In many dynamic processes like corrosion, optical devices are used
to learn more about their workings. One such instrument is the ellipsometer.
Ellipsometers have been around for about 100 years. Radical new
thinking has come up with not one, but two patented, ultra fast
ellipsometers that are streets ahead of the competition. Fast processes
once beyond the realm of traditional ellipsometers can now be measured
accurately and rapidly, thanks to these two new revolutionary designs.
Often the properties of metal surfaces are studied by optical techniques. Even processes such as corrosion or electrochemical film growth can be tracked. For these fields of research, reflectometry and ellipsometry are two commonly used sensitive techniques. The first measures the amount of reflected light while the second also deals with phase differences between two polarised light beams upon reflection. By calculating the ellipsometric angles, these techniques allow the measurement of such optical characteristics as the indices of refraction and layer thickness.
One area of research that uses this type of information is the silicon oxide (SiO2) industry. The SiO2 thickness is very important in the proper function of devices such as silicon computer chips, an important industry for Europe.
In essence, the principle on which the ellipsometer is built consists of splitting one laser beam into two, one for reference and the other for measurement. Simple reflection then picks up the required information from the sample and is the measurement beam. In an older version of the ellipsometer, the frequency of the reference beam is altered via a moving mirror that causes an interference signal when later recombined with the other beam.
Here, interference is the optical equivalent of an out-of-tune guitar - the humming sound is the interference signal, the so-called beat frequency. This is what holds the ellipsometric information. Converting the optical information into digital ouptut allows subsequent analysis.
However, one of the limitations of the old ellipsometer was the technicality of moving the mirror. It was just too slow. This called for a radical new idea.
One step beyond
After a sailing expedition, a chance conversation between researchers from Brussels and Delft led to the suggestion of improving the existing design with an alternative laser. The basis of this new design was a Zeeman two-frequency laser. Again the underlying principle of operation is interference but, unlike its predecessor, the two frequencies are present from the start. This eliminates the need for the moving mirror.
Theoretical discussion on the best way forward led to the design and patent of the first prototype. As before, the laser beam is split and later recombined with great precision. The oscillating signal in the resulting interference contains all the ellipsometric information.
Initially there were drawbacks, such as increased complexity and sensitivity. This is a particular hindrance for many measurements that take place in vacuums created by vibrating pumps. However, overall the improvement in design led to a large increase in the speed of measurement giving it its name, the Ultra Fast Ellipsometer or UFE.
Not one but two...
Building on this achievement the team patented a less complex two-laser ellipsometer using even fewer components. The difference is that it uses two lasers instead of one as in the previous case. This circumvented the problem of unwanted mixing of the two frequencies. It brought with it all the advantages of the first UFE, but it also provided better time resolution that can be tuned according to the needs of the application. This ellipsometer has other uses
outside the remit of ellipsometry such as optical recording as phase differences rather than amplitude are measured. In general, phase differences are less noise sensitive and higher signal to noise ratios are expected (compare the audio quality of your amplitude modulated (AM) radio against frequency modulated (FM) radio).
From theory to practice
With such an innovative new design, the partners had to write new computer programmes to meet the demands of the system. At the same time they successfully tackled the theoretical and practical difficulties of actually building the prototype.
Once assembled, this prototype obviously needed to be evaluated and benchmarked against existing techniques. Two of the project partners specialise in electrochemical research, providing the opportunity to test the UFE thoroughly.
For example, it allowed an important surface treatment of
aluminium films to be continuously and easily monitored in situ.
This project shows not only the feasibility of such an instrument but also the advances and improvements made compared with existing technology. It is currently the fastest ellipsometer in the world with a high level of accuracy. The potency of the technique can be seen in its ability to measure the optical response of a well-known electro-optical element, the Pockels cell. This cell gives an almost instantaneous change to the applied voltage. Even with this cell, the ellipsometer developed here fared well, being both highly accurate and capable of operating with a time resolution of 1 microsecond. This shows that the two-laser set-up can even be tuned to operate with time resolutions that are orders of magnitude better.
Both designs are so ahead of their time that their full potential has yet to be realised. Specialist market niches such as the switching of liquid crystals or even faster processes are currently being sought. The two UFEs developed here prove that to stay ahead of the competition, you have to be quick.