Designing in-vacuum applications can become quite tricky as soon as in-vacuum automation is involved. Johannes Schmid explains.
Sophisticated in-vacuum processes like sputtering are the basis for a variety of high-tech products. Molecular impurities compromise the surface’s nanostructure, affect the conductivity or interfere with the application of additional layers. In many cases an impurity makes the surface chemically vulnerable, thus reducing the life span considerably.
Similar problems arise for molecular analysis devices: In order to analyse nothing but the sample an ultra high vacuum has to be maintained (10-11hPa), to avoid molecular contamination of the sample.
To precisely adjust samples or to create efficient and flexible production processes, in-vacuum automation is needed.
While standard motors with axis feed-through seem to save space inside the chamber, they are not without risk. The feed-through seal itself is a wear point and can cause severe delays besides destroying an entire batch of product. The leaking implement has to be replaced and the vacuum in the chamber needs to rebuild.
Especially when multi-dimensional movements are needed inside the chamber axis feed-throughs come with limitations Complex transmissions such as magnetic couplings or bellows do not only require more space, they also need additional expensive power transmission in the vacuum.
“In order to analyse solid samples in mass spectrometry in vacuum, they have to be fed to a desorption laser one by one. For this purpose, a two-axis precision positioning stage is required. Moving seals, whether against vacuum or as a radial shaft seal on motors, have always been the Achilles heel for finding a lasting solution,” explains Dr Jens Höhndorf, Head of Development for Time-of-flight mass spectrometry at Bruker Daltonics GmbH, Bremen.
In-vaccum motors
With in-vacuum motors, even complex multi-axis mechanisms are easily implemented, eg for use in electron microscopes for x, y, z-adjustment of the samples.
“Vacuum motors installed directly at the point where the torque is needed provide a more elegant, simple and reliable solution. We have had very positive experiences using Phytron’s vacuum motors,” says Dr Höhndorf.
In terms of cost, reliability and durability the use of motors directly in the vacuum comes with benefits, but creates additional requirements for the motor. When organic substances such as lubricants, adhesives, insulation and board material are outgassed, they run the risk of contaminating workpieces or sensitive instruments within the chamber.
The degree of outgassing depends on the motor materials, the vacuum class and the operating temperature of the motor. HV and UHV motors contain only materials that also meet the requirements of the ECSS (European Space regulations) with a maximum TML (Total Mass Loss) value of 1 per cent and a maximum CVCM (Volatile Mass Losses) value of 0.1 per cent. As UHV motors are designed to withstand a short-term winding temperature of up to 300°C the fully assembled motor can be pre-conditioned with at least 200°C in a vacuum chamber. (The rule of thumb: the outgassing rate decreases with a decimal power for every 100 K increase of the outgassing temperature. In the actual application the motor should always be driven at least 40K below the outgassing temperature.)
The motors should be vacuum baked, robust and highly accurate – if possible even without feedback electronics, to avoid further outgassing. High-resolution stepper motors position accurately and dynamically with high torque and without the need for additional feedback electronics. Driven with a modern micro stepping power stage they run smoothly with low noise.
Their special material and robust design prepares them ideally for duty in extreme environments. A UHV motor may be safely operated up to a radiation dose of 106J/kg. A motor not designed for radiation will suffer degradation of the insulation, the adhesives and the grease, reducing the efficiency and ultimately blocking the motor. This is exactly why in-vacuum motors are used in the large vacuum facility at the Max Planck Institute for Extraterrestrial Physics or a large variety of particle accelerators with temperatures down to -269°C or experiments with high radiation levels: the tight schedule of the research facility enforces the use of fail-safe solutions.
Temperature management
Special attention should be given to the temperature management in a vacuum. Due to the lack of convection in vacuum, the motor can heat up very quickly and often work at a high temperature level - depending on the duty-cycle. To avoid overheating of the motor, the configuration should include safety margins. In addition, a temperature sensor should be integrated directly into the winding to monitor the motor temperature. By these means, a long lifetime of the system can be ensured.
With proper care, there is practically no limit to the potential life span of the high-quality motor. For controlling the motor from outside the vacuum chamber, some essential functions such as temperature monitoring should be considered from the start.
As many customers asked for a controller suitable to their very specific needs Phytron designed the modular controller phyMOTION for up to 18 axes to include all necessary features for in-vacuum motors. High resolution power stages with optional temperature, encoder and resolver evaluation. The integrated fieldbus interface allows for control by PLC systems, via the supplied software, LabView or EPICS interface.
Dipl Ing Johannes Schmid is the technical director of Phytron GmbH, Gröbenzell, Germany. www.phytron.de
