Abstract:
A memory device, and method of make same, having a substrate of semiconductor material of a first conductivity type, first and second spaced-apart regions in the substrate of a second conductivity type, with a channel region in the substrate therebetween, a conductive floating gate over and insulated from the substrate, wherein the floating gate is disposed at least partially over the first region and a first portion of the channel region, a conductive second gate laterally adjacent to and insulated from the floating gate, wherein the second gate is disposed at least partially over and insulated from a second portion of the channel region, and a stressor region of embedded silicon carbide formed in the substrate underneath the second gate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to split-gate, non-volatile Flash memory cells and methods of making the same, and more particularly memory cells having a stressor region in the substrate under the word line gate. 
     BACKGROUND OF THE INVENTION 
     Split gate non-volatile Flash memory cells having a select gate, a floating gate, a control gate and an erase gate are well known in the art. See for example U.S. Pat. Nos. 6,747,310, 7,868,375 and 7,927,994, and published application 2011/0127599, which are all incorporated herein by reference in their entirety for all purposes. Such split gate memory cells include a channel region in the substrate that extends between the source and drain. The channel region has a first portion underneath the floating gate (hereinafter called the FG channel, the conductivity of which is controlled by the floating gate), and a second portion underneath the select gate (hereinafter the “WL channel” (wordline), the conductivity of which is controlled by the select gate). 
     In order to increase performance and reduce operating voltages for read, program and erase, various insulation and other thicknesses can be optimized. However, there is a need for further cell optimization not achievable by cell geometry optimization alone. 
     BRIEF SUMMARY OF THE INVENTION 
     Superior cell optimization has been achieved in a memory device having a substrate of semiconductor material of a first conductivity type, first and second spaced-apart regions in the substrate of a second conductivity type, with a channel region in the substrate therebetween, a conductive floating gate over and insulated from the substrate, wherein the floating gate is disposed at least partially over the first region and a first portion of the channel region, a conductive second gate laterally adjacent to and insulated from the floating gate, wherein the second gate is disposed at least partially over and insulated from a second portion of the channel region, and a stressor region of embedded silicon carbide formed in the substrate underneath the second gate. 
     A method of forming a memory device includes providing a substrate of semiconductor material of a first conductivity type, forming first and second spaced-apart regions in the substrate of a second conductivity type, with a channel region in the substrate therebetween, wherein the channel region has first and second portions, forming a stressor region of embedded silicon carbide in the substrate, forming a conductive floating gate over and insulated from the substrate, wherein the floating gate is disposed at least partially over the first region and the first portion of the channel region, and forming a conductive second gate laterally adjacent to and insulated from the floating gate, wherein the second gate is disposed at least partially over and insulated from the second portion of the channel region and over the stressor region. 
     Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross sectional view of a four gate memory cell with the stressor region of the present invention. 
         FIGS. 2A to 2M  are side cross sectional views illustrating the steps in the process to make a non-volatile memory cell according the present invention. 
         FIG. 3  is a side cross sectional view of a three gate memory cell with the stressor region of the present invention. 
         FIG. 4  is a side cross sectional view of a two gate memory cell with the stressor region of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a cross-sectional view of a non-volatile memory cell  10  of the present invention. While the memory cell  10  of  FIG. 1  is exemplary of the type that can benefit from the techniques of the present invention, it is only one example and should not be deemed to be limiting. The memory cell  10  is made in a substantially single crystalline substrate  12 , such as single crystalline silicon, of a first conductivity type (e.g. P conductivity type). Within the substrate  12  is a region  14  of a second conductivity type. If the first conductivity type is P then the second conductivity type is N. Spaced apart from region  14  is another region  16  of the second conductivity type. Between the regions  14  and  16  is a channel region  18  which comprises the WL channel  18   a  and the FG channel  18   b , and which provides for the conduction of charges between region  14  and region  16 . 
     Positioned above, and spaced apart and insulated from the substrate  12  is a select gate  20 , also known as the word line  20 . The select gate  20  is positioned over a first portion of the channel region  18  (i.e. the WL channel portion  18   a ). The WL channel portion  18   a  of the channel region  18  immediately abuts the region  14 . Thus, the select gate  20  has little or no overlap with the region  14 . A floating gate  22  is also positioned above and is spaced apart and is insulated from the substrate  12 . The floating gate  22  is positioned over a second portion of the channel region  18  (i.e. the FG channel portion  18   b ) and a portion of the region  16 . The FG channel portion  18   b  of the channel region  18  is distinct from the WL channel portion  18   a  of the channel region  18 . Thus, the floating gate  22  is laterally spaced apart and is insulated from and is adjacent to the select gate  20 . An erase gate  24  is positioned over and spaced apart from the region  16 , and is insulated from the substrate  12 . The erase gate  24  is laterally insulated and spaced apart from the floating gate  22 . The select gate  20  is to one side of the floating gate  22 , with the erase gate  24  to another side of the floating gate  22 . Finally, positioned above the floating gate  22  and insulated and spaced apart therefrom is a control gate  26 . The control gate  26  is positioned between and insulated from the erase gate  24  and the select gate  20 . 
     The WL channel  18   a  includes a stressor region  19  of embedded silicon carbide in the substrate  12  underneath the select gate  20 . The stressor region  19  induces a tensile strain in the WL channel  18   a  in the form of a strained silicon layer  19   a  above stressor region  19 . The stressor region  19  and strained silicon layer  19   a  enhances electron mobility, which in turn allows for a higher threshold voltage (Vt) to be used to reduce the off read current (Ioff) while maintaining a target read current (Iread). In addition, with its wide bandgap, the silicon carbide stressor region  19  introduces an energy barrier against electron transport, which further reduces Ioff. 
     The present invention is important as cell size is scaled down. Specifically, as the length of the WL channel portion  18   a  becomes shorter, and the word line threshold voltage (Vtwl) is optimized for the desired cell current, the leakage during the read operation can increase as much as four times. Table 1 below illustrates the changes in operational performance parameters of the memory cell  10  (without the presence of stressor region  19 ) when the word line critical dimension is scaled from 0.15 μm to 0.11 μm. 
                                                                                                           TABLE 1                           Memory cell 10   Memory cell 10               (without stressor 19)   (without stressor 19)               WL CD = 0.15 μm   WL CD = 0.11 μm                                        WL tox   32   Å   22   Å                Iread   ≈30 μA   ≈22 μA               Vwl = Vcg = 1.8 V,   Vwl = Vcg = 1.2 V,               Vbl = 0.6 V, Vsl = 0 V   Vbl = 0.6 V, Vsl = 0 V                Ioff   ≈55   nA/kbit   ≈226   nA/Kbit           Vt   0.42   V   0.37   V                        
The smaller memory cell dimensions result in the off read current (Ioff) quadrupling, along with Vt dropping over ten percent.
 
     However, Table 2 below illustrates the operation performance parameters of memory cell  10  with a 0.11 μm word line critical dimension, without and then with stressor region  19 . 
                                                                                                           TABLE 2                           Memory cell 10   Memory cell 10               (without stressor 19)   (with stressor 19)               WL CD = 0.11 μm   WL CD = 0.11 μm                                        WL tox   22   Å   22   Å                Iread   ≈22 μA   ≈22.5 μA               Vwl = Vcg = 1.2 V   Vwl = Vcg = 1.2 V               Vbl = 0.6 V, Vsl = 0 V   Vbl~0.6 V, Vsl = 0 V                Ioff   ≈226   nA/Kbit   ≈57   nA/Kbit           Vt   0.37   V   0.52   V                        
The inclusion of stressor region  19  results in dropping the off read current (Ioff) to essentially that of a 0.15 μm memory cell (i.e. a 4 factor drop), while maintaining a high read current (Iread), and a high voltage Vt. Therefore, stressor region  19  significantly enhances the performance of the memory cell  10  (allows for higher Vt to be used to reduce Ioff while maintaining the target Iread, and reduces Ioff further by introducing an energy barrier against electron transport).
