JPH1126703A - Ferroelectric memory - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control degradation particularly of remanence polarization QSW by blocking an oxide system ferroelectric film from being reduced when exposed to a reducing atmosphere produced in a device forming process and the like. SOLUTION: This memory comprises a lower electrode, a ferroelectric film 3 and an upper electrode laminated sequentially on a substrate. The ferroelectric film 3 is formed of a perovskite oxide or the like exhibiting ferroelectricity, such as, for example PZT. A grain boundary 4a and a triple point 4b of a crystal grain 3a constituting such a ferroelectric film 3 have an insulating phase 5 containing at least one kind of material selected among Si, B and P, which is formed of an amorphous phase using a glass forming component such as, for example a borosilicate glass.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ferroelectric memory.

[0002]

2. Description of the Related Art A DRAΜ (dynamic random access memory) IC memory which is widely used at present is capable of electrically writing and erasing data, but has a serious disadvantage that all stored data is lost when the power is turned off. Have. For this reason, while maintaining the same high speed, large capacity, and low power consumption as the DR @ M, a nonvolatile ferroelectric memory (FRAM) in which stored data is not erased even when the power is turned off.
(Ferroelectric random access memory)).

An FRAM has a function of retaining data by replacing a capacitor portion of a DRAM with a ferroelectric material. A ferroelectric is a crystal that has spontaneous electric polarization, and the direction of the spontaneous polarization is reversed by applying an electric field.
By switching the applied voltage between positive and negative, + or-
Can be induced on the crystal surface. Even when the voltage is turned off, the positive or negative charge is retained, and thus the nonvolatile memory can be used. The memory is configured by associating this state with 0 and 1.

Such a ferroelectric film for FRAM has a large charge (residual polarization) when a voltage is cut off. For example, lead zircon titanate (PZT (Pb (Z
r, Ti) Ο ₃₎₎ perovskite oxide exhibiting ferroelectric properties such as are used. Methods for forming a ferroelectric film such as PZT include sputtering, OCVD, sol-gel, and OD (metal organic deposition).
There are various methods such as a laser ablation method, an ion beam sputtering method, and the like, but in any case, it is necessary to perform a heat treatment for crystallization.

[0005] For example, in the sputtering method, two types of heat can be applied. One is to maintain the substrate temperature at or above the crystallization temperature during film formation, and to form a perovskite structure in the film formation stage (as depo state). Is a method of performing a heat treatment for In the method of forming a film by heating the substrate, the perovskite structure crystal grows from the substrate surface, so that epitaxial growth is easy. However, since the PZT-based ferroelectric material is very sensitive to temperature, even if the temperature deviates even slightly, the crystal grows. May have a change in the anisotropy or the crystal structure itself. Further, since it is very difficult to stabilize the time required for the substrate temperature to stabilize after loading the substrate and to stabilize the heat uniformity within the substrate surface, generally, a crystallization heat treatment is performed after film formation at a low temperature. The method is adopted in terms of stability and reproducibility.

A general process for producing FRA # including the above-mentioned crystallization heat treatment is described below. First, Si /
A lower electrode is formed on a SiO ₂ substrate, and a ferroelectric film made of PZT or the like is formed thereon. Next, the PZT film is crystallized by rapid thermal processing (RTA). When film formation and crystallization are performed simultaneously by substrate heating sputtering, a rapid heat treatment process may not be necessary.

Next, an upper electrode is formed into a film,
The ferroelectric film and the lower electrode are sequentially etched. After forming a passivation film on it and etching the upper electrode and contact etching of the upper electrode,
An interconnection (Ti / TiN) is deposited and etched on the upper electrode. Further, a passivation film is formed, contact etching, Ti / TiN film formation, and Al wiring film formation are performed, and after these are etched, a passivation film is formed to obtain a desired FRA.
M is obtained.

Here, in the above-described FRAM forming process, the passivation film is generally formed by TEO.
S (tetraethoxysilane) gas is used. That is, TEOS gas flows at a temperature of about 673K,
Forming a Siomikuron ₂ film as a passivation film. At this time, since a large amount of H ₂ gas is generated along with the thermal decomposition of TEOS, a strong reducing atmosphere is obtained. Due to the strong reducing atmosphere that accompanies the formation of this passivation film, PZ
There is a problem that an oxide ferroelectric film such as T is reduced and its electrical characteristics deteriorate.

