Abstract:
A resonator having temperature and electronic compensation. The resonator has several layers on a substrate having opposite thermal coefficients of the sound velocity for temperature compensation. Also, the frequency of the resonator is adjusted in accordance with an external time reference. The resonator has a high quality factor and a very small size.

Description:
BACKGROUND  
         [0001]    This application claims the benefit of U.S. Provisional Application No. 60/315,862, entitled “Bulk Resonator”, filed Aug. 29, 2001, wherein such document is incorporated herein by reference.  
           [0002]    The invention pertains to resonators and particularly to compensated resonators.  
           [0003]    There is a need of very stable resonators for oscillators, filters and other components, specifically in the frequency range of about 0.3 GHz to 5 GHz. Also, small size and high quality factor (Q) are desired.  
         SUMMARY  
         [0004]    The invention is a resonator that may be compensated in structure and electronically, e.g., with respect to the resonant frequency drift with temperature. The resonator may be of a bulk nature and be designed to have a high Q. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIGS. 1 a  and  1   b  reveal illustrative embodiments of a resonator with structural compensation.  
         [0006]    [0006]FIGS. 2 a  and  2   b  show other illustrative embodiments of the resonator with structural compensation.  
         [0007]    [0007]FIG. 3 is a schematic of electronic compensation for resonator. 
     
    
     DESCRIPTION  
       [0008]    [0008]FIG. 1 a  shows an illustrative example of a bulk resonator  10 . An SiO 2  layer  12  is part of or may be put on a silicon support  13 . On layer  12  may be a layer  15  of silicon. On silicon layer  15  is a layer  16  of a piezoelectric material. Layer  16  may be GaN or AlN or a composite of AlGaN or a stack of multiple layers. Layer  16  may be made with an AlN seed layer, GaN, an alternating superlattice of AlN and GaN, then with more GaN on top. Or the layer may be a gradient which starts as AlN and ends up as GaN. On layer  16  is a layer  17  of an amorphous material. This material may be amorphous silicon, SiO 2  or silicon nitride. In an alternative embodiment  45 , the layer  17  may be deposited as a layer  47  at the bottom of layer  12  instead, as shown in FIG. 1 b . In resonators  10  and  45 , on layer  15  is a layer  18  of SiO 2 . In FIG. 1 a , on and bridging layers  17  and  18  is a layer  19  of contact material for a contact for resonator  10 . For resonator  45  of FIG. 1 b , layer  19  bridges layers  16  and  18 . This contact is an ohmic contact that may have material which is aluminum, doped silicon or amorphous silicon, or any electrically conductive material. A second contact is a layer  20  on the conducting layer  15 . This contact is an ohmic contact that may have material which is aluminum, doped silicon or amorphous silicon, or any electrically conductive material. Between layer  18  and layers  16  and  17  of FIG. 1 a  is a space  21  which may be filled with air or a dielectric. Similarly, in FIG. 1 b , space  21  between layers  16  and  18 , under layer  19 , may be filled with air or a dielectric. Or in resonators  10  and  45 , space  21  may be a vacuum.  
         [0009]    The size  22  of resonator may be about 75 microns. This dimension generally would be less than 2200 microns. In certain instances, dimension  22  could be more or less of the mentioned magnitudes. The ridge support  13  of layer  12  may be a square or a circle having a side dimension  22  or diameter  22 , respectively. Layers  16  and  17  have opposite thermal coefficients of change of the sound velocity and thus reduce the change of frequency of resonator  10  due to temperature change of the resonator.  
         [0010]    The thickness of layers  16  and  17  may each be between about one and ten microns. The dimensions for layer  47  may be about the same as either of these layers. In an illustrative example, each thickness may be about two microns for a possible resonator frequency of around 1.0 GHz. The thickness of layer  12  may be between 0.1 and 50 microns. Layer  15  may have a thickness between 0.01 and 10 micron. A value may be about 0.3 micron for layer  12  and about 0.2 micron for layer  15  of resonators  10  and  45 . Length  23  of layer  18  may be between 20 and 50 microns with a value of 30 microns for resonators  10  and  45 . Length  23  may be from the outside edge of the device to the inside edge of support  13 . Length  25  of layers  16  and  17  may be between 20 and 2000 microns with a value of 50 microns for resonator  10 . These dimensions may be applicable to layers  16  and  47  for resonator  45 .  
         [0011]    The thickness of contact  19  may between 0.1 and 0.5 micron. That thickness may be 0.3 if aluminum is used. If contact  19  is amorphous silicon, which may be doped to be conductive, the thickness may be 0.5 microns but could be one micron. Length  24  of space  21  between layer  18  and layers  16  and  17  may be 10 microns but could vary between 0.2 and 20 microns. Space  21  may have a similar length in resonator  45 .  
         [0012]    Contact layer  20  may have a length  26  of about 4 to 30 microns but could be another length up to 50 microns, or it could be much smaller (i.e., even sub micron) if another non-contacting metal layer (e.g., a via) is used to bring the signal out to a larger pad. A length  27  between layer  20  and layer  16  may be between 1 and 10 microns but for the illustrative examples it may be about 4 microns. Generally, lengths  27  and  24  may be about the same, and may be the length between the edge of layer  16  or  47  and the inside edge of support  13  as shown in FIGS. 1 a  and  1   b . Space  21  may be filled with air but it could be filled with a dielectric such as SiO 2 , or be a vacuum as noted above. Layer  18  may be an insulating material, such as silicon or sapphire instead of SiO 2 , since insulation is a purpose of layer  18 . Physical characteristics noted above may be applicable to all illustrative examples disclosed here where compatible.  
         [0013]    [0013]FIGS. 2 a  and  2   b  show illustrative embodiment of resonator  30  and  50  where in lieu of layer  17  and contact layer  19  of FIG. 1 a , a layer  28  may be substituted. Layer  28  may extend from the right edge of layer  16  to the left edge of layer  18 . The thickness of layer  18  may be about the same thickness as layer  16  since it appears more convenient to draw it that way, but it is not necessary for layer  18  to be the same thickness as layer  16 . This may apply to resonator  45  in FIG. 1 b , and to resonator  10  with respect to layers  16  and  17  relative to layer  18  in FIG. 1 a . The thickness of space  21  may be about the same as the thickness of layer  16  of resonators  30  and  50 . Layer  28  may be of the same material for resonators  30  and  50 , that is, amorphous silicon. Layer  28  has a thermal coefficient of variation of velocity of sound that is the opposite of the coefficient of layer  16 . The length of layer  28  may be about 90 microns but could be a length between about 20 microns and 2100 microns. Layer  28  may be utilized as a contact layer in that the top surface  29  of layer  28  or the whole thickness of layer  28  may be doped so as to be conductive for purposes of a contact for resonators  30  and  50 . Space  21  may be filled with air, SiO 2  or some other dielectric. It may be a vacuum instead. Layer  18  may have SiO 2  substituted with sapphire or silicon nitride. The other structural and material aspects not noted specifically about embodiments  30  and  50  may be similar to those aspects of embodiments  10  and  45 .  
         [0014]    A feature of resonator  50  of FIG. 2 b , not shown in resonator  30  of FIG. 2 a , is a layer  48 . Layer  48  may have length and thickness dimensions similar to those of layer  16 . It may have a vertical alignment with layer  16 . Layer  48  of resonator  50  may have certain characteristics similar to those of layer  47  of resonator  45  and/or those of layer  28  of resonator  50 .  
         [0015]    Temperature compensation of another kind besides the opposite coefficients of certain layers in resonators  10 ,  30 ,  45  and  50 , may be achieved in parallel or alternatively. That kind is electronic compensation as shown with an illustrative example  40  in FIG. 3. An output of resonator  31  may be amplified by amplifier  32 . The output of amplifier  32 , which may be a periodic waveform at the resonator frequency, may go to a frequency counter  33 . Frequency counter  33  counts the periodic cycling of the signal from amplifier  32  over a set period of time. For example, a count for a duration of one second could be a common indication of the frequency of a device such as resonator  31 . The count from counter  33  may go to a compare count and control circuit  34 . Another input to circuit  34  may be from a high resolution time reference  35 . One illustrative example of a time reference is a Global Positioning System (GPS) acquisition period or time which may occur every 30 seconds. The state of frequency counter  33  may be checked at equal time intervals, each of which may be determined by the GPS protocol. The state of the counter may be compared to a present time reference value, which is indicative of a preferred frequency of resonator  31 . If there is a difference between the state of counter  33  and the present value, then the frequency of resonator  31  may be adjusted.  
         [0016]    For adjustment of the frequency of resonator  31 , a DC current may be applied to contacts  19  and  20  of embodiments  10  and  45  or to contacts  20  and  28  of embodiments  30  and  50  of resonator  31 . Internally to the layers  16  and  17 , or layers  16  and  28 , there is a resonator resistance  37  that heats resonator  31 . In some embodiments, the piezoelectric layer may have some leakage (e.g., resistance) which may be used to resistively heat resonator  31 . Alternatively, or in addition, one or more heating resistor(s)  36  may be provided in thermal communication with resonator  31 . For example, a heating resistor may be provided on a top  29  of the top contact layer  28 . In either case, a transistor or the like may be used to adjust the current in the heating resistor(s) to tune the resonant frequency of the resonator  31 . The power requirements of resonator  31  may be reduced by providing resonator  31  on a thin diaphragm and/or in a vacuum package, both of which may reduce the thermal dissipation and mass of resonator  31 .  
         [0017]    Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.