Abstract:
A method for manufacturing a bipolar transistor, including the steps of: forming a first surface-doped region of a semiconductor substrate having a semiconductor layer extending thereon with an interposed first insulating layer; forming, at the surface of the device, a stack of a silicon layer and of a second insulating layer; defining a trench crossing the stack and the semiconductor layer opposite to the first doped region, and then an opening in the exposed region of the first insulating layer; forming a single-crystal silicon region in the opening; forming a silicon-germanium region at the surface of single-crystal silicon region, in contact with the remaining regions of the semiconductor layer and of the silicon layer; and forming a second doped region at least in the remaining space of the trench.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to bipolar transistors formed on an integrated circuit. More specifically, the present disclosure relates to a method for manufacturing such a transistor. 
     Description of the Related Art 
     In integrated circuits, it may be advantageous to integrate, on a same wafer, MOS transistors and bipolar transistors (integration better known as “BiCMOS”). Indeed, these two types of transistors have specific advantages. In particular, MOS transistors allow fast switchings for digital processings, while bipolar transistors have a particularly good performance at high frequencies, for example, higher than some hundred GHz, and may have a high output power. Thus, these last transistors may be used to form circuits for controlling optical circuits, for example, lasers. 
     Thus, methods for simultaneously manufacturing MOS transistors and bipolar transistors on a same substrate are needed. 
       FIG. 1  illustrates an example of a conventional bipolar transistor formed on a solid substrate where MOS transistors can also be formed. 
     At the surface of a solid substrate  10  is defined an active area delimited by deep insulating trenches  12 . Trenches  12  are conventionally formed and are currently known as DTI (for Deep Trench Isolation) trenches. 
     A heavily-doped region  14  forming the collector of the bipolar transistor extends in depth in the active area of substrate  10  delimited by trenches  12 . Region  14  extends in depth in substrate  10  across a thickness on the order of 1 μm, leaving a less heavily-doped layer  16  at the substrate surface. 
     Shallow trenches  18 , currently known as STI (for Shallow Trench Isolation) trenches, are provided on either side of the active area and stop deep in region  14 . In the middle of shallow trenches  18  are provided regions  20  of access to collector region  14 . Regions  20  are in practice a heavily-doped region of substrate  10 . 
     At the surface of substrate  10  is formed a stack of an insulating layer  22 , for example, an oxide, and of a heavily-doped polysilicon layer  24  (of type P if the transistor is an NPN transistor). The stack of layers  22  and  24  extends above the apparent surface of substrate  10  (region  16 ) and stops above a portion of shallow trench  18 . Opposite to region  16 , a portion of insulating layer  22  is replaced with a stack  25  of a silicon-germanium layer and of a silicon layer. Stack  25  forms the base of the bipolar transistor. 
     An opening is also provided in layer  24 , opposite to region  16  and on a smaller surface area than the opening in region  22 . In this opening defined in layer  24 , as well as at the surface of layer  25 , a heavily-doped region  26  forming the emitter region of the bipolar transistor is provided. Region  26  is separated from layer  22  by spacers  28  made of insulating material. 
     An emitter contact  29  is provided on semiconductor material  26  via a silicide layer  30  formed at the surface of semiconductor material  26 . A base contact  32  is provided on layer  24  via a silicide layer  34  formed at the surface of layer  24 , and a collector contact  36  is provided on regions  20  via a silicide layer  38  formed at the surface of these regions. 
     To obtain the device of  FIG. 1 , the following steps may be carried out. At an initial step, heavily doped region  14  is formed in depth in a semiconductor substrate  10 . A semiconductor material epitaxy may then be performed to obtain a less heavily doped region  16  of adapted thickness. Insulating trenches  12  for delimiting the active area, as well as trenches  18 , are then defined (by means of adapted masks). The dopant implantation enabling to form regions  20  is then performed. 
