Extreme UV lithography to power electronics
US computer processing company AMD, in collaboration with IBM, has announced the development of a 22x33mm working test chip using extreme ultraviolet lithography (EUVL) for the critical first layer of metal connections across the entire chip.
Extreme ultraviolet lithography for manufacturing smaller transistors for more powerful microchips has been the focus of intensive research for a decade, but previous work has only been able to cover part of the chip.
How small the transistors and the metal lines that connect them are is directly related to the wavelength of light used to project the chip design onto a silicon wafer.
Extreme ultraviolet lithography, unlike its predecessor optical lithography, relies on light shone at 13.5nm instead of 193nm, and uses mirrors in place of lenses.
‘The problem is like using a huge paintbrush to paint a tiny line. So you have to find a smaller paintbrush,’ explains an executive from rival company Intel, which is also working on EUVL.
The test chip manufactured at AMD has undergone electrical testing. The next stage is to prove the viability of EUVL for all critical layers and not just the metal interconnects.
AMD representative Julia Clark adds, ‘The most significant materials difference is the photoresist, which is optimised for EUVL performance rather than conventional 193nm light exposures, and will need further improvement to be suitable for 32nm and a half pitch and beyond’.
The photoresist is a light- sensitive material onto which the chip’s pattern transfer is placed. It usually contains a polymer matrix and photo acid generator (PAG) substrate that dissolves after exposure to EUV.
‘The challenge is to make sure the polymer matrix is strong enough before exposure to stand up to the etch, which involves bombarding it with ions,’ says Mike Mayberry, head of research at Intel. ‘But it must also be able to fall apart after exposure. If it’s too tough, you can’t get it back off the wafer.’
Adjusting the smoothness of the grain is key to fine-tuning the toughness of the matrix, and involves developing a uniform rather than chaotic molecule content.
Typical materials used in addition to the PAG/polymer substrate include conductive and absorber films made from chromium nitride and tantalum nitride. Researchers are also considering making photoresists from molecular glass instead of polymers, which allows the grains to be defined by the size of the molecules. ‘This is further away from production, but offers better granularity,’ adds Mayberry.
Materials engineering will also involve optimising the smoothness of the mirrors and incorporating multilayer coatings made of titanium, ruthenium or silicon on them.
Mayberry believes the first EUVL production tools will be available from 2010, 15 years after work first started.