Using AIMSTM to qualify repairs of defects on photomasks is the industry standard. AIMSTM provides a reasonable
matching of lithographic imaging performances without the need of wafer prints. The need of utilisation of this
capability by photomask manufacturers has risen due to the increased complexity of layouts incorporating aggressive
RET and phase shift technologies as well as tighter specifications have pushed aerial image metrology to consider CD
performance results in addition to the traditional intensity verification.
The content of the paper describes the utilisation of the AIMSTM Repair Verification (RV) software for the verification
of aerial images in a mask shop production environment. The software is used to analyze images from various AIMSTM
tool generations and the two main routines, Multi Slice Analysis (MSA) and Image Compare (IC), are used to compare
defective and non-defective areas of aerial images. It is detailed how the RV software cleans "non real" errors potentially
induced by operator misjudgements, thus providing accurate and repeatable analyses all proven against the results
A user friendly GUI drives the user through few simple, fast and safe operations and automatically provides summary
tables containing all the relevant results of the analysis that can be easily exported in a proper format and sent out to the
customer as a technical documentation. This results in a sensible improvement of the throughput of the printability
evaluation process in a mask manufacturing environment, providing reliable analyses at a higher productivity.
Using AIMSTM to qualify repairs of defects on photomasks is an industry standard. AIMSTM images match the
lithographic imaging performance without the need for wafer prints. Utilization of this capability by photomask
manufacturers has risen due to the increased complexity of layouts incorporating RET and phase shift technologies.
Tighter specifications by end-users have pushed AIMSTM analysis to now include CD performance results in addition to
the traditional intensity performance results.
Discussed is a new Repair Verification system for automated analysis of AIMSTM images. Newly designed user
interfaces and algorithms guide users through predefined analysis routines as to minimize errors. There are two main
routines discussed, one allowing multiple reference sites along with a test/defect site within a single image of repeating
features. The second routine compares a test/defect measurement image with a reference measurement image. Three
evaluation methods possible with the compared images are discussed in the context of providing thorough analysis
This paper highlights new functionality for AIMSTM analysis. Using structured analysis processes and innovative
analysis tools leads to a highly efficient and more reliable result reporting of repair verification analysis.
Moving forward to 32nm node and below optical lithography using 193nm is faced with complex requirements to be
solved. Mask makers are forced to address both Double Patterning Techniques and Computational Lithography
approaches such as Source Mask Optimizations and Inverse Lithography. Additionally, lithography at low k1 values
increases the challenges for mask repair as well as for repair verification and review by AIMSTM. Higher CD
repeatability, more flexibility in the illumination settings as well as significantly improved image performance must be
added when developing the next generation mask qualification equipment. This paper reports latest measurement results
verifying the appropriateness of the latest member of AIMSTM measurement tools - the AIMSTM 32-193i.
We analyze CD repeatability measurements on lines and spaces pattern. The influence of the improved optical
performance and newly introduced interferometer stage will be verified. This paper highlights both the new Double
Patterning functionality emulating double patterning processes and the influence of its critical parameters such as overlay
errors and resist impact. Beneficial advanced illumination schemes emulating scanner illumination document the
AIMSTM 32-193i to meet mask maker community's requirements for the 32nm node.
Flash memory is an important driver of the lithography roadmap, with its dramatic acceleration in dimensional shrink, pushing for ever smaller feature sizes. The introduction of hyper-NA immersion lithography has brought the 45nm node and below within reach for memory makers using single exposure. At these feature sizes mask topology and the material properties of the film stack on the mask play an important role on imaging performance. Furthermore, the break up of the array pitch regularity in the NAND-type flash memory cell by two thick wordlines and a central space, leads to feature-center placement (overlay) errors, that are inherent to the design. An integral optimization approach is needed to mitigate these effects and to control both the CD and placement errors tightly.
In this paper we will show that aerial image measurements at mask-level are useful for characterizing the gate layer of a NAND-Flash design before exposure. The aerial image measurements are performed with the AIMSTM 45-193i. and compared to CD measurements on the wafer obtained with an XT:1900Gi hyper-NA immersion system. An excellent correlation is demonstrated for feature-center placement errors and CD variations across the mask (see Figure 1) for several features in the gate layer down to 40nm half pitch. This shows the potential to use aerial image measurements at mask-level in combination with correction techniques on the photomask, like the CDC200 tool in combination with exposure tool correction techniques, such as DoseMapperTM, to improve both across field and across wafer CD uniformity of critical layers.
