Reduction of across-wafer CDU via constrained optimization of a multichannel PEB plate controller based on in-situ measurements of thermal time constants

Abstract

As line widths approach 90nm node in volume production, post exposure bake (PEB) uniformity becomes a much larger component of the across wafer critical dimension uniformity (CDU). In production, the need for PEB plate matching has led to novel solutions such as plate specific dose offsets. This type of correction does not help across wafer CDU. Due to unequal activation energies of the critical PEB processes, any thermal history difference can result in a corresponding CD variation. The rise time of the resist to the target temperature has been shown to affect CD, with the most critical time being the first 5-7 seconds. A typical PEB plate has multi-zone thermal control with one thermal sensor per zone. The current practice is to setup each plate to match the steady-state target temperature, ignoring any dynamic performance. Using an in-situ wireless RTD wafer, it is possible to characterize the dynamic performance, or time constant, of each RTD location on the sensing wafer. Constrained by the zone structure of the PEB plate, the proportional, integral and derivative (PID) settings of each controller channel could be optimized to reduce the variations in rise time across the RTD wafer, thereby reducing the PEB component of across wafer CDU.

Modeling and Real-Time Control of Multizone Thermal Processing System for Photoresist Processing

“…Requirements call for the temperature to be controlled within 0.1 °C across the wafer at temperatures between 70 and 150 °C. 4,5 A number of recent investigations also showed the importance of proper bake plate operation, both in steady-state and transient, on CD control. 2,6−8 Two approaches exist in addressing this issue in industry.…”

“…This is attributed to the temperature distribution while rising to the postexposure bake (PEB) temperature. 5 In this work, we propose a real-time wafer temperature control method to minimize temperature nonuniformity in the baking steps and improve the dynamic performance of the wafer temperature. In contrast, our present model adopts the full thermal diffusion formalism so that the thermal behavior during the initial transient period is properly captured.…”

“…The most temperature sensitive step in the lithography sequence is the postexposure bake step. Requirements call for the temperature to be controlled within 0.1 °C across the wafer at temperatures between 70 and 150 °C. , A number of recent investigations also showed the importance of proper bake plate operation, both in steady-state and transient, on CD control. ,− Two approaches exist in addressing this issue in industry. The first approach involves the development of less temperature sensitive photoresist.…”

“…It has also been reported that even though the resultant range of steady-state temperatures was minimized, the consequent gain in CD uniformity cannot be realized. This is attributed to the temperature distribution while rising to the postexposure bake (PEB) temperature …”