Papers

AVS2001 Session PS-WeA: Plasma Surface Interactions II

Wednesday, October 31, 2001 2:00 PM in Room 104

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		PS-WeA-1 Laser Desorption-Laser Induced Fluorescence In situ Studies of Si Etching in Inductively-Coupled Cl₂-Ar Plasmas N.C.M. Fuller (Columbia University); V.M. Donnelly (Agere Systems); I.P. Herman (Columbia University) Laser desorption-laser induced fluorescence (LD-LIF) is used to determine the surface coverage of chlorine during the steady-state etching of Si in an 18 mTorr inductively-coupled Cl₂-Ar plasma as a function of the rf power, substrate bias and Cl₂ fraction. Laser repetition rate studies, which indicate how the surface is re-chlorinated between laser pulses after each step of laser desorption of surface SiCl_x, reveal that close to steady-state chlorination is achieved in the 10 ms time between 308-nm laser pulses (at 100 Hz) even with only 6% Cl₂ (94% Ar). This is not unexpected given our prior work in neat Cl₂ plasmas, for which there is near steady-state chlorination between such laser pulses at 1 mTorr pressure. A mechanism for the competitive etching processes of chlorination (mostly by Cl atoms in the bright mode) and surface sputtering (mostly by Ar and Cl positive ions) will be presented, by coupling these surface adlayer measurements with the etch rates and the optical emission actinometry determination of the densities of the major neutral and positive ions in the plasma.
2:20 PM		PS-WeA-2 Monitoring Plasma-Wall Interactions During Etching of Thin Film Stacks S.J. Ullal (University of California, Santa Barbara); H. Singh, J. Daugherty, V. Vahedi (Lam Research Corporation); E.S. Aydil (University of California, Santa Barbara) Surface reactions on plasma etching reactor walls affect the species concentrations in the discharge and plasma properties such as electron temperature and ion flux. Despite the importance of plasma-wall interactions, reactions occurring on surfaces in contact with the plasma are poorly understood and wall conditions are uncontrolled during etching. Often, a stack of thin films of different materials must be etched sequentially in the same reactor using different gases. Complex multi-layered films are deposited on the chamber walls during the etching of the stack and interaction between successive etching steps through the changing wall conditions may have deleterious effects. Thus, it is critical to monitor the wall conditions and the nature of the films and adsorbates that are deposited on the walls. We have developed a surface probe based on in situ multiple total internal reflection Fourier transform infrared (MTIR-FTIR) spectroscopy as a diagnostic to monitor the films and adsorbates on the walls of an inductively coupled plasma etching reactor. Using this probe we studied the shallow trench isolation etching of Si where a photoresist patterned stack of anti-reflection coating, Si₃N₄, SiO₂ and Si is etched sequentially using gases as varied as fluorocarbons, Cl₂, HBr, and O₂. During the fluorocarbon etching of Si₃N₄ and SiO₂, fluorocarbon films deposit on the chamber walls. During the subsequent etching of Si by Cl₂/O₂, etch products such as SiCl_x react with the O in the plasma and deposit a silicon oxychloride layer on the reactor walls on top of the fluorocarbon layer. In order to maintain etching reproducibility, these multi-layered films must be cleaned before the next wafer is etched in the chamber. Reactions occurring on the wall surfaces and strategies to remove these complex multi-layered films to maintain reproducibility of wall conditions and etching processes will be discussed.
3:00 PM		PS-WeA-4 Controlling the Ion Flux and Energy Distributions in LAPPS¹ S.G. Walton, D. Leonhardt, D.D. Blackwell, D.P. Murphy, R.F. Fernsler, R.A. Meger (Naval Research Laboratory) In situ mass and energy resolved measurements of ion fluxes to a conducting electrode surface in NRL's Large Area Plasma Processing System (LAPPS) are presented. In LAPPS, a high-energy electron beam is used to ionize a background gas, producing a plasma over the volume of the beam. The beam is generated by a linear hollow cathode and magnetically collimated which allows for the production of uniform plasmas over areas up to 1 m² or more. Electron beams are efficient at producing high-density plasmas (10¹⁰-10¹² cm^-3) at low temperatures (Te < 1.0 eV) and are decoupled from the reactor geometry. Hence, control over the flux and incident ion energy at independently located and biased electrodes is possible and advantageous in dry processing applications. Temporally resolved ion flux and energy distributions at an electrode surface are reported for pulsed discharges in noble and molecular gases. The flux, sampled through a small orifice located in the center of the electrode, is analyzed via an energy selector in series with a mass spectrometer. Measurements are presented for a grounded and RF-biased electrode as a function of operating pressure, source-electrode separation, and the applied bias. In argon for example, the incident Ar⁺ energy is pressure dependent and found to scale with the applied RF bias. In molecular gases, the magnitude and composition of the flux is dependent upon the source-electrode separation and found to vary in time, particularly in the afterglow. The results are discussed in terms of processing applications. Additional details concerning LAPPS are presented by co-authors at this conference.² ¹ Work supported by the Office of Naval Research ² See presentation by D. Leonhardt and D.D. Blackwell. ^*S.G. Walton: SFA, Inc.; D.D. Blackwell: NRC Postdoctoral Research Associate
