CN88201030U - Overpressure-proof type pressure transducer with rectangle dual-island silicon-film structure - Google Patents

Abstract

The utility model belongs to the field of semiconductor pressure sensors. The chip structure is characterized by two symmetrical rectangular silicon islands formed by anisotropic etching on the back of the silicon film. There is a gap between the end face of the silicon island and the device substrate, and the front of the silicon film corresponds to the groove between the double islands. Force-sensitive resistors are arranged on the part and the groove between the island and the frame, and the group of resistors is connected to form a Wheatstone bridge. The pressure sensor of this structure is characterized by high sensitivity, good linearity, and overvoltage protection function, and can be widely used in various industrial and medical pressure measurement systems, especially low pressure measurement systems.

Description

The utility model belongs to the field of semiconductor pressure sensors, and is a pressure sensor with a silicon membrane with a rectangular double-island structure and overvoltage protection function.

Among the technical indicators of pressure sensors, pressure sensitivity and linearity are the most prominent. Most of the elastic bodies of existing pressure sensors adopt flat film type (or called C type) and ring groove single island type (or called E type) structures. For a given design, pressure sensitivity depends primarily on the ratio of film thickness to its size. In order to improve the pressure sensitivity, especially the sensitivity of the micro-pressure sensor, the silicon film needs to be etched very thin. But this is not only difficult in the process, but also the linearity of the device is seriously affected due to the "balloon effect" of the film. The contradiction between sensitivity and linearity is very prominent. In addition, overvoltage protection is also a difficult problem, especially for micro-pressure sensors with a range below 0.2 kg/cm ² . A pressure sensor without overvoltage protection or with a too small overvoltage protection range is extremely vulnerable to being scrapped due to accidental overvoltage in practice. The high linearity and overvoltage protection of the micro-pressure sensor are two problems that have not been well resolved internationally. The sensitivity of the piezoresistive pressure sensor can be improved by adopting the double-island structure instead of the C-type or E-type structure. However, previous reports did not solve the chamfering problem of rectangular double-island convex corners during anisotropic corrosion. The double-islands formed by corrosion are polygonal, so there are not uniform and slender grooves between the double-islands and between the double-islands and the side wall of the frame. slot, resulting in poor linearity of the device, especially at high output. In this way, the accuracy of the sensor is affected. Figure 1 is a schematic diagram of a square silicon film and a rectangular double-island structure without chamfering compensation. The solid line is the mask pattern of the double-island before etching, which are two rectangles. The dotted lines are the upper and lower edges of the double island after corrosion, and the corners of the rectangle are chamfered, and the double island becomes a polygon.

The purpose of the utility model is to improve the pressure sensor of the double-island silicon film structure, so as to improve the linearity of the device, further increase its sensitivity, and make the device have the function of overvoltage protection.

Fig. 2 is a structural diagram of the utility model. Figure 2(a) is a front view of the rectangular double-island silicon film structure, and Figure 2(b) is a cross-sectional view at A-A' of Figure 2(a). Two dotted square boxes represent the bottom edge of the rectangular or approximately rectangular silicon island 1 on the back side of the silicon film 2; The corresponding part of the edge groove between the island and the side wall of the silicon film frame 4; there is a layer of gap 6 between the end faces of the two silicon islands and the device substrate 5, and the substrate has a small hole 7.

The chip material of the utility model can adopt (001) oriented N-type silicon chip, and the silicon film can be square. At this time, the side lines of the square film and the rectangular silicon island are respectively parallel to the [110] or [110] direction. The height h of the silicon island is 150 to 250 microns, and the longitudinal width of the silicon island is 1/3 to 1/2 of the side length of the square film. The center groove width is 2.5 to 3.5 times the edge groove width.

Because of the chamfering effect when silicon islands are formed by anisotropic etching of silicon wafers, the exposed surfaces of the chamfered corners are the {212} crystal plane family, which intersect with the silicon wafer (001) surface in the <210> crystal orientation group, In order to obtain regular rectangular silicon islands through etching, when designing the pattern of the double-island etching mask, carry out figure chamfering compensation processing, that is, add a compensation angle at each right angle of the two rectangles, and the angle is defined by <210 〉Corresponding orientation composition in the crystal plane family. In this way, the pattern of the double-island etching mask is two polygons composed of the baseline segment of the rectangle in the [110] or [1 10] direction and the compensation angle on the right angle. Figure 3 is a pattern of a double-island etching mask with a compensation angle, and the dotted lines are the upper and lower edges when the etching is terminated. FIG. 4 is an enlarged view of a polygon in FIG. 3 , where the dotted line represents the chamfering line of the rectangular silicon island during anisotropic etching without compensation corners. The numbers in parentheses above each edge indicate the crystal orientation of the line. Compensation angle θ=53.13°, and angle γ=18.43° between the angle side and [110] or [1 10] direction. The distance δ _c from the vertex of the right angle to the edge of the right angle is called _the width of the compensation angle; It is required that δ _c is equal or approximately equal to δ _u . If δ _c = δ _u , the shape of the silicon island obtained by shrinking the compensation angle to one point during anisotropic etching is exactly rectangular; if δ _c < δ _u , the silicon island obtained by etching has slightly chamfered corners and is approximately rectangular , which is called undercompensation; if δ _c > δ _u , then there is a residual part of the compensation angle on the top of the silicon island corner obtained by etching, which is called overcompensation.

The most reasonable design scheme for the layout of the force-sensitive resistors on the front side of the silicon film is as follows: two symmetrical resistors are arranged at the corresponding parts of the central groove, which are R ₂ and R ₃ , and one is arranged at the corresponding parts of the two edge grooves, which are R ₁ and R 3 . R ₄ . They have the same shape, size, and orientation, and they are connected to form a Wheatstone bridge. R ₁ , R ₄ and R ₂ , R ₃ each constitute a pair of bridge arms of the electric bridge.

A gap is reserved between the end face of the silicon island and the surface of the device substrate. Its function is to make the deflection displacement of the silicon film limited by the substrate surface when the front of the device is overpressured, thereby protecting the silicon film and preventing the silicon film from breaking. Preferably, the height of the gap is 0.4% to 0.8% of the side length of the silicon film. The gap can be formed by the following methods: one more layer is etched on the end surface of the silicon island than the frame, or a flat shallow pit is first formed by etching or other methods on the substrate corresponding to the silicon island. The substrate material is glass or silicon wafer, which is electrostatically sealed with the silicon film frame.

Since the front side of the silicon membrane corresponding to the bottom of the rectangular silicon island is basically not deformed due to the influence of external pressure, a zero-adjusting compensation resistor or a signal processing circuit can be arranged on it to form an integrated pressure sensor.

Because the utility model adopts the new design of the graphic compensation angle, the chamfering of the protruding corners produced by the anisotropic corrosion is compensated, and the rectangular double-island structure is realized. The grooves are more uniform and slender than without chamfer compensation, which further improves the pressure sensitivity and linearity of the device.

Figure 5 shows the stress distribution when the silicon film is a flat film (C-type) structure, a single-island (E-type) structure and a rectangular double-island structure. Among them, (a), (b), and (c) in Figure 5 are the front views of the silicon membranes of the three structures; (a'), (b'), and (c') are the middle parts of the three structures Cross-sectional view; (a″), (b″), and (c″) are the stress distribution diagrams of the three structures respectively. σx and σy represent the stress in the x and y directions respectively. The stress of the double-island structure is highly concentrated in the force sensitive resistor in the groove in which it is located.

（σx－σy），π₄₄为压阻系数，当硅膜厚度与外围尺寸一定时，C型结构平膜上的有效应力最小，因而灵敏度低，线性度也较差。单岛E型结构中沟槽两侧应力符号相反，中间有零应力点，故力敏电阻的几何尺寸以及双面套准光刻的偏差对灵敏度和线性度的影响均很大。双岛结构中边缘沟槽与中心沟槽内应力远离零点且符号相反，但同一沟槽内表面应力变化较平缓，也不变符号，故力敏电阻的几何尺寸对灵敏度的影响就小得多，这样就有利于改善器件的失调电压和稳定性。而且窄长沟槽内主要受横向应力作用，即σx＞＞σy，σx本身的数值又比前二种结构大得多，所以双岛结构压力传感器的压力灵敏度比C型或E型结构要高好几倍。而本实用新型实现矩形双岛结构后应力更为集中而均匀，所以灵敏度更高。The rate of change of the force sensitive resistor R on the silicon film (ΔR)/(R) ≈

(σx－σy), π ₄₄ is the piezoresistive coefficient. When the thickness of the silicon film and the peripheral size are constant, the effective stress on the C-type flat film is the smallest, so the sensitivity is low and the linearity is poor. In the single island E-type structure, the stress signs on both sides of the groove are opposite, and there is a zero stress point in the middle, so the geometric size of the force sensitive resistor and the deviation of the double-sided registration lithography have a great influence on the sensitivity and linearity. In the double-island structure, the internal stress of the edge groove and the central groove is far away from zero and the sign is opposite, but the surface stress in the same groove changes more gently and does not change the sign, so the geometric size of the force sensitive resistor has much less influence on the sensitivity , which is beneficial to improve the offset voltage and stability of the device. Moreover, the narrow and long groove is mainly affected by lateral stress, that is, σx>>σy, and the value of σx itself is much larger than the previous two structures, so the pressure sensitivity of the double-island structure pressure sensor is higher than that of the C-type or E-type structure. several times. However, after the utility model realizes the rectangular double-island structure, the stress is more concentrated and uniform, so the sensitivity is higher.

