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Reducing Radiation Damage in Insulated Gate Field Effect Transistors

IP.com Disclosure Number: IPCOM000075199D
Original Publication Date: 1971-Aug-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Grosewald, P: AUTHOR [+3]

Abstract

It is known that the silicon dioxide sputtering process used to passivate IGFETs, results in a shift in the threshold (V(t)) or turnon voltage and increases leakage currents of these devices. These effects are believed to result from radiation damage during the sputtering process. It is also known that such damage in MOS structures is concentrated at the oxide-silicon interface, and that the damage at low-radiation dosages is related to the degree of disorder present at and near that interface. Described are several methods which minimize the disorder or modifies it in such a way that irradiation during the sputtering process has a reduced effect on the oxide charge, which in turn has an effect on device properties.

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Reducing Radiation Damage in Insulated Gate Field Effect Transistors

It is known that the silicon dioxide sputtering process used to passivate IGFETs, results in a shift in the threshold (V(t)) or turnon voltage and increases leakage currents of these devices. These effects are believed to result from radiation damage during the sputtering process. It is also known that such damage in MOS structures is concentrated at the oxide-silicon interface, and that the damage at low-radiation dosages is related to the degree of disorder present at and near that interface. Described are several methods which minimize the disorder or modifies it in such a way that irradiation during the sputtering process has a reduced effect on the oxide charge, which in turn has an effect on device properties.

Thermally grown silicon dioxide on silicon has a band structure such that electrons are lost to the silicon leaving a net trapped positive oxide charge. This electron transfer process is one that tends toward equilibrium. The rate of approach to equilibrium is governed by a time factor and various ambient conditions including temperature. In current FET processing, it is probable that oxide growth and subsequent anneals are of such character that quasi-equilibrium and not true equilibrium is obtained. The net effect of this is a large number of (unfilled) hole trapping states. Irradiation, during sputtering, creates electron-hole pairs. The electrons being quite mobile leave the oxide, while many of the holes fill the traps resulting in an increase in net oxide positive charge.

This increase in trapped positive oxide charge shows up as a lower threshold voltage, increased leakage and a larger V(t) shift, as compared to devices in which the oxide has come to equilibrium. To achieve equilibrium conditions more closely, both longer oxide growth times and final anneal times at a temperature greater than 850 degrees C but equal to or less than 1100 degrees C is carried out, resulting in a closer approach to equilibrium and devices which are more radiation resistant.

This approach results in a post-glass decrease in the density of holes in the oxide. The net effect is to provide radiation hard...