  The invention of scanning tunneling microscope (STM) dates back to the work
of Binnig and Rohrer in the early 1980s, whose seminal contribution was
rewarded by the 1986 Nobel Prize in Physics for the design of the scanning
tunneling microscope. Forty years later, the STM remains the best existing tool
for studying electronic, chemical, and physical properties of conducting and
semiconducting surfaces with atomic precision. It has opened entirely new
fields of research, enabling scientists to gain invaluable insight into
properties and structure of matter at the atomic scale. Recent breakthroughs in
STM-based automated hydrogen depassivation lithography (HDL) on silicon have
resulted in the STM being considered a viable tool for fabrication of
error-free silicon-based quantum-electronic devices. Despite the STM's unique
ability to interrogate and manipulate matter with atomic precision, it remains
a challenging tool to use. It turns out that many issues can be traced back to
the STM's feedback control system, which has remained essentially unchanged
since its invention about 40 years ago. This article explains the role of
feedback control system of the STM and reviews some of the recent progress made
possible in imaging, spectroscopy, and lithography by making appropriate
changes to the STM's feedback control loop. We believe that the full potential
of the STM is yet to be realized, and the key to new innovations will be the
application of advanced model-based control and estimation techniques to this
system.
