JPS6349385B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a thin film capacitor mainly composed of Ta ₂ O ₅ , and is particularly suitable for a capacitive element in an integrated circuit using Si as a semiconductor. The present invention relates to a thin film capacitor that has large capacitance per area, low leakage current, and high dielectric strength.

[Prior art]

Thin film capacitors formed on semiconductor substrates such as Si play an important role as storage capacitors in semiconductor devices such as active matrix drive circuits for displays and high-density storage integrated circuits. In particular, as the degree of integration of circuits increases, the area occupied by storage capacitors is becoming non-negligible. The need for thin film capacitors with high dielectric strength is increasing.

Until now, thermal oxidation method and
SiO ₂ formed by CVD has been used.
When SiO ₂ is used, its dielectric constant is as low as 3.4, so there is a drawback that the capacitance per unit area is not large. Although it is possible to increase the capacitance per unit area by reducing the thickness of the SiO ₂ film, this has the disadvantage that the dielectric strength of the thin film capacitor decreases.

Until now, in order to obtain thin film capacitors with large capacitance per unit area, materials such as Ta ₂ O ₅ ,
Attempts have been made to use a material with a dielectric constant about 7 to 8 times that of SiO ₂ as an insulating layer material for thin film capacitors. Ta ₂ O ₅ can be formed by a method such as a CVD method or thermal oxidation or anodic oxidation of a previously formed Ta film. However, a thin film capacitor using Ta ₂ O ₅ formed by such a method has a drawback of large leakage current due to the presence of a large number of trap levels in Ta ₂ O ₅ . In addition, as the leakage current increases, the dielectric strength decreases, making it necessary to increase the thickness of the Ta ₂ O ₅ film, which essentially has the disadvantage that the capacitance per unit area does not increase. be.

On the other hand, when applied to semiconductor devices such as integrated circuits, in addition to leakage current and dielectric strength immediately after film formation,
It is also important that the deterioration of these characteristics due to heat treatment used in the semiconductor device manufacturing process is small. In particular, even now that the manufacturing process has become lower in temperature, heat treatment at 400°C to 500°C after the metal electrode formation process is essential, so the deterioration of characteristics due to heat treatment at about 500°C is important for application to semiconductor devices. It becomes a problem. Ta ₂ O ₅ formed by the above-mentioned CVD method, thermal oxidation method, etc. suffers from a significant drop in insulation properties even after heat treatment at about 200°C, making it difficult to apply it to semiconductor devices.

[Purpose of the invention]

The purpose of the present invention has been made to eliminate these drawbacks, and uses Ta ₂ O ₅ formed by a sputtering method as the main constituent material, and has a large capacitance per unit area, a low leakage current, and a high It is an object of the present invention to provide a thin film capacitor having a dielectric strength and high insulation properties even after heat treatment.

[Structure of the invention]

To summarize the present invention, the present invention relates to a thin film capacitor, which is a dielectric capacitor formed on a Si substrate by a sputtering method using an inert gas containing oxygen, targeting a Ta or Ta ₂ O ₅ sintered body. A thin film capacitor having a body thin film as an insulating layer is characterized in that the insulating layer is composed of two layers: a mixed layer of Ta ₂ O ₅ and SiO ₂ and a Ta ₂ O ₅ layer.

FIG. 1 is a schematic cross-sectional view showing one embodiment of the present invention. In Figure 1, 1 is a Si substrate, 2 and 3 are
This is an insulating layer mainly composed of Ta ₂ O ₅ and consists of two layers: 2, which is a mixed layer of Ta ₂ O ₅ and SiO ₂ , and 3, which is Ta ₂ O ₅ . 4 is an Al electrode. To fabricate this thin film capacitor, Ta or
An insulating layer mainly composed of Ta ₂ O ₅ is formed by magnetron sputtering using a Ta ₂ O ₅ sintered body as a target and an inert gas containing oxygen as a sputtering gas. The sputtering method used here involves generating an electrical discharge in a low vacuum to ionize the gas, and accelerating the ions with an electric field so that they collide with the target, causing constituent atoms to be repelled from the target and deposited on the substrate. In particular, the magnetron sputtering method is a technique in which a magnetic field is applied at the same time so that a discharge occurs under a lower pressure than in the general sputtering method.

In the present invention, by using an inert gas containing oxygen as a sputtering gas, oxygen ions are generated during discharge, and this oxidizes the Si substrate surface and deposits the target constituent atoms on the substrate at the same time. , First, form a mixed layer _{of Ta2O5} and _SiO2 . After the mixed layer of Ta ₂ O ₅ and SiO ₂ is formed on the Si substrate surface, only Ta ₂ O ₅ is deposited because the oxidation of the Si substrate surface does not proceed. As a result, the insulating layer consists of two layers: a Ta ₂ O ₅ and SiO ₂ mixed layer and a Ta ₂ O ₅ single layer. Therefore, the dielectric constant of the insulating layer is
Higher than SiO ₂ . After forming the insulating layer, an Al electrode is deposited to complete the thin film capacitor.

The present invention will be explained below using quantitative experimental data.

FIG. 2 is a graph showing an example of the change in conductivity of a thin film capacitor before and after heat treatment at 450° C. in a hydrogen atmosphere. In FIG. 2, the vertical axis shows the logarithm of the conductivity of the thin film capacitor (Ω ^-1 cm ^-1 ), and the horizontal axis shows the square root of the electric field strength (10 ³ V ^1/2 cm ^-1/2 ). The thin film capacitor was formed by magnetron sputtering using a Ta ₂ O ₅ sintered body as a target in an argon gas atmosphere mixed with 20% oxygen. Further, the heat treatment was performed after forming the Al electrode. As shown in Figure 2, although an increase in conductivity (decrease in insulation) is observed due to heat treatment, high insulation (1×10 ⁶ V/cm
When an electric field is applied, the conductivity remains at 10 ^-14 Ω ^-1 cm ^-1 ). When Ta ₂ O ₅ is formed by CVD or thermal oxidation, a mixed layer of Ta ₂ O ₅ and SiO ₂ cannot be formed, and the trap level density in Ta ₂ O ₅ is 10 ¹⁸ cm
Due to its high value of ^-3 or higher, it no longer exhibits insulating properties after such heat treatment.

