Abstract:
A method for manufacturing a semiconductor array, particularly a high-frequency bipolar transistor, is provided, the method includes process steps, so that a dielectric is produced on a mono-crystalline, first semiconductor region of a first conductivity type, a silicide layer is deposited and patterned in such a way that the silicide layer is insulated from the first semiconductor region by the dielectric, and, to form a base region, a second semiconductor region of a second conductivity type is applied to the first semiconductor region and to the silicide layer in such a way that the second semiconductor region lies with a first interface on the first semiconductor region and with a second interface on the silicide layer.

Description:
This nonprovisional application claims priority under 35 U.S.C. § 119(a) on German Patent Application No. DE 102004053394.6, which was filed in Germany on Nov. 5, 2004, and which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor array and to a method for manufacturing a semiconductor array. 
     2. Description of the Background Art 
     DE 43 01 333 A1, which corresponds to U.S. Pat. No. 5,424,227, discloses different process steps in a manufacturing process for an NPN-silicon-germanium heterobipolar transistor with a P-silicon-germanium base layer. Reference is made to the entire scope of this publication and the process steps in  FIGS. 1   a  to  1   h  are explained. 
       FIG. 1   a : On the surface of a P-doped silicon substrate wafer  1   s , a (masking) oxide layer  10   s  is formed by thermal oxidation and patterned with photoresist; an N +  collector connection layer  2   s  (“buried layer”) is formed by implantation and post-diffusion of arsenic ions. 
       FIG. 1   b : After removal of the masking oxide layer  10   s , the layer sequence of the SiGe HBT is grown in a single process (for example, using molecular beam epitaxy MBE) without interruption, with simultaneous doping of the layers. The layer sequence comprises the N −  collector layer  3   s  (for example, with a layer thickness of 300 nm and a doping concentration of 10 16  to 10 17  cm −3 ), the P +  SiGe base layer  4   s  (germanium proportion of, for example, 20%, layer thickness of 50 nm, and doping concentration of 10 19  cm −3 ), the N −  emitter layer  5   s  (for example, with a layer thickness of 100 nm and a doping concentration of 10 18  cm −3 ), and the N +  emitter contact layer  6   s  (for example, layer thickness of 50 nm and doping concentration exceeding 10 20  cm −3 ). 
       FIG. 1   c : An auxiliary layer  11   s  (for example, of silicon nitride Si 3 N 4 ) is deposited over the full surface of the N +  emitter contact layer  6   s  (for example, by a CVD method), with a layer thickness of approximately 0.3 μm and patterned with photoresist; part of the Si 3 N 4  auxiliary layer  11   s  and the Si emitter contact layer  6   s  is etched off anisotropically—the emitter layer  5   s  continues to remain above the SiGe base layer  4   s.    
       FIG. 1   d : A CVD oxide (TEOS)  12   s  (for example, having a layer thickness of 0.3 μm) is conformally deposited over the surface of the semiconductor array and etched back anisotropically in such a way that an oxide layer  12   s  (spacer) remains only at the vertical edges of the auxiliary layer  11   s  and of the emitter contact layer  6   s , and the remaining surface (for the base connection) is exposed; BF2 ions (dose, for example, 4×10 15  cm −2 , acceleration voltage, for example, 40-80 kV) are implanted in the exposed surface to a depth just below the SiGe base layer  4   s  and hence the emitter region  5   s  at the side of the oxide spacer  12   s  is redoped to provide the P +  base connection region  7   s , and the base collector PN junction  9   s  is moved to underneath the base/collector interface  9   as , and the emitter/base/PN junction  8   s  to underneath the spacer oxide layer  12   s.    
       FIG. 1   e : The final mesa-type structure is formed by the base connection region  7   s  and the collector layer  3   s  is patterned by anisotropic etching; by deposition and patterning of a CVD oxide (layer thickness, for example, 0.5 μm), field oxide regions  14   s  are formed that separate the various elements or active structures of the integrated circuit from one another. 
       FIG. 1   f : By thermal oxidation at a temperature of, for example, 700° C., the exposed surface of the semiconductor array is covered with an SiO 2  layer  13   s  with a thickness of, for example, 10 nm. Both the base/emitter/PN junction  8   s  and the collector/base/PN junction  9   s  are passivated on the surface and hence protected from leakage currents. 
       FIG. 1   g : A CVD oxide  15   s  (TEOS) is deposited over the entire surface of the semiconductor array (for example, with a layer thickness of 0.5 μm) and etched back anisotropically in such a way that, on the one hand, the collector contact opening  20   s  is formed and, on the other, oxide spacers  15   s  remain, which protect the surface of the base/emitter/PN junction  8   s  over a length d 2 . The distances d 1  (from the edge of the emitter contact layer  6   s  to the base connection region  7   s )and d 2  (from the base connection region  7   s  to the later base metal layer  16   s ) are self-aligned and minimal thanks to the spacer technique described and use of the oxide layers  12   s  and  15   s.    
       FIG. 1   h : The remainder of the Si 3 N 4  auxiliary layer  11   s  is removed, and the emitter contact opening  21   s  is exposed. By vapor deposition of a metal layer  16   s  (for example, titanium) and temperature treatment of said layer, a metal silicide (for example, titanium silicide) is formed on the surface of the silicon areas  2   s ,  6   s ,  7   s , whereas the pure metal remains on the surface of the oxide areas  13   s ,  14   s ,  15   s ; the metal layer above the oxide areas  13   s ,  14   s ,  15   s  is selectively removed using an etchant solution. It is then possible, after application of an oxide layer and the manufacture of contact openings for emitter, base, and collector, to apply a metallized layer using conventional methods. SiGe HBTs of this type of  FIG. 1   h , however, can be produced not only on simply patterned substrates as described, but also, for example, on substrates already containing finished components in pure silicon technology (e.g., MOS or bipolar). 
     The base of the previously described transistor can be conceptually subdivided into an intrinsic base and an extrinsic base. The intrinsic base with the adjacent emitter and collector regions forms level and substantially parallel pn junctions, whereas the extrinsic base is made to provide the electrical connection of the intrinsic base to at least one external contact. 
     For many applications of bipolar transistors, thus, for example, for high-frequency power amplification, the base resistance is an important parameter, which limits the electrical properties of the transistors, particularly their cutoff frequency. Frequently, the base resistance is dominated by the resistance of the extrinsic base. 
     To reduce the resistance of the extrinsic base, the extrinsic base in a high-doped semiconductor material is made of the same type of dopant as the intrinsic base, but with a much greater layer thickness, in order to achieve a low layer resistance in this way. In the method taught by the publication “IEEE-IEDM 2002, Article 31.6.1, ” for example, the extrinsic base is made of polysilicon with a layer thickness that is far greater than the width of the intrinsic base. Moreover, the extrinsic base can be made as a high-doped implanted region. 
     In order to reduce the resistance of the extrinsic base of bipolar transistors in silicon technology, the extrinsic base is silicidized, as set forth, for example, in the publications U.S. Pat. No. 5,459,084, U.S. Pat. No. 6,177,717, US 2002/0168829, or U.S. Pat. No. 6,465,317. Here, a layer of a metal capable of reacting with silicon is applied to the extrinsic base and said layer is caused to react with the silicon material of the extrinsic base. By annealing above the transformation point, the forming suicides are converted to a modification with a low specific resistance. 
     It is desirable for many applications, particularly in the high-frequency range, to introduce another semiconductor material, for example, germanium or carbon, in addition to silicon, into the intrinsic base of a bipolar transistor to manufacture a heterobipolar transistor. During the epitaxial deposition of the semiconductor material for the intrinsic base, the additional semiconductor material is also introduced into parts of the extrinsic base in addition to silicon. Germanium and carbon behave chemically like silicon, so that a metal capable of reacting with silicon also reacts with germanium or carbon under the same reaction conditions. 
     Particularly compounds of titanium and germanium exhibit a lower thermal stability than homologous compounds of titanium and silicon and dissociate at the transition point of the suicides. The germanium and titanium separation products formed during the dissociation act to increase the resistance. 
     In the presence of another semiconductor material (germanium) in the intrinsic base, it can therefore be desirable to carry out the silicidization reaction in such a way that the silicidation front does not penetrate deep into the intrinsic base. Nevertheless, a high silicide thickness is desirable to reduce base resistance. 
