Infrared optical germanium wafer CMP polishing liquid and application thereof

Abstract

This invention discloses a CMP polishing slurry for infrared optical germanium wafers and its applications. The polishing slurry comprises modified nano-cerium oxide abrasive with surface covalently anchored pyromellitic acid (PMA), an oxidant, a complexing agent, a corrosion inhibitor, a pH adjuster, and deionized water. The modified nano-cerium oxide abrasive has a core-shell structure with nano-cerium oxide as the core and a PMA functional layer anchored to the surface via Ce-O-C covalent bonds, and a pH value of 7.5-8.5. This invention utilizes the steric hindrance and covalent anchoring stability provided by the rigid aromatic ring shell of PMA, combined with a mild and synergistic polishing system constructed from ammonium persulfate, glycine, 5-aminotetraazole, and tetramethylammonium hydroxide. This significantly inhibits particle agglomeration, mechanical scratches, and post-polishing organic residues while ensuring controllable removal capabilities. The polished germanium wafer achieves a synergistic balance of high removal rate, low damage, and low residue, making it suitable for high-end precision polishing of infrared optical germanium wafers.

Description

Technical Field

[0001] This invention relates to the field of precision surface processing technology for semiconductor and optical materials, specifically to CMP polishing slurry for infrared optical germanium wafers and its applications. Background Technology

[0002] Germanium, with its excellent infrared transmittance, high refractive index, and good optical homogeneity, is a crucial fundamental material for fabricating infrared windows, infrared lenses, infrared imaging components, and high-end infrared optical systems. As infrared detection, thermal imaging, and high-precision optoelectronic systems continue to evolve towards higher resolution, higher sensitivity, and higher stability, higher demands are placed on the surface quality of germanium wafers for infrared optics. Especially during the final polishing or fine polishing stage, not only is extremely low surface roughness required, but also low levels of surface micro-pits, scratches, subsurface damage, and post-polishing residues are necessary to reduce the adverse effects of light scattering, haze, and localized defects on infrared optical performance.

[0003] In the CMP process of germanium wafers, the oxidant in the polishing slurry typically induces the formation of a relatively soft reaction layer on the germanium surface, which is then removed by the action of abrasives, polishing pads, and relative motion. Theoretically, when chemical and mechanical actions reach equilibrium, gentle removal of the germanium surface can be achieved, resulting in good surface quality. However, in actual fine polishing processes, simultaneously ensuring removal stability, low damage, and low residue remains a significant challenge. Existing technologies mainly face the following core issues: (1) Damage control during the fine polishing process of germanium surface is difficult. Germanium is a typical brittle material, which is prone to surface micro-scratches, micro-pits and subsurface damage under mechanical action. If the abrasive hardness is too high, the particle morphology is sharp or locally agglomerated, stress concentration is easily formed during the fine polishing process, resulting in fine scratches, local brittle peeling or even subsurface damage expansion on the germanium surface, which is difficult to meet the requirements of infrared optical components for high surface integrity.

[0004] (2) Conventional free abrasive systems cannot simultaneously achieve both gentle removal and high surface quality. If mechanical action is simply increased to enhance removal capacity, it can easily lead to increased surface damage; if strong chemical oxidation or corrosion is relied upon to soften the surface layer, it can easily cause local over-corrosion, fogging, and micro-pit defects. Traditional polishing slurry systems that rely on strong mechanical removal or strong chemical corrosion are difficult to achieve stable processing with high quality and low damage.

[0005] (3) Post-polishing residue problem is prominent. During the fine polishing process, abrasive particles, dispersants, surface active components and reaction byproducts are easily adsorbed or retained on the germanium surface, resulting in particle residue, organic residue or local contamination, which not only affects the surface cleanliness, but may also further increase light scattering, reduce transmittance and affect the efficiency of subsequent cleaning.

[0006] (4) Conventional abrasive systems have limited adaptability in the fine polishing of infrared optical germanium wafers. Traditional silica abrasives rely more on mechanical action and conventional interface interaction, making it difficult to highlight the synergistic advantages of low damage and low residue in high-end fine polishing scenarios; hard abrasives such as alumina and diamond have strong removal capabilities, but they are more likely to introduce scratches, micro-pits and subsurface damage under free abrasive fine polishing conditions.

