Patent Publication Number: US-7591201-B1

Title: MEMS structure having a compensated resonating member

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The invention is in the field of Microelectromechanical Systems (MEMS). 
     2) Description of Related Art 
     For the past several years, MEMS structures have been playing an increasingly important role in consumer products. For example, MEMS devices, such as sensors, detectors and mirrors, can be found in products ranging from air-bag triggers in vehicles to displays in the visual arts industry. As these technologies mature, the demands on precision and functionality of the MEMS structures have escalated. For example, optimal performance may depend on the ability to fine-tune the characteristics of various components of these MEMS structures. Furthermore, consistency requirements for the performance of MEMS devices (both intra-device and device-to-device) often dictate that the processes used to fabricate such MEMS devices need to be extremely sophisticated. 
     MEMS resonators are also becoming more prevalent. For example, motion sensors may be fabricated from a resonant accelerometer.  FIG. 1  illustrates a cross-sectional view representing a MEMS structure having a resonant accelerometer, in accordance with the prior art. 
     Referring to  FIG. 1 , a MEMS structure  100  comprises a resonating member  102  coupled to an anchor  104  and a mass  106 . In response to a motion, i.e. a positional displacement of MEMS structure  100 , mass  106  oscillates in the direction of arrow  108 . The oscillation of mass  106  alters the rigidity of resonating member  102 , thus modifying the resonant frequency of resonating member  102 . This change in resonant frequency may be detected to indicate that a positional displacement has occurred. 
     In another example, a clocking device may be fabricated from a MEMS resonator. For a reliable clocking device, however, very little to no change of the vibrational frequency of the MEMS resonator may be acceptable. This can prove difficult because many MEMS resonators undergo undesirable frequency fluctuations in response to environmental changes. Thus, a MEMS structure having a compensated resonating member is described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view representing a MEMS structure having a resonant accelerometer, in accordance with the prior art. 
         FIG. 2  illustrates a correlation plot representing the frequency of a resonating member as a function of temperature, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates a cross-sectional view representing a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIGS. 4A-B  illustrate cross-sectional views representing the operation of a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIGS. 5A-F  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIGS. 6A-C  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIGS. 7A-E  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIGS. 8A-C  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
         FIG. 9  illustrates a cross-sectional view representing a MEMS structure having a centrally-anchored compensated resonating member, in accordance with an embodiment of the present invention. 
         FIG. 10  illustrates a cross-sectional view representing a MEMS structure having a compensated resonating member and a dynamic mass-load comprising more than two materials with mismatched thermal coefficients of expansion, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A MEMS structure having a compensated resonating member and a method to form such a structure are described. In the following description, numerous specific details are set forth, such as material compositions and chemical regimes, in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known features, such as lithographic parameters and patterning procedures, are not described in detail in order to not unnecessarily obscure the present invention. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. 
     Disclosed herein is a MEMS structure having a compensated resonating member. In an embodiment, a MEMS structure comprises a resonating member coupled to a substrate by an anchor. A dynamic mass-load is coupled with the resonating member. The dynamic mass-load is for compensating a change in frequency of the resonating member by altering the moment of inertia of the resonating member by way of a positional change relative to the anchor. In a specific embodiment, the dynamic mass-load compensates the resonating member in response to an environmental change selected from the group consisting of temperature, pressure, light, electrical and chemical. 
     A MEMS structure having a compensated resonating member may enhance the performance and reliability of a MEMS clocking device that incorporates such a MEMS structure. For example, in accordance with an embodiment of the present invention, a MEMS resonator is comprised of a resonating member having a resonant frequency that is a function of temperature. In one embodiment, the resonant frequency of the resonating member decreases in correlation with an increase in temperature. A dynamic mass-load coupled with the resonating member may displace the moment of inertia of the resonating member such that the effective mass of the resonating member is altered. In an embodiment, the effective mass is reduced in response to an increase in temperature and, hence, the relative resonant frequency at a given temperature is increased for a resonating member having a resonant frequency that would otherwise decrease with increasing temperature. Thus, a frequency compensation may be invoked for a resonating member having a resonant frequency that decreases in response to an increase in temperature. In an example,  FIG. 2  illustrates a correlation plot representing the frequency of a resonating member as a function of temperature, in accordance with an embodiment of the present invention. Referring to  FIG. 2 , the resonant frequency of a resonating member with a fixed effective mass decreases with increasing temperature, as represented by the solid line. However, the frequency of the resonating member for a given temperature may be manipulated by altering the effective mass of the resonating member. Thus, in one embodiment, the effective mass of the resonating member is decreased in correlation with an increasing temperature. In a specific embodiment, this dynamic mass compensation provides for a substantially consistent resonant frequency of the resonating member, regardless of temperature, as depicted by the dashed line in  FIG. 2 . 
