Failure Modes of Electronics [part 2]

<< cont. from part 1

Electrostatic discharge

Electrostatic discharge (ESD), a subclass of electrical overstress (EOS), may cause immediate failure of the semiconductor device, a permanent shift of its parameters, or a latent damage causing increased rate of degradation. ESD failure has at least one of three components: localized heat generation, high current density, and high electric field gradient. Currents of several amperes can be present for several hundreds of nanoseconds; the energy deposited to the device structure then causes the damage.

ESD discharge in a real circuit containing capacitances and/or inductances causes a "ringing" waveform, a damped wave with rapidly alternating polarity. Affected junctions are rapidly stressed in alternating forward and reverse polarization.

ESD damage has four basic mechanisms:

• Oxide rupture/breakdown, occurring at field strengths above 6-10 MV/cm.

Designing the circuit so the oxide layers are protected by junctions with lower avalanche breakdown thresholds than the oxide breakdown voltage can protect against this failure mode.

• Junction damage manifests as increased reverse-bias leakage, up to total shorting. o Junction filamentation, also known as second or thermal breakdown.

Localized overheating causes melting of the silicon; molten silicon has 30 or more times lower resistance; the current through the junction therefore flows in few narrow filaments of high current, resulting in a thermal runaway. In MOSFET devices, the filamentation region is usually near the surface, in the gate-drain overlap region, where the insulator layer serves as a thermal insulation. Devices where the hot spots occur deeper in the structure are less susceptible and are often used as ESD protection structures. Dopants diffuse easily in the molten region; shorter events lead to localized thinning of the base region, resulting in increased leakage, longer events may provide enough time for formation of an ohmic channel across the base, resulting in emitter-collector (or source-drain) short.

-- Junction spiking, where metallization is involved; the aluminum metalized devices are more susceptible as the eutectic melting point of aluminum-silicon alloy is 577 °C instead of 1415 °C for silicon.

Refractory barrier metals inhibit this effect. When the molten silicon region reaches the metallization, a violent exchange of materials occurs.

• Metallization and polysilicon burn-out, where the damage is focused to the conductive and resistive elements - metal and polysilicon interconnects, thin film and diffused resistors. Thin film resistors are the most susceptible. The thermal damage causes localized degradation to destruction of the conductive structure.

The critical current densities can be lower in structures surrounded by thermal insulators where the Joule heat can not easily dissipate.

• Charge injection, where the hot carriers generated by an avalanche breakdown are injected into the oxide layer. This mechanism is the same as for hot carrier injection. The result is an increased leakage current from the FETs prebiased by the injected charges in the gate oxide.

For catastrophic failures, there are three basic ESD-related mechanisms:

• Junction burnout, formation of a conductive path through the junction, shorting it

• Metallization burnout, melting or vaporizing part of the metal interconnect, interrupting it

• Oxide punch-through, formation of a conductive path through the insulating layer between two conductors or semiconductors; the gate oxides are thinnest and therefore most sensitive. The damaged transistor shows a low-ohmic junction between gate and drain terminals.

ESD can cause a parametric performance failure; the device still operates, but its parameters are shifted. The failure may manifest in stress testing. In some cases, the degree of damage can lower over time (so called cold healing). ESD can also cause latent failures, which manifest themselves in a delayed fashion and are difficult to impossible to test for. They have these main reasons:

• Damage to insulators: weakening of the insulator structures, leading to accelerated breakdown and/or increased leakage

• Damage to junctions: lowering the lifetime of minority carriers with consequent bipolar transistor gain loss; increasing resistance in forward biased state; increasing leakage in reverse biased state

• Damage to metallization: weakening of the conductor, leading to increased resistance or increased rate of electromigration Catastrophic failures require the highest discharge voltages; they are easiest to test for, and rarest to occur. Parametric performance failures occur at intermediate discharge voltages and occur more often. Latent failures occur at low voltages and are the most common; for each parametric performance failure there are 4-10 latent ones.

Modern high-integration circuits are more ESD sensitive. The features are smaller, their capacitance is lower and for the same amount of charge the deposited voltage is higher.

The silicidation of the conductive layers makes them more conductive, reducing the ballast resistance that can have a degree of protective role.

