David A. Horsley

Mapping protein collapse with single-molecule fluorescence and kinetic synchrotron radiation circular dichroism spectroscopy

We have used the combination of single-molecule Förster resonance energy transfer and kinetic synchrotron radiation circular dichroism experiments to probe the conformational ensemble of the collapsed unfolded state of the small cold shock protein CspTm under near-native conditions. This regime is physiologically most relevant but difficult to access experimentally, because the equilibrium signal in ensemble experiments is dominated by folded molecules. Here, we avoid this problem in two ways. One is the use of single-molecule Förster resonance energy transfer, which allows the separation of folded and unfolded subpopulations at equilibrium and provides information on long-range intramolecular distance distributions. From experiments with donor and acceptor chromophores placed at different positions within the chain, we find that the distance distributions in unfolded CspTm agree surprisingly well with a Gaussian chain not only at high concentrations of denaturant, where the polypeptide chain is expanded, but also at low denaturant concentrations, where the chain is collapsed. The second, complementary approach is synchrotron radiation circular dichroism spectroscopy of collapsed unfolded molecules transiently populated with a microfluidic device that enables rapid mixing. The results indicate a β-structure content of the collapsed unfolded state of ≈20% compared with the folded protein. This suggests that collapse can induce secondary structure in an unfolded state without interfering with long-range distance distributions characteristic of a random coil, which were previously found only for highly expanded unfolded proteins.

Batch transfer of microstructures using flip-chip solder bonding

This paper describes a novel method for transfer and assembly of microstructures using sacrificial-layer micromachining and flip-chip bonding. The technique is performed at room temperature (cold weld) and at the back end of the process flow and may thus provide a commercially viable alternative to monolithic integration and costly hybrid packages. The transfer is achieved using break-away tethers and by cold welding (compression bonding) electroplated indium solder bumps to electroplated copper pads. Both high-aspect-ratio MEMS devices as well as surface-micromachined devices have been successfully transferred using this method with no observable misalignment between moving and stationary parts. The maximum tensile and shear stress the solder bond can withstand before failure is measured to be 11/spl plusmn/3 MPa and 9/spl plusmn/1 MPa, respectively. The contact resistance is measured to be of the order of 1.5 m/spl Omega/ for a 65 /spl mu/m/spl times/65 /spl mu/m/spl times/4-/spl mu/m indium bump.

Precision positioning using a microfabricated electrostatic actuator

This paper presents a microfabricated actuator designed for high precision servo-positioning in a magnetic hard disk drive. The device is actuated using electrostatic force generated with parallel-plate capacitive electrodes. The displacement of these electrodes is measured using a dedicated capacitive sensing interface, allowing closed-loop control to be used to extend the servo bandwidth. Using the sensing electronics and a simple phase-lead compensator, a prototype device was used to actuate a 1.6 mg ceramic slider over a 1.2 kHz bandwidth. Using optical position measurements, the same actuator was used to achieve a 2.5 kHz bandwidth.

Three-Axis Lorentz-Force Magnetic Sensor for Electronic Compass Applications

A low-power microelectromechanical-systems (MEMS) three-axis Lorentz-force magnetic sensor is presented. The sensor detects magnetic field in two axes with a single MEMS structure. Three-axis sensing is performed using two perpendicular structures on the same die. The MEMS device is a micromechanical resonator, and sensing is conducted using excitation currents at the devices in-plane and out-of-plane mechanical resonant frequencies which are 20.55 and 46.96 kHz, respectively. A die-level vacuum seal results in in-plane and out-of-plane mechanical quality factors of 1400 and 10000, current, the sensors noise is equivalent to 137 nT/√Hz for the respectively. With 0.58 mW used to provide the two-axis excitation z-axis magnetic field inputs and 444 nT/√Hz for the x-and y-axis fields. For the z-axis field measurements, Brownian noise is the dominant noise component, while the xand y-axis field measurements are limited by the electronic noise in the discrete capacitive-sensing electronics. The major source of offset error is residual motion induced by electrostatic force. The offset is reduced to 14 μT using a dc compensation voltage applied to the MEMS structure to null the electrostatic force. After compensation, the offset stability is 400 nT with a 0.7-s averaging time.

High-bandwidth high-accuracy rotary microactuators for magnetic hard disk drive tracking servos

Reports on the design, fabrication, and testing of an electrostatic microactuator for a magnetic hard disk drive (HDD) tracking servo. The design requirements for a microactuator are investigated. These include high Z-directional stiffness, low in-plane stiffness, high structural aspect ratio, large output force, high area efficiency, low cost, and mass batch production. An area-efficient rotary microactuator design was devised, and microactuators were successfully fabricated using innovative processing technologies. The microactuator has a structural thickness of 40 /spl mu/m with a minimum gap/structure width of approximately 2 /spl mu/m. Its frequency response was measured and it was determined that it can be modeled as a second-order linear system, up to the 26-kHz frequency range. Moreover, the microactuator will enable the design of a servo system that exceeds a 5-kHz servo bandwidth, which is adequate to achieve a track density of more than 25 kilotrack per inch (kTPI). The microactuator/slider assembly was also tested on a spinning disk, with its position controlled by a PID controller using the magnetic position error signal written on the disk. An accuracy of about 0.05 /spl mu/m was observed when the servo controller was turned on. Continuous-time dual-stage servos were designed and simulated using the /spl mu/-synthesis technique. A sequentially designed SISO and a MIMO control design method have been shown to be capable of meeting prescribed uncertainty and performance specifications.

Angular micropositioner for disk drives

Rotary electrostatic microactuators suitable for use in a two-stage servo system for magnetic disk drives have been fabricated using the HexSil process. A 2.6 mm diameter device is shown to be capable of producing 0.17 mN-mm of output torque, corresponding to a predicted actuation bandwidth of 1.6 kHz. The structures are formed from LPCVD polysilicon deposited into deep trenches etched into a silicon mold wafer. Upon release, these structures are transferred to a target wafer using a solder bond. The solder bonding process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be re-used for subsequent polysilicon depositions, amortizing the cost of the deep trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Further, 100 /spl mu/m high structures may be made from a 3 /spl mu/m deposition of polysilicon, increasing overall fabrication speed.

Design and fabrication of an angular microactuator for magnetic disk drives

Angular electrostatic microactuators suitable for use in a two-stage servo system for magnetic disk drives have been fabricated from molded chemical-vapor-deposited (CVD) polysilicon using the HexSil process. A 2.6-mm-diameter device has been shown to be capable of positioning the read/write elements of a 30% picoslider over a /spl plusmn/1-/spl mu/m range, with a predicted bandwidth of 2 kHz. The structures are formed by depositing polysilicon via CVD into deep trenches etched into a silicon mold wafer. Upon release, the actuators are assembled onto a target wafer using a solder bond. The solder-bonding process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be reused for subsequent polysilicon depositions, amortizing the cost of the deep-trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Furthermore, 100-/spl mu/m-high structures may be made from a 3-/spl mu/m deposition of polysilicon, increasing overall fabrication speed.

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

Batch transfer of microstructures using flip-chip solder bump bonding

This paper describes a novel method for transfer and assembly of microstructures using sacrificial-layer micromachining and flip-chip bonding. The technique is performed at room temperature (cold weld) and at the back end of the process flow, and may thus provide a commercially viable alternative to monolithic integration and costly hybrid packages. The transfer is achieved using break-away tethers and by cold welding electroplated indium solder bumps to thick electroplated copper pads. Both high aspect ratio MEMS devices as well as surface micromachined devices have been successfully transferred using this method with no observable misalignment between moving and stationary parts. The ultimate tensile and shear strength of the solder bond is measured to be 11/spl plusmn/3 MPa and 9/spl plusmn/1 MPa respectively. The contact resistance is measured to be of the order of 1.5 m/spl Omega/ for a 65 /spl mu/m/spl times/65 /spl mu/m/spl times/4 /spl mu/m indium bump.

In-Air Rangefinding With an AlN Piezoelectric Micromachined Ultrasound Transducer

An ultrasonic rangefinder has a working range of 30 to 450 mm and operates at a 375-Hz maximum sampling rate. The random noise increases with distance and equals 1.3 mm at the maximum range. The range measurement principle is based on pulse-echo time-of-flight measurement using a single transducer for transmit and receive. The transducer consists of a piezoelectric AlN membrane with 400-μm diameter, which was fabricated using a low-temperature process compatible with processed CMOS wafers. The performance of the system exceeds the performance of other micromechanical rangefinders.