Effect of BCl3 in chlorine-based plasma on etching 4H-SiC for photoconductive semiconductor switch applications

Abstract

Inductively coupled plasma reactive ion etching (ICP-RIE) of n-type SiC epitaxial layers grown on (0001¯) 4H-SiC semi-insulating substrates has been investigated using chlorine-based plasma. The etch rate and postetching surface morphology have been studied as functions of the plasma composition, ICP power, RIE power, and process pressure. The authors found that the surface smoothness of the epitaxial layer was increased by introducing BCl3 into Cl2/Ar plasma. An optimized process has been developed yielding etch rates of ∼220 nm/min and very smooth surfaces with root mean square roughness of ∼0.3 nm. The spatial-frequency dependence of the one dimensional power spectral density was interpreted using the surface height function h(x) including a low-frequency range, which exhibits saturation and a high-frequency range, which exhibits scaling properties. Through this etching process, the effects of subcontact doping on 4H-SiC photoconductive semiconductor switch (PCSS) performance were investigated. A PCSS was fabricated using this etching process with a 1 μm heavily doped (1.6 × 1018 cm−3 n-type) epitaxial layer beneath the device contacts and compared with a PCSS fabricated with a subcontact doped layer created through laser enhanced diffusion (∼50 nm depth, 2.0 × 1018 cm−3 n-type). The PCSS with the epitaxial layer demonstrated on average a 30% reduction in minimum on-state resistance, and eliminated cracking of the bulk material when switching currents ≤38 A.