Abstract:
A process for fabricating a BiCMOS device, on a semiconductor substrate, featuring PFET and NFET devices, and an NPN bipolar junction transistor, has been developed. The process features the integration, or the sharing of process steps, used for both the CMOS and bipolar devices, such as the creation of an N type buried layer, used in one region for isolation of PFET devices, and used in a second region, of the semiconductor substrate, as a subcollector region, for the bipolar device. Features of the BiCMOS process include the formation of N well, and P well regions, for CMOS device, as well as the use of an epitaxial silicon layer, to allow optimum bipolar characteristics to be achieved.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to processes used to fabricate semiconductor devices, and more specifically to a process used to fabricate a bipolar—complimentary metal oxide semiconductor, (BiCMOS), device, on a semiconductor substrate. 
     (2) Description of Prior Art 
     The addition of bipolar junction transistors, to CMOS designs, results in a BiCMOS device, superior in performance to CMOS counterparts, as a result of the inclusion of the higher performing bipolar junction devices. An objective of the semiconductor industry has been to develop a process fabrication sequence, that allows integration of the bipolar, and CMOS devices, using shared process steps, and without sacrificing the performance of the bipolar junction transistor, as a result of having to use basically CMOS materials and processes. 
     This invention will describe a process for fabricating a BiCMOS device, in which a novel twin well, and epitaxial silicon layer, are featured, to arrive at a BiCMOS chip, formed using many shared, (bipolar and CMOS), process steps, and formed using an N type epitaxial layer, at a concentration, that allows the bipolar device, to achieve the desired performance requirements. Prior art, such as Ronkainen et al, in U.S. Pat. No. 5,776,807, describes a process for the fabricating a BiCMOS device, however this prior art does not describe the integration of the N type epitaxial layer, using a specific dopant level, needed to optimize bipolar performance, described in this present invention. 
     SUMMARY OF THE INVENTION 
     It is a principal object of this invention to design a BiCMOS structure which employs separate masks for the P well, as well as for the N well CMOS regions, while choosing the ideal N type epitaxial silicon layer for the NPN bipolar devices. 
     It is an object of this invention to integrate the fabrication of twin wells, a P well for the N type CMOS devices, and an N well for the P type CMOS devices, into the BiCMOS fabrication process. 
     It is another object of this invention to create a buried sub-collector region, for the bipolar device, while creating a buried N type layer, for the P type CMOS devices, using the same masking and ion implantation procedures. 
     It is still another object of this invention to integrate an N type, epitaxial layer, into the BiCMOS fabrication sequence, to be used as the collector region of the bipolar device. 
     It is still yet another object of this invention to use a split polysilicon layer, to protect the CMOS gate insulator layers from specific bipolar fabrication processes. 
     The design concept invention for BiCMOS is to form an N well region, for the P channel devices, and to form a P well region, for the N channel devices, and use an N type, epitaxial silicon layer for the NPN bipolar device. The N type epitaxial layer, in terms of dopant concentration, is specifically designed for the NPN bipolar devices. In prior BiCMOS designs the same N well, used for P channel devices, was used for the NPN bipolar device, particularly as CMOS fabrication is using a feature size of 0.25 uM, or less, and with an increased N well dopant concentration. The use of increased N well dopant concentration, for the NPN bipolar devices, is unsatisfactory, in terms of performance. In conventional BiCMOS designs the epitaxial layer, which could be either N type, or P type, is very low. However in this invention, using a feature size of 0.25 uM, or less, the epitaxial silicon layer must be N type, with the doping level designed to satisfy the NPN bipolar performance criteria. This will result in optimum CMOS, as well as NPN bipolar devices. In addition the process described in this present invention does not increase process steps or process complexity. 
