Phase change memory cell with large electrode contact area

A phase change memory cell and a method for fabricating the phase change memory cell. The phase change memory cell includes a bottom electrode and a first non-conductive layer. The first non-conductive layer defines a first well, a first electrically conductive liner lines the first well, and the first well is filled with a phase change material in the phase change memory cell.

BACKGROUND

This invention relates to computer memory, and more particularly to a method for fabricating phase change memory cells.

Phase change memory cells are known to have failure modes. One example is an endurance failure that occurs after cycling a phase change memory cell many times between the crystalline (SET) and amorphous (RESET) states. Phase change memory cycling results in the phase change material becoming disconnected from an electrode and causing the cell to be electrically open with no current passing through. A root cause of this failure is a separation of the phase change material from a bottom electrode material.

Another example is a process induced failure leading to delamination of a phase change material. In this failure mode, high temperature processes throughout the processing of an integrated structure cause the phase change material shrink and form voids at corners. This causes the phase change material in the phase change memory cell to delaminate at its interfaces leaving the cell electrically open with no current passing through.

BRIEF SUMMARY

Accordingly, one example aspect of the present invention is a phase change memory cell. The phase change memory cell includes a bottom electrode and a non-conductive layer deposited above the bottom electrode. The non-conductive layer defines a well. The well is positioned above the bottom electrode. An electrically conductive liner lines the well. The electrically conductive liner at least partially lines the well and is in electrical contact with the bottom electrode. A phase change material is deposited in the well. The phase change material is in electrical contact with the electrically conductive liner.

Another example of the present invention is a method for fabricating a phase change memory cell. The method includes forming a bottom electrode and depositing a non-conductive layer above the bottom electrode. A defining step defines a well of arbitrary shape in the non-conductive layer and a lining step is used to line an electrically conductive liner along the side(s) and bottom of the well. A depositing step deposits a phase change material in the well such that the phase change material is in electrical contact with the electrically conductive liner. The electrically conductive liner is also in electrical contact with the bottom electrode.

DETAILED DESCRIPTION

The present invention is described with reference to embodiments of the invention. Throughout the description of the invention reference is made toFIGS. 1-6B. When referring to the figures, like structures and elements shown throughout are indicated with like reference numerals.

FIG. 1Ashows a phase change memory cell102according to one embodiment of the present invention. The phase change memory cell102includes a bottom electrode104and a first non-conductive layer106. The first non-conductive layer106is deposited above the bottom electrode104and defines a first well108. As used herein, the term first well may also be referred to as a lower pore. The first well of arbitrary shape108is positioned directly above the bottom electrode104. A first electrically conductive liner110lines at least one wall of the first well108. As used herein, the term electrically conductive liner may also be referred to as an electrode material layer. The first electrically conductive liner110partially fills the first well108and is in electrical contact with the bottom electrode104.

A phase change material112is deposited in the first well108and is in electrical contact with the electrically conductive liner110. In phase change memory, information is stored in materials that can be manipulated into different phases. Each of these phases exhibit different electrical properties which can be used for storing information. The amorphous and crystalline phases are typically two phases used for bit storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.

In one embodiment, glass chalcogenides are utilized as phase change material. This group of materials contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a glass chalcogenide when creating a phase change memory cell. An example of this would be Ge2Sb2Te5(GST), SbTe, and In2Se3. However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used as phase change material cell as long as they can retain separate amorphous and crystalline states.

The phase change memory cell102may further include a top electrode114. The first electrically conductive liner110may be in physical contact with both the bottom electrode104and the top electrode114. The phase change material112may also be in physical contact with both the bottom electrode104and the top electrode114.

In one embodiment, the phase change material112is in physical contact with both the first electrically conductive liner110and the bottom electrode104. Alternatively, as shown inFIG. 1B, the conductive liner110may line the bottom of the first well108. Thus, the conductive liner110is physically positioned between the phase change material112and the bottom electrode104.

FIG. 2shows the phase change memory cell according to another embodiment of the present invention. As shown, the phase change memory cell102includes a second non-conductive layer116deposited above the first non-conductive layer106. The second non-conductive layer116defines a second well118. It is noted that the second non-conductive layer116may be the same material as the first non-conductive layer106, thereby constituting a single unitary layer. Alternatively, the second non-conductive layer116and first non-conductive layer106may be composed of different materials. The wells formed in these layers (or layer) can be defined with an etch or any other method to form a well of arbitrary shape.

The second well118is positioned directly above the first well108. The phase change material112is deposited in the second well118. The phase change memory cell102may further include a top electrode114positioned above the second well118. The top electrode114may be in physical contact with the phase change material112in the second well. Alternatively, phase change material is deposited to fill the first well and above the well, followed by the top electrode material deposition, and is then pattered with Reactive Ion Etching (RIE) and encapsulated with insulating layer116.

