In recent years, a few companies around the world have been looking for the next generation technology for producing semiconductor devices with features size <14nm. The first stage in a semiconductor device manufacturing process is the lithography step. In this process, a radiation source is used to photoengrave a pattern in a silicon wafer. Until recently a UV source at wavelength of ~190nm was used. However, creating features of sizes smaller than 14nm becomes extremely difficult when using this source. The leading technology contender today employs Extreme Ultraviolet (EUV) radiation sources. These sources use high intensity lasers to irradiate small liquid tin droplet targets producing a plasma which has a strong emission feature at a 13.5nm wavelength.
Livermore Lab researchers have developed a new EUV target design that replaces liquid tin droplets with tin microbeads embedded in a low Z tamping fluid. The use of low Z liquid tamped targets can solve several problems that are currently faced by the industry. It can increase the total operational uptime from 80% to close to 100%. It can simplify EUV source design and reduce operating costs by eliminating the need for some major components and their associated maintenance requirements.
LLNL’s new target design uses smaller mass targets that can be fully ionized leaving no neutral debris to contaminate system optics and reduce the possibility of metal particulates reaching the mask/wafer.
Tamped targets generate a Lambertian radiation pattern which allows for a mostly axial direction of emission without significant loss. Combined with an off-axis laser excitation prevents the laser from blocking the line of sight and as a result eliminates the need to use a large, expensive multilayer collector mirror. Current EUV systems are expensive to run and suffer from significant downtime as their collector mirror needs frequent maintenance. The new design does not use multilayer mirrors allowing other target materials to be employed for future radiation sources with even shorter wavelengths.
As an added advantage emission occurs from both sides of the target potentially doubling EUV output and by sizing targets to be completely consumed leads to even further conversion efficiency improvements.
These novel target sources can be used for efficiently generating EUV light for photolithographically imprinting the smallest critical dimension very-large-scale integrated circuit elements on silicon wafers for fabricating next generation ultra-high-density microelectronics, exascale microprocessors and terabyte capacity memory chips.
LLNL’s liquid-tamped targets for EUV lithography technology is patent pending. LLNL assesses the TRL at 2-3.