Method for releasing sacrificial layer of MEMS structure and MEMS structure

Abstract

The invention provides a method for releasing a sacrificial layer of an MEMS structure and the MEMS structure, and the method comprises the following steps: providing a substrate on which the sacrificial layer is to be released, and sequentially forming a structure layer, the sacrificial layer, a device layer and a release hole communicated with the sacrificial layer on the substrate; carrying out pre-etching treatment on the substrate by adopting a fluorine-containing and oxygen-containing dry etching process; and releasing the sacrificial layer through a dry etching process by using the release holes. According to the method, the pre-etching treatment is added before the sacrificial layer is released, so that residues in a previous layer process are removed, the surface state of the sacrificial layer is improved, efficient sacrificial layer release is further achieved, the sacrificial layer can be removed in a shorter time and at a lower temperature, damage of the release process to the surfaces of structures such as a structural layer and a device layer is reduced, and the yield of the device is improved. Therefore, the MEMS device with a clean surface, good uniformity and excellent roughness is obtained.

Description

Technical Field

[0001] The present application relates to the field of semiconductor technology, and in particular to a method for releasing a sacrificial layer of a MEMS structure and a MEMS structure. Background Art

[0002] MEMS (Micro-Electro-Mechanical Systems) structures have been widely used in the field of modern technology. Taking MEMS speakers as an example, they are widely used in medical devices, mobile phone and tablet electronic devices, automotive electronics and other fields due to their advantages such as small size, low cost, high integration, low power consumption and good audio quality. With the development of digital signal processing technology and micro speakers, the miniaturization trend of electronic devices is significant, and the demand for MEMS speakers is also increasing. In the silicon micromachining process of MEMS speakers, the sacrificial layer release technology is one of the most critical process steps, which directly affects the sound pressure, sound quality and reliability of the finished product.

[0003] The sacrificial layer release technology is a micromachining technology that utilizes the difference in corrosion rates of different materials in the corrosive liquid (gas) to selectively remove the sacrificial layer (the material between the structural pattern and the substrate) to release the thin film suspended structure or form a cavity in the surface silicon process. Silicon nitride and silicon oxide are the most commonly used structural layer and sacrificial layer materials in the prior art, respectively, and are usually etched using vapor phase hydrofluoric acid (VHF). In this process, since the surface of the silicon oxide will inevitably contain residues from the previous process, the corrosion rate is low and the effect is poor. As the process time increases and the process temperature increases, there will be varying degrees of damage on the surface of other metal films in the structural layer and device layer, resulting in a rough surface and affecting the reliability of the device, or even a short circuit causing the product to completely fail.

[0004] It should be noted that the above introduction to the technical background is only for the convenience of providing a clear and complete description of the technical solutions of the present application and for the convenience of understanding by those skilled in the art. It cannot be considered that the above technical solutions are well known to those skilled in the art simply because they are described in the background technology section of the present application. Summary of the invention

[0005] The purpose of the present invention is to provide a method for releasing a sacrificial layer of a MEMS structure and a MEMS structure, so as to quickly release the sacrificial layer and obtain a MEMS device with a clean surface, good uniformity and excellent roughness.

[0006] In order to solve the above problems, in a first aspect, a method for releasing a sacrificial layer of a MEMS structure is provided below, comprising the following steps:

[0007] S1. Provide a substrate for releasing a sacrificial layer, on which a structural layer, a sacrificial layer and a device layer are sequentially formed, as well as a release hole connected to the sacrificial layer.

[0008] S2. Pre-etching the substrate using a fluorine-containing and oxygen-containing dry etching process.

[0009] S3. Using the release hole, release the sacrificial layer through a dry etching process.

[0010] The present invention adds a pre-etching treatment before releasing the sacrificial layer to remove residues in the previous layer process and improve the surface state of the sacrificial layer, thereby achieving efficient sacrificial layer release. The sacrificial layer can be removed in a shorter time and at a lower temperature, and the damage caused by etching to the surface of structures such as structural layers and device layers is reduced, thereby obtaining a MEMS device with a clean surface, good uniformity and excellent roughness.

[0011] In step S2, a CF 4 and O 2 The dry etching process is used for pre-etching. This dry etching process does not damage the film structure of the MEMS device, and the CF 4 It can effectively remove residual by-products from the previous process and increase the subsequent sacrificial layer etching rate.

[0012] In step S2, the pre-etching temperature is 220-250°C, the reaction pressure is 0.95-1 torr, and the 2 Flux is 2000-3000sccm, CF 4 The flux is 10-30sccm. This process can remove as many residual byproducts as possible without causing too much impact on the device structure, thus ensuring the reliability of the device.

[0013] It includes multiple pre-etching processes, each of which has a reaction time of 90-120 seconds; the number of pre-etching processes is 3-5 times. Multiple pre-etching processes are used, and the process reaction time is reasonably controlled for each etching, which can effectively remove process residues while reducing damage to MEMS devices.

[0014] The sacrificial layer is a silicon dioxide layer, which is a common sacrificial layer material in MEMS devices, and the use of silicon oxide etching solution has a good selectivity ratio for silicon nitride and silicon-based substrates.

[0015] In step S3, the dry etching process is a gas phase hydrofluoric acid etching process. Using gas phase hydrofluoric acid to etch and release the silicon oxide sacrificial layer has a higher etching rate and etching selectivity, ensuring the uniformity and integrity of the etching.

[0016] In step S3, in the gas phase hydrofluoric acid etching process, HF and C 2 H 5 The flow ratio of OH is between 1:3 and 1:4. In the gas phase hydrofluoric acid etching process, using ethanol as a catalyst can further accelerate the etching reaction, increase the corrosion rate and reduce the liquid water generated during the reaction, thereby avoiding the adhesion of the liquid to the suspended micro-nano structure. The reasonable ratio of the two can increase the etching rate of silicon dioxide and obtain a more uniform device surface.

[0017] The release holes are formed in the substrate and the device layer. By providing the release holes on both the upper and lower sides, the contact area between the etching gas and the sacrificial layer can be increased, thereby increasing the etching rate.

[0018] On the other hand, the present application provides a MEMS structure obtained according to the method for releasing a sacrificial layer of a MEMS structure described in any one of the first aspects. Through the above method, the release efficiency of the sacrificial layer can be improved, and a MEMS device with a clean surface, good uniformity and excellent roughness can be obtained.

[0019] The substrate is a silicon substrate, the structural layer is a silicon-rich low-stress silicon nitride layer, the sacrificial layer is a silicon dioxide layer, and the device layer is a polysilicon layer.

[0020] Compared with the prior art, the beneficial effects of the present invention mainly include the following: 1) the pre-etching treatment does not damage other film layers of the device, and fluorine can effectively remove residual by-products of the previous process; 2) the release rate of the sacrificial layer can be increased, the etching process time is short, and the effect is good, especially for large-sized cavities; 3) after release, the device surface is clean, uniform, and has excellent roughness; 4) the process is repeatable and stable, and is suitable for mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings required for use in the description of the embodiments 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 these drawings without paying creative work.

[0022] Figure 1 A flow chart of a method for releasing a sacrificial layer of a MEMS structure provided by the present invention.

[0023] Figure 2 A schematic diagram of a MEMS structure sacrificial layer releasing method provided by the present invention. DETAILED DESCRIPTION

[0024] The above and other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of a preferred embodiment with reference to the drawings. The directional terms mentioned in the following embodiments, such as up, down, left, right, front or back, etc., are only referenced to the directions of the attached drawings. Therefore, the directional terms used are used to illustrate and not to limit the present invention.

[0025] The following will describe the various embodiments of the present application in detail with reference to the accompanying drawings. However, it will be appreciated by those skilled in the art that in the various embodiments of the present application, many technical details are provided in order to enable the reader to better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solution claimed in the present application can be implemented.

[0026] The steps in the following embodiments do not correspond one to one with the summary of the invention.

[0027] Embodiment 1

[0028] like Figure 1 Shown is a flow chart of a method for releasing a sacrificial layer of a MEMS structure provided in this embodiment. Figure 2 A schematic diagram of a sacrificial layer release method for a MEMS structure provided by the present invention. It is understood that in this application, the release of the sacrificial layer of a MEMS speaker cavity is only used as an example, and is not limited to this field. In fact, the sacrificial layer release method can also be used in other MEMS structures.

[0029] refer to Figure 1 and Figure 2 The present invention provides a method for releasing a sacrificial layer of a MEMS structure, the method comprising the following steps:

[0030] Step 1: providing a substrate for releasing a sacrificial layer, on which a structural layer, a sacrificial layer and a device layer are sequentially formed, as well as a release hole connected to the surface of the sacrificial layer.

[0031] refer to Figure 2 As shown in a in FIG. 1 , a substrate 1 to be released from a sacrificial layer is provided, a structural layer 2 is formed on the surface of the substrate 1, a sacrificial layer 3 is formed on the structural layer 2, and a device layer 4 is formed on the structural layer 2 and the sacrificial layer 3; in addition, a release hole 5 is formed up to the surface (including the upper surface and the lower surface) of the sacrificial layer 3. In this embodiment, the substrate 1 may be a silicon substrate, a silicon germanium substrate, or a silicon on insulator (SOI), or other semiconductor substrates known to those skilled in the art.

[0032] For ease of understanding, this embodiment provides a formation process of the substrate 1 to be released from the sacrificial layer, including: first, depositing a layer of silicon-rich low-stress silicon nitride film on a silicon substrate by a low-pressure chemical vapor deposition (LPCVD) method, and patterning it, and then depositing another layer of silicon nitride film by the same method. In this embodiment, the thickness of silicon nitride is 0.2-0.4 μm, as a structural layer 2; next, deep silicon etching is performed on the surface of the substrate 1 by an etching process to form a release hole 5 in the substrate 1; then, a silicon dioxide layer is formed on the surface of the structural layer 2 by a thermal oxidation or low-pressure chemical vapor deposition process, as a sacrificial layer 3, in this embodiment, the thickness of silicon oxide is 2.5-3.5 μm. Patterning is performed on the surface of the sacrificial layer 3, and the portion connected to the release hole 5 is the area where the cavity needs to be formed. Finally, polysilicon is deposited on the surface of the sacrificial layer 3, and the polysilicon is patterned by photolithography and etching processes to serve as a device layer 4, and a release hole 5 is also reserved in the device layer 4.

[0033] In this embodiment, according to the electrical and acoustic performances of the MEMS speaker, the preferred thickness of the structural layer 2 is 0.25 μm, and the preferred thickness of the sacrificial layer 3 is 3.25 μm.

[0034] Step 2: Perform pre-etching treatment using a dry etching process.

[0035] In the existing release process of the sacrificial layer 3, there are usually defects such as low corrosion rate and poor effect. In order to solve this problem, the present application pre-etches the substrate 1 through a fluorine-containing and oxygen-containing dry plasma etching process before the formal release process to remove the residues of the previous layer process, thereby improving the release efficiency of the sacrificial layer and reducing damage to the rest of the device.

[0036] Specifically, in this embodiment, fluorine (CF 4 ) oxygen (O 2 ) is used to perform a pre-etching treatment on the substrate 1 to be released from the sacrificial layer by a dry plasma etching process; the pre-etching treatment temperature is 250° C., the reaction pressure is 1 torr (torr, pressure unit), the oxygen flux is 3000 sccm (standard cubic centimeters per minute), and the CF 4 The flux was 30 sccm, the single process reaction time was 90 s, and the process was repeated 5 times.

[0037] In other embodiments, 3-5 pre-etching processes may be performed, the pre-etching temperature is 220-250° C., the reaction pressure is 0.95-1 torr, the oxygen flux is 2000-3000 sccm, and the CF 4The flux is 10-30sccm, and the single process reaction time is 90-120s. The pre-etching process does not damage other film layers in the MEMS device, and fluorine can effectively remove residual byproducts of the previous process; multiple pre-etching processes are used, and the process reaction time is reasonably controlled for each etching, which can effectively remove process residues while reducing damage to the MEMS device.

[0038] Step 3: Release the sacrificial layer using gas phase hydrofluoric acid.

[0039] In this embodiment, VHF etching is used to release the sacrificial layer 3, wherein C 2 H 5 OH acts as a catalyst, HF and C 2 H 5 The ratio (flow ratio) of HF to OH is 1:4, and the release time is 10 min. 2 H 5 The OH ratio (flow ratio) is 1:3 to 1:4.

[0040] refer to Figure 2 As shown in b, the etching gas enters through the reserved release hole 5 and reacts with the sacrificial layer 3. Since the pre-etching treatment in step 2 removes the residual byproducts and changes the surface state of the sacrificial layer 3, the VHF at this time can react with silicon dioxide more quickly and fully, greatly improving the release rate of silicon dioxide (about 3-5 times that of the existing process), without increasing the process temperature or extending the process time to achieve 100% release of the sacrificial layer. Compared with the prior art, the VHF etching process of this application is short in time and good in effect, especially for large-sized cavities; and after release, the device surface is clean, uniform, and has excellent roughness.

[0041] In order to facilitate the description, some common English nouns or letters used in the present invention are only used for exemplary reference rather than restrictive interpretation or specific usage, and the protection scope of the present invention should not be limited by their possible Chinese translations or specific letters.

[0042] It should also be noted that, in this document, relational terms such as "first" and "second" are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

Claims

1. A method for releasing a sacrificial layer of a MEMS structure, characterized in that: The steps include: S1, providing a substrate for releasing a sacrificial layer, wherein a structural layer, a sacrificial layer and a device layer are sequentially formed on the substrate, and a release hole connected to the sacrificial layer; S2, pre-etching the substrate using a fluorine-containing and oxygen-containing dry etching process; S3. Using the release hole, release the sacrificial layer through a dry etching process.

2. A method for releasing a sacrificial layer of a MEMS structure according to claim 1, characterized in that: In step S2, a pre-etching process is performed using a dry etching process containing CF4 and O2.

3. A method for releasing a sacrificial layer of a MEMS structure according to claim 2, characterized in that: In step S2, the pre-etching treatment temperature is 220-250°C, the reaction pressure is 0.95-1 torr, the O2 flux is 2000-3000 sccm, and the CF4 flux is 10-30 sccm.

4. The method for releasing a sacrificial layer of a MEMS structure according to claim 2, characterized in that: It includes multiple pre-etching processes, each of which has a reaction time of 90-120 seconds; and the number of pre-etching processes is 3-5 times.

5. The method for releasing a sacrificial layer of a MEMS structure according to claim 1, characterized in that: The sacrificial layer is a silicon dioxide layer.

6. A method for releasing a sacrificial layer of a MEMS structure according to claim 5, characterized in that: In step S3, the dry etching process is a gas phase hydrofluoric acid etching process.

7. A method for releasing a sacrificial layer of a MEMS structure according to claim 6, characterized in that: In step S3, in the gas phase hydrofluoric acid etching process, the flow ratio of HF to C2H5OH is between 1:3 and 1:

4.

8. The method for releasing a sacrificial layer of a MEMS structure according to claim 1, characterized in that: The release holes are formed in both the substrate and the device layer.

9. A MEMS structure, characterized in that: The method for releasing a sacrificial layer of a MEMS structure is obtained according to any one of claims 1-8.

10. A MEMS structure according to claim 9, characterized in that: The substrate is a silicon substrate, the structural layer is a silicon-rich low-stress silicon nitride layer, the sacrificial layer is a silicon dioxide layer, and the device layer is a polysilicon layer.

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