Abstract:
This invention describes fabrication procedures to construct MEMS devices, specifically band-pass filter resonators, in a manner compatible with current integrated circuit processing. The final devices are constructed of single-crystal silicon, eliminating the mechanical problems associated with using polycrystalline silicon or amorphous silicon. The final MEMS device lies below the silicon surface, allowing further processing of the integrated circuit, without any 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:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of application Ser. No. 09/375,940, filed Aug. 17, 1999 now U.S. Pat. No. 6,238,946. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to systems and methods for fabricating integrated circuit resonant devices, and particularly a process for manufacturing integrated circuit (IC) band-pass filters using micro electromechanical system (MEMS) technology on single crystal silicon-on-insulator (SOI) wafers in a manner consistent with current integrated circuit fabrication techniques. 
     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  12  input 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, 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 micromechanical 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, above-mentioned references 1) and 2) both detail the problems encountered 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 an isotropic 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). This makes 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 immediately destroyed by either of the above planarization processes. 
     Additional prior art 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. 4,232,265A (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. 4,232,265A 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 band-pass filter device in a manner consistent with current integrated circuit fabrication techniques that avoids completely or reduces significantly all of the above-described limitations. 
     SUMMARY OF THE INVENTION 
     It is an object of Lhe present invention to provide an improved IC MEMS resonator band-pass filter device of a construction that lends itself to manufacture in accordance with current IC manufacturing techniques and that overcomes the fundamental weaknesses as outlined in the above-mentioned references. 
     Particularly, according to one aspect of the invention, there is provided a resonatored MEMS bandpass filter device that is constructed of single-crystal silicon, eliminating the mechanical problems associated with using polycrystalline or amorphous materials. The final MEMS device lies below the silicon surface, allowing further processing of the integrated circuit, without any 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. 
     According to another aspect of the invention, there is provided a novel resonatored MEMS bandpass filter device fabrication technique for constructing such MEMS devices in a manner compatible with current integrated circuit processing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
     FIG.  2 ( a ) is a schematic isometric diagram of a MEMS bandpass filter fabricated with a buried planar input contact according to a first embodiment of the invention. 
     FIG.  2 ( b ) is a side view of this same device. 
     FIGS.  3 ( a ) and  3 ( b ) are schematic isometric and side view diagrams of a MEMS bandpass filter fabricated with the input contact in a sunken well according to a second embodiment of the invention. 
     FIGS.  4 ( a ) and  4 ( b ) are schematic diagrams of a MEMS bandpass filter fabricated with the input contact causing horizontal oscillation of the resonator according to a third embodiment of the invention. 
     FIGS.  5 ( a )- 5 ( k ) illustrate the various masks used in construction of the device, and also depicts intermediate structures during the fabrication process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS.  2 ( a ) and  2 ( b ) illustrate respective isometric and side views of a novel resonatored 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 is fabricated to have an input contact  103  for diverting a received electrical signal downwards through connection  106  to a bottom planar contact  102 . A well  108  is created in the silicon surface, and a resonator  101  straddles this well and is free to vibrate. The resonator  101  is electrically connected to an output pad  105 , which propagates the final filtered signal. The input contact  103  is capacitively coupled to the resonator  101 , so that the input signal will cause the resonator to vibrate-in the vertical direction as indicated by the arrow A in FIG. 2 ( b ) . The resonator has a natural frequency of vibration, based on its dimensions and material, and signals of this frequency (or its harmonics) are preferentially propagated through the resonator to the output terminal  105 . The natural frequency of the device may be tuned by heating the resonator, and changing its elastic constant. This may be accomplished by fabrication of a pad  104  which functions to enable a current to be sent through the resonator to pad  105  and consequently heat up the resonator. 
     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, 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. 
     FIGS.  3 ( a ) and  3 ( b ) illustrate respective isometric and side views of a novel resonatored MEMS bandpass filter device  110  manufactured according to a second embodiment of the invention. In FIG.  2 ( a ), above, the resonatored MEMS bandpass filter device  100  was of a construction in which the input contact was connected to the lower contact plane with a conductive via. In FIG.  3 ( a ), a metal contact  117  is dropped down from the surface to the bottom of the well  108  holding the resonator  111 . The output signal pad  105  and tuning pad  104  are similar to those shown in FIG.  2 ( a ). Again, as in FIG.  2 ( b ), the resonator vibrates in the vertical direction as indicated by the arrow B in FIG.  3 ( b ). 
     FIGS.  4 ( a ) and  4 ( b ) illustrate respective isometric and side views of a novel resonatored MEMS bandpass filter device  120  manufactured according to a third embodiment of the invention. In the embodiment illustrated in FIG.  4 ( a ), input contact  129  and input contact extension  130  are formed in the same plane of the resonator  121 , thus, eliminating the need to make a contact plane below the resonator, as is needed for the designs shown in FIGS.  2 ( a ),  2 ( b ) and FIGS.  3 ( a ) and  3  ( b ). Here, the resonator  121  vibrates horizontally rather than vertically as depicted by the arrow C in FIG. 4 ( b ). This design is the simplest of the three variations to fabricate, however mechanical performance is reduced because of the edge surface roughness of the resonator in the direction of vibration. In designs of FIGS.  2 ( a ) and  3 ( a ), the resonator vibrates perpendicular to the surface of the substrate, and the top and bottom surfaces are as smooth as the SOI process can produce (normally&lt;20 nm) However, the resonator design of FIG.  4 ( a ) requires these surfaces to be defined by photolithography, which currently limits the roughness of edge definition to about 100 nm. 
     In accordance with the invention, the process used to fabricate each of the MEMS resonator bandpass filter devices utilizes 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. 
     Typically, SOI wafers are 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 MEMS device, e.g., the resonator structure  100  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 Si 0   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 illustration. Next, as shown in FIG.  5 ( b ) , a thick photoresist layer  223  is applied to the silicon surface, and implementing photolithography, a long rectangle  225  is opened, that is, for example, about 4 mm×1 mm in size. Then, as shown in the cross-sectional view of FIG.  5 ( c ), phosphorus ions are implanted, for example, at 440 keV to a dose of 10 15 /cm 2  through the opening  225  to create an n −  layer  224  in the substrate  222 , just below the SiO 2  layer  212 , and spatially limited by the mask  223 . The n +  phosphorus layer  224  forms the buried conductive layer  224  of the resultant resonator bandpass filter device. Then, as indicated in FIG.  5 ( d ), the old photoresist layer  223  (FIG.  5 ( c )) is removed, and a new photoresist coating is applied so that a second opening  235  may be created using photolithography. This second opening  235  corresponds to the resonator  226  and its electrical contacts,  227  and  228  and is related to the prior opening  225  as illustrated by the dotted-line rectangle. Next, as illustrated in FIG.  5 ( e ), boron ions are implanted at 15 keV to a dose of 10 15 /cm 2  through the opening  235  to create a p +  layer in the silicon layer  202  where the resonator  226  is to be constructed. At this point, the old photoresist is removed. Furthermore, at this point, the wafer may be annealed to remove any radiation damage from the implants, and to activate the B (boron) and P (phosphorus) impurities. A typical anneal process may be implemented in forming gas at 950° C. for 30 minutes. 
     The next step requires the application of a new photoresist coating so that a photolithography technique may be used to open three rectangles  230 ,  231  and  232  at the surface as illustrated in FIG.  5 ( f ). These three holes fit inside the opened rectangle  225 . The relationship of the three holes to the resonator is such that, in a subsequent etch process performed through the surface silicon  202  exposed by presence of the three holes  230 - 232 , the resonator Boron implant region  226  is sandwiched between two holes  231  and  232  at the silicon surface layer  202  such as shown in FIG.  5 ( g ) . A liquid silicon etch such as Ethylene-Dimene-PyroCatehcol Pyrozine (EPPW) may be used, however, according to a preferred embodiment, a reactive ion silicon etch (RIE) using CF 4 ÷O 2  (10%) is used because it will leave more abrupt edges. The structure after this step is illustrated in FIG.  5 ( g ) , which shows the relationship of the three holes to the resonator  216  and the buried conductive layer  224 . 
     Next, as shown in FIG.  5 ( h ), the old photoresist is removed, and a new photoresist coating is applied so that a photolithography technique may be used to open a rectangle  233  that is substantially aligned with the original rectangle  230  (see FIG.  5 ( f )). Further, an etch process is performed to etch through opening  233 , removing the Si 0   2  layer using an etchant such as buffered HF, down to the phosphorus implant layer  224 . 
     As shown in FIG.  5 ( i ), a conducting metal, typically Ti (50 nm thick) followed by Al (550 nm thick) is deposited on the wafer to form the metal contact  234 . Specifically, the prior photoresist layer is removed which enables all of the Ti and Al to be removed from the wafer except for that portion which was deposited within the hole  233  etched in the prior step. Thus, the hole  233  is filled with metal  234 , enabling electrical contact from the surface  201  to the buried phosphorus implant layer  224 . 
     Next, as shown in FIG.  5 ( j ), a new photoresist coating is applied so that a photolithography technique may be used to open two rectangles, substantially aligned with the remaining two prior fabricated rectangular openings  231  and  232  (see FIG.  5 ( f )). 
     Finally, as indicated in FIG.  5 ( k ), an etching process is performed to etch down through holes  231  and  232 , through the SiO 2  layer, utilizing an etchant, e.g., buffered HF. Preferably, the etching continues until the Si 0   2  under the resonator  226  (between the two open rectangles,  231  and  232 ) is fully removed, leaving a resonator structure as shown in the cross-sectional view of FIG.  5 ( k ). Except for connection to other circuit elements, the basic band-pass filter structure  100  of FIG.  2 ( a ) is completed. 
     In operation, as shown in FIG.  5 ( k ), an input signal is conducted down the metal layer  234  to the deep contact  224 . Specifically, the input is the reach-through contact  234 , which transmits the signal to the buried phosphorus layer  224 . This layer is n-type (phosphorus doped silicon) and has junction isolation from the p-type substrate  222 . Layer  224  capacitively couples the input signal to the resonator  226 , and enables the resonator to vibrate at its natural mechanical frequencies, filtering signals which will transmitted to the output electrical pad  228 . Specifically, the signal propagates through the buried layer  224  until it is under the resonator  226 . An image charge is induced in the resonator, and it will mechanically distort towards the buried layer. For electrical signals in resonance with the natural mechanical frequencies of the structure, the resonator will vibrate and capacitively propagate the signal through the P +  doped layer to the output contact  228 . As shown in FIG.  5 ( d ), a second contact  227  is placed at the other end of the resonator  226 , which may be used for frequency tuning. For example, a small current, e.g., of about 10 mA, injected at second contact  227 , will raise the temperature of the resonator to about 150° C., changing the resonator natural vibrational frequency and allowing the band-pass filter to be tuned. 
     In accordance with the principles of the invention described herein, similar procedures may be used to construct the variations on the above MEMS resonator device, such as shown in FIGS. 3 and 4. It should be apparent that manufacture of the resonator device structure  110  of FIG.  3 ( a ) is the same but, does not require the phosphorus implant steps as depicted in FIGS.  5 ( b ) and  5 ( c ) above, nor, the reach-through etch and metallization steps as depicted in FIGS.  5 ( h )- 5 ( i ). Rather, the final bottom contact  117  is formed by depositing a metal layer using a technique such as electroplating to cover the bottom of the well  108  beneath the resonator. 
     Additionally, as mentioned, the MEMS resonator device  120  of FIG.  4 ( a ) vibrates parallel to the wafer surface, and innovates in the inclusion of the single-crystal silicon resonator constructed in accordance with the processes described above. 
     Furthermore, as mentioned, the natural frequency of the resonator structures described herein may be altered by ion implantation into the resonator. Such an implant may be done using the same mask as described with respect to FIG.  5 ( d ), above, and may follow the boron implant process step depicted in FIG.  5 ( e ). 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 polycrstalline 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. 
     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.