Abstract:
The invention relates to a method for producing silicon semiconductor wafers and components having layer structures of III-V layers for integrating III-V semiconductor components. The method employs SOI silicon semiconductor wafers having varying substrate orientations, and the III-V semiconductor layers are produced in trenches ( 28, 43, 70 ) produced by etching within certain regions ( 38, 39 ), which are electrically insulated from each other, of the active semiconductor layer ( 24, 42 ) by means of a cover layer or cover layers ( 29 ) using MOCVD methods.

Description:
FIELD OF THE INVENTION 
     The invention relates to the production of silicon semiconductor wafers having (patterned) III-V semiconductor layers in the context of silicon CMOS process technology, and in particular also to group III nitride layers (for example GaN, AlN or InN), and thus to the monolithic integration of III-V semiconductor elements with silicon semiconductor devices or components by using these silicon wafers with the possibility of combining Si-based logic and individual III-V components for high-voltage, high-power and optoelectronic applications. 
     BACKGROUND OF THE INVENTION 
     The pure deposition or formation of layers of group III nitride layers on silicon wafers having in particular a (111) orientation by using buffer layers is described in DE 102 06 750 A1, DE 102 19 223 A1 and WO 2008 132204 A2. These cases deal with blanket depositions without any patterning and exposure of the initial Si surface. The big challenge of the method is to avoid layer stress caused by the different lattice constants and structure by using appropriate buffer layers such that cracks within the layers and an increase of lattice defects, respectively, are avoided. 
     SUMMARY OF THE INVENTION 
     WO 2006 138378 A1, US 2006/0284247 A1 and U.S. Pat. No. 7,420,226 B2 disclose a bonded multilayer wafer that is used to integrate the silicon CMOS technology with III-V semiconductors on a single wafer. The multilayer wafer consists of a substrate wafer of a material having a high thermal conductivity (for example SiC or diamond) having formed thereon continuous layers: a single-crystalline layer (for example (111) oriented silicon), formed thereon the III-V layer (for example AlGaN/GaN), followed by a passivation layer (for example formed from nitride), followed by a silicon layer. In a first area CMOS transistors are formed in the silicon layer, in a second area the silicon layer is etched away and for example a High Electron Mobility Transistor (HEMT) is formed in the lower lying exposed III-V layer. 
     US 2007 0105274 A1 (or US 2007 0105335 A1 and US 2007 0105256 A1) disclose that further single-crystalline semiconductor and insulator layers are applied to the silicon substrate wafer. This multilayer wafer is produced by bonding. There are also shown structures in which different semiconductor materials are present on the surface of different regions. As an example it is referred to  FIG. 8  of this document, in which the multilayer wafer consists of silicon areas and single-crystalline semiconductor areas at the surface, wherein these areas are separated from each other by insulator layers. In  FIG. 9  of this document a manufacturing method is described, which initially uses a multilayer wafer as a starting wafer, subsequently forms silicon components in a first area (however only by so-called Front End steps, i.e. process steps up to the contact level without metallisation), thereafter etches a second area into the depth down to a crystalline semiconductor layer and re-fills the produced cavity by an epitaxially grown single-crystalline semiconductor layer. Thereafter the Front End process steps for structures in the single-crystalline semiconductor layer and the Back End steps (application of the metallisation) follow. 
     FIG. 8 of US 2007 0105274 A1 was taken as prior art of  FIG. 1  of the present application. The semiconductor assembly shown as a structure consists of two areas  18  and  19  and uses a multilayer wafer as starting material. The first area  18  consists of a single-crystalline silicon layer  14  that has been deposited above an insulation layer  13 . Under the insulation layer  13  there are a single-crystalline semiconductor layer  12  (consisting of a germanium layer and/or a silicon-germanium layer) and a silicon substrate layer  11 . The second area consists of a second single-crystalline semiconductor layer  16  and  17  that is formed on an area section  12   a  of the single-crystalline semiconductor layer  12 . The two areas  18  and  19  are insulated from each other by an insulation layer  15  (oxide, nitride or a combination thereof). 
