Patent Application
20130082230
The present invention relates to a method of manufacturing a variable resistance nonvolatile memory element that changes in resistance value according to an applied electrical signal.
With advances in digital technology, electronic appliances such as mobile information appliances and home information appliances are increasingly sophisticated in recent years. Such sophistication of electronic appliances is accompanied by rapid miniaturization and acceleration of semiconductor elements used. In particular, large-capacity nonvolatile memories, represented by flash memories, are finding rapidly expanding use. Moreover, nonvolatile memory devices including variable resistance nonvolatile memory elements are under research and development as new, next-generation nonvolatile memories that can replace flash memories. A variable resistance nonvolatile memory element mentioned here is such an element that has a property of reversibly changing in resistance value according to an applied electrical signal and enables information corresponding to the resistance value to be written in a nonvolatile manner.
For example, the variable resistance nonvolatile memory element has a structure in which a variable resistance layer comprising a variable resistance material is placed between a pair of electrodes, as disclosed in Patent Literature (PTL) 1. There are two main types of the variable resistance nonvolatile memory element: bipolar type and unipolar type, according to differences in electrical characteristics.
A nonvolatile memory element of bipolar type (hereafter referred to as “bipolar type element”) is an element of a type that uses voltages of different polarities as voltages for changing a resistance state between a high resistance state and a low resistance state.
A nonvolatile memory element of unipolar type (hereafter referred to as “unipolar type element”) is an element of a type that uses voltages of the same polarity as voltages for changing the resistance state. In the unipolar type element, for example, a single transition metal oxide such as nickel oxide (NiOx), or titanium oxide (TiOx) is used as the variable resistance material. Of the above-mentioned two types of nonvolatile memory elements, the unipolar type element has the following problems. In the unipolar type element using a transition metal oxide such as NiOx, the variable resistance material can be changed from the high resistance state to the low resistance state by a short electrical pulse of about 100 ns, as disclosed in Non Patent Literature (NPL) 1. To change the variable resistance material from the low resistance state to the high resistance state, however, a long pulse of the order of microseconds is needed, which makes it difficult to accelerate operations. Besides, in the unipolar type element, the resistance state is hard to be changed immediately after the structure in which the variable resistance layer is placed between the upper and lower electrodes is formed.
Typically, in operations of the variable resistance nonvolatile memory element, an initial breakdown step similar to dielectric breakdown of an insulator needs to be performed until a steady state of resistance change is reached. In the initial breakdown step, a voltage higher than a voltage required for a steady resistance change is applied to the element. This hinders low voltage operations. Thus, the unipolar type element has a demerit of requiring a high voltage for the initial breakdown step.
The initial breakdown step hinders low voltage operations of the nonvolatile memory element, as described above. Especially when the initial breakdown step is performed in a state where a load resistance element such as a diode or a transistor is connected to the variable resistance nonvolatile memory element, there are cases where, if a large current flows upon initial breakdown, an effective applied voltage to the nonvolatile memory element decreases due to an IR drop (voltage drop) in the load resistance element, as a result of which initial breakdown fails to occur. To ensure that initial breakdown occurs, it is necessary to increase the applied voltage so as to compensate for the IR drop in the load resistance element.
For example, in the unipolar type element, the variable resistance layer comprises a comparatively thick high-oxygen-concentration metal oxide of 10 nm or more. Accordingly, the current upon breakdown is extremely small though the breakdown voltage of the element alone is high, and so it is hardly necessary to increase the applied voltage in order to compensate for the IR drop in the load resistance element.
In the bipolar type element whose variable resistance layer has a stack structure of a high resistance layer and a low resistance layer, however, the high resistance layer has a small film thickness. Accordingly, the current upon breakdown is large though the breakdown voltage of the element alone is low. This can pose a problem as there is a need to increase the applied voltage in order to compensate for the IR drop in the load resistance element.
The present invention has been made in view of such circumstances, and has an object of providing a method of manufacturing a variable resistance nonvolatile memory element capable of reducing a voltage upon initial breakdown.
To solve the problems stated above, a method of manufacturing a nonvolatile memory element according to one aspect of the present invention includes: forming a first electrode on a substrate; forming a high resistance layer on the first electrode, the high resistance layer comprising a transition metal oxide;
modifying at least one part of the high resistance layer to a modified layer by reducing an oxygen deficiency of the at least one part, the modified layer having a higher oxygen content atomic percentage than the high resistance layer; forming a low resistance layer on the modified layer, the low resistance layer comprising a transition metal oxide having a lower oxygen content atomic percentage than the high resistance layer; and forming a second electrode on the low resistance layer.
According to this manufacturing method, a current flowing in a variable resistance nonvolatile memory element upon initial breakdown can be reduced by controlling a state of an interface between a first oxide layer and a second oxide layer.
To solve the problems stated above, a method of manufacturing a nonvolatile memory element according to one aspect of the present invention includes: forming a first electrode on a substrate; forming a low resistance layer on the first electrode, the low resistance layer comprising a transition metal oxide; forming a high resistance layer on the low resistance layer, the high resistance layer comprising a transition metal oxide having a higher oxygen content atomic percentage than the low resistance layer; modifying at least one part of the high resistance layer to a modified layer by reducing an oxygen deficiency of the at least one part, the modified layer having a higher oxygen content atomic percentage than the high resistance layer; and forming a second electrode on the high resistance layer or the modified layer.
Preferably, the modifying includes modifying the whole high resistance layer to the modified layer.
Moreover, the modifying may include modifying a part of the high resistance layer to the modified layer, wherein the nonvolatile memory element includes a variable resistance layer including: the low resistance layer; the high resistance layer; and the modified layer located between the low resistance layer and the high resistance layer.
Preferably, the modifying includes oxidizing the at least one part of the high resistance layer.
Here, the oxidizing may include plasma oxidizing the at east one part of the high resistance layer.
Preferably, the nonvolatile memory element changes between a high resistance state and a low resistance state according to an applied electrical pulse.
Moreover, the high resistance layer may comprise a tantalum oxide having a composition expressed as TaOx where 2.1≦x, wherein the low resistance layer comprises a tantalum oxide having a composition expressed as TaOy where 0.8≦y≦1.9.
Preferably, the variable resistance layer has a thickness not less than 5 nm and not more than 1 μm, and the high resistance layer has a thickness not less than 1 nm and not more than 8 nm,
Moreover, the first electrode (or the second electrode) may have a flat surface with no projection of 2 nm or larger, in an interface of the first electrode (or the second electrode) with the high resistance layer or the modified layer. For example, the first electrode (or the second electrode) may comprise platinum with a film thickness not less than 1 nm and not more than 8 nm, and the first electrode or the second electrode may comprise iridium,
Moreover, the nonvolatile memory element may be manufactured to further include a current steering element that is electrically connected to the first electrode or the second electrode. For example, the current steering element may be a transistor, and the current steering element may be a diode.
To solve the problems stated above, a method of manufacturing a nonvolatile memory element according to one aspect of the present invention includes: a variable resistance layer that changes between a high resistance state and a low resistance state according to an applied electrical pulse; and a first electrode and a second electrode that are connected to the variable resistance layer, wherein the variable resistance layer includes: a high resistance layer comprising a transition metal oxide; a low resistance layer comprising a transition metal oxide having a lower oxygen content atomic percentage than the high resistance layer; and a modified layer located between the high resistance layer and the low resistance layer, and comprising a transition metal oxide having a higher oxygen content atomic percentage than the high resistance layer.
To solve the problems stated above, a method of manufacturing a nonvolatile memory element according to one aspect of the present invention is a method of manufacturing a nonvolatile memory element including: a variable resistance layer that comprises a metal oxide and changes between a high resistance state and a low resistance state according to an applied electrical pulse; and a first electrode and a second electrode that are connected to the variable resistance layer, the method including: forming the first electrode on a substrate; forming a high resistance layer on the first electrode, the high resistance layer comprising a transition metal oxide having a predetermined oxygen content atomic percentage; forming an intermediate layer on the high resistance layer, the intermediate layer comprising a transition metal oxide that has an oxygen deficiency reduced from an oxygen deficiency of the transition metal oxide of the high resistance layer and has a higher oxygen content atomic percentage than the high resistance layer; forming a low resistance layer on the intermediate layer, the low resistance layer comprising a transition metal oxide having a lower oxygen content atomic percentage than the high resistance layer; and forming the second electrode on the low resistance layer, wherein the variable resistance layer includes the high resistance layer, the intermediate layer, and the low resistance layer.
To solve the problems stated above, a method of manufacturing a nonvolatile memory element according to one aspect of the present invention is a method of manufacturing a nonvolatile memory element including: a variable resistance layer that comprises a metal oxide and changes between a high resistance state and a low resistance state according to an applied electrical pulse; and a first electrode and a second electrode that are connected to the variable resistance layer, the method including: forming the first electrode on a substrate; forming a low resistance layer on the first electrode, the low resistance layer comprising a transition metal oxide having a predetermined oxygen content atomic percentage; forming an intermediate layer on the low resistance layer, the intermediate layer comprising a transition metal oxide having a higher oxygen content atomic percentage than the low resistance layer; forming a high resistance layer on the intermediate layer, the high resistance layer comprising a transition metal oxide having a higher oxygen content atomic percentage than the low resistance layer and a lower oxygen content atomic percentage than the intermediate layer; and forming the second electrode on the high resistance layer, wherein the variable resistance layer includes the high resistance layer, the intermediate layer, and the low resistance layer.
To solve the problems stated above, a nonvolatile memory element according to one aspect of the present invention includes; a variable resistance layer that changes between a high resistance state and a low resistance state according to an applied electrical pulse; and a first electrode and a second electrode that are connected to the variable resistance layer, wherein the variable resistance layer includes: a high resistance layer comprising a transition metal oxide; a low resistance layer comprising a transition metal oxide having a lower oxygen content atomic percentage than the high resistance layer; and an intermediate layer located between the high resistance layer and the low resistance layer, and comprising a transition metal oxide having a higher oxygen content atomic percentage than the high resistance layer,
According to the present invention, a method of manufacturing a variable resistance nonvolatile memory element capable of reducing a voltage upon initial breakdown can be provided. Even when a load resistor is connected to a variable resistance nonvolatile memory element such as a variable resistance element, there is no need to increase a voltage for an initial breakdown step, so that a high-density memory cell array can be realized without an increase in size of a transistor and the like.
The following describes a preferred embodiment of the present invention with reference to drawings.
A structure of a variable resistance element according to the present invention is described first.
The substrate 1 is composed of a silicon substrate, as an example.
The first electrode 2 and the second electrode 4 are both physically and electrically connected to the metal oxide layer 3. In the case where the variable resistance element to includes the load resistor 6, the load resistor 6 is connected in series with at least one of the first electrode 2 and the second electrode 4. The first electrode 2 and the second electrode 4 may have the same size.
For example, the first electrode 2 and the second electrode 4 comprise one or more materials out of Au (gold), Pt (platinum), Ir (iridium), Pd (palladium), Cu (copper), Ag (silver), TaN (tantalum nitride), Ta (tantalum), Ti (titanium), and TiN (titanium nitride).
In detail, the first electrode 2 is placed in contact with the first metal oxide layer 31. The first electrode 2 comprises any of Au, Pt, Ir, Pd, Cu, Ag, and the like, as an example. In the case of using Ta as the transition metal constituting the metal oxide layer 3, the first electrode 2 preferably comprises one or more materials that are higher in standard electrode potential than Ta. Meanwhile, the second electrode 4 preferably comprises any of materials (e.g. W (tungsten), Ni (nickel), Ta, TaN, and the like) that are lower in standard electrode potential than the material(s) constituting the first electrode 2. In other words, when the standard electrode potential of the first electrode 2 is denoted by V1, the standard electrode potential of the second electrode 4 by V2, and the standard electrode potential of tantalum as VTa, it is preferable to satisfy relations VTa<V1 and V2<V1. This makes it possible to stably induce a resistance change phenomenon in the first metal oxide layer 31 which is in contact with the first electrode 2.
The first electrode 2 preferably comprises Ir, or Pt with a film thickness not less than 1 nm and not more than 23 nm. In the case where the first electrode 2 comprises Pt, the film thickness is more preferably not less than 1 nm and not more than 13 nm, still more preferably not less than 1 nm and not more than 10 nm, and most preferably not less than 1 nm and not more than 8 nm,
The metal oxide layer 3 is a variable resistance layer formed by stacking a plurality of metal oxide layers (tantalum oxide layers here) that differ in oxygen content atomic percentage. In detail, the metal oxide layer 3 is formed by stacking the first metal oxide layer 31 and the second metal oxide layer 32. The first metal oxide layer 31 has a higher oxygen content atomic percentage than the second metal oxide layer 32. Preferably, when the first metal oxide layer 31 has a composition expressed as TaOx and the second metal oxide layer 32 has a composition expressed as TaOy, x is not less than 2.1 and y is not less than 0.8 and not more than 1.9. The metal oxide layer 3 may be a transition metal oxide layer other than a tantalum oxide layer. As an example, in the case where the metal oxide layer 3 is formed by stacking a first hafnium oxide layer (first metal oxide layer 31) and a second hafnium oxide layer (second metal oxide layer 32), it is desirable that, when the first hafnium oxide layer has a composition expressed as HfOx and the second hafnium oxide layer has a composition expressed as HfOy, x is more than 1.8 and y is not less than 0.9 and not more than 1.6. As another example, in the case where the metal oxide layer 3 is formed by stacking a first zirconium oxide layer (first metal oxide layer 31) and a second zirconium oxide layer (second metal oxide layer 32), it is desirable that, when the first zirconium oxide layer has a composition expressed as ZrOx and the second zirconium oxide layer has a composition expressed as ZrOy, x is more than 1.9 and y is not less than 0.9 and not more than 1.4. In the case where x and y are in the above-mentioned ranges, the resistance value of the metal oxide layer 3 can be stably changed at high speed. This will be described in detail later.
The metal oxide layer 3 has a thickness that is preferably not less than 10 nm, and is not more than 1 μm and preferably not more than 200 nm. When the thickness of the metal oxide layer 3 is not more than 1 μm, a change in resistance value is observed. A thickness not more than 200 nm is preferable because manufacture is facilitated in the case of using lithography in a patterning process, and also a voltage value of a voltage pulse necessary for changing the resistance value of the metal oxide layer 3 can be reduced. A thickness not less than 10 nm is preferable because the variable resistance element 10 is more reliably kept from breakdown upon voltage pulse application.
The first metal oxide layer 31 is a layer, in the metal oxide layer 3, where a resistance change phenomenon substantially occurs, and is a high resistance layer provided in order to protect the variable resistance element 10 from breakdown or writing due to charge-up or the like during manufacturing. In other words, the first metal oxide layer 31 has a very important role in achieving stable resistance change operations of the variable resistance element 10. In the case where the first metal oxide layer 31 comprises tantalum oxide, the thickness of the first metal oxide layer 31 is preferably about not less than 1 nm and not more than 8 nm. In the case where the first metal oxide layer 31 comprises hafnium oxide, the thickness of the first metal oxide layer 31 is preferably about not less than 4 nm and not more than 5 nm. In the case where the first metal oxide layer 31 comprises zirconium oxide, the thickness of the first metal oxide layer 31 is preferably about not less than 1 nm and not more than 5 nm. An excessively thick first metal oxide layer 31 is disadvantageous because an initial resistance value is too high, and an excessively thin first metal oxide layer 31 is equally disadvantageous because a stable resistance change cannot be achieved.
The variable resistance element 10 has the structure described above.
Moreover, a modified layer or an intermediate layer having a higher oxygen content atomic percentage (higher resistance) than the first metal oxide layer 31 is formed between the first metal oxide layer 31 (high resistance layer) and the second metal oxide layer 32 (low resistance layer). In detail, the modified layer is formed in at least one part of the first metal oxide layer 31, by performing modification that reduces an oxygen deficiency of the at least one part of the first metal oxide layer 31. The modified layer may be the whole first metal oxide layer 31 (high resistance layer). The intermediate layer is a layer formed on the first metal oxide layer 31 and having an oxygen deficiency reduced from that of the first metal oxide layer 31.
As mentioned above, the transition metal oxide (preferably oxide of Ta, Hf, Zr, or the like) is used as the material of the metal oxide layer 3, i.e. the variable resistance layer, in the variable resistance element 10. In particular, the second metal oxide layer 32 preferably comprises an oxygen-deficient transition metal oxide. The oxygen-deficient transition metal oxide is an oxide that is lower in oxygen content (atomic ratio: a proportion of the number of oxygen atoms to the total number of atoms) than an oxide having a stoichiometric composition. Typically, an oxide having a stoichiometric composition is an insulator or has a very high resistance. For example, in the case of Ta as a transition metal, Ta2O5 is an oxide having a stoichiometric composition, where a ratio (O/Ta) in the number of atoms between Ta and O is 2.5. Accordingly, oxygen-deficient Ta oxide has an O/Ta atomic ratio that is more than 0 and less than 2.5. In the present invention, the oxygen-deficient transition metal oxide is preferably oxygen-deficient Ta oxide. More preferably, the second metal oxide layer 32 has at least a stack structure formed by stacking a second tantalum-containing layer (second metal oxide layer 32) having a composition expressed as TaOy (0<y<2.5) and a first tantalum-containing layer (first metal oxide layer 31) having a composition expressed as TaOx (y<x), as described earlier. One or more other layers such as a third tantalum-containing layer and a layer of another transition metal oxide may be provided according to need.
The following describes reasons why the above-mentioned film thickness ranges of the first electrode 2 are preferable.
The first electrode 2 is an electrode in contact with the first metal oxide layer 31 having a higher oxygen content atomic percentage than the second metal oxide layer 32. By performing the below-mentioned modification process for the film of the first metal oxide layer 31 according to the present invention in a state where the first electrode 2 is an electrode having a flat surface without small projections that are for example not more than 2 nm, preferably not more than 1 nm, and more preferably not more than 0.5 nm, it is possible to not only reduce a breakdown voltage but also reduce variations in initial resistance. This advantageous effect is described in detail first,
It has hitherto been difficult to both improve reproducibility of electrical characteristics (in particular, initial resistance) and operational reliability (durability) and reduce the breakdown voltage in the initialization process. In detail, in terms of electrical characteristics and reliability, it is preferable that a flat electrode (first electrode 2) without small projections forms an interface with the first metal oxide layer 31 having a high initial resistance. However, the use of such an electrode has a problem that the initial resistance increases and the breakdown voltage in the initialization process increases because the breakdown voltage is uniformly applied to the first metal oxide layer 31 in the initialization process. In terms of reducing the breakdown voltage in the initialization process, on the other hand, an electrode with small projections is preferable. However, such an electrode cannot be employed because of a problem that it is difficult to attain desired electrical characteristics and ensure reliability.
In view of this, a predetermined modification process for the film of the first metal oxide layer 31 according to the present invention is performed to reduce the breakdown voltage in the initialization process while using a flat electrode (first electrode 2) without small projections that are for example not more than 2 nm, preferably not more than 1 nm, and more preferably not more than 0.5 nm. The problem of the increase of the breakdown voltage in the initialization process is solved in this way. That is, even in the case of using a flat electrode (first electrode 2) without small projections, the breakdown voltage can be reduced while ensuring desired electrical characteristics and reliability.
The first electrode 2 can be formed as a flat electrode without small projections, by using an Ir electrode as the first electrode 2 or by using a Pt electrode whose film thickness is not less than 1 nm and not more than 23 nm as the first electrode 2 as mentioned earlier. A smaller film thickness of the Pt electrode contributes to reduced stress, with it being possible to suppress the formation of projections in the interface. Accordingly, the film thickness of the Pt electrode (first electrode 2) is more preferably not less than 1 nm and not more than 13 nm, still more preferably not less than 1 nm and not more than 10 nm, and most preferably not less than 1 nm and not more than 8 nm.
These findings about the first electrode 2 result from the below-mentioned experiment conducted by the inventors of the present invention. Experimental results leading to the findings and considerations about the experimental results are described in Experiment 1 below.
In the element shown in
As is clear from a close study of
Most of these projections extend from around grain boundaries of the upper and lower Pt layers. It is especially to be noted that the projections extending from the first electrode layer 120a reach approximately half the thickness of the first oxygen-deficient tantalum oxide layer 131a.
The manufacturing method of the element shown in
Projections comprising Pt were not formed in this element, as shown in
The initial resistance of each of the above-mentioned elements (elements shown in
Here, the first oxygen-deficient tantalum oxide layer (131a, 131b) is provided to induce a resistance change phenomenon of the element shown in
This being so, the presence of projections as shown in
Besides, when a voltage is applied across the first electrode layer 120a and the second electrode layer 140a in the state shown in
This suggests that the reproducibility of electrical characteristics and the operational reliability of the variable resistance element are expected to be improved if the formation of projections from the electrodes toward the oxygen-deficient tantalum oxide layers can be suppressed.
The following is a projection formation mechanism, as an example. A change in the Pt layer in the heating step in the manufacturing process is one probable cause. For instance, migration of Pt atoms when the Pt layer reaches a high temperature can cause projection formation. A possible reason why the projections develop from the grain boundaries of the Pt layer is that such migration tends to occur along the grain boundaries of the Pt layer.
