This PDF file contains the front matter associated with SPIE Proceedings Volume 8973, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Historically, spectroscopy has been a cumbersome endeavor due to the relatively large sizes (3ft – 100ft in length) of modern spectroscopy systems. Taking advantage of the photoacoustic effect would allow for much smaller absorption chambers since the photoacoustic (PA) effect is independent of the absorption path length. In order to detect the photoacoustic waves being generated, a photoacoustic microphone would be required. This paper reports on the fabrication efforts taken in order to create microelectromechanical systems (MEMS) cantilevers for the purpose of sensing photoacoustic waves generated via terahertz (THz) radiation passing through a gaseous sample. The cantilevers are first modeled through the use of the finite element modeling software, CoventorWare®. The cantilevers fabricated with bulk micromachining processes and are 7x2x0.010mm on a silicon-on-insulator (SOI) wafer which acts as the physical structure of the cantilever. The devices are released by etching through the wafer’s backside and etching through the buried oxide with hydrofluoric acid. The cantilevers are placed in a test chamber and their vibration and deflection are measured via a Michelson type interferometer that reflects a laser off a gold tip evaporated onto the tip of the cantilever. The test chamber is machined from stainless steel and housed in a THz testing environment at Wright State University. Fabricated devices have decreased residual stress and larger radii of curvatures by approximately 10X.

Negative stiffness can provide a method of altering the stiffness of a device without changing its geometry. The silicon/ silicon dioxide (Si/SiO₂) membrane presented in this research utilizes buckling resulting from compressive residual stress. A transversely actuated buckled membrane displays properties similar to a linear regressive spring, which include a positive and negative stiffness region. Cantilever beams were used to restrict the outward displacement of the membrane and force it to actuate only in its negative stiffness region. Analytical equations were utilized to estimate the amount of outward deflection by the membrane and to estimate the amount of reduced deflection required for the device to display only negative stiffness characteristics. Devices were tested using a force sensor actuated by a piezo controller. Interferometric imaging confirmed the cantilevers ability to reduce the buckling displacement in the membrane up to 30%.

An electromagnetic microactuator, for controlling of a tuneable lens, with an integrated electrostatic element is fabricated by thin film technology. The actuator consists of two parts: the first part with microcoil and flux guide and the second part with a ring shaped back iron on a polyimide membrane. The back iron is additionally useable as electrode for electrostatic measurement of the air gap and for electrostatic actuation. By attracting the back iron an optical liquid is displaced and forms a liquid lens inside the back iron ring covered by the membrane. For testing the thin film fabrication sequence, up-scaled systems are generated in a test run. To fabricate the flux guide in an easy and quick way, a Ni-Fe foil with a thickness of 50 μm is laminated on the Si-wafer. This foil is also utilized in the following fabrication sequence as seed layer for electroplating. Compared to Ni-Fe structures deposited by electroplating, the foil is featuring better soft magnetic properties. The foil is structured by wet chemical etching and the backside of the wafer is structured by deep reactive ion etching (DRIE). For post fabrication thinning, the polyimide membrane is treated by oxygen plasma etching. To align the back iron to the microcoil and the flux guide, a flip-chip-bonder is used during test run of system integration. To adjust a constant air gap, a water solvable polymer is tested. A two component epoxy and a polyimide based glue are compared for their bonding properties of the actuator parts.

Microfabricating high resolution micro-optics structures requires shape control to <1/8th wavelength (~60nm) in both vertical and horizontal surface precision. Grayscale bimetallic photomasks are bi-layer thermal resists consisting of two thin layers of Bi-on-Indium or Tin-on-Indium. A focused laser spot creates a thermal metal oxide with a controllably transparency set by the beam power of optical density from ~3OD (unexposed) to <0.22OD (fully exposed). A directwrite raster-scan photomask laser system with a CW Argon-ion laser at 514nm for the bimetallic writing and 457nm line for measuring the OD change used a feedback-controlled Gaussian beam to achieve 256-level grayscale masks. Setting the graylevels required to achieve uniform vertical steps in the photoresist requires adjustment in transparency based on the exact response curves of a given resist/development process. An initial model is developed using the classic resist threshold dose exposure D₀ and dose to clear Dc creating a power law relation between the required exposure dose for each thickness step and the mask transparency. However real resists behave differently than the simple model near the threshold requiring careful calibrating of mask graylevel transparencies with the photoresist response curve for a given resist/development process. Test structures ranging from steps to ramps and complex patterns were examined via both SEM and profilometry from the resulting bimetallic grayscale masks. Secondary corrections modify the needed bimetallic OD due to the exposure source spectrum differences from the 457nm measurement. This enhances the patterning of micro-optic and 3D MEMS structures.

Deep Reactive Ion Etching (DRIE) has revolutionized a wide variety of MEMS applications since its inception nearly two decades ago. The DRIE technology has been largely responsible for allowing lab scale technology demonstrations to become manufacturable and profitable consumer products. As applications which utilize DRIE technologies continue to expand and evolve, they continue to spawn a range of new requirements and open up exciting opportunities for advancement of DRIE. This paper will examine a number of current and emerging DRIE applications including nanotechnology, and DRIE related packaging technologies such as Through Silicon Via (TSV) and plasma dicing. The paper will discuss a number of technical challenges and solutions associated with these applications including: feature profile control at high aspect ratios, causes and elimination of feature tilt/skew, process options for fragile device structures, and problems associated with through substrate etching. The paper will close with a short discussion around the challenges of implementing DRIE in production environments as well as looking at potentially disruptive enhancements / substitutions for DRIE.

Chemical mechanical polishing (CMP) is a technique which helps to print a smaller depth of focus and smoother surface in micro fabrication industry. In this project, boron doped polysilicon is used as a fill material for Through Silicon Vias (TSV) creating a 3D package. It is shown that the presence of boron as dopant suppresses the polysilicon polish rate. To increase the polish rate, understanding the mechanism of polish rate retardation is essential. We believe that the electrical effects play the major role in this phenomenon and by reducing this effect we are able to increase the polish rate.

A method for fabricating structures half the size of the listed minimum feature size of a direct-write laser lithography system was demonstrated by taking advantage of the offset spacing allowed between write paths of the machine. This unique process allows microelectromechanical systems (MEMS) structures to be fabricated with minimum features smaller than equipment specifications. This method provides an increase in the capability of a lab without having to go through the cost and effort of re-tooling in order to provide the same capabilities. This ability will allow for the design and fabrication of structures, such as sensors, with an increased degree of sensitivity over those previously designed and fabricated with the same equipment. This new process resulted in 500 nanometer wide beams which half the size of the minimum feature size specified for our equipment.

A table top nanofabrication system which combines the classic Talbot imaging effect and a compact table top soft-x ray laser is described in this paper. Periodic nanostructures on millimeter square are fabricated using this robust, simple and defect tolerant fabrication method. Talbot lithography allows for a complete coherent extreme ultraviolet lithography process in a compact table top system. Double exposure allowed for the reduction of the feature sizes.

The ability to design and develop 3D microstructures is important for microelectromechanical systems (MEMS) fabrication. Previous techniques used to create 3D devices included tedious steps in direct writing and aligning patterns onto a substrate followed by multiple photolithography steps using expensive, customized equipment. Additionally, these techniques restricted batch processing and placed limits on achievable shapes. Gray-scale lithography enables the fabrication of a variety of shapes using a single photolithography step followed by reactive ion etching (RIE). Micromachining 3D silicon structures for MEMS can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study, we investigated: using MATLAB for mask designs; feasibility of using 1 μm Heidelberg mask maker to direct write patterns onto photoresist; using RIE processing to etch patterns into a silicon substrate; and the ability to tailor etch selectivity for precise fabrication. To determine etch rates and to obtain desired etch selectivity, parameters such as gas mixture, gas flow, and electrode power were studied. This process successfully demonstrates the ability to use gray-scale lithography and RIE for use in the study of micro-contacts. These results were used to produce a known engineered non-planer surface for testing micro-contacts. Surface structures are between 5 μm and 20 μm wide with varying depths and slopes based on mask design and etch rate selectivity. The engineered surfaces will provide more insight into contact geometries and failure modes of fixed-fixed micro-contacts.

This paper presents the growth and release process effects on the deformation of suspended gold micro-structures. Cantilever type test structures, typically used for RF MEMS devices have been examined. The structures have a thickness of 2μm, produced by patterned gold electro deposition above a sacrificial photoresist layer, then removed by dry release in oxygen plasma ashing and wet release using critical point dryer (CPD). The growth process of gold electroplating is optimized for low residual stress using pulse power supply. Minimum stress 35-60 MPa is obtained at grain size 30-45nm and RMS roughness of the order of 5-8nm. The growth mechanism of structural layer and releasing methods are optimized to obtain planar MEMS structures. The main parameters considered are the initial stress during the growth of electroplated gold and the release process recipes developed for fabrication of metallic structural layer.

Germanium Telluride (GeTe) can be described as a non-volatile (latching state) phase change material (PCM) in memory applications. GeTe also exhibits a volatile (reversible state) region when heated and cooled between 100-180 °C. At temperatures higher than 185 °C the material crystallizes and “latches” until a temperature near to its melting point (725 °C) is reached and cooled rapidly (quenching). Germanium Antimony Telluride (GeSbTe) or also known as GST has similar characteristics as GeTe. GST also exhibits a volatile (reversible state) region when heated and cooled between 100-150 °C. GST crystallizes at 155 °C and its melting point is 600 °C. This paper demonstrates the feasibility of fabricating radio frequency (RF) devices of phase change materials (PCM) and it also presents a comparison between amorphous and crystalline PCMs in the RF spectrum. Previous work focuses on exploiting GeTe and GST as nonvolatile materials in memory applications, and also on characterizing them for their electrical and mechanical properties. The approach here focuses on fabricating RF devices and analyzing their responses. A simulation with resistor-capacitor (RC) and resistor-inductor (RL) circuits is presented to represent the response of the RF devices under testing. The fabrication process includes two-layer and four-layer devices on the Si wafer. PCMs are sputtered and the test pads are deposited using electron beam evaporation. Results show that these RF devices alone can serve as a low pass filter with a cutoff frequency of 10 MHz.

Flexible electronic circuitry is an emerging technology that will significantly impact the future of healthcare and medicine, food safety inspection, environmental monitoring, and public security. Recent advances in drop-on-demand printing technology and electrically conductive inks have enabled simple electronic circuits to be fabricated on mechanically flexible polymers, paper, and bioresorbable silk. Research has shown that graphene, and its derivative formulations, can be used to create low-cost electrically conductive inks. Graphene is a one atom thick two-dimensional layer composed of carbon atoms arranged in a hexagonal lattice forming a material with very high fracture strength, high Young’s Modulus, and low electrical resistance. Non-conductive graphene-oxide (GO) inks can also be synthesized from inexpensive graphite powders. Once deposited on the flexible substrate the electrical conductivity of the printed GO microcircuit traces can be restored through thermal reduction. In this paper, a femtosecond laser with a wavelength of 775nm and pulse width of 120fs is used to transform the non-conductive printed GO film into electrically conductive oxygen reduced graphene-oxide (rGO) passive electronic components by the process of laser assisted thermal reduction. The heat affected zone produced during the process was minimized because of the femtosecond pulsed laser. The degree of conductivity exhibited by the microstructure is directly related to the laser power level and exposure time. Although rGO films have higher resistances than pristine graphene, the ability to inkjet print capacitive elements and modify local resistive properties provides for a new method of fabricating sensor microcircuits on a variety of substrate surfaces.

This paper discusses research being conducted on aluminum nitride (AlN) as a pyroelectric material for use in detecting applications. AlN is being investigated because of its high pyroelectric coefficient, thermal stability, and high Curie temperature. In order to determine suitability of the pyroelectric properties of AlN for use as a detector, testing of several devices was conducted. These devices were fabricated using microelectromechanical systems (MEMS) fabrication processes; the devices were also designed to allow for voltage and current measurements. The deposited AlN films used were 150 nm – 300 nm in thickness. Thin-films were used to rapidly increase the temperature response after the thermal stimulus was applied to the pyroelectric material. This is important because the pyroelectric effect is directly proportional to the rate of temperature change. The design used was a face-electrode bridge that provides thermal isolation which minimizes heat loss to the substrate, thereby increasing operation frequency of the pyroelectric device. A thermal stimulus was applied to the pyroelectric material and the response was measured across the electrodes. A thermal imaging camera was used to monitor the changes in temperature. Throughout the testing process, the annealing temperatures, type of layers, and thicknesses were also varied. These changes resulted in improved MEMS designs, which were fabricated to obtain an optimal design configuration for achieving a high pyroelectric response. A pyroelectric voltage response of 38.9 mV_p-p was measured without filtering, 12.45 mV_p-p was measured in the infrared (IR) region using a Si filter, and 6.38 mV_p-p was measured in the short wavelength IR region using a long pass filter. The results showed that AlN’s pyroelectric properties can be used in detecting applications.

An application friendly technique to increase the surface area of the ionomer membrane such as Aquivion™ has been developed. By utilizing existing micro-fabrication technologies, square pillars were fabricated onto glass and silicon substrates. In combination with a low cost heat press, the glass and silicon stamps were used to successfully hot emboss micro-features onto the ionomer membrane. Consequently, the surface area of the Aquivion™ membrane was drastically increased enabling potential improvement of sensing and energy storage technologies. Preliminary results show successful fabrication of devices with systematic higher surface area and an improved capacitance.

Non-conductive graphene-oxide (GO) inks can be synthesized from inexpensive graphite powders and deposited on functionalized flexible substrates using inkjet printing technology. Once deposited, the electrical conductivity of the GO film can be restored through laser assisted thermal reduction. Unfortunately, the inkjet nozzle diameter (~40μm) places a limit on the printed feature size. In contrast, a tightly focused femtosecond pulsed laser can create precise micro features with dimensions in the order of 2 to 3 μm. The smallest feature size produced by laser microfabrication is a function of the laser beam diameter, power level, feed rate, material characteristics and spatial resolution of the micropositioning system. Laser micromachining can also remove excess GO film material adjacent to the electrode traces and passive electronic components. Excess material removal is essential for creating stable oxygen-reduced graphene-oxide (rGO) printed circuits because electron buildup along the feature edges will alter the conductivity of the non-functional film. A study on the impact of laser ablation on the GO film and the substrate are performed using a 775nm, 120fs pulsed laser. The average laser power was 25mW at a spot size of ~ 5μm, and the feed rate was 1000-1500mm/min. Several simple microtraces were fabricated and characterized in terms of electrical resistance and surface topology.

A novel method for laser assisted room temperature bonding of two substrates is presented. The method enables the packaging of delicate (bio)structures and/or finished (MEMS) devices, as there is no need for a high temperature annealing process. This also allows the bonding of two substrates with non-matching thermal expansion coefficients. The basis of the presented technology is the ability to create a direct pre-bond between two substrates. These can be two glass substrates, of which one has a thin film metal coating (e.g. Cr. Ti, Ta, Au…), or a silicon-glass combination. After (aligned) pre-bonding of the two wafers, a laser (e.g. a Nd:YAG laser) is used to form a permanent bond line on the bond interface, using the metal layer as a light absorber (or the silicon, in the case of a glass-silicon combination). The permanent bond line width is in the order of 10-50μm. The use of a laser to form the permanent bond ensures a hermetic sealing of the total package; a distinctive advantage over other, more conventional methods of room temperature bonding (e.g. adhesive bonding). He-leak testing showed leak rates in the order of 10^-9 mbar l/s. This meets the failure criteria of the MIL-STD-883H standard of 5x10-8 mbar l/s. An added functionality of the proposed method is the possibility to create electrical circuitry on the bond interface, using the laser to modify the metal interlayer, rendering it electrically non-conductive. Biocompatible packages are also possible, by choosing the appropriate interlayer material. This would allow for the fabrication of implantable packages.

