Two basic pattern generator architectures have been used for mask fabrication: vector scanning architectures, using either a shaped or Gaussian beam exposure strategy, and raster scanning architectures, using either a binary or gray exposure strategy. The architectures differ in their means of pointing the exposing beam. The various exposure strategies are efforts to improve the rate of pattern data delivery.

The vectoring architecture was originally developed for direct-write-on-wafer applications and has been adapted with limited success for mask making. This strategy is inherently an analog process like that used in the first generation of computer displays and printers.

The raster architecture was developed specifically for mask making, the original binary exposure strategy having been superseded in recent years by the modern gray exposure strategy. This architecture is inherently a digital process and is like that employed in state-of-the-art computer graphics displays and printers.

A raster scanning system patterns a mask by scanning the exposing beam in one direction at a fixed amplitude while the mask is moved under the beam. The beam is turned off and on thousands of times during each scan in order to compose the pattern. The result is very like the raster scanning of a TV screen. The vector architecture attempts to improve throughput by deflecting the exposing beam only to those parts of the pattern requiring exposure, the assumption being that significant time can be saved by skipping over blank areas. In practice, improved throughput is seldom realized and pattern quality problems are introduced. The quality problems arise because of the continuously varying nature of the analog vectoring process.

Beam scanning over the distances of concern (on the order of 1 millimeter) is performed at about 30KHz for a raster scan. To reduced pointing errors, 10KHz is typical for a vector scan. The technology for large-field beam deflection has remained essentially the same for many years and is unlikely to improve significantly. Thus, all else being equal, a vectoring strategy can only improve throughput if a pattern contains more than about two-thirds blank area. This potential for additional speed is available only at the risk of degraded pattern accuracy for the following reasons: Raster scanning is independent of the actual pattern being exposed, the scan need not vary in amplitude or timing within a mask or for different masks. Vector scanning is pattern-dependent; beam deflection amplitude and timing vary continuously. The path of the beam through a mask is essentially random and is different for every mask. The result is that at an equivalent level of technical implementation, raster scanning produces a more accurate pattern. Because the raster scan is invariant, it has fewer sources of pointing error than vector scanning. The pointing errors present in a raster scan can be compensated by fixed variation produced by the actual patter, or, more typically, the exposure is slowed down to minimize the errors.

Exposure Strategy

In raster scanning systems, the rate at which the beam can be turned off and on to compose the pattern, the blanking rate, has increased over the last two decades by more than an order of magnitude, from about 20-500MHz. In a raster scanning system using a simple binary exposure strategy, the resolvable pattern element (pixel) is the size of the exposing beam, and the pixel delivery rate is the same as the blanking rate.

The rate of improvement in blanking rates has been outstripped by the number of pixels necessary to reproduce integrated circuit patterns. The number of pixels in a given pattern area varies as the inverse square of the pixel dimension. Thus, if the pixel size is reduced by one-half, the pattern area will contain four times the number of pixels, and four times the blanking rate would be required to expose the pattern area in the same elapsed time. Rather than simply increasing the blanking rate, far greater rates of pixel delivery are achieved using a gray exposure strategy. The strategy applies the same principles used for resolution enhancement of digital printers and scanners. The total exposure dose delivered at each beam location is modulated to intermediate levels. The result is that the edges of pattern images can be placed with a resolution increment smaller than the size of the beam. With only 16 levels of grayscale, instantaneous pixel delivery rates greater than 10GHz can be achieved using current blanking rates. Gray exposure can only be implemented in a raster scanning architecture. This is the reason modern scanning, display and printing devices have adopted raster architecture.

Variable shaped-beam exposure was developed to improve upon the pixel delivery rate in vector scanning systems. The pattern is composed in a series of "flashes" using a beam shaped into rectangles and triangles of variable size--on the order of 0.2-micron to 20 microns. In principle, this approach should significantly improve the throughput, because each flash can contain many pixels exposed in parallel. However, the flash rate of shaped beams cannot approach the blanking rate of raster systems, so relative throughput may actually be reduced. Furthermore, additional adverse effects on pattern quality are introduced.