MEMS resonator having a released electrode

Abstract

A MEMS, microelectromechanical systems, resonator (100) and method for manufacturing MEMS resonators. The MEMS resonator having a resonating element (101) comprising: a substrate (110), a piezoelectric layer (120) on the substrate (110), and an electrode layer (130) on the piezoelectric layer (120) opposite the substrate (110), wherein there is at least one space (125) between the electrode layer (130) and substrate (110).

Description

[0001] MEMS RESONATOR HAVING A RELEASED ELECTRODE

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to microelectromechanical systems, and to MEMS resonators which comprise a released electrode.

[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, which 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 objective of certain embodiments of the invention to improve frequency stability of MEMS resonators or at least to provide an alternative to existing technology. 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 objectives are obtained in certain embodiments by providing MEMS resonators that comprise an at least partially released resonating element and / or electrode layer. In certain embodiments, the electrode layer is released such that there is at least one space between the electrode layer and substrate layer of the MEMS resonator.

[0010] According to a first example aspect of the invention there is provided a MEMS resonator comprising:

[0011] - a resonating element comprising:

[0012] - a substrate,

[0013] - a piezoelectric layer on the substrate, and

[0014] - an electrode layer on the piezoelectric layer opposite the substrate, wherein there is at least one space between the electrode layer and substrate.

[0015] Within at least some embodiments, the space is at least partially formed by at least one gap in the piezoelectric layer.

[0016] Certain embodiments further comprise a sacrificial layer between the electrode layer and piezoelectric layer, the space being at least partially formed by at least one gap in the sacrificial layer.

[0017] In certain embodiments, there are a plurality of gaps in the piezoelectric layer, the plurality of gaps forming the at least one space. While in some embodiments, there are a plurality of gaps in the sacrificial layer, the plurality of gaps forming the at least one space.

[0018] Within at least some embodiments, the electrode layer comprises perforations. In certain embodiments the gap(s) in the piezoelectric layer and / or sacrificial layer are centered around perforations of the electrode layer. In some embodiments the gaps in the piezoelectric layer and / or sacrificial layer are undercut in relation to the perforations of the electrode layer. In some embodiments, the MEMS resonator comprises supports within the space between the electrode layer and substrate.

[0019] Certain embodiments of the MEMS resonator further comprise a porous layer between the electrode layer and piezoelectric layer. In some embodiments comprising a porous layer between the electrode layer and piezoelectric layer, the supports are comprised of the same material as the porous layer.

[0020] Within at least some embodiments, the piezoelectric layer comprises aluminum nitride. In certain embodiments, the electrode layer comprises metal, preferably gold.

[0021] In some embodiments the substrate acts as a second electrode layer comprising 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.

[0022] In this context, UHD doping refers to a doping level above 1020cm“3. In certain embodiments, the doping level of the resonating element is above 1019cm“3.

[0023] According to some embodiments, the resonating element comprises a resonating beam. In certain embodiments the resonating element comprises a plurality of resonating beams.

[0024] At least some embodiments comprise an electrode layer having perforations, or perforated electrode. In certain embodiments, the perforations of the perforated electrode are uniformly spaced, creating a mesh. In certain embodiments, the perforations are comprised in columns along the perforated electrode and adjacent columns of perforations are off set from each other. In some embodiments, the perforations are equally spaced throughout the perforated electrode.

[0025] According to a second example aspect of the invention there is provided a method for manufacturing a MEMS resonator comprising:

[0026] - providing a wafer comprising a piezoelectric layer on a substrate and an electrode layer on the piezoelectric layer opposite the substrate, and

[0027] - forming at least one space between electrode layer and substrate. Within at least some embodiments, providing the wafer comprises first providing the piezoelectric layer on the substrate and forming the at least one space comprises forming a gap in the piezoelectric layer prior to forming the electrode layer on the piezoelectric layer.

[0028] In certain embodiments, the electrode layer is perforated and forming the at least one space comprises etching via the perforations of the electrode layer.

[0029] In certain embodiments of the second example aspect, the wafer further comprises a porous layer, and the wafer is provided by depositing the electrode layer onto the porous layer.

