Abstract
A novel approach of chemical mechanical polishing (CMP) is developed for cadmium zinc telluride (CdZnTe or CZT) wafers. The approach uses environment-friendly slurry that consists of mainly silica, hydrogen peroxide, and citric acid. This is different from the previously reported slurries that are usually composed of strong acid, alkali, and bromine methanol, and are detrimental to the environment and operators. Surface roughness 0.5 nm and 4.7 nm are achieved for Ra and peak-to-valley (PV) values respectively in a measurement area of 70 × 50 μm2, using the developed novel approach. Fundamental polishing mechanisms are also investigated in terms of X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. Hydrogen peroxide dominates the passivating process during the CMP of CZT wafers, indicating by the lowest passivation current density among silica, citric acid and hydrogen peroxide solution. Chemical reaction equations are proposed during CMP according to the XPS and electrochemical measurements.
Highlights
Increase in the scanning area to 5 × 5 μm[2], Ra and PV values increase to 0.94 and 20.2 nm, correspondingly[14]
Surface roughness Ra reaches to 1.8 nm measured by atomic force microscopy (AFM) with a scanning area of 2 × 2 μm[2], which is produced after mechanical polishing by free abrasives of alumina and diamond, followed by chemical etching of BM20
Iodine is dissolved in methanol replacing bromine in chemical etching of CZT wafers after mechanical polishing by diamond pastes, and Ra and PV values are 1.563 and 15.85 nm[21], respectively with a measurement area of 180 × 130 μm[2]
Summary
The as-received Cd0.96Zn0.04Te (111) wafers were 10 mm in length, 10 mm in width, and 1.5 mm in thickness, which were grown by the modified Bridgman method[1]. SiC waterproof papers were put on a stainless steel plate as lapping pads. CZT wafers were lapped using SiC papers with mesh sizes in a sequence of 2500, 5000, and 8000, and lapping time was set at 3, 2, 2 min, respectively. The pressure of the lapping plate was 20 kPa, and rotation speeds of both the CZT wafers and SiC papers were 65 rpm. The CZT wafers were cleaned using deionized wafer and dried by compressed air for further characterization by an optical microscope (Olympus). The SiC papers were replaced by floss polishing pads on the stainless steel plate. The pH value of the CMP slurry varied from 4 to 4.5, which was decreased by the citric acid. During CMP, the rotation speeds of both the CZT wafers and floss polishing pads were 65 rpm. The pH values of H2O2, citric acid, and mixed slurry consisting of H2O2, SO2, and citric acid were 2.89, 4.45, 7.61, respectively
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