Thin film bulk acoustic wave (BAW) resonator and method for manufacturing

Abstract

A thin film bulk acoustic wave (BAW) resonator (100) com- prises a piezoelectric stack structure having a piezoelectric layer (120) sandwiched between a top conductive electrode layer (118) and a bottom conductive electrode layer (122) on the substrate (112, 114). The top and bottom conductive elec- trodes (118, 122) are detached from the vibrating piezoelec- tric layer (120) by intermediate gaps (130, 132) formed on both sides of the piezoelectric layer so that the vibrating pie- zoelectric layer (120) is suspended over the space between the top and bottom conductive electrodes. A resonating vol- ume of the resonator includes solely the vibrating piezoelec- tric layer the resonance frequency is solely defined by the thickness of the piezoelectric layer.

Description

[0001] THIN FILM BULK ACOUSTIC WAVE (BAW) RESONATOR AND METHOD FOR MANUFACTURING

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to thin film bulk acoustic resonators and manufacturing thereof.

[0004] BACKGROUND OF THE INVENTION

[0005] Microwave frequency range spans from about 300 MHz to about 30 GHz corresponding to a wavelength range of 1 m to 10 mm, respectively. Milli-metre waves occupy the frequency range from about 30 GHz to about 300 GHz corresponding to a wavelength range of 10 mm - 1 mm, respectively. The milli-metre- wave spectrum has been receiving more and more attention, and many applications, such as 5G, are under development.

[0006] Acoustic wave resonators based on bulk piezoelectric materials such as quartz crystal have been used for many decades of timing and filtering devices in RF systems and in various sensing applications. Quartz-based resonators have advantages in terms of temperature stability and high Q-factor. However, one major limitation is that they cannot readily achieve frequencies in the range of hundreds of MHz. In addition, the manufacturing cost and size of these bulk piezoelectric devices remains relatively large versus other system components.

[0007] The size and manufacturing limitations have led to the development of MEMS (Micro-Electro-Mechanical-Systems) versions of piezoelectric resonators, such as silicon-based MEMS resonators using either electrostatic actuation or piezoelectric thin film actuation. However, most quartz and Si-MEMS timing resonators still have at most 10’s of MHz or up to 100 MHz frequency, and higher oscillator output frequencies are generated by multiplication with detrimental effects to noise characteristics.

[0008] One type of piezoelectric resonator is a bulk acoustic wave (BAW) resonator. A well-known technology for 1 - 6 GHz frequency range resonators is thin film bulk acoustic wave resonators exciting bulk acoustic wave resonances in the thickness of piezoelectric thin films, such as AIN, AlScN, ZnO, etc. With the thin film technology, it is easy to realize sub-micron resonant dimensions suitable for GHz- range frequencies and provide electrodes for efficient excitation and connections. Therefore, the technology is widely used in RF filters for mobile communication. As compared to the Si-MEMS technology, the thin film BAW technology 1) provides significantly larger effective coupling (k2) (several percentages and more, which is decades better than the coupling in MEMS); 2) has a lower Q-value due to the high frequency, polycrystalline thin films, and a large fraction of a resonant volume being made of polycrystalline metal electrodes; 3) is much less sensitive to surrounding conditions such as pressure; 4) can be temperature compensated to at least first order (while MEMS and quartz resonators can be temperature compensated up to second order); and 5) utilizes a simple thin film manufacturing technology.

[0009] Thus, although thin film bulk acoustic wave resonators are easy to manufacture at the GHz range, they suffer from a low Q-value due to losses in polycrystalline metal electrodes and radiation of acoustic energy to substrate and surroundings.

[0010] Examples of film bulk acoustic wave resonators are disclosed in US20050140247A1 and US9209776B2.

[0011] BRIEF DESCRIPTION OF THE INVENTION

[0012] An object of the present invention is to provide a thin film bulk acoustic wave (BAW) resonator having a higher Q- value and allow higher frequencies at the GHz range. The object of the invention is achieved by a thin film bulk acoustic wave (BAW) resonator and a manufacturing method according to the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

[0013] An aspect of the invention is a thin film bulk acoustic wave (BAW) resonator, comprising a substrate, a piezoelectric stack structure having a piezoelectric layer sandwiched between a top conductive electrode layer and a bottom conductive electrode layer on the substrate, wherein the top and bottom conductive electrodes are detached from the vibrating piezoelectric layer by intermediate gaps formed on both sides of the piezoelectric layer, the vibrating piezoelectric layer being suspended between the top and bottom conductive electrodes.

[0014] In an embodiment, a resonance frequency of the BAW resonator is defined solely by the thickness of the vibrating piezoelectric layer.

[0015] In an embodiment, a resonating volume of the resonator includes solely the vibrating piezoelectric layer and optionally one or more temperature compensating layers deposited on the piezoelectric layer. In an embodiment, the resonator comprises a top temperature compensating layer and a bottom temperature compensating layer deposited on the top surface and on the bottom surface, respectively, of the vibrating piezoelectric layer, wherein a top gap is between the top temperature compensating layer and the top conductive electrode, and a bottom gap is between the bottom temperature compensating layer and the bottom conductive electrode, and wherein the top and bottom temperature compensating layer induce opposite direction of temperature drift of resonance frequency as the piezo layer, being preferably silicon dioxide (SiO2) layers.

[0016] In an embodiment, the piezoelectric layer may comprise two piezoelectric layers having a temperature compensating layer sandwiched between them.

[0017] In an embodiment, the intermediate gaps are formed by a sacrificial layer removal process.

[0018] In an embodiment, the conductive top and bottom electrode layers are made of metal, a metal alloy, a sufficiently conducting semiconductor, a conductive oxide, or a layered stack of materials providing sufficient conductivity and structural integrity.

[0019] In an embodiment, the piezoelectric layer comprises a deposited thin film layer or a single crystal layer of a piezoelectric material, the piezoelectric material preferably being one of aluminum nitride (AIN), scandium aluminium nitride (ScAlN), lithium niobate (LiNbO3), and lithium tantalate (LiTaO3).

[0020] In an embodiment, the substrate comprises a silicon (Si) substrate, a glass substrate, a quartz substrate, or a sapphire substrate, and wherein the substrate preferably is a high electrical resistivity substrate.

[0021] In an embodiment, the resonator comprises at least one thin film protective layer deposited on top of the piezoelectric stack structure.

[0022] In an embodiment, the resonator comprises an energy confining edge provided at suspension points of the piezoelectric layer, the energy confining edge preferably comprising thickening or mass loading structure on the piezoelectric layer at the suspension points.

[0023] In an embodiment, the energy trapping edge comprises a stack of an insulating layer, the conductive top electrode, and optionally a temperature compensating layer on the piezoelectric layer at the suspension points.

[0024] Another aspect of the invention is a method of manufacturing a thin film bulk acoustic wave resonator having a piezoelectric stack structure having a piezoelectric layer sandwiched between a top conductive electrode layer and a bottom conductive electrode layer on a substrate, the top and bottom electrodes are detached from the vibrating piezoelectric layer by top and bottom gaps formed on top and bottom sides, respectively, of the piezoelectric layer, the method comprising: i) forming an insulating layer on the substrate; ii) forming the bottom conductive electrode on the insulating layer; hi) forming a bottom sacrificial layer on the bottom conductive electrode: iv) forming the piezoelectric layer on the bottom sacrificial layer; v) forming a top sacrificial layer on the piezoelectric layer; vi) forming the top conductive electrode on the top sacrificial layer; vii) removing the top and bottom sacrificial layers by etching with an etching agent introduced through the one or more vias and thereby forming the top and bottom gaps.

[0025] In an embodiment, the top and bottom sacrificial layers are made of molybdenum (Mo) or silicon (Si), and / or wherein the etching agent is xenon difluoride (XeF2) vapor.

[0026] In an embodiment, the resonator comprises the top and bottom sacrificial layers are made of silicon dioxide (SiOz) and / or the etching agent is hydrofluoric acid (HF) vapor.

[0027] In an embodiment, the step iv) of forming the piezoelectric layer comprises forming the piezoelectric layer with a temperature compensating layer on at least one side or forming two piezoelectric layers with a temperature compensating layer sandwiched therebetween.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the following the invention will be described in greater detail by means of exemplary embodiments with reference to the attached drawings, in which

[0030] Figure 1 is a cross-sectional view illustrating an example of a prior art film bulk acoustic resonator (FBAR);

[0031] Figure 2 is a top view illustrating an example layout of a film bulk acoustic resonator;

[0032] Figure 3A is a cross-sectional view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment;

[0033] Figure 3B is an enlarged cross-sectional view of a portion of the FBAR shown in Fig. 3A;

[0034] Figure 4 is a top view illustrating an example layout of a film bulk acoustic resonator (FBAR) according to an embodiment;

[0035] Figures 5, 6A-6B, and 7A-7B are cross-sectional views illustrating examples sacrificial layers and sacrificial etch vias for forming top and bottom air gaps in a film bulk acoustic resonator (FBAR) according to an embodiment;

[0036] Figure 8 is a cross-sectional view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment having a thick capping layer or a system / stack of capping layers;

[0037] Figure 9 is a cross-sectional partial view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment having temperature compensating layers;

[0038] Figure 10 is a cross-sectional partial view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment having temperature compensating layers and a thin film capping layer;

[0039] Figure 11A is a cross-sectional view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment having temperature compensating layer between piezoelectric layers as well as a thin film capping layer;

[0040] Figure 11B is an enlarged cross-sectional view of a portion of the FBAR shown in Fig. 11A;

[0041] Figures 12A and 12B are exemplary graphs illustrating the resonance frequency / in function of the thickness t of an AIN piezoelectric layer not loaded with electrodes;

[0042] Figures 13A and 13B are exemplary graphs illustrating the resonance frequency / in function of the thickness tof an Sc0.2AI0.8N (20 % ScAlN) piezoelectric layer not loaded with electrodes;

[0043] Figure 14A shows an exemplary graph illustrating the amplitude of the admittance Y as function of the frequency for different thicknesses of the air gaps with an AIN piezoelectric layer;

[0044] Figure 14B shows an exemplary graph illustrating the pole zero distance PZD as function of the thickness of the air gaps where PZD is defined as PZD=( / - / a) / / *100% and is a measure of the electromechanical coupling K2of a resonator, / being the resonance frequency and being the antiresonance frequency of the resonator;

[0045] Figure 15A shows an exemplary graph illustrating the amplitude of the admitance Y in function of the frequency for different thicknesses of the air gaps with temperature compensating 100 nm thick SiOz layers on both sides of an AIN piezoelectric layer 1844 nm thick;

[0046] Figure 15B shows an exemplary graph illustrating the PZD as function of the thickness of the air gaps for an AIN piezoelectric layer 1844 nm thick with temperature compensating 100 nm thick SiOz layers on both sides;

[0047] Figures 16A and 16B show first dispersion curves at suspension point and at the vibrating resonator, respectively; and

[0048] Figures 17A and 17B show second dispersion curves at suspension point and at the vibrating resonator, respectively.

[0049] DETAILED DESCRIPTION

[0050] The bulk acoustic wave (BAW) resonator is basically constituted by a piezoelectric layer sandwiched between two electrodes. The application of an electric field between the two electrodes generates a mechanical stress that is further propagated through the bulk of the structure (acoustic wave). The resonance condition is established when the acoustical path (in thickness direction) corresponds effectively to odd integer multiples of the half acoustic wavelength between the free surfaces of the resonator. In a crystal BAW the piezoelectric layer is a thin slab of piezoelectric crystal which is metallized on both sides for applying an exciting voltage. The most common material used for crystal resonators is quartz. The traditional method of fabricating crystal oscillators includes fabrication of bulk crystals, from which thin plates are diced, ground and polished down to a desired thickness. The dimensions produced by this manufacturing process that determine the resonance frequency of the resonators, followed by a possible trimming step. It is difficult to thin down a crystal plate down to the micrometer scale, limiting the quartz resonators’ frequencies typically in the range below 100 MHz.

