Some Properties of an Anisotropic Thin-Film-Waffle-Iron Memory Cell

Coupled-film arrays have been fabricated on insulated Cu ground plane with 0.004-in.-wide bit lines on 0.006-in. centers and 0.007-in.-wide word lines on 0.014-in. centers, resulting in an unusually high packing density of 12 000 bits/in.2 A sense signal of 2 mV under worst-case testing conditions has been measured at 200-mA word current (6 nsec risetime) and 15-mA bit current. The low word and bit currents have been achieved by miniaturizing the devices while providing flux closures. The etched Permalloy-Cu-Permalloy bit/sense lines are plated with Permalloy at the edges. The word lines are embedded in ferrite. Permalloy backing on word line and Permalloy embedded in ground further improve flux closure. Available Permalloy materials have intrinsic limitations on the obtainable Hk, Hc, α and the ratio Hc/α Hk (a measure of disturb immunity). The device operating margin can be extended by using doublet word and/or bit currents. The doublet bit current increases the bit disturb threshold, while the doublet word current decreases the bit write threshold. With both doublet bit and word currents, both beneficial effects are achieved.

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