer  80 , away from lower insulator  50  and along a direction perpendicular to the direction of row lead  44 . Suitable materials and thicknesses for column lead  46  are generally similar to those described above for row lead  44 . In a present implementation, column lead  46  is formed by a 100 Å Cr/4000 Å Au/300 Å Cr stack. 
     A first sacrificial layer  82  is formed over the contact surface of lower electrode  50 , and over part of insulator layer  80  and column lead  46 . Sacrificial layer  82  includes a generally circular central part  86  extending over the entire extent of the contact surface of lower electrode  50  and over a part of insulator layer  80  surrounding the contact surface. Central part  86  of sacrificial layer  82  will form part of the air chamber of switch  40 , between lower electrode  50  and the upper electrode. Sacrificial layer  82  also includes a tunnel part  90   a-b , which extends away from central part  86  in a direction parallel to row lead  44 . Tunnel part  90 a-b will form an inter- cell air tunnel for facilitating the equalization of pressure between different switch chambers. Sacrificial layer  82  further includes a set of four vent extensions  88  extending away from central part  86 , at 45° relative to row lead  44  and column lead  46 . Vent extensions  88  serve as a channel area that provides access to sacrificial layer  82  during a subsequent step in which sacrificial layer  82  is removed. In general, one or more (e.g. more than four) vent extensions may be used. 
     The thickness of sacrificial layer  82  defines the spacing between the contact surfaces of lower electrode  50  and the upper electrode to be subsequently deposited over sacrificial layer  82 . The thickness of sacrificial layer  82  is preferably between 500 Å and 1 μm, in particular between 0.1 and 0.5 μm. Suitable materials for sacrificial layer  82  include Cu, Al, or any other etchable materials. In a present implementation, sacrificial layer  82  is formed by 2000 Å-thick Cu. In alternative embodiments, the sacrificial layer may include an organic release material such as a polyimide. 
     A second, annular sacrificial layer  84  is deposited onto first sacrificial layer  82 , along an annular portion flanking the contact surface of lower electrode  50  and extending generally over the annular protrusion  81 . Sacrificial layer  84  serves to provide additional annular creasing to the upper electrode to be subsequently deposited, in order to reduce residual stresses in the upper electrode. Suitable thicknesses and materials for second sacrificial layer  84  are similar to those described above for first sacrificial layer  82 . In a present implementation, second sacrificial layer  84  is formed by 3000 Å-thick Cu. 
       FIGS. 6-A ,  6 -B, and  6 -C show isometric, top, and side sectional views, respectively, of the structure resulting after an upper electrode  66  is formed above sacrificial layers  82 ,  84 , according to the preferred embodiment of the present invention. Upper electrode  66  is deposited over the entire extents of sacrificial layers  82 ,  84 , as well as over insulator layer  80  and column lead  46 . As is clear to the skilled artisan, upper electrode  66  extends over only parts of insulator layer  80  and column lead  46 . Upper electrode  66  includes a central part  92  extending above lower electrode  50  and sacrificial layers  82 ,  84 , an edge anchor part  94  extending over insulator layer  80  but not over sacrificial layers  82 ,  84 , and a tunnel and column contact part  96  extending over column lead  46 . Upper electrode  66  does not extend over the tips of the vent extensions  88  defined by sacrificial layer  82 , in order to provide access to sacrificial layers  82 ,  84  through the vent extensions  88 , which serve as a channel area during subsequent steps in which sacrificial layers  82 ,  84  are removed. 
     Edge anchor part  92  anchors the edges of upper electrode  66  to the fixed insulator layer  80 . Central part  92  is designed to be capable of downward flexing motion after the removal of sacrificial layers  82 ,  84 . Central part  92  includes annular creases for decreasing stresses within upper electrode  66 . Central part  92  further includes a planar inner contact surface facing the contact surface of lower electrode  50 . Tunnel and column contact part  96  defines the electrical contact between column lead  46  and upper electrode  66 , as well as the top part of the intercell tunnel extending over column lead  46 . 
     Preferred materials for upper electrode  66  include Au, Cr, Mo, Ru, and ITO. The thickness of upper electrode  66  is chosen so as to produce desired stiffness and stress characteristics. Preferably, the thickness of upper electrode  66  is between 800 and 4000 Å, in particular between 1000 and 2000 Å. In a present implementation, upper electrode  66  is formed by a 500 Å Au/1000 Å Cr stack, with the gold layer stacked below the chromium layer. The free part of upper electrode  66  preferably has an overall size or diameter between 1 and 5 μm. In a present implementation, the diameter of the free part of upper electrode  66  is about 3.2 μm. 
       FIGS. 7-A  and  7 -B illustrate, in top and side sectional views, respectively, the formation of a field oxide insulator and impact-absorbing support  100  over the structure of  FIGS. 6-A  through  6 -C, according to the preferred embodiment of the present invention. Support  100  is deposited over the entire extent of the underlying structure, except over the center of upper electrode  66  and over the tips of vent extensions  88 . Support  100  can extend over the anchor part  94  of upper electrode  66 . Support  100  does not extend over the central part of upper electrode  66 , in order to allow the flexure of upper electrode  66 . Support  100  does not extend over the tips of vent extensions  88 , in order to provide vents for removing the sacrificial layers from switch  40 . Support  100  serves to electrically insulate column lead  46  and upper electrode  66  from the external environment. Support  100  also provides a robust external surface for switch  40  outside the area defined by the flexible membrane structure that drives the motion of upper electrode  66 . The robust external surface provided by support  100  is capable of absorbing external shocks and hits to the switches  40 , minimizing the damage to the flexible membrane structures of switches  40 . Such hits can occur during normal operation of a fingerprint sensor, for example if a user bumps or drops an object onto the surface of the sensor. 
     Preferred materials for support and insulator layer  100  include silicon dioxide, silicon nitride, and silicon oxynitride. The thickness of layer  100  is preferably between 1000 Å and 3 μm. In a present implementation, layer  100  is formed by 0.75 μm-thick field silicon dioxide. 
     After layer  100  is formed, a wet chemical etch is used to remove the sacrificial layers present between lower electrode  50  and upper electrode  66 . The sacrificial layers are removed through the vents defined over the tips of vent extensions  88 . Following the removal of the sacrificial layers, lower and upper electrodes  50 ,  66  are separated by the air gap or chamber  54 . 
       FIGS. 8-A  and  8 -B illustrate, in top and side sectional views, respectively, the formation of a flexible top diaphragm  68  and vent seals  102  on the structure of  FIGS. 7-A ,  7 -B, according to the preferred embodiment of the present invention. Diaphragm  68  is formed over upper electrode  66  and over the inner edge of layer  100 . Together with upper electrode  66 , diaphragm  68  forms a flexible membrane structure  52  capable of flexing to establish contact between upper electrode  66  and lower electrode  50 . Diaphragm  68  adds thickness and stiffness to membrane structure  52 . Anchoring the external edge of diaphragm  68  to layer  100  reduces the stresses caused within upper electrode  66  by diaphragm  68 . 
     Preferably, the height of membrane structure  52  is approximately equal to the height of the support layer  100 . It is preferred that the top surface of membrane structure  52  be within less than 4 μm higher and 0.5 μm lower than the top surface of support layer  100 . If the top surface of membrane structure  52  is too low relative to support layer  100 , support layer  100  can obstruct the penetration of fingerprint ridges to membrane structure  52  and thus prevent the closing of switch  40 . If the top surface of membrane structure  52  is too high relative to support layer  100 , switch  40  can become unnecessarily vulnerable to external impact forces capable of stressing or damaging membrane structure  52  or lower electrode  50 . 
     A set of four vent seals  102  are deposited over the sacrificial layer vents defined in support layer  100 , for closing the internal chamber of switch  40  to outside particles that could otherwise contaminate switch  40 . Vent seals  102  preferably are not air-tight, such that the air pressure within the internal chamber of switch  40  can equalize with the air pressure in the external environment of switch  40 . Preferred materials for vent seals  102  include silicon oxide, silicon nitride, metals, or other materials that will not leak into the internal chamber of switch  40 . The thickness of vent seals  102  is preferably sufficiently high so that vent seals  102  cover the aperture left behind by the vent extensions of the sacrificial layers, but not so high that vent seals  102  interfere with the sensing of fingerprints by blocking access to the membrane structure  52 . In a present implementation, vent seals  102  include a lower layer of 0.6 μm-thick silicon dioxide, and a polymer cap stacked over the silicon dioxide lower layer. In this implementation, it was observed that the air pressure within the internal switch chamber substantially equalizes with the atmospheric air pressure outside of the switch within a time period on the order of half an hour or less. In alternative implementations, suitable vent seals may be formed by a single layer of a material such as silicon nitride or a metal. 
