Selective Dry Etching of the GaN/InN/AlN, GaAs/AlGaAs and GaAs/InGaP Systems
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ABSTRACT Selective etching of InN over GaN and AIN, and of GaAs over both AIGaAs and InGaP was examined with a number of different plasma chemistries under inductively coupled plasma conditions. Selectivities up to 55 for InN/GaN and 20 for InN/AIN were achieved in IC1/Ar discharges. For GaAs/AIGaAs, maximum selectivities of 75(with BCI3 /SF 6) were obtained while for GaAs/InGaP values of 80(with BC13/SF 6) and 25(with BC13 /NF 3) were achieved. Selective etching of InGaP over GaAs is possible with either CH4/H 2 or B13. The selectivity is a strong function of ion flux and ion energy, and can result from two factors - either formation of a nonvolatile etch product, or a difference in bond strength between the two materials.
INTRODUCTION There are a number of device processing steps where high selectivity etch removal for GaAs over AlGaAs or InGaP is required[ I]. The first is the gate recess for high electron mobility transistors, where the ohmic contact layer must be removed to expose the donor layer for subsequent gate metal deposition. The second is in the emitter mesa process for heterojunction bipolar transistors, where it is desirable to remove the GaAs contact layer to the wide bandgap emitter first, prior to etching the emitter and exposing the base for contacting. There is an extensive literature on selective wet and dry etching of GaAs over A1GaAs and InGaP[2-1 1]. For wet etching, most attention for the GaAs/AlGaAs system has focused on the N1 4 0H/H 2 0 2 , the peroxide ammonia, or PA etch[2,5,6], and citric acid/H 20 2 formulations[ 1,2,8]. The advantage of the latter is that it does not attack the copolymer resist often used to produce the T-gate pattern for submicron HEMT devices. For the GaAs/InGaP system, H3PO 4/HCI/H 20 solutions provide selective removal of InGaP[ 10], while virtually any other set of acids that do not contain HCI will selectively etch GaAs from an underlying InGaP layer[10]. For dry etching, GaAs can be removed from AIGaAs using plasma chemistries involving a combination of C12, which produces the etching reaction and F2, which provides the etch stop reaction with AI[2-4,11-8]. Typical gas chemistries include BCI3/SF 6, SiC14/SiF 4 and, before its restriction due to its ozone depleting properties, CCI 2F 2. There is currently no selective etch for AIGaAs over GaAs. For the GaAs/InGaP system, GaAs can be selectively etched in any C12-based plasma[1O], while CH 4/H2 can remove InGaP from GaAs. Virtually all of the selective dry etching has been performed under reactive ion etching(RIE) conditions, and there is little information available for the newer high density tools, particularly Inductively Coupled Plasma(ICP) systems. The III-nitrides are of great current interest for applications in full-color displays, undersea communications, data storage, UV solar-blind detectors and high power/high temperature electronics[ 1215]. Little attention has been paid to development of selective dry etching processes for nitride materials, since most of the work to date has focused on fabrica
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