LaVern A. Starman

Stress Characterization of MEMS Microbridges by Micro-Raman Spectroscopy

Abstract In this research, micro-Raman spectroscopy is employed to examine, and characterize the residual stress in MUMPs polysilicon, micro-electro-mechanical systems (MEMS) microbridge structures. Currently, few techniques are available to measure the residual stress in MEMS devices. The residual stresses from the deposition processes can have a profound effect on the functionality of the fabricated MEMS structures. Typically, material properties of thin films used in surface micromachining are not controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition methods. Several post-fabrication processes are available to reduce the inherent residual stress from these deposition methods. In an attempt to reduce the residual stress in MEMS microbridges, a phosphorous diffusion and accompanying anneals were performed. Residual stress profiles obtained through micro-Raman spectroscopy are presented, illustrating stress reduction is possible through these post-processing techniques. The stress profiles presented demonstrate the variations between the MUMPs structural layers (Poly1 and Poly2) for different microbridge widths. The improved stress levels could significantly increase device performance, reliability, and yield.

Improved Micro-Contact Resistance Model that Considers Material Deformation, Electron Transport and Thin Film Characteristics

This paper reports on an improved analytic model for predicting micro-contact resistance needed for designing microelectro- mechanical systems (MEMS) switches. The original model had two primary considerations: 1) contact material deformation (i.e. elastic, plastic, or elastic-plastic) and 2) effective contact area radius. The model also assumed that individual aspots were close together and that their interactions were dependent on each other which led to using the single effective aspot contact area model. This single effective area model was used to determine specific electron transport regions (i.e. ballistic, quasi-ballistic, or diffusive) by comparing the effective radius and the mean free path of an electron. Using this model required that micro-switch contact materials be deposited, during device fabrication, with processes ensuring low surface roughness values (i.e. sputtered films). Sputtered thin film electric contacts, however, do not behave like bulk materials and the effects of thin film contacts and spreading resistance must be considered. The improved micro-contact resistance model accounts for the two primary considerations above, as well as, using thin film, sputtered, electric contacts

Increasing stability and distinguishability of the digital fingerprint in FPGAs through input word analysis

Field Programmable Gate Arrays (FPGAs) have become increasingly popular in circuit development due to their rapid development times and low costs. With their increased use, the need to protect their Intellectual Property (IP) becomes more urgent. The digital fingerprint accomplishes this by creating a unique identification (ID) for each FPGA. In this research, we propose methods to dramatically increase the stability and robustness of the digital fingerprint ID by the proper choice of input sequences. We also show that by properly choosing the input word, we can significantly increase the DF resistance to operating temperature changes.

Electrostatically Tunable Meta-Atoms Integrated With In Situ Fabricated MEMS Cantilever Beam Arrays

Two concentric split ring resonators (SRRs) or meta-atoms designed to have a resonant frequency of 14 GHz are integrated with microelectromechanical systems cantilever arrays to enable electrostatic tuning of the resonant frequency. The entire structure was fabricated monolithically to improve scalability and minimize losses from externally wire-bonded components. A cantilever array was fabricated in the gap of both the inner and outer SRRs and consisted of five evenly spaced beams with lengths ranging from 300 to 400 μm. The cantilevers pulled in between 15 and 24 V depending on the beam geometry. Each pulled-in beam increased the SRR gap capacitance resulting in an overall 1-GHz shift of the measured meta-atom resonant frequency.

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.

