Abstract: Techniques are provided herein for forming multi-tier memory structures with graded characteristics across different tiers. A given memory structure includes memory cells, with a given memory cell having an access device and a storage device. The access device may include, for example, a thin film transistor (TFT) structure, and the storage device may include a capacitor. Certain geometric or material parameters of the memory structures can be altered in a graded fashion across any number of tiers to compensate for process effects that occur when fabricating a given tier, which also affect any lower tiers. This may be done to more closely match the performance of the memory arrays across each of the tiers.

Abstract: Techniques are provided for making asymmetric contacts to improve the performance of thin film transistors (TFT) structures. The asymmetry may be with respect to the area of contact interface with the semiconductor region and/or the depth to which the contacts extend into the semiconductor region. According to some embodiments, the TFT structures are used in memory structures arranged in a two-dimensional array within one or more interconnect layers and stacked in a vertical direction such that multiple tiers of memory structure arrays are formed within the interconnect region. Any of the given TFT structures may include asymmetric contacts, such as two contacts that each have a different contact area to a semiconductor region, and/or that extend to different depths within the semiconductor region. The degree of asymmetry may be tuned during fabrication to modulate certain transistor parameters such as, for example, leakage, capacitance, gate control, channel length, or contact resistance.