We report on advanced defect classification using TNO’s RapidNano particle scanner. RapidNano was originally designed for defect detection on blank substrates. In detection-mode, the RapidNano signal from nine azimuth angles is added for sensitivity. In review-mode signals from individual angles are analyzed to derive additional defect properties. We define the Fourier coefficient parameter space that is useful to study the statistical variation in defect types on a sample. By selecting defects from each defect type for further review by SEM, information on all defects can be obtained efficiently.
Particle defects are important contributors to yield loss in semi-conductor manufacturing. Particles need to be detected
and characterized in order to determine and eliminate their root cause. We have conceived a process flow for advanced
defect classification (ADC) that distinguishes three consecutive steps; detection, review and classification. For defect
detection, TNO has developed the Rapid Nano (RN3) particle scanner, which illuminates the sample from nine azimuth
angles. The RN3 is capable of detecting 42 nm Latex Sphere Equivalent (LSE) particles on XXX-flat Silicon wafers. For
each sample, the lower detection limit (LDL) can be verified by an analysis of the speckle signal, which originates from
the surface roughness of the substrate. In detection-mode (RN3.1), the signal from all illumination angles is added. In
review-mode (RN3.9), the signals from all nine arms are recorded individually and analyzed in order to retrieve
additional information on the shape and size of deep sub-wavelength defects. This paper presents experimental and
modelling results on the extraction of shape information from the RN3.9 multi-azimuth signal such as aspect ratio,
skewness, and orientation of test defects.
Both modeling and experimental work confirm that the RN3.9 signal contains detailed defect shape information. After
review by RN3.9, defects are coarsely classified, yielding a purified Defect-of-Interest (DoI) list for further analysis on
slower metrology tools, such as SEM, AFM or HIM, that provide more detailed review data and further classification.
Purifying the DoI list via optical metrology with RN3.9 will make inspection time on slower review tools more efficient.
The Rapid Nano is a particle inspection system developed by TNO for the qualification of EUV reticle handling equipment. The sensitivity of this system has been improved by model based design. Our model identified two parameters that could be tuned to be able to detect smaller particles. The first step is a multi azimuth illumination mode and the second parameter is the illumination wavelength. Here we report on the results of the Rapid Nano 4, which has both of these parameters optimized to have a sub 20 nm LSE detection limit on EUV mask blanks.
is a well-known detection method which is applied in many different scientific and technology domains including atmospheric physics, environmental control, and biology. It allows contactless and remote detection of sub-micron size particles. However, methods for detecting a single fast moving particle smaller than 100 nm are lacking.
In the present work we report a preliminary design study of an inline large area detector for nanoparticles larger than 50 nm which move with velocities up to 100 m/s. The detector design is based on light scattering using commercially available components.
The presented design takes into account all challenges connected to the inline implementation of the scattering technique in the system: the need for the detector to have a large field of view to cover a volume with a footprint commensurate to an area of 100mm x 100mm, the necessity to sense nanoparticles transported at high velocity, and the requirement of large capture rate with a false detection as low as one false positive per week. The impact of all these stringent requirements on the expected sensitivity and performances of the device is analyzed by mean of a dedicated performance model.
Particle free handling of EUV reticles is a major concern in industry. For reaching economically feasible yield levels, it is reported that Particle-per-Reticle-Pass (PRP) levels should be better than 0.0001 for particles larger than 18 nm. Such cleanliness levels are yet to be reported for current reticle handling systems. A reticle handler was built based on a modular concept with three uniform linked base frames. In the first stage of the project a dual pod loading unit, two exchange units for opening inner pods and a reticle flip unit are installed on the base frames. In the near future improvements on cleanliness will be tested and particle detection equipment will be integrated. The system will act as a testing platform for clean handling technology for industry.
The background in simple dark field particle inspection shows a high scatter variance which cannot be distinguished from signals by small particles. According to our models, illumination from different azimuths can reduce the background variance. A multi-azimuth illumination has been successfully integrated on the Rapid Nano particle scanner. This illumination method reduces the variance of the background scattering on substrate roughness. It allows for a lower setting of the detection threshold, resulting in a more sensitive inspection system. By implementing this system the lower detection limit of the scanner was reduced from 59 nm to 42 nm LSE. A next improvement, a change of the inspection wavelength to 193 nm will bring the detection limit to sub 20 nm.
