I-Fei Tsu

Electromigration-induced failure of single layered NiFe Permalloy thin films for a giant magnetoresistive read head

Electromigration-induced failures have been investigated in single layered NiFe Permalloy thin films, which are used in giant magnetoresistance (GMR) read heads in high-density magnetic recording disk drives. The time-to-failure and median-time-to-failure (MTTF, t50) of NiFe thin films were found to be strongly dependent on the applied current density, film thickness, and ambient temperature. The activation energy (ΔQ) of NiFe films derived from the “Black equation” was found to be 0.8 eV. The MTTF and standard deviation (σ) of a NiFe thin film electromigration test stripe with 10 nm thickness, 5 μm width, and 20 μm length were 3.74 and 1.74 h, respectively, at 160 °C ambient temperature. Typical electromigration failures such as “voids and cracks” were observed at the cathode or center region of NiFe film test stripes. These results suggest that the electromigration-induced failures are one of the crucial factors in determining the reliability of GMR spin-valve read heads used in high-density magnetic re...

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 de...

Diffusion behavior of the spin valve structure

NiMn/NiFe/Co/Cu/Co/NiFe/seed layer (sample No. 1) and NiFe/CoFe/Cu/CoFe/Ru/CoFe/NiFe/NiMn/Seed layer (sample No. 2), are investigated by using high resolution analytical transmission electron microscopy and an imaging filter. The compositional analysis demonstrated that the diffusions of the Mn and Ni into the Cu/Co bilayer are only observed in sample No. 1. This result indicated that the Ru layer in sample No. 2 might not only act as the spacer of the synthetic antiferromagnet but also behaves as a good diffusion barrier for the Ni and Mn element in the spin valve structure. The diffusion coefficients of constituent elements are simply investigated using the Matano–Boltzmann method. The diffusion mechanisms of Cu in Co layer and Co in Cu layer were primarily dominated by the grain boundary.

Degradation of spin valve heads under accelerated stress conditions

The failure mode of spin valve head was studied at various oven temperature and applied current density conditions. The effects on device reliability due to Joule-heat temperature rise, current density, and stress current polarity were investigated by in-situ quasistatic transfer curve measurements. Arrhenius plots of the time to failure (TTF) data show similar activation energy of /spl sim/1 eV for failure criteria of 0.2% resistance decrease and 10% amplitude loss. The device reliability is shown to depend predominantly on the peak sensor temperature. The magnetic field produced by the bias current affects primarily magnetic instability and causes mainly the scattering of life test results.

Diffusion behavior in spin valves studied by high resolution transmission electron microscopy and nanobeam technique

Abstract x4282 NiMn/NiFe/Co/Cu/Co/NiFe/seed layer and NiFe/CoFe/Cu/CoFe/Ru/CoFe/NiFe/NiMn/seed layer structures have been investigated using high resolution transmission electron microscopy (HRTEM) coupled with nanobeam energy dispersive X-ray (EDX) and electron energy loss spectroscopy (EELS). An accumulated sum technique was applied to analyse the lattice spacing variation across the ultra-thin film from HRTEM micrographs. A Wiener filter was used to remove the electron beam broadening effect from nanobeam EDX data to improve the spatial resolution of composition profiles. A combination of these two techniques (lattice variation measurement and the deconvolution of nanobeam EDX) allows any interdiffusion effects between the ultra-thin films in the spin-valve devices to be revealed. The compositional analysis shows that diffusion of Mn and Ni into the Cu/Co bilayer is only observed in the Co/Cu/Co sample. This result indicates that the Ru layer in the CoFe/Cu/CoFe sample might not only act as a spacer of the synthetic antiferromagnet but also behaves as a good diffusion barrier for Ni and Mn elements in the spin valve structure. The diffusion coefficients of constituent elements are simply determined by using the Matano–Boltzmann method from the deconvoluted compositional profiles. Diffusion of Cu in Co and Ni in Co is primarily by the grain boundary mechanism. In the CoFe/Cu/CoFe/Ru/CoFe sample, diffusivity of Cu and Ni is smaller than that in the Co/Cu/Co sample.

Thermal reliability of NiMn-based spin valves

The thermal reliability of NiMn-based top spin valve was studied at sensor temperatures between 200 and 300 °C by furnace annealing and thermal-electrical stressing. Sensor resistance was measured as a function of heating time up to 1000 h. The sensor resistance change (dR/R) vs time (t) curves showed diffusion-like characteristics. Sensor lifetime for 0.5% resistance increase was determined from the Arrhenius plot to be more than 5 years at 200 °C anneal condition. Temperature- independent linear relationship between resistance change and time was observed in both anneal and bias stress conditions in dR/R=0.2%–3% range. It suggests an interface-controlled kinetics for the sensor failure process. Comparison between the reliability results from two types of stress suggests that device degradation is accelerated by the electrical bias when at the same sensor temperature.

Dependence of electromigration-induced failure lifetimes on NiFe thin-film thickness in giant magnetoresistive spin-valve read heads

The physical mechanisms responsible for the dependence of electromigration-induced failure lifetimes on NiFe thin film thickness in giant magnetoresistive spin-valve read heads have been studied to determine the maximum allowable current density. Based on measured median-times-to-failure, a maximum current density of about 1–2×108 A/cm2 was found to be safely used in 3–5 nm thick NiFe films. Grain size analyses using a transmission electron microscopy suggests that the longer lifetimes of thinner films are mainly due to the smaller number of grain boundaries and fewer triple points which result in less atomic flux divergence.

Contact-dependent relfability of spin valve heads

Contact-dependent failure modes were observed in spin valve sensors in nominal current density range of 2-5/spl times/10/sup 7/ A/cm/sup 2/ at 70/spl deg/C ambient. At high bias condition, continually decreased quasistatic test (QST) transfer curve amplitude with negligible resistance change was observed in sensors abutted with Ta contact. In contrast, steady resistance rise with relatively unchanged amplitude was seen in Ta/Au/Ta-contact device prior to catastrophic burnout. Void formation is apparent in the Au layer of the burnout device. Locations of the voids show bias polarity dependency, suggesting an electromigration mechanism in the Au layer.

Resistivity reliability of tunneling magnetoresistive heads

Summary form only given. The electrical reliability of tunneling magnetoresistive (TMR) read heads has been investigated in terms of breakdown voltage[ 11 and electrical open failures[2] under constant biasing-current. The breakdown voltage is strongly dependent on duration of the applied voltage step, and ambient temperature.

Electromigration of magnetic thin films for predicting electrical-failured lifetime of GMR read heads

Summary form only given. Electromigration-induced failures of magnetic thin films have been investigated and analyzed to predict the electrical reliability of giant magnetoresistance (GMR) read heads. Magnetic thin films showed the larger failure lifetime (median time to failure, MTTF) compared to the high conductivity metallic thin films such as Cu and Al thin films under the same constant current biasing conditions. The increase of resistance due to void formation and increase of shunting current or interdiffusion paths due to hillock formation are considered to be serious potential problems in the degradation of the electrical reliability of GMR spin-valve heads.