Radioisotope Thin Films for Microsystems

Abstract

Precise timing and frequency sources are vital in a wide range electronic-based systems such as communication networks and global positioning systems. These applications constantly demand reductions in size, weight and power (SWaP) while improving the precision of time or frequency references. Historically, clocks based on electromagnetic oscillations of atoms have provided the most precise method of timing events lasting longer than a few minutes. These oscillations are so precise that in 1967 the unit of time – the second – was redefined to be the time taken for a Cs atom in a particular quantum state to undergo exactly 9, 192, 631, 770 oscillations. While the long-term precision of atomic clocks is un-surpassed, the size and power required to run these devices has prevented their use in a variety of areas, particularly in those ap-plications requiring portability or battery operation. The NIST F-1 primary standard, for example, occupies a large optical table and requires many hundreds of watts to operate. The state-of-the-art in compact commercial atomic frequency references are Rb vapor-cell devices with volumes near 100 cm 3 that operate on a few tens of watts of power and cost about 1–3 thousand dollars. The long-term stability of atomic clocks including is based on the ability to interrogate a fundamental time constant – the hy-perfine resonance frequency of ground level transitions 1 . It is thus natural to extend this idea of interrogating other time constants to realize clocks with good long-term stabilities. We can identify three important characteristics necessary to realize a " good " time base or clock. These are 1 For example, 6.84 GHz in the case of rubidium atomic clocks R. Duggirala et al., Radioisotope Thin-Film Powered Microsystems, 127 MEMS Reference Shelf 6, DOI 10.1007/978-1-4419-6763-3 7, c Springer Science+Business Media, LLC 2010 128 7 Radioisotope Decay Rate Based Counting Clock 1. Long and short-term frequency stability, usually measured in Allan variance and phase noise of the frequency source 2. The physical size of the clock, and 3. The power consumed by the clock Recent trends in miniaturization of atomic clocks – most no-tably, the chip-scale atomic clocks (CSACs) [43] – are based on Micro-electromechanical systems (MEMS) which offers advan-tages such as smaller size, an improvement in the device power dissipation (as the heat lost to the environment via the device sur-face is smaller) and enable high-volume, wafer-based production of atomic clocks, which would substantially reduce cost. In spite of the advantages offered such reduced power due to parasitic heat dissipation, the power consumed by such an envisioned MEMS-based atomic clocks hasn't shown reductions to orders in which these systems would be portable battery operated systems for long-term operations (such a week-long missions for the military, a few months-long working of communication based units or even year-or decade-long operations for sensor node applications). 7.2 Background