PRINCIPLE
The Chell Knudsen or K-Cell
utilises the principle of molecular effusion (demonstrated by Knudsen
in 1909).
Material to be deposited is heated to a suitable vapour pressure in a isothermal
enclosure with an aperture in its wall. Molecular effusion occurs when the
mean free path is large compared with the dimensions of the aperture. This
gives rise to a cosine intensity distribution which can be collimated to form
a beam of electrically neutral material.
The advantage of this type of source is its ability to reproducibly deposit
material with coverages ranging from sub-monolayer to continuous films.
Chell has engineered these principles into a compact, self-contained
cell, enabling the researcher to have a convenient, controllable
deposition source.
The furnace housing, which
may be cooled by water or liquid nitrogen,is mounted together with
the electrical
leadthrough and the bellows-sealed
shutter assembly on a 35CF (2.75”OD) flange.
This arrangement allows the cell to pass down a 35mm bore tubulation.
The furnace of the Knudsen Cell is designed as a removable cartridge
which contains the crucible, heater element and heat shields. The
crucible is heated by a Tantalum foil element which is isolated with
Pyrolytic Boron Nitride (PBN) shields. The central crucible is graphite
with an embedded thermocouple for accurate temperature measurement.
The position of this thermocouple
has been selected to follow the internal furnace temperature as
accurately
as possible.The design,
developed by Chell’s engineers, gives a stable high temperature
furnace for controlled deposition of a wide range of materials. For
maximum versatility a PBN liner may be fitted into the main furnace.
Both the graphite and PBN crucibles are supplied with removable apertures
to define and limit the output beam. Chell’s unique cartridge
design gives maximum flexibility to the user since complete cells
of just the PBN liner can easily be replaced when the deposition
of a different material is required. |
