CH 4 :H 2 :Ar rf/ECR Plasma Etching of GaAs and InP
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CH 4 :H2 :Ar rf/ECR PLASMA ETCHING OF GaAs AND InP VICTOR.J.LAW, S.G.INGRAM*, G.A.C.JONES, R.C.GRIMWOODt H.ROYALt. Cavendish Laboratory, Madingley Rd, Cambridge, CB3 OHE, UK. *Also at GEC-Marconi Ltd. Hirst Research Centre, East Lane Wembley, Middlesex HA9 7PP, UK tOxford Plasma Technology, Yatton, Bristol, Avon, BS19 4AP, UK. ABSTRACT A comparative study of CH 4 :H 2 , and CH 4 :H 2 :Ar rf-plasma and microwave electron cyclotron resonance (ECR) plasma etching of GaAs and InP is presented. The study is in two parts; (i) Kinetic studies of GaAs and InP etch rates as a function of the constituent gas flow rates, applied rf and microwave powers, substrate temperature and position. The results indicate that CH4 :H2 :Ar ECR etching of GaAs is lOx more efficient in the utilisation of the CH 4 precursor gas than rfplasmas. However, the absolute etch rates are lower (70 nm min- 1 for rf and 25 nm min- 1 for rf biassed ECR-plasmas) .The effect of etching conditions on InP morphology is also examined. (ii) The study of electrical "damage" in GaAs/AlGaAs high electron mobility transistor (HEMT) Hall bar structures, was investigated by ECR-plasma etching off the top GaAs capping layer. Results indicate that ECR-plasma etching with an rf-bias between OV and -30V does not significantly effect the electrical characteristics of such devices at 300K, with some degredation at 1.2 K. Introduction GaAs/AlGaAs and related InP semiconductor materials have all been rf-plasma etched using methane (CH4 ) [1,2,3], ethane (C 2 H6 ) (4,5] and propane (C3H 8 ) [6), with admixtures of H2 and Ar. More recently CH 4 with the substitution of the admix H2 by the noble gases; He, Ne, and Ar has been demonstrated to etch GaAs, the etch rate scaling with the ionisation efficiency of the admix gas [7] . ECR-plasma etching has also been shown to etch these materials (8], although at a reduced etch rate. The related electrical "damage" associated with the CH 4 :H 2 rf-plasma etching of GaAs/AlGaAs can be used to advantage when fabricating double barrier resonant tunneling structures (RTS) [9] . The active quantum well layers are shielded from the rfplasma by the ohmic contact metals which are employed as the etchant mask. In contrast, devices such as the two dimensional electron-gas (2DEG) base hot-electron transistor [10], requires the active layers to be exposed to the plasma during the etching process. The resultant "damage" to the active layers impairs the device performance when compared with standard wet chemical etching [11]. However, to realise the high frequency operation of such devices, lateral etch features of 111m have to be achieved with good anisotropy, which requires directional plasma etching. In this work, these fabrication requirements are investigated using a microwave ECR-plasma to etch bulk GaAs and InP samples. The induced "damage" is examined by etching the GaAs capping layers of HEMT Hall bars, and measuring their source-drain resistance. Mat. Res. Soc. Symp. Proc. Vol. 223. 01991 Materials Research Society
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