Stiction Problems Research Articles

Regimes in the axisymmetric stiction of thin elastic plates

This work considers the adhesion of a thin, prestressed elastic plate to the bottom of a microcavity – a scenario that can be found frequently in thin-film devices from pressure sensors to microfluidics. This adhesion phenomenon is also referred to as stiction in the field of nano/microelectromechanical systems (N/MEMS); the geometry we consider is axisymmetric (thereby we term this problem axisymmetric stiction). Motivated by the extreme thinness of increasingly exploited nanofilms such as 2D materials in functional devices, various limiting regimes of the axisymmetric stiction problem that arise due to the interplay of the bending, stretching, and pretension effects are discussed. Specifically, key dimensionless physical parameters in this problem are discussed and the range of these parameters for the classification of different regimes is outlined. This classification allows for analytical/asymptotic solutions for the critical adhesion conditions and the adhesion length in different regimes, many of which are not yet available in the literature. These analytical results are verified numerically and also compared with experiments based on 3-500 nm thick 2D materials. As such, this work provides a complete overview of the physically relevant regimes associated with axisymmetric stiction, establishing a regime diagram that can be directed used for the evaluation of the structural reliability of rapidly emerging thin plate devices.

Investigation of Vapor HF Sacrificial Etching Characteristics Through Submicron Release Holes for Wafer-Level Vacuum Packaging Based on Silicon Migration Seal

Vapor hydrogen fluoride (vHF) sacrificial SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> etching is a crucial process for wafer-level packaging based on silicon migration seal (SMS) technology. In this study, by using test samples with several kinds of test patterns and structures, the characteristics of vHF etching through release holes with a diameter of 0.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> m were investigated. The results showed that through-hole etch rate had some dependence on the number and distribution of release holes, distance from the release holes, and dimension of sealed cavity, which is much different from normal vHF etching. The stiction problem in such a special case was also tested by releasing cantilever structures of different length. They were more likely to bend during through-hole etching than during normal etching. This work reveals the role of the water as both a by-product and catalyst during through-hole vHF etching. The results of this study provide important design guidelines for SMS-based MEMS packaging as well as other similar vacuum packaging technology and improve the understanding of vHF etching mechanisms. 2023-0065

Contact time dependence of adhesion force at silica/silica interface on AFM: Influence of relative humidity and contact history

In the micro-nano adhesion force study, discrepancies in contact time dependence require clarification. Adhesion forces between hydrophobic silica cantilevers and partially hydrophobic silica were measured to investigate the dependence on an atomic force microscope (AFM) at 10%∼90% relative humidities (RH). Experimental results demonstrate that the dependence is related to RH and contact history. The behavior with dwell time is logarithmically increasing with saturation time of 80 ∼ 200 s at 10%∼80% RHs and independent at ∼90% RH. However, once the interface is exposed at ∼90% RH for some time, the instability (the change of magnitude with repeated contacts) occurs, and the behavior can change to logarithmical increase or larger forces with long dwell time. These behaviors at ∼90% RH were attributed to the condensation polymerization of orthosilicic acid to form silica sols and gels. With the exposure, the subsequent behaviors can also be changed to: (1) increasing–decreasing or stable-increasing-stable at 60%∼80% RHs, and (2) short saturation time at 10%∼30% RHs. The effect of the exposure can be eliminated with repeated contacts at a low RH. Therefore, the contact history matters. The results may deepen the understanding of adhesion mechanisms and provide solutions to stiction problems for small-scale silicon-based structures.

Stiction Mitigation of Nanoparticles in Magnetically Targeted Drug Delivery System

This article addresses the stiction issue of magnetic nanoparticles (MNPs) in a targeted drug delivery system (TDDS). In a magnetically actuated TDDS, the MNPs are guided to the desired blood vessel by steering them from the bifurcation points, using an external electromagnetic actuation (EMA) system. During the steering process, some of the MNPs get stuck to the vessel wall. To overcome this problem, an EMA system is designed using four coils and the corresponding coil parameters are computed to efficiently steer the MNPs in a Y-shaped microchannel. The proposed EMA system operates in two different modes alternately, by switching on and off a pair of coils. In the first mode, the system generates magnetic force responsible for steering the MNPs. In the second mode, it generates a small magnetic force in the reverse direction, which pulls back the MNPs stuck to the vessel walls, thus mitigating the stiction problem. The MNPs that are separated from the sidewalls move ahead with the fluid flow to the desired channel and this guidance mechanism is repeated till the MNPs reach the target point. The experimental validation of the proposed EMA system highlights its practical feasibility to mitigate the stiction issue. Results demonstrate that the magnetophoretic force produced to release the adhered particles in our system is around 99.5% lower than that of an existing EMA system, which aims to address the same issue. Thus, our system provides enhanced efficacy in mitigating the stiction issue by alleviating the detrimental effect of the MNPs getting steered to the undesired outlet.

