| By storing more than one bit per memory cell, MultiLevel Dynamic Random-Access Memory (MLDRAM) explores an additional dimension to increase the per-cell storage capacity over conventional two-level DRAM. A well-balanced and robust MLDRAM scheme was proposed previously by Birk, Elliott and Cockburn. We designed and implemented a test chip for this MLDRAM in TSMC's 0.18-micron CMOS technology. The test chip has an adjustable cell capacity that can be selected from 2, 3, 4 and 6 levels per cell, corresponding to 1, 1.5, 2 and 2.5 bits per cell. Prototypes of the test chip were verified using an Agilent 81200 digital IC tester. Most of the cells in operational chips were found to work. However, small voltage offsets affecting the signal and reference cells cause read errors for some cells. A follow-up project would be to characterize the offset problem in greater detail and to design an improved test chip. |
