Abstract:
A plurality of core gate stacks and periphery gates on the substrate, each core gate stack and periphery gate having at least one side and first and second protective shoulders formed on said plurality of core gate stacks and periphery gates, such that a dopant can be implanted sequentially into source and drain regions of a substrate supporting the stacks to establish transistors and such that charge migration into said at least one side of the gate stacks during interlayer dielectric (ILD) formation and device metallization is prevented, at least the second shoulder being frabricated from at least one material selected from a group consisting essentially of nitride and silicon oxynitride (SiON).

Description:
RELATED APPLICATION(S) 
     This application is a divisional patent application of U.S. patent application Ser. No. 09/376,658, entitled “METHOD FOR PROTECTING GATE EDGES FROM CHARGE GAIN/LOSS IN SEMICONDUCTOR DEVICE”, filed Aug. 18, 1999, now U.S. Pat. No. 6,248,627 by the same applicants. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to fabricating semiconductor memory devices such as EEPROM or flash EEPROM, and more particularly to methods for reducing mobile ion migration into transistor gate sides during semiconductor manufacturing. 
     BACKGROUND OF THE INVENTION 
     Semiconductor chips or wafers are used in many applications, including as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. A common circuit component of flash memory devices is the transistor. In these devices, a transistor is established by forming a gate stack including a control gate and a floating gate on a silicon substrate, and then forming a source region and a drain region in the substrate beneath the gate stack by implanting ion dopants into the areas of the substrate that are to become the source and drain regions. This generally-described structure cooperates to function as a transistor. 
     After the gate stacks have been formed, subsequent manufacturing steps are undertaken to complete the semiconductor device. These subsequent steps include, among other things, the formation of additional layers of transistors along with interlayer dielectrics (ILD), followed by metallization and external electrical contact formation. It happens, however, that during these subsequent steps, mobile ions and/or other processed-induced charges can migrate into the sides of the floating gate, which undesirably can alter an electrical characteristic of the device, such as the threshold voltage or current, from its design value. This in turn adversely affects the reliability of the transistor. The present invention recognizes this prior art drawback and provides the below-noted solutions. 
     BRIEF SUMMARY OF THE INVENTION 
     A method is disclosed for establishing plural core gate transistors on a semiconductor substrate. The method includes forming plural core gate stacks on the substrate. The core gate stacks are covered with a first protective layer, portions of which are etched away such that at least intended source regions of the substrate are exposed. Dopant is next implanted into the intended source regions. A second protective layer made of nitride or silicon oxynitride (SiON) is deposited onto the first layer and portions etched away such that at least intended drain portions of the substrate are exposed. Then, dopant is implanted into the intended drain regions to thereby establish plural core transistors. Subsequent manufacturing acts can be undertaken with the first and second layers protecting at least the sides of the core gate stacks from ion migration and, hence, from unwanted charge gain or loss from the floating gate of the gate stack. 
     In a preferred embodiment, periphery gates are formed on the substrate, and the periphery stacks are covered with the first protective layer. However, etching away of the first protective layer on the periphery gates is prevented during the act of etching away portions of the first layer to expose the intended source regions. 
     As intended in the preferred embodiment, the first and second layers cover only the sides of the core gate stacks after the second etching act, as well as portions of the periphery gates. The first protective layer can have a thickness of between three hundred Angstroms (300 Å) and one thousand Angstroms (1000 Å) immediately subsequent to forming the first protective layer, and it can be made of a variety of materials such as nitride, SiON, etc. 
     As set forth in greater detail below, each core gate stack includes a source side and a drain side, and the protective layers on the source sides are thinner than the protective layers on the drain sides. Moreover, the protective layers on the source sides are shorter than the protective layers on the drain sides. A semiconductor device made according to the present method, as well as a computer incorporating the device, are also disclosed. 
     In another aspect, a method for making a flash memory device includes forming first and second protective shoulders on core gate stacks, such that dopant can be implanted into a substrate supporting the stacks to establish transistors. Charge migration into sides of the gate stacks during interlayer dielectric (ILD) formation and device metallization is prevented, however, by the protective shoulders. 
     In still another aspect, a semiconductor device includes plural gate stacks, each defining a source side and a drain side. The device also includes inner and outer protective shoulders on both sides of the stacks, whereby migration of charges into the sides is impeded. 
     Other features of the present invention are disclosed or apparent in the section entitled “DETAILED DESCRIPTION OF THE INVENTION”. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram of a semiconductor device made according to the present invention, shown in combination with a digital processing apparatus; 
     FIG. 2 is a flow chart showing the steps of the present invention; 
     FIG. 3 is a side view showing gate stacks in the periphery region and core region, prior to formation of the source and drain regions below the gate stacks; 
     FIG. 4 is a side view showing the gate stacks after deposition of the first protective layer; 
     FIG. 5 is a side view showing the gate stacks after the first layer is partially etched back over the regions into which source dopant is to be implanted; and 
     FIG. 6 is a side view showing the gate stacks after deposition and etch-back of the second protective layer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, a semiconductor device  10  is shown incorporated into a digital processing apparatus such as a computer  12 . The device  10  is made in accordance with the below disclosure. 
