A Single-Chip Finger Print Sensor And Identifier

A chip architecture that integrates a fingerprint sensor and an identifier in a single chip is proposed. The Fingerprint identifier is formed by an array of pixels, and each pixel contains a sensing element and a Processing element. The sensing element senses capacitances formed by a finger surface to capture a Fingerprint image. Identification is performed by the pixel-parallel processing of the pixels. The sensing element is built above the processing element in each pixel. The chip architecture realizes a wide-area sensor without a large increase of chip size and ensures high sensor sensitivity while maintaining a high image density. The sensing element is covered with a hard film to prevent physical and chemical degradation and surrounded by a ground wall to shield it. The wall is also exposed on the chip surface to protect against damage by electrostatic discharges from the finger contacting the chip. A 15* 15 mm2 single-chip fingerprint
Sensor/identifier LSI uses 0.5um standard CMOS with the sensor process. The sensor area is 10.1 * 13.5 mm2: The sensing and identification time is 102 ms with power consumption of 8.8 mW at 3.3 V. Five hundred tests confirmed a stranger-rejection rate of the chip of more than 99% and a user-rejection rate of less than 1%.

Portable and mobile types of equipment, like IC cards, notebook computers, and cellular phones, are becoming increasingly popular. Consequently, user authentication to prevent unauthorized use of such equipment has become an important issue. For consumer products, the authentication process should be simple and reliable. Methods of user authentication include the use of passwords, personal identification numbers (PIN’s), and verification using the iris in the human eye. One of the simplest and most reliable authentication method is the fingerprint. For use in portable and mobile equipment, the fingerprint identification unit should be compact and inexpensive and must guarantee security of the user’s data and privacy. A fingerprint identification unit is composed of a fingerprint sensor, a fingerprint identification device, and a memory that stores a template of the user’s fingerprint. Fingerprint identification requires extensive image processing, performed by a high performance microprocessor, and a large memory. The template is also stored in this memory. The size and cost of the components make such a unit unsuitable for application to portable equipment. Recently, some small, thin, and inexpensive direct-touch semiconductor fingerprint sensors have been proposed. These devices sense the capacitance formed by the finger when it makes direct contact with the chip.

A. Requirements for Fingerprint Chip Architecture: Recently, some architecture for the integration of the sensor and processing unit in a single chip has been reported. Fig. 1 shows the conventional photo image sensing chip architecture. In the architecture for a sensor embedded chip, the sensor and processing unit are integrated on a single chip by placing them side by side, as shown in Fig. 1(a). The photo sensor does not require large area, but a rather large number of pixels are needed for capturing an array. On the other hand, the sensing area of a direct-touch fingerprint sensor should be as large as the finger because the sensor senses the fingerprint of a finger in contact with the chip. Recently, a small-area slim fingerprint sensor was proposed. It senses fractional images of a fingerprint as the finger is slid across it. The architecture for the sensor-embedded pixel array employs a pixel array; a sensing element and processing element are placed side by side in each pixel, as shown in Fig 1(b).
In this architecture, the array of pixels forms the sensor and processing unit. Image processing is carried out by parallel processing of the processing elements, a direct-touch fingerprint sensor requires a large sensing element, for example over 30 m because the sensor senses the capacitance of the finger on the sensing element,
And its sensitivity depends on the size of the detection plate within the sensing element... If these architectures were employed to provide a single chip fingerprint sensor and identifier, the characteristics would be as summarized in Table I. The fingerprint chip architecture requires not only a large-area sensor in a small chip but also an expanded sensing element with higher image density. The sensor-embedded chip can provide a large sensing element and high sensitivity. It can also enhance the density of the sensed fingerprint image.

