As extreme ultraviolet lithography (EUVL) enters high volume manufacturing (HVM), the integrated circuit (IC) industry considers actinic patterned mask inspection (APMI) to be the last major EUV mask infrastructure gap. For over 20 years, there have been calls for an APMI tool for both the final qualification of EUV masks in the mask shop and for the requalification of EUV masks in the wafer fab1. Actinic, in this context, is matching the 13.5 nm scanner wavelength to that of the inspection tool so that all types of EUV mask defects can be detected. In order to enable EUVL HVM, we have developed and introduced the world’s first commercially available APMI tool. Actinic inspection enables HVM EUVL by ensuring that the EUV mask going to the EUV scanner is free from EUVprintable defects that may have been overlooked during EUV blank manufacturing or occurred during EUV mask manufacturing, cleaning and use. In this paper we will review EUV mask defect requirements from the maskshop and fab perspective, as well as capabilities of different inspection methods available for HVM. Further, we will provide an overview of the history of APMI tool development and highlight challenges and successes made when designing major components for the tool. APMI enables reliable detection of all classes of EUV-printable mask defects: small absorber defects, phase and amplitude defects in the multi-layer, In this paper, inspection performance of the APMI tool will be reviewed using representative cases from programmed defect masks with designs resembling real production cases. Finally, we will provide an outlook for the next steps in tool development including Die-to-Database inspection, throughpellicle inspection and platform extendibility to high NA EUVL.
As extreme ultraviolet (EUV) lithography enters high volume manufacturing, the semiconductor industry has considered a lithography-wavelength-matched actinic patterned mask inspection (APMI) tool to be a major remaining EUV mask infrastructure gap. Now, an actinic patterned mask inspection system has been developed to fill this gap. Combining experience gained from developing and commercializing the 13.5nm wavelength actinic blank inspection (ABI) system with decades of deep ultraviolet (DUV) patterned mask defect inspection system manufacturing, we have introduced the world’s first high-sensitivity actinic patterned mask inspection and review system, the ACTIS A150 (ACTinic Inspection System). Producing this APMI system required developing and implementing new technologies including a high-intensity EUV source and high-numerical aperture EUV optics. The APMI system achieves extremely high sensitivity to defects because of its high-resolution, low noise imaging. It has demonstrated a capability to detect mask defects having an estimated lithographic impact of 10% CD deviation on the printed wafer.
Since defense technologies greatly depend on Low Earth Orbit (LEO) military surveillance satellites, anti-satellite (ASAT) operation is the first reaction after a war breaks out. Surveillance satellites with the altitude of <400 km can be small and low cost, which realizes many deployments like drones in the sky. Therefore, ASAT operation targeting such a low-cost satellite by a multi-stage missile is costly. This gives an expectation for a high energy laser (HEL) as an ASAT weapon. However, the use of a ground-based laser (GBL) is limited to use only in clear sky conditions. A space-based laser (SBL) seems to realize an ideal ASAT weapon, but many SBLs are necessary to disable even a few satellites because the SBLs need to go around the Earth. Meanwhile a ballistic-missile defense system using a Chemical Oxygen-Iodine Laser (COIL) carried by a high-altitude airship (HAA) was proposed to intercept a ballistic missile. Since an HAA can always stay in a clear sky, it should also be used for ASAT operations. Only a single system can disable many satellites since it can stay at a fixed location. The beam profile is simulated to estimate the beam intensity at the target satellite. Since the laser beam propagates upward obliquely to the target, altitude-dependent atmospheric turbulence must be considered. The calculation results show that the beam intensity can be high enough to damage the solar panels of the satellite.
New designs of a defense system using a chemical oxygen iodine laser (COIL) are presented to realize a boost-phase interception of a ballistic missile. Although a space-based laser (SBL), in which the Hydrogen-Fluoride chemical laser is the primary candidate, can realize such a defense system, many SBLs are necessary to cover even a single missile site because they need to continuously go around the earth. This is an expensive system if the potential enemy is a small country. Meanwhile a high energy laser (HEL) carried by a high-altitude airship (HAA) can realize a geostationary defense system if the HEL is quite lightweight. A chemical oxygen-iodine laser (COIL) is suitable since it does not require a heavy electric power supply. But since the COIL should be as light as possible, it would be more advantageous if it can operate without a vacuum pump that requires a large electric-power supply and cooling water. Rate-equation based simulations have been performed to see if it can operate without a vacuum pump by filling a buffer gas at the pressure higher than the outside. The simulation results indicate that it can operate continuously at an altitude of 20 km where the atmospheric pressure becomes ~5,400 Pa (~40 Torr). Moreover, since atmospheric turbulence is greatly reduced at that altitude, adaptive optics is also not necessary for focusing the beam after a long propagation. A simple focusing mirror can focus the beam tightly enough to destroy the target of >100 km away.
A new concept of a space-based-laser (SBL) defense system is proposed. It is based on a chemical oxygen laser
(COL) which has been investigated to achieve its oscillation 1-3). A COL is suitable as a high energy laser (HEL)
directed energy weapon (DEW) 4) because it could produce a giant pulse of ~0.1 ms which can damage a target by a
single shot without producing plasma during the propagation. However since the beam cannot propagate for a long
distance due to the absorption in air, it should be used in space considering the capability of operation without
electric power supply. Therefore a new SBL defense system using a COL is proposed in order to destroy a ballistic
missile in its boost phase. It is based on an SBL at geostationary Earth orbit (GEO) with the altitude of ~36,000 km.
Since the beam needs to propagate for a long distance, the focused beam diameter is ~8 m even if the initial beam
diameter is 8 m. Therefore an 8 m-diameter focusing mirror, carried by a high altitude airship (HAA) flying at the
altitude of more than 20 km, could be used to focus the beam at the target. Although such a large focusing mirror is
necessary, the focused spot size can be <1 cm at 30 km away. Thus, much less than 100 kJ pulse can cause a fatal
damage. Unlike a conventional SBL defense system based on SBLs and/or relay-mirror satellites in low Earth orbit
(LEO), the new defense system needs only a single SBL and a single relay mirror HAA (RM HAA) to intercept a
ballistic missile if the enemy is a small country since the HAA can always stay close to the enemy’s missile site.
Another concept of the defense system is also proposed, which is based on a COL equipped with anther HAA
because a COL can be lightweight. These geostationary defense systems can also intercept a submarine-launched
ballistic missile (SLBM) if the submarine’s location is monitored.
Improvements in the detection capability of a high-volume-manufacturing (HVM) actinic blank inspection (ABI) prototype for native defects caused by illumination numerical aperture (NA) enlargement were evaluated. A mask blank was inspected by varying the illumination NA. The defect signal intensity increased with illumination NA enlargement as predicted from simulation. The mask blank was also inspected with optical tools, and no additional phase defect was detected. All of the printable phase defects were verified to have been detected by the HVM ABI prototype.
Ship defense system with a pulsed COIL (Chemical Oxygen-Iodine Laser) has been considered. One of the greatest
threats for battle ships and carriers in warfare are supersonic anti-ship cruise missiles (ASCMs). A countermeasure is
considered to be a supersonic RAM (Rolling Airframe Missile) at first. A gun-type CIWS (Close-In Weapon System)
should be used as the last line of defense. However since an ASCM can be detected at only 30-50km away due to radar
horizon, a speed-of-light weapon is desirable as the first defense especially if the ASCM flies at >Mach 6. Our previous
report explained several advantages of a giant pulse from a chemical oxygen laser (COL) to shoot down supersonic
aircrafts. Since the first defense has the target distance of ~30km, the use of COIL is better considering its beam having
high transmissivity in air. Therefore efficient operation of a giant-pulsed COIL has been investigated with rate-equation
simulations. The simulation results indicate that efficient single-pass amplification can be expected. Also a design
example of a giant-pulsed COIL MOPA (master oscillator and power amplifier) system has been shown, in which the
output energy can be increased without limit.
A high volume manufacturing (HVM) model of EUV Actinic Blank Inspection (ABI) tool has been developed for the purpose of detecting phase defects on EUV masks. Simulation has been carried out as to how defect aspect ratio (height/width) and illumination numerical aperture (NA) affect defect signal intensity (DSI). It shows that a higher illumination NA leads to a higher DSI for defects with low-aspect ratios. For example, if the illumination NA is changed from 0.07 to 0.1, DSI is expected to increase 20% or more for defects with an aspect ratio lower than 0.015. The ABI tool has shown an enhanced sensitivity, especially for low-aspect ratio defects, after its NA illumination is raised from its original 0.07 NA to 0.1 NA. Actual inspection results using programmed-defect masks show that DSI has increased significantly for defects with low aspect ratios while the signal intensities for defects with high aspect ratios remain the same.
