JPH07183165A - Thin-film capacitor - Google Patents

Abstract

PURPOSE:To reduce the leakage current density of a thin-film capacitor to a practical level while maintaining large capacitance by providing multiple inorganic polycrystalline dielectric layer of different average grain sizes between electrodes. CONSTITUTION:A thin-film capacitor comprises a lower electrode 2 formed on a substrate 1, a dielectric layer 3 on the lower electrode 2, and an upper electrode on the dielectric layer. The dielectric layer includes a first layer 4 of large average grain size and a second layer 5 of small average grain size. The layer 5 is preferably thinner than 1/10 of the total thickness of the layer 3. When the capacitor is constituted in such a way, the leakage current density the capacitor can be reduced to a practical level while the capacitance of the capacitor is maintained at a high level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film capacitor used in DRAM memory cells, mobile communication MMICs and the like.

[0002]

2. Description of the Related Art With the increase in performance and capacity of electronic devices, complex oxide thin films having a perovskite structure such as SrTiO ₃ and (BaSr) TiO _{3 have been} replaced by conventional SiO
From the viewpoint of having a two-digit large dielectric constant as compared with _2, DRAM
Are being applied to thin film capacitors used in memory cells and MMICs for mobile communication. The characteristics required for these thin film capacitors include large capacity, low leakage current, high breakdown voltage, low dielectric loss, and the like.

These thin film capacitors have a structure in which a dielectric film is sandwiched between electrodes made of a dielectric film. As factors that affect the characteristics, the film material and element structure of the upper or lower electrode (of each layer). Layer thickness and electrode shape)
The film quality of the dielectric film is important, and a high-quality film that can obtain characteristics such as high relative permittivity, low dielectric loss, and low leakage current density in a desired temperature range is required.

Although the correlation between the film quality of a complex oxide thin film such as SrTiO ₃ and the above characteristics is still in the research stage and there are many unclear points, one of the obtained views is the crystal grain size of a polycrystalline film. There is a correlation with the relative dielectric constant or the leakage current density.
That is, the smaller the crystal grain size, the smaller the leak current density, and by obtaining the microcrystalline film, the leak current density of the film can be suppressed. However, there is a problem that not only the leakage current density decreases but also the relative dielectric constant of the film becomes smaller as the crystal grain size becomes smaller. The trade-off relationship between the relative permittivity of the film and the leakage current density has been confirmed when the film composition and the like are changed in addition to the crystal grain size, and this has become a major problem in improving the film characteristics. There is.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and is a thin film in which the leakage current density is reduced to a practically satisfactory level while maintaining a large capacity. It is intended to provide a capacitor.

[0006]

A thin film capacitor according to the present invention is characterized by having an inorganic polycrystalline dielectric layer having a layer having a small average crystal grain size in the film thickness direction. Examples of the inorganic polycrystal dielectric include Sr
TiO ₃ , BaTiO ₃ , (BaSr) TiO ₃ , Pb
Examples thereof include complex oxides having a perovskite structure such as TiO ₃ , Pb (ZnTi) O ₃ , and CaTiO _3, and TiO ₂ having a rutile structure.

The layer having a large average particle diameter in the inorganic polycrystalline dielectric layer preferably has an average particle diameter of 60 nm or more. The layer having a small average crystal grain size preferably has an average grain size of 1/2 or less of that of the layer having a large average grain size.

The thickness of the layer having a small average crystal grain size is preferably 1/10 or less of the thickness of the dielectric layer itself. The method for forming the inorganic polycrystalline dielectric layer is not particularly limited, and a vacuum method such as vapor deposition or sputtering, or a wet method such as anodic oxidation, a sol-gel method, or the like can be employed. In particular, a vacuum process is desirable for controlling the crystal grain size of the inorganic polycrystalline dielectric layer with high accuracy.

There are several methods for controlling the crystal grain size of the inorganic dielectric layer. For example, in the case of forming a film by the RF sputtering method, a dielectric layer having a larger crystal grain size can be obtained as the film forming speed becomes lower and the substrate temperature during film forming becomes higher. By changing the film conditions to change the film formation rate during film formation with time, or by changing the substrate temperature with time during film formation, it is possible to form a dielectric layer having a layer with a small average grain size. With this method, it is possible to form a dielectric layer having a gradient of crystal grain size in the film thickness direction.

[0010]

According to the thin film capacitor of the present invention, a layer having a small average crystal grain size is provided in the film thickness direction of the inorganic polycrystalline dielectric layer sandwiched between the electrodes, thereby maintaining a large capacity and practical use. It is possible to reduce the leakage current density to the extent that it is satisfactory.