The inventors of the present invention then examined whether or not the same problem arises in the case of using hafnium instead of tantalum as the transition metal included in the variable resistance layer.
In the element shown in
As is clear from a close study of
In the element shown in
These results suggest that, in the variable resistance element 10 (nonvolatile memory element) including a Pt layer (electrode layer) of a large film thickness and an oxygen-deficient transition metal oxide as its components, Pt projections tend to be formed when exposed to a high temperature of about 400° C., regardless of the type of transition metal.
It can be understood from the above experimental results that the formation of projections is suppressed if the heating step is omitted when forming the variable resistance element 10. In the ordinary semiconductor process, however, the heating step of about several hundred degrees is essential to improve wiring reliability, and therefore it is impractical to set an upper limit of the heating temperature in the manufacturing process to about 100° C.
As a result of conducting further study based on these findings, the inventors of the present invention have found out that projections can be suppressed by reducing the film thickness of the electrode layer comprising Pt. This is described below.
A first electrode layer 320a comprising a Pt layer in the element A shown in
A close study of
These results suggest that projections are significantly suppressed by reducing the film thickness of the Pt layer (first electrode layer). The suppression effect decreases as the film thickness of the Pt layer (first electrode layer) increases.
The initial resistance mentioned here means a resistance value (resistance value between the first electrode layer 320 and the second electrode layer 340) immediately after element formation. That is, for an element that has undergone the manufacturing process including the heating step, the initial resistance is a resistance value measured in a state where no electrical pulse (high-voltage electrical pulse that causes a change in resistance value) is applied yet. The initial resistance was measured by applying a weak voltage of 50 my across the first electrode layer 320 and the second electrode layer 340 and measuring current flow,
As shown in
These results suggest that the decrease in resistance value with the increase in film thickness of the Pt layer is closely correlated with the formation of projections or irregularities in the Pt layer. When the film thickness of the Pt layer increases, Pt projections (irregularities) develop toward the inside of the first oxygen-deficient tantalum oxide layer, as a result of which the film thickness of the first oxygen-deficient tantalum oxide layer substantially decreases in some parts. The first oxygen-deficient tantalum oxide layer has a higher resistance than the second oxygen-deficient tantalum oxide layer. Accordingly, when Pt projections enter into the first oxygen-deficient tantalum oxide layer, the initial resistance of the element decreases significantly. Conversely, a higher initial resistance of the element indicates a greater effect of suppressing Pt projections.
As can be seen from
The following is a projection suppression mechanism in the case of reducing the film thickness of the Pt layer. As mentioned earlier, it is estimated that Pt projections are caused by Pt atoms migrating along grain boundaries present in the Pt layer. If no grain boundary is present, migration is unlikely to occur, se that no projection is formed.
Typically, in the case of depositing a metal or the like on a substrate, when the film thickness is larger, crystal grains (grains, crystalline lumps) grow more and as a result grain boundaries become clearer. When the film thickness is smaller, on the other hand, the grain growth is insufficient and grain boundaries do not appear clearly. This is demonstrated by the fact that clear grain boundaries as seen in
It is also estimated that, since projections are caused by migration of Pt atoms, projections are less likely to be formed if there are fewer Pt atoms which migrate. That is, the number of Pt atoms is also a probable contributory factor in suppressing projections. Reducing the film thickness of the Pt layer is equivalent to reducing the number of Pt atoms. In the elements A to C, the formation of projections is suppressed because there are fewer Pt atoms which migrate.
Whether or not the elements A, B, and C perform resistance change operations was examined next. This is described below.
Here, a voltage polarity is represented by a voltage of the first electrode layer 320 with respect to the second electrode layer 340. In detail, a voltage in the case where a higher voltage is applied to the first electrode layer 320 than the second electrode layer 340 is regarded as a positive voltage, whereas a voltage in the case where a lower voltage is applied to the first electrode layer 320 than the second electrode layer 340 is regarded as a negative voltage. The voltage application was performed using electrical pulses of 100 ns in pulse width. The resistance value was measured by measuring the current when applying a weak voltage of 50 mV across the electrodes (first electrode layer 320 and second electrode layer 340) before the application of each electrical pulse.
In
In
In
Though not shown, the element of the comparative example (film thickness of Pt layer=80 nm) was observed to perform substantially same resistance change operations as the elements A to C.
From these results, it can be understood that whether or not the elements A to C perform resistance change operations hardly depends on the film thickness of the Pt layer,
The experimental results and study results described above can lead to the conclusion that, though the film thickness of the Pt layer does not affect resistance change phenomena, there is a preferable range (upper limit) of the film thickness of the Pt layer in terms of reducing variations in initial resistance and improving reliability.
In detail, as shown in
As shown in
Thus, the results obtained in terms of instrumental analysis in
The following examines whether or not the Pt film thickness has a lower limit.
As mentioned earlier, Pt is a material that has a high standard electrode potential and accordingly facilitates a resistance change of the variable resistance layer in contact with Pt (i.e. facilitates a change in resistance value when an electrical pulse is applied). There is a possibility that this resistance change is induced by movement of oxygen atoms near the interface between the electrode and the oxygen-deficient tantalum oxide layer. Pt is also well known as a material that acts as a catalyst for an oxidation reduction reaction.
Considering all of these factors, it can be assumed that, in the variable resistance element 10 in this embodiment, Pt acts as a catalyst on the oxygen-deficient tantalum oxide layer to promote movement of oxygen atoms, as a result of which a resistance change operation is facilitated. That is, in the variable resistance element 10 in this embodiment, the resistance value increases as a result of oxygen being supplied (oxidation) to the oxide layer near the interface between the Pt layer (first electrode 2) and the oxygen-deficient tantalum oxide layer (first metal oxide layer 31), and decreases as a result of oxygen being lost (reduction) from the oxygen-deficient tantalum oxide layer (first metal oxide layer 31) near the interface. Here, Pt acts to decrease activation energy for causing the oxidation reduction reaction of the oxide layer, i.e. acts as a catalyst.
In view of this, the Pt layer (first electrode 2) needs to cover the entire surface of the oxygen-deficient tantalum oxide layer (first metal oxide layer 31) without any gap. In the case where the Pt layer (first electrode 2) that is not continuous but separated like islands partially covers the oxygen-deficient tantalum oxide layer (first metal oxide layer 31), the area exhibiting the resistance change in the oxygen-deficient tantalum oxide layer varies depending on the size or density of each island-like Pt layer (first electrode 2), causing variations in resistance value.
Hence, the extent to which Pt needs to be deposited to form a film (continuous film) covering the entire surface without any gap was examined using X-ray photoelectron spectroscopy (XPS).
