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MEMS: huge success with tiny sensors

Microscopic yet indispensable to modern-day transportation and communication, microelectromechanical systems, or MEMS for short, are sure to figure prominently in our future. Their success story began in the mid-1990s with a revolutionary Bosch invention.

Synthetic sensory organs

When an airbag opens precisely and quickly, a skidding car recovers stability and traction, or a tilted smartphone screen rotates, it is a microelectromechanical system, or MEMS sensor, pulling the strings.

The internal silicon structures of these sensors, often just a fraction of a human hair across, can convert microscopic motions into electrical signals, process this information, and transmit it. In a manner of speaking, they are the sensory organs of the technical world.

What’s inside a MEMS sensor?

Decoupling unit

Ensures the MEMS element is suspended in such a way as to minimize stress. Without this decoupling, the MEMS element would be subjected to compressive or tensile stress in the event of temperature fluctuations, which in turn could distort measurement signals.

MEMS element

Depending on the type of sensor, it detects physical variables such as acceleration, yaw rate, and pressure, as well as chemical variables such as volatile organic compounds (VOC).

Bonding wires

Provide the electrical connection between the MEMS element and the evaluation circuit (ASIC).

ASIC

The application specific integrated circuit amplifies and evaluates measurements taken by the MEMS chip to deliver the appropriate output signal.

Printed circuit board

Routes out the ASIC’s output signals and feeds in the power supply via its internal conductor tracks.

The breakthrough

MEMS sensors from Bosch are like the sensory organs of technology. — The comb-like structures of a three-channel sensor that measures acceleration along three axes

As early as the 1970s, small instruments had been installed in devices and machines that could, for example, gage pressure or measure acceleration rates. But it was not until the mid-1990s, when Bosch achieved an engineering breakthrough, that MEMS sensor technology triumphed. Deep reactive-ion etching (DRIE) created comb-like structures with steep-sided walls. It also enabled engineers to downsize sensors to cut costs and scale up precision. Bosch licensed the process to competitors, but the invention was so inextricably linked with its inventor that DRIE became better known as the Bosch process.

What’s so special about the Bosch process?

Fine comb-like structures are etched into the accelerometer’s silicon surface of Bosch's MEMS sensor. — Fine comb-like structures are etched into the accelerometer’s silicon surface. In nature, the impact of raindrops on soft rock can create raised surfaces. This process bombards silicon with ions to the same effect. © Martin Oeggerli, supported by C-CINA, University Basel

The Bosch process achieves an aspect ratio of 1:20 with just one micrometer horizontal displacement at a depth of 20 micrometers. A real breakthrough in terms of size and production costs, this method enables manufacturers to densely populate the silicon wafer with structures or components.

Some of these comb-like structures of Bosch's MEMS sensor are many times thinner than a human hair. — Some of these comb-like structures are many times thinner than a human hair. Less than a millimeter in height, MEMS sensors for consumer electronics are nonetheless remarkably powerful and robust. This also has to do with silicon.

The Silicon in Bosch's MEMS sensors is a material with superior quality and readily variable electronic properties are the products of millions of man-years of research. — This material’s superior quality and readily variable electronic properties are the products of millions of man-years of research. Silicon is a third of the weight and thermal expansion coefficient of steel, yet has four times the elasticity.

As the new millennium approached, Bosch laid the foundation for market leadership among MEMS producers. In 1998, the company unveiled its first silicon MEMS yaw-rate sensor for use in the ESP^® electronic stability program. Installed as a standard feature, this driver assistance system would become practically ubiquitous. Studies suggest that such systems save more than 10,000 lives a year, with much of the credit going to MEMS sensors.

ESP^® applications initially tipped the scales of Bosch’s MEMS manufacturing brief toward solutions for the automotive industry. Keen to advance the state of the art, the company’s engineers sought to improve sensors’ ability to work at engine compartment temperatures ranging from 40 below zero to 135 degrees Celsius, while holding up to mechanical stress and electrical interference.

5 Bosch MEMS sensors

feature on average in every new car.

Boldness pays off

Manufacturing Bosch MEMS sensors, a robot arm immerses wafers in a chemical bath.. — A robot arm immerses wafers in a chemical bath.

Bosch was quick to spot the growing potential of MEMS sensors for other applications — particularly for consumer products. In 2005, it launched a startup, Bosch Sensortec GmbH, to develop this line of business. This was a bold move, as the market for entertainment devices has a voracious appetite for new products. To win the race to market leadership, the company was compelled to step up its pace to satisfy this demand and go all-out in its efforts to reduce size, weight, power consumption, and cost. That boldness paid off. Bosch now manufactures more than four million MEMS sensors a day.

A positive influence

70 percent of the MEMS sensors made by Bosch are used for consumer electronics. — A wafer carrier is loaded to a wet cleaning system for a preparatory cleanse.

Consumer electronics now account for the biggest slice of the MEMS pie — 70 percent of the MEMS sensors made by Bosch. Sensors once found exclusively in upscale products now feature in simple pedometers used by recreational joggers.