     In accordance with the present invention a method of fabricating a BICMOS device, on a semiconductor substrate, featuring the use of an epitaxial silicon layer, for the bipolar device, and featuring the use of twin wells, for the CMOS devices, is described. After forming buried N type layers, in a first region of the semiconductor substrate, to be used for isolation of P type, (PFET), CMOS devices, and in a third region, to be used for the buried subcollector region, of the bipolar devices, a buried P type layer is formed in a second region of the semiconductor substrate, to be used for isolation of N type, (NFET), CMOS devices. An N type epitaxial silicon layer is grown, followed by the formation of an N well region, overlying the buried N type layer, in the first region, or in the PFET region, of the semiconductor substrate. After the creation of a heavily doped reach through region, contacting the buried subcollector layer, in the third region of the semiconductor substrate, a pattern in an oxidation resistant, composite layer, is formed on the regions of the semiconductor substrate, to be protected from a subsequent oxidation procedure, used to form isolation regions. After formation of a P well region, overlying the buried P type layer, in the second region, or in the NFET region of the semiconductor substrate, an oxidation procedure is performed, creating isolation regions, in areas not protected by the oxidation resistant, composite insulator layer, leaving subsequent active device regions, unoxidized. 
     Removal of the oxidation resistant, composite layer, is followed by the growth of a gate insulator layer, on the surface of all active device regions, followed by the deposition of a thin, first polysilicon layer. Conventional photolithographic and ion implantation procedures, are used to create a P type base region, in the collector region, located in the third region, or in the bipolar region, of the semiconductor substrate. Conventional photolithographic and dry etching procedures, are used to create an emitter opening in the thin, first polysilicon, followed by the formation of an N type, self-aligned collector region, ion implanted through the emitter opening, and located in the collector region, underlying the P type base region, and overlying the buried subcollector region. After removal of the gate insulator layer, exposed in the emitter opening, a thick, second polysilicon layer, is deposited, doped, and along with thin, first polysilicon layer, is patterned to create polysilicon gate structures, in the NFET and PFET regions, as well as creating a polysilicon emitter structure, contacting the base region, in the emitter opening. Conventional photolithographic block out masking, is used to allow a lightly doped, N type source/drain regions, to be formed in an area of the NFET region, not covered by the polysilicon gate structure, while similar photolithographic block out masking is used to allow a lightly doped, P type source/drain region to be formed in an area of the PFET region, not covered by a polysilicon gate structure. After formation of insulator spacers, on the sides of the polysilicon gate structures, and on the sides of the polysilicon emitter structure, an emitter drive-in cycle is performed, allowing dopant from the polysilicon emitter structure to diffuse into the top portion of the P type base region, creating an emitter region. Conventional photolithographic block out procedures, and conventional ion implantation procedures, are used to create the heavily doped N type, source/drain region, in an area of the NFET region, not covered by the polysilicon gate structure, or by the insulator spacers, followed by additional photolithographic block out masking, and ion implantation procedure, used to create the heavily doped, P type source/drain region, in an area of the PFET region, not covered by the polysilicon gate structure, or by insulator spacers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other advantages of this invention are best explained in the preferred embodiment with reference to the attached drawings that include: 
     FIGS. 1-16, which schematically, in cross-sectional style, describe key stages of fabrication of a BiCMOS device, featuring twin wells, and an N type, epitaxial silicon layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process used to fabricate a BICMOS device, featuring an N type epitaxial silicon layer, for the bipolar components, and twin well regions, for the CMOS elements, will now be described in detail. A P type, single crystalline silicon substrate  1 , with a &lt;100&gt; crystallographic orientation, and a resistivity between about 15 to 25 ohm-cm, is used, and shown schematically in FIG. 1. A silicon dioxide layer  2 , to be used as a screen oxide layer, for subsequent ion implantation procedures, is thermally grown, to a thickness between about 120 to 160 Angstroms, on semiconductor substrate  1 . Conventional photolithographic block out masking is next used, to allow N type buried layer  3 , to be formed in region  60 , and to allow N type buried subcollector  5 , to be formed in region  62 , of