The production of high-resolution flat panel displays (FPDs) for mobile phones today requires the use
of high-quality large-size photomasks (LSPMs). Organic light emitting diode (OLED) displays use
several transistors on each pixel for precise current control and, as such, the mask patterns for OLED
displays are denser and finer than the patterns for the previous generation displays throughout the entire
mask surface. It is therefore strongly demanded that mask patterns be produced with high fidelity and
free of defect. To enable the production of a high quality LSPM in a short lead time, the manufacturers need
a high-sensitivity high-speed mask blank inspection system that meets the requirement of advanced LSPMs.
Lasertec has developed a large-size blank inspection system called LBIS, which achieves high
sensitivity based on a laser-scattering technique. LBIS employs a high power laser as its inspection light
source. LBIS’s delivery optics, including a scanner and F-Theta scan lens, focus the light from the source
linearly on the surface of the blank. Its specially-designed optics collect the light scattered by particles
and defects generated during the manufacturing process, such as scratches, on the surface and guide it to
photo multiplier tubes (PMTs) with high efficiency. Multiple PMTs are used on LBIS for the stable
detection of scattered light, which may be distributed at various angles due to irregular shapes of defects.
LBIS captures 0.3mμ PSL at a detection rate of over 99.5% with uniform sensitivity. Its inspection time
is 20 minutes for a G8 blank and 35 minutes for G10. The differential interference contrast (DIC)
microscope on the inspection head of LBIS captures high-contrast review images after inspection. The
images are classified automatically.
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.
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.
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.
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.
Extreme ultraviolet lithography (EUVL) is a leading technology to succeed optical lithography for high volume
production of 22 nm node and beyond. One of the top risks for EUVL is the readiness of defect-free masks, especially
the availability of Mo/Si mask blanks with acceptable defect level. Fast, accurate and repeatable defect inspection of
substrate and multi-layer (ML) blank is critical for process development by both blank suppliers and mask makers. In
this paper we report the results of performance improvements on a latest generation mask blank inspection tool from
Lasertec Corporation; the MAGICS M7360 at Intel Corporation's EUV Mask Pilot Line. Inspection repeatability and
sensitivity for both quartz substrates (Qz) and ML blanks are measured and compared with the previous Phase I tool
M7360. Preliminary results of high speed scan correction mirror implementation are also presented
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.
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.
Extreme ultraviolet lithography (EUVL) mask blanks must be free of printable defects. The SEMATECH Mask Blank
Development Center (MBDC) is focused on driving down the defect density of EUVL mask blanks by providing a
collaborative environment for EUVL mask substrate and equipment suppliers and a state-of-the-art analytical toolset for
them to improve their products. Multilayer (ML) coating, substrate cleaning, and substrate suppliers are on site
improving their products with a toolset that includes defect inspection, multilayer deposition, and substrate cleaning
capabilities. X-ray diffraction (XRD) and EUV reflectance measurement capability as well as focused ion beam
scanning electron microscopy/energy-dispersive X-ray (FIBSEM/EDX) and atomic force microscopy (AFM) for defect
characterization are on site.
The SEMATECH MBDC has just installed a Lasertec M7360, an advanced EUV mask blank inspection tool. The
M7360 operates at a much shorter wavelength than the previous generation of confocal scanning inspection tools
(266nm vs. 488nm for the M1350). The M7360 represents a significant improvement in our defect detection
sensitivity. This paper will center on the capabilities of this new tool and show initial inspection results on EUV
multilayer at sensitivities well below those that have been previously reported.
