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Z-correction, a method for achieving ultraprecise self-calibration on large area coordinate measurement machines for photomasks
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, 055002- p.Article in journal (Refereed) Published
Abstract [en]

High-quality photomasks are a prerequisite for the production of flat panel TVs, tablets and other kinds of high-resolution displays. During the past years, the resolution demand has become more and more accelerated, and today, the high-definition standard HD, 1920 x 1080 pixels(2), is well established, and already the next-generation so-called ultra-high-definition UHD or 4K display is entering the market. Highly advanced mask writers are used to produce the photomasks needed for the production of such displays. The dimensional tolerance in X and Y on absolute pattern placement on these photomasks, with sizes of square meters, has been in the range of 200-300 nm (3 sigma), but is now on the way to be <150 nm (3 sigma). To verify these photomasks, 2D ultra-precision coordinate measurement machines are used with even tighter tolerance requirements. The metrology tool MMS15000 is today the world standard tool used for the verification of large area photomasks. This paper will present a method called Z-correction that has been developed for the purpose of improving the absolute X, Y placement accuracy of features on the photomask in the writing process. However, Z-correction is also a prerequisite for achieving X and Y uncertainty levels <90 nm (3 sigma) in the self-calibration process of the MMS15000 stage area of 1.4 x 1.5 m(2). When talking of uncertainty specifications below 200 nm (3 sigma) of such a large area, the calibration object used, here an 8-16 mmthick quartz plate of size approximately a square meter, cannot be treated as a rigid body. The reason for this is that the absolute shape of the plate will be affected by gravity and will therefore not be the same at different places on the measurement machine stage when it is used in the self-calibration process. This mechanical deformation will stretch or compress the top surface (i.e. the image side) of the plate where the pattern resides, and therefore spatially deform the mask pattern in the X- and Y-directions. Errors due to this deformation can easily be several hundred nanometers. When Z-correction is used in the writer, it is also possible to relax the flatness demand of the photomask backside, leading to reduced manufacturing costs of the plates.

Place, publisher, year, edition, pages
2014. Vol. 25, no 5, 055002- p.
Keyword [en]
ultra-precision, metrology, Z-correction, self-calibration, overlay, large area, absolute accuracy
National Category
Production Engineering, Human Work Science and Ergonomics
URN: urn:nbn:se:kth:diva-122272DOI: 10.1088/0957-0233/25/5/055002ISI: 000334352000002ScopusID: 2-s2.0-84898452513OAI: diva2:621651

QC 20140611. Updated from submitted to published.

Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2014-06-11Bibliographically 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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