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Low Impedance Creep-Free Room Temperature Mechanical Pump Trapping Technique

IP.com Disclosure Number: IPCOM000042731D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 38K

Publishing Venue

IBM

Related People

O'Hanlon, JF: AUTHOR

Abstract

Concepts of viscous flushing and sorption of oil on a suitable material are incorporated in a novel technique so as to produce a trap which will not allow oil to backstream through the volume or creep along the wall. This technique may have particular application for low pressure chemical vapor deposition and reduced pressure epitaxy of materials, such as gallium arsenide, where high gas flows are required, but where contamination due to the small creep of oil cannot be tolerated. Currently there are no room temperature traps for stopping the backstreaming of oil from a mechanical pump to a vacuum chamber which will stop creep and allow high gas flow.

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Low Impedance Creep-Free Room Temperature Mechanical Pump Trapping Technique

Concepts of viscous flushing and sorption of oil on a suitable material are incorporated in a novel technique so as to produce a trap which will not allow oil to backstream through the volume or creep along the wall. This technique may have particular application for low pressure chemical vapor deposition and reduced pressure epitaxy of materials, such as gallium arsenide, where high gas flows are required, but where contamination due to the small creep of oil cannot be tolerated. Currently there are no room temperature traps for stopping the backstreaming of oil from a mechanical pump to a vacuum chamber which will stop creep and allow high gas flow. Those traps which will stop oil creep do so by (1) filling the entire cross-section of the line with absorbent granules or pellets, or absorbent sheets arranged in a labyrinth, or (2) immersing the entire trap in liquid nitrogen. Traps incorporating design (1) have very high impedances to the flow of gases because the pumping line is completely filled to a depth of 2 - 6 inches, while those of design (2) allow creep if inadvertently warmed. Baker, et al., [1] have shown that viscous flushing is effective in reducing chemical pump oil backstreaming. However, the data given in their paper show backstreaming to decrease with increasing pressure until, at a pressure of 0.2 torr, the backstreaming becomes a constant. Jones and Tsonis [2] have calculated the backstreaming through a pipe with perfectly absorbing walls. Their calculations predict the backstreaming to decrease as e-P or faster. The experimental and calculated data are shown in Fig. 1. The difference between these two rates is due to oil creeping along the wall and desorbing into the chamber at the point where the pipe joins the chamber wall. Santeler [3] has also observed this and showed that the volume backstreaming is reduced by a continual flow of gas through the piping to the mechanical pump. Gas flushing to prevent volume backstreaming is now described in tutorial textbooks, and widely used in industry. For example, vacuum systems used in some production lines must be fitted with a leak which provides gas flow to keep the roughing line at or above 100 microns Hg (10-2 Torr). This article embodies two concepts in a unique way. The physical design of the trap is shown in Fig. 2. The active med...