Electron microscopy
(Dry)-Scroll Pumps
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Scroll vacuum pumps are a type of compressive (mechanical) pumps. These pumps have been introduced into EMs in recent years as a replacement of conventional oil-filled rotary pumps when an absolutely oil-free pumping system is needed. Table 3177 lists the vacuum of various pumping configurations with scroll pumps and their applications.

Table 3177. Vacuum of various pumping configurations with scroll pumps and their applications.

Pumping configuration

Best reachable vacuum

Figure 3177c (a)

~1×10-5 - 1×10-10 Torr

UHV for epitaxial graphene by annealing SiC; Film deposition [2]; Thermal evaporator; Sputtering system; During pump down and bake out of UHV for laser vaporization [1]; Airlocks; Tabletop SEM; Cleanest SEM system for microelectronics industry; Gas-fed pulsed plasma thrusters (GFPPTs) [4]
Figure 3177c (b)
~10-10 - 10-11 Torr UHV for MBE system
Figure 3177c (c)
~10-9 - 10-10 Torr Electron gun for EMs; Upper portion of the column for cleanest SEM system for microelectronics industry
Figure 3177c (d)
Low vacuum Tribology equipment [3]

A typical scroll pump is mainly composed of a few parts, including fixed scroll, an orbiting scroll, a drive shaft and an electric motor as shown in Figure 3177a. In this system, the two scrolls are identical and are assembled at a relative angle of 180°, and have rotation axes that do not meet each other. The orbiting scroll rotates around its own axis with the gas trapped in between the fixed and orbiting scrolls, where a series of crescent-shaped "chambers" are formed because the two scrolls contact each other at several touch lines, as shown in Figure 3177b. During pumping process, the outer chamber becomes smaller and smaller, resulting in compression of the volume of gas.

Schematic illustration of scroll pump

Figure 3177a. Schematic illustration of scroll pump.

Working process of scroll pumps

Figure 3177b. Working process of scroll pumps: (a) At 0°, (b) At 90°, (c) At 180°, and (d) At 270°. The fixed scroll is in red, while the orbiting scroll is in black. The green circle with an arrow indicates the clockwise-moving path of the orbiting scroll.

The matching accuracy of the two scrolls needs to be very high. Scroll pumps normally use Teflon strips to make the sill between the scrolls. Comparing with oil-sealed pumps, scroll pumps are more vulnerable to debris that is introduced into the scrolls. In general, scroll pumps are also more expensive because of more work on design, manufacturing and assembly.

In a typical TEM system, rotary or scroll pump are used to evacuate the compressed air molecules from the exhaust port of the turbo-molecular pump as shown in Figure 3177c (a).

Mostly, microelectronics industry has the need of the cleanest microscope columns. For instance, after the semiconductor wafers, containing numerous microelectronic devices, are observed with SEMs, various fabrication processes will continue. Therefore, the wafers cannot be contaminated during SEM observation. The vacuum configuration of the microscopes for this application would probably have a scroll pump used to rough out and to back a turbomolecular pump for the specimen chamber as shown in Figure 3177c (a). In addition, ion getter pumps are typically attached to the electron electron gun and the upper portion of the column as shown in Figure 3177c (c). In this case, the three types of pumps can provide the cleanest SEM system.

Turbo-molecule pump + Scroll pump Three cryogenic pumps and an ion pump (~10-11 Torr) + Scroll pump (~10-2 - 10-5 Torr) Ion getter pump + Turbo-molecule pump + Scroll pump Scroll pump only

Figure 3177c. (a) Turbo-molecule pump + Scroll pump; (b) Three cryogenic pumps and an ion pump (~10-11 Torr) + Scroll pump (~10-2 - 10-5 Torr); (c) Ion getter pump + Turbo-molecule pump + Scroll pump; (d) Scroll pump only. The arrows indicates the direction of gas flow.

The main properties of dry-scroll pump are:
        i) High pumping speed. The typical pumping speed is ~4 l/s.
        ii) Oil free operation, ensuring no back-streaming of oil would contaminate the chamber.[3]
        iii) Scroll pumps require minimum maintenance due to the fact that there are less moving parts than rotary pumps.




[1] Paul Kemper, Andrei Kolmakov, Xiao Tong, Yigal Lilach, Lauren Benz, Manuel Manard, Horia Metiu, Steven K. Buratto, Michael T. Bowers, Formation, deposition and examination of size selected metal clusters on semiconductor surfaces: An experimental setup, International Journal of Mass Spectrometry 254 (2006) 202–209.
[2] Michael S. Doescher, Una Evans, Paula E. Colavita, Paul G. Miney, and Michael L. Myrick, Construction of a Nanowell Electrode Array by Electrochemical Gold Stripping and Ion Bombardment, Electrochemical and Solid-State Letters, 6 ~8! C112-C115 (2003).
[3] J. F. Ohanlon, Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 2, 174 (1984).
[4] James E. Cooley, Edgar Y. Choueiri, Fundamentals of Discharge Initiation in Gas-Fed Pulsed Plasma Thrusters, The 29th International Electric Propulsion Conference, 2005.


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