15 October 1986
The Royal Swedish Academy of Sciences has decided to
award the 1986 Nobel Prize in Physics by one half to
Professor. Ernst Ruska, Fritz-Haber-Institut der
Max-Planck-Gesellschaft, Berlin, Federal Republic of Germany,
for his fundamental work in electron optics, and for the
design of the first electron microscope
and the other half, jointly to
Dr Gerd Binnig and Dr Heinrich Rohrer, IBM Research
Laboratory, Zurich, Switzerland, for their design of the
scanning tunnelling microscope.
One half of this year's Nobel Prize in
Physics has been awarded to Ernst Ruska for "his
fundamental work in electron optics and for the design of the
first electron microscope". The significance of the electron
microscope in different fields of science such as biology and
medicine is now fully established: it is one of the most
important inventions of this century.
Its development began with work carried out by Ruska as a young
student at the Berlin Technical University at the end of the
1920's. He found that a magnetic coil could act as a lens for
electrons, and that such an electron lens could be used to obtain
an image of an object irradiated with electrons. By coupling two
electron lensed he produced a primitive microscope. He very
quickly improved various details and in 1933 was able to build
the first electron microscope with a performance clearly superior
to that of the conventional light microscope. Ruska subsequently
contributed actively to the development of commercial
mass-produced electron microscopes that rapidly found
applications within many areas of science.
Electron microscopy has since been developed through technical
improvements and through the advent of entirely new designs,
among them the scanning tunnelling electron microscope. A number
of researchers have taken part in both this and the earlier
development, but Ruska's pioneering work is clearly the
outstanding achievement.
The other half of this year's prize has been awarded to Gerd
Binnig and Heinrich Rohrer for "their design of the
scanning tunneling microscope". This instrument is not a true
microscope (i.e. an instrument that gives a direct image of an
object) since it is based on the principle that the structure of
a surface can be studied using a stylus that scans the surface at
a fixed distance from it. Vertical adjustment of the stylus is
controlled by means of what is termed the tunnel effect - hence
the name of the instrument. An electrical potential between the
tip of the stylus and the surface causes an electric current to
flow between them despite the fact that they are not in contact.
The strength of the current is strongly dependent on the
distance, and this makes it possible to maintain the distance
constant at approximately 10-7 cm (i.e. about two atom
diameters). The stylus is also extremely sharp, the tip being
formed of one single atom. This enables it to follow even the
smallest details of the surface it is scanning. Recording the
vertical movement of the stylus makes it possible to study the
structure of the surface atom by atom.
The scanning tunneling microscope is completely new, and we have
so far seen only the beginning of its development. It is,
however, clear that entirely new fields are opening up for the
study of the structure of matter. Binnig's and Rohrer's great
achievement is that, starting from earlier work and ideas. they
have succeeded in mastering the enormous experimental
difficulties involved in building an instrument of the precision
and stability required.
Background information
The invention of the conventional microscope represented a great
step forward for science, particularly in biology and medicine.
As better and better microscopes were built, it was discovered
that there exists a limit that cannot be exceeded. This is
connected with the wave characteristics of light. Using light
waves, it is impossible to distinguish details smaller than the
wavelength of the light. The term "resolution" refers to the
distance between two details of an image that can just be
distinguished. For a conventional microscope using visible light,
the resolution is some 4 000 Å (1 Å, ångstrom =
l0-8cm).
The great breakthrough in microscopy came when it was found
possible to produce an image of an object using an electron beam.
The starting point was the discovery that a magnetic coil can
function like an optical lens. A divergent bundle of electrons
passing through the coil is focused to a point. A suitable
electric field can also act as an electron-optical lens. Using a
lens of this type, an enlarged image can be obtained of an object
irradiated with electrons. the image is recorded on a fluorescent
screen or a photographic plate. It also proved possible to
combine two or more lenses to increase the magnification. The
work was carried out at the Technical University of Berlin at the
end of the 1920's.
The scientist who has made the greatest contribution to this
development is Ernst Ruska. As a young student together with his
supervisor Max Knoll, he began studying simple magnetic coils, He
found that the use of suitably-designed iron encapsulation
improved their electron-optical properties. Above all, it now
became possible to build a lens of short focal length. This is
essential if high magnification is desired. Using two coils in
series, Ruska achieved a magnification of fifteen times. Even
though this was a modest result, it nevertheless represents the
first prototype of an electron micrcscope. Ruska subsequently
worked purposefully to improve the details, and in 1933 he built
what can be described as the first electron microscope in the
modern sense - an instrument with considerably better performance
than a conventional light microscope 's. He was then appointed by
Siemens and took part in the development of the first
commercially-available, mass-produced electron microscope, which
entered the market in 1939. This event may be considered the real
breakthrough for electron microscopy.
