Reduction of defect density on blanks:: application to the extreme ultraviolet lithography

Our laboratory is involved in the French EUV (Extreme Ultra-Violet) program PREUVE to develop Mo/Si mirrors for blanks free of defects by using an ion beam sputtering deposition technique. This paper illustrates a reduction strategy used to lead to an EUV mirror with a defect density as low as possible. One of the methods adopted is the analysis of the process step by step, therefore, the defect number added by each process step has been quantified. It appears that the most critical step is substrate cleanness. Today, our best performance for a final mirror is 1.2 def/cm(2) with a defect size > 200 nm. This value has been measured on our home-made counting device COMNET. Our counting device has been validated by comparison with a commercial tool. Two improvements have been implemented on COMNET to increase its routine performance which allows to detect particles of 200 nm. The first one is the sample illumination with a laser in oblique incidence (60degrees) which increases the signal-to-noise ratio. The noise is the roughness of the sample. The second one is the implementation of a cooled CCD camera with a variable exposure time. Thanks to these improvements, the detection of particles with a diameter size of 155 nm has been demonstrated. On the basis of the experimental results and calculations, the detection of 100 nm particles on silicon and EUV blanks can be reasonably predicted. (C) 2002 Elsevier Science B.V. All rights reserved.