Chlorine atom recombination coefficient (γCl) measurements are reported for a variety of surfaces and at a range of surface temperatures. The surfaces include crystalline silicon, quartz, anodized aluminum, tungsten, stainless steel, polycrystalline silicon, and photoresist. Surface temperatures ranged from about −90 °C up to 85 °C. Measurements were made in a vacuum chamber with chlorine atoms and molecules effusing from an external discharge source as a molecular beam and impacting a selected surface. The incident and reflected beam compositions calculated using a modulated beam mass spectrometer were used to infer the recombination coefficient. At room temperature, the values of γCl ranged from below the detection sensitivity (about 0.01) for crystalline silicon to ∼0.85 for stainless steel. Other surfaces displayed intermediate values between these extremes. For example, γCl for polycrystalline silicon is about 0.2–0.3 at room temperature. All surfaces, except stainless steel, displayed increasing values of γCl as surface temperature was lowered below room temperature, down to the freezing temperature of chlorine (−101 °C). The γCl for stainless steel appeared to saturate at 0.85 as temperature was lowered. All surfaces displayed decreasing values for the recombination coefficient as surface temperature was raised above room temperature. The γCl data as a function of temperature were fit to a phenomenological model. The phenomenological model assumes Cl atoms adsorb into a weakly bound physisorbed, state on at least 1 monolayer of strongly bound, chemisorbed chlorine. After adsorption, the model assumes that thermally activated diffusion and atomic recombination occur with a rate that is first order in physisorbed chlorine. Thermal desorption competes with diffusion and reaction, and is also thermally activated. Fits to the data were made, and the physical interpretation of the model parameters is discussed.
Surface reactions of atomic halogen atoms play important roles in various plasma etching processes, commonly used in microlectronics manufacturing. However, relatively little is known about the surface chemistry of these key reactive intermediates. Previous measurements of the recombination coefficients of Cl, Br, and F on various surfaces in a molecular beam apparatus indicated that the recombination reaction is pseudofirst order [G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci. Technol. A 16, 270 (1998); 16, 2215 (1998)]. One mechanism that would result in pseudofirst order kinetics is a two-step process in which the first halogen atom adsorbs into a relatively strongly bound chemisorbed state, and the second atom reacts with it either through a direct reaction, or after being physisorbed onto the halogenated surface. In this article, we report experiments in which surfaces are first exposed to a molecular beam of one type of halogen atom, then the surface is exposed to a second type of halogen. During the second exposure, the heteronuclear reaction product is monitored with a mass spectrometer. Finally, the surface is sputtered and the mass spectrometer is used to detect any remaining presence of the original halogen atom. Analogous experiments were also performed with isotopically enriched mixtures of chlorine. These experiments unambiguously demonstrate that halogen atom surface recombination involves a two step adsorption-abstraction mechanism. Under all conditions studied, the surface recombination reactions proceeded at rates on the order of surface collision frequencies. The relative magnitudes of the heteronuclear rates (as a function of surface composition and halogen atom type) scaled in the same way as the homonuclear recombination probabilities measured previously. In every case examined, after the second halogen exposure, the surface retained a significant coverage of the halogen that had been originally exposed to the surface. This leads to the conclusion that only a fraction of the strongly bound surface sites are available for abstraction by free radical attack. Absolute calibration of the incident and evolved species fluxes allowed an estimate to be made of the reactive site densities for several surfaces. These ranged from 1012 to 1015 cm−2 depending on the surface.
Recombination coefficients (γ) of Br and F atoms have been measured for crystalline Si, quartz, photoresist, anodized aluminum, poly-Si, WSix, tungsten and stainless steel surfaces for a range of temperatures. The γBr and γF values are compared to our previously reported measurements of γCl [G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci. Technol. A 16, 270 (1998)]. In general, the Br-, Cl- and F-atom recombination coefficients decrease as the surface temperature increases. The γBr values are similar to the γCl values for the various surfaces. At room temperature, γBr is highest (>0.4) for stainless steel and tungsten, moderate (0.1–0.4) for poly-Si, WSix and anodized Al, and lowest (<0.05) for c-Si, quartz and photoresist. However, γF, at room temperature, is no greater than 0.05 for all the surfaces. γF increases slightly as the temperature is decreased to 80 K but is still below 0.1 for all the surfaces. The recombination coefficient data as a function of temperature for all surfaces are fit to a phenomenological model developed previously for γCl (see the above reference). The model assumes that the incident halogen atoms physisorb on a surface that is saturated with chemisorbed halogen atoms. The physisorbed atoms are assumed to diffuse on the surface and either desorb before recombining or recombine and then desorb. The recombination rate is assumed to be first order in physisorbed atom coverage.
Advanced integrated metrology capability is actively being pursued in several process areas, including etch, to shorten process cycle times, enable wafer-level advanced process control (APC), and improve productivity. In this study, KLATencor's scatterometry-based iSpectra Spectroscopic CD was integrated on a Lam 2300 Versys StarTM silicon etch system. Feed-forward control techniques were used to reduce critical dimension (CD) variation. Pre-etch CD measurements were sent to the etch system to modify the trim time and achieve targeted CDs. CDs were brought to within 1 nm from a starting CD spread of 25 nm, showing the effectiveness of this process control approach together with the advantages of spectroscopic CD metrology over conventional CD measurement techniques.
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