     A MEMS structure may be fabricated having a resonating member coupled with a dynamic mass-load.  FIG. 3  illustrates a cross-sectional view representing a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 3 , a MEMS structure  302  comprises a resonating member  304  and a base  306 . Resonating member  304  is coupled with a dynamic mass-load  308  and base  306  is coupled with substrate  300  by an anchor  310 . Resonating member  304  is suspended between a drive electrode  312  and a sensor electrode  314 . Dynamic mass-load  308  comprises a pair of mass units  316  and a two-component arm  318  comprised of a first material  320  and a second material  322 . 
     MEMS structure  302  may be any device that falls within the scope of MEMS technologies. For example, MEMS structure  302  may be any mechanical and electronic structure having a critical dimension of less than approximately 250 microns and fabricated using lithography, deposition, and etching processes above a substrate. In accordance with an embodiment of the present invention, MEMS structure  302  is a device selected from the group consisting of a resonator, a sensor, a detector, a filter and a mirror. In a particular embodiment, MEMS structure  302  is a resonator for use in a clocking circuit. Thus, resonating member  304  may be any suspended feature having a resonant frequency. For example, in an embodiment, resonating member  304  is a feature selected from the group consisting of a beam, a plate and a tuning fork. In a specific embodiment, resonating member  304  is a cantilever arm, as depicted in  FIG. 3 . 
     Resonating member  304  may have any dimensions suitable for a desired MEMS function. For example, in accordance with an embodiment of the present invention, MEMS structure  302  is a resonator comprised of a resonating cantilever arm  304 . The height of resonating member  304  is in the range of 0.1-10 microns and the width is in the range of 0.1-100 microns. In one embodiment, the length of resonating member  304  is in the range of 70-90 microns, the height of resonating member  304  is in the range of 0.5-5 microns and the width of resonating member  304  is in the range of 0.5-5 microns. The distance that resonating member  304  is suspended above substrate  300  may be selected to mitigate the acoustic back-scattering for a desired MEMS function. In an embodiment, MEMS structure  302  is a clocking device and resonating member  304  is suspended at a distance in the range of 0.1-5 microns above substrate  300 . The spacing between resonating member  304  and electrodes  312  and  314  may be sufficient to generate and collect high quality signals without interfering with a resonating mode of resonating member  304 . In one embodiment, the spacing between resonating member  304  and each of electrodes  312  and  314  is in the range of 100-500 nanometers. 
     Resonating member  304 , and thus base  306 , may be comprised of any material suitable to withstand a MEMS fabrication process. For example, in accordance with an embodiment of the present invention, resonating member  304  is comprised of a material selected from the group consisting of an insulator, a semiconductor and a conductor. In one embodiment, resonating member  304  is comprised of an insulating material selected from the group consisting of silicon dioxide, silicon nitride, silicon oxy-nitride and a high-K dielectric material. In one embodiment, resonating member  304  is comprised of a semiconducting material selected from the group consisting of silicon, germanium, silicon/germanium, carbon-doped silicon, carbon-doped silicon/germanium and a III-V material. The semiconducting material may also be comprised of dopant impurity atoms. For example, in a specific embodiment, resonating member  304  is comprised of polycrystalline silicon/germanium with a germanium atomic concentration in the range of 50-70% and boron dopant impurity atoms with a total atomic concentration in the range of 1×10 18 -5×10 20  atoms/cm 3 . In one embodiment, resonating member  304  is comprised of a conductor and is formed from a material selected from the group consisting of copper, aluminum, a metal alloy and a metal silicide. 
     Dynamic mass-load  308  comprises a pair of mass units  316  and a two-component arm  318  having a first material  320  and a second material  322 . Mass units  316  and first material  320  may be comprised of any material described in association with resonating member  304 . In one embodiment, mass units  316 , first material  320  and resonating member  304  are all comprised of the same material, as depicted in  FIG. 3 . In a specific embodiment, mass units  316 , first material  320  and resonating member  304  are comprised of polycrystalline silicon/germanium with a germanium atomic concentration in the range of 50-70% and boron dopant impurity atoms with a total atomic concentration in the range of 1×10 18 -5×10 20  atoms/cm 3 . 
     Second material  322  may also be comprised of any material described in association with resonating member  304 . However, in accordance with an embodiment of the present invention, second material  322  is comprised of a material having a different thermal coefficient of expansion (TCE) than first material  320 . In one embodiment, this TCE mismatch between first material  320  and second material  322  is at least 1.0×10 −6 /° C. In an embodiment, the first material/second material  320 / 322  pairing is a pairing of an insulator and a semiconductor. In a specific embodiment, first material  320  is comprised of silicon/germanium while second material  322  is comprised of silicon dioxide. In an alternative embodiment, first material  320  is comprised of silicon dioxide while second material  322  is comprised of silicon/germanium. Second material  322  need not cover the entire length of first material  320 . However, in accordance with an embodiment of the present invention, in order to maximize the performance of dynamic mass-load  308 , second material  322  is continuous and covers a substantial portion of the length of first material  320  of dynamic mass-load  308 . Although not shown, it is to be understood that second material  322  may be encapsulated by protective layers, including additional layers comprised of the same material as first material  320 . 
