Display manufacturing: Particles on display
12/01/2005
As display panels get bigger, thinner and more expensive, manufacturers look to the semiconductor industry for guidance on how to get particle contamination under control
By Sarah Fister Gale
In the electronics industry where the never-ending goal is usually to shrink the size of products and technology, display manufacturing is an anomaly. It is one of the few sectors struggling to meet the demand for bigger technology, bigger tools, and bigger production facilities. In a world where smaller equals better, display manufacturers are often pushing the envelope in the opposite direction.
As consumer demand for giant screen TVs, advanced gaming devices, and huge computer monitors grows, the display industry rushes to advance its technology and fabs to accommodate the need. New glass technology and world-class processes are constantly moving forward to address the continual development of larger, thinner, lighter, sharper and more cost-efficient displays.
In the beginning, consumers were happy with boxy 30-inch TVs and would tolerate a few blank spots on an otherwise working screen, but today expectations are much higher. Liquid crystal displays (LCDs), which offer several advantages over traditional cathode-ray tube (CRT) displays, are becoming the norm for today’s TV and computer screens. Since 2000, glass substrate size has approximately doubled every 1.5 years. It took ten years for the display industry to migrate from Generation One glass sizes to Generation Four, but it took only four years to get from Generation Four to Generation Seven glass sizes. Generation Five and larger size substrates are expected to account for nearly 80 percent of all glass substrates produced by 2007.
This fast migration to larger-generation glass is driven both by product applications, such as bigger, flatter TVs and desktop monitors, and by economies of scale. Large generation glass offers dramatic manufacturing efficiencies, allowing display makers to produce more panels at lower costs with less waste. While a typical Generation Four sheet yields four 17-inch panels, a Generation Five sheet increases that yield threefold, to twelve 17-inch panels. A Generation Six substrate can produce eight larger panels of 32 inches, and a Generation Seven substrate can produce twelve 32-inch panels.
The industrywide migration by LCD manufacturers to large-generation substrates is expected to reduce prices to consumers, further driving the adoption of LCD technology in the desktop monitor and television market segments. LCD desktop monitors accounted for more than 50 percent of all monitors sold in 2004 and are projected to account for nearly 80 percent in 2007, which is twice the penetration rate of 2003. In 2004, LCD TVs represented only 5 percent of the color television market, but new screen sizes, falling prices and expanding availability are expected to drive market penetration to approximately 20 percent by 2007.
Lessons from semi
Ironically, even though display screens and the tools and fabs that make them are expanding in size with every new product generation, display manufacturers face the same challenges that the semiconductor industry faced as its products and technology continued to shrink.
Contamination control, for example, is becoming more critical, and the need for standards to manage the production process is increasing, just as it did for the semi industry several years ago. As a result, display manufacturers often look to the semi industry for guidance, says Mark Merrill, vice president of Photon Dynamics, a San Jose, California-based yield management company that offers test, repair and inspection tools throughout array, cell and module fabs for the display-panel manufacturing industry (see Fig. 1). “The display manufacturing process is a lot like the semiconductor manufacturing process, it’s just simpler and bigger.”
For display manufacturers, how well they manage contaminants in the processing environment determines increases or decreases in their yield. Even though display products keep increasing in size, particle contamination is becoming more of a problem, with tolerances for particulate size and counts continually decreasing.
As in the semi industry, where one particulate can destroy an entire chip, in large-display manufacturing one unfortunately placed particle can ruin an entire large, flat screen, which is a much more costly yield loss than the smaller panels of previous generations.
If the manufacturer is making smaller displays, a single panel of glass might be cut into 16 individual panels, whereas if it’s making larger displays, it may only be cut into four panels. “Imagine 100 particles falling on that piece of glass, and five of them cause pixel damage,” Merrill says. “That might kill five of the 16 smaller panels, but it could kill all of the four larger panels. That has a huge impact on yield.”
Managing yields through process control is an essential component of optimizing costs and time-to-market throughout the electronics industry. In the flat-panel display sector, the industry’s migration toward high-volume production of larger display panels is driving the need to maximize manufacturing yields. “They are bigger panels, but they have the same defect density,” Merrill points out, which means the environment can support fewer particles just to achieve the same yields.
While there are no conclusive statistics about the average yield of display manufacturers, Merrill estimates the industry is achieving 70 to 90 percent yields. The numbers improve as a fab matures and kinks are worked out of the system, he says.
