A low-power-consumption silicon-based MEMS gas sensor chip and a preparation method thereof

Abstract

The application discloses a low-power-consumption silicon-based MEMS gas sensor chip and a preparation method thereof. The application utilizes the design of a pair of needle-shaped heating electrodes, so that the heating resistor is concentrated on the connection area of the top of the pair of needles. The size of the central area is extremely small, usually only tens to hundreds of nanometers, and enough heat can be generated at extremely low power consumption to stimulate the sensitive characteristics of the gas-sensitive material. Compared with the traditional snake-shaped coiled heating electrode, the structure not only has lower power consumption, but also has smaller deformation of the electrode during heating due to the small working area, the gradually changing width and the annular support structure, which is beneficial to the stability of the overall structure. The preparation of the detection electrode is simpler due to the design of the pair of needle-shaped electrodes, and the yield can be greatly improved. Meanwhile, the overall volume of the sensor can be greatly reduced by using the structural characteristics of the new type of heating electrode and sensing electrode, so that the sensor has higher integrability and wider application scenarios.

Description

Technical Field

[0001] This invention belongs to the field of MEMS gas sensor technology, specifically relating to a low-power silicon-based MEMS gas sensor chip and its fabrication method. Background Technology

[0002] With the increasing environmental awareness of modern people, the monitoring of air quality and the control of pollution sources are becoming increasingly important. A growing variety of gas sensors are being widely used in air quality monitoring, VOCs detection, fire detection, and industrial process control, and will also find widespread application in future smart homes, the Internet of Things, and artificial intelligence.

[0003] Traditional gas sensors, such as electrochemical sensors, infrared sensors, and thermal conductivity sensors, are large in size, complex, and require external power supplies, which limits their application in miniaturization and intelligent applications. MEMS (Micro-Electro-Mechanical Systems) gas sensors, on the other hand, not only have a smaller size but also offer higher reliability, stability, and response speed, enabling real-time monitoring and online analysis of various gases.

[0004] Currently, mainstream MEMS gas sensor hotplate structures typically employ a disc-shaped resistance wire design. A disc-shaped resistance wire, ranging from tens to hundreds of micrometers in size, is fabricated on a chip as a thermistor, with its electrodes and thin-film resistors forming an electrothermal hotplate sensor. However, this structure requires high-power heating during operation, consuming a significant amount of energy. This structural characteristic also results in a large temperature gradient in the heated portion of the device. Furthermore, due to the suspended structure of the hotplate, its weight is entirely supported by the bridge-shaped metal wire, making it prone to deformation or failure during operation. Simultaneously, the large size of the heated portion limits the miniaturization and integration of the sensor.

[0005] For example, patent application CN1 14487036A discloses a MEMS gas sensor with gas enrichment function and its working method. This MEMS gas sensor with gas enrichment function includes a substrate, a first insulating support layer, a heating resistor layer, a second insulating support layer, a sensitive electrode layer, a semiconductor gas-sensitive material layer, a gas enrichment material layer, and a suspended membrane via. The heating resistor layer (3) includes: at least one heating resistor wire disposed between the first insulating support layer (2) and the second insulating support layer (4), its path passing through the projection area of ​​the gas enrichment material layer (7) on the first insulating support layer (2); and at least one first welding part, the two ends of the heating resistor wire being electrically connected to the first welding part, the first welding part being used to electrically connect the working and temperature sensing circuits of the heating resistor.

[0006] The literature (Shen Weiqiang et al., Research Progress of MEMS Micro Hot Plates, Sensors & Microsystems, Vol. 42, No. 7, 2023, pp. 6-10) discloses that the performance of gas sensors, such as sensitivity, selectivity, and response time, depends on the thermal characteristics of the micro hot plate. When designing heating electrodes, power consumption, temperature stability, temperature uniformity, and stress are key parameters, which are generally adjusted by changing the electrode geometry. Researchers have so far studied heating electrodes with different geometries, including serpentine, loop-shaped, drive wheel-shaped, and honeycomb-shaped electrodes; among them, serpentine and loop-shaped structures have better temperature distribution uniformity, structural stability, and more flexible adjustment of electrode width and spacing, making them the most widely used. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention discloses a low-power silicon-based MEMS gas sensor chip structure and its fabrication method. By optimizing the micro-hot plate structure and utilizing the size effect of metal resistance, a nanoscale heating structure is designed, which significantly reduces the volume of the heating unit, increases structural stability, and greatly reduces the power consumption of the sensor. This achieves a reduction in sensor size and an improvement in reliability, enabling it to meet more diverse application scenarios.