[0009]

The ferroelectrics used in the FRAM include (1) large remanent polarization Q _sw , (2) small coercive electric field E _c , (3) phase transition temperature and Curie point T Characteristics such as high _c , (4) low crystallization temperature, (5) low leakage current (leak current) LC, and (6) excellent fatigue characteristics are required. Among these required properties, especially residual polarization Q _sw is deteriorated by the reducing atmosphere due to the formation or the like of the passivation film, i.e. characteristic deterioration due to the reduction of the oxide-based ferroelectric film such as PZT is intense, for example, residual polarization Q _sw is 60%
There is a problem that the above is reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and particularly, by preventing reduction of a ferroelectric film when exposed to a reducing atmosphere generated in a device forming step, the remanent polarization Q _An object of the present invention is to provide a ferroelectric memory capable of suppressing deterioration of _sw .

[0011]

According to a first aspect of the present invention, there is provided a ferroelectric memory including a lower electrode, a ferroelectric film, and an upper electrode which are sequentially stacked on a substrate. In the memory, the ferroelectric film is characterized in that an insulating phase containing at least one selected from Si, B and P exists at a grain boundary and a triple point of a crystal grain constituting the ferroelectric film. The ferroelectric memory according to the present invention is characterized in that the insulating phase is an amorphous phase.

In the ferroelectric memory of the present invention, an insulating phase containing at least one selected from Si, B and P is present as a second phase at the grain boundaries and triple points of the ferroelectric film. Since the insulating phase existing at the grain boundaries and at the triple points functions as a barrier layer, the ferroelectric particles can be prevented from being reduced even when exposed to a reducing atmosphere in the device forming step. Therefore, it is possible to suppress the deterioration of the electric characteristics due to the reduction of the ferroelectric film, particularly the deterioration of the remanent polarization _Qsw . When the insulating phase as the second phase is an amorphous phase, the periphery of the ferroelectric particles can be uniformly and reliably covered with the insulating phase, so that a more remarkable effect can be obtained.

[0013]

Embodiments of the present invention will be described below.

FIG. 1 shows a ferroelectric memory (FRA) of the present invention.
FIG. 3M is a cross-sectional view illustrating a main part configuration of one embodiment, that is, a charge accumulation unit of the FRAM. In FIG. 1, reference numeral 1 denotes a semiconductor substrate such as a Si substrate on which a thermally oxidized SiO ₂ film is formed, or a substrate formed of an MgO single crystal substrate, a SrTiO ₃ single crystal substrate, or the like.

A lower electrode 2 is formed on a substrate 1. Although various conductive materials can be used for the lower electrode 2, noble metal materials such as Pt and Ru which are hardly oxidized,
It is preferable to use a conductive oxide such as an oxide of Ru and a conductive perovskite oxide such as SrRuO ₃ .
In addition, between the substrate 1 and the lower electrode 2, TiN or (Ti,
A compound layer such as Al) N may be interposed.

A ferroelectric film 3 is formed on the lower electrode 2. For the ferroelectric film 3, for example, an oxide having ferroelectricity, for example, a perovskite oxide can be used. Examples of the perovskite-type oxide exhibiting ferroelectricity include Pb (Zr, Ti) O ₃ (PZT) and (P
b, La) (Zr, Ti ) O 3 (PLZT) or the like, or SrBi ₂ Ta SrBi-Ta-O-based oxide such as _{2 O 9, Bi-Ti-} O system such as Bi ₄ Ti ₃ O ₁₂ Oxides,
Bi-Sr-Ti-O-based oxides are exemplified. In addition, Ba-rich Ba _1-x Sr _x TiO ₃ or BaTiO
_Using a perovskite-type oxide such as ₃ and utilizing the strain-induced ferroelectricity caused by the lattice mismatch with the lower electrode 2,
It is also possible to configure the ferroelectric film 3. The thickness of the ferroelectric film 3 is not particularly limited.
Like M, it can be about 50 to 300 nm.

