Yongwei Zhu

Using Taguchi Method to Optimize Polishing Parameters in Ice Fixed Abrasive Polishing

Ice fixed abrasive polishing (IFAP) was used to polish single crystal silicon wafer. Polishing parameters were polishing pressure, table velocity, eccentricity, and polishing time. Using Taguchi method, the influences of the polishing parameters on material removal rate (MRR) and surface roughness (Sa) were invested. The results show that polishing pressure plays the most significant role on MRR followed by table velocity; as far as Sa is concerned; polishing time is the most important one, followed by table velocity. In order to get high MRR during IFAP of silicon wafer, the optimal processing parameters are: polishing pressure 0.1 MPa, table velocity 400 r/min, eccentricity 30 mm, and polishing time 60 min. The best Sa can be obtained when the optimal processing parameters are: polishing pressure 0.075 MPa, table velocity 300 r/min, eccentricity 20 mm, and polishing time 40 min. The experimental results illustrate that the Taguchi method is a viable way to obtain the optimum conditions for high MRR and best Sa.

Basic research on polishing with ice bonded nanoabrasive pad

New approach to polish silicon wafers was proposed in this study, where nanoabrasives were bonded by ice to make ice bonded fixed abrasive pads (IFA) and the wafer was polished with the IFA under cryogenic conditions. After the introduction of the preparation process of the IFA with nanosized CeO2 and Al2O3 abrasives, polishing experiments of silicon wafers were carried out, and polishing mechanism was discussed. The results show that a supersmooth surface with roughness of Ra 0.568nm is obtained when polished with Al2O3 IFA and Ra 0.369nm when polished with CeO2 IFA. No microcracks are found in the subsurface of the workpiece. Since the grain size is about 80–100nm in diameter and there exists a soft corrosion layer and a stable thin liquid film between the silicon wafer and the IFA during polishing, the real cutting depth of the abrasive decreases and the material is removed in a ductile mode. The chemical corrosion and mechanical removal simultaneously exist during the cryogenic polishing of silicon wa...

Surface Formation of Single Silicon Wafer Polished with Nano-sized Al2O3 Powders

Ice polishing single silicon wafers with nano-sized Al2O3 abrasives can be known as ice fixed abrasives chemical mechanical polishing (IFA-CMP). TAn abrasive slurry was made of nano-sized Al2O3 particles dispersed in de-ionized water with a surfactant and the slurry was frozen to form an ice polishing pad. Then polishing tests of blanket silicon wafers with the above ice polishing pad were carried out. The morphologies and surface roughness of the polished silicon wafers were observed and examined on an atomic force microscope. The subsurface damage was assessed by means of cross-section transmission electron microscopy. The surface chemical constituents of the polished silicon wafers were characterized using X-ray photoelectron spectroscopy in order to gain insight into the chemical mechanisms in the process. Scratch resistance of the single silicon wafer was measured by nanoscratching using a nanoindenter to explore the mechanical removal mechanism. The results show that a super smooth surface with an average roughness of 0.367 nm is obtained within 1000 nm × 1000 nm and there is a perfect silicon diamond structure without any microcracks in the subsurface. The removal of material is dominated by the coactions of ductile regime machining and chemical corrosion. In the end, a model of material removal of IFA-CMP is built.

Optimization of FAP in Nano Machining Process

To achieve the nano precision surface quality of LBO crystal, the fixed abrasive polishing technology was adopted to realize the nano machining process. The machining tool plays a key role in nano machining process and the same as the fixed abrasive polishing pad (FAP) for the polishing process. The effect of the matrix hardness and polishing powder concentration of the FAP on material removal rate, surface topography, microscopic appearances and surface roughness were investigated in nano machining LBO crystal. The results show that the matrix hardness B and the concentration of the polishing powder 150% of FAP are the optimization characteristics for the maximum material removal rate and the best surface quality in fixed abrasive polishing of LBO crystal. The maximum material removal rate is 71.4 nm/min and the optimal surface roughness Sa is 0.657 nm. The nano precision surface quality with nanoscale material removal was obtained in nano machining LBO crystal.

Effect of alkaline slurries on nano machining CaF2 crystal

ABSTRACT The slurry chemical action affects chemical reaction between the wafer and slurry, and self-conditioning performance of the pad in nano machining process. Fixed abrasive polishing, one of important nano machining technologies, was adopted to achieve a nano precision surface quality of CaF2 crystal. Five kinds of alkaline regulators, triethanolamine, sodium citrate, sodium carbonate, ethylenediamine and sodium phosphate in slurries with pH 10, were screened in nano machining CaF2 crystal. The effect on material removal rate (MRR), surface topography and surface roughness was investigated in fixed abrasive polishing of CaF2 crystal. The results indicated that surface quality is getting better with MRR decreasing. The optimal surface quality of CaF2 crystal with surface roughness Sa 4.13 nm can be obtained by sodium phosphate slurry with MRR 224 nm/min in fixed abrasive polishing of CaF2 crystal. The nanometer precision surface quality with high material removal was achieved in nano machining CaF2 crystal.

Grid and Substrate Bias Effects on Mechanical Properties of Diamond Films Prepared by HFCVD

Diamond films were prepared at different grid bias and substrate bias in hot filament chemical vapor deposition (HFCVD) system. The Raman and SEM results show that grain size decreases and non-diamond impurities increase for applying grid and substrate bias currents. The defects and impurities in the film increase with the decrease of grain size, which causes the decrease of hardness and elastic modulus of diamond films. The fracture toughness of film increases because of the grain size effects by applying bias. The grid bias and substrate bias make the friction coefficient smaller because of the smaller grain size and lubricating effect of graphite in the film. But the excessively high substrate bias current will lead to the dramatic decrease of mechanical properties of CVD diamond as a lot of non-diamond impurities appear in the film.

Relationship between coefficient of friction and surface roughness of wafer in nanomachining process

Fixed abrasive polishing technology can obtain a nanoscale surface and is one of the future nano machining directions. The coefficient of friction between the pad and the wafer in the polishing process can influence on the surface quality of the wafer. The relationship between the coefficient of friction and surface roughness of the wafer was investigated to improve the efficiency and surface quality. Based on the Florida model, the adhesion, asperity plough and abrasive plough from the pad in the polishing process was analyzed. The friction force per unit area was calculated by the properties of the pad and wafer. Based on the rod model, the actual contact area was calculated by the surface roughness and the properties of the pad and wafer. The relational model between the surface roughness of the wafer and the friction coefficient was established. The model was verified by the experiments of fixed abrasive polishing of BK7 glass. When the friction coefficient is less than 1.9, the data of the experiment and theory match very well in the comparison process.

Temperature Distribution of IFA Polishing Single Silicon Wafer

The ice fixed abrasives (IFA) polishing is a potential polishing process in the semiconductor industry to realize superior surface finish and planarity for semiconductor wafers. The key question in IFA polishing is how to keep suitable ambient temperature and melting rate in production process in order to avoid premature failure of the IFA pad. In this paper, effects of ambient temperature (T), pressure in cylinder (Pc), rotary speed of IFA pad (v) and eccentricity of pressure head (e) on temperature distribution and melting rate of the IFA pad are researched. The results show that T should be kept at about 10 °C in order to control the melting rate of the IFA pad effectively and keep longer polishing time. And suitable Pc, e can be kept at 0.075 MPa or 0.1 MPa and 20 mm or 30 mm, respectively. In order to increase IFA polishing efficiency, the rotary speed of IFA pad can be increased appropriately. All the results provide the basis for choosing suitable processing parameters in IFA polishing.

Influence of slurry pH on material removal rate and surface roughness of super-precision polishing of LBO crystal

LBO crystal with high quality surface, which must be defect-free and super smooth, is urgently needed because of its applications in high energy laser system. Chemical mechanical polishing (CMP) is adopted to raise surface quality and processing efficiency in super precision polishing of LBO crystal. The polyurethane pad and colloidal SiO2 slurry are chosen and the polishing experiments are performed on Logitech PM5 Precision Lapping & Polishing Machine. The slurry pH is changed and its influence on material removal rate (MRR) and surface roughness is studied. The polished surface roughness is measured by using atomic force microscope. MRR is calculated through the difference of the crystal thickness between before and after polishing by polishing time. In the pH range from 2 to 6, MRR of LBO crystal increases with pH decreasing and there is an optimal pH for surface roughness. While in the pH range from 7 to 13, MRR and surface roughness vibrate with pH. The maximal MRR reaches 758 nm/min when slurry pH is kept at 2 and the best surface roughness reaches 0.197 nm RMS when it at 4.

Characterization of Surface Layer of Silicon Wafer by Using Nano Indenter

Mechanical properties of the silicon wafer are evaluated by a nano indenter system with the continuous stiffness measurement (CSM) technique. Contact stiffness, hardness and elastic modulus of the silicon wafer are continuously measured during the loading in an indentation test. The results show that when the contact depth is between 20 and 32 nm, its contact stiffness is linear with the contact depth, and its hardness and elastic modulus keep constant at 10.2 GPa and 140.3 GPa respectively, which belong to the oxide coating of the silicon wafer. When the contact depth is between 32 and 60 nm, its contact stiffness is not linear with the contact depth, and the hardness and elastic modulus increase rapidly with the contact depth, because they are affected by the bulk material. When the contact depth is over 60 nm, the contact stiffness of the silicon wafer is linear with the contact depth again, and the hardness and elastic modulus keep constant at 12.5 GPa and 165.6 GPa respectively, which belong to the silicon wafer, the bulk material.Copyright