[0007] Cerium oxide abrasives have unique surface chemical activity. After surface modification, the surface functional groups, surface potential, dispersion stability and interfacial wetting characteristics of nano-cerium oxide particles can be controlled. In the fine polishing process of germanium wafers, a gentler interfacial interaction can be achieved, reducing particle agglomeration and hard contact probability, and reducing post-polishing adsorption residue.

[0008] Therefore, developing a low-damage, low-residue CMP polishing slurry with surface-modified nano-cerium oxide as the core abrasive is of great significance for improving the surface quality, processing stability, and subsequent performance of infrared optical germanium wafers. Summary of the Invention

[0009] In view of this, the first aspect of this application provides a chemical mechanical polishing slurry for fine polishing of infrared optical germanium wafers, comprising modified nano-cerium oxide abrasive, oxidant, complexing agent, corrosion inhibitor, pH adjuster, dispersant stabilizer and water; the modified nano-cerium oxide abrasive is a shell with nano-cerium oxide as the core and asymmetric functional sites extending from pyromellitic tetracarboxylic acid molecules, and there is a dense chemical bonding interface layer between the core and the shell, which is generated by hydrothermal dehydration condensation reaction; the pH value of the polishing slurry is 7.5~8.5.

[0010] Furthermore, by weight percentage: modified nano-cerium oxide abrasive 0.1~5.0wt%, oxidant 0.1~5.0wt%, complexing agent 0.05~3.0wt%, corrosion inhibitor 0.005~0.5wt%, pH adjuster 0.01~1.0wt%, dispersant stabilizer 0~0.5wt%, balance deionized water.

[0011] Furthermore, in the modified nano-cerium oxide abrasive, the mass ratio of pyromellitic acid to nano-cerium oxide is 0.5%~5.0%, the average particle size of the abrasive is 20~60nm, and the particle morphology is near-spherical or quasi-spherical.

[0012] Furthermore, the oxidant is ammonium persulfate, the complexing agent is glycine, the corrosion inhibitor is 5-aminotetrazole, the pH adjuster is tetramethylammonium hydroxide, and the dispersing stabilizer is polyethylene glycol.

[0013] Furthermore, by weight percentage: 1.0~3.0wt% modified nano-cerium oxide abrasive, 0.5~2.0wt% ammonium persulfate, 0.2~1.0wt% glycine, 0.01~0.1wt% 5-aminotetrazole, 0.02~0.1wt% polyethylene glycol, appropriate amount of tetramethylammonium hydroxide to adjust pH to 7.5~8.5, and the balance being deionized water.

[0014] The second aspect of this application provides a method for preparing a chemical mechanical polishing slurry, comprising the following steps: a. Dissolve the complexing agent, corrosion inhibitor, dispersant stabilizer, and oxidant in a portion of deionized water to form a basic clear solution; b. Add the modified nano-cerium oxide abrasive concentrated dispersion to the base clear liquid under stirring, and then perform ultrasonic or high-shear dispersion treatment; c. Adjust the pH of the system to 7.5-8.5 using tetramethylammonium hydroxide; d. Add the remaining deionized water, filter through a 0.1μm microporous membrane, and obtain the chemical mechanical polishing solution.

[0015] Furthermore, the modified nano-cerium oxide abrasive is prepared through the following steps: (1) Select high-purity near-spherical nano CeO2 (particle size 20–60 nm), dry powder is pre-dried to remove impurities, or directly take water dispersion; (2) Disperse nano-cerium oxide in deionized water, add pyromellitic acid, the mass ratio of pyromellitic acid to nano-cerium oxide is 0.5%~5.0%, adjust the pH to 3.0~5.5, and ultrasonically or high shear dispersion for 30~120 min to obtain pre-dispersed slurry; (3) Transfer the pre-dispersed slurry to a high-pressure reactor, heat it to 120-150℃ at a rate of 2-5℃ / min, and keep it at that temperature for 6-24 hours; (4) After the reaction is completed, cool the product, centrifuge or ultrafiltration wash to remove unreacted substances and by-products, redisperse the washed product in deionized water, and grade it through a 0.2~0.5μm microporous filter membrane to obtain modified nano-cerium oxide abrasive.

[0016] The third aspect of this application provides a chemical mechanical polishing method for infrared optical germanium wafers, which uses a polishing slurry to perform fine polishing on the infrared optical germanium wafers. The polishing process parameters are: polishing head pressure 1.0~3.0psi, polishing head rotation speed 40~80rpm, polishing disc rotation speed 50~90rpm, and polishing slurry flow rate 100~250mL / min.