 
       FIGS. 2A-2M  illustrate cross-sectional views of the steps in the process to make a 4-gate non-volatile memory cell  10 . Commencing with  FIG. 2A , a layer of silicon dioxide  40  is formed on substrate  12  (e.g. P type single crystalline silicon). For 90-120 nm processes, the layer  40  of silicon dioxide can be on the order of 80-100 angstroms. Thereafter a first layer  42  of polysilicon (or amorphous silicon) is deposited or formed on the layer  40  of silicon dioxide. The first layer  42  of polysilicon can be on the order of 300-800 angstroms. The first layer  42  of polysilicon is subsequently patterned in a direction perpendicular to the select gate  20 . 
     Referring to  FIG. 2B , another insulating layer  44 , such as silicon dioxide (or even a composite layer, such as ONO) is deposited or formed on the first layer  42  of polysilicon. Depending on whether the material is silicon dioxide or ONO, the layer  44  can be on the order of 100-200 angstroms. A second layer  46  of polysilicon is then deposited or formed on the layer  44 . The second layer  46  of polysilicon can be on the order of 500-4000 angstroms thick. Another layer  48  of insulator is deposited or formed on the second layer  46  of polysilicon and used as a hard mask during subsequent dry etching. In a preferred embodiment, the layer  48  is a composite layer, comprising silicon nitride  48   a , silicon dioxide  48   b , and silicon nitride  48   c , where the dimensions can be 200-600 angstroms for layer  48   a , 200-600 angstroms for layer  48   b , and 500-3000 angstroms for layer  48   c.    
     Referring to  FIG. 2C , photoresist material (not shown) is deposited on the structure shown in  FIG. 2B , and a masking step is formed exposing selected portions of the photoresist material. The photoresist is developed and using the photoresist as a mask, the structure is etched. The composite layer  48 , the second layer  46  of polysilicon, the insulating layer  44  are then anisotropically etched, until the first layer  42  of polysilicon is exposed. The resultant structure is shown in  FIG. 2C . Although only two “stacks”: S1 and S2 are shown, it should be clear that there are number of such “stacks” that are separated from one another. 
     Referring to  FIG. 2D , silicon dioxide  49  is deposited or formed on the structure. This is followed by the deposition of silicon nitride layer  50 . The silicon dioxide  49  and silicon nitride  50  are anisotropically etched leaving a spacer  51  (which is the combination of the silicon dioxide  49  and silicon nitride  50 ) around each of the stacks S1 and S2. The resultant structure is shown in  FIG. 2D . 
     Referring to  FIG. 2E , a photoresist mask is formed over the regions between the stacks S1 and S2, and other alternating pair stacks. For the purpose of this discussion, this region between the stacks S1 and S2 will be called the “inner region” and the regions not covered by the photoresist, shall be referred to as the “outer regions”. The exposed first polysilicon  42  in the outer regions is anisotropically etched. The oxide layer  40  is similarly anisotropically etched. The resultant structure is shown in  FIG. 2E . 
     Referring to  FIG. 2F , the photoresist material is removed from the structure shown in  FIG. 2E . A layer of oxide  52  is then deposited or formed. The oxide layer  52  is then subject to an anisotropical etch leaving spacers  52 , adjacent to the stacks S1 and S2. The resultant structure is shown in  FIG. 2F . 
     Referring to  FIG. 2G , photoresist material is then deposited and is masked leaving openings in the inner regions between the stacks S1 and S2. Again, similar to the drawing shown in  FIG. 2E , the photoresist is between other alternating pairs of stacks. The polysilicon  42  in the inner regions between the stacks S1 and S2 (and other alternating pairs of stacks) is anisotropically etched. The silicon dioxide layer  40  beneath the polysilicon  42  may also be anisotropically etched. The resultant structure is subject to a high voltage ion implant forming the regions  16 . The resultant structure is shown in  FIG. 2G . 
     Referring to  FIG. 2H , the oxide spacer  52  adjacent to the stacks S1 and S2 in the inner region is removed by e.g. a wet etch or a dry isotropic etch. Referring to  FIG. 2I , the photoresist material in the outer regions of the stacks S1 and S2 is removed. Silicon dioxide  54  is deposited or formed everywhere. The resultant structure is shown in  FIG. 2I . 
