Leading-edge technologies require continually shrinking design grids due to the industry demands for decreasing minimum feature size and higher resolution. Using conventional raster-scanned exposure tools to place these patterns on photomasks results in longer writing times, because linear decreases in address result in exponential increases in writing time. This phenomenon can be compensated for by making changes in writing strategies. Multipass gray (MPG) is one method of drastically improving throughput at small addresses while retaining lithographic quality.MPG was introduced with the MEBES 4500S system and takes advantage of the gray-level writing strategies of the CORE® and ALTA® exposure systems. MPG combines the use of multiple exposure passes and offset scan voting (OSV) to reduce placement, butting, and scan linearity errors. Additionally, this multipass strategy allows the exposure of higher doses without impacting throughput. The added dose allows use of less sensitive resists, such as DNQ/novolacs, and high-molecular weight, chain scission resists, such as ZEP 7000. This paper describes the MPG writing strategy and presents lithographic results. Results are based on a process using GHOST proximity effect correction (PEC), ZEP 7000 resist, and dry etch of the chrome film. Placement and butting error reduction with multipass writing, critical dimension (CD) control, CD linearity, and overall lithographic quality will be discussed.
MULTIPASS GRAY WRITING STRATEGYRaster-scan writing techniques have evolved through several stages, beginning with simple single-pass printing (SPP). The latest stage in this evolution for MEBES systems, multipass gray (MPG), is built on the successful implementation of gray-level writing strategies in the CORE and ALTA laser systems. As pattern addresses become smaller with each device generation, the writing time of systems operating with SPP becomes impractical, because the writing time increases quadratically with linear decreases in address size. Compared to writing equivalent patterns with SPP, MPG delivers four times the dose with approximately four times the throughput. Notably, at small address sizes, CD quality is not lost, and often it is better than that obtained with SPP. Combined with GHOST PEC, 180-nm devices can be generated with good CD uniformity and linearity.To understand how the MPG process can produce such detailed features with high throughput and dose, a brief review of writing strategies is in order. SPP is the original writing strategy for MEBES systems, and it is the most intuitive. Analogous to single-pass scanning in a cathode ray tube, the electron beam (e-beam) is scanned in raster fashion across the entire quality area of the plate. The full-width, half-maximum of the beam is, nominally, equal to the grid spacing in the raster. Given a sharply focused beam, SPP can produce detailed features but with unacceptable time penalties at small addresses. In addition, minor, normal perturbations of beam quality during the single scan negatively impact CD...