Since then, development of the electron microscope has been very
extensive. Its resolving power could be considered theoretically
unlimited, since the electron is a pointlike particle, However,
according to quantum mechanics, every particle has wave
characteristics which introduce an uncertainty into the
determination of its position. This sets a theoretical limit to
resolution for the acceleration potentials normally used of the
order of 0.5 - 1 Å. In practice, resolutions down to about 1
Å have been achieved.
The type of elect on microscope developed by Ruska is called the
transmission microscope. The object to be examined is in the form
of a thin section. The electron beam goes right through this in
the same way that light pierces the object in a light microscope.
There are, however, several other types of electron microscope,
the most important apart from the transmission microscope being
perhaps the scanning electron microscope. In this extremely
sharply focused electron beam strikes the object The secondary
electrons emitted are collected by a detector and the current is
recorded. Magnetic coils cause the electron beam to scan the
object in the same way as the beam of a TV tube. The variations
in the emission of secendary electrons carn be used to build up
an image. The advantage is the large depth of focus which gives a
three-dimensional image as opposed to the sectional image
obtained with a transmission microscope. However, the resolution
is poorer. These two types of microscope thus complement each
other.
Electron microscopy has developed extremely over the last few
decades, with technical improvements and entirely new designs.
Its importance can scarcely be exaggerated and, against this
background, the importance of the earliest, fundamental work
becomes increasingly evident. While many researchers were
involved Ruska's contributions clearly predominate. His
electron-optical investigations and the building of the first
true electron microscope were crucial for future
development.
The latest contribution to the development of microscopy is what
is termed the scanning tunneling microscope. Its principle
differs completely from that of other microscopes. A mechanical
device is used to sense the structure of a surface. To this
extent, the principle is the same as that of braille-reading. In
braille, it is the reader's fingers that detect the impressed
characters but a much more detailed picture of the topography of
a surface can be obtained if the surface is traversed by a fine
stylus, the vertical movement of which is recorded. What
determines the amount of detail in the image - the resolution -
is the sharpness of stylus and how well it can follow the
structure of the surface. Obviously if the tip of the stylus is
too sharp, it rapidly becomes destroyed. At the same time, small
structural details of the surface can be damaged by mechanical
contact, One solution to this problem would be to maintain the
stylus at a small, constant distance from the surface The first
to succeed in doing this was the American physicist Russel Young
at the National Bureau of Standards in the USA. He used the
phenomenon known as field emission. If a sufficiently high
potential is applied between stylus and surface, a current flows
with a strength depending on the stylus-surface distance. If
regulated by a servo mecanism controlled by the current, this
distance can be kept constant without mechanical contact. Young
succeeded in building an instrument that worked on this
principle. The distance between the stylus tip and the surface
was approximately 200 Å. Its resolution was thus
considerably poorer than that of an electron microscope
However, Young realised, that it should be possible to achieve
better resolution by using the so-called tunnel effect This is a
quantummechanical effect that allows an electron (and also other
particles to cross an area where, according to classical physics
it cannot exist since it lacks sufficiently high energy. It makes
its way so to speak, through a potential mountain by
quantum-mechanical tunneling; hence the name tunneling
microscope. This means here that if the tip of the stylus is near
enough to the surface (10 Å, i.e. 1-2 atom diameters) a
current flows even at low voltages. In the same way as field
emission, it should be possible to control the stylus without
mechanical contact. However, Young was unable to convert this
idea into practice owing to the exceptionally large experimental
difficulties involved
The first researchers to succeed in building a scanning tunneling
microscope were Gerd Binnig and Heinrich Rohrer at the IBM
Research Laboratories in Zürich, Switzerland. The reason for
their success was the exceptional precision of the mechanical
design One example of this is that disturbing vibrations from the
environment were eliminated by building the microscope upon a
heavy permanent magnet floating freely in a dish of
superconducting lead. Less bulky but equally effective devices
for stable, disturbance-free suspension of the microscope have
now been developed. Piezoelectrical elements are used to control
the horizontal movement of the stylus in two perpend icular
directions so that it scans the surface a long parallel lines -
hence the name scanning microscope The vertical movement of the
stylus is controlled and measured using another piozoelement.
Using a special technique it has been possible to produce
styluses with tips consisting of a single atom. Consequently, the
precision of the image is particularly great. Horizontal
resolution is approximately 2 Å and vertical resolution.
approximately 0.1 Å. This makes it possible to depict
individual atoms, that is, to study in the greatest possible
detail the atomic structure of the surface being examined.
It is evident that this technique is one of exceptional promise,
and that we have so far seen only the beginning or its
development. Many research groups in different areas of science
are now in using the scanning tunneling microscope. The study of
surfaces is an important part of physics, with particular
applications in semiconductor physics and microelectronics In
chemistry, also, surface reactions play an important part, for
example in connection with catalysis. It is also possible to
fixate organic molecules on a surface and study their structures.
Among other applications, this technique has been used in the
study of DNA molecules.