AVS1997 Session PS1-ThM: Plasma Surface Interactions I

Thursday, October 23, 1997 8:20 AM in Room A5/6

Thursday Morning

Start	Invited?	Item
8:20 AM		PS1-ThM-1 Beam Studies of Halogen Atom Homonuclear and Heteronuclear Surface Reactions G.P. Kota, D.B. Graves (University of California, Berkeley) Radical-surface reactions play an important role in plasma processing both at the reactor scale and at the feature scale. Surface recombination coefficients of halogen atoms (fluorine, chlorine and bromine) have been measured for a variety of surfaces over a range of temperatures. The surfaces studied are: crystalline-Si, poly-Si, W, WSi, quartz, anodized-Al, photoresist, and stainless steel. In general, all the halogen atoms show an increase in the recombination coefficient (γ) as the temperature decreases. In detail, the halogen atoms behave very differently. Fluorine recombination coefficients for all the surfaces are below our experimental detection level (≤ 0.02) at room temperature. At low temperatures (~100K), γ_F is between 0.05 and 0.1 for all the surfaces. In contrast, the recombination coefficients for chlorine and bromine on most surfaces are significantly higher. Chlorine recombination coefficients vary over a wide range for the different surfaces. The γ_Cl and γ_Br versus temperature data are fit to a phenomenological model that assumes incident atoms physisorb with a high probability, then either recombine or thermally desorb. The major difference between Cl and Br recombination is that γ_Cl decreases more rapidly with increasing temperature than does γ_Br. For most surfaces γ_Br is higher than γ_Cl. We have also conducted mixed halogen recombination experiments in which a selected surface is initially exposed to one type of halogen, and then exposed to another type of halogen. The transient heteronuclear reaction product is detected. In general, surfaces that display high probability for homonuclear recombination also display high probability for heteronuclear reactions.
8:40 AM		PS1-ThM-2 Measurement of the Reactivity of CF₂ in Saturated Fluorcarbon Plasmas using Imaging of Radicals Interacting with Surfaces N.M. Mackie, V.A. Venturo, E.R. Fisher (Colorado State University) Fluorocarbon plasmas are used in a wide variety of processes, from deposition of thin, non-wettable polymer films to etching of Si and SiO₂ in the semiconductor industry. There has been a great deal of work studying either the gas-phase chemistry of fluorocarbon plasmas or the resulting deposited or etched material. There has been little work performed probing the interface between the fluorocarbon plasma and the surface of the material being processed. Using the Imaging of Radicals Interacting with Surfaces (IRIS) method we can determine directly the reactivities of small radicals such as CF and CF₂under a variety of etching and deposition conditions. IRIS combines spatially resolved laser-induced fluorescence (LIF) with molecular beam techniques. We have observed fluorescence of CF₂ (A¹B₁ - X¹A₁) in C₂F₆ and C₂F₆/H₂ plasmas. For the C₂F₆ system, we have modeled our experimental reactivity data and found that the best fit to the data assumes a combination of adsorption/desorption and specular scattering mechanisms. Results will be presented detailing the reactivity of CF₂ under different plasma conditions, vibrational state and different substrates (Si (100), polyimide, and polytetrafluoroethylene). With 100%C₂F₆ plasma, the reactivity of CF₂ is ~0, but increases when hydrogen is added to the feed and deposition conditions are achieved. Films deposited during the IRIS experiment have been analyzed with FTIR and XPS and show that fluorocarbon polymer films are similar to those deposited directly in the plasma. Implications for possible deposition mechanisms will be discussed.
9:00 AM		PS1-ThM-3 A Meso-scale Model for Bulk Plasma and Surface Chemistry in Cl₂ Etching of poly-Si¹ M.J. Grapperhaus, M.J. Kushner (University of Illinois, Urbana) The consequences of plasma-surface interactions on plasma and etch properties have been well characterized on reactor and feature scales. The observation that the patterning of the wafer can cause localized changes in etch properties emphasizes the need to investigate the consequences of meso-scale (pattern or die scale) phenomena on plasma-surface interactions. These processes include mask erosion, redeposition and re-emission which produce changes in reactive sticking coefficients which depend on, for example, the fractional open area of the mask. To investigate these effects, a model has been developed which uses Monte Carlo (MC) techniques to simulate quasi-steady state die scale surface chemistry. This model is integrated within the Hybrid Plasma Equipment Model (HPEM)² which resolves 2-dimensional reactor scale conditions. Results from the surface MC simulation modify the boundary condition at the wafer surface in the HPEM to provide effective reactive sticking coefficients and evolved species. This yields surface conditions resolved in two dimensions on the die scale. Results from the model will be used to discuss pattern dependent poly-Si etch properties in inductively coupled plasma etching tools reactor sustained in Cl₂ mixtures. As pressure is varied from 10 to 100 mTorr and the effect of local photoresist sputter and redeposition on nearby exposed etch areas is shown to increase which leads to different etch rates near the boundaries of etching regions versus unexposed regions. ¹Work supported by SRC, NSF, DARPA and the University of Wisconsin ERC for Plasma Aided Manufacturing. ²M. J. Grapperhaus, J. Appl. Phys., v. 81, 569 (1997).
