Abstract:
A method of fabricating an oxide-nitride-oxide (ONO) layer in a memory cell to retain charge well in the nitride layer includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer and oxidizing a top oxide layer, thereby causing oxygen to be introduced into the nitride layer. Another method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer and oxidizing a portion of a top oxide layer, thereby causing oxygen to be introduced into the nitride layer and depositing a remaining portion of the top oxide layer, thereby assisting in controlling the amount of oxygen introduced into the nitride layer. A farther method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer, depositing a portion of a top oxide layer and oxidizing a remaining portion of the top oxide layer, thereby causing oxygen to be introduced into the nitride layer.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to methods of fabrication of nitride read only memory (NROM) cells and arrays.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 1, to which reference is made, illustrates a typical prior art NROM cell. This cell includes a substrate  10  in which are implanted a source  12  and a drain  14  and on top of which lies an oxide-nitride-oxide (ONO) structure  16  having a layer of nitride  17  sandwiched between two oxide layers  18  and  20 . On top of the ONO structure  16  lies a gate conductor  22 . Between source  12  and drain  14  is a channel  15  formed under ONO structure  16 .  
           [0003]    Nitride section  17  provides the charge retention mechanism for programming the memory cell. Specifically, when programming voltages are provided to source  12 , drain  14  and gate conductor  22 , electrons flow towards drain  14 . According to the hot electron injection phenomenon, some hot electrons penetrate rough the lower section of silicon oxide  18 , especially if section  18  is thin, and are then collected in nitride section  17 . As is known in the art, nitride section  17  retains the received charge, labeled  24 , in a concentrated area adjacent drain  14 . Concentrated charge  24  significantly raises the threshold of the portion of the channel of the memory cell under charge  24  to be higher than the threshold of the remaining portion of the channel  15 .  
           [0004]    When concentrated charge  24  is present (i.e the cell is programmed), the raised threshold of the cell does not pert the cell to be placed into a conductive state during reading of the cell. If concentrated charge  24  is not present, the read voltage on gate conductor  22  can overcome the much lower threshold and accordingly, channel  15  becomes inverted and hence, conductive.  
           [0005]    U.S. application Ser. No. 08/861,430 filed Jul. 23, 1996 and owned by the common inventor of he present invention describes an improved NROM cell, which is programmed in one direction and read in the reverse direction.  
           [0006]    It is noted that the threshold voltage Vth of NROM cells is generally very sensitive to the voltages Vdrain and Vgate provided on the drain  14  and on the gate  22 , respectively. Furthermore, U.S. application Ser. No. 08/861,430 selects the voltages Vdrain and Vgate are selected in order to ensure that the charge trapped in a portion of the nitride layer  17  remains localized in that portion.  
           [0007]    Read only memory cells, including a nitride layer in the gate dielectric (NROM) are described, inter alia, in U.S. Pat. No. 5,168,334 to Mitchell et al. and U.S. Pat. No. 4,173,766 to Hayes.  
           [0008]    Mitchell et al. describe two processes to produce the NROM cells. In the first process, bit lines are first created in the substrate, after which the surface is oxidized. Following the oxidation, the ONO layers are added over the entire array. Polysilicon word lines are then deposited in rows over the ONO layers. Unfortunately, when an oxide layer is grown (typically under high temperature), the already present bit lines will diffuse to the side, an undesirable occurrence which limits the extent to which the cell size can be shrunk.  
           [0009]    In the second process, the ONO layers are formed over the entire array first, on top of which conducive blocks of polysilicon are formed. The bit lines are implanted between the blocks of polysilicon after which the ONO layers are etched away from on top of the bit lines. Planarized oxide is ten deposited between the polysilicon blocks after which polysilicon word lines are deposited. Mitchell et al. utilize a planarized oxide since such can be deposited rather than grown. Mitchell et al. cannot grow an oxide over the bit lines since such an oxidation operation would also grow oxide over the polysilicon blocks and the latter must be left with a very clean surface in order to connect with the polysilicon word lines. Unfortunately, planarized oxide is not a clean oxide nor does it seal around the edges of the ONO sections Furthermore, the plied oxide adds complexity and cost to the process.  
