Rapid Verticai Processor for Fast-Ramp Small-Batch Diffusion and Oxidation Applications
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Pedestal Wafer Boat
Figure 1. Cross Section of Atmospheric Reactor 207 Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
which directs process gasses into the process tube. Furnace temperature is monitored and controlled via "profile" thermocouples located strategically inside the process chamber in each of five control zones of the heater element. These profile thermocouples provide an approximation of the wafer stack temperature. "Spike" thermocouples are positioned outside of the quartz process chamber in close proximity to the element windings in each of the five control zones. These spike thermocouples provide an approximation of the heater element temperature. FAST-RAMP FURNACE DEVELOPMENT A new high watt density element was designed to deliver the high power levels required for fast-ramp up in a batch furnace. This new element consists of a stack of helical coils. The individual coils are optimized into coil groups which are mapped together to form the 5 control zones of the furnace element. Nine solid state power controllers are used to modulate power delivered to the coil groups. Each controller is capable of delivering up to 13.8 kilowatts. Instantaneous ramp up rates of over 100°C/min. are achievable. An innovative quartz boat (54-200mm wafer capacity) was designed to allow for maximum
temperature ramp rates while maintaining minimum radial delta temperature (AT) within each
wafer. The unique feature of this boat that reduces thermal stress is a series of semi-transparent quartz bands encircling each wafer in the load. These bands reduce the intensity of radiation incident on the edge of the wafer and ultimately result in a more uniform heat flux across the wafer surface. During ramp down, the bands act in the converse mode providing a thermal mass buffer between the edge of the wafer and the rapidly cooling reactor walls. The result is a more uniform temperature across the wafer while ramping down. In conjunction with increased wafer spacing, the bands minimize radial AT on each wafer and thus minimize within wafer stress during periods of rapid heating and cooling. The final feature incorporated into this new system is a forced air cooling system which enables rapid cooling. Air is drawn through the furnace and flows through the annular space between the process tube and the furnace element coils. In this manner heat is drawn from the heating elements and quartz process tube and is carried out the upper openings and into the exhaust system. Cool down rates of up to 40'C/min. are achieved. This is a significant improvement over conventional furnace ramp down rates of 2 to 4°C/min. TEMPERATURE CONTROL Model Based Temperature Control (MBTC) is used in conjunction with Proportional Integral Derivative algorithms (PID) to control furnace and wafer temperature. See MBTC/PID control mode for each process step in Table I. The term Model Based Control is used to denote a broad class of control approaches based on systematic use of a mathematical model of the process in determini
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