Ratan K. Choudhury

Physically-based simulation of the early and long-term failures in the copper dual damascene interconnect

We have developed a novel physical model and a simulation algorithm capable of predicting electromigration (EM) induced void nucleation and growth in an arbitrary interconnect segment. Incorporation of all important atom migration causes into the mass balance equation and its solution together with solution of the corresponding electromagnetic, heat transfer and electromagnetic, heat transfer and elasticity problems, in a coupled manner, has allowed to discriminate an early failure from a long term one taking place in a via containing copper dual damascene (DD) structure.

Electromigration reliability of low capacitance air-gap interconnect structures

The electromigration lifetimes of aluminum lines with low capacitance air-gap structures is evaluated and compared to the case of traditional gapfill passivation. Electromigration lifetime of air-gap interconnect structures is determined to be significantly higher than that of the gapfill case. Failure analysis indicates that the elasticity of the airgap sidewall passivation reduces the line stress incurred during electromigration and increases the time necessary to reach the critical stress for void nucleation. Simulations support the experimental results. Capacitance measurements and simulations show the air-gap structures reduce capacitance by as much as 40%.

Comparison of the Ti/TiN/AlCu/TiN stack with TiN/AlCu/Ti/TiN stack for application in ULSI metallization

The relative advantages and disadvantages of the two ULSI metallization stacks viz., Ti/TiN/AlCu/TiN and TiN/AlCu/Ti/TiN are described. The electromigration and thermal stability characteristics are compared using grain size of the Al metal, the XRD rocking curve peak and FWHM of the Al (111) orientation, SWEAT lifetime and electrical characteristics before and after thermal cycling of the test structures. In the literature, there exists work that report an improvement in the electromigration lifetime in the situation where Ti is in direct contact with Al due to the formation of TiAl3. The results for submicron dimensions presented in this paper clearly point to very important requirements for those results to hold and highlight the danger in assuming its validity for submicron lines. The W- plug via schemes resulting from use of both these metallization stacks will be compared from both the electrical and reliability standpoint. The conflict arising between the functionality and reliability of the vias with the reliability of the metallization lines is discussed.

Electromigration simulation in Cu-low-K multilevel interconnect segments

We present a physically based simulation model of electromigration (EM) induced failure development. Transient, 3-D, fully linked multiphysics simulation scheme has been developed on the basis of the proposed model. Simulations have been done on the realistic interconnect structures. Obtained simulation results have been found to fit well to available experimental data regarding the location of void nucleation sites and grow kinetics.

A candid comparison of the SWEAT technique and the conventional test procedure for electromigration study in sub-half micron ULSI interconnects

The thicknesses of the various layers forming the TiN-AlCu-TiN stack were varied and the impact on reliability was studied using the SWEAT technique (Root and Turner, Int. Reliability Physics Symp., 1985) along with the conventional electromigration procedure for 0.35 /spl mu/m SWEAT structures and interconnects respectively. The SWEAT results indicated that the lifetime correlates well with the AlCu thickness, while the TiN thicknesses were relatively unimportant. The conventional test procedure indicated that the lifetime correlates well with the bottom TiN thickness or even the sum of the bottom and top TiN thicknesses, while the AlCu thickness was relatively less important. It is understood that when the AlCu layer opens up during the electromigration test, the TiN acts as a shunt to prolong conductor life and that a thicker TiN layer does this better, provided that the current density is not extremely high. The relatively high current density associated with the SWEAT procedure does not allow for this mechanism, and the lifetime is almost entirely dependent on the AlCu properties. Therefore, the conventional test procedure is good for reliability estimation, while process monitoring (especially AlCu properties) may be done using the SWEAT procedure.

3-D Physically-Based Electromigration Simulation in Copper - Low-K Interconnect

We have developed a novel physical model and a simulation algorithm capable of predicting electromigration (EM) induced void nucleation and growth in an arbitrary interconnect segment. Incorporation of all important atom migration causes into the mass balance equation and its coupled solution with the corresponding electromagnetics, heat transfer and elasticity problems has provided a capability for the EM design rules generation/optimization with the physically based simulations. Simulations have been done on the realistic interconnect structures. Simulation results have been found to fit well to available experimental data regarding the location of void nucleation sites and growth kinetics.

Development of reliable multilayer metallization for submicron ULSI technology

In this paper, various multilayer metallization schemes for 0.5 micrometers CMOS technology are studied. Experimental results show that multilayer interconnect consisting of AlCu on Ti/TiN barrier layer has superior electromigration resistance as compared to that deposited on single TiN film. The application of an advanced wafer-level reliability test enables us to investigate grain boundary diffusion controlled electromigration phenomenon. The microstructural properties of the metallizations are also characterized by x-ray diffraction and scanning electron microscope. The stress migration resistance is also studied using high temperature storage at 175 degree(s)C and thermal cycle treatment at 450 degree(s)C.