Richard Hernandez

Electromigration of Cu-Sn-Cu micropads in 3D interconnect

There is significant interest in 3D interconnect technology due to its capability to provide fast, efficient inter-die interconnects at a minimum package footprint. Intermetallic Cu-Sn bonding has been widely investigated for 3D interconnects. However, the electromigration (EM) intrinsic reliability of the 3D Cu-Sn die-to-die microconnects has not been reported. In this paper the EM performance of 3D Cu-Sn microconnects formed by thermo-compression bonding is investigated and the failure mechanisms are discussed. The 3D stacked dice were assembled in wire bond ceramic packages and EM tests were conducted in both air and nitrogen ambient at various temperatures. Microconnect chain and Kelvin structures failure lifetime and the mean time to failure (MTTF) were measured. The failure analysis has been conducted and the possible failure mechanism has been proposed.

Statistical analysis of electromigration lifetimes and void evolution

Electromigration failure statistics and the origin of the log-normal standard deviation for copper interconnects were investigated by analyzing the statistics of electromigration lifetime and void size distributions at various stages during testing. Experiments were performed on 0.18μm wide Cu interconnects with tests terminated after certain amounts of resistance increase, or after a specified test time. The lifetime and void size distributions were found to follow log-normal distribution functions. The sigma values of these distributions decrease with increasing test time. The statistics of resistance-based void size distributions can be simulated by considering geometrical variations of the void shape. In contrast, the characteristics of time-based void size distributions require consideration of kinetic aspects of the electromigration process. The sigma values of lifetime distributions can be adequately simulated by combining the statistics of both types of void size distributions. Thus, a statistical...

Analysis of electromigration statistics for Cu interconnects

Electromigration failure statistics and the origin of the lognormal standard deviation for copper interconnects were investigated by analyzing the statistics of electromigration lifetimes and void size distributions at various stages during testing. A statistical correlation between electromigration lifetimes and void evolution was established. Using simulation to fit the experimental data, the parameters influencing the electromigration lifetime statistics were identified as variations in void sizes, geometrical and experimental factors of the electromigration experiment, and kinetic aspects of the mass transport process, such as differences in the interface diffusivity between the lines.

Large-scale statistical analysis of early failures in Cu electromigration, Part I: Dominating mechanisms

With continuing scaling of Cu-based metallization, the electromigration (EM) failure risk has remained one of the most important reliability concerns for advanced process technologies. The main factors requiring attention are the activation energy related to the dominating diffusion mechanism, the current exponent as well as the median lifetimes and lognormal standard deviation values of experimentally acquired failure time distributions. In general, the origin and scaling behavior of these parameters are relatively well understood. However, the observation of strong bimodality for the electron up-flow direction in dual-inlaid Cu interconnects has added complexity. The failure voids can occur both within the via (“early” mode) or within the trench (“late” mode). Over the last few years, bimodality has been reported also in down-flow EM, leading to very short lifetimes due to small, slit-shaped voids under vias. These voids, requiring only a very limited amount of mass movement, are generally causing conce...

Examination of Critical Length Effect in Copper Interconnects With Oxide and Low‐k Dielectrics

As technology moves toward faster microelectronic devices with smaller feature sizes, copper is replacing aluminum‐copper alloy and low‐k dielectric is replacing oxide as the materials of choice for advanced interconnect integrations. Copper not only brings to the table the advantage of lower resistivity, but also exhibits better electromigration performance when compared to Al(Cu). Low‐k dielectric materials are advantageous because they reduce power consumption and improve signal delay. Due to these advantages, the industry trend is moving towards integrating copper and low‐k dielectric for high performance interconnects. The purpose of this study is to evaluate the critical length effect in single‐inlaid copper interconnects and determine the critical product, (jl)c, for a variety of integrations, examining the effect of ILD (oxide vs. low‐k), geometry, and stress temperature.

Large‐Scale Statistical Study of Electromigration Early Failure for Cu/low‐k Interconnects

Even after the successful introduction of Cu‐based metallization, the electromigration (EM) failure risk has remained one of the important reliability concerns for advanced process technologies. The main factors requiring attention are the activation energy related to the dominating diffusion mechanism, the current exponent as well as the median lifetimes and lognormal standard deviation values of experimentally acquired failure time distributions. In general, the origin and scaling behavior of these parameters are relatively well understood. However, the observation of strong bimodality for the electron up‐flow direction in dual‐inlaid Cu interconnects is adding complexity. The failure voids can occur both within the via (“early” mode) or within the trench (“late” mode). Very recently, bimodality has been reported also in down‐flow EM, leading to very short lifetimes due to small, slit‐shaped voids under vias. For a more thorough investigation of these early failure phenomena, specific test structures we...

