Power MOSFET devices are extensively used in the automotive industry, but their modes of ageing are still poorly understood. Here we focus on the physical degradation mechanisms that occur in the upper Al-based metallization layer (source). This layer undergoes thermo-mechanical structural modifications due to the combination of electrical pulses and differences between the various coefficients of thermal expansion. Using electronic and ionic microscopy, we show that ageing can be divided in 2 phases where dislocation-based plasticity and then grain boundary diffusion become predominant. As a result, grain boundary grooving and surface roughening follows a partial division of the later in disconnected Al grains. Such a degradation of the metallization has been widely observed in various devices. It may lead to the observed augmentation of resistivity and also to the focusing of the various current paths, promoting hot spots and subsequent failure. (C) 2014 Elsevier Ltd. All rights reserved.

The fatigue-induced modifications of the structure of power devices on components that underwent extreme electro-thermal fatigue were investigated, with special attention to the behaviour of the Al metallization layer. Electrical cycling induced a moderate grain growth in the metallization layer, except for the regions located beneath the connection wires. There the wire bonding process is associated with severe plastic deformation, leading to a fragmentation of the initial grains and significant grain growth. Grain-boundary grooving is another example of fatigue-induced structural changes: it reveals an intense stress- and temperaturedriven grain boundary diffusion process.

Extreme electro-thermal fatigue tests have been performed to failure on power MOSFET devices that were later observed using electron and ion microcopy. At variance with devices from the former technology generation, fatigue-induced ageing of these components is observed only in the source metallization zone. An increase in drain–source resistance may originate from both a loss of contact between the wire bondings and the Al layer and/or an extensive decohesion between the metal grains. Failure modes include local melting of the Al and creation of eutectic alloys.

Strain relaxation in low-mismatch ZnSe/GaAs heterostructures is studied by transmission electron microscopy. The early stages of plastic relaxation proceeds by activation of secondary 1/2 < 011 >{133}slip systems, leaving an array of misfit dislocations aligned along < 310 > directions in the interface. Threading dislocations originate from randomly distributed, highly strained single-or multi-twinned regions in the ZnSe layer, that are probably due to growth accidents. The influence of cross slip on the propagation of threading dislocations is discussed. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A method based on the failure analysis of power MOSFET devices tested under extreme electrothermal fatigue is proposed. Failure modes are associated to several structural changes that have been investigated through acoustic, electron and ion microscopy. The main aging mode is related to the exponential increase in drain resistance due to delamination at the die attach. Earlier failures are observed when very local defects due to electrical over stresses (EOS) occur at the source metallization or at the wire bonding. Aging models were elaborated to account for the die attach delamination, but are still lacking to take in account the structural evolution of the Al metallization. This new methodology, based on accelerated tests and structural observations aims at designing a new generation of power components that will be more reliable.

Microstructural analysis of power devices were carried out on components from Freescale Semiconductor that underwent extreme electrothermal fatigue. Several destructive and non destructive techniques were used. Altered layers are located on both sides of the Si substrate : at the heat sink (drain) and the Al metallization (source). Here, we have tried to establish a link between the increase of the transistor resistance and the amount of delamination of the solder at the die attach on one side and to the evolution of grain size and intergranular grooving on the other. Concerning the Al grain size, we will show that this parameter can be taken as a fatigue marker only in definite regions of the metallization. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In situ straining experiments in a transmission electron microscope provide a unique way to investigate in real time the influence of various parameters (temperature, electron beam intensity, etc.) on the dislocation behaviour in semiconductors. A systematic study of the influence of electronic excitation on the dislocation behaviour in single phase ZnS crystals is reported. The observed radiation enhanced dislocation motion is attributed to a lowering of the lattice friction, due to non-radiative recombination of carriers at electronic levels associated with dislocations. Analysis of the results makes it possible to determine which elementary mechanism of dislocation glide is affected by this effect. The defect dynamics in a ZnSe/GaAs heterostructure in the course of in situ heating experiments is investigated. Different dislocation mechanisms are analyzed, which emphasize the influence of electronic excitation on the dislocation behaviour. The contribution of these mechanisms to the strain relaxation is discussed.

