Current Research Projects:
MEMS RF-to-Millimeter-Wave Switches, Filters, Varactors,Oscillators and Antennas

I. WHAT IS MEMS? WHAT IS MICROMACHINING?

II. ELECTROMAGNETIC AND MECHANICAL MODELING OF MEMS SWITCHES, HIGH-ISOLATION MEMS SWITCH CIRCUITS

PRESS HERE FOR SOME COOL PICTURES ON MEMS SWITCHES. (120 K)

We have completed a detailed electromagnetic modeling of MEMS switches. The up-state capacitance can be accurately modeled using 3-D static solvers, and full-wave solvers (HFSS, Sonnet, etc.) are used to predict the current distribution and the inductance of the switch. The loss in the up-state position is equivalent to the CPW line loss and is 0.01-0.02 dB at 10-30 GHz for a 2 µm thick Au MEMS shunt switch. It is also shown that dramatic increase in the down-state isolation (20 dB) can be achieved with the choice of the correct LC series resonant frequency of the switch in the down-state position. The accurate MEMS switch model is used for the construction of a variety of high-isolation circuits, from 10 GHz to 40 GHz. Some of these circuits are: 1) The inductive tuned switch, 2) the cross-switch, 3) the tuned double-switch and 4) the distributed switch. The cross switch attained an insertion loss of less than 0.3-0.6 dB, a return loss below ­20 dB from 22-38 GHz in the up-state, and a down-state isolation of 45-50 dB with only 1.5 pF of down-state capacitance (Cd). Also, an X-band MEMS switch with an insertion loss of less than 0.2 dB and an isolation of 35 dB is presented. This is done by inductively tuning the the LC series resonance of the shunt switch. Application areas are in low-loss high-isolation communication and radar switches.

We have also developed low-loss absorptive switches at 30 GHz, and very low loss X-band microstrip shunt MEMS switches. The 30 GHz CPW SPST absorptive MEMS switch is implemented using capacitive shunt bridges with fixed-fixed beams. A return loss of better than 15 dB and an insertion loss of 0.8-1.0 dB is achieved in the up-state position. The return loss is better than 20 dB and the isolation is 25-30 dB at 30 GHz in the down-state position. Potential application areas include switch matrix communication systems. The microstrip X-Band shunt switch bandwidth are limited by the radial stubs, and careful design must be done in order to get a wideband design. A microstrip pi-network X-band switch resulted in an isolation of 30 dB with an insertion loss of 0.3 dB from 8-12 GHz.

All of the above work can be found in the papers below. Also, check the figures. There is a lot of information there!!

- J.B. Muldavin and G.M. Rebeiz, "High isolation MEMS shunt switches; Part 1: Modeling," Accepted for publication in the IEEE Trans. Microwave Theory Tech. To appear June 2000. (File: pdf 540K)

- J.B. Muldavin and G.M. Rebeiz, "High isolation MEMS shunt switches; Part 2: Design," Accepted for publication in the IEEE Trans. Microwave Theory Tech. To appear June 2000. (File: pdf 240K)

- J. Rizk, G.L. Tan, J. Muldavin and G.M. Rebeiz, "High-isolation W-band shunt MEMS switches," 10th Symp. on Space Terahertz Technology, Ann Arbor, MI, May 2000.

- J.B. Muldavin and G.M. Rebeiz,"High-isolation inductively-tuned X-band MEMS shunt switches," Accepted for the IEEE-MTT Int. Microwave Symp., Boston, June 2000.

III. MEMS PHASE SHIFTERS FOR X-BAND TO W-BAND APPLICATIONS
(AND THE FIRST MECHANICAL 35 GHz BPSK MODULATOR)

PRESS HERE FOR SOME COOL PICTURES ON MEMS PHASE SHIFTERS. (120 K)

Wideband switches and true-time delay phase shifters have been developed using distributed microelectromechanical system (MEMS) transmission lines for applications in phased array and communication systems. The design consists of a CPW transmission line (W=G=100 µm) fabricated on a 500um quartz substrate with fixed-fixed beam MEMS bridge capacitors placed periodically over the transmission line, thus creating a slow-wave structure. A single analog control voltage applied to the center conductor of the CPW line can vary the phase velocity of the loaded line by pulling down on the MEMS bridges to increase the distributed capacitive loading. The resulting change in the phase velocity yields a true-time delay (TTD) phase shift. Alternatively, the control voltage can be increased beyond the pull-down voltage of the MEMS bridges such that the capacitive loading greatly increases and shorts the line to ground. The pull-down voltage is 10-23 V depending on the residual stress in the MEMS bridge. To our knowledge, this work presents the first wideband true-time delay MEMS phase shifters and wideband switches to date.