semiconductor substrate  1 , with buried layer  3 , used for the P type, of PFET, CMOS devices, while buried layer  5 , is to be used for the bipolar device. The photoresist block out shape, (not shown in the drawings), protected region  61 , to be used for the N type, of NFET, CMOS devices, from the buried layer, antimony, or arsenic, ion implantation procedure, performed at an energy between about 60 to 80 KeV, at a dose between about 4E15 to 6E15 atoms/cm 2 . After removal of the photoresist block out shape, used to define N type buried layer  3 , and N type buried subcollector layer  5 , via plasma oxygen ashing and careful wet cleans, an anneal procedure, performed at a temperature between 1150 to 1200° C., for a time between about 50 to 70 min., is employed to finalize the N type buried layer regions. This is schematically shown in FIG.  1 . Another photoresist block out shape, (not shown in the drawings), is next used to protect region  60 , and region  62 , from an ion implantation procedure, using boron ions, at an energy between about 30 to 70 KeV, and at a dose between about 5E12 to 5E13, to create P type, buried layer  4 , in region  61 , to be used for the NFET CMOS devices. After removal of the photoresist block out shape, used to create P type buried layer  4 , via plasma oxygen ashing and careful wet cleans, another anneal cycle, performed at a temperature between about 875 to 925° C., for a time between about 25 to 35 min., is used to finalize P type, buried layer  4 , schematically shown in FIG. 2. A hydrofluoric acid procedure is next used to remove silicon dioxide layer  2 , followed by the growth of epitaxial layer  6 , shown schematically in FIG. 3. N type, epitaxial silicon layer  6 , is grown at a temperature between about 700 to 1150° C., to a thickness between about 1.0 to 1.5 uM, in a silane ambient, with the addition of arsenic, or phosphorous, to result in a surface concentration between about 1.0E16 to 3.0E16 atoms/cm 3 . The surface concentration of N type epitaxial silicon layer  6 , about 1.6E16 atoms/cm 2 , is needed to obtain the desired device characteristics of subsequently formed, bipolar devices. 
     Silicon dioxide screen oxide layer  7 , is next thermally grown, to a thickness between about 225 to 275 Angstroms, in an oxygen—steam ambient, at a temperature between about 910 to 930° C. Photoresist block out shape  8 , is next used as a mask to allow N well region  9 , shown schematically in FIG. 4, to be formed in PFET region  60 , via ion implantation of phosphorous ions, at an energy between about 150 to 170 KeV, at a dose between about 3E12 to 5E12 atoms/cm 2 . After removal of photoresist block out shape  8 , via plasma oxygen ashing and careful wet cleans, photoresist block out shape  10 , is formed, and used to allow the formation of N type, collector reach through region  11 , to be created in a portion of N type epitaxial silicon layer  6 . Collector reach through region  11 , schematically shown in FIG. 5, is formed via a first ion implantation of phosphorous ions, at an energy between about 150 to 170 KeV, and at a dose between about 0.9E14 to 1.1E14 atoms/cm 2 , followed by a second ion implantation procedure of phosphorous ions, at an energy between about 70 to 90 KeV, and at a dose between about 4E15 to 6E15 atoms/cm 2 . 
     After removal of block out shape  10 , via plasma oxygen ashing and careful wet cleans, and after the removal of silicon dioxide layer  7 , via a buffered hydrofluoric acid procedure, silicon oxide pad layer  12 , is thermally grown, to a thickness between about 325 to 375 Angstroms, in an oxygen—steam ambient, at a temperature between about 975 to 1025° C. A silicon nitride layer is next deposited, via low pressure chemical vapor deposition, (LPCVD), or plasma enhanced chemical vapor deposition, (PECVD), procedures, to a thickness between about 1400 to 1600 Angstroms. Photoresist shapes  14 , are then used as a mask to create silicon nitride shapes  13 , via an anisotropic RIE procedure, using CHF 3  and CF 4  as an etchant. This is schematically shown in FIG.  6 . After removal of photoresist shapes  14 , via plasma oxygen ashing and careful wet cleans, photoresist shape  15 , is formed, and used as a block out mask to allow P well region  16 , to be formed in NFET CMOS region  61 , via a first ion implantation procedure, using boron ions, at an energy between about 25 to 35 KeV, at a dose between about 4E13 to 6E13 atoms/cm 2 , and a second ion implantation procedure, again using boron ions, at an energy between about 150 to 170 KeV, at a dose between about 0.9E12 to 1.1E12 atoms/cm 2 . This is schematically shown in FIG.  7 . After removal of photoresist shape  15 , via plasma oxygen ashing and careful wet cleans, an oxidation procedure is performed, in an oxygen—steam ambient, at a temperature between about 970 to 1000° C., to create silicon dioxide isolation regions  17 , shown schematically in FIG. 8, in areas not protected by silicon nitride shapes  13 . Silicon nitride shapes  13 , protected subsequent CMOS, and bipolar, active device regions, from the LOCcalized Oxidation of Silicon, (LOCOS), procedure. 
