A new mask lithography tool for advanced mask manufacturing

Abstract

In this paper, we will describe the design and performance of a new mask lithography tool, the ALTA-3000 system produced by Etec System, Inc. The introduction of both step-and-repeat reduction wafer lithography and raster-scanned electron beam mask lithography in the early 1980s effectively minimized mask error contributions in IC manufacturing such that masks are now generally viewed as having only one distinguishing characteristic - price. The continual demand for greater performance has naturally led to smaller integrated circuit linewidths and tighter dimensional tolerances as well as greater data volume and density, which present new challenges to the mask maker. While smaller linewidths on the wafer do not directly place limits on the mask maker (mask dimensions are 4 to 5 times the size of the corresponding wafer dimensions), lithography limitations at the wafer level require operating steppers at lower k1 factors, causing wafer lithography to be more sensitive to mask CD errors. In addition, techniques to enhance resolution and depth of focus at the wafer, including optical proximity correction and phase-shifting masks, add complexity to the mask in the form of increased data volume and density, multiple aligned levels, smaller dimensional addressability and tightened CD control. Finally, tighter placement and level-to-level overlay on the mask are needed to achieve acceptable yield as device scale decreases and density increases. The Etec ALTA-3000 is a new scanned laser mask lithography system that has been designed to address these issues in the mask manufacturing environment. This system employs a greatly enhanced raster printing scheme using 32 beams scanned in parallel, an 8-pass print strategy, interstitial printing grids and 5 levels of beam intensity control per beam to achieve high accuracy and high throughput simultaneously. Thirty-two exposure grey-levels provide a grid addressability of 8.333 nanometers independent of throughput. Advanced multi-pass averaging schemes are used to reduce systematic errors. A new expandable parallel datapath architecture has a raw rasterization rate of 80 million geometries/second, and a peak transfer rate of over 2 billion pixels/second to accommodate large, dense data files. An alignment system based on CCD camera image capture can be used to register the printing of multiple levels on the same mask. A new stage transport system can handle plate sizes up to 9″ by 9″ by 0.25″. The ALTA-3000 uses an argon-ion laser source operating at a wavelength of 383.8 nm, making it compatible with exisiting laser mask manufacturing processes. © 1995 Elsevier Science B.V. All rights reserved.