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.

POWER AND TEMPERATURE
CONTROL

Since the characteristics of the heater element change with temperature both input power and temperature must be regulated.

A microprocessor managed three term PID (Proportional, Integral, Differential) controller determines the power level necessary to reach and maintain the required temperature, within the specified limits. Changes in the cell temperature are simply achieved by changing the set temperature which is digitally displayed on the controller.