Projection reduction exposure with variable axis immersion lenses: Next generation lithography

Projection reduction exposure with variable axis immersion lenses (PREVAIL) represents the high throughput e-beam projection approach to NGL, which IB M is pursuing in cooperation with Nikon as alliance partner; another e-beam projection approach is SCALPEL pursued by Lucent Technologies. This articl e discusses the challenges and accomplishments of the PREVAIL project. It w ill focus on the results obtained with the proof of concept (POC) system. T his system was developed to demonstrate key technical building blocks requi red for high throughput, high resolution e-beam step, and scan projection l ithography. 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 t o 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 wit h the deflected beam, so that the beam effectively remains on axis. This te chnique developed by IBM has been successfully applied to probe-forming sha ped 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 i n projection reduction systems. The POC system projects sequentially 1 x 1 mm(2) subfields, selected at the reticle, in 4:1 reduction mode onto the wa fer, exposing and resolving patterns of 80 nm lines and spaces in resist; e ach sub field contains 10(7) pixels. The deflection capability demonstrated permits electronic selection of 20 1 mm subfields at the reticle and proje ction 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 undefle cted images are indistinguishable. PREVAIL also controls off-axis aberratio ns affecting placement accuracy of pixels, since distortions of the deflect ed subfield are corrected to within 12 nm. A high emittance gun has been de veloped to provide uniform illumination of the patterned subfield, and to f ill the large numerical aperture of the projection optics required to signi ficantly reduce beam blur caused by Coulomb interaction. (C) 1999 American Vacuum Society. [S0734-211X(99)03806-8].