Raman spectroscopy-based measurements of thin film thickness

Silicon-on-sapphire (SOS) devices are used in a range of defense and aerospace radiation-hardened communications applications. Metrology of the Si film thickness is needed during the fabrication of these devices for quality and process control purposes. However, due to the nature of the sapphire substrate, conventional thin metrology tools such as ellipsometry are challenging to use.

In a recent research study funded by the Defense Microelectronics Activity (DMEA) (Contract No. HQ072718C008), Advanced Cooling Technologies, Inc. (ACT), in collaboration with Iowa State University, has developed a new thin-film metrology tool based on the principles of Raman spectroscopy.

What is Raman Spectroscopy?

Raman spectroscopy is a measure of the inelastic scattering of light, in which incident photons exchange energy with vibrational modes of the material. When light is incident on a crystalline material, it sends the vibrational modes of the material into a higher energy “virtual state” before being reemitted, i.e., scattered. The majority of photons scatter at the same frequency as the incident light (Rayleigh scattering). However, around one in a million photons scatter at a different frequency as the molecules settle into a different vibrational state. This is known as Raman scattering (Figure 1)

Figure 1: Energy level diagram illustrating transitions involved in Rayleigh (elastic) and Raman (inelastic) scattering

Calculating the Relative Raman Intensity

Raman spectroscopy is commonly used for identifying materials or probing their molecular structures. In our research, we have shown that if the experimental conditions are adequately controlled, the intensity of the Raman signal from a thin film material can be correlated to its thickness. However, due to interference by the multiple reflections of both the incident and Raman scattered light, the relation between Raman signal intensity and thin film thickness is complicated.

A theoretical model was developed for calculating the relative Raman intensity as a function of film thickness [link to paper]. To account for the uncertainty associated with the complicated Raman signal intensity trend with thickness, a dual-laser approach was developed. A dual-laser Raman spectrometer, equipped with 532 nm and 785 nm lasers, along with associated LabVIEW-based controls, was setup to demonstrate the technique (Figure 2).

Figure 2: Dual-laser Raman spectrometer setup, highlighting key components

The developed model was verified with Raman measurements of SOS thin films with thicknesses from 80-500 nm (Figure 3). For comparison of the measurements to the model, an instrument-specific scaling factor is determined using a least-squares fitting of the measured intensities (Figure 4).

Based on these results, ACT has successfully demonstrated the determination of SOS thin film thicknesses using this dual-laser Raman intensity approach. This Raman-based technique may also be extended to other semiconductor materials and thin-film systems, as well as next-generation two-dimensional atomic layer materials. This technology may be used for semiconductor device process quality control where alternative metrology techniques such as ellipsometry may not be feasible.

Figure 3: Raman spectra obtained from SOS thin film samples using 532 nm laser. Spectra offset on y-axis for clarity. Notice the non-monotonic trend of peak intensity with thickness

Figure 4: Comparison of predicted intensity to measured intensity of the 521 cm-1 Si Raman peak for SOS thin films, taking into account instrument-specific scaling factor.

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