     Then, an insulating material layer (having region  22  forming a portion thereof at the end of the manufacturing) is formed over the entire active region, after which a heavily-doped polysilicon layer (having region  24  forming a portion thereof at the end of the manufacturing) is formed at the surface of the substrate. A dopant implantation is then performed in region  16 , through the insulating material present above this region, to form a collector region localized in this region. An opening is then formed in heavily-doped polysilicon layer  24  opposite to region  16 , this opening corresponding to the final opening defined in layer  24 . An insulating material layer is then formed at the surface of layer  24  and on the walls of the previously-defined opening. 
     An etching is then performed from the bottom of the opening defined in layer  24  to remove the material of insulating layer  22  under the opening, but also to laterally define a cavity in the layer of material  22 , under layer  24 . 
     A silicon-germanium growth is then carried out in the cavity thus defined. Silicon-germanium  25  grows from the lower surface of polysilicon layer  24  as well as from the upper surface of region  16 , to fill the cavity formed in insulating layer  22 . Then, spacers  28  are formed at the surface of silicon-germanium region  25 . The opening remaining at the surface of silicon-germanium layer  25  is then filled with material  26  forming the transistor emitter. 
     A last step comprises performing etchings to obtain the topology of the transistor of  FIG. 1  and thus to expose the upper surfaces of regions  20  and  24 , after which a silicidation of the device is carried out to form silicide regions  30 ,  34 , and  38 . 
     A first disadvantage of a bipolar transistor such as that in  FIG. 1  is its bulk. Indeed, in order to operate properly, collector region  14  typically has, in substrate  10 , a depth on the order of one micrometer. Such a depth is not compatible with recent methods for manufacturing MOS transistors on substrates of silicon-on-insulator type (SOI) where the upper substrate is very thin (thickness smaller than 15 nm). Such substrates, currently used in new semiconductor technologies, are called FD-SOI (fully depleted semiconductor on insulator). 
     Further, with the device of  FIG. 1 , the access to the base is performed via a layer  24  of heavily-doped polysilicon, the contact between layer  24  and silicon-germanium region  25  being achieved on a horizontal surface. This contact is illustrated in  FIG. 1  by a region in dotted lines  39 . 
     The use of a polysilicon layer to access the base alters the transistor performance. Indeed, polysilicon has a higher resistivity than, for example, a metal or heavily-doped single-crystal silicon. Thus, there is a significant access resistance between base contact  32  and base  25 , which is not desired. It should be noted that the forming of single-crystal silicon for the access to the base is not compatible with the above method, a growth or a deposition of single-crystal silicon being impossible to perform on an insulating material. 
     Further, with the device of  FIG. 1 , the junction between base region  25  and collector region  16  has a relatively extensive surface area, which implies a significant junction capacitance between these two regions. To obtain a bipolar transistor having a satisfactory performance, it is desired for the junction capacitances to be as low as possible. 
     Thus, the bipolar transistor of  FIG. 1  has junction capacitances and access resistances which are generally not compatible with a high-performance bipolar transistor. 
     Thus, there is a need for a method for manufacturing a high-performance bipolar transistor on a substrate of FD-SOI type. 
     BRIEF SUMMARY 
     One or more embodiments provide a method for manufacturing an integrated bipolar transistor having a very high frequency performance. 
     One embodiment provides such a method compatible with substrates currently used for the forming of MOS transistors. 
     Another embodiment relates to a bipolar transistor obtained by this method, and at an integrated circuit comprising such a bipolar transistor as well as conventional MOS transistors. 
     Thus, an embodiment provides a method for manufacturing a bipolar transistor, comprising the successive steps of: forming a first surface-doped region of a semiconductor substrate having a semiconductor layer extending thereon with an interposed first insulating layer; forming, at the surface of the device, a stack of a silicon layer and of a second insulating layer; defining a trench crossing the stack and the semiconductor layer opposite to the first doped region, and then an opening in the exposed region of the first insulating layer; forming a single-crystal silicon region in the opening; forming a silicon-germanium region at the surface of single-crystal silicon region, in contact with the remaining regions of the semiconductor layer and of the silicon layer; and forming a second doped region at least in the remaining space of the trench. 
     According to an embodiment, the semiconductor layer has a thickness ranging between 5 and 15 nm and the first insulating layer has a thickness ranging between 10 and 50 nm. 
     According to an embodiment, the method comprises an initial step of forming shallow insulating trenches which extend in the semiconductor layer, the first insulating layer, and the semiconductor substrate to delimit active areas. 
     According to an embodiment, the step of defining an opening is preceded by a step of forming a third insulating layer on the walls of the trench and the step of forming a single-crystal silicon region in the opening is followed by a step of removal of the third insulating layer. 
     According to an embodiment, the step of forming a second doped region at least in the remaining space of the trench is preceded by a step of forming spacers on the remaining walls of the trench. 
     According to an embodiment, the method further comprises a final step of defining openings of access to the first doped region and to the silicon layer. 
     According to an embodiment, the openings of access to the silicon layer and to the first doped region are obtained by performing a first etching of a portion of the second insulating layer and a second etching of the silicon layer and of the semiconductor layer. 
     According to an embodiment, the method further comprises a final step of annealing the structure. 
     According to an embodiment, the method further comprises a final step of silicidation of the device. 
     An embodiment further provides a bipolar transistor formed in a structure comprising a semiconductor layer extending on a semiconductor substrate with an interposed insulating layer, the transistor comprising a collector region defined at the surface of the semiconductor substrate, a buffer region between base and collector defined in an opening formed in the insulating layer opposite to the collector region, and base and emitter regions formed at the surface of the buffer region. 
     According to an embodiment, the semiconductor layer has a thickness ranging between 5 and 15 nm and the insulating layer has a thickness ranging between 10 and 50 nm. 
     According to an embodiment, the transistor further comprises a region of access to the base made of a single-crystal semiconductor material. 
     An embodiment further provides an integrated circuit comprising an association of at least one MOS transistor and of at least one bipolar transistor such as defined hereabove. 
     The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1 , previously described, illustrates a bipolar transistor formed on a solid substrate by a known method; and 
         FIGS. 2 to 18  illustrate results of steps of a method for manufacturing a bipolar transistor according to an embodiment. 
         FIG. 19  is a schematic view of an integrated circuit that includes a MOS transistor and a bipolar transistor. 
     
    
    
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
     DETAILED DESCRIPTION 
     A method for manufacturing a bipolar transistor on a FD-SOI-type substrate is here provided.  FIGS. 2 to 18  illustrate results of steps of such a method. 
     At a step illustrated in  FIG. 2 , it is started from a structure of FD-SOI type comprising an upper semiconductor layer  40  which extends on a semiconductor substrate  42  with an interposed insulating layer  44 . Conventionally, such structures have an insulating layer  44  with a thickness ranging between 10 and 50 nm, for example, 25 nm, and a fully-depleted upper layer  40  with a thickness ranging between 5 and 15 nm, for example, 10 nm. 
     At a step illustrated in  FIG. 3 , shallow insulating trenches  46  (STI trenches) which cross semiconductor layer  40 , insulating layer  44 , and which penetrate in depth into semiconductor substrate  42  are formed. Trenches  46  extend down to a total depth ranging between 150 and 350 nm, for example, a depth equal to 250 nm. 
     At a step illustrated in  FIG. 4 , a dopant implantation has been performed through semiconductor layer  40  and insulating layer  44 , to form a heavily-doped region  48  at the surface of substrate  42  and in the active area laterally defined by insulating trenches  46 . Region  48  may extend in substrate  42  across a thickness ranging between 100 and 300 nm and is doped at a dopant concentration ranging between 5.10 18  and 5.10 19  at/cm 3 , for example, on the order of 10 19  at/cm 3 . Region  48  forms the collector region of the bipolar transistor. As an example, region  48  may be obtained by an arsenic implantation if the desired bipolar transistor is of type NPN. 
     At a step illustrated in  FIG. 5 , a heavily-doped single-crystal silicon layer  50  of the conductivity type desired for the transistor base, for example, heavily doped with boron if an NPN bipolar transistor is desired, has been formed. Single-crystal silicon layer  50  is preferably formed by selective epitaxial growth (SEG), which enables the growth of a heavily-doped single-crystal silicon layer  50  only at the surface of layer  40 , and not at the surface of insulating trenches  46 . Silicon layer  50  has a thickness ranging between 20 and 60 nm, so that the stack of semiconductor layer  40  and of layer  50  has a total thickness ranging between 25 and 75 nm. 