[0017] The fourth aspect of this application provides the application of a chemical mechanical polishing slurry in the final polishing or fine polishing process of infrared optical germanium wafers.

[0018] The beneficial effects of this invention are: (1) Low surface damage. This invention uses modified nano-cerium oxide abrasive with surface covalently anchored pyromellitic acid (PMA). Its rigid aromatic ring shell forms a micro-stress buffer barrier in the mechanical contact area, effectively softening the rigid mechanical stress of the abrasive on the surface of the germanium wafer. Compared with conventional SiO2 abrasive, the surface roughness Ra is reduced from 0.419 nm to 0.220 nm, a reduction of 47.5%; the number of scratches on the entire wafer is reduced from 22.8 to 2.0, a reduction of 91.2%. Compared with unmodified cerium oxide, the surface roughness is reduced by 42.9%, and the number of scratches is reduced by 87.2%.

[0019] (2) Low post-polishing residue. The PMA covalent anchoring layer exhibits extremely high chemical stability in the alkaline polishing system, effectively preventing the desorption and re-adsorption of the organic layer. Combined with the use of 5-aminotetrazole (a small molecule corrosion inhibitor) and tetramethylammonium hydroxide (a metal ion-free organic base), post-polishing organic residue and metal ion contamination are significantly reduced. Compared to citric acid-modified cerium oxide through physical adsorption, surface carbon residue decreased from 3.03 at.% to 1.22 at.%, a reduction of 59.7%; compared to the BTA+KOH system, surface carbon residue decreased from 5.06 at.% to 1.22 at.%, a reduction of 75.9%.

[0020] (3) High removal efficiency. PMA-CeO2 abrasive retains the chemical activity of the cerium oxide fluorite crystal structure, and works synergistically with ammonium persulfate-glycine to achieve highly efficient chemical-mechanical synergistic removal. Under low pressure conditions of 1.5 psi, the germanium removal rate can reach 232.0 nm / min, which is 31.1% higher than that of conventional SiO2 abrasive and 12.1% higher than that of the BTA+KOH system.

[0021] (4) Excellent abrasive dispersion stability. The free carboxyl groups on the PMA molecule that do not participate in interfacial anchoring endow the abrasive surface with a high negative Zeta potential (-45.2mV), constructing a dual anti-agglomeration mechanism of "electrostatic repulsion + rigid steric hindrance" between particles. After 30 days in an alkaline polishing slurry system with pH=8.5, the modified abrasive maintained an almost constant average particle size; while the physically adsorbed modified abrasive showed obvious agglomeration after 7 days and severe precipitation after 30 days. Excellent dispersion stability ensures the consistency of the polishing process and the long-term use of the polishing slurry.

[0022] (5) Low defects and high surface cleanliness. Through the steric hindrance of the rigid shell of pyromellitic acid and the synergistic effect of the absence of metal ions and small molecule additives in the formula, the germanium surface after polishing is free of obvious micro-pits, corrosion pits and fogging defects, and the surface quality meets the stringent requirements of infrared optical components for high surface integrity.

[0023] (6) Applicable to high-end precision polishing of infrared optical germanium wafers. The polishing slurry of this invention can achieve a synergistic balance of low damage, low residue, and low defects while ensuring controllable removal capabilities. It is particularly suitable for the final polishing process of infrared windows, infrared lenses, and high-end infrared optical systems that are sensitive to surface light scattering, haze, and local defects. After polishing, the surface roughness Ra≤0.22nm, the number of scratches on the entire wafer≤2.0, and the surface carbon residue≤1.22at.%, with comprehensive performance significantly superior to existing technologies. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The image shows a comparison of the FTIR spectra of the modified abrasive (PMA-CeO2) and pure PMA.

[0026] Figure 2 This is the O1sXPS high-resolution spectrum of the modified abrasive of this invention.

[0027] Figure 3 This is a graph showing the particle size variation over time of the modified abrasive and the physical adsorption comparison sample of this invention under pH=8.5 conditions. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The following embodiments are used to illustrate this invention, but cannot be used to limit the scope of this invention.

[0029] In the following examples and comparative examples, the performance testing methods are as follows: I. Testing Methods Germanium removal rate: Calculated by weighing method, with each group tested 3 times and the average value taken.

[0030] Surface roughness: Atomic force microscopy (AFM) was used to scan a 10μm×10μm area, and five locations were randomly selected for testing each sample to obtain the average value.

[0031] Surface scratches: The entire sheet is scanned using a laser surface defect detector to count the number of scratches.

[0032] Surface carbon residue: The percentage of carbon atoms on the wafer surface was determined by X-ray photoelectron spectroscopy (XPS), with three points tested for each sample and the average value taken.

[0033] Polishing slurry stability: The precipitation formation was observed by standing at room temperature, and the average particle size of the abrasive (dynamic light scattering method) and the change of Zeta potential were measured at different time points.

[0034] Abrasive characterization: Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the chemical bonding state of the abrasive surface; dynamic light scattering method was used to determine the hydrodynamic diameter and zeta potential.

[0035] II. Standardized Polishing Process Conditions To objectively verify the polishing performance, the following examples and comparative examples all adopted the same CMP test conditions: Polishing station: A standard CMP device suitable for 8-inch wafers; Polishing pad: SUBA IV polyurethane polishing pad; Polished wafer: 8-inch infrared optical pure germanium (Ge) wafer; Process parameters: polishing head pressure 1.5psi, polishing head speed 60rpm or polishing disc speed 65rpm, polishing fluid flow rate 150mL / min; Post-cleaning process: High-purity deionized water and PVA sponge are used for double-sided brushing, supplemented by mega-sonic cleaning for 3-5 minutes.

[0036] Preparation Example 1: Preparation of Modified Nano-Cerium Oxide Abrasive (PMA-CeO2) 1.1 The PMA covalently anchored nano-cerium oxide abrasive used in this invention is prepared according to the following steps: Step 1: Raw Material Preparation. Nano-cerium oxide powder or nano-cerium oxide aqueous dispersion is selected as the initial abrasive precursor. The average particle size of the nano-cerium oxide particles is 35 nm; the particle morphology is preferably near-spherical or quasi-spherical to reduce the local stress concentration effect of sharp edges on the polished surface. Preferably, the purity of the initial nano-cerium oxide particles is not less than 99.0%, and the content of coarse particles is low. When using nano-cerium oxide powder, it is first dried at 100℃ for 6 hours to remove adsorbed water and light volatile impurities; when using commercial nano-cerium oxide aqueous dispersion, it can directly proceed to the next dispersion treatment.

[0037] Step 2: Pre-dispersion under weakly acidic conditions. The nano-cerium oxide (CeO2) prepared in Step 1 is dispersed in deionized water, and a specific proportion of pyromellitic acid (PMA) is added as a rigid polycarboxyl group modifier. Preferably, the mass ratio of pyromellitic acid (PMA) to nano-cerium oxide is controlled at 3.0%. The pH of the system is adjusted to a weakly acidic environment of 4.0 using dilute acid or dilute alkali. Under these weakly acidic conditions, ultrasonic dispersion (500W power) or high-shear emulsification is performed for 75 minutes. The purpose is to utilize the partial carboxyl groups in the pyromellitic acid molecules to form preliminary hydrogen bonds and weak coordination with the active hydroxyl groups on the surface of the nano-cerium oxide particles, breaking the original soft agglomeration state between the particles. The pyromellitic acid (PMA) molecules, acting as a spatial "occupier," uniformly coat the surface of individual particles, enabling the abrasive to achieve a highly "monodispersed" state at the microscopic level, providing an ideal spatial arrangement basis for subsequent high-temperature chemical anchoring.

[0038] Step 3: Solvent-thermal high-temperature and high-pressure in-situ covalent coordination. The highly monodisperse slurry obtained in Step 2 is transferred to a high-pressure reactor with a polytetrafluoroethylene liner, sealed, and then heated. Preferably, the heating rate is controlled at 3.5℃ / min, heated to 135℃, and reacted under these high-temperature, high-pressure, and constant-temperature conditions for 15 hours.

[0039] The core objective of this step is to overcome the reaction energy barrier under the combined drive of thermodynamics and high pressure, promoting an interfacial esterification and dehydration condensation reaction between the carboxyl groups of pyromellitic acid (PMA) and the hydroxyl groups on the cerium oxide surface, forming a highly thermodynamically stable Ce-OC covalent bond. Because pyromellitic acid has a rigid benzene ring, the steric hindrance that is difficult to overcome at room temperature is broken under these extreme conditions, thus solidifying in situ on the abrasive surface to form an outer organic shell. Simultaneously, since the preceding step has achieved monodisperse isolation of the particles, this in-situ solidification process fundamentally prevents the irreversible "hard agglomeration" caused by violent particle collisions and bonding in conventional hydrothermal reactions.

