New technologies drive demand for advanced FPD tools
03/01/2006
Griff Resor III, Resor Associates, Boxborough, Massachusetts
Flat screen displays keep getting bigger, better, and cheaper. Currently, the market for large, flat TVs is dominated by plasma displays, but active matrix liquid crystal displays (AMLCD) are rapidly gaining market share. Less noticed, but equally important, AMLCDs are replacing monochrome supertwist-nematic (STN) displays in cell phones and in other mobile displays, as buyers demand high-resolution color pictures. A handful of US-based companies are using these market changes to increase sales of their flat panel display (FPD) manufacturing equipment.
Inspection and image processing tools
Photon Dynamics Inc. (PDI) has recently received multiple orders for its new PanelMaster. This product automatically inspects displays at the end of the cell-assembly process, after they have been filled and cut out of the large glass sheets used for production. The inspection system includes a calibrated back-light, polarizing films, and probe fixtures to power up each display and automatically look for point, line, and so-called “Mura” defects. Defect detection and classification is done with a vision system and image processing software. An array of cameras provides high throughput even on LCD-TVs as large as 57 in. Given that major FPD factories process 90,000 glass sheets/month, with each sheet containing eight or more displays, a huge amount of data is generated, on the order of terabytes per hour. This raw data is analyzed, condensed, and fed to a yield management system. Automated inspection data is inherently more accurate than manual data, and so provides better guidance for yield improvements.
Zygo Corp. makes a line of One Shot image-processing tools to measure flat-panel structures in 3D. The tools use an optical-based, noncontact technology developed in-house to make rapid 3D measurements of key structures. It could be thought of as a very fast noncontact atomic force microscope (AFM) tool, except that it is an image-based tool. It acquires data over an area in one shot, without needing to scan the area the way an AFM tool must.
 Figure 1. MVA structures and a photospacer on a color filter. Source: Zygo Corp. |
LCD-TV makers have added 3D structures in red, green, and blue sub-pixels (Fig. 1). These structures improve display contrast and viewing angle and are needed to make LCD-TV competitive with plasma TVs. Spacers (the small post seen in Fig. 1) are used to hold the two glass sheets apart after cell assembly. These spacers - formed by photolithography and thus called “photospacers” - are made with a compliant organic material. The new image-processing machine measures the critical dimensions of photospacers to nanometer tolerances. The volume of all spacers in one display is calculated and subtracted from nominal fill volume so just the right amount of liquid crystal fills each display.
If you go to your favorite TV store, you will find that the viewing angle problem in LCD-TVs has been solved. In many LCD-TVs, this result has been achieved by using 3D structures like those shown in Fig. 1. Zygo’s machine measures the size and placement of these 3D structures at the same time that it acquires data on spacers. Again, the volume is accurately calculated and used to adjust the amount of liquid crystal used.
Processing tools
Azores Corp. is building a line of compact steppers dedicated to manufacturing color displays for cell phones and other mobile electronic products. With the help of Corning Tropel Corp., Azores has created a giant lens that can print nine cell phone displays in just one shot, using a 1.25× enlarging lens. Cell phone display makers don’t want to use the giant masks that are used to make LCD-TVs because they are too expensive. The 1.25× enlarging lens lets manufacturers use simple, low-cost IC mask blanks. It also resolves 1.5µm patterns with >9µm depth of focus, which allows end users to integrate control logic on the display glass, saving cost and space in ultra-thin cell phone packages. The company is using a 10,000W mercury arc lamp at i-line to meet customer throughput requirements. Only 12.5 sec are needed to expose a full 730×920mm Gen 4 sheet. When load, align, step-and-repeat, and unload times are added, the Model 9200 processes one Gen 4 sheet every 70 sec.
TCZ GmbH, a joint venture of Cymer Inc. and Carl Zeiss SMT AG, introduced a demonstration version of the TCZ 900× in July of 2005 and will begin full production mid-year 2006. The new production tool converts amorphous silicon (a-Si) to polysilicon. The first unit is at a customer site in Korea.
Polysilicon can be used to make smaller and faster transistors. Cell phone display makers need polysilicon to build higher resolution color displays. Since the transistors are smaller, more light passes through the display. This feature can extend the useful battery life or reduce the phone’s size. Polysilicon is being used in R&D labs to integrate control logic on the display glass, which will further reduce cell phone cost and size, and improve the reliability of the display by reducing the number of electrical connections.
The present method for converting a-Si to polysilicon is the excimer laser annealing (ELA) method. Small grains, ~100nm dia., grow up from nucleation sites on the glass substrate. A 99% overlap of laser flashes is used to melt the a-Si layer many times, providing a nearly uniform polysilicon film, but at slow throughput. Moreover, the small grains limit transistor performance. The insert in Fig. 2 shows typical crystals made with the ELA method.
 Figure 2. Very long polysilicon grains made with the TCZ 900X. (The picture of the ELA process is at the same scale.) Source: TCZ GmbH |
The laser beam used in the TCZ tool covers 5µm along the scan axis and the full 730mm dimension of a Gen4 glass sheet along the other axis. By flashing the laser rapidly, a full sheet is converted in one continuous scan. Only a 50% overlap in the scan direction is needed to grow long polysilicon crystals, which are the key to making better polysilicon (i.e., for higher mobility). The technique, which the company calls the TDX method, causes crystals to seed at the edge of the 5µm beam and grow parallel to the glass surface and to the scan axis. As shown in Fig. 2, very long crystal grains (>10µm) can be made.
During each laser flash, a small amount of a-Si is converted. The melt and recrystallization happens in nanoseconds and control is critical. This new machine uses four built-in metrology packages to ensure optimum setup and consistent operation. The Cymer laser monitors every flash and uses a feed-forward algorithm to deliver <±1% (1σ) pulse-to-pulse control. A raw beam profiler measures the beam as it enters the optical module and is used to simplify laser setup and maintain constant illumination. A high-resolution CCD camera and energy detector located at the substrate plane are provided so users can see and modify the beam shape in the scan axis. The CCD sensor can be scanned along the 730mm axis to provide beam profile data in that axis. Finally, a long array of sensors looks at each flash in the 730mm axis to make sure the system delivers the same profile with every flash. The result is a uniform, well-controlled beam.
As flat-display technologies compete with each other, more process changes will provide opportunities for more companies to enter FPD equipment and materials markets. The four examples reported here may well inspire others to make their move.
Acknowledgments
PanelMaster is a trademark of Photon Dynamics Inc. One Shot is a trademark of Zygo Corp. PanelPrinter is a trademark of Azores Corp.
Griff Resor III is president of Resor Associates, Boxborough, MA; ph 978/263-7826; e-mail [email protected].