Ichiro Mukouda

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

Abstract In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu concentration=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.

Microstructure in pure copper irradiated by simultaneous multi-ion beam of hydrogen, helium and self ions

Abstract Pure copper was irradiated at 300–500°C by 5 MeV Cu ions (single beam) and Cu ions plus gas atoms (H and He) (dual beam irradiation) simultaneously. The high energy ion irradiation was carried out with the accelerator TIARA at the Takasaki-establishment of JAERI. The ions stop within a few microns from surface level and damage was formed up to this depth. The damage structure was observed as a function of the depth utilizing a focused ion beam (FIB) device. Below 300°C irradiation with a single beam produced a high density of stacking fault tetrahedra (SFT) but void formation was not observed. Large voids were observed with single beam irradiation at 500°C. In specimen irradiated with a dual beam of helium and Ni ions, the number density of voids was increased significantly. In copper irradiated with hydrogen and Ni ions, the number density of voids was not so large. Experimental results show that helium atoms promote void formation. Hydrogen atoms have less effect on void formation than helium atoms in pure copper.

The influence of dynamical structural relaxation of point defect clusters on void formation in irradiated copper

Abstract In the neutron-irradiation experiment with a temperature controlled capsule at JMTR, residual-gas-free copper was irradiated at 200°C and 300°C together with as-received copper. The fluences were 5 × 10 18 n/cm 2 (the low fluence) to 1 × 10 20 n/cm 2 (the high fluence). TEM observation of the irradiated specimens showed that interstitial clusters form a colony at the low fluence which develops into a dislocation structure at the high fluence. Between the colonies only vacancy clusters in the form of voids and stacking fault tetrahedra (sft) were observed. There are no effects of residual gas atoms on the formation of voids at the low fluence although the effects become appreciable at the high fluence. The number of vacancies which are accumulated in a void is 350 times larger than that in a sft at the low fluence. The number density of voids decreased with increasing neutron fluence while the number density of sft increased. The voids form uniformly in copper irradiated to the low fluence while they were observed along dislocations at the high fluence. Computer simulations by molecular dynamics show that small interstitial clusters relax to a bundle of 〈110〉 crowdions and move long distances in response to small strain fields. Interstitial clusters move along a 〈110〉 direction and can switch to other 〈110〉 directions, and form groups of clusters. At high temperature, a dense colony of the clusters forms and develops into a dislocation structure. It is shown that small vacancy clusters relax to movable structures at high temperature. The structure consists of vacancies which are connected in a curved string shape. Along the vacancy strings, many relaxations of a tri-vacancy of Damask- Dienes-Weizer type (3v-sft) were observed. Such a relaxation to the 3v-sft type makes it difficult for a single vacancy evaporation. Small vacancy clusters move and coalesce into larger vacancy clusters. The linkage of the results of experiments and computer-simulations suggests that voids nucleate as a metastable defects at coalesced vacancy clusters at high temperature. The nucleation of voids is not affected by the influence of gas atoms dissolved in the material. Micro-voids migrate in the specimens after their nucleation. During their movement, gas atoms are trapped in the voids. The trapping of a larger number of gas atoms limits the movement of voids. This leads to a higher number density of voids in the as-received copper than in residual-gas-free specimens at the high fluence. Voids form uniformly in specimens at the low flunce and they migrate to dislocation lines. Dislocations are also trapped at voids during climbing by absorbing interstitial clusters. These finally lead to the preferential formation of voids along dislocation lines.

Computer simulation study of the atomistic mechanism of deformation and fracture initiation in thin fcc metal films

Abstract A molecular dynamics computer simulation was performed using a potential of embedded atom method (EAM) of copper so as to study the atomistic mechanism of plastic deformation in thin metal films. When a thin film of Cu crystal is elongated by more than 8%, small islands of surplus atoms start to nucleate between (111) planes. The formation of new (111) islands occurs by the movement of atoms along the 〈111〉 direction from two successive (111) planes. Multiplication of these (111) planes serves as the mechanism of elongation, releasing the accumulated elastic stress. Vacancies and small clusters, thereof, are left behind at the positions from which atoms have migrated and also at the part of newly nucleated (111) planes in which atoms were not filled completely. Stacking fault tetrahedra (sft) are nucleated directly in the deformed region as a result of movement of atoms along 〈111〉 on the tetrahedral (111) cap.

Microstructure in vanadium irradiated by simultaneous multi-ion beam of hydrogen, helium and nickel ions

Abstract Pure vanadium was irradiated at 500 and 600 °C by either 5 MeV Ni ions (single beam) or Ni+H and He ions simultaneously. The pure vanadium was of nominal 99.8% purity. For the quantitative investigation of damage structure as a function of the depth, we utilized focused ion beam (FIB) microscopy. To preserve the surface of ion-irradiated metals, we deposited tungsten on the irradiated surface. The specimens were electro-polished to remove the damaged region by FIB. When only nickel ions were used, voids formed in the region from the surface to a depth of ∼0.5 μm when irradiated at 500 and 600 °C. However, in the region of the damage peak, voids were not observed. Needle-like precipitates of about 100 nm of length were observed for any specimen covering the full ion penetration depth. It is thought that the precipitate is a carbide. Moreover, in the specimen irradiated at 600 °C, the granular precipitates were over the region of the 1.0–1.5 μm depth. Void formation was observed over the whole ion penetration depth when the specimen was subjected to Ni+He simultaneous irradiation. Needle-like precipitates were observed.

