IN-SITU PULSED LASER-INDUCED THERMAL-DESORPTION STUDIES OF THE SILICON CHLORIDE SURFACE-LAYER DURING SILICON ETCHING IN HIGH-DENSITY PLASMAS OF CL2 AND CL2 O2 MIXTURES/

We have used laser-induced thermal desorption, combined with laser-ind uced fluorescence of SiCl(g) to study, in real time, the Si-chloride ( SiClx(ads)) layer that is present on the surface during Si etching in a high-plasma density, low pressure Cl2 helical resonator plasma. The SiClx(ads) layer that builds up during etching contains about twice as much Cl as the saturated layer that forms when Si is exposed to Cl2 g as. By varying the laser repetition rate we determined that the surfac e is chlorinated with an apparent first-order time contant of approxim ately 6 ms at 1.0 mTorr, and 20 ms at 0.3 mTorr. Therefore in the plas ma at pressures above approximately 0.5 mTorr, the SiClx(ads) layer re aches saturated coverage on a time scale that is short compared to the time required to etch one monolayer (40 ms). From the weak dependence of the SiClx(ads) layer coverage on discharge power (0.2-1 W/cm3), su bstrate bias voltage (from 0 to -50 V dc), and pressure (0.5-10 mTorr) , we conclude that ion flux, and not neutral etchant flux (i.e., Cl an d Cl2), limits the etch rate, even in a low pressure, high-charge-dens ity plasma. The chemically enhanced Cl2+ sputtering yield is 0.38 at a n ion energy of 50 eV and 0.60 at 125 eV. Because of the relatively lo w neutral-to-ion flux ratios (approximately 2:1 at the lowest pressure s) compared to reactive ion etching conditions, a substantial portion of the chlorine needed to form volatile products can be provided by th e impinging ions. The SiClx(ads) layer does not change appreciably (< 10% decrease in Cl coverage) after the plasma is extinguished and the gas is pumped away. Consequently, post-etching surface analysis measur ements on samples that are transferred under ultrahigh vacuum to an an alysis chamber provide information on the surface as it was during etc hing. The SiClx(ads) coverage and etch rate decreases with increasing addition of O2 to Cl2, due to the competition for adsorption sites by O.