           [0010]    Hayes et al. describe au NROM cell having only an oxide-nitride (ON) layer. The cells in the array are created by forming layers of oxide, nitride and polysilicon (the latter to produce the gate) one after another and then patterning and etching these layers to form the on cells. The uncapped nitride in each cell does not hold charge well in both the vertical and lateral directions. Due to hole and hot electron conduction within the nitride, the charge to be stored will flow vertically towards the gate coveting it unless the nitride is tick and will flow laterally in the nitride in response to lateral electric fields.  
         SUMMARY OF THE PRESENT INVENTION  
         [0011]    It is an object of the present invention to provide a method of fabricating NROM cells and NROM cell arrays with improved data retention.  
           [0012]    There is therefore provided, in accordance with a preferred embodiment of the present invention, a method of fabricating an oxide-nitride-oxide (ONO) layer in a memory cell to retain charge in the nitride layer. The method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer and oxidizing a top oxide layer, thereby causing oxygen to be introduced into the nitride layer.  
           [0013]    Alternatively, in accordance with a preferred embodiment of the present invention, the method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer, oxidizing a portion of a top oxide layer thereby causing oxygen to be introduced into the nitride layer aid doting a remaining portion of the top oxide layer, thereby assist in controlling the amount of oxygen introduced into the nitride layer.  
           [0014]    Further, in accordance with a preferred embodiment of the present invention, the method includes the steps of for a bottom oxide layer on a substrate, depositing a nitride layer, depositing a portion of a top oxide layer and oxidizing a remaining portion of the top oxide layer, thereby can oxygen to be introduced into the nitride layer.  
           [0015]    There is provided, in accordance with a preferred embodiment of the present invention, a method for improving the charge retention in a nitride layer of a memory chip. The method includes the steps of deposit a nitride layer and introducing oxygen into the nitride layer.  
           [0016]    Alternatively, in accordance wit a preferred embodiment of the present invention, the method includes the steps of depositing a nitride layer, controlling the thickness of the deposited nitride layer and introducing oxygen into the nitride layer.  
           [0017]    Further, in accordance with a preferred embodiment of the present invention, the method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer at a thickness approximate to the final thickness after fabrication, depositing a portion of a top oxide layer and oxidizing a remaining portion of the top oxide layer, thereby assisting in controlling the introduction of oxygen into the nitride layer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
         [0019]    [0019]FIG. 1 is a schematic illustration of a prior art NROM memory cell;  
         [0020]    [0020]FIG. 2 is a schematic illustration of the NROM memory chip after an oxide-nitride-oxide layer has been laid down;  
         [0021]    [0021]FIG. 3A is a schematic illustration in top view of a bit line implant mask;  
         [0022]    [0022]FIG. 3B a cross section of a portion of the memory array of the chip of FIG. 2 after the mask of FIG. 3A is laid down and after etching away the exposed portions of the ONO layer leaving part of the bottom oxide layer;  
         [0023]    [0023]FIG. 3C shows the cross section of FIG. 3B after an implant of an impurity to form the bit lines in the memory away portion of the chip of FIG. 3B;  
         [0024]    [0024]FIG. 4 shows in cross section the memory array portion of the chip of FIG. 3C after oxidation of the bit lines;  
         [0025]    [0025]FIG. 5 is a schematic illustration of an ONO protect mask for the memory array and periphery sections of the chip; and  
         [0026]    [0026]FIGS. 6A and 6B are schematic illusions of the memory array portion of the chip of the present invention after a polysilicon or polysilicide layer  60  has been laid down, in top and side views, respectively.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    Reference is now made to FIGS. 2, 3A,  3 B,  3 C,  4 ,  5 ,  6 A and  6 B, which illustrate the NROM fabrication method of the present invention. Similar reference numerals herein refer to similar elements. It is noted that the present invention covers the fabrication of the entire chip, which includes the NROM memory array portion and the complementary metal oxide semiconductor (CMOS) periphery devices.  
         [0028]    In the following discussion, the process of etching a layer, which includes placing photoresist on the layer, placing a mask on the photoresist, etching wherever the mask is not and removing the photoresist, will not be detailed.  
         [0029]    The method begins with a standard complementary metal oxide semiconductor (CMOS) initial process for preparing the substrate  10  including N well formation and field oxide formation. A screen oxide layer is then grown (not shown) on substrate  10  after which it is removed, typically with a wet etch thereby to remove any residual nitride at the edge of the field. A typical thickness of the screen oxide layer is 200-400 Å.  
