Comprehensive reliability analysis of CoWP Metal Cap unit processes for high volume production in sub-μm dimensions

Abstract

INTRODUCTIONReliability of copper interconnect systems is very sensitive to the maximum current used in the product. Shrinking the metal dimensions from technology node to technology node yields a need for higher current density to fulfill the product needs. Since the electromigration performance is predicted to decrease for 45nm and beyond [1], higher electromigration robustness in smaller dimensions is necessary. A main driving force for electromigration improvement is engineering the interface quality/composition between Cu and Cap layer [2]. Two ways to achieve this are discussed so far. Producing a layer between Cu and Cap by silicidation of the Cu surface [3] or introducing a metal cap below the dielectric cap [1].The latter has the advantage of better resistance control but is very challenging with respect to cleaning processes and leakage/breakdown behavior.In this paper we present the results of advanced process improvements leading to very reliable full-build interconnect systems ready for high volume production. Besides the electromigration investigation, we studied stress migration performance as well as BTS and leakage measurements. We found behavior changes in almost all reliability tests suggesting differences in physical behavior, while exhibiting very strong lifetime performance. EXPERIMENTAL SET UP AND RESULTSWe evaluated a wide set of reliability measurements on 65nm and 45nm technology node material. Electromigration testing was performed in commercially available oven systems on package level. The conventional test conditions did not yield any electromigration failures. Even higher acceleration with four times higher current density did only yield a small resistance increase after 1000h of testing. For comparison non-CoWP sample on the same technology node have been tested at the same conditions. Fig. 1 (right) provides an example of the electromigration performance gain due to CoWP capping.To evaluate the electromigration kinetics of the metal cap, we accelerated the electromigration testing extremely by using the maximum available oven temperature (350°C) and very high current densities. For this investigation we used current densities of 4MA/cm2 up to 20MA/cm2 (Tab. 1). The tested structure is a single link structure connected with one via on each end to a wide feeder line. In this investigation we tested via 1 in up stream and down stream direction.The tests showed significant resistance increases above 12MA/cm2. At higher current densities the typical electromigration breakdown behavior (Step-like resistance increase) was eventually observed. Knowing that this very high current density causes significant joule heating the temperatures have been corrected by either calculating the average temperature using the TCR [4] as well as by a simulation of the exact temperature profile using 3D-FEM modeling (Fig. 2) [5]. These results give fairly good activation energy values (Tab. 2) and n-values of ~2.2 (TCR temperature corrected). Applying advanced failure analysis techniques, such as OBIRCH ...

Grain Size And Cap Layer Effects On Electromigration Reliability Of Cu Interconnects: Experiments And Simulation

“…This result supports the observation of highly disordered Cu/SiN x interfaces and highly ordered, dominantly metallic bonding Cu/Co interfaces 12 . The EM lifetime improvement by using the CoWP cap observed in this study is consistent with the results reported previously 7,8,13 . In addition, the results showed a clear grain size effect on EM lifetime for both cap layers.…”

Process options for improving electromigration performance in 32nm technology and beyond

“…The results show an improved lifetime by a factor of two for ACP and a factor of 100 for metal cap samples compared to standard processing. Additionally to that we found an activation energy improvement of 0.1eV for ACP and > 0.4eV for MC compared to standard processing [11].…”

Reliability of silicon integrated circuits

“…To compensate, analogs of the techniques used to improve Al The data indicate that current density exponent can vary between 1 and 2 depending on the relative importance of void nucleation and growth to the failure time. electromigration performance have been introduced for Cu: namely, Cu alloys to limit fast grain boundary diffusion [79][80][81][82][83][84], which controls the rate of electromigration in highly scaled Cu, and the incorporation of metallic cap layers at the Cu surface to impede diffusion along the top surface of the Cu [85,86].…”