Abstract:
In accordance with the invention, a process for forming a dual gate structure for CMOS devices comprises the steps of a) providing a semiconductor workpiece including n-type and p-type regions and a gate dielectric region for a dual gate structure formed over the regions, b) forming over the gate dielectric region a thin layer of semiconductor doped to one type of conductivity, c) selectively removing the doped semiconductor overlying the workpiece region of like conductivity doping and d) forming a thin layer of semiconductor doped to the opposite kind of conductivity. The doped layers are then planarized as by chemical-mechanical polishing (CMP). An additional layer of undoped semiconductor can optionally be applied to bury the doped layers, and the device can be finished by coating with metal silicide in the usual fashion. This process can be completed with only one photolithography step, simplifying device fabrication by several operations.

Description:
FIELD OF THE INVENTION 
     This invention relates to fabrication of integrated circuits, and in particular, to the fabrication of dual gate structures for CMOS devices. 
     BACKGROUND OF THE INVENTION 
     A variety of applications utilize CMOS (Complimentary Metal Oxide Semiconductor) integrated circuits containing a dual-gate structure. FIG. 1 illustrates a structure used in making CMOS circuits. This structure comprises a silicon substrate  100  including an n-doped region  102  and an adjacent p-doped region  104 . A gate dielectric region  108  overlies regions  102  and  104 , and a field dielectric  106  is disposed overlying the boundary of regions  102  and  104  to assist in their isolation. A polysilicon gate region  110  overlies the gate dielectric, and a metal silicide region  112  is formed on the polysilicon gate to enhance conductivity. 
     Typically, fabrication of a dual-gate structure begins by forming a field dielectric region  106  over the boundary between n-doped region  102  and a p-doped region  104 . (A dielectric material is an electrically insulating material, i.e., a material having a resistivity of about 10 6  ohm-cm or greater.) The field dielectric  106  isolates the oppositely-doped regions of the device. The gate dielectric region  108  is then formed over regions  102  and  104 . The polysilicon region  110  is typically deposited over the gate dielectric  108  and field dielectric  106 . The portion of the polysilicon  110  overlying the n-doped region  102  is provided with a p-type dopant such as boron or BF 2 , and the portion of the polysilicon  110  overlying the p-doped region  104  is provided with an n-type dopant such as phosphorus or arsenic. The refractory metal silicide layer is typically formed by the silicide process (direct deposit of metal silicide) or the salicide process (deposit of metal followed by heating). 
     A difficulty in the fabrication of such structures is that n-type and p-type dopants tend to diffuse more readily in refractory metal silicides than in polysilicon. Dopants thus tend to diffuse, for example, from a region of the polysilicon  110  overlying doped silicon region  102  into the silicide layer  112 , laterally in the silicide layer  112 , and then back into the polysilicon  110  at a region overlying the oppositely-doped region  104 . Thus, n-type dopants move into a p-doped polysilicon region and vice versa. This phenomenon, referred to as cross-doping, causes undesirable shifts in threshold voltage, an important parameter in CMOS design and operation. Moreover, the problem of cross-doping is becoming more severe as the industry moves toward smaller CMOS devices. The smaller the devices, the larger the effect of cross-dopants on properties such as threshold voltage, and the closer the devices, the less distance the dopants have to laterally travel to interfere with adjacent devices. 
     Problems are also created by the distribution of dopants in the implanted regions of the polysilicon  110 . Advantageously, the implanted dopants in the final device are located near the underlying gate dielectric  108 . Typically, however, the majority of dopants lie close to the top of the polysilicon  110 , and an anneal is used to diffuse the dopants toward the gate dielectric  108 . However, the anneal time and temperature required to diffuse the dopants across this distance will often undesirably allow diffusion of some of the dopants laterally within the polysilicon  110  into an oppositely-doped region of the polysilicon  110 , causing cross-doping. This lateral diffusion within the polysilicon  110  is a problem regardless of whether a silicide layer is present. This mechanism of cross-doping is particularly problematic where half the distance between the active regions of adjacent devices becomes comparable to the thickness of the doped regions of the polysilicon  110 . In addition, the use of thinner gate dielectric layers improves device performance, but only where a relatively large concentration of dopants, advantageously about 10 20  dopants/cm 3  or greater, is located adjacent to the gate dielectric (resulting in what is known in the art as low poly-depletion). If sufficient dopants are not located adjacent to the dielectric layer, the use of a thinner gate dielectric will at best only marginally improve device performance. 
