Abstract:
A method of depositing polycrystalline silicon exclusively on monocrystalline first silicon surface portions of a substrate surface which besides the first surface portions additionally has insulator surface portions, comprising the steps of depositing boron on the first silicon surface portions in an amount which in relation to the first silicon surface portions respectively corresponds to more than a monolayer of boron, and depositing silicon on the first silicon surface portions treated in that way.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is for entry into the U.S. national phase under §371 for International Application No. PCT/EP2008/051875 having an international filing date of Feb. 15, 2008, and from which priority is claimed under all applicable sections of Title 35 of the United States Code including, but not limited to, Sections 120, 363 and 365(c), and which in turn claims priority under 35 USC §119 to German Patent Application No. 10 2007 010 563.2 filed on Feb. 22, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The invention concerns a method of depositing polycrystalline silicon-bearing semiconductor material exclusively on monocrystalline first silicon-bearing semiconductor surface portions of a substrate surface which in addition has insulator surface portions. 
         [0004]    2. Discussion of Related Art 
         [0005]    Depositing polycrystalline silicon on monocrystalline silicon surface portions of a substrate surface is at the present time successful only if the monocrystalline silicon surface portions are covered with a thin silicon dioxide layer. For many component applications however the presence of an oxide layer between monocrystalline silicon and polycrystalline silicon causes problems. 
         [0006]    The problem also correspondingly arises in regard to depositing silicon germanium or carbon-doped silicon or silicon germanium. It does not arise in connection with silicon surfaces but also in the case of surfaces of silicon germanium or carbon-doped silicon or silicon germanium. 
         [0007]    Therefore the underlying technical object of the invention is that of providing a method with which polycrystalline silicon-bearing semiconductor material can be deposited directly on monocrystalline silicon-bearing semiconductor surface portions of a substrate surface. 
         [0008]    That technical object is attained by a method of depositing polycrystalline silicon-bearing semiconductor material exclusively on monocrystalline first silicon-bearing semiconductor surface portions, hereinafter referred to as first surface portions, of a substrate surface which besides the first surface portions additionally has insulator surface portions, comprising the steps:
       depositing boron on the first surface portions in an amount which in relation to the first surface portions respectively corresponds to more than a monolayer of boron; and   depositing silicon-bearing semiconductor material on the first surface portions treated in that way.       
 
         [0011]    The method according to the invention is successful in depositing polycrystalline silicon-bearing semiconductor material, for example silicon or silicon germanium, selectively on the monocrystalline first surface portions of the substrate surface. “Selective” signifies in this context that deposition is effected exclusively on the monocrystalline first surface portions and not on the insulator surface portions of the substrate surface. 
         [0012]    The method is based on the realization that the deposition of boron on the first surface portions in an amount which in relation to the first surface portions respectively corresponds to more than a monolayer of boron provides a substrate surface suitable for the growth of polycrystalline silicon-bearing semiconductor material. The boron deposition covers the monocrystalline first surface portions so that the lattice structure of the surface portions, in the subsequent deposition of silicon-bearing semiconductor material on the boron deposition, can no longer affect the structure of the growing silicon. Non-epitaxial growth of polycrystalline silicon-bearing semiconductor material therefore takes place. 
         [0013]    The method according to the invention permits a selection of the surface portions on which polycrystalline silicon-bearing semiconductor material is to be deposited. It therefore enhances the flexibility in regard to the configuration of the layer structures of semiconductor components with silicon-bearing semiconductor layers. A specific embodiment by way of example is described in that respect hereinafter. 
         [0014]    In accordance with the inventors&#39; findings the deposit of boron in an amount which in relation to the first surface portions respectively corresponds to less than a monolayer of boron leads not to polycrystalline but to epitaxial growth. 
         [0015]    As explained hereinbefore the term “first surface portions” is used here for silicon-bearing semiconductor surface portions. These therefore involve portions of the substrate surface, which are formed by a silicon-bearing semiconductor material. The first surface portions can also be carbon-doped. The proportion of carbon in that respect can be between values of unintentional background doping and some percent, that is to say in the region of up to some 10 22  cm −3 . For the purposes of this application silicon carbide is not deemed to be embraced by the term “silicon-bearing semiconductor material”. 
