In the 5G era, the filter market is being restructured

Source: Content from Wang Li's team at Soochow Electronics , Thanks.

1. No matter how things change, they still remain essentially the same. Filters carry four basic functions

1.1. Low-pass, high-pass, band-pass, and band-stop, the four basic functions of filters

Since the beginning of telecommunications, filters have played an important role in circuits and have made continuous progress with the development of communication technology. New communication systems require the development of a new technology that can extract and detect signals within a specific frequency band, and the development of this new technology has further accelerated the research and development of filter technology.

In the field of modern communication technology, there is almost no branch that is not affected by digital filtering technology. Source coding, channel coding, modulation, multiplexing, data compression and adaptive channel equalization all widely use digital filters, especially in digital communication, network communication, image communication and other applications. Without digital filters, it is almost impossible to move forward.

We believe that filters play a huge role in the field of communications. First of all, we need to have a basic understanding of filters: filters are frequency-selective devices that allow specific frequency components in the signal to pass through while greatly attenuating other frequency components. Broadly speaking, any channel for information transmission can be regarded as a filter. There are many ways to classify filters, which can be classified by band and application field, or by other methods. Filters are classified into analog filters and digital filters according to the type of signal they process. Analog filters are further divided into active filters and passive filters. Passive filters are filters composed of passive devices, which are generally composed of discrete components such as RC and LC. Commonly used passive filters include Bessel filters, Butterworth filters, Cherchev filters, elliptical filters, etc. Active filters are composed of active devices, and commonly used active devices include op amps. Filters are classified into acoustic filters, crystal filters, ceramic filters, etc. according to material technology. The surface acoustic filter is a representative of acoustic filters. It is made on single crystal materials using semiconductor planar technology. It has good consistency and repeatability, extremely high temperature stability, strong radiation resistance, large dynamic range, and does not involve electron migration. Crystal filters have the advantages of high quality factor, good attenuation characteristics, low loss, and high selectivity. Ceramic filters are a solid circuit with good filtering characteristics, no need for tuning, no interference from magnetic fields, and low cost. They are often used as intermediate frequency filter devices in mobile communication terminals such as mobile phones to make intermediate frequency signals stable and not easily affected by external magnetic fields. According to the frequency band of the passing signal, they are divided into four types: low-pass, high-pass, band-pass, and band-stop filters.

Regardless of how filters are classified and what technical solutions are used, filters in any circuit carry four basic functions: low-pass, high-pass, band-pass, and band-stop. Low-pass: Blocks all frequencies above a certain frequency and allows all other frequencies to pass (compared to high-pass). High-pass: Allows all frequencies above a certain frequency to pass and blocks all other frequencies (compared to low-pass). Band-pass: Allows all frequencies between two frequencies to pass and blocks all other frequencies (compared to band-stop). Band-stop: Blocks all frequencies between two frequencies and allows all other frequencies to pass (compared to band-pass). Among them, band-pass filters are widely used in signal circuits.

1.2. RF front end, filters play an important role

1.2.1. Wireless communication is inseparable from the RF front-end system

RF front end is the core of wireless communication

Regardless of the communication protocol, whether the communication frequency is high or low, the configuration of RF device modules is a necessary basic component of the system. Whether it is an NFC system that uses 13.56Mhz signals as a transmission carrier; or a GSM communication system that uses 900/1800Mhz signals as a transmission carrier; or an unmanned millimeter-wave radar that uses 24Ghz and 77Ghz electromagnetic wave signals as a transmission carrier, RF device modules are required. The RF front end is an indispensable part of wireless communication.

From 2G, 3G, 4G to 5G, the requirements for RF front-end are getting higher and higher

From the perspective of mobile networks, from 2G GSM to 3G WCDMA and then to 4G LTE-Advanced, each generation of upgrades has brought new communication protocols, and the complexity has increased exponentially, and the requirements for the RF system in the mobile phone have also become higher and more stringent.

In terms of bandwidth, the 2G channel bandwidth is 200kHz; 3G is generally around 5MHz; the LTE-Advanced protocol in 4G has a bandwidth of 100MHz. Higher bandwidth brings faster speed, but bandwidth is currently a scarce resource. In addition to the occupation of various other dedicated communication bandwidths (such as military, civil aviation communications and other dedicated networks), in order to avoid interference, a certain amount of bandwidth must be vacated between the spectrums for isolation, and the available bandwidth for mobile phone communications is very limited. All parties have tried every means to save spectrum resources, such as the carrier aggregation (CA) technology used in 4G, which can aggregate 2-5 component carriers in LTE (with small bandwidth, usually 20M) to achieve a maximum transmission bandwidth of 100MHz. There is also orthogonal frequency division multiplexing technology (OFDM), which divides the channel into several orthogonal sub-channels, divides the communication signal into multiple groups, and transmits them in parallel in each sub-channel at the same time, which greatly saves bandwidth utilization. In addition, there are technologies such as MIMO. The implementation of these technologies also puts higher requirements on the performance and parameters of mobile phone RF devices.

1.2.2. RF front end, filters are crucial

RF front-end systems such as smartphones, satellite navigation, and satellite TV all require filters to work properly. Filters can filter out-of-band interference and noise to meet the signal-to-noise ratio requirements of RF systems and communication protocols. Let's look at the importance of filters in the fields of smartphones and satellite navigation.

