The rapid development of electronics and manufacturing technologies has led to the widespread adoption of Micro-Electro-Mechanical Systems (MEMS). While MEMS-based accelerometers , pressure sensors, and gyroscopes have been used for over 30 years, their application was primarily limited to industrial, military, automotive, instrumentation, and medical fields due to technological and cost constraints, rather than entering the consumer market. However, this situation has changed dramatically. MEMS technology is no longer as expensive as it was decades ago; low-cost, small-size, low-power, and high-performance MEMS sensors have sparked a new wave of design and consumer adoption.
Multi-axis accelerometers based on MEMS technology are currently mainly used in game consoles and mobile phones, drop protection for hard drives, step counting in portable devices, and image stabilization in digital cameras/camcorders. In addition, physical sensors for measuring physical quantities such as thermal effects, light intensity, and pressure are also a key area for future development.
STMicroelectronics has consistently invested in the widespread application of MEMS technology and the development of new products and technologies. It pioneered the investment in establishing an advanced eight-inch wafer production line while simultaneously fostering complementary partnerships. ST's goal is to popularize and mass-produce MEMS products in the consumer sector, becoming a driving force behind this consumer technology wave.
1. Overview of MEMS
Microelectromechanical systems (MEMS) are three-dimensional mechanical and electronic structures manufactured using silicon wafer micromachining technology. They began to appear in semiconductor factories in the 1960s. People frequently encounter physical sensors based on MEMS technology in their daily lives, used to sense acceleration, angular velocity, pressure, and sound pressure levels.
Automotive electronics is a rapidly developing market, and MEMS technology is ubiquitous within it. For example, all active and passive safety systems, such as vehicle dynamic control and airbags, use acceleration or yaw rate sensors to protect passenger safety; pressure sensors are also used in engine manifolds and fuel lines to reduce fuel consumption.
MEMS technology has sparked a surge in product design innovation in the dynamic consumer electronics market. For the consumer market, MEMS products are often the optimal solution considering both technical and economic factors, offering a blueprint for future miniaturization and advanced technology. One of the most successful applications is the revolutionary control experience brought by accelerometers in Nintendo's Wii and Sony's PS3 game consoles. Previously, accelerometers were only used in active and passive safety systems in automobiles, with safety regulations driving their application in automotive electronics. Today, "sensing and simplification" has become the value proposition of MEMS in the consumer product market, and eliminating all barriers between users and the complex world of electronic devices is a dream of design master Naoto Fukusawa-san. Furthermore, three-axis accelerometers can also be used to design drop protection devices for hard drives to safeguard data; and they can be used to achieve more user-friendly UI controls in portable devices.
MEMS devices, like CMOS in chips, are manufactured using semiconductor wafer fabs. However, unlike CMOS devices, MEMS devices are not merely electronic products; they also incorporate numerous mechanical structures, such as movable mechanisms like silicon springs, electrodes, membranes, and cantilever beams. Furthermore, silicon microfabricated devices often compete with traditional quartz or piezoelectric products in terms of price, size, and performance.
Motion sensors such as accelerometers and gyroscopes are bringing motion detection capabilities to silicon components. Their use in the automotive market will continue to increase, driven by regulatory requirements; their application in the consumer market will also increase significantly, with a rapid growth rate. Multi-axis accelerometers, previously used only in active and passive safety systems in automobiles, are now more widely used in laptops, hard drives, mobile phones, and game controllers. In addition to automotive dynamic control systems, offset sensors (gyroscopes) are also used to improve image stabilization in digital cameras and camcorders. Furthermore, motion sensors and magnetometers are expected to be integrated into motion measurement units to provide personalized navigation for handheld devices, enabling location-based services (LBS) offered by telecommunications operators.
Industry experts predict that the use of motion sensors such as accelerometers and gyroscopes in the consumer electronics market will continue to increase, and the growth will be very rapid.
