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Diffusion Furnaces are tube furnaces used in the manufacturing process of semiconductor components. They are used to add doping impurities into high purity silicon wafers, thereby creating embedded semiconductor devices. This process occurs at high temperatures and demands a high degree of measurement accuracy and control stability.
Most diffusion furnaces have 3 heating zones along with some gas flow control loops. Additionally some processes are conducted under vacuum. Diffusion furnaces are installed in “Stacks”, which simply means they are mounted one above the other. A stack will usually have 4 or 5 furnaces, each requiring a 3 loop temperature controller.
A three loop 2604/2704 controller is used. Loop 1 controls the centre and loops 2 and 3 control the outer zones, control outputs are normally 4-20mA to thyristor units. The same program setpoint is used in all loops and the programmer also has the capability of generating additional setpoints for remote gas controllers.
Some applications require 3 cascade loops and this becomes possible by using the dual analogue input (DP) module together with external Action Instruments signal conditioners (For example Q488). Such a setup allows 6 analogue inputs, three of which are high precision thermocouple inputs along with 3 analogue outputs.