Reticles are contaminated during its lifetime and can catch particles as large as several tens of microns. Such defects on
the backside of photomasks are usually considered as uncritical and thus do not receive much attention. Backside defects
are out of focus by the mask thickness during wafer exposure and cannot be directly imaged on wafer. However, the
shadow of the defects changes the local illumination of the mask patterns and may result in spatial variation of critical
dimension (CD) on wafer depending on numerical aperture (NA) and pupil shape.
There have been only a few investigations on printability of backside defects in the past, and no data are available for the
most advanced technology nodes. Reticles are regularly inspected for particles on the glass side in the wafer fab but
limits for acceptable defect size are based on estimations. Detection of non-acceptable particles causes exposed wafers
being either delayed or reworked with impact on throughput and cost performance. It is therefore important to gain better
understanding of critical sizes of backside defects and of appropriate detection capabilities.
We have designed and manufactured a test mask with repeating patterns of 20nm, 28nm and 40nm technology node
ranging from contact and line/space critical layers to non-critical implant layers. Programmed chrome defects of varying
size are placed on the backside of the mask opposite to the repeating front side patterns in order to measure the spatial
variation of transmission and wafer CD caused by the backside defects. The test mask was printed on a bare Silicon
wafer and the size of printed features was measured by spatial sampling. Wafer CD variation for different backside
defect sizes are demonstrated and compared for 28nm node first metal layer.
Although the opaque chrome defects on the backside do not behave like real particles they aim on deriving a print
threshold for backside particles based on actual wafer data. After such critical size of backside defects is obtained the
reticle was also utilized to investigate the detection ability of backside defects by defect inspection of the reticle.
For the high volume manufacturing at the 45nm node and beyond it is crucial to match the OPC behaviour
of all scanners used at a given process step. For this task the ASML LithoTuner PatternMatcher software was
used. LithoTuner PatternMatcher is a tool to improve the proximity differences between a reference
scanner and one or more so called 'to be matched' scanners. The optimization uses the concept of
sensitivities of CDs of critical features towards adjustable scanner parameters in combination with the delta
CD's of those critical features.
To perform the scanner matching it is very important to have accurate and repeatable CD data. Therefore
we investigated the use of scatterometry as a replacement for the traditional CDSEM measurement.
Scatterometry significantly enhances the measurement precision while simultaneously reduces the
measurement time. In a first step we determined the sensitivities of the structures by measuring the CD
response to small perturbations of the individual scanner parameter settings. CD through pitch and
repeating 2 dimensional line end structures were measured using the ASML YieldStar tool and a Hitachi
CDSEM. The scatterometry- and CDSEM based sensitivities of the scanner parameter settings are compared.
Finally a scanner matching based on both sets of sensitivities has been performed.
In this article we will show that both methods are suited to perform the scanner matching. We will also
present the differences between the two sets of sensitivities obtained with scatterometry and CDSEM. At
the end we will present the results of the tool matching and show the results of a cross check. In the cross
check sensitivities obtained with the use of scatterometry were used for the scanner matching next to SEM
metrology used for verification.
Differences in imaging behaviour between lithographic systems of the same wavelength result in variations of optical
proximity effects (OPE). A way to compensate these irregularities is through scanner tuning. In scanner tuning, scanner
specific adjustments are obtained through the determination of scanner knob sensitivities of relevant structures followed
by an optimization to adjust the structure CD values to be close to the desired values.
Traditionally, scanner tuning methods have relied heavily on wafer-based CD metrology to characterize both the initial
mismatch as well as the sensitivities of CDs to the scanner tuning knobs. These methods have proven very successful in
reducing the mismatch, but their deployment in manufacturing has been hampered by the metrology effort. In this paper,
we explore the possibility of using ASML's LithoTuner PatternMatcher FullChip (PMFC) computational lithography
tool to reduce the dependence on wafer CD metrology.
One tuning application using flexray illumination instead of traditional scanner knobs is presented in this work; in this
application individual critical features in wafer printing are improved without affecting other sites. The limited impact of
tuning on other structures is verified through full-chip LMC runs. Potential uses of this technology are for process
transfers from one fab to another where the OPC signature in the receiving fab is similar but not identical to the signature
of the originating fab.