A detailed experimental method in
First, an oxygen-deficient tantalum oxide layer was deposited on a substrate, and its surface was naturally oxidized in atmosphere. A Pt layer was then deposited on the oxygen-deficient tantalum oxide layer by sputtering with different film thicknesses, and an XPS spectrum in each film thickness was measured. The film thickness of the Pt layer was adjusted by a sputtering time. Note that the term “converted film thickness” means a virtual film thickness calculated from the sputtering time based on an assumption that the film thickness is proportional to the sputtering time. When the film thickness is large (when a continuous film is formed), the converted film thickness and the actual film thickness match. When the film thickness is small, the film does not have a uniform thickness and is separated like islands. It is difficult to define the film thickness in such a case. The “converted film thickness” in the case where the film thickness is small substantially matches an average film thickness of the Pt layer separated like islands,
As shown in
Thus, the Pt layer on the oxygen-deficient tantalum oxide is a continuous film when its film thickness is not less than 1 nm. Since transition metal oxides have substantially similar characteristics, the Pt layer is a continuous film with the substantially same film thickness even in the case where Pt is deposited on a transition metal oxide other than tantalum oxide.
Therefore, according to this example, the film thickness range of the Pt layer is not less than 1 nm and not more than 23 nm, preferably not less than 1 nm and not more than 23 nm, more preferably not less than 1 nm and not more than 13 nm, still more preferably not less than 1 nm and not more than 10 nm, and most preferably not less than 1 nm and not more than 8 nm. The first electrode 2 comprising the Pt layer thus has the preferable film thickness range.
Besides Pt, palladium (Pd) shows the same characteristics as Pt. Accordingly, the same applies even when Pd is used for the first electrode 2, and so its description is omitted.
In the case of using iridium (Ir) as the material of the first electrode 2, on the other hand, Ir projections were not formed in the interface with the first metal oxide layer even when heated at 400° C. after electrode formation, regardless of the film thickness of Ir. This is probably because, as compared with Pt and Pd, Ir has a low thermal expansion coefficient, a high melting point, and a high Young's modulus, and therefore does not thermally expand much by annealing, resists thermal diffusion due to its high melting point, and is not deformed much by applied stress.
In the experiments in this example, each element (sample) corresponding to the variable resistance element was produced by forming the second tantalum oxide layer (second metal oxide layer 32), the first tantalum oxide layer (first metal oxide layer 31), and the first electrode 2 in this order on the second electrode 4. However, given that Pt migration occurs regardless of the vertical positional relation of the first electrode 2 and the second electrode 4 as shown by the results in
The following describes a method of manufacturing the variable resistance element 10 having the above structure.
First, the first electrode 2 (Ir of 80 nm in film thickness here) is formed in the above-mentioned preferable film thickness range, on the substrate 1 by sputtering (
Next, the first tantalum oxide layer (first metal oxide layer 31) is formed on the first electrode 2 by RF sputtering in argon gas, using a tantalum oxide (e.g. Ta2O5) target having a high oxygen content atomic percentage (
Next, the modification step of forming the modified layer by modifying the surface part (31a in
Though this modification step uses (shares) a step of stabilizing discharge when sputtering a tantalum target before the formation of the second tantalum oxide layer in the next step starts, the modification step may instead use an oxidation step (in detail, plasma oxidation step) conducted in another apparatus. This will be described in detail later.
Next, the shutter is opened, and the tantalum target is sputtered in argon gas and oxygen gas under the same conditions that are: no substrate heating; power of 1000 W; a film formation pressure of 0.05 Pa; and a gas flow rate of Ar/O2=20/23 sccm (reactive sputtering). As a result, the second tantalum oxide layer is formed on the first tantalum oxide layer (
The oxygen content atomic percentage of the second tantalum oxide layer can be easily adjusted by changing the flow rate of oxygen gas to argon gas. Moreover, the substrate temperature can be set to an ambient temperature with no particular heating.
Thus, the first tantalum oxide layer (high resistance layer) having the modified layer and the second tantalum oxide layer (low resistance layer) are stacked to form the metal oxide layer 3.
Next, the second electrode 4 of 50 nm in thickness is formed on the formed metal oxide layer 3 by sputtering (
Next, a pattern 7 is formed on the second electrode 4 by a photoresist in a photoresist step (
The variable resistance element 10 is manufactured in this way.
The size and shape of the variable resistance element region, i.e. the size and shape of the first electrode 2, the second electrode 4, and the metal oxide layer 3, can be adjusted by masking and lithography. In this embodiment, the size of the second electrode 4 and the metal oxide layer 3 is 0.5 μm×0.5 μm (an area of 0.25 μm2), and the size of the contact part of the first electrode 2 and the metal oxide layer 3 is also 0.5 μm×0.5 μm (an area of 0.25 μm2).
Moreover, in this embodiment, the composition of the first tantalum oxide layer is TaOx (x=2.47), and the composition of the second tantalum oxide layer is TaOy (y=1.54).
Note that the values of x and y are not limited to x=2.47 and y=1.54, so long as x is not less than 2.1 (2.1≦x) and y is not less than 0.8 and not more than 1.9 (0.8≦y≦1.9) as mentioned earlier. With this range, a stable resistance change can be realized as in the resistance change characteristics in this embodiment.
The modification of the surface part 31a of the first tantalum oxide layer (first metal oxide layer 31) by the above-mentioned modification step is described using experimental results below.
First, a sample of a film formed under the same conditions as the above-mentioned first tantalum oxide layer was subjected to plasma oxidation of the same conditions as the film formation conditions of the second tantalum oxide layer (second metal oxide layer 32) except that the shutter is closed, and a film quality change was analyzed.
Film structure analysis was conducted first by a grazing incidence X-ray reflective (GIXR) technique. This is a method of making X-rays incident on a sample surface at a very shallow angle and measuring an intensity of reflected X-rays (measurement apparatus: ATX-E by Rigaku Corporation). Fitting was performed for the spectrum by assuming an appropriate structural model, and a film thickness, a density (refractive index), and surface roughness were evaluated. Here, fitting parameters are a refractive index, a film thickness, and surface roughness.
In
As shown in
Table 1 shows results of calculating a δ value, a film thickness, and surface roughness by least square fitting for the measurement results shown in
As shown in Table 1, the film thickness slightly increased from 3.55 nm to 3.58 nm by plasma oxidation. In addition, the surface roughness increased from 0.49 nm to 0.54 nm by 0.05 nm by plasma oxidation. The δ value (density) showed a tendency of being smaller when the processing time is longer.
In the case of tantalum oxide, a higher oxygen composition contributes to a smaller δ value (density), and also there is a slight increase in film thickness. This indicates that the oxygen concentration of the first tantalum oxide layer increases by plasma oxidation.
Next, a Ta4F spectrum and an O1s spectrum were measured by X-ray photoelectron spectroscopy, in order to examine the oxygen composition of each of the above samples (samples a to d).
From integrated intensities of the Ta4f peak shown in
Table 2 shows results of calculating the composition ratio of each sample and its increase rate.
As shown in Table 2, an increase by 2.1% was observed at the maximum, showing a tendency that the oxygen composition increases when the processing time is longer, as compared with when the process is not performed.
These results are in agreement with the tendency of the 5 value in X-ray reflectivity measurement. This demonstrates that the oxygen composition of the first tantalum oxide layer increases by plasma oxidation. To be more precise, the oxygen composition of the surface part 31a of the first tantalum oxide layer increases,
By forming the metal oxide layer 3 in this way, the film quality of the first tantalum oxide layer can be controlled so that more oxygen is present.