Defense application for a chemical oxygen laser (COL) is explained. Although a COL has not yet been successful in lasing, the oscillator was estimated to produce a giant pulse with the full width at half maximum (FWHM) of ~0.05ms which makes the damage threshold for the mirrors several-order higher than that for a typical solid-state laser with a ~10ns pulse width. Therefore it has a potential to produce MJ class output considering the simple scalability of being a chemical laser. Since within 0.05ms a supersonic aircraft can move only a few centimeters which is roughly equal to the spot size of the focused beam at ~10km away using a large-diameter focusing mirror, a COL has a potential to make a damage to an enemy aircraft by a single shot without beam tracking. But since the extracted beam can propagate up to a few kilometers due to the absorption in the air, it may be suitable to use in space. While a chemical oxygen-iodine laser (COIL) can give a pulsed output with a width of ~2 ms using a high-pressure singlet oxygen generator (SOG). Therefore a pulsed COIL may also not require beam tracking if a target aircraft is approaching. Another advantage for these pulsed high-energy lasers (HELs) is that, in case of propagating in cloud or fog, much less energy is required for a laser for aerosol vaporization (LAV) than that of a LAV for a CW HEL. Considerations to use a COL as a directed energy weapon (DEW) in a point defense system are shown.
New concepts are presented to realize a chemical oxygen laser (COL) based on the transition from O2(1Δg) to O2 (3Σg).
The chemical oxygen iodine laser (COIL) utilizes the energy transfer from the chemically generated O2(1Δg) to iodine I (2P3/2) because the stimulated emission cross section of O2(1Δg) is too small to give a direct oscillation. But since extractable laser energy has no relation to the stimulated emission cross section, a COL has a potential to produce a high energy laser output if it has a long enough active medium to give a positive gain. The intrinsically long upper-state life time enables the storage of large energy, which has a potential give a giant pulsed laser. Since the previous report elucidated the problems 1), the proposed concepts are based on the consideration of them. Also a Q switched COL oscillator is simulated with a rate equation. The simulation results show that a giant pulse of ~0.05ms width can be obtained with the extraction efficiency of 10-20%.
A high-volume manufacturing (HVM) actinic blank inspection (ABI) prototype has been developed, of which the inspection capability for a native defect was evaluated. An analysis of defect signal intensity (DSI) analysis showed that the DSI varied as a result of mask surface roughness. Operating the ABI under a review mode reduced that variation by 71 %, and therefore this operation was made available for precise DSI evaluation. The result also indicated that the defect capture rate was influenced by the DSI variation caused by mask surface roughness. A mask blank was inspected three times by the HVM ABI prototype, and impact of the detected native defects on wafer CD was evaluated. There was observed a pronounced relationship between the DSI and wafer CD; and this means that the ABI tool could detect wafer printable defects. Using the total DSI variation, the capture rate of the smallest defect critical for 16 nm node was estimated to be 93.2 %. This means that most of the critical defects for 16 nm node can be detected with the HVM ABI prototype.
While extreme ultraviolet lithography (EUVL) is the leading candidate of the next generation lithography, the challenge of managing blank defects must be overcome before EUVL being put to practical use. Besides the efforts of manufacturing defect free blanks, the use of mitigation technique called “pattern shift” is now considered to be a more feasible solution. Whether we aim for defect free blanks or use pattern shift, however, it is quite important to understand the properties of the defects on EUV masks. Of particular interest is to distinguish phase defects from amplitude defects, and pits from bumps. To address the need to understand defect properties, the Actinic Blank Inspection (ABI) high volume manufacturing (HVM) model has acquired a review function using a 1200x magnification optics capable of accurately measuring the size and shape of defects. In this paper, we will discuss how the ABI HVM model classifies defects into pits and bumps.
