MEMS @ BOSCH: Automotive applications and beyond

Dr. Peter Ernst

Robert Bosch GmbH Automotive Electronics

Abstract BOSCH has been a supplier for MEMS automotive sensors for more than a decade. After starting in the early ‘90s with some ground-lying basic technology development (the BOSCH process for DRIE, release etch technology, and others), BOSCH introduced pressure sensors, air mass flow sensors, accelerometers and gyroscopes into the automotive industry. These MEMS products have become enablers for several advanced automotive systems that help to make cars safe, clean and economical: Advanced fuel injection systems, passive and active safety systems, and others. This talk reviews the key role of MEMS technology products for the progress of automotive electronics and gives an outline for their use in a world of applications in the consumer electronics industry.

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The BOSCH Group BOSCH is a leading global supplier of automotive and industrial technology and of consumer goods and building technology. The Bosch group comprises 280 subsidiary companies, just under 250 of which are located outside Germany. In 2005, worldwide sales of the Bosch Group amounted to 41.5 billion euros. The number of associates employed around the globe in our three business sectors – Automotive Technology, Industrial Technology, and Consumer Goods and Building Technology – rose by 13,300 to approximately 251,000. We want to consistently pursue our strategy of further expanding our presence in international markets. Our objective is to make use of our opportunities in the world’s future growth markets, and to increase the competitiveness of our entire organization via international expertise and a balanced cost structure. We underpinned this strategy in 2005 with investments in locations such as China, India, and eastern Europe. Our sales outside Germany rose by 8.5 % in 2005, to 30.4 billion euros. This represents 73 % of the total sales figure. In 2005, our Automotive Technology business sector generated sales of 26.3 billion euros. Within this sector, Bosch has a long tradition of technology milestones. These include inventions of systems like Diesel injection pumps (1927), Antilock braking systems (ABS, 1978), Electronic diesel injection and Traction control systems (1986), common rail injection systems with piezo in-line injectors (2003), and most recently, an enhanced night-vision system (2005). Today’s perspective in the automotive industry Safe, clean, and economical: This is today’s perspective of progress in automotive systems. This trend is supported by electronically controlled chassis systems, powertrain management and assistance and information systems. All of these systems rely strongly on the input from sensors, and a growing share of them is manufactured using MEMS technology [1, 2].

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Safe (Figure 1): Cars have become safer. The blue line shows the development of the traffic density since the early 1970s – it is today almost three times as high as thirty years ago. Accordingly the total number of accidents has grown, although not as much as the traffic density. As can be seen from the yellow and red line, the total number of injured or killed persons in traffic accidents have even decreased. To a great extend, this is due to the introduction of a number of safety systems like seat belts and antilock brakes, airbags and ESP.

Figure 1: Statistical data about the traffic density, number of accidents and injured or killed persons. Clean: Figure 2 displays the development of the emission of particles, hydrocarbons, nitrogen oxides and carbon monoxide: all have decreased considerably – again due to improvements in electronically controlled and sensor-based automotive systems.

Figure 2: Reduction of particles and gas emission over years.

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Economical: Figure 3 shows the fuel consumption of average new cars. This number has also clearly improved, again driven by the introduction of ever more advanced powertrain systems. In the following sections, we discuss sensor product lines that support these systems. It will be shown that MEMS technology has been crucial to make these sensors reliable, precise, and cost-efficient enough to help increasing the installation rates of the above-mentioned automotive systems. Sensors are the automobile’s “organs of perception” and form the basis for a multitude of control, regulation, information and monitoring functions in the vehicle. Modern vehicles can contain up to 100 different MEMS sensors. Since introducing the products ten years ago, Bosch has manufactured over 500 million micromachined sensors.

Figure 3: Development of the average fuel consumption of new cars over years.

