Thermal SiO2films on n+polycrystalline silicon: Electrical conduction and breakdown

The conduction and breakdown properties of thermally grown SiO2films on amorphous-deposited n⁺polycrystalline silicon (polysilicon) are evaluated using ramped current-voltage ( ) measurements. It is shown that the inferior insulating properties of oxides on polysilicon (polyoxides), when compared to SiO2on bulk silicon, can be directly attributed to oxidation-induced interface roughness leading to localized enhancement of the oxide electric field. For example, 16.7-nm-thick polyoxides approach bulk SiO2properties since a breakdown field EBDof approximately 9.5 MV . cm^-1and an effective barrier height for Fowler-Nordheim tunneling, φBeffas high as 2.78 eV were measured. Both of these parameters are progressively degraded by increasing polyoxide thickness DOXsuch that for 165-nm-thick polyoxides MV . cm^-1and φBeffis reduced to as low as 0.83 eV. The measured curves are found to become more polarity dependent with increasing DOXdue to a comparatively higher degree of oxidation-induced surface roughening at the lower interface, which renders it more "conductive," with regard to Fowler-Nordheim electron injection, than the upper oxide-polysilicon interface. Certain specific device applications require a relatively "conductive" polyoxide capable of carrying high current densities before failure. Consequently, a polysilicon texturing procedure was developed that has the effect of decreasing φBeff, increasing breakdown current JBD, and eliminating the polarity dependence of curves for any subsequently formed thin polyoxide. The particular process entails growing a predetermined thickness of "texturing" oxide Dteox, and removal prior to formation of the device polyoxide of approximately 25-nm thickness. As Dteoxis increased from zero to 103 nm, the resultant JBDis found to increase by more than an order of magnitude for both polarities of bias. The corresponding decrease in φBeffis from 1.7 and 2.4 eV for positive and negative gate bias, respectively, to a polarity- -independent value of approximately 1.3 eV.