Photostimulated desorption in laser-assisted etching of silicon

Abstract

Photodesorption of SiF3 groups, which are the principal adsorbates on a silicon surface during etching by XeF2, is found to be responsible for the etch-rate enhancement observed under illumination by lowpower, cw band-gap radiation. It is proposed that desorption is stimulated by a photogenerated-chargecarrier-mediated chemical reaction, and not the simple charge trapping and recombination mechanism usually invoked for desorption from semiconductor surfaces. 73.25.+i, 82.65.My When surface reactions produce volatile products, particularly under bombardment by energetic particles such as photons, it can be difficult to identify the role of radiation in the promotion of both reaction and desorption steps. This is particularly so when pulsed lasers are involved, since steep temporal and spatial gradients in temperatures 1 and collisions above the surface 2 can complicate the identification of intrinsic surface-photochemical and surface-physical processes tremendously. Despite the complexities, it has been learned that substrate excitations, direct and indirect, are central to reaction and desorption whether pulsed or continuous light sources are used. Resonant photodesorption occurs when a specific transition of an adsorbed species is excited with sufficient energy to result in desorption after the energy has been delocalized to vibrations of atoms involved in bonding to the substrate, 3 and to the substrate itself. Photodesorption also occurs as a result of substrate heating, 1,a as well as much lower levels of phonon excitation. 4 Substrate electronic excitations by x rays or high-energy electrons are known to lead to desorption via electronic transitions of adsorbates to states which are not strongly bonded to the surface. 5 Desorption induced by valence-band electron-hole pair excitations in semiconductors, of specific interest in this work, has not been as well studied. Desorption of NO from Si (Ref. 6) and CO2 from metal-oxide surfaces 7,8 by low-power, cw band-gap radiation has been ascribed to trapping of photogenerated charge carriers, thus weakening surface bonding. Subband-gap extrinsic excitations may also be quite important, particularly at photon energies near the desorption threshold. 9,10 Desorption is not always the only process stimulated by the laser, and the interplay of carrier-induced chemical reaction and desorption, explored for two systems, NO (Ref. 6) and XeF2 (Ref. 11) on silicon, is particularly interesting. Dissociation and desorption of molecularly adsorbed NO both occur under illumination, appearing to be independent, competing channels. Photoreaction and desorption are tightly coupled in the XeF2-Si system, however, and it is the nature of that coupling which is discussed in this Letter.XeF2 reacts spontaneously with silicon in the dark, first forming a thick (10-20 A) layer whose main component is SiF3, 12 then volatile products, chiefly SiF4 but also including SiF3 under certain conditions. 13 Illumination of this surface with low-power, cw 515-nm light enhances the etch rate by...

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