WO2003043188A1

WO2003043188A1 - Passivated baw resonator and baw filter

Info

Publication number: WO2003043188A1
Application number: PCT/EP2002/011425
Authority: WO
Inventors: Robert Aigner; Guenter Ehrler; Lueder Elbrecht; Andreas Meckes; Winfried Nessler; Alfred Niklas; Hans-Joerg Timme
Original assignee: Infineon Technologies Ag
Priority date: 2001-11-14
Filing date: 2002-10-11
Publication date: 2003-05-22
Also published as: DE10155927A1

Abstract

The invention relates to a BAW resonator comprising a substrate (102), a first electrode (108) which is arranged on a surface (106) of the substrate (102), a piezoelectric layer (112) which is at least partially arranged on the first electrode (108), and a second electrode (114) which is arranged on at least part of the piezoelectric layer (112) in such a way that it at least partially overlaps the first electrode (108). Furthermore, a passivation layer (128) is provided on the second electrode (114) in order to protect the same.

Description

description

Passivated BAW resonator and BAW filter

The present invention relates to BAW resonators

(BAW = Bulk Acoustic Wave = acoustic bulk waves). In particular, the present invention relates to a passivated BAW resonator. Furthermore, the present invention relates to BAW filters which include such BAW resonators.

BAW resonators can be used in filters in high-frequency technology, for example, where they can replace existing SAW filters (SAW = Surface Acoustic Wave = acoustic surface wave) or ceramic filters. An exemplary filter configuration is the bandpass filter, which is used, among other things, in mobile communication devices. For this application, it is necessary for SAW filters to be hermetically installed in a housing.

This applies equally to BAW filter arrangements, such as BAW bandpass filters. Hermetic housings, e.g. Ceramic housings, however, are disadvantageous for BAW filter arrangements, since they can only be manufactured with great effort. Therefore, non-hermetic housings, e.g. Plastic housings common in HF applications, preferred to hermetic housings, since they are much easier, smaller and cheaper to manufacture. Since it is the housing that significantly influences the filter production costs, the use of non-hermetic housings is desirable and imperative in order to be able to further save production costs in the future.

BAW filters consist of circuits that were built using BAW resonators. A BAW resonator is basically a piezoelectric layer that is between two E- electrodes is arranged. Both electrodes consist of a single metal layer or a multilayer metallization. Typical piezoelectric materials for the piezoelectric layer (active layer) are PZT (lead zirconium titanate), ZnO (zinc oxide) and A1N (aluminum nitride). Typical metals used for the electrodes include e.g. B. AI (aluminum) and W (tungsten). The electrodes of a BAW resonator preferably have a high conductivity in order to conduct a resonator current without significant ohmic losses (parasitic effects). However, since the electrodes not only have an electrical function, but at the same time also determine the acoustic properties of the resonator, they cannot be optimized solely from an electrical point of view. So would be B. Sufficiently thick Al layers are suitable for minimizing ohmic (parasitic) losses if, on the other hand, they do not have important properties such as, for example, the load on the resonator. B. would worsen the bandwidth. The electrodes also influence the resonance frequencies of a BAW resonator. If the BAW resonator is used in a non-hermetic housing, reliability problems can arise. This is especially true when the top layer of the top electrode is made of Al or another base metal. In contrast to hermetic housings, moisture enters into non-hermetic housings. , This moisture itself does not have to attack the electrode. However, condensation of moisture occurs in the non-hermetic housing, e.g. B. causes water droplets to form on the electrode. With AI electrodes z. B. there is a reaction (corrosion) which leads to decomposition of the electrode, which in turn leads to a change in the resonator properties (frequency, quality, etc.).

In order to avoid the problems just mentioned, only filter arrangements, BAW filters or SAW filters are known in the prior art, in which the filter circuit have a hermetic housing, which results in the problems set out above brings with it, namely the high production costs, as well as the effort associated with the production.

Another problem with water droplets arises during the manufacture of the BAW resonators, in which a large number of individual BAW resonators are generally formed on a wafer and are separated at the end of the manufacture. Here, the wafers are sawn using cooling or rinsing water, which causes the problems outlined above when it hits the electrodes.

