the following description of the invention will typically be with reference to specific structural embodiments and methods . it is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments and methods but that the invention may be practiced using other features , elements , methods and embodiments . like elements in various embodiments are commonly referred to with like reference numerals . fig1 is a simplified cross - sectional view of a phase change memory device 10 made according to one embodiment of the invention . device 10 comprises broadly a memory cell access layer 12 formed on a substrate , not shown , and a memory cell layer 14 formed on top of access layer 12 . access layer 12 typically comprises access transistors ; other types of access devices may also be used . access layer 12 comprises first and second polysilicon word lines acting as first and second elements 16 , 18 , first and second plugs 20 , 22 and a common source line 24 all within a dielectric film layer 26 . phase change memory device 10 and its method of manufacturer will be described with reference to fig2 - 11 . referring now to fig2 , memory cell access layer 12 is seen to have a generally flat upper surface 28 , the upper surface being interrupted by voids 30 formed in plugs 20 , 22 and by void 32 formed in common source line 24 . voids 30 , 32 , or other surface imperfections , are formed as an artifact of the deposition process used for formation of tungsten plugs within small dimension vias . deposition of , for example , a phase change material directly onto the upper surfaces 33 of plugs 20 , 22 can create a distribution problem , that is create an increased variance in the operational characteristics of the devices , due to the existence of voids 30 . fig3 illustrates the results of tin deposition to create a first electrode layer 34 and chemical mechanical polishing cmp of layer 34 to create a planarized surface 36 . layer 34 is preferably about 100 to 800 nm thick , typically about 500 nm thick after planarization . first electrode layer 34 fills voids 30 , 32 to effectively eliminate the distribution problem that could be created by the voids or other surface imperfections . planarization removes artifacts of the voids that result from deposition of the electrode material layer 34 . an inter - electrode layer 38 , see fig4 , is deposited on layer 34 . layer 38 may comprise one or more layer of an electrical insulator such as silicon dioxide , or variants thereof , is preferably about 40 to 80 nm thick , typically about 60 nm thick for the illustrated example . vias 40 , see fig5 , are formed in inter - electrode ayer 38 , typically using an appropriate lithographic mask , not shown , generally centered , within alignment tolerances of the manufacturing processes , above plugs 20 , 22 . vias 40 have a diameter of about the technology node , that is about 90 to 150 nm , typically about 130 nm for a technology node having a minimum lithographic feature size of 0 . 13 microns . a thermal insulator 42 is formed within each via 40 , using a conformal deposition process such as chemical vapor deposition ( cvd ). thermal insulator 42 is a better thermal insulator than the material of inter - electrode layer 38 , preferably at least 10 % better . therefore , thermal insulator 42 has a thermal conductivity value kappa of less than 0 . 014 j / cm / k / sec . representative materials for thermal insulator 42 include materials that are a combination of the elements silicon si , carbon c , oxygen o , fluorine f , and hydrogen h . examples of thermally insulating materials which have a thermal insulation value kappa of greater than 0 . 014 j / cm /° k / sec . and are candidates for use as thermal insulator 42 include sicoh , polyimide , polyamide , and fluorocarbon polymers . other examples of materials which are candidates for use for thermal insulator 42 include fluorinated sio 2 , silsesquioxane , polyarylene ethers , parylene , fluoro - polymers , fluorinated amorphous carbon , diamond like carbon , porous silica , mesoporous silica , porous silsesquioxane , porous polyimide , and porous polyarylene ethers . in other embodiments , the thermally insulating structure comprises a gas - filled void lining the walls of via 40 . a single layer or combination of layers can provide thermal insulation . thermal insulator 42 is preferably formed as sidewall stucture to create the generally conical , downwardly and inwardly tapering central open region 44 shown in fig6 . open region 44 could have other constricting shapes , such as an hourglass shape , a reverse conical shape or a staircase or otherwise stepped shape . it may also be possible to make open region with a constant , appropriately small cross - sectional size and thus without a constricted area . the shape of open region 44 may be the result of the deposition process chosen for the deposition of thermal insulator 42 ; the deposition of thermal insulator 42 may also