MEMS resonator having a perforated resonating element

Abstract

A MEMS, microelectromechanical systems, resonator (100) comprising a perforated resonating element (101) comprising: a piezoelectric layer (110), an electrode layer (120) on the piezoelectric layer (110), a second electrode layer (130) on the piezoelectric layer (110) opposite the electrode layer (120), and at least one anchor (160) configured to connect the perforated resonating element (101) to, and suspend the perforated resonating element (101) from, surrounding layers, wherein the electrode layer (120) comprises perforations (150 / 250 / 350 / 450 / 650).

Description

[0001] MEMS RESONATOR HAVING A PERFORATED RESONATING ELEMENT

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to microelectromechanical systems, MEMS, resonators which comprise a perforated resonating element.

[0004] BACKGROUND OF THE INVENTION

[0005] This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

[0006] Microelectromechanical systems, MEMS, resonators are being developed to provide the same functionality as quartz resonators with benefits such as smaller chip size, reduced cost, and increased robustness against shock and vibrations.

[0007] A key performance parameter in MEMS resonators is equivalent series resistance, ESR. ESR is inversely proportional to a quality factor Q of the resonator, and the minimization of ESR is often desirable. Another key performance parameter in MEMS resonators is the stability of the resonance frequency. In certain MEMS resonators, especially resonators constructed of dissimilar layers, such as resonators with metallic electrodes, the instability of the metallic layer may lead to frequency instability.

[0008] SUMMARY

[0009] It is an object of certain embodiments of the invention to reduce negative impacts of a resonator electrode, for example a metal electrode, without degradation of its main function or at least to provide an alternative to existing technology. It is a further object of certain embodiments to reduce or negate reliability issues, increase or maintain a quality factor, and / or improve frequency variation over temperature characteristics while retaining low ESR. These objects are obtained in certain embodiments by providing MEMS resonators that comprise a perforated resonating element. Such perforations allow for the electric field used for electromechanical transduction to be equivalent, or close to equivalent, to the electric field of a solid electrode due to the fringing field effect, allowing the retention of a low ESR.

[0010] According to a first example aspect of the invention there is provided a MEMS, microelectromechanical systems, resonator comprising: a perforated resonating element comprising: a piezoelectric layer, an electrode layer on the piezoelectric layer, a second electrode layer on the piezoelectric layer opposite the electrode layer, and at least one anchor configured to connect the perforated resonating element to, and suspend the perforated resonating element from surrounding layers, wherein the electrode layer comprises perforations. Within certain embodiments the at least one anchor comprises portions of the piezoelectric layer, electrode layer and second electrode layer.

[0011] Some embodiments further comprise a handling layer affixed to the at least one anchor.

[0012] Within at least some embodiments, the piezoelectric layer comprises aluminum nitride. In certain embodiments, the electrode layer comprises metal, preferably gold.

[0013] In certain embodiments, the second electrode layer is referred to as a substrate. In some embodiments the second electrode layer comprises silicon, preferably doped silicon, such as ultra-heavily doped, UHD, silicon, more preferably single crystal silicon. In certain embodiments, there is an ultra-heavily doped, UHD, second electrode of N- type or P-type.

[0014] In this context, UHD doping refers to a doping level above 1020cm“3. In certain embodiments, the doping level of the UHD silicon is above 1019cm“3.

[0015] According to some embodiments, the perforated resonating element comprises a resonating beam. In certain embodiments the perforated resonating element comprises a stacked beam resonator having a plurality of adjacent beams connected by connection element(s). In at least some embodiments comprising a resonating beam, at least 10% of the length of each resonating beam comprises perforations. In certain embodiments at least 20% of the length of each resonating beam comprises perforations. Within certain embodiments, the perforations are longitudinally centered along each resonating beam. In some embodiments the perforations are centered around an axis that crosses perpendicularly the beam length at the center of the beam.

[0016] Within some embodiments, the perforations of the perforated resonating element continue through at least the piezoelectric layer. In certain embodiments, the perforations of the perforated resonating element continue through both the piezoelectric layer and the second electrode layer.

