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Ultra precision metrology: the key for mask lithography and manufacturing of high definition displays
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Metrology and Optics.
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 [en]
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: urn:nbn:se:kth:diva-33788ISBN: 978-91-7415-959-2OAI: oai:DiVA.org:kth-33788DiVA: diva2:417639
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
List of papers
1. Recent developments in large-area photomasks for display applications
Open this publication in new window or tab >>Recent developments in large-area photomasks for display applications
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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
Keyword
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:nbn:se:kth:diva-33786 (URN)10.1889/1.1844659 (DOI)
Note
QC 20110517Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2013-05-16Bibliographically approved
2. Ultra-precision geometrical measurement technique based on a statistical random phase clock combined with acoustic-optical deflection
Open this publication in new window or tab >>Ultra-precision geometrical measurement technique based on a statistical random phase clock combined with acoustic-optical deflection
2010 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 21, no 12, 125103- p.Article in journal (Refereed) Published
Abstract [en]

Mask writers and large area measurements systems are key systems for production of large liquid crystal displays (LCD) and image devices. With position tolerances in the sub-mu m range over square meter sized masks, the metrology challenges are indeed demanding. Most systems used for this type of measurement rely on a microscope camera imaging system, provided with a charge coupled device, a complementary metal-oxide-semiconductor sensor or a time delay and integration sensor to transform the optical image to a digital gray-level image. From this image, processing algorithms are used to extract information such as location of edges. The drawback of this technique is the vast amount of data captured but never used. This paper presents a new approach for ultra-high-precision lateral measurement at nm-levels of chrome/glass patterns separated by centimeters, so called registration marks, on masks used for the LCD manufacturing. Registration specifications demand a positioning accuracy <200 nm and critical dimensions, i.e. chrome line widths, which need to be accurate in the 80 nm range. This accuracy has to be achieved on glass masks of 2.4 x 1.6 m(2) size. Our new measurement method is based on nm-precise lateral scanning of a focused laser beam combined with statistical random phase sampling of the reflected signal. The precise scanning is based on an extremely accurate time measuring device controlling an acousto optic deflector crystal. The method has been successfully applied in measuring the 4 mu m pitch of reference gratings at standard deviations sigma of 0.5 nm and registration marks separated by several cm at standard deviations of 23 nm.

Keyword
metrology, nm-resolution, large area, random phase measurement, ultra-precision, scanning, acousto-optic deflection, mask
National Category
Engineering and Technology Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-27066 (URN)10.1088/0957-0233/21/12/125103 (DOI)000284261900013 ()2-s2.0-78649831988 (ScopusID)
Note
QC 20101210Available from: 2010-12-10 Created: 2010-12-06 Last updated: 2013-05-16Bibliographically approved
3. A Large-area ultra-precision 2D geometrical measurement technique based on statistical random phase detection
Open this publication in new window or tab >>A Large-area ultra-precision 2D geometrical measurement technique based on statistical random phase detection
2012 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 23, no 3Article in journal (Refereed) Published
Abstract [en]

The manufacturing of high-quality chrome masks used in the display industry for the manufacturing of liquid crystals, organic light emission diodes and other display devices would not be possible without high-precision large-area metrology. In contrast to the semiconductor industry where 6' masks are most common, the quartz glass masks for the manufacturing of large area TVs can have sizes of up to 1.6 x 1.8 m(2). Besides the large area, there are demands of sub-micrometer accuracy in 'registration', i.e. absolute dimensional measurements and nanometer requirements for 'overlay', i.e. repeatability. The technique for making such precise measurements on large masks is one of the most challenging tasks in dimensional metrology today. This paper presents a new approach to two-dimensional (2D) ultra-precision measurements based on random sampling. The technique was recently presented for ultra-precise one-dimensional (1D) measurement. The 1D method relies on timing the scanning of a focused laser beam 200 mu m in the Y-direction from an interferometrically determined reference position. This microsweep is controlled by an acousto-optical deflector. By letting the microsweep scan from random X-positions, we can build XY-recordings through a time-to-space conversion that gives very precise maps of the feature edges of the masks. The method differs a lot from ordinary image processing methods using CCD or CMOS sensors for capturing images in the spatial domain. We use events grabbed by a single detector in the time domain in both the X-and Y-directions. After a simple scaling, we get precise and repeatable spatial information. Thanks to the extremely linear microsweep and its precise power control, spatial and intensity distortions, common in ordinary image processing systems using 2D optics and 2D sensors, can be practically eliminated. Our 2D method has proved to give a standard deviation in repeatability of less than 4 nm (1 sigma) in both the X-and Y-directions over an area of approximately 0.8 x 0.8 m(2). Only feature edges are recorded, so all irrelevant information in areas containing constant intensity are filtered out already by the hardware. This relaxes the demands and complexity of the data channel dramatically compared to conventional imaging systems.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2012
Keyword
metrology, nm-resolution, large area, random phase measurement, acousto-optic deflection, scanning, 2D measurement, mask, ultra precision
National Category
Engineering and Technology Production Engineering, Human Work Science and Ergonomics
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-33787 (URN)10.1088/0957-0233/23/3/035007 (DOI)000300614800008 ()2-s2.0-84857422289 (ScopusID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20120315

Updated from submitted to published

Available from: 2011-05-17 Created: 2011-05-17 Last updated: 2016-04-25Bibliographically approved

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