Abstract:
A capacitor device with a capacitance is introduced. The capacitor device includes at least one capacitive element. The at least capacitive element comprises a pair of first conductive layers being opposed to each other, at least one first dielectric layer formed on a surface of at least one of the first conductive layers, and a second dielectric layer being sandwiched between the first conductive layers. The first dielectric layer has a first dielectric constant and the second dielectric layer has a second dielectric constant. The capacitance of the capacitor device depends on dielectric parameters of the first dielectric layer and the second dielectric layer. The dielectric parameters comprise the first dielectric constant and thickness of the at least one first dielectric layer and the second dielectric constant and thickness of the second dielectric layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention generally relates to capacitors, and more particularly, to capacitors that may be embedded within printed circuit boards or other microelectronic devices. 
         [0003]    2. Background of the Invention 
         [0004]    Capacitors are devices used for introducing capacitance into a circuit. Capacitors function primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. They comprise a layer of dielectric material sandwiched between a pair of spaced conductive metal layers, such as copper foils. 
         [0005]    Capacitors are common elements on printed circuit boards (PCBs) and other microelectronic devices. In recent years, substantial efforts have been expended in the design of such PCBs and devices arranged thereupon to compensate for voltage fluctuations arising between the power and ground planes in the PCBs. One common type of voltage fluctuations include “switching noises,” which may be caused by switching operation of transistors in the integrated circuits. A common solution to this problem is to place one or more capacitors serving as a decoupling capacitors or bypass capacitors, which may be coupled between the power and ground terminals in proximity to the integrated circuits. 
         [0006]    Capacitors may be electrically connected either as discrete elements on a circuit board, or may be embedded within the circuit boards. Of these options, forming embedded capacitors within the circuit boards allows increased surface area of the board for other purposes. 
         [0007]    Two main factors for selection of a capacitor include the capacitance and the frequency bandwidth of a capacitor. The frequency bandwidth of a capacitor depends on its self-resonance frequency because a capacitor behaves properly when it operates in a frequency below the self-resonance frequency. Equation (1) below shows the relationship between capacitance and self-resonance frequency of a capacitor 
         [0000]    
       
         
           
             
               
                 
                   fr 
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where f r  represents the self-resonance frequency, L represents the parasitic inductance (i.e., equivalent series inductance “ESL”), and C represents the parasitic capacitance (i.e., equivalent series capacitance “ESC”). According to Eq. (1), a capacitor with smaller capacitance may have higher self-resonance frequency, thereby having a broad frequency bandwidth. On the other hand, a capacitor with larger capacitance may have lower self-resonance frequency, thereby having a narrow frequency bandwidth. However, for decoupling capacitors, it is highly desirable to have a high self-resonance frequency and high capacitance. 
         [0008]    Capacitance, in general, can be determined by the equation below: 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     ɛ 
                      
                     
                         
                     
                      
                     
                       A 
                       d 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C represents the capacitance of the capacitor in Farads, ∈ represents the dielectric constant of the dielectric material, and A represents the surface area of the dielectric material held between two conducting plates and d represents the distance between the plates. According to Eq. (2) above, capacitance is proportional to the surface area of the conducting plates and the dielectric constant, of the dielectric material, and inversely proportional to the distance between the plates. Thus, in order to increase the capacitance of a capacitor, one may increase the area of the conducting plates or select an extremely thin layer of a dielectric material with a high dielectric constant. However, each of these approaches presents difficulties. First, increasing the area of the conducting plates departs from the object of compact designs. In addition, the selection of the dielectric material is often limited by many production and configuration limitations. Additional difficulties arise when the thickness of a dielectric layer is reduced. In particular, the thickness of a dielectric layer on a circuit board can be difficult to control because dielectric thickness may be dramatically changed due to the shapes and dimensions of the patterned features (e.g., capacitor electrodes) over which dielectric is deposited. A thin-dielectric layer design usually comes with the danger of having metal-to-metal shorting through the thin dielectric layer and of having microscopic voids or other structural defects in the layer that may impact capacitive effects and characteristics. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Examples consistent with the present invention may provide a capacitor device with a capacitance and a method of fabricating a capacitor. 
