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UNIQUE SURFACE TREATMENT OF 304 SS VACUUM

CHAMBER REDUCES PUMP DOWN TIME 

A unique surface treatment for 304 stainless steel vacuum components and chambers has been developed by the Pernicka Corporation. This process is considered “environmentally correct” since no chemical waste or byproduct are generated as a result of the treatment. Three features of the treatment are: 1) reduction of the effective surface area; 2) reduction of the sticking coefficient for a water molecule on the surface; 3) reduction in the diffusion rate for hydrogen coming out of the bulk material. The result of applying the treatment to vacuum chambers is generally faster pump down times and lower base pressures with the same pumping system. Data is presented comparing the performance of two identical systems: one with a treated chamber; and one with a normal, chemically cleaned chamber. Although the process is somewhat costly, the performance benefits achieved and the elimination of toxic waste chemicals make this surface treatment very practical in today's economy.

The following experiment was performed to evaluate the effectiveness of the surface treatment on a typical vacuum chamber. The 2.0-liter chamber selected for this experiment was manufactured of 304 stainless steel by Balzers and was subjected to a standard chemical cleaning and passivation process. The chamber was connected to a 1E-10 Torr Turbomolecular Pumped Vacuum System by a six-inch valve. Next, the chamber was evacuated and baked at 400 C for six hours and then cooled to room temperature.

The chamber was vented to room air (30%RH), allowed to stabilize for 1.0 hour, pumped down to 50mTorr using a dual seal oil mechanical pump, and them transitioned to a 170L/sec turbomolecular pump. The pump down curve was recorded. Similarly, the chamber was tested after backfilling with grade 4.8 Argon. The chamber was then removed from the Vacuum System and the internal surface was modified. The chamber was reinstalled on the Vacuum System and the above pump down test was repeated.

UNIQUE SURFACE TREATMENT OF 304 SS VACUUM CHAMBER

REDUCES PUMP DOWN TIME-CONTINUED

Table (1) summarized these test results.

TIME (minutes) 0 10 40 70 B.P
untreated chamber
with room air
661 1.15x10-6 2.23x10-7 1.1x10-7 4.5x10-8
untreated chamber
with 4.8 Argon
661 1.56x10-7 7.0x10-8 5.1x10-8 4.5x10-8
treated chamber
with room air
661 3.97x10-7 7.8x10-8 3.2x10-8 1.2x10-9
treated chamber
with 4.8 Argon
661 8.65x10-8 1.91x10-8 7.1x10-9 1.2x10-9

Table (1), Pressure in Torr as a function of pump down time.

The treated chamber pumped down faster than the untreated chamber in both test cases.

In fact, the treated chamber filled with room air out performed the untreated chamber filled with Argon after 40 minutes of pumping. The base pressure of the system was over 10 times lower for the treated chamber.

If we assume that the limitation of the base pressure is a direct result of the hydrogen-outgassing rate, we can conclude that the treated chamber must have a lower outgassing rate for Hydrogen. If we assume that the initial pump down rate of the room air filled chamber is directly influenced by the sticking coefficient of water, then we can conclude the treated chamber must have a lower sticking coefficient.

If we assume the initial pump down of the Argon filled chamber is directly influenced by the effective surface area, then we can conclude that the treated chamber effective surface area was less than the untreated chamber.

The obvious benefit of the surface treated chamber is a cleaner vacuum environment. This process has been applied to 316 stainless steel surfaces with similar results.

For further information regarding the application of this process, contact the author.

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