The gate oxide of some MOSFET transistors can be damaged by as little as 50 volts potential. The gate is isolated from the junction; potential accumulated on it causes extreme stress on the thin dielectric layer. Stressed oxide can shatter and fail immediately (gate rupture - occurs in nanoseconds, does not require a sustained electric current, and is irreversible; usually the gate and backgate of the affected transistor end up connected together). The gate oxide does not have to fail immediately; the gate leakage can increase by stress induced leakage current, the oxide damage can lead to a delayed failure after hundreds or thousands of operation hours. On-chip capacitors using oxide or nitride dielectric are vulnerable to the same kind of damage. Small structures are more vulnerable than large ones by the virtue of their lower capacitance; the same amount of charge carriers will charge the capacitor the structure forms to a higher voltage. All thin layers of dielectric, e.g. the protective oxide layers over emitter regions of transistors or the insulation between two interconnections, are vulnerable; not just the MOS gates.

Chips made by processes employing thicker oxide layers are less vulnerable.

The degree of gate oxide damage depends on the size of the gate. Older transistors had larger gate regions with higher capacitance; a discharge of the accumulated potential frequently caused a gate rupture, causing a hard breakdown. Newer ultrathin oxide layers are more commonly damaged by a soft breakdown; while the damage is still irreversible, its most significant effect is an increase of the noise voltage of the gate, by up to 4 orders of magnitude. Gate oxide breakdown does not always have to lead to failure; gate-to-channel breakdowns usually do not lead to a hard failure, unlike the gate to-source or gate-to-drain breakdowns.

Gate oxides can also be stressed during manufacture, by so called antenna effect; charges introduced during e.g. ion implantation or dry etching accumulate in conductive structures, causing a voltage buildup across the vulnerable dielectrics.

Breakdown of the gate oxide can form a number of different structures with varying voltage-current characteristics. The simplest case is a connection between two regions with the same doping; a resistive path is then formed. In case of opposite doping regions, the resulting structure is a diode. A metal-to-semiconductor connection may create an ohmic contact or a Schottky diode.

The formation of an ohmic path across an insulator or a junction may not always lead to a hard failure. If the resistance of the path is higher than a critical value, only a degradation of parameters occurs.

Interruption of a conductor may also not always lead to a hard failure. Capacitive coupling can still occur between the structures, maintaining partially degraded functionality, reducing signal strength or affecting gate output voltage range. Faults in drain or source connections of CMOS transistors may lead to formation of quasi-memory cells, making the defect manifestation dependent on previous states of the logic circuit.

Loss of an output driver can lead to high-impedance output at some circuit states; the load capacitance then can maintain previous state for some time as a dynamic memory cell.

Semiconductor junctions are more robust than thin dielectrics. A sufficiently high voltage leads to an avalanche breakdown. The electric current is usually concentrated to a small area, due to current crowding; a semiconductor heated above certain limit starts losing resistance with increasing temperature and the region concentrates more current in itself, leading to more heat production and a thermal runaway. The extreme current density can migrate the metallization and short the junction, the heat can melt and recrystallize or even shatter the junction. Such damage usually manifests as a shorted junction.

Current-induced failures are more common in bipolar junction devices, where Schottky or pn junctions are predominant. The high power of the discharge (often above 5 kilowatts for less than a microsecond) is capable of melting and vaporizing silicon and metal. Thin-film resistors can have their value altered by a discharge path forming a shunt across part of their length (value decrease) or get part of the layer vaporized (value increase); this can be problematic in precision analog applications where the values are critical.

A junction which suffered an avalanche breakdown typically has increased leakage.

Avalanching the base-emitter junction of a NPN transistor permanently lowers its beta.

Device terminals connected only to MOS gates or capacitors are the most vulnerable to the effects of electrostatic discharge. In case both insulated gates and junctions are connected, avalanching usually takes precedence before gate oxide damage. Substrates of chips and large diffusion areas in the semiconductors, e.g. collectors of high-power transistors, are less vulnerable due to their large size and ability to dissipate more energy.

Small diffusion areas, e.g. base and emitter regions of small NPN transistors, are more vulnerable.

Newer CMOS output buffers using a lightly doped drain and silicide source/drain are more ESD sensitive. The N-channel driver usually suffers damage, usually in the oxide layer or n+/p well junction. The damage is caused by current crowding during the snapback of the parasitic NPN transistor. In PMOS-NMOS totem-pole structures, the NMOS transistor is virtually always the one damaged.

Typical ESD-related failure distribution for bipolar/Schottky devices is 90% junction burnout and 10% metallization burnout. For MOSFET devices it is 63% metallization burnout and 27% oxide punch-through. MIL-HDBK-263 offers more complete discussion.

The structure of the junction influences its ESD sensitivity. Corners and defects can lead to current crowding, reducing the damage threshold. Forward-biased junctions are less sensitive than reverse-biased; in the first case the Joule heat is dissipated through the thicker layer of the material while in the latter it is concentrated in the narrow depletion region.

