46 Integral Passives

4.6.1 Advantages and Limitations of Integral Passives

Discrete components continue to offer low cost and value in area array interconnection solutions. An increasing number of applications can take advantage of solutions where miniaturization and/or functionality require integral passive technology or integrated technology solutions.

Integrated and integral passive components are defined by the Passive Components Technology Working Group of the National Electronics Manufacturing Initiative (NEMI). Although some definitions may change, those given by NEMI generally apply to all chip carriers and package solutions [49, 50]

Integral passive or integrated packages can use thick- or thin-film processes and can be fabricated using ceramic technology, thin film, laminate technologies or within silicon die. Utilizing buried integral passives permits increased space on boards or packages that can not be integral. Most approaches to date have utilized thick-film paste printed on layers. More recent developments include sputtered or evaporated metals on dielectric films.

4.6.1.1 Resistors

Resistors have been integral to ceramic packaging since the 1960s where palladium-silver-palladium oxide-glass thick films were screen printed on chip carriers. Today, resistors support small or preferably no change of properties with time. Resistor properties are defined as positive or negative temperature coefficient of resistance (TCR), which relates to changes in resistance with temperature. Resistors ranging from a few ohms to thousands of ohms are available using ruthenium oxides and lead borosilicates resistor pastes as examples. In addition, a wide range of compositions are used, depending upon the temperature coefficient of resistance desired. Increase control of tolerances in advanced applications are continuing to be sought. Laser trimming of resistors after sintering is utilized to fine tune the desired resistor values.

Thin-film resistors have also been widely used on top of blank ceramic carriers, but the major focus in recent years has been on the integration of resistors into multilayer, multichip thin-film modules. Materials such as copper-nickel alloys, tantalum-nitride, tantalum-silicide, ruthenium-dioxide, and titanium-tungsten have been vacuum deposited on ceramic and/or polymer layers to integrate both within and on the surface of a package. Most of these applications are in the development or prototyping stage.

4.6.1.2 Capacitors

Integral capacitors have been used in multilayer ceramics where thin dielectric tape layers (sometimes with increased dielectric constant for increased capacitance) are incorporated during the building of a chip carrier. Cavity wire bond packages and area array flip-chip packages used from one to about five thin dielectric layers to provide capacitance near or directly under a die. High temperature co-fired ceramic (HTCC) with integral capacitors uses dielectric thickness down to ~20 ^m fired and a dielectric constant of 9-14 for alumina ceramic. Integral capacitors in LTCC can provide higher capacitance values where dielectric constants of 8 -40 have been utilized with similar dielectric thickness compared to HTCC. Potential integral capacitor solutions for LTCC are being developed with dielectric constants of over 200.

Thin-film capacitor development is also primarily driven by the need to provide high decoupling capacitance coupled with a low-impedance path as close as possible to the chip. These factors become more and more important as chip frequencies increase and chip voltages decrease. Several materials and techniques are being investigated to produce thin, high-dielectric-constant, defect-free films compatible with multilayer copper/polyimide thin-film structures. Materials such as barium titanate, aluminum oxide, and tantalum pentoxide either deposited as the oxide or subsequently anodized have been used to achieve capacitance values as high as 70 pF/cm2.

This work continues at the exploratory and protype stage. As the performance of discrete capacitors continues to improve, the performance requirements for economic use of integral capacitors also continue to increase.

4.6.1.3 Other Applications

As discussed in the introduction, communications is becoming an increasingly important area for ceramic applications. A wireless radio product utilizes hundreds of discrete components. Increasing functionality along with manufacturing cost and miniaturization requirements are causing increased focus on low-temperature co-fired ceramic technology with integral passives. Motorola has reported on the development of multilayer ceramic integrated circuits (MCIC) where low-temperature co-fire ceramic (LTCC) could be used to integrate radio frequency functions for wireless communications application.

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