Oversized cleanrooms assume huge role in emerging flat-panel display production
08/01/2002
Flat-Panel
Producers have been struggling to sort through and refine production methodologies that can deliver high volume at low cost. The cleanroom is evolving to meet these needs
by Michael O'Halloran, Richard Grout and Nancy Pettengill
It's hard to believe, but as late as the 1980s, "cleanroom" was not a familiar term in much of the manufacturing world.
In the time since, the cleanroom concept has never remained static. It has continued to evolve and respond to the dynamically changing needs of high-technology production environments around the world.
One glance at a latest-generation flat-panel manufacturing facility will convince the most casual observer that this new breed of cleanroom is anything but business as usual.
In broad terms, cleanrooms assume two distinctly different responsibilities and forms in the flat-panel industry-as new technology incubator and high-volume producer. As in any emerging industry, flat-panel producers have been struggling to sort through and refine production methodologies that can deliver high volume at low cost.
Small-scale flat-panel products, such as miniature televisions, handheld games and laptop computers, have been successfully penetrating the consumer market for some time. This nascent industry's biggest challenge, however, will be achieving heavy market penetration for larger projects, primarily big-screen televisions.
 AKT Inc., an Applied Materials Company, offers the 10K tool as its current generation (Generation 5) of AKT PECVD tools. |
Attaining this goal would have a revolutionary impact on the industry's growth, but this is a vision whose fate is largely predicated on price. The generally accepted consumer price at which flat-panel TVs will achieve mass-market penetration is $100 per diagonal inch of display dimension.
This translates to $3,200 for a 32-inch diagonal flat-panel TV or $4,200 for a 42-inch diagonal unit. Actual retail prices are currently running just less than $5,000 for the least-expensive major brand 32-inch flat panel, and about $6,500 for 42-inch units. These products involve plasma display panel (PDP) technology.
Flat-panel producers are basing their production efficiency goals on values that retail customers are willing to assign to large-scale flat-panel products. The industry knows that its future prospects depend on its ability to lower production costs until the consumer's "dream" price points can be met. Some U.S. discount retailers are approaching that threshold by offering 32-inch units for $3,999, and you can get a 20-inch diagonal active matrix liquid crystal display (AMLCD) TV in the North American market for $2,000.
However, penetration of the mainstream mass market for these products still has a way to go.
Steady improvements in flat-panel production technology have allowed the industry to continue to edge closer to that desired level of manufacturing cost effectiveness, and cleanrooms have been a central ingredient in that advancement. However, the cleanroom requirements of the flat-panel industry demand more than a "plug-and-play" adaptation of "off-the-shelf" cleanroom approaches standard in the microelectronics industry.
Key flat-panel technologies that have already graduated from the R&D and pilot plant echelons to high-volume production status include AMLCD and plasma. Leading manufacturers in these technologies are primarily in Japan, Korea and, more recently, Taiwan.
In the industry's emergent tier are such developing flat-panel technologies as field-emission display (FED) and electroluminescent (EL). Within the EL family are such offshoot approaches as thin film, organic light-emitting diode (OLED) and inorganic light-emitting diode (ILED). These technologies are in various stages of development, from pure R&D to small-scale production.
Size issues
The large size of the end product is a key driver in the planning of flat-panel cleanrooms, which accommodate mammoth production tools 20 or 30 meters in length.
Glass sizes for flat-panel display (FPD) production facilities range from approximately 400x400 mm to 1.2x1.6 meters, depending on product type. Glass sizes for pilot facilities range from 370x470 mm to 600x720 mm. The size of these products dwarfs the deliverables of the microelectronics industry's latest-generation product, the 300-mm wafer.
 AKT Inc.'s PECVD 4300 tool represents the Generation 3 glass size, another generation of the company's AKT PECVD tools. |
The much larger size of the end product is reflected in every aspect of a flat-panel plant's design and operation-most notably in cleanroom criteria. In one Generation 5 flat-panel plant currently in design in Asia, the cleanroom space is five times larger than the largest U.S. 300-mm chip plant. For comparison, this facility encompasses more square footage than a Boeing 777 hanger of approximately one million square feet, except that it stacks that immense quantity of space in three 350,000-square-foot levels.
The primary focus of R&D operations is understandably to advance new product technology. However, it is important for such operations to share some portion of the focus with the advancement of their manufacturing technology as well. Many promising new products have been needlessly doomed because they failed to successfully transition from the R&D or pilot-plant stages to a high-volume production status.
This peril threatens the flat-panel industry at a time of particular vulnerability. It is a time when progressive new production approaches could be dismissed simply because they lack an "exit strategy" to smoothly scale them up from the research lab to economical high-volume output on the factory floor.
Pilot facilities can usually handle 4,000 to 8,000 sheets per month. However, the throughput capacity for FPD production facilities jumps up to 25,000 to 90,000 sheets (mother glass) per month. As manufacturing technologies prove out at the R&D level, production capacities must have a practical, efficient and achievable path forward to smoothly ramp up and meet commercial volume requirements without massive factory modifications.
