Gas Detection Strategies

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Gas Detection Strategies

• Overview of Selection Criteria for Gas Detection
  Systems

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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?

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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

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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

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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?

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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

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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

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Developing Selection Criteria
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Developing Selection Criteria
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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?

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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.

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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

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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

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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.

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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.

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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.

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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

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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)

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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!

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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.

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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.
• Dont forget to add time to suit up and move
  around in the fab!

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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

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Understand your Trade Offs!
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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

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