While electroceramics such as piezo materials with flexure guides remain the gold standard for breaking the resolution nanometer barrier, there are several other commercial solutions available today providing repeatable single-digit nanometer step resolution including linear motors, voice-coil drives, and frictionless guides such as air bearings and magnetic bearings.

Flexure guides are also employed in the vast majority of piezo-driven nanopositioning systems. Piezoelectric technologies play a foundational role in positioning applications with nanometer resolution and below requirements. The direct piezo effect was discovered more than 100 years ago and today’s nanopositioning devices capitalize on the inverse effect, using a voltage to provide motion via material expansion. Most such devices use a polarized ferroelectric ceramic material made from lead, zirconium, and titanium. Consequently, they are often called piezoelectric transducers, or PZTs — an acronym which reflects their chemical constituents. (see sidebar below.) The benefits of piezo-based devices include: • Unlimited resolution: Positioning increments well below 1 nm are possible. This is critically important for applications ranging from semiconductor lithography to superresolution microscopy and optical tweezers. However, precision performance requires carefully designed and manufactured mechanisms; otherwise incremental motion can be limited by stick-slip effects, mechanical losses, parasitic motion errors, and suboptimal metrology implementation. • Fast expansion and response (microsecond time constants). • Maintenance-free, solid-state construction that reduces wear and eliminates scheduled maintenance, lubrication, or adjustment even with heavy usage. • High efficiency: Electrically, piezoelectric elements resemble capacitors, in that energy is absorbed only to perform movement, not to maintain position. • Inherent vacuum-compatibility (especially for the newest ceramic-encapsulated piezo stacks). • Non-magnetic and magnetic-insensitive construction. Being ceramic devices, piezoelectric elements are impervious to magnetic fields and produce none of their own. When paired with appropriate structural materials and bearings, virtually fieldless and field-insensitive mechanisms can be constructed. • High throughput and dynamic accuracy, especially when newer controls technologies are leveraged. • Finally, piezoelectric technology has been applied in novel mechanisms that provide many millimeters of travel while maintaining better than nanometer resolution. Industrial reliability Most piezo ceramic actuators are constructed of many layers sandwiched between electrodes, similar to ceramic capacitors and traditionally made using polymer coatings on the outside as insulation. But polymer coatings also trap moisture, leading to limited lifetime. Then, with the advent of high-duty-cycle, high-dynamic, uptime-critical industrial applications, traditional encapsulation began proving inadequate to protect sensitive mechanisms. High-dynamic industrial applications often resulted in heating which could be problematic for polymer coatings, and nanopositioning was increasingly important in vacuum applications such as e-beam lithography and microscopy.

Throughput, speed, and precision: electroceramics If very rapid movements are needed, piezoelectric devices are often the only solution. Piezoceramic positioning devices can have bandwidths of tens of kilohertz or more, and they lack the responsiveness-limiting inertia of leadscrews and other conventional mechanisms. Unfortunately, advancing resolution needs in industry and research are physically at cross-purposes to advancing speed needs. In this way, applications’ increasing need-for-speed coupled with unabated pressure on resolution capabilities exposed fundamental physical limitations of traditional nanopositioning technologies, especially for controls: • Electronic bandwidths, limited by amplifier, sensor, and servo-processing capabilities, cause “rounding of corners” in motion-waveform generation, phase-lags, and nonlinearities in high-dynamic applications. • Mechanical stiffness, characterized by the lowest resonant frequency (Fres) of the mechanism, limits the accurate controllability of a mechanism in high-dynamic applications. Traditional approaches to improving matters include using higher power amplifiers, faster servo update rates, stiffer materials, and so on. However, these merely push the margins of system performance back incrementally, and sometimes with a very poor cost/benefit ratio. In addition, applications requiring varying loads often force compromises, such as servo settings which are safe for all intended loads but optimum for none.

Most piezo actuators are designed to provide motion ranges of 5 to 300 μm. This range is determined by the composition, length of the material, maximum applied voltage and other factors. Typical PZTs feature a 0.13 percent strain, meaning a 100-mm-long stack can expand to 130 μm when the maximum allowable voltage is applied. The basic piezo actuator stack is the heart of the nanopositioning stage. High-precision, stiction-free flexure lever amplification and guidance mechanisms allow stage travels to 1 mm. Stiction, looseness, rigidities and guidance quality all factor heavily in the device’s overall performance and require meticulous and highly specialized design and manufacturing skills. For reproducible nanometer-scale accuracy and reliability in industrial-class applications, extraordinary mechanical quality must be maintained throughout the mechanism. However, stage design and construction quality can vary greatly, particularly in multiaxis designs where traditional stacking and nesting techniques can compromise performance and throughput. Caution dictates a close collaboration between engineer and vendor to evaluate all aspects of performance, such as out-of-plane motion, step/settle response, and EMI immunity, rather than just relying on nominal specifications. Linear piezo actuators are typically built from thin, stacked PZT layers.