Advanced photolithography tools use 193 nanometer wavelength light for conventional and immersion printing. The
increased energy of 193 nm (ArF) light coupled with the higher absorption cross section of most materials has lead to a
dramatic increase in the rate of haze formation as compared to previously used lithographic wavelengths (248 KrF and
365 nm i-line systems). It is well known that at this short wavelength photochemical reactions are enhanced leading to
progressive defect formation, or haze, on optical surfaces within microlithography tools. Therefore, strict contamination
control of the optics environment is needed to avoid cumulative effects. Such measures have been implemented in
lithography tools both for the optics and for the reticle during exposure. However, the patterned side of the photomask is
the most sensitive element in the litho optical path for haze growth, because it is in focus and small defects will show up
as printing defects. Moreover, the reticle life time depends both on rigorous contamination control for expose and
transport/storage conditions (both inside and outside of the lithography tool). The litho operating cost depends directly
on reticle life time. It is imperative that the industry takes the required measures to improve the airborne molecular
contamination levels both in the storage part of the photolithography tool and in devices used to transport reticles outside
of the tool to slow down reticle haze
Past studies have shown the large effects of humidity and AMC on haze growth during storage and exposure. Therefore,
significant improvements in storage and exposure environment have been implemented by many fabs to reduce the
frequency of haze failures. It has also been shown that outgassing from materials surrounding the mask can influence or
cause haze. It is clear that the reticle must be adequately protected from contamination sources throughout the life cycle
of the reticle (both inside and outside of the lithography tool). In this paper we examine improvements in the storage
conditions of reticles inside the lithography tool as well as improvements in commercial SMIF pods used in fab storage
and automated handling of reticles.
While significant progress has been made in reducing the occurrence rate of progressive
defect growth on photomasks used at 193nm, the issue continues to be a problem for
many semiconductor fabs. Increasing evidence from multiple sources indicates that
further reduction in haze risk involves closely controlling the storage and exposure
environment of the photomask. Further controlled testing is necessary to characterize the
impact of environment and individual components on growth. In this way, photomask
users, equipment and material providers may be better prepared to ensure the proper
storage and use of photomasks in order to reduce the risk of haze growth.
In continuation of work previously reported by Toppan Photomasks, advanced test
apparatus, recently designed and built, now enables researchers to generate and maintain
stable and controlled levels of multiple impurities which potentially effect haze growth.
Supported by on-line and off-line analytical methods and instrumentation, new
experimental set-up enables accuracy in the testing and validation of the impacts of
Different classes of pollutants in multiple combinations have been studied to more
precisely characterize environmental sensitivity of varying types of 193 nm reticles.
Authors report further on the study of the effect of environmental conditions on severity
and rate of haze formation to provide insight into the requirements for reducing or even
preventing such conditions.
An accurate optical model is the foundation of an accurate optical proximity correction (OPC) model, which has always been the key for successful implementation of model-based OPC. As critical dimension (CD) control requirements become severe at the 45- and 32-nm device generations, OPC model accuracy and hence optical model accuracy requirements become more stringent. In previous generations, certain optical effects could be safely ignored. For example, the transmission attenuation particularly at high spatial frequencies caused by lens apodization effects and organic pellicle films was ignored or not accurately modeled in conventional OPC simulators. These effects are now playing a more important role in OPC modeling as technology scales down. Our simulations indicate these effects can cause CD modeling errors of 5 nm or larger, at the 45-nm technology node and beyond. Therefore, they must be accurately modeled in OPC modeling. In our OPC modeling methodology, we propose two novel low-pass-filter models to capture the frequency-dependent transmission attenuation due to lens apodization and to pellicle films. These parameterized novel low-pass-filter models ensure that lens apodization and pellicle-film-induced transmission attenuation can be appropriately account for through regression during the experimental OPC model calibration stage in the case where no measured transmission data are available, thus enabling physics-centric OPC model building with considerably higher accuracy. We can then avoid overfitting the OPC model, which could cause instability in the OPC correction stage. The validity and efficiency of the proposed novel models are also verified using an industry-standard lithography simulator as well as an experimental OPC model calibration at the 45-nm technology node.
