Elementary processes in alkane-based reactive ion etching of III-V semiconductors
Creators: name: Meharg, Paul F. A.
Publisher: name: University of British Columbia
Year: 2008

description: Reactive ion etching (RIE) is a process used extensively in the microelectronics industry. Recently, the use of alkane-based plasmas has been developed for the etching of compound semiconductors. However, due to the complex nature of the plasma environment, relatively little is known about the etching mechanisms involved. To gain some understanding of the elementary processes involved in this technology, the interactions of some relevant ions and free radicals with these semiconductors were examined. To simulate the low energy ion bombardment that occurs in RIE using alkanes, GaAs and InP were exposed to 20-500 eV carbon ions, using a mass-separated carbon ion beam in an ultrahigh vacuum chamber. The changes induced by ion bombardment and the effects of subsequent damage-removal treatments were determined by angle-dependent X-ray photoelectron spectroscopy (XPS). To enable the effects of sputtering and ion penetration to be quantified with XPS, an InP wafer consisting of an ultrathin (4 nm) epitaxial In Player on InGaAs was used. Initially, carbon ion irradiation caused minor sputtering of the semiconductors and preferential removal of the group V constituents, with concurrent formation of carbon-semiconductor phases. The extent of the interaction increased with increasing bombardment energy. A chemically resistant, amorphous carbon residue formed after further bombardment. Whereas heating or ultraviolet light/ozone oxidation did not remove the damage, removal was effected by exposure to hydrogen ions. Similar results were observed when these semiconductors were exposed to low energy methyl ions (&lt;100 eV), except that the deposited amorphous hydrocarbon was readily removed by ozone oxidation treatments. Methyl radicals are suspected as likely etchant species. The methyl ion impact energy was therefore lowered to 3 eV, an energy where the CH3+ species should remain intact on the surface. No sputtering or etching was observed at room temperature. However, when the surface was heated to 350 °C during the bombardment with 3 eV methyl ions, etching of InP was observed. In addition to methyl radicals, hydrogen atoms are believed to be the major etchant species in alkane plasmas. Therefore, GaAs samples were exposed to thermalised hydrogen atoms produced in a remote microwave-driven molecular hydrogen discharge. The absolute atom pressures were measured by a calorimetric technique, and varied from 20 to 50 mTorr. GaAs was found to etch continuously with etch rates between 10 and 40 nm min-1 and at temperatures ranging from 180 to 375 °C. An Arrhenius activation energy of 20.2 ± 3.9 kJ mol-1 was determined. The resultant surfaces were rough, exhibited crystallographic features, and were arsenic deficient. Addition of methane into the hydrogen atom stream increased the etchrate of GaAs by up to an order of magnitude. Using the well established rate constants for the reactions of hydrogen atoms and methane and the flow parameters of the apparatus, a model was developed to calculate the concentration of methyl radicals at the etching surface. The etch rates obeyed a first order dependence on the calculated methyl radical concentration. The first order rate constants were large 5x105 nm min-1 Torr-1) but were limited by the diffusion of the methyl radicals to the surface. The resultant surface morphology was rough and crystallographic etch features were always observed. Arsenic deficient surfaces also resulted, but were less depleted than hydrogen atom etched surfaces. Gallium-containing residues were deposited on the reactor surfaces. The implications of these results for alkane-based RIE are discussed.