Masahiko Hasunuma

Electromigration‐induced void growth in bamboo structures

A novel resistometric technique enables the investigation of single void nucleation and growth induced by electromigration (EM) for aluminum (Al) lines having a perfect bamboo structure in comparison with single‐crystal Al lines. Fine tungsten (W) voltage probes were fabricated at every 4 μm along the Al line with grain sizes of 10 μm or more. Local electrical resistance changes have confirmed that a void nucleated only at the grain boundary and no damage appeared within the grains. The measured values of the local electrical resistance changes were converted to EM‐induced void growth rates. The vacancy flux was deduced from the void growth rate under the assumption that a vacancy volume is equal to the atomic volume. It has been clarified that the vacancy fluxes for a bamboo‐structured Al line were about one order magnitude smaller than the ideal vacancy fluxes in the Al lattice derived from the Nernst–Einstein relation. The vacancy fluxes for single‐crystal Al lines were also quantified under an acceler...

Single crystal aluminum lines with excellent endurance against stress induced failure

An excellent ability to withstand stress-induced failure and electromigration failure is demonstrated for single-crystal Al lines. In both single-crystal and polycrystalline Al lines, voids were surrounded by low surface energy planes, so that the shape of the voids would be affected by crystal orientation. The results suggest that single-crystal Al is a potential candidate for submicron lines in ULSIs.<<ETX>>

Improvement in the electromigration lifetime using hyper-textured aluminum formed on amorphous tantalum-aluminum underlayer

A new fabrication technique for hyper-textured aluminum (Al) films has been developed by using an amorphous tantalum-aluminum (Ta-Al) underlayer. The full width at half maximum (FWHM) value of the (111) rocking curve for Al film has been attained to be less than 1 degree. It has been clarified that the empirical relation between the electromigration (EM) lifetime of an Al interconnection /spl tau/ and the FWHM value /spl omega/ is described as /spl tausup /spl prop/spl omegasup -2/. This result has confirmed that a hyper-texture is a promising approach to withstand higher current densities required in future ULSIs.<<ETX>>

A newly developed model for stress induced slit-like voiding

Thermodynamic analysis of stress-induced voiding has indicated that a slit-like void is the origin of metal line open failures. Wedge-shaped voids nucleate initially at specific grain boundaries where

Reliability Improvement by Adopting Ti-barrier Metal B for Porous Low-k IL Structure

This paper elucidated for the first time that titanium (Ti) is an excellent barrier metal (BM) material from the stand point of cost and performance, especially for the porous low-k ILD materials. Both stress induced voiding (SIV) suppression and one order longer electromigration (EM) lifetime were obtained by introducing Ti instead of the conventional tantalum (Ta). It has been considered that the smaller volume change when oxidized and the existence of metallic Ti-O solid-solution phase for Ti would be the reason for its control of moisture penetration from the low-k ILD materials which resulted in excellent SIV suppression. No electrical resistance increase due to intermetallic reaction between Cu and Ti was observed. Furthermore, the suppression of Cu grain boundary migration was attributed to the segregation of Ti atoms at the Cu grain boundaries. This resulted in higher interconnect reliability

Impact of Residual Impurities on Annealing Properties of Vacancies in Electroplated Cu Studied Using Monoenergetic Positron Beams

Positron annihilation was used to probe vacancies in Cu films deposited by Ta/SiO2/Si using electroplating and sputtering techniques. During room temperature grain growth (i.e., self-annealing) of the Cu films, two different types of vacancies (small vacancy clusters such as divacancies and large vacancy agglomerates) were introduced into grains; the formation of such defects was enhanced by residual impurities. For electroplated Cu, isochronal annealing experiments revealed further agglomeration of vacancies when annealing was done below 300 °C, and these agglomerates started to dissociate above 350 °C. The effect of impurities on the vacancy agglomerates disappeared in the defect recovery stage (≥350 °C).

Electromigration induced aluminum atom migration retarding by grain boundary structure stabilization and copper doping

