Integrated MEMS Is Powering the Internet of Moving Things



Connected and not tied to one place? Then we’re talking the Internet of Moving Things and this sector of the IoT is only going to get hotter, spurring MEMS and sensor innovation in sectors like gaming. 9DoF sensor functionality on your mobile, anyone?

While motion sensors first became popular in high-end smartphones and tablets, they are now rapidly becoming key requirements in every moving consumer device on the market. This world of connected and interactive electronic devices not fixed in one place is now known as the Internet of Moving Things (IoMT). Through the IoMT, we have the ability to connect anything that moves to monitor, analyze and deliver real-time insights from the resulting data. Because motion sensors provide an intuitive way for consumers to interact with their electronic devices, the market applications are endless. Recently, the need to meet size, power and functionality requirements for the new IoMT markets such as wearable devices and gaming consoles has fueled sensor innovation. MEMS devices are presenting designers with novel methods of user interaction and a self-aware quality that is compelling and driving new use cases in the IoMT.

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Figure 1. Optimizing design and packaging has led to shrinking accelerometer footprint.

Accelerating Sensor Size Reduction

In Figure 1 you can see the evidence of this trend come to life with the steep reduction in accelerometer sensor size over the years. These reductions in size were largely achieved by optimizing existing designs and packaging. Minimum features have been pushed to physical limits and further advancement is now limited by the overhead of chip interconnectivity. Another method for reduction in size involved combining multiple degrees of freedom (DoF). While 6-axis accelerometer-gyroscope, accelerometer-magnetic sensors, and even 9-axis accel-gyro-mag products are now available, the majority of them use the same MEMS processing at the wafer level, with integration and size reduction accomplished by chip stacking of discrete sensor and CMOS die in assembly.

To truly accelerate the trend toward substantial reductions in size requires real innovations in the underlying fabrication technology. The next phase in MEMS sensor evolution will require integration of MEMS sensors with electronics in a competitive, monolithic process. Not only will this improve economy, performance and functionality, it also will enable a chip-on-board (COB) approach not possible with solutions requiring multiple die.

Integrating MEMS
Most motion sensors available in today’s market are based on multi-chip technology developed in the 1980s. Using this approach, the mechanical sensor (MEMS) and the drive electronics (IC) are manufactured in different facilities using different processing techniques. The two chips are tested separately and then wired together during packaging. The disadvantages of this approach include the size and cost of two chips, the cost and capacity constraints of specialized MEMS fabrication and the performance limitations of interconnecting two separate chips into a single package.

Recently, a method for producing MEMS has emerged that uses a monolithic single-chip MEMS technology. With this approach, the MEMS sensors are fabricated directly on top of the IC electronics in a standard CMOS fabrication facility. Advantages of this monolithic approach include smaller size, higher performance, lower cost, and the ability to integrate multiple sensors onto a single-chip.

Additional benefits include:

  • High yields
  • Very efficient area usage
  • Superior interconnectivity between MEMS and CMOS
  • Significantly smaller footprint for devices with equivalent transducers
  • Complete functionality at wafer level
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Figure 2. MEMS fabrication on top of standard CMOS.

A schematic cross-section is shown in Figure 2. Released MEMS structures are fabricated directly on top of standard CMOS, integrating the two efficiently.

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Table 1. An overview of the process in which MEMS sensors are fabricated directly on top of the IC electronics in a standard CMOS fabrication facility.

How Integrated MEMS Works
Table 1 outlines the major steps involved in the process of using monolithic single-chip MEMS technology. Several key features allow the process to overcome the historical drawbacks of an integrated MEMS process.

  • Vertical integration—MEMS structures are processed directly on top of CMOS, unlike side-by-side or adjacent of area approaches
  • Minimal size of interconnection—the MEMS via in the mCube accelerometer is only 3µm in diameter.
  • An example device is shown in Figure 3. The MEMS area has been uncapped to show the underlying structure. The complete interface to the device, including all testing, is accomplished with eight bond pads. The process has several benefits that are particularly critical in the consumer markets of phones and wearables.

    MEMS Figure 3
    Figure 3. An example device with the MEMS area uncapped to reveal structure.

    Size
    Integrated MEMS benefit from a significant size reduction by 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 directly connect the MEMS to the underlying CMOS. The vias shown in Figure 4 are only 3µm in diameter. In a typical comparison shown in Table 2, the integrated approach can have four times fewer bond pads than a two-chip approach.

    MEMS Figure 4
    Figure 4. MEMS vias ohmically directly connect the MEMS to the underlying CMOS.

    Cost
    Figure 5 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.

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    Figure 5. 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 sensitive to power consumption is to measure a change in capacitance. Approaches measuring 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.

    Chip-on-Board
    One particular benefit that is unique to the mCube monolithic, single chip device is that it is a fully functional, self-contained product at the die level. Customers can also consider using a chip-on-board (COB) assembly. The overhead of trying to do this with a multi-chip product would eliminate any potential advantage of this approach, particularly in the design of wearables and other space-constrained devices. The single-chip MEMS+ASIC design makes this unique form factor possible.

    In addition, because it is not possible to know in advance which ASIC is married to which MEMS at a customer’s site, no wafer-level trilling can be performed prior to assembly with the multi-chip approach. In contrast, the monolithic, single-chip approach can test and trim many of the analog functions at the wafer level prior to shipping known good die. This is because the MEMS and ASIC are a single chip. Chip ID numbers are also programmed into one-time programmable (OTP) memory, providing full traceability for both MEMS and ASIC processes back to the foundry.

    The Next Generation of MEMS Sensors
    While MEMS sensors have been used in industrial, automotive and even printer applications for many years, these first generation solutions are expensive and complex to manufacture, and are produced using proprietary manufacturing processes. Packaged in large multi-chip modules, first generation motion sensors require higher power and often have reduced reliability due to the packaging process. With the advent of smartphones and gesture-controlled gaming, second generation MEMS sensors have emerged, featuring smaller size and lower power. These second generation devices are still manufactured on proprietary processes and feature discrete products packaged with stacked die. As a result, they continue to be relatively large and consume considerable power.

    In contrast, a monolithic single-chip MEMS design approach represents the next generation of sensors—very small, single-chip MEMS+ASIC devices that are cost effective, consume low power, and feature very high performance. These advancements make it possible to place one or more motion sensors onto nearly any object or device. In some cases, the single-chip MEMS motion sensor silicon can be attached directly onto a printed circuit board, without requiring chip packaging, saving real estate and reducing height to enable thinner and smaller system designs.

    Sensors in Everything
    Motion sensors are key components in consumer, mobile and wearable devices, including smartphones and tablets that typically incorporate multiple MEMS accelerometers, gyroscopes and magnetic sensors. Through innovation the applicability of these sensors can be expanded to virtually anything that moves. Example use cases include:

    • Smartphones and tablets for 3D gaming, indoor navigation and augmented reality applications;
    • Smart watches, wearable devices and smart clothing that can detect how far you walked or ran, the rate at which you moved, and how many calories you burned;
    • Automotive or commercial trucking where motion sensors can be connected to video cameras in rear-view mirrors to better assess the cause of an accident and driving conditions;
    • Shipping, where motion sensors can be embedded directly onto packages to record jarring motions or accidents in order to determine when and how contents were damaged;
    • Sensor tags to monitor farm animals for abnormal activity, patterns of grazing, potential illness and herd behavior.

    Motion Sensing for Gaming
    The advent of mobile devices as preferred gaming platforms, combined with an explosion of virtual reality applications, has made motion-sensing technology a critical requirement. The ability to rotate your device with a precise rotation rate while playing a game, and have it sense that movement, is something every high-end smartphone and tablet user expects. The problem, however, is the hardware component needed for true immersive gaming, known as a gyroscope or gyro, is expensive and consumes too much power to be offered in mainstream phones and tablets.

    Software-based gyroscopes that delivers immersive, 9DoF motion gaming and augmented reality experiences to mainstream phone and tablet users around the world have become available, including offerings that make motion gaming and augmented reality for Android mobile devices possible.

    One example of a software-based gyroscope is the mCube iGyro. The iGyro, as compared to a hardware gyro:

    • Delivers 80 percent reduction in power consumption
    • Costs 50 percent less than discrete hardware 9DoF solutions
    • Features pin compatibility to simplify customers’ board designs with flexible configurations

    Conclusion:
    The next generation of motion sensors will have to be simple to manufacture and easy to design into a broad range of applications. And the innovation is not just on the hardware side. Companies are also delivering software innovations that complement hardware motion sensing solutions, including software-based gyroscopes that enable 9DoF sensor functionality on all mobile devices. Breakthroughs like these allow users to play sophisticated motion games on their phones, with very low power consumption, low cost and a small footprint.


    ben_leeBen Lee is president and Chief Executive Officer of mCube, the first company to integrate a MEMS sensor with an ASIC onto a single die using standard CMOS process. He has over 20 years of senior management experience in the semiconductor industry with a successful track record of driving rapid revenue and profit growth.

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