1. Field of the Invention
The present invention relates to a film bulk acoustic resonator (FBAR) and a duplexer using the FBAR. More particularly, the present invention is directed to an air-gap type FBAR and a duplexer fabricated through a simple substrate-securing process, and fabricating methods thereof.
2. Description of the Related Art
As mobile communication devices, such as cellular telephones, become increasingly prevalent, demand for compact, lightweight filters used in such devices is also rising. Film bulk acoustic resonators (FBARs) can be used as such compact, lightweight filters and can be inexpensively mass-produced. Also, the FBAR can obtain a high quality factor (Q) value, can be used in a micro-frequency band, and, more particularly, can be implemented up to PCS (Personal Communication System) and DCS (Digital Cordless System) bands.
Generally, the FBAR is implemented by laminating a first electrode, a piezoelectric layer, and a second electrode on a substrate in that order. In operation, an electric field, which varies with time, is induced in the piezoelectric layer by applying electric energy to the electrodes. This electric field causes a bulk acoustic wave to be generated in a vibrating direction of the piezoelectric layer, thereby generating resonance.
FIGS. 1A to 1C illustrate cross-sections of a Bragg reflector type FBAR, a bulk micro-machining type FBAR, and a surface micro-machining type FBAR, respectively. Each of these types of FBARs provides separation between the substrate and the multi-layer resonance section in different manners. The separation is needed so that acoustic waves generated from the piezoelectric layer are not affected by the substrate.
The Bragg reflector type FBAR in FIG. 1A includes a substrate 10, a reflector structure 11, a lower electrode 12, a piezoelectric layer 13, and an upper electrode 14 in that order. The reflector structure 11 includes a plurality of dielectric layers alternating between low impedance and high impedance materials to insure efficient confinement of the acoustic energy in the resonator. Thus, elastic-wave energy, which has passed through the piezoelectric layer, is not transferred to the substrate 10, but is wholly reflected by the reflector structure 11 to generate an efficient resonance. The Bragg reflector type FBAR is robust and is subject to no bending stresses. However, precise thickness control of the layers of the reflection structure 11 for the total reflection in such a Bragg reflector type FBAR is not easy and increases the manufacturing cost.
The bulk micro-machining type FBAR in FIG. 1B includes a membrane 21 of a material, such as SiO2, on a substrate 20, a cavity 23, formed by anisotropic-etching the rear surface of the substrate 20, and an acoustic resonator 22 on the membrane. Again, the acoustic resonator 22 includes a lower electrode, a piezoelectric layer and an upper electrode. The bulk micro-machining type FBAR is very weak structurally, making its implementation impractical.
The surface micro-machining type FBAR in FIG. 1C includes a substrate 30, an air gap 31, a dielectric layer 32, and a first electrode 33, a piezoelectric layer 34 and a second electrode 35 on the dielectric layer 32 in sequential order. This FBAR is fabricated using a sacrificial layer on the substrate 30 and forming the dielectric layer 32, on the sacrificial layer, forming the resonator structure on the dielectric layer 32, and then removing the sacrificial layer to form the air gap 31. That is, the air gap 31 is formed in a position where the sacrificial layer existed. The surface micro-machining type FBAR, has a complicated fabricating process and the resonance structure may cave in or peel off during the fabricating process.
Also, the above conventional FBARs have the common problem in that separate packaging, which requires lots of time and expense, is needed after the FBAR has been fabricated, and the FBAR may be damaged due to heat generated during the packaging process.
A duplexer is a representative element that uses multiple filters. The duplexer properly branches signals transmitted/received through one antenna in a frequency division type communication system so that the same antenna can efficiently both transmit and receive. The duplexer basically includes a transmitter filter and a receiver filter in addition to an antenna. The transmitter filter is a band pass filter for passing only a frequency to be transmitted, and the receiver filter is a band pass filter for passing only a frequency to be received. The duplexer can perform the signal transmission/reception through one antenna by adjusting the frequencies passing through the transmitter filter and the receiver filter. The duplexer can improve its performance using the FBAR as its transmitting/receiving filter.
Since the difference between the frequencies of the signals transmitted/received through the transmitter filter and the receiver filter is small, the signals influence each other giving rise to interference. Accordingly, an isolation part that isolates the transmitter filter and the receiver filter from each other to prevent mutual interference may be incorporated in the duplexer to improve performance. The isolation part includes a phase shifter using a capacitor and a resistor, and prevents the mutual interference by introducing a phase difference of 90° between the frequencies of the transmitted signal and the received signal.
The duplexer in FIG. 2A includes a transmitter filter 41, a receiver filter 42, and an isolation part 43 for isolating the two filters from each other on a printed circuit board (PCB) 40. The transmitter filter 41 and the receiver filter 42 are wire bonded to the PCB 40, creating a hybrid design. This duplexer can be fabricated using the air-gap type FBAR as shown in FIGS. 1B and 1C. However, the separate packaging for each FBAR increases the size of the duplexer, reducing the usefulness of the duplexer in miniaturized devices. Also, losses due to the wire bonding may occur.
FIG. 2B shows a duplexer fabricated on one substrate 50 using the Bragg type FBAR of FIG. 1A. Only details of a representative FBAR 60 will be discussed below. The FBAR 60 includes a reflector layer 64 formed by alternating layers of materials having greatly different acoustic impedances, a lower electrode 63, a piezoelectric layer 62, and an upper electrode 61 in that order, as a filter. As shown in FIG. 2B, the receiver filter includes a serial resonator 60 and a parallel resonator 70, and the transmitter filter includes a serial resonator 80 and a parallel resonator 90, all of which are integrated onto one substrate 50. The Bragg type duplexer is fabricated on one substrate to effect a one-chip fabrication. The Bragg type duplexer has a strong structure, but it is difficult to accurately adjust the width of the respective layers, and the thin film is easily cracked due to the stress caused during formation of the thick reflector layer. Further, the Bragg type duplexer has a greatly lowered Q value compared to a duplexer using an air gap type FBAR.