A piezoelectric crystal photolithography etching process

Abstract

This invention relates to a piezoelectric crystal photolithography etching process, belonging to the technical field of quartz micromachining. The process includes the following steps: cleaning the quartz substrate, depositing a barrier film, double-sided etching, wet etching, boss photolithography, boss etching, and frequency fine-tuning. The quartz wafer obtained after processing the quartz substrate is measured using a quartz wafer size measuring instrument. The standard deviation σ of the wafer length is measured to be 1.10 μm, and the standard deviation σ of the wafer width is 1.52 μm. These indicators meet the process target requirement of dimensional accuracy σ < 2 μm.

Description

Technical Field

[0001] This invention belongs to the technical field of quartz micromachining, and relates to a piezoelectric quartz wafer processing method, especially a piezoelectric crystal photolithography etching process. Background Technology

[0002] Quartz crystals are the core component of all frequency control devices (filters, oscillators, etc.). Quartz is the only material that simultaneously possesses piezoelectric properties, low loss, high Q value, and stable physicochemical properties. Furthermore, the AT-cut type of quartz crystal has a zero temperature coefficient. Combined with the mature artificial growth technology for quartz crystals and the advantages of mass production at low cost, it is now widely used in resonators and oscillators. By encapsulating quartz crystals, various quartz crystal oscillator products can be manufactured for use as signal frequency sources, time bases, and frequency control devices in receiving systems.

[0003] Traditional quartz oscillator wafer manufacturing processes employ mechanical machining methods. The main process flow is: quartz crystal rod → crystal orientation scribing → crystal rod cutting → wafer grinding → wafer dimensional machining → wafer shape machining → wafer etching.

[0004] The increasing demands for size and precision in quartz oscillator wafers are making traditional machining processes increasingly difficult. QMEMS photolithography, similar to semiconductor processes, has emerged to address this challenge. A passivation layer is physically deposited on a polished quartz substrate using magnetron sputtering. Then, a photolithography layer is created on the passivation layer using semiconductor photolithography. High-precision photolithographic exposure forms the designed pattern on the quartz wafer. An etchant is then used to remove the deposited material not covered by the photolithography layer. Finally, a wet etching step etches the exposed quartz material in the depth direction. By controlling the etchant ratio and etching temperature, quartz MEMS structures can be fabricated. The resulting quartz oscillator products exhibit excellent dimensional consistency.

[0005] While QMEMS (Quartz Membrane Electron System) fabrication technology enables high-quality production of quartz crystal wafers, the passivation layer material used in its fabrication primarily consists of metallic chromium and gold. Although gold can be chemically recycled, reducing production costs, the production cost of QMEMS quartz crystal wafers remains high. According to relevant data analysis, the production cost of quartz crystal wafers fabricated using QMEMS technology is 3 to 5 times that of those fabricated using traditional methods. This higher production cost is one of the factors limiting the mass production of this technology. Summary of the Invention

[0006] This invention aims to achieve finer processing precision in quartz wafer manufacturing processes while minimizing the amount of precious metal Au used. It only requires one Au barrier layer coating, avoiding the drawback of multiple Au barrier layer coatings required in QMEMS processes, thus reducing enterprise production costs and meeting the processing precision requirements of quartz oscillator wafers at a lower cost.

[0007] The technical solution adopted by this invention to achieve its purpose is as follows:

[0008] This invention employs the following processes: cleaning the quartz substrate, depositing a barrier film, double-sided etching, wet etching, boss photolithography, boss etching, and frequency fine-tuning. The larger quartz substrate can be processed to separate smaller quartz wafers (i.e., the desired finished product). Specific operation steps are as follows:

[0009] S1. Cleaning the quartz substrate: Immerse the quartz substrate in the cleaning solution and heat it to remove organic matter and other impurities adhering to the surface of the quartz substrate; then clean it by ultrasonic cleaning with pure water and ultrasonic cleaning with ethanol in sequence; finally, dehydrate and dry it in an oven and take it out for use.

[0010] S2. Deposition of barrier film: The cleaned quartz substrate is placed into the magnetron sputtering coating machine via a mounting bracket, and the vacuum chamber is evacuated to 1×10⁻⁶. -5 For a thickness of less than Pa, the surface of the quartz substrate is first pretreated with plasma, and then a Cr film with a thickness of 10 nm and an Au film with a thickness of 300 nm are deposited by magnetron sputtering. The coating process requires double-sided coating.