The ferroelectric film 3 as described above, for example, as schematically shown in FIG. 2, has Si, a grain boundary 4a and a triple junction 4b of crystal grains (ferroelectric particles) 3a constituting the film.
An insulating phase 5 containing at least one selected from B and P is present as a second phase. In other words, the periphery of the ferroelectric particles 3a is covered with the insulating phase 5 existing at the grain boundaries 4a and the triple points 4b.

As described above, by covering the periphery of the crystal grains (ferroelectric particles) 3a constituting the ferroelectric film 3 with the insulating phase 5, for example, a strong reducing atmosphere generated when a passivation film 7 described later is formed. Thus, the ferroelectric particles 3a made of a ferroelectric perovskite oxide or the like can be protected. That is, since the insulating phase 5 existing around the ferroelectric particles 3a (the grain boundaries 4a and the triple junctions 4b) functions as a barrier layer against a reducing atmosphere, an oxide ferroelectric such as a ferroelectric perovskite oxide is used. The reduction of the particles 3a can be suppressed. As a result, it is possible to effectively suppress the deterioration of the electric characteristics of the ferroelectric film 3, particularly the reduction of the remanent polarization _Qsw .

The insulating phase 5 as the second phase may be any insulating material containing at least one selected from Si, B and P, for example, an oxide. Since each element of Si, B and P hardly penetrates or substitutes into the crystal grains 3a of the ferroelectric perovskite oxide used as the ferroelectric film 3, the use of the oxide containing these elements makes the ferroelectric film 3 Can be satisfactorily covered with the insulating phase 5 without impairing the essential characteristics of the ferroelectric particles 3a.

As a material for forming the insulating phase 5, it is particularly preferable to use a glass-forming component having a low melting point. That is, the insulating phase 5 is preferably made of an amorphous phase containing at least one selected from Si, B and P. Si, B
And glass-forming components containing at least one selected from P include silicate glass (SiO ₂ glass), borate glass (B ₂ O ₃ glass), and phosphoric glass (P ₂ O ₅ glass). , Borosilicate glass (B ₂ O ₃ −
SiO ₂ -based glass) and the like, which may contain various fluxes.

At least one selected from Si, B and P
A glass-forming component such as an oxide containing one kind is melted by heat treatment for crystallization of the ferroelectric film 3 (heating during film formation, rapid heat treatment after film formation, etc.), and becomes a ferroelectric material as a liquid phase. Since the particles are uniformly wet around the particles 3a, the insulating phase 5 can be uniformly formed at the grain boundaries 4a and the triple points 4b. That is, the periphery of the crystal grains 3a constituting the ferroelectric film 3 can be more uniformly covered with the insulating phase 5 made of an amorphous phase. Thereby, the ferroelectric perovskite oxide used as the ferroelectric film 3 can be more effectively protected from the reducing atmosphere.

The average thickness of the insulating phase 5 is preferably 50 nm or less. If the average thickness of the insulating phase 5 exceeds 50 nm, the share of the electric field on the ferroelectric particles 3a becomes small, so that the ferroelectric material is difficult to sufficiently polarize and the ferroelectric film 3
_May be reduced. However, if the average thickness of the insulating phase 5 is too small, for example, the coating state of the ferroelectric particles 3a may be partially reduced. Therefore, the average thickness of the insulating phase 5 is preferably 1 nm or more.

When the ferroelectric film 3 in which the insulating phase 5 is present at the grain boundaries 4a and the triple points 4b is used, for example, when the ferroelectric film 3 is formed by a sputtering method, the ferroelectric film 3 is used as a target made of a ferroelectric perovskite oxide. An oxide containing at least one selected from the group consisting of Si, B and P serving as the insulating phase 5, in particular, a glass forming component is added, and the ferroelectric film 3 is formed by using such a sputter target. Can be

Here, when the ferroelectric film 3 is formed by sputtering at a low temperature (without substrate heating), the film at the beginning of the film formation is an amorphous phase, which is subjected to rapid thermal annealing (RTA). The ferroelectric film 3 is crystallized. At this time, the insulating phase 5 composed of a glass forming component and the like is melted and uniformly wets around the ferroelectric particles 3a as a liquid phase. Thereafter, by cooling, the insulating phase 5 composed of an amorphous phase can be uniformly present at the grain boundaries 4a and the triple points 4b of the ferroelectric particles 3. When the substrate is heated to form a sputter film, the grain boundaries 4a and the triple points 4b of the ferroelectric particles 3 are similarly formed.
The insulating phase 5 composed of an amorphous phase can be uniformly present.