     At a step illustrated in  FIG. 6 , a deposition, over the entire structure, of a first insulating layer  52  and of a second insulating layer  54  has been performed, second insulating layer  54  being made of a material different from that of insulating layer  52 . As an example, first layer  52  may be made of a dielectric material such as tetraethoxysilane (TEOS) having a thickness ranging between 5 and 10 nm, for example, 8 nm, and layer  54  may be a silicon nitride layer having a thickness ranging between 30 and 80 nm, for example, equal to 50 nm. Layers  52  and  54  extend at the surface of heavily-doped single-crystal silicon layer  50 , on the walls of this layer, and cover insulating trenches  46 . 
     At a step illustrated in  FIG. 7 , a first trench  60 , at the center of the active area, which crosses the stack of layers  54 ,  52 ,  50 , and  40 , has been defined to expose a portion of the upper surface of insulating layer  44 . The etching enabling to define trench  60  may of course be, in practice, formed in several steps for etching the different materials of the above-mentioned layers. 
     At a step illustrated in  FIG. 8 , on the walls of trench  60 , an insulating material  62 , for example, made of silicon nitride, has been formed. To form region  62  on the walls of trench  60 , a nitride layer may be conformally deposited over the entire structure, after which an anisotropic etching is performed to remove the horizontal portions of the layer thus formed. Only regions  62  thus remain on the walls of trench  60 . After, insulating layer  44  has been etched through the mask delimited by walls  62  to form a second trench  64  which extends through this layer to expose semiconductor substrate  42 , at the level of heavily-doped region  48  formed at the surface of this substrate. The etching performed to remove layer  44  is selective over the nitride of walls  62  and/or of layer  54 . 
     At a step illustrated in  FIG. 9 , a single-crystal silicon layer  66  has been grown from heavily-doped region  48 . The growth of layer  66  is performed by low-temperature selective epitaxy, which provides a single-crystal silicon layer  66  having a well-controlled profile at the surface of region  48 . The upper surface of layer  66  is provided to be flush with the surface of insulating layer  44 . It should be noted that a slight misalignment between the surfaces of layers  66  and  44  is not critical, as long as this misalignment does not exceed some ten nanometers. Single-crystal silicon layer  66  forms a buffer area between base and collector. 
     At a step illustrated in  FIG. 10 , insulating regions  62  have been etched. To achieve this, the silicon nitride has been selectively etched, this etching also eliminating an upper portion of nitride layer  54 . As an example, the etching may be an isotropic plasma etching. 
     At a step illustrated in  FIG. 11 , on single-crystal silicon  66  and at the bottom of trench  60 , a layer  70  has been grown. Layer  70  is formed of a stack of several layers, for example, a silicon-germanium layer and a silicon layer. The silicon-germanium layer contains the dopant of the base (boron if the transistor is an NPN transistor). The silicon-germanium layer may also contain carbon atoms to decrease the boron diffusion during subsequent anneals. The growth of layer  70  advantageously is a selective growth, easy to control, so that the upper surface of layer  70  extends under the upper surface of single-crystal silicon layer  50 , or is flush with the upper surface of layer  50 . 
     At a step illustrated in  FIG. 12 , conventional in the forming of vertical bipolar transistors, spacers  72  made of an insulating material, for example, an oxide, have been formed at the surface of layer  70 . Spacers  72  extend on the contour of the upper surface of layer  70  and on the walls of trench  60 . Thus, spacers  72  cover the edges of insulating regions  52  and  54 , as well as the remaining edge, if present, of single-crystal silicon layer  50 . Conventionally, spacers  72  may be formed by deposition of an oxide, followed by the deposition of amorphous silicon over the entire structure. An anisotropic etching of the amorphous silicon, followed by an etching of the material forming the spacers via the mask formed by the amorphous silicon, are then performed. The amorphous silicon may then be removed, which enables to obtain “L” shapes, characteristic of spacers, above layer  70 . The amorphous silicon may also be kept above layer  70 , this material mixing afterwards with the material deposited to form the transistor emitter. 