EUVL scanner throughputs are calculated considering a higher mask magnification. The calculation
results show that the throughput of 8X mask system is 60-70% of that of 4X mask system. However the
relative throughput compared to the 4X is higher if the duty cycle is considered as the input EUV power.
The throughput is also estimated considering a 450mm wafer. Additionally the throughput for a twin
reticle stage system using two 8X 6” masks is estimated for the case of stitching exposure.
One of the most challenging tasks to make EUVL (Extreme Ultra Violet Lithography) a reality is to achieve zero
defects for mask blanks. However, since it is uncertain whether mask blanks can be made completely defect-free, defect
mitigation schemes are considered crucial for realization of EUVL. One of the mitigation schemes, pattern shift, covers
ML defects under absorber patterns by device pattern adjustment and prevents the defects from being printed onto wafers.
This scheme, however, requires accurate defect locations, and blank inspection tools must be able to provide the
locations within a margin of the error of tens of nanometers. In this paper we describe a high accuracy defect locating
function of the EUV Actinic Blank Inspection (ABI) tool being developed for HVM hp16 nm and 11 nm nodes.
The availability of actinic blank inspection is one of the key milestones for EUV lithography on the way to high volume
manufacturing. Placed at the very beginning of the mask manufacturing flow, blank inspection delivers the most critical data set for the judgment of the initial blank quality and final mask performance. From all actinic metrology tools proposed and discussed over the last years, actinic blank inspection (ABI) tool is the first one to reach the pre-production status. In this paper we give an overview of EIDEC-Lasertec ABI program, provide a description of the system and share the most recent performance test results of the tool for 16 nm technology node.
Because the realization of defect-free Extreme Ultra-violet Lithography (EUVL) mask blanks is uncertain, the defect
mitigation techniques are becoming quite important. One mitigation technique, "Pattern shift", is a technique that places a
device pattern to cover multilayer (ML) defects underneath the absorber pattern in such a way that the ML defects are not
printed onto wafers. This mitigation method requires the defect coordinate accuracy of down to tens of nanometers.
Consequently, there is a strong demand for a Blank Inspection tool that is capable of providing such defect coordinate
To meet such requirement, we have started to develop a high accuracy defect locating function as an optional feature to
our EUV Actinic Blank Inspection (ABI) system which is currently being developed aiming at HVM hp16 nm-11 nm node.
Since a 26x Schwarzschild optics is used in this inspection tool, it is quite difficult to pinpoint defect location with high
accuracy. Therefore we have decided to realize a high magnification review optics of 600x or higher by adding two mirrors
to the Schwarzschild optics. One of the additional two mirrors is retractable so that the magnification can be switched
according to the purpose of inspections. The high magnification review mode locates defect coordinates accurately with
respect to the fiducial position. We set the accuracy target at 20 nm so that the mitigation technique can be implemented
successfully. The optical configuration proposed in this paper allows both a high speed inspection for HVM and a high
accuracy defect locating function to be achieved on one inspection system.
A new direct Phase-shift/Transmittance measurement tool "MPM193EX" has been developed to respond to the
growing demand for higher precision measurements of finer patterns in ArF Lithography. Specifications of MPM193EX
are listed below along with corresponding specifications of the conventional tool MPM193.
1) Phase-shift [3 Sigma]: 0.5 deg. (MPM193) => 0.2 deg. (MPM193EX)
2) Transmittance [3 Sigma]: 0.20 % (MPM193) => 0.04 % (MPM193EX)
3) Minimum measurement pattern width: 7.5 μm (MPM193) => 1.0 μm (MPM193EX)
Furthermore, new design optics using an ArF Laser and an objective lens with long working distance allows
measurements of masks with pellicles.
The new method for improving the measurement repeatability is based on elimination of influence from instantaneous
fluctuation in interferometer fringes by scanning two adjacent areas simultaneously. Also, MPM193EX is equipped with
high-resolution and stable optics. The newly employed auto-focus system in MPM193EX accurately adjusts, by a new
image processing method using high-resolution optics, the focus height that is one of the most important factors for
measurements in a micro pattern.
