Chemical polishing and annealing method of quartz wafer

Abstract

The quartz wafer chemical polishing annealing method disclosed by the embodiment of the present application comprises the following steps: step 1, placing a quartz wafer to be polished in an annealing furnace, and heating to a first preset temperature under a vacuum environment or under an inert gas; step 2, continuing to heat to a second preset temperature, and filling fluorine-containing gas, and reacting for a first preset time, so that the fluorine-containing gas is pyrolyzed to generate fluorine radicals to perform chemical polishing on the surface of the quartz wafer; step 3, performing purging by using an inert gas, and removing SiF4 and residual fluoride generated in the reaction; and step 4, maintaining an inert gas atmosphere, keeping warm for a second preset time, and then cooling, to complete the chemical polishing annealing on the surface of the quartz wafer. The present application is a full dry process, and the quartz wafer is not contacted with any liquid from the wafer loading to the furnace discharge, so that mechanical damage and secondary pollution of the 3215 wafer are avoided, and surface purification can be completed at the same time of annealing.

Description

Technical Field

[0001] This invention relates to the field of crystal oscillator manufacturing technology, and in particular to a chemical polishing and annealing method for quartz wafers. Background Technology

[0002] To reduce the equivalent series resistance (ESR), the substrate thickness of the 3215 tuning fork type quartz wafer (3.2mm × 1.5mm) has been reduced to 80-100μm or even thinner. During wet etching, microcracks, amorphous layers, and residual photoresist ash easily form on the surface. Traditional vacuum annealing can only relieve stress but cannot remove these surface chemical defects, resulting in high contact resistance and low Q value after coating. Summary of the Invention

[0003] The technical problem to be solved by the embodiments of the present invention is to provide a chemical polishing and annealing method for quartz wafers to eliminate chemical defects on the surface of quartz wafers and improve surface micro-roughness.

[0004] To address the aforementioned technical problems, this invention provides a chemical polishing and annealing method for quartz wafers, comprising: Step 1: Place the quartz wafer to be polished in an annealing furnace and heat it to the first preset temperature under vacuum or inert gas conditions; Step 2: Continue heating to the second preset temperature and introduce fluorine-containing gas. React for the first preset time to allow the fluorine-containing gas to undergo pyrolysis and generate fluorine free radicals to chemically polish the surface of the quartz wafer. Step 3: Purging with inert gas to remove the SiF4 generated in the reaction and residual fluorides; Step 4: Maintain the inert gas atmosphere, keep warm for the second preset time, and then cool to complete the chemical polishing and annealing of the quartz wafer surface.

[0005] Furthermore, the first preset temperature is 550℃-650℃.

[0006] Furthermore, in step 1, the heating rate is 1-3℃ / min.

[0007] Furthermore, in step 1, nitrogen or argon gas is introduced during the heating process at a flow rate of 10-20 L / min to maintain positive pressure.

[0008] Furthermore, the second preset temperature is 800℃-1000℃, and the concentration of fluorine gas is 0.01%-2%.

[0009] Furthermore, the fluorine-containing gas is one or a mixture of two of CF4 and SF6, and the volume percentage of the fluorine-containing gas is 0.1%-2%.

[0010] Furthermore, the fluorine-containing gas is HF vapor, and the volume percentage of the fluorine-containing gas is 0.01%-0.1%.

[0011] Furthermore, the first preset time is 20-40 minutes.

[0012] Furthermore, the second preset time is 1-2 hours.

[0013] Furthermore, in step 4, the cooling rate is ≤2℃ / min.

[0014] The beneficial effects of this invention are as follows: 1) By removing the surface damage layer and impurities, this invention reduces acoustic wave scattering and electrode contact resistance, making the 3215 wafer more likely to meet the requirements of low-power RTC chips.

[0015] 2) This invention uses CF4 or SF6, which fundamentally solves the corrosion problem of high-temperature fluorine-containing gas on the quartz tube, heating wire and sealing ring of the annealing furnace, extending the service life of the equipment by 3-5 times and reducing maintenance costs.

[0016] 3) The wafer of the present invention has high surface activity and is clean after polishing, with strong adhesion of silver / gold plating and no peeling or flaking.