[0051] The thin film BAW resonator technology is based on the same principal structure, but the piezoelectric material as well as the electrodes, are produced by vacuum deposition of thin films. Standard lithography and etching methods used in microelectronic circuit fabrication are employed to pattern the films into devices. This allows for micrometre-range dimensions and subsequently GHz-range resonance frequencies.

[0052] Thin film technology requires a substrate such as silicon or glass wafer to provide a mechanical support for the films. The vibration and acoustic waves, however, need to be contained within the thin film resonator structures. To keep the acoustic waves from escaping into the substrate there are typically two types of thin film BAW resonators: a Film Bulk Acoustic Resonator (FBAR) and a solidly mounted bulk acoustic resonator (SMR). The FBAR resonator is often called a membrane type resonator. Both the FBAR and the SMR comprise acoustic stacks that are disposed over a reflective element. The reflective element of an SMR is a Bragg reflector comprising alternating layers of high acoustic impedance and low acoustic impedance. The reflective element of an FBAR is a cavity, normally in a substrate over which the acoustic stack is mounted, creating a suspended acoustic stack. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the acoustic waves being determined by the materials in the acoustic stack. The application of an electric field between the two electrodes generates a mechanical stress that is further propagated through the bulk of the structure (acoustic wave).

[0053] FIG. 1 is a cross-sectional view illustrating an example of a prior art film bulk acoustic resonator (FBAR) and FIG. 2 is a top view illustrating an example of a film bulk acoustic resonator. The cross-sectional view of Fig. 1 is taken along line A-A in Fig. 2. Referring to FIG. 1, the prior art FBAR 10 may include a substrate 12, an insulating layer 14, an air, gas or vacuum cavity 16, and a piezoelectric stack structure (the resonating part) having a piezoelectric layer 20 sandwiched between a top conductive electrode layer 18 and a bottom conductive electrode layer 22. Further, a contact 26 for the top electrode layer and a contact 28 for the bottom electrode 22 as well as an insulating layer 24 for separating the contacts 26 and 28 from the piezoelectric layer 20 are shown. The bottom electrode layer 22, the piezoelectric layer 20, and the top electrode layer 18 are sequentially stacked and are suspended over a cavity 16, which functions as a reflective element. An active region of the resonator is a region that vibrates and resonates in a predetermined direction due to the piezoelectric phenomenon occurring in the piezoelectric layer 20 when an electric field is applied between the top electrode layer 18 and the bottom electrode layer 22, and may correspond to a region in which the top electrode layer 18, the piezoelectric layer 20, and the bottom electrode layer 22 are overlapped in a vertical or thickness direction above the air cavity 16. Thus, the resonating volume includes the piezoelectric layer and the electrode layers. An inactive region of the resonator is a region of where voltage is not applied over the piezoelectric layer or where the layer stack is such that the resonance frequency is significantly different from that in the active region.

[0054] According to an aspect of the invention, a piezoelectric layer sandwiched is between a top conductive electrode layer and a bottom conductive electrode layer on the substrate, but the top and bottom conductive electrodes are detached from the vibrating piezoelectric layer by intermediate gaps formed on both sides of the piezoelectric layer. Herein a term air gap is used generically to refer to spaces (gaps) filled with air, vacuum, or other gases. Thereby the vibrating piezoelectric layer is suspended over a space between the top and bottom conductive electrodes. A resonating volume of the resonator includes solely the vibrating piezoelectric layer. Acoustic losses are not caused by the conductive electrodes. As result, a higher Q-value of the resonator can be achieved due to having no resonance vibration in lossy electrode materials (e.g., metals) which are not part of the resonating volume at all. Further, the resonance frequency is solely defined by the thickness of the piezoelectric layer. This gives freedom to optimize the material and thickness of the electrode layers for resistance and other goals. This allows also reaching extremely high frequencies with manufacturable thicknesses of the piezoelectric layer. For example, for a piezoelectric layer of aluminum nitride (AIN), a resonance frequency of 14.3 GHz can be achieved with the layer thickness of 300 nm and 4.4 GHz can be achieved with the layer thickness of 1 gm (micrometre).

[0055] Figs. 12A and 12B are exemplary graphs illustrating the resonance fre- quency / in function of the thickness tof an AIN piezoelectric layer not loaded with electrodes. Figs. 13A and 13B are exemplary graphs illustrating the resonance fre- quency / in function of the thickness t of a 20% ScAlN piezoelectric layer not loaded with electrodes. Figs. 12A and 13A show a thickness range from 0.2 gm to 2 gm, while Figs. 12B and 13B show a shorter thickness range from 1 gm to 2 gm. The resonance frequencies are calculated by approximating an unloaded (sole) piezoelectric layer in a conventional stacked resonator model with a piezoelectric layer sandwiched between Al top and bottom electrodes having thickness of 1 nm. Resonance frequencies from 2.5 GHz to 18 GHz are achieved. For example, an AIN piezoelectric layer having thickness 2 gm yields resonance at 2.77 GHz, and an AIN piezoelectric layer having thickness 1.844 gm yields resonance at 3 GHz. Similarly, a 20 % ScAlN piezoelectric layer having thickness 2 gm yields resonance at 2.385 GHz, and a 20 % ScAlN having thickness of 1.59 gm yields resonance at 3 GHz.

[0056] Electromechanical coupling is reduced by the capacitance of the air gaps with low permittivity but due to resonator volume consisting of solely piezoelectric material can still readily be much larger than that of a Si MEMS resonator consisting mostly of non-piezoelectric silicon. The fundamental meaning of the electromechanical coupling coefficient is to measure the efficiency of the piezoelectric device in converting electrical energy to mechanical energy and vice versa which is manifested in the frequency difference of resonance frand antiresonance / a. Fig. 14A shows an exemplary graph illustrating the amplitude of the admittance Y as function of the frequency for different thicknesses of the top and bottom air gaps with an AIN piezoelectric layer having thickness 1.844 gm. Fig. 14A reveals that the resonant frequency / ris shifted towards the anti-resonant frequency / aof the resonator 2 with increasing thickness of the air gaps indicating as shown in Fig. 14A that the pole zero distance (PZD) of the resonator becomes all the smaller, the greater are the air gaps. The pole zero distance of a resonator 2 is defined as the relative distance between resonant frequency and anti-resonant frequency. The PZD with no air gaps (i.e., with air gap thickness 0) was 2.5 %. Fig. 14B shows an exemplary graph illustrating the PZD in function of the thickness of the air gaps. PZD, i.e., coupling is reduced by the air gaps, but it is still very good for oscillator or narrow band filter purposes. Air gaps having thickness of 50 nm still yields PZD of about 1.6 %. PZD of quartz based piezoelectric resonators is less than 1 %. Coupling can further be enhanced by using piezomaterials with stronger piezo effect, such as ScAlN or single crystal LiNbO3 or LiTaO3.

[0057] FIG. 3A is a cross-sectional view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment. FIG. 3B is an enlarged cross-sectional view of a portion of the FBAR shown in Fig. 3A. Fig. 4 is a top view illustrating an example of a film bulk acoustic resonator (FBAR) according to an embodiment. The cross-sectional views of Figs. 3A and 3B are taken along line A-A shown in in Fig. 4. The exemplary FBAR resonator 100 may include a substrate 112, an insulating layer 114, and a piezoelectric stack structure having a piezoelectric layer 120 sandwiched between a top conductive electrode layer 118 and a bottom conductive electrode layer 122. Further, there may be a metal contact, such metal contact 126, for the top electrode layer 118 and a metal contact, such as metal contact 128, for the bottom electrode 122 as well as at least one insulating layer 124 for separating the metal contacts 126 and 128 from the piezoelectric layer 120. The top and bottom conductive electrodes 118 and 122 are detached from the vibrating piezoelectric layer 120 by intermediate air gaps 130 and 132 formed on both sides of the piezoelectric layer 120. The top air gap 130 is provided between the piezoelectric layer 122 and the top electrode 118. The top air gap 130 may preferably extend laterally beyond the peripheral edge of the top electrode 118 under the insulating layer 124, so that the top electrode 118 is suspended over the top air gap 130 by the insulating layer 124 and that the piezoelectric layer 120 is planar between the suspension points. The bottom air gap 132 is provided between the piezoelectric layer 122 and the bottom electrode 122. The bottom air gap 132 preferably extends laterally beyond the edge of the top air gap 130, thereby having larger lateral diameter than the top air gap 130. The bottom electrode layer 122 is formed on top of the insulating layer 114 and the substrate 112. Thereby the piezoelectric layer 120 is effectively suspended over air between the top and bottom conductive electrodes 118 and 122. The air gaps 130 and 132 function as reflective elements that prevent the acoustic waves from escaping from the piezoelectric layer 120 into the top and bottom electrodes 118 and 120 and further to the substrate. Therefore, the top and bottom electrodes 130 and 132 are configured to be static and not to vibrate, so that a resonating volume of the resonator includes solely the suspended portion of the piezoelectric layer 120. No reflective element, such as an air cavity, is needed below the bottom electrode layer 122. An active region of the resonator is a region of the piezoelectric layer 120 that vibrates and resonates in a predetermined direction due to the piezoelectric phenomenon occurring in the piezoelectric layer 120 when an electric field is applied between the top electrode layer 118 and the bottom electrode layer 122, and may correspond to a region in which the top electrode layer 118, the top air gap 130, the piezoelectric layer 120, the bottom air gap 132 and the bottom electrode layer 122 are overlapped in a vertical or thickness direction. An inactive region of the resonator is a region of where voltage is not applied over the piezoelectric layer or where the layer stack is such that the resonance frequency is significantly different from that in the active region.

[0058] The inactive region may correspond to an outer region surrounding the active region of the piezoelectric layer 120.

[0059] The piezoelectric layer 120 may comprise a deposited thin film layer of a suitable piezoelectric material, for example aluminum nitride (AIN), scandium aluminium nitride (ScAlN), lithium niobate (LiNbOs), or lithium tantalate (LiTaOs). In an embodiment, the piezoelectric layer may be a single crystal layer of a piezoelectric material.

[0060] The conductive top and bottom electrodes 118 and 120 may be made of suitable metal, for example aluminium (Al), platinum (Pt), molybdenum (Mo) or tungsten (W) or a metal alloy such as titanium-tungsten (TiW) or a sufficiently conducting semiconductor such as doped silicon (Si) or a conductive oxide such as Indium Tin Oxide (ITO) or of a layered stack of materials providing sufficient conductivity. The electrode material shall not be of material that does not stand etching carried out during the manufacture, or it needs to be protected from it, as will be discussed below.

[0061] The insulating layers 114 and 116 may be made of silicon dioxide (SiOz), for example. The substrate 112 may comprise a silicon (Si) substrate, a glass substrate, a quartz substrate, or a sapphire substrate, for example. The substrate is preferably a high resistance substrate.

[0062] In embodiments, the top and bottom air gaps 130 and 132 are formed by a sacrificial layer removal process during a manufacturing process of the resonator. For example, a bottom sacrificial layer corresponding to the bottom air gap 132 is formed on the bottom electrode 122, and the piezoelectric layer 120 is formed on the bottom sacrificial layer. Further, a top sacrificial layer corresponding to the top air gap 130 is formed on the piezoelectric layer 120 and the top electrode layer 118 is formed on the top sacrificial layer. Thereafter, further layers may be formed. At a subsequent stage of the manufacturing process, the top and bottom sacrificial layers are removed and the top and bottom air cavities are opened by etching through one or more sacrificial etch vias, such as sacrificial etch vias 142, 144, 146 and 148 illustrated in Figure 4. The resulting air gaps can be very thin, the thinner the better as long as the electrodes can be kept from touching the piezoelectric layer. The thickness of the sacrificial layers defines the thickness of the air gaps. Basically, the lower limit to the thickness of the air gaps may be set by the thickness of the sacrificial layer that can be removed by the etching process. Depending on the material of the sacrificial layer, it can be removed from very narrow gaps over large horizontal distances and via very small holes or holes that are easy to seal afterwards by depositing a layer of material to them. For example, a 50 nm air gap may be easy to release (i.e., to remove a 50 nm sacrificial layer by etching), while thicknesses below 30 nm are more challenging or require longer etching times. In embodiments, the thickness of the sacrificial layers is only some tens of nanometres, preferably 50 nm or less, more preferably between 10 nm and 50 nm, and even more preferably between 30 nm and 50 nm.

[0063] Figures 5, 6A-6B, and 7A-7B are cross-sectional views illustrating examples sacrificial layers and sacrificial etch vias for forming top and bottom air gaps in a film bulk acoustic resonator (FBAR) according to an embodiment. The cross-sectional view of Fig. 5 is taken along line C-C shown in in Fig. 4, the cross- sectional views of Fig. 6A and 6B are taken along line B-B, and the cross-sectional views of Fig. 7A and 7B are taken along line D-D. FIG. 6B is an enlarged cross-sectional view of a portion of the FBAR shown in Fig. 6A. FIG. 7B is an enlarged cross- sectional view of a portion of the FBAR shown in Fig. 7A. Same reference symbols in Figures 5, 6A-6B, 7A-7B and Figs. 3A-3B, 4 refer to same or similar elements or structures.

[0064] In Fig. 4, a dashed line 130 may define the region of the top air gap layer within the FBAR, and a dashed line 144 may define the region of a lateral etch via or channel that connects the top air gap layer 130 to a vertical etch via or opening 142. Similarly, a dashed line 132 may define the region of the bottom air gap layer within the FBAR, and a dashed line 148 may define the region of a lateral etch via or channel that connects the bottom air gap layer 132 to a vertical etch via or opening 146. When layers of the FBAR 100 are deposited, a bottom sacrificial layer 131 is deposited and patterned in the regions of the bottom air gap layer 132 and the etch channel 148, and a top sacrificial layer 131 is deposited and patterned in the regions of the top air gap layer 130 and the etch channel 144, as shown in Figs. 5, 6A, and 7A. At a subsequent stage of the manufacturing process, vertical etch via openings 142 and 144 may be formed to extend from a top surface to the sacrificial layer 128 and 132, respectively. An etching agent is fed to the etch via openings 142 and 146 to remove the sacrificial layers 129 and 131 to first open the lateral etch channels 144 and 148 and then the top and bottom air gaps 130 and 132. In embodiments, the lateral etch channels 144 and 148 may be aligned and share a common vertical etch via opening, such as the etch via opening 146. The etch vias can be sealed afterwards by depositing a layer of material to them. Sacrificial layers 129 and 131 may be, for example, made of deposited Si by sputtering, low pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD), or of sputtered molybdenum (Mo). The sacrificial layers 129 and 131 may be released by a vapor etching, for example. For releasing the sacrificial layers made of sputtered Mo or Si, the etching agent may be xenon difluoride (XeFz), for example. The sacrificial layers made of SiOz may be released by hydrofluoric acid (HF) vapor, for example. The metal electrodes 118 and 122 shall be resistant to the etching agent. For example, the electrodes 118 and 122 cannot be made of Mo, if the etching agent is XeFz, or must be protected by an additional layer resistant to etching.

[0065] An exemplary process flow for manufacturing a film bulk acoustic resonator (FBAR) according to embodiments may be as follows: i) provide a substrate (a wafer) 112, e.g., Si or glass substrate; ii) oxidize the substrate (wafer) 112 or deposit a SiOz layer 114 on top of the substrate 112; hi) deposit a bottom metal electrode 122 on top of the SiOz layer 114, e.g., sputter a smooth layer of metal (not Mo); iv) deposit a bottom sacrificial layer 131 on top of the bottom metal electrode 122, e.g., sputter a thin layer of Mo or Si, preferably 10-50 nm thick layer; v) optionally pattern the bottom sacrificial layer 131, if needed, e.g., to define regions 132 and 148 shown in Fig. 4; vi) deposit a piezoelectric layer 120 on top of the patterned bottom sacrificial layer 131 and the surrounding bottom metal electrode 122, e.g., sputter an AIN or ScAlN layer; vii) deposit a top sacrificial layer 129 on top of the piezoelectric layer 120, e.g., sputter a thin layer of Mo or Si, preferably 10-50 nm thick layer; viii) optionally pattern the top sacrificial layer 129, if needed, e.g., to define regions 130 and 144 shown in Fig. 4; ix) deposit and pattern a top metal electrode 118 on top of the top sacrificial layer 129, e.g., sputter a smooth layer of metal (not Mo); x) deposit an insulating layer 124 on top of the patterned top metal electrode 118, and the surrounding sacrificial layer 129 and the surrounding piezoelectric layer 120, e.g., deposit SiO2 layer by PECVD or sputtering; xi) open a vertical etch via 142 to the top sacrificial layer 129; xii) vapour etch by XeFz the sacrificial layer 129 to open the lateral etch channel 144 and the top air gap 130; xiii) open a vertical etch via 146 to the bottom sacrificial layer 131; xiv) vapour etch by XeFz the sacrificial layer 131 to open the lateral etch channel 146 and the bottom air gap 132; xvi) provide metal contacts 126 and 128 for the top metal electrode 118 and the bottom metal electrode 122; and xv) optionally apply thin film capping layers.

