Surface Physics

Yesterday, Birk Reichenbach gave his dissertation defense at 10:30am.  I would like to congratulate Birk on an excellent presentation, and in my opionion, one of the best presentations I have seen him give.

I have watched Birk for the last 5 months slowly putting together his dissertation.  The hours have appeared long and stressful, but the pieces have been long falling in place, and now the puzzle is almost done.  Only a few more pieces left, and then it will be Dr. Reichenbach.  Once again, congradulations Birk.  You deserve it.

Birk during his defense.

The Reichenbach family at the conclusion of the defense.

Join us next Thursday as Birk Reichenbach gives his dissertation defense.

Location: Physics and Astronomy

Room: 184

Time: 10:30 am

Date: Thursday, September 24th, 2009

One of my current projects consists of plasma cleaning our micro arrays. Our micro arrays consist of ~10k molybdenum tips that operate as field emitters in reverse, and the cleaner they are the better they work. Prior to the plasma cleaning, we would heat the array and contaminants on the tips would desorb into the atmosphere and be removed by the ion pump. However, we ran into problems with a glass part on our T-05 headers getting too hot, expanding, and breaking our electrical contacts. This set an upper limit on how hot we could get, and unfortunately we need to get hotter. One solution that is being pursued is switching to ceramic headers to mount the arrays. Without the problematic glass, we should be able to get quite a bit hotter. March 20th’s post describes brazing Kovar rings on these ceramic headers.

The second solution consists of using hydrogen plasma to clean the surface. A DC glow discharge generates, among other things, hydrogen atoms and ions, which chemically clean the surface of the arrays by bonding with carbon and oxygen contaminants. These resulting molecules are then desorbed into the atmosphere and removed with an ion pump (and turbo molecular pump).

This shows a tungsten coil acting as the cathode surrounding a stainless steel wire anode separated by glass bead insulators. The plasma discharge takes place inside the coil, and the ions are drawn into the micro array (center) by applying a negative bias.

• Remotely generated DC glow discharge hydrogen plasma.
• Biasing array helps control incoming ion energy
• Better energy control helps avoid implanting and sputtering.
• Reduced operating pressure by:
  • Decreased discharge distance
  • Increased area of cathode relative to anode.
• Compatible with our neutron generating systems
• High voltage inputs more convenient.
• Array can act as a probe to better determine plasma characteristics

This shows the bottom of a cylindrical aluminum cathode with an aluminum mesh covering the end. The micro array is contained inside the cylinder. The anode is stainless steel in the bottom right. A hole in the cylinder near the anode exposes the array to the plasma.

• DC hydrogen glow discharge.
• Hydrogen ions are below sputtering range. (But may be implanting?)
• Data suggests the arrays are being cleaned.
• Heating while Plasma cleaning possible.
• Seems to operate consistently.

Problems to fix:

• Not currently compatible in our neutron systems.
• Not much control over ion energies.

Overall the plasma cleaning has been progressing steadily over the last few months. I’d have been very disappointed if it hadn’t considering all the papers and book that I have been reading. So far, though, the data suggests that the plasma is actually cleaning the arrays. Yay!

The overall progress is as follows:

• No Idea what I’m doing, but gotta start somewhere.
• Completely destroying the array via arc discharge. Oops!
• Destroying the array and seeing visible damage, but no arc discharges.
• Destroying it so that it no longer works but no visible damage.
• It works after I clean it, but did i actually ‘clean’ anything?
• Data suggests array is being cleaned.
• Some experiments with heating and plasma cleaning at the same time.
• Release of beta revision! (Today)

Hopefully, the beta version will help reveal some light on a few things. The more intelligent design will hopefully allow us to characterize the plasma a bit better, using the array as an (approximate) Langmuir probe. This will help determine ion current and narrow in on the most efficient cleaning time, etc. Also, being able to bias the sample and control the ion energy will rule out the possibility of implanting ions, which I suspect may be causing some funny business.

Well, there is certainly a lot of work to be done now.
Ill keep you posted,
~Ben

Undergraduate student Paul Whiteley has been working to braze kovar rings to ceramic headers.  Over the past few weeks the process has been fine tuned, and the results are turning out quite nicely.

Induction Heating works by electromagnetically inducing eddy currents in a conductive material, which in turn joule heat the material due to its internal resistance.

For us induction heating is desirable due to its ability to attain high temperatures in a very clean atmosphere.

In the photos above a hanging metal basket is used to hold the ceramic header and kovar ring, and is the primary object that is inductively heated. This alows objects inside to be heated up to a desired temperature. The red glow is due to blackbody radiation emitted by the metal basket during the process. The photo on the right shows the end result of the brazing.