WO2007060557A1

WO2007060557A1 - Bulk acoustic wave resonator device

Info

Publication number: WO2007060557A1
Application number: PCT/IB2006/053974
Authority: WO
Inventors: Robert F. Milsom
Original assignee: Nxp B.V.
Priority date: 2005-11-25
Filing date: 2006-10-27
Publication date: 2007-05-31

Abstract

A bulk acoustic wave resonator device comprises a piezoelectric resonator (10, 20, 30) mounted on an acoustic reflector (50), the acoustic reflector (50) comprising a plurality of layers having alternating relatively low and high acoustic impedance. The thickness of the layers is arranged to provide maximum reflection at a frequency offset from the resonant frequency of the piezoelectric resonator (10, 20, 30). The offset may be positive or negative.

Description

DESCRIPTION

BULK ACOUSTIC WAVE RESONATOR DEVICE

The invention relates to a bulk acoustic wave (BAW) resonator device, a method of manufacturing a BAW device, a filter comprising the device, and an electronic apparatus comprising the filter.

Thin-film bulk-acoustic wave (BAW) filters are becoming the technology of choice for the radio-frequency (RF) selectivity function in applications above about 1 GHz. Applications include mobile phones and wireless connectivity devices. The basic building blocks of these filters are BAW resonators using either Film-Bulk-Acoustic-wave-Resonator (FBAR) or Solidly-mounted-Bulk- Acoustic-wave-Resonator (SBAR or SMR) technology. Ideally these resonators should excite a single acoustic mode. In thin-film BAW devices the wanted mode employed is usually the fundamental resonance of the thickness-extensional (TE) mode. For this mode the acoustic energy is incident normally on the thin-film layers. In practice other acoustic waves are generated through scattering of energy at electrode edges into laterally-guided surface acoustic waves (SAW). These can form unwanted standing-wave modes (resonances) which degrade the electrical responses of both resonators and filters, for example by causing ripple in the responses. Typically both these types of BAW resonator employ a silicon substrate. To provide acoustic isolation from the substrate the FBAR incorporates a free- space region, while an SBAR incorporates an acoustic Bragg reflector comprising several additional thin-film layers, the number depending on the choice of materials. The latter therefore has a more complex, but also more physically robust construction.

Figure 1 shows a cross-section of the layer structure of a typical SBAR, and indicates the principal acoustic waves. The parts of the resonator which are most active acoustically are the overlapping area of the two electrode layers 10, 20, and that portion of the piezoelectric layer 30 which lies within this area i.e. between the dashed lines in Figure 1. The Bragg reflector 50 in an SBAR prevents the normally-incident TE wave 90 from penetrating into the substrate 60 over a range of frequencies defined as its reflection band. The reflector 50 comprises alternate layers of high and low acoustic impedance with thickness chosen to be a quarter of a wavelength at the resonance frequency of the resonator. With sufficient layers this structure reflects almost 100% of the energy of a normally-incident TE wave. The larger the ratio of acoustic impedances the fewer the number of layers required. Silicon dioxide (Siθ2) is an example of a material having relatively low acoustic impedance, while tungsten (W) and tantalum pentoxide (Ta2O₅) have relatively high acoustic impedance. Tungsten has a higher impedance than tantalum pentoxide, so a tantalum pentoxide and silicon dioxide combination requires about 11 layers for close to 100% reflection while a tungsten and silicon dioxide combination requires only about 5 layers. In both these examples the reflection bandwidth is much wider than a typical filter bandwidth, but in the latter example it is about twice that in the former. A small number of layer is desirable because it is simpler to manufacture.

SBARs achieve a very good suppression of unwanted resonances, although with few (typically 5) reflector layers, the strongest unwanted modes are due to laterally-guided surface acoustic waves (reference 70 in Figure 1 ), rather than the laterally-guided scattered TE wave 80, as described in, for example, "Solidly mounted bulk acoustic wave (BAW) filters for the GHz range", H-P. Lόbl et al, IEEE Ultrasonics Symposium, Munich, p. 897, 2002.

Figure 2 illustrates the reflection coefficient, as predicted by modelling, of a 5-layer reflector having two layers of tungsten and three layers of silicon dioxide. The reflection coefficient is better than 0.001 dB (i.e. > 99.99% reflectivity) over a >20% band centred on 1.95 GHz, so energy loss associated with the normally-incident wave is negligible.

