Detecting Hazardous Gases in the Fab

As new materials are increasingly being evaluated and adopted for new technologies to satisfy IC performance requirements, semiconductor fabs continue to carefully scrutinize the emissions and byproducts of these materials to ensure that no significant health or environmental issues arise.

One simple emission monitoring method is paper tape-based colorimetric gas detection technology, which has been used by the semiconductor industry since 1985. In spite of rapid advancements made in other gas-sensing technologies, such as infrared (IR) spectroscopy and electrochemical sensors, paper tape-based colorimetric technology is still the preferred method when addressing the most demanding requirements. Applications where colorimetric technology can be used include semiconductor manufacturing, nanotechnology, research labs and government/high-technology uses.

Some of the advantages to colorimetric technology include:

• Visible proof of a gas release event (via paper stain)
• Great sensitivity (detection down to ppb levels)
• Superior selectivity (little cross-interference with other gases)
• Fast response time
• Stability (consistent, repeatable operation with no calibration required)

The primary advantage of colorimetric detection is that it offers visible proof of a gas emission and the concentration level of the gas. Detection relies on a paper tape (impregnated with a dry reagent medium to collect an air sample), combined with an optical scanner, to detect the presence of gas (Fig. 1). When the tape is exposed to a target gas, it changes color in direct proportion to the concentration of gas present. The tape serves as a real-time log of the presence of toxic gases in the fab's air; any deviation from acceptable limits registers as a color change on the paper tape. A multipoint detection methodology is also possible if air samples are desired from more than one area (Fig. 2). In this example, colorimetric technology offers the advantage of speed of earlier response, which enables the fab safety manager to make a better, more informed decision on whether to shut down the process. For a post-event gas release, the fab manager can also choose to view the tape through a remote camera operation, if desired. Paper tape gives the most reliable confirmation of an actual toxic gas release at the lowest possible detection limit.

1. A colorimetric detection method works by collecting fab air and exposing it to chemically treated paper that changes color in proportion to gas concentration. The monitoring system’s firmware uses concentration vs. optical density response curves to calculate, record and display the gas concentration.

When used correctly, paper tape technology meets global health and safety requirements and best practice guidelines, and protects workers from the risks of toxic gases. Fab managers need proof of hazardous gas emissions to protect workers, as well as for preventive maintenance and legal reasons. If the average productivity lost during a semiconductor plant shutdown is ~$40,000/min, and the average downtime incident is 30 min, then each false alarm costs up to a million dollars or more in productive manufacturing time, not including the cost of product lost to scrap.

2. A multipoint detection methodology has sample tubes running into a multipoint toxic gas monitoring system (with an internal power module and pump that can draw from sampling points as far as 400 ft away from the system).

Classic colorimetric technology

3. This tape has been used to detect phosgene gas, with >98% reproducibility.

Changes in gas detection/monitoring requirements have inspired a number of innovations within the colorimetric R&D lab. Chemists have created new chemical formulations to achieve new flavors for more gases, chemicals and hybrids. The sensitivity for certain gases detectable by colorimetric technology typically cannot be equaled by other technologies.

Colorimetric paper-based technology also happens to be the preferred gas detection methodology used in many applications by NASA (Washington, D.C.), FEMA (Hyattsville, Md.), the Environmental Protection Agency (EPA, Washington, D.C.) and other government agencies. During the recent, well-publicized takedown of a U.S. rogue spy satellite, FEMA regional offices deployed additional monitors equipped with the colorimetric technology to monitor and detect any hydrazine-contaminated debris that fell to Earth.

Sensor technologies do not typically make like-for-like comparisons. The major technologies developed for gas monitoring — electrochemical, catalytic bead, IR, photoionization detector and colorimetric — has its own distinct advantages and disadvantages. For example, electrochemical cells (ECs) offer end users an attractive entry-level pricing per point of gas monitoring, are generally built within a small footprint and are easy to install and connect to. On the downside, EC is prone to cross-sensitivities to gases in addition to the target gases; manufacturers generally recommend periodic bump testing of EC monitors to maintain the sensor's optimal sensitivity and workability. It also depends on frequent calibration intervals and may not be best suited to detect certain gases produced in the fab environment. The question faced by fab safety managers when evaluating detection technologies often comes down to the cost of initial purchase vs. cost of ownership.

Colorimetric detection is a calibration-free method of toxic gas monitoring; this drives down the cost-per-detection-point over the life of the analyzer. Some tapes, for example, are manufactured to traceable ISO 9000 manufacturing standards by trained chemists, and each batch is calibrated with ultrapure gas that is traceable to nationally accepted standards. The end user does not need to provide calibration facilities, thereby saving additional cost and reducing the risks of chemical exposure.

The colorimetric paper-based medium and instrumentation and software that control the detection principle are uniquely integrated. Together, they can provide a low point-per-detection cost, a higher degree of reliability and a higher selectivity of gases monitored — with visible proof of detection.