Abstract:
A method for fabricating a first memory cell and a second memory cell having floating gates electrically isolated from each other. A first polysilicon (poly I) layer is formed on an oxide coated substrate, portions of the poly I layer to serve as future floating gates for the first and second memory cells. An interpoly dielectric layer is formed over the poly I layer. At least a portion of the interpoly dielectric layer is etched to expose at least a portion of the poly I layer so as to pattern the floating gates on either side of the exposed portion of the poly I layer. The exposed portion of the poly I layer is transformed into an insulator via thermal oxidation such that the insulator electrically isolates a floating gate of the first memory cell from a floating gate of the second memory cell. A second polysilicon (poly II) layer is formed substantially free of abrupt changes in step height.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to integrated circuits and, in particular, to a method of memory device fabrication which improves memory cell reliability and manufacturability by preventing formation of poly stringers caused by an oxide-nitride-oxide (ONO) fence. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices typically include multiple individual components formed on or within a substrate. Such devices often comprise a high density section and a low density section. For example, as illustrated in prior art FIG. 1, a memory device such as a flash memory 10 comprises one or more high density core regions 12 and a low density peripheral portion 14 on a single substrate 16. The high density core regions 12 typically consist of at least one M×N array of individually addressable, substantially identical memory cells and the low density peripheral portion 14 typically includes input/output (I/O) circuitry and circuitry for selectively addressing the individual cells (such as decoders for connecting the source, gate and drain of selected cells to predetermined voltages or impedances to effect designated operations of the cell such as programming, reading or erasing). 
     The memory cells within the core portion 12 are coupled together in a circuit configuration, such as that illustrated in prior art FIG. 2. Each memory cell 20 has a drain 22, a source 24 and a stacked gate 26. Each stacked gate 26 is coupled to a word line (WL 0 , WL 1 , . . . , WL N ) while each drain 22 is coupled to a bit line (BL 0 , BL 1 , . . . , BL N ). Lastly, each source 24 is coupled to a common source line CS. Using peripheral decoder and control circuitry, each memory cell 20 can be addressed for programming, reading or erasing functions. 
     Prior art FIG. 3 represents a fragmentary cross-sectional diagram of a typical memory cell 20 in the core region 12 of prior art FIGS. 1 and 2. Such a memory cell 20 typically includes the source 24, the drain 22 and a channel 28 in a substrate 30; and the stacked gate structure 26 overlying the channel 28. The stacked gate 26 includes a thin gate dielectric layer 32 (commonly referred to as the tunnel oxide) formed on the surface of the substrate 30. The tunnel oxide layer 32 coats a portion of the top surface of the silicon substrate 30 and serves to support an array of different layers directly over the channel 28. The stacked gate 26 includes a lower most or first film layer 38, such as doped polycrystalline silicon (polysilicon or poly I) layer which serves as a floating gate 38 that overlies the tunnel oxide 32. On top of the poly I layer 38 is an interpoly dielectric layer 40. The interpoly dielectric layer 40 is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer having two oxide layers sandwiching a nitride layer, or in the an alternative can be another dielectric layer such as tantalum pentoxide. Finally, the stacked gate 26 includes an upper or second polysilicon layer (poly II) 44 which serves as a polysilicon control gate overlying the ONO layer 40. The control gates 44 of the respective cells 20 that are formed in a given row share a common word line (WL) associated with the row of cells (see, e.g., prior art FIG. 2). In addition, as highlighted above, the drain regions 22 of the respective cells in a vertical column are connected together by a conductive bit line (BL). The channel 28 of the cell 20 conducts current between the source 24 and the drain 22 in accordance with an electric field developed in the channel 28 by the stacked gate structure 26. 
     According to conventional operation, the memory cell 20 (e.g., flash memory cell) operates in the following manner. The memory cell 20 is programmed by applying a relatively high voltage V G  (e.g., approximately 12 volts) to the control gate 38 and a moderately high voltage V D  (e.g. approximately 9 volts) to the drain 22 in order to produce &#34;hot&#34; (high energy) electrons in the channel 28 near the drain 22. The hot electrons accelerate across the tunnel oxide 32 and into the floating gate 34 and become trapped in the floating gate 38 because the floating gate 38 is surrounded by insulators (the interpoly dielectric 40 and the tunnel oxide 32). As a result of the trapped electrons, a threshold voltage (V T ) of the memory cell 20 increases by about 3 to 5 volts. This change in the threshold voltage (and thereby the channel conductance) of the memory cell 20 created by the trapped electrons is what causes the memory cell 20 to be programmed. 
