Selection of measurement locations for the control of rapid thermal processor

Rapid thermal processing (RTP) offers a growing potential in integrated circuit manufacturing as the feature sizes in ULSI move further down into the subhalf micrometer technology. The main problem of RTP control lies in temperature uniformity. In this work, process characteristics of RTP systems are explored and difficulties in control are explained. Two important factors have to be taken into account in the RTP temperature control. They are: temperature uniformity across the wafer and the controllability of the resultant control structure. Measurement selection criteria are devised to achieve both these objectives. The conventional equal-spaced measurement selection criteria in general cannot guarantee temperature uniformity. A nonlinear measurement selection criterion is proposed to overcome this problem. An alternative procedure is also explored to achieve both objectives by varying the temperature set points. Once the control structure is determined, internal model control (IMC) principle is employed for the design of multivariable controllers. As a result of ramp inputs, this leads to a PID type of controller with double integrators: a PI^2D controller. Nonlinear compensation is also implemented to deal with a wide range of set point changes. Simulation results show that a 42% improvement in temperature uniformity can be achieved by using the proposed design procedure.

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