Abstract:
The present invention provides a fabrication method of a pressure type fingerprint sensor, which uses the commercial integrated circuit process to form the sensor and the processing circuit together on the same chip. The present invention comprises a plurality of capacitive pressure sensors arranged in a 2-D array and applies the charge sharing principle to each capacitive pressure sensor for signal reading. The main structure of each pressure sensor is a pair of plate electrodes with an air gap between them to form a plate sensor capacitor, wherein the plate electrodes comprise a floating electrode and a fixed electrode. When the finger ridge contacts the floating electrode, the pressure from the finger changes the spacing of the air gap so as to change the capacitance of the plate sensor capacitor. The 2-D sensor array can read the 2-D pressure distribution pressed by the finger ridge to construct the fingerprint pattern.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a fabrication method of a sensor device, and more particularly relates to an integrated capacitive pressure sensor array manufactured by micromachining technology and CMOS process for fingerprint sensing. 
     2. Description of the Prior Art 
     There are many known techniques of identifying an individual through the identification of the individual&#39;s fingerprint. The use of an ink pad and the direct transfer of ink by the thumb or finger from the ink pad to a recording card is the standard way of making this identification. Then, an optical scanner scans the recording card to get an image, which is then compared to fingerprint images in the computer database. However, the most serious drawback of the above-mentioned method is that the fingerprint identification cannot be processed in real-time, and thus cannot satisfy the requirement of real-time authentication, such as network authentication, e-business, portable electronics products, personal ID card, security system, and the like. 
     The method for reading a fingerprint in real-time has become the important technology in the biometrics market. Conventionally, an optical fingerprint sensor may be used to read a fingerprint in real-time, which can be referred to in U.S. Pat. Nos. 4,053,228 and 4,340,330, and the development is quite mature and accurate. However, the optical fingerprint sensor has a drawback because it is large in size. 
     Consequently, silicon fingerprint sensors, which overcome the drawbacks of the optical sensor and are formed by silicon semiconductor technology, have been developed. According to the consideration of silicon integrated circuit (IC) processes, the capacitive fingerprint sensor has become the most direct and simple product, which is referred to in U.S. Pat. Nos. 4,290,052 and 4,353,056. However, the problem with the capacitive fingerprint sensor is that it does not effectively overcome the interference problem caused by moisture on the finger and ESD damage to the sensor circuits. 
     The latest method utilizes capacitive pressure sensor array fabricated by micromachining technology, as the detecting method of the fingerprint. The related material is referred to in appendix  1  “A High Density Capacitive Pressure Sensor Array For Fingerprint Sensor Application” disclosed by Rey et al.; appendix  2  “A Very High Density Bulk Micromachined Capacitive Tactile Imager” disclosed by De Souza et al.; and appendix  3  “MEMS Fingerprint Sensor With Arrayed Cavity Structures” disclosed by Sato et al., which utilize the pressure pressed from the ridge of the fingerprint for the sensing principle to effectively overcome the above mentioned moisture problem of the capacitive fingerprint sensor. However, the methods disclosed by Rey, De Souza, et al. can not be effectively integrated into the integrated circuit process. Hence, it is not feasible for actual use. The post-IC method disclosed by Sato et al. utilizes an electroplated gold material and sacrificial layer technology to form the pressure sensor structure. However, it increases the process complexity, reduces the yield, and increases the cost. Furthermore, the gold material is not compatible with silicon integrated circuit processing and causes pollution problems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fabrication method of a pressure type fingerprint sensor, which comprises a plurality of capacitive pressure sensors arranged in a 2-D array for reading the fingerprint, to overcome interference problems caused by the moisture of a finger and to overcome the ESD damage to the sensor circuit. The fabrication method of the present invention is completely compatible with commercial integrated circuit processes, especially the CMOS process and material. Additionally, the present invention can improve the yield and reduce the cost. 
     Another object of the present invention is to form a pressure type fingerprint sensor, which applies the charge sharing principle to detect the capacitance variation of each capacitive pressure sensor. 
     In order to achieve these and other objects, the present invention discloses a fabrication method of a pressure type fingerprint sensor comprising a pressure sensor array and a set of processing circuitry. Wherein, the pressure sensor array comprises a plurality of pressure sensors arranged in a 2-D array. Each of the pressure sensors further comprises a plate sensor capacitor comprising a floating electrode, an air gap, and a fixed electrode; a reference capacitor connected to the fixed electrode; and a signal reading unit arranged beside the sensor capacitor for reading the sensor capacitance and connecting to the processing circuitry. Wherein, the floating electrode is used as a contacting surface on which the finger is positioned. The pressure pressed from the ridge of the fingerprint changes the spacing of the air gap so as to change the sensor capacitance. The sensor capacitor further includes a protrusion arranged at a central portion of the contacting surface as a stress concentration point to enhance the displacement of the floating electrode when contacted by the finger ridge. There is further a protection layer formed on the most outer surface for wearing and chemical resistance purposes. 