B. Fingerprint Chip Architecture: The present fingerprint sensor/identifier chip architecture is proposed to satisfy all requirements for the integration of the fingerprint sensor and identifier in a single chip. Fig. 2 is a block diagram of the chip architecture. The innovative features of the proposed architecture are that a
Fingerprint is sensed and identified by the array of pixels and the sensor is stacked above the identifier to integrate them in each pixel. The chip is composed of a identifying array and a small Embedded controller.The array consist of pixels,each of which has a sensor
and a processing element and handles one pixel of fingerprint image data. The pixel parallel structure of the identifier provides a large sensor area in a small chip. In the architecture, the pixel size would depend on the size of the processing element. For a high-density sensor, the processing element has to be small, and its function also has to be limited. In order to provide an identifier using such pixels, the architecture employs an optimized fingerprint identification algorithm, which uses the combination of simple image processing to identify a fingerprint. On the other hand, Stacking the sensor element above circuits increases the influence of parasitic capacitance. In the proposed architecture, the sensing circuit is designed to suppress the influence of parasitic capacitance. In this chip architecture, the user’s template fingerprint data are stored in the chip, and sensing and identification are Completely performed without any other chips. There is no transmission of user fingerprint data between the chip and external devices. Therefore, the user template cannot be replaced with different one, and the user’s fingerprint image cannot be stolen. Moreover, high-speed and low-power fingerprint identification can be achieved.
C. Fingerprint Identification: In the proposed architecture, the fingerprint is identified by the parallel processing of the pixels. To enhance the density of the sensor, the size of the processing circuit has to be limited, and an algorithm suitable for such processing circuit is required. In our pixel array, we adopt the fingerprint verification method based on thinned-image pattern matching. This algorithm employs a large number of simple image processing, such as image pattern matching, for fingerprint identification, so it is quite suitable for the array of the pixels, which can handle simple image processing at high speed and low power.      In this algorithm, a user’s original fingerprint image is thinned down to make a user’s template. For the identification, the sensed fingerprint is binarized to a black-and-white image. The identification is carried out on the basis of image pattern matching between the thinned template and the binarized fingerprint images. If the maximum matching ratio of the two images was higher than a threshold, identification would be achieved and the fingerprint authenticated. Using a thinned image as a template allows for fluctuation and rotation (up to 10 of the fingerprint image.Fig.3 shows the process for fingerprint registration and identification. For registration, the sensor produces an image of the user’s fingerprint, and the image is sent to an external PC. The PC thins the lines in the image down to a width of 1 pixel and writes this thinned image to the memory of the identifier as the user template. Then, the interface circuit, which communicates with the external PC, is shut off by breaking it. The identifier cannot make a template by itself, and cannot input and output fingerprint images. This prevents illicit registration of a different template and theft of the user’s template and sensed image. After registration, the chip alone senses and identifies the fingerprint. For identification, the sensor again produces an image of the user’s fingerprint when the finger makes contact with the chip.The identifier compares that image with the template using pixel-parallel processing, and produces an index indicating the closeness of the match. If the index is above a threshold, a positive result is output. The details of how the chip carries out the fingerprint identification algorithm are as follows. The embedded controller collects the results of the comparison from all pixels and evaluates the matching ratio. In practical use, it cannot be guaranteed that the finger is put at the same position on the sensor every time. So the pixel array, using communication among neighboring pixels, shifts the sensed image in order to allow for deviation of the finger location. The sensed image is shifted in all of the defined directions (i.e., horizontal and vertical axis from 20 to 20 pixels) and compared with the template at each shifting to produce the maximum matching ratio.

For the fingerprint identification, the array of identifying pixels employs pixel parallel image processing. For such processing, each processing element in the identifying pixels has to perform image processing on a 1-pixel datum of a fingerprint image in parallel. Hence, an identifying pixel must:

• store the 1-pixel datum of the fingerprint image as a user template;
• sense the shape of a fingerprint and produce the fingerprint image;
• binarize the sensed image into a 1-bit datum for digital processing;
• shift the image to allow variations in finger position;
• compare the sensed datum with the template datum and transmit the result to the controller.
The identifying pixel is devised to handle all these functions while maintaining small pixel size. It consists of a sensor plate, a sensing circuit, a 1-bit memory, and a processing circuit, as shown in the block diagram in Fig. 4.The memory stores a 1-pixel datum of the user template fingerprint image made by the external PC at registration. The sensor plate is the fingerprint sensing element and is built above the processing element in each pixel. Stacking the sensor plate above logic circuits achieves the vertical integration of the sensing element and processing element in the pixel while providing a larger sensing element and a higher image density. The sensing circuit is connected to the sensor plate and detects the fingerprint on the plate. In order to suppress the influence of the parasitic capacitance, the differential charge- transfer amplifier technique is applied to the sensing circuit.
When the finger touches the chip, capacitance is formed between the surface of the finger and the sensor plate. It depends on the distance between the finger and the plate, and its value reflects the shape of the fingerprint. Then, the sensing circuit converts the sensed value into a 1-bit digital signal indicating whether the sensed point is a ridge or a valley and outputs it. The processing circuit performs image processing according to the control signals from the controller. It is connected to the sensing circuit and the memory and gets data from them to compare a sensed image datum produced with the template image datum stored in the memory. The comparison result is sent to the controller for the evaluation of the matching ratio. The processing circuit is also connected to neighboring pixels to communicate the pixel datum. Communication among neighboring pixels enables shifting of a fingerprint image. Fig. 5 shows the circuit diagram of an identifying pixel. It consists of a sensing circuit, a memory circuit, and a processing circuit containing a selector, register, and comparator. In the processing circuit, the selector inputs the signals from the sensing circuit and the neighboring pixels. It selects these input signals according to the control signal from the controller and outputs the selected signal to the register. The register stores the signal from the selector with the control signal and outputs the stored data to the comparator and the neighboring pixels. If all of the pixels transfer the stored data to neighboring pixels in the same direction, the
stored fingerprint image can be shifted. This direction depends on the signal election of the selector. The comparator compares the sensed and shifted image datum with the template datum stored in the memory and sends the result of the comparison to the controller. The memory circuit stores the pixel datum of the user template fingerprint, and it is connected to the comparator. It is also connected to the register through a switch circuit. One of the pixels in the array is connected to the controller. At registration, the controller sends the template datum to this pixel, and then that datum is transferred pixel
by pixel pixel to neighboring pixels using the image-shift operation. As a result, the user template data are spread over the registers of all pixels. When the switch circuit turns on, the register writes the transferred template datum into the memory circuit. As described above, the identifying pixel can perform all of the functions required for fingerprint-identification processing. The array of identifying pixels enables pixel parallel processing for fingerprint identification and achieves a large-area sensor in a small chip and high speed, low-power operation