     Drive electrode  312  and sensor electrode  314  may be comprised of any material described in association with resonating member  304 . In accordance with an embodiment of the present invention, drive electrode  312  and sensor electrode  314  are comprised of substantially the same material as resonating member  304 . Drive electrode  312  and sensor electrode  314  may be coupled with substrate  300  by conductive couplers, as depicted in  FIG. 3 . The conductive couplers may be comprised of any conductive material suitable to withstand a MEMS fabrication process. For example, in accordance with an embodiment of the present invention, the conductive couplers are comprised of a material selected from the group consisting of a semiconductor material heavily doped with charge-carrier impurity atoms and a conductor. In one embodiment, the conductive couplers are comprised of a heavily doped semiconducting material selected from the group consisting of silicon, germanium, silicon/germanium, carbon-doped silicon, carbon-doped silicon/germanium and a III-V material. In a specific embodiment, the conductive couplers are comprised of a group IV material heavily doped with charge-carrier impurity atoms selected from the group consisting of boron, indium, phosphorus, arsenic and antimony. For example, in a particular embodiment, the conductive couplers are comprised of polycrystalline silicon/germanium with a germanium atomic concentration in the range of 55-95% and boron dopant impurity atoms with a total atomic concentration in the range of 1×10 −20 -5×10 22  atoms/cm 3 . In one embodiment, the conductive couplers are comprised of a conductor material and are formed from a material selected from the group consisting of copper, aluminum, a metal alloy and a metal silicide. In another embodiment, the conductive couplers are comprised of substantially the same material as drive electrode  312  and sensor electrode  314 . In accordance with an embodiment of the present invention, the conductive couplers are for electrically coupling drive electrode  312  and sensor electrode  314  with a plurality of interconnects housed in substrate  300 . 
     Anchor  310  may be comprised of any material described in association with resonating member  304  or electrodes  312  and  314  and their respective conductive couplers. In accordance with an embodiment of the present invention, anchor  310  is comprised of substantially the same material as the conductive couplers of electrodes  312  and  314 . Anchor  310  may be comprised of a material suitable to affix resonating member  304  to substrate  300 . Thus, in accordance with an embodiment of the present invention, anchor  310  is comprised of the same material as the conductive couplers for electrodes  312  and  314 , but anchor  310  is provided for anchoring resonating member  304  while the conductive couplers are provided for electrically coupling drive electrode  312  and sensor electrode  314  with a plurality of interconnects housed in substrate  300 . In one embodiment, anchor  310  is electrically isolated from a plurality of interconnects. 
     Substrate  300  may be comprised of any material suitable to withstand a MEMS fabrication process and to provide structural integrity for a MEMS structure having a suspended member. In an embodiment, substrate  300  is comprised of group IV-based materials such as crystalline silicon, germanium or silicon/germanium. In another embodiment, substrate  300  is comprised of a III-V material. Substrate  300  may also comprise an insulating layer. In one embodiment, the insulating layer is comprised of a material selected from the group consisting of silicon dioxide, silicon nitride, silicon oxy-nitride and a high-k dielectric layer. Substrate  300  may be an insulator. In one embodiment, substrate  302  consists of glass, quartz or sapphire. Substrate  300  may house a fabricated integrated circuit. For example, in accordance with an embodiment of the present invention, substrate  300  comprises an insulator layer above a plurality of interconnect structures connecting a plurality of micro-electronic devices. In one embodiment, the plurality of micro-electronic devices is a plurality of N-type and P-type transistors and the plurality of interconnect structures is a plurality of metal interconnects that tie the plurality of N-type and P-type transistors into an integrated circuit. Substrate  300  may comprise a top isolation stack comprised of any material suitable to electrically isolate resonating member  304  of MEMS structure  302  from a plurality of interconnects housed in substrate  300 . For example, the isolation stack may be comprised of an insulating top layer. In one embodiment, the insulating top layer is comprised of a material selected from the group consisting of silicon dioxide, silicon nitride, silicon oxy-nitride and a high-k dielectric layer. 
     A dynamic mass-load that alters the moment of inertia of a resonating member may do so by deformation of a two-component arm in response to a TCE mismatch.  FIGS. 4A-B  illustrate cross-sectional views representing the operation of a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 4A , a dynamic mass-load  408  is comprised of a first material  420  and a second material  422 , each having a different TCE. Referring to  FIGS. 4A and 4B , the two materials are arranged such that the dynamic mass-load deforms in response to a change in temperature—a consequence of the TCE mismatch. In one embodiment, the dynamic mass-load deforms to shift the moment of inertia of resonating member  404 , thus altering the effective mass of resonating member  404 . The dynamic mass-load may be comprised of any suitable pairing of TCE mismatched materials that alters the moment of inertia of resonating member  404  by an amount sufficient to compensate for a change in resonant frequency of resonating member  404  that results from a change in temperature. For example, the displacement of a dynamic mass-load comprised of a two-component arm may be represented by equation (1). 
