Christopher D. Nordquist

Voltage-Controlled Ku-Band and X-Band Tunable Combline Filters Using Barium-Strontium-Titanate

Tunable X-band and Ku-band combline bandpass filters using barium-strontium-titanate capacitors fabricated on alumina substrates with through-substrate CuW vias are reported. Under a 0-100 V bias, the X-band filter changes center frequency from 8.75 GHz to 10.96 GHz with 4-8 dB of loss while the Ku-band filter changes center frequency from 11.7 GHz to 14.3 GHz with 6-10 dB of loss. Advances in processing and integration related to the filter fabrication and design are discussed.

A DC to 10-GHz 6-b RF MEMS time delay circuit

A 6-b radio frequency (RF) microelectromechanical system (MEMS) time-delay circuit operating from dc to 10 GHz with 393.75-ps total time delay is presented. The circuit is fabricated on 250-/spl mu/m-thick alumina and uses metal contacting RF MEMS switches to realize series-shunt SP4T switching networks. The circuit demonstrates 1.8+/-0.6 dB of loss at 10 GHz and has linear phase response across the entire band with accuracy of better than a least significant bit for most states.

Lifetime limitations of ohmic, contacting RF MEMS switches with Au, Pt and Ir contact materials due to accumulation of ‘friction polymer’ on the contacts

We present lifetime limitations and failure analysis of many packaged RF MEMS ohmic contacting switches with Au–Au, Au–Ir, and Au–Pt contact materials operating with 100 μN of contact force per contact in hermetically sealed glass wall packages. All metals were tested using the same switch design in a controlled environment to provide a comparison between the performance of the different materials and their corresponding failure mechanisms. The switch lifetimes of the different contact materials varied from several hundred cycles to 200 million cycles with different mechanisms causing failures for different contact materials. Switches with Au–Au contacts failed due to adhesion when thoroughly cleaned while switches with dissimilar metal contacts (Au–Ir and Au–Pt) operated without adhesion failures but failed due to carbon accumulation on the contacts even in a clean, packaged environment as a result of the catalytic behavior of the contact materials. Switch lifetimes correlated inversely with catalytic behavior of the contact metals. The data suggests the path to increase switch lifetime is to use favorable catalytic materials as contacts, design switches with higher contact forces to break through any residual contamination, and use cleaner, probably smaller, packages. (Some figures may appear in colour only in the online journal)

Impact of in situ oxygen plasma cleaning on the resistance of Ru and Au- Ru based rf microelectromechanical system contacts in vacuum

Contact resistance measurements are reported for radio frequency microelectromechanical system switches operating in an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Ru-based contacts were prepared by means of standard sputtering techniques, sputtering followed by postdeposition oxidation, (surface RuO2) or reactive sputtering in the presence of oxygen (bulk RuO2). In situ oxygen plasma cleaning lowered the resistance of Ru contacts by two or more orders of magnitude but not lower than Au contacts, irrespective of whether the Au contacts were cleaned. The time dependence of the resistance was fit to power law extrapolations to infer contact creep properties and resistance values at t=∞. Time-dependent creep properties of mixed Au-Ru contacts were observed to be similar to those of Au-Au contacts, while the absolute value of the resistance of such contacts was more comparable to Ru-Ru contacts. Prior to, and for short oxygen plasma exposure times, bulk RuO2 resistance v...

A low loss RF MEMS Ku-band integrated switched filter bank

A switched Ku-band filter bank has been developed using two single-pole triple-throw (SP3T) microelectromechanical systems (MEMS) switching networks, and three fixed three-pole end-coupled bandpass filters. A tuning range of 17.7% from 14.9 to 17.8 GHz was achieved with a fractional bandwidth of 7.7 /spl plusmn/2.9%, and mid-band insertion loss ranging from 1.7 to 2.0 dB.

