Page 97 - Semiconductor Manufacturing Handbook

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PHOTOMASK

8.4 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

8.3 PHOTOMASK EQUIPMENT

The principal tool, and consequently one of the most expensive, in the photomask manufacturing
facility is the pattern generator. As mentioned previously, there exist two broad categories of tools—
those that utilize beams of energetic electrons and those that use energetic photons. Electron beam
tools were among the first to come into the marketplace and the design aesthetic of these machines
influenced succeeding generations.
One of the original ebeam tools was the electron beam exposure system (EBES) developed by
Bell Labs in Murray Hill, New Jersey. This system utilized a raster-scanning system similar in prin-
ciple to the television picture tube. The electron optics scanned a beam in one direction, while a
mechanical stage moved the resist-coated mask in the perpendicular direction. The photomask
image was constructed of “stripes” of variable lengths in the direction of stage travel and a fixed
height that depended on the scan length of the electron beam. This simple raster scan approach
exposed pixels on the resist by turning the beam on or off at the appropriate times. The photomask
design was laid out on a square grid structure and the writing grid of the ebeam system, as well as
the spot size of the beam, matched this grid. Simple raster scanning suffers a fundamental through-
put flaw. As the smallest feature on the mask decreases in a linear fashion, the spot size of the beam
must decrease in a quadratic fashion. One way to tackle the problem was to develop faster elec-
tronics and speed up the exposure rate. The MEBES system developed by ETEC Systems (an
Applied Materials company) evolved through several models with exposure rates increasing from
40 MHz for the original MEBES tool up to 320 MHz for the MEBES 5500. Even an eightfold
increase in speed could not keep pace with the reduction in feature sizes and writing time quick-
ly drove production costs higher and higher. To remedy this problem, alternative writing strategies
were developed. Mask lithographers recognized that the important parameter was not so much the
minimum feature size, as the placement of the edges of a given feature. Schemes using writing
grids and spot sizes larger than the design grid, which patterned the mask with multiple passes,
were developed. Rather than simply exposing a pixel once, developers found that feature edges
could be controlled very precisely by exposing them in several print passes. Exposures could be
averaged and this averaging reduced printing errors.
Optical pattern generators are also widely used in the photomask industry. The Altas family of
optical pattern generators from ETEC Systems is a very common production tool. These tools are
raster-scanning systems that utilize multipass printing. The Altas are laser-based systems that split the
laser into 32 separate beams. The beams are then raster scanned across the mask using a 24-faceted
rotating cylindrical mirror called a polygon. These systems are capable of very high throughput.

8.4 OPERATIONS, ECONOMICS, SAFETY, AND MAINTENANCE
CONSIDERATIONS

Operations and economics issues in photomask manufacture tend to be driven by the mask yield at
a given technology node. An industry survey conducted by International SEMATECH in 2003 pro-
duced the following summary observations: 4

• Logic designs account for 61 percent of the mask volume.
• While technologies for 130 nm node and below capture the attention of the technical community,
they only account for 3 percent of the mask volume. Nearly half of all masks are built to a 500 nm,
or larger design node.
• Binary masks account for 95 percent of the total volume.
• Six-inch photoblank usage exceeds that of 5 in by a factor of 3.4 to 1. All other glass sizes account
for 7 percent of the total volume.

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