The era of EUV technology is approaching and use of EUV lithography in chip manufacturing process was reported. The EUV technology has still serious challenges to overcome, to which belong defectivity, source power and throughput of the exposure tool, to name the most obvious. Important part of the lithography, which differs significantly from previous optical technology, is the mask. The mask stack, especially the multilayer (ML) mirror surface and its protection is of high importance, determining the reflectivity of the mask. The ML mirror is protected by a thin Ru capping layer, which however is very sensitive to oxidation and damage during mask manufacturing processes and its use. Also estimation of the capping layer thickness is not trivial, and is unreliable by damage free analytical methods. In our work, we focus on the capping layer integrity and assess it as function of several applied cleaning processes. The integrity is examined via e-beam repair process and AFM measurement of the feature height. As identified in previous experiments, the UV exposure used in manufacturing processes has significant influence on the Ru layer at some conditions. However, there is good chance to find conditions at which the Ru layer is not attacked by the UV exposure, and removed by the subsequent wet process in which the products of Ru oxidization are diluted. Above mentioned procedure we intend to identify EUV mask manufacturing conditions, at which the capping layer is not impacted by the clean process. At the end of the manufacturing process, the EUV mask has to have a thick enough capping layer to perform the repair process and protect the ML mirror during the mask lifetime. Currently available processes allow us to manufacture EUV masks with a remaining capping layer up to five times thicker than required for the e-beam mask repair. This result confirms readiness of the mask manufacturing process for HVM from perspective of the mask health and integrity of the ML mirror and Ru capping layer.
For certain designs, we observe a rather peculiar defect during phase-shift mask production. At distinct positions on the mask, the chrome disappears within the second level process in almost perfect half circles. This effect can even be observed if no etching is applied at all. The root cause of this defect is electrochemical dissolving of chrome in DI water during the development rinse process, which appears at locations where the chrome is in contact to the developer rinse medium. In this publication we describe the experimental set-up to investigate the root cause mechanism and propose solutions to overcome the effect.
EUV technology is according to current trend approaching the final development phase in which defect free EUV masks
are of key importance for development and optimization of the lithography process. This task consists of three
contributing aspects- defect free multilayer blank, mask manufacturing process with very low defect formation
probability and availability of repair process for EUV mask.
In comparison to optical mask, development of the repair process for EUV mask is different in several aspects. The fact,
that the TaN absorber is placed on top of Mo/Si mirror is making the process very sensitive to variation of the mask
material, as the etch rate of the mirror is significantly higher, than that of absorber, when no capping layer is present
between the absorber and ML mirror. The presence of the Ru capping layer increases the process window due to
significant selectivity improvement by one or two orders of magnitude, however, the capping layer is very sensitive to
damage by preceding manufacturing processes.
Its thickness and also it chemical purity - lack of modification by incorporation of impurities is crucial for successful
The repair process for optical masks is typically optimized using AIMS for both development and qualification of the
process. The availability of EUV AIMS system is very limited, for what reason we have to rely on other measures
during the process development and use the AIMS for process qualification only, or use correlation between e.g. CD
SEM or AFM measurement and AIMS data for selection and qualification of the repair process.
Also the usage of mask – exposure on the scanner is modifying the mask surface. Therefore the impact of the mask
exposure needs to be investigated, when EUV gets in HVM stage.
In the past, the influence of the mask cleaning process on the integrity of EUV mask was investigated, with respect to
several lithography-critical parameters as actinic reflectivity, critical dimension (CD) shift, edge roughness and surface
roughness. The reparability of the mask was so far not in focus, assuming, that mask can be repaired anytime during its
lifetime. This missing item needed for the successful usage of EUV mask needs to be checked and status confirmed
prior start of the HVM.