FIG. 3Ashows the phase change memory cell according to another embodiment of the present invention. As shown, the phase change memory cell102includes a second electrically conductive liner120. The second electrically conductive liner120may line at least one wall of the second well118. The second electrically conductive liner120may not be in physical contact with the first electrically conductive liner110. In another embodiment, the second electrically conductive liner120may be in physical contact with the first electrically conductive liner110.

In one embodiment, the phase change material112is deposited in the first well108and defines a cavity within the second well118. The phase change memory cell102further includes a third non-conductive material layer122. The third non-conductive layer122is deposited above the phase change material112and inside the second well118. The phase change memory cell102may further include a top electrode114. The top electrode114is positioned above the third non-conductive material layer122. The top electrode114may be in physical contact with the third non-conductively material layer122, the phase change material112layer, and the second electrically conductive liner116. In one embodiment, the top electrode114may include materials such as Ti, TiN and W.

In one embodiment, the phase change material112is in physical contact with both the first electrically conductive liner110and the bottom electrode104. Alternatively, as shown inFIG. 3B, the conductive liner120bmay line the bottom of the first well108. Thus, the conductive liner120bis physically positioned between the phase change material112band the bottom electrode104. Alternatively, the conductive liner120bmay also line the bottom of the first well108. Thus, the conductive liner120bis physically positioned between the phase change material112band the bottom electrode104.

The conductive liner120bmay be in physical contact with both the bottom electrode104and the top electrode114b. Thus, the conductive liner120band the phase change material are arranged in parallel circuit between the top and bottom electrodes. The resistance of the conductive liner material may be at least two times the resistance of the phase change material when in the fully crystalline (SET) state. The conductive liner thickness may be in the range of 0.5 nm to 5 nm and may be comprised of TiN, TaN, TaAlN, TaSiN, TiAlN, TiSiN, TiCN, or TaCN.

It is noted that the second non-conductive layer116may be the same material as the first non-conductive layer106, thereby constituting a single unitary layer. Alternatively, the second non-conductive layer116and first non-conductive layer106may be composed of different materials. The wells formed in these layers (or layer) can be defined with an etch or any other method to form a well of arbitrary shape.

FIG. 4shows a method for fabricating a phase change memory cell in accordance with one embodiment of the present invention. The method includes forming step404. During forming step404, a bottom electrode is formed. The bottom electrode may comprise materials such as Ti, TiN and W. After forming step404is complete, the method continues to depositing step406.

During depositing step406, a first non-conductive layer is deposited above the bottom electrode. In one embodiment, the first non-conductive layer is made of SiN. After the depositing step406is complete, the method continues to defining step408.

During defining step408, a first well is defined in the first non-conductive layer. The first well is positioned directly above the bottom electrode. This step may include transferring a keyhole cavity down to the first non-conductive layer to form the first well. After the defining step408is complete, the method continues to lining step410.

At lining step410, a first electrically conductive liner is lined along at least one wall of the first well. The electrically conductive liner partially fills the first well and is in electrical contact with the bottom electrode. In one embodiment, the electrically conductive liner is TiN. In other embodiments the electrically conductive liner is TaN, TaAlN, TaSiN, TiAlN, TiSiN, TiCN, or TaCN. After lining step410is complete, the method continues to depositing step412.

During depositing step412, a phase change material is deposited in the first well. The phase change material may be in electrical contact with the electrically conductive liner. As discussed above, a variety of materials may be used for the phase change material. The phase change material may include Ge2Sb2Te5. The phase change material may include SbxTe1-xmaterial, where 0.4 (Sb2Te3)<=x<=0.7 (Sb7Te3). The phase change memory cells may include In2Se3. After depositing step412is finished, the method proceeds to forming step414.

At forming step414, a top electrode is formed. The first electrically conductive liner may be in physical contact with both the bottom electrode and the top electrode. In one embodiment, the top electrode is TiN. The phase change material may be in physical contact with both the bottom electrode and the top electrode. In another embodiment the top electrode and the bottom electrode are comprised of TiAlN, or TaAlN, or TaN, or TiSiN, or TaSiN.

FIGS. 5A and 5Bshow another method for fabricating a phase change memory cell in accordance with one embodiment of the present invention. The method includes forming step404, depositing step406, defining step408, lining step410, and depositing step412discussed above.

After depositing step406is performed, the process continues to depositing step502. At depositing step502, a second non-conductive layer is deposited above the first non-conductive. In one embodiment, the second non-conductive layer is SiO2. After depositing step502is finished, the method continues to defining step504.

During the defining step504, a second well is defined in the second non-conductive layer. In one embodiment, the second well is formed before the first well is defined at defining step408. This step may include performing a lithographic etch and a buffered oxide etch (BOE) to create the second well, with an overhang above the second non-conductive layer. The second well is then filled with a conformal material such that a keyhole cavity is formed within the via. After defining step418is finished, the method continues to defining step408.

As mentioned above, defining step408may include transferring the keyhole cavity down to the first non-conductive layer to form the first well. After the defining step408is complete, the method continues to lining steps506and410.

