WO2013189631A1

WO2013189631A1 - Micro-acoustic component comprising a tcf compensation layer

Info

Publication number: WO2013189631A1
Application number: PCT/EP2013/058385
Authority: WO
Inventors: Matthias Honal; Günter SCHEINBACHER; Ulrich Knauer
Original assignee: Epcos Ag
Priority date: 2012-06-18
Filing date: 2013-04-23
Publication date: 2013-12-27
Also published as: DE102012105286B4; DE102012105286A1

Abstract

The invention relates to a micro-acoustic component comprising a piezoelectric substrate, an electrode for exciting an acoustic wave, and a compensation layer that has an abnormal thermomechanical behavior. According to the invention, the compensation layer is an SiO₂-based vitreous material in which the Si-O-Si bond angle in the glass network is larger than the 144° bond angle of pure SiO₂.

Description

description

The invention relates to a microacoustic component having a TCF compensation layer (TCF = temperature coefficient of frequency).

Frequency-determined properties of acoustic wave devices, e.g. SAW components (SAW = surface acoustic wave) usually show a dependence on the temperature. For example, the temperature coefficient of the center frequency (TCF) of SAW devices is typically at e.g. 40 ppm / K. This is because at

Temperature increase usually takes place, a thermal expansion of the substrate, which leads to an increase in the electrode spacing in interdigital transducer structures. Since this distance determines the center frequency of the transducer and thus of the SAW device, so does the wavelength, whereby the center frequency is lowered.

However, associated with the thermal expansion is also a change in the speed of sound, since change with ther ^¬ mix expansion and the elastic properties of the piezoelectric material. In addition, most of the commonly used piezoelectric wafer materials exhibit strong anisotropy and have a crystal axis-dependent temperature response of their properties.

US Pat. No. 7,589,452 B1 proposes a component which operates with acoustic waves and which has various components

Measures for lowering the temperature response (TC compensation) in particular the resonance frequency combined. The component has on the upper side electrically conductive component structure. structures and on the underside of a compensation layer, which is mechanically firmly connected with the substrate so that a mechanical stress is created, or builds up in Temper ^¬ aturänderung. Above the component structures, a relatively thick Si0 ₂ layer is arranged. A disadvantage of this solution is that the required reflectivity of Elek ^¬ electrodes is obtained only with heavy electrodes. In the S1O ₂ layer there is also a bad waveguide, so that the maximum achievable TC compensation is limited. This is unsatisfactory especially for SAW devices and insufficient for some applications. In addition, the known measures for TC compensation are associated with partly unacceptable losses in coupling, bandwidth and attenuation.

Object of the present invention is to provide a micro-acoustic device, which has an improved

Compensation for the temperature coefficient TCF of the frequency (TC compensation) has.

A micro-acoustic device is proposed that comprises a piezoelectric substrate, an acoustic wave excitation electrode, and a compensation layer exhibiting abnormal thermomechanical behavior. As a compensation layer, a glassy material based on S1O _{2 is} proposed, in which the Si-O-Si angle in the glass network is increased relative to the bond angle of 144 ° of pure S1O ₂ . In this case, a piezoelectric substrate is understood as meaning a substrate which comprises at least one piezoelectric layer as a functional layer for exciting acoustic waves. For a SAW device, the piezoelectric substrate is usually a single crystal of a piezoelectric

Materials.

It has been shown that the TK compensation in the

Component, which usually has a negative temperature ^¬ coefficient of frequency, with such

Compensating layer, which has an improved (higher) negative temperature coefficient of frequency, can be improved over a device with a compensation layer of pure Si0 ₂ . The device thus exhibits a reduced or even a fully compensated temperature response of the frequency.

The positive effect of the Si0 ₂ layer is that it has the desired positive temperature coefficient of acoustic deceleration while the piezoelectric substrates usually exhibit negative temperature coefficients of acoustic deceleration. The SAW velocity is less than in the Si0 ₂ layer

for example in LT. Therefore, the wave is preferably at the interface between Si0 ₂ layer and substrate.

Another advantage of this measure is that the Si0 ₂ as a good dielectric has a high breakdown strength of about 1 MV / mm ^. The ESD strength is thus increased. This is also due to the "rounding" of the finger edges by the layer applied over it. In addition, the dielectric constant of Si0 ₂ with 3.9 is about 4 times as large as that of air, which further increases the dielectric strength. In addition, the hard Si0 ₂ layer also acts as a particle protection, for example against a short circuit, which may result from metal particles from a production process. With glassy Si0 ₂ , which has an increased bond angle, these effects are now improved again. It is even possible for the first time to cause an overcompensation of the temperature variation in the component, so that the component according to the invention has a sign from the sign

having reverse temperature coefficient.

A direct observation of the Si-O-Si bond angle in the S1O _{2 of} the compensation layer is possible via IR spectroscopy. As a measure of the deviation of the bond angle can be in the FT IR spectroscopy, the co3 band and the co4 band

used in pure, by means of PECVD

deposited S1O ₂ at about 815 cm ^-1 or at about 1055 cm ^-1 lie. If the bond angle deviates from its "natural" position in pure Si0 ₂ in the doped material and increases, the co3 band shifts towards smaller wavenumbers, whereas the co4 band shifts towards larger ones

Wavenumbers. The compensation layer can be next to the S1O ₂ in one

Comprise embodiment as a further constituent glass formers and / or conventional glass aggregates, which are selected from B ₂ O _3, Si0 _2, P2O5, As2O3, Sb ₂ 0 _3, ln ₂ 0 _3, Sn ₂ 0 _3, Pb0 _2, Li ₂ 0, CaO, Na ₂ 0, K ₂ 0, MgO, Rb ₂ 0, Cs ₂ 0, SrO, Te0 _2, Se0 _2, Mo0 _2, W0 _3, Bi0 _3, A1 ₂ 0 _3, BaO, V ₂ O and S0 ₃ , Although these additions do not necessarily change the bond angle, they can further improve the thermo-mechanical glassy properties of the compensation layer. An effective expansion of the Si-O-Si bond angle succeeds in one embodiment certainly by means of a doping, wherein as effective dopants boron, phosphorus, fluorine or additional OH groups may be contained in the S1O ₂ . Such Dopants are also known as network-changing dopants. By incorporating them into the glass structure of the S1O ₂ , the regular quartz lattice is disturbed or interrupted. The effect with respect to the widening of the mentioned bond angle and the temperature compensation increases with it

increasing dopant concentration.

In one embodiment, the compensation layer in S1O _{2 contains} at least one dopant in a proportion of 0.5-8.0 atom%, selected from boron and phosphorus. The

optionally obtained with further additions glass can also be called boron or phosphorus silicate glass.

In one embodiment, the compensation layer has a relative thickness of 5-30% relative to the wavelength of the

Device at center frequency on. This layer thickness is usually sufficient to achieve a complete compensation of the temperature coefficient of the device. In one embodiment, the component is designed as a SAW component and has one as an interdigital electrode

formed electrode deposited on the substrate, e.g.

is disposed between the compensation layer and the piezoelectric substrate.

In one embodiment, the compensation layer is conformally deposited. As a result, in the SAW device the

Increased reflectivity per electrode finger of the interdigital electrode. This is because now also the surface of the Si0 ₂ layer has a topography that at least

partially follows the topography of the electrode fingers and thus has its own reflectivity, or increases the overall reflectivity. In a further embodiment of the invention, the topography on the surface of the Si0 ₂ layer by subsequent

Structuring and other measures are designed and structured with a grid of finger-like elevations.

The period, the relative finger width, the edge angle or the edge shape of the elevations on the

Surface of the Si0 ₂ layer / compensation layer can be varied. The surveys can match the structure of

Electrode fingers match or deviate from this in the above points.

It was also found that over a variation of the

Edge angle of the electrode fingers itself improved layer deposition of the compensation layer and in particular a better edge coverage is possible. While known compensation layer of Si0 ₂ tend to cracks from a certain thickness ^¬ thick, is a deviating from 90 ° edge angle of the metallization and thus a this

Corner angle following topography of the compensation layer counteracted, thus avoiding cracks. A compensation layer above ^¬ component structures with oblique and for example 75 ° inclined edge angles has significantly less cracks than one layer of the same thickness with conventional

Edge angles of 90 °. Alternatively, with an edge angle of the metallization smaller than 90 °, the layer thickness of the compensation layer can be increased and the compensation of the TCF can be improved without causing increased cracks. Generally, however, glassy S1O ₂ is less prone to cracking. In one embodiment, a trimmable layer is disposed over the compensation layer. This allows the

Layer thickness by Schichtdickenabtrag that of the

Layer thickness dependent properties of the device in to readjust or to trim in a certain frame. Such a trimmable layer may comprise, for example, silicon nitride. This can be applied in a controlled manner and can be selectively etched against S1O ₂ . The trimming process can be carried out uniformly at wafer level for the entire wafer in which the components are formed. However, it is also possible to carry out location-selective trim processes in order to compensate for inhomogeneities within a wafer.

