Acoustic Isolator Having a High Impedance Layer of Hafnium Oxide

The present invention relates to resonant acoustic devices that are commonly used as high frequency radio components and, in particular, to a resonant acoustic isolator having a high impedance layer formed of hafnium oxide.

Electrically driven piezoelectric acoustic resonators have utility in a wide range of radio frequency applications. Piezoelectric acoustic resonators comprise a layer of piezoelectric material that has a thickness of 1/2 the acoustic wavelength at resonant frequency. Drive electrodes are formed on the top and bottom surfaces of the piezoelectric layer. A discrete piezoelectric resonator may be used, for example, as a frequency reference in an oscillator circuit. Several resonators can be combined in a ladder network to form a band pass filter.

For resonators, an important figure of merit is the quality factor (Q), defined as: ##EQU1## where f.sub.0 is the center frequency and .DELTA.f is the frequency width of the resonance. For a parallel plate resonator, ##EQU2## where Q.sub.m is the inherent Q of the resonator material, and R.sub.1 and R.sub.2 are the reflectivities of the two surfaces. For a free air (or vacuum) interface, the reflectivity R is very nearly 1. Thus, a piezoelectric thin film resonator can have a very high Q when freely suspended in air. Such resonators have been demonstrated in discrete form and as integrated filters.

Thin film resonators, however, are difficult to fabricate, very sensitive to stress, and very fragile. An alternative is to fabricate thin film resonators on solid supporting substrates. In this case, R.sub.1 is essentially 1 for the top interface with air, but R.sub.2 is determined by the acoustic impedance mismatch between the resonator material and the supporting substrate as follows: ##EQU3## and where .rho. is the material density and C is the acoustic velocity in the material. The larger the mismatch in acoustic impedance between the resonator material and supporting substrate, the larger the reflection. Unfortunately, the range of acoustic impedance values in available materials is not large enough to yield a very high Q for a single interface. However, if a sequence of layers of alternating high and low impedance materials is used, with each layer having a thickness of 1/4 acoustic wavelength at the resonant frequency, the reflections from each pair of layers (high and low impedance material) combine in phase at the resonant frequency. For normal incidence, the one-dimensional case yields a total reflectivity of

where

R' is the reflectivity of each pair of layers and N is the number of pairs. Thus, the Q of a resonator mounted on such a resonant acoustic isolator can be expressed as ##EQU4##

If the material losses are low, nearly arbitrary reflectivity can be achieved by using a sufficiently large number of layers. However, to minimize the number of pairs of layers required, one should choose materials with a large acoustic mismatch. For ease in fabricating a thin film piezoelectric resonator (including top and bottom electrodes) directly on the supporting layers of the acoustic isolator, it is highly desirable that reflector materials comprise electrical insulators. Thus, devices can be isolated by patterning the drive electrodes without the necessity of patterning the supporting layers of the acoustic isolator.

Silicon dioxide (SiO.sub.2) can be effective as the low impedance layer (Z.sub.a .apprxeq.13) in a resonant acoustic isolator because it is a low loss material, it can be deposited by a variety of techniques, and it is ubiquitous in the semiconductor world. There is a need, however, for a high acoustic impedance material that can be readily deposited using a technique compatible with SiO.sub.2 deposition to form a multilayer resonant acoustic isolator.

The present invention comprises a multilayer resonant acoustic isolator that may be used for supporting a thin film acoustic resonator. The acoustic resonator typically comprises a film of piezoelectric material having top and bottom electrical contacts. The resonator can be formed atop the resonant acoustic isolator, which comprises at least one pair of layers of materials having different acoustic impedances. The pairs of layers are formed atop a substrate in the form of alternating layers of high and low impedance material, with each layer having a thickness of 1/4 acoustic wavelength at the resonant frequency. Reflectivity of the isolator is increased by using pairs of layers because the reflections from each pair of layers combine in phase at the resonant frequency. Silicon dioxide (SiO.sub.2) is preferred for the low impedance material because it is ubiquitous in the semiconductor industry, it has relatively low acoustic impedance (approximately 13) with very low intrinsic acoustic loss, and it can be deposited using a variety of convenient methods. Hafnium oxide (HfO.sub.2) is preferred for use as the high acoustic impedance material because it can be deposited by evaporation to form a hard, dense dielectric having a relatively high acoustic impedance (approximately 40). This structure has an advantage for low cost production because multiple wafers can be coated at one time, with an entire stack of alternating SiO.sub.2 and HfO.sub.2 layers deposited in the same chamber using electron beam deposition techniques.

A principal object of the invention is an effective resonant acoustic isolator that can support a thin film acoustic resonator. A feature of the invention is at least one pair of 1/4 acoustic wavelength layers comprising a layer of low acoustic impedance material and a layer of high acoustic impedance material consisting of hafnium oxide. An advantage of the invention is ease of deposition of high impedance hafnium oxide in conjunction with commonly used low impedance materials.

