Abstract:
In semiconductor devices and methods for their manufacture, the semiconductor devices are arranged as a trench-Schottky-barrier-Schottky diode having a pn diode as a clamping element (TSBS-pn), and having additional properties compared to usual TSBS elements which make possible adaptation of the electrical properties. The TSBS-pn diodes are produced using special manufacturing methods, are arranged in their physical properties such that they are suitable for use in a rectifier for a motor vehicle generator, and are also able to be operated as Z diodes.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a semiconductor device as well as to a method for its manufacture. With regard to the semiconductor device of the species, a trench-Schottky-barrier-Schottky diode is involved. 
       BACKGROUND INFORMATION 
       [0002]    In modern motor vehicles, ever more functions are implemented using electrical components. This causes an ever rising requirement for electrical power. In order to cover this requirement, the efficiency of the generator system in the motor vehicle has to be increased. 
         [0003]    Up to now, pn diodes have been used, as a rule, as Z diodes in the motor vehicle generator system as rectifiers. The advantages of the pn diodes are low blocking-state current and robustness. The main disadvantage of pn diodes is the high forward voltage UF. At room temperature, the current only begins to flow at UF=0.7V. And under normal operating conditions, for instance, at a current density of 500 A/cm 2 , UF rises to over 1 V, which means a nonnegligible loss in efficiency. 
         [0004]    In theory, the Schottky diode is available as an alternative. Schottky diodes have a clearly lower forward voltage than pn diodes, such as 0.5 V to 0.6 V at a high current density of 500 A/cm 2 . In addition, Schottky diodes, as majority carrier components, offer advantages in rapid switching operation. The use of Schottky diodes in motor vehicle generator systems has, however, still not taken place. This must be attributed to a few decisive disadvantages of Schottky diodes: 1) higher blocking-state current in comparison to pn diodes, 2) strong dependence of the blocking-state current of the reverse voltage, and 3) poor robustness, especially at high temperature. 
         [0005]    The usual Schottky diodes can be changed in their electrical properties by applying trenches implemented by etching. Such further developed Schottky diodes are also designated as trench-Schottky-barrier-Schottky diodes (TSBS). An example of such a conventional semiconductor device is shown in  FIG. 1 . However, the conventional trench-Schottky-barrier-Schottky diodes do not have the requisite robustness they need in order to be used as Z diodes in motor vehicle generator systems. 
       SUMMARY 
       [0006]    Semiconductor devices according to example embodiments of the present invention, on the other hand, have the advantage of high robustness, and are therefore suitable for being used as Z diodes in motor vehicle generator systems. 
         [0007]    These advantages are achieved by furnishing a trench-Schottky-barrier-Schottky diode (TSBS diode) with an integrated pn diode as clamping element. The additional p wells, in this context, are dimensioned so that the breakdown voltage of the pn diode is lower than the breakdown voltage of the Schottky diode. 
         [0008]    By arranging the trench-Schottky-barrier-Schottky diode (TSBS diode) to have an integrated pn diode as clamping element (TSBS-pn), it is possible to operate the diode as a Z diode. At a breakdown voltage of the Z diode of approximately 20 Volt, it is suitable for use in a motor vehicle generator system and can be used as a rectifying element for the rectifier of the generator. 
         [0009]    The manufacture of the semiconductor devices according to variant 1 or variant 2 is accomplished using suitable methods that are adapted to the different variants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  shows a conventional trench-Schottky-barrier-Schottky diode (TSBS). 
           [0011]      FIG. 2  shows an exemplary embodiment of the present invention having a trench-Schottky-barrier-Schottky diode having an integrated pn diode as clamping element (TSBS-pn) having p-doped silicon or polysilicon in the lower region of the trenches. 
           [0012]      FIG. 3  shows an exemplary embodiment of the present invention having a trench-Schottky-barrier-Schottky diode having an integrated pn diode as clamping element (TSBS-pn) having a-barrier oxide and p-doped silicon or polysilicon in the lower region of the trenches (TSBS-BOpn). 
       
    
    
     DETAILED DESCRIPTION  
       [0013]    As  FIG. 1  shows, the trench-Schottky-barrier-Schottky diode (TSBS) is made up of an n +  substrate  1 , an n-doped epilayer, at least two trenches  3  of a depth Dt, implemented in the n-epilayer  2  by etching, and a metallic layer  4  on the back surface of the chip as ohmic contact or cathode electrode. The two trenches  3  are first filled in using a metal  5  of a thickness Dm 1  and this is subsequently covered using a second metal  6 . Second metal  6  fills the rest of the trenches at a thickness Dm 2 . The two metals  5  and  6  on the front side of the chip are used as Schottky contacts or an anode electrode. Second metal  6 , as a rule, has a smaller barrier height than first metal  5 . Therefore, from an electrical point of view, the TSBS is a combination of two Schottky diodes having different barrier heights: a Schottky diode having a Schottky barrier between metal  5  as anode and n-epilayer  2  as cathode, and a second Schottky diode having a Schottky barrier between metal  6  as anode and n-epilayer  2  as cathode. 
