EUV Lithography

Prototype of an EUVL wafer stepper by the Lawrence Livermore National Laboratory, California, USA.
Dr. Lutz Aschke EUVL, Project Manager,

Fit for Future Chip Technology

SCHOTT LITHOTEC is a world leader in high-tech optical materials for lithography. In a new project the company is now also developing key materials and components for Extreme Ultraviolet Technology (EUVL).

According to Moore’s Law the power of computer chips doubles approximately every 18 months by continually miniaturizing their structure. This is only possible if appropriate lithography processes are available to treat the silicon wafers. Inevitably continuously smaller light wavelengths are used in the exposure machines (wafer steppers) which transfer the complicated design of the electronic components and circuits. Nowadays mass production is handled by eximer lasers which work with wavelengths of 248 and 193 nanometers (nm = one-billionth of a meter) and thus belong in the invisible, short-wave UV range. For the next generation of chips, which will be going into production beginning in 2005, wavelengths of 157 nm are foreseen, which can be produced using fluorine lasers. Back in 1999, a German consortium started work on 157 nm lithography. As the world’s leading manufacturer of calcium fluoride crystals, SCHOTT LITHOTEC is playing an important role in the consortium.

Innovation Tempo High

The use of ever smaller light wavelengths reduces the structure gaps on computer chips and thus increases their performance.
To maintain the tempo of the International Technology Roadmap for Semiconductors, i.e. the planning agreed between the chip manufacturers, the foundations must be laid down now for “Extreme Ultra Violet Lithography” (EUVL). EUVL is operated with an extremely short-wave UV radiation of only 13 nm and is set to lead to structure spacings of 50 nm to start with, reducing to spacings of less than 35 nm in due course. This is the most likely variant of lithography after the introduction of the 157 nm version. The changeover to EUVL technology means a shift from the transfer of images from masks to the wafer by using lenses. The use of EUV light requires the exclusive use of reflecting optical elements instead of transmission optics, in essence mirrors. These consist of a substrate with very special properties, to which the reflective layers are applied.

Masks, Lenses, Mirrors

Test structure with “ZERODUR®” glass-ceramic for mechanical EUVL components.
EUV lithography marks a change in technology in which lenses are completely replaced with mirrors.
Lithography is not just a question of the radiation sources, but above all the suitable materials for components and systems. SCHOTT LITHOTEC AG of Jena is the only manufacturer at the present time supplying all the technologies currently in use. These materials include high homogeneity optical glasses, fused silica and calcium fluoride. The substrate requirements for 13 nm lithography represent a further challenge. For example the material must have an extremely low thermal expansion so that it does not distort when warmed up. Based on optical simulations, the expansion would have to remain below just a few ppb (parts per billion) per Kelvin range. One ppb per Kelvin means that a body 100 millimeters long may not expand more than 0.1 nanometers when heated up by one degree – this greatly surpasses all previous demands on so-called zero expansion materials. It calls for the development of a new material and enhanced measurement techniques if the required material properties, with regard to expansion behavior, flatness, surface finish and freedom from imperfections, are to be verified with absolute certainty.

Extremely Clean Rooms

The fused silica substrates are lapped, then polished and then under clean room conditions finally coated with more than 100 layers, each only a few atoms thick.
Yet virtually undetectable thermal expansion is only one of several requirements that the substrate material for the so-called mask blanks has to meet. Another indispensable aspect is that it must be capable of being polished to an exceptionally high degree to give a final surface roughness of only a few tenths of a nanometer, which is in the range of a few atoms. To avoid even the slightest imperfections, the preparation, polishing and coating stages are carried out in extremely clean rooms. Every minute imperfection on the mask is reproduced on the chip, just like every speck of dust on a slide stands out when it is projected onto a wall. On a square mask blank with a 15 cm edge, not a single imperfection is permitted that is larger than 50 nm. Even verifying this by measurement is an enormous challenge. SCHOTT LITHOTEC, in cooperation with appliance manufacturers and institutes such as the IOM (Institut für Oberflächenmodifikation = Institute for Surface Modification of Leipzig), is investigating various correction processes to further improve the overall surface of the masks by carefully removing the minutest amounts of material from it.
A further prerequisite is a degree of material homogeneity which can only be achieved with the special processes used for the manufacture of optical glasses. With its “ZERODUR®” glass ceramic and its proven performance in scientific and industrial applications, SCHOTT is in an excellent starting position, since this material already comes very near to meeting these requirements. Nevertheless some fundamental work still needs to be done.

Sights on Higher Market Shares in Lithography

The goal of SCHOTT LITHOTEC and SCHOTT GLAS is clear. If the broad industrial introduction of the EUV technology is imminent from 2006 onwards, key materials and components should come from SCHOTT. These include the substrate material for masks and optical elements plus the complete mask blanks. The share of the lithography market is set to be expanded continuously. SCHOTT LITHOTEC plans to transfer its current world leadership in the manufacture of calcium fluoride to the technologies of future lithography generations too.