Thickness mode resonator

Abstract

A resonator includes a substrate having a cavity extending over a major surface thereof, and a vibration resonator disposed above the major surface of the substrate, the vibration resonator including bottom and top electrodes and a piezoelectric layer disposed therebetween. Furthermore, a silicon dioxide layer is disposed above the substrate and below the vibration resonator to cover the cavity in the substrate, and the silicon layer is disposed between the silicon dioxide layer and the vibration resonator. The bottom electrode, the top electrode, and the piezoelectric layer of the vibration resonator each have a thickness configured to accommodate approximately half the wavelength λ / 2 of the resonator, and the silicon layer has a thickness that accommodates approximately a multiple of the half wavelength λ / 2 of the resonator.

Description

[Technical field]

[0001] This application relates to high performance resonators, and more particularly to thickness mode overtone resonators. [Background technology]

[0002] High speed communication systems require high performance resonators with good stability and quality factor. Currently, these resonators can be provided as quartz crystal resonators that generally operate at frequencies between 10 MHz and 100 MHz. Furthermore, for digital networks such as Ethernet networks, surface acoustic wave (SAW) resonators that operate at frequencies higher than 100 MHz can be used, but these resonators are relatively large and expensive. It is expected that future communication systems and technologies will increase the requirements to use frequencies higher than 100 MHz. Therefore, there is an increasing demand for high frequency resonators with small size and good performance.

[0003] One type of resonator that can be used as a filter at frequencies above 1 GHz is a thin film bulk acoustic resonator (FBAR). For example, FIG. 1 shows an exemplary conventional air-gap FBAR. As shown, FIG. 1 shows a cross-sectional view of an air-gap FBAR 10 formed from a silicon substrate 12 in the thickness direction. A cavity 14 (i.e., an air gap) extends through the center of the silicon substrate 12. The vibration resonator further comprises a bottom electrode 16, a piezoelectric layer 18, and a top electrode 20. Reference numeral 22 further indicates the shape of the vibration mode. The vibration of the air-gap FBAR 10 is mainly in thickness mode. In this design, the thickness of the resonator is equal to a half wavelength. In addition, in such a conventional design, the piezoelectric layer 18 is generally formed from aluminum nitride (AlN), and the bottom and top electrodes 16 and 20 can be formed from metals such as molybdenum, aluminum, tungsten, platinum, or the like.

[0004] Another type of conventional resonator is the solidly mounted acoustic resonator (SMR). For example, FIG. 2 shows an exemplary conventional SMR 30. The SMR 30 is also formed from a silicon substrate 32 and includes a bottom electrode 34, a piezoelectric layer 36, and a top electrode 38. Similar to the FBAR 10 shown in FIG. 1, reference numeral 40 denotes the shape of the vibration mode of the SMR 30. This vibration is also predominantly thickness mode, with the thickness of the resonator equal to one-half wavelength. However, unlike the FBAR 10, the SMR 30 shown in FIG. 2 does not have a cavity below the vibrating bottom electrode 34. Instead, the resonator 30 includes an acoustic reflector 42 formed on the silicon substrate 32. The reflector 42 is provided to minimize energy leakage from the resonator 30 to the silicon substrate 32, thereby reducing the amount of vibration that can occur. Bottom electrode 34 This eliminates the need for a cavity underneath.

[0005] Although FBAR10 has good characteristics for filter applications, such conventional designs are not well suited for making oscillators. In particular, the resonant frequency is high (above 1 GHz) but the quality factor is low (e.g., between 1,000 and 2,000). In addition, the temperature coefficient of frequency (TCF) of FBAR resonators is significantly larger than that of quartz resonators. Therefore, there is a need for compact resonators with good temperature characteristics. Summary of the Invention [Means for solving the problem]

[0006] Each exemplary embodiment of the present invention provides a resonator that enables high performance and is compact, and therefore suitable for high frequency oscillator applications required for high speed communication systems.

