Abstract:
The present invention provides a method for making high-frequency piezoelectric resonators so that constants of the resonator can be measured precisely. A cavity is formed at a central section of an AT-cut crystal substrate. Two grooves are formed at predetermined distances from the left and right of the cavity, and two more grooves are formed at predetermined distances outward from these two grooves. Two more grooves perpendicular to the first set of grooves are formed. A pair of main electrodes and a pair of secondary electrodes shorted to ground and surrounding the main electrodes are,disposed at roughly the center of the crystal substrate. One main electrode and one secondary electrode are used as inputs and the other main electrode and secondary electrode are used as outputs, with these two terminal pairs being used to measure and adjust a frequency.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a piezoelectric resonator, especially to an improved fundamental wave piezoelectric resonator in which precise measurements for frequency and constants can be measured.  
         BACKGROUND OF THE INVENTION  
         [0002]    Piezoelectric resonators are widely used in electric circuits because, not only are they small and light, but also they are more stable in frequency and suffer less change over time than other electric parts. Among piezoelectric resonators, high-frequency crystal resonators, in which a cavity is formed in a part of a crystal substrate, are currently used especially for VHF and UHF bands.  
           [0003]    [0003]FIG. 3( a ) shows a plan drawing of a conventional high-frequency quartz resonator  30  and FIG. 3( b ) is a cross section figure of the conventional high-frequency quarts resonator  30  at the  3 ( b )- 3 ( b ) plane. A cavity  32  is formed by photolithography and etching in the center of one of the main facets of an AT-cut crystal substrate  31  wherein the cavity  32  is a resonator. An electrode  33   a  is formed in the flat side of the crystal substrate  31 , and a lead electrode  34   a  extends from the electrode  33   a  to the edge of the crystal substrate  31  and connects to a pad electrode  35   a.  Further an electrode  33   b  is formed in the cavity  32  facing against to the electrode  33   a,  wherein a lead electrode  34   b  extends from the electrode  33   b  to the edge of the crystal substrate and connects to a pad electrode  35   b  to form a high-frequency resonator  30 . It has been known that the resonant frequency of a high-frequency crystal resonator  30  is inversely proportional to the thickness of the vibrate portion of the cavity  32 , and the levelness and the flatness of the cavity  32  are known to have great influences on the various characteristics of the high-frequency crystal resonator  30  and the spurious output near the resonance frequency.  
           [0004]    When a high-frequency crystal resonator is used in a voltage-controlled crystal oscillator (VCXO), it is preferable to drive the resonator on the fundamental mode in order to widen the variable range of the frequency and not to deteriorate the capacitance ratio of the crystal resonator.  
           [0005]    [0005]FIG. 4 shows another conventional high-frequency crystal resonator  30 ′ that has been improved to suppress spurious appearing near the resonance frequency of the high-frequency crystal resonator shown in FIG. 3. A pair of secondary electrodes  36   a  and  36   b  are formed with a gap around a pair of main electrodes  33   a  and  33   b.  The secondary electrodes  36   a  and  36   b  are short-circuited each other, and may be grounded so that the shield effect between input and output terminals are brought about by the secondary electrode  36   a  and  36   b.    
           [0006]    In these high-frequency vibratos  30 ′, it was more difficult to measure various constants accurately at higher frequencies because the various constants are measured by a method using a π circuit through the pair of electrode pads  35   a,    35   b  connecting with the lead electrodes  34   a,    34   b,  and the extending from the main electrodes  33   a,    33   b.  The measurements are prone to the influence of floating capacitance and the like. For example, the IEC standards for π circuit measurements set the upper limit of the measurement to be 125 MHz, and it is not possible to precisely measure for higher frequencies beyond that upper limit.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0007]    The object of the present invention is to overcome these problems and to provide a piezoelectric resonator that allows precise measurement of resonance frequencies of high-frequency crystal resonators such as 600. MHz crystal resonators.  
           [0008]    In order to achieve this object, the present invention provides a method for making crystal resonators including an AT-cut crystal substrate, a pair of electrodes disposed roughly at the center thereof, and a pair of secondary electrodes, which are formed in a shape surrounding the primary electrodes and are electrically short-circuited. The method of the present invention includes: a step for grounding the secondary electrodes and measuring a frequency of a two-terminal pair circuit, with a primary electrode and the secondary electrodes serving as output terminals and another primary electrode and the secondary electrodes serving as output terminals; and a step for performing frequency adjustments when there is a difference between a measured frequency and a desired frequency.  
           [0009]    An embodiment of the present invention provides a method for making crystal resonators including an AT-cut crystal substrate including a cavity formed on one main surface thereof, a pair of primary electrodes disposed roughly at the center of the cavity, and a pair of secondary electrodes, which are formed in a shape surrounding the primary electrodes and are electrically short-circuited. This method includes: a step for forming on one main surface of the AT-cut crystal substrate a cavity, first and second grooves disposed rightward and leftward from the cavity, third and fourth grooves disposed on either outer side of the first and second grooves, and fifth and sixth grooves formed above and below the cavity; a step for grounding the secondary electrodes; and a step for measuring a frequency of a two terminal pair circuit and performing frequency-adjustment if there is a difference between a measured frequency and a desired frequency, an input terminal for the measuring being formed by respectively connecting two pad electrodes disposed at positions between the first and third grooves with one primary electrode and the secondary electrodes, and an output terminal for the measuring being formed by respectively connecting two pad electrodes disposed at position between the second and fourth grooves with another primary electrode and the secondary electrodes.  
           [0010]    In another embodiment of the present invention, individual crystal resonators are obtained by dividing along the first, second, fifth, and sixth grooves.  
           [0011]    The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1( a ) is a front plan view drawing of high-frequency crystal resonators arranged as a matrix on an AT-cut crystal substrate according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 1( b ) is a front plan view drawing of a high-frequency crystal resonator which is separated to an individual piece of the AT-cut crystal substrate of FIG. 1( b );  
         [0014]    [0014]FIG. 1( c ) is a back view of the high-frequency crystal resonator of FIG. 1( b );  
         [0015]    [0015]FIG. 1( d ) is a cross sectional view taken at the  1 ( d )- 1 ( d ) plane of the high-frequency crystal resonator of FIG. 1( b );  
         [0016]    [0016]FIG. 2( a ) is a front plan view drawing of high-frequency crystal resonators arranged as a matrix on an AT-cut crystal substrate according to another embodiment of the present invention;  
         [0017]    [0017]FIG. 2( b ) is a front plan view drawing of a high-frequency crystal resonator which is separated to an individual piece of the AT-cut crystal substrate of FIG. 2( a );  
         [0018]    [0018]FIG. 2( c ) is a back view of the high-frequency crystal resonator of FIG. 2( b );  
         [0019]    [0019]FIG. 2( d ) is a cross sectional view taken at the  2 ( d )- 2 ( d ) plane of the high-frequency crystal resonator of FIG. 2( b );  
         [0020]    [0020]FIG. 3( a ) is a front plan view drawing of an example of a conventional high-frequency crystal resonator;  
         [0021]    [0021]FIG. 3 ( b ) is a cross sectional view taken at the  3 ( b )- 3 ( b ) plane of the conventional high-frequency crystal resonator of FIG. 3( a );  
         [0022]    [0022]FIG. 4( a ) is a front plan view drawing of another example of a conventional high-frequency crystal resonator; and  
         [0023]    [0023]FIG. 4( b ) is a cross sectional view taken at the  4 ( b )- 4 ( b ) plane of the conventional high-frequency crystal resonator of FIG. 4( a ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The embodiments of th e present invention are explained using the figures.  
         [0025]    FIGS.  1 ( a ),  1 ( b ),  1 ( c ),  1 ( d ) show the composition of a high-frequency crystal resonator according to the present invention, wherein FIG. 1( a ) is a plan drawing of an AT-cut crystal wafer  2 , on which many high-frequency crystal resonators  1  are formed in a matrix-like arrangement. FIG. 1( b ) is a plan drawing (front view) of a single high-frequency crystal resonator  1 β which is cut out from the crystal wafer  2  individually and FIG. 1( c ) is a back view of the high-frequency crystal resonator  1 β of FIG. 1( b ). FIG. 1( d ) is a cross section of the high-frequency crystal resonator  1 β taken at the  1 ( d )- 1 ( d ) plane.  
