ON-CHIP MICRO BATTERIES FOR MILLIMETER-SCALE WIRELESS NODES.
Graduate Student: Dan Steingart
Under-Graduate Student: Christine Ho
This is a joint project between Prof. Evans and Prof. Paul Wright of Mechanical Engineering.
  
    The full utility of wireless devices can only be realized if both data transmission and supply of power to a node is wireless. The next generation of wireless sensor networks requires the energy production and storage mechanism to be integrated on-chip. We are designing a micro-scale lithium secondary battery to be deposited on the backside of a millimeter scale wireless sensor. The battery will continuously be charged by energy scavenging devices such as MEMS piezo-benders or heat engines. The battery novelty comes from designing the cell around the duty cycle for a given application. The material novelty in the system lies in adaptation of Dunn's V2O5 aerogel cathode and Kostecki's pyrolyzed carbon anode for micro scale implementation. The figure below shows one possible way that a cell of this type might be assembled.

    1. A layer of SiO2 will be deposited onto a single crystal Si substrate.
    2. The pyrolyzed carbon cathode will be placed onto the SiO2 and patterned via the process described by Kostecki (LBNL).
    3. A layer of negative photoresist will be spun on and patterned to form the area for the placement of the cathode.
    4. Previously prepared V2O5 sol-gel will be applied to the substrate so that it fills the trenches and the excess physically removed. While this methodology may be suspect for sub-micron fabrication, the features sizes of the initial battery (50 to 100 µm) would allow it. The sol-gel will be turned to an aerogel in place using a supercritical CO2 drier. Earlier work on these aerogels has been described by Dunn (UCLA).
    5. A low molecular weight, photosensitive PEO will be placed in the area between the electrodes and allowed to infiltrate the porous regions of the electrode before being cured with a UV laser source.
    6. A layer of SiO2 will be grown over the entire exposed structure by chemical vapor deposition methods.
    7. Aluminum will be sputtered or evaporated onto the electrodes after the oxide over the electrodes has been etched away. The aluminum itself will serve as the current collector as well as the contacts to the rest of the circuit.

Research supported by an Intel Fellowship for Dan Steingart.