Stress-voiding of narrow conductor lines

The growth of voids in conductor lines that have no applied voltage nor imposed thermal or concentration gradients is examined. Such voids can cause narrow aluminium conductors in silicon ICs to fail spontaneously. Recent attempts to describe void growth mathematically as a stress-driven diffusive phenomenon are reviewed, and an expression for the time dependent void size is derived. The equation is used to explore the many variables of the void-growth problem     

Electromigration failure distributions of dual damascene Cu /low – k interconnects

The reliability of dual damascene Cu/low – k interconnects is limited by electromigration – induced void formation at vias. In this paper we investigate via void morphologies and associated failure distributions at the low percentiles typical of industry reliability requirements. We show that Cu/low – k reliability is fundamentally limited by the formation of slit – voids under vias. Using experimental and simulation approaches we clarify the practical importance of apparent incubation phenomena associated with this failure mode.     

A study of void defects in metalorganic molecular-beam epitaxy grown HgCdTe

Void defects that occur under Hg deficient conditions during the metalorganic molecular beam epitaxy (MOMBE) growth of HgCdTe have been characterized using secondary electron microscopy (SEM) and energy dispersion spectrometry (EDS) mapping as well as by EDS quantitative analysis. For a set of HgCdTe samples grown under a range of Hg fluxes, it was found that the surface morphology had a significant dependence on the Hg flux. An optimum growth window defined by a narrow range of Hg fluxes was identified in which there exists a smooth surface with few voids, whereas at either side of the Hg window surfaces were rough. This surface morphology correlated very well with a minimum in the x-ray line widths and maximum hole concentration and mobility values. This correlation is important for the growth of HgCdTe materials and subsequent device fabrication. Several types of void morphologies have been observed with different correlation to Te and Hg. It was found that there is a pronounced Te enrichment and Hg deficiency associated with most of the developed voids, as compared to the composition of the HgCdTe films. It was also found that most of the voids originated within the HgCdTe film. A mechanism for void formation and growth is proposed. In addition, it was found that annealing caused the voids to separate from the HgCdTe film.     

Electromigration behaviour of SiO2-covered,large-grained,narrow AlSi lines with ohmic contacts

Starting from large-grained, narrow AlSi-lines which were characterized by homogeneous bulk mass flow unde rstress conditions, electromigration tests were performed with 8 × 10⁵ A/cm² at 200°C, using an additional SiO₂ passivation layer. The covering film drastically degraded electromigration behaviour. Premature failures were produced by contact openings, increased contact resistances and stripe interruptions. On the one hand, the covering films increased the Al drift velocity substantially. On the other hand, AlSi-compounds or Si-nodules were formed, which favoured void growth and stripe interruptions. Those interruptions even occurred if the net Al mass flow seemed to be negligible. When Si was omitted, failure times increased considerably.     

Electromigration—A brief survey and some recent results

Recently, electromigration has been identified as a potential wear-out failure mode for semiconductor devices employing metal film conductors of inadequate cross-sectional area. A brief survey of electromigration indicates that although the effect has been known for several decades, a great deal of the processes involved is still unknown, especially for complex metals and solute ions. Earlier design equations are improved to account for conductor film cross-sectional area as well as film structure, film temperature, and current density. Design curves are presented which permit the construction of high reliability "infinite life" aluminum conductors for specific conditions of maximum current and temperature stress expected in use. It is also shown that positive gradients, in terms of electron flow, of temperature, current density, or ion diffusion coefficient foreshorten conductor life because they present regions where vacancies condense to form voids.     

Correlation of electromigration lifetime distribution to failure mode in dual Damascene Cu/SiLK interconnects

The electromigration cumulative percent lifetime probability of dual Damascene Cu/SiLK interconnects was fitted using three, individual lognormal functions where the functional populations were grouped by void growth location determined from focused ion beam failure analysis of all 54 of the stressed structures. The early, first mode failures were characterized by small voids in the bottom of the vias. The intermediate mode failures had voids in the line and via bottom while the late mode failures had voids that formed in the line only. The three, individual lognormal functions provided good fits of the data. Failure mode population separation using comprehensive failure analysis suggested that only the first mode failures should be used in the prediction of the chip design current.     

Formation and control of defects during molecular beam epitaxial growth of HgCdTe

Void defects were demonstrated to form away from the substrate-epifilm interface during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride substrates. These were smaller in size compared to voids which nucleated at the substrate-epifilm interface, which were also observed. Observations of void nucleation away from the substrate-epifilm interface were related to the respective growth regimes active at the time of the void nucleation. Once nucleated, voids replicated all the way to the surface even if the flux ratios were modified to prevent additional nucleation of voids. For a significant number of films, void defects were observed co-located with hillocks. These voids were usually smaller than 1 μm and appeared almost indistinguishable from unaccompanied simple voids. However, these void-hillock complexes displayed a nest of dislocation etch pits around these defects upon dislocation etching, whereas unaccompanied voids did not. The nests could extend as much as 25 μm from the individual void-hillock complex. The density of dislocations within the nest exceeded 5×10⁶ cm⁻², whereas the dislocation density outside of the nest could decrease to <2×10⁵ cm⁻². The void-hillock complexes formed due to fluctuations in growth parameters. Elimination of these fluctuations drastically decreased the concentrations of these defects.     

A compact model for early electromigration failures of copper dual-damascene interconnects

A compact model for early electromigration failures in copper dual-damascene interconnects is proposed. The model is based on the combination of a complete void nucleation model together with a simple mechanism of slit void growth under the via. It is demonstrated that the early electromigration lifetime is well described by a simple analytical expression, from where a statistical distribution can be conveniently obtained. Furthermore, it is shown that the simulation results provide a reasonable estimation for the lifetimes.     

Adhesion,phase morphology,and bondability of reacttvely-bonded and frit-bonded gold and silver thick-film conductors

The phase morphologies of reactively-bonded commercial gold and similarly bonded silver thick-film conductors have been characterized as a function of firing conditions and material types. Adhesion strengths of the samples were measured by a solderless technique, and the strengths are correlated in this presentation with the development of phase morphologies in the film and with the adhesion data of other investigators. The bondability-adhesion strength trade-off for frit-bonded and reactively-bonded conductors is discussed in view of an analysis for potential bondability which was made using a recently developed technique. First-order theories of adhesion development in reactively-bonded and frit-bonded conductors are briefly discussed, and conclusions are drawn for optimizing firing parameters in the processing of frit-bonded and reactively-bonded thick film conductors.     

Acceleration of the growth of Cu3Sn voids in solder joints

Soldering to Cu surface finishes usually leads to the formation of a bi-layer intermetallic structure, Cu₃Sn/Cu₆Sn₅, that provides a more robust bond than common alternatives. Occasionally, and so far unpredictably, voids may however grow within the Cu₃Sn over time and allow for premature failure of microelectronics products in service. A quantitative assessment of the reliability risk of voids observed after accelerated aging requires the knowledge of the variation of void growth with temperature and time. It is argued that in the case of realistic solder joints diffusion controlled void growth kinetics are unlikely to follow simple Arrhenius and parabolic dependencies, respectively. Nevertheless, three very different sets of samples were all shown to exhibit void growth that could be well approximated by a parabolic time dependence and an effective activation energy of 0.65–0.80 eV.     

Electromigration lifetime sudies of submicrometer-linewidth Al-Cu conductors

Electromigration in metal conductors used in VLSI circuits raises important concerns especially at submicrometer dimensions. In this paper, we show that the current-carrying capability required in submicrometer MOS technology can be quite severe. We show experimentally that the mean time to fail for Al-Cu conductors increases as the linewidth decreases below about 2 µm and well into the submicrometer regime. Concomitant with this increase in the mean time to fail, there is an increase in σ, the spread of the failure distribution as well, leading to decreased reliability at early times for very narrow lines. Grain size and geometry are used to explain our results. Our studies also show that the applicability of an unenhanced "lift-off" defined Al-Cu metallurgy for submicrometer NMOS application needs careful examination.     

The effects of hydrogen ambients on electromigration kinetics in A1–2% Cu thin film conductors

Surface chemical phenomena are likely to dominate the overall behavior of a system when the majority of the mass in the system is present as surface mass. Thin film interconnections of small dimensions [Large Scale and Very Large Scale Integration (LSI and VLSI)]fall into this class due to the presence of external and internal (including grain and inter-phase boundaries) surfaces. Previous investigations have strongly suggested that the presence of H₂, presumably through surface interactions, can greatly reduce electromigration damage (EMD) leading to higher reliability for device operation. Yet, no systematic studies have been reported.We have applied the Temperature-ramp Resistance Analysis to Characterize Electromigration (TRACE) technique to examine the effect of hydrogen ambients on EMD kinetics and its effect on solid-state transport and on the void morphology of A1–2% Cu thin film conductors.Systematic variation of H₂ ambients of varying composition and variation of the heating rate during the TRACE experiment show that the apparent activation energy for EMD remains constant while the pre-exponent changes. The results confirm a substantial improvement in EMD resistance in H₂ containing ambients. The variation in the pre-exponential is consistent with a transport process controlled by the nucleation of voids and their subsequent growth.     

The influence of TiN ARC thickness on stress-induced void formationin tungsten-plug vias

This paper describes a new failure mechanism in W-plug vias, and the process conditions which enhance it. For submicron technologies, the limiting factor in interconnect reliability performance is increasingly dominated by the electromigration resistance of tungsten-plug vias. We have observed that under certain experimental conditions, early electromigration failures can be induced in via-chain test structures. We have demonstrated that these are caused by stress-induced void formation in the metal line immediately beneath the tungsten plug. This is thought to be due to highly localized film stress around the base of the plug, which can be minimized by increasing the thickness of the TiN anti-reflective coating (ARC). This has the effect of reducing the incidence of early failure by suppressing the stress-induced failures     

Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.     

