Abstract:
An intermediate semiconductor device for use in making surface channel MOS transistors is disclosed. The intermediate semiconductor device includes a semiconductor substrate having a top surface, a bottom surface, a plurality of doped isolation regions and a first surface channel. A first dielectric layer overlies a first portion of the top surface of the semiconductor substrate and a portion of at least one of the plurality of doped isolation regions. A first polysilicon layer overlies the first dielectric layer, and a second dielectric layer overlies the first polysilicon layer and a second portion of the top surface of the semiconductor substrate. The second dielectric layer is overlaid with a second polysilicon layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/153,931, filed Sep. 16, 1998, now U.S. Pat. No. 6,110,788 issued Aug. 29, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to methods for fabricating integrated circuit and semiconductor devices and the resulting structures. More particularly, the present invention relates to metal-oxide-silicon (MOS) transistor devices for use in memory arrays, methods for making the same, and semiconductor devices containing the same. 
     One common device in integrated circuits (ICs) is a MOS transistor, such as those described in U.S. Pat. Nos. 5,658,811, 5,585,302, 5,668,394, 5,633,522, 5,567,647, 5,605,854, and 5,627,393, the disclosures of which are incorporated herein by reference. One type of MOS transistor, a MOS field-effect-transistor (MOSFET), can be characterized in one manner as either “buried channel” or “surface channel” depending on the location of the channel. In surface channel (SC) devices, the channel is located near the surface of the substrate where the gate oxide of the transistor is disposed, generally at a depth of about 100 Å. In buried channel (BC) devices, the channel is located deeper in the substrate and further away from the gate oxide, generally at a depth of about 1000 Å. 
     The performance of buried-channel and surface-channel MOSFET devices also differs. For example, the mobility of carriers (holes or electrons) in buried channels is about 15% higher than carriers in surface channels. Unfortunately, the advantages of BC MOSFETs are often outweighed by some of their disadvantages. As the size of MOSFET devices shrinks and gate lengths become smaller, the breakdown voltage (BVdSS) from the drain to the source, as well as control of the threshold voltage (V t ) in BC MOSFET devices become worse, as described in pages 294-303 of  Silicon Processing For The VLSI Era  by Wolf et al., the disclosure of which is incorporated herein by reference. 
     To better control BVdSS and V t  as device dimensions shrink, SC MOSFET devices have begun replacing BC MOSFET devices. SC MOSFET devices are easy to fabricate with salicide processes since the implanting step used to form the source and drain regions also implants the polysilicon gate. SC MOSFET devices, however, typically operate with a dual-gate operation that can require either a thick oxide layer during fabrication so p-dopants (such as boron) do not diffuse quickly into the surrounding areas and destroy the device performance or a hardened thin oxide layer where nitrogen is incorporated into the oxide layer. As device dimensions of SC MOSFET devices shrink into the submicron dimensions, these oxide layers unfortunately become too thin to prevent this out-diffusion of boron. Moreover, the fabrication processes for SC MOSFET devices often require additional masking steps during implantation of the channels, making manufacture more complex and costly. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides methods for forming IC devices and the structures formed from these methods. Specifically, the present invention provides methods for forming IC devices containing SC MOS transistors. In particular, the present invention provides methods for fabricating SC MOSFET devices—including both SC P-MOSFET and SC N-MOSFET devices—without the additional masking steps that would conventionally be required during manufacture of SC MOSFET devices. 
     The methods of the present invention are practiced by the steps of providing a substrate with at least one isolation region and then forming a first dielectric layer over the substrate, and then forming a first polysilicon layer over the first dielectric layer. A portion of the first polysilicon layer is then removed to expose a portion of the first dielectric layer and at least one diffusion region is formed in the substrate underlying the exposed portion of the first dielectric layer. The exposed portion of the first dielectric layer is then removed, a second dielectric layer is then formed over the first polysilicon layer and the at least one diffusion region, and a second polysilicon layer is formed over the second dielectric layer. Next, the portions of the second dielectric layer and second polysilicon layer overlying the first polysilicon layer are removed, a conductive layer is deposited over the first and second polysilicon layers, and a third dielectric layer is formed over the conductive layer. Finally, an undesired portion of the third dielectric layer, conductive layer, first and second polysilicon layers, and first and second dielectric layers is removed. The first and second dielectric layers may comprise the same or different materials and/or may be the same or different thickness. The first and second polysilicon layers may be doped independently with different dopant species. The conductive layer may be tungsten silicide. 
     The present invention fabricates SC MOSFET devices without some of the masking steps that are conventionally required, thus lowering the cost for fabricating SC MOSFET devices. The present invention also provides flat polysilicon typography during fabrication of SC MOSFETS, allowing easier masking and etching of the polysilicon and easier fabrication of smaller device features. The present invention also fabricates a tungsten silicide strapped gate that is scalable to less than 0.25 micrometers with a low resist level. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The figures presented in conjunction with this description are not actual views of any particular portion of an actual semiconducting device or component, but are merely representations employed to more clearly and fully depict the present invention. 
     FIGS. 1-12 are cross-sectional views of the steps of one exemplary process for making SC MOSFET devices according to the present invention and the resulting structures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description provides specific details, such as material thicknesses and types, in order to provide a thorough understanding of the present invention. The skilled artisan, however, would understand that the present invention may be practiced without employing these specific details. Indeed, the present invention can be practiced in conjunction with fabrication techniques conventionally used in the industry. 
     The process steps and structures described below do not form a complete process flow for manufacturing IC devices or a completed device. Only the process steps and structures necessary to understand the present invention are described below. 
     FIGS. 1-12 illustrate one exemplary SC MOSFET device according to the present invention and the steps in making this device. It will be understood, however, by those skilled in the art that other SC MOSFET devices could be formed by slight modifications of the illustrated method, such as substituting other polarities to those illustrated. 
     As shown in FIG. 1, substrate  2  is first provided. Substrate  2  may be any surface suitable for device formation, such as a semiconductor wafer, and may be doped and/or include an epitaxial layer. Preferably, substrate  2  is a silicon wafer or bulk silicon region, such as a silicon-on-insulator or silicon-on-sapphire structure. More preferably, substrate  2  is a silicon wafer lightly doped with p-type dopants, such as boron, to a concentration of 1×10 15  atoms/cm 3 . 
     Next, active device regions and isolation regions are defined in the upper surface of substrate  2  by any suitable process known in the art. One preferred process for defining these regions begins by forming pad oxide layer  4  as a stress relief layer over the surface of substrate  2 . Pad oxide layer  4  is then thermally grown or deposited by chemical vapor deposition (CVD) to a thickness of about 50 to about 150 angstroms on substrate  2 . At least one p-well region  6  is then formed to a depth of about 10,000 to about 20,000 angstroms in substrate  2  by a blanket implant of p-type dopant, such as boron, through pad oxide layer  4  to a concentration of 2×10 16  atoms/cm 3 . Silicon nitride layer  8  is then deposited over pad oxide layer  4 . Any suitable process known in the art, such as a CVD process, can be employed to deposit silicon nitride layer  8  to a thickness ranging from about 500 to about 2000 Å. 
     Next, as shown in FIG. 2, a pattern and etch process is employed to remove portions of silicon nitride layer  8  and pad oxide layer  4  above portions of substrate  2  where isolation regions will be formed. Silicon nitride layer  8   a  and pad oxide layer  4   a  will remain over regions of substrate  2  where active devices will be formed. Any suitable pattern and etch process known in the art, such as a photolithographic pattern and etch process, can be used to remove the portions of silicon nitride layer  8  and pad oxide layer  4 . For example, a photoresist film can be spun on silicon nitride layer  8 , developed, and portions thereof removed to leave photoresist mask  9  (shown by the broken line in FIG. 1) above silicon nitride layer  8 . Using photoresist mask  9 , the undesired portions of silicon nitride layer  8  and pad oxide layer  4  are then removed by any suitable anisotropic etching process to obtain silicon nitride layer  8   a  and pad oxide layer  4   a . Photoresist mask  9  may then be removed by any suitable process known in the art which does not attack or degrade silicon nitride layer  8   a  or substrate  2 . 
     As shown in FIG. 3, isolation regions  10  are then formed in substrate  2 . Isolation regions  10  may be formed by any suitable process which employs silicon nitride layer  8   a  as a mask, such as a trench-and-refill or local oxidation of silicon (LOCOS) process. Preferably, as illustrated in FIG. 3, isolation regions  10  are field oxide regions formed by a shallow trench isolation process. The thickness of isolation regions  10  may range from about 2000 to about 4000 angstroms. After forming isolation regions  10 , silicon nitride layer  8   a  and pad oxide layer  4   a  are removed by any suitable process known in the art. Preferably, silicon nitride layer  8   a  and pad oxide layer  4   a  are removed by a wet etch process using phosphoric acid and/or hydrofluoric acid to leave substrate  2  with isolation regions  10 . 
     Then, as illustrated in FIG. 4, sacrificial oxide layer  12  is formed over substrate  2 . Sacrificial oxide layer  12  may be formed by any suitable thermal oxidation process which grows the sacrificial oxide layer to a thickness of about 100 to about 400 angstroms. Next, a field implant step is performed to implant dopants into isolation regions  10 . In this field implant step, portions of the structure of FIG. 4 not containing isolation regions  10  are masked by implant mask  11  (shown by the broken line in FIG. 4) using any suitable masking process in the art. The desired dopants are then implanted selectively into the isolation regions  10  using implant mask  11 . Preferably, boron ions are implanted at an energy ranging from about 50 to about 150 keV. 
     After the field implant step, implant mask  11  is removed by any suitable process in the art and a blanket enhancement implant performed. The blanket enhancement implant is performed across the entire surface of the structure of FIG.  4 . Any suitable implantation process known in the art can be employed, such as implanting boron ions at an energy ranging from about 15 to about 100 keV. The enhancement implant step, also known as an adjust implant, is performed to enhance the implantation steps previously performed in the fabrication process and regulate the dopant concentration and control the threshold voltage of the MOSFET. During the enhancement implant step, sacrificial oxide layer  12  reduces the channeling effects of the dopants in substrate  2 . Once the enhancement implant has been performed, sacrificial oxide layer  12  is removed by any suitable removal process, such as an etching process, which does not degrade isolation regions  10  or substrate  2 . 
     Next, as depicted in FIG. 5, dielectric layer  14  is formed over substrate  2  and, optionally, isolation regions  10 . Any dielectric material suitable as a gate dielectric, such as silicon oxide or silicon nitride, can be used as dielectric layer  14 . Preferably, dielectric layer  14  is a silicon oxide layer formed by thermally oxidizing the preferred silicon substrate  2  to form a high-quality silicon oxide layer with little to no contamination. The preferred silicon oxide layer is formed primarily over the exposed regions of substrate  2 , but may also be formed over isolation regions  10 , especially if the silicon oxide layer is deposited, rather than thermally grown. The thickness of dielectric layer  14  may range from about 30 to about 150 angstroms. 
     Next, polysilicon layer  16  is formed over dielectric layer  14 . Polysilicon layer  16  may be formed by any suitable deposition method known in the art, such as physical or chemical vapor deposition (CVD). Preferably, polysilicon layer  16  is deposited by CVD to a thickness ranging from about 500 to about 2000 angstroms. Polysilicon layer  16  is then doped with an n-type dopant, such as arsenic, by any suitable ion implantation process known in the art. Alternatively, polysilicon layer  16  can be in-situ doped during deposition by including a gas containing the desired n-type dopant in the deposition atmosphere. 
     Optionally, as illustrated in FIG. 6, silicon nitride layer  18  can then be deposited over polysilicon layer  16 . Silicon nitride layer  18  can be deposited by any suitable method known in the art, such as a CVD process, to a thickness ranging from about 100 to about 300 angstroms. As explained below, silicon nitride layer  18  serves as an etch stop during a later planarization process and prohibits subsequent oxidation steps from oxidizing the polysilicon. 
     Next, as shown in FIG. 7, photoresist mask  20  (represented by the broken line) is formed over polysilicon layer  16 . Any suitable process known in the art can be employed to form photoresist mask  20  to a thickness ranging from about 5000 to about 10,000 angstroms. Preferably, photoresist mask  20  is formed by depositing a photoresist layer, developing the photoresist layer, and removing portions of the developed layer by any suitable process. Using photoresist mask  20 , the exposed portions of polysilicon layer  16  are removed, preferably by using an etching process, such as a dry etch process, using chlorine-based chemistry. 
     As shown in FIG. 8, removing a portion of polysilicon layer  16  exposes a portion of dielectric layer  14 . N-well  22  is then formed in the surface of substrate  2  underlying the exposed portion of dielectric layer  14 . N-well  22  may be formed by any suitable process known in the art which yields the desired impurity profile for n-well  22 . Preferably, n-well  22  is formed by high-energy ion implantation of the desired n-type dopant, such as arsenic or phosphorous, through dielectric layer  14  at an energy level of 150 keV to 1000 MeV. Alternatively, to obtain the desired n-well profile, multiple implants of arsenic or phosphorous can be performed at multiple energies ranging from about 100 KeV to about 3.0 MeV. To compensate for the enhancement implantation performed after the field implantation, a low energy n-type ion implantation process is then performed using arsenic at an energy of 5 to 100 keV in order to obtain the desired threshold voltage for the device that will be formed in this region. 
     Following the high-energy and low-energy implantation steps, photoresist mask  20  is then removed. Photoresist mask  20  can be removed by any suitable etching process known in the art which does not degrade polysilicon layer  16 . The portion of dielectric layer  14  remaining on substrate  2  after the high-energy and low-energy implantation steps is then removed. Any suitable removal process which does not degrade polysilicon layer  16 , substrate  2 , or isolation regions  10  can be employed to remove this remaining portion of dielectric layer  14 . 
