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Technical Notes
Bakeout Heaters
When pumping a high-vacuum chamber, the pressure decreases exponentially. That is, if it takes 5 minutes for the pressure to drop from 8 x 10-5 to 4 x 10-5 Torr, then it takes 5 minutes to drop from 4 x 10-5 to 2 x 10-5 Torr even though the latter interval is only half the absolute pressure difference of the first. 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, which removes most of the surface adsorbed materials. 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 because the dissolved gas migrates through the steel faster at higher temperatures.
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
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, 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 mounted at 2-3/4" CF ports is an effective way of raising the chamber and contents to high temperatures, particularly if the exterior is wellinsulated. Quartz tubular lamps with reflectors 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 roughly 4.75" to 6" long with wattages 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 temperature depends on the sample’s emissivity, the distance between lamp and sample, 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 operating 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, to which they are capable of delivering 3-5 A/cm2.
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 (between the atmosphere and the main chamber) with its own pumping system. Its purpose is to enable sample introduction without breaking vacuum in the main chamber. The load lock is connected to the chamber by a gate valve of sufficient size that samples can pass through. The load lock is vented, its door opened, and the sample introduced. The door is closed and the load lock evacuated to a reasonably low pressure. The gate valve is then opened and the sample transferred to the main chamber. The major advantage of this is that the amount of gas transferred from load lock to main chamber is small and quickly removed. If the main chamber were vented, its walls would be covered with adsorbed gas that would take much longer to remove. (See sidebar below for related information.)
Frames & Hoists
As vacuum processes become more complex, chambers become both larger and heavier. The support structure for a vacuum system must be carefully designed for strength and accessibility. We offer both carbon steel square cross-section frames in appropriate “bay” and “half-bay” increments and extruded aluminum “open-style” frames to suit most research and batch production systems. In addition, we offer a variety of yokes and hoists. The yokes are custom designed to fit the particular dimensions of the chamber lid and attach to the selected hoist. This combination, when correctly frame-mounted, lifts the chamber (or just the lid) and rotates through a full 360°, giving the necessary clearance for internal maintenance.
Constructing a Load Lock AssemblyThis sidebar should serve as an invaluable guide to anyone wishing to combine various standard components into a load lock built for their specific criteria.
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.
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).
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.
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.
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.
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. |
