High Energy Density Capacitors Fabricated by Thin Film Technology
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filter capacitors, variable speed electric motors, and pulsed power sources demand low loss, low inductance capacitors that tolerate high temperature. Nanostructure multilayer capacitors with interleaved films of conductor and dielectric make it possible to pack many thin dielectric layers in parallel. The key to achieving high energy density is making a dielectric layer that can support a very high field strength, since the energy density of the capacitor is proportional to the square of the field strength. To realize the high field strength of materials in a device, it is necessary to make large-area, defect free films, since any defect will lower the voltage at which the device will fail. While the electronics industry continues to develop multilayer ceramic capacitors by powder processing of ferroelectric materials we take a different approach and grow high quality insulators by physical vapor deposition. The aluminum oxide films discussed here are all made by reactive magnetron sputtering. The dielectric constant of A120 3 is about 9 which is over twice that of silicon dioxide. Like Si0 2, A120 3 deposits as an amorphous film. This eliminates the possibility of through film columnar grain boundaries becoming a breakdown pathway. Also polycrystalline films are more difficult to keep smooth during deposition, and roughness at the interface between the dielectric and the contact can lead to field intensification and breakdown. Although ferroelectric materials have a much higher dielectric constant and are receiving much attention in the literature, there a number a of reasons we have chosen not to work with them. First, ferroelectric materials must be crystalline to achieve high dielectric constant. 219 Mat. Res. Soc. Symp. Proc. Vol. 574 ' 1999 Materials Research Society
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Figure 1:a) Schematic top view of a Cu / A120 3 / Cu capacitor structure deposited though a diamond shaped shadow mask aperture. With this shape one mask can be used to deposit dielectric and contacts allowing the whole capacitor to be deposited without breaking vacuum. b) Cross section of a single layer capacitor. c) The single layer subunit can be repeated to form a multiple layer, stacked capacitor. (Vertical exaggeration 1000x.)
Second, ferroelectric materials have a large piezoelectric effect leading to fatigue and high losses. Finally, the high dielectric constant of ferroelectric materials saturates at high field lowering their potential energy storage density. The primary goal of this work is to develop an efficient high rate sputtering process for depositing thin film capacitors, while meeting the stringent quality requirements necessary to achieve high capacitance and high energy density. The primary challenges include making fully oxidized very high resistivity films keeping the deposition region particle free to eliminate defects in the films and maintaining smooth and abrupt interfaces with the capacitor contacts. EXPERIMENTAL PROCEDURE A small clean room has been constructed around the magnetron sputter deposition system
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