Patent Grant
11765910
This application claims foreign priority to European Application EP 19208590.0, filed on Nov. 12, 2019, which is incorporated herein by reference in its entirety.
The disclosed technology relates to the field of memory devices, and more particularly, to memory devices including memory arrays. The disclosed technology provides a selector device for a memory array, a method of forming the selector device, and a memory device comprising a memory array and a plurality of the selector devices. The selector device is, in some embodiments, a bipolar selector device comprising a double-sided Schottky diode.
High-density memory arrays, e.g., magnetic random access memory (MRAM) arrays can suffer from the so-called sneak path problem. This problem arises from cross-talk interference between adjacent memory cells caused by a sneak path current, e.g., between a read-out cell and a neighboring cell. This can result in misinterpretation of the read-out signal of the read-out cell.
Selector devices can be used to protect the read-out signals. In particular, the sneak path problem can be mitigated by inserting selector devices next to the memory cells. The selector devices provide a threshold (diode behavior) for device selection based on word-line and bit-line (source line) voltage.
In spin-torque-transfer (STT)-MRAM array, for instance, the selector devices should be bipolar (a write operation needs current in an opposite direction than a read operation), i.e., there is need for bipolar diodes with a threshold behavior in opposite directions. In particular, bipolar, highly non-linear, selector devices should be inserted next to the memory cells.
One candidate for such selector devices is a double-sided indium-gallium-zinc-oxide (IGZO) Schottky diode, as it is illustrated in
The selector device requires both contacts of the vertical IGZO channel 93 to be of the Schottky type, i.e., a Schottky interface needs to be formed between the IGZO 93 and the top electrode 91, as well as between the IGZO 93 and the bottom electrode 94. However, it is difficult to obtain a Schottky contact at the top interface of the IGZO channel 93, i.e., the interface to the top electrode 91, because a plasma-assisted process (e.g., physical vapor deposition (PVD) that is typically involved in the deposition of the top electrode 91, damages the IGZO 93. Thereby, defects are created that make the contact of the IGZO 93 to the top electrode 91 Ohmic instead of Schottky. This leads to the sneak path problem described above.
Attempts have been made to reduce the power of the plasma, which is involved in the deposition of the top electrode 91, but that attempt was still not sufficient to obtain a good Schottky contact between the IGZO 93 and the top electrode 91.
In view of the above, one objective of the disclosed technology aims to provide an improved solution to the sneak path problem. Another objective is to provide a selector device and an integration flow for fabricating the selector device, which results in a bipolar selector device having good Schottky contacts. The selector device should comprise a double-sided Schottky diode. To this end, defects that make the contacts Ohmic instead of Schottky should not be created. The selector device should in some embodiments be based on a semiconductor material comprising a metal oxide material, like IGZO.
The objectives are achieved by the embodiments of the invention provided in the enclosed independent claims. Advantageous implementations of these embodiments are defined in the dependent claims.
A first aspect of the disclosed technology provides a selector device for a memory array, the selector device comprising: a first electrode layer embedded in an oxide, a second electrode layer arranged above the first electrode layer and separated from the first electrode layer by the oxide, a semiconductor material forming a semiconductor layer on the top surface of the second electrode layer and extending through the second electrode layer and the oxide onto the top surface of the first electrode layer, wherein the semiconductor material contacts the first electrode layer and the second electrode layer.
With the selector device of the first aspect, a double-sided Schottky barrier is obtained, which is necessary to counter the sneak path problem. In some embodiments, a Schottky contact/interface is formed between the semiconductor material and the first electrode layer, as well between the semiconductor material and the second electrode layer. Due to the arrangement of the semiconductor material and how it contacts the first and second electrode layer, respectively, any defects that are created do not impact the Schottky interfaces. This is, because any defects that are created are remote from the Schottky interfaces. Thus, the selector device of the first aspect provides an improved solution to the sneak path problem.
In an embodiment of the selector device, the semiconductor material comprises a metal oxide material, in some embodiments indium gallium zinc oxide or indium tin oxide, or comprises amorphous silicon.
These materials form particularly good Schottky contacts, and therefore improve the selector device further.
In an embodiment of the selector device, the first electrode layer, the semiconductor material, and the second electrode layer form a double-sided Schottky diode.
Thus, the selector devices provides a bipolar threshold (diode behavior), which is well suitable for device selection based on word-line and bit-line (source line) voltage in a memory array.
In an embodiment of the selector device, a first Schottky interface is formed between the semiconductor material extending through the oxide and the top surface of the first electrode layer, and a second Schottky interface is formed between the bottom surface of the semiconductor layer and the top surface of the second electrode layer.
In an embodiment of the selector device, the first electrode layer and/or the second electrode layer comprises platinum, palladium, and/or gold.
