referring now to the drawings , in particular to fig3 a - 3 h there is shown steps of forming a split - gate flash memory cell with silicon nitride spacer that prevent inadvertent reverse tunneling between the floating gate and the control gate of the cell . it is preferred that the sidewall of the floating gate have a negative slope in order to affect successfully the forming of the nitride spacer . first , current manufacturing practice of forming floating gates with straight walls are described in fig2 a - 2 g . in fig2 a , pad oxide layer ( 120 ) is first thermally grown over substrate ( 100 ) using conventional methods . next , a first polysilicon layer ( 130 ) is formed followed by the deposition of nitride layer ( 140 ). a photoresist layer ( 150 ) is then formed over the substrate and then patterned with a floating gate pattern ( 155 ) as shown in the same fig2 a , which in turn , is etched into the nitride layer ( 140 ) to replicate opening ( 145 ) as shown in fig2 b . the photoresist layer , which is no longer needed , is removed . next , first polysilicon layer ( 130 ) that is exposed in the pattern openings ( 145 ) in the nitride layer is oxidized to form polyoxide ( 135 ) as shown in fig2 d . subsequently , the nitride layer is removed where now polyoxide ( 135 ) serves as a hard mask to remove all the first polysilicon portions except those that are covered by the overlying polyoxide layer as shown in fig2 d . normally , the next step involves the forming of an intergate or interpoly oxide between the formed floating gate ( 130 ) shown in fig2 d , and the control gate ( 170 ) to be formed subsequently as shown in fig2 g . it is preferred that the interpoly oxide comprise high temperature oxide ( hto ). further , it is a key aspect of the present invention that a silicon nitride spacer be formed between the floating gate and the control gate in order to prevent inadvertent reverse tunneling . this is accomplished by forming silicon nitride layer ( 170 ) as shown in fig2 e and anisotropically etching to form spacer ( 175 ) as shown in fig2 f . however , with conventional vertical sidewalls of the floating gate , it has been the experience of the present manufacturing line that it is very difficult to form sustainable nitride spacers . they are usually small and fragile as shown in fig2 f . control gate ( 170 ) is formed by depositing a second polysilicon layer covering the interpoly oxide layer ( 160 ) including the nitride spacer ( 175 ) and then patterning with another layer of photoresist mask ( not shown ) and etching through the patterns in the photoresist mask . fig2 g depicts the completed split - gate flash memory cell with the small nitride spacer ( 175 ) formed on the vertical wall of floating gate . because of the fragility of the spacer formed on the vertical wall of the floating gate , the spacer as well as the adjacent interpoly oxide become degraded . as a result , during programming , reverse tunneling , or , write disturb , as is commonly known , occurs in the following way : each time flash cells are to be programmed to an “ 0 ” state , the word line ( formed over the respective control gate ) for those cells are selected as shown in fig2 g . here , the selected cell on the left side in fig2 g is shown with a gate voltage v g of 2 . 0 volts , drain ( 115 ) voltage vd of 0 . 5 volts while the common source ( 110 ) has a vs voltage of 12 volts . however , cells that are not to be programmed to “ 0 ” state must be unselected so that channel hot electrons are not injected into the floating gate . this is shown on the right side of fig2 g where the v g is 0 volts . with poor or degraded spacer such as ( 175 ) shown in fig2 g , however , it is found that the unselected cell with 0 v g experiences reverse tunneling as shown by arrow ( 190 ) in fig2 g . in other words , the well - known programming “ write disturb ” occurs due to the unsatisfactory barrier to reverse tunneling provided by the ineffective silicon nitride spacer ( 175 ) of the present manufacturing line . the main feature and key aspect of the present invention is to provide a simplified method for forming a reliable spacer for split - gate flash memory cells . now , therefore , referring to the preferred embodiment shown in fig3 a - 3 h , and in particular to fig3 a , pad oxide layer ( 220 ) is formed over substrate ( 100 ), preferably silicon , to serve as an etch stop when etching nitride layer ( 230 ) to