The acoustic filter is one of the core components in the current RF front-end module. With the in-depth advancement of the fifth generation (5G) mobile communication technology, the technical requirements for filters are also increasing. Acoustic filters can be subdivided into surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters. Let's take a look together.
Ⅰ. What is a SAW filter?
SAW filter is a special filtering device made of piezoelectric materials such as quartz crystal and piezoelectric ceramics, using its piezoelectric effect and the physical characteristics of surface acoustic wave propagation. It is widely used in the intermediate frequency circuits of televisions and video recorders to replace LC intermediate frequency filters, which greatly improves the quality of images and sounds. The surface acoustic wave is an elastic wave that is generated and propagated on the surface of the piezoelectric substrate material, and whose amplitude decreases rapidly as the depth of the substrate material increases. In addition, the SAW filter is manufactured using a semiconductor planar process, which has good consistency and repeatability, and can achieve low-cost mass production.
2. Working principle
Surface acoustic waves were first discovered by Rayleigh, so they are also called Rayleigh waves. In 1885, Lord Rayleigh discovered the mode of propagation of surface acoustic waves and predicted the properties of these waves in his classic treatise. SAW has longitudinal and vertical shear components, which can be coupled with the medium in contact with the surface of the device, and its energy is limited to propagate on the contact surface. Since there are electrostatic waves associated with the SAW on the contact surface substrate, we can perform electroacoustic conversion on the transducer. Because the transducer is shaped like a pair of crossed fingers, it is often referred to as an interdigital transducer (IDT). The figure below shows an example of an interdigital transducer.
Interdigital transducers convert electroacoustic signals through the piezoelectric effect. As shown in the figure above, a SAW signal can be excited by applying a suitable oscillating signal (AC voltage) across the gates of a set of specially designed interdigital transducers. It can be seen from the figure that the interdigitated type is composed of multiple pairs of intersecting electrodes, forming a grating-like structure, and the spacing between the gratings determines the SAW wavelength. The conversion of an electrical signal to a SAW acoustic signal is accomplished by grounding one side of the transducer and applying a signal at a frequency obtained by dividing the SAW velocity (approximately 2700m/s for GaAs) by the spacing of the transducers.
A SAW filter consists of at least two transducers. The figure below shows a schematic diagram of a simple SAW bidirectional filter.
On the transducer at one end, the excited SAW signal propagates forward on the substrate surface, and at the same time, the elastic deformation generated by the piezoelectric substrate is converted into electrical energy through the piezoelectric effect, and then can be easily transmitted by the fingers at the other end type transducer capture. Because the surface acoustic wave and the substrate are strongly coupled, the amplitude and speed during its propagation will be affected by the substrate parameters, such as substrate quality, width, thickness, dielectric coefficient, insertion loss, etc. When the SAW passes under the transducer with a grating-like structure with appropriate spacing, an alternating potential will be generated at both ends of the transducer. So far, the whole system has completed the transmission process from electrical signal to acoustic signal and then to electrical signal.
Considering that the device system composed of two transducers and a substrate has a specific frequency response characteristic, and the frequency response characteristic can be manipulated by the preparation of the transducer and the substrate, and the device has high stability, small size, high selectivity, high Q value, etc., so the system is successfully used as a filter for various functions. It is conceivable that the process and design of the SAW filter are equally important as the manufacture of the chip. Because the parameters of the substrate and the IDT transducer have a huge impact on the performance of the entire device. The process is particularly important for the performance of the device.
The three common types of SAW filters are IIDT type filter, DMS type filter and Ladder type filter.
(1) IIDT type filter
IIDT is an acronym for Inter-digitatedInter-Digital Transducer, which uses an interdigitated IDT transducer. This type of filter usually has a loss of 3-4db and requires an external matching circuit for impedance matching.
In order to solve the shortcomings of the IIDT filter, people have developed a DMS filter and a Ladder filter. The loss of the DMS filter and the Ladder filter is relatively small, and they have performance comparable to that of the BAW filter at low frequencies.
