In this chapter, a novel cascade electrostatic comb-drive actuator is proposed. The simulation result, the fabrication process, and the experiment results are given to verify the proposed device. With this novel cascade configuration, the stable traveling range is extended successfully. Measurement results indicated that the proposed cascade actuator extends the stroke near 200% as compared with the actuator with single stage. This cascade electrostatic comb-drive actuator will benefit the microsystems that requires large traveling stroke such as micro x-y stages and variable optical attenuators.
CHAPTER IV
CASCADE VERTICAL ELECTROSTATIC COMB- DRIVE ACTUATOR
4.1INTRODUCTION
In this chapter, a vertical electrostatic comb-drive actuator with a novel cascade configuration called cascade vertical comb-drive actuator (CVCA) is developed to magnify the actuation stroke. The CVCA combines vertical comb-drive actuators and multi-stage structures to form a stacked actuation configuration that can be used to overcome the stroke constraint of the traditional vertical comb-drive actuator. Due to the multilayer characteristic of the microstructure, the proposed CVCA can achieve the vertical motion and cascade structures. The experimental results indicated that the stroke of the proposed CVCA can be magnified three times as compared with that of the standard vertical comb drive with the same chip size.
4.2 SIDE INSTABILITY OF VERTICAL ELECTROSTATIC COMB-DRIVE
ACTUATOR
The vertical motion of a verticval electrostatic comb-drive actuator occurs when a driving voltage is applied between the fixed comb fingers and the movable comb fingers. The electrostatic force of a comb finger pair is
V2
g F = εl
, (4.1)
where ε is the dielectric constant in air, l is the finger length, g is the finger gap spacing, and V is the applied voltage. Fig. 4.1(a) shows the schematic drawing of a vertical electrostatic comb-drive actuator without applying driving voltage. As the driving voltage between the fixed comb fingers and the movable fingers increases, the comb fingers become partially engaged, as shown in Fig.
4.1(b). As the driving voltage further increases, the comb fingers become fully engaged, as shown in Fig. 4.1(c). Therefore, the comb finger thickness d is an important factor to improve the stroke of the vertical electrostatic comb-drive actuator and that actuation stroke increases while the comb finger thickness d increases.
Fig. 4.1 Schematic drawing of comb fingers (a) not engaged, (b) partially engaged, and fully engaged.
To increase the stroke of typical vertical electrostatic comb-drive actuators, high-aspect-ratio micro-machining processes using electroplating, deep reactive iron etching (DRIE) or inductively coupled plasma (ICP) have been demonstrated [83–86]. Although these approaches can increase the actuation stroke, they often
need a complex fabrication process. In addition, vertical electrostatic comb-drive actuators with thick comb fingers easily result in side pull-in (side instability) [87].
Side pull-in means the situation in which the movable comb fingers move perpendicular to the stroke direction and then make contact with the fixed comb fingers. When comb fingers are increasingly engaged, the electrostatic force between the fixed comb fingers and the movable comb fingers becomes larger. If the movable comb fingers move with a small displacement along the x-direction, the movable comb fingers will suddenly snap to the fixed comb fingers. This so-called side pull-in phenomenon constrains the actuation stroke of the verticval electrostatic comb-drive actuator. Fig. 4.2 shows a cross-sectional view of the side pull-in condition. Therefore, increasing the comb finger thickness d does not increase the actuation stroke with sufficient efficiency due to the side pull-in phenomenon.
Fig. 4.2 Cross-sectional view of side pull-in condition.
4.3DESIGN CONCEPT AND FEMSIMULATION
4.3.1DESIGN CONCEPT AND FEMSIMULATION
To magnify the stroke of a vertical electrostatic comb-drive actuator, a vertical comb-drive actuator with a cascade configuration is developed. Fig. 4.3 shows a schematic diagram of a CVCA. This actuator comprises four stages, where the first stage is fixed to the anchor and second to fourth stages are connected to the neighbor stage. Each stage contains a fixed comb finger structure, a movable comb finger structure, and suspension springs. The neighboring two stages are linked by the connection frame. This indicates that the movable comb finger structure of one stage is connected with the fixed comb finger structure of the next stage. Consequently, all of the four stages are constructed with a multi-stage cascade configuration. Notably, each comb-drive actuator could be driven independently. By applying driving voltage to each stage simultaneously, the stroke of the cascade actuator is the summation strokes of the first, second, third, and fourth stages. Thus, the proposed CVCA is capable of performing a stable extended traveling range. Figure 4.4 shows the finite element method (FEM) simulation results of the simplified CVCA model.
Fig. 4.3 Schematic drawing of CVCA with four stages.
Fig. 4.4 FEM simulation results of simplified CVCA model (a) without driving voltage and (b) with driving voltage.
4.3.2ACTUATION PRINCIPLE
As shown in Fig. 2.4, the microstructure can have several metal layers embedded, which is a major difference from homogeneous silicon counterparts.
These characteristic multilayer structures enable the CVCA to have the vertical motion capability. Fig. 4.5 shows cross-sectional views of the fixed comb fingers and movable comb fingers. The fixed comb fingers comprise M3, M4, via, and silicon dioxide, and the movable comb fingers comprise Poly2, M1, M2, M3, via, and silicon dioxide. By applying driving voltage between the movable comb fingers and the fixed comb fingers, the actuator can achieve vertical motion due to the electrostatic force.
Fig. 4.5 Cross-sectional view of fixed comb fingers and movable fingers.
4.4EXPERIMENT RESULTS
Fig. 4.6 shows a scanning electron microscopy (SEM) image of the fabricated CVCA. The proposed CVCA consists of four stages. The width and length of the comb fingers are 3.6 and 40 μm, respectively. The finger gap spacing and finger overlap are 3 and 35 μm, respectively.
Fig. 4.6 SEM image of fabricated device.
By applying driving voltage between the movable comb fingers and the fixed comb fingers, we are able to measure the performance of the fabricated device using an optical interferometric profiler (WYKO). Fig. 4.7 shows the static characteristic of the fabricated device. The result indicates that the maximum total displacement that the CVCA (first stage + second stage + third stage + fourth
stage) can achieve is 2.6 μm by applying a 90 V driving voltage. Note that the displacement of the first stage is equivalent to the traditional vertical electrostatic comb-drive actuator, which can achieve 0.9 μm displacement with a 90 V driving voltage. In conclusion, the actuation stroke of the proposed CVCA can be magnify three times higher than that of the standard vertical comb drive actuator.
Fig. 4.7 Static characteristic of fabricated device.
4.5REMARK CONCLUSION
A novel cascade configuration for magnifying the stroke of a vertical electrostatic comb-drive actuator is proposed in this paper. This novel design can be used to overcome the stroke constraint of a traditional vertical electrostatic comb-drive actuator. The proposed CVCA is implemented using TSMC 0.35 μm 2p4m CMOS fabrication processes and post-CMOS micromachining steps. The design, simulation and measurement are also presented. The experimental results
indicate that the stroke of electrostatic actuator can be effectively magnified using the cascade configuration. The CVCA will benefit microsystems such as an optical pickup and a variable optical attenuator.
CHAPTER V CONCLUSIONS
5.1SUMMARY OF THIS DISSERTATION
In this dissertation, the electrostatic comb-drive actuator with multi-stage configuration is proposed. With this cascade structure, the stroke of the actuator can be greatly improved without adding extra driving voltage. This cascade structure can be utilized in lateral electrostatic comb-drive actuators; furthermore, it can also be employed in vertical electrostatic comb-drive actuators.
Commercialized FEM tools have been employed to simulate actuator characteristics. To realize the designed actuators, a CMOS process and post-CMOS micromachining steps are employed. Experimental results had demonstrated our design concept. The major contributions of this dissertation are summarized in the following section.
5.2CONTRIBUTIONS
The major contributions of this dissertation are summarized in the following:
(1) Enlarge the stroke of actuator
The proposed cascade electrostatic comb-drive actuator can enlarge the stroke of the actuator. Comparing with traditional actuator (without cascade configuration), the proposed lateral actuator can improve the stroke up to 180%, and the proposed vertical actuator can improve the stroke up to 188%. With more cascade stages, the stroke can be improved more.
(2) Extending actuation stroke without adding extra driving voltage
Although the actuation stroke can be extended by utilizing the pre-bent suspensions or adding the second electrode, the actuators still need higher driving voltage to extend the actuation stroke. However, with this novel cascade configuration, the actuation stroke can be extended greatly without extra driving voltage.
(3) Capability of integrating with on-chip circuitry
In this dissertation, the proposed two actuators were fabricated through standard CMOS processes and post-micromachining steps. Hence, the actuators not only can be fabricated through commercial processes, but also can on-chip integrate with circuitry.
(4) Characteristics of multilayer structures
Unlike the general MEMS devices made of homogeneous polysilicon, the proposed actuators can have several embedded metal layer. This multi-layer structure characteristic can benefit MEMS devices have more advantages such as the electric signal isolation and the vertical motion capability. Therefore, the MEMS devices can be applied in more fields.
5.3SUGGESTIONS FOR FUTURE RESEARCH
In this dissertation, the cascade electrostatic comb-drive actuators are designed, developed, and implement. With the cascade configuration, the actuation stroke can be improved greatly without extra driving voltage. For the future research, a few suggestions are provided as following:
(1) Improve the fabrication process to reduce the residual stress.
(2) Develop the X-Y-Z stage by combine the proposed lateral actuator and vertical actuator.
(3) Integrate the proposed actuators with other CMOS components such as CCD to form a complete MEMS system for practical applications.
(4) Develop other cascade actuators such as cascade thermal actuators and cascade magnetic actuators.
APPENDIX A: CMOS PROCESS FLOW
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