Hybrid Nanopositioning Actuators with Large Travel and Extremely High Resolution

For the last decades ultra-precision positioning technology was mainly driven by the semiconductor industry. In the 1990’s The opto-electronic telecommunications boom was the starting point for radical new technologies such as very compact multi-axis systems capable of nanometer level resolution and automated alignment tools. With the emergence of nanotechnology came the need for new types of precision positioning tools that go beyond the requirements of the conventional semiconductor sector. But also the roadmap of the semiconductor industry requires novel nanopositioning systems capable of even higher resolution over larger travel ranges because the wafer sizes keep going up.

Traditional motor drives can be used for large movements, but the resolution is far from the requirements for nanotechnology. This paper describes new hybrid systems designs such as the combination of piezoelectric and motorized systems with a long travel ranges and extremely high resolution, controlled by one sensor and one servo system.

Serial stacked Hybrid Systems with Separated Sensor Design

With the emergence of nanotechnology came the need for new types of precision positioning tools that go beyond the requirements of the conventional semiconductor sector. The functional structures of nano-devices are on the nanometer or even picometer range. Yet the dimensions of the entire devices are in most cases much larger. Thus, there are two new mechanical requirements: large travel ranges (up to one inch or more) and – at the same time – extremely accurate positioning with nanometer or sub-nanometer resolution.  Furthermore, a higher number of axes are often necessary.

Specially designed piezo positioning systems meet the above requirements. Paired with capacitive sensors, they allow for controlled sub-nm motion.

Figure 1. Example of a serial stacked hybrid system featuring a high-speed coarse positioning over a large range: 25 x 25 mm in x and y (a) and piezo stage for fine positioning: Up to 6 axes (2 axis stage shown here), down to sub-nm resolution (b).

Table 1.Comparison of piezoelectric and motorized motion systems

Serial Stacked Hybrid Systems with one Common Position Sensor

Hybrid systems consist on the combination of both:

• Piezo actuators for extremely high accuracy and
• Motorized drives for long travel ranges.

For the maximum absolute positioning accuracy, the controller should rely on only one position sensor for both the coarse and fine positioning ranges. The following sketch shows a variety of hybrid designs with sensors, that measure the complete travel range.

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Figure 2. (a) Mechatronic Hybrid system featuring high resolution optical incremental sensor providing 2 nm resolution and a motor driven stage combined with flexure guided piezo actuator for 100mm travel range. (b) Hybrid leadscrew (spindle) / nut systems with piezo fine adjustment.

Promising solutions for hybrid actuators are the combination of:

• Leadscrew/nut or ultrasonic piezo drives & PZT linear actuators in the strut
• Leadscrew/ nut & PZT linear actuators in the nut (stack or tube)
• Piezo ultrasonic drives with additional analog mode
• PiezoWalk® drives

The motor driven unit can be described as a combination of a resonant system and the motor part. Both the motor driven part and the piezo actuators have resonant properties. For fast response these resonant terms should be cancelled by notch filters.

Controller Structure

Figure 3 shows the control structure of the hybrid system.

Figure 3. Structure of the hybrid controller.

The control structure consists of two paths: One to control the motor position using the incremental optical sensor; a PID and a notch filter; and a PWM driver to provide power to the motor.

The piezo path consists of the same optical sensor as reference and uses a PI filter and a notch filter.

Some level limits are necessary to stabilize the control function and prevent overflow errors in the D/A or D/PWM parts.

An additional I – term was added in the motor path, which is designed to drop the piezo voltage, when the system approaches the target position. Because both the motor and the piezo actuator operate on the same position target, it would be possible for the piezo voltage to max out when the stage reaches the target position. The voltage can be reduced by this additional I- term with a lower time constant than the piezo part.

The controller outputs a voltage range of +/- 10V to the piezo in static mode, a few milliseconds after a step.

Measurement Results

The controller shares/splits the response between the piezo actuator and the motorized stage. The piezo actuator is driven at a higher bandwidth than the motorized actuator. Otherwise both actuators would try to compensate for their respective motion.

The controller is based on a two hardware-synchronized PC cards. One card reads and buffers the encoder pulses for the PC access, the other one works as the D/PWM output and drives the amplifier. An incremental optical encoder with 4µm pitch and 1000 times interpolation module was used to achieve a resolution of 4nm over a travel range of 100mm. The controller concept shows that the actual position is stable to one encoder count of 4nm.

New encoders with sub-nanometer resolution are now available and can be used with the system.

Conclusion

The research & development of nanotechnology systems requires new positioning tools with extremely high resolution combined with a large travel range. New controller and actuator designs show that a hybrid approach with electromagnetic and piezoelectric actuators together with high resolution incremental encoders and capacitive sensors provide the most flexible technology.

This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.

For more information on this source, please visit PI (Physik Instrumente) LP, Piezo Nano Positioning.