Surface chemistry during plasma etching of silicon

Angle-resolved x-ray photoelectron spectroscopy ( X P S ) and laserinduced thermal desorption (LD), combined with laser-induced fluorescence (LIF) detection, were used to study the etching of olycrystalline Si (poly-Si) and single crystal Si(lO0) in high density (1-2 x 10fl ions/cm3), low presswe (0.510 mTorr) C12MBr-containing, helical resonator plasmas. The XPS measurements on both unmasked Si(100) and fine-line masked poly-Si samples were performed after the sample was etched and then transferred under high vacuum from the plasma reactor to the ultrahigh vacuum 0analysis chamber. The LD-LIF measurements on unmasked Si(lO0) samples were performed in-situ during etching. In these experiments, XeCl excimer laser pulses rapidly heat the Si surface to near the melting point, causing thermal desorption of SiCl. The tail of the same laser pulse excites Sic1 to the (B2C+) state in the gas-phase near the surface. The subsequent fluorescence signal from this state is proportional to C1-coverage, verified by XPS. In HBrcontaining plasmas, analogous LD-LIF detection was used for SiBr, providing a measure of Br coverage. The major findings of these studies are that Si surfaces rapidly become covered with a stable (in vacuum), saturated layer of about 2 monolayers of halogens during plasma etching. The layer consists of silicon mono-, di-, and tri-halides. In C12 plasmas, the C1 coverage increases with increasing ion energy, but is nearly independent of pressure (0.5-20 mTorr). Chlorination occurs rapidly with respect to the time required to etch one monolayer, at pressures as low as 0.5 mTorr. Consequently, the etching rate is limited by the ion flux, and not the neutral flux under these conditions. In mixed C12/HBr plasmas, the coverages of C1 and Br are simply proportional to the total respective halogen content of the feed gas. Other implications for etching mechanisms are discussed. INTRODUCTION Despite considerable effort, plasma etching mechanisms are still not well understood. In particular, deviations from ideal anisotropic etching are often observed and are the subject of many current investigations. Most experimental and theoretical studies have dealt with the gas-phase plasma physics and chemistry. Fewer studies have been performed on plasma-surface interactions, and most of these have either carried out surface analysis after etching, or have simulated plasma conditions in high-vacuum beam experiments. This high vacuum, non-plasma environment is necessary to allow conventional electron-detection-based surface analysis techniques to be used. To understand and better control etching processes, real-time surface diagnostic techniques are required that are not based on electron spectroscopy. Infrared (IR) optical absorption is one possible technique that can be applied, and one study of etching of Si in a chlorine plasma has recently been published (1). Surface IR absorption is weak, however, and Si-Cl vibrational frequencies are low, so that special wedged substrates with buried metal layers are required to obtain sufficient sensitivity. Recently, we reported a laser desorptionlaser induced fluorescence (LD-LIF) technique for measuring Cl and Br coverages on Si during etching in high-density HBr/C12 plasmas (2)-(4). Laser pulses are used to rapidly heat the surface and desorb Simonohalide which is in turn detected by laser-induced fluorescence. The LD-LIF technique can be