Principle of 5G RF Filter Technology

(1) SAW surface acoustic wave filter

SAW is a type of sound wave that propagates along a solid surface, and a basic SAW filter consists of a piezoelectric material and two IDTs (interdigital transducers). The IDT core functions in energy conversion, converting received sound waves into electrical signals at the output end and electrical signals into sound waves at the input end. This transformation mainly relies on the intermediate piezoelectric material. When the crystal of the piezoelectric material is subjected to external pressure, it undergoes deformation, and the distance between atoms inside the crystal changes, breaking the original balance of positive and negative charges and generating voltage on the crystal surface. Conversely, when the two ends of the crystal are subjected to voltage, the crystal also undergoes deformation.

SAW is an elastic wave that is generated and propagated on the surface of a piezoelectric substrate material, and its amplitude rapidly decreases with increasing depth into the substrate material. The basic structure of a SAW filter is to fabricate two piezoelectric transducers - an interdigital transducer - on a polished surface of a substrate material with piezoelectric properties, which are used as the transmitting transducer and the receiving transducer, respectively. The transmitting transducer converts RF signals into surface acoustic waves, which propagate on the substrate surface. After a certain delay, the receiving transducer converts the acoustic signals into electrical signals and outputs them. The filtering process is achieved through the conversion from electrical to acoustic and from acoustic to electrical.

SAW filters use crystals such as quartz, lithium tantalate, and lithium niobate as piezoelectric materials, combining low insertion loss and good suppression performance. They not only achieve wide bandwidth, but also have a much smaller volume than traditional cavities or even ceramic filters. Because SAW filters are fabricated on wafers, they can be mass-produced at low cost. SAW technology also supports the integration of filters and duplexers for different frequency bands on a single chip, with minimal or no additional process steps required. However, SAW filters have limitations, as their selectivity decreases above approximately 1GHz; At around 2.5GHz, SAW filters reach their maximum operating frequency. SAW devices are sensitive to temperature changes and their performance decreases with increasing temperature. When processing high-frequency signals, their performance is poor. Therefore, SAW is suitable for use below 1.5GHz. However, when the operating frequency exceeds 1.5GHz, the Q value of SAW begins to decrease. As the temperature increases, the stiffness of the substrate material tends to decrease and the sound velocity also decreases. Therefore, an alternative method is needed to improve the heat dissipation and Q value stability of SAW filters. TC-SAW filters were developed based on this premise.

TC-SAW filter, temperature compensation filter, is a filter developed to compensate for the weakness of SAW filter affected by temperature. It is a coating on the substrate that enhances the stiffness when the temperature rises, reducing its temperature coefficient and increasing the Q value. Its cost is higher than SAW filter, but lower than BAW filter. The frequency temperature coefficient (TCF) of temperature uncompensated SAW devices is typically around -45ppm/℃, while TC-SAW filters drop to -15 to -25ppm/℃. However, due to the need for doubled mask layers in temperature compensation processes, TC-SAW filters are more complex and have higher manufacturing costs.

(2) BAW (Body Acoustic Wave Filter)

Although SAW filters and TC-SAW filters are highly suitable for applications within approximately 1.5 GHz, BAW filters have significant performance advantages above 1.5 GHz. The maximum frequency of BAW filter can reach 20GHz, and its size also decreases with increasing frequency, making it very suitable for demanding 3G and 4G applications. In addition, even in high bandwidth designs, BAW filters are not sensitive to temperature changes, and they also have extremely low losses and very steep filter skirts. Unlike SAW filters, sound waves propagate vertically within BAW filters. For BAW resonators using quartz crystals as substrates, the metal embedded on the top and bottom sides of the quartz substrate excites sound waves, causing them to bounce back from the top surface to the bottom to form standing sound waves. The thickness of the slab and the mass of the electrode determine the resonance frequency. In order to extend to high-frequency applications, BAW filters use MEMS technology to significantly reduce the size of quartz lamp piezoelectric crystals. The thickness of the piezoelectric layer material is in the range of single digit micrometers, such as a quartz substrate with a thickness of about 2um at 2GHz. Therefore, it is necessary to use thin film deposition and micro mechanical processing techniques on the carrier substrate to achieve the resonator structure. To prevent sound waves from scattering onto the substrate, a Bragg reflector is formed by stacking thin layers of different stiffness and density. This method is called the BAW or BAW-SMR device for firmly installing resonators. Another method, called Thin Film Bulk Acoustic Resonator (FBAR), is to etch a cavity below the active region to form a suspended film.

(3) FBAR (Thin Film Bulk Acoustic Resonator Filter)

FBAR is a resonant technology based on bulk acoustic wave (BAW), which utilizes the inverse piezoelectric effect of piezoelectric thin films to convert electrical energy (signal) into sound waves, thereby forming resonance. When a direct current electric field is applied to both ends of a material, the deformation of the material will change with the magnitude of the electric field, and when the direction of this electric field is opposite, the direction of deformation of the material will also change accordingly. When an alternating electric field is applied, the deformation direction of the material will undergo an interactive change of contraction or expansion with the positive and negative half cycles of the electric field. This is called the inverse piezoelectric effect. Unlike SAW, this vibration occurs within the cavity of piezoelectric materials and can therefore withstand greater power. This is also one reason why FBAR technology is superior to SAW.

Three types of FBAR structures:

1) Air gap type: Based on MEMS surface microfabrication technology, an air gap is formed on the upper surface of a silicon wafer to confine sound waves within the piezoelectric oscillator stack. Prepare an air chamber by first filling the sacrificial material and then removing it to form an air metal interface. This method is compatible with traditional silicon technology.

2) Silicon reverse etching type: Based on MEMS bulk silicon (Si) microfabrication technology, the reverse side of the Si wafer is etched. An air metal interface is formed on the lower surface of the piezoelectric oscillator stack to confine sound waves within the piezoelectric oscillator stack. The disadvantage of this technology is that the mechanical fastness is reduced due to the extensive removal of Si substrates.

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3) Solid state assembly type: using Bragg reflector technology to confine sound waves within the piezoelectric oscillator stack. Composed of alternating layers of high acoustic impedance material with a quarter wavelength thickness and low acoustic impedance material with a quarter wavelength thickness. The more layers there are, the higher the reflection coefficient and the higher the Q value of the resulting device.