From the Desk of Lewis Bass

Welcome back to “Ask Lewis Bass,” our ongoing series where we answer real-world questions from engineers, compliance teams, and safety professionals working on SEMI standards projects.

This week’s submission comes from someone we’ll call Nano-machining Engineer Nick, who is deep into a SEMI S2 project and wants clarity around when an exhaust ventilation study is actually required and especially for systems using non-toxic gases like CO₂ and nitrogen.

Question:

Hi Lewis Bass,

I’m working on a new precision intelligent nano-machining system that uses CO₂ at or near the triple point, and air or N₂ as a carrier gas. We’ll have exhaust ventilation active during normal operation.

Do I need an exhaust ventilation study for SEMI S2 compliance on this tool, even though CO₂ and N₂ aren’t “toxic” per se?

– Nano-machining Engineer Nick

Lewis Bass:

Hi Nick,

Great question, Nick, and it is also one we see surprisingly often in early SEMI S2 scoping discussions. The short answer is: yes, you most certainly do need to treat exhaust ventilation as a study-worthy item in this case. Now, let’s get to the bottom of the “why”.

1. SEMI S2 treats ventilation as a safety control, not just a comfort feature.

Even though CO₂ and nitrogen are not classified as chemical toxics in the traditional sense, both gases can displace oxygen, creating asphyxiation hazards if allowed to accumulate. SEMI S2 doesn’t focus only on toxicity, it cares about any hazard that could impact personnel safety. Because your system uses active exhaust during normal operation, the ventilation system becomes part of the engineered safety control for the process.

That means you can’t just document that exhaust exists…you also need to demonstrate how well it works.

2. Active exhaust during normal operation means you must verify its performance.

Tools with exhaust ventilation that run continuously or during normal process cycles can’t rely on assumption or facility nominal specs alone. SEMI S2 expects:

• Quantified airflow (CFM) and static pressure

• Verification of capture efficiency of any enclosures or hoods

• Demonstration that the exhaust meets the design expectations even under worst-case scenarios

This becomes even more important when the exhaust is the means by which a gas like CO₂, which can be generated in multiple phases, is kept from accumulating in an enclosed workspace.

3. Phase changes complicate things.

Carbon dioxide near its triple point (where solid, liquid, and gas can coexist) can create unpredictable local gas releases. These phase transitions aren’t “chemical hazards” in the traditional SEMI S2 sense, but they are physical events with safety implications because they can drive rapid volume change and local high concentrations. That’s exactly the sort of scenario that makes exhaust ventilation performance verification essential.

4. SEMI S2 Section 22 explicitly covers exhaust ventilation.

Among the many sections of SEMI S2 explored during a full evaluation, Section 22 is dedicated to exhaust ventilation not as an optional add-on, but as a named safety category inspectors review. When exhaust is part of the safety case, that section gets applied.

So what does the study actually need to show?

At a minimum, a compliant ventilation study should include:

• Measured exhaust airflow rates at tool interfaces

• Static pressures and fan performance curves

• Hood face velocities (if applicable)

• A worst-case release scenario evaluation

• Interlock/alarm logic tied to loss of exhaust

• A margin of safety between tool requirement and available exhaust capacity

This allows the safety evaluation to show that even if something isn’t “toxic,” the tool still manages a credible hazard and the controls you’re applying are effective.

For a system like yours with active exhaust and gases that can displace oxygen or phase transition unpredictably, yes, an exhaust ventilation study is required for SEMI S2 compliance.

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