JP3689674B2 - Dielectric capacitor and method of manufacturing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dielectric capacitor, and more particularly to improvement of its ferroelectricity.
[0002]
[Prior art]
A conventional ferroelectric capacitor is shown in FIG. A silicon oxide layer 4 is formed on the silicon substrate 2. On top of that, a lower electrode 6 made of platinum is provided. On the lower electrode 6, a ferroelectric layer PZT (PbZr _X Ti _1-X O _Three ) A film 8 is provided, and an upper electrode 10 made of platinum is provided thereon. In this way, a ferroelectric capacitor is formed by the lower electrode 6, the PZT film 8, and the upper electrode 10.
[0003]
Here, the reason why platinum is used for the lower electrode 6 is as follows. The PZT film 8 must be formed on the alignment film. This is because, if formed on an amorphous film, the ferroelectricity is impaired because it is not oriented. On the other hand, the lower electrode 6 must be formed in a state insulated from the silicon substrate 2. For this reason, the silicon oxide layer 4 is formed on the silicon substrate 2. This silicon oxide layer 4 is amorphous. In general, a film formed on an amorphous film becomes a non-oriented film, but platinum has a property of becoming an oriented film even on an amorphous film. For this reason, platinum is used as the lower electrode.
[0004]
[Problems to be solved by the invention]
However, the conventional ferroelectric capacitor as described above has the following problems.
[0005]
Since platinum easily permeates oxygen, there is a problem that the ferroelectricity decreases due to the escape of oxygen in the ferroelectric (PZT), aging, and repeated polarization inversion. That is, as shown in FIG. 30, oxygen in the ferroelectric material may escape from between the platinum columnar crystals.
[0006]
In addition, such a problem occurs in a capacitor using a dielectric having a high dielectric constant.
[0007]
An object of the present invention is to solve the above-described problems and to provide a ferroelectric capacitor or a dielectric capacitor having a high dielectric constant with little deterioration due to aging and repeated polarization inversion.
[0008]
[Means for Solving the Problems]
The dielectric capacitor of the present invention includes a substrate, a lower electrode formed on the substrate, a dielectric formed on the lower electrode, and configured by an oxide ferroelectric film or an oxide dielectric film having a high dielectric constant. The upper electrode is formed so as to face the layer and the lower electrode, and the upper electrode includes an iridium layer and an iridium oxide layer formed thereon.
The dielectric capacitor manufacturing method of the present invention includes a step of forming a lower electrode on a substrate, and an oxide ferroelectric film or an oxide dielectric film having a high dielectric constant is formed as a dielectric layer on the lower electrode. And a step of forming an iridium layer on the dielectric layer by sputtering, and a step of forming an iridium oxide layer by oxidizing the surface of the iridium layer.
In the present invention, the “capacitor” refers to a structure in which electrodes are provided on both sides of an insulator, and includes a structure having this structure regardless of whether it is used for storing electricity.
[0009]
Other examples of the present invention include the following.
A lower electrode having at least an iridium oxide layer, a dielectric layer formed on the lower electrode and made of a ferroelectric or a dielectric having a high dielectric constant, and an upper electrode formed on the dielectric layer ing.
[0010]
Preferably, the lower electrode is formed of an iridium oxide layer formed by sputtering.
[0011]
Preferably, a conductive layer having a good crystal orientation is formed on the iridium oxide layer to form a lower electrode, and a ferroelectric layer is formed on the conductive layer. Yes.
[0012]
Preferably, the conductor layer is a platinum layer.
[0013]
Preferably, the conductor layer is an iridium layer.
[0014]
Preferably, the conductor layer is an alloy layer of platinum and iridium.
[0015]
Preferably, the lower electrode is formed on a silicon oxide layer formed on a substrate, and the lower electrode has a bonding layer in contact with the silicon oxide layer.
[0016]
Preferably, the lower electrode includes an iridium layer and an iridium oxide layer formed thereon.
[0017]
Preferably, the lower electrode is formed by oxidizing the surface of the iridium layer.
[0018]
Desirably, a conductor layer with good crystal orientation is formed on the iridium oxide layer to form a lower electrode, and a ferroelectric layer is formed on the conductor layer. It is characterized by.
