Donald S. Gardner

Mechanical stress as a function of temperature in aluminum films

Mechanical stress in interconnection is a problem of growing importance in VLSI devices. Open circuits due to metal cracking and voiding and short circuits due to hillocks are stress-related phenomena. The origins of this stress are discussed including intrinsic stresses from the synthesis of the films and thermally induced stresses. A measurement technique based on the determination of wafer curvature with a laser scanning device is utilized to directly measure the film stress in situ as a function of temperature during thermal cycling. The changes in stress observed during thermal cycles are interpreted quantitatively and mechanisms that lead to plastic deformation and their relationship to hillocks are discussed. In the stress vs. temperature measurements, several regions have been identified including elastic and plastic behavior both under compression and tension, the yield strength, recrystallization, gain growth, hardening, and solid-state reactions. The effects of deposition conditions on these regions are also examined. >

Mechanical stress as a function of temperature for aluminum alloy films

Aluminum alloys have virtually replaced aluminum thin films for interconnections in very large‐scale integration because of their improved reliability. Mechanical stress is a problem of growing importance in these interconnections. Stress as a function of temperature was measured for thin aluminum films on an oxidized silicon substrate and several aluminum alloys and layered films consisting of silicon, copper, titanium, tungsten, tantalum, vanadium, and TiSi2. Solid‐state reactions of the aluminum with the additives and with the ambient during thermal cycling will occur, and depending on what compounds have formed and at what temperature, this will determine the morphology and reliability of the metallization. The measurement technique, based on determination of wafer curvature with a laser scanning device, directly measures the total film stress and reflectivity in situ as a function of temperature during thermal cycling. Changes in stress were detected when film composition and structure varied and wer...

Interconnection and electromigration scaling theory

Interconnections will become the limit in performance and reliability at submicrometer dimensions. Long-distance interconnections are defined using models based on resistivity, and it is found that more than half of the interconnections will become categorized as such at 0.5-µm feature sizes. The resistivity of even less resistive materials, therefore, will become important. A model for analyzing the trends of material usage for interconnections and for projecting design rules is presented. Electromigration is the driving force away from the lowest resistivity silicon compatible material, namely aluminum. Replacements such as gold, however, have technological problems and the resistivity of refractory metals will be too high for a large fraction of the interconnections. Layered structures are one possible solution to the problems of electromigration and hillocks.

Layered and Homogeneous Films of Aluminum and Aluminum/Silicon with Titanium and Tungsten for Multilevel Interconnects

Layered structures and homogeneous alloy films synthesized by sputter deposition were investigated for use in a VLSI multilevel interconnect technology. Major areas studied include hillock formation, resistivity before and after annealing, film composition and structure, reproducibility, interlevel shorts, and dry etching. It has been demonstrated in this work that aluminum alloyed with silicon and titanium and layered with titanium offers advantages over current technological materials for interconnections in integrated circuits. Measurements of surface roughness and electrical shorts between two levels of metal showed that the hillock densities in the films are significantly reduced when small amounts (one to three atomic percent) of titanium and silicon are present. The resistivity of such homogeneous films, however, is 4.5 to 5.5 /spl mu//spl Omega//spl dot/cm, which is higher than standard metallization alloys. When Al/Si was layered with Ti, no hillocks were observed and the resistivity of the composite films was comparable to standard metallization alloys.

Aluminum–samarium alloy for interconnections in integrated circuits

Thin films of aluminum alloyed with copper, silicon, or titanium have been conventionally used as interconnection materials in integrated circuits to reduce hillock growth, electromigration, and junction spiking. The interconnection resistivity of these homogeneous alloys is, however, too high for maximum performance integrated circuits. In this work, hillock growth, resistivity, and stress were investigated for aluminum alloyed with samarium (Al‐1 wt. % Sm) as an alternative interconnection material. The results indicated that Al–Sm metallization exhibits very favorable properties, namely, low resistivity and good thermal stability including hillock growth resistance, for potential integrated circuit applications. Other property measurements and process and material compatibility studies are, however, desired prior to its application.

Fundamental factors governing improved performance of Al–Si/Ti multilayer metallization for very large scale integration

Fundamental factors governing the previously observed improvements in conductivity, hillock growth resistance, and thermal stability of the Al–Si/Ti multilayer metallization system have been investigated using various bulk characterization methods and surface analytical techniques (Auger electron spectroscopy and x‐ray photoelectron spectroscopy). Si partitioning from the Al–Si into Ti layer duirng annealing at 450 °C was found to be associated with the formation of a ternary phase similar to TiAl3, but enriched in silicon. In the Al–Si/Ti–Si multilayer metallization system, Si migrates into the Al–Si layer and Al migrates into the Ti–Si layer during annealing. These results suggest that the TiAl3(–Si) phase is thermodynamically more stable than the TiSi2 and TiAl3 phases and that its formation is not kinetically limited by diffusion of Si and Al in either Al or Ti at 450 °C. Formation of TiAl3(–Si) between Al layers is an important factor contributing to improved conductivity of the Al–Si/Ti layered meta...

Layered and homogeneous films of aluminum and aluminum/silicon with titanium, zirconium, and tungsten for multilevel interconnects

Layered structures and homogeneous alloy films synthesized by sputter deposition were investigated for use in a VLSI multilevel interconnect technology. Major areas studied include hillock formation, resistivity before and after annealing, film composition and structure reproducibility, interlevel shorts and dry etching. We have demonstrated in this work that aluminum alloyed with silicon and titanium and layered with titanium offers advantages over current technological materials for interconnections in integrated circuits. Measurements of surface roughness and electrical shorts between two levels of metal showed that the hillock densities in the films are, significantly reduced when small amounts (one to two atomic percent) of titanium and silicon are present However, the resistivity of such homogeneous films is 4.5 to 6 µΩ-cm which is higher than standard metallization alloys. When Al/Si was layered with Ti, no hillocks were, observed and the resistivity of the composite films was comparable to standard metallization alloys.