     To read the memory cell 20, a predetermined voltage V G  that is greater than the threshold voltage of an unprogrammed memory cell, but less than the threshold voltage of a programmed memory cell, is applied to the control gate 44. If the memory cell 20 conducts, then the memory cell 20 has not been programmed (the memory cell 20 is therefore at a first logic state, e.g., a zero &#34;0&#34;). Conversely, if the memory cell 20 does not conduct, then the memory cell 20 has been programmed (the memory cell 20 is therefore at a second logic state, e.g., a one &#34;1&#34;). Thus, each memory cell 20 may be read in order to determine whether it has been programmed (and therefore identify the logic state of the memory cell 20). 
     In order to erase the memory cell 20, a relatively high voltage V S  (e.g., approximately 12 volts) is applied to the source 24 and the control gate 44 is held at a ground potential (V G  =0), while the drain 22 is allowed to float. Under these conditions, a strong electric field is developed across the tunnel oxide 32 between the floating gate 38 and the source region 24. The electrons that are trapped in the floating gate 38 flow toward and cluster at the portion of the floating gate 38 overlying the source region 24 and are extracted from the floating gate 38 and into the source region 22 by way of Fowler-Nordheim tunneling through the tunnel oxide 32. Consequently, as the electrons are removed from the floating gate 38, the memory cell 20 is erased. 
     Having described a structural arrangement of the memory cell 20, attention is now brought to fabrication of the memory device 10. FIG. 4 illustrates an overall arrangement of the memory device 10 at an early stage of formation. A substrate 30 is shown which comprises regions of thick oxide (field oxide) 34 and thin oxide (tunnel oxide) 32. The field oxide 34 provides for electrically insulating transistors from one and other. A poly I layer 38 has been laid down over the substrate 30, and sections of the poly I layer 38 have been patterned and masked such that an unmasked portion 42 is etched away using convention photolithographic techniques so as to form a series of poly I layer rows 38. FIG. 5 illustrates an ONO layer 40 laid down over the poly I layer rows 38 and the partially exposed field oxide regions 34 between the rows of poly I layer 38. More particularly, since sections of the poly I layer 38 have been etched away, gaps 42 exist between the rows of poly I layer 38 such that sidewalls of the poly I layer rows become coated with the ONO layer material 40 as it is being deposited. The etching step of the poly I layer 38 causes the ONO layer 40 being deposited thereon to be non-uniform in step height. More specifically, since there are gaps 42 between the rows of poly I layer 38, and since the ONO layer 40 conforms to the topography on which it is deposited, the ONO that lies along the sidewalls of the etched poly I lines is significantly thicker that the ONO on top of either the flat portion of the poly I or the flat portion of the field oxide. It is to be appreciated that the thickness of the ONO layer 40 in the figures is shown to be relatively the same as the other layers for ease of understanding, however, the ONO layer 40 is actually very thin relative to the poly I layer 38 and poly II layer 44 (FIG. 6a). 
     After application of the ONO layer 40, the poly II layer 44 is laid down over the ONO layer 40 as shown in FIG. 6a. Like the ONO layer 40, the poly II layer 44 also includes undulations as a result of the gaps 42 between rows of the poly I layer 38. The gaps 42 result in the poly II layer 44 being undulated such that portions of the poly II layer 44 adjacent an edge of a respective poly I layer row 38 (where the ONO layer 40 is thickest) is greater in height with respect to the substrate surface 30 than a portion of the poly II layer 44 which lies relatively over other areas. As will be discussed in greater detail below, the gaps 42 may lead to discontinuity in ONO 40 and poly II 44 thickness and even possibly film cracks or breaks. 
     FIG. 6b illustrates a substantially large maximum step height (y M ) that results because of the undulating poly II layer 44. In particular, the step height of a portion of the poly II layer that lies respectively over a poly I layer row 38 has a step height of y 1 , and a portion of the poly II layer that lies respectively over the gap 42 between adjacent poly I layer rows has a step height of y 2 . However, the portion of the poly II layer 44 which represents an undulation (i.e., the transition from the poly II layer lying over the poly I layer row 38 and over the gap 42 between poly I layer rows 38) has a step height of y M , where y M  is substantially greater in height than y 1  or y 2  and results in problems relating to overetch requirements and the formation of an ONO fence as will be discussed in greater detail below. 