[0017] In certain embodiments, at least 5% of electrode layer has been removed by perforations within the perforated resonating element. In some embodiments, at least 9% of electrode layer has been removed by perforations within the perforated resonating element. In certain embodiments, at least 15% of the electrode layer has been removed by perforations. For example, in some embodiments 30% of the electrode layer is removed by perforations providing for a 30% improvement in frequency stability.

[0018] In some embodiments, the diameter of the perforations of the perforated resonating element are less than 3 times the thickness of the electrode layer. In certain embodiments, the diameter of the perforations of the perforated resonating element are less than 3 times the thickness of the piezoelectric layer. In some embodiments, the diameter of the perforations of the perforated resonating element are less than twice the thickness of the electrode layer. In certain embodiments, the diameter of the perforations of the perforated resonating element are less than twice the thickness of the piezoelectric layer. In some embodiments, the perforations of the perforated resonating element are less than 4.5 pm in diameter, preferably less than 3.5 pm, most preferably 2.5 pm.

[0019] In certain embodiments, the perforations of the perforated resonating element have the shape of hexagons. In certain embodiments, the perforations of the perforated resonating element have the shape of hexagons with triangle extensions. In some embodiments, the perforations of the perforated resonating element have the shape of crosses. In a number of embodiments, the perforations of the perforated electrode take the shape of rectangles. In some embodiments, the perforations of the perforated resonating element have the shape of a circle with a number of ellipses placed at the edge of the circle.

[0020] In some embodiments, the perforated resonating element is configured to resonate in a length-extensional, LE, resonance mode. In certain embodiments the perforated resonating element is configured to resonate in a width-extensional, WE, resonance mode.

[0021] In certain embodiments, the resonating beam(s) are longitudinally aligned within 25 degrees of a <100> crystal direction of the silicon of the second electrode layer.

[0022] In at least some embodiments, the perforations of the perforated resonating element are uniformly spaced, creating a mesh. In certain embodiments, the perforations are comprised in columns along the perforated resonating element and adjacent columns of perforations are off-set from each other. In some embodiments, the perforations are equally spaced throughout the perforated resonating element.

[0023] In certain embodiments, the electrode layer(s) are implemented by a layer of metal. In certain embodiments, the electrode layer is implemented by a layer of doped silicon. In certain embodiments, the second electrode layer is implemented by an UHD silicon layer, preferably of single-crystal silicon.

[0024] In certain embodiments, the MEMS resonator comprises a plurality of resonating elements, one of the resonating elements being configured to resonate in a resonance mode of a first type, and another of the resonating elements being configured to resonate in a resonance mode of another type, different from the first type. At least one of the resonating elements is a perforated resonating element.

[0025] At least some embodiments of the present invention find use in transduced devices having a moving body which is non-conductive or electrically insulating. MEMS resonators according to certain embodiments may be employed, for example, in microphones.

[0026] Examples of the types of resonance modes are an in-plane length extensional (LE) mode, a width extensional (WE) mode, Lame or square extensional (SE) modes, and a flexural mode.

[0027] Different non-binding example aspects and embodiments have been presented in the foregoing. The above embodiments and embodiments described later in this description are used to explain selected aspects or steps that may be utilized in implementations of the present invention. It should be appreciated that corresponding embodiments apply to other example aspects as well. Any appropriate combinations of the embodiments can be formed.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0030] Fig. 1A shows a schematic top view of a MEMS resonator in accordance with certain embodiments;

[0031] Fig. 1 B shows a cross-section of the resonator of Fig. 1A along line BB’;

[0032] Fig. 1C shows a cross-section of the resonator of Fig. 1A along line CC’;

[0033] Figs. 1 D & 1 E show cross-sections of further embodiments along line BB’;

[0034] Fig. 2A shows a schematic top view of a MEMS resonator having a handling layer and surrounding layers in accordance with some embodiments;

[0035] Fig. 2B shows a cross-section of the resonator of Fig. 2A along line BB’;

[0036] Fig. 2C shows a cross-section of the resonator of Fig. 2A along line CC’;

[0037] Figs. 3A-3D show schematic top views of further MEMS resonators in accordance with some embodiments;

[0038] Fig. 4 shows a schematic top view of a portion of a MEMS resonator in accordance with certain embodiments; Fig. 5A-5F show the shape and arrangement of perforations according to certain embodiments; and

[0039] Fig. 6 shows a schematic top view of a MEMS resonator having rectangular perforations according to some embodiments.

[0040] DETAILED DESCRIPTION

[0041] In the following description, like numbers denote like elements.

[0042] A MEMS resonator according to certain embodiments of the present invention is illustrated within Figures 1A, 1 B and 1C where Figure 1 A shows a schematic top view of the MEMS resonator, Figure 1 B shows a cross section of the MEMS resonator along line BB’ of Figure 1A and Figure 1C shows a cross section of the MEMS resonator along line CC’. As illustrated, the MEMS, microelectromechanical systems, resonator 100 comprises: a perforated resonating element 101 comprising: a piezoelectric layer 110, an electrode layer 120 on the piezoelectric layer 110, a second electrode layer 130 on the piezoelectric layer 110 opposite the electrode layer 120 and at least one anchor 160 configured to connect the perforated resonating element 101 to and suspend the perforated resonating element from surrounding layers, wherein the electrode layer comprises perforations 150.

[0043] In some embodiments the second electrode layer 130 comprises silicon, preferably doped silicon, such as ultra-heavily doped silicon, more preferably single crystal silicon. In certain embodiments, there is an ultra-heavily doped, UHD, second electrode of N- type. In this context, UHD doping refers to a doping level above 102°cm-3. In certain embodiments, the doping level of the perforated resonating element is above 1019cm“3.

[0044] As seen within Figure 1 C, in certain embodiments, the at least one anchor comprises portions of the piezoelectric layer, electrode layer and second electrode layer. As seen in Figure 1A, within at least some embodiments the perforated resonating element comprises a plurality of anchors configured to connect and suspend the perforated resonating element from surrounding layers. As described herein, in at least some embodiments, layers are on a further layer or substrate such that the layer is deposited on the further layer or substrate. The layer may be adhered to the further layer or substrate, for example by an adhesive layer. The layer may be on the further layer or substrate such that the layer is grown directly on the further layer or substrate substrate or on a piezoelectric layer. In certain embodiments, layers are on a further layer or substrate such that they are positioned above or below the further layer or substrate. Within some embodiments one component is on another component such that they are positioned next to each other. In at least some embodiments, layers are on another layer such that there is an intermediate layer between the layer on another layer.

[0045] In at least some embodiments the electrode layer 120 is on the piezoelectric layer 110 such that the electrode layer is deposited on the piezoelectric layer. The electrode layer may be adhered to the piezoelectric layer, for example by an adhesive layer. The electrode may be on the piezoelectric layer such that the electrode is grown directly on the piezoelectric layer or on an intermediate layer. As illustrated within Figure 1A, the electrode layer 120 may be on the piezoelectric layer 110 such that it is above the piezoelectric layer. Within some embodiments one component is on another component such that they are positioned next to each other.

[0046] Perforations according to embodiments of the present invention may be provided in a variety of fashions. For example, certain embodiments employ lithographic patterning. Etching may be employed with at least some embodiments.

[0047] As seen within Figure 1 B, in at least some embodiments, the electrode layer 120 is the topmost layer. In certain embodiments, the perforated electrode layer is the topmost electrode layer or first electrode layer with the opposing electrode layer being the bottommost or second electrode layer.

[0048] Within at least some embodiments, the piezoelectric layer comprises aluminum nitride. In certain embodiments, the electrode layer 120 comprises metal, preferably gold.