[0019]
Preferably, the conductor layer is a platinum layer.
Preferably, the conductor layer is an iridium layer.
[0021]
Preferably, the conductor layer is an alloy layer of platinum and iridium.
[0022]
Preferably, the lower electrode is formed on a silicon oxide layer formed on a substrate, and the lower electrode has a bonding layer in contact with the silicon oxide layer.
[0023]
Preferably, a dielectric comprising a lower electrode, a dielectric layer formed on the lower electrode and made of a ferroelectric or a dielectric having a high dielectric constant, and an upper electrode formed on the dielectric layer In the capacitor, the lower electrode is formed by forming a conductive thin film having good crystal orientation on the surface of the iridium layer and then oxidizing the conductive thin film.
[0024]
Preferably, the conductive thin film is platinum.
[0025]
Preferably, the conductive thin film is an alloy of iridium and platinum.
[0026]
Preferably, the lower electrode is formed on the lower electrode, and is formed of a ferroelectric or a dielectric having a high dielectric constant. The upper electrode is formed on the dielectric layer and has at least an iridium oxide layer. It is equipped with.
[0027]
Preferably, the upper electrode is formed of an iridium oxide layer formed by sputtering.
[0028]
Preferably, the upper electrode includes an iridium layer and an iridium oxide layer formed thereon.
[0029]
Preferably, the upper electrode is formed by oxidizing the surface of the iridium layer.
[0030]
Desirably, a lower electrode having at least an iridium oxide layer, a dielectric layer formed on the dielectric layer having a ferroelectric or high dielectric constant, formed on the dielectric layer, and at least oxidized An upper electrode having an iridium layer.
[0031]
Since this dielectric capacitor has an iridium oxide layer and iridium on the lower electrode, it is possible to prevent the escape of oxygen from the dielectric layer and to suppress the secular change of the dielectric characteristics.
That is, the iridium oxide layer does not have orientation regardless of whether the underlying layer is oriented, but when the iridium layer is further formed on the iridium oxide layer, the iridium layer has orientation, The dielectric layer formed on this upper layer is well oriented, and the current integral value hardly changes and good characteristics can be obtained.
Further, since iridium is formed on the iridium oxide layer, the orientation of the ferroelectric layer can be improved while preventing escape of oxygen.
Furthermore, since the lead component in the ferroelectric layer does not penetrate into the lower electrode, the imprint characteristics are improved.
Further, the lower electrode is composed of an iridium layer formed on the substrate surface via a silicon oxide film and an iridium oxide layer formed thereon.
Therefore, since the dielectric layer is formed on the lower electrode, the entire surface of the dielectric layer is in contact with the lower electrode, and oxygen permeation from the dielectric layer becomes a problem. Then, iridium oxide is formed so as to fill in the space between the iridium columnar crystals. Therefore, the permeation of oxygen from the dielectric layer can be prevented very well.
For this reason, it is possible to prevent permeation of oxygen, and it is possible to provide a dielectric capacitor having excellent remanent polarization characteristics.
Furthermore, the substrate is composed of an iridium layer made of columnar crystals formed on an insulating surface on the substrate, and an iridium oxide layer formed between the columnar crystals by oxidation of the iridium layer. Since the lower electrode is provided, iridium oxide is formed so as to fill the space between the iridium columnar crystals. Therefore, the permeation of oxygen from the dielectric layer can be prevented very well.
For this reason, it is possible to prevent permeation of oxygen, and it is possible to provide a dielectric capacitor having excellent remanent polarization characteristics.
In addition, since the upper electrode includes an iridium layer made of columnar crystals and an iridium oxide layer formed between the columnar crystals by oxidation of the iridium layer, the iridium columnar crystals Iridium oxide is formed so as to fill the space between the crystals. Therefore, the permeation of oxygen from the dielectric layer can be prevented very well.
For this reason, it is possible to prevent permeation of oxygen, and it is possible to provide a dielectric capacitor having excellent remanent polarization characteristics.
[0032]
Furthermore, in this dielectric capacitor, a conductive layer having a good crystal orientation is provided on the iridium oxide layer, and a dielectric layer is provided on the conductive layer. Therefore, the orientation of the dielectric layer is improved, and the secular change of the dielectric characteristics can be suppressed.
