Low Energy, High Density Plasma (ICP) for Low Defect Etching and Deposition Applications on Compound Semiconductors
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InP family are fragile, it is important to reduce the structural and electrical damages produced by chemically or physically active species in the plasma. Among all the plasma setting parameters, most of the time the damages are attributed to the high ion energy [1]. Several plasma etching techniques have been proposed to minimize the energy and still keep anisotropy and acceptable etching rates. The Inductively Coupled Plasma (ICP) that has been utilized in this study is a good candidate, since it can provide extremely high ion density independently of the ion energy. Using a hydrogen free chemistry with SiCI 4 and very low ion energy, the problem of the gate recess in the InGaAs/InAIAs/InP HEMT has been addressed since the electrical and structural properties of the surface of the AlInAs barrier after recess are extremely important for the performances of the HEMT. Furthermore the use of a dry process for this technological step is a warrant of uniform threshold voltage throughout a complete wafer and thus open the field of digital electronics for the InP HEMT[2]. 1.2 ICP Process The experiments are performed in an ALCATEL METLAB ICP source operating at 13.6MHz. The ICP etching process on InP has been recently studied [3], with a SiCI4 chemistry to avoid hydrogen diffusion and dopant compensation as well as sidewall polymer redeposition that are observed with the more conventional CH 4/H 2 chemistry. The surface stoechiometry of InP has been studied with Auger spectrometry and the electrical properties 189 Mat. Res. Soc. Symp. Proc. Vol. 573 ©1999 Materials Research Society
with photoluminescence. The variation of these properties are shown in figure 1 as a function of ion energy (the plasma potential is not accounted for) for a fixed ion density of around 1010 cm 3 . As can be seen from this figure, the structural and electrical disorders are located in a surface region that is only 3 nm thick when the ion energy is set at 30 eV. Most of the work that has been reported to date on ICP process utilizes bias voltages on the order of 30 to 50 V combined with high process pressure (over 1 Pa) in order to obtain high etch rates. Actually etch rates as high as 1ýl/mn have been reported on InP [3]. In this work the lowest bias voltage that can be set on the plasma has been used in order to slow down the rate and thus obtain a controlable etching depth with a minimun amount of ion induced damages. ? 10
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Figure 1: surface stoichiometry and photoluminescence recovery as a function of ion energy of a SiC14 ICP plasma on an InP substrate. A HEMT like structure was designed with a 10 nm GaInAs cap layer and a 800nm AlInAs barrier layer MOVPE grown on a S doped InP substrate. The recess of the 1Onm thick cap layer has been attempted with an ICP process where the ion energy was reduced to 13 eV. The ion density was kept at around 1010 cm 3 , and the substrate held at room temperature. Since the gate recess of the HEMT requires a rep
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