[0007] Thus, according to one exemplary embodiment, a resonator for vibrating in the thickness direction is provided. In this aspect, the resonator includes a substrate having a cavity extending over its main surface, a piezoelectric film stack disposed above the main surface of the substrate, the piezoelectric film stack including a bottom and top electrode and a piezoelectric layer disposed therebetween, and a thickness approximately corresponding to a half wavelength λ / 2 of the resonator, and a positive temperature coefficient stack including at least a silicon layer disposed between the substrate and the piezoelectric film stack to cover the cavity of the substrate, the positive temperature coefficient stack having a thickness approximately corresponding to a multiple of the half wavelength λ / 2 of the resonator. Furthermore, the piezoelectric film stack has a negative temperature coefficient, and impurities are added to the silicon layer to configure the positive temperature coefficient stack so that the positive temperature coefficient stack has a positive temperature coefficient, so that the positive temperature coefficient stack combined with the piezoelectric film stack configures the resonator as a zero temperature coefficient overtone resonator vibrating in the thickness direction.

[0008] In another exemplary embodiment, the positive temperature coefficient stack further comprises at least one silicon dioxide layer disposed between the silicon layer and the piezoelectric film stack.

[0009] In another exemplary embodiment, the piezoelectric film stack further comprises at least one silicon dioxide layer disposed between the bottom and top electrode layers.

[0010] In another exemplary embodiment, phosphorus is added to the silicon layer to construct a positive temperature coefficient stack to balance the positive temperature coefficient of the piezoelectric film stack. In a refinement of this embodiment, the resistivity of the phosphorus-doped silicon layer is 0.8 mΩ·cm or less.

[0011] In another exemplary embodiment, the cavity has a first width extending in a direction parallel to a major surface of the cavity, and the top electrode has a second width extending in a direction parallel to the major surface of the cavity. The second width is smaller than the first width. Moreover, in one embodiment, the second width of the top electrode is between 50 μm and 500 μm.

[0012] In another exemplary embodiment, the resonator includes a plurality of etch trenches. The plurality of etch trenches extend through the bottom electrode and the piezoelectric layer of the piezoelectric film stack, and through the positive temperature coefficient stack, such that the plurality of etch trenches extend to a cavity in the substrate. Furthermore, in one embodiment, the top electrode has a surface area such that each of the plurality of etch trenches is disposed equidistant from the top electrode. In another embodiment, the top electrode has a surface area such that each of the plurality of etch trenches is between 10 and 100 μm away from the top electrode.

[0013] In another exemplary embodiment, the thickness of the silicon layer comprises one of the wavelengths of the resonator, λ / 2, λ, 3λ / 2, or 2λ, such that the resonator is configured as a thickness mode overtone resonator.

[0014] In another exemplary embodiment, the top electrode has a surface area that is 80% or less of the protrusion of the cavity region over which it is disposed. Furthermore, in one refinement of this embodiment, the distance between the top electrode and one edge of the cavity in the width direction of the resonator is between 10 and 100 μm from the top electrode.

[0015] In another exemplary embodiment, the top electrode comprises a plurality of segmented top electrodes, the bottom electrode being grounded, such that the resonator is configured as a differential oscillator, with at least two of the plurality of segmented top electrodes forming adjacent regions of a differential oscillator that vibrate 180 degrees out of phase with each other.

[0016] According to another exemplary embodiment, a resonator that vibrates in a thickness direction is provided. In this aspect, the resonator includes a substrate having a cavity extending over its main surface, a piezoelectric resonator disposed above the main surface of the substrate, the piezoelectric resonator including lower and upper electrodes and a piezoelectric layer disposed therebetween, and a positive temperature coefficient resonator disposed between the substrate and the piezoelectric resonator to cover the cavity of the substrate, the positive temperature coefficient resonator including at least a silicon layer doped with impurities to configure the positive temperature coefficient resonator so that the positive temperature coefficient resonator has a positive temperature coefficient. Furthermore, the cavity has a first width extending in a direction parallel to the main surface of the cavity, and the upper electrode has a second width extending in a direction parallel to the main surface of the cavity. The second width is smaller than the first width.

[0017] In another exemplary embodiment, the piezoelectric resonator has a negative temperature coefficient. Phosphorus is added to the silicon layer to balance the positive temperature coefficient resonator with the piezoelectric resonator such that the resonator is configured as a zero temperature coefficient overtone resonator vibrating in the thickness direction. Furthermore, in one embodiment, the piezoelectric resonator has a thickness that corresponds approximately to a half wavelength λ / 2 of the resonator. The positive temperature coefficient resonator has a thickness that corresponds approximately to a multiple of the half wavelength λ / 2 of the resonator. Furthermore, in another exemplary embodiment, the thickness of the silicon layer comprises one of the wavelengths λ / 2, λ, 3λ / 2, or 2λ of the resonator such that the resonator is configured as a thickness mode overtone resonator.