Using aerial image metrology to qualify repairs of defects on photomasks is an industry standard. Aerial image
metrology provides reasonable matching of lithographic imaging performance without the need for wafer prints.
Utilization of this capability by photomask manufacturers has risen due to the increased complexity of layouts
incorporating RET and phase shift technologies. Tighter specifications by end-users have pushed aerial image
metrology activities to now include CD performance results in addition to the traditional intensity performance results.
Discussed is the computer implemented semi-automated analysis of aerial images for repair verification activities.
Newly designed user interfaces and algorithms could guide users through predefined analysis routines as to minimize
errors. There are two main routines discussed here, one allowing multiple reference sites along with a test/defect site on
a single image of repeating features. The second routine compares a test/defect measurement image with a reference
This paper highlights new functionality desirable for aerial image analysis as well as describes possible ways of its
realization. Using structured analysis processes and innovative analysis tools could lead to a highly efficient and more
reliable result reporting of repair verification metrology.
The AIMTM45-193i is the established tool for mask performance qualification and defect printing
analysis in the mask shop under scanner conditions. Vector effects are taken into account by the
proprietary Zeiss vector effect emulator. In several studies an excellent correlation to wafer prints has
been reported. However, a systematic offset to wafer prints in terms of mask error enhancement factor
(MEEF) and exposure latitude has been observed which is attributed to well known resist effects.
The AIMSTM measures the aerial image in resist whereas in a real lithography process further image blur
of the latent image is caused by photo acid diffusion during wafer processing and resist development. To
explain the gap between the AIMTM and wafer prints we have investigated aerial images in combination
with an easy to use resist model. It does take resist effects into account with sufficient accuracy to
explain printing behavior of photo masks but without the need to calibrate lots of parameters of the
actually used resist which usually are not known to a mask shop.
The resist effects predominantly reduce the image contrast and thus increase the MEEF and the
sensitivity to mask defects. This somewhat counterintuitive behavior is labeled "contrast enhancement
by contrast reduction". Additionally application of the resist model improves the agreement of e.g. the
exposure latitude or MEEF measured by the AIMSTM compared to wafer prints.
Recently more and more mask designs for critical layers involve strong OPC which increases the complexity for standard
CD SEM mask measurements and conclusive interpretation of results. For wafer printing the wafer level CD is the
crucial measure if the mask can be successfully used in production. Recent developments in the AIMSTM software have
enabled the user to use the tool for wafer level CD metrology under scanner conditions. The advantage of this
methodology is that AIMSTM does see the CD with scanner eyes. All lithographic relevant effects like OPC imaging
which can not be measured by other tools like mask CD SEM will be captured optically by the AIMSTM principle.
Therefore, measuring the CD uniformity of the mask by using AIMSTM will lead to added value in mask metrology. With
decreasing feature sizes the requirements for CD metrology do increase. In this feasibility study a new prototype
algorithm for measuring the lithographically relevant AIMSTM CD with sub pixel accuracy has been tested. It will be
demonstrated that by using this algorithm line edge and line width roughness can be measured accurately by an AIMSTM
image. Furthermore, CD repeatability and tool matching results will be shown.
Flash memory has become one of the most important segments of the semiconductor industry in recent years. Flash
memory is also an important driver of the lithography roadmap, with its dramatic acceleration in dimensional shrink,
pushing for ever smaller feature sizes. The introduction of the XT:1700Fi and XT:1900Gi have brought the 45nm node
and below within reach for memory makers. At these feature sizes mask topology and the material properties of the film
stack on the mask play an important role on imaging performance. Furthermore, the break up of the array pitch
regularity in the NAND-type flash memory cell by two thick wordlines and a central space, leads to feature-center
placement (overlay) errors, that are inherent to the design. An integral optimization approach is needed to mitigate these
effects and to control both the CD and placement errors tightly.