The ability of a confocal microscope to inspect for defects on EUVL mask blanks has been investigated both experimentally and theoretically. A model was developed to predict the image contrast of confocal microscope. Measurements were made on PSL spheres and programmed multilayer defects using a Lasertec M1350 operating with 488 nm light. The images obtained of PSL spheres on both fused silica and multilayer-coated blanks are found to be accurately predicted with the model using no adjustable parameters. Good agreement is also demonstrated for the modeling of multilayer defects. Predictions are made for the expected increase in contrast at the shorter wavelength of 266 nm. Substrate roughness contributes to the "noise" which limits the sensitivity to small defects. The contrast fluctuations due to roughness have been modeled using a simple single surface approximation. The model has been validated with measurements on substrates with varying degrees of roughness. The contribution of mask roughness to the sensitivity of a 266 nm tool is estimated.
Direct phase-shift measurement is one of the key technologies to realize Phase-Shift-Mask (PSM) application. Most mask makers are developing practical PSMs for 157nm lithography. Final tuning of the optical parameters and quality assurance of them require accurate measurement tool of phase-shift and transmittance with 157nm light
illumination. In this paper, we will report the development of the system, which measures the phase-shift and transmittance of 157nm PSM at wavelength. This system has a 157nm F<sub>2</sub> laser as a light source of the illumination and CaF<sub>2</sub> optics with a CCD camera for the imaging. Key component is the interferometer, which has a function of lateral image shearing and phase modulation. The same technology is used in the current UV and DUV tools already exist. N<sub>2</sub> purge and vacuum environments are newly introduced for the optical path to minimize attenuation of 157nm light by O<sub>2</sub> and H<sub>2</sub>O. A fluctuation of the attenuation in the optical path significantly affects the short-term measurement repeatability. A new measurement algorithm, which uses two measurement spots on a PSM image, gives better repeatability than using single measurement spot under such unstable condition. Because most fluctuations are common elements to both of the two spots, they can be canceled out by the new calculation algorithms for phase-shift and transmittance measurements. The system with new techniques shows enough performance for the requirement of 157nm PSM measurements with new techniques.
One of the key challenges for the successful implementation of EUV Lithography (EUVL) is the supply of defect free mask blanks. Obviously a reliable defect inspection is a prerequisite to achieve this goal. We report results from a EUVL blank inspection tool developed by Lasertec. The inspection principle of this tool is
based on confocal microscopy at 488nm inspection wavelength. On quartz substrates a sensitivity of 60nm is demonstrated. On buried defects in the multilayer stack a reasonable capture rate down to approximately 25nm defect height has been measured. We compare these results to previously reported data on the wafer version
(M350) of the current M1350.
Transmittance measurement of small object such as Ga-stain of repair or particle on photomask is getting to be important. This paper. describes the characteristic of transmittance measurement with a shearing interferometer microscope comparing with a conventional method. Measurement wavelength are 436, 365 and 248nm. In this system the transmittance is calculated from interference signal amplitude that is free from a flare light caused by reflection of optical parts.
This paper describes a direct phase-shift measurement system with transmitted deep-UV illumination for phase shifting mask (PSM) using a lateral shearing interferometer system. This interferometer has new structure developed for this purpose. The mirror mount of the interferometer is made of SiC ceramics that promote stability against vibration and ambient temperature drift. The illumination employs a xenon mercury arc lamp that has a spectrum close to the wavelength of KrF excimer laser. The repeatability of measurements is 0.5 degree in 3 sigma. The system can measure a small pattern down to 1 μm with an alternating type PSM with the objective of N.A.=0.4. Influence of incident angle of illumination on phase-shift measurement is investigated by experiment. The results show similar effects with simulation for circular illumination. The phase-shift measurement results on quartz step meet well with a calculation from step height and known refractive index including the effect of incident angle of illumination. The deep-UV measurement results also have good correlation with calculations from the results with another direct phase-shift measurement system that wavelength is 365nm. The simulation for focus latitude of alternating type PSMs agree with the experimental results of wafer exposure and the phase measurement. The accuracy of this system is sufficient for application to development of phase shift mask process.
The performance of an i-/g-line direct-phase measurement system Lasertec MPM- 100 has been evaluated. The minimum measurable pattern sizes is 2.5 .μm for holes on an 8%-i-line transmittance halftone phase shift masks (HPSMs). The effect of the focus position is not significant for hole pattern of above 3.5 μm. Both short-term repeatability and long-term stability are excellent, being less than 0.5 deg. The effect of the illumination NA has been investigated theoretically and experimentally, and the use of correction factors based on experiment is proposed for estimating effective phase shifts from phase shifts obtained by MPM- 100.
This paper describes a direct phase measurement system with transmitted UV-light for phase shifting mask (PSM) inspection using a shearing interferometer microscope. Measurements were made with 365 nm monochromatic light of mercury arc lamp. The accuracy of this system is sufficient for the application for phase shifting mask inspection. The measurement results are in good agreement with the calculation based on quartz step height measurement and refractive index. Wafer exposure results of attenuating-type PSM also agree with the phase measurement results.
Attenuated phase-shifting mask with a single-layer absorptive shifter of CrO, CrON, MoSiO or MoSiON films has been developed. The optical parameter of these films can be controlled by the condition of sputtering deposition. These films satisfy the shifter requirements, both the 180-degrees phase shift and the transmittance between 5 and 20% for i-line. MoSiO and MoSiON films also satisfy the requirement for KrF excimer laser light. Conventional mask processes, such as etching, cleaning, defect inspection and defect repair, can be used for the mask fabrication. Defect-free masks for hole layers of 64 M-bit DRAM are obtained. Using this mask, the focus depth of 0.35-micrometers hole is improved from 0.6 micrometers to 1.5 micrometers for i-line lithography. The printing of 0.2-micrometers hole patterns is achieved by the combination of this mask and KrF excimer laser lithography.
Phase-shifting needs the critical dimension (CD) accuracy to be less than 0.05 micrometers for the metal and shifter pattern on a phase-shifting mask. Thus we have investigated a new etching process using magnetically enhanced reactive ion etching (MERIE). A magnetic field was provided by two pairs of solenoid coils outside the chamber. By using this MERIE system, the etching characteristics of chromium (Cr) and spin on glass (SOG) were evaluated. A Cl<SUB>2</SUB> and O<SUB>2</SUB> gas mixture was used for Cr etching. The etching selectivity had a maximum when the concentration of O<SUB>2</SUB> was 20%. The etching selectivity increased with an increase in the magnetic field and gas pressure as well as with a decrease in the rf power. High etching selectivity and anisotropic etching features were obtained when the magnetic field was 100 G, the gas pressure 10 - 30 Pa, and the rf power density 0.18 - 0.22 W/cm<SUP>2</SUP>. Phase-shifting masks fabricated with this system show a CD accuracy of better than 0.05 micrometers , so 64 MB DRAM phase-shifting masks can be successfully fabricated with this MERIE system.
In this new process for phase-shifting mask fabrication, molybdenum silicide (MoSi) is used as an optical shield layer and spin-on glass (SOG) as a phase-shifter layer. Chromium is employed as an etch-stopper during SOG etching. Cr etch-stopper will be removed at the end of tiie process, therefore all optical problems related to an etch-stopper are avoided. This Cr etch-stopper is also useful in inspection and repair of shifter remaining defects. At first, we will describe the fabrication process including the shifter-defect inspection and repair. Secondary, we will discuss the phase-shifting mask accuracy and its influence to the printed resist pattern when using the alternating type phase-shifting mask. Lastly,we will mention the application result of development of lithography for 64Mbit DRAM using this process.
By using two-dimensional simulation, dependence of light intensity contrast on numerical aperture (NA),
coherence factor (a) of i-line stepper, shifter-width, phase error and slope angle of phase shifter edges have been
investigated. For the slope angle of 90° and the shifter phase of 180°, the highest contrast is obtained for NA=0.65 and a=0.3. As the slope angle becomes to be small, contrast degrades remarkably for high NA(=0.65). On these simulation, 0.18pm resolution limit of isolated space pattern is successfully realized using an image reversal resist process.