The amount of the material for forming the insulating phase 5, for example, the glass forming component, to be added to the target made of a ferroelectric perovskite oxide is preferably, for example, about 50 to 3000 ppm. If the amount of the glass-forming component or the like is 50 ppm or less, it becomes difficult to uniformly wet the periphery of the ferroelectric particles 3a with a liquid phase, and the function of the insulating phase 5 as a barrier layer may be reduced. On the other hand, when the addition amount of the glass forming component and the like exceeds 3000 ppm, the thickness of the insulating phase 5 becomes too thick, and the sharing of the electric field with the ferroelectric particles 3a becomes small as described above. Becomes difficult to polarize,
The remanent polarization Q _sw may be reduced.

An upper electrode 6 is formed on the ferroelectric film 3 described above.
And these constitute a charge storage section of the FRAM. The upper electrode 6 may be made of Pt, Ru, Ru oxide, conductive perovskite oxide, or the like, similar to the lower electrode 2, and the thickness thereof is not particularly limited. Preferably, the thickness is about 100 nm.

The specific device structure of the charge storage portion of the FRAM is not particularly limited, and various structures such as a planar type, a stack type, and an internal trench type can be applied. The method of forming the layers 2, 3, and 6 constituting the charge storage portion of the FRAM described above is not limited to the sputtering method, and various film forming methods such as a CVD method, a MOD method, and a laser ablation method may be used. Can be applied.

In the figure, reference numeral 7 denotes a passivation film, for example, a SiΟ ₂ film formed using a TEOS (tetraethoxysilane) gas is applied. The present invention, for example,
The oxide ferroelectric film 3 such as a ferroelectric perovskite oxide is protected from a reducing atmosphere generated when the passivation film 7 is formed using the EOS gas, that is, the reduction of the oxide ferroelectric film 3 is prevented. Thereby, excellent electrical characteristics, particularly excellent remanent polarization Q _sw can be obtained.

However, the reducing atmosphere that adversely affects the remanent polarization Q _{sw of} the oxide-based ferroelectric film 3 is not limited to that generated by the decomposition of the TEOS gas. For example, a reducing atmosphere is similarly generated when forming the W wiring on the transistor side. The insulating phase 5 according to the present invention is effective against the reducing atmosphere generated by such various element forming steps.

The present invention is effective not only for suppressing the deterioration of the initial remanent polarization Q _sw but also for suppressing the long term remanent polarization Q _sw called fatigue characteristics. That is, according to the ferroelectric memory of the present invention, a good remanent polarization Q _sw can be obtained over a long period of time.

[0031]

Next, specific examples of the present invention and evaluation results thereof will be described.

Example 1 First, a Ru film having a thickness of 180 nm was formed as a lower electrode by a sputtering method on a Si substrate having a thermally oxidized SiO ₂ film having a thickness of 100 nm formed on its surface. Sputtering 50 sccm Ar gas
Introduced and performed at an output of 300W. Next, Pb (Zr _0.52 Ti
_0.48 ) A 250 nm thick ΡZT film was formed by sputtering using a PZT target in which 100 ppm of borosilicate glass was added to O ₃ (PZT). After the film formation, rapid heat treatment was performed at 873K for 30 seconds. Thereafter, a Ru film having a thickness of 180 nm was formed by a sputtering method, and this was lifted off to a 100 nm square to form an upper electrode.

When the microstructure of the PZT film as a ferroelectric film was observed with a scanning transmission electron microscope (STEM), the average grain thickness containing B and Si at the grain boundaries of the PZT crystal grains was about 1 nm. And the triple point has about 10 sides
There was a ~ 30 nm triangular heterophase. When this hetero phase was analyzed by electron beam diffraction, a halo pattern was obtained, thereby confirming that it was an amorphous phase.

Next, a passivation film was formed on the upper electrode by using TEOS gas, and an element forming step was further performed to manufacture an FRAM element. The obtained FRAM element was placed in an electric furnace maintained at 673K at an oxygen partial pressure of 10 ^-8 Pa.
After holding for 1 hour, the sample was taken out into the air and a voltage of 5 V was applied to measure a hysteresis curve. As a result, the remanent polarization Q _sw was 47 μC / cm ² .

On the other hand, as a comparative example with the present invention, Pb (Z
r _0.52 Ti _0.48 ) In the same manner as in Example 1 except that a target having no glass component added to O ₃ was used,
An FRA element was manufactured. In the PZT film of Comparative Example 1, no heterophase was present at the grain boundaries or triple points. The FRAM element of Comparative Example 1 has the same oxygen partial pressure as in Example 1.
After holding for 1 hour in an electric furnace maintained at 673K at 10 ^-8 Pa, the sample was taken out to the atmosphere and a hysteresis curve was measured by applying a voltage of 5V. As a result, the remanent polarization Q _sw
Was 11 μC / cm ² .

Next, the fatigue characteristics of Example 1 and Comparative Example 1 were compared. That is, each FRA of Example 1 and Comparative Example 1
A rectangular voltage of ± 5 V and 5 msec was applied 10 ¹⁰ times to the M element, and then the remanent polarization Q _sw was measured. As a result, Example 1 whereas decreasing rate of the Q _sw was -5%, Q _sw in Comparative Example 1 was greatly reduced and -60%.

Example 2 On a Si substrate having a thermally oxidized SiO ₂ film having a thickness of 100 nm formed on its surface, a Ti film having a thickness of 20 nm and a thickness of 180 nm were formed as lower electrodes.
Was formed by a sputtering method. Ar sputtering
The gas was introduced at 50 sccm and the output was 300 W. Next, SrB
Using a target obtained by adding 100 ppm of borosilicate glass to i ₂ Ta ₂ O ₉ , a 250 nm thick SrBi ₂ Ta ₂ O ₉
The film was formed by a sputtering method. Sputtering Ar gas
50 sccm, O ₂ gas introduced at 10 sccm, output 300 W, substrate heating
I went at 873K. Thereafter, a Pt film having a thickness of 180 nm was formed by a sputtering method, and this was lifted off to a 100 nm square to form an upper electrode.

SrBi ₂ T as the above ferroelectric film
The microstructure of the a ₂ O ₉ film was examined using a scanning transmission electron microscope (STE).
Observed in (M), there is a hetero phase having an average thickness of about 1 nm containing B and Si at the grain boundary, and about three
There was a triangular heterophase of 10-30 nm. When this hetero phase was analyzed by electron beam diffraction, a halo pattern was obtained, thereby confirming that it was an amorphous phase.

Next, a passivation film was formed on the upper electrode by using TEOS gas, and an element forming step was further performed to manufacture an FRAM element. The obtained FRAM element was placed in an electric furnace maintained at 673K with an oxygen partial pressure of 10 ^-8 Pa.
After holding for a time, the sample was taken out into the air and a voltage of 5 V was applied to measure a hysteresis curve. as a result,
The remanent polarization Q _sw was 18 μC / cm ² .

On the other hand, as a comparative example with the present invention, SrBi
FRAΜ was prepared in the same manner as in Example 2 except that a target in which no glass component was added to ₂ Ta ₂ O ₉ was used.
An element was manufactured. SrBi ₂ Ta ₂ O _{9 of} Comparative Example 2
No heterophase was present in the grain boundary or triple point in the film.
The FRAM element of Comparative Example 2 was held in an electric furnace maintained at 673 K at an oxygen partial pressure of 10 ⁻⁸ Pa for 1 hour, taken out to the atmosphere, and a voltage of 5 V was applied in the same manner as in Example 2. Thus, a hysteresis curve was measured. As a result, the remanent polarization Q _sw was ² μC / cm ² .

Next, the fatigue characteristics of Example 2 and Comparative Example 2 were compared. That is, each FRA of Example 2 and Comparative Example 2
A rectangular voltage of ± 3 V and 5 msec was applied 10 ¹² times to the M element, and then the remanent polarization Q _sw was measured. As a result, in Example 2, the rate of decrease in Q _sw was -3%, whereas in Comparative Example 2, Q _sw was significantly reduced to -51%.

