JPH10335596A - Ferroelectric element and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the polarization inversion resistance and stability of operation by providing a ferroelectric layer adjacent conductive electrodes, having a tetragonal system or such system having specified a-axis to c-axis length ratio and ferroelectric layer not adjacent thereto. SOLUTION: A ferroelectric layer has a tetragonal system having an a-axis to c-axis length ratio of less than 1.00-1.02 and is made by forming a Pt film 3 on an Si oxide film 2 formed on an Si single crystal substrate 1, and forming a ferroelectric thin film 4, using a sol-gel soln. of ferroelectric material contg. Pb, La, Zr, and Ti at a mol ratio of 1.05:0.05:0.65:0.35 at a concn. of 0.25 mol/lit. for a first and fourth steps and sol-gel soln. contg. Pb, Zr and Ti at a mol ratio of 1.1:0.65:0.35 at a concn. of 0.25 mol/lit. for a second and third steps, thereby forming a ferroelectric element superior in polarization inversion resistance and operative stably.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor memory device, and more particularly to a ferroelectric device comprising a transistor and a ferroelectric capacitor and a method of manufacturing the same.

[0002]

2. Description of the Related Art A volatile memory capable of storing information only while a power is turned on is provided in a semiconductor memory element.
There is a nonvolatile memory that can store information even when the power is turned off. Among them, a non-volatile memory is a mask ROM (Mask-Read-O).
nly Memory), PROM (Programm
able ・ Read ・ Only ・ Memory) 、 EP
ROM (Erasable / Programmable)
・ Read ・ Only ・ Memory) 、 EEPROM
(Electrically / Erasable / an
d ・ Programmable ・ Read ・ Only ・
Memory).

[0003] As a future form of these nonvolatile memories,
Recently, a ferroelectric memory which has a high operation speed and can be reduced in area has been proposed. An FRAM (Fe) having a structure in which a DRAM capacitor is replaced with a ferroelectric capacitor
rroelectric, Random, Access
(Memory) (Ramtron Corporation, Japanese Patent Application Laid-Open No. Hei 2-113496) is one of the closest to realization.

[0004]

Generally, lead-based PbZrTiO ₃ or PbLaZrTiO ₃ is used as a material of the ferroelectric capacitor of the FRAM.
_In No. ₃ , since the transition from cubic to tetragonal occurs during the ferroelectric manufacturing process, cracks are likely to occur, and as a result, the fatigue properties are slightly inferior. On the other hand, since PbLaZrTiO ₃ has a structure close to a cubic crystal, it has few cracks and the like, has good fatigue properties, but has a disadvantage of low spontaneous polarization strength. The present invention has been made in order to solve the unresolved problems of such a conventional technology, a ferroelectric element with less fatigue of the ferroelectric material, less leakage current due to the occurrence of cracks, and the like. It is intended to provide a manufacturing method.

[0005]

According to a first aspect of the present invention, there is provided a ferroelectric element having a plurality of ferroelectric layers sandwiched between conductive electrodes. ,
The layer in contact with the conductive electrode is a cubic or tetragonal ferroelectric layer in which the ratio c / a of the length of the a-axis and the c-axis of the crystal structure is 1.00 or more and less than 1.02, The layer not in contact with the conductive electrode is a tetragonal ferroelectric layer. The ferroelectric element according to claim 2 is
The ferroelectric element according to claim 1, wherein the cubic ferroelectric layer is a ferroelectric layer containing lead, lanthanum, zirconium, and titanium, and the tetragonal ferroelectric layer is lead, It is a ferroelectric layer containing zirconium and titanium. A method of manufacturing a ferroelectric element according to claim 3 is a method of manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, wherein a liquid containing lead, zirconium, and titanium is provided. The ferroelectric raw material is applied three times or more, and the first and final times are added to the raw material, and a liquid ferroelectric raw material containing lanthanum is used.

According to a fourth aspect of the present invention, there is provided a method of manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, comprising lead, zirconium and titanium. A ferroelectric layer is formed by laminating three or more times by sputtering using a target containing, and the first and final times are added to the above target, and a target containing lanthanum is used. A method of manufacturing a ferroelectric element according to claim 5 is a method of manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, the method including an organic material containing lead, zirconium, and titanium. A ferroelectric material is laminated three or more times by chemical vapor deposition using a raw material to form a film, and the first and last rounds are added to the organic raw material and an organic raw material containing lanthanum is used. is there.

