DE102016103834B4 - BAW device - Google Patents

Abstract

BAW device with a layer sequence comprising:
a first electrode layer (El),
a dielectric buffer layer (BL), wherein the dielectric buffer layer (BL) is selected from a layer of SiO ₂ , Si ₃ N ₄ , AlN, SiC and Al ₂ O ₃ or of an AlN-based material,
a piezoelectric layer (PL),
a second electrode layer (E2),
and wherein the dielectric buffer layer (BL) covers the entire surface of the underlying layer, the piezoelectric layer (PL) adhering directly to the buffer layer.

Description

Bulk Acoustic Wave (BAW) devices generally include a first electrode, a second electrode, and a piezoelectric thin film deposited between the two electrodes.

Filters may be formed from BAW devices by electrically connecting several BAW resonators in series or in parallel. In this case, the first and second electrodes are patterned, while the piezoelectric layer may be a continuous layer, so that all the BAW resonators of such a BAW device use the same continuous layer which may be etched away in some specific areas to contact from above to get with the lower electrode.

During patterning of the first electrode by applying the electrode in a structured manner or by removing portions of the deposited electrode layer, the surface of the layer on which the first electrode is disposed is uncovered and exposed. Since the piezoelectric layer is deposited on the entire surface, the piezoelectric layer must cover both the surface of the first electrode and exposed portions of the surface underlying the electrode layer. In addition, the piezoelectric layer must cover the edges of the structured first electrode. Accordingly, the piezoelectric layer must be deposited on two different materials and also on an uneven topography. Another structure may be present under the first electrode, which has further topographic steps.

All of these factors are unfavorable to the growth of a piezoelectric layer having a unitary structure without any defects that may be due to the different growth rate and to the topographic steps. Such steps may result in an offset in the layer thickness of the piezoelectric layer growing above and above such a step.

Since the piezoelectric effect of the piezoelectric layer mainly depends on the crystallographic orientation, the piezoelectric property of the piezoelectric layer may suffer from defects and steps of the underlying layer. At least the piezoelectric properties are not homogeneous over the piezoelectric layer. Further, the piezoelectric layer may have cracks in its crystallographic structure which may reduce the reliability of the BAW device by introducing moisture or other chemicals through these cracks.

So far, there have been efforts to avoid these disadvantages. These efforts are focused primarily on providing planar or planar surfaces on which a planar and a more homogeneous piezoelectric layer can be grown.

It has already been proposed to use a chemical-mechanical polishing method, such as in US 6,542,054 B2 described. However, this method can not completely avoid the topographic steps resulting from patterning of the first electrode under the piezoelectric layer. And even with this solution, the piezoelectric layer still has to grow on two different surfaces.

Therefore, it is an object of the present invention to provide a BAW device which avoids the above-mentioned disadvantages and which has a more homogeneous piezoelectric layer which is free of crystallographic defects and which can be manufactured without too much additional effort.

This object is achieved by a BAW device according to claim 1. A method for producing such a BAW device and advantageous embodiments are specified in further claims.

The gist of the invention is to provide a BAW device in which a dielectric buffer layer is interposed between the first electrode layer and the piezoelectric layer. Therefore, such a BAW resonator comprises a layer sequence of a first electrode layer, a dielectric buffer layer, a piezoelectric layer and a second electrode layer. Such a layer sequence is present at least in those areas in which a lower electrode is present. In other areas, the layer sequence may include another lower material, such as a substrate material.

It is advantageous that the buffer layer completely covers the surface under the piezoelectric layer, so that the entire piezoelectric layer can be grown on the buffer layer. As the growth of the piezoelectric layer progresses on a uniform surface of a unitary material, the major disadvantage of two different seed layers is eliminated. Therefore, the growth rate of the piezoelectric layer can be controlled to be uniform over the entire surface, including all topographic steps and all transitions between regions a first electrode layer and regions in which, in addition to the electrode layer, another surface material is exposed where parts of the first electrode layer have been removed or where the first electrode layer has not been deposited. Therefore, different growth rates and different orientations of the growing layer are avoided, which may otherwise lead to the above-mentioned cracks. As a result, a crack-free and homogeneous piezoelectric layer is achieved.

The piezoelectric layer may include a piezoelectric material that may be deposited as a thin film. The material can be selected from PZT (lead zirconium titanate - lead zirconium titanate), zinc oxide or AlN (aluminum nitride). Doped AlN, LiNbO ₃ and LiTaO ₃ are other possible materials.

