The analysis of in-situ ellipsometry data by a digital signal processing technique is described. The deposition rate and thickness of LP-VPE arsenic-doped silicon ([As] less than or equal to 1 x 10(18) cm(-3)) are estimated in real time at 700 degrees C. The effect of arsine pre-exposure upon dopant uniformity and deposition rate is demonstrated. The estimator is shown to be sufficiently robust and accurate under a range of growth conditions to be suitable for automatic control applications.