RF MEMS phase shifters

We recently spoke with Dr. Van Caekenberghe, author of several articles on RF MEMS technology for radar sensors, about RF MEMS phase shifters. Koen shares his thoughts on the small but growing RF MEMS phase shifter market including applications, market developments, pricing and vendors of RF MEMS phase shifters as well as alternative technologies.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. In Koen's opinion, approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors -- and during the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

MEMS Investor Journal: Please provide a general description of RF MEMS phase shifters.  

Koen: RF MEMS phase shifters alter the phase of an RF signal by means of RF MEMS switches, switched capacitors, and varactors [1, 2]. Phase shifters are used in radars based on electronically scanned arrays. 

MEMS Investor Journal: How do radars work?

Koen: Radars sense angle, range and velocity of (moving) scatterers in the environment. Radar figures of merit include field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. The angle of a target is detected by scanning the field of view with a directive beam. Scanning is done electronically, by scanning the beam of an array, or mechanically, by rotating an antenna. The range and radial velocity of a target are detected through frequency modulation (FM) ranging and range differentiation (frequency modulated continuous wave radar), or through pulse delay ranging and the Doppler effect (pulse-Doppler radar). The angular resolution is inversely related to the half power beamwidth of the antenna or the array, whereas the range resolution is inversely related to the signal bandwidth. 

MEMS Investor Journal: As you mentioned, RF MEMS phase shifters are used in radars based on electronically scanned arrays. What are the main advantages of using them?

Koen: Electronically scanned arrays, or phased arrays, offer several advantages over mechanically scanned antennas such as multiple agile beams and interleaved radar modes. Figures of merit of an electronically scanned array, as shown in Fig. 1, are the bandwidth, the effective isotropically radiated power (EIRP) times the G_r/T product, the field of view, the half-power beamwidth, the pointing error, the polarization purity and the sidelobe level. EIRP is the product of the transmit gain, G_t, and the transmit power, P_t. G_r/T is the quotient of the receive gain and the antenna noise temperature. G_r and G_t are linearly related to the aperture area, whereas the half power beamwidth is inversely related to the largest aperture dimension. The field of view is limited by the antenna element spacing, d, and the pointing error is inversely related to the phase shift resolution (number of effective bits of the phase shifter).

Figure 1: Figures of merit of an electronically scanned array set the radar sensor's ability to search and track targets. 

MEMS Investor Journal: What is the history of RF MEMS phase shifters and where were they first developed? 

Koen: RF MEMS phase shifters were pioneered by HRL, Malibu, CA [3], Raytheon, Dallas, TX [4], Rockwell Science, Thousand Oaks, CA [5], and the University of Michigan, Ann Arbor, MI [6], during the nineties. Since then loaded-line, reflection, switched LC network and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors, as shown in Fig. 2. The switched LC network phase shifter is the most common phase shifter. RF MEMS distributed loaded-line and switched-line true-time-delay phase shifters will enable ultra wideband (UWB) radar sensors, whereas RF MEMS reflection phase shifters will find application in reflect arrays; a reflect array is a particular embodiment of a PESA.

Figure 2: Loaded-line, reflection, switched LC network, and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors. 

MEMS Investor Journal: Are RF MEMS phase shifters an extension of or an improvement on an existing technology? If so, can you describe for our readers the features and benefits as compared with existing systems?

Koen: While most RF MEMS switches, switched capacitors and varactors are biased electrostatically instead of magnetostatically, RF MEMS technology can be thought of being a microscopic extension of electromechanical relay and switch technology, which dates back to the 19th century [7]. The application of electromechanical relay technology is limited to the VHF band (30-300 MHz), which confines its application to tunable filters for multi-band VHF communication equipment such as used in public safety 2-way radio networks. RF MEMS technology enables the use of a broader RF spectrum, ranging from the VHF band to the W-band (75-110 GHz), with a corresponding increase in communication and sensing applications.

RF MEMS phase shifters offer lower insertion loss, and higher linearity and power handling than semiconductor phase shifters, enabling passive electronically scanned arrays (PESAs) with higher EIRP x G_r/T product and longer range detection. They do not consume prime power, but require a high control voltage and wafer-level packaging.

MEMS Investor Journal: How are RF MEMS phase shifters used today and what are the various markets in which they find application?

Koen: RF MEMS phase shifters will find application in airborne and space-borne PESA radar sensors, which require low prime power consumption but do not require long-range search and track capability. The low-altitude unmanned aerial vehicles (UAV) radar sensor market, for example, offers potential for RF MEMS phase shifters.

In general, the choice between an active electronically scanned array (AESA) and a PESA is determined by the range requirement. An AESA has distributed power amplification because every antenna is connected to a T/R module. An AESA therefore has a higher EIRP x G_r/T product (dynamic range) and better search and track capabilities than a PESA. A PESA has centralized power amplification, but offers cost, prime power consumption, size and weight savings, as shown in Fig. 3.

Some airborne platforms, such as fighter jets, have a dual need. For example they have a high-performance nose-cone AESA radar sensor to search and track agile targets, and a low-power pod-mounted PESA radar sensor underneath to measure the height, to follow (avoid) terrain, or to map the ground during a low fly over. The use depends on the range of the envisioned target.

Figure 3: AESA (left) versus PESA (right).

MEMS Investor Journal: Can you comment on the relative importance of these markets?

Koen: RF MEMS phase shifter product announcements remain rare to non-existent, and R&D budgets are under pressure nowadays. Table 1 summarizes all publicly funded programs on RF MEMS phase shifters and RF MEMS electronically scanned arrays.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. I believe approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors. During the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

Table 1: RF MEMS Electronically Scanned Array Programs

Program	Awardee	Description	Duration	Funding
European Union
ARASCOM [8]	Consortium	MEMS and liquid crystal based, agile reflectarray antennas for security and communication	2008 - 2011	5.02M USD
MEMS4MMIC [9]	Consortium	Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration	2008 - 2011	4.88M USD
Radarauge [10]	Consortium	Phasengesteuertes Radar Modul bei 79 GHz auf organischen und keramischen Substraten	2005 - 2008	4.75M USD
TUMESA [11]	Consortium	MEMS tuneable metamaterials for smart wireless applications	2008 - 2011	3.25M USD
United States
AESLA [12]	Raytheon & ONR	Active electronically scanned lens array	2008-2012	14M USD
Total: 31.9M USD (1 EURO = 1,2838 USD)

MEMS Investor Journal: Within these applications, how are the products being priced? Do you see price as an impediment to wider application or are these products priced more favorably than the technology they replace?

Koen: Bricks and tiles are the physical building blocks of an electronically scanned array. Hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks and RF MEMS phase shifters will have a resale value of $100-$250 depending on the frequency band. The price setting cannot exceed the price of a packaged AESA transmit/receive T/R module chip set, which is $250-$750 and which will continue to drop as silicon germanium bipolar and complementary metal oxide semiconductor (BiCMOS) implementations become available.

MEMS Investor Journal: What companies are producing RF MEMS Phase Shifters and what markets are they addressing?

Koen: Unlike RF MEMS resonators, RF MEMS phase shifters have not yet been commercialized. Radant MEMS demonstrated world's first RF MEMS X-band PESA in 2006 [13], and corporate research and development centers of large aerospace and defense companies such as Raytheon [12], EADS [14] and Thales continue working on RF MEMS phase shifters. Raytheon and the Office of Naval Research are developing an active electronically scanned lens array featuring gallium nitride power amplifiers and RF MEMS phase shifters. Companies that have commercialized RF MEMS switches are MEMtronics, Radant MEMS, Wispry, and XCOM Wireless.

MEMS Investor Journal: What is required for a new market entrant to gain market share?

Koen: Successful commercialization of RF MEMS phase shifters requires access to RF MEMS foundries providing packaged and reliable RF MEMS products. Access to RF MEMS foundries or products is subject to International Traffic in Arms Regulations (ITAR).

MEMS Investor Journal: What are the challenges being faced by the industry in terms of product improvements, manufacturing, pricing, applications?

Koen: Product improvement and manufacturing challenges being faced by potential RF MEMS phase shifter vendors are the integration of multi-level plating and wafer-level packaging in the RF MEMS fabrication process while maintaining wafer yield. RF MEMS phase shifters are electrically large structures mainly consisting of transmission lines. In order to fulfill the low insertion loss promise of RF MEMS technology, multi-level plated transmission lines with a thickness in excess of 5 micron and large hermetic wafer-level packages are required. Pricing and application challenges include the development of a market for mobile or portable PESA radar sensors.

MEMS Investor Journal: What do you see as the next major development in RF MEMS Phase Shifters? Where is this technology headed?

Koen: The emphasis will shift toward aforementioned hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks, and RF MEMS phase shifters. It is unlikely that unpackaged RF MEMS phase shifters will have a resale value because of their fragility.

MEMS Investor Journal: Are there competing technologies and how do RF MEMS phase shifters compare with competing technologies?

Koen: AESA radar sensors are accommodated by high-performance monolithic microwave integrated circuits (MMICs) based on gallium arsenic, gallium nitride, and indium phosphide III-V compound semiconductor technology. MMICs allow for integration of attenuators, drivers, duplexers, low noise amplifiers, and phase shifters on a single chip. Future AESA radar sensors may be accommodated by low-cost radio frequency integrated circuits (RFICs) based on silicon germanium BiCMOS technology.

PESA radar sensors are best accommodated with RF MEMS phase shifters in order to avoid the need for electronic power conditioning. As already mentioned, RF MEMS switches are biased electrostatically by a high control voltage, but they do not draw current from the supply. Therefore, there is no prime power consumption. Besides planar RF MEMS phase shifters, waveguide ferrite and ferroelectric phase shifters could be used in PESA radar sensors. Waveguide ferrite and ferroelectric phase shifters have a high power handling, but are heavy and voluminous.

A comparison of state-of-the-art switched LC network phase shifters is made in Table 2. Figures of merit of phase shifters can be related to radar sensor performance as follows:

• A large bandwidth enables radar sensors with a high range resolution.
• A high number of effective bits and low RMS errors enable radar sensors with low pointing error.
• A low insertion loss, a high linearity, a high phase shift / noise figure, and a high power handling enable long-range radars.
• A low prime power consumption, size and weight enable low-power mobile and portable radar sensors.

One might state that for a given amount of effective bits, advantages of RF MEMS phase shifters are lower insertion loss, higher linearity, lower noise figure, and no prime power consumption. Disadvantages of RF MEMS phase shifters are higher RMS errors due to process variations on the RF MEMS switches, larger size and slower switching. Well-designed wafer-level packages are required to preserve the low insertion loss of RF MEMS phase shifters.

Table 2: Comparison of State-of-the-Art Switched LC Network (High-Pass/Low-Pass) Phase Shifters

	MMIC		RFIC	RF MEMS
Reference	Bahl, et al.	MIMIX XS1000	Min, et al.	Morton, et al.
	2008 [14]	2008 [15]	2008 [16]	2008 [17]
RF Technology	0.4 μm GaAs MESFET	0.5 μm GaAs PHEMT	0.12 μm SiGe BiCMOS	240 μm capacitive cantilever switch
Foundry	M/A-COM	WIN	IBM	Georgia Tech
Design characteristics
Bus	parallel	parallel	parallel	parallel
Differential or single-ended	single-ended	single-ended	differential	single-ended
Frequency	10-16 GHz	7-13 GHz	31-38 GHz	8-12 GHz
Packaging requirement	none	none	none	hermetic
Metal thickness	4.5 μm	3.1 μm	4 μm	1.5 μm
Substrate	125 μm GaAs	100 μm GaAs	9.25 μm SiO2	10 μm SiO2
	εr = 12.9	εr = 12.9	εr = 4.2	εr = 4.2
	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001
Supply voltage	-5/0/5 V	-7.5/0 V	0/1.5 V	-22/0/22 V
Transmission line	microstrip	microstrip	11/12/11 μm GCPW	microstrip
Figures of merit
Bandwidth	6 GHz	6 GHz	7 GHz	4 GHz
Effective bits	4	6	4	5
Insertion loss	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
	1.25 dB/bit	1.03 dB/bit	3.25 dB/bit	0.9 dB/bit (1.6 dB/bit packaged)
Linearity (IP1dB)	29-30 dBm	25 dBm	10 dBm	> 30 dBm
Noise figure	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
Power consumption	20 mW	< 100 mW	0 mW	0 mW
Power handling	-	30 dBm	-	30 dBm
RMS amplitude error	0.3 dB	0.9 dB	2 dB	1.29 dB
RMS phase error	4°	2.5°	11°	10°
Size	2.6 mm2	3 x 2.1 mm2	530 x 220 μm2	7.1 x 1.3 mm2
Switching time	< 20 ns	< 45 ns	- ns	13 μs

