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The development of thin film coating technology promotes the performance improvement of thin film pressure sensor

Published on: September 12, 2019 by Zetian company

From 1950's to 1980's, vacuum evaporation coating technology has developed for nearly 30 years Thin film pressure sensor The manufacture of the film has been restricted by the quality of the film. In early 1980, NaF, a material company in Sweden and Switzerland, undertook the development plan of replacing the old mechanical pressure sensor with a new type of pressure sensor. They began to use vacuum evaporation coating technology, and came to the conclusion that it is not feasible to use vacuum evaporation to deposit Ni Cr films for high resistance films. The reason is that the film resistance of the sensor should reach 5K Ω and the film should be very thin. The discontinuity of the thin film leads to poor stability and repeatability of the sensor.

From the 1980s to the 1990s, Thin film pressure sensor With the development of magnetron sputtering technology, the technical performance of the film pressure sensor has been improved continuously. However, due to the complexity of the technology, only a few companies have mastered this technology, and the yield is not high about 30%. At the same time, there are a few domestic units using magnetron sputtering technology to manufacture thin-film pressure sensors. Due to technical reasons, it has basically stopped.

Physical vapor deposition (PVD) methods include vacuum evaporation, magnetron sputtering, ion beam sputtering, etc. These are film deposition technologies. The quality of films varies greatly because the formation of thin films strongly depends on various process conditions. In the process of coalescence and coalescence, many particles continue to grow up on the surface of PVD, which are connected to each other in the process of coalescence Membrane. When the average thickness of the film reaches a certain value, the discontinuous film forms a continuous film with stable performance. At present, thin film pressure sensors are continuous thin films. Because they tend to be stable after heat treatment. Obviously, the size of the initial nucleation, density, adhesion and impurity contamination are important factors to determine the film quality. The main defect of the film is dislocation. It is caused by the stress in the film, the aggregation of the nucleation islands, the formation of the quasi boundary, the difference of thermal expansion coefficient between the film and the substrate, etc. Other defects are vacancies, voids, voids and grain boundaries. Filling defects, surface roughness, impurity contamination, etc. These defects exist in the way of physical vapor deposition, but the degree of defects is different in different methods.

The vacuum evaporation vapor phase process takes place very quickly, the deposition rate is also fast, the initial formation of gas phase particles is also large, the defects of the film are characterized by a large number of vacancies, followed by more holes, dislocations, impurities pollution, etc., which lead to the film surface rough and bulky, poor adhesion. Therefore, the thickness of the film used in the medium is limited to 1000 nm, which is easy to crack. The average thickness of the continuous film is about 500 nm. The resistance of the thin film can only be made up of several hundred ohm. In particular, these defects cause the internal stress and thermal stress of the film, leading to the large zero drift. The alloy composition of strain resistance varies greatly, the bridge resistance is unstable and does not repeat, and the strain coefficient is difficult to control.

Due to the relatively slow gas phase process, the composition ratio of the alloy film is easy to control, the crystal nucleus of the film is smaller, the film is denser and the adhesion is higher. The film defects are also greatly reduced. These make the insulation performance of the dielectric film greatly improved, the stability of the bridge arm resistance also improved, and the working temperature range was widened. However, the film is formed in the plasma region above 300 ℃. Therefore, the bad environment in the plasma region limits the further improvement of the film quality. The main defect is the contamination of impurities in the plasma potential region. The contamination of solid impurities changes the performance of the strain resistance film. The dielectric film reduces the insulation performance, especially the local high pressure gas Ar causes the film adsorption. A large amount of gas adsorbed in the film gradually diffuses and escapes during the growth of the film, forming a lot of voids. Due to impurity pollution and voids, the increase of pinholes leads to the decrease of insulation performance. In order to increase the film thickness and improve the insulation strength, it is also limited by the cracking of thick film higher than 2000 nm. The insulation performance is generally 100m Ω / 50V. The average thickness of the resistance film is about 250 nm in the 1980s and 100 nm in the late 1990s. So the bridge arm resistance can reach about 2K Ω, but the defects of the resistance film, especially the contamination of impurities, have poor thermal stability. At high temperature, the zero point output drift is large. Generally, it can only be controlled in a few tens of thousands. And the modern requirements to achieve a few parts per 100000, or even a few parts per million. It is reported that the yield of foreign companies is about 30%, and the failure is mainly due to poor insulation, which may be the limit of DC and HF sputter deposition technology.

Ion beam sputtering technology and magnetron sputtering film technology developed at the same time, but it was not until Kaufman and others invented the ion source to generate low energy ion beam that the ion beam sputtering technology was applied in practice. Using this low energy ion beam to bombard the solid surface produces kinetic energy conversion, which makes the atoms on the target surface escape, which is called ion beam sputtering. Generally, the energy of ion beam is about 1 kev-2 keV. The deposition rate of PVD film is higher than that of the film deposited on the substrate at low temperature. Therefore, the dielectric film can be deposited more than 4000 nm thick, and its insulation performance is greatly improved. Generally, it reaches 500m Ω or even 1000m Ω at 100VDC. Not only the withstand voltage is twice as high as that of magnetron sputtered films, but also the insulation resistance is increased by 5 to 10 times. The average thickness of Ni Cr Thin Films as strain resistance is generally in the range of 100 nm to 150 nm. The bridge arm resistance can be about 4K Ω. The defects of ion beam sputtered films are mainly manifested in the inherent defects of PVD films. Therefore, the performance difference is mainly manifested in the poor thermal stability of the sensor at high temperature, that is, the thermal zero drift is large, which is controlled in the range of ± 0.2% f · s. In order to further reduce the zero drift, it is necessary to modify the metamaterials. at present Zetian sensing The thermal zero drift has been reduced to 0.0002% FS / ℃, which is the leading one in the world.

With the development of thin film technology from vacuum evaporation to magnetron sputtering and ion beam sputtering, modern thin film pressure sensors have developed to a very high level. In magnetron sputtering and ion beam sputtering, the number of atoms escaping from the target is several to dozens of atomic layers. The deposition rate is relatively fast, and the deposition atoms first vaporize and then condense, this process is very complex. With the advent of very low energy accelerator, the deposition mechanism of atomic layer becomes a reality. This article comes from Zetian sensor, please keep the source.