Image analysis – Applications – Personnel identification
Reexamination Certificate
1999-07-02
2002-08-20
Tran, Phuoc (Department: 2621)
Image analysis
Applications
Personnel identification
C382S125000, C382S126000, C382S127000, C382S108000
Reexamination Certificate
active
06438257
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a small capacitance change detection device and, more particularly, to a small capacitance change detection device for detecting a surface shape having a small three-dimensional pattern of, e.g., the skin surface of a human finger or nose of an animal as a small change in capacitance.
As sensors for recognizing a surface shape having a small three-dimensional pattern, especially, devices aiming at fingerprint detection have been reported. As a technique of detecting a fingerprint pattern, a capacitive detection type sensor using the LSI manufacturing technology has been proposed. This is described in, e.g., “ISSCC DIGEST OF TECHNICAL PAPERS”, FEBRUARY 1998 pp. 284-285.
A capacitive detection type sensor senses the three-dimensional pattern of the skin surface of a finger by detecting an electrostatic capacitance generated between the electrodes of small sense units two-dimensionally arrayed on an LSI chip and the skin of a finger in contact with the electrodes via an insulating film. Since the capacitance value changes depending on the three-dimensional pattern on the skin surface of a finger, the three-dimensional pattern of the skin surface of a finger can be sensed by detecting the small capacitance difference.
FIG. 54
shows the basic arrangement of a conventional small capacitance change detection device using this principle. This small capacitance change detection device has a detection element
310
formed from an electrostatic capacitance between an electrode and skin of a finger in contact with the electrode via an insulating film, signal generation circuit
320
for generating a voltage signal corresponding to the electrostatic capacitance value of the detection element
310
, and output circuit
340
for converting the voltage signal from the signal generation circuit
320
and outputting a signal.
FIGS. 55A and 55B
show the layout of the conventional small capacitance change detection device. This small capacitance change detection device has a plurality of detection elements
310
, a plurality of signal generation circuits
320
, and a plurality of output circuits
340
. One detection element
310
and one signal generation circuit
320
construct a sense unit
301
. The sense units
301
are two-dimensionally arrayed on an LSI chip to form a sensor array
302
. The output circuits
340
are arranged near the sensor array
302
to form an output section
304
.
Since the electrostatic capacitance value of each detection element
310
is determined depending on the distance between the electrode of the sense unit
301
and skin surface of a finger, the electrostatic capacitance value of the detection element
310
changes depending on the three-dimensional pattern of the skin surface of the finger. When a finger is depressed against the sensor array
302
, each sense unit
301
outputs a voltage signal corresponding to the three-dimensional pattern of the skin surface of the finger. This voltage signal is converted into a desired signal reflecting the three-dimensional pattern of the skin surface of the finger, so the fingerprint pattern is detected.
The arrangement and operation of the conventional small capacitance change detection device shown in
FIG. 54
will be described below in more detail.
FIG. 56
shows the circuit arrangement of the conventional small capacitance change detection device. Referring to
FIG. 56
, reference symbol Cf denotes an electrostatic capacitance formed between the electrode of the sense unit
301
and the skin surface of a finger in contact with the electrode via an insulating film. The electrode of the sense unit
301
is connected to the input side of a current source
321
of a current I through an NMOS transistor Q
3
. A node N
1
between the electrode and transistor Q
3
is connected to the input side of the output circuit
340
. A power supply voltage VDD is applied to the node N
1
through a PMOS transistor Q
1
. The node N
1
has a parasitic capacitance Cp
1
. Signals {overscore (PRE)} and RE are supplied to the gate terminals of the transistors Q
1
and Q
3
, respectively.
The capacitance Cf forms the detection element
310
. The current source
321
and transistor Q
3
construct the signal generation circuit
320
.
FIGS. 57A
to
57
C explain the operation of the small capacitance change detection device shown in FIG.
56
.
First, the signal {overscore (PRE)} of high level (VDD) is supplied to the gate terminal of the transistor Q
1
, and the signal RE of low level (GND) is supplied to the gate terminal of the transistor Q
3
. Hence, both the transistors Q
1
and Q
3
are OFF.
In this state, when the signal {overscore (PRE)} changes from high level to low level, the transistor Q
1
is turned on. Since the transistor Q
3
is kept off, the node N
1
is precharged to VDD.
After precharge, the signal {overscore (PRE)} goes high, and simultaneously, the signal RE goes high. The transistor Q
1
is turned off, and the transistor Q
3
is turned on. Charges stored at the node N
1
are removed from the current source
321
. As a result, the potential at the node N
1
lowers.
Letting &Dgr;t be the period while the signal RE is at high level, a potential drop &Dgr;V at the node N
1
after the period &Dgr;t elapses is given by I&Dgr;t/(Cf+Cp
1
).
Since the current I, period &agr;t, and parasitic capacitance Cp
1
are constant, the potential drop &Dgr;V is determined by the capacitance Cf. Since the capacitance Cf is determined by the distance between the electrode of the sensor and the skin surface of a finger, the value of the capacitance Cf changes depending on the three-dimensional pattern of the skin surface of a finger. This means that the magnitude of the potential drop &Dgr;V changes reflecting the three-dimensional pattern of the skin surface of a finger. This potential drop &Dgr;V is supplied to the output circuit
340
as an input signal. The output circuit
340
identifies the magnitude of the potential drop &Dgr;V and outputs a signal reflecting the three-dimensional pattern of the skin surface of a finger.
In the conventional small capacitance change detection device, however, when the parasitic capacitance Cp
1
at the node N
1
is large, the potential drop &Dgr;V becomes small. When the circuit shown in
FIG. 56
is arranged using the LSI manufacturing technology in practice, the parasitic capacitance Cp
1
becomes larger than the capacitance Cf.
The potential drop &Dgr;V can be made large by increasing the current I of the current source
321
or period &Dgr;t of the signal RE at high level. However, when the current I is large, the sense units
301
with manufacturing variations are hard to control. For this reason, the current I is preferably relatively small to obtain high detection accuracy. Also, the period &Dgr;t cannot be made so long from the viewpoint of the detection time.
Consequently, the potential drop &Dgr;V as a signal to be input to the output circuit
340
becomes small, and the output varies due to noise margin or manufacturing variations, resulting in a decrease in surface shape detection accuracy.
Hence, as described above, a signal change reflecting the three-dimensional pattern of a skin surface of a finger decreases due to the influence of a parasitic element such as the parasitic capacitance Cp
1
formed in the manufacturing process, and the detection accuracy of the small capacitance change detection device becomes low.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to accurately extract a small change in capacitance by a small capacitance change detection device.
It is another object of the present invention to increase the design margin of the output circuit of a small capacitance change detection device.
In order to achieve the above objects, according to the present invention, there is provided a small capacitance change detection device comprising a capacitance detection element for detecting a small capacitance change, a signal generation circuit having an output side connected to the capacitanc
Hirata Akihiko
Machida Katsuyuki
Morimura Hiroki
Shigematsu Satoshi
Bayat Ali
Blakely & Sokoloff, Taylor & Zafman
Nippon Telegraph and Telephone Corporation
Tran Phuoc
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