References

[1] Radant MEMS president discusses RF switches. Available: http://www.memsinvestorjournal.com/ 2006/02/radant_mems_ceo.html

[2] RF MEMS: a brief history and future trends. Available: http://www.memsinvestorjournal.com/ 2006/10/rf_mems_a_brief.html

[3] C. Quan, J. J. Lee, B. M. Pierce, and R. C. Allison, "Wideband 2-D electronically scanned array with compact CTS feed and MEMS phase shifters," U.S. Patent 6,822,615, February 25, 2003.

[4] B. Pillans, S. Eshelman, A. Malczewski, J. Ehmke, and C. Goldsmith, "Ka-band RF MEMS phase shifters," IEEE Microwave Wireless Compon. Lett., vol. 9, no. 12, pp. 520-522, December 1999.

[5] J. B. Hacker, R. E. Mihailovich, M. Kim, and J. F. DeNatale, "A Ka-band 3-bit RF MEMS true-time-delay network," IEEE Trans. Microwave Theory Tech., vol. 51, no. 1, pp. 305-308, January 2003.

[6] N. S. Barker and G. M. Rebeiz, "Distributed MEMS true-time-delay phase shifters and wideband switches," IEEE Trans. Microwave Theory Tech., vol. 46, no. 11, pp. 1881-1890, November 1998.

[7] Relay - Wikipedia, the free encyclopedia. Available: http://en.wikipedia.org/wiki/relay

[8] MEMS and liquid crystal based, agile reflectarray antennas for security and communication. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-arascom_en.pdf

[9] Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ ict/docs/micro-nanosystems/20080630-rf-mems-cluster-workshop-report_en.pdf

[10] Phase steered radar module at 79 GHz on organic and ceramic substrates. Available: http://www.radarauge-project.com

[11] MEMS tuneable metamaterials for smart wireless applications. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-tumesa_en.pdf

[12] Raytheon technologies promise to improve radar affordability. Available: http://investor.raytheon.com/phoenix.zhtml?c=84193&p=irol-newsArticle&ID=1176149

[13] World's first demonstration of microelectromechanical systems-based X-band radar. Available: http://radantmems.com/radantmems/04-06-06.html

[14] A. Stehle, G. Georgiev, V. Ziegler, B. Schinlinner, U. Prechtel, H. Seidel, and U. Schmid, "RF MEMS switch and phase shifter optimized for W-band," European Microwave Conference, pp. 104-107, October 2008.

[15] I. J. Bahl and M. Dayton, "A Ku-band 4-bit compact octave bandwidth GaAs MMIC phase shifter," Microwave Journal, vol. 51, no. 6, pp. 30-42, June 2008.

[16] XS1000-BD 6-bit digital phase shifter. Available: http://www.mimixbroadband.com/defense.asp

[17] B. Min and G. M. Rebeiz, "Single-ended and differential Ka-band BiCMOS phased array front-ends," IEEE J. Solid-State Circuits, vol. 43, no. 10, pp. 2239-2250, October 2008.

[18] M. A. Morton and J. Papapolymerou, "A packaged MEMS-based 5-bit X-band high-pass/low-pass phase shifter," IEEE Trans. Microwave Theory Tech., vol. 56, no. 9, pp. 2025-2031, September 2008.

**************************************************

Koen Van Caekenberghe received the Ph.D. degree in electrical engineering from the University of Michigan, Ann Arbor, in 2007. His doctoral research involved RF MEMS technology for radar sensors.

Copyright 2009 MEMS Investor Journal

We recently spoke with Dr. Van Caekenberghe, author of several articles on RF MEMS technology for radar sensors, about RF MEMS phase shifters. Koen shares his thoughts on the small but growing RF MEMS phase shifter market including applications, market developments, pricing and vendors of RF MEMS phase shifters as well as alternative technologies.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. In Koen's opinion, approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors -- and during the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

MEMS Investor Journal: Please provide a general description of RF MEMS phase shifters.

Koen: RF MEMS phase shifters alter the phase of an RF signal by means of RF MEMS switches, switched capacitors, and varactors [1, 2]. Phase shifters are used in radars based on electronically scanned arrays.

MEMS Investor Journal: How do radars work?

Koen: Radars sense angle, range and velocity of (moving) scatterers in the environment. Radar figures of merit include field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. The angle of a target is detected by scanning the field of view with a directive beam. Scanning is done electronically, by scanning the beam of an array, or mechanically, by rotating an antenna. The range and radial velocity of a target are detected through frequency modulation (FM) ranging and range differentiation (frequency modulated continuous wave radar), or through pulse delay ranging and the Doppler effect (pulse-Doppler radar). The angular resolution is inversely related to the half power beamwidth of the antenna or the array, whereas the range resolution is inversely related to the signal bandwidth.

MEMS Investor Journal: As you mentioned, RF MEMS phase shifters are used in radars based on electronically scanned arrays. What are the main advantages of using them?

Koen: Electronically scanned arrays, or phased arrays, offer several advantages over mechanically scanned antennas such as multiple agile beams and interleaved radar modes. Figures of merit of an electronically scanned array, as shown in Fig. 1, are the bandwidth, the effective isotropically radiated power (EIRP) times the G_r/T product, the field of view, the half-power beamwidth, the pointing error, the polarization purity and the sidelobe level. EIRP is the product of the transmit gain, G_t, and the transmit power, P_t. G_r/T is the quotient of the receive gain and the antenna noise temperature. G_r and G_t are linearly related to the aperture area, whereas the half power beamwidth is inversely related to the largest aperture dimension. The field of view is limited by the antenna element spacing, d, and the pointing error is inversely related to the phase shift resolution (number of effective bits of the phase shifter).

Figure 1: Figures of merit of an electronically scanned array set the radar sensor�s ability to search and track targets.