The design and optimization of analog distributed micromechanical systems (MEMS) transmission-line phase shifters at both 40-60 GHz and 75-110 GHz has been also completed. The 40-60 GHz design results 90 degree phase shift per dB loss at 60 GHz with a 17% change in the MEMS bridge capacitance. The W-band design results in 70 degree/dB from 75-110 GHz with a 15% change in the MEMS bridge capacitance.

A wideband distributed coplanar-waveguide (CPW) phase shifter has been developed for X-band operation. In this case, the varactors are fabricated using a series combination of MEMS bridges and fixed-value MIM capacitors. A high-capacitance ratio varactor (1.5 to 2.5) results in a large loading on the CPW line and therefore a large phase shift. A distributed phase shifter was fabricated on a 500 um quartz substrate, and achieved a true-time delay operation from 1-10 GHz with a reflection coefficient less than -15 dB, and 180 degree/dB of insertion loss at 8-10 GHz. It is possible with this design to cascade the DMTL to result in 2 and 3-bit phase shifters with excellent wideband performance at X-band frequencies.

Summary of achieved results in Distributed Transmission-Line Phase Shifters:

Analog Version:
4 dB loss at 60 GHz for 360 deg. phase shift
5 dB loss at 90 GHz for 360 deg. phase shift

Digital Version:
1.8 dB loss at 10 GHz for 360 deg. phase shift
and improving every day!!!

- J.S. Hayden and G.M. Rebeiz, "Low-loss cascadable MEMS distributed X-band phase shifters," Accepted for publication in the IEEE Microwave and Guided Wave Letters. To appear April 2000. (File: pdf 360 K)

- N.S. Barker and G.M. Rebeiz, "Distributed MEMS true-time delay phase shifters and wideband switches," IEEE Trans. Microwave Theory Tech., Vol. 46, pp. 1881-1890, Nov. 1998. (IEEE 2000 Microwave Prize) (File: pdf 360 K)

- N.S. Barker and G.M. Rebeiz, "Distributed MEMS transmission line phase shifters," Submitted for publication in the IEEE Microwave Theory and Techniques, October 1999.

Micromachined BPSK Modulator:
The distributed phase shifter using an analog approach can also be made to be an extremely low power, low data rate BPSK modulator. We have fabricated a 10 Kbps modulator at 35 GHz which consumes less than 1 uW of DC power using this technique. We are pushing this design to 1 Mbps. Best of all, in this design, the MEMS varactors never touch the circuit and therefore have near-infinite cycle reliability.

- N.S. Barker and G.M. Rebeiz, "Distributed Ka-band MEMS transmission line BPSK modulator," Accepted for publication in the IEEE Microwave and Guided Wave Letters, To appear May 2000. (File: pdf 240 K)

IV. HIGH-Q MICROMACHINED RESONATORS, LOW-LOSS FILTERS AND LOW-PHASE NOISE OSCILLATORS (AND HIGH ISOLATION MICROPACKAGING)

CLICK HERE TO SEE COOL PICTURES ON MICROMACHINED RESONATORS, FILTERS, DIPLEXERS AND OSCILLATORS. (120 K)

Micromachined Resonators and Low-Loss Filters:
This work, in my personal opinion, is very neat and the closet to be applied to the real world. It details the fabrication, design and test of very high-Q micromachined resonators at 20-60 GHz and their use in very low-loss 4-5 pole filters at 28 GHz, 35 GHz, and 60 GHz. Also a 28/31 GHz planar high performance diplexer for LMDS communication systems has been constructed using this technology. The idea is simple: Suspend the resonators on thin silicon-nitride membrane and micro-package the resonators using silicon micromachining techniques. The resonator is limited by the Ohmic losses of the resonator only (no radiation, no dielectric losses), and can easily result in a resonator Q of 500-800 at 20-60 GHz.

