by Benedetto Vigna, VP and GM, MEMS and healthcare, RF and sensors product division, STMicroelectronics, Castelletto, Italy
MEMS are manufactured in semiconductor fabs much like the CMOS transistors we find in any electronic chip, but in this case not only electrons are moving, but also silicon springs, electrodes, membranes, and cantilevers. Silicon micromachined products, which compete quite often with quartz- and piezoelectric-based products in price, size, and performance, usually represent the best technical and economical solution for the consumer market with a clear roadmap towards miniaturization and integration.
Motion sensors, such as accelerometers and gyroscopes, are bringing the movement detection dimension to the inside of the silicon. Their penetration will continue to increase in the automotive market, but their presence in the consumer market will happen at a higher rate. Multi-axis accelerometers, for example, once used only for active and passive safety systems in the car, are finding a place in laptops, hard-disk-drives, mobile phones, and game controllers. In addition to their use in vehicle dynamic control systems, yaw rate sensors are helping to improve image stabilization in camcorders and digital still cameras. Moreover, motion sensors and geo-magnetometers are expected to cluster together in motion measurement units to enable personal navigation in portable devices.
Tiny pressure sensors today are widely used in the automotive and the medical markets, and their penetration in the automotive area will rapidly increase thanks to tire pressure monitoring applications. Recently developed thin, small, and inexpensive pressure sensors will also appeal to the big consumer market by enabling new applications. Capacitive silicon microphones are also competing adequately with non-surface mountable electret-condenser microphones in mobile phones and laptops.
Let’s take a look at some well-identified consumer applications for MEMS devices, and have a peek at emerging opportunities.
Mobile phone applications
The combination of micro-machined accelerometers and the appropriate application software eliminates the need for conventional switches or button and thumb wheels for the scrolling, zooming, and panning of Web pages, e-books, and spreadsheets. This is an innovative way to solve the well-known “small button/big finger” problem that plagues many users. In fact, while small cell phones are convenient and easy to carry, their small display screens and limited graphic capabilities reduce the total user experience. The sensor can detect basic human movements and use them as the input for display orientation, which in turn simplifies how the user views the downloaded pages. The user can then navigate through Web pages or pan through maps by simply tilting the device in the desired direction. Apple’s i-Phone is an excellent example of MEMS adoption in this market segment.
Remote game controller
A user-friendly interface is very attractive for gaming in portable devices, and it is a must for any company targeting the teenage end-user market. By making the motion of the hand become the “mouse” for handheld devices, single-handed operations and gesture recognition can be added to gaming devices. Accelerometers allow this easy-to-use interface by sensing hand/fingers/wrist motion and translating that motion to an action in the game. The Nintendo Wii is one of the most successful examples of this usage.
Mouse and 3D pointer
A computer mouse is the most common interface between a person and a computer or any computer-controlled device. Hand movement across a single plane or two-dimensional surface controls a cursor or pointer to activate a particular task. A typical mouse contains two or three buttons for entering commands as well as a communications interface for connecting with the computer system. In this application, inertial sensors are a good alternative to the optical solution, which suffers from high power consumption especially in a wireless solution. A user-controlled device accommodating an accelerometer can detect 3D movements and send corresponding control signals to an electrical appliance such as a computer system. The combination of gyroscopes and accelerometers enhances equipment performance while enriching user experience and usability.
Small and safe
In hard disk drive (HDD)-based devices such as MP3 and MP4 players, laptops, mobile phones, camcorders, and digital still cameras, the use of three-axis accelerometers can help protect the HDD from any potential loss of stored data in case of freefall. When located on the device’s board, three-axis accelerometers guarantee freefall protection along all three axes (x, y, z). If the computer falls, the accelerometer senses the zero-gravity, and the dedicated microcontroller signals the read/write head to park away from the sensitive disks, before the head can crash onto the disk causing the loss of data or damage to the drive.
