Yohan Barbarin

Low field TDDB of BEOL interconnects using >40 months of data

Over 40 months of low field BEOL TDDB data obtained on different test vehicles with spacings ranging from 90-30nm and OSG low-k dielectrics with k-values ranging from 3.22.0 are summarized. For the dielectrics with k≥2.5, a simultaneous maximum likelihood fit with a fixed acceleration factor and varying distributional shapes is performed. By considering the log-likelihood of each model fit, this approach allows a comparison of fitted lifetime models. This approach also allows estimating the parameters of the impact damage model, which is more difficult to fit due to its multiple acceleration factors. From a statistical point of view and by using a 95% significance level, the results show that the power law and the impact damage model equally outperform all other proposed models and that their prediction to lower fields are very similar. As from a practical point of view the power law model is much more easy to use due to its limited number of fitting parameters, we propose to use the power law model for low-k dielectrics with k-value between 2.5 and 3.2. Regardless of the presence of a protection film, our low-field data obtained on the k=2.0 material show different acceleration factors at high and low fields. This suggests that different breakdown mechanisms are present at different fields and that, in order to allow reliable predictions to operating fields, future TDDB tests of highly porous films will require stresses at much wider field ranges.

Role of copper in time dependent dielectric breakdown of porous organo-silicate glass low-k materials

The role of copper in time dependent dielectric breakdown (TDDB) of a porous low-k dielectric with TaN/Ta barrier was investigated on a metal-insulator-metal capacitor configuration where Cu ions can drift into the low-k film by applying a positive potential on the top while they are not permitted to enter the low-k dielectric if a negative potential is applied on the top. No difference in TDDB performance was observed between the positive and negative bias conditions, suggesting that Cu cannot penetrate TaN/Ta barrier to play a critical role in the TDDB of porous low-k material.

Influence of porosity on dielectric breakdown of ultralow-k dielectrics

The effect of porosity on the electrical properties of porogen-free ultralow-k dielectric materials was demonstrated using a special curing process that allows a separate control of porosity and matrix properties. It is shown that the leakage current was insensitive to porosity, suggesting a bulk conduction mechanism. On the other hand, higher porosity leads to lower breakdown voltage, indicating that porosity can degrade the electrical reliability performance of the dielectric material. The observed lower breakdown field is explained in terms of the amount of cage structure in the film, the exacerbation of strain in the Si–O–Si backbone structure by an external electric field, and local field enhancements near the pores, thereby making the Si–O bond highly susceptible to breakage.

Reliability concerns in copper TSV's: Methods and results

Due to their large volume and close proximity to devices, the reliability of copper TSVs is a concern, both with respect to mechanical stresses induced by the TSV in the Si and with respect to copper drift into the liner and the Si. This abstract summarizes recent achievements obtained in imecs 3D-reliability work package where above mentioned reliability concerns are evaluated in detail. To study the impact of mechanical stresses induced by the TSV in the Si, the saturation drain currents Id of transistors have been used as stress sensors. The offset of the Id of transistors closer to a TSV with respect to transistors far away from a TSV has been studied, both directly after processing and after thermal storage and thermal shock. It is shown that stresses generated by the TSV in the Si increase after thermal storage above certain temperatures while thermal shock reduces these stresses. The first is attributed to stress relaxation at high temperatures, while the latter is attributed to cracking/delamination at critical interfaces. To study continuity in TSV-barriers, a method, further referred to as dual ramp rate IVctrl, is introduced. The method consists of controlled current-voltage sweeps at different rates. The difference in breakdown fields for different ramp rates allows estimating TDDB (=Time Dependent Dielectric Breakdown) field acceleration parameters. Applying a negative voltage to the TSV (-V) does not allow copper to drift into the liner, while when applying a positive voltage (+V) to the TSV, copper can drift into the liner in case of a defective, non-continuous barrier. Comparing TDDB field acceleration parameters of -V versus +V tests gives insight in barrier properties. In our study, weak reliability is observed in systems where the TSV-barriers are not continuous.

Towards the understanding of intrinsic degradation and breakdown mechanisms of a SiOCH low-k dielectric

Degradation and breakdown mechanisms of a SiOCH low-k material with k=2.3 (25% porosity) and thicknesses ranging from 90nm to 20nm were investigated. By combining time dependent dielectric breakdown (TDDB) data at positive/negative bias stress with thickness scaling results, dielectric failure is proven to be intrinsic and not influenced by copper drift or barrier deposition induced dielectric damage, which is further demonstrated by a very low TDDB thermal activation energy. During dielectric degradation, low field leakage current increase is suggested to be caused by donor type trap generation. It is shown that stress induced leakage current (SILC) can be used as a measure of dielectric degradation. Therefore, by monitoring SILC, low field lifetime can be safely estimated using extrapolation. Based on our experimental results, it is suggested that the impact damage model has a better accuracy for low field lifetime prediction.

Reliability characteristics of thin porous low-K silica-based interconnect dielectrics

The dielectric breakdown field (EBD) and the time-dependent-dielectric-breakdown (TDDB) of eight different low-K films with porosities between 3% (K=3.2) and 50% (K=1.8) and thicknesses between 15 and 60 nm were investigated using imecs planar capacitors (p-cap) test vehicle. EBD values decrease linearly with porosity to reach 6MV/cm at 50% porosity. The analogous Organo-Silicate Glass (OSG) films show a similar field acceleration factors independently of porosity. An OSG 2.0 film with 45% porosity and a periodic mesoporous organosilica (PMO) 1.8 film, both sealed with 12-nm OSG 3.0 sealing also showed the same field acceleration factor. On the other hand, the corresponding Weibull slopes vary and decrease linearly with porosity, which is in agreement with the percolation model. Also, the Weibull slopes decrease linearly with dielectric thickness. Extrapolating those data and analyzing the maximum allowed electrical fields to meet 10-years lifetime (EMAX), critical dielectric spacing are discussed as a function of porosity. It is shown that for 20-nm spacing remedial measures are required for porosities >30%.

Electrical improvement of CNT contacts with Cu damascene top metallization

We discuss the improvement in the electrical characterization and the performance of 150 nm diameter contacts filled with carbon nanotubes (CNT) and a Cu damascene top metal on 200mm wafers. The excellent agreement between the yield curves for the parallel and single contacts shows that a reliable electrical characterization is obtained. We demonstrate that integration changes improved the resistivity of the CNT contact significantly by reducing it from 11.8·10³ μΩ·cm down to 5.1·10³ μΩ·cm. Finally, a length scaling of the CNT contacts was used to find the individual contributors to the lowering of the single CNT contact resistance.

Correlation between field dependent electrical conduction and dielectric breakdown in a SiCOH based low-k (k = 2.0) dielectric

The electrical conduction of a SiCOH based ultralow-k (k = 2.0) dielectric is investigated over an electric field range from 1.0 MV/cm to breakdown. Below 4.0 MV/cm, space-charge-limited current dominates the leakage. Above 5.0 MV/cm, a transition is found from trap-assisted Fowler-Nordheim (F-N) tunneling to F-N tunneling. It is hypothesized that under F-N tunneling stress, intrinsic material degradation causes positively charged defects generated in the dielectric. Moreover, this change of the dominant conduction path has a significant impact on the time dependent dielectric breakdown lifetime behavior.

Impact of carbon-doping on time dependent dielectric breakdown of SiO2-based films

Impact of carbon-doping on time dependent dielectric breakdown (TDDB) of three SiO2-based films was investigated under two different breakdown mechanisms, one involving Cu ion injection and the other caused by intrinsic dielectric degradation without Cu injection. In the case of breakdown dominated by dielectric degradation, an undoped SiO2 film shows better TDDB performance than the two other carbon-doped SiO2 or organo-silicate glass films, suggesting that carbon-doping makes the films weaker for dielectric breakdown. In contrast, in the case of breakdown involving Cu ion injection, the two carbon-doped films show better TDDB performance than the undoped SiO2, suggesting that the presence of the carbon slows down Cu ion injection and therefore leads to less TDDB degradation.

CVD Mn-based self-formed barrier for advanced interconnect technology

CVD Mn-based self-formed barrier (SFB) has been evaluated and integrated for reliability and RC delay assessment. Intrinsic TDDB lifetimes were extracted from planar capacitor measurement. A comparable lifetime as the TaN/Ta reference was obtained on SiO2 and porous low-k with a thin oxide liner. Good reliability performance was demonstrated after integration. Compared to conventional barrier, significant RC reduction (up to 45% at 40nm half pitch) and lower via resistance which become more beneficial upon scaling present CVD Mn-based SFB as an attractive candidate for future interconnect technology.