Ex situ variable angle spectroscopic ellipsometry studies of electron cyclotron resonance etching of Hg 1−xCd xTe

Abstract

The development of low damage anisotropic etching is important for the fabrication of mercury cadmium telluride (Hg 1− x Cd x Te) based IR devices. Typical methane-hydrogen chemistries have been shown to etch Hg 1− x Cd x Te causing changes in the surface composition and roughness. To investigate these changes, Hg 1− x Cd x Te samples with (111)A and (111)B orientations were systematically etched in hydrogen and methane mixtures, with 0%, 5%, 10%, and 20% methane in hydrogen using an electron cyclotron resonance plasma etching tool. Results were compared with reactive ion etched samples. Ellipsometry, atomic force microscopy (AFM) and scatterometry show that as etching proceeds there is considerable surface roughening. Quantitative measurements of roughness as measured by the three techniques reveal that etching causes vertical roughening between 2 nm and 20 nm. AFM measurements correlated well with scatterometry measurements. Ellipsometry measurements only weakly correlated with the other techniques owing to the sensitivity of ellipsometry to roughness features less than the wavelength of light as analyzed using effective medium theory. Ellipsometry results show that etching changes the surface alloy composition as well as roughness, and in several cases resulted in apparent vertical grading of composition. The addition of a few per cent methane in hydrogen reduced surface roughness.