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HEPA vs ULPA Filter: Efficiency Classes, MPPS and Cleanroom Use

HEPA and ULPA filters are both high-performance air filters made of fine glass-fiber (or similar) media folded into pleats. They are mounted in cleanroom ceilings or fan filter units to remove airborne particles. The difference is efficiency. The Japanese JIS terminology standard (JIS Z 8122) defines a HEPA filter as capturing at least 99.97% of 0.3 µm particles at rated airflow, and an ULPA filter as capturing at least 99.9995% of 0.15 µm particles. The European standard EN 1822-1 instead grades filters by their efficiency at the most penetrating particle size (MPPS): classes H13 and H14 are HEPA, and U15 to U17 are ULPA. ULPA filters stop more particles but, at the same airflow, have a higher pressure drop and cost more, so the choice follows the required cleanliness.

AspectHEPA filterULPA filter
What the name meansHigh Efficiency Particulate AirUltra Low Penetration Air
JIS definition (JIS Z 8122)At least 99.97% of 0.3 µm particles captured at rated airflowAt least 99.9995% of 0.15 µm particles captured at rated airflow
EN 1822-1 classes (overall efficiency at MPPS)H13: at least 99.95%; H14: at least 99.995%U15: at least 99.9995%; U16: at least 99.99995%; U17: at least 99.999995%
Share of particles that get throughAt most 0.05% for H13 and 0.005% for H14 (5 in 10,000 and 5 in 100,000)At most 0.0005% for U15 (5 in a million); each class step cuts it by a factor of 10
Most penetrating particle size (MPPS)Depends on the media, typically about 0.1 to 0.3 µm; larger particles are caught by inertia and interception, smaller ones by diffusionSame mechanisms, typically about 0.1 to 0.3 µm; the denser media lets fewer particles through even at this size
Pressure drop and costLower pressure drop at the same airflow, so fan power and price are easier to keep downTends to have a higher pressure drop at the same airflow, so fan power and price rise
Typical cleanroom useAreas around ISO Class 5 to 8, operating rooms, pharmaceutical plants; in chip fabs, bays with relaxed cleanlinessISO Class 4 and cleaner, and places that need especially clean air, such as the mini-environments of fab tools
What they cannot removeGaseous airborne molecular contamination (AMC) such as ammonia and organics; a separate chemical filter is neededLikewise cannot remove gases; where boron released from glass-fiber media is a problem, low-boron or PTFE media are used

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

HEPA filter
For areas where cleanliness around ISO Class 5 or looser is enough. The lower pressure drop keeps fan power down even when many filters cover a large area.
ULPA filter
For especially clean places that must reduce even very small particles, such as semiconductor steps with fine patterns. The higher efficiency comes with higher pressure drop and cost.
The filter alone does not set cleanliness
Room cleanliness depends not only on the filter class but also on how much of the ceiling is covered by filters, the downflow airflow, particles shed by people and tools, and leaks around filter frames. A higher-class filter does not help if the airflow is turbulent.

FAQ

What does "99.97% at 0.3 µm" mean for a HEPA filter?

When air containing 0.3 µm particles passes through the filter at rated airflow, it captures at least 99.97% of them, so no more than 3 in 10,000 get through. 0.3 µm is close to the size that fibers capture least well (the MPPS), which is why it has been used to state performance. Particles larger or smaller than that are generally captured with even higher efficiency.

What are H13 and U15?

They are classes in the European standard EN 1822-1. The overall efficiency of the filter is measured with particles at the most penetrating particle size (MPPS): H13 is at least 99.95%, H14 at least 99.995% and U15 at least 99.9995%, so each step up cuts the share that gets through by a factor of 10. The international standard ISO 29463 grades filters in much the same way. The test particle size differs from the JIS definition, so the numbers should not be compared directly.

Does an ULPA filter make any room very clean?