Design concerns
This scale of building poses formidable structural challenges, but structural issues pale in comparison to such a facility's cleanroom design concerns. The management of cleanroom airflows, difficult enough in the "normal" cleanrooms of the microelectronics industry, becomes a colossal consideration in sprawling flat-panel facilities.
Airflows in microelectronics cleanrooms are carefully fine-tuned to accommodate cleanroom-processing tools. The same adjustments are required in flat-panel cleanrooms, but the potential for airflow irregularities is greatly magnified by the volumetric dynamics of production tools, which are much larger than their microelectronics counterparts. Even the cassettes used to transport flat-panel substrates are roughly the size of an office desk.
Further aggravating airflow management concerns are the extensive networks of material handling systems that are typical in latest-generation flat-panel facilities, where material handling is virtually 100 percent automated. Automation includes equipment interface systems and floor-supported conveying systems, stockers and rail-guided vehicles (RGVs). Integration of these systems with the total cleanroom design is key to a successful project. Ground-based transport systems, automated guided vehicle (AGV) or manual guided vehicle (MGV), are used for glass handling in pilot facilities.
By contrast, the portion of the industry engaged in the research and development of new flat-panel technologies requires relatively small cleanroom facilities-much smaller than those found in a typical semiconductor operation. Typically, custom-designed processing equipment supports such cleanrooms, because these facilities are involved in pioneering production processes for which there are currently no standardized production tools.
In these facilities, cleanroom system components are also largely custom-designed to meet specific process requirements. Experience with such operators underscores the importance of building flexibility into the front end of the cleanroom design process. A risk to innovators of new flat-panel technologies is a risk shared by those on the leading edge of other high-tech manufacturing processes: The evolution of their process technology may render their manufacturing plants extinct.
Process and environment
The production processes for flat panels are similar to the ones we are familiar with from the semiconductor industry, but there are some new ones evolving. There are three sets of processes that go into making a flat panel:
- Thin-film transistors (TFT) or "array": Creates an array of TFTs and capacitors on a sheet of glass.
- Color filter (CF): Creates an array of CFs on another sheet of glass.
- Liquid crystal display (LCD) or "cell": Puts the two sheets of glass together, applying polarizing film to the outer surfaces, filling the space between the glass sheets with liquid crystal material and then slicing the panels into smaller pieces or cells.
The TFT process includes photomasking, chemical vapor deposition (CVD) and etch. CF involves a repetitive series of coating, baking, etching and cleaning steps. LCD is more of a light industrial assembly process. Many flat-panel companies farm out the color filter operation.
A ballroom-type design is common for both production and pilot FPD cleanrooms. The ballroom configuration provides the flexibility to accommodate the variety of tool configurations and sizes found in FPD manufacturing, as well as simplifies the installation of very large tools. Individual tools for state-of-the-art TFT LCD and PDP facilities are 20 to 40 meters long and 7 to 10 meters wide. Fully integrated lines for these facilities may be up to 100 meters in length.
Cleanliness classes range from ISO Class 4 in localized areas where the product is exposed to air to ISO Class 7 space, which is typical for cleaning and polishing the glass substrate before it enters the production fabs and for packaging the finished product. The general production environment typically is ISO Class 6.
Feature sizes for FPD products are two to three orders of magnitude larger than those found in state-of-the-art semiconductors. The smallest horizontal geometries are associated with TFT and OLED manufacturing, where critical feature sizes are in the 5- to 10-micron range. Minimum geometries for typical color filter and PDP products range from 50 to 300 microns.
Despite the relatively coarse feature sizes, particle contamination and yield are still major concerns in the manufacturing of large-area TFT's. A large-area TFT LCD will contain more than one-million gate structures, while a single substrate may represent only four to six finished products of this type, with a value in excess of $1,000 each. Target yields for manufacturing facilities of this type range from 80 percent to 90 percent.
Most of the major production tools in TFT and CF are housed in minienvironments. However, many of these minienvironments rely on house fan filter unit (FFU) systems for clean air. The area inside the minienvironments typically gets 50 percent filter coverage. Total coverage is applied outside the mini environments where the product is exposed to room air, for example, at transfer paths between stockers and minienvironments.
The product stockers are numerous and large (many as long as 50 m). These can be equipped with FFUs, drawing air from the overhead air plenum and discharging it to the subfab or beneath the raised floor, as the case may be. This air adds to the amount the house system, in particular the dry coils, must handle.
The flat-panel industry supports its cleanrooms with many of the same utilities that the semiconductor industry uses. Its quantities are different, though. On a unit-area basis, the figures are much lower for flat panel. However, because the square foot component is huge, some of the central systems end up being quite large. Acid exhaust for a Generation 5 facility might be on the order of 120,000 cfm, which is less than a typical semiconductor facility. But total ammonia exhaust is greater, perhaps in the range of 50,000 cfm.
Other exhaust streams of note include stripper exhaust from the TFT process, which contains volatile organic compounds (VOCs) and Tetramethylammonium Hydroxide (TMAH). The VOCs are removed in point-of-use condensers (which need chilled water); the rest of the exhaust goes to dedicated wet scrubbers. The other unusual exhaust comes from the CVD tools in TFT.
Emissions management
The CVD process uses a lot of silane, so the exhaust passes through point-of-use "burn box" type treatment units. The decomposition process produces SiO2 dust particles averaging one micron in size. Silica dust has to be removed before the CVD exhaust, which is acidic, can be scrubbed. Baghouses equipped with polytetrafluoroethylene (PTFE) filter bags can be used for this purpose.
However, recent advances in emissions abatement technology have produced systems that can scrub both gases and particles from exhaust streams at levels not previously attainable.
One new approach uses huge bursts of energy to break the molecular bonds of contaminating molecules in exhaust gases. Another of the new emissions abatement technologies uses a mist of fine water droplets, which are charged with high voltage and released into an open chamber that contains waste flow streams. These scrubbers treat gas-phase species and sub-micron particles simultaneously, with no packing and at a pressure drop of less than one inch of water.
Due to the low wall-to-floor area ratio in FPD cleanrooms, the pressurization component of makeup air can be relatively low. Adding process exhaust can drive the need for makeup air to levels approaching one million cfm.
Architecturally, a flat-panel facility's structural systems are similar to those of microelectronics fabs, although the vibration specifications are less stringent. The typical process support systems, ultrapure water (UPW), CDA, vacuum, process cooling water (PCW), gas and chemical storage and distribution, are present. Some of the chemical supply systems require mixing or dilution equipment in addition to the usual storage and pumping modules.
Process gas types for TFT LCD and low-temperature polysilicon (LTPS) facilities are similar to those for semiconductor manufacturing, although purity and particle count specifications are less stringent than those found in semiconductor manufacturing. Gases for PDP and passive matrix OLED manufacturing are limited to typical house gases such as nitrogen, oxygen, hydrogen, argon and helium.
The variety of processes found in FPD facilities drives requirements for large numbers of waste collection and treatment systems as well as bulk chemical dispensing systems. As many as 15 to 20 different waste collection and treatment systems are needed for a complete TFT LCD manufacturing facility.
Materials that must be handled include concentrated and dilute acids and caustics, solvents, slurries, pigments and dyes. Most of the wastewater is simply neutralized and discharged to the sewer, although additional treatment processes are required for some waste streams. Chemicals can be drained to waste collection tanks for temporary storage prior to removal from the manufacturing site.
Meeting future needs
The growing FPD industry benefits from the many lessons that have been learned in the microelectronics industry over the past 20 years.
In addition to sharing many of the same technologies, these industries also face many of the same potential threats. As the semiconductor industry has matured, it has shaken out manufacturers less capable of meeting "time-to-market" windows of opportunity than their competitors.
FPD manufacturers are entering a new era in which many of the same dynamics related to output and cost control will increasingly take hold as differentiators that will define this emerging industry's leaders.
As in the semiconductor wafer fab, the cleanroom will be at the center of the action as the most vital organ in the massive FPD machine. Successful FPD cleanrooms will be those that are planned and designed with the flexibility to accomplish long-term competitive success-not just meet short-term production goals.
Michael O'Halloran, Industrial Design & Construction's (IDC-Portland, OR), director of technology, has more than 25 years of experience in engineering design, cleanroom design, high-purity gas and liquid system design, project management, processing equipment design and hookup, production control, process and equipment automation, material handling, material flow, facilities control, energy conservation, ergonomic design and industrial safety and hygiene.
Richard Grout, a member of IDC's mechanical department, is a specialist in the design of advanced air-handling systems for high-technology manufacturing facilities, and is a leading expert in the specific requirements of flat-panel facilities.
Nancy Pettengill, also a member of IDC's technology staff, has more than twenty years of experience in manufacturing technology and equipment engineering, and has particular expertise in advanced technology involving flat-panel manufacturing.
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Generational categorization in brief
A general history of flat-panel display's (FPD) generational categorization is below. The ranges cited for each generation are approximations, because different manufacturers in each generation varied the sizes of their substrate, or "mother glass," to meet the specific needs of their individual products. In general, though, these are the size ranges attached to each generation.
Generation 2 substrate:
370x470 mm
Generation 3:
550x650 mm
Generation 3.5:
590x670 mm or 600x720 mm
Generation 4:
750x900 mm
Generation 5:
This moved substrate sizes into a range in which the smaller dimension was one meter or greater. 1x1.25 m has been the general de facto Generation 5 standard.
Generation 6:
There is discussion of Generation 6 dimensions, not in effect yet, which are projected to be 1.2x1.6 m.