       FIG. 9-A  shows a side sectional view of a membrane switch  240  according to an alternative embodiment of the present invention. Switch  240  is shown in its quiescent, open state. Switch  240  is part of a larger two-dimensional array, and is connected to row and column leads (not shown) as described above. Switch  240  includes a membrane structure (membrane)  252  comprising an upper electrode  266 . Upper electrode  266  is attached to the underside of a flexible, insulative, protective flat membrane  268 . Membrane  268  is anchored around its edges to the fixed structure of switch  240 . Upper electrode  266  is capable of establishing contact with a fixed lower electrode  250  when membrane  268  flexes downward in response to pressure applied by a texture protrusion or ridge. Switch  240  differs from the switch  40  described above in that the upper electrode of switch  240  is not anchored around its edges to the fixed structure of the switch. The stiffness properties of membrane  252  are determined primarily by the properties of the protective, insulative part  268 , rather than by the properties of upper electrode  266 . 
       FIG. 9-B  shows a side sectional view of a membrane switch  340  according to another alternative embodiment of the present invention. Switch  340  is shown in its quiescent, open state. Switch  340  is part of a larger two-dimensional array, and is connected to row and column leads (not shown) as described above. Switch  340  includes a membrane structure (membrane)  352  including an upper electrode membrane  366  anchored around its edges to the fixed structure of the switch, and an insulative, protective, coupling button  368  disposed above upper electrode  366 . Button  368  extends above the fixed surfaces of switch  340 , so as to couple the downward pressure applied by texture ridges to upper electrode  366 . The downward pressure results in contact between the movable upper electrode  366  and a fixed lower electrode  350 . Switch  340  differs from the switch  40  described above in that the button  368  is not anchored around its edges to the fixed part of switch  340 . 
       FIG. 9-C  shows a side sectional view of a membrane switch  440  according to yet another alternative embodiment of the present invention. Switch  440  is shown in its quiescent, closed state. Switch  440  is part of a larger two-dimensional array, and is connected to row and column leads (not shown) as described above. Switch  440  includes a fixed, annular upper electrode  466  facing downward into a first generally annular cavity  454   a . Upper electrode  466  is rigidly coupled to a base  462  of switch  440 , and does not move substantially during the operation of switch  440 . Upper electrode  466  is disposed on the bottom side of a cantilevered, annular support  480  which is rigidly coupled to base  462 . Upper electrode  466  extends around a vertical aperture  496  defined in the center of support  480 . 
     A flexible membrane structure  452  comprises a flexible lower electrode membrane  450  anchored around its edges to base  462 . The middle part of membrane  450  extends upward from base  462 , and is separated from base  462  by a disk-shaped second cavity  454   b . A coupling button  468  is disposed above the middle part of lower electrode  450 , through vertical aperture  496 . Coupling button  468  extends above the fixed surfaces of switch  440 , so as to couple applied downward pressure to lower electrode membrane  450 . A thin, flexible sheet  498  is disposed over the entire switch array, above the corresponding coupling buttons of all the switches in the array. Flexible sheet  498  serves to keep particulate matter away from the contact surface between the fixed upper electrode  466  and the movable lower electrode  450 . 
     Switch  440  is closed in its quiescent state, when no pressure or texture is applied. In the quiescent state, upper electrode  466  is in electrical contact with lower electrode  450 . When a texture ridge or protrusion applies downward pressure to lower electrode  450  through sheet  498  and coupling button  468 , lower electrode  450  flexes downward and breaks its electrical contact to upper electrode  466 . Switch  440  is then open. When the applied pressure is removed, lower electrode  450  returns to its quiescent state and switch  440  becomes closed again. 
     The structure described with reference to  FIG. 9-C  may be modified in a manner similar to the one described above with reference to  FIG. 9-A . An insulative lower membrane may be used instead of a conductive one, and a conductive lower electrode is disposed onto the top surface of the insulative membrane. 
     The use of the suspended-membrane switch designs described above allows enhanced reliability and relative insensitivity to the amount of pressure applied by the individual. The flexible membrane is capable of closing each switch in response to relatively light pressure applied by a user. Since the lower and upper electrodes are spaced closely apart relative to their in-plane extents, excessive application of force to the upper electrode does not generally cause fracture of the membrane. The switch and associated passive resistor design minimizes the need to use transistors to address the different cells using expensive CMOS processes. The switch is relatively insensitive to electrostatic discharge or other voltage spikes that would otherwise damage silicon-based sensors. 
