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.