With the use of 193nm lithography, haze growth has increasingly become a critical issue for
photomask suppliers and wafer fabs. Recent photomask industry surveys indicate the occurrence
rate of haze is 10 times higher on 193nm masks compared to 248nm masks. Additionally, work has
been presented that shows strong relationship between environmental conditions around the
photomask and the occurrence of haze at 193nm. This underscores the need to better understand
the basic mechanisms of haze and the measures such as environmental airborne molecular
contamination (AMC) control which can be employed to reduce the occurrence of haze in use.
A custom excimer laser test system capable of 193nm and 248nm wavelengths was built to
accelerate haze growth and to better understand haze formation mechanisms. Work on materials
impact on haze growth, such as pellicles and reticle compacts, as well as preliminary findings on
environmental impacts have been presented previously. Results indicate even on pristine
surfaces haze can grow when contaminants are present in the storage and use environment. The test
system has been upgraded to include tight control on the concentration of specific airborne
contaminants of concern. The impact of these contaminants and their relative concentrations will be
examined in this paper and are presented to aid the industry in determining the level of
environmental control needed over the life of a reticle.
The semiconductor industry will soon be putting >=1.07NA 193nm immersion lithography systems into production for
the 45nm device node and in about three years will be putting >=1.30NA systems into production for the 32nm device
node. For these very high NA systems, the maximum angle of light incident on a 4X reticle will reach ~16 degrees and
~20 degrees for the 45nm and 32nm nodes respectively. These angles can no longer be accurately approximated by an
assumption of normal incidence. The optical diffraction and thin film effects of high incident angles on the wafer and
on the photomask have been studied by many different authors. Extensive previous work has also investigated the
impact of high angles upon hard (e.g., F-doped silica) thick (>700μm) pellicles for 157nm lithography, e.g.,.
However, the interaction of these high incident angles with traditional thin (< 1μm) organic pellicles has not been
widely discussed in the literature.
In this paper we analyze the impact of traditional thin organic pellicles in the imaging plane for hyper-NA
immersion lithography at the 45nm and 32nm nodes. The use of existing pellicles with hyper-NA imaging is shown to
have a definite negative impact upon lithographic CD control and optical proximity correction (OPC) model accuracy.
This is due to the traditional method of setting organic pellicle thickness to optimize normally incident light
transmission intensity. Due to thin film interference effects with hyper-NA angles, this traditional pellicle optimization
method will induce a loss of high spatial frequency (i.e., high transmitted angle) intensity which is similar in negative
impact to a strong lens apodization effect. Therefore, using simulation we investigate different pellicle manufacturing
options (e.g., multi-layer pellicle films) and OPC modeling options to reduce the high spatial frequency loss and its
With the use of 193nm lithography, haze growth has increased and become a critical issue for photomask suppliers and wafer fabs. Currently, the industry uses various test methods to measure known contributions to crystal growth, such as ion chromatography of cleaning residues and environmental monitoring in steppers. The understanding of the conditions that create haze is limited to end user photomask lifetime experience, which is gathered under varying environmental conditions. A better method to understand the formation of haze is to create a controlled environment and vary experimental conditions. Once experimental factors are understood, product reliability can be verified through end-user feedback. A custom excimer laser test system capable of 193nm and 248nm wavelengths was built to accelerate haze growth and to better understand haze formation. A photomask is enclosed in a test chamber where the environmental atmosphere and exposure conditions are controlled and monitored throughout testing. The system is used to test various elements important to mask fabrication and use, including materials, mask fabrication processes, and environmental operating conditions. This paper details the investigation of haze performance with commercially available pellicles using controlled environmental conditions and varying exposure parameters, such as pulse rate, energy density, and exposure dose. Using this methodology, the conditions that create haze growth were identified.
