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

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.     

Electromigration lifetimes and void growth at low cumulative failure probability

We have investigated electromigration (EM) lifetimes and void formation at cumulative failure probability of around 50 ppm. We carried out EM test in damascene Cu lines using sudden-death test structures. Cumulative failure probability of the test ranges from 50 ppm to 90%. To investigate the void nucleation and growth behaviour, Cu microstructures were investigated by using scanning transmission electron microscopy (S-TEM) and electron backscatter diffraction (EBSD) technique. EM lifetime shows strong correlation with the void nucleation site and the void volume. In addition, the worst case for EM lifetime is that wide angle grain boundary exists just under the via as a void nucleation site.     

Improving Reliability of Copper Dual-Damascene Interconnects by Impurity Doping and Interface Strengthening

Electromigration (EM)-derived void nucleation and growth in 65-nm-node dual-damascene interconnects were investigated, and the effects of impurity doping as well as copper adhesion strength to a capping-dielectric layer (CAP) are clarified. It is found that surface-reductive treatment of the copper line improves its adhesion to the SiCN-CAP, elongating the incubation time of voiding at the via bottom. An aluminum doping is effective in suppressing both the void nucleation and growth. Consequently, an aluminum-doped copper alloy with the strong copper/CAP interface improves the EM lifetime by 50 times compared to that of a conventional pure copper. These results clearly indicate that blocking migration paths of vacancies through both grain boundaries and the copper/CAP interface is essential in improving the EM reliability.     

Electromigration studies on Sn(Cu) alloy lines

The Cu alloying effect in the Sn(Cu) solder line has been studied. The Sn0.7Cu solder line has the most serious electromigration (EM) damage compared to pure Sn and Sn3.0Cu solder lines. The dominant factor for the fast EM rate in Sn0.7Cu could be attributed to the relatively small grain size and the low critical stress, i.e., the yielding stress of the Sn0.7Cu solder line. Also, we found that the shortest Sn0.7Cu solder line, 250 μm, has the most serious EM damage among three solder lines of different lengths. The back stress induced by EM might not play a significant role on the EM test of long solder lines. A new failure mode of EM test was observed; EM under an external tensile stress. The external stress is superimposed on the stress profile induced by EM. As a result, the hillock formation was retarded at the anode side, and void formation was enhanced at the cathode.     

Electromigration failure in ultra-fine copper interconnects

This paper presents experimental evidence suggesting that electromigration (EM) can be a serious reliability threat when the dimension of Cu interconnects approaches the nanoscale range. To understand the failure mechanism prevailing in nanoscale Cu interconnects, single-level, 400-μm long interconnects with various effective widths, ranging from 750 nm to 80 nm, were made, EM tested, and characterized in this investigation. The results indicate that interface EM (Cu/barrier) may be the predominant EM mechanism in all line widths. The evidence supporting the active Cu/barrier interface EM includes the fact that the EM lifetime is inversely proportional to the interface area fraction. Microscopic analysis of the failure sites also supports the conclusion of interface EM because voids and hillocks are found at the ends of the test strip, which is not possible if lines fail by grain-boundary EM in the test structure used in this study. In addition, our study finds evidence that failure is assisted by a secondary mechanism. The influence of this factor is particularly significant when the feature size is small, resulting in more uniform distribution of failure time in narrower lines. Although limited, evidence suggests that the secondary factor is probably attributed to pre-existing defects or grain boundaries.     

热退火对多晶硅特性的影响

为研究热退火对太阳电池用多晶硅的影响,在750～1150℃,N2和O2环境下分别对硅片进行热处理.用傅里叶红外光谱仪和准稳态光电导衰减法测量退火前后的氧碳含量和少子寿命的变化.为了比较,对有相同氧碳含量的直拉硅片进行同样处理.结果发现:在多晶和单晶片中氧碳含量下降很小,意味着没有氧沉淀产生,晶界对碳行为影响不大.多晶硅片在N2和O2环境下,850、950和1150℃下退火,少子寿命都有很大提高,并且在O2中退火比N2中退火少子寿命上升得更多,可能由于在高温退火时大量杂质扩散到晶界处,减少了复合中心.另外,间隙硅原子填充了空位或复合中心从而导致寿命提高.     

Analysis of electromigration induced early failures in Cu interconnects for 45 nm node

Bi-directional current stressing was used for monitoring electromigration (EM) lifetime evolution in 45 nm node interconnects. Experimental results show that an initial bimodal distribution of lifetimes can be modified into a more robust mono-modal distribution. Since the bi-directional tests provide successive void nucleation and void healing phases, the Cu microstructure is thought to evolve once the formed void is filled thanks to EM induced matter displacement. FEM modeling is used to compare the predicted location of void nucleation for given microstructures at the cathode end: a multigrain structure is compared to a perfect bamboo microstructure. Experimental and modeling results let us assume that small grains (<linewidth or via diameter) at the cathode end present a risk of EM induced early fails. Indeed at this location void nucleates and grows nearby the via opening it shortly. On the contrary, the bamboo microstructure is thought to provide more robust lifetime because voids nucleate a few hundred nanometers in the line and grow down reaching the bottom diffusion barrier of the line. This latter case provides larger void size before circuit opening.     

具有冗余设计的铜互连线电迁移可靠性评估

铜互连的电迁移可靠性与晶粒结构、几何结构、制造工艺以及介质材料等因素有着密切的关系。分别试制了末端有一定延伸的互连线冗余结构设计的样品,以及无冗余结构的互连线样品,并对样品进行了失效加速测试。测试结果显示,采用冗余结构设计的互连线失效时间更长,具有更好的抗电迁移可靠性。对冗余结构的失效模式进行了讨论,并结合互连线的制造工艺,指出采用冗余结构设计的互连线可以在有效改善互连线的电迁移特性,而且不会引入其他影响可靠性的因素,是一种有效提高铜互连电迁移可靠性的方法。     

Electromigration-induced failures in thin-film Al-Cu conductors

The effect of CuAl₂ precipitates on the electromigration lifetime of thin film Al-Cu conductors was studied by varying the Cu concentration in the 0–12 wt per cent range. Experiments show that the conductor lifetime does not increase monotonically with an increase in precipitate content. It is shown that the precipitates, when present in sufficient quantity, can greatly modify the grain size distribution and hence can affect the electromigration lifetime. These results are interpreted in terms of a model relating the vacancy and the copper atom flux along the grain boundaries. The roles of annealing treatments, copper concentration, structural non-homogeneities and test temperatures are explained by the present model.     

Impact of thermal cycling on the evolution of grain, precipitate and dislocation structure in Al, 0.5% Cu, 1% Si thin films

Thermal cycles have been performed both outside and inside a transmission electron microscope (TEM) in order to analyze the evolution of the microstructure of Al, 0.5% Cu, 1% Si thin films deposited onto oxidized Si substrates. It is shown that grain growth and dislocation activity start almost simultaneously and cooperate throughout the plastic regime of the stress–temperature curve to generate bamboo-type grains with low dislocation density. Si precipitates serve as anchoring points for dislocations and grain boundaries. Thermal cycling and diffusion cause the growth of these precipitates and a diminution of their number. Diffusion also proves to play an important role regarding plastic relaxation at the Al/SiO_x interface and at the grain boundaries where an intense hillock and whisker formation has been observed in scanning electron microscope (SEM). The stress–temperature evolution is discussed in light of these observations.     

Al-Mg-Si-Cu合金中晶界和晶内析出相粗化规律的研究

采用显微硬度测试仪、扫描电镜和透射电镜观察及能谱分析,研究了Al-0.5Mg-1.0Si-0.8Cu(wt.%)合金的晶界析出规律和晶内析出相的粗化机制。结果表明:180℃时效处理的Al-0.5Mg-1.0Si-0.8Cu(wt.%)合金晶界处存在富Si相和Q相两类不连续分布的析出相,它们的尺寸分别约为1 mm和几十纳米;时效0.5 h时晶界处有少量富Si相,时效5 h时晶界富Si相明显增多,时效36 h时富Si相开始粗化且间距变大,再进一步时效其形貌和分布变化不大;晶界Q相与相邻晶粒铝基体的界面取向关系是:[510]Al//[1120]Q;时效36 h晶内开始出现粗化析出相,且随时效继续进行合金晶内析出大量粗化相,存在明显的晶界无析出带现象。晶内粗化析出相主要含有Si元素,呈片状、球状和棒状3种形貌。其中,棒状Si析出相是沿〈001〉Al方向生长的,且〈112〉Si//〈001〉Al,{111}Si//{010}Al。     

A grain structure based statistical simulation of electromigration damage in chip level interconnect lines

