Question:
I recently asked for the vapor pressure of Si because we were trying to make a thin film by electron beam deposition in my laboratory. However, we have decided to change to Germanium and I was wondering at which pressure it will evaporate. We use a machine which has 10kV voltage difference. I have been reading some articles an I have seen they talk about beam current (between 25 and 100 mA), but I am more interested in pressure. We have tried with 1.5 e-5 torr but it started to spread and we had to stop the measurement
Has anybody tried to do this with Germanium or could help me? I think may by 1e-5 mbar (which could be rounded to 1e-5torr) could be ok as I read from some papers but I would appreciate any help!
Answer1:
It recomends 10-6 Torr or lower with an evaporation temperature of 1400ºC. It is important how much you fill the crucible, they recomended 67-75% of crucible volume, to avoid spills.
They also shows a couple of pressures more and the temperatures but the previous is the best recipe.
Answer2:
The base pressure is the one that really matters to achieve evaporation of the material at the conditions of operation. However, your question is totally reasonable, when the material starts to evaporate the pressure inside the chamber rises (the vacuum drops). How much? In my experience around 1 order of magnitude, but probably it depends on the operation conditions, material to be deposited etc. Initially the change is more strong, but then it recovers a bit.
So, if your base pressure is 10-5 torr the operative pressure while deposition will be higher -say 10-4 torr- .
After a quite extensive search I didn´t find any concern about the pressure rise during evaporation, both in scientific papers or technical brochures from the manufacturers.
Most e-beam deposition systems include a PID controller (Proportional-Integral -Derivative controller, or three-term controller, that is a feedback loop, to keep the conditions of operation constant during the process. I thought it could also deals with the chamber´s pressure, but apparently it pay attention mainly to the evaporation rate and beam conditions.
Some companies make vacuum controllers that allow automatic vacuum control, for example:
I found a 2016 PhD thesis from UPM by Penyun Huo, where he deals with e-beam deposition of multiple materials, including Ge, that could be useful for your work too. They used a Ferrotec machine and it was what lead me to the previous link.
About the relationship between VP, Temp and Voltage, my be this link might be useful:
Here you also can see the tables with evaporation conditions for different elements.
About depositing Ge at 10-5 I have found some e-beam sources that apparently can do the job, they use sources with arc-suppression:
Solved:
We tried it with base pressure of 2E-7 torr and it increased up tu E-5. Thank you also for the curves, they have been very useful!
