Patent Publication Number: US-9431177-B2

Title: Interface for communication between voltage domains

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application Ser. No. 13/478,737 filed on May 23, 2012, the contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Aspects of the present disclosure relate to apparatuses, devices, and methods involving capacitive isolation. 
     Aspects of the present disclosure relate to the transmission of data between circuits that operate in different voltage domains. For example, circuits may be galvanically isolated using capacitive coupling on signal paths between the circuits. As a result of such isolation, the circuits operate in separate voltage domains that are not referenced to one another by a common ground voltage level. As such, large voltage differences may arise between the corresponding voltage domains. Galvanic isolation has been used for a variety of different applications. For instance, galvanic isolation can be provided between multiple integrated circuit chips, which can be located within the same package or in different packages. Signals can be passed between the integrated circuits using galvanic isolation techniques. 
     One method of galvanic isolation uses capacitors on signal paths between two circuits as a means of blocking DC voltages and attenuating low-frequency signals while transmitting high-frequency signals. Due to large voltage differences that may arise between isolated voltage domains for some applications, capacitors having high breakdown voltage may be required. However, physical space constraints may make it difficult to implement capacitors having the required breakdown voltage. For example, a parallel plate capacitor may be implemented alongside other circuitry in an integrated circuit (IC) using conventional processes (e.g., CMOS, complementary metal-oxide-semiconductor). Two capacitive plates are implemented in different metallization layers of the IC and are separated by a dielectric layer. The breakdown voltage of the parallel plate capacitor is dependent on the thickness of the dielectric layer. For higher voltage applications, the thickness of the dielectric layer is increased to provide a higher breakdown voltage. However, in certain CMOS processes, the maximum dielectric thickness that can be implemented is limited to about 5-10 um. For some applications, this thickness is not sufficient to guarantee sufficient breakdown voltage. 
     Aspects of the present disclosure relate to the transmission of data between circuits that are isolated from one another. For example, circuits may be galvanically isolated using capacitive coupling on signal paths between the circuits. As a result of such isolation, the circuits operate in separate voltage domains that are not referenced to one another by a common ground voltage level. As such, voltage differences may arise between the corresponding voltage domains. In certain instances, the voltage differences can be relatively large in comparison to the voltages within each of the separate voltage domains. 
     SUMMARY 
     In one or more embodiments, circuitry is provided for isolation and communication of signals between circuits operating in different voltage domains using capacitive coupling. The embodiments utilize capacitive structures that can be useful for providing high breakdown voltages. The capacitive structures include three capacitive plates arranged to have two plates located in an upper layer and one plate located in a lower layer. In operation, the capacitive structure functions as two capacitors coupled in series (e.g., an input plate and an intermediate plate forming a capacitor and the plate and the output plate for another capacitor). This configuration can be particularly useful for providing a high overall breakdown voltage of the structure. 
     Embodiments of the present disclosure are directed toward an isolation circuit having a first capacitive structure in a first voltage domain. The first capacitive structure can include a first conducting substrate that is electrically connected to a reference voltage of the first voltage domain, such as ground. For instance, the substrate may also have active CMOS components (e.g., transistors) that require that conductive substrate to be connected to ground. The input and intermediate capacitive plates function as a first capacitor. The intermediate and output capacitive plates function as a second capacitor arranged in series with the first capacitor. A first dielectric layer defines/provides a first breakdown voltage by way of the physical separation between the intermediate capacitor plate and each of the input and output capacitor plates. The intermediate capacitor plate can be located below each of the input and output capacitor plates (e.g., in a lower stack layer). A second dielectric layer is configured and arranged to provide substantially the same breakdown voltage as the first breakdown voltage. This is accomplished by setting the distance of the physical separation provided between the conducting substrate and the first intermediate capacitor plate accordingly. Although the two dielectric layers can be part of the same dielectric stack, they are referred to as different layers due to the stacked relationship between the respective plates and/or conductive substrate. The above arrangement of plates can then be repeated for a second capacitive structure in a second voltage domain, which is then arranged in series with the first capacitive structure. 
     In some embodiments, the capacitive structures may be implemented on a substrate having various portions connected to one or more reference voltages (e.g., ground voltages). Each capacitive structure includes a first dielectric layer separating the input and output capacitive plates from the intermediate capacitive plate. Each capacitive structure also includes a second dielectric layer separating the intermediate capacitive plate the substrate. The first and second dielectric layers have thicknesses such that a first breakdown voltage between the capacitive plates is approximately equal to a second breakdown voltage between the lower capacitive plate and the reference voltage tied to the substrate. In some embodiments, the capacitive structures may be implemented on a silicon-on-insulator (SOI) substrate, in which a silicon layer is separated from a silicon handle wafer by an insulating oxide layer. The capacitive structures are arranged such that both the second dielectric layer and the buried oxide provide isolation between the intermediate lower capacitive plate and the substrate. Use of the buried oxide layer of the SOI substrate in this manner provides allows the thickness of the first dielectric layer to be increased compared to a standard silicon wafer substrate. As a result, the breakdown voltage between the capacitive plates is increased. 
