Novel actuators achieve greater stability and precision
In the optics world, however, where applications have more to do with optimizing and holding a signal-delay lines, fiber alignment and positioning, and photolithography-attributes such as minimum step size and long-term stability are often more important than repeatable motion.
Innovative piezo-based motion-control systems allow nanoscale handling with no applied voltage or brake.
Picomotor-driven translation stage provides the coarse (30-nm), long-range travel motion. The flexure design coupled with the single-degree-of-freedom contact of the piezo actuator allows the stage to deliver 1-nm positioning accuracy with 1 nm out-of-plane motion and a 4-kHz resonant frequency.
Recent advances in the optoelectronics, mass-storage, semiconductor, and biomedical industries have placed higher demands on motion-control systems, requiring higher resolution and long-term stability. In the past, optical applications have relied on traditional motion-control systems-stepper motors, dc servomotors, and ferroelectric (piezoelectric or electrostrictive) actuators. These devices were designed mainly for the manufacturing environment, where they brought repeatability and automation to processes such as in assembly lines or machining processes.
In the optics world, however, where applications have more to do with optimizing and holding a signal-delay lines, fiber alignment and positioning, and photolithography-attributes such as minimum step size and long-term stability are often more important than repeatable motion. While traditional motion-control devices are good at providing accurate and repeatable motion, they do require an external brake or an applied voltage to hold a specified position accurately and have, therefore, provided an unsatisfactory solution. Moreover, traditional devices have only been able to provide resolution on the order of a few tens of nanometers, which has proven inadequate for next-generation nanoprecision systems.
Nonetheless, through novel innovations, motion-control solutions have been developed to address these issues. Unique systems that can hold their position with no applied voltage or brake yet can provide tens of nanometer step sizes and other systems that provide sub-10-nm resolution are commercially available today.
Classic motion-control systems
In the optics world, motorized micrometers-or "motor mikes"-integrated into translation or rotation stages have dominated classic motion control. These devices typically consist of a stepper or dc servomotor coupled to a precision screw-and-nut set to create a linear pusher. While dc motors provide smooth continuous motion, steppers rotate in discrete steps in response to electrical pulses. In both cases, motor mikes suffer from backlash.
Stepper motors use the principle of magnetic attraction and repulsion to move a screw (see Fig. 1). By alternately applying current to the individual windings in the motor stator, a torque is created that turns a permanent magnet and/or iron rotor. When the windings of the stepper motor are energized, a holding torque is generated; the motor moves only when that current is switched from winding to winding. Because digital pulses must be used to provide the rotation, stepper motors rotate in discrete steps. By interpolating between steps (called half-stepping and microstepping), resolutions down to 10 nm and absolute accuracy to a micron have been achieved.
Unlike dc motors, stepper motors have an inherent holding, or detent, torque that can be used to maintain position in the power-off state for a period of time. The higher-resolution microstepping motors, however, will jump to the nearest full-step location when the power is turned off. A stepper motor can achieve 10-nm resolution over 25-mm travel range with
1-µm absolute accuracy. Moreover, it can be powered down and left overnight, but it can hold its position with an accuracy of 100 nm. Because steppers provide an inexpensive open-loop method to achieve relatively good accuracy, they have been widely used in the optics world.
Stepper motors, however, are bulky, noisy (have inherent vibrations), generate a significant amount of unwanted heat, and do not provide smooth continuous motion. Moreover, they provide no manual-adjustment capability. The large size of stepper motors also makes them difficult to incorporate into mirror mounts and scanning stages.
On the other hand, dc motors provide smooth, continuous motion as well as high speeds and submicron accuracy when used with an encoder. The bulky dc motor consists of an armature-coils of wire around a metal core-inside a magnetic field. When current is applied to the windings, the armature interacts with the magnetic field causing the armature to turn. Because the dc motor requires constant power or an external brake to maintain position, it is not an ideal solution for set-and-hold applications. Moreover, it generates a significant amount of unwanted heat and requires a feedback mechanism for controlling position and velocity. Even when holding a specified position, because of stiction or hysteresis, these motors often dither or oscillate around a position.
Finally, to achieve finer control, several companies build stepper/servo-type actuators with a ferroelectric tip. These actuators combine the best of both worlds: long travel range with fast high-resolution scanning. Ferroelectric actuators are made with piezoelectric or electrostrictive materials. These are materials that expand and contract in response to the square of an applied electrical voltage. While the range of travel is limited and measured in microns, ferroelectric materials provide practical resolutions of tens of nanometers and extremely high-speed motion. Piezos, however, cannot maintain position without electrical power (the expansion of the material is a direct measure of the voltage ap plied), and they exhibit nonlinearity, creep under power, and hysteresis.
Accuracy over distance with a piezo
The Inchworm motor from Burleigh Instruments Inc. (Fishers, NY) uses piezos to achieve 4-nm resolution with travel ranges up to 200 mm. Three piezo elements move a shaft with a force greater than 15 N (see Fig. 2). Two clamps and one extension actuator move in a synchronized "clamp-extend-clamp-retract" motor cycle. The motor is also quite fast-typical motor cycles produce >2 µm of movement which, at a motor frequency of 750 Hz, corresponds to a velocity greater than 1.5 mm/s. Power is required to hold the position, but the motor does not produce heat when holding a position, thus minimizing the effects of thermal drift.
To close the loop, the motor can be used with the newest glass-scale encoder technologies that provide nanometer to subnanometer resolution. Says David Henderson, director of positioning products at Burleigh, "The high stiffness and nanometer resolution of the Inchworm motor make it ideal for use with ad vanced glass-scale-en coder technologies. Performance is not limited by friction, external vibrations, and control-loop stability, enabling us to achieve the encoder`s theoretical resolution. In addition, the solid-state design en ables stages to be used in high-vacuum, ultrahigh-vacuum (UHV), and highly magnetic environments."
