Recent Improvements in Dry Etching of Hg 1-x Cd x Te by CH 4 Based Electron Cyclotron Resonance Plasmas
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A.STRACT Significant improvements of a previously reported etching process [1] for Hg IxCdxTe have been achieved with respect to etch rate, surface morphology and surface stoichiometry by optimization of the process parameters. The gas phase and surface reactions driving the etching process have been analyzed by combined optical and electrical characterization of the plasma and surface analyses of the samples. A reaction scheme is suggested which allows to model and upscale the process in a consistent manner. INTRODUCTION Dry etching is an important processing step improving the performance of infrared focal plane arrays [2-6] and required to design novel nanostructure IR devices [7] based on Hgl-xCdxTe. Electron cyclotron resonance (ECR) plasmas of CH 4 /H 2 mixtures are commonly used for etching of HgCdTe which proceeds by reaction of the CH 3 radical to form volatile compounds. According to CdTe + 4 CH 3
=* Cd(CH 3)2 (gas) + Te(CH 3)2 HgTe + 2 CH 3 = Te(CH 3 )2 (gas) + Hg (gas)
(gas)
(1) (2)
material is removed in the stoichiometric proportions of the Hgl-xCdxTe sample. Beside Cd(CH 3)2 and Te(CH 3)2 , residual gas analysis showed TeH 2 as an additional etching product of Hgl-xCdxTe [8]. Produced in the plasma from H2 or occuring as byproduct of methane dissociation atomic hydrogen etches HgTe according to HgTe + 2H => TeH 2(gas) + Hg (gas) CdTe + 2H =>' TeH 2 (gas) + Cd (solid)
(3) (4)
Since atomic hydrogen can break the bonds of HgTe, reaction (3) occurs at a rapid rate and leads to highly non-stoichiometric etching accompanied with considerable surface roughening. Moreover, hydrogen atoms diffuse rapidly in the Hgl-xCdxTe matrix and may cause a type conversion in p-material [9]. Hence, an optimization of the etching conditions should aim at supporting reactions (1) and (2) and inhibiting reactions (3) and (4). An additonal boundary condition for the etching process exists as to polymer deposition has to be avoided. The addition of hydrogen to the plasma is usually required since atomic hydrogen has an inhibiting effect on polymer deposition. Polymer formation is initiated by the precursors CH and CH 2 , which occur as additional byproducts of methane dissociation. 259 Mat. Res. Soc. Symp. Proc. Vol. 484 © 1998 Materials Research Society
Regarding the detrimental effects of hydrogen and polymers considerable improvements had been already achieved [1] by addition of nitrogen to the etching gas, by which polymer formation could be suppressed and smooth surfaces were obtained. The objective of this investigation was to get insight into the microscopic reaction mechanisms and their control by the external process parameters. EXPERIMENTAL The plasma reactor chamber [8] is equipped with an ASTEX compact electron cyclotron resonance (ECR) source (250 W) and a temperature controlled sample stage (30°C
•
(7)
(8) (9)
The beneficial effect of the nitrogen addition was traced back to the reactions (CH) + N 2ms (H)+N 2 m s
=> =
Nad + Had+ (H2) ='
CN + NH (NH)+N
(10) (11)
NH 3
(12)
Reactions such as (10)
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