     As described previously in DE 43 01 333 A1, a thick emitter layer  5   s  is provided for this purpose, which has the function of keeping the silicon material available for a thick silicide layer  16   s , without the silicidation front penetrating deep into the layer  4   s  of the silicon-germanium composition. A pn junction with a relatively large area is formed at the edge of the thick emitter area, the pn junction lying completely within the silicon. A relatively large silicon diode is connected in particular to the base/emitter diode of the heterobipolar transistor. This influences the electrical properties of the forming element and limits its geometric scaleability. 
     DE 198 45 790 A1 discloses a manufacturing process for SiGe heterobipolar transistors without large silicon edge transistors, in that for the wet-chemical thinning of silicon layers in the active emitter region of a bipolar transistor, additional doping, having a thickness less than 3 nm, introduced by means of “atomic layer doping” (ALD) in a cover layer, acts as an etch stop layer for wet-chemical etchant. The etch stop layer is subsequently removed with a wet-chemical etchant. 
     The layer growth of the cover layer occurs further above the etch stop layer as single crystals, so that with the aid of a silicon etchant, the single-crystal silicon of the cover layer can be removed highly selectively to the etch stop layer and to a dielectric. In an embodiment of the method of DE 198 45 790 A1, the epitaxial deposition of the thick emitter layer is interrupted by the application of the thin etch stop layer, for example, consisting of high-p-doped silicon, which acts as an etch stop, at the site of the emitter window during selective, wet-chemical thinning of the thick emitter layer for deepening the emitter window. 
     The publication “IEEE IEDM 2003, Technical Digest, Article 5.3.1” also describes a manufacturing method with relatively large edge transistors in which a high silicide thickness is achieved on the extrinsic base, whereas simultaneously the silicidation front above the germanium-containing layer is brought to a standstill. To that end, after the completion of the emitter contact, the silicon material is deposited exclusively above the extrinsic base by selective epitaxy and then silicidized. 
     A further disadvantage of the prior art arises from the fact that a silicide-silicon interface, forming during the silicidation, is generally formed unevenly. There is the particular risk of the formation of needle-shaped or pyramidal silicide crystallites, which project into the silicon area. To avoid a short circuit (“spike”) between the silicide layer on the extrinsic base and the emitter region, accordingly, a sufficient lateral distance must be provided between the silicide edge and the emitter edge. As a result, the bulk resistance of the extrinsic base is detrimentally increased. 
     In particular, in the methods as taught in the publications U.S. Pat. No. 6,518,111 and U.S. Pat. No. 6,239,477, it is endeavored to reduce the base bulk resistance portion due to the non-silicidized region of the extrinsic base. In this regard, a large minimum distance is maintained between the emitter window and the extrinsic base to prevent base-emitter short circuits through laterally growing silicide crystals, the so-called spikes. To reduce the resistance, the region of the extrinsic base, which is situated between the silicide and the intrinsic base, is doped as highly as possible by dopants from a solid dopant source, without detrimentally affecting the properties of the base/emitter PN junction. 
     U.S. Pat. No. 6,518,111 provides a dielectric diffusion source layer (borosilicate glass) as a dopant source to reduce the layer resistance of the contact region between the intrinsic base and an extrinsic base of polysilicon, whereas U.S. Pat. No. 6,239,477 provides doping between the intrinsic base and a silicidized extrinsic base. US 2004/0014271 provides a selectively epitaxially deposited, highly doped silicon layer as the solid source. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to reduce the base resistance as much as possible, with the disadvantages of large lateral expansion and the effects of parasitic edge transistors, which can form between contact regions, being reduced to the greatest extent possible. 
     Accordingly, a method for manufacturing a semiconductor array is provided. Preferably, an integrated high-frequency circuit with high-frequency bipolar transistors or high-frequency bipolar transistors as single components are manufactured. 
     In this manufacturing method, at least the following process steps occur in the listed sequence; here, a sequence is not understood to be a stringent succession, so that other process steps, such as lithographic process steps, cleaning steps, etc., can occur between the process steps of the invention as listed in the sequence. 