     Next, as shown in FIG. 9, dielectric layer  24  is formed over polysilicon layer  16  and substrate  2 . Any dielectric material suitable as a gate dielectric, such as silicon oxide, silicon nitride, or silicon oxynitride, can be used as dielectric layer  24 . Dielectric layer  24  may be the same or a different material than dielectric layer  14 . Preferably, dielectric layer  24  is a silicon oxide layer formed by a conformal deposition process to form a high-quality silicon oxide layer with little to no contamination with impurities. The preferred silicon oxide layer  24  is formed over the exposed regions of substrate  2  and over polysilicon layer  16  if silicon nitride layer  18  has not been formed over polysilicon layer  16 . The thickness of dielectric layer  24  can be the same or different than the thickness of dielectric layer  14 , and may range from about 30 to about 100 angstroms. 
     Polysilicon layer  26  is then formed over dielectric layer  24 . Polysilicon layer  26  may be formed by any suitable deposition method in the art, such as physical or chemical vapor deposition. Preferably, the polysilicon is deposited by CVD to a thickness ranging from about 500 to about 2000 angstroms. Polysilicon layer  26  is then doped with a p-type dopant, such as boron, by a suitable ion implantation process known in the art, such as the processes used to dope polysilicon layer  16 . Alternatively, polysilicon layer  26  can be in-situ doped during deposition of polysilicon layer  26  by including a gas containing the desired p-type dopant in the deposition ambient. 
     Next, as depicted in FIG. 10, polysilicon layer  26  and dielectric layer  24  above polysilicon layer  16  and dielectric layer  14  are removed. Any suitable process which removes polysilicon layer  26  and dielectric layer  24  without removing the polysilicon layer  16  and dielectric layer  14  can be employed in the present invention. Preferably, this removal process is performed by planarization, such as an abrasive planarization process. More preferably, the planarization is chemical mechanical planarization using a peroxide based slurry or fixed abrasives. The planarization proceeds until the surface of polysilicon layer  16  is exposed. If silicon nitride layer  18  has been deposited (see FIG.  6 ), the planarization proceeds until the surface of silicon nitride layer  18  is exposed, after which the silicon nitride layer is removed by a suitable etching process which does not attack polysilicon layers  16  or  26  or dielectric layer  24 . 
     As shown in FIG. 11, conductive layer  28  is then deposited over polysilicon layers  16  and  26 . Conductive layer  28  acts as a metal contact, or conductive bridge, connecting polysilicon layers  16  and  26 . Accordingly, conductive layer  28  can comprise any conductive material known in the art, such as metals and metal alloys. Preferably, conductive layer  28  is tungsten silicide since tungsten silicide, unlike other conductive materials like titanium silicide, is scalable to less than 0.25 micrometers with a low resist level. The preferred tungsten silicide layer may be deposited by CVD to a thickness ranging from about 200 to about 1500 angstroms. 
     Next, dielectric layer  30  is deposited over conductive layer  28 . Dielectric layer  30  may comprise any suitable dielectric material known in the art, such as silicon nitride or silicon oxide. Dielectric layer  30  is deposited by a suitable CVD process known in the art to a thickness ranging from about 1000 to about 4000 angstroms. 
     A photoresist layer is then deposited and patterned similar to the patterning described above to form photoresist mask  31  (shown by the broken line in FIG.  11 ). Portions of conductive layer  28 , polysilicon layers  16  and  26 , and dielectric layers  14 ,  24 , and  30  not needed for the desired MOSFET device are then removed by a suitable etching process. For example, as shown in FIG. 12, this etching process could remove such layers where subsequent metal contacts are to connect with n-well  22  and p-well  6 . Photoresist mask  31  could then be removed and subsequent processing performed to complete the integrated circuit device. 
     By using the above process, additional mask steps need not be employed. Conventionally, using the structure in FIG. 12 as an example, surface channels (implanted areas immediately below dielectric layers  14  and  24 ) in MOSFET devices would have been separately formed by: first, fabricating the first transistor (dielectric layer  14  and polysilicon layer  16 ) and second transistor (dielectric layer  24  and polysilicon layer  26 ); second, masking the first transistor while implanting the second transistor to form the surface channel under the second transistor; and third, masking the second transistor while implanting the first transistor to form the surface channel under the first transistor. The above process allows these additional masking steps to be eliminated, thereby decreasing the cost and complexity for SC MOSFET device fabrication. In addition, the inventive process uses—as an example—the mask that defines the SC P-MOSFET as the n-well mask, rather than two separate masks. Further, both polysilicon layers can be in situ doped and, therefore, the inventive process eliminates the need of implanting two polysilicon layers, cleaning steps, and activation steps. 
     Further enhancements can be performed in the process. Dielectric layers  14  and  24  can be doped. Moreover, dielectric layers  14  and  24  can be made of the same or different materials. Further, dielectric layer  24  can inhibit boron punch-through often exhibited during boron doping by either being a hardened oxide layer or by using silicon oxynitride as the material for dielectric layer  24 . 
     Having thus described in detail the preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.