Semiconductor Industrial Hygiene Monitoring: New Challenges and a Few Old Favorites in the World of Hazard Identification and Exposure Evaluation
What are the industrial hygiene concerns in a basic semiconductor fabrication facility? This paper highlights key industrial hygiene monitoring strategies for selected normal production operations and maintenance tasks. Monitoring for airborne and surface chemicals, ionizing radiation, radiofrequency and microwave radiation, ultraviolet and infrared light, static magnetic fields, and ventilation effectiveness is discussed. Areas of traditional industrial hygiene focus, such as photolithography, are addressed, with an emphasis on best available assessment approaches. Maintenance tasks, such as wet etch/deposition tool chamber cleans and ion implanter parts cleaning, often present the greatest potential for employee exposure to health hazards. Specific exposure issues are presented, such as cyanide compounds in metal etch chamber residues and arsenic surface contamination from ion implanter parts, with a discussion of commonly reported monitoring results.
The art and science of industrial hygiene involves the recognition, evaluation, and control of workplace hazards. At first glance, the typical semiconductor fabrication facility (fab) appears to be a pristine, clean environment. Great care is taken to ensure ambient air purity, with newer processing tools isolated in mini-environments to minimize potential product contamination. Fab employees appear well protected in uniformly white head-to-toe garments. The fab itself is quiet relative to a typical heavy manufacturing environment, and production employees are rarely exposed to extreme safety conditions.
So the new industrial hygienist gowns up once, wanders around, becomes entranced by the coaters for a while, and leaves thinking all is well and she better get back to that flickering computer monitor problem near the electrical panel…
Scratching the surface a bit, a more complex world unfolds. The typical fab presents a variety of chemical and radiation hazards, many of which are unique to the semiconductor industry. In many cases, these hazards are well recognized before the equipment is installed. The processing tools use an array of toxic gases, solvents, and metals during normal production. Maintenance activities may introduce additional chemicals, and often create opportunities for employee exposure that do not exist during normal production. Radiation sources include ionizing radiation generated from high voltage/current processes, radiofrequency (Rf) and microwave (MW) radiation from plasma operations, ultraviolet (UV) and infrared (IR) radiation often as secondary hazards from plasma and other processes, and static magnetic fields from tools with large magnets.
This paper reviews hazard evaluation techniques for the semiconductor facility, then presents examples of some specific exposure situations.
Industrial Hygiene Evaluation
Before conducting any field monitoring, the industrial hygienist must create an evaluation strategy. Instead of classifying employees into similar exposure groups only by job classification, it is especially important in the fab environment to assess exposures by task. Such a strategy should include information on the tasks conducted, tools and equipment used, associated hazards, and controls in place. Each item on the strategy should be prioritized, and the monitoring plan designed to capture the high priority tasks first. In reality, industrial hygiene monitoring is often strongly focused on employee concerns and odor complaints, in addition to the prioritized tasks.
Industrial hygiene monitoring in the fab typically includes some or all of the following equipment:
- Air sampling pumps and media or passive dosimeters for integrated personal and area samples
- Direct-reading equipment such as the MDA TLD-1 or colorimetric indicator tubes
- Filter media for collecting surface wipe samples
- Geiger-Mueller meter and ion chamber for ionizing radiation
- Rf/MW meter
- UV/IR radiometer
- Static magnetic field meter
At the same time that chemical and radiation hazards are evaluated, ventilation controls are typically evaluated, too. This will almost always include evaluation of wet bench face velocity, which is quite different from laboratory fume hood face velocity evaluation. The tools required include a thermal anemometer or velometer, a tape measure, and a vapor visualization device.
Normal Production Monitoring
The “normal production” environment refers to the (sometimes rare) state of production tools processing wafers under standard operating conditions. In this environment, the tool enclosures minimize chemical and radiation exposures to the fab operators. Some direct chemical exposures may still exist, as when using open chemical baths or performing routine cleaning steps, but these exposures are usually of short duration. Radiation exposures would typically only result during normal production from tool leakage due to inadequate tool maintenance.
One frequent area of odor generation, and resulting employee concern, is photolithography. Newer photolithography equipment is designed to keep contaminants out of the process, which conveniently keeps odors in. Many facilities, though, are still plagued by recurring nuisance solvent odors. Because of the toxic nature of photoresist solvents previously used in the industry, media attention, and the relative lack of odors in other areas of the fab, among other factors, this area frequently induces employee concerns and, therefore, ends up near the top of the industrial hygiene priorities list.
