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Session 2259

Programmable PID Temperature Control of Multi-Tube Multi-Zone Diffusion Furnaces M.G. Guvench, R. Stone, S. Pennell and R.Worcester University of Southern Maine

Abstract

This paper describes the design, operation and performance results obtained with a programmable temperature and gas flow controller designed to control a multi-tube multi-zone diffusion system. The system was built and used for 4” silicon wafer processing at University of Southern Maine’s MicroFabrication laboratory. The diffusion furnace employed is a four-tube stacked Thermco furnace, with its each tube operating in three temperature zones powered from a bank of 3-phase transformers. The power delivered to each zone is controlled separately by varying the duty cycle of 40VRMS transformer outputs via zero-angle firing thyristors. The control system was built with off-the-shelf DAQ, digital and analog IO cards housed in a 586-class PC. Simultaneous PID control of twelve temperature zones and twelve switched gas lines was achieved with LabView operating under Windows’95. The controller is designed to operate as a slave to a programmable master scheduler which generates for each tube up to ten intervals of time in each of which temperature is varied linearly. In this way a piecewise linear variation of diffusion/oxidation temperature is obtained, allowing the wafer to go through a recipe of diffusion/oxidation/annealing sequence at various temperatures and in different gas compositions with fully controlled rates of ramp up or ramp down. A temperature control of ± 1 C RMS is shown to be achievable which is mostly limited by noise in the temperature readings.

1. Introduction

In the university operated microfabrication laboratories, unlike an industrial production setting, the diffusion/oxidation furnaces employed for semiconductor device and integrated circuit fabrication have very small average usage time due to the inherent low-volume of such operations[1,2]. In these laboratories the standard practice is to dedicate each tube of a diffusion/oxidation furnace to a different process, set the temperature of each tube at its dedicated process temperature and maintain the temperature at its constant value day and night. The wafer batch is loaded into the appropriate tube operating at the appropriate temperature with appropriate gas flows for a prescribed amount of time for each individual diffusion/oxidation cycle. Considering a four-tube set-up with each tube running at 10 KW, the amount of the electrical power wasted by such an operation is enormous.

The goal of this project was to control a diffusion system in a programmable way via a PC so that once the silicon wafers are loaded, a sequence of thermal processes could be commanded by the computer to the furnace tubes and the gas controllers to go through a recipe of temperature and gas compositions starting from and ending at a low stand-by temperature, thus achieving automation as well as conserving energy.

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Pennell, S., & Worcester, R., & Stone, R., & Guvench, M. (1998, June), Programmable Pid Temperature Control Of Multi Tube Multi Zone Diffusion Furnaces Paper presented at 1998 Annual Conference, Seattle, Washington. 10.18260/1-2--7364