Hidekazu Okabayashi

Stress-induced void formation in metallization for integrated circuits

Abstract The term “stress migration” in Al lines in ICs is used in a broad sense to described thermally induced void formation and resulting failures, i.e. open circuits and increases in resistance, which occur in high-temperature processes, such as insulating film deposition on Al lines in IC fabrication, and during storage tests at elevated temperatures for evaluating the reliability of fabricated ICs. Stress migration degrades the IC production yield and reliability. The problem becomes more serious as the linewidth decreases. This paper reviews stress migration-induced void formation and resulting failures in Al lines, and is intended to clarify current experimental and theoretical knowledge of their characteristics. The main topics discussed are classification and characteristics of stress migration, accelerated life test results and failure prediction, Al line stress, void formation models and countermeasures for reducing failures.

Process of forming electrodes and interconnections on silicon semiconductor devices

A process of forming electrodes and interconnections in a silicon semiconductor device comprises the steps of forming an insulating film on a silicon substrate, defining an opening in the insulating film, depositing a layer of metal having a high melting point on the insulating film, implanting ions to mix an interface between the metal layer and the silicon substrate, heating the construction in a temperature in the range of from 400 to 650 degrees Celsius to form a silicide of the metal layer in the opening, and selectively etching away an unreacted metal layer so as to self-align the silicide metal layer with the opening. The silicide metal layer is then annealed in a non-reducing gas atmosphere at a temperature ranging from 800 to 1,100 degrees Celsius.

Mo- and Ti-silicided low-resistance shallow junctions formed using the ion implantation through metal technique

Mo-and Ti-silicided junctions were formed using the ITM technique, which consists of ion implantation through metal (ITM) to induce metal-Si interface mixing and subsequent thermal annealing. Double ion implantation, using nondopant ions (Si or Ar) implantation for the metal-Si interface mixing and dopant ion (As or B) implantation for doping, has resulted in ultrashallow ( ≤ 0.1-µm) p⁺-n or n⁺-p junctions with ∼30-Ω sheet resistance for Mo-silicided junctions and ∼5.5-Ω sheet resistance for Ti-silicided junctions. The leakage current levels for the Mo-silicided n⁺-p junctions (0.1-µm junction depth) and the Mo-silicided p⁺-n junction (0.16-µm junction depth) are comparable to that for unsilicided n⁺-p junction with greater junction depth ( ∼0.25 µm).

Low-resistance MOS technology using self-aligned refractory silicidation

A new low-resistance MOS technology has been developed for use in VLSIs with scaled MOSFETs. A new MOSFET is featured by gate and source-drain being refractory silicided in self-alignment and isolated from one another, even without any insulating spacers on gate sides. An essential part of the MOSFET fabrication process is the ion implantation through metal (ITM) silicidation technique, which consists of ion-beam-induced metal-silicon interface mixing and appropriate annealings to form high-quality refractory metal silicides in self-alignment with silicon patterns and with good reproducibility.

Behavior of electromigration‐induced gaps in a layered Al line observed by in situ sideview transmission electron microscopy

We observed the detailed behavior of electromigration‐induced gaps (voids that cause an open circuit) in a submicron‐wide Al line layered on a Ti/TiN conductor by in situ sideview transmission electron microscopy. Two types of gaps were observed. The first type is characterized by extensive growth, which may make the Ti/TiN shunt ineffective under use conditions. The gap causes a decrease in the Al drift velocity upstream in the electron flow. This decrease probably results from mass transport through the exposed TiN surface into the gap. The second type of gap did not grow and later healed; therefore, the shunt seems to be effective. The gap had little influence on the drift velocity. Mass was transported directly from the cathode‐side Al segment through the gap to the anode‐side segment. These mass transport processes may be the causes of resistance oscillation in layered lines.

Recent progress in refractory-metal silicide formation by ion beam mixing and its applications to VLSL

Abstract Recent progress in refractory-metal silicide formation by ion beam mixing and subsequent annealing (the ITM technique) is reviewed in terms of ion beam mixing (IM), silicidation and device applications. Important items for further investigations to improve the technique to the level required for VLSI production applications are pointed out; they are clarification of the mechanism for lateral silicide suppression and clarification of the effects of recoiled metal atoms and IM induced defects on junction leakage current and MOS properties.

Schottky barrier inhomogeneity caused by grain boundaries in epitaxial Al film formed on Si(111)

Epitaxial Al contacts on Si(111) are fabricated by electron beam evaporation at various substrate temperatures around 250 °C. They are observed by high‐resolution transmission electron microscopy. Schottky barrier heights (SBHs) of the contacts are measured using current‐voltage and capacitance‐voltage methods. In the case of single‐crystalline Al film, the SBH does not change and its spatial distribution remains homogeneous up to an annealing temperature of 550 °C. In contrast with this, for an epitaxial Al film containing grain boundaries, the spatial distribution of the SBH becomes inhomogeneous above 400 °C. This is attributed to Si diffusion along the grain boundaries in the Al film.

A Self-Aligned Mo-Silicide Formation

Mo-silicides, formed by a new technique combining ion implantation through metal film (ITM) to induce metal/Si interface mixing and also to form doped layers with appropriate subsequent annealings, were found to have excellent properties in film uniformity and self-aligned formation for exposed Si areas (contact holes). These excellent silicidation properties in the ITM technique were also confirmed for patterned poly-Si silicidation. Lateral silicide growth out of contact holes, usually observed in silicidation using mere thermal reaction of refractory-metal/Si structures, was markedly suppressed in the ITM silicidation.

Electromigration in layered Al lines studied by in-situ ultra-high voltage electron microscopy

Abstract In-situ side-view transmission electron microscopy (TEM) observations of electromigration in Al-on-TiN lines with a drift velocity measurement structure have been carried out using an ultrahigh voltage (2 MV) electron microscope. Thick chips as-diced from silicon substrates served as TEM samples. The observations revealed the dynamic behavior of electromigration-induced voids and hillocks during forward and reverse current feeding through the Al lines. The results include vertical growth of voids bounded by faceted Al, refilling of voids, void growth in a hillock upon current reversal, and whisker growth.

An analytical open-failure lifetime model for stress-induced voiding in aluminum lines

An analytical model is derived for determining the time before which LSI aluminum (Al) lines with a bamboo-grain structure undergo open failures caused by stress-induced voiding. Derivations are based on the following assumptions: that the driving force for void growth is relaxation of Al-line tensile stress, that the mass transport rate is proportional to the nth power of Al-line stress, and that the void will maintain its prism shape throughout its growth. The critical minimum initial tensile stress value for producing open failure is calculated, as are the respective dependencies of lifetime on void tip angle, temperature, line dimensions, and line tensile stress. By adjusting the values of certain parameters within the model, it is possible to derive temperature, line-dimension, and tensile-stress dependencies that fall within the range of experimentally obtained results. >