JPH0755919A - Ultrasonic sensor - Google Patents

Abstract

PURPOSE:To improve sensitivity and to further prevent entrance of noise. CONSTITUTION:A cantilever beam type piezoelectric film 3 is supported to a silicon semiconductor substrate 2. A pair of electrodes 5, 6 are formed on both surfaces of the film 3 in a thickness direction in such a manner that any of the electrodes is formed only in the vicinity of a base end 4 thereby to lead a piezoelectric detection voltage near the end 4 to be operated by a large stress as it is, thereby improving sensitivity. A source follower using a field effect transistor 7 is provided on the substrate 2, and entrance of noise such as induction noise and popcorn noise is prevented.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an ultrasonic sensor.

[0002]

2. Description of the Related Art A typical prior art is JP-A-61-220.
596. In this prior art, a piezoelectric film is formed in a cantilever shape on a semiconductor substrate, and a pair of electrodes are formed on the entire surfaces of both surfaces in the thickness direction of the piezoelectric film.

[0003]

In such a prior art, since the electrodes are formed over the entire surfaces of the piezoelectric film in the thickness direction as described above, there is a problem of low sensitivity. A large stress acts on the piezoelectric film in the vicinity of its base end, and a large voltage is generated in the vicinity of its base end. Nevertheless, as described above, since electrodes are formed over the entire surface of the semiconductor film in the thickness direction, the output voltages derived from these electrodes are, as it were, averaged, and the output voltage becomes low. , Therefore the sensitivity is low.

A main object of the present invention is to provide an ultrasonic sensor having improved sensitivity.

Further, in such a prior art, it is desired to reduce induction noise and popcorn noise,
This makes it possible to manufacture a high-quality ultrasonic sensor.

Another object of the present invention is to provide an ultrasonic sensor designed to reduce noise contamination.

[0007]

According to the present invention, a piezoelectric film is supported by a support in a cantilever manner, and a pair of electrodes are formed on both surfaces in the thickness direction including the vicinity of the base end of the piezoelectric film. In the ultrasonic sensor described above, at least one of the electrodes is formed only near the base end portion.

The present invention also provides an ultrasonic sensor in which a piezoelectric film is supported on a semiconductor substrate in a cantilever manner and a pair of electrodes are formed on both surfaces in the thickness direction including the vicinity of the base end of the piezoelectric film. An ultrasonic sensor is characterized in that a circuit connected to the electrode and reducing noise is formed on the semiconductor substrate.

[0009]

According to the present invention, at least one of the pair of electrodes formed on both surfaces in the thickness direction of the piezoelectric film is fixed to a support such as a semiconductor substrate of the piezoelectric body. The large output voltage due to the large stress acting on the piezoelectric film in the vicinity of the base end can be derived, which is related to the above-mentioned prior art. As mentioned above, by arranging electrodes on the part of the piezoelectric film where only a small amount of stress acts or where no stress acts, it is possible to prevent the output voltage from being averaged, so to speak, and to improve the sensitivity. Become. Only one of the pair of electrodes may be formed only near the base end portion, but both electrodes may be formed only near the base end portion.

Further, according to the invention, a circuit for reducing noise, for example, a source follower circuit using a field effect transistor is provided on the electrode to prevent noise from being mixed in a signal taken out between both electrodes. You can The source follower circuit can reduce the output impedance, thereby preventing induction noise from entering. Further, since these electrodes have a structure directly connected to the circuit unlike a structure connected to the circuit for reducing the noise through a lead wire or the like provided separately, mixing of popcorn noise or the like also occurs. Can be prevented.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 (1) is a longitudinal sectional view of an embodiment of the present invention, and FIG. 2 is a simplified plan view of the embodiment. The ultrasonic sensor 1 has a piezoelectric film 3 supported on a p ⁻ -type semiconductor substrate 2 in a cantilever form in a shape having a width W1 and a length L1 so that the piezoelectric film 3 can resonate by a received ultrasonic wave. A pair of electrodes 5 and 6 are formed on both surfaces in the thickness direction including
The output from these electrodes 5, 6 is
Are derived from a source follower circuit realized by a field effect transistor 7 which is directly connected. Piezoelectric film 3
The width W1 of the vibrating part of is, for example, several tens of μm,
The length L1 of the vibrating portion may be several tens to several hundreds μm. One electrode 5 is formed over substantially the entire length of this length L1. This electrode 5 may be Pt or the like, for example, and is 2000 to 3000 Å. The piezoelectric film 3 may be, for example, PbTiO ₃ , or may be ZnO, PZT (lead zirconate titanate), A
1N or vinylidene fluoride polymer (abbreviated as PVD)
F) or the like, and the thickness of the piezoelectric film 3 is, for example, 2 μm. The other electrode 6 is, for example, aluminum and has a thickness of, for example, 2000 to 3000.
According to the present invention, this electrode 6 is formed only near the base end portion 4 of the piezoelectric film 3.

The SiO ₂ film 8 on which the electrode 5 is formed has a thickness of, for example, 5000 Å and is formed on the semiconductor substrate 2. Instead of the SiO ₂ film 8, S heavily doped with B
It may be i or the like.

FIG. 1B shows the piezoelectric film 3 supported by the semiconductor substrate 2 in a cantilever manner in the longitudinal direction (FIG. 1B).
3A and 3B show the output voltage distribution at the time of ultrasonic reception at each position along the horizontal direction of FIG. The stress acting on the piezoelectric film 3 at the time of resonance, that is, the piezoelectric voltage output is large in the vicinity of the base end portion 4 and is very small in the vicinity of the free end portion 9. In the present invention, the electrode 6 is formed only in the vicinity of the base end portion 4 over the length L2 (see FIG. 2), and therefore an output can be obtained in a region where the output voltage of the piezoelectric film 3 is large. Therefore, the sensitivity can be improved. L2 / L1 may be, for example, 20 to 30%.

FIG. 3 is an equivalent electric circuit diagram of the ultrasonic sensor shown in FIGS. 1 and 2. The electrode 6 is connected to the gate 10 of the field effect transistor 7, and the source electrode 1
From 1, the output signal is derived. A resistor 12 may be formed on the semiconductor substrate 2 and connected to the source 11 to be grounded. The electrode 5 is grounded. Thus, a source follower circuit using the field effect transistor 7 is realized. The drain 13 is connected to the power supply. The output impedance of the source 11 can be lowered to, for example, about several kΩ, which can prevent induction noise from being mixed into the output signal. Further, by directly connecting the electrodes 5 and 6 to the resistor 12 and the gate 10 of the field effect transistor 7, it is possible to prevent popcorn noise which may be mixed when a lead wire or the like is used. In this way, it is possible to prevent the mixing of noise and obtain an ultrasonic detection signal with a good S / N. The resistor 12 is drawn for convenience of illustration, and by forming the electrode 5 on the source 11 of the field effect transistor 7, the resistor 12 is formed.
May be formed simultaneously, and a resistor may be connected from the outside. Reference numerals 31 and 32 indicate terminals.

The procedure for manufacturing the ultrasonic sensor 1 will be described with reference to FIGS. First, referring to FIG. 4A, an n ^{− type} Si layer 14 epitaxially formed is formed on the p ^{− type} silicon semiconductor substrate 2, and a SiO ₂ layer 35 is further formed thereon. . Using this,
As shown in a), the p regions 33 and 3 having the role of element isolation are formed by the photolithography and ion implantation methods.
4 is formed, and the SiO ₂ layer 15 is further formed thereon. Next, as shown in FIG. 4B, the SiO ₂ layer 15 is formed.
In addition, the field effect transistor 7 is formed by the photolithography method.
Holes 16 and 1 corresponding to the source 11 and drain 13 of
7 is formed, and impurities are diffused to form n ^{+ type} regions 18 and 19 deeply.

Then, next, as shown in FIG. 4C, a photoresist film 20 is formed, a hole 21 corresponding to the gate electrode 10 of the field effect transistor 7 is formed in the SiO ₂ layer 15, and the ion implantation is made shallow. Then, the p ⁺ region 22 is formed as shown in FIG. 4 (4).

Next, referring to FIG. 5A, the electrode 5 extending from the region near the field effect transistor 7 to the width W1 × length L1 shown in FIG. 2 is formed. In forming the electrode 5, first, Pt which is a material of the electrode 5 is formed.
Are formed over the entire surface by a method such as sputtering or electron beam evaporation, and then the electrodes 5 are selectively formed and patterned by a method of photolithography and etching. This electrode 5 is grounded. The SiO ₂ layer 15 is indicated by reference numeral 8 in the vibrating portion.

Therefore, as shown in FIG.
A piezoelectric film 3 made of PbTiO ₃ is selectively formed on the upper surface. At this time, reference marks 23,
A natural oxide film 24 made of SiO ₂ is formed.

Further, subsequently, as shown in FIG. 5C, S is provided near the portion of the electrode 5 protruding from the piezoelectric film 3.
An iO ₂ layer 25 is selectively formed, and then, as shown in FIG. 5 (4), a hole is formed in the oxide film 24 by a photolithography / etching method to form aluminum electrodes 6, 2
6 is formed. The electrode 26 is connected to the region 19 which is the drain of the field effect transistor 7.

Next, as shown in FIG. 6, after selectively forming a passivation layer 27 for environmental stabilization, anisotropic etching of the p ^{− type} semiconductor substrate 2 is performed, as shown in FIG. The monolithic ultrasonic sensor 1 is completed.
The etching solution may be (a) a mixed solution of ethylenediamine, pyrocatechol and water (abbreviated as EPW), (b) KOH or (c) hydrazine, and these etching solutions may be used in the plane orientation of the semiconductor substrate 2 ( The etching rate for (100) etc. is significantly higher than that for the (111) plane. In this way, the concave opening 28 is formed into a truncated pyramid or a truncated pyramid. The inner surface of 28 is the (111) surface.

FIG. 8 is a simplified plan view of another embodiment of the present invention. On one semiconductor substrate 2, a plurality of ultrasonic sensors 1a having the same configuration as the ultrasonic sensor 1 described above.
˜1d are formed, the field effect transistors 7a to 7d are individually formed, the ultrasonic reception output is derived by the source follower, and the processing circuits 30 are processed through the gate circuits 29a to 29d that perform individual switching operations. Given. Directivity can be improved by arranging a plurality of such ultrasonic sensors 1a to 1d to form an array. The gates 29a-given to the processing circuit 30
Each signal through 29d is delayed, and each delay time ΔTa to ΔTd thereof is set by deriving such that the phases of the received ultrasonic waves are shifted and derived, and their gates 29a are obtained.
The outputs of .about.29d can be added to improve the sensitivity. Thus, the plurality of ultrasonic sensors 1a to
The output of 1d can be selectively delayed and electronically scanned to form a phased array, and the sensitivity can be further improved.

[0022]

As described above, according to the present invention, at least one of a pair of electrodes formed on both surfaces in the thickness direction of a cantilever type piezoelectric film is used as a base end. By forming only in the vicinity of the portion, a large output voltage in the vicinity of the base end portion of the piezoelectric film can be derived as it is, and the sensitivity can be improved.

Further, according to the present invention, the semiconductor film is supported on a semiconductor substrate, and a circuit for reducing noise is formed on the semiconductor substrate by connecting to the electrode, whereby various kinds of noise such as induction noise and popcorn noise are formed. It is possible to prevent the noise from being mixed.

[Brief description of drawings]

FIG. 1 is a diagram showing an ultrasonic sensor 1 according to an embodiment of the present invention and a distribution of generated voltage.

2 is a simplified plan view of the ultrasonic sensor 1 shown in FIG. 1. FIG.

FIG. 3 is a simplified electric circuit diagram of the ultrasonic sensor 1.

FIG. 4 is a cross-sectional view showing a part of the manufacturing process of the ultrasonic sensor 1.

FIG. 5 is a cross-sectional view showing another part of the manufacturing process of the ultrasonic sensor 1.

FIG. 6 is a sectional view showing a completed state for explaining the manufacturing process of the ultrasonic sensor 1.

7 is a perspective view of the ultrasonic sensor 1. FIG.

FIG. 8 is a simplified plan view of another embodiment of the present invention.

[Explanation of symbols]

1 Ultrasonic Sensor 2 p ^- Type Silicon Semiconductor Substrate 3 Piezoelectric Film 4 Base End 5, 6 Electrode 7 Field Effect Transistor 8 SiO ₂ Layer 9 Free End 10 Gate 11 Source 12 Resistance 13 Drain 28 Opening

Claims (2)

[Claims] 1. An ultrasonic sensor in which a piezoelectric film is supported by a support in a cantilever manner, and a pair of electrodes are formed on both surfaces in the thickness direction including the vicinity of the base end portion of the piezoelectric film. The ultrasonic sensor, wherein one of the electrodes is formed only near the base end portion. 2. An ultrasonic sensor in which a piezoelectric film is supported on a semiconductor substrate in a cantilever form, and a pair of electrodes are formed on both surfaces in the thickness direction including the vicinity of the base end of the piezoelectric film, wherein the semiconductor An ultrasonic sensor characterized in that a circuit connected to the electrodes and reducing noise is formed on a substrate.