Protected resonator

Cross-reference of related applications This application claims the benefit of US Patent Application No. 12 / 321,860, filed January 26, 2009, entitled "Protected Resonator," the disclosure of which is incorporated herein by reference. ..

Highly stable resonators that are largely unaffected by environmental effects and aging, more specifically thin film bulk sonic resonators, are disclosed herein.

The response of bulk sonic resonators (FBAR, SMR, HBAR, etc.) exhibits long-term drift in their characteristics, especially in frequency. This long-term change in time dependence is known as resonator drift. Drift is caused by both intrinsic and extrinsic factors, and intrinsic instability is often referred to as resonator aging. Aged deterioration occurs even when external environmental factors are kept constant. Figure 1 shows an example of the SMR structure according to the prior art.

In the literature, Walls and Vig, "Fundamental Limits on the Frequency Stabilities of Crystal Oscillators," IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, 42 (4): 576-589, July 1995 (Walls herein). And Vig, 1995), as well as Vig and Meeker, "The Aging of Bulk Acoustic Wave Resonators, Filters, and Oscillators", Proc.45th Ann.Symp.Frequency Control, IEEE Cat.No.91 CH2965-2, 77- Page 101, (1991) (Vig and Meeker herein) (Called 1991) presents a classification of the mechanisms that cause aging of resonators and oscillators. These mechanisms include mass transfer to and from the surface of the resonator due to contaminant deposition or delamination, stress relaxation in crystal mounting structures, changes in electrodes, leaks in packages, and changes in piezoelectric materials. Other mechanisms include external environmental effects such as temperature and stress cycling (hysteresis) and inertial effects.

Overall, previous attempts to stabilize the resonator against environmental effects and aging have focused on frequency stability. This effort has focused on packaging design and mounting structural design.

Resonator packaging has typically been the primary method of protecting resonators against aging caused by contamination and leakage. Packaging also partially isolates the resonator from external environmental effects.

To protect the quartz resonator from certain aging phenomena, the quartz resonator has traditionally been packaged in a container. Many examples exist in the prior art. See, for example, US Pat. No. 5,640,746 entitled "Method of Herm etically Encapsulating a Crystal Oscillator Using a Thermoplastic Shell" by Knecht et al., Published June 24, 1997 .

Micromechanized thin film resonators are packaged using wafer-scale or device-scale encapsulation techniques. Many examples exist in the prior art. Micromechanized thin film resonators such as silicon resonators and thin film bulk sonic resonators (FBARs) use micromechanical support structures such as struts and suspensions. These structures are also designed to minimize stress transfer, including temperature-induced stress in the crystal resonator. See, for example, Kim et al., "Frequency stability of wafer-scale film encapsulated silicon-based MEMS resonators," Sensors and Actuators A 136 (2007), pp. 125-131. See also US Pat. No. 7,153,717, entitled "Encapsulation of MEMS Devices Using Pillar-Supported Caps," published December 26, 2006 by Carley et al.

Recent methods of packaging make it difficult to integrate them into the product with high profiles (in the case of quartz), or those methods are released of inertial resonators or FBARs. Such thin film structures are either encapsulated and susceptible to other forms of instability, such as acceleration or impact.

The mounting structure is another location that may deteriorate over time. The stress induced by packaging and transmitted to the crystal causes long-term frequency aging.

The quartz resonator was attached via a support leg before being sealed under the cap. The support structure is carefully designed to minimize stress transfer to the crystal (and therefore aging). Many examples exist in the prior art. See, for example, US Pat. No. 4,642,510, entitled "Mount for quartz crystal oscillator device," by Yamashita, issued February 10, 1987. See also US Pat. No. 5,030,875, entitled "Sacrificial Quartz Crystal Mount," by Knecht, issued July 9, 1991. It has also been advocated to use electrostatic levitation to levitate crystals so as to minimize the aging effects associated with mechanical mounting structures. See Wall and Vig, 1995.

Recent methods of resonator mounting are susceptible to inertial and thermal fatigue, and thus aging. However, none of these approaches address the protection of the crystal and / or electrode material itself. Recent approaches do not protect crystals or electrode materials from environmental effects, including aging.

<p num="0013"><patcit num="1"><text>U.S. Pat. No. 5,640,746</text></patcit><patcit num="2"><text>U.S. Pat. No. 7,153,717</text></patcit><patcit num="3"><text>U.S. Pat. No. 4,642,510</text></patcit><patcit num="4"><text>U.S. Pat. No. 5,030,875</text></patcit><patcit num="5"><text>U.S. Patent Application No. 12 / 002,524</text></patcit></p>

<p num="0014"><nplcit num="1"><text>Walls and Vig, "Fundamental Limits on the Frequency Stabilities of Crystal Oscillators", IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, 42 (4): 576-589, July 1995</text></nplcit><nplcit num="2"><text>Vig and Meeker, "The Aging of Bulk Acoustic Wave Resonators, Filters, and Oscillators", Proc.45th Ann.Symp.Frequency Control, IEEE Cat.No.91 CH2965-2, pp. 77-101, (1991)</text></nplcit><nplcit num="3"><text>Kim et al., "Frequency stability of wafer-scale film encapsulated silicon-based MEMS resonators," Sensors and Actuators A 136 (2007), pp. 125-131.</text></nplcit></p>

<p num="0015"> Focusing research on the date of frequency stability is appropriate for certain applications. However, it is desirable to have a more overall focus on the overall behavior of the resonator near the primary resonance, both in frequency (f) and in time.</p>

<p num="0016"> The present invention relates to a bulk sonic resonator structure that isolates the core resonator from both environmental and aging effects. This structure still provides excellent inertial resistance while protecting against changes to the piezoelectric material due to contamination, package leakage, and external effects such as ionizing radiation and package stress. In a preferred embodiment of the invention, the structure has one or more protective elements that limit the aging effect below a defined threshold. The threshold is expressed herein as a sub-value shift in the impedance response of the resonator as a function of frequency. It is well understood by those skilled in the art that acoustic resonator devices have impedance values for each frequency value. As the resonator ages, the impedance value associated with the frequency value can change over time. Therefore, the designed impedance value (Z) associated with frequency f for the acoustic resonator is the value Z over time._iMay change to. The present invention limits the rate of frequency change below a certain level.</p><p num="0017"> More specifically, when the acoustic resonator is constructed in an oscillator, the frequency f associated with the complex impedance Z = (Za, Zφ)._OSCOscillates at. The impedance of this complex value is the design value g for which the value of the function g (Z) is specified.₀First determined when is equal to. The drift in the behavior of the resonator within that bandwidth, the complex impedance is g₀Occurs when changing from a value associated with. This drift is the oscillator frequency (f) associated with the complex impedance._OSC), G₀Related to complex value impedance in (f)_OSC) Appears as a transition. f_OSCDrift in is expressed herein as a change in ppm at a frequency associated with the impedance of a particular complex number per unit time. As used herein, "ppm" is Hz relative to the MHz scale. That is, a change of 5 Hz is f of 1 MHz._OSCIt is a change of 5ppm in. f_OSCThe rate of change in is less than about 5 ppm / year.</p><p num="0018"> In other embodiments, this drift can be measured as a change in the series or parallel resonance of the resonator. It will be appreciated by those skilled in the art that the impedance response is minimal at the amplitude associated with a particular frequency over the bandwidth of the resonator. The frequency associated with this minimum amplitude is called series resonance. The frequency associated with the maximum amplitude is parallel resonance. Also, the effect of aging appears as a change in frequency associated with series and parallel resonances. In these preferred embodiments, the transition at at least one of the series or parallel resonant frequencies of the device is about 5 ppm / year or less.</p><p num="0019"> According to the embodiments described herein, mitigating the effects that cause aging is more than simply minimizing the frequency drift of series or parallel resonances. For example, the series resonance frequency is determined by the acoustic path and any parasite. Therefore, the electromechanical coupling coefficient and permittivity of the resonator material (and changes in electromechanical coupling coefficient and permittivity over time) have no effect in determining the series resonant frequency and subsequent drifts at that frequency. ..</p><p num="0020"> As a result, in certain embodiments, the behavior of the resonator is stabilized not only in series resonance, but over the entire bandwidth of the resonator. To achieve such stability, the electromechanical coupling coefficient and dielectric constant of the resonator material must be stable over the long term. Embodiments of the present invention address not only the long-term aging effect of frequency drift, but also the short-term environmental instability. However, in one preferred embodiment, the behavior of the resonator is stabilized such that the transition at resonance frequency for at least one of series and parallel resonances is less than 5 ppm / year of device operation.</p><p num="0021"> The resonator has a protective element that is the color of the material, in one particular embodiment, the piezoelectric material, so as to keep environmental and aging phenomena associated with the periphery and edges away from the core resonator. By providing it around, it is protected from the effects of material degradation due to environmental and / or aging processes.</p><p num="0022"> In one embodiment, the resonator is protected from the effects of surface contamination by surrounding the entire surface of the resonator with a protective element that is a Bragg layer that traps multiple energies.</p><p num="0023"> In some embodiments, package stresses and environmental stresses are damped, thereby preventing them from reaching the main cavity structure. Attenuation is achieved by providing one or more layers of protective elements formed above and around the cavity functional site. These layers are Bragg layers, spacer layers, stress relaxation layers or sealing layers, or any combination thereof, as described in more detail herein.</p>

<figref num="1">It is sectional drawing of SMR (solid-state mounting type resonator) of the prior art of thin film solid-state mounting.</figref><figref num="2">It is a frequency spectrum of the electrical impedance of the resonator.</figref><figref num="3">FIG. 3 is a cross-sectional view of an alternative embodiment of the present invention having a core resonator surrounded by a collar.</figref><figref num="4">FIG. 5 is a cross-sectional view of an alternative embodiment having a core resonator surrounded by a collar.</figref><figref num="5">It is a figure of the frequency spectrum of the prior art SMR (Fig. 1).</figref><figref num="6">It is a figure of the frequency spectrum of an alternative embodiment.</figref><figref num="7">It is a figure of the frequency spectrum of the prior art SMR (FIG. 1) of the same material as the SMR of FIG.</figref><figref num="8">It is a figure of the frequency spectrum of the alternative embodiment of the same material as SMR of FIG.</figref><figref num="9">FIG. 5 is a cross-sectional view of an alternative embodiment having a core resonator surrounded by a collar.</figref><figref num="10">FIG. 6 is a cross-sectional view of another alternative embodiment of an FBAR configuration having a core resonator, collar and spacer under the cap.</figref><figref num="11">FIG. 5 is a cross-sectional view of an alternative embodiment in which the FBAR is supported in the center of the structure in an FBAR configuration having a core resonator, collar and spacer under the cap.</figref><figref num="12">FIG. 6 is a cross-sectional view of an alternative embodiment having a core resonator and collar and having a reversing SMR configuration encapsulated under a cap constructed as a Bragg reflector.</figref><figref num="13">FIG. 6 is a cross-sectional view of an alternative embodiment having an inverted SMR configuration encapsulated under a cap constructed as a Bragg reflector having a core cavity and collar and having an alternative electrode configuration.</figref><figref num="14">f_OSCIt is a graph which shows the complex impedance response of a resonator as a function of.</figref>

For clarity and brevity, the same elements and parts have the same symbols and numbers throughout the drawings.

FIG. 1 is a cross-sectional view of a prior art SMR (solid-state resonator) 10 for thin-film solid-state mounting. The SMR has a lower Bragg reflector 12, a lower electrode 14, a layer of piezoelectric 16 and an upper electrode 18. The lower Bragg reflector 12 has a plurality of pairs of high acoustic impedance layers 24 and low acoustic impedance layers 22. Each layer (22, 24) has a thickness approximately equal to an odd multiple of the 1/4 acoustic wavelength of the mode of operation in the material. The operating mode is structurally supported and is one of several resonance modes, including thickness vibration mode, thickness slip mode, and others. For each of these modes, a resonator spectrum as shown in FIG. 2 is feasible. Choosing a thickness that is an odd multiple of the 1/4 acoustic wavelength based on a particular mode results in the maximum reflection of total internal reflection of sound energy in that mode. It is also possible to optimize the thickness to a value different from the thickness of the quarter wavelength in order to obtain good reflection of sound energy in two or more different modes.

An embodiment of the present invention reduces the impedance drift of all resonators near the primary resonance. The primary resonance is the region of the spectrum from just before the series resonance 28 of the device to just after the peak of the parallel resonance 30, with reference to FIG. Series resonance is usually defined as the frequency with the lowest impedance. Parallel resonance is often defined as the frequency with the highest impedance. The frequency range between the two is known as the resonator bandwidth.

Embodiments of the invention provide protective elements that mitigate long-term drift, preferably at least one or both of series and parallel resonances. A preferred embodiment of the invention has at least one of three features collectively referred to as the protective element, which is the main contributor to these unwanted transitions in series and / or parallel resonances. Address some of the most important environmental effects and aging mechanisms. One such embodiment is shown in FIG. The first exemplary protective element is the collar 44 formed around the core resonator 42. The collar 44 shown in FIG. 3 is a region surrounding the core resonator and includes a layer of material forming the core region. For example, the peripheral portion of the piezoelectric layer 16 extends into the collar 44 and forms a part thereof. The collar 44 ensures that environmental and aging phenomena associated with the periphery and edges are kept away from the core resonator 42.

