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    2021-10-21 17:38:36
    Introduction to BAW Filter

    Topics covered in this article:

    What is a BAW filter?

    Filter definition

    SAW, BAW, and FBAR

    Following the 5G high-frequency trend, BAW is expected to replace SAW in 2022

    What is a BAW filter?

    The BAW filter (Bulk Acoustic Wave) converts an electrical input signal to an acoustic wave using interdigital transducers (IDTs) on a piezoelectric substrate like quartz. The IDTs are made up of interleaved metal electrodes that launch and receive waves, converting an electrical signal to an acoustic wave and then back to an electrical signal.

    Filter definition

    The power filter is a filter circuit composed of capacitors, inductors, and resistors. The filter can effectively filter out the frequency point of a specific frequency in the power line or the frequency other than the frequency point to obtain a power signal of a specific frequency, or eliminate a power signal of a specific frequency.

    A filter is a device that filters waves. "Wave" is a very broad physical concept. In the field of electronic technology, "wave" is narrowly confined to specifically describe the process of fluctuations in the values of various physical quantities over time. This process is converted into a time function of voltage or current through the action of various sensors, which is called the time waveform of various physical quantities or called signals.

    Radiofrequency interference has always been the natural enemy of wireless communication. It requires designers to take drastic measures to restrain them. With the increasing number of frequency bands supported in each device, today's wireless devices must simultaneously guard against interference signals from other devices and themselves.

    A high-end smartphone must filter the transmission and reception paths of up to 15 frequency bands of 2G, 3G, and 4G wireless access methods. At the same time, the filtering also includes Wi-Fi, Bluetooth, and GPS receiver reception paths. The signal of each receiving path must be isolated. It is also necessary to suppress other external signals that have many sources and are difficult to cite. To do this, a multi-band smartphone requires eight or nine filters and eight duplexers. Without acoustic filtering technology, this would be difficult to achieve.

    Many communication systems will have a trend of miniaturization to a certain extent. On the one hand, miniaturization can make the system more portable and effective. On the other hand, the ever-developing IC manufacturing technology can produce large quantities of small products at a lower cost.

    MEMS is one of the related technologies of this small product. MEMS can detect changes in the environment and generate related reactions through microcircuits. The main parts of MEMS include sensor (microsensor) or actuator (microactuator) and transducer (transformer), in which the sensor can detect the presence or intensity of a certain physical, chemical or biological, such as temperature, pressure, sound, or chemical composition. A transducer converts one kind of energy into another (for example, from electrical signals to mechanical waves). At present, MEMS is widely used in many fields.

    SAW, BAW, and FBAR

    1. SAW filter

    Surface Acoustic Wave (SAW) Filter is a kind of transduction passive band made by using the piezoelectric effect of piezoelectric ceramics, lithium niobate, quartz, and other piezoelectric quartz crystal oscillator materials and the physical characteristics of surface acoustic wave propagation. The passed filter is a special filtering device made of piezoelectric materials such as quartz crystals and piezoelectric ceramics, using its piezoelectric effect and the physical characteristics of surface acoustic wave propagation.

    Surface Acoustic Wave (SAW) technology is an emerging field of science and technology that was developed in the late 1960s. It is a fringe subject that combines acoustics and electronics. The development of surface acoustic wave technology is quite rapid. Its application field has been developed from the beginning of military radar to almost the entire radio communication, especially the rapid development of mobile communication technology, which has further promoted the development of surface acoustic wave technology.

    SAW filters are widely used in electrical equipment such as color TVs, mobile phones, GPS positioning, satellite communications, and cable television.

    Basic SAW filter.jpg

    Basic SAW filter


    2. BAW filter

    Compared with the SAW filter, the BAW filter is more suitable for high frequency. Like the SAW/TC-SAW filter, the size of the BAW filter decreases as the frequency increases. In addition, the BAW filter has the advantages of being insensitive to temperature changes, low insertion loss, and large out-of-band attenuation (steep filter skirts).

    BAW filter.jpg

    BAW filter 

    Although SAW and TC-SAW filters are very suitable for applications within 1.5GHz, BAW filters have performance advantages above 1.5GHz, as shown in the figure. The size of the BAW filter also shrinks with increasing frequency, which makes it very suitable for very demanding 3G and 4G applications. In addition, even in a high-bandwidth design, BAW is not so sensitive to temperature changes, and it also has extremely low loss and a very steep filter skirt.

    Unlike SAW filters, sound waves propagate vertically in BAW filters, as shown in the figure below. It can also be seen from the name that SAW is a surface, which spreads along the surface, and BAW is a bulk, which spreads within an object. For BAW resonators using quartz crystal as the substrate, the metal attached to the top and bottom sides of the quartz substrate excites the sound wave, so that the sound wave bounces from the top surface to the bottom to form a standing sound wave. The slab thickness and electrode mass determine the resonance frequency. In the high frequency where BAW filters show their talents, the thickness of the piezoelectric layer must be on the order of a few micrometers. Therefore, thin-film deposition and micro-machining techniques must be used to realize the resonator structure on the carrier substrate.



    In order to prevent sound waves from spreading to the substrate, an acoustic Bragg reflector is formed by stacking thin layers of different stiffness and density. This method is called a BAW or BAW-SMR device with a firmly mounted resonator.

    Principle analysis of bulk acoustic wave (BAW) technology

    BAW is called a bulk acoustic wave, and this technology has been used in filters for a long time. However, in the process of wireless communication developing towards high communication frequency, high transmission rate, high-density multiplexing, and high integration, the resonator technology also has a bottleneck, and the integration of the resonator is the key to the problem. In the past, BAW resonator technology was often used to filter signals in communication technologies such as smartphones.

