Embodiments described herein relate generally to a semiconductor memory device applied to, for example, a magnetoresistive random access memory (MRAM), and a method of manufacturing the same.
MRAM is a generic term for a nonvolatile semiconductor memory which utilizes that the resistance of a barrier layer varies by the magnetization direction of a ferromagnetic material. A memory cell of an MRAM comprises a magnetic tunnel junction (MTJ) which utilizes the tunneling magnetoresistive (TMR) effect and a transistor. The MTJ element is a three-layered thin film comprising a recording layer and a reference layer, which are formed of magnetic materials, and an insulating layer interposed therebetween. The MTJ element stores data based on the magnetization states of the recording layer and reference layer.
In order to achieve a large capacity by miniaturizing the cell size and also a low current, a spin injection MRAM which employs a spin transfer torque (STT) write mode has been proposed. In the spin injection MRAM, data is written to the MTJ element when a current flows in a vertical direction with respect to a film surface of the MTJ element. As the magnetic layer used for the MTJ element, a vertical magnetization film in which the magnetization direction is set in, for example, the vertical direction with respect to the film surface has been proposed.
The MTJ element is processed by the following procedure. That is, magnetic layers and insulating layers are stacked one on another. Then, a hard mask is formed, and the magnetic layers and insulating layers are processed by ion beam etching (IBE) using the hard mask all at once. IBE is physical sputtering using an ion beam. By this means, it is difficult to process a high-density MTJ element due to the shadowing effect. As a masking material for processing an MTJ element, diamond-like carbon (DLC), which has a high selection ratio in physical sputtering, has been receiving attention.
In general, according to one embodiment, a semiconductor memory device comprises a lower electrode, an MTJ element, a cap layer and an upper electrode. The lower electrode is provided above a semiconductor substrate. The MTJ element is provided above lower electrode. The cap layer is provided above the MTJ element and does not contain oxygen. The upper electrode is connected to the cap layer.
Embodiments will now be described with reference to the drawings. Throughout the drawings, the same parts are designated by the same reference numbers.
On the substrate 13, an interlayer insulating film 16 which covers the transistor 11 is formed, and in the interlayer insulating film 16, a lower contact plug 17 serving as a contact layer and electrically connected to one side of the diffusion layers 15 constituting the S/D regions is formed. A lower electrode 18 is formed on the lower contact plug 17. The lower electrode 18 is formed of, for example, tantalum (Ta). For example, a lower layer 19 is formed on the lower electrode 18. The lower layer 19 is formed of, for example, a hafnium boride (HfB). But, it is not limited to HfB, but also, aluminum nitride (AlN) can be used. The MTJ element 12 is formed on the lower layer 19.
The MTJ element 12 comprises, for example, a magnetic layer 12a, a barrier layer 12b as an insulating layer, and a magnetic layer 12c. The magnetic layers 12a and 12c are formed of, for example, CoFeB, whereas the barrier layer 12c is formed of, for example, MgO. Of the magnetic layer 12a and 12c, one whose magnetization direction is fixed is called a fixed layer (reference layer), and one whose magnetization direction is reversed by STT is called a free layer (storage layer). In this embodiment, the magnetic layer 12a is, for example, a fixed layer, and the magnetic layer 12c is, for example, a free layer.
In this embodiment, the MTJ element 12 is not limited to the above-described structure, but it may be modified into various versions. For example, the element may have such a structure that the fixed layer further comprises an interference layer in contact with the barrier layer, or the fixed layer comprises the first magnetic layer, ruthenium (Ru) and the second magnetic layer. Further, the MTJ element 12 may have such a structure that comprises a first fixed layer, a first barrier layer, a free layer, a second barrier layer and a second fixed layer.
An upper layer 20 is formed on the MTJ element 12. The upper layer 20 is formed of, for example, ruthenium (Ru). Note that it is not limited Ru, but also, for example, tungsten (W) or titanium nitride (TiN) can be applied. A cap layer 21 is formed on the upper layer 20. The cap layer 21 serves to prevent degradation of DLC used as a hard mask when processing the material of the MTJ element 12 by IBE, and also oxidation of the MTJ element 12 when removing DLC. For this reason, the cap layer 12 comprises a material free of oxygen. However, it can be considered that in a manufacturing process, slight oxygen is mixed in the cap layer 12 unintentionally. Such the cap layer 12 including the slight oxygen is also defined as comprising the material free of oxygen.
That is, the cap layer 21 is formed of, for example, one of Ta, tungsten (W), titanium (Ti) and silicon (Si), or one of carbides of Ta, W, Ti and Si, or one of nitrides of Ta, W, Ti and Si. Further, the cap layer 21 may have a multi-layer structure comprising one of Ta, W, Ti and Si and a carbide thereof, or one of Ta, W, Ti and Si and a nitride thereof.
On sidewalls of the MTJ element 12, the upper layer 20 and the cap layer 21, slight oxide films 22 are formed. The oxide films 22 comprise oxides of the material of the MTJ element, which is re-deposited when etching the material of the MTJ element 12. With the oxide films 22, a shunt error of the sidewalls of the MTJ element 12 can be prevented.
On each of the oxide films 22, a side-wall insulating film 23 comprising, for example, silicon nitride, is formed. On the silicon nitride film 23, a protective film 24 comprising, for example, silicon nitride, is formed. An insulating film 25 is formed on the protective film 24. An upper electrode 26 is formed in parts of the insulating film 25 and the protective film 24 and is connected to the cap layer 21. A bit line BL is formed on the upper electrode 26. The bit line BL is placed in a direction orthogonal to the word line WL.
Meanwhile, a contact 27 is formed in the interlayer insulating film 16 corresponding to the other side of the diffusion layer 15 constituting the S/D regions, the protective film 24 and the insulating film 25. The contact 27 is electrically connected to the other side of the diffusion layer 15 constituting the S/D regions. A source line SL is formed on the contact 27 is arranged along the bit line BL.
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The cap layer 21, as mentioned above, comprises a material free of oxygen, which is one of Ta, W, Ti and Si, or a carbide or nitride of one of Ta, W, Ti and Si. The carbide or nitride material is an extremely thin layer of a thickness of, for example, 2 to 3 nm. Further, the cap layer 21 may be of a multi-layer structure comprising, for example, one of Ta, W, Ti and Si and a carbide of one of Ta, W, Ti and Si, or one of Ta, W, Ti and Si and a nitride of one of Ta, W, Ti and Si.
In order to increase the adhesive force between the cap layer 21 and a DLC film 31 to be formed thereon, and also to prevent the oxidization of the MTJ element 12 when removing the hard mask comprising the DLC film 31, the surface of the DLC film 31 should preferably be slightly nitrided.
After that, the DLC film 31 serving as the mask material is formed on the cap layer 21. The DLC film 31 is formed of amorphous carbon comprising carbon containing, for example, both sp3 bond of diamond and sp2 bond of graphite. The DLC film 31 has a hardness of 80 GPa or less. The DLC film 31 is formed by, for example, a physical vapor deposition (PVD) method or a chemical vapor deposition (CVD) method. As to the PVD, sputtering, ion beam deposition, cathodic arc ion plating, laser ablation or the like is carried out using a solid raw material, such as graphite, to form a DLC film. The DLC film formed by PVD contains a very smaller amount of hydrogen as compared to that of the CVD, and in fact, does not substantially contain hydrogen. In the meantime, as to the CVD method, plasma-enhanced CVD which uses a gaseous material containing carbon, for example, C2H2, may be used to form a DLC film. The DLC film formed by CVD contains hydrogen.
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After that, for example, water vapor (H2O) is introduced into the chamber in which the IBE was carried out, and the deposit re-deposited on the sidewalls of the MTJ element 12 and the sidewalls of the MTJ element 12 are oxidized. That is, the process by IBE and the in-situ oxidization process by H2O are carried out continuously. The oxidation process is carried out by exposing a wafer to H2O at room temperature, for example, and for 2 to 3 minutes, for example. With this oxidization process, the sidewalls of the MTJ element 12 are oxidized, and thus passivated. In other words, an oxide film 22 is formed on the sidewalls of the MTJ element 12 including the lower layer 19, the upper layer 20 and the cap layer 21, and is prevented from a shunt error.
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According to the first embodiment, the cap layer 21 formed underneath the DLC mask 31a comprises an oxygen-free material. With this structure, it is possible to prevent the generation of oxygen radicals from the cap layer 21 during the processing of the material of the MTJ element 12 using the DLC mask 31a as a mask by IBE, thereby making it possible to reduce the damage of the DLC mask 31a. Thus, the shape of the DLC mask 31a can be stably maintained, and therefore the MTJ element 12 having a fine structure can be processed with high accuracy.
Further, the cap layer 21 can prevent the oxidization of the MTJ element 12 when the DLC mask 31a is ashed by oxygen plasma or oxygen radicals so as to be removed. Therefore, it is possible to prevent the degradation of the magnetic characteristics of the MTJ element 12.
In addition, when the cap layer 21 comprises a carbide of one of Ta, W, Ti and Si, or a nitride of one of Ta, W, Ti and Si, the oxidization of the surface of the cap layer 21, which may occur when the DLC mask 31a is removed, can be prevented. Therefore, here, it is not required to subject the inside of the opening 25a to a reduction process when forming the upper electrode 26, and thus the manufacturing process can be simplified.
In the first embodiment, the DLC mask 31a is formed directly on the cap layer 21. In contrast, according to the second embodiment, an anti-oxidization layer is provided between a cap layer 21 and a DLC mask 31a.
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The DLC mask 31a is formed on the anti-oxidization layer 41. Using the DLC mask 31a, the anti-oxidization layer 41, a cap layer 21, an upper layer 20, a magnetic layer 12c, a barrier layer 12b, a magnetic layer 12a and a lower layer 19 are etched by IBE. The manufacturing process from this step on is the same as that of the first embodiment, and therefore the explanation thereof will be omitted.
Note that the anti-oxidization layer 41 remains even after the removal of the DLC mask 31a, and as shown in
According to the second embodiment, the anti-oxidization layer 41 and the cap layer 21 each comprise an oxygen-free material. With this structure, it is possible to prevent generation of oxygen radicals from the anti-oxidization layer 41 and the cap layer 21 during the processing of the material of the MTJ element 12 using the DLC mask 31a as a mask by IBE. In this manner, it is possible to prevent the damage of the DLC mask 31a, and therefore the MTJ element 12 having a fine structure can be processed with high accuracy.
Further, the oxidization of the cap 21 and the MTJ element 12 can be prevented by the anti-oxidization layer 41 when the DLC mask 31a is ashed by oxygen plasma or oxygen radicals. Therefore, it is possible to prevent the degradation of the magnetic characteristics of the MTJ element 12.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
This application claims the benefit of U.S. Provisional Application No. 62/047,584, filed Sep. 8, 2014, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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62047584 | Sep 2014 | US |