Design of Super-Miniature Bandpass Filters Embedded in the Organic Substrate without Using Large-Value Capacitors

Abstract

Very small-size bandpass filters embedded in the organic substrate for wireless front-end applications are presented. Due to the lack of fabrication process for large-value capacitors in the organic substrate, a novel filter prototype using an inductive coupling of series resonators is proposed to achieve high performance within a stringent size constraint. In implementation, the organic bandpass filters are composed of the spiral inductors and parallel-plate capacitors that are embedded in a 4-layer BT laminate, one of today's most popular package substrate. The final realized performance and size are competitive to the LTCC filters but with a much lower cost. IntroductionThe currently available low-temperature cofired ceramic (LTCC) technology can implement high-performance miniature embedded filters quite successfully because of the capability to realize large-value capacitors in a large number of thin substrate layers. To fully employ this capability, most of the LTCC bandpass filter designs adopt a capacitive coupling of parallel resonators as a lumped-element prototype [1]- [3], which mainly uses the large-value vertically interdigitated capacitors in conjunction with the small-value meander-line inductors. However, in system-on-package applications, the LTCC technology has two major disadvantages that it has a high fabrication cost and also has a difficulty in dealing with the high-density interconnects. On the other hand, the currently most popular low-cost package substrate is an organic laminate on which the high-density interconnects as well as the high-Q embedded passives are feasible [4]- [8]. But it has only a limited number of layers and the layer thickness is not small enough for realizing largevalue capacitors. In our literature review, due to the lack of small-volume large-value capacitors, the previously designed bandpass filters embedded in the organic substrate [9]- [12] were inevitably big in size with moderate to high insertion losses in passband. Although a thin high-k dielectric buried in the organic substrate [13], [14] for realizing large-value capacitors is available nowadays, the cost is still too high.A new lumped-element design for the organic bandpass filters without using the large-value capacitors is proposed in this paper. It is based on an inductive coupling of series resonators with components suitable for the large-value spiral inductors in conjunction with the small-value parallel-plate capacitors. In our previous works [15], [16], we have established the scalable model libraries for both types of passive components embedded in a 4-layer BT laminate substrate. When used to design the proposed bandpass filter in the 2.5-2.9 GHz WiMax band, the typical component values are in the range of several nH for the spiral inductors and in

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