LEDs and lasers grown on sapphire substrate are more susceptible to ESD damage.

In NMOS transistors, the ESD pulse can lead to formation of a metal filament between source and drain, by a phenomenon called electro-thermo-migration. The junction heat can also melt the polysilicon gate, forming polysilicon filaments between gate-source and gate-drain.

The damage can be observed on the I/V curve; undamaged devices show sharp knees, while damaged ones are significantly softened.

Optoelectronics

• List of LED failure modes

• Catastrophic optical damage of semiconductor lasers at high power, when the part is overstressed

• Hydrogen darkening of optical fibers in presence of hydrogen

• Phosphor degradation, in white LEDs, cathode ray tubes, plasma displays, fluorescent lights, etc.

• Corrosion of optical materials; silicate glasses are attacked by moisture. Lenses and optical fibers are prone to deterioration by condensed moisture. Phosphate glasses are more susceptible to corrosion than silica-based glasses, different formulations of glasses can have vastly different.

MEMS

Microelectromechanical systems suffer from specific types of failures.

• Stiction causes the moving parts stick to other surfaces on contact. An external impulse can sometimes release the adhesion and restore functionality. Non-stick coatings, reduction of surface contact area, and increased awareness virtually eliminated the problem in contemporary systems.

• Particle contamination can cause failures by particles lodged between the elements and blocking their movements. Conductive particles may short out circuits, namely electrostatic actuators.

• Wear damages the surfaces; debris, removed from the surfaces during their mutual movements, can be a source of particle contamination.

• Fractures may cause loss of mechanical parts

• Material fatigue may induce cracks in moving structures

• Electrostatic discharge

• Oxide charging, causing electrostatic attraction between parts, may lead to electrostatically mediated stiction

• Dielectric breakdown, causing a short circuit and irreversibly damaging the MEMS structure

Vacuum tubes

• Vacuum tubes and fluorescent lights are susceptible to degradation of hot cathodes, leading to gradual loss of emission of electrons. The vacuum inside the tubes can be compromised by outgassing of materials inside, diffusion of gases through the envelope, or envelope failure. Burnout of the hot cathode filaments leads to a sudden failure. Cold cathode devices tend to be more reliable.

• Multipactor effect

• Phosphor degradation

Passive elements

Resistors

Resistors can fail open (going to infinite resistance), fail short (going to close to zero resistance), or their value can increase or decrease under environmental conditions (e.g.

corrosion, material aging) or because of exceeding of performance limits (e.g. overheating). Thin film resistors are formed from a thin film of a suitable material, e.g. chromium or tantalum nitride.

• Resistor--Failure modes

• The value of certain thin film metal resistors can change when they overheat, due to annealing of their crystalline structure. Extreme overheating may lead to open circuit failure. In integrated circuits, diffused resistors are preferable for applications where high transient currents are to be encountered, e.g. ESD protection, as they are in close contact with the semiconductor substrate which serves as additional heatsinking.

• Mechanical defects from manufacturing can cause intermittent problems or failures. Improperly crimped caps on carbon or metal resistors can become loose and lose constant permanently or intermittently, or the resistor-to-cap interface resistance increase can shift the value of the resistor. Leads to the through-hole resistors are welded to the caps by spark or by butt weld; a faulty weld can become loose. Rough handling can cause such defects or lead to manifestation of latent defects. Thin cracks in the ceramic substrate may cause an open fault, often only annoyingly intermittent.

• Deformation of wire-wound resistors can lead to shorting of adjacent loops of resistive wire, causing partial loss of resistance.

• Operation in oxidizing atmosphere causes oxidation to the outer layer of the resistive wire, reducing its active diameter and increasing its resistance. Operation in reducing atmosphere, e.g. in presence of hydrogen, has the opposite effect.

• Ceramic and carbon resistor cores are prone to cracking under mechanical loads and shocks.

• Under overload, the power dissipated on the resistor can cause heating above the maximum rating, and melting or oxidizing of the resistive element. The protective lacquer or polymer is charred and pyrolyzed, with release of the characteristic smell and possible formation of a conductive path. The resistive element may be interrupted or weakened.

• At high voltages, arcing can occur between parts of the resistor surface, with possible formation of conductive paths or vaporization of the resistive material.

• Damage to insulation layer of the end caps can lead to short between the cap and an underlying circuit board trace.

• Carbon composition resistors, rods of carbon/ceramic composite with embedded leads, may absorb water in humid environments during operation or storage, with resistance changes of as much as 15% up or down.

