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-induced 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 gas. By varying the laser repetition rate we determined that the surface is chlorinated with an apparent first-order time contant of approximately 6 ms at 1.0 mTorr, and 20 ms at 0.3 mTorr. Therefore in the plasma at pressures above approximately 0.5 mTorr, the SiClx(ads) layer reaches 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), substrate 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 and Cl2), limits the etch rate, even in a low pressure, high-charge-density plasma. The chemically enhanced Cl2+ sputtering yield is 0.38 at an ion energy of 50 eV and 0.60 at 125 eV. Because of the relatively low neutral-to-ion flux ratios (approximately 2:1 at the lowest pressures) compared to reactive ion etching conditions, a substantial portion of the chlorine needed to form volatile products can be provided by the 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 measurements on samples that are transferred under ultrahigh vacuum to an analysis chamber provide information on the surface as it was during etching. The SiClx(ads) coverage and etch rate decreases with increasing addition of O2 to Cl2, due to the competition for adsorption sites by O.