Precision positioning systems for optimized performance in ultra-high vacuums
Sophisticated, vacuum-compatible motion devices are increasingly being used by product designers and incorporated into a widening range of ultra-high vacuum applications. Now, the latest vacuum-compatible motion control devices deliver an even broader product development capability for OEM designers of ultra-high vacuum systems.
Specialized motion control products constructed for safe operation in vacuums provide critical performance for today's demanding vacuum applications in markets such as semiconductor manufacturing and inspection, fabrication, aerospace, life sciences, medical, nanotechnology, pharmaceuticals, photonics, telecommunications, semiconductors, astronomy, and microscopy. Key attributes of these systems include materials and assembly details selected for use in vacuum, motor, and electronic subassemblies with appropriate material and heat performance, bake-out compatibility, and clean preparation and shipping practices that avoid contamination.
Vacuum applications are increasingly important in industrial manufacturing and many fields of research. Many processes can only be performed in a vacuum, such as thin-film sputtering and ion milling. Technologies formerly reserved for exotic materials processing are seeing application in new fields. Furthermore, the march of Moore's Law (stating that the number of transistors on an integrated circuit doubles every two years) means new types of equipment have been developed that impose vacuum requirements in systems where none existed before; for example, as extreme ultra-high vacuum (EUHV) reticle illumination supplants previous technologies.
Five tips to specify vacuum-safe equipment
When specifying equipment for use in a required vacuum level, it is important to work with a supplier who can help analyze the application and ensure no details are overlooked. With ultra-high vacuums (UHV) for example, the following factors influence product suitability:
- Lubricant: choice, reduction, or elimination
- Surface area minimization: including elimination of rough or micro-cavitated coatings and finishes
- Suitability for operation under specific conditions: such as magnetic fields, radiation resistance (particle-radiation, X-ray radiation, EUHV), or cryogenic environments at -269 to 40 C (-452 to 104 F).
- Particular process: sensitivity to specific trace species
- Pump: capacity and bake-out practices.
Engineering a system for UHV
UHV is characterized by pressures lower than about 10−7 Pascal or 100 nanopascals (10−9 mbar, ~10−9 Torr). To achieve the goal of an optimized, reliable UHV positioning system, electrical and electronic equipment, such as motors, position feedback sensors, and limit/home switches, has to be suitable for the specified environment, including such details as UV illumination and electromagnetic field compatibility. But other critical factors must be considered, such as material selection, positioning system design, outgassing, thermal management, and cleaning and assembly. Vacuum-facing components should undergo a three-stage cleaning process:
- Ultrasonically clean the components.
- Dry the components in a climate cabinet under a nitrogen atmosphere.
- Assemble the stage in a cleanroom or in a flow box. The system should then be packed in vacuum-sealed bags and protected against dirt, air, and humidity prior to shipping to the site.
The requirements for a vacuum-compatible material include:
- No particle emission
- No outgassing
- Bakeable and temperature resistant.
This limits the choice of materials for the body of any vacuum-compatible product. For example, CuZn (brass) alloys cannot be used in vacuum systems, and standard plastic parts must be exchanged with polyether ether ketone (PEEK) or metal components to eliminate outgassing of organic contaminants. Preferred materials for vacuum positioning systems are stainless steel, titanium, bronze, aluminum, ceramic, sapphire, Viton, Teflon, PEEK, Kapton, and Macor.
For vacuum-compatible precision positioning stages, suitable materials are aluminum alloys, stainless steel, or titanium. Surface treatment is adapted to the vacuum class; for example, the surfaces of the higher vacuum classes are not coated but electro-polished. Anodizing of parts is routinely eliminated, since its intricate cavitation of the surface on a microscopic scale can increase the effective surface area of the part by a factor of six, and the dyes used to color ordinary parts can generate significant outgassing.
Vacuum pressure vs. residual materials
The pressure level alone is not the only critical parameter. Crystallography and optical coating manufacturing have different requirements, not only with regard to overall chamber pressure, but also regarding specific residual materials in the vacuum chamber. Frequently, the partial pressure of carbon hydrides is decisive. As part of lubricants and plastics, these hydrocarbons can be a source of contamination of surfaces. This is especially damaging in laser applications in the UV range, because the hydrocarbons can be split into reactive fragments, which subsequently can deposit on optics. Using a chamber with a connected mass spectrometer enables verification of the suitability of possible materials, but vigilant selection of components used in in-chamber subsystems is critical.
The general prohibition of ordinary plastics inside a vacuum chamber means conventional wiring (with its plastic insulation) cannot be used in vacuum-compatible motion hardware, either in external cables or in internal wiring. Cable management system construction and materials typically use Teflon insulated wires along with specialized electrical connectors that employ a variety of materials including PEEK. Other cable and connection options are available depending on the application requirements.
Positioning system design
An experienced motion-control provider will dive deeply into all aspects of a vacuum application to ensure that all details are considered. For one thing, vacuum chambers offer limited space and require a compact design. Electronic subsystems, such as controllers and amplifiers, will not survive vacuum use, so they must be placed outside the vacuum chamber, which requires consideration of feed-throughs and cabling design. Unavoidably, some electronic components such as motors, end and home switches, and encoder read-heads must reside in-chamber and must be specifically constructed for vacuum compatibility and suitable for the wide temperature variations encountered during in-chamber use.
Beyond the basic choice of materials and surface finish, the body of a positioning stage must be assembled with an emphasis on eliminating trapped pockets of air. Trapped air pockets can gas out very slowly. These are called virtual leaks and can considerably delay, or even make generating a stable vacuum impossible. Holes and screws need to be vented. The formation of pockets between surfaces when bolted together (such as when a structure with stiffening ribs is bolted to plate) must be avoided, or provisions for rapid evacuation of the pocket (such as a drilled vent) must be incorporated.
The selection of suitable components, especially the drive elements, is very important. In particular, it is critical to take heat generation during operation into account, as heat can only be dissipated with great difficulty under vacuum. Knowledge of the planned working cycles is, therefore, useful for selection. This allows advance testing of the heat behavior of the selected individual parts.