FEATURE NANOTECHNOLOGY


The Incredible World of Nanopositioning


resistance to roll, pitch and yaw. An actuator gives control along its axis but should only normally be used so that side loads are minimised; domed ends are often used to help minimise non-axial loading. A typical piezo stack will only produce


around 1micron per mm in length. When ranges in the hundreds of microns are required, mechanical amplification of the motion is usually applied. The key to good stage and actuator positioning resolution is minimising friction, which causes the ‘stick slip’ phenomena that prevents positioning to the nano meter. For this reason, flexures are used instead of slide ways and bearings. Environmental changes can have


John Andrew Clarke, MEng DipHE AMIMechE, from Queensgate Instruments looks at the shrinking world of nanopositioning and asks how far it can go in the future


W


ith data and processing demands increasing year on year, there is a


constant drive to make technology ever smaller. As the compactness increases, new challenges arise for manufacture and inspection. These systems often require the use of devices with accuracies in the nanometre level, a nanopositioner. In most fields of engineering, working to


micrometre accuracy is considered to be the standard, with typical high quality bearings being tolerant to within hundreds or even tens of microns. But the pioneers in nanopositioning are now working at hundreds or even tens of pico meters, an incredible million times smaller. For comparison, an atom is around one to five hundred pico meters in diameter and a human hair is 100,000,000 pico meters across. So, what is a nanopositioner? Firstly, it is based on a motor that can move nanometers at a time. For this, the reverse piezoelectric effect is used to create a motion in a piezo stack, with many layers of a piezoelectric material laminated with electrodes. By varying the voltage that is applied to this stack, small movements can be achieved. The key to the highest levels of nanopositioner accuracy is the control of this voltage and to get best control, a closed loop system is required using a positioning sensor. This overcomes the piezo’s natural non-linearity, hysteresis and position drift. Strain gauges offer a low-cost solution that is adequate where very high accuracy


14 SPRING 2015 | MICROMATTERS


is not required. But for the highest accuracy, optical or capacitive sensors are used to measure the position. The most accurate systems typically use capacitive devices to sense the position of the nanopositioner. The sensors can also be used separately for monitoring distance and vibration on mechanical systems. When mounted parallel to each other these can measure ranges from 20µm with a linearity of up to 0.01% before electronic compensation. Capacitive sensors have the advantage of


being non-contact and there are variants suitable for precision measurement in UVAC, radiation environments and cryogenic temperatures. Systems like these have been used in many applications and environments including semiconductor lithography, beamlines and in outer space. The Dextre International SpaceStation Robot employs capacitive sensors to monitor stresses and prevent damage being caused. When it comes to nanopositioning technology, a common question is what is the difference between a stage and an actuator? There is no definite industry standard, but in general a stage is always flexure guided to provide good directional stability and gives some mechanical


Figure 1:


The Dextre International SpaceStation Robot employs capacitive sensors to monitor stresses and prevent damage being caused


significant effects on the nanopositioner performance. Where practical, it is best to keep environmental conditions including temperature and relative humidity controlled. The use of low thermal expansion Super Invar in the stage construction helps to improve position stability over standard aluminium or stainless steel. When a nanopositioner is used for rapid scanning, drift is usually less of an issue as the stage does not have time to drift significantly between relative positions. A high stage resonant frequency allows high speed stability, making it possible for step times smaller than a millisecond to be achieved. As nanotechnology applications increase


“As nanotechnology


applications increase the need for accurate


control of dimensions of objects and accurate positioning at the nanoscale increases”


the need for accurate control of dimensions of objects and accurate positioning at the nanoscale increases. Nanometrology has a crucial role in producing devices with a high degree of accuracy and reliability in high technology fields such as semiconductor test and measurement, optical alignment, nanoimprinting, scanning microscopy and microlithography. Third and Fourth Generation Synchrotrons have increased demand for alignment and positioning technology as they permit high brightness and spatially resolved information. The demand for smaller and smaller measurement and control is growing rapidly. The question is, how small can we go? The answer is limited mainly by the environmental stability and electrical system noise. But with gradual improvements in technologies and design refinements, we may reliably be able to deliver stability to a few 10s of piccometers within the next 10 years. But this will also require the


environment to be carefully controlled.


Ten years ago we would not have predicted reaching the nanoscales we have reached today so time will tell just how far we can go.


Queensgate Instruments www.nanopositioning.com 01803 407 701


Enter 204 / MICROMATTERS


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