Large-scale statistical analysis of early failures in Cu electromigration, Part II: Scaling behavior and short-length effects

The first part of this study, presented in a separate paper, focused on the early failure mechanisms in down-flow electromigration. Since bimodality can occur at very small percentage levels, specific test structures were designed based on the Wheatstone Bridge technique. The use of these structures enabled a tested sample size past 800,000 for the 90 nm technology node, allowing a direct analysis of electromigration failure mechanisms at the single-digit ppm regime. The activation energy for the down-flow early failure mechanism was determined to be 0.83±0.01u2002eV, significantly lower than the usually reported activation energy of about 0.90 eV for electromigration-induced diffusion along Cu/SiCN interfaces. Very short experimental lifetimes due to small, slit-shaped voids under vias were found to control the chip lifetime at operating conditions. In this second part of our large-scale, statistical study, we will discuss the electromigration scaling behavior across 90, 65, and 45 nm technologies. Results i...

The influence of process parameters on electromigration lifetime statistics

Even after the successful introduction of Cu-based metallization, the electromigration failure risk has remained one of the important reliability concerns for advanced process technologies mostly due to ever increasing operating current densities. The main factors that require understanding are the activation energy related to the dominating diffusion mechanism, the median lifetimes, and the lognormal standard deviation sigma of experimentally obtained lifetime distributions. This study investigates the effect of different process parameters on electromigration lifetime statistics in Cu interconnects. First, the failure distributions of single damascene interconnects with smaller line height are examined, followed by an analysis of the influence of different passivation layers on electromigration statistics. A third part focuses on samples with dual damascene technology. It is observed that the first two process modifications change the median time to failure but do not alter the sigma value. Geometrical ...

Large-Scale Electromigration Statistics for Cu Interconnects

Even after the successful introduction of Cu-based metallization, the electromigration failure risk has remained one of the important reliability concerns for advanced process technologies. The observation of strong bimodality for the electron up-flow direction in dual-inlaid Cu interconnects has added complexity, but is now widely accepted. More recently, bimodality has been reported also in down-flow electromigration, leading to very short lifetimes due to small, slit-shaped voids under vias. For a more thorough investigation of these early failure phenomena, specific test structures were designed based on the Wheatstone Bridge technique. The use of these structures enabled an increase of the tested sample size past 1.1 million, allowing a direct analysis of electromigration failure mechanisms at the single-digit ppm regime. Results indicate that down-flow electromigration exhibits bimodality at very small percentage levels, not readily identifiable with standard testing methods. The activation energy for the down-flow early failure mechanism was determined to be 0.83 ± 0.01 eV. Within the small error bounds of this large-scale statistical experiment, this value is deemed to be significantly lower than the usually reported activation energy of 0.90 eV for electromigration-induced diffusion along Cu/SiCN interfaces. Due to the advantages of the Wheatstone Bridge technique, we were also able to expand the experimental temperature range down to 150 °C, coming quite close to typical operating conditions up to 125 °C. As a result of the lowered activation energy, we conclude that the down-flow early failure mode may control the chip lifetime at operating conditions. The slit-like character of the early failure void morphology also raises concerns about the validity of the Blech-effect for this mechanism. A very small amount of Cu depletion may cause failure even before a stress gradient is established. We therefore conducted large-scale statistical experiments close to the critical current density-length product (jL)*. The results indicate that even at very small failure percentages, this critical product seems to extrapolate to about 2900 ± 400 A/cm for SiCOH-based dielectrics, close to previously determined (jL)* products of about 3000 ± 500 A/cm for the same technology node and dielectric material, acquired with single link interconnects. More detailed studies are currently ongoing to verify the extrapolation methods at small percentages. Furthermore, the scaling behavior of the early failure population was investigated.

Statistical Analysis of Electromigration Lifetimes for Cu Interconnects

Electromigration (EM) failure statistics and the origin of the lognormal standard deviation (σ) for Cu interconnects have been investigated by analyzing the statistics of lifetime and void size distributions at various stages during EM testing. Experiments were performed on 0.18μm wide, single damascene Cu interconnects with tests terminated after certain amounts of resistance increase, or after a specified test time. Void size distributions of resistance‐based and time‐based EM tests were obtained. The σ values of lifetime and void size distributions showed a significant decrease with increasing resistance failure criterion. The statistics of resistance‐based void size distributions can be explained by considering geometrical variations of the void shape, while the characteristics of time‐based void size distributions require consideration of kinetic aspects of the EM process. The σ values of EM lifetime distributions can be adequately simulated by combining the statistics of both types of void size dist...