Burleigh`s TSE-150H crossed-roller bearing stage, for example, offers 20-nm closed-loop resolution over a 25-mm travel range. This product has the resolution of a linear interferometer in a smaller and less-expensive package, making it particularly attractive for use in vacuum chambers for scanning-electron-microscopy (SEM) applications. For UHV applications, the company recently introduced a closed-loop glass-scale encoder prototype stage with 50-nm resolution making in situ metrology practical for scanning-tunneling microscopy, secondary ion-mass spectrometry, and other critical-surface UHV research and processes. In the future, the company expects to offer Inchworm systems with 1-nm closed-loop resolution using glass-scale encoders.
One example of how the motor has been used is in active alignment of telescope mirror segments. NASA Goddard Space Flight Center (Greenbelt, MD) is currently using 42 of the motors in the Developmental Comparative Active Telescope Testbed (DCATT). Each segment in this seven-segment telescope is supported by a six-degree-of-freedom hexapod stage that resembles a flight-simulator motion system. Each hexapod has six support legs, and each leg uses an IW-701 motor to change length and align the mirror segment. One application of the DCATT will be the Next Generation Space Telescope (NGST), which will operate in deep space at
20 K. To develop a new cryogenic motor for the NGST, NASA has awarded Burleigh a Phase II Small-Business Innovation Research (SBIR) grant.
Ultrafast nanoscale piezo-driven stages
Another method of achieving ultrahigh-precision motion that operates at high frequencies was demonstrated by Piezomax Technologies Inc. (Madison, WI). This company, as part of a Defense Advanced Research Projects Agency SBIR contract for fast, high-resolution profilometry, developed the fastest guided-motion linear actuator yet and is now offering it for other positioning applications requiring 1-nm accuracy and <1-nm out-of-plane motion. Such applications include laser positioning, scanning probe microscopy, cell inspection, electron-beam positioning, and mask and wafer alignment.
The Piezomax linear-motion stages use an innovative flexure design in combination with a low-voltage piezostack actuator (see photo on p. 189). The company offers a wide range of stages with travel ranges up to 100 µm and resonant frequencies up to 7 kHz. To increase the available travel range, the stages can be used in combination with longer-range scanning stages.
"To produce true single-plane parallel motion, we used a finite-element analysis of scanning stages and designed the stage from the ground up," says Max Lagally, president of Piezomax. Parasitic-coupled motions were identified and then eliminated through design optimization. Moreover, because these parasitic-coupled motions introduce extra settling times, removing all the unwanted parasitic motion from the system allows the stages to be run at the highest speeds possible. The special design includes a combination of flexure hinges with an innovative single-degree-of-freedom contact between the piezo actuator and scanning stage that decouples the parasitic motions of typical piezoactuators from the stage.
The stage, flexure hinges, and frame are all constructed out of a single piece of high-performance aluminum-alloy material using electric-discharge machining, enabling the hinges to be made virtually identical. The special aluminum alloy has more than twice the fatigue strength of standard aluminum alloys, allowing these systems to be much stiffer and therefore less prone to coupled motions. Unlike bearing stages, the flexure system has no parts that move against each other, so the motion is smoother and more reproducible. Moreover, these stages can be easily integrated into a closed-loop system because they have an internal capacitance or strain-gauge sensor.
The velocity-observer controller allows these stages to be operated up to their resonant frequency, decreasing the settling time to the minimum possible and therefore making the stage motion as fast as possible. Such a stage has been used to move the optical fiber in a near-field scanning optical probe for near-field confocal microscopy of real-time biological processes.
To address the issue of long-term stability, New Focus Inc. (Santa Clara, CA) used piezos in a new manner in its Picomotor actuator. Although this actuator does use a piezo, it does not rely on the expansion and contraction of the piezo to act as the positioning element. The actuator works much like human fingers-two jaws grasp an 80-pitch screw and a piezoelectric transducer slides the jaws in opposite directions, just as a thumb and forefinger would (see Fig. 3).
Operation relies on the difference between static and dynamic friction. Slow action (high static friction) causes a screw rotation, while fast action (low dynamic friction) resulting from rotational inertia causes no rotation. An electronic driver, equipped with full GPIB/RS232, analog, and TTL input capabilities, generates the high-voltage pulses necessary to activate the piezo. In this manner, the compact (0.75 in.) actuator delivers better than
30-nm resolution over an unlimited travel range. The compact size allows it to be integrated into mounts and stages with both remote-control and manual-adjustment capability.
The piezo is used only to turn the screw and not to hold the adjusted position, so it does not suffer from the typical piezo problems of hysteresis and creep and can maintain its position with no applied voltage, leaving the mechanical stability of the device uncompromised. The drawbacks are that they tend to be slower than the other types of actuators and have lower open-loop accuracy because of frictional uncertainties.
One industrial set-and-hold application in which the Picomotor has been used is in the z/tip/tilt platform of a scanning xy stage for semiconductor-wafer inspection and processing and microscopy. The wafer is scanned for inspection by a fast xy stepper or servo system. The sample is supported by three actuators for aligning orientation and focal height. This adjustment is periodically updated and then held constant over significant time intervals.
Innovative solutions have been able to address the issues of higher resolution and improved long-term stability. As next-generation systems require even smaller and faster throughput, they may be based on microelectromechanical technology for applications such as barcode readers and fiberoptic switches. o