Migration of mobile ions in dielectric semiconductor compounds

Abstract

Mobile ions are found in every environment and in almost every materialused in the manufacture of semiconductor devices. Even very smallamounts of mobile ions like sodium or chloride ions can affect the lifetimeof many products we use every day. Today, the risk of failure due tomobile ions has been reduced using clean rooms, ultra-pure materials,and new manufacturing techniques. However, mobile ions still pose a riskto semiconductor devices. With ever-smaller feature sizes and everincreasingstress levels, the problems caused by mobile ions will be thesubject of research for many decades to come. Therefore, the preventionof mobile ion migration is an important challenge for semiconductorindustry. In this work, different dielectric layers were tested for barrierproperties against mobile ion migration.To begin with, a method for recording artifact-free depth profiles of alkaliions in non-conducting materials is described. In contrast to othermethods, the described method does not rely on a specially equippedToF-SIMS. Instead, only the heating-cooling stage and the cesium sputtergun were used to record depth profiles of alkali metals, which do not showany profile alteration due to surface charging.Next a new method for determination of the diffusion coefficient andactivation energy of silver in molding compounds is described. Althoughonly silver migration is described in this work, this method can also beapplied to a variety of other metals.Using the methods mentioned before, aluminum oxide is investigated asa barrier to mobile ion migration because these layers can be depositedin very thin layers with good quality by atomic layer deposition. Aluminumoxide is shown to be an exceptionally good barrier to sodium migration.Neither high voltage nor high temperature cause sodium to migratethrough the entire aluminum oxide layer.For comparison, many of the experiments are repeated with samplescontaining silicon nitride or borophosphosilicate glass instead ofaluminum oxide, as both materials are the current industry standard. Themeasurements clearly show that aluminum oxide exhibited the bestbarrier properties in all experiments.