     Now referring to FIGS. 2 and 3, as indicated at block  14  in FIG.  2  and as shown in FIG. 3, periphery gates  16  are formed on a semiconductor substrate  18  around core area gate stacks  20  in accordance with principles known in the art. In one preferred embodiment, each periphery gate  16  includes a respective polysilicon “2” layer  22  that supports a tungsten silicide layer  24 , labelled “W si ” in FIG.  3 . On top of the tungsten silicide layer  24  is a capping layer  26 , preferably made of polysilicon, and on top of that, an anti-reflective coating layer  28  such as silicon oxynitride (SiON) or nitride. 
     In contrast to the periphery gates  16 , each core gate stack  20  includes a respective floating gate  30  and a control gate  32  disposed above the floating gate  30 , with a very thin layer  34  of, e.g., oxide nitride oxide (ONO) sandwiched therebetween. The gates  30 ,  32  can be made of polysilicon. On top of the control gate  32  is a tungsten silicide layer  36  and a polysilicon cap layer  38 , and on top of that, a silicon oxynitride (SiON) layer  40 . 
     As is known in the art, very thin oxide layers (not shown in the Figures) can be sandwiched between the substrate  18  and periphery gates  16  and core gate stacks  20 . Also, the gate stacks  16 ,  20  can be covered with a thin oxide layer, also not shown but discussed here for completeness. 
     Having thus set forth one preferred configuration of gate stacks  16 ,  20 , attention is now drawn to block  42  of FIG.  2  and to FIG. 4. A first protective layer  44  is deposited over the gate stacks  16 ,  20  as shown in FIG.  4 . In the presently preferred embodiment, the first protective layer  44  is made of nitride or SiON or other appropriate charge-blocking material. The first layer  44  preferably is deposited using plasma deposition principles, although other methods of deposition (e.g., chemical vapor deposition (CVD), low pressure CVD, PECVD) can be used. As envisioned in one embodiment of the present invention, the first protective layer  44  has a thickness “t 1 ” of between three hundred Angstroms (300 Å) and one thousand Angstroms (1000 Å). 
     A photoresist mask is next deposited over areas of the device  10  that are not to function as source dopant implantation regions, as also indicated at block  42  of FIG.  2 . Proceeding to block  46  of FIG.  2  and now referring to FIG. 5, the first protective layer  44  is partially etched back over the core stacks  20  using wet or dry chemical etch principles, with the photoresist layer masking certain regions of the device  10 . More particularly, source side portions of the first layer  44 , i.e., the portions that cover intended source regions  48  in the substrate  18 , are etched away, with drain side portions  44   d  of the first layer  44  that cover intended drain regions  50  being masked by the photoresist as shown. However, source side portions  44   s  of the first layer  44  that cover the source sides  20   s  of the core gate stacks  20  remain in the shoulder configuration shown. Although not envisioned in the preferred embodiment, the masking and implantation order of the source and drain regions can be reversed if desired. 
     Moving to block  52  of FIG. 2, dopant is implanted into the now-exposed source regions  48 . The source dopant, however, is masked from the stacks  16 ,  20  and intended drain regions  50  by the first protective layer  44 . 
     At block  54  of FIG. 2, and now referring to FIG. 6, a second protective layer  56  is deposited over the first protective layer  44  over both the periphery gates  16  and core gate stacks  20 , and then both layers  44 ,  56  are etched back to the configuration shown in FIG.  6 . Specifically, the second layer  56  conforms to the first layer  44 . Accordingly, source side portions  56   s  of the second layer  56  and the source side portions  44   s , of the first layer  44  are shorter and thinner (looking down on FIG. 6) than drain side portions  56   d ,  44   d  of the respective layers  56 ,  44 , with both protective layers  44 ,  56  after etching covering the sides of the periphery and core gate stacks  16 ,  20  but not the tops thereof. In one preferred embodiment, the second protective layer  56  is made of nitride or SiON. 
     Completing the description of FIG. 2, at block  58  drain dopant is implanted into the now-exposed drain regions  50 . Then, at block  60 , subsequent manufacturing steps including ILD deposition and metallization can be undertaken, with mobile ions and/or other process-induced charges being blocked from entering the sides of the gate stacks  16 ,  20  by the protective layers  44 ,  56 . Consequently, unwanted charge gain and/or loss from the gate stacks  16 ,  20  and other transistor characteristic changes are prevented. 
     While the particular METHOD FOR PROTECTING GATE EDGES FROM CHARGE GAIN/LOSS IN SEMICONDUCTOR DEVICE as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and is thus representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. Indeed, although a single transistor structure is shown in the drawings for clarity, the skilled artisan will appreciate that the chip  10  can include plural transistors, each substantially identical to that shown, as well as other circuit components. All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for”.