Optical emission spectroscopy was used to investigate the effect of Y2O3, YOF, and YF3 chamber wall coatings on the relative number densities of gaseous species during etching of Si in Cl2/Ar inductively coupled plasmas. Etching plasmas were alternated with NF3/Ar plasma chamber-cleaning steps. Small differences were found for the three materials. Si-to-Cl emission ratios were similar for Y2O3 and YOF, and 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. 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 the O2 flow was extinguished, etching continued with much less changes in Cl and SiClx relative number densities.
Time-dependent studies of power-modulated chlorine inductively-coupled plasmas are presented. Power at 13.56 MHz applied to the plasma was modulated between high and low power states. Time-resolved optical emission, power delivery, and Langmuir probe measurements revealed at least two conditions upon switching from high to low power: a 'normal' mode in which electron temperature (T e ) remains constant, while electron and ion number densities (n e and n + ) and optical emission spectroscopic (OES) intensities smoothly drop to a level roughly equal to the fractional drop in power, and an 'abnormal' mode in which n e , n + and OES intensities plummet before the plasma re-ignites and these values rise to levels more commensurate with the drop in power. Whether the plasma operates in the normal or abnormal mode is sensitive to impedance matching conditions and is also a function of pressure and pulsing parameters. This ignition delays in the abnormal mode can be qualitatively understood in terms of a power balance model commonly used to explain instability-induced, self-modulation in highly electronegative plasmas, caused by the slower time response of negative ions compared with electrons. The study indicates that power modulation for added control in processes such as plasma etching will require careful measurement and possibly control of power with microsecond resolution.
Time-dependent behavior of chlorine inductively coupled plasmas is presented for Si etching, following NF3-Ar plasma cleaning of a chamber coated with Y2O3. 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. Time-dependent Langmuir probe measurements of ion and electron number densities and electron energy distributions were also carried out. Ex situ x-ray photoelectron spectroscopy 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.
Studies of power-modulated chlorine inductively coupled plasmas (ICPs) bounded by yttria-coated chamber walls are presented. Time-resolved optical emissions from Cl and Xe actinometry trace gas were recorded over the 740–920 nm region as power at 13.56 MHz was modulated between high power and no power. The intensity ratio of Cl-to-Xe emission, proportional to Cl number density, nCl, followed the modulation in power, allowing Cl heterogeneous loss coefficients, γCl, to be obtained from a simple time-resolved, 0-dimensional model of the afterglow period that best matched computed relative changes in nCl at the beginning and end of the powered period, with γCl as the only adjustable parameter. This approach only requires a treatment of diffusion and avoids complications introduced by attempting simulations of the full modulation period. Cl recombination coefficients were determined on the mostly yttria surfaces for Cl2 ICPs (a) immediately after NF3 plasma cleaning (γCl = 0.20), (b) during long exposure to the Cl2 plasma with no substrate bias (γCl = 0.11), and (c) during Si etching with substrate bias (γCl = 0.055-0.070). For Cl2/5% O2 ICPs, these values are 0.28, 0.17, and 0.030, respectively. These results compare favorably to qualitative behavior reported previously for continuous Cl2 and Cl2/O2 ICPs in this yttria-coated chamber.
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