The core resonator 42 is surrounded on all sides by Bragg layers (22, 24) and is therefore unaffected by contaminant deposition. Such a Bragg layer is described in previously cited U.S. Patent Application No. 12 / 002,524.

In one alternative embodiment (FIG. 4), the overall structure 52 is encapsulated within a protective element, such as airgel, which is a low acoustic impedance, low density capsule material 50. In this embodiment, package and external stresses are not transmitted to structure 52.

With reference to FIG. 3 again, the cross section of 52 has a core resonator 42 surrounded by collar 44. The structure 52 has a Bragg reflector 48 that traps energy both below (12) and above (40) the piezoelectric layer 16. The structure also has a lower electrode 14, a layer of spacers 38, an optional temperature compensation layer 32, an upper electrode 18, an optional interconnect 34 layer, and a passivation 36 layer. Each reflector has a plurality of pairs of high acoustic impedance layers 24 and low acoustic impedance layers 22. Each of these Bragg material layers (22, 24) has a thickness equal to an odd multiple of the 1/4 acoustic wavelength of the mode of operation in the material. The layer of spacer 38 can have a thickness equal to an odd multiple of the 1/4 acoustic wavelength of the mode of operation in the material. The optional interconnect 34 layer electrically connects the resonator electrodes (14, 18) to the pad (49) or through a via 46 to a CMOS circuit (not shown) in the lower substrate 20. The present invention contemplates both single-ended and differential embodiments. As used herein, a single-ended embodiment is one in which the upper and lower electrodes are each electrically connected to a circuit. In a differential embodiment, both electrodes connected to the circuit are placed on one side of the device. The electrodes on the other side remain floating.

The structure shown in FIG. 3 is formed by the method shown below. The layered structure 52 is formed on the substrate 20 by depositing or patterning layers of material as follows. The layers are deposited to form a lower Bragg reflector 12 (formed by alternating high acoustic impedance layers 24 and low acoustic impedance layers 22) and a lower electrode 14. The material of the piezoelectric 16 is deposited on the lower electrode 14. The piezoelectric 16 material is patterned by photolithography and etching. The electrodes 14 and, in some cases, reflectors 12 are then similarly patterned by photolithography and one or more subsequent etching steps. A layer of insulating spacer 38 is then deposited and patterned, whereby the spacer material above the portion of the piezoelectric 16 material is removed from the core resonator region 42 where the device is formed. Therefore, the spacer layer defines the core and collar areas in this embodiment. The top electrode 18 is formed over the exposed area of the piezoelectric 16 material to create the active part of the device. In products where it is desired to integrate the resonator on a substrate with a CMOS device, an ohm contact to the lower electrode 14 is provided. In the illustrated embodiment, photolithography and etching form a via 46 through the insulating spacer layer 38 down to the lower electrode 14. Conductive interconnects 34 are then formed to electrically connect electrodes 14 and 18 to CMOS devices formed within the substrate, if necessary. The energy-confining Bragg reflector layers 22 and 24, which form the upper Bragg structure, are then deposited in sequence, followed by pattern formation. Preferably, such patterning provides access to the interconnect layer 34 for later packaging. The sealing layer 36 is then formed on top of the structure to further protect and isolate the active structure from the environment and environmental influences. Layer 36 is referred to herein as a passivation layer or sealing layer. As a passivation layer or a sealing layer

FIG. 4 is a cross-sectional view of a preferred embodiment of the invention, which also has a structure 52 having a core resonator 42 surrounded by a collar 44 of protective elements. A protective element, which is the low acoustic impedance material 50, is installed over the entire structure 52. This material encapsulates the device and dampenes the transfer of external and package stresses to the device. Similar to the embodiment shown in FIG. 3, this embodiment may also be single-ended or differential. To further reduce the transfer of stress in the package, which changes over time, after the resonator has been assembled into the package, before overmolding the package with plastic, it is commonly referred to as a "glop top". An additional layer of flexible material, such as a known encapsulating resin (eg, silicon), may be deposited on top of the resonator. Encapsulating resins are well known to those of skill in the art and are not described in detail herein.

The material formed on the structure 52 is a low density, low acoustic impedance encapsulating material 50 such as airgel. Layer 50 is deposited on top of structure 52 prior to plastic packaging. Layer 52 ensures that external and package stresses are not transmitted to the core cavity structure 42.