3:20 PM		PS-WeA-5 High-Density Plasma-Based Etching of Organosilicate Glass (OSG) in C₄F₈/Ar and C₄F₈/O₂ Gas Mixtures: Process Results and Diagnostics M. Fukasawa, X. Li, X. Wang, L. Ling, G.S. Oehrlein (University of Maryland, College Park); F.G. Celii, K.H.R. Kirmse (Texas Instruments, Inc.) We report gas phase and surface studies of high-density plasma etching processes of organosilicate glass (OSG), a low-k oxide, and Si₃N₄ and SiC etch stop materials, in C₄F₈/Ar and C₄F₈/O₂ gas mixtures. Owing to the presence of methyl groups in the SiO₂ backbone the etching behavior of OSG differs significantly from that of conventional SiO₂. The addition of O₂ can be used to increase the OSG etching rate (e.g. from 900 nm/min for a 1400 W 6 mTorr C₄F₈ discharge and a selfbias voltage of -85 V to 1100 nm/min for C₄F₈/20% O₂), but can modify the OSG material by oxidizing methyl groups and reduce the selectivity to the etch stop material. The goal of this work was to establish the key variables that can be used to maximize the etch selectivity of OSG with respect to the etch stop materials while minimizing the OSG modifications. An inductively coupled high-density plasma etching reactor equipped with in situ ellipsometry, optical emission spectroscopy (OES) and mass spectrometry was used. Measurements were made as a function of C₄F₈/Ar and C₄F₈/O₂ gas composition for pressures ranging from 6 to 20 mTorr, source power levels up to 1400 W, and as a function of RF bias. Both blanket film etching of OSG, SiO₂, Si₃N₄ and SiC and transfer of hole/trench patterns into OSG were studied as a function of process conditions. We utilized the gas phase characterization results, and X-ray photoelectron spectroscopy (XPS) data of etched films after vacuum transfer, to explain the observed etching behavior, evaluate surface/bulk modifications of the OSG vs. process conditions, and identify the critical factors that enable high quality pattern transfer processes of OSG over Si₃N₄ and SiC.
3:40 PM		PS-WeA-6 Plasma Deposition of Silicon Thin Films: Atomic-Scale Modeling of Radical-Surface Interactions D. Maroudas (University of California, Santa Barbara) Hydrogenated amorphous silicon (a-Si:H) thin films grown by plasma-assisted deposition from silane-containing discharges are used widely in technological applications. A fundamental understanding of the interactions of radicals, such as SiHx (0
4:20 PM		PS-WeA-8 Molecular Dynamics Simulations of Ar⁺-Si and Si:F Interactions D.B. Graves, D. Humbird (University of California at Berkeley)
4:40 PM		PS-WeA-9 Atomic-Scale Simulation Study of the Role of H Atoms in the Amorphous to Nanocrystalline Transformation in Plasma-Deposited Silicon Thin Films S. Sriraman, E.S. Aydil, D. Maroudas (University of California, Santa Barbara) Hydrogenated amorphous (a-Si:H) and nanocrystalline (nc-Si:H) silicon thin films grown by plasma deposition from SiH₄ and H₂ containing discharges are widely used in photovoltaic and flat-panel display technologies. When an a-Si:H thin film is exposed to a H₂ plasma, its nanostructure changes from amorphous to nanocrystalline. Though several hypotheses have been proposed, the fundamental mechanisms behind this transformation are still not well understood. Molecular dynamics (MD) simulations of the interactions of thermal and energetic H atoms with a-Si:H films and their surfaces are used to elucidate the nanoscopic mechanisms behind the amorphous to nanocrystalline transformation. a-Si:H films are deposited through MD simulations of repeated impingement of individual SiH₃ precursors on an initial H-terminated Si(001)-(2x1) surface. H₂ plasma exposure is simulated through repeated impingement of individual H atoms onto these a-Si:H films grown by MD. Of the many elementary surface reactions that were identified, Eley-Rideal type H abstraction reactions are believed to mediate strain relaxation processes and promote amorphous to nanocrystalline transformation. The effects of abstraction reactions on the growth surface are examined by analyzing their influence on both local and overall film structure. The surface hydride compositions in the deposited films before and after exposure are compared with experimental data and the comparisons are used to discuss our current understanding of the amorphous to nanocrystalline transformation.
5:00 PM		PS-WeA-10 A Fast Computational Model for Study of Coupled Bulk Plasma-Sheath-Bias Circuit Phenomena and its Effect on Plasma-Surface Interactions L.L. Raja (Colorado School of Mines); E. Meeks (Reaction Design, Inc.) High-density plasma reactors are used extensively in the etching and deposition of thin films in the manufacture of large-scale integrated circuits. With feature sizes approaching 0.1 microns and lower, it is increasingly important to develop quantitative understanding of plasma-surface interactions and their dependence on plasma reactor geometry, operating conditions, and bias-circuit settings. Of critical importance is the relationship between reactor controls and ion impact phenomena such as the Ion Energy and Angular Distribution Functions (IEDF and IADF). We have developed a new fast computational software tool that enables prediction of IEDF and IADF in high-density plasma reactors through coupling of bulk plasma, RF sheath, and bias circuit models. We simulate the bulk plasma using the well mixed reactor model, AURORA,¹ while the RF sheath sub-model uses a multiple-ion extension of the Riley sheath model^2,3 coupled to a typical bias circuit model, based on first-principles. The coupled model handles detailed gas-phase chemical reactions that are characteristic of process plasmas and can predict multiple ion IEDFs and IADFs as a function of reactor geometry, and reactor and bias circuit settings. The general surface-chemistry capability allows for specification of ion-energy dependent yields for ion-enhanced surface reactions. The coupled model executes within minutes on a personal computer, providing a fast simulation tool for quickly exploring alternative process conditions and reactor designs. Example results in a fluorocarbon plasma etching system will be reported. ¹R. J. Kee, F. M. Rupley, J. A. Miller, M. E. Coltrin, J. F. Grcar, E. Meeks, H. K. Moffat, A. E. Lutz, et al., Chemkin Collection 3.6, (Reaction Design, Inc., San Diego, CA, 2000) ²M. E. Riley, Sandia National Laboratories Report No. SAND95-0775, 1995. ³M. E. Riley, Sandia National Laboratories Report No. SAND96-1948, 1996.