text: the method of forming cmos devices , in which specific sram cell , nfet devices , are fabricated for high performance , while other peripheral nfet devices are created to withstand greater reliability demands than the sram counterparts , will now be covered in detail . this process can also be used for other cmos applications , such as logic / asic circuits , now being currently manufactured in industry , therefore only the specific areas unique to understanding this invention will be covered in detail . a substrate , 1 , composed of n type , single crystal silicon with a & lt ; 100 & gt ; orientation is used , and shown in fig1 . a thin silicon dioxide film , 2 , to be used as a screen oxide for subsequent ion implantation processes , is thermally grown at a temperature between about 900 ° to 1200 ° c ., to a thickness between about 100 to 300 angstroms . a masking photoresist pattern , 3 , is then obtained followed by a phosphorous ion implantation step , at an energy between about 100 to 200 kev ., at a dose between about 5e12 to 1e13 atoms / cm2 , used to create n well region 4 , also shown in fig1 . after photoresist removal , using oxygen plasma ashing , another photoresist mask , 5 , is created followed by ion implantation of boron , at an energy between about 70 to 120 kev ., at a dose between about 5e12 to 1e13 atoms / cm2 , used to create p well region 6 , shown schematically in fig2 . fig3 shows the creation of thick field oxide regions , 7 , used for isolation purposes . silicon area , 8 , will be used to create pfet devices , to be used in an sram , while silicon region , 9 , will be used to fabricate the nfet counterparts of the sram cell . silicon region , 10 , will be used to construct the peripheral nfet structures . briefly the method used to form the field oxide , ( fox ), regions , is to create an oxidation mask of silicon nitride , on a silicon oxide layer , 2 , on regions where the devices are to be built . this is accomplished via lpcvd deposition of silicon nitride , at a temperature between about 600 ° to 900 ° c ., to a thickness between about 200 to 500 angstroms , followed by photoresist patterning and rie using cf4 and h2 . after photoresist removal , followed by careful wet chemical cleans , the fox region is created via thermal oxidation , in an oxygen -- steam ambient , at a temperature between about 900 ° to 1200 ° c . to a thickness between about 4000 to 6000 angstroms . after removal of the oxidation mask via a hot phosphoric acid for the silicon nitride layer , a blanket boron ion implantation step is performed at an energy between about 20 to 80 kev ., at a dose between 5e11 to 5e12 atoms / cm2 . this step adjusts the threshold voltage , ( vt ), of the nfet devices , increasing the vt of the nfet devices to be created in the p well region for both the sram and peripheral devices . this ion implantation step also adjusts the vt for the pfet devices , in the n well region , resulting in a lowering of the pfet vt . a buffered hydrofluoric acid solution is then used to remove silicon oxide , 2 , followed by another wet chemical clean and a carefully grown gate oxide , 11 , shown in fig4 . the gate oxide is grown in an oxygen -- steam ambient , at a temperature between about 800 ° to 1000 ° c ., to a thickness between about 50 to 200 angstroms . next a polysilicon layer , 12 , is deposited using lpcvd processing , at a temperature between about 580 ° to 650 ° c ., to a thickness between about 1000 to 3000 angstroms . the polysilicon layer is doped via ion implantation of phosphorous , at an energy between about 40 to 150 kev ., at a dose between about 1e15 to 5e15 atoms / cm2 . polysilicon doping can also be accomplished via poc13 processing at a temperature between about 850 ° to 1000 ° c . the polysilicon gate structures are then formed using conventional photolithographic processing , followed by anisotropic rie procedures using a c12 chemistry , using gate oxide 11 , for endpoint . photoresist removal , via oxygen plasma ashing , and careful wet cleans , are then performed . fig5 - 6 , describe the critical fabrication process used to create the ldd regions for the sram nfet , as well as for the peripheral nfet . it will be shown that the process used for ldd formation , is highlighted by an arsenic ldd for the sram cell design , while a phosphorous ldd is used for the peripheral nfet device . concentrating on the sram cell nfet , 9 , photoresist masking , 13 , is used to block out all regions but region 9 . an arsenic ion implantation is performed at an energy between about 30 to 50 kev ., at a dose between about 1e13 to 1e14 atoms / cm2 , resulting in a sram cell nfet ldd region , 14 . the use of the slower diffusing arsenic dopant allows for a more conductive ldd region , then for example a faster diffusing phosphorous dopant . therefore the use of arsenic , for the sram cell nfet results in a lower series resistance and a higher idsat , then would have been