the present invention provides a key hole structure and method for forming a key hole structure used to form a pore of a phase change memory cell . with reference now to fig2 , a substrate 100 including a bottom electrode 102 is provided . the bottom electrode 102 may be formed of titanium nitride ( tin ) or tungsten ( w ), for example . the bottom electrode 102 is connected to access circuitry ( not shown ) such as an access transistor or diode . a first dielectric layer 104 is formed on the substrate 100 and an isolation layer 106 is formed on the first dielectric layer 104 . according to an embodiment of the present invention , the first dielectric layer 104 may be formed of silicon nitride ( sin ), for example . the isolation layer 106 may be formed of silicon dioxide ( sio 2 ), for example . further , shown in fig2 , a second dielectric layer 108 is formed on the isolation layer 106 . the second dielectric layer 108 may be formed of silicon nitride ( sin ), for example . an alternative material for the second dielectric layer may be titanium nitride ( tin ). the choice of the materials is based on the material system . for instance , if silicon dioxide ( sio2 ) is used for the first dielectric layer 104 , and silicon ( si ) for the isolation layer 106 , then silicon dioxide ( sio2 ) may be used for the second dielectric layer 108 as well . as long as the wet etch can selectively recess the isolation layer 106 and the planarization stop layer 110 , and not the second dielectric layer 108 and the first dielectric layer 104 . also shown in fig2 , a third dielectric layer 110 e . g ., a planarization stop layer 110 is formed on the second dielectric layer 108 . according to an embodiment of the present invention , the planarization stop layer 110 may be formed of a low temperature silicon nitride ( sin ), silicon dioxide ( sio 2 ), oxy - nitride , oxide or other suitable material that has a faster buffered oxide etch ( boe ) etch rate compared to approximately 400 degree celsius silicon nitride ( sin ). according to an embodiment of the present invention , the planarization stop layer 110 is formed of a material having a faster etch rate than that of the second dielectric layer 108 and a similar etch rate as that of the isolation layer 106 . according to one embodiment of the present invention , the second dielectric layer 108 and the planarization stop layer 110 may both be formed of silicon nitride ( sin ) based on different deposition conditions ( e . g ., temperature ) thereby having different etch rates . for example , the second dielectric layer 108 may be formed of silicon nitride ( sin ) formed at a 400 degree celsius processing temperature and the planarization stop layer 110 may be formed of silicon nitride ( sin ) formed at a 200 degree celsius processing temperature . according to an embodiment of the present invention , the planarization stop layer 110 may be of a predetermined thickness equal to or greater than that of the second dielectric layer 108 . for example , the second dielectric layer 108 may be of a predetermined thickness of approximately 30 nanometers ( nm ) while the planarization stop layer 110 may be of a predetermined thickness of approximately 30 nanometers ( nm ) or greater . according to another embodiment of the present invention , the second dielectric layer 108 may be of a predetermined thickness ranging from approximately 10 nanometers ( nm ) to 50 nanometers ( nm ) and the planarization stop layer 110 may be of a predetermined thickness ranging from approximately 30 nanometers ( nm ) to approximately 60 nanometers ( nm ). further , in fig2 , a via 112 is formed extending to the first dielectric layer 104 using lithographic techniques . according to an embodiment of the present invention , a minimum thickness of the planarization stop layer 110 (“ third dielectric layer ”) is determined by the material recess amount after wet etch and cmp stop range . the thickness of the third dielectric layer should not be too thick that it changes the aspect ratio of the via 112 and changes the subsequent conformal film deposition profile . fig3 is a diagram illustrating a recessing operation of the via 112 that can be implemented within embodiments of the present invention . in fig3 , according to an embodiment of the present invention , since the isolation layer 106 and the planarization stop layer 110 have a similar etch rate , both are recessed a predetermined distance when an etching operation is performed , for example , a 500 : 1 buffered oxide etch ( boe ) operation , thereby creating overhang portions 108 a , 108 b of the second dielectric layer 108 . the present invention is not limited to any particular type of etching process and may vary accordingly . as shown in fig3 , the isolation layer 106 and the third dielectric layer 110 may be etched back at approximately a same predetermined distance . in the case of forming a 30 nanometers ( nm ) key hole , the isolation layer 106 may be etched back approximately 15 nanometers ( nm ) on each side while the third dielectric layer 110 is etched back approximately 5 to 10 nanometers ( nm ) on each