Abstract In order to clarify the relationship between Al line reliability and film microstructure, most notably grain boundary structure, we have tested three kinds of highly textured Al lines, namely a single-crystal Al line, a quasi single-crystal Al line and a hyper-textured Al line. Consequently, it has been shown that these kinds of lines have excellent endurance against electromigration (EM), compared with conventional Al lines deposited on TiN/Ti and SiO2. The improvement of Al line reliability is attributable to the following factors; firstly, homogeneous microstructure and high activation energy, 1.28 eV, of the single-crystal Al line (ω=0.18°); secondly, subgrain boundaries, consisting of dislocation arrays found in the quasi single-crystal Al line (ω=0.26°), have turned out to be no more effective mass transport paths because dislocation lines are perpendicular to the direction of electron wind; finally, the decrease of the (1 1 1) full width at half maximum (FWHM) value promotes the formation of subgrain boundaries and low-angle boundaries, which have small grain boundary diffusivity, as revealed by the detailed orientation analysis of individual grains in the hyper-textured line (FWHM=0.5°) formed by using an amorphous Ta–Al underlayer (Toyoda H, Kawanoue T, Hasunuma M, Kaneko H, Miyauchi M. Proc. 32nd Ann. Int. Reliab. Phys. Symp., IEEE, 1994;178). Moreover, the diffusivity reduction and the uniformity of atomic flux result in the suppression of void/hillock pair in the Al lines. It has been clarified that a FWHM value is a useful criterion of reliability for an interconnection. Also, the Cu doping effect against EM endurance by using Cu implantation of the single-crystal Al lines has been examined. It has been clarified that EM lifetime is lengthened by about one order of magnitude for the Cu concentration of 0.1 at% in spite of almost the same diffusion coefficients. Moreover, the incubation time for a void nucleation has been observed even in the case of a pure-Al line. Thus, in accordance with the stress evolution model, it is concluded that the mechanism of lifetime improvement by Cu doping is such that critical stress for EM void nucleation is increased by the Cu doping. These results have confirmed that control of texture and/or grain boundary structure so as to suppress EM induced metal atom migration is a promising approach for the development of Al lines and Cu lines capable of withstanding the higher current densities required in future ULSIs.

Significant improvement in electromigration of reflow-sputtered Al-0.5wt%Cu/Nb-liner dual damascene interconnects with low-k organic SOG dielectric

Reflow-sputtered Al-0.5wt%Cu/Nb-liner dual damascene interconnects with low-k organic spin-on glass (SOG) passivation were fabricated for the first time. A significant improvement in median time-to-failure (MTF) for electromigration (EM) was observed when the SOG was compared to a standard TEOS passivation. This improvement is due to the low Youngs modulus of the SOG, which suppresses stress evolution in the Al interconnects near the via during EM testing.

A Highly Reliable Al Line with Controlled Texture and Grain Boundaries

In order to clarify the relationship between Al line reliability and film microstructure, especially grain boundary structure and crystal texture, we have tested three kinds of highly textured Al lines, namely, single-crystal Al line, quasi-single-crystal Al line and hypertextured Al line, and two kinds of conventional Al lines deposited on TiN/Ti and on SiO2. Consequently, the empirical relation between the electromigration (EM) lifetime of Al line † and the (111) full width at half maximum (FWHM) value ω is described by † ∝ ω -2 [1]. This improvement of Al line reliability results from as following reasons; firstly, homogeneous microstructure and high activation energy of 1.28eV for the single-crystal Al line (ω=0.18°); secondly, sub-grain boundaries which consisted of dislocation arrays found in the quasi-single-crystal Al line (ω=0.26°) has turned out to be no more effective mass transport paths because dislocation lines are perpendicular to the direction of electron wind. Although there exist plural grain boundary diffusion paths in the newly developed hypertextured Al line (ω=0.5°) formed by using an amorphous Ta-Al underlayer {1], the vacancy flux along the line has been suppressed to the same order of magnitude of single crystal line. It has been clarified that the decrease of FWHM value has promoted the formation of sub-grain boundaries and low-angle boundaries with detailed orientation analysis of individual grains in the hypertextured film. The longer EM lifetime for the hypertextured Al line is considered to be due to the small grain boundary diffusivities for these stable grain boundaries, and this diffusivity reduction resulted in the suppression of void/hillock pair in the Al lines. These results have confirmed that controlling texture and/or grain boundary itself is a promising approach to develop reliable Al lines which withstand higher current densities required in future ULSIs.

Effects of aluminum texture on electromigration lifetime

A hyper-textured aluminum (Al) film has been fabricated by using a newly developed amorphous tantalum-aluminum (Ta-Al) underlayer. The full width at half maximum (FWHM) value of the (111) rocking curve of the Al film has been attained to be 0.5 degrees. It has been shown that the electromigration (EM) lifetime of Al interconnections increased inversely proportional to the square of the FWHM value. This lifetime improvement has been attributed to the reduction of the vacancy flux along the grain boundaries. The analysis of the grain boundary structure in the hyper-textured Al film with the transmission electron microscope (TEM) has revealed that the texture improvement not only eliminates the twist component of the boundary mismatch, but also controls the tilt one. It has been also clarified that the compressive stress relaxation and the hillock formation of Al film during the thermal cycling were restrained by the hyper-texture. This phenomenon has been well explained by the grain boundary diffusion suppr...