Tunable lithium niobate resonators and filters using lithiation and delthiation

Abstract

Acoustic surface wave (SAW) device (100) comprising the following: a silicon substrate (108) with a main surface; a piezoelectric substrate (102) formed from lithium niobate; a metal oxide layer (110) that lies between the silicon substrate (108) and the piezoelectric substrate (102); at least one electrode (106) on the piezoelectric substrate (102); and a tuning circuit (120) configured to modify a charge center of the piezoelectric substrate (102).

Description

TECHNICAL FIELD

[0001] The present application relates generally to surface acoustic wave devices and, more particularly, to systems and methods for tuning surface acoustic wave devices. BACKGROUND

[0002] SAW (surface acoustic wave) devices are a class of microelectromechanical systems (MEMS) based on the modulation of surface acoustic waves to detect a physical phenomenon. The sensor converts an input electrical signal into a mechanical wave, which, unlike an electrical signal, can be easily influenced by physical phenomena. The device then converts this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.

[0003] SAW technology utilizes the piezoelectric effect in its operation. A basic SAW device consists of a piezoelectric substrate and one or more interdigital transducers (IDTs). The piezoelectric substrate is made of a piezoelectric material, such as quartz, lithium tantalite (LiTaO3), or lithium niobate (LiNbO3). An IDT is made of two metallic comb-like structures arranged in a finger-like interdigitated manner on the surface of the piezoelectric substrate.

[0004] SAW devices typically have two IDTs, an input IDT and an output IDT. Surface acoustic waves are generated by applying an alternating voltage across the input IDT. The input IDT is fed with a sinusoidal electrical input signal, which creates alternating polarity between the fingers of the interdigitated transducer. This creates alternating regions of tensile and compressive stress between the fingers of the electrode through the piezoelectric effect, generating a mechanical wave at the surface known as a surface acoustic wave.

[0005] The wave is received by the output IDT, which converts the mechanical energy from the wave back into an electric field using the piezoelectric effect. Any changes the mechanical wave has undergone are reflected in the electrical output signal because the characteristics of the surface acoustic wave are modified by changes in the surface properties of the piezoelectric substrate.

[0006] SAW devices have a resonant frequency that is a function of the spacing between the input and output IDTs and the material's acoustic wave velocity. When a surface acoustic wave is generated at or near the resonant frequency of the piezoelectric material, the insertion loss of the SAW device can be minimized. However, this also means that the frequency response of the device is set during fabrication. The ability to adjust, or "tune," the frequency response of SAW devices after fabrication is limited. Consequently, the use of SAW devices is typically limited to applications where the frequency response is fixed. US 2005 / 0 096 230 A1 describes a device based on a perovskite-type oxide thin film epitaxially grown on an oxide thin film on an inorganic amorphous layer or an organic solid layer.EP 1 515 436 A2 discloses a surface acoustic wave element. US 2008 / 0042 517 A1 describes the control of the speed of sound in a surface acoustic wave device. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view of a tunable SAW device. Fig. 2 is a schematic elevation view of the SAW device of Fig. 1. Fig. 3 is a graph showing a frequency response curve for a SAW device having a silicon substrate such as in Fig. 1, and shows a SAW device without a silicon substrate. Fig. Figure 4 is a schematic perspective view of a tunable SAW device such as in Fig. 1, with a tuning circuit. Fig. Figure 5 is a graph showing the direction of change in resonant frequency for a tunable SAW device such as in Fig. 4, due to the tuning circuit for a delithiation and a lithiation process. DETAILED DESCRIPTION

[0007] For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It should be understood that no limitation of the scope of the disclosure is intended thereby. It is further understood that the present disclosure includes any variations and modifications to the illustrated embodiments and includes other applications of the principles of the disclosure as would normally occur to one of ordinary skill in the art to which this disclosure belongs.

[0008] Fig. 1 illustrates a SAW (surface acoustic wave) device 100 capable of being tuned in a simple and cost-effective manner. The tunable SAW device 100 includes a piezoelectric substrate 102, at least one interdigital electrode 106, a silicon substrate 108, and a metal oxide layer 110. The piezoelectric substrate is formed of lithium niobate. One or more interdigital electrodes are formed on the piezoelectric substrate for generating and / or receiving surface acoustic waves in the piezoelectric substrate. The electrodes are formed of a conductive metal, such as aluminum, an aluminum alloy, platinum, tantalum, or any other suitable conductive metal. When illustrated in Fig. 1, a single interdigital electrode is shown. The interdigital electrodes include nodes 104 for connecting the electrodes 106 to other circuitry, such as signal generation and -receiving circuits and signal evaluation circuits, which are not shown.

[0009] The carrier substrate is made of silicon. Silicon substrates have a lower acoustic impedance and are known to conduct acoustic waves, resulting in a higher Q factor and lower insertion loss. The silicon substrate also contributes durability to the device design and enables the integration of other circuit elements into the SAW device, such as resonators and filters. Fig. Figure 3 is a graph showing the change in frequency response of a SAW device with a silicon substrate such as in Fig. 1, and a SAW device without a silicon substrate.

[0010] As in Fig. As can be seen in Figure 3, the curve labeled 112, which represents the frequency response for a SAW device with a silicon substrate, has a narrower width than the curve labeled 114, which represents the frequency response of a SAW device without a silicon substrate, indicating that the SAW device with a silicon substrate has a higher quality factor (Q) than the device without a silicon substrate. The peaks of the curves are centered at the resonant frequency for the respective devices. The higher peak of the frequency response curve 112 for the SAW device with a silicon substrate correlates with a lower insertion loss for the SAW device with a silicon substrate than for the SAW device without a silicon substrate.

[0011] Again with reference to Fig. 1 and Fig. 2, to enable tuning of the SAW device 100, a metal oxide layer 110 is interposed between the silicon support substrate 108 and the piezoelectric substrate 102. In one embodiment, the metal oxide layer 110 is formed from aluminum oxide, or "alumina." The aluminum oxide layer 110 may be deposited in any suitable manner. In one embodiment, the aluminum oxide layer is deposited using an ALD process. The aluminum oxide layer may be deposited to a thickness ranging from approximately 10 nm to 1000 µm, although any suitable thickness may be used, including thicknesses outside this range.

[0012] The aluminum oxide layer 110 between the lithium niobate substrate 102 and the silicon substrate 108 creates a structure similar to a lithium-ion battery, with the lithium niobate acting as the cathode, the silicon acting as the anode, and the aluminum oxide acting as the electrolyte, more specifically, a solid electrolyte. When lithium-ion batteries are charged, a positive (+) voltage source applies a positive (+) voltage potential to the cathode. The positive (+) voltage potential forces the Li ions from the LiCoO2 complex to migrate across the electrolyte and intercalate (insert themselves into interstitial or other vacancies) into the anode material. This ionic movement from the cathode across the electrolyte to the anode is balanced by electronic movement from the cathode to the anode via an external circuit. This electrochemical process is called delithiation.Similarly, when a lithium-ion battery is discharged, an electrical load is connected between the anode and cathode. Upon connecting an electrical load, the lithium ions diffuse from the anode through the electrolyte back to the cathode, balanced by an electronic flow through the electrical load from the anode to the cathode. This electrochemical process is called lithiation.

[0013] With reference to Fig. 1, when a positive (+) DC bias is applied to the lithium niobate substrate 102, lithium ions migrate from the lithium niobate to the silicon of the silicon substrate 108, similar to the lithiation process for lithium-ion batteries. When an electrical load, e.g., a resistive load, is connected between the lithium niobate substrate 102 and the silicon substrate 108, lithium ions move from the silicon substrate 108 back to the lithium niobate substrate 102, similar to the delithiation process for lithium-ion batteries. By causing Li ions to move from the lithium niobate substrate into the silicon substrate (i.e., lithiation), positive and negative charge centers in a piezoelectric material, such as lithium niobate, are shifted. This shift of the positive and negative charge centers changes the electrochemical coupling of the piezoelectric lithium niobate substrate.This, in turn, changes the acoustic resonant frequency of the lithium niobate substrate and, as a consequence, changes the resonant frequency of the SAW device 100. Configuring the SAW device 100 in the manner of a lithium-ion battery accordingly allows the same electrochemical processes used to charge and discharge a lithium-ion battery, i.e., delithiation and lithiation, to be used to controllably and selectively adjust the resonant frequency of the SAW device.

[0014] With reference to Fig. Figure 4 shows a schematic illustration of the SAW device 100 configured for tuning via delithiation and lithiation processes. To tune the SAW device 100, the SAW device 100 is provided with a tuning circuit 120 connected between the lithium niobate substrate 102 and a reference potential, e.g., ground (GND). The tuning circuit 120 can be implemented in the silicon substrate or on a separate substrate and can be electrically connected to the SAW device 100 in any suitable manner.

[0015] The tuning circuit 120 may include a voltage source 122 (for delithiation) and / or an electrical resistance element 124 (for lithiation). The voltage source 122 may be electrically connected to the lithium niobate substrate 102 and configured to apply a positive (+) DC bias to the lithium niobate substrate 102. The voltage source 122 may be implemented in any suitable manner and may be configured to apply any suitable voltage level to the substrate to achieve a desired shift of the charge centers of the lithium niobate substrate. The delithiation caused by the positive (+) DC bias results in an increase, e.g., "tuning up," of the resonant frequency f R of the SAW device, as shown by reference to the curve labeled 130 in the Fig. 5 can be seen in the graph shown.

[0016] The electrical resistance element comprises one or more resistive circuit elements configured to have a predetermined electrical resistance when connected to the lithium niobate substrate. In one embodiment, the electrical resistance element comprises a tunable resistor, although any suitable type of resistive element or resistor may be used. The predetermined resistance of the electrical resistance element may be any suitable resistor for achieving a desired displacement of the charge centers of the lithium niobate substrate. The lithiation caused by the electrical resistance element results in a decrease, e.g., "tuning down," of the resonant frequency f R ' of the SAW device, as shown by reference to the curve labeled XXX in the figure shown in Fig. 5 can be seen in the graph shown.

Claims

[1] A surface acoustic wave (SAW) device (100) comprising: a silicon substrate (108) having a main surface; a piezoelectric substrate (102) formed of lithium niobate; a metal oxide layer (110) interposed between the silicon substrate (108) and the piezoelectric substrate (102); at least one electrode (106) on the piezoelectric substrate (102); and a tuning circuit (120) configured to change a charge center of the piezoelectric substrate (102). [2] The SAW device (100) according to claim 1, wherein the metal oxide layer (110) is formed of aluminum oxide. [3] The SAW device (100) of claim 2, wherein the at least one electrode (106) comprises at least one pair of interdigital transducers. [4] The SAW device (100) of claim 1, wherein the tuning circuit (120) comprises a resistor (124) and / or a voltage source (122) electrically connected to the piezoelectric substrate (102). [5] The SAW device (100) of claim 4, wherein the resistor (124) is a tunable resistor. [6] The SAW device (100) of claim 1, wherein the tuning circuit (120) includes the voltage source (122), the voltage source (122) being configured to apply a predetermined voltage to the piezoelectric substrate (102). [7] The SAW device (100) of claim 3, further comprising a pair of reflectors on the piezoelectric substrate (102) positioned on opposite sides of the at least one pair of interdigital transducers. [8] A method of manufacturing a tunable surface acoustic wave (SAW) device (100), the method comprising: Depositing a metal oxide layer (110) on a silicon substrate (108); Attaching a piezoelectric substrate (102) to the metal oxide layer (110), the piezoelectric substrate (102) being formed of lithium niobate; Forming at least one electrode (106) on a side of the piezoelectric substrate (102) opposite the metal oxide layer (110); and Connecting a tuning circuit (120) to the piezoelectric substrate (102), wherein the tuning circuit (120) is configured to selectively change a charge center of the piezoelectric substrate (102). [9] The method of claim 8, wherein the metal oxide layer (110) is an aluminum oxide layer. [10] The method of claim 9, wherein the aluminum oxide layer (110) is deposited using an atomic layer deposition process. [11] The method of claim 10, wherein the at least one electrode (106) comprises at least one pair of interdigital transducers. [12] A method of operating a tunable surface acoustic wave (SAW) device (100), the method comprising: electrically connecting a tuning circuit (120) to the tunable SAW device (100), the tunable SAW device (100) comprising: a silicon substrate (108); a piezoelectric substrate (102) formed of lithium niobate; and an aluminum oxide layer (110) interposed between the silicon substrate (108) and the piezoelectric substrate (102); at least one electrode (106) on the piezoelectric substrate (102) on a side of the piezoelectric substrate (102) opposite the aluminum oxide layer (110), Activating the tuning circuit (120) to selectively change a charge center of the piezoelectric substrate (102) so as to change a resonant frequency of the SAW device (100). [13] The method of claim 12, wherein the tuning circuit (120) comprises a resistor (124) and / or a voltage source (122) electrically connected to the piezoelectric substrate (102).

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