     First, a dielectric is produced on a monocrystalline, first semiconductor region of a first conductivity type. The monocrystalline semiconductor region is advantageously a silicon substrate with preferably a &lt;100&gt; orientation. 
     A silicide layer is deposited and patterned in such a way that the silicide layer is insulated from the first semiconductor region by the dielectric. The silicide layer can thereby be preferably deposited by sputtering a suicide target. Alternatively, for depositing the silicide layer, a silicon layer and a metal layer, for example, titanium, cobalt, or tantalum, can be deposited and caused to react with one another. 
     To form a base region, a second semiconductor region of a second conductivity type, opposite to the first conductivity type, is applied to the first semiconductor region and to the silicide layer. The application occurs in such a way that the second semiconductor region lies with a first interface on the first semiconductor region and with a second interface on the silicide layer. Because the second semiconductor region is applied to the silicide layer to form the base region, the base can be connected from underneath, in that the size of the second interface decisively influences the contact resistance between the semiconductor material of the second semiconductor region of the base and the silicide layer. 
     The contact resistance between the silicide layer and the semiconductor material of the second semiconductor region significantly shapes the base resistance and thereby the high-frequency response of the semiconductor array. The connection of the base by the silicide layer placed below the semiconductor material, moreover, enables free spaces for process design of the functional layer(s) of the semiconductor array, placed above the second semiconductor region of the base. 
     Before the production of the dielectric, a hard mask can be produced on a first silicon surface of the first semiconductor region and parts of the first semiconductor region, not covered by the hard mask, are selectively removed. This enables placing the silicide layer to the side of the active collector-base region with the collector-base space-charge zone, and making the second semiconductor region substantially planar. Moreover, the selective removal of parts of the first semiconductor region may be utilized advantageously to form a trench structure, which enables a distancing of the first semiconductor region from the connection regions of the base. The aim is to reduce a parasitic capacitance between the first semiconductor region, which advantageously functions as a collector, and the base. 
     It is advantageous in an embodiment of the invention that the dielectric is produced by thermally oxidizing the surface, exposed after the selective removal, of the first semiconductor region and/or depositing one or more dielectric layers. The thermal oxide or the deposited dielectric layers can, moreover, be advantageously patterned afterwards. 
     In an embodiment of the invention, the thermal oxide and/or one or more dielectric layers are applied and patterned before the application of the second semiconductor region. 
     Furthermore, it is advantageous to planarize the surface of the wafer to the hard mask and to remove the hard mask before applying the second semiconductor region. The planarization enables a substantially planar layout of the second semiconductor region and the additional layer applied thereon, to produce preferably substantially planar PN junctions. 
     A first embodiment variant of the invention provides that the surface of the wafer is planarized by chemical/mechanical polishing (CMP) of the wafer. In this embodiment variant, the hard mask can be used advantageously as a planarization stop. 
     A second, alternative embodiment variant of the invention provides a planarization of the surface of the wafer by spin coating a resist with a suitable surface tension and thereby forming a level surface. By dry etching the wafer is uniformly thinned preferably to the first silicon surface of the first semiconductor region. In this regard, for example, the etching can be carried out by time. 
     Further, a silicon-germanium layer can be applied epitaxially to form the second semiconductor region. Advantageously, the first conductivity type is the n-conductivity type and the silicon-germanium layer is p-doped, if particularly an npn-high-frequency bipolar transistor is to be manufactured. For a pnp transistor, the assignment is appropriately inverse, accordingly for a pnp transistor the first conductivity type would be the p-conductivity type and the silicon-germanium layer n-doped. 
     For an npn transistor, to form the second semiconductor region an n-doped silicon layer and on said layer a p-doped silicon-germanium layer are applied. In so doing, the silicon layer is redoped by an annealing step by dopants for the p-conductivity type, proceeding from the silicide layer acting as the dopant source. 
     In this regard, for example, boron or another dopant for silicon is introduced into the silicide layer beforehand, for example, implanted or added in situ during the deposition of the silicide layer. These foreign substances in the silicide layer act as dopants after their diffusion out into the n-doped silicon layer. Due to a higher dopant concentration of the outwardly diffused dopants of the p-conductivity type versus the dopant concentration of the dopants of the n-conductivity type, this silicon layer is redoped and thereby forms a low-resistance junction between the silicide layer and the silicon-germanium layer of the second semiconductor region. 