FIG. 3 is a graph showing an example of the influence of sputtering pressure during Ta ₂ O ₅ formation on the conductivity of a thin film capacitor.

In FIG. 3, the vertical axis shows the logarithm of the conductivity of the thin film capacitor (Ω ^-1 cm ^-1 ), and the horizontal axis shows the square root of the electric field strength (10 ³ V ^1/2 cm ^-1/2 ). Note that thin film capacitors are manufactured in an argon gas atmosphere containing 20% oxygen.
It was formed by magnetron sputtering using a Ta ₂ O ₅ sintered body as a target. Furthermore, after forming the Al electrode, heat treatment was performed at 450°C in a hydrogen atmosphere. As shown in FIG. 3, as the sputtering pressure increases, the conductivity of the thin film capacitor increases and the insulation properties decrease. This means that the trap level density in Ta ₂ O ₅ formed under low sputtering pressure is 10 ¹⁵
This is because the trapped level density increases to the 10 ¹⁶ cm ^-3 level as the sputtering pressure increases, whereas it is on the cm ^-3 level. As described above, the sputtering pressure is a dominant factor in the density of trapped levels in Ta ₂ O ₅ and therefore has a large effect on the conductivity of thin film capacitors. The range of sputtering pressures capable of forming Ta ₂ O ₅ is from 2×10 ⁻³ Torr to 2×10 ⁻¹ Torr. In order to obtain a highly insulating thin film capacitor, it is necessary to form Ta ₂ O ₅ with a low trap level density under conditions of low sputtering pressure. The low sputter pressure limit of 2×10 ⁻³ Torr is a value determined because discharge cannot be maintained below this value.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 Single crystal Si [p-type, plane index (100),
Using a specific resistance of 1.2 to 1.8 Ω・cm], 20% oxygen was used to target Ta ₂ O ₅ sintered body (purity 99.99%).
An insulating film mainly composed of Ta ₂ O ₅ was formed by magnetron sputtering in a mixed argon gas atmosphere. The magnetron sputtering was performed under a pressure of 5×10 ⁻³ Torr. In the insulating film thus formed, the thickness of the mixed layer of Ta ₂ O ₅ and SiO ₂ was 15 nm, and the thickness of Ta ₂ O ₅ was 85 nm. Following the formation of the insulating film, an Al electrode is formed,
Heat treatment was performed at 450°C in a hydrogen atmosphere. The leakage current characteristics of the thin film capacitor thus formed are shown in FIG. That is, FIG. 4 is a graph showing a comparison of the leakage current characteristics of thin film capacitors depending on the Ta ₂ O ₅ formation method. In FIG. 4, the vertical axis shows the logarithm of the leakage current (Acm ^-2 ), and the horizontal axis shows the square root of the electric field strength (10 ³ V ^1/2 cm ^-1/2 ). The Ta ₂ O ₅ formation methods shown for comparison are a thermal oxidation method and a CVD method. No heat treatment is applied to these cases formed by thermal oxidation or CVD. Despite being heat-treated, the thin-film capacitor of the present invention has a lower leakage current when 1×10 ⁶ V/cm is applied than when Ta ₂ O ₅ is formed by thermal oxidation or CVD. is 4 to 6 orders of magnitude lower. Also, the dielectric constant of the insulating layer is 12.5
The value is about four times higher than that of SiO ₂ , and the dielectric strength is 5×10 ⁶ V/cm, which is more than twice that of conventional Ta ₂ O ₅ .

In the above embodiment, argon gas mixed with 20% oxygen was used as the sputtering gas, but the same effect can be obtained even if the oxygen content is varied within the range of 5% to 70%. Further, although a Ta ₂ O ₅ sintered body was used as the target, a similar thin film capacitor can be obtained by using Ta.

〔Effect of the invention〕

As explained above, according to the present invention, Ta ₂ O ₅
By forming an insulating layer consisting of a mixed layer of SiO ₂ and Ta ₂ O ₅ , it is suitable for capacitive elements in integrated circuits, and has a large capacitance per unit area, low leakage current, and high dielectric strength. A remarkable effect is achieved in that a thin film capacitor can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing one embodiment of the present invention;
Figure 2 is a graph showing the dependence of the conductivity of a thin film capacitor on heat treatment, Figure 3 is a graph showing the dependence of the conductivity of a thin film capacitor on sputtering pressure, and Figure 4 is a graph showing the Ta ₂ O ₅ leakage current of the thin film capacitor. It is a graph showing a comparison between different formation methods. 1: Si substrate, 2: Ta ₂ O ₅ and SiO ₂ mixed layer, 3:
Ta ₂ O ₅ , 4: Al electrode.

Claims (1)

[Claims] 1 In a thin film capacitor whose insulating layer is a dielectric thin film formed on a Si substrate by sputtering using an inert gas containing oxygen and targeting Ta or Ta ₂ O ₅ sintered body, the insulating layer is Ta or Ta 2 O 5 sintered body. ₂ o ₅
A thin film capacitor consisting of two layers: a mixed layer of SiO ₂ and ₅ layers of Ta ₂ O.