     Continuous layers on a substrate, for example substrates formed from single-crystalline silicon, having expansion coefficients deviating from those of the substrate, as are used in the known methods, involve difficulties during the fabrication of the layer configuration, which are caused by the elastic stress of the layer assembly and the risk of the generation of structure stacking faults in the active single-crystalline semiconductor layer, thereby resulting in degradation of the characteristic data, yield loss and a reduction of the reliability of the components fabricated in the faulty layers, in addition to the increased efforts in terms of processes and materials. 
     Starting from this prior art it is an object of the present invention to configure an improved method for producing semiconductor wafers and components based on silicon with III-V layer structures for integrating III-V semiconductor components such that drawbacks of the prior art will be overcome, and in particular a method for producing structures should be provided that enables a substantially defect-free growth of III-V semiconductor materials on a specific area or region of a silicon wafer, which is to be processed by, for instance, CMOS technology or which is already partially processed. A planar or substantially planar surface and an electric insulation of the III-V semiconductor component with respect to the remaining wafer are intended. On the one hand an influence on or damaging of the III-V layers by silicon process steps, for example CMOS steps, and, on the other hand, damaging of silicon structures by the III-V process steps are to be avoided. 
     In order to allow cost-effective production the integration may also be accomplished for wafer diameters of 6 inch and greater as are usually applied for the silicon technology, for instance, for the CMOS technology. In this manner, advanced manufacturing tools available also for these wafer diameters may be used for the manufacturing process. 
     According to one aspect of the present invention the above object is solved by means of to a method for producing semiconductor wafers (claim  1 ). In this case, the semiconductor wafer comprises an active silicon layer and at least one III-V layer for integrating III-V semiconductor components with silicon semiconductor components by applying a silicon process technology. In the method an SOI silicon wafer having a buried insulation layer and an active silicon layer formed thereon is used, wherein one or more first and one or more second areas of the active silicon layer are formed by the buried insulation layer and a trench isolation so as to be electrically insulated from each other. The first (insulated) area of the active silicon layer is covered with a mask and a cavity is formed in the second area of the active silicon layer by using the mask as an etch mask. A single-crystalline III-V layer is formed in the cavity by a selective epitaxy technique in the presence of the mask. 
     According to the present invention the technical problem is therefore solved in this aspect such that one starts from an SOI wafer (silicon on insulator) as a starting material. In this case the buried insulation layer, for example a silicon dioxide layer, serves as a vertical insulation. A horizontal insulation of the various areas of the active layer is obtained by the presence of the trench isolation. By combining the vertical insulation (the buried insulation layer) with the horizontal insulation (the isolation trenches or their insulating filling, respectively) thus areas of the wafer may specifically be electrically insulated with respect to each other. In this manner the areas, in which III-V semiconductor elements are to be formed, may be determined in their lateral position and size by silicon technology without additional process steps. By a specific dimensioning of these areas the stress caused by the application of the III-V layer(s) may be maintained at a low level, since material is grown at the required locations only. 
     The deposition of the at least one III-V semiconductor layer occurs at least in a specific area only, for example by MOCVD methods, wherein silicon is the substrate base, that is, the “template” for the selective epitaxy growth of the III-V semiconductor layer. Other areas in the silicon, in which a deposition is not to be accomplished, are covered by the mask, for example in form of an oxide layer and/or a nitride layer, or by the isolation trenches. 
     In one embodiment the cavity extends laterally to the (horizontal) trench isolation such that a precisely defined lateral size of the area for the III-V semiconductor is achieved already by the manufacturing process for the trench without requiring a corresponding precise alignment during the lithography required for producing the mask. 
     The used silicon materials have an appropriate crystallographic surface orientation, for to example a (100) or a (111) orientation, whereby an adaptation is achieved as an appropriate template material (substrate base) and/or as a base material appropriate for the silicon process. The cavity may be etched by means of an isotropically acting etchant, if an appropriate stop layer is present at the bottom of the cavity and a lateral etch rate is restricted by the isolation trenches. To this end a plurality of plasma assisted etch processes with comparable vertical and lateral etch rate or also wet chemical etch processes for etching silicon are available. 
