Mark W. Jenkins

A simple ionizing radiation spectrometer/dosimeter based on radiation sensing field effect transistors (RadFETs)

This paper reports on the processing steps in a silicon foundry leading to improved performance of the Radiation Sensing Field Effect Transistor (RadFET) and the use of multiple RadFETs in a handheld, battery operated, combination spectrometer/dosimeter.

Accelerating aging failures in MEMS devices

The feasibility of using temperature and humidity to age vapor-deposited SAM-coated electrostatic-actuated MEMS devices with contacting surfaces was determined. Failures were dependent on both temperature and humidity. The trend indicated longer life at both lower temperatures and lower humidity levels. Using cantilever beams, measurements reveal degradation of the VSAM (vapor-deposited self assembled monolayer) surface coating when stressed at 300/spl deg/C with controlled humidity environments of 500 and 2000 ppmv. In particular, we have seen the surface adhesion change for these beams stressed at 300/spl deg/C for time intervals of 10, 24, 50, 100, and 200 hours. However, there is no measurable change after 2 hours. The higher humidity case promotes the same surface adhesion change in a factor of ten less time. The complex MEMS devices tested followed the same trends as the beam test structures. We definitely observe a failure of the MEMS devices due to the environment with most failures occurring at 300/spl deg/C and some failures at 200/spl deg/C. These failures are due to an adhesion site in the hub of the load gear where the typical gap is 0.3 /spl mu/m.

Viologen polymer-coated impedance sensors for midrange humidity levels and other volatile organic compounds

The authors have demonstrated a sensor based on an interdigitated electrode platform coated with a conductive viologen polymer that has excellent moisture sensitivity, response time, and low power consumption. Work has been done to explain the limits of sensitivity and characterize the response of the sensor in low-humidity environments and also characterize the response of the device to various organic interferents such as alcohols and organophosphonates. Using lumped-circuit models, the nature of the response is explained and design parameters are isolated that will allow future performance improvements. In addition, the temperature dependence of moisture sensitivity is presented.

Sensitivity of piezoresistive readout device for microfabricated acoustic spectrum analyzer

A readout mechanism has been developed for measuring the response of mechanical microresonators to be used in an array for a microfabricated acoustic spectrum analyzer. It is based on the piezo-resistive property of polysilicon. The piezo-resistive readout mechanism is constructed in a quarter Wheatstone bridge fashion in which four equal serpentine polysilicon patterns are fabricated on top of a dielectric layer of silicon nitride. Microresonator devices using cantilever and clamped-clamped beam types with piezo-resistive readout mechanisms are fabricated using the surface micromachining technology of SUMMiTTM. The sensitivity of the piezo-resistive mechanism is characterized using 10 volts as supply on the Wheatstone bridge and no amplification of signal. The testing is conducted with electrostatic drive potentials 0-75 volts. Sensitivity of 1-5 millivolts per micron of beam deflection was observed by the characterization.

Packaging Dissimilar Materials for Microfluidic Applications

In this paper we address the problem of assembling several different microfluidic devices, often made from different materials, into a hybrid packaged system. The focus will be on integration of silicon microfluidic die into larger hybrid systems. Details of three different approaches are presented: 1) a plastic flow manifold with tape die attach, 2) multiple capillary die insertion utilizing PDMS (Polydimethylsiloxane – silicone adhesive) for sealing and structural support, and 3) die attachment to a glass flow manifold utilizing anodic bonding. The unique tools required for each of these three techniques will be described. Sealed microfluidic connections (>10 ATM pressure) between silicon microfluidic chips (die) and flow manifolds are demonstrated.Copyright

Dry actuation testing of viscous drag micropumping systems for determination of optimal drive waveforms

This paper presents the dry actuation testing procedures and results for novel viscous drag micropumping systems. To overcome the limitations of previously developed mechanical pumps, we have developed pumps that are surface micromachined for efficient mass production which utilize viscous drag (dominant at low Reynolds numbers typical of microfluidics) to move fluid. The SUMMiT (www.sandia.gov/micromachine) fabricated pumps, presented first by Kilani et al., are being experimentally and computationally analyzed. In this paper we will describe the development of optimal waveforms to drive the electrostatic pumping mechanism while dry. While wet actuation will be significantly different, dry testing provides insight into how to optimally move the mechanism and differences between dry and wet actuation can be used to isolate fluid effects. Characterization began with an analysis of the driving voltage waveforms for the torsional ratcheting actuator (TRA), a micro-motor that drove the gear transmission for the pump, actuated with SAMA (Sandia’s Arbitrary waveform MEMS Actuator), a new waveform generating computer program with the ability to generate and output arbitrary voltage signals. Based upon previous research, a 50% duty cycle half-sine wave was initially selected for actuation of the TRA. However, due to the geometry of the half-sine waveform, the loaded micromotor could not transmit the motion required to pump the tested liquids. Six waveforms were then conceived, constructed, and selected for device actuation testing. Dry actuation tests included high voltage, low voltage, high frequency, and endurance/reliability testing of the TRA, gear transmission and pump assembly. In the SUMMiT process, all of the components of the system are fabricated together on one silicon chip already assembled in a monolithic microfabrication process. A 40% duty cycle quarter-sine waveform with a 20% DC at 60V has currently proved to be the most reliable, allowing for an 825Hz continuous TRA operating frequency for the micropumps. This novel waveform allowed for higher TRA actuation frequencies than those obtained in prior research of the pumps.