[0030] Within at least some embodiments the at least one gap is formed in an etching process. In at least some embodiments the porous layer is polycrystalline silicon.

[0031] The embodiments of the first aspect are applicable to the second aspect. For example, at least some embodiments of the second aspect are for manufacturing the MEMS resonator of the first aspect.

[0032] 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.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0035] Fig. 1A shows a schematic top view of a MEMS resonator in accordance with certain embodiments;

[0036] Fig. 1 B shows a cross-section of the resonator of Fig. 1 A along line AA’; Fig. 1C shows a cross-section of a resonator comprising a sacrificial layer;

[0037] Fig. 2A shows a schematic top view of a MEMS resonator in accordance with some embodiments;

[0038] Fig. 2B shows a cross-section of the resonator of Fig. 2A along line BB’;

[0039] Fig. 2C shows a cross-section of the resonator of Fig. 2A along line CC’;

[0040] Fig. 2D - 2F show a schematic top view and cross sections of a MEMS resonator comprising a sacrificial layer in accordance with some embodiments;

[0041] Fig. 3A shows a schematic top view of a MEMS resonator in accordance with some embodiments;

[0042] Fig. 3B shows a cross-section of the resonator of Fig. 3A along line BB’;

[0043] Fig. 3C shows a cross-section of the resonator of Fig. 3A along line CC’;

[0044] Fig. 3D shows cross-sections of a further embodiment;

[0045] Figs. 3E-3G show an embodiment further comprising a sacrificial layer;

[0046] Figs. 4A-4D show schematic top views of further MEMS resonators in accordance with some embodiments;

[0047] Fig. 5 shows a schematic top view of a portion of an electrode layer in accordance with certain embodiments;

[0048] Fig. 6A-6F show the shape and arrangement of perforations according to certain embodiments;

[0049] Figs. 7 shows a schematic top view of a MEMS resonator having rectangular perforations according to some embodiments;

[0050] Fig. 8A-8C show a process for manufacturing MEMS resonators according to certain embodiments;

[0051] Fig. 9A-9D show a process for manufacturing MEMS resonators according to some embodiments;

[0052] Fig. 10A-10E show MEMS resonators comprising supports according to certain embodiments;

[0053] Fig. 11 illustrates a method for manufacturing MEMS resonators according to some embodiments, and

[0054] Fig. 12A, 12B show a schematic top view and detail of said top view illustrating a MEMS resonator in accordance with certain embodiments. DETAILED DESCRIPTION

[0055] In the following description, like numbers denote like elements.

[0056] A MEMS, microelectromechanical systems, resonator according to certain embodiments of the present invention is illustrated within Figures 1A and 1 B where Figure 1 B shows a cross section of the MEMS resonator along line AA’ of Figure 1A. As illustrated, the MEMS resonator 100 comprises: a resonating element 101 comprising: a substrate 110, a piezoelectric layer 120 on the substrate 110, and an electrode layer 130 on the piezoelectric layer 120 opposite the substrate 110, wherein there is at least one space 125 between the electrode layer 130 and substrate 110. Figures 1 A and 1 B also illustrate optional connecting elements 160 which may be used, for example, to suspend the resonating element. As seen within Figure 1 B, within at least some embodiments, the space 125 is at least partially formed by a gap in a layer of the MEMS resonator, for example, the piezoelectric layer. That is, according to certain embodiments, gaps are present in an otherwise solid layer of material. For example, the resonating element of the MEMS resonator according to certain embodiments may have a solid piezoelectric layer within the resonating element save for gaps within the piezoelectric layer which serve to create the space between the electrode layer and substrate. Such gaps may, as seen in Figure 1 B, extend through the entire height of the layer, effectively eliminating the layer in the location of the gap. According to some embodiments, the gap extends only partly through the height of the layer, for example, forming a divot or indentation within the layer comprising the gap.

[0057] Within at least some embodiments the term space is synonymous with void. In certain embodiments, within the space there is no material, for example, no material from any layer of the resonator. In some embodiments the space is also known as a cavity.

[0058] As can be seen in Figures 1A and 1 B, within at least some embodiments, the gap in the piezoelectric layer 120 may be such that the space 125 extends across the entire resonating element, thus releasing the electrode layer from the substrate completely within the area of at least a resonating beam. Within the embodiment of Figures 1 A and 1 B the gap of the piezoelectric layer is such that the piezoelectric layer is only present within the connecting elements 160 as illustrated by the reference line B showing the edge of the connecting element 160.

[0059] As can be seen in Figure 1 C, at least some embodiments of MEMS resonator further comprise a sacrificial layer 140. The embodiment of Figure 1 C would have the same schematic top view of Figure 1A, but as seen in the cross section of Figure 1C the MEMS resonator further comprises a sacrificial layer 140, the gap is in the sacrificial layer 140 and may be such that the space 125 extends across the entire resonating element, thus releasing the electrode layer from the substrate completely within the area of at least a resonating beam. In at least some embodiments, the sacrificial layer is an oxide layer or a porous layer.

[0060] A MEMS resonator 200 according to certain embodiments is illustrated within Figures 2A - 2C with Figure 2B showing a cross section of the MEMS resonator along line BB’ of Figure 2A and Figure 2C showing a cross section along line CC’. Within Figure 2A the dotted lines illustrate the extent of the gaps in the piezoelectric layer 220. As seen, in at least some embodiments there are a plurality of gaps in the piezoelectric layer 220. As shown in Figures 2B and 2C, the space 225 is now such that it does not extend across the entire resonating element 201. Instead, the piezoelectric layer 220 provides a rectangular shaped support between the electrode layer 230 and substrate 210. Such a support may be referred to as a support frame. In at least some embodiments the support is comprised of any portion of the piezoelectric layer 220 which remains. For example, the support may take on a non-rectangular shape.

[0061] A MEMS resonator 200’ according to at least some embodiments is illustrated within Figures 2D - 2F with Figure 2E showing a cross section of the MEMS resonator along line EE’ of Figure 2D and Figure 2F showing a cross section along line FF’. Compared with the MEMS resonator 200 of Figures 2A - 2C, the MEMS resonator 200’ further comprises a sacrificial layer 240. Within Figure 2D the dotted lines illustrate the extent of the gap in the sacrificial layer 240. Similar to Figure 2A, in at least some embodiments there are a plurality of gaps in the sacrificial layer 240. As shown in Figures 2E and 2F, the space 225 between the electrode layer 230 and substrate 210 may be comprised in the sacrificial layer 240. In this manner, the sacrificial layer 240 may act as a support of the electrode layer 230. The sacrificial layer 240 being arranged between the piezoelectric layer 220 and electrode layer 230. In other words, the sacrificial layer 240 provides a support, such as a rectangular shaped support, between the electrode layer 230 and piezoelectric layer 220. Such a support may be referred to as a support frame.

[0062] 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 or on a piezoelectric layer. As illustrated within Figure 1 A, the electrode layer 130 may be on the piezoelectric layer 120 such that it is above the piezoelectric layer 120. 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.

[0063] Figures 3A - 3D illustrate MEMS resonators 300 according to certain embodiments wherein the electrode layer 330 comprises perforations 350. Once again there is a substrate 310, a piezoelectric layer 320 on the substrate 310, an electrode layer 330 on the piezoelectric layer 320 opposite the substrate 310 and at least some space 325 between the electrode layer 330 and substrate 310. Figure 3B shows a cross section along line BB’ and Figure 3C shows a cross section along line CC’. As seen, the electrode layer 330 of the resonating element 301 comprises a plurality of perforations 350. While in certain embodiments the electrode layer comprises only a single perforation.

[0064] As seen within Figures 3E - 3G, certain embodiments further comprise a sacrificial layer 340 between the electrode layer and piezoelectric layer, the space 325 being at least partially formed by a gap in the sacrificial layer. Within at least some embodiments, there are a plurality of gaps in the sacrificial layer, the plurality of gaps forming the at least one space. At least some embodiments comprising a sacrificial layer find use without the need for a perforated electrode, for example, by directly etched during a manufacturing step, or as a further example, by employing a porous layer.

[0065] As seen in Figure 3B, within certain embodiments the gap(s) in the piezoelectric layer 320 are centered around perforations 350 of the electrode layer 330. Within certain embodiments comprising a sacrificial layer, the gaps in the sacrificial layer are centered around perforations of the electrode layer. While in some embodiments the gaps in the piezoelectric layer 320 are undercut in relation to the perforations 350 of the electrode layer 330 as also shown in Figure 3B and 3C. Similarly, in certain embodiments comprising a sacrificial layer, the gaps in the sacrificial layer are undercut in relation to the perforations of the electrode layer.

[0066] As seen in Figures 3B and 3C, within certain embodiments there are a plurality of gaps in the piezoelectric layer the plurality of gaps forming a plurality of spaces 325.

[0067] Figure 3D illustrates a further embodiment wherein a single gap 325 spans multiple perforations 350 of the electrode layer. Within some embodiments there is a single gap for a plurality of perforations. In certain embodiments there is a single gap spanning all of the perforations. In some embodiments the plurality of gaps form a single space wherein in other embodiments the plurality of gaps form a plurality of spaces.

[0068] Figures 3E - 3G show an embodiment further comprising a sacrificial layer 340. The schematic top view of the MEMS resonator of Figures 3E - 3G would be the same as in Figure 3A with Figure 3E showing a cross section along line CO’, Figure 3E a cross section along line BB’ and Figure 3G showing a cross section along line CO’. As seen in Figures 3E - 3G, the space 325 may be formed in the sacrificial layer. Such a space may be formed in a singular gap, as in Figure 3E or in a plurality of gaps as seen in Figure 3G. Although, in at least some embodiments, as will be appreciated by comparing Figures 3E and 3F, two long gaps may be formed if portions of the sacrificial layer are left between the two rows of perforations. In at least some embodiments, the space is solely comprised in the sacrificial layer as seen.

[0069] As seen within Figures 1 - 3, in at least some embodiments, the electrode layer is the topmost layer. In certain embodiments comprising multiple electrode layers, 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. In at least some embodiments the substrate acts as the second electrode layer.

[0070] Within at least some embodiments, the piezoelectric layer comprises aluminum nitride. In certain embodiments, the electrode layer comprises metal, preferably gold.

[0071] According to some embodiments, the resonating element comprises a resonating beam. For example, certain embodiments comprise a singular resonating beam such that the resonating element comprises only a single resonating beam such as those illustrated within Figures 1 - 3.

[0072] In certain embodiments the resonating element comprises a plurality of resonating beams. For example, the MEMS resonators 400 of Figures 4A - 4D. 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.

[0073] Within some embodiments the plurality of resonating beams 401 are separated by trenches 481. Within at least some embodiments comprising a plurality of resonating beams the resonating beams are connected by connection elements 402 between the beams. In certain embodiments having connection elements, the connection elements comprise at least one perforation. Also illustrated within Figures 4A - 4D are the surrounding layers 470 according to at least some embodiments. As shown, the surrounding layers 470 are separated from the resonating element via trenches 480. Further illustrated are anchors 460 which serve to connect the resonating element to and suspend the resonating element from surrounding layers. The trenches 480 serve to separate the perforated resonating element comprised of the perforated resonating beams 401 , or resonating area, from the non-resonating area, of the resonator 400 or wafer.

[0074] Figures 4A - 4D show certain embodiments having differing arrangements of perforations 450 and a plurality of resonating beams 401 . 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 4A. In other embodiments, the mesh may be comprised only in a portion of the resonating element as seen in Figure 4B 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 4C, 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.

[0075] While some resistive loss may result from perforating the electrode layer, arranging and / or 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 substantially avoided if not negated.

[0076] In certain embodiments, the perforations are comprised in columns. For example, the perforations of the embodiments of Figures 4A - 4D may be considered to be arranged in columns. In some embodiments, the perforations are equally spaced throughout the resonating element. Figures 4A - 4D also provide examples of Length Extensional, LE, MEMS resonators 400 comprising LE beams 401 having perforations 450 according to certain embodiments. Within at least some embodiments, a certain portion of the length L of each beam comprises perforations 450. For example, Figure 4B wherein 33% of the length of each beam 401 comprises perforations 450, which may be, for example, centered along the length of each beam 401 . As another example, Figure 4D shows an embodiment wherein 66% of each beam 401 comprises perforations 450. Alternatively, 100% of each beam 401 may comprise perforations 450 as shown in Figure 4A. In at least some embodiments 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.

[0077] Perforations of the electrode layer 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 in at least some embodiments. In certain embodiments, at least 5% of the electrode layer has been removed by perforations. In some embodiments, at least 9% of the electrode layer has been removed by perforations. 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.

[0078] 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 electrode layer 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 are less than twice the thickness of the piezoelectric layer. In some embodiments, the perforations of the electrode layer 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. While the columns of Figures 4A - 4D are substantially aligned, in certain embodiments adjacent columns of perforations are off-set from each other as can be seen in Figure 5. Within Figure 5, a portion of the electrode layer 501 having perforations 550 is shown to illustrate an arrangement of columns of perforations 550. As can be seen, the first column of perforations 551 and second column of perforations 552 are not aligned such that perforations of the second column 552 are centered at a height which is halfway between the centers of perforations of the first column 551. In other terms, the perforations of the first column 551 of Figure 5 have a pitch, that is a distance between the center points of perforations within the same column. The second column 552 is then offset by a half of this pitch such that the center points of the perforations of the second column 552 begin half a pitch further down that the perforations of the first column 551. Within at least some embodiments, the columns are off-set by at least a fraction of the pitch, for example, a quarter pitch.

[0079] As shown in Figures 6A - 6F, shapes and arrangement of perforations may vary. Within Figures 6A - 6F the perforations are shown as the lighter portions of the figure.

[0080] As seen, in certain embodiments, the perforations take a circular shape as in Figure 6A. While in some embodiments the perforations take an octagonal shape, in certain embodiments forming a honeycomb like structure as in Figure 6B. In terms of pitch as discussed above, such a honeycomb structure may be formed by shifting every column of perforations by half a pitch.

[0081] As shown in Figure 6C, in some embodiments the perforations have the shape of octagons with triangle extensions. Within Figure 6C, the annotated shape 605 is provided with dashed lines to delineate between the octagon 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 layer.

[0082] Illustrated within Figure 6D 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.

[0083] As seen in Figure 6E, in some embodiments, the perforations of the electrode layer 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.

[0084] Illustrated within Figure 6F 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 6F, 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.

[0085] 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.

[0086] In several embodiments, the perforations of the electrode layer take the shape of rectangles. For example, the embodiment illustrated within Figure 7 which shows another type of resonating element 701 having a perforated electrode layer. The resonating element 701 of Figure 7 may be a portion of a stack of resonating elements 701 forming a stacked resonating element, for example the perforated resonating element 701 could be a resonating beam. As can be seen, the perforations 750 of Figure 7 are of different shapes and sizes.

[0087] Within embodiments comprising rectangular perforations the design may be discussed in terms of lines and nodes as labelled within Figure 7. In such embodiments the lines running along the electrode, 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 comprising a mesh of perforations or top electrode mesh.

[0088] Also illustrated within Figure 7 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 7. 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.

[0089] In some embodiments, the resonating element is configured to resonate in a lengthextensional, 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 certain embodiments, at least 15% of the length of each resonating beam comprises perforations, for example, within certain embodiments, at least 20% of each resonating beam comprises perforations. Within certain embodiments, the perforations are longitudinally centered along each resonating beam.

[0090] In some embodiments the substrate acts as a second electrode layer comprising 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 resonating element is above 1019cm“3.

[0091] According to at least some embodiments there is provided a method for manufacturing a MEMS resonator comprising: - providing a wafer comprising a piezoelectric layer on a substrate and a perforated electrode on the piezoelectric layer opposite the substrate,

[0092] - forming at least one gap in the piezoelectric layer to form at least one space between the substrate and the perforated electrode.

[0093] According to certain embodiments there is provided a method for manufacturing a MEMS resonator comprising:

[0094] - providing a wafer comprising a piezoelectric layer on a substrate, and a sacrificial layer on the piezoelectric layer opposite the substrate,

[0095] - forming at least one gap in the sacrificial layer

[0096] - depositing an electrode layer onto the sacrificial layer, opposite the piezoelectric layer such that at least one space is formed between the substrate and the electrode layer.

[0097] In such methods, the electrode layer may be deposited, for example, prior to formation of the at least one gap in the sacrificial layer. For example, the electrode layer may comprise perforations which allow for an etch process to remove portions of the sacrificial layer even after deposition of the electrode layer. As another example, in at least some embodiments, a porous layer is further provided on top of the sacrificial layer.

[0098] As seen in Figures 8A - 8C, at least some embodiments comprise a perforated electrode layer 830. Within the process of Figures 8A - 8C there is first provided a wafer 800 comprising a piezoelectric layer 820 on a substrate 810, an electrode layer 830, and a sacrificial layer 840 between the electrode layer 830 and piezoelectric layer 820 as shown in Figure 8A. After the provision of the wafer within Figure 8A, perforations 835 are formed in the electrode layer. In at least some embodiments an etching process etches perforations 835 into the electrode layer 830. The electrode layer may be, in certain embodiments, a metallic electrode layer. In at least some embodiments, the method may begin, as shown in Figure 8B with a perforated electrode. The next step, between Figures 8B and 8C is to form at least one gap in the sacrificial layer 840 to form at least one space 845 between the substrate and the perforated electrode. The formation of the at least one space partially releases the electrode layer around the perforations. The formation of gaps in the sacrificial layer may be achieved through an etch step, for example an HF vapor etch step, which selectively etches the sacrificial layer. The extent, for example the lateral extent, and size of the gap(s), and thus the space(s) can be controlled by adjusting the etch time.

[0099] In certain embodiments of the second example aspect, the wafer comprises the porous layer, and the method further comprises the step of depositing a top electrode layer onto the wafer after forming the at least one gap in the piezoelectric or sacrificial layer.

[0100] Within at least some embodiments the at least one space is formed in an etching process. In at least some embodiments, the at least one gap in a layer is formed in an etching process. In certain embodiments the sacrificial layer is an oxide layer. In certain embodiments the sacrificial layer is a sacrificial layer such that it is more susceptible to an etching process than at least one other layer. In some embodiments the sacrificial layer is a sacrificial layer in that it is more susceptible to an etching process than all other layers. In at least some embodiments the porous layer is polycrystalline silicon.

[0101] Figures 9A - 9D illustrate a process for manufacturing MEMS resonators according to at least some embodiments. As can be seen in cross section of a portion of the resulting MEMS resonator of Figure 9D, certain embodiments further comprise a porous layer 950 between the electrode layer 930, or top electrode, or metallic top electrode, and piezoelectric layer 920. As also illustrated within Figure 9D, there is the substrate 910. Figures 9B shows the sacrificial layer 940 which is removed to form the space between the electrode layer 930 and substrate layer 910. Within at least some embodiments the sacrificial layer is an oxide layer. In certain embodiments the substrate is an aluminum nitride layer.

[0102] Within the process of Figures 9A - 9D, first a starting material, or wafer, comprising the substrate 910 and piezoelectric layer 920 is provided as shown in Figure 9A. Then, as shown in Figure 9B, the sacrificial layer 940 and porous layer 950 are formed, for example by deposition. Then, within Figure 9C, at least a portion of the sacrificial layer 940 is removed. This removal of the sacrificial layer, for example via etching, forms the at least one space between the electrode layer 930 and substrate 910 in the final product as seen in figure 9D. For the sake of illustration of the process, the entire sacrificial layer 940 is shown as removed within Figure 9C, but it will be appreciated that portions of the sacrificial layer not shown remain as discussed above with regards to certain embodiments. Finally, the electrode layer 930 is formed, for example via metallization. As can be seen, in at least some embodiments it is not necessary to employ a perforated electrode.

[0103] Within at least some embodiments the sacrificial layer comprises silicone dioxide. In some embodiments the sacrificial layer comprises poly(methyl methacrylate) (PMMA). In certain embodiments the porous layer comprises polycrystalline silicon.

[0104] In certain embodiments the MEMS resonator comprises supports within the space between the electrode layer and substrate. For example, portions of the sacrificial layer may remain to support the electrode layer relative to the substrate. In embodiments comprising a porous layer between the electrode layer and piezoelectric layer, the supports may be comprised of the same material as the porous layer.

