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Group to develop low-energy e-beam litho system

Posted: 23 Sep 2002 ?? ?Print Version ?Bookmark and Share

Keywords:Toshiba? Ebara? Tokyo Electron? Dai Nippon Screen? soc?

Toshiba Corp. and three partners have joined forces to develop a maskless, low-energy electron-beam direct-writing system for system-on-chip devices. The work could yield the best lithography solution for fabricating SoC devices in small lots, said Toshiba and partners Tokyo Electron Ltd, Ebara Corp. and Dai Nippon Screen Mfg. Co. Ltd.

Toshiba reported that 100nm and 80nm line and space patterns could be formed by exposure to an electron beam with energy as low as 2keV. The company will transfer its e-beam lithography technology to a joint venture, tentatively named E-Beam Corp. that the partner trio will own. All are newcomers to the lithography business.

E-beam direct-writing systems have the advantages of high resolution and need no costly masks, but are seen as having a low-throughput problem. Currently it takes about three hours for an e-beam system using a variable-shape beam to fully write on one wafer.

"Unless e-beam systems have throughput of at least five to 10 wafers per hour, there is not a cost merit in using (them)," said Kazuyoshi Sugihara, a group manager of lithography process development at Toshiba's process and manufacturing engineering center. "Toshiba can provide the solution to realize throughput of several wafers per hour," he said. That rate would be 20 to 30 times higher than comparable KrF lithography systems in relatively small lots.

The low-energy e-beam system business that Toshiba and its partners are planning will take a unique form. Toshiba will transfer its e-beam lithography technology to a joint-venture company to be established by Tokyo Electron, Ebara and Dai Nippon Screen. The venture intends to introduce a maskless lithography system built on the Toshiba technology that targets low cost and quick turnaround time.

Toshiba has been fostering electron-beam lithography technology for more than 20 years. The research team turned to low-energy e-beams around 1996 and has already verified their operation, Sugihara said.

Based on its recent rigorous selection and concentration policy, however, Toshiba is opting to transfer the technology rather than keep it in-house. Once that happens, the company said it will reduce e-beam system development activity.

E-Beam Corp. is to set up shop in Tokyo by the end of this month with a mission to develop, manufacture and market low-energy e-beam systems. Toshiba will design the direct-writing system, Tokyo Electron will provide marketing and process technology in lithography, Ebara will build the system, and Dai Nippon Screen will contribute its software technology.

The company will be capitalized with $3.7 million. Tokyo Electron and Ebara will each own 40 percent of shares with Dai Nippon Screen having the rest. The alliance will give three shareholders a chance to enter the lithography market, while Toshiba can reap the results without any further investment.

With the key technology from Toshiba in hand, the joint venture plans to develop related technologies and conduct process verifications. The first target is to conduct verifications using the fully built system by March 2004. Then, in 2005, E-Beam will start offering a system capable of 65nm resolution at around $8.5 million, with a sales target of 10 units in the first year.

Low-energy advantages

Low-energy electron-beam lithography has advantages over conventional high-energy e-beam systems in terms of shorter exposure time and less susceptibility to so-called proximity effects. Because the resist's sensitivity increases in inverse proportion to exposed electron energy, exposure time can be dramatically reduced with lower-energy beams.

For conventional high-energy e-beam systems, compensation or countermeasures are mandatory to deal with the proximity effect, in which an incident electron beam scatters inside the silicon substrate and returns to the surface to expose an undesired part of the resist. To form a desired pattern despite the proximity effect, it is necessary to precisely control the shots and to take extra shots as necessary. But the energy in low-energy e-beams is small enough to essentially render the effect negligible. No compensatory shots are necessary.

On the other hand, low-energy electron beams have some disadvantages too. They cannot expose a resist as thick as 1 micron all the way to the bottom. However, resist is becoming thinner and thinner with every process generation. In today's KrF generation, it is about 0.3?m; in the argon-fluoride lithography generation it will drop to roughly 0.15?m, which can be penetrated with 5keV energy.

To take advantage of low-energy e-beam's throughput boost, Toshiba engineers adopted what they call the "character projection" method. There are repeatedly appearing patterns in LSIs that represent circuit blocks. Each of these patterns is picked up as a single character (fixed pattern) and patterned onto a stencil. Using the stencil, the character pattern is exposed with one shot. The same pattern may require more than 10 shots if the variable-shape beam (VSB) method is used.

Toshiba engineers propose compiling a "compact cell library," which is a set of a limited number of general-purpose characters. Logic synthesis is basically possible using these cells, Toshiba said, and the number of lithographic shots can be greatly reduced. When a full-custom device needs 2 billion shots using VSB, a corresponding device designed using only the characters in the compact cell library will need 120 million, by Toshiba's calculations.

"This is a proposal from the e-beam lithography process side. To realize really high throughput, collaboration from the design side is necessary," Sugihara said.

- Yoshiko Hara

EE Times

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