A 90 percent yield may sound impressive for some industries, but in display manufacturing, where one panel costs close to $1000, a yield loss of 2 percent can cost a company several million dollars. Because of the high cost of materials used in the manufacture of LCDs, defective LCDs, especially larger displays, can be quite costly and ultimately drive up both panel and end-product costs. “That’s the reason why, in this industry, you test 100 percent of the product,” Merrill says. “We don’t just sample. We need to know that every single panel is going to work.”
As a result of the growing cost of yield loss, display fabs are fighting to achieve cleaner environments, and manufacturers are paying closer attention to the tools used to monitor and control contaminants throughout the manufacturing process (see Fig. 2).
 Figure 2. Pictured here is the latest-generation ArraySaver™ system from Photon Dynamics, Inc. used in the manufacture of LCDs. Photo courtesy of Photon Dynamics, Inc. |
“Yield management becomes increasingly critical with each successive product generation,” Merrill says. “As manufacturers bring sixth-, seventh- and eighth-generation fabs online, yield management will emerge as one of the chief gating factors.”
Manufacturing high performance LCDs for TV applications further increases process complexities, especially when dealing with color filters on array, viewing angle technologies, and photo spacers on color filters that are applied to the glass during processing.
Adding to the challenge of controlling particulate contamination is the fact that the fabs, the tools and the glass keep getting bigger. Every new generation of fab increases in size to accommodate the equipment needed to manage the product. Fabs today span several football fields in size, and the glass panels may be two to three meters across. Because of its size and weight, the glass is transported on ever bigger conveyers, which require larger open air spaces and broader chamber doors to move the glass in and out of the work space. The fab itself continues to expand, requiring more floor and air space, making environmental control processes more complex.
New equipment is also much heavier, creating greater stress on the environment and tools, both when it’s moved and used. “It’s also more complicated to install,” adds Merrill who notes that the photo equipment alone can weigh 60,000 kilograms and take months to install.
The purest surface imaginable
Once a fab is up and running, the display manufacturing process begins with the production of the glass, explains Peter Bocko, division vice president of commercial technology for Corning Display Technologies, maker of pristine flat glass used in LCDs for computer and electronics companies. These glass substrates are the foundation for active matrix LCDs.
The display manufacturing process has greatly improved over the last two decades, according to Bocko. He remembers a time when glass furnaces were made out of brick, and they often sat outside a machine shop perpetually encased in a cloud of dust. When the glass hardened, a worker would manually etch and break it and carry it to the next step in the process. “In the early days it was amazing what you’d find on the surface of the glass-cigarette smoke particles, lubricants, binder materials from gloves, skin flakes. Anything you can imagine,” Bocko says.
The manufacturing process has evolved dramatically since the early days, with much stricter monitoring of the process and environment, from raw material to finished product. “Today, the entire process, from melting on, is in a controlled environment,” he says.
Most display fabs today are Class 10,000 [ISO Class 7] environments, and for the last 10 years they’ve been managed mostly by robotics. The automation is both to eliminate human error and because the glass is too heavy for human handling (see Fig. 3).
At Corning, source material is fed into a furnace through a platinum tube where it is blended into a precise glass composition, which is then melted and conditioned to be homogenous and virtually defect-free.
When the molten glass achieves the right viscosity and temperature, it is fed into an arrowhead-shaped trough called an Isopipe. The glass flows evenly over both sides of the trough, meeting at the pointed bottom where it forms into a continuous sheet of viscous glass that is 0.7 to 0.5 millimeter in thickness. These thinner glass panels dramatically reduce the overall weight of the end product, but create a more delicate surface with which to work.
Because the glass sheet is formed in air, its surface is pristine and flat; no subsequent grinding or polishing, which could damage the glass, is required. The fusion process also maintains tight control over the thickness of the glass, leading to a consistent product. This is critical in the production of panels for LCD televisions, where viewing angle technology sensitizes the image to thickness differences in the glass.
“At this point in the process, the glass has the purest surface imaginable,” Bocko says. “There are no particulates or organic content anywhere. From this fusion step until it reaches the customer, it’s all about contamination control. Everything you do to it from here on only degrades the surface.”
How much degradation occurs depends on how the glass is handled and how clean the environment is in which it is processed. Every step in the process creates potential contaminants. When the glass is etched, broken and polished, particulate material is generated; the tools used to process the glass create dust and metal particles; cleaning chemicals and water used during beveling can leave residues; and transportation exposes the glass to potential damage from contaminants and movement. “There are two things that are especially bad for an LCD surface-water and particulates,” Bocko says.
