Electroacoustic resonator and method for manufacturing the same

Abstract

An electroacoustic resonator and a method for making the same. An electroacoustic resonator comprises an acoustic reflector (120), a bottom electrode (130) and a piezoelectric layer (140) arranged on a carrier substrate (110). A structured silicon dioxide petal layer (150) is arranged on the piezoelectric layer (140), the two layers having a common contact surface. Arranging the silicon dioxide (150) directly on the piezoelectric layer (140) increases the quality factor of the resonator and leads to enhanced RF filter performance.

Description

Technical Field

[0001] The present disclosure relates to an electroacoustic resonator. Specifically, the present disclosure relates to an electroacoustic resonator comprising a piezoelectric layer sandwiched between a bottom electrode and a top electrode. The present disclosure also relates to a method of manufacturing such an electroacoustic resonator. The present disclosure also relates to an RF filter comprising several resonators. Background Art

[0002] Electroacoustic resonators are widely used in electronic devices to perform frequency selection functions. A bulk acoustic wave (BAW) resonator comprises a piezoelectric layer sandwiched between a bottom electrode and a top electrode. An electrical signal applied to the electrodes generates a resonant acoustic wave in the piezoelectric layer that is frequency selective to the electrical signal. An RF filter comprising several electroacoustic resonators can be used in communication devices to select a desired signal from the received signal spectrum or to shape the spectrum to be transmitted.

[0003] A dielectric layer disposed on the piezoelectric layer surrounding the acoustically active region creates step features on the surface of the piezoelectric layer to substantially confine the acoustic energy within the active region and prevent acoustic waves from escaping the active region.

[0004] An RF filter comprising a plurality of BAW resonators arranged in a trapezoidal structure can be particularly useful for prior art communication services, such as the 4G (LTE) communication standard. As communication equipment manufacturers are constantly seeking to improve the quality of their equipment, there is a need to reduce the passband characteristics of the RF filter, for example, to increase transmission or reduce attenuation within the passband portion of the RF filter. Communication equipment manufacturers would even welcome a modest improvement in filter performance. Thus, there is a need to increase the quality factor of the BAW resonator and improve the passband performance of the RF filter.

[0005] An object of the present disclosure is to provide an electroacoustic resonator of the bulk acoustic wave type exhibiting an improved quality factor.

[0006] Another object of the present disclosure is to provide a method for manufacturing an electroacoustic resonator exhibiting an improved quality factor.

[0007] Yet another object of the present disclosure is to provide an RF filter with improved passband performance. Summary of the invention

[0008] One or more of the above-mentioned objects are achieved by the electroacoustic resonator according to the embodiments of the present disclosure.

[0009] According to various embodiments, an electroacoustic resonator includes a carrier substrate and an acoustic reflector disposed thereon. A sandwich of a bottom electrode, a piezoelectric layer, and a top electrode is disposed on the acoustic reflector, thereby forming an acoustic active region in an overlapping region of the bottom electrode and the top electrode. A silicon dioxide layer is disposed on a portion of the piezoelectric layer. The silicon dioxide layer is structured and surrounds the acoustic active region, wherein a portion of the silicon dioxide layer in the active region has been removed.

[0010] The structured silicon dioxide layer and the piezoelectric layer have a common contact surface. The silicon dioxide layer is directly disposed on the piezoelectric layer without an intermediate layer, such as a seed layer, for example, a metal or titanium seed layer. The piezoelectric layer and the structured silicon dioxide layer are in direct mechanical contact with each other. By direct deposition of the silicon dioxide layer on the piezoelectric layer, the electroacoustic characteristics of the resonator are improved. The results show that the quality factor of the electroacoustic resonator is improved so that an RF filter with several of these resonators (for example, in a ladder filter structure) has less attenuation and higher transmission in the passband frequency region of the filter.

[0011] A silicon dioxide layer is deposited on a wafer and structured using a mask and photolithography step to remove portions that subsequently form a top metal electrode. In addition, an overlapping layer may extend into the region of the removed portion of the silicon dioxide layer. The structured silicon dioxide layer surrounds the region in which the top electrode is disposed. The structured silicon dioxide layer is often referred to as a petal layer. One of the functions of the structured silicon dioxide layer is to confine acoustic energy within the active region and substantially prevent acoustic energy from leaking out of the active region. The structured silicon dioxide layer adds additional mass to the piezoelectric layer surrounding the active region so as to change the acoustic characteristics in the region where the silicon dioxide petals are present, thereby causing an energy confinement effect.