The tuning application is investigated with respect to its applicability in a production environment, including further
metrology effort reduction by using scatterometry tools.
Pellicles are mounted on the masks used in ArF lithography for integrated circuit manufacturing to ensure defect-free printing. The pellicle, a thin transparent polymer film, protects the reticle from dust. But, as the light transmittance through the pellicle has an angular dependency, the pellicle also acts as an apodization filter. In the current work, we present both experimental and simulation results at 1.35 numerical aperture immersion ArF lithography showing the influence of two types of pellicles on proximity and intra-die critical dimension uniformity (CDU). To do so, we mounted and dismounted the different pellicle types on one and the same mask. The considered structures on wafer are compatible with the 32-nm logic node for poly and metal. For the standard ArF pellicle (thickness 830 nm), we experimentally observe a distinct effect of several nm due to the pellicle presence on both the proximity and the intra-die CDU. For the more advanced pellicle (thickness 280 nm), no signature of the pellicle on proximity or CDU could be found. By modeling the pellicle's optical properties as a Jones Pupil, we are able to simulate the pellicle effects with good accuracy. These results indicate that for the 32-nm node, it is recommended to take the pellicle properties into account in the optical proximity correction calculation when using a standard pellicle. In addition, simulations also indicate that a local dose correction can compensate to a large extent for the intra-die pellicle effect. When using the more advanced thin pellicle (280 nm), no such corrections are needed.
A host of complementary imaging techniques (Scanning Electron Microscopy), surface
analytical technique (Auger Electron Spectroscopy, AES), chemical analytical and
speciation techniques (Grazing Incidence Reflectance Fourier-Transform Infrared
Spectroscopy, GIR-FTIR; and Raman spectroscopy) have been assessed for their
sensitivity and effectiveness in analyzing contamination on three EUV reticles that were
contaminated to varying degrees. The first reticle was contaminated as a result of its
exposure experience on the SEMATECH EUV Micro Exposure Tool (MET) at Lawrence
Berkeley National Laboratories, where it was exposed to up to 80 hours of EUV radiation.
The second reticle was a full-field reticle, specifically designed to monitor molecular
contamination, and exposed to greater than 1600J/cm2 of EUV radiation on the ASML Alpha Demo Tool (ADT) in Albany Nanotech in New York. The third reticle was intentionally contaminated with hydrocarbons in the Microscope for Mask Imaging and
Contamination Studies (MIMICS) tool at the College of Nanoscale Sciences of State
University of New York at Albany. The EUV reflectivities of some of these reticles were
measured on the Advanced Light Source EUV Reflectomer at Lawrence Berkeley
National Laboratories and PTB Bessy in Berlin, respectively. Analysis and
characterization of thin film contaminants on the two EUV reticles exposed to varying
degrees of EUV radiation in both MET and ADT confirm that the two most common
contamination types are carbonization and surface oxidation, mostly on the exposed areas
of the reticle, and with the MET being significantly more susceptible to carbon
contamination than the ADT. While AES in both surface scanning and sputter mode is
sensitive and efficient in analyzing thin contaminant films (of a few nanometers), GIRFTIR
is sensitive to thick films (of order of a 100 nm or more on non-infra-red reflecting
substrates), Raman spectroscopy is not compatible with analyzing such contaminants because of laser-induced evaporation of the contaminant film. SEM and EUV reflectometry are effective in quantifying the impact of contamination on imaging performance and reflectivity, respectively.