In Experiment 5, the electrical characteristics of the variable resistance element 10 formed as described above were measured.
First, three types of samples (hereafter referred to as samples 1 to 3) that each have the metal oxide layer 3 of 50 nm in total thickness and that differ in thickness of the first tantalum oxide layer (first metal oxide layer 31) and the second tantalum oxide layer (second metal oxide layer 32) were prepared as samples of the variable resistance element 10.
In detail, in the sample 1, the first tantalum oxide layer is 3 nm in thickness, and the second tantalum oxide layer is 47 nm in thickness. In the sample 2, the first tantalum oxide layer is 4 nm in thickness, and the second tantalum oxide layer is 46 nm in thickness. In the sample 3, the first tantalum oxide layer is 5 nm in thickness, and the second tantalum oxide layer is 45 nm in thickness.
Three types of samples (hereafter referred to as comparative samples 1 to 3) of a variable resistance element 20 whose film structure is upside down as shown in
The comparative sample has the upside-down structure for the following reason. In the structure of the variable resistance element 10, the surface of the first tantalum oxide layer is exposed to mixed gas plasma of Ar and oxygen when forming the second tantalum oxide layer. Hence, the comparative sample that is completely free from such an effect is provided.
A method of manufacturing the variable resistance element 20 as the comparative sample is described below.
First, the second electrode 4 (TaN film here) of 50 nm in thickness is formed by sputtering.
Next, the second tantalum oxide layer is formed on the second electrode 4 by reactive DC sputtering of a tantalum target in argon gas and oxygen gas. Film formation conditions used here are: no substrate heating; power of 1000 W; a film formation pressure of 0.05 Pa; and a gas flow rate of Ar/O2=20/23 sccm. These film formation conditions are exactly the same as those in the step in FIG. 8B.
Next, the first tantalum oxide layer is formed on the second tantalum oxide layer by RF sputtering in argon gas, using a tantalum oxide (e.g. Ta2O5) target having a high oxygen content atomic percentage. Film formation conditions used here are: no substrate heating; power of 1000 W; a film formation pressure of 0.05 Pa; and a gas flow rate of Ar=50 sccm. These film formation conditions are exactly the same as those in the step in
The second tantalum oxide layer and the first tantalum oxide layer constitute a metal oxide layer 30.
Next, the first electrode 2 (Ir film here) of 50 nm in thickness is formed on the formed metal oxide layer 30 by sputtering.
Lastly, the pattern 7 is formed on the first electrode 2 by a photoresist in a photoresist step, and dry etching is performed so as to leave a desired region (element region). The variable resistance element 20 can be manufactured in this way.
In the comparative samples, i.e. the comparative samples 1 to 3, the size of the second electrode 4 and the metal oxide layer 30 is 0.5 μm×0.5 μm (an area of 0.25 μm2), and the size of the contact part of the first electrode 2 and the metal oxide layer 30 is also 0.5 μm×0.5 μm (an area of 0.25 μm2), as in the samples 1 to 3.
Moreover, in the comparative samples, i.e. the comparative samples 1 to 3, the composition of the first tantalum oxide layer is TaOx (x=2.47), and the composition of the second tantalum oxide layer is TaOy, (y=1.54), as in the samples 1 to 3. The comparative samples 1 to 3 each have the metal oxide layer 30 of 50 nm in total thickness, and differ in thickness of the first tantalum oxide layer and the second tantalum oxide layer. In detail, in the comparative sample 1, the first tantalum oxide layer is 4.5 nm in thickness, and the second tantalum oxide layer is 45.5 nm in thickness. In the comparative sample 2, the first tantalum oxide layer is 5.5 nm in thickness, and the second tantalum oxide layer is 44.5 nm in thickness. In the comparative sample 3, the first tantalum oxide layer is 6.5 nm in thickness, and the second tantalum oxide layer is 43.5 nm in thickness.
The comparative samples (comparative samples 1 to 3) have the film structure that involves no modification process of the first tantalum oxide layer. That is, the comparative samples 1 to 3 have the film structure including the first tantalum oxide layer which is unmodified.
The following describes comparison in electrical characteristics between the samples 1 to 3 and the comparative samples 1 to 3 produced as described above.
Definitions of terms used below are described first.
A state in which the metal oxide layer 3 (or the metal oxide layer 30) has a predetermined high resistance value (e.g. several 10 kΩ) is referred to as a high resistance state, and a state in which the metal oxide layer 3 (or the metal oxide layer 30) has a predetermined low resistance value (e.g. several kΩ) is referred to as a low resistance state.
In the variable resistance element 10 shown in
In the variable resistance element 10 shown in
Note that, in the variable resistance element 10 shown in
The terms defined above equally apply to the variable resistance element 20 shown in
The following examines results of measuring the initial resistance value as the characteristics of the samples 1 to 3 and the comparative samples 1 to 3.
As shown in
The first tantalum oxide layer comprises tantalum oxide having a high oxygen content atomic percentage close to Ta2O5 and behaves like a semiconductor, so that its resistance is determined by a Schottky junction with the first electrode 2. That is, an interface resistance between the first electrode 2 and the first tantalum oxide layer is dominant in the initial resistance of the variable resistance element 10 or 20.
Since the samples 1 to 3 and the comparative samples 1 to 3 are on the same curved line regarding the initial resistance value, the interface between the first tantalum oxide layer (first metal oxide layer 31) and the first electrode 2 and the interface between the first tantalum oxide layer (first metal oxide layer 231) and the first electrode 2 are equal to each other.
The following examines initial breakdown characteristics as the characteristics of the samples 1 to 3 and the comparative samples 1 to 3.
As shown in
When soft breakdown starts, the resistance of the variable resistance element (samples 1 to 3 and comparative samples 1 to 3) decreases and so the voltage distributed to the series-connected load resistor 6 (load resistance value of 5 k) increases, making it difficult to exert a voltage on the variable resistance element. This leads to a phenomenon that, even when the voltage applied across the variable resistance element and the load resistor 6 connected in series is increased, the voltage on the variable resistance element (samples 1 to 3 and comparative samples 1 to 3) hardly changes, and only the current increases.
When the applied voltage is further increased, the resistance value sharply decreases at a second voltage. This phenomenon is called hard breakdown, and the second voltage is called a hard breakdown voltage. The current flowing in the variable resistance element (samples 1 to 3 and comparative samples 1 to 3) to which the hard breakdown voltage is applied is called a hard breakdown current.
The phenomenon of dielectric breakdown as mentioned above can also be seen in a SiO2 thin film which is a gate oxide film of a semiconductor, for example in the case where the current is steered by a series resistance load. That is, soft breakdown occurs first, and then hard breakdown is reached when a high stress voltage is applied. This is called progressive breakdown and reported, for example, in V. L. Lo etc., IEEE IRDS pp. 602, 2005 (Non Patent Literature).
The hard breakdown voltage and the hard breakdown current of the variable resistance element 10 (samples 1 to 3) can be read from
It can be understood from
The soft breakdown point can also be regarded as a point at which the initial resistance starts to change slightly, that is, a point at which the state of the first tantalum oxide layer (31, 213) in the interface between the first tantalum oxide layer (31, 213) and the first electrode 2 starts to change.