A planar diplexer integrated on a single silicon substrate has been developed. The diplexer channels are 5% and 6.5% relative bandwidth at 28 and 31 GHz, respectively. The diplexer is based on a micropackaged, membrane supported, capacitively coupled microstrip structure and is 1.5 x 1.6 cm and only 1.4 mm thick. The weight of the diplexer is less than 1 gram. The measured insertion loss is 1.3 dB (5%) and 0.9 dB (6.5%) for the two channels with better than 40 dB isolation in the 28 GHz band and better than 50 dB isolation in the 31 GHz band. The measured results include all transition and packaging effects. The diplexer has CPW ports and can easily be integrated with other elements such as planar antennas, LNAs, and power amplifiers. This diplexer is ideal for LMDS applications, or point-to-point communication systems.

Micromachined Filter Performance

Freq (GHz)	# of Poles	Bandwidth (%)	Insertion Loss (dB)	Q of Resonator
20	4	12	1.1	330^
28	4	5	0.9	460
28	3	6	0.6	460
35*	4	3	2	450
60*	4	8	1.1	500
60*	4	3	2.7	500

^ Not optimized in resonator Q.
*: In collaboration with Univ. de Limoges, Dr. Pierre Blondy and Prof. Dominque Cros.

Micromachined Diplexer Performance

Channel (GHz)	# of Poles	Bandwidth (%)	Insertion Loss (dB)	Isolation dB (S23)
28.2	4	5	1.3	-40
31.7	3	6.5	0.9	-50

- A.R. Brown and G.M. Rebeiz, "A high-performance integrated K-band diplexer," IEEE Trans. Microwave Theory Tech., Vol. 47, pp. 1477-1481, August 1999. (File: pdf 780 K)

- A.R. Brown, P. Blondy and G.M. Rebeiz, "Microwave and millimeter-wave high-Q micromachined resonators," Int. J. of RF and Microwave Computer-Aided Engineering, Vol. 9, pp. 326-337, July 1999. (File: pdf 840 K)

- P. Blondy, A.R. Brown, D. Cros and G.M. Rebeiz, "Low loss micromachined filters for millimeter-wave communication systems," IEEE Trans. Microwave Theory Tech., Vol. 46, pp. 2308-2316, Dec. 1998. (File: 300 K)

- C.Y. Chi and G.M. Rebeiz, "Conductor-loss limited stripline resonators and filters," IEEE Trans. Microwave Theory Tech., vol. MTT-44, pp. 626-630, April 1996. (File: pdf 840 K)

Micropackaging:
Notice that the isolation between the input and output port, on the same wafer, is around 80 dB at 28 GHz. This means that micropackaging has achieved an immense amount of isolation, equivalent to metal cavity isolation, but on the same wafer!!!

We have also micropackaged filters together for applications in filter banks, and achieved an isolation better than 50 dB at X-band. In this case, the feedlines for filter #1 and filter #2 were coupling to each other and therefore limited the isolation. This work can be found in the paper below.

- A.R. Brown and G.M. Rebeiz, "Micromachined micropackaged filter banks," IEEE Microwave Guided Wave Lett., Vol. MGWL-8, pp. 158-160, April 1998. (File: pdf 120 K)

Micromachined Oscillator:
The high-Q resonators have also been used in the development of a micromachined HEMT oscillator at 28.65 GHz using a planar resonator. The resonator is micromachined close to the transistor and has an unloaded Q of 460. The oscillator design is based on the series-feedback technique which is a standard design used in most dielectric resonator oscillators (DRO). The transistor was a Fujitsu FLR20X. The oscillator performance is summarized below. The performance of the micromachined oscillator was very similar to the DRO by Funabashi with absolutely no tuning. This represents a significant improvement in low phase noise oscillator design at mm-wave frequencies.

Micromachined LMDS Oscillator

Design Freq. (GHz)	Oscillation Freq. (GHz)	Output Power (mW)	DC-RF Efficiency (%)	Phase Noise (dBc/Hz)
28.7000	28.6536	1.4	5.7	- 92 @ 100 KHz - 122 @ 1 MHz

A recent 30GHz HEMT DRO published by Funabashi in the 1995 IEEE MTT-Symposium resulted in a phase noise of -117 dBc/Hz @ 1MHz from the carrier. This means that micromachined oscillators are equivalent to DRO's at mm-wave frequencies.

- A.R. Brown and G.M. Rebeiz," A Ka-band micromachined low phase-noise oscillator," IEEE Trans. Microwave Theory Tech.,Vol, 47, pp. 1504-1508, August 1999. (File: pdf 660K)

V. HIGH-Q MEMS VARACTORS, MEMS TUNABLE FILTERS AND LOW-PHASE NOISE TUNABLE OSCILLATORS

PRESS HERE FOR SOME COOL PICTURES ON TUNABLE FILTERS. (120 K)

The field of tunable filters is ideal for high-Q MEMS varactors. We are currently developing high-Q MEMS varactors with a capacitance ratio of 1.5:1 and 2:1 and will use them in tunable filters and low phase-noise oscillators. We are also working on low loss tunable filters using Schottky varactor diodes. In this application, an 700 MHz to 1.3 GHz filter was developed with a silicon varactor with a Q of 150-50, depending on the bias voltage. The tunable filter performance is summarized in the table below.

More papers and results will be placed on this website in Summer 00.

Varactor Tunable RF Filter

Tuning Range (MHz)	# of Poles	Bandwidth (%)	Loss (dB)	Q of Resonator/Q of Diodes
700-1300	4	10-15	2-3	1,030 / 50-150

- A.R. Brown and G.M. Rebeiz, "A varactor tuned RF filter," Accepted for publication in the IEEE Trans. Microwave Theory Tech. To appear June 2000. (File: pdf 540 K)

VI. FIRST GENERATION MICROMACHINED FILTERS, TRANSMISSION LINES, COUPLERS, AND HIGH-Q LUMPED ELEMENTS

PRESS HERE FOR SOME COOL PICTURES ON MICROMACHINED CIRCUITS. (120 K)

The first generation of micromachined components were not optimized for high-Q and low loss, but were a good demonstration of the technology. Most of the work was done by Dr. Chen-Yu Chi, now at Agilent, Santa Rosa, CA (former HP), Dr. Tom Weller, now at the Univ. of Florida, and by Dr. Steve Robertson, now at Lucent Technologies, New Jersey. The papers below summarize the work done on the microshield line, interdigital filters, high rejection SSB mixers, coupled-line filters at W-band frequencies, and coupled-line couplers. Also, micromachined inductors suspended on thin dielectric membranes and with resonant frequencies up to 70 GHz have been fabricated by Chen-Yu Chi. Download the papers for a good introduction to the field.

-G.M. Rebeiz, L.P. Katehi, T.M. Weller, C.Y. Chi and S.V. Robertson, "Micromachined filters for microwave and millimeter-wave applications," Invited paper, Special Issue on Passive and Active Filters, Int. J. of Microwave and Millimeter-Wave Computer Aided Engineering, vol. 7, pp. 149-166, Feb. 1997.

- C.Y. Chi and G.M. Rebeiz, "Planar microwave and millimeter-wave lumped elements and coupled-line filters using micro-machining techniques," IEEE Trans. Microwave Theory Tech., vol. MTT-43, pp. 730-738, Apr. 1995. (File: pdf 1.1 MB)

- C.Y. Chi and G.M. Rebeiz, "Conductor-loss limited stripline resonators and filters," IEEE Trans. Microwave Theory Tech., vol. MTT-44, pp. 626-630, April 1996. (File: pdf 840 K)

- S.V. Robertson, L.P. Katehi and G.M. Rebeiz, "Micromachined W-band filters," IEEE Trans. Microwave Theory Tech., vol. MTT-44, pp. 598-606, April 1996. (File: pdf 1.1 MB)

- C.Y. Chi and G.M. Rebeiz, "Design of lange-couplers and single-sideband mixers using micromachining techniques," IEEE Trans. Microwave Theory Tech., vol. MTT-45, pp. 291-294, Feb. 1997. (File: pdf 120 K)

- S.V. Robertson, A.R. Brown, L.P. Katehi and G.M. Rebeiz, "A 10-60 GHz micromachined directional coupler," IEEE Trans. Microwave Theory Tech., Vol. 46, p. 1845-1849, Nov. 1998. (File: pdf 600 K)

- T.M. Weller, L.P. Katehi and G.M. Rebeiz, "High performance microshield line components," IEEE Trans. Microwave Theory Tech., vol. MTT-43, pp. 534-543, Mar. 1995. (File: pdf 1.3 MB)

- T.M. Weller, L.P. Katehi and G.M. Rebeiz, "A 250 GHz microshield band-pass filter," IEEE Microwave Guided Wave Lett., vol. MGWL-5, pp. 153-155, May 1995. (File: pdf 300 K)

VII. MICROMACINED ANTENNAS: 10 GHz TO 100 GHz.

PRESS HERE FOR SOME COOL PICTURES ON MICROMACHINED ANTENNAS. (120 K)

CLICK HERE FOR A PRESENTATION ON A 94 GHZ MICROMACHINED MICROSTRIP ANTENNA. (File: pdf 120 K)

Micromachining can also be applied to antennas in order to increase their efficiency. There are two ways to achieve this: 1) reduce the effective dielectric constant below the antenna by removing part of the dielectric using wet or dry etching techniques, or 2) reduce the surface wave loss in the dielectric substrate using a periodic set of holes etched in the wafer using micromachining techniques. The first method has been mostly applied to microstrip antennas at 10 GHz and 94 GHz, while the second method has been applied tapered slot antennas at 10 GHz, 35 GHz and 94 GHz. The following papers summarize the results achieved using these techniques, and work is currently being done on a micromachined tapered slot antenna at 94 GHz on a 6 mil high-resistivity silicon wafer.

- G.P. Gauthier, A. Courtay and G.M. Rebeiz, "Microstrip antennas on synthesized low dielectric constant substrates," IEEE Trans. Antennas Propag., vol. AP-45, pp. 1310-1314, Aug. 1997. (File: pdf 180 K)

- G.P. Gauthier, J.-P. Raskin, L.P. Katehi and G.M. Rebeiz, "A 94 GHz aperture-coupled micromachined microstrip antenna," Accepted for publication in the IEEE Trans. Antennas Propag., To appear Nov. 1999. (File: pdf 360 K)

- J.B. Muldavin and G.M. Rebeiz, "Millimeter-wave tapered-slot on synthesized low-permittivity substrates," IEEE Trans. Antennas Propag., Vol. 47, pp. 1276-1280, Aug. 1999. (File: pdf 180 K)

- T.J. Ellis and G.M. Rebeiz, "MM-wave tapered slot antennas on micromachined photonic bandgap dielectrics," IEEE MTT-S Int. Microwave Symp., pp. 1157-1160, June 1996. (Student Paper Award.) (File: pdf 300 K)

Related Transitions:
- K.J. Herrick, J.-G. Yook, S.V. Robertson, G.M. Rebeiz and L.P. Katehi, "W-band micromachined vertical interconnection for three-dimensional microwave ICs", European Microwave Conference, Oct. 1998.

- J.-P. Raskin, G. Gauthier, L.P. Katehi, and G.M. Rebeiz, "Mode conversion at GCPW-to-microstrip line transitions," IEEE Trans. Microwave Theory Tech., Vol. 48, pp. 158-161, Jan. 2000.

- J.-P. Raskin, G. Gauthier, L.P. Katehi and G.M. Rebeiz, "W-band single layer vertical transitions," IEEE Trans. Microwave Theory Tech., Vol. 48, pp. 161-164, Jan. 2000.

VIII. MICROMACHINING FOR TERAHERTZ APPLICATIONS:

- V. Lubecke, K. Mizuno and G.M. Rebeiz, "Micromachining for Terahertz application," Invited paper, IEEE Trans. Microwave Theory Tech., Vol. 46, pp. 1821-1831, Nov. 1998. (File: pdf 60 K)