The Investigation of Fluorine Effects on Charge Trapping and Interface State Generation in Mos Structures
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THE INVESTIGATION OF FLUORINE EFFECTS ON CHARGE TRAPPING AND INTERFACE STATE GENERATION IN MOS STRUCTURES DUNXIAN D. XIE, TA-CHENG LIN, AND DONALD R. YOUNG Sherman Fairchild Laboratory, Lehigh University, Bethlehem, PA 18015 ABSTRACT The bulk and interface charge trapping phenomena of fluorinated oxides have been studied by various electronic measurements. Fluorine is introduced into dry oxides by low energy (25kev) implantation followed by a 1000°C N2 ambient anneal to remove physical damage. Both the flat band and the mid gap voltage shifts of such MOS devices are measured during avalanche electron injection. We have developed techniques to separate effects due to interface state generation from bulk trapping effects. The bulk electron traps in the fluorinated oxides have a different cross section from the known water-related traps in conventional oxides. The generation of fast and slow interface states for different dosages of fluorine implantation is discussed based on Q-V and C-V measurements. The fast interface donor states, generated during avalanche injection, are charged at flat band but discharged at mid gap and beyond. An optimum dosage of fluorine implantation is found to suppress the so called turn-around effect during avalanche injection due to the formation of slow donor states. Finally, injection under high temperature (120°C-150'C) anneals out most of these donor states. INTRODUCTION In recent years, the incorporation of F into SiO 2 has been tried to improve the quality of MOS devices. Different techniques have been proposed to search for an optimum quantity of F which could reduce interface states between silicon and silicon dioxide. Nishioka, Ma et al [1],[2] reported that the generation of interface states was reduced either by immersing wafers in a diluted HF solution prior to oxidation or by purging NF3 during oxidation. Saraswat and Wright [3],[4] of a research group in Stanford University found the hot-electron immunity by F implantation. They also observed physical damage and positive charge in fluorinated oxides during high field current injection. It seems that more information is needed to understand the effects of F on MOS structures. In this paper, we report our results on bulk and interface effects of fluorinated oxides by avalanche injection technique and some other electrical measurements SAMPLE PREPARATION MOS capaciors are fabricated on p-type < 100 >, 0.1-0.2 0-cm B-doped silicon wafers. Dry oxidation at 1000°C grows the oxide thickness around 540-880A. Fluorine (10"310-'cm-2) is then implanted, with low energy (25kev), into oxides. After implantation, samples receive a 1000°C N2 ambient anneal for 30 minutes to remove physical ion damage. Gate contacts are formed by aluminum evaporation in high vacuum chamber. Finally, PostMetal-Anneal in a forming gas (20%H 2/80%N 2 ) mixture is done at 400'C for 30 minutes. Control samples are fabricated in the same processing procedure except for ion
Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
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implantation.
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