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TECHNICAL PAPERS

# Change in Radiative Optical Properties of $Ta2O5$ Thin Films due to High-Temperature Heat Treatment

[+] Author and Article Information
Ramesh Chandrasekharan, Shaurya Prakash

Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801

Mark A. Shannon1

Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801mshannon@uiuc.edu

R. I. Masel

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews, Urbana, IL 61801

1

Corresponding author.

J. Heat Transfer 129(1), 27-36 (May 03, 2006) (10 pages) doi:10.1115/1.2401195 History: Received October 14, 2005; Revised May 03, 2006

## Abstract

Thin films ($0.85μm$, $3μm$) of $Ta2O5$ deposited on Si and $SiO2$ were heated to $900°C$. Their reflectance in the infrared was measured using a Fourier transform infrared spectrometer equipped with a multiple angle reflectometer before and after exposure to the high-temperature heat treatment. An interfacial layer $(TaSixOy)$ formed by the diffusion of Si from the substrate into the deposited film was observed using Auger depth profiling, and the effect of this interfacial layer on the reflectance was measured. Using a least squares optimization technique coupled with an optical admittance algorithm, the multiple angle reflectance data were used to calculate the optical constants of the as deposited $Ta2O5$ film, crystalline $Ta2O5$, and the interfacial layer in the 1.6 to $10μm$ range. The interfacial layer formed due to exposure to high temperature was found to be more absorptive than the crystalline $Ta2O5$.

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## Figures

Figure 1

Schematic of a multilayer thin film stack for Eqs. 1,2,3,4,5. Each thin film layer has its own admittance as given by Eq. 1 and the whole stack’s reflectance is given by Eq. 5. The individual layers could be a homogenous film or be the interfacial layer between two different films.

Figure 2

Example of reflectance curve (specular reflection, s polarization 45deg). For clarity the plot shows reflection as a function of the wavenumber (instead of wavelength). The positions of the interference peaks and troughs from plot (a) are used to plot (b).

Figure 3

(a) Auger depth profile of an amorphous Ta2O5 film on Si (as deposited). The Auger technique does not give exact concentration information and needs a standard sample for calibration. Here the calibration was done against a thermally grown Ta2O5 thin film sample whose composition was verified independently using XPS and XRD. Similarly, the Auger sputtering time is a rough indicator of the depth and needs a standard for calibration. Note the sharp transition from film to substrate and the homogeneous material composition with depth. (b) Auger depth profile of a Ta2O5 film on Si heated to 900°C. Note the formation of a TaSixOy interfacial layer between the Ta2O5 film and Si substrate. Also note the inhomogeneous nature of the film and the interfacial layer.

Figure 4

(a) Specular reflectance (s polarization, 45deg) of the as-deposited Ta2O5 film on Si. Note the absorption peak at 2.82μm (absorption peaks positions are confirmed from measurements at different angles). The reflectance maxima from 1.6μm to 2.7μm all equal reflectance of bare substrate. Also note the decreasing value of the minima from 4μm onward (indicating changing n and k). Linear plots similar to Fig. 2 show an approximately constant value of n from 1.6μm to 2.7μm and then from 3μm to 10μm. (b) Specular reflectance (s polarization, 45deg) of heated (900°C)Ta2O5 film on Si. Note the large change in reflectance values at lower wavelengths and the introduction of absorption peaks at higher wavelengths. The monotonic increase in the reflectance from 1.6 to ∼7μm indicates a constant value of k. Linear plots of peak positions similar to Fig. 2 show an approximately constant value of n from 1.6μm to 10μm wavelength.

Figure 5

Auger depth profile of a heated Ta2O5 film on SiO2. Comparing with the heated Ta2O5 film on Si (Fig. 3), note that the interfacial layer formed between the SiO2 and the tantalum pentoxide is much thinner and also the film itself is more homogenous in material composition.

Figure 6

(a) Specular reflectance (s polarization, 45deg) of as-deposited Ta2O5 film on thermally grown SiO2. Note the absorption peak at 2.82μm (this is the same peak as seen in Fig. 4 for as-deposited Ta2O5 on Si). The values of the maxima do not correspond to the bare substrate given the thermal oxide layer present, giving rise to multilayer reflection enhancement. (b) Specular reflectance (s polarization, 45deg) of heated (900°C)Ta2O5 film on thermal oxide. Note the change in reflectance values at lower wavelengths compared to the as-deposited Ta2O5.

Figure 7

(a), (b) Comparison of the measured (dotted) and fitted (complete line) reflectance for the as-deposited Ta2O5, (3μm thick, amorphous). The figures are shown for 30deg (on the right) and 45deg incident and reflected angles (c)n and k values as a function of wavelength for the as-deposited Ta2O5 film. Note that the k values are shown as negative to signify that the N=n−ik notation was used in this study.

Figure 8

(a) Comparison of the measured (dotted) and fitted (complete line) reflectance for Ta2O5 heated to 900°C for 9h, (overall film 2.81μm thick, crystalline). Also, the optimization routine gave the thickness of the interfacial layer to be 0.41μm. The figures are shown for 30deg (on the right) and 45deg incident and reflected angles (b)n and k values as a function of wavelength for the Ta2O5 film. (c) n and k values for the interfacial layer between film and substrate.

Figure 9

Comparison of measured (dotted) and fitted (complete line) reflectance for a 0.85-μm-thick Ta2O5 film (as-deposited) on 0.1μmSiO2. The fitted reflectance was calculated from the n and k values shown in Fig. 7. This plot provides independent verification of the n and k values for the as-deposited film.

Figure 10

Comparison of measured (dotted) and fitted (complete line) reflectance for a Ta2O5 film subjected to 900°C for 9h on 0.1μmSiO2. The fitted reflectance was calculated from the n and k values shown in Fig. 9. Note that the interfacial layer formed between the Ta2O5 and SiO2 films is not significantly thick (Fig. 5) and hence only the crystalline Ta2O5 film optical properties were substituted to calculate the fitted reflectance here. This plot provides independent verification of the n and k values for the crystalline Ta2O5 film (without the interfacial layer).

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