(2) DMS type filter
DMS type filter is DualModeSAW filter. The DMS filter constructs the filter by setting the IDT between the gratings, realizes a wide operating bandwidth through the combination of each resonance mode, and matches the input or output of the balanced amplifier through the balance setting of the input and output. And it couples in the longitudinal direction through two identical resonant modes, so as to achieve low insertion loss and good out-of-band suppression characteristics.
(3) Ladder type filter
The Ladder filter is composed of a one-port ladder SAW resonator, which is generally used for unbalanced input and output, and uses two kinds of one-port SAW resonators (one is a series structure and the other is a parallel structure). These two kinds of SAW resonators have different resonant frequencies and are realized by electrical coupling. Ladder type filters have lower loss characteristics, higher power handling and similar levels of out-of-band rejection.
A piezoelectric crystal such as quartz, lithium niobate or brazing lead titanate is used as the substrate, and a metal film is covered on it after surface polishing. Two groups of interdigitated metal electrodes with energy conversion function were prepared by using photolithography technology, and named as "input interdigitated transducer" and "output interdigitated transducer".
Surface acoustic wave devices have the advantages of light weight, small size, high reliability, good consistency, flexible design, and can be manufactured by microelectronic processing technology, suitable for mass production, etc., and have been used in mobile communications, radio and television, non-destructive testing, identification Positioning, navigation and telemetry and many other fields. Now the operating frequency of SAW devices has covered 10MHz ~ 5GHz, which is an indispensable key component of the modern information industry.
6. Comparison of ordinary SAW, TC-SAW and I.H.P-SAW filters
SAW filter products include ordinary SAW filters, TC-SAW filters with temperature compensation characteristics and high-performance high-frequency SAW filters. The latter two are upgraded products of ordinary SAW filters. The operating frequency of ordinary SAW is generally below 2.5GHz, and the cost is low, but its thermal stability and high frequency performance are poor. In order to improve the thermal stability of common SAW, the TC-SAW filter scheme was developed. At this stage, TC-SAW technology is becoming more and more mature. Overseas manufacturers have successively launched products applied to the RF front-end of mobile phones, and achieved good application results. However, China still needs further exploration in this field. In order to overcome the weakness of ordinary SAW low frequency and poor heat dissipation performance, Japan's MuRata has developed the I.H.P-SAW filter, whose operating frequency can reach 3.5GHz, and has both the temperature characteristics and high heat dissipation advantages of BAW, which can partially replace the BAW filter.
Ⅱ. What is a BAW filter?
The operating frequency range of bulk acoustic wave filter is 1.5GHz-6GHz, up to 10GHz. The size of the BAW filter will shrink as the frequency increases, and it is suitable for 4G and 5G communications with higher requirements. Compared with SAW filters, BAW filters are more suitable for high frequencies, and have the advantages of insensitivity to temperature changes, small insertion loss, and large out-of-band attenuation.
Unlike SAW filters, sound waves in BAW filters propagate vertically. For a BAW resonator that uses a quartz crystal as a substrate, the metal attached to the top and bottom sides of the quartz substrate excites the sound wave, causing the sound wave to bounce from the top surface to the bottom to form a standing acoustic wave. The slab thickness and electrode mass determine the resonant frequency. In order to extend to high frequency, BAW filter applies MEMS technology, which greatly reduces the size of piezoelectric crystal of quartz lamp. The thickness of piezoelectric layer material is in the single-digit micron level. For example, the thickness of quartz substrate is about 2um at 2GHz. Therefore, the resonator structure is implemented on a carrier substrate using thin film deposition and micromachining techniques. To keep the sound waves from spreading to the substrate, an acoustic Bragg (Bragg) reflector is formed by stacking thin layers of different stiffness and density. This approach is known as BAW or BAW-SMR devices with firmly mounted resonators. Another approach is called a film bulk acoustic resonator (FBAR). It is to etch a cavity under the active area to form a suspended film.
2. Working principle
(1) Piezoelectric crystal sheet: The core component of the BAW filter is the piezoelectric crystal sheet. It converts electrical signals into mechanical vibration signals, and then converts mechanical vibration signals into electrical signals.
(2) Oscillation drop: The piezoelectric crystal sheet is placed in the oscillation circuit. When the external signal passes through the oscillating circuit, the capacitance and inductance in the oscillating circuit will resonate with the piezoelectric crystal sheet to generate mechanical vibration.
(3) Filtering effect: The mechanical vibration in the oscillating circuit will change the piezoelectric characteristics of the piezoelectric crystal sheet, thereby changing the electrical characteristics of the circuit and realizing the filtering effect. Signals of different frequencies will be attenuated to different degrees, and only signals that meet the specific frequency range of the filter can pass.
According to different acoustic wave reflection structures, BAW filter product types include Solid Mounted Bulk Acoustic Wave Filter and Film Bulk Acoustic Wave Filter.
There are currently two implementations of the BAW process: film bulk acoustic resonator (FBAR) and solidly mounted resonator (BAW-SMR ).
The main difference between the two lies in the way of reflection of acoustic energy: FBAR relies on an air cavity between a support layer and the substrate to achieve energy reflection, while BAW-SMR relies on a Bragg reflector on the substrate to achieve energy reflection. In terms of technology, FBAR is closer to MEMS, while SMR is closer to the implementation of integrated circuits. The FBAR process can provide relatively good energy reflection, so FBAR can provide a larger bandwidth, that is, slightly better filtering performance. As for the BAW-SMR type filter, because there is a thermal conduction path leading to the substrate in its structure, it can dissipate heat well through the substrate. FBAR, on the other hand, has a weaker thermal conductivity due to the air gap on each side of the resonator. BAW-SMR filter suppliers represented by Qorvo can provide filters with close to zero temperature drift.
BAW filters consist of two metal electrodes sandwiching a piezoelectric film. The working principle of the BAW filter is similar to that of the SAW filter. The difference is that the sound waves of the BAW filter propagate in the vertical direction, and the metal electrodes embedded on the top and bottom sides of the quartz substrate excite the sound waves. The shock on the top surface bounces to the bottom to form a standing wave larger than the 2.5GHz band.
The resonant frequency of a BAW filter depends on the electrode mass and the thickness of the thin-film piezoelectric layer. The thickness of the piezoelectric substrate of the BAW filter used in high-frequency scenarios must be on the order of microns, so the carrier substrate needs to adopt difficult film deposition and micromachining technology (MEMS), and the manufacturing cost is high.
In terms of piezoelectric materials, the piezoelectric materials of BAW filters need to have high electromechanical coupling coefficient, low electromechanical loss, high thermal stability and other characteristics, and must meet the semiconductor manufacturing process technology. At present, the piezoelectric materials commonly used in BAW filters mainly include aluminum nitride (ALN), lead zirconate titanate (PZT) or zinc oxide (ZnO).
Ⅲ. Development of SAW and BAW filter technology
There are three mainstream ways to reduce the volume of SAW/BAW filters in the market:
（1）Improve the packaging form of the device;
（2）Optimize the device chip design to make it smaller;
（3）Package SAW/BAW filters with different functions together to form a combined device to reduce the PCB footprint.
2. High frequency and wide bandwidth
The high-frequency and wide-band trend of electronic equipment requires that SAW/BAW filters must increase their operating frequency and expand their bandwidth. For SAW filters with lower applicable frequencies, the common improvement methods are as follows:
(1) Improve exposure equipment and lithography technology capabilities;
(2) Optimal design of the electrode structure of IDT;
(3) Use piezoelectric materials with higher surface acoustic wave propagation speeds.
Using advanced packaging integration technology (SiP), based on the characteristics of various components, multiple component chips are packaged and integrated in one housing. For example, SOI technology is used to monolithically integrate the filter module and other RF front-end modules.
Ⅳ. What is the difference between a SAW filter and a BAW filter?
The two main types of AWFs—bulk acoustic wave (BAW) and surface acoustic wave (SAW) filters—convert electrical and acoustic signals using interdigital transducers. BAW filters direct the signal energy through the bulk of the substrate, while SAW filters direct the signal energy along the surface of the substrate.