     It is also possible for dopant distribution to cause problems when forming a refractory metal silicide by a salicide process. Growth of the silicide layer in the salicide process is detrimentally affected if too many dopants, or dopant-based precipitates, are located in the top region of the polysilicon gate structure, where the silicide is formed. In addition, because the polysilicon region is typically thicker when using a salicide process, the dopant diffusion distance to the gate dielectric is often increased, thereby allowing encroachment of the underlying channel region that often leads to shorts in the device. 
     A process that places dopants deep within the polysilicon layer is desired. However, such a deep implant is difficult to attain. Typically, the majority of dopants will lie close to the top surface of the polysilicon regions. It is difficult to implant dopants deeper in the polysilicon without encountering undesirable effects. For example, it is possible for dopants, particularly boron, to penetrate the polysilicon during ion implantation and move into the underlying silicon substrate, or to move along certain crystallographic orientations of polysilicon—a phenomenon known as channeling. (Both mechanisms are referred to herein generally as penetration.) The presence of the boron in the channel region of the silicon substrate detrimentally affects the threshold voltage. Thus, implantation is performed at energies low enough to reduce penetration. Yet, where lower implantation energies are used, the concentration profile will often not be deep enough to avoid the problems discussed above. 
     An improved process is described in applicant&#39;s copending application Ser. No. 08/902,044, filed on Jul. 29, 1997 and entitled “Process for Device Fabrication”. In essence, devices are prepared by forming a first, relatively thin (e.g., about 300-1000 Å) amorphous silicon region over the gate dielectric region. An n-type dopant is implanted at a first portion of the first amorphous silicon region, typically over the p-type region of the substrate. The n-type dopant is advantageously implanted such that substantially all of the dopant remains in the first amorphous silicon region and does not penetrate into the underlying dielectric region or the substrate. A p-type dopant species is then implanted at a second portion of the first amorphous silicon region, typically over the n-type region of the substrate. 
     Once the desired dopants are implanted into the first silicon region, a second amorphous silicon (or polysilicon) region is formed over the first silicon region, in essence burying the implanted dopants. Typically, a refractory metal silicide layer is formed over the second amorphous silicon region. Devices are then formed on the structure in accordance with conventional processing techniques known to one skilled in the art. 
     The creation of the buried implant layer hinders cross-doping that occurs through the silicide. In order for such detrimental cross-doping to occur, the dopant must diffuse from the p-doped region of the first amorphous silicon region into and through the second amorphous silicon region into the metal silicide layer, diffuse laterally within the silicide layer to the area over the oppositely-doped amorphous silicon region, diffuse back through the second amorphous silicon region into the opposite-doped region of the first amorphous silicon region, and move through the first amorphous silicon region to an area along the underlying gate dielectric. 
     While this process works well, it involves two implantations, each of which requires a separate photolithography step. Each photolithography requires several different operations to complete. It would be advantageous to have a new process providing protection from cross doping which involved fewer operations. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a process for forming a dual gate structure for CMOS devices comprises the steps of a) providing a semiconductor workpiece including n-type and p-type regions and a gate dielectric region for a dual gate structure formed over the regions, b) forming over the gate dielectric region a thin layer of semiconductor doped to one type of conductivity, c) selectively removing the doped semiconductor overlying the workpiece region of like conductivity doping and d) forming a thin layer of semiconductor doped to the opposite kind of conductivity. The doped layers are then planarized as by chemical-mechanical polishing (CMP). An additional layer of undoped semiconductor can optionally be applied to bury the doped layers, and the device can be finished by coating with metal silicide in the usual fashion. This process can be completed with only one photolithography step, simplifying device fabrication by several operations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings: 
     FIG. 1, which is prior art, is a schematic cross section of a typical structure for making a CMOS device. 
