Abstract:
Integrated circuit fabrication technique for constructing novel MEMS devices, specifically band-pass filter resonators, in a manner compatible with current integrated circuit processing, and completely encapsulated to optimize performance and eliminate environmental corrosion. The final devices may be constructed of single-crystal silicon, eliminating the mechanical problems associated with using polycrystalline or amorphous materials. However, other materials may be used for the resonator. The final MEMS device lies below the substrate surface, enabling further processing of the integrated circuit, without protruding structures. The MEMS device is about the size of a SRAM cell, and may be easily incorporated into existing integrated circuit chips. The natural frequency of the device may be altered with post-processing or electronically controlled using voltages and currents compatible with integrated circuits.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to systems and methods for fabricating integrated circuit band-pass filters, and particularly a process for manufacturing integrated circuit (IC) filters using micro electro-mechanical system (MEMS) technology that involves the encapsulation of a single-crystal MEMS band-pass filter in a vacuum environment, at or below the substrate surface using standard processing, compatible with other standard integrated circuit devices, and, in a manner that eliminates the need for precision lithography. 
     2. Discussion of the Prior Art 
     Micro Electro-Mechanical Systems (MEMS) technology is currently implemented for the fabrication of narrow bandpass filters (high-Q filters) for various UHF and IF communication circuits. These filters use the natural vibrational frequency of micro-resonators to transmit signals at very precise frequencies while attenuating signals and noise at other frequencies. FIG. 1 illustrates a conventional MEMS bandpass filter device  10  which comprises a semi-conductive resonator structure  11 , e.g., made of polycrystalline or amorphous material, suspended over a planar conductive input structure  12 , which is extended to a contact  13 . An alternating electrical signal on the input  12  will cause an image charge to form on the resonator  11 , attracting it and deflecting it downwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the transmitted AC signal. The meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation. It is understood that the input and output terminals of this device may be reversed, without changing its operating characteristics. 
     Typically, MEMS resonator filter devices  10  are fabricated by standard integrated circuit masking/ deposition/etching processes. Details regarding the manufacture and structure of MEMS band-pass filters may be found in the following references: 1) C. T. -C. Nguyen, L. P. B. Katehi and G. M. Rebeiz “Micromachined Devices for Wireless Communications”, Proc. IEEE, 86, 1756-1768; 2) J. M. Bustillo, R. T. Howe and R. S. Muller “Surface Micromachining for Microelectromechanical Systems”, Proc. IEEE, 86, 1552-1574 (1998); 3) C. T. -C. Nguyen,“High-Q Micromechanical Oscillators and Filters for Communications”, IEEE Intl. Symp. Circ. Sys., 2825-2828 (1997); 4) G. T. A. Kovacs, N. I. Maluf and K. E. Petersen, “Bulk Micromachining of Silicon”, Proc. IEEE 86, 1536-1551 (1998); 5) K. M. Lakin, G. R. Kline and K. T. McCarron, “Development of Miniature Filters for Wireless Applications”, IEEE Trans. Microwave Theory and Tech., 43, 2933-2939 (1995); and, 6) A. R. Brown, “Micromachined Micropackaged Filter Banks”, IEEE Microwave and Guided Wave Lett.,8, 158-160 (1998). 