MEMS Investor Journal: What is the history of RF MEMS phase shifters and where were they first developed?

Koen: RF MEMS phase shifters were pioneered by HRL, Malibu, CA [3], Raytheon, Dallas, TX [4], Rockwell Science, Thousand Oaks, CA [5], and the University of Michigan, Ann Arbor, MI [6], during the nineties. Since then loaded-line, reflection, switched LC network and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors, as shown in Fig. 2. The switched LC network phase shifter is the most common phase shifter. RF MEMS distributed loaded-line and switched-line true-time-delay phase shifters will enable ultra wideband (UWB) radar sensors, whereas RF MEMS reflection phase shifters will find application in reflect arrays; a reflect array is a particular embodiment of a PESA.

Figure 2: Loaded-line, reflection, switched LC network, and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors.

MEMS Investor Journal: Are RF MEMS phase shifters an extension of or an improvement on an existing technology? If so, can you describe for our readers the features and benefits as compared with existing systems?

Koen: While most RF MEMS switches, switched capacitors and varactors are biased electrostatically instead of magnetostatically, RF MEMS technology can be thought of being a microscopic extension of electromechanical relay and switch technology, which dates back to the 19th century [7]. The application of electromechanical relay technology is limited to the VHF band (30-300 MHz), which confines its application to tunable filters for multi-band VHF communication equipment such as used in public safety 2-way radio networks. RF MEMS technology enables the use of a broader RF spectrum, ranging from the VHF band to the W-band (75-110 GHz), with a corresponding increase in communication and sensing applications.

RF MEMS phase shifters offer lower insertion loss, and higher linearity and power handling than semiconductor phase shifters, enabling passive electronically scanned arrays (PESAs) with higher EIRP x G_r/T product and longer range detection. They do not consume prime power, but require a high control voltage and wafer-level packaging.

MEMS Investor Journal: How are RF MEMS phase shifters used today and what are the various markets in which they find application?

Koen: RF MEMS phase shifters will find application in airborne and space-borne PESA radar sensors, which require low prime power consumption but do not require long-range search and track capability. The low-altitude unmanned aerial vehicles (UAV) radar sensor market, for example, offers potential for RF MEMS phase shifters.

In general, the choice between an active electronically scanned array (AESA) and a PESA is determined by the range requirement. An AESA has distributed power amplification because every antenna is connected to a T/R module. An AESA therefore has a higher EIRP x G_r/T product (dynamic range) and better search and track capabilities than a PESA. A PESA has centralized power amplification, but offers cost, prime power consumption, size and weight savings, as shown in Fig. 3.

Some airborne platforms, such as fighter jets, have a dual need. For example they have a high-performance nose-cone AESA radar sensor to search and track agile targets, and a low-power pod-mounted PESA radar sensor underneath to measure the height, to follow (avoid) terrain, or to map the ground during a low fly over. The use depends on the range of the envisioned target.

Figure 3: AESA (left) versus PESA (right).

MEMS Investor Journal: Can you comment on the relative importance of these markets?

Koen: RF MEMS phase shifter product announcements remain rare to non-existent, and R&D budgets are under pressure nowadays. Table 1 summarizes all publicly funded programs on RF MEMS phase shifters and RF MEMS electronically scanned arrays.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. I believe approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors. During the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

Table 1: RF MEMS Electronically Scanned Array Programs

Program	Awardee	Description	Duration	Funding
European Union
ARASCOM [8]	Consortium	MEMS and liquid crystal based, agile reflectarray antennas for security and communication	2008 - 2011	5.02M USD
MEMS4MMIC [9]	Consortium	Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration	2008 - 2011	4.88M USD
Radarauge [10]	Consortium	Phasengesteuertes Radar Modul bei 79 GHz auf organischen und keramischen Substraten	2005 - 2008	4.75M USD
TUMESA [11]	Consortium	MEMS tuneable metamaterials for smart wireless applications	2008 - 2011	3.25M USD
United States
AESLA [12]	Raytheon & ONR	Active electronically scanned lens array	2008-2012	14M USD
Total: 31.9M USD (1 EURO = 1,2838 USD)

MEMS Investor Journal: Within these applications, how are the products being priced? Do you see price as an impediment to wider application or are these products priced more favorably than the technology they replace?

Koen: Bricks and tiles are the physical building blocks of an electronically scanned array. Hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks and RF MEMS phase shifters will have a resale value of $100-$250 depending on the frequency band. The price setting cannot exceed the price of a packaged AESA transmit/receive T/R module chip set, which is $250-$750 and which will continue to drop as silicon germanium bipolar and complementary metal oxide semiconductor (BiCMOS) implementations become available.

MEMS Investor Journal: What companies are producing RF MEMS Phase Shifters and what markets are they addressing?

Koen: Unlike RF MEMS resonators, RF MEMS phase shifters have not yet been commercialized. Radant MEMS demonstrated world's first RF MEMS X-band PESA in 2006 [13], and corporate research and development centers of large aerospace and defense companies such as Raytheon [12], EADS [14] and Thales continue working on RF MEMS phase shifters. Raytheon and the Office of Naval Research are developing an active electronically scanned lens array featuring gallium nitride power amplifiers and RF MEMS phase shifters. Companies that have commercialized RF MEMS switches are MEMtronics, Radant MEMS, Wispry, and XCOM Wireless.

MEMS Investor Journal: What is required for a new market entrant to gain market share?

Koen: Successful commercialization of RF MEMS phase shifters requires access to RF MEMS foundries providing packaged and reliable RF MEMS products. Access to RF MEMS foundries or products is subject to International Traffic in Arms Regulations (ITAR).

MEMS Investor Journal: What are the challenges being faced by the industry in terms of product improvements, manufacturing, pricing, applications?

Koen: Product improvement and manufacturing challenges being faced by potential RF MEMS phase shifter vendors are the integration of multi-level plating and wafer-level packaging in the RF MEMS fabrication process while maintaining wafer yield. RF MEMS phase shifters are electrically large structures mainly consisting of transmission lines. In order to fulfill the low insertion loss promise of RF MEMS technology, multi-level plated transmission lines with a thickness in excess of 5 micron and large hermetic wafer-level packages are required. Pricing and application challenges include the development of a market for mobile or portable PESA radar sensors.

