Abstract:
The invention relates to a bipolar transistor and a method for producing same. The aim of the invention is to provide a bipolar transistor and a method for producing same, which during the use of a single-process poly-silicon technology with differential epitaxis for the production of bases overcomes the disadvantages of conventional systems, so as notably further to improve the high-speed properties of a bipolar transistor, provide the most conductive connections possible between the metal contacts and the active (internal) transistor region as well as a minimized passive transistor surface, while at the same time avoiding greater process complexity and increased contact resistances. To this end a surface relief is produced in the active emitter region by a wet-chemical process. A single-process poly-silicon bipolar transistor having a base produced by epitaxis in accordance with the invention permits a reduction in external base resistance without causing a deterioration in emitter properties. Because the internal and external base regions are deposited continuously no interface problems arise during connection of the base. Base-collector capacity can also be lowered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The invention relates to a bipolar transistor as well as to a method of its fabrication. 
     An important field of application of bipolar transistors are high speed applications. In order to improve the efficiency of transistors in the range of highest speeds, the influence of parasitic components, i.e. resistances and capacitances, must be reduced. For this reason, not only are conductive connections, if possible, required between the metal contacts and the active (internal) region of the transistor but also a minimized passive transistor surface. 
     2. The Prior Art. 
     To satisfy these requirements, laterally scaled, so-called “double-process poly-silicon technologies” are used in modern methods of fabrication of vertical bipolar transistors. It is possible in such technologies to arrange the base contact and parts of the highly conductive poly-silicon connection between contact and internal base above insulated regions. A special arrangement of this kind in which the base has been fabricated epitaxially over an etched semiconductor region is described in U.S. Pat. No. 5,137,840. Relative to “single-process poly-silicon technologies” these structural advantages unfortunately entail such drawbacks as additional processing complexity and heightened contact resistances. These drawbacks are rooted in the required etching of the poly-silicon in the active transistor region as well as in the out-diffusion of the dopants from the highly doped poly-silicon layer into the monocrystalline base connection region. Since the poly-silicon for the base connection region over the active transistor region is removed by dry etching techniques and since there is no selectivity as to the monocrystalline silicon therebelow, the exposed silicon surface is likely to be damaged. Surface roughness, malfunctions of the lattice structure and penetration of foreign substances are the result. 
     There have been a variety of proposals to overcome these problems. For instance, etch stop layers are used to protect the emitter region in order to solve problems during dry etching. Additional efforts are needed to ensure a self-adjustment of emitter region and etch stop layer. 
     Lately, it has been possible further to improve the high speed characteristics by the use of epitaxial processes. To this end use is made of in situ doping during precipitation to achieve small base widths, i.e. small thicknesses of the base layers and low base layer resistances. 
     A further degree of freedom during setting of the base resistance and current amplification and, hence, optimization of the high speed characteristics is obtained by the precipitation of hetero layers. 
     The concept of double-poly-silicon technology including etch stop layer has also been realized in the case of epitaxially incorporated base layers by so-called selective epitaxy. In epitaxy that epitaxial growth on exposed semiconductor surfaces only is ensured by the conditions of precipitation. If differential epitaxy is used during which silicon material is precipitated on semiconductor as well as insulating regions, it is possible simultaneously to produce the internal base and the connection to a base contact positioned in an insulating region (base connection region). In general, this avoids the need for an additional poly-silicon layer. The resultant quasi double-poly-silicon arrangement makes it possible to simplify the process. 
     However, compared to a complete double-poly-silicon process one is confronted by the disadvantage of it not being possible to set the thickness of the epitaxial layer in the active transistor region independently of the thickness of the silicon layer. For two different requirements exist as regards the thickness of the epitaxial layer. That is to say that within the active emitter region the layer thickness between the highly doped emitter and the base should be sufficiently thin. In the outer base region a greater thickness is advantageous to provide a low resistance base connection. In order simultaneously to satisfy these opposing requirements, recourse may be had to increasing the thickness of the epitaxial layer above the base (cover thickness) in connection with a selective implantation in the active emitter region. Without appropriate measures the annealing of implantation damage, for instance in the case of epitaxially produced doping of the base in a silicon-germanium layer, leads to an unacceptable widening of the base profile, however. If the cover layer is doped in situ, undesired consequences result in respect of the conventionally used poly-emitter structures: An emitter zone of reduced conductivity or even reduced charge carriers is the usual result below the passivation between the margin of the emitter and the highly doped base connection. This may lead to a deterioration of the static as well as dynamic characteristics of the transistor. A thicker buffer layer between substrate and base may provide partial relief if in situ doping of the collector doping kind is incorporated during epitaxy. 
