Degradation of Memory Retention Characteristics in DRAM Chip by Si Thinning for 3-D Integration

The Young's modulus (E) of Si substrate begin to noticeably decrease below 50-mu m thickness. The Young's modulus in 30-mu m thick Si substrate decreased by 30% compared to the modulus of 50-mu m thickness. In 30-mu m thick Si substrate, the lattice structure of Si atom is highly distorted. Large distortion of the lattice structure induces the Young's modulus reduction, consequently weakens the mechanical strength. A DRAM chip of 200-mu m thickness is bonded to a Si interposer and thinned down to 50/40/30/20-mu m thickness, respectively. The retention characteristics of DRAM cell are degraded depending on the decreasing of the chip thickness, especially dramatically degraded below 50-mu m thickness. The retention time of DRAM cell in the 20-mu m thick chip is shortened by similar to 40% compared to the 50-mu m thick chip, regardless of the well structure (triple-well, twin-well). The distortion of the lattice structure in the thin chip effects carrier recombination rates, consequently a shortening retention time of DRAM cell.