D. B. Knorr

Texture and microstructure of thin copper films

Microstructure is an important factor influencing the reliability of thin film interconnects. The microstructure of copper films is of particular interest because of its use in numerous electronic applications. Pole figure x-ray diffraction and transmission electron microcopy were conducted on copper films deposited by several techniques: sputtering, partially ionized beam deposition, chemical vapor deposition, evaporation, and electroplating. Quantitative texture data are determined from fiber texture plots. A typical copper film consists of three texture components: (111), (200), and random. (220) and (511) texture components are possible under some deposition conditions. Compared to aluminum films, the fraction of the random texture component and the distribution of the (hkl) components in copper films are relatively large. Bimodal grain size distributions are observed in some films.

TEXTURE IN MULTILAYER METALLIZATION STRUCTURES

The effects of thin Ti, TiN, or Ti/TiN underlayers on the development of the crystallographic texture and the grain structure are explored. Metal layers ∼0.5 μm in thickness of Al‐0.5Cu or of Cu are deposited on these underlayers and on amorphous SiO2 as a reference. A strongly textured underlayer such as Ti〈0002〉 or Ti〈0002〉/TiN〈111〉 induces a similarly strong 〈111〉 texture in the AlCu. In copper with 〈111〉, 〈200〉, and random texture components, an underlayer induces a stronger 〈111〉 component compared to an analogous film deposited on SiO2. A nearly random texture in TiN significantly weakens the texture in subsequent metal films. Grain size distributions in all AlCu films are monomodal reflecting a process of normal grain growth. The grain size distribution for Cu sometimes deviates from lognormal. The bimodal distribution implies that grain growth is abnormal even though the median grain size does not exceed a low multiple of the film thickness.

The effect of aging on microstructure, room temperature deformation, and fracture of Sn-Bi/Cu solder joints

The effects of isothermal aging on the microstructure and mechanical behavior of Sn-Bi/Cu solder joints are reported. Lap shear solder joints of eutectic Sn-Bi solder were aged for 3 to 30 days at 80°C and then loaded to failure in shear. Changes in the joint microstructure including interphase coarsening, intermetallic growth, and evolution of the intermetallic/solder interface are documented. The aging experiments reveal the segregation of the Bi-rich phase of the solder to the intermetallic/solder interface. The ultimate shear strength and ductility of the joints are reported at strain rates of 4.0 × 10−1 to 4.0 × 10−5 S−1 for 3 and 30 days aging. The strength of the joints decreases with strain rate for both aging conditions; the ductility is low and independent of strain rate for the joints aged three days and increases considerably with reduced strain rate for joints, aged 30 days. Fractographs and cross sections of the failed joints detail the effect of aging on the fracture mechanism.

The role of texture in the electromigration behavior of pure aluminum lines

The effects of microstructure on electromigration behavior were evaluated in three nominally 1 μm thick pure aluminum films, which were tested at temperatures from 423 to 523 K. The three Al films had essentially the same grain structure but different variants of an 〈111〉 texture. Texture had a very strong effect on the electromigration behavior in ∼2 μm wide polycrystalline lines, where both a reduced fraction of randomly oriented grains and a tighter 〈111〉 distribution increased the electromigration lifetime. The apparent activation energy for electromigration decreased as the texture strengthened. The near bamboo microstructure of 0.5 μm narrow lines showed extensive orientation clustering with an unusually high proportion of low angle boundaries in the most strongly 〈111〉 textured film. The electromigration damage in both 2 and 0.5 μm wide lines was correlated with the types of flux divergence sites in each film. The texture impacts the character of the grain boundaries and interfaces which control th...

THE PROPERTIES OF TIN-BISMUTH ALLOY SOLDERS

Tin-bismuth alloys may be an alternative to lead-based solders for low-temperature applications, but very little is known about their manufacturability and reliability. This article presents an overview of these issues. First, experiments to determine the wetting properties of the Sn-Bi solder are presented. The results show that Sn-Bi solders do not wet bare copper well, but that they do wet copper having a hot-dipped Sn-Bi coating. Next, the effects of aging on the microstructure of Sn-Bi solders are described. The results show that during aging, tin is de-pleted from the solder/base metal interface. The two-phase Sn-Bi microstructure coarsens during aging; the rate of coarsening can be slowed by adding 1.0 wt. % Cu to the solder. The aging also affects the shear strength of the solder joints, where aged joints show an increase in maximum shear stress and ductility at failure.

Relationship between texture and electromigration lifetime in sputtered Al-1% Si thin films

The relationship among the grain structure, texture, and electromigration lifetime of four Al-1% silicon metallizations produced under similar sputtering conditions was explored. The grain sizes and distributions were similar and the grain structure was near-bamboo for all metallizations. All metallizations exhibited a near-(111) fiber texture, as determined by the pole figure technique. Differences in electromigration behavior were noted. Three of the metallizations exhibited a bimodal failure distribution while the fourth was monomodal and had the longest electromigration lifetime. The electromigration lifetime was directly related to the strength of the (111) fiber texture in the metallization as anticipated. However, whereas the grain size distribution has an effect on the electromigration lifetime when metallization lines are several grains wide, the electromigration lifetime of these near-bamboo metallizations appeared independent of the grain structure. It was also observed that a number of failures occurred in the 8 μm interconnect supplying the 5 μm wide test lines. This apparently reflects an increased susceptibility of the wider interconnect lines to electromigration damage.

