WO2002095941A1

WO2002095941A1 - Surface acoustic wave device

Info

Publication number: WO2002095941A1
Application number: PCT/AU2002/000617
Authority: WO
Inventors: Kourosh Kalantar-Zadeh; Wojtek Wlodarski
Original assignee: Microtechnology Centre Management Limited
Priority date: 2001-05-21
Filing date: 2002-05-20
Publication date: 2002-11-28
Also published as: AUPR511401A0

Abstract

A love mode surface acoustic wave device is provided having a piezoelectric material deposited on a piezoelectric substrate. The preferred structure is a zinc oxide film on 90° rotated ST-cut quartz crystal. The device is particularly suitable for use as a filter for mobile phones.

Description

Surface Acoustic Wave Device

This invention relates to improvements in Surface Acoustic Wave [SAW] devices and particularly SAW devices used as filters.

Background to the invention

SAW devices have been proposed as filters and for use in other acousto- electronic devices. USA patent 4194171 discloses a SAW device for use in a convolver. The surface acoustic waves propagate in 3 directions and can be classified as longitudinal wave motion, normal waves and shear waves. A class of shear horizontal waves are called Love waves which are propagated in layered devices that concentrate the wave energy in a highly confined region near to the surface. USA patent 4544857 discloses a Love mode SAW filter which utilises a lithium niobate piezo crystal. An advantage of SAW filters is that they are smaller in size than alternative filter types and are therefore useful in hand held devices. USA patent 5130681 discloses the use of a SAW filter in a mobile phone and uses an aluminium film on a lithium tantalate piezo substrate. The minaturising of saw filters is desirable in the production of smaller mobile phones. The speed of the acoustic wave propagation determines the filter centre frequency. To decrease the size of the filter the speed of the acoustic wave propagation should be as high as possible.

Love mode SAW filters have also been proposed in USA patent 5847486 using a tungsten or Tantalum layer on a lithium niobate substrate. USA patent 5432392 discloses a surface wave device using a lithium Niobate substrate with a thin film of zinc oxide on the interdigital transducers to generate love waves. The main disadvantage of Lithium niobate SAW filters is the low speed of acoustic wave propagation and high temperature coefficient. USA patent 4757283 disclosed that the surface wave velocity can be adjusted by applying a thin layer of elastic material onto the substrate.

The speed can be increased by depositing piezoelectric films on non piezoelectric high propagation speed substrates such as zinc oxide piezo layer on a diamond substrate. These structures are expensive and difficult to fabricate. USA patent 6127768 sought to improve the performance by using a metal doped Zinc oxide layer on a diamond or fused quartz wave propagation layer. This device operates in Rayleigh mode.

Another difficulty with many of these prior art devices is that they couple with other wave propagating modes meaning that other pass bands exist. It is an object of this invention to provide a love mode SAW filter with improved performance.

Brief description of the invention

To this end the present invention provides a surface acoustic wave filter in which a layer of a piezoelectrc material lies on the surface of a piezoelectric substrate cut for propagating a love mode surface acoustic wave. This structure provides the following advantages:

1- High Electromechanical Coupling Coefficient (K²).

2- Small Temperature Coefficient. 3- High confinement of energy on the surface.

4- As the Surface Skimming Bulk Wave SSBW mode is used, speed of acoustic wave propagation is high (about 40% higher than Rayleigh mode), which means the dimension of the device fabricated will be smaller. This feature is quite useful for acoustic devices (such as filters, resonators, duplexers etc.) used in handset equipment like mobile phones.

There are two types of Shear Horizontal wave that can be changed to Love Wave:

Leaky Surface Acoustic Wave (Leaky-SAW) and Surface Skimming Bulk Wave

(SSBW).

Leaky SAW is a shear horizontal wave which propagates mainly beneath the piezoelectric surface, as a result it is less sensitive to contamination of the surface.

However for transforming such a wave to Love mode larger thicknesses of guiding layers are required.

SSBW are mainly longitudinal with shear horizontal polarization. They also can be converted to Love waves but this transformation occurs at smaller thicknesses of guiding layer comparing to Leaky-SAW which reduces the cost of fabrication. Both Leaky-SAW and SSBW have high speed of acoustic wave propagation. About 1.6 times that of Rayleigh waves.

A preferred piezo substrate is 90° rotated ST-cut quartz crystal which has a propagation speed of 5000m/s and the dominant wave is SSBW and has zero coupling to other modes. It is dominantly a Shear Horizontal [SH] bulk wave and has a low temperature coefficient, Its major disadvantage is a high insertion loss as it changes from SSBW to love mode. When a film material is deposited on the surface it should load the substrate which means the speed of propagation in the film is less than in the substrate. In this case, the mode of propagation changes to Love mode. When metal oxides or polymer films are deposited on the substrate the insertion loss is decreased as the mode of operation changes from SSBW to Love mode. Its major advantage is a lower insertion loss as it decreases from SSBW to Love mode. Quartz crystal also has a low relative permittivity and the SAW filters made from the SAW device of this invention results in a small input capacitance filters. This is an important advantage as in circuits this capacitance must be compensated with an inductance and inductances are large in size and generally undesirable. The preferred piezo layer is zinc oxide [ZnO]. ZnO is the best candidate for fabricating Love mode devices. It has a porous surface and is a piezoelectric material with a low phase velocity (2650 m/s). This implies that ZnO can increase the electromechanical coupling coefficient more than other deposited materials. Furthermore, ZnO consists of hexagonally shaped cylinders with gaps in between them, making the guiding layer porous. In another aspect this invention provides a surface acoustic wave device which incorporates a substrate of a piezoelectric quartz crystal at least one interdigital transducer formed on said quartz crystal a piezo electric film of zinc oxide deposited on said crystal and transducer ZnO has a positive temperature coefficient whereas 90° rotated ST-cut quartz crystal has a negative temperature coefficient. The combination of positive and negative temperature coefficients assists to reduce the temperature coefficient of the whole structure. Around room temperature (25°C) the temperature coefficient remains relatively lower than that of the blank SSBW structures. Compared to the love mode devices disclosed in USA patent 5432392 the temperature coefficient of frequency for the device of this invention is much lower and approaches zero.

