text: referring now to the drawing , in which like reference numbers refer to like elements throughout , fig1 - 12 show various aspects of an exemplary dram cell of the present invention and intermediate steps in the process for making the cell . as shown in fig1 and 3 , a typical deep trench storage capacitor 10 is formed into a pad 22 and a substrate 24 by conventional processing techniques well known in the art . for example , an optical lithography step may be used to form a pattern on pad 22 . then a dry etching step such as reactive - ion etching ( rie ), may be used to create a trench 20 to a desired depth through pad 22 and into substrate 24 . the cross - sectional pattern of deep trench 20 is typically an ellipse that cuts across crystal axes a and b . for example , as shown in fig1 axis a may have a [ 011 ] crystallographic orientation and axis b may have a [ 001 ] orientation . substrate 24 is typically silicon and pad 22 is typically a silicon nitride ( sin ) layer having a thickness of about 10 nm to about 100 nm . there may also be a thin thermal oxide layer ( not shown ), typically about 3 nm to about 10 nm thick , between substrate 24 and pad 22 . deep trench 20 generally has a depth of about 3 μm to about 10 μm and a diameter or maximum width that is a function of the lithographic ground rule , typically about 0 . 5 μm to less than 0 . 1 μm . trench 20 has side walls 32 and a bottom 33 . as shown in fig2 and 4 , in accordance with the present invention , an isolation collar 26 is formed in upper region 28 of trench 20 . upper region 28 typically comprises 10 to 20 % of the total depth of trench 20 . collar 26 may be formed using local thermal oxidation ( locos ), such as by the exemplary process explained below , or by other physical and chemical mechanisms , as also indicated below . before the oxidation step , a barrier film ( not shown ) may be formed along the exposed surfaces of trench 20 and pad 22 such as by a low - pressure chemical vapor deposition ( lpcvd ) of a sin film having a thickness of about 2 nm to about 10 nm . the barrier film is then removed from upper region 28 , for example by filling trench 20 with photoresist ( not shown ) and partially etching the photoresist down into trench 20 to a depth controlled by the amount of overetch time . this step exposes the barrier film in upper region 28 while leaving the lower region 30 covered . the exposed barrier film may then be removed in upper region 28 of trench 20 and from pad layer 22 , for example , by chemical or dry etching , and then the photoresist stripped away . other processes for isolating side wall 32 in upper region 28 while protecting side wall 32 in lower region 30 may also be used . the local oxidation step is then performed at oxidation conditions that promote the oxidation rate along one family of crystal axes over another , such as , for example , promoting oxidation along & lt ; 011 & gt ; axes over & lt ; 001 & gt ; axes . for instance , the oxidation step may comprise the use of oxygen ( o2 ) or water ( h2o ) at a temperature of between about 800 ° c . and about 1 , 100 ° c . for between about 2 minutes and about 10 minutes , not including ramping time , to achieve an oxide isolation collar 26 having a thickness of between about 10 nm to about 50 nm . such oxidation conditions induce faceting of the underlying silicon substrate 24 during growth of collar 26 . thus , the curved trench wall 32 having an elliptical cross section , as shown in fig1 facets into a polygonal cross section having distinct planar walls 32 ′ and 32 ″ aligned with crystal planes ( 001 ) and ( 011 ), respectively , as shown in fig2 . faceted walls 32 ′ and 32 ″ thus have consistent physical and electrical properties along the faceted structure , providing improved transistor electrical , physical , and reliability characteristics as compared to transistors built on unfaceted trench walls . the thermal oxide collar 26 and associated faceting are formed only on side walls 32 in upper region 28 of trench 20 ; the barrier film protects side walls 32 in lower region 30 of trench 20 . faceted side walls 32 ′, 32 ″ may be formed , as described above , by thermal oxidation . faceted side walls 32 ′, 32 ″ may instead be formed by other physical or chemical mechanisms . such mechanisms include , for example , preferential crystal axis etching , such as etching with potassium hydroxide ( koh ), as is well - known in the art . the remaining dram structure may be constructed according to processes well known in the art , such as the exemplary process described below . the process provided below is not intended to be a limitation of the present invention , but rather is included for illustration . in such an exemplary process , the barrier film in lower region 30 is stripped via a process that selectively leaves thermal oxide isolation collar 26 in upper region 28 of trench 20 . buried plate 34 is then created in lower region 30 , leaving the configuration shown in fig4 . buried plate 34 may be created by doping lower region 30 of trench 20 to form an out - diffusion in substrate 24 using collar 26 as a mask for upper region 28 . the out - diffusion may be formed using arsenosilicate glass ( asg ) drive - in , plasma doping ( plad ), plasma ion implantation ( piii ), gas - phase diffusion of arsenic ( as ) or phosphorus ( p ), or other techniques known in the art . next , as shown in fig5 a thin node dielectric 35 is created , such as by thermal nitridation , for example with ammonia ( nh3 ), followed by lpcvd of sin . finally , trench 20 is filled , such as with an n + doped lpcvd polysilicon 36 , and recessed to a desired depth d 1 . depth d 1 is typically about 300 mn to about 700 nm , preferably between 300 to 450 nm . isolation collar 26 is then etched away , such as with a wet etch using a solution containing hydrogen fluoride ( hf ), to expose side walls 32 in the area where the collar 26 is not covered by polysilicon 36 and below the polysilicon level to a depth d 2 , as shown in fig6 . d 2 is typically about 10 nm to about 50 nm . next , as shown in fig7 a buried strap 40 is formed . typically of lpcvd silicon , buried strap 40 is formed in a layer having a thickness of about 10 mn to about 50 mn . as shown in fig8 buried strap 40 is then removed from side walls 32 of trench 20 above polysilicon 36 and above pad 22 , such as by an isotropic wet chemical or dry etching step . then , as shown in fig9 trench - top dielectric 42 or trench - top oxide ( tto ) is formed , such as by an anisotropic high - density plasma ( hdp ) or other bias - assisted oxide deposition step . the creation of trench - top dielectric 42 typically forms a corresponding layer ( not shown ) atop pad 22 , which is removed by a chemical mechanical polishing ( cmp ) step as is known in the art . thus exposed , pad 22 is then stripped , typically by a wet chemical etch step selective to trench - top dielectric 42 , and a sacrificial oxide 44 is grown on the exposed surface of substrate 24 and exposed side wall 32 of trench 20 , as shown in fig9 . ion implantation may then be used to create a p - well 50 and an n - band 52 below p - well 50 in substrate 24 . similarly , ion implantation of as or p may be used to create diffusion region 62 . another diffusion region 62 ′ is created by out - diffusion from n + doped polysilicon region 36 through buried strap 40 . such process steps yield the structure shown in fig9 . other device - threshold - tailoring implants may also be created at this time . next , as shown in fig1 , sacrificial oxide 44 is removed , such as by a chemical wet etch process with an hf - containing solution . then , gate oxide 45 is grown and a conductive gate layer 48 , such as polysilicon having a thickness approximately equal to the diameter of trench 20 , is formed . a nitride pad ( not shown ) is formed having a thickness of approximately half to approximately equal to the thickness of gate layer 48 . an active area 54 is patterned ( see fig1 a and 11 b ), typically by photolithography , and an etching step , such as rie , is performed to etch shallow trench isolation ( sti ) regions 46 everywhere except in active area 54 . sti regions 46 are filled , typically with an oxide , and planarized by a cmp step down to the pad nitride . the pad nitride is then stripped away , leaving the structure shown in fig1 . next , a thin seed layer of polysilicon is typically deposited , extending polysilicon gate layer 48 over the edge of trench 20 and over top diffusion region 62 in p - well 50 of substrate 24 . middle layer 56 , which typically comprises a higher conductivity material than polysilicon gate layer 48 , such as tungsten ( w ) or tungsten silicide ( ws ), is then formed . finally , gate cap layer 58 , comprising sin or silicon oxide , is formed . the gate conductor layers are then patterned by lithography and dry etched , leaving the gate conductor 59 ( comprising gate layer 48 , middle layer 56 , and gate cap 58 ) shown in fig1 . next , side wall isolation spacers 70 , typically comprising silicon nitride , silicon oxide , or a combination of those materials , are created by processes well - known in the art to electrically isolate the wordline ( gate conductor 59 ) from the bitline ( the diffusion contact 64 ). spacers 70 are typically created by depositing a conformal coating of sin of about 10 nm to about 100 nm , and performing an anisotropic dry spacer etch process to leave the spacers 70 only on the side walls of gate conductor 59 . at this time , optional added implants may be performed to tailor source and drain regions ( diffusion regions 62 and 62 ′) of the transistor . next , the isolating regions between multiple gate conductors 59 on the wafer are typically filled with an interlevel dielectric 63 , and contact holes are etched via lithography and dry etching to create the holes in which to form diffusion contact 64 . diffusion contact 64 typically comprises doped polysilicon or a tungsten stud . the overlapping region 72 of diffusion contact 64 and of gate cap layer 58 of gate conductor 59 , as shown in fig1 , is typical of diffusion contacts known as borderless contacts . thus , as shown in fig1 a , 11 b , and 12 , an exemplary trench - side wall array device 60 results from the exemplary process described above . as shown , n + diffusion regions 62 , 62 ′ under diffusion contact 64 adjacent to trench 20 serve as the source and drain of device 60 . a channel 66 results in substrate 24 ( p - well 50 ) adjacent faceted side wall 32 of trench 20 . although device 60 as shown in fig1 a has been fabricated across the ( 001 ) plane , device 60 may also be fabricated across the ( 011 ) plane as shown in fig1 b . the surface of substrate 24 is typically along the ( 100 ) plane . thus , device 60 may be along a crystallographic plane in the same family as the substrate surface { 001 }, as shown in fig1 a , or may be on a different crystallographic plane ( 011 ) in a different family , as shown in fig1 b . although illustrated and described herein with reference to certain specific embodiments , the present invention is nevertheless not intended to be limited to the details shown . rather , various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention .