Reduction in Polysilicon Oxide Leakage Current by Annealing prior to Oxidation

Abstract

The mechanism of leakage current reduction in phosphorus-doped po]ysilicon oxide by high temperature annealing prior to low temperature oxidation was investigated by bias polarity dependence of oxide leakage current, polysilicon grain size measurement, and polysilicon surface observation. Increased phosphorus concentration in polysilicon makes the grains larger and smoothens the surface roughness; consequently, the highest electric field is obtained at the phosphorus concentration of about 6 • 102~ cm-'~. Even at the optimum concentration, leakage current due to field enhancement at the interface between polysilicon oxide and polysilicon is still high. Leakage current can be reduced remarkably by high temperature annealing prior to oxidation, especially at the optimum concentration. The annealing effect is explained by interface flatness improvement due to grain growth during annealing and increase of phosphorus atoms in grains. Phosphorus atom migration from the grain boundaries inside the grains is considered to enhance the grain growth.Thermal oxide grown on heavily doped polysilicon is commonly used as the dielectric of MOS LSI's. Particularly, in a floating polysilicon gate EPROM's and EEPROM cells polysilicon oxide (poly-oxide) hardness to the electric field plays an important role in determining data retention characteristics. However, poly-oxide is well known to be more conductive than bulk silicon oxide, which has been attributed mainly to surface roughness at the poly-oxide/polysilicon interface.A smoother interface is obtained by oxidizing polysilicon at higher temperatures (1), where the viscous flow of the oxide and the small dependence of oxidation rate on polysilicon orientation can moderate the surface roughness generated by oxidation. A similar improvement in the surface roughness is attained by using polysilicon with amorphous structure (2). Also, high temperature annealing just after polysilicon oxidation improves the breakdown field (3). This improvement is explained by the release of stress in the structure, which prevents further flawing induced after the oxidation and thermal cycling.In this paper, the role and mechanism of high temperature annealing prior to oxidation of phosphorus-doped polysilicon is investigated as an effective means of improving the reliability of low temperature oxidized polyoxide. The bias polarity dependence of Fowler-Nordheim tunneling current through poly-oxide, measurement of the polysilicon grain size, and observation of the oxidized polysilicon surface using TEM and replica techniques are reported. ExperimentalPolysilicon films with thickness of 0.40 ~m were deposited by low pressure chemical vapor deposition (LPCVD) at 620~ onto thermally oxidized Si substrates and were doped with phosphorus by diffusion at 900~ from a POC13 source. Prior to oxidation, 0.30 ~m thick CVD oxide films were deposited onto the doped polysilicon films, and annealing was performed at 950 ~ and ll00~ for 10 min in N~ atmosphere. Some of the Si substrates remained unannealed, as shown ...

Evolution de la taille des grains du silicium polycristallin pendant des traitements thermiques ou oxydation

“…Celle-ci évolue jusqu'à près de 70 nm après un traitement de recuit ou d'oxydation à haute température. Notons que cette taille initiale des grains est plus petite que celle d'échantillons de silicium polycristallin dopés par diffusion, à partir d'une source de POCI3 (50 nm environ [13,18]). Par ailleurs, la température de dopage par diffusion est nettement supérieure (900 °C-1 000 °C) à la température du dopage in situ (670 °C), ce qui entraîne la présence de contraintes thermiques.…”

Surface topography of phosphorus doped polysilicon

“…11 The poly-Si deposition and doping processes establish the microstructure, dopant concentrations, and surface topography of these films. [2][3][4][13][14][15][16][17][18] The purpose of this work was to evaluate the surface topography of poly-Si films that are used for double-poly-Si capacitor applications. [2][3][4][13][14][15][16][17][18] The purpose of this work was to evaluate the surface topography of poly-Si films that are used for double-poly-Si capacitor applications.…”

Improvement of polysilicon oxide characteristics by fluorine incorporation

“…It is seen that the effective barrier heights were almost the same for these samples. Since the barrier height value is very sensitive to the interface texture structure [8], this implied that the fluorine-induced improvement of polyoxide characteristics was not caused by the change of the polysilicodpolyoxide interface texture. We would then suspected that the improvement of the polyoxide property is caused by the oxide stress relaxation due to the incorporation of fluorine.…”