be formed next . pad oxide ( 220 ) can be formed by conventional thermal oxidation process at a temperature between about 800 to 1000 ° c . pad oxide layer ( 220 ) has a thickness between about 150 to 500 angstroms ( å ). nitride layer ( 230 ) shown in fig3 b is formed by reacting either dichlorosilane ( sicl 2 h 2 ), or silane ( sih 4 ), with ammonia ( nh 3 ) in an lpcvd at a temperature between about 700 to 800 ° c . the preferred thickness of the nitride layer is between about 1000 to 2000 å . this is followed by forming a photoresist mask ( 240 ) having patterns ( 245 ) over the nitride layer corresponding to areas where floating gates are to be defined as shown in both fig3 a and 3 b . the patterns are then etched into the nitride layer ( 230 ) forming openings ( 235 ) where the underlying pad oxide layer ( 220 ) is exposed . subsequently , the photoresist layer is removed using oxygen plasma ashing . the etching of nitride layer ( 230 ) is important because the resulting opening must have tapered wall ( 233 ) with negative slope as shown in fig3 b . the degree of the tapered wall can be adjusted through controlling process parameters of either patterning or etching , both of which are known to those skilled in the art . thus , accordingly , the slope of the tapered wall ( 233 ) is preferably to have an angle ( 239 ) between about 45 to 85 degrees from the horizontal , as shown in fig3 b to be effective in the forming of the nitride spacer of this invention . the etching of nitride layer ( 230 ) is continued until it stops on pad oxide layer ( 220 ) in opening ( 235 ), also shown in fig3 b . pad oxide is next removed until the underlying silicon substrate is exposed using conventional wet etch methods . then , a high - temperature oxide ( hto ) layer ( 250 ) is formed over the substrate , including the tapered wall of opening ( 235 ) as shown in fig3 c . this is accomplished by depositing a layer of conformal oxide at a temperature between about 750 to 850 ° c ., and to a thickness between about 50 to 150 å . this is followed by depositing a first polysilicon layer ( 260 ) over the substrate , including oxide layer ( 250 ) in opening ( 235 ), through reduction of sih 4 using lpcvd at a temperature between about 530 to 630 ° c . the preferred thickness of the first layer of polysilicon is between about 500 to 2000 å . next , the substrate is subjected to chemical - mechanical polishing ( cmp ) using nitride layer ( 230 ) as a polishing stop , resulting in the structure shown in fig3 e . it will be noted that the substrate is planarized and the surface of the first polysilicon layer ( 260 ) in opening ( 235 ) is recessed due to the polishing action . next , the first polysilicon so formed in opening ( 235 ) with tapered wall is next oxidized , preferably , by wet oxidation at a temperature between about 850 to 950 ° c . to form poly - oxide ( 265 ), as shown in fig3 f . the thickness of poly - oxide is between about 1000 to 2000 å . subsequently , nitride layer is etched anisotropically such that nitride spacer ( 237 ) is formed with good adhesion in the space formed by the negatively sloped wall of the floating gate as shown in fig3 g . å . it will be noted in fig3 g - 3 h that the nitride spacers assume a triangular shape after the etching . furthermore , the silicon nitride spacer has a thickness between about 250 to 1000 . after having formed the nitride spacer , ( 237 ), of this invention , the completion of the split - gate is accomplished by first blanket depositing ( not shown ) over the substrate a second layer of polysilicon and patterning it to form control gate , ( 270 ), shown in fig3 h . it is preferred that the second layer of polysilicon is formed using silicon source sih 4 in an lpcvd chamber at a temperature between about 530 to 630 ° c ., and that it has a thickness between about 500 to 2000 å . thus , in the programming of the split - gate flash cell shown in fig3 h , the nitride spacer ( 237 ) of this invention provides the necessary barrier to prevent reverse tunneling , or “ write disturb ” of unselected cells . the nitride spacer is formed along the negatively sloped wall of a floating gate . the preferred embodiment also eliminates from the current practice on the manufacturing line the extra step of forming a conformal nitride layer . at the same time , the manufacturability is improved by providing a better etch control in the forming of a more robust nitride spacer .