     Referring now to FIG. 7, a resist 50 is lithographically patterned over portions of the poly II layer 44. Then, the poly II layer 44 is etched away at portions not covered by the resist 50, the etched away portion of poly II layer is generally designated at 54. 
     FIG. 8 is a partial cross-sectional view of the memory device 10 taken at the portion 54. As is seen, the poly II layer 44 has been etched away leaving an ONO layer 40 laid down atop and along vertical sidewalls of the poly I layer 38. The field oxide 34 and tunnel oxide 36 of the substrate 30 are not shown for ease of understanding. In FIG. 9, the ONO layer 40 is shown being substantially etched away using conventional etching techniques. The ONO layer 40 has a substantially greater step height at side wall portions 60 of the poly I layer 38. As a result, these side wall portions of ONO do not become completely etched away and leave what is coined an ONO fence 64 (FIG. 10) along the sidewalls of the poly I layer 38. 
     In FIG. 11, the poly I layer 38 is substantially etched away using conventional etching techniques. However, a problem often occurs at this step involving formation of poly stringers. Poly stringers result from incomplete removal of poly I from the unmasked portions of the wafer during etch. The poly stringers of concern here are created during the self-aligned etch (SAE). During the SAE, the ONO 40 and then the poly I 38 between adjacent second gate lines is etched away. In the SAE, the second gate lines act as a mask. This results in substantially perfect alignment of the first gate with the second gate along a direction perpendicular to the second gate lines--hence, the name self-aligned etch. During the SAE, the ONO 64 along the sidewalls of the poly I is only partially removed, resulting in the ONO fence. When the poly I 38 is etched, for some memory cells a small &#34;string&#34; of polysilicon is hidden from the etch by the ONO fence. If this happens to even a few cells in the memory the memory chip will not function properly. As shown in FIG. 12, the ONO fence 64 acts as an umbrella and shields portions of the poly I layer 38 from being etched away. These remaining portions of poly I material are known as poly I stringers 70a and 70b as shown in FIG. 13, which may result in electrically shorting adjacent memory cells 20. In other words, the poly I etching step of FIG. 11 serves in part to isolate one memory cell 20 from another. However, if a portion of the poly I layer 38 is not etched away and forms a conductive path (e.g., poly stringer 70) from one memory cell 20 to another, the memory cells 20 will become electrically shorted. 
     FIG. 13 illustrates in perspective view the ONO fences 64a, 64b that have lead to the formation of poly stringers 70a, 70b which may cause shorting of poly I layers 38a and 38b of two memory cells 20a and 20b, respectively. The polysilicon floating gates 38a and 38b rest on the oxide coated substrate 30. The ONO fences 64a and 64b remain along the sidewalls of the poly I layers 38a and 38b and in the region 80 between the two memory cells 20a and 20b. The additional layers that make up the stacked gate structure 26 of the respective memory cells 20a and 20b are not shown in prior art FIG. 13 for sake of simplicity. 
     As long as the initial etching of the polysilicon floating gate 38 (which delineates cells 20 along a single word line) occurs in an ideally anisotropic manner, no poly stringers are formed during the second etching of the floating gate 38 (which delineates separate word lines). It is well known, however, that anisotropic etch processes do not repeatably provide ideally anisotropic profiles. Instead, most anisotropic etch processes provide non-ideal profiles in the range of about 85-95° (wherein 90° is ideal). A non-ideal anisotropic etch profile as is illustrated in prior art FIG. 12 leaves an angled ONO fence 64 which acts as an umbrella (or shield) to the poly I etch. 
     More specifically, when the polysilicon gate 38 is subsequently etched (in an anisotropic manner via, e.g., reactive ion etching (RIE)), as illustrated in prior art FIG. 11, the angled ONO fence 64 shields a portion of the polysilicon gate 38, resulting in remnants of polysilicon, which are the poly stringers 70. Transposing the non-ideally anisotropic etched polysilicon gate 38 and the resulting poly stringers 70 into their macroscopic context (as illustrated in prior art FIG. 13), it is clear that the poly stringers 70 pose a substantial reliability problem since the poly stringers 70 in the etched region 80 can short out the word lines in regions 82 and 84, respectively. That is, instead of the etched region 80 electrically isolating the word lines in regions 82 and 84 from one another, the poly stringers 70 (which are conductive) span the etched region 80 and cause the poly I layers (ie., floating gates) 38a and 38b in the regions 82 and 84 to be shorted together. 
