Page 85 - Semiconductor Manufacturing Handbook

Geng(SMH)_CH07.qxd 04/04/2005 19:40 Page 7.6

VACUUM TECHNOLOGY

7.6 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

The motion of the diaphragm opens an inlet port, isolates a volume of gas, compresses the gas, and
then expels this gas to the atmosphere. These pumps are available as multiple stage units that can
attain pressures as low as 1 torr. Diaphragm pumps are well suited for intermittent-use applications
and can be oil-free.
Scroll Pumps. In this pump design there are two interleaved helical scrolls—one that is stationary
and one that moves in a circular oscillation relative to the stationary scroll. During each oscillation
of the moving scroll, a crescent-shaped volume is created at a port connected to the pump inlet. As
oscillation of the moving scroll continues, the crescent-shaped volume increases in size, thus local-
ly reducing pressure and gas flows into this volume from the pump inlet. With further travel of the
oscillating scroll, the volume of gas drawn into the crescent-shaped volume is isolated from the
pump inlet and is compressed. In the last stage of the pump cycle, the size of the crescent-shaped
volume is reduced, compressing the gas to slightly above atmospheric pressure and the gas is
exhausted through a spring-loaded valve. Scroll pumps use no lubricants in the swept volume and
are well suited for use in primary evacuation of vacuum vessels and load vacuum locks.
Screw Pumps. The concept of moving fluids using a screw mechanism dates back over 2000 years
and is attributed to Archimedes. In the modern implementation of this design for vacuum applica-
tions, two parallel counter-rotating close-tolerance screws are fitted within a stator housing. As the
screws rotate at approximately 6000 rpm, gas is drawn into the pump inlet and forced along the axis
of the pump by the screw threads. Most of the compression occurs at the exhaust end of the pump
and gas temperatures can rise to 300°C. The screws of these pumps are often coated with Teflon
(PTFE) to reduce friction and to protect the base metal of the pumps’ internal components from
chemical attack by the gases being pumped. Screw pumps are used for primary evacuation of a ves-
sel from atmospheric pressure to a base pressure of approximately 10 mtorr.

Sorption Pumps. Sorption pumps remove gases from a vacuum vessel by cryosorption and cry-
ocondensation. These pumps typically consist of an aluminum cylinder internally filled with a sor-
bent such as zeolite. The exterior of the pump body is cooled to below room temperature often
through immersion in a cryogenic fluid such as liquid nitrogen. The cooled sorbent material will cry-
ocondense gases that have a boiling point above that of liquid nitrogen; other gases are cryosorbed
onto the very fine pore structure of the zeolite. In either case, the gas molecules entering the
cryosorption pump are effectively removed from the vacuum vessel thus reducing pressure in the
vessel. Gases not efficiently pumped using a cryosorption pump are helium, hydrogen, and neon.
Sorption pumps are used for primary evacuation of a vessel from atmospheric pressure to a base
pressure of approximately 100 mtorr.

7.3.2 Secondary Vacuum Pumps
Secondary vacuum pumps are used to further reduce pressure in a vacuum vessel following the pri-
mary evacuation from 760 torr to approximately 10 mtorr. Secondary vacuum pumps if operated cor-
−8
rectly can routinely achieve pressures in the range of 10 torr and with extra care and good vessel
design, pressures as low as 10 −11 torr.
Momentum Transfer Pumps. Momentum transfer vacuum pumps reduce pressure in a vacuum ves-
sel by compressing gas and expelling it to the inlet of a primary vacuum pump that further compresses
the gas and expels it to the atmosphere. During operation, momentum transfer pumps require the pres-
sure at their exhaust port (also called the foreline) to be maintained at a pressure below their critical fore-
line pressure. The value of the critical foreline pressure is a function of pump design; manufacturers
clearly state the value of the critical foreline pressure for each pump in the published specifications.
Oil-Vapor Diffusion Pumps. In the oil-vapor diffusion pump, a supersonic-speed jet of vapor is
created by controlled boiling of the pump fluid inside the pump body. The oil vapor rising up the
internal stack of the diffusion pump body is forced out of jets that are directed downward and toward

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.