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 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.