Electromigration Of Cu Research Articles

Cu Wettability and Diffusion Barrier Property of Ru Thin Film for Cu Metallization

The main issue of Cu metallization is the electromigration of Cu through the interface between Cu and the barrier or capping layer. To improve electromigration resistance at the Cu and barrier metal interface, insertion of a glue layer which enhances the adhesion of Cu onto the under layer may be effective. The wettability of Cu on Ru and Ta glue layers was evaluated as the index of Cu adhesion strength onto glue layers. The wetting angle of Cu (43°) on a Ru substrate was three times lower than that of Cu (123°) on a Ta substrate after annealing. Lower wetting angle of Cu on a Ru substrate indicates a good adhesion property between Cu and Ru and may imply a high electromigration resistance. The better Cu wettability of Ru compared to Ta can be explained by the concept of lattice misfit. A Ru(002) plane has lower lattice misfit, which suggests lower interface energy, and enhanced the adhesion of Cu onto Ru. However, the Ru film showed poor Cu diffusion barrier properties, which suggests Ru should be used as a glue layer in combination with another barrier layer.

Atom motion of Cu and Co in Cu damascene lines with a CoWP cap

Electromigration of Cu and diffusion of Co in Cu damascene bamboo-like grain structure lines capped with CoWP have been studied for sample temperatures between 350 and 425 °C. Void growth from the Cu line/W via interface was observed. Bulk-like activation energy for electromigration of 2.4±0.2 eV was obtained for these samples suggesting that electromigration damage is greatly diminished for these on-chip Cu interconnections. The solubility and diffusivity of Co in Cu was determined from line resistance measurements of thermally annealed Cu lines which were affected by Co diffusion into the Cu line.

Electromigration in sputtered copper film on plasma-treated hydrogen silsesquioxane dielectric

Electromigration (EM) damage is one of the major causes for the failure of interconnects. Plasma treatment, such as dry etching, is frequently employed in the fabrication of multilevel interconnection patterns. This work investigates the hydrogen silsesquioxane (HSQ) and copper integrated systems and the effect of H2 plasma treatment on the EM of Cu. Hydrogen plasma bombardment induces a rough HSQ surface and results in a coarse morphology of the Cu film deposited on HSQ. The crystallographic texture of Cu is also affected by the plasma treatment. A decrease in the Cu I(111)/I(200) peak ratio is observed for a specimen treated with H2 plasma. The activation energy for EM in Cu and the EM lifetime of the Cu interconnect decreases with an increased degree of plasma treatment. The activation energies obtained, ranging from 0.76 eV to 0.94 eV, suggest that the electromigration in copper proceeds via an interfacial diffusion path. Possible mechanisms for the effects of plasma treatment are explored. The rough surface and the retarded Cu (111) orientation induced by H2 plasma bombardment are the major causes for the decrease of activation energy and EM lifetime.

Effects of Underlayer Dielectric on the Thermal Characteristics and Electromigration Resistance of Copper Interconnect

The effects of underlayer dielectric on the thermal characteristics and electromigration resistance of copper interconnects are investigated. It is shown that the thermal impedance of metal lines on polyimide is about three times that of metal lines on plasma-enhanced chemical vapor deposition of tetraethylorthosilicate (PETEOS) oxide. The open-circuit failure occurs after 75 s stressing at a current density of 1.4×107 A/cm2 for polyimide underlayer specimens due to the high joule heating effect which causes the extreme temperature rise at interconnects. Hence, decomposition of polyimide occurred which further accelerated the breakdown of interconnects. The thermal impedance is one of the major factors causing the electrical resistance to change during the electromigration (EM) test. The activation energies for electromigration of Cu are 0.87 eV for SiO2/Cu/PI2610 and 0.65 eV for SiO2/Cu/SiO2. After 500 thermal cycles, both the mean time-to-failure (MTF) and activation energy Ea decrease for EM, and lognormal standard deviation increases. At 225°C, the MTF of SiO2/Cu/PI2610 is about two times longer than that of SiO2/Cu/SiO2.

Recent advances on electromigration in very-large-scale-integration of interconnects

Today, the price of building a factory to produce submicron size electronic devices on 300 mm Si wafers is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from fabricating the very-large-scale-integration of the interconnect metallization. The most serious and persistent reliability problem in interconnect metallization is electromigration. In the past 40 years, the microelectronic industry has used Al as the on-chip conductor. Due to miniaturization, however, a better conductor is needed in terms of resistance–capacitance delay, electromigration resistance, and cost of production. The industry has turned to Cu as the on-chip conductor, so the question of electromigration in Cu metallization must be examined. On the basis of what we have learned from the use of Al in devices, we review here what is current with respect to electromigration in Cu. In addition, the system of interconnects on an advanced device includes flip chip solder joints, which now tend to become weak links in the system due to, surprisingly, electromigration. In this review, we compare the electromigration in Al, Cu, and solder on the basis of the ratio of their melting point to the device operating temperature of 100 °C. Accordingly, grain boundary diffusion, surface diffusion, and lattice diffusion dominate, respectively, the electromigration in Al, Cu, and solder. In turn, the effects of microstructure, solute, and stress on electromigration in Al, Cu, and solder are different. The stress induced by electromigration in Cu/low-k interconnects will be a very serious issue since the low-k dielectric (with a value of k around 2) tends to be weak mechanically. In a multilevel interconnect, a electromigration force due to current crowding, acting normal to current flow, has been proposed to explain why many electromigration induced damages occur away from the high current density region. In mean-time-to-failure analysis, the time taken to nucleate a void is found to be much longer than the growth of the void in Al and solder interconnects. This is not the case for Cu interconnects for the nucleation of a void on a surface. On accelerated tests of electromigration in Cu interconnects, the results gathered above 300 °C will be misleading since the mass transport will have a large contribution of grain boundary diffusion, which is irrelevant to electromigration failure in real devices induced by surface diffusion.

Curing of polyimide and the effect of the TEOS SiO2 barrier layer on the electromigration of sputtered Cu with polyimide passivation

The curing process of polyimide and the electromigration of copper films with polyimide (PI) passivation are studied. Thermal analysis of polyimide suggests that imidization completes at ∼200°C with an endothermic reaction associated with the breaking of the C-OH and N-H bonds as revealed by Fourier transformation infrared spectroscopy (FTIR). Although there is 89.8% weight loss when PI is heated from 20°C to 200°C, outgassing of PI passivation is still observed at higher temperatures. Carbon, nitrogen, and oxygen atoms diffuse into Cu during thermal processing of PI/Cu films. The tetraethyl orthosilicate (TEOS) SiO2 films are used as the barrier layer between PI and Cu to retard the poisoning of Cu. The effect of TEOS SiO2 film on electromigration of Cu is investigated.

Electromigration in Cu thin films with Sn and Al cross strips

Electromigration in Cu thin films is studied in a cross-strip configuration. Cu lines with isolated areas of Cu(Al) or Cu(Sn) are tested between 250 and 390 °C with the following results. The hillock and void marker motion indicates that Sn moves in the direction of electron flow. The marker polarity indicates that it decreases the grain boundary electromigration of Cu, in agreement with previous studies. This study also finds evidence of active surface migration in Cu. During tests in forming gas, hillocks and voids form adjacent to a native Al2O3 layer at all temperatures, indicating the likelihood that Cu migrates faster through the Cu free surface than the interface between the surface layer of Al2O3 and Cu(Al). Active surface migration in Cu thin films is also evidenced by the growth of hillocks with highly developed facets, most of which are attached to the underlying film by narrow necks.

Electromigration of Cu and Ti atoms and dopant junction profiles in the p+-Si implanted channel under high-density current

Ultrafast diffusion of Cu and Ti atoms in p+-Si channel was achieved in samples stressed with high current density. The junction was changed under high current stress and copper silicide was formed in the junction. The current effects of high current on the silicide line formation and symmetrical end-of-range defects elimination near the center of p+-Si channel are discussed.

Diffusion and electromigration of copper in SiO2-passivated single-crystal aluminum interconnects

We have experimentally studied the diffusion and electromigration of Cu in passivated single-crystal Al interconnects, fabricated on oxidized Si wafers. The test structure consisted of parallel lines (0.4 μm thick and 5 μm wide), with alternating lines terminating in shared contact pads. Cu was locally added to the same regions in all the lines, and the effects of temperature and electric fields on diffusion along the lines were characterized by analyzing the Cu concentration profile measured by electron-probe microanalysis. The activation energy for diffusion was found to be 0.8±0.2 eV in the temperature range 320–400 °C, which suggests that the path for diffusion of Cu is along the Al/AlOx/SiOy interface for these single crystals. The apparent effective valence (Z*) characterizing electromigration was measured to be −5.4±1.2 at 350 °C.

Electromigration in sputtered copper films on polyimide

Electromigration damage (EMD) is one of the major causes for the failures of interconnects. In this study, the electromigration of Cu on polyimide is investigated. An empirical formula is used to evaluate the EMD of Cu film. The activation energy (Q) obtained is significantly less than those of lattice and grain boundary diffusion. This suggests that the electromigration in copper proceeds via an interfacial diffusion path. The geometry of the metallization also affects the activation energy. Any abrupt change in the interconnect direction and metal width causes current crowding and should be avoided. The current exponents (n), calculated from EMD data, are 3.58 and 3.35 for straight and zig-zag Cu films, respectively.

Reliability Studies of Cu using Wafer Level Joule Heated Electromigration Test

AbstractIn this paper, we describe some of the results that we have obtained using Wafer Level Joule Heated Electromigration Test and Current Ramp Test. The results indicate that the Cu we tested is more resistive to open failure due to current ramp than Al-1%Si-0.5%Cu. The value of the activation energy for electromigration of Cu using fixed current under wafer level joule heated electromigration test is 0.59 eV. The activation energy due to the effect of temperature gradient failure mechanism is 0.82 eV.

Diffusion and Electromigration of Cu in Single Crystal Al Interconnects

AbstractThe electromigration-induced transport properties of Cu in Al-Cu alloys, and their effect on electromigration lifetimes in interconnects with bamboo grain structures are not well understood. To isolate and study the mechanisms and kinetics of Cu diffusion and electromigration in interconnects for which grain boundary transport is not dominant, we have developed a test structure consisting of parallel Al single crystal lines, with every alternate line terminating in contact pads. Cu is locally added to the same regions in all the lines, and the effect of temperature and electric field can be simultaneously characterized by analyzing the Cu concentration profile measured using electron-probe microanalysis. Comparison of the calculated values of diffusivities with the diffusivity of Cu through the Al lattice, and through dislocation cores in Al, suggests that the path of diffusion of Cu in Al single crystals is along the Al/AlOx interface.

Direct evidence for diffusion and electromigration of Cu in CuInSe2

Cu diffusion in chalcopyrite CuInSe2 was studied directly, using 64Cu as a radioactive tracer. For diffusion from a thin surface layer, the Cu diffusion coefficients at 380 and 430 °C, were found to vary from 10−8 to 10−9 cm2/s. In case of diffusion from a volume source at 400 °C, a value of 10−10 cm2/s was calculated from diffusion profiles. Electromigration of Cu was demonstrated, by applying a strong electric field to a sample and following the redistribution of 64Cu, that had been thermally diffused into the sample, prior to electric field application.

On the unusual electromigration behavior of copper interconnects

The present uncertainty in the ability of Cu to substitute for Al as the conductor material in very large scale integration arises from the perplexing electromigration behavior of Cu interconnects: the electromigration activation energy in multigrained lines is often about two times lower than for grain-boundary diffusion, while the pre-exponential factor in the electromigration rate expression is several orders of magnitude smaller than that characteristic of electromigration along grain boundaries. Using literature data, in particular those of drift velocity experiments, we show that regardless of these unusual facts, grain boundaries are still most likely the major electromigration diffusion pathways in Cu interconnects. Based upon recent progress in the theory of grain-boundary grooving with an arbitrary grain-boundary flux [Klinger et al. J. Appl. Phys. 78, 3833 (1995)], and the specific model applying the general theory to electromigration [Glickman, Phys. Low-Dim. Struct. 11/12, 69 (1994)], we explain the major features of electromigration in Cu in terms of the extension of slit-like grooves along the interconnect line, followed by their merging. Fitting the electromigration activation energy reported for pure Cu into the model suggests that surface diffusion along freshly created groove walls is slow, with an activation energy above 2 eV. Most likely, this is due to trace surface contaminations. Having this new key element in the grain-boundary grooving model, with surface diffusion acting in effect as a ‘‘healing’’ mechanism rather than as an independent pathway parallel to grain-boundary diffusion, and using empirical surface diffusion parameters for Cu, enable us to rationalize the major features of Cu electromigration behavior reported in the literature: the values of the electromigration pre-exponents, the island-like morphology of the electromigration displacement region, the current density exponent, and the origin of the Sn effect on electromigration parameters. Our main conclusion is that the full advantages of Cu as the most promising electromigration resistant interconnect material can be realized, provided that trace contaminations responsible for suppressing diffusion along freshly created surfaces are identified, controlled and eliminated. This will allow unalloyed, multigrained Cu interconnects to have an electromigration resistance four orders of magnitude larger than Al(2% Cu) at 100 °C.

