A pressure sensor and a preparation method thereof

Abstract

The present invention provides a pressure sensor and a preparation method thereof, which includes a glass base and a silicon strain diaphragm located on the glass base. A concave cavity is provided on one side of the glass base. The silicon strain diaphragm includes an insulating dielectric layer on the front surface and a silicon substrate covered by the insulating dielectric layer. A beam film structure with a cross beam structure is provided on the front surface of the silicon strain diaphragm facing the cavity. A group of piezoresistors, a group of heavily doped contact regions and a pair of metal leads are provided at each end of the beam film structure. The piezoresistors and the heavily doped contact regions are connected in series, and both ends are led out from the heavily doped contact regions by the metal leads. An ohmic contact is formed between the metal leads and the heavily doped contact regions on the front surface of the silicon strain diaphragm, and a Wheatstone bridge is formed between the piezoresistors. The beam film structure includes a plurality of groove structures provided on the cross beam for adjusting the performance of the pressure sensor to adapt to different ranges; the linearity is higher, the structure is more stable, and it is applicable to different ranges.

Description

Technical Field

[0001] The present invention relates to the technical field of sensors, and in particular to a pressure sensor and a preparation method thereof. Background Art

[0002] The MEMS piezoresistive pressure sensor converts the change of external pressure into a corresponding electrical signal based on the piezoresistive effect of single crystal silicon, and measures the external pressure through a Wheatstone bridge composed of four equal-value resistors. The MEMS piezoresistive pressure sensor is mainly applied to related fields such as industrial control, automotive electronics, consumer electronics, medical electronics, and aerospace. The MEMS piezoresistive pressure sensor is designed and developed by using MEMS technology, and its internal structure is composed of a silicon diaphragm obtained from a silicon wafer as a force-sensitive element, four pairs of equal-value resistors and low-resistance interconnections fabricated by MEMS processes such as doping and etching, and a multi-functional layer integrated with various materials such as evaporated metal leads.

[0003] For the MEMS piezoresistive pressure sensor, in order to pursue high-sensitivity performance requirements, the strain diaphragm of the pressure sensor chip is designed to be thinner and thinner. However, the thinner strain diaphragm will cause a larger membrane deflection, resulting in the maximum displacement value of the diaphragm exceeding the general design standard (the principle of one-fifth of the beam membrane thickness), thereby reducing the linearity of the sensor. Although the beam membrane structure pressure sensor has excellent linearity, its structural stability is poor, and its plasticity and flexibility are relatively poor, which is not suitable for working conditions with large vibration and impact loads. Summary of the Invention

[0004] The purpose of the present invention is to provide a pressure sensor and a preparation method thereof, which can overcome the problems of poor linearity, unstable structure, poor plasticity and flexibility, and not being suitable for working conditions with large vibration and impact loads.

[0005] To achieve the above purpose, the solution of the present invention is as follows:

[0006] A pressure sensor includes a glass base and a silicon strain diaphragm located on the glass base. A concave cavity is provided on one side of the glass base. The silicon strain diaphragm includes an insulating dielectric layer on the front surface and a silicon substrate covered by the insulating dielectric layer.

[0007] A beam membrane structure with a cross beam structure is provided on the front surface of the silicon strain diaphragm facing the cavity. A group of piezoresistors, a group of heavily doped contact regions, and a pair of metal leads are provided at each end of the beam membrane structure. The piezoresistors and the heavily doped contact regions are connected in series, and both ends are led out from the heavily doped contact regions by the metal leads. The metal leads and the heavily doped contact regions form an ohmic contact on the front surface of the silicon strain diaphragm, and a Wheatstone bridge is formed between the piezoresistors.

[0008] It also includes a plurality of groove structures provided on the cross beam for adjusting the performance of the pressure sensor to adapt to different ranges.

[0009] Further, the groove structures are provided on the cross beam at equal intervals or unequal intervals.

[0010] Further, the shape of the groove structure where the beam film structure is connected to the edge film region is a straight bar shape or a linearly tapered shape.

[0011] Further, each group of the varistors includes a plurality of varistor strip numbers.

[0012] Further, the groove structure includes a rectangular groove, a trapezoidal groove, a triangular groove, a rhombic groove, and a circular groove.

