Page 87 - Semiconductor Manufacturing Handbook

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VACUUM TECHNOLOGY

7.8 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

Sputter-Ion Pumps. These pumps reduce the pressure inside a vacuum vessel by removing mole-
cules from the gas phase. There are three methods by which sputter-ion pumps remove gas mole-
cules from a vacuum vessel—gettering, ion burial, and physical burial. Gettering is a chemical
process in which a gas molecule reacts with an active metal to form a solid product. In most sputter-
ion pumps, the active metal that is utilized is titanium. During operation of the sputter-ion pump, a
thin film of titanium is deposited onto the internal surfaces of the pump. Titanium is a chemically
reactive metal that will readily react with atmospheric gases to form stable solid compounds such as
titanium dioxide, titanium hydride, and titanium nitride. In addition to gettering, sputter-ion pumps
can also remove noble gases such as helium, neon, argon, krypton, xenon and radon. These gases
tend not to form chemical compounds with reactive metals. During operation of the sputter-ion
pump, all gases including the noble gases may be ionized by the electrons traversing the space
between the cathode and anode. If a collision between an electron and a gas molecule occurs, the gas
molecule may become ionized. This ionized gas molecule will then move under the applied electric
field of the pump toward the cathode. Upon impact with the cathode, the ion will lose its electric
charge and may have sufficient kinetic energy to become implanted within the bulk of the cathode.
This process is called ion implantation. It should be noted that as pump operation continues, the
exposed surface of the cathode will continue to be eroded and some of the implanted gas molecules
may be liberated. The last process by which sputter-ion pumps can remove molecules from the gas
phase is called physical burial. In this process gas molecules that land on the internal surfaces of the
sputter-ion pump body and remain on that surface for a residence time may be overcoated with
titanium.

7.4 VACUUM SYSTEM COMPONENTS

7.4.1 Flanges with Demountable Seals
Components such as vacuum pumps, pressure gauges, and virtually every component of a vacuum
system are typically connected to a vacuum vessel using a flange system incorporating some type of
demountable seal. These joints are made up using mechanical devices such as threaded fasteners or
clamps and form a relatively leak-tight seal that allows the vacuum system to achieve its design base
pressure. The seals used may be polymeric (O-rings) or metallic.

7.4.2 Valves
Valves serve to control the flow of gas into or out of a vacuum vessel. While some vacuum systems
are constructed without valves for simplicity, robustness, or cost savings, most vacuum systems
employ several types of valves. Isolation valves are generally placed between the primary roughing
pump inlet and the vacuum vessel as well as between the inlet of a secondary vacuum pump and
the vacuum vessel. If a process gas is injected into a vacuum system, a metering valve is often used
to control either the flow rate or the pressure, based on feedback from a pressure gauge and flow
controller.

7.4.3 Feedthroughs
Feedthroughs allow for the transmission of mechanical motion, radiation, or fluids into or out of
vacuum vessels. Mechanical feedthroughs allow us to manipulate objects inside a vacuum system
under vacuum; examples of this are the multiple axis stages of electron microscopes that allow
simultaneous tip, tilt translation, and rotation of a sample under study. Viewports on a vacuum sys-
tem are a type of optical feedthrough. Viewports permit us to observe the vacuum environment
inside a vessel or to transmit radiation such as a beam of laser light generated outside the vessel to
the vessel interior while the vessel is evacuated. Electrical feedthroughs permit the transmission of
electrical power into an evacuated vessel for the operation of devices such as deposition sources and

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