As mentioned above, the collar 44 around the core resonator 42 shown in FIGS. 3 and 4 separates the core resonator 42 from most environmental effects. The collar attenuates thermal, stress or aging effects before they reach the core resonator 42. Applicants do not want to be bound by any particular theory, but consider the collar 44 to act like a parasitic resonator in the core resonator 42. Advantageously, the magnitude of the parasitic resonance can be customized during device design to meet the performance requirements of the product.

Also, as mentioned above, the use of the Bragg reflector 48, which traps energy, not only separates the core resonator 42 from external contaminants released by the environment, but also resists thermal and / or stress effects. It is a protective element that provides additional sealing for the core resonator 42. The protective element 50 is a stress barrier that protects the core resonator from external sources (eg, packaging, environment) to prevent these stresses from propagating to the core resonator 42.

Another spacer 38 is also used to electrically insulate the lower electrode 14 from the upper electrode 18 in addition to supplying additional material into the area of the collar 44 to dampen environmental effects. It is a protective element. In this respect, it is advantageous if the spacer layer 38 is a dielectric material (eg, silicon dioxide). In a preferred embodiment, the layer thickness of the spacer 38 is approximately equal to an odd multiple of the Bragg wavelength in the material. Bragg wavelength is the following formula v = v (c33 / d) (1) Defined in, where v is the speed of sound in the material, c33 is the stiffness factor of the material in the wave propagation direction, and d is the material density. The layer thickness is the following formula t = Nx (v / f) / 4 (2) Calculated in, where t is the layer thickness and f is the Bragg center frequency. N is an odd number greater than or equal to 1.

Referring to FIG. 14, the impedance response to the oscillator is f._OSCShown as a function of. The impedance response is shown for oscillator-type resonator devices. The device has a frequency f determined by the complex impedance Z = (Za, Zφ)._OSCOscillates at. This impedance value is the design value g for which the value of the function g (Z) is specified.₀It is confirmed when it is equal to. The drift within its bandwidth in the behavior of the resonator is such that the impedance of the resonator is equal to (Za, Zφ) f_OSCCauses a drift in the value of. f_OSCDrift in can be quantified at ppm / hour. According to the present invention, the aging effect is controlled so that the change in the frequency related to the complex impedance Z = (Za, Zφ) changes at a rate of 5 ppm / year or less.

External stresses and / or ionizing radiation are examples of environmental conditions that can alter the material properties of the resonator. In particular, the characteristics of piezoelectric 16 are susceptible to changes due to external factors. FIG. 5 shows the resonance spectrum of SMR according to the prior art (SMR shown in FIG. 1). As mentioned above, ionizing radiation or other external processes can change the material structure of the resonator layer and therefore the electromechanical properties of the resonator layer. These changes appear as changes or transitions in the resonator spectrum. When there is a 10% reduction in the coupling coefficient of the piezoelectric 16 in the SMR as shown in FIG. 1, there is a large transition in the resonance spectrum. The structure of FIG. 1 has a 3.5 pair of tungsten / oxide Bragg reflector layers, an AlN (aluminum nitride) piezoelectric layer 16, and molybdenum (MO) electrodes (14, 18). Series and parallel resonances are Agilent Advanced Design A long-term model for this structure was created using the 1-D acoustic model and 3-D electrical model implemented in System (ADS) 2006. The effect of a 10% reduction in the coupling coefficient of the resonant spectrum on the prior art structure shown in Figure 1 was modeled. For this structure of FIG. 1, as shown in FIG. 5, FIG. 5 shows a -510 ppm transition at series resonance 54 (ie, a downward transition of 870,570 Hz) and a -2200 ppm transition at parallel resonance 56 (ie, a downward transition of 870,570 Hz). That is, it indicates that there is a downward transition of 3,790,600 Hz). A larger reduction in parallel resonance 30 compared to series resonance 28 results in a reduced k2 or bandwidth for the resonator. For many applications, such transitions in behavior can have a significant negative impact on performance.

FIG. 6 has the same material as the prior art example of FIG. 5 with respect to the Bragg layer, piezoelectric layer and electrodes, but with a 10% reduction in the binding of the piezoelectric 16 of the color 44 material 52 (eg, eg The figure of the frequency spectrum of FIG. 3 and FIG. 4) is shown. This change, caused by the same environmental conditions that caused the transition shown in FIG. 5, in the resonance spectrum is + 32 ppm transition at parallel resonance 56 (ie, upward transition at 54,624 Hz) and 0 ppm at series resonance 28. The result is a negligible transition of the transition (0 transition). The reason for the difference observed in the effect of the coupling coefficient on the frequency transition of the two devices is that in the devices of FIGS. 3 and 4, the change in the coupling coefficient of the piezoelectric material was mostly confined to the color region 42. .. The piezoelectric material in the core resonator 44 had a very small change in coupling coefficient.

In the example shown in FIG. 6, the resonant transition is barely visible in narrowband frequency sweeps. Sweeping a very narrow band makes it clear that there is a negligible transition at parallel resonance 56 and no transition at series resonance 28. This provides the structural capabilities described herein to protect the core resonator 42 without affecting the performance of the core resonator 42 by providing a collar 44 that mitigates the adverse environmental impact on material properties. Shown.

FIG. 7 is a diagram of the frequency spectrum of the SMR of FIG. 1 using the same material as identified in the discussion of FIG. FIG. 7 shows the transition in the resonance spectrum of SMR by a 0.1 μm thick oxide contaminated layer on the top surface of the device (piezoelectric layer 16 in FIG. 1). Series and parallel resonances have been modeled over time for this structure using the software tools described above. As an example of the physical effect of aging in one scenario over the prior art structure shown in Figure 1, a 0.1 μm thick oxide contaminated layer on the top of the device is used for the first year of use. It is expected to be formed over. The contaminant can be any material that deposits at any rate over any period of time. It was used solely to explain the concept and benefits of upper Bragg. This causes a -1143 ppm transition (ie, 1,939,671 Hz downward transition) at series resonance 54 and a -2461 ppm transition (ie, 4,199,697 Hz downward transition) at parallel resonance 56. In addition, spurious mode 58 is triggered near the principal resonance of the spectrum.

Figure 7 shows the large changes that occur in the resonator response as a result of the deposition of contaminants on the structure. A large negative transition at the series resonance 54 and a larger negative transition at the parallel resonance 56 result in a reduced k2 or bandwidth for the resonator. In addition, spurious mode 58 is triggered near the main resonance, further changing the behavior from baseline. For many applications, such a shift in performance is unacceptable.

FIG. 8 is a diagram of the frequency spectrum for structure 52 made of the same material as described in FIG. The structure has an oxide contaminated layer with a thickness of 0.1 μm on the top surface of the device. In contrast to the spectral transitions caused by this layer on prior art devices, oxide contamination causes a negligible transition to the resonant spectrum of the device having structure 52.

Specifically, resonance transitions are barely visible in narrowband frequency sweeps when structure 52 is affected by the same environmental pollution as the prior art structure of FIG. As shown in FIG. 8, there is a + 1 ppm transition (ie, 1,697 Hz upward transition) at series resonance 54 and a + 1.2 ppm transition (ie, 2,047.8 Hz upward transition) at parallel resonance 56. This indicates that the resonator is protected by an energy trapping Bragg structure that attenuates the acoustic and electrical effects of contaminants that deposit on the surface of the device.

By introducing an airgel-like capsule material 50 around the device (Figure 4) as an additional protective element, both stresses are further dampened by moving the package and inertial stresses further away from the resonator device.

FIG. 9 is another embodiment of the present invention. This embodiment is also configured to protect the resonator from external stress. The SMR structure is equipped with a protective element, the collar 44, as explained earlier. The resulting structure is then covered with another protective element. Its protective element is the structure of a device-level thin film cap 60 that can not only isolate the resonator from the effects of contaminants, but also attenuate package stress. In this structure, the stress management function of the structure is separated from the function of the acoustic resonator.

The structure of FIG. 9 is produced in the following order. The lower Bragg reflector 12 (alternate stacking of high acoustic impedance layers 24 and low acoustic impedance layers 22) is formed by alternately stacking and depositing a plurality of layers 22 and 24 on a substrate 20. A material layer of the lower electrode 14 and the piezoelectric 16 is deposited on the Bragg reflector 12. The piezoelectric 16 material is patterned by photolithography and etching. The electrode 14, and in some embodiments, the reflector 12 is then patterned by photolithography and one or more etching steps. A layer of insulating spacer 38 is then deposited and patterned to remove the layer of insulating spacer 38 from the portion of the piezoelectric 16 material within region 42. An upper electrode 18 is formed over the exposed area of the piezoelectric material 16 to create an active part of the device. In products where it is desirable to integrate the resonator with a CMOS device (and therefore require an ohm contact to the bottom electrode 14), the via 46 is photolithographically and etched into the bottom electrode 14 through the insulating layer. Is formed up to. Conductive interconnect layers 34 are formed to electrically connect to each of the electrodes. Up to this point, the manufacturing process described for the embodiments shown in FIGS. 3 and 4 continues.

