JP2000031411A - Manufacture of ferroelectric thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a dense and uniform thin film by using laser annealing for laser processing by depositing ferroelectric material on a base to form a pre- ferroelectric thin film and then annealing to crystallize the pre-ferroelectric thin film into the thin film. SOLUTION: Ferroelectric material is deposited on a base 10 to form a pre- ferroelectric thin film 12a. By annealing, the pre-ferroelectric thin film 12a is crystallized to be transformed into a ferroelectric thin film 12b. The annealing is laser annealing, the beam size being 0.47 mm×150 nm, and 95% overlapping. Laser beam is emitted on the pre-ferroelectric thin film 12a by means of an excimer laser line annealing system to rapidly heat the thin film 12a into the ferroelectric thin film 12b. After forming an upper electrode 14 on the ferroelectric thin film 12b, second annealing is conducted. Then, a structure constituted of the base 10, the ferroelectric thin film 12b, and the upper electrode 14 is heated at 800 deg.C for 30 min. in an oxygen atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a ferroelectric thin film used for a ferroelectric memory.

[0002]

2. Description of the Related Art In recent years, the densification of semiconductor memories has been promoted, and recently, Reference 1, “Ceramics, Vol.
0 (1995), no. 6, pp. 499-507, attention has been paid to those using a ferroelectric thin film. As a technical problem when using this ferroelectric thin film, there is a problem of deterioration of polarization history characteristics due to repeated application of a voltage to the ferroelectric thin film, so-called film fatigue. In order to improve this fatigue property, improvement of thin film making method,
Selection of materials, improvement of electrode materials, and the like have been proposed.

[0003] Bi-layered compounds have attracted attention as a new material for a thin film having fatigue resistance, particularly SrBi ₂ Ta.
Active research is being conducted on ₂ O ₉ -based substances. As a method for producing this SrBi ₂ Ta ₂ O ₉ (SBT) thin film, for example, reference 2 “Jpn.J.Appl.Phys.vol.3”
4, No. 9B, (1995) pp. 5096-5099 "discloses a method of forming by using a sol-gel method.

Further, in order to exhibit the advantages of the non-volatile memory, it is required to lower the driving voltage (the lowering of the voltage is also advantageous from the viewpoint of driving). In order to polarize the ferroelectric at a low voltage, it is necessary to increase the electric field strength of the film, and the simplest method can be achieved by reducing the thickness of the ferroelectric thin film.

[0005]

However, when the film thickness is reduced, an increase in leakage current and an increase in in-plane non-uniformity are caused. Was the limit.

On the other hand, when a compound containing Bi is formed from a solution, the Bi content in the starting material for production is often excessive in order to improve the characteristics of the ferroelectric thin film. In particular, when a SrBi ₂ Ta ₂ O ₉ thin film is formed, as described in Reference 2, the formation is performed in a state where the Sr content is reduced and Bi is excessive. When Bi is excessive, a film having good crystallinity and excellent electric characteristics can be formed.

However, this film has a structure in which bale-shaped particles are stacked and is not dense. In order to promote further thinning, it is necessary to make the film dense and uniform.

In order to achieve a dense and uniform film, it is preferable to promote nucleation (generation of crystal nuclei) during crystallization. However, since heating is performed gradually in normal heat treatment,
Crystal growth is prioritized over nucleation, and nucleation is suppressed. It is important to raise the temperature rapidly to promote nucleation. In ferroelectrics, rapid thermal annealing (RTA) commonly used in semiconductor processes
) Is not enough for nucleation.

Therefore, there has been a demand for a method of producing a ferroelectric thin film that can be made dense and uniform by preferentially promoting nucleation during crystallization.

[0010]

Therefore, according to the method of manufacturing a ferroelectric thin film of the present invention, a preliminary ferroelectric thin film is formed by depositing a ferroelectric material on a base, and annealing is performed. When crystallizing the preliminary ferroelectric thin film to obtain a ferroelectric thin film, an annealing process is performed by laser annealing.

As described above, by using laser annealing, the temperature of the preliminary ferroelectric thin film can be increased relatively quickly. Therefore, nucleation can be promoted during crystallization, and the ferroelectric thin film can be made dense and uniform.

In the method of manufacturing a ferroelectric thin film according to the present invention, it is preferable to perform annealing again after the laser annealing.

As described above, by performing the annealing again, the damage caused by the laser irradiation at the time of the laser annealing can be recovered.

