Technology Competence

The use of more and more powerful lasers leads to increasing challenges for the optics. Besides the already known demands for optical homogeneity and stress-induced birefringence the bubble and inclusion content is gaining importance. Defect sizes down to 10µm begin to become important in the damage behaviour of fused quartz and fused silica. The wavefront deviation caused by a defect can create local spikes in the wavefront. The energy thus deposited by the spikes in the glass is above the damage threshold of the material, so the glass can fracture.

Heraeus Quarzglas has developed a system to detect defects down to 10µm in diameter. With this automated bubble and inclusion detection system Heraeus Quarzglas can now certify its inclusion free fused silica. The picture shows the system at work.

The use of lasers with increasing powers, e.g. in the field of laser material processing, is putting higher and higher demands on the optics. Especially in the infrared region of the spectrum the absorption due to the water content in the glass, i.e. OH-groups, is very important. Modern laser systems working at a wavelength of 1064 nm need optics with a very low absorption, especially for increasing laser powers. The reason is that absorption causes lens heating. The heating causes a change in the index of refraction which in turn creates a thermal lens. The thermal lens will then cause a shift in the focus spot of the optic. The focus-shift will be bigger the higher the absorption of the system.

Thus, Heraeus Quarzglas has developed a nearly water-free synthetic fused silica with an extremely low absorption. This synthetic fused silica is ideally suited for this purpose. It has an absorption of typically 0.3ppm/cm, which has been verified by especially equipped research institutes.

In microlithography tiny structures (down to 45 nm) of highly complex integrated circuits are projected on to silicon wafers from different photomasks. This is accomplished by lens systems made from bubble free and optically extremely homogenous fused silica. The storage density and performance of microchips doubles every two to three years. This necessitates new gener- ations of lens systems being able to image the constantly shrinking structures. The synthetic fused silica used in these optics has to follow this development as well.

With Suprasil 501 Dr Stephan Thomas and his team from Heraeus Quarzglas have developed an extremely radiation resistant generation of fused silica. Fused silica during its lifetime as a lens material in stepper-optics, has to survive over 200 billion laser pulses undamaged and may show only the slightest deviation in the index of refraction or in transmission losses.

This requirement is fulfilled by Suprasil 501. This is achieved by hydrogen molecules existing in the glass which may cure laser-induced defects. The defects would otherwise cause a degradation of the outstanding optical properties of the material.