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The Advantages of
Integrated MEMS to Enable
the Internet of Moving Things WHITE PAPER September 2019
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The availability of contextual information regarding motion is transforming several consumer device applications.
Wearables, Hearables, Medical devices, Smart Cities/Homes, Games, 3D user interfaces, contextual awareness, and
navigation applications have increased the demand for MEMS sensors to deliver real-world, motion-aware applications
that enrich the consumer experience. While these devices started in automotive and industrial applications, they have
quickly become ubiquitous in gaming controllers, smartphones and tablets. That wave is now growing into an expanded
Internet of Moving Things (IoMT) including smart wearables, hearables, medical devices, and sports equipment where
the size and weight requirements have previously limited options for interfacing to these devices through motion
sensing. Recently the need to meet size, power, and functionality requirements for these new markets has fueled a
new wave of innovation in sensors. In fact, a new era of sensors is emerging where MEMS devices can provide designers
with novel methods of user interaction and a self-aware quality that is compelling and driving new use cases in the
Internet of Moving Things.
Figure 1: Accelerometer MEMS size evolution (source: Yole)
Figure 1 shows the evidence of this trend with the steep reduction in accelerometer sensor size over the years. Over
years, some part of size reductions were achieved by optimizing designs and chip stacking of MEMS sensor die and
CMOS die in assembly. Minimum feature size of technology node has been reduced and further advancement is now
limited by the overhead of chip interconnectivity.
To truly accelerate the trend toward substantial reductions in size requires real innovations in the underlying
fabrication technology and integration of MEMS sensors with electronics in a competitive, monolithic process. These
innovations not only improve economy, performance, and functionality, but it also enables a Known Good Die (KGD)
and Chip Scale Package (CSP) approach not possible with solutions that require multiple die.
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Integrated MEMS Integrating microelectromechanical structures (MEMS) with electronics is not new. Commercial production of a
monolithic MEMS device was first achieved in the mid 1990’s. Despite the technical achievements, the market, in
particular the consumer device market, preferred two-chip solutions, due to limitations in the original monolithic
approach:
▪ Area inefficiencies: When MEMS design is fabricated adjacent to CMOS, it results in extra overhead in silicon area
when MEMS processing is done before CMOS or vice-versa
▪ Lower yields: The higher level of complexity in an integrated process led to lower yields.
▪ Advances in packaging: Wafer thinning and stacking eroded the potential advantages of early attempts of
integrated products compared to two-chip solutions.
For these reasons, multi-chip solutions for accelerometers, gyroscopes, and pressure sensors have continued in the
consumer market, but they are increasingly challenged to meet the size and cost reduction requirements without
further sacrificing features and performance. mCube is the first company to successfully bring to market an integrated
MEMS+ASIC that does not suffer from the drawbacks of previous approaches. Fig.2 summarizes technology evolution
leading to miniaturization of MEMS motion sensors.
Figure 2: Technology drivers for size evolution of MEMS Sensors
The mCube monolithic single-chip structural design offers:
▪ High yields
▪ Very efficient silicon area usage
▪ Superior interconnectivity between MEMS and CMOS
▪ Significantly smaller footprint for devices with equivalent transducers
▪ Complete functionality at wafer level
▪ Increased quality and reliability
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As indicated by benchmarking report shown in Figure 1, with the monolithic 3D integration of MEMS+CMOS, mCube
has been a consistent leader in delivering smallest size accelerometer.
A schematic cross-section is shown in Figure 3. Released MEMS structures are fabricated directly on top of standard
CMOS, integrating the two more efficiently than in any previous commercial MEMS process.
Figure 3: The mCube monolithic, single-chip
platform, shown above in a schematic cross-
section, integrates MEMS with CMOS more
efficiently than in any other commercial MEMS
product
Process Overview An overview of the major steps in the process is shown in Table 1.
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The process for the mCube accelerometer is able to overcome the historical drawbacks of an integrated MEMS process
because of several key features:
Vertical integration: MEMS structures are processed directly on top of
CMOS. Unlike side-by-side or adjacent approaches, there are no significant
keep-out rules or reserved processing areas that lead to an inefficient use
of area.
Truly monolithic: Because the MEMS features of the mCube accelerometer
are defined lithographically, the alignment tolerance between MEMS and
CMOS in the mCube accelerometer is 0.1 µm. The overhead is much less
compared to MEMS wafer bonding, where the alignment tolerance of 3-5
µm must be accommodated in every feature.
Minimal size of interconnection: The MEMS via in the mCube
accelerometer is only 3 µm in diameter.
