MEMS Microscopic eyes to observe the heartbeat of the Earth – Turning an impossible wish into an everyday reality. –

The pressure sensor built into OMRON blood pressure monitors creates a repeating vibration that sounds remarkably like a heartbeat. The sensor is a mere 1.36mm square.

Sensors are incorporated in virtually all kinds of devices, from daily-use appliances to sophisticated industrial equipment. These sensors are constantly at work detecting various phenomena in the natural world, such as pressure, inertia, flow, temperature, sound, and light.

For example, infrared sensors that detect the presence of humans by evaluating body temperature can save electricity by turning on lights or sending cool air only where a person is present. Seismic sensors can detect intense seismic vibrations and stop machines or equipment, in order to protect them from failure or fire. Microphones built into smartphones are also a type of sensor. Capable of detecting a wide range of audio frequencies and clearly transmitting sound, this sensor makes it possible to talk on a smartphone or use voice input.

MEMS microphone mechanism

These sensors are best characterized by their miniaturization. Hardware with dimensions of only several millimeters is packed with components such as a detection mechanism, as well as a controller and circuitry for converting detected data into electrical signals for output. These miniaturized machines are called "MEMS (Micro Electro Mechanical Systems)."

MEMS is a key next-generation technology with steadily expanding applications. In particular, the use of MEMS is growing for smartphones and wearable devices, which is a direct result of the development of increasingly smaller and more precise MEMS devices. In fact, MEMS devices can do a lot. For example, tilting your smartphone rotates the monitor, and the map app tells you exactly where you are. It is even possible to check the intensity of physical activity you perform simply by carrying it around.

What makes all these functions possible are MEMS devices. Recently, MEMS sensors have become able to collect "altitude" information, which has attracted much attention. Adding altitude data to positional information will enable navigation systems to recognize the floor of a building and the area on the floor where a person is present. This also enables more accurate calculation of physical activity, such as going up and down stairs.

MEMS sensors can be used for navigation within a shopping mall

In what way can "altitude" be evaluated? The key point is the different in atmospheric pressure.

It's a well-known fact that atmospheric pressure decreases with increasing altitude. It is believed that the air pressure on the summit of Mt. Everest is a mere 30% of that on the ground surface. But these differences occur not just on top of a mountain. In fact, we are constantly exposed to variable pressure induced by an extremely small mass of air molecules.

Absolute pressure sensors are designed to measure miniscule amounts of air pressure relative to a perfect vacuum.

OMRON absolute pressure sensor with detection accuracy among the world's highest

Its pen tip-sized package measuring only 2.0 x 2.5 x 0.85mm contains a sensor that detects air pressure, and an IC chip that serves as a controller.

This sensor can detect a miniscule pressure change of +/- 0.6Pa, which is equivalent to an altitude difference of +/-5cm. The capability to assess +/-5cm altitudinal variations makes it possible to accurately detect the movement of a person going up or down a single step of stairs. Achieving this high level of accuracy in an extremely small package makes this MEMS sensor truly the "best of breed."

People in varied positions join together to meet the challenge presented by limitless combinations.

Creating a machine that can detect a miniscule change in air pressure, yet still fits in a package as small as 2mm square—this actually was an unexpectedly tough challenge even for highly experienced OMRON developers.

Inside the package is a MEMS sensor that consists of a thin silicon film (diaphragm) suspended over a vacuum cavity. This sensor determines atmospheric pressure by the deformation of the diaphragm caused by air entering from outside. To increase the detection accuracy of the sensor, the silicon diaphragm must be made as thin as possible so that it deforms to a large extent even when exposed to miniscule air pressure. As for the circuit to process and convert the diaphragm's deformation into an electrical output, developers also faced the challenge of how to reduce noise for more accurate transmission of electrical signals.

Although the developers succeeded in improving the accuracy of sensors, another problem occurred—as the sensor accuracy increased, the sensor became more susceptible to other effects, such as a slight deformation (stress) of the casing caused by temperature changes.

Cross-sectional view of the absolute pressure sensor

What would be necessary to create a high-sensitivity sensor that is still immune to stress components other than air pressure? The development team members thoroughly sought and reviewed the most appropriate materials, parts and configurations.

Kanemoto Kagoshima and Katsuyuki Inoue of the development team worked hard to review designs

The team had to find the most suitable materials for the cap that covers the IC chip, the wire connecting the sensor and IC chip, and adhesives to bond parts, as well as the optimal layout for individual parts. There are countless numbers of combinations that they came up with, and which they had to test.

"Our goal was to enhance the performance of the sensor, while also making stable production possible at a reasonable cost," says Katsuyuki Inoue. The team was made up of members with varied specialties and tasks. "This diversity of people contributed ideas about the structure and materials, and we conducted repeated prototyping and evaluation to locate configurations that were promising. We put out an incredible number of ideas and persistently tested different combinations until we reached our target configuration," Inoue remembers. "We even questioned what were considered common notions, and tried materials that were said to be impossible to use. In this way, we have moved forward together with manufacturers of materials.

Koji Sano and Shigeyuki Ukai, in charge of performance tests

It was also not an easy task to make a prototype. Assembling parts, each measuring less than 1mm square, and fine-tuning their positions with micron-scale precision requires highly advanced packaging technology. What enabled the team to overcome all these difficulties was the expertise that OMRON has accumulated in over 20 years of MEMS development.

According to one developer, OMRON has manufactured more than 1 billion units of MEMS microphones built into smartphones, as well as the sophisticated MEMS manufacturing technology. MEMS devices are difficult to assemble and its production requires many adjustment steps. "The development of this absolute pressure sensor was possible only because we could fully leverage the microfabrication and packaging technologies and quality inspection knowhow that OMRON has cultivated over many years, and that includes our experience of repeated failures," said one developer speaking for the team.

Shinichi Terasaka and Akira Kita, in charge of manufacturing, diligently applied their skills to prototyping

The achievement that the team enjoys today is the result of not only successes but repeated failures as well. By getting rid of all the assumable effects of stress, the team finally succeeded in detecting a change of +/-0.6Pa in air pressure— a "best of breed" performance.

"OMRON MEMS is the result of combining the company's technological legacy and innovative technologies," Yoshitaka Adachi, product manager, remarks. A mere 1.5 years after the launch of the project, the team completed and released a sensor featuring one of the highest performance levels in the world. Adachi explains how this was achieved: "Each member involved in planning, development, technology, quality assurance and sales sectors continued to face challenges, with none of them saying 'I cannot.'"

Finally, the absolute pressure sensor incorporating the aspirations and commitments of all members came into existence. "I think this sensor is the fruition of all members' determination to offer new value to the world by creating an innovative sensor," Adachi concluded confidently.

Yoshitaka Adachi, product manager who led the planning of the product

MEMS turns an impossible wish into an everyday reality.

The high level of performance required to detect a miniscule +/-0.6Pa difference in air pressure with a miniaturized design will broaden the scope of potential applications for the absolute pressure sensor to an unprecedented degree.

This sensor can identify human actions such as stand up, sit down, lie down, and fall down, and recognize movements of climbing up or down stairs, or even detect positions in three-dimensional space by precise measurement of altitudinal variation. By taking advantage of these capabilities, more and more applications will be developed for the sensor, especially in the area of smartphones and various wearable devices.

Shuichi Wakabayashi is a salesperson in charge of developing applications together with his customers such as device manufacturers worldwide. He points out that the sensor can also be used for 3D positioning indoors where GPS radio waves do not reach. "It can locate the floor of a department store where a lost child is present, for instance, or identify the floor of a building in which disaster victims are waiting to be rescued," says Wakabayashi. "With these features, this MEMS sensor can be used for crime prevention, watching over children, and rescuing disaster victims, thus contributing to enhanced safety and security for society."

Shuichi Wakabayashi remains committed to expanding the sensor's applications in cooperation with customers

This sensor can also help promote health when it is incorporated into high-precision wearable activity monitors. For example, it can provide a more accurate assessment of physical activity, by taking into account altitudinal changes such as climbing up and down stairs or hills.

Around 2020 when Tokyo will host Olympic Games, it may be even possible that OMRON MEMS absolute pressure sensors will be installed in innovative 3D navigation systems, which could help guide tourists from all over the world.

Product Manager Adachi sums up by saying, "By detecting various phenomena in the natural world, such as pressure, inertia, flow, temperature, sound, and light with OMRON's sensing technology and converting the data into high-value information, we can create more people-friendly products that were not available in the past." He added, "All members of our team will work together to realize a future in which 'an impossible wish turns into an everyday reality' through the development of MEMS devices."

What do you wish for in the future? Our future will be truly exciting if many of the things that people are wishing for become a reality. Beyond OMRON technologies is a future that awaits us, in which all these exciting fulfillments will be brought within the reach of everyone in their daily lives.

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