A PROM array based on the PAF will suffer yield loss if the programmed fuse does not have a high enough resistance to be properly sensed. Programming yield depends on the fuse design (see Fig. 8), array size, and circuit design. Even after optimizing the element and circuit, the resulting yield may not be as high as expected. In that case, redundant fuse elements are needed such that if programming of a given fuse in a given memory bit is not successful, an additional fuse is available in that bit for an extra attempt. In this work, for a 64 bit array, a programming yield loss of less than 1 in 10,000 was achieved using two fuses per bit (a programmed state in either fuse resulted in a programmed bit).

The reliability of this element was characterized by placing a large number of samples (programmed and unprogrammed) under thermo-mechanical stress (1000 cycles of condition 'C' temperature shock) and in bake (300 hours, 250C). The element was found to be quite stable under these conditions (see Fig. 11). Additional testing was done to characterize the stability of the unprogrammed fuse under bias temperature stress. Results indicated that as long as the sensing current is significantly less than the current at the onset of programming, the device will remain stable.

 

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