An example device is shown in Figure 4. The MEMS area has been de-
capped to show the underlying structure. The complete interface to the
device, including all testing, is accomplished with as few as six bond pads. The process has several benefits that are
particularly critical in the consumer markets of phones and wearables.
Size
Size reduction is achieved by significantly reducing the bond pads and their
required overhead, (e.g. ESD protection) from the die real estate. This is
accomplished with MEMS vias that ohmically connect the MEMS to the
underlying CMOS directly. The vias shown in Figure 5 are only 3 µm in
diameter. In a typical comparison as shown in Table 2, the integrated
approach can have five times fewer bond pads than a two-chip approach.
Table 2: An integrated device can have four times fewer the number of bonded connections as a two chip approach
for an accelerometer
Figure 5: These vias in a mCube device are
directly connected to CMOS underneath
Figure 4: SEM of the mCube accelerometer
shows the MEMS structure monolithically
integrated with ASIX
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Cost
Figure 6 compares the cost vs. aggregate yield of a two-chip MEMS solution
with an integrated device of comparable technology and performance. It
shows that at lower yield, the two-chip approach has an advantage,
primarily because of the ability to sort and pair known good MEMS with
known good CMOS in assembly. In an integrated approach, if either the
MEMS or the CMOS portion is defective, the entire product is lost. The
integrated device, however, has a steeper reduction in cost owing to the
smaller area in silicon for interconnecting the MEMS and CMOS, reduced
test cost (one wafer load vs. two), and significantly lower assembly costs.
At higher yields, an integrated device can have a lower total cost.
The advantage of bond pad savings for MEMS-CMOS interconnections
becomes even more pronounced when considering devices with multiple
degrees of freedom (DoF) such as a 6-axis accel-gyro combination.
Performance
In sensors that respond with a change in position (e.g. accelerometer, gyroscope, pressure), the preferred method of
measuring that change in applications that are sensitive to power consumption is to measure a change in capacitance.
Approaches that measure a change in resistance or frequency tend towards higher power consumption.
A second consideration in the performance of the device is the parasitic coupling of interfering signals. Whether the
objective is to reduce the EMI cross section or shield from coupling to undesired signals like clock or communication,
the MEMS via approach has a significant advantage over running long traces to bond wires between two chips. The
intimate coupling of the MEMS to CMOS is inherently much easier to safeguard against interference.
Power Consumption
The key requirements for sensors in most applications are low power and low noise or high signal to noise ratio which
translates into effective high resolution. However, minimizing noise requires more current. Integrated MEMS
technology described in this paper, minimizes parasitic at the input of sensor amplifier which reduces noise of sensor
compared to the sensor with larger capacitance. This allows the design to consume less power compared to alternative
technologies.
The mCube accelerometer using Integrated MEMS Technology benefits from low parasitic due to process technology
and efficient design architecture which results in low power consumption in active sensing modes. For instance, the
MC3672, a three-axis accelerometer with a digital output, draws just 2.8µA at an output data rate of 100Hz and only
0.9uA at 25Hz.
Figure 6: The cost vs. yield curve is steeper for
and integrated device
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But even more important is the provision of low-power modes for applications which are mostly dormant, such as a
smart door lock. The MC3600 accelerometer series have a Sniff mode in which much of their internal circuitry is
switched off, and they only maintain sufficient functions to detect motion in one or more dimensions. On detecting a
motion event, the device wakes up the rest of the circuit and quickly goes into normal operating mode. In Sniff mode,
the MC3672 draws just 400nA even while maintaining a sampling rate of 6Hz. Extremely space-constrained products
which operate on battery power, or even on harvested energy, can also now use a MEMS motion sensor for the first
time as a wake-up device. User-generated motion, turning the wrist to look at a watch, or gently tapping an earbud can
replace an on/off button or switch, and provides a convenient and unobtrusive alternative to other forms of user input
such as voice activation
Chip Scale Package (CSP)
Wafer Level Chip Scale Package (WLCSP) technology is another disruptive technology
for MEMS sensors where there have been rare examples of commercially produced
CSP MEMS inertial sensors. While TSV is getting adopted in CMOS industry, the
complexity of integrating a moving MEMS element and manufacturing in an cost
effective manner have been two major roadblocks faced by companies. mCube
adopted a unique approach of TSV through top of MEMS thereby taking full advantage
of single monolithic technology platform to come up with an innovative WLCSP solution.
MC3672 is world’s smallest 3-axis accelerometer package of 1mm3 which is 62% smaller
(or approx. factor 3 smaller) than most competitors. For Wearable applications, the size
& profile height are very critical factors. This product is also a key development to enable
Medical applications such as disposable pills and robotic surgery. Applications requiring small form factor & weight
such as Hearing aid customers can enormously benefit from the smallest size Accelerometer.
Known Good Die (KGD)
The next step in miniaturization is the movement from mounting packaged sensors on
boards to directly placing the die directly onto the PCB or flex materials without any
packaging. mCube’s unique monolithic, single-chip device combined with recent
advancements in inertial sensor testers, has enabled mCube to start sampling KGD for the
latest low-power accelerometers. The KGD are first electrically tested on wafer level to
determine functionality. After singulation via a wafer saw, the die is fully calibrated and
analog trimmed on this new test platform. KGD product enables
customers to directly place the sensor directly into a board knowing that the device is fully
tested and mechanically calibrated with the same quality and reliability as a packaged
product. This technology also enables next-generation System-in-Package (SIP) products
that combine the accelerometer functionality with other sensors and/or processors. In
addition to enabling SIP products, the KGD can be used for Chip-onboard (COB) assembly.
Figure 7: MC3672, the world’s
smallest & lowest power
accelerometer in 1.1x1.3 mm CSP
Figure 8: Known Good Die
on PCB with half glob top
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Chip-on-Flex
Sensors are increasingly getting used in applications that require flexible surfaces or flex
material. mCube’s unique 3D single chip technology allows the sensor die to be mounted
on a flex. Stress due to bending of the flexible surface is often a big concern for chip on flex.
mCube’s smaller die size and certain patented design techniques enable very low offset
solution using mCube sensors on flex.
Applications of the mCube Technology
Motion sensing implemented with Integrated MEMS Technology based accelerometer provides a valuable and user-
friendly solution to address wide variety of applications in smart or connected devices
• In a smart watch, an accelerometer can detect rotation in one dimension, when the user turns his/her wrist
to look at the display. This can trigger the watch to wake up the display and other components from power-
down mode. Enabling the watch to operate for long periods in power-down helps extend battery run-time.
• In wireless earbuds, an accelerometer can enable tap and double-tap detection, to provide a means for the
user to switch them on or off without the need for buttons.
• Smart city applications are benefiting from the implementation of motion sensing. For instance, buildings in
zones prone to earthquakes are starting to integrate a low-power accelerometer in safety systems that
automatically switch off utilities such as electricity and water supplies on detection of an earthquake. Battery-
powered for ease of installation, such systems maintain the accelerometer in always-on mode while every
other component is in power-down until triggered to wake up when the accelerometer detects a motion
event. Such systems can be designed to operate on a single-cell primary battery for longer than 10 years
between battery replacements.
• Smart home applications can use an always-on accelerometer in the same way to wake up the host system
from power-down when triggered by motion. A smart front door lock is an example of this type of application
in which the system is almost continuously in power-down mode.
• Industrial 4.0 application utilize sensors with ultra-low power to track machine health without battery
replacement for multiple years
• Medical pills for detecting and treating various ailments can use sensors for tracking location & orientation of
the pill
• Catheters used for Robotic surgery can accommodate small sensor to allow tracking position within body
• Sensors can be part of electronics used on body patches or even implanted in body to monitor biometrics and
detect health conditions
• Energy harvesting can be used with ultra-low power sensors for battery free operation of a device. Examples
are watches, smart home gadgets, environmental sensing solutions etc.
Figure 9: Chip on Flex
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A common feature of all these use cases is the need for an accelerometer to be always on while drawing an extremely
low current, to extend battery run-time.
Many including wearable devices, hearables, and medical devices are space-constrained, and can only accommodate
an accelerometer if it has a very small footprint.
Conclusion
The integrated monolithic, single-chip process and structural design enables mCube to ship the world’s smallest
integrated accelerometer in volume. It can achieve this size without sacrificing performance or features. The savings
from reduced size, lower testing costs, and lower assembly costs also enable this integrated approach to be very cost
effective. While this approach has some advantages in the smartphone and gaming market, it offers an attractive path
of continued innovation for new consumer, fitness and medical devices in The Internet of Moving Things (IoMT).
ABOUT MCUBE mCube makes the smallest motion sensors in the world. As a technology leader, mCube aspires to be the enabler for the Internet of Moving Things by putting a MEMS motion sensor on anything that moves, improving the way consumers live and interact with technology. mCube is backed by leading investors and has already shipped over 500M units. For more information, visit www.mcubemems.com. Copyright 2019. All rights reserved. mCube, Inc., the mCube logo and certain other mCube trademarks and logos are trademarks and/or registered trademarks of mCube, Inc.
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