Semiconductor fabrication facilities consume much hazardous gases in their production processes. These gases are stored, distributed or used in fabrication throughout the processes. The primary hazards arise from these gases are fire, explosion, intoxication and corrosion resulting in immeasurable losses or unscheduled shutdown. These gases must be continuously monitored to ensure the health and safety of employees, to protect property, as well as to maintain regulatory compliance.
Continuous gas monitoring in semiconductor facilities is compulsory in most cases by Occupational Health & Safety Administration (OSHA), the FM Approvals, the NFPA Fire Protection standards and some regional specific standards. These regulations and standards offer guidelines concerning the proper design, installation and operation of hazardous gas detection systems.
A reliable and proven gas detection and monitoring systems or Life Safety System (LSS) is crucial for the semiconductor plant Process Safety Management (PSM) which consisted of gas detector, gas indicator, process controller and annunciation with safety interlocking mechanism.
A reliable LSS start from a reliable and proven gas detector. The main reason is the toxic and combustible gases are very deadly that could create fatalities and cause serious damages to asset in split second and small dosage. It is important for the gas detector to be able to detect toxic gases in very low ppm (part per million) or ppb (part per billion) in some cases. The gas detector must also be able to respond to the presence of toxic gases within seconds without subjecting to cross interference from some other gases. False alarm can be a nuisance to the production and in severe cases interrupted the production. These can be achieved by a membrane separated electrochemical sensor within the gas detector.
Membrane Separated Electrochemical Sensor features gas permeable membrane on the side of the container filled with electrolyte. An electrolyte layer is positioned between the working electrode and the gaseous electrode.
A potential circuit maintain a constant potential difference between the working electrode and reference electrode. In the normal condition (no gas presence), the milivoltage is maintain constant. When the toxic gas molecule in contact with the gas permeable membrane, the gas molecule will permeate through the membrane and react with the electrolyte. The gas molecule has been reduced by the electrolyte and the ion generates milivoltage, The milivoltage is being infer to the ppm/ppb for the targeted gas.
When HCL molecule in contact with the electrolyte,
HCL ------> H+ + CL-
H+ will further react with I- and IO3- from the electrolyte,
6H+ + IO3- + 5I- --------> 3 I2 + 3 H2O
That also explained why Membrane Separated Electrochemical Sensor will not subjected to cross interference from hydrogen and alcohol.
Alarm Management is also an issue for process safety management where there is reliance on human response to an alarm to prevent or mitigate incidents, as well as major hazards. False alarms can be a nuisance at the beginning to the operators and eventually create the habits of ignorance in the long run. Although a good gas detector can minimize or eliminate false alarm from gas cross interference, false alarms can also attributed to the plant maintenance when the maintenance crew is maintaining and testing the Life Safety System (LSS) at the process controller level. The situation will become worst if the LSS is under maintenance and all the safety alarms are being isolated. In the event of actual gas leakage, the LSS system will not be able to receive the alarm signal from the gas detector. It is important to install a local gas indicator and annunciator to localize the alarms to warn the operators of immediate danger.
It is crucial to connect the gas detectors to the gas indicators having individual indication channel to represent the gas detection status of the targeted gas. Some plant owner might prefer to use Programmable Logic Controller (PLC) which will work fine as long as the failure of the single controller shall not affect the functional integrity of the system or a failure of an Input/Output (IO) card affecting the signal input and output of a few number of gas detectors.
The ideal gas indicator should be simple and easy to maintain without requiring periodic maintenance. It should be installed and forget as long as the consistent power supply provided. Each of the gas indicator should have the intuitive indication features by indicating the the level of gas leakage in a rising color bar with green, yellow and red which are universal indication for safe, alarming and danger.
The indicator card should be plug and play without required much skills and knowledge on part replacement.
A gas detection and monitoring system or Life Safety System (LSS) should be open, scalable and easily configurable without depending heavily on the service provider. The hardware failure from one area should not be affecting the functionality of the complete system. Hardware redundancy might help to enhance the system uptime but segregation or segmentation can help to minimize the risk of complete system shut down.
Having a good range of portable gas detectors in the plant is also important for complimenting the LSS. The placement of the fix gas detectors might not be the most ideal and need to be evaluated from time to time. Semiconductor industry is quite a versatile industry where the tools placement and production layout might change due to new tooling installation. Portable gas detectors can further assist the plant operator for further assess the potential hazard due to gas leakage.