At Corning, to reduce the risk of both kinds of contamination, once the glass is cut and broken using robotic devices, it is covered in a protective polymer coating sheet, which prevents environmental contamination from adhering to the glass, Bocko says. This coating, specially designed for Corning’s glass-manufacturing process, took years to develop because it required a surface that would adhere to the glass to protect it during processing but would also be easy to peel off without leaving surface material behind. “This is a highly engineered polyethylene-based adhesive that won’t leave a residue when it’s removed,” he says.
The coated glass is taken to a finishing area where it is etched, cut to fit, and the raw edges are polished. The coating is then removed so the glass can be washed and dried.
Washing is done through ultrasonic mechanical agitation, using pure water and chlorine-based detergents. It includes a soft-brush scrub, sonic rinsing processes, and drying using compressed air to push the water off the back of the glass, eliminating the risk of water residue.
Once the glass is cleaned, a worker uses a computer monitor to inspect it for defects. If particulates are discovered, the inspector must judge whether they can be removed during processing or whether the glass should be scrapped (see Fig. 4).
Although loss of glass impacts yield, the farther a faulty piece makes it through processing the more expensive the loss is. Making correct judgments during this inspection process is, therefore, highly important.
“We monitor particulates to one micron,” Bocko says. “And if any particulate larger than three to five microns remains on the glass after it is washed, the glass can be rejected.” The size limit of three to five microns is critical because this is the thickness of the LCD layer that’s sandwiched between the layers of glass.
Liquid crystal is a state that exists between solid and liquid, which certain kinds of matter can achieve under the right conditions. The molecules in liquid crystal do not all point in the same direction all the time as they do in solids; however, over time, they tend to point more in one direction than others. This direction is referred to as the director of the liquid crystal. The “amount” of order is measured by the order parameter of the liquid crystal, which in turn is highly dependent on temperature.
LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light. They are flat, and use only a fraction of the power required by CRTs. Thus, a particle thicker than the liquid center will create a cell gap in the final product, knocking out pixels and leaving dark spots on the screen.
Glass to go
If a panel is approved, the finished glass is then packed for delivery to the customer (see Fig. 5). Packaging and transporting large-generation substrates present another set of challenges.
In the conventional system, glass panels are packed in slotted crates to prevent them from touching as a result of vibration during transport. The air gap needed between each substrate in a slotted crate limits the number of substrates per case to twenty.
Corning recently switched to a new technique that protects the glass and allows for greater numbers of panels to be shipped in a smaller amount of space. Called the “DensePak” system, it allows for the safe transport, storage and staging of up to 500 sheets per case, in the same footprint as a 20-substrate case.
In the DensePak, another layer of polymer film is adhered to each panel, so they can be packed side-by-side in a vertical glass brick. “The polymer surface is enough to protect the glass, and it’s more efficient because it doesn’t require huge shipping boxes full of air,” Bocko says.
Once the glass is received, the film is removed and any remaining residue is cleaned away. “That cleaning process brings the glass back to the original surface quality,” Bocko says.
The glass is then reinspected and prepped for LCD applications. At this stage in the process, the two layers of glass are sandwiched together with a thin layer of LCD material between them.
Particulates continue to be an increasing risk as the display-manufacturing industry strives to master thinner LCD layers in an effort to create clearer, sharper pictures. “With thinner technology, even smaller particulates will cause problems,” Merrill says. “A half-micron particle in the right spot could take out a whole panel.”
Particle control methodologies
Because the challenges of managing particulates in such a large environment are so great, and the impact on yield so significant, reliable, repeatable methods of analyzing production-line data and repairing process-related defects are essential for the production of larger, higher-quality LCDs at affordable price points. This means testing the glass at key steps in the manufacturing process to ensure that particulates have been removed before panels are permanently affixed with LCD material.
When the pixels are in place on a display prior to its completion, they can be powered up using an LCD sensor that determines whether they will turn on. If the test reveals a dark pixel due to a microscopic particle, it can often be vaporized or knocked out with a laser to fix the problem, Merrill says.
If particulate is inside, however, once the LCD material is sandwiched between the glass, it can’t be repaired. “At that point, the best you can do is try to determine the source of the contamination so you can fix it before it does more damage,” he says.
Fortunately, there are only four or five steps in the display-manufacturing process, making it easier to find the contamination culprit. The industry doesn’t always analyze the make-up of any given particulate, instead manufacturers look back over the life of the panel to identify the problem. It may be caused by failure in a process tool, human error, or a cleaning problem. “Finding the source requires a lot of excursion control,” Merrill says.
To determine whether tools are emitting high particulate counts, process inspectors may use a laser particle counter. They also perform regular airflow monitoring and use HEPA filters to remove low concentrations of airborne molecular contaminants generated during the manufacturing process or by the equipment, says Manjaya Hegde, continuous improvement leader in the Silicon Valley office of PerkinElmer, a global technology leader for health sciences, optoelectronic and photonics markets. “We have 100 percent HEPA coverage in our fabs,” he says.
At PerkinElmer, the tools, ovens and vacuums all have HEPA filters, which are changed on a set schedule, and the overall environment also has a laminar airflow control system. The airflow is maintained at 90 FPM (+/-20 FPM), Hegde says. “If it goes above 110 FPM, it will generate turbulence that can cause particles to swirl through the room.”
In the scribing and breaking areas, which generate a lot of larger glass particles, HEPA-filtered vacuums are used to clean the glass surfaces and remove particles from the air and equipment surfaces.
Strict gowning techniques are followed, and personnel are prohibited from wearing makeup or perfume in the fab. All consumables, including wipes, swabs, and filters, are sterilized and double-bagged before they are brought into the fab. The first bag is opened in the first chamber of the cleanroom, after gowning is completed. To prevent any external contaminants entering the cleanroom environment, the outer bag is left in the transition chamber, and the inner sterile bag is opened in the fab. “Fab personnel go through retraining every six months to reinforce these techniques,” Hegde says.
Cleanroom gowns are washed weekly and are tested for particulates before being bagged and returned to the fab for use. PerkinElmer conducts audits of its garment suppliers twice a year, testing the water, equipment, preventive maintenance techniques, and environmental particulates to ensure they meet cleanroom compliance standards.
To further manage contamination before it gets out of control, monitors in the fab run checks every five seconds on temperature and humidity, and manual checks are conducted monthly in eight zones in the fab to verify that the environment is clean and tools are running according to spec.
Regular, systematic cleaning, sanitation and preventive maintenance strategies also help to reduce particulates in the fab environment. During cleaning, the tool components are regularly wiped down to eliminate rogue particles, and the quality-control team monitors cleaning chemicals and the chemical filters, changing them when the liquid particle counts become too high.
Through an environmental monitoring program, Hegde’s team also regularly counts and measures particle contaminants in 500 locations in the fab, looking for changes in trends throughout the processing steps.
He’s found that the best way to monitor actual particles in the fab is to use a witness panel, which is a glass substrate with a metalized layer, left at workbench level in a working fab for up to 10 days. Hegde moves the witness panel around the fab, scanning the particles accumulated in each area. “Even with HEPA filters there are going to be a lot of particles in the environment,” he admits. “But if they get outrageous, we analyze them and try to eliminate the problem.”
Quality versus quantity
The display industry is still easily a decade behind semconductor in its ability to control contaminants, and it continues to be an issue. As the display industry moves toward the future, fabs will get bigger and processors will struggle to bring yields under control. Consumers will continue to demand bigger and higher-quality screens, and their tolerance for any faults is already minimal, notes Bob Pinnel, chief technology officer for the U.S. Display Consortium in San Jose, Calif. “In the first laptops, six faulty pixels were considered acceptable,” he says. “Now it’s zero in almost all high-quality display products.”
Because the environments are so large, Pinnel suggests that the way the industry will manage its contamination challenges is to eliminate risks as early as possible and improve its inspection and repair technology. “Quality control tools are the way manufacturers will manage yield,” he says.
Merrill agrees: “In the future, even more emphasis will be placed on cleaner environments and on the ability to measure the quality of the contamination-control techniques,” he says, but he notes that quality and quantity go hand-in-hand. “You can spend the time to create the perfect clean environment, but while you do that, your competition will leapfrog ahead of you,” he says. “It’s a constant balancing act.”
He also suspects fabs will move toward vertical glass handling, more minienvironments within the fab, and pick-in-place robot cassettes to streamline processes. “When you get every processing step in line, you can deal with it more predictably,” Merrill says. “It’s a cleaner manufacturing process.”
While the equipment manufacturers would like to see standards developed for manufacturing technology, display manufacturers are typically so large and self-contained that this has, thus far, been a tough sell. “For larger groups, it’s cost effective to create their own in-house handling systems,” Merrill says. Even though standards would ultimately bring the costs of equipment down, the companies would have to change the way they do things internally to meet the standards, which is unlikely to happen unless external forces in the industry demand it. “It’s a Catch 22, but eventually standards will come.”