[0017] 4) This invention allows for thickness variations of <0.5μm, eliminating the need for extremely precise pre-thinning control, as subsequent laser frequency modulation can easily cover this error, thus improving the tolerance of the preceding process. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of a chemical polishing and annealing method for quartz wafers according to an embodiment of the present invention. Detailed Implementation

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] In this embodiment of the invention, directional indicators (such as up, down, left, right, front, back, etc.) are only used to explain the relative positional relationship and movement of each component in a specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0021] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features.

[0022] Please refer to Figure 1 The chemical polishing and annealing method for quartz wafers in this embodiment of the invention includes steps 1 to 4.

[0023] Step 1: (1) Pretreatment and wafer loading: Place the quartz wafer to be polished in the reaction chamber of the annealing furnace. The quartz wafer to be polished is a 3215 quartz wafer that has completed photolithography to define the electrode and has undergone preliminary wet coarse etching (with a large fundamental frequency deviation and a margin for laser frequency modulation). Place the 3215 quartz wafer in a high-purity quartz boat, ensuring that the wafer is horizontal and avoiding overlap, and push it into the constant temperature zone of the annealing furnace; (2) Heating and degassing (inert gas protection): Heate to the first preset temperature under vacuum or inert gas. The first preset temperature is 550℃-650℃, and the heating rate is 1-3℃ / min. Seal the furnace body and evacuate to 10⁻²Pa or fill with high-purity nitrogen for replacement. During this stage, high-purity nitrogen (N2) or argon (Ar) is introduced at a flow rate of 10-20 L / min to maintain positive pressure. The purpose is to remove the physical water and organic volatiles adsorbed on the wafer. No fluorine-containing gas is introduced during this stage to prevent low-temperature condensation.

[0024] Step 2, High-Temperature In-Situ Chemical Polishing: Continue heating to the second preset temperature and introduce fluorine-containing gas. React for the first preset time (20-40 minutes) to allow the fluorine-containing gas to pyrolyze, generating fluorine free radicals that chemically polish the quartz wafer surface (removing micro-protrusions, impurities, etc.). The second preset temperature is 800℃-1000℃, and the fluorine gas concentration is 0.01%-2%.

[0025] Preferably, the temperature is further increased to 850℃-950℃ (the optimal temperature for quartz lattice repair). The second preset temperature is preferably 900℃. Gas switching: Open the fluorine-containing gas branch and introduce CF4 or SF6 (CF4 is preferred due to its low cost and easy availability) into the main gas flow. Concentration control: The volume percentage of fluorine-containing gas is 0.1% - 2% (preferably 0.5%). If HF vapor is used, it must be strictly controlled at 0.01%-0.1% to protect the equipment; fluorocarbons are preferred. Reaction mechanism: CF4 at high temperature C + 4F (active free radical); SiO2 (impurities / protrusions) + 4F → SiF4↑ + O2↑. Time control: Maintain this mixed atmosphere for 20-40 minutes. Under these conditions, the surface micro-protrusions are vaporized and removed, while the loss of the quartz matrix is ​​minimal, and the total thickness reduction is controlled between 0.1-0.5μm (corresponding to a frequency drift of approximately 100-500ppm, this deviation is subsequently corrected by laser).

[0026] Step 3, In-situ purging and shut-off: Use inert gas to purge and remove the SiF4 generated in the reaction and residual fluorides. After the set time is reached, immediately cut off the supply of fluorine-containing gas. Then switch to high-flow-rate high-purity nitrogen (>30 L / min) to purge for 5-10 minutes to completely remove the SiF4 generated in the reaction and residual fluorides from the reaction chamber, preventing reverse corrosion or adsorption on the cold wall during cooling.

[0027] Step 4, Annealing and Cooling: Maintain an inert gas atmosphere, hold at the temperature for the second preset time, then cool to complete the chemical polishing annealing of the quartz wafer surface (subsequent process is laser frequency modulation). Maintain a pure nitrogen atmosphere and hold at approximately 900℃ for 1-2 hours (this stage is mainly to eliminate lattice stress). Cut off heating and cool with the furnace to below 100℃ at a rate of ≤2℃ / min. Example

[0028] (1) Equipment: Continuous atmosphere annealing furnace, equipped with MFC mass flow meter, the reaction tube is a high-purity quartz tube (inner wall coating is optional, it can withstand CF4 even without coating).