side . next , the deposition of a conformal film is described below with reference to fig4 . fig4 is a diagram illustrating a deposition operation of a conformal film that can be implemented within embodiments of the present invention . as shown in fig4 , a conformal film 120 is deposited within the via 112 and pinched to form a void ( i . e ., a key hole 130 ) in the via 112 . the conformal film 120 may be made of amorphous silicon ( a - si ) or polysilicon , for example . the conformal film 120 may be deposited by a chemical vapor deposition ( cvd ) or atomic layer deposition ( ald ) deposition , for example or any other suitable deposition process . as shown in fig4 , a tip 130 a of the key hole 130 is formed at an upper surface of the second dielectric layer 108 . fig5 is a diagram illustrating a planarization operation of the conformal film shown in fig4 that can be implemented within embodiments of the present invention . as shown in fig5 , the conformal film 120 is polished via a chemical mechanical polishing ( cmp ) process , for example , to an upper surface of the planarization stop layer 110 . therefore , during the cmp process , the planarization stop layer 110 acts as a cmp stop layer to stop the cmp before it approaches the tip 130 a of the key hole 130 . since the tip 130 a of the key hole 130 is formed at the upper surface of the second dielectric layer 108 , the tip 130 a remains closed and cmp slurry does not enter the key hole 130 and block a subsequent reactive ion etching ( rie ) process ( as depicted in fig6 ) for transferring the key hole 130 to the first dielectric layer 104 to form a pore of a memory cell . the formation of a pore of a memory cell will now be described below with reference to fig6 . fig6 is a diagram illustrating the formation of a pore that can be implemented within embodiments of the present invention . as shown in fig6 , the conformal film 120 is recessed via a rie process and the keyhole 130 is transferred down into the first dielectric layer 106 to form a pore 135 of a memory cell . further , phase change material ( not shown ) is then deposited within the pore 135 and a top electrode is formed on the phase change material , resulting in the formation of a phase change memory cell . according to an embodiment of the present invention , the pore 135 may be filled with tin or w to form the sub - lithography diameter bottom electrode for mushroom phase change memory cell . embodiments of the present invention provide a key hole structure and method for forming a key hole structure to form a pore of a memory cell . the present invention uses a planarization stop layer which provides the advantage of forming a key hole structure that prevents cmp slurry from approaching a tip of the key hole thereby preventing interference of a key hole transfer during a rie process for forming a pore within a memory cell . the present invention further minimizes the time of the rie process which contributes to the non - uniformity across the wafer , thereby improving the uniformity across the wafer . the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention . as used herein , the singular forms “ a ”, “ an ” and “ the ” are intended to include the plural forms as well , unless the context clearly indicates otherwise . it will be further understood that the terms “ comprises ” and / or “ comprising ,” when used in this specification , specify the presence of stated features , integers , steps , operations , elements , and / or components , but do not preclude the presence or addition of one or more other features , integers , steps , operations , element components , and / or groups thereof . the corresponding structures , materials , acts , and equivalents of all means or step plus function elements in the claims below are intended to include any structure , material , or act for performing the function in combination with other claimed elements as specifically claimed . the description of the present invention has been presented for purposes of illustration and description , but is not intended to be exhaustive or limited to the invention in the form disclosed . many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention . the embodiment was chosen and described in order to best explain the principles of the invention and the practical application , and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated the flow diagrams depicted herein are just one example . there may be many variations to this diagram or the steps ( or operations ) described therein without departing from the spirit of the invention . for instance , the steps may be performed in a differing order or steps may be added , deleted or modified . all of these variations are considered a part of the claimed invention . while the preferred embodiment to the invention had been described , it will be understood that those skilled in the art , both now and in the future , may make various improvements and enhancements which fall within the scope of the claims which follow . these claims should be construed to maintain the proper protection for the invention first described .