Optical Lithography Creates Subangstrom Features in Silicon

R. Winn Hardin

A group of researchers at the University of Texas has shown the world that conventional optical lithography can continue to produce ever smaller features on computer microchips without resorting to next-generation plasma laser, ion or electron beams.
Grant Willson and his students Kyle Patterson and Sungseo Cho generated 0.08-µm features on a microchip using an ArF excimer laser from Cymer Inc. of San Diego. Previously, semiconductor experts expected that 193-nm ArF lasers would be "pushing the hairy limit" to create 0.1-µm features.
The feat was a mixture of cutting-edge optics including a catadioptric lens system from Tropel Inc. of Fairport, N.Y., advanced stepper designs from Integrated Solutions Inc. in Austin and a quartz-chromium photomask from DuPont Photomasks Inc. of Round Rock, Texas.
The photomask, first developed by Mark Levinson at IBM, provides high-contrast patterning of photoresist material, Grant said. Catadioptric lens designs use mirrors to correct for color aberrations induced by quartz optical systems, and the highly sensitive photoresist developed with support from the microchip consortium SEMATECH retains sharp feature control through subsequent etching processes.
Other companies such as stepper-maker Canon Inc., Lucent Technologies and IBM Corp. have produced feature sizes at the 0.08-µm level; however, these systems depend on soft x-rays and complicated reflective optics and photomasks. Several companies continue to pursue this process, pouring time and research into reflective optics that offer greater than 13 magnifications and even smaller feature sizes.

Coherent UV light
A similar effort, supported by a consortium led by Intel, Motorola and Advanced Micro Devices Inc., uses an Nd:YAG laser to pump a supersonic jet of gas to create extreme-UV coherent light. Beta systems for these methods are not expected before 2008.
Although Willson's group uses conventional technology, "in order for it to work in manufacturing, we need some major changes to the equipment," he said. The largest field or chip size supported by a prototype 193-nm ministepper is only a few millimeters wide, whereas production chips measure several centimeters wide with critical dimensions in the 0.25-µm range.
Willson added that the manufacturing process for the photoresist, while impressive, has low yields, and much about the fundamental chemical reactions is not understood. "There's still a great deal of fundamental work to be done," he said.