EUV developed in the last decade to the most promising <7nm technology candidate. Defects are considered to be one of the most critical issues of the EUV mask. There are several contributors which make the EUV mask so different from the optical one. First one is the significantly more complicated mask stack consisting currently of 40 Mo/Si double layers, covered by Ru capping layer and TaN/TaO absorber/anti-reflective coating on top of the front face of the mask. Backside is in contrary to optical mask covered as well by conductive layer consisting of Cr or CrN. Second contributor is the fact that EUV mask is currently in contrary to optical mask not yet equipped with sealed pellicle, leading to much higher risk of mask contamination. Third reason is use of EUV mask in vacuum, possibly leading to deposition of vacuum contaminants on the EUV mask surface. Latter reason in combination with tight requirements on backside cleanliness lead to the request of frequent recleaning of the EUV mask, in order to sustain mask lifetime similar to that of optical mask. Mask cleaning process alters slightly the surface of any mask - binary COG mask, as well as phase shift mask of any type and naturally also of the EUV mask as well. In case of optical masks the changes are almost negligible, as the mask is exposed to max. 10-20 re-cleans within its life time. These modifications can be expressed in terms of different specified parameters, e.g. CD shift, phase/trans shift, change of the surface roughness etc. The CD shift, expressed as thinning (or exceptionally thickening) of the dark features on the mask is typically in order of magnitude 0.1nm per process run, which is completely acceptable for optical mask. Projected on the lifetime of EUV mask, assuming 100 clean process cycles, this will lead to CD change of about 10nm. For this reason the requirements for EUV mask cleaning are significantly tighter, << 0.1 nm per process run. This task will look even more challenging, when considering, that the tools for CD measurement at the EUV mask are identical as for optical mask. There is one aspect influencing the CD shift, which demands attention. The mask composition of the EUV mask is significantly different from the optical mask. More precisely there are 2 materials influencing the estimated CD in case of EUV mask, whereas there is one material only in case of optical masks, in first approximation. For optical masks, the CD changes can be attributed to modification of the absorber/ARC layer, as the quartz substrate can be hardly modified by the wet process. For EUV Masks chemical modification of the Ru capping layer - thinning, oxidization etc. are rather more probable and we need to take into account, how this effects can influence the CD measurement process. CD changes measured can be interpreted as either change in the feature size, or modification of the chemical nature of both absorber/ARC layer stack and the Ru capping layer. In our work we try to separate the effect of absorber and Ru/capping layer on the CD shift observed and propose independent way of estimation both parameters.
The EUV mask readiness is ranked under the top three challenges for successful introduction of EUV into high volume manufacturing. Whereas the basic mask manufacturing processes can be principally taken over from optical mask manufacturing, the big amount of new materials incorporated in the relatively complicated EUV mask stack is causing new effects either by their chemical and physical nature, or by their interaction with the processes. Some of the major challenges for EUV mask manufacturing compared to the optical mask is the EUV mask lifetime. First of those is the high illumination energy, which is expected to introduce changes in the mask stack, degradation of the pattern fidelity and reflectivity of the EUV mask. Also EUV mask manufacturing processes influence the mask lifetime. Few of those processes used over decade for successful manufacturing of optical mask can be used for manufacturing of EUV mask, which however will not perform very well and will impact mask lifetime. Interactions between unit processes were identified, influencing not only the pristine mask performance, but impacting their lifetime as well. In our work mainly the interaction between EUV etch and dedicated EUV cleaning process is investigated. Mainly the absorber etch process is dominantly determining the actinic reflectivity and its uniformity across the mask. Additionally the etch process is interacting with the clean process and limits the threshold for mask properties change due to clean process. Finally modification of surface layer and the presence (or absence) of Ru-based capping layer are critical factors for overall EUV mask properties and stability.
EUV lithography is currently the favorite and most promising candidate among the next generation lithography (NGL) technologies. Decade ago the NGL was supposed to be used for 45 nm technology node. Due to introduction of immersion 193nm lithography, double/triple patterning and further techniques, the 193 nm lithography capabilities was greatly improved, so it is expected to be used successfully depending on business decision of the end user down to 10 nm logic. Subsequent technology node will require EUV or DSA alternative technology. Manufacturing and especially process development for EUV technology requires significant number of unique processes, in several cases performed at dedicated tools. Currently several of these tools as e.g. EUV AIMS or actinic reflectometer are not available on site yet. The process development is done using external services /tools with impact on the single unit process development timeline and the uncertainty of the process performance estimation, therefore compromises in process development, caused by assumption about similarities between optical and EUV mask made in experiment planning and omitting of tests are further reasons for challenges to unit process development. Increased defect risk and uncertainty in process qualification are just two examples, which can impact mask quality / process development. The aim of this paper is to identify critical aspects of the EUV mask manufacturing with respect to defects on the mask with focus on mask cleaning and defect repair and discuss the impact of the EUV specific requirements on the experiments needed.