Fluid Stiction From a Contact Condition

This paper considers modeling of fluid stiction between two separating plates that start from a mechanical contact condition. Published experimental work on initially contacting plates showed significant variations in stiction force peak values. In order to describe the observed strong force variations with mathematical models, the models should be quite sensitive to some of the input parameters of the stiction problem. The model in this paper assumes that small air bubbles are entrapped between the contact areas of the asperity peaks and that the fluid film flow between the cavitation bubbles is guided by Reynolds equation. The proposed model exhibits high sensitivity to initial bubble size and initial contact force compared to state-of-the art models. A delay of about 1 ms in the simulated stiction force evolution and the experiments was found. Potential causes for this discrepancy are discussed at the end of this paper and an outlook to future work, which can reduce the discrepancy between the model and experimental results is given.

An intensive approach to optimize capacitive type RF MEMS shunt switch

Optimization of the RF MEMS switch is essential to enhance its performance characteristics at high frequency Millimeterwave applications. Many optimization techniques have been proposed based on design factors and output responses but not confined with the resonant frequency of RF transmission signal. In this paper, a novel optimization technique is proposed using Multiphysics FEM simulations based on electromagnetic and electromechanical studies. A novel Capacitive type of RF MEMS shunt switch is designed by using iterative meander technique and optimized at the resonant frequency of 41 ​GHz which can efficiently used for millimeter wave applications up to 60 ​GHz. The proposed optimization model which is a novel bottom – up approach consists of intermediate steps for optimization of various layers of switch from substrate to beam. The CPW transmission line with 60/100/60 as G/S/G and having 0.5 ​μm as thickness is considered to allow the frequencies up to 60 ​GHz. The width of the suspended Beam of switch is optimized to obtain up-state capacitance of 37 ​fF which allows the RF signal at 41 ​GHz. Parametric analysis has been carried out to obtain optimized thickness of each layer. Two types of switches other than Fixed – Fixed beam are designed using meandering technique to reduce pull in voltage. Among these the proposed Iterative meander switch shows very low pull in voltage of 1.4 ​V and also shows good return loss of −48.9 ​dB at 41 ​GHz and high isolation of −48.42 ​dB at 38 ​GHz. The proposed switches which are optimized at 41 ​GHz does not contains the perforations and suffers from serious stiction problems at downstate. Hence, perforations are introduced in switch membrane and fabricated using surface micromachining technology. The optimized iterative meander switch shows low pull – in voltage of 1.85 ​V which is closely approximated to the simulated value.

Electrodynamic Force, Casimir Effect, and Stiction Mitigation in Silicon Carbide Nanoelectromechanical Switches.

Logic switches enabled by nanoelectromechanical systems (NEMS) offer abrupt on/off-state transition with zero off-state leakage and minimal subthreshold swing, making them uniquely suited for enhancing mainstream electronics in a range of applications, such as power gating, high-temperature and high-voltage logic, and ultralow-power circuits requiring zero standby leakage. As NEMS switches are scaled with genuinely nanoscale gaps and contacts, quantum mechanical electrodynamic force (EDF) takes an important role and may be the ultimate cause of the plaguing problem of stiction. Here, combined with experiments on three-terminal silicon carbide (SiC) NEMS switches, a theoretical investigation is performed to elucidate the origin of EDF and Casimir effect leading to stiction, and to develop a stiction-mitigation design. The EDF calculation with full Lifshitz formula using the actual material and device parameters is provided. Finite element modeling and analytical calculations demonstrate that EDF becomes dominant over elastic restoring force in such SiC NEMS when the switching gap shrinks to a few nanometers, leading to irreversible stiction at contact. Artificially corrugated contact surfaces are designed to reduce the contact area and the EDF, thus evading stiction. This rational surface engineering reduces the EDF down to 4% compared with the case of unengineered, flat contact surfaces.

Seesaw Capacitive Structure as an Electrostatically Actuated Nonlinear Impact Resonator

In this paper, we present a new design of electrostatically actuated nonlinear impact resonator with a capacitive seesaw structure to solve the problem of short circuit, stiction and chaotic motion. The device is driven by electrostatic force on the capacitors, which utilizes nonlinear behavior and creates a pull-in effect. The seesaw structure can be set into oscillation with only a DC power source and a resistor, which is proved by a prototype device. No stiction occurred in over 10 h of experimental time, thus demonstrating the feasibility of solving the breakdown and stiction problem. A static mathematical model was established, solved, and verified by the experiment results, and a dynamic model with floating charge was analyzed. The analysis reveals the working principle of the proposed seesaw capacitive structure as an electrostatically actuated nonlinear impact resonator, and indicates that the working voltage can be decreased to less than 25 V if the oscillator dimensions are decreased to micrometers. The seesaw structure has considerable potential application for autonomous sensors.

Review on the Bubble Dynamics Based Cavitation Dynamics for the Negative Squeeze Motion in Lubricated Contacts

Simulation models for the cavitation dynamics in lubricated contacts can be roughly clustered into two groups: either without or with bubble dynamics, the first one being the standard case for most fluid film bearing calculations. The approach with bubble dynamics has been introduced to the lubrication community about 20 years ago by Someya, and it is based on the coupling of Reynolds equation and Rayleigh-Plesset equation. It has been used for journal bearings, squeeze film dampers, and it is essentially required for correct numerical calculations of the negative squeeze motion (i. e. the separation of two plates) or the oil stiction problem. More than a decade ago, in 2009, the first paper on the negative squeeze motion with bubble dynamics -- allowing numerical calculations of tensile stresses in the lubricant -- had been published. The application in mind is the simulation of mixed lubrication for rough surfaces. The negative squeeze motion is then understood as the motion of asperities (on smaller length scales). The paper at hand summarizes some of the research on the dynamics of cavitation in lubricated contacts from different research groups from the last 10 to 15 years and sketches key topics for further research on the topic. The roadmap is centered around the three key issues that remained from the previous research of the author: (a) numerical stability of the calculations for curved plates, (b) characteristic time scale for separation of plates and (c) experimental evidence for validating the calculation results.

Large aperture surface-micromachined rotating micromirror with the majority of dimples removed

This paper presents a surface-micromachined repulsive-force driven 1D rotating micromirror with a large aperture (1 mm). Two rotating beams are located on the bottom side of the mirror plate, while two repulsive-force actuators are connected to two middle sides to push the mirror plate up for out-of-plane rotation. Thus, no complex self-assembly structure is required to raise the mirror plate for rotation as most conventional surface-micromachined rotating micromirrors do. In order to increase the yield rate for the large aperture micromirror and avoid dimples used for preventing stiction during fabrication, six post-fabrication melted supporting beams are used along the circumference to increase the stiffness in fabrication, which are electrically melted and broken after fabrication to restore the initial stiffness. Only 12 dimples in the center of mirror plate are used for preventing operational stiction. The mirror’s reflectivity is increased to 25% from 20% after removing the majority of dimples. 36 prototypes are fabricated and tested and none of them suffer from stiction problem. The 6.3° optical rotation is achieved with a settling time of 26 ms.

Beam and dielectric materials impact on improvement of the performance of a novel capacitive shunt RF MEMS switch

AbstractThis paper presents a novel radiofrequency micro‐electro‐mechanical systems (RF MEMS) shunt capacitive switch having high isolation and low insertion loss for microphone applications. The device optimization and various electromechanical analysis are performed using a finite element modeler (FEM) solver. The proposed switch results in a pull‐in voltage of 36.88 V to displace movable beam 2/3rd of its initial gap. By varying beam material, dielectric material, beam‐dielectric thickness, and air gap between movable beam and dielectric layer, various electromechanical and electromagnetic analyses are carried out. For better isolation, silicon nitride (Si3N4) is used for the dielectric layer than hafnium oxide (HfO2) and silicon carbide (SiC). To achieve maximum displacement of the beam, various materials such as aluminum, gold, nickel, and copper are chosen; among all, this gold shows high displacement because of its good conductivity. By implementing various meandering techniques along with perforations, pull‐in voltage and stiction problems are reduced. The switch exhibits good return loss of −28 dB and insertion loss of −0.017 dB during on‐state at 5 GHz and high isolation of −41.26 dB at 10.5 GHz during off‐state. The proposed switch is used at the subsystem/device level in the future microphone, aircraft navigation, and satellite applications.

Post-Fabrication Melting Procedure With I-Shaped Beams for Stiction-Free Release of 2-D Surface-Micromachined Micromirrors Equipped With Repulsive-Force Actuators

This paper presents a post-fabrication melting procedure with an I-shaped beam design to solve the problem of stiction in surface-micromachined 2-D micromirrors equipped with repulsive-force actuators. This design adds extra I-shaped beams to the free-standing mirror plate to increase its stiffness about one thousandfold and allow it to survive the wet release process. Then, those beams are melted and broken electrically on-chip after fabrication to restore the original stiffness. With the aid of repulsive force, the I-shaped beam design can use a low current for melting. A high current was used in the previously presented T-shaped fusible tether beam design, but it causes splattering and residue. The I-shaped beam design has been successfully applied to a repulsive-force 2D micromirror that is integrated with head-up display and stiction is eliminated. The modeling, simulation, and prototyping processes are presented, and the ways in which it can be used in a general surface-micromachined free-standing structure to eliminate stiction are discussed. [2017-0299]

Fabrication and Characterization of Capacitive RF MEMS Perforated Switch

In this paper, we have designed, simulated, fabricated, and characterized a clamped-clamped micro mechanical structure-based shunt capacitive RF MEMS switch. The clamped–clamped micromechanical structure is micromachined using a gold metal thickness of 500 nm. AlN is used as a dielectric material, and it is deposited using the dc sputtering PVD process. In the MEMS technology, particularly in devices fabrication, releasing the membrane is a difficult task, and here, we have presented a novel wet process to release the membrane. Primarily, the S1813 sacrificial layer is etched by using the piranha solution and cleaned with the IPA solution. Critical point drying is done after fabrication to reduce the stiction effect on the switch. Overall, the switch requires the pull-in voltage of 5.5 V for 1.8- $\mu \text{m}$ displacement. In the process of optimization, primarily, the switch is designed and simulated using finite-element method tools. The reliability of the capacitive RF MEMS switches depends on the stiction problem caused by dielectric charging, and the proposed capacitive switch dielectric charging behavior is characterized using the CV curve method.

A study on the compensation of secondary distortion effect in the cutting process of a welded structure

Lug structures are commonly used in heavy industries. Welded lugs with holes could be hooked onto and transported by a crane. After setting up, the lugs are typically cut away using a flame cutting process. As undesirable damage is caused by concentrated heat input, the band saw cutting process is more appropriate. However, re-establishment of equilibrium within the remaining part of the structure causes secondary distortion owing to the redistribution of the residual stresses. Additionally, in the cutting procedure, the stiction problem always occurs because of the squeezing of the saw blade between the materials. A compensation method, which uses an additional procedure before the cutting process, is proposed in this study. The top side of the lug is thermally heated to induce the redistribution of residual stress and make the amount of distortion same as that in the final state after cutting. Comparing the results from experiment sets, the stiction problem in the band sawing of a lug was alleviated using this additional procedure. The additional heating procedure did not intensify the final welding distortion. The compensation effect of thermal heating was confirmed in this study.

Dicing‐free SOI process based on wet release technology

A simple, low-cost and reliable dicing-free silicon-on-insulator (SOI) process is presented for solving three major challenges in manufacturing the microelectromechanical systems devices, i.e. stiction, notching and dicing damage. In this process, the cavity is used and patterned on the handle layer to solve the stiction problem, and the exposed oxide is removed in hydrofluoric acid (HF) solution before deep reactive ion etching (DRIE) on the structure layer to eliminate the notching effect's impact. The dies are attached temporally to a designed frame by the silicon dioxide. After removing the oxide using HF solution, the dies are separated from the wafer cleanly without dicing damage. The layout design rules on the front side and backside patterns are established. Furthermore, a grooved carrier wafer with specific design rules was introduced to enhance the yield rate of the process. Finally, a tuning fork gyroscope was fabricated to demonstrate the proposed fabrication process and a yield rate of over 81% was achieved. The process solves the stiction, notching and dicing damage problems only involving the apparatus associated with lithography and DRIE, offering an economic and complete SOI process solution.

An Approximate Computational Method for the Fluid Stiction Problem of Two Separating Parallel Plates With Cavitation

Stiction forces exerted by a fluid in a thin, quickly widening gap to its boundaries can become a strongly limiting factor of the performance of technical devices, like compressor valves or hydraulic on–off valves. In design optimization, such forces need to be properly and efficiently modeled. Cavitation during parts of a stiction process plays a strong role and needs to be taken into account to achieve a meaningful model. The paper presents an approximate calculation method which uses qualitative solution properties of the non cavitating stiction problem, in particular of its level curves and gradient lines. In this method, the formation of the cavitation boundaries is approximated by an elliptic domain. The pressure distribution along its principle axis is described by a directly integrable differential equation, the evolutions of its boundaries is guided just by pressure boundary conditions when the cavitation zone expands and by a nonlinear differential equation when it shrinks. The results of this approximate model agree quite well with the solutions of a finite volume (FV) model for the fluid stiction problem with cavitation.

Actuator stiction compensation via model predictive control for nonlinear processes

The problem of valve stiction is addressed, which is a nonlinear friction phenomenon that causes poor performance of control loops in the process industries. A model predictive control (MPC) stiction compensation formulation is developed including detailed dynamics for a sticky valve and additional constraints on the input rate of change and actuation magnitude to reduce control loop performance degradation and to prevent the MPC from requesting physically unrealistic control actions due to stiction. Although developed with a focus on stiction, the MPC‐based compensation method presented is general and has potential to compensate for other nonlinear valve dynamics which have some similarities to those caused by stiction. Feasibility and closed‐loop stability of the proposed MPC formulation are proven for a sufficiently small sampling period when Lyapunov‐based constraints are incorporated. Using a chemical process example with an economic model predictive controller (EMPC), the selection of appropriate constraints for the proposed method is demonstrated. The example verified the incorporation of the stiction dynamics and actuation magnitude constraints in the EMPC causes it to select set‐points that the valve output can reach and causes the operating constraints to be met. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2004–2023, 2016

The Repulsive Casimir Force with Metallic Ellipsoid Structure

We propose a new structure, one plate with a hole above the ellipsoid and the other plate with a hole below the ellipsoid, to obtain a repulsive Casimir force. The force was obtained numerically by using the in-house FDTD method, based on Maxwell’s stress tensor and harmonic expansion. The code can be verified by calculating the force of a perfect-metal ellipsoid centered above a perfect-metal plate with a hole. Our numerical method can effectively simulate the Casimir force by reducing the total simulated time. The further numerical results of realistic dielectric material immersing in fluids or adding other plates above the ellipsoid are also presented. It is not surprising to find that the larger differences can be achieved by varying the parameters such as the center-center separation, medium immersed, and the dielectric material of the structure. Thus, it is possible to tune these parameters relatively in the realistic microelectromechanical systems to overcome stiction and friction problems.

Regularized model of post-touchdown configurations in electrostatic MEMS: bistability analysis

This paper considers the problem of stiction in electrostatic–elastic deflections whereby elastic surfaces adhere to one another after coming into physical contact under attracting Coulomb interactions. This phenomenon is studied in a family of recently derived models which account for forces which become important when the elastic surfaces are in close proximity. The presence of bistability in these models results in hysteresis, or nonreversibility, which accounts for the difficulty in achieving separation after an initial contact event. We use singular perturbation techniques to derive explicit formula for the critical parameters over which bistability occurs and discuss new operational modes which arise when bistability is present.

Comparative Study of Perforated RF MEMS Switch

Abstract MEMS are the Micro Electronic mechanical system or in general terms it is also known as Micro electronic mechanical switch. MEMS have classified in two types of switch that is series switch and shunt switch .The cantilever is a series type switch whereas the fixed- fixed beam is shunt type switch. Fixed- Fixed beam is the element that is fixed at both anchor ends. The electrostatic actuation process occurs on the switch due to which switch deflects from its original position. The stiction problem occurs in MEMS switches which have been reduced by the proposed design. The perforation is used to reduce the squeeze film damping by decreasing the mass of the switch. As the voltage increases the switch moves to downward z-direction .The displacement is produced in the switch as direction of movement is towards negative z-axis. When the beam contacts with electrode, pull in voltage is achieved. This paper explores the perforation and meander concept with Fixed -fixed switch, which increases the flexibility, low actuation voltage and switching speed. The various types of perforations provide discrete displacement corresponding to voltage. In this paper we represent the design and simulation of Fixed-Fixed switch using perforation of size 2 μm-5 μm. The electrostatic actuation mechanism is applied on the Fixed-fixed switch which has a serpentine meanders and perforation at different voltages. The switch is designed and simulated by using COMSOL®MULTIPHYSICS 4.3b software.