Abstract:
The invention relates to a method for producing high-speed vertical npn bipolar transistors and complementary MOS transistors on a chip. In order to produce these high-speed vertical npn bipolar transistors and complementary MOS transistors on a chip, all technological method steps for producing the vertical structure of the collector, base and emitter in the active region of the npn bipolar transistors as well as for laterally structuring the collector regions, base regions and emitter regions are performed before the troughs and the gate insulating layer for the MOS transistors are produced.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention concerns a process for the production of high-speed vertical npn-bipolar transistors and complementary MOS-transistors on a chip. 
   2. Description of Related Art 
   BiCMOS-technologies are used for the production of integrated circuits or circuit blocks which include both MOS-transistors with a p- and an n-channel and also bipolar transistors. Modern BiCMOS-technologies aim specifically at combining circuit blocks with very high-speed bipolar transistors and very highly integrated CMOS logic on a chip in order to achieve powerful technological platforms for wireless and optical waveguide-based telecommunications. In consideration of the immense expenditure in terms of development and qualification of a very highly integrated CMOS technology, the modular integration of bipolar transistors into a qualified CMOS-technology is the most economical way of implementing such a modern BiCMOS-technology. Modular integration denotes in that respect that the influence of the additional process steps which are required for manufacture of the bipolar transistors on the parameters and yield of the CMOS transistors is kept so low that the circuitry libraries of the CMOS-technology can also be used for the BiCMOS-technology which is derived therefrom. 
   The best high-frequency characteristics of Si-based bipolar transistors are achieved at the present time with SiGe-hetero-bipolar transistors (SiGe-HBT). For that reason SiGe-HBT are also used for high-power BiCMOS-technologies, as described for example in G. Freeman: ‘A 0.18 μm 90 GHz f T  SiGe HBT BiCMOS, ASIC-Compatible, Copper Interconnect Technology for RF and Microwave Applications’ Technical Digest of the 1999 International Electron Devices Meeting, pages 569–572 and R. Tang et al.: ‘A Low-Cost Modular SiGe BiCMOS Technology and Analog Passives for High-Performance RF and Wide-Band Applications’ Proceedings of the 2000 Bipolar/BiCMOS Circuits and Technology Meeting, pages 102–105. It is characteristic in respect of those technologies that essential process steps for integration of the HBTs into the original CMOS-process and for production of the HBTs themselves, such as the deposit of various protective layers, the SiGe-base layer and the emitter layer, are effected after production of the CMOS-wells, gate oxidation, gate structuring and (in the first publication quoted) doping and restoration of the MOS-transistors with an n-channel. That specific procedure for the production of bipolar transistors within a BiCMOS-technology has the disadvantage that it is not possible to completely exclude influencing of the vertical and lateral doping profiles of the CMOS-transistors by HBT-process steps, in particular by virtue of the given thermal budget thereof. The consequence of this can be that the original CMOS-transistor parameters cannot be exactly reproduced in the BiCMOS-process. 
   Theoretically that disadvantage can be avoided if all technological process steps which are employed for integration and production of the bipolar transistors were effected prior to the production of the wells and the gate insulating layer for the MOS-transistors. It will be noted that with that operating procedure the CMOS-process steps then act on the bipolar structures, which causes extreme difficulty in implementation of the steep doping profiles necessary for very good high-frequency properties, in particular in the base of the bipolar transistors. That applies in particular in regard to SiGe-HBTs, where a widening of the base doping profile (boron) beyond the edges of the SiGe-layer results in the formation of parasitic conduction band barriers which make it impossible to achieve very good high-frequency properties. CMOS-process steps which are particularly critical in terms of their effect on the doping profiles of bipolar transistors are gate oxidation and, when employed, reoxidation of the polysilicon gates, both just by virtue of the thermal budget which is usually involved in that respect. When those steps have an effect on unprotected single-crystal regions of the bipolar structures, the so-called OED effect is additionally involved (OED—Oxidation Enhanced Diffusion). A further critical CMOS-process step is restoration of the implant which is used for doping of the gate and source-drain regions of the MOS-transistors with an n-channel. 