MEMS Investor Journal: What do you see as the next major development in RF MEMS Phase Shifters? Where is this technology headed?

Koen: The emphasis will shift toward aforementioned hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks, and RF MEMS phase shifters. It is unlikely that unpackaged RF MEMS phase shifters will have a resale value because of their fragility.

MEMS Investor Journal: Are there competing technologies and how do RF MEMS phase shifters compare with competing technologies?

Koen: AESA radar sensors are accommodated by high-performance monolithic microwave integrated circuits (MMICs) based on gallium arsenic, gallium nitride, and indium phosphide III-V compound semiconductor technology. MMICs allow for integration of attenuators, drivers, duplexers, low noise amplifiers, and phase shifters on a single chip. Future AESA radar sensors may be accommodated by low-cost radio frequency integrated circuits (RFICs) based on silicon germanium BiCMOS technology.

PESA radar sensors are best accommodated with RF MEMS phase shifters in order to avoid the need for electronic power conditioning. As already mentioned, RF MEMS switches are biased electrostatically by a high control voltage, but they do not draw current from the supply. Therefore, there is no prime power consumption. Besides planar RF MEMS phase shifters, waveguide ferrite and ferroelectric phase shifters could be used in PESA radar sensors. Waveguide ferrite and ferroelectric phase shifters have a high power handling, but are heavy and voluminous.

A comparison of state-of-the-art switched LC network phase shifters is made in Table 2. Figures of merit of phase shifters can be related to radar sensor performance as follows:

• A large bandwidth enables radar sensors with a high range resolution.
• A high number of effective bits and low RMS errors enable radar sensors with low pointing error.
• A low insertion loss, a high linearity, a high phase shift / noise figure, and a high power handling enable long-range radars.
• A low prime power consumption, size and weight enable low-power mobile and portable radar sensors.

One might state that for a given amount of effective bits, advantages of RF MEMS phase shifters are lower insertion loss, higher linearity, lower noise figure, and no prime power consumption. Disadvantages of RF MEMS phase shifters are higher RMS errors due to process variations on the RF MEMS switches, larger size and slower switching. Well-designed wafer-level packages are required to preserve the low insertion loss of RF MEMS phase shifters.

Table 2: Comparison of State-of-the-Art Switched LC Network (High-Pass/Low-Pass) Phase Shifters

	MMIC		RFIC	RF MEMS
Reference	Bahl, et al.	MIMIX XS1000	Min, et al.	Morton, et al.
	2008 [14]	2008 [15]	2008 [16]	2008 [17]
RF Technology	0.4 μm GaAs MESFET	0.5 μm GaAs PHEMT	0.12 μm SiGe BiCMOS	240 μm capacitive cantilever switch
Foundry	M/A-COM	WIN	IBM	Georgia Tech
Design characteristics
Bus	parallel	parallel	parallel	parallel
Differential or single-ended	single-ended	single-ended	differential	single-ended
Frequency	10-16 GHz	7-13 GHz	31-38 GHz	8-12 GHz
Packaging requirement	none	none	none	hermetic
Metal thickness	4.5 μm	3.1 μm	4 μm	1.5 μm
Substrate	125 μm GaAs	100 μm GaAs	9.25 μm SiO2	10 μm SiO2
	εr = 12.9	εr = 12.9	εr = 4.2	εr = 4.2
	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001
Supply voltage	-5/0/5 V	-7.5/0 V	0/1.5 V	-22/0/22 V
Transmission line	microstrip	microstrip	11/12/11 μm GCPW	microstrip
Figures of merit
Bandwidth	6 GHz	6 GHz	7 GHz	4 GHz
Effective bits	4	6	4	5
Insertion loss	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
	1.25 dB/bit	1.03 dB/bit	3.25 dB/bit	0.9 dB/bit (1.6 dB/bit packaged)
Linearity (IP1dB)	29-30 dBm	25 dBm	10 dBm	> 30 dBm
Noise figure	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
Power consumption	20 mW	< 100 mW	0 mW	0 mW
Power handling	-	30 dBm	-	30 dBm
RMS amplitude error	0.3 dB	0.9 dB	2 dB	1.29 dB
RMS phase error	4°	2.5°	11°	10°
Size	2.6 mm2	3 x 2.1 mm2	530 x 220 μm2	7.1 x 1.3 mm2
Switching time	< 20 ns	< 45 ns	- ns	13 μs

References

[1] Radant MEMS president discusses RF switches. Available: http://www.memsinvestorjournal.com/ 2006/02/radant_mems_ceo.html

[2] RF MEMS: a brief history and future trends. Available: http://www.memsinvestorjournal.com/ 2006/10/rf_mems_a_brief.html

[3] C. Quan, J. J. Lee, B. M. Pierce, and R. C. Allison, �Wideband 2-D electronically scanned array with compact CTS feed and MEMS phase shifters,� U.S. Patent 6,822,615, February 25, 2003.

[4] B. Pillans, S. Eshelman, A. Malczewski, J. Ehmke, and C. Goldsmith, �Ka-band RF MEMS phase shifters,� IEEE Microwave Wireless Compon. Lett., vol. 9, no. 12, pp. 520�522, December 1999.

[5] J. B. Hacker, R. E. Mihailovich, M. Kim, and J. F. DeNatale, �A Ka-band 3-bit RF MEMS true-time-delay network,� IEEE Trans. Microwave Theory Tech., vol. 51, no. 1, pp. 305�308, January 2003.

[6] N. S. Barker and G. M. Rebeiz, �Distributed MEMS true-time-delay phase shifters and wideband switches,� IEEE Trans. Microwave Theory Tech., vol. 46, no. 11, pp. 1881�1890, November 1998.

[7] Relay - Wikipedia, the free encyclopedia. Available: http://en.wikipedia.org/wiki/relay

[8] MEMS and liquid crystal based, agile reflectarray antennas for security and communication. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-arascom_en.pdf

[9] Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ ict/docs/micro-nanosystems/20080630-rf-mems-cluster-workshop-report_en.pdf

[10] Phase steered radar module at 79 GHz on organic and ceramic substrates. Available: http://www.radarauge-project.com

[11] MEMS tuneable metamaterials for smart wireless applications. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-tumesa_en.pdf

[12] Raytheon technologies promise to improve radar affordability. Available: http://investor.raytheon.com/phoenix.zhtml?c=84193&p=irol-newsArticle&ID=1176149

[13] World's first demonstration of microelectromechanical systems-based X-band radar. Available: http://radantmems.com/radantmems/04-06-06.html

[14] A. Stehle, G. Georgiev, V. Ziegler, B. Sch�nlinner, U. Prechtel, H. Seidel, and U. Schmid, �RF MEMS switch and phase shifter optimized for W-band,� European Microwave Conference, pp. 104�107, October 2008.

