Silicon hairspring and preparation method and application thereof

Abstract

The invention discloses a silicon hairspring and a preparation method and application thereof, and belongs to the technical field of clock precision machinery manufacturing. At present, a silicon hairspring is mainly prepared based on an SOI wafer, but the preparation of the SOI needs special equipment such as ion implantation and a complex process, so that the production cost of the silicon hairspring is relatively high. The silicon hairspring is prepared by using a monocrystalline silicon wafer as a raw material instead of an SOI wafer in the prior art. The preparation method comprises the following steps: etching the polished surface of the monocrystalline silicon wafer to form a silicon hairspring structure, and then carrying out thermal oxidation; one surface of the silicon hairspring structure is uniformly coated with liquid resin, then a layer of hard material is pasted, and after the liquid resin is completely cured, temporary bonding of the monocrystalline silicon wafer and the hard material layer is completed; and thinning one surface, deviating from the silicon hairspring structure, of the monocrystalline silicon wafer until the silicon hairspring structure is exposed, and then removing the cured resin to obtain the free silicon hairspring. According to the preparation method, the preparation cost of the silicon hairspring is remarkably reduced, and large-scale use of the silicon hairspring in the mechanical watch industry is facilitated.

Description

Technical Field

[0001] The present application relates to a silicon hairspring and a preparation method and application thereof, belonging to the technical field of precision machinery manufacturing for watches. Background Art

[0002] In mechanical watches, the hairspring is the core component that cooperates with the balance wheel. Its function is to control the swing frequency of the balance wheel, so as to achieve accurate timing of the mechanical watch. Therefore, the quality of the hairspring directly determines the travel accuracy. The watchmaking industry has long used metal hairsprings, such as low-carbon steel hairsprings. However, metal hairsprings have many disadvantages. For example, they are extremely sensitive to magnetic fields and temperatures, and are easily affected by magnetic interference, which leads to increased travel errors. In addition, thermal deformation caused by temperature changes will also affect the travel accuracy of mechanical watches. It was not until the emergence of silicon hairsprings that the above problems were perfectly solved. Silicon hairsprings are a major technological innovation of traditional metal hairsprings.

[0003] Silicon is one of the most abundant elements on earth. Crystalline silicon has the advantages of high hardness, good wear resistance, strong anti-magnetism, low thermal expansion coefficient and good corrosion resistance, and can be prepared into precise shapes through semiconductor micro-nano processing technology. These characteristics enable silicon hairsprings to effectively overcome the shortcomings of traditional metal hairsprings, improve the stability and anti-interference ability of mechanical watches, and thus improve the travel accuracy. Specifically, first, silicon hairsprings can effectively resist the interference of external magnetic fields and avoid the existence of magnetic fields affecting the travel accuracy of watches; second, the thermal expansion coefficient of silicon is much smaller than that of traditional metal hairsprings, so in an environment with changing temperature, the size change of silicon hairsprings is relatively small, so it can better resist the influence of changes in external temperature on travel accuracy; third, the crystalline silicon material itself has high hardness and wear resistance, so the silicon hairspring is not easy to wear during the long-term use of mechanical watches, and can maintain the stability of its initial shape and performance, reducing the problems such as the increase in the travel error of mechanical watches caused by hairspring wear. Finally, the silicon hairspring also has sufficient strength and low hysteresis elasticity to accurately control the swing frequency of the balance wheel during the operation of the movement, thereby ensuring the travel accuracy of the mechanical watch. Its elastic characteristics enable the silicon hairspring to quickly return to its original state after being subjected to external forces, thereby ensuring the normal operation of the mechanical watch. Therefore, the preparation of high-quality silicon hairsprings has become particularly important in the mechanical watch industry.

