Physical Principles of Electron Microscopy: An Introduction to TEM ...

152 Chapter 5
One example of this process is electron-beam contamination (Section 3.6), in
which hydrocarbon molecules adsorbed onto a surface are polymerized into
a material of high molecular weight that is impossible to remove by most
solvents. Once again, this generally-unwanted effect can be put to good use,
by using the polymerized layer as an ion-beam resist; see Fig. 5-22. As in
case of positive resists, many types of negative resist are commercially
available. The choice of the tone (negative or positive) and the chemical
nature of the resist are dictated by the application.
Figure 5-22. (a) Hydrocarbon-contamination line written onto a thin substrate by scanning a
focused probe of 200-keV electrons. (b) A broader line transferred to polycrystalline bismuth
by argon-ion etching. The arrow indicates the position of a grain boundary in the bismuth.
Courtesy of M. Malac, National Institute of Nanotechnology, Canada.
Figure 5-23. (a) Central region of a zone plate, fabricated by e-beam lithography and imaged
with secondary electrons. Bright rings are topographical contrast due to the step between bare
Si substrate and PMMA-covered Si, but weak materials contrast is also present: the bare Si
appears brighter because of its higher backscattering coefficient, giving a larger SE2 signal.
(b) Outer region of the zone plate, where the spatial resolution and accuracy of the pattern
represent an engineering challenge. Courtesy of Peng Li and Mirwais Aktary, Applied
Nanotools Inc., Edmonton, Canada.