In dark-field particle inspection, the limiting factor for sensitivity is the amount of background scatter due to substrate roughness. This scatter forms a speckle pattern and shows an intensity distribution with a long tail. To reduce falsepositives to an acceptable level, a high detection threshold should be chosen such that the tail of the background distribution is avoided. We have modeled an optimized illumination mode, that reduces the variance in the background distribution. This illumination mode illuminates the substrate from multiple azimuth angles. We show that the speckle patterns generated by each azimuth angle can be independent from each other. Therefore by combining the angles, the variance of the background signal is reduced. We show that for the parameters of our inspection system the detection threshold can be reduced by a factor three, resulting in a lower detection limit that is 20% smaller in particle size. The change in the background scattering distribution was confirmed by experiments.
TNO has developed the Rapid Nano scanner to detect nanoparticles on EUVL mask blanks. This scanner was designed to be used in particle qualifications of EUV reticle handling equipment. In this paper we present an end-to-end model of the Rapid Nano detection process. All important design parameters concerning illumination, detection and noise are included in the model. The prediction from the model matches the performance that was experimentally determined (59 nm LSE). The model will be used to design and predict the performance of future generations of particle scanners.
With the introduction of 450 mm wafers, which are considerably larger than the currently largest wafers of 300mm,
handling with side grippers is no longer possible and backside grippers are required. Backside gripping increases the
possible buildup of particles on the backside of the wafers with possible cross-contamination to the front-side. Therefore,
regular backside cleaning is required. Three vacuum compatible cleaning methods were selected. Tacky rollers and highvoltage
cleaning were selected for particles and plasma cleaning for molecular layers. A test-bench was designed and
constructed implementing these three cleaning methods. The first experiments show promising results for the plasma
cleaner and the tacky roller.
The introduction of EUV Lithography for the next node has two major obstacles at the moment; the first is source power
and reliability and the second is defect free reticles and damage free cleaning of reticles. We present our results on our
investigation for damage free cleaning of EUV reticles with remote plasma cleaning for molecular (carbon)
contamination and nanobubbles for particle removal. We believe that a multi step approach is necessary for cleaning of
reticles as a single cleaning step will not be sufficient for the efficient removal of molecular as well as particle
contamination. Remote plasma seems to be the favorable technique for carbon cleaning and repeated cleaning up to 85
nm of carbon removal shows no degradation of the reticle material.
With the market introduction of the NXE:3100, Extreme Ultra Violet Lithography (EUVL) enters a new stage. Now
infrastructure in the wafer fabs must be prepared for new processes and new materials. Especially the infrastructure for
masks poses a challenge. Because of the absence of a pellicle reticle front sides are exceptionally vulnerable to particles.
It was also shown that particles on the backside of a reticle may cause tool down time. These effects set extreme
requirements to the cleanliness level of the fab infrastructure for EUV masks. The cost of EUV masks justifies the use of
equipment that is qualified on particle cleanliness.
Until now equipment qualification on particle cleanliness have not been carried out with statistically based qualification
procedures. Since we are dealing with extreme clean equipment the number of observed particles is expected to be very
low. These particle levels can only be measured by repetitively cycling a mask substrate in the equipment. Recent work
in the EUV AD-tool presents data on added particles during load/unload cycles, reported as number of Particles per
Reticle Pass (PRP). In the interpretation of the data, variation by deposition statistics is not taken into account. In
measurements with low numbers of added particles the standard deviation in PRP number can be large.
An additional issue is that particles which are added in the routing outside the equipment may have a large impact on the
testing result. The number mismatch between a single handling step outside the tool and the multiple cycling in the
equipment makes accuracy of measurements rather complex.
The low number of expected particles, the large variation in results and the combined effect of added particles inside and
outside the equipment justifies putting good effort in making a test plan. Without a proper statistical background, tests
may not be suitable for proving that equipment qualifies for the limiting cleanliness levels. Other risks are that a test may
requires an unrealistic high testing effort or that equipment can only pass for a test when it meets unrealistic high
TNO developed a testing model which enables setting up a qualification test on particle cleanliness for EUV mask
infrastructure. It is based on particle deposition models with a Poisson statistics and an acceptance sampling test method.