In the RF front-end system of smartphones, the RF front-end includes SAW filters, duplexers, low pass filters (LPF), power amplifiers, switches, etc. SAW filters are responsible for filtering RF signals in the receiving channel of the TDD system, duplexers are responsible for duplex switching in the FDD system and filtering RF signals in the receiving/transmitting channels; power amplifiers are responsible for amplifying RF signals in the transmitting channel; switches are responsible for mutual conversion between the receiving channel and the transmitting channel; and antennas are responsible for mutual conversion between RF signals and electromagnetic signals.

In the satellite navigation RF front-end system, the positioning receiver needs to be able to receive signals transmitted by two or more different navigation satellite systems. To process all these signals, the GNSS receiver has higher requirements for bandwidth, linearity and anti-interference performance than traditional consumer GPS receivers, which requires filters to process the corresponding signals.

It can be seen that filters play an important role in RF front-end systems. In modern communication systems, in order to suppress the impact of external interference signals on the sensitivity of terminal receiving signals and suppress out-of-band interference of RF signals in the transmission path, it is usually necessary to configure SAW filters and low-pass filters on the receiving channel and transmitting channel of the RF front-end respectively, and at the same time, a duplexer is needed to solve the filtering problems of the receiving channel and transmitting channel of the RF front-end.

1.2.3. Duplexers, coexistence filters, and carrier aggregation are all applications of filters in the RF front end

Duplexer, Triplexer: Integration of Multiple Filters

In FDD systems, a duplexer is required. A duplexer is an important application of filters. Its function is to isolate the transmit and receive signals to ensure that both the receive and transmit signals can work normally at the same time. A duplexer consists of two sets of stopband filters with different frequencies to prevent the local transmit signal from being transmitted to the receiver. A general duplexer consists of six stopband filters (notch filters), each resonating at the transmit and receive frequencies. The receive filter resonates at the transmit frequency and prevents the transmit power from being connected to the receiver. The transmit filter resonates at the receive frequency. A triplexer consists of three filters that share a node. The passband loading and isolation targets of the triplexer are the same as those of the duplexer.

Coexistence filters: Solving adjacent band interference issues

In the 2.4 GHz Wi-Fi band, interference with cellular communications (especially LTE networks) using adjacent frequency bands is becoming more and more likely, requiring RF filters that can suppress adjacent frequencies. At the same time, the filter must also minimize insertion loss in the Wi-Fi transmit path to ensure the high signal-to-noise ratio and corresponding low error vector magnitude (EVM) required by the 802.11n and 802.11ac standards. BAW filters are effective in meeting these requirements and have significant advantages over traditional SAW and ceramic filters used in cellular Wi-Fi applications.

Carrier aggregation: Boosting throughput speeds with filters

Network operators are working hard to improve network performance in the face of growing data demand. To maintain customer experience, higher and higher data rates need to be provided, and the direct way to do this is to increase bandwidth. Carrier aggregation is a 4G LTE Advanced feature that allows service providers to combine multiple spectrum blocks into a wider channel to provide higher data rates. There are three types of carrier aggregation. The first type is "inter-band aggregation", which refers to the aggregation of spectrum in different frequency bands. These bands can be far apart or close together. Aggregation of far-apart bands is the simplest and only requires a diplexer. For closely adjacent bands, quadplexers or multiple antenna solutions may be required. The other two types involve combining spectrum within the same band.

Carrier aggregation has several implications for filter design. For aggregation of distant bands, the diplexers that split the signals incur additional losses that must be compensated by low-loss filters. Additionally, the attenuation of the filter stopbands must be planned to ensure adequate attenuation of the other aggregated bands. Finally, for adjacent bands, more complex multiplexers are required.

2. 5G communication, SAW/BAW high-performance filters shine

2.1. SAW and BAW, mainstream technologies for high-performance RF filtering

As mentioned above, there are many types of filters (dielectric filters, LC filters, SAW filters, BAW filters, etc.), each with its own application field. In the current communication protocol, the frequency difference between different frequency bands is getting smaller and smaller, so the signal selectivity of the filter is extremely high, allowing the signal in the passband to pass and blocking the signal outside the passband. The larger the Q, the narrower the passband bandwidth the filter can achieve, that is, the better the selectivity can be achieved. In addition to the quality factor Q, insertion loss is also an important parameter. Insertion loss refers to the attenuation of the passband signal by the filter, that is, the signal power loss. If the insertion loss is 1dB, the signal power is attenuated by 20%; when the insertion loss reaches 3dB, the signal power is attenuated by 50%. In the 4G era, signal power amplification is not simple, and low insertion loss is very important for the processing of RF signals. It is precisely because of these characteristics that SAW/BAW filters have become the mainstream technology of RF filters with excellent band selectivity, high Q value, low insertion loss and other characteristics.

SAW filters combine low insertion loss and good suppression performance. They can not only achieve wide bandwidth, but also are much smaller than traditional cavity and even ceramic filters. However, SAW filters also have limitations and are generally only suitable for applications below 1.5GHz. In addition, they are also susceptible to temperature changes. When the temperature rises, the stiffness of the substrate material tends to decrease and the speed of sound decreases. Above 1.5GHz, TC-SAW and BAW filters have more performance advantages. The size of the BAW filter also decreases with increasing frequency, which makes it very suitable for very demanding 3G, 4G and 5G applications. Even in high-bandwidth designs, BAW is not so sensitive to temperature changes, and it also has extremely low insertion loss and very steep filter edges.

