Protect your yield from molecular contaminants
By Sarah Fister Gale
Counting and measuring particles in the cleanroom has never been a satisfactory way to manage airborne molecular contamination (AMC), as particles are different from gases. While controlling the number and size of particles is an obvious and necessary part of maintaining a clean environment, cleanroom managers now realize that this alone does not deliver enough information about what’s in the air or how to remove it. As geometries shrink, cleanroom operators need to know the exact makeup of their environment in order to eliminate contaminants.
Left unchecked, chemical contaminants can have a huge impact on productivity. Organic amines and ammonia, for example, even in parts per billion (ppb) concentrations, can alter the chemical properties of DUV photoresist. Condensation of organic species on wafer surfaces can lead to deterioration of gate formation or optical hazing. Photolithography optics also can be caused by organics in the air recondensing onto a surface. Fortunately, chemical filters and other techniques can help solve these problems.
Image 1: This photo shows optical hazing on a final lens element. Image courtesy of PMS.
The biggest concerns currently driving molecular contamination monitoring is optics and reticle hazing in 193 nm lithography, according to Steven Rowley, molecular product manager for Particle Measuring Systems (Boulder, CO). “Gas-phase acids and bases react and combine to form damaging deposits in the presence of 193 nm radiation,” he says. “Even organic and refractory compounds will form irreversible deposits on these critical components.”
Avoiding this damage has become a high priority for manufacturers. “I think that the biggest advancement in the management of AMC is that this issue is now being monitored at the highest levels in the corporations,” Rowley says. “It has evolved in such a way that fab managers now actively track this aspect of contamination control, as the repercussions for not monitoring are orders of magnitude higher and much more severe than the actual cost of implementing a monitoring program.”
Allyson Hartzell, managing scientist at Exponent, Inc. (Natick, MA) agrees. “The issue of AMC gets bigger and bigger as geometries get smaller and smaller,” she says. “As surface area-to-volume ratios increase, what sits on the surface becomes more important than ever.”
The February 2006 release of International Organization for Standardization (ISO) ISO/FDIS 14644-8, Cleanrooms and associated controlled environments-Part 8: Classification of airborne molecular contamination, underscores the growing importance of controlling chemical contaminants. Now available from the Institute of Environmental Sciences and Technology (IEST), the new document assigns ISO classification levels to specify the limits of AMC concentrations within a cleanroom and associated controlled environments where the product or process is deemed to be at risk from such contamination. ISO/FDIS 14644-8 defines AMC in terms of specific chemical species (individual, group, or category), and considers concentrations of AMC between 100 and 1012 g/m3 under cleanroom operational conditions. The document provides a protocol to include test methods, analysis, and time-weighted factors within the specification for classification.
“In the past, we always classified particles in the air in cleanrooms by number and size,” says Hartzell, US delegate to ISO 14644-8. “But there was never a way to quantify the air for molecular contamination, for the chemical makeup and mass of airborne chemicals per unit of volume.”
And that measurement, according to Hartzell, is now critical. “You have to understand what is deleterious to your product, and what contaminating chemicals cause problems before you can fix it,” she says.
That is why Hartzell is especially excited about the informational annexes included in ISO/FDIS 14644-8 that cover parameters for consideration, typical contaminating chemicals and substances, typical methods of measurement and analysis, and considerations of specific requirements for separative enclosures. The annexes give examples of the eight contamination material categories and how they can interact with the product or surface of interest.
“The new standard is very versatile in this respect,” Hartzell says. “It will give people a metric for their product manufacturing environment. They can use it to set goals and identify methods to remove contaminants from the cleanroom or controlled environment in a cost-effective manner to improve yield.”
Staying ahead of the game
Knowing you need to control chemical contaminants is only the first in a long series of steps to preventing and controlling molecular contamination before it impacts yield.
“The best AMC control system is a proactive one,” Rowley says. “Companies should utilize appropriate technology, such as chemical filtration and ultra-high purity purge gasses, and rely on real-time monitoring techniques to identify when contamination levels are approaching levels that would damage critical product or equipment.”
Contaminants can come from so many sources in the cleanroom-the fab itself, the wet bench, etching steps, people, polymers, carriers, pods, shippers, or mini-environments, as well as processing and cleaning chemicals and solvents.
“You have to pay attention to everything,” says Chris Muller, technical director for Purafil, a manufacturer of air-filtration systems (Doraville, GA).
