Piezoelectric substances, which have piezoelectric properties that allow them to convert applied pressure to voltage and applied voltage to pressure, are fabricated in thin form to make piezoelectric thin films, which generally incorporate a material called lead zirconate titanate (PZT)^*1.

These films are currently widely used in angular rate sensors that detect digital camera shake and the position of vehicles, as well as in inkjet printer heads. Their uses are expected to expand to include micro-electromechanical system (MEMS) mirror devices^*2 used in projectors and heads-up displays, as well as in energy harvesters^*3.

With the increasing awareness of the need to preserve the Earth's environment, it is necessary to eliminate the use of lead. However, it has been difficult to achieve sufficient piezoelectric properties^*4 in piezoelectric thin films using lead-free materials, and micromachining has also been difficult.

Hitachi Metals and Wacoh have succeeded in developing a 3-axis angular rate sensor that employs lead-free piezoelectric thin film. Potassium-sodium niobate (KNN)^*5, which is highly environment-friendly, was selected as a lead-free replacement material, and sensors incorporating it achieve the same level of angular rate sensing properties as PZT thin film sensors. During the development phase, Hitachi Metals handled the processes from piezoelectric thin film to sensor device production, while Wacoh designed the devices and evaluated the sensor's properties.

In 2010, Hitachi Cable, Ltd. (now part of Hitachi Metals) succeeded in developing lead-free KNN piezoelectric thin film with piezoelectric properties of more than 100 pm/V, which is sufficient for practical applications. This time, the elemental technologies required to manufacture angular rate sensor devices have been developed, including a thin film-making technology for six-inch wafers, a thin film-micromachining technology, and an electrode-forming technology. Based on these successes, the 3-axis angular rate sensor was developed.

In the existing PZT thin film sensor production process, depolarization occurred when the temperature exceeded 260℃, adversely affecting device properties. However, the newly-developed lead-free KNN piezoelectric thin film sensor has excellent temperature tolerance, and it was confirmed that its properties do not deteriorate even when the sensor is exposed to 400℃. This will enable the selection of environment-friendly soldering materials for reflow soldering processes. Hitachi Metals has established technology for mass-production and commercial use of the lead-free piezoelectric thin film wafer.

Hitachi Metals plans to present this development at PiezoMEMS 2014, an international scientific conference to be held at Kobe University's Port Island Campus on October 28 (Tue.) and 29 (Wed.) (our company's presentation number is I-8).

For inquiries from customers
Kenji Shibata, Cable Materials Company, Hitachi Metals, Ltd.
Tel: +81-29-826-7654

For inquiries from the press
Masayuki Yoshihara, Corporate Communications Office, Hitachi Metals, Ltd.
hmcc.sa@hitachi-metals.com