A scientific study of the problems
of digital engineering for space flight systems,
with a view to their practical solution.
Ionizing Radiation Response of an Amorphous Silicon Based Antifuse
J. M. Benedetto and C. C. Hafer
UTMC Microelectronic Systems
4350 Centennial Blvd.
Colorado Springs, CO 80907
Abstract
The ionizing radiation response of Ti/W metal electrode amorphous silicon (a-Si) antifuses is examined. It is shown that the resistance of unprogrammed a-Si antifuses improve (i.e. increase resistance) with increasing radiation dose when irradiated with a positive 3 or 5V bias. The resistance of unprogrammed a-Si antifuses irradiated with zero bias and the resistance of fully programmed antifuses are insensitive to total ionizing radiation dose. The radiation response of partially programmed antifuses (as programmed resistances between 2000 and 4000W) is also examined.
Table of Contents
I. Introduction
II. The Metal Electrode a-Si Antifuse Structure
III. Experimental Techniques
IV. Results and Discussion
A. Unprogrammed Antifuses
B. Low Resistance Programmed Antifuses
C. Weakly or Partially Programmed Antifuses
V. Addressing the Radiation Sensitivity of Partially Programmed Antifuses in Programmable Read-Only Memories
VI. Summary
VII. References
List of Figures
Figure 1. Schematic cross section of an a-Si antifuse structure.
Figure 2. Current-voltage characteristics of unprogrammed antifuse irradiated with 0V bias.
Figure 3. Current-voltage characteristics of unprogrammed antifuse irradiated with 3V bias.
Figure 4. Current-voltage characteristics of unprogrammed antifuse irradiated with 5V bias.
Figure 5. Resistance of fully programmed antifuse as a function of total dose.
Figure 6. Resistance of "weakly" programmed antifuse as a function of total dose.
Summary
This work examined the ionizing radiation response of Ti/W metal electrode a-Si antifuses in three states, (1) the unprogrammed state (resistance >109W), (2) the fully programmed state (resistance <100W) and (3) in a less than optimum programmed state (resistances between 2000 and 4000W). It was shown that the resistance of an unprogrammed a-Si antifuse increases, i.e., the resistance characteristics improve with increasing total dose. The resistance of a fully programmed fuse, as expected, is insensitive to ionizing radiation. For an antifuse which was programmed to a high resistance state, i.e., the resistance is on the order of 2000-4000W instead of less than 100W (as is the case for a fully programmed antifuse), ionizing radiation can increase the resistance of the antifuse stack sufficiently to cause a bit flip in a PROM.
Because of the radiation sensitivity of partially programmed bits an improved programming algorithm was developed to reduce the percentage of partially programmed antifuses. A proprietary post-programming conditioning technique was also developed to strengthen any weak antifuses that remained with the new programming algorithm. Following implementation of this new programming flow 44 256K PROMs have been irradiated to over 1Mrad(Si) with no bit or functional failures (equal to over 11 million programmed a-Si antifuses with no failures). It appears that the radiation hardness of an a-Si based antifuse can be improved dramatically by eliminating any partially programmed antifuses.
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