ABSTRACT
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%.
INTRODUCTION:
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.
FINGERPRINT SENSOR/IDENTIFIER 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.
IDENTIFYING PIXEL:
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
SENSOR STRUCTURE:
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.
IMPLEMENTATION:
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.
CONCLUSION:
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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