FIELD OF THE INVENTION
The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the tuning of such resonators and filters.
BACKGROUND OF THE INVENTION
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the acoustically generated wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonant frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonant frequency using this fabrication method. Fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonant frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). The difference between these two types of devices lies mainly in their structures. An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one shunt FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, a FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIG. 1. As shown in FIG. 1, the FBAR device 1 comprises a substrate 2, a bottom electrode 4, a piezoelectric layer 6, a top electrode 8 and a passivation layer 10. The FBAR device 1 may additionally include an acoustic mirror layer 12, which is comprised of a layer 16 of high acoustic impedance sandwiched between two layers 14 and 18 of low acoustic impedance. The mirror usually, but not always, consists of pairs of high and low impedance layers (even number of layers). Some mirrors consist of two pairs of such layers arranged in a sequence like SiO2, W, SiO2, W. Instead of the mirror, an FBAR device may additionally include one or more membrane layers of SiO2 and a sacrificial layer. The substrate 2 can be made from silicon (Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass, or ceramic materials. The bottom electrode 4 and top electrode 8 can be made from gold (Au), molybdenum (Mo), aluminum (Al), titanium (Ti) or other electrically conductive materials. The piezoelectric layer 6 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer can be made from SiO2, Si3N4 or polyimide. The low acoustic impedance layers 14 and 18 can be made from Si, SiO2, poly-silicon, Al or a polymer. The high acoustic impedance layer 16 can be made from Au, Mo or tungsten (W), and in some cases, dielectric such as AlN to make a number of layer pairs. FBAR ladder filters are typically designed so that the series resonators yield a series resonance at a frequency that is approximately equal to, or near, the desired, or designed, center frequency of the respective filters. Similarly, the shunt, or parallel, resonators yield a parallel resonance at a frequency slightly offset from the series FBAR resonance. The series resonators are usually designed to have their maximum peak in transmission at the center frequency, so that signals are transmitted through the series resonators. In contrast, the shunt resonators are designed to have their minimum in transmission so that signals are not shorted to ground. FBARs yield parallel resonance and series resonance at frequencies that differ by an amount that is a function of a piezoelectric coupling coefficient of the piezoelectric materials used to fabricate the devices, in addition to other factors such as the types of layers and other materials employed within in the device. In particular, FBAR ladder filters yield passbands having bandwidths that are a function of, for example, the types of materials used to form the piezoelectric layers of the resonators and the thickness of various layers in the device.
The difference in the thickness of various layers in the device can be achieved during the fabrication of the device. Presently, FBARs are fabricated on a glass substrate or a silicon wafer. The various layers in the FBAR-based device are sequentially formed by thin-film deposition. In an FBAR-based device, the resonant frequency of the device usually has to be controlled to within a 0.2-0.5% tolerance. This means that, if no tuning is used, the thickness of each layer in the device must be controlled in a similar way. It is known that, however, the deposition of thin-film layers is difficult to control to yield a thickness within such tolerance when the area of substrate or wafer is large. With a small wafer or substrate, certain thickness non-uniformity can be accepted without losing many components due to the operation frequency being out of specification. However, fabricating devices on small wafers or substrates is less cost-effective than doing the same on large substrates. In the case of using large substrates, the problem associated with thickness non-uniformity becomes acute.
Thus, it is advantageous and desirable to provide a method and system to solve the problem associated with thickness non-uniformity in the fabrication of FBAR-based devices on large substrates or wafers.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide a method and system for achieving a desired resonant frequency of the bulk acoustic wave device within a given tolerance. This object can be achieved by reducing the thickness non-uniformity of the device on a substrate. The thickness non-uniformity can be reduced by selectively and locally removing material from the topmost surface layer of the wafer, or die, before the wafer is cut into a plurality of device chips. Thus, the wafer has one or more bulk acoustic wave generating and controlling layers formed thereon. In that context, the bulk acoustic wave device, as described herein, refers to the entire wafer or substrate that has one or more layers fabricated thereon to form one or more individual device chips, or part of such wafer or substrate. Moreover, the bulk acoustic wave devices referred to herein include bulk acoustic wave resonators, bulk acoustic wave filters, stacked crystal filters, any combination of resonators and filters, and the structural variations of the resonators and filters. Furthermore, although one or more layers are already formed on the wafer or substrate when the thickness non-uniformity of the topmost layer is reduced, the device may or may not have all the necessary layers or the patterns of the layers. For example, the topmost layer of the device can be a piezoelectric layer. In that case, one or more layers are added on to the adjusted layer to complete the device.