     In one or more embodiments, a method of communication between first and second voltage domains is provided. Communication signals are transmitted from a transmitter circuit located in a first voltage domain on a first substrate. Capacitive isolation is provided for the communication signals between the first voltage domain and a second voltage domain by communicating the transmitted communication signals through an isolation circuit. The isolation circuit includes a first capacitive structure located on the first substrate to provide the communication signal from the first voltage domain to an unreferenced voltage domain. The isolation circuit includes a second capacitive structure located on a second substrate that is used to provide the communication signals from the unreferenced voltage domain to a second voltage domain. A receiver circuit is located on the second substrate and is configured to receive the communication signals at an input in the second voltage domain from the second capacitive structure. 
     Consistent with certain embodiments, a device includes a transmitter circuit that is in a first voltage domain and that is configured and arranged to transmit communication signals. A receiver circuit is located in a second voltage domain and has an input that is configured and arranged to receive the communication signals in the second voltage domain. An isolation circuit is configured and arranged to provide capacitive isolation for the communication signals between the first and second voltage domains. The isolation circuit includes a first and second capacitive structure. The first capacitive structure is located on a first substrate in a first voltage domain. The first capacitive structure includes a first/input capacitive plate configured and arranged to receive the communication signals from the transmitter circuit and in the first voltage domain. A second/intermediate capacitive plate is configured and arranged to receive the communication signals from the first capacitive plate at a first floating node of the isolation circuit. A third/output capacitive plate is configured and arranged to receive the communication signals from the second capacitive plate at a second floating node of the isolation circuit. A first dielectric layer is configured and arranged to provide first breakdown voltages of the first capacitive structure by providing an electrical and physical separation between the second capacitive plate and each of the first and third capacitive plates, the physical separation having a first distance. A second dielectric layer provides an electrical and physical separation between the first substrate and the second capacitive plate to provide substantially the same breakdown voltages as the first breakdown voltages. 
     The second capacitive structure is located on a second substrate in a second voltage domain. This second capacitive structure includes a fourth (input) capacitive plate configured and arranged to receive the communication signals from the third capacitive plate at a third floating node of the isolation circuit. A fifth (intermediate) capacitive plate is configured and arranged to receive the communication signals from the fourth capacitive plate at a fourth floating node of the isolation circuit. A sixth (output) capacitive plate is configured and arranged to receive the communication signals from the fifth capacitive plate and to provide the communication signals to the input of the receiver. A third dielectric layer defines second breakdown voltages of the second capacitive structure by providing an electrical and physical separation between the fifth capacitive plate and each of the fourth and sixth capacitive plates, the physical separation having a second distance. A fourth dielectric layer provides an electrical and physical separation between the second substrate and the fifth capacitive plate to provide substantially the same breakdown voltages as the second breakdown voltages. 
     Various embodiments are directed toward methods that include transmitting communication signals from a transmitter circuit located on a first substrate that is in a first voltage domain. Capacitive isolation is provided for the communication signals between the first voltage domain and a second voltage domain by communicating the transmitted communication signals through the isolation circuit by capacitively coupling the communication signals from the first voltage domain to an unreferenced domain using a first capacitive structure located on the first substrate. The isolation circuit can include the first and second isolation structures described in the previous embodiments. The communication signals are then received from the second capacitive structure at an input of a receiver circuit that is in the second voltage domain. 
     The above summary is not intended to describe each embodiment or every implementation of the present disclosure. The figures, detailed description, and claims that follow more particularly exemplify various embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure may be more completely understood in consideration of the detailed description of various embodiments of the present disclosure that follows in connection with the accompanying drawings, in which: 
         FIG. 1  depicts a block diagram of a system for communicating between two isolated voltage domains, consistent with one or more embodiments of the present disclosure; 
         FIG. 2  shows a top view of a high breakdown voltage capacitive structure, consistent with one or more embodiments of the present disclosure; 
         FIG. 3  shows a cross sectional view of the capacitive structure shown in  FIG. 2 , consistent with one or more embodiments of the present disclosure; 