At lining steps506and410, a second electrically conductive liner is lined along at least one wall of the second well and a first electrically conductive liner is lined along at least one wall of the first well, respectively. The electrically conductive liner partially fills the first and second wells. These steps may be followed with a reactive ion etch (RIE) to remove the second liner along the bottom of the second well. In one embodiment, the first and second electrically conductive liners are TiN. In other embodiments, the first and second electrically conductive liners are TaN, TaAlN, TaSiN, TiAlN, TiSiN, TiCN, or TaCN. After lining step410is complete, the method continues to depositing steps508and412.

At depositing steps508and412, phase change material is deposited in the first and second wells, respectively. The phase change material may be in electrical contact with the electrically conductive liner. As discussed above, a variety of materials may be used for the phase change material. The phase change material may include Ge2Sb2Te5. The phase change material may include SbxTe1-xmaterial, where 0.4 (Sb2Te3)<=x<=0.7 (Sb7Te3). The phase change memory cells may include In2Se3. After depositing steps508and412are completed, the method proceeds to forming step510.

During forming step510, a top electrode is formed above the second well. In one embodiment, the top electrode is TiN. The top electrode may be in physical contact with the phase change material in the second well.

FIGS. 6A and 6Bshow a method for fabricating a phase change memory cell in accordance with a further embodiment of the present invention. The method includes forming step404, depositing step406, depositing step502, defining step504, defining step408, lining step506, lining step410, depositing step508, and depositing step412, as discussed above. After depositing step508is complete, the method continues to depositing step602.

During depositing602, a third non-conductive material layer is deposited above the phase change material and within the second well. In one embodiment, the third non-conductive material layer is comprised of Si3N4.

Depositing step602may include spinning on a NFC (near frictionless carbon) material which will planarize the surface. Next, a RIE etch is performed to etch back the NFC. The exposed SiN layer is then selectively etched using DHF or hot phosphoric acid. Next, the exposed phase change material is etched using RIE or wet etch, for example. Finally, the NFC is removed using RIE or wet etch. After depositing step602is complete, the method continues to forming step604.

During the forming step604, a top electrode material is formed above the third non-conductive material layer. The top electrode material may be physical contact with the third non-conductively material layer, the phase change material layer, and the second electrically conductive liner. In one embodiment, forming step604may include an Ar sputter clean, a Ti deposition, a CVD and TiN deposition, and a CVD and W deposition.

Accordingly, one embodiment of the invention is a phase change memory cell with a volume of phase change material connecting the top electrode and the bottom electrode. The phase change material is programmable between the fully crystalline and fully amorphous states, and may also be programmed in a partially amorphous state. In one embodiment of the invention, the phase change material may be in parallel to a conductive liner material. In this arrangement, the non-phase change material also makes a connection between the top electrode and bottom electrode. The effective resistance of the phase change memory cell is also the resistance of the phase change material in parallel with the conductive liner.

If the conduction path has the conductive liner material and the phase change material in parallel, the cell may require that the resistivity of the conductive liner material be greater than the resistivity of the phase change material when in the crystalline state (SET state). For example, the resistance of the conductive liner material may be at least two times the resistance of the phase change material when in the fully crystalline (SET) state.

Embodiments of the invention include a phase change memory cell structure and a method to build said structure. The phase change memory cell advantageously includes a large surface area of contact between the phase change material and the electrode material while to maintaining a small effective electrode cross-sectional area.

In one embodiment, the first well is filled partially with the phase change material and partially with an electrode material layer. TiN or TaN may be used for the electrode material layer. Al, Si, C, or N doping may also be used inside of the electrode material layer to increase the resistivity. In one embodiment of the invention, a reactive ion etch (RIE) may be used to remove the electrode material layer in physical contact with and parallel to the first non-conductive layer. In this embodiment, the phase change material may be deposited to completely fill the lower pore.

In one embodiment, the phase change material of the phase change memory cell fills the second well entirely and the electrically conductive liner is disconnected between the first well and the second well, the thermodynamic properties and electro-conductive properties may be tuned to prevent an increased programming current that may result from heat conduction along the lower pore. This embodiment may have cell characteristics in between a standard mushroom cell with a lower pore completely filled with a non-phase change conductive material, and a cell with no electrically conductive liner such that the lower pore is filled with a phase change material only.

In one embodiment, the phase change material fills the lower pore and lines a cavity in the second non-conductive layer. In this embodiment, there can be a third non-conductive layer deposited above the phase change material layer. Si3N4 may be used for the third non-conductive layer. In another embodiment the third non-conductive layer may be selectively etched using compositions such as DHF or phosphoric acid. The phase change material of this embodiment may also be etched using RIE or wet etch.

In one embodiment, a top electrode material may be deposited above the third non-conductive layer such that it is also in contact with the phase change material and the electrically conductive liner. The top electrode material may include Ti, TiN, TaN, and W.