A method for preparing a micro MEMS piezoresistive pressure sensor

Abstract

The present invention provides a method for preparing a micro-MEMS piezoresistive pressure sensor, in which a varistor is prepared by ion implantation in the top silicon layer, a cavity is prepared by etching in the back silicon layer, and bonding is performed twice on the front and back sides of the SOI wafer, which respectively serve as a temporary bonding sheet for protection and a substrate for cavity sealing, and the whole structure is a sandwich structure. This structure ensures the rigidity of the device, reduces the probability of wafer breakage in subsequent processing, and can also achieve a thinner device thickness. Miniaturization is achieved, the thickness of the device is reduced, and the protection of subsequent devices is achieved. The micromechanical piezoresistive pressure sensor using temporary bonding can play a role of positive protection in the grinding and thinning of the SOI wafer. At the same time, due to the presence of the upper temporary bonding sheet after bonding, the overall rigidity is guaranteed, and the breakage caused by the wafer being too thin in the subsequent process is avoided, and it is easy to separate later.

Description

Technical Field

[0001] The present invention relates to the technical field of sensor manufacturing, and in particular to a method for preparing a micro MEMS piezoresistive pressure sensor. Background Art

[0002] Micro MEMS pressure sensors have the advantages of low power consumption, light weight, small size, high measurement accuracy, low cost, and little impact on the detection target. They are widely used in aerospace, medical treatment, automobile and consumer electronics. The working principle of piezoresistive pressure sensors is that the elastic diaphragm deforms under pressure, causing the resistance value of the varistor to change. Compared with capacitive / resonant pressure sensors, piezoresistive pressure sensors have simpler manufacturing processes and subsequent circuit processing, better output linearity, and are easier to produce small-sized devices, so they have attracted more attention.

[0003] The direction of miniaturization of piezoresistive pressure sensors is mainly focused on reducing the area of ​​piezoresistive film, optimizing circuit layout and thinning bottom silicon. Among them, the reduction of film length and the optimized circuit layout are to shrink the sensor in a two-dimensional plane. In the thickness direction, a combination of reactive ion deep etching (DRIE) and chemical mechanical polishing (CMP) is usually used to etch a cavity with a depth of more than 100um, thereby producing a pressure sensor with a thickness of more than 100um. However, the continuous thinning of the bottom silicon will cause the wafer to inevitably break in the subsequent processing process, thereby affecting the device yield. Summary of the invention

[0004] The main purpose of the present invention is to provide a method for preparing a micro MEMS piezoresistive pressure sensor, aiming to improve the technical problem in the prior art that continuous thinning of the bottom silicon may cause wafer breakage in subsequent processing steps.

[0005] To achieve the above object, the present invention provides a method for preparing a micro MEMS piezoresistive pressure sensor, comprising the following steps:

[0006] a: Use etching process to etch the alignment mark on the top silicon layer of SOI wafer;

[0007] b: Using two ion implantations and annealing, an ohmic contact region, a varistor, and a reference resistor are formed on the top silicon layer of the SOI wafer;

[0008] c: fabricating a metal electrode on the top silicon layer of the SOI wafer, and performing ohmic contact annealing on the metal electrode and the ohmic contact region;

[0009] d: forming a first metal film on the top silicon layer of the SOI wafer;

[0010] e: preparing a second metal film on the first glass sheet by using a patterning process;

[0011] f: thermally pressing and bonding the first glass sheet and the SOI wafer through the second metal film and the first metal film;

[0012] g: Use chemical mechanical polishing to thin the back silicon layer of the SOI wafer;

[0013] h: preparing a patterned photoresist on the back silicon layer of the SOI wafer at a position corresponding to the pressure-sensitive film, and using the patterned photoresist as a mask, etching the back silicon layer of the SOI wafer using a BOSCH process until the etching reaches the buried oxide layer;

[0014] i: Etch the buried oxide layer, remove the photoresist mask, and clean the SOI wafer;

[0015] j: Bonding the back silicon layer of the SOI wafer to the second glass sheet using an anodic bonding method;

[0016] k: thinning the second glass sheet by chemical mechanical polishing;

[0017] l: Slice according to the size of the pressure sensor;

[0018] m: Place the split MEMS device in a chemical solution to corrode the second metal film and the first metal film used for thermal compression bonding, and remove the first glass sheet.