     Consequently, in light of the above, it would be desirable to have a method for fabricating a memory cell such that the formation of an ONO fence and resulting poly stringers is eliminated or otherwise substantially reduced. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a method of manufacturing a memory cell which prevents the formation of poly stringers resulting from an ONO fence. ONO fences typically result from an anisotropic etching step which leaves an ONO fence on sidewalls of a poly I layer. As noted above, the ONO fence can result in the formation of poly stringers which may short adjacent memory cells. The present invention removes the need for an initial poly I etching step which in turn avoids formation of an ONO fence and which in turn prevents poly stringers from forming. 
     According to the present invention, an ONO layer is deposited/grown over a poly I layer. The ONO layer is patterned and etched so as to define floating gates of future memory cells. In other words, the ONO layer is etched so as to configure the underlying poly I layer such that floating gate regions of memory cells are isolated from one another in a desired manner. The exposed portions of the poly I layer (i.e., portions of the poly I not covered by ONO are transformed into insulating portions (e.g., silicon oxide, silicon dioxide) by a suitable technique in accordance with the present invention. The resulting insulating portions serve to isolate the floating gates of memory cells from one another. 
     The present invention eliminates the need to perform an initial etch of the poly I layer as is done conventionally, which in turn eliminates formation of an ONO fence and subsequent formation of poly stringers. More specifically, since the poly I layer is not etched, ONO is never formed adjacent a row or column of poly I because the area between rows or columns of poly I is occupied by an insulating medium (e.g., poly I transformed into silicon dioxide). In the present invention, the ONO is deposited on a wafer surface that is free of changes in height that occur over angles greater that ˜60°. That is, changes in the height of the surface of the wafer result from gentle undulations rather than from substantially abrupt 90° steps. In particular, since there is no initial poly I etch step there is no formation of gaps between poly I rows as which result using conventional memory cell fabrication techniques. Rather, the areas that conventionally exist as gaps between rows of poly I are occupied by the insulating medium (e.g., silicon dioxide) in accordance with the present invention. Therefore, an ONO fence is prevented from forming which thus avoids the subsequent formation of poly I stringers which may lead to shorting floating gates among conventionally fabricated memory cells. 
     Furthermore, because there are no open gaps between rows of poly I layer, the ONO layer portions and poly II layer deposited over the poly I layer do not have substantially abrupt 90° steps. As a result of forming a poly II layer without abrupt undulations, a maximum step height of the poly II layer is reduced. The reduction in maximum step height of the poly II layer affords for reducing overall over etch requirements for the poly II material. In other words, since the maximum step height of the poly II layer is reduced, as compared to conventionally fabricated memory devices, less etching of the poly II layer is required. 
     In addition, the mitigation of abrupt step heights of the various layers by the present invention also results in a second gate stack (e.g., comprising the ONO layer, poly II layer and topside layer) of low resistance (i.e., less cracks) as compared to second gate stacks fabricated in accordance with conventional techniques. There is an increasing demand for miniaturization in the memory cell industry. This demand has led to an ever constant reduction in separation between memory cells in order to reduce chip size and/or increase density. As a result, gaps between floating gates of adjacent memory cells have become increasingly smaller. However, such gaps still may include steep sidewalls. Consequently, the combination of steep sidewalls and small gap width leads to difficulty in depositing layer material within the small gaps as can be seen in prior art FIG. 14. As a result, portions of layers deposited over such gaps may be weak. In other words, in conventionally fabricated memory devices, breaks, cracks or holes may result in a topside layer 45 and possibly underlying layers at portions lying over the gaps between poly I lines. Therefore, by reducing or eliminating the abrupt step heights the respective layers are less susceptible to cracks and as a result exhibit low resistance. 
     Thus, the present invention improves memory cell reliability and manufacturability by preventing formation of poly I stringers caused by an ONO (oxide-nitride-oxide) fence, provides for reduction of over etch requirements of gate structure layer materials (e.g., poly II layer) and affords for a second gate stack having low resistance. 
