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Structure and technical requirements of sensitive core film for sputtering pressure sensor

Published on: June 18, 2020 by Zetian sensing

The functional films of the sensing core of sputtered thin film pressure sensor generally adopt multilayer thin film structure. These functional films are generally produced by vacuum ion beam sputtering process. Because the thickness of sensitive film is generally less than 100nm, we also call it nano film. The pressure sensor manufactured by ion beam sputtering process is sometimes called nano film pressure sensor. Zetian sensing is the specialty of this kind of thin film pressure sensor Manufacturer

 Sputtering pressure sensor

The substrate of sputtered thin film pressure sensor is a special round metal elastomer. The first layer of functional thin film is a dielectric insulating film, which usually adopts SiO2, Al2O3 or composite structure. The second film is a strain sensitive metal film. The strain resistance of the bridge is formed by photolithography, which becomes the core of the pressure sensitive core. The material industry generally adopts Ni Cr alloy and nicrmnsi alloy. The third layer is passivation protection medium film, which is mainly used to protect strain resistance and isolate air and water vapor, and prevent the instability of strain resistance caused by oxidation and corrosion of strain film. SiO2, Al2O3 and other materials are usually used as passivation and protection medium films. The fourth film is the window gold-plated film for gold wire lead. It contacts with the strain resistance film to realize electrical extraction.

according to Thin film pressure sensor For the insulating film, the insulation resistance between the strain resistance and the shell of the sensor should be more than 100000 m Ω, the withstand voltage should be more than 100 VDC, and the insulation resistance should be more than 5000 m Ω. At the same time, the surface adhesion between the insulation film and the elastomer is high. In the range of measuring range, bending deformation over 10000000 cycles does not fail. It is required that the thermal expansion coefficient of the insulating film and the elastomer should be basically the same, so that the internal stress will not be caused by the difference between them, thus the output of the sensor will be unstable, and the creep of the film pressure sensor made of it should be small. In addition to the strict heat treatment of metal elastomer, the dielectric insulation film deposited on it is required to have high adhesion, compactness and no pinholes and cavities. If there is creep, it will increase the zero drift error and reduce the nonlinearity of the sensor. The insulating film contains less impurity and has no adsorption gas. In this way, the insulation performance is reduced, the leakage current increases and even insulation failure caused by the migration of impurities and gas release. In a word, dielectric insulation film should have high resistance coefficient, high breakdown voltage, excellent insulation performance, good adhesion to elastomer, good transmission of elastic deformation and high thermal stability.

For strain sensitive metal films, due to the use of a variety of material composition, it is generally required that the composition of the film is consistent with the bulk material, so as to ensure that its temperature coefficient and sensitivity remain unchanged. For Ni Cr Thin Films, the residual stress of strain sensitive films should be as small as possible after heat treatment. The existence of residual stress will cause zero drift error. It is required that the thermal expansion coefficient of the strain metal sensitive film and the dielectric insulation film should be as consistent as possible; the thickness of the film should be controlled as far as possible on the premise of ensuring a stable average thickness of continuous film. If the film is too thick, the lattice defects in the film increase the distortion and internal stress, and the resistance value is unstable; if the film is too thin, the film is discontinuous and cannot be stabilized by heat treatment. Considering the high resistance, the bridge resistance should be controlled at (3 ~ 6) k Ω, and the sheet resistance of the strain sensitive metal film should be about 50 Ω. Its thickness is about 90 nm. High resistance and low power consumption can reduce the instability of resistance caused by self heating. The resistance value of the strain resistance is stable in a wide temperature range. For example, if the stability of the sensor is required to be 0.1% FS, then the change of resistance should be less than 0.05%. For the long-term stability requirements, the resistance stability requirement is less than 0.001%. It is very difficult to ensure such high resistance stability. It is necessary to use some special material modification technology to make the resistance stability as small as possible. In short, the strain sensitive metal film layer should have high sensitivity of resistivity to strain; strain detection is less affected by temperature; high resistivity; low temperature resistance coefficient; reliable chemical and structural stability.

In addition to the thermal expansion coefficient and adhesion requirements, the thickness of the film should not be too thick, which is easy to crack and damage. Generally, in the case of very dense film, it should be as thin as possible, about 300nm. The passivation protective medium film layer should have high electrical insulation, good moisture resistance and adhesion, high temperature resistance (above 300 degrees), high resistance to ion penetration and low stress.

For the gold-plated film on the window of lead area, the adhesion of the film is required to be good, and it is not easy to peel off and damage the insulation layer during ball welding. Therefore, the gold-plated film on the window is generally about 1500 nm thick. The film is made of gold, which is a heavy metal and easily diffuses through Ni Cr and SiO2 layers, causing insulation damage. Therefore, it is necessary to increase the thickness of Ni-SiO2 barrier layer. The gold-plated film layer of lead window should have good electrical connection with strain layer, low resistivity, compatibility with subsequent processing, high thermal stability, good adhesion, and matching resistance temperature coefficient with strain layer.

To sum up, the general requirements of sputtering film pressure sensor for the quality of each layer of film are mainly mechanical and electrical characteristics. The mechanical properties include: adhesion, internal stress, thermal stress, good elasticity after stress deformation, small thermal expansion coefficient, etc. The main electrical characteristics are that the dielectric film has good insulation performance, high breakdown strength, small leakage current, moderate square resistance of metal film and good stability of resistance value. The mechanical and electrical properties of thin film pressure sensor mainly depend on the film preparation equipment and preparation technology. Because the film formation process is very complex, even if the film is made by the same production equipment, the quality of the film sometimes varies greatly. This article originates from Zetian sensing All rights reserved. Please keep the source.