A new surface-flattening method using single-point diamond turning (SPDT) and its effects on LPE HgCdTe photodiodes

Abstract

Excellent surface flatness of HgCdTe wafers is essential for fabricating large infrared focal plane arrays (IRFPAs) within the framework of a flip-chip bonding yield. However, liquid-phase epitaxy (LPE) HgCdTe wafers, which are widely used for IRFPAs, have inherent problems pertaining to surface flatness. In this study, we introduced a single-point diamond turning (SPDT) method for use in fabricating LPE HgCdTe photodiodes. The cutoff wavelength of the wafers is ∼5 µm. The surface roughness of the LPE HgCdTe wafer has been substantially reduced but a type-converted defective layer was formed on the surface of the HgCdTe wafer after SPDT, which was confirmed using Hall and capacitance–voltage (C–V) measurements. The defective layer, however, was easily removed by bromine in methanol (Br–MeOH) etching. The fabricated photodiode showed a dynamic resistance–area product at the zero bias (R0A) value of ∼1 × 104 Ω cm2 for a junction area of 30 × 30 µm2 at 80 K, which is equivalent to that of a conventional photodiode. The flip-chip bonding efficiency has been remarkably improved from 89.43% to 99.99% for 320 × 256 IRFPA at room temperature after SPDT.