[0018] In another exemplary embodiment, at least one silicon dioxide layer is disposed at least between the silicon layer and the piezoelectric resonator or between the bottom and top electrode layers.

[0019] In another exemplary aspect, the resonator includes a plurality of etch trenches that extend through the bottom electrode and the piezoelectric layer of the piezoelectric resonator and through the positive temperature coefficient resonator such that the plurality of etch trenches extend to a cavity in the substrate.

[0020] The above simplified summary of several exemplary embodiments is helpful in providing a basic understanding of the present disclosure. This summary is not an extensive overview of all possible aspects, and its purpose is not to identify key or essential elements of all aspects, nor to delineate the scope of any or all aspects of the present disclosure. Its sole purpose is to present one or more aspects in a simplified form as a prelude to the more detailed description of the present disclosure that follows. To accomplish the above, one or more aspects of the present disclosure include the features that are particularly pointed out and recited in the claims. [Brief description of the drawings]

[0021] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more exemplary embodiments of the disclosure and, together with the detailed description, serve to explain the principles and implementations thereof. The drawings are for illustrative purposes only and therefore are not drawn to scale.

[0022] [Figure 1] 1 shows a cross-sectional view of an exemplary conventional air-gap thin film piezoelectric resonator.

[0023] [Diagram 2] 1 shows a cross-sectional view of an exemplary conventional multilayer resonator.

[0024] [Diagram 3] 1 illustrates a thickness mode overtone resonator according to a first exemplary embodiment.

[0025] [Figure 4A] 4 shows a cross-sectional view of a thickness mode overtone resonator according to a first exemplary embodiment taken along line AA in FIG. 3.

[0026] [Figure 4B] 4B illustrates an improvement of the exemplary embodiment of the thickness mode overtone resonator as shown in FIG. 4A.

[0027] [Figure 4C]4B illustrates an improvement of the exemplary embodiment of the thickness mode overtone resonator as shown in FIG. 4A.

[0028] [Diagram 5] 1 illustrates a thickness mode overtone resonator according to a second exemplary embodiment.

[0029] [Figure 6] 6 shows a cross-sectional view of a thickness mode overtone resonator according to a second exemplary embodiment taken along line BB in FIG. 5.

[0030] [Figure 7] 13 illustrates a thickness mode overtone resonator according to a third exemplary embodiment.

[0031] [Figure 8] 8 shows a cross-sectional view of a thickness mode overtone resonator according to a third exemplary embodiment taken along line CC in FIG. 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Several exemplary aspects are described herein in the context of thickness-mode overtone resonators that overcome many of the technical shortcomings of existing devices described above. In particular, each exemplary embodiment disclosed herein provides a resonator that enables high performance and is compact, and thus suitable for high frequency oscillator applications required for high speed communication systems.

[0033] Those skilled in the art will recognize that the following description is merely illustrative and is not intended to be limiting in any way. Other embodiments will readily occur to those skilled in the art having the benefit of this disclosure. Reference will now be made in detail to the illustrative embodiments, which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the following description to refer to the same or similar items.

[0034] Thus, Figure 3 illustrates a thickness mode overtone resonator 100 according to a first exemplary embodiment. Figure 4A illustrates a cross-sectional view of the thickness mode overtone resonator 100 along line AA in Figure 3.

[0035] As shown, according to a first exemplary embodiment, the thickness mode overtone resonator 100 can be formed on a silicon substrate 110 having a cavity 120 formed therein. In this embodiment, the cavity 120 is formed in a central portion or section of a major surface (i.e., parallel to the X and Y axes) of the silicon substrate 110, and each of the silicon substrates 110 is formed with a cavity 120. above Each side edge 112 extending thicknesswise from the face effectively defines a cavity 120. It should be appreciated that the formation of the cavity 120 in the silicon layer 110 can be accomplished using any conventional technique, such as etching.