Molecular contamination risk to an EUV reticle exposed to up to 1600J/cm2 of 13.5 nm
EUV radiation in ASML Alpha Demo Tool (ADT) is negligible. Carbon film (< 0.5 nm)
deposition and oxidation (surface oxides ~1 nm) are the two main molecular
contaminants observed on this EUV reticle. These results run counter to recent empirical
results obtained from EUV micro-exposure tools (MET) which suggest that molecular
contamination of EUV reticles, even at the very low partial pressures expected in the
exposure chamber of EUV exposure tools, poses challenges in the implementation of
EUV lithography in large-scale volume manufacturing of devices. To assess the
molecular contamination risk to the use and lifetime of a given EUV reticle, we
monitored the contamination buildup on a specially designed reticle during one year as it
was exposed on ASML ADT located in Albany, New York. The reticle was analyzed
with a suite of complementary surface analytical technique (such as Auger Electron
Spectroscopy, AES) and chemical analytical techniques (such as Grazing Incidence
Reflection Fourier Transform Infra-red Spectroscopy, GIR-FTIR), as well as imaging
technique (such as Scanning Electron Microscopy). The influence of molecular
contamination on the reflectivity of this reticle was measured at the Lawrence Berkeley
Advanced Light Source EUV reflectometry. The differences in the contamination
outcome of EUV reticles exposed in ASML ADT and MET may be related to the
implementation of active contamination mitigation schemes in ADT and the lack of similar schemes in METs.
This paper assesses the readiness of EUV masks for pilot line production. The printability
of well characterized reticle defects, with particular emphasis on those reticle defects that
cause electrical errors on wafer test chips, is investigated. The reticles are equipped with
test marks that are inspected in a die-to-die mode (using DUV inspection tool) and
reviewed (using a SEM tool), and which also comprise electrically testable patterns. The
reticles have three modules comprising features with 32 nm ground rules in 104 nm pitch,
22 nm ground rules with 80 nm pitch, and 16 nm ground rules with 56 nm pitch (on the
wafer scale). In order to determine whether specific defects originate from the substrate,
the multilayer film, the absorber stack, or from the patterning process, the reticles were
inspected after each fabrication step. Following fabrication, the reticles were used to print
wafers on a 0.25 NA full-field ASML EUV exposure tool. The printed wafers were
inspected with state of the art bright-field and Deep UV inspection tools. It is observed
that the printability of EUV mask defects down to a pitch of 56 nm shows a trend of
increased printability as the pitch of the printed pattern gets smaller - a well established
trend at larger pitches of 80 nm and 104 nm, respectively. The sensitivity of state-of-the-art
reticle inspection tools is greatly improved over that of the previous generation of
tools. There appears to be no apparent decline in the sensitivity of these state-of-the-art
reticle inspection tools for higher density (smaller) patterns on the mask, even down to
56nm pitch (1x). Preliminary results indicate that a blank defect density of the order of
0.25 defects/cm2 can support very early learning on EUV pilot line production at the 16nm node.
Pellicles are mounted on the masks used in ArF lithography for IC manufacturing to ensure defect-free printing. The
pellicle, a thin transparent polymer film, protects the reticle from dust. But, as the light transmittance through the pellicle
has an angular dependency, the pellicle also acts as an apodization filter.
In the current work, we present both experimental and simulation results at 1.35 NA immersion ArF lithography showing
the influence of two types of pellicles on proximity and intra-die Critical Dimension Uniformity (CDU). To do so, we
mounted and dismounted the different pellicle types on one and the same mask. The considered structures on wafer are
compatible with the 32 nm logic node for poly and metal. For the standard ArF pellicle (thickness 830 nm), we
experimentally observe a distinct effect of several nm's due to the pellicle presence on both the proximity and the intradie
CDU. For the more advanced pellicle (thickness 280 nm) no signature of the pellicle on proximity or CDU could be
By modeling the pellicle's optical properties as a Jones Pupil, we are able to simulate the pellicle effects with good
accuracy. These results indicate that for the 32 nm node, it is recommended to take the pellicle properties into account in
the OPC calculation when using a standard pellicle. In addition, simulations also indicate that a local dose correction can
compensate to a large extent for the intra-die pellicle effect. When using the more advanced thin pellicle (280 nm), no
such corrections are needed.
