Investigations of mobile ion transport processes in thin layers upon bias-temperature stress

Mobile ions can alter the properties of a MOSFET device e.g. by contamination of the gate oxide. Mobile sodium ions (Na+) are thereby a particular danger, since encountered in comparable large amounts. In order to investigate mobile ion incorporation from an external source into an underlying thin insulating or conductive material film and the subsequent transport therein, a developed methodology is presented. This comprises a stressing procedure by applying a bias voltage, while keeping the investigated sample at a constant temperature (B-T stress) and a final physical analysis by time of flight - secondary ion mass spectrometry (ToF-SIMS). But the determination of mobile ion distributions in insulating samples by means of ToF-SIMS depth profiling has always been a challenge. Such depth profiles exhibit severe artifacts in the mobile ion depth distribution, due to the appearance of an electric field within the insulator, arising by means of sample charging during the sputter process. This problem is addressed by investigation of the temperature dependence of Na migration. Including this feature, insulating and conductive films of high importance for the semiconductor devices are investigated: silicon dioxide (SiO2), silicon oxynitride (SiOxNy), silicon nitride (Si3N4), boron-phosphosilicat glass (BPSG) and an aluminum silicon-copper alloy. In particular SiO2 is examined in detail, both qualitatively and quantitatively: For the example of Na+, a comparison is given between the calculated provided amount in the host matrix with potentiostatic data from B-T stress and spectrometric data from quantification of ToF-SIMS depth profiles. Appropriate physico-chemical models are adapted on the present samples and thus parameters describing mobile ion transport in SiO2 are condensed out of potentiostatic data (activation energy, incorporation rate and diffusion constant). (author)