[0029] (2) Raw materials: 1000 3215 wafers with photolithography rough etching were selected, with an initial average ESR of 90kΩ, surface roughness Ra=1.5nm, and thickness of 80μm.

[0030] (3) Process parameters: Heat to 900℃, introduce CF4, and control the flow rate to 0.8% of the main N2 flow (e.g., N2 10L / min, CF4 80sccm).

[0031] React for 30 minutes, purge with high-flow-rate N2 for 10 minutes, anneal at room temperature for 90 minutes, and then slowly cool.

[0032] (4) Results: 1) Thickness change: The average thickness of the wafer decreased by 0.3μm (frequency drift of about 300ppm), which is within the effective compensation range of laser frequency modulation.

[0033] 2) Surface quality: AFM testing showed that the surface roughness Ra was reduced to 0.4 nm, and the micro-pits and scratches were "ironed out".

[0034] 3) Electrical performance: After silver plating, the average ESR dropped to 60kΩ and the Q value increased by 30%.

[0035] 4) Equipment condition: After 100 furnaces of continuous operation, there was no obvious atomization or etching pits on the inner wall of the quartz tube, and the heating wire was not broken (proving that CF4 is friendly to the equipment).

[0036] 5) Comparative Example: Annealing was performed using pure N2 at 900℃. Results: ESR only decreased to 85kΩ, surface roughness was not significantly improved, and Ra remained at 1.4nm.

[0037] This invention uses CF4 or SF6, both of which are chemically stable and do not corrode stainless steel and quartz at room temperature. They only undergo pyrolysis in the high-temperature annealing zone (>800℃) to generate fluorine radicals, enabling low-temperature storage and transportation while maintaining high-temperature reaction, significantly reducing equipment wear. It utilizes the high reactivity of fluorine atoms with defect sites and impurity clusters. For flat quartz substrates, the reaction activation energy is high, resulting in an extremely slow etching rate; for surface micro-protrusions and impurity particles, the reaction activation energy is low, allowing for preferential etching removal. The total thickness variation is allowed to be <0.5μm, a variation well within the compensation range of subsequent laser frequency modulation. Therefore, unlike traditional processes, there is no need to strictly limit the etching amount, allowing for greater focus on improving surface quality. The entirely dry process eliminates liquid contact from wafer loading to unloading, avoiding mechanical damage and secondary contamination of the 3215 wafers, and enabling surface purification during annealing.

[0038] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A chemical polishing and annealing method for quartz wafers, characterized in that, include: Step 1: Place the quartz wafer to be polished in an annealing furnace and heat it to the first preset temperature under vacuum or inert gas conditions; Step 2: Continue heating to the second preset temperature and introduce fluorine-containing gas. React for the first preset time to allow the fluorine-containing gas to undergo pyrolysis and generate fluorine free radicals to chemically polish the surface of the quartz wafer. Step 3: Purging with inert gas to remove the SiF4 generated in the reaction and residual fluorides; Step 4: Maintain the inert gas atmosphere, keep warm for the second preset time, and then cool to complete the chemical polishing and annealing of the quartz wafer surface.

2. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, The first preset temperature is 550℃-650℃.

3. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, In step 1, the heating rate is 1-3℃ / min.

4. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, In step 1, nitrogen or argon gas is introduced during the heating process at a flow rate of 10-20 L / min to maintain positive pressure.

5. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, The second preset temperature is 800℃-1000℃, and the concentration of fluorine gas is 0.01%-2%.

6. The chemical polishing and annealing method for quartz wafers as described in claim 5, characterized in that, The fluorine-containing gas is one or a mixture of two of CF4 and SF6, and the volume percentage of the fluorine-containing gas is 0.1% - 2%.

7. The chemical polishing and annealing method for quartz wafers as described in claim 5, characterized in that, The fluorine-containing gas is HF vapor, and the volume percentage of the fluorine-containing gas is 0.01%-0.1%.

8. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, The first preset time is 20-40 minutes.

9. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, The second preset time is 1-2 hours.

10. The chemical polishing and annealing method for quartz wafers as described in claim 1, characterized in that, In step 4, the cooling rate is ≤2℃ / min.

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