Cl atom recombination on silicon oxy-chloride layers deposited on chamber walls in chlorine–oxygen plasmas

Abstract

Chlorine atom recombination coefficients were measured on silicon oxy-chloride surfaces deposited in a chlorine inductively coupled plasma (ICP) with varying oxygen concentrations, using the spinning wall technique. A small cylinder embedded in the walls of the plasma reactor chamber was rapidly rotated, repetitively exposing its surface to the plasma chamber and a differentially pumped analysis chamber housing a quadruple mass spectrometer for line-of-sight desorbing species detection, or an Auger electron spectrometer for in situ surface analysis. The spinning wall frequency was varied from 800 to 30 000 rpm resulting in a detection time, t (the time a point on the surface takes to rotate from plasma chamber to the position facing the mass or Auger spectrometer), of ∼1–40 ms. Desorbing Cl2, due to Langmuir–Hinshelwood (LH) Cl atom recombination on the reactor wall surfaces, was detected by the mass spectrometer and also by a pressure rise in one of the differentially pumped chambers. LH Cl recombination coefficients were calculated by extrapolating time-resolved desorption decay curves to t = 0. A silicon-covered electrode immersed in the plasma was either powered at 13 MHz, creating a dc bias of −119 V, or allowed to electrically float with no bias power. After long exposure to a Cl2 ICP without substrate bias, slow etching of the Si wafer coats the chamber and spinning wall surfaces with an Si-chloride layer with a relatively small amount of oxygen (due to a slow erosion of the quartz discharge tube) with a stoichiometry of Si:O:Cl = 1:0.38:0.38. On this low-oxygen-coverage surface, any Cl2 desorption after LH recombination of Cl was below the detection limit. Adding 5% O2 to the Cl2 feed gas stopped etching of the Si wafer (with no rf bias) and increased the oxygen content of the wall deposits, while decreasing the Cl content (Si:O:Cl = 1:1.09:0.08). Cl2 desorption was detectable for Cl recombination on the spinning wall surface coated with this layer, and a recombination probability of γCl = 0.03 was obtained. After this surface was conditioned with a pure oxygen plasma for ∼60 min, γCl increased to 0.044 and the surface layer was slightly enriched in oxygen fraction (Si:O:Cl = 1:1.09:0.04). This behavior is attributed to a mechanism whereby Cl LH recombination occurs mainly on chlorinated oxygen sites on the silicon oxy-chloride surface, because of the weak Cl–O bond compared to the Cl–Si bond.

Plasma etching: Yesterday, today, and tomorrow

“…It is much better to detect these species directly by UV absorption spectroscopy 120 or mass spectrometry. 121,122 Weak emission from Cl þ in the UV and visible regions is also observed in high-density Cl 2 plasma emission spectra, as shown in the expanded spectrum in Fig. 11 (blue, upper trace).…”

“…Another approach, called the "spinning wall" method inserts a cylindrical substrate into the reactor wall. 109,121,122,[140][141][142][143][144][145][146] Part of the cylinder is in the plasma while another section is in a differentially pumped chamber with a mass spectrometer and Auger electron spectrometer (AES) facing the surface. By rapidly rotating the substrate, portions of the surface that were in the plasma as little as 1 ms ago can be diagnosed.…”

“…449 While pulsed gas valves and fast switching mass flow controllers have improved in recent years, the cycle time is too low for this process to be widely applicable. Even with very fast, stable flow switching, the sticky halogen compounds [etching products 121,122 and even Cl 2 (Ref. 143)] demand long purging times.…”

Interactions of chlorine plasmas with silicon chloride-coated reactor walls during and after silicon etching

“…38 Even though there is only a slow etching of the Si-covered electrode, the trace amount of oxygen coming from erosion of the quartz discharge tube surface is insufficient to heavily oxidize the SiCl x layer depositing on the spinning wall; hence, the L-H Cl recombination coefficient is low. A low Cl-recombination could be due to a high reaction probability of impinging Cl with a relatively thin SiCl x layer to form volatile products such as SiCl 4 , rather than Cl 2 .…”

“…38 The mass spectrometer signal, Q Cl 2 , produced by this recombined Cl 2 at m/e ¼ 70 can be converted to an absolute Cl 2 desorption flux, C Cl 2 , by the calibration procedure given in a prior publication. To do this, we temporarily established conditions that favor substantial recombination of Cl and observe desorption of recombined Cl 2 .…”