Double patterning (DPT) lithography is seen industry-wide as an intermediate solution for the 32-nm node if high index immersion as well as extreme ultraviolet lithography are not ready for a timely release for production. Apart from the obvious drawbacks of the additional exposure, the processing steps, and the resulting reduced throughput, DPT possesses a number of additional technical challenges. This relates to, e.g., exposure tool capability, the actual applied process in the wafer fab, but also to mask performance and metrology. In this work we address the mask performance. To characterize the mask performance in an actual DPT process, conventional mask parameters need to be re-evaluated. Furthermore, new parameters might be more suitable to describe mask capability. This refers to, e.g., reticle to reticle overlay, but also to CD differences between masks of a DPT reticle set. For the 32-nm node, a DPT target of reticle to reticle induced overlay of 6 nm, 3 at mask level, was recently proposed. We report on the performance of a two-reticle set based on a design developed to study the impact of global and local mask placement errors on double patterning using a dual-line process. In a first step we focus on reticle to reticle overlay based on conventional mask metrology. The overlay between two masks evaluated for standard wafer overlay targets is compared with measurements on actual resolution structures, contributions of displacements on different spatial scales are discussed, and mask to mask CD variations are addressed. In a second step, we compare reticle data with experimental intrafield overlay data obtained from wafers on an ASML XT:1700i using the IMEC dual-line double patterning process. Reticle to reticle overlay contribution is studied on the wafers for both standard overlay targets and dedicated DPT features. The results of this study show...
Double Patterning Technology (DPT) is considered the most acceptable solution for 32nm node lithography. Apart from
the obvious drawbacks of additional exposure and processing steps and therefore reduced throughput, DPT possesses a
number of additional technical challenges. This relates to exposure tool capability, the actual applied process in the
wafer fab but also to mask performance. This paper will focus on the latter.
We will report on the performance of a two-reticle set based on a design developed to study the impact of mask global
and local placement errors on a DPT dual line process. For 32 nm node lithography using DPT a reticle to reticle
overlay contribution target of ≤ 1.5nm has been proposed. Reticle based measurements have shown that this
proposed target can be met for standard overlay features and dedicated DPT features. In this paper we will present
experimental intra field overlay wafer data resulting from the earlier mentioned reticle set.
The reticles contain a 13x19 array of modules comprising various standard overlay features such as ASML overlay
gratings and bar-in-bar overlay targets. Furthermore the modules contain split 40nm half pitch DPT features. The
reticles have been exposed on an ASML XT:1700i on several wafers in multiple fields. Reticle to reticle overlay
contribution has been studied in resist (double exposure) and using the IMEC dual line process (DPT). We will show
that the reticle to reticle overlay contribution on the wafer is smaller than 1.5nm (1x). We will compare the wafer data
with the reticle data, study the correlation and show that reticle to reticle overlay contribution based single mask
registration measurements can be used to qualify the reticle to reticle overlay contribution on wafer.
Double patterning (DPT) lithography is seen industry-wide as an intermediate solution for the 32nm node if high index
immersion as well as extreme ultraviolet lithography are not ready for a timely release for production. Apart from the
obvious drawbacks of additional exposure, processing steps and the resulting reduced throughput, DPT possesses a
number of additional technical challenges. This relates to, e.g., exposure tool capability, the actual applied process in the
wafer fab but also to mask performance and metrology. In this paper we will address the mask performance.
To characterize the mask performance in an actual DPT process, conventional parameters need to be re-evaluated.
Furthermore new parameters might be more suitable to describe mask capability. This refers to, e.g., reticle to reticle
overlay but also to CD differences between masks of a DPT reticle set. A DPT target of reticle to reticle induced overlay
of 6nm, 3σ at mask level was proposed recently for the 32nm node. The results show that this target can be met.
Besides that, local CD variations and local displacement become critical. Finally, the actual mask metrology for
determination of these parameters might not be trivial and needs to be set up and characterized properly. In this paper
we report on the performance of two-reticle sets based on a design developed to study the impact of mask global and
local placement errors on a DPT dual line process.
In a first step we focus on reticle to reticle overlay. The overlay between two masks evaluated for different wafer
overlay targets is compared with measurements on actual resolution structures. In a second step, mask to mask CD
variations are addressed. Off-target CD differences as well as variations of CD signatures on both reticles of a set are
investigated. Finally, local CD variations and local displacements are examined. To this aim, local variations of adjacent
structures on the reticle are characterized. The contribution of local effects to the overall CD and registration budget is
Chromeless Phase Lithography (CPL) is discussed as interesting option for the 65nm node and beyond offering high resolution and small Mask Error Enhancement Factor. However, it was shown recently that at high NA CPL masks can exhibit large polarization and also phase effects. A well known phase effect occurring for CPL semi dense lines are through focus Bossung tilts.
