Y.B. Tian

A study on the diamond grinding of ultra-thin silicon wafers:

The demand for ultra-thin silicon wafers has escalated in recent years with the rapid development of miniaturized electronic devices. In this work, diamond grinding for thinning silicon wafers was carried out on an ultra-precision grinding machine. The thinning performance and the minimum wafer thickness were investigated under different grinding conditions. It was found that the grain depth of cut that was used to characterize the overall grinding conditions played an important role in the determination of the final grinding performance. The relationship between the subsurface damage of the ground wafer and the minimum wafer thickness achieved was also revealed.

Modeling and Analyzing on Nonuniformity of Material Removal in Chemical Mechanical Polishing of Silicon Wafer

Chemical mechanical polishing (CMP) has already become a mainstream technology in global planarization of wafer, but the mechanism of nonuniform material removal has not been revealed. In this paper, the calculation of particle movement tracks on wafer surface was conducted by the motion relationship between the wafer and the polishing pad on a large-sized single head CMP machine. Based on the distribution of particle tracks on wafer surface, the model for the within-wafer-nonuniformity (WIWNU) of material removal was put forward. By the calculation and analysis, the relationship between the motion variables of the CMP machine and the WIWNU of material removal on wafer surface had been derived. This model can be used not only for predicting the WIWNU, but also for providing theoretical guide to the design of CMP equipment, selecting the motion variables of CMP and further understanding the material removal mechanism in wafer CMP.

Chemical Mechanical Polishing of Glass Disk Substrates: Preliminary Experimental Investigation

Glass disk substrates are used in a wide range of portable devices because of their relatively high resistance to heat and shocks compared with aluminum substrates. Chemical mechanical polishing (CMP) is an available method to eliminate residual surface defects induced by grinding/lapping to meet final product requirements. In this work, CMP experiments with a low-cost fabric cloth pad were performed to study the effects of four key process factors on the material removal rate (MRR), flatness, and surface finish of polished glass disk substrates, and to explore the feasibility to achieve high machining efficiency, sub-nanometric surface roughness, and micrometric flatness of polished glass disk substrates simultaneously under optimized CMP conditions. The experimental results and analyses reveal that the interaction of pad rotational speed and polish head rotational speed had significant effect on the roughness (Ra), down force, and the interaction of down force and pad rotational speed had significant effects on the flatness, and down force and pad rotational speed had significant effects on the MRR. After compromise and prioritization analyses are performed, a combination of the four variables, which is the combination of 50 g/cm2 for down force, 40 rpm for pad rotational speed, 20 rpm for polish head rotational speed, and 100 ml/min for slurry supply velocity, is obtained to achieve high machining efficiency, sub-nanometric surface roughness and micrometric flatness of polished glass disk substrates simultaneously.

Finite element analysis of deflection and residual stress on machined ultra-thin silicon wafers

The demand for ultra-thin silicon wafers has escalated in recent years with the rapid development of miniaturized electronic devices. Residual stress generated in the thinning process has a great influence on the machining quality of ultra-thin wafers. This work has developed a 2D axisymmetric finite element (FE) model to predict the deflection and full-field residual stress of ground ultra-thin wafers. The FE model consists of two-layer structures, i.e. a damage layer induced by the thinning process and a bulk silicon crystal layer without defects. A series of uniform in-plane strains is applied to the damage layer to simulate machining-generated initial stress. A full-field residual stress distribution in a machined ultra-thin wafer is predicted with the developed FE model after the initial stress is released. Based on the FE model, effects of wafer geometrical dimensions and loaded initial strain (or stress) on the maximum compressive/tensile residual stress and the maximum wafer deflection are revealed. The model is finally verified by comparing the simulated wafer deflection with the measured value. Based on this work, the deflection and residual stress of a machined ultra-thin wafer can be conveniently predicted.

Material Removal Mechanism of Chemo-Mechanical Grinding (CMG) of Si Wafer by Using Soft Abrasive Grinding Wheel (SAGW)

An innovative fixed abrasive grinding process of chemo-mechanical grinding (CMG) by using soft abrasive grinding wheel (SAGW) has been recently proposed to achieve a damage-free ground workpiece surface. The basic principle, ideas and characteristics of CMG with SAGW are briefly introduced in this paper. The CMG experiments using newly developed SAGW for Si wafer are conducted at the condition of dry grinding. The grinding performances are evaluated and analyzed in terms of surface roughness, surface topography and surface/subsurface damage of ground wafer by use of Zygo interferometer, Scan Introduction ning Electron Microscope (SEM) and Cross-section Transmission Electron Microscope (Cross-section TEM). The component of product of ground Si surface is studied by X-ray Photoelectron Spectroscopy (XPS) to verify chemical reaction between the abrasive / additives of grinding wheel and Si wafer. The CMG process model by using SAGW is developed to understand the material removal mechanism and generation principle of damage-free surface. The study results show that the material removal mechanism of CMG by using SAGW can be explained as a hybrid process of chemical and mechanical action.