[0033]
This dielectric capacitor has an iridium layer on the iridium oxide layer, and a dielectric layer on the iridium layer. Therefore, the orientation of the dielectric layer is further improved, and the secular change of the dielectric characteristics can be suppressed.
[0034]
This dielectric capacitor is formed by providing an iridium surface with a thin film conductor having good crystal orientation and then performing an oxidation treatment. Therefore, escape of oxygen from the dielectric layer can be prevented, and a dielectric layer having good orientation can be obtained. Accordingly, the secular change of the dielectric characteristics can be extremely suppressed.
[0035]
This dielectric capacitor has an iridium oxide layer on the upper electrode. Therefore, the escape of oxygen from the dielectric layer can be prevented, and the secular change of dielectric characteristics can be suppressed.
[0036]
This dielectric capacitor has an iridium oxide layer on both the upper electrode and the lower electrode. Therefore, the escape of oxygen from the dielectric layer can be reliably prevented, and the secular change of the dielectric characteristics can be suppressed.
[0037]
That is, it is possible to provide a dielectric capacitor with good ferroelectricity and high dielectric property.
[0038]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a composition of a ferroelectric capacitor according to an embodiment of the present invention. A silicon oxide layer 4, a lower electrode 12, a ferroelectric film (ferroelectric layer) 8, and an upper electrode 15 are provided on the silicon substrate 2. The lower electrode 12 is made of iridium oxide, and the upper electrode 15 is also made of iridium oxide.
[0039]
FIG. 2 compares the physical properties of platinum and iridium. As is clear from this table, the resistivity of iridium oxide is 49 × 10 ^-6 Since it is Ωcm, there is no problem as an electrode material.
[0040]
As shown in FIG. 30 of the conventional example, since platinum is a columnar crystal, oxygen in the ferroelectric film 8 is transmitted. In this embodiment, iridium oxide is used as the lower electrode 12. Since this iridium oxide layer 12 is not a columnar crystal, it hardly transmits oxygen. Therefore, oxygen deficiency in the ferroelectric film 8 can be prevented. The same applies to the upper electrode 15. Thereby, the ferroelectricity of the ferroelectric film 8 was improved. This point will be described later together with experimental data.
[0041]
In the above embodiment, since both the lower electrode 12 and the upper electrode 15 are formed of iridium oxide, the permeation of oxygen can be reliably prevented. However, a certain degree of effect can be obtained with only one of them. This point will also be described later.
[0042]
The ferroelectric capacitor as described above can be used as a nonvolatile memory in combination with the transistor 24 as shown in FIG. 3, for example.
[0043]
FIG. 4 shows a manufacturing process of a ferroelectric capacitor according to an embodiment of the present invention. The surface of the silicon substrate 2 is thermally oxidized to form a silicon oxide layer 4 (FIG. 4A). Here, the thickness of the silicon oxide layer 4 was 600 nm. Next, iridium oxide is formed on the silicon oxide layer 4 by reactive sputtering using iridium as a target, and this is used as the lower electrode 12 (FIG. 4B). Here, it was formed to a thickness of 200 nm.
[0044]
Next, a PZT film is formed as the ferroelectric layer 8 on the lower electrode 12 by a sol-gel method (FIG. 4C). As a starting material, Pb (CH _Three COO) ₂ ・ 3H ₂ O, Zr (t-OC _Four H ₉ ) _Four , Ti (i-OC _Three H ₇ ) _Four A mixed solution of was used. After spin-coating this mixed solution, it was dried at 150 degrees (Celsius, hereinafter the same), and pre-baked at 400 degrees for 30 seconds in a dry air atmosphere. After repeating this 5 times, O ₂ Heat treatment at 700 ° C. or higher was performed in the atmosphere. In this way, a 250 nm ferroelectric layer 8 was formed. Here, PbZr _x Ti _1-x O _Three , X is set to 0.52 (hereinafter referred to as PZT (52 · 48)) to form a PZT film.
[0045]
Further, iridium oxide is formed on the ferroelectric layer 8 by reactive sputtering to form the upper electrode 15 (FIG. 4D). Here, it was formed to a thickness of 200 nm. In this way, a ferroelectric capacitor can be obtained.