[0019] Optionally, the top silicon layer is an n-type high-resistance silicon material with a thickness between 0.1um and 5um;

[0020] The ohmic contact region, the varistor, and the reference resistor are made of p-type silicon material, and the two ion implantations used are boron implantations.

[0021] Optionally, the pressure-sensitive film is located on the top silicon layer of the SOI wafer, and the shape of the pressure-sensitive film is square or circular;

[0022] The varistor is located at the inner edge of the pressure-sensitive film, and the shape of the varistor is square, Π-shaped or U-shaped;

[0023] The reference resistor has the same shape and size as the varistor, and is located outside the pressure-sensitive film.

[0024] Optionally, the ohmic contact region is located at both ends of the varistor and the reference resistor, and is connected to the varistor and the reference resistor.

[0025] Optionally, the metal electrode is located on the upper surface of the top silicon layer and is connected to the ohmic contact region, the thickness of the metal electrode is 50nm-2um, and the material of the metal electrode is a metal material that can be commonly used to generate ohmic contact with P-type silicon;

[0026] The first metal film is located on the upper surface of the top silicon layer and is not in contact with the metal electrode. The thickness of the first metal film is greater than the metal electrode. The material of the first metal film is a metal material commonly used in wafer thermal compression bonding.

[0027] Optionally, the thickness of the first glass sheet is between 100 micrometers and 1 millimeter, and at least one side is a polished surface with a surface roughness of less than 2 nm.

[0028] Optionally, the second metal film is located on the polished side surface of the first glass sheet, and the second metal film is made of a metal material commonly used in wafer thermal compression bonding.

[0029] Optionally, the thickness of the back silicon layer after thinning and polishing is between 10 microns and 100 microns, and the roughness of the surface of the back silicon layer is less than 2 nm.

[0030] Optionally, the bonding pressure of the anodic bonding between the second glass sheet and the SOI wafer is 0.001 mbar-1 bar, and the pressure of the sealed cavity between the second glass sheet and the back silicon layer and the pressure-sensitive film after bonding is 1 mbar-1 bar.

[0031] Optionally, the chemical solution in step m has selective corrosion capability to different metal materials;

[0032] The chemical solution is corrosive to the first metal film and the second metal film, but is not corrosive or has weak corrosiveness to the metal electrode.

[0033] The preparation method of the micro-MEMS piezoresistive pressure sensor based on temporary bonding provided by the present invention temporarily bonds the SOI wafer 1 and the first glass sheet 8 into a whole by thermal compression bonding, thereby increasing the rigidity of the structure and facilitating further thinning of the device. On the one hand, it ensures the integrity of the wafer and the yield rate of the device in the subsequent process, and on the other hand, the thinned pressure sensor can produce a shallower cavity, which can provide protection for the device film under overload conditions. At the same time, the micro-MEMS piezoresistive pressure sensor has a simple manufacturing process, small size, low cost, and easy integration. It is widely used in aerospace, medical testing, automotive industry / consumer electronics and other fields. The present invention has considerable economic and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0035] Figure 1A schematic structural diagram of an embodiment of a micro-MEMS piezoresistive pressure sensor provided by the present invention;

[0036] Figure 2 A top view of the micro MEMS piezoresistive pressure sensor provided by the present invention;

[0037] Figure 3 A schematic diagram of the SOI wafer of the micro MEMS piezoresistive pressure sensor provided by the present invention after the piezoresistor, ohmic contact area, and lead connection process are completed;

[0038] Figure 4 A schematic diagram of a micro-MEMS piezoresistive pressure sensor provided by the present invention after the SOI wafer and the first glass sheet are thermally pressed and bonded;

[0039] Figure 5 This is a schematic diagram of the micro-MEMS piezoresistive pressure sensor provided by the present invention after the SOI wafer is thermocompressed and bonded to the first glass sheet and then the cavity is etched;

[0040] Figure 6 This is a schematic diagram of the final structure of the micro MEMS piezoresistive pressure sensor provided by the present invention after removing the first glass sheet.

