Microelectromechanical Systems Packaging Research Articles

Comparative annealing effect on bonded wafers in air and ultrahigh vacuum for microelectromechanical systems/microfluidics packaging

The fundamentals of room temperature bonding methods-surface activated bonding (SAB) and sequentially plasma-activated bonding (SPAB)-are reviewed with applications for packaging of microelectromechanical systems (MEMS) and microfluidic devices. The room temperature bonding strength of the silicon/silicon interface in the SAB and SPAB is as high as that of the hydrophilic bonding method, which requires annealing as high as 1000°C to achieve covalent bonding. After heating, voids are not observed and bonding strengths are not changed in the SAB. In the SPAB, interfacial voids are increased and decreased the bonding strength. Water rearrangement such as absorption and desorption across the bonded interface is found below 225°C. While voids are not significant up to 400°C, a considerable amount of thermal voids above 600°C is found due to viscous flow of oxides. Before heating, interfacial amorphous layers are observed both in the SAB (8.3 nm) and SPAB (4.8 nm), but after heating these disappear and enlarge in the SAB and SPAB, respectively. This enlarged amorphous layer is SiO2, which is due to the oxidation of silicon/silicon interface after sequential heating. The bonding strength, sealing, and chemical performances of the interfaces meet the requirements for MEMS and microfluidics applications.

Study on localized induction heating for wafer level packaging

Micro-electro-mechanical systems (MEMS) are being developed as a new multi-disciplinary technology, which will undoubtedly have a revolutionary impact on the future of human life. However, with the development of MEMS technology, the packaging has become the main technical obstacle to the commercialization of MEMS. An approach to MEMS packaging by high-frequency electromagnetic induction heating at wafer level is presented in terms of numerical simulation and experimental study. The structure of inductor is firstly designed and optimized. Then the heating situation of PCB board is verified. The results indicate that the heat impact on the chip during the packaging process can be effectively reduced by local induction heating packaging, therefore the thermal stress on the chip is considerably lowered. This method can effectively improve the reliability of the MEMS devices.

Reliability Verification of Hermetic Package With Nanoliter Cavity for RF-Micro Device

With the advance of high-performance and small-size microelectromechanical systems (MEMS) devices, wafer-level packaging has gained increased attention over the past few years. Most MEMS packages must protect the often-fragile mechanical structures against the environment and provide the interface for the interaction with the next level in the packaging hierarchy. It is obvious that stable performance and high reliability are essential requirements of a packaged device. In this paper, a novel hermetic package, called the WL-¿P, recently developed for radio-frequency (RF)-filter and RF-duplexer, will be reviewed in terms of its construction, fabrication process, and electrical/mechanical performance. The package consists of a device wafer for a MEMS device and a cap wafer that has a micromachined cavity and through-wafer vias for electrical connections. The cap and device wafers are bonded to each other through a closed square loop of gold/tin eutectic solder at the peripheral edge. The via-in-cavity structure is designed in the cap substrate, with vertical via holes fabricated and fully electroplated with copper. The detailed design and fabrication technology of this new type of hermetically sealed package are presented with process flow. The performance evaluation and reliability results of a hermetic package will also be presented. The developed wafer-level hermetic package technology is able to fulfill today's requirements for hermetic and cost-effective packaging of high-speed RF-MEMS applications.

A Method for Wafer-Scale Encapsulation of Large Lateral Deflection MEMS Devices

Packaging of microelectromechanical systems (MEMS) is a critical step in the transition from development to commercialized product. This paper presents a thin-film encapsulation process that allows varying trench widths suitable for MEMS devices with lateral deflections as large as 20 ?m. The process involves the deposition and planarization of a sacrificial-oxide layer of up to 23 ?m thick, the deposition of a 20 ?m epitaxial-silicon sealing cap, the release of structures using hydrofluoric acid (HF) vapor, and the sealing of the structure at low pressure. Devices produced using this encapsulation method are capable of surviving standard backend processes such as wafer singulation and wire bonding. Among the numerous types of devices encapsulated, two different types of silicon MEMS resonators were fabricated. These functioning resonators demonstrate the ability of the process to successfully encapsulate devices, taking advantage of both large and small trench widths. Such a generalized fabrication platform greatly expands the possibilities of the wafer-scale encapsulation to numerous MEMS devices and retains the robustness necessary for backend processing.

