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Astronomy

Large-stroke actuators have applications in adaptive optics and vision science

Using electrochemical fabrication, comb-drive and vertical parallel plate actuators have been created with calculated deflections of 24 and 4.67μm, respectively.
10 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0173

Adaptive optical (AO) systems are used to enhance optical signal quality by compensating for aberrations caused by fabrication errors, thermal effects, and atmospheric turbulence.1 The latter is a particularly important distortion when using astronomical telescopes. To correct for this in both astronomy and vision science, deformable mirrors that can rapidly change their surface geometry are used. These consist of thousands of actuators that deform the mirror by pulling on the surface, and the component actuators must have a deflection > 10μm.

Other research efforts in such ‘large-stroke’ actuators have used the Sandia ultra-planar, multi-level MEMS technology (SUMMiT) to fabricate segmented AO mirrors and the MEMSCAP polysilicon surface micromachining process (MEMSCAP is a company based in France and the US) to fabricate continuous-face-sheet AO mirrors.2 These methods are basic modifications of the two-dimensional surface micromachining process developed by Howe and Muller3 and consist of 1-2μm thick layers of structural polysilicon and sacrificial oxide. The modifications needed to fabricate large stroke actuators, e.g. thicker sacrificial oxides and additional polysilicon layers, have required a large amount of process development, costing time and money. Nonetheless the goal of 10μm of stroke for continuous-face-sheet mirrors has yet to be achieved by this route.

Instead, we have used an electrochemical fabrication process4—which can have structural heights of up to 1mm—to fabricate comb-drive and vertical plate actuators with calculated deflections of 24 and 4.67μm, respectively.5 Called EFAB™, this is a true three-dimensional fabrication process that can be used for rapid prototyping, since the actuators can be designed with computer-aided-design software such as SolidWorks™. Thus, what you see is what you get.

The EFAB™ process consists of multi-layer electrodeposition and planarization of metal layers.5 The fabrication process starts with a thick alumina substrate (1mm), followed by the deposition of a patterned sacrificial metal, copper, and a blanket deposition of a structural nickel-cobalt layer. These layers are then planarized using chemical-mechanical polishing. The patterned deposition, blanket deposition, and planarization of layers are repeated multiple times with different layer thicknesses to fabricate structures of the required height.

Table 1.

Calculated pull-in voltages for vertical parallel actuators

Table 2.
Calculated pull-in voltages for comb-drive actuators
 
To address the needs of both astronomical and vision science, we designed two types of large-stroke actuator: a vertical ganged parallel-plate actuator and a comb-drive actuator (as shown in Figures 1 and 2, respectively).

Figure 1. A vertical parallel-plate actuator can be used in deformable mirrors for adaptive optical systems.

Figure 2. Comb-drive actuators have the advantage of providing larger deflections.
 

Both types of actuator have a low pull voltage (less than 200V) and provide both a large stroke (∼10μm) and a high bandwidth (∼10kHz). Created using SolidWorks™ software and fabricated via the EFAB™ process, the results are shown in Figures 34.


Figure 3. A vertical parallel plate actuator5 fabricated with EFAB™.

Figure 4. An EFAB™-fabricated comb-drive actuator.5
 

Parallel-plate actuators of 600μm, 500μm, and 400μ;m were designed to have a 4.67μm stroke before reaching a pull-in instability under voltage control at one third of the initial 14μm gap.5Table 1 shows the calculated pull-in voltage values for the three scaled parallel actuators. In addition, four scaled models of comb-drive actuators with lengths of 800μm, 900μm, 1120μm, and 1280μm were designed to have a maximum stroke of 24μm. Table 2 contains their calculated pull-in voltage values using a displacement of 20μm. With a laser vibrometer we measured that the largest comb-drive actuator was deflected by 28μm with an applied voltage of 300V.

Conclusion

Vertical parallel-plate and comb-drive actuators designs can be used in applications such as astronomy and vision science, where large stoke is required. They can be applied to deformable mirrors that can help enhance an optical system's signal quality by compensating for atmospheric turbulence.1 Other research programs have used surface micromachining for fabrication of AO mirrors that consists of thin layers were the layer thicknesses are set by the fabrication design rules. EFAB™ allows for rapid prototyping and the structural and sacrificial layers are left to the designer rather being set by design rule restrictions. With this manufacturing process, vertical parallel-plate and comb-drive actuators designs were fabricated with deflections of 4.67μm and 24μm,5 respectively.

This work has been supported in part by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under Cooperative Agreement No. AST-9876783. This research was also supported in part by a Special Research Grant from the University of California, Santa Cruz. The authors would also like to acknowledge the help of Oscar Azucena, Stacy Barbadillo and Dmitry Kozak of the UCSC MEMS group.


Authors
Bautista Fernandez
Electrical Engineering, University of California, Santa Cruz
Santa Cruz, CA
Joel A. Kubby
Electrical Engineering, University of California, Santa Cruz
Santa Cruz, CA

References:
1. R. K. Tyson,
Adaptive Optics Engineering Handbook,
2000.