• Wire-wound resistors are prone to corrosion of the resistive wire. Cap crimping and welds between the wire, caps, and leads are also weak points of this type.

• Metal and carbon film resistors, composed of a thin film of resistive material (typically cut into a spiral) on a (typically) ceramic core, are prone to corrosion and electrolysis of the resistive layer, when the protective layer is penetrated by moisture and contaminants. The electrolytic damage can erode the layer up to an open circuit failure; externally applied voltage is needed for this deterioration mode. Conductive contaminants may form bridges between the loops of the spiral and lead to resistance drop.

• Surface-mount resistors can suffer delamination of their structure where dissimilar materials join, e.g. between the ceramic substrate and the resistive layer, or between the resistive layer and the contact terminations.

• Nichrome thin-film resistors in integrated circuits can be attacked by phosphorus from the passivation glass, which corrodes them and increases their resistance.

• Laser-trimmed resistors may develop instabilities due to the heat damage and microcracks in the resistive layer adjacent to the site of the kerf.

• SMD resistors with silver metallization of termination contacts may suffer open circuit failure in sulfur-rich environment (e.g. close to sulfur-vulcanized rubber), due to buildup of silver sulfide. Presence of either sulfur dioxide (emitted e.g. by heated rubber) and hydrogen sulfide can be the cause. Conformal coatings do not prevent the failure. The failure is however fairly rare.

• Materials may migrate through the resistor structure and alter resistance or cause shorts. Silver can migrate from the electrodes of thick film chip resistors. Silver atoms were detected as far as 100 micrometers from the electrodes. In addition to lowering the resistance, the silver atoms create nonhomogenities in current distribution and lead to current crowding, instability of characteristics over time, and increase of noise.

• Copper dendrites may grow from copper oxide present in some materials (e.g. from the layer facilitating adhesion of metallization to a ceramic substrate), and bridge the trimming kerf.

• The resistive layer of thick film chip resistors may degrade when subjected to overvoltage or overcurrent.

• The electrodes may crack or corrode.

Potentiometers and trimmers

Potentiometers and trimmers are three-terminal electromechanical parts, containing a resistive path with a wiper contact with adjustable position. All the failure modes listed for resistors apply to these parts too. Additionally, mechanical wear on the wiper and the resistive layer, together with corrosion, surface contamination, and mechanical deformations, may lead to intermittent path-wiper resistance changes. These are especially annoying with audio amplifiers. Many types are not perfectly sealed, and contaminants and moisture may readily enter the parts. An especially common contaminant is the solder flux. Mechanical deformations, with impaired wiper-path contact, can occur by warpage of the part housing during soldering or mechanical stress during mounting. Excess stress on leads can cause substrate cracking and open failure when the crack penetrates the resistive path.

Capacitors

Capacitors are characterized by their capacitance, parasitic resistance in series (equivalent series resistance) and parallel (leakage) - both often frequency-dependent and voltage dependent, breakdown voltage, and dissipation factor. Structurally, capacitors consist of electrodes separated by the dielectric (sometimes soaked with liquid electrolyte), the leads, and the housing. Deterioration of any of these structures may cause shift of parameters or open or short failure. Short failures and increase of leakage are the most common failure modes of capacitors, followed by open failures.

• Dielectric breakdown occurs due to overvoltage or aging of the dielectric material leading to breakdown voltage falling below the operating voltage; some types of capacitors are "self-healing", the internal arcing vaporizes parts of the electrodes around the failed spot of the dielectric and breaks the contact, others form a conductive pathway through the dielectric, leading to a short or to partial loss of dielectric resistance.

• Electrode materials may migrate across the dielectric, forming conductive paths.

• Leads can be separated from the capacitor by rough handling during storage, assembly or operation; this leads to an open failure. The failure can occur inside the packaging, invisible from the outside but measurable.

• Dissipation factor may increase due to contamination of the capacitor materials, whether from manufacture or by penetration along the leads or through the packaging. Flux and solvent residues are a common source of problems.

• Leakage can result from contaminants forming conductive paths across the capacitor plates or from altering the dielectric parameters. Moisture or solvents can be absorbed by the dielectric if the coating cracks. Cracks in dielectric can also form leakage paths.

• Excessive charging or discharging current may cause partial fusing of the electrodes, leading to open failure or to capacitance shift. Excessive charging or discharging currents may also fuse the leads to the electrodes.

• Partial de-bonding of leads can cause loss of capacitance in multilayer capacitors.

• Delamination of multilayer capacitors may cause partial capacitance loss and deterioration of other electrical parameters.

• Voids, cracks, pores and other defects may impair the parameters of the dielectric, causing leakages, loss of capacitance, and loss of maximum breakdown voltage.

• Cracks in the dielectric may present pathways for metal migration and arcing.

• Chemical attack on the dielectric may degrade the insulation resistance.

• Thermal shocks, especially during soldering, may cause mechanical defects in the parts, and alter (temporarily or permanently) the properties of the dielectrics.

• Capacitors in low-voltage high-impedance applications generally fail due to lowered dielectric resistance. Capacitors in high voltage and/or low impedance applications tend to fail due to dielectric breakdown.

• Pinholes and voids cause increased leakage in film capacitors. When sufficient current is available, the defect site can be burned away and the capacitor "self heals". When insufficient current is available, the current can cause migration of electrode material through the defect, gradually decreasing the leakage path resistance.

• Film capacitors are sensitive to excessive ripple voltage; the thin metallization on the dielectric may not sustain high currents, and partially fuse with a loss of capacitance.

• Polymer film dielectrics tend to become brittle with age, especially at increased temperature. Fractures may develop. Moisture may cause degradation of the polymer.

• Corrosion and cracking of the internal leads can cause open failure.

• Overvoltage in a ceramic capacitor may cause an avalanche breakdown in a weak spot of the dielectric, resulting in a thermal runaway. The damaged area may show localized melting and cracking. The energy deposited in the failure point can be high enough to cause the capacitor to explode, create a miniature fireball, and char the encapsulation and possibly the underlying circuitboard.

• Mica capacitors may show intermittent open failures from separation of the electrode plates from the termination at certain temperatures. Reconstituted mica capacitors may fail short due to weaknesses in the dielectric. As mica capacitors are typically used in high voltage applications, the results may be severe.

Electrolytic capacitors

• Aluminum electrolytic capacitors suffer from gradual increase of leakage and equivalent series resistance, and loss of capacitance due to drying out of the electrolyte. Power dissipation due to high ripple currents and high internal resistance cause rise of the internal temperature of the capacitor, which accelerates the deterioration rate, especially when the internal temperature exceeds maximum design temperature. Such capacitors usually fail short.

• Electrolyte contamination, especially with moisture, can lead to corrosion of the electrodes. The deterioration of their area can lead to capacitance loss, loose particles of conductive material can cause shorts.

• Electrolyte may under certain conditions evolve gas. This leads to increased pressure inside the capacitor housing, in extreme but frequent cases leading to an explosion.

-- Capacitor plague is an extreme kind of premature electrolytic capacitor degradation and failure.

• Tantalum capacitors are susceptible to electrical overstress. Exceeding of the maximum surge voltage may permanently degrade the dielectric and even cause open or short failure.

• The most common failure mode of solid tantalum capacitors is an electrical short.

The most common manufactured-related cause of this failure is a defect in the dielectric oxide.

• The failure locations may be visible as discolored dielectric, or as locally melted anode.

• Defects in the tantalum chip anode material, e.g. impurities, are transferred to the dielectric oxide layer formed during the capacitor manufacture. These locations then present weak spots that may fail prematurely. Thermal shock e.g. during soldering may generate new defects.

• The thickness of the oxide layer is limited by the size of the powder from which the tantalum slug is made.

• On voltage transients, tantalum capacitors may show a momentary short or increase of leakage current, called "scintillation". If the current through the failure site is small, the site self-heals; if the current is sufficient to heat the site above 400 °C, thermal runaway occurs and the part shorts.

Crystals

Crystals for crystal oscillators are thin slabs of a piezoelectric material, typically quartz, with deposited electrodes on the slab surfaces, mounted in a hermetically sealed housing, providing the circuit with oscillations at a stable frequency.

• Cracks of crystal slab can occur at extremely high drive levels (especially with low-frequency crystals), or due to mechanical shocks strong enough to make the crystal impacts the inside of the housing.

• Failure of lead connection can lead to an open circuit failure.

• Outgassing of materials inside the housing, most often improperly cured epoxy, can lead to frequency shifts of the crystal.

• Moisture present in the housing can condense on the crystal at low temperatures, causing significant frequency shifts.

• Ionizing radiation and neutron radiation can cause frequency shifts. Swept quartz, from which the mobile alkali metal ions were removed, is used for rad-hard crystals.

• De-hermetization of the housing, by rough handling or corrosion, may lead to penetration of contaminants and moisture into the housing, with consequent changes in resistance and frequency, and possible lead-to-lead and lead-to packaging leakage paths.