Electrical and Reliability Improvement in Polyoxide by Fluorine Implantation

Abstract

We show that the incorporation of fluorine into the oxide grown on polysilicon ͑polyoxide͒ not only improves the electrical characteristics ͑i.e., lower leakage current, higher electrical breakdown field͒, but also improves the reliability ͑lower electron trapping rate, larger Q bd ͒. This improvement is believed to be due to the stress relaxation of the polyoxide and smoother polysilicon/polyoxide interface by the fluorine incorporation. The optimum fluorine dose ͑2 ϫ 10 14 ͒ shows the best characteristics such as E bd over 12 MV/cm and Q bd ϳ 2 C/cm 2 . However, excessive fluorination ͑1 ϫ 10 15 ͒ seems to result in performance degradation due to the generation of nonbridging oxygen centers.Fluorinated gate oxide dielectrics have attracted considerable attention over the past years. 1-8 The incorporation of approximate amounts of F into the oxide near the Si/SiO 2 interface was shown to improve the oxide breakdown distribution and the interface hardness against hot-electron and radiation damages of metal-oxidesemiconductor ͑MOS͒ devices. 9-14 The improved reliability is that fluorine can break strained Si-O-Si bonds to cause local strain relaxation, and fluorine in the oxide can replace weak Si-H bonds to form strong Si-F bonds. 15 The oxides grown incorporated with fluorine had also been reported to cause local strain-stress relaxation and to exhibit less interface state density because of the passivation of interface defects such as dangling bonds and strain bonds. 16 Recently, the effects of fluorine on the defect passivation and the hot carrier endurance of polysilicon thin-film transistors ͑TFTs͒ due to the similarity between the poly-Si/SiO 2 interface and the polysilicon grain boundaries were investigated. 17-19 The fluorine implantation not only improved the electrical characteristics such as on current, subthreshold swings, and mobility, but also improved the hot carrier immunity of the device 20,21 due to the fluorine passivation more uniformly reducing the trap states in the band tail and those in the midgap.The quality of polysilicon oxide ͑polyoxide͒ plays an important role in the performance of polysilicon TFTs 22 and nonvolatile memories. 23 The electrical properties of the polyoxide are mainly determined by the polysilicon/polyoxide interface roughness. 24 It had been demonstrated that the rough and nonuniform surface at the polysilicon/polyoxide interface enhanced the local electric field and resulted in low breakdown strength as compared to single crystalline silicon oxide. 25 It had also been reported that a compressive stress two or more times larger than the stress in the thermal oxide of single crystal was formed on the thermally grown polyoxide. This increased stress in thermal polyoxide is due to the oxidized polysilicon at grain boundaries enhancing the interface roughness, and causing an additionally compressive stress. 26 In this work, we show that the fluorine ions are directly implanted into the bottom polysilicon and the fluorinated polyoxides are grown in dilute ͑N 2 + O 2 ͒ ambie...

Fluorine Effects on NMOS Characteristics and DRAM Refresh

“…For the commonly used polycide process, fluorine is inadvertently introduced into the gate oxide from WSix deposition using WF 6 gas. The effects of fluorine have been studied intensively such as reliability issues [1][2][3], device performance [4,5], and the physical properties of gate oxide [6]. However, few studies have focused on the fluorine effect as the function of the channel length.…”