9:20 AM		PS1-ThM-4 Characterization of Si(100) Surfaces after High Density Plasma HBr/Cl₂/O₂ Etching by AFM and XPS C.H. Low, W.S. Chin (National University of Singapore); M.S. Zhou, Q.H. Zhong, L.H. Chan (Chartered Semiconductor Manufacturing Ltd., Singapore) In this study, Atomic Force Microscopy (AFM) was used to capture the changes of Si(100) surface morphology after high density plasma (HDP) HBr/Cl₂/O₂ etching and dilute HF (1% HF) treatment. X-ray Photoelectron Spectroscopy (XPS) was also employed for chemical species identification of residues generated during the silicon etching. The shape of the Si2p peak, the presence of the O1s peak and a small Br3d peak shows that the Si(100) surface layer of the etched sample without dilute HF treatment consists mainly of different oxides of silicon ¹^,²^,³ and small amounts of adsorbed Br ⁴^,⁵^,⁶. The AFM image shows a much roughened surface as compared to that of untreated Si(100) surfaces. With 10 seconds of dilute HF treatment, the Si2p(Si-O) peak shows a dramatic reduction in intensity while that of elemental Si emerged, suggesting that dilute HF is effective in removing any surface etch residues. This is further evidenced by the change in the morphology of the Si(100) surface as observed in the AFM image which shows the decrease of the Si grain size. In addition, the prominent change in the Si-O peak shape confirms the presence of different oxide species. The relative intensity of the elemental Si peak was found to increase with the duration of dilute HF treatment whereas that of Si-O showed an inverse relationship with the treatment time. The AFM image also shows a flattened Si(100) surface indicating that some surface layers have been removed by the dilute HF treatment. The effect of dilute HF in cleaning the surface is estimated to be complete after 40 seconds as shown by the XPS spectra and AFM images. The underlying etching mechanism will be discussed ⁷^,⁸. ¹F.H. Bell and O. Joubert, J. Vac. Sci. Technol. B 15(1), 88 (1997). ²K. Nishikawa, T. Oomori, K. Ono and M. Tuda, Jpn. J. Appl. Phys. Part 1 35(4B), 2421 (1996). ³K. Koshino, J. Matsuo and M. Nakamura, Jpn. J. Appl. Phys. Part 1 32(6B), 3063 (1993). ⁴Tim D. Bestwick and Gottileb S. Oehrlein, J. Vac. Sci. Technol. A 8(3),1696 (1989). ⁵M. Haverlag, G.S. Oehrlein and D. Vender, J. Vac. Sci. Technol. B 12(1), 96 (1994). ⁶C.C. Cheng, K.V. Guinn and V.M. Donnelly, J. Vac. Sci. Technol. B 14(1), 85 (1996). ⁷T. Tsuchizawa, C. takahashi and S. Matsuo, Jpn. J. Appl. Phys. Part 1 33(10), 6019 (1994). ⁸N. Ozawa, T. Matsui and J. Kanamori, Jpn. J. Appl. Phys. Part 1 34(12B), 6815 (1995).
9:40 AM	Invited	PS1-ThM-5 Gas Phase and Surface Diagnostics for Understanding the Etching and Polymer Deposition Mechanisms on Si Surface K. Tachibana (Kyoto University) The higher selectivity and reproducibility in SiO₂/Si etching are key issues for the 300 mm wafer process with high aspect ratio patterns. For understanding and control of the involved reactions we have been developing in situ gas-phase and surface diagnostics tools. Firstly, FT-IR PMSE (Phase-Modulated Spectroscopic Ellipsometry) has become an effective method for the diagnostics of chemical bonds on the surface with mono-layer sensitivity using the advantage of higher sensitivity in Im(D) signal than in Re(D), where D is the optical density of overlayer. By the method surface bonds such as -CF, -CF₂, -CHF, -CHF₂, and C-C in CF_x polymer were observed on the Si surface in an ICP plasma with CHF₃ source gas. These bonds predominated at lower substrate bias voltage, but decreased toward zero or a certain small level over 50-60V. The SiF₂ bond increased to have a peak at about 30 V and then decreased to a certain level over 70V, where the appearance of Si-C bond was noticed. These changes suggest that the ion bombardment strongly affects the reactions in the formation and destruction of these surface bonds. Secondly, EAMS (Electron Attachment Mass Spectrometry), has been developed to be a powerful method for the study of highly associated species in the gas phase. This method has advantages over a traditional ionization mass spectrometry, since the fragmentation probability is lower and the attachment cross section has the resonance-like peak for each species. Species such like C_nF_m with n up to 9 were observed in a CF₄ plasma with the RIE configuration, and the higher mass species increases when C₂F₆ and C₄F₈ gases are used instead of CF₄. These species can be precursors for the polymer deposition on the surfaces. The selective etching and polymerization mechanisms will be argued using these data combined with the laser spectroscopic measurements of smaller radicals such as CF_x (x = 1-3). This work has been done with the collaboration of Drs. G. Kroesen, E. Stoffels, W. Stoffels, H-J.Lee, and T. Shirafuji.
