Haze, Still Misunderstood, Costing Industry $1B a Year
Aaron Hand, Executive Editor, Electronic Media -- Semiconductor International, 5/7/2008 11:10:00 AM
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Grenon made these comments at ESTECH 2008, the Institute of Envinromental Sciences and Technology’s (IEST) multi-industry contamination-focused conference going on this week in Bloomingdale, Ill. Speaking in a session devoted to the topic of time-dependent haze, Grenon noted that fabs still see haze as a problem that their suppliers need to take care of.
The problem, specifically, is that haze is forming on wafers, photomasks and optical elements, degrading surfaces, causing increased downtime for cleaning, or even catastrophic failure. Some of the micro-contaminants are being deposited by cleaning processes, and many are deposited from the environment. Still others are introduced by the packaging used to transport the elements. Photomasks are more susceptible to haze than are the optics within a lithography scanner, according to Oleg Kishkovich of Entegris (Chaska, Minn.), largely due to construction materials, environment and purge dynamics.
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| Optical and SEM images show ammonium carbonate haze formation. (Source: Grenon Consulting) |
Dominion Semiconductor was the first to report yield loss from ammonium sulfate haze around 1997, when the company lost some $25M in one day. Since then, essentially every semiconductor fab has experienced some form of haze contamination, according to Grenon, who showed a global map of haze contamination. In some cases, the loss actually outstripped revenue, with the largest loss reported to date being $100M. The worst effects have been seen in Taiwan and Shanghai, China, where environmental factors figure prominently, he said. And the problem is not getting any better. The higher energy of the shorter-wavelength lasers being used in lithography, as well as larger wafer sizes, are aggravating the issue. The most recent major incident was only about three months ago in Korea.
The issues have been difficult to quantify because nobody wants to admit that they actually have a haze problem. In the photomask industry, where extreme competition makes maskmakers hold every bit of information close to their chests, it is difficult to get data, for example, on how many times masks are cleaned. But in fact, the surfactants that maskmakers use to clean the photomasks are a significant cause of haze, and Grenon contends that he can tell how many times a mask has been cleaned by how many layers of contamination are built up on it. “We find that the more you clean, the dirtier it gets,” he said.
Surfactants have been identified as a key ingredient to avoid in cleaning processes because of the haze they leave behind. “The worst possible clean you can do with an optical element is put it in a surfactant,” Grenon said.
Wafer supplier MEMC Electronic Materials Inc. (St. Peters, Mo.) stopped using surfactants to clean the packages used to transport its wafers, and found that they were able to keep haze much more under control, according to Larry Shive, MEMC Science Fellow. Because haze continues to grow over time, it can greatly reduce the storage shelf life of a wafer, for example. But with changes it has made to its processes, MEMC has been able to extend the storage time for its wafers from six months to 18 months.
As Shrive explained, time-dependent haze (TDH), also known as degradation haze, is formed in the following way:
- The wafer is contaminated with water-soluble ions and organic molecules (other organic molecules also deposit on the wafer, making it more hydrophobic).
- A change in humidity causes water to condense on the wafer surface.
- The surface water dissolves the water-soluble contaminants.
- The hydrophobic surface causes the water to form microscopic droplets.
- The micro-droplets evaporate and leave residual TDH defects.
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| Wafer maps at MEMC showed that packaging issues were causing the growth of haze on wafers placed in storage. In this case, a rough estimate puts particle density at more than a million defects per square centimeter. (Source: MEMC) |
Besides doing away with its use of surfactants, the company focused on moisture barriers for its packages (including adding desiccants to the package) and maintaining the package integrity. MEMC has found instances, for example, in which the aluminum packaging has been torn by the metal shelves where they are placed for storage. With even the slightest pinhole in a package, moisture can enter, causing haze formation. In the past, the best way to find a pinhole in a package was to put the package over your head and see if any light came through — “not very elegant.” Instead, MEMC has developed oxide thickness determination method that measures the oxidation inside the package.
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| Ammonium sulfate haze (shown) was first reported as a yield issue around 1997. Although sulfate has largely been removed from environments, other compounds continue to create haze issues. (Source: Grenon Consulting) |
Ammonium sulfate, which caused the first reported haze, has become widely known as a micro-contaminant, so is largely under control because the industry has removed sulfate from the environment. But that doesn’t mean haze has been solved — only that the industry has since encountered new contaminants. Despite all of the changes, the organics are still there. “All you’re going to do is change the molecules that get adsorbed on the surface,” Grenon said. The latest haze, he noted, is ammonium oxalate, which is created by carbon dioxide and water in the air, along with ammonium (which will always find its way into a facility, according to Mark Camenzind, Air Liquide – Balazs Analytical Service).
There’s really no such thing as a clean surface, as Grenon noted, so the best that can be done is to keep the contamination under control. “The potential for TDH is always huge,” MEMC’s Shrive said. “So what we have to do is do the things that we can to avoid formation of the particles.”
The tool and materials suppliers are well aware of the issues, but still learning what to do about them. But if a wafer supplier, for example, takes the care necessary to keep haze of its products, it doesn’t help that the chipmaker then “essentially drags it across floor,” Grenon quipped. As it stands, however, the fabs expect the suppliers to provide the solutions — scanner manufacturers should provide a tool environment that keeps anything in the fab’s environment from getting in.
While haze can mean millions of lost dollars for a single fab, maskmakers are happy to take the reticles back for cleaning because it means more income for them. And because of haze, fabs are now ordering three masks where they used to order two: After the first one develops haze, it goes back to the mask shop to get cleaned and re-pelliclized, the second mask goes into production, and the third is there for the backup that the second used to be able to provide. And companies like Entegris see haze as a happy occurrence because they’re in the business of providing contamination solutions.
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