         FIG. 4  shows a cross sectional view of the capacitive structure shown in  FIG. 2 , consistent with one or more embodiments of the present disclosure; 
         FIG. 5  shows a top view of the capacitive structure formed on a partially floating substrate, consistent with one or more embodiments of the present disclosure; 
         FIG. 6  shows a cross sectional view of the capacitive structure shown in  FIG. 5 , consistent with one or more embodiments of the present disclosure; 
         FIG. 7  shows a cross sectional view of the several series capacitive structures, consistent with one or more embodiments of the present disclosure; 
         FIG. 8  shows a method of manufacturing a capacitive structure, consistent with one or more embodiments of the present disclosure; and 
         FIG. 9  illustrates a method of communication between first and second voltage domains, consistent with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure including aspects defined in the claims. 
     Aspects of the present disclosure relate to the transmission of data between circuits that are isolated from one another. For example, circuits may be galvanically isolated using capacitive coupling on signal paths between the circuits. Particular embodiments use parallel plate capacitor structures to provide capacitive coupling. 
     In one or more embodiments, an isolation circuit is used to provide capacitive coupling for communication between circuits operating in different voltage domains. In some particular embodiments of the present disclosure, the capacitive isolation circuits include three capacitive plates arranged to have two (e.g., input and output) plates located in an upper layer and another (e.g., intermediate) plate located in a lower layer. The arrangement of the capacitive plates creates a capacitive field between the plate in the lower layer and each of the plates in the upper layer, respectively. 
     In operation, the isolation circuit functions as two capacitors connected together in series. As such, breakdown voltage measured between the two upper plates, is effectively doubled in comparison to a two-plate capacitor having the same the dielectric thickness constraints. However, even where capacitors are connected in series on the same substrate, breakdown voltage can be reduced due to breakdown between one of the capacitive plates and the substrate. This breakdown can be particularly problematic for CMOS applications, where the substrate may be connected to a ground voltage. In some embodiments, thickness of dielectric layers is such that the breakdown voltage between the intermediate (lower) capacitive plate and each of the input/output (upper) capacitive plates is nominally equal to half of the overall breakdown voltage of the capacitive structure (i.e. is nominally equal to the vertical breakdown voltage between the bottom electrode and a grounded substrate). Similarly, to prevent lateral breakdown between the input and output capacitive plates (located in the upper layer), these two plates are separated by a lateral distance such that the lateral breakdown voltage between the input and output plates is nominally equal to the overall breakdown voltage of the capacitive structure (i.e., lateral breakdown is nominally equal to twice the breakdown voltage between either/each of the upper plates and the lower plate, respectively). 
     In some particular embodiments, the capacitive structure is used to provide galvanic isolation between transmitter and receiver circuits operating in different voltage domains. A first capacitive structure is located in a first voltage domain of the transmitter and a second capacitive structure is located in a second voltage domain of the receiver circuit. Each of the capacitive structures includes three capacitive plates arranged, as described above, to provide a high breakdown voltage. 
     In a first one of capacitive structures, a first/input capacitive plate is configured to receive the communication signal in the first voltage domain from the transmitter. A second/intermediate capacitive plate is configured as an intermediate node of the capacitive structure and operates at a first floating voltage. A third/output capacitive plate is configured to provide the communication signals to the second capacitive structure in the second voltage domain and operates as a second floating node between the two voltage domains. Accordingly, the input and output capacitor plates share the intermediate capacitor plate and provide the function of two series capacitors. A first dielectric layer separates the intermediate capacitive plate, which is located in a first horizontal layer, from each of the input and output capacitive plates, which are located in a second horizontal layer. A second dielectric layer separates the intermediate capacitive plate from the substrate. 
     In the second capacitive structure, a fourth capacitive plate (e.g., a second input capacitive plate) is configured to receive the communication signals from the first capacitive structure. The second input capacitive plate operates as a third floating node between the first and second voltage domains. A fifth capacitive plate (e.g., a second intermediate capacitive plate) is configured as a floating node in the second capacitive structure. A sixth capacitive plate (e.g., a second output capacitive plate) is configured to provide the communication signals in the second voltage domain to the input of the receiver circuit. The second input and output capacitor plates share the second intermediate capacitor plate and provide the function of two series capacitors. A third dielectric layer separates the second intermediate capacitive plate located in a third horizontal layer and each of the second input and output capacitive plates, which are located in a fourth horizontal layer. A fourth dielectric layer separates the second intermediate capacitive plate from the substrate. 
       FIG. 1  shows a system for galvanic isolation between transmitter and receiver circuits operating in different voltage domains. A transmitter circuit  102  operates in a first voltage domain and a receiver circuit  110  operates in a second voltage domain. Isolation circuit  104  provides a communication signal path between the transmitter  102  and receiver  110  while providing galvanic isolation between the two circuits. The isolation circuit  104  includes two capacitive structures  106  and  108 . 
