Characteristics Analysis of Different Pressure Sensor Core Materials

At present, there are many kinds of pressure sensor core materials. The following briefly introduces the performance of several core materials

1. monocrystalline silicon

Silicon has a wide range of applications in the production of integrated circuits and microelectronic devices, mainly using the electrical properties of silicon; in the MEMS micro-mechanical structure, it is using its mechanical properties, and then produce a new generation of silicon electromechanical devices and devices. Silicon material is abundant and low cost. Silicon crystal growth is easy, and there are ultra-pure and non-miscellaneous materials, not purity in the order of one billion, so its own internal friction is small, mechanical quality factor can be as high as 10 ^ 6 orders of magnitude. A properly designed micro-active structure, such as a micro-sensor, can achieve minimal hysteresis and creep, excellent repeatability and long-term stability, and high reliability. Therefore, the use of silicon to make silicon piezoresistive pressure sensors is conducive to solving the three problems that have plagued the sensor field for a long time-hysteresis, repeatability and long-term drift.

The density of silicon material is 2.33 g/cm ^ 2, which is 1/3.5 of the density of stainless steel, while the bending strength is 3.5 times that of stainless steel, with higher strength/density ratio and higher stiffness/density ratio. Monocrystalline silicon has good thermal conductivity, which is 5 times that of stainless steel, and the thermal expansion coefficient is less than 1/7 of stainless steel. It can be well connected with low expansion Invar alloy and avoid thermal stress. Monocrystalline silicon is a cubic crystal and is an anisotropic material. Many mechanical and electronic properties depend on the crystal orientation, such as elastic modulus and piezoresistive effect.

The resistance strain sensitivity coefficient of single crystal silicon is high. Under the same input, a higher signal output than a metal strain gauge can be obtained, generally 10-100 times that of a metal, and the input signal can be sensitive at 10 ^-6 or even 10 ^-8 levels. The manufacturing process of silicon material has good compatibility with integrated circuit technology, which is convenient for miniaturization, integration and mass production. The silicon can be covered with a number of materials, such as silicon nitride, thereby obtaining excellent protection against corrosion media. It has good wear resistance.

In summary, the advantages of silicon materials can be attributed to: excellent mechanical properties; easy batch of micro-mechanical structures and micro-electromechanical components; compatible with micro-electronic integrated circuit technology; micro-mechanical and micro-electronic circuits are easy to integrate.

It is these advantages that make silicon the most preferred material for the fabrication of microelectromechanical and micromechanical structures. However, silicon material is extremely sensitive to temperature, and its resistance temperature system is close to the order of 2000 × 10 ^-6/K. Therefore, all sensors based on the piezoresistive effect of silicon as the measurement principle must be temperature compensated, which is a disadvantage, while the available side is that the temperature can be measured directly while measuring other parameters.

2. polysilicon

Polycrystalline silicon is a polymer of many single crystals (grains). The arrangement of these grains is disordered, different grains have different single crystal orientations, and each grain has the characteristics of a single crystal. The part between the grains is called the grain boundary, and the influence of the grain boundary on its electrical characteristics can be adjusted by the doping atom concentration. Polysilicon films are generally fabricated by low pressure chemical vapor deposition (LPVCD), and their resistivity varies greatly with the concentration of boron-doped atoms. The resistivity of the polycrystalline silicon film is higher than that of the single crystal silicon, and the resistivity of the polycrystalline silicon increases rapidly especially at a low doping atom concentration. The resistivity can vary over a wide range of values depending on the concentration of the dopant atoms.

Polysilicon has a piezoelectric effect: the resistance decreases when compressed, and the resistance increases when stretched. The strain sensitive system of polysilicon resistance decreases slightly with the increase of doping concentration. The maximum value of the longitudinal strain sensitivity coefficient is about 30 times of the maximum value of the metal strain gauge, which is 1/3 of the maximum value of the strain sensitivity coefficient of monocrystalline silicon resistance. In addition, the polysilicon piezoresistive film can be fabricated on a different material substrate, such as on a dielectric (SiO2, Si3N4), as compared to the single-crystal silicon piezoresistive film. Its preparation process is compatible with conventional semiconductor technology, and there is no PN junction isolation problem, so it is suitable for higher operating temperature (t≥ 200 ℃). Compared with the single-crystal silicon piezoresistive film, the polycrystalline silicon piezoresistive film can suppress the temperature drift more effectively under the same working temperature, which is beneficial to the realization of long-term stability. An accurate resistance value of the polysilicon resistive film can be obtained by a photolithographic means.

In summary, the polysilicon film has a wide operating temperature range (-60~300 ℃), adjustable resistivity characteristics, adjustable temperature coefficient, high strain sensitivity coefficient and can achieve accurate adjustment of resistance characteristics. Therefore, in the development of micro sensors and micro actuators, the use of polysilicon film these electrical characteristics, sometimes more valuable than only single crystal silicon. For example, the pressure-sensitive diaphragm is made of monocrystalline silicon with excellent mechanical properties, covered with a layer of dielectric film SiO2, and then a layer of polysilicon piezoresistive film is deposited on SiO2. This kind of micro pressure sensor with hybrid structure plays the advantages of monocrystalline silicon and polycrystalline silicon materials, and its working temperature can reach at least 200 ℃, even 300 ℃, and the low temperature is -60 ℃.