By first depositing the sacrificial layer on top of the structure, forming the structure of the cap 60 on it, and then patterning the structure to remove the sacrificial layer, the structure of the cap 60, which is a protective element, is on top. It is formed on the structure described in. The resulting space accommodates a vacuum or rare gas in the finished device. The sacrificial layer may be made of any material that can use the underlying exposed material as an etch stop. Silicon is an example of a suitable sacrificial layer. The cap layer as shown has a two-layer structure of an internal protective layer 62 and an external sealing layer 64. The internal protective layer 62 is deposited on top of the sacrificial layer described above. The sacrificial layer has a via formed therein, and the via is filled with the material of the protective layer when the protective layer is formed on the sacrificial layer. Since these vias are filled with the protective layer material, the protective layer material remains when the sacrificial layer is removed. The inner protective layer is patterned so as to remain on the device region 42 fixed to the substrate 20 by the protective layer material that remains after the sacrificial layer is removed. The sacrificial layer is then completely etched off, leaving the device and protective layer 62 as a freestanding, unconstrained structure. This is observed by the gap 63 between the device structure and the protective layer 62. The sealing layer 64 is deposited over the structure and encapsulates the device under the cap 60. The layer 62 may be any structurally rigid material such as alumina, silicon nitride, gold, etc. In one embodiment, layer 62 is configured as a Bragg structure as described above. Layer 64 may be a material that provides an airtight seal to the structure of the underlying layer. Alumina and silicon nitride are examples of suitable materials.

FIG. 9 is a cross-sectional view of alternative embodiment 66 described above, having a core resonator 42 surrounded by collar 44. The structure 66 has a lower Bragg reflector 12, a lower electrode 14, a piezoelectric 16 layer, a spacer 38 layer, an optional temperature compensation layer 32, an upper electrode 18, and an optional interconnect 34 layer. The Bragg reflector 12 has a layer in which a high acoustic impedance layer 24 and a low acoustic impedance layer 22 are alternately overlapped. The Bragg reflector has multiple pairs of these layers. Each of these material layers has a thickness equal to 1/4 acoustic wavelength of the mode of operation in the material. The cap 60 formed on the resonator structure has an internal protective layer 62 and an outer covering layer 64. Similar to the other embodiments described herein, the structure of FIG. 9 may be single-ended or differential.

For some applications, it is well suggested that the stress due to the wafer itself requires an unconstrained (FBAR) structure, even if the unconstrained (FBAR) structure exhibits reduced inertial resistance. 10 and 11 show another alternative embodiment 66. The unconstrained FBAR has a collar 44 and a spacer 38 that protect the core resonator 42. The FBAR is covered with the structure of the cap 60, as described above for FIG.

The structures shown in FIGS. 10 and 11 are formed as follows. A sacrificial exfoliation layer is deposited and patterned to remain beneath the device area 42. A temperature compensation layer 32, a lower electrode 14, a material of piezoelectric 16 and an upper electrode 18 are formed on the substrate 20. The top electrode 18 is patterned by photolithography and etching. The piezoelectric 16 material, bottom electrode 14, and temperature compensating layer are all patterned (preferably with the same mask) using photolithography and a series of etchings. A second temperature compensating layer 32 is deposited and patterned to seal both regions 42 and 44 of the device, except for the electrical interconnection to the top electrode 18. The conductive layer 70, which electrically connects the devices and mechanically supports the peeled structure, is then deposited and patterned.

The structure of the cap 60 is formed over the upper structure. A second sacrificial layer (not shown) is deposited and patterned. The internal protective layer 62 is deposited and remains over the device area and is patterned so that it is secured to the substrate 20 through holes only in the second sacrificial layer. The sacrificial layer is etched off, leaving the device and protective layer as an unrestrained structure that stands unsupported. The sealing layer 64 is deposited and the device is encapsulated under the protective membrane.

With reference to FIG. 10, the structure has a core resonator 42, a collar 44, and a spacer 38 under the cap 60. The structure 66 has a lower electrode 14, a piezoelectric 16 layer, a spacer 38 layer, an optional temperature compensation layer 32, an upper electrode 18, and an interconnect layer 70. Also, the layer of interconnect 70 is a suspension support. The suspension 70 supports the FBAR and is connected to the substrate 20 by an anchor 72. The interconnect / suspension 70 is made of a metal such as aluminum or copper. In the situation where the resonator is built directly on top of the previously made circuit wafer, the material of the fixture 72 is also metal to facilitate interconnection with the underlying circuit. The cap 60 formed on the resonator structure is composed of an internal protective layer 62 and a sealing layer 64.

FIG. 11 shows a variant of the FBAR embodiment shown in FIG. 10, where the FBAR is supported by a middle column 72 instead of a side support. The advantage of this embodiment is that temperature-related stresses and other environmental effects (one of the main contributors to device aging) do not induce changes in the performance of the FBAR. This is because the temperature coefficient incompatibility of different materials in the cavity structure, especially piezoelectric materials and oxides, cannot cause stress at the material interface.

FIG. 11 is a cross-sectional view of an embodiment of this alternative middle pillar. FIG. 11 shows the core resonator 42, collar 44 and spacer 38 formed under the cap 60, with the FBAR supported in the center of the structure. The structure includes a lower electrode 14, a piezoelectric 16 layer, a spacer 38 layer, an optional temperature compensation layer 32, an upper electrode 18, and an interconnect (not shown) layer. The layers of the top electrode 18 and the interconnect 34 form the suspension 70 in the illustrated embodiment. The suspension 70 supports the FBAR and is connected to the substrate 20 by a fixture 72. The substrate 20 has a barrier layer 78 formed on the substrate 20. The barrier layer is any suitable material, such as silicon nitride. The cap 60 is formed on top of the resonator structure. The cap 60 has an internal protective layer 62 and a sealing layer 64.

An alternative FBAR configuration is shown in Figure 13. In this embodiment, the interconnect 76 is placed under the cap 60, especially under the protective layer 62 of the cap 60. In this embodiment, the top electrode 18 is capacitively or inductively coupled to an interconnect 76 located under the cap. An alternative configuration in which the interconnect 76 is located on the barrier layer 78 is not shown. In this configuration, the lower electrode 14 is capacitively or inductively coupled to the interconnect 76. Therefore, the lower electrode 14 is a two-part electrode in this alternative configuration. The upper electrode in the alternative configuration is one part (as in the lower electrode 14 shown in FIG. 13). Also, the interconnection is two parts in this alternative configuration, one part of each being placed on either side of the temperature compensation layer placed on the barrier layer. The advantage of this structure is that neither the lower electrode 14 nor the upper electrode performs a supporting function. Therefore, deterioration of their mechanical properties due to environmental effects or aging does not affect the performance of the resonator.

FIG. 12 is a cross-sectional view of the alternative embodiment 74. This embodiment is an inverted SMR with core resonator 42 and collar 44, both core resonator 42 and collar 44 are encapsulated under a cap 40 constructed as a Bragg reflector. The structure 74 has a lower electrode 14, a layer of piezoelectric 16 and an upper electrode 18. The temperature compensation layer is optional and is not shown. The cap 40 formed on the resonator structure is composed of an internal protective layer 62 and a sealing layer 64. The sealing layer 64 is configured as an upper Bragg reflector that traps energy. As described above, such layers have a plurality of layer pairs, each pair having a high acoustic impedance layer 24 and a low acoustic impedance layer 22. As explained above, each of these materials has a thickness equal to the 1/4 acoustic wavelength of the mode of operation in the material. The individual reflector layers are not shown, but are shown as one monolithic layer 40. As with other embodiments, this embodiment may be single-ended or differential.

This embodiment is considered to be an inverted SMR hanging from a cap 40 constructed as a Bragg reflector. There is no lower Bragg reflector (eg, 12 in the structure shown in FIG. 9), instead there is a cavity 80 created when the sacrificial layer is etched off in the stripping process.

Airgel-like capsule materials in other embodiments may not be suitable for products that require very low profile packaging. Unconstrained FBAR construction may not be suitable for other products that require a high degree of inertial impact resistance. The cap 40 is capable of protecting the device 42 from packaging and external stresses, but still provides a very low profile. Since the resonator is part of the cap 40, it is still acoustically separated from the environment by the Bragg reflector characteristics of the cap 40, while benefiting from the mechanical strength of the cap 40.

Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely exemplary of the principles and uses of the invention. Therefore, a number of modifications can be made for exemplary embodiments without departing from the spirit and scope of the invention as defined by the appended claims, and other structures can be recalled. I want you to understand.

Other modifications and modifications modified to suit specific operating requirements and operating environments will be apparent to those skilled in the art, and the present invention will be limited to the examples selected for disclosure. Is not considered and is intended to include all modifications and modifications that do not deviate from the true purpose and scope of the invention.