A method for releasing MEMS ultra-thin suspended membrane

Abstract

The purpose of the present invention is to provide a method for releasing ultra-thin suspended films that is suitable for batch production, low cost, and relatively simple, and belongs to the field of MEMS sensor technology. In the dry etching process, the present invention introduces a patch ring, and the patch ring photoresist forms a step, so that the front protection companion film is not in direct contact with the membrane structure, thereby avoiding external force damage during subsequent membrane separation. The patch ring is located in a circle of the wafer and is a closed ring to prevent etching gas from entering the gap and causing damage. It is suitable for batch production, low cost, and relatively simple ultra-thin suspended film release.

Description

Technical Field

[0001] The invention belongs to the technical field of MEMS sensors, and in particular relates to a MEMS ultra-suspended membrane release method. Background Art

[0002] In the MEMS field, more and more MEMS sensor structures use supporting structures such as suspended membranes, closed membranes, and cantilever beams. Due to the miniaturization, miniaturization, and high sensitivity development trend of MEMS devices, the thickness of their supporting structures is getting thinner and thinner. The morphology of the supporting membrane after release plays a crucial role in the performance of MEMS sensors.

[0003] At present, many methods of supporting film release based on MEMS technology have been reported, including the release of the suspended film front opening window by direct etching of XeF2 gas, and the release of the closed film structure by wet etching and dry deep etching on the back. However, these methods have certain disadvantages. The yield rate of the front opening window by direct etching of XeF2 gas is low, the cost is high, and it is not suitable for mass production; the wet etching methods such as KOH have slow rates, and the front needs direct protection, which is relatively cumbersome. The thin film is easy to break in the high-temperature etching liquid, and is only suitable for the release of the thicker back cavity; the most common is the back cavity dry etching, which directly sticks the wafer and the companion wafer through wax, pump oil, etc. to protect the front structure. However, the wax, pump oil, etc. in this method are also in direct contact with the wafer structure. When released later, the thinner film is easy to break, and it is also only suitable for the release of the thicker back cavity. Summary of the invention

[0004] In view of the above problems, in order to meet the development trend of miniaturization, miniaturization and high sensitivity of MEMS devices, the purpose of the present invention is to provide a method for releasing ultra-thin suspended films that is suitable for batch production, low cost and relatively simple. During the dry etching process, a patch ring is introduced, and the patch ring photoresist forms a step, so that the front protection companion film is not in direct contact with the membrane structure, thereby avoiding external force damage during subsequent membrane separation. The patch ring is located in a circle around the wafer and is a closed ring to prevent etching gas from entering the gap and causing damage.

[0005] The present invention adopts the following technical solution:

[0006] A method for releasing a MEMS ultra-thin suspended film comprises the following steps:

[0007] In the first step, a support film layer is prepared by a thin film deposition method, and a suspended film is pre-prepared on the support film layer to obtain a wafer structure;

[0008] The second step is to pattern the back cavity etching area on the back of the wafer after the wafer front structure is prepared;

[0009] The third step is to perform dry etching directly through deep silicon etching;

[0010] The fourth step is to design and prepare a patch ring mask, take a silicon wafer of the same size as the supporting film layer as a companion wafer, and pattern the patch ring on its polished surface, including cleaning, baking adhesive, spin coating photoresist, front baking, back alignment exposure, development, microscope inspection, plasma scanning bottom film, and post baking to form a patch ring photoresist step. Before the patch ring is post-baked, the previously etched film structure wafer is placed face down, the cut edges are aligned, and it is attached to the companion wafer through the patch ring photoresist. At this time, the membrane structure wafer and the companion wafer are attached through the patch ring photoresist, but the middle structure area is in a suspended state and not in direct contact. It is placed on a hot plate and baked;

[0011] Step 5: Continue to dry etch the wafer protected by the patch ring through deep etching, and observe until it is completely etched clean and the transparent film layer is exposed;

[0012] In the sixth step, wipe the edge patch ring photoresist with a cotton swab dipped in an organic solvent to separate the film structure wafer from the companion wafer and complete the release of the film structure.

