Contamination and especially Airbone Molecular Contamination (AMC) is a critical issue for mask
material flow with a severe and fairly unpredictable risk of induced contamination and damages
especially for 193 nm lithography. It is therefore essential to measure, to understand and then try to
reduce AMC in mask environment.
Results and assessment of mask pod environment in term of molecular contamination was presented
in a first step (1<sup>1</sup>). Then in second step further studies was carried out within European CRYSTAL
project in order to reduce mask pod influence and contamination due to material out gassing. Results
are shown here. These studies were carried out in the frame of the European R&D project, the so
called "CRYSTAL" project, focusing on photomask defect reduction.
Within the frame of the European R&D project the so called "HYMNE" project, lead by STM, advanced vacuum decontamination processes had been demonstrated to be efficient on wafer substrates in order to remove airborne molecular contamination (moisture, VOC..), to avoid crystalline defects after dry etching process and to improve yield for sub 90 nm technologies.
Further to these significant results on wafers, a pool of partners investigated new methods and processes based on vacuum technology for photomask decontamination. These studies were carried out in the frame of the European R&D CRYSTAL project, focusing on photomask defect reduction.
Today, vacuum process is not very widespread in photomask environment: in fab environment nor in mask manufacturing cycle. However such vacuum substrate decontamination could be also efficiently applied in order to reduce AMC contamination, which is one of the root causes of haze and crystalline defects. In this paper, we report for the first time, vacuum process investigations on pellicled photomasks that could be applied in fab environment, as well as vacuum process investigations on patterned blank that could be integrated into mask manufacturing cycle.
First, vacuum process had been investigated on pellicled photomasks, including parameter influences. Goal is to renew and replace the environment under the pellicle by clean environment. During the process, specific care has to be taken on pellicle behavior under vacuum. The challenge is indeed to manage the pellicle during the vacuum process without damaging it, especially after several decontamination cycles. Finally, repeatability tests have also been successfully carried out and will be reported.
We also report advanced vacuum process on patterned blank that could be integrated into mask manufacturing flow. Such procedure is an efficient complementary process in order to outgas contaminants from photomasks, and in order to reduce AMC residues (especially sulfate) in mask manufacturing cycle. Experimental results will be reported.
Traditionally, definition of mask specifications is done completely by the mask user, while characterization of the mask
relative to the specifications is done completely by the mask maker. As the challenges of low-k1 imaging continue to
grow in scope of designs and in absolute complexity, the inevitable partnership between wafer lithographers and mask
makers has strengthened as well. This is reflected in the jointly owned mask facilities and device manufacturers'
continued maintenance of fully captive mask shops which foster the closer mask-litho relationships. However, while
some device manufacturers have leveraged this to optimize mask specifications before the mask is built and, therefore,
improve mask yield and cost, the opportunity for post-fabrication partnering on mask characterization is more apparent
The Advanced Mask Technology Center (AMTC) has been investigating the concept of assessing how a mask images,
rather than the mask's physical attributes, as a technically superior and lower-cost method to characterize a mask. The
idea of printing a mask under its intended imaging conditions, then characterizing the imaged wafer as a surrogate for
traditional mask inspections and measurements represents the ultimate method to characterize a mask's performance,
which is most meaningful to the user. Surrogate wafer print (SWaP) is already done as part of leading-edge wafer fab
mask qualification to validate defect and dimensional performance.
In the past, the prospect of executing this concept has generally been summarily discarded as technically untenable and
logistically intractable. The AMTC published a paper at BACUS 2007 successfully demonstrating the performance of
SWaP for the characterization of defects as an alternative to traditional mask inspection . It showed that this concept is
not only feasible, but, in some cases, desirable.
This paper expands on last year's work at AMTC to assess the full implementation of SWaP as an enhancement to mask
characterization quality including defectivity, dimensional control, pattern fidelity, and in-plane distortion. We present a
thorough analysis of both the technical and logistical challenges coupled with an objective view of the advantages and
disadvantages from both the technical and financial perspectives. The analysis and model used by the AMTC will serve
to provoke other mask shops to prepare their own analyses then consider this new paradigm for mask characterization
Context/ study Motivation:
Contamination and especially Airbone Molecular Contamination (AMC) is a critical issue for mask material
flow with a severe and fairly unpredictable risk of induced contamination and damages especially for 193 nm
lithography. It is therefore essential to measure, to understand and then try to reduce AMC in mask environment.
Mask material flow was studied in a global approach by a pool of European partners, especially within the frame
of European MEDEA+ project, so called "MUSCLE". This paper deals with results and assessment of mask pod
environment in term of molecular contamination in a first step, then in a second step preliminary studies to
reduce mask pod influence and contamination due to material out gassing.
Approach and techniques:
A specific assessment of environmental / molecular contamination along the supply chain was performed by all
partners. After previous work presented at EMLC 07, further studies were performed on real time
contamination measurement pod at different sites locations (including Mask manufacturing site, blank
manufacturing sites, IC fab). Studies were linked to the main critical issues: cleaning, storage, handling,
materials and processes.
Contamination measurement campaigns were carried out along the mask supply chain using specific Adixen
analyzer in order to monitor in real time organic contaminants (ppb level) in mask pods. Key results would be
presented: VOC, AMC and humidity level on different kinds of mask carriers, impact of basic cleaning on pod
outgassing measurement (VOC, NH3), and process influence on pod contamination...
In a second step, preliminary specific pod conditioning studies for better pod environment were performed based
on Adixen vacuum process. Process influence had been experimentally measured in term of molecular
outgassing from mask pods. Different AMC experimental characterization methods had been carried out leading
to results on a wide range of organic and inorganic contaminants: by inline techniques based on Adixen
humidity, also VOC and organic sensors, together by off-line techniques already used in the extensive previous
mask pods benchmark (TD-GCMS & Ionic Chromatography). Humidity and VOC levels from mask carriers had
shown significant reduction after Adixen pod conditioning process. Focus had been made on optimized vacuum
step (for AMC) after particles carrier cleaning cycle. Based upon these key results new procedures, as well as
guidelines for mask carrier cleaning optimization are proposed to improve pod contamination control.
Summary results/next steps:
This paper reports molecular contamination measurement campaigns performed by a pool of European partners
along the mask supply chain. It allows us to investigate, identify and quantify critical molecular contamination in
mask pod, as well as VOC and humidity, issues depending on locations, uses, and carrier's type. Preliminary
studies highlight initial process solutions for pods conditioning that are being used for short term
industrialization and further industrialized.
In the frame of the European Medea+ 2T302 MUSCLE project, an extensive mask carriers benchmark was carried out in
order to evaluate whether some containers answer to the 65nm technology needs. Ten different containers, currently used
or expected in the future all along the mask supply chain (blank, maskhouse and fab carriers) were selected at different
steps of their life cycle (new, aged, aged & cleaned). The most critical parameters identified for analysis versus future
technologies were: automation, particle contamination, chemical contamination (organic outgassing, ionic
contamination), cleanability, ESD, airtightness and purgeability. Furthermore, experimental protocols corresponding to
suitable methods were then developed and implemented to test each criterion. The benchmark results are presented
giving a "state of the art" of mask carriers currently available and allowing a gap analysis for the tested parameters
related to future needs. This approach is detailed through the particular case of carrier contamination measurements.
Finally, this benchmark / gap analysis leads to propose advisable mask carrier specifications (and the test protocols
associated) on various key parameters which can also be taken as guidelines for a standardization perspective for the
65nm technology. This also indicates that none of tested carriers fulfills all the specifications proposed.
Mask defects are of increasing concern for future lithography generations. The improved resolution capabilities of immersion and EUV systems increase also the sensitivity of these systems with respect to small imperfections of the mask. Advanced mask technologies such as alternating phase shift masks (AltPSM), chromeless phase shift lithography (CPL), or "thick" absorbers on EUV masks introduce new defect types. The paper presents an application of rigorous electromagnetic field modeling for the study of typical defect printing mechanisms in ArF immersion lithography and in EUV lithography. For standard imaging and mask technologies, such as binary masks or attenuated phase shift masks, small defects usually print as linewidth or critical dimension (CD) errors with the largest effect at best focus. For AltPSM, CPL masks, and EUV masks this is not always the case. Several unusual printing scenarios were observed: placement errors due to defects can become more critical than CD-errors, defects may print more critical at defocus positions different from the center of the process window, the defect printing may become asymmetric through focus, and the risk of defect printing depends on the polarization of the used light source. Several simulation examples will demonstrate these effects. Rigorous EMF simulations in combination with vector imaging simulations are very useful to understand the origins of the observed defect printing mechanisms.
This paper intends to develop a measurement system to characterize photomasks for 193 nm lithography applications. Based on the VUV spectrophotometer at the Fraunhofer IOF institute, some modifications have been addressed to fulfil these special measurements. Characterizations on photomasks have been successfully carried out, which show good correlations to simulations.
In the process of discussion of possible mask-types for the 5x nm node (half-pitch) and below, the alternating phase-shifting mask (AltPSM) is a potential candidate to be screened. The current scenario suggests using 193 nm immersion lithography with NA values of up to 1.2 and above. New optical effects from oblique incident angles, mask-induced polarization of the transmitted light and birefringence from the substrate need to be taken into account when the optical performance of a mask is evaluated. This paper addresses mask induced polarization effects from dense lines-and-space structures on a real mask. Measurements of the polarization dependent diffraction efficiencies have been performed on AltPSM masks. Experimental results show good agreement with simulations. A comparison with Binary Masks is made.
As the lithographic projection technology of the future will require higher numerical aperture (NA) values, new physical effects will have to be taken into consideration. Immersion lithography will result in NA values of up to 1.2 and above. New optical effects like 3D shadowing, effects from oblique incident angles, mask-induced polarization of the transmitted light and birefringence from the substrate should be considered when the masks optical performance is evaluated. This paper addresses mask induced polarization effects from dense lines-and-space structures of standard production masks. On a binary and on an attenuated phase-shifting mask, which were manufactured at the Advanced Mask Technology Center (AMTC) transmission experimental investigations were performed. Measurements of diffraction efficiencies for TE- and TM-polarized light using three different incident angles are presented for all considered mask types and compared to simulations. The structures under investigation include line-space-pattern with varying pitches as well as varying duty cycles. Experimental results show good agreement with simulations.
As microlithography moves to smaller critical dimensions, structures on reticles reach feature sizes comparable to the operating wavelength. Furthermore, with increasing NA the angle of incidence of light illuminating the mask steadily increases. In particular for immersion lithography this will have severe consequences on the printing behavior of reticles. Polarization effects arise which have an impact on, among other things, the contrast of the printed image. Angular effects have to be considered when aggressive off-axis illumination schemes are used. Whereas numerous articles have been published on those effects and the underlying theory seems to be understood, there is a strong need for experimental verification of properties of real masks at the actinic wavelength. This paper presents measurements of polarization effects on different mask blank types produced at Schott Lithotec including chrome and alternative absorber binary mask blanks, as well as phase shift mask blanks. Thickness and optical dispersion of all layers were determined using grazing incidence x-ray reflectometry (GIXR) and variable angle spectroscopic ellipsometry (VASE). The set of mask blanks was patterned using a special design developed at the Advanced Mask Technology Center (AMTC) to allow measurements at different line width and pitch sizes. VUV Ellipsometry was then used to measure the properties of the structured materials, in particular the intensities in the 0<sup>th</sup> and 1<sup>st</sup> diffraction order for both polarization directions and varying angle of incidence. The degree of polarization of respective mask types is evaluated for dense lines with varying pitches and duty cycles. The results obtained experimentally are compared with simulations based on rigorous coupled wave analysis (RCWA).