[0049] Figure 1 D illustrates certain embodiments wherein the perforations 150 of the perforated resonating element 101 continue through at least the piezoelectric layer 110. As seen in Figure 1 E, within certain embodiments, the perforations 150 of the perforated resonating element continue through both the piezoelectric layer 110 and the second electrode layer 130. Such embodiments may be achieved, for example, by using a trench mask, such as a single trench mask, and lithographic patterning to remove all layers in successive etching process steps. In certain embodiments the diameter of the holes left in the trench mask prior to etching controls the depth of the etching and thus how many layers are removed. For example, after lithography, three successive etching steps are employed to achieve certain embodiments. A wet or dry etch to remove the electrode layer, a wet or a dry etch to remove the piezoelectric layer and then a deep reactive-ion etching for the second electrode layer or substrate.

[0050] Figures 2A - 2C illustrate a MEMS resonator according to at least some embodiments, with Figure 2B showing a cross section of the resonator along line BB’ of Figure 2A and Figure 2C showing a cross section along line CC’. As illustrated, the MEMS, microelectromechanical systems, resonator 200 comprises: a perforated resonating element 201 comprising: a piezoelectric layer 210, and an electrode layer 220 on the piezoelectric layer 210, wherein the electrode layer comprises perforations 250; and a second electrode layer 230 on the piezoelectric layer 210 opposite the electrode layer 220, at least the first electrode layer 220 comprising perforations 250.

[0051] As also seen in Figures 2A - 2C, the resonating element 201 is surrounded by surrounding layers 270 and supported by a handle layer 240 affixed to the at least one anchor 260. In at least some embodiments the surrounding layers 270 are formed of the same layers as the resonating element. That is, the piezoelectric layer 210, electrode layer 220 and second electrode layer 230 extend through the resonator 200 in at least the surrounding layers 270 and the perforated resonating element 201 such that the perforated resonating element 201 , or resonator area, is defined and separated from the surrounding layers 270, or non-resonator area, by trenches 280.

[0052] As seen within Figures 2A - 2C, within at least some embodiments the electrode layer 220 is present within the area of the resonating element 201 and in the area of the at least one anchor 260. The electrode layer 220 present in the area of the at least one anchor 260 may act as a trace extending from the resonating element to a pad located elsewhere on the resonator for connecting a signal to the electrode layer of the resonating element. While shown as extending across to an anchor on each side of the resonating element within Figure 2C, in at least some embodiments having more than one anchor, the electrode layer is present in the area of only one of the anchors.

[0053] Figures 2B and 2C further illustrate the handle layer 240 upon which all other layers are deposited within at least some embodiments. As illustrated, the handle layer 240 may be used as a carrier and functions to allow mechanical handling of the wafer comprising the resonator. In this handle layer, cavity 290, or a plurality of cavities within certain embodiments, are patterned to allow resonating element 201 , comprising piezoelectric layer 210, first electrode layer 220 and substrate or second electrode layer 230 to be suspended. As seen, the first electrode layer 220 comprises perforations 250.

[0054] Within at least some embodiments, such as that seen within Figure 2C, the at least one anchor 260, comprising layers 210, 220 and 230, extends to suspend the resonating element from the handling layer. As also illustrated, within at least some embodiments, there is an insulating layer 245, preferably silicon dioxide, between the handle layer 240 and substrate 230, the insulating layer acting as an electrical insulator between the handle layer and the substrate.

[0055] In addition to the provision of cavities 290, trenches 280 provide for release of the resonator structure 201 from the surrounding layers within at least some embodiments.

[0056] In some embodiments the second electrode layer 230 comprises silicon, preferably doped silicon, such as ultra-heavily doped silicon, more preferably single crystal silicon. In certain embodiments, there is an ultra-heavily doped, UHD, second electrode of N- type. In this context, UHD doping refers to a doping level above 1020cm“3. In certain embodiments, the doping level of the perforated resonating element is above 1019cm“3.

