WO2004045073A1

WO2004045073A1 - Device for filtering electrical high frequency signals

Info

Publication number: WO2004045073A1
Application number: PCT/CH2002/000605
Authority: WO
Inventors: Jacqueline Vollmann; Jürg Dual; Dieter M. Profunser
Original assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Current assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Priority date: 2002-11-11
Filing date: 2002-11-11
Publication date: 2004-05-27
Anticipated expiration: 2005-05-11
Also published as: AU2002340692A1

Abstract

A device for filtering electrical high frequency signals comprises a substrate (10) carrying mechanical waves. At least one first transmitting transducer (1) translating an electrical signal into mechanical waves (2) is mounted on the substrate (10). A smooth acoustic interface layer (3) having a frequency dependent reflection and transmission behaviour by means of amplitude or direction or phase is provided on the substrate (10). At least one second receiving transducer (11, 21) translating the mechanical waves into electrical signals is mounted either on the same side of the substrate (10) as one first transmitting transducer (1) and/or one second receiving transducer (21) is mounted on the side opposite to the smooth acoustic interface layer (3). The smooth acoustic interface layer (3) has a gradually varying acoustic impedance representing a filter for induced mechanical waves.

Description

_Device for filtering electrical high frequency signals

Field of the Invention

The invention relates to a device for filtering electrical high frequency signals using acoustic waves and its manufacture.

Background of the Invention

There are several reasons for the technological evolution of acoustic wave devices in radio frequency (RF) applications: First of all, is the acoustical wavelength 4 to 5 orders of magnitude smaller than the corresponding electromagnetic wavelength for any given frequency, which leads to a tremendous reduction of size.

Initially, surface acoustic wave (SAW) devices appeared, due to the fact that the production of such surface acoustic wave fil- ters takes advantage of lithographic techniques which permit the mass production of high quality and low cost narrow band RF filters. The idea of using surface acoustic waves for signal filter purposes was first published by R.M. Arzt and K. Dransfield, "Excitation of Rayleigh Waves at High Frequencies and at Low Temperatures", Applied Physics Letters, Vol. 7, No. 6, pp. 156 - 158, American Institute of Physics, 1965. Corresponding patents are from Ralph Frank Mayo, "Electroacoustic Wave Shaping Device", US 3 '376'572, of April 1968 and Pierre Hartmann, "Energy- Weighted Dispersive Acoustic Delay Line of the Surface Wave Type", US 3 '633 '132, of March 1970. The commercial success of the SAW filter during the passed decade was - and still is - driven by the growing demand of mobile communications products, such as cellular phone handsets, which need to be small, light, robust, and economical by means of energy consumption.

_Nevertheless SAW devices show some drawbacks especially at fre^¬ quency ranges beyond 2 GHz (Gigahertz) . Higher frequency means smaller distances between the electrodes as well as smaller dimensions of the electrodes themselves. In frequency ranges above 2 GHz, the lithographic techniques become very challenging, a fact that impedes a low cost mass production. Furthermore a small electrode gap increases the chance of electric puncture, thus reducing the power transmitted through the filter. Since the frequency bands below 2 GHz are already very well used, new frequency bands had to be accessed technologically in order to fulfil the demands of further wireless applications. This led to the introduction of so called bulk acoustic wave (BAW) devices .

Apart from the higher frequency range, BAW filters have the advantage to be smaller than SAW filters and can directly be mounted on an IC chip. BAW devices usually consist of a piezoelectric layer which vibrates in one of its lower resonant modes in the thickness direction. Electrodes driving the vibration and electrodes for the detection of the vibration are laterally separated. In order to enhance the quality factor of the resonator, the piezoelectric layer is usually mounted on a so called reflection layer, which consists of a number of thin films hav- ing alternating high and low acoustic impedances, thus reflecting the acoustic energy into the resonator.

When compared with SAW devices, the mass production of BAW devices has a number of disadvantages: Instead of only one elec- trode layer, several layers of different materials need to be deposited. The resonant frequencies are mainly determined by the thickness of the piezoelectric layer which is harder to control than the gap between two lithographically mounted electrodes. A typical BAW device is presented in the patent of Robert F. Milsom, "Bulk Acoustic Wave Device", US 6 '448'695 B2, of Septem^¬ ber 2002.