[0011] S3. Double-sided etching: A double-sided coated quartz substrate is coated with a uniformly thick negative photoresist on both sides. A double-sided etching machine is used to align and expose both sides of the quartz substrate to create the pattern required for wet etching. After inspection, the etched pattern is etched with a mixed solution of iodine and potassium iodide to remove Au metal for 3 minutes (i.e., using the Au etching solution to etch areas not protected by the photoresist, thus removing excess Au film). Then, a mixed solution of cerium ammonium nitrate and glacial acetic acid is used to etch Cr metal for 2 minutes (i.e., using the Cr etching solution to etch areas not protected by the photoresist, thus removing excess Au film). The area protected by photoresist and Au film is etched to remove excess Cr film. After etching, optical microscopy is used for inspection, requiring complete etching of the metal barrier film with neat and straight edges. This invention uses a mixed solution of iodine and potassium iodide to etch Au metal, resulting in good dissolution and convenient industrial operation, allowing for control of etching depth. This invention uses a non-oxidizing acid to provide an acidic environment for cerium ammonium nitrate, avoiding the problem of reduced oxidizing power due to hydrolysis, thus improving the high surface roughness and poor etching effect of the substrate after etching. Simultaneously, the etching depth can be controlled to achieve ideal etching precision and effect.

[0012] S4. Wet etching: Quartz substrates are individually spaced and placed into a special etching fixture, and the quartz material is etched in an etching solution; different etching solution concentrations and temperatures have a great influence on the edge of the final quartz oscillator wafer; after wet etching, the dimensions of the quartz oscillator wafer are completed, and negative photoresist is removed using a photoresist remover.

[0013] S5. Protrusion photolithography: After wet etching, the dimensions of the quartz wafer are completed. The negative photoresist on the quartz wafer is removed using photoresist remover, and a new photoresist layer is applied to both sides. The quartz substrate is then exposed to both sides using a photolithography machine to create the protrusion photolithography pattern and the wafer cleavage pattern. Au metal etching solution and Cr metal etching solution are used in sequence to remove the metal barrier film that is not protected by the photoresist, exposing the area to be etched.

[0014] S6. Boss Etching: The quartz substrate is placed separately into a special etching fixture again, and the entire substrate is immersed in the etching solution for a second double-sided etching of the quartz. The temperature of the etching solution is lower than that of the wet etching in step S4. The height of the boss is controlled by etching. After the quartz etching is completed, the photoresist layer on the surface is removed with photoresist remover, and the barrier metal layer is removed with Au metal etching solution and Cr metal etching solution. The quartz wafer is separated from the quartz substrate using a dicing machine.

[0015] S7. Frequency fine-tuning: Place the quartz crystal wafers into the quartz crystal wafer sorting machine, perform frequency sorting tests on the quartz crystal wafers, and use NH4F etching solution to fine-tune the etching of quartz crystal wafers of different frequency grades. After etching, the frequency dispersion of the batch of quartz crystal wafers is narrowed.

[0016] The key technologies that need to be mentioned in particular are:

[0017] In step S1, the cleaning solution is [C3H4O2] measured by molar ratio. n (Acrylic polymer): C6H8O7 (citric acid): CHN2O8 (edeamic acid): H2O (deionized water) = 25:22:53:1500, temperature condition: 51.2℃;

[0018] In step S4, the etching solution is the mother solution diluted with deionized water. The mother solution accounts for 55% of the weight of the etching solution. The mother solution has a molar ratio of HF:HNO3:H2SO4:H2O2 = 16:1:35:6 and the temperature is 58.3℃.

[0019] In step S6, the etching solution is a mother solution diluted with deionized water. The mother solution accounts for 55% of the weight of the etching solution. The mother solution has a molar ratio of HF:HNO3:H2SO4:H2O2 = 16:1:35:6, and the temperature is 55.1℃.

[0020] The beneficial effects of this invention are:

[0021] The processing technology of this invention minimizes the amount of precious metal Au used. By using a single coating and two photolithography steps, it avoids the drawback of requiring multiple coatings for the Au barrier layer in QMEMS processes, thus reducing enterprise development costs.

[0022] Tests have proven that, using the piezoelectric crystal photolithography etching process described in this invention, the frequency-temperature characteristics of the quartz crystal oscillator are within the test range of -40 to 95℃, with the test frequency fluctuating within ±8ppm, and the maximum resonant resistance is 28.1Ω. This meets the product setting requirements (test frequency drift not greater than ±12ppm, resonant resistance not greater than 40Ω), and the product yield is 96.45%, significantly reducing product costs and enhancing the market competitiveness of manufacturing enterprises. Attached Figure Description

[0023] Figure 1 It is a process flow diagram.

[0024] Figure 2 This is the relationship between the concentration of the corrosive solution and the corrosion rate at 58.3℃.