     Aside from process related technological difficulties, this variant suffers from the disadvantage, among others, of subsequent redoping of the buffer layer outside the active transistor increasing the base-collector capacitance. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention, to propose a bipolar transistor and a method of its fabrication by which, in a single-process, poly-silicon technology using differential epitaxy to produce the base, the described disadvantages of conventional arrangements are overcome, thereby further improving the high speed characteristics in particular, providing connections as conductive as possible between the metal contacts and the active (internal) transistor region as well as a minimized passive transistor surface, while at the same time avoiding additional process complexity and increased contact resistances. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention the object is accomplished by precipitating a cover layer ( 17 ,  119 ) or a combination of layers ( 117 ,  118 ,  119 ) over the base layer ( 16 ,  116 ) and complete or partial removal thereof by means of a partial mask in the active emitter region, whereby a surface relief is wet-chemically formed in the active emitter region. 
     A single-process poly-silicon bipolar transistor with an epitaxially produced base in accordance with the invention makes reduced external base resistances possible without deterioration of the emitter characteristics. As a result of the uninterrupted precipitation of internal and external base region no interfacial problems occur at the base connection. Moreover, the base-collector capacitance can also be reduced. 
     An additional in situ doping or hetero layer sequence in the cover over the precipitated base layer offers further advantages. As etch stop layers such intermediate layers affect a balancing of thickness variations of the cover in the active emitter region. 
     An appropriate doping of the cover layer of the conductive kind of the base which in the active emitter is removed during re-etching of the cover leads to a reduced external base resistance. The self-adjusting arrangement makes possible a low-ohmic base connection free of implantation damage up to very small emitter widths. Moreover, such additional doping of the base connection makes possible smaller distances between the active emitter region and to siliconize the base connection. 
    
    
     DESCRIPTION OF THE SEVERAL DRAWINGS 
     The novel features which are considered to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, in respect of its structure, construction and lay-out as well as manufacturing techniques, together with other objects and advantages thereof, will be best understood from the following description of preferred embodiments when read in connection with the appended drawings, in which: 
     FIG. 1 is a schematic presentation of a bipolar transistor; 
     FIG. 2 is a schematic presentation of a bipolar transistor in accordance with FIG. 1 before epitaxy; 
     FIG. 3 is a schematic presentation of a bipolar transistor in accordance with FIG. 1 during fabrication; 
     FIG. 4 is a schematic presentation of a bipolar transistor; 
     FIG. 5 is a schematic presentation of a bipolar transistor in accordance with FIG. 4 before epitaxy; and 
     FIG. 6 is a schematic presentation of a bipolar transistor in accordance with FIG. 4 during fabrication. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Example 1 
     The invention will now be described in connection with a single-process poly-silicon method with an epitaxially produced base. Modifications of this process, such as, for instance, hetero epitaxy, or incorporation in a bipolar CMOS-technology (BiCMOS) are also possible. 
     FIG. 1 schematically depicts a bipolar transistor  10  in accordance with the invention. On the semiconductor substrate region  11  of circuit type I there has been fabricated a collector region of circuit type II. Where emitter and collector are, for instance, n-conductive, the base will be p-conductive, and vice versa. Several methods have become known for providing suitable collector doping. Among these are, for instance, the structure shown in FIG. 1 with a highly doped buried layer  12  and a less doped epitaxial layer  13 , as well as implanted retrograde wells. In the embodiment here shown, a field insulating region  14  separates the bipolar transistor from other components not shown in FIG. 1, as well as the collector connection region from the active transistor region. Other suitable technologies are also known, such as, for instance, spaced mesa arrangements. A buried implant  20  may optionally be incorporated to reduce the resistance between the contact layer  21  consisting of highly doped poly-silicon and the buried layer  12 . 
     A sequence of epitaxial layers consisting of a buffer layer  15 , in situ doped base layer  16  of circuit type I as well as a cover layer  17 , covers the emitter region in the active transistor area and at least a part of the insulating region. The epitaxial layer structured outside of the active transistor region is covered by a non-conductive material  18 . While the buffer layer  15  is a possible component, it is not essential for purposes of the present invention. The specific values of thickness, dopant content as well as material composition of the base are to be selected according to the requirements of function of the bipolar transistor and in terms of the invention are not subject to any special requirements. In the described example the base layer consists of silicon, is p-doped with 2·10 18  cm −3  and is assumed to have thickness of 40 nm. Other material compositions and doping profiles may also be used. In terms of the invention, the use of a cover layer  17  above the base is essential. In the active emitter region it is partially or completely removed. Doping of the emitter in the monocrystalline silicon is ensured by outdiffusion of dopant  22  from the highly doped poly-silicon contact layer  21 . The precipitated thickness of the cover layer may typically be 50 to 150 nm. A thickness of 100 nm is assumed to be the preferred value. The exact depth of the re-etched part  19  of the cover layer  17  depends upon the structure of the base doping and material composition as well as on the desired dopant profile at the base-emitter junction. In the described example there has been predetermined a remaining thickness of 50 nm for the cover layer. While the buffer, base and cover layers grow as monocrystals over the silicon substrate, polycrystalline layers  15 / 1 ,  16 / 1 ,  17 / 1  are formed over the insulating region  14 . Outside of the poly-silicon contact layer  21  overlapping the active transistor region the doping of the base connection region has been additionally doped by implantation  23 . The insulating layer  24  separates emitter, base and collector contact. The structure of the transistor is completed by metal contacts for the emitter  25 , base  26  and collector  27 . 
