Patent Number: 056420145
Section: summary

BACKGROUND OF THE INVENTION 1. Field of invention This invention relates to a beta voltaic power source integrated with a substrate as a power source for integrated circuits formed on the substrate. 2. Description of related art Radio isotopic power sources convert radiation from radioactive isotopes directly into electrical energy. Devices, such as artificial cardiac pacemakers, utilize the radio isotopic power sources for sustained long term power which allow the devices to function for many years without any other source of energy. Tritium is an isotope of hydrogen having a half life of 12.5 years. Because tritium emits only beta particles and the intensity of the beta particles is limited, tritium is an excellent source of radiation for radio isotopic power source applications. Beta voltaic power sources incorporate tritium together with a pn junction to directly convert the emitted beta particles into electrical energy. The beta particles emitted by the tritium is absorbed by the pn junction generating electron-hole pairs. The electron-hole pairs are separated by the built in electric field of the pn junction producing an electric current. Relatively high efficiencies are possible because each high energy beta particle produces many electron-hole pairs. Current applications of the beta voltaic power source are in the form of a battery component. The battery is connected to a separate device such as the artificial cardiac pacemaker. SUMMARY OF THE INVENTION An object of the invention is to provide a self-powered device integrating a radioactive power source with integrated circuits including an least one substrate, at least one radioactive power source formed over the at least one substrate generating electric current, and integrated circuits formed over the at least one substrate. The integrated circuits are adapted to receive power from the radioactive power source. The radioactive power source includes a first active layer having a first conductivity type formed over the substrate. An active layer is a semiconductor doped with an impurity to form either a p-type or n-type region. The substrate has a second conductivity type. A second active layer having the second conductivity type is formed over the first active layer forming a depletion region at the boundary between the first and second active layers. The interface between the first and second active layers forms either a pn or an np junction. A tritium containing layer is provided which supplies beta particles to the depletion region. A metal tritide layer is an example of the tritium containing layer. Another embodiment of the self-powered device includes an integrated circuit substrate and at least one cap substrate. The integrated circuit substrate includes a plurality of integrated circuits and at least one power source portion. Each of the power source portion includes a first active layer having a first conductivity type formed over the integrated circuit substrate and a second active layer having the second conductivity type formed over the first active layer. The cap substrate includes a fourth active layer having the first conductivity type formed over a bottom surface of the cap substrate. The cap substrate has the second conductivity type. A fifth active layer having the second conductivity type is formed over a top surface of the cap substrate. The cap substrate is placed over a corresponding power source portion on the integrated circuit substrate. A tritium containing layer is placed between the cap substrate and the power source portion. The cap substrate, the power source portion and the tritium containing layer together form a beta voltaic power source. When several of the beta voltaic power sources are connected either in series and/or in parallel, a wide range of voltage and current values can be obtained. The beta voltaic power source of the self-powered device is enhanced by trench structures formed by the first, second or fourth active layers. The trench structures allow the beta particles to be more efficiently converted into electric current. Another object of the invention is to provide a method for producing the self-powered device. The method includes providing at least one substrate, forming at least one radioactive power source over the substrate and forming integrated circuits over the substrate. The radioactive power source is provided by forming a metal layer and diffusing tritium into the metal layer. The metal layer is comprised of metal that forms stable metal tritides with tritium.