Raman Spectroscopy: A Unique Tool for the Study of Thin Films
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Raman Spectroscopy: A Unique Tool for the Study of Thin Films Ingrid De Wolf IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium [email protected] ABSTRACT In this paper, the different applications of Raman spectroscopy for the study of thin films is briefly discussed, using examples from microelectronics. Special attention is given to the application of micro-Raman spectroscopy for the measurement of local stress in and near films. INTRODUCTION Raman spectroscopy is a non-destructive optical technique, which can give information on different properties of films such as composition, crystal structure (crystalline, poly-crystalline, amorphous), crystal orientation, crystal phase, mechanical stress, temperature, and in some cases doping [1]. As such, this technique is unique for the study of various properties in different kinds of films. The spatial resolution of a conventional micro-Raman spectroscope is about 1 µm. Mechanical stresses develop at various stages of IC processing. These stresses depend not only on the mechanical and thermal properties of the different films, such as their intrinsic stress and their thermal expansion coefficient, but also on the dimensions and pitch of the film. With the continued miniaturization and densification of IC devices, the effects of local mechanical stress on the reliability of the device often become more important. For example, metal films will introduce stress in the substrate. This stress is especially high near the side and edges of the lines. Reduction of the spacing between two metal films will result in an overlap of these stress fields, with possible defect generation in the substrate as a result. Conventional techniques to measure stress, such as X-ray diffraction, can not really be used to look at these local stress fields. Raman spectroscopy, on the other hand, is very useful if a Raman signal of the film or of the surrounding substrate can be measured. The effect of miniaturization on local stress, as studied with Raman spectroscopy, is discussed in this paper using examples from metal lines with different size and pitch. THEORY Raman scattering in crystals arises from an interaction of light with lattice vibrations (phonons). These vibrations are classically described in terms of collective motions in the form of waves. Each possible vibration is characterized by a wavevector qj, a frequency ωj, and an amplitude Qj (normal coordinate). The vibration amplitude, at position r, is given by: Qj = Aj exp[±i(qj.r -ωjt)]
(1)
G8.1.1
When monochromatic light of frequency ωj is incident on a crystal in a direction kj, the associated electric field E will induce at position r an electric moment P: P = εo χ E = εo χ Eo exp[i (ki.r - ωi t)]
(2)
where χ is the susceptibility tensor. If the atoms are vibrating, χ may change as a function of these vibrations. This can be expressed by expanding χ , for each mode of vibration j, in a Taylor series with respect to the normal coordinate of this vibration Qj: ∂χ ∂χ Qj + Q j Q k + ... χ = χo + ∂Q j o ∂Q j ∂Q k
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