Example 3 A Pt film having a thickness of 180 nm was formed as a lower electrode by a sputtering method on a Si substrate having a thermally oxidized SiO ₂ film having a thickness of 100 nm formed on its surface. The sputtering was performed at an output of 300 W by introducing 50 sccm of Ar gas. Next, using a target obtained by adding 100 ppm of borosilicate glass to Bi ₄ Ti ₃ O ₁₂ ,
A 250 nm Bi ₄ Ti ₃ O ₁₂ film was formed by sputtering. The sputtering was performed by introducing Ar gas at 50 sccm and O ₂ gas at 10 sccm, outputting 300 W and heating the substrate at 873 K. Then the thickness
A 180 nm Pt film was formed by sputtering, and this was lifted off to a 100 nm square to form an upper electrode.

Bi ₄ Ti ₃ as the above ferroelectric film
Observation of the microstructure of the O ₁₂ film by a scanning transmission electron microscope (STEM) revealed that a heterogeneous phase containing B and Si having an average thickness of about 1 nm was present at the grain boundaries, and about 10 nm on one side at the triple point. ~ 30n
There was a m-triangular heteromorph. When this hetero phase was analyzed by electron beam diffraction, a halo pattern was obtained, thereby confirming that it was an amorphous phase.

Next, a passivation film was formed on the upper electrode by using TEOS gas, and an element forming step was further performed to manufacture an FRAM element. The obtained FRAM device was placed in an electric furnace maintained at 673 K at an oxygen partial pressure of 10 ⁻⁸ Pa for 30 minutes.
After holding for 1 minute, the sample was taken out into the air and a voltage of 5 V was applied to measure a hysteresis curve. as a result,
The remanent polarization Q _sw was 25 μC / cm ² .

On the other hand, as a comparative example with the present invention, Bi ₄ T
An FRAΜ device was produced in the same manner as in Example 3 except that a target in which no glass component was added to i ₃ O ₁₂ was used. In the Bi ₄ Ti ₃ O ₁₂ film of Comparative Example 3, no heterophase was present at the grain boundaries or triple points. The FRAM device of Comparative Example 3 has the same oxygen partial pressure as in Example 3.
After holding in an electric furnace maintained at 673K at 10 ^-8 Pa for 30 minutes, the sample was taken out to the atmosphere and a hysteresis curve was measured by applying a voltage of 5V. As a result, the remanent polarization Q _sw
Was 5 μC / cm ² .

Next, the fatigue characteristics of Example 3 and Comparative Example 3 were compared. That is, each FRA of Example 3 and Comparative Example 3
A rectangular voltage of ± 3 V and 5 msec was applied 10 ¹² times to the M element, and then the remanent polarization Q _sw was measured. As a result, in Example 3, the rate of decrease in Q _sw was -5%, whereas in Comparative Example 3, Q _sw was significantly reduced to -58%.

[0047]

As described above, according to the ferroelectric memory of the present invention, the ferroelectric particles can be prevented from being reduced by the reducing atmosphere generated in the element forming step and the like.
Deterioration of the remanent polarization Q _sw can be suppressed. That is, it is possible to provide a ferroelectric memory having excellent remanent polarization _Qsw , which is an important characteristic for a ferroelectric memory, with good reproducibility.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a main configuration of a ferroelectric memory according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a microstructure of a ferroelectric film in the ferroelectric memory shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Lower electrode 3 ... Ferroelectric film 3a ... Ferroelectric particle 4a ... Grain boundary 4b ... Triple junction 5 ... Insulating phase (amorphous phase) 6 ... …… Top electrode 7 ……… Passivation film

Claims (2)

[Claims] 1. A ferroelectric memory comprising a lower electrode, a ferroelectric film, and an upper electrode sequentially laminated on a substrate, wherein the ferroelectric film has a grain boundary and a triple point of crystal grains constituting the ferroelectric film. At least one selected from Si, B and P
A ferroelectric memory characterized by the presence of a seed-containing insulating phase. 2. The ferroelectric memory according to claim 1, wherein said insulating phase is an amorphous phase.