         [0030]    Substrate  10  is then overlaid with an ONO layer. A bottom oxide layer  30  is grown over substrate  10  typically to a thickness of between 50 Å and 150 Å in a low temperature oxidation operation. A typical oxidation temperature is about 800° C. but it can vary between 750-1000° C. A preferred thickness of the bottom oxide layer  30  is 80 Å.  
         [0031]    A nitride layer  32  is then deposited over bottom oxide layer  30  to a thickness of between 20 Å and 150 Å where a preferred thickness is as thin as possible, such as 10 Å-50 Å.  
         [0032]    Applicant notes that a thin layer of the nitride prohibits lateral movement of the charge retained within the nitride, and hence, it is beneficial to control the thickness of nitride layer  32 .  
         [0033]    Top oxide  34  is then produced either through oxidation of the nitride (i.e. growing of the oxide), or by deposition or by a combination thereof. It is noted that top oxide  34  consumes nitride during oxidation, where typically half of the oxide thickness comes from the consumed nitride. Thus, if it is desired to have a top oxide which is 100 Å thick, the nitride layer  32  should be at least 50 Å thicker than the final desired nitride thickness, with this extra nitride being for consumption in the formation of tie top oxide layer.  
         [0034]    It is also noted that, during oxidation of top oxide layer, some of the oxygen is introduced into the non-consumed nitride layer.  
         [0035]    Ultimately, as is described hereinbelow, nitride layer  32  is transformed into nitride section  17 , which provides the charge retention mechanism for the memory cell. Nitride, particularly due to its structure, traps the electrons, which are introduced into nitride section  17 . Oxygen however, is a better insulator than nitride and helps to minimize the lateral movement of electrons in nitride layer  32 . It is thus an important element for effective retention of the charge. It is therefore noted tat one of the factors effecting the quality of retention ability of nitride section  17  is the concentration of oxygen within nitride layer  32 . The oxygen concentration is defied as the percentage of oxygen atoms relative to the nitride atoms, irrespective of the type of molecule in which the oxygen atoms are found. The concentration can range from a low of 10% to a high of 80%.  
         [0036]    Hence, in order to produce a retention layer, which provides effective charge retention it is recommended to introduce a high percentage of oxygen into the nitride. Nonetheless, if the oxide-nitride composition is too oxygen rich, even though nitride is essentially an oxidation barrier, a run-away situation is produced whereby nitride layer  32  absorbs too much oxygen and ceases to act as a barrier for oxygen diffusion. In such an instance, the oxygen introduced into the oxygen rich nitride layer  32  reaches the silicone oxide layer  18 , and become SIO 2 .  
         [0037]    In summary, in order to produce a nitride section  17  with maximum retention qualities, it is desirable to make nitride layer  32  as thin as possible, with the maximum oxygen concentration, without inducing a run-away situation. Consequently, it is critical to control the fabrication the ONO structure, and specifically, the manner in which the top oxide  34  is produced.  
         [0038]    The top oxide is typically of a thickness of between 50 Å and 150 Å. Three alternative operations for creating a top oxide  34  of 100 Å are described hereinbelow.  
         [0039]    The first method involves depositing nitride layer  32  of approximately 150-160 Å, growing 120-130 Å of top oxide  34 , (which includes consuming 60-65 Å of nitride layer  32 ) and removing 20-30 Å of oxide layer  34  during cleaning. Since a large portion of nitride layer  32  is consumed, it is difficult to control tie amount of oxygen introduced into nitride layer  32 . Thus, in order to avoid a possibility of run-away conditions in the nitride layer, it is essential to “leave” a thicker nitride layer. This alternative produces a thicker nitride layer; however it provides for high introduction of oxygen into the nitride and is a simple process to perform.  
         [0040]    The second method involves depositing nitride layer  32  at a thickness of approximately 60 Å, growing a tin layer of oxide layer  34  (approximately 40 Å) while consuming about 20 Å of nitride, depositing 80-90 Å and removing 20-30 Å during cleaning. Since depositing oxide is a quicker process than growing oxide, this alternative is quicker than the first alternative and it offers marginally better control over the amount of oxygen introduced into nitride layer  32 .  
         [0041]    It is noted that the longer the oxidation process continues the greater the effect on previously produced layers. Therefore, in order diminish the effect on previous layers, it is desirable to create the top oxide layer as quickly as possible.  