3. Silicon-Sapphire

The silicon-sapphire material is formed by growing a silicon crystal on a sapphire (α-Al2O3) substrate by an epitaxial growth technique. The silicon crystal can be considered an extension of the sapphire, and both make up a silicon-sapphire SOS(Silicon On Sapphire) wafer. The sapphire material is an insulator, and the electrical properties of each resistor deposited on it are completely independent. This can not only eliminate the drift caused by PN junction leakage, but also provide high strain effect and operation stability in high temperature (≥ 300 ℃) environment. The hysteresis and creep of sapphire material are negligible, so it has excellent repeatability; sapphire is an inert material with good chemical stability, corrosion resistance and strong radiation resistance; sapphire has high mechanical strength.

In summary, by making full use of the characteristics of silicon-sapphire, sensors and circuits with superior performance such as high temperature resistance, corrosion resistance and radiation resistance can be produced; but to obtain high-precision, stable and reliable indicators, it is also necessary to solve the overall structure The thermal matching between materials in the middle, otherwise it is difficult to achieve the expected goal. Because the silicon-sapphire material is brittle and hard, its hardness is second only to diamond, the production technology is more complex.

4. compound semiconductor materials

Silicon is the main material for making micro-electromechanical devices and devices. In order to improve the performance of devices and systems and expand the scope of application, compound semiconductor materials play an important role in some specialized technologies. For example, PbSe, InAs, Hg1-xCdxTe(x represents the percentage of Cd) and other materials are increasingly widely used in infrared, visible and ultraviolet light imagers and detectors.

The infrared detector is illustrated as an example. The practical photosensitive detector developed by using various effects produced by infrared radiation and matter is mainly developed for the three bands (1-3μm,3-5μm,8-14μm) with the clearest transmittance of infrared radiation in atmospheric transmission. Detectors sensitive to wavelength of 1-3 μm include PbS, InAs and Hg0.61Cd0.39Te; Detectors sensitive to wavelength of 3-5 μm include InAs, PbSe and Hg0.73Cd0.27Te; For explorers sensitive to wavelength of 8-14 μm, there are Pb1-xSnxTe, Hg0.8Cd0.2Te and extrinsic (doped) semiconductors Ge:Hg,Si:Ga and Si:Al, etc. The three-element alloy Hg1-xCdxTe is an intrinsic absorption material, by adjusting the composition of the material, not only can be made suitable for three bands of devices, but also can be developed for longer working band (1-30μm) applications, which has attracted much attention.

It should be noted that the above materials need to work at low temperatures (e. g., 77K). Because at room temperature, due to the interaction of lattice vibration energy and impurity energy, the number of thermally excited carriers increases, while the number of excited photons decreases significantly, thus reducing the detection sensitivity in the wavelength region.

5. SiC thin film materials

SiC is another compound semiconductor used in special environments. It consists of carbon atoms and silicon atoms. SiC with high quality cubic structure can be obtained by implanting carbon atoms into single crystal silicon by ion implantation doping technology. The crystal structure obtained with the difference of doping concentration is different, which can be expressed as β-SiC. β represents the crystal structure of different morphologies. The SiC material obtained by ion implantation has excellent physical, chemical and electrical properties, showing high strength, large stiffness, low internal residual stress, extremely strong chemical inertness, wide band gap (nearly 1-2 times that of silicon) and high piezoresistive coefficient. Therefore, SiC material can resist corrosion and radiation at high temperature, and has stable electrical properties. It is very suitable for micro-electromechanical selection and use in high temperature and harsh environment.

Due to the high cost, high hardness and difficult processing of SiC single crystal materials, SiC thin films based on silicon single crystal wafers have become an ideal choice for research and use. By ion implantation, chemical vapor deposition (VCD) and other technologies, it is made on a Si substrate or an insulator substrate (SiCOI) for designers to choose. For example, for the pressure measurement of heat-resistant cavities and surface parts such as aero engines, rockets, missiles and satellites, SiC thin films with insulator as substrate can be selected as pressure sensing elements (diaphragms), and high temperature pressure micro sensors can be made to realize the pressure measurement on the above occasions. The working temperature during pressure measurement can reach above 600 ℃.

In addition to the use of single-crystal SiC(Single-SiC) films, in many applications of MEMS, polycrystalline SiC(Poly-SiC) films can also be used. Compared with single crystal SiC thin films, polycrystalline SiC has wider applicability. It can be in a variety of substrates (such as monocrystalline silicon, insulator, SiO2 sacrificial layer and amorphous silicon, etc.), the use of plasma enhanced vapor deposition, physical sputtering, low pressure vapor deposition and electron beam radiation technology to grow into thin films for different occasions to choose to use.

In short, SiC is a material with excellent properties, such as wide band gap, high breakdown field strength, high thermal conductivity, high electron saturation velocity and excellent mechanical and chemical properties. These characteristics make SiC materials suitable for the manufacture of high-temperature, high-power and high-frequency electronic devices; also suitable for the manufacture of high-temperature semiconductor pressure sensors.