Next generation integral passives: materials, processes, and integration of resistors and capacitors on PWB substrates

Integral passives are becoming increasingly important in realizing next gen eration electronics industry needs through gradual replacement of discretes . The need for integral passives emerges from the increasing consumer deman d for product miniaturization thus requiring components to be smaller and p ackaging to be space efficient. In this paper, the feasibility of integrati on of polymer/ceramic thin film (similar to 5 mu m thick) capacitors (C) wi th other passive components such as resistors (R) and inductors (L) has bee n discussed. An integrated RC network requiring relatively large capacitanc e and resistance is selected as a model for co-integration of R and C compo nents using low temperature PWB compatible fabrication processes. This test vehicle is a subset of a large electrical circuit of a functional medical device. In order to produce higher capacitance density and reduce in-plane device area, multi-layer (currently two-layer) capacitors are stacked in th e thickness direction. A commercially available Ohmega-Ply resistor/conduct or material is selected for integral resistors. Resistors were fabricated u sing a multi-step lithography process with the utilization of two separate masks. Bottom copper electrodes for capacitors were also defined during the resistor fabrication process. Photodefinable epoxies filled with a high pe rmittivity ceramic powder were used for fabrication of thin film capacitors . Epoxy and ceramic powders were mixed in the required proportion and blend ed using a high shear apparatus. The coating solution was homogenized in a roll miller for 3 to 5 days prior to casting in order to prevent settling o f the higher density ceramic particles. Capacitors were fabricated by spin- coating on the sub-etched copper electrodes. The deposited dielectric layer s were dried, exposed with UV radiation, patterned, and thermally cured. To p capacitor electrodes (copper) were deposited using a metal or an e-beam e vaporator. The electrodes were patterned using the standard photolithograph y processes. Selected good samples were used for depositing the second capa citor layer. The RC network is extended to incorporate electroplated polyme r/ferrite core micro-inductors through the fabrication of an industry proto type low pass RLC filter. Meniscus coating was evaluated for large area man ufacturing with high process yield. A capacitance density of similar to 3 n F cm(-2) was obtained on a single layer capacitor with similar to 6 mu m th ick films. The capacitance density was increased to similar to 6 nF cm(-2) with the two-layer deposition process. The capacitors were relatively stabl e up to a frequency range of 120 Hz to 100 KHz. Meniscus coating was qualif ied to be a viable manufacturable method for depositing polymer/ceramic cap acitors on large area (300mm x 300mm) PWB substrates. Dielectric constant v alues in the range of 3.5 to 35 with increase in filler loading up to 45 vo l% were achieved in the epoxy nanocomposite system where the dielectric con stant of the host polymer was limited to similar to 3.5. Higher dielectric constant polymers are required to meet the increasingly higher capacitance needs for the next generation electronics packaging. Possible avenues for a chieving higher capacitance density in polymer/ceramic nanocomposite system have been discussed.