EUV lithography is the leading candidate for chip manufacturing at sub 10nm technology node. EUV specifications for
mask were derived from those of optical masks and are currently adjusted according to knowledge collected with respect
to specific behavior of EUV masks and exposure process.
Difference in application of optical and EUV masks - transmission vs. reflection lead to addition of parameters
characterizing surface roughness of both capping layer covered multilayer mirror as well as absorber, tight specification
of line edge roughness (LER), actinic reflectivity of the mask as well as critical dimension (CD) mean to nominal.
Absence of pellicle at EUV masks is expected to result in more frequent cleaning of the mask and hence more
pronounced changes of the mask properties due to the mask cleaning process. These assumptions are reflected in tighter
specifications for clean process influence.
Investigation of the clean process influence on the EUV relevant parameter identifies differences in CD shift between
optical and EUV masks. The chemical composition of EUV mask surfaces; absorber and capping layer; not used at
optical masks is reason, why change of line width can be observed, and is possibly responsible for differences in CD
stability of commercially available EUV absorbers too.
Combined with narrowing specifications for CD shift due to cleaning, the material properties may result in the need of
material specific clean processes. In our work we investigate the root cause of the differences and check if one process
can be used which covers both EUV material stacks at given CD shift specifications.
This paper presents results of the optimization of an EUV mask cleaning process and compares the results to data
obtained on COG and EPSM masks using processes specifically designed for such masks. The key parameter
investigated was cleaning efficiency, as measured in terms of Particle Removal Efficiency (PRE), CD shift and actinic
reflectivity change. The PRE of 100%, 84%, and 80% was obtained for COG, EUV and HT-PSM masks, respectively.
The CD change per clean cycle was 0.07nm. The feature damage limit was 50nm. Actinic reflectivity change in the
range <0.1% per clean cycle was obtained for the process.
After decades of binary mask manufacturing using Cr absorber the material spectrum was extended by phase shift
material in late 90's during introduction of Half Tone Phase Shift Masks (HT-PSM). This change had strong impact on
manufacturing flow as well as several unit processes.
A consequences of phase shifter introduction was the necessity of introducing a second level litho process, as well as
introducing of dry etch processes due to poor etch properties of MoSi using wet chemistry. Less obvious and rather
unremarkable was the impact of this change to clean processes, except the impact of the clean process on the phase
In recent years we've seen several new materials based on varying chemical composition as well as thickness of the
absorber developed by various mask blank vendors namely Hoya and ShinEtsu. These materials are improving
resolution, pattern fidelity and to some degree also mask lifetime. Adding the EUV mask blank materials increases
further the spectrum of materials, taking into account all the absorber stacks available today on market.
Thorough investigation of the clean process performance as a function of surface material shows significant variation in
the critical parameters as defectivity, susceptibility to recontamination and relative cleaning efficiency.
Goal of this work is to
1) Compare the already mentioned clean related properties together with feature damage and impact on the critical
dimension (CD) shift for different materials.
2) Find a compromise between the technology requirements and process limitations resulting from the combination of
available processes with material properties.
Some aspects of the new materials such as stack height and interface between absorber and substrate are making this
task easier, especially with respect to feature damage. On the other hand the most critical parameter - the cleaning
efficiency, dropped due to the introduction of the new materials, mainly due to unfavorable sticking coefficients of
Pending the availability of actinic inspection tools, optical inspection tools with 193 nm DUV
illumination wavelength are currently used to inspect EUV masks and EUVL-exposed wafers.
