Abstract:
A piezoelectric cantilever pressure sensor array is disclosed. The piezoelectric cantilever pressure sensor array contains a substrate, a readout circuit, and piezoelectric cantilever pressure sensors electrically connected to the readout circuit. Each piezoelectric cantilever pressure sensor contains an elongate piezoelectric cantilever mounted at one end on the substrate and extending over a cavity. The piezoelectric cantilever contains a piezoelectric layer sandwiched between two electrodes and generates a measurable voltage when deformed under pressure. The piezoelectric cantilever pressure sensor array can be manufactured at low cost and used in various applications including fingerprint identification devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. patent application Ser. No. 10/792,777, entitled “PIEZOELECTRIC CANTILEVER PRESSURE SENSOR” to Jun AMANO, et al.; and U.S. patent application Ser. No. 10/792,778, entitled “METHOD OF MAKING PIEZOELECTRIC CANTILEVER PRESSURE SENSOR ARRAY” to Jun AMANO, et al., both applications of which are concurrently herewith being filed under separate covers, the subject matters of which are herein incorporated by reference in their entireties. 
   TECHNICAL FIELD 
   The technical field is pressure sensors and, in particular, piezoelectric cantilever pressure sensors. 
   BACKGROUND 
   Fingerprint identification involves the recognition of a pattern of ridges and valleys on the fingertips of a human hand. Fingerprint images can be captured by several types of methods. The oldest method is optical scanning. Most optical scanners use a charge coupled device (CCD) to capture the image of a fingertip that is placed on an illuminated plastic or glass platen. The CCD then converts the image into a digital signal. Optical fingerprint scanners are reliable and inexpensive, but they are fairly large and cannot be easily integrated into small devices. 
   In recent years, new approaches using non-optical technologies have been developed. One approach uses capacitance, or an object&#39;s ability to hold an electric charge, to capture fingerprint images. In this approach, the finger skin is one of the capacitor plates and a microelectrode is the other capacitor plate. The value of the capacitance is a function of the distance between the finger skin and the microelectrode. When the finger is placed on a microelectrode array, the capacitance variation pattern measured from electrode to electrode gives a mapping of the distance between the finger skin and the various microelectrodes underneath. The mapping corresponds to the ridge and valley structure on the finger tip. The capacitance is read using a integrated circuit fabricated on the same substrate as the microelectrode array. 
   A slightly different approach uses an active capacitive sensor array to capture the fingerprint image. The surface of each sensor is composed of two adjacent sensor plates. These sensor plates create a fringing capacitance between them whose field lines extend beyond the surface of the sensor. When live skin is brought in close proximity to the sensor plates, the skin interferes with field lines between the two plates and generate a “feedback” capacitance that is different from the original fringing capacitance. Because the fingerprint ridge and fingerprint valley generate different feedback capacitance, the entire fingerprint image may be captured by the array based on the feedback capacitance from each sensor. The capacitance sensors, however, are vulnerable to electric field and electrostatic discharge (ESD). The capacitance sensors also do not work with wet fingers. Moreover, the silicon-based sensor chip requires high power input (about 20 mA) and is expensive to manufacture. 
   Another approach employs thermal scanners to measure the differences in temperature between the ridges and the air caught in the valleys. The scanners typically use an array of thermal-electric sensors to capture the temperature difference. As the electrical charge generated within a sensor depends on the temperature change experienced by this sensor, a representation of the temperature field on the sensor array is obtained. This temperature field is directly related to the fingerprint structure. When a finger is initially placed on a thermal scanner, the temperature difference between the finger and the sensors in the array is usually large enough to be measurable and an image is created. However, it takes less than one-tenth of a second for the finger and the sensors to reach an equal temperature and the charge pattern representing the fingerprint will quickly fade away if the temperature change is not regularly refreshed. 
   Yet another approach is to use pressure sensors to detect the ridges and valleys of a fingerprint. The sensors typically include a compressible dielectric layer sandwiched between two electrodes. When pressure is applied to the top electrode, the inter-electrode distance changes, which modifies the capacitance associated with this structure. The higher the pressure applied, the larger the sensor capacitance gets. Arrays of such sensors combined with a read-out integrated circuit can be used for fingerprint acquisition. The pressure sensors may also be made of piezoelectric material. U.S. Patent Application Publication No. 20020053857 describes a piezoelectric film fingerprint scanner that contains an array of rod-like piezoelectric pressure sensors covered by a protective film. When a finger is brought into contact with such an array, the impedance of the pressure sensor changes under pressure. Fingerprint ridges correspond to the highest pressure point, while little pressure is applied at points associated with the fingerprint valleys. A range of intermediate pressures can be read for the transition zone between fingerprint ridge and valleys. The pattern of impedance changes, which is recorded by an impedance detector circuit, provides a representation of the fingerprint structure. The pressure sensing methods provide good recognition for wet fingers and are not susceptible to ESD. However, the major problem with the pressure based-detection method is the low sensor sensitivity. A certain amount of pressure is required for a sensor to generate a signal that is above the background noise. In order to reach this threshold pressure, the finger often needs to be pressed hard against the scanner to a point that the ridges and valleys are flattened under pressure, which may result in inaccurate fingerprint representation. 
   Thus, a need still exists for a fingerprint identification device that is accurate and sensitive, has a compact size, requires low power input, and can be manufactured at low cost. 
   SUMMARY 
   A piezoelectric cantilever pressure sensor array is disclosed. The piezoelectric cantilever pressure sensor array contains a substrate, a readout circuit, and piezoelectric cantilever pressure sensors electrically connected to the readout circuit. Each piezoelectric cantilever pressure sensor contains an elongate piezoelectric cantilever mounted at one end on the substrate and extending over a cavity. The piezoelectric cantilever contains a first electrode, a second electrode, and a piezoelectric element between the first electrode and the second electrode and electrically connected thereto. 
   The piezoelectric cantilever pressure sensor array can be manufactured at low cost and used in various applications including fingerprint identification devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The detailed description will refer to the following drawings, in which like numerals refer to like elements, and in which: 
       FIGS. 1A and 1B  are schematic cross-sectional views depicting a first embodiment of a piezoelectric cantilever pressure sensor in quiescent state and stressed state, respectively. 
       FIGS. 1C and 1D  are schematic cross-sectional views depicting a second embodiment and a third embodiment, respectively, of a piezoelectric cantilever pressure sensor. 
       FIG. 2  is a cross-sectional view depicting a fourth embodiment of a piezoelectric cantilever pressure sensor. 
