Abstract:
A micromechanical resonator device and a micromechanical device utilizing same are disclosed based upon a radially or laterally vibrating disk structure and capable of vibrating at frequencies well past the GHz range. The center of the disk is a nodal point, so when the disk resonator is supported at its center, anchor dissipation to the substrate is minimized, allowing this design to retain high-Q at high frequency. In addition, this design retains high stiffness at high frequencies and so maximizes dynamic range. Furthermore, the sidewall surface area of this disk resonator is often larger than that attainable in previous flexural-mode resonator designs, allowing this disk design to achieve a smaller series motional resistance than its counterparts when using capacitive (or electrostatic) transduction at a given frequency. Capacitive detection is not required in this design, and piezoelectric, magnetostrictive, etc. detection are also possible. The frequency and dynamic range attainable by this resonator makes it applicable to high-Q RF filtering and oscillator applications in a wide variety of communication systems. Its size also makes it particularly suited for portable, wireless applications, where, if used in large numbers, such a resonator can greatly lower the power consumption, increase robustness, and extend the range of application of high performance wireless transceivers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefits of U.S. provisional patent applications Serial No. 60/227,505 filed Aug. 24, 2000 and entitled “Micromechanical Disk Resonators” and Serial No. 60/227,507 also filed Aug. 24, 2000 and entitled “Process Technology For Lateral Small-Gap Micromechanical Structures.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] The invention was made with Government support under DARPA Contract No. F30602-97-2-0101. The Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention relates to micromechanical resonator devices and micromechanical devices utilizing same.  
           [0005]    2. Background Art  
           [0006]    Vibrating mechanical tank components, such as crystal and SAW resonators, are widely used for frequency selection in communication sub-systems because of their high quality factor (Q&#39;s in the tens of thousands) and exceptional stability against thermal variations and aging. In particular, the majority of heterodyning communication transceivers rely heavily upon the high-Q of SAW and bulk acoustic mechanical resonators to achieve adequate frequency selection in their RF and IF filtering stages and to realize the required low phase noise and stability in their local oscillators. In addition, discrete inductors and variable capacitors are used to properly tune and couple the front end sense and power amplifiers, and to implement widely tunable voltage-controlled oscillators. At present, the aforementioned resonators and discrete elements are off-chip components, and so must interface with integrated electronics at the board level, often consuming a sizable portion of the total sub-system area. In this respect, these devices pose an important bottleneck against the ultimate miniaturization and portability of wireless transceivers. For this reason, many research efforts have been focused upon strategies for either miniaturizing these components or eliminating the need for them altogether.  
           [0007]    Recent demonstrations of micro-scale high-Q oscillators and mechanical bandpass filters with area dimensions on the order of 30 μm×20 μm now bring the first of the above strategies closer to reality. Such devices utilize high-Q, on-chip, micromechanical (abbreviated “μmechanical”) resonators constructed in polycrystalline silicon using IC-compatible surface micromachining fabrication techniques, and featuring Q&#39;s of over 80,000 under vacuum and center frequency temperature coefficients in the range of −10 ppm/° C. (several times less with nulling techniques). To date, resonators based on freely-supported, vibrating prismatic beams have achieved frequencies of up to 92 MHz. For use in many portable communications applications, however, higher frequencies must be achieved and are thus important to the success of this technology.  
           [0008]    Much like the case for transistors, extending the frequency of μmechanical resonators generally entails scaling of resonator dimensions. Some of the previous VHF demonstrations with clamped-clamped boundary conditions actually used submicron dimensions to avoid Q-limiting anchor losses. Unfortunately, smaller size often coincides with smaller power handling and increased susceptibility to environmental effects, such as contamination or thermal fluctuations. Although recently demonstrated free-free beam μmechanical resonators have been able to achieve frequencies up to 92 MHz with Q&#39;s around 8,000 while avoiding submicron dimensions, as shown in U.S. Pat. No. 6,249,073, whether or not they can maintain their size and Q at UHF frequencies has yet to be seen.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the present invention is to provide a micromechanical disk resonator and micromechanical device utilizing same.  
           [0010]    In carrying out the above object and other objects of the present invention, a micromechanical resonator device having at least one mode shape is provided. The device includes a substrate and a disk-shaped resonator disposed above the substrate and having at least one nodal point.  
           [0011]    The device may include a support structure anchored to the substrate to support the resonator at the at least one nodal point above the substrate. Both the resonator and the support structure are dimensioned and positioned relative to one another so that the resonator is substantially isolated during vibration thereof. Energy losses to the substrate are substantially eliminated and the resonator device is a high-Q resonator device.  
           [0012]    The at least one mode shape may include a radial-contour mode shape and/or a flexural mode shape.  
           [0013]    The device preferably further includes a drive electrode structure formed on the substrate at a position to allow electrostatic excitation of the resonator so that the resonator is driven in the at least one mode shape and the resonator and the drive electrode structure may define a capacitive gap therebetween.  
           [0014]    The drive electrode structure may be disposed about a periphery of the resonator and the at least one mode shape may include a radial-contour mode shape.  
           [0015]    The capacitive gap is preferably a sub-micron, lateral, capacitive gap.  
           [0016]    The drive electrode structure may include a plurality of split electrodes.  
           [0017]    The device may have a single electrode which not only allows electrostatic excitation of the resonator, but also to sense output current based on motion of the resonator. Then the device has two terminals, one on the electrode, one on the resonator, and both are used for both driving and sensing.  
           [0018]    The at least one nodal point may correspond to a center of the resonator and the support structure may be a single anchor positioned at the center of the resonator.  
           [0019]    The device may further include a sense electrode structure formed on the substrate at a position to sense output current based on motion of the resonator.  
           [0020]    The drive electrode structure may include a plurality of separate input drive electrodes and the sense electrode structure may include a plurality of separate output sense electrodes.  
           [0021]    The drive electrode structure may be positioned beneath the resonator and the at least one mode shape may include a flexural mode shape.  
           [0022]    The device may be diamond-based or silicon-based.  
           [0023]    Further in carrying out the above object and other objects of the present invention, a micromechanical device is provided. The device includes a substrate, a disk-shaped input resonator disposed above the substrate and having at least one nodal point, and a disk-shaped output resonator disposed above the substrate and coupled to the input resonator and having at least one nodal point.  
           [0024]    The device may also include support structures anchored to the substrate to support the input and output resonators at their respective nodal points above the substrate.  
           [0025]    The micromechanical device may be a filter such as a bandpass filter or an integratable filter.  
           [0026]    The resonators may be mechanically coupled together or electrically coupled together.  
           [0027]    The device may further include a coupling spring for mechanically coupling the resonators together. The coupling spring can be an extension mode spring. Furthermore, the spring can also be flexural, or even combine two different types of modes (e.g., flexural or torsional).  
           [0028]    The device may further include a drive electrode structure formed on the substrate at a position to allow electrostatic excitation of the input resonator and a sense electrode structure formed on the substrate at a position to sense output current based on motion of the output resonator.  
           [0029]    The micromechanical disk resonators presented here have the potential to extend the frequency of micromechanical devices well into the GHz range, making them viable in all stages of wireless systems (including cellular phones) from the RF front-end down to IF filtering and mixing and enabling a completely integrated, single chip transceiver.  
           [0030]    Disk resonators of the present invention have advantages over the freely supported and clamped-clamped beams used in previous HF designs, including the capability to reach UHF frequencies using low numbered mode shapes, typically leading to improved Q. In addition, their larger size at a given frequency improves their power handling capacity, making them more appropriate for RF front-ends where dynamic range is an important parameter. This size also leads to a larger electromechanical coupling area, which improves the series resistance of electrostatically-driven devices. The larger size also makes these devices easier to manufacture repeatedly (i.e., with repeatable frequency, etc.). The larger size also makes disk resonators less susceptible to “scaling-induced” degradation mechanisms, in the manner of contamination or thermal fluctuations.  
           [0031]    This work presents an alternative, perhaps superior, design based on the radial contour mode, or areal dilation, of a μmechanical disk structure, and capable of attaining very high frequencies, while retaining relatively large dimensions. Using this disk design, a frequency of 156 MHz has been attained with a Q exceeding 9,400 and with a diameter of 34 μm—substantially larger than the 11.3 μm required to attain only 92 MHz in previous free-free beam resonators.  
           [0032]    The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 a  is a combined electrical and perspective schematic view of a preferred embodiment of a disk resonator constructed in accordance with the present invention;  
         [0034]    [0034]FIG. 1 b  is a cross-sectional view of the disk resonator of FIG. 1 a  taken along lines  1   b - 1   b  in FIG. 1 a;    
         [0035]    [0035]FIG. 2 is a graph of predicted resonant frequency versus radius for radial mode disk resonators in both polysilicon and polydiamond;  
         [0036]    [0036]FIG. 3 is a graph illustrating measured frequency spectrum of a 157 MHz disk resonator;  
         [0037]    [0037]FIG. 4 a  is a combined electrical and perspective schematic view of a mechanically-coupled, two resonator, bandpass filter of the present invention;  
         [0038]    [0038]FIG. 4 b  is a top plan schematic view, partially broken away, of a flexural-mode “U” coupler for coupling disk-shaped resonators together;  
         [0039]    [0039]FIG. 5 is a top plan schematic view of a disk-shaped resonator illustrating its node and anti-node portions;  
         [0040]    [0040]FIG. 6 is a side elevational view, partially broken away and in cross-section, of a disk-shaped resonator including fins and a number of electrodes for sensing output of the resonator at its outer rim and the anti-node portion;  
         [0041]    [0041]FIG. 7 is a top plan schematic view of another embodiment of the resonator device;  
         [0042]    [0042]FIG. 8 is a sectional view taken along lines  8 - 8  of FIG. 7 of the device wherein the disk-shaped resonator and its support are made at the same time with different material;  
         [0043]    [0043]FIG. 9 is a top plan schematic view of a bridged filter of the present invention;  
         [0044]    [0044]FIGS. 10 a - 10   e  are side schematic views illustrating micromechanical disk resonator fabrication; and  
         [0045]    [0045]FIGS. 11 a - 11   c  are graphs of displacement versus radial coordinates for first, second and third modes, respectively, of a disk-shaped resonator of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    [0046]FIGS. 1 a  and  1   b  present a preferred embodiment of a micromechanical resonator device of the present invention, generally indicated at  10 . In FIG. 1 a , the perspective view shows the major features of the preferred embodiment. The resonator device  10  includes a disk  12  supported above a ground plane  14  and supported by a single anchor post  16  at its center  18 . A drive electrode structure, generally indicated at  20 , surrounds the perimeter of the disk  12 , separated by a narrow air gap  22 . The electrode structure  20  is split in half to form two input electrodes  21  in order to allow routing to the ground plane  14  and maintain symmetry. In this configuration, the resonator disk  12  is designed to move in a purely radial mode, expanding and contracting along its radius at resonance ideally with no motion perpendicular to a substrate  24  having isolation and passivation layers  23  and  25  or rotation around the anchor point or center  18 . In this mode, the center  18  of the disk  12  is a nodal point, ideally devoid of any motion at resonance. Anchoring at the natural node, a very important part of this design, minimizes anchor dissipation to the substrate  24 , greatly enhancing Q.  
         [0047]    However, it is to be understood that the disk could alternatively be levitated either electrically or magnetically above the substrate wherein no physical or mechanical support is needed.  
         [0048]    In order to operate the device  10 , a DC bias voltage V P  is applied to the structure, while an AC input signal, V i , is applied to the electrodes  21 , resulting in a time varying electrostatic force acting radially on the disk  12 . When the input signal, and hence the force, is acting at the resonant frequency of the device  10 , the response to that force is multiplied by the Q factor of the resonator device  10  and produces expansion and contraction of the disk  12  along its radius. This motion in turn results in a time-varying, DC-biased capacitor and generates an output current which can be measured through a sense or output electrode or structure  26  attached to the structure.  
         [0049]    A cross-section of the device  10  is shown in FIG. 1 b  and more clearly illustrates the capacitive gap  22  and spacing above the ground plane  14  as well as the anchor  16 . Preferably, the structural material of the disk  12  is polysilicon, but the structural material may include crystalline silicon, polydiamond, silicon carbide, and metals.  