   SUMMARY OF THE INVENTION 
   Particularly when using arsenic as the doping element, in order to avoid gate depletion, it is necessary to usually involve a thermal budget which can greatly widen the base doping profile of bipolar transistors and in the case of SiGe-HBTs displace it beyond the edges of the SiGe-layer. 
   Therefore the object of the invention is to eliminate the described disadvantages of the state of the art and to propose a process for the production of high-speed vertical npn-bipolar transistors and complementary MOS-transistors on a chip, and in addition to avoid any influencing of the vertical and lateral doping profiles of the CMOS-transistors by the process steps which are used for production of the bipolar transistors. 
   In accordance with the invention that object is attained in that all technological process steps for the production of the vertical structure of collector, base and emitter in the active region of the npn-bipolar transistors and for lateral structuring of the collector, base and emitter regions are effected prior to production of the wells and the gate insulating layer for the MOS-transistors. The technological process steps for production of the vertical structure of collector, base and emitter in the active region of the npn-bipolar transistors and for lateral structuring of the collector, base and emitter regions are carried out prior to production of the wells and/or the gate insulating layer for the MOS-transistors. The finished structures of the npn-bipolar transistors, prior to the production of the gate insulating layer for the MOS-transistors, are provided with a protective layer which, during execution of technological process steps for production of the MOS-transistors, prevents conversion of silicon into silicon oxide within the finished structures of the npn-bipolar transistors. A relief is rounded between the structured emitter and base regions of the npn-bipolar transistors prior to production of the gate insulating layer for the MOS-transistors, to such an extent that the formation of spacers is avoided at that relief during structuring of the gates of the MOS-transistors. Not more than 4 lithographic steps are used for the provision of the vertical structure of collector, base and emitter in the active region of the npn-bipolar transistors and the lateral structuring of the collector, base and emitter regions. The doping profile for the base of the bipolar structures in the vertical direction is enclosed with a C-atom profile which has a concentration in the range of between 1×10 19  and 1×10 20  cm −3 . In spite of the action of the CMOS process steps on the bipolar structures the bipolar structures produced using BiCMOS-technology have a vertical doping profile, the process according to the invention makes it possible to achieve a product of collector-emitter breakdown voltage (BV CEO ) and transit frequency (f T ) of at least 150 VGHz. The production according to the invention of single-polysilicon npn SiGe-HBTs in a BiCMOS-technology is set forth hereinafter by reference to an example illustrated by  FIGS. 1(   a ) through  1 ( p ). 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1(   a ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   b ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   c ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   d ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   e ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   f ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   g ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   h ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   i ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   j ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   k ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   l ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   m ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   n ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   o ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
       FIG. 1(   p ) is a diagram of a semiconductor structure in accordance with a respective step in the process of the present invention. 
     The operating procedure set forth in that respect can equally well be applied to double-polysilicon npn SiGe-HBTs and to npn Si-homo-bipolar transistors, the base of which is produced by implantation or epitaxy. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1(   a ) shows the initial structure. Insulating regions  1  comprising SiO 2  were introduced into an Si wafer  0  with methods which are usual in semiconductor technology. The wafer is covered over its entire area with a further SiO 2  layer  2  and a layer  3  comprising Si 3 N 4 . A first mask of photolacquer  4  was produced using the methods usual in semiconductor technology. The nitride layer was preferably dry-chemically etched out in the regions freed of photolacquer and then an n-conductive region  5  produced by implantation with ions. The damage produced in that implantation operation was restored after removal of the lacquer mask and suitable cleaning preferably by means of an RTA-step (RTA=Rapid Thermal Annealing). The subjacent SiO 2  layer is then removed by means of a suitable hydrofluoric acid-bearing solution, in which respect the previously structured nitride layer serves as a mask. 
