Improve Moisture Dry-Down and Corrosion Resistance

Restek treated tubing and system components offer improved analytical reliability and longer lifetimes.

Introduction

Gas transfer systems serving the semiconductor industry often require low moisture content and retention, and high resistance to corrosion. The current substrate of choice in semiconductor manufacturing, electropolished VIM/VAR (vacuum induction melt/vacuum arc melt) 316L stainless steel, frequently is insufficient in these capacities, increasing periodic maintenance, prolonging equilibration times, and allowing system contamination and inaccurate analytical results. In contrast, surface treatments available through the Restek Performance Coatings Group provide significant added value to conventional stainless steel substrates, by greatly accelerating wet-up and dry-down times and dramatically improving corrosion resistance.

Restek surface treatments deposit an amorphous silicon based layer onto, and into, the steel surface through a chemical vapor deposition (CVD) process in vacuum at 400°C. There are no line of sight limitations; all exposed surfaces are coated. Layer depth is well controlled and, for corrosion resistance, is optimized at 5 to 10 microns. The amorphous silicon layer can be further functionalized using the patented Siltek® process (US Patent #6,444,326), which has been optimized to reduce moisture hold-up and improve surface inertness.

Data for wet-up and dry-down experiments, measuring the relative response time for moisture content change in treated electropolished stainless steel tubing, untreated electropolished stainless steel tubing, and standard 316L stainless steel tubing, demonstrate a significant advantage in treated versus untreated substrates.(1) Tubing used in the wet-up / dry-down experiments was supplied by Cardinal UHP (St. Louis, MO). All tubing was tested as 100 foot coils of 1/4" OD x 0.020" wall 316L stainless steel. Electropolished tubing had a surface roughness of 10 to 15 microinches. Siltek® treated tubing (Restek Performance Coatings, Bellefonte, PA) is finished with 5µm of amorphous silicon, followed by a surface functionalization that increases inertness and hydrophobicity.

Wet-up curves for Siltek® treated electropolished, electropolished, and standard tubing are compared in Figure 1. Treated electropolished tubing reached the 98% saturation limit in 30 minutes, compared to 60 minutes for electropolished tubing. Standard tubing could only achieve a 96% uptake, after 180 minutes.

After the tubing was stabilized with 1ppm of moisture, dry-down properties were measured. Moisture dry-down curves for the three tubing treatments show treated electropolished tubing achieved dry-down in 35 minutes, electropolished tubing required 65 minutes, and standard tubing required 175 minutes (Figure 2). Table 1 compares time to various dry-down levels for tubing saturated with 10ppm of moisture.

In addition to rapid wet-up and dry-down, the other key advantage of Restek treatment for 316L stainless steel is a dramatic improvement in corrosion resistance. The amorphous silicon layer is insoluble in many of the acidic environments encountered in the semiconductor industry. Figures 3, 4, and 5 briefly summarize the results of corrosion testing by ASTM methods. Comparisons between treated and untreated test samples illustrate the improvements in corrosion resistance offered by Silcosteel®-CR treatment. To learn more, visit our Silcosteel®-CR web page or contact our technical service group at 814-353-1300 or 800-356-1688, ext. 4.

When moisture considerations and corrosion concerns arise in transfer of ultra-high purity gas streams, Restek treated tubing and system components will dramatically improve dry-down, reduce contamination from moisture carryover, and extend periodic maintenance cycles.