[0008] This invention first provides a low-power MEMS gas sensor chip, comprising:

[0009] Substrate,

[0010] A first oxide layer is disposed on the surface of the substrate, serving as both a dielectric layer and a support layer.

[0011] A first adhesive layer is disposed on the surface of the first oxide layer.

[0012] A heating electrode layer, disposed on the surface of the first adhesive layer, includes heating electrodes and corresponding pads. The heating electrodes include two electrodes with a paired pin-type structure.

[0013] The second adhesive layer is disposed on the surface of the heating electrode layer and the surface of the first adhesive layer not covered by the heating electrode layer.

[0014] A second oxide layer is disposed on the surface of the second adhesive layer.

[0015] A third adhesive layer is disposed on the surface of the second oxide layer.

[0016] A measurement electrode layer, disposed on the surface of the third adhesive layer, includes measurement electrodes and corresponding pads. The measurement electrodes include two electrodes with a paired pin-type structure.

[0017] A gas-sensitive material layer fills and covers the third adhesive layer between the two measuring electrodes in the central working area that is not covered by the measuring electrode layer.

[0018] Preferably, the axial length of the contact area between the tips of the two heating electrodes is 10 nm to 1 μm. The central working area of ​​the heating electrode refers to the area where the tips of the two heating electrodes touch. The axial length of the contact area refers to the dimension along the axial direction of the heating electrode.

[0019] The distance between the tips of the two measuring electrodes, which are pin-shaped structures, is 10 nm to 1 μm. The central working region of the measuring electrodes refers to the area where the tips of the two measuring electrodes are located.

[0020] More preferably, the two heating electrodes gradually widen from the tip of the needle outwards;

[0021] The two measuring electrodes gradually widen from the tip of the needle outwards.

[0022] Preferably, the thickness of the heating electrode is 30-300 nm; the thickness of the measuring electrode is 30-300 nm.

[0023] Preferably, the pads corresponding to the heating electrode and the pads corresponding to the measuring electrode do not overlap.

[0024] Preferably, the substrate is made of a material selected from: single-crystal silicon, ceramic, porous silicon, or silicon nitride;

[0025] The materials for the first oxide layer and the second oxide layer are independently selected from: silicon oxide and silicon nitride;

[0026] The materials for the first adhesive layer, the second adhesive layer, and the third adhesive layer are each independently selected from: titanium and aluminum oxide;

[0027] The gas-sensitive material layer is made of tin oxide or tungsten oxide.

[0028] The materials for the heating electrode and the measuring electrode are independently selected from platinum and gold.

[0029] Preferably, the thickness of the first oxide layer is 500 nm-3 μm; the thickness of the second oxide layer is 50 nm-1 μm.

[0030] The thicknesses of the first adhesive layer, the second adhesive layer, and the third adhesive layer are all 5-10 nm.

[0031] The thickness of the gas-sensitive material layer is 5nm-1μm.

[0032] This invention further provides a method for fabricating the low-power MEMS gas sensor chip, comprising the following steps:

[0033] Step S1: Prepare the substrate and prepare the first oxide layer on the substrate surface.

[0034] Step S2: A first adhesion layer is prepared on the surface of the first oxide layer, and a heating electrode layer is prepared on the first adhesion layer to form a heating electrode and a corresponding pad.

[0035] Step S3: Prepare a second adhesion layer on the surface of the heated electrode layer, and then prepare a second oxide layer on the surface of the second adhesion layer.

[0036] Step S4: A third adhesion layer is prepared on the surface of the second oxide layer, and then a measurement electrode layer is prepared on the surface of the third adhesion layer to form the measurement electrode and the corresponding pad.

[0037] Step S5: Prepare a gas-sensitive material layer on the surface of the measuring electrode layer.

[0038] Step S6: Remove silicon oxide outside the effective electrode area.

[0039] Step S7: Thin the entire substrate from the back side.

[0040] Step S8: Etch the substrate from the back side to form a cavity, so that the substrate corresponding to the working area is completely etched away, and the working area is suspended.