For a more complete understanding of the present invention and for further advantages thereof, the following Detailed Description of the Preferred Embodiments makes reference to the accompanying Drawings, in which:

FIG. 1 is a schematic cross section of an acoustic resonator of the present invention; and

FIG. 2 is a graph of Limiting Q versus number of Pairs of Layers using high impedance material having values of Z.sub.a .apprxeq.25 to 40.

The present invention comprises a multilayer resonant acoustic isolator. An embodiment of an acoustic isolator 10 combined with a thin film acoustic resonator is shown in schematic cross section in FIG. 1. Acoustic isolator 10 is formed on a substrate 11, which may comprise an insulator, such as glass, for example. Acoustic isolation materials having high and low acoustic impedance are deposited in pairs of alternating layers, such as layers 12 and 13 (and optional layers 14 and 15, for example), on substrate 11.

As illustrated in FIG. 1, acoustic isolator 10 provides support for an acoustic resonator comprising a layer of piezoelectric material 18 having bottom and top electrical contacts 17 and 19, respectively. The layer of piezoelectric material 18 typically has a thickness of 1/2 the acoustic wavelength at resonant frequency. Layers 12 and 13 of acoustic isolator 10 are formed atop substrate 11 in alternating layers of high and low impedance material, with each layer having a thickness of 1/4 acoustic wavelength at resonant frequency. It is advantageous to use paired layers (such as layers 12 and 13) comprising materials having a significant difference in acoustic impedance, with the low impedance material proximate the acoustic resonator, to produce high reflectivity and low acoustic loss with a small number of layer pairs. In a preferred embodiment of the present invention, the high impedance material (layers 12 and 14 in FIG. 1, for example) comprises hafnium oxide (HfO.sub.2) and the low impedance material (layers 13 and 15 in FIG. 1, for example) comprises silicon dioxide (SiO.sub.2).

FIG. 2 is a graph showing the effective Q (as defined above) of a thin film piezoelectric acoustic resonator (having a top free air surface) mounted on a resonant acoustic isolator 10 comprising various numbers of pairs of 1/4 acoustic wavelength layers (as described above). Silicon dioxide is assumed to be the low impedance material for all curves and the high impedance material ranges from Z.sub.a =25 to 40. The substrate is silicon and the thin film piezoelectric resonator itself is assumed to have impedance Z.sub.a =35. As can be seen from FIG. 2, higher Q can be achieved with fewer pairs of acoustic isolator layers when the difference between the impedances of the high and low impedance materials is greatest.

Silicon dioxide is preferred for the low impedance material because it is ubiquitous in the semiconductor industry, it has very low intrinsic acoustic loss with relatively low acoustic impedance (Z.sub.a of approximately 13), and it can be deposited using a variety of convenient methods. Hafnium oxide is particularly suited for use in the alternate layers of high acoustic impedance material of because it can be deposited by sputtering or electron beam evaporation with concurrent ion bombardment, for example, to form a hard, dense dielectric having a relatively high acoustic impedance (Z.sub.a of approximately 40).

Alternative high acoustic impedance materials, such as TiO.sub.2 and ZrO.sub.2, have nearly the same acoustic impedance as HfO.sub.2 and can be evaporated, but because they have lower density and higher acoustic velocity, thicker films are required to reach 1/4 acoustic wavelength. Various metals have higher acoustic impedance, but because thin film acoustic resonators must be confined in area, the multiple layers of the acoustic isolator would have to be patterned to electrically insulate them from the electrical contacts 17 and 19 of piezoelectric resonator 18. Aluminum nitride and zinc oxide (Z.sub.a of approximately 35) may be used as the high impedance material, but because of their lower impedance compared with hafnium oxide, more pairs of layers are needed to reach the same reflectivity, as illustrated in FIG. 2. Furthermore, aluminum nitride and zinc oxide cannot be deposited effectively by evaporation. Layers of these materials can be formed by laser assisted deposition or sputtering, but these non-batch processes are inherently more expensive.

In fabricating resonant acoustic isolator 10, HfO.sub.2 films evaporated at low temperatures have been found to be acoustically lossy. Raising the substrate temperature above 400.degree. C. increases the density, but the films become polycrystalline with rough surfaces that produce unacceptable acoustic scattering. However, ion assisted electron beam evaporation (i.e., electron beam evaporation with concurrent ion bombardment, as known in the prior art), has been used to achieve the required bulk density and hardness while maintaining a smooth surface. Such films have been found to exhibit low acoustic loss. Very high reflectivity layers have been produced by this method, resulting in high performance resonant acoustic isolators.

Although the present invention has been described with respect to specific embodiments thereof, various changes and modifications can be carded out by those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.