     The cavity may be formed such that it terminates in the active silicon layer (claim  7 ). In this manner relatively short etch times are realised. Moreover, advantageous characteristics of silicon may also be exploited for the III-V areas. In some illustrative embodiments the cavity is additionally or alternatively laterally surrounded or enclosed by material of the active silicon layer of the first area (claim  1 ) so that, if desired, a lateral embedding of the III-V material is achieved, for instance with respect to the thermal conductivity characteristics. 
     In illustrative embodiments the cavity is formed by means of an etch process, which comprises at least one crystallographically anisotropically acting etch step (claim  9 ). In this manner the etch process may precisely be controlled and, compared to the surface orientation, different crystal planes may be provided as growth planes for the selective epitaxy. The cavity may be formed such that {111} oriented side faces are formed in the cavity (claim  10 ). 
     In further embodiments the cavity is formed such that it extends through the buried insulation layer and terminates in or on a crystalline semiconductor material, on which the buried insulation layer is formed (claim  7 , claim  11 ). By this approach different crystallographic surface orientations may be used for the crystalline substrate material and the active layer such that an appropriate crystalline growth is obtained for the III-V layer(s), while at the same time the appropriate orientation for the silicon technology may be selected for the active silicon layer. 
     The III-V layer may grow as a single layer or as a layer stack with two or more III-V sub layers (claim  12 ) in order to obtain the desired electronic and crystal characteristics. The III-V layer may advantageously be provided as a III-nitride layer, that is, as a nitrogen-containing layer, as it is advantageous for many optoelectronic applications (claim  13 ). 
     In a further aspect the above-indicated object is solved by a method for producing to semiconductor elements in an active silicon layer and in a III-V layer. The method comprises the usage of a substrate having a buried insulation layer formed above a crystalline substrate material and an active silicon layer comprising electrically insulated areas and being formed on the buried insulation layer. Isolation trenches effect this electrical insulation of said areas. A first area of the active silicon layer that is not to be etched is covered with a mask, and a cavity is formed in a second area that is not covered by the mask. A single-crystalline III-V layer is formed in the cavity by a selective epitaxy process, and a III-V semiconductor component is formed in the second area and a silicon semiconductor component is formed in the first area by using silicon process technology. 
     In illustrative embodiments the cavity may be formed such that it extends to the crystalline substrate material, while in other cases the cavity terminates in the active silicon layer above the buried insulation layer or ends in the depth. Also the above identified embodiments may also be advantageously used for the fabrication of a semiconductor wafer. 
     Further advantages embodiments are defined in the dependent claims and also in the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now explained by referring to embodiments using the schematic cross-sectional drawings. In the drawings 
         FIG. 1  shows a layer assembly of a semiconductor wafer in cross-sectional view according to the prior art, 
         FIG. 2  shows a cross-sectional view of a layer assembly of a semiconductor wafer as an intermediate step for producing a III-V semiconductor layer that is grown in an electrically insulated to area  38  of an active silicon layer; 
         FIG. 3  shows a layer assembly according to  FIG. 2  after completing the III-V semiconductor layer  30 ; 
         FIG. 4  shows a cross-sectional view of a layer assembly of a semiconductor wafer as an intermediate step for producing a III-V semiconductor layer that is to be grown in an electrically insulated area within which the surface of the substrate wafer is exposed by means of a cavity  43  formed by etching; 
         FIG. 5  shows the layer assembly according to  FIG. 4  after completing the III-V semiconductor layer  31  grown on the silicon substrate, 
         FIG. 6  shows a cross-sectional view of a layer assembly of a semiconductor wafer as an intermediate step for producing a III-V semiconductor layer that is to be grown in an electrically insulated area of the active silicon layer, which comprises two {111} oriented side faces as crystallographically (100) oriented layers formed by an alkaline etching, 
         FIG. 7  shows the layer assembly according to  FIG. 6  after completing the III-V semiconductor layer  32  grown on the {111} oriented side faces. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a first embodiment. Starting point as an SOI wafer  1 , consisting of a silicon carrier wafer  20  (briefly denoted as: carrier or substrate), which is also denoted as crystalline substrate material and comprises an appropriate crystalline configuration, a buried insulation layer, for instance an oxide layer  22 , and an active silicon layer  24 , which also has an appropriate crystalline configuration that may be identical to or different from the crystalline configuration of the substrate material  20 . 