Portable and vehicle navigation systems use GPS receivers to determine position and provide route guidance. With any GPS system, the signal reception is not always 100% reliable—e.g., in urban areas where GPS signals are blocked due to underground tunnels, bridges, and skyscrapers, making accurate navigation difficult. In this context, micro-machined motion sensors can assist and substitute for the GPS. If there is signal loss, a dead reckoning system continues tracking movements when satellite signals are not visible or where they are not sufficiently accurate. Furthermore, to implement dead reckoning, it is necessary to know the distance and direction travelled. Therefore, a motion measurement unit, including an accelerometer, a gyroscope, and often a magnetometer, is needed. It is important to note that because battery-operated GPS devicesconsume a lot of power, dead reckoning is a vital feature enabled by low-power motion sensors.
Pedometers are used to measure burnt calories, the speed and distance travelled by an individual on foot. For this application, an accelerometer detects the motion of a walking person—specifically, the accelerometer output is a periodic signal describing the vertical plane motion. The wireless pedometer can be worn on the shoe and communicate with another personal device such as a stopwatch to display the measurements, providing athletes with a complete training tool. Pedometers also represent an important building block for personal navigation devices, and they have begun to be integrated in MP3 players and media phones.
Weather stations and altimeters
Pressure sensors allow the integration of a “weather station” in portable handsets and to assist GPS devices track the altitude of the end-user. For instance, when an emergency number such as 911 is dialed, a GPS equipped with a pressure sensor will automatically signal the position and the floor of the building in which the end-user is located.
Currently, piezoelectric vibrating gyroscopes are used for image stabilization in camcorders and digital still cameras. Silicon micro-machined gyroscopes offer the advantages of reduced dimensions and lower power consumption. Moreover, they can measure angular rate along pitch and roll axes simultaneously and can be integrated more easily with other motion sensors. The increasing number of mobile phones with cameras (currently ~80%) represents a market opportunity of several hundreds million devices per year.
Emerging applications for MEMS
MEMS suppliers must focus on miniaturization and multi-sensor clustering to address customer requests. STMicroelectronics is pursuing device miniaturization (Figure 1), while also structuring a technology platform and device roadmap to enable sensor fusion in custom and standard packages (Figure 2). Moreover, many companies and research centers are actively working in the field of wireless sensor networks. It’s hard to predict the market success of these applications, since some technological hurdles still exist for a big volume take-off. However, it’s clear that all the applications could benefit from the tiny and low-power micro-machined sensors that companies are developing now for the consumer market. With these new applications, the MEMS of the “ consumerization wave” could literally generate a “MEMS tsunami”.
Fig.1. Three-axis accelerometers in full molded plastic Land Grid Array packages with a thickness less than 1.0 mm. From left to right, packages measure 7×5mm2, 5×5 mm2 , 3×5 mm2, 4×4 mm2, and 3×3 mm2.
This is already the case with the tire pressure monitoring system, a simple five-node wireless sensor network. The system is already on the market, and it relies on a micro-machined pressure sensor and acceleration switch.
Fig.2. A schematic block diagram of a multi-sensor module comprised of 3-axis accelerometers, 3-axis gyroscopes, pressure sensors, magnetic sensors, and other technologies.
Other “motes”—wireless sensor modules consisting of some combination of a sensor, controller, receiver, battery, and antenna—are now yielding commercial results. The potential market for motes is limited only by the imagination. Once certain technical challenges are overcome, motes will ultimately become a regular part of our lives. They could, for example, find many applications in consumer markets, with solutions ranging from security and bio-detection to building and home automation, industrial control, pollution monitoring, and agriculture. Also, rising concerns for safety, convenience, entertainment, and efficiency factors, coupled with worldwide government mandates, could boost sensor usage to unprecedented levels, although not all of them necessarily silicon micro-machined.
In fact, motes will have to measure real-world variables like pressure, temperature, heat, flow, force, vibration, acceleration, shock, torque, humidity, strain, and images. Some of them will use micro-machined solutions while others will use conventional sensors that have been available for decades.
Benedetto Vigna, is VP and GM, MEMS and Healthcare, RF and sensors product division, at STMicroelectronics, Castelletto, Italy