Thin-Film Vacuum Deposition Sub-Systems & Components

Bakeout Heaters

When pumping a high-vacuum chamber, the pressure decreases exponentially. The reason is that the forces binding an adsorbed gas molecule to a surface depend, in part, on how many molecular layers separate that molecule from the surface. Molecules nearer the surface are bound more firmly than outer layers.

In any vacuum system, a molecule cannot be pumped until it enters the pumping mechanism, which only happens if the molecule is in the gas phase. Increasing the desorption rate is a major issue in achieving low chamber pressures in a reasonable time. The common method of increasing desorption rate is to raise the chamber temperature.

The typical bakeout temperature for a high vacuum chamber is between 150°C and near 200°C. However, to reach UHV pressures in the 10-11 Torr range, hydrogen diffusing from the stainless steel matrix is the major gas load source and the chamber must be baked to 400°C for many hours to speed up H atom migration through the steel’s matrix.

External Bakeout Heaters

These devices are mounted outside the chamber, on a structural worktop below the chamber, and apply heat to the airside surfaces only. They are augmented, as appropriate, by a shaped insulating blanket or tent built around the system. The four heater types used for this application are resistive fin, ceramic, tape, and sleeve.

The resistive fin is, in effect, a normal cartridge heater mated to a number of fins that provide a large surface area for convection-driven heating of the chamber.

The ceramic heater is a serpentine rod heater potted in a ceramic material that relies more on radiation than convection for heat exchange.

Heater tapes are resistance wires enmeshed in highly flexible woven fiberglass. They are wrapped around the chamber surfaces, transferring heat by conduction.

Sleeve heaters have resistance wires in 1/2" thick silicon rubber "boots" or sleeves that are molded to fit the size and shape of the specific ports and part of the chamber. Heat transfer is mostly conduction.

The highest chamber temperatures are probably obtained using the first two heater types. However, all types will usually give a local chamber surface temperature within the 150°C to 200°C range.

Internal Bakeout

Photo bakeoutheater TNInternal bakeout heaters are mounted inside the chamber but are designed to heat the chamber walls, not specifically a substrate or sample stage. A primary requirement for this type of heater is vacuum compatibility. They must have minimum outgassing when at temperature and cannot have volatile metals, such as cadmium or sinc, used anywhere in the structure or in the braze used to make electrical connections.

The flange mounted stab-in heaters use vacuum-compatible quartz IR lamps supported by the power feedthrough. Using a number of stab-in heaters mounted on 2-3/4" CF ports is an effective way of raising the chamber and contents to high temperatures, particularly if the exterior is well-insulated. Quartz tubular lamps with reflectors (see below) directed at the walls are also used as internal chamber heaters.

 

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