Abstract:
A manufacturing method having the steps of: depositing an upper layer of polycrystalline silicon; defining the upper layer, obtaining LV gate regions of low voltage transistors and undefined portions; forming LV source and drain regions laterally to the LV gate regions; forming a silicide layer on the LV source and drain regions, on the LV gate regions, and on the undefined portions; defining salicided HV gate regions of high voltage transistors; and forming HV source and drain regions not directly overlaid by silicide portions.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a method for manufacturing electronic devices having high voltage (HV) transistors and low voltage (LV) transistors with salicided junctions.  
         BACKGROUND OF THE INVENTION  
         [0002]    In advanced processes (gate lengths of 0.35 μm or less), the need has recently arisen to integrate HV transistors in high-speed devices which use the technique of saliciding the diffusions. As is known, this technique is based on the use of a layer of self-aligned silicide (“salicide”), which reduces the resistivity of the junctions. The salicide layer (typically of titanium, but also cobalt or another transition metal) is obtained by depositing a titanium layer on the entire surface of the device, and performing a heat treatment that makes the titanium react with the silicon, which is left bare on the junctions and the gate regions, such as to form titanium silicide. Subsequently, the non-reacted titanium (for example that deposited on oxide regions), is removed by etching with an appropriate solution, which leaves the titanium silicide intact. Thereby, both the gate regions and the junctions have in parallel a layer of silicide with low resistivity (approximately 3-4 Ω/square), which makes it possible to reduce the resistance in series at the transistors. The salicide technique is described for example in the article “Application of the self-aligned titanium silicide process to very large-scale integrated n-metal-oxide-semiconductor and complementary metal-oxide-semi-conductor technologies” by R. A. Haken, in J. Vac. Sci. Technol. B, vol. 3, No. 6, November/December 1985.  
           [0003]    The HV transistors are formed without intensive implanting doping ionic species, to obtain lightly doped junctions, which thus have a high breakdown voltage. The saliciding process is difficult if the silicon beneath is lightly doped, and this means that it is necessary to avoid saliciding the junctions of the HV transistors.  
           [0004]    Process flows are thus being designed which permit integration of HV transistors and LV transistors with saliciding; however this integration is made difficult by the fact that these components have different characteristics, and require different process steps.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention described herein provides a method for manufacturing high-speed HV transistors and LV transistors that is simple and has the lowest possible costs.  
           [0006]    According to the invention, a method is provided for manufacturing electronic devices comprising high-speed HV transistors and LV transistors with salicided junctions.  
           [0007]    Hereinafter, a production process will be described, aimed to produce EEPROM memory cells, besides LV and HV transistors; however, the invention relates in general to the production of LV and HV transistors, irrespective of the memory cells and the specific process described. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For the understanding of the present invention, a preferred embodiment is now described, purely by way of non-limiting example, with reference to the attached drawings, in which:  
         [0009]    [0009]FIG. 1 shows a cross-section through a silicon wafer, in an initial step of the manufacturing method according to the invention;  
         [0010]    [0010]FIG. 2 shows a view from above of the wafer of FIG. 1;  
         [0011]    FIGS.  3 - 7  show cross-sections similar to FIG. 1, in successive manufacturing steps;  
         [0012]    [0012]FIG. 8 shows a view from above of the wafer of FIG. 7;  
         [0013]    FIGS.  9 - 11  show cross-sections similar to FIG. 7, in successive manufacturing steps;  
         [0014]    [0014]FIG. 12 shows a view from above of the wafer of FIG. 11;  
         [0015]    [0015]FIG. 13 shows a cross-section similar to FIG. 11, in a successive manufacturing step;  
         [0016]    [0016]FIG. 14 is a cross-section, taken along lines XIV-XIV of FIG. 13;  
         [0017]    [0017]FIG. 15 shows a view from above of the wafer of FIG. 13;  
         [0018]    FIGS.  16 - 19  show cross-sections similar to FIG. 13, in successive manufacturing steps;  
         [0019]    [0019]FIG. 20 shows a view from above of the wafer of FIG. 19; and  
         [0020]    FIGS.  21 - 23  show cross-sections similar to FIG. 19, in successive manufacturing steps. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The following description relates to an embodiment for forming LV (low voltage and high speed) and HV (high voltage) NMOS transistors, LV and HV PMOS transistors, and EEPROM memory cells, comprising a selection transistor and a memory transistor. In particular, in view of the duality in manufacturing NMOS and PMOS transistors, the drawings show only the steps relating to NMOS transistors, and the steps relating to PMOS transistors are described in words alone. The EEPROM cells form a memory array, and are produced in a part of the wafer which is also known hereinafter as array zone  15 .  
