3.4. Vacuum System

A pumping system is employed to remove air from the column. In most microprobes, vacuum is achieved through a combination of two types of pumps, mechanical and diffusion pumps. High vacuum is maintained by using O-rings at all mechanical fittings and constant pumping by mechanical and oil diffusion pumps. Various valves allow sequential pumping of the microprobe and pumping of isolated portions (Figure 3.4a).

Vacuum system schematic

 Figure 3.4a. Schematic diagram of the MBX vacuum system. (1) Electron gun isolation valve; (2) Window separating spectrometers from column; (3) Sample change airlock; (4) Ballast tank; (5) Mechanical pumps; (D) Oil diffusion pump.

The most familiar vacuum units are atmosphere and Torr (mm Hg); however, the proper MKS unit is a Pascal (1 Pa = 1 newton/meter2 = 10 dynes/cm2). One atmosphere = 760 Torr, 1 Torr = 1 mm Hg = 133.3 Pa, 1 Pa = 7.5 x 10-3 Torr (7.5 millitorr), and 1 Pa = about 10-5 atmosphere.

The microprobe column and spectrometers must be operated under high vacuum for four reasons:

  1. To produce a mean-free path for electrons greater than the length of the electron column. This corresponds to a vacuum of better than about 10-4 Torr.
  2. To avoid arcing between the cathode (filament) and the anode plate. There is very high voltage between these two components and stray air or gas molecules can cause electrical arcing between them.
  3. To avoid collisions between electrons of the beam and stray molecules. These collisions can result in spreading or diffusing of the electron beam or, more seriously, can result in volatization event if the molecule is organic in nature (for example, vacuum oil). Volatizations can severely contaminate the microscope column, especially apertures, and degrade the image.
  4. To avoid damaging the filament. The volume around the electron gun must be kept free of gas molecules especially oxygen, which will greatly shorten the filament life.
  5. To prevent absorption of X-rays produced from the sample by air molecules. At high vacuum, even soft X-rays (such as B-Ka) are transmitted without loss.

 

The MBX microprobe is divided into high- and low-vacuum  sections. The spectrometers are evacuated by a mechanical pump alone and kept at relatively high pressure (about 10-3 Torr = 1 Pa). They are separated from the electron column, which is kept at about 10-6 Torr (10-3 PA) by thin windows. These windows are made of polypropylene or Mylar, the former used on the light-element spectrometer because it absorbs fewer X-rays. These windows allow a smaller volume to be kept at high vacuum and help keep the column cleaner by limiting out-gasses gear and fitting oils and leaking detector gas to the spectrometers.

A mechanical pump initially evacuates the column and spectrometers and pressure is monitored using a thermocouple gauge (Figure 3.4b). Thermocouple gauges work down to a pressure of about 10-3 Torr. Once the vacuum is sufficiently good, the "gate" valve to the diffusion pump is opened, allowing it to pump on the column. A cold-cathode (ion) gauge, which operates down to pressures of 10-6 Torr, monitors the pressure at higher vacuums. The cold-cathode gauge gets dirty with time and reads a better vacuum than actually exists.

Figure 3.4b. Schematic diagram of the MBX vacuum system. During initial pumping (green and blue) the column is evacuated using mechanical pump 1. Once a vacuum of about 1 Torr is achieved the gate valve opens to access the oil diffusion pump (green and red), isolating mechanical pump 1. The diffusion pump is kept at operating vacuum by mechanical pump 2. The ballast tank serves as a vacuum reservoir and allows the diffusion pump to function for short intervals with pumps 1 and 2 turned off. This minimizes mechanical vibration and permits high magnification operation.

In addition to the airlock, described in the with the sample stage, the electron gun can be isolated from the rest of the column and independently pumped and vented. This allows filament changes to occur without having to let the entire electron column come to atmospheric pressure.

Back: 3.3.1. Samples | Next: 3.4.1. Mechanical Pump | Home: Course Overview

Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM