Abstract:
A structure including at least two neighboring components, capable of operating at high frequencies, formed in a thin silicon substrate extending on a silicon support and separated therefrom by an insulating layer, the components being laterally separated by insulating regions. The silicon support has, at least in the vicinity of its portion in contact with the insulating layer, a resistivity greater than or equal to 1,000 ohms.cm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure comprising high-frequency components, for example a protection structure. 
     2. Discussion of the Related Art 
     Generally, electronic components are formed either in a solid substrate semiconductor wafer, or in a semiconductor-on-insulator (SOI) layer. 
       FIG. 1  shows the upper portion of a doped solid substrate  1  of a first conductivity type, for example, P, in which are formed several doped wells  3  of a second conductivity type, for example N. High-frequency electronic components can then be formed in wells  3 . The isolation between wells  3  results from an appropriate biasing of the PN junctions between wells  3  and substrate  1 . The capacitances between a well  3  and substrate  1  and between two neighboring wells  3  are often non-negligible as compared with the specific capacitances of the components formed in wells  3  and this is a problem, especially in the field of high-frequency components. 
       FIG. 2  shows an example of an SOI type structure. This structure comprises, on a semiconductor support  5 , a thin layer of a semiconductor substrate with an interposed silicon oxide layer  7 . Semiconductor wells  9 , in which are formed electronic components, are formed side by side in the substrate. The free space between the wells is occupied by an insulator, for example, silicon oxide  11 . The silicon oxide of layer  7 , as for itself, enables insulating the electronic components in the vertical direction. This structure has the advantage over the structure of  FIG. 1  of decreasing the values of the stray capacitances between neighboring components. However, these capacitances are not negligible in certain cases, especially at high frequency. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a structure comprising several electronic components operating at high frequency, insulated with respect to one another, in which the stray capacitances between components are decreased. 
     To achieve all or part of these objects as well as others, one embodiment of the present invention provides a structure comprising at least two neighboring components, capable of operating at high frequencies, formed in a thin silicon substrate extending on a silicon support of a first conductivity type and separated therefrom by an insulating layer, the components being laterally separated by insulating regions, wherein the silicon support has, at least in the vicinity of its portion in contact with the insulating layer, heavily-doped islands of a second conductivity type, the distance between islands being smaller than twice the extent of the space charge area created by the junction with the silicon support. 
     According to an embodiment of the present invention, the components are two diodes for protecting a line, the first diode having its anode connected to ground and its cathode connected to the line to be protected and the second diode having its cathode connected to a supply voltage and its anode connected to the line to be protected. 
     According to an embodiment of the present invention, each diode comprises a silicon well of a first conductivity type having its bottom and its lateral walls bordered by an area of the first conductivity type with a strong doping level and in the surface of which is formed a region of the second conductivity type. 
     The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2 , previously described, are cross-section views illustrating two known electronic component forming modes; 
         FIG. 3  is a cross-section view of a structure comprising two diodes according to an embodiment of the present invention; 
         FIG. 4  is an equivalent electric diagram of the structure of  FIG. 3 ; 
         FIG. 5  illustrates the stray capacitances present in the structure of  FIG. 3 ; 
         FIG. 6  is an equivalent electric diagram of the structure of  FIG. 5 ; and 
         FIG. 7  illustrates a structure according to an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of semiconductor components, the various representations of semiconductor components are not drawn to scale. 
       FIG. 3  shows two adjacent diodes D 1  and D 2  of protection against overvoltages. These diodes are placed side by side and are formed on an SOI-type structure comprising a thin semiconductor substrate formed on an insulating layer  7 , itself formed on a semiconductor support  5 . Diodes D 1  and D 2 , which are identical, are laterally insulated by insulating regions  11 , for example made of silicon oxide. Each diode comprises an N-type doped well  13  in which is formed a P-type doped region  15 . The well is surrounded (bottom, lateral walls and part of its upper surface) with a heavily-doped N-type region  17 . Anode and cathode contacts  21  and  22  are formed on region  15  and on region  17  in contact with well  13 . Anode  21  of diode D 1  is grounded. Cathode  22  of diode D 2  is connected to a voltage source Vcc. Cathode  22  of diode D 1  and anode  21  of diode D 2  are connected to a line I/O to be protected. 
       FIG. 4  shows the equivalent electric diagram of the structure shown in  FIG. 3 . This circuit comprises diode D 1  having its anode connected to ground and its cathode connected to a line I/O to be protected, and diode D 2  having its cathode connected to voltage source Vcc and its anode connected to line I/O. 
     As described previously, components D 1  and D 2  are insulated from each other by insulating layer  7  and by insulating regions  11 . Now, this insulation is not perfect and stray capacitances still exist between the various components, which capacitances must be minimized for a high-frequency operation. 
       FIG. 5  shows a modeling of the different capacitances of the structure of  FIG. 3 . The stray capacitance horizontally created between the two wells  13  is called C 1 . The stray capacitances vertically formed between wells  13  and support  5  are called C 2  and C 2 ′. The impedance of support  5  between the areas underlying components D 1  and D 2  is formed of a resistor Rs in parallel with a capacitor Cs. 
       FIG. 6  illustrates the way in which the various high-frequency capacitances are associated, in which case it can be considered that terminals Vcc and the ground are connected. This drawing relates to the case where the diodes are off. The diode capacitances are designated as C D1  and C D2 , respectively. Capacitances C D1 , C 1 , and C D2  are in parallel, the assembly being in parallel with the series assembly of capacitances C 2  and C 2 ′ and of impedance (Cs/Rs) of support  5 . 
     The applicant has analyzed the circuit operation at 1 MHz. This analysis is summed up by table 1 hereafter. This table indicates the values of the various capacitances for various resistivity values of the material of support  5 . The capacitance values are given in arbitrary units, stating that, for a relatively conductive substrate of a resistivity lower than 1 ohm.cm, the assembly of the capacitances of both diodes D 1  and D 2  and of the associated parasitic elements brings the capacitance on terminal I/O (see  FIG. 6 ) to a value of 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 1 ohm · 
                   
                   
                 5,000 
               
               
                   
                 cm 
                 10 ohms · cm 
                 1,000 ohms · cm 
                 ohms · cm 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 D1//D2 
                 0.64 
                 0.64 
                 0.64 
                 0.64 
               
               
                 C1 
                 0.08 
                 0.08 
                 0.08 
                 0.08 
               
               
                 D1//D2//C1 
                 0.72 
                 0.72 
                 0.72 
                 0.72 
               
               
                 C2 + C2′ 
                 0.28 
                 0.28 
                 0.28 
                 0.28 
               
               
                 Cs 
                 High 
                 High 
                 0.8 
                 0.37 
               
               
                 C2 + C2′ + Cs 
                 0.28 
                 0.28 
                 0.21 
                 0.16 
               
               
                 Total Cap. 
                 1 
                 1 
                 0.93 
                 0.88 
               
               
                   
               
             
          
         
       
     
     It is considered that diodes D 1  and D 2  each have a 0.32 capacitance and that capacitance C 1  of the oxide walls is 0.08, which brings the parallel value of the capacitances of diodes D 1 , D 2 , and C 1  to 0.72. Capacitances C 2  and C 2 ′ corresponding to oxide  7  of the SOI structure have in series a value equal to 0.28. All the above-mentioned capacitances have constant values, independently from the resistance of support  5 . However, the equivalent capacitance Cs of the impedance of support  5  depends on the resistivity of this support. Thus, it can be seen that for a frequency of approximately 1 MHz, the contribution of the impedance of support  5  decreases from 0.28 to 0.16 when the resistivity of support  5  increases from 1 to 5,000 ohms.cm. This difference is not very significant and this may be the reason why prior art studies for improving the influence of stray capacitances have come to nothing. 
     The applicant has carried out the same study in the context of the same circuit operating at a 1 GHz frequency. The results of this study are provided in table 2 hereafter. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 1 ohm · 
                   
                   
                 5,000 
               
               
                   
                 cm 
                 10 ohms · cm 
                 1,000 ohms · cm 
                 ohms · cm 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 D1//D2 
                 0.64 
                 0.64 
                 0.64 
                 0.64 
               
               
                 C1 
                 0.08 
                 0.08 
                 0.08 
                 0.08 
               
               
                 D1//D2//C1 
                 0.72 
                 0.72 
                 0.72 
                 0.72 
               
               
                 C2 + C2′ 
                 0.28 
                 0.28 
                 0.28 
                 0.28 
               
               
                 Cs 
                 High 
                 1.6 
                 0.01 
                 0.01 
               
               
                 C2 + C2′ + Cs 
                 0.28 
                 0.24 
                 0.01 
                 0.01 
               
               
                 Total Cap. 
                 1 
                 0.96 
                 0.73 
                 0.73 
               
               
                   
               
             
          
         
       
     
     It can be seen that for most of its lines, table 2 corresponds to table 1. Especially, the capacitances of diodes D 1  and D 2 , capacitance C 1  and capacitances C 2  and C 2 ′ practically do not vary along with the frequency. However, equivalent capacitance Cs of the substrate depends a lot on the frequency. While it was relatively high at a 1-MHz frequency, when operating at frequencies close to 1 GHz, its value considerably decreases when the substrate resistivity increases. Thus, as shown by the table, as soon as the substrate resistivity exceeds 1000 ohms.cm, the contribution of capacitances C 2 , Cs, and C 2 ′ altogether becomes negligible: the total capacitance varies from 0.72 in the case where the substrate would have been perfectly insulating to 0.73 as soon as the substrate reaches a resistivity greater than 1,000 ohms.cm, that is, the influence of the substrate becomes negligible. It should however be noted that this could not be observed at frequencies on the order of one megahertz. 
     Thus, the present invention provides using a substrate of a resistivity equal to or greater than 1000 ohms.cm to reduce the inter-component capacitance of a high-frequency circuit intended to operate a values on the order of one gigahertz or more. 
     A way to obtain this performance increase without requiring an additional increase in the substrate resistivity is illustrated in  FIG. 7 . Heavily-doped islands  23  of the conductivity type opposite to that of the substrate, for example, N +  islands in a P substrate, are formed in the upper substrate portion. This results in the creation of a depleted area 25 free of any carrier, which can be considered as equivalent to an insulator. In the case of a substrate of a doping level on the order of 5.10 12  atoms/cm 3 , the extent of the depleted area is on the order of from 10 to 15 μm. A structure having a heavily-insulating upper portion is thus obtained, which enables improving the performances displayed in table 1, achieving the advantages previously discussed in relation with table 2. 
     With the above-indicated method, it should be noted that the frequency for which a decrease in the stray capacitance is obtained is decreased. Thus, low stray capacitance values may be provided, such as indicated in the right-hand column of table 2, at frequencies much lower than 1 GHz. The present invention can then usefully be applied to devices operating, for example, at frequencies on the order of one megahertz. 
     The present invention has been described in the context of the association of two protection diodes for high-frequency circuits. It should be understood that the present invention may find other applications and generally applies in the high-frequency field when the stray capacitances between two components are desired to be as negligible as possible. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.