Investigation Of Environment-friendly Chemical Mechanical Polishing For Soft-brittle HgCdTe Crystals

Mercury cadmium telluride (HgCdTe or MCT) is a representative for third-generation soft-brittle semiconductors, and has excellent optoelectronic properties. MCT is widely used in infrered detectors, such as aeronautic, astronautic, military and economic fields. Ultrasmooth and damage-free wafers are required for high performance infrared MCT based detectors.MCT has soft and brittle nature, and is a kind of difficult-to-machine mateirials. Machining processes of MCT are different from both conventional plastic crystals, e.g. copper and steel, and hard-brittle ones, such as silicon, germanium and gallium arsenide. Ultraprecision cutting, grinding, and chemical mechanical polishing (CMP) are the main ultraprecision machining methods for MCT crystals. Among them, high precision setups are demanded for ultraprecison cutting and grinding, however whose pricsion of machining is not well as that of CMP. Free abrasives are widely used in traditional CMP for MCT wafers, which is easy to result in the embedding of free abrasives and chipping on MCT wafers, due to their soft and brittle nature. Bromine, methanol, lactic acid, and nitric acid are usually employed in the following CMP of MCT wafers. Furthermore, organic solution is used for the cleaning of MCT wafers. These chemical reagents used in the machining of MCT wafers have potential damages to both the environment and operators.Processes and mechanism of CMP for MCT wafers were focused in our work, which was based on the machining of substrates, cadmium zinc telluride, for MCT films, owing to the siginifcance of MCT and disadvantages of conventional machining on them. Fixed abrasive lapping and environment-friendly CMP were developed to machine MCT wafers, effectively eliminating the embedding of free abrasives and chipping of MCT wafers, conducted by fixed abrasive lapping. Environment-friendly CMP solution consisted mainly of silica and hydrogen peroxide. Maclic acid was employed to regulate the pH value of CMP solution developed. Material removal rate was increased and potential damages both to the environment and operators were avoided, using the developed environment-friendly CMP solution. During the final cleaning process, deionized water was used to flush the MCT wafers, followed by drying of compressed air, which is environment-friendly as well. Arithmetic average surface roughness, Ra of0.5nm was achieved by developed CMP solution, which is lower than that of1.4nm published in previous reports. A model was proposed to present the effects of pressure, rotational speed, and size of abrasives on the material removal rate. To investigate the CMP mechanism, X-ray photoelectron spectroscopy (XPS) and electrochemical workstation were applied to analyze the active function between MCT crystals and CMP solution.