Yuli Sun

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.

Research on Subsurface Damage After Abrasives and Fixed-Abrasive Lapping of K9 Glass

Fixed-abrasive lapping (FAL) is a new machining technology and is adopted to manufacture hard brittle materials to obtain the high surface quality. In the same machining condition, K9 glasses are lapped by abrasives and fixed-abrasive, respectively. Two grain sizes of diamond abrasives are adopted in every lapping means. Differential chemical etch method (DCEM) is employed to measure the depth of subsurface damage (SSD) of different lapping means. Surface damages are compared by Microscope. The results show that the depth of SSD is 53 and 15.2μm after abrasives lapping (AL) by 40 and 28μm diamond abrasives. FAL with 40 and 28μm diamond abrasive leads to 4.5 and 3.4μm subsurface damage depth, respectively. FAL can get smaller surface damage and shallower depth of SSD than AL. And FAL can obtain the higher surface quality than AL.

Effects of rare earth additions during surface boriding on microstructure and properties of titanium alloy TC21

Abstract In the present paper, the effects of rare earth (RE) additions to the solid state boriding of titanium alloy TC21 have been studied. The microstructural evolution and phase transformations of the borided layers were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction. Moreover, the microhardness for the borided layer was also determined by Vickers hardness test. The results showed that the addition of a small amount of RE elements in the boriding process can lead to an increased boron concentration in the surface layer coupled with the improved surface hardness and coating layer thickness. Furthermore, the presence of trace quantities of RE oxide (Ce2O3) in boride layers indicated that the RE elements as catalysts could not only influence but also accelerate boriding process.

Effect of rare earth addition on diffusion kinetics of borided TC21-DT titanium alloy

In this study, the properties and growth kinetics of boride layers, generated on the surface of TC21-DT alloy using appropriate amount of powders of boron carbide (B4C) and rare earth oxide (CeO2) were investigated. By conducting a series of experiments at different temperatures of 1123, 1223 and 1273 K for periods up to 10 h, the effects of rare earth (RE) addition on the growth kinetics of boride layers were studied. The characteristics of the boride layers were examined by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction and micro-Vickers hardness tester. The results showed that the boron diffusion in boride layers was obviously accelerated by RE addition and the activation energy for RE addition-boriding in TC21-DT alloy was greatly decreased to 58.13kJ/mol, which was approximately 40% lower than that of conventional one, without RE addition.

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.

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

Study on dispersion property of sub-micro α-Al2O3 powders in water suspension by physical-chemical process

ABSTRACT Sub-micro α-Al2O3 polishing liquid is often used as the slurry in chemical mechanical polishing (CMP). At present, water is usually used as dispersion medium to disperse sub-micro α-Al2O3 powders, but the sub-micro α-Al2O3 powders easily subside in aqueous solution. In previous studies, researchers usually used single method to disperse superfine powders, however, the dispersion effect was non-ideal. In order to solve the difficult problem of sub-micro α-Al2O3 powders dispersed in water suspension, firstly, using physical method, the effects of milling time, ultrasonic time on the dispersion property of sub-micro α-Al2O3 powders in water suspension was studied by single factorial test. Then, using chemical method, the effects of dispersant type, dispersant concentration and pH value on the dispersion property of sub-micro α-Al2O3 powders in water suspension was studied by orthogonal test. Absorbance was used to evaluate the dispersion property of sub-micro α-Al2O3 powders. The results show that the optimal dispersing conditions are milling time 60 min, ultrasonic time 20 min, mass percent concentration of STPP 1% and pH value 9.

Free and Fixed Abrasive Lapping of BK7 Glass

Fixed abrasive technology which has many advantages is one of the future machining directions. Free and fixed abrasive lapping of BK7 glass was investigated and different material removal modes and surface damage categories by lapping were discussed. The results show that material removal rate is larger for free abrasive lapping than that of fixed abrasive lapping with four abrasive sizes and decreases with diamond size decreasing in two lapping processes. Surface quality is better for fixed abrasive lapping than that of free abrasive lapping at the same diamond size and gets better with the decreasing of diamond size. Fixed abrasive lapping can achieve simultaneously high MRR and good surface quality.