[0013] A method for manufacturing a pressure sensor as described in any one of the above, comprising the following steps:

[0014] 1) Fabricate interconnected varistors and heavily doped contact regions on the front surface of the silicon substrate to obtain a silicon strain diaphragm (2);

[0015] 2) Fabricate lead holes and metal leads on the front surface of the silicon strain diaphragm;

[0016] 3) Define the shape of the cross beam structure of the beam film structure on the front surface of the silicon strain diaphragm by photolithography, and then etch to fabricate the cross beam structure;

[0017] 4) Photolithographically etch to fabricate the groove structure on the upper layer of the cross beam;

[0018] 5) Back-etch the SOI wafer / silicon wafer until a preset thickness is reached and stop;

[0019] 6) Anodic bond the silicon strain diaphragm with the cross beam structure obtained in step 5) to the glass with or without a cavity;

[0020] 7) Dice to fabricate the pressure sensor.

[0021] Further, in step 1), the varistors and the heavily doped contact regions are fabricated by ion implantation;

[0022] In step 2), a metal layer is deposited by processes such as evaporation or sputtering, etched to obtain the metal leads, and annealed and alloyed to form an ohmic contact;

[0023] In step 3), the shape of the cross beam is defined by front-side photolithography, and the cross beam structure is obtained by shallow etching with a thickness of d1;

[0024] In step 4), the groove structure is fabricated by photolithographic etching with a depth of d2;

[0025] In step 5), reverse and forward nested deep silicon etching is used to etch the back cavity until the buried oxide layer is encountered.

[0026] Further, in step 2), obtaining a SiO2 layer or a SiN layer by using the LPCVD process, and performing photolithography and etching to obtain electrode holes.

[0027] Further, in step 1), a B+ ion is implanted and annealed to obtain a varistor and a heavily doped contact region.

[0028] Further, the thickness d1 of the cross beam is equal to or greater than the depth d2 of the groove structure.

[0029] After adopting the above technical solution, the linearity is improved through the groove structure, and the overall structure is more stable, with better plasticity and flexibility, and is suitable for working conditions with large vibration and impact loads; and through different numbers of groove structures and different spacings between the groove structures, the performance of the sensor can be adjusted to make it suitable for different ranges. Description of the Drawings

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings based on these drawings without creative efforts.

[0031] Figure 1 It is a schematic structural diagram of a pressure sensor provided in Embodiment 1 of the present invention;

[0032] Figure 2 It is a partial cross-sectional view of a pressure sensor provided in Embodiment 1 of the present invention;

[0033] Figure 3 It is a top view of a pressure sensor provided in Embodiment 1 of the present invention;

[0034] Figures 4A - 4I It is a preparation flow chart of the pressure sensor in Embodiment 2 of the present invention, where:

[0035] Figure 4A It is a schematic diagram of the wafer preparation and mark alignment process;

[0036] Figure 4B It is a schematic diagram of the varistor production;

[0037] Figure 4C It is a schematic diagram of the heavily doped contact region production;

[0038] Figure 4D It is a schematic diagram of the electrode hole production;

[0039] Figure 4E Schematic diagram for fabricating metal leads;

[0040] Figure 4F Schematic diagram for fabricating beam film structure;

[0041] Figure 4G Schematic diagram for fabricating groove structure;

[0042] Figure 4H Schematic diagram for back cavity etching;

[0043] Figure 4I Schematic diagram for silicon - glass bonding;

[0044] Figures 5A - 5C Front - view schematic diagram of silicon strain diaphragms with various structural forms;

[0045] Figures 6A - 6C Front - view schematic diagram of groove structures with various structural forms. Detailed implementation manners

[0046] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0047] Embodiment 1:

[0048] As shown in Figure 1 , a pressure sensor, which is a MEMS piezoresistive pressure sensor designed and developed using MEMS technology, includes a glass base 1 and a silicon strain diaphragm 2 located on the glass base 1. The silicon strain diaphragm 2 is a silicon film with a front - side beam film and a back - cavity structure formed by front - side etching and back - cavity etching processes on an SOI wafer / silicon wafer.