[0057] In at least some embodiments the second electrode layer 230, otherwise known as a substrate in at least some embodiments, is on the piezoelectric layer 210 such that the piezoelectric layer is deposited on the second electrode layer or vice versa. The second electrode layer may be adhered to the piezoelectric layer, for example by an adhesive layer. The second electrode layer may be on the piezoelectric layer such that the piezoelectric layer is grown directly on the second electrode layer or on an intermediate layer. In certain embodiments the second electrode layer is on the piezoelectric layer such that there are intermediate layer(s) between the piezoelectric layer and the second electrode layer. In some embodiments such an intermediate layer is an insulator layer. In at least some embodiments the second electrode layer is a bottom electrode. Within some embodiments one component is on another component such that they are positioned next to each other.

[0058] According to some embodiments, the perforated resonating element comprises a perforated resonating beam. For example, certain embodiments comprise a singular resonating beam such that the perforated resonating element comprises only a single perforated resonating beam such as those illustrated within Figures 1A and 2A.

[0059] In certain embodiments the perforated resonating element comprises a plurality of resonating beams. For example, the MEMS resonators 300 of Figures 3A - 3D. In certain embodiments the plurality of resonating beams are arranged to form a stacked beams resonating element or stacked beam resonator. A stacked beam resonator comprises a plurality of resonator beams positioned side-by-side in a plane, separated by trenches and connected by connection elements. In at least some stacked beam resonators, the resonator beams are positioned side-by-side in a plane such that at least a portion of each resonator is in the same plane. In certain stacked beam resonators, no two resonator beams are positioned atop each other.

[0060] Within some embodiments the plurality of resonating beams 301 are separated by trenches 381. Within at least some embodiments comprising a plurality of resonating beams the resonating beams are connected by connection elements 302 between the beams. In certain embodiments having connection elements, the connection elements comprise perforation(s).

[0061] Also illustrated within Figures 3A - 3D are the surrounding layers 370 according to at least some embodiments. As shown, the surrounding layers 370 are separated from the resonating element via trenches 380. Further illustrated are anchors 360 which serve to connect the resonating element to, and suspend the resonating element from, the surrounding layers 370. The trenches 380 serve to separate the perforated resonating element comprised of the perforated resonating beams 301 , or resonating area, from the non-resonating area, of the resonator 300 or wafer.

[0062] Figures 3A - 3D show certain embodiments having differing arrangements of perforations 350 and a plurality of resonating beams 301 . As can be seen, within some embodiments the perforations are uniformly spaced. Within certain embodiments the perforations are arranged to create a mesh, such as a mesh comprising uniformly spaced perforations. In at least some embodiments, a meshed electrode is formed by the perforations, such a mesh may be comprised throughout the resonating element as in Figures 3A. In other embodiments, the mesh may be comprised only in a portion of the resonating element as seen in Figure 3B wherein the mesh is centered on the plurality of resonating beams such that each beam comprises a mesh of perforations centered along the beam longitudinally. In other embodiments, as seen in Figure 3C, the mesh is present primarily at the ends of the beams. In certain embodiments, the longitudinal center of each resonating beam is free of perforations. In at least some embodiments, the perforations are concentrated towards the ends of each beam.

[0063] While some resistive loss may result from perforating the electrode layer, controlling the size and placement of the perforations can limit said resistive loss. For example, by limiting the perforations to certain portions of the resonating element, for example a central portion of the resonating beam as described herein, resistive losses can be negated if not substantially avoided.

[0064] In certain embodiments, the perforations are comprised in columns along the resonating element. For example, the perforations of the embodiments of Figures 3A - 3D may be considered to be arranged in columns. In some embodiments, the perforations are equally spaced throughout the resonating element.

[0065] Figures 3A - 3D also provide examples of LE MEMS resonators 300 comprising LE beams 301 having perforations 350 according to certain embodiments. A certain portion of the length L of each beam comprises perforations 350. For example, Figure 3B wherein 33% of the length of each beam 301 comprises perforations 350, which may be, for example, centered along the length of each beam 301. As another example, Figure 3D shows an embodiment wherein 66% of each beam 301 comprises perforations 350. Alternatively, 100% of each beam 301 may comprise perforations 350 as shown in Figure 3A.

[0066] In certain embodiments the perforated resonating element is configured to resonate in a width-extensional, WE, resonance mode.

[0067] In certain embodiments, at least 5% of electrode layer has been removed by perforations within the perforated resonating element. For example, in some embodiments 30% of the electrode layer is removed by perforations providing for a 30% improvement in frequency stability.

[0068] While the columns of Figures 3A - 3D are substantially aligned, in certain embodiments adjacent columns of perforations are off-set from each other as can be seen in Figure 4. Within Figure 4, a portion of a perforated resonating element 401 having perforations 450 is shown illustrating an arrangement of columns of perforations 450. As can be seen the first column of perforations 451 and second column of perforations 452 are not aligned such that perforations of the second column 452 are centered at a height which is halfway between the centers of perforations of the first column 451. In other terms, the perforations of the first column 451 of Figure 4 have a pitch, that is a distance between the center points of perforations within the same column. The second column 452 is then offset by a half of this pitch such that the center points of the perforations of the second column 452 begin half a pitch further down that the perforations of the first column 451. At least some embodiments comprise off-set perforations such that one column is at least partially off-set from the first. For example, a column can be one quarter pitch off-set, or half pitch off-set as shown in Figure 4.

[0069] As shown in Figures 5A - 5F, shapes and arrangement of perforations may vary. Within Figures 5A - 5F the perforations are shown as the lighter portions of the figure.

[0070] As seen, in certain embodiments, the perforations take a circular shape as in Figure 5A. While in some embodiments the perforations take an octagonal shape, in certain embodiments forming a honeycomb like structure as in Figure 5B. In terms of pitch as discussed above, such a honeycomb structure may be formed by shifting every column of perforations by half a pitch.

[0071] As shown in Figure 5C, in some embodiments the perforations have the shape of octagons with triangle extensions. Within Figure 5C, the annotated shape 505 is provided with dashed lines to delineate between the octogen and the triangle extension. Such dashed lines are merely illustrative and do not alter the shape of the perforation. Such embodiments provide for a more uniform width of electrode material across the electrode.

[0072] Illustrated within Figure 5D are perforations having a hexagonal shape as in certain embodiments. Embodiments having hexagonal perforations may also form a honeycomb like structure as seen. Once again, the perforations are shifted half a pitch from column to column.

[0073] As seen in Figure 5E, in some embodiments, the perforations of the perforated resonating element have the shape of crosses. Employing perforations in the shape of crosses provides for easier process steps as it is easier to pattern longer strips as in the cross than to pattern the comparatively small holes of the circles and hexagons of other embodiments.

[0074] Illustrated within Figure 5F are perforations according to certain embodiments. As illustrated, within at least some embodiments, the perforations are composed of several shapes. Within the embodiment of Figure 5F, ellipses are formed at the circumference of a circle to form an irregular shape. Put another way, ellipses are added to the top left, top right, bottom right and bottom left of a circle. Such embodiments allow for maintenance of a uniform perforation distance across the electrode while using aligned columns.

[0075] Within certain embodiments the shapes of the perforations are combined. For example, a portion of perforations may be circular while some are hexagonal. As another example, a portion of perforations may be in the shape of crosses and others rectangular. In several embodiments, the perforations of the perforated electrode take the shape of rectangles. For example, the embodiment illustrated within Figure 6 which shows another type of perforated resonating element 601. The perforated resonating element 601 of Figure 6 may be a portion of a stack of resonating elements 601 forming a stacked beams resonating element, for example the perforated resonating element 601 could be a resonating beam. As can be seen, the perforations 650 of Figure 6 are of different shapes and sizes.

[0076] Within embodiments comprising rectangular perforations the design may be discussed in terms of lines and nodes as labelled within Figure 6. In such embodiments the lines running along the resonator, for example along the surface, may be vertical or horizontal and may have different spacings. Said lines are interconnected via the nodes. Within some embodiments the lines and nodes comprise the top electrode, for example a top electrode mesh comprising an array or matrix of perforations.

