Ravi Achanta

Line edge roughness and spacing effect on low-k TDDB characteristics

The study of low-k TDDB line space scaling is important for assuring robust reliability for new technologies. Although spacing effects due to line edge roughness (LER) on low-k TDDB lifetime were reported previously (Chen et al., 2007; Lloyd et al., 2007; Kim et al., 2007), there has been a lack of an analytical model with which to link line edge roughness to experimental TDDB data in a simple quantitative format. This work reports a thorough investigation into the low-k SiCOH line LER effect on low-k TDDB covering both experimental results and finite element modeling (FEM) simulations. The maximum electric field intensity as a result of sidewall LER bump was found to depend on the bump curvature. The decrease of low-k line spacing that resulted in a shorter TDDB lifetime even under the same applied electric field was then carefully analyzed. A simple analytical model of the effect of line edge roughness on TDDB failure time reduction is presented. This model was verified by experimental results. Additionally, a method to electrically quantify an overall line edge roughness is introduced.

A time dependent dielectric breakdown model for field accelerated low-k breakdown due to copper ions

We have previously shown that the choice of J(t,L)=0 boundary condition in the model of copper ion drift causes an internal field enhancement at the cathode due to the accumulation of copper ions. The ultimate breakdown of the dielectric occurs due to a combination of the field increase exceeding the intrinsic breakdown strength as well as the intrinsic bond breakage due to thermal and field effects. Here, we show that the field dependence of the intrinsic bond breakage shows an E2 dependence in the presence of copper ions enabling an excellent fit to the failure data on Cu∕SiO2∕Si devices.

Copper ion drift in integrated circuits: Effect of boundary conditions on reliability and breakdown of low-k dielectrics

Copper drift in current interconnect structures is a reliability issue leading to premature low-k dielectric breakdown. Copper ions drift through insulating low-k dielectrics leading to an increase in the local electric field that, if high enough, could lead to device failure. We have solved the coupled continuity and Poisson’s equation using the boundary condition that the flux of copper ions equals zero [J(t,L)=0] at the cathode or dielectric-semiconductor interface. This condition closely mirrors the actual physics in interconnects and leads to a buildup or accumulation of copper ions at the dielectric-semiconductor interface. Significant differences are observed in the copper concentration and local electric field behavior near the interface when the condition of a blocking electrode [J(t,L)=0] compared to the concentration equals zero [C(t,L)=0] with implications for the reliability of the low-k dielectric and time to failure for the interconnect structure. The time to failure (TTF) is shown to depen...

Critical ultra low-k TDDB reliability issues for advanced CMOS technologies

During technology development, the study of ultra low-k (ULK) TDDB is important for assuring robust reliability. As the technology advances, several critical ULK TDDB issues were faced for the first time and needed to be addressed. First, the increase of ULK leakage current noise level induced by soft breakdown during stress was observed. Second, it was found that ULK had lower field acceleration than dense low-k. Such process and material dependences of ULK TDDB kinetics were investigated, and an optimal process to improve ULK voltage acceleration was identified. Last, as the reliability margin for ULK TDDB of via-related structures is greatly reduced at advanced CMOS technologies, a systematic study of via TDDB regarding area scaling and test structure design was conducted. It was found that only a portion of the total vias possibly determines the low-k via TDDB. A new “fatal” via ratio concept is introduced to replace the as-designed area ratio for TDDB area scaling in structures with vias, and a methodology called shift and compare (S&C) is proposed to determine the “fatal” via ratio.

Polymer Penetration and Pore Sealing in Nanoporous Silica by CHF3 Plasma Exposure

The polymerization and pore sealing that occurs during fluorocarbon plasma treatment of nanoporous silica xerogel was investigated experimentally by Rutherford backscattering spectroscopy and successfully modeled using a diffusion-reaction analysis. CHF 3 was used as a reactant gas to expedite the rate of polymerization due to the presence of hydrogen in its structure and its low C/F ratio. Knudsen diffusion was assumed to be the dominant mechanism for the motion of polymer precursor species through the nanoporous material over the range of pressures used in the plasma experiments. The amount of fluorine atoms deposited on the sidewalls of the pores was measured as a function of depth in the dielectric film and that amount was assumed to correspond with the mass of the polymer layer formed inside the pores. By applying a Thiele-type analysis to this system, we successfully matched model calculations with measured fluorine amounts, predicted the time required to reach a steady-state concentration of the polymer precursor in a pore (∼10 - 7 s) and predicted the time required to seal off pore necks at the surface of the dielectric (∼70 s). Both the model and experimental results show a greater depth of penetration and an enhanced deposition of polymer at higher porosities, confirming the need for pore sealing during back-end-of-the-line processing of nanoporous materials.

