Effective Cryogenic Cooling for MBE Systems
Molecular-beam epitaxy — a method for thin-film deposition of single crystals — is a growing technology, but proper calculation and component selection are critical.
An epitaxy is deposition of a crystalline overlayer on a crystalline substrate. In the late 1960s, J.R. Arthur and Alfred Y. Cho, working at the Bell Telephone Laboratories, developed molecular-beam epitaxy (MBE) as a method for thin-film deposition of single crystals. Today, MBE is used widely in the manufacture of semiconductor devices, including transistors for cellular phones and wireless equipment. According to Riber, a major manufacturer of production MBE equipment, the science of MBE and applications will drive a high expected growth rate over the next few years.
MBE utilizes highly controlled conditions during the process of making a crystal. No dirt particles or unwanted gas molecules can interfere with or contaminate the crystal growth. Therefore, the conditions incorporate extreme cleanliness and an ultra-high vacuum (UHV). Extreme cleanliness conditions are even cleaner than the conditions used in normal semiconductor manufacture. An ultra-high vacuum means the pressure is so low that it is at the limit of what is easily measurable.
In some cases, cryogenic temperatures act as a sink for impurities in the vacuum during the MBE process. As a result, vacuum levels must be several orders of magnitude better to deposit films under these conditions. In systems where the substrate must be cooled, the ultra-high vacuum environment within the growth chamber is maintained by a system of cryopumps and cryopanels, chilled using liquid nitrogen or cold nitrogen gas to a temperature very close to 77 Kelvin (-320°F [-196°C]).
Designing Cryogenic Supply Piping for MBE
The challenge for a system engineer designing the cryogenic supply piping for an MBE machine is to conceive a liquid nitrogen delivery system that will provide liquid saturated very near 0 psig at extremely stable pressure. In the case of a cryogenic fluid, keeping the pressure stable is required to maintain the temperature within a close range: The temperature changes with variations in the fluid pressure. Often, this is a huge problem due to the fluctuating nature of commercially contained cryogenic liquids.
Liquid nitrogen typically is transported over the highway at about 25 psig saturated pressure/temperature and delivered to a customer’s bulk-storage tanks. After delivery, and over time, the saturated pressure of the liquid nitrogen (LN2) in the bulk tank can rise to the relief-valve setting of the bulk vessel due to heat leak. The relief-valve settings vary from tank to tank, and many range from 40 to 150 psig. Different bulk-storage tanks have different heat leak rates and, as a result, the pressure and temperature of the liquid nitrogen inside raises at different rates. In old, poorly insulated tanks, the saturated pressure of the LN2 can rise to 150 psig in a few hours. In other tanks, it can take much longer.
Nearly all of these tanks have pressure builders that vaporize a small amount of liquid nitrogen to pressurize the vapor space of the tank. This pressure is used to cause liquid to flow to delivery points of lower pressure. The saturated pressure of the liquid, and the pressure in the vapor space, are independent and — usually — not the same. The saturated pressure of the liquid is the pressure at which it will boil, and it determines the liquid nitrogen’s temperature.
For any given saturated pressure, there is a corresponding fluid temperature. Simply put, this means the temperature of the available liquid nitrogen is constantly changing, and it is never the same from one instant in time to another. If an application such as an MBE machine requires constant pressure and temperature, another means must be employed rather than relying on bulk-storage tank delivery. Typically, the other means is a phase separator placed between the storage tank and the delivery point.
In every case, it is best to place the phase separator as close as possible to the end-use point. This minimizes pressure drop and downstream heat leak. It also is important to place the phase separator above the use points in elevation to prevent the possibility of backfilling due to gravity. Placing the phase separator above the use points also allows vapor created by heat leak from the piping and fittings to migrate back to the phase separator vessel, thereby avoiding delivering it to the MBE machine.
One caveat: to minimize pressure head, the phase separator should be positioned as low as possible while still meeting the requirement of being placed above the use points. In the case of an MBE machine, very cold liquid is needed. As such, the phase separator must be vented to atmosphere to cause the liquid saturation pressure to approach 0 psig and -320°F.
Generally, MBE machines require the liquid nitrogen to be as close as possible to -320°F while maintaining pressure within ±5 psig to control the temperature range. These requirements make necessary the use of a special phase separator.
The internal liquid level of the typical atmospheric pressure phase separator is controlled by a large-inlet vacuum-jacketed valve. These large valves almost always demonstrate some amount of hysteresis that hampers precise liquid-level control. These pneumatic-actuated valves also add to the imprecision problem due to compressible actuation fluid. During filling, the pressure temporarily rises inside the phase separator even if it is vented directly to atmosphere because some vent line resistance to flow always exists. The pressure rise is due to a decrease in the vapor space available, and to the creation of vapor resulting from an energy release of the incoming cryogen. The result is internal pressure changes during filling and depletion.
When the pressure of a cryogenic fluid changes its density, the volume also changes. This results in additional liquid-level changes as the pressure changes. By example, when the pressure rises, the fluid quickly warms, and its density decreases. This results in a volume increase and a liquid-level rise.
Dealing with Cryogen Pressure and Temperature Fluctuations
When the application requires very consistent output pressure, as noted previously, a special liquid-level option must be used. Adding closed-loop proportional-inlet valve control to the phase separator provides the liquid-level control needed. While this approach is more expensive, the PID controls the pressure well. With this option, the phase separator programmable logic controller (PLC) includes a proportional-integral-derivative (PID) logic path to control liquid level very near the setpoint. This results in a nearly constant liquid level and a smooth-acting inlet valve that does not open or close abruptly.
The valve control is programmed to open only as much as required to maintain downstream flow demand. A maximum valve-opening control is manually placed into the program to prevent overshoot and liquid-level oscillations. This control system limits downstream pressure spikes and unstable delivery pressure. Output pressure from the phase separator then can be maintained within ±5 psig, thus meeting the MBE machine’s requirement.
Finally, the cryogenic piping system supplying the MBE machine must be of high quality vacuum, both initially and over the long term. The high quality vacuum limits heat leak that can cause vaporization of a portion of the liquid supplying the MBE machine. Vapor in the supply pipe causes erratic liquid flow that can cause pressure fluctuation. It also causes temperature changes in the fluid.
The MBE machine supply and vent piping works best as a closed-loop flow to and from the phase separator. By closing the loop, the vaporized cryogen is returned to the phase separator through the vacuum-jacketed pipe along with any liquid not vaporized, which is returned by the process of thermosiphon. To get the proper liquid thermosiphon requires knowledge of the flow requirements of the particular MBE machine being supplied and its heat-absorption needs. Then, the piping must be sized to provide the needed flow rate considering the head pressure available.
In conclusion, paying attention to these issues and executing the calculations needed to size the pipe, along with the selection of the proper components, will result in a high performance system with no surprises after installation.