       FIG. 3A  is a schematic representation of a piezoelectric cantilever pressure sensor array. 
       FIGS. 3B and 3C  are schematic representations of a piezoelectric cantilever pressure sensor in on-state and off-state, respectively. 
       FIG. 3D  is a schematic representation of a detection circuit for a piezoelectric cantilever sensor array. 
       FIGS. 4A–4F  are schematic cross-sectional views depicting a first layer structure from which the first and second embodiments of the piezoelectric cantilever pressure sensor are fabricated at different stages of its manufacture. 
       FIGS. 5A and 5B  are schematic top views of the layer structure before and after, respectively, the second etching in fabricating the first embodiment. 
       FIG. 5C  shows a cross-sectional view of the partially completed piezoelectric cantilever along the line  5 C— 5 C in  FIG. 5B . 
       FIG. 6A  is a schematic top view of the layer structure after the third etching in fabricating the first embodiment. 
       FIG. 6B  shows a cross-sectional view of the partially completed piezoelectric cantilever along the line  6 B— 6 B in  FIG. 6A . 
       FIG. 7A  is a schematic top view of the layer structure after the fourth etching in fabricating the first embodiment. 
       FIGS. 7B and 7C  show cross-sectional views of the partially completed piezoelectric cantilever along the lines  7 B— 7 B and  7 C— 7 C in  FIG. 7A . 
       FIG. 8A  is a schematic top view of the layer structure after depositing the X-line metal layer in fabricating the first embodiment. 
       FIGS. 8B and 8C  show cross-sectional views of the partially completed piezoelectric cantilever along the lines  8 B— 8 B and  8 C— 8 C in  FIG. 8A . 
       FIG. 9A  is a schematic top view of the layer structure after the fifth etching in fabricating the first embodiment. 
       FIGS. 9B and 9C  are cross-sectional views of the partially completed piezoelectric cantilever along the lines  9 B— 9 B and  9 C— 9 C in  FIG. 9A . 
       FIG. 10A  is a schematic top view of the layer structure after formation of the second protective coating in fabricating the first embodiment. 
       FIG. 10B  shows a cross-sectional view of the partially completed piezoelectric cantilever along the line  10 B— 10 B in  FIG. 10A . 
       FIG. 11A  is a schematic top view of the layer structure after the sixth etching in fabricating the first embodiment. 
       FIG. 11B  shows a cross-sectional view of the piezoelectric cantilever along the line  11 B— 1 B in  FIG. 11A . 
       FIG. 12  is a schematic top view of the layer structure after the seventh etching in fabricating the first embodiment. 
       FIG. 13A  is a schematic top view of the layer structure in  FIG. 4D  after the second etching in fabricating the second embodiment. 
       FIG. 13B  is a schematic cross-sectional view of the partially completed piezoelectric cantilever along the line  13 B— 13 B in  FIG. 13A . 
       FIG. 14A  is a schematic top view of the layer structure after the third etching in fabricating the second embodiment. 
       FIG. 14B  is a schematic cross-sectional view of the partially completed piezoelectric cantilever along the line  14 B— 14 B in  FIG. 14A . 
       FIG. 15A  is a schematic top view of the layer structure after the fourth etching in fabricating the second embodiment. 
       FIGS. 15B and 15C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  15 B— 15 B and  15 C— 15 C in  FIG. 15A . 
       FIG. 16A  is a schematic top view of the layer structure after depositing the X-line metal layer in fabricating the second embodiment. 
       FIGS. 16B and 16C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  16 B— 16 B and  16 C— 16 C in  FIG. 16A . 
       FIG. 17A  is a schematic top view of the layer structure after the fifth etching in fabricating the second embodiment. 
       FIGS. 17B and 17C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  17 B— 17 B and  17 C— 17 C in  FIG. 17A . 
       FIG. 18A  is a schematic top view of the layer structure after the sixth etching in fabricating the second embodiment. 
       FIG. 18B  is a schematic cross-sectional view of the completed piezoelectric cantilever along the line  18 B— 18 B in  FIG. 18A . 
       FIG. 19A  is a schematic top view of a layer structure after the formation of the second protective coating in fabricating the second embodiment. 
       FIG. 19B  is a schematic cross-sectional view of the completed piezoelectric cantilever along the line  19 B— 19 B in  FIG. 19A . 
       FIG. 20  is a schematic top view of the layer structure after the seventh etching in fabricating the second embodiment. 
       FIGS. 21A–21K  are schematic cross-sectional views depicting a second layer structure at different stages of its manufacture in fabricating the third embodiment. 
       FIGS. 22A and 22B  are schematic top views of the layer structure before and after the third etching, respectively, in fabricating the third embodiment. 
       FIG. 22C  is a schematic cross-sectional view of the partially completed piezoelectric cantilever along the line  22 C— 22 C in  FIG. 22B . 
       FIG. 23A  is a schematic top view of the layer structure after the fourth etching in fabricating the third embodiment. 
       FIG. 23B  is a schematic cross-sectional view of the partially completed piezoelectric cantilever along the line  23 B— 23 B in  FIG. 23A . 
       FIG. 24A  is a schematic top view of the layer structure after the fifth etching in fabricating the third embodiment. 
       FIGS. 24B and 24C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  24 B— 24 B and  24 C— 24 C in  FIG. 24A . 
       FIG. 25A  is a schematic top view of the layer structure after the deposition of the X-line metal layer in fabricating the third embodiment. 
       FIGS. 25B and 25C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  25 B— 25 B and  25 C— 25 C in  FIG. 25A . 
       FIG. 26A  is a schematic top view of the layer structure after the sixth etching in fabricating the third embodiment. 
       FIGS. 26B and 26C  are schematic cross-sectional views of the partially completed piezoelectric cantilever along the lines  26 B— 26 B and  26 C— 26 C in  FIG. 26A . 
       FIG. 27A  is a schematic top view of the layer structure after the seventh etching in fabricating the third embodiment. 
       FIG. 27B  is a schematic cross-sectional view of a completed piezoelectric cantilever along the line  27 B— 27 B in  FIG. 27A . 
       FIG. 28A  is schematic top view of the layer structure after the formation of the second protective layer in fabricating the third embodiment. 
       FIG. 28B  is a schematic cross-sectional view of the completed piezoelectric cantilever along the line  28 B— 28 B in  FIG. 28A . 
       FIG. 29  is a schematic top view of the layer structure after the eighth etching in fabricating the third embodiment. 