     The power consumption of the sensor device is relatively low, since a relatively small amount of current is used to test for the state of each electrical circuit when a finger or texture is placed on the sensor. Additionally, the sensor is on only when a fingerprint is being acquired, which reduces the drain on the energy source used for polling the sensor. In its quiescent state, the sensor draws no current. Low power consumption is particularly useful in portable devices such as cellular phones and laptop computers. 
     The sensor is relatively insensitive to the choice of materials, and thus can be made relatively robust through the use of materials having relatively high corrosion and abrasion resistance. The device can be made relatively inexpensively, since its fabrication does not require expensive processing of silicon wafers. The relative ease of processing allows for the fabrication process to be applied to large area substrates, yielding more sensors per processed wafer and decreased manufacturing costs. 
     The use of a membrane allows eliminating the lateral motion of the upper electrode relative to the lower electrode. The membrane design facilitates the sealing of the chamber containing the contact surface of the switch, thus preventing external particles from contaminating the contact surface and blocking the switch in an open or closed state. Such particles could in principle contaminate the contact surface during normal operation of the sensor, or during manufacturing steps used to fabricate the sensor. The chamber sealing can be appropriately tailored to allow the passage of air in and out of the chamber while preventing the entry of contaminants. It is thought that the preferred manufacturing process and structure described above allows keeping out contaminants that are larger than on the order of hundreds or thousands of Angstroms. Allowing the pressure inside the switch chambers to become equal to the pressure outside the switches reduces the dependence of the switch operation on environmental pressure. 
     The individual membrane sealing reduces the need for a global cover sheet applied over all the switches for mechanical protection. A thick or inflexible cover sheet can lead to cross-talk between adjacent switches, as switches disposed under valleys are pressed downward by the downward motion of the cover sheet pressed down by adjacent ridges. Providing inter-switch tunnels allows the pressure within different switch chambers to equalize. Equalizing the pressures in different switch chambers leads to reduced variability in the force required to close different switches. A flexible, global cover sheet applied over all the switches can be used in a sensor according to alternative embodiments of the present invention. 
     The profile and relatively compact size of the flexible membrane structure allows the use of thick, hard field regions between adjacent membrane structures. The field regions protect the membrane structures from high impact forces and lateral shear forces caused by scratching, aggressive wiping, rubbing or other forces. 
     A global sealed vent may be used at the end of each row or column of switches, in order to facilitate the equalization of pressure between the interior chambers of multiple sensor switches and the external environment. Such a global vent may include an aperture, facing upward, that communicates with one or more interswitch tunnel(s) of the sensor. The aperture can be sealed by a structure such as a polymer film, so as to prevent the entry of particulate contaminants into the interswitch tunnels. 
     In alternative embodiments, a substrate made of a insulator such as a plastic may be employed to support the switch array. Various conductors can be deposited on the insulator, and insulative sheets may be laminated onto the substrate to provide desired insulation between the conductors. 
     In an alternative embodiment, the quiescent state of each state is closed rather than open. A flexible membrane can then include a movable lower electrode of the switch disposed on the upper surface of a membrane. The membrane is separated from the base by a cavity allowing downward flexure of the membrane into the cavity. A fixed upper electrode of the switch is disposed along a protrusion which forms part of the base or is rigidly attached to the base. The upper electrode faces downward. In the closed quiescent state, the fixed and movable electrodes are in contact along an annular region around the cavity. When the membrane is pressed down by an applied ridge, the flexure of the membrane causes the lower electrode to move downward so as to break the contact between the fixed and movable electrodes. 
     It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, the edges of the upper electrode need not be anchored to the fixed switch structure. An unanchored upper electrode can be stacked underneath another membrane layer which is anchored around the edges to the fixed switch structure. Devices as described above can be used in robotic control applications, on the tips of robotic arms, for sensing textures of objects rather than skin. A button disposed on each flexible membrane structure can extend above the other structures of the sensor, in order to facilitate the coupling of texture ridges to the flexible membranes. The switch chambers can be sealed to be air- or vacuum-tight, in order to maintain a vacuum or given amount of air within the chambers. Inter-chamber tunnels can be provided along both orthogonal directions (row and column) of the sensor, as well as along other directions. The vents used for removing the sacrificial layers can be positioned in various places, such as above an inter-chamber tunnel. Various materials, layer thicknesses and other structural dimensions are given for illustrative purposes. It is understood that other dimensions and materials can be suitable for use with the present invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.