obtained if phosphorous had been used . additional performance benefits , via the use of arsenic for the sram cell nfet , are arrived at by minimizing gate to source and drain overlap capacitance . if the faster diffusing phosphorous dopants were used more of the ldd would be located under the gate . the slower diffusing arsenic region also reduces the short channel effect , resulting in slightly longer channel lengths then phosphorous counterparts for a specific gate dimension . finally subthreshold leakage is less of a concern with the longer channel lengths obtained via the use of arsenic . thus an extra cell photomasking , and ion implantation step , that may have been needed if phosphorous were used for the sram nfet , now can be eliminated with arsenic ldds . however , the higher concentration gradient , existing at the aresenic ldd -- channel region interface , may aggravate hot carrier electron , ( hce ) injection , effects . this is not a serious concern for the sram nfet , due to the operating conditions of the cell , but would adversely effect the peripheral nfet , 10 , if that device was fabricated using an arsenic ldd . therefore the photoresist blockout , 13 , used to create the arsenic ldd in the sram nfet , is removed via oxygen plasma ashing , and an anneal is performed at a temperature between about 850 ° to 950 ° c ., for a time between about 20 to 40 min , in a nitrogen ambient . next another photoresist blockout , 15 , is formed to allow only the peripheral nfet device to receive a phosphorous ion implantation at an energy between about 30 to 80 kev ., at a dose between about 1e13 to 1e14 atoms / cm2 , to form phosphorous ldd region , 16 . the faster diffusing phosphorous dopant will result in a less severe concentration gradient at the ldd -- channel region interface , and thus be able to withstand the more severe operating conditions , then devices made with an arsenic ldd . finally the ldd process for the sram pfet is addressed by again stripping photoresist mask 15 , and forming photoresist blockout mask 17 , allowing the pfet to experience a bf2 ion implantation at an energy between about 20 to 60 kev ., at a dose between about 8e12 to 1e14 atoms / cm2 , to form the sram pfet ldd region 18 , shown in fig7 . after photoresist blockout mask 17 , is removed via oxygen plasma ashing , followed by careful wet cleans , a silicon oxide layer is deposited using lpcvd or plasma enhanced chemical vapor deposition , ( pecvd ), using tetraethylorthosilicate , ( teos ), as a source , at a temperature between about 650 ° to 750 ° c ., to a thickness between about 1000 to 3000 angstroms . a blanket , anisotropic rie procedure is then employed , using chf3 , to form an silicon oxide spacer , 19 , on the sidewalls of all polysilicon gate structures . this is shown in fig8 . next the heavily doped n type source and drain regions are formed for both the sram and peripheral nfets . this is accomplished by using photoresist blockout 20 , allowing an arsenic ion implantation to be performed to only the sram and peripheral nfets , at an energy between about 30 to 60 kev ., at a dose between about 2e15 to 5e15 atoms / cm2 , to form the heavily doped n type source and drain regions , 21 , shown in fig9 . the heavily doped region , 21 , results in a lowering of resistance , and since this region does not interface with the channel region of the peripheral nfet device , the hce phenomena is not influenced . the heavily doped p type source and drain regions are next formed . again photoresist removal of blockout mask 20 is accomplished via oxygen plasma ashing , followed by photoresist application and patterning to obtain blockout mask 22 . this mask allows only the pfets to experience a bf2 ion implantation , at an energy between about 25 to 60 kev ., at a dose between about 2e15 to 5e15 atoms / cm to form the heavily doped p type source and drain regions , 23 , for the sram pfet , shown in fig1 . fig1 , shows the passivation and metallization sequence used for this cmos structure . for purposes of clarity only the peripheral nfet device will be used . first a layer of silicon oxide , 24 , is grown using pecvd processing at a temperature between about 400 ° to 600 ° c ., to a thickness between about 4000 to 8000 angstroms . photolithographic and rie processing are used to create contact holes , 25 . after photoresist removal , via oxygen plasma ashing , followed by careful wet cleans , an al -- cu -- si film is deposited using r . f . sputtering or evaporation processes , to a thickness between about 4000 to 6000 angstroms . photolithographic and rie processing are then used to create metal structure , 26 , shown in fig1 . while this invention has been particularly shown and described with reference to , the preferred embodiments thereof , it will be understood by those skilled in the art that various changes in form and details may be made without departing form the spirit and scope of this invention .