MEMS integrated metamaterial structure having variable resonance for RF applications

Metamaterial structures for RF applications are becoming essential in the race to reduce the footprint of antenna and components necessary for RF systems. Metamaterials provide a viable option to engineer structures from commonly used materials and processes to reduce the weight and size requirements for systems that normally operate at ¼ wavelength or greater in size for optimal performance. The Split ring resonators (SRR) first developed by Pendry, et al., has proven to be a viable component necessary to create negative index material structures. A fabricated SRR has a specific

Effects of hydrogen pretreatment on physical-vapor-deposited nickel catalyst for multi-walled carbon nanotube growth

Physical vapor deposited nickel catalyst layers of 10, 50, 100, 200, 350, and 500  were granulated using hydrogen plasma for varying times to determine an effective carbon nanotube (CNT) growth process using microwave plasma enhanced CVD (MPECVD). Nickel was deposited via sputtering or evaporation. The catalyst granule size, density, and resulting CNTs were analyzed. Sputtered nickel of 50  with 5 minutes of hydrogen plasma pretreatment resulted in the most effective CNT growth.

Study on the effects of hydrogen pretreatment on nickel catalyst used for multi walled carbon nanotube growth

We investigated the effects of hydrogen pretreatment on nickel catalyst of different thicknesses and deposition methods on a silicon substrate and how it will affect the growth of carbon nanotubes using microwave plasma enhanced chemical vapor deposition (MPECVD). Nickel catalyst of 10, 50, 100, 200, 350 and 500 Å thickness was treated with hydrogen flowing at 135 standard cubic centimeter per minute (sccm), substrate temperature of 400 °C, microwave power of 400 W, and pressure of 20 torr. The treated catalyst granule size and density was determined optically through scanning electron microscope (SEM) images and atomic force microscope (AFM) measurements. We found that sputtered catalyst needs a longer pretreatment than evaporated catalyst. As expected, the pretreatment time must be increased as the catalyst thickness increases to get granule sizes and densities favorable for carbon nanotube (CNT) growth. CNT growth took place with a hydrogen flow of 120 sccm, methane flow of 15 sccm, substrate temperature of 650 °C, microwave power of 1000 W and a pressure of 20 torr. We determined the catalyst can be over treated causing catalyst conglomeration that result in poor CNT growth.

Utilizing micro-electro-mechanical systems (MEMS) micro-shutter designs for adaptive coded aperture imaging (ACAI) technologies

Coded aperture imaging (CAI) has been used in both the astronomical and medical communities for years due to its ability to image light at short wavelengths and thus replacing conventional lenses. Where CAI is limited, adaptive coded aperture imaging (ACAI) can recover what is lost. The use of photonic micro-electro-mechanical-systems (MEMS) for creating adaptive coded apertures has been gaining momentum since 2007. Successful implementation of micro-shutter technologies would potentially enable the use of adaptive coded aperture imaging and non-imaging systems in current and future military surveillance and intelligence programs. In this effort, a prototype of MEMS microshutters has been designed and fabricated onto a 3 mm x 3 mm square of silicon substrate using the PolyMUMPSTM process. This prototype is a line-drivable array using thin flaps of polysilicon to cover and uncover an 8 x 8 array of 20 μm apertures. A characterization of the micro-shutters to include mechanical, electrical and optical properties is provided. This prototype, its actuation scheme, and other designs for individual microshutters have been modeled and studied for feasibility purposes. In addition, microshutters fabricated from an Al-Au alloy on a quartz wafer were optically tested and characterized with a 632 nm HeNe laser.

Flip bonding with SU-8 for hybrid AlxGa1-xAs-polysilicon MEMs-tunable filter

We report the influence of bonding temperature on SU-8 to SU-8 bonding and report fabrication of a hybrid microelectromechanical-tunable filter (MEM-TF) using SU-8 bond pads. We demonstrate use of 2-μm-thick 50×50-μm 2 SU-8 bond pads to attach 4.92-μm-thick 250×250-μm 2 Al 0.4 Ga 0.6 As-GaAs distributed Bragg reflectors (DBR) to polysilicon MUMPs ® piston actuators. Advantages of this process include compatibility with hydrofluoric-acid-release chemistry, low-temperature/low-pressure bonding, simple bond-pad photolithography, 57% flip-bonded DBR yield, and 30% electrostatically actuatable hybrid MEM-TF yield.