     The reference 7) N. Cleland and M. L. Roukes, “Fabrication of High Frequency Nanometer Scale Mechanical Resonators from Bulk Si Crystals”, Appl. Phys. Lett, 69, 2653-2655 (1996) describes the advantages of using single crystal resonators as band-pass filters. The references 8) C. T. -C. Nguyen, “Frequency-Selective MEMS for Miniaturized Communication Devices”, 1998 IEEE Aerospace Conf. Proc., 1, 445-460 (1998) and 9) R. A. Syms, “Electrothermal Frequency Tuning of Folded and Coupled Vibrating Micromechanical Resonators, J. MicroElectroMechanical Sys., 7, 164-171 (1998) both discuss the effects of heat on the stability of micro-mechanical band-pass filters. Of particular relevance as noted in these references is the acknowledgment that the existing processes for making MEMS bandpass filters have serious drawbacks. For instance, as most resonators are made of polycrystalline or amorphous materials to simplify fabrication, there is exhibited an increase in mechanical energy dissipation which softens the natural frequency of oscillation, as noted in above-mentioned references 1)-3). Etching polycrystalline materials does not allow for device features smaller than the polycrystalline grain size, which creates rough surfaces and prevents precise mechanical characteristics. For example, references entitled “Micromachined Devices for Wireless Communications”, “Surface Micromachining for Microelectromechanical Systems”, both detail the problems when polycrystalline material is used in MEMS resonators. Additionally, in reference 7) there is described the construction of resonators made of single-crystal silicon including a description of an attempt to use complex dry-etch techniques to obtain single-crystal resonators. The reference reports such resonator structures having scalloped edges, which reduces the precision of the final mechanical performance to that of polycrystalline structures. That is, their etch-process produced surface roughness that was similar to that of polycrystalline materials. 
     Other attempts to use single-crystal silicon have been reviewed in reference 4). However, these attempts were made to eliminate the poor device performance when polycrystalline materials were used for construction. Most used anisotropic etches to undercut single-crystal silicon surfaces and construct resonators (and other structures). In all cases, the structures were quite large, in part to minimize the effects of surface roughness and non-parallel surfaces on the device performance. Since the devices were very large, they were useful only for low-frequency applications (below 100 MHz), which is of limited usefulness as a communication frequency filter in the commercial band of 300-6000 MHz. 
     A further limitation of all MEMS band-pass structures is that they are formed above the silicon surface (see references 1-9), making the structures incompatible with standard integrated circuit fabrication, since it prevents “planarization”. After the devices of an integrated circuit have been fabricated, the wafer enters its final processing which is called “metallization” and “planarization”. Before this step, all the devices on the wafer are isolated, and for integration they must be connected together with metal wires. In modern devices, the wiring is done as a series of layers, each containing wiring in certain directions (i.e., metallization). After each layer is deposited, the wafer surface is smoothed, i.e., is planarized so that subsequent layers of wiring may be deposited on a smooth surface. Planarization is typically done by chemical-mechanical polishing (CMP processing) or by melting a thin layer of glass over the surface. If there is a micro-mechanical device protruding up above the surface, it would be destroyed by either of the above planarization processes. 
     Finally, most MEMS band-pass filter structures have been “open” structures, i.e. on top of the substrate and operating in air. References 1)-3) report that operating resonators in air adds significant friction to the system, reducing the device efficiency. 
     Prior patented devices such as described in U.S. Pat. No. 3,634,787 (1972), U.S. Pat. No. 3,983,477 (1976), and U.S. Pat. No. 4,232,265 A (1980) describe similar mechanical resonatored structures, but which are incompatible with integrated circuit processing. 
     For instance, U.S. Pat. No. 3,634,787 describes an electro-mechanical resonator band-pass filter device having a mechanical component consisting of a support being a unitary body of semiconductor material and having a piezoelectric field effect transducer therein. Thus, its electrical operation relies upon the piezoelectrical effect. U.S. Pat. No. 3,983,477 describes a ferromagnetic element tuned oscillator located close to a high-voltage current carrying conductor, however, as such, its electrical operation relies on the ferromagnetic effect. U.S. Pat. No. 4,232,265 A describes a device for converting the intensity of a magnetic or an electromagnetic field into an electric signal wherein movable elements are made as ferromagnetic plates. Likewise, its electrical operation relies upon the ferromagnetic effect. U.S. Pat. No. 5,594,331 describes a self-excitation circuitry connected to a resonator to process induced variable frequency voltage signals in a resonant pass band and is of exemplary use as a power line sensor. Likewise, U.S. Pat. No. 5,696,491 describes a microelectromechanical resonating resonator which responds to physical phenomenon by generating an induced variable frequency voltage signal corresponding to the physical phenomenon and thus, does not lend itself to manufacture by current integrated circuit fabrication technology. 