Transmission electron microscopy is used to investigate the microstructure evolution of a ZnSe/GaAs layer under the simultaneous influence of electron beam irradiation (in the range 1000–5000A/m2) and specimen heating (in the range ambient—250°C). Degradation of the layer is connected to the nucleation and growth of dislocation loops. At 150°C, loops nucleate on pre-existing misfit dislocations lying in 〈310〉 directions by point defect accumulation consistent with a climb mechanism. At 250°C, large zones of very tangled dislocations propagate rapidly mainly by climb under electron beam excitation leading to a complete degradation of the ZnSe layer. The elementary mechanisms involved in dislocation multiplication are enhanced by non-radiative recombination of electron hole pairs.

In this paper, an innovative methodology for predictive reliability of intelligent power devices used in automotive applications is considered. Reliability management is done at all levels of the technological process. This method is based on the failure analysis along with electro-thermo- mechanical modeling and on extreme fatigue testing. A new power MOS device has been electrically fatigued in order to evaluate its failure modes. Using a thermally regulated test bench, electrical pulses were applied to the device until failure. This failure is associated to several structural changes that have been investigated through acoustic and electron microscopy. Delamination was observed preferentially at the solder between the copper heat sink and the die.

Strain relaxation in ZnSe/GaAs(001) heterostructure grown by molecular beam epitaxy is studied by transmission electron microscopy. In as-grown samples, an array of perfect misfit dislocations, lying along 310 directions, with Burgers vector (1/2)011 inclined to the interface is observed. The corresponding threading segments propagate by glide in {331} planes, leaving misfit segments in the interface. From a mechanical equilibrium analysis, it is concluded that, in the case of low misfit (0.27%), the critical thickness for {331} planes is less than for {111} glide. Dislocations with the (1/2)011 Burgers vector lying along 310 directions are more efficient at relaxing the misfit strain than dislocations lying along 110 directions.

A novel convergent-beam-electron-diffraction method is discussed, which permits us to determine unambiguously the polarity of {001}-oriented semiconductor compounds with sphalerite structure. It is based on a comparison between experimental patterns and simulated patterns. At variance with {110}-oriented crystals, the polarity cannot be uniquely determined from observed intensity asymmetry in the ±{200} discs. Additional features have to be considered, such as the geometry and intensity of Kikuchi lines in the transmitted disc. Examples from ZnSe crystals are given.

The dynamic behaviours of several types of defects (single stacking faults, paired stacking faults, half loops of Shockley partial dislocation, misfit dislocations aligned along <310>) are analysed. Heating the sample up to 300degreesC causes the formation of a grid of edge dislocations. Finally, both heating and electron irradiation induce rapid degradation of the active layer.

In situ transmission electron microscopy experiments provide a unique way to investigate in real time the dislocation behaviour at a microscopic scale and to decide which elementary process controls the dislocation glide in semiconductors. In this review the experimental results obtained on different semiconductors are presented and discussed. Particular attention is devoted to the radiation-enhanced glide process.

TEM in situ straining experiments provide a unique way to investigate in real time the behaviour of individual dislocations under applied stress. The results obtained on a variety of semiconductors are presented: numerous dislocation sources are observed which makes it possible to measure the dislocation velocity as a function of different physical parameters (local shear stress, temperature, dislocation character, length of the moving dislocation,...). The experimental results are consistent with a dislocation glide governed by the Peierls mechanism, even for II-VI compounds which have a significant degree of ionic character.For compounds, a linear dependence of the dislocation velocity on the length of the moving segment is noticed, whereas for elemental semiconductors a transition between a length-dependent and a length-independent velocity regime is observed. Analysed in the framework of the kink diffusion model (Hirth and Lothe theory), these results allow an estimation of the kink formation and migration energies.For a variety of semiconductors, the dislocation behaviour is sensitive to electronic excitations. A strong increase of dislocation mobility with increasing electron beam intensity is observed (radiation-enhanced dislocation glide). It is attributed to a lowering of the lattice friction, due to non-radiative recombinations of electronic carriers at dislocation sites.

The repetitive energy discharge test (power cycling) is an accelerated stress test (AST) that can be used to characterize the long-term behavior of power transistors taking into account stress on customer final application. This paper describes the application of this test to an LDMOS transistor in order to optimize both design and size while preserving final reliability of the product.We will detail this reliability characterization program, emphasizing the power cycling tests performed in extreme conditions in order to reach and study physical limit of LDMOS devices.Analysis of LDMOS devices under these extreme conditions indicated evidence of metal voiding: the conditions of formation of these thermal fatigue induced voids will be discussed.The effect of major parameters like power cycling energy, ambient temperature will also be discussed. Electrical characterization of stressed devices will be presented. Finally, the results of the Transmission Electron Microscopy characterization of the metal microstructure will be discussed.

Vickers indentation tests have been performed on the (001) faces of GaAs single crystals, in darkness and under laser light illumination with a wavelength close to the band absorption edge. When low loads (0.196 N or less) are applied to the indenter, illumination results in a decrease in the Vickers hardness. This confirms the softening effect of photonic excitation (negative photoplastic effect), as previously reported by Mdivanyan and Shikhsaidov (1988, Phys. Stat. sol., (a), 107, 131) from compression experiments. However, the effect of illumination is less marked on microhardness than on plastic flow. The spectral dependence of the negative photoplastic effect has been investigated; it is shown that the mechanism responsible for the illumination-induced softening is operative on both sides of the band absorption edge. The defect structure around the microindents has been studied by transmission electron microscopy (200 kV and 1 MV), with particular attention to indentation rosettes. Rosette arms (which expand along perpendicular (110) directions) contain perfect dislocations with Burgers vector parallel to the surface, but microtwins are formed only in (111) planes in zone with [110]. Perfect dislocations nucleate in the bulk as elongated half-loops; in contrast, twinning dislocations nucleate on the indented surface. In darkness, perpendicular rosette arms have approximately the same length; the alpha-beta asymmetry is not observed in the experimental conditions (light applied load, room temperature). Under infrared illumination the rosette pattern presents a well marked twofold symmetry; the movement of alpha dislocations is enhanced under photonic excitation whereas the movement of beta dislocations appears rather insensitive to illumination. These results are discussed in connection with the radiation-enhanced dislocation glide mechanisms.

ZnSe thin films grown on GaAs(001) substrate by molecular beam epitaxy to a thickness of 2500 Å have been studied by transmission electron microscopy (TEM). Three types of structural defect have been observed: (i) Triangle-shaped stacking faults, with the apex close to the interface, either isolated or paired. They are bounded by two different Shockley dislocations. (ii) Stacking faults generated from the surface of the ZnSe epilayer by movement of a Shockley half-loop. (iii) An array of perfect misfit dislocations. Their Burgers vectors are inclined to the interface. Most of them lie along ⟨310⟩ directions; only a few are parallel to ⟨110⟩.

Transmission electron microscope in situ deformation experiments have been performed on ZnS and ZnSe single crystals to get quantitative information on the effect of electronic excitation on dislocation movement. The dislocation mobility is strongly enhanced by electron irradiation as a result of the lowering of the lattice friction. The observed reduction in activation energy is discussed in terms of the radiation-enhanced dislocation glide mechanism, due to nonradiative recombination of injected carriers at electronic levels associated with dislocations. (C) 1999 Elsevier Science B.V. All rights reserved.

The characteristic features of the dislocation microstructure that develops around indentations performed at room temperature on (001) GaAs single crystals are analysed by High Voltage Transmission Electron Microscopy. It is shown that perfect rosette dislocations (with Burgers vector parallel to the surface) are nucleated deeply in the crystal and never merge at the sample surface. Perpendicular rosette arms have similar lengths, which raises the question of the a/b asymmetry. Twins are present in one rosette arm only; twinning dislocations are nucleated on ± or very close to ± the indented surface. These results are discussed in connection with previously reported results.