Introduction

Because the elastic modulus of the elastic body sensitive to the pressure sensor is easily affected by temperature changes, the output sensitivity of the sensor when the temperature changes are different from that at room temperature, which can not truly reflect the sensitivity of the sensor. Therefore, the temperature sensitivity of the pressure sensor needs to be compensated to keep the sensor's accurate output sensitivity. 

The traditional compensation process uses a nickel resistance element to compensate outside the sensing element. The strain resistance and compensation resistance are not located in the same temperature zone, and the temperature gradient generated between them makes the compensation result of the sensor error. This paper provides a temperature sensitivity compensation process based on thin film technology. By depositing high-purity nickel film through ion beam sputtering, the compensation resistance and strain resistance are made on the same surface of the sensitive elastomer, so that they are located in the same temperature zone, improving the compensation accuracy of the pressure sensor, while making its structure more simple and convenient for assembly.

The precondition of using thin film nickel resistance to compensate for the sensor temperature sensitivity is to ensure that the temperature coefficient of thin film nickel resistance meets the compensation requirements. The process parameters that the temperature coefficient of thin film nickel resistance reaches the standard are obtained through the following tests.

Temperature sensitivity compensation principle

After the sensor is assembled, a preliminary test and temperature compensation test shall be carried out to provide necessary data for various compensations in the future. The compensation method is to add various compensation resistors on the measuring line. There are three purposes to use line compensation in sensors: one is to standardize the parameters of sensors; Second, to improve the output characteristics; third to improve the adaptability to environmental temperature changes. Comprehensive line compensation includes initial imbalance compensation (zero point correction), temperature compensation, nonlinear compensation, output sensitivity compensation, and input, and output impedance compensation. In this passage, temperature sensitivity compensation is mainly studied. The temperature sensitivity compensation accuracy of the sensor is improved through the thin film technology compensation process.

The elastic modulus E of the elastomer material and the sensitivity coefficient K of the strain gauge change with temperature. Generally, the elastic modulus E decreases when the temperature rises. According to Hooke's law, when the external force is constant, the strain ε To increase, the bridge output increases, making the sensitivity of the sensor larger.

FIG1
Where: α L-thermal expansion coefficient of elastomer material;
α E - temperature coefficient of elastic modulus of elastomer material;

α M - temperature coefficient of compensation resistance RE.

It can be seen from the above formula that the temperature sensitivity compensation resistor should choose a resistor with a high-temperature coefficient and good stability. Although the temperature coefficient of copper can meet the standard, copper is easy to oxidize and has poor stability. Therefore, we choose pure nickel as the compensation resistor.

In order to make the sensor reach certain accuracy requirements, there are certain requirements for the temperature coefficient of the sensor temperature sensitivity compensation resistance, which is at least 5000ppm Ω/℃. In this article, the thickness of the film when the temperature coefficient reaches the standard is determined by the nickel coating test. Secondly, according to the compensation schematic diagram of the sensor bridge, the film nickel resistance graph suitable for the sensitive core with a diameter of 7mm is designed. The resistance calculation is shown in Formula (2).

Temperature sensitivity compensation nickel resistance temperature coefficient test

The precondition of using thin film nickel resistance to compensate for the sensor temperature sensitivity is to ensure that the resistance temperature coefficient of nickel film meets the compensation requirements. In this paper, the temperature coefficient of thin film nickel resistance that meets the requirements is gradually obtained through tests of different process parameters (the nickel resistance temperature coefficient that meets the sensor compensation requirements is ≥ 5000ppm Ω/℃).

Relationship between nickel film thickness and resistance temperature coefficient

The thickness of the film also affects the temperature coefficient of the nickel resistance. The thicker the film is, the closer the temperature coefficient is to the bulk. However, due to the influence of the photoresist resolution during the test, the film thickness has a certain range. The coating time corresponding to the photoresist resolution limit is 30min, so the coating time should be less than 30min. For the temperature coefficient of the nickel resistance corresponding to different coating times, select 5 test pieces for the same coating time, and take the average temperature coefficient. The resistance temperature coefficient increases with the increase in film thickness, but it remains unchanged when the film thickness reaches a certain value.

Relationship between heat treatment temperature and resistance temperature coefficient

Heat treatment can stabilize the structure and performance of nickel film, reduce the defects in the film deposition process, complete the transformation from amorphous to crystalline, and stabilize the film. Therefore, the appropriate heat treatment temperature should be selected. When the film thickness is the same, the temperature coefficient of the film resistance will increase with the increase of heat treatment temperature but will remain unchanged when reaching a certain temperature.

Through the above analysis of the factors affecting the temperature coefficient of nickel resistance, it is finally obtained that the temperature coefficient of nickel resistance meeting the sensor compensation requirements is 5000ppm. The process requirements are: there is no transition layer of titanium before nickel coating, the nickel coating time is 12min, and the heat treatment temperature is 400 ℃. However, the problem to be solved at present is how to improve the adhesion of nickel film.

Process for improving adhesion of nickel film

Previous experiments showed that due to the influence of the photoresist resolution, the nickel resistance coating is easy to remove the film when cleaning the photoresist. Therefore, the process was improved to change the previous nickel resistance line plating into line etching, and the film removal phenomenon was eliminated. This is because the photoresist protection area is different. The improved protection area and the improved nickel resistance wire grid line position are complementary, and the exposure window is wider than before, within the resolution range of the photoresist, Therefore, the nickel resistance pattern can be fully exposed and a clean and complete nickel resistance pattern can be obtained.

After the improved process, the temperature coefficient of the film nickel resistance meets the requirements of sensor compensation, and the film adhesion is also improved. After the photoresist is cleaned, there is no film peeling phenomenon. 

This process has been verified in small batch production, and the subsequent sensor installation and adjustment can meet the accuracy requirements of temperature sensitivity compensation.

Through the calculation of the temperature sensitivity compensation formula of the sensor and the actual test verification, the temperature sensitivity compensation coefficient is 4.67%. According to a certain bridge input resistance value, corresponding to a certain compensation resistance value, the compensation resistance is fine-adjusted by the laser resistor adjuster to make the compensation accuracy of the sensor more accurate.

Temperature performance test of the pressure sensor

After the sputtered film sensor is assembled, the temperature aging test is carried out first to compensate for the temperature zero point of the sensor with large temperature drift, and then to compensate for the temperature sensitivity.

The nickel resistance was fabricated by a specific process, and the temperature performance of the sensor was verified. The temperature sensitivity index after the compensation of the thin film nickel resistance was 0.003% F S/℃, compared with the traditional nickel resistance element, the compensation accuracy has been greatly improved, which simplifies the subsequent sensor installation and adjustment, and optimizes the overall structure and size of the sensor.

Conclusion

In this article, the temperature sensitivity compensation resistor is made on the surface of the sensitive element by the sputtering coating method, and various factors that affect the temperature coefficient of the temperature sensitivity compensation resistor are analyzed. A certain process improvement scheme is adopted to make the temperature coefficient of the compensation resistor meet the requirements. A reasonable compensation resistor grid pattern is designed through theoretical calculation and experimental verification, and the integrity of the process and design is verified through small batch production.

The temperature sensitivity of the pressure sensor is compensated according to the determining process, and the temperature sensitivity drift of the pressure sensor after compensation is 0.003F S/℃, which is superior to the traditional compensation results using nickel resistance elements. After the improvement of the temperature sensitivity compensation process, the advantages of sputtered thin film sensors can be expanded, and the temperature sensitivity compensation technology of thin film resistors can be applied to more types of sensors.