     Generally the crystalline configuration is indicated on the basis of the crystalline orientation of a surface of the carrier  20  or of the active layer  24 . The surface of these materials (and thus any parallel cross-section thereof) corresponds to a (certain) crystal plane or to a physically equivalent plane. For example a (100) surface orientation is to be understood such that the surface corresponds to a (100) plane so that for a cubic shape of the unit cell in silicon a &lt;100&gt; orientation is perpendicular to the surface. Furthermore, the orientation of the wafer is such that typically the transistors and other components are oriented along a &lt;100&gt; or &lt;110&gt; crystal axis. In a corresponding orientation there are also (100) or (110) planes as boundary faces for perpendicularly etched trenches or cavities when they are arranged according to the alignment of the transistors in the above sense. 
     By incorporating isolation trenches  26 ,  26 ′ or  26 ″ the active layer  24  is divided into individual areas, for example a first area  38  and a second area  39  that are electrically insulated from each other. A plurality of such insulated areas may be provided. The isolation trenches may be formed in a desired “silicon technology” in which trenches are etched in the active layer  24  for laterally dividing the layer into active areas and in which the trenches are then filled with at least a partially insulating material. By this measure also the areas  38  and  39  are formed with an appropriate lateral size by this silicon process technique, as is required for the silicon components (transistors and the like) and for the III-V semiconductor components (transistors of increased mobility, optoelectronic components in the form of LEDs and laser diodes). 
     In selected areas, that is, in the embodiment illustrated, in the area  39 , a mask  29 , for instance an oxide mask, a nitride mask or the like is formed by usual process steps of a desired silicon process technology, such as CVD layer deposition, photomask processing, plasma etching or reactive ion etching, respectively, removal of the photo resist mask, wherein only specific areas, here the area  38 , are exposed by etching. Other areas, i.e. the area  39  and, if required, the isolation trenches remained masked. In the exposed areas  38  a part of the silicon layer  24  may be removed by etching without any further masking steps by using the mask  29 , thereby creating a cavity  28  in which one or more desired III-V semiconductor layer(s) are grown in a later phase. There remains the rest  24 ′ in the area  38 . The cavity  28  is formed above the rest  24 ′. 
       FIG. 3  depicts the semiconductor wafer  1  after an epitaxy step, for instance by a MOCVD (metal-organic CVD) process, by which a III-V semiconductor layer  30  is formed within this epitaxy cavity  28 . The epitaxy process used is a selective method, in which the layer growth occurs on the exposed crystalline silicon surface only with no growth on the mask  29  and on the insulating material (for example the silicon oxide) of the side faces of the isolation trenches  26 ′ and  26 ″. Hence, a selective and defect-depleted epitaxy may be obtained at the bottom of the epitaxy cavity  28 . Since in the embodiment shown the cavity  28 , whose side faces do not act as growth faces due to the selectivity of the deposition process, extends laterally to the isolation trenches  26 , the layer  30  that may also be grown as a layer stack of several layers may be formed by using the surface orientation of the remaining active layer  24 ′. 
     By tuning the depth of the epitaxy cavity  28  with respect to the required layer thickness of the III-V semiconductor layer  30  a planar surface is formed, if required. 
     After the removal of the oxide mask  29  the structure is obtained as illustrated in  FIG. 3 . In other areas of the active layer  24  usual silicon components, for instance CMOS transistors, diodes, resistors, and the like, may be placed. The electrically insulated III-V semiconductor layer  30  may be provided, for example, as a Al x Ga 1-x N/GaN hetero-layer and may form the basis for an electrically insulated High Electron Mobility Transistor (HEMT). 