Due to strong optical absorption, DUV photons can penetrate only a few surface layers of EUV
masks, making them sub-optimal for detecting hidden defects embedded within the sub-layers of
the mask, the so-called phase defects. Although these phase defects may not be detected by
optical inspection tools, they may print on the wafer. Conversely, false and nuisance defects
which may not print on the wafer may be detected by optical inspection tools, and by so doing,
degrade the inspection sensitivity of the tool to real and critical defects. This paper discusses
approaches to optimizing the optical inspection sensitivity of EUV masks, with a view to
overcoming some of the absorption limitations of the inspection wavelength and also with a view
to enhancing the imaging contrast of the reflected light between the low reflective absorber/antireflection
coating stack and the moderately reflective mirror surface of Mo/Si bilayers, capped
with a thin Ru layer, and which serves to protect the mirror surface from damage and
contamination during mask fabrication and wafer printing processes. The effects of mask
absorber/ARC stack thickness on optical inspection contrast are simulated using rigorous
coupled wave analysis (RCWA), and compared to experimental results. EUV masks with thin
absorber/ARC stacks are observed to have higher inspection contrast, up to 15 % higher than
their thicker counterparts, especially as the feature pitch gets smaller. Blank defect inspection
performance of tools such as the Siemens DFX40 tool and KLA 617 Teron tool equipped with
Phasur module are compared, and correlated with patterned mask inspection data generated from
KLA 617 Teron tool. Patterned mask defect sensitivities to the tune of 40 nm and 90 nm were
obtained on thin and thick absorber/ARC stacks, respectively. The defect location accuracy of
the Teron 617 tool is better than 250 nm (3σ), while the alignment repeatability of the Teron 617
on the fiducials is better than 60 nm (3σ). Printability of mask blank and patterned mask defects
on exposed wafers in terms of what and where the defects print, are also presented. Four masks
with different absorber and antireflection coating thicknesses, some with substrate and absorber programmed defects of different types and sizes, were fabricated and used to expose resistcoated
SiN substrate wafers on full field ASML EUV scanners.
Photomask technology has attained feature sizes of about 50nm and below. Whereas the main feature size is still above
70-80nm at 20nm technology node recently reported e.g. by Toppan Printing Company as developed, assist features for
this node are in the range of 50-60nm.
One of the critical aspects of this technology development is the cleaning process. Processes are supposed to clean off
contamination and particles down to a defect size of about 40nm and at the same time prevent damage to assist features
in the same size range. Due to obvious trade offs between cleaning power and Feature Damage Probability (FPD), this
task becomes tricky. Improvement of cleaning processes by raising the power of megasonic (MS) cleaning, or adjusting
the speed and size of droplets for spray cleaning occurs at the expense of increased feature damage. Prolongation of
physical cleaning steps does not necessarily leads to improvement of the cleaning as shown previously. Susceptibility
to feature damage occurs predicatively according to dimension and orientation. This allows us to extrapolate a Feature
Damage Limit (FDL) which approximates the smallest feature size for which a process has an acceptable probability of
In a practical sense, the most advantageous approach seems to be to adjust the cleaning power to the maximum allowed
by the FDP and then optimize to the lowest process time necessary to reach expected cleaning efficiency.
Since there are several alternative physical cleaning principles, we have to pick the best one for a given application.
At this point we have to raise the question of how to compare the cleaning efficiency of processes. The goal of this
work is to provide a method for evaluation and comparison of cleaning efficiency between physical cleaning processes
and demonstrate the method on an example. We will focus on comparing two physical cleaning processes 1MHz
megasonic and binary spray process.
Since introduction of 193nm exposure wavelength, the haze formation becomes a serious challenge especially at mask
used for big number of exposures. Several compounds present in air as low concentration contaminants are leading to
haze building. Well understood is the sulphate based haze formation, however, still causing significant losses and
demanding for re-cleaning of the mask during mask life time.
There are plenty of publications taking different approaches to reduction of the final sulphate concentration on the mask
and reduction of the use of sulphuric acid during the mask manufacturing. Beside traditional process as hot water
extraction, UV exposure, baking, IR exposure at vacuum, Ammonia solution treatment more exotic method were
published as surface treatment preventing migration of the sulphate ions on mask surface. The number of exposures till
haze crystals growth prevents further use of the mask is not solely dependent on the sulphate concentration on the freshly
manufactured mask. Additional factors as storage and use conditions are significantly influencing the period till re-clean
of the mask is needed.
