Nano-precision stages are vacuum-compatible

Nano-positioning equipment has to meet tough specifications that are often further complicated when the management of motion is in a vacuum, or when there is a requirement for extended travel, finer repeatability, higher speed or greater uptime - all at the right price. The challenges of engineering and selecting components that must work together in harmony to achieve such precision should not be under-estimated. The drive mechanism, bearings, feedback system, motion controller, kinematic structure and the environmental conditions must be considered if the design is to meet this accuracy in atmospheric or vacuum environments. These vacuum challenges can vary from 10e-3 Torr to 10e-10 Torr, with positioning performance and outgassing challenges.

The various ALIO stages primarily use linear ceramic servo motors for demanding UHV applications, with linear servo motors used for less demanding HV applications. Ceramic servo motors feature a spiralling friction action that allows for unlimited travel without mechanical hysteresis while maintaining nanometre precision. Other beneficial performance features from using piezoelectric motor-driven stages include shorter settling times, constant velocity range (from less than 1um/s to 250mm/s with less than 0.5 per cent variation), no drive inertia, no servo dither and no hysteresis. They are also suitable for ultra-high vacuum environments (10e-10 Torr) due to the materials' minimal heat generation and operating temperature range.

Three approaches to feedback

Position feedback systems used in a vacuum chambers have special designs to ensure performance and no outgassing. There are three main approaches that have been tried and proven to perform at single-nanometre resolutions in UHV. Optical encoders based on reading a physical scale can resolve down to the nanometre level. Although the scale has a 20um pitch, the signal has a sufficient signal-to-noise ratio to allow it to be interpolated down to the single-digit nanometre (2.5-5nm resolutions, depending on interpolator). These encoders work well for most applications where low cost, accuracy and repeatability are needed. The next level of performance to an optical encoder with tape or glass scale utilises a similar read head with a novel scale. Although using the same 20um pitch, it is etched directly into the stainless steel of a ring, for rotary applications, or onto a nickel-plated invar spar for linear applications.

Beyond optical scale encoders, a laser interferometer can be used to provide resolutions to 38pm, which provides positioning stability on a suitable mechanical system to the sub-nanometre level. Using a plane mirror optical scheme in two axes also allows the Abbe error to be eliminated. The added advantage of the interferometer is that only the plane mirror would reside in the vacuum chamber.

Depending upon the required measurement, a single mirror can be placed in the chamber to measure from the stage to the chamber wall. Alternatively, a differential measuring scheme can be employed to measure the distance between two plane mirrors within the vacuum chamber. This eliminates all common-mode noise sources between the stage and instrument.

The nanometre management of motion has become more pervasive throughout industry, as leading-edge technology pursues a common trend of working on smaller and smaller scales. This trend is prevalent in numerous positioning-sensitive industries, such as fibre optics, biomedical, micro-machines, electronics, semiconductor, energy, optics, aerospace and synchrotrons, as well as in other research and development establishments.

Micromech is the exclusive UK distributor for ALIO's nano-precision stages.