（σx－σy）在本实用新型设计中σx＞＞σy，R₁和R₄为位于边缘沟槽的力敏电阻，σx∝P；R₂和R₃为位于中心沟槽的力敏电阻，σx∝－P，P为外界压力。In order to improve the linearity of the pressure sensor, the traditional approach is to try to reduce the nonlinearity of each force sensitive resistor. However, as the sensitivity requirements increase, the thickness of the silicon film becomes thinner and the deflection becomes larger, which increases the nonlinearity accordingly. In the design of the utility model, the force sensitive resistors of the two pairs of bridge arms are all located in the groove, and are mainly subjected to only lateral tension or compression force, which is equivalent to a one-dimensional force situation. Moreover, because one pair of them is subjected to transverse tension and the other pair is subjected to transverse compression, their nonlinear coefficients are just opposite in sign and similar in value, thus playing an internal compensation role and greatly reducing the total nonlinearity of the whole bridge. The analysis is as follows: the change rate (ΔR)/(R) of the force sensitive resistance R of the bridge arm ≈

(σx－σy) In the design of this utility model, σx>>σy, R ₁ and R ₄ are force sensitive resistors located in the edge groove, σx∝P; R ₂ and R ₃ are force sensitive resistors located in the central groove, σx∝－P, P is the external pressure.

So (△R ₁ )/(R ₁ ) ＝ (△R ₄ )/(R ₄ ) ≈K ₁ P＋N ₁ P ² ＝β

(△R ₂ )/(R ₂ ) = (△R ₃ )/(R ₃ ) ≈－(K ₂ P－N ₂ P ² )＝α

In the formula, K ₁ and K ₂ respectively reflect the pressure sensitivity of force sensitive resistors R ₁ , R ₄ and R ₂ , R ₃ , and N ₁ and N ₂ reflect their nonlinearity. They are all related to the piezoresistive coefficient π ₄₄ , the geometric size of the device structure, the thickness of the silicon film, Young's modulus and Poisson's ratio of silicon.

It can be proved that the force-sensitive full-bridge output voltage V _c∝ can be calculated by the following formula

＝ (β－α)/(2＋α＋β) ≈ (K₁+K₂)/2 P－ 1/4 （K₁ ²－K₂ ²）P²

= (β－α)/(2+α+β) ≈ (K ₁ +K ₂ )/2 P－ 1/4 (K ₁ ² －K ₂ ² )P ²

+ 1/2 (N ₁ -N ₂ ) P ²

Higher-order nonlinear terms are omitted here, where V _B is the bridge excitation voltage.

It can also be proved that when K ₁ =K ₂ and N ₁ =N ₂ are properly selected related parameters such as device structure size, the non-linear items can be completely ignored at this time. Since the utility model realizes a regular square membrane rectangular island structure, it is very beneficial to use a computer to carry out numerical analysis on the stress distribution to obtain the best matching geometric size.

Since the utility model uses the gap between the end faces of the double islands and the surface of the substrate as the limiting amount of the deflection displacement of the silicon film, when the middle edge of the double islands is displaced to contact with the surface of the substrate due to pressure, the displacement at this point will be terminated. . At this time, the stress variation of the silicon film at the central trench begins to transform into a monolithic silicon film mode. Since the ratio of its width to thickness is much smaller than that of the entire silicon film, the stress change that occurs as the pressure continues to increase is also significantly reduced. The higher pressure makes the edge groove continue to deform, but because one side of the island can no longer be displaced and becomes a fixed end, the increase in the internal stress of the edge groove is also greatly eased, so that the failure pressure corresponding to the fracture stress is also corresponding significantly improved. The limit situation is that the end faces of the double islands are flat against the surface of the substrate and cannot be further displaced. If the stress in the trench does not exceed the fracture stress at this time, the device will tolerate a greater overvoltage load. Figure 6 is a schematic diagram of overvoltage protection. Figure 6(a) shows that the device is in the linear operating range when it is under pressure; Figure 6(b) shows that the inner side of the double island is in contact with the substrate surface after excessive pressure, and the overvoltage protection begins to work; Figure 6(c) shows that Greater pressure makes the double-island end face flattened with the substrate surface. FIG. 7 shows the pressure signal voltage output characteristics of the pressure sensor designed in the present invention. P _B and P _C in the figure are pressure values corresponding to the states in Figure 6(b) and (c).

