Session 3A – Dielectric Reliability (Front-end and Back-end)
Session 3A – Dielectric Reliability (Front-end and Back-end)
Session Co-Chairs: Shou-Chung Lee, TSMC, Shinji Yokogawa, Polytechnic University of Japan
1:55 p.m. – Session Introduction
3A.1 Fundamental Statistical Properties of Reconstruction Methodology for TDDB with variability (BEOL/MOL/FEOL) Applications
E. Wu, J. Stathis, B. Li*, A. Kim*, B. Linder*, R. Bolam*, G. Bonilla, IBM Research, *IBM System Division
Recently, a sampling-based technique has received a great deal of attention as a methodology for the reconstruction of the Weibull distribution [1-3] to solve variability issues [4-7] related to breakdown (BD) statistics. While this method has been successful in SiO2 with small thickness (TOX) variation [1,3], its applicability remains questionable  for MOL/BEOL dielectrics with substantial spacing variation and intrinsic line-edge roughness (LER) [4-6]. A sampling-number dependence of Weibull slope is recently reported but its root-cause is not understood yet . Thus, a large sampling number (n) is required to obtain an accurate beta value. Moreover, it has been shown that the reconstructed distribution with a small n can be misleading as it masks the non-Weibull/non-Poisson area-scaling nature of underlying in-die TBD distributions . This suggests a lack of fundamental understanding of this methodology can lead to its incorrect usage. In this work, we show statistical scaling property of the Beta-sampling curve, along with new results of sampling-number dependence of the T63 variation. For the first time, we report a fundamental mathematical formulation for these two sampling-number dependencies of beta and Simga_T63. Finally, we propose a quantitative criterion for the applicability of reconstruction method for its correct usage.
3A.2 New Breakdown Mechanism Investigation: Barrier Metal Penetration Induced Soft Breakdown in Low-k Dielectrics
C. Wu, Y. Li, J. Bömmels, I. De Wolf, Z. Tőkei, K.Croes, imec
A Soft Breakdown (SBD) phenomenon happening in porous low-k dielectrics during time dependent dielectric breakdown measurements was investigated. The early formation of local conductive paths was identified by monitoring leakage currents and capacitance data in the SBD phase. The nature of this conductive path was demonstrated to be related to intrinsic dielectric degradation. By comparing samples with different process conditions, we found that barrier metal penetration is an important root cause of SBD initiation. Our study of the voltage and temperature acceleration of the SBD phenomenon shows that these acceleration factors, m=22 and Ea=0.2eV, are at a reasonable level. However, further investigations on large size devices illustrate that the difference in barrier metal penetration depth between different samples could lead to a large decrease of Weibull slopes and degrade the overall reliability performance. Therefore, innovations of metal barrier deposition on porous low-k dielectrics to avoid barrier metal penetration are required for advanced technology nodes.
3A.3 On Why Dielectric Breakdown Strength Reduces With Dielectric Thickness
J. McPherson, McPherson Reliability Consulting LLC
A fundamental-physics reason is presented for why the dielectric breakdown strength Ebd reduces with increases in dielectric thickness tdiel. Through the extensive use of planar dipolar summations, it is shown that the Lorentz factor L tends to increase with dielectric thickness. The increase in L with dielectric thickness produces a higher local electric field (Eloc) in thicker dielectrics. This higher Eloc produces more polar-bond distortion (stretching, compressing, bending, etc.) and this leads to a reduction in bond strength. A reduction in bond strength causes Ebd and TDDB reductions regard-less of the actual bond breakage mechanism (standard Boltzmann processes, current driven processes, or hydrogen release processes). This work shows that, while thicker SiO2 has lower Ebd, all SiO2 thicknesses tend to breakdown at roughly the same local electric field (Eloc)bd ≈ 40MV/cm.
3A.4 A New Aspect of Time-dependent Clustering Model for Non-uniform Dielectric TDDB
T. Shimizu, N. Suzumura, K. Ohgata, H. Tsuchiya, H. Aono, M. Ogasawara, Renesas Electronics Corporation
In this study, we developed the theory of the time-dependent clustering (TDC) model for time-dependent dielectric breakdown (TDDB) of non-uniform dielectrics and shed light on a new aspect. We found that TDC model can be derived from the Weibull model with scale parameter variation and that the corresponding electric field distribution is a kind of extreme value distributions. Using Metal-Insulator-Metal (MIM) capacitor TDDB data, we demonstrated the distributions of electric filed and space variations can be obtained from lifetime data by our approach.
3A.5 Time-Dependent Series Resistance and Implications for Voltage Acceleration Models in BEOL TDDB
A. Kim, B. Li, P. McLaughlin, C. J. Christiansen*, E. Wu**, IBM Research, *GlobalFoundries
A continuous down-scaling of BEOL pitch for advanced process technologies has drastically increased the resistance of Cu interconnects. A concern of the voltage drop of TDDB stress voltage along resistive metal electrodes due to a series resistance effect or IR drop has been reported [1,2]. Such an IR drop is believed to cause non-Poisson area scaling , lower-than-expected voltage acceleration parameter values and erroneous choice of models for TDDB
lifetime projections . For accurate TDDB lifetime projections at operating voltages, it is of a critical importance to understand the series resistance effect in BEOL devices under high voltage stresses to accurately estimate the voltage acceleration parameter of a chosen TDDB model. Our results show that IV characteristics of BEOL ILD changes drastically over time due to charge trapping and defect creation under voltage stresses, rather than assumed to be constant as reported in [1,2]. In contrast to previous work, we present four independent experiments and fully transient simulation results to show that IR drop due to series resistance has a negligible effect on voltage acceleration models. Finally, our long-term TDDB results along with a long-term TDDB results (> 2 years) of 64-pitch interconnect made of Cu/ULK (k=2.55, SiCOH) with IR drop corrections on high stress voltages show that the power law model best describes the voltage acceleration behavior of the ULK dielectric material used for our studies.
3A.6 A New Model for Dielectric Breakdown Mechanism of Silicon Nitride Metal-Insulator-Metal Structures
K. Okada, Y. Ito, S. Suzuki, TowerJazz Panasonic Semiconductor
For the further advance of Si RF devices and also GaAs/GaN MMIC devices, highly reliable silicon nitride MIM structure has to be realized on the basis of deeper understandings of its degradation/breakdown mechanisms. Through the study on TDDB reliability of MIM structures with silicon nitride films having various thicknesses by three deposition processes, it has been revealed that QBD (charge to breakdown) strongly depends on the stress condition and film deposition process and that the electric field shift until breakdown is constant independent of deposition process, stress condition, thickness, and temperature. Furthermore, the field shift at the anode is the key for breakdown. Based on these results, a new TDDB model for silicon nitride MIM structure has been proposed.