Diffusion though small pore membranes
We have previously discussed how the membranes near the center of the wafer have very small pores, while as you progress outward larger pores begin to appear. I tested one of these central membranes from wafer 304 (TEM sample, position -1,0) using our standard fluorescence microscope based assay.
I observed obvious rhodamine diffusion through this sample, indicating small but open pores within this membrane. There were no pinholes in this sample, although a small bit of wrinkling was observed at the corners of the membrane.
Its good to know that these things in the background are open pores because the density of small pores in the background seems similar across the wafer.
We might think about two contributing mechanisms of pore formation. The first one – perhaps volume contraction – gives a constant density of small pores across the wafer. The second one creates the large pores and has a radial dependence. Perhaps these pores result from macroscale film stresses.
Jess- based on the the rate of diffusion, can you say anything about the porosity of this center membrane? It’s good to know that some of them are open, but do you think it’s possible for them ALL to be open?
Regarding the formation mechanism, I still believe that the fundamental phenomenon is the same for both pore types. However, some local condition is causing groups of pores to merge into much larger pores toward the outside of the wafer. Personally, I think this local variation is a deposition issue…
It will be interesting to see Maryna’s air permeability measurements for the different membranes across the wafer. Her data should give us an idea of the porosity, which we can then compare to the rate of diffusion?