[0041] Description of Figure Numbers:

[0042] Label name Label name 1 SOI Wafer 11 Buried Oxide Layer 2 Top silicon layer 12 Pressure sensitive film 3 Registration mark 13 Reference resistor 4 Ohmic contact area 14 Second glass sheet 5 Varistor 6 Metal Electrode 7 The first metal film 8 First glass sheet 9 Second metal film 10 Back silicon layer

[0043] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] It should be noted that if a directional indication is involved in the embodiment of the present invention, the directional indication is only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0046] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the meaning of "and / or" appearing in the full text includes three parallel schemes. Taking "A and / or B" as an example, it includes scheme A, or scheme B, or a scheme that satisfies both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0047] See also Figure 1-Figure 2 As shown, the present invention provides a micro-MEMS piezoresistive pressure sensor structure, the first glass sheet 8 used for temporary bonding on the top is specifically pyrex7740, the thickness can be 300um-1mm, and it is thermocompressively bonded to the top silicon layer 2 with a cavity on the back through a temporary bonding material, and the bonding material includes a second metal film 9 and a first metal film 7. A high-doped ohmic contact region 4 and a varistor 5 are formed on the front of the top silicon layer 2. The varistor 5 is formed by etching the back silicon layer 10 and corroding the buried oxide layer 11 to obtain a pressure-sensitive film 12. The varistor 5 can be one, two (not shown) or four (not shown) to form a Wheatstone bridge. The varistor 5 forms an ohmic contact with the metal electrode 6 through the high-doped ohmic contact region 4, and the formed ohmic contact serves as an extraction electrode. The back silicon layer 10 thinned by CMP, its thickness can be 30um-400um, and is bonded to the pyrex7740 substrate 14 through silicon-glass anodic bonding.

[0048] The embodiment of the present invention combines the MEMS silicon processing technology, prepares the varistor 5 by ion implantation on the top silicon layer 2, prepares the cavity by etching on the back silicon layer 2, and performs two bondings on the front and back sides of the SOI wafer 1, which respectively serve as a temporary bonding sheet for protection and a substrate for cavity sealing, and the whole is a sandwich structure. This structure ensures the rigidity of the device, reduces the probability of wafer breakage in subsequent processing, and can also achieve a thinner device thickness.

[0049] See also Figure 3-Figure 6 As shown, the specific process of the preparation process is as follows:

[0050] a: Select SOI wafer 1, which is composed of three parts: a 2um thick top silicon layer 2, an 800nm ​​buried oxide layer 11 and a 450um bottom silicon layer 10. Use ICP etching process to etch the alignment mark 3 on the top silicon layer 2.

[0051] b: Two ion implantations are used to form a highly doped ohmic contact region 4 and a varistor 5 in the top silicon layer 2 of the SOI wafer 1, respectively. The target doping concentration is achieved by reasonably adjusting the implantation energy and implantation dose. The ion implantation dose of the ohmic contact region 4 is 5E15cm-3, and the ion implantation dose of the varistor 5 region is 2E14cm-3. Both implantation energies are 30keV.

[0052] c: A metal electrode 6 with a thickness of 1 um is produced by magnetron sputtering aluminum in the ohmic contact region 4 of the top silicon layer 2 of the SOI wafer 1 .

[0053] d: A first metal film 7 for thermocompression bonding is fabricated on the top silicon layer 2 of the SOI wafer 1. The specific material is copper and the thickness is 500 nm.

[0054] e: A second metal film 9 for hot pressing bonding is made on the first glass sheet 8 by magnetron sputtering copper with a thickness of 500 nm, and the metal film is graphically etched to ensure that the coordinate position area on the first glass sheet 8 is the same as the alignment mark 3 and is not covered by the metal film.

[0055] f: Bond the first glass sheet 8 to one side of the top silicon layer 2 of the SOI wafer 1 by thermal compression, ensuring that the alignment mark 3 on the SOI wafer 1 can be seen from the side of the first glass sheet 8. The thermal compression bonding strength is adjusted by bonding time, temperature and pressure.