     After removal of silicon nitride shapes, via use of a hot phosphoric acid solution, and the removal of silicon oxide pad layer  12 , via use of a buffered hydrofluoric acid solution, silicon dioxide layer  18 , is thermally grown, at a thickness between about 130 to 150 Angstroms, in an oxygen—steam ambient, at a temperature between about 910 to 930° C. Silicon dioxide layer  18 , will be used as the gate insulator layer, in PFET CMOS region  60 , and in NFET CMOS region  61 , while silicon dioxide layer  18 , schematically shown in FIG.  9 . will be used as part of a base oxide layer, in bipolar region  62 . A thin polysilicon layer  19 , is next deposited via LPCVD procedures, to a thickness between about 450 to 550 Angstroms, using silane as a source, with thin polysilicon layer  19 , to be used to protect silicon dioxide layer  18 , located in the CMOS regions, from subsequent bipolar processing sequences. Photoresist shape  20 , is then used as a mask, to allow an ion implantation procedure, using boron ions, at an energy between about 30 to 80 KeV, and at a dose between about 1E13 to 5E13 atoms/cm 2 , to create base region  21 , in bipolar region  62 . This is schematically shown in FIG.  9 . 
     After removal of photoresist shape  20 , using plasma oxygen ashing and careful wet cleans, photoresist shape  22 , is formed, allowing emitter opening  23 , shown schematically in FIG.  10 . to be formed via an anisotropic RIE procedure, performed to thin polysilicon layer  19 . A selective collector, (SIC), region  24 , is next placed, underlying base region  21 , and overlying N type buried subcollector region  5 , using photoresist shape  22 , as a mask, allowing an ion implantation procedure, using phosphorous ions, at an energy between about 200 to 500 KeV, at a dose between about 1.0E12 to 3.0E13 atoms/cm 2 , to create SIC region  24 . SIC region  24 , with an N type doping level greater then the doping level of N type epitaxial silicon layer  6 , restricts the width of a subsequent base width region, thus improving the performance of the bipolar device. 
     After removal of photoresist shape  22 , again using plasma oxygen ashing and careful wet cleans, silicon dioxide layer  18 , exposed in emitter opening  23 , is removed using a buffered hydrofluoric acid solution. A thick polysilicon layer  25 , shown schematically in FIG. 11, is deposited, via LPCVD procedures, to a thickness between about 2000 to 3500 Angstroms, overlying thin polysilicon layer  19 , in all regions, except in emitter opening  23 , in which thick polysilicon layer  25 , directly contacts the exposed portion of base region  21 . An ion implantation procedure, using arsenic ions, at an energy between about 40 to 80 KeV, and at a does between about 1E16 to 2E16 atoms/cm 2 , is used to dope thick polysilicon layer  25 . Photoresist shape  26   a , and  27   a , to be used as etch mask for subsequent CMOS gate structure definition, and photoresist shape  28   a , to be used for emitter structure definition, are next formed on thick polysilicon layer  25 . This is schematically shown in FIG.  11 . 
     An anisotropic RIE procedure, using Cl 2  as an etchant, and using photoresist shapes  26   a ,  27   a , and  28   a , as etch masks, are used to define: PFET polysilicon gate structure  26   b , comprised of thick polysilicon layer  25 , and underlying thin polysilicon layer  19 ; NFET polysilicon gate structure  27   b , comprised of thick polysilicon layer  25 , and underlying thin polysilicon layer  19 ; and emitter structure  28   b , comprised of thick polysilicon layer  25 , contacting base region  21 , in emitter opening  23 , and comprised of underlying thin polysilicon layer  19 , residing on silicon dioxide layer  18 , in regions adjacent to emitter opening  23 . This is schematically shown in FIG.  12 . After removal of the photoresist shapes, used to define the gate structures and the emitter structure, via plasma oxygen ashing and careful wet cleans, an oxidation procedure, performed at a temperature between about 900 to 940° C., in an oxygensteam ambient, is used to create silicon oxide layer  40 , at a thickness between about 80 to 100 Angstroms, on emitter structure  28   b , as well as formation of silicon oxide layer  41 , on exposed surfaces of the polysilicon gate structures. This is schematically shown in FIG.  12 . 
     FIG. 13, schematically shows the formation of lightly doped, N type source/drain regions  30 , in NFET CMOS region  61 . Photoresist shape  29 , is used to block out PFET CMOS region  60 , and bipolar region  62 , from an ion implantation procedure, using phosphorous ions at an energy between about 40 to 60 KeV, at a dose between about 1.4E13 to 1.6E13 atoms/cm 2 , creating lightly doped, N type source/drain regions  30 . After removal of photoresist shape  29 , via plasma oxygen ashing and careful wet cleans, photoresist shape  31 , is employed to block out NFET CMOS region  61 , and bipolar region  62 , from an ion implantation procedure, performed using BF 2  ions, at an energy between about 40 to 80 KeV, and at a dose between about 0.9E13 to 1.1E13 atoms/cm 2 , creating lightly doped, P type source/drain regions  32 , in PFET CMOS region  60 . This is schematically shown in FIG.  14 . 
     After removal of photoresist shape  31 , via plasma oxygen ashing and careful wet cleans, a deposition of silicon oxide, is achieved via LPCVD or PECVD procedures, at a thickness between about 2500 to 3500 Angstroms, using tetraethylorthosilicate, (TEOS), as a source. An anisotropic RIE procedure, using CHF 3  as an etchant, is used to create silicon oxide spacers  33 , on the sides of the polysilicon gate structures, as well as on the sides of the emitter structure. This is schematically shown in FIG.  15 . 
     FIG. 16, schematically shows heavily doped, N type source drain region  35 , in NFET CMOS region  61 , as well as P type, heavily doped source/drain region  36 , in PFET CMOS region  60 . First a photoresist block out shape, (not shown in the drawings), is used to protect PFET CMOS region  60 , and bipolar region  62 , from an ion implantation procedure, using arsenic ions, at an energy between about 60 to 80 Kev, at a dose between about 5E15 to 7E15 atoms/cm 2 , creating heavily doped, N type source/drain region  35 . After removal of the NFET photoresist block out shape, photoresist shape, (not shown in the drawings), is formed, and used to block out NFET CMOS region  61 , and bipolar region  62 , from an ion implantation procedure, performed using BF 2  ions, at an energy between 40 to 60 Kev, and at a dose between about 4E15 to 6E15 atoms/cm 2 , to create heavily doped, P type source drain region  36 . The photoresist block out shape is again removed using plasma oxygen ashing and careful wet cleans. 
     A rapid thermal anneal, (RTA), procedure is next performed at a temperature between about 1000 to 1060° C., for a time between about 5 to 20 sec., to drive dopant from emitter structure  28   b , into an area of P type base region  21 , exposed in emitter opening  23 , creating emitter region  34 , located in a top portion of P type base region  21 . This is schematically shown in FIG.  16 . The space between emitter region  34 , and SIC region  24 , is the base width of the NPN bipolar device. 
     Creation of metal contact structures, (not shown in the drawings), to underlying elements of the CMOS and bipolar devices, are formed in contact holes, created in conventional passivation layers, such as boro-phosphosilicate glass, (BPSG). Metal interconnect structures are formed in via holes, created in an interlevel dielectric layer, with the metal interconnect structure, overlying and contacting the underlying metal contact structure. 
     An additional embodiment of this invention, can be a third well, an N well, located in bipolar region  62 , used to modify the dopant concentration of N type epitaxial silicon layer  6 . 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.