Accordingly, in the initial state, the state of the interface between the first tantalum oxide layer (31, 213) and the first electrode 2 is substantially the same in the variable resistance element 10 (samples 1 to 3) and the variable resistance element 20 (comparative samples 1 to 3),
As shown in
In detail, when compared using the same film thickness of the first tantalum oxide layer (31, 213), the variable resistance element 10 has a small hard breakdown voltage at most ½ that of the variable resistance element 20, and a small hard breakdown current at most about ¼ that of the variable resistance element 20. For example, in the case where the film thickness of the first tantalum oxide (31, 213) is 5 nm, the sample 3 has a hard breakdown voltage of 3.4 V and a hard breakdown current of 345 μA, while the comparative sample 1 has a hard breakdown voltage of 8.6 V and a hard breakdown current of 1.4 mA.
That is, in the initial state (i.e. until the start of soft breakdown), the state of the interface between the first tantalum oxide layer (first metal oxide layer 31, 231) and the first electrode 2 is the same. Once soft breakdown starts to occur, however, the effects of not only the state of the interface between the first tantalum oxide layer and the first electrode 2 but also the film quality of the first tantalum oxide layer and the state of the interface between the first tantalum oxide layer and the second tantalum oxide layer (second metal oxide layer 32) emerge. In the variable resistance element 10 (samples 1 to 3), after the first tantalum oxide layer (first metal oxide layer 31) is formed, plasma oxidation is performed to modify the first tantalum oxide layer before forming the second tantalum oxide layer (second metal oxide layer 32).
On the other hand, in the variable resistance element 20 (comparative samples 1 to 3), after the second tantalum oxide layer (second metal oxide layer 32) is formed, the first tantalum oxide layer (first metal oxide layer 231) is formed without the modification process (plasma oxidation) of the first tantalum oxide layer. Its interface is therefore expected to be in a state of being higher in oxygen deficiency than the first tantalum oxide layer (first metal oxide layer 31) of the variable resistance element 10. Hence, after soft breakdown, the current flows comparatively easily (low resistance) due to oxygen deficiency, which causes a relative increase in proportion of the voltage on the load resistor and makes it difficult to exert the voltage on the variable resistance element 20, so that hard breakdown is not easily reached. This leads to increases in hard breakdown voltage and current ensue.
Moreover, the oxygen concentration distribution of the second tantalum oxide layer near the interface between the modified first tantalum oxide layer (first metal oxide layer 31) and the second tantalum oxide layer (second metal oxide layer 32) is not affected by the modification step, and also the oxygen concentration of the first tantalum oxide layer near the interface between the first tantalum oxide layer and the second tantalum oxide layer is higher than the oxygen concentration near the interface between the first electrode 2 and the first tantalum oxide layer.
These results demonstrate that the voltage and the current upon initial breakdown can be reduced by performing such a modification process of the first tantalum oxide layer that reduces the oxygen deficiency of the first tantalum oxide layer near the interface between the first tantalum oxide layer (first metal oxide layer 31) and the second tantalum oxide layer (second metal oxide layer 32) without affecting the oxygen concentration distribution of the second tantalum oxide layer near the interface between the first tantalum oxide layer and the second tantalum oxide layer, as in the variable resistance element 10 in this embodiment.
Resistance change operations after initial breakdown are described below, using the sample 3 as an example of the variable resistance element 10 and the comparative sample 2 as an example of the variable resistance element 20. The sample 3 and the comparative sample 2 have substantially the same film thickness of the first tantalum oxide layer, i.e. about 5 nm.
First, −3.5 V was applied across the first electrode 2 and the second electrode 4 of the variable resistance element 10 (sample 3) as a voltage pulse that causes the first electrode to be relatively negative, to perform an initial breakdown operation. As a result, the resistance value of the variable resistance element 10 (sample 3) decreased from 20 MΩ to 14 kΩ.
Following this, +3.1 V was applied across the first electrode 2 and the second electrode 4 as a voltage pulse that causes the first electrode to be relatively positive, as a result of which the resistance value increased to 54 kΩ.
After this, −1.5 V and +2.5 V were repeatedly applied as lower voltages, so that the variable resistance element 10 (sample 3) performed operations of repeatedly changing between the low resistance state with the resistance value decreased to about 1.1 kΩ and the high resistance state with the resistance value increased to about 50 to 150 kΩ.
When −3.5 V same as in the sample 3 was applied across the first electrode 2 and the second electrode 4 of the variable resistance element 20 (comparative sample 2) as a voltage pulse that causes the first electrode to be relatively negative, initial breakdown was unable to be induced. −7.0 V was eventually applied to induce initial breakdown. As a result, the resistance value of the variable resistance element 20 (comparative sample 2) decreased from 33 MΩ to 7 kΩ.
Following this, +6.1 V was applied across the first electrode 2 and the second electrode 4 as a voltage pulse that causes the first electrode to be relatively positive, as a result of which the resistance value increased to 54 kΩ.
After this, −1.5 V and +2.5 V were repeatedly applied as lower voltages, so that the variable resistance element 20 (comparative sample 2) performed operations of repeatedly changing between the low resistance state with the resistance value decreased to about 1.2 kΩ and the high resistance state with the resistance value increased to about 40 to 110 kΩ,
As shown in
Thus, in the case where such a modification process that reduces the oxygen deficiency of the first tantalum oxide layer is performed on the first tantalum oxide layer when forming the variable resistance element 10 according to the manufacturing method in this embodiment, the voltage and the current upon initial breakdown can be reduced and also stable resistance change operations can be maintained.
As described above, according to the present invention, a method of manufacturing a variable resistance nonvolatile memory element capable of reducing a current upon initial breakdown can be provided. Even when a load resistor (on resistor of a selection transistor or a diode, wiring resistor, or the like) is connected to a variable resistance nonvolatile memory element such as a variable resistance element, there is no need to increase a voltage for an initial breakdown step, so that a high-density memory cell array can be realized without an increase in size of a transistor and the like or an increase in withstand voltage.
The same advantageous effects can be achieved even in the case where the above-mentioned modification process performed on the first tantalum oxide layer to reduce its oxygen deficiency is applied to the variable resistance element 20 shown in
Though the method of manufacturing the variable resistance nonvolatile memory element according to the present invention has been described by way of the embodiment, the present invention is not limited to the embodiment. Modifications obtained by applying various changes conceivable by those skilled in the art to the embodiment and any combinations of components in different embodiments are also included in the present invention without departing from the scope of the present invention.
For example, though the modification process of the film of the first metal oxide layer 31 is performed by plasma oxidation in the sputtering apparatus immediately before the film formation of the second metal oxide layer 32 starts according to the present invention, other oxidation processes such as ozone oxidation and thermal oxidation in oxygen atmosphere are also applicable. The method of modification is not limited so long as the film quality of the first metal oxide layer 31 or 231 can be modified to reduce the oxygen deficiency.
