Materials options for dielectrics in integrated capacitors

Value requirements for capacitors in modern microelectronic assemblies extend over six orders of magnitude from about 1 pF to 1 μF. For many applications this entire range must be present on the same substrate with the lower end providing a variety of filtering, timing, RF and A/D purposes and the upper end serving decoupling and other energy-storage functions. The smaller capacitors generally require stricter tolerances and tighter stabilities than the larger units. There are many candidate dielectric materials, deposition/etching processes and plate configurations for integrating these devices. It is difficult, if not impossible, for any single dielectric material and capacitor configuration to provide this entire range without either the highest or lowest-valued components occupying too large an area, exhibiting excessively high series resistance, or being too small to fabricate with acceptable tolerance. Therefore, it will often be necessary to mix integrated capacitor technologies on a single substrate. BCB, polyimide and SiO₂ provide a small enough capacitance and sufficient tolerance for the bottom end of the range while anodized metals or ferroelectric powders in epoxy thin films can cover decoupling termination and some energy storage into the range of hundreds of nF. Ferroelectric thin films provide much higher dielectric constants but are currently difficult to integrate, especially for small values, and have less temperature, frequency and bias stability than paraelectrics. Once it becomes possible to form ferroelectric thin films at temperatures low enough for organic substrates these films will be very useful for energy storage