JPH0561966A - Fingerprint sensor - Google Patents

Abstract

PURPOSE:To detect pressure distribution in accordance with a fingerprint pattern by a piezoelectric thin film sensor with high accuracy by arranging a sensor element to which polarization processing is applied in matrix shape via upper and lover plane electrodes. CONSTITUTION:A sensor element 1 is formed in such a manner that a piezoelectric thin film 5 with at least width size W of a lower plane electrode 4 is formed on the lower plane electrode 4. The element is comprised by forming an upper plane electrode 6 retracted by size W2 of 1/2 the piezoelectric thin film 5 from the boundary side of the lower plane electrode 4 on the piezoelectric thin film, and applying the polarization processing via the lower plane electrode 4 and the upper plane electrode 6. The piezoelectric sensor element 1 consisting of the piezoelectric thin film 5 is brought into contact with the chevron part (ridge) of a fingerprint, and the pressure of each piezoelectric element at a part coming in contact with the chevron part is converted to an electrical output signal by a piezoelectric effect. Only a piezoelectric element at a part to which the polarization processing is applied via the lower plane electrode 4 and the upper plane electrode 6 is provided with the piezoelectric effect, and no influence on a neighboring element is applied, thereby. the pressure distribution in accordance with the fingerprint pattern of a finger can be accurately detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fingerprint sensor using a piezoelectric thin film, and more particularly, to a structure of a sensor unit for detecting a fingerprint pattern of a finger.

[0002]

2. Description of the Related Art In recent years, with the development, popularization and diversification of information systems, information security using fingerprint information such as "everyone is different" and "lifetime invariant" has been attracting attention. The fingerprint collation system currently put into practical use has a large shape and is expensive to be incorporated in various terminals, and in the future, there is a strong demand for a miniaturized or card-shaped fingerprint sensor that can be incorporated in an information system device.

In the conventional fingerprint detecting method, a fingertip is pressed against a glass surface or the like, the portion is irradiated with a light source, the reflected light is photoelectrically converted by a CCD or the like into an electric output signal, and the electric output signal is processed. By doing so, the fingerprint is detected (see, for example, JP-A-60-114979).

In addition to such an optical fingerprint sensor, a method of pressure-sensitively detecting a fingerprint pattern by forming a matrix electrode on a pressure-sensitive sheet and electrically taking out the resistance value of the pressure-sensitive sheet (for example, JP-A-63-204374
No. gazette) is also proposed.

[0005]

However, in the above-mentioned optical fingerprint sensor, an optical system including a light source and its power source, a lens, and the like, and a photoelectric conversion element such as a CCD are required, so that the sensor section becomes large in size. It had a problem of high cost. Further, in the pressure-sensitive fingerprint sensor described above, since the pressure-sensitive sheet is used, the structure of the sensor unit is simple and can be made thin, but since pressure-sensitive conductive rubber is used for sensing, reproducibility, creep, etc. are reliable. It had a problem of lack of sex.

Further, as a practical problem of the pressure-sensitive fingerprint sensor, there is an influence on an adjacent element at the time of pressure detection. As a solution to this problem, a method of insulating the pressure-sensitive sheet via a small piece or a method of forming a slit in the pressure-sensitive sheet for insulation has been proposed, but the manufacturing process becomes complicated.

An object of the present invention is to solve the above-mentioned conventional problems, and an object thereof is to provide a fingerprint sensor which does not affect adjacent elements, can be miniaturized, has high resolution and is highly reliable.

[0008]

In order to solve the above problems, in the fingerprint sensor of the present invention, a piezoelectric thin film is formed on the lower surface electrode, and then at least ½ of the thickness of the piezoelectric thin film is formed on the piezoelectric thin film. It has a configuration in which a sensor element configured by forming an upper surface electrode that is recessed by a dimension and performing polarization processing through the lower surface electrode and the upper surface electrode is arranged in a matrix.

[0009]

According to the above means, the piezoelectric sensor element formed of the piezoelectric thin film is in contact with the ridge (ridge) of the fingerprint, and each piezoelectric element in the portion in contact with the ridge of the fingerprint by the (reverse) piezoelectric effect. The pressure is converted as an electrical output signal. This piezoelectric element is polarized through the lower surface electrode and the upper surface electrode, and only the piezoelectric element in the polarized portion has the piezoelectric effect.
The pressure distribution according to the fingerprint pattern of the finger can be accurately detected without affecting adjacent elements.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view for explaining the structure of one unit, that is, one sensor element portion of the fingerprint sensor matrix of the present invention. In FIG. 1, reference numeral 1 denotes a piezoelectric thin film 5 having a width W ₁ of at least the width of the lower surface electrode 4 formed on the lower surface electrode 4, and then at least the thickness of the piezoelectric thin film 5 from the boundary side of the lower surface electrode 4 on the piezoelectric thin film 5. Is a sensor element formed by forming the upper surface electrode 6 which is recessed by half the dimension W ₂ and subjecting it to polarization through the lower surface electrode 4 and the upper surface electrode 6.

Further, 2 is a switching MOS field effect transistor (hereinafter referred to as FET) for scanning the sensor element 1, and 3 is signal processing by lowering the impedance of the output signal of the sensor element 1. It is an impedance conversion FET to make it easier
May be an FET for amplification that amplifies the output signal of the sensor element 1.

Since the output of the sensor element 1 formed of the piezoelectric thin film 5 is very small and the output impedance is extremely large, in order to read the output of the sensor element 1 accurately, amplification of the output, conversion of impedance, or the like is necessary. Both are required.

A feature of the fingerprint sensor of the present embodiment is that the piezoelectric thin film 5 only in the sensor element 1 part is polarized. With this configuration, only the piezoelectric element in the detection part has a piezoelectric effect. Because of this, the generated charge due to pressure does not leak to the adjacent element, and an accurate pressure distribution according to the fingerprint pattern of the finger can be detected.

In order to explain the structure of the sensor element 1 in this embodiment in more detail, FIG. 2 is a sectional view taken along the line AB in FIG. In the figure, first, in order to adjust the height of the sensor element 1 on the single crystal silicon substrate 8,
In the process of forming the FET, the single crystal silicon substrate 8 is oxidized to form the oxide film 7, and then the lower electrode 4 made of aluminum or the like is formed by a vacuum evaporation method or a sputtering method using a mask.

Next, a piezoelectric thin film 5 made of lead zirconate titanate having a width W ₁ of at least the width of the lower electrode 4 and a thickness t is formed on the lower electrode 4 by a sputtering method, and the piezoelectric thin film 5 is further formed. An upper surface electrode 6 made of aluminum or the like is formed on the boundary surface of the lower surface electrode 4 by a vacuum evaporation method or a sputtering method so as to recede from the boundary surface of the lower surface electrode 4 by a dimension W _{2 which is} at least half the thickness t of the piezoelectric thin film 5.

Then, a DC voltage is applied to the lower surface electrode 4 and the upper surface electrode 6 to polarize the piezoelectric thin film 5, and then an insulating protective film (not shown) is formed on the upper surface electrode 6 to form the sensor element 1. I am configuring.

Here, the potential difference, that is, the output voltage, generated between the lower surface electrode 4 and the upper surface electrode 6 when the sensor element 1 comes into contact with the peak portion of the fingerprint pattern and pressure is applied to the piezoelectric thin film 5, Since it is proportional to the piezoelectric constant of the piezoelectric material and inversely proportional to the thickness of the piezoelectric material, the material of the piezoelectric thin film needs to be a material having a uniform thickness, a small thickness, and a large piezoelectric constant. In addition to this, in consideration of easiness of film formation, film formation temperature, film uniformity, and the like, a lead zirconate titanate-based material is preferable, and a thickness t of about 1 μm is suitable.

The switching FET 2 and the amplifying or impedance converting FET 3 are composed of an n-type or p-type source and drain formed by ion implantation on a single crystal silicon substrate 8 and a polycrystalline silicon gate, The lower surface electrode 4 of the sensor portion is taken out from the gate 10 of the amplification or impedance conversion FET 3, the upper surface electrode 6 of the sensor portion is taken out from the source 11, and the drain 12 is connected to the drain 14 of the switching FET through the electrode 13. The drain resistance 15 is connected via the electrode 13, and the electrode 16 is connected to the positive side of a power source (not shown).

Depending on the size of the drain resistance 15, amplification of the sensor output or impedance conversion can be realized (both functions of amplification and impedance conversion can also be realized). A control signal is applied to the gate 17 of the switching FET 2 via the electrode 18, and an output signal from the source 19 is detected via the electrode 20.

Here, the structure of the mountain portion of the fingerprint (ridge structure)
Is a fine pattern with a spatial frequency of 2-3 lines / mm, and the width of the valley portion (valley line) of the fingerprint is about 100 μm in a narrow place. Therefore, in order to input the fingerprint pattern with good reproducibility, A resolution of about 50 μm (20 lines / mm) is required as a sensor. Therefore, the detection area is 2
Assuming that the resolution is 0 × 20 to 25 × 25 mm and the resolution is 50 μm, 400 × 400 to 500 to 500 (160,000 pixels to
It is necessary to configure a sensor unit of 250,000 pixels).

That is, the sensor element 1, the switching FET 2 and the amplifying or impedance F shown in FIG.
By arranging a plurality of one sensor unit (hatched area in the figure), which is composed of ET3 and has a side of about 50 μm, in a matrix as shown in FIG. I am configuring.

With this configuration, it is possible to detect the peaks and valleys of the fingerprint, and it is possible to accurately detect the pressure distribution according to the fingerprint pattern of the finger. In the present embodiment, one sensor unit is arranged in a matrix, but the present invention is not limited to this, and it is also possible to arrange in a zigzag shape, for example.

In this embodiment, the FET array of single crystal silicon is used as the means for detecting the electric output signal. The reason for this will be described.

As a detection method other than the above-mentioned FET array, there is a method of transferring an output signal by a CCD, but since the minimum handling charge is about 6.3 × 10 ^-17 C, a small output charge by the piezoelectric thin film can be accurately measured. However, there is a problem in that a sufficient gradation cannot be obtained for detection.

Further, there is a method of forming a switching element by using a thin film transistor (TET) made of amorphous silicon or polycrystalline silicon. However, both of them have a large leak current as compared with a FET made of single crystal silicon, so that they are on the order of μsec. There is a problem that it is necessary to read the output signal in a very short time.

In comparison with this, by forming an amplifying element or an impedance conversion element and a switching element on single crystal silicon, single crystal silicon has a leakage current of 10 ^{2 as} compared with amorphous silicon or polycrystalline silicon. Since the output signal reading time is on the order of msec to sec, which is about 10 ⁴ times less, it is possible to accurately detect a small output charge due to the piezoelectric thin film.

Next, a method of detecting the output signal of this fingerprint sensor will be described. For simplification of the description, FIG. 3 shows a circuit diagram of a fingerprint sensor including detection elements in a matrix of 3 rows × 3 columns. In the figure, _first , when a positive voltage is applied to one of the control terminals C ₁ , C ₂ and C ₃ , for example C ₁ , and the other control terminals C ₂ and C ₃ are set to 0 voltage,
Switching is connected to the control terminal C ₁ FET2
Only the elements 11, 212, 213 are turned on, the impedance conversion FETs 311, 312, 313 perform impedance conversion on the outputs of the sensor elements 111, 112, 113, and the sensor elements 111, 112 are drained via the drain resistors 411, 412, 413. , 113, the output signals can be read.

Next, by selecting one of the output terminals R ₁ , R ₂ and R ₃ such as R ₁ , only the output signal of the sensor element 111 is read out.

Thus, by selecting a row using the control terminals C ₁ , C ₂ and C ₃ and a column using the output terminals R ₁ , R ₂ and R ₃ , this row and column are It becomes possible to sequentially read the output signal of the specific sensor element connected to the intersecting portion of the.

Although only the structure of the sensor unit main body is described in the above embodiment, a circuit for selecting a row using the control terminal and a circuit for selecting a column using the output terminal are provided in addition to the above circuit structure. Although a separate output signal processing circuit is required, the configuration of these peripheral circuits is not particularly limited. In the present embodiment, since the semiconductor substrate is used as the sensor substrate, the peripheral circuit section Can also be integrally formed on the peripheral portion of the sensor.

[0032]

As described above, according to the present invention, after the piezoelectric thin film is formed on the lower surface electrode, the upper surface of the piezoelectric thin film is receded from the boundary of the lower surface electrode by at least half the thickness of the piezoelectric thin film. By forming the electrodes and arranging the sensor elements, which are configured by performing polarization processing through the lower surface electrode and the upper surface electrode, in a matrix, the fingers of the piezoelectric thin film sensor element can be provided without affecting adjacent elements. Since the pressure distribution according to the fingerprint pattern can be accurately detected, it is possible to realize an excellent fingerprint sensor that can be downsized and can detect the fingerprint pattern with high resolution.

[Brief description of drawings]

FIG. 1 is a plan view of a sensor element portion in an embodiment of a fingerprint sensor of the present invention.

FIG. 2 is a sectional view taken along the line AB in the embodiment of FIG.

FIG. 3 is a plan view showing the overall configuration of the same embodiment.

FIG. 4 is a circuit diagram of the fingerprint sensor of the same embodiment.

[Explanation of symbols]

1 Sensor element 2 FET for switching 3 FET for impedance conversion 4 Lower surface electrode 5 Piezoelectric thin film 6 Upper surface electrode 7 Oxide film 8 Single crystal silicon substrate 10 Gate of impedance conversion FET 11 Source of impedance conversion FET 12 Drain of impedance conversion FET 13, 16, 18, 20 Electrodes 14 Drain of switching FET 15 Drain resistance 17 Gate of switching FET 19 Source of switching FET 111, 112, 113 Sensor element 121, 122, 123 Sensor element 131, 132, 133 Sensor element 211, 212, 213 Switching FETs 221, 222, 223 Switching FETs 231, 232, 233 Switching FETs 311, 312, 313 Impedance ET 321, 322, 323 impedance for FET 331, 332, 333 impedance for FET 411, 412, 413 drain resistance 421, 422, 423 drain resistance 431, 432, 433 drain resistance C _1, C _2, C ₃ control terminal R ₁ , R ₂ , R ₃ output terminal V power supply terminal G ground

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Kawasaki 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. A fingerprint sensor for detecting fingerprint information of a finger by arranging a plurality of piezoelectric thin film elements in a matrix and converting the pressure applied to each piezoelectric thin film element into an electric output signal. After forming the piezoelectric thin film on the electrode, at least 1 / thickness of the piezoelectric thin film is formed on the piezoelectric thin film.
A fingerprint sensor, wherein a sensor element is formed by forming an upper surface electrode that is recessed by a dimension of 2 and performing polarization processing through the lower surface electrode and the upper surface electrode. 2. An amplification element or impedance conversion element for amplifying or impedance-converting an output signal of each piezoelectric thin film element, and a switching element for sequentially scanning the output of the amplification element or impedance conversion element. The fingerprint sensor according to claim 1. 3. The fingerprint sensor according to claim 1, wherein the piezoelectric thin film is a lead zircon titanate thin film.