[0036] As further shown, a silicon dioxide layer 130 is disposed over the silicon substrate 110, and in particular on each side of the silicon substrate 110, to effectively cover and seal the cavity 120. edge 112, defining an air gap therebetween. In one exemplary embodiment, the silicon dioxide layer 130 has a thickness between 100 nm and 2 μm. Furthermore, the single crystal silicon layer 140 is formed on (and disposed on) the silicon dioxide layer 130, such that the silicon dioxide layer 130 is formed between the single crystal silicon layer 140 and the silicon substrate 110. The single crystal silicon layer 140 and the silicon dioxide layer 130 together form a positive temperature coefficient stack, also referred to as a positive temperature coefficient resonator, which will be described in more detail below. Furthermore, according to one exemplary embodiment, the single crystal silicon layer 140 may have a thickness between 2 μm and 50 μm.

[0037] As further shown, a metallic bottom electrode 150, a piezoelectric layer 160, and a top electrode 170 are provided to form a piezoelectric film stack (also referred to as a piezoelectric resonator) on the single crystal silicon layer 140. In one exemplary embodiment, the piezoelectric layer 160 can be formed from aluminum nitride (AlN) and have a thickness in the range of 0.4 μm to 3 μm. Additionally, the bottom and top electrodes 150 and 170 are typically metal, such as molybdenum, and have a thickness of 0.1 μm to 1 μm.

[0038] According to the exemplary embodiment shown in FIG. 3 and FIG. 4A, the thickness mode overtone resonator 100 is configured to vibrate in a thickness mode (i.e., in the Z-axis or vertical direction). In other words, the vibration mode is shown by the dotted line 180. The vibration mode extends through the piezoelectric film stack into the silicon layer 140. At the resonant frequency, the mode shape is a multiple of half wavelengths. It is noted that, according to several variations of this exemplary embodiment, there are many possible choices for the thickness of the piezoelectric film stack and the thickness of the silicon that accommodate multiple half wavelengths in the thickness direction. For timekeeping applications, it is preferable to maximize the electromechanical coupling, resulting in the minimum electrical impedance and the maximum resonator current. The optimal coupling is achieved when the thickness of the piezoelectric film stack (including the metal electrodes 150 and 170 and the piezoelectric thin film 160) is configured to approximately accommodate a half wavelength λ / 2 at the resonant frequency of the resonator 100. In this design configuration, the single crystal silicon layer 140 has a thickness configured to accommodate a multiple of the half wavelength λ. For example, the thickness of the single crystal silicon layer 140 can be selected as, for example, λ / 2, λ, 3λ / 2, or 2λ. Based on these configurations, the overtone resonator 100 is configured as a thickness mode overtone resonator with a high quality factor. It should be noted that the term "substantially" used herein is added to take into account slight variations in the thickness of each layer that may occur during fabrication, as will be understood by those skilled in the art.

[0039] In one exemplary embodiment, a thickness overtone resonator 100 is provided having a configuration as shown in Figures 3 and 4A. In this configuration, the silicon layer 140 is 6 μm thick, the bottom electrode 150 is 0.2 μm thick, the piezoelectric layer 160 formed from aluminum nitride is 1.2 μm thick, and the top electrode 170 is 0.6 μm thick. With the exemplary overtone resonator 100 having these dimensions, the overtone resonator 100 would be configured to operate at a resonant frequency of approximately 1.3 GHz. These dimensions therefore accommodate approximately two half wavelengths in the silicon and one half wavelength in the layers above the silicon layer 140 (i.e., the bottom electrode 150, the top electrode 170, and the aluminum nitride layer 160).

[0040] According to another exemplary embodiment, another thickness overtone resonator 100 is provided having a configuration as shown in FIG. 3 and FIG. 4A. In this configuration, the silicon layer 140 is 5 μm thick, the bottom electrode 150 is 0.4 μm thick, the piezoelectric layer 160 made of aluminum nitride is 2.2 μm thick, and the top electrode 170 is 1 μm thick. With the exemplary overtone resonator 100 having these dimensions, the exemplary overtone resonator 100 will be configured to operate at a resonant frequency of approximately 800 MHz. Thus, these dimensions accommodate approximately two half wavelengths in silicon and one half wavelength in the layers above the silicon layer 140 (i.e., the bottom electrode 150, the top electrode 170, and the aluminum nitride layer 160).

[0041] According to this exemplary embodiment, the piezoelectric film stack formed by the piezoelectric layer 160 and the metal electrodes 150 and 170 has a negative frequency temperature coefficient. In other words, the resonant frequency decreases with temperature. This technical effect is highly undesirable for timekeeping applications. Therefore, in this exemplary embodiment, the negative frequency temperature coefficient can be reduced by incorporating silicon dioxide between the bottom electrode 150 and the top electrode 170 as shown in FIG. 4C (below). Although this approach reduces the temperature coefficient, the resulting resonator typically has a relatively large quadratic temperature coefficient, resulting in a frequency drift of more than 50 ppm over the typical temperature range of −40° C. to 85° C.

[0042] In one refinement of this exemplary embodiment, if the thickness overtone resonator 100 is configured for timekeeping, the temperature compensation of the single crystal silicon layer 140 can be achieved by adding phosphorus to the single crystal silicon layer 140. In one preferred aspect of this exemplary embodiment, the temperature compensation is performed at 5-10 19 1 / cm 3 The doping of silicon with phosphorus at a doping density of greater than 0.015 nm makes the temperature coefficient of silicon positive. Additionally, in a refinement of this exemplary embodiment, the resistivity of the phosphorus doped silicon can be made less than or equal to 0.8 mΩ·cm.

[0043] Advantageously, the slightly positive first order temperature coefficient of silicon balances the negative temperature coefficient of the metal electrodes 150 and 170 and the piezoelectric layer 160. If the thickness overtone resonator 100 is implemented in an application requiring good temperature stability, it is advantageous to use silicon 140 as a compensation material to reduce the temperature dependence. In applications requiring exceptional frequency stability, the phosphorus doping can be increased to less than 0.65 mΩ·cm. This also minimizes the second order temperature coefficient of the resonator. However, very high doping concentrations are known to reduce the magnitude of the positive temperature coefficient of phosphorus doped silicon. Silicon doped with very high doping concentrations of phosphorus may not be able to compensate for the negative temperature dependence of the piezoelectric film stack.

[0044] In a further refinement of this exemplary embodiment, one or more silicon dioxide layers can be added to make the first order temperature coefficient more positive. This allows the use of higher loading concentrations to minimize the second order temperature dependence. For example, in one embodiment as shown in FIG. 4B, an additional silicon dioxide layer 190B can be incorporated between the single crystal silicon layer 140 and the bottom electrode 150. In another exemplary embodiment as shown in FIG. 4C, an additional silicon dioxide layer 190C is below the single crystal silicon 140. In this exemplary embodiment, the single crystal silicon 140 and the silicon dioxide layer 130 or layers 130 and 190B and / or 190C form a temperature compensation stack with a positive temperature dependence that compensates for the temperature dependence of the piezoelectric film stack (i.e., electrodes 150 and 170 and piezoelectric layer 160). Furthermore, the thickness of this temperature compensation stack layer is configured so that the stack accommodates multiples of half wavelengths λ at the resonant frequency. By combining a temperature compensation stack (i.e., a single crystal silicon layer and a silicon dioxide layer(s)) with a piezoelectric film stack (i.e., top and bottom electrodes and a piezoelectric layer), a total frequency shift of less than 50 ppm can be achieved over a temperature range of -40°C to 85°C.

[0045] According to this exemplary embodiment, an additional advantage of using an overtone resonator 100 with one or more half wavelengths housed in silicon is that the thickness of the monocrystalline silicon layer 140 can be increased without changing the resonant frequency in order to adjust the electrical and mechanical properties of the resonator. For example, in the second example above, the thickness of the monocrystalline silicon layer 140 can be a multiple of a half wavelength. Possible thicknesses are 5 μm, 10 μm, 15 μm, etc. Increasing the thickness of the monocrystalline silicon layer 140 makes the resonator 100 mechanically more robust, as will be appreciated by those skilled in the art. Furthermore, by increasing the thickness of the silicon, the temperature characteristics of silicon play a larger role in determining the frequency temperature coefficient of the entire resonator 100. Since the temperature dependence of silicon can be precisely tuned and controlled, a larger thickness of the silicon layer compared to the thickness of the metal electrodes 150 and 170 and the piezoelectric layer 160 is advantageous for fabricating a high performance resonator with good temperature characteristics.

[0046] In addition to good temperature characteristics, it is important, as noted above, that the thick overtone resonator 100 have a high quality factor. Movement resonator 100 It is necessary to confine the energy within the cavity 112 to minimize energy leakage into the substrate 110. Figures 4A, 4B and 4C In, for example, the thickness vibration mode (i.e., reference numeral 180) is confined below the region defined by the top electrode 170. In one exemplary embodiment, the top electrode 170 can have a planar (i.e., in directions parallel to the X and Y axis directions) surface area of ​​2,500 μm2 to 250,000 μm2. Additionally, the top electrode 170 can be formed with an electrode width (i.e., in the X and Y axis directions) of 50 μm to 500 μm.

[0047] 4, the width (e.g., diameter in the X-axis direction) of the cavity 120 is greater than the corresponding width of the top electrode 170. In particular, outside the area of ​​the top electrode 170, the vibration amplitude decays with increasing distance from the edge 172 of the top electrode. Thus, the resonator is configured such that the cavity 120 has a first width extending in a direction parallel to the major surface of the cavity 120, and the top electrode 170 has a second width extending in a direction parallel to the major surface of the cavity, the second width being narrower than the first width.

[0048] Therefore, to minimize energy losses and thus maximize the quality factor of the resonator 100, a cavity 120 is formed in the silicon substrate 110 whose widths in the X and Y directions are greater than the corresponding widths in the X and Y directions of the surface area of ​​the top electrode 170. Furthermore, in one exemplary embodiment, the distance (e.g., extending in the X direction) between the electrode edge 172 and the cavity edge 112 should ideally be several wavelengths. For example, the distance (in the X-axis direction) from the electrode edge 172 to the cavity edge 112 can be between 10 μm and 100 μm. Thus, the area defined by the cavity 120 (i.e., from a top view) can be between 4,900 μm and 5,000 μm. 2 ~490,000μm 2According to this configuration, the top electrode 170 has an area above the cavity 120 that is 80% or less of the area of ​​the resonator.

[0049] In another exemplary aspect, Fig. 5 shows a thickness mode overtone resonator according to a second exemplary embodiment. Fig. 6 shows a cross-sectional view of the thickness mode overtone resonator according to the second exemplary embodiment along line BB in Fig. 5.

[0050] It should be noted that the thickness mode overtone resonator 200 according to the second exemplary embodiment includes many of the same components as those shown in Figures 3 and 4. For example, the thickness mode overtone resonator 200 typically includes a silicon substrate 210, a silicon dioxide layer 230 formed on the silicon substrate 210, a single crystal silicon layer 240 disposed thereon, and a stack of a bottom electrode 250, a piezoelectric layer 260, and a top electrode 270 to form a vibration resonator. Furthermore, each of these components may be provided in the same configuration and with the same dimensions (e.g., thickness) as described above with respect to the components of the thickness mode overtone resonator 100 according to the first exemplary embodiment. It should also be noted that the thickness mode overtone resonator 200 is configured to vibrate in a thickness mode, i.e., as indicated by the dotted line 280.

[0051] In addition, according to a second exemplary embodiment, Figures 5 and 6 As shown in FIG. 1, the plurality of etch trenches 262 extend through the silicon dioxide layer 230 and the silicon layer 240 (as well as the vibration resonator formed by the electrodes 250 and 270 and the piezoelectric layer 260) so that the plurality of etch trenches 262 extend to the cavity 220 formed in the silicon substrate 210. To further reduce energy losses and improve the quality factor, the plurality of etch trenches 262 extend through the resonator. 200Advantageously, the etched grooves are etched at the periphery of the resonator 260. These etched grooves can be defined by etching through the thin films and the single crystal silicon 240. Additionally, corner connections 264 around the periphery of the resonator are not etched to effectively form one or more narrow anchors 264 for attaching the resonator (i.e., electrodes 250 and 270 and piezoelectric layer 260) to the silicon substrate 210.

[0052] That is, in one refinement of this exemplary embodiment, even though the etched trench 262 minimizes energy loss in the resonator, the distance from the side edge 272 of the top electrode 270 to the edge 266 (as shown in FIG. 6) of the groove or trench 262 is a multiple of wavelengths, preferably in the range of 10 μm to 100 μm. Furthermore, in one similar configuration as described above, the area of ​​the top electrode 270 (i.e., in the X and Y axis directions) is 80% or less of the area of ​​the resonator surrounded by the etched trench 262.

[0053] In a further refinement of this exemplary embodiment, the top electrode 270 includes a polygonal (e.g., rectangular or square) surface shape (i.e., parallel to the X and Y axis directions). It is further contemplated that the multiple etch trenches 262 can be disposed equidistant from the top electrode 270, as further shown in FIG.