In this work we try to assess above mentioned approaches and provide rough estimate of their limits.
In our paper we make an analysis of conditions for the haze development on photomask fabricated on Mo-Si
containing substrates. We bring in focus cases of haze formation on masks with intrinsically very low contaminants level
and being exposed in very well controlled atmosphere. There are clear indications that this new type of haze formation
deviates from the generally accepted models not only with respect to the formation mechanisms but also with regard to
the chemical composition of the haze products. In our analysis we speculate that the new haze type formation is closely
related to the earlier reported CD degradation observed on Mo-Si masks. We also analyze the hypothesis that the
ingredients for the haze formation are not only airborne contaminants and/or traces on the mask surface, but are also
provided by the substrate material. Finally we present and discuss experimental data in the view of the advanced models.
Large error bars in cleaning experiments are commonly accepted in mask making but such errors restrict potential
improvements in cleaning and restrict the uniform delivery of megasonic (MS) energy. Hence, large error limits in
particle removals have an impact to operational costs based on contamination and breakage. New data handling
methods are developed here, which exceed the current capability scatterometric particle measurement methods and
which create a better statistical basis for interpretation. These improved data treatment methods employ subdivisions of
the mask into regions as small as mm<sup>2</sup>. The effective number of runs becomes many thousands of time greater which
can compensate for the small number of blanks available for tests due to restricted costs. This new technology is
combined with a precise modeling of the MS tracking patterns on a plate and allows better comparisons between
theoretical modeling and experimentally observed cleans. The combination of these two methods yields an improved
determination of rate kinetics for particle removal. Collectively, these methods provide the basis for better interpretation
of the spatial non-uniformities seen in MS spin cleaning methods with obvious consequences to manufacturing costs.
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.
The cleaning processes used today for photomasks were developed over decades and optimized to fulfill customer specifications. Some mask procedures were adapted from wafer cleaning technology. A principal technique, megasonic (MS) cleaning, yields high particle removal efficiencies (PRE). However, MS can frequently cause feature damage, and so damage becomes the principle limitation to MS power levels applied to small feature sizes. The use of lower MS power levels can benefit from a better understanding of removal mechanisms. In several publications the effects influencing the mechanisms of particle cleaning were discussed <sup></sup>. Particle transfer was investigated experimentally on wafer surfaces using bath tools and was tracked using fluorescent optical microscopy <sup></sup>. The goal of our investigation is to test the validity of the aforementioned models for mask cleaning using a spinning mask and a megasonic head mounted on a arm swinging over the mask surface, which is the most common hardware setup used for mask cleaning tools. While this equipment setup provides a useful variability, it also introduces disadvantages e.g. non-equal distribution of the megasonic power across the cleaned surface as will be shown. We will focus on some of the main parameters e.g. chuck speed, arm swing speed and media flow, which are strongly coupled by the fluid dynamics and cannot be treated separately. All three parameters influence particle-mask decoupling and reattachment during particle transport by the media stream across the mask surface. The approach to estimate the particle removal and reattachment rate is illustrated. The experiments performed allow the conclusion that the reattachment rate on a flat spinning mask surface is lower than previously assumed and the most critical part of the cleaning process is the detachment of the particle from the surface.
In order to fulfil the upcoming requirements for photomasks there is a need for improving the process stability
(reproducibility) of the unit processes in photomask fabrication. In order to understand and minimize the etch
contribution to the CD stability impedance sensors integrated into the capacitively coupled radio frequency (RF) circuit
(bias circuit) have shown a big potential.
The last step towards a full characterization of the RF properties is the integration of impedance sensors in the
inductively coupled RF circuit (source). This kind of sensor measures voltage, current and phase angle for the
fundamental (13.56 MHz) and higher harmonics (up to the 5th harmonic).
In this paper we are describing the integration of the Z-Scan sensors into the source RF matchbox and its impact on the
RF and CD characteristics of the mask etcher. The central point is the correlation of impedance data to CD data. We will
also compare the responses for bias and source impedance measurements.