       FIG. 30  is a flow-chart of a process for manufacturing a piezoelectric cantilever pressure sensor array. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a first embodiment of a piezoelectric cantilever pressure sensor  100  in its quiescent state. The piezoelectric cantilever pressure sensor  100  includes a piezoelectric cantilever  150  having a base portion  152  and a beam portion  154 , and an access transistor  160  having a gate contact  161 , a drain contact  163 , and a source contact  165 . The piezoelectric cantilever  150  further includes, from top to bottom, a top electrode  104 , a piezoelectric element  106 , a bottom electrode  108 , and an elastic element  110 . The electrodes  104  and  108  are electrically coupled to the piezoelectric element  106 . The bottom electrode  108  is also connected to the drain contact  163  of the access transistor  160 . The base portion  152  of the piezoelectric cantilever  150  is supported by a substrate  120 , while the beam portion  154  of the piezoelectric cantilever  150  is suspended above a cavity  130 . 
   In this first embodiment, the piezoelectric element  106  and the elastic element  110  form a asymmetrical piezoelectric bimorph, i.e., a two-layered structure having a piezoelectric element and a non-piezoelectric element. When the bimorph is bent, one element elongates and is under tensile stress while the other element contracts and is under compressive stress. In the quiescent, zero stress state of the piezoelectric cantilever pressure sensor  100 , there is no voltage difference between the electrodes  104  and  108 . When a finger touches the piezoelectric cantilever pressure sensor  100 , direct contact between a finger ridge and the beam portion  154  of the piezoelectric cantilever  150  (shown as arrow A in  FIG. 1B ) will deflect the beam portion  154  of the piezoelectric cantilever  150 . This causes tensile stress in the piezoelectric element  106  and compressive stress in the elastic element  110 . The stress in the piezoelectric element  106  produces a proportional output voltage V between the electrodes  104  and  108 . The elastic element  110  offsets the neutral axis  140  of stress in the piezoelectric cantilever  150  so that strain produced by piezoelectric effect is translated into an output voltage in the piezoelectric element  106 . Typically, the piezoelectric cantilever pressure sensor  100  is capable of generating a voltage in the range of 100 mV to 1.0 V with a typical finger touch. A detailed description on the mathematical modeling of the piezoelectric cantilever  150  can be found, for example, in “Modeling and Optimal Design of Piezoelectric Cantilever Microactuators” (DeVoe and Pisano, IEEE J. Microelectromech. Syst., 6:266–270, 1997), which is incorporated herein by reference. 
   The material of substrate  120  is any etchable material. The material of substrate  120  is additionally selected based on its thermal stability, chemical inertness, specific coefficients of thermal expansion, and cost. In one embodiment, the material of the substrate is glass. Examples of glasses include, but are not limited to, borosilicate glasses, ceramic glasses, quartz and fused silica glasses, and soda lime glasses. The thickness of the substrate  120  may vary depending on the substrate material and the manufacturing process. In an embodiment, the material of the substrate  120  is a borosilicate glass and the substrate has a thickness of about 0.5 mm to about 1 mm. In this disclosure, the major surface of the substrate  120  on which the piezoelectric cantilever  150  is located will be called the top surface of the substrate and the major surface of the substrate opposite the top surface will be called the bottom surface. 
   The material of the piezoelectric element  106  is a piezoelectric material. Examples of the piezoelectric material include, but are not limited to, lead zirconate titanate (PZT), lead magnesium niobate-lead zirconate titanate (PMN-PZT), lead zirconate niobate-lead zirconate titanate (PZN-PZT), aluminum nitride (AlN), and zinc oxide (ZnO). The thickness of the piezoelectric element  106  depends on the piezoelectric material and the specific requirement of a particular application. In an embodiment, the piezoelectric element  106  has a thickness of about 0.5 μm to about 1 μm and is composed of PZT with a zirconium/titanium molar ratio of about 0.4 to about 0.6. 
   The electrodes  104 ,  108 , and  112  are typically composed of one or more thin layers of a conducting material. The thickness of the electrodes is typically in the range of 20–200 nm. In one embodiment, at least one of the electrodes  104 ,  108 , and  112  is composed of one or more layers of metal such as gold, silver, platinum, palladium, copper, aluminum or an alloy comprising one or more of such metals. In another embodiment, the top electrode  104  is composed of platinum and the bottom electrode  108  is composed of a layer of platinum and a layer of titanium or titanium oxide (TiO x ). 
   The elastic element  110  is typically composed of a silicon-based material. Examples include, but are not limited to, silicon, polycrystalline silicon (polysilicon), and silicon nitride (SiN x ). The thickness of the elastic element  110  is typically in the range of 0.2–1 μm. In an embodiment, the elastic element  110  is composed of silicon or silicon nitride and has a thickness of about 0.3–0.7 μm. 
   In all the embodiments described herein, the beam portion of the piezoelectric cantilever, e.g., the beam portion  154  of the piezoelectric cantilever  150 , is designed to have a rigidity that would allow a deflection large enough to generate a measurable voltage under the pressure from a finger. Typically, the load applied by an individual&#39;s finger on a fingerprint sensor surface is in the range of 100–500 g. A fingerprint sensor surface is approximately 15 mm×15 mm in dimensions. Assuming the fingerprint sensor has an array of piezoelectric cantilever pressure sensors with a standard pitch (i.e., distance between two neighboring sensors) of 50 μm, which corresponds to at least 500 dot per inch (dpi) specified by the Federal Bureau of Investigation, there will be a total of 90,000 sensors in the fingerprint area. As a first order approximation, one can assume that the area of the fingerprint ridges is equal to that of the fingerprint valleys. Accordingly, approximately 45,000 sensors will bear the applied load from the fingerprint. If one conservatively assumes an applied load of 90 grams from the fingerprint, then each beam portion  154  of the piezoelectric cantilever  150  bears a load of about 2 mg. Since the beam needs to fit within the array pitch dimensions of a maximum of 50 μm×50 μm, the length and width of the beam portion  154  of the piezoelectric cantilever  150  need to be less than the array pitch. Based on the length, width, thickness, and Young&#39;s Modulus for the beam material, the possible deflection of the beam portion  154  of the piezoelectric cantilever  150  under a given load and the voltage generated by the deflection can be determined. In an embodiment, the piezoelectric cantilever  150  is capable of producing a maximum voltage in the range of 500–1,000 mV under normal pressure from a finger. 
   The cavity  130  under the piezoelectric cantilever  150  is deep enough to allow maximum deflection of the cantilever  150 . In the first embodiment shown in  FIGS. 1A and 1B , the cavity  130  extends through the thickness of the substrate  120  and is formed by etching from the bottom surface of the substrate  120 . 
     FIG. 1C  shows a second embodiment of piezoelectric cantilever pressure sensor  100  in which the cavity  130  extends into the substrate  120  from the top surface of the substrate. Typically, the cavity  130  does not extend all the way to the bottom surface of the substrate  120  in this embodiment. In this embodiment, releasing holes  503  extend through the thickness of the beam portion  154  of the piezoelectric cantilever. The releasing holes permit etching of the cavity  130  from the top surface of the substrate to release the beam portion  154  of the piezoelectric cantilever from the substrate. 
     FIG. 1D  shows a third embodiment of piezoelectric cantilever pressure sensor  100  in which the elastic element  110  is shaped to define a pedestal  111  that spaces the substrate-facing surface of the beam portion  154  of the piezoelectric cantilever  150  from the major surface of the substrate  120 . In this embodiment, the cavity  130  is located between the substrate-facing surface of the beam portion  154  and the top surface of the substrate  120 . The elastic element is shaped with the aid of a sacrificial mesa, as will be described in detail below. 
   The second and third embodiments shown in  FIGS. 1C and 1D  are otherwise similar to the first embodiment shown in  FIGS. 1A and 1B , and will not be described further here. Exemplary methods that can be used to fabricate all three embodiments will be described below. 
     FIG. 2  shows a fourth embodiment of a piezoelectric cantilever pressure sensor  200  in which the piezoelectric cantilever incorporates a symmetrical piezoelectric bimorph. Piezoelectric cantilever pressure sensor  200  is based on the first embodiment of the piezoelectric cantilever pressure sensor described above with reference to  FIGS. 1A and 1B . The second and third embodiments of the piezoelectric cantilever pressure sensor described above with reference to  FIGS. 1C and 1D , respectively, may be similarly modified to incorporate a symmetrical piezoelectric bimorph. 
   The piezoelectric cantilever pressure sensor  200  includes a piezoelectric cantilever  250  having a base portion  252  and a beam portion  254 , and the access transistor  160  having the gate contact  161 , the drain contact  163 , and the source contact  165 . The piezoelectric cantilever  250  incorporates a symmetrical piezoelectric bimorph composed of, from top to bottom, the top electrode  104 , the piezoelectric element  106 , a middle electrode  112 , an additional piezoelectric element  107 , and the bottom electrode  108 , all of which are supported by the substrate  120 . The electrodes  104  and  112  are electrically coupled to the piezoelectric element  106 . The electrodes  112  and  104  are electrically coupled to the piezoelectric element  107 . The bottom electrode  108  is connected to the drain contact  163  of the access transistor  160 . 
   In this fourth embodiment, the piezoelectric elements  106  and  107  and their respective electrodes form a symmetrical piezoelectric bimorph that generates a measurable output voltage in response to finger pressure. When the bimorph structure is bent, the piezoelectric element  106  elongates and is under tensile stress while the piezoelectric element  107  contracts and is under compressive stress. 
     FIG. 3A  shows a highly simplified example of a piezoelectric cantilever pressure sensor array  300  composed of four piezoelectric cantilever pressure sensors in a two-by-two matrix. In the example shown, the piezoelectric cantilever pressure sensors are the first embodiment of the piezoelectric cantilever pressure sensors  100  described above with reference to  FIGS. 1A and 1B . However, the piezoelectric cantilever pressure sensor array  300  can incorporate any of the above-described piezoelectric cantilever pressure sensor embodiments. The piezoelectric cantilever pressure sensors  100  are connected to a grid of X-axis contact lines (X-lines)  302  and Y-axis contact lines (Y-lines)  304 . Each line  302  or  304  is connected to an exposed X-contact pad  306  (X-pad) or Y-contact pad (Y-pad)  308 , respectively. Specifically, the top electrodes of the piezoelectric cantilever pressure sensors  100  in each row of the array are connected to a respective X-line and the gates of the access transistors  160  of the piezoelectric cantilever pressure sensors  100  in each column of the array are connected to a respective Y-line. Additionally, the sources of the access transistors  160  of the piezoelectric cantilever pressure sensors  100  in each column of the array are connected to a respective reference voltage contact line (reference line)  312 . The reference lines  312  are connected to an exposed reference voltage contact pad (reference pad)  310 . Typically, the piezoelectric cantilever pressure sensor array  300  has a pitch of 50 μm and an array size of 300×300 or 256×360. 
   The state of each piezoelectric cantilever pressure sensor  100  in the piezoelectric cantilever pressure sensor array  300  is read out by the access transistor  160  connected to the piezoelectric cantilever  150  and typically located adjacent the base portion  152  of each piezoelectric cantilever  150  as shown in  FIG. 1A . As shown in  FIGS. 3B and 3C , the gate contact  161  of the access transistor  160  is connected to the Y-line, the drain contact  163  of the access transistor  160  is connected to the bottom electrode  108  of the piezoelectric cantilever  150 , and the source contact  165  of the access transistor  160  is connected to a reference voltage V ref  by the reference line  312  shown in  FIG. 3A . The piezoelectric cantilever  150  is accessed through the access transistor  160  by providing an activation signal on the Y-line and detecting the voltage signal output by the piezoelectric cantilever  150  on the X-line. The access signal causes the access transistor  160  to connect the bottom electrode  108  to the reference voltage, typically ground, applied to the reference pad  310 . The piezoelectric cantilever  150  bent by a fingerprint ridge will be said to be in an on state. A piezoelectric cantilever  150  in the on state delivers the output signal, typically in the range of 500–1000 mV, to the X-line ( FIG. 3B ), when the piezoelectric cantilever pressure sensor  100  is accessed through its access transistor  160  by the activation signal. On the other hand, the piezoelectric cantilever  150  under a fingerprint valley is not bent and will be said to be in an off state. A piezoelectric cantilever  150  in the off state generates no voltage difference between the electrodes  104  and  108 . Accordingly, when the piezoelectric cantilever pressure sensor  100  is accessed through its access transistor  160  by the activation signal, no output signal is generated on the X-line ( FIG. 3C ). A typical capacitance of the piezoelectric cantilever pressure sensor  100  is from 0.5 to 2 pF. The parasitic capacitance of the X-line is typically in the range of 1 to 5 pF and the sensing current in the X-line is in the order of 1–10 μA. The resistance of the X-line is in the order of few hundred Ohms, which results in a very fast operation of the piezoelectric cantilever pressure sensor  100 . 
