Pressure differential is a key ingredient in cleanrooms and clean zones. Differences inpressure can help control airborne contaminants both in clean rooms, clean zones and cleanroom hoods (laminar and non-laminar flow hoods). I am surprised how often I see pressure differential used improperly in some settings. It’s important to understand pressure differential, how and when to use it.
Negative Pressure is acheived when air is evacuated from an area, room or hood. Laminar flow hoods often use negative pressure to sweep away contaminants from a specific processor in areas where particles and/or other contaminants are being generated. The principle is simple, and when designed with the correct airflow, quite effective. Once the air is removed from the area, it’s important to have a clean source of make-up air to replace the air that is being drawn from the area. Evacuated air is either exhausted or run through a series of filters and returned to the room. If the air is exhausted, it is very important to have filtered air returning to the area to avoid pulling in outside contaminants. This ensures a clean work area and contributes to worker safety while cutting down on cross contamination.
Negative pressure hoods or booths are popular in the microelectronics industry and is becoming popular in government mailrooms where they are pre-screening mail for biological contamination.
Positive pressure areas are generally used in cleanrooms as away to keep particle contamination from entering the room or area. The principle of positive pressure is to supply an area with enough clean, filtered air to keep contaminants from entering the room or area. Positive pressure areas are an effective method for reducing cross-contamination and is often misapplied in areas where contaminants are being produced by a process.
The general rule of thumb is to use positive pressure to keep contaminants out of an area, and negative pressure to capture contaminants and keep them from contaminating surrounding areas or worker breathing zones. An important thing to keep in mind when designing or applying pressure differential is to account for the make-up air. For every cubic foot of air that is moved, it will be replaced, if this air needs to be clean, you must filter it.
Minienvironments and isolation technology are common elements in semiconductor and other manufacturing industries that rely on cutting-edge technology. The use of minienvironments and the associated automation can not only enhance control, but also improve process cleanliness.
Minienvironments can come in a number of configurations, varying in shape and size. Conventional minienvironments were fashioned around the process tool after the tool was positioned in place. However, more modern minienvironments are incorporated as part of the process equipment. The toolâ€™s design, airflow patterns and pressurization are important factors in maintaining the cleanliness classification at which the tool is designed to operate.
Using particle counters to evaluate and keep track of minienvironments is an established practice in the cleanroom manufacturing process. Generally, a particle counter is employed as the principle device to complete the testing, characterization and fine tuning process. This is normally the same particle counter used to monitor the main cleanroom
Increase in Continuous Monitoring
Increasingly, certain risk factors are causing manufacturers to opt for continuous particle counting to monitor their minienvionments. Continuous particle counting methods include dedicated particle counting and sequential sampling system.
Dedicated particle counting typically involves a remote particle counter. Lacking many of the features of a portable device, a remote particle counter has its data recorded and displayed by an external computer or other mechanism. The dedicated particle counter is connected to the minienvironment internally or externally with the sample probe situated at a specific location inside the minienvironment.
A sequential sampling system is often referred to as Manifold Particle Counting System or Multiport Sampling System. As its name indicates, it allows a single particle counter to sample many locations in sequence.
Hereâ€™s how it works: The particle counter is positioned in a set location attached to the Multiport Sampling System. Tubes connect to multiple areas in the cleanroom or minienvironments. An external blower draws air from the tubes on a continuous basis.
Ultimately, the particle counter samples from each location one tube at a time. Sampling times typically represent the sampling of one cubic foot of air. Multiport Sampling Systems generally can connect up to 32 sample locations to a particle counter.
MiniManifold Provides a New Solution
There are several disadvantages with dedicated and manifold particle counting that have given rise to a new solution. A single particle counter or particle sampling location canâ€™t adequately monitor the entire minienvironment. Adequately monitoring the full environment requires multiple particle countersâ€”which adds considerable expense.
However, a MiniManifold has been developed to solve this dilemma. Essentially, it allows a single particle counter to monitor up to six locations continuously in a single minienvironment or process tool, while correctly counting particles from each location.
The MiniManifold is made up of a manifold sequencing system with a built-in controller and external blower. The controller communicates with the particle counter and can interface with a computer system or process tool. The blower, which can be remotely installed from the manifold, draws from all six locations at the same time.
The MiniManifold also has a buffer to store data records for each location. This greatly increases the coverage of a single particle counter inside this type of equipment. The enhanced coverage, in turn, increases the chance of detecting a contamination event.