[0013] Furthermore, the thin film deposition method in the first step includes any one or more combinations of thermal oxidation, LPCVD, and PECVD.

[0014] Furthermore, the supporting film layer in the first step includes a single layer of silicon oxide, a single layer of silicon nitride, a composite film layer of silicon oxide and silicon nitride, or a closed film layer of other materials or a suspended film layer with openings.

[0015] Furthermore, the graphic method in the second step includes cleaning, adhesive baking, photoresist spin coating, pre-baking, back-side alignment exposure, development, microscope inspection, plasma bottom film scanning, and post-baking in sequence.

[0016] Furthermore, the dry etching in the third step is stopped when the thickness of the support film layer is 30-50 μm.

[0017] Furthermore, the patch ring described in the fourth step is a closed structure, the outer diameter of the patch ring is the same as the wafer diameter, and the width of the patch ring is not less than 1 cm; the thickness of the prepared patch ring photoresist step is not less than 8 μm.

[0018] The beneficial effects of the present invention are as follows:

[0019] 1. The method of the present invention is suitable for a batch, low-cost, ultra-thin suspended film release method and is easy to operate;

[0020] 2. In the dry etching process of the present invention, a patch ring is introduced, and the patch ring photoresist forms a step, so that the front protection companion sheet is not in direct contact with the membrane structure, thereby avoiding external force damage during subsequent membrane separation.

[0021] 3. The patch ring is located around the wafer and is a closed ring to prevent etching gas from entering the gap and causing damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a structural schematic diagram of a MEMS ultra-thin suspended membrane during the release process proposed by the present invention.

[0023] Figure 2 A schematic diagram of a patch ring mask used in the release process of a MEMS ultra-thin suspended membrane proposed by the present invention.

[0024] Figure 3 A suspended membrane is prepared in advance on a silicon substrate.

[0025] Figure 4 Patterning of the back cavity photoresist structure.

[0026] Figure 5 It is part of direct deep silicon dry etching.

[0027] Figure 6 To pattern the die ring on the companion wafer.

[0028] Figure 7 The half-etched wafer is connected to the companion wafer with the front side facing down through the bonding ring.

[0029] Figure 8 The wafer protected by the die ring is further subjected to dry deep etching.

[0030] Fig. 9 The etching is completed to separate the patch ring and obtain a suspended membrane structure.

[0031] Among them: 1-wafer that needs to release the film; 2-chip ring photoresist step; 3-slave wafer; 4-suspended film; 5-silicon substrate; 6-photoresist; 7-cavity formed by etching; 8-chip ring mask. DETAILED DESCRIPTION

[0032] The present invention will be further described with reference to the accompanying drawings.

[0033] Figure 1 This is a structural schematic diagram of a MEMS ultra-thin suspended membrane during the release process proposed by the present invention.

[0034] like Figure 3 As shown, first, a support film layer is prepared by thin film deposition methods such as thermal oxidation, LPCVD, PECVD, etc. The support film layer can be a single layer of silicon oxide, a single layer of silicon nitride, a composite film layer of silicon oxide and silicon nitride, or a closed film layer of other materials or a suspended film layer with openings. The thickness is generally at the nanometer or micron level and can meet the stress support requirements.

[0035] like Figure 4 As shown, after the wafer front structure is prepared, the back cavity etching area is first patterned on the back of the wafer, which mainly includes cleaning, adhesive baking, photoresist spin coating, front baking, back side alignment exposure, development, microscope inspection, plasma bottom film scanning, and post baking.

[0036] like Figure 5 As shown, dry etching is performed directly through deep etching. At this time, there is no need for front-side companion chip protection. Etching is stopped when only about 50μm is left.

[0037] like Figure 6-Figure 7 As shown, a patch ring mask is designed and prepared.

[0038] Take a silicon wafer or glass wafer of the same size as a companion wafer, and pattern the patch ring on its polished surface, which mainly includes cleaning, baking adhesive, spin coating photoresist, front baking, back alignment exposure, development, microscope inspection, plasma scanning bottom film, and post baking to form a patch ring photoresist step. It is worth noting that before the patch ring is post-baked, the half-etched film structure wafer is placed face down, the cut edges are aligned, and it is attached to the companion wafer through the patch ring photoresist. At this time, the film structure wafer and the companion wafer are attached through the patch ring photoresist, but the middle structure area is in a suspended state and not in direct contact. Place it on a hot plate for post-baking. During post-baking, an appropriate mass block can be added to ensure that the wafer and the photoresist are tightly attached. The mass block should not be too heavy, otherwise it will cause the wafer middle film structure to contact the companion wafer, and it will be easily damaged when released later. If the mass is too light, the patch ring photoresist will not be firmly attached and will separate. In addition, the post-baking time should not be too long, otherwise it will cause the patch ring photoresist to be low in humidity, dry and non-sticky.

[0039] like Figure 8 As shown, the wafer protected by the patch ring is further dry-etched by deep etching until it is completely etched and the transparent film layer is exposed.

[0040] like Fig. 9 As shown, finally, the edge patch ring photoresist is wiped by a cotton swab dipped in acetone or other organic solvents to separate the film structure wafer from the companion wafer, thereby completing the release of the film structure.

Claims

1. A method for releasing a MEMS ultra-thin suspended film, characterized in that: The steps include: In the first step, a support film layer is prepared by a thin film deposition method, and a suspended film is pre-prepared on the support film layer to obtain a wafer structure; The second step is to pattern the back cavity etching area on the back of the wafer after the wafer front structure is prepared; The third step is to perform dry etching directly through deep silicon etching; The fourth step is to design and prepare a patch ring mask, take a silicon wafer of the same size as the supporting film layer as a companion wafer, and pattern the patch ring on its polished surface, including cleaning, baking adhesive, spin coating photoresist, front baking, back alignment exposure, development, microscope inspection, plasma scanning bottom film, and post baking to form a patch ring photoresist step. Before the patch ring is post-baked, the previously etched film structure wafer is placed face down, the cut edges are aligned, and it is attached to the companion wafer through the patch ring photoresist. At this time, the membrane structure wafer and the companion wafer are attached through the patch ring photoresist, but the middle structure area is in a suspended state and not in direct contact. It is placed on a hot plate and baked; Step 5: Continue to dry etch the wafer protected by the patch ring through deep etching, and observe until it is completely etched clean and the transparent film layer is exposed; In the sixth step, wipe the edge patch ring photoresist with a cotton swab dipped in an organic solvent to separate the film structure wafer from the companion wafer and complete the release of the film structure.

2. A method for releasing a MEMS ultra-thin suspended film according to claim 1, characterized in that: The thin film deposition method in the first step includes any one or more combinations of thermal oxidation, LPCVD, and PECVD.

3. A method for releasing a MEMS ultra-thin suspended film according to claim 1, characterized in that: In the first step, the supporting film layer includes a single layer of silicon oxide, a single layer of silicon nitride, a composite film layer of silicon oxide and silicon nitride, or a closed film layer of other materials or a suspended film layer with openings.

4. A method for releasing a MEMS ultra-thin suspended film according to claim 1, characterized in that: The graphic method described in the second step includes cleaning, baking adhesive, spin coating photoresist, pre-baking, back-alignment exposure, development, microscope inspection, plasma scanning bottom film, and post-baking in sequence.

5. A method for releasing a MEMS ultra-thin suspended film according to claim 1, characterized in that: In the third step, the dry etching is stopped when the thickness of the support film layer reaches 30-50 μm.

6. A method for releasing a MEMS ultra-thin suspended film according to claim 1, characterized in that: The patch ring described in the fourth step is a closed structure, the outer diameter of the patch ring is the same as the wafer diameter, and the width of the patch ring is not less than 1 cm; the thickness of the prepared patch ring photoresist step is not less than 8 μm.

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