[15] I. J. Bahl and M. Dayton, �A Ku-band 4-bit compact octave bandwidth GaAs MMIC phase shifter,� Microwave Journal, vol. 51, no. 6, pp. 30�42, June 2008.

[16] XS1000-BD 6-bit digital phase shifter. Available: http://www.mimixbroadband.com/defense.asp

[17] B. Min and G. M. Rebeiz, �Single-ended and differential Ka-band BiCMOS phased array front-ends,� IEEE J. Solid-State Circuits, vol. 43, no. 10, pp. 2239�2250, October 2008.

[18] M. A. Morton and J. Papapolymerou, �A packaged MEMS-based 5-bit X-band high-pass/low-pass phase shifter,� IEEE Trans. Microwave Theory Tech., vol. 56, no. 9, pp. 2025�2031, September 2008.

**************************************************

Koen Van Caekenberghe received the Ph.D. degree in electrical engineering from the University of Michigan, Ann Arbor, in 2007. His doctoral research involved RF MEMS technology for radar sensors.

Copyright 2009 MEMS Investor Journal

We recently spoke with Dr. Van Caekenberghe, author of several articles on RF MEMS technology for radar sensors, about RF MEMS phase shifters. Koen shares his thoughts on the small but growing RF MEMS phase shifter market including applications, market developments, pricing and vendors of RF MEMS phase shifters as well as alternative technologies.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. In Koen's opinion, approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors -- and during the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

MEMS Investor Journal: Please provide a general description of RF MEMS phase shifters.

Koen: RF MEMS phase shifters alter the phase of an RF signal by means of RF MEMS switches, switched capacitors, and varactors [1, 2]. Phase shifters are used in radars based on electronically scanned arrays.

MEMS Investor Journal: How do radars work?

Koen: Radars sense angle, range and velocity of (moving) scatterers in the environment. Radar figures of merit include field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. The angle of a target is detected by scanning the field of view with a directive beam. Scanning is done electronically, by scanning the beam of an array, or mechanically, by rotating an antenna. The range and radial velocity of a target are detected through frequency modulation (FM) ranging and range differentiation (frequency modulated continuous wave radar), or through pulse delay ranging and the Doppler effect (pulse-Doppler radar). The angular resolution is inversely related to the half power beamwidth of the antenna or the array, whereas the range resolution is inversely related to the signal bandwidth.

MEMS Investor Journal: As you mentioned, RF MEMS phase shifters are used in radars based on electronically scanned arrays. What are the main advantages of using them?

Koen: Electronically scanned arrays, or phased arrays, offer several advantages over mechanically scanned antennas such as multiple agile beams and interleaved radar modes. Figures of merit of an electronically scanned array, as shown in Fig. 1, are the bandwidth, the effective isotropically radiated power (EIRP) times the G_r/T product, the field of view, the half-power beamwidth, the pointing error, the polarization purity and the sidelobe level. EIRP is the product of the transmit gain, G_t, and the transmit power, P_t. G_r/T is the quotient of the receive gain and the antenna noise temperature. G_r and G_t are linearly related to the aperture area, whereas the half power beamwidth is inversely related to the largest aperture dimension. The field of view is limited by the antenna element spacing, d, and the pointing error is inversely related to the phase shift resolution (number of effective bits of the phase shifter).

Figure 1: Figures of merit of an electronically scanned array set the radar sensor�s ability to search and track targets.

MEMS Investor Journal: What is the history of RF MEMS phase shifters and where were they first developed?

Koen: RF MEMS phase shifters were pioneered by HRL, Malibu, CA [3], Raytheon, Dallas, TX [4], Rockwell Science, Thousand Oaks, CA [5], and the University of Michigan, Ann Arbor, MI [6], during the nineties. Since then loaded-line, reflection, switched LC network and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors, as shown in Fig. 2. The switched LC network phase shifter is the most common phase shifter. RF MEMS distributed loaded-line and switched-line true-time-delay phase shifters will enable ultra wideband (UWB) radar sensors, whereas RF MEMS reflection phase shifters will find application in reflect arrays; a reflect array is a particular embodiment of a PESA.

Figure 2: Loaded-line, reflection, switched LC network, and switched-line phase shifter designs have been implemented using RF MEMS switches, switched capacitors and varactors.

MEMS Investor Journal: Are RF MEMS phase shifters an extension of or an improvement on an existing technology? If so, can you describe for our readers the features and benefits as compared with existing systems?

Koen: While most RF MEMS switches, switched capacitors and varactors are biased electrostatically instead of magnetostatically, RF MEMS technology can be thought of being a microscopic extension of electromechanical relay and switch technology, which dates back to the 19th century [7]. The application of electromechanical relay technology is limited to the VHF band (30-300 MHz), which confines its application to tunable filters for multi-band VHF communication equipment such as used in public safety 2-way radio networks. RF MEMS technology enables the use of a broader RF spectrum, ranging from the VHF band to the W-band (75-110 GHz), with a corresponding increase in communication and sensing applications.

RF MEMS phase shifters offer lower insertion loss, and higher linearity and power handling than semiconductor phase shifters, enabling passive electronically scanned arrays (PESAs) with higher EIRP x G_r/T product and longer range detection. They do not consume prime power, but require a high control voltage and wafer-level packaging.

MEMS Investor Journal: How are RF MEMS phase shifters used today and what are the various markets in which they find application?

Koen: RF MEMS phase shifters will find application in airborne and space-borne PESA radar sensors, which require low prime power consumption but do not require long-range search and track capability. The low-altitude unmanned aerial vehicles (UAV) radar sensor market, for example, offers potential for RF MEMS phase shifters.

