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Quality of Wafers in Semiconductor Devices

In semiconductor device fabrication, it is essential that wafers are micro-scratch free and uniformly flat. However, such quality is hard to achieve, and requires a sophisticated polishing procedure known as chemical-mechanical planarization (see Figure 1):




Figure 1  Schematic diagram of chemical-mechanical planarization


Scientists and engineers have long noticed that the polishing procedure creates micro-scratches, a serious defect that can ultimately lead to the wafer being scrapped. Although much effort had been devoted to understanding this issue, deeper insight was still needed.

The reference presents a study showing that the main cause of micro-scratches is not the moving abrasive particles introduced by the slurry, but the particles that are trapped in the polishing pad. ADINA was used in this study to simulate how the particles interact with the wafer and the pad.

The simulation uses four parts: the wafer, the particle, the pad, and the slurry. Fluid-structure interaction conditions were applied to the surface of the particle, wafer, and pad that were in contact with the slurry fluid.

The simulation revealed that the particles could be completely stuck in the pad, resulting in structural changes on the pad surface. This in turn changes the mechanical properties of the pad (such as its stiffness and hardness) and also the frictional properties, causing the micro-scratches. The formation of the pad-particle mixture can be seen in the movie on top and the snapshots in Figure 2, where we only show the results on the solid model. A snapshot of the slurry flow around the particle is shown in Figure 3.





Figure 2  Snapshots of the pad-particle mixture formation (wafer, particle, and polyurethane pad)






Figure 3  Snapshot of slurry flow



The study also tackled the wafer non-uniformity problem, another serious defect during wafer preparation. Most scholars in previous research applied macro-scale models (no particle included) to simulate the wafer-pad interaction and employed the contact pressure between the wafer and the pad as an indicator of the wafer material removal rate. However, as observed in experimental tests, the material removal rate is directly related to the stress distribution in the abrasive layer, not the contact pressure on the wafer surface. A macro-scale approach fails to predict the higher stresses at the edge of the wafer, and is not able to explain the wafer non-uniformity problem — a micro-scale model is necessary.

In the reference, a model is given that represents the particles in detail and takes the fluid-structure interaction effect into account. Two cases are simulated, one with moving particles and one with trapped particles. It is found that the condition of trapped particles correlates better with the experimental data, see the movie below





This study is an example as to how the use of ADINA can help identify and study important physical phenomena that include multiphysics effects — there are of course numerous most valuable research studies in which ADINA has been and is being used.

Reference

  • Sung, I.-H., Kim, H.J., and Yeo, C.D. "First observation on the feasibility of scratch formation by pad-particle mixture in CMP process", Applied Surface Science, 258(2012): 8298-8306. All figures and movies were kindly provided by the authors.


Keywords:
Wafer, semiconductor, chemical-mechanical planarization, fluid-structure interaction, multiphysics, slurry, abrasive


Courtesy of I.-H. Sung et al, Hannam University, Republic of Korea

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