Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/205,791 filed Jul. 26, 2002, now U.S. Pat. No. 6,849,485, the entirety of which is hereby incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method, and a design layout used, to optimize the yield of dual gate metal oxide semiconductor field effect transistor (MOSFET) devices. 
   (2) Description of Prior Art 
   Specific devices designed to provide dual voltage applications, particularly for the deep sub-micron technology, are achieved using two different gate insulator layer thicknesses, sometimes referred to as a dual gate oxide technology. However the process sequences used to form dual gate insulator layers can result in unwanted device leakage phenomena. For example a process used to form two different gate insulator layers entails removal of a first insulator layer, exposing first portions, or active device regions of a semiconductor substrate in which a thin gate insulator layer will be regrown on. Portions of unremoved first insulator layer located on second areas of the semiconductor substrate, now subjected to conditions used to grow the thin gate insulator layer will increase in thickness resulting in a second gate insulator layer located on second portions, or portions of the semiconductor substrate to be used for thicker gate insulator devices, with the second gate insulator layer greater in thickness than the thin gate insulator layer, again located on the thin gate insulator active device portions of the semiconductor substrate. 
   Although the process sequence described above can be used to obtain two gate insulator thicknesses, several vulnerable situations can be present. First, shallow trench isolation (STI) regions comprised of insulator filled shallow trench shapes is subjected to the process used to remove the first insulator layer. In addition to global recessing of the STI regions, local recessing or notching of the STI region, at the interface of the STI—active device region, can result in regions in which conductive materials used for gate and silicide regions, can be difficult to remove, resulting in gate to substrate leakage or shorts. In addition the dry etch definition of a gate structure, performed exposing the thin gate insulator in the active device region for the devices employing thin gate insulator layers, can result in silicon loss if the thin gate layer is removed during the gate definition procedure, again resulting in unwanted leakages as a result of substrate damage. 
   The present invention will describe a novel process sequence and mask layout in which the amount of STI area subjected to the process used to clear a subsequent active device region for thin gate insulator formation, is reduced. In addition this invention will describe a novel process sequence and mask layout, in which the amount of thin gate insulator area subjected to the gate structure definition procedure, is reduced. Prior art, such as: Yu et al, in U.S. Pat. No. 6,225,167 B1; Yu, in U.S. Pat. No. 6,171,911 B1; Fang et al, in U.S. Pat. No. 5,668,035; and Chen et al, in U.S. Pat. No. 6,074,915; all describe processes used to form dual gate insulator layers. However these prior arts do not describe the novel mask layout—process sequence combination disclosed in this present invention, in which STI recess and silicon damage is reduced during the process sequence used to fabricate dual gate insulator layers. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to fabricate a semiconductor device featuring two gate insulator thicknesses, a thin gate insulator layer for low voltage applications, and a thicker gate insulator layer for higher voltage applications. 
   It is another object of this invention to reduce STI recessing and notching occurring during insulator removal in the active device region used for the thin gate insulator, or low voltage application, via use of a novel photolithographic mask layout. 
   It is still another object of this invention to prevent silicon damage in the active device region of the thin gate insulator layer application, during definition of a gate structure, via use of a novel process sequence allowing the presence of thicker insulator layer to be used as an etch stop layer. 
   In accordance with the present invention a method of reducing silicon damage and reducing notching and recessing of insulator filled shallow trench regions, during the fabrication of a thin gate insulator gate layer, of dual gate insulator layer device, is described. A first insulator layer is grown on the portion of semiconductor substrate exposed in an active device region to be used for the thin gate insulator layer of the dual gate insulator device, with the active device region located between shallow trench isolation (STI) regions. Photolithographic procedures result in exposure of a first portion of the first gate insulator layer, while a second portion of the first gate insulator layer, and portions of the STI regions are not exposed. Removal of exposed portions of the first insulator layer via wet etch procedures result in an active device region now comprised of a first area of exposed semiconductor substrate and second areas of unetched first insulator layer located adjacent to the first area of exposed semiconductor substrate. Thermal growth of the thin gate insulator layer on the exposed first area of the active device region also results in additional insulator growth on portions of first insulator layer located in second areas of the active device region, resulting in a thicker second insulator layer. Deposition of, and patterning of a conductive layer, results in definition of a conductive gate structure overlying the thin gate insulator layer in the first area of the active device region. The thicker second insulator layer, located in the second areas of the active device region, exposed during the gate structure definition procedure, prevented damage to, and loss of, underlying semiconductor material. A thicker gate insulator layer, of the dual gate insulator layer, previously grown and located in a different active device region of the semiconductor substrate, is also overlaid with a conductive gate structure during gate structure definition procedure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other advantages of this invention are best described in the preferred embodiments with reference to the attached drawings that include: 
       FIGS. 1A and 2A , which schematically show top views of the active device region used to accommodate the thin gate insulator layer, at various stages of the fabrication; and 
       FIGS. 1B ,  2 B, and  3 – 6 , which schematically, in cross-sectional style, show key stages of fabrication use to form the thin gate insulator layer of a dual gate insulator device. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method of forming a thin gate insulator layer for a device comprised with dual gate insulator layers, featuring a reduction of silicon damage, and a reduction of STI recessing and notching, both occurring during the thin gate insulator fabrication sequence, will now be described in detail. Semiconductor substrate  1 , comprised of single crystalline silicon, featuring a &lt;100&gt; crystallographic orientation, is used and schematically shown in  FIG. 1B . Shallow trench isolation (STI) regions  2 , are next formed in portions of semiconductor substrate  1 . This is accomplished via definition of shallow trench shapes in semiconductor substrate  1 , via photolithographic and dry etch procedure, to a depth between about 2000 to 6000 Angstroms, followed by filling of the shallow trench shapes with silicon oxide obtained via chemical vapor deposition procedures, using tetraethylorthosilicate (TEOS), as a source. Unwanted portions of the silicon oxide are removed via chemical mechanical polishing procedures resulting in the attainment of STI regions  2 . Portions of semiconductor substrate  1 , not occupied by STI regions  2 , are used as active device regions  20 , the regions of the dual gate insulator device used to accommodate the thin gate insulator component. First insulator layer  3 , comprised of silicon dioxide, is next thermally grown on active device region  20 , to a thickness between about 50 to 250 Angstroms, at a temperature between about 800 to 1100° C., in an oxygen-steam ambient. This is schematically shown in cross-sectional style in  FIG. 1B , and schematically shown as a top view in  FIG. 1A   
   A photolithographic plate is next used to create photoresist shape  4 , featuring opening  5 , which exposes the portion of first insulator layer  3 , located on a first area of active device region  20 . This is shown schematically in top view  2 A, and in cross-sectional style in  FIG. 2B . The novel layout of the photolithographic plate minimized the exposure of STI regions  2 , during the subsequent removal of the portions of first insulator layer  3 , exposed in opening  5 . First insulator layer  3 , exposed in opening  5 , of photoresist shape  4 , is removed from a first area of active device region  20 , via wet etch procedures using a buffered, or dilute hydrofluoric acid solution, exposing a portion of the surface of semiconductor substrate  1 , in a first area of active device region  20 . The novel feature of the photolithographic plate is the allowance of only a first area of active device region  20 , to be cleared of first insulator layer  3 , maintaining first insulator layer  3 , in second areas of active device region  20 . In addition the design layout of this photolithographic plate minimized the amount of STI area exposed to the HF procedure, thus minimizing notching at the active device region—STI interface, and limiting the amount of STI area subjected to recessing during the HF procedure. 
   After removal of photoresist shape  4 , via plasma oxygen ashing procedures, thin gate insulator layer  7 , is formed on the portion of semiconductor substrate  1 , exposed in the first area of active device region  20 . Thin gate insulator layer  7 , comprised of silicon dioxide, is thermally grown to a thickness between about 50 to 200 Angstroms, at a temperature between about 800 to 1100° C., in an oxygen-steam ambient. The same thermal oxidation procedures results in additional growth of first insulator layer  3 , resulting in second insulator layer  6   a , on second areas of active device region  20 . Second insulator layer  6   a , is comprised of silicon dioxide at a thickness between about 100 to 400 Angstroms. This is schematically shown in  FIG. 3 . 
   Conductive layer  8   a , such as an in situ doped polysilicon layer, is next deposited via low pressure chemical vapor deposition (LPCVD), procedures, to a thickness between about 1000 to 2500 Angstroms. The polysilicon layer can also be deposited intrinsically rather than doped via implantation procedures. In addition if decreased word line resistance is desired, conductive layer  8   a  can be comprised of materials comprised of lower resistivity such as metal silicide or metal layers. Photoresist shape  9 , is then formed on the portion of conductive layer  8   a , which overlays only thin gate insulator layer  7 , in the first area of active device region  20 . This is schematically shown in  FIG. 4 . 
   An anisotropic reactive ion etching (RIE), procedure is next employed to remove regions of conductive layer  8   a , not covered by photoresist shape  9 , resulting in conductive gate structure  8   b , shown schematically in  FIG. 5 . The anisotropic RIE procedure is performed using Cl 2  or SF 6  as an etchant for conductive layer  8   a , and although a high etch rate ratio of conductive layer  8   a , to silicon dioxide, between about 10 to 1, to 20 to 1, does exist, at the conclusion of the conductive gate definition procedure portions of thin gate insulator layer  7 , can be exposed and subjected to the RIE procedure resulting in partial or complete removal of exposed portions of thin gate insulator layer located adjacent to conductive gate structure  8   b . However it should be noted that the thicker, second insulator layer  6   a , located in second areas of active device region  20 , exposed at the conclusion of the conductive gate definition procedure, protected all of the second area of active device region  20 , from possible silicon damage. This was made possible via the previous implementation of the novel photolithographic plate design which allowed first insulator layer  3 , to be protected from the wet etch removal procedure used to clear first insulator layer  3 , from only a first area of active device region  20 . Second insulator layer  6   a , subjected to the gate structure definition procedure is now reduced to insulator layer  6   b , at a thickness between about 150 to 300 Angstroms. Thus the use of the novel photolithographic plate design, in addition to the above described process sequence, allowed reduced areas of STI notching and recessing, as well as reduced areas of possible silicon damage of active device region  20 , reducing the risk of junction leakage. 
   The conclusion of the process sequence, and novel photolithographic plate design layout, employed in the fabrication of the thin gate insulator layer of a dual gate insulator device, is next described. The thick gate insulator layer of the dual gate insulator layer, at this stage of processing may have been formed in other active device regions, prior to the formation of the thin gate insulator layer described in the previous sections of this invention, or second insulator layer  6   b , can be used as the thick gate insulator layer. In either case conductive gate structure  8   b , was defined overlying each gate insulator layer. The conclusion of this process sequence however will be applied to, and described for the thin gate insulator layer component of the dual gate insulator device. After removal of photoresist shape  9 , via plasma oxygen ashing procedures, lightly doped source/drain region  10 , is formed via implantation of ions into portions of semiconductor substrate  1 , or into portions of active device region  20 , not covered by conductive gate structure  8   b . If the dual gate device is to be an N channel device, lightly doped source/drain region  10 , is formed in a P well region, (not shown in the drawings), in semiconductor substrate  1 , via implantation of arsenic or phosphorous ions, at an energy between about 10 to 70 KeV, at a dose between about 1E13 to 1E14 atoms/cm 2 . If the dual gate device is to be a P channel device, lightly doped source/drain region  10 , is formed in an N well region, (not shown in the drawings), in semiconductor substrate  1 , via implantation of boron or BF 2  ions, at an energy between about 10 to 70 KeV, at a dose between about 1E13 to 1E14 atoms/cm 2 . Insulator spacers  11 , comprised of silicon oxide or silicon nitride, are next formed on the sides of conductive gate structure  8   b . This is accomplished via deposition of silicon oxide or silicon nitride at a thickness between about 1000 to 3000 Angstroms, via LPCVD or via plasma enhanced chemical vapor deposition (PECVD) procedures, followed by a blanket, selective, anisotropric RIE procedure using CHF 3  or CF 4  as an etchant. The anisotropic RIE procedure also results in removal of second insulator  6   b , from the top surface of the second areas of active device region  20 . Heavily doped source/drain region  12 , is next formed via ion implantation of ions into portions of the active device region not covered by conductive gate structure  8   b , or by insulator spacers  11 . Again if the dual gate device is to be an N channel device, heavily doped source/drain region  12 , is formed in a P well region located in semiconductor substrate  1 , via implantation of arsenic or phosphorous ions, at an energy between about 20 to 100 KeV, at a dose between about 5E14 to 1E16 atoms/cm 2 . If the dual gate device is to be a P channel device, heavily doped source/drain region  12 , is formed in an N well region in semiconductor substrate  1 , via implantation of boron or BF 2  ions, at an energy between about 20 to 100 KeV, at a dose between about 5E14 to 1E16 atoms/cm 2 . The result of these procedures is schematically shown in  FIG. 6 . The portion of the dual gate insulator device comprised with the thick gate insulator layer, is subjected to the same source/drain, insulator spacer formation procedures just described and used for the thin gate insulator layer component. 
   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.

Technology Category: 5