Fabrication Of MEMS Devices Research Articles

Investigation of the Contact Characteristics of Silicon-Gold in an Anodic Bonding Structure.

Anodic bonding is broadly utilized to realize the structure support and electrical connection in the process of fabrication and packaging of MEMS devices, and the mechanical and electrical characteristics of the bonded interface of structure exhibit a significant impact on the stability and reliability of devices. For the anodic bonding structure, including the gold electrode of micro accelerometers, the elastic/plastic contact model of a gold–silicon rough surface is established based on Hertz contact theory to gain the contact area and force of Gauss surface bonding. The trans-scale finite element model of a silicon–gold glass structure is built in Workbench through the reconstruction of Gauss surface net by the reverse engineering technique. The translation load is added to mimic the process of contact to acquire the contact behaviors through the coupling of mechanical and electrical fields, and then the change law of contact resistance is obtained. Finally, the measurement shows a good agreement between the experimental results, theoretical analysis and simulation, which indicates there is almost no change of resistance when the surface gap is less than 20 nm and the resistance is less than 5Ω, while the resistance changes rapidly after the gap exceeds 20 nm.

Integration Technology for Wafer-Level LiNbO3 Single-Crystal Thin Film on Silicon by Polyimide Adhesive Bonding and Chemical Mechanical Polishing.

An integration technology for wafer-level LiNbO3 single-crystal thin film on Si has been achieved. The optimized spin-coating speed of PI (polyimide) adhesive is 3500 rad/min. According to Fourier infrared analysis of the chemical state of the film baked under different conditions, a high-quality PI film that can be used for wafer-level bonding is obtained. A high bonding strength of 11.38 MPa is obtained by a tensile machine. The bonding interface is uniform, completed and non-porous. After the PI adhesive bonding process, the LiNbO3 single-crystal was lapped by chemical mechanical polishing. The thickness of the 100 mm diameter LiNbO3 can be decreased from 500 to 10 μm without generating serious cracks. A defect-free and tight bonding interface was confirmed by scanning electron microscopy. X-ray diffraction results show that the prepared LiNbO3 single-crystal thin film has a highly crystalline quality. Heterogeneous integration of LiNbO3 single-crystal thin film on Si is of great significance to the fabrication of MEMS devices for in-situ measurement of space-sensing signals.

Performance Investigation of Carbon Nanotube Based Temperature Compensated Piezoresistive Pressure Sensor

In silicon-based piezoresistive pressure sensor, the accuracy of the sensor is affected mainly by thermal drift and the sensitivity of the sensor varies with the rise in temperature. Here, the temperature effects on the desired representation of the sensor are analysed. Use of smart material Carbon nanotubes (CNT) and a few effective temperature compensation techniques are presented in this study to reduce the temperature effect on the accuracy of the sensor. Resistive compensation employed extra piezoresistors with Negative Temperature Coefficient of Resistivity (TCR) for temperature compensation. The attainment of the desired compensation techniques is highly compatible with the MEMS device fabrication. The compensated pressure sensor is supremacy for pressure measurement with temperature variations. Though various techniques have been suggested and put into actuality with successful attainment, the techniques featuring easy implementation and perfect compatibility with existing schemes are still blooming demanded to design a piezoresistive pressure sensor with perfect comprehensive performance. In this paper, CNT piezoresistive material has been employed as sensing elements for pressure sensor and compared with silicon in terms of output voltage and sensor performance degradation at higher temperature. Pressure sensors using CNT and silicon piezo resistive sensing materials were simulated on silicon (100) diaphragm by ANYSIS. Based on simulation results, silicon and CNT both pressure sensor also shows better results at near room temperature. With the increasing temperature it is observed that silicon pressure output underestimated by 23%.

Cavity-BOX SOI: Advanced Silicon Substrate with Pre-Patterned BOX for Monolithic MEMS Fabrication

Several Silicon on Insulator (SOI) wafer manufacturers are now offering products with customer-defined cavities etched in the handle wafer, which significantly simplifies the fabrication of MEMS devices such as pressure sensors. This paper presents a novel cavity buried oxide (BOX) SOI substrate (cavity-BOX) that contains a patterned BOX layer. The patterned BOX can form a buried microchannels network, or serve as a stop layer and a buried hard-etch mask, to accurately pattern the device layer while etching it from the backside of the wafer using the cleanroom microfabrication compatible tools and methods. The use of the cavity-BOX as a buried hard-etch mask is demonstrated by applying it for the fabrication of a deep brain stimulation (DBS) demonstrator. The demonstrator consists of a large flexible area and precisely defined 80 µm-thick silicon islands wrapped into a 1.4 mm diameter cylinder. With cavity-BOX, the process of thinning and separating the silicon islands was largely simplified and became more robust. This test case illustrates how cavity-BOX wafers can advance the fabrication of various MEMS devices, especially those with complex geometry and added functionality, by enabling more design freedom and easing the optimization of the fabrication process.

