Nanopositioners are fundamental to practical research in the field of nanotechnology. Whether used for microscopic imaging or actual manipulation of molecular level objects via optical tweezers, the nanopositioner forms the mechanical basis for work in the sub-micron world.
The Importance of Precision with Nanopositioners
Not too surprisingly, work at this minute level places demands on equipment which are quite different than mechanical systems operating at a larger scale. Unlike most mechanical devices containing bearings or slides to reduce friction, the nanopositioner must have completely frictionless and repeatable motion in order to produce useful results in the nanometer and sub-nanometer range. This stringent mechanical requirement is matched by equally stringent electronic control requirements. Continuous position sensing and feedback to the electronic controller must form a closed loop system capable of locating and maintaining a user chosen position without errors caused by position measurement, drift, or hysteresis. Only when both sets of requirements are met can nanometer and sub-nanometer controlled motion be reliably achieved.
Achieving Zero Friction in Nanopositioners
The first mechanical requirement of zero friction, while seemingly impossible, is the easier of the two. Given that the total range of motion of the nanopositioner is in the range of only a few microns to a couple of hundred microns, it is possible to support the moving structure by use of machined flexures. Flexures operate essentially as springs having only one axis of allowable motion. By virtue of their mechanical shape, flexures can bend slightly along one axis but, due to their shape, are highly resistant to bending in any other direction. Motion in more than one axis can be achieved either by stacking single axis nanopositioners or by embedding one inside of another so that their combined motion produces the desired number of degrees of freedom.
Zero Friction in The Drive Mechanism
Likewise, it is very important that the internal driving mechanism is designed with an eye towards minimizing friction while simultaneously maintaining a high level of control. Most nanopositioning devices use the electro-mechanical characteristics of piezo ceramics to create the driving force. When a voltage is applied to a piezo ceramic actuator it increases its physical length along one axis. Other unwanted motions, such as corkscrewing and twisting, are also produced by the actuator. Carefully controlled mounting of the piezo actuator inside of the nanopositioner provides the means to directionally limit the forces it places on the moving parts of the nanopositioner and produce motion only in the desired direction.
Precise Position Sensing in Nanopositioners
The second basic requirement, precise position sensing and control feedback, is more difficult but can be effectively resolved by use of piezoresistive sensors. While many types of displacement sensors exist, few have the necessary linearity, thermal stability, measurement speed, and small physical size necessary for use in a nanopositioner design.
Mechanisms for Position Sensing
Simple balanced bridge strain gauges mounted to internal bending elements can effectively measure the motion of a nanopositioner but suffer from inherent nonlinearity and susceptibility to thermal drift. Variable capacitance sensing of displacement is widely used in nanopositioners but falls short in a couple of important respects. A perfect capacitance sensor would be infinitely large (to eliminate fringe effects and maximize sensitivity) and would also have conductive plates which are perfectly parallel to each other. In the real world, capacitance sensors must be sized small enough to fit into positioning systems and have conductive plates which are not perfectly parallel. These inherent manufacturing problems produce nonlinearity errors. If the errors are assumed to be well defined and consistent, the nanopositioner with capacitance sensors can be calibrated and shipped with an error table. The error look up table can be used to calculate the actual position from the raw capacitance measurement. A third method, recently developed by Mad City Labs, uses piezoresistive sensors to directly measure displacement at the nanometer level with inherent linearity and temperature compensation. During fabrication, tiny piezoresistive sensors are integrated into the stage at specific locations. As the nanopositioner moves, the induced strain on the piezoresistive devices affects the crystal structure which in turn causes a change in electrical resistance. The change in resistance is constantly monitored by the connected controller as position feedback into the control loop. Throughout their range, piezoresistive sensors inherently exhibit less than 0.05% measured nonlinearity. Position creep is virtually nonexistent. Translated into a term called “position noise”, a nanopositioner with piezoresistive sensors can be shown to have less than 0.1 nanometer (1 angstrom) of position noise. Measurements of full range bidirectional repeatability have shown that any chosen physical position can be located to within less than one nanometer – regardless of the nanopositioner’s direction of travel. In short, nanopositioners equipped with piezoresistive position sensing and closed loop control can fully meet the requirements necessary to produce reliable, stable, and repeatable positioning down into the nanometer world.