   After the application of suitable cleaning procedures, production of a single-crystal layer stack is effected, comprising an Si-buffer layer  6   a , the SiGe-base layer  6  doped with boron atoms and the Si-cover layer  6   b  ( FIG. 1(   b )). A suitable configuration for the deposition process provides that, during the epitaxial growth of the layers  6   a  and  6   b , a polycrystalline Si/SiGe/Si-layer  7  is produced at the same time on the regions covered with SiO 2  or Si 3 N 4 . What is essential for manufacture in accordance with the invention of the SiGe-HBTs is that in the SiGe-layer deposition process the doping profile for the base of the bipolar structures, in the vertical direction, is enclosed by a C-atom profile which has a concentration in the range of between 1×10 19  and 1×10 20  cm −3 . That measure ensures that the thermal budget which is later applied for formation of the CMOS-structures does not result in a widening of the base doping profile beyond the edges of the SiGe-layer and thus the formation of parasitic conduction band barriers which would make it impossible to achieve the desired high-frequency properties. 
     FIG. 1(   c ) shows the production of the emitter windows of the HBTs with the photolacquer mask  10 . That mask is used in order preferably by means of dry-chemical etching in the lacquer-free regions to remove a layer  9  consisting of amorphous or polycrystalline Si and a part of the SiO 2 -layer  8  and to locally produce an n-conducting region  11  by means of ion implantation. After removal of the lacquer mask, the remainder of the SiO 2 -layer  8  is removed in the etched window by means of hydrofluoric acid-bearing solution and an Si-layer  12  is deposited. That layer is doped during the deposition procedure preferably with As, an As-concentration of at least 10 20  cm −3  being set. 
     FIG. 1(   d ) shows the structuring of the layers  9  and  12  and the auxiliary layers  13  consisting of SiO 2  and  14 , preferably comprising Si-rich silicon nitride, which is effected by means of a photolacquer mask  15  and dry-chemical etching at a defined distance in relation to the emitter window. After the etching operation, the mask  12  also serves for the local introduction of a B- or BF 2 -implant for doping of the outer base regions of the HBTs. 
   After removal of the lacquer mask, an Si 3 N 4 -layer is deposited and removed again without using a lacquer mask by means of an etching step having a strongly anisotropic action, wherein formed at the previously produced edges of the layer stack  9 ,  12 ,  13  and  14  is an Si 3 N 4 -spacer  16  ( FIG. 1(   e )) which rounds those edges. That rounding is intended to prevent the formation of spacers at those edges in the subsequent operation of structuring the CMOS-gates. After the formation of the Si 3 N 4 -spacers  16 , the layer  8  is removed by means of hydrofluoric acid-bearing solution and implantation with B- or BF 2 -ions is again optionally implemented, which serves to further increase the doping level of the outer base regions of the HBTs. 
   The procedure then involves depositing on the resulting structure an Si 3 N 4 -layer  17  and an SiO 2 -layer  18  ( FIG. 1(   f )). 
   The following lacquer mask  19  defines the outside edge of the outer base regions of the HBTs. With that lacquer mask, the layers  18 ,  17  and  7  are removed preferably by means of dry-chemical etching from all regions in which the CMOS-transistors are later produced ( FIG. 1(   g )). 
   After removal of the lacquer mask  19  the Si 3 N 4 -layer  3  is then preferably removed by means of phosphoric acid, in which respect there is no etching attack on the subjacent SiO 2 -layer  2  by virtue of the selectivity, which is known to be high, of the etching rates ( FIG. 1(   h )). It is at that stage of the BiCMOS-process that production of the CMOS-transistors begins. Production of the n- and p-wells for the MOS-transistors with a p- or n-channel respectively is firstly implemented by using various lacquer masks, implantation and restoring steps. 
