Special Devices Presentation

7/25/2019 Special Devices Presentation

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Failure Analysis of

MEMS


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Micro-Electro-Mechanical

Systems (MEMS)technology that in its most general formcan be defined as miniaturized mechanical

and electro-mechanical elementsmade using the techniques ofmicrofabrication


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Known as Micromachines in Japan and MicroSystems Technologies in Europe

Made up of components between 1 to 100micrometres in size (i.e. 0.001 to 0.1 mm)

generally range in size from 20 micrometres to amillimetre (i.e. 0.02 to 1.0 mm).

usually consist of a central unit that processes data(the microprocessor) & several components thatinteract with the surroundings such as microsensors.

Micro-Electro-Mechanical Systems(MEMS)


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Micro-Electro-Mechanical

Systems (MEMS)Accelerometer

Gyroscope

Microphonefor Mobile

Phones


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MEMS RELIABILITY

One of the most critical points in developing areliability analysis is to understand the way inwhich a system can fail, or commonly known asits root cause.

For that reason, afailure mode is defined as theapparent failure of a system, and thefailure

mechanism as the physical cause (mechanical,chemical, or thermal) of the failure modes in thesystem.


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Failure mechanisms of

MEMSMechanical Fracturedefined as the breaking of a uniform

material into two separate sections.


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Mechanical Fracture

Ductile Fracture - occurs in ductilematerials. It is characterized by almostuninterrupted plastic deformation of a

material.

Brittle fracture - occurs along crystal

planes and develops rapidly with littledeformation


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Mechanical Fracture

Intercrystalline fracture - brittlefracture that occurs along grain

boundaries in polycrystallinematerials, often beginning at a pointwhere impurities or precipitates

accumulate.


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STICTION EFFECT

The moving partsof micromechanical

machines tend toseize up underforce of stickingand friction.


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TWO STAGES OF STICTION

Release Related StictionIt occurs during sacrificial

layer removal in fabrication

and such stiction is cause by

capillary forces

In Use Stiction

It usually occurs when successfully released microstructuresare exposed to humid environment


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STICTION CATEGORIESMechanical Collapes due to Capillary Force

Due to fabrication of MEMS. If

etching is performed in liquid

environment, a bridge of liquid

will be formed from the

suspended member and

substrate. When the liquid

is removed during dehydration cycle yielding to an attractive

capillary force which strong enough to make it collapse


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STICTION CATEGORIESStiction by van der Waals and Casimir Forces

If the gap between contiveler and sustrate is a fewmicrometer, a Stiction sometimes cause by the force

we called van der Waals and Casimir Force.


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WEAR EFFECTWear may be defined as damage to a solidsurface caused by the removal ordisplacement of material by the mechanicalaction of a contacting solid, liquid, or gas.


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TYPES OF WEAR

Adhesive WearAbraisive Wear

Corrosive Wear

Surface Fatigue Wear


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ADHESIVE WEARAdhesive wear can befound betweensurfacesduring frictional cont

act and generallyrefers to unwanteddisplacement andattachment of weardebris and materialcompounds from onesurface to another.
https://en.wikipedia.org/wiki/Frictionhttps://en.wikipedia.org/wiki/Friction


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Abraisive Wear

Abrasive wearoccurs when ahard roughsurface slidesacross a softer

surface.


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Corrosive WearThis kind of wear occur in a variety ofsituations both in lubricated andunlubricated contacts. Thefundamental cause of these forms ofwear is chemical reaction between

the worn material and the corrodingmedium


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SURFACE FATIGUE WEAR

occurs mostly inrollingapplications, such

as bearings andgears. It affectshighly polished

surfaces that rollinstead of sliding.


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DELAMINATIONIt occurs when a material loses its adhesivebond due to strong force. It can also arise dueto thermal expansion and a result of fatigue.


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VIBRATION AND SHOCKSDue to the sensitivity and fragilenature of many MEMS externalvibrations can cause either throughinducing surface adhesion or throughfracturing device support structures,

external vibration can cause failure.Long-term vibration can alsocontribute to fatigue.


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Shock is somehow different with

vibration.Shock is a single mechanical impactinstead of repeated movement event.

It is a direct transfer of force in adevice and it is much stronger thanvibration.


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ELECTROSTATIC DISCHARGE AND DIECHARGING

It occurs when a device is improperly handled.A human body routinely develops an electricpotential in excess of 1,000V. Upon contactingan electronic device, this buildup will discharge,which will create a large potential difference

across the device. The effect is known to havecatastrophic effects in circuits and could havesimilar effects in MEMS.

Dielectric charging and breakdown is the charging that may occur inthe dielectric layer. Sensors are known to drift over time due to chargeaccumulating at the surface.


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RADIATION EFFECTS

The field of radiation effects on MEMS is becoming increasingly important.It has long been known that electrical systems are susceptible to radiation,and recent research has raised the possibility that mechanical devices mayalso be prone to radiation-induced damage. Especially sensitive toradiation are devices that have mechanical motion governed by electricfields across insulators, such as electrostatically positioned cantileverbeams. Insulators can fail under single event dielectric rupture. A furthercomplication is the fact that radiation can cause bulk lattice damage andmake materials more susceptible to fracture.


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TEMPERATURE

The temperature range in which a device will operate within acceptableparameters is determined by the coefficient of linear expansion. Indevices where the coefficients are poorly matched, there will be a lowtolerance for thermal variations.

Thermal effects cause problems in metal packaging, as the thermalcoefficient of expansion of metals can be greater than ten-times that ofsilicon.


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HUMIDITY

Humidity is considered another serious concern for MEMS. Surfacemicromachined devices are extremely hydrophilic for reasons related toprocessing. In the presence of humidity, water will condense into smallcracks and pores on the surface of these structures.


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PARTICULATES

Particulates are fine particles that are prevalent in the atmosphere. Theseparticles have been known to electrically short out MEMS and can alsoinduce stiction. While these particles are normally filtered out of the cleanroom environment, many MEMS are designed to operate outside the

confines of the clean room and without the safety of a hermetically sealedpackage.


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Failure Analysis TechniqueOptical Microscopy

one of the most valuable and widely used

tools in the FA of MEMS.

referred to as light microscope, is a typeof microscope which uses visible light and a

system of lenses to magnify images ofsmall samples.


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Optical MicroscopyThe features that can be observed optically

include:

textures,

stains,

debris,

fracture, and

abnormal displacements.


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Optical Microscopy


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Scanning Laser

Microscopy (SLM)It is a technique used for obtaininghigh-resolution optical imageswith depth selectivity.

A confocal image is an image witha very limited depth of field (depthof focus) created by inserting anaperture in the optical path.

By taking a series of confocal

images at different focal planes,an extended depth-of-focus imagecan be constructed

i


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Scanning Laser

Microscopy (SLM)

i l


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Scanning Electron

Microscopy (SEM)A method for high-resolutionimaging of surfaces. The SEM use selectrons for imaging, much as alight microscope uses visible light.

The SEM has been useful forimaging defects at highmagnification as well asdetermining electrical continuity instatic and operating micro engines

i l


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Scanning Electron

Microscopy (SEM)Passive voltage contrast is defined as contrast whicharises from voltage differences induced by rasteringthe beam causing various elements reach an

equilibrium potential through self-charging.

Active voltage contrast is defined as that arisingfrom external application of voltage on differentstructures.


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Acoustic Microscopy

A high frequency ultrasound transducer emits soundwaves that can be received back (echo) or transmittedthrough a material. The acoustic signature or waveform maythen be interpreted to determine variations of acoustic

impedance within a sample. The difference in acousticimpedance may indicate a change in material densities orseparation at an interface. The transducer may also bemechanically scanned across the sample in a raster patternemitting and receiving the ultrasound signal to generate an

image (pulse-echo). An immersion fluid medium, typically DIwater, is used to acoustically couple the sample whileperforming the analysis.


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Acoustic Emission

Acoustic Emission (AE) testing is a powerful method forexamining the behavior of materials deforming understress. Acoustic Emission may be defined as a transient

elastic wave generated by the rapid release of energywithin a material. Materials "talk" when they are introuble: with Acoustic Emission equipment you can"listen" to the sounds of cracks growing, fibers breakingand many other modes of active damage in the stressed

material.


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Laser Cutting

The laser is used for failure analysis of new andreturned devices to isolate faulty components by cuttingthe traces (metal lines) that connect them to the rest of

the circuit, to remove passivation over the circuit toprovide access to circuit traces and connections, and todig through multiple layers of interlayer dielectrics andeven metal allowing electrical contact to be made toburied circuit paths.


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Lift Off Technique

Careful removing elements of a microengine witha conductive laboratory adhesive tape used for SEMmounting has added another dimension to the analysis

of microengines. The lift-off technique allowsexamination of the bottom surfaces of engines (Fig. 23)that provide additional information (such as theaccumulation of wear debris or evidence of damage topin receiver holes) for determining failure modes of thedefective and failed microengines


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Focused Ion Beam (FIB)

Resembles a Scanning Electron Microscope Uses a focused beam of positive gallium ions to

irradiate the surface of the sample in a definedarea. This irradiation causes surface charging,which can be neutralized by a flow of low energy

electrons from a flood gun


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Focused Ion Beam (FIB)


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Focused Ion

Beam (FIB)

FIB workstation


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Focused Ion

Beam (FIB)Principlethe gallium (Ga+) primary ion beamhits the sample surface and sputtersa small amount of material, which

leaves the surface as eithersecondary ions (i+ or i-) or neutralatoms (n0). The primary beam alsoproduces secondary electrons (e). Asthe primary beam rasters on thesample surface, the signal from the

sputtered ions or secondaryelectrons is collected to form animage


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Focused Ion

Beam (FIB)Usage Micro imaging used as a micro machining tool, to

modify or machine materials at the

micro- and nanoscale to cut unwanted electrical

connections to deposit conductive material in

order to make a connection


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Focused Ion

Beam (FIB)

Sample image milled

by FIB


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Atomic Force Microscopy (AFM)

Also called scanning-force microscopy (SFM), isa very high-resolution type of scanning probe

microscopy (SPM), with demonstratedresolution on the order of fractions of ananometer, more than 1000 times better thanthe optical diffraction limit.

The atomic force microscope (AFM) providesvery detailed topographic images and surfacetraces.


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Atomic Force Microscopy (AFM)


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Atomic Force Microscopy (AFM)PrincipleThe AFM consists of a cantilever with asharp tip (probe) at its end that is used toscan the specimen surface. The cantileveris typically silicon or silicon nitride with atip radius of curvature on the order ofnanometers. When the tip is brought intoproximity of a sample surface, forces

between the tip and the sample lead to adeflection of the cantilever. Along withforce, additional quantities maysimultaneously be measured through theuse of specialized types of probes.Typically, the deflection is measured usinga laser spot reflected from the top surface

of the cantilever into an array ofphotodiodes. Other methods that areused include optical interferometry,capacitive sensing or piezoresistive AFMcantilevers

Usage

the identification of atoms at asurface

the evaluation of interactions

between a specific atom and itsneighboring atoms

the study of changes in physicalproperties arising from changes inan atomic arrangement throughatomic manipulation.


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Atomic Force Microscopy (AFM)Imaging Modes

Contact Mode- the tip is "dragged" across the surface of the sample and thecontours of the surface are measured either using the deflection ofthe cantilever directly or, more commonly, using the feedback signalrequired to keep the cantilever at a constant position the evaluationof interactions between a specific atom and its neighboring atoms

Non-Contact Mode- the tip of the cantilever does not contact the sample surface. Thecantilever is instead oscillated at either its resonant frequency(frequency modulation) or just above (amplitude modulation) wherethe amplitude of oscillation is typically a few nanometers (


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Failure Analysis of

PassiveComponents


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Resistors

Resistors can fail open or short, alongside their value changing underenvironmental conditions and outside performance limits. Examples of resistorfailures include:

Manufacturing defects causing intermittent problems. For example,improperly crimped caps on carbon or metal resistors can loosen and losecontact, and the resistor-to-cap resistance can change the values of theresistor

Surface-mount resistors delaminating where dissimilar materials join, likebetween the ceramic substrate and the resistive layer.

Nichrome thin-film resistors in integrated circuits attacked by phosphorus

from the passivation glass, corroding them and increasing their resistance. SMD resistors with silver metallization of contacts suffering open-circuit

failure in a sulfur-rich environment, due to buildup of silver sulfide.

Copper dendrites growing from Copper(II) oxide present in some materials(like the layer facilitating adhesion of metallization to a ceramic substrate)and bridging the trimming kerf slot.


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Resistors


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Capacitors

Capacitors are characterized by their capacitance, parasitic resistance in seriesand parallel, breakdown voltage and dissipation factor; both parasitic parametersare often frequency- and voltage-dependent. Structurally, capacitors consist ofelectrodes separated by a dielectric, connecting leads, and housing; deterioration

of any of these may cause parameter shifts or failure. Shorted failures and leakagedue to increase of parallel parasitic resistance are the most common failure modesof capacitors, followed by open failures. Some examples of capacitor failuresinclude:

Dielectric breakdown due to overvoltage or aging of the dielectric, occurringwhen breakdown voltage falls below operating voltage. Some types ofcapacitors "self-heal", as internal arcing vaporizes parts of the electrodes around

the failed spot. Others form a conductive pathway through the dielectric,leading to shorting or partial loss of dielectric resistance.

Electrode materials migrating across the dielectric, forming conductive paths.

Leads separated from the capacitor by rough handling during storage,assembly or operation, leading to an open failure. The failure can occur invisiblyinside the packaging and is measurable.

Increase of dissipation factor due to contamination of capacitor materials,particularly from flux and solvent residues.


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Electrolytic capacitors

In addition to the problems listed above, electrolytic capacitors suffer fromthese failures:

Aluminum versions having their electrolyte dry out for a gradual leakage,equivalent series resistance and loss of capacitance. Power dissipation byhigh ripple currents and internal resistances cause an increase of thecapacitor's internal temperature beyond specifications, accelerating thedeterioration rate; such capacitors usually fail short.

Electrolyte contamination (like from moisture) corroding the electrodes,leading to capacitance loss and shorts.

Electrolytes evolving a gas, increasing pressure inside the capacitor housingand sometimes causing an explosion; an example is the capacitor plague.

Tantalum versions being electrically overstressed, permanently degradingthe dielectric and sometimes causing open or short failure. Sites that havefailed this way are usually visible as a discolored dielectric or as a locallymelted anode.


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MULTI-LAYER CHIP CAPACITORS

(MLCCS)


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TANTALUM CAPACITORS


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ALUMINUM ELECTROLYTIC

CAPACITORS


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INDUCTORS


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Diodes

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