Analysis of applying time-dependent clustering model to BEOL TDDB

Recently, a time-dependent clustering model has been reported showing good agreement with multiple sets of experimental TDDB data by proper consideration of the percentile scaling of different areas (vertical translation in the Weibull scale) [1,2]. In this work, we investigate the scaling property (horizontal translation) of time-to-fail (TFAIL) for different areas. Our results indicate that both the horizontal and vertical scaling properties for area transformation are preserved in the clustering model, showing its potential to replace the Weibull model. Moreover, we demonstrate the applicability of the time-dependent clustering model to bi-modal TDDB data, often encountered in practice. Finally, we develop a successive breakdown theory in the framework of the clustering model and compare it with experimental BEOL TDDB data.

Predicting the lifetime of copper/barrier/dielectric systems: Insights for designing better barriers for reducing copper ion drift/diffusion into the dielectric

Our previous drift/diffusion model describing the effect of the transport and accumulation of copper ions on the time to failure of dielectrics was extended here to include metallic barriers and the effect of an elastic drift term. The results of the model were compared with experimental data on the time to failure of SiO2 dielectrics that included Ru(P) and TaN barriers. Excellent agreement was found, and the success of the Ru(P) compared to the TaN barrier was attributed to the former’s lower copper ion solubility and diffusivity. There is a change in the value of the field acceleration parameter (γ) with barriers that leads to an increase in the projected lifetime. We link this change in γ to a number of factors including the transport of oxidizing species through the barrier responsible for copper ion formation, and the injection and motion of electrons and holes, which are altered in the presence of a barrier between the metal and dielectric.

A time-dependent clustering model for non-uniform dielectric breakdown

We report a time-dependent clustering model for non-uniform dielectric breakdown. While at high percentiles non-Possion area scaling dominates, the model restores the weakest-link characteristics at low percentiles relevant for reliability projection. Its validity is demonstrated by area scaling and excellent agreement with multiple experimental data sets. We show the clustering model can replace Weibull model with largely improved reliability margins.

Post-breakdown statistics and acceleration characteristics in high-K dielectric stacks

Contrary to recent claims, experimental results obtained in thin and thick Hafnium-based high-K gate dielectric stacks demonstrate that progressive breakdown is relevant in these insulators. For thin and thick stacks and both in NFETs and PFETs, the residual time distributions are found to be non-Weibull with two regions: a universally shallower slope at long times and a steeper slope at short times. The shallow distributions favour the coexistence of single-spot BD and multiple competing spots in different samples. Contrary to what happens in the case of SiON dielectrics, the final failure distribution is reported to be strongly dependent on the threshold current IF used to define device failure. Also contrary to what found for SiON single-layer dielectrics, the voltage acceleration and temperature activation energy of the residual time is reported to be much stronger than that of the first breakdown time. All these results emphasize the important role of progressive breakdown for high-K reliability assessment methodology.

Copper ion transport induced dielectric failure: Inclusion of elastic drift and consequences for reliability

The authors have modeled the copper ion concentration and internal electric field profiles in a SiO2 dielectric by solving the transient nonlinear continuity/Poisson equations. The predicted time-to-failure of the dielectric correlates well with the theory that failure occurs once copper ions accumulate to the point where the induced electric field at the cathode exceeds a critical value. However, the copper ion concentration at the cathode required for failure was much higher than the reported ion solubility, raising doubts about the validity of the model. More realistic values for the required copper ion concentration were obtained by incorporating an “elastic” diffusion term in the continuity equation. Adding this new term and increasing solubility, Ce, to the maximum value reported in the literature reduces the concentration at the cathode to a logically consistent value but does not significantly alter the predicted time-to-failure. The new formulation predicts a higher minimum applied electric field...