Scientists at the Naval Research Laboratory (NRL) are challenging the performance limitations stated in Moore’s Law by pursuing research on semiconductor devices that rely on electron spin rather than electron charge. Traditionally, semiconductor devices used charge flow through device junctions and size scaling to achieve higher speeds and frequencies in devices.
But, as the size of semiconductor devices shrinks to atomic proportions, new device design approaches are needed to sustain performance improvements. Research efforts at NRL and elsewhere have shown that electron spin angular momentum can be used to store information in metal and semiconductor-based devices.
Thus, an electron’s spin, rather than its charge, would be used as a state variable. Known as “semiconductor spintronics,” the new approach has been realized in III-V compound semiconductors such as gallium-arsenide (GaAs) devices, but with little success in lower-cost silicon materials.
Yet recent work by NRL researchers shows that high electron spin polarizations can be achieved in silicon by electrically injecting current from a ferromagnetic metal, such as an iron film. Their process includes vacuum deposition of a contact area after a simple wet chemical cleaning of the silicon wafer, achieving performance with low-0 bias voltages (approximately 2 eV) compatible with CMOS technology.
The branches of the armed forces maintain their own research laboratories, such as the NRL and the Army Research Laboratory (ARL). However, the Defense Advanced Research Projects Agency (DARPA) is the main research and development organization for the U.S. Department of Defense (DoD).
DARPA not only supports the electronic technology needs of major programs, such as FCS and the Cruiser Modernization Program, it also manages and publishes a “wish list” of current technology requirements on its Web site (www.darpa.mil).
Its Defense Sciences Office (DSO), for example, is attempting to develop hyperspectral radiography sources that can convert short laser pulses into other forms of energy, such as x-radiation or high-energy particles for spectroscopy and tomography applications.
The DSO also is pursuing applications for molecular electronics, including a programmable nanoprocessor system with high-density memory array. Targeting reduced-geometry gate sizes in the 10-nm range, the molecular electronics program aims at creating high-speed processors with reduced leakage current. The same technology could be applied to developing a nanosensor based on nanowires. Such a device would feature much greater chemical/biological sensing capabilities than current detectors.
Yet another DSO program involves the development of an optical arbitrary waveform generator ultimately capable of 500-THz bandwidth and subfemtosecond timing resolution. The first step is the design and development of an optical waveform generator with coherent analog signal bandwidth exceeding 1 THz and a 10-GHz refresh rate.
Three-dimensional (3D) ICs are the focus of DARPA’s Microsystems Technology Office (MTO). These circuits embody the overall push in military electronics for higher levels of integration at lower power levels and in smaller packages. The MTO’s 3D IC concepts allow for the integration of mixed technologies such as analog, digital, optical, and microelectromechanical systems (MEMS) on miniature modules formed from stacked IC layers that are bonded together.
DARPA has embraced the term “polylithic integration” to refer to microsystems that incorporate circuits and devices in dissimilar materials, such as GaAs on silicon, to achieve “the best junction for the function.” They also reduce signal delays by eliminating off-chip interconnections.
DARPA conintues to pursue a variety of circuit-level technologies, such as low-temperature-cofired-ceramic (LTCC) processes, in its quest for smaller size and higher levels of integration. It also has shown interest in proprietary multilayer circuit technologies like the MultiMix technology developed by Merrimac Industries.
Based on fusion-bonded layers of polytetrafluoroethylene (PTFE) substrate materials, MultiMix is used to fabricate basic RF components (filters and power dividers) as well as more complex subsystems, such as instantaneous-frequency-measurement (IFM) receiver modules and beam-forming networks for phased-array radars, in a fraction of the size of conventional circuit technologies.