Thus, according to the first aspect of the present invention, a method of tuning a bulk acoustic wave device made of a plurality of acoustic wave generating and controlling layers formed on a substrate, wherein the bulk acoustic wave device has a surface layer and a surface layer thickness having a non-uniformity profile defining a plurality of locations at which the surface layer requires thickness adjustment, and wherein the bulk acoustic wave device has an operating frequency which varies partly with the surface layer thickness, and the operation frequency can be adjusted by adjusting the surface layer thickness. The method comprises the steps of:
providing an etching medium over the surface layer at one section thereof for locally adjusting the thickness of at least one location of the surface layer at a time;
relocating the etching medium to another section of the surface layer for adjusting the thickness of at least one other location of the surface layer; and
repeating the relocating step if at least one remaining location of the surface layer requires thickness adjustment.
Preferably, the etching medium is a laser beam.
When the surface layer comprises a plurality of individual bulk acoustic wave components each having a top surface located at one of the locations, it is preferable to use a laser beam to trim the top surface of the individual bulk acoustic wave components, one at a time. The top surface can be a piezoelectric layer, a top electrode layer, a bottom electrode layer, a passivation layer overlapping the active area of the device, or a combination of thereof. For example, the top surface may include a piezoelectric layer and a top electrode layer.
Each of the individual bulk acoustic wave components may be a part of resonator, filter, stacked crystal filter or a combination thereof.
Preferably, the method further comprises the step of mapping the thickness non-uniformity of the surface layer prior to adjusting the thickness thereof.
According to the second aspect of the present invention, a system for tuning a bulk acoustic wave device made of a plurality of acoustic wave generating and controlling layers formed on a substrate, wherein the bulk acoustic wave device has a surface layer made of a surface material, and a surface layer thickness having a non-uniformity profile defining a plurality of locations at which the surface layer requires thickness adjustment, and wherein the bulk acoustic wave device has an operating frequency, which varies partly with the surface layer thickness, and the operation frequency can be adjusted by adjusting the layer thickness. The system comprises:
means, positioned above a section of the surface layer, for adjusting locally the thickness of at least one location of the surface layer at a time; and
means for relocating the thickness adjusting means, relative to the device, to another section of the surface layer for adjusting the thickness of at least one other location of the surface layer.
Preferably, the thickness adjusting means comprises a laser for providing a laser beam for removing the surface material.
Preferably, the system further comprises means for mapping the thickness non-uniformity profile of the surface layer prior to adjusting the thickness thereof.
It is understood that if the thickness of the piezoelectric layer is adjusted according to the above-described method, then a top electrode layer is deposited on the piezoelectric layer after the thickness of the piezoelectric layer is adjusted. It may also be necessary to adjust the thickness of the top electrode layer using the same method. Additionally, a patterning step is usually necessary to produce a desired pattern for the electrode layer. The patterning step can be carried out before or after the thickness of the electrode layer is adjusted. The patterning step is not part of the present invention. Furthermore, if a passivation layer is deposited on top of the top electrode layer, it may be necessary to adjust the thickness of the passivation layer. Thus, the thickness adjustment steps, according to the present invention, may be carried out one or more times for tuning the entire device, if necessary.
It is understood that, the relocation means can also be used to relocate the device, relative to the thickness adjusting means.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 2 to 7.