Radak
What crucible is recommended for my material?
Please download our latest crucible selection guide and see our growing table of crucible / liner recommendations for depositing common materials using Radak sources.
Another helpful resource is the table of deposition techniques on Kurt Lesker’s website here. This table lists the boat or crucible typically used with most materials. The table is based on nominal chemical compatibility and availability of standard crucible/boat materials. The table also lists the melting point and temperature where useful vapor pressures are achieved for deposition. We tend to run near the temperature which corresponds to a partial pressure of 10-6 Torr; higher temperature results in a higher deposition rate. If the evaporation temperature is below the melt temperature the source sublimes, greatly reducing the risk of crucible cracking. If the material melts below the evaporation temperature, a liner may be required to ensure the crucible does not crack.
How do I install a Radak furnace in my system?
Our sources mount with a single 1/4 -20 or optional 6 mm bolt (please specify metric with your request for quote). In addition you will need power (40VAC/ 40A) and thermocouple (C or K) feedthroughs. Furnaces are supplied with vacuum side thermocouple lead wire, you will need to supply the vacuum side power leads (10 AWG solid copper recommended). Power controllers ship with both air side power and thermocouple leads. You can find the instructions for installation and operation of our furnaces on our website here.
We have measured the flux distribution from a Radak II furnace, which closely follows a Cos4 distribution; please see our data sheet on uniformity available on our website here.
For further detail, an excellent paper describing the theory and modeling of flux distribution from Knudsen cell sources was published in the Journal of Vacuum Science and Technology:
“Monte Carlo calculations of the beam flux distribution from molecular-beam epitaxy sources”, Adamson, S.; OCarroll, C.; McGilp, J. F., Journal of Vacuum Science & Technology B, May 1989, Volume: 7 Issue:3, Pages: 487 – 490.
Do you make larger capacity evaporation sources like a Radak V, or bigger?
Yes! Luxel works with customers to develop furnaces to meet their special requirements, including even higher-volume sources, water-cooled fast cool-down furnaces, and many other special requests. Luxel’s technical support staff is ready to help you with your application; please call us to discuss your furnace requirements (360-378-4137).
What are the latest design changes to Radak furnaces?
In response to end-user requests to operate our furnaces under more demanding environments (specifically CuInGaSe2 solar cell production) we’ve made a few changes in our furnaces. If you’re familiar with our earlier design, the primary difference you'll notice is changing the top from a 45 deg cone to the flat top we use in our OLED sources. The flat top reduces material build up on the vapor shield and has a pressed ring which indexes the crucible for proper alignment. The second change you'll notice is the use of a ceramic plug for the thermocouple. The ceramic plugs make installation and removal much easier. On the Radak/OLED I, the ceramic plug extends slightly from the side of the furnace, which may present an interference problem in some installations. For these cases an optional 90° mounting adapter is available to turn the plug downwards, eliminating protrusion from the side of the furnace. Upon request, the old-style copper bosses are still available for thermocouple connection. Optional stainless steel boss style terminals are now available for Se compatibility.
Internally, the Radak II furnaces are constructed with a profiled ceramic insulator post eliminating the Ta braces used in the previous design. The profiled ceramic enhances the structure of the cage and eliminates the potential shorting and breakage issues of the Ta supports. Finally, all Ni has been removed from the furnace construction to make the furnaces less susceptible to damage from over-temperature transients.
Crucibles are now being manufactured with a notched lip. The lip projects slightly beyond the vapor shield to promote better sealing between the vapor shield and crucible to guard against evaporant entering the furnace. The lipped crucibles are compatible with existing furnace designs. These crucibles became standard in 2009.
Will the presence of Oxygen damage the furnace during oxide depositions?
Many oxide materials require a background of Oxygen to ensure proper stoichiometry. The oxygen can attack the W filament used to heat the furnace. As a result, the filament may become brittle and prone to breakage. As long as the oxygen level is low enough (around the 10-3 Torr range), the degradation of the filament should be relatively slow. If the filament is being attacked by the oxygen, WO3 will deposit inside the furnace and will appear as a blue hue on the crucible and ceramics inside of the furnace. If the filament does fail, it can be replaced by the user (with purchase of a heater replacement kit).
What if my material deposits at temperatures above 1500C?
The 1500°C limit includes a considerable safety margin and the furnace design has been tested to 1700°C; however, operation above 1500°C is not covered under warranty and may result in a shortened filament life. If, for example, your material begins to evaporate at 1650°C (Yttrium) you may be not be able to get a usable deposition rate within the range of the Radak furnace.
Where can I find a copy of the Radak Operating Manual?
Please download the latest version of the Radak operating manual from the Radak webpage here. Likewise, the Radak Power Controller manuals are available on the controller webpage here.
Remove as much material as you can with a soft tool, then use an oxygen plasma to remove the remaining organics. The main reason for the anodizing is to prevent shorting of the heater coils. Oxygen plasma may damage the anodizing but will work to passivate the surface so no harm done.
When should the radiation baffle be used?
With the radiation baffle installed on Radak furnaces, it can be hard to get good PID temperature control below about 500°C. Removing the baffle greatly improves radiative cooling, hence improving response at lower temperatures. With a properly tuned PID you should be able to control the furnace to within a degree at low temperatures.
