XPS and Electrochemical Studies on Tungsten-Oxidizer Interaction in Chemical Mechanical Polishing
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passivation to understand passivity and dissolution modes in tungsten in both static and CMP conditions. EXPERIMENTAL XPS studies were performed on 1.5cm X 1.5cm pieces cut from a 8" diameter silicon wafer coated with 4000 A0 thick LPCVD tungsten layer. Most of the electrochemical studies were performed on a 1" diameter bulk tungsten electrode (99.95% purity, purchased from Target Materials). This electrode was mirror polished for use in electrochemical studies. Using XPS it was observed that as received tungsten coated wafers had an approximately 15 to 20 A0 thick native oxide layer present on the surface. Sample preparation, therefore involved immersing the samples in 45% KOH solution for 5 minutes to remove these native tungsten oxides from the surface. All the chemicals used in this study were of reagent grade purchased from Fisher
Scientific. The pH of the solutions used in these experiments was maintained using various buffer solutions purchased from Fisher Scientific. We used both in-situ and ex-situ electrochemical measurements to understand tungstenoxidizer interaction. Ex-situ electrochemical studies were performed in model EG&G Electrochemical Flat Cell using EG&G potentiostat model 273. CMP of tungsten was performed using a Buehler Minimet 1000 system. In this set-up, polishing pad is stationary and the polishing piece is moved relative to pad causing the abrasion. Polishing experiments were carried out at 4 psi pressure and 30 rpm speed (equivalent to approximately 20 cm/sec linear velocity). The polisher was modified to incorporate in-situ electrochemical measurements. A 1" diameter bulk tungsten sample was used as the working electrode, whereas the stainless steel polishing platen was used as the counter-electrode. Ag/AgCI reference electrode with a vycor tip was used as a reference electrode. The slurries used in these experiments were prepared by diluting the abrasive component of RODEL MSW2000 slurry to 4.3 wt % using buffer solutions containing oxidizers with desired concentrations. XPS was performed using a PHI 5400 ESCA system. Samples were transferred in an inert argon atmosphere to the XPS analysis chamber to minimize the oxide formation on surface due to exposure to air. Non-monochromatic Al Kcu X-ray source (hv= 1486.6 eV) at a power of
350 watts was used for the analysis. The spectrometer was calibrated using Au 4f 7/2 peak at 84±0.1 eV (FWHM of 0.8 eV). Standard data fitting procedures were observed for fitting various core level peaks with component peaks having a Gaussian-Lorentzian distribution. Binding energy reference has been made with respect to C (Is) peak at 284.6 eV.6 RESULTS AND DISCUSSION Macdonald et al. 7 have developed an elaborate theory for formation of oxide films on various
metals based on movement of point defects (ionic vacancies) through the lattice. There are five different processes that need to take place during steady state oxide formation (1) Generation of cation vacancies at the oxide/solution interface because of injection of cations into the solution. These
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