Electronic effects of ion mobility in semiconductors: Mixed electronic–ionic behavior and device creation in Si:Li

Micrometer-sized homojunction structures can be formed by applying strong electric pulses, at ambient temperatures, to Li-doped, floating zone n-Si. Two such junctions, arranged back to back, act as a transistor, as evidenced by electron-beam-induced current and current–voltage measurements. The structures are created during a time ranging from ∼100 ms to a few seconds, depending on the size of the structure. The phenomenon is similar to what was observed earlier in CuInSe2 and was explained there by thermally assisted electromigration of Cu. In the case of Si doped with Li we can use secondary-ion-mass spectrometry to detect the redistribution of Li after electric-field application. Such a redistribution is indeed found and corresponds to an n+-p-n structure with the p region extending at least ∼20 μm into the bulk of Si. Structures created in Si doped with Li in this way are stable for at least 13 months after their creation. We ascribe this to the large difference between Li diffusivity at the local temperature that is reached during structure formation (∼400 °C; 10−8 cm2/s) and at room temperature (∼10−15 cm2/s).

ＣｕドープＣｄＳ蒸着膜の光学透過率に対する電界の影響

The optical transmittance of cells with Cu-Doped CdS thin films (CdS (Cu) films) was studied to investigate the electromigration of Cu in the CdS films. Cells with SnO2/CdS (Cu) /Au structure were fabricated by successive deposition of Cu (30 nm) and CdS (500 nm) on the SnO2 (F) film at 200°C, and deposition of semitransparent Au film at room temperature. Cu atoms diffused thermally into CdS films. The transmittance of the cells changed upon appling dc voltage to the cells. The change of transmittance of the cells was remained after a high electric field (about 106V/m) was removed from the cells. When the bias was positive for the SnO2 electrode relative to the Au electrode, the transmittance increased. However, the transmittance decreased when the bias was negative for the SnO2 electrode. These characteristics suggest that Cu+ ions migrate to the negative electrode in CdS films under high electric field at room temperature.

Activation Energy of Electromigration in Copper Thin Film Conductor Lines

AbstractActivation energy of electromigration damage was determined as 1.2 eV for Cu lines, indicating grain boundary paths for electromigrationo The surface diffusion was found to play a role during electromigration in Cu and impeded the failure process.

Activation Energy of Electromigration in Copper Thin Film Conductor Lines

AbstractActivation energy of electromigration damage was determined as 1.2 eV for Cu lines, indicating grain boundary paths for electromigration. The surface diffusion was found to play a role during electromigration in Cu and impeded the failure process.

Hillock formation during electromigration in Cu and Al thin films: Three-dimensional grain growth

The evolution of microstructure in Al and Cu thin film lines during electromigration has been studied using a transmission electron microscopy. Grain boundary migration was found to be critically involved in the electromigration induced hillock formation that can be described as a three-dimensional growth of a single grain.

Theory of surface electromigration on metals: application to self-electromigration on Cu(111)

We present a calculation of the force, due to electron scattering, acting upon an adatom at the surface of a current-carrying metal, i.e., the so-called “wind” force in surface electromigration. The force is calculated from the Feynman-Hellmann theorem in which the electron scattering states are computed with full multiple scattering corrections by a layer-KKR method. In contrast to previous jellium models, this approach allows the proper treatment of electron scattering by both the surface barrier and the substrate and its effect upon the current-carrying states at the surface. The electron scattering force is computed for the self-electromigration of Cu adatoms on Cu(111). At room temperature, the effective valence of the adatom is found to be ≈ −21. We conclude that surface electromigration of Cu on Cu(111) is dominated by the electron scattering force and that any direct force originating from charge transfer to the adatom is likely to be unimportant in this case. The implications of the computed value of the wind force for the mass transport at a current-carrying metal surface are discussed.