Method of manufacturing a bulk acoustic wave resonator and bulk acoustic wave resonator device

Technical Field

The present disclosure relates to electro-acoustic

resonators. Specifically, the present disclosure relates to a method of manufacturing a bulk acoustic wave resonator including forming a metal electrode layer, a piezoelectric layer, another metal electrode layer and an acoustic

reflection element. The present disclosure also relates to a bulk acoustic wave resonator manufactured according to the method .

Background

Bulk acoustic wave (BAW) resonators are widely used in electronic systems to realize RF filters. A piezoelectric layer is sandwiched between a pair of electrodes. By the application of an electrical signal to the electrodes an acoustic resonating wave is established in the piezoelectric layer. BAW resonators can be of a solidly mounted resonator (SMR) type where the acoustically active region is mounted on a substrate including an acoustic reflector such as a Bragg mirror to prevent the acoustic waves from escaping into the substrate, or of a film or freestanding bulk acoustic

resonator (FBAR) type where a cavity serving as an acoustic reflector is disposed underneath the acoustically active region . Radio frequency (RF) BAW resonators require high acoustic and electromagnetic quality factors. The acoustic quality may depend on the quality of the material such as the

piezoelectric layer and the vertical and lateral acoustic energy confinement features. The electromagnetic quality may depend on the conductivity of the metal electrodes.

With increasing operational frequencies of the BAW resonators the design feature sizes of the resonators decrease.

Specifically, future high frequency applications require decreasing thicknesses of the electrodes of the resonators which leads to substantial ohmic losses in the metal

electrode layers.

There is a need for future RF bulk acoustic wave resonators that exhibit high quality factors. Specifically, there is a need for RF BAW resonators with reduced ohmic losses in the metal electrodes.

It is an object of the present disclosure to provide a method of manufacturing a bulk acoustic wave resonator that has reduced ohmic losses.

It is another object of the present disclosure to provide a method of manufacturing a bulk acoustic wave resonator that comprises electrodes that have enhanced conductivity.

It is yet another object of the present disclosure to provide a bulk acoustic wave resonator for RF frequency applications with reduced ohmic losses in the electrodes. Summary

One or more of the above-mentioned objects are achieved by a method of manufacturing a bulk acoustic wave resonator that comprises the steps of: forming a workpiece, comprising:

providing a substrate; forming a separation layer in the substrate; forming a rare earth metal oxide layer on the substrate; forming a metal electrode layer on the rare earth metal oxide layer; forming a piezoelectric layer on the metal electrode layer; and forming another metal electrode layer on the piezoelectric layer; forming an acoustic reflection element; bonding another substrate to the workpiece; and splitting the substrate along the separation layer and removing a split portion of the substrate.

According to an embodiment, a workpiece wafer is provided that includes a separation layer within a substrate. A rare earth metal oxide layer is formed on the separation layer so that the active area of the BAW resonator can be formed on the rare earth metal oxide layer. The forming of the active area layer stack comprises the forming of a metal electrode layer on the rare earth metal oxide layer, the forming of a piezoelectric layer on the metal electrode layer and the forming of another metal electrode layer on the piezoelectric layer. The rare earth metal oxide layer serves as a seed layer for the deposition of the metal electrode layer. In this case, the deposition of a metal layer on a rare earth metal oxide layer generates a highly crystalline metal layer. The subsequently deposited layers inherit the crystalline structure of the firstly deposited metal electrode layer so that also the piezoelectric layer and the other metal

electrode layer exhibit a highly crystalline structure. As a result, the metal electrodes are highly crystalline caused by the initial deposition of a rare earth metal oxide layer so that the ohmic resistance of the metal electrode layers is relatively low and the electric conductivity is relatively high. The ohmic losses are decreased when compared to conventional solutions where the metal electrodes exhibit only little or no crystalline structure. Also the

piezoelectric layer exhibits improved crystallinity as it is deposited on a highly crystalline first metal electrode layer .

Further following the course of the method of manufacturing the BAW resonator, an acoustic reflection element is formed which may be a Bragg mirror layer stack for an SMR-type BAW resonator or a cavity containing air for an FBAR-type BAW resonator. A carrier substrate is bonded to the workpiece. While the other carrier substrate remains with the produced BAW resonator device, the original substrate of the workpiece is split along the separation layer to remove a split portion of that substrate. The split portion of the substrate may be refurbished and reused for the production of additional resonators. On the workpiece remains a portion of the

separation layer and the rare earth metal oxide layer that are removed from the workpiece to expose the second metal electrode layer. The second metal electrode layer is

structured to finish the BAW resonator device.