[0046]
FIG. 5 and FIG. 6 show fatigue of the remanent polarization Pr when a ferroelectric capacitor is formed using PZT (52/48) as the ferroelectric film 8. The test was performed by applying a voltage between points a and b with the structure shown in FIG. Voltages of 5V and -5V as shown in FIG. 7 were applied between the upper electrode 15 and the lower electrode 12, and the degradation of the remanent polarization Pr was measured. In addition, the application of the voltage of 5V and -5V was made into 1 cycle. The voltage was applied at a frequency of 500 KHz.
[0047]
The vertical axis in FIGS. 5 and 6 represents the value of Pr / Po, where Po is the initial residual polarization and Pr is the residual polarization after fatigue. That is, the larger the value, the more significant the deterioration. The horizontal axis represents how many cycles the voltage of FIG. 7 was applied. In the figure, a curve 50 indicates that the upper electrode 15 and the lower electrode 12 are both made of iridium oxide, a curve 52 indicates that the upper electrode 15 is formed of platinum, and a lower electrode 12 is formed of iridium oxide. When the lower electrode 12 is made of platinum with iridium oxide, the characteristic changes when both the upper electrode 15 and the lower electrode 12 are made of platinum.
[0048]
As is clear from this figure, it is clear that the deterioration of the remanent polarization Pr is considerably improved when either the upper electrode 15 or the lower electrode 12 is made of iridium oxide. In particular, when both the upper electrode 15 and the lower electrode 12 are made of iridium oxide, 10 ^Ten It is clear that there is little degradation until the cycle.
[0049]
By the way, iridium oxide has no orientation regardless of whether or not the underlayer is oriented. For this reason, the ferroelectric film 8 formed on the iridium oxide is not oriented.
[0050]
In order to verify that iridium oxide has no orientation regardless of the orientation of the underlying layer, the following test was conducted. The characteristics of a ferroelectric capacitor in which iridium oxide is directly formed on the silicon substrate 2 as the lower electrode 12 and a ferroelectric capacitor in which iridium oxide is formed on the silicon oxide layer 4 as the lower electrode 12 were compared. . FIG. 9 shows the hysteresis characteristics. FIG. 9A shows the case where the lower electrode 12 is formed on the silicon oxide layer 4. 9 B is a case where iridium oxide is directly formed on the silicon substrate 2 as the lower electrode 12. As is apparent from this graph, the characteristics of the ferroelectric film 8 are the same regardless of whether the underlying layer of iridium oxide is silicon or silicon oxide. This seems to be because iridium oxide has no orientation regardless of whether or not the underlayer is oriented.
[0051]
If the iridium oxide layer of the lower electrode 12 is not oriented, the orientation of the ferroelectric film 8 formed thereon is also deteriorated. Therefore, if the orientation of the ferroelectric layer 8 is improved while using the iridium oxide layer, the fatigue characteristics of the remanent polarization Pr are further improved.
[0052]
For this purpose, the lower electrode 12 may be formed by forming a platinum layer on the iridium oxide layer. FIG. 10A shows a change in characteristics in such a case. In this graph, the vertical axis represents the magnitude of polarization, and the horizontal axis represents the same voltage application cycle as in FIG. In addition, this graph shows changes in the characteristic values Pr, Pmax, P, and N of the hysteresis characteristics shown in FIG. 10B. As is apparent from the graph, the deterioration of characteristics is further improved by providing a platinum layer on the iridium oxide layer. In other words, there is almost no deterioration until 1011 cycles.
[0053]
Instead of the platinum layer, a conductor layer with good orientation such as an iridium layer or an alloy of platinum and iridium may be provided (as will be described later, the structure shown in FIG. 27 may be used). In particular, an alloy of platinum and iridium can select the lattice constant by changing the compounding ratio, and can easily match the lattice constant with the ferroelectric layer.
[0054]
In FIG. 11, the residual polarization Pr and the coercive electric field when PZT (52/48) is used as the ferroelectric layer 8 and PtxIr1-x is used as the lower electrode 12 and the composition ratio x of platinum and iridium is changed. The change in Ec is shown graphically. As is clear from this figure, the remanent polarization Pr is higher when an alloy with iridium is used as the lower electrode 12 than when only platinum is used (when x = 1.0). It is clear to show the value. That is, it can be said that the ferroelectricity is improved. A remarkable improvement in characteristics is obtained in the range of 0% to 50% of platinum, and particularly excellent characteristics are obtained at a mixing ratio of 20% to 30% with a peak at about 25% of platinum. Therefore, if the alloy is formed on the iridium oxide layer, the ferroelectric film 8 having excellent characteristics can be obtained.