     Hereafter, there will be described the fabrication of a bipolar transistor in accordance with the invention. The method in accordance with the invention proceeds from the structure before epitaxy shown in FIG.  2 . After photolithographic structuring, a highly doped n-layer  12  is incorporated by implantation and annealed into p-doped silicon substrate  11 . Thereafter, a weakly doped n-layer  13  is epitaxially precipitated. Conventional process steps define the active region and form insulating regions  14  (e.g. LOCOS) in the remaining regions. 
     The result of the ensuing process steps is shown in FIG.  3 . Buffer layer  15 ,  15 / 1 , base layer  16 ,  16 / 1  and cover layer  17 ,  17 / 1  are precipitated by means of differential epitaxy. They grow as monocrystals on the silicon region and as polycrystals on the insulating region. After photolithographic structuring of a mask, the precipitated silicon or poly-silicon layers on the insulating region outside of the subsequent transistor and base connection regions are removed with an etch stop by means of a plasma etching step. A subsequently applied dielectric substance  18 , preferably an oxide, is initially opened in the emitter region by common lithographic or wet etching processes. 
     The surface relief shown in FIG. 3 may be produced by means of known wet chemical etching agents which remove silicon in a well-controlled and highly selective manner with respect to the masking dielectric substance. In this process, the cover layer in the active emitter region is partially or completely removed. The desired surface relief may, however, also be produced by opening a nitride mask over the precipitated layer sequence  15 ,  15 / 1 ;  16 ,  16 / 1 ;  17 ,  17 / 1  above the active emitter, subsequently converting the cover layer  17  partially or completely at thermal oxidation in SiO 2  and, finally, by wet-chemical removal of the oxide formed in the active emitter relative to the mask and etch stop on the monocrystalline silicon. 
     The collector connection region is now exposed by photolithographic structuring of a lacquer mask, and the buried implant is incorporated. Before removal of the lacquer mask, the oxide layer  18  in the collector connection region is also etched, preferably wet-chemically, as previously in the emitter region. The process continues by precipitation of an amorphous silicon layer. This layer may be doped by implantation in situ, during or after precipitation. Emitter and collector contact regions are masked by a lithographic step. In the remaining regions the amorphous silicon is removed by an etching step with a stop on the SiO 2  layer. During the following implantation of the base connection region the emitter and collector connection regions are protected by the present masking. After removal of this masking and covering of the formed surface with an oxide, tempering will take place for annealing implantation damage as well as for forming of the poly-emitter. The process is completed by opening the via holes for emitter, base and collector and a standard metallization for the transistor contacts. 
     Example 2 
     FIG. 4 depicts a second embodiment of a bipolar transistor  100  in accordance with the invention. In a manner similar to the first example, there has been formed a collector region of circuit type  11  on the semiconductor substrate region  111  of circuit type  1 . Doping of the collector consists of a highly doped buried layer  112  and a lesser doped epitaxial layer  113 . An insulating region  114  separates the bipolar transistor from other components not shown in the drawing as well as collector connection region from the active transistor region. A buried implant  123  may optionally be applied for reducing the resistance between the contact layer  124  made of highly doped poly-silicon and the buried layer  112 . 
     A sequence of epitaxial layers consisting of a buffer layer  115 , in situ doped base layer  116  of circuit type  1 , the weakly doped intermediate layer  117 , an etch stop layer  118  and, finally, a preferably doped cover layer  119  (of the circuit type of the base), covers the emitter region in the active transistor region and at least a part of the insulating region. While the buffer, base, intermediate, etch stop and cover layers grow in a monocrystalline manner, polycrystalline layers  115 / 1 ,  116 / 1 ,  117 / 1 ,  118 / 1  and  119 / 1  are formed over the insulating region  114 . The epitaxial layer structured outside of the active transistor region is covered by a dielectric substance  120 . The buffer layer is optional but not essential for purposes of the invention. The specific values as regards thickness, dopant content as well as material composition of the base are selected in accordance with requirements of the function of the bipolar transistor and in respect of the invention they are not subject to any special requirements. A highly doped silicon-germanium layer may be used, for instance. To prevent the outdiffusion of boron from the silicon-germanium layer, carbon may be appropriately incorporated in the base layer. Other material compositions and doping profiles may also be used for the base. 