Design and control of a high-acceleration precision positioning system using a novel flexible decoupling mechanism

This article presents a high-acceleration precision positioning system to improve the efficiency and precision of micro-electro-mechanical system (MEMS) packaging. The positioning system is direct driven by linear voice coil actuators (LVCAs), and the mass and inertia of the motion system are significantly reduced by using a novel flexible decoupling mechanism based on flexure hinges. The decoupling principle of the mechanism is introduced, and the flexure hinges are designed by using an analytical method. Through dynamic analysis, the stiffness and preload of the spring are determined. With the aid of finite-element method and dynamic analysis, the influences of flexure hinges on the characteristics of the positioning system are investigated. Based on the dynamic characteristics of LVCA and the developed mechanism structure, the electromechanical coupled models are established. Minimum deadbeat response control strategy with a pre-filter is implemented to control the positioning system, and the parameters of the controller are optimized. Experimental tests are carried out to examine the characteristics of the positioning system. The results show that the positioning system provides good performances and satisfies the requirements of MEMS packaging operations.

NEG thin films for under controlled atmosphere MEMS packaging

Non-evaporable getter (NEG) thin films for residual gases control in micro-electro-mechanical systems (MEMS) cavities after sealing are considered. For this study, several getters are deposited and analyzed in terms of sorption performances, microstructure and activation mechanism. Compared to single layer evaporated titanium reference, the addition of a sub-layer is shown to decrease the activation temperature of the NEG. Effective pumping of nitrogen is monitored by Residual Gas Analysis (RGA). Microstructure investigation by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) reveals no apparent modification in NEG morphology due to sub-layer addition. X-ray Photoelectron Spectroscopy (XPS) and depth profiling monitoring of NEG activation under high vacuum conditions (10−8 mbar) shows the dissolution of the NEG surface oxide in the getter bulk, along with the formation of carbide and nitride compounds. These results are in good agreement with already published results on getter alloys.

Wafer-Level Hermetic Bonding Using Sn/In and Cu/Ti/Au Metallization

Low-temperature hermetic wafer bonding using In/Sn interlayer and Cu/Ti/Au metallization was investigated for microelectromechanical systems packaging application. In this case, the thin Ti layer was used as a buffer layer to prevent the diffusion between solder interlayer and Cu after deposition and to save more solders for diffusion bonding process. Bonding was performed in a wafer bonder at 180 and 150degC for 20 min with a pressure of 5.5 MPa. It was found that bonding at 180degC voids free seal joints composed of high-temperature intermetallic compounds were obtained with good hermeticity. However, with bonding at 150degC, voids were generated along the seal joint, which caused poor hermeticity compared with that bonded at 180degC. After four types of reliability tests-pressure cooker test, high humidity storage, high-temperature storage, and temperature cycling test-dies bonded at 180degC showed good reliability properties evidenced by hermeticity test and shear tests. Results presented here prove that high-yield and low-temperature hermetic bonding using Sn/In/Cu metallization with thin Ti buffer layer can be achieved.

<italic>In situ</italic> measurement of gas diffusion properties of polymeric seals used in MEMS packages by optical gas leak testing

A novel inverse approach is proposed for in situ measurement of gas diffusion properties of polymeric seals used in microelectromechanical systems (MEMS) packages. The cavity pressure evolution of a polymer-sealed MEMS package subjected to a constant bombing pressure is documented as a function of time using classical interferometry, and the diffusion properties of the polymeric seal are subsequently determined from the measured pressure history. A comprehensive numerical procedure for the inverse analysis is established considering three diffusion regimes that characterize the leak behavior through a polymeric seal. The method is implemented to determine the helium diffusivity and solubility of a polymeric seal.