[0012] Thus, the top electrode is made of metal and can be a stack of layers. The top electrode can include a layer stack, which includes a bottom layer, which is opposite and adjacent to the piezoelectric tungsten layer; an intermediate aluminum-copper layer disposed thereon; and a top metal nitride (e.g., titanium nitride) layer disposed thereon. The aluminum-copper layer can be formed by sputtering technology using an AlCu target. In one example, the bottom electrode can have the same layer as the top electrode. The layer stack of the top electrode is surrounded by a structured silicon dioxide layer. The electrode forms an acoustically active region within the piezoelectric layer, wherein the piezoelectric layer is sandwiched by the bottom electrode and the top electrode.

[0013] According to various embodiments, an overlay layer may be disposed on a structured silicon dioxide layer. The overlay layer may extend from the top surface of the silicon dioxide layer to the portion of the silicon dioxide layer removed in the acoustically active region. The overlay layer extends from the top surface of the silicon dioxide layer to the piezoelectric layer in the acoustically active region along the vertical sidewalls of the silicon dioxide layer. In the acoustically active region, a metal overlay layer is disposed between a top electrode and the piezoelectric layer. The top electrode covers the active region, which includes a portion of the petal layer and a portion of the overlay layer. The metal overlay layer may include a layer stack of a titanium layer disposed on the silicon dioxide layer and a tungsten layer disposed on the titanium layer. Both layers of the layer stack may be removed within a portion of the acoustically active region. It is also possible to remove only the top tungsten layer and maintain the bottom titanium layer of the overlay layer as an adhesion promoter for subsequently forming a top electrode.

[0014] The piezoelectric layer can be made of piezoelectric aluminum nitride, which can be crystalline columnar aluminum nitride; or can be made of aluminum scandium nitride. Other piezoelectric materials are also useful. The scandium portion within the aluminum scandium nitride piezoelectric layer can be in the range of 0 weight % to about 35 weight %. Specifically, the scandium portion can be in the range of 5 weight % to 15 weight %, and more specifically, in the aluminum scandium nitride layer, the scandium portion can be 7 weight %.

[0015] One or more of the above-mentioned objects are also achieved by a method according to an embodiment of the present disclosure.

[0016] The manufacture of an electroacoustic resonator includes: providing a carrier substrate on which an acoustic reflector is arranged. A structured bottom electrode is formed on the acoustic reflector by depositing a layer or layer stack of a metal bottom electrode and forming an electrode structure by masking and photolithography steps. A piezoelectric layer is deposited and extends on the workpiece as a bulk layer. Then, a silicon dioxide layer is formed on the piezoelectric layer, thereby establishing a common contact surface between the piezoelectric layer and the silicon dioxide layer. A portion of the silicon dioxide layer is removed in a structuring step using a mask and photolithography step in an area opposite to the bottom electrode. The piezoelectric layer is exposed at the portion where the silicon dioxide layer is removed. A top electrode is formed on the piezoelectric layer in an area where the piezoelectric layer is arranged so that the acoustic active area is established by the layer stack of the bottom electrode, the piezoelectric layer and the top electrode, the layer stack including a portion of the electrode above the petal layer.

[0017] According to various embodiments, a silicon dioxide layer is formed on the piezoelectric layer without an intermediate layer such as a metal seed layer, for example, a titanium seed layer as may be used in conventional resonators.

[0018] The silicon dioxide layer can be formed by physical vapor deposition, in which argon ions are used to bombard a target such as a silicon target. The reaction chamber includes oxygen as a reaction gas to deposit silicon dioxide on the surface of the piezoelectric layer. Other deposition techniques are also useful, such as chemical vapor deposition, which uses TEOS or silane gas in a reaction chamber to deposit a silicon dioxide layer on the surface of the piezoelectric layer. The density of the PVD deposited silicon dioxide layer is higher than the density of the CVD deposited silicon dioxide layer, so that the speed of sound within the PVD silicon dioxide layer is different from the speed of sound in the CVD silicon dioxide layer. The speed of sound in the PVD layer is higher than the speed of sound in the CVD layer. The results show that PVD deposition of the silicon dioxide layer improves the quality factor of the resonator when compared to a resonator using a CVD silicon dioxide layer and the resonator is used to enhance the performance of a ladder-type RF filter.

