Surface Microstructural Evolution of Ultrathin films by Real time Spectroscopic Elupsometry
- PDF / 581,053 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 74 Downloads / 144 Views
SURFACE MICROSTRUCTURAL EVOLUTION OF ULTRATHIN FILMS BY REAL TIME SPECTROSCOPIC ELLIPSOMETRY R. W. COLLINS,* ILSIN AN,* Y. M. LI,* AND C. R. WRONSKI** * The Pennsylvania State University, Department of Physics and Materials Research Laboratory, University Park, PA 16802. ** The Pennsylvania State University, Department of Electrical and Computer Engineering University Park, PA 16802. ABSTRACT Vapor deposition of smooth, microstructurally uniform amorphous films on dissimilar substrates requires coalescence of clusters that form during initial nucleation. We have developed techniques that provide sub-monolayer sensitivity to this phenomenon, relying on real time spectroscopic ellipsometry observations during ultrathin film growth (thicknesses < 50 A). An investigation of tetrahedrally-bonded amorphous semiconductors lends insights into the role of nucleation density and adatom surface diffusion in determining the ultimate atomic-scale roughness on the film. INTRODUCTION The morphological evolution of thin films is of great scientific and technological interest. In continuum models of film growth, many authors have studied the stability of one-dimensional surface profiles in response to imposed sinusoidal perturbations of wavelength, ?•.r.[1- 4 ] In models of ballistic deposition, effects of finite atomic size[2] and shadowing[4] have been proposed to enhance the perturbations, whereas adatom surface diffusion damps them. A smooth profile is regained for •. less than the adatom diffusion length, X,.o;however, when r>oa modulated profile ultimately develops that appears analogous to experimentally-observed columnar microstructure.[2] In models of chemical vapor deposition (CVD), perturbations are enhanced by the gas phase concentration gradient, whereas they are damped via processes of evaporation and condensation.[1,3] In experimental situations, the surface roughness associated with initial cluster nucleation is a dominant perturbation for thin film deposition on dissimilar substrates. It is technologically important to determine and control the evolution of surface morphology with subsequent growth. For example, when multilayered amorphous films are prepared on smooth bulk substrates, initial growth-related roughness may persist for 1000/ , or more.[5] Of direct importance is the ability to fabricate multilayered structures with atomically smooth interfaces, e.g. for applications in x-ray optics. Of indirect importance is the fact that surface processes leading to complete relaxation, or coalescence, of the nucleation-related surface microstructure often lead to other favorable properties such as low densities of voids or electronically-active defects. In fact, a high adatom diffusion length, which leads to rapid surface smoothening in many film growth models, also enters into a specific model for hydrogenated amorphous silicon (a-Si:H) growth[6] to explain the optimum plasma-enhanced CVD (PECVD) conditions for electronic device quality material. Non-invasive, real time measurements are required to observe the thin
Data Loading...