[0007]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more detail. In the present invention, the conductive electrode is a metal, an electrode made of a good conductor such as an oxide, preferably, platinum, gold, iridium, iridium oxide, rhodium, palladium, or
A mixed material of two or more of these materials, a laminated structure of these two types, or the like is used. Ferroelectric, lead, lanthanum, zirconium, a layer in contact with the conductive electrode is a ferroelectric containing titanium, a layer not in contact with the conductive electrode, that is,
The central portion of the ferroelectric layer is a ferroelectric containing lead, zirconium, and titanium. The amount of lanthanum added to the ferroelectric layer in contact with the conductive electrode is preferably 0.0
It is preferably about 1 to 0.1. Usually, the names of the crystal axes of the crystal structure are called a, b, c, etc., and as shown in FIG. 7, a = b, c in a cubic system and a = b in a tetragonal system. In the present invention, c / a means a-axis (= b) length, c / a
Indicates the ratio to the axial length. As a manufacturing method, a sol-gel method, a sputtering method, a chemical vapor deposition method (CVD method), or the like is used.

The sol-gel method is a process in which a ferroelectric material is manufactured by applying a liquid ferroelectric material containing lead, zirconium, and titanium at least three times and performing heat treatment. A liquid ferroelectric material containing lanthanum in addition to the material is used. The liquid ferroelectric material preferably includes an organic acid salt, an organic metal compound, an alkoxide,
Use a carbonate or the like. For example, PZT (PbZrTiO ₃ ) or the like can be obtained from this ferroelectric material. The application method is to apply the ferroelectric material by dripping the material onto a rotating wafer or spraying the material onto the surface of the wafer. The heat treatment includes a step of drying after each application and a step of baking and heating after drying of the final application. Generally, these heat treatments are preferably performed in an atmosphere containing oxygen. The coating process is performed three times or more,
The first and final rounds are added to the above raw materials, and the liquid ferroelectric raw material obtained by adding lanthanum to the constituent elements is used.

In the sputtering method, a ferroelectric material sandwiched between conductive electrodes is formed by laminating a ferroelectric material three or more times using a target containing lead, zirconium, and titanium. In the final round, a target containing lanthanum is used in addition to the previous target. Preferably, two types of targets are switched during film formation to form a film. CV
Method D is a process in which a ferroelectric material sandwiched between conductive electrodes is formed by stacking a ferroelectric material three or more times using an organic material containing lead, zirconium, and titanium to form a film. And an organic material containing lanthanum is used. Preferably, a method of switching between two kinds of organic materials or adding an organic material containing lanthanum during film formation is used.

As a result, the ferroelectric layer adjacent to the lower electrode and the upper electrode has a structure close to a cubic crystal containing lanthanum (for example, PLZT), and is a film with less cracks and the like. A structure close to a cubic system is a tetragonal system and c / a =
It means a structure of less than 1.02, preferably c / a = 1.01 or less. Further, a large spontaneous polarization of PZT can be obtained at the center of the film. As described above, by making the ferroelectric material near the electrodes PLZT or the like, it is possible to suppress the deterioration of the electric characteristics of the ferroelectric material due to the influence of cracks, grain boundaries, etc., and to maintain a large spontaneous polarization. In particular, it is effective in improving the number of times of inversion polarization and maintaining the memory retention time.

[0011] These features make it possible to improve the electrical characteristics of the ferroelectric element, and to obtain a highly reliable ferroelectric element. FIG. 1 shows a case where it is specifically used for a memory element. FIG. 1 shows a structure in which a SiO ₂ film 2 is provided on a Si single crystal substrate 1, and a ferroelectric thin film 4 having the structure of the present invention is sandwiched between a platinum electrode 3 and a platinum electrode 5. I have. Further, the platinum electrode 5 of the upper electrode is connected to the source or the drain of the transistor.