According to an embodiment, the separation layer is composed of a porous layer disposed in the substrate. The substrate may be a doped silicon wafer so that the porous layer can be formed by electrochemical anodization. The parameters of the electrochemical anodization process can be varied to form porous layer sections with different characteristics. For example, a first porous layer disposed in a larger depth within the doped silicon substrate has a larger density of pores and has the function of a separation layer. A second porous layer disposed above the first porous layer at a lower depth within the doped silicon substrate is composed of pores of smaller density. The second porous layer serves as a seed layer for the deposition of the rare earth metal oxide layer. The doped silicon wafer may be a doped crystalline silicon wafer so that the second porous layer close to the wafer surface provides superior conditions for the deposition of the rare earth metal oxide layer.

The silicon wafer may be annealed after the forming of the first and second porous layers to passivate and repair any defects generated by the electrochemical anodization process, to reorganize the generated pores and to recrystallize the surface of the doped silicon wafer. The doping may be in the range of about 10 ¹⁸ atoms/cm ³ by a p-doping agent such as boron .

During electrochemical anodization, the current density may be varied as a function of time to generate layers of

different porosity in different depths within the silicon wafer. For example, a low porosity layer at a lower depth may need a current density in the range of 5 to 7 mAcrn ^-2 ,

preferably 6 mAcrn ^-2 for about 10 seconds and a large porosity layer at a larger depth may require a current density of about 100 mAcrn ^-2 for about 1 second. The electrolyte may be hydrofluoric acid in water and ethanol. The low porosity layer includes pores of lower density and the large porosity layer includes pores of higher density. Also, the pores of the low porosity layer may be smaller than the pores of the large porosity layer. The rare earth metal oxide layer that is a seed layer for the subsequent deposition of a crystalline metal electrode may be made of any rare earth metal useful for that purpose.

Preferably, the rare earth metal may be selected from one of Gadolinium or Erbium. Both rare earth metals are commercially available and can be deposited in a controlled way and are useful for the subsequent deposition of a crystalline

molybdenum electrode.

In an embodiment, a thin epitaxial silicon layer may be deposited on the surface of the wafer after the anneal process and before the deposition of the rare earth metal oxide layer to facilitate nucleation and adhesion of the rare earth metal oxide layer.

The bonding of the workpiece to a carrier substrate allows the use of a variety of substrates. The additional carrier substrate to which the workpiece is bonded may be a wafer of silicon, a wafer of glass or a wafer of plastic so that the BAW resonator offers more flexibility for further processing and the use of materials that are less expensive. The

additional carrier substrate may be a flexible substrate. The flexible substrate may be made of polyimide.

One or more of the above-mentioned objects are also achieved by a bulk acoustic wave resonator device that is manufactured according to the above-described manufacturing process and comprises: a bottom electrode and a top electrode; a

piezoelectric layer disposed between the bottom electrode and the top electrode; an acoustic reflection element disposed at the bottom electrode; wherein at least one of the bottom and top electrodes has a thickness of 200 nanometers or less and comprises a metal material having a crystalline structure. According to an embodiment, the device features may allow a resonance frequency in the range of 3 GHz or more,

specifically in the range from 3 GHz to 8 GHz. The thickness of the metal electrodes required to achieve such an operating frequency is below about 200 nm, specifically between 50 nm to 200 nm. A most typical thickness may be in the range of 100 nm. The BAW resonator manufactured according to the present manufacturing process comprises top and bottom electrodes of a metal material such as molybdenum. The molybdenum electrode has a highly crystalline structure. Due to the crystalline structure, the top and bottom metal electrodes of the active area of the BAW resonator include less than usual grain boundaries and contact surfaces between the crystals so that they have a relatively low ohmic resistance so that the losses during electro-acoustic operation are relatively low. Less grain boundaries enable also a high reflection of the acoustic waves within the piezelectric material at the surface of the electrodes. This enables a satisfactory performance at high frequencies in the range of 3 GHz to 8 GHz.