[0056] g: The back silicon layer 10 of the SOI wafer 1 is thinned by chemical mechanical polishing, and the thinning thickness is controlled by the particle size of the abrasive and the polishing time.

[0057] h): Using ultraviolet lithography, patterning photoresist is performed on the back silicon layer 10 of the SOI wafer 1 , and etching the back silicon layer 10 of the SOI wafer 1 using the photoresist as a mask, with the etching depth reaching the buried oxide layer 11 .

[0058] i): The buried oxide layer 11 is etched with a BOE solution, and the photoresist used as a back silicon etching mask is removed to produce a pressure-sensitive film 12.

[0059] j: The back silicon layer 10 of the SOI wafer 1 is bonded to the second glass sheet 14 using a silicon-glass anodic bonding method.

[0060] k: The second glass sheet 14 is thinned by chemical mechanical polishing to a thickness of 400 um, with a remaining thickness of 50 um.

[0061] l: Use a grinding wheel dicing machine for dicing, the device size is 1600um×500um×100um.

[0062] m: Place the split MEMS device in a release solution to corrode the first metal film 7 and the second metal film 9 used for hot pressing bonding, remove the first glass sheet 8, and complete the manufacture of the MEMS piezoresistive pressure sensor. By adjusting the etching solution ratio (H2O2:H2SO4:H2O=1:1:40), the etching temperature is 70°C, and the etching time is 30 minutes, the bonding material and the SOI pressure sensor are separated.

[0063] It should be noted that: the top silicon layer 2 is an n-type high-resistance silicon material with a thickness between 0.1um and 5um; the ohmic contact area 4, the varistor 5, and the reference resistor 13 are p-type silicon materials, and the two ion implantations used are boron implantations. The pressure-sensitive film 12 is located on the top silicon layer 2, and the shape of the pressure-sensitive film 12 is square or circular; the varistor 5 is located at the inner edge of the pressure-sensitive film 12, and the shape of the varistor 5 is square or Π-shaped or U-shaped; the reference resistor 13 has the same shape and size as the varistor 5, and the reference resistor 13 is located outside the pressure-sensitive film 12. The ohmic contact area 4 is located at both ends of the varistor 5 and the reference resistor 13, and is connected to the varistor 5 and the reference resistor 13. The metal electrode 6 is located on the upper surface of the top silicon layer 2 and is connected to the ohmic contact area 4. The thickness of the metal electrode 6 is 50nm-2um. The material of the metal electrode 6 is a metal material that can be commonly used to produce ohmic contact with P-type silicon, specifically an electrode material commonly used in MEMS devices such as aluminum and titanium.

[0064] The first metal film 7 is located on the upper surface of the top silicon layer 2 and does not contact the metal electrode 6. The thickness of the first metal film 7 is greater than that of the metal electrode 6. The material of the first metal film 7 is a metal material commonly used for wafer hot pressing bonding. The thickness of the first glass sheet 8 is between 100 microns and 1 mm, and at least one side is a polished surface with a surface roughness of less than 2nm. The second metal film 9 is located on the polished side surface of the first glass sheet 8. The material of the second metal film 9 is a metal material commonly used for wafer hot pressing bonding. The thickness of the back silicon layer 10 after thinning and polishing is between 10 microns and 100 microns, and the roughness of the surface of the back silicon layer 10 is less than 2nm. The bonding pressure of the anodic bonding between the second glass sheet 14 and the SOI wafer 1 is 0.001mbar-1bar, and the air pressure of the sealed cavity between the second glass sheet 14 and the back silicon layer 10 and the pressure-sensitive film 12 after bonding is 1mbar-1bar. The chemical solution in step m has the ability to selectively corrode different metal materials; wherein the chemical solution is corrosive to the first metal film 7 and the second metal film 9, and has no corrosiveness or weak corrosiveness to the metal electrode 6.

