Elsevier

Microelectronics Journal

Volume 45, Issue 8, August 2014, Pages 1093-1102
Microelectronics Journal

Design, fabrication and characterization of miniature RF MEMS switched capacitor based phase shifter

https://doi.org/10.1016/j.mejo.2014.05.009Get rights and content

Abstract

This paper aims to present in detail the design, fabrication and the various characterization techniques adopted in realizing a novel miniature-size switched capacitor based phase shifter. The work strives to provide an all-round development of a DMTL (Distributed MEMS Transmission Line) based phase shifting unit yielding an overall phase shift of 15° at a frequency of 15 GHz. The RF MEMS (Radio Frequency Micro Electro Mechanical Systems) Switched Capacitor based phase shifter has highly miniaturized dimensions, and is capable of solving many limitations to which the conventional Switched Capacitors are mostly susceptible. This miniature RF MEMS switched capacitor actuates at a low actuation voltage of 12 V, exhibits a fundamental frequency of vibration as high as 1.468 MHz and a switching time of 1.4 µs which is an improvement over the other reported designs. Various characterization results seem to validate the simulations.

Introduction

Recent advancement in the area of RF MEMS technology has proved to be highly beneficial in the fields of satellite communication, radar and military applications [1], [2], [3], [4], [5], [6], [7]. The most prominent and simplest unit among all other RF MEMS based devices is the switch or rather the Switched Capacitor (SC), which is of primary importance so far, the phase shifter design, is concerned. SC is the basic building block or the smallest entity which can be periodically replicated on a Coplanar Waveguide (CPW) based transmission line structure so as to yield an appreciable amount of phase shift between the input and output ports. This is the basic concept behind the realization of DMTL phase shifters which has been utilized for the task to be discussed in the present context. Although some literature is available on MEMS phase shifters, primarily by Hayden et al. [8], [9], [10], [11], our work is unique in obtaining a very large capacitance ratio and an extreme miniaturized design. SCs are bridge/beam-like structures which are fixed at the two extremities whereas, the central part of the beam is free to actuate on application of an external voltage. SCs having conventional dimensions, for e.g., beam length (l)=120–260 µm, beam width (w)=80–150 µm, beam thickness (t)=1–3 µm, electrode width (W)=60–100 µm and air gap height (g0)=1–3 µm have been widely employed for the realization of phase shifters [3]. However, such SCs having standard dimensions are prone to several inherent limitations such as, requirement of high actuation voltage (~20–40 V) to achieve down-state configuration, slower switching rate (~10–30 µs), lower frequency of vibration (~a few kHz), sensitivity to residual stress and temperature variations and a few reliability issues such as stiction and buckling effects [3]. These shortcomings can be mitigated with the employment of a SC having miniature-size dimensions as compared to the conventional equivalent. In the present context, we report the complete design, fabrication and characterization of a miniaturized SC as stated above and one major application of the device, i.e., as Phase Shifters. As evident from [3], scaling down the lateral dimensions of the conventional SC, i.e., beam length, beam width, beam thickness, electrode width and air gap height, each by 10 times (approximately), would lead to the development of a miniaturized SC, which is capable of solving most issues commonly associated with that of a conventional SC. Although, miniature-size switched capacitors are known to have a much lower Capacitance Ratio (Cr) (~2–4) as compared to the conventional counterpart, this design reports a higher Capacitance Ratio (Cr) than the ones already available in literature and hence, can be considered to be a major improvement.

It should be specified here that the paper has been categorized into two broad sections. The first part will concentrate on the design, fabrication and characterization of the miniature-size switched capacitor, whereas, the second part will deal with one major application of the above-mentioned miniature-size SC, i.e., the phase shifter unit cell.

The value of differential phase shift (in degrees) and the corresponding loss-performances (Return Loss (S11 in dB) and Insertion Loss (S21 in dB)) for the up and down-state configuration of the SC have been obtained by simulations in Ansoft HFSS v. 13 [12]. Estimation of the actuation voltage to achieve down-state configuration of the bridge from the up-state and Eigen-frequency analysis leading to generation of the fundamental and higher-order modes of vibration have also been analyzed by simulations in COMSOL Multiphysics v. 4.2a [13]. The values of actuation voltage and switching time have been verified analytically. The phase shifting miniature unit cell structure has been fabricated employing a novel technique and the various simulated and measured results have been compared and contrasted in order to validate the expected outcome.

Section snippets

Miniature-size switched capacitor design and properties

The CPW based t-line structure has been housed in low-resistivity (~20 Ω-cm) silicon substrate (due to unavailability of high-resistivity silicon substrates), 275 µm in thickness. The CPW t-line, fabricated of aluminum is 1 µm thick. At the region, where the four miniature-size SCs are placed; the CPW maintains a G/W/G spacing of 9.5 µm/5.5 µm/9.5 µm (please see Fig. 1) so as to ensure a characteristic impedance of 70 Ω at the central part of the CPW based t-line [14]. However, the CPW based t-line at

Fabrication process flow

Fabrication of the unit cell phase shifter structure employs the standard silicon surface micromachining technique which requires four broad steps in order to complete the entire fabrication procedure. The fabrication process starts with a 2-in., p-type, 〈100〉, low-resistivity (~20 Ω-cm) silicon wafer having a single-side polished surface. The steps of fabrication have been elaborated as under:

  • Wafer is cleaned to remove all possible traces of organic and inorganic contaminants [Fig. 2(a)].

  • Wet

Capacitance vs. voltage measurements

Fig. 5 shows the measured capacitance vs. voltage (CV) characteristic curve for the SC showing the transition from up-state to down-state configuration. Capacitance vs. voltage measurements have been recorded from the VEGA standard DC four-probe station equipped with Keithley 236 SMU (2 nos.), Agilent LCR Meter (1 no.) and Keithley 708A Switch Matrix (1 no.). The graph itself indicates that at a voltage of 10.5 V, the bridges begin to undergo actuation. From [3], it has been noted that

Application of the miniature-size switched capacitor

RF MEMS based switched capacitors have many applications and by far, the most prominent of them is the development of DMTL (Distributed MEMS Transmission Line) based phase shifters. Major publications relating to the development of DMTL based phase shifters have been reported in [8], [9], [10], [11], but the work discussed in the present context is novel with respect to its highly miniature-size dimensions and fabrication technique employed. Fig. 1 refers to the miniature size phase shifting

Conclusions

A detailed design, fabrication and characterization procedure of a miniature SC based phase shifting unit cell has been presented in details in the paper. The work aims at simplifying the fabrication process by reducing a major lithography step yet maintaining the overall cost within limits. The use of polymer as posts/support for the bridge structure proves to be beneficial and can have diversified applications in other device fabrication as well. Also, the paper reports to have a higher

Acknowledgments

Authors would like to express their gratitute to Indian Nanoelectronics User׳s Program (INUP (Grant No. 20(8)/2007 – NANO (Vol. III))), IIT Bombay since the entire fabrication/characterization was carried out at the CEN, IIT Bombay under INUP, which is sponsored by DeitY, MCIT, and Government of India. The authors would also like to thank the members of MNCF (Micro and Nano Characterization Facility), CeNSE (Centre of Nano Science and Engineering), Indian Institute of Science, Bangalore for

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