  We present TEM images of the interior of metal whiskers (MW) grown on
electroplated Sn films. Along with earlier published information, our
observations focus on a number of questions, such as why MWs' diameters are in
the micron range (significantly exceeding the typical nano-sizes of nuclei in
solids), why the diameters remain practically unchanged in the course of MW
growth, what is the nature of MW diameter stochasticity, and what is the origin
of the well-known striation structure of MW side surfaces. In an attempt to
address such questions we performed an in-depth study of MW structure at the
nanoscale by detaching a MW from its original film, reducing its size to a thin
slice by cutting its sides by a focused ion beam, and performing TEM on that
structure. Our observations revealed a rich nontrivial morphology suggesting
that MW may consist of many side by side grown filaments. This structure
appears to extend to the outside whisker surface and be the reason for the
striation. In addition, we put forward a theory where nucleation of multiple
thin metal needles results into micron-scale and larger MW diameters. This
theory is developed in the average field approximation similar to the
roughening transitions of metal surfaces. The theory also predicts MW
nucleation barriers and other observed features.
