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Organic “Wafer-Scale” Packaged Miniature 4-bit RF MEMS Phase Shifter
Nickolas Kingsley, Student Member, IEEE, and John Papapolymerou, Senior Member, IEEE
Sep 29, 2008:
Abstract—This paper presents for the first time a 4-bit microelectromechanical systems (MEMS) phase shifter fabricated on, integrated, and packaged into an organic flexible low-permittivity material. A microstrip switched-line phase shifter has been optimized at 14 GHz for small size and excellent performance. In addition, the MEMS phase shifter was packaged in an all-organic flexible lowpermittivity liquid-crystal polymer (LCP) package. The improved geometry of the reduced size phase shifter is 2.8 times smaller than a traditional switched-line phase shifter and is much less lossy. For the 4-bit phase shifter, the worst case return loss is greater than 19.7 dB and the average insertion loss is less than 0.96 dB (0.24 dB/bit or 280 /dB). The average phase error is only 3.96 . It has been demonstrated that the addition of the LCP package has a negligible effect on the phase-shifter performance, but will enable the device to remain flexible and protected against various environmental conditions.
Index Terms—Liquid crystal polymer (LCP), miniature, multibit, organic, packaged, phase shifter, reduced size, RF microelectromechanical systems (MEMS), system-on-package (SOP), tree junction, wafer scale.
I. Introduction
To date, 4-bit phase shifters have been documented in various system-on-chip (SOC) devices. They have been published on various materials, including silicon [1] and GaAs [2]. Various switching elements have been used including field-effect transistors (FETs) [2], p-i-n diodes [3], and in recent years, microelectromechanical systems (MEMS) [1]. Currently published 4-bit phase shifter papers have several shortcomings. First, they are all fabricated on nonorganic substrates. Some of these substrates are costly, such as GaAs. Second, many of them use solid-state switching elements. p-i-n diodes, FETs, and other solid-state switches are typically lossier, consume more power, and have more distortion at high frequencies than MEMS switches. Third, none of the published 4-bit phase shifters are packaged. This paper presents a solution to all of these issues.
There is an increased interest in moving toward system-onpackage (SOP) RF front-end technologies. SOP offers design simplicity, lower cost, higher system function integration, better electrical performance, and various three-dimensional (3-D) packaging capabilities [4]. This paper presents for the first time an SOP RF front-end 4-bit MEMS phase shifter on an organic substrate. This paper furthers the all-organic flexible packaging technology by housing more than one MEMS devices in a single package and presents, for the first time, a packaged phase shifter on an organic flexible low-permittivity substrate. The organic substrate serves as both the RF substrate and the packaging material (superstrate).
RF MEMS have previously been integrated monolithically on printed circuit board (PCB) substrates. In [5], the authors integrated RF MEMS switches with a diversity antenna on an FR4 substrate. FR4 is an organic low-permittivity substrate like liquid-crystal polymer (LCP), but it does not have the same flexibility and low loss at high-frequency characteristics like LCP. Wafer-scale packaging is discussed in [5], but it has been realized in this paper.
Phase shifters are an integral part of RF systems. Microwave devices on nonsemiconductor substrates (i.e., LCP) have been explored due to their low-cost, low-loss, flexibility, and near-hermetic nature [6]. The first MEMS phase shifter on a flexible organic substrate was presented at the 35th European Microwave Conference, Paris, France [7]. 1- and 2-bit phase shifters were presented. However, those phase shifters were neither miniaturized, nor packaged. This paper continues the work presented in [7] by greatly shrinking the size, lowering the loss, expanding to 4 bits, and integrating into an all-organic package.