Overview of a 65-Degree ATR

ATR PRO 650G has an incident angle set at 65 degrees. By increasing the angle of incidence, it is possible to measure high-refractive-index samples such as carbon-containing rubber and Si wafers. It is also possible obtain information on the outermost surface of the sample.

The difference between 65-degree incident and 45-degree incident is the
viewpoint of sample refractive index .The total reflection condition is expressed by the formula shown below.

Reflection condition formula
Figure 1 Reflection condition formula

When germanium (n1 = 4.0) is used as the prism , the equation gives n2 = 3.6 for 65 degree incidence and n 2 = 2.8 for 45 degree incidence. Therefore, a sample with a higher refractive index can be measured when it is incident at 65 degrees.

The depth of digging is expressed by the formula below.
Formula of depth
Figure 2. Formula of depth

From the equation, it can be seen that the depth of penetration depends on the refractive index, the wavelength of the incident light, and the angle of incidence.

Focusing on the inside of the route, we can see that the larger the angle, the larger the value inside the route, and as a result, the shallower the depth of penetration.

Angle of Incidence PrismLower limit of measurement on the low wavenumber siden 1n 2Depth of digging (when 1000 cm −1 , n 2 = 1.5)
65°Ge~ 700cm - µm
45°Ge~ 700cm -µm

Measurement of Si Wafer

The surface of the Si wafer on which the natural oxide film was formed was measured using a 45-degree incident ATR and a 65-degree incident ATR.

It can be seen that the spectrum is significantly distorted because the 45 degree incident ATR does not meet the conditions required for total internal reflection.

On the other hand, at 65 degree incident ATR, shows there is no distortion in the spectrum and the baseline is flat.

Two ATR Spectra showing the difference between 45-degree incident ATR and 65-degree incident ATR
Figure 3. Two ATR Spectra show the difference between 45-degree incident ATR and 65-degree incident ATR

In the spectrum acquired by the 65-degree incident ATR, not only the absorption peak can be confirmed near 1235 cm- 1 but also the peak of 3000-2800 cm- 1 attributed to CH absorption is observed, and a slight amount of organic matter adheres to the surface.

From this, it is possible to obtain information on the outermost surface of the sample, which cannot be confirmed by the transmission method, by using the 65-degree incident ATR.

The Two ATR Spectra  show Oxide film on a Si wafer (standardization/offset display)
Figure 4. Oxide film spectrum on Si wafer (standardization/offset display)
ATR PrismGe
ATR/Sample Contact Area3.0 mm diameter
No. of Reflections1
Angle of Incidence65°
Maximum Sample Size6 inches
* A polarizer and attenuator mesh are optional
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