The Intercorrelation Between Microstructure and Chemicalmechanical Polish of Metal Thin Films
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sensitive to the deposition processi and pattern geometry 2 and density.3 These variations in microstructure would have certain impacts on any subsequent manufacturing process (such as CMP) following metal deposition. Wang et al. found that the CMP removal rate increases with increasing Cu content and decreasing grain size in Al thin films. 4 In this study, we examine the microstructure of metal thin films after polish and report a CMP-induced microstructure alteration phenomenon, which may have certain impacts on the reliability of interconnects. EXPERIMENT Both sputter-deposited Al and chemical-vapor deposited W thin films were chosen for investigation. During deposition, the temperature is varied to induce different microstructure and grain size. Al films containing 1% Cu were sputtered onto 150 mm oxidized Si wafer (oxide thickness = 50 nm) to a thickness of 600 rim. W films were grown on 150 mm oxidized Si wafers using WF 6 + Sill4 gas chemistry to a thickness about 600 rnm. After deposition, both Al and W wafers were annealed in Ar atmosphere at 400° for 1 hour to establish the microstructure. CMP experiments were performed on an IPEC 372M polisher with Rodel XARS 1627 composite pad. For Al CMP, the slurry consists of 5 wt.% 0.05 4tm A120 3 abrasive particles and 5 vol. % hydrogen peroxide. Citric acid and phosphoric acid were added to adjust the pH to -3.0. For W CMP, alumina-based peroxide-added slurry with pH -2.5 was used. The head speed was fixed at 42 rpm, whereas the table speed and down force were varied. After CMP, the wafers were buffed 459 Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
in D.I. water for 10 seconds. Removal rate was measured at 49 points across the wafer and the average was taken for reference. AFM and SEM were employed to characterize the.roughness and grain size, while X-ray diffraction (Bragg-Brentano method) was used to determine the crystallographic orientation of the metal films. The resistivity of metal films was derived from measurements of sheet resistance and SEM cross-section. Light reflectance of metal films is determined using i-line (wavelength = 365 rnm). Resistivity and roughness measurements were conducted at same spots for a total of 9 spots across the wafer. RESULTS
The grain size and resistivity of the various Al and W films after deposition and anneal are summarized in Table I. As expected, grain size increases with increasing substrate temperature. The increase in grain size also increases the roughness, which in turn reduces the reflectance. For Al films deposited at 400. , Cu precipitates can be identified at grain boundaries after anneal. The resistivity, on the other hand, does not increase significantly with substrate temperature. However, the change in resistivity is more distinct for W films. This may result from the impurities incorporated during deposition at higher temperatures. Table I: Grain size (d), 1-a grain size distribution, i-line reflectance (R), root-mean-square roughness (R,) and resistivity (p) of the Al and W fil
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