[0004] In the prior art, the silicon hairspring preparation technology based on SOI (Silicon on Insulator) wafers, the SOI wafer used includes a surface silicon "device" layer, an intermediate silicon oxide bonding layer and a lower silicon substrate layer. Specifically, first, the pattern of the hairspring is formed on the upper surface of the silicon "device" layer using photolithography technology, and then the silicon hairspring structure is formed on the "device" layer through deep reactive ion etching technology (Deep Reactive Ion Etching, DRIE), and finally the unnecessary parts are removed by wet etching (Wet Etching) technology to release the silicon hairspring from the lower silicon substrate, followed by surface oxidation to finally obtain a high-quality silicon hairspring. The preparation process of this SOI wafer is complex and the conditions are harsh, requiring high-end and expensive equipment such as ion implanters and bonding machines. Therefore, the price of SOI wafers has remained high for a long time, resulting in a high production cost of silicon hairsprings. Therefore, the production cost problem has always been the primary reason why silicon hairsprings are difficult to use on a large scale in the mechanical watch industry. In addition, the wet etching technology used to separate the silicon hairspring from the substrate has problems such as uniformity and environmental safety, and there is also a lot of room for improvement. In summary, it can be seen that there is an urgent need to develop a new silicon hairspring preparation method with cheap raw materials, fewer preparation steps, simple equipment operation, and environmental friendliness. Summary of the invention

[0005] In order to solve at least one of the problems existing in the current silicon hairspring preparation technology, namely, high cost, easy introduction of defects, difficulty in accurately controlling the uniformity of silicon hairspring size, and safety of the process environment of the preparation method, the present application proposes a new technical solution for the preparation of silicon hairsprings, namely, using single crystal silicon wafers as raw materials, directly etching to form a silicon hairspring structure, then inverting and temporarily bonding it, and thinning the back side of the single crystal silicon wafer until the silicon hairspring structure is exposed.

[0006] This application adopts the following technical solutions:

[0007] According to a first aspect of the present application, a method for preparing a silicon balance spring is provided, comprising the following steps:

[0008] Provide monocrystalline silicon wafers;

[0009] Etching a silicon hairspring structure on a surface of one side of a single-crystal silicon wafer, and then performing a thermal oxidation treatment on the single-crystal silicon wafer to form a protective layer on the silicon hairspring structure;

[0010] Bonding one side of the silicon balance spring structure of the single crystal silicon wafer to the hard material layer by curing resin;

[0011] Thinning a side of the single crystal silicon wafer away from the silicon hairspring structure until the silicon hairspring structure is exposed;

[0012] The cured resin is removed to obtain the free silicon balance spring.

[0013] In one embodiment, the step of etching to form a silicon balance spring structure comprises:

[0014] The silicon balance spring structure is obtained on the polished surface of the single crystal silicon wafer by photolithography and deep reactive ion etching.

[0015] In one embodiment, the curing resin fills the internal gap of the silicon balance spring structure and covers the silicon balance spring structure.

[0016] In one embodiment, the step of removing the cured resin to obtain the free silicon balance spring comprises:

[0017] The cured resin containing the exposed silicon hairspring structure is placed in a treatment liquid to separate the silicon hairspring from the cured resin, thereby releasing the free silicon hairspring.

[0018] In one embodiment, the thickness of the protective layer is 1 μm to 4 μm.

[0019] In one embodiment, the protective layer is a silicon oxide protective layer;

[0020] The step of performing thermal oxidation treatment on the single crystal silicon wafer to form the protective layer on the silicon balance spring structure comprises:

[0021] At 800° C. to 1000° C., flowing oxygen or water vapor is brought into contact with the surface of the silicon hairspring structure to grow and form a silicon oxide protective layer on the surface of the silicon hairspring structure.

[0022] In one embodiment, the bonding step includes: evenly coating liquid resin on one side of the silicon hairspring structure, then laminating a hard material layer on the liquid resin, curing the liquid resin, and after the liquid resin is completely cured to form the cured resin, temporarily bonding the single crystal silicon wafer to the hard material layer.

[0023] In one embodiment, the thickness of the single crystal silicon wafer is 500 μm to 750 μm.

[0024] In one embodiment, the thickness of the silicon hairspring structure is 50 μm to 200 μm.

[0025] In one embodiment, the thickness of the hard material layer is 750 μm to 2000 μm.

[0026] According to a second aspect of the present application, a silicon hairspring obtained by the above-mentioned preparation method is provided.

[0027] According to a second aspect of the present application, there is provided an application of the above-mentioned preparation method in the preparation of a measuring hairspring, a contact hairspring or a mechanical watch.