     The isolation circuit  104  is configured to receive data signals from the transmitter  102 . The received data signals can be referenced to the first voltage domain (e.g., reference to a ground). The data signals are eventually received at the receiver circuit  110 ; however, receiver circuit  110  operates in a second voltage domain, (e.g., with reference to a second ground). Intermediate nodes of the isolation circuit, e.g., nodes between circuits  106  and  108 , may be floating voltages that are not referenced to the ground voltage of either voltage domain. 
     Consistent with embodiments of the present disclosure, the capacitive structures of the isolation circuit  104  may be implemented on the same or different substrates of the system. For example, the first capacitive structure  106  may be integrated with the transmitter  102  on a first IC and the second capacitive structure  108  may be integrated with the receiver on a second IC. 
     The first and second capacitive structures  106  and  108  each include a respective set of three capacitive plates (e.g.,  107  and  109 ), which are arranged to function as two series-connected capacitors. For instance, each capacitive structure can include a set of three capacitive plates: a first/input plate, a second/intermediate plate, and a third/output plate. The input and output plates can each be configured to function as a respective a capacitor with the intermediate plate acting as the other plate of the respective capacitor. In the first capacitive structure  106 , one of the capacitive plates of the set  107  is connected to the transmitter  102  in the first voltage domain, and second and third capacitive plates of the set  107  operate as floating nodes in the unreferenced voltage domain. The second capacitive structure  108 , also includes a set of three capacitive plates (e.g., fourth, fifth, and sixth capacitive plates)  109  arranged to function as two capacitors connected in series. The fourth and fifth capacitive plates (e.g., second input and intermediate capacitive plates) operate as floating nodes in the unreferenced voltage domain. The sixth capacitive plate (e.g., the second output capacitive plate) of the series connected capacitor arrangement  109  is connected to the receiver  110  in the second voltage domain. The third and fourth capacitive plates are connected by a conductor that provides a current path, e.g., a bondwire between the first and second capacitive structures. 
       FIGS. 2 and 3  show a capacitive structure formed on a conductive substrate in accordance with one or more embodiments of the present disclosure.  FIG. 2  shows a top view of a capacitive structure in accordance with one or more embodiments. In some embodiments, for instance, the capacitive structure  200  may be used, e.g., to implement capacitive structures  106  and  108  in  FIG. 1 . 
       FIG. 3  shows a cross section A of the capacitive structure shown in  FIG. 2 , in accordance with one or more embodiments. The capacitive structure includes three capacitive plates arranged to function as two series coupled capacitors. The capacitive structure includes capacitive plates  212  and  216  (i.e., input and output capacitive plates) located in an upper layer and a capacitive plate  214  (i.e., intermediate capacitive plate) located below capacitive plates  212  and  216  in a lower layer. Capacitive plates  212  and  214  form a first capacitor that is connected in series by plate  214  with a second capacitor formed by capacitive plates  214  and  216 . A first layer of dielectric material  218  separates the capacitive plate  214  from each of capacitive plates  212  and  216 . Thickness d 1  of the dielectric  218  determines the vertical breakdown voltage of the capacitors. The series-coupled arrangement allows overall breakdown voltage (i.e., V Max  between  220  and  222 ) to be double the breakdown voltage typically provided by a typical two plate capacitor having the same thickness d 1  of dielectric material. 
     Embodiments of the present disclosure are directed toward an isolation circuit having a first capacitive structure  200  in a first voltage domain. The first capacitive structure can include a first conducting substrate  210  that is electrically connected to a reference voltage of the first voltage domain, such as ground. For instance, the substrate  210  may also have active CMOS components (e.g., transistors) that require that conductive substrate to be connected to ground. A first dielectric layer, having a thickness d 1 , defines/provides a breakdown voltage by way of the physical separation between the intermediate capacitor plate  214  and each of the input  212  and output  216  capacitor plates. A second dielectric layer, having a thickness d 2 , is configured and arranged to provide substantially the same breakdown voltage as the first breakdown voltage. These breakdown voltages can be achieved by setting the distance of the physical separation provided between the conducting substrate  210  and the first intermediate capacitor plate  216  accordingly. For instance, if the first and second dielectric layers have identical breakdown fields E bd =E1 bd =E2 bd , for example because they have the same material composition, this can be accomplished by selecting thickness d1 to be equal to d2. If, however, the first dielectric layer has a breakdown field E1 bd  and the second dielectric layer has a different breakdown field E2 bd , this can be accomplished by setting the layer thicknesses such that d1*E1 bd =d2*E2 bd . Although the two dielectric layers can be part of the same dielectric material, they are referred to as different layers due to their respective relationships between the respective plates and/or conductive substrate. The above arrangement of plates can then be repeated for a second capacitive structure in a second voltage domain. 
     Consistent with embodiments of the present disclosure, capacitive plate  212  has an area A 1  and the capacitive plate  216  has an area A 2 . The voltage (VB) on the lower capacitive plate  214  will be V B =(V Max /2). There are two conditions in which breakdown may occur. First, breakdown occurs between plates  212  and  214  and/or between  214  and  216  when
 
 V   Max   −V   B   =V   Max /2&gt; d 1 *E   bd .
 
Second, breakdown occurs between plate  214  and substrate  210  when,
 
 V   B   =V   Max /2&gt; d 2* E   bd .
 
Dielectric layers thicknesses d 1  and d 2  are selected to maximize V Max  without exceeding the above breakdown limits.
 
     In certain CMOS-based manufacturing processes, the total dielectric stack has a constant thickness (D), where D=d 1 +d 2 . The distances d 1  and d 2  can be varied by choosing different layers in the CMOS backend for capacitive plate  214  (a CMOS backend often has 5 or 6 metal layers at different heights). Under the constraints that D=d 1 +d 2 , the distance between substrate  210  and capacitive plate  214  (d 2 ) is given by,
 
 d 2= D−d 1.
 
where d 2  is the distance between the substrate  210  and the capacitive plate  214 , and d 1  is the distance between capacitive plate  214  and capacitive plates  212  and  216 .
 
     When the two layers have the same breakdown field Ebd, the overall breakdown voltage (V max ) of the series capacitor arrangement  200  is then given by:
 
For  d 1≧ d 2: V   max   =D*E   bd  
 
For  d 1&lt; d 2: V   max =2* d 1* E   bd   &lt;D*E   bd  
 
Accordingly, when d 1 ≧d 2 , there is a larger overall breakdown voltage than when d 1 &lt;d 2 . However, larger dielectric thicknesses d 1  can result in a decrease in capacitance. In some particular embodiments, the thicknesses of the first and second dielectric layers are therefore substantially equal (d 1 =d 2 ). For d 1 &gt;d 2 :
 
 V   max   =D*E   bd  
 
where D=d 1 +d 2  is the total dielectric thickness, E bd  is the breakdown field, d 1  is the distance between capacitive plate  214  and capacitive plates  212  and  216 , and V 1   bd =d 1 *E bd  is the breakdown voltage of a conventional two-plate capacitor having dielectric thickness d 1 .
 
     It has been recognized that simply increasing d 1  as much as possible is not acceptable for many applications. Rather, a minimum dielectric thickness d 2  may be required in order to prevent shorting of the lower capacitive plate and the substrate  210 . Therefore the value for d 1  can be limited by d 1   max =D−d 2   min  and the corresponding breakdown voltage for the conventional two-plate capacitor is V 1   bd =d 1   max *E bd . Such a capacitive structure can exhibit a breakdown voltage that is higher by a factor D/d 1   max  when compared to a corresponding two plate capacitive structure. 
     In some embodiments, a capacitive structure is formed on a silicon-on-insulator (SOI) substrate.  FIG. 4  shows a cross section A of the capacitive structure shown in  FIG. 2  formed on an 
     SOI substrate, in accordance with one or more embodiments. The cross section shown in  FIG. 4  is similar to the cross section shown in  FIG. 3  in many respects. For ease of explanation, reference numbers of  FIG. 3  are used to describe similar aspects. The SOI substrate  430 , includes a handle wafer  436 , a silicon layer  432 , and a buried oxide layer  434  providing isolation between the handle wafer the silicon layer. For example, a buried oxide layer with thickness d_box=1 micrometer may be used to isolate the handle wafer  436  from the 1.5 micron thick silicon layer  432 . In such embodiments, the breakdown voltage may be further improved using the buried oxide layer  434  to provide additional isolation between plate  214  and the handle wafer  436 , which is tied to a ground voltage. The buried oxide adds to the effective dielectric thickness below plate  214 . As a result, d 2  can be reduced and d 1  increased to further increase the breakdown voltage of the capacitive structure. If the buried oxide has the same breakdown field Ebd as the other layers in  218 , the breakdown voltage can be improved with respect to the design in  FIG. 3  by a factor (D+d_box)/D. 
     In some SOI implementations, SOI process may form the dielectric  218  from several different dielectric layers having different dielectric constants and, thus, different breakdown fields. The bottom electrode might be made in the silicon of the SOI or in a conducting layer above it. In this case, the thickness d 2  is the sum of the thicknesses of all dielectrics below the lower capacitive plate  214  and d 1  is the sum of the thicknesses of all dielectric layers between the lower capacitive plate  214  and the upper capacitive plates  212  and  216 . In such an implementation, where different dielectric layers have different E bd  (assuming the layers have the same dielectric constant), the breakdown occurs first in the dielectric layer having the smallest breakdown field. The maximum breakdown voltage can be obtained by an approximate ratio of d 1  to d 2  that satisfies the relation,
 
 d 1* E 1 bd   =d 2* E 2 bd .
 
where E bd,1  is the breakdown field of dielectric layer above plate  214  having the weakest breakdown field, and E2 bd  is breakdown field of dielectric layer above plate  214  that has the weakest breakdown field.
 
     In some applications, the silicon layer (e.g.,  432 ) of a SOI substrate may be connected to a reference voltage (e.g., ground voltage). In order for the insulation buried oxide layer  434  to provide additional isolation between the capacitive plate  214  and the ground voltage, the portion of the silicon layer  432  below the capacitive plate  214  may be insulated from the ground voltage. 
       FIGS. 5 and 6  show a capacitive structure with an oxide ring  502  provided to insulate the portion of the silicon layer of the SOI substrate from the ground voltage.  FIG. 5  shows a top view of the capacitive structure in accordance with one or more embodiments. In some embodiments, for instance, the capacitive structure  500  may be used, e.g., to implement capacitive structures  106  and  108  in  FIG. 1 .  FIG. 6  shows a cross section B of the capacitive structure shown in  FIG. 5 , in accordance with one or more embodiments. The capacitive structure shown in  FIGS. 5 and 6  is similar to the capacitive structure shown in  FIGS. 2 and 4  in many respects. For ease of explanation, reference numbers of  FIGS. 2 and 4  are used to describe similar aspects. 
     As a result of the oxide ring  502 , the voltage of the insulated portion of the silicon layer is floating with respect to the ground voltage. The oxide ring  502  may be implemented in some embodiments using medium trench isolation (MTI). To fully utilize the isolation of the oxide layer  434 , the breakdown strength of the oxide ring  502  should be greater than or equal to that of the oxide layer  434 . In some implementations, the oxide ring  502  may be implemented using multiple concentric oxide rings. 
     Some applications may require breakdown voltages greater than those achieved by the above capacitive structures. It is recognized that multiple capacitive structures may be implemented on one or more substrates and connected together in series to increase the overall breakdown voltage. In such arrangements, the breakdown voltage between the endpoint contacts of the series coupled capacitive structures is equal to the sum of the individual breakdown voltages (V_Total max ) of the capacitive structures so connected. 
       FIG. 7  shows three series-connected capacitive structures located on separate substrates, consistent with one or more embodiments. In this example, the three capacitive structures are similar in structure and operation to the cross section of the capacitive structure  200  shown in  FIG. 3 . Substrates  710 ,  720 , and  740  correspond to substrate  210 , and dielectrics  718 ,  728 , and  748  correspond to dielectric  218 . Capacitive plates  712 ,  722 , and  742  each correspond to capacitive plate  212  shown in  FIG. 3 . Likewise, capacitive plates  714 ,  724 , and  744  each correspond to capacitive plate  214 , and capacitive plates  716 ,  726 , and  746  each correspond to capacitive plate  216 . 
     The three capacitive structures are connected together in series to provide a series coupled capacitance. The connections between pads  716  and  722  can, for example, be made using bondwires. This can be particularly useful for providing a large breakdown voltage V_Total max  between nodes  760  and  762 , relative to a single capacitive structure (e.g., V 1   max ). By placing multiple capacitive structures in series, the effective breakdown voltage V_Total max  is the sum of the individual breakdown voltages of the series coupled capacitive structures (e.g., V_Total max =V 1   max +V 2   max +V 3   max ). 
     In some embodiments, the substrates  710 , 720  and  740  can be tied to a reference voltage, such as ground, as shown in  FIG. 7 . For instance, various types of digital circuitry can require, or otherwise benefit from, a grounded substrate. In other embodiments, one or more of the substrates  710 , 720  or  740  can be left at a floating voltage potential. For these floating substrates, breakdown voltage between the intermediate capacitive plate and the substrate may be less of a concern (e.g., due to the respective voltage differentials that may then occur between an intermediate plate and the substrate). For such applications, the overall breakdown voltage may be increased by increasing the dielectric thickness d 1  relative to the dielectric thickness d 2 . 
       FIG. 8  shows a method for manufacture of a capacitive structure consistent with one or more embodiments. An SOI substrate is provided at block  802 . Optionally, in some embodiments, an isolation ring may be formed at block  804  by forming an oxide ring (e.g.,  502 ) in a top silicon layer (e.g.,  230 ) of the SOI substrate. For instance the oxide ring may be formed using medium trench isolation (MTI). As described with reference to  FIGS. 5 and 6 , the addition of the isolation trench can be used to insulate a portion of the silicon layer that is floating with respect to a ground voltage. 
     A lower dielectric layer (e.g., the dielectic layer having thickness d 2 ) is formed over the SOI substrate at block  806 . An intermediate capacitive plate is formed on the dielectric layer at block  808 . An upper dielectric layer (e.g., the dielectric layer having thickness d1) is formed on the intermediate capacitive plate at block  810 . Input and output capacitive plates (e.g.,  212  and  216 ) are formed on the upper dielectric layer at block  812 . 
     The lower dielectric layer separates the SOI substrate from the intermediate capacitive plate by a distance (e.g. d 2 ) that influences the breakdown voltage between the capacitive plate and the SOI substrate. Similarly, the upper dielectric layer separates each of the input and output capacitive plates (e.g.,  212  and  216 ) from the intermediate capacitive plate (e.g.,  214 ) by a distance (e.g., d 1 ) that influences the breakdown voltage between the intermediate capacitive plate and the input or output capacitive plates. As described with reference to  FIGS. 2 and 3 , the overall breakdown voltage of the capacitive structure is determined by the smaller of the two breakdown voltages through the respective first and second dielectric layers. One or more embodiments adjust the overall breakdown voltage by configuring the respective thicknesses d 1  and d 2  of the dielectric layers, such that the first and second breakdown voltages are (substantially) equal. 
     In certain embodiments, the breakdown voltages are substantially equal relative to the desired application. For instance, the breakdown voltages may vary depending upon the manufacturing processes. In such instances, the precise breakdown voltages can vary from device to device. The breakdown voltage, however, is still substantially equal despite such manufacturing variations. In another instance, minor differences in the breakdown voltages are possible such that the overall breakdown voltage of the device or structure is within an acceptable margin relative to having identical breakdown voltages (e.g., 5%). 
       FIG. 9  illustrates a method of communication between first and second voltage domains, consistent with one or more embodiments. Communication signals are transmitted from a transmitter circuit  902  located on a first substrate  906  that is in a first voltage domain. Capacitive isolation is provided for the communication signals between the first voltage domain and a second voltage domain by communicating the transmitted communication signals through an isolation circuit that includes a first capacitive structure  904  located on the first substrate  906  to provide the communication signal from the first voltage domain to an unreferenced voltage domain. A second capacitive structure  922  located on a second substrate  924  is used to provide the communication signals from the unreferenced voltage domain to a second voltage domain. A receiver circuit  920  is located on the second substrate and is configured to receive the communication signals at an input in the second voltage domain from the second capacitive structure  922 . 
     The capacitive structures  904  and  922  illustrated in  FIG. 9  are each structurally and operationally similar to the capacitive structure discussed, for instance, with reference to  FIGS. 2 and 3 . For instance, capacitive structure  904  includes three capacitive plates  910 ,  912 , and  914 , which are configured and arranged in a similar manner as capacitive plates  212 ,  214 , and  216 . Likewise, capacitive structure  904  includes three capacitive plates  930 ,  934 , and  932 , which are configured and arranged in a similar manner as capacitive plates  212 ,  214 , and  216 . 
     Capacitive plate  910  is configured to receive the communication signals from the transmitter  902  in the first voltage domain. The communication signals are capacitively communicated (using capacitive coupling between plates of the capacitor) from the plate  910  to plate  914 , which operates as a floating node in the unreferenced voltage domain. The communication signals are capacitively communicated from plate  914  to plate  912 , which operates as a second floating node in the unreferenced voltage domain. The communication signals are passed from plate  912  to plate  930 , which operates as a third floating node in the unreferenced voltage domain. The communication signals are capacitively communicated from the plate  912  to plate  930  with operates as a third floating node in the unreferenced voltage domain. The communication signals are capacitively communicated from the plate  930  to plate  934 , which operates as a fourth floating node in the unreferenced voltage domain. The communication signals are capacitively communicated from the plate  934  to plate  932 , which operates as in the second voltage domain. The receiver  920  is receives the communication signals in the second voltage domain from capacitive plate  932 . 
     Certain embodiments are directed toward controlling the breakdown voltage by designing the different capacitive plates to have different sizes and/or shapes. For instance, it has been recognized that the electric field can be strongest near the edge of a capacitive plate and or at a corner of a capacitive plate. Accordingly, the breakdown voltage is sometimes limited by the electric field strength at such locations (where it is the strongest). Accordingly, embodiments contemplate offsetting the edges of the respective plates from one another (relative to a horizontal/lateral position). For instance, the intermediate (bottom) capacitive plate can be configured to extend beyond the edges of each of the input and output (upper) capacitive plates. 
     In certain instances, one or more embodiments can use different coding techniques and different types of circuits communicating data through the isolation region. The data communications can use analog, digital, RF, serial and/or parallel communication techniques. For certain high-speed applications, different types of modulation schemes can be used for carrying information across the isolation region, including but not limited to OOK (on-off keying), amplitude, phase-based and/or frequency-based. In some instances, communications can be carried out between multiple circuits placed within a single chip-package (e.g., BGA package) and also having galvanic isolation there between. In other instances, the communications can be carried out between multi-chip modules (MCM) having multiple integrated circuits (ICs), semiconductor dies or other discrete components contained within a common housing. In some instances, part of the signals reaching the receiver can be used for powering the IC, or waking it up out of a low-power mode. 
     The various communications can be carried out using different isolation buffer circuits and amplifiers. Various applications are also contemplated, including but not limited to applications in which small voltage differences exist between transmitters and receivers, and applications in which large voltages can exist (e.g., hundreds of volts as can be used in automotive applications where electric motors are used in place of (or in combination with) combustion engines). Consistent with one or more embodiments discussed herein, U.S. Pat. No. 6,920,576 (filed May, 31, 2001; Ehmann, Gregory E.), U.S. Pat. No. 6,882,046 (filed Dec. 18, 2001; Davenport, et al.) and “Signal Isolation Buffer Amplifiers” Burr-Brown, ISO102, ISO106, January 1995, each describe useful technical details, applications and various background information, and each of these documents is fully incorporated herein by reference. 
     The embodiments are thought to be applicable to a variety of applications using galvanic isolation. Other aspects and embodiments will be apparent to those skilled in the art from consideration of the specification. While the present disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in further detail. It should be understood that the intention is not to limit the disclosure to the particular embodiments and/or applications described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.