Plasma Wall Interactions and Recombination Coefficients Measurement on Yttrium-based Coated Chamber Wall Surfaces

Plasma etching is a widely used method to pattern materials in the fabrication of microelectronic devices. As the minimum feature sizes, or so-called critical dimensions, shrink beyond 14 nm, plasma etching processes need to be ever more tightly controlled. At low pressures, the ~cm mean free path of species ranges are comparable to reactor dimensions. Consequently, gas phase reactions (especially three-body processes) become less likely and heterogeneous reactions on chamber walls become increasingly important. These issues are particularly important in plasma etching processes at lower pressures (typically 1 – 100 mTorr) where heterogeneous reactions of neutrals on the chamber walls are important and can often dominate gas-phase reactions in establishing the number density of species in the plasma. The surface layers formed on the reactor walls become a source of production or loss of species. As a result, shifting plasma composition leads to process drifts, leading to changes in etching rates, profiles, selectivity, and yields. Hence, it is of prime importance to understand the interactions of plasmas with the dynamic chamber wall surfaces. Plasma-surface interactions were investigated for Y2O3, YOF and YF3 chamber wall coatings. Time-dependent behavior of chlorine inductively-coupled plasmas was studied for Si etching, following NF3 plasma cleaning of different chamber coatings. Optical emission intensities were recorded throughout the processes for Cl, O, F, Si, SiClx=1-3, SiF, and N2, as well as from added trace rare gases Xe and Ar for determination of number densities for selected species by actinometry. Small differences were found for the three materials. Si-to-Cl emission ratios were similar for Y2O3 and YOF, and ix somewhat larger for YF3. SiClx=1-3 emissions were similar for the Y2O3 and YOF-coated liners, but significantly less stable with time for YF3. Compared with Cl2/Ar plasmas, Cl2/O2/Ar plasmas produced nearly time-independent and much more consistent Cl number densities during etching. This takes place despite a consistent upward drift in SiClx=0-3 emissions for all three materials. Time-dependent Langmuir probe (LP) measurements of ion and electron number densities and electron energy distributions were also carried out. Ex-situ X-ray photoelectron spectroscopy (XPS) measurements of the surface composition of Y2O3 coupon pieces after different etching and clean processes were also performed. Initially fluorinated yttria surfaces are shown to have a relatively high probability for loss (“recombination”) of Cl through formation of both Cl2 and SiClx. As etching proceeds, SiClx abstracts F from the surface and deposits Si and Cl, lowering of the heterogeneous recombination of Cl. The initially high recombination coefficient for Cl is explained by the weakening of the surface binding energy for Cl and SiClx at YFx sites, due to the highly electronegative nature of F, allowing recombination reactions forming Cl2 and SiClx to become energetically favorable. A conditioning procedure for the YOF coating was shown to reduce drift during Si etching in Cl2 plasmas. Specifically, a Cl2/O2/Ar plasma pretreatment was briefly operated with substrate bias, generating SiClx etching products that rapidly remove F from the liner surface. When O2 flow was extinguished, etching continued with much less changes in Cl and SiClx relative number densities. x Heterogeneous loss coefficient were also investigated in power-modulated chlorine inductively-coupled plasmas (ICPs) bounded by yttria-coated chamber walls. Power at 13.56 MHz applied to the plasma was modulated between high power and no power. Time-resolved optical emissions from Cl and Xe actinometry trace gas were recorded over the 740 to 900 nm region. The intensity ratio of Cl-to-Xe emission, proportional to Cl number density, nCl, was modulated with power, allowing Cl heterogeneous loss coefficients, Cl, to be obtained from a simple time-resolved, 0-dimensional model that best matched computed absolute and relative changes in nCl as a function of the modulation period, with Cl as the only adjustable parameter. Cl2 or Cl2/O2 ICPs were studied for surfaces (a) immediately after NF3 plasma cleaning (Cl = 0.30), (b) during long exposure to Cl2 ICPs with no substrate bias (Cl = 0.09), (c) during exposure to SiClx etching products in Cl2 ICPs with Si substrate bias (Cl = 0.07), and during exposure to SiClx etching products in Cl2/O2 ICPs with Si substrate bias (Cl = 0.03). These results compare favorably to qualitative behavior reported previously for continuous Cl2 ICPs in this yttria-coated chamber.