Specialty Stainless Solves Contamination Problems with Threaded Parts for Semiconductor Industry

Technical Guide

Threaded components and fittings of stainless steel are viewed by many in the semiconductor industry as potential headaches waiting to inflict damage when least expected.

They are either subject to metal-to-metal galling and difficult disassembly or, if the threads are plated or coated to avoid seizure, the protective layer is at risk to flake and contaminate the high purity gases used in manufacturing. In either case, the prospects for costly failures or losses in a highly competitive business are very real.

Integrated Flow Systems, Inc., designer and manufacturer of regulators, fittings and gas delivery components for the industry, has avoided both unpleasant consequences. IFS has employed a new split-nut design for the end connections in its unique set pressure regulator, along with a specialty stainless steel that resists galling.

The Scotts Valley, CA, firm makes the well known VCR1 fitting used in ultra-high purity applications in semiconductor fabricating facilities and in OEM tools used to make microchips.

A male and a female nut in the fitting connect two face-seal metal glands.

The split-nut concept developed by IFS offers safe, trouble-free regulator performance, along with several other conveniences and cost savings for the end user.

In conventional design, valves must be sexed and the nut installed before the gland is welded to the tubing. For each fitting, inventory must carry a male inlet, male outlet, female inlet and female outlet valve. Then, when a nut has to be changed, it has to be cut off and a new one welded back in place.

With the patented split-nut design, two split inserts of Custom 630 (17-4PH) stainless steel from Carpenter Technology Corp., Reading, PA, fit snugly around the gland shaft so their ends are flush. An identical pair of split halves fits around the same gland, mating with the first pair. Most of the outside diameter on all four inserts is threaded.

Now connections can be changed instantly and easily from female-to-male or male-to-female, eliminating much inventory. Glands can be integrally machined on to the regulator, then electropolished.

The split nuts can be installed after fabrication and welding. Since clearance for a weld head is no longer needed, a shorter, rotatable male end connection is now possible. This feature, in turn, permits improved component flexibility.

Of major significance is the internally threaded female hex collar, made of Carpenter Gall-Tough® stainless steel, which fits over the threaded male split nut on the end connector. This specialty alloy is a high-silicon, high-manganese, nitrogen strengthened, austenitic stainless steel which possesses superior self-mated galling and metal-to-metal wear resistance.

The Carpenter alloy has displayed higher strength and elevated temperature oxidation resistance than Type 304 stainless, with comparable corrosion resistance, depending on the environment.

It is common practice in the industry to plate the threads of these VCR nuts with silver, or coat them with wax to prevent galling or fusion with the male nut. This practice may eliminate galling for awhile, but it unveils a more serious downside.

As the mating nuts are tightened, both the plating and the coating are prone to flake and generate fine particles that can flow into the ultra-pure gas, insidiously contaminating the chips and the expensive tools used to make them. Those particles are also capable of causing an imperfect seal and subsequent leakage. That can spell disaster because not only are the gases ultra pure, but some are even toxic and/or pyrophoric (subject to exploding when exposed to oxygen).

IFS, whose fittings are widely accepted for high purity usage, warns that if threaded fittings are not plated or coated, and they seize in service, retrofitting can be very costly.

Best Solution

The ideal solution, suggested Joe Williams, IFS engineering manager, is to use gall-resistant stainless steel nuts that "provide a tight, reliable and repeatable sealing force."

Williams, who said that Carpenter Gall-Tough stainless has been in the original fitting design specifications, reported that he knew of no Gall-Tough stainless fittings that have seized in service.

The Gall-Tough stainless VCR collars are made from 3/4" hex bar. They are turned on a two-headed CNC lathe which removes the inside diameter of the nuts and threads the OD. All the collars are electropolished to a fine finish for added system purity.

IFS turned to Carpenter also for a specialty alloy needed to satisfy rigid requirements for pressure-containment parts in its set pressure regulator. The company selected Carpenter's Type 316L-SCQ® stainless steel for the regulator's diaphragm support housing, diaphragm face plate and the entire body shell.

This is a premium, vacuum melted alloy with improved microstructure that can be electropolished to an extremely fine finish. It can achieve the chrome-to-iron ratio of at least 1.5 to 1 required by IFS to resist the attack of toxic chemicals and prevent contamination in semiconductor manufacture.

Working with 1/2", 1-1/2" and 2" bar stock, IFS fabricated its parts primarily on a CNC lathe. It then mechanically polished and electropolished outside surfaces to a bright 7RA finish.

Special care was given the interior surfaces, which also were electropolished to a 7RA finish. The manufacturer could not tolerate even microscopic inclusions or pockets that could harbor particles with the potential to contaminate downstream processes.

1 Registered trademark of Swagelok Co.