     Other aspects, features, and advantages of the present invention will become apparent, as the invention becomes better understood by reading the following description in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the accompanying advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a 3-D schematic representation of the pressure type fingerprint sensor, in accordance with the present invention; 
     FIG. 2 is a block diagram of the pressure type fingerprint sensor, in accordance with the present invention; 
     FIG. 3 is a schematic representation of reading the fingerprint by the pressure type fingerprint sensor, in accordance with the present invention; and 
     FIG. 4A to FIG. 4C are schematic representations of fabrication flows of the pressure type fingerprint sensor, in accordance with one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer to the FIG. 1, which is a 3-D schematic representation of a portion of a pressure type fingerprint sensor  2  in accordance with one embodiment of the present invention. It comprises a plurality of pressure sensors  20  in 2-D array formed on a silicon substrate  200 . The structure of each pressure sensor  20  is a plate sensor capacitor  21  comprising a fixed electrode at the bottom (not shown in the figure) and a floating electrode  21   a . The floating electrode  21   a  is a diaphragm structure with its periphery fixed on the silicon substrate  200  and there is an air gap between the floating electrode  21   a  and the fixed electrode (not shown in the figure). In addition, on the central portion of the floating electrode  21   a , at least, one protrusion  23  is formed as a stress concentration point by contact with the finger ridge to enhance the displacement (the change of the capacitance will become bigger) of the floating electrode  21   a  to improve the sensitivity. Simultaneously, there is further a protection layer formed on the outer most layer for wearing and chemical resistance purposes. 
     In each pressure sensor  20 , a reference capacitor is designed under each sensor capacitor  21  (not shown in the figure) and a signal reading unit  22  based on charge sharing configuration is positioned by the sensor capacitor  21  to in situ process and output the detected signal. A detailed description regarding signal reading unit  22  of the pressure sensor  20  of this invention may be found in commonly-owned, co-pending U.S. patent application Ser. No. 10/403,052, filed Apr. 1, 2003 and entitled “CAPACITIVE FINGERPRINT SENSOR,” the disclosure of which is incorporated by reference as if fully set forth herein. 
     In order to more clearly explain the architecture of the pressure type fingerprint sensor shown in FIG. 1, please refer to FIG. 2, which is a block diagram of the system configuration of the pressure type fingerprint sensor in accordance with an embodiment of the present invention. The pressure type fingerprint sensor mainly consists of a pressure sensor array  201 . A set of processing circuitry is composed of a row decoder  202 , a column decoder  203 , a correlated double sampler (CDS)  204  (the column decoder  203  combining the CDS  204  is named column multiplexer hereafter). The row decoder  202  is arranged beside the sensor array  201 . The column multiplexer is arranged beside the sensing members array  201  and at a side perpendicular to the row decoder  202 . 
     The row decoder  202  controls the charging and charge-sharing in a specific pressure sensor  20 ′ through a specific set of control line  202   a . Then, a voltage signal output from the pressure sensor  20 ′ is obtained by the column multiplexer via a specific signal line  203   a . The obtained voltage signals may be sequentially amplified and converted into digitally gray-scale image by an analog signal processing unit including a programmable gain amplifier  205  and an analog-to-digital converter  206 . The actions mentioned above are all controlled by a controlling and interface circuit  210 . 
     At the same time, a trigger electrode layout  207  is designed amid the pressure sensor array  201 . This layout of the trigger electrode ensures at least a portion of the finger can contact over the trigger electrode surface to switch on the power-controlling circuit  208 . The power-controlling circuit  208  will turn on the power of this fingerprint sensor while the finger contacts the sensor surface. A detailed description regarding the trigger design of this invention may also be found in commonly-owned, co-pending U.S. patent application Ser. No. 10/403,052, filed Apr. 1, 2003 and entitled “CAPACITIVE FINGERPRINT SENSOR,” the disclosure of which is incorporated by reference as if fully set forth herein. 
     Refer to FIG. 3, which is a schematic representation of reading the fingerprint by the pressure type fingerprint sensor. Wherein, the plate sensor capacitor  21  is composed of a floating electrode  21   a  and a fixed electrode  21   b . There is an air gap  24  between these two electrodes  21   a  and  21   b . There is a protrusion  23  formed on the central region of the floating electrode  21   a  as a stress concentration point by contact with finger ridge to enhance the displacement (the change of the capacitance will become bigger) of the floating electrode  21   a  to improve the sensitivity. 
