Tod Laurvick

Improved terahertz modulation using germanium telluride (GeTe) chalcogenide thin films

We demonstrate improved terahertz (THz) modulation using thermally crystallized germanium telluride (GeTe) thin films. GeTe is a chalcogenide material that exhibits a nonvolatile, amorphous to crystalline phase change at approximately 200 °C, as well as six orders of magnitude decreased electrical resistivity. In this study, amorphous GeTe thin films were sputtered on sapphire substrates and then tested using THz time-domain spectroscopy (THz-TDS). The test samples, heated in-situ while collecting THz-TDS measurements, exhibited a gradual absorbance increase, an abrupt nonvolatile reduction at the transition temperature, followed by another gradual increase in absorbance. The transition temperature was verified by conducting similar thermal tests while monitoring electrical resistivity. THz transmittance modulation data were investigated between 10 and 110 cm−1 (0.3–3.3 THz). A peak modulation of approximately 99% was achieved at 2.3 THz with a 100 nm GeTe film on a sapphire substrate. After isolating the...

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...

Experimental validation of external load effects on micro-contact performance and reliability

This paper presents a follow-on study previously presented at the Holm Conference. In the previous work, it was theorized that micro-switch performance and reliability was directly related to the type of external load that was connected. In particular, unintended capacitive loads may discharge at unpredictable times during switch operation and severely degrade or destroy micro-contact surfaces while properly configured loads may actually enhance performance. The severity of this potential vulnerability can be mitigated by purposely including specific circuit elements in various load configurations. This current study is to experimentally investigate and analyze this phenomenon. Using microelectromechanical systems (MEMS) based devices, we have the ability to efficiently and inexpensively fabricate large numbers of identical micro-contact pairs and then connect them to external loads of interest. Using this approach, it was demonstrated that both performance and reliability can be drastically affected by loading. In all cases tested, series inductance and parallel capacitance resulted in premature failure of the micro-contacts tested. Various protective configurations were also tested and all such devices lasted to the targeted 10M cycles of operation with little sign of imminent failure.

Contact resistance evaluation of micro-contacts with upper hemisphere and lower planar or engineered surfaces

This paper presents a comparison of the resistance performance of Au-Au micro-contacts fabricated with planar and engineered lower contacts. Gray-scale lithography was used to construct 3D structures into photoresist. The structures were then etched into a silicon wafer using a Trion reactive ion etch (RIE) system. The engineered lower contact surfaces consisted of 2D pyramids and 3D pyramid patterns paired with a hemispherical upper contact. A microelectromechanical systems (MEMS) micro-contact support structure, consisting of a fixed-fixed beam, was micro machined as the upper contact. The micro-contact support structure was used as the platform for a hemisphere shaped upper contact. The micro-contacts were actuated using an external, calibrated load. To observe micro-contact performance, the contact resistance and force required to close the contact, were monitored throughout testing. Next the micro-contacts underwent contact resistance testing to evaluate how the engineered lower contacts affected performance. Results show that the 3D pyramid design closely matched the hemisphere/planar contact data with a contact resistance of 0.7Ω after 107 cycles. The 2D pyramid pattern resulted in a higher contact resistance during initial testing and then ended with a contact resistance of 1.083Ω after 107 cycles.

Experimental investigation of thin film spreading resistance in micro-contacts

This paper experimentally explores thin film micro-contact pairs of Au-Au, Au-Ru and Au-RuO2 through the utilization of thin-film contact resistance and spreading resistance theory. Contacts of various combinations of material pairs, including evaporated Au lower planar contact and sputtered Au, Ru or Au-RuO2 upper hemispherical contact were fabricated and their performance monitored through many cycles of actuation. The micro-contacts were actuated using an external, calibrated point load and cycled between 1 and 107 cycles in a controlled atmosphere. To examine the micro-contact performance, the contact resistance and force required to close the contacts were monitored simultaneously throughout testing. Overall performance of these devices followed current models reasonably well, but did show some degree of variation from predicted behavior using standard elastic and plastic material deformation model based predictions of contact resistance. Thin film spreading resistance theories provide a possible explanation for these observed variations and using this theory may also allow for explanations of other observations such as material transfer and micro-contact failure. Better understanding of the physics driving this and the manner in which these devices behave is a necessary step for optimizing micro-switch designs to survive greater cycling and provide more predictable and reliable operations. When compared to data that is free from error induced by other factors (such as contamination film) addition of spreading resistance theory improves the ability to predict contact resistance. In the case of Au-Ru contact pairs values have been measured as low as ~ 0.2Ω. Without spreading resistance, current theory predicts values for this experiment to be around 1.5-3.5Ωs. Addition of spreading resistance reduces the expected range to 0.8-1.5Ω. Similar results were shown with Au-RuO2 pairs while Au-Au were inconclusive.

Improving Gold/Gold Microcontact Performance and Reliability Under Low-Frequency AC Through Circuit Loading

This paper investigates the performance and reliability of microcontacts under low-frequency and low-amplitude ac test conditions. Current microcontact theory is based on dc tests adapted to RF applications. To help better apply dc theory to RF applications, frequencies between 100 Hz to 100 kHz were experimentally investigated. Microcontacts designed to conduct performance and reliability measurements were used, which in prior dc testing typically lasted for 100 million cycles or more. Under ac loads, at similar power levels, eight devices were tested under cold-switching conditions, and only one was still operational at 10 million cycles. The effect of external circuitry on dc loaded devices was also considered. The experimental data were presented for dc conditions, which demonstrated that both a parallel capacitance with a microcontact and a series inductance were highly detrimental. For all six tested devices, failure occurred typically in 100 thousand cycles or less. However, utilizing series resistive/capacitive circuits as well as parallel resistor/inductive resulted in improved performance, with only one device of the four tested failing prematurely, but those that lasted showed less variation in measure contact resistance throughout the lifetime of the device. Two devices were tested with passive contact protection using parallel and series resistances, and both devices lasted for the full test duration. Finally, the effects of applying circuit protection to microcontacts and repeating ac test conditions were investigated. Reliability and device lifetime were extended significantly (9.1% success rate without protection was increased to 87% success rate). It was also observed in several instances that devices that failed showed subtle signs of variance during contact closure measurements in the range of 5–30

Engineered surfaces to control secondary electron emission for multipactor suppression

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Integrating nanosphere lithography in device fabrication

N, indicating a possible means for accurately predicting device failure. For these failed devices, notable physical damage was observed using a scanning electron microscope.

Surface feature engineering through nanosphere lithography

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.

Micro-contact performance and reliability under low frequency, low amplitude, alternating current (AC) test conditions

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.