ar X iv : c on d - m at / 0 30 61 53 v 1 6 J un 2 00 3 Diagnosis and Location of Pinhole Defects in Tunnel Junctions using only Electrical Measurements

In the development of the first generation of sensors and memory chips based on spin-dependent tunneling through a thin trilayer, it has become clear that pinhole defects can have a deleterious effect on magnetoresistance. However, current diagnostic protocols based on Andreev reflection and the temperature dependence of junction resistance may not be suitable for production quality control. We show that the current density in a tunnel junction in the cross-strip geometry becomes very inhomogeneous in the presence of a single pinhole, yielding a four-terminal resistance that depends on the location of the pinhole in the junction. Taking advantage of this position dependence, we propose a simple protocol of four four-terminal measurements. Solving an inverse problem, we can diagnose the presence of a pinhole and estimate its position and resistance. PACS ’03: 73.40.Rw, 73.40.Jn, 85.75.Dd, 85.75.Mm Semiconductor manufacturers are currently developing magnetic-random-access-memory elements 1–3 based on magnetic tunnel junctions; such junctions separate two ferromagnetic metallic leads by a thin insulting layer, 4–6 often made by oxidizing a film of Al or other suitable metal. Both because of the thinness of the insulating layer and because of the possibility of inadequate oxidation, “pinhole” defects—direct metal-metal shorts through the nominal insulator—have attracted significant attention. 7–13 A single pinhole can also be generated in a previously pinhole-free junction carefully through a voltage ramp 14–17 or, by implication, inadvertently. Generally, the parasitic current through pinholes detracts from a junction’s magnetoresistance, 18 so methods for diagnosing and locating such defects become important during the development of practical devices. Surprisingly, a fit of differential conductance to the Simmons form 19 fails unambiguously to guarantee the absence of pinholes. 11,20 Surer methods include the use of an integrated superconducting electrode, 20 the temperature dependence of device resistance, 20–22 and surface decoration. 7,14 These diagnostics may not be integrated easily into a development or manufacturing process. We therefore propose a very simple test that not only determines the presence of a pinhole with high confidence but also can typically locate the pinhole to within 7% of the junction area and estimate the pinhole resistance. We propose four four-terminal measurements. A discrete three-dimensional resistor model allows us to compute the result of each measurement for an assumed pinhole position; working backward from a set of four experimental measurements to the pinhole position therefore constitutes an inverse problem, to which we demonstrate a solution. Since the method applies to non-magnetic as well as to magnetic tunnel junctions, we shall generally treat the junctions without reference to magnetic properties.