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In the beginning of this section we offer complete sub-assembly packages that consist of a chamber, frame (hoist where applicable), both high vacuum and backing pump, gauging, valves, and load lock assemblies. The technical notes below focus on the individual components offered in the remainder of this section that are not included elsewhere in the catalog. These include both bakeout and sample heaters, load locks, and chamber frames.
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.
Back to TopInternal Bakeout
Internal 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.
Sample Heaters
Quartz Lamp Heaters
The quartz tubular lamps with reflectors are popular sample heaters. Depending on the sample temperature required, two or four lamps are arranged around the sample’s back-side, heating it by radiation.
Lamps are 4.75" to 6" long with a wattage from 200 to 1,000 watts, enabling them to heat multiple small samples, or larger single samples from 4" to 12" in diameter. Temperatures are controlled by thermocouple feedback to an SCR controller supplying the power to the lamps. But the actual maximum sample temperature depends on its emissivity, the distance from lamp, the illuminated area, and various geometric considerations, including the angle at which the radiation strikes the sample surface.
Temperature uniformity depends on the sample’s thermal conductivity, the illuminated area, and the sample’s rotational speed. With some samples, it’s possible to reach backside temperatures of 900°C.
Button Heaters
These small cylindrical heaters (up to 2.5 cm diameter) have maximum surface temperatures between 950°C and 1,200°C at UHV pressures. The resistance wire is potted in alumina and sheathed in molybdenum. They are used for contact or radiation heating of small diameter samples.
Pyrolytic Boron Nitride Heaters
Discs of pyrolytic boron nitride (about 2 mm thick) are coated with a layer of pyrolytic graphite that is then cut in a (continuous) serpentine fashion to give it a long, uniform width path length. Finally, the graphite conductor is coated with a sealing layer of BN to reduce the heater chemical reactivity. The heater is capable of reaching a surface temperature of 1,200°C over disc diameters ranging from 1.8 cm to 5 cm, by applying power to each end of the serpentine.
EpiCentre® Heater & Rotator
The EpiCentre is a combined sample heater and rotator. The heater element is a serpentine machined from a graphite disc. While the heater element can reach 2,000°C, the construction of the sample holder and rotating components of the EpiCentre require the maximum operating temperature to be limited to 1,200°C to 1,400°C. The sample is heated from the back-side, and EpiCentres are constructed for sample diameters ranging from 5 cm to 15 cm.
Load Locks
The load lock is an intermediate vacuum chamber with its own pumping system and a quick opening door mounted between atmosphere and the entry point to the main chamber. It is connected to the main chamber by a gate valve large enough to allow transport of samples through it.
The load lock allows samples to be placed in the main chamber without breaking its high vacuum condition. With the gate valve closed, the load lock is vented, opened, and the sample placed on a linear motion device (LMD). The load lock’s door is closed and it volume evacuated into the high vacuum range. When the gate valve is then opened the sample is moved into the main chamber by the LMD. Since both chambers are in the high vacuum range, only a small quantity of gas is transferred (usually from load lock to main chamber). Once the sample is in its correct position, the LMD is detached and removed from the main chamber. The gate valve is then closed and the main chamber returns to its base pressure.
Constructing a Load Lock Assembly
This sidebar should serve as an invaluable guide to anyone wishing to combine various standard components into a load lock built for their specific criteria.
Load Lock Vessels
KJLC offers a variety of styles of load lock vessels. Choose a vessel with flanges that match those of the gate valve. Vessels are predesigned with ports to accept most linear transporters, pumps, and gauge tubes. Their compact design results in decreased pump down time. Mount the load lock in a “convenient” position, easily enabling you to reach through the door to attach the sample to the transporter.
Gate Valve
The valve must be compatible with the main chamber’s flange type, size, pressure requirements, and temperature requirements. Do not select a manual version just to reduce costs because this markedly increases the chances of accidentally venting the main chamber. Choose a thin valve rather than a thick one to reduce the transporter travel length required. We recommend a high-quality thin gate valve (a pneumatically operated, bellows-sealed actuator) with fluorocarbon flapper seal (if it can withstand the chamber bakeout) as the best choice. The valve bore must accept the wafer, sample, sample stage, or transporter probe. For trouble-free operation, interlock the valve with the load-lock pressure (so it cannot open prematurely) and with the transporter’s position (so it cannot close prematurely).
Transporters
When choosing a transporter, consider whether it travels far enough, whether it moves with sufficient precision to position the sample once in the chamber, and whether sample transfer with this transporter requires rotation (about the transporter axis). Also, when calculating the required transporter movement, remember that from its start position (just recessed in its mounting flange) it must traverse the thickness of the adapter flange to the cross, the width of the cross, the thickness of the gate valve, the chamber port length, and the distance from the chamber side-wall to the sample transfer point. Always select a transporter with a longer than minimum travel. Adding a permanent extender to the probe does not increase the travel, but starts the probe closer to its target. If you need only “general area’’ positioning, with or without rotation, use a magnetically coupled transporter. For precise positioning, however, choose a rack-and-pinion transporter, making sure it offers rotation if needed.
Doors & Viewports
If the load lock must reach UHV pressures, you must use a CF flange sealed with a copper gasket for the door. This is inconvenient but necessary. Frequently, however, the user can accept a rise in the main chamber’s pressure to the 10-8 Torr range as the gate valve opens. Therefore, taking Boyles law into account, the load lock’s base pressure need only be 10-7 Torr because its volume is much smaller than the chamber’s. A more convenient arrangement is an o-ring sealed door with a single-point locking mechanism. For a visual check on the sample, or the transporter probe’s position, you will need a viewport. Doors with built-in viewports serve a dual purpose.
Pumps
For the high-vacuum pump, the turbo—with its fast start and stop action, makes an ideal load lock. For ultra-clean systems, a turbo with magnetically levitated bearings works best, although grease-lubricated bearing pumps will probably not contaminate the chamber if operated correctly. However, Cryopumps need an isolation valve and separate rough line, making operation more error prone. To rough the load lock and to back the HV pump, we strongly recommend oil-free mechanical pumps. They have an unmatched combination of high pumping speed, cleanliness, and low price.
Pressure Measurement
As its primary function, a load lock protects the main chamber from contamination. No load lock is complete without gauges to indicate acceptable pressure before opening the door or the gate valve. Make sure the gauge controller has setpoints in the 10-7 to 10-10 Torr range to pressure interlock the gate valve. With hundreds of vacuum systems in service, we have a wealth of load lock knowledge that we will gladly share. Please contact us with your questions and application requirements.