In general, the choice between an active electronically scanned array (AESA) and a PESA is determined by the range requirement. An AESA has distributed power amplification because every antenna is connected to a T/R module. An AESA therefore has a higher EIRP x G_r/T product (dynamic range) and better search and track capabilities than a PESA. A PESA has centralized power amplification, but offers cost, prime power consumption, size and weight savings, as shown in Fig. 3.

Some airborne platforms, such as fighter jets, have a dual need. For example they have a high-performance nose-cone AESA radar sensor to search and track agile targets, and a low-power pod-mounted PESA radar sensor underneath to measure the height, to follow (avoid) terrain, or to map the ground during a low fly over. The use depends on the range of the envisioned target.

Figure 3: AESA (left) versus PESA (right).

MEMS Investor Journal: Can you comment on the relative importance of these markets?

Koen: RF MEMS phase shifter product announcements remain rare to non-existent, and R&D budgets are under pressure nowadays. Table 1 summarizes all publicly funded programs on RF MEMS phase shifters and RF MEMS electronically scanned arrays.

The radar sensor market has a global turnover of about $6.25 billion annually according to Defense Industry Daily. I believe approximately 50% of the budget is spent on airborne, ground-based, and naval AESA radar sensors, and approximately 25% of the budget is spent on mechanically scanned radar sensors. During the next decade, 20% of the mechanically scanned radar sensors might be replaced by PESA radar sensors based on RF MEMS shifters, resulting in a potential global market of $300 million annually.

Table 1: RF MEMS Electronically Scanned Array Programs

Program	Awardee	Description	Duration	Funding
European Union
ARASCOM [8]	Consortium	MEMS and liquid crystal based, agile reflectarray antennas for security and communication	2008 - 2011	5.02M USD
MEMS4MMIC [9]	Consortium	Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration	2008 - 2011	4.88M USD
Radarauge [10]	Consortium	Phasengesteuertes Radar Modul bei 79 GHz auf organischen und keramischen Substraten	2005 - 2008	4.75M USD
TUMESA [11]	Consortium	MEMS tuneable metamaterials for smart wireless applications	2008 - 2011	3.25M USD
United States
AESLA [12]	Raytheon & ONR	Active electronically scanned lens array	2008-2012	14M USD
Total: 31.9M USD (1 EURO = 1,2838 USD)

MEMS Investor Journal: Within these applications, how are the products being priced? Do you see price as an impediment to wider application or are these products priced more favorably than the technology they replace?

Koen: Bricks and tiles are the physical building blocks of an electronically scanned array. Hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks and RF MEMS phase shifters will have a resale value of $100-$250 depending on the frequency band. The price setting cannot exceed the price of a packaged AESA transmit/receive T/R module chip set, which is $250-$750 and which will continue to drop as silicon germanium bipolar and complementary metal oxide semiconductor (BiCMOS) implementations become available.

MEMS Investor Journal: What companies are producing RF MEMS Phase Shifters and what markets are they addressing?

Koen: Unlike RF MEMS resonators, RF MEMS phase shifters have not yet been commercialized. Radant MEMS demonstrated world's first RF MEMS X-band PESA in 2006 [13], and corporate research and development centers of large aerospace and defense companies such as Raytheon [12], EADS [14] and Thales continue working on RF MEMS phase shifters. Raytheon and the Office of Naval Research are developing an active electronically scanned lens array featuring gallium nitride power amplifiers and RF MEMS phase shifters. Companies that have commercialized RF MEMS switches are MEMtronics, Radant MEMS, Wispry, and XCOM Wireless.

MEMS Investor Journal: What is required for a new market entrant to gain market share?

Koen: Successful commercialization of RF MEMS phase shifters requires access to RF MEMS foundries providing packaged and reliable RF MEMS products. Access to RF MEMS foundries or products is subject to International Traffic in Arms Regulations (ITAR).

MEMS Investor Journal: What are the challenges being faced by the industry in terms of product improvements, manufacturing, pricing, applications?

Koen: Product improvement and manufacturing challenges being faced by potential RF MEMS phase shifter vendors are the integration of multi-level plating and wafer-level packaging in the RF MEMS fabrication process while maintaining wafer yield. RF MEMS phase shifters are electrically large structures mainly consisting of transmission lines. In order to fulfill the low insertion loss promise of RF MEMS technology, multi-level plated transmission lines with a thickness in excess of 5 micron and large hermetic wafer-level packages are required. Pricing and application challenges include the development of a market for mobile or portable PESA radar sensors.

MEMS Investor Journal: What do you see as the next major development in RF MEMS Phase Shifters? Where is this technology headed?

Koen: The emphasis will shift toward aforementioned hermetically packaged bricks and tiles comprising monolithically integrated antennas, feed networks, and RF MEMS phase shifters. It is unlikely that unpackaged RF MEMS phase shifters will have a resale value because of their fragility.

MEMS Investor Journal: Are there competing technologies and how do RF MEMS phase shifters compare with competing technologies?

Koen: AESA radar sensors are accommodated by high-performance monolithic microwave integrated circuits (MMICs) based on gallium arsenic, gallium nitride, and indium phosphide III-V compound semiconductor technology. MMICs allow for integration of attenuators, drivers, duplexers, low noise amplifiers, and phase shifters on a single chip. Future AESA radar sensors may be accommodated by low-cost radio frequency integrated circuits (RFICs) based on silicon germanium BiCMOS technology.

PESA radar sensors are best accommodated with RF MEMS phase shifters in order to avoid the need for electronic power conditioning. As already mentioned, RF MEMS switches are biased electrostatically by a high control voltage, but they do not draw current from the supply. Therefore, there is no prime power consumption. Besides planar RF MEMS phase shifters, waveguide ferrite and ferroelectric phase shifters could be used in PESA radar sensors. Waveguide ferrite and ferroelectric phase shifters have a high power handling, but are heavy and voluminous.

A comparison of state-of-the-art switched LC network phase shifters is made in Table 2. Figures of merit of phase shifters can be related to radar sensor performance as follows:

• A large bandwidth enables radar sensors with a high range resolution.
• A high number of effective bits and low RMS errors enable radar sensors with low pointing error.
• A low insertion loss, a high linearity, a high phase shift / noise figure, and a high power handling enable long-range radars.
• A low prime power consumption, size and weight enable low-power mobile and portable radar sensors.

