H. Niwa

Stress distribution in an aluminum interconnect of very large scale integration

Thermal stress‐induced void formation in aluminum interconnect has become a major reliability problem in the usage of very large scale integration circuits. The purpose of this work is to analytically evaluate stresses in the Al line. By applying Eshelby’s method in micromechanics, the stresses in the Al line were estimated analytically as a function of the aspect ratio of the Al line cross section. The yielding criteria in plasticity were applied to examine whether the calculated stresses can induce relaxation by plastic deformation. The analytically calculated results were compared with previous results of numerical calculation and experimental observation.

Diffusional relaxation and void growth in an aluminum interconnect of very large scale integration

Using the previously obtained stress distributions in an Al line after relaxation by plastic deformation, another possible relaxation process by diffusion was analyzed. Even after this relaxation occurs, some stresses still remain in the Al line. If these remaining stresses are large enough, they can be responsible for the growth of voids causing line failure. Using a theory for diffusional growth of grain‐boundary voids, the time to failure of the Al line was estimated analytically. The previous and present papers together constitute a full analysis of the so‐called ‘‘stress migration’’ phenomenon.

Effect of Al3Ti intermetallic compound on electromigration lifetime of Al alloy interconnections

The dependence of the electromigration (EM) lifetime and the cross-sectional structure of interconnections after EM tests on linewidths was investigated in multi-level and single-level Al alloy interconnections consisting of a top–TiN/Ti/Al–0.5%Cu/TiN/Ti–bottom stack. In this study, an almost uniform Al3Ti intermetallic compound layer was formed by a well-known reaction between the Ti and Al. We found the following anomalous behavior: the mean time to failure (MTF) of EM in the multi-level interconnections with tungsten diffusion barriers decreased by increasing the linewidth. We also found that in the multi-level interconnections after EM tests, independent of linewidths, a local Al thickening formed near the anode end of the line and voids formed near the cathode end. On the other hand, in the single-level interconnections with bonding pads, the MTF of EM increased by increasing the linewidth and, after EM tests, a local Al thickening formed near the anode end even though no voids were observed near the...

Effects of line size on thermal stress in aluminum conductors

The authors evaluate the aspect ratio dependence of thermal stress distribution in passivated aluminum conductors on line width and thickness as directly determined using X-ray stress measurement. Measured stresses are compared with the results of analytical and numerical simulations. It was found that they differ significantly. This difference is ascribable to the dependence of the yield stress on the line width and thickness. The yield stress increases with decreasing width and thickness. The dependence of aluminum lifetime on the width and thickness of aluminum conductors was also evaluated. This dependence is discussed with a diffusional void growth model. The experimentally obtained lifetime is well described by this model with directly measured stress.<<ETX>>

Epitaxial growth of Al on Si(001) by sputtering

Epitaxy of Al deposited by sputtering on Si(001) substrates was investigated in detail by transmission electron microscopy studies. Heat treatment during or after deposition and growth to greater thickness enlarged the Al grains in the film and made the epitaxial relationship more distinct. It is confirmed that the Al epitaxy formed by the sputtering method is as perfect as that by other techniques.

Epitaxial growth of sputtered A1 films on Si(001) substrates

Abstract When Al is deposited on to Si(001) by sputtering, the Al(110)/Si(001) epitaxial relationship is realized. Its crystallography has been analysed from the viewpoint of lattice match geometry and misfit strain energy. From these analyses, it has been found that the adoption of the idea of superlattice unit cells is useful in considering the lattice correspondence between A1 Si planes at the interface. With this idea, together with previously-proposed simple criteria, the crystallography of the Al(110)/Si(001) epitaxy has been explained successfully.

Single‐crystal Al films grown by sputtering on (111)Si substrates

Single‐crystal growth of Al films deposited by the sputtering method on (111)Si substrates was investigated in detail by transmission electron microscopy studies. A single‐crystal epitaxial Al film 500 nm thick was obtained by conducting an appropriate heat treatment during and after the deposition.

Ti layer thickness dependence on electromigration performance of Ti-AlCu metallization

Electromigration lifetime tests on TiN-Ti-AlCu-TiN-Ti stacked structures with various upper Ti film thicknesses have been carried out on two-level interconnect structures connected with W-plugs. We found that a high electromigration resistance was obtained with thin Ti, resulting in an island shaped Al/sub 3/Ti intermetallic. This result is inconsistent with a well-known bypass model. We propose a new model in which the Al/sub 3/Ti-Al interface mass transport is faster than that of others.

Crystallography and electromigration resistance of epitaxial Al films grown on (011)Si substrates by the sputtering method

From transmission electron microscopy observations, Al films sputtered on (011)Si substrates were found to grow epitaxially, similar to those on (001)Si and (111)Si substrates. The condition for single crystallization of the epitaxial Al films was obtained by changing heat treatments and Al film thickness. In addition, a simple acceleration test revealed that the electromigration resistance of Al interconnects on a (011)Si substrate was much higher than that on a SiO2 film.

Morphology and crystallography of single crystal precipitates on Al alloy films grown by the sputtering method

When Al alloys such as Al‐1%Si or Al‐2%Cu are sputtered on Si substrates at elevated temperatures, extruded single crystal precipitates are observed on the alloy films. The morphology and crystallography of the precipitates are studied in detail using scanning electron microscopy and transmission electron microscopy. The precipitates on the Al‐2%Cu films contain a few percent Cu and are single crystals with crystallographically low‐index facet planes. The temperature profiles during and after the deposition for the favorable occurrence of the precipitation are also investigated.