[0041] Preferably, after thinning in step S7, the substrate retains a thickness of 100-300 μm.

[0042] Compared with the prior art, the advantages of the present invention are:

[0043] Utilizing an innovative needle-type heating electrode design, the heating resistance is concentrated in the connection region at the needle apex. This central region is extremely small, typically only 10nm-1μm in size, generating sufficient heat to excite the sensitive properties of the gas-sensitive material with very low power consumption. Compared to traditional serpentine coiled heating electrodes, this structure not only consumes less power, but its extremely small working area, gradually changing width, and ring-shaped support structure also ensure less electrode deformation during heating, contributing to the overall structural stability. The fabrication of the needle-shaped detection electrode is simpler, and the yield rate is significantly improved. Furthermore, the structural characteristics of this novel heating and sensing electrode allow for a substantial reduction in the overall size of the sensor, resulting in higher integrability and a wider range of applications. Attached Figure Description

[0044] Figure 1 This is a flowchart of the MEMS sensor fabrication method provided in the embodiments.

[0045] Figure 2 This is an illustration of the heating electrode structure and central region obtained after step S2 in the MEMS sensor fabrication method provided in the embodiment.

[0046] Figure 3This is an illustration of the measurement electrode structure and central region obtained after step S4 in the MEMS sensor fabrication method provided in the embodiment.

[0047] Figure 4 This is an analytical illustration of the central structure after etching in step S6 of the MEMS sensor fabrication method provided in the embodiment.

[0048] Figure 5 This is an analytical illustration of the suspended support structure after etching in step S8 of the MEMS sensor fabrication method provided in the embodiment.

[0049] Figure 6 This is a perspective view of the complete sensor structure obtained after completing steps S1-S8 in the MEMS sensor fabrication method provided in the embodiment.

[0050] Explanation of reference numerals in the attached figures:

[0051] 1: Substrate; 2: First oxide layer; 3: First adhesive layer; 4: Heating electrode layer; 5: Second adhesive layer; 6: Second oxide layer; 7: Third adhesive layer; 8: Measurement electrode layer; 9: Gas-sensitive material layer; I: Central working area of ​​the heating electrode; J: Central partition area of ​​the measurement electrode; 10: Cavity area. Detailed Implementation

[0052] Please refer to Figure 1 This invention provides a low-power silicon-based MEMS gas sensor chip structure and its fabrication method. The cross-sectional positions shown in the figures for each step may differ, so the regions of the same layer may differ between the preceding and following steps. The fabrication method includes the following steps:

[0053] Step 1) Provide a substrate 1, which is made of single-crystal silicon, ceramic, porous silicon or silicon nitride; prepare a first oxide layer 2 on the surface of the substrate 1 as a dielectric layer and support layer. The first oxide layer 2 can be silicon oxide, silicon nitride or other dielectric materials.

[0054] Step 2) Prepare a first adhesion layer 3 on the surface of the first oxide layer 2. The material of the first adhesion layer can be titanium or aluminum oxide. Prepare a heating electrode layer 4 on the surface of the first adhesion layer 3. The heating electrode layer 4 has a heating electrode and a corresponding pad. The heating electrode and the pad can both be platinum (pt), or both can be gold, or the pad can be gold coated on the pt surface of the heating electrode. There are two pads on the heating electrode.

[0055] Step 3) Prepare a second adhesion layer 5 on the surface of the first oxide layer 2 exposed outside the heating electrode layer 4 and the electrode area (i.e., the area not covered by the electrode). The material of the second adhesion layer can be titanium or aluminum oxide. Prepare a second oxide layer 6 on the surface of the second adhesion layer 5 to serve as a dielectric layer and support layer. The second oxide layer 6 can be silicon oxide, silicon nitride, or other dielectric materials.

[0056] Step 4) Prepare a third adhesion layer 7 on the surface of the treated second oxide layer 6. The material of the third adhesion layer can be titanium or aluminum oxide. Prepare a measurement electrode layer 8 on the surface of the third adhesion layer 7. The measurement electrode layer 8 has measurement electrodes and corresponding pads. The measurement electrodes and pads can both be platinum (pt), or both can be gold, or the measurement electrodes can be platinum-coated with gold as pads. There are two pads on the measurement electrodes. The pattern of the third adhesion layer 7 is consistent with that of the measurement electrodes.