[0055]
Even when the conductive layer is formed on the iridium oxide layer and used as an electrode, the upper and lower electrodes 15 and 12 are formed so as to have the iridium oxide layer as in the case of FIGS. Is preferred.
[0056]
FIGS. 12 and 13 show hysteresis characteristics when an iridium oxide layer and an iridium layer are used for the upper electrode 15 (FIGS. 12A and 13A), and hysteresis characteristics when only an iridium layer is used for the upper electrode 15 (FIG. 12B). FIG. 13B) is shown. In both cases, the lower electrode was formed of an iridium oxide layer and an iridium layer. Both initial characteristics (FIGS. 12A and 12B) are the same, but 10 ⁸ The characteristics (FIG. 13A and FIG. 13B) after applying a pulse over a number of times (cycles) are clearly superior when the iridium oxide layer and the iridium layer are also used for the upper electrode 15. This is also considered to be an effect of preventing the escape of oxygen by the iridium oxide layer.
[0057]
Note that there was no difference in the aging characteristics of the remanent polarization Pr and the coercive electric field Ec as described above when platinum was provided on the iridium oxide layer and when iridium was provided. However, there is a difference in imprint characteristics in which, when a pulse in a certain direction is continuously applied to the ferroelectric layer 8 or is kept in a polarization state in a certain direction for a long time, the polarization characteristics are affected.
[0058]
FIG. 14 shows the characteristics of the ferroelectric capacitor when the lower electrode 12 is formed by providing platinum on the iridium oxide layer. This graph shows the case where a voltage different from the polarization direction is applied to the ferroelectric capacitor (ferroelectric film 8) in the polarization state (inversion mode shown in FIG. 15A) and the case where the same voltage as the polarization direction is applied. FIG. 15 is a graph showing temporal changes in current (current per unit area) in the non-inversion mode shown in FIG. 15B. FIG. It can be seen that a larger current flows in the reverse mode.
[0059]
This characteristic is used when a ferroelectric capacitor is used as a memory. That is, it is used to read the recorded information by judging the magnitude of the integrated value of the current when the voltage is applied compared to the threshold value. Therefore, as the usage period elapses, the current integrated value in the inverting mode QP decreases (that is, approaches the threshold value), or the current integrated value in the non-inverting mode increases (that is, the threshold value). ), There is a risk of erroneous reading.
[0060]
In FIG. 16, a pulse in the same direction is applied to the ferroelectric film 8 (when the upper electrode 15 and the lower electrode 12 are both formed by providing platinum on the iridium oxide layer). ⁹ The current in the inversion and non-inversion modes after the application is applied once. As is clear from the experimental results, the inversion mode QP ⁺ Current integration value at the time of the decrease, almost the first non-inverting mode QU ^- It approaches to the point where it becomes equal to the integral value of.
[0061]
FIG. 17 shows changes in the current integration value in the above case. The horizontal axis represents the pulse application cycle (number of times), and the vertical axis represents the current integration value. As can be seen from the graph, it can be seen that the current integrated value has changed greatly from around 10 4 times.
[0062]
FIG. 18 shows changes in the current integral value of the ferroelectric film 8 when both the upper electrode 15 and the lower electrode 12 are formed by providing an iridium layer on the iridium oxide layer. As is apparent from the graph, the current integral value hardly changes and a good characteristic is selected. That is, regarding the imprint characteristics as described above, it has been clarified that an electrode in which iridium is formed on iridium oxide is superior to an electrode in which platinum is formed on iridium oxide.
[0063]
FIG. 19 shows the result of analyzing the element content in each layer of the ferroelectric capacitor used in the above experiment. FIG. 19A shows a case where platinum is formed on iridium oxide, and FIG. 19B shows a case where iridium is formed on iridium oxide. The horizontal axis indicates the depth from the surface of the upper electrode, and the vertical axis indicates the content of each element.
