An X-Band to Ku-Band RF MEMS Switched Coplanar Strip Filter

RF MEMS

Abstract—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 1.5 mm and

demonstrates less than 2-dB of loss, making it promising for numerous

applications within these critical frequency bands.

INTRODUCTION

Reconfigurable microwave circuits have generated great

interest for both military and commercial applications because

these networks allow increased system functionality with

lower weight and cost than existing systems [1]. Due to

their low-loss and other attractive characteristics, radio

frequency microelectromechanical systems (RF MEMS) are

key to meeting these objectives [1], [2]. One of the most critical

elements enabled by RF MEMS is the tunable filter, of which

there have been several examples. These filters have

included both lumped designs [3]–[5] and distributed designs

[6]–[9] at frequencies ranging from L-band to millimeter-wave.

However, there have been few examples of RF MEMS-enabled

filters in the 8 to 18 GHz range where numerous radar and

communications applications reside. In this work, we present

an RF MEMS switched filter operating in this important

frequency range.

RF MEMS SWITCHED CAPACITOR

This filter design is enabled by a low-loss RF MEMS

switched capacitor. The device used in this work is optimized

for low loss and high-Q at microwave frequencies. To enable

post-processing on arbitrary substrates ranging from quartz

to InP MMICs, the device was fabricated using a l

ow-temperature surface micromachining process described

in detail previously [10]. This process uses a polymer

sacrificial layer and evaporated gold films to fabricate the

capacitor, and a low-temperature dry etch process to release

the devices. In the future, we anticipate adding thin-film

resistors using a high-resistivity layer to allow individual

biasing of the switches, enabling more complex and

higher-order filter designs.

The switched capacitor consists of a 1.2 m-thick gold

cantilever suspended 5 m over a ground pad coated

with silicon oxynitride dielectric. A cantilever device is

used to reduce sensitivity due to thermal mismatches

between the substrate and device, but has not been

characterized over a broad temperature range. The

device is switched by increasing the actuation voltage

until the plate pulls down onto the substrate at

approximately 15 V. The up-state capacitance is

approximately 150 fF while the down-state capacitance

is approximately 400 fF at 20 V. Additionally, the downstate

capacitance can be tuned between approximately 350 and

400 fF by varying the holding voltage between the

pull-off voltage of 10 V and maximum voltage of 20 V,

allowing approximately 10% tuning in the downstate.

Even though this slight tuning is possible, in this filter

application the device is used as a two-state switch for

improved stability and reproducibility in the presence of

vibration or bias voltage noise. This MEMS capacitor has

a-factor of over 100 through the entire band of operation

up to 25 GHz, and an extrapolated minimum self-resonant

frequency of over 60 GHz in the highest capacitance state.

Additionally, switching times between the down-state and

up-state were measured to be under 100 s. Finally, while

the device has been operated to over a billion cycles

without any charging or stiction-related failures, during

the lifetime test the pull-in voltage gradually shifted from

15 to 7 V due to metal fatigue at the anchor. This reliability

limitation will be addressed in future designs.

FILTER DESIGN AND FABRICATION

The filter was designed as a second-order capacitively-loaded

interdigital filter [11]. The resonators were grounded at one

end by a large grounding capacitor for bias isolation, their

lengths were chosen to be approximately 45 long at the center

frequency of 12.5 GHz to allow for maximum tuning range, a

nd the tap locations were chosen for optimum filter

characteristics at 12.5 GHz. The filter elements, to take full

advantage of the high performance of the switched MEMS

capacitor, were fabricated in a coupled coplanar strip (CPS)

configuration on a GaAs substrate [12]. Coplanar technology,

rather than microstrip, was:

 Chosen both for process simplicity and the ability to fabricate these

components on arbitrary substrates. This filter differs from

previously reported RF MEMS switched CPS-filters [9]

in that this design is switched by changing the loading

capacitors rather than by physically changing the length of the

line. This approach allows for a filter layout that is up to 50%

smaller at a given frequency than previous examples.

 After using standard filter design techniques [13] and

software to design a 15% bandwidth, 0.25-dB ripple filter,

the layout accuracy was verified with a commercial 2.5D

method-of-moments simulator [14] and a commercial

three-dimensional (3-D) finite element simulator [15].

An optical micrograph of the filter, which occupies only

1.5 2.2 mm of chip area, is shown in Fig. 1, which also includes

a simple sketch of the equivalent circuit as an inset.

The coplanar waveguide (CPW) feedlines are designed for

50 impedance, while the CPS resonators are 80 m wide

with a 35 m gap between the signal line and the ground

plane, producing a characteristic impedance of 58 and an

effective dielectric constant of 6.85. The resonators are

1100- m long and separated by 450 m, producing even and

odd mode impedances of approximately 62ohms and 52

ohms. The switched capacitors are 300 300 m with an initia

l air gap of 5 m. Because of the short resonators

(about 45 ohms), this filter occupies less than half the

chip area as earlier designs.

RESULTS

The filter was tested using coplanar Cascade probes and

an HP8510C network analyzer from 50 MHz to 25 GHz.

For testing, wirebonds were added across the CPW feed

to suppress spurious modes at the feed junctions.

The control voltage, ranging from 0 to 20 V, was introduced

to the CPW center conductor through the internal bias

tees on the network analyzer. The transmission and return

loss properties of the filter at voltages of 0 and 20 V are

shown in Fig. 2, which also shows the circuit model response.

At 0 V, the filter's center frequency is 15.5 GHz, with a

bandwidth of 2.2 GHz. The minimum passband loss is

approximately 1.4 dB in that state, with the return loss

better than 20 dB in the passband. When the switches are

pulled in at 20 V, the filter center frequency is 10.7 GHz with

1.8 GHz bandwidth. In this state, the insertion loss is less

than 2 dB, with in-band return loss better than 10 dB. This

37% center frequency switching is achieved with only two

simple RF MEMS devices.

 The rejection of the filter at two bandwidths above the

center frequency is greater than 15 dB for both of the

states, as expected for a two-pole filter. In similar filters

operating at lower frequency, we have observed a slight

decrease in stop-band rejection at three times the center

frequency, but the rejection of this particular filter remains

greater than 20 dB because of the electrically short resonators.

 While the measured and modeled responses correspond well,

there is a discrepancy between the measured and modeled

bandwidths which can be accounted for by several factors.

 First, the circuit-level simulation does not accurately account

for the coplanar-strip tees, which impact the performance

of the resonators. Second, accurately accounting for the

coupling between the coplanar strip resonators and end ground

planes requires full 3-D electromagnetic simulation. Finally,

the resonator grounds are not exactly fixed at the end of the

resonator, but instead distributed across the large area of the

grounding capacitor. All of these effects were captured by full

electromagnetic simulation and considered in the final design,

but were not incorporated into the circuit model. The filter

properties change with frequency because the physical

locations and lengths of the resonators and taps are fixed.

Therefore, as the filter is switched, the effective tap position

and coupling coefficient vary, deviating from the ideal desired

filter response. The filter was optimized for a response at

12.5 GHz, resulting in larger insertion and return losses at

lower frequencies and merging of the poles at the upper frequency.

Except for these slight departures from an ideal filter shape, the

filter response is symmetric about the center frequency. The loss

of the filter is limited by the low (30–40) of the coplanar strip line,

rather than by losses in the switched capacitors. By using a

transmission line technology with improved unloaded, the loss

of this filter can be significantly improved.

 CONCLUSION

In this work, we have presented an RF MEMS enabled

switched filter with 37% tuning from a center frequency of

10.7 to 15.5 GHz. The filter has a bandwidth of approximately

15% and less than 2 dB of loss in the passband. The monolithic

design takes full advantage of the RF MEMS device by

eliminating losses associated with off-chip connections and

allowing the total filter to occupy only 3.3 mm. This filter has

lower loss, smaller size, and a larger tuning range than previous

RF MEMS filters of this type, and switches between two

critical radar bands, enabling frequency agile radars and

other systems. Future work to improve the filter passband

properties is ongoing, and is expected to make this design and

fabrication technique a strong candidate for insertion into

advanced RF system designs.

Página: www.emo.org.tr

Realizado: Franco A Rivera C.

Asignatura: CRF