Photolithography solvents can be monitored in a variety of ways, from traditional air sampling pumps and media to portable gas chromatography units (ref. Gunderson, Fall 1998 SSA Journal). Gas chromatography has low enough detection limits to record background solvent concentrations in the ppb range. These detection limits are required to detect and track concentrations of chemicals with low odor thresholds, such as those present in photolithography areas.
Wet benches containing either solvent or acid chemistry are prevalent throughout most facilities, presenting some of the most common direct chemical exposures to fab operators. The primary exposure controls are covers on the chemical baths themselves and ventilation. In order to assess airborne exposure, air sampling pumps can be used to collect both personal and area samples.
In addition to chemical monitoring, the effectiveness of the ventilation system should be periodically checked. Wet bench local exhaust ventilation functions differently from a traditional laboratory-type fume hood. The typical wet bench design uses the room laminar flow to increase capture efficiency at the chemical baths. This design moves the capture plane from the lab-type hood “face” to the chemical bath surface (see Figures).
Although a wet bench is not the same as a lab hood, many facilities in California, as well as facilities located in other states, use the 100 fpm face velocity requirement from the California regulation “Ventilation Requirements for Laboratory-Type Hood Operations” (8 CCR 5154.1) as an internal wet bench ventilation standard. This standard can be used as a guideline, but should be supplemented with qualitative capture efficiency data such as that obtained with a vapor visualization device.
If equipment is well maintained, it is unusual for fab operators to be exposed to any type of radiation during normal production. Ion implanters are periodically surveyed, by running the survey meters along all surfaces of the tool, to ensure that no ionizing radiation leakage is present. Plasma tools are periodically surveyed for Rf or MW radiation to ensure that no leakage is present at the back of the tool (many instances of Rf leakage result from missing screws in the matching network or other parts of the tool directly exposed to Rf). Viewports of plasma tools are surveyed for UV and/or IR exposure. As long as the viewports have UV shielding, exposures are unlikely. IR radiation may also be present at horizontal diffusion furnaces. That warm, orange “glow” actually presents a significant source of IR exposure. Finally, any tool with a static magnet should be surveyed to determine the extent of the magnetic field, so that hazard alerts can be posted for pacemaker wearers.
Maintenance tasks, particularly those that require opening enclosures or tools, can lead to employee exposures not present during normal operation. The loss of ventilation and the influx of room air create chemical exposures for maintenance personnel (ref. Frist, Spring 1996 SSA Journal). Maintenance exposures may arise during routine preventative maintenance and during trouble shooting.
Etch and deposition chamber tools are at the forefront of maintenance exposures. The chemicals used in these tools often leave highly corrosive residues of fluoride or chloride compounds. Because of the nature of plasma processing, the residues left in these chambers are of unknown chemical composition, so the industrial hygienist needs to do some predictive chemistry, look at previous monitoring reports, and use professional judgement in the hazard recognition stage of sampling strategy development.
For example, over the past year or so, several reports of cyanide and cyanogen compound presence in the air during manual metal etch wet chamber cleans have been reported (ref. Pais, 1998 SSA proceedings). The typical metal etch process gases include some combination of chlorinated compounds (Cl2, BCl3), halogenated organics (CF4, CHF3), fluorinated compounds (SF6, NF3), and inerts (N2, Ar). While integrated personal air samples are almost always non-detectable for most potential process by-products, area samples located in a worst-case position and direct-reading measurements have shown halogen (HF, HCl) and cyanide compound (HCN, CNCl) concentrations above the American Conference of Governmental Industrial Hygienists (ACGIH) exposure limits. This is significant in that the exposure limits for these four compounds are all Ceiling values, which should not be exceeded during any part of the work day.
Ion Implanter Tasks
Ion implanters use materials such as arsenic, arsine gas, boron, boron trifluoride, and antimony in semiconductor wafer processing. Exposure to arsenic, a carcinogen, is a particular concern during maintenance activities. Some activities, such as source chamber cleaning, present significant arsenic exposure to maintenance technicians. Other activities, such as source removal or parts cleaning, may present lesser exposures. In most cases, arsenic surface contamination is a concern (ref: Roberge, 1998 SSA proceedings).
Industrial hygiene monitoring provides a means to evaluate potential hazards to human health in the semiconductor manufacturing environment. The monitoring techniques available continue to evolve, as with portable gas chromatography, and decreasing chemical detection limits. Even with state-of-the-art monitoring equipment, the industrial hygienist must use professional judgment in evaluating results and making recommendations, particularly when multiple monitoring techniques are used. As new semiconductor processes are introduced, industrial hygiene techniques can be used to help identify potential hazards before introduction of the process to the manufacturing environment, in addition to monitoring in the production fab.