     In accordance with one specific aspect of the present invention, a method for fabricating a first memory cell and a second memory cell having floating gates electrically isolated from each other is provided. A first polysilicon (poly I) layer is formed on an oxide coated substrate, portions of the poly I layer to serve as future floating gates for the first and second memory cells. An interpoly dielectric layer is formed over the poly I layer. At least a portion of the interpoly dielectric layer is etched to expose at least a portion of the poly I layer so as to pattern the floating gates on either side of the exposed portion of the poly I layer. The exposed portion of the poly I layer is transformed into an insulator via thermal oxidation such that the insulator electrically isolates a floating gate of the first memory cell from a floating gate of the second memory cell. 
     Another specific aspect of the present invention provides for a group of memory cells. The group includes a first memory cell and a second memory cell, the first and second memory cells each including a poly silicon (poly I) layer, the poly I layers serving as floating gates. An electrically nonconductive medium isolates the floating gate of the first memory cell from the floating gate of the second memory cell, the electrically nonconductive medium being poly I material, not covered by an interpoly dielectric layer, converted into nonconductive material via thermal oxidation. 
     In accordance with still another aspect of the present invention, a method for fabricating a first memory cell and a second memory cell having floating gates electrically isolated from each other is provided. A first polysilicon (poly I) layer is formed on an oxide coated substrate, portions of the poly I layer to serve as future floating gates for the first and second memory cells. An interpoly dielectric layer is formed over the poly I layer. At least a portion of the interpoly dielectric layer is etched to expose at least a portion of the poly I layer so as to pattern the floating gates on either side of the exposed portion of the poly I layer. The exposed portion of the poly I layer is transformed into an insulator via thermal oxidation such that the insulator electrically isolates a floating gate of the first memory cell from a floating gate of the second memory cell. A second polysilicon (poly II) layer is formed substantially free of abrupt changes in step height. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view schematically illustrating a prior art layout of a memory device; 
     FIG. 2 is a schematic diagram illustrating a prior art core portion of a memory circuit; 
     FIG. 3 is a partial cross-sectional view of a prior art stacked gate memory cell; 
     FIG. 4 is a perspective illustration of a portion of a prior art memory device at an early stage in fabrication; 
     FIG. 5 is a perspective illustration of the prior art memory device of FIG. 4 after formation of an ONO layer; 
     FIG. 6a is a perspective illustration of the prior art memory device of FIG. 5 after formation of a poly II layer; 
     FIG. 6b is a cross-sectional view showing the variation in thickness of the poly II layer in the vicinity of the step in poly I; 
     FIG. 7 is a perspective illustration of the prior art memory device of FIG. 6a after a resist layer has been laid down and portions of the poly II layer have been etched away; 
     FIG. 8 is a cross-sectional view of the poly I layer, having the ONO layer thereon, of the prior art memory device of FIG. 7; 
     FIG. 9 is a cross-sectional view of the prior art memory device of FIG. 8, wherein the ONO layer is being etched away; 
     FIG. 10 is a cross-sectional view of the prior art memory device of FIG. 9, depicting an ONO fence remaining along sidewalls of the poly I layer after the ONO etch step; 
     FIG. 11 is a cross-sectional view of the prior art memory device of FIG. 10 wherein the poly I layer is being etched away; 
     FIG. 12 is a cross-sectional view of the prior art memory device of FIG. 11 wherein the ONO fence shields poly I portions from being etched away during the poly I etch of FIG. 11; 
     FIG. 13 is a perspective illustration of the prior art memory device of FIG. 12 depicting ONO fences and poly stringers electrically shorting floating gates of adjacent memory cells; 
     FIG. 14 is a cross-sectional view of a prior art memory device where breaks have occurred in a topside layer; 
     FIG. 15 is a perspective illustration of a portion of a memory device at an early stage in fabrication in accordance with the present invention; 
     FIG. 16 is a cross-sectional illustration of the memory device of FIG. 15 wherein an ONO layer is deposited/grown over a poly I layer in accordance with the present invention; 
     FIG. 17 is a cross-sectional illustration of the memory device of FIG. 16 after portions of the ONO layer have been etched away in accordance with the present invention; 
     FIG. 18 is a cross-sectional illustration of the memory device of FIG. 17 wherein the ONO layer portions and the exposed portions of the poly I layer are oxidized in accordance with the present invention; 
     FIG. 19 is a cross-sectional illustration of the memory device of FIG. 18 after a poly II layer has been deposited over the ONO layer portions and the oxidized poly I layer portions in accordance with the present invention; 
     FIG. 20 is a perspective drawing illustrating the memory device of FIG. 19 after an unmasked portion of the poly II layer, ONO layer portions and the oxidized poly I layer portions have been etched away in accordance with the present invention; and 
     FIG. 21 is a perspective drawing illustrating electrically isolated memory cells after a substantially final etching step in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. 