     FIG. 3D  shows a circuit  400  that serves to record the status of the sensors of the piezoelectric cantilever pressure sensor array  300 . Each piezoelectric cantilever pressure sensor  100  in the circuit  400  has a unique X-Y address based on its position in the X-line/Y-line matrix. The read-out circuits  170  scan the matrix by sequentially sending out activation signals to Y-lines. The status of each piezoelectric cantilever pressure sensor  100  is determined on the X-line to which it is connected based on its response to the activation signal. Typically, to distinguish between a real signal and an aberrant voltage fluctuation, the scan is repeated hundreds of times each second. Only signals detected for two or more scans are acted upon by the read-out circuits  170 . Such read-out circuits and the scanning mechanism are known in the art. 
   In addition to fingerprint detection, the piezoelectric cantilever pressure sensor array  300  has utility in many other applications. The piezoelectric cantilever pressure sensor array  300  may be used for tactile imaging of lumps in soft tissue in medical devices. For example, the piezoelectric cantilever pressure sensor array  300  can be used in ultrasound imaging devices to provide a three-dimensional image of breast cancer or as an electric “fingertip” in remote surgery. The piezoelectric cantilever pressure sensor array  300  may also be used to detect nano- or micro-movement. For example, the piezoelectric cantilever pressure sensor array  300  can be used in automobile electronics as a tire pressure sensor or an impact sensor and in microphones and micro-speakers as an acoustic sensor. The piezoelectric cantilever sensors can also be used as microactuators or nanopositioners by applying a drive voltage to them. 
     FIGS. 4A–4F ,  5 A– 5 C,  6 A,  6 B,  7 A– 7 C,  8 A– 8 C,  9 A– 9 C,  10 A,  10 B,  11 A,  11 B, and  12  illustrate a first embodiment of a method of making an array of piezoelectric cantilever pressure sensors that incorporates piezoelectric cantilever pressure sensors  100  in accordance with the first embodiment described above with reference to  FIGS. 1A  and  1 B. The piezoelectric cantilever sensor array made by the method is otherwise similar to the array  300  described above with reference to  FIGS. 3A and 3D . 
   The first embodiment of the method starts with the fabrication of a layer structure that can also be used in a second embodiment of the method, to be described below. The second embodiment of the method is for making an array of piezoelectric cantilever pressure sensors that incorporates piezoelectric cantilever pressure sensors  100  in accordance with the second embodiment shown in  FIG. 1C . 
     FIGS. 4A–4F  show the fabrication of a layer structure  180  by mounting prefabricated access transistors  160  on the top surface of the substrate  120  ( FIG. 4A ); forming the reference pad  310  ( FIG. 3A ) and reference lines  312  ( FIG. 3A ) connecting the reference pad (not shown in  FIG. 4A ) to the source contacts  165  of the access transistors; depositing the elastic layer  410  on the substrate  120  ( FIG. 4B ); forming contact holes  171  and  173  extending through the elastic layer  410  to the gate contact  161  and drain contact  163 , respectively, of each access transistor  160  ( FIG. 4C ); depositing a bottom electrode layer  408  on the elastic layer  410  ( FIG. 4D ); depositing a piezoelectric layer  406  on the bottom electrode-layer  408  ( FIG. 4E ); and depositing a top electrode layer  404  on the piezoelectric layer  106  ( FIG. 4F ). 
   The elastic layer  410 , the electrode layers  408  and  404 , and the piezoelectric layer  406  are deposited by a process such as sputtering, chemical vapor deposition (CVD), plasma CVD, physical vapor deposition (PVD) or the like. The contact holes  171  and  173  are formed by a first etching process that uses a first mask. The layer structure  180  fabricated as just described is shown in  FIG. 4F . The layer structure  180  is then subject to additional processing to form the array of piezoelectric cantilever pressure sensors. 
   As described above, the thickness of each layer of the layer structure  180  depends on the specific requirements of a particular application. Either or both of the electrode layers  404  and  408  may also be a layer structure. In one embodiment, the substrate  120  is composed of borosilicate glass with a thickness of about 0.5 mm; the elastic layer  410  is composed of silicon nitride with a thickness of about 500 nm; the bottom electrode layer  408  has a two-layered structure composed of a platinum layer with a thickness of about 100 nm and a titanium oxide layer with a thickness of about 50 nm; the piezoelectric layer  406  has a thickness of about 500 nm to about 1,000 nm and is composed of PZT with a zirconium/titanium ratio of 0.4 to 0.6; the top electrode layer  404  has a thickness of about 100 nm and is composed of platinum. 
   Next, as shown in  FIGS. 5A–5C , the layer structure  180  is subject to a second etching process that uses a second mask.  FIG. 5A  shows the layer structure  180  before the second etching process is performed. The locations on the surface of the substrate of the access transistors  160 , the reference lines  312  and the reference pad  310  are shown by broken lines. The second etching process defines partially completed piezoelectric cantilevers  501  in the top electrode layer  404  and the piezoelectric layer  406 . In an embodiment, the partially completed piezoelectric cantilever  501  has dimensions of 25 μm×10 μm (top view) to conform to the standard sensor pitch of 50 μm. As shown in  FIGS. 5B and 5C , the second etching process removes part of the top electrode layer  404  and the piezoelectric layer  406  to define the top electrode  104  and the piezoelectric element  106  of the partially completed piezoelectric cantilevers in these layers, and additionally exposes part of the bottom electrode layer  408  for the next etching process. 
   After the second etching process, the layer structure  180  is subject to a third etching process that uses a third mask. As shown in  FIGS. 6A and 6B , the third etching process removes the unmasked portion of the bottom electrode layer  408  to define the bottom electrodes  108 , the Y-lines  304  and Y-pads  308 , and the electrical connection between the bottom electrodes and the drains of the respective access transistors  160 . The third etching process additionally removes the unmasked portion of the elastic layer  410  to define the elastic element  110  and to expose the access transistors  160 , the prefabricated reference pad  310  and the reference lines  312 , which are connected to the source contacts  165  of the access transistors  160 . The Y-lines  304  are connected to the gate contacts  161  of the access transistors  160 . One of the Y-lines is shown as part of the bottom metal layer  408  on the gate contact  161  in  FIG. 6B . The third etching process forms partially completed piezoelectric cantilevers  601 . 
