Positioning in High Vacuum (HV) and Ultrahigh Vacuum (UHV)

Providing precision positioning equipment for a vacuum environment follows clearly defined conditions. First, contamination of the vacuum chamber with particles through abrasion or outgassing but also through excessive heat input must be avoided. In addition, the installation space is usually limited. PI has many years of experience in vacuum technology and applications as well as a high vertical range of manufacturing. Therefore, PI is able to offer standard and customized products that are precisely tailored to the requirements of the vacuum application, e.g. by appropriate drive components, material selection, design, and accessories as well as suitable manufacturing processes and quality control.

PI offers various motion technologies that are suitable for vacuum applications: This includes piezo actuators, which can also work in strong magnetic fields and cryogenic environment, piezo systems with travel ranges of less than 1.5 mm and subnanometer precision, piezo motors in different designs with respect to force, dynamics, and travel range, motorized positioners with specially designed DC or stepper motors that allow larger travel ranges as well as parallel kinematic hexapods for positioning in six degrees of freedom.

Tab. 1: Different measures taken to achieve the corresponding vacuum class of motorized positioners.

>> Linear, rotation, Z stages and accessories suitable for vacuum

>> Hexapod series suitable for vacuum

To determine the required vacuum class, it is necessary to know the application as well as possible. For example, EUV lithography, the production of microelectronic circuits, or material analysis using SEM (scanning electron microscopy) place different demands on the pressure range, but also on the permissible residual materials in the vacuum chamber. Therefore, the outgassing rate is another relevant specification. Often the partial pressure of HCs (hydrocarbons) is decisive. Because it determines the pressure in the system (together with the capacity of the vacuum pump), outgassing prohibits fast achievement of low pressure values. In addition, the outgassing compounds may deposit on surfaces of optical elements or other sensitive devices and can obscure or damage them. Therefore, the residual gas must contain very little or no HCs and no metals with high vapor pressure such as zinc, lead, or cadmium.

Tab. 2: Definition of vacuum classes

Because outgassing is a challenge for creating and maintaining clean high-vacuum environments, the right choice of materials and treatments is compulsory in the design and production of vacuum systems. To manufacture products for high (HV) or ultrahigh (UHV) vacuum the following has to be considered: material selection, design, manufacturing and commissioning processes.

The material may not outgas and must be resistant to heat in order to overcome the bakeout temperatures necessary for higher vacuum classes. However, the required precision and high position resolution as well as flatness must be maintained.
For example, common electrical and electronic equipment contains components that either do not suit vacuum applications or only to limited extent. These comprise cables, motors, scaling systems, connectors or limit switches. PI implements specific product features for all these components to strongly reduce outgassing from these components and creating a clean vacuum environment.

Therefore, the materials used for vacuum positioning systems are aluminum alloys, stainless steels, titanium, or bronze. Surface treatment is adapted to the vacuum class, for example, the surfaces of the higher vacuum classes are not coated but blanc aluminum or electro polish. For use in HV and UHV, special vacuum lubricants are used. On request, the lubricant may be defined when placing the order. Vacuum cable insulation is made of PTFE or FEP (Teflon), and also of polyimide (Kapton) or PEEK. The use of adhesives is reduced as far as possible.

Substituting standard materials with vacuum-suitable materials, is a strict requirement of vacuum stage design. Another requirement is the reduction and minimization of the surface of the system.
In addition, it is important to avoid air inclusions since these act like virtual leaks in a vacuum.
The gas in this trapped volume cannot be pumped out easily due to only narrow paths connecting to the vacuum chamber. This delays reaching the required vacuum level.
Trapped volumes are usually caused by unvented or poorly vented blind tapped holes. These are either at the tip of a screw or under the rim of the screw head. Furthermore, holes that are covered either when mounting the positioner on a base plate or fixing a sample on the positioner, often cause virtual leaks. When virtual leaks are caused by screws, the use of vented screws, for example, is recommended.

The conditions and handling regulations for vacuum positioning systems are just as important as the design principles. Vacuum stages are mounted under cleanroom conditions. All components are cleaned in an ultrasonic bath. They are shipped in a packaging free from particles. Before a vacuum positioner is assembled, all pure metal parts undergo a cleaning process in an ultrasonic bath. Electrical and electronic units are wipe-cleaned. Standard, lubricated components such as bearings and guides are degreased, cleaned, and lubricated with special vacuum grease. Ultrasonic-cleaned components are dried in a climate chamber.

The assembly of a stage is carried out in a >> cleanroom or in a flow box. After assembling, the stage must pass a performance test in a clean environment. Vacuum tests are performed for each type of stage and when vacuum-critical changes are made to the product. After assembling, the system is packed in vacuum-sealed bags protected against dirt, air, and humidity: First, the stage undergoes a baking process in a climate chamber. After packing and sealing in an inner vacuum bag, the stage is then put into a second, outer vacuum bag that is then vacuum sealed completely.