Robert G. Walmsley

Noise properties of multilayered Co-alloy magnetic recording media

The results of the medium-noise characterization of multilayered magnetic recording films having thin nonmagnetic isolation layers are presented. A substantial reduction of all types of medium noise was found for several Co alloys when multilayered. This reduction in noise power is approximately proportional to 1/(number of magnetic layers) but also depends on film thickness and other factors. The primary cause of the reduction is evidently the breaking of exchange between magnetic layers. It is also found that the intrinsic noise power per unit varies with film thickness, more in some alloys than in others, altering the noise reduction for multilayers. Evidence also suggests that magnetostatic interactions between layers further reduce the noise power. A phenomenological model explaining the observed effects is presented. The evidence suggests that a similar effect would be observed in any metal-film recording medium. >

Sub-10 nm nanoimprint lithography by wafer bowing.

We introduce the concept of wafer bowing to affect nanoimprinting. This approach allows a design that can fit the key imprinting mechanism into a compact module, which we have constructed and demonstrated with an overlay and resolution of <0.5 microm and <10 nm, respectively. In the short term, this wafer bowing approach makes nanoimprint lithography much more accessible to a broad range of researchers. More importantly, this approach eliminates machine movement other than wafer bowing and shortens the mechanical path; these will enable the achievement of excellent patterning and overlay at a much lower cost. In the long term, wafer bowing is extensible to step-and-repeat printing for volume manufacturing.

Gaseous wear products from perfluoropolyether lubricant films

Abstract Degradation products of perfluoropolyether lubricants are observed during friction tests on thin film magnetic recording disks using a mass spectrometer. The friction tests are conducted in high vacuum using thin film read-write heads as sliders. Fomblin Z-dol and Krytox 143AX are used as lubricant. Observed removal rates are significant compared with the lubricant film thickness used. The data suggest that the degradation reaction is activated by low energy electrons produced at the tribological interface.

Hewlett packard's seismic grade MEMS accelerometer

HP has recently made its plans to develop an ultrahigh-resolution seismic sensing solution public. This solution is being developed with Royal Dutch Shell Corporation for oil and gas exploration [1]. Central to delivering this system is HPs new single axis seismic grade MEMS accelerometer. The HP device uses both bulk micromachining methods and standard thin film technologies as well as HPs new three phase sensing technology. This sensor detects motion when an array of electrodes located on the proof mass moves relative to a stationary array of electrodes across a fixed gap. Initial characterization shows a flat noise power spectral density of &#60; 100 nG/vHz over a bandwidth of DC-200 Hz. The device exhibits a linear response to +/− 150mG with a sensitivity of over 25 V/g within the specified bandwidth.

An ultra-low noise MEMS accelerometer for seismic imaging

A new MEMS capacitive accelerometer has been developed to meet the requirements for oil and gas exploration, specifically for imaging deep and complex subterranean features. The sensor has been optimized to have a very low noise floor in a frequency range of 1–200 Hz. Several design and process parameters were modified from our previous sensors to reduce noise. Testing of the sensor has demonstrated a noise floor of 10ng/√Hz, in agreement with our predictive noise models. The sensor has a dynamic range of 120db with a maximum acceleration of +/− 80mg. In addition to the performance specifications, automated calibration routines have been implemented, allowing bias and sensitivity calibrations to be done in the field to ensure valid and accurate data. The sensor frequency and quality factor can also be measured in the field for an automated sensor health check.

Thermal variations in switching fields for sub-micron MRAM cells

Thermal effects in switching of sub-micron tunnel junctions are investigated. The switching field is shown to he inversely dependent on temperature, and switching field jitter is shown to be strong function of temperature. Micromagnetic modeling is used to understand thermal effects. In some instances a stability factor (defined as KV/kT) of 100 or more may be required for acceptable switching field jitter, while with proper optimization of FM layers, stability factors of 50 or 60 may suffice.

Micro-G silicon accelerometer using surface electrodes

We present a new technology platform for silicon inertial sensors. The platform combines three technology features to set new performance and manufacturability standards for MEMS sensors. First, bonding three silicon wafers creates wafer level packaging and a homogenous stack of silicon material improving device temperature stability. Second, through-wafer etching is used to define the mechanical structure creating a proof mass with 1000x larger mass than a typical MEMS sensor. Finally, we use surface electrode technology to create a lateral capacitance-based transducer enabling large capacitance change per acceleration and allowing a large dynamic range without electrode contact. The large mass together with reduced damping of a lateral sensor result in substantially reduced thermal-mechanical noise. We present a two axis, in-plane, MEMS accelerometer having nG/√Hz noise performance, over 130 dB dynamic range, 300 Hz bandwidth, and a chip size comparable to other MEMS accelerometers. The platform is extensible to gyroscopes and single chip IMU.

Gaseous wear products from lubricated thin film disks

Degradation products of perfluoropolyether lubricants were observed during friction tests on thin film magnetic recording disks using a mass spectrometer. The friction tests were conducted in high vacuum using thin-film read/write heads as sliders. Fomblin Z-dol and Krytox 143AX were used as lubricants. The removal rates observed are significant compared to the lubricant film thickness used. The data suggest that the degradation reaction is activated by low-energy electrons produced at the tribological interface. >

Tribochemical wear of carbon films in oxygen

Gases sampled directly from the sliding interface of a carbon‐coated thin‐film magnetic recording disk and a conventional read/write head show that carbon dioxide is generated during sliding in both dry‐nitrogen and dry‐oxygen environments. The generation rate is approximately ten times greater when sliding in oxygen rather than nitrogen. A novel pressure interface and sampling system was developed for the experiments.

Integrated position sensing for control of XY actuator

We present an integrated capacitance based sensor and electronics system for measuring the position of a two axis microelectromechanical (MEMS) stage. Integrated position sensing enables closed-loop operation of the stage, greatly increasing system performance compared to previous micro-stages. The 2 mm square stage is fabricated in a bonded wafer process and is actuated using a bipolar, electrostatic surface drive. Sensor and actuator electrodes are positioned on the bottom of the stage increasing the available moving area of the stage to 50% of the chip size. Individual sensors for each axis and high aspect ratio flexures minimize sensor crosstalk from in-plane motion. The sensor electronics rejects signals from out-of-plane motion of the stage or sensors. The stage is capable of 50 /spl mu/m of travel in each axis with maxima for velocity and acceleration of 40 mm/s and 200 m/s/sup 2/, respectively. Sensor resolution has been measured at 2 /spl Aring/ in a 10 Hz bandwidth, giving the system near part-per-million position resolution.