Christine K. Eun

Exploring microdischarges for portable sensing applications

AbstractThis paper describes the use of microdischarges as transducing elements in sensors and detectors. Chemical and physical sensing of gases, chemical sensing of liquids, and radiation detection are described. These applications are explored from the perspective of their use in portable microsystems, with emphasis on compactness, power consumption, the ability to operate at or near atmospheric pressure (to reduce pumping challenges), and the ability to operate in an air ambient (to reduce the need for reservoirs of carrier gases). Manufacturing methods and performance results are described for selected examples. FigureSide-view photograph of an ultraviolet light source that uses microdischarges

A microfabricated steel and glass radiation detector with inherent wireless signaling

This paper describes an investigation of the performance compromises imposed by a manufacturing approach that utilizes lithographic micromachining processes to fabricate a wireless beta/gamma radiation detector. The device uses in-package assembly of stainless steel electrodes and glass spacers. These elements are micromachined using photochemical etching and powder blasting, respectively. The detector utilizes a commercial, TO-5 package that is hermetically sealed at 760 Torr with an Ar fill-gas. Gas microdischarges between the electrodes, which are initiated by the radiation, transmit wideband wireless signals. The detector diameter and height are 9 and 9.6 mm, respectively, and it weighs 0.97 g. The device performance has been characterized using various sealed, radioisotope sources, e.g., 30‐99 μCi from 137 Cs (which is a beta and gamma emitter) and 0.1 μCi from 90 Sr (which is a pure beta emitter). It has a measured output of >15.5 counts s −1 when in close proximity to 99 μCi from 137 Cs. The wireless signaling spans 1.25 GHz at receiving antenna-to-detector distances >89 cm, when in close proximity to a 0.1 μCi 90 Sr source. The estimated intrinsic detection efficiency (i.e. with the background rate subtracted) is 3.34% as measured with the biasing arrangement described in the paper. (Some figures in this article are in color only in the electronic version)

A Wireless-Enabled Microdischarge-Based Radiation Detector Utilizing Stacked Electrode Arrays for Enhanced Detection Efficiency

This paper describes a wireless gas-based beta/ gamma radiation detector that uses an arrayed electrode structure to demonstrate a scalable path for increasing detection efficiency. The device uses an assembly of stainless-steel electrodes and a glass spacer structure within a TO-5 package. The components are manufactured by commercial micromachining methods, e.g., the electrodes are photochemically etched whereas the spacer structure is ultrasonically machined. Two different fill gases are evaluated near 760 torr-i.e., Ar and P-10. The detector diameter and height are 9 and 9.6 mm, respectively, and its weight is 1.01 g. With a 99-μCi Cs-137 source (which is a beta and gamma emitter), the detector provides >; 78 cpm to a hardwired inter face at a source-detector distance of 30.5 cm. Receiver operating characteristics evaluated for integration times ranging from 30 to 180 s have shown to improve with longer integration time. The estimated intrinsic detection efficiency (i.e., with the back ground rate subtracted) is ≈4%, as measured with the biasing arrangement described in this paper. Portable powering modules developed for these detectors are also presented. During operation, gas microdischarges between the electrodes, which are initiated by incident radiation, transmit wideband wireless signals. Wireless signaling has been demonstrated to exhibit fast transient durations on the order of tens of nanoseconds. Wireless-enabled radiation sensors are envisioned for use in rapidly deployable mobile net work configurations.

Wireless Signaling of Beta Detection Using Microdischarges

This paper explores the possibility of using microdischarges to generate broadband radio-frequency (RF) signaling from gas-based microdetectors of beta radiation. The concept is evaluated using two types of lithographically manufactured test structures: 1) a silicon/glass stack with etched detection cavities and 2) a planar metal-on-glass structure. The test structures include electrodes that bias a fill-gas region with a high electric field, in which incident beta particles initiate avalanche-driven microdischarge pulses that inherently transmit RF spectra with frequency content extending into the ultrawideband (UWB) range of communication. The discharge gaps range from 165 to 500 μm. The impact of operating pressure, fill gases (which are typically a mixture of Ne and N2), and electrode materials (Ni and Cu) on operating voltage and wireless signaling performance is evaluated. Tests are performed in the proximity of weak (0.1-1.0-μCi) beta sources (90Sr and 204TI). Both types of test structures are capable of producing UWB signals spanning > 1 GHz. Measurements in an anechoic chamber using various receiver antennas show that microdischarges can produce field strengths up to 90 dB · μV/m measured at 1.67 m from the test structure.