[0025] Figure 3 This describes the relationship between corrosion temperature and corrosion rate under the condition of a 55.0 wt% concentration of corrosive solution. Detailed Implementation

[0026] The present invention will now be described in detail with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0027] Example 1

[0028] This invention employs the following processes: cleaning the quartz substrate, depositing a barrier film, double-sided etching, wet etching, boss photolithography, boss etching, and frequency fine-tuning. The specific operation steps are as follows:

[0029] S1. Cleaning the quartz substrate: Immerse the quartz substrate in a cleaning solution and heat to clean it, removing organic matter and other impurities adhering to the surface of the quartz substrate. The cleaning solution, in molar ratio, is [C3H4O2]. n (Acrylic polymer): C6H8O7 (citric acid): CHN2O8 (edeamic acid): H2O (deionized water) = 25:22:53:1500, cleaning temperature is 51.2℃; then it is ultrasonically cleaned with pure water (40kHz, 10 minutes) and ultrasonically cleaned with ethanol (900kHz, 8 minutes), and finally dehydrated and dried in an oven and taken out for use.

[0030] S2. Deposition of barrier film: The cleaned quartz substrate is placed into the magnetron sputtering coating machine via a mounting bracket, and the vacuum chamber is evacuated to 1×10⁻⁶. -5 For a thickness of less than Pa, the surface of the quartz substrate is first pretreated with plasma, and then a Cr film with a thickness of 10 nm and an Au film with a thickness of 300 nm are deposited by magnetron sputtering. The coating process requires double-sided coating.

[0031] S3. Double-sided etching: A double-sided coated quartz substrate is coated with a uniformly thick negative photoresist on both sides. A double-sided etching machine is used to align and expose both sides of the quartz substrate to create the pattern required for wet etching. After inspection, Au metal is etched using a mixed solution of iodine and potassium iodide (mass ratio: iodine:potassium iodide:deionized water = 1:3:30) for 3 minutes. Then, Cr metal is etched using a mixed solution of cerium ammonium nitrate and glacial acetic acid (mass ratio: cerium ammonium nitrate:glacial acetic acid:deionized water = 10:1:30) for 2 minutes. After etching, the metal barrier film is inspected under an optical microscope, ensuring complete etching and neat, straight edges.

[0032] S4. Wet Etching: The quartz substrate is individually spaced and placed into a special etching fixture. Double-sided etching of the quartz material is performed in an etching solution. The etching solution in wet etching is a mother solution diluted with deionized water. The mother solution has the following composition: HF:HNO3:H2SO4:H2O2 = 16:1:35:6, and the weight percentage of the mother solution in the etching solution is 55%. The temperature is 58.3℃. Different etching solution concentrations and temperatures have a significant impact on the edge of the final quartz wafer. After wet etching, the quartz wafer size is completed, and negative photoresist is removed using a photoresist remover.

[0033] S5. Protrusion photolithography: After wet etching, the dimensions of the quartz wafer are completed. The negative photoresist on the quartz wafer is removed using photoresist remover, and a new photoresist layer is applied to both sides. The quartz substrate is then exposed to both sides using a photolithography machine to create the protrusion photolithography pattern and the wafer cleavage pattern. Au metal etching solution and Cr metal etching solution are used in sequence to remove the metal barrier film that is not protected by the photoresist, exposing the area to be etched.

[0034] S6. Boss Etching: The quartz substrate is again individually placed into a special etching fixture, and the entire substrate is immersed in the etching solution for a second double-sided quartz etching. The temperature of the etching solution is lower than that of the wet etching in step S4. The height of the boss is controlled by slow etching. The etching solution is a mother solution diluted with deionized water. The mother solution is HF:HNO3:H2SO4:H2O2 = 16:1:35:6, and the weight percentage of the mother solution in the etching solution is 55%. The temperature condition is 55.1℃. After the quartz etching is completed, the surface photoresist layer is removed using a photoresist remover, and the barrier metal layer is removed using Au metal etching solution and Cr metal etching solution. The quartz oscillator wafer is separated from the quartz substrate using a dicing machine.

[0035] S7. Frequency fine-tuning: Place the quartz crystal wafers into the quartz crystal sorting machine, perform frequency sorting tests on the quartz crystal wafers, and use NH4F etching solution to fine-tune the quartz crystal wafers of different frequency grades. After etching, the frequency dispersion of the batch of quartz crystal wafers is narrowed.

[0036] The quartz wafers obtained after processing the quartz substrate were measured using a quartz wafer size measuring instrument. The quartz wafer size test data shown in Table 1 are as follows: the standard deviation of wafer length σ is 1.10 μm, and the standard deviation of wafer width σ is 1.52 μm, which meets the process target requirements. The dimensional accuracy of the product after processing is σ < 2 μm.