Figure 3 shows dispersion curves for the laterally-guided waves in the electroded region of such a 5-layer reflector SBAR. The convention in dispersion curves is to display frequency on the y-axis, real part of the normalized wave-number on the positive x-axis and imaginary part of the normalized wave-number on the negative x-axis. In Figure 3 there are curves for eight SAW modes, and the laterally-guided TE wave. The laterally-guided TE wave actually includes a significant parallel (xrdirected) as well as normal (x3-directed) component of motion, and is pure TE only at its cut-off frequency, in this case 1.95 GHz. All the other guided waves also have both parallel and normal components of motion. Most of the input electrical energy is converted to the required normal-incidence TE wave, however a significant proportion of the energy is converted to the laterally-guided TE wave. There is also conversion of energy to the other guided waves which are SAW. It can be seen, from the dispersion curve, that the laterally-guided TE wave has a real wave-number over a limited frequency range (in this case below cut-off) and the SAW modes are unattenuated at all frequencies, which means they are unattenuated in the xrdirection over this range. Standing waves are therefore formed over this range due to partial reflections at the electrode edges. These are the unwanted anharmonic modes of the resonator, and they occur at frequencies for which there is an integral number of half-wavelengths of the mode concerned in one of the lateral dimensions. Figures 4 and 5 show, respectively, the predicted magnitude and phase of the admittance of a 100 μm wide 1.95 GHz SBAR reflector having five layers of tungsten and silicon dioxide, for one dimensional (1 D) and two dimensional (2D) models.

Figure 6 shows, as a function of depth, the magnitudes of components of motion of the forced vibration parallel to, |ui(x₃)|, and normal to, |u₃(x3)|, the layers at a typical unwanted resonance frequency for an SBAR with a 5-layer reflector. For the piezoelectric layer deposited with its C-axis normal to the layers the parallel component |ui(x3)| is zero. Figure 7 shows the equivalent magnitudes for the dominant SAW mode. In these figures, the vertical lines are the layer boundaries. |U3(X3)| decays to near zero in the Bragg reflector as required.

A manufacturing process for known BAW devices is described by R. Aigner in "Volume manufacturing of BAW filters in CMOS", R. Aigner, 2^nd Int. Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, Chiba University, Japan, p. 127, March 2004, and by H-P. Lόbl et al in "Solidly mounted bulk acoustic wave (BAW) filters for the GHz range", IEEE Ultrasonics Symposium, Munich, p. 897, 2002.

An object of the present invention is to reduce unwanted modes in SBARs, particularly but not exclusively those with few reflector layers.

According to a first aspect of the invention there is provided a bulk acoustic wave resonator device comprising a piezoelectric resonator mounted on an acoustic reflector, the acoustic reflector comprising a plurality of layers having alternating relatively low and high acoustic impedance, wherein the thickness of the layers is arranged to provide maximum reflection at a frequency offset from the resonant frequency of the piezoelectric resonator.

According to a second aspect of the invention there is provided a method of manufacturing a bulk acoustic wave resonator device comprising a piezoelectric resonator mounted on an acoustic reflector, the acoustic reflector comprising a plurality of layers having alternating relatively low and high acoustic impedance, the method comprising forming the layers with a thickness arranged to provide maximum reflection at a frequency offset from the resonant frequency of the piezoelectric resonator.

The invention is based on the realisation that unwanted modes can be suppressed by introducing a significant offset between the frequency of maximum reflection of the reflector and the resonance frequency of the resonator. Moreover, the manufacture of such a BAW device involves no more complexity than a resonator without any special measure for suppression of unwanted modes. The frequency of maximum reflection of the acoustic reflector may be either higher or lower than the resonant frequency of the piezoelectric resonator. Preferably the offset is in the range 2%-30%, for example 5%.

The invention also provides a filter comprising a bulk acoustic wave resonator device according to the first aspect of the invention, and an electronic apparatus comprising such a filter. The invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 is a schematic cross-section through a layer structure of a BAW resonator device; Figure 2 is a graph of reflection coefficient as a function of frequency for a prior art BAW resonator device;

Figure 3 illustrates dispersion curves for a prior art BAW resonator device;

Figure 4 is a graph of the magnitude of the admittance as a function of frequency for a prior art BAW resonator device;

Figure 5 is a graph of the phase of the admittance as a function of frequency for a prior art BAW resonator device;

Figure 6 is a graph of magnitude of components of motion of the forced vibration for a prior art BAW resonator device at a typical unwanted resonance frequency;

Figure 7 is a graph of magnitude of components of motion of the dominant SAW mode for a prior art BAW resonator device at a typical unwanted resonance frequency;

Figure 8 is a graph of reflection coefficient as a function of frequency for a BAW resonator device in accordance with the invention;

Figure 9 illustrates dispersion curves for a BAW resonator device in accordance with the invention;

Figure 10 is a graph of the magnitude of the admittance as a function of frequency for BAW resonator device in accordance with the invention; Figure 11 is a graph of the phase of the admittance as a function of frequency for a BAW resonator device in accordance with the invention;

Figure 12 is a schematic diagram of a filter comprising a BAW resonator; and

Figure 13 is a schematic diagram of an radio transceiver employing a filter comprising a BAW resonator. The layer structure of a BAW device in accordance with the invention is as illustrated schematically in Figure 1 (not to scale) so will not be described again, but is distinguished from the prior art by having the thickness of the layers of the reflector arranged to introduce a significant offset between the frequency of maximum reflection of the reflector and the resonance frequency of the resonator.

The frequency of maximum reflection of the reflector may be made higher than the resonance frequency of the resonator by arranging for a plurality of each of the layers of the reflector having relatively low and high acoustic impedance to have a thickness less than one quarter of a wavelength of the resonance frequency of the resonator. The wavelength λ and the resonance frequency f_r of a reflector are related by the equation K = Z/ (p. f_r), where Z is the acoustic impedance of the material of a layer and p is the density of the material of that layer. So a plurality of each of the layers of relatively low and high impedance are arranged to have a thickness less than Z/ (4p. f_r).

Alternatively, the frequency of maximum reflection of the reflector may be made lower than the resonance frequency of the resonator by arranging for a plurality of each of the layers of the reflector having relatively low and high acoustic impedance of the reflector to have a thickness greater than one quarter of a wavelength of the resonance frequency of the resonator, i.e. a thickness greater than Z/ (4p. f_r).

The offset between the frequency of maximum reflection of the reflector and the resonance frequency of the resonator is preferably in the range 2% to 30%. If the offset is less than 2%, insufficient suppression of SAW resonances will occur. If the offset is greater than 30%, the reflection of the wanted normally incident TE mode will be insufficient, giving a reduced quality factor. A preferred value of the offset is approximately 5%.

The manufacturing process for a BAW in accordance with the invention is the same as for known BAW devices, for example as referenced above, except that the thicknesses of the layers of the reflector are arranged as described herein to introduce an offset between the frequency of maximum reflection of the reflector and the resonance frequency of the resonator.

Figure 8 shows the reflection coefficient of a 5-layer tungsten and silicon dioxide reflector having a 1.85 GHz frequency of maximum reflection, and used with a piezoelectric resonator having a resonance frequency of 1.95 GHz. Despite the frequency offset, it can be seen that the reflection coefficient is still better than 0.001 dB (i.e. > 99.99% reflectivity) over a >20% band centred on 1.95 GHz, so energy loss associated with the normally incident TE wave is still negligible when the resonance of the resonator is 1.95 GHz. The wanted mode is therefore substantially unchanged. Figure 9 shows the dispersion curves for the laterally-guided waves for a BAW resonator in accordance with the invention. The normalized wavenumber of the laterally- guided TE wave now has a larger imaginary component than illustrated in Figure 3, indicating more leakage through the reflector. The penetration depth of the forced vibration is greater than in a reflector that does not employ the offset in accordance with the invention, the effect of which is to introduce less coupling to SAW modes whose penetration depth is lower. Further the greater leakiness of the laterally-guided TE wave has an increased damping effect on the SAW modes to which it is coupled. Figures 10 and 11 show the magnitude and phase respectively of the admittance of a 100 μm wide SBAR employing a piezoelectric resonator having a resonance frequency of 1.95 GHz and employing a 5-layer tungsten and silicon dioxide reflector having a 1.85 GHz frequency of maximum reflection. Comparison with Figures 4 and 5 respectively shows that the spurious resonances are largely suppressed.

Figure 12 is a schematic diagram of a filter comprising a plurality of series connected and shunt connected BAW resonators 102, 104, and input port 100 and an output port 110. The difference between the series and shunt resonators is in their resonance frequencies. The shunt resonators 104 are mass loaded such that their anti-resonance frequency nearly coincides with the resonant frequency of the series resonators 102. An incoming electrical signal at this particular frequency will observe nearly zero impedance when passing the series resonator 102, and a very high impedance to ground. In effect almost the full signal will be delivered to the output port 1 10. For input signals with different frequencies, only a small fraction of the signal reaches the output port 110 because either the series resonators 102 have a high impedance or current is drained through the low impedance shunt resonators 104 to ground.

Figure 11 illustrates a block schematic diagram of a radio transceiver employing a filter comprising one or more BAW resonators made in accordance with the present invention. The transceiver comprises an antenna 120 which is connected to a duplexer or diplexer 122 which has branches coupled to a superheterodyne receiver section Rx and a transmitter section Tx. The receiver section Rx comprises a RF filter 124 formed by BAW resonators having an input coupled to the diplexer/ duplexer 122 and an output coupled to a low noise amplifier (LNA) 126. The RF signal from the LNA 126 is frequency down converted to an intermediate frequency (IF) in a mixer 128 to which a local oscillator 130 is connected. An IF signal output from the mixer 128 is filtered in an IF filter 132 and applied to a second mixer 134 to which a second local oscillator 136 is connected in order to mix the frequency of the IF signal down to baseband. The baseband signal is low pass filtered in a low pass filter 138 and is subsequently processed in a baseband stage 140 to provide an output on a terminal 142. In an alternative, non-illustrated embodiment the receiver section Rx is implemented as a direct conversion receiver.

The transmitter section Tx comprises an input terminal 144 for information, for example speech or data, to be transmitted which is coupled to a baseband section 146. A baseband signal output from the baseband section 146 is frequency up-converted in a mixer 148 to which an oscillator 150 is connected. The output signal from the mixer 148 is filtered in a bandpass filter 152 formed by BAW resonators, and applied to a power amplifier 154. An output of the power amplifier 154 is applied to the diplexer/duplexer 122.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of BAW filters and component parts therefor and which may be used instead of or in addition to features already described herein.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed.

The use of reference signs placed between parentheses in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A bulk acoustic wave resonator device comprising a piezoelectric resonator (10, 20, 30) mounted on an acoustic reflector (50), the acoustic reflector (50) comprising a plurality of layers having alternating relatively low and high acoustic impedance, wherein the thickness of the layers is arranged to provide maximum reflection at a frequency offset from the resonant frequency of the piezoelectric resonator (10, 20, 30).

2. A bulk acoustic wave resonator device as claimed in claim 1 , wherein a plurality of each of the layers of relatively low and high acoustic impedance have a thickness exceeding a quarter of the wavelength at the resonant frequency of the piezoelectric resonator (10, 20, 30).

3. A bulk acoustic wave resonator device as claimed in claim 1 , wherein a plurality of each of the layers of relatively low and high acoustic impedance have a thickness less than a quarter of the wavelength at the resonant frequency of the piezoelectric resonator (10, 20, 30).

4. A bulk acoustic wave resonator device as claimed in claim 1 , 2 or 3, wherein the offset is less than 30%.

5. A filter comprising a bulk acoustic wave resonator device 102, 104 as claimed in any one of claims 1 to 4.

6. An electronic apparatus comprising the filter of claim 5.

7. A method of manufacturing a bulk acoustic wave resonator device comprising a piezoelectric resonator (10, 20, 30) mounted on an acoustic reflector (50), the acoustic reflector (50) comprising a plurality of layers having alternating relatively low and high acoustic impedance, the method comprising forming the layers with a thickness arranged to provide maximum reflection at a frequency offset from the resonant frequency of the piezoelectric resonator.

8. A method as claimed in claim 7, comprising forming a plurality of each of the layers of relatively low and high acoustic impedance with a thickness exceeding a quarter of the wavelength at the resonant frequency of the piezoelectric resonator (10, 20, 30).

9. A method as claimed in claim 7, comprising forming a plurality of each of the layers of relatively low and high acoustic impedance with a thickness less than a quarter of the wavelength at the resonant frequency of the piezoelectric resonator (10, 20, 30).

10. A method as claimed in claim 7, 8, or, wherein the offset is less than 30%.