   Next, the layer structure  180  is coated with a first protective layer  114 , as shown in  FIGS. 7A and 7B , followed by a fourth etching process that uses a fourth mask. The protective layer  114  prevents hydrogen or water penetration. The protective layer  114  is composed of aluminum oxide or any other suitable material. The protective layer  114  is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. The fourth etching process forms contact openings  703  in the protective layer  114 . As shown in  FIGS. 7A and 7C , the contact openings expose part of the top electrodes  104  of the partially completed piezoelectric cantilevers  601 . 
   After the fourth etching process, an X-line metal layer  116  is deposited on the first protective layer  114 , as shown in  FIGS. 8A and 8B . The X-line metal layer  116  is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. The X-line metal layer  116  is typically composed of aluminum or an aluminum alloy. As shown in  FIG. 8C , the X-line metal layer  116  fills the contact opening  703  in the first protective layer  114  and is thus electrically connected to the top electrode  104  of the partially completed piezoelectric cantilever  801 . 
   Next, a fifth etching process that uses a fifth mask is performed to define the X-lines  302  and X-pads  306  in the X-line metal layer  116 . As shown in  FIGS. 9A–9C , the fifth etching process removes the unmasked portion of the X-line metal layer  116  to define the X-lines  302  and the X-pads  306  and additionally exposes the first protective layer  114 . 
   After the fifth etching process, a second protective layer  118  is deposited on the layer structure  180  by spin coating, as shown in  FIGS. 10A and 10B . The second protective layer prevents direct contact between the fingertip and the X-lines  302 . The second protective layer  118  is composed of any material that meets the heat resistance, chemical resistance, and insulation requirement. The second protective layer  118  is also flexible enough to allow repeated deformation. In one embodiment, the second protective layer  118  is composed of polyimide and has a thickness of about 2–7 μm. 
   After the second protective layer  118  is deposited, the layer structure  180  is subject to a sixth etching process that uses a sixth mask. The sixth etching process is performed by applying the etchant to the bottom surface of the substrate  120 . The sixth etching process forms a cavity  130  that extends through the substrate  120  to the elastic element  110  of each completed piezoelectric cantilever  150 , as shown in  FIGS. 11A and 11B . Forming the cavity  130  releases the beam portion  154  of each piezoelectric cantilever  150  from the substrate to complete the fabrication of the piezoelectric cantilevers. 
   Next, a seventh and final etching process that uses a seventh mask is performed. The seventh etching process removes portions of the first protective layer  114  and the second protective layer  118  to expose the X-pads  306 , the Y-pads  308 , and the reference pad  310 , as shown in  FIG. 12 . 
   The method just described fabricates a piezoelectric cantilever pressure sensor array  300  with piezoelectric cantilever pressure sensors  100  in accordance with the first embodiment connected to the X-lines  302 , the Y-lines  304 , and the reference lines  312 , as shown in  FIG. 12 . As is known in the art, the piezoelectric cantilevers  150 , the X-lines  302  and X-pads  306 , the Y-lines  304  and Y-pads  308 , the reference lines  312  and reference pad  310 , and the cavities  130  may differ in size, shape and layout from the example shown in the figures. For example, the shape of the cavities  130  can be round, oval, or rectangular. 
   Alternatively, the access transistors  160  can be fabricated after the piezoelectric cantilevers  150  have been defined in the layer structure  180  and the cavities  130  have been etched. The drain contacts  163  of the access transistors  160  are connected to the bottom electrodes  108  of the piezoelectric cantilevers  150  by a metallization process. The reference pad  310  and reference lines  312  are fabricated and connected to the gate contact  161  of the access transistor  160  by the same or another metallization process. The fabrication process for access transistors  160  is known in the art. For example, the process is described in detail in the book, “Thin Film Transistors” by C. R. Kagan and P. Andry, Marcel Dekker (New York, 2003), which is hereby incorporated by reference. 
     FIGS. 13A ,  13 B,  14 A,  14 B,  15 A– 15 C,  16 A– 16 C,  17 A– 17 C,  18 A,  18 B,  19 A,  19 B, and  20  illustrate the above-mentioned second embodiment of a method of making an array of piezoelectric cantilever pressure sensors that incorporates piezoelectric cantilever pressure sensors  100  in accordance with the second embodiment described above with reference to  FIG. 1C . The piezoelectric cantilever sensor array is otherwise similar to the array  300  described above with reference to  FIGS. 3A and 3D . This second embodiment of the method fabricates the piezoelectric cantilever pressure sensor array using the layer structure  180  whose fabrication is described above with reference to  FIGS. 4A–4F . 
   This second embodiment begins with the fabrication of the layer structure  180  as described above with reference to  FIGS. 4A–4F . The layer structure  180  is then subject to a second etching process that uses a second mask. The second mask is similar to that used in the second etching process described above with reference to  FIGS. 5A–5C  except that it additionally defines releasing holes  503  in the beam portion  154  of each partially completed piezoelectric cantilever  1301 . The releasing holes are used later to facilitate etching part of the cavity under each beam portion. 
   As shown in  FIG. 13B , the second etching process removes part of the top electrode layer  404  and the piezoelectric layer  406  to define the top electrode  104  and the piezoelectric element  106  of the partially completed piezoelectric cantilevers  1301  and to define the releasing holes  503  that extend through the top electrode layer and the piezoelectric layer. The second etching process additionally exposes part of the bottom electrode layer  408  for the next etching process. 
   After the second etching process, the layer structure  180  is subject to a third etching process that uses a third mask. As shown in  FIGS. 14A and 14B , the third etching process removes the unmasked portion of the bottom electrode layer  408  to define the bottom electrodes  108 , the Y-lines  304  and Y-pads  308  and the electrical connection between the bottom electrodes and the drains of the respective access transistors  160 . The third etching process additionally removes the unmasked portion of the elastic layer  410  to define the elastic element  110  and to expose the prefabricated reference pad  310  and reference lines  312 , which are connected to the source contacts  165  of the access transistors  160 . The Y-lines  304  are connected to the gate contacts  161  of the access transistors  160 . One of the Y-lines  304  is shown as part of the bottom electrode layer  408  on the gate contact  161  in  FIG. 14B . The third etching process forms a partially completed piezoelectric cantilever  1401 . 
   Next, the layer structure  180  is coated with a first protective layer  114 , as shown in  FIGS. 15A and 15B , followed by a fourth etching process that uses a fourth mask. The protective layer  114  prevents hydrogen or water penetration. The protective layer  114  is composed of aluminum oxide or any other suitable material. The protective layer  114  is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. 
   The fourth etching process forms in the protective layer  114  contact openings  703  and additionally forms a second set of releasing holes  705  around each partially completed piezoelectric cantilever  1501 . The fourth etching process also re-opens the releasing holes  503  that extend through the beam portion  154  of each partially completed piezoelectric cantilever  1501 . As shown in  FIGS. 15A–15C , the contact openings  703  expose the top electrode layer  104 , while the releasing holes  503  and  705  expose the top surface of the substrate  120 . The interior wall  505  of the releasing holes  503  remains covered by the protective coating layer  114  after the fourth etching process. 
   Next, an X-line metal layer  116  is deposited on the first protective layer  114 , as shown in  FIGS. 16A and 16B . The metal layer  116  is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. The X-line metal layer  116  is typically composed of aluminum or an aluminum alloy. As shown in  FIG. 16C , the X-line metal layer  116  fills the contact opening  703  in the first protective layer  114  and is thus electrically connected to the top electrode  104  of the partially completed piezoelectric cantilever  1601 . 
   Next, a fifth etching process that uses a fifth mask is performed to define the X-lines  302  and X-pads  306  in the X-line metal layer  116 . As shown in  FIGS. 17A–17C , the fifth etching process removes the unmasked portion of the X-line metal layer  116  to define the X-lines and X-pads and exposes the first protective layer  114 . The fifth etching process additionally re-opens the releasing holes  503  and  705 , as shown in  FIG. 17B . 
   After the fifth etching process, the layer structure  180  is subject to a sixth etching process that creates a cavity  130  under the beam portion  154  of each piezoelectric cantilever  150 , as shown in  FIGS. 18A and 18B . The sixth etching process releases the beam portions  154  from the surface of the substrate  120 . No mask is needed for this etching process. Etchant flows through the releasing holes  503  and  705  shown in  FIG. 18B  to the portion of the top surface of the substrate  120  under the elastic element  110  and etches away this portion of the substrate to form the cavity  130 . Typically, the sixth etching process etches the cavity  130  to a depth that is larger than the maximum possible deflection of the piezoelectric cantilever  150  under the pressure from a fingertip, but is substantially less than the total thickness of the substrate  120 . Consequently, the sixth etching process is substantially shorter in duration than the etching process performed from the bottom surface of the substrate to form the cavity in the first embodiment of the method described above. 
   After the sixth etching process, a second protective layer  118  is deposited on the layer structure  180  by spin coating, as shown in  FIGS. 19A and 19B . The second protective layer prevents direct contact between the fingertip and the X-lines  302 . 
   Finally, after the second protective layer  118  has been deposited, a seventh etching process that uses a seventh mask is performed. The seventh etching process removes portions of the first protective layer  114  and the second protective layer  118  to expose the X-pads  306 , the Y-pads  308 , and the reference pad  310 , as shown in  FIG. 20 . 
   The method just described fabricates a piezoelectric cantilever pressure sensor array  300  with piezoelectric cantilever pressure sensors  100  in accordance with the second embodiment connected to the X-lines  302 , the Y-lines  304 , and the reference lines  312 , as shown in  FIG. 20 . As is known in the art, the piezoelectric cantilevers  150 , the X-lines  302  and X-pads  306 , the Y-lines  304  and Y-pads  308 , the reference lines  312  and reference pad  310 , and the cavities  130  may differ in size, shape and layout from the example shown in the figures. 
     FIGS. 21A–21K ,  22 A– 22 C,  23 A,  23 B,  24 A– 24 C,  25 A– 25 C,  26 A– 26 C,  27 A,  27 B,  28 A,  28 B and  29  illustrate a third embodiment of a method of making a piezoelectric cantilever sensor array that incorporates piezoelectric cantilever pressure sensors  100  in accordance with the third embodiment described above with reference to  FIG. 1D . The piezoelectric cantilever sensor array made by the method is otherwise similar to the array  300  described above with reference to  FIGS. 3A and 3D . 
   The third embodiment of the method starts with the fabrication of a layer structure  180 , as shown in  FIGS. 21A–21K . The layer structure  180  is made by depositing the coating layer  122  on the top surface of the substrate  120  ( FIG. 21A ); mounting prefabricated access transistors  160  on the coating layer  122  and forming on the coating layer  122  the reference pad  310  and reference lines  312  connecting the reference pad  310  to the source contacts  165  of the access transistors  160  ( FIGS. 21B and 21C ); depositing a sacrificial layer  426  typically of phosphosilicate glass (PSG) on the coating layer  122  ( FIG. 21D ); etching the sacrificial layer  426  using a first mask to define a sacrificial mesa  126  adjacent each of the access transistors  160  and to expose the access transistors  160 , the reference pad  310 , the reference lines  312 , and the coating layer  122  ( FIGS. 21E and 21F ); depositing the elastic layer  410  ( FIG. 21G ); etching the elastic layer  410  using a second mask to create contact holes  171  and  173  extending through the elastic layer  410  to the gate contact  161  and drain contact  163 , respectively, of each access transistor  160  ( FIG. 21H ); depositing a bottom electrode layer  408  on the elastic layer  410  ( FIG. 21I ); depositing a piezoelectric layer  406  on the bottom electrode layer  408  ( FIG. 21J ); and depositing a top electrode layer  404  on the piezoelectric layer  406  ( FIG. 21K ). The coating layer  122 , sacrificial layer  124 , elastic layer  410 , electrode layers  408  and  404 , and the piezoelectric layer  406  are deposited by a process such as sputtering, chemical vapor deposition (CVD), plasma CVD, physical vapor deposition (PVD) or the like. The layer structure  180  fabricated as just described is shown in  FIG. 21K . As will be described in the following paragraphs, the sacrificial mesas  126  will be etched away to form a cavity under the beam portion  154  of each piezoelectric cantilever  150 . Accordingly, the sacrificial mesas  126  typically have dimensions that are slightly larger than the dimensions of the beam portion  154  of the piezoelectric cantilever  150 , as shown in  FIG. 5B . The thickness of the sacrificial mesas  126  is typically larger than the maximum possible deflection of the beam portion  154  of the piezoelectric cantilever  150 . In other words, the cavity created by etching away the sacrificial mesa  126  typically has a depth that accommodates the maximum possible deflection of the beam portion  154  of the piezoelectric cantilever  150 . 
   Next, as shown in  FIGS. 22A–22C , the layer structure  180  is subject to a third etching process that uses a third mask. The third etching process partially define the piezoelectric cantilevers in the top electrode layer  404  and the piezoelectric layer  406 .  FIG. 22A  shows the layer structure  180  before the second etching process is performed. The locations on the surface of the substrate of the access transistors  160 , the reference lines  312 , the reference pad  310 , and the sacrificial mesas  126  are shown by broken lines. The third etching process defines partially completed piezoelectric cantilevers  2201  in the top electrode layer  404  and the piezoelectric layer  406 . As shown in  FIGS. 22B and 22C , the third etching process removes part of the top electrode layer  404  and the piezoelectric layer  406  to define the top electrode  104  and the piezoelectric element  106  of the partially completed piezoelectric cantilevers in these layers, and additionally exposes part of the bottom electrode layer  408  for the next etching process. 
   After the third etching process, the layer structure  180  is subject to a fourth etching process that uses a fourth mask. As shown in  FIGS. 23A and 23B , the fourth etching process removes the unmasked portion of the bottom electrode layer  408  to define the bottom electrodes  108 , the Y-lines  304  and Y-pads  308 , and the electrical connection between the bottom electrodes and the drains of the respective access transistors  160 . The fourth etching process additionally removes the unmasked portion of the elastic layer  410  to define the elastic element  110  and to expose the access transistors  160 , part of the sacrificial mesas  126 , the prefabricated reference pad  310  and reference lines  312 . The part of the elastic element  110  that later becomes part of the beam portion of the completed piezoelectric cantilever extends over the sacrificial mesa  126 . The reference lines are connected to the source contacts  165  of the access transistors  160 . The Y-lines  304  are electrically connected to the gate contact  161  of the access transistors  160  in each column. One of the Y-lines is shown as part of the bottom electrode layer  408  on the gate contact  161  in  FIG. 23B . The fourth etching process forms partially completed piezoelectric cantilevers  2301 . 
   Next, the layer structure  180  is coated with a first protective layer  114 , as shown in  FIGS. 24A and 24B , followed by a fifth etching process that uses a fifth mask. The protective layer  114  prevents hydrogen or water penetration. The protective layer  114  is composed of aluminum oxide or any other suitable material. The protective layer  114  is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. The fifth etching process forms contact openings  703  in the protective layer  114  on each partially completed piezoelectric cantilever  2401 . The fifth etching process additionally forms release openings  707  around each partially completed piezoelectric cantilever  2401 , as shown in  FIG. 24B . As shown in  FIGS. 24B and 24C , the release openings  707  expose part of the sacrificial mesas  126  and the contact openings  703  expose the top electrodes  104 . 
   After the fifth etching process, an X-line metal layer  116  is deposited on the first protective layer  114 , as shown in  FIGS. 25A and 25B . The X-line metal layer is deposited by a process such as sputtering, CVD, plasma CVD, PVD or the like. The X-line metal layer  116  is typically composed of aluminum or an aluminum alloy. As shown in  FIG. 25C , the X-line metal layer  116  fills the contact opening  703  in the first protective layer  114  and is thus electrically connected to the top electrode  104  of the partially completed piezoelectric cantilever  2501 . 
   Next, a sixth etching process that uses a sixth mask is performed to define the X-lines  302  and X-pads  306  in the X-line metal layer  116 . The sixth etching process additionally reopens the release openings  707 . As shown in  FIGS. 26A–26C , the sixth etching process removes the unmasked portion of the X-line metal layer  116  to define the X-lines  302  and X-pads  306 , and additionally exposes the first protective layer  114  and re-opens the release openings  707  to expose part of the sacrificial mesas  126 . 
   After the sixth etching process, a seventh etching process is performed to create the cavities  130  by removing the sacrificial mesas  126 , as shown in  FIGS. 27A and 27B . No mask is used in the seventh etching process. The etchant flows through the release openings  707  and etches away the sacrificial mesa  126  from between the beam portion of each piezoelectric cantilever  150  and the top surface of the protective layer  122 . The seventh etching process releases the beam portion  154  from the substrate  120 . 
   Next, a second protective layer  118  is deposited on the layer structure  180  by spin coating, as shown in  FIGS. 28A and 28B . The second protective layer prevents direct contact between the fingertip and the X-lines  302 . 
   Finally, an eighth and final etching process that uses a seventh mask is performed. The eighth etching process removes portions of the first protective layer  114  and the second protective layer  118  to expose the X-pads  306 , the Y-pads  308 , and the reference pad  310 , as shown in  FIG. 29 . 
   The third embodiment of the method just described fabricates a piezoelectric cantilever pressure sensor array  300  with piezoelectric cantilever pressure sensors  100  in accordance with the third embodiment connected to the X-lines  302 , the Y-lines  304 , and the reference lines  312 , as shown in  FIG. 29 . As is known in the art, the piezoelectric cantilevers  150 , the X-lines  302  and X-pads  306 , the Y-lines  304  and Y-pads  308 , the reference lines  312  and reference pad  310 , and the cavities  130  may differ in size, shape and layout from the example shown in the figures. 
     FIG. 30  shows a method  3000  for manufacturing the piezoelectric cantilever pressure sensor array  300 . In the method  3000 , there is formed ( 3001 ) a layer structure having, in order, a substrate, an elastic layer, a bottom electrode layer, a piezoelectric layer, and a top electrode layer, piezoelectric cantilevers are defined ( 3003 ) in the layer structure, Y-lines and Y-pads are defined ( 3005 ) in the bottom electrode layer, X-lines and X-pads are formed ( 3007 ), and a cavity is created ( 3009 ) under each piezoelectric cantilever. 
   In an embodiment, the layer structure additionally has a prefabricated access transistor adjacent each piezoelectric cantilever. Defining the piezoelectric cantilever forming an electrical connection between the bottom electrode of the piezoelectric cantilever and the drain of the access transistor. 
   In an embodiment, the cavity is created by etching the substrate from the bottom surface thereof. In another embodiment, the cavity is created by etching the substrate from the top surface thereof. In a third embodiment, the layer structure additionally has a sacrificial mesa and the piezoelectric cantilever partially overlaps the sacrificial mesa. In this embodiment, the cavity is created by removing the sacrificial mesa from under the piezoelectric cantilever. 
   In yet another embodiment, the process of forming X-lines and X-pads includes forming a first protective coating, creating contact openings in the first protective coating, depositing an X-line metal layer on the first protective coating, and defining the X-lines and X-pads in the X-line metal layer. In another embodiment, the layer structure is covered by a flexible protective layer. 
   Although preferred embodiments and their advantages have been described in detail, various changes, substitutions and alterations can be made herein without departing from the scope of the invention defined by the appended claims and their equivalents.