         [0016]    Other dopants such as oxygen or dopants for achieving a desired conductivity such as for example phosphorus, arsenic, boron or aluminum can be included in the usual levels of concentration, in which respect however neither the silicon-bearing semiconductor surface portions nor the deposited material involve degeneracy (metallic conductivity). 
         [0017]    The semiconductor material is for example silicon or silicon germanium, with or without carbon doping. The proportion of germanium is typically up to 20% but can also be higher than that. The method according to the invention, in a particular embodiment, can even be used for the deposition of germanium on a germanium or silicon germanium surface. It would therefore be alternatively possible in the description of the method according to the invention to use for the deposited semiconductor material as for the material of the substrate surface to be covered therewith, the term “silicon and/or germanium-bearing semiconductor material” which includes silicon, silicon germanium and germanium each with or without carbon doping, but is intended to exclude silicon carbide. 
       DISCLOSURE OF INVENTION 
       [0018]    Embodiments by way of example of the method according to the invention are described hereinafter. The embodiments can be combined together if they are not described expressly as alternatives to each other. 
         [0019]    It is not necessary, and in the foregoing description of implementation of the method, it is not to be interpreted as a necessary condition, for boron to be deposited exclusively on the first surface portions. In an embodiment boron is deposited on the substrate surface in a non-masked procedure, which includes the first silicon surface portions and the insulator surface portions. Then, in that embodiment, silicon or silicon germanium is deposited, leading to the selective formation of a polycrystalline layer of that material on the first surface portions. In this embodiment, no deposition takes place on the insulator surface portions. Alternatively, it is also possible to effect masked deposition of boron exclusively on the first silicon surface portions, which however is more complicated and expensive. 
         [0020]    In an embodiment the step of depositing silicon-bearing semiconductor material is carried out under deposition conditions of epitaxial deposition. That method, in a development, permits the simultaneous deposition of polycrystalline and monocrystalline semiconductor material. Such a method in which the substrate surface additionally includes second silicon-bearing semiconductor surface portions, referred to for brevity as “second surface portions”, for the monocrystalline deposition, can comprise for example the following additional steps:
       covering the second surface portions with a mask prior to the deposition of boron on the first surface portions;   depositing a polycrystalline silicon germination layer on the first surface portions;   removing the mask from the second surface portions; and   simultaneously depositing the polycrystalline silicon-bearing semiconductor material on the first surface portions and epitaxial silicon on the second surface portions.       
 
         [0025]    The deposition of a polycrystalline silicon germination layer on the first surface portions uses the advantageous effect of boron deposition in this embodiment in the presence of a mask. Polycrystalline silicon can then selectively grow on the silicon germination layer in the later deposition layer, as is predetermined by the structure of the silicon germination layer. The silicon germination layer is typically some monolayers thick. The silicon germination layer also prevents the deposited boron being removed at the first silicon surface portions which are provided for a polycrystalline deposition, upon removal of the mask. 
         [0026]    The method of this embodiment can also be carried out with a germination layer of silicon germanium or carbon-doped silicon or silicon germanium. 
         [0027]    For a more detailed discussion of the term “second surface portion” reference can be made to the foregoing discussion of the term “first surface portion”. 
         [0028]    What is stated there in respect of the “first surface portion” also correspondingly applies here to the “second surface portion”. 
         [0029]    In a further embodiment boron is deposited in an amount which in relation to the first surface portions respectively corresponds to more than two monolayers of boron. For example the amount of boron can correspond to 3, 7 or more monolayers of boron. In that case the amount of boron can be so selected as requires a desired boron concentration in the finished component. To produce a desired boron concentration in the deposited polycrystalline layer and the subjacent monocrystalline layer, an embodiment provides for performing a boron diffusion step after the deposition of silicon-bearing semiconductor material. 