     
       
         
           
             
               
                 
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     Referring to equation (1), d is the displacement distance, k is a constant of proportionality that depends on the Young&#39;s moduli of the two materials and is ˜0.2 in many cases, L is the length of the two-component arm and h is the thickness of the two-component arm. The expression (T−T 0 ) represents a change in temperature of dynamic mass-load  408  and, hence, of resonating member  404 . The expression (α material #1 −α material #2 ) represents the TCE mismatch between first material  420  and second material  422 . In one embodiment, first material  420  is comprised of silicon/germanium and second material  422  is comprised of silicon dioxide. Thus, since α SiGe =4.52×10 −6 /° C. and α SiO2 =0.5×10 −6 /° C., a 30 μm long two-component arm will have a displacement of approximately 0.07 μm over a 100° C. temperature range. 
     The deformation of a two-component dynamic mass-load may compensate for a change in resonant frequency of a resonating member. For example, in one embodiment, first material  420  is comprised of silicon/germanium, second material  422  is comprised of silicon dioxide, resonating member  408  is approximately 82 μm in length, 1.8 μm in width and 2 μm in height, the two-component arm is approximately 35.4 μm in length, and second material is 0.6 μm wide. The frequency variation of resonating member  408  is compensated (i.e. mitigated) to approximately 50 ppm over a 120° C. temperature range. In the absence of a compensating dynamic mass-load, resonating member  408  has a frequency variation in the range of 1000-5000 ppm over the same temperature range. 
     Thus, the frequency of a resonating member may be compensated by a dynamic mass-load. In accordance with an embodiment of the present invention, the resonant frequency of a resonating member decreases with increasing temperature. A dynamic mass-load compensates for the decreasing frequency by reducing the effective mass of the resonating member in response to the same temperature change, i.e. by moving the moment of inertia closer to an anchor of the resonating member. In an alternative embodiment, the resonant frequency of a resonating member increases with increasing temperature. A dynamic mass-load compensates for the increasing frequency by increasing the effective mass of the resonating member in response to the same temperature change, i.e. by moving the moment of inertia further from an anchor of the resonating member. It is to be understood that the present invention is not limited to a dynamic mass-load that responds to a change in temperature. In accordance with an embodiment of the present invention, the dynamic mass-load responds to an environmental change selected from the group consisting of pressure, light, electrical and chemical. Furthermore, it is to be understood that a dynamic mass-load may be positioned relative to a resonating member in any location suitable for the desired compensating performance of the dynamic mass-load. In another embodiment, the performance of a dynamic mass-load is optimized by selecting an appropriate mass for the mass-units of the dynamic mass-load. 
     A MEMS structure having a resonating member coupled with a dynamic mass-load may be fabricated by any technique suitable to pair materials having different TCEs. In one embodiment, for optimal integration of the MEMS structure with a pre-fabricated CMOS circuit, all process steps used to form the MEMS structure are carried out at a temperature less than approximately 450° C. In a specific embodiment, a damascene process is used to fabricate a MEMS structure having a resonating member coupled with a dynamic mass-load.  FIGS. 5A-F  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 5A , a stacked structure is provided comprising a substrate  500  having a release layer  540  and a first structural layer  550 . Substrate  500  and first structural layer  550  may be comprised of any material and have any feature described in association with substrate  300  and resonating member  304  described in association with  FIG. 3 . 
     First structural layer  550  may be deposited by any technique suitable to generate a uniform material layer of consistent composition. In accordance with an embodiment of the present invention, first structural layer  550  is deposited by a process selected from the group consisting of chemical vapor deposition, physical vapor deposition, atomic layer deposition, electroplating and electro-less plating deposition. In a specific embodiment, first structural layer is deposited by a low-pressure CVD process that utilizes the precursor gases SiH 4  and GeH 4  at a combined pressure in the range of 200-1000 mTorr. In one embodiment, first structural layer  550  is formed by first blanket depositing a material layer above release layer  340  and subsequently planarizing the material layer. First structural layer  550  may be comprised of a material that is formed by a low temperature process. Thus, in accordance with another embodiment of the present invention, first structural layer  550  is comprised of a material formed at a temperature less than approximately 450° C. 
     Release layer  540  may be comprised of any material suitable to withstand a MEMS fabrication process. For example, in accordance with an embodiment of the present invention, release layer  540  is comprised of a material selected from the group consisting of an insulator and a semiconductor. In one embodiment, release layer  540  is an insulating material and is comprised of a material selected from the group consisting of silicon dioxide, silicon nitride, silicon oxy-nitride and a high-K dielectric material. In one embodiment, release layer  540  is a semiconducting material and is selected from the group consisting of silicon, germanium, silicon/germanium, carbon-doped silicon, carbon-doped silicon/germanium and a III-V material. The semiconducting material may also be comprised of dopant impurity atoms. For example, in one embodiment, the semiconducting material is substantially comprised of germanium atoms and the concentration of dopant impurity atoms is selected to optimize the germanium nucleation at a temperature in the range of 300-400° C. In a specific embodiment, release layer  540  is comprised of greater than 98% germanium atoms along with boron dopant impurity atoms having a total atomic concentration in the range of 5×10 19 -5×10 20  atoms/cm 3 . Release layer  540  may be comprised of any material that can be removed with high selectivity to first structural layer  550 . For example, in accordance with an embodiment of the present invention, first structural layer  550  is comprised of silicon/germanium and release layer  540  substantially comprises germanium. In a specific embodiment, both the silicon/germanium structural layer and the germanium release layer are doped with boron dopant impurity atoms. In another embodiment, first structural layer  550  is comprised of silicon/germanium and release layer  540  substantially comprises silicon. 
     Release layer  540  may be formed on substrate  500  by any suitable deposition process that generates a uniform material layer of consistent composition and is conformal to any patterning already present on the surface of substrate  500 . For example, in accordance with an embodiment of the present invention, release layer  540  is deposited by a process selected from the group consisting of chemical vapor deposition, physical vapor deposition and atomic layer deposition. Release layer  540  may be deposited by a low temperature deposition process. In a specific embodiment, substrate  500  is comprised of an insulator layer above an integrated circuit and release layer  540  is deposited by a low-pressure chemical vapor deposition process at a temperatures less than 450° C. The thickness of release layer  540  may be any thickness suitable to provide a suspension height necessary for a desired MEMS application. For example, in accordance with an embodiment of the present invention, the thickness of release layer  540  determines the height at which a MEMS resonating member is subsequently suspended above substrate  500 . In one embodiment, the thickness of release layer  540  is in the range of 0.1-5 microns. 
     Referring to  FIG. 5B , trenches  560  are formed in first structural layer  550 . In accordance with an embodiment of the present invention, release layer  540  acts as an etch stop during the patterning of first structural layer  550  to form trenches  560 . First structural layer  550  may be patterned by any lithographic/etch process suitable to provide an appropriately sized trench  560  and suitable to not etch a significant portion of release layer  540 . For example, in accordance with an embodiment of the present invention, first structural layer  550  is patterned by first patterning a positive photo-resist layer above first structural layer  550  by exposure to a wavelength of light selected from the group consisting of 248 nm, 193 nm and 157 nm. In another embodiment, an e-beam direct-write process is used to pattern the positive photo-resist layer. An etch process may then be used to pattern first structural layer  550  with selectivity to release layer  540 . In one embodiment, a dry etch process is used to etch first structural layer  550  with a selectivity to release layer  540  of at least 10:1. In a particular embodiment, first structural layer  550  is comprised substantially of silicon/germanium, release layer  540  is comprised substantially of germanium, and the dry etch process comprises an anisotropic plasma etch process wherein the plasma is generated from gases selected from the group consisting of SF 6  and the combination of HBr, Cl 2  and O 2 . In one embodiment, although not depicted, a hard-mask layer is utilized in between the positive photo-resist layer and first structural layer  550 . 
     Referring to  FIG. 5C , trenches  560  are filled with a second structural layer  570 . Second structural layer  570  may be formed from any material suitable to withstand a MEMS fabrication process, to substantially fill trenches  560 , and to satisfy the required characteristics of a dynamic mass-load. For example, in accordance with an embodiment of the present invention, second structural layer  570  is comprised of a material selected from the group consisting of an insulator, a semiconductor and a conductor. In one embodiment, second structural layer  570  is comprised of any material described in association with second material  322  from  FIG. 3 . In a specific embodiment, the first/second structural layer pairing  550 / 570  may be any TCE-mismatch pairing described in association with the first/second material pairing  320 / 322  from  FIG. 3 . Second structural layer  570  may be deposited by any technique described for the deposition of first structural layer  550 . Thus, in accordance with an embodiment of the present invention, second structural layer  570  is comprised of a material formed at a temperature less than approximately 450° C. 
     Referring to  FIG. 5D , the portion of second structural layer  570  not in trenches  560  is removed. Thus, regions of second material  522  are formed. Removal of this portion of second structural layer  570  may be completed by using any suitable removal technique that has high selectivity to first structural layer  550 . For example, in accordance with an embodiment of the present invention, the portion of second structural layer  570  not in trenches  560  is removed by a dry etch process wherein first structural layer  550  is used as an etch stop layer. In one embodiment, second structural layer  570  is an insulator layer and the etch gases used to generate a plasma for the dry etch process are selected from the group consisting of CF 4  or the combination Cl 2 , HBr, O 2  and BCl 3 . In accordance with another embodiment of the present invention, second structural layer  570  is planarized with a chemical-mechanical process step with an end-point determined by exposure of the top surface of first structural layer  550 . In one embodiment, the portion of second structural layer  570  not in trenches  560  is removed by polishing with a slurry comprised of species selected from the group consisting of silica beads, ammonium hydroxide and water. 