[0065] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a micro MEMS piezoresistive pressure sensor, characterized in that: The steps include: a: using an etching process to etch an alignment mark (3) on a top silicon layer (2) of an SOI wafer (1), wherein the SOI wafer is composed of a top silicon layer (2), a buried oxide layer (11) and a back silicon layer (10); b: using two ion implantations and annealing to form an ohmic contact region (4), a varistor (5) and a reference resistor (13) on the top silicon layer (2) of the SOI wafer (1); c: manufacturing a metal electrode (6) on the top silicon layer (2) of the SOI wafer (1), and performing ohmic contact annealing on the metal electrode (6) and the ohmic contact region (4); d: fabricating a first metal film (7) on the top silicon layer (2) of the SOI wafer (1); e: preparing a second metal film (9) on the first glass sheet (8) using a patterning process; f: photolithographically aligning and thermally pressing the first glass sheet (8) and the SOI wafer (1) using the second metal film (9) and the first metal film (7); g: thinning the back silicon layer (10) of the SOI wafer (1) using a chemical mechanical polishing method; h: preparing a patterned photoresist at a position corresponding to the pressure-sensitive film (12) on the back silicon layer (10) of the SOI wafer (1), and using the patterned photoresist as a mask to etch the back silicon layer (10) of the SOI wafer (1) using a BOSCH process until the etching reaches the buried oxide layer (11); i: etching the buried oxide layer (11), removing the photoresist mask, and cleaning the SOI wafer (1); j: bonding the back silicon layer (10) of the SOI wafer (1) to the second glass sheet (14) using an anodic bonding method; k: thinning the second glass sheet (14) by using a chemical mechanical polishing method; l: Slice according to the size of the pressure sensor; m: placing the split MEMS device in a chemical solution to corrode the second metal film (9) and the first metal film (7) used for thermal compression bonding, and removing the first glass sheet (8); The ohmic contact region (4) is located at both ends of the varistor (5) and the reference resistor (13), and is connected to the varistor (5) and the reference resistor (13); The chemical solution in step m has the ability to selectively corrode different metal materials; wherein the chemical solution is corrosive to the first metal film (7) and the second metal film (9), and is not corrosive or has weak corrosiveness to the metal electrode (6).

2. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The top silicon layer (2) is an n-type high-resistance silicon material with a thickness between 0.1 um and 5 um; The ohmic contact region (4), the varistor (5), and the reference resistor (13) are made of p-type silicon material, and the two ion implantations used are boron implantations.

3. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The pressure-sensitive film (12) is located on the top silicon layer (2) of the SOI wafer (1), and the shape of the pressure-sensitive film (12) is square or circular; The varistor (5) is located at the inner edge of the pressure-sensitive film (12), and the shape of the varistor (5) is square, Π-shaped or U-shaped; The reference resistor (13) has the same shape and size as the varistor (5), and the reference resistor (13) is located outside the pressure-sensitive film (12).

4. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The metal electrode (6) is located on the upper surface of the top silicon layer (2) and is connected to the ohmic contact area (4); the thickness of the metal electrode (6) is 50 nm-2 um; the material of the metal electrode (6) is a metal material commonly used to generate ohmic contact with P-type silicon; The first metal film (7) is located on the upper surface of the top silicon layer (2) and is not in contact with the metal electrode (6); the thickness of the first metal film (7) is greater than that of the metal electrode (6); and the material of the first metal film (7) is a metal material commonly used in wafer thermal compression bonding.

5. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The thickness of the first glass sheet (8) is between 100 micrometers and 1 millimeter, and at least one side is a polished surface, wherein the surface roughness of the polished surface is less than 2 nm.

6. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 5, characterized in that: The second metal film (9) is located on the surface of the polished side of the first glass sheet (8), and the material of the second metal film (9) is a metal material commonly used in wafer thermal compression bonding.

7. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The thickness of the back silicon layer (10) after thinning and polishing is between 10 micrometers and 100 micrometers, and the surface roughness of the back silicon layer (10) is less than 2 nm.

8. The method for preparing a micro-MEMS piezoresistive pressure sensor according to claim 1, characterized in that: The bonding pressure of the anodic bonding between the second glass sheet (14) and the SOI wafer (1) is 0.001 mbar-1 bar, and after bonding, the air pressure of the sealed cavity between the second glass sheet (14), the back silicon layer (10) and the pressure-sensitive film (12) is 1 mbar-1 bar.

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