         [0014]    In the forward direction, at least when the barrier heights of the two metals are clearly different, currents flow chiefly to upper metal  6  that has the lower barrier, even at the corresponding sidewalls of the trenches. Provided Dm 2 &gt;Wt applies, the effective surface for the current flow in the flow direction in a TSBS is greater in comparison to a conventional planar Schottky diode. 
         [0015]    In the blocking direction, first metal  5 , with its greater barrier height, provides great expansion of the space charge region. The space charge regions expand with increasing voltage and, at a voltage that is less than the breakdown voltage of the TSBS, they collide in the middle of the region between adjacent trenches  3 . The Schottky effects responsible for high blocking-state currents are screened off thereby, and the blocking-state currents are reduced. This screening effect is greatly dependent on structural parameters Dt (the depth of the trench), Wm (the distance between the trenches), Wt (the width of the trench), as well as on Dm 1  (the thickness of first metal  5 ), see  FIG. 1 . The expansion of the space charge regions in the mesa region between trenches  3  is virtually unidimensional, if Dt is clearly greater than Wm. 
         [0016]    The specialty of a TSBS is the combination of the two metals, which makes possible a certain separation of the designs with respect to requirements on forward voltage and screening response. Forward voltage UF and the initial value of blocking-state current IRO are influenced primarily by second metal  6  that has the lower barrier. The greater the proportion of second metal  6 , the lower is UF, and the higher is IRO. On the other hand, first metal  5 , having the greater barrier, determines the voltage dependence of the blocking-state current, the breakdown voltage and the current distribution at high blocking-state currents. Therefore, TSBS offers a possibility of optimization by combination of the two metals. Both thicknesses DM 1  and DM 2  and the barrier heights of the two metals can be used as parameters. 
         [0017]    However, a certain disadvantage of TSBS is the weakness of the Schottky contacts at the trench floor, since there, strong electric fields, and consequently also high blocking-state densities are created. Because of the inhomogeneity of the boundary layer at the trench floor, it is not recommended to use TSBS as Z diodes and to use them in the breakdown range (drifting, Q risk). 
         [0018]    Such TSBS elements are manufactured as follows: Implementation of trenches  3  by etching n-epilayer  2 , filling in trenches  3  with first metal  5 , and, if necessary, etching back the first metal to a specified thickness, and filling in the rest of the trenches using second metal  6 . 
         [0019]    Example embodiments of the present invention are shown in  FIGS. 2 and 3 . 
         [0020]    Variant  1 : A trench-Schottky-barrier-Schottky diode having an integrated pn diode as clamping element (TSBS-pn), having p-doped silicon or polysilicon in the lower region of the trenches is shown in  FIG. 2 . 
         [0021]    As shown, the TSBS-pn of an exemplary embodiment of the present invention is made up of an n +  substrate  1 , an n-epilayer  2 , at least two trenches  3  of depth Dt etched into n-epilayer  2 , and a metal layer  4  on the back surface of the chip as ohmic contact or cathode electrode. Lower region  7  of trenches  3  is filled in using p-doped Si or poly-Si up to a height of Dp. The trenches are then filled in using a metal  5  of a thickness Dm 1 , using an ohmic contact to p-doped Si or poly-Si  7 , and using a Schottky contact to n-epilayer  2  (anode electrode) and subsequently covered using second metal  6 . Second metal  6  fills in the rest of the trenches at a thickness of Dm 2 , having Schottky contact to n-epilayer  2 , and is also used as an anode electrode. From an electrical point of view, TSBS-pn is a combination of two Schottky diodes having different barrier heights and a pn diode having p-wells  7  as anode and n-epilayer  2  as cathode. 
         [0022]    In the TSBS-pn, the currents flow in the forward direction only through the upper Schottky diode. The lower Schottky diode and the pn diode are not available. The Schottky contacts at the sidewalls of the trenches take care of a greater effective surface for the current flows in the forward direction, in comparison to conventional planar Schottky diodes, provided 2Dm 2 &gt;Wt applies. In the blocking direction, space charge regions develop, both in the case of the Schottky barriers and the p-n junction. The space charge regions expand with increasing voltage and, at a voltage that is less than the breakdown voltage of the TSBS-pn, they collide in the middle of the region between adjacent trenches  3 . The Schottky effects responsible for high blocking-state currents are screened off thereby, and the blocking-state currents are reduced. This screening effect is greatly dependent upon structural parameters Wm (clearance between the trenches), Wt (width of the trenches or the p-wells), Dp (depth of the trench proportion having p-doped Si or poly-Si; corresponds to the thickness of the p-wells), as well as DM 1  (thickness of first metal  5 ), see  FIG. 2 . 
         [0023]    TSBS-pn has similar screening effects of Schottky effects as does TSBS, but in addition, it offers great robustness because of the integrated clamping function of the pn diode. The breakdown voltage of the pn diode, BV_pn is configured such that BV_pn is lower than the breakdown voltage of the Schottky diodes BV_schottky. The breakdown takes place at the trench floor, and the breakdown voltage of TSBS-pn is determined by BV_pn. Therefore, there is no high field strength in the vicinity of Schottky contacts, and the blocking-state currents in breakdown operation then only flow through the p-n junction, and not through the Schottky contacts, as in a conventional TSBS. With that, TSBS-pn has the robustness of a pn diode. As a result, TSBS-pn is quite suitable as a Z diode. 
         [0024]    Manufacturing Method of TSBS-pn, Variant  1   
         [0025]    Step  1 : monocrystalline n +  substrate is used as starting material. 
         [0026]    Step  2 : the also monocrystalline n-doped layer  2  is applied in an epitaxy method. 
         [0027]    Step  3 : trenches  3  are etched into layer  2 , using trench etching. 
         [0028]    Step  4 : filling in trenches  3  takes place using p-doped Si or poly-Si. 
         [0029]    Step  5 : if necessary, partial etching of a part of the p-doped Si or poly-Si in trenches  3 . 
         [0030]    Step  6 : filling in trenches  3  with first metal  5 . 
         [0031]    Step  7 : if necessary, etching first metal  5  down to a specified thickness. 
         [0032]    Step  8 : filling in the rest of trenches  3  with second metal  6 . 
         [0033]    Step  9 : metalizing on the back surface, that is, applying metal layer  4  to the back surface of the chip. 
         [0034]      FIG. 3  shows an exemplary embodiment of the present invention, according to variant  2 : having a trench-Schottky-barrier-Schottky diode having an integrated pn diode as clamping element (TSBS-pn) having a barrier oxide and p-doped silicon or polysilicon in the lower region of the trenches (TSBS-BOpn). 
         [0035]    As shown in  FIG. 3 , the difference from Variant  1  is that, in variant  2  an oxide layer  8  is introduced between first metal  5  and the sidewalls of trenches  3 . With that, an additional barrier-oxide structure (trench-MOS structure) is implemented. That is why this variant is given the name TSBS-BOpn. Oxide thickness To now also belongs to the important structural parameters. 
         [0036]    Second metal  6  having the lower barrier, as in variant  1 , provides the lower forward voltage. The pn junction at the trench floor determines the breakdown voltage and is used as clamping element. 
         [0037]    An advantage of variant  2  over variant  1  is, on the one hand, a better screening effect and therefore a lower blocking-state current. This advantage is conditioned upon the trench-MOS structure, which now replaces the lower Schottky contact. With increasing barrier height of metal  5 , this effect is increased somewhat more. 
         [0038]    Variant  2  includes an MOS structure, but within the sensitive oxides and in the adjacent Si region, no great field strength appears. Therefore, in the case of a TSBS-BOpn, one does not have to be afraid of the injection of “hot” charge carriers. 
         [0039]    As a result, TSBS-BOpn is robust like a pn diode, and is quite suitable for operation as a Z diode. 
         [0040]    Manufacturing Method of TSBS-pn (TSBS-BOpn), Variant  2 : 
         [0041]    Step  1 : monocrystalline n +  substrate is used as starting material. 
         [0042]    Step  2 : n-doped layer  2  is applied in an epitaxy method. 
         [0043]    Step  3 : trenches  3  are etched using trench etching. 
         [0044]    Step  4 : filling in trenches  3  using p-doped Si or poly-Si. 
         [0045]    Step  4 : partial etching of the p-doped Si or poly-Si in trenches  3 . 
         [0046]    Step  5 : oxidation of the surface of trenches  3 . 
         [0047]    Step  6 : etching of the oxides on the floor and at the sidewalls of trenches  3  to a specified height. 
         [0048]    Step  7 : filling in trenches  3  with first metal  5 . 
         [0049]    Step  8 : if necessary, etching of the first metal down to a specified thickness. 
         [0050]    Step  9 : filling in the rest of trenches  3  with second metal  6 . 
         [0051]    Step  10 : metallization on the back surface of the chip. 
         [0052]    Both variant  1  and variant  2  may have additional structures in the edge region, for the reduction of the fringing field strength. These may be, for example, low-doped p regions, magnetoresistors, etc., e.g., corresponding to conventional arrangements.