Precision Laser Annealing of Focal Plane Arrays

We present results from laser annealing experiments in Si using a passively Q-switched Nd:YAG microlaser. Exposure with laser at fluence values above the damage threshold of commercially available photodiodes results in electrical damage (as measured by an increase in photodiode dark current). We show that increasing the laser fluence to values in excess of the damage threshold can result in annealing of a damage site and a reduction in detector dark current by as much as 100x in some cases. A still further increase in fluence results in irreparable damage. Thus we demonstrate the presence of a laser annealing window over which performance of damaged detectors can be at least partially reconstituted. Moreover dark current reduction is observed over the entire operating range of the diode indicating that device performance has been improved for all values of reverse bias voltage. Additionally, we will present results of laser annealing in Si waveguides. By exposing a small (<10 um) length of a Si waveguide to an annealing laser pulse, the longitudinal phase of light acquired in propagating through the waveguide can be modified with high precision, <15 milliradian per laser pulse. Phase tuning by 180 degrees is exhibited with multiple exposures to one arm of a Mach-Zehnder interferometer at fluence values below the morphological damage threshold of an etched Si waveguide. No reduction in optical transmission at 1550 nm was found after 220 annealing laser shots. Modeling results for laser annealing in Si are also presented.

Precision laser annealing of silicon devices for enhanced electro-optic performance

We present results from laser annealing experiments in Si using a passively Q-switched Nd:YAG microlaser. Exposure with laser at fluence values above the damage threshold of commercially available photodiodes results in electrical damage (as measured by an increase in photodiode dark current). We show that increasing the laser fluence to values in excess of the damage threshold can result in annealing of a damage site and a reduction in detector dark current by as much as 100x in some cases. A still further increase in fluence results in irreparable damage. Thus we demonstrate the presence of a laser annealing window over which performance of damaged detectors can be at least partially reconstituted. Moreover dark current reduction is observed over the entire operating range of the diode indicating that device performance has been improved for all values of reverse bias voltage. Additionally, we will present results of laser annealing in Si waveguides. By exposing a small (<10 um) length of a Si waveguide to an annealing laser pulse, the longitudinal phase of light acquired in propagating through the waveguide can be modified with high precision, <15 milliradian per laser pulse. Phase tuning by 180 degrees is exhibited with multiple exposures to one arm of a Mach-Zehnder interferometer at fluence values below the morphological damage threshold of an etched Si waveguide. No reduction in optical transmission at 1550 nm was found after 220 annealing laser shots.

Stabilizing the response of Pd/Ni alloy films to hydrogen with Ti adhesion layers

We have studied catalytic thin film resistors made from a Pd and Ni alloy, and propose a method for dramatically reducing the drift of the measured resistance. The resistances of Pd films increase monotonically when exposed to hydrogen, however a stable baseline is difficult to achieve and alpha to beta phase transitions result in hysteresis. It is known that at high hydrogen concentrations, the Pd film cracks and delaminates, however long-term exposures to low concentrations of hydrogen can also result in delaminations. Studies using Pd/Ni alloys show that the phase transition can be suppressed. High temperature anneals in 2 % hydrogen, and the addition of a Ti adhesion layer is shown to reduce drift. Usually long term studies on films are conducted in an ordinary air (oxidizing) atmosphere, however, we report here on studies carried out in a reducing atmosphere of 0.1% hydrogen in nitrogen for 6 months on two sensor structures, field effect transistors (FETs) and resistors. The Sandia Robust Hydrogen Sensor platform containing integrated heaters, temperature sensors, and hydrogen sensitive resistors and FETs was compared to a Sandia Wide Range Sensor containing a 10 atomic percent Ni/Pd (1000A) alloy resistor with a (1000A) Ti adhesion layer. After six months the two hydrogen sensing resistors on the Robust platform, without an adhesion layer, read a hydrogen concentration of 61% and 2.3%, while the Wide Range Sensor read a hydrogen concentration of 0.102%, which is a dramatic improvement in limiting baseline drift.

Long term drift studies of Sandia H{sub 2} sensor in reducing atmospheres

A study of the drift in Pd/Ni alloy hydrogen sensitive resistor and transistor responses is presented. The sensors were monitored for a period of 6 months in a reducing atmosphere of 0.1% H{sub 2} in N{sub 2} with periodic calibration exposures. A comparison of a resistor film with an adhesion layer showed considerable improvement in diminishing the drift.