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Recent developments in large-area photomasks for display applications
KTH, School of Industrial Engineering and Management (ITM).
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).
Show others and affiliations
2001 (English)In: Journal of the Society for Information Display, ISSN 1071-0922, Vol. 9, no 1, 3-8 p.Article in journal (Refereed) Published
Abstract [en]

One of the most critical areas in the manufacturing process for FPD panels or shadow masks for CRTs is lithography. Most existing lithography technologies require high-quality large-area photomasks. The requirements on these photomasks include positioning accuracy (registration) and repeatability (overlay), systematic image quality errors ("mura" or display quality), and resolution (minimum feature size). The general trend toward higher resolution and improved performance, e.g., for TFT desktop monitors, has put a strong focus on the specifications for large-area-display photomasks. This article intends to give an overview of the dominant issues for large-area-display photomasks, and illustrates differences compared with other applications. The article will also present state-of-the-art methods and trends. In particular, the aspects of positioning accuracy over large areas and systematic image-quality errors will be described. New qualitative and objective methods have been developed as means to capture systematic image-quality errors. Results indicating that errors below 25 nm can be found early in the manufacturing process is presented, thus allowing inspection for visual effects before the actual display is completed. Positioning accuracy below 400 nm (3 sigma) over 720 × 560 mm have been achieved. These results will in the future be extended up toward 1 × 1 m for generation 4 in TFT-LCD production.

Place, publisher, year, edition, pages
San Jose, CA, 2001. Vol. 9, no 1, 3-8 p.
Keyword [en]
CRTs, FEDs, FPDs, Lithography, Manufacturing, Photomask, Shadow mask, TFT-LCDs, Cathode ray tubes, Electron device manufacture, Image quality, Liquid crystal displays, Masks, Photolithography, Photomasks, Positioning accuracy, Shallow masks, Systematic image quality errors, Flat panel displays
National Category
Production Engineering, Human Work Science and Ergonomics Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-33786DOI: 10.1889/1.1844659OAI: oai:DiVA.org:kth-33786DiVA: diva2:417623
Note
QC 20110517Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2013-05-16Bibliographically approved
In thesis
1. Ultra precision metrology: the key for mask lithography and manufacturing of high definition displays
Open this publication in new window or tab >>Ultra precision metrology: the key for mask lithography and manufacturing of high definition displays
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Metrology is the science of measurement. It is also a prerequisite for maintaining a high quality in all manufacturing processes. In this thesis we will present the demands and solutions for ultra-precision metrology in the manufacturing of lithography masks for the TV-display industry. The extreme challenge that needs to be overcome is a measurement uncertainty of 10 nm on an absolute scale of more that 2 meters in X and Y. Materials such as metal, ceramic composites, quartz or glass are highly affected by the surrounding temperature when tolerances are specified at nanometer levels. Also the fact that the refractive index of air in the interferometers measuring absolute distances is affected by temperature, pressure, humidity and CO2 contents makes the reference measurements really challenging. This goes hand in hand with the ability of how to design a mask writer, a pattern generator with a performance good enough for writing masks for the display industry with sub-micron accuracy over areas of square meters.

 As in many other areas in the industry high quality metrology is the key for success in developing high accuracy production tools. The aim of this thesis is therefore to discuss the metrology requirements of mask making for display screens. Defects that cause stripes in the image of a display, the so called “Mura” effect, are extremely difficult to measure as they are caused by spatially systematic errors in the mask writing process in the range of 10-20 nm. These errors may spatially extend in several hundreds of mm and are superposed by random noise with significantly higher amplitude compared to the 10-20 nm.

 A novel method for measuring chromium patterns on glass substrates will also be presented in this thesis. This method will be compared to methods based on CCD and CMOS images. Different methods have been implementedin the Micronic MMS1500 large area measuring machine, which is the metrology tool used by the mask industry, for verifying the masks made by the Micronic mask writers. Using alternative methods in the same system has been very efficient for handling different measurement situations. Some of  the discussed methods are also used by the writers for calibration purposes.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. viii, 65 p.
Series
Trita-IIP, ISSN 1650-1888 ; 2011:04
Keyword
Ultra precision metrology, LCD-display, OLED-display, nm-resolution, large area, random phase measurement, acousto-optic deflection, scanning, 2D measurement, mask, CCD, CMOS, image processing, edge detection
National Category
Engineering and Technology Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-33788 (URN)978-91-7415-959-2 (ISBN)
Presentation
2011-05-13, KTH, Stockholm, 10:00
Opponent
Supervisors
Funder
XPRES - Initiative for excellence in production research
Note
QC 20110517Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2012-06-19Bibliographically approved
2. 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.
Series
Trita-IIP, ISSN 1650-1888 ; 13:04
Keyword
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
Identifiers
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)
Opponent
Supervisors
Funder
XPRES - Initiative for excellence in production research
Note

QC 20130515

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

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