[0066] It shall be appreciated that the present invention is not limited to the illustrated exemplary manufacturing processes and arrangements to make the air gaps. Upon reading the present specification, a person skilled in the art will readily infer various alternatives and variations to the make the air gaps on both sides of the piezoelectric layer to detach the top and bottom metal electrodes. For example, the order of process steps may vary. For example, steps xi)-xiv) may be carried out after the step xv). In embodiments, the thin film bulk acoustic resonator (FBAR) comprises a thin film packaging on top of the resonator. Thereby the packaging of the FBAR can be like in e.g. CMOS circuits. A separate packaging may be avoided. The thin film packaging is possible in FBAR resonators according to embodiments of the invention, as the vibrating part of the resonator the is isolated from the top and bottom electrodes by the air gaps. In embodiments there are one or more thin film capping layers deposited on top of the resonator. The capping layer or layers may comprise one or more protection, support and / or passivation layers deposited on top of the resonator. Openings for accessing the top and bottom contacts may be etched through the capping layer (s). FIG. 8 is a cross-sectional view illustrating an example of a film bulk acoustic resonator (FBAR) 200 according to an embodiment formed by depositing a thick capping layer 138 on top of the FBAR 100 shown in Fig. 3A and making openings through it for accessing top and bottom contacts 126 and 128.

[0067] In embodiments, a temperature compensating layer is stacked on one or both sides of a piezoelectric layer, thereby the resonator volume comprises the piezoelectric layer and one or more temperature compensating layers. The top or bottom air gap is between the temperature compensating layer and the facing top or bottom electrode. The temperature compensating layer has a temperature coefficient opposite in sign to the temperature coefficient of the piezoelectric layer. The temperature compensation layer(s) can be used to reduce the temperature drift of the resonator. In embodiments, the temperature compensating layer(s) may be an oxide layer, preferably silicon dioxide (SiOz) layer (s). The air gap is between each temperature compensating layer and the facing top or bottom electrode.

[0068] Electromechanical coupling is further reduced by the temperature compensating layers, but it is still very good for oscillator purposes. Fig. 15A shows an exemplary graph illustrating the amplitude of the admittance Y in function of the frequency for different thicknesses (widths) of the top and bottom air gaps with an AIN piezoelectric layer of thickness 1.844 gm and temperature compensating SiOz layers of lOOnm on both sides. Fig. 15B shows an exemplary graph illustrating the PZD in function of the thickness (width) of the air gaps. Air gaps having thickness of 50 nm still yields PZD of about 1.4 %.

[0069] FIG. 9 is a cross-sectional partial view illustrating an example of a film bulk acoustic resonator (FBAR) 300 according to an embodiment having temperature compensating layers. Fig. 10 is a cross-sectional partial view illustrating an example of a film bulk acoustic resonator (FBAR) 400 according to an embodiment having temperature compensating layers and a thin film capping layer. Figs. 9 and 10 are taken along line A-A shown in in Fig. 4. Same reference symbols in Figs. 3A- 3B, 4-5, 6A-6B, 7A-7B and Figs. 9-10 refer to same or similar elements or structures.

[0070] The exemplary FBAR resonator 300 or 400 may include a substrate 112, an insulating layer 114, and a piezoelectric stack structure having a piezoelectric layer 120 sandwiched between a top conductive electrode layer 118 and a bottom conductive electrode layer 122. The piezoelectric layer 120 comprises a bottom temperature compensating layer 136 on its bottom surface and a top temperature compensating layer 134 on its top surface. The temperature compensating layers 134 and 136 extend at least over the active region of the resonator, preferably at least over the length of the air gap 130 and 132, respectively. In the illustrated examples, the temperature compensating layers 134 and 136 extend over the entire piezoelectric layer 120. Further, there are a metal contact 126 for the top electrode layer 118 and a metal contact 128 for the bottom electrode 122 as well as an insulating layer 124 for separating the contacts 126 and 128 from the piezoelectric layer 120. The top and bottom conductive electrodes 118 and 122 are detached from the vibrating stack of the layers 134, 120 and 136 by intermediate air gaps 130 and 132 formed on both sides of the layer stack. The air gap 130 is provided between the top temperature compensating layer 134 and the top electrode 118. The air gap 130 preferably extends laterally beyond the peripheral edge of the top electrode 118 under the insulating layer 124, so that the top electrode 118 is suspended over the air gap 130 by the insulating layer 124. The air gap 132 is provided between the bottom temperature compensating layer 136 and the bottom electrode 122. The air gap 132 preferably extends laterally beyond the edge of the air gap 130, thereby having larger lateral diameter than the air gap 130. The vibrating stack of the temperature compensating layers 134, 136 and the piezoelectric layer 120 is effectively suspended over air between the top and bottom conductive electrodes 118 and 122. The air gaps 130 and 132 function as reflective elements that prevent the acoustic waves from escaping from the vibrating layer stack into the top and bottom electrodes 118 and 120 and further to the substrate. Therefore, the top and bottom electrodes 130 and 132 are configured to be static and not to vibrate, so that a resonating volume of the resonator includes solely the suspended portion of the vibrating stack of piezoelectric and temperature compensating layers. The active region of the resonator is a region of the stack of piezoelectric and temperature compensating layers that vibrates and resonates in a predetermined direction due to the piezoelectric phenomenon occurring in the piezoelectric layer 120 when an electric field is applied between the top electrode layer 118 and the bottom electrode layer 122, and may correspond to a region in which the top electrode layer 118, the air gap 130, the top temperature compensating layer 134, the piezoelectric layer 120, the bottom temperature compensating layer 136, the air gap 132 and the bottom electrode layer 122 are overlapped in a vertical or thickness direction. An inactive region of the resonator is a region of where voltage is not applied over the piezoelectric layer or where the layer stack is such that the resonance frequency is significantly different from that in the active region. The inactive region may correspond to an outer region surrounding the active region.

[0071] The air gaps 130 and 132 can be manufactured in a similar manner as described above. Now there may be an additional step between steps v] and vi] for depositing the bottom compensating layer 136 on top of the sacrificial layer, and an additional step between steps vi] and vii] for depositing the top compensating layer 134 on top of the piezoelectric layer 120. When the sacrificial layers are made of Mo or Si, the airgaps 130 and 132 can be released with XeFz vapor etching. However, in the case the sacrificial layers are made of SiOz, HF vapor etching is not possible, because also the compensating layers would be etched.

[0072] In embodiments, the piezoelectric layer may comprise two piezoelectric layers having a temperature compensating layer sandwiched between them. Thereby, it is possible to achieve smaller overall thickness of the sandwiched piezoelectric structure. Further, the HF vapor etching is now possible.

[0073] Figs. 11A and 1 IB illustrate an example of a film bulk acoustic resonator (FBAR) according to an embodiment having temperature compensating layer 137 sandwiched between two piezoelectric layers 120A and 120B. Figs. 11A and 11B are taken along line A-A shown in in Fig. 4. Same reference symbols in Figs. 3A-3B, 4-5, 6A-6B, 7A-7B, 9-10 and Figs. 11A-11B refer to same or similar elements or structures.

[0074] The exemplary FBAR resonator 500 may include a substrate 112, an insulating layer 114, and a piezoelectric stack structure having a piezoelectric layer 120 sandwiched between a top conductive electrode layer 118 and a bottom conductive electrode layer 122. The piezoelectric layer 120 comprises a bottom piezoelectric layer 120B, a top piezoelectric layer 120A, and temperature compensating layer 137 sandwiched between the two piezoelectric layers 120A and 120B. The intermediate temperature compensating layer 137 extends at least over the active region of the resonator, preferably at least over the length of the air gap 130 and 132, respectively. In the illustrated example, the temperature compensating layer 137 extends over the entire piezoelectric layers 120A and 120B. Further, there are a contact opening 126 for the top electrode layer 118 and a contact opening 128 for the bottom electrode 122 as well as an insulating layer 124 for separating the contacts 126 and 128 from the top piezoelectric layer 120A. The top and bottom conductive electrodes 118 and 122 are detached from the vibrating stack of the layers 120A, 137 and 120B by intermediate air gaps 130 and 132 formed on both sides of the layer stack. The air gap 130 is provided between the top piezoelectric layer 120A and the top electrode 118. The air gap 130 preferably extends laterally beyond the peripheral edge of the top electrode 118 under the insulating layer 124, so that the top electrode 118 is suspended over the air gap 130 by the insulating layer 124. The air gap 132 is provided between the bottom piezoelectric layer 120B and the bottom electrode 122. The air gap 132 preferably extends laterally beyond the edge of the air gap 130, thereby having larger lateral diameter than the air gap 130. The vibrating stack of the piezoelectric layers 120A, 120B and the temperature compensating layer 137 is effectively suspended over air between the top and bottom conductive electrodes 118 and 122. The air gaps 130 and 132 function as reflective elements that prevent the acoustic waves from escaping from the vibrating layer stack into the top and bottom electrodes 118 and 120 and further to the substrate. Therefore, the top and bottom electrodes 130 and 132 are configured to be static and not to vibrate, so that a resonating volume of the resonator includes solely the suspended portion of the vibrating stack of piezoelectric and temperature compensating layers. The active region of the resonator is a region of the stack of piezoelectric and temperature compensating layers that vibrates and resonates in a predetermined direction due to the piezoelectric phenomenon occurring in the piezoelectric layers 120A and 120B when an electric field is applied between the top electrode layer 118 and the bottom electrode layer 122, and may correspond to a region in which the top electrode layer 118, the air gap 130, the top temperature compensating layer 134, the piezoelectric layers 120A-120B, the bottom temperature compensating layer 136, the air gap 132 and the bottom electrode layer 122 are overlapped in a vertical or thickness direction.

[0075] The air gaps 130 and 132 can be manufactured in a similar manner as described above. The bottom piezoelectric layer 120B may be deposited in step v). Now there may be additional steps between steps v] and vii] for depositing the intermediate temperature compensating layer 137 on top of the bottom piezoelectric layer 120B, and for depositing the top piezoelectric layer 120A on top of the intermediate temperature compensating layer 137.

[0076] In a BAW resonator, the vibration needs to be contained in the resonator volume (i.e., in the active region) not only in the vertical direction, where it is done by the air gaps 130 and 132, but also in the lateral direction. Therefore, in embodiments, energy trapping edge is provided at suspension points of the piezoelectric layer 120 to provide acoustic energy trapping conditions at the suspension points. Thereby, maximal Q-value and minimized acoustic losses can be achieved. Suspension or anchoring points comprise the points or locations where active region of the vibrating piezoelectric layer 120 is connected and suspended to the surrounding inactive region or support layers. In the exemplary rectangular layout shown in Fig. 4, the active region of the piezoelectric layer 120 may be suspended along all edges.

[0077] In embodiments, the energy trapping edge is provided by thickening or mass loading at suspension points of the suspended vibrating piezoelectric layer so as to have acoustic energy trapping conditions at the suspension points. Energy trapping condition means that there is no horizontally propagating vibration modes similar to the resonance mode available at or near the resonance frequency.

[0078] In embodiments, the energy trapping edge is provided by stacking the insulating layer and the conductive top electrode on the piezoelectric layer at the suspension points. Examples of locations of energy trapping edges are indicated by arrows in Fig. 4. The energy trapping edge is preferably located at the end of the top air gap. Similarly, energy trapping edges can be found in other exemplary embodiments illustrated.

[0079] Figs. 16A and 16B show first dispersion curves at suspension point and at the vibrating resonator, respectively. The resonator is an AIN piezoelectric layer 120 of thickness 1.844 gm. The suspension point comprises the AIN piezoelectric layer 120 of thickness 1.844 gm, the SiOz insulating layer 124 of thickness 0.5 gm, and the top metal electrode layer 118 of thickness 0.3 gm.