Study of aluminum nitride films deposited on silicon for fabrication of MEMs devices

In this work, aluminum nitride (AlN) thin films deposited on the silicon (100) by RF magnetron-sputtering were analyzed. Nitrogen and argon plasmas were used in a vacuum system technique, being possible to obtain films oriented to the (oo2) crystallographic direction analyzed by X-ray diffraction (XRD) technique. Scanning electronic microscopy (SEM) was used to obtain the chemical composition (% at.) of AlN thin films. SEM analyses were accomplished to verify the images of the AlN films. Raman spectroscopy was used to obtain the Raman displacement as a function of the light intensity of the beam incident on the AlN films. Therefore, it was possible to reach the peaks of laser radiation absorption (λ = 514 nm) during Raman scattering. Ellipsometry was required to obtain: the roughness (Rz), optical gap (E04), and films thickness. Optical properties of the films depend on the temperature during the deposition. COMSOL software was required to simulate the performance of MEMS device, operating in the match circuit on a few ten of MHz resonance frequency.

Study of aluminum nitride films deposited on silicon for fabrication of MEMs devices

In this work, aluminum nitride (AlN) thin films deposited on the silicon (100) by RF magnetron-sputtering were analyzed. Nitrogen and argon plasmas were used in a vacuum system technique, being possible to obtain films oriented to the (oo2) crystallographic direction analyzed by X-ray diffraction (XRD) technique. Scanning electronic microscopy (SEM) was used to obtain the chemical composition (% at.) of AlN thin films. SEM analyses were accomplished to verify the images of the AlN films. Raman spectroscopy was used to obtain the Raman displacement as a function of the light intensity of the beam incident on the AlN films. Therefore, it was possible to reach the peaks of laser radiation absorption (λ = 514 nm) during Raman scattering. Ellipsometry was required to obtain: the roughness (Rz), optical gap (E04), and films thickness. Optical properties of the films depend on the temperature during the deposition. COMSOL software was required to simulate the performance of MEMS device, operating in the match circuit on a few ten of MHz resonance frequency.

A Cellular Automata Based Multi-Process Microfabrication Simulator

We describe a 3D simulator for several fabrication techniques utilized to create MEMS. The software is based on a cellular automata model and allows the user to simulate several processes, such as anisotropic wet etching in alkaline solutions and deep reactive-ion etching (DRIE) on (100) oriented Si substrates. The simulator allows for arbitrarily shaped masking materials and several processes can be applied in sequence to the same substrate. This enables the software to simulate the fabrication of complex MEMS devices which require more than one etching step. So, we show examples of fabrication processes involving different combinations of substrate wet and plasma DRI etching. Although relatively simple automata were utilized for the simulations, the results are in excellent accordance with reported experimental results. At this moment the simulations do not consider physical parameters affecting the fabrication process, the results shown here are important from an engineering point of view for qualitative analyses. At this moment more sophisticated automata are in development to simulate other processes, like film deposition (with different degrees of anisotropy) on previously etched substrates.

Plasma Etching of Polycrystalline Silicon using Thinning Technology for Application in CMOS and MEMS Technologies

This work presents results of the study of profile evolution for Si-poly structures during plasma etching using the thinning technology in SF6/CF4/CHF3 gas mixtures. Structures with an aspect ratio (height/width) up to 5, widths end in the range of 0.3 – 0.1 μm and 0.3μm thick, were produced. Sipoly structures with high anisotropy (anisotropy factor up to 0.92-0.98) after etching were demostrated. The method can be used for fabrication of sub-micron Si-poly gates in CMOS and in fabrication of MEMS devices.

Modeling of anodic bonding with SiO2 dielectric as interlayer

An anodic bonding model of Si/SiO2-glass is established and demonstrated. In the theoretical part, charge movement in Si/SiO2-glass anodic bonding is studied in detail. On this basis, an equivalent circuit model of anodic bonding with SiO2 structure is established. By solving the corresponding Poisson equation, the formulas for the width of the depletion layer and the electrostatic force during Si/SiO2-glass anodic bonding are obtained. In addition, the breakdown mechanism of SiO2 in anodic bonding is explained with 1/E model, and the breakdown type of SiO2 is analyzed. In the experimental part, through the analysis of the anodic bonding curve, we have concluded that soft breakdown of SiO2 occurs in the Si/SiO2-glass structure for the first time. At a temperature of 340 °C, for SiO2 with a thickness of 2000 Å and 3400 Å, the soft breakdown voltages are measured as 223.4 V and 424.3 V, respectively. At the same time, through experiments of small-area bonding SiO2 structures, the torsional bonding strength of the Si-glass structure and Si/SiO2-glass structure are measured to be 4.14 GPa and 4.94 GPa, respectively. The proposed soft breakdown model of SiO2 provides a theoretical basis for reliable fabrication and package of MEMS devices.

Physical Simulation and Visualization of the Marangoni Convection inside Meniscus Region under IPA Vapor in Wafer Drying Process

With the recent rapid progress of higher integration of semiconductor devices, mechano-chemical polishing process is playing a very important role. Along with this trend, cleaning and drying processes after the mechano-chemical process, have been regarded as more and more important technologies for ensuring surface cleanliness of the wafers. As a drying process after cleaning, there is a method of flowing pure water to a rotating wafer, and simultaneously supplying IPA (Iso-Propyl Alcohol) vapor to the meniscus region of the liquid layer (this is known as Rotagoni drying). This method causes a surface tension difference on the liquid surface and then drives fluid convection by the Marangoni effect, which is thought to efficiently remove impure particles on the wafer surface. During the impingement of IPA vapor to the meniscus region, in-situ observation of Marangoni convection is so difficult that details of the flow velocity and flow pattern are not well understood. In order to further improve this method to efficiently remove ultrafine foreign particles on wafer surface in future, the authors believe that the observation and the understanding of the Marangoni convection are indispensable. In this study, as a first step to observe the flow, rather than treating the complicated three-dimensional flow on actual rotating wafer from the beginning, physical simulation by creating a simplified two-dimensional flow was performed. Using photoetching method and PDMS material as in the fabrication of MEMS devices, a two-dimensional flow channel with a depth of about 600 mm and a width of about 1000 mm was created, and pure water and IPA vapor (mixture of nitrogen and IPA in which concentration was controlled.) was supplied to reproduce the situation where IPA vapor collided with pure water in the meniscus region. Pure water was provided from one direction of the T-shaped flow channel by a syringe pump, and IPA vapor was supplied from the other direction. By setting a minute step for locking the position of the liquid, the stable fluid meniscus in an oblique direction to the channel width was realized in the channel. The Marangoni convection was generated in the meniscus where the IPA impinged, and the flow was visualized by utilizing PIV (Particle Image Velocimetry) method with tracer particles. From the observations, the flow velocities and the flow patterns were successfully obtained from the PIV image analysis. It was found that a rapid convection of 20 mm/s occurred at the maximum velocity when IPA vapor was introduced. This was about ten times of the velocity when only nitrogen gas was provided. In the case with only nitrogen gas, the liquid convection was generated only by the shear stress to the liquid surface caused by the gas flow. However, the Marangoni convection caused by the IPA vapor was about ten times larger than the flow velocity with only nitrogen, of which difference was unexpectedly large. Moreover, the direction of the Marangoni convection was not as simple as anticipated, which could change depending on the impinging position of the IPA vapor. In an experimental condition of the present study, the Marangoni convection was directed from the gas-liquid interface to the triple point on the wall. It is thought that these flow velocities and directions would be strongly related to the removal of foreign particles on the wafer surface. Videos taken in the observation will be also shown in the presentation. END Figure 1

A Proactive Plastic Deformation Method for Fine-Tuning of Metal-Based MEMS Devices After Fabrication

An innovative plastic deformation method is proposed to tune metal-based MEMS devices after fabrication. The plastic deformation is a well-known failure mechanism in MEMS devices, which includes a phenomenon that the devices do not return original position after repeated actuation. Here, in contrast to this common experience, we exploit the plastic deformation for the first time to tune the performance of MEMS devices, by producing deliberate, controlled plastic deformation with the help of Joule heating. The proposed plastic deformation method is precisely and predictably controlled by the model we developed here, and it was successfully demonstrated and evaluated using a fabricated MEMS device. Unlike conventional plastic deformation methods, the proposed method was accomplished solely by electrical control, which means that the method can be employed in integrated devices. Tuning resolution was demonstrated at a 10-nm level, which is remarkably smaller than that of conventional plastic deformation methods, where the tuning resolution is typically several micrometers. We confirmed that the proposed method does not deteriorate the basic performance of the fabricated MEMS devices. Finally, we proposed and demonstrated a method for calibrating MEMS devices after fabrication and suggested an interesting application, which is recovering the performance of aged MEMS devices. [2017-0129]

Photoresist materials comprising photobase generators and epoxy resins bearing carboxylic acids to lead to marked enhancement of base‐catalyzed cross‐linking reactions

Most of the photopatterning materials based on epoxy resins utilize photoacid generators (PAGs), which generate superacids as catalysts. They have been used for high aspect ratio photoresists in the fabrication of MEMS devices. However, there is a drawback, in that the acidic species from PAGs will induce metal corrosion. One of the approaches to overcome this problem is the use of photobase generators (PBGs) because organic bases would induce no corrosion. Although there have been many previous investigations of PBGs, only a few articles have mentioned photoreactive materials relying on PBGs because of their low photosensitivity. We report here highly sensitive photopatterning materials comprising PBGs and an epoxy resin bearing carboxylic acid groups. As a result, the photopatterning materials showed higher photosensitivity than conventional epoxy resin systems. We obtained high‐photosensitivity (up to 900 mJ/cm2), high‐resolution (10‐μm line‐and‐space) patterning materials in films, 10 μm in thickness.

Material selection for Micro-Electro-Mechanical-Systems (MEMS) using Ashby's approach

A key aspect in design optimization of a product or a system is the selection of materials that best meet the design needs, ensuring maximum performance and minimum cost. Ashby's approach, originally introduced for macro-systems and products, has been very successfully employed for Micro-Electro-Mechanical-Systems (MEMS)/micromachined sensors, actuators and devices. This paper presents a comprehensive review and critical analysis of MEMS material selection studies using Ashby's approach reported in the literature during the last two decades. Performance and Material Indices derived for various microsystems and MEMS devices have been summarized. Moreover, all MEMS materials reported in the literature and the most suitable materials proposed for a variety of MEMS systems and devices have also been consolidated. A material selection case study utilizing micro-scale properties of 51 MEMS compatible materials has been presented to demonstrate that the use of different materials' bulk properties is not the best choice for MEMS materials selection. This paper will serve as a reference guide and useful resource for researchers and engineers engaged in the design and fabrication of various microsystems and MEMS sensors, actuators and devices.

Contributions of network topological structures to the mechanical properties of PDMS elastomers

Silicone elastomers (poly dimethyl siloxane, PDMS) are widely used in the fabrication of MEMS devices and in mechanobiological studies. Mechanical properties of the elastomers may be varied by changing the crosslinking density and physical entanglements in the networks. Other factors like dangling chains and uncrosslinked sol fractions also influence the network mechanics. We investigated the role of crosslinks, entanglements and sol fraction in the mechanical properties of PDMS prepared with different ratios of base to crosslinker. Results from compression tests show greater elastic modulus for the highly crosslinked networks. We removed uncrosslinked sol, altered the entanglement density with xylene extraction, and used data from mechanical tests to assess predictions from the Frenkel-Flory-Rehner model. These studies show that the polymer-solvent interaction parameter, χ, varied linearly with the equilibrium volume fraction. Dynamic mechanical analyses of extracted PDMS samples showed that crosslinking density, solvent presence, and sol fraction affect their overall viscoelasticity properties. We also compared experimental data with scaling laws and demonstrate that trapped entanglements in the networks contribute to the mechanical properties of PDMS. Finally, we show from these studies that swelling behaviors of PDMS in xylene follow affine deformation.

Fabrication of Multilayer 3D MEMS Device for Optimizing Pico Electric Field Thermal Effect on Cancer Cell Nanoporation

Fabrication of Multilayer 3D MEMS Device for Optimizing Pico Electric Field Thermal Effect on Cancer Cell Nanoporation

吸引プラズマエッチング法を用いたSiO<sub>2</sub>ダイアフラム構造作製技術の開発

We propose a simple method to fabricate SiO2 diaphragms supported on a Si substrate by using the inward plasma etching. In this method, a Si substrate covered with SiO2 was locally etched with the inward plasma from the Si side. When using a SiO2 (280 nm)/Si (380 μm) substrate, a SiO2 diaphragm with a diameter of ∼50 μm was fabricated at the bottom of the bowl-like hole with an opening diameter of ∼1.2 mm. This method does not require lithographic processes which are inevitable for the conventional microfabrication techniques and may be used to fabricate MEMS devices in the lab.

An Investigation of Structural Dimension Variation in Electrostatically Coupled MEMS Resonator Pairs Using Mode Localization

If a pair of MEMS resonators is electrostatically coupled together, the vibration amplitude ratios at the resonant frequencies of the resulting coupled system are sensitive to stiffness perturbation. An imbalance between the two resonators causes the confinement of vibration energy when the system is resonating, an effect known as mode localization. The degree of localization can be determined by extracting the amplitude ratio of the resonators through capacitive transduction. In this paper, we have fabricated MEMS devices, using a dicing-free silicon-on-insulator process, consisting of pairs of closely spaced microresonators. Each resonator consists of a clamped–clamped beam with a wider section in the middle, which is the location of the electrostatic coupling, instituted through the dc biasing of the resonators. Several devices have been fabricated, with the length of the anchor beams being varied, which influences the frequency of resonance. Stiffness imbalance between the resonators has been introduced through electrostatic spring softening, with the sensitivity of the amplitude ratio of the resonant-mode shape being greater for the higher frequency, shorter anchor devices. The sensitivities of the devices in this paper have been found to be nine times greater than the state-of-the-art two-degree-of-freedom mode-localized sensors.

Transfer Methods of PZT Thin Capacitor to Flexible Polymer by Self‐Assembled Monolayers

SUMMARYThis paper describes the fabrication method that a lead zirconate titanate (PZT) thin film capacitor, which is formed on a Si substrate by metal organic decomposition (MOD), can be easily transferred to flexible polymer (polydimenthylsiloxane; PDMS) without additional chemical contamination. The thin film capacitor is composed of sandwich structure of PZT between two metal layers. To embed the capacitor structure in the polymer substrate by dry peel‐off process, three kinds of self‐assembled monolayers (SAMs) were investigated for adhesion or antiadhesion layer between metal, polymer, and Si substrate, respectively. Also, the MOD's annealing process with SAMs for PZT lamination was investigated. As a result, the PZT capacitors of 1 μm thickness with a perovskite structure were successfully transferred and embedded in PDMS substrate. These results will be useful for fabricating MEMS devices and integration technology of electrical device to a polymer substrate such as capacitors in three‐dimensional integration or for flexible electronics.

Process Integration of Electroformed MEMS Variable Capacitors for Magnetostriction Measurements

Electrochemically deposited CoFe and CoFeB films are being studied for use as wireless remote sensors. Coded signals can be achieved through passive devices and interrogated to produce electronic signatures that are nearly impossible to replicate or counterfeit. A MEMS variable capacitor is being fabricated from a novel electroformed process with CoFe and CoFeB magnetostrictive films. This approach to MEMS fabrication is being used to incorporate electroplated ferric alloys into a variable capacitor, to study Joule magnetostriction. Such alloys are capable of a large change in material shape when subjected to a magnetic field. Integrating these films with a variable capacitor MEMS device allows for a unique approach to provide accurate microstrain measurements in low magnetic fields up to saturation, to provide feedback for the optimization without the use of expensive commercial setups. This process is unique to MEMS device fabrication and is differentiated by the use of a planar layered approach. Maintaining planarity by manipulating oxides to form electroplating molds, as well as provide a sacrificial material, results in an electrochemically deposited cantilever structure with an integrated magnetostrictive sensing element. This presentation will discuss the fabrication process of these devices. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

MEMS Fabrication on LTCC Substrates for RF Applications: Challenges and Perspectives

Abstract Microelectromechanical Systems (MEMS) are often used in transceiver modules, especially for telecommunication and radar applications. In this paper, we present recent progress in the development on a MEMS-on-LTCC process. We focus on the Low Temperature Co-fired Ceramic (LTCC) substrate issues and we present a successful solution for overcoming the substrate challenges through surface pre-treatment using a chemical mechanical surface polishing (CMP) process which allows us to reach the required smoothness for the fabrication of MEMS devices. We discuss various process parameters such as slurry type, rotating pad and rotation speed, and show their impact on the final surface finish. With an optimized process, the maximum roughness was decreased from more than 10μm to less than 0.5 μm over a 640 × 640 μm2 LTCC sample. Also, we present the various MEMS process steps starting with the deposition and patterning of various layers to a prototype switch highlighting the validated steps and the challenges encountered. A brief discussion of the perspectives for the integration of MEMS and LTCC technologies is also presented.