Starting from this prior art, the present invention has for its object to provide a BAW resonator that has a high level of reliability without a hermetic housing.

This object is achieved by a BAW resonator according to claim 1.

The present invention provides a BAW resonator with

a substrate,

a first electrode arranged on a surface of the substrate;

a piezoelectric layer that is at least partially arranged on the first electrode;

a second electrode which is arranged at least partially on the piezoelectric layer and at least partially overlapping with the first electrode; and

a passivation layer which is arranged on the second electrode in order to protect the second electrode. According to one aspect of the present invention, a BAW filter is created which comprises one or more of the BAW resonators according to the invention.

The present invention is based on the finding that the required protection for the electrode can be achieved by adding an acoustically thin passivation film to the surface of the upper electrode of a BAW resonator. This additional layer is not required to achieve proper functioning of the BAW resonator, nor is it necessary to achieve reliable operation when the BAW resonator is housed in a hermetic housing, such as in a ceramic housing with a soldered or welded metal lid.

According to the invention, a hermetic housing can be dispensed with, and nevertheless optimal protection of the BAW resonator can be achieved by protecting the upper electrode of the BAW resonator with a thin passivation film. Silicon oxide or silicon nitride or TiN (titanium nitride) is advantageously used as the material for this passivation film. Precious metals, such as B. gold or platinum can also be used. It is important that the passivation film is quite thin in order to avoid a deterioration in the resonator behavior, in particular in the bandwidth (mass charge effect). The passivation film is preferably already taken into account in the design of the BAW resonator in order to keep its acoustic influence on the resonator behavior low or to take it into account. The thickness of the passivation layer is preferably between 20 nm and 200 nm.

According to a preferred exemplary embodiment of the present invention, aluminum nitride (A1N) is used as the piezoelectric material for the piezoelectric layer, which is provided with opposite aluminum electrodes (where both electrodes can also be multi-layer electrodes in which different materials are used). A silicon nitride layer is provided as the upper passivation layer, a typical thickness of this silicon nitride layer preferably being between 20 nm and 100 nm.

The present invention offers the advantage that the upper electrode of a BAW resonator withstands environmental influences even in the unhoused or in the non-hermetically sealed state. Furthermore, due to the protective effect of the upper layer, the high conductivity of the electrode underneath is retained, so that the resonator current can be conducted without significant losses.

In addition, the upper electrode is also protected during process steps during the manufacture of the individual BAW resonators, such as, for example, against electrochemical corroding due to the water that is used when sawing the wafers.

According to a further exemplary embodiment of the present invention, the applied passivation layer can be used to set a frequency of the BAW resonator to a desired target frequency. , Likewise, the passivation layer can be used to set a detuning with respect to a desired frequency in a BAW resonator, as is required for filter arrangements that use a plurality of BAW resonators. One example is the so-called ladder topology for bandpass filters, in which all parallel resonators are detuned from the series resonators in order to achieve the desired bandpass filter effect. Essentially, the so-called parallel resonance of the parallel resonators must correspond to the so-called series resonance of the series resonators, ie the frequency detuning between the series and parallel resonators essentially corresponds to the resonator Bandwidth (the frequency distance between the two resonance frequencies of a resonator).

Preferred developments of the present invention are defined in the subclaims.

Preferred exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. Show it:

1 shows a first exemplary embodiment of a BAW resonator according to the invention; and

2 shows a second exemplary embodiment of a BAW resonator according to the invention.

1 shows a BAW resonator according to the invention, which is designated in its entirety by reference number 100. The BAW resonator 100 comprises a substrate 102, which has a first, lower main surface 104 and a second, upper

Main surface 106 includes. A first, lower electrode 108 is formed on the second main surface 106 and is made of aluminum in the exemplary embodiment shown. Furthermore, an insulating section 110 is shown, which is also on the upper main surface 106 of the

