# He diffusion through solids

1. May 19, 2005

### kawikdx225

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?

Thanks

2. May 19, 2005

### Gokul43201

Staff Emeritus
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.

3. May 19, 2005

### kawikdx225

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.

4. May 20, 2005

### Astronuc

Staff Emeritus
Try this reference - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000041000004001018000001&idtype=cvips&gifs=yes [Broken]

At 20°C, I doubt the He atom would diffuse, at least thermally. It might move with dislocation glide, but this requires application of stress.

As Gokul alludes to, there is usually a threshold temperature, however I am not sure what it might be.

Are you working with polycrystalline or amorphous Si?

Last edited by a moderator: May 2, 2017
5. May 23, 2005

### Gokul43201

Staff Emeritus
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 !

6. May 30, 2005