     FIG. 2 is a flow chart of the steps involved in fabricating a dual gate structure in accordance with the invention; and 
     FIGS. 3A-3E illustrate the dual gate structure at various stages of the fabrication process of FIG.  2 . 
    
    
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION 
     Referring to the drawings, FIG. 1 is a schematic cross section of a dual gate structure to be fabricated and is described above in the Background of the Invention. 
     FIG. 2 is a flow diagram of the steps in fabricating a dual gate structure. As shown in block A of FIG. 2, the first step is to provide a workpiece for a dual gate structure including the n-type and p-type regions and the overlying gate dielectric regions. FIG. 3A schematically illustrates such a workpiece comprising a semiconductor substrate  100 , n-type region  102 , adjacent p-type region  104  and dielectric regions  106 ,  108 . Dielectric region  106  is a field dielectric and region  108  is a gate dielectric. Field dielectric  106  is formed overlying the boundary between the n-type region  102  and the p-type region  104 . Field dielectric  106  constitutes, for example, a surface isolation region or a trench isolation region. Typically, the field dielectric  106  is a shallow trench isolation region and has a thickness of about 1200 to about 3000 Å. Advantageously, the semiconductor is silicon. These regions can be formed in accordance with standard processing techniques well known in the art, such as the twin tub process described in U.S. Pat. No. 4,435,596 to Parillo et al., the disclosure of which is hereby incorporated by reference. 
     The gate dielectric region  108  is typically silicon dioxide formed over the portions of the n-type region  102  and p-type region  104  not covered by the field dielectric  106 . The gate dielectric  108  is formed in accordance with standard processing techniques and, when formed from silicon dioxide, is advantageously about 15 to about 100 Å thick. The combination of the field dielectric  106  and gate dielectric  108  can be considered a single dielectric material region. 
     The next step shown in Block B of FIG. 2 is to form over the gate dielectric region a thin layer of doped semiconductor. The semiconductor can be amorphous silicon. It typically has a thickness in the range 300-2000 Å. Preferably the semiconductor is grown with in situ doping of one type of conductivity, preferably n-type, with a concentration in the range 10 20 -10 21  atoms/cm 3 . The n-type semiconductor can be grown over both the underlying p-type region of the workpiece and the underlying n-type region. It is noteworthy that in the growth of in situ doped silicon, the initially deposited silicon (e.g. the first 50-200 angstroms, is substantially free of dopant). Methods for growing thin layers in in situ doped semiconductors are well known in the art. See, for example, U.S. Pat. No. 5,607,511 issued to B. Meyerson on Mar. 4, 1997 and U.S. Pat. No. 5,256,566 issued to D. Baily on Oct. 26, 1993, both of which are incorporated herein by reference. Alternatively, the semiconductor can be grown as an undoped layer and subsequently be doped, as by ion implantation. 
     The third step (Block C) is to selectively remove the grown, doped semiconductor overlying the workpiece region of like conductivity doping. In the present example, this involves masking the n-type silicon overlying the p-type region and selectively removing the n-type semiconductor overlying the n-type region  102 , as by wet etching or reactive ion etching. The mask is conveniently formed of photoresist using conventional photolithgraphic techniques. 
     It should be noted that this step does not require removal of all semiconductor overlying the n-type region. Only the upper portion of the layer containing dopant need be removed. A small undoped thickness can be left to protect the gate dielectric region. 
     FIG. 3B shows the structure with the top (doped) portion of the in situ doped semiconductor layer  30  removed over the n-doped region of the workpiece. The semiconductor layer over the p-doped region  104  is protected from removal by mask  31 . 