     The application of the layer combination  117 ,  118  and  119  above the base layer  116  is to be considered essential to the invention, the cover layer  119  being completely and the etch stop layer  118  being completely or partially removed in the active emitter region. Immediately over the base there is a weakly doped intermediate layer  117 . The thickness of this intermediate layer is selected so as to avoid tunneling currents between the high doping of base and emitter and advantageously to affect the dynamic transistor characteristics. The characteristics of the etch stop layer  118  and cover layer  119  are designed so that during etching of the cover layer  119  the selectivity relative to the stop layer  118  is sufficient. In addition, the etch stop layer is preferably designed so that it may itself be removed in a second etching step. In this manner it is possible strongly to dope the etching stop layer with the circuit type of the base layer in the same manner as the cover layer  119 . These requirements may be satisfied by very strongly doping a silicon etching stop layer  118  with boron, especially more than 3·10 19  cm −3  and by doping the superposed cover layer at boron concentration less than 10 19  cm 3 . It is also possible to achieve an etch stopping effect relative to the cover layer  119  by other material compositions of the etch stop layer  118 , such as, for instance, silicon-germanium or carbon enriched silicon. 
     The precipitated thickness of the intermediate layer  117  may typically be between 30 and 50 nm. A preferred value may be assumed to be 40 nm. The thickness of the etching layer may be between 5 and 15 nm. A value of 10 nm is preferred. The thickness of the cover layer may be between 30 and 100 nm; a preferred value is assumed to be 50 nm. A sufficient lateral space between the preferably used high level doping of the etch stop layer  118  and cover layer  119  on the one hand and, on the other hand, the emitter doping formed by outdiffusion from the highly doped poly-silicon contact layer is ensured by the formation of an inside poly-silicon spacer  122 . During fabrication of the inside spacer the SiO 2  insulating layer serves as an etch stop layer for the required plasma etching step. Outside of the poly-silicon layer  124  overlapping the active transistor range doping in the base connection region has been additionally increased by implantation  125 . The accelerated diffusion originating from this implantation is preferably to be used for bringing about spreading of the doping incorporated in cover layer  119  and etch stop layer  118  in the direction of the base while spreading of the inner base layer is prevented by incorporation of carbon. The insulating layer  126  separates emitter, base and collector contacts. The transistor structure is completed by metal contacts for the emitter  127 , base  128  and collector  129 . 
     The fabrication of the bipolar transistor  100  shown in FIG. 4 will be described hereafter. The process steps in the second embodiment up to the differential epitaxy are identical to the steps of the first embodiment. Instead of the layer sequence described in the first embodiment, the process continues with the following stack of layers: The buffer layer  115 ,  115 / 1  and base layer  116 ,  116 / 1  are followed by a weakly doped intermediate layer  117 ,  117 / 1 , and etch stop layer  118 ,  118 / 1  as well as a cover layer  119 ,  119 / 1 . Monocrystalline growth occurs in the silicon region and polycrystalline growth occurs in the insulating region. The process steps proceed from the structure shown in FIG.  5 . 
     After photolithographically structuring a mask the precipitated silicon or poly-silicon layers outside of the subsequent transistor and base connection regions are removed by a plasma etching step with an etch stop on the insulating region  114 . A subsequently applied dielectric material  120  (FIG.  6 ), e.g. 50 nm oxide, is initially opened in the emitter region by common lithographic and wet etching processes. 
     The cover layer  119  is completely removed down to the etch stop layer  118  by means of conventional wet-chemical etching agents of the kind capable of removing a boron doped silicon layer in a controlled and sufficiently select manner with respect to the underlying etch stop layer or SiO 2 . There is no need for selectivity with respect to the cover layer exposed at the side walls. Thereafter, a double layer consisting of 20 nm SiO 2  and 100 nm amorphous silicon is precipitated. Inside spacers are formed following a plasma etching step with an etch stop on the last-precipitated oxide layer. The silicon surface will thereafter be exposed by a wet-chemical etching step. 
     This leads to the formation of the surface relief shown in FIG.  6 . The collector connection region is then opened by photolithographically structuring of a lacquer mask, and the buried implant is incorporated. Prior to removing this lacquer mask the oxide layers  120 ,  121  in the collector connection region are preferably wet-chemically etched. To fabricate a complete transistor, the process is then continued as in the first embodiment. 
     A bipolar transistor and a method of its fabrication has been described in the present invention on the basis of concrete embodiments. It is to be mentioned, however, that the present invention is not limited by the details of the embodiments in the description as changes and variations are being claimed within the scope of the patent claims.