the following description is of the best - contemplated mode of carrying out the invention . this description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense . the scope of the invention is best determined by reference to the appended claims . embodiments of phase - change memory devices and methods for manufacturing the same are described as below incorporating fig2 - 18 . as shown in fig2 , a substrate 100 such as a semiconductor substrate or a p type silicon substrate is provided . active devices such as transistors , diodes or thin films such as dielectric layers may be formed on the substrate 100 but are not illustrated here . the substrate 100 is merely illustrated as a planar substrate without showing the layers or active devices formed thereon for the sake of simplicity . a plurality of n + buried address lines or n + buried base lines 102 are formed in the substrate 100 . the formation of the n + buried base lines 102 may be accomplished by conventional lithographic processes and ion implantation process . for example , a mask such as a patterned photoresist layer ( not shown ) defining the n + buried base lines 102 is first formed on the substrate 100 . an ion implantation process is then carried out to implant n type dopants such as arsenic or phosphorus into the substrate 100 through the openings in the patterned photoresist layer . after removing the patterned photoresist layer , an anneal process may be carried out to activate the dopants . after the formation of the n + buried base lines 102 , a dielectric layer 103 is then deposited over the substrate 100 . the dielectric layer 103 may comprise borophosphosilicate glass ( bpsg ), silicon oxide , spin - on glass ( sog ) or silicon nitride formed by , for example , chemical vapor deposition or spin - on methods . as shown in fig3 , subsequently , a photolithographic process ( bit line photo patterning ) and an etching process are carried out to form openings 103 a in the dielectric layer 103 . the openings 103 a is formed through the dielectric layer 103 and each of the openings 103 a partially exposes a portion of the underlying n + buried base lines 102 . as shown in fig4 , a layer of conductive material is then blanketly deposited over the dielectric layer 103 and fills the openings 103 a . the portion of the conductive material outside the openings 103 a and above the dielectric layer 103 is then removed by a planarization process such as a chemical mechanical polishing ( cmp ) process , thereby leaving a conductive electrode 104 in each of the openings 103 a and exposing a top surface of the conductive electrode 104 . according to this embodiment , the conductive electrode 104 may comprise doped polysilicon , doped si , doped sige , tiw , or tin . as shown in fig5 , an etching process is then performed to remove portions of the dielectric layer 103 and exposes a top portion 104 a of each of the conductive electrodes 104 . according to this embodiment , the top portion 104 a of each of the conductive electrodes 104 is exposed with a height h of about 10 - 5000 angstroms , preferably of about 100 - 4000 angstroms . in addition , the conductive electrode 104 is formed with a diameter d 1 that is determined by the process capability of the photolithography process for forming the openings 103 a . the conductive electrodes 104 may function as a heating electrode for heating up a sequentially formed phase change material layer . as shown in fig6 and fig7 , an oxidation process is then performed to partially oxidize the exposed top portion 104 a of each of the conductive electrodes 104 , thereby forming an oxide layer 104 b over the surface of the exposed top portion 104 a of each of the conductive electrodes 104 . the oxidation process may be a furnace oxidation process which is performed under a temperature of about 500 - 1000 ° c . for a time period of about 1 - 600 minutes . process time and temperature of the oxidation process is not limited to those disclosed above and may vary according to materials used in the conductive electrode 104 . the oxide layer 104 b is formed on a top surface and sidewall surfaces of the exposed top portion 104 a of each of the conductive electrode 104 . the oxide layer 104 b may slightly penetrate downward to a portion of the conductive electrode 104 below the top surface of the dielectric layer 103 . the oxide layer 104 b is then removed or stripped by methods known in the art such as wet etching , as shown in fig7 . at this point , the top portion 104 a is shrunk with a reduced dimension d 2 . alternatively , to shrink the top portion 104 a of each of the conductive electrodes 104 , a wet etching process may be used instead of the oxidation process . as shown in fig8 , a dielectric layer 105 is conformally deposited over the dielectric layer 103 and over the shrunk top portion 104 a of the conductive electrode 104 . the dielectric layer 105 forms a recess 105 a between neighboring top portions 104 a of the conductive electrodes 104 . the recess 105 a is formed around each of the top portion 104 a of the conductive electrode 104 . according to the embodiment , the dielectric layer 105 may comprise silicon nitride or silicon oxide and may have a thickness ranging between 50 angstroms and 500 angstroms . the dielectric layer 105 may be deposited by chemical vapor deposition methods such as pecvd . as shown in fig9 , a dielectric layer 106 is deposited over the dielectric layer 105 and the dielectric layer 106 fills the recess 105 a . according to this embodiment , the dielectric layer 106 and the dielectric layer 105 are made of different dielectric materials . for example , the dielectric layer 105 is composed of silicon nitride and the dielectric layer 106 is composed of silicon oxide , whereby the dielectric layer 106 can be removed selectively from the top surface of the dielectric layer 105 . in another embodiment , the dielectric layer 105 is composed of silicon oxide , while the dielectric layer 106 is composed of silicon nitride . likewise , the dielectric layer 106 may be deposited by chemical vapor deposition methods such as pecvd . as shown in fig1 , a planarization process such as a chemical mechanical polishing ( cmp ) process is carried out to remove a portion of the dielectric layer 106 and a portion of the dielectric layer 105 , thereby exposing a top surface of the top portion 104 a of each of the conductive electrodes 104 and