TMR angle sensors acquire a large share of the EPS motor sensor market
and are expected to be used as rotation and position sensors
Interview with engineers "TMR angle sensors"
TDK's TMR angle sensors are developed using its thin-film processing technology, which has been accumulated through the production of HDD heads. Highly sensitive, accurate, and stable, TMR angle sensors are the focus of the automotive market's attention as the replacements of Hall Effect sensors and resolvers. In 2016, TDK received the Semi-Grand Prix in the Green Innovation category of the CEATEC AWARD 2016 as well as the Automotive Parts Award of the "CHO" MONODZUKURI (Ultra Craftsmanship) Award, which was co-hosted by the MONODZUKURI Nippon Conference and the Nikkan Kogyo Shimbun Ltd. We interviewed the development leaders of the Asama Techno Factory (ATF) who solved a wide variety of technical issues and launched a new product about the technical process toward the birth of the TMR angle sensors.
Head of Design Team,
MR Sensors R&D Group,
Magnetic Heads and Sensors BC,
Head of MR Sensors Operation and R&D Group,
Magnetic Heads and Sensors BC,
"If we're given an opportunity to develop a new product, we'll tackle the most difficult technical problem."
――When did you start to develop TMR angle sensors?
Sakai We started in 2009, when ATF (Asama Techno Factory) was already a globally recognized production base for HDD heads. Mr. Ishiguro (current president) and I wanted to use its technology and facilities to launch a new product that would be a key pillar of our business in the future. We agreed to take the opportunity to tackle the most difficult technical problem.
Automotive electronics is included in one of TDK's growing fields, so we decided to develop automotive angle sensors by using the TMR element, which was used as a cutting-edge reading element for HDD heads, as a magnetic sensor.
Hirabayashi At that time, I was engaged in the design of HDD heads, but I was suddenly invited and got to join the development group. It is currently large and consists of multiple teams, including design and evaluation teams, but used to be only three people including me.
Sakai I was looking for someone who was competent as well as unique and who thought outside the box because we had to address extremely difficult technical issues. I saw him working at the forefront of HDD head production and thought that he was perfect to lead the design team.
――Mr. Hirabayashi, what type of engineer do you think Mr. Sakai is?
Hirabayashi We had to develop TMR angle sensors from scratch, and taking the first step forward bravely was the key to the success of new product development. Both thoughtful and bold, Mr. Sakai was the leader of the group and the pioneer in the development of TMR angle sensors. This led to the commercialization of TMR angle sensor technology.
――Did you use your TMR element technology for HDD heads without changing it?
Sakai While the element development process, which uses thin-film processing technology, is the same as the process for HDD heads, the concept of the product is fundamentally different from that of HDD heads. In addition, automotive sensors must operate properly over a wide range of temperatures from low to high.
――Specifically, what is the most difficult part?
Hirabayashi The wafer process involves as many steps as the semiconductor device manufacturing process, and this costs time. Even after successfully developing sensor elements, we still faced a pile of technical issues, such as incorporating elements into a circuit, IC assembling, and packaging. I am often asked what I struggled with most when developing. I cannot summarize it as there were too many challenges. Luckily, however, I am curious by nature, and I resolved the issues one by one through trial and error. After several years there were good prospects for achieving our goal, and in 2014 we finally started to mass produce TMR angle sensors.
"Actually, our TMR angle sensors were difficult to damage, which was another issue"
――There have been sensors that use magneto-resistive (MR) elements. What differentiates TDK's TMR angle sensors from the others?
Sakai Above all, their output is extremely high. TMR sensors are magnetic ones that use tunnel magnetoresistance (TMR). Whereas the MR ratio, which shows resistance change in an element, is 3% for the AMR element and 12% for the GMR element, it is close to 100% for the TMR element. Therefore, TMR sensors' output is 20 times higher than AMR sensors and 8 times higher than GMR sensors. In addition, with a high degree of angle accuracy, they are highly resistant to temperature drift and aging, which make the TMR sensor highly stable, and it is an important feature for an automotive sensor.
Hirabayashi We repeated reliability tests, in particular, again and again as a failure in an automotive sensor can lead to a serious car accident. Our customers are mainly Tier 1 manufacturers, who sell components to car manufacturers. Tier 1 manufacturers want to fully understand how automotive electronic components, including sensors, will be damaged. However, our TMR angle sensors would not be damaged in thermal shock and temperature cycle tests under severe conditions far beyond the specified requirements. We didn't know what to do that time.
Sakai In a temperature cycle test, we increased the number of cycles from 100 to 1,000, but they were not damaged. When we increased the number to 8,000, we thought that they were finally damaged. We were pleased, but that was only a wire disconnection caused by a cracked resin in the package, which was a disappointment for us. Anyway, the sensor elements are incredibly durable.
――Specifically, where are TMR angle sensors used on a car?
Sakai Firstly, we started to sell them for electronic power steering (EPS) motors. We have received many inquiries for several years after mass production.
They can also be used as rotation sensors. For example, in ABS, a slip ratio is calculated to adjust the brake pressure. In order to so, Hall or MR rotation sensors are used as wheel speed sensors, which measure the rotation speeds of wheels. TMR angle sensors can replace them. Equipment on a car, including a windshield wiper, power mirror, and automatic air conditioner, uses many DC motors. TMR angle sensors will gain popularity because of their compact size and low power consumption. Additionally, due to its high accuracy, TMR sensors are expected to be used not only for angle detection but also for position detection.
Of course, all of the sensors will not be replaced by TMR sensors. There is a tradeoff between the quality and price of a sensor. I assume that customers will choose the right sensor depending on the application. For example, they will use TMR sensors when high accuracy is required and other sensors for other occasions.
"ATF's culture allows us to challenge ourselves"
――What are your future plans about the sensor business?
Sakai As well as selling our TMR angle sensors as automotive ones, we would like to develop applications in the fields of wearable technology and IoT by using their best attributes: small, lightweight, and low power consumption.
――What technical resources did TDK have to produce innovative TMR angle sensors?
Sakai It was the 1980s that TDK started the development of thin-film magnetic heads for HDDs. At that time, there were many rival companies and a fierce competition for the best technical developer among them.
The development of thin-film magnetic heads involves comprehensive technology, including thin-film processing technology, whose level of sophistication should be the same as that of semiconductor device manufacturing technology, as well as technology for magnetic material handling, evaluation, and simulation. While our competitors, one after the other, withdrew from the competition or were weeded out, TDK has survived by launching cutting-edge HDD head products, such as AMR, GMR, and TMR heads. We have become one of the world's best HDD head manufactures. This is because our manufacturing personnel have continued the tradition of great engineering spirit and development capability based on core technology since the foundation.
Hirabayashi Furthermore, ATF's culture, in particular, allows us to challenge ourselves. Also, it has enough facilities and talented personnel to achieve significant growth as the development base for a wide variety of sensors based on thin-film processing technology.
【TMR angle sensors】
The structure of TDK's TMR angle sensors has a barrier layer, a nanometer-order thin insulator, between two strong magnetic layers (free and pinned layers). The direction of magnetization is set in the pinned layer. However, that changes in the free layer affected by where an external magnetic field is, and the elements' electric resistance changes accordingly. By using these features, TDK has successfully developed TMR angle sensors that are high-output, high-accuracy, and high-safety.