6.5.
MECHANICS OF CARBON EVAPORATION
201
be too delicate to be useful. It should be realized that pure plastic films
(Parlodion, Formvar, or methacrylate itself) tend to drift or creep in the
electron beam, and it is this movement during photography that often
destroys resolution. Thus, carbon used in various ways has proved to be
a boon to electron microscopists. The carbon evaporating apparatus has
become a useful piece of equipment in most laboratories doing section
work. The apparatus ordinarily is the same as that used for metal shadowing which has its major application in other areas of electron microscopy.
Film deposition by vacuum evaporation has come to have a number of
industrial applications, most notably the "coating" of optical lenses and
prisms to minimize surface reflections. Thus, there are commercially
available both large and small equipment packages for this which are
easily adaptable to carbon deposition. These units contain a pumping
system consisting of a mechanical forepump and a high-capacity diffusion
pump. They contain suitable valving systems arranged to protect the
diffusion pump, and they have gauges which inform the operator of the
vacuum. All of these features are desirable, but they are costly. Satisfactory carbon films can be produced in what the physicist would regard as
a very moderate vacuum. The vacuum produced by a high quality mechanical pump actually is all that is needed. If necessary to economize, one
can dispense with gauges and valves and use the mechanical pump directly to evacuate the chamber in which the carbonizing is performed.
Carbon is evaporated by heating it to a brilliant white incandescence
in the vacuum. In the absence of oxygen, atomic carbon flies in all directions until colliding with some part of the apparatus or with any residual
gas molecules in the chamber. Thus, films are built up on surfaces one
atom at a time. Since carbon does not melt, and sublimes only at 3560°C,
it is not affected by any heat generated by electron bombardment in the
electron microscope.
Carbon has sufficient electrical resistance so that thin rods of it can be
heated until they vaporize by passing moderate currents through them.
In practice, one or two carbon electrode rods are given tips of small dimensions [Figs. 23(a) and (b)]. A spring holds one rod against the other,
and high amperage, low voltage current is passed through the system. If
short tips about 1 mm in diameter are used it will require about 40-60
amps of 12-volt current (or the equivalent at somewhat higher or lower
voltage) to evaporate the carbon. The current is supplied by a heavy duty
MECHANICS OF CARBON EVAPORATION
201
be too delicate to be useful. It should be realized that pure plastic films
(Parlodion, Formvar, or methacrylate itself) tend to drift or creep in the
electron beam, and it is this movement during photography that often
destroys resolution. Thus, carbon used in various ways has proved to be
a boon to electron microscopists. The carbon evaporating apparatus has
become a useful piece of equipment in most laboratories doing section
work. The apparatus ordinarily is the same as that used for metal shadowing which has its major application in other areas of electron microscopy.
Film deposition by vacuum evaporation has come to have a number of
industrial applications, most notably the "coating" of optical lenses and
prisms to minimize surface reflections. Thus, there are commercially
available both large and small equipment packages for this which are
easily adaptable to carbon deposition. These units contain a pumping
system consisting of a mechanical forepump and a high-capacity diffusion
pump. They contain suitable valving systems arranged to protect the
diffusion pump, and they have gauges which inform the operator of the
vacuum. All of these features are desirable, but they are costly. Satisfactory carbon films can be produced in what the physicist would regard as
a very moderate vacuum. The vacuum produced by a high quality mechanical pump actually is all that is needed. If necessary to economize, one
can dispense with gauges and valves and use the mechanical pump directly to evacuate the chamber in which the carbonizing is performed.
Carbon is evaporated by heating it to a brilliant white incandescence
in the vacuum. In the absence of oxygen, atomic carbon flies in all directions until colliding with some part of the apparatus or with any residual
gas molecules in the chamber. Thus, films are built up on surfaces one
atom at a time. Since carbon does not melt, and sublimes only at 3560°C,
it is not affected by any heat generated by electron bombardment in the
electron microscope.
Carbon has sufficient electrical resistance so that thin rods of it can be
heated until they vaporize by passing moderate currents through them.
In practice, one or two carbon electrode rods are given tips of small dimensions [Figs. 23(a) and (b)]. A spring holds one rod against the other,
and high amperage, low voltage current is passed through the system. If
short tips about 1 mm in diameter are used it will require about 40-60
amps of 12-volt current (or the equivalent at somewhat higher or lower
voltage) to evaporate the carbon. The current is supplied by a heavy duty