Products in Action

Acoustic Sensor Improves Hazardous Gas Monitoring in Cypress's Round Rock Facility

To ensure fab safety, semiconductor industry environmental, health, and safety (EHS) personnel must find suitable monitoring technologies for hazardous gases. Controlling monitoring costs is a constant challenge. However, finding a reliable and specific detection method for hydrogen, which is widely used in semiconductor processing, is the primary concern. Aside from the obvious safety issues, the lack of an effective monitoring approach can mean disruption of operations caused by false alarms. SonoSense acoustic sensing technology (TeloSense, Fremont, CA) reliably and specifically monitors hydrogen gas in the fab without the costly personnel intensive upkeep of typical discrete sensors.

Hydrogen gas is explosive in the range of 4­75% by volume in air and is commonly used with diffusion, epitaxy, and CVD processes as well as emissions abatement equipment. It is often mixed with other process gases and is a process by-product. Potential sources of hydrogen leaks within a fab include bulk hydrogen delivery tanks, gas delivery lines, gas manifolds, valve boxes, regulators, the point at which a gas line enters a tool, and exhaust ducts.

If hydrogen concentrations reach the flammable range, an ignition source could cause a fire or explosion. To reduce this risk, cleanroom safety requirements generally state that hydrogen must be detected at well below its lower explosive limit (LEL) of 4% by volume in air. Gas monitoring system alarms are usually set to trigger at concentrations of 10­20% LEL. Some fabs are increasing their internal standards and requesting monitors sensitive enough to detect <1% LEL. Between 100 and 200 sensors may be installed at gas monitoring locations, which are often linked to a centralized data monitoring system that notifies safety personnel of a potential problem.

Hydrogen monitoring technologies used in the semiconductor industry -- such as electrochemistry, catalytic combustion, and bulk modulus (solid-state sensors) -- have typically been adapted from other industries. Since each has specific strengths and limitations, a given technology may be preferable for a specific application. There is some overlap in the different sensors' capabilities, and all may be operated simultaneously since safety engineers prefer to have backup systems in case one type fails.

Hydrogen monitoring is usually based on individual distributed sensors that are subject to degradation and zero drift, so the units must be periodically challenged, recalibrated, or replaced. Accurate calibration procedures are time-consuming and must be performed by a technically skilled staff. If exposed to a rapid flow rate or excessive concentration of gas during calibration, the sensors can take hours to restabilize.

David Morris, safety and environmental engineer at the Cypress Semiconductor fab in Round Rock, TX, uses these monitoring technologies. The fab does not have a full-time gas detection/maintenance staff and outsources quarterly maintenance of monitoring equipment. (Larger fabs often dedicate technicians to the ongoing maintenance of the electrochemical sensors.) Because these services are purchased, he was able to quantify costs for monitoring hydrogen. "Maintenance personnel have to physically take a calibration standard to each sensor, expose the sensor to the standard, and adjust its gain and zero offset," explains Morris. "With this procedure, two people needed one full day to perform maintenance on 16 sensors. The cost was several thousands of dollars per year."

These sensors are also subject to RF interference from handheld radios, are temperature sensitive, and can generate false alarms because of cross-sensitivity to isopropyl alcohol, ammonia, and other chemicals. These false alarms can cost fabs more than $20,000 per hour in lost production costs alone and have occurred an average of two to three times per year at his plant, says Morris. Added inconvenience and associated costs caused by false alarms include the ungowning and regowning of cleanroom staff during the evacuation procedure, the large size of the units, parts replacement, and supplies consumption.

In addition, existing sensing technologies do not specifically detect hydrogen. The SonoSense acoustic sensing technology, however, overcomes this deficiency. Ultrasound and a computing process are used to sense hydrogen through measurements of physical properties versus reactive chemical properties. This technology is based on hydrogen's ability to increase the speed of sound in air, a characteristic few gases possess. It detects hydrogen concentrations down to parts-per-million levels, and alarms may be set as low as 1% LEL. An acoustic analyzer senses the time-of-flight of sound waves through air samples. Standard thermodynamics computes hydrogen concentration.

