1830 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 9, SEPTEMBER 1999 A Novel Low-Loss Low-Crosstalk Interconnect for Broad-Band Mixed-Signal Silicon MMIC’s Juno Kim, Student Member, IEEE, Yongxi Qian, Member, IEEE, Guojin Feng, Pingxi Ma, Member, IEEE, Jack Judy, Member, IEEE, M. Frank Chang, Fellow, IEEE, and Tatsuo Itoh, Life Fellow, IEEE Abstract— A novel RF interconnect configuration for high- density broad-band mixed-signal silicon monolithic microwave integrated circuits (MMIC’s) is presented. The proposed sili- con–metal–polyimide (SIMPOL) structure is based on multilayer polyimide technology with self-packaging features, and is ex- tremely effective in reducing the noise crosstalk as well as overall size of MMIC chips. Moreover, since the SIMPOL interconnect can be built on low-cost silicon substrate using standard CMOS processing techniques, it is very cost-effective and applicable to current products without major cost addition. Measured results of a prototype test wafer demonstrate that the SIMPOL intercon- nect has reasonably low insertion loss (0.62 dB/mm at 30 GHz), which agrees well with theoretical prediction (0.5 dB/mm). The line loss can be reduced significantly ( 0.1 dB/mm) by a using thicker dielectric layer. The measured crosstalk is at the same level as the background noise floor up to 30 GHz ( 60 dB), and limited primarily by imperfect termination of idling ports in the test structure. Full-wave finite-difference time-domain simulations indicate that SIMPOL could achieve an extremely high level of signal isolation, above 100 dB, at frequencies up to 50 GHz or beyond. Index Terms— Crosstalk, insertion loss, interconnect, mixed- signal MMIC, noise isolation, silicon IC. I. INTRODUCTION M IXED-SIGNAL integrated circuits (IC’s) have been pursued with great interest in recent years, due to their significant benefits such as overall chip size reduction, lower fabrication cost and power consumption, as well as enhanced system performance [1]. This is particularly true for mobile wireless communication systems because it offers the possibility of convenient integration of all RF/analog and digital circuits into a single chip. Meanwhile, significant progress in silicon devices, such as high-speed ( and GHz), high-power, and low-noise SiGe heterojunction bipolar transistors (HBT’s) [2], [3], has made it possible to realize silicon-based mixed-signal monolithic microwave integrated circuits (MMIC’s) even at high frequencies up to the millimeter-wave region. However, applications of these high-frequency devices to mixed-signal MMIC’s have not yet been demonstrated adequately, mainly due to the significant transmission-line loss on conductive silicon substrates and serious noise crosstalk between digital and RF/analog circuits. Manuscript received February 5, 1999; revised May 12, 1999. This work was supported by the Army Research Office Multidisciplinary Research Initiative under Contract DAAH04-96-1-0005. The authors are with the Electrical Engineering Department, University of California at Los Angeles, Los Angeles, CA 90095-1594 USA. Publisher Item Identifier S 0018-9480(99)07156-2. For low-frequency applications ( 10 GHz) many tech- niques have been suggested to reduce the noise crosstalk [4], [5]. However, these approaches are inadequate at higher microwave and millimeter-wave frequencies where most noise coupling occurs through radiation in addition to the substrate. One technique to reduce radiation coupling is to use buried microstrip lines [6]. However, since this approach is also based on open microstrip-type configuration, it also becomes less effective for higher frequency applications. A potentially ulti- mate solution is the micropackaging where individual circuit components on the same chip are shielded from each other by using the latest micromachining technology [7], [8]. This approach has successfully demonstrated substantial reduction in noise crosstalk along with low line insertion loss up to 40 GHz, although its cost effectiveness has yet to be fully addressed. Multilayer technology using polyimide has been developed intensively in recent years [9]–[16]. This technology is very effective in reducing the size and cost of IC’s by using three- dimensional (3-D) configuration. This 3-D MMIC technology has shown the possibility of making use of polyimide-based passive components and silicon devices at high frequencies. The multilayer polyimide technology has also been success- fully applied to hybrid-type RF circuits combined with ceramic substrates. However, detailed potential crosstalk problems related to these structures have yet to be studied in details since the top-layer circuit configurations are essentially based on open microstrips. Recently, we proposed a novel interconnect concept for broad-band ( 50 GHz) mixed-signal silicon MMIC implementation using multilayer polyimide technology, called silicon–metal–polyimide (SIMPOL) [17], [18]. Based on a stripline-like configuration with self-packaging characteristics, the proposed SIMPOL interconnect is extremely effective in reducing the noise crosstalk, EMI problems, and in increasing the MMIC integration density. It is also very cost-effective since realized on low-cost CMOS grade silicon substrates, and requires only existing standard silicon processes, which we have used to fabricate our test wafer. In this paper, we present a detailed description of the implementation of the SIMPOL structure, as well as full-wave simulation results by employing a simplified numerical model. A prototype test wafer has been designed and fabricated. Initial measurement up to 40 GHz reveals that the SIMPOL interconnect has reasonably low insertion loss, which agrees well with the theoretical prediction. The crosstalk between 0018–9480/99$10.00  1999 IEEE