10:20 AM		PS1-ThM-7 X-ray Photoelectron Spectroscopy Analyses of Metal Stacks Etched in Cl₂/BCl₃ and Cl₂/HCl/N₂ Chemistries : Investigation of Sidewall Passivation Mechanisms and Anti-Corrosion Treatments Efficiency P. Czuprynski, O. Joubert, L. Vallier (France Telecom CNET); M. Heitzmann (CEA-LETI, France); M. Puttock (TRIKON Technologies, United Kingdom) We have used X-ray Photoelectron Spectroscopy (XPS) to determine the chemical elements present on the tops, sidewalls and bottoms of submicron photoresist masked TiN/AlCu/TiN/Ti features etched according to 0.25 µm design rules in a high density ICP source using Cl₂/BCl₃ and Cl₂/HCl/N₂ gas mixtures. In-situ XPS analyses have shown that aluminun oxide is deposited on all the surfaces of the features exposed to the plasma due to erosion of the alumina liner located in the source region. A chlorine rich carbon film is formed on the sidewalls and at the bottom of the aluminum features during the etching process. At the bottom of the features, chlorine species must diffuse through the carbon layer to etch aluminum whereas spontaneous reactions between chlorine and aluminum are blocked on the sides of the features. This polymer film enhances anisotropic etching by providing a thin protective layer against spontaneous etching reaction of chlorine with aluminum. X-ray Photoelectron Spectroscopy (XPS) has also been used combined with Wide Dispersive X-Ray Fluorescence (WDXRF) to evaluate the efficiency of metal stack anticorrosion treatments by measuring the chlorine concentration left in the metal stack. Anti-corrosion treatments usually consist in water-based plasmas followed by depolymerizing wet chemistry. Analyses demonstrate that whatever the anti-corrosion treatment performed, residual chlorine is still present in the metal stack showing that corrosion could occur in the subsequent technological steps of the process. In particular, XPS analyses have shown that, after anticorrosion treatments, residual chlorine species are mainly located on the aluminum sidewalls of the features. These techniques can be useful to improve anticorrosion treatments or, may help in selecting an etch chemistry which leaves a minimal amount of chlorine in the metal stack after anti-corrosion treatment. ^ext This work has been carried out within the GRESSI consortium between CEA-LETI and France Telecom-CNET.
10:40 AM		PS1-ThM-8 Etching of Thin Films in High Density Fluorocarbon Plasmas: Mechanism of Etching through a Steady State Fluorocarbon Layer T.E.F.M. Standaert, M. Schaepkens, N.R. Rueger, G.S. Oehrlein (State University of New York, Albany); J.M. Cook (Lam Research Corporation) The etching of various substrates has been studied in an Inductively Coupled Plasma (ICP) reactor using different fluorocarbon feedgases as a function of operating conditions and a consistent model is developed. Without supplying rf bias power, fluorocarbon material is deposited at a high rate. Fluorocarbon deposition can be decreased by increasing the amount of applied rf bias power, until ultimately etching occurs above a (discharge parameter-dependent) critical rf bias voltage. Above this voltage a steady state fluorocarbon film develops on the substrate. The etching of the substrate has to occur through this layer, which depends on the substrate and, in our work, varies in thickness from a few monolayers up to 25 monolayers. Since ion penetration is limited to only a few monolayers, we propose that neutral fluorine transport through the fluorocarbon layer determines the layer thickness and the etching of the underlayer. The fluorine reacts in the fluorocarbon layer resulting in desorption of fluorocarbon species. This happens in such a way that the fluorocarbon deposition during the steady state etching process is balanced. In addition, fluorine may be consumed by the etching of the substrate. A model including Fickian diffusion of fluorine leads to a value for both the etch rate and the fluorocarbon layer thickness. We assigned two possible roles of the ions in the etching process that were found to be consistent with our experimental data on Si etching. First of all, fluorine from the gas phase may be transported through the fluorocarbon film. This transport can be enhanced by the ion energy deposition in the film. Secondly, fluorine may be created by ion impact fragmentation and dissociation of fluorocarbon-bonds. Finally, it will be shown that our model can also succesfully be applied for the etching of other substrates, such as SiO₂ and Si₃N₄.
