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.
A system for monitoring the transient and steady state temperature profiles during the deep UV (DUV) post exposure bake (PEB) is described. The system, called Accura°C, consists of a sensor wafer, a wireless electronics unit and software on a laptop computer. To monitor temperature platinum resistance temperature detectors (RTDs) are embedded into silicon wafers. A flexible high temperature printed circuit (PC) ribbon cable connects the wireless electronics unit to the wafer. The system robot moves both the sensor wafer and electronics unit through the system. Communication between the electronics unit and a laptop computer is accomplished by a Bluetooth RF link. The RF link enables the laptop computer to analyze the temperature measurements in real time. The rechargeable batteries in the electronics unit allow detailed examination of all process chambers. Further the long operating time and real time data stream provide for bake chamber optimization such as tuning. The sensor integration into the wafer provides accurate, artifact free measurements of the rapid temperature changes during PEB ramps.
A system for monitoring the dynamic temperature profile during the deep UV (DUV) post exposure bake (PEB) is described. Platinum resistor temperature detectors (RTDs) are embedded into silicon wafers. Hardware and software convert the resistances to temperatures. The RTD calibration is National Institute of Standards and Technology (NIST) traceable. The wafers are tested on both a thermally uniform hot plate and a production PEB chamber. The temperature profiles for the PEB are fitted to a heat transfer model allowing heating and cooling time constants to be determined. High accuracy and precision are demonstrated.