Characterization of Surface Roughness on Aluminum, Amorphous Germanium, Amorphous GERMANIUM:HYDROGEN, and Amorphous Silicon Thin Films by Spectroscopic Ellipsometry.

Spectroscopic ellipsometry (SE) has been used to detect and characterize roughness on the surface of aluminum (Al), amorphous germanium (a-Ge), hydrogenated amorphous germanium (a-Ge:H), and amorphous silicon (a -Si) thin films. The measurements were carried out at an angle of incidence of 70 degrees and at a number of discrete wavelengths in the spectral range 300-700 nm for all the materials. In addition, the Al and a-Si samples were measured at a number of angles of incidence in the range 30-80 degrees over the same spectral range. Due to the high precision of the SE measurements, rms surface roughness on the order of 10 (ANGSTROM) was detected on the Al samples. The rms surface slope of these samples was calculated from the measured ellipsometric parameters (DELTA) and (psi) using the scalar diffraction theory of Ohlidal and Lukes (OL). The results at a 70 degree angle of incidence were compared with those obtained from total integrated scattering (TIS) and profilometry measurements. A good qualitative agreement was found. The calculated rms surface slope, however, was found to be a function of the angle of incidence and wavelength at angles of incidence between 30 and 50 degrees. This result was interpreted as indicative of the lack of generality of the OL theory. The amorphous semiconductor samples were also measured using SE. The measured (DELTA) and (psi) parameters were analyzed using effective medium theories and standard n-layer models. These models showed that these amorphous thin films exhibited a top layer surface roughness consisting of a mixture of bulk material and void and/or an oxide layer. The material below this top surface region for some a-Ge films was found to exhibit a density greater than that of crystalline Ge, when compared to the directly measured density of a-Ge films of Paul, Connell, and Temkin (1973). The a-Ge:H films, however, were found to exhibit a density-deficient region. This deficiency, expressed as % void, was found to correlate 1:1 with the % H content of these films up to about 10% H. The validity of these models was reinforced by the results obtained from Auger electron spectroscopy (AES), scanning electron microscopy (SEM), infrared (IR) spectroscopy, and cross-section transmission electron microscopy (XTEM).