     It would thus be highly desirable to construct an IC MEMS filter device in a manner consistent with current integrated circuit fabrication techniques that avoids completely or reduces significantly all of the above-described limitations. 
     Furthermore, it would be highly desirable to construct an IC MEMS filter device in a manner consistent with current integrated circuit fabrication techniques including features for removing problems of environmental contamination and also the deleterious effects of air on the vibrating resonator. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved IC MEMS resonator device of a construction that lends itself to manufacture in accordance with current IC manufacturing techniques and that overcomes the fundamental weaknesses as outlined above. 
     Particularly, according to one aspect of the invention, there is provided a fabrication technique for constructing novel MEMS devices, specifically band-pass filter resonators, in a manner compatible with current integrated circuit processing, and completely encapsulated to optimize performance and eliminate environmental corrosion. The final devices may be constructed of single-crystal silicon, eliminating the mechanical problems associated with using polycrystalline or amorphous materials. However, according to another aspect of the invention, other materials such as polycrystalline or amorphous silicon may be used for the resonator. The final MEMS device lies at or below the substrate surface, enabling further processing of the integrated circuit, without protruding structures. 
     Advantageously, the MEMS device of the invention is about the size of a SRAM cell, and may be easily incorporated into existing integrated circuit chips. The natural frequency of the device may be altered with post-processing, or electronically controlled using voltages and currents compatible with integrated circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further features, aspects and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 is a schematic diagram of a conventional MEMS bandpass filter device of a suspended resonator design. 
     FIGS.  2 ( a )- 2 ( c ) illustrate respective top, side and end views of an encapsulated MEMS bandpass filter  100  having a suspended resonator with a third contact for possible electrical frequency tuning. 
     FIGS.  3 ( a )- 3 ( c ) illustrate respective top, side and end views of an encapsulated MEMS bandpass filter  110  having a suspended resonator without a third contact for electrical tuning. 
     FIGS.  4 ( a )- 4 ( c ) illustrate respective side, top and end views of an encapsulated MEMS bandpass filter  200  with the resonator transverse to the input conductor, with a third contact for possible electrical frequency tuning. 
     FIGS.  5 ( a )- 5 ( i ) illustrate the various steps and masks used in construction of the encapsulated MEMS bandpass filter device, and also depicts intermediate structures during the fabrication process. 
     FIGS.  6 ( a ) and  6 ( b ) illustrate the initial cross-section of the layers used for the fabrication of poly crystalline resonators and multi-layer bonded resonator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS.  2 ( a ),  2 ( b ) and  2 ( c ) illustrate respective top, side, and edge views of the novel encapsulated resonator MEMS bandpass filter device  100  manufactured according to a first embodiment of the invention. As shown in FIGS.  2 ( a ) and  2 ( b ), the device  100  includes a conducting wire input or contact terminal  101  that is suspended over a vacuum cavity  103  containing a conducting resonator  102  which is constructed at or below a primary surface  104  of the device. As shown, the resonator is connected to output wire or contact terminal  105 . It should be understood that the input ( 101 ) and output ( 105 ) terminals of this device may be reversed, without changing its operating characteristics, but for simplicity the input and output contacts will be discussed as shown in FIG.  2 ( b ). In operation, an alternating signal on the input  101  causes an image charge to form on the resonator  102 , attracting it and deflecting it upwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the capacitively coupled AC signal. This meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation. 
     As shown in the end view of the device in FIG.  2 ( c ), the single-crystal resonator  102  suspended in the vacuum cavity  103  under input conductor  101 . The side view of FIG.  2 ( b ) shows the same resonator  102  clamped at both ends, under the input conductor. As further shown, all conductors are electrically isolated from the substrate by an insulating layer  108 , except where a contact purposely is made. The resonator  102  is made electrically conductive by a buried implantation layer  106 , which also connects it to the output conductor  105 . With resonator dimensions of 1 mm×1 mm, 0.3 mm thickness, for example, the resonant frequency may range from about 500 MHz-800 MHz, with a first harmonic in the band of 3-5 GHz, using standard theory of vibrating bar frequencies when clamped at both ends (see Ref. 11). 
