A method, system and medium are provided for enabling improved feedback
and feedforward control. An error, or deviation from target result, is
observed during manufacture of semi conductor chips. The error within
standard deviation is caused by two components: a white noise component
and a signal component (such as systematic errors). The white noise
component is random noise and therefore is relatively non-controllable.
The systematic error, in contrast, may be controlled by changing the
control parameters. A ratio between the two components is calculated
autoregressively. Based on the ratio and using the observed or measured
error, the actual value of the error caused by the signal component is
calculated utilizing an autoregressive stochastic sequence. The actual
value of the error is then used in determining when and how to change the
control parameters. The autoregressive stochastic sequence addresses the
issue of real-time control of the effects of run-to-run deviations, and
provides a mechanism that can extract white noise from the statistical
process variance in real time. This results in an ability to provide
tighter control of feedback and feedforward variations.