Plasma Etching of InP and Related Materials in Electron Cyclotron Resonance CH 4 /H 2 /Ar Discharges
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PLASMA ETCHING OF InP AND RELATED MATERIALS IN ELECTRON CYCLOTRON RESONANCE CH 4 /H 2 /Ar DISCHARGES S. J. PEARTON*, U. K. CHAKRABARTI*, D. JOHNSON** and C. CONSTANTINE** *AT&T Bell Laboratories, Murray Hill, NJ "**PlasmaTherm, Inc., Voorhees, NJ
A. P. KINSELLA*,
A. B. EMERSON*,
ABSTRACT We have investigated the etch rates, residual lattice damage, surface morphologies and chemistries of InP, InGaAs, AlinAs and GaAs plasma etched in electron cyclotron resonance (ECR) CH 4 /H 2 /Ar discharges. The etch rates of InP and InGaAs increase linearly with additional RF biasing of the substrate, and are approximately a factor of two faster than for GaAs. Under our conditions the etch rate of Al0 .52Ga 0 .48As is very low (-25 A - min-' ) even for the addition of 100 V RF bias. In all of these materials the residual damage layer remaining after dry etching is very shallow (-20 A) as evidenced from Schottky barrier height and photoluminescence measurements combined with wet chemical etching. InP shows significant P depletion with the addition of RF biasing during the ECR etching while GaAs retains a near-stoichiometric surface. Hydrogen passivation of shallow donors in n-type GaAs occurs to a depth of -3000,X during exposure to the CH 4 /H 2 /Ar discharge for long periods (60 mins). The surface morphologies of the In-based materials become roughened for etching with the addition of RF biasing while GaAs displays smooth, residue-free surfaces under these conditions. INTRODUCTION The increasing requirement for sub-micron feature sizes in advanced Ill-V semiconductor devices has led to renewed emphasis on developing dry-etch processesIll. Low pressure reactive ion etching (RIE) has demonstrated the ability to produce highly anisotropic features of this type but the relatively high ion energies (> 100 eV) required often lead to problems with low selectivity for different materials and the introduction of lattice disruption up to 1000 X from the surface. 121 For these reasons there has been intense recent interest in the use of various magnetically enhanced discharges to provide high density plasmas with low self-biases13 1. Prominent among this type of dry etching is the use of microwave electron cyclotron resonance (ECR) discharges in which free electrons in the plasma are forced to orbit about magnetic field lines while absorbing microwave energy. At the cyclotron resonance condition outer shell electrons from gas molecules in the discharge may also be liberated, leading to a very high degree of ionization in the plasma.1 41 Since the motion of the electrons is constrained by the external magnetic field, fewer are lost by collisions with the reactor walls than in a conventional RF plasma and therefore the plasma potential relative to ground is much lower. The resultant energies of ions reaching the sample to be etched are typically < 15 eV. Since this is less than the displacement threshold for damage in most semiconductors, ECR etching should lead to much lower levels of damage than conventional RIE processes. It is particu
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