The introduction of 157 nm as the next optical lithography wavelength has created a need for new soft (polymeric)
or hard (quartz) pellicle materials. Pellicles should be > 98% transparent to incident 157 nm light and, ideally, sufficiently
resistant to photochemical damage to remain useful for an exposure lifetime of 7.5 kJ/cm2.
The transparency specification has been met. We have developed families of experimental Teflon™AF (TAFx)
polymers with > 98% transparency which can be spin coated and lifted as micron-scale, unsupported membranes. Still higher
transparencies should be possible once optimization of intrinsic (composition, end groups, impurities, molecular weight) and
extrinsic (oxygen, absorbed hydrocarbons, contaminants) factors are completed. The measured transparencies of actual
pellicle films, however, are affected by many factors other than absorption. Film thickness must be precisely controlled so as
to allow operation at the fringe maxima for the lithographic wavelength. Roughness and thickness uniformity are also
critical. An important part of our program has thus been learning how to spin membranes from the solvents that dissolve our
Meeting the durability specification at 157 nm remains a major concern. The 157 nm radiation durability lifetime of
a polymer is determined by two fundamental properties: the fraction of 157 nm radiation absorbed and the fraction (quantum
efficiency) of this absorbed radiation that results in photochemical darkening. Originally it was assumed that lifetime
increases uniformly with increasing transparency. We now have cases where materials with very different absorbances
(TAFx4P and 46P) have similar lifetimes and materials with similar absorptions (TAFx46P and 2P) have very different
lifetimes. These findings demonstrate the importance of the relative quantum efficiencies as the 157 nm light energy
distributes itself along degradative versus non-degradative pathways. In an effort to identify chemical and structural features
that control lifetime, we have been studying model molecular materials, some quite similar to the monomer units used to
make our pellicle candidates. Several of these models have shown transparencies much higher and lifetimes far longer than
our best pellicle candidates to date.
The introduction of 157 nm as the next optical lithography wavelength has created a need for new soft (polymeric) or hard (quartz) pellicle materials optimized for this wavelength. Materials design and development of ultra transparent fluoropolymers suitable for 157 nm soft pellicle applications has produced a number of promising candidate materials with absorbances below 0.03/micrometer as is necessary to achieve pellicle transmissions above 95%. We have developed 12 families of experimental TeflonAF<SUP>R</SUP> (TAFx) materials which have sufficient transparency to produce transmissions above 95%. For the successful fabrication of 157 nm pellicles from these materials, the fluoropolymers must have appropriate physical properties to permit the spin coating of thin polymer films and their lifting and adhesive mounting to pellicle frames, the processes which produce free standing pellicle membranes of micron scale thickness. Relevant physical properties include molecular weight, glass transition temperature, and mechanical strength and toughness. We have successfully developed various of the ultra transparent TAFx polymer families with these physical properties. Upon irradiation these 157 nm pellicle polymers undergo photochemical darkening, which reduces the 157 nm transmission of the material. Measurements of the photochemical darkening rate allow the estimation of the pellicle lifetime corresponding to a 10% drop in 157 nm transmission. Increasing the 157 nm lifetime of fluoropolymers involves simultaneous optimization of the materials, the pellicle and the end use. Similar optimization was essential to achieve the desired radiation durability lifetimes for pellicles successfully developed for use with KrF (248 nm) and ArF (193 nm) lithography.
The semiconductor industry continues to push the resolution capability of lithographic processes in order to produce increasingly smaller device geometry at higher densities. To achieve these advances corresponding changes are occurring in the lithography equipment used to manufacture these devices. The wavelengths used for exposure are decreasing, numerical apertures are increasing and new off axis illumination systems are being introduced. These all have ramifications on the performance, effect and proper use of pellicles in the lithography system. At the same time the available process budgets are decreasing thereby increasing the relative effect of the pellicle contribution towards those budgets. Many of the traditional pellicle designs are no longer the optimum choice for use in high performance lithography. This study examines the effects of pellicles in high performance lithography systems.