Vacuum applications present a unique set of challenges for electromagnet actuation. Brushless DC motors, whether rotary or linear, are composed of materials and built by processes that do not readily make them vacuum compatible. When considering a motion system requiring vacuum, partnering with a trusted motor/motion system supplier, with a proven history in high vacuum applications, will save you time and money. Allied Motion products from ThinGap, ALIO Industries, Sierramotion and Airex have all been successful in vacuum applications within the space and semi segments for the past twenty years. This tech note will share some of their application knowledge regarding motor and motion system in vacuum environments.
Vacuum Level
Every vacuum conversation begins with defining or stating the needed vacuum level. With this known, decisions and processes can start taking shape to be successful with the desired vacuum level. There are many levels of vacuums (see Table 1), and we will briefly describe a few and focus on the challenges of Ultra-High Vacuum (UHV).
Table 1. Vacuum levels (Pumps for High and Ultra-High Vacuum (vacaero.com))
For references, atmospheric pressure is about 14.7 PSI or 760 Torr with 1 Torr being (about) 1mm of mercury. “Partial Vacuum” begins in the 10E-1 Torr range and is typical of special processing environments and very high altitudes. A unique challenge has termed this region as the “Paschen-zone”, a troublesome partial vacuum level where low-pressure gasses ionize at low voltages forming plasma that can erode electrical insulation materials.
“Low Vacuum” is defined ad 1E-3 to 1E-6 Torr. Here, outgassing is mostly water vapor. This is the vacuum zone where space just starts at or near 100km above earth at about 3E-6Torr.
“High Vacuum” (HV), or sometimes called “Hard Vacuum” is typically 1E-6Torr to 1E-9Torr. As a reference, the orbit of the ISS is about 400km, where vacuum approaches 1E-10Torr.
“Ultra-High” Vacuum (UVH), is generally less than 1E-9 Torr. Once past HV, the outgassing is mostly hydrogen.
Ultra-High Vacuum
Ultra-High vacuum compatibility is a critical feature when providing motors for semiconductor and space applications. Low outgassing materials are required, as vacuum environments cause the release of VOCs and other dissolved gas particles which in semiconductor applications, can adhere to in-process wafers or scatter electron beams, causing poor imaging or high voltage arching, both limiting yield and throughput. Figure 1 below is an image created from an electron beam microscope, imaging features with nanometer level dimensions, all made possible with a UHV environment. In space, outgassing can result in substances vaporizing and condensing in the spacecraft, leading to fogged optics or premature component failure. Components and motion systems delivered into UHV applications require the elimination of real and virtual leaks and need high cleanliness levels. In these systems, one fingerprint can cost hours or days of pumping down a wafer inspection/process chamber or contaminate lenses and mirrors in space.
Figure 1. Electron beam lithography made possible utilizing an UHV environment.
Contamination
Absorption and desorption are both common challenges for Ultra-High vacuum environments (see Figure cc). While the term “absorption” is commonly understood, that being the process of one substance being fully incorporated into another, “desorption” is the release of an absorbed substance from a surface. Even at Ultra-High vacuum levels, gas molecules are present and can adhere to material surfaces, especially when rough surface finishes are present. In vacuum, gasses slowly release from surfaces and produce out-gassing. These gases can then be absorbed and desorbed from material surface all throughout the vacuum volume. Temperature, time, material history and other factors matter greatly in determining the level of contamination impact. Figure 2 below is an image of dust particles deposited on a wafer surface, rendering this area un-useable.
Figure 2. Semiconductor wafer image with deposited particles
Low surface out-gassing
Low surface out-gassing depends on how smooth surfaces are. Improving surface finish reduces molecular sticking and shrinks the effective surface area. It is critical to select the right materials for the vacuum level desired, as some choices absorb less gas than others.
Outgassing
Material out-gassing is dependent on cleaning, the material history, and any pre-treatment bake-out. As an example, an anodized aluminum part can greatly increase the storage and out-gassing of water vapors, as they can be trapped within the surface finish (see Figure 3 below). Sulfur containing metals, such as Zinc Sulfide, a common pigment found in paints, plastics, and rubber also have increased outgassing levels.
