Advances in Precision Positioning Stage Design-Optimized Outgassing and Motion Performance in Ultra-High Vacuum Applications

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.

Vacuum-Compatible Motors

Motorization provides ready adjustability of in-chamber elements and allows scanning, seeking and patterned-motion operations to be performed. Depending on the application’s requirement for such things as absolute positioning accuracy, minimum incremental motion, bi-directional repeatability and in-position stability, the optimal type of motion system will be evident. It is crucial, then, to work with a motion subsystem provider with access to a broad and deep toolkit of motion technologies.

Stepper motors are a versatile type of magnetic motor that require no brushed commutation, a significant advantage for outgassing and motor lifetime in a vacuum application. They provide a native holding force and can be operated in many modes depending on application needs. Stepper-motor vacuum stages typically employ special two-phase stepper motors that operate reliably up to 10-9 hPa and eliminate vacuum-incompatible construction details common among everyday stepper motors. For example, two-phase stepper motors for vacuum operation typically utilize a stainless steel housing, actively heated up to 120° C for outgassing, and of course plastic and other incompatible elements are eliminated.

Other common types of motors are less suitable for in-chamber use. For example, brushed DC servo motors are usually a poor choice due to unavoidable brush arcing, which contributes to outgassing and can present hazardous current paths in the corona regime. Similarly, magnetic linear motors present significant heat-management problems in vacuum use and are typically avoided.

Reduction or elimination of lubricants means that in general, motorized vacuum stages offer lower velocities and a shorter service life than equivalent models configured for lab or fab use.

Piezoelectric technology offers another class of motion drive that is exceptionally well suited for vacuum use as a matter of principle, as piezo drive elements (and the flexure mechanisms in which they are often deployed) do not require any lubricants for operation. PI’s PICMA®piezo actuator, for example, shows controllable nanoscale displacements well into the cryogenic range, down to -271° C. This is an example of a piezoelectric nanopositioning stack, a ceramic structure formed by sintering thin layers of specialized ceramic between metallic electrodes. These actuators eliminate the vacuum-incompatible polymer insulation ordinarily applied to piezoelectric stacks, substituting an advanced ceramic encapsulation that is inherently vacuum-compatible. These stacks are at the heart of many ultra-precision applications like microlithography, as application of a voltage produces fast material expansion that can be precisely controlled well below the nanoscale, although travel ranges are necessarily small.

Piezo actuator single-axis and multi-axis scanning stages combine nanometer-precision resolution and guiding precision with minimum crosstalk. This makes them particularly suitable for reference applications in metrology, for microscopic processes, for interferometry or in inspection systems for semiconductor chip production.

Beyond the familiar piezoceramic stacks, piezoelectric ceramics have been configured in a variety of novel motor designs which eliminate the small-travel drawback. Some of these piezomotors provide many millimeters of travel yet retain the sub-nanometer position resolution for which piezoelectric stacks are renowned.

Regardless of the drive principle, all piezomotors can be configured for use in a vacuum environment with simple attention to details like wire insulation. Piezomotors can also be used in high-field applications like magnetic resonance imaging, because they do not generate magnetic fields nor are they affected by them. Piezomotors are an enabler in UHV, especially when radiation resistance and travel ranges in the millimeter range are required.

PiezoWalk® stepping drives are a type of piezomotor based exclusively on static friction contacts of several piezo actuators on a rod. For feed forward, the actuators are lifted physically from the rod, minimizing wear and abrasion. These drives have high resolution of less than one nanometer. Their small size makes them ideally suitable as drive systems for stages and actuators in a vacuum down to 10-9 hPa, allowing them to be used for 6-axis hexapod systems, such as specialized units constructed for high-field applications.

Multi-Axis Motion in a Vacuum

Multi-axis motion can be accomplished using classical stacks of individual linear and rotary stages (serial kinematics) or with a more compact and elegant parallel kinematics approach. Two basic designs are common: Hexapod positioners (Greek for “six-legged”) and planar parallel-positioners (SpaceFabs). Hexapods are also known as Stewart Gough platforms.

A particularly demanding application is low-temperature spectroscopy utilizing the ultra-short-wavelength probe beam provided by large synchrotron accelerators. These are the most brilliant light sources on the planet and are instrumental in delving ever deeper into the structural intricacies of micro-electronic materials and biological specimens. Often significant travels in multiple degrees of freedom are necessary. A popular approach to providing stable, high-resolution, 6-DOF positioning in these and other applications is the novel planar parallel positioner (SpaceFab) mechanism manufactured by PImiCos of Eschbach, Germany. These are tri-strut assemblies with fixed-length struts actuated in a common plane by three XY stacks of ultraprecision stages. This assembly provides long transverse travel in the XY plane together with precision adjustability in the three angular axes and Z via differential actuation of the stage stacks. A noteworthy capability of this mechanism is the option to place the rotation centerpoint anywhere in space, such as a focal point or other optical sweet spot. With this flexible, modular and extensible principle, a wide variety of motion technologies can be utilized, including UHV-compatible magnetic and piezo stepper motors.

Hexapods offer less planar XY travel than a SpaceFab but even higher stiffness, greater load capacity, larger angular range and a smaller surface area. Benefits in common with SpaceFab include the virtual rotational centerpoint. Some manufacturers integrate especially tidy cable management that eliminates moving/sweeping cables. An example is PI’s M-824 Vacuum hexapod, a 6-Axis micropositioning and alignment system which provides high resolution submicron motion in six degrees of freedom. It is designed to work in a vacuum and provides up to 45 mm (linear) and 25 degrees (rotation).

The greatest hexapod advantage is their compact size compared to conventional six-axis stage stacks, a fact that is especially beneficial in applications in vacuum chambers, where space is at a premium. Many have through-apertures as well, which facilitates optical designs. Typical applications focus on semiconductor technology, multi-axis alignment of optics, X-ray microscopy, and X-ray monochromators.

Thermal Management and Outgassing

Thermal management is key in vacuum systems because they cannot rely on convection or fans for the removal of heat from the motors, electronic subsystems and bearings. Without careful thermal management, stage performance, reliability and life can be reduced significantly. Development of thermal isolation methods and passive and active cooling techniques help to maximize conduction modes of cooling and reduce or eliminate heat sources inside the chamber.

Similarly, outgassing is a challenge to creating and maintaining clean high-vacuum environments. NASA maintains a list of low-outgassing materials (http://outgassing.nasa.gov) to be used for spacecraft, as outgassing products can condense onto optical elements, thermal radiators, or solar cells and obscure them. Materials not normally considered absorbent can release enough light-weight molecules to interfere with industrial or scientific vacuum processes. Even metals and glass can release gases from cracks or impurities, but moisture, sealants, lubricants, and adhesives are the most common sources. The rate of outgassing increases at higher temperatures because the vapor pressure and rate of chemical reaction increases. For most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly. Cleaning surfaces, or baking individual components or the entire assembly before use, can drive off volatiles.