Tracking Nanoposition Advances
Parallel kinematics, active trajectory control, vibration suppression and tracking-error elimination promote precision.
What works in the microworld very often doesn’t work in nanopositioning applications such as
nano-imprinting, scanning microscopy, microlithography and automated alignment.
End users will often need to rethink positioning strategies, including the way they
define basic device parameters such as resolution.
When the term “nanopositioning” was
coined, some companies advertised open-loop, stepper-driven leadscrew devices as
nanopositioners. Although such simple drives have since been replaced by more sophisticated
systems, how some companies define accuracy and resolution still can be elusive.
However, regardless of a device’s stated resolution, repeatable nanometer-range
motion is impossible as long as there is friction, which all sliding or rolling
True nanopositioning devices provide
frictionless motion, virtually instantaneous response, high linearity and stiffness,
and trajectory control on top of subnanometer resolution. They do so with help from
flexure design, multiaxis low-inertia parallel kinematics, and active trajectory
and high-bandwidth control.
The first nanopositioning design rule is to eliminate
friction. This rules out all devices with ball, roller or sliding bearings, leaving
air bearings and flexures. Air bearings are suitable for long travel ranges but
can be bulky, feature high inertia and are costly to operate. They also do not work
in a vacuum, as is increasingly required.
While flexures work only over short
travel ranges, this is hardly a disadvantage in nanopositioning. These frictionless,
stictionless, hingelike devices rely on elastic deformation (flexing) of a solid
material to permit motion (Figure 1). Available in multiaxis designs if needed,
they provide trajectory control with excellent straightness and flatness. The devices
also exhibit no wear, involve no operating costs, and are very stiff and maintenance-free.
Figure 1. Anti-arcuate-motion
flexure designs provide guiding precision in the low-nanometer range.
Drive friction also is unacceptable.
Lead- or ball screws — even friction-based, ultrasonic, linear piezo motor
drives — cannot surpass submicron precision. Electromagnetic linear motors,
voice-coil drives and solid-state piezo actuators are the most common frictionless
drives. The first two options are fine for larger distances but have the disadvantages
of magnetic fields (not tolerable in applications such as electron-beam lithography),
heat generation and moderate stiffness and acceleration, resulting in low bandwidth.
Although piezoelectric drives are limited
to small distances, they are extremely stiff, and they can achieve accelerations
up to 10,000 g, a requisite for millisecond or submillisecond step-and-settle and
high scanning rates. Such devices neither produce magnetic fields nor are they influenced
by them. A recent breakthrough in production technology has eliminated the need
for polymer insulation, thereby increasing drive lifetime even under extreme conditions
Figure 2. Ceramic-insulated
piezoelectric actuators have extended lifetimes, even under extreme conditions,
and exhibit no outgassing in vacuum applications.
End users also need to examine other features
of nanopositioning systems. For example, indirect motion metrology devices, such
as motor-mounted rotary encoders and actuator or flexure-mounted piezoresistive
strain sensors, are cheap but do not qualify for state-of-the-art nanopositioning.
High-performance nanopositioning systems employ noncontact direct metrology, placed
to measure motion where it matters most to the application. Examples include capacitive
sensors, laser interferometers and noncontact optical incremental encoders.
Incremental encoders are excellent
for long-distance measurements. Most are based on a grating pitch of 20, 10 or 2
μm. To get from there to the published 10- or 5-nm resolution usually requires
interpolation. Although many encoders are very linear at multiples of the pitch,
linearity at the nanometer scale can be as poor as 20 percent. In addition, if they
are not mounted coaxially with the drive, any tilt in the guiding system caused
by motion reversal will further increase error.
Often overlooked are the small forces
induced by the moving cable of an encoder read head, which can cause friction and
hysteresis on the order of several tens of nanometers. For processes requiring repeatable
nanometer-scale step widths, there are better solutions.
Laser interferometers are the accepted
standard in position measurement. However, the output of a heterodyne interferometer
is not perfectly linear. This nonlinearity, caused mainly by polarization ellipticity
or nonorthogonality of laser beams, is also influenced by imperfections in optics.
The best commercial interferometers thus offer linearity of 2 to 5 nm — not
good enough in some high-end nanopositioning applications. Users also must have
profound knowledge of interferometry and special equipment to maximize performance
of an interferometer, as either a feedback or calibration device.
The highest performance is delivered
by absolute-measuring, two-plate capacitive sensors. Working best over small ranges,
these devices provide a perfect match for flexure-guided piezoelectric drives. Capacitive
sensors are compact, vacuum-compatible and insensitive to electromagnetic interference.
Good designs also provide extremely high linearity with resolution of 0.1 nm and
below. The absolute measuring principle eliminates the need for a homing procedure,
and there is no bandwidth-limiting interpolator or counter circuit prone to lose
motion in high-speed applications or when there is ringing at the end of a fast
In industrial production and testing processes,
throughput and time matter more than they ever have before. Head/media test applications,
for example, require movement in the form of subnanometer steps to reach a new position
and to hold it to nanometer tolerances in a matter of milliseconds or less.
It does not matter how rapidly the
positioning stage can stop, but how fast the load reaches a stable position —
something often overlooked. Piezoelectric drives can respond to input in less than
0.1 ms, often more than the payload or the supporting structures are designed for.
The ultrafast step time of the nanopositioning stage, however, can excite vibrations
in its load or in neighboring components.
One way to prevent structural resonances
involves real-time feedforward technology called Input? Shaping, which was commercialized
by Convolve Inc. of New York. Now an integrated option for Polytec PI digital piezo
nanopositioning controllers, it eliminates motion-driven excitation of resonances
throughout the system, including all fixturing and ancillary components (Figure
3). Input Shaping also requires no feedback because it works with a priori knowledge
of multiple resonances throughout the system (see “Eliminating Vibration in the Nano-World,” July 2002, p. 60).
