The Influence of pH and Temperature on Polish Rates and Selectivity of Silicon Dioxide and Nitride Films
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Mat. Res. Soc. Symp. Proc. Vol. 566 0 2000 Materials Research Society
were used in the polishing experiments. The silica slurry used was SS-25 (Cabot Corporation) diluted 1:1 with DI water. Adjustments of the slurry pH was carried out with either 11.6 N HCI or 40% KOH solution. Ceria polishing was carried out with a 30 rpm table and spindle speed, a 240 mLimin slurry flow rate and a 5.6 psi down force. Silica polishing was carried out with a table and spindle speed of 75 rpm each, 200 mL/min slurry flow rate, and with a 6 psi down force and a 1.5 psi wafer back pressure. The effect of the temperature of ceria slurry on polishing rate was investigated using heated slurry and measuring the temperature of the slurry as it flowed onto the polishing table. Warm water at the temperature of the slurry was flowed over the pad for three minutes before each run to ensure that the pad does not reduce the temperature of the slurry appreciably. The effect of pH on patterned TEOS oxide wafers was also investigated using the MIT CMP Characterization Mask Set pitch mask [1]. The pitch mask is a 6 x 6 array of various equal width lines and spaces. The lines and spaces have widths ranging from 2 jim to 1000 Rm. The 12 mm pitch pattern was first created in photoresist across the entire wafer. The exposed oxide was plasma etched to a depth of about 7000/ Abelow the surface. This created an array of 36 subarrays with equal width lines and spaces so that half the area was lines and the other half spaces. The wafers were polished with silica slurry at 3 different pH values. The oxide thickness at the plateau area (starting surface of the oxide) for 10 sub-patterns of line widths 40, 60, 80, 100, 125, 150, 180, 200, 250, and 500 gim were measured before and after CMP. The measurements were made in each sub-array in increasing order of line/space width. Five dies were measured along the diameter of the wafer. RESULTS AND DISCUSSION Effect of Temperature on Polish Rate for Ceria Slurry The effect of temperature of slurry containing ceria particles (average diameter 440 nm and IEP 8.5), on the removal rates of TEOS oxide and silicon nitride was found to be weak as seen in Figure 1. This is similar to the effect of temperature of a silica based slurry on silica polishing in the temperature range of 20-50 °C for the slurry with the pad maintained between 25-30 'C [2]. The pad hardness decreases with an increase in temperature [2] and the associated reduction in removal rate may have compensated for any increased chemical removal caused by the increased temperature. Effect of pH on Polish Rate for Ceria and Silica Slurries Figure 2. shows the change of polish rate of silicon dioxide and silicon nitride as the pH of the silica slurry changes from 9.7 to 11.3. There is a steady increase in the TEOS polish rate as the pH increases which is expected since silicon dioxide is known to have an increasing dissolution rate with increasing pH. The silicon nitride, on the other hand, shows a drop in polish rate as the pH increases, changing from about 7
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