The test model combines the single contribution of the routing outside the equipment and contribution of multiple
cycling in the equipment. This model enables designing a test with minimal testing effort that proves that equipment
meets a required cleanliness level. Furthermore, it gives insight in other equipment requirements on reliability.
Extreme Ultraviolet Lithography (EUVL) is the most promising solution for technology nodes 16nm (hp) and below.
However, several unique EUV mask challenges must be resolved for a successful launch of the technology into the
market. Uncontrolled introduction of particles and/or contamination into the EUV scanner significantly increases the risk
for device yield loss and potentially scanner down-time. With the absence of a pellicle to protect the surface of the EUV
mask, a zero particle adder regime between final clean and the point-of-exposure is critical for the active areas of the
mask. A Dual Pod concept for handling EUV masks had been proposed by the industry as means to minimize the risk of
mask contamination during transport and storage.
SuSS-HamaTech introduces MaskTrackPro InSync as a fully automated solution for the handling of EUV masks in and
out of this Dual Pod System and therefore constitutes an interface between various tools inside the Fab. The intrinsic
cleanliness of each individual handling and storage step of the inner shell (EIP) of this Dual Pod and the EUV mask
inside the InSync Tool has been investigated to confirm the capability for minimizing the risk of cross-contamination.
An Entegris Dual Pod EUV-1000A-A110 has been used for the qualification. The particle detection for the qualification
procedure was executed with the TNO's RapidNano Particle Scanner, qualified for particle sizes down to 50nm (PSL
It has been shown that the target specification of < 2 particles @ 60nm per 25 cycles has been achieved. In case where
added particles were measured, the EIP has been identified as a potential root cause for Ni particle generation. Any direct
Ni-Al contact has to be avoided to mitigate the risk of material abrasion.
Before new equipment for handling of EUV reticles can be used, it should be shown that the apparatus is qualified for
operating at a sufficiently clean level. TNO developed a qualification procedure that is separated into two parts: reticle
handling and transport qualification and the qualification of the equipment. A statistical method was developed to include
the results of the handling and transport qualification into the qualification criterion for the equipment. As a result we are
able to calculate the minimum required experimental effort to prove that the particle contamination levels of the
equipment are within the requirements. The qualification procedure was applied to the TNO EUV reticle load port
module of the HamaTech MaskTrack Pro cleaning tool.
A Particle per Reticle Pass (PRP) between 0.005 and 0.076 for particles ≥ 80nm was measured for the reticle load port
module including handling and transport contribution. However, a high number of particles were found in the transport
test. As a result a much higher number of repeat cycles (more than a factor 6) were required to reduce the confidence
interval. Therefore, elimination of the transport step is absolutely required for a good qualification procedure. This can
be obtained by placing the inspection tool close to the equipment to be qualified. In this way, the required experimental
effort can be reduced significantly, saving both machine time and costs.
Since 2006 EUV Lithographic tools have been available for testing purposes giving a boost to the development of fab
infrastructure for EUV masks. The absence of a pellicle makes the EUV reticles extremely vulnerable to particles.
Therefore, the fab infrastructure for masks must meet very strict particle requirements. It is expected that all new
equipment must be qualified on particles before it can be put into operation. This qualification requirement increases the
need for a low cost method for particle detection on mask substrates.
TNO developed its fourth generation particle scanner, the Rapid Nano. This scanner is capable of detecting nanometer
sized particles on flat surfaces. The particle detection is based on dark field imaging techniques and fast image
processing. The tool was designed for detection of a single added particle in a handling experiment over a reticle sized
substrate. Therefore, the Rapid Nano is very suitable for the validation of particle cleanliness of equipment. During the
measurement, the substrate is protected against particle contamination by placing it in a protective environment. This
environment shields the substrate from all possible contamination source in the Nano Rapid (stages, elevator, cabling).
The imaging takes place through a window in the protective cover. The geometry of the protective environment enables
large flexibility in substrate shape and size. Particles can be detected on substrates varying from 152 x 152 mm mask
substrates to wafers up to 200 mm. PSL particles of 50 nm were detected with signal noise ratio of 26. Larger particles
had higher signal noise ratios. By individually linking particles in two measurements the addition of particles can be
detected. These results show that the Rapid Nano is capable of detecting particles of 50 nm and larger of a full reticle