[0028] The beneficial effects of this application include:

[0029] (1) The present application uses single crystal silicon wafers as raw materials to replace the SOI wafers used in the prior art to prepare silicon hairsprings. Since the preparation of SOI wafers requires special equipment such as ion implantation and complex process steps, the production cost of SOI wafers is much higher than that of silicon wafers. Therefore, the technology of the present application reduces the preparation cost of silicon hairsprings, making it easier for silicon hairsprings to be used on a large scale in the mechanical watch industry.

[0030] (2) The present application adopts temporary bonding, back thinning and degumming processes to prepare silicon hairsprings. Compared with the multiple wet etching techniques used in the prior art solutions, this method is simpler, safer to operate and reduces pollution to the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the process flow of preparing the final silicon hairspring using single crystal silicon wafer as raw material in this application. DETAILED DESCRIPTION

[0032] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0033] Unless otherwise specified, the raw materials in the examples of this application were purchased through commercial channels.

[0034] Unless otherwise specified, conventional methods were used for testing and instrument settings were those recommended by the manufacturer.

[0035] SOI technology is the technology for preparing silicon on insulating substrates. Although it has many advantages, it also has some challenges and limitations. The first is the cost issue. The preparation process of SOI wafers requires ion implanters and long-term high-temperature thermal annealing processes, and also requires a complex post-processing process. Therefore, SOI wafers are much more expensive than silicon wafers. For example, the current price of 8-inch SOI wafers is more than 10 times that of ordinary single-crystal silicon wafers. This has limited the widespread use of SOI wafers in the field of cost-sensitive products to a certain extent; the second is the problem of interface defects and injection damage. The injection of high-energy ions or high-temperature bonding will cause damage below the surface of the silicon wafer, which is easy to introduce defects, and the uniformity of the silicon film is difficult to accurately control. Therefore, it is necessary to solve the above problems through continuous technological innovation and process improvement. Wet etching technology is a technology that uses chemical reactions to selectively process materials. Although it has the advantages of simple operation and low cost, it also has some challenges and problems. For example, the etching accuracy and uniformity are poor, the toxicity and environmental pollution of chemical reagents, and the process conditions are difficult to control. Therefore, it is necessary to explore new wet etching recipes and optimize etching process parameters to improve the efficiency, selectivity and safety of wet etching.

[0036] In view of the cost and technical problems existing in the preparation of the above-mentioned existing silicon hairsprings, this application proposes a new method for preparing silicon hairsprings using single crystal silicon wafers as raw materials and temporary bonding and back thinning as the main technical means. This application uses photolithography and deep reactive ion etching technology on the surface of single crystal silicon material to obtain the structure of the hairspring. Since the bottom of the hairspring is still connected to the silicon wafer, we first evenly apply liquid resin on the upper surface of the silicon hairspring to fill it into the gap between the inside of the hairspring, and then temporarily bond the coated upper surface to a layer of hard material layer. After the liquid resin is completely cured, the back of the silicon wafer is thinned until the silicon hairspring structure is "exposed", and finally the cured resin is washed off to completely release the silicon hairspring. Compared with the method for preparing silicon hairsprings based on SOI wafers, this invention method is simple in process, does not require special equipment and complicated process steps, is not only low in cost, but also does not require the use of multiple wet etching techniques. Therefore, this application provides a new idea for the preparation of silicon hairsprings.

[0037] In one embodiment, a method for preparing a silicon balance spring comprises the following steps:

[0038] S1. Provide single crystal silicon wafers;

[0039] S2, etching a silicon hairspring structure on a surface of one side of the single crystal silicon wafer, and then performing a thermal oxidation treatment on the single crystal silicon wafer to form a protective layer on the silicon hairspring structure;

[0040] S3, bonding one side of the silicon hairspring structure of the single crystal silicon wafer to the hard material layer by curing resin;

[0041] S4, thinning a side of the single crystal silicon wafer away from the silicon hairspring structure until the silicon hairspring structure is exposed;

[0042] S5, removing the solidified resin to obtain the free silicon hairspring.

[0043] In one embodiment, in step S2, the step of etching to form a silicon balance spring structure includes:

[0044] The silicon balance spring structure is obtained on the polished surface of the single crystal silicon wafer by photolithography and deep reactive ion etching.