Specifically for an FBAR-type BAW resonator, the second substrate bonded to one of the electrodes includes a cavity that serves as a reflector for the acoustic resonating waves within the active area. The cavity is achieved by etching into the substrate to a first depth, wherein the substrate may be a silicon wafer. The substrate itself has a thickness larger than the first depth so that it encloses the cavity and the web portion of the substrate opposite the cavity and positioned opposite the active area stabilizes the structure of the resonator. The process according to the present disclosure concerning the FBAR-type resonator generates a relatively robust BAW resonator of the FBAR type. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or

framework to understand the nature and character of the claims. The accompanying drawings are included to provide a further understanding and are incorporated in, and constitute a part of, this description. The drawings illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments. The same elements in different figures of the drawings are denoted by the same reference signs.

Brief Description of the Drawings

In the drawings:

Figures 1A through II show consecutive process steps during the manufacturing of SMR- and FBAR-type BAW resonators;

Figures 2A through 2D show additional consecutive process steps for the manufacturing of a SMR-type BAW resonator; and

Figures 3A through 3C show additional consecutive process steps for the manufacturing of a FBAR-type BAW resonator.

Detailed Description

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings showing embodiments of the disclosure. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will fully convey the scope of the disclosure to those skilled in the art. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the

disclosure .

Figures 1A through II show an exemplary sequence of process steps to manufacture a bulk acoustic wave resonator. Figure 1A shows a substrate wafer A comprised of a doped crystalline silicon wafer. The silicon wafer may be boron doped at a concentration of 10 ¹⁸ atoms/cm ^-3 . Other crystalline,

conductive substrates may also be useful.

Figure IB shows wafer A after a first electrochemical anodization process which generates a porous layer 110 close to the top surface of wafer A. The electrochemical

anodization process generates pores at a certain depth within the wafer. The depth, size and density of the pores can be controlled by the current density and the time period during which the current density is applied. For example, the current density is 100 mAcrn ^-2 applied for about 1 second to generate pores of a relatively large density and size at a relatively large depth within the substrate. The chemistry comprises an electrolyte of hydrofluoric acid in water and ethanol .

Figure 1C shows wafer A after another electrochemical anodization process that generated another porous layer 120 having smaller density of pores or porosity at a smaller depth within wafer A when compared to porous layer 110 which has a higher density of pores or porosity. The pores of layer 120 may be smaller than the pores of layer 110. The porous layer 120 is closer to the top surface of wafer A than the first porous layer 110 and has smaller pores than the pores of layer 110. In an example, layer 120 may have a porosity of 20 to 30 % and layer 110 may have a porosity of 60 to 75 %.

In an example, the thickness of layer 120 may be 1 to 3 ym and the thickness of layer 110 may be 0.2 to 0.3 ym.

Figure ID shows wafer A after a high temperature annealing step which subjects wafer A to a high temperature. The anneal temperature may be in the range of about 1000 °C. The defects in layers 110 and 120 are passivated and the pores are reorganized so that lower porous layer 110 is transformed to porous layer 111 and upper porous layer 120 is transformed to porous layer 121. The pores of layer 120 may grow to even larger pores establishing layer 111 and the surface on porous layer 120 may be reconstructed and recrystallized generating porous layer 121. Porous layer 111 serves as a separation layer to separate the wafer along the line established by layer 111 in response to a mechanical impact and porous layer 121 serves as a seed layer for the subsequent deposition processes as explained in more detail herein below.

Figure IE shows wafer A after the growth of a rare earth metal oxide layer 130. The annealed surface of porous layer 121 is a good seed layer for the deposition of rare earth metal oxide layer 130. The rare earth metal may be Erbium or Gadolinium. Other rare earth metals are also useful.

Optionally, a thin epitaxial silicon layer (not shown in Figure 1A) may be grown on the surface of porous layer 121 by a CVD process prior to the deposition of the rare earth metal oxide layer 130 to enhance the surface quality.

Figure IF shows wafer A after the deposition of a metal electrode layer 140 on rare earth metal oxide layer 130.

Layer 130 serves as a seed layer for the metal deposition process which facilitates the nucleation of crystals to establish a highly crystalline metal electrode layer 140. The metal material of layer 140 may be molybdenum which grows in a highly crystalline structure on a rare earth metal oxide such as layer 130. The growth methods to deposit a molybdenum electrode layer 140 include molecular beam epitaxy (MBE) , metal oxide chemical vapor deposition (MOCVD) , pulsed laser deposition (PLD) , sputtering and atomic layer deposition (ALD) . Most preferably, the molybdenum metal electrode layer 140 may be deposited by sputtering using a molybdenum target or by an MOCVD process. Depending on the growth method and parameters the epitaxial molybdenum electrode can exhibit large-scale crystallinity or smaller grain sizes.

Figure 1G shows wafer A after the forming of a piezoelectric layer 150. The piezoelectric layer material may be aluminum nitride in crystalline or columnar form so that it exhibits piezoelectric properties. Piezoelectric layer 150 may be sputter-deposited in a sputtering process using an aluminum target in nitrogen atmosphere or by an epitaxial growth process. Alternatively, the piezoelectric layer 150 may be made of aluminum scandium nitride, Al(l - x)Sc(x)N (0 < x < 0.3) . As the underlying metal electrode layer 140 has a highly crystalline structure, the nucleation process of a highly crystalline aluminum nitride layer 150 is also facilitated. The crystalline structure provided at the surface of the electrode layer 140 is inherited by and transferred to the piezoelectric layer 150 to generate a piezoelectric layer 150 of an even more enhanced level of piezoelectric property.

Figure 1H shows wafer A with another electrode 160 deposited on piezoelectric layer 150. Electrode 160 may be also comprised of a layer of molybdenum which is highly crystalline, inheriting the crystalline orientation provided by piezoelectric layer 150.

Figure II shows workpiece wafer A with a structured electrode 161. Portions of electrode 160 have been removed so that the gap portions are filled with dielectric layer 170, for example, made of a silicon dioxide. The top surface of the so far processed wafer A may be planarized by a chemical

mechanical polishing (CMP) process to obtain a planarized top surface of electrode 161 and silicon dioxide layer 170.

Figures 2A through 2D show subsequent process steps to be performed after the step of Figure II to manufacture a BAW resonator of the SMR type. Figure 2A shows a Bragg mirror layer stack 210 that serves as a vertical acoustic barrier to prevent the acoustic wave from escaping through metal

electrode layer 161 out of the acoustic active region of the resonator. The forming of a Bragg mirror layer stack is well- known to the skilled person. The Bragg mirror includes at least two layers 212, 213 of a material of acoustic high impedance such as tungsten or (amorphous) aluminum nitride. The layers 212, 213 are embedded in a material of low

acoustic impedance 211 such as silicon dioxide. The exposed surface 214 of the Bragg mirror may be planarized to achieve an even and uniform surface 214.

Figure 2B depicts the layer stack of Figure 2A bonded to another substrate such as carrier substrate B. The surface of substrate B is bonded to the surface 214 of Bragg mirror 210. As shown in Figure 2B workpiece wafer A has been flipped so that the layer stack has upside-down orientation. Bonding may be achieved by cleaning the to be bonded surfaces and bonding them with adhesion forces. Also a glue material may be used to bond substrate B to surface 214. Substrate B may be made of an insulating material such as silicon that is doped to compensate intrinsic conductivity or glass or plastic

material or a flexible material. In an example, the flexible material may be polyimide. A great variety of materials is possible for wafer B to be bonded to surface 214 of Bragg mirror 210. This offers more options for the use of the BAW resonator in an electronic RF filter or offers the

possibility to use cheaper materials such as glass, plastic or polyimide.

Figure 2C shows that a portion 220 of wafer A is split from the layer stack along the separation layer 111. A portion 222 of porous separation layer 111 resides at the split-off portion 220 of wafer A and another portion 221 resides at the resonator layer stack. The split process is initiated with the application of a mechanical impact which may be a

thermally induced mechanical stress and/or a mechanical shock, e.g. through a bolt. Portion 220 of wafer A is detached from the resonator layer stack and may be reused after removal of the portion 222 of the porous separation layer 111 residing at the detached portion of wafer A.

As shown in Figure 2D, the residual layers on the surface of metal electrode 140 have been removed. That is that the portion 221 of separation layer 111 remaining on the

resonator layer stack after detachment of wafer A, the second porous seed layer 121 and the rare earth metal oxide layer 130 are removed exposing the surface of molybdenum metal electrode 140. These layers may be removed by etching, mechanical milling or mechanical grinding. The remaining molybdenum layer 140 is structured to form electrode 141. The overlapping area of top metal electrode 141 and bottom metal electrode 161 defines the active area of the BAW resonator.