Review of vacuum packaging and maintenance of MEMS and the use of getters therein

Key types of microelectromechanical systems (MEMS) sensors need vacuum or other controlled ambients in order to operate properly. Examples include MEMS infrared sensors and sensitive MEMS gyros. Steps to attain and maintain the vacuum ambient include: (i) Proper processing to reduce trapped gasses in the package, (ii) hermetically sealing the package, and (iii) providing a means to pump away gasses that outgas into the package. Proper package processing and getter technology are key to success in this endeavor. Although many aspects of package processing and assembly are proprietary, the key items to control include leaks (i.e., obtaining a hermetic package) and outgassing from materials within the vacuum cavity. Once gas loads are minimized as much as possible, the actual service life of a package depends on pumping away any internal gases built up through outgassing, leaks, or permeation. This pumping is done by a class of materials called getters. Gettering technology is discussed in relation to vacuum-packaged MEMS/MOEMS. With proper materials and processes, controlled ambients to include vacuum can be obtained in MEMS/MOEMS packages. Not only can they be obtained, they can be maintained for the desired system lifetime.

A Microfabricated Planar Electrospray Array Ionic Liquid Ion Source With Integrated Extractor

This paper reports the design, fabrication, and experimental characterization of a fully microfabricated planar array of externally fed electrospray emitters that produces heavy molecular ions from the ionic liquids EMI-BF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and EMI-Im. The microelectromechanical systems (MEMS) electrospray array is composed of the following two microfabricated parts: 1) an emitter die with as many as 502 emitters in 1.13 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 2) an extractor component that provides assembly alignment, electrical insulation, and a common bias voltage to the emitter array. The devices were created using Pyrex and silicon substrates, as well as microfabrication techniques such as deep reactive ion etching, low-temperature fusion bonding, and anodic bonding. The emitters are coated with black silicon, which acts as a wicking material for transporting the liquid to the emitter tips. The extractor electrode uses a 3-D MEMS packaging technology that allows hand assembly of the two components with micrometer-level precision. Experimental characterization of the MEMS electrospray array includes current-voltage characteristics, time-of-flight mass spectrometry, beam divergence, and imprints on a collector. The data show that with both ionic liquids and in both polarities, the electrospray array works in the pure ionic regime, emitting ions with as little as 500 V of bias voltage. The data suggest that the MEMS electrospray array ion source could be used in applications such as coating, printing, etching, and nanosatellite propulsion.

Hermeticity Evaluation of Polymer-Sealed MEMS Packages by Gas Diffusion Analysis

A gas transport mechanism is studied to characterize the hermetic behavior of polymer-sealed microelectromechanical systems packages. Diffusion-based governing equations, which are fundamentally different from the conduction-based governing equations used for metallic seals, are proposed to predict a change in cavity pressure. An effective numerical scheme is developed to implement the governing equations. The validity of the governing equations is corroborated by the optical leak test. The verified gas diffusion model is utilized to investigate the effect of the diffusion properties and geometries of polymeric seals on the gas leak behavior.

A Cost-Effective MEMS Cavity Packaging Technology for Mass Production

Cost effective microelectromechanical system (MEMS) packaging methods have been required, because the cost portion of the MEMS package is more than 80% of the manufacturing cost of a MEMS device. For this reason, cost-effective MEMS packaging is proposed in this paper for mass production using copper (Cu) lead frames (L/F) as a preplated frame (PPF). Package types include an epoxy molding compound (EMC) cavity wall and an on-frame type. The EMC-cavity package consists of a substrate, a cavity wall and a flat lid on top of the cavity. The on-frame package has a folded lid without a cavity wall. Finite element method (FEM) numerical modeling is performed to anticipate the mechanical warpage and stress of the packages. Assembled MEMS cavity packages were tested for wire pulling, lid pulling, hermetic test, and reliability tests in order to prove the feasibility of this packaging. The wire bonding strength was improved by 40% using plasma cleaning before wire bonding. Through a lid pulling test, a lid bonding strength of 2.40 kgf on average was obtained using an epoxy adhesive. Finally, all samples of the packages passed the reliability tests of the TC, HAST, and HTST, standardized by Joint Electron Device Engineering Council (JEDEC). Also, this cavity package showed excellent hermeticity through leak tests.

Flexible Chip-Scale Package and Interconnect for Implantable MEMS Movable Microelectrodes for the Brain

We report here a novel approach called MEMS microflex interconnect (MMFI) technology for packaging a new generation of Bio-MEMS devices that involve movable microelectrodes implanted in brain tissue. MMFI addresses the need for (i) operating space for movable parts and (ii) flexible interconnects for mechanical isolation. We fabricated a thin polyimide substrate with embedded bond-pads, vias, and conducting traces for the interconnect with a backside dry etch, so that the flexible substrate can act as a thin-film cap for the MEMS package. A double gold stud bump rivet bonding mechanism was used to form electrical connections to the chip and also to provide a spacing of approximately 15-20 µm for the movable parts. The MMFI approach achieved a chip scale package (CSP) that is lightweight, biocompatible, having flexible interconnects, without an underfill. Reliability tests demonstrated minimal increases of 0.35 mΩ, 0.23 mΩ and 0.15 mΩ in mean contact resistances under high humidity, thermal cycling, and thermal shock conditions respectively. High temperature tests resulted in an increase in resistance of > 90 mΩ when aluminum bond pads were used, but an increase of ~ 4.2 mΩ with gold bond pads. The mean-time-to-failure (MTTF) was estimated to be at least one year under physiological conditions. We conclude that MMFI technology is a feasible and reliable approach for packaging and interconnecting Bio-MEMS devices.

Improved MEMS structure for stress-free flip-chip packaging

We present a new method for stress-free microelectromechanical systems (MEMS) flip-chip packaging. Residual stress, which is mainly generated during the reflow process, is a notorious problem in flip-chip packaging. The residual stress reduces the bump fatigue life and device performance owing to the deformation. Underfill encapsulation is a common way to reduce the residual stress in a flip-chip integrated circuit (IC) packaging process. However, it cannot be applied to MEMS packaging because MEMS devices usually include moving microstructures. We intend to resolve this problem by improving the MEMS structure, designing spring beams to introduce electrical pads. The residual stress, which is caused by the mismatch of the coefficient of thermal expansion (CTE) between the MEMS device and the package substrate, can be absorbed through the deflection of spring beams. By using this idea, a quartz MEMS-based capacitive tilt sensor, which was bulk etched and composed of vertical comb electrodes in wafer thickness for achieving large initial capacitance, was successfully packaged. A high melting point alloy Au80Sn20 was used as the solder joint material. The thermal cycling test and sensitivity evaluation experimental results demonstrated the effectiveness of the proposed method.

Pirani Vacuum Gauges Using Silicon-on-Glass and Dissolved-Wafer Processes for the Characterization of MEMS Vacuum Packaging

This paper presents the design and implementation of Pirani vacuum gauges for the characterization of vacuum packaging of microelectromechanical systems (MEMS). Various Pirani vacuum gauges are fabricated with two different standard in-house fabrication processes, namely the silicon-on-glass (SOG) process and dissolved-wafer process (DWP). The Pirani gauges utilize meander-shaped suspended silicon coils as the heaters and two isolated silicon islands in the close proximity of the heater that function as dual-heat sinks to enhance the sensitivity and dynamic range as compared to a microbridge with a single heat sink. The gauges are designed to occupy an area of 4 mm × 1.5 mm. The DWP Pirani gauge fabricated with a structural thickness of 14 mum and a gap of 2 mum shows a measured sensitivity of 4.2×10⁴ (K/W)/Torr in a dynamic range of 10-2000 mTorr. The SOG Pirani gauge fabricated with a structural thickness of 100 mum and a gap of 3 mum shows a lower measured sensitivity of 3.8×10³ (K/W)/Torr in a dynamic range of 50-5000 mTorr; however, the 100 mum-thick structural layer results in a much more robust process against stress-based deformations in suspended silicon compared to the DWP Pirani gauges. Each gauge is used to monitor the pressure of a different packaging approach. The DWP Pirani gauge is used to detect the pressure of a wafer-level vacuum package, where the pressure inside the cavity is measured to be about 2.4 mTorr. The SOG Pirani gauge is used the monitor the pressure inside a hybrid platform package which is vacuum-sealed using a projection welder, where the pressure is measured to be about 1400 mTorr. These measurements verify that the DWP and SOG Pirani gauges can be used for the characterization of wafer-level or hybrid platform vacuum packages for MEMS devices.

System on Wafer: A New Silicon Concept in SiP

System integration is clearly a driving force for innovation in packaging. The need for miniaturization has led to new architectures that combine disparate technologies and materials. Today several different approaches have been developed. These include technologies like system in package. In this way, a new concept for heterogeneous integration is currently being developed at CEA-LETI and is called system on wafer (SoW). This concept is based on a chip to wafer approach. Every component is achieved by using wafer-level technologies, and the final system is performed by single component mounting on a silicon substrate. The main strength of this approach is to use silicon as a substrate for components and for basic support. To perform the SoW, a generic technological toolbox is needed. This includes every standard packaging technology such as flip chip, signal rerouting, and passive component integration as well as new advanced technologies such as microelectromechanical systems packaging, advanced interconnections, energy source integration, integrated cooling, or silicon through vias. In this paper, the SoW concept will be presented and the generic toolbox for SoW achievement will be described.

Study of Low-Temperature Thermocompression Bonding in Ag-In Solder for Packaging Applications

Low-temperature solders have wide applications in integrated circuits and micro-electromechanical systems packaging. In this article, a study on Ag-In solder for chip-to-chip thermocompression bonding was carried out. The resulting joint consists of AgIn2 and Ag9In4 phases, with the latter phase having a melting temperature higher than 400°C. Complete consumption of In solder into a Ag-rich intermetallic compound is achieved by applying a bond pressure of 1.4 MPa at 180°C for 40 min. We also observe that the bonding pressure effect enables a Ag-rich phase to be formed within a shorter bonding duration (10 min) at a higher pressure of 1.6 MPa. Finally, prolonged aging leads to the formation of the final phase of Ag9In4 in the bonded joints.

Bump-less Wafer Level Bonding Experiment for Vertical Integration MEMS Device

In the future, it is desired that high performance sensing module in small volume is realized. However, conventional MEMS (Micro Electro Mechanical Systems) packaging technology is becoming difficult to meet its desire (Especially Bulk MEMS). Small and package-less 3D sensing module, which consist of MEMS and IC chips bonded by SAB (Surface Activated Bonding) solves this problem (Fig. 1). But we have many technological opportunities in small and package-less 3D sensing module; Low damage bonding, electrical contact, and bonding alignment. This paper reports 'Bump-less wafer level bonding' which is needed for vertical integration MEMS device and the 3 wafer bonding experiment results of bonding strength, electrical contact, and alignment.

An Investigation Into the Effect of the PCB Motion on the Dynamic Response of MEMS Devices Under Mechanical Shock Loads

We present an investigation into the effect of the motion of a printed circuit board (PCB) on the response of a microelectromechanical system (MEMS) device to shock loads. A two-degrees-of-freedom model is used to model the motion of the PCB and the microstructure, which can be a beam or a plate. The mechanical shock is represented as a single point force impacting the PCB. The effects of the fundamental natural frequency of the PCB, damping, shock pulse duration, electrostatic force, and their interactions are investigated. It is found that neglecting the PCB effect on the modeling of MEMS under shock loads can lead to erroneous predictions of the microstructure motion. Further, contradictory to what is mentioned in literature that a PCB, as a worst-case scenario, transfers the shock pulse to the microstructure without significantly altering its shape or intensity, we show that a poor design of the PCB or the MEMS package may result in severe amplification of the shock effect. This amplification can cause early pull-in instability for MEMS devices employing electrostatic forces.

Assessment of testing methodologies for thin-film vacuum MEMS packages

In this paper, a summary of the most relevant failure mechanisms of thin-film vacuum microelectromechanical systems (MEMS) packages and existing testing techniques will be presented. Then, based on analytical models for thin-film vacuum MEMS packages (volume in the order of 10E−11 l), a feasibility study on options for thin-film vacuum MEMS package testing will be presented. This feasibility study leads to new insights and suggestions for future thin-film vacuum MEMS package testing.