Electromigration Damage Research Articles

Relationship between copper concentration and stress during electromigration in an Al(0.25 at. % Cu) conductor line

Synchrotron-based x-ray microbeam fluorescence and diffraction have been used for in situ measurements of Cu concentration and biaxial stress in a 200-μm-long, 10-μm-wide Al(0.25 at. % Cu) conductor line with 1.5-μm-thick SiO2 passivation during electromigration. Measurements over 48 h with T=300 °C and j=1.5×105 A/cm2 show that a stress gradient of 3 MPa/μm develops over the upstream 130 μm of line length where Cu concentration drops below 0.15 at. %, and a 10-μm-long void develops at the cathode end of the line, but little change in stress occurs over the downstream 70 μm of line length where Cu concentration remains above 0.15 at. %. These experimental results have been reproduced by a finite element model in which the downstream Cu transport is accompanied by a counter flow of Al in the upstream direction, and downstream Al motion is blocked where the local Cu concentration is above ∼0.15 at. %. Defect mediated coupling between Al and Cu diffusive flows, e.g., Cu–vacancy binding, is proposed as the cause for the counterflow of Al when the Cu concentration is above the critical concentration, and as the mechanism by which Cu reduces the rate of electromigration damage in Al(Cu) conductor lines.

Real-time observation of electromigration-induced stress changes with imaging x-ray topography

We have observed the evolution of stress in Al interconnect lines during the electromigration process using x-ray topography in a new imaging configuration. Monochromatic x rays from a synchrotron source were reflected in a symmetric geometry by the Si substrate of the interconnect structure. We used a Fresnel zone plate in an off-axis imaging geometry to obtain magnified images of the sample. The results show the establishment of a constant stress gradient in the steady state where the electromigration damage is halted as well as time-dependent behavior of stress leading to that state. The imaging x-ray topography technique offers a number of advantages including better spatial resolution potential by eliminating the blur that occurs in conventional topography due to the finite distance between the sample and the detector.

737 配線端部におけるエレクトロマイグレーション損傷支配パラメータを用いた配線物性値の導出

737 配線端部におけるエレクトロマイグレーション損傷支配パラメータを用いた配線物性値の導出

1833 配線端部におけるエレクトロマイグレーション損傷の支配パラメータ

1833 配線端部におけるエレクトロマイグレーション損傷の支配パラメータ

Onset of long-range diffusion and exponent of 1/fα noise in metal films with electromigration damage

Investigations of low-frequency conductance fluctuations have been done on silver films which have been made to undergo electromigration damage. The system shows a clear increase in noise magnitude after electromigration damage. The noise spectral power shows a distinct presence of 1/f3/2 component arising out of long-range diffusion. The temperature dependence of noise (150 K<T<350 K) shows a marked deviation from the Dutta–Horn type behavior with the 1/f3/2 term showing a strong dominance at higher temperatures. We propose that the increase of noise in metal films after electromigration damage arise predominantly from this spectral component.

In situ SEM observation of electromigration phenomena in fully embedded copper interconnect structures

An experimental set-up is presented, that allows in situ scanning electron microscope (SEM) investigations of the progress of electromigration damage in fully embedded copper interconnect structures. A LEO Gemini 1550 SEM has been equipped with a heating stage and electrical connections for the experiment. The studied interconnect structures are usually used for reliability testing in electromigration ovens. These test structures are located within the scribelines of wafers. Therefore, they allow the characterization of the electromigration behaviour of products on the wafer. To enable the SEM observation, focused ion beam (FIB) was used to prepare cross-sections of the samples maintaining their electrical functionality. Thereby, a thin layer of passivation was left over in front of the interconnects to keep them fully embedded. The SEM images which were taken at an angle of 60° allow the observation of both the entire via/contact and the connecting lines. Multiple images were recorded during the degradation experiments. The resulting video sequences provide a good visualization of the formation, growth and motion of voids at the stressed interconnects. The dominant diffusion path has been identified.

Prediction of electromigration failure in passivated polycrystalline line

Mechanical stress or atomic density distribution in passivated metal lines is induced by electromigration, and plays an important role in the mechanism of electromigration damage. Recently, a governing parameter for electromigration damage in passivated polycrystalline lines, atomic flux divergence (AFDgen*), was formulated by adding the effect of the atomic density gradient to the governing parameter for electromigration damage in unpassivated polycrystalline lines, AFDgen. The latter has already been utilized to construct a prediction method for electromigration failure in unpassivated lines, as the first step toward the development of a practical method. In this article, a prediction method for electromigration failure in a passivated polycrystalline line is proposed using AFDgen*. It is shown that the proposed method is able to predict the failure location as well as the lifetime of the passivated polycrystalline lines. Both the lifetime and the failure location in a passivated polycrystalline line are predicted by means of numerical simulation of the failure process covering the building up of atomic density distribution, void initiation, void growth, and ultimately—line failure. The usefulness of this method is verified by an experiment where two passivated straight lines with different lengths are treated.

Governing parameter for electromigration damage in the polycrystalline line covered with a passivation layer

A governing parameter, called AFDgen*, which reflects electromigration damage in a polycrystalline line covered with a passivation layer is proposed. The formulation is based on the parameter AFDgen previously introduced in our studies. With the help of AFDgen we can calculate the atomic flux divergence due to electromigration by considering two-dimensional distributions of current density and temperature and also by simply considering the microstructure of polycrystalline lines and bamboo lines. AFDgen has been identified as a governing parameter for electromigration damage in unpassivated polycrystalline lines and bamboo lines through experimental verification. As the first step in the development of a practical and universal prediction method for electromigration damage, we treated metal lines not covered with a passivation layer. On the other hand, metal lines used in packaged silicon integrated circuits are covered with passivation. Electromigration induces a mechanical stress (atomic density) gradient in such lines. This gradient plays an important role in the mechanism of electromigration damage. The new parameter proposed here, AFDgen*, includes the effect of the atomic density gradient. We develop also an AFDgen*-based method for determination of film characteristics. This method is applied to both covered and uncovered metal lines made of the same Al film. The film characteristics of both line types are obtained experimentally. Based on a discussion about the validity of the obtained characteristic constants, we were finally able to conclude that the AFDgen* parameter and the proposed method for deriving film characteristics are useful.

The effect of contact resistance on current crowding and electromigration in ULSI multi-level interconnects

Electromigration (EM) has been the most persistent interconnect reliability issue over the decades. In general, EM damages tend to occur at atomic flux divergence sites. The EM failure rate can be further accelerated by current crowding, which occurs when current flows between inter-level wires. In this work, we used two different via etch schemes to study the effect of contact resistance on current crowding and EM. We found that the etch stop structures show longer EM lifetimes than the over etch. The contact resistance of the etch stop is higher than that of the over etch. Two-dimensional simulation results show that the higher contact resistance in the etch stop can suppress current crowding and improve EM lifetimes. Differences in void morphology between the over etch and etch stop as a result of current crowding are discussed.

Effects of corner position and operating condition on electromigration failure in angled bamboo lines without passivation layer

The atomic flux divergence due to electromigration, AFD gen, has been formulated considering two-dimensional distributions of current density and temperature within polycrystalline and bamboo line structures without passivation. The divergence AFD gen has been identified as a governing parameter of electromigration damage by experimental verification. Recently, an AFD gen-based method for predicting the lifetime and failure site in an unpassivated bamboo line was proposed based on numerical simulation of the failure process of a metal line. Availing of the advantage of the universal method, the effects of corner position, corner angle and operating conditions on the lifetime and failure location in angled metal lines are discussed. The lifetime and failure location varied with the line shape and operating conditions. Understanding these effects on metal line failure gives important knowledge for enhancing the electromigration endurance of metal lines.

Electromigration of eutectic SnPb and SnAg3.8Cu0.7 flip chip solder bumps and under-bump metallization

The electromigration damage in flip chip solder bumps of eutectic SnPb and SnAg3.8Cu0.7 was studied after current stressing at 120 °C with 1.5 A. The diameter of the bumps was about 200 μm. The under-bump metallization (UBM) on the chip side was electroless Ni and on the board side was electroplated Cu. Surface marker motion was used to measure the atomic flux driven by electromigration and to calculate the effective charge number, Z*, of the solder. For eutectic SnPb, Z* is about 36–100 after 39.5 h of electromigration, but for SnAg3.8Cu0.7 the marker movement was too small to measure Z* even after 200 h of current stressing. While the effect of electromigration in SnAg3.8Cu0.7 is much smaller than that in eutectic SnPb, hillocks of intermetallic compounds at the anode have been observed in the former. An extensive growth of Ni–Cu–Sn intermetallic compounds in the matrix of both solder bumps was found. A localized dissolution of electroless Ni UBM was observed under current stressing, yet no dissolution of the Cu UBM by electromigration was found.

Investigation of the role of compositional effects on electromigration damage of metallic interconnects

Compositional effects are usually observed in the early stages of electromigration of interconnects made of Al alloys like Al–Cu and Al–Si. These effects are due to a variation of the fraction of the solute species (Cu or Si) dissolved into the Al matrix mainly due to heating effects. Here we investigate the role of compositional effects on electromigration phenomena by using a dynamical percolation model that we have recently developed. Our results well reproduce several phenomenological aspects typical of electromigration experiments. This agreement, together with the flexibility of our approach, suggests wide possibilities of further extending the model to study also the geometrical and structural effects.

A percolative approach to electromigration in metallic lines

We present a stochastic model which simulates electromigration damage in metallic interconnects by biased percolation of a random resistornetwork in the presence of degradation and recovery processes. The main features of experiments including Black's law, times to failure distribution, current threshold for the onset of electromigration,etc are properly reproduced.Compositional effects showing up in early resistance changes measuredon Al-0.5%Cu and Al-1%Si lines are also studied.

The effect of polyimide passivation on the electromigration of Cu multilayer interconnections

Electromigration damage (EMD) is one of the major causes for the failures of interconnect. In this study, the electromigration (EM) of Cu multilayer (TiWN/Cu/TiWN) with polyimide passivation is investigated with an isothermal resistance change method. The EM measurements were carried out on a wafer level at various temperatures (170–230 °C) and current densities (2.28–4.0\( MA/cm^2 \)). The activation energy for passivated Cu multilayer is larger than that of the unpassivated ones. The lifetime of passivated specimens are from two to 16 times those of the unpassivated ones. Resistance oscillation, which is attributed to the formation and closing of Cu gap, is observed during EM test. The TiWN interlayer helps to maintain the electrical continuity when a local Cu gap is formed. Hence, the lifetime of Cu metallization is further enhanced by the presence of the interlayer. Copper multilayer interconnect has better EMD resistance than Cu monolayer interconnect does.

440 エレクトロマイグレーション損傷の支配パラメータを用いた表面に保護膜を有する多結晶配線の断線予測とその検証(OS01-5 電子デバイス実装・電子材料と計算力学(5))(OS01 電子デバイス実装・電子材料と計算力学)

440 エレクトロマイグレーション損傷の支配パラメータを用いた表面に保護膜を有する多結晶配線の断線予測とその検証(OS01-5 電子デバイス実装・電子材料と計算力学(5))(OS01 電子デバイス実装・電子材料と計算力学)

528 表面に保護膜を有する多結晶配線のエレクトロマイグレーション損傷支配パラメータの実験的検証(OS7-3 損傷・破壊)(OS7 微視構造を有する材料の力学)

528 表面に保護膜を有する多結晶配線のエレクトロマイグレーション損傷支配パラメータの実験的検証(OS7-3 損傷・破壊)(OS7 微視構造を有する材料の力学)

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.

Relationship between microstructure and electromigration damage in unpassivated PVD copper damascene interconnects

The microstructure of unpassivated PVD copper interconnects has been determined by electron backscatter diffraction technique (EBSD) inside a scanning electron microscope (SEM), and the appearance and growth of voids and hillocks during the electromigration testing has been observed in situ inside the SEM. The EBSD measurement indicates a strong <111 > texture for the tested line and a high angle boundary fraction of more than 70%. The comparison of the EBSD maps and the SEM images of the defect formation due to electromigration shows that the voids are formed mainly at the sidewall and after blocking grains. These images indicate that the diffusion paths are both the interface and the grain boundaries.

Study of Electromigration Damage in Al Interconnect Lines inside a SEM

The mechanisms of electromigration, i.e. mass transport induced by high electric current, were studied in thin AlSi1Cu0.5 interconnects using in situ experiments in a Scanning Electron Microscope (SEM). The influence of grain boundaries as important paths of diffusion could be shown because the initial grain structure was recorded in detail by an orientation mapping with high lateral resolution over the whole interconnect. That was done by the Electron Back Scatter Diffraction technique in the SEM. The role of grain boundaries with high misorientation angles and of large blocking grains was investigated in detail by comparison of the localised damages with the corresponding part of orientation map. The formation of fatal voids was found to take place at the end of a large blocking grain followed by a high angle grain boundary directed parallel to the current flow. Hillocks were seen to be formed at such grain boundary triple junctions where a high flux divergence occurs due to different misorientation angles of the joined grain boundaries, and due to their direction with respect to the current flow. Additionally an increased content of the alloying element Cu was found in some of the hillocks.

Electromigration damage in mechanically deformed Al conductor lines: dislocations as fast diffusion paths

The role of dislocations in the generation of electromigration damage in model experiments is considered. Continuous segments of polycrystalline and single-crystal pure aluminum conductor lines were plastically deformed using nanoindentation methods. The lines were subsequently subjected to accelerated electromigration tests, in part in-situ in a scanning electron microscope. Electromigration damage was generated at plastically deformed segments in the single-crystal conductor lines, but not in polycrystalline lines. Diffusion paths and the origins of flux divergences that lead to electromigration damage are discussed. Investigations of the microstructure using focused ion beam (FIB) and electron back-scattered diffraction (EBSD) techniques did not show signs of fine-grain recrystallization in the indented regions. This and numerical estimates of the diffusivities involved suggest that electromigration damage arises by fast diffusion along dislocations with reasonable densities (of order 10 15/m 2).