[0019] According to various embodiments, the deposition of the piezoelectric layer and the deposition of the silicon dioxide layer are performed in situ, where both deposition processes occur without breaking the vacuum. The deposition process can use two chambers connected under vacuum. The wafer can be transferred without breaking the vacuum between the chambers. The in-situ process avoids the parasitic formation of a native oxide layer on the piezoelectric layer and ensures that the silicon dioxide layer is directly disposed on the piezoelectric layer without any intermediate layer. The results show that the ex-situ process also obtains good results comparable to the in-situ process and increases the quality factor of the resonator and the performance of the ladder-type RF filter, wherein the processed workpiece is transferred from the chamber for depositing the piezoelectric layer to another chamber for depositing the silicon dioxide layer.

[0020] According to various embodiments, after structuring of the silicon dioxide layer and before formation of the top electrode, an overlay layer is formed. The overlay layer may be made of a metal, such as a layer stack of titanium and tungsten. At least a portion of the overlay layer is removed in the region opposite the bottom electrode, wherein another portion of the overlay layer remains in the region opposite the bottom electrode, so that the overlay layer is disposed between the piezoelectric layer and the top electrode.

[0021] One or more of the above-mentioned objects are also achieved by the RF filter according to the embodiments of the present disclosure.

[0022] The RF filter includes a series path coupled between a first filter port and a second filter port. The series path includes a series connection of several electroacoustic resonators described above. One or more shunt paths or parallel paths are provided, which are coupled between at least one resonator in the resonator of the series path and a terminal for a reference potential such as a ground potential. The shunt path includes at least one electroacoustic resonator as described above. The RF filter exhibits a ladder structure. The series path may include four resonators connected in series, wherein four shunt paths are provided. The resonators of the series path and the shunt path may have different resonant frequencies. In fact, the resonator may have three different resonant frequencies. Compared with conventional resonators, the RF filter exhibits increased transmission within the filter passband or reduced attenuation within the filter passband. Improvements to filter performance are achieved when the silicon dioxide layer is directly disposed on the piezoelectric layer without any intermediate layer (preferably, by a PVD deposition process). As an example, the filter may be a transmission (Tx) filter for LTE band 25, the passband of the Tx filter being between 1.85 GHz and 1.915 GHz.

[0023] It should be understood that both the foregoing general description and the following detailed description are exemplary only and are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide additional understanding and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate one or more embodiments and, together with the description, are used to explain the principles and operation of the various embodiments. The same elements in different figures of the drawings are represented by the same reference numerals. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the attached figure

[0025] Figure 1 A cross section of a portion of a bulk acoustic wave resonator is shown.

[0026] Figure 2 shows an RF filter schematic; and

[0027] Figure 3 Shows Figure 2 Transmission diagram of the RF filter. DETAILED DESCRIPTION

[0028] Now, the present disclosure is described more fully below with reference to the accompanying drawings showing embodiments of the present disclosure. However, the present disclosure can be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure fully conveys the scope of the present disclosure to those skilled in the art. The accompanying drawings are not necessarily drawn to scale, but are configured to clearly illustrate the present disclosure.

[0029] Figure 1 A cross-sectional view of a portion of a bulk acoustic wave (BAW) resonator according to the principles of the present disclosure is depicted. The resonator is a solid mounted resonator (SMR) type that includes an acoustic reflector, such as a Bragg reflector, on which an acoustic active region is disposed. In more detail, the depicted BAW resonator includes a carrier substrate 110, such as a silicon wafer. A Bragg reflector arrangement 120 is disposed on the carrier substrate 110. The Bragg reflector 120 includes several layers with higher acoustic impedance, such as layers 122, 124, which may be made of tungsten; and other layers with lower acoustic impedance, such as layers 121, 123, 125, which may be made of a dielectric material such as silicon dioxide. In the present example, the topmost layer 125 of the Bragg reflector 120 is made of silicon dioxide. A bottom electrode 130 is disposed on the Bragg reflector 120. The electrode 130 extends on the left side beyond Figure 1 The portion depicted in FIG. 1 reaches other circuit elements, such as bump contacts, to connect the electrode 130 to other elements of the electronic circuit. A piezoelectric layer 140 is disposed on the Bragg reflector 120 and on the bottom electrode 130. In this example, the piezoelectric layer 140 may be made of aluminum scandium nitride with 7 wt % scandium. Other piezoelectric materials such as aluminum scandium nitride or aluminum nitride or another piezoelectric material with any scandium portion are also possible.

[0030] A dielectric petal layer such as a silicon dioxide layer 150 is disposed on the top surface of the piezoelectric layer 140. The silicon dioxide petal layer 150 is constructed by masking and photolithography steps to form a portion in which the silicon dioxide layer 150 is removed, which is opposite to the bottom electrode 130 and accommodates the top electrode 170. The silicon dioxide layer 150 surrounds and encloses the removed portion in which the top electrode 170 is disposed. For example, for a resonator of a band 25 filter, the thickness of the silicon dioxide petal layer can be in the range of 140 nm. In a resonator for a filter according to the 5G standard, the thickness can be lower, for example, as low as 20 nm.

[0031] It should be noted that the silicon dioxide petal layer 150 is disposed directly on the piezoelectric layer 140. The bottom surface of the silicon dioxide layer 150 is adjacent to and opposite the top surface of the piezoelectric layer 140. The layers 150, 140 have a common contact surface, so no additional layers are disposed between the silicon dioxide layer 150 and the piezoelectric layer 140. The silicon dioxide layer 150 is in direct mechanical contact with the top surface of the piezoelectric layer 140.

[0032] An overlay layer 160 made of a metal or a metal layer stack is disposed on the silicon dioxide layer 150 and extends into the acoustically active region where a portion of the silicon dioxide 150 is removed. The overlay layer 160 may include a bottom titanium layer and a top tungsten layer. The overlay layer 160 extends over the vertical sidewalls of the silicon dioxide layer 150 and contacts the top surface of the piezoelectric layer 140. The overlay layer 160 is removed from the inner portion of the acoustically active region to allow contact between the top electrode 170 and the piezoelectric layer 140. Specifically, for example, the top tungsten layer of the overlay 160 may be removed, wherein the bottom titanium layer 160 of the overlay 160 may still exist as a seed layer in the acoustically active region to enable the top electrode 170 to be properly formed in the active region on the piezoelectric layer 140.

[0033] The silicon dioxide layer 150 may be referred to as a petal layer, which covers the top surface of the piezoelectric layer, except for the portion where the electrode contacts the piezoelectric layer 140 such as the top electrode 170 in the acoustically active region. The acoustically active region is formed in the overlapping region of the bottom electrode 130 and the top electrode 170. By applying an electrical RF signal to the electrodes 130, 170, an acoustic resonance wave is generated in the piezoelectric layer 140 between the electrodes 130, 170. The petal layer 150 generates a step feature at its vertical sidewalls, which has the function of an energy confinement ring surrounding the acoustically active region, thereby preventing the acoustic energy concentrated in the acoustically active region from escaping laterally from the region into the region of the piezoelectric substrate 140 outside the acoustically active region and outside the removed portion of the petal layer 150.

[0034] In manufacturing Figure 1During the depicted BAW resonator, a carrier substrate 110 and an acoustic mirror 120 arranged thereon are provided, exposing a top dielectric layer 125 of the Bragg mirror arrangement 120. A bottom electrode layer 130 is deposited on the top layer 125 of the Bragg mirror 120 and is configured to form a bottom electrode of the acoustically active region. Then, a piezoelectric layer 140, such as a scandium aluminum nitride layer with 7 wt % scandium, is deposited. According to one embodiment, the workpiece can be transferred to another deposition chamber under vacuum conditions for silicon dioxide deposition, and the process continues with PVD deposition of a silicon dioxide layer 150. During the PVD process, a silicon target is bombarded by argon ions in an oxygen atmosphere so that silicon dioxide of about 140 nm thickness is deposited on the surface of the piezoelectric layer 140. During the in-situ deposition of the piezoelectric layer 140 and the silicon dioxide layer 150, no additional layer is generated between the two layers. The silicon dioxide layer 150 is then structured to remove a portion of the layer opposite the bottom electrode 130 to generate a petal layer 150 that forms an energy confinement ring feature for acoustic resonant waves. An overlay layer 160 is deposited, wherein at least a portion of the overlay layer 160 is removed within a portion of the active region. A top electrode layer 170 is deposited opposite the bottom electrode 130 to form an active region in the overlapping region of the bottom electrode 130 and the top electrode 170. The overlay layer 160 may extend from the top surface of the silicon dioxide petal layer 150 into the active region below the top electrode 170 along a defined length amount.

[0035] The etching of the silicon dioxide layer 150 can be performed by a dry etching process using a suitable reagent for dry etching silicon dioxide (such as gases CF4, CHF4, Ar, O2) to generate a flap structure. The etching is performed in the area opposite to the bottom electrode 130. The etching process is continued until the piezoelectric substrate is reached and the piezoelectric substrate acts as an etching stop. The detection of the reactive gas (such as aluminum fluoride AlF2) from the piezoelectric layer can be used as a detection means to stop the etching.

[0036] In one exemplary process, the PVD sputtering process used to deposit the silicon dioxide layer 150 may use the following parameters:

[0037] Substrate temperature: 100°C

[0038] Target power: 2.25kW

[0039] Oxygen flow rate: 100SCCM

[0040] Argon flow rate: 20SCCM

[0041] Chamber pressure: 6.7 to 6.9 10 -3 Torr (0.89Pa to 0.92Pa).

[0042] Platen RF forward power: 325W

[0043] Direct PVD deposition of the petal layer 150 on the piezoelectric layer 140 without a seed layer in between realizes a resonator with an increased quality factor. As explained below, the performance of an RF filter including several of such resonators is improved.

[0044] Figure 2 A schematic diagram of an RF filter is depicted. The RF filter comprises a first input / output port 201 and a second input / output port 202, between which four series resonators 210, 211, 212, 213 are connected in series. The node between the resonators 210, 211 is coupled to a shunt path comprising a resonator 214. The resonator 214 is connected between a node 221 and a node 222 for a ground potential terminal. Other shunt paths, each comprising a resonator 215, 216, 217, are provided between a corresponding node of the series resonator and the ground potential. Figure 2 The filter topology depicted in FIG. 1 is generally referred to as a ladder filter. All resonators 210, . . . , 217 have Figure 1 The depicted structure, in which the petal layer 150 is directly disposed on the piezoelectric layer 140. According to the ladder circuit concept, the resonators 210, ..., 217 can have different resonant frequencies. The three different resonant frequencies are Figure 2 The resonator of the ladder type filter is useful.

[0045] Figure 2 The size of the filter can be designed so that it forms a Figure 3 Depicted is the passband of Band 25. The uplink (Tx) passband of Band 25 has a lower edge 310 at 1.85 GHz and an upper edge 302 at 1.915 GHz. Figure 3 The graphs in FIG. 3 show various transmission curves 310 from RF filters with comparative resonators that exhibit a 5 nm thick titanium seed layer between the piezoelectric layer and a 140 nm thick CVD deposited silicon oxide petal layer; and Figure 1 Various transmission curves 320 from RF filters with BAW resonators including a 140 nm thick PVD deposited silicon dioxide layer without a seed layer illustrate the principles of the present disclosure. Figure 3 The curves depicted in are based on actual measurements of RF filters where the resonators are placed on wafers from the same manufacturing batch. Different curves may result from natural non-uniform variations across the wafer.

[0046] according to Figure 3, it can be clearly seen that the RF filter with BAW resonators according to the principles of the present disclosure shows a transmission curve 320 arranged above the transmission curve 310 according to the comparative RF filter. The transmission curve 320 exhibits higher transmission and lower attenuation than the comparative transmission curve 310. Specifically, the various curves 320 show an improvement of about 0.1 dB in region 331 at the lower edge of the passband, which is an improvement of about 5%. The improvement at the upper passband edge of region 332 is about 0.3 dB, which is an improvement of about 10%. The improvement in the center region is about 0.1 dB, which is equivalent to an improvement of about 5%.

