Integration of a Cr–N Thin-Film Displacement Sensor into an XY Micro-stage for Closed-Loop Nano-positioning
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In this paper, an attempt is made to integrate a compact strain gauge-type displacement sensor referred to as the Cr–N thin-film displacement sensor into the XY micro-stage designed in a small size of 25 mm (X) × 25 mm (Y) × 6.5 mm (Z) to achieve closed-loop nano-positioning of the stage. In the XY micro-stage, which has been developed in the previous study by the authors, Cr–N strain gauges for the Cr–N thin-film displacement sensors are directly fabricated on elastic hinges integrated into the stage structure so that the elastic hinges can be employed as sensors for stage positioning as well as guides for a stage table. In this paper, two types of the Cr–N thin-film displacement sensor having different layouts of the strain gauges, line patterns and land patterns are designed and fabricated. Computer simulation based on the finite element method is also carried out to evaluate the robustness of sensors against the external disturbances from electrical wires soldered onto a sensor surface. After some basic experiments, each of the developed Cr–N thin-film displacement sensors is integrated into the XY micro-stage, and its feasibility is evaluated in experiments.
KeywordsXY micro-stage Cr–N thin-film displacement sensor Positioning Sensor
Nano-positioning is one of the key technologies required not only in state-of-the-art scientific instruments but also in measuring instruments and machine tools in production engineering [1, 2, 3]. With the enhancement of precision positioning sensors such as linear/planar encoders [4, 5, 6, 7] or laser interferometers , highly precise positioning can be carried out in many kinds of positioning systems [9, 10].
Among the positioning systems, recently, micro-stages designed in a compact size are of great importance especially in industrial fields such as micro-optics , biology  and precision measurement . In these applications, a micro-stage is required to satisfy multi-axis motion, a compact size of several cubic centimeters, a millimetric wide travel range and a nanometric positioning resolution simultaneously. Many efforts have therefore been made so far to design positioning systems in a compact size. With the employment of piezoelectric (PZT) actuators combined with hinge mechanisms [14, 15, 16] or driving mechanisms [17, 18, 19], some of the positioning systems have achieved a long travel range and a high positioning resolution simultaneously, while packing all the stage structures in a compact size. The author’s group has also developed the XY micro-stage  designed in a compact size of 24 mm (X) × 24 mm (Y) × 5 mm (Z) that has achieved a long travel range of 1 mm in both the X- and Y-directions based on the friction drive mechanism with a unique driving unit composed of a permanent magnet and a pair of PZT actuators.
For such micro-stage systems, compact displacement sensors capable of being integrated into the stage structure are necessary to carry out closed-loop nano-positioning. In most of the cases, capacitive-type displacement sensors are employed in micro-stage systems due to their high resolutions . However, on the other hand, it is difficult to design a capacitive-type displacement sensor in a compact size while achieving a high resolution, which prevents a micro-stage system to be designed in a compact size. Meanwhile, a strain gauge-type displacement sensor, which is designed in a compact size and is easy to be integrated into a limited space, is a candidate for displacement measurement in a micro-stage system. Meanwhile, low resolutions of conventional strain gauge-type displacement sensors with Cu–Ni- or Si-based strain gauges have prevented the employment of conventional strain gauge sensors from being applied to micro-stage systems.
In responding to the background described above, a highly sensitive thin-film displacement sensor based on Cr–N thin-film strain gauges (referred to as the Cr–N sensor in the following) has been developed [22, 23, 24]. Since the Cr–N thin film, which is made of chromium with a small quantity of nitrogen deposited by reactive sputtering in a RF magnetron sputtering device , has a much higher gauge factor compared with the conventional Cu–Ni-based strain gauges as well as much lower temperature coefficient of resistance (TCR) compared with the conventional Si-based strain gauges, a highly sensitive and stable displacement measurement can be expected to be realized by the Cr–N sensor. The results of basic experiments have revealed that the developed Cr–N sensor has a possibility of achieving a nanometric resolution for displacement measurement . Meanwhile, performance verification of the developed Cr–N thin-film displacement sensor in the XY micro-stage has remained as a challenge to be addressed. In this paper, for the verification of the achievable positioning resolution by the XY micro-stage with the Cr–N sensors, an integration of the developed Cr–N sensor into the XY micro-stage is carried out as the next step of research. Two types of the Cr–N sensor are prepared, and their basic performances in the XY micro-stage are investigated in experiments.
2 Principle of the Cr–N Sensor for Measurement of the Displacements in the XY Micro-stage
Design parameters for the Cr–N sensors
Gain of the gauge amp
Size of strain gauges
1.56 mm × 0.72 mm
1.56 mm × 0.62 mm
1.50 mm × 0.22 mm
Line pattern width
3 Verification of the Basic Performances of the Developed Cr–N Sensors Before the Sensor Integration into the Stage
3.1 Basic Experiments
3.2 Computer Simulation Based on Finite Element Method
4 Integrations of the Cr–N Sensors into the XY Micro-stage
It should be noted that, as the second step of research following the development of mechanical structures for the XY micro-stage, this paper has focused on the integration of Cr–N sensor into the XY micro-stage. Further detailed investigation for a closed-loop control of the stage system as well as the evaluation of dynamic characteristics of the developed sensor is the next step of research and will be carried out in future work.
Aiming to achieve the closed-loop nano-positioning control of the XY micro-stage having a travel range of 1 mm , an attempt has been made to integrate a Cr–N thin-film displacement sensor (Cr–N sensor) into the stage. Two types of electrical layout design have been prepared for the Cr–N sensor, and the performances of fabricated Cr–N sensors have been compared in experiments. In addition, computer simulation based on the finite element method (FEM) has also been carried out to evaluate the robustness of sensors against the external disturbances from electric wires soldered onto a sensor surface. The results of basic experiments before the sensor integration and the simulation have revealed that the signal quality of the sensor output has degraded due to the external disturbances in the case where land patterns being placed among the strain gauges on an elastic hinge surface in the Cr–N sensor. Each of the designed Cr–N sensors has been integrated into the XY micro-stage, and some experiments have been carried out to evaluate the performances of sensor in the stage system. Experimental results have demonstrated that the sensor having land patterns at the edges of elastic hinges can achieve a resolution of 5 nm for displacement measurement of the XY micro-stage.
It should be noted that an effort has been made in this paper focusing on the integration of Cr–N sensor into the XY micro-stage as the second step of research following the development of mechanical structures of the XY micro-stage. Further detailed investigation for a closed-loop control of the stage system as well as the evaluation of dynamic characteristics of the developed sensor will be carried out in future work.
This work was supported by Japan Society for the Promotion of Science (JSPS) and the NSK Foundation’s Research Grant for Advancement of Mechatronics (2016–2017).
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