Abstract:
Methods for integrating quartz-based resonators with electronics on a large area wafer through direct pick-and-place and flip-chip bonding or wafer-to-wafer boding using handle wafers are described. The resulting combination of quartz-based resonators and large area electronics wafer solves the problem of the quartz-electronics substrate diameter mismatch and enables the integration of arrays of quartz devices of different frequencies with the same electronics.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    Subject matter disclosed in this specification was supported at least in part through governmental grant no. DAAB07-02-C-P613 awarded by the Defense Advanced Research Projects Agency, and is subject to certain governmental rights and interests. 
       FIELD 
       [0002]    The present disclosure relates to nanoresonators and methods for the large-scale integration of nanoresonators with electronics. 
       BACKGROUND 
       [0003]    The use of quartz substrates in a microelectromechanical systems (MEMS) process provides for the fabrication of high quality (Q) factor and thermally compensated resonators. For thickness shear mode resonators, the thickness of the substrate determines the resonant frequency of the resonator. The thinner the quartz substrate, the higher the resonant frequency. Therefore, by varying the thickness of the substrate over a broad range, the resonant frequency can be adjusted over a broad range. Having a quartz substrate with a thickness on the order of 10 microns or less can result in resonant frequencies greater than 100 MHz, which is desirable for high frequency applications. 
         [0004]    By combining several quartz based resonators having different resonant frequencies with a radiofrequency (RF) MEMS switch on the same chip, frequency hopping and filter reconfiguration can occur on the microsecond time scale. In frequency hopping and filter reconfiguration the desired frequency in a band of frequencies is selected by using the RF MEMS switch to activate the quartz resonator having a resonant frequency equal to the desired frequency. The spectral band for most radio frequency hopping and filter reconfiguration applications is 20 MHz to 3 GHz. The low frequency part of the band is extremely difficult to cover with conventional capacitive-based filters since capacitive-based filters are larger in size. Frequency hopping and filter reconfiguration applications would benefit from temperature compensated, stable, high-Q (in the amount of about 10,000), arrays of resonators and filters. 
         [0005]    However, present quartz fabrication techniques for oscillators or filters do not allow quartz resonators to be integrated on a chip with other electronics. The inability to integrate the quartz resonators on a chip with other electronics is a significant contributing factor to the size and cost of a device due to the need to use separate off-chip components. 
         [0006]    MEMS devices which consist of silicon-based nanoresonators have been fabricated in an attempt to integrate nanoresonators or microresonators with other electronics. Nanoresonators and microresonators are resonators which have linear dimensions on the order of nanometers and micrometres or microns, respectively. These silicon-based nanoresonators have shown resonant frequencies as high as 1 GHz, and quality factors in the range of 1000-2000. However, the problem with silicon-based nanoresonators is that they have high electrical impedances and high temperature drift. 
         [0007]    An alternative solution is known which makes use of non-MEMS quartz resonators. Such resonators usually consist of shear strip individual resonators operating in ranges of about 10 MHz to about 250 MHz. These resonators are packaged as discrete devices and mounted as hybrids to other RF circuits. The problem with non-MEMS quartz resonators is that they are non-integrable, they have higher costs, lower frequencies, and they are physically larger in size. 
         [0008]    U.S. patent application Ser. No. 11/426,931 for “Quartz-Based Nanoresonators and Method of Fabricating Same,” published as 2004/0211052 A1, is co-owned with and a parent to the current application. The disclosure of U.S. patent application publication 2004/0211052 is hereby incorporated by reference in this specification for all purposes allowed by law and regulation. This application is addressed to a method for fabricating a quartz nanoresonator that can be integrated on a substrate along with other electronics. It teaches a method for fabricating and integrating quartz-based resonators on a high speed substrate for integrated signal processing that utilizes a combination of novel bonding and etching steps to form ultra thin quartz based resonators. Thinning the quartz substrate in the quartz resonator provided the desired resonant frequency. The quartz resonators made by this process may achieve a frequency in excess of 1 GHz. 
         [0009]    The first embodiment of the process for forming quartz nanoresonators disclosed in U.S. patent application publication 2004/0211052 is shown in general outline in the diagrams of  FIGS. 1-13  of that patent application publication. Referring now to the drawings of the present specification,  FIGS. 1-8  summarize this process. The step shown in  FIG. 1  shows the provision of the starting materials, namely, wafers or substrates of single-crystal quartz  2  and silicon  4  (the “silicon handle”). The quartz wafer  2  has a first surface  3  and a second surface  5 . 
         [0010]    The next step is shown in  FIG. 2 , which is to define and etch a cavity  7  in the silicon handle wafer  4 . A third step, shown in  FIG. 3 , is to deposit the top-side electrode  10  and the interconnect bond metal  8  metal onto the quartz wafer  2  using known methods of patterning and metalizing. The electrode  10  is a “top-side” electrode because ultimately first surface  3  will be the top surface and the electrode  10  will be on top. 
         [0011]    In a fourth step, shown in  FIG. 4 , the quartz wafer  2  is reversed and then brought together with the silicon handle wafer  4  using a direct bonding process. 
         [0012]    A fifth step, shown in  FIG. 5 , uses lapping and reactive ion etching to thin and polish at least part of the quartz wafer  2  into a to a precisely measured thickness suitable for the desired resonant frequency. It will be appreciated that second surface  5  is now closer to first surface  3  due to the thinning of the quart wafer  2 . Next, in a sixth step, shown in  FIG. 6 , photolithography techniques are used to pattern and metallize via holes  12  in the quartz wafer. In a seventh step, shown in  FIG. 7 . bottom-side bonding pads  14 , and bottom side electrode  16 , are deposited. 
         [0013]    The contact vias  12  will provide electrical access to the top-side electrodes of the resonator from the bottom-side bonding pads  14  that will be in contact with probe pads on the subtrate or host wafer  6  that will support the quartz resonator. The host wafer  6  could contain high-speed RF electronics, thus eliminating the need for lengthy bond wires and facilitating on-chip integration. 
         [0014]    In an eighth step, see  FIG. 8 , the quartz wafer  2  is patterned and etched into a modified quartz substrate  2   a,  thus forming the final resonator  20 . The resonator  20  is still attached to the silicon handle  4 . 
         [0015]    In a later step shown in U.S. patent application publication 2004/0211052, but not shown in the drawings of this application, the quartz resonator is later transferred and attached to a base substrate of about the same diameter. Since quartz wafers are typically grown in sizes up to four to five inches (10.2 to 12.7 centimeters), bonding a four inch quartz wafer to a twelve inch (30.5 centimeters) CMOS wafer would not utilize all the electronic components on the CMOS wafer. One would prefer to bond a twelve inch quartz wafer to a tweleve inch CMOS wafer but twelve inch quartz wafers are not available.  FIGS. 10 and 11  of U.S. patent application publication 2004/0211052 show the shaping and addition of probe pads to the base substrate and  FIG. 12  of that publication shows the attachment of the quartz resonator to the pobe pads on the base substrate. In a still later step (shown in  FIG. 13  of U.S. patent application publication 2004/0211052) the silicon handle wafer  4  is removed from the quartz resonators  20 . 
         [0016]    The purpose of the first and second bonding metals  8  and  14  is to receive an electrical signal from the probe pads which can bias or drive the resonator  20  with an electric field. The electrical signal is preferably an AC signal. When the electrical signal is received by the first and second electrodes  10  and  16  a stress is placed on the modified quartz substrate  2   a.  This stress stimulates the mechanical resonant frequency of the modified quartz substrate  2   a  by the piezoelectric effect, thereby causing the modified quartz substrate  2   a  to oscillate at its resonant frequency. Additionally, it is also possible to use the first and second electrodes  10  and  16  to sense the movement of the modified quartz substrate  2   a  relative to a specified plane (not shown). Once the modified quartz substrate  2   a  is oscillating at its resonant frequency, it can be used to drive other electrical components at a frequency equal to its resonant frequency. a  8  and testable. By ablating a portion of the bottom electrode  16 , the resonant frequency of the quartz resonator  20  can be adjusted before final integration with the CMOS substrate. This ablation can be performed with known techniques such as focussed ion beam milling or laser ablation finally, the wafer is diced for final assembly onto the electronic substrate. 
         [0017]    U.S. patent application publication 2004/0211052 addressed direct wafer-to-wafer bonding of quartz devices to electronics and was only applicable to electronics wafers with diameters of about four inches or less. Since there is low demand for large quartz wafers, wafers of crystalline quartz larger than about four to five inches in diameter are not available. In addition, the packaging density of the quartz resonators on the final CMOS wafer will not in general provide for optimal useage of the quartz wafer and can result in wasted quartz. 
         [0018]    Present quartz fabrication techniques for oscillators or filters do not allow the resonators to be integrated on-chip with associated electronics. Only individual oscillators can be purchased for hybrid integration with RF circuits for wireless applications. U.S. patent application 2004/0211052 describes a process to integrate the quartz devices on electronic substrates, but only for wafers which are four to five inches in diameter. This limits the large volume production on CMOS wafers since most high voltage CMOS fabrication is performed on larger wafers. Only compound semiconductor processing of Group mn-V electronics is currently being manufactured with four inch wafers. Precise wafer-to-wafer bonding can only be performed in current aligners with wafers of similar size. In addition, no solutions for integrating quartz devices or pre-testing and screening individual resonators for optimized yield were known. 
         [0019]    As a result, a new process for integrating quartz-based resonators with electronics on a large area wafer is desired in order to solve the aforementioned problems. 
       SUMMARY 
       [0020]    The methods disclosed and claimed herein provides for attaching quartz-based devices such as quartz resonators to host electronics wafers of any size. This may be accomplished through direct pick-and-place and flip-chip bonding or handle wafer bonding. 
         [0021]    The methods disclosed and claimed herein for integrating quartz-based devices such as quartz resonators to large area electronic wafers have the following advantages: 
         [0022]    1. a fabrication process which allows on-chip integration of quartz-based devices, such as quartz resonators, with electronics made on large area substrates (diameter greater than four inches) through direct pick-and-place and flip-chip bonding or handle wafer bonding; 
         [0023]    2. the ability to pre-test and select known-good-dies from the quartz-based devices wafer and bin together devices of similar performance; and 
         [0024]    3. the ability to integrate quartz-based devices of different charactaristics such as frequencies onto the same electronic wafer. This is useful for frequency selection applications. 
         [0025]    4. The ability to reduce the cost of fabrication by optimizing the packaging density of the quartz resonators on the quartz substrate while still allowing for arbitrary placement on the CMOS wafer. 
         [0026]    The method leads to the integration of quartz-based devices with electronics wafers or substrates of any size. The quartz-based devices may be resonators of any frequency. 
         [0027]    In an aspect of the methods disclosed herein, a method for large scale integration of quartz-based devices with an electronic host wafer is provided comprising the steps of:selecting a plurality of quartz-based devices according to their possesion of desired characteristics, wherein each selected quartz-based device is attached to an individual handle wafer; attaching the quartz-based devices to the electronics host wafer to form an array of quartz-based devices on the electronics host wafer; bonding the quartz-based devices to the electronics host wafer; and removing the individual handle wafers from the quartz-based devices. The quartz-based devices may be attached serially to the electronics host wafer using pick-and-place and flip-chip techniques. Alternatively, the quartz-based devices may be attached to a group handle. The group handle may be moved so that the quartz-based devices are in position to be attached to the electronics host wafer. The group handle may be removed afterwards. 
