Issues in MEMS Testing: A ReveiwMuhammad Shoaib1, Nor Hisham B Hamid2 Department of Electrical & Electronic Engineering, Universiti Teknologi PETRONASBandar Seri Iskandar, 31750 Tronoh, Perak Malaysia1shoaib-shafi@hotmail.com, 2 hishmid@petronas.com.my,

Abstract This paper presents a review about testing of MEMS devices. It highlights the current issues of testing and test time consumption in detail. A comprehensive process flow is presented to tackle the issues. Different proposed test techniques have been discussed to achieve high degree of testing parallelism in order to grab the serious challenges of huge test cost and time consumption.

KeywordsMEMS, ATE, Parallel TestingI. INTRODUCTIONSince the mid of 1970 MEMS (Micro-Electro-Mechanical Systems) have emerged as an innovative technology by creating new opportunities in physical [1], chemical [2] and biological [3] sensor and actuator applications. Although MEMS technology emerges from IC fabrication techniques but test methods [4] of both technologies significantly differ from each other. This is because MEMS devices respond to both electrical and non-electrical (physical, chemical, biological, optical) stimuli. Comment by Aamir F. Malik: MEMS devices are tested at two stages, the production and the packaging stage. This testing is essential to verify the performance metrics of the device, parametrically and functionally. In the 1st stage, MEMS are tested by measuring all AC and DC parameters at wafer level [5, 6] using an ATE (Automatic Test Equipment) [7]. This test phase sorts out the wafer for good and bad die by exploiting Design for Testability circuitries within chip, for instance self test mechanisms and scan chains, similarly to common integrated circuits. Then dicing is performed and good devices are packaged. In the 2nd and final stage, these packaged devices are retested parametrically to confirm their overall functionalities. Table.1 shows the testable parameter of few devices. After the parametric testing, calibration and functional testing is performed to assure the confirmation of the device to mechanical and electrical requirements. Functional testing is essential for every MEMS sensor before proper utilization. Therefore some intrinsic constants or trimming values related to the device working principle are captured during calibration as reported in [8-10]. These trimming values vary from device to device and are stored into the device under test (DUT) by using ad-hoc registers, fuse transistors, trimmed resistors and a flash memory. Functionality of the device is measured by applying a known physical and electrical stimulation and comparing the device output. If the measured values are different from the estimated one the device is considered fail, otherwise it is accepted as a good device. Comprehensive details of infrastructures about Automatic Test Stations and methods for MEMS testing are reported in [11, 12].Comment by Aamir F. Malik: Self-test techniques for several commercial MEMS are reported in [13-15], these are based on BIST (built in self-test) mechanism capable for testing device with electrical stimuli. However, previous MEMS BIST techniques cannot be used as a substitute of traditional manufacturing test. The reason is that due to the variations in device fabrication process, the electrostatic force of self-test mode become different for individual MEMS device and the validation process requires the complete device testing using external test equipment [15]. The BIST methods are used for online, offline and in-field testing. Therefore BIST mechanism increases the time and process of testing because the device cannot perform if the BIST itself isnt tested and verified initially. Consequently, testing become essential for each part of the device using automatic test equipment before shipping it to the market.Comment by Aamir F. Malik: In MEMS manufacturing process, testing has an important role to verify performance and reliability, however this testing process consumes a huge cost [16]. A plethora of work is reported on the cost of MEMS testing process. C. G. Masi and M. F. Cortese [17, 18] reported that 25-35% cost is consumed in overall testing process. According to Texas Instruments [19], in case of testing digital micromirror device, the cost of final test (Packaged device testing) is 14% whereas the cost of wafer level test is 8%. MEPTEC (Microelectronics Packaging and Test Engineering Council) [20] reported, the cost of MEMS testing is 20-45% of the total device manufacturing cost. On the other hand, in MEMS test standard report of MIG (MEMS Industry Group) [21] testing cost at manufacturing level can accede up to 50% due to device complexity and maturity. Recently, V. Henttonen et al. [22] reported more than 60% testing cost of the device, the reason of this huge cost is that testing systems are mostly associated with automotive sensor testing which can be costly to be adopted in as testing systems for ordinary consumer MEMS devices mentioned in [23]. Comment by Aamir F. Malik: Therefore it is utmost important to find a suitable alternate to reduce this huge cost. A part work has been reported on the reduction of cost in final testing. One way of cost reduction technique is to increase the test throughput by testing multiple DUTs (device under test) in parallel [24, 25]. Substantial research efforts have been practiced to reduce test time and cost by examining various aspects of test [26]. These techniques of parallel testing have widely used in VLSI test areas [27]. However these testing techniques are not fully applicable to MEMS because MEMS devices operate under working principle of different domains (electrostatic, electromagnetic, electrothermal, piezoelectric etc.). Comment by Aamir F. Malik: Recently available technique is parallel testing of single domain using electrical and non-electrical stimuli for DUT response [28-31]. In this work, enhancing the parallelism of test system has been tried to reduce the cost. On the other hand, L. Ciganda, et al [29] introduced a technique of parallel testing of different devices i.e. accelerometer and gyroscope in parallel. However the mentioned in the next section is applicable to test the devices which belong to single domain only. This review motivates to come up with a cost effective system for parallel testing of MEMS. The parallel testing mechanism, capable of testing devices using electrical or physical test stimuli can prove to be efficient and cost effective. Literature shows the customized methodologies of testing a variety of MEMS devices [32-39] that are available in the market. Table 1 represents the customized testing techniques for different sensors.Comment by Aamir F. Malik: Table 1: customized test techniques for MEMS.DeviceTesting ParametersTest TechniqueRef

AccelerometerForce, capacitance, displacement, Sensitivityusing the Extended Kalman Filter and pseudo random signal to capture the low and high frequency dynamics of the system,[40]

GyroscopeDisplacement, rotation, Vout,A Novel Vibration Mode Testing Method[41]

Pressure sensorCapacitance, pressure, voltageUsing thermal inducing system (Thermostream TP04100A, Temptronic Corp., Mansfield, MA, USA)[42]

Gas sensorResponse, capacitance, frequency, temperatureScanning electron microscopy (SEM), Energy dispersive spectrometry (EDS)[43]

Microphone[44]

II- MEMS testing in manufacturing processIn the previous research work, a lot of discussions [18, 45-51] have been done to cope with the challenges and issues in MEMS testing and its requirement. MEMS final testing has limited visibility in the literature from industries that have successfully manufactured MEMS devices such as humidity sensors, pressure sensors, magnetic field sensor etc. This this type of trend shoes an indication of custom nature of test for MEMS. According to MIGs METRIC (MEMS Technology Roadmap and Industry Congress) [21] there are no agreed testing standards and that is the major limitation for the industries growth and innovation.Most of literature shows a few test systems that are commercially available [52] but these are limited to test specific commercial devices i.e. accelerometers, gyroscope, pressure sensors, microphones, combo sensor, proximity sensor and magnetic sensor. This section discusses the limitation and issues of test systems reported in literature. An attempt was made by Theresa Maudie, et.al [53] to improve ATE for testing accelerometers in parallel. Variations in electrical parameters were being occurred from device to device in case of producing MEMS devices in bulk. ATE was unable to perform proper calibration of these devices due to lack of modularity and flexibility. It was improved by changing electrical and nonelectrical interfaces of DUT. Moreover these efforts and test system were limited to accelerometers only.Various approaches to test MEMS optical devices and flow sensors were reported by Hans G. Kerkhoff [10]. Packaging influence alters the device specifications due to change in environment. Therefore test system had the issues in handling mechanism of non-electric input for flow sensor. That increased the test time, therefore novel test handlers were developed to reduce time and enhance parallelism. The test systems are also limited to the specific devices. Berny Chen reported an effort of improving traditional ATE in [54]. The technique was able to test only 16-sites in parallel. Therefore QSPI (quad serial port interface) was used to enhance the parallelism. However ATE was confined to test the accelerometers only.A case study based on accelerometers testing was introduced by L. Ciganda et al [28]. In conventional tester, interfaces of DUTs were connected through wires, which were being stretched and twisted due to mechanical movement of test stage. Some DUTs were being suffered through the lack of signal communication during testing process, resulting in raise the test time and reducing the parallelism. Then an enhanced architecture was proposed that was able to implement calibration and testing process by hardware, serial interface module (SIM), reducing the amount of tester internal wires. Their efforts and test system was also restricted to accelerometers only. In [29] L. Ciganda et al. also used the FPGA technique in conventional testing machine of MEMS devices . Lengthy communication distance and wires between tester and DUT interface were the limiting issues in ATE, tending to limit the electrical stimulation frequency, which ultimately slowed down the test process. High testing parallelism was achieved to overcome such limitations using FPGA module. These techniques were being used on commercial gyroscopes and accelerometers.Several MEMS test systems introduced by [55] have the ability to test multiple MEMS applications like accelerometer, gyroscope, pressure sensor, magnetic sensor and microphone. These systems are only capable of testing single domain devices in parallel. However their modification is required to test different domain devices in parallel. C. Schaeffel et al in [56] introduced the design of interferometry test station for parallel testing of optical MEMS. The inspection system has the ability of testing up to 100 DUTs simultaneously. As commercial optical test systems available in the market have ability of testing devices serially, which consume a lot of testing time and costs. Hence they used the concept of parallel measurement of multiple dies to obtain a significant reduction of measurement time. Another technique has been reported by Florian Oesterle et al [57] for a massive parallel test of MEMS microphones. A reconstruction method was adopted by utilizing techniques of tomographic imaging. The specific parameters of all parallel connected DUTs are superimposed through one signal and the DUT response is read-out with the reference of single measurement of device.

