Introduction

In high-purity gas systems, transporting gas from the supply source to the point of use (POU) while maintaining quality is critical. Achieving this requires a well-designed supply system, correct selection of pipes and components, proper installation, and rigorous testing and validation.

High-purity gas pipelines must be selected based on the process requirements for gas purity, allowable impurity levels, and particulate content. For example, the semiconductor industry demands extremely clean production environments and high-precision processes. As technology advances from micrometer to sub-micrometer and deep sub-micrometer scales (<0.35 µm), the requirements for water and impurity levels in process gases become exceedingly strict, moving from ppm to ppb or even ppt levels, while particle size must be controlled to 0.05–0.1 µm.Therefore, pipeline materials must inherently meet the requirements for high-purity and ultra-high-purity gas transport.

Key Material Characteristics

The main material characteristics affecting gas quality in high-purity pipelines include:

 • Gas permeation

 • Outgassing rate

 • Adsorption properties

 • Surface roughness and wear resistance

 • Corrosion resistance 

Gas Permeation

Gas permeation is the phenomenon where gas molecules move from a region of high pressure (or partial pressure) to a region of lower pressure through a material. Air components, primarily oxygen and nitrogen, have the highest partial pressures, and their small molecular size makes their permeation a critical consideration.

Common materials’ oxygen permeation (ppm in air) are roughly:

 • Stainless steel: 0

 • Copper: 0

 • Polypropylene: 11

 • PTFE: 12

 • Carbon steel: 203

Insight: For gases requiring ppm-level or higher purity, stainless steel or steel must be used. Using unsuitable materials will negate any purification measures applied downstream.

Outgassing Rate

Pipeline materials contain impurities such as nitrogen or hydrocarbons trapped within intergranular or lattice structures from the smelting process. These impurities may slowly release during high-purity gas transport, especially critical for ppb-level gas. Low-carbon stainless steel exhibits extremely low outgassing rates, making it ideal for high-purity applications.

Adsorption Properties

Polar molecules such as water are strongly adsorbed by materials. Rubber, plastics, and rough-surfaced materials readily adsorb water and other impurities, contaminating high-purity gas. Copper has particularly strong water adsorption. Therefore, except for certain ppm-level oxygen applications, copper and plastic pipes are avoided in high-purity systems. Elastomeric or plastic gaskets and packing, including PTFE, are also prohibited in ultra-high-purity pipelines.

Surface Roughness and Wear Resistance

Smooth, wear-resistant internal surfaces prevent particulate accumulation and moisture retention. Stainless steel produces fewer particles than copper and withstands high-velocity gas flow better.

Corrosion Resistance

In semiconductor production, corrosive gases are common. Pipelines must resist corrosion to prevent rust or perforation, which could contaminate gas and create safety hazards.

Surface Treatment

To further enhance pipeline properties, internal surfaces can be treated via:

 • Acid Pickling & Passivation (AP)

 • Bright Annealing (BA)

 • Electropolishing (EP)

Surface treatment reduces roughness, lowers outgassing, and minimizes adsorption.

Recommended Pipeline Materials and Connections

Valves and Connections

Valve selection depends on gas purity, toxicity, and flammability:

 • Ball valves: 304 stainless steel, suitable for purity ≤99.999%, with soft-metal or PTFE gaskets.

 • Diaphragm valves & bellows valves: Required for ≤10 ppm, flammable, or highly toxic gases. Diaphragm valves offer minimal dead space, easier purging, and lower contamination.

 • Connection methods: VCR compression fittings with soft-metal gaskets for systems ≤1 ppm.

Conclusion:

Maintaining high-purity gas quality requires attention to pipeline material, valve selection, connection method, and internal surface treatment. Only by integrating all these factors can gas purity be reliably delivered from source to point of use (POU). 
Therefore, pipeline materials must inherently meet the requirements for high-purity and ultra-high-purity gas transport.