Over the past few years, with the development of radio frequency integrated circuit technology and system architecture, many discrete components of the RF portion of mobile phones have been replaced. Most notably, discrete low noise amplifiers (LNAs) and intermediate frequency (IF) filters in receivers have been integrated into RF integrated circuits. It is expected that each RF module will be gradually integrated into a standard BiCMOS or CMOS integrated circuit, but there are still several types of RF components that are not easily integrated, including RF filters. All mobile phones require an RF filter to protect the sensitive receive (Rx) channel from other users' transmit (Tx) signals and noise from various RF sources. Mobile phones may require that the Rx signal still function when it is 120 dB below the interference signal strength. The preamplifier does not provide enough small intermodulation to meet this requirement.
Bulk acoustic wave (BAW) and thin film cavity acoustic resonator (FBAR) filters are used to replace traditional RF filters in mobile phones, respectively, because their performance has exceeded surface wave (SAW) filters and can be passed through standard integrated circuits. Technical production, very competitive price.
The highly selective RF filter between the antenna and the preamplifier ensures that only the correct Rx band is amplified. The frequency bands allocated to the mobile telephone system range from 400 MHz to 2.2 GHz; the bandwidth is typically between 20 and 75 MHz. The Tx band is lower than the Rx band, but there is only a gap of 20 MHz between them. In a narrow transition band of 20 MHz, the Rx filter must have an attenuation of more than 15 dB from the edge at the corresponding Tx band, with an insertion loss of less than 3 dB at the edge of the Rx band. To achieve such a steep edge, the filter components need to have very low losses and a high quality factor (Q). For reactive components, Q ≥ 400 is necessary. A selective RF filter is also required in the Tx channel of the mobile phone to avoid RF power being emitted outside of the specified band. The main consideration of these Tx filters is that they do not allow the power amplifier to amplify noise and signals outside the Tx band. The GSM system is time division multiplexed. The antenna of the GSM handset uses an RF switch to switch back and forth between the Rx and Tx channels. Due to this switching, in the GSM system, the received and transmitted signals are relatively easy to isolate from each other. Unlike GSM, both CDMA and W-CDMA and third generation (UMTS) standards operate in full-duplex mode, where the phone is simultaneously receiving and transmitting signals. This mode of operation makes the so-called antenna duplexer a necessary component. The antenna duplexer includes a highly selective filter for the Rx and Tx bands. It ensures that the power delivered from the amplifier is fed back to the receive channel as little as possible, and the signal received from the antenna is as small as possible. Attenuation is introduced into the preamplifier. The use of a SAW filter in such a duplexer is difficult because it can handle up to 2 watts of output power and maintains normal operation as the temperature rises due to self-heating. The BAW/FBAR filter is well suited for these applications because it has a quality factor of up to 1500, can handle up to several watts of power, and has a temperature coefficient that is significantly lower than the SAW filter.
Fundamental
The BAW resonator uses a MEMS process to extend the working mechanism of the quartz crystal to higher frequencies. Typical thicknesses of piezoelectric layers are on the order of a few microns or less. The piezoelectric layer can drive a standing acoustic wave with a wavelength that is twice the total thickness of the piezoelectric layer and the electrode. The standing acoustic wave propagates in the vertical direction. This mode is employed because the direction of the deposited piezoelectric layer is best supported by the thickness epitaxial mode (TE). Near the resonant frequency, the electrical impedance will change strongly. In BAW, the pressure field appears to be very similar to a (single crystal) quartz crystal, but a larger portion of the standing wave is located in the electrode and support layer. To extend the working mechanism of the thickness epitaxial quartz crystal to the GHz range, the most straightforward method is to form the piezoelectric layer and the electrode into a film structure, or to make a thin supporting film layer.
The BAW device produced by this method of film structure requires a minimum number of layers to be deposited. The disadvantage of this method is that it is difficult to handle the wafer due to the fragile film on the top, and there are other robustness issues.
In order to isolate the sound waves from the substrate, it is also possible to use acoustic mirrors. A plurality of layers of high and low acoustic impedance are alternately stacked, and the thickness of these layers is equal to 1/4 of the wavelength of the main resonance, thus constructing an effective acoustic mirror. This mirroring mechanism is very common in optics. At the interface between each of the high-impedance layer and the low-impedance layer, most of the sound waves are reflected, and since the thickness of these layers is /4, the reflected waves are superimposed in an appropriate phase. This type of BAW is known as a solid state assembled resonator (SMR). In terms of robustness, SMR is much better than the BAW of the membrane structure. There is no risk of mechanical damage in the various standard procedures required for dicing and assembly. The layer pressure received on the piezoelectric layer and the electrode layer also does not cause a problem. For BAWs that require a large power capability, it is advantageous to have a vertical heat transfer path directly through the mirror to significantly reduce the thermal impedance to the surrounding environment. SMR-like FBARs have significant advantages for IC integration because they can be embedded in alternating metal-oxide stacks that are generally available in advanced IC processes. In fact, by integrating SMR into the IC process, the overall process and mask layers are saved.
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