Gas Detection Strategies
Overview of Selection Criteria for Gas
Detection Systems
How do I choose?
• Which Gas Detection Technology is best for my application?
• What criteria is important to me?
• How do I understand the specifications that are given to me?
Which Technology is Best for my Application?• Understand that there are different
technologies for most gases
• Understand that some technologies do not work for certain gases– Examples: FTIR cannot detect diatomic
molecules (Cl2, Br2)– Example: Paper Tape (Chemcassettes) cannot
detect Hydrogen
Which Technology is Best for my Application?• Develop a selection criteria and use it to
evaluate the different technologies
• Make sure you are comparing apples with apples – many suppliers call data by the same name but calculate it differently
Developing Selection Criteria
Questions to Consider Before We Begin
• Review Fab Monitoring Locations-- Tool Exhaust -- Fab Ambient Air
-- VMB -- Gas Cabinet-- Subfab Ambient -- Pump Housings
-- Process Protection Applications
• Do You Want the Same Criteria for Each Location?
Developing Selection Criteria
What is Important for the Application(Usually Same List / Sometimes Different Priorities)
• Specificity• Speed of Response• Lower Detectable Limits• Reliability • Accuracy• Cost of Ownership• Initial Cost of Capital Investment
Developing Selection Criteria• Establish a Spreadsheet for each Monitoring
Application (Ambient, Exhaust, Gas Cabinet, etc.)
• List your Criteria• Rank your Criteria• Sort your list by Rank• Evaluate each technology by the criteria• Choose the technology that best meets your
needs
Developing Selection Criteria
Monitoring Location: Ambient Monitoring Location: AmbientCriteria Rank Criteria Rank
Specificity 1 Specificity 1Speed of Response 2 Speed of Response 2Lower Detectable Limits 6 Reliability 3Reliability 3 Accuracy 4Accuracy 4 Cost of Ownership 5Cost of Ownership 5 Lower Detectable Limits 6Initial Cost of Capital Investment 7 Initial Cost of Capital Investment 7
Developing Selection Criteria
Monitoring Location: Gas Cabinet Exhaust
Monitoring Location: Gas Cabinet Exhaust
Criteria Rank Criteria Rank
Specificity 7 Cost of Ownership 1Speed of Response 4 Initial Cost of Capital Investment 2Lower Detectable Limits 3 Lower Detectable Limits 3Reliability 5 Speed of Response 4Accuracy 6 Reliability 5Cost of Ownership 1 Accuracy 6Initial Cost of Capital Investment 2 Specificity 7
Developing Selection Critera
You May Ask Yourself…….Specificity? Speed of Response?
Reliability? Accuracy? Lower Detectible Limits?
What Does Do These Terms Really Mean to Gas Detection?
Specificity
• No cross sensitivities to other substances commonly used in a semiconductor fab– Review manufacturer data – this information
should be published and readily available– Be sure to include non toxics in your review such
as cleaning solvents!!! They may have undesirable effects on your gas detection system.
Speed of Response
• Speed of response is the most difficult data to compare between technologies
• Several factors affect speed data– Data often stated differently from
technology to technology, makes comparison difficult
– Hidden technical aspects unique to each technology
Speed of Response
• Statements of Response– 30 seconds at TLV
• Response time usually varies over the range of the technology, slower at lower concentrations, faster at higher concentrations
– T90 in 30 seconds• Sensors responds to 90% of the concentration in 30
seconds, hits a response peak, levels out and very slowly responds to the last 10%.
• If the leak concentration borders around the TLV, you may only detect 90% of TLV, and not actuate an alarm
Speed of Response
• Statements of Response– Other Factors to Consider
• Some technologies respond to a leak at the end of a fixed period
• Some have to verify the leak if other cross interferences are present, adding to the time to alarm
• Many technologies do not have the same response time over the lifespan – they are quick when first installed but become slower, more sluggish the longer they are installed in the field. When recalibrated at specific intervals, concentration response is measured but many test companies do not log the time to response, showing that the sensor is responding much slower than expected.
Speed of Response
• Statements of Response– Other Factors to consider (continued)
• Some technologies are not continuous, so the time it takes to scan between points needs to be added in
• Beware of suppliers who tell you their response is “instantaneous” – that is NOT data! Ask them to define instantaneous with data. You will find it means 10, 20, 30 seconds.
Lower Detectable Limit
• LDL is important to semiconductor industry due to high air flow rats
• High air flows can dilute gases to a level undetectable by some technologies
• No monitor starts at “zero” – every has a level between zero and LDL which is electronic noise where they cannot reliably detect. Question 0-100 as a monitoring range.
Reliability
• High air flows can dry out /cause false detections on some technologies
• Cross-sensitivities
• Drift (caused by temperature, humidity, air flow)
• RF interference can cause false positive detections
Reliability
• Look for extensive self-diagnostics that can compensate for known problems – Can the detector compensate for drift and
translate it as a maintenance issue not a false gas alarm?
– Can the detector identify problems that could cause it to miss a gas detection (paper tape – out of tape, electrochemical – dried or failed sensors, FTIR – spectrums overlapping the target gas detection wave length area)
Accuracy
• Gas detection should have a stated accuracy specification
– Example: +/- 10% the direct reading
– Example: +/- 10% at TLV
• These statements do NOT mean the same thing!
Accuracy
• Example: minimum of 2% full scale drift per month – Point: Understand what this means – if
the technology has a narrow monitoring range, and is maintained every 6 months, the sensor could easily drift into the alarm range. If the sensor has a large scale, probably not a big problem.
Cost of Ownership
• Cost of consumables• Cost of labor – ask for an estimated # of
hours per point and compare by using a standard labor rate
• Example: .25 hr per point x 500 points x $25.00 a hr.
• Don’t forget to add time to suit up and move around in the fab!
Initial Investment Cost
• When comparing this cost it is helpful to average it with the cost of ownership over 5 years.
• Many technologies are expensive up front but inexpensive to maintain
• Many technologies are inexpensive up front but expensive to maintain
Understand your Trade Offs!
How Do I Choose?
Back To Our Initial Question!
• Establish what your corporate requirements and priorities are
• Work with suppliers to understand the data you are being asked to review
• Correlate the data with your priorities to make your selection