[0049] As shown in Figure 1 or Figure 2 , one side of the glass base 1 is provided with a sunken cavity. The silicon strain diaphragm 2 includes an insulating dielectric layer on the front side and a silicon substrate covered by the insulating dielectric layer. The silicon substrate is an N - type <100> - plane SOI (Silicon - On - Insulator) silicon wafer or an N - type silicon wafer. The silicon strain diaphragm 2 and the glass base 1 with the sunken cavity are bonded by anodic bonding. The non - bonded surface of the silicon wafer can be thinned. Since the cavity is made on the glass base 1, the thinning thickness is not affected by the cavity, which can reduce the thickness and other dimensions of the chip and lower the cost of the chip. And because the silicon - glass anodic bonding process is adopted, the glass plays a stress - buffering role for the silicon strain diaphragm 2, improving the stability of the sensor in subsequent packaging and testing, and having broad application prospects.

[0050] As shown inFigure 1 As shown, on the front side of the silicon strain diaphragm 2 facing the cavity, there is a beam diaphragm structure 3 with a crossbeam structure, and the beam diaphragm structure 3 is arranged at the center of the silicon strain diaphragm 2, which inhibits the large deformation of the silicon strain diaphragm 2 and at the same time enables the pressure sensor to have high linearity;

[0051] As Figure 1 or Figure 3 shown, specifically, the beam diaphragm structure 3 has four ends, and a set of piezoresistors 4, a set of heavily doped contact regions 5 and a pair of metal leads 6 are provided at each end. The piezoresistors 4 and the heavily doped contact regions 5 at each end are connected in series, and the two ends are led out from the heavily doped contact regions 5 by the metal leads 6. The metal leads 6 and the heavily doped contact regions 5 form an ohmic contact on the front side of the silicon strain diaphragm 2, and a Wheatstone bridge is formed between the piezoresistors 4; the beam diaphragm structure 3 further includes a plurality of groove structures 7 provided on the crossbeam for adjusting the performance of the pressure sensor to adapt to different ranges.

[0052] Specifically, the metal lead 6 can be made of materials such as Al, Cr / Au, Ti / Au, etc. In this embodiment, Cr / Au is used as the metal lead to prevent the oxidation of the metal lead material by the oxygen generated during anodic bonding.

[0053] As Figure 1 shown, further, the groove structures 7 are arranged on each beam of the crossbeam at equal or unequal intervals, and between the crossbeam of the beam diaphragm structure 3 and the edge film region, the groove structures 7 are in the shape of straight bars or linearly tapered shapes; in this embodiment, the groove structures 7 are in the shape of straight bars, making the whole more stable, and while ensuring the sensitivity, improving the linearity of the sensor;

[0054] Specifically, the depth of the groove structure 7 is lower than the thickness of the crossbeam of the beam diaphragm structure 3. The use of the groove structure 7 can ensure the sensitivity of the pressure sensor while improving the linearity of the sensor and avoiding the risk of fragmentation; and the groove structure 7 has excellent performance in bearing bending moment, pressure load and eccentric load; by adjusting the number, spacing and depth of the groove structures 7, not only can the performance of the sensor be adjusted, but also it is applicable to different ranges.

[0055] As Figure 1 or Figure 3 shown, further, each group of piezoresistors 4 includes a plurality of piezoresistor strips. Specifically, four groups of piezoresistors 4 are symmetrically distributed at the ends of the crossbeam, and each group of piezoresistors 4 is not limited to the number of piezoresistor strips, generally 2 - 4, and 4 are used in this embodiment.

[0056] Further, the groove structure 7 includes a rectangular groove, a trapezoidal groove, a triangular groove, a rhombic groove, and a circular groove. Specifically, in this embodiment, a rectangular groove is adopted, which is arranged on the beam of the cross beam at equal intervals and is straight bar-shaped. The structure is more stable, and the plasticity and flexibility are better, suitable for different ranges; see Figure 6A , the groove structure 7 adopts a rectangular groove, and the groove structures 7 are arranged staggeredly, and the structure is more stable; see Figure 6B , the groove structure 7 adopts a circular groove, and the groove structures 7 are arranged at equal intervals, and the overall plasticity and flexibility are better; Figure 6C is a triangular groove, and the groove structures 7 are arranged at equal intervals, and the whole is more stable.

