The drive towards ever smaller details on integrated electronic circuits (IC's) has followed a pace, which asks for a reduction in linear dimension by a factor of two every five to six years. Optical projection lithography is felt to be inadequate for this task, even if the wavelength is reduced to the deep UV region (e.g. 193 or even 157 nm). By reducing the wavelength towards the extreme UV (EUV) region, a new era in reduction projection lithography with dimensions well below 100 nm becomes possible. Reduction systems at EUV-wavelengths (typically 13 nm) are mirror-based optical systems. The surface finishing of the mirrors (shape and roughness) should obey extremely severe specifications in the nanometer range. A direct machining of mirror surfaces with this geometric accuracy is very difficult. However, by the deposition of relatively thick corrective layers with a refractive index close to unity at the wavelength of interest, the effective reflecting surface can be tailored so that the required shape is approached with nanometer accuracy. The deposition of the corrective layer with locally varying thickness follows from a measurement of the surface shape defect or from a quality test of the complete mirror system.
The adoption of the correction scheme described above enables a fine-tuning of the surface shape which does not seem technically feasible at this moment, even with advanced polishing means. In this way, a prototype of an EUV mirror projection system can be thought of in a few years from now. Note that the maximum correction layer thickness can be reduced when the polishing techniques are gradually improved and that in an ultimate phase the correction layer could even become superfluous or will only be needed over a limited area of the total mirror surface.
The full implementation of the correction technique asks for a material choice, an interferometric measurement of the index data of the material at the wavelength of interest and for the development of a calibrated deposition technique. The detailed influence of the introduction of the correction layers on the imaging properties of the mirror system wills also to be studied (system study).
The preparation of the testsubstrates and the system study will be done at Delft University. The Institut d'Optique (IOTA) brings in its experience in the field of at-wavelength EUV-interferometry. Carl Zeiss, the final producer of EUV mirror projection systems, will study the calibrated deposition technique for the correction layers with locally varying thickness.
Prof. Joseph Braat
Technische Univ. Delft
Depart. of Applied Physics
Faculty of Applied Sciences
NL - 2628 CJ Delft
e-mail: (email removed)
+31 / 15 2786146
fax: +31 / 15 2788105
15 October 98
Duration: 24 months
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