This study investigates warping of silicon wafers in ultra-precision grinding-based back-thinning process. By analyzing the interactions between the wafer and the vacuum chuck, together with the machining stress distributions in damage layer of ground wafer, the study establishes a mathematical model to describe wafer warping during the thinning process using the elasticity theory. The model correlates wafer warping with machining stresses, wafer final thickness, damage layer thickness, and the mechanical properties of the monocrystalline silicon. The maximum warp and the warp profile are measured on the wafers thinned to various thicknesses under different grinding conditions, and are used to verify the modeling results.

Warping of silicon wafers subjected to back-grinding process

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.

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

Although diamond grinding is the most commonly used machining technique in silicon wafer thinning, it often induces edge chipping which leads to wafer breakage. This study investigates edge chipping of silicon wafer in diamond grinding. The study correlates edge chipping with the crystallographic orientation and thickness of a silicon wafer, as well as grinding process conditions, such as wheel grit size, grinding mode and feed rate. It identifies edge chipping in terms of critical thickness, geometry and dimensions. The study discusses the mechanisms of edge chipping based on machining mechanics and energy theories. Conclusions are drawn to summarize the study.

Edge chipping of silicon wafers in diamond grinding

Elimination of surface scratch/texture on the surface of single crystal Si substrate in chemo-mechanical grinding (CMG) process

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.

http://iopscience.iop.org/article/10.1088/0268-1242/26/10/105002/pdf

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

Grinding process on the Si wafer develops subsurface damage, which remarkably degrades deflective strength of the wafer and constitutes a barrier against producing a thin wafer for low-profile packaging. In this paper, the authors propose a new index for evaluation of the Degree of Subsurface Damage (DSD). Requiring no costly instrument, the new index is easily calculated via the external geometry of the ground wafer. With the new index, it is able to quantitatively evaluate the subsurface damage introduced by different processes (or wheel) and to estimate the minimally achievable thickness of the wafer by each process. Also, a novel fixed abrasive process of Chemo-Mechanical Grinding (CMG) has been proposed for stress relief. All results indicate that the subsurface damage after CMG is nearly zero.

Fabrication and evaluation for extremely thin Si wafer

In this paper, we study and evaluate the subsurface damage of the ground wafers to understand the effect of residual stress on the wafer deflection. The experimental results show that two indexes of depth of the damaged layer and degree of the residual stress are directly associated with the warpage of wafer. The degree of the damage decreases with an increasing in grit size of diamond wheel. The theoretical analysis suggests that the minimally achievable thickness of wafer is proportional to the degree of the damage introduced by respective process.

Bahman Soltani Hosseini

Papers

Fabrication and evaluation for extremely thin Si wafer

Study on Subsurface Damage Generated in Ground Si Wafer