[0040] Step 4: Aging, Washing, Purification, and Gradient Redispersion. After the reaction vessel has cooled naturally to room temperature, the modified nano-cerium oxide slurry is removed for aging. Subsequently, the slurry is repeatedly washed using purification methods such as high-speed centrifugation (11500 rpm), ultrafiltration, or dialysis until the conductivity of the washing solution stabilizes and becomes nearly neutral, ensuring that there are no free pyromellitic acid molecules or reaction byproducts remaining in the supernatant. After purification, the centrifuged product or concentrate is redispersed in deionized water and fractionated using a microporous membrane with a pore size of 0.35 μm. For long-term storage or long-distance transportation, the aqueous dispersion can be converted into a dry powder using freeze-drying technology. When using, adding deionized water in the appropriate ratio will quickly restore its excellent monodisperse state.

[0041] The key to this step is to thoroughly wash away unbonded free impurities and precisely remove large particles (LPCs) from the system. The resulting modified nano-cerium oxide abrasive has a strong bond between its outer organic shell and the particle matrix, an extremely narrow particle size distribution, and a highly pure system.

[0042] 1.2 Abrasive Characterization: Characterization revealed that the functionalized nano-cerium oxide abrasive (PMA-CeO2) prepared in this invention has an average hydrodynamic diameter of 38.5 nm and a PDI < 0.1; its Zeta potential at pH=10 is -45.2 mV (its absolute value > 40 mV). These data not only demonstrate the abrasive's excellent monodispersity and colloidal stability, but also provide a physical basis for achieving low-damage polishing of germanium wafers.

[0043] 1.3 Characterization and analysis of abrasive microstructure: The chemical bonding state of the abrasive surface before and after modification was analyzed by FTIR. Figure 1 The results showed that pure PMA was at 1715. The characteristic absorption peak of the free carboxyl group (-COOH) at 1562°C is significantly weakened or even disappears in the spectrum of the modified abrasive (PMA-CeO2); at the same time, the peak at 1562°C is significantly weakened or even disappears. and 1405 A new peak belonging to the asymmetric and symmetric stretching vibrations of the carboxylate group (-COO-) appeared. The significant shift in peak position and the emergence of the new peak strongly prove that the PMA molecule has formed a stable Ce-OC covalent bond with the cerium oxide surface through a dehydration condensation reaction.

[0044] Further verification was performed using XPS, such as... Figure 2 The results show a distinct CO-Ce binding energy peak at 531.8 eV; simultaneously, the Ce3d spectral line exhibits a binding energy shift towards a higher field direction by approximately 0.4 eV due to the electron cloud shielding effect of the organic ligands. These microscopic evidences synergistically confirm that this invention successfully overcomes the reaction energy barrier via a solvothermal method, transforming the modifier from a fragile "physically adsorbed state" into a robust "surface covalently anchored state," thus constructing a core-shell structure with microscopic stress buffering capabilities.

[0045] Example 1 This embodiment provides a low-damage, low-residue CMP polishing slurry for fine polishing of infrared optical germanium wafers, with the following specific components: Abrasive: 2.0 wt% PMA covalently anchored modified nano-cerium oxide (PMA-CeO2, average particle size approximately 38.5 nm) Oxidizing agent: 1.0 wt% ammonium persulfate Complexing agent: 0.5wt% glycine Corrosion inhibitor: 0.05wt% 5-aminotetrazole (5-ATA) Dispersant stabilizer: 0.05wt% polyethylene glycol (PEG-1000) pH adjuster: Add appropriate amount of tetramethylammonium hydroxide (TMAH) to adjust pH to 8.0. Solvent: The balance is high-purity deionized water (resistivity > 18.2 MΩ·cm) The modified nano-cerium oxide abrasive (PMA-CeO2) was prepared according to the method in Preparation Example 1: Polishing slurry preparation steps: a. Under constant temperature conditions of 20℃, ammonium persulfate, glycine, 5-ATA and PEG are dissolved in approximately 80% of the formula volume of deionized water in sequence, and stirred to form a basic clear solution; b. Add the modified nano-cerium oxide abrasive concentrated dispersion to the base clear liquid while stirring at 500 rpm, and then perform ultrasonic or high-shear dispersion treatment for 30 min. c. Adjust the pH of the system to 7.5-8.5 using tetramethylammonium hydroxide; d. Add the remaining deionized water, filter through a 0.1μm microporous membrane, and obtain the chemical mechanical polishing solution.

