Abstract:
A high-voltage semiconductor device is disclosed. The HV semiconductor device includes: a substrate; a well of first conductive type disposed in the substrate; a first doping region of second conductive type disposed in the p-well; a first isolation structure disposed in the well of first conductive type and surrounding the first doping region of second conductive type; and a first drift ring of second conductive type disposed between the first doping region of second conductive type and the first isolation structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a high voltage diode (HV Diode), and more particularly, to a HV diode using conductive drift rings for providing stable breakdown voltage. 
         [0003]    2. Description of the Prior Art 
         [0004]    Double diffuse drain (DDD) is a diode structure that is applied to HVMOS transistors. The high-voltage diode structure is able to provide the HVMOS transistor with a higher breakdown voltage. This can prevent a high voltage, such as electrostatic discharge (ESD), from damaging the transistor. Furthermore, the hot electron effect resulting from short channel is avoided. 
         [0005]    Conventional HV diode typically includes a substrate, a n+ doping region disposed in the substrate and a field oxide layer surrounding the n+ doping region. However, the conventional HV diode framework has some drawbacks. For instance, a breakdown is often found at the junction between the n+ doping region and the field oxide layer after long period of operation to cause a serious shift to the breakdown voltage of the device. Hence, how to provide a stable and reliable structure for resolving this issue has become and important task in this field. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an objective of the present invention to provide a HV diode for resolving the aforementioned issue caused by conventional architecture. 
         [0007]    According to a preferred embodiment of the present invention, a high-voltage semiconductor device is disclosed. The HV semiconductor device includes: a substrate; a p-well disposed in the substrate; a first n-type doping region disposed in the p-well; a first isolation structure disposed in the p-well and surrounding the first n-type doping region; and a first n-type drift ring disposed between the first n-type doping region and the first isolation structure. 
         [0008]    According to a preferred embodiment of the present invention, a high-voltage semiconductor device is disclosed. The HV semiconductor device includes: a substrate; a well of first conductive type disposed in the substrate; a first doping region of second conductive type disposed in the p-well; a first isolation structure disposed in the well of first conductive type and surrounding the first doping region of second conductive type; and a first drift ring of second conductive type disposed between the first doping region of second conductive type and the first isolation structure. 
         [0009]    Another aspect of the present invention disclosed a high-voltage semiconductor device, which includes: a substrate; a well of first conductive type disposed in the substrate; a first doping region of second conductive type disposed in the well of first conductive type; a first isolation structure disposed in the well of first conductive type and surrounding the first doping region of second conductive type; and a first drift ring of second conductive type disposed directly under the first isolation structure. 
         [0010]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a top view of a HV diode according to a preferred embodiment of the present invention. 
           [0012]      FIG. 2  illustrates a cross-section view of  FIG. 1  along the sectional line AA′. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIGS. 1-2 ,  FIG. 1  illustrate a top view of a HV diode according to a preferred embodiment of the present invention and  FIG. 2  illustrates a cross-section view of  FIG. 1  along the sectional line AA′. As shown in the figures, a substrate  12 , such as a p-type substrate is provided, in which the substrate  12  could be a silicon substrate or a silicon-on-insulator substrate. 
         [0014]    An ion implantation and a thermal oxidation process are then performed to form a plurality of doping regions and isolation structures, such as by first using a thermal oxidation process to form a first field oxide layer  14  and a second field oxide layer  16  shown in the figures, and then performing a n-type ion implantation to form a deep n-well  18  in the substrate  12 . A p-type ion implantation is then conducted thereafter to form a p-well  20  in the deep n-well  18 . The first field oxide layer  14  and the second field oxide layer  16  are used for increasing breakdown voltage of a single device, hence other structures such as shallow trench isolations (STI) could be used to replace the field oxide layers disclosed in this embodiment, which are also within the scope of the present invention. It should be noted that regions other than field oxides or STIs disclosed in this embodiment are in the active region. 
         [0015]    A patterned resist (not shown) is then formed on the substrate  12 , and a low concentration n-type ion implantation is performed by using the patterned resist as mask to form at least a n-type drift ring, such as a first n-type drift ring  22  and second n-type drift ring  24  in the p-well  20 . In this embodiment, the first n-type drift ring  22  is formed in the interior and surrounding by the second n-type drift ring  24 , in which the doping concentration of the first n-type drift ring  22  and the second n-type drift ring  24  is about 1E12. 
         [0016]    Next, two n-type ion implantations are conducted sequentially by using two different patterned resist (not shown) to form a first n-grade region  26  in the p-well  20  and a n+ region  28  in the n-grade region  26 , in which the doping concentration of the n-grade region  26  is substantially higher than the first n-type drift ring  22  and the second n-type drift ring  24  and the doping concentration of the n+ region  28  is substantially higher than the n-grade region  26 . In this embodiment, the doping concentration of the n-grade region  26  is about 1E13 while the doping concentration of the n+ region  28  is about 1E15. Preferably, both the n+ region  28  and the n-grade region  26  are disposed in the substrate  12  of the first field oxide layer  14 , such as disposed in the active region surrounded by the first field oxide layer  14 . 
         [0017]    It should be noted that the first n-type drift ring  22  is preferably disposed between the first field oxide layer  14  and the n+ region  28 , and specifically at the junction of the two regions, in which the first n-type drift ring  22 , the n-grade region  26  and the n+ region  28  are substantially overlapping each other. The second n-type drift ring  24  on the other hand is disposed in the substrate  12  directly under the first field oxide layer  14  and completely surrounded and isolated by the p-well  20 . 
         [0018]    A p-type ion implantation is then performed by using another patterned resist (not shown) as mask to form two p-type drift rings  30 ,  31  between the first field oxide layer  14  and the second field oxide layer  16 . Next, two p-type ion implantations are conducted sequentially by using two different patterned resist (not shown) to form a p-grade region  32  in the p-well  20  and a p+ region  34  in the p-grade region  32 , in which the doping concentration of the p-grade region  32  is substantially higher than the p-type drift rings  30 ,  31  and the doping concentration of the p+ region  34  is substantially higher than the p-grade region  32 . In this embodiment, the p+ region  34  and p-grade region  32  are disposed between the first field oxide layer  14  and the second field oxide layer  16  and the two p-type drift rings  30 ,  31  are also disposed in the junction between the p+ region  34  and the first field oxide layer  14  and the second field oxide layer  16 , in which the p-grade region  32  and p+ region  34  substantially overlap the two p-type drift rings  30 ,  31 . This completes the fabrication of a HV diode according to a preferred embodiment of the present invention. It should be noted that the process for forming the doping regions is not limited to the order disclosed in the aforementioned embodiment, and the conductive types (such as n-type and p-type) of the regions could also be switched according to the demand of the product, which is also within the scope of the present invention. 
         [0019]    As conventional HV diode typically induces a breakdown at the junction between the n+ region and the field oxide layer so that a shift in breakdown voltage is resulted after long period of operation, the present invention preferably forms a n-type drift ring at the junction between the n+ region and the field oxide layer while not adding any extra mask during the fabrication process. By using the n-type drift ring to enforce the junction between these two regions, currents are induced into the substrate instead of concentrating on the surface between the n+ region and the field oxide layer so that damage to the substrate surface if prevented. 
         [0020]    In addition to forming the n-type drift ring between the n+ region and the field oxide layer, the n-type drift ring could also be formed directly under the field oxide layer as disclosed by another embodiment of the present invention. Alternatively, the present invention could first form a n-type drift ring between the n+ region and the field oxide layer, and then form another n-type drift ring directly under the field oxide layer to improve the stability of the HV diode structure, which is also within the scope of the present invention. 
         [0021]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.