     In the present invention an initial poly I layer etch step is not performed which avoids formation of an ONO fence that may be formed under conventional memory cell fabrication techniques. The elimination of the ONO fence prevents the formation of poly stringers which as mentioned above may short out adjacent memory cells. Additionally, the present invention provides a substrate for deposition of the poly II layer that is substantially free of abrupt 90° steps. This results in reduced maximum film thickness or step height for the poly II layer as compared with that of memory cells fabricated in accordance with conventional processes. The reduction in maximum step height of the poly II layer affords for reduced over etching requirements of the poly II layer. 
     Turning now to FIG. 15, an overall arrangement of a memory device 100 at an early stage of formation is shown in accordance with the present invention. In particular, a silicon substrate 112 is shown comprising field oxide regions 114 and tunnel oxide regions 116. It should be appreciated that although specific layering materials are identified in the preferred embodiment, any materials suitable for carrying out the present invention may be employed and fall within the scope of the claims. A doped polycrystalline silicon (polysilicon or poly I) layer 120 is laid down over the prepared substrate 112. In the present invention, the poly I layer 120 is not etched as is done conventionally. Rather, as will be discussed in greater detail below, portions of the poly I layer 120 are transformed into insulating portions such as for example silicon dioxide. The insulating portions serve as nonconductive isolators of floating gates (i.e., poly I layers) of memory cells of the memory device 100. 
     Turning now to FIGS. 16-18, steps of depositing and etching an ONO layer 130 are described. FIG. 16 illustrates deposition/growth of the ONO layer on top of the poly I layer 120. Next, suitable lithography steps (not shown) are carried out so as to define areas of the ONO layer 130 that are to be etched away at areas not covered by photoresist (not shown). FIG. 17 illustrates the ONO layer 130 etched away at portions not covered by the photoresist so as to expose portions of the underlying poly I layer 120. The exposed portions 136a, 136b and 136c (collectively referred to by reference numeral 136) of the poly I layer 120 will be transformed into insulating material (e.g., silicon dioxide) as will be discussed in greater detail below. Thus, the exposed portions 136 will serve to isolate floating gates of adjacent memory cells. As noted above, such isolating was achieved conventionally by etching of the poly I layer to form gaps between floating gate lines. However, such etching of the poly I layer contributed to the formation of poly stringers. In the present invention, such open gaps between adjacent floating gate lines do not exist thus avoiding the formation of an ONO fence which might lead to the formation of polystringers. 
     After the ONO is suitably etched, the photoresist is stripped and suitable pre-oxidation cleaning steps are performed. For example, one method for stripping the photoresist might include employing a dry photoresist strip in an O 2  plasma or oxygen plasma strip and/or a wet clean using sulfuric acid or ammonium hydroxide mixed with ionized water and hydrogen peroxide. It will be appreciated that any suitable method or means for stripping the photoresist and performing pre-oxidation cleaning may be employed and fall within the scope of the present invention. 
     Preferably, the oxidation of the exposed poly I portions 136a, 136b and 136c is performed via employment of suitable thermal oxidation techniques. For example, according to one specific aspect of the invention an entire wafer from which the memory device 100 is to be fabricated is placed in a quartz tube in a vertical or horizontal type heat treatment furnace. An oxidizing source such as oxygen and water vapor is fed into the quartz tube, the wafer is heated up (i.e., annealed) to approximately 900° C. for less than about 30 minutes and thus the unmasked portions 136a, 136b and 136c of the poly I layer 120 are oxidized. It is to be appreciated that any suitable oxidation techniques (e.g., dry oxidation, and wet oxidation) for oxidizing the poly I layer in accordance with the present invention may be employed and fall within the scope of the present invention. 
