Nanovoids in thin films
I’ve been doing some reading, and came across this article by researchers at Sandia Nat’l Labs. They are looking at the stress evolution in thin films, and have observed “nanovoids” in their amorphous silicon. Here is a cross-sectional TEM image (XTEM) from the article.
Floro et. al. PRL 2003
There are three layers of a-si with thickness ~3.5 nm separated by amorphous germanium (darker bank). The white “whiskers” are the nanovoids. The researchers attribute the nanovoid formation to a transition between compressive/tensile stress during growth. Perhaps these nanovoids are present after deposition, and the subsequent thermal treatment induces further stress which “rips” these voids further apart to form our pores.
Interestingly, poly-silicon thin films are at maximum tensile-stress when the film is 10 nm thick. A while back, I deposited a range of silicon thicknesses and noticed that an 8 nm membrane (wafer 170) had pores noticeable larger (and rounder) than films of 15 nm (wafer 167). Perhaps this was because of the higher tensile stress in the thinner film?
Is there a radial dependence on our deposited layer thicknesses to support your idea?
Unfortunately, it is difficult to relate any of our work to this paper, because the deposition techniques are so different. For example, I have never seen any tensile stress ever reported for a sputtered film, due to the ion bombardment of the growing film.