The implementation process of the utility model is as follows: the double-sided polished (001) crystal-oriented N-type silicon chip is formed into four force-sensitive resistors by conventional integrated circuit processes such as thermal oxidation, photolithography, and boron ion implantation. The main edges of the graphics are parallel to the [110] and [1 10] directions. The metal wiring is formed by conventional vacuum evaporation process and photolithography process to connect the above four force sensitive resistors in the form of a Wheatstone bridge. Form the entire square silicon film pattern on the back of the silicon wafer with a double-sided registration photolithography process, perform pre-etching with ethylenediamine-catechol aqueous solution or other silicon anisotropic etchant, and then oxidize the photolithography to form a silicon film and double-island pattern, continue anisotropic etching with the above-mentioned etching solution until the predetermined thickness of the silicon film is reached. Divide the chips and electrostatically seal them with polished substrate glass or silicon wafers coated with a thin layer of glass. The gap formed by pre-corrosion serves as overvoltage protection. Finally, it is tested and packaged.

The semiconductor pressure sensor made according to the utility model has excellent technical performance; high sensitivity and high output, when the excitation voltage is 10 volts, the output is as high as 400mv; extremely high linearity, when the measuring range is 100mmHg, the nonlinearity is less than 1 × 10 ^-3 FS; effective overvoltage protection function, its overvoltage protection capability is greater than 20 times the range and so on. Its main technical indicators exceed any known domestic and foreign similar products.

The utility model can be widely used in wind tunnel pressure measurement, ship pool pressure measurement, liquid level pressure measurement and other various industrial and medical pressure measurement systems, especially for low pressure measurement, and can also be used for altimeter, water depth gauge, low vacuum gauge and electronic Various secondary pressure measuring instruments such as hydrometers.

Claims (7)

1. A semiconductor pressure sensor consisting of a silicon film and a silicon island on it, a force sensitive resistor, and a substrate, characterized in that: (1) The silicon island 1 is on the back of the silicon film 2, and is formed by anisotropic etching to form two symmetrical rectangular or approximately rectangular masses; (2) The force sensitive resistor 3 is arranged on the front of the silicon film 2, at the corresponding part of the central groove between the two silicon islands and at the corresponding part of the edge groove between the silicon island and the side wall of the silicon film frame 4; (3) There is a gap 6 between the end faces of the two silicon islands and the device substrate 5 . 2. The semiconductor pressure sensor according to claim 1, characterized in that the silicon film is square, and the material is an N-type silicon wafer with (001) crystal orientation, and the film edge and the edge of the rectangular silicon island are along the [110] and [1 10] directions , the height of the silicon island is 150-250 microns, the longitudinal width of the silicon island is 1/3-1/2 of the side length of the square film, and the width of the central groove is 2.5-3.5 times that of the edge groove. 3. The semiconductor pressure sensor according to claim 1, characterized in that: (1) The etching mask pattern corresponding to the rectangular or approximately rectangular silicon island on the (001) oriented silicon wafer is a polygon 7 with a compensation angle added to each right angle of the rectangle, and these compensation angles are defined by <210 〉The corresponding crystal orientation composition in the crystal orientation family, the size is 53.13°, and the angle between the corner side and the [110] or [1 10] direction is 18.43°; (2) The compensation angle width δ _c is equal to the beveled angle width δ _u of the rectangular right angle formed by anisotropic corrosion. 4. The semiconductor pressure sensor according to claim 2, characterized in that two force-sensitive resistors are symmetrically arranged on the front of the corresponding part of the above-mentioned central groove, and one force-sensitive resistor is respectively arranged on the front of the corresponding part of the two edge grooves, and the four resistors have the same shape, size and orientation, and are connected to form a Wheatstone bridge. 5. The semiconductor pressure sensor according to claim 1, 2 or 4, characterized in that the height of the gap 6 between the end face of the silicon island and the substrate surface is 0.4-0.8% of the side length of the silicon film. 6. The semiconductor pressure sensor according to claim 5, characterized in that the device substrate is made of glass or silicon, and the substrate is electrostatically sealed with the silicon film frame. 7. The semiconductor pressure sensor according to claim 6, characterized in that a zero adjustment compensation resistor or a signal processing circuit is arranged on the front side of the silicon chip corresponding to the bottom of the two silicon islands.

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