     Referring to  FIG. 5E , first structural layer  550  is patterned to expose the top surface of release layer  540 . Thus, features for a MEMS resonator including regions of first material  520  directly adjacent regions of second material  522  are provided. First structural layer  550  may be patterned by any suitable patterning process that provides the required dimensions of a MEMS resonator. In one embodiment, First structural layer  550  is patterned by any lithographic/etch process described in association with the patterning of trenches  560  from  FIG. 5B . 
     Referring to  FIG. 5F , release layer  540  is removed to provide a MEMS structure  502  having a resonating member  504  and a base  506 . Resonating member  504  is coupled with a dynamic mass-load  508  and is suspended between a drive electrode  512  and a sensor electrode  514 . Dynamic mass-load  508  is comprised of a pair of mass units  516  and a two-component arm  518  comprised of first material  520  and second material  522 . Release layer  540  may be removed by any technique that enables removal without significantly impacting first and second materials  520  and  522 . For example, in accordance with an embodiment of the present invention, first material  520  comprises silicon/germanium, second material  522  comprises silicon dioxide, and release layer  540  substantially comprises germanium and is removed by an oxidizing etchant. In a specific embodiment, release layer  540  is comprised of germanium with an atomic concentration of greater than 98% germanium atoms and a wet etchant comprising an aqueous solution of H 2 O 2  with a concentration in the range of 25-35% by volume and a temperature in the range of 80-95° C. is used to remove release layer  540 . In a particular embodiment, release layer  540  is further comprised of boron dopant impurity atoms with a concentration in the range of 5×10 19 -5×10 20  atoms/cm 3 . In accordance with another embodiment of the present invention, first material  520  comprises silicon/germanium, second material  522  comprises silicon dioxide, and release layer  540  substantially comprises silicon and is removed by a high pH etchant. In a specific embodiment, release layer  540  is comprised of silicon with an atomic concentration of greater than 99% silicon atoms and a wet etchant comprising an aqueous solution of NH 4 OH with a concentration in the range of 1-20% by volume and a temperature in the range of 15-40° C. is used to remove release layer  540 . In a particular embodiment, release layer  540  is further comprised of phosphorus and/or arsenic dopant impurity atoms with a concentration in the range of 5×10 18 -1×10 20  atoms/cm 3 . In an embodiment, release layer  540  is removed with a selectivity greater than 20:1 over first and second materials  520  and  522 . 
     Anchor  510  that couples base  506  with substrate  500  along with conductive couplers for drive electrode  512  and sensor electrode  514  are also exposed upon the removal of release layer  540 , as depicted in  FIG. 5F . These conductive couplers and anchor may be fabricated by any suitable technique that substantially aligns them with drive electrode  512 , sensor electrode  514  and resonating member  504 , respectively. For example, in accordance with an embodiment of the present invention, release layer  540  is first patterned with trenches that expose a portion of substrate  500  prior to the deposition of first structural layer  550 , i.e. prior to the process steps described in association with  FIGS. 5A-F , thus enabling formation of the conductive couplers and anchor by way of a preliminary damascene process. 
     Thus, as illustrated in  FIGS. 5A-F , the second material of a dynamic mass-load may be fabricated by a damascene process. Alternatively, the second material of a dynamic mass-load may be fabricated by a subtractive etch process.  FIGS. 6A-C  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with another embodiment of the present invention. 
     Referring to  FIG. 6A , a second structural layer  670  is formed above a release layer  640  on a substrate  600 , prior to the formation of a first structural layer. Second structural layer  670 , release layer  640  and substrate  600  may be comprised of any material and may be formed by any technique described in association with second structural layer  570 , release layer  540  and substrate  500 , respectively, from  FIGS. 5A and 5C . 
     Referring to  FIG. 6B , second structural layer  670  is patterned to provide regions of second material  622 . Second structural layer  670  may be patterned by any process suitable to remove portions of second structural layer  670  with high selectivity to release layer  640  and to provide the desired dimensions of regions of second material  622 . For example, in accordance with an embodiment of the present invention, second structural layer  670  is patterned by first patterning a positive photo-resist layer above second structural layer  670  by exposure to a wavelength of light selected from the group consisting of 248 nm, 193 nm and 157 nm. In another embodiment, an e-beam direct-write process is used to pattern the positive photo-resist layer. An etch process may then be used to pattern second structural layer  670  with selectivity to release layer  640 . In one embodiment, a dry etch process is used to etch second structural layer  670  with a selectivity to release layer  640  of at least 10:1. In a particular embodiment, second structural layer  670  is comprised of silicon dioxide, release layer  640  is comprised substantially of germanium, and the dry etch process comprises an anisotropic plasma etch process wherein the plasma is generated from gases selected from the group consisting of CF 4  or the combination Cl 2 , HBr, O 2  and BCl 3 . 
     Referring to  FIG. 6C , a first structural layer  650  is formed above regions of second material  622  and above the top surface of release layer  640 . First structural layer  650  may be deposited by any process that provides a conformal layer over regions of second material  622 . In accordance with an embodiment of the present invention, first structural layer  650  is comprised of any material and is deposited by any technique described in association with first structural layer  550  from  FIG. 5A . Following the deposition, a portion of first structural layer  650  may be removed to provide a stack having a two-component structural layer above a release layer, similar to the stack depicted in  FIG. 5D . Removal of this portion of first structural layer  650  may be carried out using any suitable removal technique that planarizes first structural layer  650  with high selectivity to regions of second material  622 . For example, in accordance with one embodiment of the present invention, first structural layer  650  is planarized with a chemical-mechanical process step having an end-point determined by exposure of the top surface of regions of second material  622 . In one embodiment, first structural layer  650  is planarized by polishing with a slurry comprised of species selected from the group consisting of silica beads, ammonium hydroxide and water. Following planarization of first structural layer  650 , process steps described in association with  FIGS. 5E and 5F  may be employed to provide a MEMS structure having a resonating member coupled with a dynamic mass-load, wherein the dynamic mass-load is comprised of materials having different TCEs. 
     Thus, as illustrated in  FIGS. 6A-C , the second material of a dynamic mass-load may be fabricated by a subtractive process. Alternatively, the second material of a dynamic mass-load may be provided in the form of a ribbon fabricated by a spacer process.  FIGS. 7A-E  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an alternative embodiment of the present invention. 
     Referring to  FIG. 7A , a first structural layer  750  is formed above a release layer  740  on a substrate  700 . First structural layer  750 , release layer  740  and substrate  700  may be comprised of any material and may be formed by any technique described in association with first structural layer  550 , release layer  540  and substrate  500 , respectively, from  FIGS. 5A and 5C . First structural layer  750  is patterned to form trenches  760 , which expose the top surface of release layer  740 , as depicted in  FIG. 7A . Trenches  760  may be formed by any process described in association with the formation of trenches  560  from  FIG. 5B . 
     Referring to  FIG. 7B , a second structural layer  770  is formed above first structural layer  750  and conformally in trenches  760 . Second structural layer  770  may be comprised of any material and may be formed by any technique described in association with second structural layer  570  from  FIG. 5C . As a spacer etch approach is subsequently used to form a second material region for a dynamic mass-load, the thickness of second structural layer  770  may be substantially equivalent to the desired width of the ribbon subsequently formed. That is, in accordance with an embodiment of the present invention, the width of the portion of second structural layer  770  on the sidewalls of trenches  760  is substantially the same as the thickness of second structural layer  770 . In one embodiment, the thickness of second structural layer  770  is in the range of 500-2000 nanometers. 
     Referring to  FIG. 7C , second structural layer  770  is anisotropically patterned to provide a ribbon  722  comprised of the material of second structural layer  770 . A portion of release layer  740  is also exposed. Thus, in accordance with an embodiment of the present invention, a spacer etch process is used to provide a ribbon of structural material different from first structural layer  750 . Ribbon  722  may be formed from second structural layer  770  by any anisotropic process that removes surface portions of second structural layer  770  selective to first structural layer  750 . Thus, in accordance with an embodiment of the present invention, ribbon  722  is formed during a plasma etch process step. In one embodiment, second structural layer  770  is comprised of silicon dioxide and the gases used to generate the plasma are selected from the group consisting of CF 4  and the combination Cl 2 , HBr, O 2  and BCl 3 . In a specific embodiment, an end point for the etch process is detected when the surface of at least one of first structural layer  750  or release layer  740  is exposed. 
     Referring to  FIG. 7D , first structural layer  750  is patterned to expose the top surface of release layer  740  and to provide features for a MEMS resonator including regions of first material  720  directly adjacent to ribbons  722 . First structural layer  750  may be patterned by any process described in association with the patterning of first structural layer  550  from  FIG. 5B . Referring to  FIG. 7E , release layer  740  is removed with high selectivity to the patterned structural layer having regions of first material  720 , to ribbon  722  and to substrate  700 . Release layer  740  may be removed by any technique described in association with the removal of release layer  540  from  FIG. 5F . Thus, a MEMS structure having a resonating member coupled with a dynamic mass-load, wherein the dynamic mass-load is comprised of a first material and a ribbon having different TCEs, may be provided. The shape of ribbon  722  may include a spring extension  780 . In one embodiment, spring extension  780  is an artifact of the patterning process used to form ribbon  722 . In a specific embodiment, spring extension  780  minimizes the restoring stiffness of ribbon  722 . 
