Abstract:
The invention relates to a vertical-type single-pole component, comprising regions with a first type of conductivity which are embedded in a thick layer with a second type of conductivity. Said regions are distributed over at least one same horizontal level and are independent of each other. The regions also underlie an insulating material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of U.S. application Ser. No. 10/168,040, entitled “SINGLE-POLE COMPONENT MANUFACTURING,” filed on Sep. 16, 2002 now U.S. Pat. No. 6,903,413, which prior application is herein incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the manufacturing of single-pole components in vertical monolithic form. The following description more specifically relates to components of Schottky diode type made in vertical form in silicon substrates. 
   2. Discussion of the Related Art 
     FIG. 1  illustrates a conventional Schottky diode structure. Such a structure includes a semiconductor substrate  1 , typically made of heavily-doped single-crystal silicon of a first conductivity type, generally N type. A cathode layer  2  covers substrate  1 . It is N-type doped, but more lightly than substrate  1 . A metal layer  3  forms a Schottky contact with N-type cathode  2 . 
   The thickness of layer  2  is chosen to determine the reverse breakdown voltage of the Schottky diode. 
     FIG. 2  illustrates the variation of the electric field E across the thickness of the structure shown in  FIG. 1 , along an axis A–A′. For clarity, the different portions of curve  10  of  FIG. 2  have been connected by dotted lines to the corresponding regions of  FIG. 1 . 
   In such a homogeneous structure, the field variation per thickness unit is proportional to the doping level. In other words, the field decreases all the faster as the doping is heavy. It thus very rapidly drops to a zero value in substrate  1 . Since the breakdown voltage corresponds to the surface included between the axes and curve  10 , to obtain a high breakdown voltage, the doping of layer  2  must be minimized and its thickness must be maximized. 
   In the manufacturing of single-pole components, opposite constraints have to be considered. Single-pole components, such as the diode shown in  FIG. 1 , must indeed have as small a resistance (Ron) as possible, while having as high a breakdown voltage as possible when reverse biased. Minimizing the on-state resistance of a single-pole component imposes minimizing the thickness of the most lightly doped layer (layer  2 ) and maximizing the doping of this layer. 
   To optimize the breakdown voltage without modifying resistance Ron, structures of the type of that in  FIG. 3  have been provided. In  FIG. 3 , a vertical Schottky diode includes a single-crystal silicon semiconductor substrate  31 , heavily doped of a first conductivity type, for example, type N, and coated with a layer  32 . Layer  32  is formed of the same semiconductor material as substrate  31  and is of same doping type, but more lightly doped. Layer  32  is intended for forming the cathode of the Schottky diode. A metal layer  33  covers layer  32 . The metal forming layer  33  is chosen to form a Schottky contact with N-type silicon  32 . 
   Layer  32  includes very heavily-doped P-type silicon regions or “islands”  34 . Islands  34  are distributed over at least one horizontal level (over two levels in the example of  FIG. 3 ). 
   Islands  34  are separate and buried in layer  32 . The islands  34  of different horizontal levels are substantially distributed on same vertical lines. 
     FIG. 4  illustrates the variation profile of electric field E across the thickness of a structure similar to that in  FIG. 3 . More specifically, the profile of  FIG. 4  is observed along axis A–A′ of  FIG. 3 . 
   As appears from the comparison of  FIGS. 2 and 4 , the insertion of heavily-doped P-type “islands”  34  in the structure of  FIG. 3  modifies the variation of field E per thickness unit. Since islands  34  are much more heavily doped than N-type layer  32 , there are more negative charges created in islands  34  than there are positive charges in layer  2 . The field thus increases back in each of the horizontal areas including islands  34 . By setting the doping and the number of islands  34 , the space charge area can be almost indefinitely widened. In reverse biasing, the cathode formed by layer  32  and islands  34  thus generally behaves as a quasi-intrinsic layer. In average, the electric field variation per thickness unit thus strongly decreases. Thus, for a given doping level of layer  32 , the breakdown voltage is increased, as illustrated by the increase of the surface delimited by the axes and the curve of  FIG. 4  as compared to the corresponding surface of  FIG. 2 . 
   Accordingly, the structure of  FIG. 3  enables obtaining single-pole components of given breakdown voltage with a resistance Ron smaller than that of a conventional structure. 
   The practical implementation of such a structure with islands is described, for example, in German patent 19,815,907 issued on May 27, 1999, in patent applications DE 19,631,872 and WO99/26,296, and in French patent 2,361,750 issued on Mar. 10, 1978. These different documents provide obtaining a structure similar to that in  FIG. 3  by performing implantations/diffusions during a growth epitaxy of layer  32 . 
   The repeated interruptions of the epitaxial growth are a disadvantage of such an implementation. Indeed, thick layer  32  thus obtained has an irregular structure. Such structure irregularities alter the performances of the final component. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a novel method for manufacturing single-pole components of vertical type having a determined breakdown voltage and having a reduced on-state resistance. The present invention also aims at the obtained components. 
   To achieve these objects, the present invention provides a single-pole component of vertical type, including regions of a first conductivity type buried in a thick layer of a second conductivity type, said regions being distributed over at least one same horizontal level and being independent from one another, and the independent regions of which underlie an insulating material. 
   According to an embodiment of the present invention, the component includes at least two levels, the independent regions of successive levels being substantially vertically aligned. 
   According to an embodiment of the present invention, the independent regions are rings. 
   According to an embodiment of the present invention, the deepest level includes non ring-shaped regions. 