Substrate 100 is arranged. A piezoelectric layer 112, which is an AIN layer in the exemplary embodiment described, is applied to a section of the lower electrode 108 and to the insulating section 110. A second, upper electrode 114, also made of aluminum, is formed on a section of the side of the piezoelectric layer 112 facing away from the substrate 100. The BAW resonator or the active region thereof is formed by the region of the piezoelectric layer 112 in which the lower electrode 108 and the upper electrode 114 overlap. As can also be seen in FIG. 1, the lower electrode 108 comprises a section which extends from the piezoelectric layer 112, that is to say is not covered by the same. In this area, a first connection 116 (input or output) is provided, via which the BAW resonator 100 can be connected with a wire 118 (optional). Like the lower electrode 108, the upper electrode 114 is also pulled out in a section, this section lying opposite the insulating section 110. In this area, a second connection 120 (output or input) is provided, via which the BAW resonator 100 can be connected via a wire 122 (optional). The BAW resonator is electrically connected to other components via the connections 116 and 120.

As can also be seen in FIG. 1, the substrate comprises a reflector section 124, in which an acoustic reflector 126 is arranged, which has a plurality of individual layers 126a to 126c, which alternately have a high acoustic impedance and a low acoustic impedance. By means of the acoustic reflector 126, the BAW resonator arrangement arranged above is acoustically decoupled from the portions of the substrate 102 lying under the reflector 126.

In order to protect the upper electrode 114 and to thereby be able to dispense with the use of hermetic housings according to the invention, the surface of the upper electrode 114 facing away from the piezoelectric layer 112 is covered with a passivation layer 128. Basically, it is sufficient to use only the upper surface of the electrode 114

Cover passivation layer. Due to the dimension of the BAW resonator, the layer thicknesses of the layers 108, 112, 114 are usually in the um or nm range, the flank regions are not very critical and do not necessarily have to be covered by the passivation layer. However, as in the exemplary embodiment shown in FIG. 1, the passivation layer 124 can also cover the entire exposed surface of the Cover layer sequence 110, 112. The contacts 116, 120 are preferably formed after the passivation layer has been applied by exposing corresponding areas in the same. The passivation layer 128 is a silicon nitride layer in the exemplary embodiment shown.

In the exemplary embodiment shown in FIG. 1, an aluminum nitride material was used for the piezoelectric layer, which does not deteriorate in moist environments, and is in particular corrosion-resistant. Aluminum is also used as the material for the electrode layers, which is readily available in standard semiconductor manufacturing processes, offers high conductivity and can also be used as pad metallization (see FIG. 1).

Other exemplary embodiments include the cases in which the lower electrode 108 is made of a different material or is a multilayer electrode, and the cases in which the upper electrode 114 is made of a different material or is a multilayer electrode.

A typical thickness of the silicon nitride film is between 20 nm and 200 nm.

In order to ensure the reliability of the passivation layer 128, comparative tests were carried out (pressure cooker test). Here, a BAW resonator or a BAW filter arrangement, which comprises a plurality of BAW resonators, each comprising upper electrodes made of aluminum without a passivation layer, was initially enclosed in a non-hermetic housing. The non-hermetic isolation of the individual BAW resonators led to corrosion of the upper aluminum electrodes due to the precipitation occurring in the housing, so that the BAW filters housed in this way were no longer functional because their filter characteristics had changed. With an identical BAW Filters with a plurality of BAW resonators without a passivation layer, which was arranged in an open, non-hermetic housing, the above-described corrosion of the upper electrodes was not ascertained, since there were no formation of droplets due to condensation. In the case of a BAW filter of the same construction with a plurality of BAW resonators with the passivation layer according to the invention, which was arranged in a non-hermetic housing, the above-described corrosion of the upper electrodes was not found.

In general, the present invention offers the advantage that only a thin film, e.g. B. of silicon nitride, is sufficient to ensure very good protection of the upper electrode. This thin passivation film is also acoustically thin, i.e. it only influences the resonance behavior of the resonator to a small extent. Another advantage of using the very thin passivation film is that it can be used specifically to improve the temperature coefficient for temperature shifts (TCF).

