Printer-friendly versionSpectrometer
A spectrometer is a device that can separate the various wavelengths of the electromagnetic spectrum and is used to measure their relative or absolute intensities.
Spectrometers come in many forms. Some can detect a very large spectral range while others are specialized to look at a very small part but with very high resolution.
To measure the intensity of the light, a spectrometer needs an detector. Depending on the wavelength range required, various types of detectors have to be used. There is no ’magic device’ or a superior universal detector that can cover everything. At omt we use two different types of detectors. So called CCD or CMOS detectors made out of silicon are used for the wavelength range from 200 nm to 1000 nm. Detectors made out of the compound InGaAs (indium gallium arsenide) are used for the infrared part of the spectrum from 1000 nm to 2500 nm. Within these classes of detectors there are various specialized detector versions and at omt we know exactly which ones to choose to obtain the best possible solution for a given task.
There are also different methods for separating and detecting light. At omt we use so called linear detector arrays. In such an array there are many photodetectors packed next to each other - so called pixels. In our spectrometer each pixel measures a different part of the spectrum. This way our device can measure the whole required spectra range at once. There are other systems that use detectors with only one pixel which measure only a single wavelength at a time. Seemingly less sophisticated at first glance, there are applications - mostly in the analytical and scientific range - that still require such a design.
Reflection/transmission systems and ellipsometers
Ellipsometers perform the same job as ordinary reflection transmission systems.
So why is it they look different from the standard omt TFA system? The reason lies in the way light interacts with materials. This interaction depends on the angle of incidence under which the light hits the interface. As an electromagnetic wave, the plane in which the wave propagates in space can have various orientations. Physicists call this phenomenon ‘polarization’.
At an angle of incidence close to normal incidence, the polarization of the light does not exhibit a significant effect on how the light is transmitted or reflected. At oblique incident angles, the polarization becomes more and more important. The reflection and transmission of waves of the same wavelength, but different polarization can vary dramatically depending on the given polarization. Hence a measurement of reflectivity or transmission for a range of polarizations is extremely sensitive to the properties of thin films.
An instrument capable of performing these measurements is called an ellipsometer. The name comes from the fact that the reflection from sample often results in a light wave that physicists call ‘elliptically’ polarized. An ellipsometer can be used to analyze this state and provides not only a single spectrum, as with reflection or transmission, but two. This additional information can be very helpful in analyzing the properties of thin films deposited on substrates.