Break through large astronomical telescopes of the near future (ELT, TMT, MSE, LOT, etc.) will be comprised of extremely large optical elements, requiring segmented primary mirrors. Optical performance of the primary mirror requires an optical shape with an accuracy of a fraction of a wavelength over the whole radius. This can be practically achieved by composing the largest primary mirrors of scores to several hundreds of individually actuated hexagonal mirrors. Multi-layered control of these actuators is performed over sensors observing the position of non-surface parts of these mirror elements. Alignment calibration or co-phasing to the front-surface is required. This procedure needs to be performed at the initial assembly and updated after each (scheduled) daily mirror replacement and to compensate for residual drift. Capital investment costs of these telescopes create the necessity to minimize the duration of maintenance during the valuable night time hours. TNO has developed an affordable instrument methodology, capable of delivering coordinates of each mirror element with respect to its neighbors during day time. This instrument combines nanometer accuracy/precision with low latency and meets the strict requirements of co-phasing the large multi-segmented telescopes of the future. This article describes the requirements, design and specifications of this instrument. TNO benefits from a rich heritage of designing world class optical instruments for science and industry. This metrology system takes advantage of the development of fast, full-field, time-domain, white-light interferometry, which has been demonstrated previously for in-line quality inspection at harsh industrial machining workshops.
Contemporary production systems of mechanical precision parts show challenges as increased complexity, tolerances
shrinking to sub-microns and yield losses that must be mastered to the extreme. More advanced automation and process
control is required to accomplish this task. Often a solution based on feedforward/feedback control is chosen requiring
innovative and more advanced in line metrology. This article concentrates first on the context of in line metrology for
process control and then on the development of a specific in line height profiling sensor. The novel sensor technology is
based on full field time domain white light interferometry which is well know from the quality lab. The novel metrology
system is to be mounted close to the production equipment, as required to minimize time delay in the control loop, and is
thereby fully exposed to vibrations. This sensor is innovated to perform in line with an orders of magnitude faster
throughput than laboratory instruments; it’s robust to withstand the rigors of workshops and has a height resolution that
is in the nanometer range.
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
substrate.
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