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I recently had a customer ask us some questions about certifying and testing two cleanzone tents in which he wants to validate to ISO 14644 class 8. As he did not have prior experience validating to ISO 14644 standards, he purchased the ISO 14644-1 and ISO 14644-2 books from the IEST. As with most people the first time through, he has struggled to understand exactly what he needs to do in order to validate. Below is a question and answer session that transpired between the customer, myself and Jim Akey from LWS.
Q. – Based on the area (in meters) of each room (15â€™ X 18â€™), the minimum sample point locations is 5 per room. Using a DPC to determine room particle counts, a sample volume is needed. Based on ISO 14644-1:1999(E) Annex D, and an ISO Class 8 requirement, Cn is very large, and Vs is small (less then 2 liters). Do I just take 28 liters over 1 minute as a standard? Why? If not, how do I determine the sample volume?
A. – This is a two part answer:
1. Sample Locations – 14644 states that to determine the minimum number of Sample Locations (B.4.1.1) you take the square root of the Area (in m2). If the room is 15′ x 18′, this equates to 25.08382 square meters. The Square Root of 25.08382 (room area) = 5.0083749 (sample locations).
ISO 14644 states that you must round up, so 5.0083749 = 6 Locations.
2. Sample Volume per Location – Vs = 20/Cn,m x 1000 (ISO 14644 B.4.2.1)
Vs = “is the minimum single sample volume per location expressed in liters (except see B.4.2.2)”
Cn,m = “is the class limit (number of particles per cubic meter) for the largest considered particle size specified for the relevant class”
20 = “is the defined number of particles that could be counted if the particle concentration were at class limit”
For the above room and ISO class (I am using 0.5 microns and greater) the formula would be:
Cn,m = 3,520,000 (number of particles at 0.5 micron and greater that are allowed per cubic meter – this is determined by the chart in the standard)
20 / 3,520,000 = 0.0000056
0.0000056 x 1,000 = 0.0056
0.0056 liters would be the minimum sample volume, except that B.4.2.2 states “The volume sampled at each location shall be at least 2 liters, with a minimum sampling time at each location of 1 min.”
Since 0.0056 liters is less than the 2 liters required in B.4.2.2 – the minimum sample volume at each location would be 2 liters.
Q. – How is the number of particles per cubic meter calculated (ISO 14644-1:1999(E) Annex D.1.7)? I need the number of particles per cubic meter to calculate the 95% upper confidence limits since the number of sampling locations is less then 10.
A. – The easiest way to see if you are within classification is to compare the particle counts to the chart (Table 1) in the ISO standard. There is a formula to determine this – Section 3.2 of ISO 14644-1
ISO 14644-1 Cleanroom Standards
Notes: Number of particles per cubic meter (m 3 )
Q. – I would like to assess the air change rate â€“ which I think is â€œAirflow Volumeâ€™ or perhaps â€˜Airflow Velocityâ€™? According to ISO 14644-3:2000(E), I think I would reference Annex B.4 Airflow test? What minimum air change rate should I use as the acceptance criteria? I have heard 6 to 20/hour, but have not seen the number in print any where. Is there a formula? Can you provide me a reference? Is Airflow Velocity the same as Airflow Volume?
A. – Air Velocity is not addressed in sections one or two. I would suggest you purchase the ISO 14644-3 standard which specifies test methods for characterizing the performance of cleanrooms and clean zones. If you are only interested in calculating air exchanges per hour, the formula is very simple. Calculate total volume of air (cubic feet) by multiplying length, width and height of the room. The resulting number is your volume of air in cubic feet. Divide your cubic footage by 60, the resulting number is the amount of cubic feet per minute required for one complete air exchange. Divide this number into the amount of CFM your air handler is delivering, the resulting number is your air exchanges per hour!
L X W X H = Cf
Cf/60 = X
CFM/X = air exchanges per hour Q. – I am also proposing to perform:
a. A filter leakage test to verify filter integrity; b. Airflow direction test and visualization (in the operational state only) to verify the required airflow pattern; and c. Containment leakage to verify non-cross contamination.
Are all three of these tests applicable to the â€˜clean zonesâ€™ we have, and is this overkill for ISO Class 8?
A. – The more testing done, the better. When Validating a clean room all of the tests mentioned should be performed. However, once the room has been validated, doing these tests on a weekly basis is overkill. The standard tells you how often these tests need to be performed.
I hope that if anyone else has these types of issues, this blog will help provide some direction. If you have other questions or comments about validating and testing to ISO 14644 standards, we are available to answer your questions. There are 3 ways you can get assistance:
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