Research Introduction(for Business)

Support for the development of filters (BAW filters) for 5G and later smartphones

Currently, smartphones are equipped with various filters, which transmit and receive frequencies within a certain range and reject frequencies outside that range. Since the frequencies in low-GHz range are not only used by smartphones but also occupied by other information communication applications such as WLAN, the available bandwidth (frequency resources) is narrow. The wireless bandpass filters are commonly constructed from inductors and capacitors (LC filter) are not adequate, because the frequency bands space extremely close from each other, and the low Q-factor (the sharpness of the filter) of LC filters cannot prevent signal interference. For example, some 2 GHz band mobile phone system in United State has 20 MHz frequency range between uplink and down link, as a result, signals may easily interfere with each other.

Smartphones adopt filters that use ultrasound wave (elastic wave) instead of using LC filter used by wireless network to achieve better accuracy. This is because the mechanical resonance has a higher Q-factor than the electrical resonance. Piezoelectric materials are used in the filters for extract mechanical resonance as electrical resonance. After the electromagnetic wave that used for communication input into the piezoelectric materials, the electrical energy can be converted into the acoustic wave by the piezoelectric effect. The acoustic waves resonate at the resonance frequency which determined by the dimensions of the material, and then be transformed to electrical signal by the piezoelectric effect to realize the filter function. That means comparing to the electrical resonance, the acoustic resonance has higher Q-factor (sharpness) and temperature stability (smaller TCF). As you can see, using piezoelectric materials speciality, high-performance filters can be produced.

At present, SAW (surface acoustic wave) propagating on a piezoelectric single crystal substrate is almost exclusively used in filters. Since smartphone communication has large quantities of data, which can be handled in extra ultra-high frequencies, oscillate the acoustic waves in GHz band is necessary. According to the analysis, in 5G communications, SAW filters will not be available when the frequency reaches 4 GHz or higher. Therefore, piezoelectric thin film resonance-based BAW filters (also called FBAR, film bulk acoustic resonator), which available in high frequencies, is gradually increasing its market share. In order to increase the resonance frequency of the BAW filters, the piezoelectric films used in the filters must be thinned. However, at present, there is no good piezoelectric material can achieve this goal. The PZT ceramic which is considered as the king of the piezoelectric materials, is difficult to be used in GHz bands due to its large dielectric loss and acoustic attenuation.

Under these circumstances, our lab has discovered new nitride-based piezoelectric materials (non-publicized) with three advantages of high piezoelectricity, low dielectric loss and low acoustic attenuation. In addition, we also discovered a new method of extracting the electromechanical coupling coefficient of the piezoelectric film when the film still retained on the substrate before the device is made, and we are aiming to standardize this method. Currently, we have been commissioned by many equipment manufacturers and substrate manufacturers to evaluate piezoelectric films and explore new materials, and also conduct joint research and information exchange with them.

Most of the components mounted on smartphones are about a few yen, and the price of wireless filters is dozens of times more of the common components, which is a large business opportunity. Affected by the international roaming, the number of filters on a mobile phone has reached about 50, and the requirement of the piezoelectric material which can respond to various wireless standards is also increasing. In the field of piezoelectric and elastic wave devices, high technical level is required for development, and it is difficult to be imitated by countries with low prices and low labor costs. For this reason, Japan manufacturers in this field still occupy first place in the world. The field of piezoelectric device is not well-known, but as a domestic technology of Japan with advantages, I wish to get everyone’s support.

The following studies can be jointly researched

（１）Determination of the electromechanical coupling coefficient, sound velocity, and dielectric constant of the piezoelectric thin film retained on the substrate.

（２）Research on new doping or new materials of piezoelectric thin film (oxides, nitrides).

Supply and application device development of the ScAlN freestanding substrate (0.1 mm)

The kt2 (electromechanical coupling coefficient towards substrate) of ScAlN fabricated by our lab, has reached 22% with conservative estimate. ScAIN is lead-free and its performance is comparable to the PZT ceramic thin film, which kt2 = 25%. When compare with LiNbO3 and PMN-PT single crystal thin film, the advantages of ScAlN is the possibility of large-area production in the future. Furthermore, the ScAlN can be fabricated as curving thin film, making it useful for focusing ultrasonic transducers.