Most of efforts are being emphasized on modification of conventional test systems. Final testing of MEMS devices is still in its formative stage, the techniques proposed in [28, 29] are limited to parallel testing of single domain (Electrostatic) MEMS accelerometer and gyroscope. The current test systems [10, 54, 55] utilize both electrical and nonelectrical stimuli for final testing and calibration. But the non-electrical stimuli such as humidity, pressure, motion, and other are intrinsically slower than electrical signals and as a result, it takes long test time and it is not cost effective. Ongoing changes in MEMS market are the warnings for future testing issues and challenges. A lack of test standards has a negative indirect effect on the business of MEMS. There is a need of cost effective test methodology. Testing imposes large direct costs on the product as test equipment is expensive and test programs are limited to test specific devices. The flow chart in fig.1 shows the efforts that are being performed to improve the test systems to reduce the cost of testing.

Figure 1. Efforts of reducing MEMS testing cost.

III- Comparison between ICs and MEMS testingMEMS have to face many testing challenges [58] due to their multi physics behavior. Similar to analog electronic systems the mechanical components of MEMS have non-linear nature. Therefore analog and mixed signal tests [59] including verification and calibration are essential for MEMS. The test problem for MEMS is exacerbated due to their inherent diverse properties. For example, the output of a MEMS based device is electrical in nature; however, these signals come from mechanical actuations under different domains. Thus the use of electrical signals for test can be helpful to an extent for controllability and observability of mechanical components. Field of MEMS is relatively not as advanced as ICs. Therefore techniques of MEMS fabrication, packaging, and testing are at their initial stage. There is requirement of improvements and constant revision for next several years. MEMS testing and fault modeling [60] is a new area as compared to MEMS manufacturing and designing. Even though MEMS devices have been introduced since 1970, but testing MEMS remained a big challenge. Testing MEMS at the early stage can support in production, fabrication and packaging flow. The testing tools required to diagnose the origins of MEMS failures are essential to upgrade and design.Causes of failures [61] are as distinctive as the MEMS devices. In ICs manufacturing, significant efforts are being done in testing and handling to appropriately characterize and judge the device performance and weigh against to device specifications. The main differences in testing MEMS and ICs are the environmental circumstances. For several cases, integrated circuits are being analyzed in a variety of environments under different conditions of temperature and moisture. Relatively analogous testing and handling measures [62] are being performed during manufacturing process, however essentially the device is operated within different environmental conditions. Any change in the test environment can considerably affect the sensitivity and functionality of the device. The additional complication regarding mechanical motion demands extra heed during testing and handling. Auspiciously, in case of MEMS analysis there is a plus point of utilizing testing techniques [63] leveraged from IC industries. Although, the variety of devices are growing because of different applications, therefore multi-disciplinary knowledge is deemed essential to properly identify the causes of failures. Table 3 briefly summarize the difference of fault modeling and testing techniques of ICs and MEMS.

Table 3: Comparison between ICs systems and MEMS []Integrated system testingIntegrated digital devices and circuitsAnalog mixed signal circuits and devicesMicroelectromechanical systems

Fault modelAssertion, gate delay, line delay, redundant, path delay, behavioral, branch, bus, cross point, stuck-at, stuck-on, stuck-open, bridging etc.Hierarchical, behavioral, macro model, transistor, physical, parametric and catastrophic faults etc.Behavioral, shorts and opens in thermal, structural defect level, parametric,

Test techniqueVHDL, HSPICE, fault dictionary, Probabilistic, signature analysis method, BIST (full/partial scan path), LFSR (Linear Feedback Shift Register) etc.Pole-Zero Analysis, Artificial neural network, HSPICE, SABER, VHDL-AMS, ATPG, Diagnosis of soft faults based on fractional correlation, BIST etc. Neural network, VHDL-AMS, device-level (FEM) and circuit-level (HDLs) and transposition of techniques developed for microelectronics, BIST

III- Study of failure mechanism in MEMS deviceMEMS devices have features of motion unlike ICs, therefore special techniques and tools are required for measuring mechanical response on micro and nano scale. The devices behavior can be analyzed in detail using these tools which helps in providing feedback to the designer to improve the design. A number of MEMS devices are used in a range of applications across the world. In the past it was considered that MEMS devices belonged to ICs family and might have the same failure issues like ICs. Conversely, MEMS devices have different failure mechanism due to complex mechanical geometry and unique material and these have different biasing techniques. The failure mechanism due to complexity these devices is categorized in the following four groups.Group 1 includes stationary MEMS devices without moving parts, such as chemical sensor, microphones, DNA sequencers etc. The major source of failure in this group is the particle contamination. The contaminated particles, which are small in size and inherently non electrical, adversely affect the device performance. The particle contaminations are insulator in nature and these are unable to bridge the device structure electrically and it becomes difficult to detect as short circuits.Group 2 contains of MEMS devices that have moving structure without rubbing surfaces, such as comb drives accelerometers, gyroscopes etc. in [] reported that hinges and microcantilever yoke regions are suffered from fatigue failure. The fatigue (structural failure) is study in this group, electrostatically actuated comb-fingers with a perforated proof mass and a microcantilever with a notch is presented in []. The increase in stress levels at the notch initializes cracks on the surface of the microcantilever that reduces device life and eventually causes the failure of the device by fracture.Group 3 consists of MEMS devices have moving structure with impact surfaces, such as relays, thermal actuators, valves, etc. These devices are easily influenced by debris created on the surfaces, fracture constituents, cracks etc. Fracture generated due to impact forces on the opposite structure causes failure in the device. Group 4 surrounds MEMS devices that have moving parts, rubbing, and impacting surfaces, for instance micromirror, optical shutter, geared devices etc. Friction is created due to rough surfaces of moving components that creates wear in material or debris initiating several failures, such as 1) stiction of rubbing or contacting surfaces, 2) wear created by particles can change the motion tolerance, 3) particle contamination. It is perceptible that all kind of MEMS devices have failure mechanisms due to particle contamination and stiction can. It is, therefore, more important to understand the origins of failure before finding the remedies of the specific failure. MEMS sensors and actuators have different and distinctive failure mechanisms by their classifications and nature. In ICs, testing only electrical parameters are measured to find the fault, while in MEMS testing both the electrical and mechanical parameters are tested to deal with the failure. MEMS causes of failures and its procurement are summarized in Table 3.Table 4. Summarizes various MEMS groups, their causes of failure and procurement of the failures.GroupCauses of FailuresTest Ref

Group 1

Chemical SensorNozzlesMicrofluidicDNA SequencersDielectric BreakdownMicrofluidic device of 2x4 array, shorted electrodes along X and Y direction; simulation analysis of effects change on the electrodes; and CCD camera was used to observe the transportation of droplets.[64]

Group 1 or 2

AccelerometersFracture, Fatigue, Mechanical Wear, Charging, Change in friction 1. Built in self-test (BIST) were fabricated within the device. Comparison is performed between BIST and Non BIST MEMS; effects of fatigue and fracture on the structure was observed through simulations.2. Accelerated life time of devices was tested experimentally for 1000 hours at about 145o to 200o C. 3. Tytron 250 was used to test fatigue. The experiment showed fatigue life between 7.78x104 to 1.48x107 cycles when the stresses increased from of 2.05 to 2.83 GPa.4. Sandia National laboratories developed the SHiMMeR (Sandia High volume Measurement of Micro machine Reliability) that can simultaneously control and test up to 256 MEMS parts.5. Fatigue and creep were observed by using X-Rays diffraction. wear was analyzed through DLC coating (Diamond Like Carbon), Stiction was observed by using MIL- STD- 883F (Military Test Standard Device) and SAM (Scanning acoustic Microscope) was used for contamination[65, 66][67, 68][69]

Group 2

Pressure SensorsVibrations, shock, Fatigue, Fracture, Change in friction The fatigue was detected in the fabricated sensors operating below the stress level; it is observed that fatigue can be occurred at equal stress and fracture levels.[70]

GyroscopesVibrations, shock, charging 1. Testing of lateral capacitive gyroscope was performed to analyze the adhesion failure mode. 2. The temperature degradation and variations in signal to noise ratio were also reported. [71, 72]

Group 3

Thermal Actuators Vibration, shock, Mechanical Wear1. Weibull statistics was used to analyze the fracture tests on the beam by applying different loads. 2. Experimental technique was proposed for micro switches to observe malfunctioning due to charging and creep.3. The effects of pull-in voltages were observed during bending and torsional modes of beams. Mechanical wear and fatigue tests were performed to predict the life time.[73, 74][75]

ValvesFatigue, fracture, Mechanical Wear, Shock, Vibrations1. Classification of design issues and device failure were reported.2. Fatigue effects were observed on thick layer of aluminum sample by placing it in the liquid. [76, 77]

Micro relaysFatigue, fracture, Mechanical Wear, Shock, Vibrations, ChargingEffects of stiction and welding failure were performed at 109 On/Off switching cycles. [78]

Group 4

Electrostatic Actuators

Charge in friction, Fatigue, fracture, Mechanical Wear, Shock, VibrationsStochastic method was used to analyze the pull-in and pull-out voltages for prediction of device life time.[79, 80]

Optical ShuttersFatigue, fracture, Mechanical Wear, Shock, VibrationsThe observations showed the imprecise data.[81]

Mirror DevicesOptical degradation, Fatigue, fracture, Mechanical Wear, Shock, VibrationsTexas Instruments (TI) developed the optically inspection tool for DMD devices that examines each pixel of the DMD array.[81]

Gear DevicesCharge in friction, Fatigue, fracture, Mechanical Wear, Shock, VibrationsSliding surfaces were analyzed through simulations to prevent adhesion and wear[82]

Micro turbines/FansFatigue, fracture, Mechanical Wear, Shock, Vibrations, Charge in frictionThe observations showed the imprecise data.[82]

MEMS structures consist of flexible or rigid membranes or beams that are clamped from one or two sided and some contains perforation on its surface. These devices also contains insulated or conductive flat layers, gears, cavities and hinges integrated with an electronic readout circuit. On the other hand more than one devices are being integrated in a single chip. Single axis moving MEMS structures have been upgraded to multi axis movements. MEMS are getting more multifaceted design features to fit for more applications. Therefore improvements are required for better performance of these devices, consequently it is deem necessary to aware about the root causes of device failure at the preliminary stage. The failure analysis can help to understand the basic reasons of behavior faulty device during the testing process from wafer level to the final device packaging. Failure mechanism due to defective interconnects and low compatibility results in poor performance of the device. Table 5 indicates the foremost mechanical failures of MEMS devices. These devices are rapidly growing in the electronic market because of improved performance. No doubt, numerous mode of failures can occur in poorly designed MEMS devices, while carefully designed MEMS products can withstand harsh reliability issues. Generally it is considered that the MEMS failures are related to only mechanical parts of the device, however, electrical failures have also been ignored occasionally. These can be originated due to contact failure, electromigration, and dielectric degradation. Table 6 shows common electrical failures of MEMS devices.

Table 5. MEMS MECHANICAL FAILURESMechanical Fracture

FailureShockOverloadFatigueCorrosion

CausesMechanical Interference disorder, Excessive Loading, dropsExcessive stressStructural damage due to cyclic loading Oxidation, Chemical reaction

Stiction

FailureCapillary forcesVander Waals forcesResidual stressChemical bondingElectrostatic charging

CausesMono layer of moisture or lubricants on the surfaces prompting capillary force Interaction of atoms or molecules at the surface of close contact

Bending and deformation of structure during the release processChemical bond between contact surfaces Potential difference at two closed surfaces

Wear

FailureSurface fatigueCorrosionAbrasionAdhesion

CausesCyclic loading instead of smooth surfacesChemical interaction between two surfacesMaterial loss due to sliding of different surfacesPull-in Interaction between two surfaces during sliding due to surface forces.

Creep and Fatigue

FailureThermal stressIntrinsic stressApplied stress

CausesOver heatingResidual stressApplied stress

Table 6. MEMS ELECTRICAL FAILURESElectric short and Open Circuit

FailureOxidationElectro-migrationESD, High Electric fieldDielectric material Degradation

CausesEnvironmentalMismatch loadExcessive loadCapacitive discharges

Contamination

FailureUsage environmentIntrinsic ( crystal growth)Manufacturing - induced

CausesLow, High temperature and humidityEnvironmentalRough handling in Industry

Testing challenges and issues in different categories of MEMS sensor and actuatorsMEMS devices are categorized in six different classes. The classification is based on their operating mechanism and applications and are discussed in this section.MEMS Sensors [83] are designed and fabricated for sensing multiple environmental changes. The sensors are capable of sensing behavior of fluids, force, inertia, gas, etc. Testing of this category of devices many challenges lay ahead the test engineer due to complex sensing functionality and a variety of designs. The test mechanism developed for theses sensors also have the issues of testing as reported in [46, 84, 85], as each devices required individual test bench to analyze the sensing environment. Therefore for MEMS fault or failure analysis [], available testing techniques [] are limited. As these tools and testing techniques have been leveraged from the IC technology, therefore these are helpful in resolving various fault mechanisms to an extent. The major dilemma is to discover the faulty origins in multiple devices that are put together on single test bench for mass production. Recently existing tools [86] have limitations to test specific MEMS devices electrically and mechanically. The emerging challenge is to analyze and identify unique source of failure during assessment of multiple die on single test stage. Unfortunately there is a lack of knowledge in the literature regarding MEMS failure mechanism.In the development of a test bench for gas sensor, the main obstacle is the chamber designing for chemical environment for the device sensibility. Mechanical sensors [87] that are designed to sense physical changes like motion, pressure, are relatively easy to test or characterize. While chemical sensors have issues of selectively in case of sensing targeted gases. Et. Al. reported in [], carbon mono oxide CO and H2 sensors are operated in a variety of challenging environments of industries. Fast response is the critical requirement for theses sensors therefore readout mechanism of testing system faces high level of noise signals because detecting device signal at ppm level to find the selectivity of these gases is very cumbersome effort. [88] reported the sensitivity issues in ethanol sensor during the device analysis, without readout circuitry interface on chip these sensors become difficult to characterize.

The packaged devices that have specific environment, any change in the environment or defective packaging may cause incorrect outcomes and induce additional faulty behavior during testing. In MEMS analysis [89], lid removing changes the device environment; causes change in functionality. Device faces malfunctioning due to contaminant particles, environmental change and other potential defect. MEMS actuators [90, 91] generate power using any electrical or physical stimulus. These mechanical components generate power and motion for other MEMS components. A variety of devices for example BioMEMS, RF MEMS, microfluidic, or optical MEMS need some forms of actuations to interact with another micro structure, in moving a fluid or a micro mirror. MEMS actuators are stimulated under different electrical domains. Electrothermal actuators [] exploit heat producing due to power dissipation in the device. The increase in temperature causes expansion in the structure, then necessary displacement is induced for motion. In electrostatic actuators [92], electric field is involved to attract other parts of device to generate motion []. Prevailing fault modes of stiction [71] and particle contamination were observed in both thermal and electrostatic actuators. Actuators can contain rubbing surfaces which may result in formation of wear or debris []. The major concerns for testing of MEMS actuators must be nondestructive regarding fault analysis of stiction films and coatings, and functional testing of multiple devices in parallel. Every type of actuator may have distinctive failure mechanisms and tribulations for fault analysis. Thermal actuators face the typical fault mechanisms of thermal degradation [93].The side effects of electrothermal cycles in these devices are under observations. Analysis under stress [94] can cause faults by introducing permanent deformation in the mechanical structure. This defect can produce out of plane nonlinear motion of thermal actuator and due to increase in temperature, actuator can be welded to the substrate. Analyzing thermal behavior of dynamic structures is a difficult task. Techniques involved to generate thermal actuations may be destructive; it can slow down the motion of actuator. The significance of understanding about heating effect of a thermal actuator and occurrence of localized heat can support in thermal behavior modeling and reducing the fault mechanisms. Electrostatic actuators typically have two sets of comb fingers; the actuator is derived due to the change in polarity of the electric fields at the opposite fingers. In multi structure MEMS devices, occurrence of defects in comb fingers may be the origins of failure. Rapid identification of failure mechanism of comb finger is essential.

Table 7CategoryDevicesSource of failureTesting IssuesObservationsRef

RF MEMSSwitches, Resonators, Lab-on-chipCapacitorVaractorSensorsFilters

stiction failure, surface contaminations, accumulation of charges in the dielectric part, A short is occurred when two parts of the device stick to one another.Major issue due to left over charge in the dielectric causes increase in voltage required for the device actuation and signal transmission.Other issues of catastrophic failure can be occurred due to constant increase in voltage that causes the dielectric breakdown.

A challenge in RF MEMS testing is to analyze stiction failure nondestructivelyTracing the genuine adhesion point without the removal any part of component is challenging.Therefore knowledge about the surface contact of the devices is essential to understand the roughness of contact area.Therefore understanding about contact mechanics is important to tackle the issues about contact area, geometry and the asperity in contact area. [95][96][97][98][99][100]

Optical MEMSSensorResonator,Optical bufferMirrorsFiltersHigh surface roughnessstiction,accumulation of charges roughness of micromirror surface, stiction due to the accumulation of charges that causes stuck in actuation.tracing faults of shorts and stiction beneath the mirror surfaceCracks during detachment of the micromirror during failure analysis under mirror surface.

surface roughness of single mirror can be determined feasibly and it takes time for analysis of various mirror locations.The surface roughness analysis of arrays of mirrors is not reasonable using current AFM techniques.to develop a technique that should be capable of determining surface roughness of entire array or collection of mirrors in parallel[101][102, 103]

MicrofluidicsLab-on-chipBiosensorsGas sensorsfluid contamination, leakageNo compatibility with MEMSshort-circuit defect,catastrophic and parametric faults due to structural defectsdevice is flushed of fluid before analysis; this effort may negotiate the failure mechanism and induce erroneous outcomes.The issues of fluid contamination, deprocessing, leak detection and compatibility with MEMS.

It has been investigated in that during typical analysis using SEM, it is necessary that the device is flushed of fluid before analysis;There is need of modifying SEM techniques to achieve high quality resolution of microfluidic systems during an analysis of pressure or flow sensor exclusive of flushing the device.Therefore in verification of device functionality, purpose of using test fluid can be helpful in tracing the movement of the fluid during operation. The approach of using diagnostic fluid can also be helpful to trace the breakage, leakage or cracks in the device.

[104, 105][106][107]

BioMEMSLab-on-chipBiosensorsfluid contamination, leakage,blockage of channel,Crackscompatibility

experience analogous tribulations with the existence of additional biological materialsChallenges during testing are device deprocessing and functional testing during biocompatibility.[108] [109][110, 111]

MEMS testing techniquesMEMS devices due to Multiphysics behaviour are hardest to test. Recently the devices are being tested on the base of functional specifications. Variations in manufacturing process of the device can alter its specs, which is the disadvantageous scenario in MEMS testing, as a number of errors can be induced due to process fluctuations. Moreover, the specifications of the device can be multifarious and its measurements at several testing stages can increase the test time. Therefore quite different test techniques are required to measure the performance parameters of various devices, which forces the manufacturer to procure different test equipment for different products. Fast transient test methodologies are required to check the DUT specifications. In multi structured complex devices, specifications are disturbed mostly due to drifts in the fabrication process and defects. For the devices that are designed near the specification limits, the violations in the specs can also be occurred due to normal process variations. Drifts in processes can be identified at initial stage in the production phase by the process monitoring systems. Defects become the reasons of catastrophic failures that can be detected by tests of parametric and functional failure analysis.The development of cost effective testing techniques is the major challenge for MEMS. A number of techniques and methodology have been reported in literature. Table 8 shows the list of destructive and nondestructive testing techniques that are being implemented for MEMS measurements electrically and mechanically. Jeffery et al. [112] reported an on-line monitoring technique which is based on bias superposition for MEMS integrated sensors. They injected the test stimuli into biased conductance sensor and analyzed the structural integrity of the device and interface on the base of signal injection and signal extraction. Tong Guo et al. [113] reported optical techniques to test MEMS structures at wafer level. They utilized microscopic interferometry and computer micro-vision to perform MEMS measurements for analyzing static and dynamic properties. Biswas et al. [114] developed statistical based -SVM model for testing MEMS accelerometer to eliminate redundancy. Their work disclosed that with the help of specification based tests, the redundancy in test can be statistically identified with minor error. When hot and cold tests removed for the accelerometer, they observed the defect escape of 0.2% and yield loss of 0.1. N. Dumas et al. [115] developed online testing technique for sensors using superposition of the test stimuli on the specifications. They utilized the signal processing technique to reduce the fluctuations of test output by encoding the test stimulus through pseudo-random sequence. They also studied the overall test time and level of perturbation rejection.The dynamic Electrostatic Force Microscopy is used in [116] to characterize the beam resonators. The resonator was actuated by placing probe cantilever above the beam. Then modulated signal was applied to the probe cantilever. The resonance frequency response of the test beams was analyzed by studying coupled electrostatic interaction between the conductive beams. A. Izadian et al. [117] developed fault diagnostic system for MEMS using multiple model adaptive estimation technique. They used Kalman filters to model the fault of the system and implemented to diagnose the fault in MEMS in real time application. Lateral comb MEMS resonators are fabricated to validate the fault diagnosis unit in multiple model adaptive estimation technique. They also developed another fault diagnosis technique [118] for MEMS resonator by combining least square forgetting-factor method and multiple-model adaptive estimation method. This technique identified the parameters of slowly time-varying systems. Another work developed by Izadian [119] is the self-tuning-based parameter estimation technique for fault diagnosis of MEMS. He used this technique to recognize the parameters of system and generation of residual signals. Alena [120] developed an anti stiction and self-recovery mechanism in order to recover the functioning of RF-MEMS switches in case of malfunctioning due to stiction. An effective heat-based mechanism was designed in order release the stuck component. Reppa [121] developed a fault detection and diagnosis (FDD) technique for MEMS. Parametric faults can be captured, isolated and identified with the help of this technique. This technique depends on estimation of parameters arrayed in a set membership identification framework.

Table 8: Destructive and non-destructive techniques for MEMS testing.TechniqueObservationModeRef

Optical Microscopystains, debris, fracture, and abnormal displacementsNon-destructive[122]

Scanning laser microscopyextended depth-of-focus, abnormal vertical displacementsNon-destructive[123]

Scanning Electron Microscopy (SEM)

imaging defects at high magnificationNon-destructive[124]

Focused Ion Beam (FIB)

to image structures, to cross section elements of concern and to cut elements free for subsequent examinationdestructive[125]

Laser-Doppler vibrometerTo measure out of plane motionNon-destructive[126]

Computed tomography (CT)Visualizing 3D internal structuresNon-destructive[127, 128]

InterferometryTo detect the tilt or deflection of a sampleNon-destructive[129]

Transmission electron microscopy (TEM)To observe thin films deposited on MEMSNon-destructive[130]

Energy dispersive X-ray spectroscopy (EDS)To analyze chemical composition of surface coatingNon-destructive[131]

Wavelength dispersive X-ray spectroscopy (WDS)To analyze chemical composition of surface coatingNon-destructive[132]

Atomic force microscopy (AFM)topographic images and surface tracesNon-destructive[133, 134]

Auger electron spectroscopy (AES)To perform qualitative and semi-quantitative compositional analysis of surface of materialsNon-destructive[135]

Secondary ion mass spectroscopy (SIMS)Compositional analysis of the sample with sensitivities in ppm to ppb, removing material from the device destructive[136, 137]

Thermally-induced voltage alteration (TIVA)To identify electrical failure modeNon-destructive[138]

Resistive contrast imaging (RCI)To identify electrical failure modeNon-destructive[139]

Infrared Microscopyto construct thermal images based on the infrared radiance emitted from the structuresNon-destructive[140]

Light Emission

To observe MEMS optical devicesNon-destructive[141]

Acoustic Emission

To observe running device within packageNon-destructive[142]

Laser Cuttingto impart to elements such as gears and linksdestructive[143]

Lift-off Technique

accumulation of wear debris at the surfacesNon-destructive[144]

VI-Summery

MEMS devices are enormously dissimilar in their application and function. Test engineers must be multi-disciplinary in the field of MEMS devices. Various limited tools and techniques leveraged from the VLSI technology are being utilized in MEMS testing, however there is need of developing new test tools for the diagnosis of different faults. Several classes of devices discussed in this literature have common issues related to MEMS testing. It is deemed necessary to share knowledge among research institutions and industries to develop tools and techniques for faults localization to speed up the MEMS manufacturing and lowering the device cost. MEMS can only be penetrated as an emerging technology until the cost of testing is reduced substantially as well as valued added technique is applied at the early stage of the market. MEMS testing is fetching difficult challenges, for the reason that conventional electrical testing is unable to comprehend the mechanical behavior of a MEMS device. In addition mechanical behavior requires different technique to test it (e.g. flow, movement, pressure etc.) Therefore integration of multi-function in devices increases the testing problems as a function of the device complexity. Techniques of MEMS testing are different in development processes i.e design, prototype, and production. In the design phase, it is essential for the engineers to know the requirements of device testing and it should be cost effective and reliable as a predictor of device performance. The Design for Test (DFT) is becoming a paramount as MEMS designers are facing the pressure of more cost and time-to-market

AcknowledgmentThe authors would like to thank the UTP GA funding agency as well as the Department of Electrical & Electronic Engineering of Universiti Teknologi PETRONAS.

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