         [0022]    In FIG. 1, a wafer  1 , formed from a monocrystalline silicon substrate  2 , which here is of P-type, has been subjected to the steps of defining the active areas. In detail, with the surface  3  of substrate  2  covered by an active area mask  4  of non-oxidizable material (typically comprising a double layer of silicon oxide and silicon nitride, defined through resist), wafer  1  has been subjected to thermal oxidation; consequently, on the parts of substrate  2  which are not covered by active area mask  4 , thick oxide (field oxide) layers  5  have been grown, which delimit between one another active areas of the substrate designed to accommodate the various components of the device to be formed. In particular, FIG. 1 shows three active areas, an active LV area  6 , which is designed to accommodate an LV NMOS transistor, an active HV area  7 , which is designed to accommodate an HV NMOS transistor, and an active array area  8 , which is designed to accommodate EEPROM memory cells.  
         [0023]    In detail, and in a known manner, active array area  8  defines a grid, of which FIG. 2 shows in full only the part of one cell, showed at  9 , which has substantially the shape of a “T” rotated by 90°, and comprises a leg  9   a  (far from active HV area  7 ) and a cross-piece  9   b . The leg  9   a  is adjacent, and is electrically connected, to corresponding legs  9   a  of other cells, which are arranged above and below the shown cell, and of which only parts are shown; in addition, leg  9   a  is connected to a leg of an adjacent cell to the right (not shown), which has a structure that is symmetrical to that shown. The legs  9   a  are designed to accommodate source regions of the memory transistors; the end of cross-pieces  9   b  are designed to accommodate drain regions of the selection transistors; and gate regions of the cells must be formed on the cross-pieces  9   b . Further active areas are generally provided in order to produce LV or HV PMOS transistors, which are not shown in the drawings.  
         [0024]    Subsequently the active area mask  4  is removed, oxidation of the free surface  3  of the substrate is carried out to form a sacrificial oxide layer  10 , and masked implanting of doping ionic species of N-type is carried out to form N-HV regions (not shown) for HV PMOS transistors; then, using an HV P-well resist mask  11  that covers the entire surface of wafer  1 , except the HV active area  7  and the array area  8 , implanting of doping ionic species of P-type is carried out, as shown schematically in FIG. 3 by arrows  12 . In the substrate  2 , P-HV regions  13  of P-type are thus formed for high-voltage transistors, and a P-array region  14 , also of P-type, is formed for the cells, as shown in FIG. 3. P-HV region  13  and P-array region  14  reproduce exactly the shape of the respective HV active area  7  and array area  8 , and thus, for each cell, legs  14   a  (corresponding to legs  9   a  of the cell active areas  9  of cell, see FIG. 8), and cross-pieces  14   b  (FIG. 8, corresponding to the cross-pieces  9   b ) are shown.  
         [0025]    After HV P-well mask  11  has been removed, masked implanting of doping ionic species of N-type is carried out, to form N-LV regions (not shown) for LV PMOS transistors; then, using an LV P-well resist mask  17  which covers the entire surface of wafer  1 , except for the LV active areas  6 , implanting of doping ionic species of P-type is carried out, as shown schematically in FIG. 4 by arrows  18 . In the substrate  2 , P-LV regions  19  of P-type are thus formed for the LV NMOS transistors, as shown in FIG. 3. Thereby, P-HV regions  13  and P-LV regions  19  are separated from one another, and their electrical characteristics can be optimised to the required electrical characteristics.  
         [0026]    After LV P-well mask  17  has been removed, a capacitor mask  20  is formed, which covers the entire surface of wafer  1 , with the exception of strips perpendicular to the cross-pieces  14   b . Implanting of doping species of N-type (for example phosphorous) is then carried out, as shown schematically in FIG. 5 by arrows  21 . In the cross-pieces  14   b  continuity regions  22 , of N-type, are thus formed which are necessary for electrical continuity between each selection transistor and the corresponding memory transistor of each cell. The structure in FIG. 5 is thus obtained.  
         [0027]    After capacitor mask  20  has been removed, wafer  1  is subjected to annealing, sacrificial layer  10  is removed, and array oxidation is carried out, which leads to the formation of an array oxide layer  25  on the surface of all the regions  13 ,  14  and  19  (FIG. 6). Then, using a tunnel mask, not shown, a small portion of the array oxide layer  25  is removed from above the continuity region  22 ; after the tunnel mask has been removed, wafer  1  is oxidized again, and in the zone where the array oxide  25  had been removed, a tunnel oxide region  26  with a thickness of approximately 80 Å is formed, in a known manner. The structure in FIG. 6 is thus obtained.  
         [0028]    A first polycrystalline silicon layer (poly1 layer  27 ) is then deposited, and is suitably doped; a floating gate mask  28  is then formed which covers all the surface of wafer  1 , except for windows that expose legs  14   a  (FIG. 8) and the field oxide regions  5 , laterally to the cross-pieces  14   b  adjacent to the legs  14   a , as shown in FIG. 8. Then, through the floating gate mask  28 , poly1 layer  27  is removed where it is exposed. In particular, the portions of poly1 layer  27  removed laterally to the cross-pieces  14   b , form vertical walls  27 ′, which are arranged on two opposite sides of a quadrilateral, and the width of which (shown vertically in FIG. 8) defines the floating gate regions of the memory transistors, and the portions of the poly1 layer  27  removed from above the legs  14   a  form a vertical wall  27 ″, which is disposed on a third side of the quadrilateral (FIG. 8). On the other hand, the poly1 layer  27  is not removed where the selection transistors are to be formed. Subsequently, implanting of doping ionic species of N-type is carried out, as shown schematically by arrows  29  in FIG. 7, to reduce the resistance of the source lines. First source regions  30  of the memory transistors are then formed, at the legs  14   a  of the P-array region  14 , as shown in FIG. 7.  
         [0029]    After the floating gate mask  28  has been removed, an interpoly dielectric layer  31  is formed, which for example comprising a triple layer of ONO (silicon oxide-silicon nitride-silicon oxide), which, inter alia, covers the vertical walls  27 ′ and  27 ″(FIG. 8) of poly1 layer  27 , for electrically isolating the floating gate regions of adjacent cells. A matrix mask  33  is then formed, which covers the surface of wafer  1 , at the array zone  15 , and leaves exposed all the N and P regions designed to accommodate LV and HV, NMOS and PMOS transistors, including regions P-HV  13  and P-LV  19 ; using the matrix mask  33 , interpoly dielectric layer  31 , poly1 layer  27 , and array oxide layer  25  are etched in succession, where they are exposed. Thus the structure of FIG. 9 is obtained.  
         [0030]    After matrix mask  33  has been removed, an HV oxidation step is carried out, thus forming an HV gate oxide layer  34  on the entire free surface of the substrate  2 , and in particular on regions P-LV  19  and P-HV  13 . A thin oxide layer (not shown) is also formed on the interpoly dielectric layer  31 . Subsequently, using an HV resist oxide mask  35 , which covers regions P-HV  13  and array zone  15 , the HV gate oxide layer  34  is removed from above the regions P-LV  19 , as shown in FIG. 10.  
         [0031]    After the HV oxide mask  35  has been removed, an LV oxidation step is carried out, thus forming an LV gate oxide layer  36  on regions P-LV  19 , increases the thickness of the HV gate oxide layer  34  on regions P-HV  13 , and (with the layer previously formed), forms a thin oxide layer  38  on the interpoly dielectric layer  31  in the array zone  15 . Subsequently, a select mask  39  is formed, which covers completely the zones designed to accommodate LV and HV, NMOS and PMOS transistors, as well as, in the array zone  15 , cross-pieces  14   b  and portions of legs  14   a , as shown in FIG. 12. In practice, select mask  39  exposes most of the first cell source regions  30  and pairs of zones  40  (FIG. 12) of wafer  1 , which are arranged on both sides of the free end portion of each cross-piece  14   b . Using select mask  39 , the exposed portions of thin oxide layer  38 , interpoly dielectric layer  31 , and poly1 layer  27 , are removed in succession. The dimensions of select mask  39  are such as to leave portions  31   a  of dielectric layer  31  on the walls  27 ″ of the poly1 layer  27 , and to remove virtually all the rest of the dielectric layer  31  from above the array oxide layer  25 . In addition, the pairs of zones  40  make it possible to obtain vertical walls  27   a  (FIG. 14), which are uncovered, for the purpose indicated hereinafter. The structure in FIG. 11 is thus obtained.  
         [0032]    After select mask  39  has been removed, a second polycrystalline layer (poly2 layer  43 ) is deposited and doped; owing to the removal of zones  40 , poly2 layer  43  is in direct contact with the walls  27   a  of poly1 layer  27 , as can be seen in the cross-section of FIG. 14. Thereby, lower and upper portions of the gate region of the selection transistor of the cell are shorted to one another. An LV gate mask  44  is then formed, which covers the regions N-HV (which are not shown), the regions P-HV  13 , and the array zone  15 , except for the first cell source regions  30 ; in addition, the LV gate mask  44  covers the poly2 layer on the regions P-LV  19 , where the gate regions of the LV NMOS transistors are to be defined, as shown in FIGS. 13 and 15, and on the N-LV regions (which are not shown), where the gate regions of the LV PMOS transistors are to be defined. The exposed portions of poly2 layer  43  and of LV gate oxide layer  36  (as well as of thin oxide layer  38 ) are then removed, providing the intermediate structure of FIG. 13, wherein the remaining portions of poly2 on the regions P-LV  19  form gate regions  43   a  of the LV NMOS transistors. As shown, while defining the gate regions of the LV transistors, the layers over the regions P-HV  13  are protected, as are the layers on the regions N-HV (which are not shown); consequently, the method described provides separate definition of the gate regions of the LV transistors and the HV transistors.  
         [0033]    After removal of LV gate mask  44 , wafer I is subjected to oxidation, such that an oxide layer  46  grows on the exposed portions of regions P-LV  19 , at the sides of gate regions  43   a , on the exposed portions of the regions N-LV (which are not shown), on the poly2 layer, and on the second cell source regions  49 . Using a resist mask, which is not shown, which covers the regions N-LV and N-HV, doping ionic species of N-type are implanted (LDDN implanting), as schematised by arrows  47  in FIG. 16. At the sides of the gate regions  43   a  (inside regions P-LV  19 ), LDD regions  48  of N-type are then formed; inside the first cell source regions  30 , aligned with the portions  31   a  of dielectric layer  31 , second cell source regions  49  of N-type are formed, which are more highly doped than first cell source regions  30 ; in addition the poly2 layer  43  is suitably doped. The structure in FIG. 16 is thus obtained.  
         [0034]    After the resist mask, not shown, has been removed, doping ionic species of P-type are implanted through a mask; in particular, during this step, regions P-HV  13  and P-LV  19 , as well as array zone  15  are covered, whereas in the regions N-LV, LDD regions of P-type (which are not shown) are formed. A dielectric layer (for example TEOS-TetraEthylOrthoSilicate) is then deposited on the entire surface of wafer  1 ; then, in a known manner, the TEOS layer is subjected to anisotropic etching and is removed completely from the horizontal portions, and remains only at the sides of the gate regions  43   a  (where it forms spacers  52 ), and on the right-hand side of the poly1 layer  27  and poly2 layer  43  (on the first and second cell source regions  30 ,  49 , where it forms spacers  53 ). On the other hand, spacers are not formed above the field oxide regions  5 , since the edges of the latter have the shape of a bird&#39;s beak (formed in a per se known manner, not shown for the sake of simplicity); in addition, no spacers are formed above regions P-HV  13 , and corresponding regions N-HV, since the gate regions of the HV transistors are not yet defined. Oxide layer  46  is also removed in this step. Subsequently, using a resist mask, not shown, which covers the regions N-LV and NHV, implanting of doping ionic species of N-type is carried out, as schematised in FIG. 17 by arrows  54 . LV-NMOS source and drain regions  55  of N+-type are then formed in regions P-LV  19 , self-aligned with the spacers  52 , and third cell source regions  56  of N+-type are formed, self-aligned with the spacers  53  in the P-array region  14 . LV-NMOS source and drain regions  55  are more doped than LDD regions  48 , and third source regions  56  are more doped than second cell source regions  49 . In addition, poly2 layer  43  and gate regions  43   a  are doped of N-type, whereas the zones where HV and LV PMOS transistors are to be formed are covered. Then the structure of FIG. 17 is obtained.  
         [0035]    After the resist mask (not shown) has been removed, a similar step of masked implanting of doping ionic species of P-type is carried out, for forming the respective source and drain regions in the N-LV regions (in a not shown manner), and for P-type doping poly2 layer  43  above the regions P-LV and P-HV. In this step, the regions P-LV, P-HV and P-array are fully covered. Saliciding of the exposed layer of poly2 is then carried out. The saliciding, which is carried out in a known manner, as already described, causes the formation of regions of titanium silicide above the source and drain regions of LV NMOS and PMOS transistors (silicide regions  57   a   1  above LVNMOS source and drain regions  55 , and similar regions for the LV PMOS transistors), above the gate regions of LV NMOS and PMOS transistors (silicide regions  57   a   2  above gate regions  43   a  for the LV NMOS transistors, and similar regions for the LV PMOS transistors), above the third cell source regions  56  (silicide regions  57   b   1 ), and above the EEPROM cells and the HV zones (silicide regions  57 , where the gate regions are not yet defined), as shown in FIG. 18.  
         [0036]    Subsequently an HV gate mask  60  is formed, which covers the entire surface of wafer  1 , with the exception of the active areas where high voltage transistors are to be formed (P-HV regions  13 , in case of HV NMOS) and the EEPROM cells; in particular, mask  60  covers the zone where the gate regions of the high voltage transistors are to be defined; the gate regions of the selection transistors and the gate and source regions of the memory transistors (in this respect see also FIG. 20, which shows HV gate mask  60  from above). The portions of silicide layer  57  and of poly2  43  layer which are not covered by the HV gate mask  60  are then etched. Thus the structure of FIG. 19 is obtained, wherein the control gate region of the memory transistor is indicated at  43   b , the upper portion of the gate region of the selection transistor (which is shorted to the lower portion, as already described) is indicated at  43   c , and the gate region of the HV NMOS transistor is indicated at  43   d ; the corresponding portions of salicide are indicated at  57   b   2 ,  57   c , and  57   d . In practice, definition of the regions  43   b ,  43   c  and  43   d  takes place after saliciding, and causes removing the salicide (with the layer of poly2  43 ), on the high voltage junctions on which silicide must not be present.  
         [0037]    Without removing the HV gate mask  60 , a self-aligned mask  61  is formed, which covers completely the zone of the LV and HV, NMOS and PMOS transistors, and the zones above the cell source regions  30 ,  49 ,  56  of the cells; using the two masks, i.e., HV gate mask  60  and self-aligned mask  61 , the exposed portions of thin oxide layer  38 , interpoly dielectric layer  31 , and poly1 layer  27  are etched. Thus floating gate regions  27   b  of the memory transistors and lower portions  27   c  of the selection transistors are formed, as can be seen in FIG. 21. In practice, while defining the gate regions  27   b  and  27   c , the cell source regions  30 ,  49  and  56  are covered, and are therefore not aligned with the gate regions  27   b  and  27   c.    
         [0038]    After HV gate mask  60  and self-aligned mask  61  have been removed, an NHV mask  62  is formed, which covers the regions N-LV and N-HV (which are not shown), and the regions P-LV  19 . Using NHV mask  62 , doping ionic species of N-type are implanted, as shown schematically in FIG. 22 by arrows  63 . In the regions P-HV  13 , at both sides of the HV gate regions  43   d , HV-NMOS source and drain regions  64  of N-type are thus formed, which are less doped than LV-NMOS source and drain regions  55 ; simultaneously, in P-array region  14 , selection source and drain regions  65   a ,  65   b  are formed on both sides of the cell, including upper portion  43   c  and lower portion  27   c  of the gate region of the selection transistors. Selection source and drain regions  65   a ,  65   b  (as well as HV-NMOS source and drain regions  64 ) have a doping level lower than LV-NMOS source and drain regions  55 , and than third cell source regions  56 , and thus they have a higher breakdown voltage, as well as greater resistivity.  
         [0039]    After NHV mask  62  has been removed, the source and drain regions of the HV PMOS transistors (which are not shown) are similarly masked implanted; a protective dielectric layer  66  is then deposited, providing the structure of FIG. 23, wherein an LV NMOS transistor  70 , an HV NMOS transistor  71 , and an EEPROM cell  72 , comprising a selection transistor  73  and a memory transistor  74 , are shown. Final steps then follow, including forming contacts and electrical interconnection lines, depositing a passivation layer etc.  
         [0040]    Thus, in the final device, EEPROM cells  72  have selection source and drain regions  65  with high breakdown voltages; third source regions  56  (which form source lines) which are planar (unlike those obtained by known self-aligned processes, wherein the etching for defining the cell gate regions gives rise to trenches in substrate  2 ); first source regions (LDD cell regions)  30 , self-aligned with the floating gate regions  27   b ; source lines  56 , control gate lines  43   b , and upper portions  43   c  of the gate regions of the selection transistors  73  with low resistivity; control gate regions  43   b  and floating gate regions  27   b  self-aligned on a single side (towards the regions  65   b  which define the drain regions of the memory transistors  74  and the source regions of the selection transistors  73 ); and gate regions of the selection transistors  73 , formed by a structure with two polysilicon levels which are shorted to one another.  
         [0041]    The LV (NMOS and PMOS) transistors have a high-speed LDD structure with a dual gate (gate region  43   a  doped with doping ionic species of the same type as source and drain regions  48 ,  55 ); with salicized source and drain regions  55  and gate region  43   a.    
         [0042]    The HV (NMOS and PMOS) transistors have a dual gate and drain extension structure, with salicized gate region  43   d  alone.  
         [0043]    The described method thus allows simultaneous production of LV, HV and memory components which have very different characteristics, optimising the number of necessary steps.  
         [0044]    Finally, it is apparent that many modifications and variations can be made to the method and the device described and illustrated here, all of which come within the scope of the invention, as defined in the attached claims.