[0045] The technical means of photolithography and deep reactive ion etching are not strictly limited in the present application, and those skilled in the art can select conditions of photolithography and deep reactive ion etching as needed to achieve etching formation of the target silicon hairspring structure.

[0046] In one embodiment, in step S2, the process of forming the silicon balance spring structure is to etch on the polished surface of the single crystal silicon wafer.

[0047] In one embodiment, in step S2, the single crystal silicon wafer is washed and dried before etching.

[0048] In one embodiment, in step S2, the thickness of the single crystal silicon wafer is 500 μm to 750 μm.

[0049] In one embodiment, in step S2, the thickness of the silicon hairspring structure is 50 μm to 200 μm.

[0050] In one embodiment, in step S2, the protective layer is a silicon oxide protective layer;

[0051] The step of performing thermal oxidation treatment on the single crystal silicon wafer to form the protective layer on the silicon balance spring structure comprises:

[0052] At 800° C. to 1000° C., flowing oxygen or water vapor is brought into contact with the surface of the silicon hairspring structure to grow and form a silicon oxide protective layer on the surface of the silicon hairspring structure.

[0053] In one embodiment, in step S3, the thickness of the hard material layer is 750 μm to 2000 μm.

[0054] In one embodiment, in step S3, the curing resin fills the internal gap of the silicon hairspring structure and covers the silicon hairspring structure.

[0055] In one embodiment, in step S3, the bonding step includes: uniformly coating a liquid resin on one side of the silicon hairspring structure, then laminating a hard material layer on the liquid resin, curing the liquid resin, and after the liquid resin is completely cured to form the cured resin, the single crystal silicon wafer and the hard material layer are temporarily bonded. During the coating of the liquid resin, the resin fills the internal gap of the silicon hairspring structure and completely covers the silicon hairspring structure.

[0056] The purpose of fully curing the liquid resin is to serve as a bonding material to temporarily bond the single crystal silicon wafer to the hard material layer. In this application, there is no strict limitation on the specific material selection and curing technical means of the liquid resin. Those skilled in the art can make selections as needed. After the liquid resin is fully cured, it becomes a resin material with higher hardness, for example, it can be selected from one of polyurethane (meth) acrylate / (meth) acrylate copolymers, epoxy (meth) acrylate / (meth) acrylate copolymers, or a mixture of at least two of them.

[0057] In one embodiment, in step S3, the material of the hard material layer is selected from one of silicon, glass, quartz and ceramic.

[0058] In one embodiment, in step S4, the thinning process specifically includes: pasting and fixing the side of the hard material layer away from the single crystal silicon wafer to the thinning machine, setting the thinning thickness and speed, and thinning from the side of the single crystal silicon wafer away from the silicon hairspring structure until the silicon hairspring structure is completely exposed, or the silicon hairspring structure is exposed to a target thickness. At this time, the silicon hairspring is not released because it is connected to other parts by solidifying the resin.

[0059] In one embodiment, in step S5, the step of removing the solidified resin to obtain the free silicon balance spring comprises:

[0060] The cured resin layer containing the exposed silicon hairspring structure is placed in a treatment liquid to completely separate the silicon hairspring from the cured resin, thereby releasing the silicon hairspring.

[0061] The function of the treatment liquid is to dissolve the cured resin and free the silicon hairspring. The specific selection of the treatment liquid is not strictly limited in the present application, and those skilled in the art can select it according to their needs.

[0062] The present application adopts single crystal silicon wafer as raw material to replace the SOI wafer used in the prior art to prepare silicon balance spring. For more specific description, in one embodiment, the preparation method of silicon balance spring specifically includes the following steps:

[0063] Step 1. Select appropriate materials; this application uses single crystal silicon wafers as raw materials to prepare silicon balance springs.

[0064] Single crystal silicon wafers are used as raw materials instead of the SOI wafers used in the prior art to prepare silicon hairsprings. Since the preparation of SOI wafers requires special equipment and complex process steps such as ion implantation and high-temperature bonding, compared with SOI wafers, this application directly uses single crystal silicon as the starting material, and the production cost is much lower.

[0065] Step 2. Photolithography: First, use photolithography technology to form the pattern shape of the hairspring on the photoresist on the surface of the silicon wafer.

[0066] Step 3. Deep reactive ion etching: Deep reactive ion etching is performed on the exposed silicon that is not protected by the photoresist in a direction perpendicular to the surface of the silicon wafer to form the structure of the designed hairspring on the upper surface of the silicon wafer, and the photoresist is removed.

[0067] Step 4. Grow silicon oxide: A silicon oxide layer is grown on the surface of the silicon wafer by thermal oxidation treatment. The oxide layer will be used as a protective layer for the final silicon balance spring.

[0068] Step 5. Temporary bonding: evenly apply a liquid resin with good fluidity on the upper surface of the silicon wafer with the hairspring structure, and then cover and paste a layer of hard material (such as quartz glass). Finally, the liquid resin is completely cured by heating or light to achieve temporary bonding of the single crystal silicon wafer and the quartz glass.

[0069] Step 6. Thinning the back side of the silicon wafer; bonding the hard material layer to the sample stage of the thinning machine, and then thinning the back side of the silicon wafer until the hairspring structure pattern is completely exposed or the silicon hairspring structure is exposed to the target thickness.

[0070] Step 7. Release the silicon hairspring; after the back of the silicon wafer is thinned, the hairspring is only connected to the other parts of the silicon wafer by the solidified resin. Use a solvent or detergent to dissolve the solidified resin, and the silicon hairspring is completely detached from the silicon substrate and released.

[0071] In step 5 and step 6, a temporary bonding hard material layer is used as a substrate and combined with back thinning and degumming processes to release the silicon hairspring connected to the silicon wafer. Compared with the multiple wet etching techniques used in the prior art solution, this method is simpler, safer to operate and reduces pollution to the environment.

[0072] Example 1

[0073] Step 1. Clean a single crystal silicon wafer with a thickness of 500 μm to 750 μm by ultrasonic cleaning with deionized water, isopropanol and acetone for 15 min in sequence, and then blow dry the silicon wafer with high-purity nitrogen for later use.

[0074] Step 2. Use a coating machine to spin-coat a 2um thick layer of positive photoresist BP 212 on the polished surface of the single crystal silicon, and then use a mask with a silicon hairspring shape pattern to expose the photoresist in a UV exposure machine. Then soak the sample in AZ726MIF developer for 3 minutes. During the development process, the photoresist exposed to ultraviolet light will be dissolved, thereby transferring the silicon hairspring shape pattern to the photoresist.

[0075] Step 3. After development, the area not protected by the photoresist directly exposes the upper surface of the silicon, and then the exposed silicon is deep reactive ion etched in the direction perpendicular to the surface of the silicon wafer, and the etching depth is set to 100μm. After the etching is completed, the final hairspring structure is formed on the upper surface of the silicon wafer.

[0076] Step 4. Place the silicon wafer after etching in step 3 in a quartz furnace, and use oxygen or water vapor (air flow rate of 1 cm / s) in the temperature range of 800-1000°C to grow a 3 μm thick silicon oxide protective layer on the surface of the silicon wafer.

[0077] Step 5. Use a coating machine to spin-coat a layer of liquid resin with good fluidity on one side of the silicon hairspring of the single crystal silicon wafer, and then stick a layer of quartz glass with a thickness of 750μm to 2000μm on the resin. Then, heat or irradiate with ultraviolet light to completely cure the liquid resin, thereby bonding the upper surface of the single crystal silicon wafer and the quartz glass together to achieve temporary bonding between the two.

[0078] Step 6. Stick the quartz glass to the sample stage of the thinning machine to thin the back of the silicon wafer, set the thinning thickness and thinning speed, and thin the silicon wafer to 100μm, that is, thin it to the point where a silicon hairspring structure appears. At this time, the silicon hairspring is not released because it is connected to other parts by solidified resin.

[0079] Step 7. After the back side thinning is completed, the silicon wafer is immersed in a suitable solvent (such as a degumming agent or other organic solvent or even hot water), and ultrasonic cleaning is performed to remove the solidified resin material according to the dissolution conditions. At this time, the silicon hairspring is completely separated and released from the single crystal silicon wafer due to the loss of supporting connection, thus completing the preparation of the final silicon hairspring.

[0080] Comparative Example 1

[0081] As a typical example of the prior art for preparing silicon hairsprings, this comparative example prepares a silicon hairspring using an SOI wafer as a starting material, wherein the SOI wafer comprises a silicon "device layer", a silicon oxide bonding layer, and a silicon "handle layer" from top to bottom.

[0082] A method for preparing a silicon balance spring from an SOI wafer comprises the following steps:

[0083] Step 1. Grow a layer of silicon oxide on the SOI wafer.

[0084] Step 2. With the silicon "device layer" as the front, first spin-coat a layer of photoresist and then perform UV exposure to form a photoresist mask with a silicon hairspring shape.

[0085] Step 3. Using a template formed by photoresist, first etch the silicon oxide grown in step 1 and transfer the pattern to the silicon oxide layer.

[0086] Step 4. Using the silicon oxide layer as a hard mask, perform deep reactive ion etching on the exposed silicon "device layer" to form the final silicon hairspring structure.

[0087] Step 5. Use wet etching technology or deep reactive ion etching technology to etch away the silicon "handle layer" to completely release the silicon balance spring.

[0088] Compared with comparative example 1, the preparation method of silicon hairspring embodiment 1 of the present application uses single crystal silicon wafers as raw materials to prepare the silicon hairspring, which greatly reduces the production cost. In addition, temporary bonding and back thinning technology are used instead of multiple wet etching technologies in the process of releasing the silicon hairspring. Therefore, the operation is simple and more environmentally friendly.

[0089] The above are only a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application is disclosed as above with preferred embodiments, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing a silicon hairspring, characterized in that: The steps include: Provide monocrystalline silicon wafers; Etching a silicon hairspring structure on a surface of one side of a single-crystal silicon wafer, and then performing a thermal oxidation treatment on the single-crystal silicon wafer to form a protective layer on the silicon hairspring structure; Bonding one side of the silicon balance spring structure of the single crystal silicon wafer to the hard material layer by curing resin; Thinning a side of the single crystal silicon wafer away from the silicon hairspring structure until the silicon hairspring structure is exposed; The cured resin is removed to obtain the free silicon balance spring.

2. The preparation method according to claim 1, characterized in that: The step of etching to form a silicon balance spring structure comprises: The silicon balance spring structure is obtained on the polished surface of the single crystal silicon wafer by photolithography and deep reactive ion etching.

3. The preparation method according to claim 1, characterized in that: The curing resin fills the inner gap of the silicon hairspring structure and covers the silicon hairspring structure.

4. The preparation method according to claim 1, characterized in that: The step of removing the solidified resin to obtain the free silicon balance spring comprises: The cured resin containing the exposed silicon hairspring structure is placed in a treatment liquid to separate the silicon hairspring from the cured resin, thereby releasing the free silicon hairspring.

5. The preparation method according to claim 1, characterized in that: The thickness of the protective layer is 1 μm to 4 μm.

6. The preparation method according to claim 1, characterized in that: The protective layer is a silicon oxide protective layer; The step of performing thermal oxidation treatment on the single crystal silicon wafer to form a protective layer on the silicon balance spring structure comprises: At 800° C. to 1000° C., flowing oxygen or water vapor is brought into contact with the surface of the silicon hairspring structure to grow and form a silicon oxide protective layer on the surface of the silicon hairspring structure.

7. The preparation method according to claim 1, characterized in that: The bonding step includes: evenly coating liquid resin on one side of the silicon hairspring structure, then laminating a hard material layer on the liquid resin, curing the liquid resin, and after the liquid resin is completely cured to form the cured resin, the single crystal silicon wafer and the hard material layer are temporarily bonded.

8. The preparation method according to claim 1, characterized in that: The thickness of the single crystal silicon wafer is 500 μm to 750 μm; Preferably, the thickness of the silicon balance spring structure is 50 μm to 200 μm; Preferably, the hard material layer has a thickness of 750 μm to 2000 μm.

9. A silicon balance spring obtained according to the preparation method according to any one of claims 1 to 8.

10. Use of the preparation method according to any one of claims 1 to 8 in preparing a measuring hairspring, a contact hairspring or a mechanical watch.

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