The approach taught by the present invention to use passivation layers in BAW resonators has never been pursued, since it has always been assumed that passivation layers influence the acoustic behavior of the BAW resonators too strongly, and in particular significantly worsen the bandwidth. The present invention teaches a very thin passivation film, which develops the required protective effect, but has essentially no influence on the acoustic properties of the BAW resonator. In contrast to thick passivation stacks made of silicon nitride and silicon oxide, as z. For example, when using standard CMOS processes, the main aspect of the passivation layers on a BAW filter is not to prevent alkali ions from diffusing into the substrate, but rather to prevent corrosion of the upper electrode. be prevented. A certain diffusion rate and even so-called pinhole defects are acceptable as long as no corrosion of the electrode is found.

A second preferred exemplary embodiment of the present invention is explained in more detail below with reference to FIG. 2, similar or identical elements which have already been described with reference to FIG. 1 being provided with the same reference numerals and not being explained again in more detail. 2 shows a BAW resonator 200, which in turn comprises a substrate 102, on the upper surface 106 of which a first electrode 108 is formed. The piezoelectric layer 112, on which the upper electrode 114 of the BAW resonator was produced, is in turn formed on this electrode. In contrast to the exemplary embodiment shown in FIG. 1, in the exemplary embodiment shown in FIG. 2, the passivation layer 128 is completely deposited on the surface of the arrangement, so that, in addition to the upper electrode 114, the exposed sections of the upper surface 106 of the substrate 102 and the side walls of the

Layer stacks 108, 112, 114 are covered by the passivation layer 128. This deposition of the passivation layer is to be preferred in terms of design and production technology, since it enables complete protection of the wafer or chip surface to be achieved in one operation (the contact surfaces of the pads of course having to be opened).

In contrast to the exemplary embodiment shown in FIG. 1, there is no acoustic reflector for decoupling the active region of the resonator from a substrate region, but rather a recess 130 is formed here in the lower surface 104 of the substrate 102 in order to fix a membrane region 132, whereby the resonator region is acoustically decoupled from the substrate 102. 2, the membrane area 132 consists of a

Support membrane made of the substrate material on which the actual resonator consists of a piezoelectric layer with lower and upper electrodes. Such arrangements can be manufactured using the so-called volume micromechanics (bulk micromachining). Alternatively, however, can also be used membrane structures, in which the support membrane consists of a thin deposited layer such as polysilicon or silicon nitride, and the ent by a thin cavity on the substrate material ^¬ are coupled. Such membrane solutions can be produced by means of surface micromachining. Furthermore, membrane structures are possible in which the membrane consists purely of the piezoelectric material including the lower and upper electrodes and dispenses with a carrier membrane.

In addition to the passivation of the upper electrode described in detail above to protect it from corrosion and other influences, the passivation layer 128 can also be used to adjust or to detune the resonance frequencies of the BAW resonator. For this it is generally necessary to adjust the thickness of the passivation layer accordingly. Although preferred exemplary embodiments of the present invention were described with reference to FIGS. 1 and 2, in which aluminum was used as the electrode material, silicon nitride as the passivation material and aluminum nitride as the piezoelectric layer material, the present invention is not restricted to these materials.

The passivation layer can generally be produced from an oxide layer, a nitride layer, a combination thereof or from a noble metal. The passivation layer preferably consists of silicon oxide, silicon nitride, A1 ₂ 0 ₃ , Ta ₂ 0 ₃ , TiN, Au or Pt. With regard to the use of titanium nitride (TiN), Au and PT, it is pointed out that these are conductive materials which, in order to avoid short circuits, correspond to the The shape of the electrode 114 to be protected must be structured.

Furthermore, it was described above that the passivation layer preferably has a thickness of approximately 20 nm to 200 nm. These thicknesses can also be larger or smaller as long as it is ensured that the applied passivation layer does not impermissibly impair the acoustic properties of the BAW resonator.

Although exemplary embodiments have been described with reference to FIGS. 1 and 2, in which the piezoelectric layer and the individual electrodes have been described as individual layers, the present invention is not restricted to this embodiment. The piezoelectric layer 112 can be formed, for example, by a first layer and a second layer, the first layer comprising a piezoelectric material with a first orientation and the second layer comprising a piezoelectric material with a second orientation u, the directions of orientation of the two materials being opposite are. The two layers in the layer sequence 112 are acoustically coupled. Instead of the two different layers, the piezoelectric layer can consist of one material, e.g. B. PZT, which was grown in such a way that an orientation is directed in a first direction in a first section, and an orientation is directed in a second direction opposite to the first direction in a second section. Instead of the only two layers, the piezoelectric layer 112 can also comprise a plurality of first and second layers or first and second sections which are alternately acoustically coupled to one another.

Furthermore, instead of the single-layer electrodes 108 and 114, multi-layer electrodes can also be used, which then have different materials, e.g. B. Materials with different acoustic impedance (e.g. AI, W) alternately.

LIST OF REFERENCE NUMBERS

100 BAW resonator

102 substrate 104 lower main surface of the substrate

106 upper major surface of the substrate

108 lower electrode

110 insulating section

112 piezoelectric layer 114 upper electrode

116 first connection

118 wire

120 second connection

122 wire 124 reflector section

126 acoustic reflector

126a - 126c individual layers of the acoustic reflector

128 passivation layer

130 recess in the substrate 132 membrane area

Claims

claims

1. BAW resonator with

a substrate (102);

a first electrode (108) disposed on a surface (106) of the substrate (102);

a piezoelectric layer (112) disposed at least partially on the first electrode (108);

a second electrode (114) arranged at least partially on the piezoelectric layer (112) and at least partially overlapping with the first electrode (108); and

a passivation layer (128) disposed on the second electrode (114) to protect the second electrode (114).

2. BAW resonator according to claim 1, wherein the passivation layer (128) is an acoustically thin layer which does not substantially affect the resonator properties.

3. BAW resonator according to claim 1 or 2, wherein the passivation layer (128) comprises an oxide layer, a nitride layer, a combination thereof or a noble metal.

4. BAW resonator according to claim 3, wherein the passivation layer (128) consists of a material selected from a group comprising silicon oxide, silicon nitride, A1 ₂ 0 ₃ , Ta ₂ 0 ₃ , TiN, gold or platinum.

5. BAW resonator according to claim 4, wherein the passivation layer (128) comprises a structured TiN, gold or platinum layer.

6. BAW resonator according to one of claims 1 to 5, wherein the passivation layer (128) has a thickness of about 20 nm to about 200 nm.

7. BAW resonator according to claim 6, wherein the passivation layer (128) has a thickness of about 20 nm to about 100 nm.

8. BAW resonator according to one of claims 1 to 7, wherein the passivation layer (128) is arranged around the second

Separate the electrode (114) from the environment.

9. BAW resonator according to one of claims 1 to 7, in which the passivation layer (128) is arranged around the exposed portions of the first electrode (108), the piezoelectric layer (112), the second electrode (114) and / or to cover the substrate (102).

10. BAW resonator according to one of claims 1 to 9, wherein the passivation layer (128) after application of the same is adjustable to a predetermined thickness in order to set a frequency of the BAW resonator to a target frequency or to detune the BAW Adjust resonators against a predetermined frequency.

11. BAW resonator according to one of claims 1 to 10, wherein the substrate (102) comprises an acoustic reflector (126) or a membrane region (132) in order to acoustically separate the piezoelectric layer (112) from the substrate (102) ,

12. BAW resonator according to one of claims 1 to 11, wherein the piezoelectric layer (112) a first layer made of a piezoelectric material, which is oriented in a first direction, and a second layer made of a piezoelectric material, which in a second direction, which is opposite to the first direction, comprises, the first and the second layer acoustically coupled to one another pelt, or in which the piezoelectric layer (112) comprises at least two acoustically coupled sections with opposite orientation.

13. BAW resonator according to claim 12, wherein the piezoelectric layer (112) comprises a plurality of first layers and a plurality of second layers which are alternately acoustically coupled to one another.

14. BAW resonator according to one of claims 1 to 13, wherein the first electrode (108) and / or the second electrode (114) comprise a plurality of layers.

15. BAW resonator according to claim 14, wherein the first electrode (108) and / or the second electrode (114) aluminum and

Include tungsten.

16. BAW filter with one or more BAW resonators (100; 200) according to one of claims 1 to 12.