JP3161461B2 - Dielectric element and semiconductor storage device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a dielectric thin film capacitor used for a large-capacity semiconductor memory device or a ferroelectric capacitor used for a nonvolatile semiconductor memory device.

[0002]

2. Description of the Related Art Conventionally, as described in International Electron Devices Meeting (IEDM) Technical Digest 1990, 417-420, ferroelectric capacitors used in ferroelectric memory devices and the like. Has a single-layer structure.

A conventional example will be described based on a sectional structural view of a ferroelectric element shown in FIG.

That is, Pb (Zr _x Ti ₁ -x) O ₃ , abbreviated as a PZT film 201, is composed of an upper electrode 202 and a lower electrode 203.
The PZT film 201 is ideally formed of a thin film having a uniform composition in a direction perpendicular to the upper and lower electrodes. Since the PZT film 201 is not doped with impurities, -It was shaped.

Generally, undoped PZT is p-type.

The writing of information in the ferroelectric memory device is performed according to the direction of polarization of the ferroelectric film in the ferroelectric capacitor.

That is, the upper electrode 202 is
When a bias equal to or higher than the coercive electric field of the PZT film 201 is applied so as to have a positive potential with respect to the above, the polarization direction is downward, and when a bias is applied in the opposite direction to the above direction, the polarization is upward.

[0008] The direction of this polarization corresponds to information 0 and 1.

[0009]

However, a ferroelectric memory device has a cycle time of 100 n in the same manner as a DRAM.
s If the warranty period is 10 years, it must be rewritten at least 10 ¹⁵ times.

In the structure of a conventional ferroelectric capacitor, repeated polarization reversal causes film fatigue, and the PZ near the electrode is reduced.
Oxygen in T diffuses and reacts with the electrode, and PZT near the electrode
Oxygen depletion occurs in the inside, the conductivity type is reversed to n-type, and PZT
There is a problem that a space charge region is generated in the film or between the electrode and the electrode, the polarization is fixed, the magnitude of the residual polarization is reduced, and the leak current is increased.

Therefore, the present invention is intended to solve such a conventional problem. The purpose of the present invention is to increase the lead composition ratio at the electrode interface by forming the ferroelectric thin film into, for example, a three-layer structure. Suppress the occurrence of oxygen diffusion, suppress the generation of space charge regions even when the number of rewrites is 10 ¹⁵ , and suppress the polarization fixation, thereby providing a ferroelectric memory device with a warranty period of 10 years or more. It is to be.

In the above description, the ferroelectric film has a three-layer structure.
Since the composition ratio of the element of the lattice A to the composition ratio of the element of the lattice may be different from that of the central portion, the composition ratio may naturally change gradually in the depth direction. Here, the atoms of the B lattice refer to zirconium (Zr) and titanium (Ti), and the atoms of the A lattice refer to lead (Pb).

[0013]

The dielectric element according to claim 1 of the present invention has a structure in which an oxide dielectric having a perovskite crystal structure represented by the general formula ABO ₃ is sandwiched between two electrodes. A dielectric element having
The composition ratio of the oxide dielectric near at least one of the two electrodes is different from the composition ratio of the oxide dielectric at a position closer to the center than the vicinity of the electrode in the depth direction of the oxide dielectric. And the composition ratio of the oxide dielectric is at least one of
It is characterized by being changed stepwise toward the electrode .
The dielectric element according to claim 2 of the present invention is a dielectric element having a structure in which an oxide dielectric having a perovskite crystal structure represented by the general formula ABO ₃ is sandwiched between two electrodes. The composition ratio of the oxide dielectric in the vicinity of at least one of the two electrodes, and the composition ratio of the oxide dielectric in the center in the depth direction of the oxide dielectric near the electrode. It is formed differently, and the oxide
The composition ratio of the electric body is gradually changed toward at least one of the electrodes. According to a third aspect of the present invention, there is provided a semiconductor memory device including the dielectric element according to the first or second aspect.

[0014]

[0015]

[0016]

[0017]

In the structure of the conventional ferroelectric capacitor, repeated polarization reversal causes film fatigue and reduces the number of rewrites to 10 ¹⁵
In other words, a ferroelectric memory device with a warranty period of 10 years or more could not be provided.

The cause is as follows.

The conductivity type of non-doped PZT is p-type, and when oxygen is deficient in the PZT film, the conductivity type becomes n-type.

When the polarization inversion is repeated, oxygen diffuses toward the electrode near the electrode and reacts with the electrode at the interface.

As a result, in the PZT film, oxygen deficiency occurs near the electrode, and the conductivity type becomes n-type.

Due to the reversal of the conduction type of the PZT film near the electrode, a pn junction is formed in the film, space charges are generated, and the polarization is fixed.

Further, space charge is generated also at the interface with the electrode, and the polarization is fixed, so that the amount of switching charge apparently decreases.

Therefore, in the present invention, the lead (Pb) composition ratio of the PZT film near the electrode is set to be larger than that at the center in advance so that oxygen depletion is hardly generated near the electrode even when the polarization inversion is repeated. Keep it.

Then, since the conduction type of the PZT film does not change and no space charge is generated in the film, the polarization is not fixed even if rewriting is repeated, and the switching charge amount does not decrease.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to a sectional structural view of a dielectric device shown in FIG. 1A and a composition distribution diagram in a depth direction shown in FIG. 1B.

Reference numeral 101 denotes a lower platinum (Pt) electrode; 102, a Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ film having a thickness of 400 Å;
3 is a Pb (Zr _0.5 Ti _0.5 ) O ₃ film having a thickness of 4000Å,
104 denotes a second Pb _1.2 (Zr _0.5 Ti
_0.5 ) O ₃ film, 105 is an upper platinum electrode.

Lower platinum electrode 101, upper platinum electrode 105
In both cases, the film thickness was 5000 °.

As can be seen clearly from FIG.
The composition ratio of Pb to the composition ratio of r and Ti is as large as 1.0 at the center of the PZT film and as large as 1.2 near the electrode.

FIG. 3 is a structural sectional view of an embodiment in which the dielectric element of FIG. 1 is integrated as a charge storage capacitor on a substrate on which a field effect transistor is integrated.

Reference numeral 301 denotes a silicon substrate; 302, a diffusion layer formed by ion implantation and heat treatment;
Is polycrystalline silicon and tungsten silicide (WS
The gate electrode formed according to i), which forms the main part of the field effect transistor.

Reference numeral 304 denotes an aluminum wiring, and the upper electrode 1
05 and the diffusion layer 302.

When rewriting is repeated, the first and second
Pb of Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ films 102 and 104
Is higher than the stoichiometric composition ratio, so that diffusion of oxygen in the PZT film to the electrode does not occur or is very unlikely to occur as compared with the related art.

As a result, the generation of space charge in the PZT film and at the interface between the PZT and the electrode is suppressed, and the polarization is not fixed. Therefore, as shown in the graph of the change in switching charge with respect to the number of rewrites in FIG. In addition, the reduction of the switching charge amount is much smaller than in the conventional example.

Here, the size of the capacitor is set to 100 μm.
× 100 μm and a bias voltage of 5V.

The open circles indicate the case where the conventional one-layer capacitor is used, and the black circles indicate the case where the three-layer capacitor shown in the embodiment of the present invention is used.

In this embodiment, it is estimated that there is almost no decrease in the magnitude of the switching charge even after rewriting 10 ¹⁵ times.

[0038] Moreover, leakage current, At a later rewriting 10 ¹² times, but was a conventional 5V at 100 .mu.A / cm ² or more,
In the present example, it was as good as 8 μA / cm ² .

The method of manufacturing the capacitor shown in the above embodiment is, for example, as follows.

When high-frequency magnetron sputtering is used, two sputtering chambers are prepared, Pb (Zr _0.5 Ti _0.5 ) O ₃ is used as a target of the first chamber, and a target of the second chamber is used.

Then, a lower Pt electrode 101 is formed on a silicon substrate on which MOS transistors are integrated,
In the first and second chambers, the PZT films 102, 1
03 and 104 are continuously formed by sputtering.

As another embodiment using high-frequency magnetron sputtering, it is possible to produce the above three-layer capacitor by changing the sputtering conditions even when using one target.