A bulk silicon micromachined structure for gas microdischarge-based detection of beta-particles

This paper describes two Si micromachined structures for sensing beta-particles. The basic device (the micro-detector) includes a square silicon cathode surrounded by a concentric anode, and is formed by stacks of glass and Si wafers. Incident beta-particles ionize the gas encapsulated between the electrodes, resulting in an avalanche current pulse or ‘count’. It is shown experimentally that devices with 8 × 8m m 2 footprint can detect radiation in the proximity of sealed sources, such as 90 Sr and 204 Tl with 0.1‐1.0 µCi strengths. The sensitivity (cpm mRad −1 h) of the micromachined device is comparable to that of commercial radiation detectors, but substantially superior when normalized to the detector volume, which is about 0.06% of the conventional detectors. An extension of the basic device, the stacked micro-detector, consists of a two-tiered arrangement of cavities separated by a thin glass intercavity attenuator that is intended to provide controlled energy absorption. Higher energy particles are detected in both cavities, while lower energy particles are detected in the first cavity and subsequently absorbed by the intercavity attenuator. This can provide initial assessment of the incident radiation without adding significant complexity to the system. Preliminary experimental validation is performed by comparing the device response to 204 Tl and 90 Sr. (Some figures in this article are in colour only in the electronic version)

Broadband wireless sensing of radioactive chemicals utilizing inherent RF transmissions from pulse discharges

This paper reports a wireless sensing scheme that exploits gas discharges in microstructures and discharge-based sensors such as micromachined Geiger counters. Experiments are conducted on devices that have a glass-Si-glass stack of 8 times 8 mm2 footprint, with discharge gaps in the range of 300-550 mum. Electrical discharges triggered by exceeding the breakdown voltage and by ionization due to beta particles provide an RF spectrum spanning a bandwidth greater than 1.2 GHz, which extends into the ultra-wideband (UWB) range of communication. These are broadband signals that can be detected by AM and FM radios at distances greater than 50 cm from the sensor. Measurements of electric field strength and audio recordings from radio receivers are reported

Exploring RF Transmissions From Discharge-Based Micromachined Radiation Detectors

This paper describes micromachined gas-based radiation sensors that are capable of radio frequency wireless signaling, and their possible utility in networks. The devices include a gas-filled region with a high electric field, in which incident beta-particles initiate avalanche breakdown. Under the proper circumstances, the resulting current pulses can inherently produce wireless transmissions. Two types of lithographically-manufactured devices are presented: (1) a silicon/glass stack with etched detection cavities and (2) a planar, 3-electrode metal-on- glass structure that uses a high-impedance electrode for increased control of discharge energy. Both are capable of producing ultra-wideband signals spanning >2.9 GHz. Permanent magnets (integrated with both structures) can enhance the RF performance by ap15-20 dBmuV. The impact of operating pressure, fill-gases (which are typically a mixture of Ne and N2) and electrode materials (Ni, Cu) on device performance is described. Tests performed in the proximity of weak (0.1-1.0 muCi) beta sources (90Sr, 204Tl), show that a 25% decrease in pressure permits a 55% decrease in operating voltage. Increasing Ne content in the fill-gas (e.g. Ne-to-N2 ratio from 1:5 to 3:5) decreases the minimum operating voltage by 200 V without loss in RF performance. Ni electrodes, which have a higher secondary electron emission coefficient than Cu, provide 30% more overall signal power.

Controlling Ultra Wide Band Transmissions from a Wireless Micromachined Geiger Counter

This paper reports a parametric study of the wireless spectrum generated by discharge-based devices with focus specifically on micromachined Geiger counters as a function of the packaging, biasing circuitry, and sample isotope. Experiments are conducted with discharge devices attached to commercial high voltage (HV) packages with hermetic sealing capabilities. Influences of packaging as well as isotope type are studied and reported. Preliminary results show that current discharges emit RF spectra spanning a bandwidth greater than 3 GHz, which extends into the ultra wideband (UWB) window (from 100 MHZ to 10.6 GHz) that decreases in intensity with increasing observer distance.

Fabrication of a monolithic microdischarge-based pressure sensor for harsh environments

This paper presents a 6-mask monolithic fabrication process for a pressure sensor that uses a differential microdischarge signal to sense diaphragm deflection. Microdischarge-based transduction is potentially advantageous for device miniaturization and harsh environments because of inherently large signals and immunity to temperature. This work reports a monolithic fabrication process that successfully addresses a number of challenges for microdischarge-based pressure sensors, such as three-dimensional (3D) electrical connection by electroplating laser-drilled through-glass vias (TGVs), and backside terminals for appropriate packages. The device has an exterior volume of 585×540×200 μm3 (0.05 mm3). Preliminary results show an estimated average sensitivity equivalent to 9,800 ppm/MPa over 0-40 MPa pressure range.

A Microdischarge-Based Monolithic Pressure Sensor

This paper describes the investigation of a microdischarge-based approach for sensing the diaphragm deflection in a monolithically fabricated pressure sensor. This transduction approach is appealing from the viewpoint of miniaturization. The device consists of a deflecting Si diaphragm with a sensing cathode, and a glass substrate with an anode and a reference cathode. The total exterior volume of the device is 0.05 mm3; typical electrode size and separations are 35 and 10 mm3. Pulsed microdischarges are initiated in a sealed chamber formed between Si and glass chips, and are filled with Ar gas. External pressure deflects the Si diaphragm and changes the interelectrode spacing, thereby redistributing the current between the anode and two competing cathodes. The differential current is indicative of the diaphragm deflection which is determined by the external pressure. A 6-mask microfabrication process is investigated for device fabrication. Electrode connections to the interior of the chamber are provided by laser drilling and copper electroplating through high aspect ratio glass vias. The Si and glass substrates are bonded by Au-In eutectic. The redistribution of plasma current between competing cathodes, as a consequence of diaphragm deflection over a range of pressure, was experimentally demonstrated. First principles modeling of transient microdischarges have provided insights to the fundamental processes responsible for the differential current and guidance for scaling the device to smaller dimension.