Sample method for formation of nanometer scale holes in membranes Page: 3 of 10
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the yield of which is proportional to the square of the atomic number of imaged materials
(~Z2) In SEM images, the Au/Pd islands appear bright, while the silicon nitride substrate
and the material that covers the holes appear dark. In TEM, contrast is based on
absorption and scattering of electrons by atoms in the films, and contrast is reversed
compared to SEM with backscattered electrons. The high Z Au/Pd film appears dark,
while the silicon nitride is lighter and the material that closed the holes is very light.
TEM contrast is a convolution of film composition and film thickness. Comparison of
SEM and TEM images makes the conclusion plausible that the holes close due to built-up
of a low Z hydrocarbon layer during electron beam exposure. The possibility of
formation of a film from the Au/Pd layer can be excluded. TEM images also confirm
that the holes have not been closed completely. Rather the original hole diameter was
reduced to about 5 nm, well below the resolution of direct FIB drilling.
Holes can also be closed by electron beam deposition of selected materials, like
TEOS oxide and many metals. Here, the to be deposited material is introduced into the
vacuum chamber through a gas needle that exposes an area of interest to the selected
compound. We have tested this for platinum deposition. The platinum containing gas
was admitted into the chamber for pulses of few seconds during which the electron beam
rastered over the region of interest. As expected, the holes closed during deposition of
the platinum film. In the examples shown in Figure 4, the diameters of two holes were
reduced from 175 nm to 30 nm by electron beam deposition of platinum. The hole
diameters were reduced at an average rate of 10 nm/s, over thirty times faster then for
deposition of hydro-carbon films. The hole closing rate was found to slow down to a rate
of 6 nm/s after 10 s, an effect that is currently under closer investigation. Reduction of
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Schenkel, T.; Stach, E.A.; Radmilovic, V.; Park, S.-J. & Persaud, A. Sample method for formation of nanometer scale holes in membranes, article, February 24, 2003; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc782311/m1/3/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.