Successful Development of High Precision Silicon-Based MEMS Resonator
(Nanowerk News) Seiko Instruments Inc. (SII) has succeeded in developing a high precision siliconbased
MEMS resonator. In addition, SII, in collaboration with the Institute of
Microelectronics (IME), a research institute of the Agency for Science, Technology
and Research (A*STAR), has also succeeded in the development of a new wafer
level packaging technology for vacuum sealing MEMS devices.
High Precision Silicon-based MEMS Resonator
The resonator is a device that is indispensable in modern electronics products such
as computers, digital appliances and telecommunications devices. Currently,
resonators using quartz crystals are the mainstream due to its light weight, highly
precise and stable frequency properties. Research for new silicon based resonators
shows that further miniaturization and reduction in cost may be possible through the
use of MEMS processes to fabricate with silicon material.
High temperature dependency of the resonant frequency of silicon-based resonator
poses a major challenge. More specifically, due to factors like thermal expansion and
Young's modulus temperature dependence, its resonant frequency fluctuates when
there is a change in temperature and generally has a temperature coefficient of -
40ppm/°C. This means that for its operating temperature limit (-40°C~80°C), the
resonant frequency will fluctuate in the 5000ppm range, making it very difficult to
ensure precise output frequency in the resonator.
In order to ensure precise output frequency, electrical circuit has to be used to
compensate for the change in resonant frequency as a result of temperature
fluctuations. This creates a problem in the design for low power consumption and
miniaturization. Furthermore, the use of electrical circuit to substantially compensate
for temperature fluctuations will also lead to the occurrence of noise in the output.
Now, SII has successfully developed manufacturing technology that can greatly
improve the problem of temperature dependence of the resonant frequency in
silicon-based MEMS resonator by adding a layer of silicon dioxide film to the MEMS
resonator fabricated on the SOI (Silicon On Insulator) substrate, and employing a
structure that would produce residual stress within the resonator.
The temperature coefficient of resonant frequency fluctuation has improved from -
40ppm/°C to ±2ppm/°C. The frequency accuracy could also be controlled to a range
with a maximum of 500ppm at operating temperature range (-40°C~80°C) (Please
refer to Annex, Figure 2 and 3.) This will reduce the need for temperature
compensation to the minimal and allow for application in low power consuming,
miniaturized and high precision resonator.
Development of new wafer level packaging technology for vacuum sealing MEMS
devices
In MEMS devices such as a resonator, a vacuum has to be present when in
operation and low cost vacuum packaging technology for devices are desired. SII, in
collaboration with IME, has developed a new vacuum sealing wafer level packaging
technology by bonding the silicon substrate which acts as a cover, to the MEMS
resonator device.
Usually, anodic bonding is used for sealing processes for MEMS devices with silicon
substrate. However, this application is limited to the high temperature (400~500°C)
and high voltage (600~1000V) needed for the bonding process, which might cause
warping of the device. In addition, gases such as oxygen are produced during anodic
bonding, making it necessary to include getters in the sealing process to remove the
gases produced during the vacuum packaging process.
Cross-section of vacuum package with Gold-Tin eutectic bonding.
SII and IME have jointly developed vacuum packaging technology using gold-tin
eutectic bonding processes. With gold-tin eutectic
bonding, the process can take place at low temperatures making it possible to inhibit
the release of gases during bonding. This indicates that a higher level of vacuum
sealing below 26Pa can be achieved without the use of a getter and external
electrode can be drawn out laterally without passing through the substrate. This
method not only improves the mechanical strength but also lowers fabrication cost.
SII and IME have successfully tested the performance of the aforementioned siliconbased
MEMS resonator vacuum-sealed with the new wafer level packaging
technology. With the newly developed silicon-based MEMS resonator and the new
wafer level packaging technology, it would be possible to achieve miniaturization and
low power consumption.
SII will conduct further performance evaluation to verify the possibility of a
miniaturized and low power consuming resonator for future application in resonator
devices. Silicon MEMS resonator devices are very compatible with ICs, making
application in high frequency resonator, which would otherwise be difficult to achieve
with quartz crystals. Its practical application will make low cost, high-mix low-volume
production and miniaturized single chip high frequency resonator possible.
Application for the packaging technology in MEMS devices with various functions is
also currently under review, with the aim to continue development for more compact
and low power consuming devices. SII has set up a representative office in
Singapore and has been collaborating with IME since 2007.
About Seiko Instruments Inc. (SII)
Seiko Instruments Inc., based on more than seven decades of dependable precision
timepiece design, development and manufacturing, has extended its business
domains by developing advanced micro mechanical, low power consumption, and
nano-scale technologies leveraging its precision instrument manufacturing expertise.
About the Agency for Science, Technology and Research (A*STAR)
The Agency for Science, Technology and Research (A*STAR) is the lead agency for
fostering world-class scientific research and talent for a vibrant knowledge-based
and innovation-driven Singapore. A*STAR oversees 14 biomedical sciences, and
physical sciences and engineering research institutes, and seven consortia & centre,
which are located in Biopolis and Fusionopolis, as well as their immediate vicinity.
A*STAR supports Singapore's key economic clusters by providing intellectual,
human and industrial capital to its partners in industry. It also supports extramural
research in the universities, hospitals, research centres, and with other local and
international partners.
About the Institute of Microelectronics (IME)
The Institute of Microelectronics (IME) is a research institute of the Science and
Engineering Research Council of the Agency for Science, Technology and Research
(A*STAR). Positioned to bridge the R&D between academia and industry, IME's
mission is to add value to Singapore's semiconductor industry by developing
strategic competencies, innovative technologies and intellectual property; enabling
enterprises to be technologically competitive; and cultivating a technology talent pool
to inject new knowledge to the industry. Its key research areas are in integrated
circuits design, advanced packaging, bioelectronics and medical devices, MEMS,
nanoelectronics, and Silicon photonics.
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