In smartphone RF front-end systems, SAW/BAW filters and SAW/BAW duplexers are sometimes used as discrete devices or as combined modules with some components. Especially with the increasing number of bands, there are increasing requirements to reduce the design load of the manufacturer's RF department and ease the actual installation accuracy of ultra-miniaturized equipment. SAW/BAW filters will be the technology with the highest penetration rate in RF front-end filter systems in the next 5-10 years.

2.2. SAW: Mainstream choice for lower frequency bands of 2G/3G/4G communications

2.2.1. SAW filter principle

SAW (Surface acoustic wave) filters are made using the piezoelectric effect of quartz, lithium niobate, and barium titanate crystals. That is, when the crystal is acted upon by an electrical signal, it will also produce elastic deformation and emit mechanical waves (acoustic waves), which can convert electrical signals into acoustic signals. Simply put, surface acoustic waves refer to waves that propagate along the surface of a solid, and the energy is concentrated on the surface. During the propagation of surface acoustic waves, signals can be accessed at will. Based on this characteristic, surface acoustic wave filters are made using integrated circuit technology.

The basic elements of SAW filters are interdigital transducers (IDTs) and reflectors (Gratings). The interdigital transducer (IDT) is a metal pattern shaped like the crossed fingers of two hands formed on the surface of the piezoelectric substrate. Its function is to achieve acoustic-electric transduction. The reflector (Grating) is set in the propagation direction of the SAW to induce resonance.

SAW filters have been widely used in mobile phones and other devices, and can be roughly divided into two types according to the structure used. One is a filter called a ladder type, which is a filter in which a single-port resonator is connected in a ladder state. The other is a filter called a DMS (Double Mode SAW) type, which is a filter in which two or more IDTs are set between reflectors, thereby achieving a broadband filter characteristic effect combining multiple wave modes. Ladder filters are generally used in unbalanced input and output states, while DMS filters can achieve balanced input or output effects by obtaining electrodes from the IDT, so they are often used when connecting to amplifiers with balanced inputs.

2.2.2. SAW filters focus on applications in the frequency band below 1.5 GHz

SAW filters are widely used in 2G/3G receiver front ends, duplexers, and receive filters. SAW filters have the characteristics of low insertion loss and excellent suppression performance. They can not only achieve wide bandwidth, but also are much smaller in size than traditional cavities or even ceramic filters. From a cost perspective, SAW filters can be made on wafers, which are low-cost and can be mass-produced. In addition, SAW technology also supports the integration of filters and duplexers for different frequency bands on a single chip without the need for additional process steps.

However, SAW filters also have limitations. SAW is very suitable for use below 1.5GHz, but when the operating frequency exceeds 1.5GHz, the Q value of SAW begins to decrease. At 2.5GHz, the selectivity of SAW can only be used in some occasions with relatively low requirements. However, the current wireless communication protocol has long been operating in frequency bands greater than 2.5GHz (such as Band 41 of 4G TD-LTE), etc. At this time, SAW is not enough and bulk acoustic wave (BAW) filters must be used.

In addition to the operating frequency requirements, SAW devices are susceptible to temperature changes. When the temperature rises, the rigidity of the substrate material tends to decrease and the speed of sound also decreases. In order to solve this problem, a temperature compensation (TC-SAW) filter solution was developed, which is to apply another layer of coating on the IDT structure that will increase the rigidity when the temperature rises. Since the temperature compensation process requires double the mask layer, the TC-SAW filter is more complicated than the ordinary SAW filter, and its manufacturing cost is relatively high.

TC-SAW has been widely used in mobile phone RF front-ends. Take Samsung S7 as an example. The US version of Galaxy S7 integrates Murata RF front-end module FAJ15, which is mainly for LTE low-frequency band. It is composed of several filter chips assembled on a ceramic substrate and contains two SAW technologies: STD-SAW (standard SAW) and TC-SAW (thermally compensated SAW). The Band 8 LTE duplexer must use thermally compensated SAW technology because its frequency band requires very low thermal drift.

2.3. BAW: 5G communications will adopt high-frequency technology, and the demand for BAW filters will increase rapidly

2.3.1. BAW filter principle and its application in mobile phones

Principle of BAW filter

BAW (bulk acoustic wave) filter is a bulk acoustic wave filter, which uses thin film cavity acoustic wave resonator (FBAR) technology. The principle is basically the same as SAW. It uses quartz crystal as the substrate, and the metal embedded on 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. Unlike SAW, the sound signal is transmitted inside the medium, so the volume can be made smaller (the dielectric constant of the medium is greater than that of air).

The most basic structure of a BAW filter is a piezoelectric film sandwiched between two metal electrodes (at 2GHz, the thickness of the Quartz substrate is 2um), and the sound waves oscillate in the piezoelectric film to form standing waves. In order to keep the sound waves in the piezoelectric film, there must be enough isolation between the oscillating structure and the external environment to obtain the minimum loss and maximum Q value. The propagation speed of sound waves in solids is about 5000m/s, which means that the acoustic impedance of solids is about 105 times that of air, so 99.995% of the sound wave energy will be reflected back at the boundary between the solid and the air, forming a standing wave together with the original wave.

