Nanoionics-based devices have shown promise as alternatives to microelectro-mechanical systems (MEMS) and semiconductor diode devices for switching radio-frequency (RF) signals in diverse systems. Examples of systems that utilize RF switches include phase shifters for electronically steerable phased-array antennas, multiplexers, cellular telephones and other radio transceivers, and other portable electronic devices.
Semiconductor diode switches can operate at low potentials (about 1 to 3 V) and high speeds (switching times of the order of nanoseconds) but are characterized by significant insertion loss, high DC power consumption, low isolation, and generation of third-order harmonics and intermodulation distortion (IMD). MEMS-based switches feature low insertion loss (of the order of 0.2 dB), low DC power consumption (picowatts), high isolation (>30 dB), and low IMD, but contain moving parts, are not highly reliable, and must be operated at high actuation potentials (20 to 60 V) generated and applied by use of complex circuitry. In addition, fabrication of MEMS is complex, involving many processing steps.
Nanoionics-based switches offer the superior RF performance and low power consumption of MEMS switches, without need for the high potentials and complex circuitry necessary for operation of MEMS switches. At the same time, nanoionics-based switches offer the high switching speed of semiconductor devices. Also, like semiconductor devices, nanoionics-based switches can be fabricated relatively inexpensively by use of conventional integrated-circuit fabrication techniques. Moreover, nanoionics-based switches have simple planar structures that can easily be integrated into RF power-distribution circuits.
Nanoionics-based switches exploit the properties of some amorphous materials (in particular, chalcogenide glasses) that can incorporate relatively large amounts of metal and behave as solid electrolytes. The ionic conductivity of such a material can be of the same order of magnitude as the electronic conductivity of a semiconductor. Under appropriate bias conditions (typically between 1 and 3 V), ions of a metal (silver) are formed at an anode made of that metal and migrate into the solid electrolyte while electrons (typically at a current of the order of microamperes to milliamperes) are injected from an electrochemically inert (nickel) cathode into the solid electrolyte. The injected electrons reduce the metal anions in the solid electrolyte, thereby causing the growth of metal nanowires through the electrolyte from the cathode to the corresponding anode.
Once a nanowire has grown sufficiently to form an electrically conductive path between the electrodes, there is no need to continue to apply electric power to maintain the connection. The process of making the connection can be easily reversed by applying a reverse bias to re-oxidize the metal atoms in the solid electrolyte to recreate the insulating amorphous layer. Thus, a nanoionics-based switch is a reversible electrochemical switch that can have geometric features as small as nanometers. The process time for making or breaking the connection is about a nanosecond.
Experimental nanoionics basedswitches having several different configurations have been built and tested in a continuing effort to gain understanding of the underlying chemical and physical principles and optimize designs. Fabrication of each experimental switch began with deposition of a binary chalcogenide glass on a high-resistivity silicon wafer. Next, a layer of silver was deposited on the glass and exposed to ultraviolet light to induce a photodissolution process in which silver ions migrated into the glass matrix. Then an electrode of silver and an electrode of nickel were deposited on the chalcogenide layer.
The figure shows plots of data from a test of one of the experimental switches. Over the frequency range from 1 to 6 GHz, the insertion loss of the switch in the “on” state was less than 0.5 dB, while the isolation in the “off” state exceeded 30 dB.
This work was done by James Nessel and Richard Lee of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
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