Patent Application
20070035364
1. Field of the Invention
The present invention relates to the field of bulk acoustic wave devices.
2. Prior Art
The present invention pertains to piezoelectric resonators and filters whose primary application is for signal filtering and reference oscillators. These resonators are commonly referred to as FBAR (film bulk acoustic resonators) or BAW (bulk acoustic wave resonators). The term BAW encompasses also stacked resonators, fully coupled (Stack Crystal Filter or SCF) or partially coupled (Coupled Resonator Filters or CRF).
The resonator must be acoustically isolated from the mechanical substrate (typically a silicon wafer). This has been accomplished by an air gap (FBAR) or Bragg mirrors of alternating high and low acoustic impedance materials designed at one fourth the wavelength of interest (BAW). A high acoustic impedance material is also desirable for the electrodes. These devices are not new and are well documented in the literature. See for instance:
Tungsten is the common Bragg reflector material for the high acoustic impedance material. It is popular because of its high acoustic impedance. The primary deposition method for tungsten is by chemical vapor deposition (CVD). CVD tungsten deposition requires adhesion, barrier, and seed layers (e.g. titanium and titanium-nitride) that complicate the processing. Also CVD tungsten typically has a rough surface, limiting its use as an electrode material. CVD tungsten film stress can also be high. Tungsten can be deposited by PVD methods, but adhesion and particles are a significant challenge.
The present invention comprises the use of TiW as the high acoustic impedance material in the Bragg mirror stack and/or as the electrode composition or as a part of the electrode stack in the fabrication of FBAR or BAW devices (i.e. resonators and filters built from resonators). Classic IC fabrication methods are used for the basic manufacturing sequences, including depositions, photolithography, and etch processes. MEMS techniques may also be employed for packaging and resonator acoustic isolation from the substrate. The low acoustic impedance material may be silicon dioxide (SiO2) though other low acoustic impedance layers could be used if desired, such as a carbon based dielectric or Silicon-based polymer, or polysilicon, or other low-loss polymers such as polyimide, among other materials. TiW refers to a binary alloy of titanium and tungsten. Typically the titanium content should not exceed 15 percent by weight. Equally effective results have been obtained with 3 percent and 10 percent titanium by weight. The TiW is deposited by physical vapor deposition (PVD) in any commercially available sputter deposition system. PVD TiW is a low cost material and has high acoustic impedance, excellent adhesion to oxide layers, tunable film stress, and relatively smooth surfaces. Resist adhesion to TiW is good, allowing long wet etch patterning. Because TiW is easily patterned by isotropic wet etch methods, a planarized architecture is not needed. Thus, TiW is found to be a good BAW Bragg mirror layer or electrode material having superior characteristics in comparison to the substantially pure tungsten (W) used in the prior art.
Thus the preferred embodiments of the invention consist of utilizing PVD TiW material as the high acoustic impedance Bragg reflector layers, electrode layers, and/or shunt loads on parallel resonators for FBAR or BAW. Compared to CVD tungsten, TiW eliminates the need for seed and adhesion layers, it results in a smooth film, and the film stress is easily tailored by common PVD process parameters (e.g. temperature, pressure, bias, etc.). Acoustic velocity of TiW is not significantly compromised, particularly when compared to the full CVD tungsten stack including adhesion and seed layers. AIN (aluminum nitride) piezoelectric quality when grown on TiW can be good. TiW is more easily patterned than CVD tungsten because there are no adhesion, barrier, or seed layers to remove. For example, Ti/TiN patterning typically requires anisotropic plasma etching and hence requires full planarization of the device. A fully planarized architecture is more complex and is less likely to produce acceptable device uniformity (i.e. die yield will suffer).
Typical structures incorporating the present invention may be the same as or similar to structures using tungsten as the high acoustic impedance layers in such devices, though the relative ease in processing with the present invention avoids some of the difficulties and necessary extra processing steps to achieve the desired result with tungsten alone. By way of example, a cross section of an exemplary structure may be seen in
Now referring to
In a typical device incorporating the present invention, the electrode layers and the piezoelectric layers will be patterned to form more than one resonant device, though for convenience, such multiple resonant devices are simply referred to herein and in the appended claims as a resonator or resonators.
Thus the present invention solves the inherent process related problems of CVD tungsten, namely a rough surface, high stress, and poor adhesion. In that regard, by using stress-tunable processed titanium-tungsten PVD films, controlling the deposition temperature, pressure and deposition rate, the stress in the titanium-tungsten PVD films may be set as desired. At the same time, the excellent acoustic properties of tungsten are fully maintained. The benefit of PVD TiW is that it presents a smooth surface, the stress can be tuned to optimize the overall integration scheme, and adhesion/seed layers are not needed. Thus, TiW offers a lower cost process with equal or better performance and with increased process integration latitude.
While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