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MEMS sensor for engine management The MEMS sensor types that are used for engine management purposes include predominantly pressure sensors (covering a wide range of pressures, from below 1 bar up to more than 1000 bar), and air mass flow sensors. Figure 4 shows a barometric air pressure sensor (BAP) in a premold package as it is today widely used in engine ECU’s. It’s purpose is to adapt the parameters of the fuel combustion process to varying atmospheric pressures. Low-pressure sensors are manufactured using so-called bulk micromachining in which cavities are etched out of the wafer. A membrane is formed under these processing conditions and pressure measurements can be taken directly from the deformation of the membrane. Recently, Bosch has developed a new pressure sensor technology: APSM (Advanced Porous Silicon Membrane), see Figure 5. This technology [3] makes use of porous silicon to form a cavity underneath the surface of the mono crystalline silicon, which helps to make the wafer manufacturing process fully CMOS compatible and turning the traditional bulk-micromachining process into a surface technology.

Figure 4: Barometric air pressure sensor (BAP) in premold housing by Bosch.

Figure 5: Formation of a cavity in monocrystalline silicon during the Bosch APSM processing.

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MEMS sensors for occupant safety systems Figure 6 shows a modern airbag system with it’s numerous different components. Accelerometers are used as crash detection sensors both in the ECU (electronic control unit) and in peripheral sensors such as upfront sensors (UFS) or pedestrian contact sensors close to the bumper, peripheral acceleration sensors (PAS) in the doors, or as rear crash sensors. Today, all of these sensors rely on a MEMS sensor elements. The advancements in this technology have enabled us to reduce these sensor elements dramatically in size. This is shown in Figure 7 as a sequence of product generations that have been introduced into the market by Bosch. The MEMS elements are not only much smaller than the formerly used fine-mechanical piezoelectric device (shown left in Fig. 7) but also much more cost efficient and yet include an increasing number of functions (like self-tests and accuracy check routines).

Figure 6: High-g acceleration sensors are one of the most important sensors in modern airbag systems.

Figure 7: Generations of high-g acceleration sensors used in automotive airbag systems.

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Another important example of a MEMS technology sensor in a restraint system are the inertial sensors used for rollover detection (Figure 8). To detect a rollover, it is essential to combine the sensor signal of a gyroscope with that of a low-g accelerometer. By means of both sensor inputs, the system’s algorithm determines the angle between the z-axis of the car and the vertical as well as the angular rate ωx of the vehicle in each moment. Thus, the rollover sensing algorithm determines the exact point in time when specific airbags or belt tensioners are being fired. Figure 9 shows an example of a recent rollover gyroscope from Bosch.

Figure 8: Modern airbag with rollover detection also rely on MEMS inertial sensors (gyroscopes and low-g accelerometers).

Figure 9: Gyroscope sensor for rollover detection, by Bosch.

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Modern inertial MEMS sensors have become able by MEMS process developments that began in the 1980’s. An important building block has been the Deep Reactive Ion Etch process (DRIE) developed in that time by Bosch (Figure 10). It consists of a sequence of alternating etch and passivation (deposition) cycles and thus allows to etch structures in silicon with a very high aspect ratio, high anisotropy and yet high and economical etch rates. It has become the basis for many MEMS product technologies in research and production and is offered by several vendors of production scale manufacturing equipment as a base technology of their tool.

Figure 10: The Bosch process (Deep Reactive Ion Etching of silicon) is one of the most important MEMS processing steps. MEMS sensors for chassis control systems Recently, Bosch celebrated the tenth anniversary of the market introduction of the Electronic Stability Program (ESP®). This system is an important, if not the most important contribution of modern vehicle electronics to road safety. It actively increases the driving stability of the vehicle in all driving situations. This is achieved by automatically braking an individual wheel and controlling the engines power if the driving situation requires it. Critical driving situations such as oversteering or understeering are determined by measuring the yaw rate of the car and comparing it with other magnitudes like the steering angle and the velocity. Sensor signals form the backbone of the working principle of the ESP® algorithm.

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A sequence of product generations of yaw rate sensors is shown in Figure 11. Again, we compare a formerly used fine mechanical solution (left) with two generations of MEMS based products. The shrink in size goes along with a reduction in cost and an increase in functionality.

