Reactive ion etching (RIE) of the III–V binaries InAs, InP, GaAs, and GaSb in methane/hydrogen (CH4/H2) plasmas was shown by Werking et al. (1991) to give slow, well-controlled, anisotropic etching, but the underlying microscopic mechanism was left unestablished: the etch products were hypothesized rather than identified, and the rate-limiting step for each material was undetermined. The problem is to convert this empirical process picture into an established microscopic mechanism by (i) identifying and quantifying the volatile and nonvolatile etch products of each binary under the RIE conditions studied, (ii) determining which microscopic step (supply of reactive radicals, thermally activated surface reaction, or product desorption) limits the etch rate of each material — in particular explaining why GaSb alone shows a strongly thermally activated etch rate while InAs, InP, and GaAs are nearly temperature-independent — and (iii) characterizing the role of hydrocarbon polymer deposition at elevated chamber pressure. Later work substantially resolved the InP part by mass spectrometry (phosphine as the major phosphorus product; an organoindium species as the indium-containing product; indium removal controlling the rate), but for GaSb, InAs, and GaAs the species-level identification and rate-limiting-step questions remain open.

Background

Reactive ion etching (RIE) is a dry patterning technique in which a radio-frequency plasma generates chemically reactive radicals and ions that remove material from a masked surface; ion bombardment directed across the plasma sheath gives anisotropic (vertical-walled) profiles that wet etching cannot. For III–V compound semiconductors, the conventional chlorine-based chemistries fail for indium-containing compounds because the indium chlorides are not volatile at ordinary substrate temperatures, giving low etch rates and sloped sidewalls. A methane/hydrogen (CH4/H2) mixture offers an alternative: methyl radicals (CH3) are expected to form volatile metal–methyl compounds with the group-III elements (e.g. trimethylindium-like species) while hydrogen radicals form volatile group-V hydrides (e.g. AsH3, PH3, SbH3), a chemistry sometimes called metal-organic RIE. Werking, Schramm, Nguyen, Hu, and Kroemer (Applied Physics Letters 58, 2003–2005, 1991) characterized CH4/H2 RIE of InAs, InP, GaAs, and GaSb — motivated by the emerging InAs/GaSb/AlSb and InP device systems — using a 1:5 CH4:H2 flow at 10 mTorr with about −500 V cathode self-bias (the dc potential the driven electrode acquires relative to the plasma) and a titanium mask. They found smooth surfaces and nearly vertical sidewalls for all four binaries, with etch rates of roughly 75 Å/min (GaAs), 150 Å/min (InAs), 280 Å/min (InP), and 60 Å/min rising to 190 Å/min between 25 °C and 175 °C (GaSb). Three mechanistic observations anchored their discussion. First, the etch rates of InAs, InP, and GaAs were nearly temperature-independent from 25 °C to 175 °C, suggesting that desorption of volatile products is not the rate-limiting step for these materials and supporting — but not proving — the metal–methyl/hydride product hypothesis. Second, GaSb behaved differently: its etch rate rose steeply with cathode temperature, suggesting a different, thermally activated controlling step such as a temperature-dependent surface reaction rate. Third, raising the chamber pressure from 10 to 40 mTorr increased all etch rates but produced visible hydrocarbon polymer deposition on the masked surfaces. The authors explicitly noted that identifying the actual etch products and the rate-limiting mechanisms would require more sensitive experiments (such as species-resolved plasma diagnostics and in-situ surface analysis) than their ex-situ rate measurements, and subsequent literature has partially answered this call: for InP, quadrupole mass spectrometry during CH4/H2 RIE established phosphine as the major phosphorus etch product and evidenced an organoindium species as the indium etch product, and combined mass-spectrometric/optical/X-ray-photoelectron-spectroscopy studies showed the InP etch to be controlled by the indium removal mechanism. For GaSb, InAs, and GaAs, no such species-resolved identification exists, and the thermally activated GaSb behavior still circulates as an unconfirmed interpretation.

Problem Statement

Establish the microscopic etching mechanism of methane/hydrogen (CH4/H2) reactive ion etching of the III–V binaries InAs, InP, GaAs, and GaSb, of the kind called for by Werking et al. (1991). Concretely: (1) identify and quantify the primary volatile and nonvolatile etch products of each still-open binary (GaSb, InAs, and GaAs) under CH4/H2 RIE conditions — the specific group-III metal–methyl species and group-V hydrides; (2) determine the rate-limiting microscopic step for each of these still-open materials — supply/flux of reactive radicals, thermally activated surface chemical reaction, or product desorption — and in particular establish the microscopic origin of the distinct thermally activated GaSb etch rate (rising from about 60 Å/min at 25 °C to about 190 Å/min at 175 °C, while InAs, InP, and GaAs are nearly temperature-independent), thereby confirming or refuting the 1991 interpretations that desorption is not rate-limiting for InAs/InP/GaAs and that a temperature-dependent surface reaction rate controls GaSb; and (3) characterize the formation and role of hydrocarbon polymer deposition at elevated chamber pressure (tens of mTorr). The InP portion of this problem has been substantially resolved by later mass-spectrometric work (phosphine as the major phosphorus product; an organoindium species as the indium-containing product; indium removal controlling the rate); this known InP mechanism is context rather than a deliverable, so the open target is the remaining materials, above all GaSb.

References

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