The Critical dimension off-target (CDO) is a key parameter for mask house customer, affecting directly the performance
of the mask. The CDO is the difference between the feature size target and the measured feature size. The change of
CD during the process is either compensated within the process or by data correction. These compensation methods are
commonly called process bias and data bias, respectively. The difference between data bias and process bias in
manufacturing results in systematic CDO error, however, this systematic error does not take into account the instability
of the process bias. This instability is a result of minor variations - instabilities of manufacturing processes and changes
in materials and/or logistics.
Using several masks the CDO of the manufacturing line can be estimated. For systematic investigation of the unit
process contribution to CDO and analysis of the factors influencing the CDO contributors, a solid understanding of each
unit process and huge number of masks is necessary. Rough identification of contributing processes and splitting of the
final CDO variation between processes can be done with approx. 50 masks with identical design, material and process.
Such amount of data allows us to identify the main contributors and estimate the effect of them by means of Analysis of
variance (ANOVA) combined with multivariate analysis.
The analysis does not provide information about the root cause of the variation within the particular unit process,
however, it provides a good estimate of the impact of the process on the stability of the manufacturing line. Additionally
this analysis can be used to identify possible interaction between processes, which cannot be investigated if only single
processes are considered.
Goal of this work is to evaluate limits for CDO budgeting models given by the precision and the number of
measurements as well as partitioning the variation within the manufacturing process. The CDO variation splits
according to the suggested model into contributions from particular processes or process groups. Last but not least the
power of this method to determine the absolute strength of each parameter will be demonstrated.
Identification of the root cause of this variation within the unit process itself is not scope of this work.
Increasing demand for high end masks with ever-narrowing specifications for critical dimension uniformity (CDU), CD linearity, etc. is the major driver for the further process development. Decreasing the main feature size and increasing the complexity of sub-resolution assist features (SRAF) are restricted by the resolution limit of the mask manufacturing process, which is determined by the resolution limit of the mask lithography process and the widening of the clear structures in etch processes - etch bias.
In order to be able to compare and develop new etch processes, a reliable and well reproducible method for etch bias estimation has to be established. Previous investigation shown, that there is a gap between etch bias estimation by mean of CD SEM and AFM methods. The measurement of the CD value in resist was identified as the major problem of the etch bias estimation for several reasons. We searched for a measurement method that minimized the disadvantages of the resist CD measurement; ideally making the resist CD measurement obsolete. Since the widening of the features can be observed at the Cr edge as well, our experiment focuses on the measurement of the upper Cr edge shift as function of time which provides the information about the etch bias as well as the time evolution of the CD and side wall angle. In our work we present a method for measurement, one can adopt as etch bias measurement and use it as a way for calibration of the easy to use etch bias measurement methods like AFM, CD SEM and optical CD measurement. The understanding of the process of structure widening gives us a confidence, that the method suggested is correct and explains well, what the limitations of the etch bias are. Last but not least the proof is given, that there is no Cr etch process with zero etch bias possible.
The ever-narrowing specifications for high-end masks can only be derived from the continuous improvement of all manufacturing processes. Here, the metrology is crucial prerequisite since the development relies almost entirely on measurement results. In this paper we will address this relation by showing how the limits of metrology repeatability and reproducibility define also the limits of process development. In particular, we will show that improved metrology tool performance on resist results in a deeper understanding for the dry etch process. This is very important since resist metrology is not part of the ITRS roadmap and serves "only" as a supporting engineering process. Better short-term repeatability results in the possibility to detect more variables that might influence the etch regime. As an example, results from two CD scanning electron microscopes (SEM) were compared with very different short-term repeatability. The better knowledge based on the more accurate metrology data allows then to optimize the process within a process space which was previously not detectable with the other tool. An estimate is given how much this influenced the final performance of the process. We conclude from these results, that metrology parameters not covered in standard roadmaps become increasingly important to achieve process development goals in other process areas.
Increasing demand for high end lithography mask especially phase shift masks and narrowing the specification, lead to development of etch processes with minimum critical dimension uniformity (CDU) and very low etch bias. The etch bias becomes one of the limiting parameters for the Cr etch process, due to strong cross links between etch bias and other etch characteristics like linearity and loading effect, thus contributing strongly to the CDU for masks with non uniform pattern distribution. The goal was to develop a Cr etch process with very low etch bias, keeping the other parameters at the same level and providing a wider process window for further optimization of the CDU, loading effect and linearity. In the paper we want to present a feasibility study of one specific approach to the mentioned methods and compare different ways for measurement of the CDU and etch bias. The work presented was done on the Applied Materials Tetra II Mask Etch system.
