Keeping nanopositioning precise

Precision motion control in general, and nanopositioning in particular, are enabling technologies in many high-technology fields such as bio-nanotechnology, semiconductor test and measurement, optical alignment, nanoimprinting, scanning microscopy and microlithography. Choosing the right nanopositioning stage depends not only on resolution and accuracy, but also on factors such as dynamics, size, the application environment and cost.

A nanopositioning stage is a motion device capable of repeatedly producing motion in increments as small as a nanometer or less. There are several ways to achieve resolutions of one nanometer or below. Some may seem as simple as bolting a microstepped motor and a reduction gearbox to a leadscrew mechanism. More sophisticated approaches use additional position feedback in the form of an encoder and interpolator circuit. However, there is more to a nanopositioning system than a high-resolution motor.

The enemy is friction

Friction leads to hysteresis and induces guiding errors such as tilt and wobble. In most positioning systems, guiding errors are not measured, and hence remain uncontrolled. Tilt, wobble and runout errors automatically contribute to positional inaccuracy. This fact is often neglected and rears its ugly head when several individual positioners are combined into one multi-axis system.

Piezoceramic drive systems have always been known for their fast response and atomic resolution, though at limited motion ranges. Progress in piezo mechanisms, as well as control technologies, has solved the travel distance/precision conundrum. Scientists and motion engineers now have access to a number of piezo systems with virtually unlimited travel, without giving up stability, precision and speed.

Non-magnetic applications in semiconductor, scanning electron microscopy (SEM), and medical design are all motion control applications that have spawned the development of these new technologies.

Positioning and alignment systems in e-beam lithography systems and SEM can be equipped with electromagnetic drive mechanisms. However, the expense to shield them and/or position them outside of the action is very high, along with the increase in size. Fieldless piezo ceramic motors are significantly smaller, and can be positioned anywhere inside of these machines without causing negative effects.

In medical design technology, active ceramic components such as piezo-ceramic sensors and actuators are already in use. They are found, for example, in micro-pumps, ultrasonic transducers, fast valves for nano-dispensing applications, and for laser beam control in eye and skin surgery.

For medical imaging applications, such as magnetic resonance imaging (MRI) systems, ultra-high field imaging can have significant advantages for cardiac imaging. However, tuning of several coils in a whole-body scanner to achieve the best performance turns out to be a lengthy process. Replacing the manual tuning with computer controlled non-magnetic piezo motors speeds up the process and provides better results at the same time.

Three-dimensional optical microscopy and optical coherence tomography (OCT) can also benefit from piezo drives due to their high-efficiency, direct-acting linear motion, high-resolution, fast response and non-magnetic characteristics.

Nanometer precision in scanning microscopy

In modern drug-discovery applications, a multitude of samples have to be examined in the shortest possible time. Techniques such as fluorescence imaging are employed and require precise focusing on small amounts of liquid, usually held in multiwell plates (see figure 1). For long range, well-to-well positioning, conventional electric motors or voice-coil drives typically can provide the required speed and precision. But focusing is best achieved with frictionless, piezo flexure stages or objective positioners. Response times on the order of a few milliseconds allow extremely fast focusing and thus rapid data acquisition.

Similar speed/resolution requirements are prevalent in near-field scanning optical microscopy (NSOM). Here, small samples are scanned, typically 100 x 100µm to 500 x 500µm, with nanometer lateral resolution. To minimize the scanning time and achieve the high resolution required, flexure-guided piezo stages are the only option. The latest designs employ a parallel-kinematic motion principle, with all actuators acting on one moving central platform, greatly reducing inertia for much improved dynamics. Capacitive sensors integrated into the stage take multi-axis measurements against a common fixed reference (parallel metrology). This approach allows drift-free positioning with nanometer straightness – not available with classical stacked/nested multi-axis designs.

The same approach yields superior surface metrology results in atomic force microscopes (AFM). An AFM’s output data is only as good as the out-of-plane motion (OOPM) of the XY scanning stage it employs. Active trajectory control approaches (compensating minute off-axis errors with integrated piezo transducers) now provide OOPM in the sub-nanometer realm, over large scanning areas to hundreds of microns.

Nanopositioning goes hybrid

Hybrid positioning systems combine the best of two worlds: long travel ranges with low power requirements and sub-nanometer resolution with very high dynamics. Progress in controller design has made possible real-time closed-loop control of an actuator consisting of a piezo-flexure arrangement in series with a servo-motor/ballscrew assembly.

The controller reads the stage position from an integrated, sub-nanometer-class linear encoder and continuously coordinates both the piezoelectric and servo-motor drives simultaneously in a way to provide the best possible overall performance, with rapid pull-in, nanometer-scale bi-directional repeatability and inherent axial stiffness.

High-force, piezo-walk linear motors

High-energy physics experiments often require components to be insensitive to strong magnetic fields and EMI. An ideal scenario for a motion-control device would be to hold a position exactly when powered down. A new robust piezo motor based on the piezo-walk principle is now available to provide backlash-free, highly stable motion over centimeters of travel with nanometer resolution.

The piezo-walk principle is based on coordinated motion of several longitudinal and lateral piezo actuators arranged about a central ceramic runner. A digital controller sequences their operation. Compact piezo walk motors can be integrated in low-profile linear translation stages (see figure 2) such as used in laser tuning and aligning applications.

Multiple piezo motors can be arranged to form compact hexapod 6-axis positioners (see figure 3). The hexapod approach, with its virtual pivot point and central aperture, is crucial for optical alignment problems as large as secondary mirrors in the latest generation earthbound telescopes, and as small as fibre-to-photonic-component-alignment in telecommunication chips.

Eliminating the travel/resolution tradeoff

Classical piezo flexure positioners excel through their frictionless, guiding systems, rapid response in the kHz range and extremely high reliability. The motion of a piezo flexure actuator is roughly proportional to the applied voltage, often generated by a digital-to-analog converter (DAC) driving an amplifier. In recent history, piezo flexure motion was limited to approximately 100 microns, but advances have pushed the limits to beyond the millimeter range. The number of addressable positions for such a piezo mechanism is 2^b – where b is the bit-width of the DAC’s digital input.

Nanopositioning sensors

High-accuracy position feedback is essential in a good nanopositioning system, and direct motion metrology is the preferred choice. Direct metrology measures motion where it matters most to the application. Examples of high-resolution, direct metrology sensors are capacitive sensors, laser interferometers and non-contact optical, incremental encoders.

For travel ranges of less than 1mm, capacitive sensors have emerged as the default choice. They are compact, high-bandwidth and absolute measuring devices providing sub-nanometer resolution. For less-demanding applications, strain gauge sensors (piezoresistive sensors) are a good alternative. Piezoresistive strain gauge sensors (PRS) are economical but temperature-sensitive devices that are easily integrated in positioning devices.

Capacitive sensors are high-value sensors composed of diamond-machined plates which directly measure the absolute position of the stage platform (see figure 4). Since the stage platform is measured directly, cross-talk and orthogonality can be eliminated. Their inherent stability makes them an ideal fine-positioning companion to ultra-stable piezo motor long-travel coarse-positioning stages.

Assessing the right option

Piezo-ceramic motion systems have long been the number one choice for ultra-high precision motion. With ever-increasing requirements from the optics, biotech and semiconductor industries in recent years, manufacturers were forced to find ways to overcome limitations such as travel range and linearity, while preserving their unmatched speed, acceleration and resolution capabilities.

With the abundance of choices available today, it is more important than ever to understand the user’s application, and its requirements on dynamics and precision, as well as the control and interfacing preferences of the user. Making the right choice in nanopositioning involves assessing multiple criteria.