[0077] Also illustrated within Figure 6 are various dimensions of rectangularly perforated embodiments. For example, the spacing between adjacent lines, or width of the gap, G1 , may be selected such that fringing fields of two adjacent lines are overlapping. Similarly, the width of the lines, E1 , may be chosen to reduce a series resistance of the perforated electrode. Within certain embodiments the width of lines E1 are uniform. In some embodiments both the width of lines E1 are uniform, and the nodes are equally spaced. The length of the gaps, G2 and G3, may also vary in certain embodiments as shown in Figure 6. Within at least some embodiments the length of the gaps within the center, G2, are greater than the lengths of the gaps on the edges, G3, of the resonating element.

[0078] In some embodiments, the perforated resonating element is configured to resonate in a length-extensional, LE, resonance mode. In some embodiments, the perforations are applied in the middle of a LE resonator while a full solid electrode is used elsewhere to provide minimum series resistance. For example, in at least some embodiments comprising a resonating beam, at least 10% of the length of each resonating beam comprises perforations. Within at least some embodiments, at least 20% of the length of each resonating beam comprises perforations. Within certain embodiments, the perforations are longitudinally centered along each resonating beam.

[0079] In some embodiments, the diameter of the perforations of the perforated resonating element are less than three times the thickness of the electrode layer. In certain embodiments, the diameter of the perforations of the perforated resonating element are less than twice the thickness of the electrode layer. In certain embodiments, the diameter of the perforations of the perforated resonating element are less than three times the thickness of the piezoelectric layer. While in some embodiments, the diameter of the perforations of the perforated resonating element are less than twice the thickness of the piezoelectric layer. In some embodiments, the perforations of the perforated resonating element are less than 4.5 pm in diameter, preferably less than 3.5 pm, most preferably 2.5 pm. By limiting the perforation size as in the embodiments here, transduction loss can be minimized.

[0080] In certain embodiments, the resonating beam(s) are longitudinally aligned along a <100> crystal direction of the silicon of the second electrode layer. Within at least some embodiments the resonating beam(s) are longitudinally aligned with a <100> crystal direction of the silicon of the second electrode layer such that a longitudinal axis of each resonating beam is within 25 degrees of the <100> crystal direction of the silicon of the second electrode layer.

[0081] In certain embodiments, the electrode layer(s) are implemented by a layer of metal. In certain embodiments, the electrode layer is implemented by a layer of doped silicon. In certain embodiments, the second electrode layer is implemented by an UHD doped silicon layer, preferably of single-crystal silicon.

[0082] In certain embodiments, the MEMS resonator comprises a plurality of resonating elements, one of the resonating elements being configured to resonate in a resonance mode of a first type, and another of the resonating elements being configured to resonate in a resonance mode of another type (different from the first type).

[0083] Examples of the types of resonance modes are an in-plane length extensional (LE) mode, a width extensional (WE) mode, Lame or square extensional (SE) modes, and a flexural mode. In certain embodiments, the movement of the resonating elements is actuated by piezoelectric actuation.

[0084] Different non-binding example aspects and embodiments have been presented in the foregoing. The above embodiments and embodiments described later in this description are used to explain selected aspects or steps that may be utilized in implementations of the present invention. It should be appreciated that corresponding embodiments apply to other example aspects as well. Any appropriate combinations of the embodiments can be formed.

[0085] In certain embodiments, the resonator is separated from its surrounding, for example a surrounding piezoelectric layer, by trenches.

[0086] In certain embodiments, the resonating elements are configured to resonate in an inplane length extensional mode, in the length direction of the resonating beams. In certain embodiments, the movement of the resonating elements is actuated by piezoelectric actuation.

[0087] Preferably, in embodiments employing a length extensional resonance mode, one of the <100> crystal orientations, most preferably

[0100] , is along the direction of the vibrations of the length extensional resonance mode.