[0064]
It is clear from this graph that in the case of FIG. 19A, the lead (Pb) component in the ferroelectric layer 8 penetrates to the platinum (Pt) of the lower electrode. On the other hand, in the case of FIG. 19B, the lead (Pb) component in the ferroelectric substance hardly penetrates into the lower electrode. This seems to affect the imprint characteristics as described above.
[0065]
FIG. 20 shows the structure of a ferroelectric capacitor according to another embodiment of the present invention. In this embodiment, a titanium layer (5 nm) is provided as a bonding layer 30 between the lower electrode 12 and the silicon oxide layer 4. In general, the adhesion between iridium oxide and silicon oxide is not very good. For this reason, the alloy layer may be partially peeled off and the ferroelectric characteristics may be deteriorated. In particular, this becomes more prominent as the ratio of iridium in the alloy increases. Therefore, in this embodiment, a titanium layer having good adhesion with the silicon oxide layer 4 is provided as the bonding layer 30. This improves the ferroelectric characteristics. Note that the titanium layer may be formed by sputtering.
[0066]
In the above embodiment, a titanium layer is used as the bonding layer 30, but any material may be used as long as the material improves bonding properties. For example, a platinum layer may be used.
[0067]
In each of the above embodiments, PZT is used as the ferroelectric film 8, but any material may be used as long as it is an oxide ferroelectric. For example, Ba _Four Ti _Three O ₁₂ May be used.
[0068]
A capacitor according to another embodiment of the present invention is shown in FIG. In this embodiment, a dielectric layer 90 having a high dielectric constant is used in place of the ferroelectric layer 8. An iridium oxide lower electrode 12 is provided on the silicon oxide layer 4, and SrTiO is provided thereon. _Three , (Sr, Ba) TiO _Three A high dielectric constant thin film having a perovskite structure was formed as the dielectric layer 90. In this case, the dielectric property was improved as in the case of the ferroelectric material. That is, it has become clear that what has been described about the ferroelectric layer can be applied to a dielectric layer having a high dielectric constant.
[0069]
FIG. 22 shows the structure of a ferroelectric capacitor according to another embodiment of the present invention. A silicon oxide layer 4, a lower electrode 12, a ferroelectric film (ferroelectric layer) 8, and an upper electrode 15 are provided on the silicon substrate 2. The lower electrode 12 includes an iridium layer 11 and an iridium oxide layer formed thereon. 13 Is formed by. The upper electrode 15 is formed by the iridium layer 7 and the iridium oxide layer 9 formed thereon.
[0070]
An enlarged view of the vicinity of the lower electrode 12 is shown in FIG. Since the iridium layer 11 is a columnar crystal, oxygen in the ferroelectric film 8 is transmitted. In this embodiment, an iridium oxide layer 13 is formed on the upper surface of the iridium layer 11. As described above, this iridium oxide layer 13 can prevent oxygen deficiency in the ferroelectric film 8. The same applies to the upper electrode 15.
[0071]
In the above embodiment, since the iridium oxide layer is formed on both the lower electrode 12 and the upper electrode 15, a ferroelectric capacitor having excellent characteristics with little secular change can be obtained. Even if either one of the lower electrode 12 and the upper electrode 15 has the above structure, a certain degree of effect can be obtained. This point will also be described later.
[0072]
FIG. 24 shows the manufacturing process of this ferroelectric capacitor. The surface of the silicon substrate 2 is thermally oxidized to form a silicon oxide layer 4 (FIG. 24A). Here, the thickness of the silicon oxide layer 4 was 600 nm. Next, the iridium layer 11 is formed on the silicon oxide layer 4 using iridium as a target (FIG. 24B). Next, heat treatment is performed at 800 ° C. for 1 minute in an O 2 atmosphere to form an iridium oxide layer 13 on the surface of the iridium layer 11. The iridium layer 11 and the iridium oxide layer 13 serve as the lower electrode 12. Here, the lower electrode was formed to a thickness of 200 nm.
