Cleanroom design for a nanofabrication facility: ISO class, air handling and the bill nobody budgets
By Vvon Engineering Team | Published | Updated
How to decide which ISO class each area of a nanofab needs, why Dhaka's climate reshapes the air handling design, what gowning and construction materials actually require, and the running cost universities consistently leave out.
A cleanroom specification should connect particle requirements with the process, occupancy and operating budget. Lower ISO class numbers indicate cleaner air at a given particle threshold. Over-specifying the room can increase cost without improving the process if humidity, airflow or contamination sources remain uncontrolled.
This article covers how to choose the class for each area, what the air handling has to do in Dhaka specifically, how gowning and construction decide whether the class survives, and the running costs that belong in a departmental budget from day one.
What the ISO classes mean, and what each is for
ISO 14644-1 classifies air by the number of particles above a given size in a cubic metre. Each step down the class number is a factor of ten fewer particles. The classes most relevant to a nanofab, with the older US Federal Standard names that vendors still quote:
ISO class
Particles 0.5 micron and larger per cubic metre
Old name
What it is used for in a nanofab
ISO 5
3,520
Class 100
Lithography bay, resist spinning and exposure, critical device work
ISO 6
35,200
Class 1,000
Lithography and deposition where features are around a micron
ISO 7
352,000
Class 10,000
General process area: etch, deposition, wet benches, metrology
ISO 8
3,520,000
Class 100,000
Gowning room, sample preparation, packaging, service chase
State the occupancy condition, particle sizes and acceptance method. An at-rest classification does not demonstrate the operational condition with people and equipment working. Select the class from process sensitivity and acceptable yield; a particular feature size alone does not mandate a universal room class.
A mixed-class layout can control cost by concentrating cleaner conditions around sensitive operations. Its class boundaries, containment and service spaces must follow the process assessment rather than a fixed template.
Air handling, and why Dhaka changes the arithmetic
Cleanliness comes from filtered air changes: supply through HEPA or ULPA filters in the ceiling, return low on the walls or through a raised floor, and enough changes per hour to dilute what people and processes generate. ISO 8 needs modest air change rates, ISO 7 several times more, and ISO 5 generally needs unidirectional flow across much of the ceiling rather than a change rate at all. Those ratios drive fan power, duct sizes and plant space, so a class decision made in a meeting becomes structural and electrical consequences on the drawing.
What is different here is the latent load. Outside air in Dhaka through most of the year is warm and close to saturated. Every cubic metre of fresh air brought in has to be cooled well below its dew point so that water condenses out, then reheated to reach the room set point. That dehumidification duty, not the temperature difference, is where the electricity goes, and it is routinely sized against a European or a temperate design day.
Size exhaust and make-up air together.Every wet bench, fume hood and process pump exhaust pulls conditioned air out of the room, and every cubic metre leaving has to be replaced with fresh air that has to be dehumidified. Designing the air handling unit first and adding the tool exhaust list later is the classic sequencing error, and it shows up as a room that cannot hold pressure once the benches are switched on.
Fix the pressure cascade on paper. Cleanroom positive to gowning, gowning positive to corridor. Then note that a wet bench area may need to be negative for chemical containment, which fights the cleanliness cascade. That conflict is resolved on the drawing with an airlock and a dedicated extract, not on site by a contractor with a damper.
Temperature and humidity control is a lithography requirement in its own right.Resist thickness, develop rate and mask aligner mechanics all move with temperature and humidity. A room that meets its particle class but swings several degrees through the day will still produce inconsistent linewidths.
Fan filter units against ducted supply. Fan filter units suit buildings with little plant space and make phased expansion easy, since you add units as you add bays. Plan a maintenance route above the ceiling grid for them. Ducted supply is quieter and simpler to service, but needs plant space many campus buildings do not have.
Recovery time matters more than peak performance. Ask for a recovery test at handover: disturb the room, then measure how long it takes to return to class. That number tells you more about how the room will behave in service than a single clean certificate does.
Gowning: the protocol is the cleanroom
People can be a major particle source; equipment, materials, cleaning and air-handling faults also matter. Excellent air handling with careless gowning will not hold a class, and extra filtration does not compensate. The gowning suite is where the class is maintained.
Step onto the tacky mat and change out of outdoor shoes at a physical bench that separates the dirty and clean sides. A line painted on the floor is not a separation.
Wash and dry hands. No cosmetics, no loose fibres, no cardboard, no ordinary paper or pencils past this point. Cleanroom notebooks and ballpoint pens only.
Gown top down: hood, coverall, boots, gloves, safety glasses. Nothing touches the floor at any stage, and a garment that does is out of service.
Enter through an airlock with interlocked doors, so both are never open together.
Exit by a defined route with a separate degowning area, so soiled garments never travel back through the clean side.
Then settle the question that sets the recurring cost: who launders the garments? Cleanroom laundering is a specialised service, and where it is not available locally the facility runs on disposable coveralls, hoods, boots and gloves. That is an imported consumable with a lead time, and it belongs in the annual budget at a realistic weekly rate. A facility that assumes reusable garments and then cannot have them cleaned properly ends up with garments that shed particles rather than contain them.
Materials of construction
Walls and ceiling: modular panel systems with flush, non-shedding surfaces, a gasketed ceiling grid rated to carry filter units, and no exposed gypsum edges. Every horizontal ledge is a particle shelf and a cleaning task.
Junctions: coved skirting at wall to floor and wall to ceiling. Coving is not decoration, it is what makes wet cleaning possible and what stops dust collecting where nobody can reach.
Flooring: welded sheet vinyl with coved edges for ISO 7 and 8, static dissipative where electronics are handled, and a raised perforated floor where unidirectional flow needs a return path under an ISO 5 bay.
Penetrations: every cable, pipe and duct entry sealed. Unsealed penetrations behind tools are the most common reason a room fails a leakage test months after handover.
Doors: flush faces, vision panels, self-closers, and interlocks on airlocks. Vision panels matter for safety too, because somebody outside must be able to see a person working alone.
Lighting: flush, sealed, cleanable fittings, and in the lithography bay filtered yellow light with everything below roughly 500 nm removed, including light spilling through a vision panel from the corridor.
A service chase. A grey space behind the tool wall where pumps, chillers, gas panels and electrical distribution live and an engineer can work without entering the clean space. It is the single best decision in a nanofab layout and the first thing cut when the budget tightens. Cutting it makes every pump service a cleanroom event.
The running cost universities routinely underestimate
A cleanroom is not a room, it is a machine that runs continuously. Switch it off overnight and you lose hours of recovery time and expose tools to humidity swings. The recurring cost is predictable, and it belongs in the departmental operating budget, not in the project budget that ends with the grant.
Electricity, continuously. Air handling or fan filter units, chillers, exhaust fans and process cooling, running around the clock through a Dhaka summer. This is the largest line and the one most often absent from the proposal.
Filters. Prefilters change often given the dust loading in Dhaka. HEPA filters last years but must be tested and eventually replaced, and they are imported.
Periodic classification and verification under the facility's approved monitoring plan. Particle counting, filter integrity testing, airflow and pressure verification, and instrument calibration. Keep the certificates: grant reviewers and equipment manufacturers both ask for them.
Water, gases and abatement media. Deionised water consumables, process gas cylinders, and scrubber media that is consumed on a schedule.
A facility technician. One person whose actual job is the room: logs, filters, water quality, gas changes, gowning discipline, maintenance calendar. A facility run by whichever PhD student is most conscientious declines the moment that student graduates.
Assign the electricity and maintenance budget before the grant ends. The case for a facility should include both capital and recurrent costs, with named owners for each.
One more Bangladesh specific point, and it is not a small one. Put the air handling and the exhaust fans on the standby generator, not just the instruments. The common pattern is a UPS on the electron microscope and nothing on the air handling, so load shedding leaves a room with no pressure cascade, no exhaust and chemistry on an open bench. Exhaust and pressurisation are safety systems, and they belong at the front of the generator schedule.
A smaller cleanroom that actually works
For a first facility, evaluate a smaller process area with local clean zones, a suitable gowning route and accessible service space. Confirm the required classes and spare capacity from the funded tools, intended occupancy and expansion plan. The ISO 5–8 examples above describe possible uses, not mandatory classifications for those activities.
Softwall enclosures and laminar flow benches have a legitimate place in that plan, particularly for sample preparation and for optical work that needs cleanliness rather than a classified room. They cannot substitute for a pressurised suite once wet chemistry and process gases are involved. Use them to start, and design the shell so hardwall bays can be added later without demolition.