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COMPETENCES IN TECHNOLOGY
This same control system originally developed for precision optics can also be used for the deposition of nanometer thin metal layers (Figure 2) achieving thickness reproducibility for optoelectronics applications not achievable by other methods. SAW and BAW filters in Wireless Communication applications also benefit largely from dielectric layers with reduced thickness variations, which allow tighter device specifications and reduced manufacturing costs.
the chamber and without the need to premeasure the sample. Test series which formerly took a day or two to be performed can now be done within some hours.
PLASMA EMISSION MONITORING
For high precision optics applications preservation of a very precise surface figure is of utmost importance, which can only be obtained with low stress coatings. We could show with a series of experiments (Figure 3) that depending on the deposition parameters the stress in a dielectric SiO2/TiO2 mirror can be tailored from tensile to compressive.
Plasma emission monitoring PEM is an APC method enabling increased deposition rates in reactive processes. Typical examples would be dielectrics like SiO2 and Nb2O5. Oxides can be sputtered by using an RF sputter technique, but with the drawback of low deposition rates. Thus, oxides are most often sputtered by pulsed DC power mode from metallic targets with the addition of oxygen to the sputtering gas. Under the appropriate deposition conditions this gas can react to form completely oxidized films. However, the reaction not only takes place in the growing film but also on the target. When the reactive gas flow exceeds a certain value, the metallic target surface is rapidly covered by oxidic reaction products, which reduce the target conductivity and results in a sudden drop of the sputtering rate. The process point in the transition region between metallic and oxidized state, which yields both highest deposition rate AND fully reacted films tends to be unstable. Plasma Emission Monitoring PEM allows us to work in a stable way at this point by measuring the intensity of a spectral emission line of the gas or the sputtered metal and keep it constant by a feedback loop to e.g. the reactive gas or the target voltage.
As an illustration, the stress in a metal contact stack needed to be reduced. Being able to see stress evolve in situ let us identify which of the materials contributed most to the stress. By adapting the process parameters the overall stress of the stack could be reduced by half.
On-line reoptimisation during deposition for improved yields
Figure 1
Deposition of thin metal films
Deposition rate can be boosted up to a factor of 4 compared to DC pulsed sputtering with constant flow. In addition to increasing rates PEM is also highly beneficial for processes which need to keep an oxidation level within a close margin, such as in the deposition of ITO or VOx.
MANAGING STRESS Stress is another important film characteristic which may need to be tightly controlled. In the in situ stress measurement system two parallel beams are reflected from a surface. Depending on the surface curvature these beams will diverge or converge after reflection. The separation of the beams is detected and used to calculate the radius of curvature and the stress in the layer provided the material properties of the substrate and the film thickness are known. Being able to measure the stress in situ brings major time savings in situations where test series have to be performed to optimize the stress. In the classical way, the surface figure of a test sample has to be measured before coating, then the coating run is made, the sample is taken out and the surface figure is measured again. From the difference between pre and post measurements the stress can be determined. In the in situ measurement however, the change of the radius of curvature and thus the stress can be observed as it changes during the process. Thus different process parameter settings can be run one after the other all on the same substrate without venting
Figure 2
Mirror stress at various deposition conditions
“APC techniques open up new possibilities in Thin Film Processes�
Figure 3