Electron beam ͑EB͒ stepper, which is the electron beam projection system ͑EBPS͒, has been developed as one of the next generation lithography ͑NGL͒ systems for the 100 nm technology generation and beyond. An EB lithography system similar to EBPS is SCALPEL ® ͑scattering with the angular limitation projection electron-beam lithography͒. EBPS and SCALPEL ® use the scattering contrast principle for the wafer exposure. The development of electron beam scattering reticles is a key issue for the EB lithography system development. The SCALPEL ® system uses the scattering membrane reticle ͑SCALPEL ® mask͒. The scattering stencil reticle ͑EB reticle͒ has been investigated as the reticle for EB steppers. 75 and 200 mm test EB reticles have been developed. The 75 mm test reticle is fabricated with a stress controlled silicon-on-insulator wafer. The fabrication process condition for the 200 mm test reticle is not fixed. The status of risk issues for the EB reticle development is presented. The image placement measurements on the membrane are in agreement with the simulation results. Stencil patterns, consisting of 70 nm lines and spaces, have been exposed with the test reticles using a 100 kV test column.
Reticle cleaning is one of the important issues in all next-generation lithography ͑NGL͒ systems. In this article, an Ar aerosol cleaning technique, which is a cryogenic cleaning technique, is described for cleaning a stencil-type reticle for an electron beam stepper. The cleaning efficiency and the redeposition rate depend on the pressure of the process chamber of cleaning. The Al 2 O 3 particles of average size 0.1 m on the reticle surface and inside the through-hole pattern were effectively removed. The gas flow conditions for damage-free cleaning were limited by the size of the stencil pattern. In the reticle made of a silicon-on-insulator ͑SOI͒ wafer, the thermal distortion of the reticle in the cleaning at low temperature was expected. However, after cleaning, the distortion measurement results did not show residual distortion of the reticle.
We have fabricated Si stencil reticles that are employed by a new type of e-beam projection lithography system ͑EB stepper͒. We applied a stress reduction technique to the Si membrane to improve the pattern placement accuracy. The residual stress of Si membranes which were fabricated by anisotropic etching of B-doped Si wafers in KOH aqueous solution was reduced by annealing at 1150°C. We carried out pattern-displacement measurements for a Si stencil reticle made of a Si membrane where the residual stress was reduced to 10 MPa, and we observed that the pattern displacement error was reduced to less than 20 nm. Furthermore, the pattern displacement in the stencil reticle had a high correlation with the displacement determined from a simulation based on a finite element model. However in the same reticle, we discovered additional, comparatively small displacements in random directions, which was not expected in a membrane that had a homogeneous tensile stress. As a cause of the pattern displacement in random directions, we identified a pattern-width broadening in the dry etching process.
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.
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