Extension of Multichannel Spectroscopic Ellipsometry into the Ultraviolet for Real Time Characterization of the Growth o
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Materials Research Laboratory and the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802. Materials Research Laboratory, the Department of Physics, and the Center for Thin Film Devices, The Pennsylvania State University, University Park, PA 16802.
ABSTRACT 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/D 2 source configuration for a usable spectral output from 1.5 to 6.5 eV, (ii) MgF 2 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 A 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 (y, A) are (0.040, 0.08') and (0.0080, 0.0150), 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 (Eg) hexagonal (4 < Eg < 5.8 eV) and cubic (Eg > 6 eV) phases of this material. INTRODUCTION AND MOTIVATION Multichannel ellipsometry has emerged as a powerful method for the analysis of thin films from spectroscopic data collected in real time [1]. Previous instruments employed the following sequence of components: (i) a Xe lamp as a broadband light source, (ii) a collimator, (iii) a continuously rotating polarizer as an incident polarization modulator, (iv) a reflecting sample, (v) a fixed analyzer, and (vi) a spectrograph and silicon photodiode array as a multichannel detection system. Such instruments are capable of acquiring spectra in the ellipsometric angles (w, A) and the polarized reflectance R over the spectral range from 1.5 to 4.5 eV with a minimum acquisition time of nt/o.-15 ms, where co is the mechanical rotation frequency of the polarizer. The upper spectral limit generally arises from the fall-off in the output of the Xe lamp in the ultraviolet (uv), sometimes compounded by a reduction in the overall spectral response of the ellipsometer with increasing photon energy due, e.g., to increased reflection losses by the optical elements, increased absorption by polarizers, and decreased spectrograph grating efficie
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