JPH1056140A - Ferroelectric memory device and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferroelectric memory element, along with its manufacturing method, in which a lower electrode doesn't come off and a deterioration in characteristics of the ferrodielectric thin film can be prevented. SOLUTION: A lower electrode, a ferroelectric tin film, and an upper electrode are sequentially deposited on a semiconductor substrate 10. The lower electrode includes a nitride thin film 4 made of a nitride material selected from Ti, Zr, Hf, V, Nb, Ta and W, and a Pt thin film 5 on the nitride thin film 4. In addition, the lower electrode includes an oxide thin film 6 on the Pt thin film 5. The oxide thin film 6 is made of an oxide material selected from Ru, Ir, Re, Os, and Rh.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric memory device having a ferroelectric thin film and a method for manufacturing the same.

[0002]

2. Description of the Related Art Ferroelectric materials have many functions such as spontaneous polarization, high dielectric constant, electro-optic effect, piezoelectric effect and pyroelectric effect, and are therefore applied to a wide range of device development. For example, an infrared linear array using the pyroelectric property, an ultrasonic sensor using the piezoelectric property, a waveguide type optical modulator using the electro-optic effect, an ultra-high density DRAM using the high dielectric constant property, It is applied in various fields such as MMIC capacitors.

[0003] With the recent development of thin film forming technology, a combination of a semiconductor memory technology and a ferroelectric non-volatile memory (FeRA) which operates at high density and at high speed has been developed.
M) has been actively developed. Furthermore, in terms of high-speed writing / reading, low-voltage operation, and high repetition durability of writing / reading of a nonvolatile memory using a ferroelectric thin film, not only replacement of a conventional nonvolatile memory but also SRAM, As a memory that can be replaced in the DRAM field, research and development for its practical use have been actively conducted. The development of these devices involves remanent polarization (Pr)
Therefore, it is necessary to use a material that has a large coercive field, a small coercive electric field (Ec), a low leakage current characteristic, and a high resistance to repetition of polarization reversal.

At present, as ferroelectric materials, 3
Component composite is perovskite oxide PZT (lead zirconate titanate _{(Pb (Ti x Zr 1-} x) O 3)) and the most widely used, PZT is also known to have a good hysteresis characteristic I have.

FIG. 8 is a sectional view of a main part of a conventional memory device having a stack structure using a ferroelectric material. As shown in FIG. 8, in this device, a CMOS transistor portion is formed on the surface of a semiconductor substrate, and polysilicon, a Ti thin film, T
An iN thin film, a Pt thin film, a PZT thin film, and a Pt thin film are sequentially formed. Like this element,
For application to an actual device, it is necessary to form an electrode of a capacitor, and platinum (Pt) has been used as a conventional electrode material. This is because Pt is
This is because ZT has high resistance to oxidation reaction during the high-temperature film forming process.

As shown in FIG. 8, when a stacked cell structure indispensable for high integration is formed, a lower CM is required.
It is necessary to electrically connect the OS transistor portion and the upper ferroelectric capacitor portion with a plug of polysilicon or the like, and a nitride as a diffusion barrier layer between the PZT thin film and the polysilicon plug. Generally, a metal nitride thin film such as titanium (TiN) is used.

[0007]

However, in the above prior art, when a PZT thin film is formed on a substrate on which a TiN thin film and a Pt thin film are formed, oxygen must be contained to obtain good ferroelectric characteristics. It is necessary to perform a heat treatment at 600 ° C. or more in an ambient atmosphere, and at this time, there is a problem that separation occurs between the TiN thin film and the Pt thin film. It is considered that the cause is that oxygen in the PZT thin film permeates the Pt thin film and oxidizes the surface of the TiN thin film, so that the volume expansion causes strain between the PtT thin film and the upper Pt thin film.

In a ferroelectric memory device using a film structure in which a PZT thin film is formed directly on a Pt thin film, when data is repeatedly rewritten, the hysteresis characteristic of the ferroelectric thin film is deteriorated. The problem of fatigue of body thin films has also been reported.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and there is provided a ferroelectric memory element which does not peel off a lower electrode and can suppress deterioration of film characteristics of a ferroelectric thin film. It is intended to provide a manufacturing method thereof.

[0010]

According to the present invention, there is provided a ferroelectric memory device comprising a semiconductor substrate, in which a lower electrode, a ferroelectric thin film, and an upper electrode are sequentially laminated. Ti, Zr, H as the lower electrode
a nitride thin film made of a material selected from nitrides of f, V, Nb, Ta, and W;
Pt thin film consisting of Ru, Ir, R on the Pt thin film
and an oxide thin film made of an oxide of a material selected from e, Os, and Rh.

