Microelectromechanical Systems Materials Research Articles

Analysis of the surface effects on adhesion in MEMS structures

One of the main failure causes in microelectromechanical systems (MEMS) is stiction. Stiction is the adhesion of contacting surfaces due to surface forces. Adhesion force depends on the operating conditions and is influenced by the contact area. In this study, the adhesion force between MEMS materials and the AFM tips is analyzed using the spectroscopy in point mode of the AFM. The aim is to predict the stiction failure mode in MEMS. The investigated MEMS materials are silicon, polysilicon, platinum, aluminum, and gold. Three types of investigations were conducted. The first one aimed to determine the variation of the adhesion force with respect to the variation of the roughness. The roughness has a strong influence on the adhesion because the contact area between components increases if the roughness decreases. The second type of investigation aimed to determine the adhesion force in multiple points of each considered sample. The values obtained experimentally for the adhesion force were also validated using the JKR and DMT models. The third type of investigation was conducted with the purpose of determining the influence of the temperature on the adhesion force.

Coupled Semianalytical and FEM Modeling of Polymer Piezoelectric Material With Interdigitated Electrodes

The piezoelectric polymers are increasingly considered as favorable materials for microelectromechanical systems due to their fast response, low operating voltages, and greater efficiencies of operation. However, the difficulty of forming structures and shapes has so far limited the range of electromechanical design. The possibility of using interdigitated electrodes (IDEs) to improve the transverse actuation of polymer film is modeled. A semianalytical model based on application of the conformal mapping methods is used to determine the electrostatic field in such configuration. A coupling with a finite-element modeling allows to obtain the piezoelectric strain field. The effect of the IDEs geometry on the developed strain was modeled. The results present simple coherent guidelines for the optimization of IDE patterns deposited onto surfaces of polyvinylidene fluoride films.

Structural study of growth, orientation and defects characteristics in the functional microelectromechanical system material aluminium nitride

The real structure and morphology of piezoelectric aluminum nitride (AlN) thin films as essential components of magnetoelectric sensors are investigated via advanced transmission electron microscopy methods. State of the art electron diffraction techniques, including precession electron diffraction and automated crystal orientation mapping (ACOM), indicate a columnar growth of the AlN grains optimized for piezoelectric application with a {0 0 0 1} texture. Comparing ACOM with piezoresponse force microscopy measurements, a visual correlation of the structure and the piezoelectric properties is enabled. With a quantitative analysis of the ACOM measurements, a statistical evaluation of grain rotations is performed, indicating the presence of coincidence site lattices with Σ7, Σ13a, Σ13b, Σ25. Using a geometric phase analysis on high resolution micrographs, the occurrence of strain is detected almost exclusively at the grain boundaries. Moreover, high resolution imaging was applied for solving the atomic structure at stacking mismatch boundaries with a displacement vector of 1/2 ⟨1 0 -1 1⟩. All real structural features can be interpreted via simulations based on crystallographic computing in terms of a supercell approach.

Development of clinically relevant implantable pressure sensors: perspectives and challenges.

This review describes different aspects to consider when developing implantable pressure sensor systems. Measurement of pressure is in general highly important in clinical practice and medical research. Due to the small size, light weight and low energy consumption Micro Electro Mechanical Systems (MEMS) technology represents new possibilities for monitoring of physiological parameters inside the human body. Development of clinical relevant sensors requires close collaboration between technological experts and medical clinicians. Site of operation, size restrictions, patient safety, and required measurement range and resolution, are only some conditions that must be taken into account. An implantable device has to operate under very hostile conditions. Long-term in vivo pressure measurements are particularly demanding because the pressure sensitive part of the sensor must be in direct or indirect physical contact with the medium for which we want to detect the pressure. New sensor packaging concepts are demanded and must be developed through combined effort between scientists in MEMS technology, material science, and biology. Before launching a new medical device on the market, clinical studies must be performed. Regulatory documents and international standards set the premises for how such studies shall be conducted and reported.

Investigation of microelectromechanical systems bimaterial sensors with metamaterial absorbers for terahertz imaging

One attractive option to achieve real-time terahertz (THz) imaging is a microelectromechanical systems (MEMS) bimaterial sensor with embedded metamaterial absorbers. We have demonstrated that metamaterial films can be designed using standard MEMS materials such as silicon oxide (SiOx), silicon oxinitrate (SiOxNy), and aluminum (Al) to achieve nearly 100% resonant absorption matched to the illumination source, providing structural support, desired thermomechanical properties and access to external optical readout. The metamaterial structure absorbs the incident THz radiation and transfers the heat to bimaterial microcantilevers that are connected to the substrate, which acts as a heat sink via thermal insulating legs, allowing the overall structure to deform proportionally to the absorbed power. The amount of deformation can be probed by measuring the displacement of a laser beam reflected from the sensor’s metallic ground plane. Several sensor configurations have been designed, fabricated, and characterized to optimize responsivity and speed of operation and to minimize structural residual stress. Measured responsivity values as high as 1.2 deg/μW and time constants as low as 20 ms with detectable power on the order of 10 nW were obtained, indicating that the THz MEMS sensors have a great potential for real-time imaging.

Fracture properties of atomic layer deposited aluminum oxide free-standing membranes

The fracture strength of Al2O3 membranes deposited by atomic layer deposition at 110, 150, 200, and 300 °C was investigated. The fracture strength was found to be in the range of 2.25–3.00 GPa using Weibull statistics and nearly constant as a function of deposition temperature. This strength is superior to common microelectromechanical systems materials such as diamondlike carbon, SiO2, or SiC. As-deposited membranes sustained high cycling pressure loads >10 bar/s without fracture. Films featured, however, significant reduction in the resistance to failure after annealing (800 °C) or high humidity (95%, 60 °C) treatments.

Emerging materials for microelectromechanical systems at elevated temperatures

Abstract

Development of polynorbornene as a structural material for microfluidics and flexible BioMEMS

ABSTRACTPolynorbornene is a class of polymer that exhibits significant potential as a structural material in microelectromechanical systems owing to its dielectric constant and compatibility with silicon‐based microfabrication processes. A commercially available version of PNB (AvatrelTM 2585P) is particularly attractive for bioMEMS applications because of its low moisture absorption characteristics, photodefinability, and potential biocompatibility. This study furthers the advancement of PNB as an enabling structural material for microfluidics and flexible bioMEMS applications by developing the following key processing techniques: (1) oxygen plasma‐based surface modification for bonding PNB layers to glass substrates, and (2) the monolithic fabrication of free‐standing, mechanically flexible electrode arrays using silicon wafers as mechanical supports during fabrication. To further develop PNB for flexible, implantable bioMEMS applications, this study also includes an evaluation of: (1) the tensile properties of free standing structures after accelerated lifetime testing in phosphate‐buffered saline, and (2) the in vitro performance of free‐standing, mechanically flexible neural microelectrode array‐based neural interfaces. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40969.

ArF Excimer Laser Micromachining of MEMS Materials: Characterization and Applications

Excimer laser ablation is a versatile technique that can be used for a variety of different materials. Excimer laser ablation overcomes limitations of conventional two-dimensional (2D) microfabrication techniques and facilitates three-dimensional (3D) micromanufacturing. Previously, we have reported a characterization study on 248 nm KrF excimer laser micromachining. This paper extends the study to 193 nm ArF excimer laser micromachining on five representative micro-electro-mechanical systems (MEMS) materials (Si, soda-lime glass, SU-8, polydimethylsiloxane (PDMS), and polyimide). Relations between laser parameters (fluence, frequency and number of laser pulses) and etch performances (etch rates, aspect ratio, and surface quality) were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but was not affected by laser frequency. Materials absorbance spectrum was found to have important influence on etch rates. Thermal modeling was conducted as well using numerical simulation to investigate how the photothermal ablation mechanism affects the etching results. Thermal properties of material, primarily thermal conductivity, were proved to have significant influence on etching results. Physical deformation in laser machined sites was also investigated using scanning electron microscopy (SEM) imaging. Element composition of redeposited materials around ablation site was analyzed using energy dispersive x-ray spectroscopy (EDXS) analysis. Combined with our previous report on KrF excimer laser micromachining, this comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the 3D microfabrication capability of ArF excimer laser, cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome was conducted successfully using the optimized laser ablation parameters obtained in the current study.

Vertically Aligned Carbon Nanotubes Transfer with Glass Frit

Nanogetters, based on carbon nanotubes (CNTs) coated Ti films, do have higher pumping speed and possibly larger sorbed quantity compared with traditional getters, such as St175 of SAES [1]. Although the nanogetter is with the outstanding performance, it is still hard to be integrated into micro electro mechanical systems (MEMS) packaging and devices due to high growth temperature of CNTs (>700°C) [2], poor adhesion with substrate [3, 4], vulnerability to pollute the wafers and difficulty to pattern. Therefore, this paper proposes a promotion of nanogetter based on transferring out vertically aligned CNTs (VA-CNTs) utilizing the glass frit. Compared with the conventional nanogetters, the promotion processes have advantages of (1) compatibility to the MEMS materials with controllable melting temperature and coefficient of thermal expansion (CTE), (2) Enhancement of the adhesion between CNTs and substrate, (3) Avoiding polluting the package wafer during CNTs growth. The results indicate that the promotion processes can be fulfilled by the silicon wafers and have wide applications such as nanogetters, field emission display, etc.

Beyond CMOS: heterogeneous integration of III–V devices, RF MEMS and other dissimilar materials/devices with Si CMOS to create intelligent microsystems

Advances in silicon technology continue to revolutionize micro-/nano-electronics. However, Si cannot do everything, and devices/components based on other materials systems are required. What is the best way to integrate these dissimilar materials and to enhance the capabilities of Si, thereby continuing the micro-/nano-electronics revolution? In this paper, I review different approaches to heterogeneously integrate dissimilar materials with Si complementary metal oxide semiconductor (CMOS) technology. In particular, I summarize results on the successful integration of III-V electronic devices (InP heterojunction bipolar transistors (HBTs) and GaN high-electron-mobility transistors (HEMTs)) with Si CMOS on a common silicon-based wafer using an integration/fabrication process similar to a SiGe BiCMOS process (BiCMOS integrates bipolar junction and CMOS transistors). Our III-V BiCMOS process has been scaled to 200 mm diameter wafers for integration with scaled CMOS and used to fabricate radio-frequency (RF) and mixed signals circuits with on-chip digital control/calibration. I also show that RF microelectromechanical systems (MEMS) can be integrated onto this platform to create tunable or reconfigurable circuits. Thus, heterogeneous integration of III-V devices, MEMS and other dissimilar materials with Si CMOS enables a new class of high-performance integrated circuits that enhance the capabilities of existing systems, enable new circuit architectures and facilitate the continued proliferation of low-cost micro-/nano-electronics for a wide range of applications.

Underlayer Influence on AlN Deposition by Reactive Magnetron Sputtering

Sputter deposited aluminum nitride (AlN) thin film is the preferred piezoelectric material for microelectromechanical systems (MEMS) devices, such as film bulk acoustic resonator (FBAR) resonators and accelerometers. Silicon dioxide (SiO2) is usually used as a sacrificial layer which will be etched with BOE eventually to release the membrane. In the article, thermal oxide, as well as several types of SiO2 with different properties made by plasma enhanced chemical vapor deposition (PECVD) were selected as the underlying film. The influence of the surface morphology of silicon dioxide underlayer on the crystallinity of reactively sputtered c-axis oriented AlN thin films was investigated. The results show that the quality of AlN films is affected by the surface roughness of the underlying SiO2 film. The full width at half maximum (FWHM) of the x-ray rocking curve for 1μm AlN film ranged from 7.8¢ª to 1.5¢ª depending on the properties of PECVD SiO2 films. The rougher surface of underlayer worsen the crystallinity of AlN films, while adding chemical mechanical planarization (CMP) step was found to improve FWHM to 1.2¢ª. This paper discusses possible mechanisms for the influence of underlayer surface morphology on the properties of sputter deposited AlN films.

Strontium Titanate Perovskite SrTiO3 as a Promising Ideal Material for MEMS Applications : First-Principles FP-LMTO + LSDA Study

Strontium titanate perovskite oxide materials offer a number of advantages in microelectromechanical systems (MEMS) due to the micro-structural, electronic, magnetic and mechanical properties. In this paper, we report theoretical study on structural, electronic, magnetic and mechanical properties of strontium titanate perovskite oxide SrTiO3. This material selected as energetic substrate primarily for its dimensional stability that is relatively insensitive to environment conditions. The calculations performed using the first-principles full-potential linear muffin-tin orbital (FP-LMTO) method, under the local spin density approximation (LSDA) and plus the on-site Coulomb interaction (LSDA+U) scheme. The analysis of densities of states, exchange interactions and magnetic moments reveals that SrTiO3 exhibits a semiconductor character with an energy gap of Eg = 2.68 eV, in excellent agreement with the previous experimental results. Keywords: MEMS material; Perovskite; Electronic properties; FP-LMTO method.

High Temperature Microelectromechanical Systems Using Piezoelectric Aluminum Nitride

We report on the efforts at Sandia National Laboratories to develop high temperature capable microelectromechanical systems (MEMS). MEMS transducers are pervasive in today's culture, with examples found in cell phones, automobiles, gaming consoles, and televisions. There is currently a need for MEMS transducers that can operate in more harsh environments, such as automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Our development focuses on the coupling of silicon carbide (SiC) and aluminum nitride (AlN) thin films on SiC wafers to form a MEMS material set capable of temperatures beyond 1000°C. SiC is recognized as a promising material for high temperature capable MEMS transducers and electronics because it has the highest mechanical strength of semiconductors with the exception of diamond and its upper temperature limit exceeds 2500°C, where it sublimates rather than melts. Most transduction schemes in SiC are focused on measuring changes in capacitance or resistance, which require biasing or modulation schemes that can withstand elevated temperatures. Instead, we are coupling temperature hardened, micro-scale SiC mechanical components with piezoelectric AlN thin films. AlN is a non-ferroelectric piezoelectric material, enabling piezoelectric transduction at temperatures exceeding 1000°C. AlN is a favorable MEMS material due to its high thermal, electrical, and mechanical strength. It is also closely matched to SiC for coefficient of thermal expansion.

History and current status of commercial pulsed laser deposition equipment

This paper will review the history of the scale-up of the pulsed laser deposition (PLD) process from small areas ∼1 cm2 up to 10 m2 starting in about 1987. It also documents the history of commercialization of PLD as various companies become involved in selling fully integrated laser deposition tools starting in 1989. The paper will highlight the current state of the art of commercial PLD equipment for R&D that is available on the market today from mainstream vendors as well as production-oriented applications directed at piezo-electric materials for microelectromechanical systems and high-temperature superconductors for coated-conductor applications. The paper clearly demonstrates that considerable improvements have been made to scaling this unique physical vapour deposition process to useful substrate sizes, and that commercial deposition equipment is readily available from a variety of vendors to address a wide variety of technologically important thin-film applications.

Paper No P30: Heterogeneous Plated Metals for Display Elements and Technologies of High Reliability

AbstractThis article describes the application of advanced technologies for up‐to‐date display systems. A method for manufacturing of holographic films with high runability for roll‐to‐roll technology is considered. The way for improving the properties of the filters, optical phase retarders, wave plates and other using nanotrench filling technology is proposed. The use of nanocomposite materials for microelectromechanical systems of high reliability and their application to improve the mechanical properties of movable micromirror arrays, switches, microactuators are investigated.

An Assessment of Bulk Metallic Glasses for Microelectromechanical System Devices

Metallic glasses were born purely of academic interest and earlier commercial attempts were limited due to the low thickness of the samples. However, with the successful fabrication of bigger samples using multicomponent system, there has been renewed interest in the commercial applicability of bulk metallic glasses (BMGs) as they offer promising mechanical, tribological and other properties which make the commercialization aspect lucrative. Through this study, an attempt has been made to assess the applicability of bulk metallic glasses as a material for Microelectromechanical System (MEMS) applications. Ranking and comparison, with the current MEMS material set, has been made using multiple criteria decision making (MCDM) approach. Performance index (PIs) was calculated using Ashby approach and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) was used to rank the materials on a collective basis. Additional features have been investigated to assess their suitability as a MEMS material. It was observed that BMGs offer promising prospects as a suitable MEMS material based on their overall performance when compared to the current material set being used for MEMS applications.

Elimination of initial stress-induced curvature in a micromachined bi-material composite-layered cantilever

Micro-devices with a bi-material-cantilever (BMC) commonly suffer initial curvature due to the mismatch of residual stress. Traditional corrective methods to reduce the residual stress mismatch generally involve the development of different material deposition recipes. In this paper, a new method for reducing residual stress mismatch in a BMC is proposed based on various previously developed deposition recipes. An initial material film is deposited using two or more developed deposition recipes. This first film is designed to introduce a stepped stress gradient, which is then balanced by overlapping a second material film on the first and using appropriate deposition recipes to form a nearly stress-balanced structure. A theoretical model is proposed based on both the moment balance principle and total equal strain at the interface of two adjacent layers. Experimental results and analytical models suggest that the proposed method is effective in producing multi-layer micro cantilevers that display balanced residual stresses. The method provides a generic solution to the problem of mismatched initial stresses which universally exists in micro-electro-mechanical systems (MEMS) devices based on a BMC. Moreover, the method can be incorporated into a MEMS design automation package for efficient design of various multiple material layer devices from MEMS material library and developed deposition recipes.

In vitro and in vivo evaluation of a photosensitive polyimide thin-film microelectrode array suitable for epiretinal stimulation

BackgroundEpiretinal implants based on microelectro-mechanical system (MEMS) technology with a polyimide (PI) material are being proposed for application. Many kinds of non-photosensitive PIs have good biocompatibility and stability as typical MEMS materials for implantable electrodes. However, the effects of MEMS microfabrication, sterilization and implantation using a photosensitive polyimide (PSPI) microelectrode array for epiretinal electrical stimulation has not been extensively examined.MethodsA novel PSPI (Durimide 7510) microelectrode array for epiretinal electrical stimulation was designed, fabricated based on MEMS processing and microfabrication techniques. The biocompatibility of our new microelectrode was tested in vitro using an MTT assay and direct contact tests between the microelectrode surface and cells. Electrochemical impedance characteristics were tested based on a three-electrode testing method. The reliability and stability was evaluated by a chronic implantation of a non-functional array within the rabbit eye. Histological examination and SEM were performed to monitor possible damage of the retina and microelectrodes. Electrically evoked potentials (EEPs) were recorded during the acute stimulation of the retina.ResultsThe substrate was made of PSPI and the electrode material was platinum (Pt). The PSPI microelectrode array showed good biocompatibility and appropriate impedance characteristics for epiretinal stimulation. After a 6-month epiretinal implantation in the eyes of rabbits, we found no local retinal toxicity and no mechanical compression caused by the array. The Pt electrodes adhesion to the PSPI remained stable. A response to electrical stimuli was with recording electrodes lying on the visual cortex.ConclusionWe provide a relevant design and fundamental characteristics of a PSPI microelectrode array. Strong evidences on testing indicate that implantation is safe in terms of mechanical pressure and biocompatibility of PSPI microelectrode arrays on the retina. The dual-layer process we used proffers considerable advantages over the more traditional single-layer approach and can accommodate much many electrode sites. This lays the groundwork for a future, high-resolution retinal prosthesis with many more electrode sites based on the flexible PSPI thin film substrate.

Near-Field Electrospray Microprinting of Polymer-Derived Ceramics

Ceramic microelectromechanical systems (MEMS) sensors are potentially game-changing devices in many applications in high-temperature and corrosive environments, where the use of conventional MEMS materials such as silicon is prohibited. However, the microfabrication of ceramic MEMS devices remains a major technical challenge. Here, we report a method to directly print micro ceramic patterns using near-field electrospray (ES) of polymer-derived ceramics (PDCs). We demonstrated that the viscous ceramic precursor liquids can be printed reliably without any clogging problem. The spray self-expansion due to Coulombic repulsion force among charged droplets can be suppressed by decreasing the droplet residence time in space. A spray expansion model is used to predict the line width, and the results are in decent agreement with the experiments. We demonstrated a 1-D printed polymer feature as narrow as 35 μm and a micro pentagram pattern. Moreover, after the pyrolysis of PDC at 1100 °C in nitrogen, amorphous alloys of silicon, carbon, and nitrogen (SiCN) are obtained. The samples show good integrity and adhesion to the substrate. The near-field ES PDC printing can become a useful addition to the toolbox of high-temperature MEMS.