Abstract
A resonator device comprising a piezoelectric material and at least one electrode, the device also provided with a material with a positive coefficient of stiffness, wherein the material is disposed in the device as an electrode or as a separate layer adjacent the piezoelectric material formed as one or more layers in the device. The material that performs the temperature compensating function is selected from the group consisting of ferromagnetic metal alloys, shape-memory metal alloys, and polymers, wherein the selected material has a temperature coefficient that varies with the relative amounts of the individual constituents of the compositions and wherein the composition is selected to provide the material with the positive coefficient of stiffness.
Claims
1. A resonator device having at least one electrode disposed to cooperate with a piezoelectric material to provide an electrical response to a mechanical state, the resonator further comprising a temperature compensation layer of a temperature compensation material, wherein the temperature compensation layer is selected from the group consisting of a first top electrode, a second bottom electrode, and a separate layer, wherein the temperature compensation material has a temperature coefficient that depends upon relative amounts of individual constituents of a composition of the temperature compensation material, wherein the temperature compensation material is a nickel titanium alloy, wherein an amount of nickel by weight in the alloy is between about 27 and 42 percent by weight and is selected such that the composition provides the temperature compensation material with a positive temperature coefficient of stiffness (TCC), and wherein the temperature compensation layer is disposed adjacent to the piezoelectric material.
2. The resonator device of claim 1 wherein the piezoelectric material of the device is formed as at least one piezoelectric layer.
3. The resonator device of claim 1 wherein the piezoelectric material of the device is formed as a plurality of piezoelectric layers.
4. The resonator device of claim 1 wherein the amount of nickel in the alloy does not exceed about 34 percent by weight.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which:
(2) FIG. 1 is a cross section view of a prior art MEMS BAW resonator, shown in an FBAR configuration.
(3) FIG. 2 is a cross section view of a prior art lumped mass-spring mechanical resonator.
(4) FIG. 3 is a view of the crystalline structure of nickel-titanium crystal.
(5) FIG. 4 is a graph of the elastic modulus of 50:50 NiTi versus temperature, reproduced from Matsumoto.
(6) FIG. 5 is a graph of the temperature coefficient of elastic modulus of NiTi versus nickel content, reproduced from specialmetals.com.
(7) FIG. 6A is a graph view of the response of a FBAR resonator, with tungsten electrodes, an AlN piezoelectric, and temperature compensation performed by SiO.sub.2.
(8) FIG. 6B is a graph view of the response of a FBAR resonator, with tungsten electrodes, an AlN piezoelectric, and temperature compensation performed by NiTi.
(9) FIG. 7 is a graph view of the response of two FBAR resonators, comparing SiO.sub.2-based temperature compensation with NiTi-based temperature compensation.
(10) FIG. 8A is a symmetric cross section view of a MEMS solidly-mounted resonator (SMR) with AlN piezoelectric, tungsten lower electrode and upper electrode, and a NiTi temperature compensating layer.
(11) FIG. 8B is a displacement view of the SMR illustrated in FIG. 8A (simulated using finite element analysis).
(12) FIG. 9A is a symmetric cross section view of a MEMS solidly-mounted resonator with AlN piezoelectric, tungsten lower electrode and a NiTi upper electrode
(13) FIG. 9B is a symmetric cross section view of a MEMS solidly-mounted resonator with AlN piezoelectric, tungsten lower electrode and upper electrode, and a NiTi layer in the Bragg acoustic reflector.
(14) FIG. 10 is a graph view of a simulated impedance response of the SMR of FIG. 8 with a NiTi temperature compensation layer at two temperature values.
(15) FIG. 11 is a cross section view of a MEMS lateral mode BAW resonator.
(16) FIG. 12 is a displacement view of a lateral mode BAW resonator of FIG. 11.
(17) FIG. 13 is a graph view of a simulated impedance response of the lateral resonator of FIG. 11, at two temperature values.
(18) For purposes of clarity and brevity, like elements and components will bear the same designations and numbering throughout the Figures.
DETAILED DESCRIPTION
(19) FIG. 1 is a cross section view of a prior art MEMS BAW resonator 10, shown in an FBAR configuration. In this configuration, the thin film is supported on an anchor 12, which in turn is supported by a substrate 11. The resonator is lower electrode 13, the piezoelectric transducer 14, and the upper electrode 16. Temperature compensation is performed by the temperature compensating layer 15. Other BAW resonator configurations including FBARs supported on a membrane or supported from above are possible, without limitation.
(20) FIG. 2 is a cross section view of a prior art MEMS lumped mass-spring mechanical resonator 30. In this configuration, the proof mass 34 (the mass used to characterize performance (i.e. the seismic mass) is connected to the thin film spring 33, which is supported on anchor 32 which rests on substrate 31. Other configurations including resonators supported on a membrane or supported from above are possible, without limitation.
(21) FIG. 3 is a view of the crystalline structure of a 50:50 nickel-titanium crystal from prior art. The structure model 50 of the 50:50 nickel-titanium crystal 51 indicates the relative positions of the nickel and titanium ions in the crystal.
(22) FIG. 4 is a graph 60 of the elastic modulus 61 of 50:50 NiTi versus temperature 62. The effect of temperature on the elastic modulus of 50:50 NiTi has previously been reported in Matsumoto, which is incorporated by reference herein. The elastic modulus of 50:50 NiTi has a positive coefficient regime that lies in the operating temperature range of many MEMS devices.
(23) It is also known that the relative amounts of nickel and titanium in NiTi affect the temperature coefficient of the NiTi material. FIG. 5 illustrates how the temperature coefficient of NiTi changes with increasing nickel content in the NiTi material. FIG. 5 is a graph 70 of the temperature coefficient of elastic modulus 71 of NiTi (on the y axis) versus nickel content 72 (expressed as a percentage is on the x axis). This graph is reproduced from specialmetals.com, which is incorporated by reference herein. From FIG. 5 it is observed that, as the nickel content 72 increases, the temperature coefficient changes from a negative value to a positive value. The maximum negative coefficient is at Point A (73), with a value of about −250 ppm/° F., or about −114 ppm/° C., and a nickel content of about 16%. The temperature coefficient is zero at Point B (74), which has a nickel content of about 27%. The maximum positive coefficient is at point C (75), with a value of about +250 ppm/° F., or about +114 ppm/° C., and a nickel content of about 34%.
(24) Current research and development into the mechanical behavior of ferromagnetic materials has been focused on optimizing materials for Point B or its equivalent, so that wires and other mechanisms made of these materials do not expand under heating. Typically, NiTi materials with compositions that yield a coefficient of about zero are used for wires and other applications where no expansion of the material is desired when the temperature of the material is elevated. In the embodiments of the present invention described herein, the relative amounts of nickel and titanium in NiTi are selected to provide a desired, non-zero temperature coefficient. More broadly, the present invention contemplates selecting a composition of NiTi or other ferromagnetic metal alloys or shape-memory metal alloys or even polymers that have temperature coefficients that vary with the relative amounts of the individual constituents of the compositions. The relative amounts of the material constituents are selected to provide a desired non-zero temperature compensation effect. In the described embodiments, resonator performance is improved by selecting a composition of the temperature-compensating layer (e.g. for NiTi, by selecting a location on the FIG. 5 curve). The composition of positive TCC material is selected to compensate for countervailing changes in device performance caused by the temperature coefficients of other materials in the relevant device. Referring to FIG. 5, NiTi has temperature coefficients that vary over a wide range of compositions. Specifically, the temperature coefficients vary from the zero-TC composition (e.g. 27% nickel at point B on FIG. 5) to the maximum-TC composition (e.g. about 34% nickel at point C on FIG. 5). The selected positive temperature coefficient that results cancels the negative temperature coefficient in the other resonator materials as described above.
(25) In certain applications, it is desirable to deposit a temperature compensating material such as NiTi with the maximum-TC composition (e.g. about 34% nickel at point C on FIG. 5). As this composition provides the maximum positive temperature coefficient, when this composition is selected, the amount of material that needs to used is less than if compositions that yield a lower positive temperature coefficient are selected. Additionally and importantly, the sensitivity of the temperature coefficient to nickel content for this particular alloy is at a minimum because the slope of the curve at point C (75) in FIG. 5 is nearly zero. Selecting the NiTi composition that corresponds to point C reduces device-to-device and wafer-to-wafer performance variations due to layer thickness manufacturing variations.
(26) FIGS. 6A, 6B and 7 illustrate simulated responses over temperature for FBAR BAW resonators (illustrated in FIG. 1) and compare the response of a SIO.sub.2-compensated BAW resonator (FIG. 6A) with a NiTi-compensated BAW resonator (FIG. 6B). The NiTi layer used in the simulation of FIG. 6B has a composition that corresponds to the NiTi at point C (75) of FIG. 5. The FBAR BAW resonators have an AlN piezoelectric and tungsten electrodes. The responses were simulated at temperatures of 0° C. and 100° C.
(27) FIG. 6A is a graph 80 of a simulated impedance response of a FBAR with tungsten electrodes and an AlN piezoelectric and an SIO.sub.2 temperature compensation layer. The simulated device has a series resonance frequency (at point 81) at 1.017 GHz. The frequency is swept from 1.00 to 1.05 GHz.
(28) FIG. 6B is a graph 85 of a simulated impedance response of a FBAR with tungsten electrodes and an AlN piezoelectric, and a NiTi temperature compensation layer. The simulated device has series resonance frequency (at point 86) at 1.017 GHz. The frequency is swept from 1.00 to 1.05 GHz, as in FIG. 6A.
(29) Comparing FIG. 6A and FIG. 6B, the parallel resonance frequency 82 of the FBAR with SiO.sub.2 compensation is significantly lower than the parallel resonance frequency 87 of the FBAR with NiTi compensation. This demonstrates that the NiTi-compensated resonator has a much higher coupling coefficient k.sup.2 than the SiO.sub.2-compensated FBAR.
(30) This comparison is best viewed in FIG. 7, which is a graph 90 of the individual simulated impedance response of the FBARs illustrated in FIGS. 6A and 6. The response of the SiO.sub.2-compensated FBAR is curve 91, and the response of the NiTi-compensated FBAR is curve 92. The impedance response curves illustrate the higher coupling coefficient k.sup.2 and the higher quality factor Q achieved for a device having a NiTi-based temperature compensation layer compared with a device having an SiO.sub.2 compensation layer. Specifically, the SiO.sub.2-compensated resonator has a series-resonance Q of about 2100 and a k.sup.2 of 3.65%. The NiTi-compensated resonator has a series-resonance Q of about 2600 and a k.sup.2 of 7.3%. Therefore, the NiTi compensated device offers an about 23.8% improvement in the Q and a 50% improvement in k.sup.2 over the SiO.sub.2 compensated device.
(31) FIG. 8A is a schematic cross section view 100 of a MEMS solidly-mounted resonator (SMR) with AlN piezoelectric 14, tungsten lower 13 and upper 16 electrodes, and a NiTi temperature compensating layer 15 located between the piezoelectric layer 14 and the upper electrode 16. A symmetric view is presented for simplicity. A symmetric view only shows one half of the device, but the un-illustrated half is identical to the illustrated half. The piezoelectric layer is constructed over an acoustic Bragg mirror 102 supported by substrate 11. In this embodiment, the resonator is surrounded by a field fill 104. Finally, a perfectly matched layer (PML) 106 surrounds the structure to simplify the complexity of the finite element simulation.
(32) FIG. 8B is a displacement view 110 of the SMR of FIG. 8A. The out-of-plane displacement 111 in the piezoelectric layer 14 (from a finite element simulation), which is illustrated as a multilayer, is shown at the series resonance of the device. The displacement profile 111 indicates that the resonance is a thickness-extensional mode resonance.
(33) FIG. 9A is a schematic cross section view 113 of a MEMS solidly-mounted resonator (SMR) with AlN piezoelectric 14, tungsten lower electrode 13 and a NiTi upper electrode 114. Since the NiTi conductive electrode provides a temperature compensation function, there is no separate temperature compensation layer. Again, the schematic view is symmetric for simplicity. The piezoelectric layer and the electrodes are constructed on an acoustic Bragg mirror 102 on a substrate 11. In this embodiment, the resonator is surrounded by a field fill 104. Finally, a perfectly matched layer (PML) 106 is illustrated to surround the structure to simplify the complexity of the finite element simulation.
(34) FIG. 9B is a schematic cross section view 116 of a MEMS solidly-mounted resonator (SMR) with AlN piezoelectric, tungsten lower electrode 13 and upper electrode 16 and a NiTi temperature compensating Bragg layer 117. A symmetric view is presented for simplicity. The electrodes and piezoelectric material are constructed on an acoustic Bragg mirror 102 on a substrate 11. In this embodiment, the resonator is surrounded by a field fill 104. Finally, a perfectly matched layer (PML) 106 surrounds the structure to simplify the complexity of the finite element simulation.
(35) FIG. 10 is a graph 120 of the simulated impedance response of the solidly-mounted NiTi-compensated resonator presented in FIGS. 8A and 8B vibrating in thickness-extensional mode. The response was simulated at temperatures of 0° C. and 100° C. by finite-element simulation. Resistive losses were disregarded. The magnitude of impedance 121 is plotted against frequency 122. The resonator has a series-resonance Q of about 2500 and a k.sup.2 of 7.4%. A comparable SiO.sub.2-compensated resonator would have a series-resonance Q of 2000 and a k.sup.2 of 3.5%. The temperature coefficient of frequency is about +0.1 ppm/K.
(36) In another embodiment, presented in FIG. 11, a lateral-mode acoustic resonator has NiTi as the upper electrode 133, AlN as the piezoelectric layer 132, and tungsten as a lower electrode 131. As an upper electrode, NiTi performs the dual functions of charge collection/distribution, and as a temperature compensating layer. FIG. 11 is a schematic cross section view 130 of a MEMS lateral mode BAW resonator with a series resonance at around 1.36 GHz. The structure is released from the substrate (as required to support the lateral mode) and the supporting substrate is not shown in this cross-section for simplicity. The lower electrode 131 of the lateral resonator is tungsten, while the upper electrode 133 is NiTi. AlN is the piezoelectric transducer 132. The NiTi performs the function of the electrode as well as the temperature compensation function. Alternative embodiments (not shown) have a bilayer or multi-layer upper electrode, with NiTi as one of the layers, with the other layer(s) of lower electrical resistivity performing charge collection and distribution functions.
(37) FIG. 12 is a displacement view 140 of a lateral mode BAW resonator of FIG. 11 (lower electrode 141 in FIG. 12 corresponds to lower electrode 141 in FIG. 11). FIG. 12 shows the in-plane displacement (142, 143) computed by finite element simulation at the series resonance frequency. The lateral resonator displacement profile indicates that the resonance is a lateral-field-extensional mode.
(38) FIG. 13 is a graph 150 of a simulated impedance response of the lateral resonator of FIG. 11, at two temperature values of 0° C. and 100° C. Resistive losses are disregarded. The temperature coefficient of frequency is about −0.1 ppm/K. The k.sup.2 is about 3.3% and the series Q is about 2700, which are much higher values of k.sup.2 and Q than would be achieved with a comparable lateral-mode BAW resonator compensated by SiO.sub.2.
(39) An analysis similar to those presented above indicates that a lumped mass-spring mechanical resonator wherein the strain energy is confined to the flexural springs can be temperature compensated by constructing the springs out of NiTi, or as a bi- or multi-layer stack that contains NiTi. In the case where NiTi is the only material in the spring, a material composition that corresponds to Point B on FIG. 5 should be selected to ensure consistent performance over the operating temperature range of the device. In multi-layer stacks, a composition have a percentage nickel that exceeds above Point B would be suitable, depending on the shape, size, temperature coefficient and other details of the other materials composing the spring.
(40) The use of the positive-TCC material described herein provides greater device design and performance flexibility. Specifically, the degree of temperature compensation provided by the positive TCC is influenced by both thickness and composition. By varying the composition and thickness of the positive-TCC material, the temperature compensation provided can be adapted to the device requirements.
(41) Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
