We propose a new concept of tuning a point-spread function (a “kernel” function) in the modeling of electron beam
lithography using the machine learning scheme. Normally in the work of artificial intelligence, the researchers focus on the
output results from a neural network, such as success ratio in image recognition or improved production yield, etc. In this
work, we put more focus on the weights connecting the nodes in a convolutional neural network, which are naturally the
fractions of a point-spread function, and take out those weighted fractions after learning to be utilized as a tuned kernel.
Proof-of-concept of the kernel tuning has been demonstrated using the examples of proximity effect correction with 2-layer
network, and charging effect correction with 3-layer network. This type of new tuning method can be beneficial to give
researchers more insights to come up with a better model, yet it might be too early to be deployed to production to give better
critical dimension (CD) and positional accuracy almost instantly.
Mask writers need to be able to write sub-50nm features accurately. Nano-imprint lithography (NIL) masters need to create sub-20nm line and space (L:S) patterns reliably. Increasingly slower resists are deployed, but mask write times need to remain reasonable. The leading edge EBM-9500 offers 1200A/cm<sup>2</sup> current density to shoot variable shaped beam (VSB) to write the masks.<p> </p>Last year, thermal effect correction (TEC) was introduced by NuFlare in the EBM-9500<sup>1</sup>. It is a GPU-accelerated inline correction for the effect that the temperature of the resist has on CD. For example, a 100nm CD may print at 102nm where that area was at a comparably high temperature at the time of the shot. Since thermal effect is a temporal effect, the simulated temperature of the surface of the mask is dynamically updated for the effect of each shot in order to accurately predict the cumulative effect that is the temperature at the location of the shot at the time of the shot and therefore its impact on CD. The shot dose is changed to reverse the effects of the temperature change.<p> </p>This paper for the first time reveals an enhancement to this thermal model and a simulator for it. It turns out that the temperature at the time each location receives backscatter from other shots also make a difference to the CD. The effect is secondary, but still measurable for some resists and substrates. Results of a test-chip study will be presented.<p> </p>The computation required for the backscatter effect is substantial. It has been demonstrated that this calculation can be performed fast enough to be inline with the EBM-9500 with a reasonable-sized computing platform. Run-time results and the computing architecture will be presented.
Semiconductor scaling is slowing down because of difficulties of device manufacturing below logic 7nm
node generation. Various lithography candidates which include ArF immersion with resolution enhancement
technology (like Inversed Lithography technology), Extreme Ultra Violet lithography and Nano Imprint
lithography are being developed to address the situation. In such advanced lithography, shot counts of mask
patterns are estimated to increase explosively in critical layers, and then it is hoped that multi beam mask
writer (MBMW) is released to handle them within realistic write time. However, ArF immersion technology
with multiple patterning will continue to be a mainstream lithography solution for most of the layers. Then,
the shot counts in less critical layers are estimated to be stable because of the limitation of resolution in ArF
immersion technology. Therefore, single beam mask writer (SBMW) can play an important role for mask
production still, relative to MBMW. Also the demand of SBMW seems actually strong for the logic 7nm
node. To realize this, we have developed a new SBMW, EBM-9500 for mask fabrication in this generation. A
newly introduced electron beam source enables higher current density of 1200A/cm<sup>2</sup>. Heating effect
correction function has also been newly introduced to satisfy the requirements for both pattern accuracy and
throughput. In this paper, we will report the configuration and performance of EBM-9500.
The specifications for critical dimension (CD) accuracy and line edge roughness are getting tighter to promote every photomask manufacturer to choose electron beam resists of lower sensitivity. When the resist is exposed by too many electrons, it is excessively heated up to have higher sensitivity at a higher temperature, which results in degraded CD uniformity. This effect is called “resist heating effect” and is now the most critical error source in CD control on a variable shaped beam (VSB) mask writer. We have developed an on-tool, real-time correction system for the resist heating effect. The system is composed of correction software based on a simple thermal diffusion model and computational hardware equipped with more than 100 graphical processing unit chips. We have demonstrated that the designed correction accuracy was obtained and the runtime of correction was sufficiently shorter than the writing time. The system is ready to be deployed for our VSB mask writers to retain the writing time as short as possible for lower sensitivity resists by removing the need for increased pass count.