[0057] Such as Figure 5A shown, the end of the beam-film structure 3 in this solution can also adopt a square boss structure, which can improve the overall sensitivity and linearity; such as Figure 5B shown, the central part of the beam-film structure 3 in this solution is wider and the stability is better; such as Figure 5C shown, the end of the beam-film structure 3 in this solution adopts an arc structure, which makes the whole more stable and suitable for different ranges.

[0058] Embodiment 2:

[0059] This embodiment discloses a preparation method of a pressure sensor. A plurality of groups of piezoresistors 4 are fabricated at the midpoint positions of the side lines of a square diaphragm fabricated by a back cavity etching process. The number of piezoresistors 4 in each group is arbitrary, and in this embodiment, it is 4. A cross beam structure is etched on the front surface of the silicon strain diaphragm 2. The piezoresistors 4 are located on the cross beam structure and at the edge of the cross beam structure, and the piezoresistors 4 are in the stress concentration area. Specifically, the steps of this method include:

[0060] 1) Wafer preparation, marker layer etching: Align and etch the zero layer of the N-type <100> crystal plane SOI (Silicon-On-Insulator) silicon wafer, and thermally oxidize silicon dioxide on the silicon wafer surface, as Figure 4A shown;

[0061] 2) Photolithography of piezoresistors 4: Photolithograph the pattern of piezoresistors 4 in the device layer region of the SOI silicon wafer, ion implant B+, and anneal to obtain the piezoresistors 4, as Figure 4B shown;

[0062] 3) Photolithography of the heavily doped contact region 5: Photolithograph the pattern of the heavily doped contact region in the device layer region of the SOI silicon wafer, ion implant B+, and anneal to obtain the heavily doped contact region 5, as Figure 4C shown;

[0063] 4) Open the electrode holes 9: Obtain the SiO2 layer or SiN layer by LPCVD process, and photolithograph and etch to obtain the electrode holes 9, asFigure 4D as shown;

[0064] 5) Fabricate the metal lead 6 and make ohmic connection: Deposit a metal layer by processes such as evaporation or sputtering, etch to obtain the metal lead 6, and perform annealing alloying to form an ohmic contact, as Figure 4E shown;

[0065] 6) Fabricate the cross-beam film structure: Define the cross-beam shape by photolithography on the front side, and obtain the cross-beam structure of the beam film structure 3 by means of shallow etching, with a thickness of d1, as Figure 4F shown;

[0066] 7) Fabricate the groove structure 7: On the basis of shallow etching on the cross-beam, further perform photolithography and etching to obtain the groove structure 7 with a depth of d2, and obtain the complete groove-cross-beam structure beam film silicon strain diaphragm 2. The depth of the groove structure 7 is d2. The use of the groove structure 7 can ensure the sensitivity of the pressure sensor while also improving the linearity of the sensor. Moreover, the groove structure 7 has excellent performance in bearing bending moment, pressure load, and eccentric load; and by adjusting the number, spacing, and depth of the groove structure 7, not only can the performance of the sensor be adjusted, but it is also applicable to different ranges, and the depth d2 of the groove structure 7 is less than the thickness d1 of the cross-beam of the beam film structure 3; in this way, the sensitivity of the pressure sensor can be ensured, and at the same time, the linearity of the sensor can be improved, and the thickness d1 of the cross-beam of the beam film structure 3 is greater than the depth d2 of the groove structure 7 to avoid the risk of chip breakage, as Figure 4G shown;

[0067] 8) Etch the back cavity 8: Perform deep silicon etching on the back cavity by reverse and forward overlay etching until the buried oxide layer is encountered, as Figure 4H shown;

[0068] 9) Punch holes, perform glass bonding, and dicing: Anodically bond the silicon strain diaphragm 2 obtained by reverse and forward overlay deep silicon etching of the back cavity to the punched glass base 1, and dice to obtain the pressure sensor, which can reduce the thickness of the chip and there is no problem of increasing the chip size, and the cavity is fabricated on the glass base 1 without being restricted by the crystal orientation of the silicon wafer, as Figure 4I shown.