In the method of manufacturing a ferroelectric thin film according to the present invention, preferably, the ferroelectric material has the following chemical formula (Bi ₂ O ₂ ) ²⁺ (A _m-1 B _m O _{3m + 1} ) ^{2 -} However, m is one of integers selected from 1, 2, 3, 4 and 5, a is Bi, Pb, Ba, Sr, Ca, N
a, K and an element selected from the group consisting of rare earth elements; and B is Ti, Nb, Ta, W, Mo, Fe,
The material is preferably selected from the group consisting of Co and Cr.

[0015] Such a Bi-containing ferroelectric is also called a Bi layered compound. When a Bi layered compound is included as a material constituting the ferroelectric thin film, the fatigue resistance of the film can be improved.

Preferably, the ferroelectric material is Sr
Bi ₂ Ta ₂ O ₉ is preferred.

SrBi ₂ Ta ₂ O ₉ is obtained from the above-mentioned (Bi
Of the Bi-containing ferroelectrics represented by the formula ₂ O ₂ ) ²⁺ (A _m-1 B _m O _{3m + 1} ) ^2- , m is 2, A is Sr, and B is Ta. This is a Bi layered compound. This S
The film composed of BT is particularly excellent in fatigue resistance.

Preferably, the ferroelectric material is Sr
_1-x Bi _{2 + y} Ta ₂ O _{9 + α} (0.7 ≦ x <1 and 0 ≦
Let y ≦ 0.4 and α be a variable depending on the manufacturing conditions)
It is good to have the composition.

Since the SBT film is formed with excess Bi in order to improve the characteristics such as the polarization of the SBT film, the SBT film stoichiometrically represented by SrBi ₂ Ta ₂ O ₉ Is actually Sr _1-x Bi _{2 + y} Ta ₂ O
It is considered to have a composition of _{9 + α} .

In the method of manufacturing a ferroelectric thin film according to the present invention, preferably, the laser annealing is performed using an excimer laser.

Since heating is carried out by an excimer laser, the temperature rises sharply, so that nucleation occurs preferentially in crystallization. Accordingly, the film is densified, and a thin film having a small leak current can be formed. Therefore, even when the same voltage is applied, the electric field strength to the film increases, and the film can be driven at a low voltage.

In the method of manufacturing a ferroelectric thin film according to the present invention, preferably, the ferroelectric material has the following chemical formula ABO ₃ , wherein A is Pb, and B is a group consisting of Ti and Zr. It is preferable to use a substance represented by:

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. It should be noted that the drawings merely schematically show the configuration, arrangement, and size to the extent that the present invention can be understood. The values and materials described below are merely examples within the scope of the present invention. Therefore, the present invention is not limited to this embodiment.

In this embodiment, a ferroelectric memory structure in which a ferroelectric thin film and an upper electrode are sequentially provided on a lower electrode provided on a semiconductor substrate will be described as an example. In this embodiment, a method of forming a SrBi ₂ Ta ₂ O ₉ thin film as a ferroelectric thin film is described with reference to FIG.
This will be described with reference to FIG. FIG. 1 is a flowchart showing a method of manufacturing a ferroelectric thin film according to the present embodiment. FIG. 2 is a cross-sectional view showing a manufacturing process of the ferroelectric thin film.

First, a process of forming a preliminary ferroelectric thin film by depositing a ferroelectric material on a base will be described. Pt (platinum) is used as the material of the lower electrode constituting the base.

First, an alkoxide solution of Sr (strontium) is prepared (S1 in FIG. 1). In a methoxymethanol (CH ₃ OC ₂ H ₄ OH) solvent, a piece of metal Sr was added little by little and dissolved to form an Sr alkoxide solution (S
r (OC ₂ H ₄ OCH ₃ ) ₂ ). Bi (O-nC ₄ H ₉ ) ₃ and Ta (O
After adding C ₂ H ₅ ) ₅ , reflux is carried out at a temperature of 80 ° C. for 20 hours. Thereby, in the solution, Ta (tantalum),
The alkoxides of Sr and Bi (bismuth) are complexed, and Ta, Sr and Bi are bonded via oxygen as in (-Bi-O-Sr-O-Ta-O-).

Next, the solution that has been refluxed is applied on the base 10 by a spin coater at a rotation speed of 2000 rpm to form a coating film (S2 in FIG. 1). Thereafter, the coating film is dried at a temperature of 150 ° C. for 30 minutes (S3 in FIG. 1) to evaporate the solvent in the coating film. Subsequently, pre-bake is performed on the coating film at a temperature of 450 ° C. for 60 minutes to burn organic functional groups (S4 in FIG. 1). In this embodiment, application of a solution (S in FIG. 1)
2), drying (S3 in FIG. 1) and calcination (S4 in FIG. 1)
Is repeated twice, so that the preliminary ferroelectric thin film 12 a having a thickness of 0.1 μm is formed on the underlayer 10.
Is formed (FIG. 2A).

