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| Embedded MEMS Filter Chip and its Fabrication for VHF Applications
Bandpass Filter Developed Using MEMS Technology
Integration with Peripheral Circuits also Possible
New technologies that will contribute to miniaturization
in mobile equipment and reduced power consumption
in stationary equipment
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Sony has developed a bandpass filter
for the VHF band using MEMS* technology.
Additionally, Sony has succeeded
in integrating a MEMS filter
with peripheral circuits on a single chip
by adding MEMS processing
into the existing BiCMOS process.
Sony has also demonstrated the possibilities
for further miniaturization and thickness reductions
in mounting and for even lower power consumption.
Sony announced these technologies
in December last year at IEDM 2005.
*MEMS: Micro Electro-Mechanical Systems
Collective term for ultraminiature systems that integrate three-dimensional
movable parts, sensors, and other functions by applying LSI fabrication
techniques to perform machining operations at the micro and nano levels. The Sony Semiconductor Group has already used this technology to
develop the GLV (Grating Light Valve) display device. Sony exhibited
the GxL imaging technology that uses these concepts and technologies
at EXPO 2005, Aichi, Japan. (See CX-NEWS, Vol. 40) |
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Figure 1 Improving the Resonator from the Two-Port to the Three-Port Structures
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MEMS Filter Development (1)
Moving up to a three-port resonator design
Currently, there are limits to the miniaturization of the widely-used low-frequency SAW
(surface acoustic wave) filters due to their physical characteristics. In contrast, even further miniaturization is expected for MEMS filters. The
bandpass filters are created by combining four
resonators electrically. Therefore, the improvements of the resonator characteristics should be
first performed.
The resonator consists of a collection of parallel-coupled micromechanical resonators. For
these resonators, Sony has adopted a structure
(the clamped-clamped beam structure) in which
a beam is held fixed at both ends and resonated.
One of Sony’s unique improvements here is to
switch from the two-port resonator, which has
been widely reported by several research organizations to the three-port resonator. (See figure
1.) The beam resonance mode was changed from
first order (a single wave) to second order (two
waves), and the input and output signal lines were
placed under the beam independently. These
design improved the signal-to-noise ratio in resonance curve by suppressing signal leakage.
A subsidiary effect here is that the influence of
manufacturing variations in the patterning is reduced because this second-order flexion mode uses
a longer beam length than the first-order vibration
mode. Figure 2 shows the frequency spectrum of
this newly-developed three-port resonator.
MEMS Filter Development (2)
Adjusting the frequency characteristics with
the parallel beam layout
Next, Sony increased the signal level by
arranging multiple resonators in parallel. (See
figure 3-a.) While in general, a resonator
using MEMS technology has a property of
high Q-value (the resonance peak is sharp),
this characteristic becomes, inversely, disadvantageous when it is necessary to pass a
wider bandwidth. Sony, however, was able
to verify that the Q-value changes with the
layout pattern. Sony proved that it is possible
to optimize the frequency characteristics for
the purpose at hand.
Integration to an Existing
Silicon Process
Embedding LSIs and MEMS devices
Sony next created an embedded MEMS filter chip that consists of this MEMS filter and
the impedance matching circuit as the next
stage.
Polysilicon resonators are formed after the
FEOL (front end of line) process in the
BiCMOS process, and after the multilayer
interconnect process has completed, the ILD
(interlayer dielectric) and the sacrificial layer
around the MEMS resonators are removed
with buffered-HF treatment. (See figure 4.)
At this time, the critical point is how well the
space for the resonator’s motion can be
acquired without damaging the metal interconnects.
The fabricated filter properties coincided with
the calculated one. (See figure 5-a.) Furthermore, no significant influence on the transistors embedded on the same chip was seen.
(See figure 5-b.) The result of this was that
Sony had shown that it was possible to manufacture an embedded MEMS filter RF IC
using an existing semiconductor process. |
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IEDM 2005
International Electron Devices Meeting
– After the Presentation –
Since this was my first presentation in English at an international conference, preparing for it was a lot of work. But I made it through to the end. From the questions
and comments after the presentation, I felt that there had been a good response.
This presentation also gave me a chance to interact with well-know researchers in
the field and was a valuable experience that expanded my perspective on my work. |
 From Micromachines to MEMS
More than just components, these devices are now integrated into System LSIs. |
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Vol.43