[0069] After the chip fabrication is completed, dicing is performed. Through this step, a large number of individual pressure sensor chips are formed on the entire silicon wafer. Each silicon wafer can obtain different numbers of pressure sensor chips according to the designed size.

[0070] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, or improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A pressure sensor, characterized in that, It includes a glass base (1) and a silicon strain diaphragm (2) located on the glass base (1). One side of the glass base (1) is provided with a sunken cavity. The silicon strain diaphragm (2) includes an insulating dielectric layer on the front side and a silicon substrate covered by the insulating dielectric layer; On the front side of the silicon strain diaphragm (2) facing the cavity, there is a beam film structure (3) with a cross-beam structure. Each end of the beam film structure (3) is provided with a group of piezoresistors (4), a group of heavily doped contact regions (5), and a pair of metal leads (6). The piezoresistors (4) and the heavily doped contact regions (5) are connected in series, and both ends are led out from the heavily doped contact regions (5) by the metal leads (6). The metal leads (6) and the heavily doped contact regions (5) form an ohmic contact on the front side of the silicon strain diaphragm (2), and a Wheatstone bridge is formed between the piezoresistors (4); It also includes a plurality of groove structures (7) provided on the cross-beam to adjust the performance of the pressure sensor and adapt to different ranges; The groove structures (7) are arranged on the cross-beam at equal or unequal intervals; The shape of the groove structure (7) where the beam film structure (3) is connected to the edge film region is a straight strip shape or a linearly tapered shape; The depth of the groove structure (7) is lower than the thickness of the cross-beam of the beam film structure (3).

2. The pressure sensor according to claim 1, wherein Each group of the piezoresistors (4) includes a plurality of piezoresistor (4) numbers.

3. A pressure sensor according to claim 2, wherein, The groove structure (7) includes any one of a rectangular groove, a trapezoidal groove, a triangular groove, a rhombic groove, and a circular groove.

4. A method for preparing a pressure sensor according to any one of claims 1-3, characterized in that, It includes the following steps: 1) Fabricate interconnected piezoresistors (4) and heavily doped contact regions (5) on the front side of the silicon substrate to obtain a silicon strain diaphragm (2); 2) Fabricate lead holes and metal leads (6) on the front side of the silicon strain diaphragm (2); 3) Define the shape of the cross-beam structure of the beam film structure (3) by photolithography on the front side of the silicon strain diaphragm (2), and then etch to fabricate the cross-beam structure; 4) Photolithographically etch to fabricate the groove structure (7) on the upper layer of the cross-beam; 5) Back-etch the N-type <100> crystal plane SOI silicon wafer until a preset thickness is reached and stop; 6) Anodic bond the silicon strain diaphragm (2) with the cross-beam structure obtained in step 5) to the perforated glass base (1); 7) Dicing to fabricate a pressure sensor.

5. The preparation method of a pressure sensor according to claim 4, characterized in that, In step 1), the piezoresistors (4) and the heavily doped contact regions (5) are fabricated by ion implantation; In step 2), a metal layer is deposited by processes such as evaporation or sputtering, etched to obtain the metal leads (6), and annealed and alloyed to form an ohmic contact; In step 3), the shape of the cross-beam is defined by front-side photolithography, and the cross-beam structure is obtained by shallow etching, with a thickness of d1; In step 4), the groove structure (7) is fabricated by photolithographic etching, with a depth of d2; In step 5), deep silicon etching of the back cavity (8) is performed by reverse and forward overlay etching until the buried oxide layer is encountered.

6. The manufacturing method of a pressure sensor according to claim 5, characterized in that, In step 2), it also includes obtaining a SiO2 layer or a SiN layer by LPCVD process, and photolithographically etching to obtain electrode holes (9).

7. The manufacturing method of a pressure sensor according to claim 6, characterized in that, In step 1), the piezoresistors (4) and the heavily doped contact regions (5) are obtained by injecting B+ ions and annealing.

8. The preparation method of a pressure sensor according to claim 7, characterized in that, The thickness d1 of the cross-beam is equal to or greater than the depth d2 of the groove structure (7).

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