When there is hydrogen in the air sample introduced to the acoustic analyzer, the time-of-flight of sound pulses will differ from those in normal air caused by changes in effective molecular weight and effective heat capacity. This results in an increase in the speed of sound in the sample and a corresponding decrease in the time-of-flight of the sound waves.

The acoustic sensing technique responds only to the presence of those gases that increase sound speed in air, such as hydrogen, helium, methane, ammonia, and water vapor. The effects of water vapor and ammonia--as well as any heavy gases--are eliminated by passing the air samples through a silica gel composition buffer prior to reaching the acoustic analyzer. These gases adsorb, then slowly desorb from the composition buffer, resulting in an equalizing effect that eliminates short-term, sample-to-sample changes in air composition. Once every 10 minutes, a zero reference or autobaseline compensates for any resulting offset.

When a response is obtained by the acoustic analyzer, a proprietary technique known as Getter Validation eliminates the chance of a false alarm caused by helium or methane. In this operation, the air sample flows through a palladium-containing getter that reduces hydrogen concentrations by oxidizing it at the palladium surface, and a diminished response confirms the presence of hydrogen, not helium or methane. The sample is then reanalyzed to obtain a final reading. (A methane-specific getter is available as an option.) The getter validation operation takes an additional four seconds to confirm hydrogen in a sample after an alarm set point is exceeded.

Twenty sample locations at distances of up to 1000 feet are continuously sampled and managed from a centralized monitor. The analysis sequencing is user programmed and occurs at a rate of every two seconds per analysis. Sample air is drawn continuously at 75 m/min using a high-reliability rotary carbon vane pump. Air is drawn from the sampling tubes into an acoustic analyzer, which can resolve hydrogen/air mixtures to low-parts-per-million levels. This device consists of two stainless-steel waveguide tubes with caps welded to both ends to provide fixed-length acoustic paths. One tube is for samples, and the second, hermetically sealed and clamped to the first in order to maintain common tube temperature, serves as a reference. Piezoelectric crystals mounted to the end caps outside the tubes serve as the ultrasonic transmitters and receivers. An electronic counter measures the time it takes for the crystal-generated sound bursts to travel from the initiating ends of the tubes to the receiving ends.

The computing process uses mathematical division to put all drift and temperature factors in both the numerator and denominator, canceling their effects. Thus the acoustic analyzer is completely drift-free and never needs to be calibrated.

Morris has used the SonoSense centralized multipoint system within Cypress's Round Rock facility for a year. "This is the most trouble-free detection device we have," says the Cypress engineer. "It has good reliability, a lack of moving parts subject to wearing out, less mechanics.... After a short burn-in period, the system has been on-line for one year with no trouble... and no false alarms. Over a 10-year period, it will be significantly cheaper to run than other systems and will have a substantially lower cost of ownership."

The simplified architecture of this monitoring system eliminates the heat, noise, and mechanical difficulties associated with pumps used in traditional systems. (The Cypress facility employs the optional low-noise Venturi pump.) It also replaces bulky sensors with smaller monitoring points, making installation easier.

A twice-yearly preventive maintenance routine is all that is required to service this monitor. "The technicians perform a routine gas-test at a selected tube," explains Morris. "If one works, then they can verify that all tubes are working and do not have to physically go and test each sensor point." Self-test routines eliminate the need for constant checking by fab personnel. If an error is detected during a self-test, then an alarm is posted on the central gas monitoring system. All alarm and maintenance events are recorded.

The design and operating principles of the acoustic sensing technology provide hydrogen specificity and reliable detection without false alarms. Operating costs and downtime are minimal since no calibration is required, and only a few hours of annual maintenance are needed. In sum, technology provides fabs with fast, reliable, cost-effective hydrogen detection at a sensitivity well below the required safety level.

MicroHome | Search | Current Issue | MicroArchives
Buyers Guide | Media Kit