|Fig. 1: V-grooves and pyramidal cavities bulk micromachining.|
Nuclear Micro-batteries are micro or even nano scale batteries powered using radioactive isotopes. The nuclear energy of emitting radioactive particles is converted to electric energy which is used to power processors. It is based on the principle of beta voltaic effect or the electron voltaic effect.
The creation of electron hole pairs by bombarding a semiconducting material with beta particles from a radioactive source is called the beta voltaic effect. It was discovered by Paul Rappaport in 1953. Thereafter a lot of research was conducted on beta voltaic use for powering batteries.
The radioisotopes have the energy density 100 to 1000 times greater than fossil or chemical fuels.  Using radioisotopes one can increase the life of batteries up to about 50 years. This is especially important for application such as in medical devices like pacemakers where one can implant such devices once and then there would be no need to replace it at frequent intervals. In many micro scale devices, conventional batteries are still used which imposes a constraint for their size specification. Since the radioactive fuels are much more powerful than the traditional sources, using small amounts of radioactive fuels in the batteries helps to reduce the size. However, a careful consideration has to be made for choosing the right kind of radioactive isotope taking into consideration many parameters like cost, half-life, energy density, type of emissions and specific application for which the battery is used. Gamma rays are very high penetrating power and hence the shielding issue becomes important from safety and cost perspective. Alpha rays severely affect the crystal structure which is one of the main reasons for Silicon semiconductivity. However beta particles have very less penetrating power and therefore do not penetrate human flesh. Radioisotopes emitting Beta particles are therefore most suited for applications. One example is 63Ni. Table 1 shows pure beta sources considered for such an application.
|Table 1: Some radioisotopes used. |
The beta particles from the radioisotope strike the n and p regions of the battery and produce electron and holes giving rise to a potential difference in the circuit. For a 4 mm × 4 mm pn junction area, the output shot circuit current is 2.86 nA, open circuit volt is 128 mV, and maximum output power is 0.32 nW. 
PN Junction Nuclear microbatteries can be classified into 3 types:
Inverted Pyramid Structure
The output power of cells and the short circuit current can be improved by increased the surface area of cell. The inverted pyramid and channel designs receive hig her current and hence higher power output in the same design space. 
V channel and inverted pyramid arrays can be fabricated by bulk micromachining principles of mems. The V shaped or pyramid shaped arrays can be generated using anisotropy etching.  The following is the fabrication process.
Take Silicon substrate as starting material.
Grow Silicon dioxide on both sides of wafer thermally.
Pattern Silicon dioxide using photoresist and a lithography mask such that only the Silicon substrate parts which are to be patterned in V-shape or pyramid shape are exposed. The photoresist is exposed to light in this area and the bonds are broken. Thus when organic solvents for photoresists are used, exposed photoresist gets dissolved. The rest of the undissolved photoresist serves as a mask for the underlying Silicon dioxide against etchants.
The pyramids or V grooves act as reservoirs for liquid 63NiCl/HCl solution. This solution is deposited using pipettes.
Only 63NiCl solution remains on the surface after evaporation.
The method to bulk machine the PN junction device is as follows:
Take the starting substrate as Silicon wafer.
Grow a thin layer of thermal oxide (SiO2) on both sides of wafer.
Pattern the oxide using photoresist and mask lithographically to that the oxide is etched away only in the regions that are to be doped with n type dopant. Remove the photoresist and deposit a thin layer of n type doped material.
Pattern Aluminium electrodes in the same way selecting the right etchant and masks on the n type material for electrical contact.
On the opposite side of wafer, repeat the above steps using p type dopant.
The above principle was used to power an acoustic transmitter in wireless sensor node.  A capacitor is formed between the collector plate at the end of a cantilever beam and a beta particle emitting radioisotope. As a result of electrostatic force, the cantilever beam gets deflected towards the radioisotope. As soon as it makes a contact, the air gap capacitor discharges releasing the cantilever. This happens repeatedly. The oscillation of cantilever induces electric charge in the piezoelectric transducer mounted on the cantilever. The mechanical energy of the cantilever vibration gets converted in to electrical energy through the piezoelectric transducer and finally to the acoustic energy in external piezoelectric speaker.
© Chaitali Dalvi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 H. Guo et al., "Nuclear Microbatteries for Micro and Nano Devices," Proc. 9th Intl. Conf. on Solid-State and Integrated-Circuit Technology, ICSICT 2008, 20 Oct 08, p. 2365.
 J. Chu et al., "Research of Radioisotope Microbattery Based on β-Radio-Voltaic Effect," J. Micro/Nanolith. MEMS MOEMS 8, 021180 (2009).
 R. J. Duggirala, H. Li and A. Lal, "An Autonomous Self-Powered Acoustic Transmitter Using Radioactive Thin Films," IEEE Ultrasonics Symposium 2004 2, 1318 (2004).