be controlled to result in the desired , typically constricting , shape for open region 44 . processing steps may be also undertaken after deposition of thermal insulator 42 to create the desired shape for open region 44 . fig7 illustrates a result of depositing a phase change material 46 within central open region 44 , followed by chemical mechanical polishing to create a surface 47 . phase change material 46 is thermally insulated from layer 38 by thermal insulator 42 . the downwardly and inwardly tapering shape of thermal insulator 42 creates a narrow transition region 48 of change material 46 to create a phase change element 49 at region 48 . phase change material 46 is typically about 130 nm wide at surface 47 and about 30 to 70 nm , typically about 50 nm , at transition region 48 . both the smaller size of phase change element 49 at transition region 48 and the use of thermal insulator 42 reduce the current needed to cause a change between a lower resistivity , generally crystalline state and a higher resistivity , generally amorphous state for phase change element 49 . fig8 illustrates the results of tin deposition and chemical mechanical polishing to create a second electrode layer 50 having a planarized surface 52 . lithographic mask 54 is shown in fig9 positioned overlying first and second plugs 20 , 22 and their associated thermal insulators 42 and phase change materials 46 . fig1 illustrates the results of etching steps in which portions of second electrode layer 50 , silicon dioxide layer 38 and first electrode layer 34 not covered by mask 54 are removed using appropriate etching recipes according to the composition of the layers to create etched regions 56 and first and second electrodes 57 , 59 . lithographic mask 54 is sized so that portions 61 of inter - electrode layer 38 are left surrounding thermal insulators 42 after the etching steps of fig1 to prevent etching of thermal insulator 42 , which could be caused by conventional tolerances associated with conventional manufacturing steps . fig1 illustrates the results of an oxide fill - in step in which an fill 58 , such as silicon dioxide , is deposited within etched regions 56 , reconstituting the inter - electrode layer 48 and filling between the memory cells , and followed by cmp to create planarized surface 60 . thereafter , an electrically conductive material 62 is deposited on surface 60 to create phase change memory device 10 , including memory cells 64 , shown in fig1 . electrically conductive material 62 is typically copper or aluminum , but it also may be tungsten , titanium nitride or other materials and combinations of materials . electrodes 57 , 59 in the illustrated embodiments are preferably tin . although other materials , such as tan , tialn or taaln , may be used for electrodes 57 , 59 , tin is presently preferred because it makes good contact with gst ( discussed below ) as phase change material 46 , it is a common material used in semiconductor manufacturing , and it provides a good diffusion barrier at the higher temperatures at which phase change material 46 transitions , typically in the 600 - 700 ° c . range . plugs 20 , 22 and common source line 24 are typically made of tungsten . embodiments of the memory cell include phase change based memory materials , including chalcogenide based materials and other materials , for phase change material 46 . chalcogens include any of the four elements oxygen ( o ), sulfur ( s ), selenium ( se ), and tellurium ( te ), forming part of group vi of the periodic table . chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical . chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals . a chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements , such as germanium ( ge ) and tin ( sn ). often , chalcogenide alloys include combinations including one or more of antimony ( sb ), gallium ( ga ), indium ( in ), and silver ( ag ). many phase change based memory materials have been described in technical literature , including alloys of : ga / sb , in / sb , in / se , sb / te , ge / te , ge / sb / te , in / sb / te , ga / se / te , sn / sb / te , in / sb / ge , ag / in / sb / te , ge / sn / sb / te , ge / sb / se / te and te / ge / sb / s . in the family of ge / sb / te alloys , a wide range of alloy compositions may be workable . the compositions can be characterized as teagebsb100 -( a + b ). one researcher has described the most useful alloys as having an average concentration of te in the deposited materials well below 70 %, typically below about 60 % and ranged in general from as low as about 23 % up to about 58 % te and most preferably about 48 % to 58 % te . concentrations of ge were above about 5 % and ranged from a low of about 8 % to about 30 % average in the material , remaining generally below 50 %. most preferably , concentrations of ge ranged from about 8 % to about 40 %. the remainder of the principal constituent elements in this composition was sb . these percentages are atomic percentages that total 100 % of the atoms of the constituent elements . ( ovshinsky &# 39 ; 112 patent , columns 10 - 11 .) particular alloys evaluated by another researcher include ge 2 sb 2 te 5 , gesb 2 te 4 and gesb 4 te 7 . ( noboru yamada , “ potential of ge — sb — te phase - change optical disks for high - data - rate recording ” , spie v . 