WO2013110041A1

WO2013110041A1 - Low insertion loss monolithic lateral-mode thin-film piezoelectric filter

Info

Publication number: WO2013110041A1
Application number: PCT/US2013/022418
Authority: WO
Inventors: Reza Abdolvand; Seyedeh Hediyeh FATEMI
Original assignee: The Board Of Regents For Oklahoma State University
Priority date: 2012-01-20
Filing date: 2013-01-21
Publication date: 2013-07-25
Also published as: WO2013110041A9

Abstract

A device has a resonant structure (106) having a diamond layer coupled to a substrate (102) with a plurality of support tethers (108). A piezoelectric material (104) overlies the resonant structure (106). The device has and input electrode (110) and an output electrode (114) overlying the piezoelectric structure for imparting an input electrical signal and receiving an output electrical signal from the piezoelectric material. The diamond layer preferrably comprises ultra- nanocrystalline diamond. The resonant structure (106) operates by acoustic coupling of two higher order mode which are two lateral-extensional-modes. Coupling of the modes can be achieved by choosing for the input and output electrodes an electrode structure with electrode fingers spaced by a distance (P) and a separation distance (D) between the opposing fingers of the input and output electrode.

Description

LOW INSERTION LOSS MONOLITHIC LATERAL-MODE THIN-FILM

PIEZOELECTRIC FILTER

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/589,014 filed January 20, 2012, herein incorporated by reference in its entirety for all purposes. STATEMENT REGARDING FEDERALLY SPONSORED

RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under NSF Grant

Number 0930676 awarded by the National Science Foundation. The Government has certain rights in the invention. FIELD OF THE I VENTION

This disclosure relates to RF filters in general and, more particularly, to microelectromechanical piezoelectric RF filters.

BACKGROUND OF THE INVENTION

High performance filters with small size and broad frequency range are highly desired in the mobile communication systems. One of the prominent filtering techniques is the surface acoustic devices (SAW) which has been widely used for various applications. Despite having a good performance, SAW devices are not integrable with other circuitries and suffer from large temperature coefficient of frequency. Thin-film bulk acoustic resonator (FBAR) filters are also successfully manufactured in a broad frequency range. These filters offer large bandwidths and very low insertion loss in addition to considerable size/cost benefits. However, with the FBAR technology being based on the thickness-mode piezoelectric resonators, it cannot be exploited to realize multi-band filtering on a single chip.

What is needed is a method and device for addressing the above, and related, issues. SUMMARY OF THE INVENTION

The invention of the present disclosure, in one aspect thereof, comprises a device having a resonant structure having a diamond layer coupled to a substrate with a plurality of tethers. A piezoelectric material overlies the resonant structure. The device has an input electrode overlying the piezoelectric structure for imparting an input electrical signal to the piezoelectric material, and an output electrode overlying the piezoelectric structure for receiving an output electrical signal from the piezoelectric material. The piezoelectric material imparts a motion into the resonator based upon the input signal and the output signal is a filtered version of the input signal resulting from a resonance within the resonant structure imparting physical motion to the piezoelectric material under the output electrode.

In some embodiments, the resonant structure comprises diamond, such as ultrananocrystaline diamond. The ultrananocrystaline diamond may have a Young's modulus of between 490GPa and 930GPa. In some embodiments, the Young's modulus is about 650GPa. The input electrode may provide a plurality of electrode fingers, each originating proximate one of the plurality of tethers. These may be separated by a distance equal to a half-wavelength at a resonance frequency of the resonant structure. The output electrode may also provide a plurality of electrode fingers, each originating proximate one of the plurality of tethers, which may also be separated by a distance equal to a half-wavelength at a resonance frequency of the resonant structure. In one embodiment, the resonant structure has a length of about 212.4μ^ηι and 1 1 pairs of tethers. In another embodiment, the resonant structure has a length of about 266.4μηι and 19 pairs of tethers.

