Spectral Ellipsometry (SE)

Spectral Ellipsometry (SE)

Spectral Ellipsometry is an optical technique that uses light to measure the composition and properties of thin films. Spectral ellipsometry can determine composition, roughness, crystalline nature, thickness, electrical conductivity, and other material properties.

    How Spectral Ellipsometry (SE) works

    Light waves can oscillate in different directions. A polarized light oscillates in only one direction. Non Polarized light oscillates in many directions. The process of transforming unpolarized light into polarized light is known as polarization (Figure 1). In Spectral ellipsometry, polarized light is shone upon a sample to measure its properties.

    Figure 1: Polarization of a light beam after passing through a polarizer

    Electromagnetic radiation is emitted by a light source and linearly polarized by a polarizer. The light falls onto the sample. After reflection, the radiation passes through a second polarizer, which is called an analyzer and falls into the detector (Figure 2).

    Figure 2: Sample analysis with polarised light

    When light interacts with a thin film, its polarization changes. The change in polarization depends on the thickness and other properties of the material. Therefore, the properties of materials can be determined by measuring the change in polarization of light. The polarization change is quantified by the amplitude ratio, Ψ, and the phase difference, Δ.

    Strengths of SE 

    Ellipsometry has several advantages compared to standard reflection intensity measurements:

    1. Ellipsometry measures at least two parameters at each wavelength and can measure up to 16 parameters at each wavelength. 
    2. Ellipsometry works even when light intensity is unstable. 
    3. No dark box is necessary. 
    4. No reference measurement is necessary.  
    Figure 3: Schematic diagram of a spectroscopic ellipsometer attached to an ALD chamber. 

    Limitations of SE

    1. Rough samples are difficult.
    2. Multilayered samples are complicated to analyze. 
    3. Expertise is required.

    Uses of SE

    Spectral Ellipsometry is used extensively in research and development. It has many industrial applications. It applies to many different fields, from semiconductors to microelectronics and biology. It is also used in thin-film testing. 

    Figure 4: Ellipsometric data at multiple wavelengths from silicon surfaces with different thicknesses of chromium (5-30 nm) on them. Adapted with attribution from Reference [1].

    Study report

    Diwan et al. studied different thicknesses of chromium on silicon, plotting the resulting ∆ and ψ values that were obtained at a single wavelength (632.8 nm), and at an angle of incidence of 70° (Figure 4) [1]. They studied other elements too as shown in Figure 5.

    Figure 5: (a) ψ, (b) ∆, and (c) thickness values obtained from spectroscopic ellipsometry for the atomic layer deposition of Er2O3 and Al2O3 Adapted with attribution from Reference [1].

    Sample requirements for SE

    The sample surface should be smooth and flat. The rear of the transparent sample should be rough. The thickness should be between 70 nm and 1 um. The area to be analyzed should be at least 1 x 1.5 mm.

    Reference: 

    [1] Diwan A. & Linford M.R. Editorial, Vacuum Technology & Coating, November 2014 issue, pages 1-7. 

    The full article may be read here- https://www.researchgate.net/publication/267098351_Models_in_Ellipsometry_The_%27No_Model%27_Model_Just_Monitoring_Psi_and_Delta 

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