According to the principles of the present disclosure, the metal electrodes 141, 161 are highly crystalline so that they exhibit enhanced electrical conductivity. The thickness of the electrode layers 141, 161 can be reduced to meet

resonating conditions of high frequencies of above about 3 GHz such as in the range of 3 to 8 GHz without suffering from ohmic losses in the electrodes. Furthermore, the

surfaces between the electrodes 141 and 161 and the

piezoelectric layer 140 include a reduced amount of grain boundaries so that a high directionality of the reflection of the acoustic resonating waves at the interface between electrode and piezeoelectric layer is achieved. Also, a low amount of mode conversion (longitudinal to shear) is

achieved. This leads to reduced acoustic losses enabling high a high quality factor Q at high acoustic resonance

frequencies. As a result, the described process using a rare earth metal oxide layer as a seed layer for the first metal electrode, bonding of a workpiece wafer to another carrier substrate and using a porous layer to split the workpiece wafer allows the fabrication of a BAW resonator of the SMR type that is adapted to operate in frequencies beyond 3 GHz without suffering from increased acoustic and electric losses .

Figures 3A through 3C, in connection with the process

depicted in Figures 1A through II, show process steps to manufacture a FBAR-type BAW resonator. Figure 3A depicts a substrate wafer B such as a lightly doped silicon wafer which includes a cavity 310 that serves as an acoustic reflection element according to the FBAR principle. The cavity 310 may be obtained by reactive ion etching or wet etching or any other useful etching process to generate a large cavity in the substrate. The lateral width of the cavity 310 is determined by the active area of the BAW resonator. The etching of cavity 310 stops at a certain depth within the substrate B and establishes a bottom surface so that a web portion 311 of the substrate remains underneath the cavity.

In principle, any other substrate such as glass or plastic or polyimide that allows the forming of the cavity in the substrate is also useful. The substrate may be a flexible substrate .

Figure 3B depicts carrier substrate wafer B bonded to

prefabricated workpiece substrate A taken from the process step depicted in Figure II. Substrate B is bonded to the workpiece substrate A in such a way that the cavity 310 is opposite the active area of the resonator that is that part of the sandwich of top and bottom electrodes 140, 161 and piezoelectric layer 150 in which the electroacoustic

oscillation takes place. The remaining substrate web portion 311 of wafer B stabilizes the layer structure of the

resonator device which leads to a robust, mechanically stable FBAR-type BAW resonator.

Similar to the process depicted in Figure 2C for the SMR-type resonator, Figure 3B for the FBAR-type resonator shows that the substrate wafer A is split at the separation layer 111 so that the split portion of wafer A can be detached and reused for the fabrication of other resonator devices. A portion of the separation layer 111, the porous seed layer 121 and the rare earth metal oxide layer 130 remains at the metal electrode layer 140. These layers may be removed by etching, milling and/or grinding to expose the top surface of

electrode layer 140.

Turning now to Figure 3C, the top electrode layer 140 is structured resulting in structured top electrode 141. The active area of the resonator device is located in the area where the structured electrodes 141, 161. The cavity 310 is also situated below this area overlap so that the cavity is aligned with the active area of the resonator. As can be gathered from Figure 3C, the left-hand side wall 312 of cavity 310 is substantially aligned with the sidewall 162 of the structured electrode 161, where electrode 161 touches the dielectric layer 170. The right-hand sidewall 313 of cavity 310 matches substantially with the structured sidewall 142 of top electrode 141. In another embodiment, the cavity within the substrate B may be larger than the active area of the resonator .

In conclusion, BAW resonators of SMR and FBAR types comprise a porous silicon separation layer and electrodes of high crystallinity having enhanced conductivity. The BAW

resonators according to the present disclosure exhibit relatively low ohmic losses although the thicknesses of the electrodes are decreased to meet the high frequency

resonating conditions. Forming the electrodes on a rare earth metal oxide seed layer generates highly crystalline metal electrodes exhibiting relatively high specific conductivity so that the ohmic losses are decreased. The fabrication process includes a bonding of a workpiece with a

prefabricated layer stack to another substrate wherein the workpiece wafer is split and detached from the resonator.

This process sequence allows the deposition of a rare earth metal oxide seed layer to enhance crystallinity of the electrode layers. The porous separation and rare earth metal oxide seed layers are removed after splitting the workpiece wafer to achieve the BAW resonator layer structure.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure as laid down in the appended claims. Since modifications, combinations, sub combinations and variations of the disclosed embodiments incorporating the spirt and substance of the disclosure may occur to the persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims.