Ellipsometry as a Sensitive Technique to Probe film -Substrate Interfaces: Al 2 O 3 on Si(100)
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Ellipsometry as a sensitive technique to probe film -substrate interfaces: Al2O3 on Si(100) M. P. Singh1, G. Raghavan2, A. K. Tyagi2, and S. A. Shivashankar1 1. Materials Research Centre, Indian Institute of Science, Bangalore, India 2. Indira Gandhi Centre for Atomic Research, Kalpakkam, India
ABSTRACT
An attempt has been made to study the film-substrate interface by using a sensitive, nonconventional tool. Because of the prospective use of gate oxide in MOSFET devices, we have chosen to study alumina films grown on silicon. Film-substrate interface of alumina grown by MOCVD on Si(100) was studied systematically using spectroscopic ellipsometry in the range 1.5-5.0 eV, supported by cross-sectional SEM, and SIMS. The (ε1,ε2) versus energy data obtained for films grown at 600°C, 700°C, and 750°C were modeled to fit a substrate\interface\film “sandwich”. The experimental results reveal (as may be expected) that the nature of the substrate -film interface depends strongly on the growth temperature. The simulated (ε1,ε2) patterns are in excellent agreement with observed ellipsometric data. The MOCVD precursors results the presence of carbon in the films. Theoretical simulation was able to account for the ellipsometry data by invoking the presence of “free” carbon in the alumina films.
INTRODUCTION
An attempt has been made to study the film-substrate interface, which as an open problem in thin film science and technology field [1,2], by using a sensitive and nonconventional tool. Because of the prospective use of Al2O3 as the gate oxide in MOSFET devices for next generation in ULSI circuitry, we have examined the growth of Al2O3 on silicon(100). Moreover, Al2O3 films are used widely in the various fields ranging from optical and electric devices to abrading coatings in cutting tool inserts and as protective layers [3- 6]. Ellipsometry [7] is a sensitive technique where the state of polarization of light reflected from a surface is analyzed in terms of the relative phase and amplitude of the parallel and perpendicular components of the electric field. These changes in the state of polarization are characteristic of the effective dielectric response of the materials. The measured ellipsometric parameters ψ and ∆ (often written as tanψ and cos∆) may be expressed as ρ = (rp/rs) = tanψ exp(i∆)
.........(1)
AA3.33.1
where ρ is the ratio of rp and rs : the complex Fresnel coefficients for parallel and perpendicular components of the light, respectively. The real and imaginary parts of the complex dielectric function can be deduced by measuring the ellipsometric data as a function of the wavelength of the incident light and deducing the date through a multi-parameter regression technique. In the case of thin films, wherein a multilayer model is assumed, the parameters are deduced through a fitting procedure. In the case of compositionally graded layers, effective medium theories are used to compute the average response of the layer. Thus it is possible to carry out a depthresolved analysis of the film-substrate interface
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