         [0042]    The third method involves depositing nitride layer  32  at a thickness close to the preferred final thickness, such as 20 Å, depositing 100-110 Å of oxide, growing 2-5 Å of oxide and removing 20-30 Å of oxide during cleaning. When growing oxide after it has been deposited, the deposited layer acts as a barrier between th growing oxide and nitride layer  32 . Hence, the oxygen is introduced slowly into nitride layer  32 . This alternative is slower than the previous alternatives; however, it provides a thin nitride layer and a more controlled manner for regulating the introduction of oxygen into the nitride layer.  
         [0043]    The process by which the nitride and top oxide layers are generated depends on the ability of the manufacturing facility to control the thickness and composition of the layers of the ONO structure.  
         [0044]    At this point, the entire substrate  10  is covered wit an ONO layer, as shown in FIG. 2. The next step involves depositing a bit line mask  40  (typically photoresist  42  patterned in a well known manner), whose layout within the memory array portion of the chip is shown in FIG. 3A, to create the bit lines, forming lines of sources and lines of drains. FIG. 3B illustrates a portion of the resultant chip within the memory array portion with the photoresist  42  patterned. FIG. 3B is a side view (similar to FIG. 2) with the columns  42  of the bit line mask in place. Photoresist columns  42  define the areas where the bit lines are not to be implanted (i.e. the locations of the channels  15  (FIG. 1)).  
         [0045]    Prior to implanting the bit lines, the top oxide and nitride layers  32  and  34 , respectively, are etched away from the areas between columns  42 . The etch operation is typically a dry etch which might also etch a portion  44  of bottom oxide layer  30  which is between columns  42 , leaving portion  44  with a predetermined thickness, such as 50 Å. The etch operation produces oxide sections  18  and  20  and nitride section  17  under each column  42 .  
         [0046]    After the etch operation, bit lines  12  are implanted (FIG. 3C) in the areas between columns  42 . A typical implant might be 2-4×10 −5 /cm 2  of Arsenic at 50 Kev. It will be appreciated that this is a self-aligned implant in which the bit lines are self-aligned to the ONO structures.  
         [0047]    The photoresist layer  42  is then removed and bit line oxides  50  (FIG. 4) are then thermally grown over the bit lines  12  in an oxidation operation. At the same time, side oxides  551 , typically of 30 Å, are grown along the sides of nitride layers  17  to improve data retention within the nitride layers. The oxidation typically occurs in the range of 800° C. to 950° C. but preferably at the lower side of this range to minimize the diffusion of the bit line impurity while maximizing the thickness of the thermal oxide. This lowers the bit line capacitance. The oxidation temperature also activates the implanted bit line impurities.  
         [0048]    Thus the typical oxidation process is a low temperature oxidation of about 800° C. which, on a P− substrate, normally is continued for a time sufficient to grow the equivalent of 100 Å of thermal oxide. On the chip of the present invention, however, top oxide sections  20  will not significantly increase in thickness during the bit line oxidation due to the close presence of nitride sections  18  while oxide layer  44  over the bit lines  12  will increase significantly due to the presence of Arsenic in the bit lines  12 . The result is that the bit line oxides  50  are typically very thick, such as 500 Å thick thereby lowering the bit line capacitance.  
         [0049]    It will be appreciated that the present invention separates the creation of bottom oxide sections  18  (and thus, of the entire ONO structure  16 ) from the creation of bit line oxides  50 . Bottom oxide sections  18  are created over the entire array as part of creating the ONO structures. Bit line oxides  50  are created during the bit line oxidation operation and this oxidation does not significantly affect the oxide layers in the ONO structures. Furthermore, bit line oxides  50  are self-aligned to the ONO structures and, since the oxidation operation is at a relatively low temperature, bit lines  12  do not significantly diffuse into substrate  10  during the oxidation operation.  
         [0050]    It will flyer be appreciated that the ONO layers have been laid down on the entire chip and thus, are present in the periphery. In accordance with a preferred embodiment of the present invention, the ONO layers can be utilized as thick gate oxides in the portions of the periphery where thicker oxides are needed. Thus, if two gate dielectric thicknesses are required in the periphery, the present invention provides one gate dielectric using the ONO layers and the second, thinner gate dielectric can be produced in a separate gate oxide production step. Furthermore, as shown in FIG. 5, a single mask  52  can be utilized to mark both the locations  54  of the thick gate oxides as well as to protect the memory array (area  56 ) while etching and oxidizing the periphery.  