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

BAW filter application: Taking FBAR-BAW in iPhone 6s Plus as an example

The RF front-end system of iPhone 6s Plus contains several filter chips, which are assembled on a coreless PCB substrate. Avago AFEM8030 is one of the filter chips. The filter in Avago AFEM8030 uses a sealed wafer-level package, which comes from Avago's Microcap wafer bonding CSP, so that the area of ​​all chips in the front-end module assembled together is less than 35mm2. In addition, through-silicon vias (TSV) are used to conduct electrical signals and special grinding processes are used to control the thickness of aluminum nitride (AlN).

2.3.2. 5G communications will adopt high-frequency technology, and the demand for BAW filters will increase rapidly

The first thing that wireless communication technology needs to determine is the radio spectrum. Radio spectrum resources are an important strategic resource for a country and an important carrier of ubiquitous information. At present, the frequency band below 3Ghz has been occupied by applications such as 2G to 4G communications, television, navigation, and satellites. Therefore, 5G will choose the spectrum above 3Ghz.

The typical candidate frequency bands for 5G currently being studied by the industry include 6GHz, 15GHz, 18GHz, 28GHz, 38GHz, 45GHz, 60GHz and 72GHz, and the test scenarios cover both outdoor and indoor hotspots. Channel tests show that the higher the frequency band, the greater the channel propagation path loss.

At present, my country has proposed some candidate frequency bands to the International Standards Organization, mainly low- and medium-frequency bands below 6GHz. In the view of domestic operators such as China Mobile, the spectrum of 5G should be a combination of high, medium and low-frequency bands, not just high-frequency bands.

In July 2016, the US Federal Communications Commission officially opened the high-frequency spectrum of nearly 11GHz to mobile, flexible and fixed wireless broadband. The new 5G network speed is expected to reach 10 to 100 times that of 4G.

High-frequency communications mean that the demand for BAW filters will increase rapidly

The applicable operating frequencies of SAW and BAW filters are also different. SAW is usually applicable below 1.5GHz, while in the field of high-frequency communications, BAW is accepted by more manufacturers. At present, when a mobile phone has 2G, 3G, 4G, Wifi, Bluetooth and other communication standards at the same time, the two filters can complement each other and be mixed and applied. For example, in LTE, there are many frequency bands, including Bands 1, 2, 3, 4, 5, 8, 13, 17, 19, 20, 25, etc. Some of the frequency bands are used at a lower frequency, such as Bands 5, with a frequency range of 824MHz-894MHz. In order to reduce the overall cost, SAW filters can be selected; while in high-frequency communications such as Bands 25, BAW filters with better performance must be used.

In the future, 5G communication will require the use of spectrum above 3 GHz, which means that ordinary SAW filters cannot meet the frequency band requirements of 5G, and BAW filters must be used. At present, major filter manufacturers are also stepping up the launch of high-performance BAW filters.

2.4. SAW/BAW trends: miniaturization, high frequency bandwidth, and integration

2.4.1. Trend 1: Small-scale chips

The miniaturization of SAW/BAW filters is a basic requirement for mobile communications and other portable products. In order to reduce the size of SAW/BAW filters, three measures are usually taken: first, optimize the design of the chip used in the device and try to make it smaller; second, improve the packaging form of the device, which has now been changed from the traditional round metal shell package to a square or rectangular flat metal package or LCCC (leadless ceramic chip carrier) surface mount form; third, package SAW/BAW filters with different functions together to form a combined device to reduce the area occupied by the PCB, such as the dual-band SAW filter used in the 60MHz bandwidth of the 1.9GHz PCS terminal and the dual-standard (supporting both analog and digital modes) SAW filter for portable mobile phones recently developed by Fujitsu, both of which are equipped with two filters.

2.4.1. Trend 2: High frequency and wide bandwidth

The arrival of 5G in the next 2-3 years will add more high-frequency bands. The Federal Communications Commission (FCC) of the United States has allocated three frequency bands, 28GHz, 37GHz and 39GHz, for the construction of 5G networks by licensed operators. In addition, the frequency bands from 64GHz to 71GHz will be used for non-licensed purposes of 5G networks.

In order to meet the requirements of high frequency and broadband of electronic equipment, SAW/BAW filters must also increase the operating frequency and expand the bandwidth. Taking SAW filters as an example, the means of increasing the operating frequency are mainly considered from two aspects: 1. Improving the equipment capacity of fine line processing; 2. Using piezoelectric materials with higher propagation speed of surface acoustic waves. Exposure equipment and photolithography technology are key equipment for making high-frequency SAW filters. With the development of communication systems, it is necessary to expand the bandwidth of SAW filters. To this end, it is usually started from optimizing the electrode structure of IDT. For example, by connecting IDT in series and parallel to form a trapezoidal structure, and using fine processing technology below 0.4μm, a 2.5GHz trapezoidal structure resonant SAW filter for wireless local area network (LAN) can be produced, with a bandwidth of 100MHz; in multi-mode filters, the bandwidth of the filter using longitudinal connection is larger than that of the transverse coupling filter, so it is widely used in RF filtering of cellular phones and pagers, while the latter has a steep narrowband characteristic and can be used for intermediate frequency (IF) filtering of personal digital cellular (PDC) and analog phones.

