MEMS microphones are extensively used in many applications that require reliability, small size, and high sound quality. For harsh environment reliability data MEMS microphones need to be monitored under conditions mimicking their areas of applications. MEMS microphones have an opening/sound port in order to interact with the environment, therefore cannot be sealed completely since the sensing mechanism requires interaction between sound waves and the sensing element. Little to no information exists on reliability data for MEMS microphones under low/high temperature operating life and temperature humidity bias condition. Our work is primarily focused on providing harsh environmental reliability data which can be useful to MEMS designers and engineers. In this paper the test vehicles with MEMS Microphones have been tested under three different harsh environmental conditions: high temperature operating life (HTOL) at 125°C at 3.3V, low temperature storage (LTS) at −35°C and temperature humidity 85°C/85%RH at 3.3V. The main motive of this study is to document the incremental shift and degradation in output parameters namely distortion, frequency response, power supply rejection capability of IC, frequency vs pressure characteristics and analog output voltage of the MEMS microphone. The survivability of MEMS microphone, ADMP401, has been demonstrated as a function of change in the output parameters. Failure analysis has been conducted on the microphone samples to study failure modes and sites using analytical methods such as SEM, EDS and X-ray.

A key component of the MEMS cryogenic cooling systems we are developing is a MEMS compressor. Its function is to compress the refrigerants used in the cooling cycle. Layers of polyimide are stacked and patterned on a silicon wafer to create micro check-valves and a compression chamber over which a diaphragm is suspended. To achieve the high (4:1) pressure ratio needed for the refrigeration cycle, the polyimide diaphragm needs to be fabricated with minimal dead volume beneath it, hence the need for a sacrificial layer with thickness of 100–300 nm. The topography created by the check-valves and valve-seats makes atomic layer deposition (ALD) ideal due to its conformality. Furthermore, following sacrificial layer release, the inside of the compression chamber will also need to be coated with a hermetic moisture barrier layer to enable the device to operate at 4 atmospheres without leaking. ALD is therefore also ideal for the final internal coating because it does not require line-of-sight. Towards this end, we demonstrate here the concept of using ALD TiO 2 as a sacrificial layer to create a 5 mm × 5 mm × 20 m-thick polyimide membrane, suspended by ∼ 100–350 nm above a silicon wafer, followed by a second thinner ALD coating of the interior surfaces bounded by wafer and membrane. The air gap under the membrane, defined by the released sacrificial layer, was measured at about 130–370 nm using two independent methods: reflectometry, and FIB cross sectioning followed by SEM imaging of the air gap’s cross section. The membrane was removed from one chip and the thickness of the internal coating on the underlying silicon was measured with the reflectometer to be about 40 nm. We thus demonstrate the use of ALD TiO 2 as both a sacrificial layer for fabricating nanoscale gaps, as well as for coating nanoscale internal cavities and channels.

The high current density induces electromigration (EM) in metal lines used for electric wirings in integrated circuits. The growth of voids formed by EM in the line material leads to the line failure. Recently, multilevel interconnections are widely used in the circuit in electronics devices and MEMS. Metal lines aligned on upper and lower layer are connecting through the vias in the multilevel interconnections. The reservoir structure is often constructed in the line structure to prevent the EM damages. There is a threshold current density relating to the EM damage of the lines in the interconnection with vias. It is important to evaluate the threshold value for determination of an allowable electric current of the line. In this study, a numerical simulation technique for analyzing the atomic density distributions in the line material under high current density was used to evaluate the EM risks of metal lines in the several cases of interconnect tree structure with reservoir. The thresholds of current density leading to EM damage were calculated in the simulations considering the reservoir locations and pattern of electric current flow in the tree.

A novel interconnect approach was developed to enable packaging of miniature (<0.5 mm) MEMS pressure sensors for minimally invasive sensing applications. The interconnect approach is a variant of traditional flexible laminate circuits but does not require expensive patterning steps. It provides tight tolerance and assembly automation at relatively lower costs. In this paper, details of the novel interconnect and cabling approaches are disclosed. Examples of applications that benefit from this approach are also discussed. Experimental results on new ultra-miniature sensors utilizing the developed interconnect approach are presented.