Multi-axis accelerometers bring motion detection capabilities to silicon components and are currently widely used in hard drives, handheld devices, laptops, mobile phones, game consoles, and other devices. Gyroscopes are also used in digital cameras and camcorders to support their image stabilization functions. In addition, motion sensors and magnetometers are expected to be integrated into motion sensing units to provide personalized navigation functions for handheld devices, thereby providing a platform for location-based services (LBS) offered by telecommunications operators.
Miniature pressure sensors have been widely used in automobiles, primarily for tire pressure monitoring; medical devices are also a major market for them. Currently, with technological advancements reducing development costs and making them thinner and lighter, miniature pressure sensors are expected to soon be accepted by the consumer market and used in new fields such as wireless communications.
Due to size limitations, the use of traditional electret capacitive microphones using surface mount technology (SMT) is restricted in mobile phones and laptops. Therefore, capacitive silicon microphones based on MEMS technology will rapidly gain popularity in this market.
The integration of multiple components such as accelerometers, gyroscopes, and pressure sensors will inevitably become the future trend. MEMS product suppliers need to develop universal technology platforms that can integrate multiple sensors based on customer needs. THELMA and VENSENS are two such technology platforms launched by STMicroelectronics for sensor integration. ST's MEMS product manufacturing and R&D are carried out in advanced eight-inch MEMS wafer fabs, which can shorten time to market and quickly meet market demands. In addition, the company is also actively developing multi-axis gyroscopes, pressure sensors, and capacitive silicon microphones, and is also actively collaborating with other companies to launch sensor products with market potential or that meet customer requirements.
2. Microfabrication technology for physical sensors
Silicon crystals have become the most popular material in MEMS manufacturing due to their excellent electrical, mechanical, and thermal properties. MEMS sensors are generally fabricated and manufactured using so-called micro-machining processes, which are the same basic processes used in chip manufacturing. However, other materials such as quartz, glass, plastics, and ceramics can also be used for micro-machining or microforming. For example, quartz and ceramics are commonly used in crystal oscillators and Coriolis gyroscopes.
Besides its superior physical properties, silicon crystals are attracting attention due to industrial structural considerations. The global microelectronics industry has invested heavily and accumulated extensive expertise to build a robust industrial foundation. Manufacturers can utilize mature manufacturing technologies developed for silicon chip production to produce MEMS devices, simultaneously manufacturing thousands of micro-processed components on silicon wafers. The sheer scale of these economies of scale was once a crucial factor in the success of the electronics industry; now, MEMS can replicate this successful technology and experience to design and mass-produce silicon microelectronic components, while also focusing on further miniaturizing transistors. Furthermore, the wafer fabrication process requires extremely rigorous procedures and workflows, resulting in higher design repeatability and production yields compared to other manufacturing methods.
Silicon possesses unique physical properties. It is relatively brittle but does not readily undergo plastic deformation; it can be harder than steel, yet weighs only one-third as much. Due to these properties, when integrated into integrated circuit designs, the electrical signals generated by moving structures such as diaphragms or cantilever arms within MEMS mechanisms can provide the sensor with the physical quantities to be measured or the control capabilities to be applied.
The primary reason for the widespread use of MEMS is its extremely small size, high reliability, and low power consumption, enabling faster and more precise operation compared to other larger competing products. On the other hand, for ordinary users, especially in the price-driven consumer sector, cost considerations cannot be ignored.
Currently, MEMS devices are still in the micrometer range in size and can be manufactured using earlier 6-inch wafer fabs. However, with the rapid growth in consumer demand for related applications and price pressures, many manufacturers are expected to shift to 8-inch production lines in the coming years. ST has already completed its transition to 8-inch production lines, and its excellent competitive advantages in both technology and cost have given the company a dominant position in the market.
Many current microfabrication processes are derived from basic IC manufacturing technologies, such as photolithography, material deposition, reactive ion exchange, and chemical etching. Although the manufacturing of more and more components is being integrated into CMOS processes, different considerations are still needed for different applications due to varying circumstances.
For example, the microfabrication scale of MEMS components is approximately between tens and hundreds of micrometers, which is still different from that of chip circuits. Therefore, wet etching, thin film formation or electroplating, wafer stacking, vias, and dry etching are common microfabrication processes today. It should be particularly mentioned that MEMS components also use materials such as gold or glass-fried dielectrics, which are completely prohibited in CMOS processes.
In order to achieve a differentiated competitive advantage, MEMS suppliers have developed proprietary micromachining process technologies suitable for their own products over the past few decades, based on their own and their equipment characteristics and the process steps they are good at.
Proprietary processes used by various manufacturers can be broadly categorized into two types: bulk micromachining and surface micromachining. Bulk micromachining, a "subtractive process," involves removing a substrate to create the desired structure. It's suitable for designing thicker structures, allowing designers to freely determine the required substrate thickness; however, the shape of the micromachined structure is limited by the crystal structure of the silicon substrate. In contrast, surface micromachining is an "additive process." Its main process involves removing or leaving specific areas of different material layers through different steps, while the substrate layers remain unchanged. Because the thickness of the thin film that can be generated or deposited on the substrate is limited, this technology was initially limited to thin components of about 2 micrometers. However, new wafer bonding technologies now facilitate the design of thicker devices. Using these photolithography techniques, previously highly complex and innovative mechanical structures can become relatively simpler.
3. Thelma and Vensens micromachining processes
ST currently has two micromachining processes in mass production: THELMA and VENSENSE, both of which are hybrid manufacturing technologies combining volumetric and surface micromachining techniques.
THELMA stands for Thick Epitaxial Layer for Microgyroscopes and Accelerometers, primarily used in high-performance and low-cost motion sensors such as accelerometers, gyroscopes, and microphones. The THELMA process begins with a standard silicon wafer, on which a first oxide layer (approximately 2 micrometers) is deposited as an isolation layer. Next, a polysilicon layer for interconnects and a second sacrificial oxide layer (approximately 2 micrometers) are deposited. Holes are then created by etching individual points in this layer that serve as support points for the stationary mechanism and anchor points for the moving mechanism. A thicker barrier layer (approximately 15 micrometers) is then formed on top, and this layer is etched using a photomask to create a structure that integrates both moving and stationary units. Finally, the sacrificial oxide layer beneath this structure is removed using isotropic etching to shape the moving units. To reduce or eliminate the effects of humidity or air density variations that could affect the resonant frequency of the device, the open space near this structure is filled with air, typically dry nitrogen. The second wafer is then bonded onto the first to protect the tiny structure from damage under the high pressure applied during the injection molding process.
Table 1. Comparison of Microfabrication Processes for CMOS, Bulk, Surface, Thelma, and Vensens
VENSENSE, short for Venice Process for Sensor, enables the creation of extremely miniaturized pressure sensors. It also starts with a standard silicon wafer, and the results are quite similar to those achieved using bulk micromachining wafer bonding processes. A proprietary combination of dry and wet silicon etching steps generates a single-crystal silicon layer, upon which a sacrificial layer less than 3 micrometers thick is formed, and the structural layer can reach a thickness of 20 micrometers. However, compared to bulk micromachining, VENSENSE can produce thinner, smaller, and mechanically more stable chips; furthermore, the tight bonding of apertures does not require any wafer-to-wafer bonding, resulting in higher reliability of the bonding.
Due to the excellent electronic properties of monocrystalline silicon, stable and reliable resistors can be integrated into the structural layer through processes such as implantation or diffusion. These resistors are then connected to an aluminum metal layer, forming the four branches of a Wheatstone bridge. This metal layer is then covered with a standard insulator, such as silicon-oxygenated silicon, to provide protection against external corrosive agents. Because the bridge exhibits excellent piezoresistive characteristics due to the monocrystalline silicon layer, it is highly sensitive to changes in pressure.
4. Motion sensors in the consumer electronics market
The consumer electronics market has its own unique characteristics; consumer products need to be low-priced, low-power, low-voltage, and miniaturized. MEMS product suppliers must accelerate the development of new products while maintaining the same level of reliability as automotive electronics.
Accelerometers and gyroscopes are widely used in the automotive and medical device markets, such as in active or passive safety systems and heart rate regulators. The manufacturing methods in the consumer market differ from those in the automotive electronics market, which typically employs large, thick, and expensive ceramic packaging technologies. Consumer markets prefer surface-mount packaging and small, thin, and low-cost solutions. For example, ST's Full-Molded Plastic Gauge (PLGA) package, introduced in 2002, is now widely used in the industry and has become a process standard. With this technology, ST miniaturized its 3-axis accelerometer series from 100 cubic millimeters to a 10 cubic millimeter package size in less than three years.
Automotive sensors are not battery-powered, so power consumption is not a major technical challenge; however, high shock resistance is crucial. Furthermore, a wider temperature range and higher product reliability are also fundamental requirements for the automotive market. For the consumer market, power consumption and voltage are key considerations. Currently, the power supply voltage for consumer products has dropped to 1.8V, and the current must be less than 1.0mA.
Because handheld devices lack a fixed frame for reference, and users expect accelerometers to detect movements in all directions and enable corresponding functions, multi-axis sensor solutions are currently the mainstream in the consumer market.
Analog-output sensor solutions are gradually being replaced by digital products because digital solutions make product integration easier and software development faster. Furthermore, adding interrupt functionality pins simplifies final product integration, which is also a customer requirement. To address these needs, ST has developed biaxial, triaxial, analog, and digital accelerometers to suit various applications. ST also provides reference designs and evaluation kits, as well as dedicated development software tools.
Monolithic and hybrid solutions, using single-chip, single-package architectures, are the two main solutions on the market. Multi-chip, single-package solutions not only offer the best cost-effectiveness but also meet the modularity and flexibility required for rapid mass production, which is crucial for the consumer market. However, given the cost and time-to-market requirements of actual systems, choosing the most suitable solution is the wisest approach. Integrating the sensing unit and interface circuitry together is currently feasible, but not necessarily the optimal solution. Sometimes, using standard CMOS technology to manufacture complex control circuits can better meet both functional and cost requirements.
STMicroelectronics' accelerometers and gyroscopes employ a system-in-package (SiP) approach, where two chips are packaged in a single unit. One chip is manufactured using THELMA micromachining technology and is highly sensitive to inertia or Coriolis forces; the other can be an analog or digital control chip, packaged side-by-side or stacked with mechanical components fabricated by THELMA.
In the SiP architecture, a micro-machined sensor chip converts acceleration into a differential capacitance change, while another interface chip converts the minute capacitance change (atto-farad range) into an output signal in analog or digital format.
SiP (System-on-Package) approaches can accelerate the development of novel motion sensors such as multi-axis gyroscopes. Because ST employs a LEGO-like modular design, the mechanical and electronic modules of the gyroscope can use the same technology platform as mass-produced multi-axis accelerometers. Designers can reuse proven functional modules from multi-axis accelerometers, accelerating development and reducing costs. Furthermore, thanks to the flexibility of the LandGridArray package configuration, ST can quickly integrate any two modules from its chip into a final product, even including pin adjustments.
5. Pressure sensors in the consumer electronics market
Traditional applications of pressure sensors include the measurement of physical quantities such as pressure and airflow, primarily in industrial, automotive, and medical fields. MEMS-based pressure sensors can be used to measure physical values such as changes in resistance or capacitance. Their manufacturing primarily employs the bulk machining or surface machining methods mentioned earlier, or a combination of both. Pressure sensor materials are generally silicon semiconductors; standard silicon substrates or more expensive silicon-on-insulator (SOI) substrates are also used as the starting layer material.
Pressure sensors can be divided into resistive and capacitive types, each corresponding to different manufacturing processes. Bulk micromachining technology is the better choice for resistive pressure sensors; while capacitive pressure sensors are generally more suitable for manufacturing using surface micromachining technology.
Resistive pressure sensors utilize the piezoresistive properties of silicon to convert minute diaphragm stress into minute resistance changes. Capacitive pressure sensors, on the other hand, use two parallel plates: one fixed, and the other a thin diaphragm that moves perpendicular to the chip plane. When movement occurs, a tiny change in capacitance occurs between the two plates, generating an output. This output resistance or voltage value is transmitted to the interface circuitry and converted into a voltage value. Similar to motion sensors, the interface circuitry can be integrated using a chip or packaged form factor. Employing a SiP (System-in-Package) structure offers greater design flexibility and accelerates time-to-market.
Customized manufacturing processes are currently the mainstream of MEMS micromachining technology, and there is no so-called ideal process. However, regardless of the number of different semiconductor wafer fabs and processes on the market, the key to consumer products has always been the trade-off between price, size, and performance. This situation has resulted in only a few manufacturers being able to provide viable solutions for the consumer market. For example, in standard bulk micromachining solutions, packaging is often the main cost component. However, VENSENS technology can manufacture low-cost, miniaturized full-silicon pressure sensors with dimensions of only 0.8mm x 0.8mm and a thickness of approximately 0.3mm. Its advantage lies in making performance independent of the packaging method, removing the barrier to entry into cost-driven consumer products. ST's recently released HLGA (Holed Land Grid Array) packaging patent technology allows its pressure sensor production to re-use existing manufacturing tools for motion sensors, enabling consumers to obtain smaller and thinner packages.
6. Consumer Applications of Motion and Pressure Sensors
cell phone
MEMS technology has revolutionized the user experience of electronic products. Traditional mobile phone screens require users to use buttons or dials for scrolling and zooming, and the miniaturization of the device limits display size and functionality, leading to awkward situations like "small buttons, large fingers." Devices incorporating MEMS accelerometers can eliminate these physical components, enabling innovative interface elements. The sensors can determine the user's intentions through their movements; users may only need to tilt the device to browse ebooks or move maps, making complex operations incredibly simple.
Wireless game controller
Nintendo's Wii is one of the most successful use cases in the market. The application of a multi-axis accelerometer transformed the user's real movements into the mouse function of the handheld device; the system could control the virtual environment and characters in the game by judging the user's movements and posture. A user-friendly interface made the game more engaging, which is crucial for any company targeting young users.
Mouse and 3DPointer
The shift from keyboard to mouse was a significant advancement in operation, making the mouse the most common interface for computers and peripherals today. A mouse typically has two or three buttons for inputting commands and serves as a communication interface with the computer system. The user's movements while holding the mouse on a two-dimensional plane control the cursor or pointer position within the graphical interface and execute specific actions. The advent of inertial sensors has revolutionized mouse operation.
Integrating an accelerometer into a mouse allows the system to monitor the user's three-dimensional control actions and send the relevant data to the computer operating system; integrating a gyroscope further enhances the device's functionality and usability. This technology is also well-suited for wireless solutions requiring low power consumption.
Hard drive drop protection function
Due to the need for massive data storage, built-in hard drives are now widely used in products such as mobile phones, digital cameras, PMPs, and DVs, in addition to laptops. However, portable devices are frequently at risk of data loss due to drops. The application of three-axis accelerometers can protect the data in these devices from damage.
The accelerometer can detect the acceleration due to gravity. When the disk is dropped, the microcontroller will issue a command to remove the read/write head from the sensitive disk to avoid possible scratches when it lands, thus providing free fall protection for the hard drive in three axes.
Location-based services (LBS) assisted navigation
While GPS is now widely used, its technology has also revealed some shortcomings. GPS devices primarily rely on receivers to receive satellite signals for positioning and route guidance. However, in areas with poor signal, such as between tall buildings, in basements, tunnels, and on bridges, it cannot achieve 100% reliability and accurate positioning is difficult. Furthermore, battery-powered GPS devices often consume significant amounts of power.
Dead Reckoning (DR) uses inertial positioning to effectively compensate for the shortcomings of GPS, thus complementing it. The DR system calculates relative position by knowing the vehicle's distance and the direction of rotational offset; therefore, accelerometers, gyroscopes, and magnetometers can be used to create a motion measurement unit to achieve this function. The low power consumption of MEMS devices also effectively saves electricity.