An adequate dielectric buffer layer that can be used for such a piezoelectric layer may include a metal oxide, a metal nitride, a metal carbide, or porous silicon. All of these dielectric materials can be deposited or grown on any surface having a homogeneous structure, thereby enabling the growth of a homogeneous piezoelectric layer on this buffer layer.

According to one embodiment, the dielectric buffer layer may be an AlN-based material, such as AlScN. Other examples of such dielectrics are known in the art.

In an advantageous embodiment, the dielectric buffer layer is selected from a layer of silicon oxide, silicon nitride, aluminum nitride, silicon carbide and aluminum oxide.

A dielectric buffer layer made of silicon oxide is most preferred. This material has the further advantage that the temperature dependence of the properties of the BAW device can be reduced with such a layer. The reduced temperature dependence of the center frequency of the BAW device is extremely advantageous.

But, as a disadvantage, any dielectric layer deposited between an electrode layer and a piezoelectric layer reduces the coupling between the electrode and the piezoelectric layer so that the effective electromechanical coupling (ie, the distance between the series and parallel resonances of the resonator) is reduced becomes.

Since all of these effects increase with increasing thickness of the dielectric buffer layer, it is necessary to balance the advantageous effects of the better structure and the lower temperature coefficient versus the reduced coupling constant.

A preferred thickness of the dielectric buffer layer, which offers a compromise between advantages and disadvantages, is between 3 nm and 50 nm. Another preferred range is between 5 nm and 45 nm. Another advantageous range is between 3 nm and 20 nm.

BAW devices may be implemented in two different structures to maintain the acoustic energy within the layer sequence of the BAW device. In a first variant, commonly referred to as a thin-film bulk acoustic resonator (FBAR) device, the layer sequence of the FBAR is placed on a membrane suspended over a depression on the substrate. Then, the layer sequence of the BAW / FBAR device deposited on and encompassing the membrane is exclusively in contact with the surrounding atmosphere so that the BAW device can vibrate freely against air and can not dissipate energy by dissipation into a bulk material a substrate is lost.

In a second variant, the acoustic energy is kept within the layer sequence by the use of an acoustic mirror, which is arranged between a substrate and the layer sequence of the BAW device. Such an acoustic mirror functions like a Bragg mirror and comprises alternating layers of a first and a second material. A first material is selected having a relatively high acoustic impedance, and a second material is selected that provides a relatively low acoustic impedance. Typically, metals are used to form the high impedance layers of the acoustic mirror, while dielectric materials are used to form the low impedance layers.

In the first variant, the first electrode (lower electrode) is deposited on the membrane, which may be a single layer or which may have a multilayer structure. The upper layer of the membrane is preferably a dielectric layer or a semiconductor layer.

In the second variant, the first electrode is arranged on the uppermost layer of the acoustic mirror. It is preferred that the uppermost layer is a dielectric layer forming a low impedance layer, that of the first Electrode layer, which usually has a high impedance is adjacent. In this case, the lower electrode layer forms part of the acoustic mirror.

The dielectric buffer layer may cover both the entire surface of the patterned first electrode layer and the areas of the underlying surface exposed during patterning of the first electrode layer. In the regions where the piezoelectric layer is deposited, at least the dielectric buffer layer is present.

• - einen akustischen Spiegel, wobei der Spiegel sich abwechselnde Schichten aus Siliziumdioxid und einem Metall umfasst, wobei die obere Schicht des akustischen Spiegels eine Siliziumoxidschicht ist,
• - eine erste Elektrodenschicht, die Wolfram umfasst,
• - eine dielektrische Pufferschicht aus Siliziumoxid mit einer Dicke zwischen 3 nm und 50 nm,
• - eine piezoelektrische Schicht, die AlN umfasst,
• - eine zweite Elektrodenschicht, die Aluminium umfasst.

In a preferred embodiment, the BAW device of the present invention comprises a silicon substrate on which the layer sequence is deposited. The layer sequence comprises:

• an acoustic mirror, the mirror comprising alternating layers of silicon dioxide and a metal, the upper layer of the acoustic mirror being a silicon oxide layer,
• a first electrode layer comprising tungsten,
• a dielectric buffer layer of silicon oxide with a thickness between 3 nm and 50 nm,
• a piezoelectric layer comprising AlN,
• a second electrode layer comprising aluminum.

