Monolithic Thin-Film Piezoelectric-on-Substrate ... - Semantic Scholar
Monolithic Thin-Film Piezoelectric-on-Substrate Filters Reza Abdolvand and Farrokh Ayazi
School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, GA 30332, USA filters is same as the stacked crystal filters. However, the fabrication process is relatively complicated as multiple precisely-controlled thin-film deposition steps are involved, and thickness deviation can substantially degrade the performance of the filter. These filters also suffer from singleband operation on a chip. In this work, we introduce a new type of acousticallycoupled piezoelectric filters called monolithic thin-film piezoelectric-on-substrate (TPoS) filters. Localized resonators on a composite resonant microstructure are coupled to achieve a higher order system. The same technique has been used widely in monolithic crystal filter technology at low frequencies. Our work is the extension of the same principle to high frequency thin-film micromachined devices. The resonant structure is a stack of piezoelectric material (e.g. ZnO) sandwiched between two metal electrode layers on top of a released substrate layer (e.g. silicon). The target resonance modes are varied from width-extensional modes for low frequency application to high order thickness-extensional modes for very high frequency application. Silicon substrate with high acoustic velocity and low acoustic loss is used to improve resonance frequency and the quality factor [3]. High energy density of the silicon substrate can also enhance the linearity of the device. Improved structural integrity is another advantage of using silicon which can elevate yield and manufacturing issues involved with other technologies that employ thin free-standing membranes of piezoelectric films. Multiple-frequency filters on a single substrate are implemented by changing the lateral geometry of the filters. We demonstrate slight frequency shift in thickness mode devices covering multiple adjacent channels in a single-band and substantial change in the resonance frequency for lateral mode devices for multiple-band operation of the filters fabricated on a single substrate.
Abstract - This paper presents monolithic piezoelectric-onsubstrate acoustic filters operating in a wide frequency range. Second order narrowband filters are realized by utilizing coupled resonance modes of a single microstructure. The substrate is selected from materials with high acoustic velocity and low acoustic loss (e.g. silicon) resulting in high Q resonant structures. The unique mechanical coupling technique employed in this work enables fabrication of narrow-band filters with a high out-ofband rejection in a small footprint. We demonstrate narrow-band filters suitable for channel-select applications in IF and RF bands. Filter Q values of 800 at 250MHz, 470 at 360MHz, and 400 at 3.5GHz for small footprint second-order filters are reported. The measured power handling of these devices is high due to the use of high energy density structural material, showing a 0.2dB compression point of >15dBm at 360MHz (limited by the measurement equipment). Index Terms - Thin-film piezoelectric, monolithic filter, channel-select filters, filter Q.
I. INTRODUCTION
Narrowband channel-select filters can bring significant power saving to RF communication systems. Large filter Q in excess of a few hundreds and small shape-factors are required to prevent cross-talk between closely-spaced channels. Thin-film piezoelectric bulk acoustic resonators (FBAR) are utilized in the front-end of some transceiver circuits at GHz frequencies [1]. Typically, a number of FBAR resonators are electrically connected in a ladder configuration to provide low-loss high-order filters with a very sharp roll-off skirt. To provide adequate out-of-band rejection, the number of resonators in the coupling chain of electrically-coupled filters should be considerably large (between 4 to 10). Given the relatively large size of each resonator, electrically coupled FBAR filters consume large area, and may not offer integrated solutions for covering dispersed frequencies in a wide band. The size of the filter is of greater importance in emerging applications where multiple-band data transfer channels are required in a small form-factor. Acoustic coupling of individual resonators can potentially offer much better out-ofband rejection in a small footprint. Electrical isolation between input and output ports of an acoustically-coupled filter is the key to reach large isolation in a low-order coupled system. Second-order stacked thin-film piezoelectric BAW filters have been demonstrated with narrow pass-bands and excellent isolation suitable for applications where small size is critical [2]. The principle of operation of these acoustically-coupled
1-4244-0688-9/07/$20.00 C 2007 IEEE
II. OPERATION PRINCIPLE
The resonant structure is a free-standing plate consisting of a stack of metal electrodes, ZnO, and silicon (Fig. 1). Both bottom and top metal layers are connected through the support beams to external GSG pads and are patterned over the plate. Incorporating two sets of isolated electrodes and separating them enables each electroded portion of the plate to resonate individually. Therefore, a dual mode-shape for each resonance mode excited by one electrode set can be excited by the second set.
509
IV. DESIGN
Depending on the frequency of interest different category of resonance modes can be employed to realize monolithic TPoS filters. For LF to IF frequency band flexural resonance modes are suitable. Lateral extensional resonance modes suit IF to low RF applications and thickness extensional modes are of interest for GHz range applications. Simulating the resonance mode-shape of a chosen structure provides considerable insight for optimization of the electrode pattern to maximize the electromechanical coupling and consequently reduce the insertion loss of the filter. Surface areas of a composite structure on which the polarity of the strain field is identical at resonance should be covered with connected pieces of metal electrode. Connecting areas with opposite polarity strain field will result in charge cancellation and reduces the coupling coefficient. Although this design method is always valid, it loses applicability very quickly when the resonant structure is enlarged in order to reduce the motional impedance. Finding resonance mode shapes of large structures requires very large number of meshing elements in finite element analysis (FEA) tools which makes the method impractical. However, some intuitive design rules extracted from simulating simple structures can be loosely applied for more complicated cases. In next sub-sections two resonant mode shapes of a simple plate are discussed to better explain some of these rules.
Figure 1: Schematic diagram of a monolithic TPoS filter.
By placing these localized resonators in close vicinity of each other, enough acoustic energy can tunnel through the unelectroded portion of the medium and it results in a coupled resonator system. Like any other coupled resonator system the frequency spacing between the resonance modes (filter BW) is determined by the coupling strength. The coupling strength in monolithic filters is dependent on the thickness and the dimension of the metal electrode and the distance between the two adjacent localized resonators. Geometry of the free-standing structure, location of the support beams and the pattern of the metal electrodes are amongst the most critical design parameters that can be employed to target specific resonance frequency and suppress other resonance modes. Thickness extensional resonance modes will always appear in these devices regardless of the design strategy. However, by carefully shaping the electrodes the electromechanical coupling for these modes can be either minimized (if not desired) or maximized (if they are targeted).
A. Lateral mode A rectangular plate (same as Fig. 1) consisting of a 5pm thick silicon layer coated with a 0.5pm ZnO layer is assumed as the resonant structure. To simplify the mode-shape simulation results, a 2D cross section of the plate is drawn in FEMLAB and the third order width extensional resonance mode of the structure is shown in Fig.2.
III. FABRICATION PROCESS
The fabrication process is a 5-mask low-temperature process. The starting substrate is a high-resistivity SOI wafer with 2-6ptm silicon device layer thickness. Using a highresistivity substrate can reduce the feedthrough signal level and improve the isolation. First, the bottom metal electrode (Gold) is evaporated and patterned in a lift-off process (Fig. 1). Next, a high quality thin ZnO film (
School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, GA 30332, USA filters is same as the stacked crystal filters. However, the fabrication process is relatively complicated as multiple precisely-controlled thin-film deposition steps are involved, and thickness deviation can substantially degrade the performance of the filter. These filters also suffer from singleband operation on a chip. In this work, we introduce a new type of acousticallycoupled piezoelectric filters called monolithic thin-film piezoelectric-on-substrate (TPoS) filters. Localized resonators on a composite resonant microstructure are coupled to achieve a higher order system. The same technique has been used widely in monolithic crystal filter technology at low frequencies. Our work is the extension of the same principle to high frequency thin-film micromachined devices. The resonant structure is a stack of piezoelectric material (e.g. ZnO) sandwiched between two metal electrode layers on top of a released substrate layer (e.g. silicon). The target resonance modes are varied from width-extensional modes for low frequency application to high order thickness-extensional modes for very high frequency application. Silicon substrate with high acoustic velocity and low acoustic loss is used to improve resonance frequency and the quality factor [3]. High energy density of the silicon substrate can also enhance the linearity of the device. Improved structural integrity is another advantage of using silicon which can elevate yield and manufacturing issues involved with other technologies that employ thin free-standing membranes of piezoelectric films. Multiple-frequency filters on a single substrate are implemented by changing the lateral geometry of the filters. We demonstrate slight frequency shift in thickness mode devices covering multiple adjacent channels in a single-band and substantial change in the resonance frequency for lateral mode devices for multiple-band operation of the filters fabricated on a single substrate.
Abstract - This paper presents monolithic piezoelectric-onsubstrate acoustic filters operating in a wide frequency range. Second order narrowband filters are realized by utilizing coupled resonance modes of a single microstructure. The substrate is selected from materials with high acoustic velocity and low acoustic loss (e.g. silicon) resulting in high Q resonant structures. The unique mechanical coupling technique employed in this work enables fabrication of narrow-band filters with a high out-ofband rejection in a small footprint. We demonstrate narrow-band filters suitable for channel-select applications in IF and RF bands. Filter Q values of 800 at 250MHz, 470 at 360MHz, and 400 at 3.5GHz for small footprint second-order filters are reported. The measured power handling of these devices is high due to the use of high energy density structural material, showing a 0.2dB compression point of >15dBm at 360MHz (limited by the measurement equipment). Index Terms - Thin-film piezoelectric, monolithic filter, channel-select filters, filter Q.
I. INTRODUCTION
Narrowband channel-select filters can bring significant power saving to RF communication systems. Large filter Q in excess of a few hundreds and small shape-factors are required to prevent cross-talk between closely-spaced channels. Thin-film piezoelectric bulk acoustic resonators (FBAR) are utilized in the front-end of some transceiver circuits at GHz frequencies [1]. Typically, a number of FBAR resonators are electrically connected in a ladder configuration to provide low-loss high-order filters with a very sharp roll-off skirt. To provide adequate out-of-band rejection, the number of resonators in the coupling chain of electrically-coupled filters should be considerably large (between 4 to 10). Given the relatively large size of each resonator, electrically coupled FBAR filters consume large area, and may not offer integrated solutions for covering dispersed frequencies in a wide band. The size of the filter is of greater importance in emerging applications where multiple-band data transfer channels are required in a small form-factor. Acoustic coupling of individual resonators can potentially offer much better out-ofband rejection in a small footprint. Electrical isolation between input and output ports of an acoustically-coupled filter is the key to reach large isolation in a low-order coupled system. Second-order stacked thin-film piezoelectric BAW filters have been demonstrated with narrow pass-bands and excellent isolation suitable for applications where small size is critical [2]. The principle of operation of these acoustically-coupled
1-4244-0688-9/07/$20.00 C 2007 IEEE
II. OPERATION PRINCIPLE
The resonant structure is a free-standing plate consisting of a stack of metal electrodes, ZnO, and silicon (Fig. 1). Both bottom and top metal layers are connected through the support beams to external GSG pads and are patterned over the plate. Incorporating two sets of isolated electrodes and separating them enables each electroded portion of the plate to resonate individually. Therefore, a dual mode-shape for each resonance mode excited by one electrode set can be excited by the second set.
509
IV. DESIGN
Depending on the frequency of interest different category of resonance modes can be employed to realize monolithic TPoS filters. For LF to IF frequency band flexural resonance modes are suitable. Lateral extensional resonance modes suit IF to low RF applications and thickness extensional modes are of interest for GHz range applications. Simulating the resonance mode-shape of a chosen structure provides considerable insight for optimization of the electrode pattern to maximize the electromechanical coupling and consequently reduce the insertion loss of the filter. Surface areas of a composite structure on which the polarity of the strain field is identical at resonance should be covered with connected pieces of metal electrode. Connecting areas with opposite polarity strain field will result in charge cancellation and reduces the coupling coefficient. Although this design method is always valid, it loses applicability very quickly when the resonant structure is enlarged in order to reduce the motional impedance. Finding resonance mode shapes of large structures requires very large number of meshing elements in finite element analysis (FEA) tools which makes the method impractical. However, some intuitive design rules extracted from simulating simple structures can be loosely applied for more complicated cases. In next sub-sections two resonant mode shapes of a simple plate are discussed to better explain some of these rules.
Figure 1: Schematic diagram of a monolithic TPoS filter.
By placing these localized resonators in close vicinity of each other, enough acoustic energy can tunnel through the unelectroded portion of the medium and it results in a coupled resonator system. Like any other coupled resonator system the frequency spacing between the resonance modes (filter BW) is determined by the coupling strength. The coupling strength in monolithic filters is dependent on the thickness and the dimension of the metal electrode and the distance between the two adjacent localized resonators. Geometry of the free-standing structure, location of the support beams and the pattern of the metal electrodes are amongst the most critical design parameters that can be employed to target specific resonance frequency and suppress other resonance modes. Thickness extensional resonance modes will always appear in these devices regardless of the design strategy. However, by carefully shaping the electrodes the electromechanical coupling for these modes can be either minimized (if not desired) or maximized (if they are targeted).
A. Lateral mode A rectangular plate (same as Fig. 1) consisting of a 5pm thick silicon layer coated with a 0.5pm ZnO layer is assumed as the resonant structure. To simplify the mode-shape simulation results, a 2D cross section of the plate is drawn in FEMLAB and the third order width extensional resonance mode of the structure is shown in Fig.2.
III. FABRICATION PROCESS
The fabrication process is a 5-mask low-temperature process. The starting substrate is a high-resistivity SOI wafer with 2-6ptm silicon device layer thickness. Using a highresistivity substrate can reduce the feedthrough signal level and improve the isolation. First, the bottom metal electrode (Gold) is evaporated and patterned in a lift-off process (Fig. 1). Next, a high quality thin ZnO film (