Thinner printed wiring assemblies (PWA) and smaller clearances are driven by the continuing increase of functionality and miniaturization in portable electronic devices. The probability of secondary impact during accidental drop and impact between a circuit card and adjacent components increases with the decrease in the size and weight of the product. In particular, compared to the initial impact, impulses caused by contact during secondary impacts are typically characterized by significant increase of amplitudes and extremely short pulse widths. As a result, stress wave transmission and reflection in printed wiring boards (PWBs) can be at a frequency range close to the resonant frequencies of PWA components with miniature internal structures, such as MEMS. This study focuses on analyzing the high frequency content of the accelerations due to stress wave propagation, reflections and dispersions in the thickness direction of multilayered PWBs, caused by secondary impact, and on the consequential effects on typical failure modes with high resonant frequencies.

Microelectromechanical system (MEMS) packages are vulnerable to stresses due to its functional structure. During the assembly process of the package, stresses stemming out of CTE mismatches of the structural elements and curing of the die attach material can cause warpage of the MEMS die [1]. Even though die attach material takes relatively small volumetric portion of the package, it plays a critical role in warpage of the die due to its location and sensitivity of a MEMS sensor. Most of virgin die attach adhesives are in a state of viscous liquid and, as it is cured the material properties such as modulus and CTE change. Accordingly, residual strain is cumulated on MEMS die after curing process and signal trim process is required. Therefore, the material properties changes depending on the curing profile is valuable information for assembly process of the MEMS package. To monitor the material properties changes and shrinkage during curing process, strain and modulus of a die attach material are measured in each curing step. Also, to investigate the material property change depending on the curing profile, two different curing profiles are used. Experimental data show that die attach materials are gradually cured after each thermal cycling, which cause the increment of the modulus and glass transition temperature (T g ) with shrinkage at elevated temperature. Using the measurement data, FEA model is built to predict the warpage of the MEMS die. In the FEA model, residual strain on MEMS die is calculated by inputting material properties of die attach in each curing step. Also, die warpage of the package during the curing process is monitored using an optical profiler for the validation of the simulation results.

Polymeric die-attach adhesives have been widely utilized for attaching the sensor die to the lead-frame substrate in the packaging of micro-electro-mechanical systems (MEMS). Although much study has been performed regarding viscoelastic behaviors of packaging materials, less literature has addressed the issue of post-cure shrinkage, which occurs in isothermal storage and in accelerated tests. However, the influence of post-cure shrinkage cannot be ignored in characterizing long-term viscoelastic properties or evaluating long-term reliability for highly sensitive automotive sensors. In this study, a non-contact digital image correlation (DIC) method was used to characterize the post-cure aging shrinkage on a commercial epoxy-based die attach adhesive. The shrinkage strain of bulk specimens was monitored over a period of isothermal aging. Analysis indicates that the process of postcure shrinkage can be expressed in terms of exponential time functions. Using hygroscopic shrinking as an analogous process, this aging was introduced into a time-dependent finite element (FE) simulation in ANSYS ® 15.0. FE case studies combining both stress and dimensional relaxation indicate that certain differences can occur in the long-term finite element analysis if the shrinkage strain is ignored.

Reliability data on MEMS accelerometers operating in harsh environments is scarce. Micro-electro-mechanical systems (MEMS) are used in a variety of military and automotive applications for sensing acceleration, translation, rotation, pressure and sound. This research work focuses on dual axis MEMS accelerometer reliability in harsh environments. Structurally an accelerometer behaves like a damped mass on a spring. Commercially there are three types of accelerometers namely piezoelectric, piezoresistive and capacitive depending on the components that go into the fabrication of the MEMS device. Previously, majority of concentration was focused on an effective internal design, performance enhancement of CMOS-MEMS accelerometers and packaging techniques Cheng [2002], Qiao [2009], Lou [2005], and Weigold [2001]. Studies have also been conducted to obtain an enhanced inertial mass SOI MEMS process using a high sensitivity accelerometer Jianbing [2013], Chen [2005]. There have been prior test(s) conducted on MEMS accelerometers, Jiang [2004], Cao [2011], Chun-Sun [2009], Lou [2009], Tanner [2000] and Yang [2010] but the availability of data on reliability degradation of such devices in harsh environments Brown [2003] is almost little to none which thereby generates the importance of this work and also makes way for a whole new path involving the reliability assessment techniques for MEMS devices. Concentration of our work is primarily on the reliability of this accelerometer upon sequential exposure to harsh environment(s) and drop-shock. Reliability of accelerometers in high G environments is unknown. The effects of these pre-conditions along with the drop test condition has been studied and analyzed. In this piece of research work, a test vehicle with a MEMS accelerometer, ADXL278 dual axis capacitive accelerometer, has been tested under high/low temperature exposure followed by subjection to high-g and low-g shock loading environments. The test boards have been subjected to mechanical shocks using the method 2002.5, condition G, under the standard MIL-STD-883H test. The stress environment and the test condition used for this paper are 1500g and 70g respectively where 70g is the full scale range output of ADXL278 in the drop direction with pulse duration set to 0.5millisecond. The deterioration of the accelerometer output has been characterized using the techniques of Mahalanobis distance and Confidence intervals. Scanning Electron Microscopy (SEM) has been used to study the different failure modes inside of the accelerometer, which were potted and polished and later de-capped. Furthermore, the non-destructive evaluations of the MEMS accelerometer have been demonstrated through X-rays and micro-CT scans.

MEMS accelerometers have found applications in harsh environments with pressure, temperatures above ambient conditions, high g shock and vibrations. The complex structure of these MEMS devices has made it difficult to understand the failure modes and failure mechanisms of present day MEMS accelerometers. Little work has been done by the researchers in investigating the high g reliability of the MEMS accelerometers by continuous high g drops and quantifying the failure modes. There is little literature addressing the multiphysics finite element modelling of MEMS accelerometers subjected to high g shocks. In defense applications, where these devices are integrated with several other compactly assembled subsystems, lack of knowledge on the physics of failure for the MEMS sensor in harsh environment operation, can be detrimental to the success of the system on the whole. Being able to successfully model inside of an accelerometer, enables the user to better understand the change in parameters like time delay induced in response of successive drops, change in pulse width that indicate failure, reduction in sensed g levels. Some researchers have subjected various accelerometers to repeated drops at their maximum sensing g(not high g) level, and used optical microscopy to detect damaged sensing elements [Beliveau, 1999]. Few researchers have modeled the internal structure of the MEMS device, along with the device packaging under the stresses of operation [Fang 2004, Ghisi 2008, Xiong 2008]. In this paper, a multiphysics model of capacitive and the moving elements of the accelerometer has been developed to model the change in capacitance with respect to stroke and understand the correlation with g-levels, in addition to the transient dynamic response of the accelerometer under high-g shock. This has not been much explored in the past. The accelerometer studied in the paper is the ADXL193, and subjected to repeated drops of 3000g in each 3 axes as per 2002.4 of MIL-STD-883 without preconditioning. A characteristic graph of capacitance vs accelerometer stroke has been obtained from a series of electrostatic simulations and is then used to relate g levels, capacitance, stroke deflection and voltage change using electromechanical transducer elements. The drift in the performance characteristics of the accelerometer have been measured versus the number of shock events. In addition, an attempt has been made to investigate the failure mode in the accelerometer.

The level of stress in silicon as a result of applying Cu-Sn SLID wafer level bonding to hermetically encapsulate a high-performance infrared bolometer device was studied. Transistors are present in the read out integrated circuit (ROIC) of the device and some are located below the bond frame. Test vehicles were assembled using Cu-Sn SLID bonding and micro-Raman spectroscopy was applied on cross sectioned samples to measure stress in the silicon near the bond frame. The test vehicles contained cavities and the bulging of the structures was studied using white light interferometry. The test vehicles were thermally stressed to study possible effects of the treatments on the level of stress in the silicon. Finite element modeling was performed to support the understanding of the various observations. The measurements indicated levels of stress in the silicon that can affect transistors in regions up to 15 μm below the bond frame. The observed levels of stress corresponded well with the performed modeling. However, no noticeable effect was found for the ROIC used in this work. The specific technology used for the fabrication of the ROIC of a MEMS device is thus decisive. The level of stress did not appear to change as a result of the imposed thermal stress. The level of stress caused by the bond frame can be expected to stay constant throughout the lifetime of a device.

A new highly sensitive strain measurement method has been developed by applying the strain-induced change of the electronic conductivity of CNTs. It is reported that most multi-walled carbon nanotubes (MWCNTs) show metallic conductivity and they are rather cheap comparing with single-walled carbon nanotubes (SWCNTs). However, it was found that the electric conductivity of MWCNTs changes drastically under uniaxial strain because of the drastic change of their band gap. Therefore, the authors have developed a highly sensitive strain sensor which can detect the local strain distribution by using MWCNTs. In order to design a new sensor using MWCNT, it is very important to control the shape of the MWCNTs under strain. Thus, a method for controlling the shape of the MWCNTs was developed by applying a chemical vapor deposition (CVD) technique. It was found that the shape of the grown MWCNT could be controlled by changing the average thickness of the catalyst and the deposition temperature of the MWCNT. The electrical resistance of the grown MWCNT changed almost linearly with the applied strain, and the maximum strain sensitivity obtained under the application of uniaxial strain was about 10%/1000-μstrain (gauge factor: 100). A two-dimensional strain sensor, which consists of area-arrayed fine bundles of MWCNTs, has been developed by applying MEMS technology. Under the application of compressive strain, the electric resistance was confirmed to increase almost linearly with the applied strain.

Three-dimensional (3D) chip stacking is a promising approach for high performance microsystems. Such vertical integration could increase the clock frequency and reduce the signal delay. During operation, the input signal and the resulting input power change with time. Realistic power maps on active chip tiers produce highly non-uniform heat flux patterns with hotspots, which change with time. In this paper, a transient compact model of a multi-layer chip stack subjected to time dependent power map was developed. Inter-tier single phase microfluidic cooling with a pin fin array was used to enhance heat transfer. The validity of the compact model was confirmed by comparison with full computational fluid dynamics/heat transfer (CFD/HT) modeling. It was found that the transient compact model ran thousand times faster than the CFD/HT model. The maximum deviation between the two models was 1 °C. The compact model was then used to analyze the transient thermal response of a 4 layer dual core stack of Penryn microprocessors when a total power of 172.4 W with a 300 W/cm 2 hot spot was suddenly applied. The hotspot temperature rise approaches 35 °C, at a steady state time of about 0.015 s. The temperature rise of bottom tier was always higher than the other three tiers during the transient process because it has only single side cooling, while other tiers have double sided cooling. Also, different power maps and flow patterns, including parallel and counter flow for different layers were investigated. It was found that controlling the flow in different layers individually instead of applying the same flow conditions could achieve better thermal performance and energy savings.

Wireless handheld communications has identified significant benefits of tuning that include fewer dropped calls, increased battery life and improved user experience. The tuning can be part of the antenna, power amplifier (PA), filtering, or part of a fully integrated radio front end (FE). RF MEMS tunable capacitors have been integrated with 0.18 μm RF HVCMOS to address the need for tuning in wireless communications. These integrated, MEMS tunable capacitors are hermetically encapsulated at the wafer level, but the integrity of the encapsulation must be maintained during BEOL operations. The BEOL operations include shipping and handling, passivation coat and cure, solder bumping (screen printed or electroplated), backside grinding (BSG), dicing, and pick and place. In this paper we will describe, the flip chip packaging of the wafer level encapsulated MEMS devices including finite element analysis. The flip chip packaging of ASIC die is primarily concerned with solder bump reliability during such qualification stresses as temperature cycling and drop testing. The flip chip packaging of a wafer level encapsulated MEMS device has additional concerns that include encapsulation integrity and device package sensitivity. The die thickness, underfill, and encapsulation dimension have been varied to minimize the deflection and stress associated with the encapsulation. The primary failure mode associated with the overstress of the encapsulation is a cracked lid that will lead to the ingress of moisture and a rise in the cavity pressure from to atmospheric conditions. The failure can be detected by an increase in the MEMS switching time and frequency response or by a return to zero failure (RTZ) associated with device stiction. A low modulus and low CTE UF has been implemented for the lowest deflection and stress. The lowest deflection and stress is provided by eliminating the UF, but this is not feasible for the purpose of solder bump reliability. In practice, the MEMS encapsulation is robust to the printed solder bumping process that includes placement and removal of the bump screen and the squeegee of solder past into the solder screen. The MEMS encapsulation is robust to the attachment and removal of BSG tape and the pressures associated with BSG. The final dicing operation has not demonstrated any detrimental impact on the MEMS encapsulation. The final demonstration of success is the assembly of the MEMS tunable capacitor die to a laminate substrate using lead-free solder and underfill.

A number of small electronic devices benefit from micro-scale low temperature operation. Recently we have developed micro cryogenic coolers (MCCs) using a low-pressure, mixed-refrigerant Joule-Thomson cycle. The cryocoolers utilizes a MEMS-enabled gas compressor coupled to a micro cold stage. Two cold stages have been developed: one which uses a fiber-enabled heat exchanger assembled to a micro-machined throttling valve, and another which uses a MEMS-based heat exchanger. A microcompressor has been developed which uses MEMS-based check valves coupled to a membrane, which is actuated with a mechanically amplified piezoelectric amplifier. The compressor measures a volume 15 mL, can generate a pressure ratio of 6:1 and a maximum flow-rate of 60 standard mL/min. The complete cryocooler has reached low temperatures of 177 K, although temperature instability has been an issue, due to 2-phase flow through the micro-channels. This paper will cover the development and testing of the micro cryogenic cooler, as well as an analysis of the micro channel flow. A proper understanding of the micro-channel flow allows us to design refrigerant mixtures to improve the cooling power, and modify the cooler to eliminate temperature instabilities.

RF-MEMS devices area complex systems governed by the interaction of a variety of forces, including electrostatics, solid deformation, fluid damping and contact. The performance and reliability of these devices is strongly dependent on device geometry and composition, and also on material microstructure and related properties. In this paper, we consider multiscale simulation of RF MEMS switched. At the device level, we introduce a comprehensive integrated numerical framework to simulate the major governing physics and their interactions. At the micron scale, we develop a mesoscale contact model to describe the history-dependent force-displacement relationships in terms of the surface roughness, the long-range attractive interaction between the two surfaces, and the repulsive interaction between contacting asperities (including elastic and plastic deformation). The inputs to this model are obtained from atomic level simulations and nanoscale surface topography characterization. The mesoscale contact model is integrated in the device-level simulation to predict the pull-in and pull-out behavior of these switches. The uncertainties associated with the simulation are quantified and propagated using a non-intrusive collocation method based on generalized Polynomial Chaos (gPC) expansions. With such a framework, we are able to predict the PDFs of pull-in and pull-out voltage, identify the critical factors that have the most influence on the quantities of interest, and therefore guide resource allocation and risk-informed decision-making.

Microphone is a critical component for seamless communication converting an acoustic signal (vocal) to an electrical signal. Traditionally Electrets Condenser Microphones (ECM) have been the primary proponent of audio component in many consumer products. With functionally rich consumer devices (example smart phones, etc) there is a growing trend to look at components with higher functionality but a smaller form factor. Microelectronic Mechanical Systems (MEMS) microphone is seen as a possible replacement to ECM due to its significant reduction in form fit with additional functionality. The paper is an effort to illustrate steps that can be considered while designing MEMS microphone in a system. This includes Design considerations, Reliability tests, Manufacturing challenges and Readiness to ensure higher yield during the final assembly. Manufacturing issues (Top 5) and guideline presented in the paper are not just to increase the assembly yield (system level), but also to increase an awareness upfront to the design phase to help create a robust system/product.

Ultrasonic flip chip bonding is one of the widely used methods in semiconductor chip or microsystem packaging and ultrasonic (US) bonding tool is important part for the bonding machine. To perform the proper operation of US bonding, the adequate vibration frequency and mode of US tool is required and the vibration design of the tool is very important. Until recent days, however, the most of practical aspect of the tool design follows the trial-and-error approach. In this study, we introduce the method of topology optimization for US bonding tools. The solid isotropic material with penalization (SIMP) method is used to formulate topology optimization and optimal criteria (OC) method is introduced for the update scheme. The objective resonance frequency and longitudinal mode is tracked using Modal Assurance Criterion (MAC). We compare between 2D and 3D finite element models, and realize two types of US tools which are based on 3D optimization results. To ensure the validity of topology optimization applied to the high frequency and tough devices such as US bonding tools, the vibration displacements at anti-nodal points of the optimized US tools are measured by laser vibrometer.

A new highly sensitive strain measurement method has been developed by applying the change of the electronic conductivity of CNTs. It is reported that most multi-walled carbon nanotubes (MWCNTs) show metallic conductivity and they are rather cheap comparing with single-walled carbon nanotubes (SWCNTs). The effect of the longitudinal axial strain on the band structures of electrons in CNTs was analyzed by applying the abinitio calculation based on the density functional theory. The change of the band structure of a MWCNT under uni-axial strain was analyzed. It was found that the electric conductivity of (MWCNTs) changes drastically because of the large change of their band gap. Therefore, the authors have focused on the possibility of the application of MWCNTs to a highly sensitive strain sensor. Multi-walled CNTs were dispersed in various kinds of resins such as epoxy, polycarbonate, and polyisoprene to form a thin film which can be easily attached to rounded surfaces. The length and diameter of the CNTs were about 5 μm and 50 nm, respectively. One of the base materials of resin employed was polycarbonate and the volumetric concentration of CNT dispersed was about 11.5%. The thickness of the film was about 500 μm. Uni-axial strain was applied to the CNT-dispersed resin by applying a 4 point bending method, and the change of the electric resistance was measured. The range of the applied strain was from −0.025% to 0.025%. The electric resistance changed almost linearly with the applied strain. The ratio of the resistance change under the tensile strain was about 400%/%strain and that under the compressive strain was about 150%/%strain. The CNTs were also dispersed in polyisoprene by about 5%. Uni-axial tesile strain was also applied to the CNT-dispersed rubber. The maximum strain was 240%. It was found that the resistance of the rubber increased monotonically with the increase of the amplitude of the applied strain. The increase rate also increased with the amplitude of the applied strain, and the maximum rate reached about 25%/%strain. Two-dimensional strain fields were evaluated by using finely area-arrayed CNT-dispersed resin made by MEMS technology with spatial resolution of 50 μm.

Reliability and long term stability are the greatest challenges for commercialization of MEMS gyroscopes. Their vast use in different applications that required MEMS gyroscopes to function from medium to harsh environments make necessary to evaluate the performance of MEMS gyroscope under those conditions. This paper focuses on the combined long term effects of temperature and humidity on the performance of MEMS vibratory gyroscope. Performance of the MEMS gyroscope was evaluated over time by conducting temperature humidity bias (THB) test on a COTS (commercial off-the-shelf) single axis MEMS vibratory gyroscope having an operating temperature range from −40°C to +85°C. The gyroscope sensors were exposed to 60°C and 90%RH (Relative Humidity) for 500 hours. Six single axis gyroscopes were tested, three with in-situ device calibration and three without in-situ device calibration. Out of three MEMS vibratory gyroscopes tested without in-situ device calibration, it was observed that samples had minimum and maximum in-situ zero rate output (ZRO) drift of 1.3°/s and 2.2°/s respectively over 500 hours. These drifts were disappeared when gyroscope sensors were tested after six months by keeping at room condition. Other three single axis gyroscopes were tested in the same chamber with in-situ device calibration which didn’t show any major performance ZRO drift.

Solders have been utilized extensively in the MEMS packaging industry to create vacuum or hermetic seals in a variety of applications. MEMS technology is finding applications in wide range of products like pressure sensors, actuators, flow control devices etc. For many harsh low temperature environment applications, like commercial refrigeration systems, MEMS based pressure sensors and flow actuators are directly mounted on to metal substrates using solders to create hermetic sealing. Solders attaching silicon devices directly to metal substrates may be subjected to very high thermal stresses due to significant difference in thermal expansion coefficients during chip operation or environment temperatures. In this paper, case study of a high powered MEMS chip (referred in the paper as die) operating in a commercial refrigeration system is presented. Accelerated test method for qualifying solder joint for high pressure applications is briefly discussed. Lab experiments showing typical refrigeration cycle thermal load on solder joint are presented. Based on the study, concepts of die power toggling and power allocation towards enhancing hermetic solder joint reliability are discussed. Detailed numerical case studies are presented to quantify the improvement in solder joint reliability due to the proposed concept.