[0037] Table 1 Quartz wafer size measurement data

[0038] Serial Number wafer length Chip width unit 1 1.8014 1.2767 mm 2 1.7978 1.2733 mm 3 1.7981 1.2725 mm 4 1.7993 1.2722 mm 5 1.8004 1.2767 mm 6 1.8001 1.2729 mm 7 1.7991 1.2748 mm 8 1.8007 1.2749 mm 9 1.7995 1.2746 mm 10 1.8002 1.2739 mm 11 1.8007 1.2733 mm AVG 1.799755 1.274164 mm Range R 3.6 4.5 μm Standard deviation σ 1.10 1.52 μm

[0039] Therefore, the advantages of this invention are as follows:

[0040] 1) Compared with the existing QMEMS process, the present invention uses a specific corrosion mixture formula and temperature conditions to achieve higher processing accuracy by controlling the corrosion process. The dimensional processing accuracy of the quartz crystal can reach <2μm, which is <10μm compared with the existing quartz crystal processing technology, and the dimensional accuracy is significantly improved.

[0041] 2) Compared to the existing QMEMS process which requires more than two Au and Cr metal barrier layers, this quartz crystal production process only requires one Au and Cr metal barrier layer coating, thus reducing production costs.

Claims

1. A piezoelectric crystal photolithography etching process, characterized in that, Includes the following steps: S1. Cleaning the quartz substrate: Clean, dehydrate, and dry the quartz substrate, then set it aside for use; S2. Depositing a barrier film: Double-sided magnetron sputtering of a Cr / Au thin film onto the quartz substrate after cleaning in step S1; S3. Double-sided etching: The double-sided coated quartz substrate is coated with negative photoresist on both sides. The double-sided etching machine is used to etch and align the two sides of the quartz substrate to create the pattern required for wet etching. After the double-sided etched pattern is inspected, Au metal is etched using a mixed solution of iodine and potassium iodide for 3 minutes. Then, Cr metal is etched using a mixed solution of cerium ammonium nitrate and glacial acetic acid for 2 minutes. S4. Wet etching: The quartz substrate treated in step S3 is individually placed into a special etching fixture, and the quartz material is etched on both sides in the etching solution. S5. Protrusion photolithography: After wet etching, the quartz wafer size is completed. The negative photoresist on the quartz wafer is removed, and then a photoresist layer is re-coated on both sides. The quartz substrate is exposed with double-sided alignment using a photolithography machine to create the protrusion photolithography pattern and the wafer cleavage pattern. Au metal etching solution and Cr metal etching solution are used in sequence to remove the metal barrier film that is not protected by the photoresist, exposing the area to be etched. S6. Boss Etching: The quartz substrate is placed into a special etching fixture again, and the whole substrate is immersed in the etching solution for a second double-sided etching of quartz. The temperature of the etching solution is lower than that of the wet etching in step S4. The height of the boss is controlled by etching. After the quartz etching is completed, the surface photoresist layer is removed. Au metal etching solution and Cr metal etching solution are used to remove the blocking metal layer. The quartz wafer is separated from the quartz substrate using a dicing machine. S7. Frequency fine-tuning: Place the quartz crystal wafers into the quartz crystal wafer sorting machine, perform frequency sorting tests on the quartz crystal wafers, and use NH4F etching solution to fine-tune the etching of quartz crystal wafers of different frequency grades. After etching, the frequency dispersion of the batch of quartz crystal wafers is narrowed.

2. The piezoelectric crystal photolithography etching process according to claim 1, characterized in that, In step S1, the quartz substrate is placed in the cleaning solution and heated for cleaning, then ultrasonically cleaned with pure water and ultrasonically cleaned with ethanol in sequence, and finally dehydrated and dried in an oven and taken out for use.

3. The piezoelectric crystal photolithography etching process according to claim 2, characterized in that, In step S1, the cleaning solution is prepared by molar ratio of acrylic polymer:citric acid:edema acid:deionized water = 25:22:53:1500, and the temperature is 51.2℃.

4. The piezoelectric crystal photolithography etching process according to claim 1, characterized in that, In step S2, the cleaned quartz substrate is placed into the magnetron sputtering coating machine, and the vacuum chamber is evacuated to 1×10⁻⁶. -5 For a thickness of less than Pa, the surface of the quartz substrate is first pretreated with plasma, and then a Cr film with a thickness of 10 nm and an Au film with a thickness of 300 nm are deposited by magnetron sputtering. The coating process requires double-sided coating.

5. The piezoelectric crystal photolithography etching process according to claim 1, characterized in that, In step S4, wet etching, the etching solution is a mother solution diluted with deionized water. The mother solution accounts for 55% of the weight of the etching solution. The mother solution has a molar ratio of HF:HNO3:H2SO4:H2O2 = 16:1:35:6, and the temperature is 58.3℃.

6. The piezoelectric crystal photolithography etching process according to claim 1, characterized in that, In step S6, the etching solution is a mother solution diluted with deionized water. The mother solution accounts for 55% of the weight of the etching solution. The mother solution has a molar ratio of HF:HNO3:H2SO4:H2O2 = 16:1:35:6, and the temperature is 55.1℃.

Images