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| Optics for EUV Lithography |
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Increasing packing density of microchips
According to the roadmap for optical lithography, known as “Moores Law”, structures below 45nm are needed from the year 2010 on. One of the most promising technologies to print these structures is EUV (Extreme Ultra Violet) lithography.
Technology
The EUV radiation (13.5nm) is strongly absorbed by all known materials. Therefore refractive optics can not be used. Optical systems can only be realized with reflecting mirrors. In addition EUV exposure tools must be operated under High Vacuum conditions to avoid absorption of EUV light by gases. Article in Innovation Magazine (602 kB).
Optical System
Three optical components are required for an EUV exposure tool: the collector which captures as much radiation from the source as possible, the illumination system which homogeneously illuminates the used field on the mask, and finally the imaging optics which demagnifies the structures from the mask to the wafer.
Basic developments
Within the framework of the European EUCLIDES project, the first development work on EUV lithography was started at Carl Zeiss SMT in the mid-1990s. Further funding was given by the joint European MEDEA project and the Federal Ministry of Education and Research (BMBF) in Germany. The initial focus was the development of the technology required to produce extremely precise mirrors. Neither the figure errors nor the roughness of mirrors for the imaging system must exceed a few angstroms (one angstrom corresponds to the diameter of a hydrogen atom). For the production of mirrors for the illumination system Carl Zeiss already had the required know-how from the production of synchrotron optics and various X-ray satellite projects such as ROSAT and XMM.
Many challenges ahead
Further challenges facing optical systems for EUV lithography lie both in the measuring technology needed for testing the surface structure of the mirrors and in the coating technology used for the reflective coatings. The mechanical construction must also meet extremely stringent requirements, as no transfer of machine vibrations to the optics is permitted during the exposure process. A further focal point of attention is the long-term stability of the optical systems. A long service life is an absolute must for a successful EUV production tool.
The Micro Exposure Tool - MET
An important collaboration partner in the development was the Lawrence Livermoore National Labs (LLNL) in the USA which conducted basic developments in the field of EUV lithography on behalf of the EUV-LLC consortium and devised a concept consisting of two mirrors for a Micro-Exposure Tool (MET in short). The MET is based on the Casse-Grain principle known from large mirrors used in astronomy. International SEMATECH placed an order for the first tool, for which Carl Zeiss SMT produced the mirror substrates. The first MET was built at LLNL. SMT then commenced the development of the complete optical system for an EUV micro-stepper on behalf of the British company Exitech. In addition to the projection optics, the MET, this contains a collector system which captures as much radiation from the lamp as possible, and the illumination system for homogenous mask illumination.
In Summer 2004, a first EUV micro-stepper has been installed at a major chipmanufacturer.
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