Current status and challenges of SAW filters and introduction of new IHP SAW filters

I've been swearing a bit, I'm sorry, I'm really sorry. IHP SAW, also known as Incredible High Performance Surface Acoustic Wave, is sometimes referred to as "Super SAW." When I first came across this name, I was genuinely surprised and couldn’t help but express my admiration in a rare moment of honesty. Let’s take a look at Murata’s promotional materials here. The radio communication terminal includes an RF filter that operates within the frequency range of 800 MHz to 2500 MHz. The most commonly used RF filter today is the Surface Acoustic Wave (SAW) filter. It typically uses piezoelectric crystals such as quartz, lithium niobate, or lead titanate. These materials are polished and coated with a thin metal film, then patterned using photolithography to create two sets of interdigitated electrodes—known as the input and output interdigital transducers. When an alternating voltage is applied to the input electrode, it excites surface acoustic waves on the piezoelectric substrate. These waves propagate along the surface and in the direction of the interdigital structure, hence the term "surface acoustic wave." One direction of the wave is absorbed by a sound-absorbing material, while the other is transmitted to the output electrode, where it is converted back into an electrical signal. In recent years, we’ve seen more and more use of Bulk Acoustic Wave (BAW) filters. Unlike SAW filters, BAW filters use vertical propagation of sound waves. For BAW resonators made from quartz, the top and bottom of the quartz are covered with metal layers that excite sound waves, causing them to bounce between the top and bottom surfaces and form standing acoustic waves. The thickness of the slab and the mass of the electrodes determine the resonant frequency. BAW filters are highly visible, and the piezoelectric layer must be on the order of a few micrometers. This requires thin-film deposition and micromachining techniques on a carrier substrate. In some high-frequency bands, BAW technology offers higher Q values and better bandwidth characteristics than traditional SAW filters. Murata has developed a new SAW filter called IHP SAW, which overcomes the limitations of conventional SAW filters and even outperforms BAW filters in certain aspects. This article introduces the new IHP SAW filter. 1. Current Status and Challenges of SAW Filters RF filters are widely used in the transmit and receive paths of RF terminals. They allow signals within a specific frequency or band to pass while filtering out unwanted interference. These filters consist of multiple resonant units connected in different circuit configurations (as shown in Figure 1, a typical ladder circuit), providing a filter with a defined bandwidth. [Image: Current status and challenges of SAW filters and introduction of new IHP SAW filters] Figure 1: Schematic diagram of an RF filter (ladder circuit) A key characteristic of RF filters is the steepness of the transition band—the curve between the passband and the stopband. If the transition bandwidth is too narrow, the design becomes significantly more complex. To improve the steepness, both the connection circuit between the resonators and the quality factor (Q value) of the resonant unit itself are crucial. To meet the demands of challenging frequency bands, manufacturers have focused on increasing the Q value. SAW filters based on single-crystal LiTaO3 substrates are widely used in RF applications. These filters have interdigital transducers on the piezoelectric substrate, and the resonant characteristics of the surface acoustic waves determine the filter's performance (Figure 2). [Image: Current status and challenges of SAW filters and introduction of new IHP SAW filters] Figure 2: Surface Acoustic Wave Device Another common RF filter on the market is the Bulk Acoustic Wave (BAW) filter. Unlike SAW filters, BAW filters use vertical propagation of sound waves. A BAW resonator using quartz as a substrate has metal layers on both the top and bottom, which excite sound waves that bounce between the surfaces, forming a standing wave. The thickness of the slab and the electrode mass determine the resonant frequency. Although BAW filters are more complex to manufacture, they offer higher Q values and narrower transition bands. In this context, Murata has successfully developed a new IHP SAW technology that overcomes some of the limitations of traditional SAW filters. The IHP SAW can achieve near or better performance than BAW, with improved temperature characteristics. 2. Features of IHP SAW The IHP SAW filter has three main features: (1) high Q value, (2) low frequency temperature coefficient (TCF), and (3) excellent heat dissipation. (1) High Q Value The IHP SAW filter has a significantly higher Q value compared to conventional SAW filters. It uses a new structure that focuses the energy of the surface acoustic wave on the substrate surface, allowing it to propagate without loss. In the 1.9 GHz band, the peak Qmax of the resonant unit reaches over 3000, while the conventional SAW filter only achieves around 1000. Figure 3 shows an example of an IHP SAW filter designed for Band25, a frequency band traditionally considered difficult to handle. The measured data is impressive. The typical insertion loss in the Tx band is 1.5 dB, and in the Rx band, it's about 2.1 dB. The isolation is 57 dB in the Tx band and 59 dB in the Rx band, meeting all requirements. [Image: Current status and challenges of SAW filters and introduction of new IHP SAW filters] Figure 3: IHP SAW Band25DPX (left: transmission characteristics, right: isolation characteristics) (2) Low Frequency Temperature Coefficient (TCF) The IHP SAW filter improves the TCF, achieving better temperature stability by controlling both the linear expansion coefficient of the substrate and the speed of sound. Figure 4 shows the filter’s performance when the temperature varies from -35°C to +85°C. Traditional SAW filters have a TCF of about 40 ppm/°C, while the IHP SAW filter achieves a TCF of ±8 ppm/°C or less. This represents an improvement of approximately 30 ppm/°C, and with proper substrate design, the TCF can be further reduced to nearly zero. [Image: Current status and challenges of SAW filters and introduction of new IHP SAW filters] Figure 4: Filter waveform under temperature variation (blue: -35°C, black: +25°C, red: +85°C) The TCF performance of IHP SAW is also superior to that of BAW filters, which typically have a TCF range of 20–30 ppm/°C. Therefore, the IHP SAW filter offers better TCF characteristics than BAW filters. (3) Good Heat Dissipation The IHP SAW filter also exhibits excellent thermal characteristics. When a high-power electrical signal is applied, the IDT generates heat. Stronger signals produce more heat, which can lead to equipment failure, including electrode damage. The IHP SAW filter efficiently dissipates heat from the electrode to the substrate, reducing the temperature rise by nearly half compared to conventional SAW filters. The low TCF and improved heat dissipation ensure stable operation at high temperatures. 3. Future Prospects The IHP SAW filter performs exceptionally well across a wide frequency range, from 800 MHz to 2.5 GHz. Recent research has also confirmed its performance at 3.5 GHz, a frequency band considered challenging for traditional SAW filters (see Figure 5 for Q-value comparison). As mobile terminals evolve toward high-speed communication devices, the demand for advanced RF components will grow. The IHP SAW filter is a strong candidate for future communication systems. [Image: Current status and challenges of SAW filters and introduction of new IHP SAW filters] Figure 5: Q-characteristic comparison of IHP SAW and traditional SAW filters Another advantage of the IHP SAW filter is the flexibility in adjusting the bandwidth. Designers can choose any desired bandwidth. Additionally, the IHP SAW filter supports miniaturization, offering significant size advantages over conventional SAW filters. With the shrinking size of mobile devices, there is growing demand for compact RF components. The IHP SAW filter is an ideal solution that meets these needs.

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