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.
1 Overview 2 Mask Error Enhancement Factor (MEEF) 3 Pellicles 4 Leading commercial photomask manufacturers 5 See also 6 References
A simulated photomask. The thicker features are the integrated circuit that is desired to be printed on the wafer. The thinner features are assists that do not print themselves, but help the integrated circuit print better out-of-focus. The zig-zag appearance of the photomask is because optical proximity correction was applied to it to create a better print.
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 (EUV), 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
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. (However, some photolithography fabrications utilize reticles
with more than one layer patterned onto the same mask). 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
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
Mask Error Enhancement Factor (MEEF)
Leading-edge photomasks (pre-corrected) images of the final chip
patterns magnified by four 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 (5% spec), 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.
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
Pellicle Mounting Machine MLI
Leading commercial photomask manufacturers
Advance Reproductions Corporation
Dai Nippon Printing
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 (in-house mask shops of major chipmakers). 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 purchase price of a photomask, in 2006, could range from $1,000 to $100,000 for a single high-end phase-shift mask. As many as 30 masks (of varying price) may be required to form a complete mask set. See also
^ Rizvi, Syed (2005). "1.3 The Technology History of Masks". Handbook