Resonant-frequency tuning of angular vertical comb-driven microscanner
© Eun et al.; licensee Springer 2014
Received: 17 March 2014
Accepted: 12 May 2014
Published: 17 July 2014
The resonant-frequency tuning of a self-aligned angular vertical comb-driven electrostatic microscanner is demonstrated by the electromechanical spring effect. The microscanner is fabricated on a silicon-on-insulator wafer using the plastic deformation of silicon. A tuning electrode is fabricated to be electrically separated from the actuation electrode to tune the resonant frequency by adjusting the applied direct-current voltage bias. The experimentally obtained maximum resonant-frequency shift was 3.2% when the resonant frequency of 3167 Hz is reduced to 3066 Hz when a tuning voltage of 30 V was applied while maintaining the actuation voltage. The method enables facile frequency tuning without any permanent geometrical modification to the microscanner.
Electrostatic actuators are widely utilized in microelectromechanical systems owing to their various advantages compared to other actuation schemes. Fast response, low power consumption, and large displacement make them suitable for applications such as microscanners and resonators. Furthermore, electrostatic vertical comb-driven actuators are widely adopted because of their large scanning range and high-frequency operation, especially for microscanners. However, it is very difficult to achieve the exact desired resonant frequency for microscanners or oscillators owing to the inevitable dimensional errors existing in the microfabrication processes. Obtaining the exact designed structural dimensions of the microdevice and maintaining the uniformity of the fabricated products are challenging issues. The deviation in the mass or geometrical errors during the microfabrication processes changes the electromechanical characteristics of the microscanner; even small microscale mass deviations or a geometrical error can drastically change the characteristics of the device. Other than the fabrication errors, mechanical fatigue of the torsional springs or changes of operation environments such as temperature and pressure could also vary the resonant frequency of the microscanner during operations. Therefore, feedback control could be adopted to obtain a stable scanning angle during the operation of a microscanner. For the feedback control, an angular displacement sensing element and a resonant frequency tuning element is required.
Because it is very difficult to achieve the exact desired resonant frequency from the fabricated device, a post-fabrication process is generally required to tune the deviated resonant frequency back to the designed value. In order to accomplish this, previous works have increased the movable mass by laser ablation deposition [], increased the stiffness of the mechanical spring by polysilicon deposition [], and decreased the stiffness of the in-plane vibrating microstructure by using a focused ion beam []. These methods cause permanent modification to the microstructure of the device to adjust the resonant frequency to the desired value.
On the other hand, other studies have been carried out to tune the resonant frequency of the device without permanent structural modifications and therefore offer active and reversible tuning capabilities. Previous efforts to tune the deviated resonant frequency have used the electrostatic spring effect, which is an electrostatic stiffness dependency on the applied voltage [,], and the geometry of the capacitors [–]. A thermal method that increases the temperature of the device to change the mechanical characteristics of the device to tune the resonant frequency was also introduced [,]. These frequency-tuning methods allow the device to perform its functions at various resonant frequencies. Although electrostatic vertical comb actuators are widely adopted for light scanning applications, only a few frequency-tuning methods such as inducing a compressive stress on the flexures [] or using an angle limiter near the torsional spring [] have been reported.
In this paper, a frequency-tuning method for an electrostatic self-aligned angular vertical comb (AVC)-driven microscanner is described. The AVC-based microscanner generates torsional motion of a micromirror with respect to the axis of rotation for optical scanning. A tuning comb electrode that is electrically separated from the driving comb electrode is designed to tune the resonant frequency. The presented frequency-tuning method is based on the electrostatic spring effect induced by the nonlinear relation between the electrostatic moment and the angular displacement of the microscanner. By independently adjusting the direct-current (dc) bias applied to the tuning comb electrode, the tuning capability was maximized under limited voltage source conditions. The microscanner tested for the resonant-frequency-tuning experiments is fabricated via the plastic deformation of single-crystal silicon, as introduced previously [].
In Figure 3(b), the tuned resonant frequency is plotted as a function of the dc tuning voltages over a range of 0 V to 30 V. It is clearly observed that the magnitude of the frequency shift is proportional to the applied tuning voltages. The maximum resonant-frequency shift achieved with a tuning voltage of 30 V was 101 Hz (3.2%) from 3167 Hz with a driving voltage consisting of a 10-V ac bias and a 20-V dc bias (Vdrive = 20 Vdc + 10 Vac). The resonant frequency shifts as a function of the applied tuning dc voltages in the range of 0 V to 30 V. A resonant-frequency shift of 100 Hz from 3158 Hz was achieved with a driving voltage of 20 Vdc + 5 Vac. In addition, resonant-frequency shifts of 76 Hz from 3152 Hz and 75 Hz from 3149 Hz were respectively achieved with a driving voltage of 10 Vdc + 10 Vac (black squares) and with a driving voltage of 10 Vdc + 5 Vac for the un-tuned resonant frequency (green triangle). In all cases, the application of the voltage to the tuning electrode reduced the resonant frequency owing to the spring-softening characteristics of the electrostatic spring effect of the given microscanner geometries.
Frequency tuning of an AVC-driven microscanner was demonstrated by utilizing the electromechanical spring effect. The resonant frequency was tuned by applying a dc tuning voltage up to 30 V to the tuning comb electrode with four different driving voltages. A maximum frequency shift of 3.2% was achieved, as the resonant frequency of 3167 Hz was reduced to 3066 Hz when a tuning voltage of 30 V was applied to the tuning comb electrode. Further, the frequency shift of the microscanner was unidirectional. The application of a tuning voltage reduced the resonant frequency because of the spring-softening characteristics of the electromechanical spring effect for the given AVC-driven geometries. By utilizing the electromechanical spring effect of the AVC drive, frequency tuning of the microscanner was achieved without adding any permanent modification to the device structures.
This research was supported by the Center for Integrated Smart Sensors as Global Frontier Project (CISS-2012M3A6A6054201), the Fusion Research Program for Green Technologies (NRF-2010-0019088) through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning, and the National Research Foundation of Korea Grant (NRF-2012R1A1A2043661) funded by the Korean Government, and supported (in part) by the Yonsei University Research Fund of 2013.
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