2.4.2. Trend 3: Integration

Mobile phones are constantly adding functional components, and mobile phone manufacturers are also increasing the requirements for RF integration in the front end to maintain the size of their products. The consequence of integration may be that all devices form a single wireless RF module. However, this module will contain a multi-chip set with a mixture of multiple technologies, rather than a single chip. Among these technologies, Si, GaAs, and SAW devices will play a pivotal role in optimizing device performance.

For SAW filters, their integration with IC will result in a device with powerful functions and smaller size. Currently, two integration technologies that are being studied very hotly abroad are: (a) technology based on various components, integrating multiple component chip packages (MCM) in one housing (SiP); (b) integrating technologies with different functions on one chip (SoP). SiP technology is simpler, more flexible and more reliable than SoP technology. Therefore, in the future, SAW device manufacturers will actively cooperate with IC manufacturers to develop new integrated packaging technologies.

Note: The three major trends of SAW/BAW are: small-scale chip-type; high frequency and broadband; integration. The core content is quoted from "The Development of Surface Acoustic Wave Filter Technology", CEC 26, author: Wujiang Caoliang.

2.5. The market size of SAW/BAW filters for mobile phones alone is worth tens of billions of dollars, and the market space is vast

2.5.1. With the increase of networked devices and frequency bands, the demand for filters is growing

As the number of connected devices increases around the world, the demand for RF front-end systems increases, and the demand for filters naturally increases. Hundreds of billions of devices are connected, which is the network demand for IoT applications. In the future, the total number of devices connected to the global mobile communication network will reach hundreds of billions. According to the IMT-2020 5G Promotion Group, the number of global mobile terminals (excluding IoT devices) will exceed 10 billion by 2020, of which China will exceed 2 billion. The number of connected IoT devices around the world will also grow rapidly, approaching the global population of 7 billion in 2020, of which China will be close to 1.5 billion. By 2030, the number of connected IoT devices around the world will be close to 100 billion, of which China will exceed 20 billion.

As the number of RF bands increases, the demand for filters naturally increases. From 1G-->2G-->3G-->4G, and then to 5G, the number of RF bands continues to increase, and the number of applications of its RF devices is also increasing rapidly. From the perspective of the number of filters used in the RF front-end, as the number of frequency bands increases, the demand for RF filter components also increases accordingly. Generally speaking, at least two RF front-end filter components are required for one frequency band. Taking TD-LTE/FDD LTE/TD-SCDMA/GSM terminals as an example, if the terminal supports 11 frequency bands, 24 RF front-end filter components are required. Generally speaking, the early 2G networks used very few frequency bands, and the demand for SAW/BAW filters was also relatively small. 2G mobile phones only needed 16 filters, 3G needed 19, and 4G needed 45. In the future, the demand for 5G is expected to increase to 67.

2.5.2. The mobile phone terminal filter market alone is worth over 10 billion US dollars

Without considering other wireless networking devices, only considering mobile phone terminals, the space for RF filters is very broad. From the perspective of the value of filters used in RF front-ends, as the number of frequency bands increases, the proportion of filters in the value of RF front-ends increases. According to Triquent's forecast, in the 4G era, the value of RF devices for a single mobile phone will increase from US$3.75 for 3G terminals to US$7.5, and the ASP of terminal equipment supporting global roaming will even reach US$12.75. At the same time, we have noticed that filters are becoming more and more important in RF devices, and the value of filters has also increased from 33% for 3G terminals to 57% for all-network LTE terminals. In the 5G era, the application of filters will further increase (especially BAW filters), and the value of filters for a single mobile phone will reach more than US$10.

According to IDC data, the total sales volume of smartphones in 2016 was 1.47 billion. In 2017, global mobile phone shipments are expected to grow by 4.2% to 1.53 billion, and will reach 1.77 billion by 2021. At present, the frequency bands of smartphones are mainly 3G and 4G, and it is expected that 5G networks will be gradually supported after 2018. Here, we assume that the global smartphone shipments in 2018 will be 1.6 billion, and the filter (SAW/BAW filter) used in each mobile phone is estimated at an average of US$7. It is expected that the global smartphone filter (SAW/BAW filter) market will reach US$11.2 billion. In the future, after the popularization of 5G mobile phones, the number of filters will further increase, which will bring a new round of high growth to this market. Technavio also pointed out in its research report that the annual compound growth rate of the RF filter market from 2016 to 2020 can reach 15%, and it has surpassed PA to become the most important component of the entire RF front-end module market.

3. Japan and the United States dominate, China rises

3.1. Japanese and American corporate monopoly

Integration and mergers have formed a monopoly pattern among giants in acoustic wave filters

Compared with traditional LC or ceramic filters, acoustic wave filters (SAW/BAW) are more difficult to manufacture and more expensive. In the past decade, global semiconductor device manufacturers have continuously integrated and merged to seek optimization of the industrial chain and use scale advantages to gain more market voice and lower manufacturing costs. In the field of acoustic wave filters, after several mergers and acquisitions, a competitive landscape dominated by giants has emerged.

Murata and TDK monopolize the SAW filter market, while Avago and Qorvo monopolize the BAW filter market

At present, the monopoly of several major international manufacturers has been formed. The main suppliers of SAW filters are Murata, TDK, Taiyo Yuden and other Japanese manufacturers, while the BAW filter suppliers are Avago (acquired Broadcom) and Qorvo, which occupy more than 95% of the global market share.

Among RF front-end device manufacturers, each manufacturer has different advantages in different segments. Murata has obvious advantages in the SAW field, and Avago has obvious advantages in the BAW field.

3.2. The monopoly lies in technology and patents

3.2.1. Process: Complex and difficult to control, foreign manufacturers control it in the IDM model

The design and manufacture of SAW/BAW filters are very complex and currently cannot be mass-produced using the most integrated CMOS process. Instead, special processes must be used to ensure performance.

Let's first look at the manufacturing process of SAW/BAW filters. Taking the production of SAW filters as an example, the development technology of SAW filters is already very mature and stable abroad, so the equipment, process, device packaging and IC integration abroad are developing rapidly and are very advanced. The production of SAW filters generally adopts a subtractive process, that is, a metal film is deposited on a substrate, and then the unnecessary parts are removed to obtain the required metal interdigital pattern.

The general process is divided into: 1) Metal film deposition: The metal materials commonly used to make the interdigital electrodes are aluminum, copper and some alloy materials. Sputtering, evaporation and other methods are often used to deposit metal films on substrates. 2) Gluing and pre-baking: Drop the photoresist on the substrate, and the substrate rotates at high speed on the spin coating table to evenly coat the photoresist on the front of the substrate. The substrate after gluing is placed in a baking oven for pre-baking to remove the solvent remaining in the photoresist and enhance the adhesion of the photoresist. 3) Exposure: Use ultraviolet light to irradiate the substrate coated with photoresist through the mask plate, and expose to obtain the required pattern. 4) Development, rinsing and post-baking: The exposed substrate is placed in the developer for development. The part of the substrate covered with the metal film where the photoresist is removed will expose the metal film, and the other parts are still covered by the photoresist. After rinsing, the substrate is placed in the baking oven for post-baking to improve the adhesion of the photoresist. 5) Etching: The etching process removes the metal film that is not covered by the photoresist. There are two methods, wet etching and dry etching. 6) Removal of photoresist: Place the substrate after the etching process in a chemical solvent to remove the photoresist. Through the above process, the core of the filter, the interdigital transducer, can be produced, and then the final surface acoustic wave filter is formed through external structure packaging.

SAW filter is a kind of semiconductor process (BAW filter process is more difficult), and the materials used play a decisive role in the performance of the device, and slight changes in exposure equipment, lithography technology, process parameters, etc. will greatly affect the performance of the filter. In order to maximize the guarantee of the optimal design results, SAW/BAW filter manufacturers mostly adopt the IDM model, so its design and manufacturing process are mainly monopolized by foreign manufacturers. For Chinese manufacturers, there are still many difficulties in the design and manufacturing of SAW/BAW that need to be overcome. Taking SAW filter as an example, in the design of SAW filter, how to solve the temperature drift problem of SAW filter (that is, how to make the frequency response of SAW filter as unchanged as possible at different temperatures) is a key problem. The design of the filter is closely related to the manufacturing process. The design must be closely combined with the manufacturing process, and the designer must also have a solid understanding of the manufacturing process; in the manufacturing process of SAW filter, the operating frequency of SAW filter is determined by the width of electrode strips and the properties of piezoelectric materials. The narrower the electrode strips, the higher the frequency. The fine processing technology of semiconductor 0.2~0.35μm level can be used to produce 2~3GHz SAW filters. In addition, the design of future SAW/BAW filters must also take integration into consideration, which further increases the process difficulty.

3.2.2. Patents: Domestic layout was late

As a basic component in the RF field, the patent layout of SAW/BAW filters is extremely critical to the application of manufacturers. The development technology of SAW/BAW filters abroad is very mature and has built a patent barrier.

Taking FBAR (based on BAW technology) patents as an example, the patents are occupied by international manufacturers.

In May 1991, WESE applied for a US invention patent for FBAR filters (titled Microwave Film Bulk Acoustic Resonator and Various Filter Groups), which marked the beginning of the application for patents related to film bulk acoustic resonators. In 1999, European countries also joined the ranks of patent applications for film bulk acoustic resonators. Subsequently, my country began to apply for patents related to film bulk acoustic resonators in 2000. In the same year, Japan also began to develop patent technology for film bulk acoustic resonators. So far, the number of patent applications for film bulk acoustic resonators has formed a situation where Japan and the United States are the main applicants.

In the patent applications for FBARs, the applicants are mainly foreign companies. In the patent application statistics from 1991 to 2013, Samsung ranked first in the number of patent applications for FBARs, followed by Agilent Technologies, Fujitsu, Japan's Panasonic, Kyocera Corporation, Toshiba, etc. In my country, the number of applications by China Electronics Technology Group Corporation ranks first, but it ranks 20th in the world, which is far behind some foreign countries.

Note: The development of FBAR patents is quoted from "Development and Analysis of FBAR Thin Film Bulk Acoustic Resonator Patent Technology", Cui Yan, "Science and Fortune" 2014 No. 11

3.3. Chinese manufacturers break through technical bottlenecks and are rising

3.3.1. Domestic surface acoustic wave technology lags behind foreign countries

The development of surface acoustic wave technology in my country began in the early 1970s and has gone through three stages: the first stage was the basic research stage (early 1970s to early 1980s). It was mainly the basic principles and manufacturing process technology of surface acoustic wave technology that were studied with the support of the state, with Nanjing University, Beijing Institute of Acoustics, and China Electronics Technology 26th Institute as the leading forces. The second stage was the stage of localization of color TV accessories (mid-1980s to mid-1990s). With the support of the national "localization of color TV" project, the domestic surface acoustic wave device industry has developed rapidly. 95% of the surface acoustic wave devices used by domestic TV manufacturers are domestically produced and exported in large quantities. The third stage is the stage of base stations and mobile phones (mid-1990s to present). The development of mobile communications has put forward higher requirements for surface acoustic wave technology. Surface acoustic wave devices have expanded production towards high frequency, diversification, and multi-levels, and the product structure has moved from low-end to mid-to-high-end. Some domestic manufacturers have made considerable progress, but compared with foreign manufacturers, the gap is still very large.

Compared with foreign manufacturers, my country's SAW technology lags far behind. In 1965, White and Voltmer jointly invented the interdigital transducer (IDT: Inter Digital Transducer), which opened the door for foreign manufacturers to develop surface acoustic wave filters. Around 1975, SAW filters were applied to the field of televisions, and around 1990, SAW filters for mobile phones were developed. However, my country started in the 1970s and did not start developing SAW filters for mobile phones until 2012, which is obviously behind foreign advanced manufacturers.

3.3.2. China Electronics 55th Institute, China Electronics 26th Institute, and Wuxi Haoda have made breakthroughs in SAW RF filters for mobile phones

At present, there are about 10 companies engaged in SAW filters in my country, half of which are engaged in military filters and half in civilian filters. The three companies that have made breakthroughs in SAW RF filters for mobile phones are Deqing Huaying (China Electronics 55th Institute), China Electronics 26th Institute, and Wuxi Haoda. They have successfully developed BAND1, BAND5, and BAND8 SAW duplexers with shell sizes of 2520, 2016, and 1814 CSP packages, as well as filters for GPS and WiFi with sizes of 1411 and 1109 CSP packages. Their products have been used by second-tier brand mobile phone manufacturers.

1) Deqing Huaying (China Electronics 55th Institute)

Founded in 1978, China Electronics Technology Deqing Huaying Electronics Co., Ltd. is one of the earliest enterprises in China to develop and produce lithium niobate piezoelectric crystal materials and surface acoustic wave filter products. Now it is a company controlled by the 55th Institute of China Electronics Technology Group Corporation, specializing in the development and manufacture of artificial crystal materials, surface acoustic wave devices and electronic products. The company specializes in the production of lithium niobate crystal products, surface acoustic wave devices and other electronic products, and has independent intellectual property rights. The main lithium niobate 3″ and 4″ crystals have an annual output of 18 tons, 1 million wafers and a series of optical crystal products; 160 million various surface acoustic wave devices, 6 million sets of lighting products such as electronic ballasts and electronic transformers, 6 million high-power lamp beads, and 600,000 COB surface light sources and integrated light sources.

2) China Electronics 26th Institute

China Electronics Technology Institute 26 is a very strong commercial SAW filter manufacturer in China. Before 1999, the products and services of China Electronics Technology Institute 26 were mainly used for military and defense customization, and then gradually became civilian. China Electronics Technology Institute 26 has successfully developed thousands of specifications of SAW filters, SAW oscillators, SAW resonators, SAW delay lines, SAW direct frequency synthesizers, SAW pulse compression components and other signal processing devices and components, which are used for GSM/CDMA and other repeaters, GSM/CDMA/WCDMA/TD-SDMA and other base stations series of intermediate frequency filters, CDMA450 fixed station system 400-700MHz series filters, WAN/WLL SAW filters, various wireless communication standard wireless transceiver RF filters.

3) Wuxi Haoda

Wuxi Haoda's main products include surface acoustic wave filters, duplexers, and resonators, which are used in mobile phones, communication base stations, radars, aerospace, automotive electronics, and other RF communication fields. The company has a production line that can produce 0.25um micro-line chips and a production line that can produce CSP flip-chip product packaging. It can produce duplexers with product sizes of 1.8*1.4 and filters of 1.1*0.9. It can produce duplexers of 1.8×1.4 mm and filters of 1.1×0.9 mm. Its HDDB01NSB-B11, HDDB03CNSS-B11, HDDB05NSS-B11 and other models of duplexers are used in bands such as BAND1, BAND3, and BAND5. The company has achieved supply to mainstream mobile phone manufacturers (main customers include ZTE, Yulong, Gionee, Samsung, Sapphire, Foxconn, Meizu, etc.).