[0073]
Next, a PZT film is formed as the ferroelectric layer 8 on the lower electrode 12 by a sol-gel method (FIG. 24C). As a starting material, Pb (CH _Three COO) ₂ ・ 3H ₂ O, Zr (t-OC _Four H ₉ ) _Four , Ti (i-OC _Three H ₇ ) _Four A mixed solution of was used. After spin-coating this mixed solution, it was dried at 150 degrees (Celsius, hereinafter the same), and pre-baked at 400 degrees for 30 seconds in a dry air atmosphere. After repeating this five times, heat treatment at 700 ° C. or higher was performed in an O 2 atmosphere. In this way, a 250 nm ferroelectric layer 8 was formed. Here, PbZr _X Ti _1-X O _Three , X is set to 0.52 (hereinafter referred to as PZT (52 · 48)) to form a PZT film.
[0074]
Further, the iridium layer 7 is formed on the ferroelectric layer 8 by sputtering. Next, heat treatment is performed at 800 ° C. for 1 minute in an O 2 atmosphere to form an iridium oxide layer 9 on the surface of the iridium layer 7 (FIG. 24D). The iridium layer 7 and the iridium oxide layer 9 serve as the upper electrode 15. Here, the upper electrode 15 was formed to a thickness of 200 nm. In this way, a ferroelectric capacitor can be obtained.
[0075]
25 and 26 show the fatigue characteristics of the remanent polarization Pr of the ferroelectric capacitor thus obtained. The measurement of this graph was performed by the same method as in FIGS.
[0076]
The vertical axis in FIGS. 25 and 26 represents the value of Pr / Po, where Po is the initial residual polarization and Pr is the residual polarization after fatigue. The horizontal axis represents how many cycles the voltage of FIG. 7 was applied. In the figure, a curve α represents a case where only the lower electrode 12 is manufactured as in the above embodiment. Here, the silicon oxide layer 4 is 600 nm, the lower electrode 12 (oxidized surface of the iridium layer 11) is 200 nm, the ferroelectric film 8 (PZT (52/48)) is 250 nm, and the upper electrode is platinum. Consists of. A curve β is a case where the surface of the iridium layer of the lower electrode 12 is not oxidized (other conditions are the same as those of the curve α). A curve γ is a case where the lower electrode 12 is made of platinum (other conditions are the same as those of the curve α).
[0077]
As is apparent from this figure, when the surface of the iridium layer 11 is oxidized to form the iridium oxide layer 13, the deterioration of the remanent polarization Pr is considerably improved. Note that iridium (when not oxidized) (curve β) is superior to platinum (curve γ) because iridium is oxidized in other steps without special oxidation treatment. It is thought that.
[0078]
Also in this embodiment, it is preferable to provide the bonding layer 30 as described in FIG.
[0079]
Further, the embodiment of oxidizing the iridium surface described here can be applied not only to the ferroelectric film but also to the above-described dielectric film having a high dielectric constant, and the same effect can be obtained.
[0080]
As described above, oxygen can be prevented from escaping from the ferroelectric film by oxidizing the surface of the iridium layer, but iridium oxide is formed on the surface and the orientation of the ferroelectric film is deteriorated. As described above, this can be solved by providing an iridium layer, a platinum layer, an alloy layer thereof, or the like on the iridium oxide layer 13 (see FIG. 10). However, this problem can be solved by forming the lower electrode as follows.
[0081]
First, as shown in FIG. 27, a platinum layer 80 (thin film conductor) is provided very thinly on the iridium layer 11. Here, it was set to 30 nm. Next, heat treatment is performed in this state. Since the platinum layer 80 on the surface does not react with oxygen, it is not oxidized. Further, since the platinum layer 80 is formed thin, the crystal between the iridium layer 11 below is oxidized, and iridium oxide is formed to prevent the permeation of oxygen. Therefore, it is possible to form the lower electrode 12 that can prevent the permeation of oxygen while the surface remains excellent in orientation. As the thin film conductor, any conductor may be used as long as it has good orientation and is difficult to oxidize.
[0082]
The iridium layer 11 oxidized after forming such a thin platinum layer 80 can be used alone as the lower electrode 12. However, the conductive layer with good orientation in the example (see FIG. 10) in which the orientation is improved by providing a conductive layer (iridium layer, platinum layer, etc.) with good orientation on the iridium oxide layer formed by sputtering. Can also be used.