3109 , pp . 28 - 37 ( 1997 ).) more generally , a transition metal such as chromium ( cr ), iron ( fe ), nickel ( ni ), niobium ( nb ), palladium ( pd ), platinum ( pt ) and mixtures or alloys thereof may be combined with ge / sb / te to form a phase change alloy that has programmable resistive properties . specific examples of memory materials that may be useful are given in ovshinsky &# 39 ; 112 at columns 11 - 13 , which examples are hereby incorporated by reference . phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase , and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell . these alloys are at least bistable . the term amorphous is used to refer to a relatively less ordered structure , more disordered than a single crystal , which has the detectable characteristics such as higher electrical resistivity than the crystalline phase . the term crystalline is used to refer to a relatively more ordered structure , more ordered than in an amorphous structure , which has detectable characteristics such as lower electrical resistivity than the amorphous phase . typically , phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states . other material characteristics affected by the change between amorphous and crystalline phases include atomic order , free electron density and activation energy . the material may be switched either into different solid phases or into mixtures of two or more solid phases , providing a gray scale between completely amorphous and completely crystalline states . the electrical properties in the material may vary accordingly . phase change alloys can be changed from one phase state to another by application of electrical pulses . it has been observed that a shorter , higher amplitude pulse tends to change the phase change material to a generally amorphous state . a longer , lower amplitude pulse tends to change the phase change material to a generally crystalline state . the energy in a shorter , higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state . appropriate profiles for pulses can be determined , without undue experimentation , specifically adapted to a particular phase change alloy . a material useful for implementation of a pcram described herein is ge 2 sb 2 te 5 , commonly referred to as gst . other types of phase change materials can also be used . other programmable resistive materials may be used in other embodiments of the invention , including n 2 doped gst , ge x sb y , or other material that uses different crystal phase changes to determine resistance ; pr x ca y mno 3 , prsrmno , zrox , or other material that uses an electrical pulse to change the resistance state ; 7 , 7 , 8 , 8 - tetracyanoquinodimethane ( tcnq ), methanofullerene 6 , 6 - phenyl c61 - butyric acid methyl ester ( pcbm ), tcnq - pcbm , cu - tcnq , ag - tcnq , c60 - tcnq , tcnq doped with other metal , or any other polymer material that has bistable or multi - stable resistance state controlled by an electrical pulse . for example , another type of memory material that in some situations may be appropriate is a variable resistance ultra thin oxide layer . for additional information on the manufacture , component materials , use and operation of phase change random access memory devices , see u . s . patent application ser . no . 11 / 155 , 067 , filed 17 jun . 2005 , entitled thin film fuse phase change ram and manufacturing method . the invention has been described with reference to phase change materials . however , other memory materials , also sometimes referred to as programmable materials , can also be used . as used in this application , memory materials are those materials having electrical properties that can be changed by the application of energy ; the change can be a stepwise change or a continuous change or a combination thereof . the above descriptions may have used terms such as above , below , top , bottom , over , under , et cetera . these terms are used to aid understanding of the invention are not used in a limiting sense . while the present invention is disclosed by reference to the preferred embodiments and examples detailed above , it is to be understood that these examples are intended in an illustrative rather than in a limiting sense . it is contemplated that modifications and combinations will occur to those skilled in the art , which modifications and combinations will be within the spirit of the invention and the scope of the following claims . any and all patents , patent applications and printed publications referred to above are hereby incorporated by reference .