RU2066856C1 - Piezoelectric pressure transducer - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: invention is used in piezoelectric and electroacoustic converters to measure noises. Piezoelectric pressure transducer has membrane 1 on which planes side compensating film 2 is placed, film 3 of piezoelectric material with two electrodes 4, 5 is located above it. First electrode is placed on to outer surface of film. Membrane 1 is made of silicon, compensating film 2 - of silicon dioxide oxidizer thermally. Second electrode 5 is put of film. Film 6 produced from silicon dioxide is located between electrode 5 and film 3 of piezoelectric material. Thickness of membrane is chosen 0.5-5.0 μm, thickness of compensating film equal 0.1-0.3 μm Thickness of film of piezoelectric material equals 1.0-1.5 μm. For some usage of specified transducer there is used silicon membrane doped with boron up to concentration 10¹⁹ см^-3. EFFECT: enhanced operational reliability and stability. 4 cl, 2 dwg

Description

 The invention relates to piezoelectric transducers of pressure to an electrical signal, in particular to piezoelectric electroacoustic transducers.

 Known piezoelectric (acoustic) sensor structure ZnO-Si (1). In this design, a textured zinc oxide film 3 μm thick is formed on a silicon membrane with a thickness of 30 μm. The zinc oxide film has aluminum electrodes on both sides. The film of zinc oxide has a hexagonal crystal lattice, consists of groups of crystallite crystals. Crystallites have a preferred orientation oriented in texture. The sensitivity of the described sensor is 0.25 mV / Pa at a frequency of 1 kHz. The disadvantage of the sensor is that it uses a rather thick (30 μm) silicon membrane, due to which the flexibility of the structure is low, which leads to a decrease in sensitivity.

The closest technical solution to the proposed one is a piezoelectric pressure sensor (2), containing a silicon substrate with a window, on the surface of which there is a layer forming a composite structure in the window, consisting of the following layers successively located one above the other: a pyrolytically grown silicon dioxide layer with a thickness of 0 , 2 microns, a layer of silicon nitride with a thickness of 2.0 microns. The zinc oxide film deposited on this complex composite structure has a thickness of 0.3 μm and is contained in a capsule of layers of pyrolytic silicon dioxide with a thickness of 0.2 μm (silicon dioxide is formed at 450 ^° C. by the CVD method). The axis C of the zinc oxide film texture is oriented perpendicular to the plane of the substrate. The upper electrode is made of aluminum with a thickness of 0.5 μm, the bottom of polysilicon. The membrane is formed by anisotropic etching of the substrate on the reverse side. The sensitivity of the sensor is 0.5 mV / Pa (at a frequency of 1 kHz), the resonant frequency is 7.8 kHz.

In this design, the total thickness of the composite structure was reduced, and the ratio of the thicknesses of ZnO-Si ₃ N ₄ was selected from the conditions for obtaining the lowest possible temperature stresses.

 The disadvantage of the sensor is that zinc oxide films with a thickness of less than 1 μm have a large loss tangent, which leads to large inherent noise of the converter. ZnO films with a thickness of less than 1 μm (in this case a film with a thickness of 0.3 μm is used) have a low coefficient of electromechanical coupling, therefore, the device has a low sensitivity (0.5 mV / Pa).

 In order to increase the sensitivity of the piezoelectric pressure sensor containing a membrane, on the planar side of which there is a compensating film, and above it a film of piezoelectric material with two electrodes, the first of which is located on its outer surface, it is proposed to make a silicon membrane that compensates the thermally film oxidized silicon dioxide, while on the last is the second electrode of the film of the piezoelectric material, and between it and the film of the piezoelectric material the introduced insulating film of silicon dioxide is located, and the membrane thickness is selected from the condition: 0.5 to 5.0 μm, the thickness of the compensating film is 0.1 to 0.3 μm, the film thickness of the piezoelectric material is 1.0 to 1.5 μm.

In addition, for a number of applications of this sensor, a silicon membrane doped with boron to a concentration of at least 7 • 10 ¹⁹ cm ^{-3 was used} .

To increase the sensitivity and simplify the design of a pressure sensor containing a membrane, on the planar side of which there is a compensating film, and above it a film of piezoelectric material with an electrode located on its outer surface, and electrical contacts, it is proposed to make a silicon membrane doped with boron to a concentration not • at least 7 October ¹⁹ cm ^-3, wherein the piezoelectric material film is arranged on a compensating film made of thermally oxidized silicon dioxide, the membrane is not osredstvenno connected to one of the electrical contacts, and its thickness is 0.5 to 5.0 microns, the thickness of a compensating film is 0.1 0.3 m, the film thickness of the piezoelectric material is selected from the condition of 1.0 1.5 microns.

 The indicated upper and lower limits of the thicknesses of the composite structure are determined by technological limitations.

The thickness ratios from the indicated ranges for a particular composite structure are selected based on the minimum internal mechanical stresses by calculation and experiment. As a result of the studies, it was found that the proposed structure has the minimum temperature stresses: zinc oxide films with a thickness (1.0 ± 0.1 μm layer of thermal silicon dioxide with a thickness (0.24 ± 0.03) μm layer of boron-doped silicon with a thickness of (1, 4 ± 0.2) microns. The tolerance on the thickness of the layers is determined by the technological spread in the temperatures of the formation of the layers. When creating a membrane by stop etching, the degree of doping of silicon is at least 7 • 10 ¹⁹ cm ^-3 .

 When the film thickness of zinc oxide is 1 μm, the resistive losses in the film are reduced, i.e., the loss tangent decreases, which means that the intrinsic noise of the sensor is reduced. The sensitivity of the sensor increases to 0.8 mV / Pa (at a frequency of 1 kHz).

 In fig. 1, 2 shows the construction of a pressure sensor according to the claims.

Legend:
1 Si membrane
2 SiO ₂ Film compensating, heat,
3 ZnO film,
4 top electrode,
5 bottom electrode
6 SiO ₂ film insulating,
7.8 electrical contacts.

 The proposed sensor can be manufactured using standard Si technology.

 Production example.

Two-sided boron diffusion to a depth of 1.4 μm with a boron concentration of 10 ²⁰ cm ^{-3 is} carried out into a silicon substrate with a diameter of 76 mm (100). Then, silicon is etched locally on the back of the substrate. The area of local etching is equal to the area of the membrane. The boron-doped silicon is etched in an acid etchant of the composition HF: HNO ₃ : CH ₃ COOH in a ratio of 1: 2: 2 for 1 5 minutes to open silicon and then etch it from the back of the substrate in an anisotropic etchant. Silicon is etched in an anisotropic etchant: ethylene diamide 70% 200 ml, catechol 75 g, water 200 ml at 106 ^o C to obtain windows in the substrate and the formation of the membrane.

A 0.24 micron thick silicon dioxide layer is formed on the front side of the substrate by thermal oxidation at a temperature of 450 ^o C. A 1.0 micron thick ZnO film is deposited _{on the} SiO ₂ layer by vacuum RF magnetron sputtering at a temperature of 650 ^o C.

 The control of the internal voltage of the composite membrane is carried out on the installation EM-679 according to the method based on the optical control of changes in the curvature of the substrate depending on the internal stresses of the applied coatings. Next, aluminum is deposited with a thickness of 0.1 μm in a URM installation.

 An example of use and the results obtained.

As a result of fabrication of the structure, a sensor with the following parameters was obtained (parameters were monitored on a measuring bench equipped with equipment from Bruhl and Kjерr (Denmark):
Static capacitance (100 300) pF.

 Loss angle tangent (.05 -.01).

 Own noise in the band (0.50 to 12 kHz) (1.5 to 2.0) μV.

 Sensitivity at 1 kHz 0.8 mV / Pa.

 The resonant frequency is 4.5 kHz.

This device can be used:
as an acoustic sensor for measuring environmental noise in the band (0.1 3) kHz;
as an acoustic indicator for determining acoustic signals of a fixed frequency (f 4.5 kHz, 6.5 kHz);
as an emitter (buzzer).

Claims (3)

 1. A piezoelectric pressure sensor containing a membrane on the planar side of which there is a compensating film, and above it a film of piezoelectric material with two electrodes, the first of which is located on its outer surface, characterized in that the membrane is made of silicon, the compensating film is made of thermally oxidized silicon dioxide, on it is located the second electrode of the film of piezoelectric material, and between it and the film of piezoelectric material is located insulated I am a film of silicon dioxide, while the membrane thickness is selected from the condition: 0.5 to 5.0 μm, the thickness of the compensating film of thermally oxidized silicon dioxide is 0.1 to 0.3 μm, and the film thickness of the piezoelectric material is 1.0 1, 5 microns. 2. The pressure sensor according to claim 1, characterized in that the silicon membrane in it is doped with boron to a concentration of at least 7 • 10 ^{1 9} cm ^{- 3} . 3. A pressure sensor containing a membrane on the planar side of which a compensating film is located, and above it a film of piezoelectric material with an electrode located on its outer surface, and electrical contacts, characterized in that the membrane is made of silicon, the film of piezoelectric material is located on a compensating film made of thermally oxidized silicon dioxide, the membrane is directly connected to one of the electrical contacts and doped with boron to a concentration of at least 7 • 10 ^{1 9} cm ^{- 3} , and its thickness is 0.5 5.0 μm, while the thickness of the compensating film of thermally oxidized silicon dioxide is 0.1 0.3 μm, the film thickness of the piezoelectric material is selected from the condition 1.0 1, 5 microns.

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