[0083]
In FIG. 28B, when the iridium layer (200 nm) and the platinum layer (30 nm) having the structure shown in FIG. Shows hysteresis characteristics. FIG. 28A shows the hysteresis characteristics of the ferroelectric 8 when an iridium layer (200 nm) is provided on the iridium oxide layer (50 nm) and oxidized. As is apparent from the figure, the hysteresis characteristic is better when the structure of FIG. 27 is used.
[0084]
The embodiment described here can be applied not only to the ferroelectric film but also to the above-described dielectric film having a high dielectric constant, and the same effect can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing a composition of a ferroelectric capacitor according to an embodiment of the present invention.
FIG. 2 is a diagram showing physical properties of platinum and iridium.
3 is a view showing a nonvolatile memory using a ferroelectric capacitor 22. FIG.
FIG. 4 is a diagram showing a manufacturing process of a ferroelectric capacitor.
FIG. 5 is a graph showing the secular change of remanent polarization when the electrode material is changed.
FIG. 6 is a graph showing the secular change of remanent polarization when the electrode material is changed.
FIG. 7 is a diagram illustrating a voltage applied when a fatigue test is performed.
FIG. 8 is a diagram showing a structure for a fatigue test.
FIG. 9 is a graph showing a result of verifying that iridium oxide is not affected by an underlayer.
FIG. 10 is a graph showing a change in remanent polarization Pr when platinum is provided on iridium oxide.
FIG. 11 is a diagram showing changes in remanent polarization Pr and coercive electric field Ec when the mixing ratio of an alloy of platinum and iridium is changed.
12 is a graph comparing a case where an iridium oxide layer is provided on both the lower electrode and the upper electrode and a case where only the lower electrode is provided. FIG.
FIG. 13 is a graph comparing a case where an iridium oxide layer is provided on both the lower electrode and the upper electrode and a case where only the lower electrode is provided.
FIG. 14 is a graph showing temporal changes in current that occur when a voltage is applied to a ferroelectric.
15 is a diagram illustrating a voltage application direction in the graph of FIG.
FIG. 16 is a graph showing a state in which the characteristics of FIG. 14 are changed by imprint characteristics.
FIG. 17 is a graph showing imprint characteristics.
FIG. 18 is a graph showing imprint characteristics.
FIG. 19 is a diagram for explaining a cause of a difference in imprint characteristics.
20 is a diagram showing an example in which a bonding layer 30 is provided. FIG.
FIG. 21 is a diagram showing an example in which a dielectric 90 having a high dielectric constant is used.
FIG. 22 is a diagram showing the structure of a ferroelectric capacitor according to another embodiment.
FIG. 23 is a diagram showing a mechanism by which an iridium oxide layer prevents escape of oxygen.
24 is a diagram showing a manufacturing process of the ferroelectric capacitor of FIG. 22; FIG.
FIG. 25 is a diagram comparing the secular change of remanent polarization.
FIG. 26 is a diagram showing a comparison of secular change of remanent polarization.
FIG. 27 is a diagram showing an example in which oxidation is performed by providing a thin film platinum on the surface of iridium.
FIG. 28 is a graph showing a comparison of effects when the electrode of FIG. 27 is used.
FIG. 29 is a diagram showing a structure of a conventional ferroelectric capacitor.
FIG. 30 is a diagram showing a state in which oxygen escapes from the lower electrode 6 made of platinum.
[Explanation of symbols]
2. . . Silicon substrate
4). . . Silicon oxide layer
8). . . Ferroelectric layer
12 . . Bottom electrode
15. . . Upper electrode
90. . . Dielectric layer having high dielectric constant

Claims (2)

substrate,
A lower electrode formed on the substrate;
A dielectric layer formed on the lower electrode and configured by an oxide ferroelectric film or an oxide dielectric film having a high dielectric constant;
An upper electrode formed to face the lower electrode ;
The dielectric capacitor according to claim 1, wherein the upper electrode includes an iridium layer and an iridium oxide layer formed thereon . Forming a bottom electrode on the substrate;
Forming an oxide dielectric film having an oxide ferroelectric film or a high dielectric constant on the lower electrode as a dielectric layer,
Forming an iridium layer by sputtering on the dielectric layer;
Forming a iridium oxide layer by oxidizing the surface of the iridium layer .

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