    The structure of the BAW device is very simple. It is usually composed of a sandwich piezoelectric stack on a silicon substrate. The sandwich piezoelectric stack is composed of a bottom electrode, a piezoelectric film, and an upper electrode. The reflection in the sandwich structure of the electrode forms a standing wave resonance.

    The central layer of TI BAW resonator technology is a piezoelectric material. When a frequency is applied to the device, it will cause the mechanical structure to vibrate. At this time, the layer structure will capture the generated energy. The key attribute of the BAW resonator is to have the maximum acoustic energy in the storage structure, which is used to obtain a high electrical Q factor value.

    3. FBAR filter

    FBAR filter is the abbreviation of film bulk acoustic resonator filter. FBAR filter is different from the previous filter, it is manufactured by using silicon base plate, with the help of mems technology and thin-film technology. The FBAR filter already has characteristics slightly higher than the ordinary SAW filter.

    FBAR filter.jpg

    FBAR filter

    The application of FBAR filter in the next generation of wireless communication and wireless access products. In addition to increasing requirements for volume and power saving, today’s wireless mobile products are also moving towards multi-function, multi-frequency band, multi-system, and multi-protocol Fusion and integration. Such as gsm, CDMA, WCDMA and GPS, Bluetooth, Wifi, WiMAX, and other different functions are combined in one product. This also poses challenges to design and mass production. Designers should consider the maturity and sustainability of the technology when choosing a solution.

    Following the 5G high-frequency trend, BAW is expected to replace SAW in 2022

    With the continuous upgrade of mobile devices' support for the number of network frequency bands, the key part of the RF radio frequency front-end filter market prospects are widely optimistic.

    As the requirements for multi-frequency bands, carrier aggregation, and coexistence interference become stricter, the number of filters will increase significantly when the adjustable broadband technology is not yet mature. Generally speaking, multiple frequency bands require multiple filters to process front-end RF signals.

    At the technical level, the importance of filters in the RF front-end is also becoming increasingly prominent. The responsibility of the filter is mainly to allow useful signals in the frequency band to reach the cellular base station through the antenna, and to filter out spurious signals outside the frequency band. These "spurious" signals include signal interference from nearby frequency bands. This is caused by the filter's "invalidity". For another example, when we are on the phone, occasionally disconnection or unclear voice, etc., are also related to the malfunction of the filter.

    From the perspective of the development process of filter technology, SAW (Surface Acoustic Wave) Filter and BAW (Bulk Acoustic Wave) Filter respectively represent technical directions at different stages.

    To analyze the pros and cons of the two filter technologies, first look at the SAW filter. From the frequency formula of SAW Filter f=v/λ (v refers to speed, about 3100m/s, λ refers to the distance between IDT electrodes), it can be seen that the higher the frequency, the smaller the distance between IDT electrodes is required. In fact, the spacing cannot be too small, which makes SAW Filters not suitable for frequencies above 2.5GHz, and too large current density (high power) at a small spacing (high frequency) will cause problems such as electromigration and heating. The SAW filter is more sensitive to temperature changes, and its performance will deteriorate as the temperature rises. Although it can be solved by adding a protective coating on the IDT, the added coating will complicate the process and the cost will increase.

    In fact, the best application range of SAW filters is within 2.0GHz, and the power is less than 33dBm. At present, the SAW process has also made great progress, and can even be applied to the 2.4 GHz frequency band. But in contrast, the BAW filter can work up to 20GHz, the power can be close to 40dBm (10W), and it is not sensitive to temperature changes. In addition, the manufacturing process of the BAW Filter is also very consistent with the existing IC manufacturing process and is suitable for integrated integration with other active circuits. The disadvantage is that the cost of BAW filters is high. Because of the high Q value, mass production consistency is a fatal challenge.

    In summary, the advantages of the SAW Filter are low cost, mature and high product consistency. The disadvantage is that the working frequency band is limited (within 2.5GHz) and it is sensitive to temperature changes. The BAW Filter is just the opposite. The advantage is that it can work at high frequencies up to 20 GHz, is not sensitive to temperature changes, has low insertion loss, and has large out-of-band attenuation. The disadvantages are high cost and poor consistency. However, from the overall technological development trend, BAW Filter is a new generation of filter technology used to replace SAW Filter and has begun to be applied to high-end mobile devices such as Apple mobile phones.

    Since the structure of BAW is divided into BAW-SMR and FBAR, BAW-SMR and FBAR, as branches of BAW, also have all the advantages of BAW, especially the FBAR filter technology is favored by international manufacturers. The application of BAW Filter on high-end smartphones is becoming a trend, but this does not mean that SAW Filter has completely lost the market. On the contrary, in a certain period of time, SAW will still occupy most of the market share in the low-end market. Except for the mobile communication field, the demand for SAW for 2.4G Wi-Fi transmission is still considerable, and there will still be considerable market share in the past few years.

    BAW will not completely replace SAW. The two will exert their best performance advantages in mid-high frequency and low frequency respectively and will coexist for a long time. Compared with SAW, the advantage of BAW lies in the performance advantages of the mid-and high-frequency bands, such as smaller insertion loss and higher out-of-band suppression. However, in recent years, SAW technology has also been continuously improved, such as high-quality factor SAW and temperature compensation SAW. In short, the market share of SAW below 2GHz is still relatively large, but the market share of BAW above 2GHz will be relatively high.

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