Method for preparing half-bridge strain gauge by using glass reflow process
The half-bridge strain gauge is prepared through the glass reflow process, which solves the problems of corrosion unevenness and insufficient breakdown voltage in the manufacturing of silicon strain gauge, and achieves a strain gauge with high yield and high sensitivity.
Patent Information
- Application Number
- CN202411626302.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-14
- Publication Date
- 2025-02-18
AI Technical Summary
The existing silicon strain gauge manufacturing processes have problems such as corrosion unevenness and insufficient breakdown voltage, resulting in low yield and limited sensitivity of silicon strain gauge.
The half-bridge strain gauge is prepared by glass reflow process. The shape of glass reflow is defined by DRIE etching of silicon wafers, fill the part of the silicon wafer etched by DRIE, thin the excess glass, and the corrosion self-stop is achieved through the back side single-side corrosion process, and control the corrosion. Strain gauge thickness uniformity.
The yield of the strain gauge plate is improved, the breakdown voltage is enhanced, and the sensitivity of the silicon strain gauge is not affected.
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Abstract
Description
Technical Field
[0001] The present invention belongs to the field of micromechanical system (MEMS) processing in the field of semiconductor technology, and specifically relates to a method for preparing a half-bridge strain gauge by using a glass reflow process. Background Art
[0002] Strain gauges are widely used in force sensors and torque sensors. The main strain gauges used today are metal strain gauges and silicon strain gauges. The metal strain gauges with more mature technology and lower cost are more widely used in the market. However, due to the small strain coefficient of the metal material itself, the metal strain gauge sensor has low sensitivity. In contrast, since the strain coefficient of the silicon material itself is dozens of times that of the metal material, the use of silicon strain gauge sensors can achieve higher sensitivity, which has broad prospects for improving the performance of strain gauge force and torque sensors. However, due to the immature manufacturing process of silicon strain gauges, the cost of silicon strain gauges has remained high. There are three main methods for manufacturing silicon strain gauges.
[0003] The first method is to manufacture directly on a silicon wafer. After ion implantation on the front side of the silicon wafer, a pad structure is sputtered on the front side of the silicon wafer, and then the structure of the strain gauge is etched on the front side of the silicon wafer by DRIE. Then the silicon wafer is thinned by CMP process. Finally, black wax is used to protect the front side of the silicon wafer and the back side is etched on one side. When the back side is etched to a certain thickness and the structure of the silicon strain gauge is revealed, the etching is stopped and the black wax is washed off to separate the strain gauge (Zhang Z, Cheng X, Gui X, et al. A steel pressure sensor based on micro-fused glass frit technology[C] / / 2012 13thInternational Conference on Electronic Packaging Technology&High DensityPackaging. IEEE, 2012: 1582-1585). There are two main problems with this manufacturing method: First, due to the unevenness of the etching, when the entire wafer is etched, the strain gauge on one side of the wafer has been etched to the corresponding thickness, and the other side needs to continue to be etched, resulting in a low yield of silicon strain gauge tape-out. Second, when the silicon strain gauge is pasted on the substrate, the breakdown voltage between the strain gauge and the substrate is not high enough.
[0004] The second method is to use SOI wafers for manufacturing. The main difference between this method and the first method is that in the step of single-sided etching on the back side, the intermediate oxide layer can have the effect of self-stopping corrosion, thereby improving the problem of corrosion non-uniformity (Ham SW, Kim KB, Kim JW, et al. Design, fabrication, and characterizationof piezoresisitve strain gage-based pressure sensors for mechatronic systems[C] / / 2015 IEEE International Workshop of Electronics, Control, Measurement,Signals and their Application to Mechatronics (ECMSM). IEEE, 2015: 1-5). However, like the first method, there is the problem of insufficient breakdown voltage.