Considering the usage extension of ArF immersion lithography down to the node of hp 22nm, EUVL
should be able to cope with at least the 16 nm and preferably the 11 nm nodes. However, numerical
aperture (NA) of projection optics in EUVL exposure tools needs to be around 0.4 which requires the
oblique angle of around 8 degrees for the illumination. When one considers that the thickness of
multilayer is around 280 nm, the reflectivity drop area at the edge of absorbers has the width of
approximately 40 nm that corresponds to the width of 10 nm on wafer. There is another serious problem.
Particle check will become extremely difficult for a pellicle-less EUVL mask with smaller feature size.
Killer particles cannot be detected by an optical inspection system any longer at 22 nm and smaller. In
consideration of all these serious problems, the only simple solution is to increase the mask
magnification factor from the conventional 4X to 8X.1-4) Although is has been reported that the
throughput of an 8X exposure too is 30-70% compared with that of 4X,5X the throughput can be increased
to be equal or even higher than that of 4X when 450 nm wafer is applied.
We demonstrate a new technique for improvement of the flatness of the EUV mask substrate by using a pulsed laser.
Laser pulses from an ArF excimer laser were focused inside a quartz mask substrate to make spots. Experiments
showed that the substrate surface was locally swelled out where spots were formed just beneath the surface without
making any damages on the surface. This surface shape control technique can be applied to the final adjustment of the
substrate flatness control since no cleaning process is necessary afterward.
Although EUVL is widely considered to be used from the node of hp32nm, there are serious problems. One of them is the defect problem at the mask blank. The defect concentration increases rapidly as its size becomes smaller, but the current defect level of around 0.1/cm2(>100nm) is orders of greater than the required level of 0.005/cm2(>32nm). Also the present defect detection limit of around 50nm is much greater than the required defect size of 23nm for hp32nm and 16nm for hp22nm. Therefore 8X mask, having double-larger patterns, is helpful because of its double-larger size for the required minimum defect. Moreover the double-larger patterns have much fewer killer particles, which is also helpful for the no-pellicle mask. However changing the mask magnification to 8X has been reported to decrease the exposure-tool throughput to around 40% of that of 4X. Since this decreased throughput was estimated for KrF/ArF, and not for EUVL, throughputs of an 8X EUVL scanner are calculated. The calculated results have cleared that an 8X scanner can give around 64% of the 4X throughput, and that a 9” 8X scanner and a double-long 8X scanner can give 89-98% of the 4X throughput at the resist sensitivity of 5mJ/cm2. This is due to a double-higher scan speed obtained by 8X. Another advantage by changing to 8X is the smaller line edge shift by the shadowing effect to 1/2-1/3 because of the higher magnification and the possibility of decreasing the illumination incident angle. Similarly the mask surface flatness requirement can be loosened by 2-3 times.
A new idea of writing a PXL (Proximity X-ray Lithography) mask is presented, in which a EUVL (extreme ultraviolet lithography) exposure tool is used as a mask repeater. EUV power of less than 1W is enough to write a PXL mask within 5 minutes, and an expensive EUV mask blank can be recycled because the mother mask is not necessary once a PXL mask is written. A EUV mask repeater especially consisting of a high-NA Micro Exposure Tool (MET) makes it possible to write a PXL mask for the 32 nm nodes and after. The new system can also be applied to other lithography tools using a 1X: such as LEEPL (Low Energy E-Beam Proximity Lithography) and imprint lithography.
The fluorine molecular laser is a very promising light source for the next generation of optical microlithography below 100 nm. The fluorine laser we developed uses a new, all solid-state pulse power module, that generates an output energy of 6 J/pulse, and an optimized RF pre-ionization. At 2000Hz, 11 mJ/pulse have been measured. Single line oscillation at 157.6299nm was obtained using prisms. Fluorine laser spectra have been measured with a high- resolution VUV spectrometer. The convoluted bandwidth was 1.08pm for 0.1 percent /balance F2/He and a total pressure of 3000 hPa. Currently, we are investigating Ultra Narrow fluorine lasers with a bandwidth below 0.2pm. This laser is aimed for exposure tools using refractive projection optics at 157nm. Evaluation tools for optical materials and coatings have also been developed. The temporal transmittance during 157 nm laser irradiation and the transmittance between the DUV and VUV region directly after laser irradiation can be measured. We have successfully demonstrated the potential of the molecular fluorine laser for microlithography and a first generation laser for 157 nm exposure tools is almost ready.