However, another manifestation of phase effects for dense lines and spaces is a reduced contrast for a symmetrical off-axis illumination due to phase errors between 0th and 1st diffraction order. In this paper it is shown that these phase effects can lead to a significant contrast loss for dense features smaller than 60nm half pitch. While also present for trench structures, the contrast reduction is more pronounced for mesa style structures. It is shown that for mesa structures an adjustment of etch depth can not recover an effective pi-phase shift. Furthermore, significant polarization effects are observed. As an example, the optimum mesa structure for TE polarization is shifted to small lines.
For an experimental validation, a CPL mask containing dense lines and spaces was fabricated. Their imaging performance was characterized with an AIMS 45i offering NA's greater than 1 and linearly polarized illumination as well as by wafer printing. Gratings with pitches down to 100 nm with varying duty cycles were measured with TE, TM and unpolarized dipole illumination. Very good agreement between measurement and simulation results confirmed the validity of theoretical predictions.
In 193nm optical lithography, immersion technology will enable numerical apertures much greater than 1.0.
Furthermore, polarized light is likely to be applied, enhancing the imaging properties of structures with dimensions near
the resolution limit. As a result, the consequences of extreme oblique angle illumination as well as polarization effects
need to be carefully evaluated for all elements of the lithographic process. This paper explores the aberrations and
apodization induced by the pellicle film in hyper NA lithography.
In a first step, the angle and polarization-dependent phase errors of a perfectly flat pellicle are investigated and
discussed for varying thicknesses. It will be shown that for NAs greater than 1.0 the pellicle induces higher order
spherical aberrations which can be in the range of today's scanner lens specifications. Also, the impact of polarizationdependent
apodization will be discussed.
In a second step, the analysis is extended to the case of a non-flat pellicle due to a given frame bow. Under these
conditions, the phase and transmission error is not radially symmetric and, furthermore, is field dependent. It will be
discussed under which conditions this effect can lead to a significant pellicle-induced CD signature over the entire
Defect-free masks are one of the top issues for enabling EUV lithography at the 32-nm node. Since a defect-free process cannot be expected, an understanding of the defect printability is required in order to derive critical defect sizes for the mask inspection and repair. Simulations of the aerial image are compared to the experimental printing in resist on the wafer. Strong differences between the simulations and the actual printing are observed. In particular the minimum printable defect size is much larger than expected which is explained in terms of resist resolution. The defect printability in the current configuration is limited by the resist process rather than the projection optics.
The use of Alternating Phase Shifting Masks (APSM) for sub 50nm half pitch pattern using 193nm lithography was evaluated. Results show that polarized illumination may be necessary for APSM to compete with Half-Tone Phase-Shifting Masks (HTPSM) when printing sub 50nm features. The low sigma illumination conditions required for APSM constraints the choice of a possible polarized illuminator to the TE polarized option therefore limiting the patterns to be oriented in one direction.
Topography effects imply the use of polarization-dependant balancing of APSM which should not be a show-stopper as long as it is properly handled at the time the mask is manufactured. Due to topography effects, the MEEF is increased if compared to thin mask approximation but the relative numbers remain manageable.
The sensitivity of CD errors with respect to polarization errors of the source is comparable to HTPSM masks. The induced displacement due to polarization errors is small compared to the CD variation of the dark line.
Alternating Phase-Shifting masks (altPSM) are known to provide high contrast imaging combined with a low Mask Error Enhancement Factor (MEEF) at low k1. At feature sizes close to 60nm half-pitch and less the impact of mask topography effects increases. This applies in particular for altPSM. This is due to the quartz etch which is required for every second mask aperture to obtain the 180 degrees phase shift. It enlarges the mask profile height significantly. The influence of the quartz trench profile on the transmission and phase balancing performance has already been studied extensively. Basically it has been shown, that tighter quartz trench profile control, specifically for etch depth and width, is required with decreasing mask feature half pitch. The desired mask pattern geometry optimization is currently based on an evaluation of the printed resist pattern over defocus. However, a mask process engineer can use instead only AIMS measurements of the mask features. Therefore there is a mature interest to check, how good such measurements can replace resist pattern measurements. In the paper therefore it is evaluated how accurate AIMS measurements can describe the real printing performance of an alternating PSM in resist. Impact of differences of the image formation is investigated by use of analytical expressions. Furthermore, the influence of tool imperfections and the presence of resist are discussed. The theoretical results are compared to experimental data taken from AIMS measurements and wafer prints.
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.
Defect disposition and qualification with stepper simulating AIMSTM tools on advanced masks of the 90 nm node and below is key to match the customer's expectations for "defect free" masks, i.e. masks containing only nonprinting design variations. For defect dispositioning usually printability studies are carried out using the same illumination settings at the AIMSTM tool as later on at the steppers in the wafer fab. These studies then establish
an AIMSTM criterion (e.g., CD variation or transmission deviation) a structure deviation must not exceed. For ever more advanced technologies the accessible process window gets smaller and thus more and more complex apertures have to be used to allow for a still suitable contrast and reliable printing of the patterns. This results in more time-consuming printability studies and tighter AIMSTM specs. Simulations of the printing of mask defects could potentially help to decrease the amount of time for printability studies and also the time for defect disposition in the production. However, usually simulations in their first approximation do not account for effects such as flare, aberrations or illumination inhomogeneities of the AIMSTM tool. This makes it difficult to derive the AIMSTM criterion by simulations. In this paper we show that a homogeneous aperture illumination is crucial for the image contrast and the defect disposition. We present a method to characterize the pupil illumination and investigate the impact of illumination inhomogeneities on various structures and their orientation employing two different aperture types. The experimental results are compared to simulations with both homogeneous illumination and the real illumination distribution. It turns out that for correct simulation predictions on experimental results it is important to provide the correct illumination distribution to the simulations.
Contaminants and residues on the mask surface are still a concern to the Microlithography industry as they influence the reticle printing properties. It is conceivable that this effect will worsen as the industry moves toward smaller nodes for the next generation lithography, i.e. 193nm immersion and/or EUV.
The AUV5500 (advanced UV-cleaning and inspection) tool provides the possibility to investigate the effect of mask contaminants from transmission and reflection measurements in the spectral range 145nm to 270nm, and to clean the mask surface as well. In this paper, we are investigating the change of optical properties with organic contaminants on mask features and the ability to clean the surface to its original optical properties. At first we discuss the behavior of the 193nm illumination of the features on the mask properties. Then, with the help of a controlled contamination method to pollute the surface, we investigate the influence of the contaminant on the features on the photomask optical properties. The impact of the contaminant on AIMS data will be discussed as well.
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.
For leading mask technologies the mask inspection for finding critical defects is always a difficult task. With the introduction of chrome-less, high-transmission and alternating mask types, new absorber material and the possibility of quartz defects the defect inspection and -classification becomes even more challenging. To decide whether a defect is critical or a repair is successful, the Zeiss AIMS tool is used to classify defects. For conventional imaging the optical settings are usually chosen such that resolution is maximized, for example a dipole illumination is used for imaging a dense line-space array at an optimum contrast. In this paper we will do the opposite and reduce the optical resolution, such that we can filter out the array pattern and study the resulting defect image. This technique allows using a simple threshold detector to find and classify defects.
The goal of the present study was to investigate and quantify reticle stress birefringence in exposure conditions. Birefringence can arise in fused silica photomask substrates due to their state of stress, and cause optical effects such as phase front distortion, ray bifurcation, and polarization changes. These effects potentially produce image blurring and illumination non-uniformity, leading to lower resolution and CD variations, respectively. The main sources of substrate stress studied were the absorber stack, the mounting of a pellicle, and the impact of initial reticle bow when chucking in an exposure tool. Jones calculus was used to relate birefringence at discrete locations in the reticle, derived from the state of stress, to the net birefringence experienced by light passing through the mask. Experimentally-obtained birefringence data as well as analytical calculations of stress birefringence caused by known states of stress were used to validate the models. These results can then be compared to photomask birefringence specifications or employed in optical simulations to determine the precise impact of this substrate stress birefringence.
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 0th and 1st 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).
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.