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PHOTOMASK

PHOTOMASK 8.5

• The 5× magnification accounts for just under half the total mask volume, with 4× at a 38-percent
share and 1× at 12 percent.
• File sizes for design data averaged 1.5 GB with a maximum observed of 76 GB.
• Data preparation times averaged 6.5 h of CPU time with a maximum of 360 h reported.
• Mask yields for binary masks were in the low 90s percent for all respondents. Phase shift average
yields ranged between 40 percent and 90 percent, with attenuated phase shifting masks averaging
65 percent and alternating aperture phase shifting masks averaging 58 percent.
• The major process related yield loss mechanism is defects, accounting for about 60 percent of yield
loss. Administrative and manufacturing errors accounted for 18 percent. CD control accounted for
16 percent.
• Delivery times average 5 days for a simple binary mask to 7 days for a binary mask with aggres-
sive optical proximity correction (OPC) applied. Attenuated phase shift masks delivery times aver-
aged 11 days. Alternating aperture phase shift masks (PSMs) average 23 days.
• Mask returns represent about 0.2 percent of production volume. The largest return reason is data
preparation errors of 19.5 percent with soft defects, “other reasons,” and “administrative errors”
accounting for roughly 17 percent each.
• Mask maintenance service is dominated by damaged pellicle replacement at 63 percent and parti-
cles under the pellicle accounting for 17 percent of service volume.

8.5 FUTURE TRENDS AND CONCLUSIONS

Each new generation of lithographic technology requires advances in photomask equipment and
materials. Mask making capability and cost escalation continue to be critical to future progress in
lithography and will require continued focus. As a consequence of prior aggressive roadmap accel-
eration—particularly the microprocessor chips (MPU) gate linewidth (postetch), and increased mask
error factors (MEFs) associated with low k lithography—mask linewidth control appears to be a
1
particularly significant challenge for the future. 3
Mask equipment and process capabilities are in place for manufacturing masks with complex
OPC and PSM, while mask processes for post-193-nm technologies are in research and develop-
ment. Mask damage from electrostatic discharge (ESD) has long been a concern, and it is expected
to be even more problematic as mask features shrink. Furthermore, masks for 157-nm lithography
will be kept in ambient atmospheres nearly free of water, so the risk of ESD damage in masks will
increase. A cost-effective pellicle solution has not yet been fully developed for 157 nm masks, fur-
ther complicating mask handling for lithography at that wavelength. 3
Because of the particular challenges associated with imaging contact holes, the contact hole size
after etch will be smaller than the lithographically imaged hole, similar to the differences between
imaged and final MPU gates. This is important to comprehend in the roadmap (Fig. 8.2) because
contacts have very small process windows and large mask error factors, and minor changes in the
contact hole size have large implications for the mask CD control requirements. 3
In fact, the most critical aspect of the roadmap to photomask manufacturers is probably the CD
uniformity specifications. Figure 8.3 shows a plot of the roadmap requirements for three categories
of CD types. Over the next five years, the 3-sigma uniformity of all feature types mentioned in the
roadmap will be halved. Meeting this demand will be a major endeavor requiring coordination
between the mask house and its suppliers. Both the litho tool and process equipment vendors will
need to collaborate with their photomask customers to reach these goals.
The 2003 Mask Industry Assessment conducted by SEMATECH noted that almost 20 percent of
all photomask returns were for data preparation errors. The authors rightly noted that this indicates
a need for better and more sophisticated software tools for photomask vendors. The International

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