Abstract:
Embodiments of the invention are related to MEMS devices and methods. In one embodiment, a MEMS device includes a resonator element comprising a magnetic portion having a fixed magnetization, and at least one sensor element comprising a magnetoresistive portion, wherein a magnetization and a resistivity of the magnetoresistive portion vary according to a proximity of the magnetic portion.

Description:
BACKGROUND 
     Silicon micro-electromechanical system (MEMS) resonators can have frequency reference applications, commonly using capacitive detection and actuation principles. For example, small electrodes positioned between the resonator and the support structure can capacitively detect relative gap changes as the resonator resonates. The resonance frequency can then be derived. 
     There are several disadvantages, however, to such configurations. One disadvantage is that the input and output impedances are high. Additionally, parasitic capacitances have an increasing influence as structure sizes decrease. 
     SUMMARY 
     Embodiments of the invention are related to MEMS devices and methods. In one embodiment, a MEMS device includes a resonator element comprising a magnetic portion having a fixed magnetization, and at least one sensor element comprising a magnetoresistive portion, wherein a magnetization and a resistivity of the magnetoresistive portion vary according to a proximity of the magnetic portion. 
     In another embodiment, a silicon MEMS device includes a support structure defining a cavity, a resonator element disposed in the cavity and comprising a magnetic portion having a fixed magnetization, at least one magnetoresistive portion disposed in the support structure and having a magnetization configured to vary according to a proximity of the magnetic portion, and circuitry coupled to the at least one magnetoresistive portion and adapted to sense a resistance of the magnetoresistive portion. 
     In yet another embodiment, a method includes measuring a resistance of a magnetoresistive portion of a resonator device, and determining a resonant frequency of a resonator element of the resonator device from the resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood from the following detailed description of various embodiments in connection with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a resonating device according to one embodiment. 
         FIG. 2A  is a model of the resonating device of  FIG. 1 . 
         FIG. 2B  is a model of a system of the resonating device of  FIGS. 1 and 2A . 
         FIG. 3A  is a cross-sectional diagram of a resonating device according to one embodiment. 
         FIG. 3B  is a top view cross-sectional diagram of the resonating device of  FIG. 3A . 
         FIG. 4  is a diagram of a resonator element in various resonance positions according to one embodiment. 
         FIG. 5  is a cross-sectional diagram of a resonating device according to one 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 of the invention relate to micro-electromechanical system (MEMS) technology, such as silicon resonator devices which comprise xMR sensor structures. Various embodiments of the invention can be more readily understood by reference to  FIGS. 1-5  and the following description. While the invention is not necessarily limited to the specifically depicted application(s), the invention will be better appreciated using a discussion of exemplary embodiments in specific contexts. 
     In  FIG. 1 , one embodiment of a silicon MEMS resonator device  100  is depicted. Resonator device  100  is formed on a substrate  102 , and an insulating layer  104 , such as silicon dioxide in one embodiment, is formed on substrate  102 . An electrode  106  is formed on layer  104 . 
     Resonator device  100  further comprises a resonating element  108  coupled to a top sealing portion  110  by an anchor  112 . Resonating element  108  is separated from electrode  106  by a cavity  114 . Resonating element  108  moves or resonates in the x-y plane, generating a reference frequency. Resonators can be designed such that they resonate in other or additional planes, as in other embodiments. 
     Resonating element  108  and electrode  106  are separated by a distance d. Resonating element  108  and electrode  106  can thus be approximated as a parallel plate capacitor having a capacitance (C). 
             C   =       ɛ   ⁢           ⁢   A       d   -   x             
In the above equation, ∈ is the dielectric constant, A is the electrode area, d is the gap distance between the two plates, and x is the movement in operation.
 
     In operation, a combination of an AC voltage and a DC voltage can be applied to the electrode:
 
 U=u   dc   +u   ac  sin(ω t )
 
This voltage forces the resonator to move. The resonator itself can be described as an actuated spring-mass-damper system, e.g. a harmonic, damped, forced oscillator:
 
             F   =       m   ⁢       ∂           2     ⁢   x         ∂     t   2           +     c   ⁢       ∂   x       ∂   t         +   kx           
where F is the actuating force, m is the mass of the resonator, c is the damping coefficient, k is the stiffness of the resonator, and x is its deflection. The force is now an electrostatic force and can be written as follows:
 
     
       
         
           
             F 
             = 
             
               
                 1 
                 2 
               
               ⁢ 
               
                 U 
                 2 
               
               ⁢ 
               
                 
                   ∂ 
                   C 
                 
                 
                   ∂ 
                   x 
                 
               
             
           
         
       
     
     If the frequency of the actuating AC voltage is now the same as the natural frequency of the resonator, the resonator will begin to move. This movement causes a current to flow through the electrode-resonator system, which can then be provided as an input for an amplifier. 
     From the above, resonator devices can be electrically modelled with a Butterworth-van-Dyke model, as depicted in  FIG. 2A . The series resonant frequency f s  of resonating element  108  is defined by the motional inductance (L m ) and capacitance (C m ) and in mechanical terms by the stiffness (k) and the mass (m) of resonating element  108 . 
               f   S     =         1       2   ⁢           ⁢   π     ⁢               ⁢     1         L   m     ⁢     C   m             =       1       2   ⁢           ⁢   π     ⁢               ⁢       k   m                 
The resonant frequency f s  is therefore not dependant on the area of electrodes  106  and resonating element  108 .
 
     In series resonance, the total impedance of the system is related to the motional resistance (R m ) of resonating element  108 . It can be described by: 
               R   m     =       km       Q   ⁢           ⁢     η   2               
where Q is the quality factor of the resonator and η is the so-called electromechanical coupling, which is related to an applied DC-bias voltage (u dc ), the distance between electrode  106  and resonating element  108  ( d ) as well as the static capacitance in between (C 0 ).
 
             η   =       u   dc     ⁢       C   0     d             
Knowing that the static capacitance C 0  is the capacitance of a parallel plate capacitor and that there is no dielectric in the gap, the electromechanical coupling can be expressed as:
 
             η   =       u   dc     ⁢     ɛ   0     ⁢       A   el       d   2               
The motional resistance which governs the impedance in series resonance then becomes:
 
     
       
         
           
             
               R 
               m 
             
             = 
             
               
                 
                   km 
                 
                 
                   Q 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     η 
                     2 
                   
                 
               
               = 
               
                 
                   
                     km 
                   
                   Q 
                 
                 ⁢ 
                 
                   
                     d 
                     4 
                   
                   
                     
                       A 
                       el 
                       2 
                     
                     ⁢ 
                     
                       u 
                       dc 
                       2 
                     
                     ⁢ 
                     
                       ɛ 
                       0 
                       2 
                     
                   
                 
               
             
           
         
       
     
     In the case of series resonance, the motional inductance and the motional capacitance cancel each other out, and the impedance seen at the input and output of the resonator is related to the motional resistance. In order to have a stable resonating system, an amplifier connected to the resonator should compensate for this motional resistance, i.e., the amplifier should have a negative resistance equal to the motional resistance in operation and higher for start-up. 
     Referring to  FIG. 2B , a system model of resonator  100  and an amplifier  120  is depicted. The input of amplifier  120  is the output of resonator  100 , and vice-versa. Thus, the current flowing through resonator  100  is the input for amplifier  120  and “generates” a sinusoidal voltage drop at its output. This voltage drop can now be used to drive resonator  100 , while also serving as a sinusoidal frequency reference signal. 
     According to various embodiments of the invention, the resonator will further comprise a magnetic material in or on the resonating element to provide a defined stray magnetic field. Magnetoresistive effect, or xMR, sensors can then be positioned in or on the resonator support structure in close distance to the magnetic field. Relative movements of the resonator element can then produce a change in the magnetic field, and thus also lead to changes that can be sensed by the xMR sensors. A rotation of a sense layer of the xMR sensor will lead to a change of the magnetoresistance, which can be detected and quantified such that a resonant frequency of the resonator element can be determined. 
     Referring to  FIGS. 3A and 3B , one embodiment of a resonator device  200  is depicted. Resonator device  200  comprises a support structure  202  and a resonator element  204 . Support structure  202  defines part or all of a cavity  205  in which resonator element  204  is partially or fully disposed. Although not shown in  FIGS. 3A and 3B , support structure  202  can comprise a lower portion, such as is depicted in  FIG. 1 , formed on a substrate. Other structures and configurations can be used in other embodiments. In one embodiment, resonator element  204  is coupled to support structure  202  by an anchor (not shown) that permits movement in resonance of resonator element  204 . A variety of anchoring structures, positions and configurations can be used in various embodiments, as appreciated by one skilled in the art. In the embodiment of  FIGS. 3A and 3B , resonance of resonator element  204  is in the x-y plane, although resonator device  200  can be configured such that resonator element  204  resonates in other or additional planes in other embodiments. 
     In one embodiment, a magnetic dipole element  206  is formed on resonator element  204 . In the embodiment of  FIGS. 3A and 3B , magnetic dipole element  206  is formed along a length and a central portion of a width of resonator element  204 . In other embodiments, magnetic dipole element  206  can comprise alternate structures and configurations, such as extending along only a portion of the length and/or over a narrower, wider, or entire width of resonator element  202 . Magnetic dipole element  206  comprises a hard magnetic material, such as a thin film or particle, in one embodiment and has a fixed magnetization direction, as shown by the solid arrow in each of  FIGS. 3A and 3B . This fixed magnetization can be in an alternate direction, such as right to left, in other embodiments. 
     Resonator device  200  further comprises sensor elements  208  formed in or on support structure  202 . In the embodiment of  FIGS. 3A and 3B , a first sensor element  208  is disposed in a left-side portion of support structure  202  and a second sensor element  208  is disposed in a right-side portion of support structure  202  (relative to the orientation depicted in  FIGS. 3A and 3B ). In other embodiments, such as an embodiment described herein below with respect to  FIG. 5 , sensor elements  208  can be disposed in other positions in or on support structure  202  or relative to resonator element  204 . In general, however, sensor elements  208  are positioned such that they can sense a magnetic field related to magnetic dipole element  206 . 
     In one embodiment, sensor elements  208  comprise magnetoresistive effect elements or portions, referred to as xMR elements  210 , in a sense layer  212 . xMR elements  210  include tunneling magnetoresistive (TMR), giant magnetoresistive (GMR), anisotropic magnetoresistive (AMR), and others. In one embodiment, magnetic dipole element  206  and sensor elements  208  are formed of the same material or materials, such as a spinvalve TMR stack, for example. 
     Magnetic dipole element  206  and sensor elements  208  provide resonance detection based on resistance, as opposed to capacitance as in other MEMS resonator structures. The magnetization of magnetic dipole element  206  is fixed according to magnetostatic laws due to the large aspect ratio between the width and the length of magnetic dipole element  206  on resonator element  204 , while the magnetization of sensor elements  208 , in particular xMR elements  210 , is easily altered by the stray field of magnetic dipole element  206  and varies because of and with the resonance of resonator element  204 . The change, or rotation, of the magnetization of xMR element  210  in sense layer  212  of sensor elements  208  results in a change of the magnetoresistance, R f , of sensor elements  208 , which can be detected. As shown in  FIGS. 3A and 3B , R f  can be measured at measurement points  214 , such as electrodes, contact points or terminals in various embodiments. Other measurement elements, circuits, positions and arrangements can be used in other embodiments. In embodiments comprising TMR elements, resistance changes on the order of about 50 to about 200 percent are possible. Hence, small magnetic field changes can be directly transferred into large resistance changes that can be easily measured. A resonance frequency of resonator element  204  can be determined from this change in resistance. Embodiments of the invention therefore provide more sensitive detection and allow for the fabrication of smaller MEMS resonator devices through the reduction or elimination of the problem of increasing parasitic capacitances that exists in standard silicon MEMS resonator devices. 
     The resistance of the left-side (again, relative to the orientation depicted in  FIGS. 3A and 3B ) sensor element  208  has a phase difference of 180 degrees as compared to the resistance of the right-side sensor element  208 , providing a differential readout arrangement. The electrical properties of sensor elements  208  can be designed independently of the properties of resonator element  204  and optimized for a desired feedback loop. 
     Referring to  FIG. 4 , resonator element  204  is depicted in three different example positions during resonance. Only one of two sensor elements  208  is depicted in  FIG. 4 , here the right-side element. Sensor element  208  includes xMR portion  210  in sense layer  212 , and illustrated within each xMR portion  210  are the reference magnetization (small arrows in dashed line) and the magnetization during resonance of resonator element  204  (small arrows in solid line). 
     At position (A), resonator element  204  is at a neutral center position, spaced apart from each sensor element  208  by a distance d 0 . In one embodiment, d 0  is less than about one micron. The resulting resistance of xMR sensor element  208  is therefore at neither a high nor a low position, as shown on the curve at the right. 
     At position (B), resonator element  204  has moved away from sensor element  208  by Δd, for a total separation distance of d 0 +Δd. In this position, the magnetization of xMR portion  210  is the same as the reference, or pre-aligned, magnetization, because the effects of magnetic dipole element  206  are minimized, i.e., the strength of the magnetic field of magnetic dipole element  206  at sensor element  208  is low. The resistance is therefore relatively low. 
     At position (C), resonator element  204  has moved closer to sensor element  208 , resulting in a separation distance of d 0 −Δd. In this position, the magnetization of xMR portion  210  opposes the reference magnetization because of the effects of now-closer magnetic dipole element  206 , i.e., the strength of the magnetic field of magnetic dipole element  206  at sensor element  208  is high. The resistance is therefore relatively high. 
     As previously mentioned, only the right-side sensor element  208  is depicted in  FIG. 4 . The effects on the left-side sensor element  208  at positions (B) and (C) would thus be the opposite of those described above, i.e., 180 degrees out of phase, as resonator element  204  resonates in the space between the two sensor elements  208 . 
       FIG. 5  depicts another resonator device  300  according to one embodiment. Resonator device  300  comprises a support structure  302 , which can be similar to as described above for resonator device  200 , and a resonator element  304 . In one embodiment, resonator  304  comprises a magnetic dipole element  306  positioned on each end, and sensor elements  308  are positioned in or on support structure  302  above cavity  305  within which resonator element  304  resonates. Sensor elements  308  comprise xMR portions or elements  310 . Other features of resonator device  300 , such as further portions of support structure  302 , measurement points  314 , anchor configuration, materials, and the like can be the same as or similar to the various embodiments of resonator device  200  described above. 
     Thus, various embodiments of the resonator devices  200  and  300  depicted and described herein, provide stable and sensitive determination of resonant frequency through detection of changes in the resistance of xMR sensor elements. Embodiments of the invention therefore reduce or eliminate the aforementioned problems associated with other MEMS resonators. 
     Although specific embodiments have been illustrated and described herein for purposes of description of an example embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those skilled in the art will readily appreciate that the invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the various embodiments discussed herein, including the disclosure information in the attached appendices. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.