Abstract:
An electric component comprising an assembly of two PIN diodes in series formed in a semiconductor substrate layer separated from a support layer by an insulating layer, the doped areas forming the electrodes of each diode having a depth equal to that of the substrate layer, the component including a first area of a first doping type surrounded with a second intrinsic area, itself surrounded with a third area of a second doping type, the third area being surrounded with a fourth area of the first doping type, the fourth area being surrounded with a fifth intrinsic area, itself surrounded with a sixth area of the second doping type, the third and fourth areas being covered and connected by a metal area, each of the first and sixth areas being connected to a contact pad on which rests a welding ball.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to rectifying diodes capable of operating at high frequency. More specifically, the present invention relates to diodes formed in a “flip-chip”—assembled integrated circuit chip. Flip-chips chips are such that the contact pads connected to the anodes and cathodes of the diodes are formed on the same side of the chip and are covered with welding balls used to attach by welding the component on a printed circuit. 
   2. Discussion of the Related Art 
   Such diodes are especially used in radio wave transceiver circuits. 
     FIG. 1  shows as an example a diagram of a portion of a transceiver circuit usable in portable phones and comprising high-frequency PIN diodes. 
   The circuit comprises an antenna  1 , two transmit branches br 1  and br 2 , and two receive branches br 3  and br 4 . Transmit branch br 1  contains a diode D 1  and a capacitor C 1  in series. The cathode of diode D 1  is connected to antenna  1  and an electrode of capacitor C 1  is connected to a signal terminal TXGSM. Transmit branch br 2  contains a capacitor C 2  in series with a diode D 2 . The cathode of diode D 2  is connected to antenna  1  and an electrode of capacitor C 2  is connected to a terminal CXPCF/PCF. Transmit branch br 3  contains a capacitor C 3  and two diodes D 31  and D 32  in series. An electrode of capacitor C 3  is connected to a terminal RXGSM. The other electrode of capacitor C 3  is connected to the anode of diode D 31 , the cathode of which is connected to the anode of diode D 32 . The cathode of diode D 32  is connected to antenna  1 . Receive branch br 4  contains a capacitor C 4  and two diodes D 41  and D 42  in series. An electrode of capacitor C 4  is connected to a terminal RXDCS. The other electrode of capacitor C 4  is connected to the anode of diode D 41 , the cathode of which is connected to the anode of diode D 42 . The cathode of diode D 42  is connected to antenna  1 . 
   Bias circuits p 1 , p 2 , p 3 , and p 4  are connected to branches br 1 , br 2 , br 3 , and br 4 . Each bias circuit pi, i ranging between 1 and 4, comprises a capacitor Cpi. A first electrode of each capacitor Cpi is grounded. The second electrode of each capacitor Cpi is connected to a first terminal of a coil Bi. The second terminal of each coil Bi is connected to the intermediary point between the capacitor and the diode(s) of branch br 1 . The second electrodes of capacitors Cp 1 , Cp 2 , Cp 3 , and Cp 4  are respectively connected to bias terminals BIAS-TXGSM, BIAS-TXDCS/PCS, BIAS-RXGSM, and BIAS-RXDCS. 
   Antenna  1  is connected to a common bias circuit  2 . Bias circuit  2  comprises a coil Bc having a first terminal connected to antenna  1 . The second terminal of coil Bc is connected to a first electrode of a capacitor Cc having its second electrode connected to ground. Two resistors R 1  and R 2  in series are placed between the first electrode of capacitor Cc and the ground. The intermediary point between resistors R 1  and R 2  is connected to a bias terminal BIAS. 
   When the circuit is in receive mode, for example on terminal RXGSM, terminals BIAS-TXGSM, BIAS-TXDCS/PCS, and BIAS-RXDCS are grounded and no current flows through the diodes of branches br 1 , br 2 , and br 4 . Terminal BIAS-RXGSM is positively biased and a current flows through coil B 3 , diodes D 31  and D 32 , coil Bc, and resistors R 1  and R 2 . Capacitor C 3  behaving as a short-circuit at high frequency, the signal received on antenna  1  is transmitted onto terminal RXGSM. 
   When the circuit is in transmit mode, for example on terminal TXGSM, terminals BIAS-TXDCS/PCS, BIAS-RXGSM, and BIAS-RXDCS are grounded. Terminal BIAS-TXGSM is positively biased. The signal to be transmitted is transmitted from terminal TXGSM to antenna  1 . 
   Such a circuit comprises a large number of diodes. Currently, these diodes are made in the form of discrete components. Their costs and the printed circuit surface area taken up by the diodes are thus significant. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to form high-frequency circuits comprising several monolithic diodes. 
   Another object of the present invention is to provide such a diode assembly which reduces high-frequency interferences. 