[0057] Step 5) Prepare a gas-sensitive material layer 9 on the surface of the measuring electrode layer 8. The gas-sensitive material layer 9 uses a gas-sensitive material, which can be tin oxide or tungsten oxide. The gas-sensitive material layer 9 fills and covers the gap between the two measuring electrodes of the measuring electrode layer 8.

[0058] Step 6) Use photoresist to cover and protect the detection and heating electrodes, the central heating area, and the area used to form the ring support structure (the structure formed by the remaining first and second oxide layers after they were not removed in this step, used to provide support, increase structural stability, and prevent the central area from collapsing after the cavity is formed). This exposes the heating electrode pads, the central area that needs to be formed into a suspension, and the remaining silicon oxide areas that need to be removed (areas in the first and second oxide layers that are not covered by the heating electrodes, test electrodes, and their pads). Dry etching is then used to remove the exposed silicon oxide areas, exposing the heating electrode pads, while leaving a certain dielectric support structure (i.e., two oxide layers, the first oxide layer 2 and the second oxide layer 6). The central heating area refers to the contact area between the tips of the two heating electrodes. Because the contact area between the tips is small, the heat generation is the greatest, and it is located in the center, making it the main working area.

[0059] Step 7) Thin the entire wafer (i.e., the whole wafer).

[0060] Step 8) Etch the material (substrate) below the working area until the silicon substrate is etched through, so that the working area on the upper surface forms a suspended structure.

[0061] Furthermore, a first oxide layer 2, a heating electrode layer 4, a second oxide layer 6, a measurement electrode layer 8, and a gas-sensitive material layer 9 are sequentially fabricated on the surface of a single-crystal silicon substrate. The heating electrode layer 4 and the measurement electrode layer 8 each contain a heating electrode and a measurement electrode, respectively, and their corresponding pads. The pads are mainly located in the central region of the four edges of the device, while the heating area and the testing area are located in the central region of each layer on the device.

[0062] Furthermore, the first oxide layer and the second oxide layer are simultaneously etched using an etching window. This process exposes the pad window of the second heating electrode, removes silicon oxide outside the front working area, and retains some of the support structure in a single etching operation.

[0063] This method can reduce the number of etching operations, save costs, and is simple, yields high finished products, and is easy to use in large-scale mass production.

[0064] Example 1

[0065] A substrate with a thickness of 500 μm was selected. <100> Crystal orientation monocrystalline silicon.

[0066] Step S1: Prepare a first oxide layer 2 on the surface of substrate 1.

[0067] First, the surface of the single-crystal silicon is oxidized to form an oxide layer with a thickness of 500nm-3μm, which serves as both a dielectric layer and a support layer. Alternatively, a first oxide layer 2 of the same thickness can be deposited on the surface of substrate 1 using vapor deposition. The material is not limited to silicon oxide; it can also be a dielectric material such as silicon nitride.

[0068] Step S2: Prepare a second heating electrode and corresponding pads on the surface of the support layer (first oxide layer 2):

[0069] A 5-10 nm layer of aluminum oxide or titanium is grown on the wafer surface as the first adhesion layer 3 using sputtering or evaporation. A heating electrode layer 4, i.e., the platinum metal layer of the heating electrode, with a thickness of 30-300 nm, is then grown on top of this. Photolithography is used to pattern the photoresist to form the heating electrode and the corresponding pad pattern. The entire heating electrode and its pad pattern are designed as pins, with the two heating electrode tips in contact. The central area is the tip contact area, which is extremely small, with an axial length of 10 nm-1 μm. It gradually widens from the center outwards, forming a triangular structure with a square electrode and pad. Figure 2 As shown, Figure 2 The area shown in Figure I is the central working area of ​​the heating electrode. The vertical diagram illustrates the contact between the tips of the two heating electrodes. Excess platinum and titanium are removed by etching to form a complete heating electrode and corresponding pads.

[0070] Step S3: Prepare a second oxide layer 6 on the surface of the heated electrode layer 4 to serve as a dielectric layer and a support layer.

[0071] To increase adhesion, a second adhesion layer 5 of 5-10 nm needs to be sputtered on the wafer surface. The second adhesion layer 5 uses aluminum oxide. A layer of silicon oxide is deposited on the aluminum oxide surface by vapor deposition as a second oxide layer 6 with a thickness of 50 nm-1 μm.

