J. W. Coburn

The recombination of chlorine atoms at surfaces

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 val...

Appearance potential mass spectrometry: Discrimination of dissociative ionization products

Appearance potential mass spectrometry (APMS) has recently gained importance for detection and quantitative measurements of reactive radical species in plasmas using line-of-sight sampling of radicals. In this work, we have investigated the assumption that the extraction efficiency of ions produced by direct ionization of radicals, and ions produced by dissociative ionization of the parent molecule used as the reference signal, are equal in the ionizer of the mass spectrometer. We find that the dissociative ionization products are extracted with much lower efficiency (2–50 times smaller for the cases studied) than the direct ionization products. This is expected due to the excess kinetic energy of the dissociatively ionized products as a result of the Franck–Condon effect. Use of this procedure will thus lead to an overestimation of the radical number density by a factor of 2–50, depending on the nature of the parent and the daughter ion. We recommend an alternate procedure for APMS that utilizes an inert...

Mass spectrometric detection of reactive neutral species: Beam-to-background ratio

Mass spectrometry and appearance potential mass spectrometry (APMS) have recently gained importance for detection and quantitative measurements of reactive radical species in plasmas using line-of-sight sampling of reactive species. In this work, we have characterized the contributions to the mass spectrometer signal from the line-of-sight “beam” component and the background component of the species in the ionizer of the mass spectrometer. The beam signal is proportional to the number density of the species in the plasma, while the background component of the signal depends on various factors like the vacuum system design and pump speeds. Single differential pumping of the mass spectrometer is found to be inadequate as the background signal dominates the beam signal for radical and stable neutral species. The beam to background ratio for CFx (x=1–3) radicals is smaller than 0.25 and the large background signals of the species of interest necessitates implementation of modulated beam mass spectrometry usin...

Heterogeneous recombination of atomic bromine and fluorine

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 t...

Recombination coefficients of O and N radicals on stainless steel

Surface recombination coefficients of O and N radicals in pure O2 and N2 plasmas, respectively, have been estimated on the stainless steel walls of a low-pressure inductively coupled plasma reactor. The recombination coefficients are estimated using a steady state plasma model describing the balance between the volume generation of the radicals from electron-impact dissociation of the parent molecules, and the loss of the radicals due to surface recombination. The model uses radical and parent molecule number densities and the electron energy distribution function (EEDF) as input parameters. We have measured the radical number density using appearance potential mass spectrometry. The parent neutral number density is measured using mass spectrometry. The EEDF is measured using a Langmuir probe. The recombination coefficient of O radicals on stainless steel walls at approximately 330 K is estimated to be 0.17±0.02, and agrees well with previous measurements. The recombination coefficient of N radicals is es...

Measurements of neutral and ion composition, neutral temperature, and electron energy distribution function in a CF4 inductively coupled plasma

In this article, we present comprehensive measurements of the neutral number densities, ion number densities, and the electron energy distribution function in a CF4 inductively coupled plasma at pressures between 1 and 30 mTorr, and deposited powers between 150 and 550 W. High degrees of dissociation are observed at the lower pressures. We believe this is a result of the large electron temperature at the lower pressures. The measurements of all the dominant radical and stable neutral species using appearance potential mass spectrometry allows the estimation of the neutral temperature at the neutral sampling aperture. The neutral temperature is also estimated from the change in the number density of a trace amount of argon added to CF4 when the plasma is turned on. Neutral temperatures up to 930 K are measured at the sampling aperture. The increase in neutral temperature with power at a constant pressure results in a decrease in the total neutral number density at a constant pressure. The electron temperat...

Comparison of model and experiment for Ar, Ar/O2 and Ar/O2/Cl2 inductively coupled plasmas

Comparisons of fluid model predictions with various measurements in an inductively coupled plasma are reported. The gas chemistries used include pure Ar, mixtures of Ar and O2 and mixtures of Ar, O2 and Cl2 for pressures ranging from 10 to 80 mTorr and plasma densities ranging from ~1010–1011 cm−3. Measurements of electron density, electron energy probability function, positive ion composition and wall flux and neutral radical concentration at the chamber wall are reported and compared with corresponding model predictions. The model treats electrons, ions and neutrals as fluids, under the assumption of quasineutrality. Power into electrons is from inductive coupling from an external coil and capacitive coupling is neglected. The non-isothermal, multicomponent reacting flow equations with jump boundary conditions are used to describe neutrals and are solved iteratively with the plasma and electromagnetic equations until a steady solution is found. Radical and ion composition predictions are shown to be in reasonable agreement for Ar and Ar/O2 plasmas, but the corresponding predictions in mixtures of Ar, O2 and Cl2 are in only partial agreement with measurements, implying that the chemical reaction database for this chemistry requires further work.

Heteronuclear and homonuclear surface abstraction reactions of Cl, Br, and F

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....

Fundamental beam studies of radical enhanced atomic layer deposition of TiN

TiN films were deposited using radical enhanced atomic layer deposition (REALD) in an ultrahigh vacuum beam system. The REALD cycle used for the film deposition consisted of separate, sequential exposures of TiCl4, deuterium atoms, and nitrogen atoms. By using this three step deposition process with independently controlled beams of TiCl4 molecules, deuterium radicals, and nitrogen radicals, it was possible to monitor each step in the REALD cycle, measuring the sticking and reaction probabilities of each of these species on silicon as a function of surface temperature using a quartz crystal microbalance. Results from the ex situ XPS analysis of the deposited films are promising and suggest that deuterium radicals can abstract Cl from TiClx surface species for substrate temperatures as low as 300 K. The films display low residual chlorine content (<0.3%) when the D atom exposure is sufficiently large. The results from the third stage in the REALD TiN process are consistent with the addition of approximatel...

Etching of ruthenium coatings in O2- and Cl2-containing plasmas

Ruthenium (Ru) film etching has been studied with O2- and Cl2-containing inductively coupled plasmas to understand the etching mechanism and the relationship between plasma characteristics and the competition between the wall deposition of etch by-products and the creation of volatile etch by-products that flow into the downstream. The ICP was characterized by multiple in situ diagnostic tools. Ru films were etched either from 6-in. wafers placed on a rf-biased substrate or from Ru-coated QCMs without a Ru-coated wafer present. Ru etches readily in O2-containing plasma. Cl2 addition resulted in significant changes in etch rate, wall deposition behavior, and the downstream etch product composition. When Ru was etched by Ar∕O2 plasmas, a positive wall deposition rate was observed and no RuO4 was observed in the foreline. The etching rate correlated well with the oxygen radical density. With Cl2 addition, our observations included the significantly increased etching rate, the detection of RuO4 downstream by ...