One of the most common XUV filters used today in high-power laser experiments is the 200nm Al flatfield filter. The difficulty in producing such a filter is for it to be pin-hole free over the complete open area of the flatfield mount (5x25mm). At this thickness the filter is only around 2000 aluminium atoms thick. The smallest piece of dust visible to the naked eye is around 50um in diameter but any dust particles larger than 100nm will be detrimental to the filter meaning that just because a slide looks clean doesn’t mean it is. It raises the need for cleanliness to a new level.
Scitech has invested significantly in improving procedures and we believe that our filters are now first class.
A high-grade of microscope slide is used that has a roughness of <5nm. This is cleaned thoroughly and oven baked before being oxygen plasma cleaned. This is then coated in a release layer which is water soluble, and then the Aluminium coating is applied to the desired thickness. The Aluminium coating takes on a surface finish equal to that of the release layer, so it is imperative that this is as flat as possible and defect free. New dip-coating methods have been developed that ensure consistent results with a roughness less than 100nm. The completed coating is then floated off onto the surface of high quality de-ionised water before being floated back onto a flatfield filter mount. The finished product is then subject to quality testing using a lightbox to back light the filter to check for integrity.