Electrical reliability of tunneling magnetoresistive read heads

The electrical reliability of tunneling magnetoresistive (TMR) read heads has been studied in terms of dielectric breakdown characteristics of tunnel barrier and electrical failures under constant biasing current. The TMR read heads showed both “intrinsic breakdown” characteristics, which is caused by the high electric field, and “extrinsic breakdown” characteristics, which is caused by defects or pinhole-induced breakdown especially in a very thin dielectric layer. The dielectric breakdown voltage (Vbd) of real TMR read head is revealed to be strongly dependent on the duration of the applied voltage step (or applied voltage ramping speed), the initial junction resistance (R×A, A: Junction area) related to pinholes in a dielectric tunnel barrier, the tunnel barrier thickness, the ambient substrate temperature during breakdown test, and the applied voltage polarities. Different from typical metallic giant magnetoresistive spin-valve read heads, TMR read heads have a weaker reliability on sensing current density and exhibit two types of electrical failures: (1) Electrical short induced by tunnel barrier breakdown, and (2) electrical open induced by possible void formation in the metallic layers, which are used for free and pinned layers in exchange biased TMR read heads. The quasi-static transfer curves of head gimble assemble TMR read head was measured under various biasing currents. Transfer curve exhibited a strong dependence on the number of measurements above the threshold bias current, which is considered to be due to the current-induced voltage breakdown in the tunnel barrier.