     At a step illustrated in  FIG. 13 , a region of a heavily-doped material of a conductivity type capable of forming the emitter region of the bipolar transistor has been formed over the entire structure, to fill the space remaining in trench  60 . Thus, if an NPN-type bipolar transistor is desired to be formed, this region may be heavily doped with arsenic atoms. An etching is then performed to only leave a heavily-doped emitter-forming portion  74  above layer  70 , as well as above a portion of insulating material layer  54 . 
     At a step illustrated in  FIG. 14 , via the mask formed of portion  74 , insulating layers  54  and  52  have been etched. Thus, the surfaces of insulating trenches  46  are exposed, as well as the surface of heavily-doped single-crystal silicon layer  50 . This etching may be of any known type capable of removing insulating layers  54  and  52 . 
     At a step illustrated in  FIG. 15 , a new etching has been performed, via a mask of adapted shape, to remove portions of layer  50 , of layer  40 , and of insulating layer  44  located on the contour of the device, that is, on the contour of the active area, for example in contact with insulating trenches  46 . Thus, an access to heavily-doped region  48  forming the bipolar transistor collector is opened. This step may be carried out in several etch steps, a first step being capable of removing the semiconductor material of layers  50  and  40 , and a second step being capable of removing the insulating material of layer  44 . 
     At a step illustrated in  FIG. 16 , via the same mask as that used to perform the etching of the step of  FIG. 15 , an implantation of dopants of the same conductivity type as that of region  48  has been performed at the surface of the exposed portions of region  48 . Thus, at the surface of the exposed regions of region  48 , heavily-doped regions  78  are formed, for example, at a dopant concentration ranging between 5.10 19  and 5.10 20  at/cm 3 , for example, at 10 20  at/cm 3 . 
     At the step illustrated in  FIG. 17 , the entire structure has been annealed. This anneal allows the diffusion of the doped regions of the different elements of the structure. In particular, this anneal enables to extend heavily-doped region  78  formed at the surface of layer  48 , to form a more extended heavily-doped region  82  at the surface of this region. This anneal further enables for the dopant atoms of heavily-doped layer  50  to partly migrate to silicon layer  40  in order to form a single region  80 . The obtained region  80  forms the region of access to the formed base of layer  70 . The diffusion anneal further develops collector region  74  so that it extends slightly at the surface of layer  70 . The anneal also implies a diffusion of dopant atoms from emitter  74  to the silicon layer comprised in the stack forming layer  70 . 
     At a step illustrated in  FIG. 18 , a silicidation of the entire device, that is, a nickel deposition followed by a heat treatment and by adapted chemical treatments, has been performed without using a mask, which enables to transform the apparent silicon regions into conductive silicide regions. Thus, region  82  sees its surface covered with a silicide region  84 , region  80  sees its surface covered with a silicide region  86 , and region  74  sees its surface covered with a silicide region  88 . Regions  84 ,  86 , and  88  respectively form the contact regions of the collector, of the base, and of the emitter. 
     Thus, a bipolar transistor is obtained, having its structure extending in depth in the substrate of FD-SOI type, and thus avoiding having too large a thickness, at the surface of the device. The method provided herein is particularly compatible with the forming, in parallel, of MOS transistors on the FD-SOI substrate. 
     Further, the material of access to base  70  of semiconductor region  80  advantageously is heavily-doped single-crystal silicon. Thus, the resistance of access to the base is smaller than in the case of prior art where the access to the base was performed by means of a polysilicon region. 
     The method provided herein also enables to finely control the thicknesses of the emitter region, of the base region, of the buffer region between the collector and the base, and of the collector region, which provides a fine-quality vertical profile of the bipolar transistor, with characteristics that can easily be adjusted. 
     Further, the junction surface area between the base and the collector region is decreased, which enables to limit the base-collector junction capacitance with respect to prior art bipolar transistors. 
     Specific embodiments of the present disclosure have been described. Various alterations and modifications will occur to those skilled in the art. In particular, it should be noted that the conductivity types provided for the different regions of the bipolar transistor may be inverted to form, instead of an NPN transistor, a PNP transistor. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.