An important process in semiconductor device manufacturing is plasma etching (or reactive ion etching). In this unit operation, reactive ions are used to selectively remove (etch) layers of solid films on a wafer. A major objective of a control system is to achieve relatively etch rates (angstroms/second) by manipulating the process inputs. Currently etch rate measurements are not readily available, so it is assumed that by measuring and controlling other variables that good control of the etch rate can be obtained.
Two important variables to control are the voltage bias of the plasma and the fluorine concentration, since they ultimately determine the etch rate. Two important manipulated variables are the RF power and the outlet valve position (throttle). A schematic of a reactive ion etcher is shown in Figure 1.
A reactive ion etcher has power and throttle position inputs (P and T) and voltage (Vbias) flourine concentration ([F]) as outputs.
Figure 1. Reactive Ion Etcher
A general review of modeling and control in semiconductor device manufacturing is provided in an article by Badgwell et al. (1995). A general overview of plasma etching is provided in a number of books (Wolf and Tauber, 1986; Lee, 1990; Sze, 1988).
Badgwell, T.A., T. Breedijk, S. G. Bushman, S.W. Butler, S. Chatterjee, T.F. Edgar, A.J. Toprac and I. Trachtenberg, "Modeling and Control of Microelectronics Materials Processing," Comp. Chem. Engng., 19(1), 1-41 (1995).
Lee, H.H., Fundamentals of Microelectronics Processing, McGraw-Hill, New York (1990).
Sze, S.M., VLSI Technology, McGraw-Hill, New York (1988).
Wolf, S. and R.N. Tauber, Silicon Processing for the VLSI Era, Lattice Press, Sunset Beach, CA (1986).
Questions to answer:
Are fundamental (first principles) models of plasma etchers easy to develop?
Will fundmental models or input/output tests (i.e. step tests) normally be used for control system design?
The nominal operating conditions for this system are:
Power: 1000 W, range from 0 to 2000 W.
Throttle: 50%, range from 0 to 100%.
Voltage, 250 V
Fluorine, 50% of range
Disturbances of up to +/- 25 W in power are possible.
A SIMULINK block diagram for a reactive ion etcher is shown below. The inputs and outputs of the etcher are in physical (not deviation) variables.