resulting in a substantially planar surface at this stage . after the cmp , the remanent dielectric layer 106 a is embedded in the recess 105 a between neighboring top portions 104 a of the conductive electrodes 104 . as shown in fig1 and fig1 , a salicide process is implemented . first , a metal layer 107 is deposited over the substrate 100 by methods known in the art such as plating , sputtering , or deposition methods . according to this embodiment , the metal layer 107 may comprise cobalt , nickel , titanium or the like . the metal layer 107 directly contacts the exposed top surface of the top portion 104 a of each of the conductive electrodes 104 . subsequently , as shown in fig1 , the metal layer 107 reacts with the top portion 104 a through a rapid thermal anneal ( rtp ) process to transform the top portion 104 a into salicide heating stem 108 that acts as a heater of the pcm cell . the un - reacted metal layer 107 is then removed to expose a top surface of the salicide heating stem 108 . the salicide heating stem 108 is connected to a bottom diode 200 consisting of the conductive electrodes 104 embedded in the dielectric layer 103 and a portion of the n + buried base lines 102 . as shown in fig1 , after salicidizing , the remanent dielectric layer 106 a inlaid in the recess 105 a between neighboring top portions 104 a of the conductive electrodes 104 is selectively removed from the surface of the dielectric layer 105 , thereby revealing the recess 105 a . the conformal dielectric layer 105 extends to the sidewalls of the salicide heating stem 108 and encompasses the salicide heating stem 108 to thereby form upward protruding cylindrical features 110 . subsequently , a non - conformal deposition process is carried out to deposit a phase - change material layer 109 over the substrate 100 . according to this embodiment , the phase - change material layer 109 is not a conformal material layer , featuring a greater thickness atop the upward protruding cylindrical features 110 and a much thinner thickness at the sidewalls of the upward protruding cylindrical features 110 and at the bottom of the recess 105 a , thereby forming a tapered sectional profile . the non - conformal phase - change material layer 109 also features an overhang 109 a at the inlet of each of the recess 105 a . such non - conformal phase - change material layer 109 may be formed by non - conformal physical vapor deposition methods . the phase - change material layer 109 may comprise chalcogenide materials such as ge — sb — te trinary chalcogenide compounds or te — sb binary chalcogenide compounds . in accordance with another embodiment of this invention , the overhang 109 a may seal the recess 105 a , thereby forming void 109 b thereto , as shown in fig1 . in accordance with still another embodiment of this invention , the overhang 109 a may seal the recess 105 a and substantially no phase - change material is deposited at the bottom of the recess 105 a , as shown in fig1 . as shown in fig1 , a self - aligned etching process such as a self - aligned dry etching process or a self - aligned wet etching process is carried out to etch the phase - change material layer 109 , thereby forming a phase - change storage cap or pcm storage cap 109 c and revealing the recess 105 a in a self - aligned fashion . according to this embodiment , the phase - change material at the bottom of the recess 105 a is completely removed . however , it is understood that in another case the phase - change material at the bottom of the recess 105 a may not be completely removed . the pcm storage cap 109 c may cover an upper portion of the sidewall of the upward protruding cylindrical features 110 . in another embodiment , the pcm storage cap 109 c may cover entire sidewall of the upward protruding cylindrical features 110 and encapsulates the upward protruding cylindrical features 110 . a plan view showing the layout of pcm storage cap 109 c and the upward protruding cylindrical features 110 after performing the self - aligned etching process is shown in fig1 . as shown in fig1 , after the self - aligned etching process , a dielectric layer 111 such as silicon oxide , silicon oxy - nitride , silicon nitride or low - k dielectrics is deposited over the substrate 100 to seal the recess 105 a , thereby forming air gap 120 inside the recess 105 a . the air gap 120 may be vacuum or near vacuum depending upon the vacuum condition during the deposition of the dielectric layer 111 . the air gap 120 surrounds the upward protruding cylindrical features 110 and may act as a heat insulator that can effectively prevent heat generated from a salicide heating stem 108 from interfering neighboring cells during the heating or setting of the phase - change material of a particular cell bit . furthermore , the phase - change material may cover the sidewall of the upward protruding cylindrical features 110 and provide better heat insulating characteristic owing to the low thermal conductivity of the phase - change material . as shown in fig1 , a plurality of bit lines 140 are formed on the dielectric layer 111 and each of the plurality of bit lines 140 is electrically connected with corresponding pcm storage cap 109 c of a pcm cell through corresponding via / contact plug 144 that is formed in the dielectric layer 111 . according to this embodiment , the bit lines 140 may be composed of metals such as aluminum , copper , tungsten , silver , gold or alloys thereof . the via / contact plug 144 may be composed of aluminum , copper or tungsten . in sum , the advantages of using this invention include : ( 1 ) at least one photo mask can be spared because the pcm storage cap is defined by self - aligned method , thus making the present invention method more economical ; ( 2 ) the space between cells can be shrunk to minimum because the pcm storage cap is formed self - aligned to the salicide heating stem 108 and the bottom diode 200 , thereby increasing the cell packing density ; and ( 3 ) the heater , i . e ., the salicide heating stem 108 , is surrounded by air gap and , optionally , the phase - change material with low thermal conductivity , thereby avoiding cross talk between cells during operation and improving the phase - change efficiency . those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention . accordingly , the above disclosure should be construed as limited only by the metes and bounds of the appended claims .