Further, according to the present invention, in the above ferroelectric memory element, Ru, Ir, Re,
It comprises an oxide thin film made of an oxide of a material selected from Os and Rh, and a Pt thin film made of Pt on the oxide thin film.

Further, according to the present invention, in the above-mentioned ferroelectric memory element, Ru, Ir, Re, Os,
And Rh, the thickness of the oxide thin film made of an oxide of a material selected from the group consisting of 500 to 3000 °.

In a method of manufacturing a ferroelectric memory element in which a lower electrode, a ferroelectric thin film, and an upper electrode are sequentially formed on a semiconductor substrate, the lower electrode forming step may include Ti, Z
forming a nitride thin film made of a material selected from nitrides of r, Hf, V, Nb, Ta, and W; and forming a Pt thin film made of Pt on the nitride thin film; Ru, Ir, Re, Os, and R
h) forming an oxide thin film made of an oxide of a material selected from h.

Further, according to the present invention, in the above-mentioned method for manufacturing a ferroelectric memory element, the step of forming an upper electrode may include the step of
forming an oxide thin film made of an oxide of a material selected from u, Ir, Re, Os, and Rh; and forming a Pt thin film made of Pt on the oxide thin film. And

Further, according to the present invention, in the above-mentioned method for manufacturing a ferroelectric memory element, the Ru, Ir, R
In the step of forming an oxide thin film made of an oxide of a material selected from e, Os, and Rh, the oxide thin film is formed at a thickness of 500 to 3000 °.

According to the present invention, since the electrodes of the ferroelectric memory element are formed as described above, when the ferroelectric thin film is made of an oxide material, oxygen is transferred from the ferroelectric thin film to the lower thin film. Since diffusion is suppressed, peeling of the Pt thin film can be prevented, the film fatigue characteristics of the ferroelectric thin film can be improved, and the life performance of the ferroelectric memory element can be significantly improved.

[0017]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
FIG. 3 is a cross-sectional view of a principal part showing a schematic structure of the ferroelectric memory element according to the embodiment; As shown in FIG. 1, this ferroelectric memory element has a polysilicon plug 2, a conductive thin film 4, a TiN thin film 4, a Pt thin film 5, a conductive oxide A RuO ₂ thin film 6 as a thin film, a PZT thin film 7 as a ferroelectric thin film, and a Pt thin film 8 as an upper electrode are sequentially formed. In this embodiment, the Ti thin film 3 is formed between the polysilicon 2 and the TiN thin film 4 in order to improve the adhesion.

Next, a method of manufacturing the ferroelectric memory device according to the first embodiment shown in FIG. 1 after the step of forming the Ti thin film 3 will be described in detail. First, the semiconductor substrate 10 (Si
A CMOS transistor portion 1 is formed on the surface of a substrate, and a polysilicon plug 2 is formed thereon. Then, a Ti thin film 3 is vapor-deposited on the polysilicon plug 2 in an argon atmosphere at 200 ° by a sputtering method. Thereafter, a TiN thin film 4 is formed on the Ti thin film 3 in a mixed gas atmosphere of argon and nitrogen by the same sputtering method.
Deposit 000Å. Then, annealing is performed in a nitrogen atmosphere at 600 ° C. in order to promote the nitridation of the TiN thin film 4.

Thereafter, a Pt thin film 5 is formed on the TiN thin film 4 by sputtering at 1000 ° C. in an argon atmosphere.
A RuO ₂ thin film 6 is formed thereon to a thickness of 1000 ° by a reactive sputtering method in a mixed gas atmosphere of oxygen and argon. Here, if the thickness of the RuO ₂ thin film is less than 500 °, the effect of suppressing diffusion of oxygen is remarkably reduced, and the TiN thin film 4 and the Pt thin film 5 are separated from each other.
If the thickness is larger than 00 °, the surface roughness of the RuO ₂ thin film increases, causing deterioration of the film characteristics of the ferroelectric thin film formed thereon, and also lowers the etching accuracy.
0 ° to 3000 ° is preferred.

It is known that the RuO ₂ thin film 6 has excellent conductivity (specific resistance: 30 to 100 μΩcm), and has thermal stability and impurity diffusion barrier properties. In particular, since it is excellent in oxygen barrier properties, the lower layer Tt of the Pt thin film 5
The diffusion of oxygen from the PZT thin film 7 into the iN thin film 4 can be suppressed.