         [0028]    In another aspect of the methods disclosed herein, a method for large scale integration of quartz-based resonators with an electronics host wafer is provided comprising the steps of: providing a plurality of quartz-based resonators possessing desired resonant frequencies, each of the resonators having at least one electrode formed on a first surface of the quartz-based resonator and being attached to an individual handle wafer; placing the plurality of quartz-based resonators, each attached to an individual handle wafer, in contact with the electronics host wafer to form a predetermined array of quartz-based resonators on a surface of the electronics host wafer wherein the electrode of each quartz-based electrode is in electrical communication with an electrode disposed on the surface of the electronics host wafer; bonding the plurality of quartz resonators to the electronics host wafer; and removing the individual handle wafers from the plurality of quartz-based resonators. The quartz-based resonators may be attached serially to the electronics host wafer using pick-and-place and flip-chip techniques. Alternatively, the quartz-based resonators may be attached to a group handle. The group handle may be moved so that the quartz-based resonators are in position to be attached to the electronics host wafer. The group handle may be removed afterwards. 
         [0029]    In an aspect of the devices disclosed herein, a composite of a plurality of quart-based devices bonded to a surface of an electronics host wafer is provided. 
         [0030]    In further aspects of the devices disclosed herein, large scale integrations of quartz-based devices with an electronics host wafer formed by the methods disclosed herein are provided. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
           [0032]      FIG. 1  shows a quartz substrate and a silicon substrate, to be used in accordance with a known process for making a quartz resonator; 
           [0033]      FIG. 2  shows the silicon substrate with a cavity used in the known process; 
           [0034]      FIG. 3  shows the electrodes and a bond metal formed on the quartz substrate used in the known process; 
           [0035]      FIG. 4  shows the quartz substrate bonded to the silicon individual handle wafer in the known process; 
           [0036]      FIG. 5  shows the quartz substrate of the known process after it is thinned while being attached to the silicon individual wafer handle; 
           [0037]      FIG. 6  shows the patterning and metallizing via holes in the quartz substrate of the known process; 
           [0038]      FIG. 7  shows the patterning and metallizing of electrodes and bond metal on the quartz substrate of the known process; 
           [0039]      FIG. 8  shows the patterning and etching of the quartz substrate of the known process; 
           [0040]      FIG. 9  shows the pick-and-place and flip-chip bonding of quartz-based resonator arrays to a electronic host wafer according to an embodiment of the process of the current invention; 
           [0041]      FIG. 10  shows the released quartz-based resonator array on the electronic host wafer after the steps of pick-and-place and flip-chip bonding have taken place as shown in  FIG. 9 ; 
           [0042]      FIG. 11  shows a temporary silicon group handle wafer having receptacles formed using silicon deep reactive ion etching; 
           [0043]      FIG. 12(   a ) shows the population of the temporary silicon group handle wafer with quartz-based resonators using a pick-and-place system; 
           [0044]      FIG. 12(   b ) shows the temporary silicon group handle wafer fully populated with quartz-based resonators; 
           [0045]      FIG. 13  shows the assembly of the quartz-based resonator array held by the temporary silicon group handle wafer onto the electronic host wafer using wafer-to-wafer bonding; 
           [0046]      FIG. 14  shows the removal of the temporary silicon group handle wafer from the quartz-based resonators, leaving the quartz-based resonators on the electronic host wafer; and 
           [0047]      FIG. 15  shows the released quartz-based resonator array on the electronic host wafer. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0048]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
         [0049]    In a preliminary step quartz-based devices are formed. In this detailed description of embodiments the quartz-based devices are quartz-based resonators  20  that are formed by methods such as by the methods taught in U.S. patent application publication 2004/0211052 and described above in the background section in connection with  FIGS. 1-8 . Just as in  FIGS. 1-8 , the quartz-based resonators  20  have first electrodes  10  and second electrodes  16  connected by metallized vias  12  to the electronic circuit and are provided with first and second bonding metals  8  and  14 . At this point, the quartz-based resonators  20  are attached to their silicon handle wafers  4 , which will be designated “individual handle wafers 4” to distinguish them from the group handle wafer  40  described below. 
         [0050]    In a second step (not shown) the resonators  20  are diced and electrically characterized and tuned while attached to their individual handle wafers  4 . Known good dies are sorted and binned according to their resonant frequencies. At this point, the individual quartz-based resonators  20  are ready to be transferred to the large area electronics host wafer  30 . 
         [0051]    The next or third step is the attachment of the quartz-based resonators  20  to the electronics host wafer. 
         [0052]    In a first embodiment of the method disclosed herein, quartz-based resonators of the desired frequencies are precisely placed and bonded to the electronics host wafer  30  serially to form the resonator array as shown in  FIG. 9 . A flip chip bonder such as Datacon&#39;s “2200 apm” multi chip die bonder has a pick and place tool  50  that may be used for this purpose. 
         [0053]    In the step shown in  FIG. 9 , the electronics host wafer  30  is heated to the desired temperature and the metal bond pads  14  of the quartz-based resonators  20  are individually compressed and bonded to electrodes on electronics wafer  30  in the form of the solder bumps  32  using a low temperature thermal compression bond such as Au/In, Au/Sn or Cu/Sn. Alternatively, the die bonder can be used to apply a small amount of conducting epoxy to the metal bond pads or the electronics host wafer and this conducting epoxy forms the electrical contact to the electronics host wafer after curing. 
         [0054]    In the first embodiment of the method disclosed herein, once the electronics host wafer  30  is fully populated with the planned number of quartz-based resonators  20 , the individual handle wafers  4  can be removed by a process such as a plasma dry etch to release the quartz-based resonators  20 , as shown in  FIG. 10 . 
         [0055]    The method disclosed herein has the advantage of being able to build, for example, RF filter arrays with quartz-based resonators  20  of various frequencies that were necessarily fabricated on different wafers. It also enables the assembly of quartz-based resonators  20  of different thickness on the same electronics host wafer  30 . 
         [0056]    A second embodiment of the method provides variant steps for the assembly of the quartz resonators onto the electronics host wafer  30 . A temporary group handle wafer  40 , preferably made of silicon, has pre-etched receptacles  45 . The receptacles  45  in the group handle wafer may be made using silicon deep reactive ion etching (DRIE). 
         [0057]    The quartz-based resonators  20  are placed into the pre-etched receptacles  45  of the group handle wafer  40  using the pick-and-place tool  50  as shown in  FIGS. 12   a  and  12   b.    
         [0058]    The group handle wafer  40  with the attached quartz-based resonators  20  is aligned with the electronics host wafer  30 . The quartz-based resonators  20  are then bonded to the electronics host wafer  30  using a wafer-to-wafer bond, as shown in  FIG. 13 . A bonder (not shown), such as EV Group&#39;s “EVG520” Semi-Automated Wafer Bonding System, may be employed for this purpose. 
         [0059]    The group handle wafer  40  is then removed, as shown in  FIG. 14 , by simply lifting off since no strong bond is formed between the individual handle wafers  4  and the group handle wafer  40 . The group handle wafer  40  is re-usable while the individual handle wafers  4  in  FIG. 2  are consumed during final release. 
         [0060]    Finally, the individual handle wafers  4  on the individual quartz-based resonators  20  are removed, preferably by dry plasma etching, leaving only the quartz-based resonators  20  attached to the electronics host wafer  30  as shown in  FIG. 15 . 
         [0061]    The second embodiment has the advantage of bonding all of the quartz-based resonators  20  simultaneously to the electronics host wafer  30 , to improve throughput. It also enables the bonding of the quartz-based resonators  20  in a controlled environment (a wafer bonder) when vacuum or backfilling of a process gas is required for the assembly. The disadvantage is that all the quartz-based resonators  20 , with individual handle wafers  4  attached, must have the same overall thickness, so using well-known mesa design technology would be necessary to define the thickness of an active part of the quartz-based resonator  20  while allowing the bonding regions to have the same or uniform thickness. 
         [0062]    While several illustrative embodiments of the invention have been shown and described in the above description, numerous variations and alternative embodiments will occur to those skilled in the art and it should be understood that, within the scope of the appended claims, the invention may be practised otherwise than as specifically described. Such variations and alternative embodiments are contemplated, and can be made, without departing from the scope of the invention as defined in the appended claims.