A dielectric buffer layer made of SiO ₂ can be easily applied at a low additional cost. Furthermore, the process of applying an SiO ₂ layer can be easily controlled, so that a very uniform layer thickness and layer structure can be achieved.

A piezoelectric layer comprising aluminum nitride (AlN) is deposited on the dielectric buffer layer. AlN grows with a columnar structure that is z-axis oriented. Such a z-axis oriented piezoelectric layer exhibits good piezoelectric properties.

The invention further provides a method of manufacturing the above BAW device. The method begins with the provision of a substrate and at least one layer on the substrate, which is selected from a membrane and an acoustic mirror, depending on the type of BAW device to be manufactured. On this upper layer, a first electrode layer is deposited. On the entire surface of the first electrode layer, a dielectric buffer layer is deposited, whereby the thickness of the buffer layer is controlled. A piezoelectric layer is deposited on the buffer layer before at least a second electrode is deposited on the piezoelectric layer.

For depositing the buffer layer, a vapor deposition method is preferable which allows the thickness to be controlled during the deposition, thereby achieving a uniform thickness of the buffer layer over the entire surface.

The deposition of the piezoelectric layer is performed so as to form a crystal structure having an oriented z-axis. This means that the z-axes of all growing crystal grains are self-aligning in parallel, resulting in columnar growth. All growing grains combine to form a uniform layer with a highly ordered crystal structure that exhibits good piezoelectric properties.

In a second embodiment, the BAW device is formed on an acoustic mirror. Here, the top layer of the acoustic mirror is polished by a chemical / mechanical polishing and planarized before the buffer layer is deposited. Such a polished surface has lower or no topographic steps and provides optimum conditions for the growth of a uniform crystal layer on such a polished surface. Even if the dielectric buffer layer does not have a highly ordered structure, the layer itself grows in a uniform thickness to give a flat surface of the buffer layer.

Controlled growth of a buffer layer is achieved through the use of a MOCVD process (Metal Organic Chemical Vapor Deposition) or a PECVD (Plasma Enhanced Chemical Vapor Deposition) process. In order to achieve uniform deposition, the deposition rate is controlled to be at a low rate.

The piezoelectric layer may be deposited by a sputtering method. Here, the deposition rate of the piezoelectric layer is also controlled so that the piezoelectric layer grows in a z-axis-oriented manner.

In the following, the invention will be explained in more detail with reference to specific embodiments in conjunction with corresponding drawings.

• 1 zeigt eine BAW-Vorrichtung, die mit einem ersten Verfahren hergestellt wurde, in einem schematischen Querschnitt.
• 2 zeigt eine BAW-Vorrichtung, die mit einem zweiten Verfahren hergestellt wurde, in einem schematischen Querschnitt.
• 3 zeigt eine BAW-Vorrichtung, die zwei Elemente umfasst, die mit einem Verbindungselement elektrisch verbunden sind.
• 4 zeigt eine BAW-Vorrichtung gemäß der Erfindung in einem schematischen Querschnitt.
• 5 ist eine Fotografie, die einen Querschnitt durch eine BAW-Vorrichtung gemäß dem Stand der Technik zeigt.
• 6 ist eine Fotografie, die einen ähnlichen Querschnitt durch eine BAW-Vorrichtung gemäß der Erfindung zeigt.

The drawings are only schematic and are not to scale. Therefore no absolute or relative measures can be taken from the drawings.

• 1 shows a BAW device, which was produced by a first method, in a schematic cross section.
• 2 shows a BAW device, which was produced by a second method, in a schematic cross section.
• 3 shows a BAW device comprising two elements which are electrically connected to a connecting element.
• 4 shows a BAW device according to the invention in a schematic cross section.
• 5 is a photograph showing a cross section through a BAW device according to the prior art.
• 6 Fig. 10 is a photograph showing a similar cross section through a BAW device according to the invention.