_The device according to US 6,448,695 comprises at least three coplanar upper electrodes formed over a plurality of so called acoustically mis-matched layers. The coplanar upper electrodes enable e.g. the construction of a filter having the characteris- tics of a higher level order Chebyshev filter.

However, US 6,448,695 has a rather complex structure due to the fact that the so called acoustically mis-matched layers have to be calculated and deposited for any different filter. The layers tend to become very thin for high frequency filters, which is related to complex manufacturing problems.

Summary of the invention

It is therefore an object of the invention to provide a device for filtering electrical high frequency signals, e.g. a BAW device of the above mentioned type, which can readily be manufactured for very high frequencies.

A device for filtering electrical high frequency signals accord- ing to the invention comprises a substrate carrying mechanical waves, at least one first transmitting transducer translating an electrical signal into mechanical waves and mounted on the substrate, at least one second receiving transducer translating the mechanical waves into electrical signals, and a smooth acoustic interface layer having a frequency dependent reflection and transmission behaviour by means of amplitude, direction, and phase, the smooth acoustic interface layer being provided on the substrate, wherein one second receiving transducer is mounted on the same side of the substrate as one first transmitting trans^¬ ducer and/or one second receiving transducer is mounted on the side opposite to the smooth acoustic interface layer.

The proposed new filter takes advantage of the fact that the in^¬ terface between two neighboring layers itself acts as an acoustic filter, provided that the change of the acoustic impedance is spatially distributed within an intermediate interface layer having a thickness of the order of magnitude of the mechanical wavelength that need to be distinguished. In other words: a thick interface layer is "ignored" by waves which are short compared with its thickness, whereas it is "considered" as a sharp acoustic impedance change by waves which are long compared with its thickness, thus causing the wave to be partly reflected. In the explanations below, such an interface layer which is characterized by a well defined, smooth transition of the acoustic impedance from one value to another, shall be called "smooth acoustic interface" within this application. As mentioned, the advantage of this filter type lies in the fact that it can be realized for very high frequencies.

Using short pulse laser acoustic methods, the filter effect could be demonstrated at frequencies up to 0,5 THz (Terahertz). These experiments were carried out on a specimen consisting of a vapour deposited gold layer of 17 nanometer thickness embedded between two aluminum layers and deposited on a silicon oxide substrate. Since the gold atoms have a strong tendency to migrate into the aluminum lattice, the gold/aluminium interface can be smoothed by exposing the specimen to higher temperatures thus inducing intermetallic diffusion. A detailed description of the experiment using short pulse laser acoustic methods to demonstrate the behaviour of bulk waves in inhomogeneous media is given by J. Nollmann, D.M. Profunser, J. Dual, „Wave propagation in inhomogeneous media, phenomena and potential applications", IEEE Ultrasonics Conference Proceedings, Atlanta 2001, pp. 411- ₄14, published April 2002, and by J. Vollmann, D.M. Profunser, _J. Dual, „Sensitivity improvement of a pump-probe set-up for thin film and micro-structure metrology", ultrasonics, Vol. 40/1-8, pp. 757 - 763, Elsevier Science, Amsterdam, May 2002. By increasing the thickness of the smooth acoustic interface, the principle can be applied to any frequency range of interest. So the proposed filter type can easily be designed for frequency ranges between 1 and 10 GHz, which are of interest for many wireless RF applications.

In general, smooth acoustic interfaces can be realized by various techniques, wherein following examples are named: - the controlled and well defined intermixing of two phases of materials, diffusion processes, controlled thermally induced diffusion processes, the alternating deposition of two different phases or mate- rials of various thickness, thus representing a discretized transition of the acoustic impedance, the alternating deposition of two materials on a molecular level applying the methods of molecular beam epitaxy (MBE) , and the three-dimensional periodical, micromechanical structur- ing of surfaces or interfaces resulting in a frequency dependent reflection and/or transmission behavior.

The terms "acoustic wave" and "mechanical wave" are identical and are both used in a very general manner in order to describe any propagating or standing elastodynamic disturbance. The terms include dilatational waves (bulk waves) , transversal waves

(shear waves), and all kinds of surface- and interface waves. The acoustic impedance is defined as the product of the velocity of dilatational waves (sound velocity) and the density. In this patent it generally also includes the shear acoustic impedance which is defined as the velocity of transversal waves and the density.

The term "transducer" is used in a very general manner in order to describe any device which translates electrical signals into acoustic signals and vice versa. Especially transducers may be piezoelectric, optical, capacitive or electromagnetic transducers.

Brief description of the drawing

The invention will now be described in connection with exemplary embodiments shown in the drawings, in which:

Fig. 1 represent a first basic configuration of the device according to the invention, Fig. 2 represent a second basic configuration of the device according to the invention, Fig. 3 represent a third basic configuration of the device according to the invention, and Fig. 4 represent a fourth basic configuration of the device according to the invention.

Fig. 1 shows a section view of a first basic configuration of a device according to the invention.

The device is based on a solid substrate 10. On said substrate 10 is mounted at least one transmitting transducer 1 for the translation of an electrical signal into mechanical waves 2, propagating into the substrate. There is provided at least one receiving transducer 11 for the translation of mechanical waves 12 into an electrical signal . Reference numeral 12 denotes the low frequency path i.e. the path of the "long" mechanical waves 12.

According to the embodiment of Fig. 1 said receiving transducer ₁1 is mounted on the same surface 20 of the substrate 10. Furthermore there is provided at least one smooth acoustic interface or surface layer 3 opposite to surface 20. In general the interface of the transducers 1 and 11 and the wave carrying medium (the substrate 10) and the plane of the smooth acoustic in- terface layer 3 do not necessarily need to be parallel as shown in Fig. 1.

Reference numeral 22 denotes the mechanical waves of the high frequency path i.e. the path of the "short" mechanical waves. A second receiving transducer 21 is provided on the surface 30 of the smooth acoustic interface layer 3.

The smooth acoustic interface layer 3 is realized by variable material properties and the receiving transducers 11 and 21 are mounted on two sides 20 and 30 of the device. The surfaces 20 and 30 may comprise further layers, e.g. to adapt the transducers 1, 11 and 21.

Fig. 2 shows the reflection type of the device according to Fig. 1. All identical features in all Fig. have identical reference numerals. Therefore in comparison to Fig. 1 it has to be noted, that the detecting transducers 11 and 31 are now mounted on the same side of the device. The high frequency mechanical wave 22 is reflected at surface- 30 and travels back as reflected me- chanical wave 32. Although only two detecting transducers 11 and 31 are shown, the number can be arbitrarily increased in order to collect the fan-like, spatially distributed mechanical signals into a finite number of transducer channels, thus repre- senting a micromachined multichannel frequency analyzer.

Fig. 3 shows a third configuration of a device according to the invention, wherein the smooth acoustic interface layer 33 is here realized by a periodical structure. Said periodical structure 33 can be grating-like. The periodicity is oriented parallel to the front and back planes 20 and 30 of the device. The device works partly in transmissive mode. A second wave 22, e.g. the high frequency part, is propagating in a wave carrying me- dium 40 provided on the other side of the smooth acoustic interface layer 33. Such an additional wave carrying medium 40 might be a thin layer or an infinite medium and might be attached to any of the devices shown in Fig. 1 to 4.

Fig. 4 shows in contrast to the device shown in Fig. 2 the smooth acoustic interface layer 33 realized by a periodical structure, working in reflective mode. The embodiments according to Fig. 3 and 4 preferably comprise a multitude of receiving transducers 21, 31 for discerning different high frequency sig- nals transmitted or reflected within different angles throughout the substrate 10 or the wave carrying medium 40 provided on the other side of the smooth acoustic interface layer 33.

The devices according to Fig. 1 to 4 were produced on a specimen consisting of a vapour deposited gold layer of 17 nanometer thickness embedded between two aluminum layers and deposited on a silicon oxide substrate. Therefore the substrate 10 comprises the silicon oxide substrate and the first aluminium layer.

Since the gold atoms have a strong tendency to migrate into the aluminum lattice, the gold/aluminium interface can be smoothed by exposing the specimen to higher temperatures thus inducing intermetallic diffusion. This region of intermetallic diffusion forms the smooth acoustic layer 3 or 33. By increasing the thickness of the smooth acoustic layer 3 or 33 the principle can be applied to any frequency range of interest. So the proposed filter type can easily be designed for frequency ranges between 1 and 10 GHz, which are of interest for many wireless RF applications .

The common feature of all configurations is the frequency sensitive reflection and transmission behaviour of mechanical waves reaching the smooth acoustic interface layer 3 or 33, respectively.

The new acoustic wave filter type (bulk waves or shear waves) can be used for the decomposition of highest frequency compo- nents as well as for the signal-noise separation on a very high frequency level. Similar to existing devices like SAW filters, mechanical wave propagation phenomena are applied. In contrast to SAW filters, the propagation of mechanical waves is not limited to the surface of the wave carrying medium. The filter ef- feet is realized by the fact that the wave carrying medium contains zones with spatially smoothly varying acoustic impedances . An acoustic impedance change of two neighbouring materials or phases leads to a partial reflection and transmission of the initial wave. If the acoustic impedance change is smoothly, spa- tially distributed, and well defined within a certain thickness, the mechanical reflection/transmission behaviour becomes wavelength dependent i.e. frequency dependent . Thus a wave carrying medium having a gradually varying acoustic impedance represents a filter for mechanical waves. In this context, "smooth" means it might be discrete as long as the discretization steps are small compared with the wavelength of the mechanical wave of interest. Media having the properties described above, can be manufactured by micro- and nano technological methods. Substrate 10 and/or 40 may be aluminium and the thickness of said substrate 10 and/or 40 may be e.g. 40 nanometer. The substrates 10 and 40 are homogeneous media. The varying acoustic impedance is provided in a second layer, e.g. grown on the substrate layer, in a thickness of e.g. 5 to 10 nanometer, comprising a continuous change of the acoustic impedance or a stepwise change in discretized layers having a thickness which is small compared to the wavelength of the mechanical wave.

Claims

₁. A device for filtering electrical high frequency signals, comprising a substrate (10) carrying mechanical waves, at least one first transmitting transducer (1) translating an electrical signal into mechanical waves (2) and mounted on the substrate (10) , at least one second receiving transducer (11, 21, 31) translating the mechanical waves into electrical signals, and a smooth acoustic interface layer (3, 33) having a frequency dependent reflection and transmission behaviour by means of amplitude and/or direction and/or phase (3) , the smooth acoustic interface layer (3, 33) being provided on the substrate (10) .

2. The device according to claim 1, wherein one second receiving transducer (11, 31) is mounted on the same side of the substrate (10) as one first transmitting transducer (1) and/or one second receiving transducer (21) is mounted on the side opposite to the smooth acoustic interface layer (3, 33).

3. The device according to claim 1, wherein the frequency dependent wave propagation behaviour and/or the frequency dependent reflection and transmission behaviour of mechanical waves is caused by spatially variable mechanical properties of the smooth acoustic interface layer (3, 33) .

4. The device according to claim 1, wherein the frequency dependent wave propagation behavior and/or the frequency dependent reflection and transmission behaviour of mechanical waves is caused by a periodically structured surface or interface of the smooth acoustic interface layer (33) .

5. The device according to claim 1, wherein the frequency dependent reflection and transmission behavior of mechanical waves is caused by a finite number of thin layers, which slightly differ in their acoustic impedance, thus representing a discretized transition of the acoustic impedance within the smooth acoustic interface layer (3, 33).

6. The device according to claim 4, wherein the thickness of every thin layer is small compared to the wavelength of the mechanical wave induced by one of the first transmitting transduc- ers (1) .

7. The device according to one of claims 1 to 6 , wherein the low frequency path (12) represents a mechanical resonator.

8. The device according to one of claims 1 to 7, wherein the detecting transducer (11, 21, 31) represents also a mechanical resonator.

9. The device according to one of claims 1 to 8, wherein the transducers (11, 21, 31) are piezoelectric, optical, capacitive or electromagnetic transducers.