As chip makers approach sub-100-nm design rules, they're actively exploring alternatives to optical lithography to meet the challenges of printing ultra-small images on future ICs. One of these new techniques, electron-beam projection technology, has been positioned on the Semiconductor Industry Association (SIA) roadmap as a next-generation lithography (NGL) option for sub-100-nm applications. There have been several developments in recent years, but due to small optical field sizes, throughput has been keeping e-beam technology from advancing any further.
Researchers at IBM's Microelectronics Semiconductor Research and Development Center in East Fishkill, N.Y., have overcome the field-limiting off-axis aberrations through the use of a variable-axis lens, which electronically shifts the optical axis simultaneously with the deflected beam. In essence, the IBM technique combines e-beam projection with e-beam scanning. This achieves simultaneous shifting of the electron optical axis along a predetermined curvature, along with deflection of the electron beam, to precisely follow the curvilinear variable axis.
Known as PREVAIL—for projection reduction exposure with variable-axis immersion lenses—this technique improves the available field size over undeflected methods by several orders of magnitude. By sequentially exposing and stitching together twenty 0.25- by 0.25-mm subfields at the target wafer, IBM has shown that PREVAIL can achieve an effective field size of 5 mm at 80-nm resolution (see the figure).
"The resist images obtained with the proof-of-concept system demonstrate that PREVAIL effectively eliminates off-axis aberrations and achieves the same image quality as the undeflected subfield pattern," says Hans C. Pfeiffer, manager of e-beam technology at the Microelectronics Semiconductor R&D Center. He also says that the loss of resolution is undetectable, and the distortion is minimal.
However, the new IBM system employs a correction technique to keep distortion for the deflected subfield within 12 nm throughout the 5-mm optical field. By comparison, the distortion of the undeflected subfield is 6 nm for the X and Y axes. Also, the study shows that the PREVAIL distortion for deflected subfields without correction is nearly 225 nm in both axes.
Pfeiffer presented the results and other details of this high-throughput e-beam system in a paper titled "PREVAIL—IBM's E-Beam Technology For Next-Generation Lithography" at last month's SPIE—the International Society for Optical Engineering's conference—in Santa Clara, Calif. According to the paper, the electron source and the projection optics were designed for minimum Coulomb interaction through a large numerical aperture and beam currents of 7.5 mA at 75 kV.
Lucent Technologies' Scalpel electron-projection lithography also is in the e-beam race. But unlike Scalpel, which uses membrane masks, PREVAIL employs stencil masks. In practice, mechanically positioning the mask and wafer to accomplish subfield stitching typically limits throughput in this system.
Next, the project will work on accurate, mechanical scanning of the reticle and wafer at high speeds. Under a joint effort, Nikon Corp. of Tokyo, Japan, is developing such a system. It will be combined with IBM's high-speed e-beam scanning in a production-worthy e-beam step-and-scan Alpha lithography system that's expected to be completed in 2002. Both developers believe that the production e-beam stepper can be used within the next two generations of IC manufacturing. Initially, they hope to accomplish throughput levels of thirty 200-mm wafers per hour.