That is, by providing a layer having a small average crystal grain size in the thickness direction of the inorganic polycrystalline dielectric layer,
A layer having a large average crystal grain size in the same dielectric layer, that is, a layer having a large relative permittivity and a leakage current density (hereinafter referred to as the first layer), and a layer having a small average crystal grain size, that is, a relative permittivity. And a layer having a small leak current density (hereinafter, referred to as a second layer) can be mixed. As a result, the first
The relative permittivity is improved by the layer and the leakage current density is reduced by the second layer, and a thin film capacitor satisfying the above characteristics can be obtained as the entire dielectric layer.

A specific thin film capacitor will be described with reference to FIG. The lower electrode 2 is arranged on the substrate 1.
The dielectric layer 3 is disposed on the lower electrode 2 and includes the first layer 4 which is a layer having a large average crystal grain size and which is disposed from the lower electrode 2 side and the layer having a small average crystal grain size described above. It is formed from a certain second layer 5. Upper electrode 6
Are arranged on the dielectric layer 3.

In the thin film capacitor having such a structure, the film thickness, relative permittivity and leakage current density of the first layer are respectively d ₁ , ε _r1 and J ₁ , as well as the film thickness, relative permittivity and the dielectric constant of the second layer. Leakage current densities are d ₂ , ε _r2 and J ₂ , respectively, the thickness of the entire dielectric layer, relative permittivity and leak current density are d,
If ε _r , J, the capacitance of the dielectric layer is equal to that of the first layer and the second layer.
Since it is considered that the layers are in series, it is obtained by the following equation (1). , 1 / C = 1 / C ₁ + 1 / C ₂ (1) Here, C ₁ , C ₂ , and C are the capacitances of the first layer, the second layer, and the entire dielectric layer, respectively.

Using the relation of the equation (1), the relative permittivity of the entire dielectric layer is obtained by the following equation (2). ε _r = d ε _r1 ε _r2 / (ε _r1 d ₂ + ε _r 2 _d1 ) (2) Therefore, the second layer is made sufficiently thin, for example, 1 / th of the first layer.
With a thickness of 10, ε _r has a value approximately close to ε _r1 . On the other hand, the leakage current density J of the entire dielectric layer is a value approximately J to J ₂ because the second layer acts as a barrier layer. As described above, the dielectric constant of the first layer is improved and the leakage current density of the second layer is reduced as described above, and a thin film capacitor having a large capacity and a low leakage current density can be realized. .

[0015]

EXAMPLES Examples of the present invention will be described in detail below. Example 1 A Pt film having a thickness of 50 nm was formed as a lower electrode on an Al ₂ O ₃ substrate at room temperature by the RF magnetron sputtering method, and then by the RF magnetron sputtering method.
SrTiO ₃ Ar / O ₂ 5 mTorr, RF input power 4
The film was formed at 00W. At that time, the substrate temperature was 400 ° C. for 170 minutes during the film formation time of 180 minutes, and 250 ° C. for the remaining 10 minutes.
And

The film thickness of the SrTiO ₃ film thus formed was about 200 nm as measured by α-step profiler, and the SrTi ₃ film was analyzed by X-ray diffraction and electron diffraction.
It was found that the entire O ₃ film was a (111) -oriented polycrystal. Furthermore, as a result of observing the cross section of the film by FE-SEM, the crystal grain size in the 190 nm thick region on the lower electrode side was 7
The crystal grain size in the region of 0 nm and the thickness of 10 nm on the upper electrode side was around 30 nm.

Then, a 100 nm thick Ni film was formed as an upper electrode on the SrTiO ₃ film by an RF magnetron sputtering method at 350 ° C., a resist pattern was formed by a PEP process, and the Ni film was masked with the resist pattern. 100μm × 100μ by etching
m upper electrode pattern was formed.

When the characteristics of the obtained capacitor were evaluated by the four-terminal method, the capacitance was about 88.5 nF and the dielectric loss tangent tan δ = 0.01 at the measurement frequency of 100 kHz, which was converted into the relative permittivity ε _r. About 200 and 4
The value was almost the same as that of the SrTiO ₃ film formed at 00 ° C. Moreover, when the IV characteristics were measured, the leakage current density J was 10 ⁻⁸ A / in the region where the DC voltage V _DC was within ± 10V.
cm ² or less, SrTiO formed at 250 ° C.
_The value was almost the same as that of the _three films.

(Example 2) A Pt film having a thickness of 50 nm was formed as a lower electrode on an AlN substrate at room temperature by an RF magnetron sputtering method, and then SrTiO ₃ was placed in an O ₂ atmosphere (5 mTorr) at a substrate temperature of 400 ° C. An RF magnetron sputter film was formed. At that time, the RF input power was 400 W during the film formation time of 400 minutes up to 380 minutes, and the remaining 20 minutes was 60 W.
It was set to 0W.

The entire SrTiO3 film thus formed is
The film is a (111) oriented polycrystalline film having a film thickness of 205 nm, and 190 n of the lower electrode side from the cross section FE-SEM observation of the film.
The region of m thickness has a crystal grain size of around 70 nm and the upper electrode side is 15 n
The region of m thickness had a crystal grain size of about 30 nm.

Next, on the SrTiO ₃ film, a 100 nm Ni film was formed as an upper electrode by RF magnetron sputtering at 350 ° C., a resist pattern was formed by a PEP process, and the Ni film was etched using the resist pattern as a mask to a thickness of 100 μm. An upper electrode pattern of × 100 μm was formed.

The obtained capacitor has a measurement frequency of 100.
Capacitance: about 86.5nF, dielectric loss tangent tan δ at kHz
= 0.01, which is about 2 in terms of relative permittivity ε _r
The value was 00, which was almost the same as that of the SrTiO ₃ film formed at 400 W (film formation rate 0.5 nm / min). Also, when the IV characteristic was measured, the leakage current density J was 10 ⁻⁸ A / cm ^{2 in} the region where the DC voltage V _DC was within ± 10 V.
The following values were shown, which were almost the same as those of the SrTiO ₃ film formed at 600 W (film formation rate 0.75 nm / min).

Example 3 First, Pt was deposited on a MgO (100) single crystal at 400 ° C. by an RF magnetron sputtering method to form a 50 nm thick Pt (100) single crystal film. A SrTiO ₃ film was formed thereon by RF magnetron sputtering with Ar / O ₂ 5 mTorr and RF input power of 400 W. At that time, the substrate temperature was 400 ° C. for 175 minutes during the film formation time of 180 minutes, and 250 for the remaining 5 minutes.
℃ was made.

The SrTiO ₃ film thus obtained had a film thickness of about 190 nm. As a result of FE-SEM observation of the film cross section, the 175 nm thick region on the lower electrode side was a single crystal film, and the 15 nm thick region on the upper electrode side was a grain. It was found that the film was a polycrystalline film with a diameter of around 30 nm. Further, from the X-ray diffraction pattern, the SrTiO ₃ film was (100) oriented in both the single crystal portion and the polycrystalline portion.

Next, a 100 nm-thick Ni film was formed as an upper electrode on the SrTiO ₃ film by an RF magnetron sputtering method at 350 ° C., a resist pattern was formed by a PEP process, and the Ni film was formed using the resist pattern as a mask. 100μm × 100μm by etching
The upper electrode pattern of was formed.

When the characteristics of the obtained capacitor were evaluated by the four-terminal method, the capacitance was about 140 nF and the dielectric loss tan δ = 0.008 at a measurement frequency of 100 kHz, which was about 300 in terms of the relative permittivity ε _r. And S
The value was almost the same as that of the rTiO ₃ single crystal film. Also, I
When the −V characteristic was measured, the leakage current density J was −10.
When V ≦ V _DC ≦ −10 V, the value was 10 ⁻⁸ A / cm ² or less, which was almost the same as that of the SrTiO ₃ polycrystalline film.

Example 4 A Pt film having a thickness of 50 nm was formed as a lower electrode on a Si substrate with a thermal oxide film, and then Sr was formed.
TiO ₃ in O ₂ atmosphere (5 mTorr), substrate temperature 400
An RF magnetron sputter film was formed at ° C. At that time, the substrate temperature was 400 ° C. during the film formation time of 400 minutes until 360 minutes,
It was gradually cooled to 200 ° C. in the remaining 40 minutes.

The entire SrTiO ₃ film thus obtained is
It is a (111) oriented polycrystalline film with a film thickness of 200 nm, and the crystal grain size is 7 from the lower electrode to the film thickness direction of 180 nm.
In the uniform region of 0 nm, the crystal gradually became finer in the film thickness direction on the side of the upper electrode, and the grain size was 25 nm at the interface with the upper electrode.

Next, a 100 nm-thick Ni film was formed as an upper electrode on the SrTiO ₃ film by an RF magnetron sputtering method at 350 ° C., a resist pattern was formed by a PEP process, and the Ni film was formed using the resist pattern as a mask. 100μm × 100μm by etching
The upper electrode pattern of was formed.

The obtained capacitor has a measurement frequency of 100.
Capacity of about 97.8 nF (converted relative permittivity ε _r
˜210), dielectric loss tangent tan δ = 0.09, leakage current density J <10 ⁻⁸ A / cm ² (−10V ≦ V _DC ≦ −10).
V) and good characteristics were exhibited.

[0031]

As described above in detail, according to the present invention, it is possible to reduce the leak current density without lowering the relative permittivity of the dielectric layer, and to provide a large capacity and high performance thin film capacitor. can do.

[Brief description of drawings]

FIG. 1 is a sectional view of a thin film capacitor according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Substrate, 2 ... Lower electrode, 3 ... Dielectric layer, 4 ... First layer,
5 ... second layer, 6 ... upper electrode.

Claims (1)

[Claims] 1. A thin film capacitor comprising an inorganic polycrystalline dielectric layer in which a layer having a small average crystal grain size exists in a film thickness direction between the electrodes in a dielectric layer sandwiched between the electrodes. .