For example, Pb _1.2 (Zr _0.5 Ti
_0.5 ) O ₃ to form a Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ film 1
02 and 104, the gas pressure during sputtering is 12 mTorr, and when the Pb (Zr _0.5 Ti _0.5 ) O ₃ film 103 is formed, the gas pressure during sputtering is 1 mTorr.
6 mTorr.

Of course, it is possible to change the composition of the film by changing other sputtering conditions such as high frequency power.

As another manufacturing method, there is a sol-gel method.

That is, as a raw material for PZT, for example, P
Although metal alkoxides of b, Zr, and Ti are used, a solution in which the composition ratio in the sol solution is changed in advance is prepared, and the application and baking may be repeated three or more times.

As another manufacturing method, there is a metal organic chemical vapor deposition (MOCVD) method.

In this case, a three-layer structure can be manufactured only by switching the gas.

An organic metal raw material containing Pb, Zr, and Ti as main raw materials is prepared in a bubbler, and the flow rate of the nitrogen gas supplied from each bubbler is controlled by a flow control device, whereby a desired three-layer PZT thin film is formed. Obtainable.

That is, the Pb (Zr _0.5 Ti _0.5 ) O ₃ film 1
When depositing 03, Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃
The flow rate of the nitrogen gas supplied to the Pb bubbler may be reduced by a desired amount as compared with the case where 102 and Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ 104 are deposited. That is, the present invention
The present invention relates to a laminated structure of a dielectric capacitor, and the manufacturing method may be any of the methods described above, or may be another method such as ion beam sputtering or laser vapor deposition.

Next, a second embodiment of the present invention will be described with reference to the sectional structural view of the dielectric element of FIG. 4A and the composition distribution diagram in the depth direction of FIG. 4B.

The difference from the first embodiment is that the composition ratio of the PZT film 401 does not change stepwise, but rather, as shown in FIG. It is a point that gradually changes toward 101.

The effect is exactly the same as that of the first embodiment.

As a manufacturing method, it is possible to apply the manufacturing method shown in the first embodiment.

For example, a case where a high-frequency magnetron sputtering method using one target is used will be described. When the target composition is set, the gas pressure may be gradually changed during the sputtering of the PZT film 401.

That is, the composition in the film is changed from Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ to Pb (Zr) by initially setting the pressure to 16 mTorr and gradually reducing the gas pressure to 12 mTorr.
_0.5 Ti _0.5 ) O ₃ can be gradually changed.

Next, while forming a uniform composition region in the center of the PZT film, the gas pressure was fixed at 12 mTorr, and then the gas pressure was gradually increased to 16 mTorr to change the composition in the film to Pb ( Zr _0.5 Ti _0.5 ) O ₃ can be gradually changed to Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ .

Of course, other methods such as MOCVD may be used.

A third embodiment of the present invention will be described with reference to the sectional structural view of the dielectric element shown in FIG. 5A and the composition distribution in the depth direction shown in FIG. 5B.

In this example, the dielectric material sandwiched between the upper and lower electrodes 101 and 105 is not PZT which is a ferroelectric, but strontium titanate (SrTiO ₃ ) which is a paraelectric.
It is.

Since it is not a ferroelectric, it cannot be recorded by polarization, but has a relative dielectric constant of 200 and silicon dioxide (Si).
Since it is larger than 3.9 of the O ₂ ) film, it can be applied as a charge storage capacitor of a 64 Mbit or 256 Mbit DRAM.

In this case, as shown in FIG.
The composition ratio of Sr to i increases at the center of the SrTiO ₃ thin film and decreases near the electrode.

That is, the composition ratio of the dielectric film is such that the Sr _0.9 TiO ₃ layer 501 and the SrTi
An O ₃ layer 502 and a Sr _0.9 TiO ₃ layer 503 are provided.

Since the electric field applied to the capacitor is used only in one direction for use as a DRAM, only the dielectric composition near one of the upper and lower electrodes may be changed.

For example, when the potential of the upper electrode 105 is set to a positive potential with respect to the lower electrode 101 to be used as a DRAM capacitor, only the Sr _0.9 TiO ₃ layer 503 is necessary, and the Sr _0.9 TiO ₃ layer 501 is not provided. You may.