11:00 AM		PS1-ThM-9 Resist Etching Mechanisms in Fluorocarbon High-Density Plasmas J.M. Mirza, T.E.F.M. Standaert, G.S. Oehrlein (State University of New York, Albany); J.M. Cook (Lam Research Corporation) Excessively high resist erosion rates in low pressure high-density plasmas (HDP) fed with fluorocarbon gases, is one important problem in the application of this technology to patterning of SiO2 in integrated circuit manufacturing. In this work, we have examined resist erosion mechanisms in an inductively-coupled high-density plasma reactor with independent rf biasing of the substrate for fluorocarbon gases with and without hydrogen admixture, e.g. C3F6 and C3F6/H2. The comparison of resist erosion rates, ion current and mass spectrometric and optical emission spectroscopic measurements and chemical characterization of processed resist surfaces as a function of process parameters enables the clarification of the respective roles of ion flux, ion energy, fluorine atom flux and surface fluorocarbon films in controlling resist erosion rates. These insights enable the identification of process conditions for which the SiO2/resist etch rate ratios are maximized.
11:20 AM		PS1-ThM-10 Laser Thermal Desorption Analysis of Surface Adlayers during Inductively-Coupled Plasma Etching of Semiconductors J.Y. Choe, I.P. Herman (Columbia University); V.M. Donnelly (Bell Laboratories, Lucent Technology) Laser-induced thermal desorption combined with optical detection was used to determine the surface coverage of chlorine during steady state plasma etching of Si in a flat-coil inductively coupled plasma (ICP) source. A pulsed XeCl excimer laser (308 nm) was used to desorb (LD) the surface layer and to excite the desorbed species, SiCl (292.4 nm), for laser-induced fluorescence (LIF) detection. Since the LD-LIF signal is proportional to the density of desorbing species, it gives quantitative information about the surface layer thickness. Optical emission actinometry and Langmuir probe measurements were used to determine the relative degree of chlorine dissociation and ion density, respectively. As the RF power was increased, the ICP operated first in the dim mode and then changed to the bright mode at higher power. The surface coverage of chlorine in the bright mode was 2.0x that with chlorine flow and the plasma off, and increased slowly with power. This coverage ratio increased from 1.6x to 2.5x as the ion energy was increased from 16 to 116 eV by increasing the substrate bias, indicating that impinging ions with higher energy caused a higher degree of surface chlorination. Laser-induced thermal desorption was also detected by a transient rise in plasma-induced emission (LD-PIE). The LD-LIF and LD-PIE signals during Si etching by a Cl₂ plasma have been compared. The extension of these methods to Cl₂ plasma etching of Ge (using LD-LIF) and InP (using LD-PIE) has also been examined. The work at Columbia was supported by NSF grant DMR-94-11504.
11:40 AM		PS1-ThM-11 Halogen Uptake by Thin SiO₂ Layers on Exposure to HBr/O₂ and Cl₂ Plasmas, Investigated by Vacuum Transfer XPS V.M. Donnelly, N. Layadi (Bell Laboratories, Lucent Technologies) Thin SiO₂ layers were subjected to short exposures (10 to 40s) to HBr/O₂ and Cl₂ plasmas, simulating the over-etching process encountered when polycrystalline Si gate electrodes are etched down to the gate oxide layer. Following this treatment, the samples were transferred under vacuum to an x-ray photoelectron spectrometer (XPS) and spectra were recorded as a function of the take-off angle between the sample surface plane and the photoelectron collection lens. These angle-resolved measurements were inverted, using a maximum entropy approach, to obtain depth profiles. After etching in Cl₂ or HBr plasmas at an ion energy of 40eV (obtained with a grounded stage and a plasma potential of 40 V DC) halogenated surface layers were formed with Cl and Br areal densities of 2.0 X 10¹⁵ and 1.4 X 10¹⁵ cm^-2, respectively, distributed over a half-depth of ~10 Å. These results (both absolute areal densities and depth distributions) are nearly the same as those found previously for etching of Si under the same conditions. For SiO₂, build up of Cl or Br near the surface is accompanied by a depletion of O. Addition of 10% O₂ to HBr plasmas has little effect on the Br content in the film, but dramatically slows the etch rate of 56Å thick films from 18Å/min to <2Å/min. Increasing the mean ion energy to ~150 eV by applying an RF bias (resulting in a DC bias of -110 V) increases the etch rate in the HBr/10%O₂ plasma to 18Å/min, and increases the Br distribution to a half depth of ~20Å, but has little or no effect on the Br areal density. Implications for gate etching will be discussed.