Robert A. Lake

Electrothermal Actuators for Integrated MEMS Safe and Arming Devices

The use of electrothermal actuators to achi eve the necessary motion of a MEMS based safe and arming device was thoroughly explored. Multiple variants of thermal actuators were designed, modeled, fabricated, and tested in order to gain a better understanding of their specific characteristics. Design variations included both single and double hot arm actuators as well as bent beam thermal actuators. Studies were performed to analyze and compare the displacement and output force of these actuators both as standalone devices as well as multiple actuators joined together. Detailed analysis of the results of the modeling and testing demonstrated the advantages and disadvantages of each style of thermal actuator. Furthermore, the specific variant of electrothermal actuator that is best suited for implementation into MEMS based safe and arming devices can be effectively determined. Finally, detailed analysis of the performance of electrothermal actuators integrated into a functioning MEMS safe and arm device will be presented. Methods in which these actuators are incorporated to best take advantage of their particular characteristics is shown as well as methods that were incorporated in order to overcome some of the shortcomings inherent with these actuators in order to provide the overall safe and arming device with reliable and efficient performance.

Modeling micro-porous surfaces for secondary electron emission control to suppress multipactor

This work seeks to understand how the topography of a surface can be engineered to control secondary electron emission (SEE) for multipactor suppression. Two unique, semi-empirical models for the secondary electron yield (SEY) of a micro-porous surface are derived and compared. The first model is based on a two-dimensional (2D) pore geometry. The second model is based on a three-dimensional (3D) pore geometry. The SEY of both models is shown to depend on two categories of surface parameters: chemistry and topography. An important parameter in these models is the probability of electron emissions to escape the surface pores. This probability is shown by both models to depend exclusively on the aspect ratio of the pore (the ratio of the pore height to the pore diameter). The increased accuracy of the 3D model (compared to the 2D model) results in lower electron escape probabilities with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-p...

Engineered surfaces to control secondary electron emission for multipactor suppression

A significant problem for space-based systems is multipactor — an avalanche of electrons caused by repeated secondary electron emission (SEE). The consequences of multipactor range from altering the operation of radio frequency (RF) devices to permanent device damage. Existing efforts to suppress multipactor rely heavily on limiting power levels below a multipactor threshold [1]. This research applies surface micromachining techniques to create porous surfaces to control the secondary electron yield (SEY) of a material for multipactor suppression. Surface characteristics of interest include pore aspect ratio and density. A discussion is provided on the advantage of using electroplating (vice etching) to create porous surfaces for studying the relationships between SEY and pore aspect ratio & density (i.e. porosity). Preventing multipactor through SEY reduction will allow power level restrictions to be eased, leading to more powerful and capable space-based systems.

Integrating nanosphere lithography in device fabrication

This paper discusses the integration of nanosphere lithography (NSL) with other fabrication techniques, allowing for nano-scaled features to be realized within larger microelectromechanical system (MEMS) based devices. Nanosphere self-patterning methods have been researched for over three decades, but typically not for use as a lithography process. Only recently has progress been made towards integrating many of the best practices from these publications and determining a process that yields large areas of coverage, with repeatability and enabled a process for precise placement of nanospheres relative to other features. Discussed are two of the more common self-patterning methods used in NSL (i.e. spin-coating and dip coating) as well as a more recently conceived variation of dip coating. Recent work has suggested the repeatability of any method depends on a number of variables, so to better understand how these variables affect the process a series of test vessels were developed and fabricated. Commercially available 3-D printing technology was used to incrementally alter the test vessels allowing for each variable to be investigated individually. With these deposition vessels, NSL can now be used in conjunction with other fabrication steps to integrate features otherwise unattainable through current methods, within the overall fabrication process of larger MEMS devices. Patterned regions in 1800 series photoresist with a thickness of ~700nm are used to capture regions of self-assembled nanospheres. These regions are roughly 2-5 microns in width, and are able to control the placement of 500nm polystyrene spheres by controlling where monolayer self-assembly occurs. The resulting combination of photoresist and nanospheres can then be used with traditional deposition or etch methods to utilize these fine scale features in the overall design.