     A further embodiment is shown in  FIGS. 4 and 5 . Due to the superior lattice adaptation a (111) oriented silicon is preferred as substrate material for the growth of the III-V semiconductor layer. For the area of CMOS technology, however, a (100) oriented silicon is advantageous. This may be realised by using a (111) oriented silicon carrier wafer  40  or a crystalline substrate material having a (111) orientation that are formed on an appropriate carrier material, and by using a (100) oriented active silicon layer  42 , wherein both layers are vertically isolated from each other by the buried insulation layer  22 . 
     Within the area  38  delineated by the isolation trenches  26 ′ and  26 ′ the (100) oriented active silicon layer  42  is completely removed by etching by using a mask  59  that is composed of the oxide mask  29  and a nitride mask  44  in the embodiment shown. Also the exposed part of the buried oxide  22  is removed by etching. Thus, the epitaxy cavity  43  is formed whose bottom  43 B consists of the (111) oriented silicon carrier wafer  40  and whose walls consist of the oxide of the isolation trenches  26 ′ and  26 ″ and of the insulation layer  22 . 
     As is shown in  FIG. 5  a III-V semiconductor layer  31  may be selectively formed within the epitaxy cavity  28  after the removal of the nitride mask  44 , since the layer growth occurs only on the exposed part of the (111) oriented silicon carrier wafer  40  and does not occur on the oxide mask  29  and on the silicon oxide of the sidewalls of the isolation trenches  26  and on the insulation layer  22 . 
     After the removal of the oxide mask  29  the structure is obtained as depicted in  FIG. 5 . By adapting the thickness of the silicon layer  42  and the thickness of the buried oxide  22  to the required layer thickness of the III-V semiconductor layer  30  a planar surface is created. In the areas of the active silicon layer  42  usual silicon components, for example CMOS transistors, diodes, resistors, and the like may be placed. The electrically insulated III-V semiconductor layer  30  may be provided, for example, as an Al x Ga 1-x N/GaN hetero-layer and may form the basis for an electrically insulated High Electron Mobility Transistor (HEMT). 
     In alternative embodiments the mask  59  may completely be formed from nitride as long as the process parameters of the selective epitaxy also result in a substantially zero deposition rate on silicon nitride. In this manner, the mask  59  may act as an etch mask and a deposition mask, wherein the removal thereof may be accomplished selectively with respect to any oxide material. 
     A third embodiment is shown in  FIGS. 6 and 7 . Starting point is an SOI wafer consisting of the silicon carrier wafer  20 , the buried oxide layer  22  and the (100) oriented active silicon layer  24 . By means of the incorporation of the isolation trenches  26  the active silicon layer  24  is divided into 2-dimensional areas  38 ,  39 , which are electrically insulated from each other, as is also discussed above. In selected areas, i.e. in this case the area  39 , the mask  29  is formed such that only a specific area  38 ′ is exposed by etching. In the illustrated example the mask  29  also covers a part of the earlier area  38  shown in  FIG. 4  such that a cavity  70  is created in the area  38 ′, which cavity is laterally enclosed by material of the remaining layer  24 ″ and wherein the cavity  70  terminates in the layer  24 . 
     In the embodiments described above the etching of the form cavity is effected by isotropically acting etch processes for example by plasma-based processes or wet chemical processes, in which the lateral etch rate is approximately equal to the vertical etch rate, wherein, however, due to the etch selectivity the lateral dimension of the cavity formed is determined by the isolation trenches  26 ′,  26 ″ and the buried insulation layer, respectively. 
     In the example of  FIG. 6 , in which the lateral dimension of the cavity  70  is to be restricted such that it is embedded in the layer  24 , wherein this may be accomplished by using an anisotropic etch process. For a boundary of the cavity  70  with steep flanks a plasma assisted anisotropic recipe may be used for this purpose, in which case many well known etch recipes are available for silicon. In the embodiment shown the etching of the cavity  70  is achieved by a strongly (crystallographically) anisotropically acting etch medium, for instance potassium hydroxide (KOH), TMAH (tetramethyl ammonium hydroxide), which has different etch rates for different crystal orientations. In the example shown the cavity  70  is delineated by {111} faces of the remaining active silicon layer  24 ″, which faces are advantageous for forming thereon the III-V semiconductor. 