         [0050]    The current material of the drive electrode structure  20  is plated metal, but the invention includes other electrode materials as well.  
         [0051]    The gaps  22  are preferably submicron, high-aspect ratio, lateral electrode-to-resonator gaps. Although 1000 Å lateral gaps have been achieved previously in μmechanical structures using e-beam lithography, this approach was found to be both time-consuming and costly. In addition, if impedances on the order of 50 Ω are desired, 1000 Å gaps may not be sufficient, and even smaller gaps on the order of 300 Å may in fact be desired. To avoid the limitations of lithography and dry etching, the fabrication technology described in detail hereinbelow with reference to FIGS. 10 a - 10 - e  combines surface micromachining, metal electroplating, and a sacrificial sidewall spacer technique to achieve lateral polysilicon disk resonators with metal electrodes and submicron lateral electrode-to-resonator gaps, all without the need for aggressive lithographic or etching capability.  
         [0052]    In general, the initial process steps are identical to those used in previous polysilicon surface micromachining processes to achieve a cross-section where a 2 μm-thick structural polysilicon layer, patterned to form the disk structure, is temporarily supported by a 5000 Å first sacrificial oxide layer. For example, and referring now to FIG. 10 a , the process starts with a 2 μm thick oxide film  110  (i.e. SiO 2 ) thermally grown on a silicon substrate  112  and a 3000 Å thick film  114  of nitride (i.e. Si 3 N 4 ) which together serve as an isolation layer. After a 3000 Å thick polysilicon layer  116  is deposited via LPCVD, doped via implantation or diffusion, and patterned via reactive ion etching (RIE), a 5000 Å thick layer  118  of sacrificial oxide (i.e. SiO 2 ) is deposited by LPCVD.  
         [0053]    A via is patterned into the sacrificial oxide layer  118  by RIE, exposing the underlying polysilicon layer  116  in a specific area to later serve as the anchor  16  for the disk  12 . A 2 μm thick structural layer  120  of low stress polysilicon is then deposited via LPCVD, doped via implantation or diffusion, and patterned also via RIE to form the disk  12  with straight side walls. An oxide mask is formed above the structure and used as a hard mask during etching.  
         [0054]    Instead of releasing the structure at this point, as would be done in a conventional surface micromachining process, this process continues with the conformal deposition of 1000 Å of LPCVD oxide to cover the sidewalls of the structure and serve as the gap-defining sacrificial layer  122 . Next, the sacrificial oxide and the underlying oxide are etched (RIE and wet etch) to open anchors to the passivation layers, and a thin metal seed layer  124  (i. e. Cr 200 Å/Au 300 Å/Cr 200 Å) is evaporated onto the wafer and removed (i.e. by wet etching) from the top and sides of the structure in order to prevent plating in these areas. A PR etchback is used to delineate the metal to be etched. A Ni-plating step is also provided to make seed layer removal easier.  
         [0055]    A thick photoresist (PR) mold  126  is then deposited by lithography and patterned, as shown in FIG. 10 b , to define the electrodes  21 . This is followed by plating of the electrodes  21 , using the PR as one portion of the mold and the sidewall of the structure as the other, plating directly against the sacrificial layer  122 , as shown in FIG. 10 c . The PR  126  and seed layer  124  are then stripped, as shown in FIGS. 10 c  and  10   d , and the structure is released in HF, as shown in FIG. 10 e.    
         [0056]    The result is the disk  12  suspended over the substrate  24  by a single anchor  16  in the middle, and separated from the plated electrodes  21  by thin air gaps  22  defined by the sacrificial layer. The minimum gap thickness in this process is limited only by the ability to control the thickness of a conformally deposited oxide film, and is independent of lithography and etching.  
         [0057]    [0057]FIGS. 7 and 8 show an alternative embodiment wherein the disk-shaped resonator  70  and its support  72  thereunder are made at the same time with one mask to form a post  72  with the resonator  70  thereabout. The post  72  is made by filing it with a material such as polysilicon which may or may not be the same material as the resonator  70 . The resulting structure is a self-aligned disk resonator  70  supported above a substrate  74  and a ground plane  76 .  
         [0058]    The frequency of the disk resonator device  10  is determined by the material properties of the disk  12  along with its radius. Neglecting some higher order effects, the resonant frequency of the disk  12  is given by  
               f   0     =       α   R            E   ρ                 (   1   )                               
 
         [0059]    where a is a constant dependent on the Poisson&#39;s Ratio of the material (0.342 for polysilicon), E is the Young&#39;s Modulus of the material, p is its density, and R is the radius of the disk  12 .  
         [0060]    A more complex form of equation ( 1 ) is given by the following equations:  
                 ζ   /   ξ     ×         J   0          (     ζ   /   ξ     )           J   1          (     ζ   /   ξ     )           =     1   -   σ             (   2   )               ζ   =     2      π                   f   0        R            ρ        (     2   +     2      σ       )       E                 (   3   )               ξ   =       2     1   -   σ                 (   4   )                               
 
         [0061]    Equations 2-4 allow the determination of higher modes as well as other modes such as “wine glass” mode.  
         [0062]    The following table provides disk radii for various frequencies and modes.  
                                               Frequency                       (MHz) Mode   400   600   800   1000                   First    6.91μ    4.6Oμ   3.45μ   2.76μ       Second   18.15μ   12.10μ   9.08μ   7.26μ       Third   28.87μ   19.25μ   14.44μ    11.55μ                   
 
         [0063]    The plot in FIG. 2 shows the predicted resonant frequency as a function of radius for both polysilicon and polydiamond as the structural material of the disk  12  for first mode resonances. Although it is only a second order effect, the thickness of the structural material will also influence the frequency but only at high thickness to radius ratios (&gt;1).  
         [0064]    Silicon disk resonator devices with plated gold electrodes have been fabricated and tested. The resonators had designed frequencies from approximately 60 MHZ to 500 MHZ. A plot in FIG. 3 is a measured spectrum of a 157 MHZ resonator, showing that the devices do indeed resonate in this configuration.  
         [0065]    Although the preferred embodiment employs a center anchored, radial contour mode (moving only within the plane of the disk), one-port design, other embodiments are possible. A two-port variation of the preferred embodiment is obtained by splitting the electrode structure  20  into an even number of smaller electrodes, placed symmetrically about the disk  12 . Then, alternating electrodes are connected together for input and output ports. In other words, if the electrodes are consecutively numbered around the perimeter, all of the even numbered electrodes would be connected to form an input drive electrode structure, while all of the odd numbered electrodes would be connected to form an output sense electrode structure. The body of the resonator would then be connected directly to V p .  
         [0066]    One can also excite many other types of modes with generalized electrode configuration (e.g., wine-glass modes). Which mode is excited depends to a good extent on the frequency of the excitation.  
         [0067]    In addition, other mode shapes are possible, such as other contour modes which add more nodal points or nodal diameters. Each different configuration favors different electrode positioning in order to ensure driving the correct mode shape, as well as different anchor locations at nodal points to promote high-Q.  
         [0068]    For example, FIG. 5 shows nodes  50  and an anti-node portion  52  where there is most of the motion of a higher order disk  53 . FIG. 6 shows electrodes  54  on a substrate  56  for sensing motion at the outer rim of the disk  53 . Inner and outer electrodes  58  and  60 , respectively, are positioned to sense motion of fins  62  which project upwardly from the anti-node portion  52  of the disk  53  (shown positioned closer to the rim of the disk  52  than shown in FIG. 5 for purposes of illustration). Rather than the electrodes  58  and  60 , mechanical links may be used to obtain the output from the anti-node. FIG. 6 shows the disk  53  levitated above the substrate  56  without the need for a mechanical support. Such levitation can be achieved either electrically or magnetically as previously noted.  
         [0069]    [0069]FIGS. 1 a - 11   c  are graphs wherein their maximums and the minimums represent the radii at which anti-nodes (i. e., areas of maximum displacement) occur for first, second and third modes of a disk resonator).  
         [0070]    It is even possible to use the disks in flexural modes which require electrodes positioned beneath the structure, rather than to the sides. The invention covers not only the lateral contour modes of a disk, but flexural and other modes as well.  
         [0071]    One target application for disk resonators is the construction of micromechanical filters. FIG. 4 a  depicts a possible mechanically-coupled, two resonator, bandpass filter device, generally indicated at  30 . The device  30  includes two mechanically coupled disk-shaped resonators, generally indicated at  32  and  34 . The bodies or disks  36  and  38  of both resonators  32  and  34 , respectively, are connected together by an extensional mode coupling spring or beam  40  and directly to the bias V P . An input signal, V i , applied to the first resonator  32  through an input electrode, generally indicated at  42 , causes the first resonator  32  to begin vibrating. The extensional mode spring  40 , which operates by expanding and contracting along its length, couples this motion to the second or output resonator  34 , forcing it to move. The motion of the output resonator  34  induces a current in an output electrode, generally indicated at  44 , which can be sensed in R L .  
         [0072]    [0072]FIG. 4 b  shows a “U”-shaped coupling spring  40 ′ which can replace the beam  40  of FIG. 4 a . Such a coupling spring is shown in U.S. Pat. No. 5,839,062 at reference numeral  104 .  
         [0073]    Rather than the single peak induced by a single resonator, the resulting frequency spectrum (v o /v i ) has two peaks in its response which can be flattened into a bandpass filter spectrum via proper termination (adjustment of the source resistance driving v i  and the load resistance R L ). A larger number of resonators results in a more ideal bandpass filter, and this structure can be expanded to n resonators generating n peaks in the spectrum by coupling longer chains of disks in the same manner. Although the extensional mode spring  40  is the easiest to design, other complex spring designs are also possible.  
         [0074]    “Bridged” filters can be conveniently made using disk resonators. Bridged filters are obtained when not only adjacent resonators are connected, but also non-adjacent resonators are connected, as well. As shown in FIG. 9, the bridged filter of FIG. 9 includes an input disk resonator  80 , an output disk resonator  82  and a middle resonator  84 . Electrodes  86 ,  88  and  90  are provided for the resonators  80 ,  82  and  84 , respectively. Adjacent couplers  92  and  94  interconnect the resonators  80  and  84 , and  84  and  82 , respectively. A non-adjacent coupler  96  interconnects the resonators  80  and  82 .  
         [0075]    Benefits accruing to the invention are numerous. For example, the main advantages and contributions of this invention include:  
         [0076]    (i) Disk resonators can achieve frequency ranges from less than 50 MHz to well into the GHz range, making them viable alternatives to discrete components in RF-sub-systems;  
         [0077]    (ii) Large size of the device relative to other micromechanical devices leads to a large electromechanical coupling coefficient in electrostatically driven devices which makes them easier to integrate into existing systems;  
         [0078]    (iii) Large effective mass and stiffness enables disk resonators to handle higher power than other micromechanical devices, increasing dynamic range and decreasing distortion;  
         [0079]    (iv) Disk resonators can be mechanically coupled using extensional mode springs or other mechanical spring types in order to produce integrated micromechanical filters;  
         [0080]    (v) Symmetry of disk resonators allows more flexible coupling schemes, such as non-adjacent resonator coupling;  
         [0081]    (vi) Disk resonators may also be electrically coupled to produce integrable filters;  
         [0082]    (vii) The small size of micromechanical resonators allows for many devices to be used in parallel in order to even further improve power handling;  
         [0083]    (viii) Small size also enables new architectures which use many small, high-Q components to obtain lower power consumption in transceivers to enhance robustness, and to enable improved capabilities (e.g., multi-band reconfigurability); and  
         [0084]    (ix) Integrated resonators have the potential to lower manufacturing costs over the long run by eliminating the need for many off-chip components, reducing circuit board area and the amount of off-chip RF routing.  
         [0085]    The disk-shaped, radial-contour mode, micromechanical resonator is capable of reaching frequencies well past GHz, with high-Q, high dynamic range, and high power handling capability, and with possibilities for extremely flexible positioning of input/output ports (both electrical and mechanical). The resonator is frequency tunable via applied voltages (through its voltage-dependent electrical spring stiffness), and thus, rendered useful for high-Q voltage-controlled oscillator applications.  
         [0086]    Filters such as the filter of FIG. 4, are made by coupling the disks either mechanically or electrically are possible as well.  
         [0087]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.