     FIG. 1(   h ) shows by way of example the produced n-well  20  of an MOS-transistor with a p-channel.  FIG. 1(   h ) also shows that a suitable lacquer mask configuration makes it possible to introduce the n-wells, in overlapping relationship with the n-conducting region  5 , into the regions on which contacting of the HBT-collectors later occurs. It is possible in that way to provide that no more than 4 lithographic steps (lacquer masks  4 ,  10 ,  15  and  19 ) are required for producing the HBT-structure. 
   Subsequently the SiO 2 -layer  2  is removed over the well regions of the CMOS-transistors and at the same time the SiO 2 -layer  18  over the HBT-structures, by means of hydrofluoric acid-bearing solution ( FIG. 1(   i )). 
   After implementation of one of the usual cleaning procedures the gate oxidation operation is then effected, in which case the thin SiO 2 -layer  21  is produced over the well regions of the CMOS-transistors and an oxide layer  22  is produced at the exposed edges of the polycrystalline SiGe/Si-layer  7  ( FIG. 1(   j )). It is essential for production in accordance with the invention of the SiGe-HBTs that the covering of the HBT-structures with the Si 3 N 4 -layer  17  prevents conversion of silicon to silicon oxide within those structures. 
     FIG. 1(   k ) illustrates the production of the gates of the CMOS-transistors, comprising a preferably amorphous Si-layer  23 , using an Si-rich nitride layer  24  and a lacquer mask  25 . In the operation of dry etching the layers  24  and  23 , spacers  23   a  are formed at the outer edges of the HBT-structure, such spacers having no negative influence at all on the function or parameters of the HBTs. 
   Unwanted spacer formation at the inner relief of the HBT-structure is prevented by the rounding of the relief, as described with reference to  FIG. 1(   e ). After removal of the lacquer mask  25  reoxidation of the gates is effected, in which case an oxide layer  26  is formed at the side walls thereof ( FIG. 1(   l )). 
   As in the case of gate oxidation the Si 3 N 4 -layer  17  again prevents conversion of silicon into silicon oxide within the HBT-structures. The production of so-called LDD-regions or the involvement of so-called pocket implantation operations, as are usually employed in modern CMOS-technologies, are not illustrated here. The technological steps required for same can also be introduced without any problems into the BiCMOS-process described herein. 
   The nitride layer  24  is now removed over the CMOS-gates and the nitride layer  17  is removed over the HBT-structures, by a subsequent etching step in phosphoric acid ( FIG. 1(   m )). 
     FIG. 1(   n ) illustrates the subsequent production of gate nitride spacers  28  on an oxide substrate  27  and (shown by reference to the example of the MOS-transistor with a p-channel), production of the highly doped source and drain regions  29  of the CMOS-transistors. A suitable configuration for the lacquer mask for implantation of the source and drain regions of the MOS-transistors with an n-channel means that the implantation operation can also be used to produce highly doped, n-conducting regions  30  in the contact region of the HBT-collectors. After removal of the lacquer masks (not shown) for the source/drain implantation operations and restoring steps which are usually implemented by means of RTA, the oxide layer  27  is then removed by means of hydrofluoric acid-bearing solution. Subsequently thereto, a contact silicide layer  31  is formed on all Si-regions not covered by an insulating layer, using the methods which are usual in semiconductor technology ( FIG. 1(   o )), and provides for contacting, wiring and passivation of the transistors. 
     FIG. 1(   p ) also illustrates production of the contact holes in the insulating layer  32  for an HBT-emitter  33   a , the HBT-base  33   b  and the HBT-collector  33   c , as well as for a source  34   a  and the drain  34   b  of the CMOS-transistors. 
   In the present description the process according to the invention for the production of high-speed vertical npn-bipolar transistors and complementary MOS-transistors on a chip was set forth with reference to a specific embodiment. It should be noted however that the present invention is not limited to the details of the description in the specific embodiment as alterations and modifications are claimed within the scope of the claims.