The invention of the present disclosure, in another aspect thereof, comprises a device having a substrate, and a resonant structure having a layer of diamond covered by a layer of a piezoelectric material. A plurality of tethers anchor the resonant structure to the substrate. The device has an input electrode with a plurality of input fingers having a predetermined pitch therebetween and affixed to a first portion of the layer of piezoelectric material. The device also has an output electrode having a plurality of input fingers with the predetermined pitch therebetween, and affixed to a second portion of the layer of piezoelectric material. The input electrode provides an input signal to the piezoelectric material that is read as a filtered output signal by the output electrodes.

In some embodiments, the fingers of the input electrode and the fingers of the output electrode are substantially parallel and pointed toward the opposite electrode. The predetermined pitch may be about a half wavelength of a resonance frequency of the resonant structure. Each of the fingers of the input and output electrodes may be associated with a tether. In some embodiments, a pair of tethers running laterally to the fingers is provided for each of the input and output electrodes. The diamond layer may comprise ultrananocrystaHne diamond having Young's modulus of between 490GPa and 930GPa.

The invention of the present disclosure, in another aspect thereof, comprises a method including providing a layer of ultrananocrystaHne diamond on a semiconductor substrate, and providing a layer of piezoelectric material on the diamond layer. The method includes patterning an input electrode and an output electrode on the piezoelectric layer, and etching through the diamond and piezoelectric layers without cutting the electrodes to define a resonant structure tethered to the substrate with a plurality of tethers.

The diamond layer may have Young's modulus of between 490GPa and 930GPa. In some embodiments the input and output electrodes are patterned as a plurality of parallel fingers traversing the layer of piezoelectric material and having a pitch therebetween corresponding to a half wavelength of a resonant frequency of the resonant structure, the fingers further having a predetermined length and distance between fingers of the input and output electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an RF filter according to aspects of the present disclosure.

FIG. 2 is a perspective diagram of mode shapes for the filter of FIG. 1 as modeled in COMSO for (a) in-phase mode, and (b) out-of-phase mode.

FIG. 3(a) is a plot of the frequency response of the filter of FIG. 1 with 50Ω tenninations. FIG. 3(b) is a perspective diagram of a strain field for the in-phase lateral- extension mode of the filter of FIG. 1.

FIG. 3(c) is a perspective diagram of a strain field for the out-of-phase lateral- extension mode of the filter of FIG. 1.

FIG. 4(a) is a plot of a frequency response of a two-pole filter with 3dB- bandwidths of two resonance peaks overlapping.

FIG. 4(b) is a plot illustrating the definition of the term Δ _ζάΒ as utilized in this disclosure.

FIG. 5 is a plot of simulated responses for 56μιη and 60μιη long 7^th-order harmonic TPoD filters.

FIG. 6 is a plot of the dependency of the frequency separation between the two resonance modes on the input/output electrode separation for a 7^th-order harmonic monolithic filter.

FIG. 7 (a) - (d) are side cutaway views of an integrated circuit fabrication process for a piezoelectric RF filter.

FIG. 8 is a plot illustrating the effect of increasing device length for a piezoelectric RF filter.

FIGS. 9 (a) - (b) provide plots of measurement results from 33^rd-order harmonic filters with different lengths from 12 dies on a fabricated wafer with a Young's modulus of 933 GPa showing how insertion loss (a) and bandwidth (b) reduce, on average, as the length is increased.

FIG. 10 is a plot of the effect of increasing the electrode separation of a piezoelectric RF filter.

FIGS. 1 1 (a) - (b) provide plots of measurement results from the devices of FIGS. 9 (a) - (b) showing how insertion loss (IL) (a) and bandwidth (b) (BW) are affected.

FIG. 12 is a plot illustrating the effect of mode number on piezoelectric RF filter performance.

FIG. 13(a) is a plot of the effect of mode number on the insertion loss of the devices tested for FIG. 9. FIG. 13(b) is a plot of the effect of mode number on the bandwidth of the devices tested for FIG. 9.

FIGS. 14 (a) - (b) is a plot illustrating the effect of different support configurations on (a) insertion loss and (b) bandwidth of a 212.4μηι long 25^th-order harmonic filter.

FIG. 15 is a perspective view of a 37^th-order monolithic filter with 19 pairs of tethers along the width and two additional pairs at the two ends of the length.

FIGS. 16 (a) - (c) provide illustrations of frequency responses with the lowest insertion losses on three diamond wafers having YM of (a) 650GPa, (b) 933GPa, and (c) 491GPa.

FIG. 17(a) is a plot of the measured and simulated frequency response of the filter of FIG. 15.

FIG. 17(b) is a circuit diagram of an equivalent circuit of the filter of FIG. 15.

FIG. 18 is a plot of a wide-span frequency response of a multi-tethered 37^th- order harmonic filter on 933GPa UNCD substrate showing very few spurious modes in the entire frequency range.

FIG. 19 is a plot of the frequency response of a 25^th order harmonic design with largest bandwidth (0.2%) among all the designs tested during the work corresponding to the present disclosure.

FIG. 20 is a plot of the overlapped frequency responses of the filter of FIG. 19 at various temperatures.

FIG. 21 is a plot of the overlapped frequency responses of the filter of FIG. 19 at various input powers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Employing lateral-mode piezoelectric resonators is a plausible solution for fabrication of multi-frequency filters on the same substrate. Filters have been realized by electrical coupling of multiple resonators (e. g. in a ladder configuration or series connection where they are coupled through their intrinsic capacitance). Another approach is mechanical coupling of the resonators. However none of these filters have been reported to yield low insertion loss (IL) values (<5dB) with the standard 50Ω termination. This is mainly because of their inherently smaller coupling coefficient compared to their thickness-mode counterparts. Reducing the IL becomes more challenging especially at high frequencies, where the feature sizes reduce considerably. One other method that has formerly been explored is to couple two different lateral- extensional modes in a resonant structure in order to realize a second order monolithic filter. This class of filters demonstrated IL values as small as 3.7dB in a 50Ω network which is the lowest value reported up to the present time for the lateral-extensional piezoelectric filters.

Piezoelectric resonator and filters can be classified into two categories: 1) those that are comprised of only a piezoelectric layer; and 2) those with a piezoelectric layer stacked on a relatively thicker substrate which forms the bulk of the device. The second category may be referred to as thin-film piezoelectric-on-substrate (TPoS) devices. In various embodiments, the focus of the present disclosure is on the second category due to advantages such as better power handling and smaller temperature coefficient of frequency (TCF). However, those of skill in the art will appreciate that the techniques discussed herein can be applied to the piezoelectric-only devices as well, and possibly leading to a larger bandwidth due to their larger electromechanical coupling coefficient.

Another advantage of TPoS technology is the possibility of extending the center frequency by incorporating a material with large Young's modulus such as nanocrystalline diamond as the structural body of the device. This enables achieving higher frequencies without compromising the IL with smaller feature sizes. In order to improve the quality of the piezoelectric film, ultrananocrystalline diamond (with less than l Onm average grain size) is deposited and then polished. Three diamond substrates are utilized for the fabrication of the filters which have Young's modulus in the range of 490GPa to 930GPa.

This disclosure provides design procedures for lateral-extensional monolithic filters and describes the effect of different geometries and physical parameters on the performance of the filter. In various embodiments, the focus is on minimizing the IL of such filters while increasing the bandwidth. The discussions are then evaluated by the measurement results. IL values as low as 3.7dB and bandwidth as large as 0.2% are demonstrated. Furthermore, the nonlinear behavior of these filters is examined for input powers up to +35dBm.

Referring now to FIG. 1 , one embodiment of a lateral-extensional thin-film piezoelectric-on-substrate filter 100 is shown. The filter 100 represents a multi-tethered 7^th-order harmonic monolithic filter. The filter 100 comprises a substrate 102 (e.g., a diamond substrate) underlying a piezoelectric material 104. A portion of the substrate 102 and piezoelectric material 104 in combination forms a resonant structure 106 supported from the remainder of the device 100 via a plurality of supports 108. Two di fferent lateral-extensional modes of the resonant structure 106 are coupled together in order to realize a two-pole filter.

A set of input electrodes 110 are provided, which further comprise a plurality of electrode fingers 112. Spaced apart on the resonant structure 106 is a set of output electrodes 114, which further comprise a set of output electrode fingers 116. The input electrodes 110 and output electrodes 114 comprise a top metal of the semiconductor. A portion of bottom metal 118 is shown as well.

In the present embodiment, there is one support 108 adjacent to each of the input electrodes 114 and output electrodes 116. Additional supports 108 may be utilized at the ends of the resonant structure 106 as shown (and discussed further below). A distance between each of the electrode fingers 112, 116 is discussed below and illustrated in FIG. 1 by distance P.

In the present embodiment, the resonant structure 106 is generally rectilinear, with a length and width larger than a thickness. The fingers 112, 116 are generally parallel and rectangular, and each set points toward the opposite electrode 114, 110. It is understood that other embodiments could alter from this configuration. However, as the geometry of the device 100 that influences its useful properties as a filter, departures from known useful geometries may not yield satisfactory results. The filters of this disclosure, such as filter 100, exhibit low insertion loss (IL) values with fractional bandwidth between 0.08 to 0.2 percent while having a very small footprint. This disclosure also describes development of the lowest currently-known insertion loss for lateral-extensional piezoelectric filters with 50Ω terminations. The narrow-band filters of this disclosure are fabricated on three ultrananocrystalline diamond substrates to achieve higher frequencies without excessive reduction in the feature size. This disclosure thoroughly outlines the parameters that affect the performance of such filters and provides an evaluation by statistical data collected from actual fabricated wafers.

The filters described in this disclosure are two-pole filters built based on the acoustic coupling of two resonance modes in a single resonant structure (e.g., resonant structure 106). The resonance modes employed to build the two-pole filters of this disclosure are the higher order harmonics of two different lateral-extensional modes.

Referring now to FIG. 2, strain field and exaggerated volume deformation are displayed for the 7^th-order harmonic of the two modes corresponding to the filter 100 of FIG. 1 . FIG. 2 was prepared in COMSOL with FIG. 2(a) illustrating an in-phase mode and FIG. 2(b) illustrating an out-of-phase mode. The areas in the same shades are under the same strain polarity. Therefore, an overlaying piezoelectric film 104 sandwiched between patterned metal electrodes 110, 114 may be employed to excite and sense these modes of oscillation. Unlike other TPoS resonators, the input/output electrodes are not interdigitated. As can be seen, for the 7th-order harmonic filter of FIG. 1 each of the input and output ports 110, 114 has four electrode fingers 112, 116, respectively. The piezoelectric 104 and the metal layers (110, 114, and 118) are stacked on top of a substrate 102 which is chosen from high-Q and low-loss materials. In the present embodiment, the substrate chosen for fabricating the TPoS filters of this work is ultrananocrystalline diamond due to its large YM.

The frequency response of TPoS filters can be predicted by finite element analysis in COMSOL. The developed model incorporates a piezoelectric layer which couples the stress/strain and the electric fields, and includes the support loss as the dominant source of energy loss. The support loss (radiation of acoustic energy to the substrate through the tethers) may be considered by embedding a perfect matching layer (PML) in the model. Even though all the loss mechanisms cannot be fully captured in the model, the resulting frequency response is a strong tool to estimate the filter center frequency and study the relative effect of device dimensions on filter specifications such as loss and bandwidth, and to predict the potential nearby spurious modes which can then be eliminated by design optimization.

In all the simulations carried out, the diamond substrate thickness is 3μη and the Young's modulus of the film is set to 650GPa, which is the actual Young's modulus of one of the diamond wafers used for fabricating the thin-film piezoelectric-on- diamond (TPoD) devices (e.g., filter 100). In addition, 50Ω terminations are adopted for both input and output ports. FIG. 3(a) plots the simulated frequency response of a 60μιη long 7th-order harmonic filter (as shown in FIG. 1) with 50Ω terminations. FIGS. 3(b) and 3(c) illustrate the strain fields for the in-phase lateral extension mode and out-of- phase lateral extension mode, respectively. At each resonance mode, part of the acoustic wave escapes the device through the tethers (e.g., supports 108) which is referred to as the support loss. This portion of the acoustic wave creates a strain field in the surrounding substrate of the resonator as displayed in FIGS. 3(b) and 3(c). The acoustic wave then gets absorbed as it impinges upon the perfectly matched layer (PML).

The frequency of the in-phase mode is defined by the finger pitch P which is equal to half-wavelength at resonance. The frequency separation between the two target modes is affected by the device geometries such as the device length L and the distance between the input and output electrodes D. Below, some of these parameters are evaluated using FEM analysis.

In order to realize a two-pole filter, the 3dB-bandwidths of the two resonance peaks should overlap as shown in FIGS. 4(a)-4(b). To study the effect of different design parameters on the bandwidth of the filter and compare different designs, a new parameter called B is defined for the present disclosure. This definition is only for the sake of comparison between the different designs where the 3dB-bandwidths of the two resonance peaks do not overlap.

The simulated frequency responses for two 7^th-order harmonic filters with two different lengths L are plotted in FIG. 5. As clear from the plots, in order to decrease the frequency separation between the two resonance modes, length L of the filter should be increased. The input/output electrode separation D was varied from 2μιη to 8μιη for a 60μιη long (e.g., distance L) 7^th-order harmonic filter. The frequency responses were simulated as shown in FIG. 6. In general, the frequency separation reduces as the distance D between the input and output electrodes increases. However, since the electrode separation D is increased by decreasing the length of the electrodes, the actuation area reduces, which results in inefficient excitation of the two modes and hence larger IL and more spurious modes.

Considering that, in the present embodiment, the filter structure is identical to the resonator 106, except for the top electrode pattern, the same techniques can be employed to improve the IL for both devices. It is beyond the focus of this paper to discuss these methods as they have been thoroughly explained in the literature. In summary, these techniques include multi-tethering, and increasing the actuation area by both increasing the length and the mode number.

Referring now to FIGS. 7(a) - 7(d), a fabrication process for manufacturing filters of the present disclosure (e.g., filter 100) is shown. The overall process may be described as the bottom metal layer as shown in FIG. 7(a); deposition of A1N (as a piezoelectric material 104) and top metal layer (electrodes 110, 114), followed by patterning the top metal (electrodes 110, 114) as shown in FIG. 7(b); sputtering gold 702 on the contact areas as shown in FIG. 7(c); and etching the stack followed by releasing the device 100 by etching handle silicon 800 from the backside in order to release the device 100.

It should be understood that other piezoelectric materials could be used in place of A1N - for example, ZnO and LiNbO. Diamond was chosen as the substrate 102 for TPoS devices of the present disclosure due to its high Young's modulus and low acoustic loss. However, other substrates such as silico can also be used. In order to improve the quality of the piezoelectric film 104 (A1N) deposited on the diamond substrate 102, the surface roughness was reduced. This was accomplished by depositing ultrananocrystalline diamond (UNCD) with average grain size of less than lOnm using hot filament chemical vapor deposition (HFCVD) technique and then two subsequent polishing steps. After UNCD deposition and chemical mechanical polishing (CMP), the stack of Mo/AIN/Mo was sputtered on top of the UNCD. The quality of the AIN films is very close to that of c-axis aligned AIN deposited on polished single crystal silicon substrates for which the full-width-half-maximum (FWHM) of the rocking curve is typically in the range of 2.0-2.5°.

Referring again to FIG. 8(a), UNCD 102 is deposited on the polished surface of the silicon wafer 800. After that the bottom metal is sputtered and then patterned by dry-etching in SFe and (¾ plasma. As shown in FIG. 8(b) the AIN and top metal (110, 114) are sputtered followed by patterning the top metal (110, 114). As shown in FIG. 8(c) the AIN 102 is wet-etched in a tetra-methyl-ammonium hydroxide (TMAH) based solution to gain access to the bottom metal 118 followed by sputtering gold 702 on the electrode areas to reduce ohmic losses. Finally, as shown completed in FIG. 8(d), the device stack is etched down to the silicon substrate 800 in an inductively coupled plasma (ICP) etcher (AIN in C12 and diamond in 02/CF4 plasma). The device 100 is released by dry etching the handle silicon 800 from the backside in a deep reactive ion etcher.

FEM simulations assisted in determining how different design parameters affect the filter performance. In order to achieve very low insertion loss values, the device size has to be increased either by increasing the length or designing higher order devices. For such larger devices, it is not possible to simulate the frequency response in a reasonable time. To evaluate the guidelines and also minimize the IL and increase the filter bandwidth, several filters were been designed and fabricated with various lengths, electrode separation, and mode number in addition to different tether configurations. To better characterize the designs, devices were measured from all of the dies on one of the UNCD wafers with the highest Young's modulus. Also, to compare the three different UNCD substrates the filter design with the lowest IL was measured on all the three UNCD wafers.

All the measurements in this disclosure were performed using an Agilent E8358A PNA Network Analyzer and a pair of GSG probes while terminated with the internal 50Ω impedance of the network analyzer. Prior to measurements, short-open- load-thru (SOLT) calibration was carried out on a reference substrate. The measurements are performed at atmospheric pressure and ambient temperature, except for the measurement of the temperature coefficient of frequency (TCF) which was performed in a vacuum probe station, again with 50Ω terminations.

All the filters in this disclosure were designed based on the multi-tethering concept, which improves the performance of the TPoS resonators such as quality factor, IL, and power handling while suppressing multiple spurious modes some of which are very close to the target mode. In order to improve the out-of-band rejection, the parasitic capacitances were reduced by removing the bottom metal layer underneath the measurement pads before the deposition of AIN/Mo stack.

The first parameter that was studied was the length L of the filter. 33rd-order harmonic monolithic filters with 15 pairs of tethers were designed and fabricated with their length varying from 133μιη to 234μηι. The frequency responses of typical designs are plotted in FIG. 8 and the filter parameters are summarized in Table 1 for comparison. For the 133.2μιη design, the second resonance mode is further from the first mode, such that the coupling between the two modes is not strong enough and the second resonance peak is hardly distinguishable. For devices longer than 133.2μιη, the two peaks get closer as the length is increased such that for 212.4μιη and 234μιη long designs the Af_3dB is equal to the BW of the filter. The first set of analytical data presented in FIG. 9 demonstrate the effect of increasing the length on the bandwidth and insertion loss of the filters measured all around the fabricated wafer with Young's modulus of 933GPa. Note that the data for the 133.2μιη designs is excluded from these plots due to the very weak coupling of the two resonance modes. As expected, the IL reduces on average as the length L of the device is increased, and so does the Af_3dB .

Table 1 : The IL and bandwidth of typical 33^rd order harmonic monolithic filters with different lengths.

The second parameter that was experimentally studied was the distance D between the input and output electrodes which is referred to as the electrode separation. The 234μιη long, 33rd-order harmonic filter just mentioned was designed with four different electrode separations. The corresponding frequency responses for typical designs are plotted in FIG. 10. From the simulation results of FIG. 6, it was observed that increasing the electrode separation would push the two peaks further from each other. This trend holds true for electrode separations smaller than one wavelength (=14.4μιη) as displayed by the measured statistical data of FIG 1 1 (a). For these three designs, the IL is almost constant on average. The larger BW for 14.4μι^η separation between the input/output electrodes is achieved at the expense of larger insertion loss (~2dB larger) which is due to the weaker excitation of the resonance modes caused by smaller electrode area shown in FIG. 1 1(b). Thus, in order to increase the bandwidth of the filter while maintaining the lowest insertion loss possible, it is best to reduce the electrode separation D between the input and output electrodes as much as the fabrication technology allows.

Increasing mode number generally improves the IL of a TPoS resonator. Hence, monolithic filters were designed in four different order harmonics while keeping the aspect ratio of the device almost constant. Typical frequency responses of such designs are plotted in FIG 12. For the 21st-order harmonic filter, the frequency separation between the two peaks is large and thus, the coupling between them is very weak. As the order of the filter is increased, the two resonance modes get closer to form a two- pole filter but the drawback is the decrease in the filter out-of-band rejection which is due to the increase in the parasitic capacitance between the input and output ports. These devices were measured all over one of the U CD substrates with Young's modulus of 933GPs (FIGS. 13(a) - 13(b). The insertion loss does not change much on average for the higher order harmonics. However, the frequency separation between the two resonance peaks decreases.

It was also determined, as illustrated in FIGS. 14(a) - 14(b) that a pair of support tethers in the middle of the length L deteriorates IL. On the other hand, adding two pairs of tethers laterally proceeding from the ends of length L increased BW in designs with 9 and 1 1 pairs of tethers. Among all the designs, a 37th-order harmonic filter rigidly supported with 19 pairs of tethers along the width has the lowest insertion loss reported to date for lateral- extensional filters with 50Ω terminations. A superior view of such a device 1500 is shown in FIG. 15. One support tether 1502 can be seen underlying each electrode finger 1504. Four additional tethers 1502 can be seen extending laterally from the direction of the fingers 1504 at each of four comers of a resonator 1506. Frequency responses for the device 1500 constructed on the three UNCD substrates are plotted in FIGS 16(a) - 16(c). Direct termination with 50Ω impedance eliminates the need for on- chip impedance matching networks.

The filter on the substrate with Young's modulus of 650GPa exhibited a record low insertion loss of 3.7dB at ~900MHz and has 0.09% fractional bandwidth (-800 kHz) with a 20dB-shape factor of 2.5 and out-of-band rejection of 34dB while maintaining a very small footprint (~266μιη χ266μιη). This filter was modeled by the 4th-order system shown in FIGS. 17(a) - 17(b) using Multisim. In the equivalent circuit (FIG. 17(b), the motional resistance of each of the comprising LC tanks is as small as 8Ω.

The wide-span transmission plot for this low-loss filter is shown in FIG. 18 for a typical device on 933GPa UNCD substrate. This multi-tethered filter has no spurious mode in a large vicinity of the passband, and only a few strong spurs from 300KHz to 2GHz (at 356MHz), which is another superior property of the multi-tethering approach. Length L for this embodiment was 266.4μπι, distance D was 266μιη.

The largest bandwidth was observed for a 212.4μηι long 25th-order harmonic design with 1 1 pairs of tethers. This filter exhibits a fractional bandwidth of 0.2% which is equivalent to 2.2MHz at 1.08GHz (see FIG. 19).

The temperature coefficient of frequency was measured for the low-loss filter on

933GPa UNCD substrate to be -l l ppm/°C for temperatures ranging from -20°C to 80°C (see FIG. 20), which is reasonably smaller than the values reported previously for lateral-extensional filters based on aluminum nitride.

The nonlinear behavior of this design was studied and results are plotted for the filter on 933GPa UNCD substrate in FIG. 21. An RF amplifier was used to apply input powers above the 15dBm-limit of the network analyzer. SOLT calibration was carried out for the measurement setup including the RF amplifier. The filter shows a very good linearity for input powers up to 20dBm.

According to various embodiments of the present disclosure, thin-film piezoelectric-on-diamond filters were designed and fabricated on ultrananocrystalline diamond substrates. The filter operation is realized by coupling two different lateral- extensional modes together in the resonant structure. These two-pole filters exhibit insertion loss values as low as 3.7dB and fractional bandwidth as large as 0.2% with 50Ω terminations.

* * * *

Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.

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Claims

CLAIMS What is claimed is:

1. A device comprising:

a resonant structure having a diamond layer and coupled to a substrate with a plurality of tethers;

a piezoelectric material overlying the resonant structure;

an input electrode overlying the piezoelectric structure for imparting an input electrical signal to the piezoelectric material; and

an output electrode overlying the piezoelectric structure for receiving an output electrical signal from the piezoelectric material;

wherein the piezoelectric material imparts a motion into the resonator based upon the input signal and the output signal is a filtered version of the input signal resulting from two or more coupled resonance modes within the resonant structure imparting physical motion to the piezoelectric material under the output electrode.

2. The device of claim 1, wherein the resonant structure comprises

ultrananocrystaline diamond.

3. The device of claim 2, wherein the ultrananocrystaline diamond has a Young's modulus of between 490GPa and 930GPa.

4. The device of claim 2, wherein the ultrananocrystaline diamond has a Young's modulus of about 650GPa.

5. The device of claim 1 , wherein the input electrode provides a plurality of electrode fingers, each originating proximate one of the plurality of tethers.

6. The device of claim 5, wherein the plurality of input electrode fingers are separated by a distance equal to a half-wavelength at a resonance frequency of the resonant structure.

7. The device of claim 5, wherein the output electrode provides a plurality of electrode fingers, each originating proximate one of the plurality of tethers.

8. The device of claim 7, wherein the plurality of output electrode fingers are separated by a distance equal to a half-wavelength at a resonance frequency of the resonant structure.

9. The device of claim 1 , wherein the resonant structure has a length of about 212.4μηι and 1 1 pairs of tethers.

10. The device of claim 1 , wherein the resonant structure has a length of about 266.4μιη and 19 pairs of tethers.

1 1. A device comprising:

a substrate;

a resonant structure having a layer of diamond covered by a layer of a piezoelectric material;

a plurality of tethers anchoring the resonant structure to the substrate;

an input electrode having a plurality of input fingers having a predetermined pitch therebetween and affixed to a first portion of the layer of piezoelectric material; and

an output electrode having a plurality of input fingers having the predetermined pitch therebetween and affixed to a second portion of the layer of piezoelectric material; wherein the input electrode provides an input signal to the piezoelectric material that is read as a filtered output signal by the output electrodes.

12. The device of claim 1 1 wherein the fingers of the input electrode and the fingers of the output electrode are substantially parallel and pointed toward the opposite electrode.

13. The device of claim 1 1, wherein the predetermined pitch is about a half wavelength of a resonance frequency of the resonant structure.

14. The device of claim 1 1 , wherein each of the finger of the input and output electrodes are associated with a tether.

15. The device of claim 14, wherein a pair of tethers running laterally to the fingers is provided for each of the input and output electrodes.

16. The device of claim 1 1 , wherein the diamond layer comprises

ultrananocrystaline diamond having Young's modulus of between 490GPa and 930GPa.

17. A method comprising:

providing a layer of ultrananocrystaline diamond on a semiconductor substrate; providing a layer of piezoelectric material on the diamond layer;

patterning an input electrode and an output electrode on the piezoelectric layer; and

etching through the diamond and piezoelectric layers without cutting the electrodes to define a resonant structure tethered to the substrate with a plurality of tethers.

18. The method of claim 17, wherein the diamond layer has a Young's modulus of between 490GPa and 930GPa.

19. The method of claim 17, wherein the input and output electrodes are patterned as a plurality of parallel fingers traversing the layer of piezoelectric material and having a pitch therebetween corresponding to a half wavelength of a resonant frequency of the resonant structure, the fingers further having a predetermined length and distance between fingers of the input and output electrodes.