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Use of Double Dielectric Structures to Protect FET Circuits Against Radiation Damage

IP.com Disclosure Number: IPCOM000048080D
Original Publication Date: 1981-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Dennard, RH: AUTHOR [+2]

Abstract

This article describes structures wherein the no-ROX (recessed oxide), semi-ROX or full-ROX isolation regions. which are commonly used in present FET (field-effect transistor) technology, are replaced by double dielectric structures to improve the immunity of integrated circuits against radiation damage. In contrast to the prior art (1-3), which applied this double dielectric concept to implement FETs for various purposes, double dielectric structures are described herein which are used to form isolations between the active FETs and not for implementing the FETs themselves.

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Use of Double Dielectric Structures to Protect FET Circuits Against Radiation Damage

This article describes structures wherein the no-ROX (recessed oxide), semi-ROX or full-ROX isolation regions. which are commonly used in present FET (field-effect transistor) technology, are replaced by double dielectric structures to improve the immunity of integrated circuits against radiation damage. In contrast to the prior art (1-3), which applied this double dielectric concept to implement FETs for various purposes, double dielectric structures are described herein which are used to form isolations between the active FETs and not for implementing the FETs themselves.

When circuits are exposed to high energy radiation, electron hole pairs are generated in the silicon dioxide. Some of the holes will be captured into deep hole traps in the oxide near the Si-SiO interface (4). This causes a negative shift in the turn-on voltage (or threshold voltage), V(T), of either active thin oxide devices or of parasitic devices associated with the isolation regions. This relationship is given by: delta T(T) proportional to N(h).t(ox) where delta V(T) is the shift in the threshold voltage, N(h) is the number of holes generated and subsequently trapped, and t(ox) is the thickness of the oxide.

Furthermore, because N(h) is directly proportional to the number of holes generated in the oxide, one finds N(h) proportional to t(ox) Combining the two relations, we have

delta V(T) proportional to t/2/ over (ox).

Based on the scaling theory, when the thickness of the gate oxide of an active FET is reduced to the range of three or four hundred angstroms (5), the primary degradation of a circuit caused by radiation will not be from the active devices but from the isolation regions where the oxide is typically 3000 Angstroms to 4,000 Angstroms thick. Assume the turn-on voltage of an n- channel FET is 0.5 volt, and that of the isolation regions is 10 volts. Also, let the thickness of the oxide of the active device and of the field region be 300 Angstrom and 3,000 angstroms respectively. Then the turn-on voltage of the active device is shifted by 0.1 volt, the same amount of radiation dose will change the threshold voltage of the isolation region from 10 volts to 0 volts. Consequently, this leaky isolation will cause a failure to the circuit prior to the active device becoming defective.

Replacing the thick oxide by double dielectric structures as isolation, the radiation resistance can be improved significantly. This double dielectric structure is made of one layer of very thin oxide, which is grown on the silicon surface to ensure a good interface with the silicon substrate, and a second layer of different material over the grown oxide. A qualified dielectric material should result in a composite structure that prevents the holes generated within the second layer from moving into the oxide underneath. For instance, (i) if many effective recombination centers exist in th...