         [0030]    A particularly advantageous situation of use of the method of the present invention concerns the production of a partially polycrystalline emitter in pnp-heterobipolar transistors. In the case of such pnp-heterobipolar transistors it is advantageous on the one hand to deposit an emitter layer adjoining the base in epitaxial fashion, that is to say monocrystalline. Particularly advantageous electrical parameters for the heterobipolar transmitter such as emitter resistance and base current are achieved with the epitaxial emitter portion. On the other hand a polycrystalline emitter portion in the direction of the emitter contact is advantageous because in that way very high levels of dopant concentration (&gt;10 20  cm −3 ) can be introduced into that portion without harmful effects on the subjacent concentration profiles. In that way the electrical contact between the emitter and the emitter contact region typically including metal silicide is improved. In this example of use therefore the method according to the invention can be used during growth of the emitter. After the deposition of a monocrystalline emitter portion growth is interrupted. The growth surface of the monocrystalline emitter portions forms the first surface portions in the sense of the method according to the invention. They are then subjected to a boron deposition operation in accordance with the method implementation according to the invention, whereupon silicon is deposited, which then grows in polycrystalline fashion. Advantageously the deposited polycrystalline silicon is used in a subsequent method step for the production of a metal silicide layer. The deposited polycrystalline silicon-bearing semiconductor material is therefore either entirely or partially silicided. For example a nickel silicide layer is appropriate. 
         [0031]    In different embodiments the insulator surface portions can contain silicon dioxide or silicon nitride. 
         [0032]    Deposition of boron on the first surface portions is preferably effected by means of the per se known atomic layer deposition (ALD). Atomic layer deposition is also known by the name atomic layer epitaxy (ALE). As is known, atomic layer deposition involves a modified CVD method (chemical vapor deposition). The number of deposited monolayers can be controlled with a particularly high degree of accuracy in that method. 
         [0033]    Preferably the deposition of boron is effected in this method by means of a gas flow of B 2 H 6  as a precursor. In that case preferably hydrogen is used as the carrier gas. It has proven to be particularly desirable for the deposition of boron to be effected in accordance with this method at a temperature of 400° C. That temperature range however can certainly be varied. 
         [0034]    The subsequent deposition of silicon is effected in an embodiment using dichlorosilane (DCS) as a precursor. A hydrogen carrier gas is also suitable here. Deposition is preferably effected at a temperature of 700° C. Markedly lower deposition temperatures however can also be used. 
         [0035]    For the deposition of silicon germanium it is possible for example to use germane (GeH 4 ) or germanium tetrachloride (GeCl 4 ) as the germanium precursor. Germane has the advantage that it can be activated like dichlorosilane at 700° C. so that the thermal budget of that procedural step can be kept down, in comparison with the use of germanium tetrachloride. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention is described in greater detail hereinafter by means of embodiments by way of example with reference to the Figures in which: 
           [0037]      FIG. 1  shows a flow chart of an embodiment of the method according to the invention, 
           [0038]      FIGS. 2-4  show diagrammatic cross-sectional views of a silicon substrate during different stages in the selective deposition of polycrystalline silicon on a silicon surface; 
           [0039]      FIG. 5  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 0.5 monolayers of boron, 
           [0040]      FIG. 6  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 3 monolayers of boron, and 
           [0041]      FIG. 7  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 7 monolayers of boron. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]      FIG. 1  shows a flow chart of an embodiment by way of example of the method according to the invention.  FIG. 1  is described hereinafter in parallel with  FIGS. 2 through 4 .  FIGS. 2 through 4  show diagrammatic cross-sectional views of a silicon substrate  200  during different stages in the selective deposition of polycrystalline silicon on a silicon surface. 
         [0043]    The method begins at a step  100  with the provision of a silicon substrate. The silicon substrate has a monocrystalline silicon surface. The silicon substrate can be present for example in the form of a wafer with a (001) silicon surface. For the purposes of the present description it is assumed that the substrate  200  is such a wafer. The silicon substrate however can also be formed by a wafer in a later stage in the production of integrated circuits. For example the wafer can already include functional layers of pnp-heterobipolar transistors. In particular an epitaxial base layer and an epitaxial, that is to say monocrystalline emitter layer portion deposited thereon. Functional layers of MOS or CMOS transistors can also alternatively or additionally already be present. 