In this embodiment, PZ was used as the dielectric thin film.
T, SrTiO ₃ was used for explanation, but BaTiO ₃ , P
bTiO ₃ , KNbO ₃ , Pb (MnNb) O ₃ , (Ba
An oxide ferroelectric or oxide paraelectric film having another ABO ₃ type perovskite crystal structure such as Sr) TiO ₃ may be used.

In addition, lanthanum (La), neodymium (Nd), bismuth (Bi), and niobium (N
b), antimony (Sb), tantalum (Ta) or the like may be used as a dopant.

Further, in the above embodiment, the composition ratio in the dielectric film is changed mainly in the vicinity of the upper and lower electrodes and in the center of the dielectric film. Changing it has an effect.

[0069]

As described above, in the semiconductor memory device of the present invention, a ferroelectric film or an oxide dielectric having a perovskite crystal structure represented by the general formula ABO ₃ having a high dielectric constant is sandwiched between two electrodes. In the dielectric element having the above structure, by changing the composition ratio of the element of the A lattice to the composition ratio of the element of the B lattice, near at least one of the two electrodes, with the central part of the dielectric, Even if rewriting is repeated, the generation of oxygen depletion near the electrodes of the dielectric film is suppressed, and a decrease in the amount of switching charges due to polarization inversion can be prevented, so that a reliable dielectric element can be provided. with the leakage current can be suppressed to about one conventional 10 minutes, by integrated on a substrate formed of an active element of the dielectric element, 10 ¹⁵ times the rewritable possible trust It has the effect of said to be able to provide a good mass semiconductor memory device.

[Brief description of the drawings]

FIG. 1 (a) is a sectional structural view of a dielectric element showing a first embodiment of the present invention. (B) is a composition distribution diagram in the depth direction.

FIG. 2 is a sectional structural view of a conventional dielectric element.

FIG. 3 is a sectional structural view of a semiconductor memory device in which the dielectric element according to the first embodiment of the present invention is integrated on a semiconductor substrate.

FIG. 4 (a) is a sectional structural view of a dielectric element showing a second embodiment of the present invention. (B) is a composition distribution diagram in the depth direction.

FIG. 5A is a sectional structural view of a dielectric element showing a third embodiment of the present invention. (B) is a composition distribution diagram in the depth direction.

FIG. 6 is a graph showing a change in a switching charge amount with respect to the number of times of rewriting of the dielectric element according to the first embodiment of the present invention.

[Explanation of symbols]

101 lower platinum electrode 102 Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ film 103 P
b (Zr _0.5 Ti _0.5 ) O ₃ film 104 Pb _1.2 (Zr _0.5 Ti _0.5 ) O ₃ film 105 Upper platinum electrode 201 PZT film 202 Upper electrode 203 Lower electrode 301 Silicon substrate 302 Diffusion layer 303 Gate electrode 304 Aluminum wiring 401 PZT film 501 Sr _0.9 TiO ₃ layer 502 SrTiO ₃ layer 503 Sr _0.9 TiO ₃ layer

Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 21/822 H01L 21/8242 H01L 27/04 H01L 27/108

Claims (3)

(57) [Claims] 1. A dielectric element having a structure in which an oxide dielectric having a perovskite crystal structure represented by a general formula ABO ₃ is sandwiched between two electrodes, wherein at least one of the two electrodes is in the vicinity of at least one of the electrodes. wherein the composition ratio of the oxide dielectric, with the composed formed differently and the composition ratio of the oxide dielectric in the middle position than the electrode neighborhood in the depth direction of the oxide dielectric in the If the composition ratio of the oxide dielectric is at least one of the electrodes
The dielectric element characterized by comprising changing stepwise me. 2. A dielectric element having a structure in which an oxide dielectric having a perovskite crystal structure represented by a general formula ABO ₃ is sandwiched between two electrodes, wherein a vicinity of at least one of the two electrodes is provided. wherein the composition ratio of the oxide dielectric, with the composed formed differently and the composition ratio of the oxide dielectric in the middle position than the electrode neighborhood in the depth direction of the oxide dielectric in the A dielectric element, wherein a composition ratio of an oxide dielectric gradually changes toward at least one of the electrodes. 3. A semiconductor memory device comprising the dielectric element according to claim 1.