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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
cleanliness levels.
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.
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