     Thus, as illustrated in  FIGS. 7A-E , a second material of a dynamic mass-load may be incorporated by a spacer process to provide a ribbon comprised of the second material. Alternatively, the features of a MEMS structure may be patterned prior to the formation of the second material component of a dynamic mass-load.  FIGS. 8A-C  illustrate cross-sectional views representing a series of steps for fabricating a MEMS structure having a compensated resonating member, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 8A , a first structural layer  850  is patterned to provide the features of a MEMS structure above a release layer  840  and substrate  800 . In one embodiment, first structural layer  850 , release layer  840  and substrate  800  are comprised of any material and are formed by any technique described in association with first structural layer  550 , release layer  540  and substrate  500 , respectively, from  FIG. 5A . Referring to  FIG. 8B , a second structural layer  870  is deposited conformally above the pattern formed in first structural layer  850  and the exposed surfaces of release layer  840 . In one embodiment, second structural layer  870  is comprised of any material and is formed by any technique described in association with second structural layer  570  from  FIG. 5C . Referring to  FIG. 8C , second structural layer  870  is planarized to expose the top surface of patterned first structural layer  850 . In one embodiment, second structural layer is planarized by any technique described in association with the planarization of second structural layer  570  from  FIG. 5D . Following planarization of second structural layer  870 , process steps described in association with  FIGS. 5E and 5F  may be employed to provide a MEMS structure having a resonating member coupled with a dynamic mass-load, wherein the dynamic mass-load is comprised of materials having different TCEs. That is, in accordance with an embodiment of the present invention, second structural layer  870  from the stack illustrated in  FIG. 8C  is patterned to provide a stack substantially similar to the stack illustrated in  FIG. 5E . In one embodiment, second structural layer  870  is patterned by any patterning process described in association with the patterning of second structural layer  670  from  FIGS. 6A and 6B . 
     The present invention is not limited to a MEMS structure having a single anchor point or a MEMS structure having only one dynamic mass-load. For example, a MEMS resonator may be fabricated wherein a resonating member is centrally anchored and two dynamic mass-loads are utilized to alter the effective mass of the resonating member in response to an environmental change.  FIG. 9  illustrates a cross-sectional view representing a MEMS structure having a centrally-anchored compensated resonating member, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 9 , a MEMS structure  902  comprises a resonating member  904  and a centrally located base  906 . Resonating member  904  is coupled with two dynamic mass-loads  908  and centrally located base  906  is coupled with substrate  900  by an anchor  910 . Resonating member  904  is suspended between a drive electrode  912  and a sensor electrode  914 . Dynamic mass-loads  908  each comprise a pair of mass units  916  and a two-component arm  918  comprised of a first material  920  and a second material  922 . In accordance with an embodiment of the present invention, the two dynamic mass-loads  908  move toward one another to reduce the effective mass of resonating member  904  in response to an increase in temperature, as depicted by the arrows in  FIG. 9 . 
     The present invention is also not limited to two-component dynamic mass-loads. For example, a dynamic mass-load may be fabricated comprising a three-component arm.  FIG. 10  illustrates a cross-sectional view representing a MEMS structure having a compensated resonating member and a dynamic mass-load comprising more than two materials with mismatched TCEs, in accordance with an embodiment of the present invention. 
     Referring to  FIG. 10 , a MEMS structure  1002  comprises a resonating member  1004  and a base  1006 . Resonating member  1004  is coupled with a dynamic mass-load  1008  and base  1006  is coupled with substrate  1000  by an anchor  1010 . Resonating member  1004  is suspended between a drive electrode  1012  and a sensor electrode  1014 . Dynamic mass-load  1008  comprises a pair of mass units  1016  and a three-component arm  1018  having a first material  1020 , a second material  1022 , and a third material  1090 . In accordance with an embodiment of the present invention, the TCE of third material  1090  is greater than the TCE of first material  1020  which is greater than the TCE of second material  1022 . Thus, dynamic mass-load  1008  moves toward anchor  1010  to reduce the effective mass of resonating member  1004  in response to an increase in temperature, as depicted by the arrows in  FIG. 10 . Third material  1090  need not extend the entire length of first material  1020 , as depicted in  FIG. 10 . 
     Thus, a MEMS structure having a compensated resonating member has been disclosed. In an embodiment, a MEMS structure comprises a resonating member coupled to a substrate by an anchor. A dynamic mass-load is coupled with the resonating member. The dynamic mass-load is provided for compensating a change in frequency of the resonating member by altering the moment of inertia of the resonating member by way of a positional change relative to the anchor. In one embodiment, the dynamic mass-load compensates the resonating member in response to an environmental change selected from the group consisting of temperature, pressure, light, electrical and chemical. In a specific embodiment, all process steps used to form the MEMS resonator are carried out at a temperature less than approximately 450° C.