Strengthening the case for Plasma Polish

Three pioneering metrology techniques confirm that the Plasma Polish production process eradicates subsurface damage in SiC substrates while improving crystal quality.

BY GRANT BALDWIN AND JAMES SAGAR, OXFORD INSTRUMENTS PLC

Back in September 2022 at the International Conference for Silicon Carbide and Related Materials (ICSCRM), held in Davos, Oxford Instruments, launched Plasma Polish. Originally conceived as either a replacement or a complementary process to a chemical mechanical polish (CMP), this alternative plasma-based process has been attracting significant interest from leading substrate manufacturers and vertically integrated device manufacturers. Wafers that have undergone Plasma Polish during processing into diodes and MOSFETs have a performance that’s in line with industry standard CMP, according to full wafer electrical measurements and yield results (see Figure 1).

The non-contact, dry Plasma Polish process has several key benefits including lower cost, a reduced environmental impact and the ability to remove subsurface damage.

Plasma Polish requires low levels of consumables and creates no toxic slurry, leading to an operating cost reduction of up to 85 percent, resulting from the removal of these process requirements and their associated costs. The absence of the slurry also means that there are no disposal issues and removes the significant water use typically associated with CMP, making Plasma Polish better for the environment. Plasma Polish uses industry standard non-toxic process gasses, making it unproblematic to install in production facilities.

The third benefit, subsurface damage removal, is unique to Plasma Polish. This great strength, discussed further below, comes without having to make any changes to either upstream or subsequent processes.

Figure 1. Plasma Polish achieves a high yield on full-wafer device tests.

Assessing crystal quality
We originally conceived of Plasma Polish as a technique for preparing SiC substrates for the subsequent growth of epitaxial layers. However, following significant market engagement, including demonstrations on customer material, more and more opportunities are emerging where Plasma Polish provides a solution to surface and subsurface damage.

At this year’s ICSCRM, held in Sorrento, Italy, we presented data that confirms that Plasma Polish removes subsurface damage. To prove this is the case, we developed breakthrough characterisation techniques by harnessing expertise from many parts of our business. Our validation of the subsurface damage removal capabilities of Plasma Polish comes from three metrology and crystal analysis techniques: contact-resonance atomic force microscopy; electron back-scatter diffraction, which is an electron microscopy-based analytical tool; and Raman microscopy.

Before setting out into the insights provided by these techniques, we’ll briefly explain how and why Plasma Polish works. It’s a technique that employs physical and chemical mechanisms able to access and remove subsurface damage, with the extent of this benefit dependent on the chamber design, its configuration and the process recipe. Wafers are processed in a reactor chamber on a radio-frequency-biased wafer stage at a low pressure – this enables the acceleration of the chemically reactive plasma gas onto the wafer surface. As any damaged or defective crystal is inherently weakly bonded, this unique form of plasma etch removes poor-quality material far faster than good-quality, strongly bonded crystal. Consequently, all that remains after a Plasma Polish is higher quality material exhibiting excellent crystallography. In stark contrast, while CMP provides planarisation (flattening), it fails to remove damaged material at a higher rate than good-quality crystal.