Study Of The Reliability Of Metal Interconnects

With the scaling down of the interconnect feature size and increase of the pattern density,metal line reliability is become a more and more serious concern of the whole device development and operation lifetime.Reliability describes the ability of a device working in the normal used condition.It is the necessary step to check if the new product gets a successful design and meets market requirement.Many steps are needed for reliability evaluation like experiments,data analysis,spice modeling,failure analysis etc.This paper is focused on the study of metal line EM(Election-Migration) effect.It includes principles of EM,test condition,some testing experiences and process improvement direction based on different failure area.We show three cases in the research of EM:1.Wafer level EM test is aiming at length and width impact of interconnection to EM lifetime.In the Al interconnect,a "bamboo" structure is formed when metal line was very small which results in maximum EM lifetime.However,in the Cu interconnect,it is hard to form "bamboo" structure due to smaller grain size of copper.So there is no obviously correlation between metal width and EM lifetime.And,no matter Al or Cu metal line,there is no strongly relationship between metal length and EM lifetime.2.Then we will discuss the "wedge-shaped structure" noise of EM test on the upstream structure with top metal of Cu process.Blech length is the critical length of interconnection EM effect.The metal wouldn't suffer EM for metal line length smaller than Blech length.But the wedge-shaped line will increase the current density.From the Blech product equation,bigger current density will sacrifice more Blech length. Smaller Blech length causes EM much easier to happen.So the improvement of top metal test structure design is very important,which helps us to avoid the noise and get the correct reliability data. 3.In traditional package level EM test,gold bonding wire is forbidden.It would induce open failure because gold would diffuse to the whole bond pad during high temperature stress and alloys with Cu.In double top metal structure,surface process condition is important to EM lifetime. The lifetime increases if surface process is improved.This experiment is to find the best improvement choice from different process conditions and then improve the BEOL of 90hm process.This paper is to find the process and test condition correlation and improvement to EM lifetime based on study of metal lines reliability.It includes introduction of typical test condition and data analysis and discussion of the experiment result.