     Additionally shown in FIGS.  2 ( a )- 2 ( c ) is a third contact  107  which is electrically connected to the resonator  102  by the same buried conducting layer  106 . The insulating layer  108  prevents electrical contact between the buried layer and input conductor  101 . Conductor  107  may be used for tuning the resonator frequency either by applying DC bias relative to the input contact  101  which will apply internal stress to the resonator, or by applying DC bias relative to the output contact  105  which will cause a current to flow through the resonator, increasing its temperature. Both types of bias change the modulus of elasticity of the resonator, resulting in a change of its fundamental natural vibrational frequency. 
     That is, as described in the reference to H. J. McSkimin, J. Appl. Phys., 24, 988 (1953), and Yu. A. Burenkov and S. P. Nikanorov, Sov. Phys. Sol. State, 16, 963 (1974) the elastic constant of single crystal silicon varies with temperature. Further as described in the reference H. Guckel, Tech. Digest, IEEE Solid-State Sensor and Actuator Workshop, June, 1988, pp. 96-99, the elastic constant of polycrystalline silicon varies with temperature. In accordance with these references, the heating of silicon by 100° C. will change its elastic constant by about 0.9%, which may modify the resonator natural frequency by about 0.4%. For a 1 GHz natural frequency, this provides a tuning band of 4 MHz by controlling the resonator temperature. Measurements of such frequency changes may be made in accordance with conventional techniques (see above-mentioned references 8 and 9). In accordance with the invention, the thermal properties are used to tune the device, and improve its performance and flexibility. 
     A variation of the device  100  is illustrated in FIGS.  3 ( a )- 3 ( c ) which show respective top, side and end views of an encapsulated resonator device structure  110  according to a second embodiment of the invention. In this embodiment, the resonator device  110  includes an input contact  111  which spans cavity  113  that contains resonator  112  which straddles the cavity and is free to vibrate. The resonator is electrically connected by a deep implant layer  116  to an output pad  115 , which propagates the final filtered signal. The input contact  111  is capacitively coupled to the resonator  112 , so that the input signal will cause the resonator to vibrate. The resonator will have a natural frequency of vibration, based on its dimensions and material, and signals with this frequency (or its harmonics) will be preferentially propagated through the resonator to the output terminal  115 . This variation does not use a separate tuning contact, and any real-time tuning (after processing is completed) must be done by a DC voltage bias between the input and output conductors, applying stress to the resonator. 
     The devices illustrated in FIGS. 2 and 3, both show the resonator perpendicular to the input conductor. A variation of these structures is now described with respect to FIGS.  4 ( a )- 4 ( c ) which illustrate a resonator device structure  200  according to a third embodiment of the invention. 
     In FIG.  4 ( a ), there is illustrated an encapsulated MEMS bandpass filter device  200  in which the resonator  202  lies at 900° with respect to the structure described and shown in FIG.  2 . Thus, input conductor  201  covers the buried cavity  203  in which the MEMS resonator  202  is suspended. The resonator is made conducting by a deep implantation  206  which electrically connects it to an output conductor  205 , and also to a tuning conductor  207 . The device  200  has the advantage over the structure illustrated in FIG. 2 in that there is less area overlap, and hence less capacitive coupling between the input conductor  201  and the buried conducting layer  206 . However, this structure may require more steps to fabricate, and hence, more costly to manufacture. 
     In accordance with the invention, the process used to fabricate each of the encapsulated MEMS resonator bandpass filter devices may utilize silicon on insulator (SOI) substrates as the starting material. This material consists of a silicon wafer with a thin layers of SiO 2  and single crystal silicon on its surface (the silicon is the outmost layer). Such wafers are commercially available and are made using a variety of techniques. It is understood that the processes described herein are also applicable to silicon wafers only partially covered with SOI material. These wafers are constructed using the widely known SIMOX process (Separation by IMplanted OXygen) wherein only small areas of the surface are converted by using masks to form isolated areas of SOI material. Further, as will be described in greater detail hereinbelow in view of FIGS.  6 ( a ) and  6 ( b ), the devices may be constructed by evacuating a trench in the surface of a silicon wafer, and depositing layers of material corresponding to those illustrated in view of FIG.  5 ( a ). The fabrication of the resonators would be similar to that described for SOI starting materials, except the final resonator will be polycrystalline or amorphous silicon. 
     Typically, SOI wafers may be constructed with the topmost single crystal silicon being about 200 nm thick, the SiO2 being 400 nm thick, and the substrate being several hundred microns thick. Other layer thickness of SOI substrates are available, and all are compatible with the processes described herein. 
     FIGS.  5 ( a )- 5 ( k ) illustrate the process steps in manufacturing a SOI encapsulated MEMS device, e.g., having the resonator structure shown in FIG.  2 ( a ). 
     As shown in the cross-sectional view of FIG.  5 ( a ), a clean p-type SOI wafer  200  is provided, having a surface silicon layer  202 , an intermediate layer of SiO 2    212  on the substrate silicon  222 . For purposes of discussion, it is assumed that the surface silicon layer  202  is about 200 nm thick, the intermediate SiO 2  layer  212  is about 400 nm thick and, the silicon substrate  222  is p-type silicon, of nominal 10 Ω-cm resistivity. It is understood that none of these thickness specifications are critical to the device construction, and are used only for purposes of illustration. Next, as shown in FIG.  5 ( a ) in a second step, a top layer  232  of SiO 2  is deposited on the wafer having a thickness of about 30 nm, for example (not shown). Then, in a third step, a thick photoresist layer (not shown) is applied, and implementing photolithographic technique, a structure  225  as shown in FIG.  5 ( b ), is opened which represents a first mask. This first mask outlines the deep implant layer  106  which connects the tuning contact  107  to the resonator  102 , makes the resonator conducting, and finally connects the resonator to the output contact  105  (FIG.  2 ( a )). Thus, in a fourth step not shown, phosphorus ions are implanted, for example, at 95 keV to a dose of 10 15 /cm 2  through the mask  225  opening. This will create an n+ layer in the substrate, just below the SiO 2  layer, and spatially limited by the mask  225  of FIG.  5 ( b ). Because the silicon layer is p-type, this n-type conducting layer will be electrically isolated from the silicon layer. 
     Then, the photoresist applied for fabricating opening structure  225  is removed in a fifth step. It may be convenient to anneal the wafer at this point to remove the radiation damage from the implants, and to activate the phosphorus. A typical anneal would be in forming gas at about 950° C. for about 30 minutes. 
     Next, in a sixth step, a new photoresist coating is applied and a photolithographic technique is implemented to open a structure  230  as shown in FIG.  5 ( c ), which represents a second mask. This mask is used to create the metal layer above the resonator, and be the self-alignment structure for the band-pass filter. That is, the resonator will be constructed just below this layer, and its width will be defined by the width of this conductor (although its final width will be slightly smaller than the width of this metal layer). The positioning of this mask is shown relative to the implant mask  225  indicated by the dotted lines in FIG.  5 ( c ). Next, in a seventh step, a layer of Molybdenum (Mo) metal, or like equivalent, is deposited to a thickness of about 100 nm according to the pattern defined second mask area. Then, the photoresist is removed, lifting off the Mo except for the area  230  exposed by the pattern of FIG.  5 ( c ). At this point, in an eighth step, the structure may be annealed to make the Mo metal adhere to the underlying SiO 2 . Typically this anneal may take place in forming gas at 400° C. for 30 minutes. 
     Then, in a ninth step, a new photoresist coating is applied and a photolithographic technique utilized to open a rectangular shape  235  shown in FIG.  5 ( d ), which represents a third mask. The positioning of this third mask  235  is shown relative to the implant mask  225  indicated by the dotted lines as shown in FIG.  5 ( b ). Also shown in FIG.  5 ( d ) is the cross-sectional plane, labeled by dashed line A—A, which is the reference plane for the cross-sectional views shown in FIGS.  5 ( e )- 5 ( i ), as will be described in greater detail herein. 
     The opening in the photoresist as depicted by the third mask of FIG.  5 ( d ) applied in the previous step, will be used to etch through the various layers, and create a resonator structure below the conductor. The resonator width will be determined by the width of the Mo metal, FIG.  5 ( c ), and hence will automatically have the same basic shape (a self-aligned structure). After application of the photoresist, the structure will have the cross-section shown in FIG.  5 ( e ), taken through the reference plane, A—A, as indicated in FIG.  5 ( d ). This cross-sectional view illustrates the photoresist layer  240 , labeled “PR”, and the central metal strip  250  defined by the third mask  235 . The substrate contains the following layers (with approximate thicknesses are shown in parenthesis): SiO 2  (30 nm), Si (200 nm), SiO 2  (400 nm) and the substrate silicon. 
     Then, in a tenth step, a reactive ion etching (RIE) technique is utilized to etch through the two rectangular areas  236   a ,  236   b  exposed on either side of the Mo metal by the third mask  235  (FIG.  5 ( e )). Typical RIE gases for this etch step is CF 4 +O 2 (10%), which plasma will etch down trenches  242 ,  244  through the surface SiO 2  (30 nm) and the underlying Si layer (200 nm) and partially into the lower SiO 2  layer. This etching step will result in the creation of a structure having a cross-section as depicted in FIG.  5 ( f ) which illustrates the extent of each etched trench  242 ,  244  formed through the upper two substrate layers  232 ,  202 , and partially into the third layer  212 . 
     The next step (step  11 ) requires a removal of the photoresist layer  240  as shown in FIGS.  5 ( e ) and  5 ( f ). Then, a twelfth step is implemented to selectively etch the Si layer under the Mo metal, so that the final resonator will be slightly narrower than the Mo metal. A liquid etch such as Ethylene-Dimene-PyroCatehcol Pyrozine (EPPW) may be used which has an etch rate of Si more than 100:1 than that of SiO 2 . The portions of silicon  255   a,b  under the Mo are etched back about 100 nm as shown in the resultant structure depicted in FIG.  5 ( g ). 
     Then, in a thirteenth step, as shown in the cross-sectional view of FIG.  5 ( h ), a photoresist layer  260  is applied and a photolithographic technique implemented to open a fourth mask structure  265 . This fourth mask  265  is similar to the third mask structure  235  as shown in top-view in FIG.  5 ( d ), except that it also covers the Mo layer  250  in the middle of the structure. 
     In the next step (step  14 ), an etchant such as buffered HF is used to etch down through the SiO 2  layers in an amount sufficient to fully remove the SiO 2  layers under and over the resonator  102 . The result of this step is to leave a “well” structure  270  as shown in cross-section in FIG.  5 ( h ), with the resonator  102  separated from the metal conductor  250  by the thickness of the SiO 2  layer  232  deposited in second step (SiO 2  application shown in FIG.  5 ( a )). The resonator  102  is now mechanically “free” with the removal of the SiO 2  layers above  271  and below  272  the resonator. 
     As a final step  15 , SiO 2  is deposited in a vacuum process onto the structure, preferably commensurate in thickness to that as the well structure  270  etched in the previous step. Thus, as shown in FIG.  5 ( i ), this results in the refill of the well  270  with SiO 2  layers  280  of approximately 730 nm in thickness, for example, on both sides of the conductor  250  and the resonator  102 . Then, the layer of photoresist  260  is removed which will lift-off all the SiO 2  except which has refilled the etched well, leaving a structure as illustrated in FIG.  5 ( i ). The resonator is now fully encapsulated in a vacuum. 
