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.
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 <i>Pro</i> 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