     The next step shown in Block D of FIG. 2 is to form over the surface a thin layer of semiconductor doped to the opposite kind of conductivity, e.g. p-type silicon. As a preliminary step, the mask  31  is removed. Then the new doped layer is formed overlying both regions  102  and  104 . It is noteworthy that no photolithography is required. The layer can be formed by growing an in situ layer or by growing an undoped layer and implanting a dopant. Typical thickness is in the range 300-2000 Å and typical doping concentration is 10 20 -10 21  atoms/cm 3 . 
     FIG. 3C illustrates the workpiece structure after the formation of the p-doped layer  32 . 
     The fifth step (Block E) is to planarize the surface. This is conveniently accomplished by chemical-mechanical polishing (CMP). 
     FIG. 3D shows the structure after planarizing. The planaization effectively removes the portion of p-doped layer  32  overlying region  104  without lithography. After planarization, the doped layers  30 ,  32  overlying regions overlying regions  102 ,  104  are of respectively opposite conductivity type. 
     The final step (Block F), which is optional, is to grow an additional undoped layer of semiconductor over the two doped layers to form the remaining thickness of the gate electrodes. Typical undoped silicon thicknesses are in the range 100-500 Å. 
     FIG. 3E shows the structure after this step. Doped semiconductor layers  30 ,  32  on the gate dielectric overlie crystalline regions of opposite conductivity and undoped semiconductor layer  33  is formed on both layers  30  and  32 . 
     The device is then finished in the conventional manner. This typically involves partitioning the layer  33  to define separate gate electrodes and forming a metal silicide layer on the silicon gate to enhance conductivity. 
     The resulting structure is as shown in FIG. 1. A refractory metal silicide layer  112  is optionally formed on the grown silicon region  110  ( 33  of FIG. 3D) by standard processing techniques such as sputtering or chemical vapor deposition. Examples of suitable refractory metal silicides include tungsten silicide, tantalum silicide, and cobalt silicide. Advantageously, the refractory metal silicide layer has a thickness of about 800 to about 2000 Å. It is also advantageous to introduce nitrogen into the refractory metal silicide layer. Where the nitrogen is ion implanted, the implantation advantageously is performed at an energy of about 10-50 keV (depending on the thickness), more advantageously 30 keV, and at a dopant implant dose of about 1×10 15  to about 2×10 15  atoms/cm 2 . The nitrogen appears to trap boron atoms in the silicide layer, and thus assists in reducing lateral diffusion and cross-doping of boron. See U.S. Pat. No. 5,851,922 issued to J. Bevk et al. on Dec. 22, 1998. It is also possible to form a silicide layer by a salicide process. Metal layers other than refractory metal silicides are also possible. 
     Advantageously, when the semiconductor is silicon, an anneal is performed after formation of the second amorphous silicon region  33  to recrystallize the second amorphous silicon region  33  and first amorphous silicon regions  30 ,  32 , i.e., to transform the regions  30 ,  32 ,  33  into polysilicon. It is possible for the anneal to be performed after formation of the second silicon region  33 , after formation of the silicide layer  34 , or after a nitrogen implant of the silicide layer. The anneal is advantageously performed at a temperature of about 580 to about 650° C., for about 1 to about 5 hours, in a nitrogen atmosphere. More advantageously, the anneal is performed at about 650° C. for about 3 hours in a nitrogen atmosphere. 
     The resulting structure is then subjected to processing steps to form gate stacks over the n-regions and p-regions of the substrate, in accordance with standard procedures known in the art. Advantageously, such steps include a rapid thermal anneal after formation of gate stacks. The rapid thermal anneal is advantageously performed such that the wafer is raised to a temperature of about 900 to about 1050° C. for 2 to about 10 seconds. More advantageously, the wafer is raised to a temperature of 1000° C. for 5 seconds. The rapid thermal anneal is useful in attaining a desirable distribution of dopants in the doped regions of the device. 
