Ultraviolet Laser Combines Steps in Wafer Doping

Kathleen G. Tatterson

SAN JOSE, Calif. -- An instrument that employs a laser doping process could combine the lithography and silicon doping steps in the manufacture of integrated circuits.
The device, a deep-UV stepper using a technique that Ultratech Stepper Inc. calls projection gas immersion laser doping, would enable simple fabrication of shallow, low-resistance junctions in titanium silicon. Scientists promise that junctions made by this process will reduce power dissipation and increase operational speeds of transistors. The machine offers a step-saving technique in the source/drain doping application, according to Kurt Weiner, program manager. The tool would replace a conventional stepper, spin and bake track systems, an ion implanter, a rapid thermal processor, a plasma asher and wet processing steps. In a conventional process, manufacturers must build a photolithographic mask, then strip away the mask after input. With Ultratech's tool, "the mask is basically in the tool," Weiner said. "You don't have to go through all the photoresist steps with the masking process."
Also, the single-wafer, field-by-field technology offers process control through multiple generations of integrated circuits, extending to 0.07-µm design rules and 12-in. wafer sizes. Laser doping gives an extra degree of freedom, he said.
The system uses a 308-nm XeCl excimer laser to selectively dope regions of an integrated circuit. Optics direct the laser's output, shaping and directing the beam onto a reticle, which patterns the light. A 43 reduction lens images the beam onto the wafer, which is immersed in the desired dopant gas. Intense laser radiation quickly heats the "exposed" areas of the silicon.
Ultratech developed the instrument through a $1.5 million partnership with the Defense Advanced Research Projects Agency and Sematech. In Phase II, the group hopes to deliver beta versions of the machine to laboratories by March 1998.