   To achieve these and other objects, the present invention provides an electric component comprising an assembly of two PIN diodes in series formed in a semiconductor substrate layer separated from a support layer by an insulating layer, the doped areas forming the electrodes of each diode having a depth equal to that of the substrate layer, the component comprising a first area of a first doping type surrounded with a second intrinsic area, itself surrounded with a third area of a second doping type, the third area being surrounded with a fourth area of the first doping type, the fourth area being surrounded with a fifth intrinsic area, itself surrounded with a sixth area of the second doping type, the third and fourth areas being covered and connected by a metal area, each of the first and sixth areas being connected to a contact pad on which rests a welding ball. 
   According to an alternative embodiment of such a component, the first area has a cylindrical shape and the second, third, fourth, and fifth areas are ring-shaped. 
   The present invention further provides an electric circuit comprising several diodes formed in a substrate layer separated from a support layer by an insulating layer, the diodes being distributed on one or several branches, first branches comprising two diodes in series formed like the above-mentioned component, second branches comprising a single diode formed of a first area of the first doping type surrounded with a second intrinsic area itself surrounded with a third area of the second doping type, the sixth areas of the diodes of the first branches and the third areas of the diodes of the second branches forming a single common area. 
   According to an alternative embodiment of such a circuit, the single common area is connected to at least one contact pad, each of the first areas of the diodes of the first and second branches being connected to a contact pad. 
   According to an alternative embodiment of such a circuit, the first areas have a cylindrical shape and the second, third, fourth, and fifth areas of the diodes of the first branches and the second area of the diodes of the second branches are ring-shaped. 
   According to an alternative embodiment of such a circuit, the single common area is covered with a metal area extending above portions of the single common area separating the other doped or intrinsic areas of the different diodes. 
   According to an alternative embodiment of such a circuit, the first doping type is a P doping and the second doping type is an N doping, said first area being an anode area and said third area being a cathode area. 
   The foregoing 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 
       FIG. 1  is an example of a known transceiver circuit; 
       FIG. 2  is a vertical cross-section view of a diode assembly according to the present invention; 
       FIG. 3  is a top view of the electric component shown in  FIG. 2 ; and 
       FIGS. 4 to 7  illustrate the structures obtained after successive steps of a method according to the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention provides forming on a single semiconductor chip an assembly of high-frequency diodes assembled in series and/or with a common cathode. 
   The applicant has selected from among the various existing structures of high-frequency diodes a “lateral” diode structure formed in a silicon-on-insulator (SOI) type support. Such diodes exhibit a very low stray capacitance. 
     FIGS. 2 and 3  show an assembly of diodes comprising diodes in series and diodes with a common cathode forming an example of application of the present invention. This specific assembly forms an electric component which corresponds to the portion of the transceiver circuit shown in  FIG. 1  comprising diode assembly D 1 , D 2 , D 31 , D 32 , D 41 , and D 42 . 
     FIG. 2  is a vertical cross-section view of the electric component along axis  22 ′ of  FIG. 3 .  FIG. 3  is a top view of the electric component without its metallizations. 
   The electric component is made in a thin substrate layer  30  separated from a support layer  31  by an insulating layer  32 . Substrate layer  30 , for example, silicon, is initially little or not doped, that is, intrinsic. Insulating layer  32  may be silicon oxide and the support layer for example silicon, glass or sapphire. In the following, each time a doped area of the component formed in the substrate will be mentioned, it will be a doped area extending over the entire thickness of substrate layer  30 . 
   Series diodes D 31 /D 32  and D 41 /D 42  are formed similarly, only diode D 31  and D 32  being shown in  FIG. 2 . The anode of diode D 31  is a P-doped cylindrical area  50 . Area  50  is surrounded with five concentric annular areas. Area  50  is surrounded with an intrinsic area  51 , itself surrounded with an N-doped area  52  forming the cathode of diode D 31 . Area  52  is surrounded with a P-doped area  53  forming the anode of diode D 32 . Area  53  is surrounded with an intrinsic area  54 . Similarly, the anode of diode D 41  is a P-doped cylindrical area  55 . Area  55  is surrounded with five concentric annular areas. Area  55  is surrounded with an intrinsic layer  56  itself surrounded with an N-doped area  57  forming the cathode of diode D 41 . Area  57  is surrounded with a P-doped area  58  forming the anode of diode D 42 . Area  58  is surrounded with an intrinsic area  59 . 
   Diodes D 1 , D 2 , D 32 , and D 42  have a common cathode corresponding to an N-doped substrate area  40 . Area  40  corresponds to the substrate area not taken up by the other doped or intrinsic areas of the diode assembly. 
   Substrate layer  30  is covered with an insulating layer  60 . Openings formed in insulating layer  60  above the anode and cathode areas of each of the diodes contain metallizations which correspond to contact pads or connection areas. The connection areas enable connecting juxtaposed anode and cathode areas belonging to two diodes in series such as diodes D 31  and D 32 . The contact pads and the connection areas are formed of a conductive material, such as aluminum or copper. 