Though the above embodiment describes the case where at least one part of the first metal oxide layer 31 is modified to the modified layer having a higher oxygen content atomic percentage (higher resistance) than the first metal oxide layer 31 by reducing the oxygen deficiency of the at least one part of the first metal oxide layer 31 by the modification process of the film of the first metal oxide layer 31 (high resistance layer), the present invention is not limited to this. The intermediate layer having a higher oxygen content atomic percentage (higher resistance) than the first metal oxide layer 31 may be formed on the first metal oxide layer 31 (high resistance layer), with the second metal oxide layer 32 (low resistance layer) being formed on the intermediate layer. In either case, as long as a transition metal oxide layer having a higher oxygen content atomic percentage than the first metal oxide layer 31 (high resistance layer) is formed between the first metal oxide layer 31 (high resistance layer) and the second metal oxide layer 32 (low resistance layer), a steep wall of an oxygen content atomic percentage profile can be provided between the first metal oxide layer 31 (high resistance layer) and the second metal oxide layer 32 (low resistance layer), with it being possible to prevent diffusion of oxygen from the first metal oxide layer 31 (high resistance layer) to the second metal oxide layer 32 (low resistance layer) (i.e. corruption of oxygen content atomic percentage profile). Thus, the advantageous effects of reducing the voltage upon initial breakdown can be achieved.
Though the above embodiment describes the example where a fixed resistor of 5 kΩ is used as the load resistor 6 connected in series with the variable resistance element, the present invention is not limited to this. A current steering element such as a transistor or a diode may equally be used, with its on resistor serving as the load resistor. The current steering element may have threshold voltages respectively in a positive applied voltage region and a negative applied voltage region, and have a nonlinear property of being in a conductive state (on) in the case where the applied voltage has a higher absolute value than the corresponding threshold voltage and in a nonconductive state (off) in the case where the applied voltage is in any other region (in the case where the applied voltage has a lower absolute value than the corresponding threshold). The use of the nonvolatile memory element manufacturing method according to the present invention enables initial breakdown to be performed with a low applied voltage even when the current steering element in the memory cell has a high on resistance. The manufacturing method in the above embodiment is applicable to a memory cell array in which memory cells described above are arranged in an array.
Though the above embodiment describes the case where the metal oxide layer 3 has a stack structure of tantalum oxide, the present invention is not limited to this, as the functional effects of the present invention are not limited to the use of tantalum oxide. For example, the metal oxide layer may be formed of hafnium (Hf) oxide in a stack structure or zirconium (Zr) oxide in a stack structure. In the case where hafnium oxide in a stack structure is employed, it is preferable, when a composition of a first hafnium oxide is expressed as HfOx and a composition of a second hafnium oxide is expressed as HfOy, that the followings are satisfied: approximately 0.9≦y≦1.6; approximately 1.8<x<2.0; and the first hafnium oxide has a film thickness not less than 3 nm and not more than 4 nm. Furthermore, in the case where zirconium oxide in a stack structure is employed, it is preferable, when a composition of a first zirconium oxide is expressed as ZrOx and a composition of a second zirconium oxide is expressed as ZrOy, that the followings are satisfied: approximately 0.9≦y≦1.4; approximately 1.9<x<2.0; and the first zirconium oxide has a film thickness not less than 1 nm and not more than 5 nm.
Furthermore, in the case where hafnium oxide in a stack structure is employed, a first hafnium oxide layer is formed on the first electrode 2 by a reactive sputtering method with which sputtering is performed in argon gas and oxygen gas, using an Hf target. A second hafnium oxide layer can be formed by exposing the surface of the first hafnium oxide layer to plasma of argon gas and oxygen gas after forming the first hafnium oxide layer. The oxygen content atomic percentage in the first hafnium oxide layer can be easily adjusted by changing the flow rate of oxygen gas to argon gas in reactive sputtering, as in the case of tantalum oxide described above. The temperature of the substrate 1 can be set to an ambient temperature with no particular heating.
In addition, the film thickness of the second hafnium oxide layer can be easily adjusted according to the time of exposure to plasma of argon gas and oxygen gas. In the case where the composition of the first hafnium oxide layer is expressed as HfOx and the composition of the second hafnium oxide layer is expressed as HfOy, it is possible to implement stable resistance change characteristics when 0.9≦y≦1.6, 1.8<x<2.0, and the film thickness of the first hafnium oxide layer is not less than 3 nm and not more than 4 nm.
In the case where zirconium oxide in a stack structure is employed, a first zirconium oxide layer is formed on the first electrode 2 by a reactive sputtering method with which sputtering is performed in argon gas and oxygen gas, using a Zr target. A second zirconium oxide layer can be formed by exposing the surface of the first zirconium oxide layer to plasma of argon gas and oxygen gas after forming the first zirconium oxide layer The oxygen content atomic percentage in the first zirconium oxide layer can be easily adjusted by changing the flow rate of oxygen gas to argon gas in reactive sputtering, as in the case of tantalum oxide described above. The temperature of the substrate 1 can be set to an ambient temperature with no particular heating, as described above.
In addition, the film thickness of the second zirconium oxide layer can be easily adjusted according to the time of exposure to plasma of argon gas and oxygen gas. In the case where the composition of the first zirconium oxide layer is expressed as ZrOx and the composition of the second zirconium oxide layer is expressed as ZrOy, it is possible to implement stable resistance change characteristics when 0.9≦y≦1.4, 1.9<x<2.0, and the film thickness of the first zirconium oxide layer is not less than 1 nm and not more than 5 nm.
Though the above describes that the transition metal oxide (metal oxide layer 3) as the variable resistance layer may comprise tantalum oxide, hafnium oxide, or zirconium oxide, the present invention is not limited to this. The transition metal oxide layer placed between the upper and lower electrodes may include an oxide layer of tantalum, hafnium, zirconium, or the like as a main variable resistance layer where a resistance change occurs, and may additionally include, for example, a slight amount of other element(s). It is also possible to intentionally include the other element(s) in a small amount, for resistance value fine adjustment and the like. Such cases are also included in the scope of the present invention. For example, by adding nitrogen to the variable resistance layer, the variable resistance layer is increased in resistance value, which contributes to an improved resistance change reaction.
Thus, regarding the variable resistance element in which oxygen-deficient transition metal oxide is used in the variable resistance layer, in the case where the variable resistance layer includes a first region comprising a second oxygen-deficient transition metal oxide having a composition expressed as MO and a second region comprising a first oxygen-deficient transition metal oxide having a composition expressed as MOx (where y<x), the first region and the second region may comprise a predetermined impurity (e.g. an additive for resistance value adjustment) in addition to the corresponding transition metal oxide.
There are cases where, when the resistance film is formed by sputtering, a slight amount of element(s) is unintentionally mixed into the resistance film due to residual gas, gas emission from a vacuum vessel wall, or the like. Such cases where a slight amount of element(s) is mixed into the resistance film are also included in the scope of the present invention.
The present invention is applicable as a method of manufacturing a variable resistance nonvolatile memory element. In particular, the present invention is applicable as a method of manufacturing a variable resistance nonvolatile memory element that operates in bipolar mode of reversibly changing in resistance value according to electrical signals of different polarities and that is used in various electronic appliances such as personal computers and mobile phones.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-223818 | Oct 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/005533 | 9/30/2011 | WO | 00 | 12/11/2012 |