These materials have beneficially a work function in a range that provides a preferable Schottky barrier height, and the desired off-current needed for selector activity in, for example, an MRAM array. Palladium may beneficially form an interfacial palladium oxide, e.g., after ozone treatment, which is oxide-rich and thus reduces surface defect concentration of the semiconductor material.
In an embodiment of the selector device, the selector device further comprises a dielectric layer, in some embodiments an aluminum oxide layer, formed on the top and on the sides of the semiconductor layer.
In an embodiment of the selector device, the selector device further comprises a metal layer, in some embodiments a tungsten layer, formed on the top and on the sides of the dielectric layer, wherein the metal layer directly contacts the top surface of the second electrode layer.
The embodiments allow forming the metal layer wrapped around the top of the semiconductor layer, but isolated by the dielectric layer, while the metal layer connects directly to the second electrode layer to ensure electrical connectivity.
A second aspect of the disclosed technology provides a method of forming a selector device for a memory array, the method comprising: forming a first electrode layer, wherein the first electrode layer is embedded in an oxide, forming a second electrode layer above the first electrode layer, wherein the second electrode layer is separated from the first electrode layer by the oxide, forming an opening in the second electrode layer and in the oxide, wherein forming the opening exposes a part of the top surface of the first electrode layer, and depositing a semiconductor material, such that it fills the opening and forms a semiconductor layer on the top surface of the second electrode layer, wherein the semiconductor material contacts the first electrode layer and the second electrode layer.
The method of the second aspect results in the selector device of the first aspect, and thus all advantages described above. Without being bound by theory, the way in which the semiconductor material is formed, leads to Schottky interfaces between this semiconductor material and the first and second electrode layer, respectively.
For forming the opening, for example, a trench may be formed, e.g., etched, through the second electrode layer and the oxide onto the top surface of the first electrode layer, such that the trench exposes a part of the top surface of the first electrode layer. Alternatively, an aperture may first be formed in the second electrode layer, and the second electrode layer can then be used as a hard mask for an oxide etch through the aperture, which lands on the first electrode layer. Alternatively, the second electrode layer could be patterned into a perforated shape, e.g., with a Damascene process, and can then be used as a hard mask for an oxide etch, which lands on the first electrode layer.
In an embodiment of the method, the bottom surface of the first electrode layer is electrically connected to a memory element of the memory array.
Thus, the selector device can function in the memory array to mitigate the sneak path problem.
In an embodiment of the method, the method further comprises patterning the semiconductor layer into a semiconductor patch arranged on the top surface of the second electrode layer.
In an embodiment of the method, the method further comprises forming a dielectric layer around the semiconductor patch, and forming a metal layer, in some embodiments forming a tungsten layer, around the dielectric layer, wherein the metal layer contacts the top surface of the second electrode layer.
This allow forming the metal layer wrapped around the top of the semiconductor patch (remainder of the patterned semiconductor layer), but isolated by the dielectric layer, while the metal layer connects directly to the second electrode layer to ensure electrical connectivity. The metal layer may provide connection to a BEOL structure.
In an embodiment of the method, the method further comprises encapsulating the semiconductor patch with the dielectric layer, anisotropically etching the dielectric layer, such that a part of the top surface of the second electrode layer is exposed without exposing the encapsulated semiconductor patch, further encapsulating the encapsulated semiconductor patch with the metal layer, such that the metal layer contacts the exposed part of the top surface of the second electrode layer.
In an embodiment of the method, the dielectric layer is an aluminum oxide layer.
In an embodiment of the method, the method further comprises forming a BEOL structure above the metal layer, wherein the BEOL structure is electrically connected to the metal layer.
A third aspect of the disclosed technology provides a selector device for a memory array, wherein the selector device is fabricated using a method according to the second aspect or any embodiment thereof.
The selector device fabricated using the method of the second aspects can be distinguishing from conventional selector devices, in some embodiments, in the resulting semiconductor material connecting the first electrode layer and the second electrode layer.
A further aspect of the disclosed technology provides a memory device, comprising: a memory array including a plurality of memory elements, and a plurality of selector devices, wherein each selector device is according to the first aspect or any embodiment thereof or according to the third aspect, wherein each selector device is electrically connected to one of the memory elements.
The memory device of the fourth aspect enjoys the advantages of the selector device of the first aspect or third aspect described above. In some embodiments, in the memory array of the memory device, the sneak path problem is solved.
The above described aspects and embodiments are explained in the following description of embodiments with respect to the enclosed drawings:
The selector device 10 comprises a first (bottom) electrode layer 11, which is embedded in an oxide 12, e.g., a silicon dioxide. The first electrode layer 11 may be made of, or may comprise, platinum, palladium, and/or gold.
Further, the selector device 10 comprises a second (top) electrode layer 13, which is arranged above the first electrode layer 11, and is separated from the first electrode 11 layer by the oxide 12, in some embodiments, by a part of the oxide 12. The second electrode layer 13 may be made of, or may comprise, platinum, palladium, and/or gold.
As used herein, the term “above” refers to a stacking direction of the layers of the selector device 10, which is in
The selector device further comprises a semiconductor material 14, which may be, or may comprise, a metal oxide material. For example, it may be, or may comprise, IGZO, or ITO, or it may be, or may comprise, amorphous silicon or a similar material. The semiconductor material 14 forms a semiconductor layer 14a on the top surface of the second electrode layer 13. Further, the semiconductor material 14 extends through, e.g., forms a semiconductor channel 14b through, the second electrode layer 13 and the oxide 12, respectively, which lands on the top surface of the first electrode layer 11. The semiconductor layer 14a and the semiconductor channel 14b may be formed integrally. The semiconductor material 14 contacts the second electrode layer 13, in some embodiments, the semiconductor layer 14a contacts a top surface of the second electrode layer 11. Further, the semiconductor material 14 contacts the first electrode layer 11, in some embodiments, the semiconductor channel 14b contacts the top surface of the first electrode layer 11.
Thereby, multiple Schottky interfaces are formed as indicated in
The formation of these Schottky interface between the semiconductor material 14 and, respectively, the first electrode layer 11 and the second electrode layer 13, is an important aspect. The materials of the first and second electrode layer 11 and 13 should thus, respectively, be selected such that the work function of the material is in a range that results in a Schottky barrier having a desired height and a desired off-current needed for selector activity in a memory array. Thus, platinum, palladium, and/or gold may be used for the first and/or the second electrode layers 11 and 13, in some embodiments, in combination with IGZO as the semiconductor material 14. Furthermore, palladium can form an interfacial palladium oxide, e.g., after ozone treatment, which is oxide-rich, and may hence advantageously reduce the surface defect concentration of the IGZO.
It is further illustrated in
The selector device 10 shown in
Further, the selector device 10 of
For fabricating the selector device 10, which is shown in
The dielectric layer 23 may then be deposited on top of the semiconductor material 14, hence, potentially creating defects at the interface of semiconductor material 14 and dielectric layer 23. However, such defects have no impact on the electrical performance of the selector device 10, because they are not located in the path of the current, or near the Schottky interface between the second electrode layer 13 and the semiconductor material 14.
A connection to the upper levels of the BEOL may be obtained. For instance, by further using an etch stop layer (e.g., Al2O3) and an anisotropic opening etch, which may expose the first electrode layer 13 without reaching the top of the semiconductor layer 14a. This can prevent that a contact between the BEOL and the semiconductor material 14 becomes Ohmic.
In the selector device 10 shown in
A detailed integration flow for fabricating the selector device 10 as shown in
In step 1a, the second electrode layer 13 is deposited. For forming the second electrode layer 13, like the first electrode layer 11, a platinum blanket deposition may be performed. This step 1a relates to step 32 of the general method 30.
In step 2a1, a lithographic etch 40 (indicated with the arrows) may be performed to create an opening 43, e.g., a trench, in the second electrode layer 13 and in the oxide 12, such that the opening 43 lands on the first electrode layer 11 and exposes a part of the top surface of the first electrode layer 11. This step 2a1 relates to step 33 of the general method 30.
In step 2a2, an alternative to step 2a1 is shown. Here, the second electrode layer 13 is patterned first, e.g., an aperture 44 is formed in the second electrode layer 13. This may be done by etching. The patterned second electrode layer 13 can then be used as a hard mask for an oxide etch 45 into the oxide 12 (as indicated with the arrows). The etch 45 in the end lands on the first electrode layer 11 and forms the opening 43 (as indicated with the dotted lines).
In step 3a, the semiconductor material 14, for instance IGZO, is deposited and fills the opening 43 created in step 2a1 or 2a2. Thereby, no damage is created to the Schottky interface, which is formed between the semiconductor material 14 and the second electrode layer 13. This step 3a relates to step 34 of the general method 30.
In step 1b, the second electrode layer 13 is deposited. For forming the second electrode layer 13, like the first electrode layer 11, a platinum blanket deposition may be performed. The second electrode layer 13 is specifically deposited over the protruding structure 50 and on the recessed surface of the oxide 12, i.e., the second electrode layer 13 encapsulates the protruding structure 50. This step 1a relates to step 32 of the general method 30.
In step 2b1, CMP 51 is performed, in order to reduce the thickness of the second electrode layer 13. In some embodiments, the thickness is reduced such that the second electrode layer 13 is removed from the top surface of the protruding structure 50, and only remains next to the protruding structure 50 on the recessed top surface of the oxide 12.
