Ultratech Steppers P-GILD to demo

07/01/1997

Ultratech Stepper`s P-GILD to demo

Ultratech Stepper is disclosing details of its projection gas-immersion laser doping (P-GILD) process to customers, and expects to have a next-generation test-system demo sample by mid-July. P-GILD creates patterned junctions in a single process module without the need for photolithography.

P-GILD is an alternative to ion implant for the creation of ultra-shallow (30 nm deep at 0.18 ?m, and 10-30 nm in succeeding generations) heavily-doped source/drain regions. Ion implant cannot easily produce < 60-nm deep junctions, since activation anneals allow dopants to move more than that even if the initial implant depth is < 60 nm. Kurt Weiner, P-GILD development manager, stated that lenses and subsystems under development should be capable of producing square 0.8?0.8-?m junctions with lenses 0.5?0.5-?m planned by 1999.

The P-GILD 308-nm laser illumination subsystem is very similar to that of a standard DUV stepper. The main difference is that the laser light is so energetic that the silicon surface actually melts in a 30 nsec pulse, with the complete process requiring around 100 nsec. Different junction depths (e.g. 30 nm and 40 nm) can be produced by varying the laser power in successive steps.

Since the silicon actually melts, doping is limited only by the laser depth and liquid solubility. 100 W/ surface resistivity has been achieved with 100 P-GILD laser pulses, the lowest possible ion implant resistivity is 1000 W/. P-GILD doping profiles are compatible with the National Technology Roadmap for Semiconductors to the 0.07-?m generation.

P-GILD process integration will require several new processes flows. Since the resolution limit is currently 0.8-?m, the laser heating during junction formation will destroy adjacent gates. A germanium implant before gate formation creates amorphous regions in the wafer with lower melting temperature and thermal conductivity.

These amorphous areas re-crystallize after one P-GILD cycle, so complete doping has to occur in a single melt
e-crystallize cycle. Photolytic pre-deposition, using 193-nm laser illumination to break apart the dopants and deposit them on the surface, results in a broader range of dose control and better ability to get adequate doping in a single pulse. Using the new process flow, time dependent dielectric breakdown (tddb) tests results were identical with standard ion-implant process flows for 3-10 nm thick gates. Weiner stated that 90% of the process integration issues are resolved.

Though the P-GILD system design is conceptually similar to a standard wafer stepper, several new subsystems needed to be developed. The optics achieve 0.22 NA 4? reduction using 13 fused silica elements. Piezo-resistive elements in a new "Stewart Truss" design create active isolation of the optical path. The stage is designed to maintain the functionality of a stepper with the physical isolation required to handle the doping gases. Interferometers in the optical path are used to maintain overlay accuracy. P-GILD reticles (originally developed for the ablation industry) using reflective dielectric stacks, are based on technology developed by IBM. - E.K.