Characterization of Thin DLC Films Using Film Stress and Raman Techniques
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383 Mat. Res. Soc. Symp. Proc. Vol. 517 01998 Materials Research Society
EXPERIMENT This study focused on identifying the structural change in the DLC bonding phases during thermal annealing using analytical means like residual stress measurement and Raman spectroscopy. The purpose of the work was to understand the mechanism of structural evolution that led to significant changes in the film stress and Raman spectra of the DLC films resulting from thermal anneal. Five substrates were deposited with a DLC film by an ion beam CVD process using methane gas. Deposition rates of approximately 1.5 nm / minute were measured in this process. The targeted total thickness of the overcoat was 10 nm consisting of 2 nm of a silicon interlayer and 8 nm of DLC. The film thickness was measured on silicon coupons that were coated along with the samples in an n&k optical analyzer. The DLC coated substrates in this experiment were 75 mm [100] silicon wafers. The Raman spectroscopy was performed by backscattering from the sample using a Renishaw spectrometer with an argon ion laser (514 nm) operating at 20 mW to avoid sample degradation. The spectra were collected with a laser spot size of approximately 1 to 2 p. The DLC film stress was measured with a KLA-Tencor FLX-2320 film stress tool. The wafer curvature before and after DLC deposition was measured, and the change in curvature caused by the deposition was used to automatically calculate the film stress at room temperature. The FLX2320 also allows sample heating to 500 'C while continuously monitoring the wafer curvature, so the film stress as a function of temperature during the thermal anneal can be obtained. The room temperature film stress was measured on all 5 samples. Two samples were thermally cycled up to 500 'C at 5 degrees / minute then down to 80 'C at -5 degrees / minute. Five scfh of nitrogen is flushed through the small slot furnace of the FLX-2320 to reduce the influence of oxygen and moisture. One wafer was heated to 300 °C at 10 degrees / minute and held for 72 hours to monitor the relaxation of the film stress over time. RESULTS Film Stress Measurement The room temperature stress measured on the 5 wafers of the study is shown in Table I where each value is the average of 2 measurements. The average stress of the entire data set is -2.24 GPa, and the standard deviation is 0.12 GPa. The minimum expected standard deviation of the stress measurement tool for a 10 nm film on a 375 g substrate is about 0.05 GPa, so the numbers show reasonably typical scatter, and the deposition process was stable and uniform. Table I. Room temperature stress measurements from 5 DLC films Wafer RTlO1 RT102 RT103 RT104 RT105
DLC Thickness
Stress
(nm)
(GPa)
9.7 9.6 9.9 9.8 9.8
-2.27 -2.39 -2.11 -2.12 -2.29
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The stress behavior of wafer RT103, monitored during a thermal cycle to 500 'C, is shown in Figure 1. While heating below 100 'C, no change in stress is seen. At about 100 'C the high compressive stress begins to quickly relax towards zero, reaching zero stress near the e
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