Chemical mechanical polishing in the dry lubrication regime: Application to conductive polysilicon

• Boron-doped polysilicon is widely used in chip fabrication, however boron as dopant hinders the CMP significantly.
• The overburden can become as thick as 10–15 μm, hence the removal rate has significant impact on the fabrication cost and time.
• Mechanical interactions are mainly responsible for the heavily boron-doped polysilicon CMP.
• Highest polish rates were achieved under dry friction while using less slurry.
• Polishing boron-doped polysilicon under dry friction increases the polish rate more than 2× and reduces the slurry consumption and waste more than 8×.

Highly boron-doped polysilicon is a fill candidate for through silicon via technology where the CVD polysilicon overburden is removed by chemical mechanical polish. However, for highly boron-doped polysilicon, increases in doping have a very large effect of polish removal rate, exemplified by a 70% increase in doping causing a >3× decrease in removal rate. In this paper we develop understanding of the role of chemical and mechanical effects on removal and advance a model to explain our results. Based on this model we develop an optimum removal process and also predict other strategies that could be used to further improve the process. The optimum polish outcome with a maximum polish rate of 0.37 μm/min, minimum slurry consumption and acceptable surface quality was achieved under maximum mechanical forces and dry friction polishing at 120 rpm, 6 psi, 50 ml/min and at room temperature. This process is twice as fast as and reduces slurry consumption by a factor of 8 when compared to prior work. We predict that cooling the slurry/pad below room temperature will further increase the removal rate.