     In other embodiments various etch techniques may be combined in order to obtain an appropriate shape of the cavity  70  or of the cavities and  29  or  43 . For example, a plasma-based process, isotropic or anisotropic, may be performed first and thereafter a crystallographically anisotropic process may be applied, or isotropic and anisotropic wet chemical processes may be combined. 
     In a subsequent epitaxy step, for example an MOCVD process, a III-V semiconductor layer  32  may be formed within the anisotropic epitaxy cavity  70 , since the layer growth occurs on the exposed {111} faces only and not on the mask  29 . The layer  32  may comprise two sections  32   a ,  32   b  that are oppositely inclined to each other, and are therefore not planar. 
     By using isolation trenches the active silicon layer  24  is divided into individual areas that are electrically insulated from each other. Therefore the III-V semiconductor  32 , for example an Al x Ga 1-x N HEMT, and the silicon of the active silicon layer  24  may be at different electric potentials. 
     In a further embodiment semiconductor wafers comprising III-V layer structures, in the special case of group III nitride layer structures, for integrating III-V semiconductor components with silicon semiconductor components are formed by using the silicon CMOS process technology with the following sequence of process steps. In this case, an SOI silicon wafer  1  is used that has areas  38 ,  39  of the active silicon layer  24 ,  42  electrically insulated from each other by isolation trenches  22 ,  26 . Certain areas of the active silicon layer  24  or  42 , which are not to be etched, are masked by etch passivation layer(s) and etch cavities  28  or  43  or  70  are formed, namely in the electrically insulated area (s) of the active silicon layer that are not masked by the etch passivation layer(s). Finally a single-crystalline III-V layer is formed in the respective cavity by a MOCVD process. 
     In a variant of this method the active silicon layer  24  has a crystallographic (100) orientation and the cavity  28  is etched with an isotropically acting etchant. 
     In a further variant the active silicon layer  24  has a crystallographic (100) orientation and the cavity  70  is etched with an anisotropically acting etchant, wherein {111} oriented (inclined) side faces are formed in the cavity  70 . 
     In a further variant the active silicon layer  24  has a crystallographic (111) orientation and the cavity  28  is etched with an isotropically acting etchant. 
     In a further variant a sequence of layers of a plurality of III-V layers is formed in the cavity. 
     In a further embodiment the method for producing a silicon semiconductor wafer having III-V layers, in the special case of group III nitride layers, for integrating III-V semiconductor components with silicon semiconductor components by using the silicon CMOS process technology comprises the following process steps: using an SOI silicon wafer  1  that comprises a substrate wafer  20  and areas  38 ,  39  of the active silicon layer  42  with (100) orientation and being electrically insulated from each other by insulation layers  22 ,  26 ; covering certain areas of the active silicon layer  42 , which are not to be etched, by means of an etch mask  59  consisting of an SiO 2  layer  29  and a nitride layer  44 ; forming an etch cavity  43  in a certain electrically insulated area of the active silicon layer  42  not covered by the etch mask by completely removing the active silicon layer  42  and the vertically insulating buried oxide  22  within the cavity that extends to the surface of the substrate silicon wafer; forming a single-crystalline III-V layer  31  in the cavity  43  by a MOCVD technique. 
     In a further variant of this method the substrate  40  has a crystallographic (111) orientation and the cavity  43  is etched with an isotropically acting etchant. 
     In a further variant the substrate has a crystallographic (100) orientation and the cavity  43  is firstly etched with an isotropically acting etchant and thereafter is etched with an anisotropically acting etchant for forming {111} oriented side faces. 
     In a further variant a sequence of layers including a plurality of III-V layers is formed in the cavity  43 .