[0012]

Embodiment 1 Hereinafter, an embodiment of the present invention will be described with reference to the drawings. First, a substrate having a platinum film 3 formed by sputtering on a silicon oxide film 2 formed on a silicon single crystal substrate 1 is used (FIG. 2). The application was performed four times using a liquid ferroelectric material. 1st and 4th
As materials to be applied for the second time, lead (Pb), lanthanum (L
a) A sol-gel solution containing zirconium (Zr) and titanium (Ti) was used. The molar ratio in the solution is Pb: L
a: Zr: Ti = 1.05: 0.05: 0.65: 0.
At 35, the concentration was 0.25 mol / l. As a material to be applied for the second and third times, a sol-gel solution containing lead (Pb), zirconium (Zr), and titanium (Ti) was used. The molar ratio in the solution is Pb: Zr: Ti = 1.
1: 0.65: 0.35, the concentration was 0.25 mol / l.

The raw material was dropped on the substrate set on a spinner and rotated at 3,000 revolutions per minute. Fifteen
Dry on a hot plate at 0 ° C for 10 minutes,
Dry on a hot plate at 50 ° C. for 10 minutes. This operation was repeated using the first to fourth materials. Then, a ferroelectric thin film 4 was formed (FIG. 3). Next, in an atmosphere of oxygen, using an infrared lamp heating device, 6
After baking at 50 ° C. for 10 minutes, a platinum film was formed as the upper electrode 5 by sputtering. At this time, the upper electrode 5 is formed into a shape having a diameter of 0.5 mm by using a metal mask.

Further, this is placed in an oxygen atmosphere at 500 ° C.
Heat treatment was performed for 10 minutes. This sample is designated as A (FIG. 4). For comparison, a sample prepared by mixing all four coating materials in a molar ratio of Pb: Zr: Ti = 1: 0.65: 0.35 (concentration: 0.25 mol / l) Was prepared. This sample is designated as B. This A,
FIG. 5 shows the measurement results of the film fatigue characteristics of the sample B obtained by repeating the inversion polarization. As a result, it was found that the sample A in which lanthanum was added to the layers in contact with the platinum electrode 3 and the platinum electrode 5 had better film fatigue properties.
A ferroelectric thin film made of Pb, Zr, and Ti;
The ratio c / a of the a-axis and c-axis lengths of the ferroelectric thin film made of b, La, Zr, and Ti was measured by an X-ray diffraction method. as a result,
The former had c / a = 1.02 and the latter had c / a = 1.00.

[0015]

Embodiment 2 First, a substrate having an iridium film formed by sputtering on a silicon oxide film is used. The application was performed four times using a liquid ferroelectric material. As a material to be applied at the first and fourth times, lead (P
b) A sol-gel solution containing lanthanum (La), zirconium (Zr), and titanium (Ti) was used. The molar ratio in the solution is Pb: La: Zr: Ti = 1.05: 0.0
5: 0.65: 0.35 and the concentration was 0.25 mol / l. As the material to be applied for the second and third time,
A sol-gel solution containing lead (Pb), zirconium (Zr), and titanium (Ti) was used. The molar ratio in the solution is P
b: Zr: Ti = 1.1: 0.65: 0.35, and the concentration was 0.25 mol / liter.

The raw material was dropped on the substrate set on the spinner and rotated at 3,000 revolutions per minute. 150
Dried on a hot plate at 10 ° C. for 10 minutes,
Dry on a hot plate at 0 ° C. for 10 minutes. This operation was repeated using the first to fourth materials. Next, baking was performed at 650 ° C. for 10 minutes in an atmosphere of oxygen using an infrared lamp heating apparatus, and then platinum was formed as an upper electrode by sputtering. At this time, the upper electrode is formed into a shape having a diameter of 0.5 mm by using a metal mask.

Further, this is placed in an oxygen atmosphere at 500 ° C.
Heat treatment was performed for 10 minutes. This sample is designated as C. For comparison, four coating materials were all used in a molar ratio of Pb: Zr:
A sample was prepared by mixing (concentration: 0.25 mol / l) Ti: 1.1: 0.65: 0.35. This sample is designated as D. FIG. 6 shows the results of measuring the fatigue characteristics of the films C and D obtained by repeating the inversion polarization. As a result, it was found that Sample C, in which lanthanum was added to the layer in contact with the iridium electrode, had better film fatigue properties. Pb, Zr, Ti
A ferroelectric thin film composed of Pb, La, Zr, and Ti was subjected to X-ray diffraction to determine the ratio c of a-axis and c-axis lengths.
/ A was measured. As a result, the former was c / a = 1.02, and the latter was c / a = 1.00.

