Water and litho
Today (November 13), ASML will be presenting to financial analysts some results from its immersion lithography development work. According to a taster from the European company, positive results include benefits from its TWINSCAN system in terms of performing the metrology step in air (focus and overlay) and easy adaptability of ASML’s lens design to immersion. Immersion is being developed to extend 193nm lithography to tighter resolutions. ASML also plans to apply immersion to 157nm to reach below 45nm resolution. However, for this wavelength water cannot be used.
Japan’s Canon is also looking to immersion lithography (IML) as a potential successor to its super high NA 193nm lithography technology for 65nm and 45nm chip production.
Dr Akiyoshi Suzuki of Canon’s core technology development group comments: "ArF immersion is an attractive solution because it enables the use of current reticle technology and the adaptation of current resist technology while providing a much shorter 134nm wavelength compared to 157nm exposure technology. 157nm systems, however, have the advantage of maturity. Today, the most important factor in making the decision to pursue immersion is timing and costs to solve technical issues."
The company points to the use of water for the immersion liquid as being perhaps the most crucial issue since it is a totally new material factor in the optical subsystem of a lithography tool. Micro-bubbles can also cause problems – at the 45nm node, a 10nm diameter microbubble can have a large effect on image formation. Work at Canon has shown that if immersion-water thickness is controlled between 0.1mm and 1mm, the effect of refractive index fluctuations is small enough to be ignored - it does not put any constraints on the optical-system configuration.