   Metal contact pads  61 ,  62 ,  63 , and  64  having in this example a circular shape in top view are placed above and at the center of anode areas  41 ,  43 ,  50 , and  55 . Metal contact pads  65  and  66  are placed above selected locations of common cathode area  40 . The contact pads are shown in dotted lines in  FIG. 3 . The contact pads have in this example a circular shape in top view. 
   A metal area  67  covers annular areas  52  and  53  to short-circuit the diode formed by the two juxtaposed areas and place diodes D 31  and D 32  in series. Similarly, a metal area  68 , not shown, covers annular areas  57  and  58 . 
   Insulating layer  60  is covered with a passivation layer  70 . Openings formed in passivation layer  70  above contact pads  61 ,  65 ,  63 ,  64 ,  66 , and  62  are respectively filled with bonding layers  72 ,  73 ,  74 ,  75 , and  76  on which are placed welding balls  81 ,  82 ,  83 ,  84 ,  85 , and  86 . These welding balls enable welding the electric component on a printed circuit. 
   According to an embodiment of the present invention, the metal layer forming circular contact pads  65  and  66  placed above cathode area  40  extends in a metal area  69  shown in doted lines in  FIG. 3 . The shape of metal area  69  is provided so that it extends above the portions of area  40  separating the other intrinsic or doped areas of the different diodes. Metal area  64  enables forming a shielding between the different diodes and thus decreasing electromagnetic interferences between these diodes. 
     FIGS. 4 to 7  are vertical cross-section views of the structures obtained at successive steps of a component manufacturing method such as described in relation with  FIGS. 2 and 3 . The same reference numerals designate the elements of the previously-described electric component and the elements of the various structures described hereafter. 
   A first step of the method of the present invention comprises forming or of using an SOI-type substrate. Such a support comprises a semiconductor substrate layer  30 , such as silicon, separated from a support layer  31 , by an insulating layer  32 , conventionally a silicon oxide layer. 
   In a next step, illustrated in  FIG. 4 , substrate layer  30  is implanted with, for example, phosphorus to form the N doped areas of the cathodes of the electric component diodes. As shown in  FIG. 4 , the areas which are not desired to be N doped are covered with protection layers  90 ,  91 , and  92 . The N doped areas shown in  FIG. 4  correspond to areas  40  and  52  of the component shown in  FIG. 2 . 
   In a next step, illustrated in  FIG. 5 , substrate layer  30  is implanted with, for example, boron to form the P doped areas of the anodes of the electric component diodes. The already N-doped areas of substrate  30  or the areas which are desired to be left in the intrinsic state are covered with protection layers  94  and  95 . The P doped areas shown in  FIG. 5  correspond to areas  41 ,  50 , and  53  of the component shown in  FIG. 2 . 
   The areas not doped after the preceding steps of the method of the present invention form intrinsic areas. In the example of  FIG. 5 , intrinsic areas  42 ,  51 , and  54  can be seen. 
   At the step illustrated in  FIG. 6 , substrate layer  30  is covered with an insulating layer  60 . Openings Op 1 , Op 2 , Op 3 , and Op 4  are formed by etching above the anode and cathode areas, that is, areas  41 ,  40 ,  50 ,  52 , and  53  in this example. A conductive material such as aluminum is then deposited above insulating layer  60 . Then, the aluminum is etched to keep aluminum in openings Op 1  to Op 4 . Aluminum contact pads  61 ,  65  ( 69 ), and  63  respectively formed above areas  41 ,  40 , and  50  as well as connection areas, area  67  in this example, are thus obtained. The connection areas short-circuit the juxtaposed anode and cathode areas of two diodes in series, that is, areas  52  and  53  in this example. 
   At the step illustrated in  FIG. 7 , insulating layer  60 , the contact pads, and the connection areas are covered with a passivation layer  70 . 
   Openings are formed by etching in passivation layer  70  above metal contact pads  61 ,  65 , and  63  and respectively filled with bonding layers  72 ,  73 ,  74 ,  75 , and  76 . Welding balls  81 ,  82 ,  83 ,  84 ,  85 , and  86  are respectively formed on bonding layers  72 ,  73 ,  74 ,  75 , and  76 . 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the implantation steps may be carried out in a different order. 
   Further, it may be provided to place several contact pads on the common cathode area and to distribute them to obtain a robust assembly of the electric component on a printed circuit. 
   Further, an electric component “complementary” to the previously-described component may be formed. Such a component is formed, as previously, of an assembly of diodes distributed over several branches, the branches containing one or two diodes in series. Conversely to the described component, the branches are interconnected by the anode of one of the diodes of each branch. Such a “complementary” component has a structure similar to that of the described component, except that the different doped areas have a doping of opposite type. 
   As an alternative, anode or cathode areas in the case of the complementary component exhibiting a square, rectangular, or other top view may be formed. 
   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.