NO OLD TECHNIQUES – Absolute purity with CO2-snow


Photolithography is a procedural area in which highly specialised expertise and the highest of standards for all process steps are required. On the one hand, cleaning needs to take place in the manufacturing machines and, on the other hand, various gases are required which are essential for manufacturing semiconductor materials. For example, GaAs (gallium arsenide) is used for the material for red LEDs in car brake lights.

High level of purity

The individual process steps are often a well-kept secret of the industry. However, all processes have one thing in common: they require absolute cleanliness. Impurities are measured in ppt in this industry, i.e. in parts per trillion: “1 ppt” means that only one single foreign particle may be present in one trillion (1,000,000,000,000) parts of material.

Gas is used to create crystals

Cleaning with CO2 snow is tried and tested when it comes to cleaning mirrors which are used in the manufacturing of so-called wafers for chip production. This is a special processing step which requires the ultimate cleanliness. At the premises of our customer in the semiconductor industry, sera compressors are used to compress high-purity, gaseous CO2, which is fed through a cooler after the compression stage and condensed there. The CO2, which is now liquid, is sent through a kind of spray gun and is transformed into very fine CO2 crystals thanks to being eased to ambient pressure. This snow then, for example, allows deflection mirrors to be cleaned without leaving any residue at all. The precision of these mirrors is just as high as the other standards in this industry are too. Here is an example: If a concave mirror, which actually has a diameter of approx. 50 cm, were as big as the Earth, the greatest unevenness on this mirror would measure just a few millimetres.

The right technology for ultrapure compression

By implication, this means that sera compressors must carry out their compression work in an extremely “clean” manner in order to meet the standards. We ensure this by having the widest technology portfolio in the field of hydrogen technology. sera can provide the most optimum technology for each respective application. In this case, this is why a metal diaphragm compressor was used because its purity optimally meets the high standards in this field. sera has been developing and producing metal diaphragm compressors for many decades now and has a great deal of experience in this field. The fact that, ever since, the compressor has been doing its job in this highly sensitive field without any errors says
a lot about the quality and reliability of this technology.

A thought experiment to illustrate this: 

  • when full, the Edersee reservoir contains 199.3 million cubic metres of water.
  • If you poured a glass (containing 0.2 l) of raspberry syrup into the full Edersee reservoir and stirred
    it in well, the resulting mixture would be around 1 ppt raspberry syrup.
  • That’s not very much… In order to be able to further process such ultrapure materials, the tools and machines also need to have a corresponding level
    of cleanliness. This means that extremely high standards are also held when it comes to the cleaning processes.