Next, a process for obtaining a ferroelectric thin film by crystallizing the preliminary ferroelectric thin film 12a by performing an annealing process will be described.

In this embodiment, the annealing is performed by laser annealing (S5 in FIG. 1). To perform laser annealing, an excimer laser line annealing system (manufactured by Japan Steel Works, Ltd.) is used. Beam size is 0.47
mm × 150mm, 95% beam overlap
And With this system, the preliminary ferroelectric thin film 12a
Is irradiated with a laser beam and heated rapidly. As a result, generation of crystal nuclei is promoted, and crystallization occurs in which nucleation is prioritized over crystal growth. As a result,
The preliminary ferroelectric thin film 12a is crystallized to form the ferroelectric thin film 1
2b is obtained (FIG. 2 (B)).

In this example, when forming the SrBi ₂ Ta ₂ O ₉ thin film as the ferroelectric thin film 12b, Bi
The amount of the starting material formed is adjusted so as to be excessive. Thus, crystallization of SrBi ₂ Ta ₂ O ₉ can be performed at a relatively low temperature. The SrBi ₂ Ta ₂ O ₉ thin film thus formed has a Sr
It is considered to have a composition of _1-x Bi _{2 + y} Ta ₂ O _{9 + α} . Where 0.7 ≦ x <1 and 0 ≦ y ≦ 0.4
And Further, the composition of oxygen O in the film is a number that changes depending on the manufacturing conditions and cannot be determined.
Therefore, here, the composition of O is set to 9 + α (α is a variable depending on manufacturing conditions). Here, α is a value that fluctuates within a range larger than −9 and smaller than 18.

Further, the upper electrode 14 is formed on the ferroelectric thin film 12b (S6 in FIG. 1, FIG. 2C). Here, by RF sputtering, through a metal mask,
The upper electrode 14 having a diameter of 0.2 μm is formed. Pt is used as the material of the upper electrode 14.

After the upper electrode 14 is formed, secondary annealing is performed (S7 in FIG. 1). The structure including the base 10, the ferroelectric thin film 12b, and the upper electrode 14 is heated at a temperature of 800 ° C. in an oxygen atmosphere for 30 minutes.

The interface between the underlayer 10 and the ferroelectric thin film 12b is given a heat history by a plurality of heat treatments, but the interface between the upper electrode 14 and the ferroelectric thin film 12b is given a heat history. Therefore, the hysteresis symmetry of the ferroelectric thin film 12b may be deteriorated.
Further, since the upper electrode 14 is formed by a sputtering method in a reducing atmosphere, the B electrode in the ferroelectric thin film 12b is formed.
Since i may be reduced, it is necessary to re-oxidize. For this reason, as described above, after the upper electrode 14 is formed, by performing the heat treatment again in an oxygen atmosphere, these concerns can be avoided.

By performing such secondary annealing immediately after the above-described laser annealing, damage due to laser irradiation during laser annealing can be recovered.

Next, the characteristics of the ferroelectric thin film 12b will be described. The measurement results of the ferroelectric properties shown below are obtained from a RT-60 ferroelectric thin film property evaluation apparatus manufactured by Radiant.
This is obtained using 00S (product name).

FIG. 3 is a graph showing the hysteresis characteristics of the Sr _0.9 Bi _2.3 Ta ₂ O _{9 + α} (hereinafter abbreviated as SBT) thin film obtained by the manufacturing method of this embodiment. In the figure, the horizontal axis indicates the applied voltage value in V units, and the vertical axis indicates the polarization amount in μC · cm ⁻² . As shown in FIG. 3, saturation characteristics with a relatively high squareness ratio (remaining polarization amount / saturation polarization amount) are obtained. Usually, in order to obtain such a saturation characteristic, a film thickness of about 0.2 μm is required. However, the reason why such a saturation characteristic was obtained although the film thickness was 0.1 μm is considered to be because a dense and uniform SBT thin film was obtained by annealing with an excimer laser.

