Kinetics of hillock growth in Al and Al-alloys

Hillock growth kinetics and size distribution were investigated in Al, Al:Si 1% and Al:Si1%:Cu 0.5% layers. Metallization surface was examined by optical, SEM and TEM microscopy, stylus profiling and an automatic method of hillock recognition from a microscope image. The method allowed for counting hillocks in a desired range of their diameter d. Surface density of hillocks was measured as a function of time of furnace annealing at 400°C and as a function of temperature of RTP annealing. A maximum hillock size was found to increase linearly with metallization layer thickness and with logarithm of annealing time. A total area occupied by hillocks was evaluated. Hillock density decreased versus 1/T with an activation energy of 0.28 eV for Al and 0.31 eV for Al:Si. It was found, that a normalized hillock density N may be expressed by a formula N=N₀ exp(−cd). Values for N₀ and c are given together with a short discussion.     

Effects of deposition conditions of the Al film in Al/glass specimens and annealing conditions on internal stresses and hillock formations

In the present study, 25 kinds of specimen with five Al-film thicknesses were prepared to investigate the relation between the internal stress formed during the annealing process and the hillocks. In the preparation of specimens, the governing factors including deposition conditions, annealing temperature, and annealing time, were arranged following the orthogonal table of five-level and six-factorial (L₂₅(5⁶)) design. Stoney's formula is applied to describe the internal stresses before and after annealing (σ₀ and σ_f), respectively. The internal stress arising during the annealing process (σ_an) is evaluated using the model developed by Flinn et al. [1]. Then, the response surface methodology (RSM) is used to express the three stress parameters in terms of influential factors. The incipient σ_an value for hillocks appearing in the specimens was found to be between − 28.7 MPa and − 32 MPa in a compressive form. The annealing temperature, time, and Al-film thickness are the three major factors, affecting internal stress σ_an. An increase in the annealing time reduces the tensile stress or increases the compressive stress, or both. The tensile stress decreases and the compressive stress increases during the annealing process with increasing Al film thickness and annealing temperature. The number of hillocks formed in a unit of area is linearly proportional to both σ_an and (σ_f − σ_an).     

Low temperature low pressure solid-state porous Ag bonding for large area and its high-reliability design in die-attached power modules

Solid-state porous Ag was utilized to achieve interface bonding under low temperature and low-pressure for large area, high-reliability designs for high performance die-attach power modules in high temperature applications. In this process, solid-state porous Ag was first fabricated by sintering Ag particles onto both a Cu substrate and an Si die with different bonding areas. After polishing the surface of the solid-state porous Ag, the Ag-Ag interface was bonded together under low pressure (0.4?MPa) and low temperature (250?°C and 300?°C). The bonding strength was greater than 30?MPa for a 15?mm x 15?mm bonding area at a bonding temperature of 300?°C. The electrical resistivity of the solid-state porous Ag was about 7 µΩ cm, or half that of Pb-free alloy solder materials. The bonding mechanism of the Ag-Ag interface was analyzed by a transmission electron microscope. It was observed that Ag hillocks and nanoparticles had been generated and bridged the Ag-Ag interface. This resulted in a large interface connection ratio and a high bonding strength. In addition, it was observed that the shear strength of the solid-state porous Ag with a thick bonding layer decreased thermomechanical stress in the die-attached power module and thus improved structural reliability. This bonding process featuring solid-state porous Ag presents an attractive technology for the fabrication of large area bonding and high-reliability die-attached module structures for high temperature applications.     