         [0044]    In the simple embodiment described here however insulator regions  202  are firstly produced in a step  102 . They separate different silicon surface portions from each other. In the preferred example there are a monocrystalline first silicon surface portion  204  and a monocrystalline second silicon surface portion  206 . The substrate surface is then cleaned in a step  104  before, in a masked deposition step, boron is deposited in an amount of more than one monolayer in the first silicon surface portion  204  (step  106 ). The deposition of boron is effected for example by means of atomic layer deposition at a temperature of 400° C. In an embodiment B 2 H 6  is used as the boron precursor in an amount of 160 sccm corresponding to an amount of 250 ppn in a hydrogen carrier gas. The hydrogen carrier gas is prepared with 20 SLM at a pressure of 80 Torrs. After the boron coating a polycrystalline Si germination layer is deposited. That polycrystalline Si layer can also serve as a seeding layer for polycrystalline growth if Si is epitaxially grown after removal of the mask  208  on a second silicon surface portion  206 . 
         [0045]    In this case a mask  208  serves for covering the second silicon surface portion  206 . No polycrystalline silicon layer is to be subsequently deposited thereon. It is also conceivable that the second silicon surface portion  206  should be covered with an n-doped silicon layer. 
         [0046]    Silicon is subsequently deposited on the substrate surface freed of the mask  208  (step  108 ). Deposition therefore takes place on the polycrystalline Si germination layer, in the first silicon surface portion. In an embodiment the deposition of silicon is effected at a temperature of 700° C. with DCS as the precursor. An amount of 50 sccm with hydrogen carrier gas of 20 SLM under a pressure of 80 Torrs are for example suitable. 
         [0047]    Those conditions are suitable for providing for growth of a polycrystalline silicon layer  210  on the germination layer in the first silicon surface portion while no silicon is deposited on the insulating regions  202 . At the same time however an epitaxial monocrystalline silicon layer  212  grows on the second silicon surface portion  206 . 
         [0048]    In a subsequent step diffusion of boron into the adjacent layers can be implemented by tempering the substrate  200 . Diffusion can be prevented by the addition of carbon if that is desired for a given layer. By way of example the substrate  200  could be substantially protected from diffusion of boron thereinto by the incorporation of carbon in layers near the surface, near the first silicon surface portion  204 . 
         [0049]      FIG. 5  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 0.5 monolayers of boron.  FIG. 6  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 3 monolayers of boron.  FIG. 7  shows a scanning electron microscope recording of a deposited silicon layer after previous deposition of 7 monolayers of boron. 
         [0050]      FIG. 5  shows a scanning electron microscope recording of an epitaxial, substantially monocrystalline silicon layer  502  beside an insulator layer  504 . That structure was not produced in accordance with a method according to the invention. For, an amount of boron was deposited on a silicon surface which is not visible in  FIG. 5 , beneath the silicon layer  502 , which only corresponds to half a monolayer. Under those conditions it is not possible to produce polycrystalline silicon. 
         [0051]      FIG. 6  shows in contrast that upon the deposition of an amount of boron corresponding to 3 monolayers a polycrystalline layer  602  can be selectively grown beside an insulator layer  604 . The polycrystalline structure of the silicon layer  602  is clearly discernible in terms of its surface roughness in comparison with the epitaxial silicon layer  502 . The epitaxial layer  502  in  FIG. 5  thus exhibits as good as no surface contrast while that is strongly pronounced in the case of the layer  602 . 
         [0052]      FIG. 7  shows a result corresponding to  FIG. 6  after using 7 monolayers of boron prior to selective deposition of a polycrystalline silicon layer  702  on the boron-covered silicon substrate  706 . In this case also no silicon is deposited on an adjacent insulator layer  704 . 
         [0053]    The above-described method therefore permits a deposition of polycrystalline silicon on monocrystalline silicon surface portions of a substrate surface. That method is suitable for various applications in component manufacture. The manufacture of a partially crystalline emitter for pnp-heterobipolar transistors was described as an example. 
         [0054]    The embodiments by way of example described with reference to the Figures are admittedly limited to the deposition of silicon on a silicon surface. That however is not to be interpreted restrictively. As explained in detail hereinbefore the method according to the invention can be used for various silicon-bearing materials to be deposited and it is possible to use for that purpose not just silicon surfaces but also other silicon-bearing semiconductor surfaces.