[0018]

Embodiment 3 Another embodiment will be described below. First, Example 1
Similarly to the above, a substrate having a platinum film formed by sputtering on a silicon oxide film is used. The application was performed four times using the liquid ferroelectric material. As materials to be applied for the first and fourth times, lead (Pb), lanthanum (L
a) A sol-gel solution containing zirconium (Zr) and titanium (Ti) was used. The molar ratio in the solution is Pb: L
a: Zr: Ti = 1.05: 0.01: 0.65: 0.
At 35, the concentration was 0.25 mol / l. As a material to be applied for the second and third times, a sol-gel solution containing lead (Pb), zirconium (Zr), and titanium (Ti) was used. The molar ratio in the solution is Pb: Zr: Ti = 1.
The concentration was 1: 0.65: 0.35 and the concentration was 0.25 mol / liter. The ratio c / a of the a-axis and c-axis lengths of the ferroelectric thin film made of Pb, Zr, and Ti and the ferroelectric thin film made of Pb, La, Zr, and Ti were measured by an X-ray diffraction method. As a result, the former was c / a = 1.02 and the latter was c / a = 1.0.
05. And it turned out that Example 3 is also excellent in film fatigue characteristics like Examples 1 and 2.

[0019]

As described above, according to the present invention,
It is possible to provide a ferroelectric element which has excellent polarization reversal resistance, operates stably, and has high reliability. Therefore, there is an effect that a ferroelectric nonvolatile memory having a long life, high reliability, high speed, non-volatile, and low power consumption can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an entire cell structure.

FIG. 2 is a view showing a platinum film as a lower electrode formed on a SiO ₂ / Si substrate of Example 1.

FIG. 3 is a diagram showing four ferroelectric layers formed on the platinum film of FIG. 2 in Example 1.

FIG. 4 is a view showing that a platinum film as an upper electrode is formed on the ferroelectric layer of FIG. 3 in Example 1.

FIG. 5 is a graph showing the results of measuring the domain inversion life of the sample manufactured in Example 1.

FIG. 6 is a graph showing the results of measuring the domain inversion lifetime of the sample manufactured in Example 2.

FIG. 7 is a diagram showing cubic and tetragonal crystal structures.

[Explanation of symbols]

1 .... Si single crystal substrate 2 .... SiO ₂ film 3 ... platinum electrode (lower electrode) 4 ... ferroelectric thin film (4 layers) 5 ... a platinum electrode (upper electrode)

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 21/8247 29/788 29/792

Claims (5)

[Claims] 1. A ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, wherein a layer in contact with the conductive electrode has a length of a-axis and c-axis of a cubic or crystalline structure. A ferroelectric layer having a ratio c / a of not less than 1.00 and less than 1.02, wherein the layer not in contact with the conductive electrode is a tetragonal ferroelectric layer. Dielectric element. 2. The ferroelectric device according to claim 1, wherein
The cubic ferroelectric layer is a ferroelectric layer containing lead, lanthanum, zirconium, and titanium, and the tetragonal ferroelectric layer is a ferroelectric layer containing lead, zirconium, and titanium. The ferroelectric element according to claim 1, wherein 3. A method for manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, wherein a liquid ferroelectric material containing lead, zirconium, and titanium is applied three times or more. A method for manufacturing a ferroelectric element, wherein a liquid ferroelectric material containing lanthanum is used in addition to the above materials at the first time and the last time. 4. A method for manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, wherein the ferroelectric layer is formed three times by sputtering using a target containing lead, zirconium and titanium. The film is formed by laminating the above,
A method for manufacturing a ferroelectric element, wherein a target containing lanthanum is used in addition to the target in the first and second rounds. 5. A method of manufacturing a ferroelectric element having a multi-layered ferroelectric layer sandwiched between conductive electrodes, wherein a ferroelectric substance is formed by a chemical vapor deposition method using an organic material containing lead, zirconium, and titanium. Are laminated three or more times to form a film, and the first and final times are used in addition to the organic material, and an organic material containing lanthanum is used.