JP2943437B2 - Fingerprint sensor - Google Patents

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, information security using fingerprint information such as "everyone is unmatched" and "lifelong invariant" has attracted attention with the development, spread and diversification of information systems. A fingerprint matching system currently in practical use has a large shape and is expensive in order to be incorporated into various terminals, and a compact or card-shaped fingerprint sensor that can be incorporated into an information system device has been desired in the future.

In the conventional fingerprint detection method, a fingertip is pressed against a glass surface or the like, the portion is irradiated with a light source, and 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, a fingerprint is detected (see, for example, Japanese Patent Application Laid-Open No. Sho 60-114979).

In addition to such an optical fingerprint sensor, a matrix electrode is formed on a pressure-sensitive sheet, and a resistance value of the pressure-sensitive sheet is electrically taken out to detect a fingerprint pattern pressure-wise (for example, JP-A-63-204374
Has been proposed.

[0005]

However, the above-mentioned optical fingerprint sensor requires a light source, an optical system including a power source and a lens, and a photoelectric conversion element such as a CCD. There was a problem that the cost was high. The pressure-sensitive fingerprint sensor described above uses a pressure-sensitive sheet, so the structure of the sensor unit is simple and can be made thin.However, since pressure-sensitive conductive rubber is used for sensing, reliability such as reproducibility and creep are high. There was a problem of lack of nature.

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 small pieces or a method of insulating the pressure-sensitive sheet with slits has been proposed, but the manufacturing process is complicated.

An object of the present invention is to solve the above-mentioned conventional problems, and it is an object of the present invention to provide a high-resolution and high-reliability fingerprint sensor which has no influence on adjacent elements, can be miniaturized.

[0008]

In order to solve the above-mentioned problems, a fingerprint sensor according to the present invention comprises, after forming a piezoelectric thin film on a lower electrode, at least half the thickness of the piezoelectric thin film on the piezoelectric thin film. It has a configuration in which a top surface electrode recessed by a dimension is formed, and sensor elements configured by performing polarization processing via the bottom surface electrode and the top surface electrode are arranged in a matrix.

[0009]

According to the above-mentioned means, the piezoelectric sensor element made of a piezoelectric thin film comes into contact with the ridge (ridge) of the fingerprint, and each of the piezoelectric elements in the portion that is 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 electrode and the upper electrode, and only the polarized part of the piezoelectric element has a piezoelectric effect.
The pressure distribution according to the fingerprint pattern of the finger can be accurately detected without affecting adjacent elements.

[0010]

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 of the fingerprint sensor matrix of the present invention, that is, one sensor element portion. In FIG. 1, reference numeral 1 denotes a piezoelectric thin film 5 having at least a width dimension W ₁ of the lower electrode 4 formed on the lower electrode 4, and a thickness of at least the thickness of the piezoelectric thin film 5 from the boundary side of the lower electrode 4 on the piezoelectric thin film 5. This is a sensor element formed by forming the upper surface electrode 6 which is set back by half the dimension W _{2 of the above,} and performing polarization processing via the lower surface electrode 4 and the upper surface electrode 6.

Reference numeral 2 denotes a switching MOS-type field effect transistor (hereinafter referred to as an FET) for scanning the sensor element 1. Reference numeral 3 denotes a signal processing circuit for lowering the impedance of the output signal of the sensor element 1. It is a FET for impedance conversion to facilitate
May be an amplifying FET that amplifies the output signal of the sensor element 1.

Since the output of the sensor element 1 composed of the piezoelectric thin film 5 is very small and the output impedance is extremely large, in order to accurately read the output of the sensor element 1, the output must be amplified or the impedance converted or converted. You need both.

The feature of the fingerprint sensor of this embodiment is that the piezoelectric thin film 5 of only the sensor element 1 is polarized. With this configuration, only the piezoelectric element of the detection part has a piezoelectric effect. Therefore, it is possible to detect an accurate pressure distribution according to the fingerprint pattern of the finger without leaking charges generated by pressure to adjacent elements.

FIG. 2 is a sectional view taken along the line AB in FIG. 1 in order to explain the structure of the sensor element 1 in more detail in this embodiment. 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, an oxide film 7 is formed by oxidizing the single-crystal silicon substrate 8, 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 zircon titanate having a thickness t of at least the width W ₁ of the lower electrode 4 is formed on the lower electrode 4 by a sputtering method. by vacuum deposition or sputtering method above to form the upper electrode 6 made of aluminum or the like which has only the dimension W ₂ 1/2 retraction of at least the thickness of the piezoelectric thin film 5 t from the boundary of the lower electrode 4.

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

Here, when the sensor element 1 comes into contact with the peak of the fingerprint pattern and pressure is applied to the piezoelectric thin film 5, the potential difference generated between the lower electrode 4 and the upper electrode 6, that is, the output voltage is: 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 must be uniform in thickness, thin, and have a large piezoelectric constant. In addition, in consideration of easiness of film formation, film formation temperature, film uniformity, and the like, a lead zirconate titanate-based material is desirable, 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 and a polycrystalline silicon gate formed on a single crystal silicon substrate 8 by an ion implantation method. The lower surface electrode 4 of the sensor section is extracted from the gate 10 of the amplification or impedance conversion FET 3, the upper electrode 6 of the sensor section is extracted from the source 11, and the drain 12 is connected to the drain 14 of the switching FET via the electrode 13. The drain resistor 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 resistor 15, amplification of the sensor output or impedance conversion is realized (of course, both functions of amplification and impedance conversion can be realized). A control signal is applied to the gate 17 of the switching FET 2 via the electrode 18, and an output signal is detected from the source 19 via the electrode 20.

Here, the structure of the ridge of the fingerprint (ridge structure)
Is a fine pattern with a spatial frequency of 2 to 3 lines / mm, and the width of the valley (valley line) of the fingerprint is about 100 μm in a narrow place. Therefore, in order to accurately 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 0 × 20 to 25 × 25 mm and a resolution of 50 μm, 400 × 400 to 500 to 500 pixels (160,000 pixels to
It is necessary to configure a sensor unit of 250,000 pixels).

That is, the sensor element 1 and the switching FET 2 shown in FIG.
By arranging a plurality of sensor units (shaded area in the figure) each having a side of about 50 μm constituted by ET3 in a matrix as shown in FIG. Make up.

With this configuration, it is possible to detect peaks and valleys of the fingerprint, and it is possible to accurately detect a pressure distribution corresponding to the fingerprint pattern of the finger. In the present embodiment, one sensor unit is arranged and arranged in a matrix. However, the present invention is not limited to this. For example, the sensor units may be arranged in a staggered shape.

In this embodiment, a single-crystal silicon FET array is used as a means for detecting an 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. However, since the minimum handled charge is about 6.3 × 10 ^-17 C, a small output charge by the piezoelectric thin film can be accurately detected. However, there is a problem that a sufficient gradation for detecting the image cannot be obtained.

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

On the other hand, 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 compared to amorphous silicon or polycrystalline silicon. 10 ⁴ times less, read time of the output signal to become msec~sec order, it is possible to accurately detect small output charge due to the piezoelectric thin film.

Next, a method of detecting the output signal of the fingerprint sensor will be described. In order to simplify the explanation, FIG. 3 shows a circuit diagram of a fingerprint sensor composed of a matrix of 3 rows × 3 columns of detection elements. In the figure, _first , when a positive voltage is applied to one of the control terminals C ₁ , C ₂ , C ₃ , for example, C ₁ and the other control terminals C ₂ , C ₃ are set to 0 voltage,
Switching FET 2 connected to control terminal C ₁
11, 212, and 213 are turned on, the outputs of the sensor elements 111, 112, and 113 are impedance-converted by the impedance-converting FETs 311, 312, and 313. , 113 can be read out.

Next, by selecting one of the output terminals R ₁ , R ₂ , R ₃ , for example, R ₁ , only the output signal of the sensor element 111 is read.

As described above, by selecting a row using the control terminals C ₁ , C ₂ , and C ₃ and selecting a column using the output terminals R ₁ , R ₂ , and R ₃ , the row and the column are selected. It is possible to sequentially read the output signals of the specific sensor elements connected to the intersections of.

In the above embodiment, only the configuration of the sensor section main body has been described. However, in addition to the above-described circuit configuration, a circuit for selecting rows using control terminals and a circuit for selecting columns using output terminals are provided. And a separate output signal processing circuit are required. However, the configuration of these peripheral circuits is not particularly limited. In this embodiment, since the semiconductor substrate is used as the sensor substrate, Can be formed integrally with the periphery of the sensor.

[0032]

As described above, according to the present invention, after the piezoelectric thin film is formed on the lower electrode, the upper surface of the piezoelectric thin film receded from the boundary of the lower electrode by at least half the thickness of the piezoelectric thin film. The electrodes are formed, and the sensor elements formed by performing polarization processing through the lower electrode and the upper electrode are arranged in a matrix, so that the finger by the piezoelectric thin film sensor element can be used without affecting adjacent elements. Since the pressure distribution corresponding to the fingerprint pattern can be detected with high accuracy, an excellent fingerprint sensor which can be downsized and which can detect the fingerprint pattern with high resolution can be realized.

[Brief description of the drawings]

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

FIG. 2 is a sectional view taken along the line AB in the embodiment of FIG. 1;

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

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

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Sensor element 2 Switching FET 3 Impedance conversion FET 4 Lower electrode 5 Piezoelectric thin film 6 Upper 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 Electrode 14 Drain of switching FET 15 Drain resistance 17 Gate of switching FET 19 Source of switching FET 111, 112, 113 Sensor elements 121, 122, 123 Sensor elements 131, 132, 133 Sensor elements 211, 212, 213 Switching FETs 221, 222, 223 Switching FETs 231, 232, 233 Switching FETs 311, 312, 313 For 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 terminal G ground

────────────────────────────────────────────────── ─── Continuation of the front page (72) Osamu Kawasaki, inventor 1006 Kazuma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-58-27277 (JP, A) JP-A-1- 253624 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G06T 1/00

Claims (3)

(57) [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 pressure applied to each piezoelectric thin film element into an electric output signal. After forming a piezoelectric thin film on the electrode, at least 1 / (th) of the thickness of the piezoelectric thin film is formed on the piezoelectric thin film.
2. A fingerprint sensor, comprising: forming a sensor element by forming an upper surface electrode receded by a dimension of 2 and performing polarization processing through the lower surface electrode and the upper surface electrode. 2. An amplifying element or an impedance converting element for amplifying or impedance converting an output signal of each piezoelectric thin film element, and a switching element for sequentially scanning an output of the amplifying element or the impedance converting element. The fingerprint sensor according to claim 1, wherein 3. The fingerprint sensor according to claim 1, wherein the piezoelectric thin film is a lead zircon titanate thin film.