No. Cleanliness depends heavily on how much of the ceiling is covered by filters, whether the downflow is uniform, how many particles people and tools shed, and whether filter frames leak. After installation, the filter faces and frames are scanned with a particle counter to check for leaks, and the room's particle counts are measured according to ISO 14644-1.

Can HEPA or ULPA filters remove gases?

No. Both capture particles, and molecules such as ammonia and organics pass through the media. Chip fabs combine them with chemical filters, such as activated carbon, to reduce airborne molecular contamination (AMC).

Check your understanding

Which high-performance filters are used to remove fine particles from cleanroom air?

  1. ALow-pass filters
  2. BColor filters
  3. CULPA filters (HEPA filters)
Show answer and explanation

Answer: C. ULPA filters (HEPA filters)

HEPA filters, and higher-performance ULPA filters, capture airborne particles with pleated media of fine glass fibers and are installed in cleanroom ceilings. Even particles smaller than the gaps between fibers are caught as inertia and diffusion make them collide with fibers. A low-pass filter is a circuit that removes high frequencies, and a color filter separates colors in image sensors.

Which is the common definition of HEPA filter performance?

  1. ACapturing at least 99.97% of particles 0.3 µm in size
  2. BCapturing at least 50% of particles 10 µm in size
  3. CRemoving at least 99.99% of gaseous molecules
Show answer and explanation

Answer: A. Capturing at least 99.97% of particles 0.3 µm in size

A HEPA filter is defined (in JIS and elsewhere) as capturing at least 99.97% of particles around 0.3 µm, close to the hardest size to filter. Larger particles are caught by inertia and smaller ones by Brownian diffusion, so intermediate sizes penetrate most easily. Higher-performance ULPA filters are rated at 99.9995% or better at 0.15 µm. Neither removes gases.

Which device, arrayed on the ceiling, integrates a filter and fan to deliver clean air?

  1. AFFU (fan filter unit)
  2. BEFEM
  3. COHT
Show answer and explanation

Answer: A. FFU (fan filter unit)

An FFU combines a fan with a HEPA or ULPA filter in one unit; many are installed across a cleanroom ceiling to blow clean air downward. Airflow is adjustable per unit, accommodating layout changes and local cleanliness requirements, so FFUs have largely replaced systems that push air through large ducts. An EFEM is a tool front-end module and an OHT an overhead transport vehicle.

  1. Which element can be released from glass-fiber filters and similar sources and cause unintended doping (contamination) of wafers?
  2. Which filter is used to remove airborne molecular contamination (AMC) such as ammonia and organics from cleanroom air?
  3. Which statement about ISO 14644-1 cleanliness classes is correct?

Practice Cleanrooms and equipment questions

Key terms explained

HEPA filter
A high-efficiency air filter that captures at least 99.97% of 0.3 µm particles. Filters that capture even finer particles are ULPA filters.
ULPA filter
A high-performance air filter that captures even finer particles than a HEPA filter. It is used in the ceilings of high-cleanliness cleanrooms.
ISO 14644-1
The international standard that defines cleanroom cleanliness classes by the number of airborne particles. The smaller the number, the cleaner.
FFU (fan filter unit)
A unit combining a fan with a high-efficiency filter. Arrays of them in the cleanroom ceiling blow clean air downward.
Downflow
A cleanroom airflow method that sends clean air in one direction from the entire ceiling to the floor, quickly carrying generated particles away under the floor.
Cleanroom
A room in which the number of airborne particles is kept below a specified level and temperature, humidity and pressure are also controlled. Semiconductor manufacturing takes place in cleanrooms so that wafers are not contaminated.
Airborne molecular contamination (AMC)
Gaseous contaminants in the air such as acids, alkalis and organics. Particle filters cannot remove them, so chemical filters are used.
Mini-environment
Keeping only the small space around the wafers highly clean. Sealed carriers such as FOUPs combined with EFEMs on the tools lower the cleanliness required of the room as a whole, reducing cost.

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