Projection reduction exposure with variable axis immersion lenses (PREVAIL) represents the high throughput e-beam projection approach to NGL, which IBM is pursuing in cooperation with Nikon as alliance partner; another e-beam projection approach is SCALPEL pursued by Lucent Technologies. This article discusses the challenges and accomplishments of the PREVAIL project. It will focus on the results obtained with the proof of concept (POC) system. This system was developed to demonstrate key technical building blocks required for high throughput, high resolution e-beam step, and scan projection lithography. The supreme challenge facing all e-beam lithography approaches has been and still is throughput. Since the throughput of e-beam projection systems is severely limited by the available optical field size, the key to success is the ability to overcome this limitation. The PREVAIL technique overcomes field-limiting off-axis aberrations through the use of variable axis lenses, which electronically shift the optical axis simultaneously with the deflected beam, so that the beam effectively remains on axis. This technique developed by IBM has been successfully applied to probe-forming shaped beam systems (EL-4). It had to be modified and extended to provide the larger beam deflections and the wider beam images at the wafer plane used in projection reduction systems. The POC system projects sequentially 1×1 mm2 subfields, selected at the reticle, in 4:1 reduction mode onto the wafer, exposing and resolving patterns of 80 nm lines and spaces in resist; each subfield contains 107 pixels. The deflection capability demonstrated permits electronic selection of 20 1 mm subfields at the reticle and projection of these 20 subfields onto the wafer exposing a field with 5 mm scan length. The resist images provide proof that PREVAIL effectively eliminates off-axis aberrations affecting resolution, since the deflected and undeflected images are indistinguishable. PREVAIL also controls off-axis aberrations affecting placement accuracy of pixels, since distortions of the deflected subfield are corrected to within 12 nm. A high emittance gun has been developed to provide uniform illumination of the patterned subfield, and to fill the large numerical aperture of the projection optics required to significantly reduce beam blur caused by Coulomb interaction.

Electron-beam writing instruments for microcircuit fabrication are currently limited by total cycle time, field coverage, automatic registration, and reliability. A fully computer-controlled electron-beam pattern generator will be described which has been developed to advance the economic feasibility of electron beam writing. The instrument incorporates a fully automated mechanical stage and pattern registration system, computer designed deflection coils and a dynamic focusing system for correction of deflection introduced aberrations. Field coverage at the short working distance is variable, up to 0.120×0.120 in. with 60 000×60 000 addressable points in the field. Resolution over the field is 3500 lines at 8 mrad. Pattern distortion is within 1.0 μ over a 0.1 in. square field. The mechanical stage employs stepping motors driving lead screws to provide 3×3 in. movement in 250 μ in.steps. Additional travel of 7 in. is available in one axis for loading the wafer. The automatic pattern registration system employs four silicon etched “L” shaped fiducial marks located in the corners of each field. The electron beam is used to measure the position of the fiducial marks before each field exposure and position the pattern correctly on the wafer. Pattern position, size, rotation, and orthogonality are all typically set up by the automatic registration system in 300 msec. A vernier test pattern has been used to test the alignment system and shows registration better than ± 0.2 μ.

Pattern generation on wafers using the electronbeam exposure system (EBES)A pattern generation technique for serial electronbeam microfabrication systems

Articles you may be interested inHigh-current electron optical design for reflective electron beam lithography direct write lithography J. Vac. Sci. Technol. B 28, C6C1 (2010); 10.1116/1.3505130 REBL: A novel approach to high speed maskless electron beam direct write lithography

The IBM/Nikon alliance is developing an EPL stepper alpha tool based on the PREVAIL technology. This article provides a status report on the alliance activity with particular focus on the electron optical subsystem developed at IBM. We have previously described design features of the PREVAIL Alpha system. The state-of-the-art e-beam lithography concepts have since been reduced to practice and turned into functional building blocks of a production level lithography tool. The electron optical subsystem has been designed, built, assembled, and tested at IBM’s Semiconductor Research and Development Center (SRDC) in East Fishkill, NY. After demonstrating subsystem functionality, the column, an interim mechanical system and all associated control electronics hardware and software have been shipped during January 2001 to Nikon’s facility in Kumagaya, Japan, for integration into the Nikon commercial e-beam stepper Alpha tool. Postshipment activity has been directed primarily toward demonstrating subfield stitching, a task which exercises system architecture, calibration, stability, and noise performance.