In this paper we will present the results of aerial image measurements on mask level of a NAND-Flash Memory Gate
layer using AIMSTM 45-193i. Various imaging relevant parameters, such as MEEF, EL, DoF and placement errors are
measured for different mask absorber materials for features sizes ranging from 39nm half pitch to 41nm half pitch
design rule on wafer level. The AIMSTM measurements are compared to experimental results obtained with a
XT:1900Gi hyper-NA immersion system. Mask optimization strategies are sought to increase Depth of Focus and
minimize feature-center placement errors.
With the introduction of the TWINSCAN XT:1900Gi the limit of the water based hyper-NA immersion lithography has
been reached in terms of resolution. With a numerical aperture of 1.35 a single expose resolution of 36.5nm half pitch
has been demonstrated. However the practical resolution limit in production will be closer to 40nm half pitch, without
having to go to double patterning alike strategies. In the relentless Flash memory market the performance of the
exposure tool is stretched to the limit for a competitive advantage and cost-effective product.
In this paper we will present the results of an experimental study of the resolution limit of the NAND-Flash Memory
Gate layer for a production-worthy process on the TWINSCAN XT:1900Gi. The entire gate layer will be qualified in
terms of full wafer CD uniformity, aberration sensitivities for the different wordlines and feature-center placement
errors for 38, 39, 40 and 43nm half pitch design rule. In this study we will also compare the performance of a binary
intensity mask to a 6% attenuated phase shift mask and look at strategies to maximize Depth of Focus, and to desensitize
the gate layer for lens aberrations and placement errors. The mask is one of the dominant contributors to the
CD uniformity budget of the flash gate layer. Therefore the wafer measurements are compared to aerial image
measurements of the mask using AIMSTM 45-193i to separate the mask contribution from the scanner contribution to the
final imaging performance.
The lithographic performance of current state-of-the-art resolution enhancement techniques (RET) will become critical at
hyper numerical aperture (NA>1) due to mask 3D effects. We have studied the impact of the mask material on the
lithographic performance at NA 1.2 and above. The assessment, both by rigorous simulations and experiments, involves
the standard mask stacks, Cr binary mask (BIM) and MoSi 6% attenuated phase shift mask (attPSM), as well as
alternatives such as thick Cr BIM, Ta/SiO2 1% and 6% attenuated PSM, and Ta/SiON 1% attenuated PSM.
Using the rigorous electro-magnetic field (EMF) and lithographic process simulations (IISB DrLiTHO) the mask
structure is optimized taking into account the trade_off with mask error enhancement factor (MEEF). Next, a throughpitch
evaluation of the 45nm half-pitch (HP) node at NA1.2-1.35 is carried out examining maximum exposure latitude
(EL), depth-of-focus (DOF), best focus shifts, and MEEF behavior for the various mask stacks.
For the validation of the simulation methodology a correlation is made between scanner (ASML XT:1700Fi), AIMS
(Zeiss AIMSTM45-193i), and simulation results indicating the importance of the mask quality and mask properties.
Based on the lithographic performance and the mask manufacturability we put together a ranking of the commercially
available mask stacks for the 45nm HP node at NA 1.2 and 1.35.
We have tested the validity of the so-called 'vector-effect emulation mode' of the newest member of the AIMSTM
family, the AIMSTM45-193i, that was recently developed for Hyper-NA applications. This vector-effect emulation mode
(also called 'scanner mode') converts the measured signals into a prediction of what the image-in-resist of a Hyper-NA
scanner would be (so including vector- and polarization effects). We've done a number of experiments that directly test
the validity of this vector-effect emulation, by comparing them to rigorous lithographic simulations and to CD-measurements
from printed NA=1.20 scanner wafers, and found that the AIMSTM 45-193i results are in fact quite
accurate. Afterwards we looked at a number of potential Hyper-NA imaging applications for the AIMSTM 45-193i, again
comparing it to rigorous simulations and wafer CD-measurements. These results indicate that, next to its traditional use
as reticle-inspection tool, the AIMSTM 45-193i has potential use also in the wafer fab as an 'imaging-inspection' or
'OPC-defect inspection' tool, especially when applied to 2D patterns.
Mask manufacturing for the 45nm node for hyper NA lithography requires tight defect and printability control at small
features sizes. The AIMSTM1 technology is a well established methodology to analyze printability of mask defects,
repairs and critical features by scanner emulation. With the step towards hyper NA imaging by immersion lithography
the AIMSTM technology has been faced with new challenges like vector effects, polarized illumination and tighter specs
for repeatability and tool stability.
These requirements pushed the development of an entirely new AIMSTM generation. The AIMSTM 45-193i has been
designed and developed by Carl Zeiss to address these challenges. A new mechanical platform with a thermal and
environmental control unit enables high tool stability. Thus a new class of specification becomes available. The 193nm
optical beam path together with an improved beam homogenizer is dedicated to emulate scanners up to 1.4 NA. New
features like polarized illumination and vector effect emulation make the AIMSTM 45- 193i a powerful tool for defect
disposition and scanner emulation for 45nm immersion lithography.
In this paper results from one of the first production tools will be presented. Aerial images from phase shifting and
binary masks with different immersion relevant settings will be discussed. Also, data from a long term repeatability
study performed on masks with programmed defects will be shown. This study demonstrates the tool's ability to
perform defect disposition with high repeatability. It is found that the tool will fulfill the 45nm node requirements to
perform mask qualification for production use.
Hyper-NA lithography with polarized light illumination is introduced as the solution of 45nm or 32nm node
technology. In that case, consideration of new characteristics of mask materials and pellicle films has been required. In
order to analyze the influence of mask material's optical characteristics, we have proposed to use the AIMSTM system
measuring diffraction intensity balance in previous work. That was enabled by acquiring pupil plane images using the
Bertrand lens in the AIMSTM system to measure selected area's diffracted light.
In this study of mask material evaluation, we used same functionality of AIMSTM system, MonoPole illumination
and Bertrand lens, as previous work but other direction's pole is also used on the illumination aperture to cover total
diffraction orders of Cross-quad illumination because this illumination is more flexible for x and y patterns. In order to
get diffracted light of 45nm half-pitch, hyper-NA e.g. NA=1.35 was applied and the AIMSTM 45-193i Alpha system was
used for this evaluation. The examinations were performed with binary and half tone PSM with half pitch 40 to 150nm
on a 1x scale and fixed half pitch 45nm with various mask bias. We confirmed the relation between diffractions' intensity
balance and wafer printing performance for each material and we compared them to 3D simulation results.
Moreover, by using the same functionality of AIMSTM system, the transmission change by pellicle film was also
examined. We have prepared two different thickness pellicles to compare transmission change and printed CD on the
wafer. Intensity profile at pupil plane on the clear region of the mask was acquired with Bertrand lens and conventional
large sigma setting for both with and without pellicle film on the mask. By comparing transmission distribution change
between with and without pellicle, we could calculate transmission loss by pellicle at large incident angles. For this
experiment, NA=1.40 was applied and the AIMSTM 45-193i Alpha system was also used. The examinations were
performed with half tone PSM at half pitch 45nm and 65nm on a 1x scale on linear polarized DiPole illumination.
As a result, we have confirmed good agreement between AIMSTM measurement data and optical 3D simulations. In
conclusion, the AIMSTMsystem is a valuable tool for analyzing diffraction efficiency or intensity distribution on the
pupil plane and comparison to wafer printing performance.
Immersion lithography offers the semiconductor industry the opportunity to extend current ArF processes before switching to shorter wavelengths. As numerical apertures of scanners for hyper NA move above 1.0 with immersion lithography, increased attention must be paid to the photomask or reticle and its wafer printability. Feature sizes on the photomask become increasingly critical as they behave more like partial wire grid polarisers, as they become comparable to, or smaller than the wavelength. Besides challenges to address reticle polarisation effects, lithographers must also consider the polarisation state of the illumination and subsequently the contrast loss for light with a TM polarisation state. Such an effect, also called the vector effect, is caused by the increasing angle of incidence of the diffracted light for larger numerical apertures on the scanner. Therefore, for wafer printing using hyper NA scanners, the industry consensus is that TE polarised illumination must be used to meet the stringent requirements of imaging contrast.
In this paper, initial results of measurements using the optical test stand and the alpha tool of a completely new AIMSTM tool for the 45nm node will be presented. The system covers all aspects of immersion and polarisation lithographic emulation. Measurements have been made on binary and phase shift masks with different sizes of features and on programmed defects.