Reliability assessment of chip level interconnects is based on accelerated testing at a higher temperature and a larger current density than expected in service conditions. The critical parameters needed to extrapolate accelerated test data to service conditions are the activation energy, Q, and the current density exponent, n. Although current density exponents and activation energies are well known for the elemental processes (like void nucleation due to electromigration (EM) generated stress), there is no consensus on which apparent activation energy or current density exponent values would be applicable in reliability estimates for realistic line structures. Here, we first review our EM simulation tool. We then apply the EM simulation tool to statistical life time analysis of realistic-like line structures generated by a Monte-Carlo algorithm. For a given grain structure distribution, the stress evolution along the line is simulated, letting voids nucleate at the sites where the stress exceeds a critical level, and the nucleated voids are then allowed to grow till the largest one reaches the preset critical size, resulting in a ‘failure’ of the particular line. By repeating this process for various current densities and temperatures, it becomes possible to extract the apparent activation energies and current density exponents from the simulation data.     

Experimental study of electromigration at bamboo grain boundarieswith a new test structure using the single-crystal aluminuminterconnection

Electromigration (EM) voids in the bamboo structure interconnect were observed by a new test structure with single-crystal aluminum leads. The new test structure consists of a single grain connected to single-crystal aluminum leads formed by lateral-solid phase epitaxial growth (L-SPE). The grain was formed by suppressing L-SPE of the single-crystal aluminum leads. Since the void nucleation sites were confined to the grain boundaries, the voids were easily located and observed. In addition, the crystal orientation of single-crystal aluminum leads could be controlled by L-SPE, and so the analysis could be performed more accurately than using traditional test structures that have a series of grains with random crystal orientation. The accelerated EM test was carried out under ideal conditions similar to that of real devices, because the temperature gradients around the test site of the bamboo grain boundaries were negligible. In our preliminary experiment, a void was observed in the grain, located next to the positive voltage lead. This seems to be contradictory to general understandings, we think this is because of the grain boundary configuration difference and/or EM induced vacancy fluxes difference     

Electromigration-induced stress in a confined bamboo interconnect with randomly distributed grain sizes

Microfailure of thin film interconnects by electromigration (EM)-induced stress is one of primary causes of degradation of the reliability of microelectronic circuits. We describe a two-dimensional model to simulate EM-induced stress in a confined bamboo interconnect line with randomly distributed grain sizes. We find that the steady-state stress distribution along the line is linear, the stress gradient being solely dependent upon the applied electric field, independent of the grain distribution and the diffusion condition at the line ends. However, the grain boundary concentration produces a significant shift in the steady-state stress along the line. When the grain boundaries concentrate in the tensile (compressive) stress zone, the steady-state stress lines shift up (down). Accordingly, the maximum tensile stress increases (decreases). The more the grain boundaries are in the tensile (compressive) stress zone, the more the maximum tensile stress increases (decreases). Thus, the maximum stress achieved at steady state is not only dependent on the line length for given applied electric field, but is also dependent on the grain boundary distribution along the line. A line microstructure with more grains in the compressive stress zone is of benefit to reduce the maximum tensile stress in the line. The preset results not only interpret why the EM-induced failure observed in experiments is sensitive to the variation in the line microstructures, but also imply that designing a line with fine grains in the compressive stress zone (near anode end) may significantly reduce the maximum tensile stress in the line, which can cause inhibition of failure due to EM-induced microcracking and void nucleation.     

Mechanism of electromigration failure in submicron Cu interconnects

This paper reports on two different electromigration-failure mechanisms competing in Cu interconnects. Accelerated electromigration tests are conducted on identical single-level, 0.25-μm Cu interconnects with SiN or SiCN passivation. The results indicate that the failure mechanism can vary with the interface condition of the capping layer. The first failure mechanism, seen primarily in SiN-capped samples, is characterized by extensive interface damage, believed to be a result of failure led by interface electromigration. In this failure mode, damage initiates at the capping interface but gradually spreads along all interfaces of the Cu to form an isolated strand. The competing failure mechanism, found in SiCN-capped samples, is characterized by the formation and growth of a localized void without extensive interface damage. The absence of interface damage, in addition to the higher activation energy for failure, suggests that the failure occurs at a more localized inhomogeneity than the interface, such as grain boundaries. While the exact mechanism of how the capping layer suppresses one mechanism and promotes the other is unknown, this study reveals that the passivation-interface material and condition have a decisive role in determining the failure mechanism in Cu interconnects.     

Impact of Al in Cu alloy interconnects on electro and stress migration reliabilities

The reliability of Cu interconnects was successfully improved by applying a CuAl alloy seed. However, the effect of additive Al on the reliability is not fully understood. In order to reveal the reliability improvement mechanism, Cu films using CuAl alloy seed were investigated in detail. As stress induced voiding (SIV) as well as electromigration is caused by migration of vacancies and/or Cu atoms, the measured activation energy value of electromigration using CuAl indicates that the fast diffusion paths are Cu grain boundaries. The analysis using high lateral resolution scanning type secondary ion mass spectrometry (nano-SIMS) clarifies that additive Al in ECP-Cu film is mainly localized at grain boundaries. Furthermore, positron annihilation was used to probe vacancy-type defects in Cu films. The CuAl films before recrystallization contain larger and higher density vacancy-type defects. Whereas, the recrystallized CuAl films after annealing above 250 °C contain smaller and lower density defects. Furthermore, CuAl films with annealing above 350 °C contain less Al inside the grains. These results represent that Al atoms in Cu films with annealing above 350 °C are exhausted from inside grains to the grain boundaries, and the spewed Al atoms existing at Cu grain boundary effectively prevents the diffusion of Cu and/or vacancies.     

Effects of Mg additions on the electromigration behavior of Al thin film conductors

Aluminum thin film conductors containing Mg alloying additions have been tested for electromigration failure by formation of electrically open circuits. The test conditions were either 2 × 10⁶ A/cm² or 4 × 10⁶ A/cm² for the current density, and either 175 or 225°C for the temperature. The median lifetimes were found to increase with increasing Mg concentrations up to the highest concentration tested, about 6%. With polycrystalline films the maximum increase in lifetime resulting from Mg additions corresponds to a factor of about 100, as compared to pure Al films. This is about equal to previously reported results obtained with Cu additions. The increase in lifetime has been shown to result from a decrease in the rate of grain boundary diffusion for the Al atoms. Magnesium atoms diffuse at approximately the same rate as Al atoms. Thus the mechanism of failure formation in Al films containing Mg is thought to be different than in Al-Cu films, where Cu atoms diffuse faster than Al atoms and failure ensues upon local Cu depletion.     

Carrier lifetime limitation defects in polycrystalline silicon ribbons grown on substrate (RGS)

Carrier lifetime limitation defects in polycrystalline silicon ribbons have been examined in samples with high oxygen and carbon content. Infrared spectroscopy showed that essentially all supersaturated oxygen impurities precipitated within 1 h annealing at over 800 °C. Preferential defect etching revealed that a much higher density of oxygen precipitates were generated in dislocation-free grains than in those highly dislocated (10⁵–10⁷ cm⁻²) ones. Correlated with electron-beam-induced current imaging, we found that oxygen precipitates are the dominant carrier recombination defects in dislocation-free grains, while dislocations are the lifetime killer for highly dislocated grains. It is suggested that eliminating dislocations alone will not improve the carrier lifetime, considering that a higher density of oxygen precipitates formed in the absence of dislocation-related heterogeneous nucleation sites will significantly degrade the carrier lifetime.     

A scanning electron microscope study of electromigration in an Al-2% Cu thin film

Electromigration in an Al-27% Cu metallization thin film deposited on a silicon transistor structure has been studied in situ using the electron beam induced current mode (EBIC) of the scanning electron microscope. In this mode electrons which are transmitted through the metallization generate a signal which depends on the thin film thickness and its mass density. During the initial stages of the electromigration experiment the copper rich precipitates on the surface coarsened. Also, there was a preferential coarsening towards the positive end. Once the negative end became depleted of these surface precipitates grain thinning occurred. Voids nucleated and grew in from the sides of the metallization in these depleted regions finally causing film failure. However almost no voids were observed at grain boundaries or triple points. The copper rich precipitates in the interior of the metal film were stationary and showed no signs of coarsening or depleting any region. These observations suggest that surface diffusion is the predominant mode of atom migration in these alloys; grain boundary diffusion has been reduced compared to pure aluminum.     

Local Melting during Electromigration in Cu Conductor Lines

We have observed abrupt, reversible resistance changes during electromigration (EM) testing of damascene Cu conductor lines. The tests were conducted at temperatures of 300°C or 350°C and with current densities from 0.6 × 10⁶ A/cm² to 1.6 × 10⁶ A/cm². In most cases, an incubation period with negligible resistance increase was followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. If the direction of current was reversed, the resistances of the Cu lines decreased, due to the refilling of voids by the back flowing atoms. With further EM, the resistance curves showed spikelike features, with a sudden resistance increase followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurred, and the line failed. We present resistance data, microstructural observations, and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating and melting of conductor line segments, and from voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases, line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to the voids.