However, inhomogeneities can also occur from wafer to wafer or from batch to batch and in a trim process

for the application of the trimmable layer over or directly on the compensation layer

Requirement is. In order to detect inhomogeneities or deviations from the desired nominal values, selected components can be tested directly on the wafer.

In one embodiment, the electrode comprises copper. Preference ^¬ wise, the electrode consists mainly to completely of copper. Copper is relatively heavy against aluminum. Therefore, a copper electrode has a larger difference in the acoustic density between the electrode and the material

Compensating on. A Cu electrode is therefore also better "seen" by an acoustic wave in the component and the reflectivity is thereby increased.

The structure of the electrode can be used as the lowest layer

Adhesive layer have, which allows a more intimate adhesion of the electrode on the substrate. Adhesion promoters ^¬ mid layers may include those of Ti, Cr, Ta, titanium oxide, chromium oxide, tantalum oxide or aluminum oxide.

Over the primer layer can one growth

accelerating layer or a buffer layer arranged be that an oriented growth of the electrode

allows or supports.

Furthermore, a diffusion barrier can be applied above or below the primer layer, which is one possible

Diffusion of metal from the electrode into the substrate and / or oxygen from the material of the substrate into the substrate

Electrode prevented. As a cover layer over the electrode, a passivation layer or alternatively further adhesion promoter layer can be applied. A passivation layer may be the

increase chemical / oxidative resistance of the electrode.

Such passivation layers may be inorganic

Dielectrics may be selected, in particular from the group of oxides, silicides, nitrides and carbides. Another

Adhesive layer can improve the adhesion between electrode and compensation layer. In one embodiment, the component is a BAW resonator (BAW = bulk acoustic wave) arranged on a substrate, in which the compensation layer is arranged between the substrate and an electrode, in particular the bottom electrode of the resonator.

In one embodiment, the compensation layer is part of a GBAW (Guided Bulk Bulk Wave) device and is above the piezoelectric piezoelectric device provided with the electrode

Substrate is arranged. There, the compensation layer can act as a waveguide layer of the GBAW device.

In a further embodiment, the component comprises a first and a second filter. The first filter is the Temperature response of the center frequency by a correspondingly adjusted compensation layer negative and thus

overcompensated. The second filter is electrically connected to the first filter and has a positive one

Temperature response of the center frequency. The interconnection of the first and second filters is carried out in such a way that a complete compensation of the temperature response of the center frequency is achieved in the component as a whole. The invention has the advantage that the proposed new material for the compensation layer by means of conventional already for the production of micro-acoustic

Components used method can be produced and thus can be integrated easily into the known manufacturing process.

The invention is further illustrated by execution ^¬ examples and accompanying schematic figures. The figures are solely for the understanding of the invention and are not executed to scale. You can therefore neither absolute nor relative dimensions

be removed.

Figure 1A shows a SAW device with a

Compensation layer,

FIG. 1B shows a variant of a SAW component,

Figure IC shows a BAW resonator with a

Compensation layer, Figure 1D shows a GBAW device with a

Compensation layer according to the invention as waveguide layer, FIG. IE shows a variant of a GBAW device with a

Compensation layer,

Figure 2 shows a SAW device with a structured

Compensation layer,

Figure 3 shows IR absorption bands more different than

Compensation layer of used materials,

Figure 4 shows a diagram of the change of the

Temperature coefficient TCF of frequency in

Dependence on the dopant content in the

Compensation layer, here from the phosphorus content.

Figure 1A shows a schematic representation of the simplest embodiment of the invention with reference to a SAW device. At least one metallic interdigital electrode is applied as the electrode EL on a piezoelectric substrate SU. This preferably comprises as main component a metal which is heavier than aluminum, in particular copper. The relative metallization of the electrode is

preferably between 4 and 10%.

On the entire surface, that is above the electrode ELI and the regions of the substrate SU lying therebetween, a compensation layer KL is applied in such a way that a planar surface results which is parallel to the surface of the substrate SU. The compensation layer KL comprises a glassy material based on SiO ₂ , which in particular has a Si-O-Si bond angle, in particular by suitable dopants and / or additives, which is above the normal Si-O-Si bond angle of 144 ° in pure SiO _{2 is} increased.

The compensation layer makes it possible to compensate for the negative temperature coefficients of the frequency due to the materials used for the "bare" SAW device.

The relative thickness required for this (based on the acoustic wave propagatable in the compensation layer) of the compensation layer KL is between 15 and 45%. FIG. 1B shows a variant of a SAW component, in which the compensation layer KL according to the invention is provided on the surface of the surface opposite the electrode ELI

piezoelectric substrate SU is arranged. For

Electrode material and thickness as well as material of

Compensation layer and its layer thickness, the same limitations apply as mentioned in connection with Figure 1A.

