2.6 SiC Patterned Etching

Wet etching of SiC has proven to not be feasible from a practical point of view, as it requires molten salts (for example, NaOH-KOH at 350$~^{\circ}$C) to be used at high temperatures. The difficulty encountered in etching SiC is a result of the high bond strength, a property which makes SiC useful for high-temperature operation, but an obstacle for fabrication. Nonetheless, numerous dry etches have been employed. Most commonly employed processes involve reactive ion etching (RIE) of SiC in fluorinated plasmas and electron cyclotron resonance (ECR) etching. Sacrificial etch masks (often composed of aluminum) are deposited and photolithographically patterned to protect desired areas from being etched. A good overview of dry SiC etching results can be found in [90].

The SiC RIE process can be implemented using standard silicon RIE hardware, and typical 4H- and 6H-SiC RIE etch rates are in the order of hundreds of ångstroms per minute. Well-optimized SiC RIE processes are typically highly anisotropic with little undercutting of the etch mask, leaving smooth surfaces. One of the keys to achieving smooth surfaces is preventing micromasking wherein masking material is slightly etched and randomly redeposited onto the sample, effectively masking very small areas on the sample that were intended for uniform etching. This can result in grass-like etch-residue features being formed in the unmasked regions, which is undesirable in most cases. In special cases, RIE etching under conditions promoting micromasking is useful in greatly roughening the SiC surface to reduce the contact resistance of subsequently deposited ohmic metalizations.
Most published RIE etches all make use of fluorinated gases to etch SiC, although etch rates of 1900 Å min$^{-1}$ have been obtained in 6H-SiC using a chloride-based (C1$_2$/SiCl$_4$/O$_2$) etch with SiO$_2$ mask. RIE etch rates of 6H-SiC and 4H-SiC are typically slow in comparison to Si (300 Å min$^{-1}$ to 2000 Å min$^{-1}$), with residue-formation problems commonly found, although not prevalent in all etches. Etching of silicon in fluorinated gas has been found to occur by the reaction Si + 4F $\Rightarrow$ SiF$_4$.

While RIE etch rates are sufficient for many electronic applications, much higher SiC etch rates are necessary to carve features on the order of tens to hundreds of micrometers deep that are needed to realize advanced sensors, microelectromechanical systems (MEMS), and some very high voltage power device structures. High density plasma dry etching techniques, such as electron cyclotron resonance (ECR) and inductively coupled plasma (ICP), have been developed to meet the need for deep-etching of SiC. Residue-free patterned etch rates exceeding a thousand ångstroms a minute have been demonstrated [90,91]. Patterned etching of SiC at very high etch rates has also been demonstrated using photo-assisted and dark electrochemical wet etching [92,93]. By choosing proper etching conditions, this technique has demonstrated a dopant-selective etch-stop capability. However, there are major incompatibilities of the electrochemical process that make it undesirable for VLSI mass-production, including extensive pre-etching and post-etching sample preparation, etch isotropy and mask undercutting, and somewhat nonuniform etching across the sample.

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation