WO2014129979A1 - Dispositif à ondes acoustiques de surface - Google Patents

Dispositif à ondes acoustiques de surface Download PDF

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Publication number
WO2014129979A1
WO2014129979A1 PCT/SG2014/000080 SG2014000080W WO2014129979A1 WO 2014129979 A1 WO2014129979 A1 WO 2014129979A1 SG 2014000080 W SG2014000080 W SG 2014000080W WO 2014129979 A1 WO2014129979 A1 WO 2014129979A1
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Prior art keywords
acoustic wave
surface acoustic
wave device
trench
piezoelectric layer
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PCT/SG2014/000080
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English (en)
Inventor
Andrew B. RANDLES
Julius Ming Lin Tsai
Piotr Kropelnicki
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Agency For Science, Technology And Research
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Publication of WO2014129979A1 publication Critical patent/WO2014129979A1/fr

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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/02535—Details of surface acoustic wave devices
    • H03H9/02818—Means for compensation or elimination of undesirable effects
    • H03H9/02834—Means for compensation or elimination of undesirable effects of temperature influence
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/02535—Details of surface acoustic wave devices
    • H03H9/02543—Characteristics of substrate, e.g. cutting angles
    • H03H9/02574—Characteristics of substrate, e.g. cutting angles of combined substrates, multilayered substrates, piezoelectrical layers on not-piezoelectrical substrate

Definitions

  • the invention relates generally to a surface acoustic wave device.
  • SAW devices are used as filters in devices such as television sets, cell phones and radar arrays. They are attractive for their relative ease of manufacture and their filtering characteristics which are almost completely controlled by device design.
  • the main drawback of these devices is the temperature instability of filters and resonators. This is a function of the temperature coefficients of elasticity (TCE), coefficients of thermal expansion (CTE) and the CTE's effect on density.
  • AW temperature compensate acoustic wave
  • One device makes use of the positive TCE of silicon dioxide to compensate the negative TCE and CTE of materials used in the AW device.
  • Oxide has been used as a cladding material with SAW devices to compensate TCE and CTE of a piezoelectric substrate.
  • Oxide has also been used to compensate aluminum nitride (AIN) based Lamb wave devices.
  • oxide is applied to the underside of a suspended AIN membrane.
  • oxide pillars were added to a bulk acoustic wave (BAW) resonator to remove the first order temperature affects of TCF.
  • BAW bulk acoustic wave
  • Another temperature compensation method involves adding a heater underneath a zinc oxide (ZnO) SAW device. With the addition of the heater, the temperature of the SAW material can be controlled and maintained.
  • This device has the disadvantage that it is not a passive method of temperature compensation; consequently it cannot be used in passive sensors and the device is not CMOS compatible.
  • a development of this method changes the properties of the AIN film by doping it.
  • the doped AIN layer has positive TCE coefficients making zero TCF structures possible.
  • the disadvantage of this method is it does not allow for custom devices on the same die with different TCF and may affect the quality of resonator produced.
  • a surface acoustic wave device comprising an electrode arrangement; a substrate; a piezoelectric layer disposed between the electrode arrangement and the substrate, wherein the substrate comprises a plurality of trench arrangements that open from portions of a surface of the substrate that faces the piezoelectric layer and wherein any two adjacent trench arrangements of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device; and insulator disposed in at least one of the plurality of trench arrangements.
  • a method of fabricating a surface acoustic wave device comprising: providing a substrate; forming a plurality of trench arrangements that open from portions of a surface of the substrate, wherein the formation of the plurality of trench arrangements comprises tuning by spacing two adjacent trench arrangements a wavelength distance of an operating frequency of the surface acoustic wave device; disposing insulator in at least one of the plurality of trench arrangements; providing a piezoelectric layer such that the plurality of trench arrangements of the substrate face the piezoelectric layer; and providing an electrode arrangement, wherein the piezoelectric layer is disposed between the electrode arrangement and the substrate.
  • Figure 1 A shows the cross-sectional structure of a surface acoustic wave device according to a first embodiment of the invention.
  • Figure 1 B shows the cross-sectional structure of a surface acoustic wave device according to a second embodiment of the invention.
  • Figure 1 C shows the cross-sectional structure of a surface acoustic wave device according to a third embodiment of the invention.
  • Figure 1 D shows the cross-sectional structure of a surface acoustic wave device according to a fourth embodiment of the invention.
  • Figure 1 E shows the cross-sectional structure of a surface acoustic wave device according to a fifth embodiment of the invention.
  • Figure 1 F shows the cross-sectional structure of a surface acoustic wave device according to a sixth embodiment of the invention.
  • Figure 1G shows the cross-sectional structure of a surface acoustic wave device according to a seventh embodiment of the invention.
  • Figure H shows the cross-sectional structure of a surface acoustic wave device according to an eighth embodiment of the invention.
  • Figure 11 shows a top view of the surface acoustic wave device of Figures 1A to 1 H.
  • Figure 2 shows a top view of a surface acoustic wave device according to a ninth embodiment of the invention.
  • Figure 3A shows simulation results of the deflection and strain energy stored in the surface acoustic wave device of Figure 1 A at resonance.
  • Figures 3B to 3D show cross-sectional structures of surface acoustic wave devices where simulation results have been obtained.
  • Figures 4 and 5 show the effects of varying the trench depth and trench width on the temperature coefficient of frequency (TCF) and resonant frequency of a surface acoustic wave device.
  • TCF temperature coefficient of frequency
  • Figure 6 shows the results from simulations run with different trench dimensions that have 0 (zero) first order TCF for two different electrode widths.
  • Figure 7 shows that the effect on first order TCF and second order TCF by changing the top and bottom width ratios of a trapezoidal profiled trench.
  • Figure 8 shows a graph that plots TCF as both a function of oxide trench depth and width for an oxide having a cross sectional rectangular shape.
  • Figure 9 shows the TCF as a function of strain energy in oxide trenches.
  • Figure 10 shows a simulation of a multi trench device and the strain energy stored in the device.
  • Figure 11 shows the response of an uncompensated strain sensor operating at 0 °C and 300 °C.
  • Figure 12 shows the simulation result of adding oxide trenches, in accordance to an embodiment of the invention, to the uncompensated device of Figure 11.
  • Figure 13 shows a flow chart of a method to fabricate a surface acoustic wave device according to an embodiment of the invention.
  • the term "surface acoustic wave device” may refer to a class of microelectromechanical (MEM) resonators which rely on the modulation of surface acoustic waves to sense a physical phenomenon.
  • the MEM resonator may include a transducer that converts an electrical signal into a mechanical wave at the surface of the MEM resonator, known as a surface acoustic wave, via the piezoelectric effect.
  • the acoustic wave travels across the surface of the MEM resonator substrate to another transducer, converting the wave back into an electric signal by the piezoelectric effect. Any changes that made to the mechanical wave will be reflected in the output electric signal.
  • Sensors can be designed to quantify the physical phenomenon which brings about these changes.
  • electrode arrangement may mean one or more electrodes, with some provided discretely, i.e. not being electrically connected with another electrode, and other electrodes arranged to be interdigitated. Such an interdigitated arrangement may be realised by disposing electrodes into two adjacent sets of fingers, which are connected to create an alternating polarity between two adjacent fingers.
  • substrate may mean a base structure of the surface acoustic wave device.
  • the base structure may be a single crystal substrate and may be fabricated from materials that include silicon (Si), gallium arsenide (GaAs), indium gallium arsenide (InGaAs) and indium phosphide (InP).
  • piezoelectric layer may mean a body inside the surface acoustic wave device within whic a surface acoustic wave propagates.
  • the piezoelectric layer propagates the surface acoustic wave by the piezoelectric effect and thus may be fabricated from piezoelectric material, such as aluminum nitride (AIN).
  • AIN aluminum nitride
  • Other materials such as lithium niobate (LiNb0 3 ), zinc oxide (ZnO), quartz or lead zirconium tintate (PZT) may also be used for the piezoelectric layer.
  • the phrase "disposed between” may mean that the resonator is located at the space that separates the electrode arrangement and the substrate.
  • one surface of the resonator is flush with and in contact with the electrode arrangement, while an opposite surface is flush with and in contact with the substrate.
  • one or more layers may be present between the resonator and the electrode arrangement and one or more layers may be present between the resonator and the substrate.
  • trench arrangements may mean a group of one or more trenches, which are cavities formed in the substrate, such cavities having a depth that is preferably contained within the thickness of the substrate.
  • the width of the opening of the trenches typically matches the width of an electrode in the electrode arrangement.
  • the trenches may also be possible for the trenches to be through holes.
  • discrete may mean individually located insulators that are disconnected to each other.
  • insulator may mean materials which have the opposite effect on TCF as the rest of the materials used in the resonant structure of the surface acoustic wave device and may be fabricated from oxides that include silicon dioxide or aluminium oxide (Al 2 0 3 ).
  • Various embodiments relate to a structure that provides temperature compensation for, for example, surface acoustic wave (SAW) devices that have a piezoelectric layer on a silicon substrate.
  • SAW surface acoustic wave
  • Figure 1 A shows the cross-sectional structure of a surface acoustic wave device 100A according to a first embodiment of the invention.
  • the surface acoustic wave device 100A includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 1 10.
  • the piezoelectric layer 06 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108A that open from portions of a surface of the substrate 104 that faces the piezoelectric layer 06 and wherein any two adjacent trench arrangements 108A of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100A.
  • the insulator 110 is disposed in at least one of the plurality of trench arrangements 108A.
  • An objective of the surface acoustic wave device 00A is to make it possible to achieve a 0 (zero) temperature coefficient of frequency (TCF) for first order effects and control second order effects by device geometry. This is accomplished by providing the plurality of trench arrangements 108A, filled with the insulator 1 0 (such as oxide), under the piezoelectric layer 106 within which the SAW propagates and controlling the shape of the trenches 108A. As the plurality of trench arrangements 108A are provided in the substrate 104 and the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104, this has the effect of providing the plurality of trench arrangements 108A under the piezoelectric layer 106. The plurality of trench arrangements 108A is also arranged as detailed below.
  • Two adjacent trench arrangements 108A may be spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100A. Such a wavelength distance spacing also allows tuning to a vibration mode frequency at which the surface acoustic wave device 100A operates. This is because the wavelength distance spacing places the plurality of trench arrangements 108A in a periodic arrangement that allows the discrete insulators 110 to be aligned to vibration nodes of a surface acoustic wave that propagates in the piezoelectric layer 106.
  • the advantage of having trench arrangements being formed such that they are periodic with the surface acoustic standing wave results in temperature compensation with minimal interference with the surface acoustic standing wave.
  • the cross- sectional view 301 shows that anti-nodes of a standing wave vibration align with where each of the plurality of trench arrangements is placed.
  • the cross-sectional view 302 plots the strain energy over the surface of the surface acoustic wave 100A undergoing the same simulation.
  • the plurality of trench arrangements 108A is at the maximal areas of strain energy.
  • the substrate 104 is flush with the piezoelectric layer 106 to have the insulator 1 0 discretely disposed in each of the plurality of trench arrangements 108A to form a plurality of discrete insulators 110, wherein the plurality of discrete insulators 110 are in contact with the piezoelectric layer 106.
  • These discrete insulators 110 allow for a specific designed TCF for cases where the wavelength of the propagating surface acoustic wave is too long to make providing an insulation layer practical.
  • Figure 1 B shows the cross-sectional structure of a surface acoustic wave device 100B according to a second embodiment of the invention.
  • the surface acoustic wave device 100B includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 110.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108A that open from portions of a surface of the substrate 104 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108A of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100B.
  • the insulator 1 0 is disposed in at least one of the plurality of trench arrangements 08A.
  • the second embodiment of the invention has the insulator 110 protrude from the plurality of trench arrangements 108A to form an insulator layer 110L between the piezoelectric layer 106 and the plurality of trench arrangements 108A.
  • Figure C shows the cross-sectional structure of a surface acoustic wave device 100C according to a third embodiment of the invention.
  • the surface acoustic wave device 100C includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 1 0.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108C that open from portions of a surface of the substrate 04 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108C of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100C.
  • the insulator 110 is disposed in at least one of the plurality of trench arrangements 108C.
  • the third embodiment of the invention has trench arrangements 108C that are also trapezoidal shaped.
  • the plurality of trench arrangements 108C has their broader width located within the substrate 104.
  • the orientation of the plurality of trench arrangements 108C is inverted when compared to that of the plurality of trench arrangements 108A. The decision on which of these two orientations to use depend on the application requirements and fabrication capabilities.
  • Figure 1 D shows the cross-sectional structure of a surface acoustic wave device 00D according to a fourth embodiment of the invention.
  • the surface acoustic wave device 100C includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 110.
  • the piezoelectric layer 06 is disposed between the electrode arrangement 02 and the substrate 04.
  • the substrate 104 comprises a plurality of trench arrangements 108C that open from portions of a surface of the substrate 104 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108C of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100D.
  • the insulator 1 10 is disposed in at least one of the plurality of trench arrangements 108C.
  • the fourth embodiment of the invention has the insulator 110 protrude from the plurality of trench arrangements 108C to form an insulator layer 110L between the piezoelectric layer 106 and the plurality of trench arrangements 108C.
  • Figures A to 1 D show a surface acoustic wave device in accordance with embodiments of the invention where their plurality of trench arrangements is realised using trenches with trapezoidal cross-sectional profiles.
  • Embodiments of the invention are not restricted to trenches with trapezoidal cross-sectional profiles, but may also include realising the plurality of trench arrangements using trenches with rectangular cross-sectional profiles, as shown in Figures 1 E to 1 H.
  • Figure 1 E shows the cross-sectional structure of a surface acoustic wave device 100E according to a fifth embodiment of the invention.
  • the surface acoustic wave device 100E includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 1 10.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108E that open from portions of a surface of the substrate 104 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108E of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100E.
  • the insulator 1 0 is disposed in at least one of the plurality of trench arrangements 108E.
  • the substrate 104 is flush with the piezoelectric layer 106 to have the insulator 110 discretely disposed in each of the plurality of trench arrangements 108E to form a plurality of discrete insulators 110, wherein the plurality of discrete insulators 110 are in contact with the piezoelectric layer 106. It is also possible to fabricate the surface acoustic wave device 100E with an insulation layer, such as shown in a sixth embodiment of the invention, described below.
  • Figure 1 F shows the cross-sectional structure of a surface acoustic wave device lOOFaccording to a sixth embodiment of the invention.
  • the surface acoustic wave device 100F includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 110.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108E that open from portions of a surface of the substrate 104 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108E of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100F.
  • the insulator 110 is disposed in at least one of the plurality of trench arrangements 108E.
  • the sixth embodiment of the invention has the insulator 110 protrude from the plurality of trench arrangements 108E to form an insulator layer 110L between the piezoelectric layer 106 and the plurality of trench arrangements 108E.
  • each of their respective trench arrangements 108A to 108F is realised by a single trench. It is also possible to have a group of several trenches realise a trench arrangement used by a surface acoustic wave device in accordance with the invention, as described with reference to Figures 1G and 1 H.
  • Figure 1 G shows the cross-sectional structure of a surface acoustic wave device 100G according to a seventh embodiment of the invention.
  • the surface acoustic wave device 100G includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 110.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 102 and the substrate 104.
  • the substrate 104 comprises a plurality of trench arrangements 108G that open from portions of a surface of the substrate 04 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 108G of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device OOG.
  • the insulator 110 is disposed in at least one of the plurality of trench arrangements 108G.
  • each of the plurality of trench arrangements 108G of the seventh embodiment is realised by more than a single trench.
  • the plurality of trench arrangements used in a surface acoustic wave device according to the invention may be realised by one or more trenches.
  • three trenches are used in the seventh and eighth embodiments (see Figure 1 H), any number of trenches may be used.
  • the substrate 104 is flush with the piezoelectric layer 106 to have the insulator 110 discretely disposed in each of the plurality of trench arrangements 108G to form a plurality of discrete insulators 1 10, wherein the plurality of discrete insulators 1 0 are in contact with the piezoelectric layer 106. It is also possible to fabricate the surface acoustic wave device 100G with an insulation layer, such as shown in an eighth embodiment of the invention, described below.
  • Figure 1 H shows the cross-sectional structure of a surface acoustic wave device 100H according to an eighth embodiment of the invention.
  • the surface acoustic wave device 100H includes an electrode arrangement 102; a substrate 104; a piezoelectric layer 106; and insulators 110.
  • the piezoelectric layer 106 is disposed between the electrode arrangement 02 and the substrate 04.
  • the substrate 104 comprises a plurality of trench arrangements 108G that open from portions of a surface of the substrate 04 that faces the piezoelectric layer 106 and wherein any two adjacent trench arrangements 08G of the plurality of trench arrangements is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 100H.
  • the insulator 110 is disposed in at least one of the plurality of trench arrangements 108G.
  • the eighth embodiment of the invention has the insulator 110 protrude from the plurality of trench arrangements 108G to form an insulator layer 110L between the piezoelectric layer 106 and the plurality of trench arrangements 108G.
  • the 102 may comprise a plurality of electrodes.
  • the plurality of trench arrangements 108A, 108C, 108E, 108G may be arranged such that each of the electrodes is disposed diagonally opposite to each of the plurality of trench arrangements 108A, 08C, 108E, 08G, with respect to the piezoelectric layer 106.
  • this diagonal disposal is observed as having each of the plurality of trench arrangements 108A, 108C, 108E, 108G being located between two adjacent electrodes.
  • Figure 11 shows a top view of the surface acoustic wave device 100A to 100H of Figures 1A to 1H respectively, where the cross-sectional view of Figures 1A to 1 H is taken along line A-A' of Figure 11.
  • top view shown in Figure 11 has the plurality of trench arrangements 108A, 108C, 108E, 108G located underneath the piezoelectric layer 106 and hidden from view, so that a dashed line is used for the boundary of the plurality of trench arrangements 108A, 108C, 108E, 108G to denote their location.
  • temperature compensation is provided to the piezoelectric layer 106.
  • This temperature compensation may also be further brought about by the insulator 110 comprising material that has a positive temperature coefficient of frequency (TCF) to offset the piezoelectric layer 106 and the substrate 104, which both may comprise material that has a negative TCF.
  • TCF temperature coefficient of frequency
  • Such an offset may occur, for example, when the insulator 110 is fabricated from silicon dioxide or aluminium oxide (Al 2 0 3 ), while the piezoelectric layer 106 and the substrate 104 are fabricated from aluminum nitride (AIN) and bulk silicon respectively.
  • Figure 11 shows that a portion of the plurality of electrodes is arranged to be interdigitated (denoted using the reference numeral 102i), while the remainder of the plurality of electrodes (denoted using the reference numeral 102d) is provided as discrete electrodes.
  • Reflectors 175 are added after the interdigitated electrodes to trap the acoustic energy within a defined region. In the region around the reflectors, the plurality of trench arrangements 108A, 108C, 108E, 108G are still needed to maintain the temperature characteristics of the surface acoustic wave device 100A to 100H.
  • Figure 11 also shows that each of the electrodes in the electrode arrangement 102 has a rectangular profile. However, it is not essential for the electrodes in the electrode arrangement 102 to have a rectangular profile. A tapered profile is also possible, as shown in Figure 2.
  • FIG 2 shows a top view of a surface acoustic wave device 200 according to a ninth embodiment of the invention.
  • the surface acoustic wave device 200 includes an electrode arrangement 202, a substrate (hidden from the top view); a piezoelectric layer 206 and insulator 210.
  • the piezoelectric layer 206 is disposed between the electrode arrangement 202 and the substrate.
  • the substrate comprises a plurality of trench arrangements 208 that open from portions of a surface of the substrate that faces the piezoelectric layer 206. Any two adjacent trench arrangements 208 of the plurality of trench arrangements 208 is spaced apart a wavelength distance of an operating frequency of the surface acoustic wave device 200.
  • the insulator 210 is disposed in at least one of the plurality of trench arrangements 208.
  • the difference between the surface acoustic wave device 200 of Figure 2 and the surface acoustic wave devices 100A to 100H of Figures 1A to 1 H respectively is that while the surface acoustic wave devices 100A to 100H have a uniform cross-sectional profile for their respective electrode arrangement 102 and plurality of trench arrangements 108, the surface acoustic wave device 200 does not have a uniform cross-sectional profile for its electrode arrangement 202 and plurality of trench arrangements 208, but a tapered profile. In this tapered profile, the width of the electrode arrangement 202 and the width of the plurality of trench arrangements 208 are not uniform, but vary along their respective lengths.
  • the surface acoustic wave devices 100A, 100C, 100E and 100G has each of their insulators 110 disposed in a respective one of the plurality of trench arrangements 108A, 108C, 108E, 108G, so that the number of insulators 110 matches the number of trenches 108.
  • only a selected number of the plurality of trench arrangements accommodates the insulator, i.e. not all of the plurality of trench arrangements is provided with an insulator.
  • each of the surface acoustic wave devices 100A, 00C, 100E and 00G of Figures A, C, 1 E and 1G is flush with their respective piezoelectric layer 106, so that the insulator 110 is in contact with the piezoelectric layer 106.
  • the electrode arrangement 102 is also flush with the piezoelectric layer 106 to place the electrode arrangement 102 in contact with the piezoelectric layer 106. In this manner, the electrode arrangement 102 and the substrate 104 are in contact with opposing surfaces of the piezoelectric layer 106.
  • the piezoelectric layer 106 and the substrate 104 may be one or more layers between the piezoelectric layer 106 and the substrate 104 (such as the insulation layer 110L shown in Figures 1 B, 1D, I F and 1H) and in other embodiments (not shown) one or more layers between the piezoelectric layer 106 and the electrode arrangement 102.
  • each of trenches of the plurality of trench arrangements 108A, 108C has a trapezoidal cross-sectional profile. Such a trapezoid cross section profile reduces second order temperature effects.
  • Equation (1) is the calculation of the TCF for a homogenous material only considering first order components. .
  • TCF [TCE - (i3 ⁇ 4i + c3 ⁇ 4 2 -f- e3 ⁇ 4 3 >] - ⁇ 3 ⁇ 4 .
  • TCF, TCE, a xx refer to temperature coefficient of frequency, temperature coefficient of elasticity and thermal expansion in a specific direction respectively.
  • Equation (1 ) is only for first order TCF effects and it is for a homogenous material.
  • equation (2) provided volume ratios to compensate for first order TCF effects.
  • equation (2) has several limitations when used to determine correct trench dimensions for a surface acoustic wave device according to an embodiment of the present invention, as follows. Firstly, most of the energy is contained in the first wavelength of a SAW and does not penetrate much into the substrate (made from, for example, silicon) of a SAW device. Therefore, the rest of the bulk silicon contributes little to the overall temperature coefficient. Secondly, equation (2) only takes into account the contribution from a substrate fabricated from silicon and insulators fabricated from silicon oxide.
  • the contribution from the piezoelectric layer fabricated from, for example, aluminum nitride (AIN) is not factored.
  • the contribution from the geometry of the trenches is not taken into account, which will be shown below, to have an impact on the TCF.
  • equations (1 ) and (2) provide a starting point to carry out simulations using COMSOL to realise a surface acoustic wave device (such as the embodiments shown in Figures 1A to 1 F) starting with component thicknesses shown in Table .
  • AIN was used for the piezoelectric layer 106, silicon used for the substrate 104 and an oxide used for the insulator 1 10.
  • Figure 3A shows simulation results for the deflection (denoted using the reference numeral 301 ) of the SAW structure in resonance and the strain energy stored (denoted using the reference numeral 302) in the SAW structure at resonance, the SAW structure being that of the surface acoustic wave device 100A of Figure 1A.
  • the deflection on the surface is at a maximum and the output of .the SAW is at its peak.
  • the trapezoid trenches were 3.75 pm deep and had a top dimension of 2.6 pm and bottom dimension of 1.4 pm.
  • the surface acoustic wave device 100A uses trench arrangements having trenches with a trapezoidal shape.
  • the dimensions of the rectangular shaped trench were 2 urn wide by 3.75 urn deep.
  • the trapezoid trenches were 3.75 pm deep and had a top dimension of 2.6 pm and bottom dimension of 1.4 pm.
  • the inverted trapezoids had the top and bottom dimensions reversed with the same depth. Simulations were also conducted on SAW structures built in accordance to the surface acoustic wave device 00A, where the trench ratio was varied, i.e. the ratio of the width b over the width c of the trapezoidal shaped trench was varied. The results are shown in Figure 7.
  • Figures 4 and 5 show the effects of varying the trench depth and trench width on the TCF and resonant frequency of a SAW structure built in accordance to the surface acoustic wave device 100E (i.e. when the trench arrangements comprise rectangular profiled trenches).
  • the width of each rectangular trench was fixed to 2 pm, with the depth varied as shown in the horizontal axis of the graph of Figure 4.
  • the depth of each rectangular trench was fixed to 4 pm, with the width varied as shown in the horizontal axis of the graph of Figure 5. From Figure 4 and Figure 5, it can be seen that an increase in trench depth and width respectively leads to both an increase in the TCF and a decrease in the resonant frequency.
  • the additional oxide with its positive TCE will increase the TCF.
  • the lower acoustic velocity of oxide also lowers the resonant frequency of the surface acoustic wave device 100.
  • a series of simulations were then run with different trench dimensions that have 0 (zero) first order TCF for two different electrode widths, with the results shown in Figure 6.
  • the surface acoustic wave device is in accordance to the surface acoustic wave device 100E where the plurality of trench arrangements has a rectangular cross-sectional profile.
  • the results show that for different operating frequencies there are different optimum oxide trench dimensions that achieve a zero TCF. This is likely due to the penetration depth of the acoustic wave into the substrate.
  • the shorter wavelengths are more influenced by the top and bottom oxides cladding the AIN.
  • Curve 602 shows exemplary trench dimensions for an electrode with a 5um width, where the surface acoustic wave device is operating at a frequency of 250MHz. Under these conditions, exemplary dimensions for the trenches with the rectangular profile are a width and a depth of respectively around any one or more the following parameter pairs: 0.2 and 1.5; 0.5 and 0.75 and 0.8 and 0.5.
  • Curve 604 shows exemplary trench dimensions for an electrode with a 10um width, where the surface acoustic wave device is operating at a frequency of 25MHz. Under these conditions, exemplary dimensions for the trenches with the rectangular profile are a width and a depth of respectively around any one or more the following parameter pairs: 0.2 and 2.3; 0.5 and " 1.1 ; and 0.8 and 0.75. For curves 602 and 604, the parameter pairs of the trench width and depth are normalised to the electrode width.
  • Simulations were also conducted on a surface acoustic wave device in accordance to the embodiment of the present invention where the plurality of trench arrangements has a trapezoidal cross-sectional profile (see Figure A), where the sidewall angle of the plurality of trench arrangements was changed. This was done while maintaining the same cross sectional area of the trenches, with the results shown in Figure 7.
  • the result for a surface acoustic wave device having trenches with a trapezoidal cross-sectional profile with a top and bottom ratio of 0.3 is a first order TCF of -0.1 ppm and a second order coefficient of 7.5 ppb (parts per billion).
  • exemplary dimensions include the trenches with the trapezoidal profile having top and bottom width ratios of around: 0.1 and 0.2; and trenches with the trapezoidal profile having bottom and top width ratios of around any one or more of the following: 0.1 , 0.2, 0.3, 0.4 and 0.5.
  • the trapezoid dimensions are on the average around 3 urn deep and 2.7 urn wide, while the surface acoustic wave device has a 5 ⁇ electrode pitch.
  • Figure 7 shows that the second order TCF was changed, along with the first order TCF, by changing the top and bottom width ratios of the trapezoidal profiled trench.
  • the results show that the second order TCF can be controlled by an additional design parameter.
  • the need for the negative slope of the oxide trench is most likely due to how the trench sidewall follows the shape of the area with minimum energy.
  • each trench of the trench arrangement 108A having the trapezoidal cross-sectional profile is not normally fabricated with RIE (reactive ion etch) technology.
  • RIE reactive ion etch
  • a normal forward taper was found to not reduce second order TCF components.
  • One possible method (not shown) to fabricate the reverse taper would be to create a normal tapered trench on a first wafer. This can be followed by bonding the first wafer to a second wafer and grind/polish down to the trench in the first wafer. The remaining components that make up the surface acoustic wave device can then be fabricated on the top surface of the ground and polished wafer.
  • the amount of oxide needed to compensate the structure is dependent on the wavelength of the propagating surface acoustic wave and how much AIN and oxide there is on the substrate 104 surface. This is expected based because most of the SAW energy is trapped in the surface of the material. Longer wavelength SAW energy penetrates deeper into a Si substrate and is less affected by the top oxide on the device. Also, the profile of the cross section of the trenches has an influence on the first and second order TCF.
  • Figure 8 shows a graph that plots TCF as both a function of oxide trench depth and width for an oxide having a cross sectional rectangular shape (from the surface acoustic wave device 100E of Figure E), based on the graphs shown in Figures 4 and 5.
  • the graph of Figure 8 can then be used to obtain a plot of the amount of strain energy stored in the oxide trench and its relation to the TCF of the SAW structure, as shown in the graph of Figure 9.
  • Strain energy simulation can be observed from the cross-sectional view 302 of Figure 3C which uses darker shades for areas with higher oxide strain energy.
  • Trench dimensions can be derived from the strain energy graph shown in Figure 9 that would minimise total variation with temperature.
  • Another feature of using strain energy in designing an optimum structure for SAW devices is that it allows for arbitrary oxide trenches to be designed, and only the strain energy in the oxide trench needs to be simulated, not the full temperature simulation.
  • strain energy stored in the oxide trench and its relationship to the TCF of the structure can be used as a further design parameter as follows. Strain energy can be integrated over the surface of a surface acoustic wave device to obtain total strain energy. The energy in oxide structures can then be divided by the total strain energy and plotted with the TCF for the surface acoustic wave device. These steps allow for the graph of Figure 8 to be simplified to Figure 9.
  • Strain energy can be used to examine more complex surface acoustic wave devices 1001 shown in Figure 10 where there is a plurality of oxide trenches in each trench arrangement 1008.
  • using three trenches in each trench arrangement 008 achieved a 0 TCF structure.
  • the use of three oxide trenches, instead of one, meets the design requirements of a process development kit (PDK).
  • the PDK design rule is the oxide trench width is limited to 2 pm. Instead of using a single trench that is 5 pm wide, three smaller trenches are made that provide similar temperature performance as one large trench.
  • Figure 11 shows the response of an uncompensated strain sensor operating at 0 °C and 300 °C.
  • Figure 12 shows the simulation result of adding oxide trenches, in accordance to an embodiment of the invention, to the uncompensated device of Figure 11. The addition of the oxide trenches reduces the sensitivity by 5%, with a significant improvement over the temperature stability of the output. This enables wireless measurement of strain over a broader temperature range because the sensor resonant frequency does not drift with temperature. This would also improve the accuracy of wireless sensor because it removes inaccuracies caused by temperature changes.
  • FIG. 13 shows a flow chart 1300 of a method to fabricate a surface acoustic wave device according to an embodiment of the invention.
  • a substrate is provided.
  • a plurality of trench arrangements is formed that open from portions of a surface of the substrate, wherein the formation of the plurality of trench arrangements comprises tuning by spacing two adjacent trench arrangements a wavelength distance of an operating frequency of the surface acoustic wave device.
  • insulator is disposed in at least one of the plurality of trench arrangements.
  • a piezoelectric layer is provided such that the plurality of trench arrangements of the substrate faces the piezoelectric layer.
  • an electrode arrangement is provided, wherein the piezoelectric layer is disposed between the electrode arrangement and the substrate.
  • the method shown in Figure 13 allows for surface acoustic wave devices to be fabricated on single crystal substrates that are difficult to etch.
  • trenches of oxide are formed in the substrate for temperature compensation.
  • Using the method shown in Figure 13 has the trenches located underneath the piezoelectric layer, so that temperature compensation is undertaken underneath the plane where a surface acoustic wave propagates, in contrast to known devices which provide temperature compensation using a layer that is provided on top of their respective piezoelectric layer/piezoelectric layer.
  • Tuning by spacing apart two adjacent trench arrangement of the plurality of trench arrangements a wavelength distance of an operating frequency of the surface acoustic wave device makes the trenches periodic with the surface acoustic wave that propagates during operation of the surface acoustic wave device, to have minimal interference with the standing waves.
  • the formation of the plurality of trench arrangements in step 1304 may also include controlling the temperature coefficient of frequency of the piezoelectric layer through adapting the dimensions of the plurality of trench arrangements.
  • the adapting of the dimensions of the plurality of trench arrangements may include forming the plurality of trench arrangements into any one or more of the following cross-sectional profiles: trapezoidal and rectangular. Trapezoidal shaped structures add a variable that may be used to fine tune the temperature coefficient and reduce second order TCF components, by for example, adjusting the trench angle, during fabrication, to control non-linearity of TCF.
  • the piezoelectric layer may be fabricated to be flush with the substrate, so that the insulator is discretely disposed in each of the plurality of trench arrangements to form a plurality of discrete insulators, wherein the plurality of discrete insulators are in contact with the piezoelectric layer.
  • the electrode arrangement may be fabricated to be flush with the piezoelectric layer.
  • the fabricated surface acoustic wave device has an array of filled trenches with a +TCF to offset -TCF of the substrate and piezoelectric layer/piezoelectric body.
  • the trenches are formed in a periodic structure to match wave propagation. Different shapes of trenches (which include trapezoidal cross-sectional profiles) with better temperature compensation may be created. For instance, trapezoidal shaped trenches may be fabricated to fine tune second order effects. In addition, certain ratio of trench dimensions achieve 0 TCF.
  • Sub PPM TCF may be achievable, which find applications in piezoelectric layers and filters.
  • the fabricated surface acoustic wave device makes use of SAW technology where frequency is determined by lithography employed during fabrication.
  • the . method is also customisable to other SAW sensors where " temperature compensation is needed.
  • the temperature compensation is also passive, in that no external circuitry is required to provide the temperature compensation.
  • Second order temperature effects may also be reduced by controlling the sidewall angle of trenches that are created during fabrication of the surface acoustic wave device.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)

Abstract

Selon un aspect, la présente invention concerne un dispositif à ondes acoustiques de surface comprenant : un agencement d'électrode; un substrat; une couche piézoélectrique disposée entre l'agencement d'électrode et le substrat, le substrat étant composé d'une pluralité d'agencements de tranchées qui s'ouvrent à partir des portions de la surface du substrat qui fait face à la couche piézoélectrique et où l'un quelconque des deux agencements adjacents de tranchées de la pluralité d'agencements de tranchées est espacé d'une distance de longueur d'onde d'une fréquence de fonctionnement du dispositif à ondes acoustiques de surface; et le matériau isolant disposé dans au moins une de la pluralité des agencements de tranchée.
PCT/SG2014/000080 2013-02-22 2014-02-24 Dispositif à ondes acoustiques de surface WO2014129979A1 (fr)

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CN107005223A (zh) * 2014-10-03 2017-08-01 芬兰国家技术研究中心股份公司 温度补偿梁谐振器
CN107005224A (zh) * 2014-10-03 2017-08-01 芬兰国家技术研究中心股份公司 温度补偿板谐振器
CN107181471A (zh) * 2016-03-09 2017-09-19 天津威盛电子有限公司 具有负截面金属结构的saw谐振器及其制造方法
JP2018528639A (ja) * 2015-07-17 2018-09-27 ソイテックＳｏｉｔｅｃ 基板を製造するための方法

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US4480209A (en) * 1981-10-09 1984-10-30 Clarion Co., Ltd. Surface acoustic wave device having a specified crystalline orientation
US5907768A (en) * 1996-08-16 1999-05-25 Kobe Steel Usa Inc. Methods for fabricating microelectronic structures including semiconductor islands

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US4480209A (en) * 1981-10-09 1984-10-30 Clarion Co., Ltd. Surface acoustic wave device having a specified crystalline orientation
US5907768A (en) * 1996-08-16 1999-05-25 Kobe Steel Usa Inc. Methods for fabricating microelectronic structures including semiconductor islands

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN107005223A (zh) * 2014-10-03 2017-08-01 芬兰国家技术研究中心股份公司 温度补偿梁谐振器
CN107005224A (zh) * 2014-10-03 2017-08-01 芬兰国家技术研究中心股份公司 温度补偿板谐振器
CN107005224B (zh) * 2014-10-03 2021-06-15 芬兰国家技术研究中心股份公司 温度补偿板谐振器
JP2018528639A (ja) * 2015-07-17 2018-09-27 ソイテックＳｏｉｔｅｃ 基板を製造するための方法
US10943778B2 (en) 2015-07-17 2021-03-09 Soitec Method for manufacturing a substrate
US11837463B2 (en) 2015-07-17 2023-12-05 Soitec Method for manufacturing a substrate
CN107181471A (zh) * 2016-03-09 2017-09-19 天津威盛电子有限公司 具有负截面金属结构的saw谐振器及其制造方法
CN107181471B (zh) * 2016-03-09 2020-09-01 天津威盛电子有限公司 具有负截面金属结构的saw谐振器及其制造方法

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