John L. Ebel

Wet Chemical Digital Etching of GaAs at Room Temperature

A new room temperature wet chemical digital etching technique for GaAs is presented which uses hydrogen peroxide and an acid in a two‐step etching process to remove GaAs in approximately 15 A increments. In the first step, GaAs is oxidized by 30% hydrogen peroxide to form an oxide layer that is diffusion limited to a thickness of 14 to 17 A for time periods from 15 to 120 s. The second step removes this oxide layer with an acid that does not attack unoxidized GaAs. These steps are repeated in succession until the desired etch depth is obtained. Experimental results are presented for this digital etching technique demonstrating the etch rate and process invariability with respect to hydrogen peroxide and acid exposure times.

Reconfigurable UWB Antenna With RF-MEMS for On-Demand WLAN Rejection

A MEMS reconfigurable ultra-wideband (UWB) antenna that rejects on-demand all WLAN signals in the entire 5.15 to 5.825 GHz range (675 MHz bandwidth) is presented. The antenna design, miniaturization procedure, and monolithic integration with the MEMS and biasing network on SiO2 Quartz substrate are described. The integration challenges are addressed and the work is presented in a way that is useful for antenna engineers. A method to vary the rejection bandwidth is also provided. The fabricated prototype is conformal and single-sided. The antenna is measured using a custom-built platform at a university laboratory. Results indicate a successful integration and minimal interference of the MEMS and biasing circuitry with the antenna, paving the road for more integrated reconfigurable antennas on SiO2 using MEMS technology. Such antennas can improve UWB, WLAN and cognitive radio communication links.

A Latching Capacitive RF MEMS Switch in a Thin Film Package

A latching capacitive RF MEMS switch has been successfully designed, fabricated and tested. The switch uses a long thin metal cantilever which is electrostatically held to an upper electrode at the free end and to a lower electrode at the beam root end forming an S-shaped actuator. The upper electrode sits on top of a thin dielectric shell which also serves as part of the package for the device. Slight dielectric charging holds the cantilever in either the on-state or off-state between switching pulses. In the latched states with no bias, insertion loss is 0.2 dB at 10 GHz and isolation is >20 dB in a narrow band around 10 GHz. Repeatable switching has been demonstrated for actuation voltages below 20 V. Hot-switched power handling has been tested up to 6 W at 10 GHz without failure

Reconfigurable Bowtie Antenna Using Metal-Insulator Transition in Vanadium Dioxide

A new method to reconfigure, tune or program an antenna is presented and validated through a fabricated proof-of-concept prototype. The method relies on the changing electrical properties of a smart material, vanadium dioxide ( VO2), across its solid-to-solid phase transition. The phase change in the material is induced thermally. The developed device is a monolithically integrated VO2-based antenna that changes resonant frequency on demand. Details and challenges related to the design, fabrication and integration of VO2 thin films with antennas, as well as the measurement setup, are discussed. Measurement results match well with theory and simulations. Results unveil a new technique for reconfiguring antennas and suggest further studies and improvements could lead to more exciting results and make vanadium dioxide thin-films an alternate solution for multi-functional antennas. This work is the first prototype to implement this new reconfiguration mechanism in antennas in the gigahertz range and to present results during the VO2 metal-to-insulator phase transition.

Atomic layer deposition of Al/sub 2/O/sub 3//ZnO nano-scale films for gold RF MEMS

Atomic layer deposition (ALD) was used to create an Al/sub 2/O/sub 3//ZnO thin film for gold capacitive RF MEMS switches. These films exhibited a widely tunable range of physical properties, allowing the creation of a material capable of dissipating trapped charges and maximizing the on-capacitance of the switch. Predicted pull-down voltages of the ALD-coated switches underestimated the experimental findings due to residual stresses in the ALD film and annealing of the gold during the ALD deposition. Switch cycles to failure were measured using a 10 dBm, 10 GHz, CW signal with a bipolar actuation voltage of 25-55 V. Preliminary testing showed lifetimes of 400 million cycles using 50/50 ALD Al/sub 2/O/sub 3//ZnO films, with ultimate failure due to moisture-induced stiction and particulate contamination, not dielectric charging. The insertion loss and isolation for the switches was typically <0.35 dB and > 25 dBm, respectively, over a 10-25 GHz frequency range.

Effect of device layout on the thermal resistance of high-power thermally-shunted heterojunction bipolar transistors

The effect of device layout on thermal impedance of thermally-shunted HBTs was investigated. A direct comparison of thermally shunted devices and standard airbridge devices is made. Changes in thermal resistance of up to 67% were observed. While thermal resistance remains sensitive to emitter element placement in thermally shunted devices, variations in the location of thermal shunt landings had little effect. These results provide a basis for optimizing thermally-shunted devices.

Cross-sectional atomic force microscopy of focused ion beam milled devices

We have developed and demonstrated new techniques for failure analysis based on focused ion beam (FIB) cross-sectioning and inspection by atomic force microscopy (AFM). Normally, inspection after FIB cross-sectioning is done by scanning electron microscopy (SEM). As features of interest shrink below limits detectable by SEM, often the next method chosen is transmission electron microscopy (TEM). However, sample preparation for site-specific TEM is difficult and time-consuming, even using newer methods based on FIB milling. AFM offers higher resolution imaging than SEM, and relaxes many of the sample preparation constraints of TEM. The AFM/FIB technique has been demonstrated on GaAs-AlGaAs and GaAs-InGaP heterojunction bipolar transistors (HBTs), including devices which have been electrically stressed to failure.

Power performance of thermally-shunted heterojunction bipolar transistors

The effects of layout and thermal shunt configuration on output power, efficiency, and gain of thermally-shunted AlGaAs/GaAs HBTs were investigated. A maximum power density of 16 mW//spl mu/m/sup 2/ at 10 GHz (CW) was observed. The power gain and power-added efficiency (PAE) at this power density were 7.8 dB and 65%, respectively.

Thermal studies on heterostructure bipolar transistors using electroluminescence

We have studied electroluminescence (EL) emission from fully fabricated GaAs based heterostructure bipolar transistors. The EL emission occurs due to minority carrier injection into the base and collector layers. Under normal device operation, i.e. with reverse bias on the collector/base junction, collector emission does not occur since holes are not injected into this layer. In this case, only base emission is observed. When a forward bias is applied to the base/collector junction, EL from both the collector and the base is observed. The spectral characteristics of the two EL signals are different since the bandgap of the heavily p-doped base is smaller than that of the lightly n-doped collector. Since the bandgap depends strongly on temperature, the spectral characteristics are used to determine the heating of the HBT due to power dissipation.

Xs-MET-a reduced complexity fabrication process using complementary heterostructure field effect transistors for analog, low power, space applications

The requirements for space-based integrated circuit applications are defined with an emphasis on being radiation tolerant and low power consuming. Flexible analog signal processors (FASPs) are outlined as a means by which effective circuit designs can be utilized to perform a multitude of tasks. The development of complementary III-V technologies have been proven to meet the demands of the space environment, and have demonstrated the potential for frequency operation beyond 1 GHz using power supply voltages at or below 1.5 Volts. The novel fabrication process known as Xs-MET (pronounced kismet, which uses the Creek letter chi, X, and stands for Complementary Heterostructure Integrated Single Metal Transistor), is introduced as a manufacturing technique to be used in FASP design. The Xs-MET fabrication process is outlined with preliminary device results presented. An example of a FASP circuit design using Xs-MET is provided. Conclusions regarding the utilization of the Xs-MET process for FASPs are outlined with comments focusing on a space-based demonstration.