Extension of multichannel spectroscopic ellipsometry into the ultraviolet for real time characterization of the growth of wide bandgap materials from 1.5 to 6.5 eV

In this article, we report the results of a successful effort to extend rotating-polarizer multichannel ellipsometry into the ultraviolet (uv) spectral region above 5 eV. Key modifications over previous system designs include (i) incorporation of a see-through deuterium (D₂) lamp that allows a tandem Xe/D₂ source configuration for a usable spectral output from 1.5 to 6.5 eV, (ii) MgF₂ Rochon polarizers for high transmission in the uv without the need for optical activity corrections, and (iii) a spectrograph with a concave grating blazed at 2500 angstrom and internally mounted order-sorting filters. With these modifications, we can collect 132-point ellipsometric spectra over the range from 1.5 to 6.5 eV with a minimum acquisition time of 24.5 ms (one optical cycle). For averages over two and eighty optical cycles (requiring 49 ms and 1.96 s, respectively), the standard deviations in (ψ, Δ) are (0.04°, 0.08°) and (0.008°, 0.015°), respectively, measured on a thermally-oxidized silicon wafer at a photon energy of 6 eV. In this first report, we briefly describe problems faced in the development of the new instrument. As an example of the applications of the instrument, we present the results of a real time study of the growth of a hexagonal boron nitride (BN) thin film by rf magnetron sputtering on a Si wafer substrate at a temperature of 250°C. The instrument is expected to be useful in BN film growth studies since the additional data in the uv assist in discriminating the wide band gap (E_g) hexagonal (4 <E_g <5.8 eV) and cubic (E_g > 6 eV) phases of this material.