Semiconductors
Contenido
Muchas fallas dan como resultado la generación de electrones calientes. Estos son observables bajo un microscopio óptico, ya que generan fotones de infrarrojo cercano detectables por una cámara CCD. Los enclavamientos "Enclavamiento (electrónica)") se pueden observar de esta manera.[8] Si es visible, la ubicación de la falla puede presentar pistas en donde se ve la sobrecarga. Los recubrimientos de cristal líquido se pueden utilizar para la localización de fallas: los cristales líquidos colestéricos son termocrómicos y se usan para visualizar las ubicaciones de producción de calor en los chips, mientras que los cristales líquidos nemáticos responden al voltaje y se usan para visualizar fugas de corriente a través de defectos de óxido y de carga. estados en la superficie del chip (particularmente estados lógicos).[2] El marcado láser que marca los paquetes encapsulados en plástico puede dañar el chip si las esferas de vidrio en el empaque se alinean y dirigen el láser hacia el chip.[3].
Ejemplos de fallas de semiconductores relacionadas con cristales semiconductores incluyen:.
• - Nucleación y crecimiento de dislocaciones "Dislocación (defecto cristalino)"). Esto requiere un defecto existente en el cristal, como lo hace la radiación, y es acelerado por el calor, la alta densidad de corriente y la luz emitida. Con los LED, el arseniuro de galio y el arseniuro de galio de aluminio son más susceptibles a esto que el fosfuro de arseniuro de galio y el fosfuro de indio; el nitruro de galio y el nitruro de galio de indio son insensibles a este defecto.
• - Acumulación de portadores de carga atrapados en el óxido de compuerta de los MOSFET. Esto introduce una corriente de polarización de compuerta permanente, que influye en el voltaje umbral del transistor; puede ser causada por la inyección de portadores calientes, radiación ionizante o uso nominal. Con las células EEPROM, este es el factor principal que limita el número de ciclos de borrado-escritura.
• - Migración de portadores de carga desde puertas flotantes. Esto limita la vida útil de los datos almacenados en estructuras EEPROM y EPROM flash.
• - Pasivación inadecuada. La corrosión es una fuente importante de fallas retrasadas; los semiconductores, las interconexiones metálicas y las gafas de pasivación son susceptibles. La superficie de los semiconductores sometidos a humedad tiene una capa de óxido; el hidrógeno liberado reacciona con capas más profundas del material, produciendo hidruros volátiles.[9].
Parameter errors
Vias are a common source of unwanted series resistance in chips; defective vias show unacceptably high resistance and thus increase propagation delays. As its resistivity decreases with increasing temperature, degradation of the chip's maximum operating frequency otherwise is an indicator of such failure. Mouse bites are regions where the metallization has a decreased width; Such defects generally do not show up during electrical testing, but present a significant reliability risk. Increased current density in the mouse bite may aggravate electromigration problems; A large degree of urination is needed to create a temperature-sensitive propagation delay.[8].
Sometimes circuit tolerances can make it difficult to track erratic behavior; For example, a weak driver transistor, higher series resistance, and subsequent transistor gate capacitance may be within tolerance, but may significantly increase signal propagation delay. These may manifest themselves only under specific environmental conditions, high clock speeds, low power supply voltages, and sometimes specific circuit signal states; Significant variations can occur on a single die.[8] Damage induced by excessive stress, such as ohmic shunts or reduced transistor output current, can increase such delays, leading to erratic behavior. As propagation delays depend largely on the supply voltage, fluctuations linked to the tolerance of the latter can trigger such behavior.
Gallium arsenide monolithic microwave integrated circuits can have these flaws:[10].
• - Degradation of I[11] due to gate collapse and hydrogen poisoning. This fault is the most common and easiest to detect, and is affected by the reduction of the active channel of the transistor in gate sag and the depletion of the donor density in the active channel for hydrogen poisoning.
• - Degradation in gate leakage current. This occurs under accelerated life or high temperature tests and is suspected to be caused by surface state effects.
• - Degradation in pinch tension. This is a common failure mode for gallium arsenide devices operating at high temperature, and primarily arises from semiconductor-metal interactions and degradation of the gate metal structures, with hydrogen being another reason. It can be hindered by a suitable barrier metal between the contacts and the gallium arsenide.
• - Increased drain-to-source resistance. It is observed in high-temperature devices and is caused by metal-semiconductor interactions, gate sag, and ohmic contact degradation.
Metallization failures
Metallization failures are more common and serious causes of FET transistor degradation than material processes; Amorphous materials do not have grain boundaries, making interdiffusion and corrosion difficult.[12] Examples of such failures include:.
• - Electromigration moves atoms out of the active regions, causing dislocations and point defects that act as non-radiative recombination centers that produce heat. This can occur with aluminum gates in MESFETs with RF signals, causing erratic drain current; Electromigration in this case is called gate sag. This problem does not occur with gold doors.[12] With structures that have aluminum over a refractory metal barrier, electromigration primarily affects the aluminum, but not the refractory metal, causing the resistance of the structure to increase erratically. Displaced aluminum can cause short circuits to neighboring structures; 0.5-4% copper in aluminum increases resistance to electromigration, copper builds up at the alloy grain boundaries and increases the energy needed to dislodge atoms from them.[13] Apart from that, indium tin oxide and silver are subject to electromigration, causing leakage current and (in LEDs) non-radiative recombination along chip edges. In all cases, electromigration can cause changes in the dimensions and parameters of transistor gates and semiconductor junctions.
• - Mechanical stresses, high currents and corrosive environments forming whiskers and short circuits. These effects can occur both within the packaging and on the circuit boards.
• - Formation of silicon nodules. Aluminum interconnects can be doped with silicon to saturation during deposition to avoid alloying peaks. During thermal cycling, silicon atoms can migrate and clump together forming nodules that act as voids, increasing local resistance and reducing device life.[2].
• - Degradation due to ohmic contact between the metallization and semiconductor layers. With gallium arsenide, a layer of gold-germanium alloy (sometimes with nickel) is used to achieve low contact resistance; An ohmic contact is formed by diffusion of germanium, forming a thin, highly doped region under the metal that facilitates the connection, leaving gold deposited on it. Gallium atoms can migrate through this layer and be removed by the gold above, creating a defect-rich gallium-depleted zone under the contact; The gold and oxygen then migrate oppositely, resulting in increased ohmic contact resistance and depletion of the effective doping level.[12] The formation of intermetallic compounds also plays a role in this failure mode.
electrical overload
Most stress-related semiconductor failures are microscopically electrothermal in nature; Locally increasing temperatures can lead to immediate failure by melting or vaporizing metallization layers, melting the semiconductor, or changing structures. Diffusion and electromigration tend to be accelerated by high temperatures, shortening the life of the device; Damage to junctions that does not lead to immediate failure may manifest as altered current-voltage characteristics of the junctions. Electrical overload faults can be classified as thermally induced, electromigration-related, and electric field-related faults; Examples of such failures include:
• - Thermal runaway, where clusters in the substrate cause localized loss of thermal conductivity, leading to damage that produces more heat; The most common causes are voids caused by incomplete soldering, electromigration effects, and Kirkendall urination. The clustered distribution of current density over the junction or current filaments leads to current crowding at localized hot spots, which can evolve into thermal runaway.
• - Reverse bias. Some semiconductor devices are based on the junction of diodes and are nominally rectifiers; However, the reverse breakdown mode can be at a very low voltage, with a moderate reverse bias voltage causing immediate degradation and greatly accelerated failure. 5V is a maximum reverse bias voltage for typical LEDs, and some types have lower figures.
• - Zener diodes severely overloaded in reverse bias short circuit. A sufficiently high voltage causes avalanche breakdown of the Zener junction; that and a large current passing through the diode causes extreme localized heating, melting the junction and metallization and forming a silicon-aluminum alloy that shorts the terminals. This is sometimes used intentionally as a method of wiring connections through fuses.[13].
• - Latching (when the device is subject to an overvoltage or undervoltage pulse); a parasitic structure acting as a triggered SCR can cause an overcurrent-based fault. In MIC circuits, latches are classified as internal (such as transmission line reflections and ground bounces) or external (such as signals introduced through I/O pins and cosmic rays); External closures can be triggered by electrostatic discharge, while internal closures cannot. Latches can be activated by charge carriers injected into the chip substrate or other latch; The JEDEC78 standard tests for snag susceptibility.[8].
electrostatic discharge
Electrostatic discharge (ESD) is a subclass of electrical overload and can cause immediate device failure, permanent parameter changes, and latent damage causing a higher rate of degradation. It has at least one of three components, localized heat generation, high current density and high electric field gradient; The prolonged presence of currents of several amperes transfers energy to the structure of the device to cause damage. ESD in real circuits causes a damped wave with rapidly alternating polarity, the junctions are stressed in the same way; It has four basic mechanisms:[14].
• - Oxide decomposition occurs at field strengths greater than 6–10 MV/cm.
• - Junction damage manifesting as reverse bias leakage increases to the point of short circuit.
Polysilicon metallization and depletion, where damage is limited to metal-polysilicon interconnects, thin film resistors, and diffuse resistors.
• - Charge injection, where hot carriers generated by avalanche decay are injected into the oxide layer.
ESD catastrophic failure modes include:
• - Junction depletion, where a conducting path forms through the junction and shorts it.
• - Depletion by metallization, where the melting or vaporization of a part of the metallic interconnection disrupts it.
• - Oxide perforation, formation of a conductive path through the insulating layer between two conductors or semiconductors; gate oxides are thinner and therefore more sensitive. The damaged transistor shows a low ohmic junction between the gate and drain terminals.
A parametric fault only changes the parameters of the device and can manifest itself in stress tests; Sometimes the degree of damage can decrease over time. Latent ESD failure modes occur in a delayed manner and include:.
• - Damage to the insulation due to weakening of the insulating structures.
• - Junction damage by reducing minority carrier lifetime, increasing forward bias resistance and increasing reverse bias leakage.
• - Damage due to metallization due to weakening of the conductor.
Catastrophic failures require the highest discharge voltages, are the easiest to test, and are rarest to occur. Parametric faults occur at intermediate discharge voltages and occur more frequently, then latent faults are the most common. For every parametric fault, there are 4 to 10 latent ones.[15] Modern VLSI circuits are more sensitive to ESD, with smaller characteristics, lower capacitance, and higher voltage-to-charge ratios. The deposition of silicon from the conductive layers makes them more conductive, reducing the resistance to ballast which has a protective role.
The gate oxide of some MOSFETs can be damaged by 50 volts of potential, the gate isolated from the junction and the potential building up on it causing extreme stress on the thin dielectric layer; Stressed rust can break down and fail immediately. The gate oxide itself does not fail immediately, but can be accelerated by stress-induced leakage current,