Plasma ignition source for microscope
We recently received a development order from one of the world’s leading manufacturers of professional microscopy systems, which we were particularly happy about. The customer has asked us to develop and manufacture a plasma ignition source. An AC high voltage source with an open circuit voltage of 500V RMS was required.
AC source for service operations
In future, the source is to be used as a compact, portable device to generate the ignition voltage for a FIB (Focused Ion Beam) module during maintenance and service operations. Such FIB units are, for example, additionally installed in the customer’s scanning electron microscopes. Thus, a pre-treatment of the sample surface can be performed before the actual microscopy of the sample. With the FIB used here, the sample surface can be cleaned, for example. With special FIBs it is also possible to process the samples in µm or even nm scale (“micromachining”)! For example, cross sections of the samples can be generated in a very targeted manner or individual layers can be exposed at depth. Pure high-tech!
For service operations, the customer would like to have a simple LED display that lights up when the plasma in the FIB unit has ignited. The mains-operated device should also have a mechanical voltage switch 115V AC | 230V AC. Thus it can also be operated abroad at other grid voltages.
Our prototype of the plasma ignition source
We decided to make a prototype first. This was to test the concept and gather valuable information. After all, we had to work out a clever evaluation circuit for the series device to reliably detect the ignited plasma. But until then, we did not know what values the process variables current and voltage would take on when the plasma ignites or extinguishes.
We deliberately designed the prototype of the plasma ignition source to be spacious so that measurements and modifications can be carried out quickly at the customer’s site without fiddling. We quickly packed it into a case that we had just “lying around”. Therefore the choice was made to install the components in a 19” subrack with 3HE. The subrack is mounted in a matching black 19″ rack case and the whole unit has a nice black anodized aluminum front panel. Pretty for a testing device, isn’t it?
Internally we have wired everything cleanly and clearly. This helps immensely with measurements and quick adjustments.
Customer-specific adaptations
For the prototype, we made the AC output separately switchable using push buttons and a contactor. Inside the device there is a customer-specific wound single-phase transformer, which is designed for 35 VA apparent power. It has a two-part primary winding to enable mains voltage switching.
The target application in the FIB unit requires that the alternating current is limited to a certain value when the plasma is ignited and the voltage drops accordingly compared to the open circuit ignition voltage. For this purpose we have provided leaded power resistors with axial aluminium housing as series resistors and connected them in such a way that we can quickly try out different constellations. Furthermore, we have integrated a current measurement with digital display into the high-voltage path, which should give us information about the average current under the different operating conditions of the FIB unit.
Last but not least we have to pay attention – the unloaded output has a peak voltage of more than 700V! We have deliberately installed isolated safety laboratory sockets for the AC output, which are absolutely safe to touch.
