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Development of ultra-precision tools for metrology and lithography of large area photomasks and high definition displays
KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Metrology and Optics.
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 [en]
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: urn:nbn:se:kth:diva-122264ISBN: 978-91-7501-768-6 (print)OAI: oai:DiVA.org:kth-122264DiVA: diva2:621606
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
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
Show others...
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 (Scopus ID)
Note
QC 20101210Available from: 2010-12-10 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically 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 (Scopus ID)
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: 2017-12-11Bibliographically approved
4. A new general approach for solving the self-calibration problem on large area 2D ultra-precision coordinate measurement machines
Open this publication in new window or tab >>A new general approach for solving the self-calibration problem on large area 2D ultra-precision coordinate measurement machines
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.

Keyword
self-calibration, algorithm, metrology, large area, ultra-precision, mask, CMM
National Category
Production Engineering, Human Work Science and Ergonomics Reliability and Maintenance
Identifiers
urn:nbn:se:kth:diva-122271 (URN)10.1088/0957-0233/25/5/055001 (DOI)000334352000001 ()2-s2.0-84898464161 (Scopus ID)
Note

QC 20140610. Updated from submitted to published.

Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2017-12-06Bibliographically approved
5. Z-correction, a method for achieving ultraprecise self-calibration on large area coordinate measurement machines for photomasks
Open this publication in new window or tab >>Z-correction, a method for achieving ultraprecise self-calibration on large area coordinate measurement machines for photomasks
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.

Keyword
ultra-precision, metrology, Z-correction, self-calibration, overlay, large area, absolute accuracy
National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-122272 (URN)10.1088/0957-0233/25/5/055002 (DOI)000334352000002 ()2-s2.0-84898452513 (Scopus ID)
Note

QC 20140611. Updated from submitted to published.

Available from: 2013-05-16 Created: 2013-05-16 Last updated: 2017-12-06Bibliographically approved

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