Polycrystalline silicon (polySi) doping is an important process in VLSI manufacturing that allows one to lower the resistivity of the polySi. Diffusion doping with a gas-phase source yields the uniform and heavily doped polySi normally required for gate electrodes. The optical properties of doped polySi are significantly different from those of as-deposited (undoped) polySi. In this work we analyze the effect of these differences on the measurement of the polySi thickness. We show that simultaneous measurement of both the physical properties and thickness of polySi is required for accurate and meaningful metrology control. This requirement identifies ellipsometry as a natural choice for doped polySi measurements, PolySi was deposited under tightly controlled process conditions in a state-of-the-art large-batch APOGEE vertical LPCVD reactor suited for high volume production. PolySi was diffusion doped at 850 degrees C in a separate APOGEE(R) vertical reactor using phosphorous oxychloride POCl3 as an n-type dopant source. Diffusion doped polySi film stack was characterized using a FOCUS dual wavelength multiple angle-of-incidence (DW-MAI) ellipsometer, a research spectroscopic ellipsometer (UV-NIR range), and a resistivity probe. The DW-MAI ellipsometer was shown to provide the required multi-parameter capability for robust measurements of the doped polySi using a simple film stack model. The results of the DW-MAI ellipsometry measurements are correlated with other measurements and are found to be consistent with those obtained on the research spectroscopic ellipsometer (UV-NIR range) and with the resistivity probe.