         [0051]    Mask  52  can be utilized in one of two alternative ways. In the first embodiment, a threshold level adjustment implant for the peripheral transistors is performed after mask  52  is laid down and patterned. This provides tie periphery with a threshold level different from that of the memory array area  56 . In the second embodiment, the threshold level adjustment implant is performed on the entire chip prior to laying down mask  52 . In this embodiment, mask  52  serves only to mark the locations where the ONO layers are to be removed.  
         [0052]    Specifically, in the first embodiment, after mask  52  is laid down, the threshold voltage level adjustment is performed. This procedure involves implanting boron through the ONO layers into the portions of the periphery of the chip not covered by mask  52 . Typically, there are two adjustment steps, one each for the n-channel and p-channel transistors. It will be appreciated that, in accordance with a preferred embodiment of the present invention, the adjustment implant is performed through the ONO layers since they are not yet capped and thus, do not block the implant operation. It will further be appreciated that, for the threshold adjustment procedure, the to-be-removed ONO layers act as a sacrificial oxide (e.g. an oxide grown for an implant operation and immediately thereafter removed).  
         [0053]    Following the threshold voltage adjustment procedure, the ONO layers on the unmasked portions of the chip are removed. Initially, a dry oxide etch is utilized to remove top oxide  34  and nitride  32  layers after which a wet etch is utilized to remove bottom oxide layer  30 . Following removal of mask  52 , a gate oxide (not shown) of typically 100-150 Å is thermally grown over the entire chip. Due to the presence of nitride in the memory array. the gate oxide step does not significantly affect the thickness of top oxide  20 . However, this step creates gate oxides for die transistors in the periphery.  
         [0054]    It will be appreciated that the gate oxide thickness is thus independent of the thicknesses of the bit line oxide  50  and top oxide  20 .  
         [0055]    In a second embodiment, mask  52  is laid down after the gate and threshold voltage level adjustment procedure is performed. Thus, the memory array portion of the chip also receives threshold level adjustments. With mask  52  in place, the ONO layers on the unmasked portions of the chip are removed, as described hereinabove. Once again, the ONO layers act as a sacrificial oxide, eliminating the necessity for the additional sacrificial oxide operations.  
         [0056]    Finally, following removal of mask  52 , a gate oxide is grown over the entire array, creating gate oxides in the periphery only.  
         [0057]    Following the gate oxide growth step, a polysilicon layer, which will create word lines for the memory array portion and will create gates for the periphery transistors, is laid down over the chip. If desired, a low resistive suicide, as is known in the art, can be deposited over the polysilicon layer in order to reduce its resistivity. This creates a “polysilicide” layer. A typical total thickness of the polysilicide might be 0.3-0.4 μm. As indicated by FIG. 6A, the polysilicide or polysilicon layer is then etched using a mask into word lines  60  within the memory array. Typically the word line etch also etches at least the top oxide  20  and the nitride  17  from between the word lines  60 . This improves the charge retention of the memory cells by isolating the nitride layers  17  of each transistor.  
         [0058]    [0058]FIG. 6B illustrates one row of the resultant memory array in side view. The polysilicide or polysilicon layer  60  lies on top of the ONO structures  16  (FIG. 4), thereby forming the gates  22  (FIG. 1) of the NROM cells. Bit line oxides  50  are thick enough to isolate neighboring ONO structures  16 .  
         [0059]    The memory chip is then fished in the standard ways, including a side wall oxidation step (typically a self-aligned step), a lightly doped drain (Ldd) implant procedure into the CMOS periphery only and a spacer deposition. FIG. 6A illustrates the location of the spacers  62  as being along the sidewalls of the polysilicon word lines  60 . The Ldd typically requires separate masks for the n-channel and p-channel periphery transistors.  
         [0060]    It will be appreciated that, in the present invention, the thicknesses of the various elements of the NROM cell are generally independent of each other. For example, the thicknesses of the bottom oxide, nitride and top oxide layers are typically selected as a function of the desired operation of the memory array, the bit line oxide is independent of the thickness of bottom oxide ONO structure and tie gate oxide of the periphery is independent of the other two oxide (i.e., the bit line oxide and the bottom ONO oxide) thicknesses.  
         [0061]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow: