Abstract:
A method of forming a resonator by providing a first layer; forming a sacrificial layer on the first layer; forming a capping layer on the sacrificial layer; forming at least one etching aperture in the capping layer; forming at least one additional aperture having a different size than the at least one etching aperture; forming a cavity and releasing a resonator structure within the cavity by removing the sacrificial layer by etching via the at least one etching aperture; sealing the at least one etching aperture; and forming a lining in the at least one additional aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/834,486, which was filed Mar. 15, 2013. This application is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to microelectromechanical system (MEMS) resonators and more particularly to MEMS resonator structures which maximize a voltage supply. 
     BACKGROUND 
     Conventional digital circuitry typically relies on a quartz crystal to provide a clock signal. Quartz crystals, however, are bulky and do not provide suitable options for size reductions. As digital devices continue to shrink in size, the use of quartz crystals for timing becomes an impediment to further volume reductions. 
     Microelectromechanical system (MEMS) resonators have been identified as a possible replacement for the quartz crystal. MEMS resonators also present their own drawbacks, including that the silicon which the resonators comprise exhibits a temperature drift caused by a temperature-dependent linear expansion coefficient. While advancements in thermal stability have been made, other issues remain. For example, the drive voltage and constant bias voltage of a MEMS resonator are defined by the geometrical layout of the resonator structure, and conventional layouts do not allow for the maximum voltage on-chip to be used as the bias voltage. 
     Challenges also exist in manufacturing MEMS resonators, particularly in integrating resonators as part of conventional CMOS processing. MEMS resonators often are provided as part of “system-in-package” products along with microchips and other related structures and components to which the resonators can provide a resonant frequency used in operation. Resonator structures as part of system-in-package configurations must be taken into consideration in package design and system production. For example, compatibility with CMOS (complementary metal-oxide-semiconductor) processing can be important as direct integration of resonator fabrication as part of CMOS processes can provide cost savings that make MEMS resonators more attractive. 
     In CMOS processing, the resonator can be encapsulated in a vacuum, with sealing or capping provided a silicon capping wafer in one embodiment, or by forming the cavity for the resonator using a sacrificial layer etch. The dimensions of the cavity and the techniques used to free the resonator within the cavity such that it can resonate are important considerations when considering compatibility with CMOS process integration. For example, if a resonator cavity is to have a clearance above the resonator structure of about 1 μm, use of conventional back-end CMOS processing techniques becomes impractical, as a wet chemical etch is often used to form the cavity but a minimum cavity height is required to prevent the resonator from adhering or bonding with the capping layer. This minimum height combined with a relatively thick ceiling (e.g., about 2-3 μm in embodiments) which seals the cavity increases the size of the resonator structure beyond what is practical and/or desired. 
     SUMMARY 
     In an embodiment, a microelectromechanial system (MEMS) resonator comprises a first drive electrode; a resonator structure having a first end spaced apart from the first drive electrode by a first gap; and a ground electrode electrically coupled to the first end of the resonator. 
     In an embodiment, a method of forming a resonator comprises providing a first layer; forming a sacrificial layer on the first layer; forming a capping layer on the sacrificial layer; forming at least one etching aperture in the capping layer; forming a cavity and releasing a resonator structure within the cavity by removing the sacrificial layer by dry etching via the at least one etching aperture; and sealing the at least one etching aperture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  depicts a resonator structure according to an embodiment. 
         FIG. 2  depicts a resonator structure comprising additional trenches according to an embodiment. 
         FIG. 3  depicts a process flowchart according to an embodiment. 
         FIG. 4A  depicts a process stage of a resonator according to an embodiment. 
         FIG. 4B  depicts another process stage of a resonator according to an embodiment. 
         FIG. 4C  depicts another process stage of a resonator according to an embodiment. 
         FIG. 5  depicts a graph of layer deflection for various layer thicknesses according to an embodiment. 
         FIG. 6A  depicts a process stage of a resonator according to an embodiment. 
         FIG. 6B  depicts another process stage of a resonator according to an embodiment. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Embodiments relate to MEMS resonator structures and methods that enable application of a maximum available on-chip voltage. In an embodiment, a MEMS resonator comprises a connection between a ground potential and the gap electrode of the resonator. Embodiments also relate to manufacturing systems and methods that are less complex and enable production of MEMS resonators of reduced dimensions. 
     Referring to  FIG. 1 , a resonator  100  according to an embodiment is depicted. In embodiments, resonator  100  comprises a free-free beam MEMS piezoresistive resonator structure anchored by anchors  102  and  104  such that the beam structure  106  is configured to resonate longitudinally, i.e., side-to-side lengthwise in the direction indicated by the arrow, in operation. Resonator  100  can comprise a “dog bone” resonator structure, or some other suitable resonator structure in other embodiments. In other words, the particular shape, layout, dimensions, and other characteristics of resonator  100  can vary from those specifically depicted in the example illustration of  FIG. 1 , as appreciated by those skilled in the art. 
     Beam structure  106  also can comprise one or more slits or trenches  107 . Trenches  107  can be arranged on beam structure  106  to direct or concentrate current flow within beam structure  106 . The particular size, arrangement and configuration of trenches  107  can vary from those depicted in  FIG. 1 . For example, in other embodiments, the center trench can be wider than the trenches on the top and bottom (as oriented on the page of the drawing), or the top and bottom trenches can be shorter such that they do not extend as close to the ends of beam structure  106  or toward the center. Multiple trenches  107  also can be formed, in parallel or in series with other trenches. Other characteristics of one or more of trenches  107  also can vary in other embodiments. 
     As depicted, resonator  100  comprises a ground electrode  108 , a sense electrode  110 , and two drive electrodes  112  and  114 . A gap or trench  116  and  118  separates each drive electrode  112  and  114 , respectively, from beam structure  106 . In embodiments, resonator  100  comprises silicon and can comprise doped silicon and/or a doped region  120  that forms a piezoresistor in beam structure  106 . For example, in one embodiment the bulk of beam resonator structure  106  can comprise p-doped silicon, while doped region  120  can be n-doped, or vice-versa in another embodiment. 
     In operation, when a bias voltage is applied to drive electrodes  112  and  114 , excitation of resonator  100  occurs such that beam structure  106  resonates or oscillates between electrodes  112  and  114 , anchored by anchors  108  and  110 . In embodiments, the bias voltage comprises both DC and AC components, such as a larger DC component (e.g., in a range of about 1V to about 20V, or less or more in embodiments) and a smaller AC component (e.g., about 100 mV, or less or more in embodiments). A sensing voltage is also applied between electrodes  108  and  110  such that current flows along a path generally defined in the structure of resonator  100  including trenches  107 , i.e., within doped region  120 , and a change in the resistance of beam structure  106  due to the piezoresistive effect can then be sensed. 
     The bias and sensing voltages are related in embodiments to the geometrical layout of resonator  100 . In a capacitively excited resonator, the width of gaps  116  and  118  between the ends of beam structure  106  and each drive electrode  112  and  114  generally should be small, on the order of about 100 nanometers (nm) or less in embodiments, which functions to reduce the bias voltage necessary to drive resonator  100 . For gap widths significantly less than 100 nm, the DC bias voltage is on the order of about 1V to about 3V in embodiments. Additionally, it can be advantageous to maximize the bias voltage, such as by making it equal to the maximum available voltage, or the maximum voltage available on-chip. This can eliminate the need to use charge pumps, voltage conversion or other techniques. 
     This can be accomplished in embodiments by providing a coupling region between ground electrode  108  and drive electrodes  112  and  114 , which in  FIG. 1  is provided by a highly doped implant region  124 . Region  124  is configured to provide the potential of ground electrode  108  at the ends of beam resonator structure  106  opposite each drive electrode  112  and  114 . In other words, region  124  can provide an electrode-ground short that can maximize a voltage difference therebetween to more efficiently drive resonator  100  and use the maximum available on-chip voltage as the bias voltage in operation. 
     Thus, in one example without region  124 , a DC voltage of 5V is available on-chip. If 3V are applied between sense electrode  110  and ground electrode  108 , then 3.5V are available across gaps  116 ,  118  to drive resonator  100  given the ground potential at the ends of beam resonator structure  106 . In another example without region  124 , if a DC voltage of 3V is available and 3V are applied between sense electrode  110  and ground electrode  108 , then 1.5V are available across gaps  116  and  118 . Thus, without region  124  providing a short between ground electrode  108  and drive electrodes  112 ,  114 , only a portion of the available voltage (e.g., 3.5V vs. 5V, and 1.5V vs. 3V, in the previous examples) is used as the DC bias voltage to drive resonator  100 . Contrarily, when an electrode-ground short is provided by region  124 , the maximum available voltage (e.g., 5V and 3V in the previous examples) can be used to drive resonator  100 . 
     Referring to  FIG. 2 , in another embodiment the isolation between the ends of the piezoresistor, or region  120 , and the ground potential of region  124  can be improved by providing additional trenches  126  and  128  therebetween. If the tips of region  120  are shorted to ground by region  124 , the resistance swing is reduced by half. Trenches  126  and  128 , on the other hand, keep the capacitive coupling between the ground potential of region  124  and the piezoresistor of region  120  low. In other embodiments, trenches  126  and  128  can be supplemented or replaced by low- or counter-doped regions between region  120  and region  124 . Trenches  126  and  128  need not connect or align with some or all of trenches  107  as depicted in  FIG. 2 . In other embodiments, trenches  126  and  128  can be spaced apart from the ends of one or more of trenches  107 , or trenches  126  and  128  can meet but extend beyond the ends of trenches  107 . In other embodiments, trenches  126  and  128  as well as trenches  107  can be filled with a dielectric material, a composite filler comprising a dielectric material, or some other suitable material. Other configurations are possible in other embodiments as well. 
     As previously mentioned, the particular layout and configuration of resonator  100  can vary in other embodiments. For example, the arrangement and configuration of trenches  107 ,  126  and  128  can vary according to other characteristics of resonator  100 . Additionally, the particular size, arrangement and configuration of region  124  can vary, though the principle of providing a coupling arrangement between a ground potential and the tips of a resonator structure at a gap between the resonator and one or more drive electrodes remains. More or fewer electrodes can be present, and the particular arrangement and characteristics of the various electrodes can vary from those depicted in  FIGS. 1 and 2 . If additional electrodes are present in embodiments, the additional electrodes also can be coupled to the ground electrode as discussed herein, or not, as can be suitable for any particular embodiment as appreciated by those skilled in the art. 
     In embodiments, novel processes also can be implemented to avoid the previously mentioned drawbacks associated with integrating resonators in system-in-package and other configurations and using CMOS and other techniques. In embodiments, a dry-etchable sacrificial layer can be used along with a capping layer and/or layer sequence which provides a minimal structural height, such that the overall process can be simplified, and therefore is less expensive, while also providing a resonator structure with reduced dimensions. The processes discussed herein are suitable, for example, for fabricating resonator structures such as those discussed herein with reference to  FIGS. 1 and 2 . 
     Referring to  FIGS. 3 and 4A , a sacrificial layer  402  is formed on top of the resonator structure or surrounding the resonator structure in and/or on a silicon layer  404  at  302 . In embodiments, the sacrificial layer  402  comprises carbon and, once removed, will free a resonator  406  to resonate within a cavity  408 . Sacrificial layer  402  is capped with a capping layer  410  at  304 . 
     Capping layer  410  can comprise a dielectric in embodiments, such as oxide or nitride. A thickness of capping layer  410  is chosen in embodiments such that it corresponds to a typical thickness of an intermediate layer, such as oxide, used in the CMOS process. The thickness of capping layer  410  also should take into consideration the pressure it will have to withstand without flexing or other deformation. In embodiments in which capping layer  410  comprises nitride, for example, the inventors have found that flexure of less than about 250 nm occurs at a pressure of one atmosphere if capping layer is about 500 nm thick or more, even for relatively large cavities of up to about 40 μm wide. Refer, for example, to  FIG. 5 , which depicts the maximum deflection of capping layers of various thicknesses for various edge lengths being capped. The thickness of capping layer  410  can vary in other embodiments, for example if a material other than nitride is used, the capping layer or resonator has some different structural features, or according to other characteristics. For example, in various embodiments capping layer  410  can be less than about 1 μm thick, such as less than about 0.5 μm thick in one embodiment. 
     At  306 , an etching hole grid or other apertures  412  are etched in capping layer  410 , and at  308  sacrificial layer  402  is removed. In embodiments, sacrificial layer  402  can be removed by a plasma etching process or some other suitable dry etching process. The use of plasma or other dry etching techniques enables cavity  408  to be kept thin as there is less chance of adhesion between resonator  406  and capping layer  410 . After removal of sacrificial layer  402 , the structure as depicted in  FIG. 4B  remains. 
     Referring to  FIG. 4C , etching hole grid  412  can be sealed by a fill layer  414  at  310 . In embodiments, fill layer  414  can comprise oxide or some other dielectric and can be available as a planar dielectric for use as an intermediate oxide in the metallization module or for other purposes. 
     In another embodiment, and referring to  FIGS. 6A and 6B , the dimensions and placement of one or more of etching holes  412  or other apertures can vary. For example, in  FIG. 6A  one aperture  416  is spaced apart from cavity  408  and is larger than etching holes  412  such that aperture  416  can later comprise a contact or other circuit element. When fill layer  414  is deposited, etching holes  412  are sealed while a lining  418  is formed in aperture  416 . Lining  418  can be partially or fully removed in later processing, if desired. Thus, joint patterning of etching holes and apertures for encapsulating resonator structures and contact holes is possible, providing reduced process costs and complexities. 
     Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, configurations and locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention. 
     Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. Moreover, elements described with respect to one embodiment can be implemented in other embodiments even when not described in such embodiments unless otherwise noted. Although a dependent claim may refer in the claims to a specific combination with one or more other claims, other embodiments can also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of one or more features with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended also to include features of a claim in any other independent claim even if this claim is not directly made dependent to the independent claim. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112 sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.