One might state that for a given amount of effective bits, advantages of RF MEMS phase shifters are lower insertion loss, higher linearity, lower noise figure, and no prime power consumption. Disadvantages of RF MEMS phase shifters are higher RMS errors due to process variations on the RF MEMS switches, larger size and slower switching. Well-designed wafer-level packages are required to preserve the low insertion loss of RF MEMS phase shifters.

Table 2: Comparison of State-of-the-Art Switched LC Network (High-Pass/Low-Pass) Phase Shifters

	MMIC		RFIC	RF MEMS
Reference	Bahl, et al.	MIMIX XS1000	Min, et al.	Morton, et al.
	2008 [14]	2008 [15]	2008 [16]	2008 [17]
RF Technology	0.4 μm GaAs MESFET	0.5 μm GaAs PHEMT	0.12 μm SiGe BiCMOS	240 μm capacitive cantilever switch
Foundry	M/A-COM	WIN	IBM	Georgia Tech
Design characteristics
Bus	parallel	parallel	parallel	parallel
Differential or single-ended	single-ended	single-ended	differential	single-ended
Frequency	10-16 GHz	7-13 GHz	31-38 GHz	8-12 GHz
Packaging requirement	none	none	none	hermetic
Metal thickness	4.5 μm	3.1 μm	4 μm	1.5 μm
Substrate	125 μm GaAs	100 μm GaAs	9.25 μm SiO2	10 μm SiO2
	εr = 12.9	εr = 12.9	εr = 4.2	εr = 4.2
	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001	tan δ = 0.001
Supply voltage	-5/0/5 V	-7.5/0 V	0/1.5 V	-22/0/22 V
Transmission line	microstrip	microstrip	11/12/11 μm GCPW	microstrip
Figures of merit
Bandwidth	6 GHz	6 GHz	7 GHz	4 GHz
Effective bits	4	6	4	5
Insertion loss	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
	1.25 dB/bit	1.03 dB/bit	3.25 dB/bit	0.9 dB/bit (1.6 dB/bit packaged)
Linearity (IP1dB)	29-30 dBm	25 dBm	10 dBm	> 30 dBm
Noise figure	5 dB	6.2 dB	13 dB	4.5 dB (8 dB packaged)
Power consumption	20 mW	< 100 mW	0 mW	0 mW
Power handling	-	30 dBm	-	30 dBm
RMS amplitude error	0.3 dB	0.9 dB	2 dB	1.29 dB
RMS phase error	4°	2.5°	11°	10°
Size	2.6 mm2	3 x 2.1 mm2	530 x 220 μm2	7.1 x 1.3 mm2
Switching time	< 20 ns	< 45 ns	- ns	13 μs

References

[1] Radant MEMS president discusses RF switches. Available: http://www.memsinvestorjournal.com/ 2006/02/radant_mems_ceo.html

[2] RF MEMS: a brief history and future trends. Available: http://www.memsinvestorjournal.com/ 2006/10/rf_mems_a_brief.html

[3] C. Quan, J. J. Lee, B. M. Pierce, and R. C. Allison, �Wideband 2-D electronically scanned array with compact CTS feed and MEMS phase shifters,� U.S. Patent 6,822,615, February 25, 2003.

[4] B. Pillans, S. Eshelman, A. Malczewski, J. Ehmke, and C. Goldsmith, �Ka-band RF MEMS phase shifters,� IEEE Microwave Wireless Compon. Lett., vol. 9, no. 12, pp. 520�522, December 1999.

[5] J. B. Hacker, R. E. Mihailovich, M. Kim, and J. F. DeNatale, �A Ka-band 3-bit RF MEMS true-time-delay network,� IEEE Trans. Microwave Theory Tech., vol. 51, no. 1, pp. 305�308, January 2003.

[6] N. S. Barker and G. M. Rebeiz, �Distributed MEMS true-time-delay phase shifters and wideband switches,� IEEE Trans. Microwave Theory Tech., vol. 46, no. 11, pp. 1881�1890, November 1998.

[7] Relay - Wikipedia, the free encyclopedia. Available: http://en.wikipedia.org/wiki/relay

[8] MEMS and liquid crystal based, agile reflectarray antennas for security and communication. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-arascom_en.pdf

[9] Enabling MEMS-MMIC technology for cost-effective multifunctional RF-system integration. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ ict/docs/micro-nanosystems/20080630-rf-mems-cluster-workshop-report_en.pdf

[10] Phase steered radar module at 79 GHz on organic and ceramic substrates. Available: http://www.radarauge-project.com

[11] MEMS tuneable metamaterials for smart wireless applications. Available: ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/micro-nanosystems/20080630-tumesa_en.pdf

[12] Raytheon technologies promise to improve radar affordability. Available: http://investor.raytheon.com/phoenix.zhtml?c=84193&p=irol-newsArticle&ID=1176149

[13] World's first demonstration of microelectromechanical systems-based X-band radar. Available: http://radantmems.com/radantmems/04-06-06.html

[14] A. Stehle, G. Georgiev, V. Ziegler, B. Sch�nlinner, U. Prechtel, H. Seidel, and U. Schmid, �RF MEMS switch and phase shifter optimized for W-band,� European Microwave Conference, pp. 104�107, October 2008.

[15] I. J. Bahl and M. Dayton, �A Ku-band 4-bit compact octave bandwidth GaAs MMIC phase shifter,� Microwave Journal, vol. 51, no. 6, pp. 30�42, June 2008.

[16] XS1000-BD 6-bit digital phase shifter. Available: http://www.mimixbroadband.com/defense.asp

[17] B. Min and G. M. Rebeiz, �Single-ended and differential Ka-band BiCMOS phased array front-ends,� IEEE J. Solid-State Circuits, vol. 43, no. 10, pp. 2239�2250, October 2008.

[18] M. A. Morton and J. Papapolymerou, �A packaged MEMS-based 5-bit X-band high-pass/low-pass phase shifter,� IEEE Trans. Microwave Theory Tech., vol. 56, no. 9, pp. 2025�2031, September 2008.

**************************************************

Koen Van Caekenberghe received the Ph.D. degree in electrical engineering from the University of Michigan, Ann Arbor, in 2007. His doctoral research involved RF MEMS technology for radar sensors.

Copyright 2009 MEMS Investor Journal