1 shows a schematic cross section through a BAW device according to the prior art. The BAW device is a resonator made in SMR (Solidly Mounted Resonator) technology. On a substrate SU is an acoustic mirror AS deposited, the at least two layers, that is at least one high impedance layer HI and a low impedance layer LI , includes. The acoustic mirror AS may comprise a larger number of such pairs of layers, each layer having a thickness of about one quarter of the wavelength. The upper layer of the acoustic mirror AS is a low impedance layer LI , On the acoustic mirror AS are a first electrode layer E1 , a piezoelectric layer PL and a second electrode layer E2 arranged on the respective preceding layer. The two electrode layers E1 and E2 and the piezoelectric layer interposed therebetween in the layer sequence PL form the piezoelectric element, which works as a resonator.

2 shows a BAW resonator according to the membrane or FBAR type (FBAR = Thin Film Bulk Acoustic Resonator - acoustic thin-film volume resonator). This resonator is on a membrane M made a depression RC covered on the substrate. The membrane is suspended above the depression. The layer sequence which forms the resonator itself corresponds to that in FIG 1 shown layer sequence.

Some BAW devices require an electrical series and parallel connection of several resonators. 3 shows a schematic cross section through a layer sequence from which two different BAW resonators can be made by structuring and electrical wiring. On the substrate SU becomes a first electrode layer E1 deposited. The layer must be patterned to form individual electrode elements for individual BAW resonators. It is possible to form the first electrode layers in a self-patterning manner by depositing the metal only in the desired regions, for example, by using a mask. According to another method, the electrode layer is structured by a lift-off technique.

The piezoelectric layer PL is deposited over the entire surface of the device to cover the individual first electrodes and the surface of the substrate exposed therebetween. Since the edges of the individual electrodes form topographic steps, the deposited piezoelectric layer follows PL these steps so that other topographic steps form on the surface of the piezoelectric layer.

Similarly, the second electrode layer becomes E2 deposited over the entire surface and then structured, for example by a lift-off technique. Since the piezoelectric layer PL On a surface having topographic steps deposited, structural defects may arise and cracks may form in the piezoelectric layer near the steps. Such a defect can interfere with the piezoelectric function of the BAW device. At the very least, moisture can be introduced through the cracks and can cause corrosion which can reduce the lifetime of the BAW device or functionality.

4 shows a cross section through a BAW device according to the invention. On the substrate SU is an acoustic mirror AS in the form of some alternating high impedance and low impedance layers HI and LI (only one layer of it is in 4 shown). The uppermost layer of the acoustic mirror is a low impedance layer LI , On this upper layer becomes a first electrode layer E1 formed in a structured manner, whereby portions of the upper layer of the acoustic mirror are exposed. The first electrode layer comprises tungsten W and may further comprise an alloy of aluminum and copper, which may form a sub-layer of the first electrode layer. A thin layer of titanium may be deposited as a sub-layer on the underside of the first electrode layer.

According to the invention, a dielectric buffer layer BL over the entire surface of the device, exposing exposed areas of the exposed acoustic mirror and the top of the first electrode layer E1 to be covered. A PECVD process or MOCVD process is used to deposit the dielectric buffer layer with well-controlled thickness and homogeneity. Preferably, silicon oxide is deposited as a dielectric. On the dielectric buffer layer BL becomes a piezoelectric layer PL deposited over the entire surface of the buffer layer. A sputtering process is used to form the piezoelectric layer PL which, in a preferred embodiment comprises aluminum nitride, to precipitate. On the piezoelectric layer PL becomes a second electrode layer E2 formed, which comprises a good conductive metal, such as aluminum.

The BAW device shown forms a resonator. In an active region of the resonator, all layers of the layer sequence overlap first electrode layer E1 , Buffer layer BL , piezoelectric layer PL and second electrode layer E2 , By structuring the first electrode layer E1 and the second electrode layer E2 individual electrodes of a plurality of BAW resonators are formed. By electrically connecting these individual resonators, a BAW device may be formed comprising a plurality of electrically connected BAW resonators. The electrical connection between adjacent BAW resonators can be carried out in an integrated manufacturing process by dividing parts of the piezoelectric layer PL are removed to reveal areas of the first electrode layer below. The electrical connection may be made, for example, by depositing another interconnection layer on the second electrode layer and the exposed first electrode layer.

5 and 6 are photographs of a cross-section through a BAW device according to the prior art (as in 3 shown) and by a BAW device according to the invention (as in 4 shown).

In both cases, the same substrate structure is used, with a topographical stage TS exists over which the piezoelectric layer has been deposited. 4 shows that a crack CR in the piezoelectric layer PL formed just above the topographic stage of the substrate.

6 differs from 5 through the additional buffer layer overlying the first electrode layer E1 was deposited before the piezoelectric layer PL is deposited. The photograph clearly shows that no crack has formed, although the same substrate with the same topographic stage TS and the same deposition conditions were used.

Consequently, the proposed BAW device according to the invention has superior crystallographic properties in combination with good piezoelectric properties and a uniform structure of the piezoelectric layer. In addition, when a dielectric layer made of silicon oxide is used, the temperature coefficient of the BAW device becomes larger than that of the BAW devices 3 and 5 reduced. Furthermore, this results in an improved lifetime of the BAW device because a defect-free BAW device is much less susceptible to corrosion than a corresponding structure with cracks and other defects.

LIST OF REFERENCE NUMBERS

AS

acoustic mirror

BL

dielectric buffer layer

CR

Crack

E1, E2

first and second electrode layers

M

membrane

PL

piezoelectric layer

RC

deepening

SU

substratum

TS

topographic level

Claims (9)

A BAW device having a layer sequence, comprising: a first electrode layer (El), a dielectric buffer layer (BL), wherein the dielectric buffer layer (BL) consists of a layer of SiO ₂ , Si ₃ N ₄ , AlN, SiC and Al ₂ o ₃ or is selected from an AlN-based material, - a piezoelectric layer (PL), - a second electrode layer (E2), and wherein the dielectric buffer layer (BL) covers the entire surface of the underlying layer, wherein the piezoelectric layer ( PL) adheres directly to the buffer layer. BAW device after Claim 1 wherein the piezoelectric layer (PL) comprises one of PZT, ZnO, AlN, doped AlN, LiNbo ₃ or LiTao ₃ , wherein the dielectric buffer layer (BL) comprises a metal oxide, a metal nitride, a metal carbide or porous silicon. BAW device according to one of the preceding claims, wherein the dielectric buffer layer (BL) has a thickness between 3 nm and 50 nm or between 5 nm and 45 nm. A BAW device according to any one of the preceding claims, comprising a substrate (SU) and an acoustic mirror (AS) disposed between the substrate and the first electrode layer (Ei), the acoustic mirror comprising alternating layers of relatively high (S). HI) and relatively low (LI) acoustic impedance, wherein the dielectric buffer layer (BL) covers the entire surface on which the piezoelectric layer (PL) has been deposited. BAW device according to one of Claims 1 to 3 comprising a substrate (SU) having a recess (RC) on its upper surface and a membrane (M) suspended to cover the recess, the layer sequence being applied to the membrane. A BAW device according to any one of the preceding claims, comprising a silicon substrate (SU) on which the layer sequence is deposited, the layer sequence comprising: - an acoustic mirror (AS) comprising alternating layers of SiO ₂ and a metal, wherein the upper layer of the acoustic mirror is a Sio ₂ layer, - a first electrode layer (E1) comprising tungsten, - a Sio ₂ dielectric buffer layer (BL) having a thickness between 3 nm and 50 nm, - a piezoelectric layer (PL) comprising AlN, - a second electrode layer (E2) comprising Al. Method for producing the BAW device according to Claim 1 comprising the steps of: depositing a first electrode layer on a membrane (M) or on the upper layer of an acoustic mirror (AS), depositing a dielectric buffer layer (BL) with a controlled thickness on the entire surface of the first electrode layer ( E1), depositing a piezoelectric layer (PL) on the buffer layer (BL), depositing a second electrode layer (E2) on the piezoelectric layer, depositing the buffer layer (BL) with a controlled uniform thickness from the gas phase, the dielectric buffer layer (BL) is selected from a layer of SiO ₂ , Si ₃ N ₄ , AlN, SiC and Al ₂ O ₃ or AlN-based material, wherein the piezoelectric layer (PL) is in a c-axis oriented manner wherein the deposited dielectric buffer layer (BL) covers the entire surface of the underlying layer, and the piezoelectric layer (PL) adheres directly to the buffer layer. Method according to Claim 7 wherein the surface to which the buffer layer (BL) is to be deposited is polished and planarized by a chemical / mechanical polishing process before the buffer layer is deposited. Method according to Claim 7 or 8th in which the buffer layer (BL) is deposited by MOCVD or PECVD at a controlled deposition rate, - wherein the piezoelectric layer (PL) is deposited by a sputtering process, - the deposition rate of the piezoelectric layer is controlled.

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