     Referring to  FIG. 2J , the structure is once again covered by photoresist material and a masking step is performed exposing the outer regions of the stacks S1 and S2 and leaving photoresist material covering the inner region between the stacks S1 and S2. An oxide anisotropical etch is performed, to reduce the thickness of the spacer  54  in the outer regions of the stack S1 and S2, and to completely remove silicon dioxide from the exposed silicon substrate  12  in the outer regions. The resultant structure is shown in  FIG. 2J . 
     Referring to  FIG. 2K , a silicon carbide region is formed by epitaxial growth to form stressor region  19  of embedded silicon carbide in the substrate  12 . Before, any WL channel implant (in the WL channel region  18   a ), a photo lithographic masking process is used to selectively etch silicon from the surface of substrate  12  to form a recess region where silicon carbide layer is intended. Then, a silicon carbide layer with the desired thickness is grown in the recess region by selective epitaxy. Next, a thin layer of Si is deposited via chemical vapor deposition on top of the silicon carbide layer (resulting in strained silicon layer  19   a ). Then, a thin layer  56  of silicon dioxide is formed on the structure. This oxide layer  56  is the gate oxide between the select gate and the substrate  12 . 
     Referring to  FIG. 2L , polysilicon is deposited everywhere, which is then subject to an anisotropical etch forming spacers in the outer regions of the stack S1 and S2 which form the select gates  20  of two memory cells  10  adjacent to one another sharing a common region  16 . In addition, the spacers within the inner regions of the stacks S1 and S2 are merged together forming a single erase gate  24  which is shared by the two adjacent memory cells  10 . 
     Referring to  FIG. 2M , a layer of insulator  62  is deposited on the structure, and etched anisotropically to form spacers  62  next to the select gates  20 . Insulator  62  can be a composite layer comprising silicon dioxide and silicon nitride. Thereafter, an ion implant step is performed forming the regions  14 . Each of these memory cells on another side share a common region  14 . Insulators and metallization layers are subsequently deposited and patterned to form bit line  70  and bit line contacts  72 . The operations of program, read and erase and in particular the voltages to be applied may be the same as those as set forth in U.S. Pat. No. 6,747,310, whose disclosure has been incorporated herein by reference in its entirety. The resulting memory cells  10  are illustrated in  FIG. 2M . 
     The formation of stressor regions  19  in the WL channel can be implemented in other split gate memory cell configurations. For example, U.S. Pat. No. 7,315,056 discloses a split gate memory cell with three gates (a floating gate, a control gate and a program/erase gate), and is incorporated herein by reference in its entirety for all purposes.  FIG. 3  illustrates the three gate memory cell modified to include stressor regions  19  in the WL channel. Specifically, this memory cell configuration includes the floating gate  80 , control gate  82  laterally adjacent to the floating gate  80  and extending up and over floating  80 , and a program/erase gate  84  on the other side of floating gate  80  and extending up and over floating gate  80 . 
     U.S. Pat. No. 5,029,130 discloses a split gate memory cell with two gates (a floating gate and a control gate), and is incorporated herein by reference in its entirety for all purposes.  FIG. 4  illustrates the two gate memory cell modified to include stressor regions  19  in the FG channel. Specifically, this memory cell configuration includes the floating gate  90  and a control gate  92  laterally adjacent to the floating gate  90  and extending up and over floating  90 . 
     It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, references to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more of the claims. Materials, processes and numerical examples described above are exemplary only, and should not be deemed to limit the claims. For example, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed, but rather in any order that allows the proper formation of the memory cell of the present invention. Lastly, single layers of material could be formed as multiple layers of such or similar materials, and vice versa. 
     It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed therebetween) and “indirectly on” (intermediate materials, elements or space disposed therebetween). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed therebetween) and “indirectly adjacent” (intermediate materials, elements or space disposed there between), “mounted to” includes “directly mounted to” (no intermediate materials, elements or space disposed there between) and “indirectly mounted to” (intermediate materials, elements or spaced disposed there between), and “electrically coupled” includes “directly electrically coupled to” (no intermediate materials or elements there between that electrically connect the elements together) and “indirectly electrically coupled to” (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements therebetween, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements therebetween.