     Depending on the thermal budget for a particular device, the anneal may be either a furnace anneal, a rapid thermal anneal (RTA) or any other suitable anneal. As a result of the anneal, the selected portions 136 of the poly I layer 120 (i.e., those portions of the poly I layer not covered by the ONO layer portions 130) are transformed into insulating material. It is to be appreciated that the aforementioned oxidation step may be suitably tailored so as to have a minimum impact on the thickness of the ONO layer portions 130 also exposed to the oxidation. Additionally, if desired, ONO oxidation time during the fabrication of the ONO layer 130 may be reduced in order to compensate for additional oxidation of the ONO layer 130 during poly I oxidation. 
     Referring now to FIG. 17, the unmasked portions 136 are shown transformed into silicon dioxide (SiO 2 ) via the aforementioned oxidation step. As a result of the oxidation, the unmasked portions 136 are transformed into silicon dioxide portions 160a, 160b and 160c (collectively identified as reference numeral 160). The silicon dioxide portion(s) 160 shall serve to insulate a floating gate of one memory cell from that of another memory cell. 
     It is to be appreciated that in oxidation of the unmasked portions 136 the ONO layer 130 is employed as a masking layer for oxidation. As a result, no separate hard mask is required which affords for the elimination of a substantial number of steps associated with applying and removing a separate hard masking layer. 
     Because the poly I layer 120 is not etched but rather a portion which would have been etched conventionally is now converted into silicon dioxide in the manner described above, the ONO layer portions 130 that are not masked by the second gate word lines will be fully exposed to plasma etch. In other words, since no discrete vertical surfaces are created in the poly I layer 120, an ONO fence 64 (see e.g., FIG. 13) will not be formed. The elimination of ONO fence formation results in the avoidance of poly I stringers being formed as a result of an ONO fence 64 shielding portions of the poly I material during an initial etching step as described above. (see, prior art FIGS. 11-13). Furthermore, as can be seen in FIG. 18, the oxidized poly I portions 160 expand such that the top surfaces of the oxidized poly I portions are continuous with the top surfaces of the ONO layer portions 130. As a result, the top surfaces of ONO layer portions 130 and the top surfaces of the oxidized poly I layer portions 160 make up a surface 170 substantially free of abrupt steps. As a result, a subsequently deposited poly II layer 180 (FIG. 19) will also be substantially free of abrupt steps. 
     Turning now to FIGS. 19-21 in consecutive order, a poly II layer 180 is shown being laid down over the ONO layer portions 130 and oxidized poly I layer portions 160. Because the surface 170 is substantially free of abrupt steps, the poly II layer 180 deposited thereon is also substantially free of large variations in thickness which reduces overetch requirements of the poly II layer 180. Thereafter, the poly II layer 180 is masked such that unmasked portions 190 of the poly II layer 180, ONO layer portions 130 and oxidized poly I portions 160 are etched away using suitable techniques. Since the unmasked oxidized poly I portions 160 are not covered by ONO 130, these unmasked portions 160 will be etched at substantially the same time as when the unmasked ONO portions 130 are etched. Thereafter, the unmasked portions of poly I 120 are etched away to leave isolated memory cells 200. 
     The present invention thus removes the need for an initial poly I etching step which in turn avoids formation of an ONO fence resulting in prevention of poly stringer formation. Additionally, by not requiring an initial poly I etch step, layers formed over the poly I layer have reduced maximum step height as compared to layers of memory cells fabricated using conventional techniques. By reducing the maximum step height, the present invention provides for a reduction in overetch requirements. 
     It will be appreciated that although the present invention is described with respect to forming silicon dioxide insulating portions, any suitable material may be employed as the insulating portions. For example, but not to be considered limiting, the insulating portions may comprise silicon oxide. An exemplary procedure for forming the silicon dioxide portions is explained above, however, any suitable technique for forming silicon dioxide or other suitable insulating material (e.g., SiO X (X≧1)) may be employed to carry out the present invention and is intended to fall within the scope of the claims. 
     Furthermore, it is to be appreciated that a partial etch of the poly I layer 120 may be performed prior to oxidation thereof in order to compensate for excessive lateral or vertical spreading of the oxidized poly I portions 160 as compared to the thickness of the poly I layer 120. 
     Those skilled in the art will recognize that the embodiment(s) described above and illustrated in the attached drawings are intended for purposes of illustration only and that the subject invention may be implemented in various ways. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.