         [0010]    One example of the present invention provides a capacitor device with a capacitance comprising at least one capacitive element. The at least one capacitive element comprises a pair of first conductive layers being opposed to each other, at least one first dielectric layer formed on a surface of at least one of the first conductive layers, and a second dielectric layer being sandwiched between the first conductive layers. The first dielectric layer has a first dielectric constant and the second dielectric layer has a second dielectric constant. The capacitance of the capacitor device depends on dielectric parameters of the first dielectric layer and the second dielectric layer. The dielectric parameters comprise the first dielectric constant and thickness of the at least one first dielectric layer and the second dielectric constant and thickness of the second dielectric layer. 
         [0011]    Another example of the present invention provides a method of fabricating a capacitor comprising providing a pair of first conductive layers, forming at least one first dielectric layer on one of the first conductive layers, and laminating the first conductive layers and the at least one first dielectric layer with a second dielectric layer. 
         [0012]    One example consistent with the present invention provides a capacitor device comprising a number of capacitive elements. At least one of the capacitive elements comprises a first conductive layer and a second conductive layer being opposed to the first conductive layer, at least one first dielectric layer formed on a surface of at least one of the first and the second conductive layers, and a second dielectric layer being sandwiched between the first and the second conductive layers via the at least one first dielectric layer. The first dielectric layer has.a first dielectric constant and the second dielectric layer has a second dielectric constant. At least one of the first and the second conductive layers of the capacitive element is coupled to a conductive layer. of another capacitive element. 
         [0013]    Another example consistent with the present invention provides a capacitor device having a number of capacitive elements. The capacitor device comprises a first capacitive element comprising a pair of first conductive layers being opposed to each other, at least one first dielectric layer formed on a surface of at least one of the first conductive layers, and a second dielectric layer being sandwiched between the first conductive layers via the at least one first dielectric layer. The capacitor device further comprises a second capacitive element comprising a pair of second conductive layers being opposed to each other, at least one third dielectric layer formed on a surface of at least one of the second conductive layers, and a fourth dielectric layer being sandwiched between the second conductive layers via the at least one third dielectric layer. The at least one first dielectric layer has a first dielectric constant and the at least one third dielectric layer has a third dielectric constant. The third dielectric constant being different from the first dielectric constant. 
         [0014]    One example consistent with the present invention provides a capacitor device with a capacitance. The capacitor device comprises a pair of first conductive layers being opposed to each other, and a dielectric layer being sandwiched between the first conductive layers. The dielectric layer comprises at least a first dielectric material with a first dielectric constant and a second dielectric material with a second dielectric constant different from the first dielectric constant to form at least two capacitive elements in parallel sharing the first conductive layers. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended, exemplary drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0016]    In the drawings: 
           [0017]      FIGS. 1(   a )-( d ) are cross-sectional views of a metal-insulator-metal capacitor in the prior art; 
           [0018]      FIGS. 2(   a )-( f ) are cross-sectional views of a metal-insulator-metal capacitor in examples consistent with the present invention; 
           [0019]      FIGS. 3(   a )-( b ) are cross sectional views of a metal-insulator-metal capacitor in examples consistent with the present invention; 
           [0020]      FIG. 3(   c ) is an equivalent electrical circuit of structure of  FIG. 3(   b ); 
           [0021]      FIG. 3(   d ) is an impedance curve of a capacitor of  FIG. 30(   b ); . 
           [0022]      FIGS. 4(   a )-( b ) are cross sectional views of a metal-insulator-metal capacitor in examples consistent with the present invention; 
           [0023]      FIG. 4(   c ) shows equivalent structure of  FIG. 4(   b ); 
           [0024]      FIGS. 5(   a )-( e ) are cross sectional views of a metal-insulator-metal capacitor in examples consistent with the present invention; 
           [0025]      FIG. 6(   a ) is a cross sectional views of a metal-insulator-metal capacitor in examples consistent with the present invention; 
           [0026]      FIG. 6(   b ) is an equivalent electrical circuit of structure of  FIG. 6(   a ); 
           [0027]      FIG. 6(   c ) is an impedance curve of a capacitor of  FIG. 6(   a ); 
           [0028]      FIG. 6(   d ) is an impedance curve of three SMD capacitors in parallel; and 
           [0029]      FIGS. 7(   a )- 7 ( c ) show a capacitive core in examples consistent with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    U.S. Pat. No. 5,800,575 describes one method of fabricating a metal-insulator-metal (MIM) capacitor. Referring to  FIG. 1(   a ), the fabrication process may start from forming an initial lamination product  50  which includes the fully cured dielectric sheet  40 ′ with conductive foils  28 ′ and  46 ′ laminated or bonded on opposite sides of the dielectric sheet  40 ′. Thereafter, the conductive foil  28 ′ is etched as indicated in  FIG. 1(   b ). Referring to  FIG. 1(   c ), another lamination product  52  is formed in a similar manner as the lamination product  50 . The lamination product  52  includes the other dielectric layer  42 ′ and the conductive foils  30 ′ and 48′. An uncured dielectric sheet  32 ′ is then arranged between the lamination products  50  and  52  so that it is adjacent to both the conductive foils  28 ′ and  30 ′. After a conventional lamination to convert the uncured dielectric sheet  32 ′ to a fully cured condition, the finished capacitive PCB  10 ′ is formed as shown in  FIG. 1(   d ). The thickness of the dielectric sheet  32 ′ is usually reduced in order to obtain large capacitance. However, a thin-dielectric sheet design may cause undesirable metal-to-metal shorting through the thin dielectric sheet. 
         [0031]    One example of the present invention provides a capacitor which comprises at least one dielectric layer coated on at least one of the conductive layers serving as electrodes of a capacitor, prior to lamination with an intermediate dielectric layer. In this manner, the conductive layers are protected by the at least one dielectric layer from contacting each other.  FIGS. 2(   a )-( f ) show methods of fabricating a metal-insulator-metal capacitor in examples consistent with the present invention. The fabrication process may include forming an initial structure  210  which includes a carrier  212  and a conductive layer  214 . In some examples, the carrier  212  may include prepreg, which may be a reinforced material impregnated with epoxy resin or fiber-reinforced material coated with epoxy. In one example, the carrier  212  may have a thickness between about 9 μm to 36 μm and is made of one or more conductive materials, such as copper. The conductive layer  214  may be etched as shown in  FIG. 2(   a ). The conductive layer appropriate for the purpose of the present invention may vary depending on the desired applications. In some examples, the conductive layer  214  may include a material selected from the group consisting of copper, zinc, brass, chrome, chromates, titanium nitride, nickel, silanes, aluminum, stainless steel, iron, gold, silver, titanium, and combinations thereof. In one example, the conductive layer  214  may include or be made of copper, and its thickness may be in the range from 5 μm to 75 μm. 
         [0032]    As shown in  FIG. 2(   a ), similar to structure  210 , another initial structure  220  is formed to include a carrier  222  and a conductive layer  224 . Prior to lamination of the structures  210  and  220  with an intermediate dielectric layer  230 , another dielectric layer is formed on at least one of the conductive layers  214  and  224 . For example, a dielectric layer  226  is formed on the conductive layers  224  as shown in  FIG. 2(   a ) and dielectric layers  216  and  226  are formed on one of the conductive layers  214  and one of the conductive layers  224  as shown in  FIG. 2(   c ). In another example, two dielectric layers  216  and  226  are formed on the both conductive layers  214  and  224  as shown in  FIG. 2(   e ). The dielectric layer may be formed by screen printing, inkjet printing, or any other technique that may provide a thin dielectric layer. The dielectric layer may include a dielectric material having a dielectric constant as high as several hundred and may have a thickness of about 5 μm, but the thickness may be varied depending on the various applications. Examples of high dielectric constant or high K materials may include epoxies, polyesters, polyester containing copolymers; aromatic thermosetting copolyesters, polyarylene ethers and fluorinated polyarylene ethers, polyimides, benzocyclobutenes, liquid crystal polymers, allylated polyphenylene ethers, amines, inorganic materials such as barium titanate (BaTiO 3 ), boron nitride (BN), aluminum oxide (Al 2 O 3 ), silica, strontium titanate, barium strontium titanate, quartz and other ceramic and non-ceramic inorganic materials and combinations thereof. 
         [0033]    After the at least one dielectric layer is applied to one of the conductive layers  214  and  224 , the two structures  210  and  220  may be pressed against the intermediate dielectric layer  230  to form a structure as illustrated in  FIGS. 2(   b ),  2 ( d ) or  2 ( f ), where portions of the intermediate dielectric layer  230  are sandwiched between the conductive layers  214  and  224  via at least one dielectric layer  216  and/or  226 . The dielectric layer  230  may be a dielectric material with a high dielectric constant as described above. In one example, the dielectric constant of the dielectric layer  230  may be lower than the dielectric constant of the dielectric layer  216  and/or  226 . The thickness of the dielectric layer  230  may be about 20 μm. 
         [0034]    With the capacitor design illustrated above, the conductive layers  214  and  224  are protected by the dielectric layer  216  and/or  226  from making contacts or shorting with each other. In addition, by having a dielectric structure comprising the dielectric layer  230  and the dielectric layer  216  or  226 , the dielectric constant of the dielectric structure may be controlled by the intermediate dielectric layer  230 , and the dielectric layers  216  and  226 . In addition, the capacitance depends on the thickness of the dielectric layers  216  and/or  226  and the intermediate dielectric layer  230 . 
         [0035]      FIGS. 3(   a ) and  3 ( b ) show fabrication of an MIM capacitor in examples consistent with the present invention. Referring to  FIG. 3(   a ), each of the structures  310  and  320  includes a carrier ( 312  or  322 ) and a conductive layer ( 314  or 324 ). On the patterned conductive layers  314  and  324 , dielectric layers are formed. The dielectric layers formed on the patterned conductive layers  314  and  324  may have different dielectric constants by having different dielectric materials or different combination of dielectric materials. In one example, the dielectric layer  316   a  has the same dielectric constant as the dielectric layer  326   a  while the dielectric layer  316   b  has the same dielectric constant as the dielectric layer  326   b . After lamination of the structures  310  and  320  with the intermediate dielectric layer  330 , capacitors C 1  and C 2  are formed as shown in  FIG. 3(   b ). Since the dielectric constant for the capacitor C 1  is different from the dielectric constant for the capacitor C 2 , the capacitors C 1  and C 2  have different capacitance. An equivalent electrical circuit of  FIG. 3(   b ) is shown in  FIG. 3(   c ) where the capacitors C 1  and C 2  are connected in parallel.  FIG. 3(   d ) is the impedance curve of capacitors of  FIG. 3(   b ), which shows that, with capacitors in parallel, the bandwidth, such as the bandwidth for reducing or eliminating noises of different frequencies, for the capacitors may become broader. 
         [0036]      FIGS. 4(   a )-( b ) show an MIM capacitor consistent with examples of the present invention. Similar to  FIG. 3(   a ), each structure ( 410  or  420 ) includes a carrier ( 412  or  422 ), a patterned conductive layer ( 414  or  424 ), and a dielectric layer ( 416  or  426 ) on the patterned conductive layer. In addition, there are thin conductive layers  418  and  428  formed on each dielectric layer as shown in  FIG. 4(   a ). After lamination of the two structures  410  and  420  with the dielectric layer  430 , a capacitor with higher capacitance as shown in  FIG. 4(   b ) may be formed. As illustrated in  FIG. 4(   c ), . the distance between the conductive layers  414  and  424  may be reduced by the thin conductive layers  418  and  428 . Accordingly, the capacitance may increase. In one example, a number of thin conductive layers may be included between the conductive layers  414  and  424  to reduce the distance between the conductive layers, thereby increasing the capacitance. The conductive layers and the thin conductive layers may include or be made of one or more of the conductive materials noted above. The thin conductive layers  418  and  428  may be formed on an underlying dielectric layer using a printing and/or coating technique. Each dielectric layer may include or be made of one or more high dielectric constant materials noted above and may be printed and/or coated on its underlying layer. 
         [0037]      FIG. 5(   a ) shows an MIM capacitor in examples consistent with the present invention. In this example, the structures  510  and  520  may include a carrier ( 512  or  522 ), a conductive layer ( 514  or  524 ) and a number of spots or other patterns ( 516  or  526 ) of a high-dielectric-constant material on the surface of the conductive layers ( 514  or  524 ). The spots may be formed by inkjet printing or other techniques. The spots may form any pattern or any combination of patterns and the pattern may be formed through the control of the formation process, such as an inkjet printing process. The structures  510  and  520  with spots may be pressed against the intermediate dielectric material  530  as shown in  FIG. 5(   b ). Where the spots  516  or  526  are formed from a dielectric material, these spots may protect the conductive layers  514  and  524  from metal-to-metal shorting. In addition, the dielectric constant for the capacitors  500   a  and  500   b  may depend on the distance between the neighboring spots. 
         [0038]      FIG. 5(   c ) shows another MIM capacitor in examples consistent with the present invention. Similar to  FIG. 5(   a ), the structures  510  and  520  may include a carrier ( 512  or  522 ), a conductive layer ( 514  or  524 ) and a number of spots ( 516  or  526 ) provided on the surface of the conductive layer ( 514  or  524 ) by inkjet printing or other techniques. The spots include dielectric spots ( 516   a  or  526   a ) of a high dielectric constant material and conductive spots ( 516   b  or  526   b ) of a conductive material. The structures  510  and  520  with the spots may then be pressed against an intermediate dielectric material  530  as shown in  FIG. 5(   d ). In one example, the dielectric spots  516   a  and  526   a  and the conductive spots  516   b  and  526   b  may form a crossed or checkered pattern. The conductive spots and dielectric spots, depending on the spot or pattern arrangements, may provide a capacitor with a wave-like dielectric layer formed by connecting the dielectric spots from the two structures, as shown in  FIG. 5(   e ). With the illustrated example, the capacitance depends on the minimum distance x between the two conductive spots as illustrated in  FIG. 5(   e ). 
         [0039]    In another example, the spots or the dielectric layer may be formed by dielectric materials with different dielectric constants.  FIG. 6(   a ) shows the structure of the capacitors after lamination. Referring  FIG. 6(   a ), capacitor  600   a  has a dielectric layer  630  having three different dielectric constants by having different dielectric materials or different combinations of dielectric materials. As a result, three capacitive elements in parallel are formed. Since these three capacitive elements share the conductive layers  614  and  624 , no additional wiring is required for connecting these capacitive elements in parallel.  FIG. 6(   b ) is an example of an equivalent electrical circuit of the structure of  FIG. 6(   a ).  FIG. 6(   c ) is the impedance curve of the capacitor of  FIG. 6(   a ), which shows a broader effective bandwidth than that of the SMD capacitors in parallel as shown in  FIG. 6(   d ). 
         [0040]    Above discussion is directed to a single MIM capacitor. In some examples; a number of capacitive elements  710   a ,  710   b ,  710   c  consistent with the present invention may form a set of capacitors  720  as shown in  FIG. 7(   a ).  FIG. 7(   b ) shows another exemplary set of MIM capacitors consistent with the present invention.  FIG. 7(   b ) includes capacitive elements  730   a  and  730   b  in parallel and a capacitive element  730   c .  FIG. 7(   c ) shows an exemplary set of MIM capacitors consistent with the present invention.  FIG. 7(   c ) includes capacitive elements  740   a ,  740   b  and  740   c . As shown in  FIG. 7(   c ), one of the electrodes of these three capacitors, such as the ground plane  750 , may be coupled together. 
         [0041]    It will be appreciated by those skilled in the artt that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.