[0046] Test results (n=3, representing 3 repeated experiments):

[0047] Stability test: After standing at room temperature for 24 hours: no precipitation, average particle size 38.8 nm (change <1%). After 7 days: No precipitation, average particle size 39.5 nm, Zeta potential -44.8 mV; After 30 days: No sedimentation, average particle size 41.2nm (an increase of 6.9%), still usable; Example 2 (Changing the abrasive concentration) The method is basically the same as in Example 1, except that the concentration of PMA-CeO2 abrasive is adjusted to 1.0 wt%, and the amount of deionized water is adjusted accordingly.

[0048] Test results (n=3):

[0049] Example 3 (Changing the abrasive concentration) The method is basically the same as in Example 1, except that the concentration of PMA-CeO2 abrasive is adjusted to 3.0 wt%, and the amount of deionized water is adjusted accordingly.

[0050] Test results (n=3):

[0051] As can be seen from Examples 1-3, good polishing results can be obtained within the abrasive concentration range of 1.0-3.0 wt%. Increasing the abrasive concentration can improve the removal rate, but the surface roughness and number of scratches increase slightly; decreasing the abrasive concentration reduces the removal rate, but the surface quality is better. Considering all factors, 2.0 wt% is the optimal concentration.

[0052] Example 4 (Changing pH value) It is basically the same as Example 1, except that the pH is adjusted to 7.5 with TMAH, while the other components remain unchanged.

[0053] Test results (n=3):

[0054] Example 5 (Changing pH value) It is basically the same as Example 1, except that the pH is adjusted to 8.5 with TMAH, while the other components remain unchanged.

[0055] Test results (n=3):

[0056] As can be seen from Examples 1, 4 and 5, the polishing performance is stable in the pH range of 7.5 to 8.5, and the overall performance is optimal at pH=8.0.

[0057] Comparative Example 1 (Compared to traditional SiO2 abrasive) This comparative example aims to verify the chemical-mechanical synergistic advantages of the PMA-CeO2 abrasive of the present invention. This comparative example is basically the same as Example 1, except that the abrasive is replaced with conventional high-purity silica sol (using conventional nano-silica) with the same particle size (about 40 nm) and the same mass fraction (2.0 wt%). All other components, concentrations and pH values ​​are exactly the same as in Example 1.

[0058] Test results (n=3):

[0059] Compared with Example 1: The removal rate decreased by 31.1%; Surface roughness increased by 90.5% (deterioration); The number of scratches on the entire piece increased more than 10 times; Surface carbon residue increased by 216%; Conclusion: Traditional SiO2 abrasives rely on mechanical action and conventional interfacial interactions, resulting in low removal efficiency, significant damage, and substantial residue in the fine polishing of germanium wafers. The PMA-CeO2 abrasive of this invention combines the chemical activity of cerium oxide with the mechanical buffering effect of the PMA shell, exhibiting significant advantages in removal rate, surface quality, and cleanliness.

[0060] Comparative Example 2 (Comparison of CeO2 modified by physical adsorption of citric acid) This comparative example aims to verify the irreplaceable nature of the PMA covalently anchored shell compared to traditional physical adsorption modification. It is basically the same as Example 1, except that commercially available citric acid-modified nano-cerium oxide (average particle size of about 40 nm) is used instead of the PMA-CeO2 abrasive of the present invention.

[0061] Test results (n=3):

[0062] Compared with Example 1: The removal rate decreased by 5.9%; Surface roughness increased by 40.9%; The number of scratches on the entire piece increased nearly fourfold; Surface carbon residue increased by 148%; Stability comparison: After 7 days of standing, the polishing slurry in Comparative Example 2 showed obvious agglomeration, with the average particle size increasing from the initial 42 nm to 185 nm; after 30 days, severe precipitation occurred (particle size > 1000 nm). In contrast, the particle size in Example 1 remained almost constant within 30 days.

[0063] The modified abrasive prepared in Example 1 of this invention and Comparative Example 2 (physical adsorption control sample) were dispersed separately in an alkaline polishing slurry system with pH=8.5, and the change trend of their particle size over time was observed. The results are as follows: Figure 3 As shown. By Figure 3 As can be seen, in Comparative Example 2, the average particle size D50 increased significantly after 7 days, and by 30 days, severe agglomeration and precipitation (particle size exceeding 1000 nm) had occurred. In contrast, the modified abrasive prepared in Example 1 maintained an almost constant particle size over 30 days, further confirming the... Figure 1 and Figure 2 Characterization conclusions: The PMA segments anchored by covalent bonds have extremely high chemical stability, effectively resisting the attack of alkaline environments and maintaining the excellent monodisperse state of the abrasive for a long time.

[0064] Conclusion: Physically adsorbed modified layers are prone to desorption under high shear stress and alkaline conditions during polishing, leading to abrasive agglomeration, increased hard contact probability, and increased organic residue. The PMA covalent anchoring layer of this invention exhibits extremely high interfacial stability and steric hindrance buffering effect, effectively inhibiting agglomeration, reducing damage, and minimizing residue.

[0065] Comparative Example 3 (Compared to the traditional BTA+KOH additive system) This comparative example aims to verify the advantages of the 5-ATA+TMAH small molecule metal ion-free system of the present invention in "ultra-clean cleaning". It is basically the same as Example 1, except that the corrosion inhibitor 5-ATA is replaced with an equal amount of benzotriazole (BTA) and the pH adjuster TMAH is replaced with potassium hydroxide (KOH) to adjust the pH to 8.0.

[0066] Test results (n=3):

[0067] Compared with Example 1: The removal rate decreased by 12.1%; Surface roughness increased by 35.5%; The total number of scratches on the entire piece increased threefold; Surface carbon residue increased by 315%; Conclusion: BTA is a strong adsorption corrosion inhibitor, which easily forms a difficult-to-clean organic residue layer on the germanium surface; KOH introduces the risk of metal ion contamination. The 5-ATA of this invention is a small molecule corrosion inhibitor with moderate adsorption and easy cleaning; TMAH is a metal ion-free organic base, which helps maintain the cleanliness of the polishing solution and the post-cleaning effect, significantly reducing post-polishing organic residue.

[0068] Comparative Example 4 (Comparison with unmodified cerium oxide abrasive) This comparative example aims to verify the necessity of PMA surface modification. It is essentially the same as Example 1, except that unmodified nano-cerium oxide abrasive (average particle size of about 40 nm) is used instead of PMA-CeO2.

[0069] Test results (n=3):

[0070] Compared with Example 1: The removal rates are roughly the same (slightly lower); Surface roughness increased by 75.0%; The number of scratches on the entire piece increased nearly sevenfold; Surface carbon residue increased by 52%; Stability comparison: Unmodified cerium oxide showed obvious precipitation after standing for 2 hours at pH=8.0, with a zeta potential of only -12.5mV.

[0071] Conclusion: Unmodified cerium oxide has high surface activity and is prone to agglomeration. Although it has certain chemical activity, it causes severe mechanical damage. Covalent anchoring modification of PMA imparts excellent dispersion stability and mechanical buffering effect to the abrasive, which is the key to achieving low-damage and low-residue fine polishing.

[0072] Overall Performance Comparison Summary Table

[0073] As can be seen from the above experimental results, Examples 1 to 5 of this invention all exhibit excellent comprehensive performance in the fine polishing of infrared optical germanium wafers: removal rate of 185~268 nm / min, surface roughness Ra of 0.220~0.245 nm, number of scratches per wafer of 2.0~3.2, and surface carbon residue of 1.18~1.35 at.%. The comparative examples fully verified the significant advantages of the PMA covalently anchored abrasive structure and the 5-ATA+TMAH small molecule metal ion-free system compared to traditional SiO2 abrasives, physically adsorbed modified CeO2, the BTA+KOH system, and unmodified CeO2. The results indicate that the CMP polishing slurry system constructed by using PMA covalently anchored nano-cerium oxide as the core abrasive, combined with ammonium persulfate, glycine, 5-aminotetrazole, and TMAH, can achieve a synergistic balance of high removal rate, low surface damage, and low residue in the fine polishing of infrared optical germanium wafers.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A chemical mechanical polishing slurry for fine polishing of infrared optical germanium wafers, characterized in that, The polishing solution comprises modified nano-cerium oxide abrasive, oxidant, complexing agent, corrosion inhibitor, pH adjuster, dispersant and stabilizer, and water. The modified nano-cerium oxide abrasive has a nano-cerium oxide core and an asymmetric functional region shell extending from pyromellitic tetracarboxylic acid molecules. Between the core and the shell, there is a dense chemically bonded interface layer formed by hydrothermal dehydration condensation reaction. The pH value of the polishing solution is 7.5~8.