Surface feature engineering through nanosphere lithography

How surface geometries can be selectively manipulated through nanosphere lithography (NSL) is discussed. Self-assembled monolayers and multilayers of nanospheres have been studied for decades and have been applied to lithography for almost as long. When compared to the most modern, state-of-the-art techniques, NSL offers comparable feature resolution with many advantages over competing technologies. Several high-resolution alternatives require scan-based implementation (i.e., focused ion beams and e-beam lithography) while NSL is much more of a batch operation, allowing for full wafer or possibly even multiple wafer processing, potentially saving time and increasing throughput in a manufacturing environment. Additionally, NSL has continued to be of interest because it does not require expensive, complex equipment to be researched and realized, which continues to fuel interest in this approach. In spite of these advantages, applying NSL to specific, realizable devices is limited in the literature. The reason for this lack of application is not only unreliability in the self-assembly process, but also control of these patterned nanospheres within larger, multistep processes often required to fabricate most devices. Both of these items are addressed in this paper. The first issue was addressed through the development of a series of custom-designed nanosphere application vessels. These were designed based on the best published results from the literature, utilizing an alternate method of dip-coating but performed through draining the carrier fluid over the substrate rather than moving the substrate across the liquid–air boundary layer. This method is in the easier to perform, but arguably less-reliable spin-coating method also commonly employed. The key enabler in this effort lies in commercially available three-dimensional (3-D) printing technology, and how it was applied to rapidly prototype-improved deposition vessels. This was accomplished primarily with a single day turn-around between each 3-D printed design iteration. Each vessel design was incrementally improved, built, and tested to optimize the best performance in achieving the most reliable, repeatable self-aligned nanosphere layer formation. With an optimized design of this vessel in hand, the second challenge was addressed by using this vessel and the patterned nanosphere layers it produced with a patterned photoresist design to capture single layers of nanospheres in specifically designed locations and orientations. The hybrid mask produced from this approach can be integrated within virtually any multistep fabrication process. Additionally, other processing steps will be discussed, such as reactive ion etching (RIE), plasma ashing, and photoresist reflowing, and how they might be combined with these hybrid masks. Various results from combinations of these steps are presented. Finally, two potential applications which could benefit greatly from the resulting, engineered surface structures are discussed. These include a small-scale device application (engineering the contacting surfaces in a microswitch), as well as a much larger scale surface study application (surface engineering for controlling secondary electron emissions). The final results from this method allow for patterning groups of 500-nm polystyrene nanospheres formed in four to eight distinct rows each. These are positioned within patterned wells created in a 650-nm thick photoresist. The size and location of these wells are as precise as the photolithography process used to form them, in this case, ∼40-nm position error in the location of the edge of the laser using a Heidelberg laser lithography system. By combining multiple wells in close proximity, virtually any combination of nanosphere clusters become possible. Once patterned, postprocessing though RIE and deposition method selection together determine the final shape of the nanoscale features which result.

Using Cross-Linked SU-8 to Flip-Chip Bond, Assemble, and Package MEMS Devices

This paper investigates using an SU-8 photoresist as an adhesive material for flip-chip bonding, assembling, and packaging microelectromechanical systems devices. An important factor, when using SU-8 as an adhesive material is to control ultraviolet (UV) exposure during fabrication to maximize bond strength due to material cross linking. This approach is much improved over previous efforts where SU-8 bake times and temperatures where changed to alter material cross-linking. In this paper, bake times and temperatures were maintained constant and total UV exposure energy was varied. Once fabricated, bond strength was systematically tested to determine the tensile loads needed to separate bonded structures. The resulting separation force was shown to increase with UV exposure and ranged from 0.25 (5-s exposure) to 1.25 N (15-s exposure). The separation test data were then analyzed to determine the statistical significance of varying UV exposure time and its effect on SU-8 cross-linking and bond strength. The data show that total UV exposure dose is directly correlated with the bond strength of SU-8 bonded structures. By varying only UV dose, the separation force data exhibited a statistically significant dependence on SU-8 cross linking with a 5% probability of error. Further, SU-8 etch resiliency increased by approximately 40%-60% as cross linking was increased with UV exposures ranging from 5 to 15 s.

Post Processed Foundry MEMS Actuators for Large Deflection Optical Scanning

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.

PVDF-TrFE Electroactive Polymer Based Micro-Electro-Mechanical Systems (MEMs) Structures

Electroactive polymer (EAP) has recently been receiving significant attention as smart materials for actuators and sensors for novel micro fabricated devices. Polymer film devices have demonstrated use as pressure sensors and shown potential for harvesting energy from the natural environment. Fabrication of sensing devices using copolymer films has been accomplished using standard lithography process. Materials such as P(VDF-TrFE) (polyvinyledenedifluoride-tetrafluoroethylene) copolymer films (1 m thick or less) were evaluated and presented a large relative permittivity and greater piezoelectric-phase without stretching. Mechanical analysis of experimental structures was also provided and led to key design rules for key post-processing steps to control the performance of the devices. Further investigations will be used to identify suitable micro-electro-mechanical systems (MEMs) structures.

Segmented Control of Electrostatically Actuated Bimorph Beams

This research focused on improving the control and sensing of electrostatically actuated, large deflection bimorph beams for optical beam steering. Current iterations of designs utilize a ‘zipper’ beam and have demonstrated large deflection angles. However, with these devices precise control and deflection measurements can be difficult to achieve. Through using segmented bias channels of doped polysilicon, modeling shows it is possible to control and measure different segments of the actuation arm, thus controlling the amount of tip, tilt, or piston deflection. This paper discusses current and future designs, along with test procedures and modeling results.

Design of FerroElectric MEMS energy harvesting devices

Waste heat is a widely available but little used source of power. Converting a thermal gradient into electricity is conventionally done using the Seebeck effect, but devices that use this effect are naturally inefficient. An alternate approach uses microelectromechanical systems (MEMS) to generate movement and time-varying temperature from a constant temperature gradient. Ferroelectric materials can harvest electricity from moving structures and temperature variations. This concept was realized using traditional silicon microprocessing techniques. A silicon on insulator (SOI) wafer was backside Deep Reactive Ion Etched (DRIE) to form a one mm2 by 7 micron thick silicon/silicon dioxide membrane. Lead zirconate titanate (PZT) was deposited on the membrane and acts as a ferroelectric material. Heating the bulk of the SOI substrate causes an increase in stress and upward deflection of the membrane. The membrane then enters into contact with a cold sink fixed above the substrate. Cooling of the membrane from contact with the cold sink causes actuation downwards of the membrane. The alternating heating and cooling of the PZT layer generates electricity from the pyroelectric effect. The actuation of the membrane generates stress on the PZT layer resulting in electricity from the piezoelectric effect.