Second-oxidation properties of thin polysilicon films grown by LPCVD and heavily in situ boron-doped

In this work. we present a comparative study between first- and second-ther mal-oxidations properties of low pressure chemical vapor deposition (LPCVD) thin polysilicon films. These films are heavily in situ boron-doped with a concentration level of around 2 x 10(20) cm(-3) The first- and second dry oxidations experiments are systematically conducted under the same conditio ns. Their properties are analysed using electrical and structural character ization means. The thermal-oxidation processes are performed on submicron l ayers of 200 nm deposited at temperatures T-d ranged between 520 and 605 de grees C and thermally-oxidized in dry oxygen ambient at temperatures T-ox = 945, 1000, and 1050 degrees C. As presented in a previous study, the first -oxidation of these layers is known to have an enhanced oxidation rate comp ared to similar undoped, boron-implanted layers, and single crystal silicon . In the used range of the deposition temperature T-d, we have found that t he second-oxidations present a completely different behaviour compared to w hat is observed in the first ones. The second-oxidation rate becomes too we ak and seems to be dependent of the first-oxidation time t(oxt). Furthermor e, if the first dry oxide is etched off and a second dry oxide is grown, it is found that the second oxide contains less dopant than the first one. Ob viously, the loss of the oxidation rate is comprehensively discussed if we assume that the film structure is subjected to a deep modification at the f irst stage of the oxidation process. These results will be also correlated with other specific phenomena that we have evidenced in a previous work. (C ) 1998 Elsevier Science S.A. All rights reserved.