A photomask is an opaque plate with holes or transparencies that allow light
to shine through in a defined pattern. They are commonly used in
photolithography. Overview
Lithographic photomasks are typically transparent fused silica blanks covered
with a pattern defined with a chrome metal-absorbing film. Photomasks are
used at wavelengths of 365 nm, 248 nm, and 193 nm. Photomasks have also been
developed for other forms of radiation such as 157 nm, 13.5 nm, X-ray,
electrons, and ions; but these require entirely new materials for the substrate
and the pattern film. A set of photomasks, each defining a
pattern layer in integrated circuit fabrication, is fed into a
photolithography stepper or scanner, and individually selected for exposure. In
double patterning techniques, a photomask would correspond to a subset
of the layer pattern. In photolithography for the mass
production of integrated circuit devices, the more correct term is
usually photoreticle or simply reticle. In the case of a photomask, there is a
one-to-one correspondence between the mask pattern and the wafer pattern. This
was the standard for the 1:1 mask aligners that were succeeded by steppers
and scanners with reduction optics. As used in steppers and scanners, the
reticle commonly contains only one layer of the chip.. The pattern is projected
and shrunk by four or five times onto the wafer surface. To achieve complete
wafer coverage, the wafer is repeatedly "stepped" from position to position
under the optical column until full exposure is achieved.
Features 150 nm or below in size generally require phase-shifting to
enhance the image quality to acceptable values. This can be achieved in many
ways. The two most common methods are to use an attenuated phase-shifting
background film on the mask to increase the contrast of small intensity peaks,
or to etch the exposed quartz so that the edge between the etched and unetched
areas can be used to image nearly zero intensity. In the second case, unwanted
edges would need to be trimmed out with another exposure. The former method is
attenuated phase-shifting, and is often considered a weak enhancement, requiring
special illumination for the most enhancement, while the latter method is
known as alternating-aperture phase-shifting, and is the most popular
strong enhancement technique. As leading-edge semiconductor features
shrink, photomask features that are 4× larger must inevitably shrink as well.
This could pose challenges since the absorber film will need to become
thinner, and hence less opaque. A recent study by IMEC has found that thinner
absorbers degrade image contrast and therefore contribute to line-edge
roughness, using state-of-the-art photolithography tools. One possibility
is to eliminate absorbers altogether and use "chromeless" masks, relying solely
on phase-shifting for imaging. The emergence of immersion lithography
has a strong impact on photomask requirements. The commonly used
attenuated phase-shifting mask is more sensitive to the higher incidence angles
applied in "hyper-NA" lithography, due to the longer optical path through the
patterned film. Mask Error Enhancement Factor
Leading-edge photomasks images of the final chip patterns magnified by 4
times. This magnification factor has been a key benefit in reducing pattern
sensitivity to imaging errors. However, as features continue to shrink, two
trends come into play: the first is that the mask error factor begins to exceed
one, i.e., the dimension error on the wafer may be more than 1/4 the dimension
error on the mask, and the second is that the mask feature is becoming
smaller, and the dimension tolerance is approaching a few nanometers. For
example, a 25 nm wafer pattern should correspond to a 100 nm mask pattern, but
the wafer tolerance could be 1.25 nm, which translates into 5 nm on the
photomask. The variation of electron beam scattering in directly writing the
photomask pattern can easily well exceed this.
Pellicles The term "pellicle" is used to mean
"film," "thin film," or "membrane." Beginning in the 1960s, thin film
stretched on a metal frame, also known as a "pellicle," was used as a beam
splitter for optical instruments. It has been used in a number of instruments to
split a beam of light without causing an optical path shift due to its small film
thickness. In 1978, Shea et al. at IBM patented a process to use the "pellicle"
as a dust cover to protect a photomask or reticle(hence will all be called
"photomask" in the rest of this chapter) In the context of this entry, "pellicle"
means "thin film dust cover to protect a photomask".
Particle contamination can be a significant problem in semiconductor
manufacturing. A photomask is protected from particles by a pellicle – a thin
transparent film stretched over a frame that is glued over one side of the
photomask. The pellicle is far enough away from the mask patterns so that
moderate-to-small sized particles that land on the pellicle will be too far out
of focus to print. Although they are designed to keep particles away,
pellicles become a part of the imaging system and their optical properties need
to be taken into account. Pellicles material are Nitrocellulose and made for
various Transmission Wavelengths. Leading commercial photomask
manufacturers The SPIE Annual Conference, Photomask
Technology reports the SEMATECH Mask Industry Assessment which includes
current industry analysis and the results of their annual photomask
manufacturers survey. The following companies are listed in order of their
global market share: Advance Reproductions Corporation
Infinite Graphics Incorporated Dai Nippon Printing
Toppan Photomasks Photronics Inc
Hoya Corporation Taiwan Mask Corporation
Nippon Filcon Major chipmakers such as Intel,
Globalfoundries, IBM, NEC, TSMC, UMC, Samsung, and Micron Technology, have
their own large maskmaking facilities or joint ventures with the abovementioned
companies. Worldwide photomask market was estimated
as $3.2 billion in 2012 and $3.1 billion in 2013. Almost half of market was from
captive mask shops. The costs of creating new mask shop for
180 nm processes were estimated in 2005 as $40 million, and for 130 nm - more
than $100 million. The cost to set up a modern 45 nm
process mask shop is $200–500 million, a very high threshold for entering this
market. The purchase price of a photomask can range from $1,000 to
$100,000 for a single high-end phase-shift mask. As many as 30 masks
may be required to form a complete mask set.
See also Mask work
Mask inspection SMIF interface
Nanochannel glass materials References
^ Rizvi, Syed. "1.3 The Technology History of Masks". Handbook of Photomask
Manufacturing Technology. CRC Press. p. 728. ISBN 9781420028782.
^ Lithography experts back higher magnification in photomasks to ease
challenges EETimes 2000 ^ Y. Sato et al., Proc. SPIE, vol. 4889,
pp. 50-58. ^ M. Yoshizawa et al., Proc. SPIE, vol.
5853, pp. 243-251 ^ C. A. Mack et al., Proc. SPIE, vol.
5992, pp. 306-316 ^ E. Hendrickx et al., Proc. SPIE 7140,
714007. ^ C-J. Chen et al., Proc. SPIE 5256,
673. ^ W-H. Cheng and J. Farnsworth, Proc.
SPIE 6607, 660724. ^ Chris A. Mack. "Optical behavior of
pellicles". Microlithography World. Retrieved 2008-09-13.
^ Hughes, Greg; Henry Yun. "Mask industry assessment: 2009". Proceedings
of SPIE 7488: 748803–748803–13. doi:10.1117/12.832722. ISSN 0277-786X.
^ Chamness, Lara. "Semiconductor Photomask Market: Forecast $3.5 Billion
in 2014". SEMI Industry Research and Statistics. Retrieved 6 September 2014.
^ Tracy, Dan; Deborah Geiger. "SEMI Reports 2013 Semiconductor Photomask
Sales of $3.1 Billion". SEMI. Retrieved 6 September 2014.
^ An Analysis of the Economics of Photomask Manufacturing Part – 1: The
Economic Environment, Weber, February 9, 2005. Slide 6 "The Mask Shop’s
Perspective" ^ Weber, C.M; Berglund, C.N.; Gabella,
P.. "Mask Cost and Profitability in Photomask Manufacturing: An Empirical
Analysis". Semiconductor Manufacturing, IEEE Transactions on 19.
doi:10.1109/TSM.2006.883577; page 23 table 1