Laser shockwave cleaning

Abstract

At this moment the prime candidate for the next generation lithographic technique is Extreme UltraViolet (EUV) lithography. With EUV lithography it will be possible to produce integrated circuits with feature sizes of 32 nm and below. This capability to produce smaller feature sizes inherently means, that the role of particle contaminations on the reticle during exposure becomes increasingly important. In the case of EUV lithography, particles as small as 30 nm can already cause serious defects in the electronic components on an integrated circuit. Conventional cleaning methods (wet cleaning, megasonics) are not able to remove such small particles fast, reliably and without damage. In search of a new cleaning method that can meet the new cleaning requirements, this thesis focuses on a novel cleaning method called Laser Shockwave Cleaning (LSC). Laser shockwave cleaning is a cleaning method which is based on the Laser Induced Breakdown (LIB) of gasses. The shockwave generated by the breakdown process is used to remove small particles from the contaminated surface. To study its properties, the shockwave produced by the LIB was made visible with visualization techniques such as time resolved shadowgraphy and Schlieren photography. Amongst other things, the recorded images revealed the size and expansion velocity of the shockwave, which were found to be consistent with the Taylor & Sedov solution for point explosions. Combining the Taylor & Sedov solution with shock theory and adhesion theory, made it possible to develop a model that describes the removal of particles by LSC. The results produced by this model were later confirmed to be in agreement with the results of the particle removal experiments. To asses the cleaning performance of LSC, a number of cleaning experiments were performed on unpatterned surfaces. The majority of these experiments were performed on 1" flat silicon wafers, which were contaminated with polysterene latex (PSL) spheres. These experiments showed that it was possible to reliably remove PSL spheres as small as 50 nm with LSC. This result proves the cleaning capabilities of LSC, and can be seen as a proof of concept for the method. The removal of 30 nm particles could not be achieved, but this is in agreement with the particle removal model developed earlier. The model predicts however, that under different circumstances (in the absence of capillary forces), it must be possible to remove particles smaller than 30 nm. It is therefore recommended to continue this research, to further ascertain the cleaning possibilities of LSC.

Date of Award	29 Feb 2008
Original language	English
Supervisor	A. Bleeker (External coach) & J.J.A.M. van der Mullen (Supervisor 1)