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Micro/Nano Lithography

Next-generation technologies vie for supremacy

Eye on Technology - microlithography

From oemagazine May 2001
30 May 2001, SPIE Newsroom. DOI: 10.1117/2.5200105.0003

As desired minimum-circuit features shrink to below 100 nm, the semiconductor industry continues its search for an affordable alternative to traditional optical lithography. At the SPIE Microlithography meeting (25 February–2 March), technologies such as extreme ultraviolet lithography (EUVL), and electron-beam (e-beam) and proximity x-ray lithography were topics of discussion at the Emerging Lithographic Technologies Conference.

Members of International SEMATECH (Austin, TX) have voted EUVL as the most likely to succeed, with forms of electron projection lithography placing second, says IBM researcher and conference chair Elizabeth Dobisz. Until the mid-1990s proximity x-ray lithography was viewed as the top contender, but it has since slipped to third choice. Ion projection lithography is being developed almost exclusively in Europe.

stopping the showstoppers

Researchers have overcome some of the showstoppers of EUVL technology, said Peter Silverman from Intel (Santa Clara, CA) at the meeting, manufacturing the required high-accuracy aspheric EUV optics, and demonstrating environmentally stable, low-stress EUV multi-layer coatings with reflectivities near 70%. Meanwhile, defect levels on multi-layer coated mask blanks have been dropping by a factor of ten annually. Silverman expects them to reach production- quality levels within the next year.

Three significant challenges remain before the technology will be ready for high-volume manufacturing, however, Silverman says. These include the development of commercial exposure tools, improvement of EUV source output, and development of a commercial supplier for masks and mask blanks.

E-beam lithography faces challenges as well, but George Gomba from the IBM Microelectronics Division thinks the technology will beat EUVL to market. In addition to tool development, there has been a great deal of focus on the infrastructure, he says. At the Next Generation Lithography (NGL) Mask Center of Competency (Burlington, VT), a joint venture between IBM and Photronics (Jupiter, FL), scientists demonstrated stencil masks and a full-membrane mask technology. "We've demonstrated that we're able to pattern below the 100-nm region," says Gomba. "Now we need to focus on the sub-70-nm region."

The throughput potential and the design of the electron optics will be the critical point in commercializing e-beam lithography, says Gomba. At the conference, Nikon Corp. (Tokyo, Japan) reported that its researchers have developed a proprietary design with sufficient throughput, but declined to elaborate. Officials did say they expect alpha tools to be ready by 2003 and production tools by 2004.

Over the past several years, most of industry shied away from proximity x-raylithography due to the difficulty in making the appropriate mask materials and achieving accuracy in critical dimension control and image placement with e-beam writing. At the conference, Hiroshi Watanabe from the Super-fine SR Lithography Lab (Kanagawa, Japan) reported the first functioning real device mask for the 100-nm node for any of the lithographic contenders for 100-nm lithography and below, says Dobisz. Watanabe's group demonstrated 4-Gb gate and contact pattern writing using a 100-kV e-beam and beam drift compensation. They obtained an image placement accuracy of better than 10 nm, a mask overlay accuracy less than 10 nm, and the critical-dimension uniformity of less than 8 nm in a 24-mm-square area.

looking ahead

Over the past decade, a combination of market pressure and competitive needs have forced the acceleration of the lithography road map. "The issue the industry faces is sustaining the pace over the next two years," says Gomba. "Infrastructure must be developed for all the NGL tools."

Dobisz agrees. "These alternatives will be competing with 193-nm and 157-nm optical lithographies using phase-shifting masks to make sub-100 nm dimensions," she says.

Gomba believes that 193 nm will be the workhorse of the industry over the next three to four years. Silverman predicts that semiconductor development requirements will outrun the capability of 193-nm and even 157-nm tools well before 2004. "EUVL development tools will be needed by 2003 or sooner, and EUVL will be needed in manufacturing by 2005," Silverman says.

Because neither e-beam nor EUV have solved all the critical issues yet, says Gomba, it is too premature to discontinue the development program for any NGL candidates.