Piezoelectric bulk acoustic wave resonators employed in radio-frequency (RF) filters for wireless communication equipments are generally characterized into two major types. One type is called film bulk acoustic resonator (FBAR). A typical FBAR consists of a piezoelectric layer and two metal electrodes sandwiching the piezoelectric layer. Both metal electrodes are interfaced with air to trap acoustic energy in the piezoelectric resonator body. In practical configurations, additional layers to the metal electrodes of an FBAR may be added to enhance functionality such as physical strength, passivation, temperature compensation and the like. The other type is called solidly mounted resonator (SMR). In an SMR, an acoustic mirror comprising alternating low and high acoustic impedance layers takes the place of air on one or both sides of an FBAR. The acoustic mirror stack exhibits extremely high or low acoustic impedance so acoustic energy is well trapped in the resonator body of an SMR.
Both types of the piezoelectric bulk acoustic wave resonators provide effective electromechanical coupling (Kt2eff) and quality factor (Q) needed for high performance filters and duplexers used in wireless communication equipments (such as cell phones). The product of the Kt2eff and Q represents the figure of merit (FOM) of the resonator. In general, the greater the FOM is the more likely the filter containing the resonator reaches desired performance.
Each piezoelectric material has an intrinsic electromechanical coupling constant (Kt2) which indicates the conversion efficiency between electric and acoustic energy in the piezoelectric material. When both sides of the piezoelectric layer are loaded with metal electrodes and additional layers, the entire device presents the Kt2eff which can be derived from formula calculation with a series resonant frequency (fs) and a parallel resonant frequency (fp). It has been demonstrated that Kt2eff is dependent on electrode thickness and at a certain thickness ratio of metal electrode and piezoelectric layer the Kt2eff could be greater than the Kt2. Generally, the resonator with a greater Kt2eff can be used to make filters of a broader bandwidth.
The Q factor is the amount of acoustic energy stored in a resonator divided by the amount of energy lost to the resonator by various means. If the resonator is operating in a purely piston mode, the Q factor of the resonator is limited only by the mechanical Q factor of materials in the resonator. In a resonator with finite size, there are other operation modes co-existing with the main piston mode. Since the piston mode is the main concern, other operation modes may be called spurious modes and lateral mode is one of the spurious modes. Lateral modes are generated with an excitation of the piston operation mode and propagated in parallel to the resonator surface from one edge to the other. Standing waves occur due to lateral modes excited at resonator edges traveling in opposite directions in the active region of resonator. Waves of the lateral modes may leak out from the sides of the resonator and escape into the substrate, resulting in reduction of the Q factor near parallel resonant frequency (Qp) of the resonator.
It is easier to form lateral modes in a smaller size piezoelectric resonator than in a larger size resonator, because the lateral waves travel a shorter path after they are reflected by the opposite side of the resonator and bounced back to its original side. One of the approaches to minimize the lateral modes for a small size resonator is to combine two double-sized resonators in series to replace one single resonator to suppress the spurious modes and increase the Qp of the resonator. Although the performance may have been improved, the dimension of each die is increased, so does the cost of manufacturing the die. This is contradicted to a common goal of reducing the cost.
An alternative approach to minimize the lateral modes is to “apodize” the edges of the resonator, namely truncating a portion of the resonator such that any two edges of the resonator may not be made parallel. In contrast to a square-edged or a rectangular-edged resonator, lateral acoustic wave in an apodized resonator travels much longer due to multiple reflections before it is bounced back to its original edge. Apodization increases the propagation path of the lateral acoustic wave and reduces the fundamental lateral mode resonant frequency. However, the minimization of the lateral modes does not necessarily mean increased Qp performance because the acoustic energy leaked to outside of the resonator may not be reduced. In fact, the Qp factor in an apodized resonator may be reduced because of apodization of the resonator. This is because the perimeter of an apodized resonator is greater than a square-edged or rectangular-edged resonator of same size, the loss of acoustic energy is actually slightly increased, indicating lower Qp.
U.S. Pat. No. 7,280,007 entitled “Thin film bulk acoustic resonator with a mass loaded perimeter” discloses a technique to increase Qp through adding a raised frame to the resonator perimeter. The raised frame creates an acoustic impedance mismatch between the active area and outside area of the resonator such that acoustic energy of lateral modes is better confined in the active area. Additional mass can also be augmented either by adding more of the same material or by adding different materials with different mass density. Although the Qp is increased through the methods mentioned above, the Kt2eff and the Q factor near series resonant frequency (Qs) are reduced. This is not ideal in some applications where both the Qs and Kt2eff should be maximized, for example, UMTS band 1 duplexer. Additionally, the spurious resonances below fs are enhanced by the raised frame. The spurious resonances cause a risk of generating strong ripples in the pass band of a filter.
Therefore, it is desirable to have a resonator structure with an enhanced Qp without compromising the Kt2eff, spurious modes and the Qs. Hence, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.