Figure 3. Piezo devices are capable of millisecond-scale step-and-settle.
At left, a Polytec laser vibrometer visualizes external resonances. At right, an
Input Shaping control process eliminates motion-driven ringing of components outside
the servo-loop. Settling after rise time completes by t ~ Fres—1.
Ultimately, it is resolution, linearity
and accuracy that qualify the static performance of a motion system. However, in
dynamic applications such as scanning or tracking, static specifications are meaningless.
One way to measure dynamic behavior is bandwidth, which specifies the amplitude
response of a system in the frequency domain. The problem is that static accuracy
and bandwidth together still do not indicate a system’s dynamic accuracy.
To qualify a system in such an application,
end users must record and evaluate target data and actual position data for a given
waveform, with the difference indicating following or tracking error. In conventional
piezoelectric nanopositioning systems with proportional, integral, derivative servo
controls, tracking error can reach double-digit percentages even at scanning rates
below 10 Hz. It also increases with frequency.
For these reasons, tracking error is
a key parameter in dynamic nanopositioning applications. Recent advances in digital
controller design have led to sophisticated adaptive digital linearization methods
that reduce dynamic errors in repetitive waveforms from the micron realm to indiscernible
levels, even with high-frequency dynamic actuation under load (Figure 4).
Figure 4. The top image shows the response to a triangular scan signal with a conventional
PID controller and piezoelectric nanopositioning system. Blue = target position;
red = actual position; green = tracking error (10x for better visibility). In the
bottom image, the same system with adaptive digital linearization has tracking error
(100x for better visibility) reduced by several orders of magnitude.
Serial or parallel kinematics?
In applications such as scanning microscopy, small
areas must be scanned in two dimensions, with a third axis controlled by an external
input; e.g., force in atomic force microscopes or current in atomic tunneling microscopes.
Subnanometer line spacing and scanning rates of hundreds of hertz are desirable,
and these are feasible only with parallel-kinematics, multiaxis closed-loop piezo-driven
Rather than stacking single-axis subassemblies,
parallel-kinematics stages are monolithic, with actuators operating in parallel
on a central moving platform (Figure 5). This not only significantly reduces inertia,
but also yields identical resonant frequencies and dynamic behavior in both the
X and Y directions. Alternative, stacked assemblies always result in different
X vs. Y behavior (though published specifications sometimes fail to reflect this).
Figure 5. A monolithic, parallel-kinematics nanopositioning stage with piezoelectric drives
and flexures uses capacitive position sensors to directly measure the central moving
platform, compensating for the slightest off-axis motion in real time (left). Stacked
serial-kinematics two-axis nanopositioning stages (right) cannot correct off-axis
Consistent X vs. Y dynamic behavior
is desirable for accurate and responsive scanning and tracking. The use of capacitive
sensors to measure the monolithic moving platform means that orthogonal axes automatically
compensate for each other’s runout and crosstalk (active trajectory control
or multiaxis direct metrology), whereas with serial kinematics, runout errors of
the individual axes accumulate. For example, tilt errors of only ±10 μrad
— caused by the bottom platform of a hypothetical 4-in. multiaxis stack of
stages — would cause a 2-μm linear error at the top platform. Other shortcomings
of serial kinematics include high inertia, a high center of gravity, and up to five
moving cables that can cause friction and hysteresis.
State-of-the-art nanometer scanning
systems based on parallel kinematics control all six degrees of freedom, automatically
compensating for unwanted out-of-plane motion and rotational errors.
Best specs or best performance?
The above discussion illustrates the complexity
often involved in quantifying performance of a nanopositioning system. To find the
highest-performing device for an application (not the one with the best specifications
on paper), the user should engage in a dialogue with the manufacturer and ask the
questions relevant to his or her application. Answers sounding too good to be true
usually are just that. It always helps to find out how long a manufacturer has been
involved in nanopositioning, what quality control system is in place, how specifications
have been measured and what equipment was used.
In the aftermath of the telecom crash,
analysts and investors are looking for promising markets, and nanotechnology seems
to be one of them. This is why we will see new companies trying to make a fortune
in this field. Start-ups that claim to have revolutionary nanopositioning solutions
may lure millions of dollars in investor funding. Let’s not forget that, in
telecom, more than 99 percent of the revolutionary concepts and ideas soon proved
worthless. The real challenge lies not in the concept, but in production, yield
and consistent quality, where delivered unit after delivered unit performs as well
as the gently assembled prototype, fine-tuned by the chief engineer.
Because nanopositioning is not as simple
as one, two, three, only companies that have experienced, well-equipped design and
production teams as well as proven quality control systems will be able to satisfy
the ever-growing demands of the market. Their product specifications may not always
seem revolutionary, but they will hold up in the application environment.
1. Scott Jordan (2000). Repealing Moore’s
law; sub-0.25μm linewidths drive metrology, trajectory-control advancements
for positioning subsystems. Semiconductor FABTECH, 12th Edition.
2. R. Gloess (1998). New methods of
signal preshaping strongly increase bandwidth of closed-loop PZT actuators. ACTUATOR
International Conference on New Actuators.
3. Ping Ge and Musa Jouaneh (1996).
Tracking control of a piezoceramic actuator. IEEE Transactions on Control Systems
Technology, 4, 3.
Meet the author
Stefan Vorndran is director of corporate product
marketing communications/nanopositioning technologies for Polytec PI Inc. in Auburn,
Mass. He holds an MS in electrical engineering from FH Dieburg in Germany.
- A technique for producing micron-size structures on surfaces by using short-wavelength light or electron beams.
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