Fig. 6 is a cross section of an identifying pixel. The sensor plate is situated above some logic circuits, which are composed of the sensing, memory, and processing circuits.When a finger comes in contact with the chip, the capacitance between the plate and the finger is either for a ridge or for a valley. These capacitances are detected and binarized by the sensing circuits directly under the plate .Each sensor plate is surrounded with a lattice-like wall. This wall is also exposed on the chip surface and connected to ground. It discharges the charge on the finger to protect the circuits from electrostatic discharge and to eliminate fluctuations in the detection voltage. The wall is made of copper and encapsulated by ruthenium. Ruthenium is used as a protective material to prevent the oxidation of the copper. The wall also shields the sensor from neighboring sensors. This reduces the noise caused by the coupling capacitance and improves the signal-to- noise ratio of sensing. To protect the surface of the chip and the logic circuits from physical and chemical damage, the sensor is covered with a hard passivation film. This structure provides robust identifying pixels.

 To check the effectiveness of the proposed architecture, a test chip was fabricated using a standard 0.5um CMOS three metal process in combination with the sensor process. Fig. 7 is a micrograph of the chip. Each pixel is 81.6 m square and contains 158 MOSFET’s. The sensor plate is built above these transistors and is 63.2 m square. The density of a fingerprint image is 311 dpi. The chip also has a 6.5-Kgate embedded controller and a 16-Kbit program memory. This memory stores only the program for the controller. Fig. 8 shows how the sensing circuit, the memory, and the processing circuits, which consist of the register, comparator, selector, and bus driver, are laid out in the identifying pixel. The sensor plate and ground wall are built above these circuits. In this layout, the circuit elements, such as transistors and lines, are placed uniformly in the pixel, in order to make the sensor plate flat and enhance the yield of the chip. Fig. 9 shows scanning electron microscope (SEM) micrographs of the pixel array and an individual pixel. A top view of n this picture that each pixel is surrounded by the ground wall, and the wall is exposed on the chip surface the identifying-pixel array is shown in Fig. 9(b) is a cross-section view of an identifying pixel. One can see that the sensor plate and the ground wall are built above the logic circuit, and the sensor plate is shielded by the ground wall. In order to on firm the effectiveness of the proposed chip architecture, the fingerprint sensor/identifier chips were designed using the sensor-embedded chip and the sensor-embedded pixel array architecture. Fig. 10 shows the estimated features of these chips. The sensor-embedded chip architecture has a tradeoff between chip size and sensor area. A high-density and High-sensitivity sensor can be fabricated, but the expansion of the sensor area greatly increases chip size. When the circuit for the processing unit is assumed to be the same as that in the fabricated chip, and the parameters, which are the sensor plate size, the image density, and the sensor area, are the same as those in the fabricated test chip, the fabricated chip is 38% smaller than the one with the Sensor-embedded chip architecture,
as shown in Fig. 0(a).
The proposed architecture can expand sensor area without a large increase of chip size. On the other hand, the sensor-embedded-pixel array architecture has a tradeoff between the sensor-plate size and image density. A large sensor area and small chip can be achieved, but the expansion of the sensor plate to enhance sensor sensitivity reduces the density of a fingerprint image.Fig.10 (b) shows the relationship between image density and sensor plate size, when the sensor plate and the sensor area are the same as those in the fabricated test chip and both chips are the same size. The density of the fabricated chip is limited to 311 dpi, because the pixel size is dominated by the size of the processing element under the plate when the sensor plate is smaller than the processing element. The image density of the fabricated chip is 40% higher than that of the chip with the sensor-embedded-pixel array. The proposed chip architecture can enhance both the image density and the sensor sensitivity.

A chip architecture that enables a fingerprint sensor and identifier to be integrated on a single chip has been presented. The key feature is a pixel architecture that incorporates a sensing element and a processing element by stacking them without a large increase of chip size. This enables a high-sensitivity
sensing element to be fabricated above the processing element while maintaining high image density. Each pixel is surrounded by a ground wall to shield it to suppress the noise. When a proposed array of identifying pixels is fabricated, not only does the top surface constitute a sensing plate, but the circuitry below can also perform both sensing and identification functions. A test chip was fabricated in a standard 0.5- m CMOS process with the sensor process. The experimental results demonstrate the effectiveness of the architecture. The proposed chip architecture enables a fingerprint sensor and identifier to be integrated on a single chip, and for the first time provides a means of user authentication for portable and mobile equipment other than PIN’s and passwords.

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