VHF and UHF mechanically coupled aluminum nitride MEMS filters

This paper reports the development of narrow-bandwidth, post-CMOS compatible aluminum nitride (AlN) MEMS filters operating in the very (VHF) and ultra (UHF) high frequency bands. Percent bandwidths less than 0.1% are achieved utilizing a mechanically coupled filter architecture, where a quarter wavelength beam attached in low velocity coupling locations is used to connect two AlN ring resonators. The filter bandwidth has been successfully varied from 0.09% to 0.2% by moving the attachment of the coupling beam on the ring to locations with different velocity at resonance. Insertion losses of 11 dB are obtained for filters centered at 99.5 MHz with low termination impedances of 200 Omega. Utilizing a passive temperature compensation technique, the temperature coefficient of frequency (TCF) for these filters has been reduced from -21 ppm/C to 2.5 ppm/C. The reduced TCF is critical for narrow bandwidth filters, requiring only 13% of the filter bandwidth to account for military range (-55 to 125 C) temperature variations compared to 100% for uncompensated filters. Filters operating at 557 MHz are realized using overtone operation of the ring resonators and coupling beam where higher insertion losses of 32 dB into 50 Omega are seen due to the finite resonator quality factor and narrow bandwidth design. Overtone operation allows for the implementation of fully differential and balun type filters where the stop-band rejection is as high as 38 dB despite the increased insertion loss.

A widely tunable RF MEMS end-coupled filter

A three-pole tunable end-coupled filter from 6 to 10 GHz was developed with a broad 35% tuning range. This tuning range was realized by switching distributed loading structures with radio frequency microelectromechanical systems (RF MEMS) capacitive switches. By tuning the coupling capacitors as well as the loading capacitors, the filter achieved a constant fractional bandwidth of 15/spl plusmn/0.3% and an insertion loss ranging from 3.3 dB to 3.8 dB over the entire band. Digital switching ensured good thermal stability, and microstrip transmission lines provided lower insertion loss than with coplanar waveguide. Future improvements are expected to decrease the insertion loss to below 2.1 dB.

An X-band to Ku-band RF MEMS switched coplanar strip filter

Radio frequency microelectromechanical systems (RF MEMS) are key enabling technologies for miniature reconfigurable circuits such as microwave filters. We present a two-pole monolithic RF MEMS switched filter, fabricated on GaAs, that employs surface-micromachined capacitors to present a variable capacitance to a coupled coplanar strip filter, thereby switching the filter center frequency 37% between 10.7 GHz and 15.5 GHz with voltages of 20 and 0 V, respectively. This 15% bandwidth filter occupies a chip area of 2.2 /spl times/1.5 mm and demonstrates less than 2-dB of loss, making it promising for numerous applications within these critical frequency bands.

Fabrication and characterization of ohmic contacting RF MEMS switches

We have fabricated and characterized radio frequency microelectromechanical systems (RF MEMS) ohmic switches for applications in discrete tunable filters and phase shifters over a frequency range of 0 to 20 GHz. Our previously reported cantilever switches have been redesigned for higher isolation and are now achieving 22 dB of isolation at 10 GHz. The measured insertion loss is 0.15 dB at 10 GHz. We have also fabricated and characterized new devices, designated “crab” switches, to increase isolation and contact forces relative to the cantilever design. The measured insertion loss and isolation are 0.1 dB per switch at 20 GHz and 22 dB at 10 GHz, respectively. A simple and accurate equivalent model has been developed, consisting of a transmission line segment and either a series capacitor to represent the blocking state or a series resistor to represent the passing state. Experimental analysis of the switch shows that high contact and substrate capacitive coupling degrades the isolation performance. Simulations indicate that the isolation improves to 30 dB at 10 GHz by reducing these capacitances. The crab switch design has a measured contact force of 120 μN, which represents a factor of four increase over the cantilever switch contact force and results in consistent, low-loss performance.

Effect of microstructure on the dielectric properties of compositionally graded (Ba,Sr)TiO3 films

The demand for better performing radio frequency (RF) and microwave devices has led to research on lower dielectric loss and more temperature-stable dielectric materials. In this work, we report on an approach to decrease the temperature coefficient of capacitance (TCC) of (Ba,Sr)TiO3, which is desirable for improved stability of phase shifter elements, voltage-controlled oscillators, and other tunable devices. We have grown and characterized a series of single-composition and compositionally-graded films with both parallel plate and interdigitated (IDC) electrodes. Graded films with larger Ba2+ concentrations in the initially-grown layers, as well those films crystallized at lower temperatures, displayed the lowest TCC. An asymmetrically graded BaTiO3∕Ba0.50Sr0.50TiO3∕SrTiO3 film on polycrystalline alumina crystallized at 750 °C yielded the lowest observed TCC of −150 ppm∕°C, a dielectric constant of 240, 20% tuning of permittivity (0−40 VDC, 80 kV/cm), and losses below 1%, measured at 100 kHz. The micro...