Outside ambient air can also play a role, if it contains AMC levels high enough to interfere with semiconductor fabrication processes. “Outdoor air contains levels of ozone, sulfur and nitrogen oxides, and volatile organic compounds (VOCs) high enough to cause problems in cleanrooms,” adds Muller. “There are elevated levels of sulfur and nitrogen oxides from automobile exhaust and combustion processes in urban areas; atmospheric chlorine and boron can be found in coastal locations; and ammonia and amines may be present from agricultural activities. AMC can also cause damage to raw materials, in-process and finished products, and manufacturing and mechanical equipment.”
The solutions to these contaminants come in many forms, but they usually require more than just another set of HEPA filters. “Though effective at removing particles, HEPA and ULPA filters do nothing to protect cleanrooms from ammonia, ozone, sulfur and nitrogen oxides, and other molecular contaminants undetected by particle counters,” Muller says, noting that chemical contaminants are more complicated to control than other contaminants in the cleanroom. “We have a good handle on humidity, particulates, and electrical,” he says. “But ‘chemical’ is always that fuzzy black box.”
Despite the complexities of controlling chemicals in the cleanroom, Muller points out that fab owners are beginning to realize that in order to protect yield, they have to take measures to manage this problem. “The number of submicron-device fabrication steps affected by these molecular contaminants is rapidly increasing, and the impact can be serious,” he says.
Organic compounds in cleanroom air create problems such as wafer streaking, wafer haze, and hydrophobic wafer surfaces, and lithography defects. Also, airborne amines and volatile organic bases degrade certain photoresists, and Boron causes unintentional doping.
While in the last five years most cleanrooms have made great strides in getting control of ammonia, they’ve had less luck with some others, like sulfur dioxide, hydrogen chloride, and hydrogen fluoride.
Adding to the problem of the impact of individual chemical contaminants on process steps and the environment is the fact that many cleanroom managers can be myopic when trying to identify and eliminate them. “Someone may have a problem with ammonia in the environment, so that’s what they want to fix,” Muller says. “But there may be other things in the air that could also affect production or that could impact the effectiveness of the ammonia control strategy.”
If you just remove the primary contaminant, you may not be fixing the whole problem, or the solution to your ammonia control could be compromised by other contaminants in the environment. “Ambient contaminants may shorten the life of your chemical filters from three years to six months if you are not careful,” Muller says. “You have to look beyond what you see.”
To do that, cleanroom managers should address everything in the cleanroom that could potentially affect processes, and create a multi-tiered control system that monitors and/or eliminates them all.
Molecular monitoring systems
Determining how to control your unique AMC conditions begins with understanding your internal and external environments. Assessing your environment is the first step in identifying where and to what degree AMC control is necessary.
Image 2: Pictured here are three primary sorbent tubes from Particle Measuring Systems: black, for acids; white with caps, for bases; and white with sealed ends, for organics. Image courtesy of PMS.
The optimum control of AMC involves three steps: assessment of the air quality both outside and inside the facility to identify target contaminants as well as those that could affect the performance of the AMC control system; selection and qualification of an AMC control strategy; and ongoing monitoring of both the controlled environment and the performance of the AMC control system.
“Follow-up monitoring is important,” Muller says. “A real-time monitoring program allows you to evaluate the success of AMC controls to establish long-term air-quality trends.”
The initial step in controlling molecular contaminants is to identify the source and possibly eliminate it through adjustments in materials or process strategy. The presence of many organic contaminants in cleanroom air can be controlled through careful selection of cleanroom materials, finishes, and assembly techniques.
The use of mini-environments to create very clean localized areas is also a popular and cost-effective approach because it shrinks the control space, says Rowley. “We have seen the move to wafer pods, reticle pods, and mini-environments not only for particulate control, but also for isolating product from molecular contaminants in cleanroom environments,” he says. “Certain tools are also being isolated from other areas of the cleanroom with dedicated air-handling systems and chemical filtration where the product is most sensitive to the effects of molecular contamination. Prevention occurs using chemical filtration, point-of-use (POU) high-purity gas purges, and POU filtration systems, and vendors designing components that are less susceptible to outgassing.”
Outgassing and molecular exchange of FOUP, SMIF, FOSB, and reticle storage pods is a serious problem. “Wafers and reticles stored in those are generally protected from most particulate contamination,” Rowley says. “However, the plastics from those containers will off-gas onto the stored products, and contamination events outside of the pods will migrate into the pods, either diffusing through the plastic material or entering via chemical or pressure-relieving filters on the pods.”