Resist heating effect which is caused in electron beam lithography by rise in substrate temperature of a few tens or hundreds of degrees changes resist sensitivity and leads to degradation of local critical dimension uniformity (LCDU). Increasing writing pass count and reducing dose per pass is one way to avoid the resist heating effect, but it worsens writing throughput. As an alternative way, NuFlare Technology is developing a heating effect correction system which corrects CD deviation induced by resist heating effect and mitigates LCDU degradation even in high dose per pass conditions. Our developing correction model is based on a dose modulation method. Therefore, a kind of conversion equation to modify the dose corresponding to CD change by temperature rise is necessary. For this purpose, a CD variation model depending on local pattern density was introduced and its validity was confirmed by experiments and temperature simulations. And then the dose modulation rate which is a parameter to be used in the heating effect correction system was defined as ideally irrelevant to the local pattern density, and the actual values were also determined with the experimental results for several resist types. The accuracy of the heating effect correction was also discussed. Even when deviations depending on the pattern density slightly remains in the dose modulation rates (i.e., not ideal in actual), the estimated residual errors in the correction are sufficiently small and acceptable for practical 2 pass writing with the constant dose modulation rates. In these results, it is demonstrated that the CD variation model is effective for the heating effect correction system.
The scaling of semiconductor devices is slowing down because of the difficulty in establishing their functionality at the nano-size level and also because of the limitations in fabrications, mainly the delay of EUV lithography. While multigate devices (FinFET) are currently the main driver for scalability, other types of devices, such as 3D devices, are being realized to relax the scaling of the node. In lithography, double or multiple patterning using ArF immersion scanners is still a realistic solution offered for the hp16nm node fabrication. Other lithography candidates are those called NGL (Next Generation Lithography), such as DSA (Directed-Self-Assembling) or nanoimprint. In such situations, shot count for mask making by electron beam writers will not increase. Except for some layers, it is not increasing as previously predicted. On the other hand, there is another aspect that increases writing time. The exposure dose for mask writing is getting higher to meet tighter specifications of CD uniformity, in other words, reduce LER. To satisfy these requirements, a new electron beam mask writer, EBM-9000, has been developed for hp16nm/logic11nm generation. Electron optical system, which has the immersion lens system, was evolved from EBM-8000 to achieve higher current density of 800A/cm2. In this paper, recent shot count and dose trend are discussed. Also, writing time is estimated for the requirements in EBM-9000.
In the half pitch (hp) 16nm generation, the shot count on a mask is expected to become bipolar. The multi-patterning
technology in lithography seems to maintain the shot count around 300G shots instead of increase in the number of
masks needed for one layer. However, as a result of mask multiplication, the better positional accuracy would be
required especially in Mask-to-Mask overlay. On the other hand, in complex OPC, the shot count on a mask is expected
to exceed 1T shots.
In addition, regardless of the shot count forecast, the resist sensitivity needs to be lower to reduce the shot noise effect so
as to get better LER. In other words, slow resist would appear on main stream, in near future. Hence, such trend would
result in longer write time than that of the previous generations. At the same time, most mask makers request masks to
be written within 24 hours. Thus, a faster mask writer with better writing accuracy than those of previous generations is
With this background, a new electron beam mask writing system, EBM- 9000, has been developed to satisfy such
requirements of the hp 16nm generation. The development of EBM-9000 has focused on improving throughput for
larger shot counts and improving the writing accuracy.
EBM-9000 equipped with new features such as new electron optics, high current density (800A/cm<sup>2</sup>) and high speed deflection control has been developed for the 11nm technology node(tn) (half pitch (hp) 16nm). Also in parallel of aggressive introduction of new technologies, EBM-9000 inherits the 50kV variable shaped electron beam / vector scan architecture, continuous stage motion and VSB-12 data format handling from the preceding EBM series to maintain high reliability accepted by many customers. This paper will report our technical challenges and results obtained through the development.
In our previous work, we reported the static portion of the surface charging on EBM-8000 and compared it with that on EBM-6000. The scope of this paper is to report the analysis of charging decay component on EBM-8000 and compare it with EBM-6000. We confirmed that our fundamental modeling scheme of the charging decay worked well on EBM-8000 as well as on EBM-6000. However, we found totally different charging decay behaviors between EBM-8000 and EBM-6000. To explain the results, we propose a conceptual model of the charging decay phenomena both on EBM-8000 and EBM-6000.
In this paper, we report our modeling results of the resist surface charging effect on our newer e-beam mask writer
EBM-8000. We show that our fundamental modeling scheme we have developed for EBM-6000 can be adapted on
EBM-8000 platform without major modifications. We also discuss the significant differences in the charging effect between
EBM-6000 and EBM-8000 in terms of its amplitude, its spatial distribution, and its dependency on the pattern density.
Many lithography candidates, such as ArF immersion lithography with double-patterning/double-exposure techniques,
EUV lithography and nano-imprint lithography, show promising capability for 22-nm half-pitch generation lithography.
ArF immersion lithography with double-patterning/double-exposure techniques remains the leading choice as other
techniques still lack the conclusive evidence as the practical solution for actual production. Each of the prospective
lithography techniques at 22-nm half-pitch generation requires masks with improved accuracy and increased complexity.
We have developed a new electron beam mask writer, EBM-8000, as the tool for mask production of 22-nm half-pitch
generation and for mask development of 16nm half-pitch generation, which is necessary for the practical application of
these promising lithography technologies.
The development of EBM-8000 was focused on increasing throughput and improving beam positioning accuracy. Three
new major features of the tool are: new electron gun with higher brightness to achieve current density of 400 A/cm<sup>2</sup>,
high speed DAC amplifier to accurately position the beam with shorter settling time, and additional temperature control
to reduce the beam drift.
The improved image placement accuracy and repeatability, and higher throughput of EBM-8000 have been confirmed
by actual writing tests with our in-house tool.
A new method to describe the resist surface charging effect more accurately is proposed. In our previous work, we handled
only the static portion of the surface charging and it was applicable only to a limited situation. The scope of this paper is to
add a new model to handle the dynamic, discharging behavior on top of the existing static model to make the whole charging
model closer to what is really happening on the plate during the exposure. With the new model, the correction accuracy has
been improved not only for the equilibrium state but also for the state when the tool is dynamically writing the main pattern.
We conclude that our Charging Effect Correction (CEC) was advanced by this new model to become completely production
Complex mask shapes will be required on critical layer masks for 20nm logic node, threatening to explode the mask
write times. Model-Based Mask Data Preparation (MB-MDP) has been introduced to reduce the shot count required to
write complex masks while simultaneously improving resolution and dose margin of sub-100nm features. For
production use of MB-MDP, a number of questions have been raised and answered. This paper summarizes these
potential issues and their resolutions. In particular, the paper takes an in-depth look at one of the questions: impact of
overlapping shots on heating effect. The paper concludes that while heating effect is an important issue for all e-beam
writing even with conventional non-overlapping shots, overall dose density per unit time over microns of space is the
principal driver behind heating effects. Highly local shot density and shot sequencing does not affect heating
significantly, particularly for smaller shots. MB-MDP does not introduce any additional concerns.
In electron beam writing on EUV mask, it has been reported that CD linearity does not show simple signatures as
observed with conventional COG (Cr on Glass) masks because they are caused by scattered electrons form EUV mask
itself which comprises stacked heavy metals and thick multi-layers. To resolve this issue, Mask Process Correction
(MPC) will be ideally applicable. Every pattern is reshaped in MPC. Therefore, the number of shots would not increase
and writing time will be kept within reasonable range. In this paper, MPC is extended to modeling for correction of CD
linearity errors on EUV mask. And its effectiveness is verified with simulations and experiments through actual writing
In order to support complex optical masks today and EUV masks in the near future, it is critical to correct mask
patterning errors with a magnitude of up to 20nm over a range of 2000nm at mask scale caused by short range mask
process proximity effects. A new mask process correction technology, MPC+, has been developed to achieve the target
requirements for the next generation node. In this paper, the accuracy and throughput performance of MPC+ technology
is evaluated using the most advanced mask writing tool, the EBM-7000<sup>1</sup>), and high quality mask metrology .
The accuracy of MPC+ is achieved by using a new comprehensive mask model. The results of through-pitch and
through-linewidth linearity curves and error statistics for multiple pattern layouts (including both 1D and 2D patterns)
are demonstrated and show post-correction accuracy of 2.34nm 3σ for through-pitch/through-linewidth linearity.
Implementing faster mask model simulation and more efficient correction recipes; full mask area (100cm<sup>2</sup>) processing
run time is less than 7 hours for 32nm half-pitch technology node.
From these results, it can be concluded that MPC+ with its higher precision and speed is a practical technology for the
32nm node and future technology generations, including EUV, when used with advance mask writing processes like the
Optical lithography is facing resolution limit. To overcome this issue, highly complicated patterns with high data volume
are being adopted for optical mask fabrications. With this background, new electron beam mask writing system, EBM-
7000 is developed to satisfy requirements of hp 32nm generation. Electron optical system with low aberrations is
developed to resolve finer patterns like 30nm L/S. In addition, high current density of 200 A/cm<sup>2</sup> is realized to avoid
writing time increase. In data path, distributed processing system is newly built to handle large amounts of data
efficiently. The data processing speed of 500MB/s, fast enough to process all the necessary data within exposure time in
parallel for hp32nm generation, is achieved. And this also makes it possible to handle such large volume dense data as
2G shots/mm<sup>2</sup> local pattern density.
In this paper, system configuration of EBM-7000 with accuracy data obtained are presented.
Nano-Imprint Lithography (NIL) is one of the leading potential solutions for next generation lithography. Obtaining full field template with fine pattern resolution and reasonable throughput are the critical challenges in NIL. In a previous study, we reported the pattern resolution capability of EBM-6000 under nominal operation conditions (Current density: 70 A/cm<sup>2</sup>) that can be applied to CMOS device fabrication of 45 nm hp generation1. Smaller blur for better resolution is necessary to make NIL templates for 32nm hp generation and beyond. Blur in patterning process can be suppressed with smaller process blur, smaller aberration of electron optics, smaller forward scattering in resist and coulomb interaction among electrons. Beam blur incurred by coulomb interaction among electrons in EBM-6000 can be reduced with lower current density. In this paper, resolution extendibility of EBM-6000 with lower current density (30 A/cm<sup>2</sup>) was tested as one of the resolution enhancement techniques. Smaller aberration of electron optics is also effective to improve the resolution. We also checked the resolution of EBM-7000 under nominal operation conditions (Current density: 200 A/cm<sup>2</sup>) for a basic study of this paper. EBM-7000, which was developed for mask fabrication of 32 nm hp generation and mask development of 22 nm hp generation, will keep using 50 kV acceleration voltage and enhanced electron optics with smaller aberration as compared with EBM-6000<sup>2</sup>.
Semiconductor scaling is expected to continue to hp32nm and beyond, accompanied by explosive data volume
expansion. Required minimum feature size at hp 32nm will be less than 50nm on the mask, according to ITRS2007<sup>(1)</sup>.
EBM 7000 is a newly designed mask writer for the hp32 nm node with an improved electron optical column providing
the beam resolution (10 nm measured in situ) and beam current density (200 A/cm<sup>2</sup>) necessary for cost effective mask
production at hp32nm node. In this paper we report on column improvements, the in situ beam blur measurement
method and writing results from EBM 7000. Written patterns show dose margin (CD change [nm] / 1 % dose change) of
.94 nm /1 % dose for line/space arrays using chemically amplified resist PRL009 and our standard processing. Using a
simple model to relate the measured beam intensity distribution to the measured dose margin, we infer an effective total
blur of 30 nm, dominated by a contribution of 28 nm from the resist exposure and development process. Further
evidence of the dominance of the process contribution is the measured improvement in dose margin to .64 nm/% dose
obtained by modifying our standard process. Even larger process improvements will be needed for successful fabrication
of hp22nm masks.
The impending need of double patterning/double exposure techniques is accelerating the demand for higher pattern
placement accuracy to be achieved in the upcoming lithography generations. One of the biggest error sources of pattern
placement accuracy on an EB mask writer is the resist charging effect. In this paper, we provide a model to describe the resist
charging behavior on a photomask written on our EBM-6000 system. We found this model was very effective in correcting
and reducing the beam position error induced by the charging effect.
Double pattering or exposure methodologies are being adopted to extend 193nm optical lithography. These
methodologies require much tighter image-placement accuracy and Critical Dimension (CD) controls on mask than the
conventional single exposure technique. Our experiments indicate that the global image placement drift induced by the
time elapsed in mask writing is the dominant factor that degrades image-placement accuracy. In-situ grid measurement
method is being proposed to suppress this time dependent drift. Resist charging effect is also an important error factor.
While it can be reduced by charge dissipation layer (CDL), further feasibility study is required for using CDL to
overcome certain side-effects pertaining to CDL. High dose resist improves local CD uniformity and pattern fidelity.
However, mask writing time becomes longer with lower sensitivity. To satisfy conflicting issues, throughput and CD
uniformity, high sensitivity CAR which has short acid diffusion length is desirable. Shortening acid diffusion length is
essential for achieving good pattern resolution as well as good CD uniformity. This paper will address the results of
error source analyses and key schemes of accuracy improvements in photo-mask manufacturing using NuFlare
Technology's EB mask writers.
Double exposure / Double pattering methodologies are being adopted to extend 193nm optical lithography until the next
generation lithography, most likely the EUV, is solidified. The Double exposure / Double patterning methodologies
require tighter image-placement accuracy and Critical Dimension (CD) controls on a mask than the conventional single
exposure technique. NuFlare Technology's mask writer, EBM-6000 (1), is capable of achieving the required CD control
and high patterning resolution as fine as 35 nm, that are required for the hp 45nm lithography with Double exposure /
Double patterning methodologies, when newly developed resist (i.e. "low-sensitivity" resist) is used, as shown at several
occasions to date. Further, image-placement control with EBM-6000 has been improved based on extensive error
budget analysis to comply with the tight image-placement specifications required by the Double exposure / Double
Patterning lithography. This paper will show the results of the analysis and improvement of the image-placement
accuracy of EBM-6000 series mask writers.
Heating effect was evaluated for EBM-6000 which is operated at high current density of 70A/cm<sup>2</sup> and acceleration
voltage of 50kV. FEP171 as widely used for current productions and lower sensitivity resists are tested. Lower
sensitivity resist is one of key items to achieve highly accurate Local critical dimension uniformity (LCDU) because of
shot noise reduction.
CD variations in experiment are compared with simulated temperature changes induced by heating effect. Then, the
ratio, ΔCD/ΔT, is found mostly constant for every resist, 0.1 nm/C°.
Writing conditions are estimated to meet CDU spec of hp45 generation for a worst case pattern, i.e. 100% density
pattern. For FEP171, the maximum shot size of 0.85 μm shot size at 2pass writing mode is sufficient. It should be
reduced to 0.5 μm at 2pass writing mode for every lower sensitivity resist. When 4pass writing mode is used, the
maximum shot size of 0.85 μm is available. Writing conditions and writing time for realistic patterns are also discussed.
EBM-5000 equipped with the new feature of high current density (50A/cm<sup>2</sup>) has been developed for 45 nm technology node (half pitch (hp) 65 nm). EBM-5000 adopts 50 kV variable shaped electron beam (VSB)/vector scan architecture and continuous motion stage, following the steps of preceding EBM series. In addition to the high current density, new technologies such as high resolution electron optics, finer increment for beam position and exposure time control, and new data format "VSB-12" to handle large data volume have been introduced on EBM-5000. These new technologies address two conflicting issues: improvement of throughput and better accuracy. This paper will report the key challenging technologies, certain results of EBM-5000 operation and findings obtained through our development efforts that can be applied to future generation tools. The fundamental local CD uniformity (LCDU) limit is also discussed.
The electron beam (EB) writing system with high acceleration voltage must be used for the mask fabrication because of its fine resolution. In this case, the resist heating effect becomes one of the serious problems in CD control. This paper discusses the controllability of the resist heating effect and shows that; (1) The CD variation caused by the effect increases with higher pattern coverage and larger shot size, which supports qualitatively results of temperature simulation based on Ralf's model. (2) The multiple exposure is effective to suppress the temperature rise in a substrate and the CD variation. The shifting-type exposure is more effective than the non-shifting-type exposure for suppression of the effect. (4) The CD variation for ZEP7000 can be suppressed to less than 5.0 [nm] (range) provided the shot size is less than or equal to 1.0 [micrometer] and the shifting-type exposure is adopted. Thus, the resist heating effect can be controlled and the CD variation by the effect can be suppressed enough for fabricating the masks to produce 0.15 micrometer devices and beyond.
Toshiba and Toshiba Machine have developed an advanced electron beam writing system EX-11 for next-generation mask fabrication. EX-11 is a 50 kV variable-shaped beam lithography system for manufacturing 4x masks for 0.15 - 0.18 micrometer technology generation. Many breakthroughs were studied and applied to EX-11 to meet future mask-fabrication requirements, such as critical dimension and positioning accuracy. We have verified the accuracy required for 0.15 - 0.18 micrometer generation.
Proximity effect correction is a key technology for fabricating reticles by electron beam writing systems. To write patterns of 1 Gbit or higher-capacity DRAMs, the dimensional accuracy required for the correction is better than about 10 nm. Conventional methods do not have sufficient accuracy at the position where pattern density changes sharply. We propose a new correction method with higher accuracy for various patterns and show that we can achieve corrections accurate to about 5 nm.