In step 2b2, a self-aligned oxide etch 52 is performed into the oxide 12 (aligned with the protruding structure 50). That is, the patterned second electrode layer 13 is used as a hard mask for the oxide etch 52 (as indicated with the arrows). The etch 52 in the end lands on the first electrode layer 11 and forms the opening 43 (as indicated with the dotted lines). This step 2b2 relates to step 33 of the general method 30.
In step 3b, the semiconductor material 14, for instance IGZO, is deposited and fills the opening 43 created in step 2b1 and 2b2. Thereby, no damage is created to the Schottky interface, which is formed between the semiconductor material 14 and the second electrode layer 13. This step 3b relates to step 34 of the general method 30.
In step 4, the semiconductor material 14 is planarized 60. For instance, chemical mechanical planarization/polishing (CMP) or CMP+etch after planarization may be performed, in order to reduce the defect density. Defects in the semiconductor material 14 may form at its top as indicated, but these defects have no impact on later performance of the selector device 10. No damage/defects are created at the interface between the semiconductor material 14 and the second electrode layer 13, so that a good Schottky contact is formed. This step 4 still relates to step 34 of the general method 30.
In step 5, a dielectric layer 23 is provided onto the semiconductor material 14. In some embodiments, an aluminum oxide layer may be deposited, e.g., by means of plasma-enhanced chemical vapor deposition (PECVD).
In step 6, the dielectric layer 23 is patterned. For example, reactive ion etching (RIE) 61 may be used to pattern the (aluminum) dielectric layer 23, and the patterning is stopped before the etching 61 reaches the semiconductor material 14.
In step 7, ion beam etching (IBE) 62, using the dielectric layer 23 as a hard mask, is performed, in order to further pattern the dielectric layer 23, and to further pattern the semiconductor material 14. In some embodiments, the semiconductor layer 14a formed on the second electrode layer 13 is patterned into a semiconductor patch. Notably, re-sputtering of the material of the second electrode layer 13, e.g., re-sputtering of platinum, is not an issue at the semiconductor material sidewall (indicated by the dashed arrow and circle).
In step 8, an oxygen anneal is performed, leading to oxygen penetration 70 into the semiconductor material 14, in some embodiments from the sides. This improves the quality of the semiconductor channel 14b, in some embodiments for IGZO as the semiconductor material 14, by reducing the density of oxygen vacancy traps.
In step 9, conformal PECVD 71 is performed to further deposition dielectric material, e.g., aluminum oxide. This is, in some embodiments, performed such that the resulting dielectric layer 23 encapsulates the semiconductor patch formed in step 7. Thus, the path for oxygen and moisture is blocked. In some embodiments, the encapsulation ensures that the oxygen inserted by the anneal (see above) does not escape.
In step 10, a sacrificial material 72 is provided over the dielectric layer 23, specifically filled as indicated by the arrow, and is then planarized. The sacrificial material 72 may be a material that is selectively removable with respect to the material of the metal layer 22, e.g., W/Cu, and/or with respect to the material of the dielectric layer 23. For instance, the sacrificial material 72 may be an oxide, like a silicon oxide.
In step 11, an opening 73 in the sacrificial material 72 is created by performing a lithographic etch, wherein the etch stops on the dielectric layer 23. The etch stop may be achieved by the sacrificial material 72 being selectively removable with respect to the dielectric layer 23.
In step 12, the dielectric layer 23 is anisotropically etched 80 through the opening 73 formed in step 11, in order to expose parts of the top surface of the second electrode layer 13.
In step 13, a metal layer 22 is filled into the openings etched in step 11 and 12. In some embodiments, a tungsten fill Damascene process can be applied, in order to fill tungsten into the openings. Further, a titanium nitride barrier may be formed in this step.
In step 14, a self-aligned etch is performed, i.e., self-aligned on the metal layer 22, in order to etch the sacrificial material 72, parts of the dielectric layer 23, and parts of the second electrode layer 13. The self-aligned etch does not need to be selective to the materials of the dielectric layer 23 and of the second electrode layer 13, because these are typically very thin layers. Thus, it is acceptable if the etch of the dielectric layer 23 and the second electrode layer 13 also etches the metal layer 22 (which is typically much thicker than the dielectric layer 23 and the second electrode layer 13, respectively). Selectivity between the sacrificial material 72 and the metal layer 22 may be utilized in some embodiments.
In step 15, a continuation to the BEOL is processed 81. In some embodiments, PECVD can be used, in order to dispose an oxide, in which BEOL can be further processed.
In summary, the integration flow, which is a specific embodiment of the general method 30, leads to a selector device 10 according to an embodiment of the disclosed technology being fabricated. This selector device 10 comprises the double-sided Schottky diode formed by first electrode layer 11, semiconductor material 14 and second electrode layer 13, and can thus help to solve the sneak path problem.
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| Number | Date | Country | |
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| 20210143213 A1 | May 2021 | US |
