Embodiments of the present invention relate to thin film batteries and their fabrication on a substrate.
Thin film batteries are used in various applications, such as portable electronics, medical devices and space systems. A thin film battery typically comprises a substrate having one or more battery component films that include an electrolyte sandwiched between electrode films such an anode, cathode, and/or current collector films, that cooperate to store electrical charge and generate a voltage. The battery component films are thinner than conventional batteries, for example, they can be less than 100 microns. The small thickness dimension allows a thin film battery to have a thickness which is less than about a hundredth that of a conventional battery. Thin films used as battery component films are formed by processes, such as physical and chemical vapor deposition (PVD or CVD), oxidation, nitridation, and electroplating processes.
Conventional substrates used in the fabrication of thin film batteries can constrain the minimum dimensions of the battery. Thin film batteries are often used in applications which require a battery with high energy density and/or specific energy. The energy density level is the fully charged output energy level per unit volume of the battery, and the specific energy level is the fully charged output energy level per unit weight of the battery. However, conventional substrates often need to have a certain thickness to provide adequate mechanical support for the thin films formed on the substrate. The relatively thick substrates, limit the maximum energy density and specific energy levels that can be obtained from the resultant and film battery. Battery performance can be improved by using thin plate-like substrates, such as for example, ceramic substrates composed of Al2O3 or SiO2, to increase the energy to volume/weight ratio of the battery.
Crystalline substrates with cleavage planes can also be used to increase the energy density and specific energy levels of thin film batteries. These crystalline materials are typically relatively strong along the direction of the cleavage plane, and can also be light weight. For example, commonly assigned U.S. Pat. No. 6,632,563 to Kraznov et al., which is incorporated herein by reference in its entirety, describes a mica substrate which meets these requirements. The mica substrate reduces the total weight and volume of the battery while providing good mechanical strength and dielectric strength, at least partly because the flat planar structure and cleavage properties of mica allow it to be split into thin foils along its cleavage planes. A mica substrate can be very thin, and can even have thicknesses of less than about 100 microns, or even less than about 25 microns.
Conventional thin film fabrication methods, which are used to shape the battery component films on the substrate to form a three-dimensional battery structure, can also have problems. Typically, the battery component films, such as the cathode, electrolyte, etc., are shaped to form particular shapes, using successive masking and deposition process steps. In these methods, a mask comprising patterned apertures is positioned or deposited as a layer on top of a mica substrate. Thereafter, a second layer is deposited onto the underlying substrate surface through the patterned apertures of the mask to form features. Successive masking and deposition steps are used to build up a three-dimensional shaped structure for the thin film battery. However, conventional masking methods can have undesirable effects when used on a crystalline substrate having cleavage planes. For instance, when the mask is peeled back from the substrate in these processes, it can cause splitting along cleavage planes or fracture across the planes at portions of the substrate adjacent to the interface between the mask aperture and material deposited through the aperture. The edge of the mask can also stick to the underlying substrate material, and when peeled back, remove a portion of a plate-like layer of a substrate at a cleavage plane, or result in a non-uniform or broken edge of the feature being formed through the aperture in the mask.
Thus it is desirable to form a three-dimensional structure of battery component films on a battery substrate without the excessive use of masks to define the shape of the features. It is further desirable to shape battery component films on the substrate without forming non-uniform or broken edges of the shaped features of the films. It is also desirable to reduce the number of handling steps in the fabrication of a thin film battery to reduce contamination and stress fractures, and increase process throughput.