Design and application of automobile tire pressure sensor chip

Preface

When a car is driving at high speed, tire failure is the most worrying and difficult to prevent for drivers, and is also an important cause of sudden traffic accidents. According to a survey by the Society of Automotive Engineers of the United States, 260,000 traffic accidents in the United States are caused by low tire pressure or leakage each year. In addition, 75% of tire failures each year are caused by tire leakage or insufficient inflation. According to experts from the National Rubber Tire Quality Supervision Center, 70% of traffic accidents on Chinese highways are caused by tire blowouts, while in the United States this proportion is as high as 80%. How to maintain tire pressure in harsh working conditions, drive normally and detect tire leakage in time is the key to preventing tire blowouts and safe driving of cars. Therefore, tire pressure detection during driving is particularly important.

Automobile The target product of tire pressure sensor IC chip is the vehicle-mounted tire pressure monitoring system TPMS (Tire Rressure Monitoring System) which combines MEMS technology and integrated circuit technology. At present, the direct tire pressure monitoring system includes 4 or 5 (depending on whether the spare tire is equipped with sensors) tire modules and a central receiver module. In the introduction of new cars such as BMW's Z8 in Germany, Citroen's C5 in France, Aston Martin's super sports car Vanquish in the UK, Lincoln Continental, Pontiac's flagship Bonneville SE, Mercedes-Benz S-Class sedan, etc., TPMS systems are also installed in the car. In addition, Chrysler and Dodge minivans and Chrysler 300M and Concorde Limited buses launched in the summer of 2002 are also equipped with TPMS systems. Most domestic automobile manufacturers are currently conducting experimental research.

This article is based on the requirements for the implementation of the National Innovation Fund project. It focuses on the use of MEMS micromachining technology to design, process, and produce tire pressure sensor IC chips. That is, through micromachining technology, low-cost tire pressure sensor IC chips with various parameter indicators and performance that can compete with similar foreign products are produced. They are used to support many domestic TPMS manufacturers and gradually provide chips for international manufacturers with superior cost-effectiveness.

Figure 1 Cross-section and bottom view of E-type chip

Figure 2 Chip bridge process layout

5 to 100 kPa 100 to 1000 kPa 3000 to 5000 kPa

Figure 3 Chip process layout designed for different ranges

Figure 4 Schematic diagram of process flow

Structural principle

The chip design adopts a single island membrane structure. The following figure shows the cross-sectional and bottom views of the single island membrane structure (also known as the E-type silicon cup structure). It is equivalent to a flat diaphragm structure with fixed support around the periphery (commonly known as the C-type structure) with a thick hard core island in the center of the diaphragm. Through calculation and experiment, the chip's overload and vibration resistance capabilities are improved, while also expanding and improving the range variety and extending the service life. The principle structure of the E-type silicon cup is shown in Figures 1 and 2.

The product technology design takes into account the universality of sensor parameter indicators, making it easier for the chip to be applied to the manifold pressure sensor of the automotive engine electronic fuel injection system (chip bridge process layout see Figure 2). This avoids the problems of non-professional matching of its parameters, high temperature coefficient, low overload capacity, and scattered sensitivity parameters; the chip substrate concentration is much greater than 103, which makes the bridge resistance value high, reduces power consumption, and extends the service life of the power supply battery.

According to the design calculation, the chip layout design E-type silicon cup structure is 2.4×2.4mm, the large film radius R is 0.8mm, the center island radius ro is 0.4mm, the resistor strip width is 4mm, the length is 80mm, and it is designed as 20 square resistors. The resistor shape is a single strip. In order to reduce the influence and error of the ends, the resistors are formed by light boron doping, the square resistor is 250 ohms, the ends are short-circuited with concentrated boron, and the square resistor is 10 ohms. The practical photolithography plate should also take into account the layout design techniques such as the influence of the alignment of the concentrated boron lead-out additional resistor on the balance when forming the bridge.

Mathematical Model and Analysis

The maximum deflection of the center of the flat diaphragm with radius R is:

The maximum deflection of the center of the single island membrane with the ratio of the center island radius ro to the total membrane radius R is C is:

When the C value is 0.5 (common design), the maximum displacement of the center of the single island membrane structure is only one-fourth of the flat membrane.

When the large membrane inscribed radius of the E-type diaphragm is R and the external radius of the hard core island is ro, the radial and tangential stresses on the upper surface of the film are:

At and r=R, and reach the maximum value, the values ​​are equal in magnitude and opposite in sign, that is:

, which is times the flat membrane edge stress .

As can be seen from the formula, stresses and are approximately symmetrical. When C=0.5, this symmetry is better. The symmetric point, that is, the =0 point is at r≈0.76R, but the =0 point is at r≈0.85R. Therefore, when this solution is adopted, the radial distribution size of the resistor should not exceed 1/10R.

Key points of process realization

Process layout design

When the chip size is increased and thickened, the chip range can be expanded. The chip is a square flat diaphragm with fixed edges and supports, and has an overload capacity of 3 to 10 times. Figure 3 is a chip process layout designed for different ranges.

Mainly solved process technology problems:

①High-quality silicon-silicon vacuum bonding process;

②Uniform and high qualified rate thinning process;

③ High accuracy, high uniformity of doping consistency and slender resistor strip consistency control to ensure low temperature drift of the sensor;

④Internal stress matching elimination technology to ensure the time stability of the sensor;

⑤Corresponding anti-electromagnetic interference design;

⑥ Measures to resist high vibration and centrifugal acceleration in packaging design and process;

The process flow diagram is shown in Figure 4.

Indicator Testing

The products of this project are based on the international standards for automobile tire pressure, combined with the product usage requirements proposed by domestic users, in accordance with the relevant product standards reviewed and approved by the Electronic Standardization Institute and the Beijing Technical Supervision Bureau. After passing the type test inspection of the 304 Institute of the Ministry of Aerospace, all performance indicators meet the design and usage indicator requirements.

Application expansion and extension

In combination with the characteristics of MEMS technology and taking into account the universality of sensor post-packaging production process equipment, in the chip structure design, considering the requirements of chip structure and parameters for different products, according to the minimum requirements and classification principles of chip size and process layout, the structural design is divided into three chip types, which greatly reduces the variety of chips and expands the application areas of chips.

Conclusion

The use of MEMS technology to produce automobile tire pressure sensors has the advantages of small size, light weight, low energy consumption, stable performance, and is conducive to mass production, reducing production costs and increasing product added value. At the same time, it broadens the scope of product application, improves the promotion value of chips and the economic benefits of products.

The development of automobile tire pressure sensor chips has important social and practical significance for reducing sudden major and vicious traffic accidents caused by tire blowouts of high-speed vehicles, ensuring the safety and smooth flow of highways, avoiding personal injuries and family tragedies, and the long-term stability of the entire national society and the development of the entire national economy.

Figure 5 Product photos