Effects of capping layer material and recrystallization conditions on the characteristics of silicon-on-insulator metal-oxide-semiconductor transistors in laser-recrystallized silicon films

Abstract

A study is presented of silicon-on-insulator (SOI) films obtained by laser recrystallization of polysilicon films using silicon nitride or oxynitride as a capping layer material and with either a continuous or a periodic antireflective capping structure. Different laser scan conditions are used. Electrical characterization of these recrystallized Si layers is performed on five-terminal SOI metal-oxide-semiconductor field-effect transistors by directly probing the front and back interface quality on the device level using the charge pumping technique. By this, a correlation between interface trap density and channel mobility could be determined. While the characteristics of both front and back Si-SiO2 interfaces depend on the crystalline quality (presence of grain boundaries) of the recrystallized film, the capping layer material used mainly affects the back interface properties. Both nitride and oxynitride capping layer materials introduce large amounts of nitrogen impurities in the molten silicon which can be expected to cause a deterioration of the interface quality. In case of a nitride capping layer, the (back) interface trap density is indeed found to be very dependent on the structure of the capping layer and on the actual laser recrystallization conditions, whereas in the case of an oxynitride capping layer no deterioration of the interface characteristics takes place for any scanning condition applied. The incorporation and the segregation of nitrogen impurities in the SOI structure is discussed supported by secondary ion mass spectrometry measurements of the impurity profiles and which qualitatively accounts for the observed dependencies. It is shown that even with a silicon nitride capping layer, laser-recrystallized SOI layers having good electrical properties can be obtained.