Alternate method of TDDB study for aluminum oxide using magneto-resistance

A sensitive and accurate method/ test to predict the lifetime of aluminum oxide used as a tunnel barrier in magnetic random access memory (MRAM) has been devised. The performance of magnetic tunnel junction is dependent upon the lifetime of aluminum oxide. Aluminum oxide has also been used in high dielectric constant gate dielectric MOS devices suitable for high speed applications. This method relies upon the measurement of anti parallel and parallel resistance of the magnetic stack in a tunnel magneto resistor using aluminum oxide as a tunnel barrier. This is a more sensitive method than the conventional methods. Introduction Aluminum oxide is the tunnel barrier in the Tunnel Magneto-resistor used in MRAM. In Magnetic Tunnel Junctions (MTJ), aluminum oxide separates the hard (pinned) magnetic layer and the soft or switching magnetic layer. Aluminum oxide is also considered for use as a high K (dielectric constant) gate oxide in high-speed CMOS application.[2] As device geometries continue to shrink, the gate oxide thickness is approaching 1 nm and leakage currents are significantly high. Since the dielectric constant of aluminum oxide is higher than that of silicon dioxide (KAl2O3 ~ 9 whereas KSiO2 = 3.9), the same capacitance can be attained with a thicker high K gate dielectric which reduces the leakage current. In both the applications, the quality and reliability of aluminum oxide determines the lifetime of the product. Therefore it is quite critical to reliably predict the lifetime of aluminum Oxide. A Tunnel Magneto resistor consists of two ferromagnetic layers separated by an insulating barrier layer. The tunneling conductance of MTJs is determined by two contributions: those arising from the tunneling barrier and those related to the spin dependent density of states on the electrodes. The total magnetic moments on the electrodes determine the total flux of electrons available for conduction through the tunnel barrier by direct tunneling. The number of carriers available for tunneling is determined by spin polarization of the electronic density of states at the Fermi energy.[3] When the spin polarization is parallel in both the soft and hard magnetic layers, both layers have the same direction of magnetic polarization. When the spin polarization is antiparallel in either of the soft and hard magnetic layers, then the layers have the opposite direction of magnetic polarization. The tunneling resistance through the barrier is low for parallel spin polarization and high for antiparallel spin polarization. This effect provides the ability to detect small changes in the tunneling current. This paper describes the use of this effect as a potentially very sensitive technique to detect dielectric degradation and breakdown mechanisms. Development of method The topographic magnetostatic coupling energy, JE,[3] between two ferromagnetic films of magnetization M and M', separated by a nonmagnetic spacer of thickness t with an interface waviness of wavelength λ amplitude h is given by: