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Vacuum Technology

Torricelli is credited with the conceptual understanding of the vacuum within a mercury column by the year 1643. It is written that his good friend Viviani actually performed the first experiment, perhaps as early as 1644 [1,2]. His discovery was followed in 1650 by Otto von Guericke's piston vacuum pump. Interest in vacuum remained at a low level for more than 200 years, when a period of rapid discovery began with McLeod's invention of the compression gauge. In 1905, Gaede, a prolific inventor, designed a rotary pump sealed with mercury. The thermal conductivity gauge, diffusion pump, ion gauge, and ion pump soon followed, along with processes for liquefying helium and refining organic pumping fluids. They formed the basis of a technology that has made possible everything from incandescent light bulbs to space exploration. The significant discoveries of this early period of vacuum science and technology have been summarized in a number of historical reviews [2,3,4,5,6,7].

The gaseous state can be divided into two fundamental regions. In one region, the distances between adjacent particles are exceedingly small compared to the size of the vessel in which they are contained. We call this the viscous state because gas properties are primarily determined by interactions between nearby particles. The rarefied gas state is a space in which molecules are widely spaced and rarely collide with one another. Instead, they collide with their confining walls. Figure 1.1 sketches this behavior. This is an extremely important distinction that will appear in many discussions throughout this material.

A vacuum is a space from which air or other gas has been removed. Of course, it is impossible to remove all gas from a container. The amount removed depends on the application and is done for many reasons. At atmospheric pressure, molecules constantly bombard surfaces. They can bounce from surfaces, attach themselves to surfaces, and even chemically react with surfaces. Air or other surrounding gas can quickly contaminate a clean surface. A clean surface, e.g., a freshly cleaved crystal, will remain clean in an ultrahigh vacuum chamber for long periods of time, because the rate of molecular bombardment is low.

Fig. 1.1 View of a viscous gas and a rarefied gas.

Molecules are crowded closely together at atmospheric pressure and travel in every direction much like people in a crowded plaza. It is impossible for molecules to travel from one wall of a chamber to another without myriad collisions with others. By reducing the pressure to a suitably low value, molecules can travel from one wall to another without collision. Many things become possible if they can travel long distances without collisions. Metals can be evaporated from pure sources without reacting in transit. Molecules or atoms can be accelerated to a high energy and sputter away or be implanted in a surface. Electrons or ions can be scattered from surfaces and be collected. The energy changes they undergo on scattering or release from a surface are used to probe or analyze surfaces and underlying layers.

For convenience the sub‐atmospheric pressure scale has been divided into several ranges that are listed in Table 1.1. The ranges in this table are not so arbitrary; rather, they are a concise statement of the materials, methods, and equipment necessary to achieve the degree of vacuum needed for a given vacuum process.

The required degree of vacuum depends on the application. Reduced pressure epitaxy and laser etching of metals are two processes that are performed in the low vacuum range. Sputtering, plasma etching and deposition, low‐pressure chemical vapor deposition, ion plating, and gas filling of encapsulated heat transfer modules are examples of processes performed in the medium vacuum range.

Pressures in the high vacuum range are needed for the manufacture of low‐ and high‐tech devices such as microwave, power, cathode ray and photomultiplier tubes, light bulbs, architectural and automotive glass, decorative packaging, and processes including degassing of metals, vapor deposition, and ion implantation. A number of medium technology applications including medical, microwave susceptors, electrostatic dissipation films, and aseptic packaging use films fabricated in a vacuum environment [8]. Retail security, bank note security, and coated laser and inkjet papers are now included in this group.

Table 1.1 ISO Definition of Vacuum Pressure Ranges and Descriptions

Source: © ISO. This material is reproduced from ISO 3529‐1:2019 with permission of the American National Standards Institute (ANSI) on behalf of the International Organization for Standardization. All rights reserved.

Note 1: While there has been some variation in the selection of limits for these intervals, the above list gives typical ranges for which the limits are to be considered approximations.

Note 2: The prevailing atmospheric pressure on ground depends on weather conditions and altitude and ranges from 31 kPa (altitude of Mount Everest, weather condition: “low”) up to 110 kPa (altitude of Dead Sea, weather condition: “high”).

The background pressure must be reduced to the very high vacuum range for electron microscopy, mass spectrometry, crystal growth, X‐ray and electron beam lithography, and storage media production. For ease of reading, we call the very high vacuum region “high vacuum” and its associated pumps “high vacuum pumps.”

Pressures in the ultrahigh vacuum range were formerly the domain of the surface analyst, materials researcher, or accelerator technologist. Today, critical high‐volume production applications, such as semiconductor devices, thin‐film media heads, and extreme UV lithography, require ultrahigh vacuum base pressures to reduce gaseous impurity contamination.

Yet another category of process takes place in the medium vacuum region using pure process gases and ultrahigh vacuum chamber starting conditions to maintain purity. Additionally, these processes must be free of particles. We call these systems ultraclean vacuum systems.

A vacuum system is a combination of pumps, valves, and pipes that creates a region of low pressure. It can be anything from a simple mechanical pump or aspirator for exhausting a vacuum storage container to a complex system such as an underground accelerator with miles of piping that must be held at an ultrahigh vacuum.

Removal of air at atmospheric pressure is usually done with a displacement pump, i.e., a pump that removes air from the chamber and expels it into the atmosphere. Rotary vane pumps are often used for this “rough pumping” purpose. Liquid nitrogen sorption pumps are used to rough pump ultraclean systems, such as those used for molecular beam epitaxy (MBE) that cannot tolerate even minute amounts of organic contamination. These pumps have finite gas sorption and require periodic regeneration.

Sorption pumps, as well as rotary vane and similar mechanical pumps, have low‐pressure limits in the range 10−1–10−3 Pa. Pumps that will function in a rarefied atmosphere are required to operate below this pressure range. The diffusion pump was the first high vacuum momentum transfer pump. Its outlet pressure is below atmosphere. The turbomolecular pump, a system of high‐speed rotating turbine blades, can also pump gas at low...