[0005] The third method is to use the method of bonding silicon wafers to glass wafers for manufacturing. After bonding the silicon wafers to the glass wafers, the front silicon is thinned to 5~20μm by CMP, and then the front silicon is ion implanted, and then the pad structure is sputtered on the front silicon layer, and then the strain gauge structure is DRIE-etched on the front silicon. Finally, the back glass is thinned to 20~80μm and the entire wafer is diced to form a single strain gauge (Kim KB, Kim J, Park CW, et al. Glass-frit bonding ofsilicon strain gages on large thermal-expansion-mismatched metallicsubstrates[J]. Sensors and Actuators A: Physical, 2018, 282: 230). This method solves the problem of uneven corrosion of strain gauges and the problem of breakdown voltage, but due to the presence of the entire glass substrate, the sensitivity of the silicon strain gauge is affected. Summary of the invention
[0006] The purpose of the present invention is to provide a method for preparing a half-bridge strain gauge using a glass reflow process to address the deficiencies in the current technology, control the uniformity of single-sided corrosion of the strain gauge, and increase the breakdown voltage without affecting the sensitivity of the silicon strain gauge.
[0007] In order to achieve the above object, the technical solution provided by the present invention is:
[0008] A method for preparing a half-bridge strain gauge using a glass reflow process, wherein the half-bridge strain gauge comprises an insulating layer, a semiconductor layer and a metal pad, wherein the semiconductor layer comprises a first resistor and a second resistor located between the metal pads, and the semiconductor layer comprises a pattern of a strain gauge device layer; the insulating layer is formed by a glass reflow process;
[0009] The method of the glass reflow process comprises the following steps:
[0010] 1) On the bottom silicon wafer, the pattern of the glass reflow process is etched by DRIE;
[0011] 2) Bonding glass sheets and performing glass reflow at high temperature to fill the bottom silicon wafer pattern;
[0012] 3) Perform chemical mechanical polishing (CMP) to remove excess glass;
[0013] 4) Bonding the doped silicon wafer to a silicon wafer with a glass reflow pattern on the bottom;
[0014] 5) Define the thickness of the front semiconductor layer by black wax protected front single-sided etching and heavy doping etching self-stop process;
[0015] 6) Sputtering aluminum pads on the semiconductor layer;
[0016] 7) Use DRIE to etch the semiconductor layer to form the pattern of the strain gauge;
[0017] 8) Thinning the bottom silicon wafer by chemical mechanical polishing (CMP);
[0018] 9) Perform single-sided etching to thin the back side. When the corrosion on the back side reaches the glass position, the corrosion on the back side stops automatically due to the corrosion resistance of the glass, thereby controlling the thickness uniformity of the strain gauge. By leaving the glass on the back side, the thickness of the strain gauge is controlled and the yield rate of the strain gauge tape-out is improved.
[0019] In step 1), the thickness of the pattern etched by DRIE in the glass reflow process is controlled to be 10-100 μm, preferably 10-60 μm.
[0020] In step 1), the shape of the pattern of the glass reflow process is usually the same as the shape of the semiconductor layer, which is the shape of the strain gauge structure.
[0021] The present invention provides a half-bridge strain gauge with low cost, high breakdown voltage resistance and thickness uniformity guaranteed by glass reflow process. The pattern of the strain gauge is filled by glass reflow process. When the silicon wafer is etched by back-side single-sided etching process, the reflowed glass can achieve the effect of self-stopping etching. The process flow of the present invention can ensure the uniformity of the overall thickness of the strain gauge, thereby greatly improving the yield of the strain gauge tape-out process, and can increase the breakdown voltage without excessively affecting the sensitivity of the strain gauge. BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figure 1 to Figure 20 1 to 20 are cross-sectional views of silicon-based MEMS devices in various process flows of manufacturing strain gauges using a glass reflow process according to an embodiment of the present invention. DETAILED DESCRIPTION
[0023] The present invention is described in detail below with reference to the accompanying drawings and by taking specific implementations as examples. However, those skilled in the art should be aware that the present invention is not limited to the specific implementations listed, and as long as they conform to the spirit of the present invention, they should be included in the protection scope of the present invention.
[0024] The strain gauge described in the embodiment of the present invention mainly includes: an insulating layer, a semiconductor layer and a metal pad. The semiconductor layer mainly includes a first resistor and a second resistor located between the metal pads, and the insulating layer is formed by a glass reflow process.
[0025] The main contents of the process include: DRIE silicon wafer defines the shape of glass reflow, then after silicon and glass are bonded, the glass reflow process is used to fill the DRIE part of the silicon wafer, and then CMP thinning is performed to remove excess glass to form the shape of the glass; then the doped silicon wafer is bonded to the front side, and then the front side CMP, front side single-sided etching and heavy doping self-stop etching process are used to define the thickness of the front semiconductor layer, and then the front thin semiconductor layer is etched into a strain gauge structure through DRIE, and finally removed through CMP thinning and back side single-sided etching process.
[0026] like Figures 1 to 20 As shown, the process of the embodiment of the present invention is to first prepare a MEMS silicon wafer, namely, silicon wafer 100 .
[0027] Step 1: Photolithography of a photoresist 101 with a pattern opposite to that of the strain gauge is performed on the silicon wafer 100, so as to protect the non-strain gauge structure parts that do not need to be etched by subsequent DRIE.
[0028] Step 2: Remove the pattern part of the strain gauge by deep reactive ion etching DRIE, use BOSH process for etching, and then use acetone to remove the photoresist 101.
[0029] Step 3: Surface oxidation of the silicon wafer 100 is performed to form an oxide layer 102 . The oxide layer 102 is used to enhance the adhesion between the glass and the silicon wafer in a subsequent glass reflow process.
[0030] Step 4: Bonding the glass sheet 103 to the silicon sheet 100 , the glass sheet 103 is used for the subsequent glass reflow process.
[0031] Step 5: Reflow the bonded silicon-glass wafer at high temperature, which is generally higher than the softening point of the glass to ensure that the glass and the silicon wafer are tightly bonded.
[0032] Step 6: After the glass is reflowed, CMP thinning is performed on the reflowed surface (the front side of the silicon wafer 100 after reflow) to make the surface of the front side of the silicon wafer 100 smooth and flat.
[0033] Step 7: Bonding the doped silicon wafer 104 to the silicon-glass wafer. The doping on the silicon wafer can be completed by ion implantation or diffusion process. The doped silicon layer 105 serves as the semiconductor layer of the silicon strain gauge.
[0034] Step 8: CMP thinning is performed on the front side of the bonded assembly wafer to further planarize the surface.
[0035] Step 9: Apply black wax 106 on the back of the combined wafer for protection, and place the entire wafer into a TMAH solution for etching. Perform water bath heating during the etching process. Since the doped part 105 of the silicon wafer bonded above is heavily doped, the corrosion can stop automatically after etching to the doped part 105. This self-stopping mechanism can also be achieved by replacing the heavily doped silicon wafer with an SOI wafer with a suitable device layer and oxide layer thickness.
[0036] Step 10: Clean and remove the black wax 106 on the back for protection;
[0037] Step 11: Photolithography of the pad structure is performed on the front side of the assembly for subsequent sputtering of the pad.
[0038] Step 12: Sputtering metal to form a pad. A commonly used metal is aluminum. After sputtering, a stripping process is used and acetone is used to clean the photoresist to form a pad 107 of the strain gauge.
[0039] Step 13: Perform photolithography of the strain gauge structure on the combined sheet, and DRIE-etch the thin semiconductor layer of the front doped part of the combined sheet to form the strain gauge structure.
[0040] Step 14: Perform CMP thinning on the back side of the combined wafer to speed up the subsequent wet etching process.
[0041] Step 15: Apply black wax 106 to the front of the thinned assembly for protection, and then place the entire wafer in the TMAH solution for etching. During the etching, it is heated in a water bath. The back side is single-sidedly etched by filling the glass 103 of the bottom silicon wafer with the glass reflow process to stop the etching. When the etching reaches the back glass 103, the etching stops automatically. The front structure is protected by black wax 106, thereby completing the etching of the entire structure of the silicon strain gauge.
[0042] Step 16: Wash away the black wax 106. Previously, the strain gauge has been separated in the black wax to form a single strain gauge. When the black wax 106 is washed away, the single strain gauge falls off from the black wax, thereby achieving the individual separation of the strain gauge. Fig. 20 shown.
[0043] The present invention is a half-bridge strain gauge with low cost, high breakdown voltage resistance and thickness uniformity through glass reflow process, which mainly includes: an insulating layer, a semiconductor layer and a metal pad. The semiconductor layer mainly includes a first resistor and a second resistor located between the metal pads, and the insulating layer is formed by a glass reflow process. The main contents of the process include: a DRIE silicon wafer defines the shape of the glass reflow, and then the glass reflow process is used to fill the part of the silicon wafer etched by DRIE, and then the excess glass is thinned to form the shape of the glass; then the doped silicon wafer is bonded to the front side, and then the thickness of the front semiconductor layer is defined by the front CMP and the single-sided etching of the front side and the heavy doping self-stop etching process, and then the thin semiconductor layer of the front side is etched into a strain gauge structure by DRIE, and finally, the excess silicon is removed by the CMP and single-sided etching process on the back side, and the thickness of the strain gauge is controlled by leaving the glass on the back side after the single-sided etching, thereby improving the yield rate of the strain gauge tape-out.
[0044] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
Claims
1. A method for preparing a half-bridge strain gauge using a glass reflow process, characterized in that The half-bridge strain gauge comprises an insulating layer, a semiconductor layer and a metal pad, wherein the semiconductor layer comprises a first resistor and a second resistor located between the metal pads, and the semiconductor layer comprises a pattern of a strain gauge device layer; The insulating layer is formed by a glass reflow process; The method of the glass reflow process comprises the following steps: 1) On the bottom silicon wafer, the pattern of the glass reflow process is etched by DRIE; 2) Bonding glass sheets and performing glass reflow at high temperature to fill the bottom silicon wafer pattern; 3) Perform chemical mechanical polishing (CMP) to remove excess glass; 4) Bonding the doped silicon wafer to a silicon wafer with a glass reflow pattern on the bottom; 5) Define the thickness of the front semiconductor layer by black wax protected front single-sided etching and heavy doping etching self-stop process; 6) Sputtering aluminum pads on the semiconductor layer; 7) Use DRIE to etch the semiconductor layer to form the pattern of the strain gauge; 8) Thinning the bottom silicon wafer by chemical mechanical polishing (CMP); 9) Perform single-sided etching to thin the back side. When the corrosion on the back side reaches the glass position, the corrosion on the back side stops automatically due to the corrosion resistance of the glass, thereby controlling the thickness uniformity of the strain gauge. By leaving the glass on the back side, the thickness of the strain gauge is controlled and the yield rate of the strain gauge tape-out is improved.
2. A method for preparing a half-bridge strain gauge using a glass reflow process as claimed in claim 1, characterized in that In step 1), the thickness of the pattern etched by DRIE in the glass reflow process is controlled to be 10-100 μm.
3. A method for preparing a half-bridge strain gauge using a glass reflow process as claimed in claim 2, characterized in that The thickness of the pattern etched by DRIE in the glass reflow process is controlled to be 10-60 μm.
4. A method for preparing a half-bridge strain gauge using a glass reflow process as claimed in claim 1, characterized in that In step 1), the shape of the pattern of the glass reflow process is the same as the shape of the semiconductor layer, which is the shape of the strain gauge structure.