Figure 11: Generations of gyroscope sensors for the Electronic Stability Program ESP®. MEMS yaw rate sensors typically consist of a capacitatively driven silicon oscillator that is suspended by just a few webs of material [4]. When subjected to an external rotary motion perpendicular to the axis of oscillation, the vibrating mass undergoes a deflection from its plane of vibration because of the Coriolis force, which in turn is evaluated capacitatively. Current Bosch yaw rate sensors for automotive applications are distinguished by their high offset stability, vibration robustness and full digital signal processing. This makes them less prone to failure than analog sensors. Permanently active internal fault monitoring which enables fault identification and an active self-test are features which help to attain increased reliability. One or more yaw rate sensors combined with accelerometers in an enclosure result in e.g. the DRS-MM3 sensor cluster which was introduced to series production by Bosch as an integrated sensor platform in 2005 (Figure 12). Flexible construction enables yaw rates and accelerations to be detected in different vehicle directions – depending on the client’s specific requirements for the system. The sensor cluster is suitable for highly dynamic and high-precision systems such as the Electronic Stability Program (ESP®), Roll Over Mitigation, and Hill Hold Control or Electronic Active Steering. Each of these systems requires different sensor signals which because of the individual configurability of the DRS-MM3 range, can all be individually provided. The sensor signals are transmitted via a standard CAN interface and are therefore available for all other functions and systems in the vehicle.

Figure 12: The sensor cluster DRS-MM3, as it is used in the most recent ESP® system by Bosch.

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MEMS sensors for consumer electronics In the past, automotive applications have been the pacemaker for increases in manufacturing volumes of MEMS sensors. The future, however, will be characterized by a different scenario: We expect huge growth rates in MEMS sensors for applications in consumer electronics products, with a focus on IT and communications, and extensions into health care, life sciences, security systems, domestic appliances and others. To address the specific requirements of this new emerging market, Bosch has founded early in 2005 a one hundred percent subsidiary named Bosch Sensortec [5]. It is dedicated to engineering, developing, and marketing micromechanical sensors, foundry services for consumer electronics, medical technology, security systems, and logistics. In doing so, Bosch Sensortec has access to the development and manufacturing competencies of the Bosch Division Automotive Electronics in Reutlingen. The short distance between Bosch Sensortec and Bosch Automotive Electronics makes for flexible and close cooperation during all project phases. Bosch Sensortec has developed a new triaxial accelerometer – the SMB365 (Figure 13). Samples are already available now; mass production is going to start in the fourth quarter of 2006. The SMB365 meets all important requirements of components for consumer electronics such as small packaging and very low power consumption. Its size is only 4x4x1.2mm³. When using the SMB365, just 3 external capacitors are needed, which saves additional board space. The sensor works at a very low power input of 600µA when activated and 5µA in standby mode. Furthermore, the SMB365 is characterized by non-linearity and low signal coupling between the different axes and comes with an integrated complete self-testing function.

Figure 13: Triaxial acceleration sensor SMB365, one of the consumer electroncs MEMS products of Bosch Sensortec. Literature [1] Kallenbach, R., Emig, R.: Automotive electronics – what makes it so special? SAE paper 2004-21-0088, Detroit, 2004. [2] Marek, J., Trah, H.-P, Suzuki, Y., Yokomori, I. (ed): Sensors for Automotive Applications. Sensors Applications, Volume 4. ISBN 3-527-29553-4. Wiley, 2003. [3] Lammel, G., et.al.: Next Generation Pressure Sensors in Surface Micromachining Technology. Transducers 2005, Seoul, Korea, June 2005. [4] Goméz, U.M., et.al.: New surface micro-machined angular rate sensor for vehicle stabilizing systems in automotive applications. Transducers 2005, Seoul, Korea, June 2005. [5] Stadler, J.P.: MST@BOSCH – from automotive to consumer applications. COMS 2005 (Commercialization of Micro and Nano Systems), Baden Baden, August 2005.

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