Semiconductor and electronics cleanrooms

Semiconductor fabrication pushed cleanroom technology further than any other industry. As circuit features shrank, the particle size that could destroy a chip shrank with them, and the industry developed ultra-clean environments, ULPA filtration, minienvironments and control of chemical contamination in the air. Electronics assembly, photonics, microelectromechanical systems (MEMS), battery manufacturing and precision optics use many of the same principles at less extreme levels.

Unlike pharmaceutical cleanrooms, which focus heavily on microbial control, these facilities care about particles, airborne chemicals, electrostatic discharge, vibration and very precise temperature and humidity. This guide explains the main HVAC considerations.

Cleanliness levels

Requirements vary widely by process.

The ISO 14644 classification guide explains the class limits.

Ballrooms, bays and minienvironments

Ballroom cleanrooms

Early fabs used large open ballroom cleanrooms with full unidirectional downflow across the entire ceiling. This delivered very clean conditions everywhere, at a very high energy cost.

Bay and chase layouts

Later designs separated clean process bays, where wafers are handled, from service chases, where tool maintenance takes place under less stringent conditions. This reduced the area requiring the highest class.

Minienvironments and FOUPs

Modern fabs move wafers in sealed carriers, called front-opening unified pods (FOUPs) for 300 mm wafers, and process them inside tool-mounted minienvironments with their own fan filter units. ISO 14644-7 covers these separative devices. Because the wafer only sees the minienvironment, the surrounding cleanroom can be several classes less clean. This is one of the largest energy savings in fab design.

Airflow and filtration

Fabs typically use a raised perforated floor with a return plenum below, fan filter units or ducted filters in the ceiling, and return air risers to a plenum above the ceiling. Downflow velocities in critical areas are often around 0.3 to 0.45 m/s (60 to 90 fpm), adjusted to the minimum that achieves the required class.

ULPA filters are common in the most critical areas, with HEPA elsewhere. Filter materials themselves can release boron or other contaminants that affect some processes, so filter media and sealants are specified to avoid outgassing. See HEPA and ULPA filter systems.

The very large recirculation airflows make fan filter units with efficient EC motors popular, along with large fan decks or fan walls feeding a pressurized plenum.

Airborne molecular contamination

As features shrink, gaseous contaminants matter as much as particles. Airborne molecular contamination (AMC) includes acids, bases such as ammonia, condensable organics and dopants. They can cause defects, haze on optics and photoresist problems. ISO 14644-8 classifies air cleanliness by chemical concentration, and SEMI standards address AMC for semiconductor processes.

Control measures include:

• Chemical filters (activated carbon, impregnated media or ion exchange) in makeup air and sometimes in recirculation or at the tool.
• Careful selection of construction materials, sealants and filter components to limit outgassing.
• Locating makeup air intakes away from exhausts, roads and industrial sources.
• Separating process exhaust streams and treating them before discharge.
• Monitoring AMC continuously in critical areas.

Temperature and humidity

Photolithography and metrology demand very stable conditions because materials expand and contract with temperature. Lithography areas may require temperature control within plus or minus 0.1 C or tighter at the tool, often achieved by tool-level environmental chambers rather than the whole room. General fab areas typically hold about plus or minus 0.5 to 1 C.

Humidity matters for photoresist behaviour and static control. Fabs commonly control relative humidity in a narrow band around the mid-40 percent range, with tight tolerances in lithography.

The standard approach is a makeup air unit that controls dew point precisely, with dry sensible cooling coils in the recirculation path. See temperature and humidity control in cleanrooms. In Canadian climates, humidification in winter is a significant load and must use high-purity water or clean steam.

Battery dry rooms

Lithium-ion cell assembly needs extremely dry air, often with dew points of minus 40 C or lower, because electrolyte reacts with moisture. Desiccant dehumidification systems, airtight construction and careful control of personnel moisture loads are essential, and energy use is high.

Process exhaust

Semiconductor tools use hazardous gases and chemicals, so exhaust is extensive and segregated by type:

• Acid exhaust, often to wet scrubbers.
• Solvent exhaust, sometimes to thermal oxidizers or carbon systems.
• Ammonia or alkaline exhaust.
• General heat exhaust.
• Toxic gas exhaust from gas cabinets, with point-of-use abatement.

Exhaust volumes drive makeup air size, which in turn drives most of the facility's heating and cooling energy. Exhaust must be reliable, often with redundant fans and emergency power, because loss of exhaust can create hazardous conditions. Design follows codes and standards including NFPA 318 for semiconductor fabrication facilities, together with provincial building and fire codes. Related topics appear in industrial process ventilation.

Electrostatic discharge

Electrostatic discharge (ESD) damages semiconductor devices and electronic assemblies. ESD control programs, commonly built on ANSI/ESD S20.20 or IEC 61340-5-1, rely mainly on grounding, conductive and dissipative materials and ionizers. HVAC contributes by avoiding very low humidity, since dry air increases charge generation. Ionizers may be installed in fan filter units or at workstations.

Vibration

Lithography, electron microscopy and metrology tools are sensitive to vibration. HVAC equipment is a major vibration source, so fans, pumps and chillers are isolated, located away from sensitive areas or on separate structures, and selected for low vibration. Facilities often specify vibration criteria (VC) curves for tool areas.

Energy

Fabs are among the most energy-intensive buildings. Main HVAC energy users are recirculation fans, makeup air conditioning, process exhaust, chilled water and process cooling water. Common strategies include minienvironments to shrink ultra-clean areas, high-efficiency fans, multiple chilled water temperatures (low for dehumidification, medium for sensible cooling and process loads), heat recovery from process cooling water and reduced airflow where monitoring shows margin.

Key takeaways

• Semiconductor fabs need ISO 3 to ISO 5 at the wafer, but minienvironments let surrounding areas be much less clean.
• Airborne molecular contamination is controlled with chemical filtration, materials selection and monitoring under ISO 14644-8 and SEMI standards.
• Lithography areas need extremely stable temperature and humidity, often achieved at the tool level.
• Segregated process exhaust drives makeup air size and facility energy use.
• Electronics assembly focuses on ESD control, where humidity is one factor among grounding and materials.