There are no hard and fast rules with the baffle. We recommend using the baffle above 500-600°C but it depends on your tolerance to heat load in the system vs. control lag /cool-down time. The furnace controls relatively well at 400°C with the baffle in, but takes quite a bit longer to cool down to 100°C which is a pain if you’re doing batch coating. Above 600°C you’re dumping a lot of heat into the system and your maximum temperature will ultimately be limited by power needed to drive the furnace. The stainless outer cover may not fair so well either if the furnace is run too hot for too long without the baffle.
What is the difference between OLED and Radak furnaces?
Our OLED furnace is the low temperature version of the Radak furnace. Since late 2008, the only difference is that the OLED furnace is not supplied with an internal radiation baffle.
How do I clean my Radak furnace?
The furnace is made of low-outgassing high-temperature materials. The furnace body is stainless steel with a molybdenum vapor shield. The interior is tantalum, molybdenum, tungsten, and high purity alumina. Cleaning, when necessary, should be restricted to the stainless-molybdenum cover and the alumina crucible. Cleaning methods will depend on the material to be removed, but acids, bases, and scouring materials, which do not strongly attack stainless, molybdenum, or alumina can be used. Cleaning of the multi-layer radiation baffle is not recommended beyond ultrasonic cleaning in hot water, followed by a rinse in clean solvent such as distilled isopropyl alcohol, to remove any water trapped in the multi-layer structure, followed by oven drying.
A word of caution about using acids to clean the furnace cover - The molybdenum vapor shield will be quickly attacked by most acids; therefore exposure to acid should be minimized.
If there is heavy contamination or material buildup due to a crucible overflow, please contact our technical staff at (360) 378 4137 for specific recommendations or repair work if necessary.
Can I replace the heater coil myself?
Yes. In the event that the tungsten heater filament should fail, you can purchase our heater replacement kit and follow the instructions here. To prolong the life of the heater filament, we recommend operating the furnace at pressures less than 5x10-6 Torr. Introduction of gases during deposition is possible, but may substantially shorten the life of the filament.
Which type of thermocouple should I use?
Type K thermocouples tend to be more robust than type C thermocouples, but are limited to a maximum of 1200°C. Type C thermocouples will operate up to 2300°C, but please remember that Radak furnaces are only warranted up to 1500°C. Another consideration is that certain automation products do not yet recognize the C-Type thermocouple standard. Be sure to choose the same thermocouple type as used in your vacuum feedthroughs, power controller(s), or other system components that interface with the furnace.
Can the furnace be run with an empty crucible?
Yes. But there must be a crucible installed. The furnace will run without issue with an empty crucible but the temperature feedback relies on the thermocouple contact with the crucible so do not operate the furnaces without a crucible. If you must operate the furnace without a crucible for some reason, run the furnace on manual with a low (e.g. 10%) power setting.
Will my power supply work with the Radak furnace?
Luxel’s power controllers are designed to drive the furnace at low voltage using a high-current transformer and phase-angle-fired SCR. This minimizes the risk of electrical shock, arcing, and ionic conduction through the crucible which becomes problematic at higher voltages. If you decide to use your own power supply we offer the following recommendations:
• Output voltage should not exceed 60VAC at 100% power.
• DC power supplies will also work, but our experience is limited to AC.
• For Radak/OLED I furnaces, output current should not exceed 30 Amps at 100% power.
• For Radak/OLED II furnaces, output current should not exceed 40 Amps at 100% power.
• If depositing a low temperature material (i.e. less than 500C), we suggest limiting the maximum power output to prevent an over-temperature situation.
• If using PID temperature control, make sure to tune your PID parameters to avoid giving the furnace 100% power when cold.
• After line connections are made, always test the furnace at a low power setting first to ensure there are no shorts in the system, and verify the furnace is heating properly.
A custom transformer can be used to accommodate your country’s line voltage (i.e. 100V, 220V, 240V, etc) but this may extend the lead time a bit. The standard 110VAC unit should be connected to a 20 amp 108-120VAC circuit in your facility.
Which power controller do I need?
We currently offer three models of power controller: the Radak II, II+, and the DUAL controller. The basic difference between the controllers is the model II is meant as a stand-alone controller, the II+ has digital I/O for working with a PLC, deposition controller, etc., and the DUAL controller has all these features plus the ability to control two furnaces simultaneously for Co-Deposition. For more information, manuals, and an example of controller automation, please refer to the downloads on the Power Controller webpage.
Do I need a separate crucible for each material to be deposited?
If your material can be easily removed with solvents or acids that do not attack the crucible then you may be able to use the same crucible for multiple materials. However, because of the risk of cross-contamination we recommend using separate crucibles for each material to be deposited. If using a liner, be aware that some deposition will occur on the upper part of the crucible due to the shorter height of the liner; for this reason we also recommend a separate crucible/liner pair for each material to avoid cross-contamination.
How accurately can you control the deposition rate from a Radak furnace?
Please see our publication "Rate Stability with an OLED I Furnace" available for download on the Radak Furnace webpage
What is the maximum operating temperature of my crucible?
| Crucible | Maximum Recommended Temperature (°C) |
| Alumina | 1500 |
| PBN | 1500 |
| Quartz | 1000 |
| Arc-Coated Stainless | 900 |
| Organic | 460 |