[0088] Without limiting the scope and interpretation of the patent claims, certain technical effects of one or more of the example embodiments disclosed herein are listed in the following. A technical effect is reduction of degradation in MEMS resonators. A further technical effect is the reduction of reflow drift and ageing. At least some embodiments provide for improved quality factor Q. Certain embodiments provide for a more positive linear temperature coefficient of frequency, TCF1 . Further, embodiments provide for a reduced trimming sensitivity and thus improved trimming accuracy.

[0089] The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention.

[0090] Furthermore, some of the features of the above-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features.

[0091] As such, the foregoing description should be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

Claims1 . A MEMS, microelectromechanical systems, resonator (100) comprising:- a perforated resonating element (101 ) comprising:- a piezoelectric layer (110),- an electrode layer (120) on the piezoelectric layer (110), wherein the electrode layer (120) comprises perforations (150 / 250 / 350 / 450 / 650),- a second electrode layer (130) on the piezoelectric layer (110) opposite the electrode layer (120), and- at least one anchor (160) configured to connect the perforated resonating element (101 ) to, and suspend the perforated resonating element (101 ) from, surrounding layers, characterized in that the diameter of the perforations (150 / 250 / 350 / 450 / 650) of the perforated resonating element (101 ) are less than 3 times the thickness of the piezoelectric layer (110).

2. The MEMS resonator of claim 1 , wherein the at least one anchor (160) comprises portions of the piezoelectric layer (110), electrode layer (120) and second electrode layer (130).

3. The MEMS resonator of claim 1 or 2, wherein the electrode layer (120) comprises metal, preferably gold.

4. The MEMS resonator of any preceding claim, the resonator (100) further comprising a handling layer (240) affixed to the at least one anchor (160).

5. The MEMS resonator of any preceding claim, wherein the second electrode layer (130) comprises silicon, preferably doped silicon, such as ultra-heavily doped silicon, more preferably single crystal silicon.

6. The MEMS resonator of any preceding claim, wherein the perforated resonatingelement (101 ) comprises a resonating beam (301 ).

7. The MEMS resonator of any preceding claim, wherein the perforated resonating element (101 ) comprises a plurality of resonating beams (301 ) forming a stacked beam resonator.

8. The MEMS resonator of claim 6 or 7, wherein at least 10% of the length of each resonating beam (301 ) comprises perforations (150 / 250 / 350 / 450 / 650).

9. The MEMS resonator of claim 8, wherein the perforations (150 / 250 / 350 / 450 / 650) are longitudinally centered along each resonating beam (301 ).

10. The MEMS resonator of any preceding claim, wherein the perforations (150 / 250 / 350 / 450 / 650) of the perforated resonating element (101 ) continue through at least the piezoelectric layer (110).

11. The MEMS resonator of any preceding claim, wherein the perforations (150 / 250 / 350 / 450 / 650) of the perforated resonating element (101 ) continue through both the piezoelectric layer (110) and the second electrode layer (130).

12. The MEMS resonator of any preceding claim, wherein at least 5% of the electrode layer (120) has been removed by perforations (150 / 250 / 350 / 450 / 650) within the perforated resonating element (101 ).

13. The MEMS resonator of any preceding claim, wherein the diameter of the perforations (150 / 250 / 350 / 450 / 650) of the perforated resonating element (101 ) are less than 3 times the thickness of the electrode layer (120).

14. The MEMS resonator of any preceding claim, wherein the perforations (150 / 250 / 350 / 450 / 650) of the perforated resonating element (101 ) are less than 4.5 pm in diameter, preferably less than 3.5 pm, most preferably 2.5 pm.

15. The MEMS resonator of any preceding claim, wherein the perforated resonating element (101 ) is configured to resonate in a length-extensional, LE, resonance mode.

16. The MEMS resonator of any preceding claim, wherein the resonating beam(s)(301 ) are longitudinally aligned within 25 degrees of a <100> crystal direction.

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