The microstructure, mechanical stress, texture, and electromigration behavior of Al-Pd alloys

As the minimum feature size of interconnect lines decreases below 0.5 urn, the need to control the line microstructure becomes increasingly important. The alloy content, deposition process, fabrication method, and thermal history all determine the microstructure of an interconnect, which, in turn, affects its performance and reliability. The motivation for this work was to characterize the microstructure of various sputtered Al-Pd alloys (Al-0.3wt.%Pd, Al-2Cu-0.3Pd, and Al-0.3Nb-0.3Pd) vs sputtered Al-Cu control samples (Al-0.5Cu and Al-2Cu) and to assess the role of grain size, mechanical stress, and crystallographic texture on the electromigration behavior of submicrometer wide lines. The grain size, mechanical stress, and texture of blanket films were measured as a function of annealing. The as-deposited film stress was tensile and followed a similar stress history on heating for all of the films; on cooling, however, significant differences were observed between the Al-Pd and Al-Cu films in the shape of their stress-temperature-curves. A strong (111) crystallographic texture was typically found for Al-Cu films deposited on SiO2. A stronger (111) texture resulted when Al-Cu was deposited on 25 nm titanium. Al-0.3Pd films, however, exhibited either a weak (111) or (220) texture when deposited on SiO2, which reverted to a strong (111) texture when deposited on 25 nm titanium. The electromigration lifetimes of passivated, ≈0.7 μm wide lines at 250°C and 2.5 × 106 A/cm2 for both single and multi-level samples (separated with W studs) are reported. The electromigration behavior of Al-0.3Pd was found to be less dependent on film microstructure than on the annealing atmosphere used, i.e. forming gas (90% N2-10%H2) annealed Al-0.3Pd films were superior to all of the alloys investigated, while annealing in only N2 resulted in poor lifetimes.

Through-thickness characterization of copper electrodeposit

Through-thickness crystallographic texture, defect structure, and tensile embrittlement of 35 μm thick electrodeposit are characterized by successive thinning. An initially random grain structure, inherited from the substrate, evolves into a strong <220> fiber texture. The random to oriented grain transformation begins at the inception of thickening and is complete after about 15 μm deposit thickness, where about 0.9 volume fraction of grains become oriented near <220>. Further thickening of the deposit sharpens the texture, reducing the scatter around the <220> ideal orientation. A duplex coarse/fine particle (coherent domain) structure is obtained. Coarse particles along <220> are less defective and have smaller lattice strains; fine particles along <200>, presumably associated with the random grains, are defect-saturated with finely spaced twins, high dislocation density and enhanced lattice strains. With increasing distance from the shiny surface (of initial film formation), especially following the initial 10 μm deposit thickness, (a) along <220>: particle size and twin spacing increase whereas dislocation density and root mean square (rms) strains decrease, (b) along <200>: particle size increases gradually, dislocation density and rms strains increase sharply and the already fine twin spacing remains unchanged, and (c) the effective particle size ratio Deff<220>:Deff<200> exceeds 1.4, suggesting a twinning-induced z-direction particle shape anisotropy. A substantial decrease in tensile elongation is observed at 180°C. The embrittlement increases with the deposit thickness, attributed to the development of low density regions in the morphological boundaries. High elongation and embrittlement directional anisotropies are observed near the shiny surface, perhaps due to preferred nucleation on the substrate asperities.

Correlation between special grain boundaries and electromigration behavior of aluminum thin films

Abstract The texture in thin films develops during processing steps such as deposition and annealing. Recent studies show that texture plays an important role in stress voiding, thermal hillock formation, grain collapse and electromigration failure. Specifically, electromigration failure depends on the grain misorientation distribution, which describes the probability of different grain boundaries and, therefore, links the grain boundary structure to the mass transport that takes place primarily along the grain boundaries. To understand the relationship between the grain misorientation and electromigration lifetime in aluminum thin films, the texture was measured on three sets of films from different manufacturing conditions. The frequency of occurrence of coincidence site lattice (CSL) grain boundaries, which represent special misorientations between grains, was obtained, and electromigration tests were done for all three conditions. Experimental results show that the lifetime of patterned films increases as the amount of 111 texture and the frequency of CSL boundaries increased.

Isothermal creep of eutectic SnBi and SnAg solder and solder joints

The isothermal tensile steady state creep rates of cast SnBi and SnAg eutectics are reported at temperatures between 20/spl deg/C and 160/spl deg/C, The SnBi eutectic has three regions of stress dependence with an extended power law region with stress exponent, n=3, at rates relevant to thermal fatigue. The SnAg eutectic has a simple power law stress dependence with a stress exponent between 6.7 (high temperatures) and 8.4 (low temperatures). These results are compared to room temperature creep rates of lap shear solder joints. The SnBi data correlate well. SnAg lap shear joints were significantly stronger than bulk samples. The difference is due to the amount of copper dissolved in the solder during reflow.<<ETX>>