3.3.3. Xinwei Communication and China Electronics Technology Research Institute 55 cooperate to lay out the 5G era

* The company cooperated with China Electronics Technology Group Corporation No. 55 and invested in Deqing Huaying, entering the 5G era

On June 16, Xinwei Communication issued an announcement: the company signed an investment intent letter with the 55th Institute of China Electronics Technology Group Corporation, and Xinwei Communication plans to invest 110 million yuan in Deqing Huaying. After the capital increase is completed, the company will become the second largest shareholder of Deqing Huaying. In addition, after this round of capital increase and share expansion, if Deqing Huaying needs to increase capital again, the 55th Institute and Deqing Huaying will guarantee that the company has the priority subscription right of no less than 50% of the amount of capital increase.

Xinwei Communication and the 55th Research Institute of China Electronics Technology Group Corporation also signed a strategic cooperation framework agreement:

1. Deqing Huaying Company, which is controlled by the 55th Institute, and Xinwei Communication will carry out technical cooperation, joint development and shared sales channels in the field of surface acoustic wave crystal materials and devices.

2. The 55th Institute and Xinwei Communications jointly invested in the construction of the 5G Communication High Frequency Devices Industrial Technology Research Institute.

3. The two parties will jointly invest in the construction of a 6-inch GaN chip line platform to carry out process development and manufacturing of GaN RF power chips, as well as packaging, testing and sales of GaN RF power devices.

4. The two parties will carry out pragmatic cooperation in the field of SiC power electronics to achieve full coverage of the industrial chain from chips to modules.

5. The two parties will jointly carry out the design and processing of RF MEMS devices and build a national RF MEMS microsystem process manufacturing platform open to the whole country.

* SAW filters are currently monopolized by Japanese and American giants due to difficult manufacturing processes, high costs, and patent blockades.

SAW filters are widely used in mobile terminals and other devices due to their miniaturization, bandwidth, and integration, and their market space is very broad. At present, the frequency bands of smartphones are mainly 3G and 4G. It is expected that 5G networks will be gradually supported after 2018. By then, the global smartphone SAW filter market is expected to reach tens of billions of US dollars. In the future, after the popularization of 5G mobile phones, the number of filters will increase further, which will bring a new round of high growth to this market. Technavio also pointed out in its research report that the annual compound growth rate of the RF filter market from 2016 to 2020 can reach 15%, and it has surpassed PA to become the most important component of the entire RF front-end module market. However, compared with traditional LC or ceramic filters, SAW filters on mobile terminals are more difficult to make and more expensive, and patents are strictly blocked. Now they are monopolized by Japanese and American companies such as Avago, Skyworks, Qorvo and Qualcomm. Compared with foreign manufacturers who developed SAW filters for mobile phones in 1990, domestic manufacturers only developed them in 2012, lagging behind foreign advanced manufacturers for 20 years.

However, domestic manufacturers are not willing to lag behind. The domestic SAW filter product structure is moving from low-end to high-end, and some manufacturers have made considerable progress. As one of the earliest companies in China to develop SAW filters, China Electronics 55th Institute has obvious advantages in technology accumulation and process precipitation. It can be said to be a strong company in this field in China, representing the highest level of domestic SAW filter production and research and development.

* With the strong alliance, Xinwei has a more obvious leading advantage in the 5G era

The cooperation between Xinwei Communication and China Electronics 55th Research Institute is a powerful combination. China Electronics 55th Research Institute is a national key electronic device research institute with solid-state power devices and radio frequency microsystems as its main business. It has achieved independent research and development and original innovation in the fields of solid-state power devices and radio frequency microsystems, optoelectronic display and detection. It has a complete scientific research and production system, and its research and development capabilities and product levels are leading in China and advanced in the world.

Deqing Huaying is one of the earliest enterprises in China to develop and produce lithium niobate piezoelectric crystal materials and surface acoustic wave filter products. Now it is a company controlled by the 55th Institute of China Electronics Technology Group Corporation, specializing in the development and manufacture of artificial crystal materials, surface acoustic wave devices and electronic products. The company specializes in the production of lithium niobate crystal products, surface acoustic wave devices and other electronic products, and has independent intellectual property rights.

As a leading domestic antenna and RF device company, Xinwei Communication has been deeply involved in the development, manufacturing and sales of RF front-end devices and modules, semiconductor materials and microelectronic products, wireless communication and IoT hardware and software, microwave and millimeter wave monolithic integrated circuits for communication equipment, multi-chip micro-assembly integrated circuits and other functional components since 2016. From that point on, the company has truly entered the field of active RF devices in accordance with its long-term development plan and in line with the demand for RF devices in the 5G era.

In addition to the production and development of SAW filters, the two parties have also carried out in-depth cooperation in the raw materials of RF devices such as GaN and SiC; and jointly invested in the construction of the 5G Communication High-Frequency Device Industry Technology Research Institute, which specializes in technical research, process development and batch production verification of high-frequency devices for 5G communications, laying a solid technical foundation for the development of both parties in the field of 5G communications; in addition, the two parties also jointly carried out the design and processing of RF MEMS devices, and built a national-level RF MEMS microsystem process manufacturing platform with international advanced level, integrating research, manufacturing and engineering, and open to the whole country.

Xinwei Communication has become a global leader in RF antenna devices, has outstanding RF technology capabilities, and has an international large customer platform. Combined with Xinwei Microelectronics' technical capabilities in RF front-end devices and modules, we believe that Xinwei Communication will make full use of its own material technology advantages and customer platform advantages, and is expected to expand high-end RF devices in the 5G era and maintain the company's leading position in the field of RF devices.

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