Monocrystalline Silicon Pressure Transducer Working Principle

Monocrystalline silicon pressure transducers are one of the most common pressure transducers in industrial production. Monocrystalline silicon is a good semiconducting material. The monocrystalline silicon pressure transducer is fabricated using a high performance piezoresistive pressure core. This paper describes the working principle of monocrystalline silicon pressure transducer and the realization process of high stability and high accuracy performance. The key points include the selection of monocrystalline silicon chips, the stress-free packaging of monocrystalline silicon wafers, the elimination of backlash errors, the reduction of static pressure effects, and the amplification of the range ratio. In these ways, enhance the full performance, accuracy level and reliability of the pressure transducer. As a precision measurement instrument, monocrystalline silicon pressure transducer is very common in the field of automation, which is of great significance.

• Working principle of the monocrystalline silicon pressure transducer

As shown in the figure below. In the sensitive element of the monocrystalline silicon pressure transducer, a P-type impurity is diffused onto the N-type silicon wafer to form an extremely thin conductive P-type layer. The medium pressure is transmitted to the positive cavity side of the silicon diaphragm through the sealing silicone oil to form a pressure difference with the medium acting on the negative side. Under their combined action, one side of the diaphragm is compressed and the other side is stretched. The differential pressure changes the resistance value. Then output a signal corresponding to the pressure change. After the circuit processing this signal, the pressure transducer produce a 4-20 mA DC standard output signal that is linear with the pressure change.

As shown in the figure below. Under the action of the pressure difference between the positive chamber and the negative chamber, there are deformation and bending to the measuring silicon diaphragm (ie, the elastic member). When the pressure difference P is smaller than the limit σp of the allowable stress ratio of the silicon diaphragm, the bending can be completely reset. When the pressure difference P is greater than the limit σp of the allowable stress ratio of the silicon diaphragm, it will reach the yielding phase of the material, even to the strengthening phase. At this time, the silicon diaphragm cannot be recovered to the original position after the pressure difference is removed, resulting in an irreversible measurement deviation. When the pressure difference P reaches or exceeds the highest stress σb that the silicon diaphragm can withstand, the silicon diaphragm will be broken. It will directly damage the pressure transducer. Therefore, by preventing or weakening the external overload pressure difference P directly transmitted to the measuring silicon diaphragm, the measurement accuracy and life of the pressure transducer can be effectively protected. It leads to the problem of overload protection design for single monocrystalline chips.

• Pressure overload protection for the monocrystalline silicon pressure transducer

To overcome the shortcomings of the monocrystalline silicon wafer with insufficient overload resistance, it is necessary to use a differential pressure transducer with one-way pressure overload protection. Such a differential pressure transducer can not only measure the pressure difference of the field operating condition within the rated pressure range, but also effectively protect itself in the event of a one-way pressure overload. It can avoid damage caused by the one-way pressure overload in the monocrystalline silicon differential pressure transducer. When there is a differential pressure exceeding the allowable range of the silicon diaphragm, the center isolation diaphragm moves toward the low pressure side. At the same time, the external isolation diaphragm on the high voltage side coincides with the inner wall of the chamber. At this time, all of the silicone oil on the high pressure side enters the chamber, and it is impossible to further transmit a higher pressure value to the monocrystalline silicon chip. Finally, the generation of ultra-high voltage is avoided on the monocrystalline silicon chip, thereby effectively achieving the purpose of protecting the monocrystalline silicon chip. This anti-overload design effectively protects the long-term stability for the monocrystalline silicon chip and provides technical support for the long-term stable operation of the monocrystalline silicon pressure transducer.

• Superior range ratio adjustable performance of the monocrystalline silicon pressure transducer

The output signal amount of the monocrystalline silicon chip is large. Therefore, under the excitation of a constant voltage source of 5V, its typical range output reaches 100mV. In this way, for the electronic circuit and software at back end, it is easier to implement signal compensation and amplification processing. Compared with metal capacitive pressure and differential pressure transducers, the monocrystalline silicon pressure and differential pressure transducers have superior range ratio performance. Conventional pressure transducers have a range adjustable ratio of 100:1. The adjustable range ratio of the minor differential pressure transducer is 10:1. The high basic accuracy can be maintained after the range compression, which greatly expands the adjustable range of the pressure transducer.

• Superior pressure hysteresis performance of the monocrystalline silicon pressure transducer

The pressure hysteresis characteristic is also called return error characteristic. It is an important performance specification for pressure and differential pressure transducers. The hysteresis error of the pressure transducer directly affects its measurement accuracy and long-term drift performance. For the linear error curve of the monocrystalline silicon pressure transducer, its hysteresis error is extremely small. The upper stroke and the lower stroke almost coincide, so the hysteresis error is basically negligible. In contrast, metal capacitive pressure transducers have large hysteresis errors. Its upper stroke and the lower stroke are open, which directly affect the output accuracy of the pressure transducer.

• Unique static pressure characteristics of the monocrystalline silicon pressure transducer

When the differential pressure transducer measures the tank liquid level or the pipeline flow rate, if the static pressure is not corrected or compensated, it will bring a large error to the measurement. Especially when the liquid level range is small or the relative flow rate is small, the impact is even greater.
For example, a capacitive differential pressure transducer is combined with a throttling device to form a differential pressure flow meter. Under the static pressure of 32MPa, the full-scale static pressure error is ≤±2% FS. Although its zero error can be eliminated by zeroing, the full-scale output error cannot be avoided. Therefore, the static pressure error will directly affect the flow measurement, and the impact is large. Under this application condition, the static pressure performance of the differential pressure transducer is particularly important.
If the static pressure error is compensated, or its own static pressure error is extremely small, the measurement accuracy of the pressure transducer will be greatly improved. The monocrystalline silicon differential pressure transducer uses a unique monocrystalline silicon chip packaging process. After encapsulation, the inner and outer chambers are pressure balanced. When a working static pressure is applied to the positive and negative chambers of the measuring silicon wafer, the working static pressure is loaded onto the measuring silicon wafer in a balanced manner. The positive cavity silicone oil outside the silicon wafer and the negative cavity silicone oil inside the silicon wafer transmit this working static pressure. At the same time, mutual cancellation is achieved, so that the bending and deformation of the measuring silicon wafer under the action of the working static pressure is extremely small. The treatment greatly enhances the static pressure effect performance of the differential pressure transducer.

In applications where the differential pressure transducer is used, the differential pressure signal amount is too small. Therefore, the differential pressure transducer is very sensitive to the effects of static pressure. The unique package design and process described above still does not completely eliminate or attenuate the effects of static pressure. Therefore, for this problem, in the differential pressure measurement application, an absolute pressure transducer capable of measuring the static pressure can be integrated inside the measurement system. The absolute pressure transducer can feed the measured working static pressure signal to the microprocessor in the system in real time. The microprocessor automatically corrects the differential pressure output signal by using the working static pressure coordinate axis. In this way, the purpose of static pressure compensation is achieved. Through the unique packaging process and the installation of absolute pressure transducers, the static pressure performance of the differential pressure transducer is greatly improved. Thereby, the measurement accuracy and high stability of the differential pressure transducer can be ensured.