FIG. 4 is a graph showing the amount of remanent polarization with respect to the applied voltage. In the figure, the horizontal axis represents the maximum value V _max of the applied voltage.
Is shown in units of V, and the vertical axis represents the amount of remanent polarization + Pr in μC · c
It is shown in units of m- ² . The measured values are indicated by dots in the graph. As shown in FIG. 4, initially, as the applied voltage increases from 0 V, the amount of remanent polarization sharply increases. The inflection point is where the applied voltage is 1.09 V, and thereafter gradually increases. Since this inflection point is considered to be the saturation voltage, it can be said that the SBT thin film formed by the manufacturing method of this embodiment is sufficiently saturated by the applied voltage of 1.1V.

As described above, to saturate, usually 2V
Where the voltage must be applied, the film thickness is halved, so that when a voltage of about 1 V is applied, saturation occurs.

As described above, according to the method of manufacturing a ferroelectric thin film of this embodiment, a ferroelectric thin film having a high density can be formed. You.

The material of the lower electrode and the upper electrode 14 is not limited to platinum (Pt) but may be Ir (iridium),
Ru (ruthenium), IrO ₂ (iridium oxide), R
uO ₂ (ruthenium oxide) or the like may be used.

The material of the ferroelectric thin film 12b is not limited to SrBi ₂ Ta ₂ O ₉ but may be, for example, PbZr.
_0.5 Ti _0.5 O ₃ may be used.

[0042]

According to the method of manufacturing a ferroelectric thin film of the present invention, the temperature of the preliminary ferroelectric thin film can be raised relatively quickly by using laser annealing. Therefore, nucleation can be promoted during crystallization, the ferroelectric thin film can be made dense and uniform, and the driving voltage can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a method of manufacturing a ferroelectric thin film according to an embodiment.

FIG. 2 is a view showing a manufacturing process of a ferroelectric thin film.

FIG. 3 is a diagram showing hysteresis characteristics of a ferroelectric thin film.

FIG. 4 is a diagram showing a residual polarization amount with respect to an applied voltage.

[Explanation of symbols]

 10: Underlayer 12a: Preliminary ferroelectric thin film 12b: Ferroelectric thin film 14: Upper electrode

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01B 3/12 318 H01L 27/10 451 H01L 27/04 27/04 C 21/822 27/10 451 (72 Inventor Hiroyo Kato 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (72) Inventor Takao Kanehara 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (72) Inventor Kazuya Sano 2-2-1 Fukuura, Kanazawa-ku, Yokohama-shi, Kanagawa F-term in Japan Steel Works, Ltd. (reference) 4G048 AA03 AA04 AA05 AB05 AC02 AD02 AE05 5F038 AC15 AC18 DF05 EZ14 EZ17 5F083 JA13 JA38 PR23 PR34 5G303 AA10 AB20 BA03 CA01 CB03 CB05 CB06 CB09 CB10 CB13 CB14 CB20 CB21 CB25 CB32 CB33 CB35 CB37 CB43 DA01 DA07

Claims (7)

[Claims] A ferroelectric material is deposited on a base to form a pre-ferroelectric thin film, and annealing is performed to crystallize the pre-ferroelectric thin film to obtain a ferroelectric thin film. A method for producing a ferroelectric thin film, wherein the annealing is performed by laser annealing. 2. The method for producing a ferroelectric thin film according to claim 1, wherein the annealing is performed again after the laser annealing. 3. The method for producing a ferroelectric thin film according to claim 1, wherein the ferroelectric material has the following chemical formula (Bi ₂ O ₂ ) ²⁺ (A _m-1 B _m O _{3m + 1} ) ^2- , where m is an integer selected from 1, ² , 3, 4 and 5, and A is a group consisting of Bi, Pb, Ba, Sr, Ca, Na, K and a rare earth element. And B is Ti, Nb, Ta, W, Mo, Fe, Co and C
A method for producing a ferroelectric thin film, characterized in that the substance is selected from the group consisting of: 4. The method of manufacturing a ferroelectric thin film according to claim 3, wherein the ferroelectric material is SrBi ₂ Ta ₂ O ₉ . 5. The method for producing a ferroelectric thin film according to claim 1, wherein the ferroelectric material is Sr _{1 -x} Bi _{2 + y} Ta ₂ O.
_{9 + α} (where 0.7 ≦ x <1, and 0 ≦ y ≦ 0.4, and α is a variable depending on manufacturing conditions). Production method. 6. The method of manufacturing a ferroelectric thin film according to claim 1, wherein the laser annealing is performed using an excimer laser. 7. The method for producing a ferroelectric thin film according to claim 1, wherein the ferroelectric material has the following chemical formula: ABO ₃ , wherein A is Pb, and B is Ti and Zr. A method for producing a ferroelectric thin film, characterized in that the substance is selected from the group consisting of: