Yesterday’s purity isn’t enough

Continually scaling circuit features have led to tightened process requirements, including the purity levels of gas and liquid chemicals used in semiconductor manufacturing. How will the industry respond to the contamination control challenges posed by gas and chemical distribution?

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By Hank Hogan
For gas and liquid distribution systems in semiconductor cleanrooms, a high level of purity is a necessity, one that’s becoming more urgent all the time. Shrinking semiconductor features are the overarching factor.

Hugh Gotts, director of research and development for Air Liquide Electronics U.S. – Balazs Analytical Services (Fremont, CA; a subsidiary of gas supplier Air Liquide), notes that smaller features mean fewer and fewer atoms constitute a process film layer. “If you have a layer that’s made out of five or ten atoms on average, and if you have one layer of atoms as a contaminant, that would certainly be too much,” he says.

The smaller width, length, and height of such critical circuit elements as transistor gates in today’s state-of-the-art 45 nm node show up in the requirements spelled out in the International Technology Roadmap for Semiconductors. A look at the industry consensus document reveals what purity levels are needed now and what will be required in the future.

For example, ultra-pure water today must have fewer than 0.2 particles above critical size per milliliter. That’s unchanged over the next half decade, but the critical size scales with the node, since, as a rule of thumb, the size of a killer particle is half that of the node. This means the size of allowable particles will drop 30 percent over the next five or so years, a test for filtration technology. Similar situations apply to liquid chemicals such as the acid hydrogen fluoride or the base ammonium hydroxide.

As for gases, nitrogen is supposed to drop from 5 ppb trace contaminants now to less than 1 ppb in 2010. Other gases such as the corrosive etchant boron trichloride likewise face tightened trace contaminant requirements.

So how is the industry confronting these and other contamination control challenges in ultra-pure gas and chemical distribution?

Millions and millions cycled

Ultra Clean Technology (Menlo Park, CA) is one of the biggest providers of critical subsystems, including fluid and gas delivery systems to the semiconductor and flat panel industries, notes vice president of technology and CTO Sowmya Krishnan, PhD. The company constructs these systems at one of four sites around the globe, each having several thousand square feet of cleanroom capacity. Cleanliness levels range from Class 1 (ISO 3) to Class 1000 (ISO 6), with the goal being to produce systems that are virtually free of contamination and particulates.

Krishnan sees several technical trends affecting fluid distribution systems. She says achieving the required cleanliness and purity isn’t a chief concern. Instead, the challenge now lies elsewhere. “It’s moved to areas such as the reliability of the gas delivery system,” says Krishnan.

Figure 1. A Balazs technician uses a high-resolution inductively coupled plasma


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