Manufacturing issues related to RTP induced overlay errors in a global alignment stepper technology

The effect of rapid thermal processing on wafer distortion and overlay accuracy in global alignment photolithography in the fabrication of 0.85 μm CMOS Flash EPROM integrated circuits was studied. Both rapid thermal process parameters and system design (single and multi-lamp processors) were evaluated for their effect on overlay accuracy. It was found that a rapid thermal process (following contact etch and ion implantation) at set temperatures greater than or equal to 950°C resulted in interconnect metallization-to-contact overlay errors in excess of 1.0 μm across the wafer, which led to a 20% functional circuit yield loss. In the case of the single lamp processor, this misalignment was attributed to wafer distortion due to the temperature overshoot during the ramp step, which subsequently resulted in an across wafer temperature range of greater than 120°C. This temperature overshoot and nonuniformity was eliminated by reducing the ramp rate below 100°C/s. This ramp rate reduction, however, decreased the system wafer throughput, and required optimization to eliminate the overlay errors and minimize the effect on throughput. In this study, a 60°C/s ramp rate was found to be optimum. For the multi-lamp RTP system, the metal-to-contact overlay error was not observed. This was believed to be due to the design of the heating mechanism in the multi-lamp processor, which did not produce the large wafer temperature overshoot and nonuniformity that was observed in the single lamp processor