Cryo-transfer TEM study of vacancy cluster formation in thin films of aluminum and copper elongated at low temperature

Abstract Kiritani et al. encountered a large number of vacancy clusters in heavily deformed thin metal films at room temperature. In the present work, thin films of aluminum and copper were loaded to fracture in liquid nitrogen and transferred directly to the TEM without warming up (Cryo-transfer TEM). In thin films of Al and Cu deformed at 78 K, many defect clusters were observed at 120 K. In thin Al films, small defect clusters disappeared by the 120 keV electron irradiation. Upon isochronal annealing, some of the dislocation loops grew in their size, which is much larger than those observed at 120 K. However, in regions that were not exposed to electron irradiation at 120 K, only a large number of stacking fault tetrahedra were observed after room temperature annealing. In copper, the 200 keV electron beam did not significantly change the defect clusters observed at 120 K.

Experiments to examine the contribution of gas atoms to void formation in irradiated metals

Abstract The use of vacuum melting has been employed to demonstrate that residual gases, especially hydrogen, strongly influence void nucleation of copper, copper binary alloys and various Fe-Cr-Ni base alloys during either neutron or electron irradiation. Void nucleation in nickel appears not to be strongly affected by residual gases, however. Solute-free and solute-bearing Fe-Cr-Ni alloys appear to respond differently to differences in gas content. When contamination of specimens with sodium occurs during neutron irradiation, void nucleation in both as-fabricated and vacuum-melted specimens is similar, suggesting that gas atoms re-enter the specimens during irradiation.

Crystal Structure of Elongated Thin Foils of Fe and V

Kiritani et al. have recently reported that a large number of vacancy clusters are formed in plastically elongated thin foil of fcc metals and that no dislocations at all are observed in the thin portion of the elongated film. These findings suggest that elongation proceeds by atom transportation accompanied by generation of vacancies. In the present work, strips of prototype bcc metals of iron and vanadium were elongated to fracture, and thin portions that can be observed under TEM; i.e. , fractured edges, were observed by electron microscopy. The results show that iron thin foils that had been elongated in pure helium gas reacted with residual oxygen atoms such that the thin portions transformed to α-Fe 2 O 3 . Iron thin foil that had been elongated under a 10 −6 Pa vacuum showed usual bcc structure, and void-like defects were observed. Strips of vanadium were elongated to fracture at 77 K and 293 K, and large number of void-like defects were observed in as-elongated specimens. In thin vanadium foil that had been elongated at 673 K, TEM visible defects were not observed initially, and void like defects appeared after 200 keV electron illumination for 60 minutes. Stereo-microscopic examination showed that void-like defects had formed inside vanadium thin foil.

Dependence of observed cascade defects on neutron spectrum and dose in Au and Ag irradiated with fission and fusion neutrons at low temperature

Abstract Cryotransfer transmission electron microscopy (TEM) experiments have been performed at about 110 K for Ag and Au irradiated with fission neutrons at 6 K and fusion neutrons at 20 K. More than 50% of the observed defect clusters were of interstitial type. The average size of defect clusters increased as neutron dose became higher. This result was explained by the effect of freely migrating interstitial atoms produced by newly developed displacement cascades. The effect of the difference in neutron spectrum was manifested in the number of defect clusters in a group of closely existing clusters (‘subcascade’ structure). There was a possibility that interstitial-type defect clusters were nucleated in the close vicinity of subcascades.

Void formation in neutron-irradiated Cu and Cu alloys

Pure copper and copper-aluminum alloys were neutron-irradiated at high temperatures in the as-received condition, and after being melted under high vacuum or in argon. Melting under high vacuum was done to reduce the residual gas amount in the specimens. The number density of voids in the vacuum-melted Cu was one tenth of that in as-received Cu after JMTR irradiation to 5.2 × 1024 n/m2 at 603K. Similarly, voids were also formed in an argon-melted Cu-1at%Al specimen but were not formed in a vacuum-melted one. Following higher dose irradiation in the JOYO reactor, nearly the same number density and size of voids were formed in both argon and vacuum-melted Cu. In Cu-5at%Al, many voids were formed in argon-melted specimens, whereas in vacuum-melted specimens voids were not formed. These results show that voids nucleate at vacancy clusters which trap gas atoms. In the JOYO irradiations, diffused-in gas atoms play an important role in the formation of voids in Cu. In Cu-5at%Al, diffused-in gas atoms were trapped by Al atoms, resulting in a difference of void formation between the two types of specimens.