Detailed description of invention

Figure 1 is a schematic illustration of the SAW device of this invention. Figures 2 a) and 2b) and 3 are frequency loss results for the devices described in the example below.

Figure 4 is a comparison of the coupling coefficient for ZnO and SiO₂ films on an ST cut quartz crystal wafer.

Figure 5 is a comparison of the temperature coefficient for ZnO and SiO₂ films on an ST cut quartz crystal wafer.

As shown in figure 1 the SAW device ofthis invention consists of a piezo substrate 5 having a piezoelectric surface film 6. The input interdigital transducers 7 and the output interdigital transducers 8 lie between the piezo substrate 5 and the piezo film 6.

This invention provides piezoelectric layers on piezoelectric substrates. The Substrate's cut belongs to a class of crystal cuts that support Surface Skimming Bulk Wave (SSBW). The layers are of different of piezoelectric materials that can be deposited as a highly directional film on the substrate, which let acoustic waves propagate on shear horizontal direction. Speed of propagation of acoustic wave in the layers must be less than in the substrate to support Love mode of propagation. The substrate must support SSBW mode of operation. A few examples suitable piezoelectric crystals are shown in tables IA and 1 B Table 1 A. Some Piezoelectric Crystals Suited to Surface Skimming Bulk Wave (SSBW) Propagation

Table 1B. Some Piezoelectric Crystals Suited to Leaky-SAW

The surface film must be a piezoelectric. A few examples of piezoelectric films are shown in table 2.

Table 2 Piezoelectric films

*) VE (Vacuum Evaporation), CVD (Chemical Vapour Deposition), (Radio Frequency Magnetron Sputtering), RF-SP (Radio Frequency Sputtering) **) PC (Poly crystal), SC (Single Crystal)

Example A comparative study was conducted using SiO₂ and ZnO deposited on 90° rotated ST-cut quartz crystal.

Love mode filters were fabricated on 90° rotated 0.5mm thick ST-cut quartz substrates. The transmit and receive interdigital transducers [IDT's] of the SAW devices consist of 64 and 16 IDT pairs respectively with 50 micron periodicity with an aperture of 3mm. SiO₂film and crystallographically c-axis oriented ZnO film were deposited using a r.f. magnetron sputterer with a thickness of 1.2 microns.

The sputtering conditions used are shown in table 3.

Table 3

Insertion losses were examined by measuring the transmission S parameters which were obtained using a network analyser using a 50Ohm line impedance. The measurements indicate that the mode of operation in 90° rotated ST-cut quartz crystal has no extra coupling mode as can be seen in figure 2 b). The frequency response of blank filters and filters with SiO₂ and ZnO deposited layers is shown in figure 3. The ZnO /ST-cut quartz crystal structure possesses a lower insertion loss which is 19 dB less than the blank quartz crystal filter and 6dB better than the SiO₂/ST -cut quartz crystal structure. The acoustic signals on both layered structures are dominantly SH since an insignificant change in insrtion loss occurs when loaded with a non viscous liquid. The ZnO based acoustic filter has less insertion loss due to the piezoelectricity of ZnO which increases the coupling coefficient of the structure. Electro-mechanical coupling coefficients (K²) were estimated for the devices of different ZnO thicknesses from 0 to 3.2 μm. For comparison, a range of different SiO₂film thicknesses up to 6 μm were sputtered and coupling coefficient were measured for them as well. A software package was used for calculation of the coupling constant curves. Both calculations and measurements are shown in Fig 4. A preferred thickness for the zinc oxide films is from 0.6 to 1.6 microns Values of Temperature coefficient were measured using an environmental chamber. It was obtained by changing the temperature from -20C to 40C,

Fig. 5 Shows the frequency shift versus temperature change at different ZnO thicknesses.

From the above it can be seen that this invention provides a unique filter structure with significant advantages. Those skilled in the art will realise that variations may be made to adapt the invention to a range of applications for which surface acoustic wave devices may be used.

Claims

1. A surface acoustic wave device which incorporates a substrate of a piezoelectric quartz crystal at least one interdigital transducer formed on said quartz crystal a piezoelectric film of zinc oxide deposited on said crystal and transducer

2. A surface acoustic wave electronic device in which a layer of a piezoelectric material lies on the surface of a piezoelectric substrate cut for propagating a love mode surface acoustic wave.

3. An electronic filter or resonator which incorporates a surface acoustic wave device as defined in claim 1

4. A surface acoustic wave device as claimed in claim 2 in which the piezoelectric substrate is a 90° rotated ST-cut quartz crystal and the piezoelectric film layer is zinc oxide.

5. A surface acoustic wave device as claimed in claim 4 in which the layer of zinc oxide is from 0.6 to 1.6 microns in thickness.