Optical modeling off-stoichiometric amorphous Al2O3 thin films deposited by reactive sputtering

Jung, Jy Yun

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Description

Title

Optical modeling off-stoichiometric amorphous Al2O3 thin films deposited by reactive sputtering

Author(s)

Jung, Jy Yun

Issue Date

2012-05-22T00:18:49Z

Director of Research (if dissertation) or Advisor (if thesis)

Brewster, M. Quinn

Doctoral Committee Chair(s)

Brewster, M. Quinn

Committee Member(s)

• Abelson, John R.
• Yu, Min-Feng
• Toussaint, Kimani C.

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• ellipsometry
• off-stoichiometric
• graded strucutre
• non-homogeneous
• intermediate oxidation state
• thin film
• alumina
• aluminum

Abstract

This work was initiated from a question brought up by previous research about how the optical properties of alumina droplets/particles found in solid rocket motors are related to their stoichiometry. We approached this question by adopting methods common in thin film technology. Alumina/aluminum/off-stoichiometric thin films were deposited by the reactive sputtering, which enabled us to vary the composition of films by controlling the oxygen flow ratio fed into the deposition system. The composition of the films was investigated with X-ray photoelectron spectroscopy (XPS). This technique provided information regarding the chemical composition as well as the intermediate oxidation states of the films. The films’ thickness was measured by X-ray reflectivity (XRR), and the inevitable surface roughness of the films was examined by atomic force microscopy (AFM). Both techniques (XRR and AFM) were useful because they allowed us to make good initial guesses (in ellipsometry) for the film thickness and the roughness layer’s thickness. We also measured the films’ transmissivity with a spectrophotometer. When a film becomes optically thick, the ellipsometer cannot see inside the film. Even so, a modeling based on the ellipsometric data works well as long as the film’s properties are homogeneous along its depth. But, when there is inhomogeneity in the optically thick film, the transmissivity is crucial to the film’s optical modeling because the spectrophotometer’s source light interacts with the film all the way down to the bottom of the film and comes to its detector. Thus, it can deliver information (transmissivity) about the inhomogeneity while the ellipsometer’s source light might be absorbed within the film. These auxiliary data from XRR, AFM, and spectrophotometry allow a tangible modeling in ellipsometry and reduce the correlations between fitting parameters, which ensures that accurate optical constants are obtained. Due to the complexity of the films, these additional data were vital when we modeled the off-stoichiometric films. Prior to a film deposition on a microscope glass substrate, we did optical measurement on the bare glass and modeled it. We found there was much higher level of absorption in the glass than a quartz glass (SiO2), whose optical constants are commonly adopted for modeling a glass. If we had not identified the microscope glass absorption and had used the optical constants of the quartz, this glass absorption would have been inaccurately attributed to a subsequent deposited film. We then modeled the alumina films deposited at 5 to 9% oxygen flow ratio. Dispersion models (Sellmeier and Lorentz) showed that as the oxygen flow ratio decreased, the resonance energy also decreased. We believe the decreased oxygen flow ratios induce more defects, which situate themselves below the band gap and eventually lowered the band gap. We then modeled the aluminum film deposited at 0% oxygen flow ratio with an ensemble of a Drude oscillator and two Tauc-Lorentz oscillators. We observed the interband absorption was reduced compared to the bulk films. The reduction was thought to be an effect of volume oxides and disorderliness. We could also see that free electrons’ optical mass was higher and their relaxation time was shorter than the bulk films; thus, the optical conductivity of the film was lower. These changes were essentially caused by residual oxygen molecules gettered to the aluminum atoms. Finally, we modeled the off-stoichiometric films deposited at 1.5, 2, 3, and 4% oxygen flow ratios. Unlike the alumina (5 to 9%) and aluminum films (0%), these films were non-intrinsic and inhomogeneous in optical properties. To address these two non- ideal properties, we have used the effective medium approximation (EMA). We found mixing intrinsic (stoichiometric) materials did not simulate the non- intrinsic properties well. Significant improvements were obtained in the modeling by mixing optically or compositionally neighboring materials. The inhomogeneity was addressed by grading the constituents of the EMA. The 1.5, 2, and 3% films were found linear in the profile of the optical properties while the 4% film was found non-linear. The latter showed an exponential variation of the optical properties; most of change was confined in the bottom of the film where aluminum particles were distributed but not agglomerated due to the oxygen contents in the film. The 4% oxygen flow ratio was the threshold oxygen flow ratio, above which films came out as oxides from the reactive sputtering.

Graduation Semester

2012-05

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Copyright 2012 Jy Yun Jung

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