Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

He diffusion through solids

  1. May 19, 2005 #1
    Hello all,
    I need to know the rate at which Helium will diffuse through a solid, specifically silicon. Assume I accelerate and implant a He+ ion into a bulk silicon substrate. How long will it take to diffuse out of the silicon? I know it will depend on the temp of Si and the depth of penetration but is there a lookup table or formula I could use?

  2. jcsd
  3. May 19, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm guessing this is either for a pre-amorphization step or for gettering transition metals from Si, by creating nanocavities below the surface.

    For a 30 keV beam at a density of about 10^17 ions/cm^2, a typical anneal profile is 900C for 1 hr.

    I do not know of any tables to look up, but I can tell you the following :

    1. The minimum anneal temperature is 700C.

    2. If you are gettering metals by making nanocavities, these are not stable over 1200C, so that sets an upper limit.

    3. The required anneal time is only a weak function of the ion density. It is a stronger function of the implantation depth (or the ion KE) and the anneal temperature. If you can not find tables, you can crudely extrapolate from the single data point, using some simplistic model for gas diffusion in a solid.
  4. May 19, 2005 #3
    The implantation of He into Si is a side effect of our tool that we are trying to characterize.

    This is done at room temperature with a 40KeV He+ ion.

    There is no annealing and we sure hope there are no nanocavities created by this process lol.

    Lets assume I have a silicon sphere at 20C with a diameter of 1mm and I place a single He atom at the center. I need to know how long will it take for the He atom to diffuse through the silicon.
  5. May 20, 2005 #4


    User Avatar

    Staff: Mentor

  6. May 23, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    At this energy, I suspect you might make nanocavities unless you do a subsequent high temperature anneal.

    Uh oh...I can keep my fingers crossed, but that's not likely to help much.

    At 900C, if the diffusion time is of order 1 hour, then at 20C the diffusion time will be of order 55 hrs !
  7. May 30, 2005 #6
    Thanks for all your help. :smile:
  8. May 30, 2005 #7
    Try looking at the Defect and Diffusion Forum website. This journal has been collecting data on diffusion in silicon since the 1960s!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: He diffusion through solids
  1. Diffusion length (Replies: 1)