[0080] Figs. 17A and 17B show second dispersion curves at suspension point and at the vibrating resonator, respectively. The resonator is an AIN piezoelectric layer 120 of thickness 1.844 gm. The suspension point comprises the AIN piezoelectric layer 120 of thickness 1.844 gm, the SiOz insulating layer 124 of thickness 1 gm, and the top metal electrode layer 118 of thickness 0.5 gm.

[0081] The dispersion curves show the possible combinations of the lateral component of the wave number kxagainst frequency as dark lines. It is a typical case that the dispersion curves form smooth continuous lines. The positive values of kxstand for real valued wave number, but the negative side of the axis is used to represent imaginary values of kx. For a real valued kxthis is a plane wave propagating in the direction of the x-axis. If, however, kxis imaginary, which is an evanescent wave. Thus, the evanescent branches of the dispersion curves represent non-prop- agating, non-energy carrying vibration. The energy trapping can be accomplished by designing the resonator so that at the resonance frequency, and in a sufficiently wide band around it, the surroundings support only an evanescent wave. In case of AIN, bringing evanescent wave behaviour to the outside region of a resonator actually means thickening or mass loading the outside area. A cutoff frequency is defined by the frequency at which the dispersion curve crosses to the imaginary side.

[0082] The description and the related drawings are only intended to illustrate the principles of the present invention by means of examples. Various alternative embodiments, variations and changes are obvious to a person skilled in the art on the basis of this description. The present invention is not intended to be limited to the examples described herein but the invention may vary within the scope and spirit of the appended claims.

Claims

CLAIMS1. A thin film bulk acoustic wave (BAW) resonator, comprising a substrate, a piezoelectric stack structure having a piezoelectric layer sandwiched between a top conductive electrode layer and a bottom conductive electrode layer on the substrate, wherein the top and bottom conductive electrodes are detached from the vibrating piezoelectric layer by intermediate gaps formed on both sides of the piezoelectric layer, the vibrating piezoelectric layer being suspended between the top and bottom conductive electrodes.

2. The thin film bulk acoustic wave resonator as claimed in claim 1, wherein a resonance frequency of the BAW resonator is defined solely by the thickness of the vibrating piezoelectric layer.

3. The thin film bulk acoustic wave resonator as claimed in claim 1, wherein a resonating volume of the resonator includes solely the vibrating piezoelectric layer and optionally one or more temperature compensating layers deposited on the piezoelectric layer.

4. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, comprising a top temperature compensating layer and a bottom temperature compensating layer deposited on the top surface and on the bottom surface, respectively, of the vibrating piezoelectric layer, wherein a top gap is between the top temperature compensating layer and the top conductive electrode, and a bottom gap is between the bottom temperature compensating layer and the bottom conductive electrode, and wherein the top and bottom temperature compensating layer induce opposite direction of temperature drift of resonance frequency as the piezo layer, being preferably silicon dioxide (SiO2) layers.

5. The thin film bulk acoustic wave resonator as claimed in any one of claims 1-3, wherein the piezoelectric layer may comprise two piezoelectric layers having a temperature compensating layer sandwiched between them.

6. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, wherein the intermediate gaps are formed by a sacrificial layer removal process.

7. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, wherein the conductive top and bottom electrode layers are made of metal, a metal alloy, a sufficiently conducting semiconductor, a conductive oxide, or a layered stack of materials providing sufficient conductivity andstructural integrity.

8. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, wherein the piezoelectric layer comprises a deposited thin film layer or a single crystal layer of a piezoelectric material, the piezoelectric material preferably being one of aluminum nitride (AIN), scandium aluminium nitride (ScAlN), lithium niobate (LiNbO3), andlithium tantalate (LiTaO3).

9. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, wherein the substrate comprises a silicon (Si) substrate, a glass substrate, a quartz substrate, or a sapphire substrate, and wherein the substrate preferably is a high electrical resistivity substrate.

10. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, comprising at least one thin film protective layer deposited on top of the piezoelectric stack structure.

11. The thin film bulk acoustic wave resonator as claimed in any one of the preceding claims, comprising energy confining edge provided at suspension points of the piezoelectric layer, the energy confining edge preferably comprising thickening or mass loading structure on the piezoelectric layer at the suspension points.

12. The thin film bulk acoustic wave resonator as claimed in claim 11, the energy confining edge comprises a stack of an insulating layer, the conductive top electrode, and optionally a temperature compensating layer on the piezoelectric layer at the suspension points.

13. A method of manufacturing a thin film bulk acoustic wave resonator having a piezoelectric stack structure having a piezoelectric layer sandwiched between a top conductive electrode layer and a bottom conductive electrode layer on a substrate, the top and bottom electrodes are detached from the vibrating piezoelectric layer by top and bottom gaps formed on top and bottom sides, respectively, of the piezoelectric layer, the method comprising: i) forming an insulating layer on the substrate; ii) forming the bottom conductive electrode on the insulating layer; hi) forming a bottom sacrificial layer on the bottom conductive electrode: iv) forming the piezoelectric layer on the bottom sacrificial layer; v) forming a top sacrificial layer on the piezoelectric layer; vi) forming the top conductive electrode on the top sacrificial layer; vii) removing the top and bottom sacrificial layers by etching with anetching agent introduced through the one or more vias and thereby forming the top and bottom gaps.

14. The method as claimed in claim 13, wherein the top and bottom sacrificial layers are made of molybdenum (Mo) or silicon (Si), and / or wherein the etching agent is xenon difluoride (XeF2) vapor.

15. The method as claimed in claim 13, wherein the top and bottom sacrificial layers are made of silicon dioxide (SiOz), and / or wherein the etching agent is hydrofluoric acid (HF) vapor.

16. The method as claimed in claim 13, 14 or 15, wherein the step iv) of forming the piezoelectric layer comprises forming the piezoelectric layer with a temperature compensating layer on at least one side or forming two piezoelectric layers with a temperature compensating layer sandwiched therebetween.

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