Application of submillimeter wave lasers to high voltage cable inspection

An experimental device designed for real-time, nondestructive inspection of cross-linked polyethylene insulation during cable manufacture has been built, analyzed, and tested under laboratory conditions. The device detects scattered far-infrared (FIR) laser energy at 118.8 μm from imperfections, such as voids and contaminants, known to have a serious deleterious influence on cable in-service lifetime. As presently configured, the device can readily detect voids as small as 100 μm in diameter in a cable 2.8 cm in diameter moving past the sensor at 5 cm/s. Better performance of the liquid helium cooled Ga-doped Ge detectors could lead to a significant improvement in void detection capability.     

Study on the Effects of Copper Oxide Growth on the Peel Strength of Copper/Polyimide

In this study, various copper oxides [CuO, Cu₂O, and Cu₂O + polybenzimidazole (PBI)] were studied as alternative adhesion layers. Specimens were aged under harsh conditions (300°C, 5% O₂ or humid condition), and then peel tests were conducted to investigate the reliability of Cu oxides/polyimide (PI). The peel strength of the bare Cu, CuO, and Cu₂O specimens dropped substantially, close to nil, due to void formation after 2 h of aging at 300°C. The degree of void formation near the Cu₂O/Cu interface showed a clear inverse relationship with the peel strength, suggesting that the formation of voids beneath the Cu₂O layer was directly responsible for the peel strength degradation. Void growth was controlled by Cu₂O layer growth, while voids originated from the difference between the diffusion rate of Cu atoms through the Cu₂O layer and Cu layers. Humidity tests did not lower the peel strength significantly in any of the specimens, none of which showed voids that were detrimental to the peel strength, in contrast to the results of the aging treatments at 300°C.     

Mechanisms and solutions to the brittle solder joint in electroless Ni plating

Brittle solder joints in Electroless Ni electroless Pd immersion Au (ENEPIG) surface finishes are one of the key reliability issues in electronics assembly. Previous characterization of the reflow process has indicated that interfacial voids formed after solder reflow are responsible for the decreases in solder joint strength. However, the mechanisms behind the formation of these voids in the ENEPIG process remain unclear. In this paper, the interaction between various aspects of the ENEPIG process and solder joint strength were investigated. Surface roughness, morphology, and nano-pitting at the interface between electroless Pd and Ni-P were characterized. The size and density of nano voids inside Ni₂SnP were measured after the specimens were reflowed with Sn4Ag0·5Cu solder ball. Additionally, high speed shear solder joint strength measurements were made. The results indicated that anion adhesion induced nano-pitting at the interface between the Ni₂SnP intermetallic and Pd, resulting in the formation of a nano void layer during reflow. These interfacial voids lead to lower solder joint strength. Based on the results, a solution to prevent the brittle solder joint failures is suggested.     

Electromigration in aluminum/silicon/copper metallization due to the presence of a thin oxide layer

The effect of a thin layer of SiO₂ (50 nm) on the electromigration behavior of Al/ 0.8wt.%Si/0.5wt.%Cu metallization, passivated by spin-on-glass, phosphorus silicate glass and silicon nitride as part of the complementary metal oxide semiconductor technology fabrication process was studied. It is found that voids were formed along the edge of the metallization line as opposed to formation at triple point of grain boundaries. At the same stress current of 1 × 10⁶ A/cm², thicker metallization layer (600 nm) showed an improvement in median time to failure (MTF) (1.4 times) with smaller void size (0.2 to 0.4 μm) over one without an underlying oxide, whereas if the metallization thickness is thin (300 nm), the MTF is degraded (0.6 times) with larger void size formed (0.3 to 1.0 μm).     

Investigation of Void Nucleation and Propagation in the Joule Heating Effect During Electromigration in Flip-Chip Solder Joints

This study analyzes the effect of void propagation on the temperature increase of solder joints by using x-ray microscopy, Kelvin probes, and infrared microscopy. It was found that the temperature rise due to void formation was less than 1.3°C when the voids depleted about 75% of the contact opening, even though bump resistance had increased to 10.40 times its initial value. However, the temperature rose abruptly with an increase of up to 8.0°C when the voids depleted 96.2% of the contact opening. A hot spot was observed immediately before the occurrence of open failure in the solder bump. The local increase in temperature was about 30.2°C at the spot. This spot may be the remaining contact area immediately before the occurrence of open failure.     

Effects of Mg additions on the electromigration behavior of Al thin film conductors

Aluminum thin film conductors containing Mg alloying additions have been tested for electromigration failure by formation of electrically open circuits. The test conditions were either 2 × 10⁶ A/cm² or 4 × 10⁶ A/cm² for the current density, and either 175 or 225°C for the temperature. The median lifetimes were found to increase with increasing Mg concentrations up to the highest concentration tested, about 6%. With polycrystalline films the maximum increase in lifetime resulting from Mg additions corresponds to a factor of about 100, as compared to pure Al films. This is about equal to previously reported results obtained with Cu additions. The increase in lifetime has been shown to result from a decrease in the rate of grain boundary diffusion for the Al atoms. Magnesium atoms diffuse at approximately the same rate as Al atoms. Thus the mechanism of failure formation in Al films containing Mg is thought to be different than in Al-Cu films, where Cu atoms diffuse faster than Al atoms and failure ensues upon local Cu depletion.