[0105] Figure 10A is a schematic top view of an embodiment having supports 1025 illustrated by squares representing portions of the sacrificial layer or porous layer which remain under the electrode layer 1030 to support the electrode layer atop the piezoelectric layer 1020. The piezoelectric layer being on the substrate 1010. Such supports could be in direct contact with the electrode layer or supporting the electrode layer via a porous layer or other intermediate layer.

[0106] Figures 10B and 10C illustrate cross sections of an area of a MEMS resonator comprising a support 1025 according to at least some embodiments. In at least some embodiments the support 1025 comprises the same material as the porous layer 1050 as seen in Figure 10B. Within certain embodiments the support 1025 comprises the same material as the sacrificial layer 1040 as seen in Figure 10C. Also illustrated within Figures 10B and 10C are the electrode layer 1030, porous layer 1050, sacrificial layer 1040, piezoelectric layer 1020 and substrate 1010. As can be seen, the support 1025 is surrounded by the gap or gaps 1045. Figures 10D and 10E illustrate schematic top views of further embodiments comprising supports 1025. Such embodiments may be alternatives to the perforated embodiments of Figures 4A - 4D. As seen, the MEMS resonators 1000 have supports 1025 positioned at various locations along resonating beams 1001 and within the anchors 1060. The resonating beams 1001 and connection elements 1002 being comprised in the electrode layer visible from this top view. Also illustrated, similar to Figure 10A, are supports 1025 illustrated by squares representing portions of the sacrificial layer or porous layer which remain under the electrode layer 1030 to support the electrode layer atop the piezoelectric layer 1020. Further shown are the surrounding layers 1070, anchors 1060, and trenches 1080 and 1081.

[0107] Figure 11 illustrates a method of manufacture according to at least some embodiments. As seen, the method comprises a step 1110 of providing a wafer comprising a piezoelectric layer on a substrate and an electrode layer on the piezoelectric layer opposite the substrate. Within step 1120, the method further comprises the step of forming at least one space between the electrode layer and substrate.

[0108] Within certain embodiments, providing the wafer comprises first providing the piezoelectric layer on the substrate and forming the at least one space comprises forming a gap in the piezoelectric layer prior to forming the electrode layer on the piezoelectric layer.

[0109] In at least some embodiments, the electrode layer is perforated and forming the at least one space comprises etching via the perforations of the electrode layer. In certain embodiments, the wafer further comprises a porous layer, and the wafer is provided by depositing the electrode layer onto the porous layer.

[0110] At least some processes according to embodiments described herein provide for manufacturing of the MEMS resonators of the other embodiments described herein.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] In certain embodiments the resonating element is configured to resonate in a widthextensional, WE, resonance mode.

[0115] 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.

[0116] 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.

[0117] In certain embodiments, the resonator is separated from its surrounding, for example a surrounding substrate, by trenches. In certain embodiments, the resonating elements are configured to resonate in an in-plane 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. 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.

[0118] Figure 12A shows a schematic top view of a MEMS resonator according to certain embodiments, with Figure 12B showing a detail of Figure 12A. The MEMS resonator

[0119] 1200 shown in Figures 12A and 12B comprises a type of perforation 1250 which may be referred to as a meander. The MEMS resonator 1200 comprises beams 1201 having perforations 1250 according to certain embodiments. As shown the resonating beams

[0120] 1201 are separated by trenches 1281 with connection elements 1202 between the beams within certain embodiments.

[0121] As shown in Figure 12B, in certain embodiments the trenches 1281 extend through all layers of the MEMS resonator.

[0122] In at least some embodiments, there is a portion of the sacrificial layer which is removed from underneath the electrode, or top metallic electrode layer in some embodiments, as shown by the darker gray segments labeled 1255. This removal of the sacrificial layer provides for release of the top electrode layer. In at least some embodiments this is referenced as an undercut. In Figures 12B the removed top electrode layer is labeled 1257 as represented by the lighter gray segments. With the light solid gray segments representing the metallic top electrode 1259.

[0123] Top electrode patterning, for example via meanders, may be employed in certain embodiments. These meanders provide perforations of the top electrode layer.

[0124] Within at least some embodiments, such as those employing a meander, the remaining sacrificial material portion is a sized version of the electrode opening geometry, the sizing depending on the etch time which defines the underetching distance. The nonopened electrode portion forms a meander pattern.

[0125] 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. A further technical effect is the maintenance of quality factor Q over time. A further technical effect is an improvement of frequency stability without an increase in ESR.

[0126] 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.

[0127] 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.

[0128] 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 resonating element (101 ) comprising:- a substrate (110),- a piezoelectric layer (120) on the substrate (110),- an electrode layer (130) on the piezoelectric layer (120) opposite the substrate (110), the electrode layer (130) comprising perforations, and- a sacrificial layer (140) between the electrode layer (130) and piezoelectric layer (120), wherein there is at least one space (125) between the electrode layer (130) and substrate (110), the space (125) being at least partially formed by at least one gap in the sacrificial layer (140), and wherein the gap(s) in the piezoelectric layer (120) and / or sacrificial layer (140) are centered around the perforations of the electrode layer (130).

2. The MEMS resonator of claim 1 , wherein the space (125) is at least partially formed by at least one gap in the piezoelectric layer (120).

3. The MEMS resonator of claim 1 or 2, comprising a plurality of gaps in the piezoelectric layer (120), the plurality of gaps at least partially forming the at least one space (125).

4. The MEMS resonator of claim 3, comprising a plurality of gaps in the sacrificial layer (140), the plurality of gaps forming the at least one space (125).

5. The MEMS resonator of any preceding claim, wherein the gaps in the sacrificial layer (140) are undercut in relation to the perforations of the electrode layer (130).

6. The MEMS resonator of any preceding claim, wherein the gaps in the piezoelectric layer (120) and / or sacrificial layer (140) are undercut in relation to the perforations of the electrode layer (130).

7. The MEMS resonator of any preceding claim, further comprising supports within the space (125) between the electrode layer (130) and substrate (110).

8. The MEMS resonator of claim 7, comprising a porous layer between the electrode layer (130) and piezoelectric layer (120), wherein the supports are comprised of the same material as the porous layer.

9. The MEMS resonator of claim 7, comprising the sacrificial layer (140) between the electrode layer (130) and piezoelectric layer (120), wherein the supports are comprised of the same material as the sacrificial layer.

10. The MEMS resonator of any preceding claim, wherein the piezoelectric layer (120) comprises aluminum nitride.

11. The MEMS resonator of any preceding claim, wherein the electrode layer (130) comprises metal, preferably gold.

12. The MEMS resonator of claim 9, wherein the substrate (110) acts as a second electrode layer, the substrate (110) comprising silicon, preferably doped silicon, such as ultra-heavily doped silicon, more preferably single crystal silicon.

13. The MEMS resonator of any preceding claim, wherein the resonating element (101 ) comprises a resonating beam.

14. The MEMS resonator of any preceding claim, wherein the resonating element (101 ) comprises a plurality of resonating beams.

15. A method for manufacturing a MEMS resonator comprising:- providing a wafer comprising a piezoelectric layer (120) on a substrate (110) and a sacrificial layer (140) on the piezoelectric layer (120) opposite the substrate (110),- forming at least one gap in the sacrificial layer (140), and- depositing an electrode layer (130) onto the sacrificial layer (140), opposite the piezoelectric layer (120) such that at least one space (125) is formed between the substrate (110) and the electrode layer (130).

16. The method according to claim 15, wherein providing the wafer comprises first providing the sacrificial layer (140) on the substrate (110) and forming the at least one space (125) comprises forming a gap in the sacrificial layer (140) prior to forming the electrode layer (130) on the sacrificial layer (140).

17. The method according to claim 15 or 16, wherein the electrode layer (130) is perforated and forming the at least one space (125) comprises etching via the perforations of the electrode layer (130).

18. The method according to any one of claims 15 - 17, wherein the wafer further comprises a porous layer deposited on top of the sacrificial layer (140).

19. The method according to any one of claims 15 - 18, wherein the method is for manufacturing the MEMS resonator (100) of any of one of claims 1 - 14.

Images