     Typical processing steps subsequent to formation and implantation of refractory silicide layer  34  would include the following: 
     Deposit of a gate hard mask. The mask is formed, for example, from silicon oxide deposited by plasma-enhanced deposition of tetraethyl orthosilicate (PETEOS), a nitride layer formed by plasma-enhanced chemical vapor deposition (PECVD), or a spin-on glass (SOG) layer; 
     Formation of a gate photoresist to allow selective etching of the gate hard mask, etching of the hard mask, and removal of the photoresist; 
     Etching of refractory silicide layer  34  and silicon regions  31 ,  32 ,  33 ; 
     Formation of a photoresist to allow implantation of a low-doped drain region (LDD), implanting of the LDD, and removal of the photoresist; 
     Deposit of a dielectric, e.g., silicon oxide by PETEOS, for gate spacer formation, anneal of the dielectric, and etch of the spacers; 
     Formation of a photoresist to allow implantation of n-type source and drain, implanting the n-type source and drain, and removal of the photoresist; 
     Formation of a photoresist to allow implantation of p-type source and drain, implanting the p-type source and drain, and removal of the photoresist. 
     The rapid thermal anneal is advantageously performed subsequent to implantation of the p-type source and drain. Where a salicide process is used, the process is typically performed subsequent to formation of the n-type and p-type source and drain, and the rapid thermal anneal is typically performed prior to depositing the refractory metal on the polysilicon gate structure. 
     The invention will be further clarified by the following examples: 
     EXAMPLE 1 
     A silicon wafer was processed using conventional expedients, well known in the art, to form a silicon substrate having lightly doped n-type and p-type regions. A field dielectric region was grown on a selected portion of the substrate by the shallow trench isolation (STI) technique to electrically isolate the n-type and p-type regions. A 30 Å gate dielectric layer of silicon oixynitride was then formed on the portions of the substrate not covered by the field dielectric region by thermal oxidation in N 2 O at 850° C. A 100 Å thick layer of undoped amorphous silicon was then formed over the field and gate dielectric layers by low pressure chemical vapor deposition, followed by deposition of 400 Å thick in situ phosphorous doped (Si 2 H 6  and PH 3 ) film. A layer of photoresist, in accordance with standard practice, was formed on the silicon and patterned to expose portions of the silicon overlying the lightly doped n-type region of the substrate. The exposed portions were then reactively ion etched until the doped portion of the poly-silicon layer was removed. The photoresist was removed and a second amorphous silicon layer having a thickness of 500 Å was formed by low pressure chemical vapor deposition and implanted with B atoms (energy: 2KeV, dose: 3×10 15 /cm 2 ). The wafer was then planarized using RODEL SDE3000 polysilicon slurry, until the silicon thickness was reduced to about 500-600 Å. This was followed by a deposition of an additional 500 Å thick undoped amorphous silicon layer by low pressure chemical upon deposition. An anneal was then performed at 650 ° C. for 3 hours in nitrogen atmosphere to crystallize the amorphous silicon. After the anneal, a 1000 Å tungsten silicide layer was formed on the second amorphous silicon region by sputtering, and nitrogen was implanted into the silicide at 30 keV and a dosage of 1×10 15  atoms/cm 2 . In accordance with the standard processing steps discussed above, gate stacks were formed. Specifically, a 1500 Å silicon dioxide hard mask was formed on the silicide layer by PETEOS; the mask was etched; the silicide and silicon regions were etched; a low-doped drain region of arsenic was implanted; a silicon dioxide layer was formed for gate spacers and the spacers were etched; an anneal was performed at 750° C. for 30 minutes in oxygen to densify the silicon dioxide spacers; n-type drain and source were implanted; and p-type drain and source were implanted. Then a rapid thermal anneal of the wafer was performed, the wafer being heated at 1000° C. for 5 seconds in an RTA tool. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be devised by those skilled in the art without departing from the spirit and scope of the invention.