Ion Beam Production Methods

Following methods of producing metal ion beams are used in our sources:

1.     Gas Fed: Bottled gases can be directly bled into the source chamber through standard tubing lines. The volume of gas entering the chamber is carefully controlled through the use of sapphire valves.

2.     High temperature oven: The high temperature ovens use ohmic heating to directly heat metal material which is placed inside a crucible. Water cooled copper leads carry the heater current through thin walled tantalum or tungsten crucibles holding the material. The material is either directly placed into the crucible or into a ceramic crucible to decouple the oven material electrically from the Ta or W oven crucibles.  Typical operating currents for the high temperature oven range from 100A to 300A (up to 600W heater power). Our high temperature ovens can reliably sustain temperatures up to 2100 C.

3.     Low temperature oven: Metals with lower temperature requirements are evaporated using our low temperature ovens. The material is placed into a (typically) copper oven which is heated using a cartridge heater. The temperature is read by a thermocouple, and a temperature controller is used for feedback controls. Low temperature materials are on the order of 200 ºC to 600ºC.

4.     Radial Probe: Also known as a “cigarette” probe, a piece of material is placed onto the end of a linear shaft. It is then lowered through a radial port towards the plasma until it is close enough to be passively sputtered into the plasma via collisions with escaping electrons or ions. The main indication that the probe is at the right distance is the appearance of the probe material in the spectrum. Although this method of sputtering is not as controlled as the use of ovens, it is an effective and simple way to introduce material into a plasma.

5.     MIVOC or Metal Ions from VOlatile Compounds: The MIVOC method is based on using chemical compounds with a high vapor pressure at room temperature. The material is contained in a small chamber and connected via a valve directly to the source plasma chamber. The compound is broken down from its molecular form, releasing the desired material for ionization. In our facility we are using this method mainly for the production of boron ions.

6.     Sputter Probe: A sputter probe has been developed for the AECR-U ion source. It is similar to the radial probe in that the material is placed on the end of a linear shaft through either a radial port, or the axial oven port. A voltage is then applied to the probe which is biased negative with respect to the plasma chamber. This voltage causes ions from the plasma to be accelerated into the material at the end of the probe, thereby “sputtering” atoms of this material into the plasma. The yield is dependent upon the distance of the probe into the plasma, the amount of voltage applied to the probe, and the physical properties of the material introduced. It is a more controlled method than the cigarette probe, as the yield is a function of applied voltage. Because the ions have a much heavier mass than electrons, it is also much more effective in freeing atoms from the probe material.