The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever-smaller electronic filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Filters select those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range.
One class of electronic filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARS). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure can be suspended in air. A sample configuration of an apparatus 10 having a resonator 12 (for example, an FBAR) is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a top view of the apparatus 10 while FIG. 1B illustrates a side view of the apparatus 10 along line 1B-1B of FIG. 1A. The resonator 12 is fabricated above a substrate 14. Deposited and etched on the substrate 14 are, in order, a bottom electrode layer 15, piezoelectric layer 17, and a top electrode layer 19. Portions (as indicated by bracket 12) of these layers—15, 17, and 19—that overlap and are fabricated over a cavity 22 constitute the resonator 12. These portions are referred to as a bottom electrode 16, piezoelectric portion 18, and a top electrode 20. In the resonator 12, the bottom electrode 16 and the top electrode 20 sandwiches the PZ portion 18. The electrodes 16 and 20 are conductors while the PZ portion 18 is typically crystal such as Aluminum Nitride (AlN).
When electric field is applied between the metal electrodes 16 and 20, the PZ portion 18 converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 micrometers in lateral extent and a few micrometers in total thickness. In implementation, for example, the resonator 12 is fabricated using known semiconductor fabrication processes and is combined with electronic components and other resonators to form electronic filters for electrical signals.
During fabrication, in some instances, the resonator 12 delaminates from the substrate 14 resulting in an undesirable configuration. Such delamination, when found, is often found proximal to edges of the cavity 22 between the substrate 14 and the bottom electrode layer 15. To determine whether or not the resonator 12 is delaminated from the substrate 14, the apparatus 10 is cut along line 1B-1B to expose its cross section. Then, the cross section is examined. However, even if delamination is discovered during the examination, it is difficult to determine whether the detected delamination occurred during the fabrication process or during the cutting step to expose its cross section. Further, the cutting step destroys the apparatus 10 rendering it useless.
Consequently, there remains a need for a better method to determine whether or not delamination has occurred between a substrate and a resonator on the substrate.