Sheila Galatowitsch
San Jose—Airborne molecular contamination requires a different set of solutions than particle contamination (the current number-one concern in semiconductor and disk drive cleanrooms) and it may prove a bigger obstacle in the short term.
While a single particle can cause a killer defect, it takes millions of molecules to wreak havoc in semiconductors and disk drives. But the damage is equally severe: corrosion on wafers, disks, heads, substrates and equipment; particle and film formation; adhesion; and problems with electrical conductivity, shorts and leakage. The yield loss from airborne molecular contamination (AMC) has even contributed to some companies going out of business, according to Dr. Mark Camenzind, senior research chemist for Balazs Analytical Laboratory (Sunnyvale, CA), and the threat is rising as geometries decrease.
Camenzind spoke to some 40 people at last month's CleanRooms West '99. His talk covered the sources and effects of AMC, analytical methods commonly used for AMC and typical AMC levels in semiconductor cleanrooms. Camenzind helped the group of cleanroom users, suppliers, lab analysts and construction personnel understand the scope of the AMC problem, the difference between a particle and a molecule, and how to control AMC in a cleanroom.
Molecular contaminants are characterized by their size: 100 angstroms (0.01 micron) or less, and by their volatility, which helps them pass through HEPA and ULPA filters into cleanrooms. The total AMC by weight per cubic meter is orders of magnitude higher than the particle weight for Class 1 to 100 semiconductor cleanrooms, Camenzind says, adding that molecules diffuse to surfaces much faster than particles. However, only some processes are affected by only some of the contaminants since most do not stick to the surface.
Key contaminants are acids, bases, condensables and dopants generated from process chemicals, wet cleaning, solvents, equipment and materials outgassing, personnel, outside and recirculating air, internal or external spills and accidents, and even minienvironments. “The number one source of AMC is generally process chemicals,” Camenzind told the group. “You are usually your own worst enemy.”
Cleanroom construction materials and process equipment can also be a significant source of AMC. Other common culprits include the potting compounds and gel seals in HEPA and ULPA filters, floor tiles, curtains, sealants, polyurethanes, silicones, insulation and flame retardants. Some of these AMC sources can outgas for years, Camenzind says.
Reducing AMC sources is the first step in controlling the problem. In the fab, specific contaminants must be identified so the sources can be determined and eliminated. Process engineers and fab owners should determine if AMC from outside air, construction materials or process chemicals will affect process yields and reliability, then investigate which compounds, and at what levels, may affect the process. Quality control technicians should identify the appropriate AMC analysis methods, builders should select least contaminating construction materials, and suppliers should ensure that their products do not contribute to AMC in the cleanroom.
The second step in AMC control is installing molecular filtration to protect specific processes from specific contaminants, followed by reassessing processes to make them less sensitive to these contaminants. Other control measures include better venting of chemical sinks and reactors; chemistry dilution; alternative chemicals; faster process times; cluster tools; gas phase adsorbers; and isolation of processes. More dilute chemistries, on the order of 100 to 1 and 200 to 1, and reduced use of volatile chemistries can significantly diminish an AMC problem, Camenzind says
For Carol Asuncion, facility manager at Applied Materials (Santa Clara, CA), AMC prevention is a high priority as the company retrofits one fab and builds another for 300 mm wafers. “Now we know what things to look for that could cause problems,” she said.
Customers of Nye Lubricants (New Bedford, MA) are concerned about filter lubricants outgassing and causing AMC. According to product manager Bill Galary, the company plans to test each batch of lubricants for volatility with vapor pressure testing equipment and post the results on the batch label.
Users concerned about AMC should build a database of potential sources, document any effects and then set specifications for each process, Camenzind suggests. Besides reducing defects and improving reliability, AMC control pays off in other ways. It can improve electrical and mechanical properties, enable longer process windows between cleans and eliminate some cleaning steps.
 Some particles can get through the filter near the most penetrating particle size (MPPS). |
Conventional cleanroom filtration technology is not effective on molecular-sized contamination. At 5 to 50 angstroms, the molecules' volatility causes them to eventually migrate through HEPA and ULPA filters. At 2 to 10 angstroms, gas molecules and small organics-like air-easily penetrate filters.