A stack of nitride/oxide/amorphous Si/oxide/nitride on a glass substrate was fabricated and subsequently analyzed with spectroscopic ellipsometry. The oxides and nitrides were deposited by plasma-enhanced chemical-vapor-deposition and the amorphous Si was deposited with sputter deposition. The optical constants of these materials depend on deposition conditions and hence handbook values cannot be used. The optical constants of the amorphous Si layer are modeled as a combination of Lorentz oscillators. A further complication is that the amorphous Si layer is opaque over about half of the spectral range. Drawbacks of the straight-forward analysis method are discussed and an enhanced analysis method is described and the results presented. The enhanced method improves the uncertainty in the thickness determination of two of the layers by an order of magnitude.