Naohito Suzumura

A New TDDB Degradation Model Based on Cu Ion Drift in Cu Interconnect Dielectrics

A new physical model of time-dependent dielectric breakdown (TDDB) in Cu interconnect dielectrics is proposed. TDDB occurs due to the drift of Cu ions under an electric field E. An activation energy analysis of the leakage current demonstrates that these injected Cu ions affect the conduction mechanism of electrons. The dominant electron conduction mechanism changes from Poole-Frenkel electron current through the Cu barrier dielectrics to Fowler-Nordheim current due to the Cu pile-up at the cathode end. We assumed two possible types of Cu ion drift mechanism, Schottky type or Poole-Frenkel type. The field acceleration model (radicE model) of the Poole-Frenkel type fits both TDDB lifetime and activation energy very well. The TDDB lifetime is proportional to the exponential of the square root of the electric field radicE

Phenomenological classification of stress-induced leakage current and time-dependent dielectric breakdown mechanism

The time-dependent dielectric breakdown (TDDB) of Cu damascene interconnects was investigated, noting the time variations in stress-induced leakage current. Copper interconnects normally have symmetric current-voltage curves, which suggests that defects are distributed symmetrically between two Cu lines. Although the impact damage model satisfies this requirement, as does the thermochemical E-model, the Cu diffusion model does not. Without the barrier metal, Cu+ ions rapidly penetrate the dielectric film and form unstable conduction filaments. The leakage current fluctuates greatly due to the rapid Cu movement in the last stage of bias temperature stressing. These current fluctuations also appear in the triangular voltage sweep so that a spurious peak emerges, which is unrelated to the ionic displacement current. The extrinsic TDDB has a small field acceleration parameter (0.5 cm/MV); however, it switches to a large one (4.5 cm/MV) at electrical fields that are higher than 2 MV/cm. Another type of degrada...

Electric-field and temperature dependencies of TDDB degradation in Cu/Low-K damascene structures

The electric field and temperature dependencies of time-dependent dielectric breakdown (TDDB) degradation in Cu/low-k damascene structures are investigated using Cu/SiOC and Cu/SiCN damascene structures. A field-dependent activation energy analysis of TDDB lifetimes demonstrates that there are multiple TDDB degradation mechanisms for a Cu/SiOC structure and that the dominant TDDB degradation mechanism is dependent on the electric field. Under higher electric fields, the SiCN film used as a Cu barrier dielectric (BD) is the main cause of the TDDB failure. As the electric field decreases, the degradation of the inter-level dielectric (ILD) or ILD/BD interface has an impact on TDDB failure. Furthermore, it was found that the field-dependency of Ea reflects the dominant TDDB degradation mechanism and is an important factor in determining a TDDB degradation model that can predict an accurate TDDB lifetime.

The observation of stress-induced leakage current of damascene interconnects after bias temperature aging

Time-dependent dielectric breakdown (TDDB) was investigated, noting the time variation of stress-induce leakage current (SILC) between copper (Cu) lines. To clarify the TDDB mechanism, triangular voltage sweep (TVS) and bias temperature aging were alternatively performed for Cu damascene structures. The SILC was largely increased with stress time; however no Cu ion peak was detected in TVS measurements. The point-symmetric TVS hysteresis clearly indicates that the distribution of stress-induced defects is independent of electric polarity, namely it is a random or symmetric distribution. In addition, the SILC of Cu interconnect increased as slow as that of tungsten (W) in the usual bias temperature aging. Since the optimize barrier process sufficiently suppresses Cu ion drift in TDDB testing, the dielectric breakdown is only controlled by intrinsic factors. When the protection against moisture is structurally insufficient, the water-related degradation is seriously pronounced instead of Cu ion drift. The water-absorbed interconnect abruptly breaks down while the current is decreasing. Just before the breakdown, the asymmetric TVS hysteresis appears. Therefore, TVS observation enables us to distinguish whether stress-induced leakage current comes from intrinsic or extrinsic causes.

