James A. Bain

Single-chip computers with microelectromechanical systems-based magnetic memory (invited)

This article describes an approach for implementing a complete computer system (CPU, RAM, I/O, and nonvolatile mass memory) on a single integrated-circuit substrate (a chip)—hence, the name “single-chip computer.” The approach presented combines advances in the field of microelectromechanical systems (MEMS) and micromagnetics with traditional low-cost very-large-scale integrated circuit style parallel lithographic manufacturing. The primary barrier to the creation of a computer on a chip is the incorporation of a high-capacity [many gigabytes (GB)] re-writable nonvolatile memory (in today’s terminology, a disk drive) into an integrated circuit (IC) manufacturing process. This article presents the following design example: a MEMS-based magnetic memory that can store over 2 GB of data in 2 cm2 of die area and whose fabrication is compatible with a standard IC manufacturing process.

Stress Determination in Textured Thin Films Using X-Ray Diffraction

Thin film stresses are important in many areas of technology. In the semiconductor industry, metal interconnects are prone to stress voiding and hillock formation. Stresses in passivation layers can lead to excessive substrate curvature which can cause alignment difficulty in subsequent lithographic processing. In other thin film applications, stresses can cause peeling from mechanical failure at the film-substrate interface. Beyond these issues of reliability, stress and the resulting strain can be used to tune the properties of thin film materials. For instance, strain, coupled with the magnetostrictive effect, can be utilized to induce the preferred magnetization direction. Also, epitaxial strains can be used to adjust the bandgap of semiconductors. Finally, the anomalous mechanical properties of multilayered materials are thought to be partially due to the extreme strain states in the constituents of these materials. To fully optimize thin film performance, a fundamental understanding of the causes and effects of thin film stress is needed. These studies in turn rely on detailed characterization of the stress and strain state of thin films. X-ray diffraction and the elastic response of materials provide a powerful method for determining stresses. Stresses alter the spacing of crystallographic planes in crystals by amounts easily measured by x-ray diffraction. Each set of crystal planes can act as an in-situ strain gauge, which can be probed by x-ray diffraction in the appropriate geometry. Hence it is not surprising that x-ray diffraction is one of the most widely used techniques for determining stress and strain in materials. (For reviews of this topic, see References 5–7.) This article is a tutorial on the use of x-ray diffraction to extract the stress state and the unstrained lattice parameter from thin films. We present a handbook of useful results that can be widely applied and should be mastered by anyone seriously interested in stresses in crystalline thin films with a crystallographic growth texture.

Influence of stress and texture on soft magnetic properties of thin films

This paper presents the relationship among stress, crystallographic texture, and soft magnetic properties of thin films. The magnetic properties considered are those affected by the formation of stripe domains and by the formation of magnetization ripple. In practice, one of these two undesirable domain structures is almost always the impediment to improving the soft magnetic properties. The theoretical analysis accounts for the contribution from stress through magnetostrictive anisotropy, and calculates the resulting total, stress-dependent anisotropy of the film. This anisotropy is then analyzed to yield the effective perpendicular and local in-plane anisotropy constants. These constants allow the calculation of the stripe domain onset thickness through Murayamas stripe domain theory, and the ripple coercivity through Hoffmanns ripple theory. The influence of stress and texture on the stripe domain onset thickness and ripple coercivity is theoretically calculated and experimentally verified for the examples of sputtered CoFe and FeAlN films, two of the most promising and widely studied materials for high-density recording head poles. The results indicate that the interactions between stress and soft magnetic properties depend on the details of composition, growth texture, growth morphology (whether the film is grown in columnar fashion or as equiaxial crystallites), grain size, and thickness of the film under consideration. On the basis of these results, the paper offers a systematic approach to choosing the appropriate composition, texture, and thickness so that low stresses simultaneously promote magnetic states in which stripe domains are suppressed and coercivities are low. The calculations presented here can be easily extended to other aspects of soft magnetic properties addressed by the ripple theory, such as initial permeability.