     The semiconductor material of the second semiconductor region can be deposited amorphously and caused to crystallize by solid phase epitaxy. The crystallization here occurs in such a way that the arising crystal structure, proceeding from a surface, acting as the crystallization nucleus, of the first semiconductor region, continues monocrystalline. The monocrystalline continuation of the crystal structure occurs here at least in the vertical direction. Advantageously, moreover, a measurable lateral monocrystalline continuation is achieved at least via the dielectric. 
     A different embodiment of the invention provides that the silicide layer can be made of boron-doped cobalt silicide, and above the material of the silicide layer, remaining after the planarization, at least part of the second semiconductor region, particularly its silicon material, is redoped by the diffusion of boron out of the cobalt silicide layer to the p-doped semiconductor material. 
     A further embodiment of the invention provides that a third semiconductor region of an intermediate base region and a fourth semiconductor region of another base region are applied above the second semiconductor region. Preferably, in so doing, the conductivity type of the first semiconductor region corresponds to the conductivity type of the third semiconductor region. Preferably, moreover, the conductivity type of the second semiconductor region corresponds to the conductivity type of the fourth semiconductor region. Another silicide layer is applied above the fourth semiconductor region of the additional base region. 
     In a further embodiment, after the application of the additional silicide layer, an opening can be created in the additional silicide layer by removing the additional silicide layer within the area of the opening. After this, an emitter region is formed within the opening. Preferably, the position of the opening here, relative to the position of the silicide layer and the position of the first semiconductor region, is aligned in such a way that the interfaces of the first semiconductor region to the second semiconductor region, of the second semiconductor region to the third semiconductor region, of the third semiconductor region to the fourth semiconductor region, and of the fourth semiconductor region to the emitter semiconductor region are arranged substantially vertically over one another and preferably parallel to one another. 
     Another embodiment of the invention provides that to apply the fourth semiconductor region of the additional base region, a silicon-germanium layer and a first silicon layer are applied; here, the first silicon layer is inserted between the silicon-germanium layer and the additional silicide layer, by applying the first silicon layer, before the application of the additional silicide layer, to a first surface of the silicon-germanium layer of the fourth semiconductor region of the additional base region. 
     A second silicon layer of the fourth semiconductor region can be inserted between the additional silicide layer and the first silicon layer by applying the second silicon layer to a second surface of the first silicon layer before the application of the additional silicide layer, the second silicon layer preferably having a higher dopant concentration than the first silicon layer. 
     A preferred embodiment of the invention provides that the opening, in which the emitter region is formed during the further course of the process, is produced with a depth of at least to the second surface of the first silicon layer or to the first surface of the silicon-germanium layer of the fourth semiconductor region of the additional base layer. 
     A further development of the invention provides that both in the silicide layer and in the additional silicide layer, foreign substances are introduced, which, for example, with the aid of an annealing step or several annealing steps are diffused at the same time into the adjacent second semiconductor region, and/or into the adjacent fourth semiconductor region and act as dopants of the same conductivity type in said two semiconductor regions. 
     Another aspect of the invention is a semiconductor array. The array can be manufactured, for example, according to the previously explained process steps. 
     The semiconductor array has a monocrystalline, first semiconductor region of a first conductivity type. This is, for example, an n-doped silicon substrate. Furthermore, the semiconductor array has a second semiconductor region of a second conductivity type, which is made monocrystalline at the interface to the first semiconductor region. Apart from additional monocrystalline regions, the second semiconductor region can also have polycrystalline or amorphous regions. Based on the different conductivity type, a PN junction forms at the interface. 
     A silicide layer of the semiconductor array has an interface to the second semiconductor region. The silicide layer can have one or more silicides, such as cobalt silicide, titanium silicide, or tantalum silicide. The interface can be made, for example, in a polycrystalline area of the second semiconductor region. 
     In this regard, both the first semiconductor region and the silicide layer are placed below the second semiconductor region and insulated from one another. The second semiconductor region forms a base region, which is connected by the adjacent silicide layer. Preferably, the base region is made substantially planar. 
     Preferably, the second semiconductor region has a SiGe layer. The germanium fraction in this case can vary over the thickness of the layer. The SiGe layer is also formed monocrystalline in the area of the interface to the first semiconductor region and preferably continues monocrystalline the crystal structure of the first semiconductor region. 
     Particularly in order to avoid a contamination of the epitaxy material by the transition metal portion of the silicide layer in an epitaxial depositing of the SiGe layer, an advantageous development of the invention provides that the second semiconductor region has a silicon layer between the SiGe layer and the interface to the silicide layer, whereby this silicon layer is produced preferably by solid phase epitaxy of amorphously deposited silicon, proceeding from the interface to the first semiconductor region as the crystallization nucleus. 
     To achieve a low contact resistance between the silicide layer and the silicon-germanium layer, another embodiment of the invention provides that the silicon layer, proceeding from the silicide layer acting as the dopant source, is doped with dopants of the same conductivity type as that of the silicon-germanium layer. 
     To connect the base within a metallization system, a metal contact is placed within an opening in the second semiconductor region on the silicide layer. This is particularly advantageous when a direct contact between the semiconductor material and the metal of the metal contact is to be avoided. 
     An alternative embodiment of the invention provides that to connect the base region, a metal contact is placed on the second semiconductor region. It is also possible to place the metal contact both on the silicide layer and on the semiconductor material of the second semiconductor region. 
     Another aspect of the invention is the use of a previously explained semiconductor array in a high-frequency semiconductor component, for example, in a high-frequency bipolar transistor and/or in a vertically integrated high-frequency cascode array of an integrated circuit. 
     Another aspect of the invention is a semiconductor array having a first base region and a second base region. In this case, the two base regions are spaced apart from one another. To connect the first base, a first silicide layer is placed below the first base region, which adjoins said first base region. To connect the second base, a second silicide layer is placed above the second base region, which adjoins said second base region. 
     This different connection of the two bases in each case enables optimized electrical properties of the connections and also as independent a process integration of the connections as possible. 
     In developing this aspect of the invention further, it is provided that the first base region and the second base region are spaced apart vertically from one another by an intermediate base region. The intermediate base region, in this case, is of a first conductivity type, for example, an n-conductivity type, and the two base regions are of a second conductivity type, for example, a p-conductivity type. Thereby, both the first base region and the second base region are adjacent to the intermediate base region. 
     The first base region and/or the second base region can have a semiconductor region with a silicon-germanium layer. Preferably, this silicon-germanium layer here is placed separated from a silicide layer by a silicon layer. 
     The first base region and the second base region can be made substantially planar and parallel to one another. Advantageously, moreover, the lattice structure of the semiconductor material of the first base region is continued monocrystalline by the lattice structure of the intermediate base region and of the second base region, so that preferably the space-charge zones, associated with the two base regions, of the PN junctions are placed within the monocrystalline lattice structure. 
     A different aspect of the invention is a use of a previously described semiconductor array in a vertically integrated cascode array, particularly a high-frequency circuit. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIGS. 1   a  to  1   h  is a schematic drawing of a manufacturing process for an HBT according to the conventional art; 
         FIG. 2  is a schematic sectional view through a wafer structure before planarization; 
         FIG. 3  is a schematic sectional view through a wafer structure after planarization and application of a silicon-germanium layer; 
         FIG. 4  is a schematic sectional view through a wafer structure after planarization and application of a silicon layer and a silicon-germanium layer; and 
         FIG. 5  is a schematic sectional view through a vertically integrated cascode array. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic sectional view through a wafer structure before a planarization step is shown in  FIG. 2 . A hard mask  6  of Si 3 N 4  and of an underlying thermal oxide is applied to an already patterned first (n-doped) semiconductor region  1 , which forms a collector region. Furthermore, a thermal oxide  51  (SiO 2 ) is formed and a first dielectric layer  52  is deposited. A silicide layer  2  and a second dielectric layer  53  are deposited on the first deposited, dielectric layer  52  and subsequently patterned. By a planarization step, the shown multilayer structure is to be planarized to the dashed line (CMP). A chemical/mechanical polishing step (CMP), for example, can be used for this. 
     An exemplary embodiment of the invention after additional process steps is shown in  FIG. 3 . After the completed planarization, a second (p-doped) semiconductor region  3  with a silicon-germanium layer is applied, which has a first interface  13  to the first semiconductor region and an interface  12  to suicide layer  2 . Within the area of first interface  13 , the silicon-germanium layer of the second semiconductor region  3  continues the monocrystalline lattice structure of the first semiconductor region  1  in the vertical direction, and in a lateral region also in a lateral direction. The monocrystalline crystallization in the lateral direction is achieved in this case by solid phase epitaxy. At least in the area of another interface to dielectric  5 , the second semiconductor region has polycrystalline or amorphous layer areas  31 . 
       FIG. 4  shows a schematic sectional view of another exemplary embodiment of the invention. Here, the second semiconductor region  3 ,  4  includes silicon-germanium layer  3  and silicon layer  4  placed below silicon-germanium layer  3 , both of which continue the monocrystalline metal lattice of the first semiconductor region. 
     Silicon layer  4  is applied first weakly n-doped and redoped in the area of silicide layer  2  in a p-doped region. The redoping can be undertaken, for example, proceeding from silicide layer  2  as a solid source; for this purpose, for example, boron can be introduced into the silicide layer  2  in situ during the production or by implantation. In order to apply silicon layer  4 , for example, a solid phase epitaxy can be used by depositing first a relatively thin (for example, 40 nm) amorphous, weakly n-doped silicon layer, followed by a relatively thick (for example, 300 nm), highly n-doped, amorphous silicon layer. Under the effect of temperature, proceeding from interface  13  to first semiconductor region  1 , the amorphous areas above interface  13  are crystallized monocrystalline vertically, as well as partly the area above interface  12  in a lateral direction, by solid phase epitaxy. 
     The highly n-doped silicon layer functions here to support the solid phase epitaxy in a lateral direction. After completed solid phase epitaxy, the highly n-doped silicon layer is removed by an etchant, which selectively removes high-doped silicon areas. Next, silicon layer  4  is superficially oxidized and the formed oxide layer is removed. The oxidation step here serves to clean the surface. Simultaneously, during the oxidation step, the boron atoms introduced into silicide layer  2  diffuse out into silicon layer  4 . Remaining amorphous or arising polycrystalline regions of silicon layer  4  are indicated in  FIG. 4  by the hatching. 
       FIG. 5  shows a schematic sectional view through a vertically integrated cascode array, which preferably is used in a high-frequency circuit. The conductivity types for a high-frequency npnpn cascode apply to the following description. The collector region is formed by an n-doped layer  1 , which may have a vertical doping gradient for a higher reverse voltage (not shown in  FIG. 5 ). 
     To simplify the presentation, it is assumed in  FIG. 5  that no amorphous or polycrystalline regions remain or arise in the solid phase epitaxy. The hatching in  FIG. 5  therefore marks regions doped by diffusion out from neighboring regions. 
     A first base region ( 3 ,  4 ) with a first, p-doped silicon-germanium semiconductor layer  3  and a second base region ( 100 ,  400 ) with a second, p-doped silicon-germanium layer  100  are provided. An n-doped intermediate base region ( 9 ), to which both base regions ( 3 ,  4 ,  100 ,  400 ) are adjacent and having a silicon layer, is placed between the first base region ( 3 ,  4 ) and the second base region ( 100 ,  400 ). 
     The second base region ( 100 ,  400 ) with silicon-germanium layer  100 , which in this exemplary embodiment for a simpler presentation has only one layer ( 100 ) in the active region below the emitter, is placed above intermediate base region  9 . Silicon layer  400 , which in contrast to silicon-germanium layer  100  has no germanium, is placed on the second, p-doped silicon-germanium layer  100 . Furthermore, a dielectric of a Si 3 N 4  layer  800  is provided. 
     A silicide layer  200 , in which an opening  300  is etched by a hard mask from Si 3 N 4  layer  800  and the spacers, not shown in  FIG. 5 , is located above silicon layer  400  and thereby also above second base region  100 ,  400 , the emitter region being formed within the opening  300  in subsequent steps. Opening  300 , within which the emitter region ( 500 ,  1100 ) is located, thereby extends through the silicide layer  200 . 
     Within opening  300 , two internal spacers  600  are each located above thermal oxide  700 . A polycrystalline or amorphous, n + -(high) doped silicon  500 , which also fills a spacer opening between two internal spacers  600 , is applied adjacent to internal spacers  600 . The remaining part of opening  300  of the emitter region is filled with a silicide  1100  and optionally other metallic conductors (not shown in  FIG. 5 ). 
     Polysilicon  500 , moreover, serves as a dopant source for doping monocrystalline semiconductor layer  400  in emitter region  450 , which here is high n + -doped by dopants from polysilicon  500  and forms active emitter region  450 . Furthermore, silicide layer  200  acts as a dopant source, the silicon layer  400  in the area  420  of the extrinsic base being high p + -doped. This enables an especially low-resistance connection of the extrinsic base ( 420 ,  100 ). 
     In regard to the areas lying below the n-doped silicon layer of the intermediate base region  9 , reference is made to the embodiments in  FIG. 4 . For a simplified presentation, the necessary metal connections are not included in the shown sectional view in  FIG. 5 . These are preferably applied directly to the silicide layers ( 2 ,  200 ), so that these metals have an interface to silicide layers ( 2 ,  200 ) to be connected. 
     The production process for manufacturing the vertical semiconductor array has the following process steps, whereby the reference numbers refer to  FIGS. 2 through 5 : 
     Production of hard mask  6  on a first silicon surface of first semiconductor region  1 , hard mask  6  covering collector region  1  of the collector layer; 
     Selective removal of the collector layer portions not covered by hard mask  6 ; 
     Thermal oxidation of the surface exposed after the selective removal of the collector layer; 
     Deposition of first dielectric layer  52 ; 
     Deposition and patterning of first silicide layer  2 ; 
     Deposition and patterning of second dielectric layer  53 ; 
     Planarization to the surface of hard mask  6  by removing the material of first dielectric layer  52 , of first silicide layer  2 , and of second dielectric layer  53 ; 
     Removal of hard mask  6 ; 
     Application of amorphous, n-doped silicon layer  4 ; 
     Crystallization of amorphous, n-doped silicon layer  4  by solid phase epitaxy, proceeding from the first silicon surface  13 , exposed by removal of the hard mask  6 , of the first semiconductor region  1  as the crystallization nucleus; 
     Epitaxial deposition of first monocrystalline semiconductor layer  3  of a second semiconductor region, first monocrystalline semiconductor layer  3  being formed preferably from first p-doped SiGe layer  3 , 
     Epitaxial deposition of a second, n-doped silicon layer of third semiconductor region  9 ; 
     Epitaxial deposition of a third and fourth monocrystalline semiconductor layer of fourth semiconductor region  100 , the third monocrystalline semiconductor layer being formed preferably from a p-doped SiGe layer and the fourth semiconductor layer from weakly p-doped silicon; 
     Application of silicide layer  200  to the fourth semiconductor region ( 100 ,  400 ), the surface of said fourth semiconductor region ( 100 ,  400 ) forming a first semiconductor surface for the subsequent process steps; 
     Application of third dielectric layer  800 ; 
     Creation of an opening  300  in third dielectric layer  800  and in silicide layer  200 , by which preferably the first semiconductor surface is exposed; 
     Creation of internal spacers  600  in opening  300 ; and 
     Formation of an emitter region ( 500 ,  1100 ) in opening  300 . 
     Furthermore, it is advantageous to follow the crystallization of the amorphous, n-doped silicon layer by solid phase epitaxy with the following process steps: 
     Formation of another thermal oxide layer by partial oxidation of the crystallized n-doped silicon layer; 
     Selective redoping of the crystallized, n-doped silicon layer, remaining after the oxidation, by selective implantation of dopants of the p-dopant type in the silicon material above the first silicide layer material, remaining after the planarization step; 
     Healing of implantation damage by a thermal process; and 
     Removal of the additional thermal oxide layer. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.