Study on vertical TDDB degradation mechanism and its relation to lateral TDDB in Cu/low-k damascene structures

We investigated the inter-level (vertical) and intra-level (lateral) Time-Dependent Dielectric Breakdown (VTDDB & LTDDB) degradation mechanisms in Cu/low-k damascene structures and the relation between the two TDDB failures. The electric-field acceleration factor of the VTDDB lifetime is smaller than that of the LTDDB lifetime at 125°C, and VTDDB has the potential to become a reliability issue for high voltage operation devices because widening a space between inter-level metals is difficult by design layout. From the temperature dependency of the VTDDB lifetime, the activation energy is larger than that of the TDDB lifetime for Cu barrier dielectric (BD) film (Cu/SiCN damascene structure). Also, the VTDDB lifetime is dependent on the pore size of the porous-SiOC between inter-level metals. Our results indicate that VTDDB is limited by the intrinsic breakdown of the inter-metal low-k dielectric (ILD) film in the Via region, not BD film breakdown. Furthermore, we found an association between a VTDDB failure mechanism and a high-temperature LTDDB failure mechanism based on the temperature and electric field characteristics.

Highly reliable 45-nm-half-pitch Cu interconnects incorporating a Ti/TaN multilayer barrier

A Ti/TaN multi-layer can achieve a highly reliable Cu interconnect with a porous SiOC (ELK; k < 2.5) structure. Ti shows good wettability with Cu and unique properties with extreme low-k (ELK)-structured interconnects. On the other hand, Ta is known to be an effective barrier to Cu diffusion. We confirmed that the Ti barrier is different from the Ta barrier from the viewpoint of metal-oxide behavior and improved electromigration (EM) and stress migration (SM). In addition, the time-dependent dielectric breakdown (TDDB) characteristic can be improved by using a Ti barrier combined with a TaN barrier.

A new aspect of time-dependent clustering model for non-uniform dielectric TDDB

We investigated the time-dependent clustering (TDC) model for time-dependent dielectric breakdown (TDDB) of non-uniform dielectrics and revealed for the first time that the TDC model is a compound Weibull model that is expressed as a superposition of Weibull distributions. The Weibull model has two statistical parameters, scale parameter η and shape parameter β We clarified the precondition that the TDC model holds when term ηβ of the Weibull model is distributed according to an inverse-gamma distribution. By using our finding, we proposed a new method to directly estimate the variations of electric field and effective space from TDDB data. We found that the corresponding electric field distribution is a generalization of extreme value distribution, which is a natural consequence since the lifetime is determined by the maximum value of the electric field.

Cu Dual-Damascene Interconnects with Direct Chemical Mechanical Polishing Process on Porous Low-k Film

To reduce the effective dielectric constant (keff) value for 32 nm node technology and beyond, the effects of a direct chemical mechanical polishing (CMP) process on porous low-k film without a protective cap layer were investigated. It was confirmed that a capless structure on porous low-k film is effective in reducing the resistance–capacitance (RC) products, but it causes degradation of wire-to-wire breakdown voltage characteristics. The most important point of a direct CMP process is to control the amount of damage to the polished surface. In this study, two types of low-k film were compared in combination with a variety of CMP process conditions. As results, we found that a direct CMP process has a positive effect on wire-to-wire current leakage and time-dependent dielectric breakdown (TDDB) reliability where a porous low-k film deposited by modified conditions is used. By optimizing the deposition and curing conditions, it is possible to control the distribution of different pore sizes in porous low-k film, which allows us to realize a highly reliable capless structure.

Effects of Ru-Ta Alloy Barrier on Cu Filling and Reliability for Cu Interconnects

A Ru-Ta alloy is applied to Cu dual damascene interconnects due to its good wettability with Cu. Using Ru-Ta alloy film as a barrier layer, filling property of ECP-Cu is improved and complete filling for trenches of 45 nm in width can be achieved. Although further optimization of CMP process is necessary, Ru-Ta alloy barrier also improves estimated lifetime of electromigration.

Suppression of Stress-Induced Voiding by Controlling Microstructure of Cu Electroplated Films

In advanced LSI devices, the filling capability for the electroplating (ECP) process, impurities in Cu film, and grain growth after ECP are important properties of Cu interconnect reliability. It has been reported that the Cu grain growth depends on the ECP-Cu film characteristics and the annealing conditions. In this study, we focus on the deposition thickness of ECP-Cu films to control their microstructure. It is revealed that a thicker ECP-Cu film enlarges the Cu grain size and reduces the number of small grains. It is also revealed that Cu grains in a wide line with a thicker ECP-Cu film are larger, leading to reliability improvement. A film with larger grains has fewer grain boundaries and fewer diffusion paths of vacancies, and a film with fewer small grains has a decreased number of vacancies. As a result, stress-induced voiding phenomena are apparently suppressed by increasing the thickness of deposited ECP-Cu film.