[0054] In another exemplary aspect, Figure 7 illustrates a thickness mode overtone resonator 300 according to a third exemplary embodiment. Figure 8 illustrates a cross-sectional view of the thickness mode overtone resonator according to the third exemplary embodiment along line CC in Figure 7.

[0055] It should be noted that the thickness mode overtone resonator 300 according to the third exemplary embodiment includes many of the same components as those shown in the first and second exemplary embodiments. For example, the thickness mode overtone resonator 300 typically includes a silicon substrate 310, a silicon dioxide layer 330 formed on the silicon substrate 310, a single crystal silicon layer 340 disposed thereon, and a stack-up for a resonator including a bottom electrode 350, a piezoelectric layer 360, and a top electrode 370. Furthermore, each of these components may be provided in the same configuration and with the same dimensions (e.g., thicknesses) as described above with respect to the components of the thickness mode overtone resonators 100 and 200 according to the first and second exemplary embodiments. Additionally, the plurality of etch trenches 362 extend through the silicon dioxide layer 330 and the silicon layer 340 (as well as the bottom electrode 350 and piezoelectric layer 360 ) such that the plurality of etch trenches 362 extend to the cavity 320 formed in the silicon substrate 310 .

[0056] In addition, as shown in FIG. 7, for example, the thickness mode overtone resonator 300 is configured to be embodied in applications where noise and vibration tolerance is required. As a result, the resonator advantageously has differential inputs and outputs to build a differential oscillator. That is, according to this exemplary embodiment, the thickness mode overtone resonator 300 includes a top electrode (i.e., a segmented top electrode portion) that is segmented into two or more regions 372 and 374. As further shown in FIG. 8, under corresponding regions 372 and 374, the vibration modes are 180 degrees out of phase, as shown by the dotted mode shape lines 380A and 380B. Thus, to form a differential oscillator, the bottom electrode 350 can be grounded and the signal output can be measured differentially between the two top electrode components / regions 372 and 374. It should be understood that the thickness mode overtone resonator 300 is also configured to vibrate in a thickness mode when embodied as a differential oscillator.

[0057] For the sake of clarity, not all routine features of the embodiments are disclosed herein. In developing any practical implementation of the present disclosure, it will be understood that numerous implementation-specific decisions must be made to achieve designer-specific goals, which will vary for different implementations and different designers. It will be understood that such a design effort may be complex and time-consuming, but is nevertheless a routine design exercise for those of ordinary skill in the art having the benefit of this disclosure.

[0058] Moreover, it is to be understood that the phrases or terms used herein are for the purposes of description and not of limitation, and should be interpreted by one of ordinary skill in the art in light of the teachings and guidance presented herein, in combination with the knowledge of the skilled artisan(s). Moreover, no term contained in the specification or claims is intended to have an uncommon or special meaning unless expressly stated therein.

[0059] Although the foregoing has been described in conjunction with several exemplary embodiments, it should be understood that the term "exemplary" is meant to be merely an example, and thus, this application is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the thickness mode resonator disclosed herein. [Explanation of symbols]

[0060] 100, 200, 300 Thickness mode overtone resonators 110, 210, 310 Silicon substrate 112 side edge 120, 220, 320 cavities 130, 230, 330, 190B, 109C Silicon dioxide layer 140, 240, 340 Single crystal silicon layer 150, 250, 350 bottom electrode 160, 260, 360 Piezoelectric layer 262, 362 Etch trench 264 Corner Connection 170, 270 Top electrode 172 Electrode Edge 180, 280, 380A, 380B vibration modes 372, 374 Segmented top electrode area

Claims

1. A resonator configured to vibrate in a thickness direction, the resonator comprising: a substrate having a cavity extending on a major surface thereof; a piezoelectric film stack disposed above the major surface of the substrate, the piezoelectric film stack including lower and upper electrodes and a piezoelectric layer disposed therebetween, the piezoelectric film stack having a thickness corresponding to a half wavelength λ / 2 of the resonator; and a positive temperature coefficient stack including at least a silicon layer disposed between the substrate and the piezoelectric film stack to cover the cavity of the substrate, the positive temperature coefficient stack having a thickness corresponding to a multiple of the half wavelength λ / 2 of the resonator, the piezoelectric film stack having a negative temperature coefficient, and a silicon layer configured to configure the positive temperature coefficient stack such that the positive temperature coefficient stack has a positive temperature coefficient, such that the positive temperature coefficient stack combined with the silicon layer configures the resonator as a zero temperature coefficient overtone resonator vibrating in the thickness direction, the cavity has a first width extending in a direction parallel to the main surface of the cavity, the top electrode has a second width extending in the direction parallel to the main surface of the cavity, the second width being smaller than the first width, and a distance between the top electrode and one edge of the cavity in the width direction of the resonator is between 10 and 100 μm.

2. The resonator of claim 1 , wherein the positive temperature coefficient stack further comprises at least one silicon dioxide layer disposed between the silicon layer and the piezoelectric film stack.

3. 2. The resonator of claim 1, wherein phosphorus is added to said silicon layer to configure said positive temperature coefficient stack to balance said positive temperature coefficient with said negative temperature coefficient of said piezoelectric film stack.

4. 4. The resonator of claim 3, wherein the resistivity of the phosphorus doped silicon layer is less than or equal to 0.8 mΩ·cm.

5. The resonator of claim 1 , wherein the second width of the top electrode is between 50 μm and 500 μm.

6. 2. The resonator of claim 1, further comprising a plurality of etch trenches extending through the bottom electrode and the piezoelectric layer of the piezoelectric film stack and further through the positive temperature coefficient stack such that the plurality of etch trenches extend to the cavity in the substrate.

7. The resonator of claim 6 , wherein the top electrode comprises a surface area such that each of the plurality of etch trenches is disposed equidistant from a side edge of the top electrode.

8. The resonator of claim 7, wherein the top electrode has a surface area such that each of the plurality of etch trenches is between 10 and 100 μm from a side edge of the top electrode.

9. 2. The resonator of claim 1, wherein a thickness of the silicon layer comprises one of a wavelength of the resonator: λ / 2, λ, 3λ / 2, or 2λ, such that the resonator is configured as a thickness-mode overtone resonator.

10. The resonator of claim 1 , wherein the top electrode comprises a surface area that is no more than 80% of a projection of a cavity region over which the top electrode is disposed.

11. 2. The resonator of claim 1, wherein the top electrode comprises a plurality of segmented top electrodes, the bottom electrode being grounded such that the resonator is configured as a differential oscillator, with at least two of the plurality of segmented top electrodes constituting adjacent regions of the differential oscillator that vibrate 180 degrees out of phase with each other.

12. 1. A resonator configured to vibrate in a thickness direction, the resonator comprising: a substrate having a cavity extending over a principal surface of the substrate; a piezoelectric resonator disposed above the principal surface of the substrate, the piezoelectric resonator including bottom and top electrodes and a piezoelectric layer disposed therebetween; and a positive temperature coefficient resonator disposed between the substrate and the piezoelectric resonator to cover the cavity in the substrate, the positive temperature coefficient resonator including at least a silicon layer to which an impurity is added to configure the positive temperature coefficient resonator such that the positive temperature coefficient resonator has a positive temperature coefficient, the cavity having a first width extending in a direction parallel to the principal surface of the cavity, the top electrode having a second width extending in the direction parallel to the principal surface of the cavity, the second width being smaller than the first width, and a distance between the top electrode and one edge of the cavity in the width direction of the resonator being between 10 and 100 μm.

13. 13. The resonator of claim 12, wherein the piezoelectric resonator has a negative temperature coefficient, and phosphorus is added to the silicon layer to balance the positive temperature coefficient resonator with the piezoelectric resonator such that the resonator is configured as a zero temperature coefficient overtone resonator vibrating in the thickness direction.

14. 14. The resonator of claim 13, wherein the piezoelectric resonator has a thickness corresponding to half a wavelength λ / 2 of the resonator, and the positive temperature coefficient resonator has a thickness corresponding to a multiple of the half wavelength λ / 2 of the resonator.

15. 15. The resonator of claim 14, wherein a thickness of the silicon layer comprises one of a wavelength of the resonator: λ / 2, λ, 3λ / 2, or 2λ, such that the resonator is configured as a thickness mode overtone resonator.

16. 13. The resonator of claim 12, wherein at least one silicon dioxide layer is disposed at least between said silicon layer and said piezoelectric resonator.

17. 13. The resonator of claim 12, further comprising a plurality of etch trenches extending through the bottom electrode and the piezoelectric layer of the piezoelectric resonator and further through the positive temperature coefficient resonator such that a plurality of etch trenches extend to the cavity in the substrate.

Images