Chemical Mechanical Polishing (CMP) Processes for Manufacturing Optical Silicon Substrates with Shortened Polishing Time

Silicon substrates are used in optical components for infrared systems and mirror systems. An alternative to processing of optical silicon substrates is chemical mechanical polishing (CMP). The conventional CMP uses a three-body abrasion. In this work, a two-body abrasion CMP, called the fixed abrasive CMP, is performed on silicon substrates. Experiments are performed and results show that the material removal rate (MRR) is largely dependent on head load. Similarly, an increase in table speed would give the same effect. Compared to the conventional CMP, MRRs for the fixed abrasive CMP process are much higher. This high processing efficiency would drastically reduce the time for polishing of silicon substrates. It has been confirmed that the total polishing time can be shortened to a few minutes from hours.

Investigation on Material Removal Rate in Rotation Grinding for Large-Scale Silicon Wafer

In this paper, the formula of material removal rate (MRR) in wafer rotation grinding process is deduced based on kinematics. The main effect on MRR of the grit size and the process parameters, including the rotational speed of the cup grinding wheel, the down feed rate of the grinding wheel spindle and the rotational speed of the chuck table, is both theoretically and experimentally investigated. The influence on MRR of the cup wheel grinding status, the geometric dimension of the cup-grinding wheel, the rigidity of the grinding machine and the coolant is also analyzed. The investigating results show that, the increase of the grit size and the down feed rate of the cup grinding wheel results in great increase of the MRR; the MRR increases as the rotational speed of the cup wheel increases whereas the MRR reduces and the ground surface becomes bad due to size effect if the rotational speed of the cup wheel is overlarge; in normal grinding, the MRR decreases as the rotational speed of the chuck table increases. The results provide a theoretical basis to improve grinding efficiency, reduce grinding cost and select the proper parameters of grinding process.

A Novel Single Step Thinning Process for Extremely Thin Si Wafers

The demand for extremely-thin Si wafers is expanding. Current manufacturing technologies are meeting great challenges with the continuous decrease in Si wafer thickness. In this study, a novel single step thinning process for extremely thin Si wafers was put forward by use of an integrated cup grinding wheel (ICGW) in which diamond segments and chemo-mechanical grinding (CMG) segments are alternately allocated along the wheel periphery. The basic machining principle and key technologies were introduced in detail. Grinding experiments were performed on 8-in. Si wafers with a developed ICGW to explore the minimal wafer thickness and grinding performance. The experimental results indicate that the proposed grinding process with the ICGW is an available thinning approach for extremely thin Si wafer down to 15μm

Development of High-Efficiency and Crack-Free Grinding Process for Chamfering of LCD Glass Edge

Glass panels are one of core components in liquid crystal displays (LCDs). Grinding is an essential edge chamfering process in the preparation of LCD glass panel. With the size of glass panel increasing, both high productivity and high quality are required in the edge chamfering process. However, surface and subsurface defects are usually introduced to the chamfered glass edge under high-efficiency grinding conditions. In this work, we explored to develop crack-free grinding process while maintaining high chamfering efficiency with two designed diamond wheels for the chamfering of LCD glass edges. The grinding performance was compared and analyzed in terms of surface roughness and morphology. Normal and tangential grinding forces were measured to characterize the material removal characteristics. It was found that crack-free grinding/chamfering of LCD glass edge was achieved under high-efficiency grinding conditions i.e. wheel speed of 52.3 m/s, feed rate of 10 m/min, depth of cut of 50 μm. The developed grinding process is potential to reduce subsequent polishing time and cost or even replace subsequent polishing process for the preparation of LCD glass edge.

Effect of Wheel Additive On Chemo-Mechanical Grinding (CMG) of Single Crystal Si Wafer

Chemo-mechanical grinding (CMG) process is a promising process for large-sized Si substrate fabrication at low cost. However, effect of additive in CMG wheel is not completely understood yet. In this paper, three different CMG wheels were developed, in which one excluded additive and the other two contained two kinds of additive i.e. silicon dioxide and sodium carbonate. Grinding experiments were conducted to explore the influence of exclusion of additive and inclusion of different kinds of additive on CMG performance. The grinding characteristics of the three wheels were also analyzed and discussed to reveal the roles of wheel compositions in CMG process. This work provides some fundamental insights for the selection of different types of additive for optimization of CMG wheel.