Mobility of oxygen vacancy in SrTiO3 and its implications for oxygen-migration-based resistance switching

Capacitance−voltage characteristics of high quality Pt Schottky diodes fabricated on oxygen-vacancy-doped SrTiO3 single crystals were used to obtain the oxygen vacancy profiles within one microns of the Pt interface. Computer simulations based on solving the drift-diffusion equations for electrons and ionized vacancies were performed to understand the experimentally observed oxygen vacancy profile’s time-evolution at room temperature and 0 V applied bias. Building upon this understanding, the diode’s room temperature profile evolution under −35 V applied bias was analyzed to yield a vacancy mobility value of 1.5 × 10−13 cm2/V·s at an electric field of 500 kV/cm. This mobility is 8 orders of magnitude too low to produce nanosecond resistance switching in thin film devices. The applicability of the results to oxygen-migration-based resistance switching is discussed relative to recent observations and modeling.

Imaging of optical field confinement in ridge waveguides fabricated on very-small aperture laser

Optical field confinement in a ridge waveguide nanostructure (“C” aperture) designed for ultrahigh-density recording was observed using an apertureless near-field scanning optical microscope. The aperture was fabricated on a commercial edge-emitting semiconductor laser as the light source. High-contrast near-field images at both 1× and 2× lock-in detection frequencies were obtained. The emission patterns are in agreement with theoretical simulation of such structures. A 90 nm×70 nm full width half maximum spot size was measured and is comparable to the ridge width of the aperture.

Ridge waveguide as a near-field optical source

We investigate the feasibility of using a ridge waveguide at optical frequencies as a near-field optical transducer, using the finite difference time domain method. The complete electromagnetic field picture of the ridge waveguide, in the absence and presence of the irradiated medium, is presented. A power efficiency of 7% and an optical spot with full width half maximum of 50 nm×80 nm is obtained in the medium. We show that impedance considerations play a major role in the transducer-medium optical coupling.

An Integrated Read/Write Head for Hybrid Recording

A proposed integrated read/write head for hybrid recording is described. This design features a thin film waveguide to deliver light to an aperture. This aperture is described in terms of the propagating modes of a ridge waveguide with a reduced cutoff frequency. Also included in this integrated head is a geometry that maintains the necessary coincidence of the optical and magnetic fields. The essential features of this design are very compatible with existing fabrication methods.

Considerations In The Design Of Probe Heads For 100 Gbit/in/sup 2/ Recording Density

Thin film perpendicular playback yoke-GMR heads capable of reading at a density of 100 Gbit/in/sup 2/ have been designed using 3D boundary element reciprocity modeling. Two basic head types are considered, a single-pole and a ring-type geometry. The influence on the playback voltage and D/sub 50/ of the throat height, fly height, and permeability are studied. Isolated pulse response, roll-off curves, and off-track profiles are calculated. Media with and without a soft underlayer are also considered. The simulations show that of the designs studied the ring head in combination with the media without a soft underlayer has playback properties suitable for this density.

Imaging of quantized magnetostatic modes using spatially resolved ferromagnetic resonance

We present a measurement technique for performing spatially resolved ferromagnetic resonance and directly imaging quantized magnetostatic modes in magnetic samples that undergo high frequency magnetic drive fields (up to 8 GHz). The dynamic response of a 50×50 μm2 permalloy structure (100 nm thick) under a 7.04 GHz highly nonuniform drive field was measured as a function of the dc bias field using this technique. The magnetization variation observed indicates that quantized magnetostatic mode waves appear at certain bias fields, with the number of nodes decreasing with an increase in the bias field. We tentatively assign the indices of each mode using the Damon–Eshbach (DE) model. Similar modes have been observed for a similar sample geometry using an inductive measurement and they showed good agreement with the DE model. However, the result measured using this technique showed some discrepancy with the DE model and the spatial patterns observed are more complicated than simple one-dimensional standing wa...

Tunable on-chip inductors up to 5 GHz using patterned permalloy laminations

This paper presents the design criteria, processing techniques, and measurement results for an on-chip inductor that uses a permalloy (Ni80Fe20) film to achieve a tunable reactance. Eddy currents and ferromagnetic resonance are suppressed in the thin permalloy film using patterning and laminations to enable operation in the RF range. Measurements results show a 40% increase in inductance, a 15% tuning range, and a Q between 5 and 11 up to 5 GHz