Plasma-assisted cleaning of extreme EUV optics

In this thesis it is shown that Extreme Ultraviolet (EUV) plasma cleaning technology is feasible. ‘An EUV pulse generates a low-temperature plasma, along with photo-induced surface activation,’ Alexandr Dolgov explains. ‘These two combine, to yield a highly reactive environment that quickly and efficiently removes amorphous carbon.’

Next generation extreme ultraviolet lithography, will make use of 13.5 nm radiation. Here, ionizing photon flux and vacuum requirements create extremely challenging operating environments. Because of high EUV absorption, the optical elements must be reflective rather than transmissive, and must use multilayer mirrors (MLM) of the highest quality.

Therefore, on this topic, the Industrial Focus XUV Optics Group, led by Professor Fred Bijkerk, collaborates with ASML in Veldhoven. ‘Our knowledge is relevant for industry. During the last year of my PhD work, I performed finely tuned, practice-driven experiments at ASML,’ Alexandr says.

MLMs consist of approximately 50 Mo/Si bi-layers, stacked to form an artificial 1D Bragg crystal. Since it is expensive to manufacture such optics, the lifetime of Mo/Si multilayers is a critical parameter, for evaluating their over-all performance.

In case of vacuum, MLMs might be oxidized or contaminated by carbon. ‘This PhD provides basic know-how, required to adopt radiation-induced plasma surface etching, for industrial multilayer mirror cleaning,’ says Alexandr. ‘EUV-induced plasma is a very effective tool. It doesn’t require installation of additional devices for plasma (or radical) generation. And, EUV-induced cleaning could be realized without interrupting production operations. This will help to extend the lifetime of EUV optics.’

In order to develop cleaning and reduction mechanisms, one should characterize the plasma chemistry induced by EUV radiation on multilayer top surface first. Therefore, in this PhD model systems were developed, both experimentally and theoretically, allowing contamination scaling laws to be studied.


‘As a scientist, I favor the experimental approach more than the modeling and code-making apects of the work,’ Alexandr says. ‘Collaborating with skillfull colleagues and technicians on performing clever experiments, is the type of teamwork and communication atmosphere, that suits me best. Also the application oriented aspects appeal to me. Therefore, I favor a future job in industry.’


Alexandr now works as an R&D consultant, as an intermediate between companies and knowledge centers and universitites.

‘I now know both cultures,’ he says. ‘As a PhD researcher you must find a balance between delivering results strategically usefull for the company, and the publication requirements set by academic standards.’

‘The third year of my PhD project was decisive, because we had to time our experiments and other work very precisely, in order to come to these two kind of results. I felt a little squeezed between two worlds, but it worked out very well at the end.’


Alexandr enjoyed the Mesa+ events in which he learnt more about hightech entrepreneurship. ‘An open atmosphere is requisite, not only in academics,’ Alexandr says.

‘In the project I also collaborated with Moscow State University and ISAN (Institute for Spectroscopy Russian Academy of Sciences). This institute is specialised in plasma physics and plasma spectroscopy.’

Alexandr: ‘At the Mesa+ XUV Group, they are great integrators, coupling scientific results and industrial needs. I also enjoyed the open attitude of the ISAN experts. Fruitful collaborations on practice oriented questions are feasible in the future on this topic. In European countries people often underestimate the openmindedness and constructive mindset of Russian researchers, as they are often considered in a somewhat caricutaral way.’