Figure IC shows a schematic representation of a BAW resonator with a compensation layer KL according to the invention. The BAW resonator consists of a piezoelectric substrate SU, which is embedded between a first electrode ELI and a second electrode EL2. The Kompen ^¬ sationsschicht KL is preferably disposed between the substrate SU and the second electrode EL2.

The layer thickness of the compensation layer is chosen such that, on the one hand, the desired center frequency of the BAW response is obtained. On the other hand, the temperature compensation takes place to a desired extent, in particular, a complete compensation is sought. Otherwise, the BAW resonator has a conventionally known construction. The substrate, in this case the piezoelectric resonator body, may comprise materials such as zinc oxide or aluminum nitride or a PZT ceramic. Preferably, at least one of the metallic electrodes ELI, EL2 is made of a metal which is heavier than aluminum and, for example, of the

Group platinum, molybdenum, tungsten and tantalum is selected.

The BAW resonator can either swing against air on both sides, can be arranged on a membrane or can rest firmly on a carrier substrate, in which case

preferably an acoustic mirror is arranged between the lower electrode EL2 and the carrier material in order to keep the acoustic energy of the shaft within the resonator.

Figure 1D shows a GBAW component (GBAW = guided bulk acoustic wave) with an inventive compensation ^¬ layer KL. The GBAW device comprises a substrate SU, which has at least one piezoelectric layer. Directly above the SU, a first electrode ELI is arranged, which is formed for example as an interdigital electrode. In addition, a compensation layer KL is arranged, which serves as waveguide layer in the GBAW device. Above the

Compensation layer KL is arranged a cladding layer ML, which is selected so that the Wellenausbreitungsgeschwin ^¬ speed in the cladding layer ML is greater than in the

Compensation layer KL. Preferably, the cladding layer is a dielectric. The cladding layer may also be a cladding wafer bonded onto the compensation layer.

The thickness of the compensation layer KL is between 0.1 and 1.0 wavelengths.

As the electrode material for SAW components

known and in particular the above-mentioned heavy metals are used. The electrode EL may comprise further sub-layers selected from adhesive layer, a diffusion barrier layer and passivation layer ^¬ addition to metallic sublayers.

FIG. 1C shows a further embodiment of a GBAW component with a compensation layer KL according to the invention serving as a waveguide. A first electrode ELI is directly on a piezoelectric substrate SU

formed and has, for example, an interdigital structure. Over the compensation layer KL is a second

Electrode EL2 As a full-surface applied layer

arranged, which represents the counter electrode to the first electrode ELI.

Figure 2 shows another SAW device with a

Compensation layer KL according to the invention. In contrast to the exemplary embodiment illustrated in FIG. 1, each compensation layer KL is not planarized, but instead

structured. The topography of the compensation layer KL follows the structure of the electrode fingers and forms recesses between the electrode fingers. The

Structuring can be produced by surface-conforming generation of the compensation layer KL. It is also possible, however, to create the structure later and thereby targeted to design. The structure of the compensation layer KL can improve the reflection at the structural edges of the first electrode ELI, or provide further reflection edges. Since the reflection at the edges of the first electrode ELI, which are covered with the compensation layer KL, is reduced compared to an uncovered electrode, this negative effect of the cover is partially compensated for. FIG. 3 shows a possibility for characterizing Si0 ₂ layers according to the invention which improve the temperature compensation. In a diagram of the FT-IR spectrum of different Si0 is shown fragmentary ₂ materials, in particular the pronounced Omega 4-band (co4 band) is viewed at approximately 1100 cm ^~ l. There are four graphs displayed, in accordance with a

Comparative Experiment V0 represents the IR spectrum of a conventional pure S1O ₂ deposited in a standard PECVD process. The curve according to experiment VI is measured with a Si0 ₂ material according to the invention which has 2 atomic% boron and 2 atomic% phosphorus as doping. The curve according to experiment V2 is measured with a Si0 ₂ material according to the invention, which is doped with 3.9 atom% boron and 4.9 atom% phosphorus. The curve according to the

Experiment V3 corresponds to an Si0 ₂ material doped according to the invention with 4 atomic% phosphorus.

It is found that the material of comparative experiment V0 exhibits its Omega 4-band at the lowest wave number of 1055 cm ^~ l. The first embodiment VI shows its omega 4 band at 1084 cm ^-1 . The second embodiment V2 shows its omega 4 band at 1092 cm ^-1 , as does the third embodiment V3. The omega 3 band in this section of the spectrum also shows a shift shifted from 808 cm ^-1 (VI) to 812 cm ^-1 (V2 and V3), respectively, from the baseline value of Comparative Example VO at 814 cm ^-1 . There

It is assumed that the shift of the omega 4 band is proportional to the expansion of the Si-O-Si bond angle, the absolute shift in the FT IR spectrum according to Figure 3 is a measure of the quality of the respective

Doping with regard to the desired success dar. Of the selected embodiments or the underlying these embodiments dopants in this regard V2 and V3 are most suitable. It is not excluded that even better results with respect to the desired TC compensation can be obtained with other dopants and / or other dopant contents

To check this statement, the measured temperature coefficient TCF of the frequency in FIG

Dependence on the content of a phosphorus doping of

Compensation layer KL based on a selected SAW

Component shown in Figure 1. The ordinate represents the TCF as a dimensionless number. The abscissa indicates the P content of the S1O ₂ layer in percent. It can be seen that the temperature coefficient TCF here in the example chosen beΪ Cd 6% phosphorus content a

undergoes complete compensation, is significantly overcompensated for further increasing phosphorus content and then goes into saturation.

The required dopant content for a complete compensation (TCF = 0) is of course still further

Parameters, in particular the layer thickness of the compensation Layer or dependent on the materials used in the component.

With other dopants or dopant combinations similar possibly slightly different results can be expected.

The invention is not limited to the exemplary embodiments illustrated in the figures. Rather, the includes

Invention arbitrary combinations of the features shown in this invention and this in different ways

Components, including all types of micro-acoustic

Components can be expected.

Reference sign list

SU piezoelectric substrate

ELI, EL2 electrode

KL compensation layer

VO reference example

V1, V2, V3 embodiments

Claims

claims

1. Microacoustic component

- With a piezoelectric substrate

- With an electrode for exciting an acoustic wave

- With a compensation layer, which has an abnormal thermomechanical behavior

wherein the compensation layer comprises glass-like material based on S 1 O 2, in which the Si-O-Si angle in the glass network is increased relative to the bond angle of 144 ° of pure S 1 O 2.

2. Component according to claim 1,

in which includes compensation layer adjacent S 1 O2 as a further constituent glass formers and / or conventional glass supplements selected from B ₂ O _3, S 1 O2, P ₂ O _5, As ₂ O _3, Sb ₂ Ü3, ln ₂ 0 _3, Sn ₂ 0 ₃ , Pb0 ₂ , Li ₂ 0, CaO, Na ₂ O, K ₂ 0, MgO, Rb ₂ 0, Cs ₂ 0, SrO, Te0 ₂ , Se0 ₂ , Mo0 ₂ , W0 ₃ , Bi0 ₃ , A1 ₂ 0 ₃ , BaO, V ₂ 0 and S0 ₃ .

3. Component according to claim 1 or 2,

wherein the compensation layer contains S 1 O 2 and at least one dopant in a proportion of 0.5-8.0 at% selected from boron and phosphorus.

4. Component according to one of claims 1-3,

in which the glassy material of the compensation ^¬ layer in the IR spectrum is characterized by a co4 band in the range between 1060 cm ^-1 and 1100 cm ^-1 .

5. Component according to one of claims 1-4,

where the compensation layer has a thickness of 5-30% based on the wavelength of the device at

Center frequency has.

6. Component according to one of claims 1-5,

designed as a SAW component, in which the as

Interdigital electrode formed electrode between the compensation layer and the piezoelectric substrate is arranged.

7. The component according to claim 6,

in which a trimmable layer is arranged above the compensation layer.

8. The component according to claim 7,

wherein the trimmable layer comprises silicon nitride.

9. The component according to one of claims 1-8, wherein the electrode comprises Cu as the main component.

10. Component according to one of claims 1-9,

in which the device is on a substrate

arranged BAW resonator is, in which the

Compensation layer between the substrate and a bottom electrode of the resonator is arranged.

11. Component according to one of claims 1-10,

where the compensation layer is part of a GBAW

Component is and above that with the electrode

provided substrate is arranged.

12. The component according to one of claims 1-11, comprising - a first filter in which the temperature response of the center frequency by a correspondingly set Waveguide layer is negative and thus overcompensated, and

a second filter electrically connected to the first filter,

in which:

the second filter has a positive temperature characteristic of the center frequency,

- The interconnection of the first and second filter is carried out so that the component has a total total compensation of the temperature coefficient of the center frequency.