Reseach On Cavity Structure Of Film Bulk Acoustic Resonator

Recently, wireless RF communication devices are developing smaller, with higher frequency, higher electrical performance, and the ability of integration. As a new kind of RF MEMS devices, bulk acoustic wave(BAW) resonator becomes the emphasis of the basic devices of RF front end. It also advances rapidly in the sensor field for its high sensitivity, small size and the function of sending and receiving wireless signals.Film bulk acoustic resonator(FBAR) is an important kind of BAW resonators with both high Q factor and effective electromechanical coupling factor. It is not easy to realize the practical FBAR because many factors have to be considered for the design, and a high demanding for the properties of materials and the device structure. There is also a high threshold for the manufacture equipment.This paper uses two kinds of CAD software to make the FBAR models, simulate and optimize the FBAR design. An effective design scheme has been summarized. The influences of materials and the structure on the property of FBAR have been revealed. As to the materials, we have chosen them according to their properties and the easiness of synthesizing. We deposit the films with various deposition parameters and characterize the films using different equipment in order to find the most appropriate deposition condition. On the manufacturing of FBAR, we find a repeatable and stable program using the simple experimental facilities through optimizing the manufacture process. We have achieved a series of systematic results and some special innovation which are summarized in the following three parts.1. FBAR SimulationWe used the Mason model to simulate the influences of different materials on the resonant properties, and confirmed the proper metal and piezoelectric layers for making a FBAR are Mo and Al N, respectively. The equivalent circuit model of FBAR is achieved by the MBVD model. For this model we emphasized the extraction of the R, L, and C elements of the circuit. Then we built a 3D model using the FEM software and simulated the effects of electrode shape to the resonant frequency and resonant modal. It is concluded that the FBAR with the triangular metal shape has the smallest energy dissipation. The energy distribution is intuitively presented both at the series and parallel resonant frequency. It is also achieved that the voltage having no effects on the resonant modal and the energy distribution by simulating the resonant characteristics under different voltage.2. FBAR materialsWe have studied the sacrificial materials. The amorphous silicon(α-Si) used as sacrificial layer synthesized by the physical vapor deposition(PVD) method was proposed. The influences of background vacuum, deposition temperature, and deposition rate on the structure, surface roughness, film density, and the residual stress of α-Si thin films have been investigated. The characteristics of uniformity, conformality, I-V property, stiffness, and Young’s modulus of α-Si thin films have been discussed and compared with the PECVD α-Si films. A proper deposition condition is concluded as: the atmosphere vacuum of 1×10-3 Pa, the substrate temperature of 150 oC and the deposition rate of 3~4 ?/s. Then another PVD method of RF magnetron sputtering for the deposition α-Si has been studied. The sputtering pressure, temperature, and power were varied to study their effects on the structure, surface microtopography, density, deposition rate and the residual stress of α-Si thin films. A best synthesizing condition is achieved as: a working pressure of 0.5 Pa, a power of 150 W and a substrate temperature of 200 oC. Comparing with the traditional PECVD method, there are little differences for the surface roughness and the uniformity of the α-Si films deposited by the PVD methods. The α-Si film synthesized by the later method is easier to be etched for its relatively loose density. Also, the equipment of the PVD method is simple, and it does not need the harmful gas like Si H4.We have studied the Al N piezoelectric thin films. A method of sputtering high-oriented c-axis Al N thin films under water cooled condition was proposed. Al N thin films deposited on different Si-based substrates such as Si(111), Si(100), Si O2, and α-Si were compared and the results showed that Al N films on Si O2 exhibited the better c-axis orientation, better columnar grain structure, and the smallest surface roughness. It represented that Si O2 was a good candidate for the growth of Al N. Then the Al N thin films were sputtered on Si O2 with various RF power and N2/Ar gas ratios. By comparing the film structure, surface topography, and the residual stress, a conclusion was drawn that the most proper RF power was 150 W and the best N2/Ar flow ratio was 3: 1.The metal Mo as electrode was systematically studied. The study method is the same as Al N thin films. The Mo thin films were first sputtered on various Si-based substrates, and also Si O2 showed a good choice. Then the effects of RF power, temperature, and the working pressure on the structure, surface feature, and the residual stress of Mo thin films were researched. At last the best deposition was obtained as: a sputtering pressure of 1.0 Pa, a substrate temperature of 200 oC, and the RF power of 150 W.3. FBAR FabricationAn innovative process was implemented by changing the step of releasing the sacrificial layer to before the fabrication of Al N piezoelectric layer. This process rearrangement avoided the etching conflict between sacrificial layer and piezoelectric layer. The hard etching problem of sacrificial layer was solved by changing the single α-Si layer to the Al N/α-Si double layers. At the process of removing the redundant sacrificial layer, a lift-off method was employed to reduce the bulge area of sacrificial layer, and thus the CMP time was reduced and a smooth surface could be obtained using the small common CMP equipment. Finally, the FBAR device with clean “swimming pool”, integrated structure, smooth film edge, and few film cracks was fabricated. The frequency characteristic was also tested.