Plasma pretreatment for gallium nitride chemical mechanical polishing

Osaka University in Japan has developed a plasma pre-treatment for chemical mechanical polishing (CMP) on gallium nitride (GaN) that avoids the formation of enlarged etch pits [Hui Deng et al., Appl. Phys. Lett., vol. 107, p051602, 2015]. Surfa

Osaka University in Japan has developed a plasma pre-treatment for chemical mechanical polishing (CMP) on gallium nitride (GaN) that avoids creating enlarged etch pits [Hui Deng et al, Appl. Phys. Lett., vol. 107, p. 051602, 2015].

Surface atomic structure, roughness, and subsurface damage can adversely affect electronic performance. Chemical-mechanical polishing (CMP) is widely used to achieve atomic-level flatness in microelectronics. However, wide-bandgap materials—such as GaN, diamond, and silicon carbide—tend to be significantly harder than the mainstream electronic material, silicon, for which CMP was originally developed.

Another challenge for GaN CMP is that GaN is typically grown on foreign substrates such as sapphire, silicon carbide, or silicon. The mismatch in lattice and thermal expansion coefficients between GaN and the substrate leads to crystal defects. Etching or CMP tends to preferentially remove material around dislocations, resulting in etch pits. Indeed, the density of etch pits is a common metric used to assess the dislocation density of GaN.

The process was performed on commercial substrates with an 8μm undoped GaN epilayer on 2-inch sapphire.

The CMP was performed with 2,000 rotations per minute and a pressure of 3.74 kPa on the suede-type polishing pad from FILWEL Co., Ltd. (NP178). Two types of commercial slurry were tested—one containing silica (SiO₂). ₂ ) and the other ceria (CeO ₂).

The plasma for the 30-minute pre-treatment (Figure 1) consisted of carbon tetrafluoride in helium carrier gas at atmospheric pressure. The distance between the plasma generation electrode and the GaN surface was 1.6 mm. The electrode was 3 mm in diameter.

Figure 1: (a) Schematic of the experimental setup for plasma irradiation. (b) Image of atmospheric-pressure CF ₄plasma.

The action of the 18W 13.56MHz radio-frequency radiation was to dissociate fluorine radicals from the reactive part of the gas. Such radicals are known to react with GaN. X-ray photoelectron spectroscopy (XPS) found that the modified layer consisted of GaF. ₃ This layer was about 30 nm thick, according to cross-sectional transmission electron microscopy (XTEM).

Without the pre-treatment, the CMP with either slurry resulted in enlarged pits at dislocation sites. The effect was particularly bad for silica slurry.

The researchers only applied ceria slurry in the CMP tests on pre-treated material, since it was found to give better results on untreated GaN. After 8 minutes the protection layer was removed, but no enlarged pits were formed around the dislocations ('just-polished'). Continuing the CMP beyond this ('over-polished') created enlarged pits. Performing CMP for shorter than 8 minutes resulted in some of the GaF ₃ Protection layer remaining on the substrate ('under-polished').

The 8-minute CMP resulted in peak-to-valley surface roughness of 1.00 nm (Figure 2). The root-mean-square roughness was 0.11 nm.

Figure 2: Change in surface roughness of GaN processed by plasma irradiation and CMP.

The researchers expect high device performance in their future work from the combination of plasma pre-treatment and time-controlled CMP.

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