[0047] It is obvious to those skilled in the art that various modifications and changes can be made without departing from the spirit or scope of the present disclosure as defined in the appended claims. Since those skilled in the art may conceive of modifications, combinations, sub-combinations and changes of the disclosed embodiments that combine the spirit and essence of the present disclosure, the present disclosure should be interpreted as including all contents within the scope of the appended claims.

Claims

1. An electroacoustic resonator, comprising: a carrier substrate (110), An acoustic reflector (120) is disposed on the carrier substrate; A bottom electrode (130) is disposed on the acoustic reflector; A piezoelectric layer (140) is disposed on the bottom electrode; a structured silicon dioxide layer (150) disposed on a portion of the piezoelectric layer, the portion of the piezoelectric layer and the structured silicon dioxide layer having a common contact surface; A top electrode (170) disposed on the piezoelectric layer; as well as A metal overlap layer (160) is disposed on the structured silicon dioxide layer (150) and extends below a portion of the top electrode (170), wherein at the portion, the metal overlap layer is disposed between the top electrode (170) and the piezoelectric layer (140).

2. The electroacoustic resonator according to claim 1, wherein the piezoelectric layer (140) and the structured silicon dioxide layer (150) have a common contact surface without a seed layer arranged therebetween.

3. The electroacoustic resonator of claim 1, wherein the piezoelectric layer (140) and the structured silicon dioxide layer (150) are in direct mechanical contact with each other.

4. The electroacoustic resonator according to any one of claims 1 to 3, wherein the structured silicon dioxide layer (150) surrounds a region in which the top electrode (170) is arranged.

5. The electroacoustic resonator according to any one of claims 1 to 3, wherein the structured silicon dioxide layer (150) surrounds a region in which the silicon dioxide layer is removed and in which the top electrode (170) is arranged.

6. The electroacoustic resonator of any one of claims 1 to 3, wherein the top electrode (170) comprises a layer stack including a bottom tungsten layer, a middle aluminum-copper composition layer, and a top metal nitride layer.

7. The electroacoustic resonator of claim 1, wherein the metallic overlay layer (160) comprises a layer stack of titanium and tungsten.

8. The electroacoustic resonator according to any one of claims 1 to 3, wherein the piezoelectric layer (140) comprises one of aluminum nitride and aluminum scandium nitride.

9. A method for manufacturing an electroacoustic resonator, comprising the steps of: Providing a carrier substrate (110) and an acoustic reflector (120) disposed on the carrier substrate; forming a structured bottom electrode (130) on the acoustic reflector; forming a piezoelectric layer (140) on the bottom electrode; forming a silicon dioxide layer (150) on the piezoelectric layer, thereby forming a common contact surface between the silicon dioxide layer (150) and the piezoelectric layer (140); removing a portion of the silicon dioxide layer (150) in a region opposite to the bottom electrode (130) to expose the piezoelectric layer; forming a top electrode (170) on the piezoelectric layer in the region of the exposed piezoelectric layer; and After the step of removing a portion of the silicon dioxide layer (150) and before the step of forming the top electrode (170), the following steps are performed: forming an overlapping layer (160) made of metal, and removing a portion of the overlapping layer in an area opposite to the bottom electrode (130) so that the overlapping layer is arranged between the top electrode (170) and the piezoelectric layer (140) in the area where the portion of the silicon dioxide layer (150) is removed.

10. The method of claim 9, wherein the silicon dioxide layer (150) is formed on the piezoelectric layer (140) without an intermediate metal seed layer.

11. The method according to claim 9 or 10, wherein the step of forming a silicon dioxide layer (150) comprises: The silicon dioxide layer is deposited by physical vapor deposition.

12. The method according to claim 9 or 10, wherein the step of forming the piezoelectric layer (140) and the step of forming the silicon dioxide layer (150) are performed without breaking a vacuum.

13. An RF filter, comprising: A first port and a second port (201, 202); a series path coupled between the first port and the second port, the series path comprising a series connection of the electroacoustic resonators (210, 211, 212, 213) according to any one of claims 1 to 8, One or more shunt paths coupled to at least one of the resonators of the series path, the one or more shunt paths each comprising at least one electroacoustic resonator (214, 215, 216, 217) according to any one of claims 1 to 8.

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