One of the major types of materials analyzed using optical spectroscopy is thin films. Unlike some thin films that are self-supporting, ultra thin films are deposited on a substrate. These ultra thin films can be as thin as a single monolayer.
In certain cases, thin films are naturally grown on a specific substrate, including an aluminum oxide film on aluminum. The spectroscopical analysis of the film can influence the ability to choose a suitable substrate for these films. These films can be deposited on a transparent substrate like calcium fluoride such that the investigation can be carried out using transmission spectroscopy.
Alternately, in order to carry out analysis by reflection spectroscopy, certain films could be deposited on a metal like platinum or gold. Diamond, titanium dioxide, gallium arsenide, germanium, silicon and glass are some of the other common substrates.
The sensitivity of spectroscopic methods, such as grazing angle reflection, grazing angle Ge-ATR and transmission, was compared in the analysis of ultra thin films on different substrates. Based on the transmission results, the relative sensitivities of the other two techniques were measured.
Based on the theoretical considerations illustrated elsewhere, the simulations were carried out. Table 1 shows the parameters used for the substrates and sample. These simulations were performed using a p- polarized light at 10µm.
The refractive indices presented in the table are at that wavelength. The results may vary when a different set of indices are used for the sample and metal. However, the nature of the absorbance vs. film thickness curves may remain the same.
Table 1. Refractive indices of materials used in the simulation
||Refractive index at 10 µm
||1.40 + 0.1i
||10 + 20i
To perform the transmission simulation, the film was considered to be free-standing and the incident radiation hit the sample at normal incidence (0°). For grazing angle ATR, the substrate used for the film can be either Si or metal.
The refractive index of Ge was considered to be 4.0. Results were obtained for two incident angles: 65°, which is above the experimental critical angle provided the typical beam spread of an FTIR spectrometer, and 60°, which is slightly over the theoretical critical angle for the Si-Ge interface.
The grazing angle specular simulation involves using a metal substrate to support the film. Results were reported for two different incident angles: 75° and 80°, provided with commercial grazing angle specular reflectance accessories.
Results and Discussion
The absorbance versus film thickness for the three methods is shown in Figure 1. A linear relationship between film thickness and absorbance was observed in transmission as expected. The highest absorbance is achieved in transmission when the thickness is over 0.5µm.
Figure 1. Absorbance vs. film thickness for three techniques. Red is normal incidence transmission of a film, blue curves are for grazing angle ATR of the film on a silicon substrate at 60° (solid) and 65°(dashed), and black curves are for the grazing angle specular reflection of the same film on metal substrate for 75° (dashed) and 80° (solid).
Both the reflection techniques are sensitive than transmission for the ultra-thin films. Grazing angle specular reflection has been a well-established method used for the analysis of ultra-thin films.
However, there is a misconception that the sensitivity increases with the increase in the angle of incidence. As shown in Figure 1, ultra-thin films have higher absorbance for an 80° angle of incidence, but barely so.
The curves reverse and higher absorbance is achieved by the 75° angle of incidence when the thickness of the film is increased to about 1000Å. Upon combining this concept with practical difficulties involved in providing a high beam spread over the sample for an 80° angle of incidence, a 75° angle of incidence may be preferred for grazing angle specular reflection spectroscopy. It has to be noted that using the grazing angle specular reflection technique over transmission does not provide large benefits as assumed.
However, grazing angle Ge-ATR provides significant benefit over transmission. For angles closer to the critical angle, the absorbance is stronger. Therefore, a 60° angle of incidence offers a 50% gain over the 65° angle of incidence.
This gain needs to be weighed for the possible spectral artifacts arisen by the beam spread of the spectrometer beam. A higher fraction of rays from the incident beam will be below the critical angle when the incident angle is nearing the critical angle.
A linear relationship between film thickness and absorbance cannot be observed in both the reflection techniques. Care should be taken while interpreting experimental results. For example, when compared to a thick film, thin film can have a high absorbance.
The same kind of absorbance spectrum can be obtained from two films of different thicknesses. However, the thickness vs. absorbance relationship is linear for really thin films, under about 200Å. Therefore, the grazing angle ATR technique can be advantageous in terms of sensitivity.
This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.
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