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A new general approach for solving the self-calibration problem on large area 2D ultra-precision coordinate measurement machines
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Metrology and Optics.
Micronic Laser Systems, Stockholm, Sweden.
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Metrology and Optics.
2014 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 25, no 5, 055001- p.Article in journal (Refereed) Published
Abstract [en]

The manufacturing of flat panel displays requires a number of photomasks for the placement of pixel patterns and supporting transistor arrays. For large area photomasks, dedicated ultra-precision writers have been developed for the production of these chromium patterns on glass or quartz plates. The dimensional tolerances in X and Y for absolute pattern placement on these plates, with areas measured in square meters, are in the range of 200-300 nm (3 sigma). To verify these photomasks, 2D ultra-precision coordinate measurement machines are used having even tighter tolerance requirements. This paper will present how the world standard metrology tool used for verifying large masks, the Micronic Mydata MMS15000, is calibrated without any other references than the wavelength of the interferometers in an extremely well-controlled temperature environment. This process is called self-calibration and is the only way to calibrate the metrology tool, as no square-meter-sized large area 2D traceable artifact is available. The only parameter that cannot be found using self-calibration is the absolute length scale. To make the MMS15000 traceable, a 1D reference rod, calibrated at a national metrology lab, is used. The reference plates used in the calibration of the MMS15000 may have sizes up to 1 m(2) and a weight of 50 kg. Therefore, standard methods for self-calibration on a small scale with exact placements cannot be used in the large area case. A new, more general method had to be developed for the purpose of calibrating the MMS15000. Using this method, it is possible to calibrate the measurement tool down to an uncertainty level of <90 nm (3 sigma) over an area of (0.8 x 0.8) m(2). The method used, which is based on the concept of iteration, does not introduce any more noise than the random noise introduced by the measurements, resulting in the lowest possible noise level that can be achieved by any self-calibration method.

Place, publisher, year, edition, pages
2014. Vol. 25, no 5, 055001- p.
Keyword [en]
self-calibration, algorithm, metrology, large area, ultra-precision, mask, CMM
National Category
Production Engineering, Human Work Science and Ergonomics Reliability and Maintenance
URN: urn:nbn:se:kth:diva-122271DOI: 10.1088/0957-0233/25/5/055001ISI: 000334352000001ScopusID: 2-s2.0-84898464161OAI: diva2:621650

QC 20140610. Updated from submitted to published.

Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2014-06-10Bibliographically approved
In thesis
1. Development of ultra-precision tools for metrology and lithography of large area photomasks and high definition displays
Open this publication in new window or tab >>Development of ultra-precision tools for metrology and lithography of large area photomasks and high definition displays
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Large area flat displays are nowadays considered being a commodity. After the era of bulky CRT TV technology, LCD and OLED have taken over as the most prevalent technologies for high quality image display devices. An important factor underlying the success of these technologies has been the development of high performance photomask writers in combination with a precise photomask process. Photomask manufacturing can be regarded as an art, highly dependent on qualified and skilled workers in a few companies located in Asia. The manufacturing yield in the photomask process depends to a great extent on several steps of measurements and inspections. Metrology, which is the focus of this thesis, is the science of measurement and is a prerequisite for maintaining high quality in all manufacturing processes. The details and challenges of performing critical measurements over large area photomasks of square meter sizes will be discussed. In particular the development of methods and algorithms related to the metrology system MMS15000, the world standard for large area photomask metrology today, will be presented.

The most important quality of a metrology system is repeatability. Achieving good repeatability requires a stable environment, carefully selected materials, sophisticated mechanical solutions, precise optics and capable software. Attributes of the air including humidity, CO2 level, pressure and turbulence are other factors that can impact repeatability and accuracy if not handled properly. Besides the former qualities, there is also the behavior of the photomask itself that needs to be carefully handled in order to achieve a good correspondence to the Cartesian coordinate system. An uncertainty specification below 100 nm (3σ) over an area measured in square meters cannot be fulfilled unless special care is taken to compensate for gravity-induced errors from the photomask itself when it is resting on the metrology tool stage. Calibration is therefore a considerable challenge over these large areas. A novel method for self-calibration will be presented and discussed in the thesis. This is a general method that has proven to be highly robust even in cases when the self-calibration problem is close to being underdetermined.

A random sampling method based on massive averaging in the time domain will be presented as the solution for achieving precise spatial measurements of the photomask patterns. This method has been used for detection of the position of chrome or glass edges on the photomask with a repeatability of 1.5 nm (3σ), using a measurement time of 250 ms. The method has also been used for verification of large area measurement repeatability of approximately 10 nm (3σ) when measuring several hundred measurement marks covering an area of 0.8 x 0.8 m2.

The measurement of linewidths, referred to in the photomask industry as critical dimension (CD) measurements, is another important task for the MMS15000 system. A threshold-based inverse convolution method will be presented that enhances resolution down to 0.5 µm without requiring a change to the numerical aperture of the system.

As already mentioned, metrology is very important for maintaining high quality in a manufacturing environment. In the mask manufacturing industry in particular, the cost of poor quality (CoPQ) is extremely high. Besides the high materials cost, there are also the stringent requirements placed on CD and mask overlay, along with the need for zero defects that make the photomask industry unique. This topic is discussed further, and is shown to be a strong motivation for the development of the ultra-precision metrology built into the MMS15000 system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xiv, 105 p.
Trita-IIP, ISSN 1650-1888 ; 13:04
Ultra precision 2D metrology, LCD-display, OLED-display, nm-resolution, random phase measurement, large area, photomask, acousto-optic deflection, self-calibration, Z-correction, absolute accuracy, uncertainty.
National Category
Reliability and Maintenance Production Engineering, Human Work Science and Ergonomics
Research subject
SRA - Production
urn:nbn:se:kth:diva-122264 (URN)978-91-7501-768-6 (ISBN)
Public defence
2013-06-03, M311, Brinellvägen 68, KTH, Stockholm, 10:00 (English)
XPRES - Initiative for excellence in production research

QC 20130515

Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2013-05-16Bibliographically approved

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