Figure 3. Mild versus hard anodizing (What is Anodized Aluminum and Can Anodized Aluminum be Coated (silcotek.com))
| Material | RGA (Torr liter/Sec/cm^2) @ 1 hour |
| Aluminum Cleaned | 1e-8 |
| Stainless Cleaned | 3e-9 |
| Stainless- 24 hr bake @150C | 4e-12 |
| Copper | 1.9e-8 |
| Teflon | 6.8e-8 |
Table 2. Material Outgassing
Outgassing is not just about the material itself, but it involves how parts are assembled, cables, connectors and the fasteners used. Any amount of trapped air will slow down the pumping process.
Residual Gas Analysis (RGA)
With outgassing such a crucial aspect of working in UHV environments, Residual Gas Analysis or RGA testing is often needed to understand what chemicals are escaping from what components and the time needed to achieve a desired vacuum level. Individual sub-component testing is an excellent way to separate and identify sources of gasses and the gas itself. The time aspect of achieving the desired vacuum level is another point of significance when understanding outgassing. Having this requirement know at the onset of the project will help in selection of materials and processes.
Design Considerations
Materials
When designing motor components and motor assemblies into UHV environments, extreme care must be taken in material selection. This includes metals, coatings, epoxies, and even the mold preparation of the epoxy process. As we mentioned earlier, the surface finish plays a large role in absorption and desorption, so all surfaces must be designed to have a smooth finish. Some forms of plating may be acceptable for the specific vacuum level, but as UHV is reached, less options are available. Magnet material is an example where plating is a must. Fasteners and some metals, such as low carbon steel, must also be properly plated. Electro-polishing is a process that can improve the surface finish of problematic metals. During bake-out, material is exposed to hours, if not days of elevated temperatures. A simple example of an overlooked material would be the lead wires of a frameless motor, having thermal limits below a suggested bake-out temperature. All material, coatings, and insulators, no matter seemingly inconsequential based on volume or mass must be selected knowing a bake-out process will be needed.
Epoxy
There are some epoxies that are compatible with vacuum applications, but care must be taken in executing the required component-level bake-out. Excessive heat can weaken epoxy, making the part unable to meet its life expectancy. In these cases, it is crucial to correctly weigh epoxy components and mix per manufactures instructions, so the full cure strength and temperature resistance are achieved. Mold releases agents associated with an epoxy process can also cause excessive and un-wanted outgassing. Be sure to identify acceptable mold release agents or be able to clean all surface of the epoxy (once cured), removing absorbed chemicals during the curing process. Lastly, the molding and curing process must avoid air pocket voids, as any trapped air slows pump down time for UHV applications. Voids occurring on the surface may need to be patched. In some cases, patching can cause more harm than good, so be sure to work with the motor supplier to understand the impact of epoxy voids and the patching process. Lastly, during dis-assembly of molded parts, allowance of proper cooling is essential to not damage the mold itself, and not adversely alter the form-factor of the molded component.
Laser Welding/Hermetic Sealing
In some instances, UHV requirements will drive the design to fully seal components inside an Ultra-High vacuum volume. As an example, often motor components such as rotor and stators, containing magnetic material and copper are required to be sealed. Laser welding is a common solution, delivering hermetically sealed components, but at the expense of less electromagnetic torque. The thickness of the encapsulating metals adds to the magnetic gap, reducing flux, resulting in less torque created. Motor designers, however, will account for this constraint when a laser welded requirement is known (see Figure 4 below).
In many Semiconductor vacuum applications, Hydrogen is used to purge or continuously mix in the environment to keep optical components clean. Permanent magnets, even when coated, cannot handle the additional Hydrogen in the environment and will decompose. Laser welding is a solution for additional containment and protection from Hydrogen. By enclosing magnets in stainless steel or equivalent materials, they protect them the harmful Hydrogen gas. In any laser welded solution, Helium leak testing the enclosure is required to ensure it meets the vacuum level required.
Figure 4. Laser welded Stators.
Motor Thermal Considerations
Motor design for any level of vacuum is challenging, as conduction, radiation and possible storage are the only coil temperature management options. Getting the heat out, and understanding the exact thermal resistance is key to success. In one worst-case scenario, the motor coil overheats and is ruined, and in another, better outcome scenario, but still not ideal case, the motor is way over-sized, and its maximum capability is never realized. The penalty for this is in cost, mass, and size of the motor.