Adopting optical technique to pursue micromachining must make a compromise between the focal spot sizes the depth of focus. The focal spot size determines the minimum features can be fabricated. On the other hand, the depth of focus influences the ease of alignment in positioning the fabrication light beam. A typical approach to bypass the diffraction limit is to adopt the near-field approach, which has spot size in the range of the optical fiber tip. However, the depth of focus of the emitted light beam will be limited to tens of nanometers in most cases, which posts a difficult challenge to control the distance between the optical fiber tip and the sample to be machined optically. More specifically, problems remained in this machining approach, which include issues such as residue induced by laser ablation tends to deposit near the optical fiber tip and leads to loss of coupling efficiency. We proposed a method based on illuminating femtosecond laser through a sub-wavelength annular aperture on metallic film so as to produce Bessel light beam of sub-wavelength while maintaining large depth of focus first. To further advance the ease of use in one such system, producing sub-wavelength annular aperture on a single mode optical fiber head with sub-wavelength focusing ability is detailed. It is shown that this method can be applied in material machining with an emphasis to produce high aspect ratio structure. Simulations and experimental results are presented in this paper.

Frequency references are used in almost every modern electronic device including mobile phones, personal computers, and scientific and medical instrumentation. With modern consumer mobile devices imposing stringent requirements of low cost, low complexity, compact system integration and low power consumption, there has been significant interest to develop batch-manufactured MEMS resonators. An important challenge for MEMS resonators is to match the frequency and temperature stability of quartz resonators. We present 1MHz and 20MHz temperature compensated Free-Free beam MEMS resonators developed using PolyMUMPS, which is a commercial multi-user process available from MEMSCAP. We introduce a novel temperature compensation technique that enables high frequency stability over a wide temperature range. We used three strategies: passive compensation by using a structural gold (Au) layer on the resonator, active compensation through using a heater element, and a Free-Free beam design that minimizes the effects of thermal mismatch between the vibrating structure and the substrate. Detailed electro-mechanical simulations were performed to evaluate the frequency response and Quality Factor (Q). Specifically, for the 20MHz device, a Q of 10,000 was obtained for the passive compensated design. Finite Element Modeling (FEM) simulations were used to evaluate the Temperature Coefficient of frequency (TCf) of the resonators between -50°C and 125°C which yielded +0.638 ppm/°C for the active compensated, compared to -1.66 ppm/°C for the passively compensated design and -8.48 ppm/°C for uncompensated design for the 20MHz device. Electro-thermo-mechanical simulations showed that the heater element was capable of increasing the temperature of the resonators by approximately 53°C with an applied voltage of 10V and power consumption of 8.42 mW.

This paper describes an integrated CMOS-MEMS inertial sensor microsystem, consisting of a 3-axis accelerometer sensor device and its complementary readout circuit, which is designed to operate over a wide temperature range from - 55°C to 175°C. The accelerometer device is based on capacitive transduction and is fabricated using PolyMUMPS, which is a commercial process available from MEMSCAP. The fabricated accelerometer device is then post-processed by depositing a layer of amorphous silicon carbide to form a composite sensor structure to improve its performance over an extended wide temperature range. We designed and fabricated a CMOS readout circuit in IBM 0.13μm process that interfaces with the accelerometer device to serve as a capacitance to voltage converter. The accelerometer device is designed to operate over a measurement range of ±20g. The described sensor system allows low power, low cost and mass-producible implementation well suited for a variety of applications with harsh or wide temperature operating conditions.

Graphene is a two-dimensional sheet of carbon atoms with exceptional electronic and mechanical properties, giving it tremendous potential in nanoelectromechanical system devices. Here, we present a method to easily and reproducibly fabricate suspended graphene nanoribbons across nanogap electrodes of various separation lengths, demonstrating a technique with aggressive gap scalability and device geometry control. Fabrication is based on using a focused gallium ion beam to create a slit between joined electrodes prepatterened on a 100 nm thick silicon nitride membrane. The transparency of the nitride membrane provides reduced ion backscattering and adds milling resolution. Large-area monolayer graphene grown by atmospheric pressure chemical vapor deposition was transferred onto the silicon nitride chip and patterned into a free-standing ribbon geometry via electron beam lithography on organic ebeam resist followed by an O₂ plasma etch. We find that commonly used inorganic negative tone resist that requires a buffered oxide etch for resist removal will attack the adhesion layer (Cr₂O₃) between the electrode and nitride membrane, which is exposed immediately after milling, so an organic resist was selected to avoid this. Using this technique, we fabricate freestanding graphene devices contacted by electrodes of sub-100 nm separation length and preform a comparative study on the effects of current annealing on device resistance. The gap resolution of this technique is limited by the gallium ion beam, which allows for sub-100 nm gaps. Sub-10 nm gaps are feasible with He ion beams, proving direct applications in probing the high field transport properties of graphene nanoribbons at post-CMOS length scales.

In this paper a method for producing resonant structures using laser micromachining is presented. In the spot of laser beam impact on AlN ceramics surface a conductive aluminum layer is formed. Compilation of process parameters allows for the fabrication of structures with resistance at Rs ~ 0.01Ω/Rs. It has been also found out that the maximum value of resistance for which spiral resonator structures manifest their unique properties is at the level of Rs = 1.43 Ω. Furthermore, the occurrence of mutual capacity which value is dependent on the arrangement of individual SR structures with respect to each other was observed and examined. Based on satisfactory results for SR structures, it has been attempted to produce a resonant structures dedicated to the THz range based on the process of direct metallization of AlN ceramics surface. As a result, the Split Ring Resonator structure whose properties were verified by using the THz -TDS method was manufactured. In case of the field E perpendicular to SRR structure and one resonance area for 0.50 THz with field E parallel to the structure, two characteristic resonant dips for 0.22 THz and 0.46 THz were obtained. The studies confirmed that the method of direct metallization of AlN ceramics allows to produce resonant structures in the THz range.

A method of providing consistent uniform and controlled zone heat at a fiber’s surface is presented using a CO₂ laser with axicon reflective elements. This optical configuration converts a laser beam into a light structure resembling a disc or cone that can band a fiber’s perimeter. The characteristic of this structure is its inherent ability to deliver increasing power density through uniform convergence toward the structure’s center thus applying the appropriate melting heat to varying sizes of fiber. By offsetting the light structure, precise localized zone heating and annealing of specific areas at a fiber’s surface can be achieved as well. This is essentially a passive devise into which active feedback elements can be incorporated to allow precise control of processes such as splicing, tapering, ball and axicon lensing, end capping, and combiner fabrication.

A complaint mechanism to extend resolution in the Fourier Transform Spectroscopy (FTS) technique has been designed, fabricated and tested. The mechanism based on the complaint mechanical design strategy has not mobile parts and was fabricated in MEMS technology in a Bosch Process. When this mechanism is used to displacing the mobile mirror in a FTS setup, an extended range travel for the reference mirror is achieved; thus, the optical path difference and hence the resolving power of the FTS system is increased. The fabricated device has dimensions of 5400x4200x400 Microns at the large, width and thickness respectively, with an aspect ratio about 10. Numerical simulations with ANSYS Software were developed to get the Stress limits and the input and output displacements, the mechanical gain and the resonance frequency of the device. Experimental results in both the forced and dynamical regime are presented. It is found that in the dynamical regime when the device is operated at its resonance frequency it exhibits a higher mechanical gain several times its gain in the forced regime.