1. Field
The present technology relates to high density memory devices in which variations in cell characteristics can vary within array, such as memory devices in which multiple levels of memory cells are arranged to provide a three-dimensional 3D array.
2. Description of Related Art
As critical dimensions of devices in integrated circuits shrink to the limits of common memory cell technologies, and arrays become very large, memory cell within an array can have characteristics that vary in a manner that affects sensing margins. In one trend to achieve high density, designers have been looking to techniques for stacking multiple levels of memory cells to achieve greater storage capacity, and to achieve lower costs per bit. For example, thin film transistor techniques are applied to charge trapping memory technologies in Lai, et al., “A Multi-Layer Stackable Thin-Film Transistor (TFT) NAND-Type Flash Memory,” IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006; and in Jung et al., “Three Dimensionally Stacked NAND Flash Memory Technology Using Stacking Single Crystal Si Layers on ILD and TANOS Structure for Beyond 30 nm Node,” IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006.
Also, cross-point array techniques have been applied for anti-fuse memory in Johnson et al., “512-Mb PROM With a Three-Dimensional Array of Diode/Anti-fuse Memory Cells” IEEE J. of Solid-State Circuits, vol. 38, no. 11, November 2003. In the design described in Johnson et al., multiple levels of word lines and bit lines are provided, with memory elements at the cross-points. The memory elements comprise a p+ polysilicon anode connected to a word line, and an n-polysilicon cathode connected to a bit line, with the anode and cathode separated by anti-fuse material.
In a 3D array, differences in the electrical characteristics of structures on the various levels can lead to differences in the dynamics of programming, erasing, and charge storage, including variations in threshold voltages corresponding to memory states of memory cells on the various levels. Thus, to achieve the same threshold voltages, within acceptable margins for every level, the programming and erasing processes have to be adapted to vary with the level of the target cell in some way. These variations can lead to endurance problems with the memory cells and to other complexities.
In a 3D array, access lines, such as global bit lines, arranged for use to access the various levels of the array can be laid out so that characteristics such as capacitance and inductance encountered by circuits coupled to the access lines can vary depending on the location, such as which level in the array, of the cell being accessed. For example, global bit lines typically extend to decoder circuitry used for reading and writing the memory cells. Differences among the vertical connections to the various levels, and other differences among the levels, can lead to variations in capacitance among the global bit lines. These variations in capacitance affect the global bit line voltages during read, program and erase operations, and can result in specification requirements, such as larger read margins between programmed and erased states, and slower sensing times to account for worst case capacitances.
It is therefore desirable to provide technologies that compensate for variations in cell characteristics within an array, and also to provide a three-dimensional integrated circuit memory that reduces complexities arising from the differences among the levels.