Nanofretting Behavior of Monocrystalline Silicon (100) Against SiO2 Microsphere in Vacuum

With an atomic force microscopy, the tangential nanofretting between spherical SiO₂ tips and monocrystalline Si(100) surface was carried out at various displacement amplitudes (0.5–250 nm) under vacuum condition. Similar to fretting, the nanofretting of Si(100)/SiO₂ pair could also be divided into stick regime and slip regime upon the transition criterion. However, it was found that the energy ratio corresponding to the transition between two nanofretting regimes varied between 0.32 and 0.64, which was higher than the normal value of 0.2 for the transition criterion to determine the partial slip and gross slip regimes in fretting. One of the reasons may be attributed to the effect of adhesion force, since whose magnitude is at the same scale to the value of the applied normal load in nanofretting. During the nanofretting process of Si(100)/SiO₂ pair, the adhesion force may induce the increase in the maximum static friction force and prevent the contact pair from slipping. The higher the applied load, or the higher the adhesion force, the larger will be the transition displacement amplitude between two regimes in nanofretting. Different from fretting wear, the generation of hillocks was observed on Si(100) surface under the given conditions in nanofretting process. With the increase in the displacement amplitude in slip regime of nanofretting, the height of hillocks first increased and then attained a constant value. Compared to chemical reaction, the mechanical interaction may be the main reason responsible for the formation of silicon hillocks during the nanofretting in vacuum. The results in the research may be helpful to understand the nanofretting failures of components in MEMS/NEMS.     

Effect of Ti Content on Properties of Al-Ti Alloy Film for Address Lines of TFT-LCDs

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Effect of SiO2 passivation overlayers on hillock formation in Al thin films

Hillock formation in Al thin films with varying thicknesses of SiO₂ as a passivation layer was investigated during thermal cycling. Based on the stress measurements and the number of hillocks, 250 nm thick SiO₂ was thick enough to suppress the hillock formation and the suppression of hillock at 250 nm passivation and the lack of suppression at thinner passivation is related to the presence/absence of protection against the diffusive flow of atoms from the surrounding area to the surface due to the biaxial compressive stresses present in the film through the weak spots in the passivation layer. The stress state of Al films measured during annealing (the driving force for hillock formation) did not vary much with SiO₂ thickness. A small number of hillocks formed during the plasma enhanced chemical vapor deposition of SiO₂ overlayers at 300 °C.     

Small-scale plasticity in thin Cu and Al films

Flow stresses in thin metal films significantly exceed the flow stresses of their bulk counterparts. In order to identify the underlying deformation mechanisms and correlate them with microstructure, we analysed epitaxial and polycrystalline Cu and Al thin films. The films (100–2000 nm thickness) were magnetron sputtered on (0001) -Al₂O₃ single crystals or on nitrided and oxidised (001) Si substrates. For epitaxial films, the flow stress measurements, which were obtained from substrate-curvature tests, agree with predictions from a dislocation-based model [1 and 2], whereas for polycrystalline films the stresses measured for film thicknesses down to 400 nm are much higher than predicted. However, thinner films reveal a plateau in room temperature flow stress. This behavior, as well as the stress–temperature evolution of the various films will be discussed in terms of existing theories for plasticity in thin metal films, and under consideration of recent in situ transmission electron microscopy studies.     

Morphology evolution in TiN/Al-0.5Cu/Ti interconnection during chamber long stay and post-deposition annealing correlated to defect formation in metallization processing

TiN/Al-0.5Cu/Ti film stacks deposited on SiO₂ substrate were studied by X-ray diffraction and electron microscopy to clarify the effects of the chamber long stay and post-deposition annealing on the morphology evolution. Experimental results indicated that the chamber idleness at 270 °C resulted in significant Al₂Cu precipitation and hillock growth for the Al-Cu films, which enhanced the occurrence rate of the microcorrosion-induced bridging defects and caused yield degradation on production line while post-deposition annealing at 400 °C for 30 min was proven to effectively regain good yield for the chamber-idled wafers. The yield recovery could be attributed to the fast Al₂Cu dissolution and hillock mitigation at the annealing temperature. The electrical sheet resistance of the Al-Cu films would somewhat increase due to the formation of the Al₃Ti phase during annealing, but the Al₂Cu precipitates and surface hillocks formed during chamber idleness would scarcely change the electrical property of the films. This study suggests that the evolutions of second phase and surface hillocks can be controlled by the processing duration and post-deposition treatment rather than the deposition temperature or Cu addition amount of Al-Cu alloy.