As a common type of microelectromechanical systems (MEMS) inertial sensors, comb-structured air-gap acceleration sensors have been applied to various industrial devices and systems. Printed electronics technology has emerged recently as an alternative for fabrication of flexible electronic devices with superior productivity and eco-friendliness to MEMS technology. However, air-gap structures are hard to realize through printing without etching process, and thus comb-structured acceleration sensors have been rarely reported in the printed electronics field in spite of many advantages. This study presents design of a comb-structured air-gap acceleration sensor and materials and processes for highly productive roll-to-roll printed electronic fabrication of the sensor. The sensor is designed to have multiple layers in two parts: fixed fingers are in the lower part while the movable mass and movable fingers in the upper part. Both parts are processed separately on different flexible PET substrates by roll-to-roll gravure printing and drying. Then the upper part is transferred and bonded to the lower one and air-gap structure is formed as a result. This paper also provides electrical characteristics of the proposed comb-structured acceleration sensor by testing capacitance change as a function of acceleration.

Fabrication of Comb-Structured Acceleration Sensors by Roll-to-Roll Gravure Printing

In this work, small signal AC photothermal actuation of a large deformation, out-of-plane thin film structure is demonstrated and the frequency response up to 1 kHz is experimentally measured for laser illumination at a wavelength of 1532 nm. Results from frequency sweeps at different AC modulated laser power levels are coincident with a sub-resonant single-pole thermal response with effective cutoff frequencies ∼50 Hz. The maximum angular motion magnitude was ∼1.5° observed at 38 W/cm2 average incident power and modulation frequencies < 10 Hz. No mechanical resonant modes were observed, which was in agreement with simulation using commercial FEA software and additional laser vibrometry performed. The photoactuation measured is attributed to the inherent photoabsorption inherent to a portion of the multilayer stack of the support arms of the structure combined with slight asymmetries in the geometry and alignment of the incident infrared laser. This works shows a viable means quantifying the precision in motion that can be expected for remote actuation of these types of structures via photo illumination.

Photothermal Frequency Response Characterization of Large Deformation Multi-layer Thin Film Structures

Photothermal actuation of microstructures remains an active area of research for microsystems that demand electrically isolated, remote, on-chip manipulation. In this study, large-deformation structures constructed from thin films traditional to microsystems were explored through both simulation and experiment as a rudimentary means to both steer and shape an incident light beam through photothermal actuation. A series of unit step infrared laser exposures were applied at increasing power levels to both uniformly symmetric and deliberately asymmetric absorptive structures with the intent of characterizing the photothermal tilt response. The results indicate that a small angle (<4° at ~74 W/cm2) mechanical tilt can be instantiated through central placement of an infrared beam, although directional control appears highly sensitive to initial beam placement. Greater responsivity (up to ~9° mechanical tilt at ~54 W/cm2) and gross directional control was demonstrated with an asymmetrical absorptive design, although this response was accompanied by a large amount (~5–10°) of mechanical tilt burn-in and drift. Rigorous device cycling remains to be explored, but the results suggest that these structures, and those similar in construction, can be further matured to achieve controllable photoactuation suitable for optical beam control or other applications.

Photothermal Optical Beam Steering Using Large Deformation Multi-Layer Thin Film Structures.

In this research effort, we developed MEMS micromirrors which are suitable for large angle deflections for optical scanning applications using the PolyMUMPs™ foundry fabrication process. This foundry process uses polysilicon and gold as the structural layers to form the bimorph beam structures which make up our actuation assembly. From both modeling and experimental testing, current micromirror designs fabricated in the PolyMUMPs™ process do not meet the deflection requirements to enable large angle scanning. As a result, we developed several post processing deposition techniques, using the PolyMUMPs™ structural layers as the baseline structure, to enable the necessary upward deflections to permit large angle scanning. In this research, we design, model, post fabricate, and test high out-of-plane MEMS actuators intended for integration with SOI micromirror arrays to enable the broadband, high fill-factor scanning applications. These arrays are designed to scan in multiple directions due to the segmented actuation design. The upward deflection is accomplished through material selection and design control (i.e., structure length, material thickness, material coefficient of thermal expansion (CTE), deposition temperature, and material layer composition) of bimorph structures. Following the post processed fabrication and sacrificial release; the initial deflection profiles are measured and compared against COMSOL™ models.

Post Processed Foundry MEMS Actuators for Large Deflection Optical Scanning

MEMS micromirrors for large angle beamsteering are needed for numerous broadband steering and imaging applications. However, current scanning micromirrors generally exhibit scanning angles (<20°) and typically in only one direction. In addition, the optical fill-factor is generally <50% which leads to significant signal loss and possible heating of the underlying mechanical structures. In this research, we design, model, fabricate, and test high; out-of-plane MEMS actuators utilizing torsional spring assemblies to enable the large angle (∼45°) tip-tilt of the actuation platform assembly. The micromirror exhibit tip, tilt, and piston motion due to the segmented actuation design approach used; thus, the torsional spring structural elements must be integrated into the design to enable the large tilt angles. The torsional structures must enable large angle bending while also being robust enough to support a micromirror structure during switching events. Through precise material selection, and design control (i.e., structure length, material thickness, Young’s Modulus, and deposition temperature), these spring-based torsional structures can be used to enable the twisting and bending of the micromirror platform to enable the large angle scanning. Several different structural designs are assessed to determine the bending and torsional capabilities necessary for reliable and repeatable switching of the micromirrors.

Torsional Structures to Enable Large Angle Deflections

This research effort is focused on the development of a micro-electro-mechanical system (MEMS) energy harvesting device using large, out-of-plane bimorph MEMS structures fabricated in the PolyMUMPs™ foundry process. In general, MEMS designers have limited options for commercial device fabrication, each bringing the advantage of quick prototyping, and repeatable results but also unique limitations. The design rules limit the designers to specific materials, structural layers, and fabrication processes to those of the fabrication facility. In previous work we have shown that the PolyMUMPs™ process will provide an adequate foundational structure for the device, meeting the initial out-of-plane deflection and flexibility requirements. However, it will require some post-processing to add the required mass and piezoelectric material layers to the structures. COMSOL™ finite element modeling software is used to evaluate various mass sizes, orientations, and materials as well as to explore different piezoelectric materials and configurations to produce optimal results from the in-house post-processing of these designs. Initial modeling of the structures illustrates unique design flexibility such that the operational frequency can be tailored to meet specific resonances enabling these devices to be adapted for a multitude of applications. Lastly, since the device provides the added benefit and flexibility, they can easily be integrated into an array for increased output voltage making them a prime candidate for micro power generation circuits.

J. P. Walton

Papers

Post Processed Foundry MEMS Actuators for Large Deflection Optical Scanning

Torsional Structures to Enable Large Angle Deflections

Modeling and Simulation of Post Processed Foundry Fabricated Large, Out-of-Plane MEMS Energy Harvester