     When the finger  1  touches the pressure sensor array, only a portion of the sensors is touched by the finger ridge  11  (a portion of the sensors is covered under the finger valley  12 ) to sense the pressure from the finger  1 . The pressure will cause a displacement d of the floating electrode  21   a  to change the sensor capacitance between two electrodes, wherein the amount of the displacement is dependent on the pressure extent. After collecting the voltage signals from the sensor array  201 , the amount of sensors touched by the finger ridge  11  will be configured to reconstruct the ridge pattern as the fingerprint data. This sensing principle completely overcomes the moisture problem mentioned above due to its discrimination of pressure from a finger or not. Simultaneously, the floating electrode  21   a  is connected to ground so that ESD from any approaching body will be directly conducted to ground to avoid damaging the sensor circuit. 
     A superior advantage of the manufacturing of the pressure type fingerprint sensor of the present invention is that the present invention fully utilizes a commercial sub-micro integrated circuit process with aluminum interconnections (n layers of Al interconnections for simplified explanation), especially the Complementary Metal Oxide Semiconductor (CMOS) process. In order to simplify the description, herein only explains how to utilize the CMOS process to complete the structure design and the material properties of a single pressure sensor  21 . Other circuit designs and manufacturing are well-known technology, so will not be discuss herein. 
     Refer to FIG. 4A, which is a semi-finished structure of a single pressure sensor  21  completed by a commercial CMOS process. The present invention utilizes the (n−1) th  metal layer  304  as a sacrificial layer material. The structure of the metal layer  304  is usually a sandwich structure of titanium  304   a , aluminum alloy  304   b , and titanium nitride  304   c . A plurality of plug metals  303   b  feedthrough the (n−2) th  inter metal dielectric (IMD) layer  303  to connect the metal layer  304  and the interconnection layer there below (not shown in the figure), such as a metal layer or a polysilicon layer. A plurality of plug metals  305   b  feedthrough the (n−1) th  IMD layer  305  to connect the metal layer  304  and the n th  metal layer  306  A passivation layer  307  covers the most outer surface of the sensor. The etching window  307   a  is formed to remove a portion of the passivation layer  307 , a portion of the (n−1) th  IMD layer  305 , and a portion of titanium nitride  304   c  to expose a portion of aluminum alloy  304   b . The protrusion  308  can be a patterned polymer, for example cured polyimide, or metal. 
     Refer to FIG. 4B, after finishing the process of the FIG. 4A, the semi-finished structure is then put into an aluminum etching solution. The chemical solution etches the aluminum alloy  304   b  away through the etching window  307   a  to form an air gap  24 . The etching solution is composed of phosphoric acid, nitric acid, and acetic acid and can rapidly removes the aluminum material at an etching rate of more than 1 micrometer per minute. At the same time, the etching solution has an excellent selectivity over the titanium  304   a  and the titanium nitride  304   c  so as to etch the aluminum alloy  304   b  only and to leave the titanium  304   a  and the titanium nitride  304   c . The remained titanium  304   a  is used as the fixed electrode  21   b  of the plate sensor capacitor  21 , and electrically connects to the interconnection layer there below (not shown in the figure) via a plurality of metal plugs  303   b . The titanium nitride  304   c  is used as the floating electrode  21   a  of the plate sensor capacitor  21 , and electrically connects to the n th  metal layer  306  via a plurality of metal plugs  305   b . A portion of the n th  metal layer  306 , a portion of the (n−1) th  IMD layer  305 , and a portion of the passivation layer  307  form the diaphragm structure of the floating electrode  21   a  of the plate sensor capacitor  21 . 
     Such as shown in the FIG. 4C, there is a protection layer  309  formed on the outer most surface of the device to seal the etching windows  307   a  and to finish the final pressure sensor structure  21 . The protection layer  309  is a dielectric material such as silicon oxide, silicon nitride, or silicon carbide. The protection layer  309  may further include a polyimide layer formed on the dielectric surface as the contacting member of the finger. 
     In the manufacturing process, the related materials, and the fabrication method of FIG.  4 A and FIG. 4B are all completely compatible with all kinds of commercial integrated circuit processing. In the manufacturing process of FIG. 4C, the sensor does not require any photo-masking process which is an important concept of the manufacturing process of the present invention. 
     Although the present invention has been described in terms of the exemplary embodiments, numerous modifications and/or additions to the above-described embodiments would be readily apparent to those skilled in the art. It is intended that the scope of the present invention extends to all such modifications and/or additions and that the scope of the present invention is limited solely by the claims set forth below.