5.

2. The chemical mechanical polishing slurry according to claim 1, characterized in that, By weight percentage: modified nano-cerium oxide abrasive 0.1~5.0wt%, oxidant 0.1~5.0wt%, complexing agent 0.05~3.0wt%, corrosion inhibitor 0.005~0.5wt%, pH adjuster 0.01~1.0wt%, dispersant stabilizer 0~0.5wt%, balance deionized water.

3. The chemical mechanical polishing slurry according to claim 1, characterized in that, In the modified nano-cerium oxide abrasive, the mass ratio of pyromellitic acid to nano-cerium oxide is 0.5%~5.0%, the average particle size of the abrasive is 20~60nm, and the particle morphology is near-spherical or quasi-spherical.

4. The chemical mechanical polishing slurry according to claim 1, characterized in that, The oxidant is ammonium persulfate, the complexing agent is glycine, the corrosion inhibitor is 5-aminotetrazole, the pH adjuster is tetramethylammonium hydroxide, and the dispersing stabilizer is polyethylene glycol.

5. The chemical mechanical polishing slurry according to claim 4, characterized in that, By weight percentage: 1.0~3.0wt% modified nano-cerium oxide abrasive, 0.5~2.0wt% ammonium persulfate, 0.2~1.0wt% glycine, 0.01~0.1wt% 5-aminotetrazole, 0.02~0.1wt% polyethylene glycol, appropriate amount of tetramethylammonium hydroxide to adjust pH to 7.5~8.5, and the balance being deionized water.

6. A method for preparing the chemical mechanical polishing slurry according to any one of claims 1 to 5, characterized in that, Includes the following steps: a. Dissolve the complexing agent, corrosion inhibitor, dispersant stabilizer, and oxidant in a portion of deionized water to form a basic clear solution; b. Add the modified nano-cerium oxide abrasive concentrated dispersion to the base clear liquid under stirring, and then perform ultrasonic or high-shear dispersion treatment; c. Adjust the pH of the system to 7.5-8.5 using tetramethylammonium hydroxide; d. Add the remaining deionized water, filter through a 0.1μm microporous membrane, and obtain the chemical mechanical polishing solution.

7. The preparation method according to claim 6, characterized in that, The modified nano-cerium oxide abrasive is prepared through the following steps: (1) Select high-purity near-spherical nano CeO2 (particle size 20–60 nm), dry powder is pre-dried to remove impurities, or directly take water dispersion; (2) Disperse nano-cerium oxide in deionized water, add pyromellitic acid, the mass ratio of pyromellitic acid to nano-cerium oxide is 0.5%~5.0%, adjust the pH to 3.0~5.5, and ultrasonically or high shear dispersion for 30~120 min to obtain pre-dispersed slurry; (3) Transfer the pre-dispersed slurry to a high-pressure reactor, heat it to 120-150℃ at a rate of 2-5℃ / min, and keep it at that temperature for 6-24 hours; (4) After the reaction is completed, cool the product, centrifuge or ultrafiltration wash to remove unreacted substances and by-products, redisperse the washed product in deionized water, and grade it through a 0.2~0.5μm microporous filter membrane to obtain modified nano-cerium oxide abrasive.

8. A chemical mechanical polishing method for infrared optical germanium wafers, characterized in that, The infrared optical germanium wafer is finely polished using the polishing slurry described in any one of claims 1 to 5. The polishing process parameters are as follows: polishing head pressure 1.0~3.0psi, polishing head rotation speed 40~80rpm, polishing disc rotation speed 50~90rpm, and polishing slurry flow rate 100~250mL / min.

9. The application of the chemical mechanical polishing slurry according to any one of claims 1 to 5 in the final polishing or fine polishing process of infrared optical germanium wafers.

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