   The present invention also provides a method for manufacturing a single-pole component of vertical type in a silicon substrate of a given conductivity type, including the steps of: 
   a) forming openings in a thick silicon layer covering the substrate, doped of the same conductivity type as said substrate, but more lightly; 
   b) coating the walls and bottoms of the openings with a silicon oxide layer; 
   c) forming, by implantation/diffusion through the opening bottoms, regions of the conductivity type opposite to that of the substrate; and 
   d) filling the openings with an insulating material. 
   According to an embodiment of the present invention, before step d) of filling the openings, steps a) to c) are repeated at least once, the initial openings being continued into the thick silicon layer. 
   According to an embodiment of the present invention, the silicon layer of the same given type of conductivity as the substrate is intended for forming the cathode of a Schottky diode. 
   According to an embodiment of the present invention, the silicon layer of same conductivity type as the substrate is intended for forming the drain of a MOS transistor. 
   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, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates, in a partial simplified cross-section view, a conventional Schottky diode structure; 
       FIG. 2  illustrates the variation of the electric field across the thickness of the structure of  FIG. 1 ; 
       FIG. 3  illustrates, in a partial simplified cross-section view, the structure of a Schottky diode having a determined breakdown voltage and a reduced on-state resistance; 
       FIG. 4  illustrates the variation of the electric field across the thickness of the structure of  FIG. 3 ; and 
       FIGS. 5A to 5D  are partial simplified cross-section views of a Schottky diode at different steps of a manufacturing process according to the present invention. 
     For clarity, the same elements have been designated with the same references in the different drawings. Further, as usual in the representation of integrated circuits, the drawings are not to scale. 
       FIGS. 5A to 5D  illustrate steps of a manufacturing method of a vertical monolithic Schottky diode according to the present invention. 
   

   DETAILED DESCRIPTION 
   As illustrated in  FIG. 5A , a substrate  61  is initially covered with a single-crystal silicon layer  62 , of same doping type, for example N, as substrate  61 . Layer  62 , intended for forming the cathode of the Schottky diode, is more lightly doped than substrate  61 . Layer  62  is etched, by means of a mask  65 , to form openings  66 . Substrate  61  and layer  62  are obtained by any appropriate method. For example, layer  62  may result from an epitaxial growth on substrate  61 , or substrate  61  and layer  62  may initially be a same semiconductor region, the doping differences then resulting from implantation-diffusion operations. 
   At the next steps, illustrated in  FIG. 5B , an insulating layer  67 , for example a silicon oxide layer (SiO 2 ), is formed on the walls and at the bottom of openings  66 . Then, a P-type dopant that penetrates into the silicon at the bottom of openings  66  is implanted, after which a heating is performed to form heavily-doped P-type regions  641 . 
   At the next steps, illustrated in  FIG. 5C , layer  67 , regions  641 , and layer  62  are anisotropically etched, to form openings  68  that continue openings  66 . The upper portion of each of openings  68  is thus surrounded with a diffused ring  641 . Then, the walls and bottoms of openings  68  are covered with a thin insulating layer  69 , for example silicon oxide. 
   The implantation operations previously described in relation with  FIG. 5B  are then repeated to form heavily-doped P-type regions  642 . 
   At the next steps, illustrated in  FIG. 5D , openings  66 – 68  are filled with an insulating material  70 . Then, mask  65  is removed and the structure thus obtained is planarized. Finally, a metal layer  63  adapted to ensuring a Schottky contact with layer  62  is deposited over the entire structure. 
   Before ending, in accordance with the steps described in relation with  FIG. 5D , the structure formation by removing mask  65 , filling openings  66 – 68  with material  70 , and depositing a metal layer  63 , the steps described in relation with  FIG. 5C  could be repeated several times, to form several horizontal levels of heavily-doped P-type rings similar to rings  641 . 
   It should be noted that the intermediary rings and the underlying regions form islands according to the preceding definition. They thus provide the corresponding advantages, previously discussed in relation with  FIGS. 3 and 4 . 
   An advantage of the method according to the present invention and of the resulting structure, previously described in relation with  FIG. 5D , is the forming of a homogeneous cathode region  62 . 
   Those skilled in the art will know how to adapt the number, the dimensions, the positions, and the doping of the different rings  641 ,  642  to the desired performances. As an example, according to prior art, to obtain a breakdown voltage of approximately 600 volts, a cathode layer ( 2 ,  FIG. 1 ) of a thickness of approximately 40 μm and of a doping level on the order of 2.2·10 14  atoms/cm 3  may be used, which results in an on-state resistance of approximately 6.7 Ω·mm 2 . According to the present invention, by using groups of three P-type rings doped at approximately 3.5·10 17  atoms/cm 3 , vertically spaced apart by 10 μm around silicon oxide columns of a 1-μm width, for a same breakdown voltage of 600 V with an epitaxied layer ( 62 ,  FIG. 5D ) of a same thickness on the order of 40 μm, the cathode doping could be increased to a value on the order of some 10 15  atoms/cm 3 , which results in an on-state resistance of approximately 3 Ω·mm 2 . 
   It should be noted that it has been chosen to describe as a non-limiting example the present invention in relation with  FIG. 6  applied to the forming of silicon islands in the cathode of a Schottky diode. It would however be possible to implement a method aiming at forming in the drain of a MOS transistor, around vertical columns of an insulating material, very heavily-doped P-type silicon rings, similarly to the method previously described in relation with  FIGS. 5A–D . 
   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 operations described in relation with  FIG. 5D  can be carried out according to any appropriate sequence. Thus, after filling openings  66 – 68 , layer  65  may be removed and the structure may be planarized in a single step by means of a chem-mech polishing (CMP) method. 
   Further, the present invention applies to the forming in vertical form of any type of single-pole component, be it to reduce its on-state resistance for a given breakdown voltage, or to improve its breakdown voltage without increasing its on-state resistance.