Pedometer
People are paying increasing attention to their health, and various assistive devices are being widely used. Pedometers measure the distance and speed a user walks to determine their energy expenditure. The output of a pedometer is a set of periodic signals perpendicular to planar motion; it is housed in a shoe and can communicate wirelessly with other devices. Currently, pedometers have become an important function of PND devices, and MP3 players and multimedia phones are also beginning to integrate this feature.
Weather forecast and altimeter
Pressure sensors allow GPS devices or handheld devices to determine a user's altitude and can also provide weather forecasts. When a user makes an emergency call, the GPS and pressure sensor can automatically transmit the user's location and floor level within a building.
Image stabilization function of digital cameras/camcorders
Image stabilization in digital cameras and camcorders eliminates the impact of camera shake during shutter release. With smartphones now widely incorporating camera functionality, gyroscopes have significant market potential in this area. Piezoelectric gyroscopes are widely used in image stabilization modules for digital cameras and camcorders. MEMS gyroscopes offer advantages such as reduced spatial dimensions and lower power consumption, can simultaneously measure angular acceleration along both the pitch and roll axes, and are easily integrated with other motion sensors.
Emerging application areas of MEMS
New applications in the consumer electronics market have further propelled the "consumer wave of MEMS" into a "MEMS frenzy." Beyond the products mentioned above, MEMS technology offers vast opportunities for many other innovative applications. MEMS must focus on customer needs, catering to miniaturization and multi-sensor clustering design trends. ST will continue its miniaturization goals in MEMS technology; on the other hand, improving its technology platform and adjusting its product roadmap are also key priorities for the company. Furthermore, enabling customized and standardized packaging for multi-sensor fusion (such as full compatibility between HLGA and standard LGA packages) is also an area of great interest to ST.
Wireless Sensor Networks (WSNs) are another market that many companies and research institutions are actively developing. WSNs have been widely used in military, security and scientific monitoring fields, and are now gradually moving towards civilian use. For example, the tire pressure monitoring system in automotive electronics is actually a simple five-node WSN, in which the sensor design includes MEMS pressure sensors and accelerometer switches.
Whether WSN can achieve commercial mass production remains to be seen due to some technical obstacles; however, manufacturers and the industry can benefit from the research and development of small, low-power MEMS sensor products for the consumer product market.
MONTEs refer to wireless sensor modules consisting of sensors, receivers, controllers, batteries, and antennas. They are primarily used to measure changes in physical quantities, such as pressure, temperature, heat, airflow, force, vibration, acceleration, impact, torque, humidity, strain, and images. This concept is gradually moving towards commercialization.
Wireless sensor modules are used in numerous applications, such as home automation, industrial control, security monitoring, construction engineering, agriculture, and environmental monitoring. Currently, market demands for security, entertainment, convenience, and efficiency, along with mandatory requirements from some governments and institutions, have created opportunities for the widespread adoption of wireless sensor modules. However, this also presents challenges for MEMS suppliers, as they need the ability to integrate different technologies into a single modular format. STMicroelectronics, recognizing this need, has invested in an advanced 8-inch wafer fab to establish a production process platform for the mass production of Motes.
The selection of sensors in wireless sensor modules depends on the specific circumstances. Sensors don't necessarily need to use MEMS technology; in some applications, traditional sensors that have been developed for a longer period may be more suitable. The potential market for wireless sensor modules is huge, and once the technological bottlenecks are overcome, they will become a part of people's daily lives.
in conclusion
MEMS technology is gradually entering the civilian and consumer markets, and we are at the beginning of a "MEMS boom." ST will leverage its robust production foundation, efficient execution capabilities, in-depth understanding of the consumer electronics market, and advanced and stable production and R&D technology platform to drive this new wave. ST believes that MEMS technologies, represented by pressure sensors and accelerometers, will play an increasingly important role in future products and bring sweeping power to the entire industry; WSNs can bring endless innovative applications and, after overcoming technical challenges, will further promote the innovation revolution of MEMS.
