Joe W. McPherson

Underlying physics of the thermochemical E model in describing low-field time-dependent dielectric breakdown in SiO2 thin films

The underlying physics behind the success of the thermochemical E model in describing time-dependent dielectric breakdown (TDDB) in SiO2 thin films is presented. Weak bonding states can be broken by thermal means due to the strong dipolar coupling of intrinsic defect states with the local electric field in the dielectric. This dipole-field coupling serves to lower the activation energy required for thermal bond-breakage and accelerates the dielectric degradation process. A temperature-independent field acceleration parameter γ and a field-independent activation energy ΔH can result when different types of disturbed bonding states are mixed during TDDB testing of SiO2 thin films. While γ for each defect type alone has the expected 1/T dependence and ΔH shows a linear decrease with electric field, a nearly temperature-independent γ and a field-independent ΔH can result when two or more types of disturbed bonding states are mixed. The good agreement between long-term TDDB data and the thermochemical model su...

Thermochemical description of dielectric breakdown in high dielectric constant materials

A thermochemical/molecular model is developed for breakdown in high dielectric constant materials and the model suggests that a fundamental relationship exists between dielectric breakdown strength (Ebd) and dielectric constant (k). The model indicates that Ebd should show an approximate (k)−1/2 dependence over a wide range of high dielectric constant materials. The model also predicts that the field-acceleration parameter (γ), from time-dependent dielectric breakdown (TDDB) testing, should increase with dielectric constant. TDDB and Ebd data are presented for model support. The thermochemical model suggests that the very high local electric field (Lorentz-relation/Mossotti-field) in high-k dielectrics tends to distort/weaken the polar molecular bonds making them more susceptible to bond breakage by standard Boltzmann processes and/or by hole capture and thus lowers the breakdown strength.

Acceleration Factors for Thin Gate Oxide Stressing

Time dependent dielectric breakdown (TDDB) data for 100Å of thermally grown SiO2 has been analyzed using an Eyring model based on thermodynamic free energy considerations. The model describes well the following features of the data: (1) an apparent activation energy which is a function of the stressing electric field and (2) a field acceleration parameter that is a function of temperature. Quantitatively, the model suggests the proper field dependence for the activation energy and the observed temperature dependence of the field acceleration in the 100Å oxide material. The apparent activation energy is found to decrease from > leV at low field stressing (Eb(50%) - Es > 5 MV/cm) to <0.3eV at higher fields Eb(50%)- Es < 3 MV/cm). Also, the field acceleration was found to be approximately 6 decades/MV/cm at room temperature but reduces to 2 decades/MV/cm at 150C.

Trends in the ultimate breakdown strength of high dielectric-constant materials

The ultimate breakdown strength E/sub bd/ of a dielectric material is found to decrease as the dielectric-constant k increases. A thermochemical description of the ultimate breakdown strength of high-k dielectrics suggests that E/sub bd/ should reduce approximately as (k)/sup -1/2/ over a wide range of dielectric materials while the field-acceleration parameter /spl gamma/ should increase in similar but inverse manner. New time-dependent dielectric breakdown (TDDB) data are presented over a wide range of dielectric materials and E/sub bd/ was found to decrease as (k)/sup -0.65/ while /spl gamma/ increases as (k)/sup 0.66/. The good agreement between thermochemical theory and high-k TDDB observations suggests that the very high local electric field (Lorentz-relation/Mossotti-field) in high-k dielectrics tends to distort/weaken the polar molecular bonds making them more susceptible to bond breakage by standard Boltzmann processes and/or by hole-capture and thus lowers the breakdown strength.

Stress-induced voiding under vias connected to wide Cu metal leads

Stress-induced voiding is observed in Cu-based, deep-submicron, dual-damascene technologies where voids are formed under the via when the via connects to a wide metal lead below it. The voiding results from the supersaturation of vacancies that develops due to grain growth when the Cu is not properly annealed prior to being fully constrained. The driving force for voiding is shown to be stress migration with a maximum in voiding rate observed at /spl sim/190/spl deg/C. A diffusional model is presented which shows that the voiding mechanism is an issue primarily for vias connected to wide Cu leads. A thermomechanical stress exponent of 3.2 and a diffusional activation energy of 0.74 eV were determined for this stress-induced voiding mechanism.

Complementary model for intrinsic time-dependent dielectric breakdown in SiO2 dielectrics

A molecular physics-based complementary model, which includes both field and current, is introduced to help resolve the E versus 1/E-model controversy that has existed for many years as to the true physics behind time-dependent dielectric breakdown (TDDB). It is shown here that either TDDB model can be valid for certain specified field, temperature, and molecular bonding-energy ranges. For bond strengths 3 eV, the bond breakage must be hole catalyzed by current-induced hole injection and capture. Under these conditions, the TDDB physics is described well by the 1/E model.

Leakage, breakdown, and TDDB characteristics of porous low-k silica-based interconnect dielectrics

The reliability physics of low-k interconnect dielectrics is of great interest. Leakage, breakdown and TDDB data are presented for fluorinated silica, porous carbon-doped silica, and very porous carbon-doped silica. The breakdown and TDDB performance of the dielectrics are observed to degrade with the degree of porosity but the failure kinetics (field acceleration parameter and activation energy) seem to rather insensitive to porosity. A percolation model has been developed whereby the pores are treated as defects. The percolation model seems to describe well the observed breakdown and TDDB behavior.

Molecular model for intrinsic time-dependent dielectric breakdown in SiO2 dielectrics and the reliability implications for hyper-thin gate oxide

SiO2 films, at constant electric field, show a significant reduction in time-dependent dielectric breakdown (TDDB) performance when the thickness tox is scaled below 4.0 nm. This reduction in TDDB performance is coincident with and scales with the increase in direct tunnelling (DT) leakage through these hyper-thin oxide films. Assuming that the increase in DT leakage leads to more hole injection and trapping in the SiO2, the enhanced dielectric degradation rate with tox reduction can be explained on the basis of an intrinsic molecular model where hole capture serves to catalyze Si-O bond breakage.

Determination of the nature of molecular bonding in silica from time-dependent dielectric breakdown data

An effective molecular dipole moment of 7–13 e A is routinely observed during time-dependent dielectric breakdown testing of silica-based dielectrics. A Mie-Gruneisen analysis of the molecular bonding states indicates that the upper end of the effective dipole moment range (13 e A) is associated with a stretched silicon-oxygen bond while the lower end (7 e A) is consistent with a hole-captured silicon-oxygen bond.

AC electromigration characterization and modeling of multilayered interconnects

The AC electromigration of multilayered interconnect systems consisting of TiW-AlSiCu and CVD W-AlSiCu is studied under stress of repetitive dual-pulse current waveforms at 2 MHz. By using a general AC waveform in which the peak current density of the first pulse j/sub 1/ is fixed and the second is varied from +j/sub 1/ to -j/sub 1/, a continuous electromigration spectrum from DC through pulsed DC to pure AC conditions is obtained. Although an average current model fits the data well, there is a singularity in the median time-to-failure at pure-AC conditions. To avoid this singularity, a modification of the average current model, called the average current recovery model, is developed. It heuristically accounts for the degree of damage recovery during opposite-polarity pulses through a single recovery parameter.<<ETX>>