Bosch’s efforts to adapt its research and development to consumer electronics in turn had a positive impact on the company’s automotive innovations. For example, a state-of-the-art ESP^® yaw-rate sensor is so sensitive that it can detect the rotational speed of an analog clock’s hour hand, a movement that is all but imperceptible.

Milestones in the history of MEMS

1970: Bosch begins research into MEMS to develop pressure sensors for engine management. — Bosch begins research into MEMS to develop pressure sensors for engine management.

1993: After six years in development, Bosch presents its first production-ready MEMS prototypes. — After six years in development, Bosch presents its first production-ready MEMS prototypes.

In 1994 the Bosch scientists Franz Lärmer and Andrea Urban advance the state of the art in surface micromachining with deep reactive-ion etching (DRIE). It goes on to become the gateway technology for Bosch’s entry into the MEMS sensor business. — The Bosch scientists Franz Lärmer and Andrea Urban advance the state of the art in surface micromachining with deep reactive-ion etching (DRIE). It goes on to become the gateway technology for Bosch’s entry into the MEMS sensor business. The research team earns the 2007 European Inventor Award for its achievements.

The first sensors in 1995 gage pressure and acceleration, paving the way for further developments that will shape modern technology. — Mass-production of automotive MEMS gets underway. The first sensors gage pressure and acceleration, paving the way for further developments that will shape modern technology.

In 1998 Bosch begins production of the first MEMS yaw-rate sensor for ESP®.. — Bosch begins production of the first MEMS yaw-rate sensor for ESP®. It goes on to become a bestseller and a lifesaver for countless drivers.

In 2005 Bosch Sensortec startup opens for business — Bosch Sensortec is set up to introduce MEMS to the consumer electronics market. Bosch Sensortec provides the lion’s share of growth as Bosch takes the lead in MEMS sensor production in 2013.

In 2008 the Bosch Team takes home German Future Prize. — In recognition of their innovative manufacturing processes for surface micromachining, the Bosch researchers Jiri Marek, Michael Offenberg, and Frank Melzer are awarded the 2008 German Future Prize for a project entitled “Smart sensors conquer consumer electronics, industry, and medicine.”

In 2009 Bosch sensors get smart. — Equipped with an integrated microcontroller (shown in green), the first sensors to process motion signals autonomously allow specific movements to be recognized automatically. This finds application in functions such as tap sensing and step counting.

In 2016 Bosch launches the world's smallest nine-axis sensor, which hits the market — Bosch launches the world’s smallest nine-axis sensor for consumer applications. Engineered to consume a minimum of electricity, it enables battery-powered devices to run longer, while providing new features such as a position detector, compass, and step counter.

Groundbreaking innovation

This story illustrates Bosch’s ability to defend its lead in the fiercely competitive market for MEMS sensors, through innovation in both products and manufacturing processes. Bosch is now one of the few manufacturers worldwide to cover the entire value chain. The company is responsible for the development of MEMS processes, sensor design, evaluation circuits, the packaging, and test procedures. It also manufactures and sells the products. By optimizing the entire chain, Bosch was able to miniaturize sensors and achieve outstanding quality, leading to further groundbreaking innovation.

Five surface micromechanical processes developed by Bosch provide the technology that underpins MEMS production. One is the milestone APSM (advanced porous silicon membrane) for pressure sensors. This process creates a precisely defined cavity with a vacuum under a monocrystalline silicon membrane — a prerequisite for pressure sensors that are ultra-precise yet small and cost-effective.

How do MEMS sensors work?

Bosch's Pressure sensors consist of a base with a thin silicon membrane and resistance structures mounted on it. — Pressure sensors consist of a base with a thin silicon membrane and resistance structures mounted on it. These are sensitive to the mechanical forces of expansion and compression. The electrical voltage changes to produce a measured value proportional to the pressure level.

In the micromechanical sensor by Bosch acceleration changes the distance between the comb-like structures. — In the micromechanical sensor shown here, acceleration changes the distance between the comb-like structures. The resulting change in capacitance generates an electrical signal. This command goes to the ECU to trigger an action, for example, to deploy the airbag when the car’s brakes are slammed.

The outer frames in the yaw-rate sensor begin to oscillate in opposite directions during a rotary movement. — The outer frames in the sensor begin to oscillate in opposite directions during a rotary movement. Parts of the inner comb-like structure are deflected when the car veers. This changes the distance between combs, and thus the capacitance. Signals sent to the electronic stability program (ESP®) report this change.

The geomagnetic sensor by Bosch — A magnetic field running perpendicular to the direction of current in a conductor generates a voltage drop that also runs perpendicular to the direction of current and the magnetic field, and can be measured. In a smartphone, this function serves as a compass.

Energy efficiency and data security

The story of MEMS innovations is far from over. A third wave of development opportunities followed in the wake of the initial groundswells of growth in automotive and consumer electronics sensors. The emergent internet of things (IoT) began to drive demand for connected sensors. Some of these sensors can actually process data, thereby reducing the data traffic in all system architectures.

And if they are battery-operated, they can also cut down transmission time and power consumption. Better energy efficiency and lighter data transmission loads are not the only benefits. These developments also enhance data security, one of the greatest challenges of our time.

Where do MEMS sensors work?

Transportation

Road traffic applications

Transportation

Today a key vehicle safety component, MEMS acceleration sensors have been triggering airbags since the 1990s. The yaw-rate sensor soon followed; it enabled the wide-scale deployment of ESP®. MEMS also provided the sensing capability for motorcycle stability control, which in turn led to cornering ABS for motorbikes. Every modern-day automobile is equipped with as many as 50 MEMS sensors, including pressure sensors in engine management systems that help conserve fuel and curb emissions. Automated driving in the years to come will require inertial sensors with unprecedented stability and precision. Aviation is another application on the horizon, particularly toy drones and flying taxis.

Smart homes

Home applications

Smart homes

The backbone of a smart home is studded with MEMS sensors. They measure air quality, keep the temperature comfortable, and detect when a window is open that should be shut. They help keep burglars at bay, recognize gestures, and adjust the lighting to the intensity of the sun’s rays. A robot vacuum cleaner fitted with MEMS sensors and a Wi-Fi module provides a running report on its position.

Entertainment

Applications for gaming fun

Entertainment

MEMS sensors enable recognition-based gesture spotting for video games and treat users to a satisfying virtual and augmented reality (VR/AR) experience. When a user straps on an AR or VR device, the virtual image’s movement has to track the motions of the user’s head with utmost precision. This presents a tremendous challenge for sensors. If the tracking is even slightly off or drift effects occur, users may experience motion sickness. Intelligent MEMS sensors are up to this challenge.

Personal assistants

Applications for people on the move

Personal assistants

A personal electronic fitness trainer equipped with MEMS sensors wakes users on time on a schedule that matches their natural biorhythm. Smart sportswear can improve workout efficiency by measuring speed and calories burned. MEMS sensors help users navigate through the city and stabilize the shot if they want to take a picture during a run. They provide the perfect solution if users want to take sharp snapshots with their head camera, smartphone, or toy drone.

Multi-sensor systems by Bosch are key to the success of industry 4.0 applications.

Manufacturing

Applications for companies

Manufacturing

Multi-sensor systems are key to the success of Industry 4.0 applications. More and more machines and even workpieces are being fitted with smart sensor systems. That way, every product can report on how it is to be assembled and its state of completion. Ongoing production uses this data to largely organize and monitor itself. It detects technical issues at an early turn and automatically performs inspections. MEMS sensors also localize the position of goods.

If present forecasts are correct, it will take hundreds of billions of MEMS devices to meet future demand. The IoT itself is giving rise to an immense market. And for automated driving, cars will need MEMS sensors for self-localization based solely on acceleration and yaw-rate data without surround sensors or GPS.

Since redundant systems are a prerequisite for safe automated driving, MEMS sensors are again the key components enabling this form of mobility. The same applies to drones, another fast-growing market. And in the future, autonomous flying taxis are not going to take to the skies without the guidance of MEMS sensors.

The world of MEMS sensors

Unflagging: Every day, Bosch produces more than four million MEMS sensors for mobility solutions and consumer electronics.

Thorough: Thin round discs of silicon known as wafers are manufactured in a complex production process lasting up to 14 weeks. Bosch turns each wafer into thousands of microchips, the MEMS sensor elements.

Up to 50,000 microchips fit on a 20-centimeter wafer by Bosch — Compact: Up to 50,000 microchips fit on a 20-centimeter wafer. A MEMS sensor comprises one chip and one evaluation circuit — an ASIC — in a single housing.

Wafer production at Bosch is carried out under cleanroom class 1 conditions. — Spotless: Wafer production is carried out under cleanroom class 1 conditions. That means a cubic foot — or about 28 liters — of air may contain at most a single 0.5 microgram particle of dust. This equates to a cherry pit in Lake Constance.

Uncontested: Bosch has been the world’s leading manufacturer of MEMS sensors since 2013.

Some components of the smallest MEMS sensors by Bosch are just four microns in size — ten times smaller than the leg of an ant. — Minuscule: Some components of the smallest MEMS sensors are just four microns in size — ten times smaller than the leg of an ant.

Artificial intelligence (AI) is also making inroads into these tiny sensors. Bosch MEMS sensors are now learning to interpret human gestures. This sets the stage for further advances in device utility. For example, smartphones could learn to respond to new gestures made by people with disabilities.

And pedometers could learn to adapt to the individual’s gait. MEMS sensors and machine learning – this pairing promises to be the next chapter in an ongoing success story. In marked contrast to their diminutive stature, the reach and impact of Bosch’s minuscule marvels is sure to grow.

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Data dwarfs make the world go round

Synthetic sensory organs