Next, the RuO ₂ formed as described above is used.
On the thin film 6, a PZT thin film 7, which is a ferroelectric thin film, is formed. The PZT thin film 7 was formed by a sol-gel method, and the annealing and baking after the film formation were performed in a mixed gas atmosphere of oxygen and nitrogen at 660 ° C. for 30 seconds using a rapid thermal annealing (RTA) apparatus. The PZT thin film 7 thus formed has a thickness of 2000
Met. Then, finally, a Pt thin film 8 is deposited on the PZT thin film 7 by sputtering in an argon atmosphere at 1000 ° to complete the fabrication of the ferroelectric memory device of the present embodiment.

Here, FIG. 2 shows the result of observing the X-ray diffraction pattern after forming the PZT thin film 7 on the RuO ₂ / Pt / TiN substrate and performing annealing and firing. FIG. 2 shows that PZT (100), PZT (110), and PZT (1) which are diffraction peaks due to the perovskite phase of PZT.
11), only PZT (200) was observed and this PZT
It can be seen that in the thin film 7, only the perovskite phase was formed, and the pyrochlore phase and other impurity phases were not formed.

Next, after a PZT thin film 7 is formed on the RuO ₂ / Pt / TiN substrate and annealed and fired,
FIG. 3A shows the result of observation with a surface scanning electron microscope (SEM). As a comparative example, a PZT thin film 7 was formed on a Pt / TiN substrate in the same manner as described above except that the RuO ₂ thin film 6 was not used, and annealing and baking were performed. FIG. 3B shows the result of observation with a microscope (SEM). As shown in FIG. 3, the PZT thin film 7 formed on the RuO ₂ thin film 6 of this embodiment has a crystal grain size of about 0.2 μm and a very uniform film having good surface properties. _{(2) In the} case of the comparative example having no thin film, peeling of the film and hillocks (projections) are observed.
From these observation results, the effect of the present invention is remarkably understood. That is, in the embodiment, the RuO ₂ thin film 6
Thus, it can be seen that diffusion of oxygen from the PZT thin film 7 is suppressed and oxygen does not reach the lower TiN thin film 4.

Next, FIG. 4 shows the results of measuring the hysteresis characteristics of the PZT thin film of the ferroelectric memory device of the present embodiment manufactured as described above. Note that the electrode area of the upper electrode was 7.8 × 10 ⁻⁵ cm ² . As shown in FIG. 4, according to the present embodiment, the remanent polarization value Pr = 29
μC / cm ² and coercive electric field Ec = 49 kV / cm were obtained. FIG. 5 shows a frequency 100
kHz, 5% duty pulse voltage 5V
The result of having measured the fatigue characteristic accompanying polarization reversal by application is shown. From Figure 5, the amount of accumulated charges ΔQ, even after ¹⁰⁸ cycles, the change compared to the initial value 0.04μC / cm
It turns out that it is ² and very small.

Next, as a second embodiment, the first
PZT thin film 7 of the embodiment and Pt thin film 8 as an upper electrode
Between, to form RuO ₂ film 9, described ferroelectric memory device that constitutes the upper electrode from RuO ₂ film 9 and the Pt thin film 8.

In the first embodiment, asymmetry is recognized in the hysteresis characteristic. One of the causes is considered to be that the electrode materials in contact with the upper and lower portions of the PZT thin film are different. Therefore, in order to improve this asymmetry, the upper and lower electrodes of the PZT thin film 7 are made the same in the second embodiment.

The fabrication of the ferroelectric memory element of the second embodiment is the same as that of the first embodiment up to the annealing and firing step of forming the PZT thin film 7, and thereafter, the RuO ₂ film is formed on the PZT thin film 7. The thin film 9 is formed by a reactive sputtering method in a mixed gas atmosphere of oxygen and argon to a thickness of 1000.
The Pt thin film 8 is formed thereon by sputtering in an argon gas atmosphere to a thickness of 1000. Here, the RuO ₂ thin film 8 constituting the upper electrode
Also, as described in the first embodiment, the film thickness is preferably from 500 to 3000 °.

With respect to the ferroelectric memory element of the second embodiment, the PZT thin film 7 is formed in the same manner as in the first embodiment.
FIG. 7 shows the results of measuring the hysteresis characteristics of the. FIG.
As shown in FIG. 5, in the second embodiment, the remanent polarization value Pr = 39 μC / cm ² and the coercive electric field Ec = 35 kV / cm.
was gotten. Comparing these results with those of the first embodiment, the symmetry of the hysteresis soup is improved,
Pr also increased by 10 μC / cm ² . Also, the second
The embodiments also, the similar to that of the first embodiment, the frequency 100kHz, results of fatigue characteristics due to polarization inversion by the duty ratio of 5% of the stress pulse voltage 5V applied was measured, after ¹⁰⁸ cycles accumulated charge The change in the amount ΔQ was 0.02 μC / cm ^{2 as} compared with the initial value, and a better value than that of the first embodiment was obtained.

In the first and second embodiments,
Although a RuO ₂ thin film was used as the oxide conductive thin film constituting the electrode, indium oxide (Ir) was used instead.
O ₂ ), osmium oxide (OsO ₂ ), rhenium oxide (R
eO ₂ ) or rhodium oxide (RhO ₂ ) provided the same effects as those of the first and second embodiments. The same effects as those of the first and second embodiments were obtained by using a reactive vapor deposition method or a CVD method as a method for producing a RuO ₂ thin film. In the first and second embodiments, a titanium nitride (TiN) thin film is used as a thin film for electrically connecting the lower CMOS transistor portion and the upper capacitor portion of the ferroelectric memory element. Instead, nitrides of Zr, Hf, V, Nb, Ta, and W have the same effect as the first and second embodiments in that problems such as peeling are suppressed.

[0030]

As described above, according to the present invention, by forming a conductive thin film such as a RuO ₂ thin film as a part of the electrode, peeling between the Pt thin film and the TiN thin film is suppressed. be able to. Further, according to the present invention, since the fatigue characteristics of the ferroelectric thin film can be improved, it is possible to realize a ferroelectric memory element having extremely high life performance.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a principal part showing a schematic structure of a ferroelectric memory element according to a first embodiment.

FIG. 2 is a diagram showing an observation result of an X-ray diffraction pattern according to the first embodiment.

FIG. 3 is an electron micrograph showing the results of SEM observation of the PZT thin film surfaces of the first embodiment and a comparative example.

FIG. 4 is a diagram showing a measurement result of a hysteresis characteristic of the PZT thin film of the first embodiment.

FIG. 5 is a diagram illustrating measurement results of fatigue characteristics of the accumulated charge amount ΔQ according to the first embodiment.

FIG. 6 is a fragmentary cross-sectional view showing a schematic structure of a ferroelectric memory element according to a second embodiment.

FIG. 7 is a view showing a measurement result of a hysteresis characteristic of the PZT thin film of the second embodiment.

FIG. 8 is a cross-sectional view of a main part showing a schematic structure of a conventional ferroelectric memory element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 CMOS transistor part 2 Polysilicon plug 3 Ti thin film 4 TiN thin film 5 Pt thin film 6 RuO ₂ thin film 7 PZT thin film 8 Pt thin film 9 RuO ₂ thin film 10 Semiconductor substrate

Claims (6)

[Claims] 1. A ferroelectric memory device in which a lower electrode, a ferroelectric thin film, and an upper electrode are sequentially laminated on a semiconductor substrate, wherein Ti, Zr, Hf, V, Nb, and Ta are used as the lower electrode. ,
, A nitride thin film made of a material selected from the nitrides of W, a Pt thin film made of Pt on the nitride thin film, and Ru, Ir, Re, Os, and Rh on the Pt thin film. A ferroelectric memory element comprising: an oxide thin film made of an oxide of a selected material. 2. The method according to claim 1, wherein said upper electrode is Ru, Ir, R
e, an oxide thin film composed of an oxide of a material selected from Os, and Rh, and a Pt composed of Pt on the oxide thin film.
2. The ferroelectric memory device according to claim 1, further comprising a thin film. 3. The lower electrode of Ru, Ir, Re, O
3. The ferroelectric memory element according to claim 1, wherein the thickness of the oxide thin film made of an oxide of a material selected from s and Rh is 500 to 3000 [deg.]. 4. A method of manufacturing a ferroelectric memory device, wherein a lower electrode, a ferroelectric thin film, and an upper electrode are sequentially formed on a semiconductor substrate, wherein the lower electrode forming step includes Ti, Zr, Hf, V, N
forming a nitride thin film made of a material selected from nitrides of b, Ta, and W; and forming a P thin film on the nitride thin film.
forming a Pt thin film composed of an oxide selected from Ru, Ir, Re, Os, and Rh on the Pt thin film. A method for manufacturing a ferroelectric memory element, comprising: 5. The method according to claim 1, wherein the upper electrode forming step includes Ru, Ir,
A step of forming an oxide thin film made of an oxide of a material selected from Re, Os, and Rh; and a step of forming a Pt thin film made of Pt on the oxide thin film. A method for manufacturing a ferroelectric memory device according to claim 4. 6. The lower electrode of Ru, Ir, Re, O
3. The method according to claim 1, wherein in the step of forming the oxide thin film made of an oxide of a material selected from s and Rh, the oxide thin film is formed at a thickness of 500 to 3000 [deg.]. A method for manufacturing the ferroelectric memory device according to claim 1.