In the larger cleanroom space, makeup air systems will control primarily atmospheric contaminants if the outside air intake locations are chosen carefully. “Zero-downtime systems should be considered for makeup air handlers,” Muller says. “A balance should be struck between the desired level of AMC control versus pressure drop, versus service life.”
Makeup air systems must typically be designed to control SOx, NOx, ozone, volatile organic compounds (VOCs), and some site-specific contaminants such as chlorine, organophosphates, and ammonia.
A filter for every chemical
Chemical filtration equipment in recirculation systems must also be designed to remove a wide array of acids, bases, hydrocarbons, and other VOCs. As a rule, organic compounds-although perhaps not the most damaging-are the most abundant types of AMC found in these facilities.
Image 3: The AiM-200 from Particle Measuring Systems monitors surface molecular contamination using surface acoustic wave sensors. Image courtesy of PMS.
The use of these chemical filters is now an indispensable part of environmental control in fabs. Chemical air filtration is installed to prevent yield losses in process areas for pre-gate oxidation, salicidation, and deep ultraviolet (DUV) lithography that are sensitive to VOC. Targeted chemical filters upstream of particle filters add to the efficiency of the system, and the more specific they are, the better. “A lot of cleanrooms have four or five levels of particle control but only one level for chemical control,” Muller says. He believes that’s a shortsighted approach.
Particle filters, which capture any material of a certain size regardless of its makeup, can actually improve over time as filters clog with material. Conversely, chemical filters are each designed to react to different chemical substances and degrade over time. Like a new car, they’re never as good as the first day you use them.
Depending on what you need to filter out, you may want three, four, or five different kinds of filters to eliminate all harmful materials and to maximize the life of your tools. For example, if you want one part per billion of ammonia in the environment, you won’t get it from a filter for acids. “It’s all about sequential classes,” Muller says. Filters can fail or last two to three times longer depending on whether or not they are monitored and contamination is adequately measured.
“It is well known that a single chemical filter media may not adequately control multiple contaminants or all classes of AMC,” Muller says. “The types and numbers of AMC one would encounter make it likely that air-cleaning systems need to be equipped with multiple stages of specialized chemical filter media. The preferred system would contain these media in discreet filter beds.”
Image 4: The AirSentry System from Particle Measuring Systems is an IMS analyzer system that monitors for bases (ammonia and amines), acids (SO2), and organics (hydrocarbons and silicone compounds). Image courtesy of PMS.
Many new construction fabs are incorporating space for chemical filters as understanding of the impact of these contaminants becomes clearer. However, in older facilities, it is not always feasible to incorporate an AMC control system with two or more filter beds. Retrofit applications, in particular, present challenges to HVAC engineers who are often limited by lack of physical space for the system, sufficient static pressure in the air-handling system, or budgetary constraints. “In these cases, two media can be blended and used as a single stage to reduce the size and/or cost of the system,” Muller says. “Properly applied, blended media should not affect the efficiency of the AMC control system; however, single-media versus media-blend performance-service life will be reduced based on the relative amount of each media being reduced.”
Physical limitations placed on AMC control systems and constant budgetary constraints have spurred the development of many new chemical filtration products. And, some manufacturers are applying dry-scrubbing media in various forms to various filter substrates, although certain processes can cause the adsorbent materials to become “blinded,” or spent, through the use of, or the reaction with, various binders and adhesives and the manufacturing techniques employed.
“Advances in filter manufacturing technology have eliminated a number of these deficiencies,” Muller says, “but the appropriateness of one filtration technology or filter type over another for specific applications remains an important question to be answered.”
Monitoring avoids mishaps
Once the control system is in place, monitoring the environment is critical, both to ensure the effectiveness of control tools and to prove they are working. “Once you’ve made the investment in chemical filtration, you have to be able to show improvement,” Muller says.
Most companies obtain the greatest monitoring value from continuous monitoring of a broad range of contaminants, generally classified as total amines, total acids, non-methane hydrocarbons, and siloxanes, among others.
Real-time monitors provide either a direct gas concentration output or a mass accumulation (rate) output that can be correlated to rates seen at different gas concentrations.
Indirect monitoring, which identifies classes of contaminants, can also alert cleanroom operators to the presence of harmful materials. Witness wafers, semi quatrain methods, and even weekly litmus paper testing offer an added dimension of data to the system.