Properly sizing the motor requires knowledge of the motion profiles, dwell times and chamber cycles. In vacuum, any type of heating must be considered, and as such, cable assemblies used at high duty cycles need to be evaluated for self-heating. This would normally not ever be considered in an atmosphere application, but when under vacuum, all heat sources matter. Heat management options when operating in vacuum include conduction to the outside environment (if not in space), conduction to other thermal sink, use of liquid cooling, implementation of “cold straps”, and thermal mass storage. In many cases, many of these options are not available, leaving only conduction to the system mass and thermal mass storage.
In all these cases, accurate, and data supported models are essential. Figure 5 below is an example of how Finite Element Analysis (FEA) is used to simulated motor heating. Vacuum chamber thermal resistance testing of stator coils to stator iron and subsequent stator iron to housing, are the first steps needed in developing an accurate thermal model, as these are often the best-case scenarios of modeling temperature rise in vacuum.
Figure 5. FEA Thermal Analysis and real test data.
A second significant thermal consideration is regarding magnets and bake-out. All magnetic material has a temperature limit and if bake-out is required to meet UHV, care must be taken to not de-magnetize motor magnets. Common Neodymium maximum magnet temperatures range from 80 to 150 C, while bake-out temperatures are often above 100C. The right grade must be chosen so to withstand the needed bake-out. Magnets producing higher field strengths are often associated with lower maximum temperatures, so trade-offs will need to be made in the electro-magnetic design. There are many ways to achieve the same force or torque with lower grade magnets (and subsequent higher maximum temperatures), but these solutions often come with added physical dimensions and associated mass.
Structure/Hardware/Cables
Even when delivering only a component into a UHV application, a system level approach must be taken. Magnet tracks, rotors, and stators, each have opportunities for trapped air and the production of real and virtual leaks. Proper design for venting and assembly of parts will improve outgassing and reduce pump-down time. As mentioned earlier, in some cases, the use of fully sealed or enclosed devices is the only acceptable approach.
As motors are electrified, cabling presents unique challenges in both delivering the power, while also shielding the signals, and maintaining proper cleanliness levels. Different strategies exist, based on signal integrity required, drive electronics, and power delivered. Consult with the motor and drive applications team to understand cabling trade-offs.
Radiation and other Space Considerations
Ultraviolet Radiation must be considered for exposed materials in space applications. Some materials such as epoxies and wire coatings, will experience photodegradation. Photodegradation is a process by which UV photons are absorbed into the epoxy chain, leading to accelerated aging and a weakened structure. Depending on the component’s locations within the system, it may need shielding via cladding or a simple enclosure.
Space applications of components will often require consultation with the Total Mass Loss (TML) database from NASA. This is a list of materials with acceptable TML ratings. Radiation is another important consideration for any electrified component in space. Proper shielding is often required for all “active” electronic components such as FPGAs and microprocessors. Use of Class III printed circuit boards (PCBs) is also a requirement for electrified components. The Class III rating PCB focuses on better processes to ensure delamination and other degradation does not occur at high temperatures, via plating at 100% coverage, signal integrity is maintained, solder mask integrity, and elevated soldering workmanship. Lastly, the use of lead solder is often required to ensure high reliability in rigorous applications.
Final Preparation
The last step in preparing any component or sub-component used in a UHV application is cleaning. Acetone, IPA and distilled water can be used to dissolve most materials. A thorough wipe-down of all exposed surfaces is considered best-practice when taking product into a clean environment. The last step is often a high-temperature purge, or Thermal Vacuum Bake-Out, and once completed, components should be bagged, then double bagged, with labels on the outside bag.
Summary
Ultra-High vacuum environments in chambers or in space, present many challenges in the form of particles, outgassing, radiation, and thermal management. Required vacuum levels at or lower than 1e-9 Torr drive coatings, materials, design, and process. Coupling these challenges with motor design only complicates matters more, and although thermal management of motor stators is a significant challenge, it is only one of many challenges. Plating of magnetic and low carbon steel material and laser sealing motor stators and rotors also create their unique difficulties. The vacuum application itself will impact choices made on materials, process, and design. In all UHV applications, however, cleanliness and process are keys to success.