     Except for connection wiring to other circuit elements, the basic band-pass filter structure  100  illustrated in FIG.  2 ( a ) is completed. The input signal is carried in the Mo conductor deposited in the seventh step. This signal is capacitively coupled to the MEMS resonator  102  constructed in step  14 . This signal is then carried through the buried conductor  106  constructed by ion implantation in the fourth step to an output connector (not shown). 
     A third contact may make contact to the buried implant layer (see FIG.  2 ( a ) and may be used for frequency tuning of the band-pass filter. A small current, for example, of about 10 mA, will raise the temperature of the resonator about 150° C., changing the resonator natural vibrational frequency and allowing the band-pass filter to be tuned. This contact may also be used to apply DC bias to pre-stress the resonator, changing its natural vibrational frequency. 
     As mentioned herein, the natural frequency of the resonator structure may be altered by ion implantation into the resonator at any stage of its construction. Such an implant may be done using the same mask  225  as described with respect to FIG.  5 ( b ), above, and may follow the phosphorus implant process step  4 . Such ion implantation may be used to alter the resonator elastic constant in two ways: (1) by changing the density of the material, or (2) by changing the internal bonding structure of the material. The general formula which describes the natural fundamental frequency of a resonator beam supported at both ends is derived in the reference entitled “Vibration and Sound”, e.g., Chapter IV “The Vibration of Bars”, by P. M. Morse, McGraw Hill Book Co., New York (1948), the contents of which are incorporated herein by reference, and set forth in equation (1) as follows:                Fundamental                 Frequency     =     K                   T     L   2                         Y   ρ                 (   1   )                                
     where K is a constant, T is the beam thickness, L is the beam length, Y is the elastic constant of the beam material, and ρ is the beam material density. Examples of. processes which may be used to alter the resonator frequency (after subsequent annealing) include the following: 
     1) Ion implantation of neutral light atoms such as carbon will, after anneal, maintain the same single-crystal structure of the resonator but lowers the resonator density, and hence raises its natural frequency of vibration. It is understood that neutral atoms are those which are chemically similar to silicon, and may be directly incorporated into the silicon crystal lattice. 
     2) Implantation of neutral heavy atoms such as germanium which raises the resonator material density, and lowers the natural frequency of vibration; and, 
     3) Implantation of dopant substitutional atoms such as B, As or P will change the local bonding of the silicon, and also effect the elastic constant of the resonator. 
     The resonator frequency may also be lowered by reducing the thickness of the resonator. This may be simply done by oxidizing and then etching the silicon prior to any processing, and reducing the thickness of the surface silicon 
     The resonator frequency may also be raised by increasing the thickness of the resonator. This may be done by growing epitaxial silicon on the wafer prior to any other processing. 
     The resonator frequency may be also raised by the deposition of any material upon the resonator structure to increase its thickness. However, any material other than single-crystal silicon will degrade the device performance by introducing internal friction losses. 
     The width of the band-pass filter may be too narrow for some applications. This frequency width may be increased (widened) by ion implantation of the resonator surface with silicon atoms, partially converting it to polycrystalline or amorphous silicon. However, as noted above, internal friction from such materials reduces the device efficiency and also widens the band-pass by distorting the natural vibrational frequency. 
     The above process steps illustrated with respect to FIGS.  5 ( a )- 5 ( i ) are for fabricating a silicon crystal encapsulated MEMS resonator. However, the process described may also be used for resonators made of other materials. Polycrystalline resonators may be fabricated at low temperatures, and the mechanical limitations of using such materials may be tolerated for some applications. Multi-layer resonators may be fabricated using mixed materials, in which the mixed properties are superior to those of any individual material. For example, diamond-like-carbon (DLC) may be chosen to make a resonator with a stiff elastic constant (and hence a large natural vibrational frequency), and combine this with a metal to obtain good electrical conduction in the resonator. The DLC has an elastic constant of about 1500 Gpa (GigaPascals), in contrast to about 200 Gpa for single crystal silicon. Hence, the natural vibrational frequency will increase by the square root of the difference, i.e., about 2.7 times larger for the DLC material. Although DLC may be deposited with doping to improve its electrical conductivity, a thin bonded metal layer may be required to provide the necessary conductivity to the resonator without compromising on the elastic constant of the DLC as it is not currently known about how such doping will affect its elastic constant. 
     For a polycrystalline resonator, or a multi-layered resonator, the fabrication process requires several additional steps at the beginning of the fabrication cycle. These process steps are identified as steps  301 - 305  hereinbelow, so they will not be confused with steps  1 - 15  for the fabrication of single crystal silicon, described above. Once these steps are completed, the rest of the MEMS fabrication follows the outline of steps  2 - 15  (above). These new steps will create a filled trench as illustrated in cross-section in FIGS.  6 ( a ) (polycrystalline resonator) and  6 ( b ) (multi-layer resonator). 
     FIG.  6 ( a ) illustrates a trench  350  etched into the substrate. This trench is slightly larger than the final MEMS device, but its surface area is not critical. Successive depositions will lay down layers of the lower release layer  351 , the polycrystalline material for the resonator  352  and the upper release layer  353 . When the MEMS device is completed, the layers  351  and  353  will be absent, and the resonator will be formed of the material  352 . 
     FIG.  6 ( b ) illustrates a trench  350  etched into the substrate. This trench is slightly larger than the final MEMS device, but its surface area is not critical. Successive depositions will lay down layers of the lower release layer  351 , several layers which will make up the resonator  354 , 355 , 356 , and the upper release layer  353 . When the MEMS device is completed, the layers  351  and  353  will be absent, and the resonator will be formed of the materials  354 , 355  and  356 . 
     According to this embodiment of the invention, typical process steps include: 
     A step  300  which utilizes photoresist to open an area on the substrate surface slightly larger than the masks shown in FIGS.  5 ( b )- 5 ( d ). Etch a trench into the silicon, for example using reactive ion etching (RIE). For example, etching a rectangle about 4 mm×4 mm in area about 1 mm deep. 
     A next step  301  which requires the deposition of a “lower release layer” which will be later etched away. For example, a layer of SiO 2 , of 670 nm thick, for example, is deposited using evaporation or sputter-deposition. 
     A next step  302  requires the deposition of the material which will be used as the resonator. For example, a layer of Si, 300 nm thick, may be deposited using evaporation or sputter-deposition. Alternatively, the resonator material may be made of several layers to obtain special material properties. 
     Next, a step  303  for depositing an “upper release layer” which will be later etched away. For example, a layer of SiO 2 , 30 nm thick, may be deposited using evaporation or sputter-deposition. 
     Finally at step  304 , the photoresist is removed, releasing the deposited materials everywhere except within the box etched in step  301  above. 
     After completing initial steps  301 - 304 , the cross-section of the device will be as illustrated in depicted in FIGS.  6 ( a ) and FIG.  6 ( b ) for both the polysilicon resonator and the DLC/metal resonator. 
     The processing of this MEMS device may then proceed as herein described for the single-crystal silicon resonator, process steps  2 - 15 . Masks  1 - 3  (FIGS.  5 ( b )- 5 ( d )) will be placed within the area fabricated above in step  301 . Only two process steps need to be changed. The deep etch described in step  10 , FIG.  5 ( f ), must be able to etch through the resonator material deposited in step  302 . Also, the side-etch used in step  12 , FIG.  5 ( g ), must be an etch which is selective in etching the resonator material, while having little etching effect on the other materials in the trench. Hence, the suggested EPPW etchant may need to be changed to an etchant material specific for the resonator material chosen. Other than these two changes, the process of the device construction is similar to that described for making a single-crystal silicon MEMS device, discussed in process steps  2 - 15  herein. 
     While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.