Patent Publication Number: US-7224008-B2

Title: Self-aligned production method for an insulated gate semiconductor device cell and insulated gate semiconductor device cell

Description:
RELATED APPLICATIONS 
     This application claims priorities under 35 U.S.C. §119 to European Application No. 02406086.5 filed Dec. 10, 2002, and Under 35 U.S.C. §371 to PCT Application No. PCT/CH2003/000809 filed as an International Application on Dec. 9, 2003 designating the U.S., the entire contents of which are hereby incorporated by reference in their entireties. 
     DESCRIPTION 
     1. Technical Field 
     The invention relates to the field of semiconductor devices. It relates in particular to a method for producing an insulated gate semiconductor device and an insulated gate semiconductor device as described in the preamble of claims  1  and  7 , respectively. 
     2. Prior Art 
     To manufacture a cathode in a state-of-the art insulated gate bipolar transistor (IGBT) cell, a shallow base region and a source region are normally formed in a semiconductor substrate by means of a self-aligned process, wherein the source region is disposed in a portion of the base region. This is achieved by using a poly-silicon layer, which is to serve as a gate in the future device, as a hard mask for doping steps required in forming those regions, i.e. by doping through a cell window that has to be formed in the poly-silicon layer before the doping steps are carried out. Various topologies are common for IGBT cells, characterized by a shape of the cell window, which may be linear, circular, square, etc. In general, a plurality of cells is formed on one common semiconductor substrate. However, if IGBT cells are required that have a size comparable to that of commonly available semiconductor wafers, only one single cell per semiconductor substrate may be formed. Doping is in general done by an implantation followed by a subsequent diffusion step, but may be done by diffusion and/or implantation alone or by other methods. 
     To provide IGBTs with an improved safe operating area (SOA), in particular for high voltage devices with ratings that exceed 2000V, a composite base region is often employed. The composite base region comprises a deep base region in addition to the shallow base region, where the shallow base region and the deep base region partially overlap one another, the deep base region being narrower and higher doped than the shallow base region, i.e. having a smaller dimension in at least one direction parallel to the poly-silicon layer. To add the deep base region, a doping mask that partially masks the cell window is therefore required. This doping mask has to be accurately aligned relative to the cell window if optimum device performance is to be achieved. Misalignment will lead to an uncontrollable shift in a threshold voltage and a potential reduction in SOA capability. 
     Another process step that requires accurate mask alignment is the formation of a source contact. If a contact mask is misaligned, a width of the source region between the source contact and a channel will deviate from a corresponding design value. However, if the width of the source region varies throughout the IGBT cell, a degradation of a short circuit capability will result. 
     DESCRIPTION OF THE INVENTION 
     It is an object of the invention to provide a method for manufacturing an insulated gate semiconductor device cell of the type mentioned initially that permits accurate alignment between a deep base region and/or a source contact on the one side and a source region and/or a shallow base region and/or a cell window on the other side and an insulated gate semiconductor device cell in which performance degradation due to misalignment, in particular source contact misalignment, will be reduced. 
     This object is achieved by a method according to claim  1  and a semiconductor device according to claim  7 . 
     In the inventive method, a cell window is formed in a layered structure, said layered structure being disposed on a top surface of a semiconductor substrate, in such a manner that, inside the cell window, at least one strip of the layered structure remains. For subsequent process steps that require partial masking of the cell window, at least a first process mask is formed in such a way that at least one first strip acts as a first edge for said first process mask. This has the advantage that a position of a first process mask edge for the at least one additional subsequent process step is accurately defined relative to the cell window. Favorably, at least one additional layer, in particular an oxide layer or a primer for adding the process mask, may be disposed so that it at least partially covers the at least one strip and/or a main portion of the layered structure surrounding the cell window before the process mask is added. 
     In a preferred variation of the inventive method, at least a first strip is formed as isolated strip, i.e. is not in touch with a main portion of the layered structure surrounding the cell window. If the layered structure comprises at least one electrically conducting layer, no electrical connection will therefore remain between a first, main portion of the electrically conducting layer that surrounds the cell window and a second portion of the electrically conducting layer comprised by the first strip. The second portion of the electrically conducting layer will therefore not be electrically active when the device is in operation. This has the advantage of improving process sensitivity for potential gate-emitter short problems during a subsequent etch step required for a formation of an emitter contact when manufacturing IGBT cells. 
     In a preferred variation of the inventive method, the first process mask serves as a doping mask for adding a deep base region. The deep base region is formed by doping with the doping mask present, with dopants of a first conductivity type, preferably by implantation with subsequent diffusion. This has the advantage that the deep base region is accurately aligned relative to the first strip. Preferably, the doping mask is subsequently removed and a shallow base region and a source region is formed in subsequent process steps, preferably by implantation with subsequent diffusion of dopants of the first and of a second conductivity type, respectively. The deep and shallow base regions and the source region will thus be accurately aligned relative to one another. 
     In another preferred embodiment of the inventive method, the first process mask serves as a contact mask. A source region is added by doping through the cell window with dopants of the second conductivity type before the contact mask is formed. A first main contact is then formed by etching through the source region once the contact mask is in place. Accurate alignment of the first main contact within the cell window results. 
     In another preferred variation of the inventive method, the first process mask serves as doping mask. After removal of the first process mask and a number of intermediate process steps, a second process mask is added in such a way that the strip again acts as an edge for said second process mask, which preferably serves as contact mask. 
     In another preferred variation of the method according to the invention, one or more, preferably all, strips are removed in a subsequent process step. 
     In the inventive semiconductor device according to claim  7 , at least one strip of a layered structure is disposed on a third area of a top surface of a semiconductor substrate between a gate edge and a first main contact. This has the advantage that a degradation of short circuit capabilities resulting from potential misalignment of the first main contact relative to the gate edge will be reduced compared to a state of the art semiconductor device. 
     In a preferred embodiment of the inventive semiconductor device, a second portion of an electrically conducting layer of the layered structure, said second portion of the electrically conducting layer being comprised by the at least one strip, is electrically connected to a first main contact. This way, a degradation of short circuit capabilities resulting from misalignment can be eliminated completely. 
     Further advantageous realizations can be found in the dependent claims. 
    
    
     
       BRIEF EXPLANATION OF THE FIGURES 
       The invention will be explained in more detail in the following text with reference to exemplary realizations and in conjunction with the figures, in which: 
         FIG. 1  shows a cross section of a semiconductor substrate with a layered structure disposed on a top surface, 
         FIGS. 2 ,  3 ,  4  and  5  illustrate how to manufacture an IGBT cell cathode, 
         FIG. 6  shows a IGBT cell cathode resulting from the manufacturing process shown in  FIGS. 2 through 5 , 
         FIG. 7  shows a cut through the structure shown in  FIG. 6 , and 
         FIGS. 8 and 9  show an insulated gate semiconductor device according to the invention. 
     
    
    
     The reference signs used in the figures are explained in the list of reference signs. In principle, identical reference symbols are used to denote identical parts. 
     APPROACHES TO REALIZATION OF THE INVENTION 
       FIG. 1   a  shows a cross section of an n-doped semiconductor substrate  1 . A layered structure  2  comprising an oxide layer  22  and a poly-silicon layer  21  was disposed on a top surface of the semiconductor substrate  1 .  FIGS. 2 ,  3 ,  4  and  5  illustrate how to manufacture an IGBT cell cathode in the semiconductor substrate  1 : In a first step, a cell window  3  is generated in the layered structure  2  as shown in a perspective representation in  FIG. 2  and in cross-section in  FIG. 3   a.  Standard methods known to a person skilled in the art of photolithography are preferably used to remove the layered structure  2  where desired. The cell window  3  is formed in such a way that a first row of isolated strips  41  and a second row of isolated strips  42  of the layered structure  2  remain inside the cell window  3 , so that at least one opening  411  and  421  results within each row. A second portion of the poly-silicon layer  21  comprised by the isolated strips  41 ,  42  is not electrically connected to a first, main portion of the poly-silicon layer  21  that surrounds the cell window  3 , and will therefore not be electrically active when the device is in operation. This has the advantage of improving a process sensitivity for potential gate-emitter short problems during a subsequent contact etch. 
     A first process mask  51 , typically photoresist, is then formed so that said first process mask  51  partially covers the cell window  3 , and at least partially covers each of the strips  41  and  42  as shown in  FIG. 3   b.  Widths w 1  and w 2  are preferably bigger than a typical uncertainty in the position of the process mask  51 , i.e. w 1  and w 2  measure preferably a few micrometers, preferably 2 to 3 micrometers. A deep p + -base region  11  as shown in  FIG. 3   d  is then formed by a first ion implantation step and a subsequent first diffusion step of dopants of a first conductivity type, e.g. Boron. This will provide the resulting IGBT cell with good safe operating area (SOA) capabilities. During the ion implantation step, the strips  41 ,  42  act as a process mask edge. A resulting first implantation profile  11   a  as shown in  FIG. 3   c  is therefore laterally confined by the strip position. The first process mask  51  can now be removed for further processing steps. 
     Additional processing steps are required to complete the IGBT cell. A shallow p-base region  12  with dopants of the first conductivity type as shown in  FIG. 4   b  is added, again by a second implantation step, resulting in a second implantation profile  12   a  as shown in  FIG. 4   a.  This time, implantation is done over the whole cross section of the cell window  3 . After a second diffusion step, the shallow p-base region  12  results. In a subsequent step, an n + -source region  13  with dopants of a second conductivity type, e.g. Arsenic or Phosphorus as shown in  FIG. 4   c  is added preferably by a third implantation step followed by a third diffusion step. 
     In a preferred embodiment of the invention, a second process mask  52  is then formed as shown in  FIG. 5   a.  In a subsequent contact etch step, the second process mask  52  serves as a contact mask, with the strips  41  and  42  again acting as mask edges. An emitter contact  6  is formed by etching through the n + -source region  13  in an area between the strips  41  and  42 , as shown in  FIG. 5   b,  and by filling a resulting contact well with metal, so that a good electrical connection is established between the emitter contact  6  and the deep p+-base region  11 . 
     A IGBT cell cathode resulting from the manufacturing process described above in connection with  FIGS. 2 through 5  is shown in  FIG. 6 . The structure shown is a so-called mini-strip design.  FIG. 7   a  shows a cut through the structure of  FIG. 6 , along the line A–A′ and perpendicular to the z-axis. An alternative design, a so-called full-strip design is shown in  FIG. 7   b.  The openings  411  and  421  ensure that an outer portion of the n+-source regions  13  located between each of the strips  41 ,  42  and the first, main portion of the layered structure  2  surrounding the cell window  3  are electrically connected to the emitter contact  6  via a distributed resistance that is in series with the emitter contact  6 , as known from emitter ballasting. The ration of the openings to the full strip lengths is an important factor in the design of a IGBT cell. The basic idea behind this approach is that if an emitter current increases locally, the voltage drop across an emitter ballast resistance will also increase, which in turn will reduce the emitter current and divert the emitter current to other regions of the n+-source region  13 . This has the advantage that it promotes a more uniform current distribution. 
     In another preferred embodiment of the invention, an electrical connection is established between the emitter contact  6  and the second portion of the poly-silicon layer  21  comprised by the strips  41 ,  42 . This is preferably achieved by also etching away such portions of the strips  41 ,  42  that are not covered by the contact mask  52  during the contact etch step, and choosing a thickness of the emitter contact  6  appropriately. 
     In another preferred embodiment of the invention, the strips  41  and  42  are removed by a dry-etch step after the deep p + -base region  11  has been formed. A dry-etch mask is formed on those portions of the poly-silicon layer  21  that are not to be removed by the dry-etch step, in particular the first, main portion of the poly-silicon layer  21  surrounding the cell window  3 , i.e. the future gate regions. This is of particular advantage in another preferred variant of the method according to the invention, in which the cell window  3  in the layered structure is formed in such a manner that the strip that remains forms a closed loop, which may, e.g., be rectangular, circular, hexagonal, etc. 
     A vertical cut through an insulated gate semiconductor device according to the invention is shown in  FIG. 8 . The device represents an IGBT structure, with a p-doped anode region  14  formed on the bottom of the semiconductor substrate  1 , said p-doped anode region  14  being electrically connected to an anode electrode  7 . A low temperature oxide layer  8  is disposed on top of and around the poly-silicon layers  21 , electrically insulating poly-silicon layer  21  from the emitter metallization  9 . In this embodiment, there is no electrical connection between the strips  41 ,  42  of the second portion of the poly-silicon layer  21  and either the emitter metallization  9  or the emitter contact  6 . Another vertical cut through one of the openings in the strips of the semiconductor device according to the invention is shown in  FIG. 9 . 
     In a preferred embodiment of the semiconductor device according to the invention an electrical connection exists between the second portion of the poly-silicon layer  21  being comprised by the strips  41 ,  42  and the emitter metallization  9  and the emitter contact  6 . 
     LIST OF REFERENCE SIGNS 
     
         
           1  Semiconductor substrate 
           11  First doped region, deep p + -base 
           11   a  First implantation profile 
           12  Second doped region, shallow p-base 
           12   a  Second implantation profile 
           13  Third doped region, n + -source 
           14  p-doped anode region 
           21  Electrically conducting layer, polysilicon layer 
           22  Electrically insulating layer 
           3  Cell window 
           41 ,  42  First, second strips 
           411 ,  421  First, second opening 
           51  First process mask, doping mask 
           52  Second process mask, contact mask 
           6  First main contact, emitter contact 
           7  Anode metallization 
           8  Low temperature oxide 
           9  Cathode metallization