Characterization of Sputtered Cerium Dioxide Thin Films
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silicon substrates have similar optical properties and surface morphology. It is known that surface or interface roughness significantly affects the performance and reliability of thin film devices. As the films become thinner, the quality of the oxide films increases in importance. Therefore, a quantitative characterization of roughness is necessary not only for the optimization of fabrication processes, but also for evaluation of device performance. In this study, the optical properties of CeO 2 films grown on sapphire and silicon substrates were investigated using variable angle spectroscopic ellipsometry (VASE) and a multiple optical model. The surface morphology of the films was characterized by both VASE and atomic force microscopy (AFM) measurements. A post-deposition heat treatment was performed to improve the crystalline quality and surface morphology of the CeO 2 layers. 2 EXPERIMENTAL DETAILS The CeO 2 films on sapphire were deposited in an rf magnetron sputtering system with an off-axis configuration 9. The CeO 2 films on silicon were fabricated with an on-axis configuration in an rf magnetron sputtering system. The sputtering conditions were as follows: rf power -100W, frequency 13.56 MHz, base pressure 2x10 7 Torr, deposition pressure 3x10 2 Torr with an Ar/0 2 ratio of -10:1 and substrate temperature -600 °C. In order to examine the effects of heat treatment, post-annealing processes were carried out at 760 Torr (1 atm) in an 02 ambient 209 at either 930 or 1100 °C for 30 min. Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
AFM measurements were done at room temperature using a commercial Nanoscope III system (Digital Instruments, Santa Barbara, CA, USA). The instrument was operated in tapping mode using silicon cantilevers (Nanoprobe, Germany) oscillating with an amplitude of typically 100 m and at a resonance frequency between 250-450 kHz. AFM measurements were carried out on lxi pm2 and 5x5 pm2 areas. The root mean square roughness, R,.,, and peak to valley roughness, R. (Fig. 1), were calculated from the 5x5 pn 2 scans. Overviews of surface morphology on different samples are shown for lxi pi 2 scan fields. X-ray diffraction (XRD) measurements were performed on a Philips PW1729 using CuIL radiation to investigate the crystalline quality of the films. A cross-section of a sapphire sample for transmission electron microscopy (TEM) was prepared by a precision polishing method'0 and examined in a JEOL 2000FX at 200 KeV. TEM studies revealed a columnar structure (Fig. 1). The top of each 'column' is pointed and therefore presents a rough and lower density film for ellipsometric examination. An ellipsometric roughness d, was defined to show the depth of change in the surface. R.=
"Rough Layer"'
U kMtm L Myer
d z
Substrate
Fig. 1. (a) A cross-sectional TEM image of a CeO 2 film and (b) a schematic diagram
showing the AFM parameters R, and R.., the ellipsometric roughness d, and the 2-layer concept.
The ellipsometric parameters, the amplitude ratio 'Nf' and the relative phas
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