[0072] Step S4: Prepare the measurement electrode layer 8 on the surface of the second oxide layer 6.

[0073] The measurement electrode layer 8 has measurement electrodes and corresponding pads. The fabrication process of the measurement electrodes is basically the same as that of the heating electrode fabrication process in step S2. A 5-10 nm layer of aluminum oxide or titanium is grown on the wafer surface as the third adhesion layer 7 using magnetron sputtering or electron beam evaporation. Then, a platinum metal layer of 30-300 nm thick for the measurement electrodes is sputtered. The measurement electrodes and corresponding pad patterns are patterned using photolithography. The entire measurement electrode and its pad pattern is a pin-shaped design with a small central area (typically only 10 nm-1 μm) with a narrow gap between the pins, gradually widening from the center outwards to form a trapezoidal and square electrode and pad structure. Figure 3 As shown in the figure, area J represents the central separation region of the measurement electrode. The vertical diagram illustrates a small gap between the tips of the two measurement electrodes. Excess platinum and titanium are etched away to form a complete measurement electrode and its corresponding pad.

[0074] Step S5: Prepare a gas-sensitive material layer 9 on the surface of the measuring electrode layer 8 to cover the gap in the middle of the measuring electrode.

[0075] Photolithography is used to prepare the patterned window of the loading region. A gas-sensitive material layer 9 is grown using methods such as sputtering or evaporation. The gas-sensitive material layer 9 can be made of tin oxide, tungsten oxide, or other gas-sensitive materials. The gas-sensitive material covers the surface of the measuring electrode, such as... Figure 4 The gap between the measuring electrodes is filled, allowing for the acquisition of effective electrical signals during operation. Further: If the sensitive material is loaded using a printing method, this step (step S5) can be omitted and performed after the entire process is completed but before the wafer is split. The thickness of the gas-sensitive material layer 9 is generally tens of nanometers or more, with no upper limit. It can be flexibly varied depending on the process, material, and gas without affecting the sensing effect; a preferred thickness is 5nm-1μm.

[0076] Furthermore: if the gas-sensitive material is loaded using a microneedle loading method, the loading can be performed after the entire process is completed but before wafer cleaving, or after wafer cleaving but before encapsulation and testing.

[0077] Step S6: Remove silicon oxide outside the effective electrode area (the area covered by the electrode and pad pattern):

[0078] When fabricating the photolithography mask for this step, it is necessary to expose the location of the heating electrode layer pads while protecting the heating and measurement electrodes. At the same time, a certain amount of silicon oxide structure needs to be left to ensure that the pad windows of the heating electrodes in heating electrode layer 4 are exposed after etching, and that silicon oxide outside the front working area is removed while retaining a certain ring support structure. Figure 5 As shown, this is to ensure the stability of the suspended electrode area after the process is completed.

[0079] Step S7, overall wafer thinning:

[0080] The wafer is thinned on the back side, retaining a wafer substrate with a thickness of 200-300μm.

[0081] Step S8: Etch the silicon substrate to form a cavity region 10, thereby suspending the working region.

[0082] When fabricating the back mask, the hole needs to be placed directly below the working area. The silicon substrate is etched to form a cavity region 10. The cavity region 10 extends from the back to the front, directly etched through, and runs through both the front and back sides, so that the working area is suspended without substrate support.

[0083] Figure 6 This is a perspective view of the complete sensor structure obtained by the MEMS sensor fabrication method provided in the embodiment.

[0084] The structure of the second silicon oxide layer 6 in the aforementioned MEMS gas sensor chip structure, preserved after etching, effectively improves the stability of the levitation structure. The specially designed pin-shaped heating electrode has an extremely small working size, typically only tens to hundreds of nanometers, enabling it to generate sufficient heat to excite the sensitive properties of the gas-sensitive material with very low power consumption. Compared to traditional serpentine coiled heating electrodes, this structure not only consumes less power, but its extremely small working area also ensures less electrode deformation during heating, contributing to the overall structural stability. The isolated pin-shaped measurement electrode is easier to fabricate, and the yield is significantly improved. Simultaneously, this structural feature allows for a substantial reduction in the overall size of the sensor, resulting in higher integrability and a wider range of applications.

Claims

1. A low-power MEMS gas sensor chip, characterized by, The application relates to a gas sensor, which comprises the following parts: a substrate, a first oxide layer arranged on the surface of the substrate as a dielectric layer and a support layer, a first adhesive layer arranged on the surface of the first oxide layer, a heating electrode layer arranged on the surface of the first adhesive layer and comprising heating electrodes and corresponding pads, the heating electrodes comprising two pairs of needle-shaped structures, the tips of the two pairs of heating electrodes being in contact, a second adhesive layer arranged on the surface of the heating electrode layer and the surface of the first adhesive layer which is not covered by the heating electrode pattern, the central working area of the heating electrode being the area where the tips of the two pairs of heating electrodes are in contact, a second oxide layer arranged on the surface of the second adhesive layer as a dielectric layer and a support layer, a third adhesive layer arranged on the surface of the second oxide layer, a measuring electrode layer arranged on the surface of the third adhesive layer and comprising measuring electrodes and corresponding pads, the measuring electrodes comprising two pairs of needle-shaped structures, the distance between the tips of the two pairs of needle-shaped structures of the measuring electrodes being 10 nm-1 mu m, the central working area of the measuring electrode being the area where the tips of the two pairs of measuring electrodes are located, the corresponding pads of the heating electrodes and the corresponding pads of the measuring electrodes not overlapping each other, and a gas-sensitive material layer filling and covering the third adhesive layer in the central working area of the measuring electrode which is not covered by the measuring electrode layer. The size of the tip contact area of the two pairs of needle-shaped structures of the heating electrodes along the axial direction of the heating electrodes is 10 nm-1 mu m. The two pairs of heating electrodes gradually widen from the tip positions to both sides. The two pairs of measuring electrodes gradually widen from the tip positions to both sides. The thickness of the heating electrodes is 30-300 nm; and the thickness of the measuring electrodes is 30-300 nm. The material of the substrate is selected from single crystal silicon, ceramic, porous silicon and silicon nitride. The materials of the first oxide layer and the second oxide layer are independently selected from silicon oxide and silicon nitride. The materials of the first adhesive layer, the second adhesive layer and the third adhesive layer are independently selected from titanium and aluminum oxide. The material of the gas-sensitive material layer is selected from tin oxide, tungsten oxide and other gas-sensitive materials.

2. The low-power MEMS gas sensor chip according to claim 1, wherein, The materials of the heating electrodes and the measuring electrodes are independently selected from platinum and gold.

3. The low-power MEMS gas sensor chip according to claim 2, wherein, The thickness of the first oxide layer is 500 nm-3 mu m; and the thickness of the second oxide layer is 50 nm-1 mu m. The thicknesses of the first adhesive layer, the second adhesive layer and the third adhesive layer are all 5-10 nm.

4. The low power MEMS gas sensor chip of claim 1, wherein, The thickness of the gas-sensitive material layer is 5 nm-1 mu m.

5. The low power MEMS gas sensor chip of claim 1, wherein, The application further discloses a preparation method of the gas sensor, which comprises the following steps: S1, preparing a substrate and preparing a first oxide layer on the surface of the substrate, S2, preparing a first adhesive layer on the surface of the first oxide layer, preparing a heating electrode layer on the first adhesive layer, and forming heating electrodes and corresponding pads, S3, preparing a second adhesive layer on the surface of the heating electrode layer, and then preparing a second oxide layer on the surface of the second adhesive layer, S4, preparing a third adhesive layer on the surface of the second oxide layer, and then preparing a measuring electrode layer on the surface of the third adhesive layer, and forming measuring electrodes and corresponding pads, S5, preparing a gas-sensitive material layer on the surface of the measuring electrode layer, S6, removing silicon oxide outside the effective electrode area, and S7, thinning the whole substrate from the back. ​ ​ ​ ​ 6. The low power MEMS gas sensor chip of claim 1, wherein, ​ ​ ​ 7. The method of claim 1-6, wherein the low power MEMS gas sensor chip is prepared by the steps of: ​ ​ ​ ​ ​ ​ ​ ​ ​ Step S8, etching the substrate from the back to form a cavity, so that the working area corresponds to the substrate is etched off, the working area is suspended.

8. The preparation method according to claim 7, characterized in that, After step S7, the substrate is left with a thickness of 100-300 μm.

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