For the last few years several different photoresists and Cr layers were used for mask making: -I line resist for 363.8 nm laser writer; -e-beam resist; -Positive CAR resist and DUV CAR resist.
Introduction of a new resist into production has several risks associated with and requires process adjustments in litho and etch process likewise. This presentation will focus on the differences in the endpoint detection using optical emission spectroscopy (OES), especially at low Cr load, when using above mentioned photo resists.
Development of the OES endpoint detection starting from single wavelength is shortly discussed and methods for endpoint detection at low Cr concentration in the gas phase caused by decreasing plasma power and increasing volume of the etch chamber are shown.
An important factor for the practical use of the endpoint detection is the reliability, scalability for different Cr loads and dependence on the chamber seasoning. These factors will be discussed finally.
One of the most critical steps for photomask CD off-target is the patterning of the mask. Here the instability of the dry etch process contributes directly to the stability of the CD value. The increasing demands on high-end masks cause a narrowing of both mask CD off-target and CD uniformity specifications, and accordingly the process stability has to be improved to fulfill these criteria. In this work we investigated the correlation between hardware parameters, basic etch process parameters and the corresponding CD mean-to-target value. Correlations between CD mean-to-target and Cr etch rate as well as effects of chamber seasoning after wet cleans are discussed.
Uniform radical distribution in the etching plasma is essential to meet chrome critical dimension (CD) uniformity for future technology nodes on chrome masks. The Etec Systems Tetra photomask etch chamber utilizes an alumina focus ring in order to optimize the etch uniformity of the chrome mask by minimizing gas flow effects and shaping the radial distribution of the etching radicals over the mask surface. This paper describes a systematic investigation to optimize the current focus ring, in order to improve etch critical dimension uniformity. The focus ring (FR) optimization work was made possible by manufacturing a modular focus ring that allowed the geometry to be varied at different heights and diameters. The circular shape of the modular focus ring, along with the height and diameter combinations, has a large influence on the etch performance at the mask corners and edges. The underlying mechanism was investigated by modeling and simulation. Based on simulation results the focus ring geometry was varied and the optimum FR configuration was found. The critical dimension uniformity could be adjusted on uniformly patterned masks with different pattern loads to meet production specifications.
Usually in photomask manufacturing, photoresists are stripped by wet processes using amineous solvents or acids. However, new photoresists and novel polymer-rich plasma etch processes in photomask manufacturing require new resist and polymer stripping techniques. The use of plasma strip processes strongly improves the stripping capability. One simple and economic solution is the microwave type reactor using oxygen plasma. As the chromium oxide antireflective coating (ARC) layer is etched in pure oxygen microwave plasma, the stripping plasma chemistry has to be modified to maintain sufficiently high selectivity towards chromium oxide. In this work a stripping process was optimized with respect to photoresist-to-chrome oxide selectivity and photoresist etch rate. The effect of the strip process on CD performance of the mask and integrity of the chromium oxide antireflective coating were investigated. Finally an endpoint detection solution was developed to optimize throughput. The described plasma stripping process proved to be fully applicable to photomask manufacturing.
CD uniformity and CD mean to target specifications nowadays can only be accomplished by mask manufacturing process using chrome dry etch. Chrome plasma etch processes tend to show a strong dependency of the chrome etch rate and thus the etch bias on the clearfield percentage of a mask resulting in varying offtarget behavior. There are various possibilities to compensate for this loading effect. In previous work the methods of using exposure dose and development time for offtarget control were investigated. In this study we examined the capability of plasma etch parameters to be used for offtarget control. The effects of oxygen concentration, pressure and overetch percentage on etch bias and CD uniformity were experiment. Two different development processes were investigated. The resulting offtarget control model was then confirmed by running additional masks at three different clearfield percentages. Measurement results showed a high confidence level for the model predicted numbers. SEM images confirmed stable behavior of chromium sidewall angles.