An effect of filler-induced stress on DRAM sense amplifiers

A new phenomenon, that the degradation of VLSI DRAM\´s operating margin can be caused by plastic encapsulation, has been investigated. The plastic encapsulation induces stress concentration on DRAM sense amplifiers through the grains of silica, which are generally referred to as fillers. From our experiments, it was revealed that the sensitivity of DRAM sense amplifiers could be degraded by this stress concentration through the fillers in plastic molded packages. We can name this mechanism "filler-induced stress." This paper also describes powerful countermeasures against the filler-induced stress. By employing smaller size fillers and/or buffer coating over the chip, it was found that stress concentration could be relaxed, resulting in realization of high-density DRAM\´s encapsulated in plastic molded packages.