         [0026]    The high-frequency crystal resonators  1  of the present invention are formed on the crystal wafer  2  in a matrix-like arrangement. Here, however, a unit of a high-frequency crystal resonator  1  is explained for the sake of simplicity. At first, a cavity  3  with a predetermined thickness is formed by photolithography and etching at roughly the center of one surface of the AT-cut crystal wafer  2 . Also, first and second grooves  4   a  and  4   b  are formed on the crystal wafer  2  at predetermined distances from the left and right of the cavity  3 . Third and fourth grooves  4   c  and  4   d  are formed at predetermined distances outward from the first and second grooves  4   a  and  4   b.  Furthermore, fifth and sixth grooves  4   e  and  4   f  are formed perpendicular to the first groove  4   a  and the second groove  4   b  to form a crystal substrate  2 ′ to be used for the single high-frequency crystal resonator  1 β. The crystal substrate  2 ′ is comprised of parts α, β, γ as shown in FIG. 1( a ) and are also referred to as crystal substrates  2 ′α,  2 ′β,  2 ′γ. The parts α, β, γ are divided by the grooves  4   a,    4   b,    4   c,    4   d,    4   e  and  4   f  formed by etching for breaking off the crystal substrates. The cavity  3  is formed by etching at the rear center of β (called crystal substrate  2 ′β).  
         [0027]    An electrode  5   a  is attached to the flat side of a crystal substrate  2 ′ made as described above, and another electrode  5   b  is attached to the cavity  3 , at the opposite side of the crystal substrate  2 ′ as electrode  5   a.  Lead electrodes  6   a  and  6   b  extend from the electrode  5   a  and  5   b  to the edges of the crystal substrate  2 ′β. Furthermore, the lead electrode  6   a  extends to the edge of the crystal substrate  2 ′γ and connects to a pad electrode  7  for measurement. The lead electrode  6   b  in the reverse side extends to the terminal electrode  6   b ′ located at the edge of the substrate  2 ′β, and the terminal electrode  6   b ′ is connected to a terminal electrodes  6   b ″ by way of the metal plating of a through hole h 1 . In addition, the terminal electrode  6   b ″ is connected to a pad electrode  8  for measurement which is formed at the edge of the crystal substrate  2 ′α.  
         [0028]    A pair of secondary electrodes  9   a  and  9   b  are formed surrounding the driving electrodes  5   a,    5   b  with a gap between each driving electrode  5   a,    5   b  and lead electrodes  10   a  and  10   b  extend from the left and right edge of the secondary electrode  9   a  to the edge of the crystal substrate  2 ′β. The lead electrodes  10   a  and  10   b  extend over the crystal substrates  2 ′α and  2 ′γ and connect to the pad electrodes  10   a ′ and  10   b ′ for measurement. Furthermore, the lead electrodes  10   c  and  10   d  extend from the secondary electrodes  9   a  and  9   b  toward the edge of the crystal substrate  2 ′β and connect to the terminal electrodes  10   c ′ and  10   d ′ which are formed at the edge of the crystal substrate  2 ′β. A through hole h 2  is formed between the terminal electrodes  10   c ′ and  10   d ′, and the terminal electrodes  10   c ′ and  10   d ′ are electrically connected through the metal plating on the through hole h 2 .  
         [0029]    By forming multiple high-frequency crystal resonators  1 , each of which serve as-the smallest unit, on the large wafer  2  as shown in FIG. 1( a ) and by grounding the terminal  10   c ′, the constants of the crystal resonators  1 , each of which serve as the smallest unit, can be measured using the S-parameter method, which is suited for high-frequency measurements. In this case, the pad electrodes  8 ,  10   a ′ serve as input terminals and the pad electrodes  7 ,  10   b ′ serve as output terminals. The resulting measurements provide improved accuracy. At the same time, the pad electrodes  8 ,  10   a ′,  7  and  10   b ′ touched by a probe for measurements are positioned away from each individual resonator  2 ′β where the resonator is formed, and the pad electrodes  8 ,  10   a ′,  7  and  10   b ′ are separated from the individual resonator  2 ′β by the grooves  4   a  and  4   b.  Thus, the stress caused by the touch of the probe is moderated, the effect on the resonator  1 β is lowered, and the frequency change due to the stress-strain on measurement can be kept very small. The reason why the input pad electrodes  8 ,  10   a ′ and the output pad electrodes  7 ,  10   b ′ are placed diagonally to each other is to accommodate the S-parameter probe.  
         [0030]    After the measurement of various constants and the adjustment of frequency, if needed, single high-frequency resonators  1 β are obtained by separating the grooves  4   a,    4   b,    4   c,    4   d  on the edges of the crystal substrates  2 ′α,  2 ′γ and the grooves  4   e  and  4   f  on the edge of the crystal substrate  2 ′β. The high-frequency crystal resonator  1 β is completed by being placed at the bottom of a cavity in a ceramic package, connecting the terminal electrode  10   c ′ of the secondary electrode  9   a  to the ground terminal, and closing the ceramic package with an airtight metal lid.  
         [0031]    Since the secondary electrodes  9   a  and  9   b  in the high-frequency crystal resonator  1 β in the present embodiment are grounded and the electric potential of the secondary electrodes  9   a  and  9   b  is the same as the ground electrode of the package, a resonator  1 β with minimum floating capacitance can be obtained. When a VCXO is made by using a high-frequency crystal resonator  1 β constructed as above, such as a 600 MHz resonator, the variable range of frequency is widened and frequency stability is improved.  
         [0032]    FIGS.  2 ( a ),  2 ( b ),  2 ( c ) show another embodiment of a high-frequency crystal resonator  11 . FIG. 2( a ) is a plan drawing of an AT-cut crystal wafer  12 , on which many high-frequency crystal resonators  11  are formed in a matrix like arrangement by photolithography and etching. FIG. 2( b ) is a plan drawing (front view) of the high-frequency resonator  11  which is cut out from the crystal wafer  12  individually. FIG. 2( c ) is a back view of the high-frequency resonator  11  of FIG. 2( b ). FIG. 2( d ) is a cross section of the high-frequency resonator  11  taken at the  2 ( d )- 2 ( d ) plane. Many of the cavities  3  and grooves  4  for cutting which surround the cavity  3  are formed in a matrix-like arrangement by using a photolithography and etching technique on the AT-cut crystal wafer  12  of a predetermined thickness. One high-frequency crystal resonator  11  on the AT-cut wafer  12  is described for the sake of simplicity. At the back of an individual piece of a crystal substrate  12 ′, a cavity  3  is formed in the center, and the grooves  4  for breaking the AT-cut wafer  12  into individual pieces  12 ′ surround the cavity  3 . In the center of the flat side of the crystal substrate  12 ′, a driving electrode  15   a  is formed and a leading electrode  16   a  extends from the electrode  15   a  toward the edge of the crystal substrate  12 ′ and connects to a pad electrode  16   a ′. An electrode  15   b  is formed in the cavity  3  facing against to the electrode  15   a,  and a lead electrode  16   b  extends from the electrode  15   b  toward the edge of the crystal substrate  12 ′ and connects to a pad electrode  16   b ′ which is formed at the edge of the backside of the crystal substrate  12 ′. In front side, a pad electrode  16   b ″ is formed facing to the terminal electrode  16   b ′. The two electrodes  16   b ′ and  16   b ″ are electrically connected through a through hole h 1 .  
         [0033]    Further, a pair of secondary electrodes.  17   a  and  17   b,  which surround the main electrodes  15   a  and  15   b,  are formed both side of the resonator  11  facing each other. Lead electrodes  18   a  and  18   b  are extend from each secondary electrode  17   a  and  17   b  toward the edge of the crystal substrate.  12 ′ and connect to the terminal electrodes  18   a ′ and  18   b ′ which are formed at the edge of the crystal substrate  12 ′. A through hole h 2  is formed between the lead electrodes  18   a ′ and  18   b ′, which are connected electronically through the metal plating on the through hole h 2 . A pad electrode  19  is formed in the middle of the electrode  18   a  on the front side wherein an angle θ between the line connecting the pad electrodes  16   b ″ and  19  and the x axis is set to be about 60°, and the angle θ between the line connecting the pad electrodes  16   a ′ and  19  and x axis is also set to be about 60°. Also, the above mentioned angle θ may be set to be 120°. The angle is determined so that the stress-frequency sensitivity is minimized in order to lessen the frequency change due to the stress caused by touching the pad electrodes  16   a ′,  16   b ″ and  19  with a probe for measurement of various constants of the high-frequency crystal resonator. It is also possible to minimize the frequency change from support stress by having the high-frequency crystal resonator  11  supported at the pad electrodes  16   b ″ and  16   a ′ when placing the high-frequency crystal resonator  11  in the ceramic package.  
         [0034]    The deterioration of the capacitance ratio due to the floating capacitance can be prevented by grounding the secondary electrodes  9   a,    9   b ,  17   a,    17   b,  and at the same time the spurious output can be effectively suppressed.  
         [0035]    The present invention provides an accurate measurement of various constants of the high-frequency crystal resonator. An embodiment of the present invention provides a crystal resonator which is suitable for high frequency voltage controlled crystal oscillators because the floating capacitance can be kept low and the capacitance ratio can be kept small. An embodiment of the present invention provides a crystal resonator with high frequency stability in which the support influence is minimized and deterioration of the capacitance ratio is prevented.  
         [0036]    Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.