Surface Cleaning And Chemical Etching Of SiC Single Crystals

High quality space optical systems put strict demands on the surface machiningquality of SiC space mirror reflectors. Chemical mechanical polishing (CMP) hasbeen traditionally employed as the final polishing step for SiC space mirrorreflectors; however, CMP produces surface and sub-surface damage layers whichare undesirable for long-term stable operation of reflectors. In this thesis, we presentresults on cleaning and chemical etching of SiC single-crystals, aiming to gainmechanistic insights into the damage-free polishing technique.In the first part, results on cleaning of6H-SiC single crystals are presented.Using atomic force microscopy, XPS and contact Angle measurements, we evaluateperformance of various cleaning methods for different samples. For samples, freshlytaken out from the package, immersion in the sulfuric acid plus hydrogen peroxidemixture (SPM) shows best cleaning performance in terms of removing organiccontaminants. For samples unpackaged and exposed to lab air, scrubbing withdetergent solution together with SPM immersion is better than other treatments. Forused samples soaked in HF, sequentially scrubbing and sonicating with detergentssolutions together with SPM immersion are required to achieve desired cleaningperformance. The results obtained lay a firm foundation for the following etchingand surface characterization studies.In the second part, results on chemical etching of SiC single crystals arepresented. Etching behaviors of samples in direct contact with Pt, Au, Ag wires andparticles in various solutions are studied. Firstly, etching behaviors with three formsof Pt–wires, powders and nanoparticles–are compared. Etching pits anddissolution of steps are found on surface areas contacting Pt wires and powders onlyin HF solution, and the HF concentration as well as immersion time effects arerevealed. The greater the HF concentration, the more obviously the SiC surfaceetching; along with the increase of immersion time, the surface morphology changegradually until etching pits occurred. During the etching research, we found that theshape and size of pits are related to the surface morphology of Pt particles; Ptpowders will be desired for site-specific etching studies. In contrast, Ptnanoparticles deposited onto samples show no etching enhancement effect.Secondly, Etching behaviors of samples in direct contact with Au and Ag powdersin various solutions are studied. No etching pits are found around these powders.The results reported in this part provide microscopic evidence of Pt promotedetching of SiC crystals, lay a firm foundation for the future damage-free polishingstudies. The research on SiC’s surface cleaning methods provides a simple and feasiblepretreatment for the research on surface properties and surcace treatment of SiC; theresearch on the precious metal catalyst dissolving etching mechanism of SiC fromthe micro level for the first time provides a crtain theoretical support for the researchon the polishing technology.