JPH028452B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is suitable for integrated circuits using Si as a semiconductor;
The present invention relates to a method for manufacturing a thin film capacitor with high capacitance per unit area and low leakage current.

[Conventional technology]

Thin film capacitors formed on semiconductor substrates such as Si play an important role as storage capacitors in semiconductor devices such as high-density memory integrated circuits and active matrix drive circuits for flat panel displays. In particular, as the degree of integration of circuits increases, the area occupied by storage capacitors is becoming non-negligible.In order to reduce the area occupied, capacitance per unit area is high and leakage current is small. There is an increasing need for thin film capacitors with high dielectric strength and high dielectric strength.

FIG. 8 shows a cross-sectional structure of a typical thin film capacitor used in the above semiconductor device. This capacitor has an insulating layer 2 and a metal electrode 3 formed in this order on a Si substrate 1. Until now, insulating layer materials for such thin film capacitors have been formed using thermal oxidation or CVD methods.
SiO ₂ has been used. SiO ₂ has a dielectric constant of
Because of its low value of 3.4, thin film capacitors using SiO ₂ have the disadvantage that the capacitance per unit area is not large. In addition, attempts have been made to reduce the thickness of the SiO ₂ film in order to increase the capacitance per unit area, but in this case, as the thickness of the insulating layer decreases, the insulation of the thin film capacitor A problem arises in that the withstand voltage is reduced.

In order to solve these problems, _{Ta 2 O 5} and
Attempts have been made to use dielectric thin films such as Nb ₂ O ₅ as insulating layer materials for thin film capacitors. These dielectric materials such as Ta ₂ O ₅ and Nb ₂ O ₅ have a melting point
Since the temperature is extremely high at over 1000℃, vacuum evaporation using resistance heating or electron beam heating is used.
It is difficult to form thin films. For this reason, the sputtering method is mainly used for its formation. In the sputtering method, gas in the atmosphere is ionized by causing an electric discharge in a low vacuum, and the ionized gas molecules are accelerated by an electric field and collided with a target, which is a sintered body of the constituent material of the thin film to be attached. This is a method in which constituent atoms are repelled from a target and a thin film is deposited on a substrate on which a semiconductor element or the like is to be formed. According to this sputtering method, it is possible to form a dielectric thin film with a high melting point while keeping the substrate temperature low.

However, so far, Ta ₂ O ₅ thin films and Nb ₂ O ₅ thin films formed on Si using the sputtering method have low leakage current and high dielectric strength. As a result, in order to keep the leakage current low and obtain sufficient dielectric strength as a thin film capacitor, the thickness of the insulating layer has to be increased, resulting in the problem that the capacitance per unit area does not become large. It is occurring.

In addition, instead of increasing the thickness of the insulating layer as a means of suppressing leakage current, the insulating layer is made of SiO _2, which has low leakage current and high dielectric strength, and materials with high dielectric constant, such as Ta ₂ O ₅ . Attempts have been made to create a two-layer structure. Although a thin film capacitor with this configuration can suppress leakage current to a low level, approximately 20% of the entire insulating layer is composed of SiO ₂ , which has a low dielectric constant, so the dielectric constant of the entire insulating layer is as small as 10 or less. In practice, there is a problem in that the capacitance per unit area is not sufficiently large.

[Problem that the invention seeks to solve]

As mentioned above, no method has been available so far for producing a thin film capacitor on Si, which is made of an insulating layer with a high dielectric constant and low leakage current.

An object of the present invention is to provide a method for manufacturing a thin film capacitor on Si, which solves these problems and consists of an insulating layer with a high dielectric constant and low leakage current.

[Means for solving problems]

The main feature of the present invention is that when forming a dielectric thin film on Si by sputtering, the thickness of the surface oxide film on the Si surface before formation is controlled to a desired thickness. At least a step of using a shutter that covers the entire Si substrate and performing pre-sputtering with the shutter closed, a step of controlling the thickness of an ultra-thin surface oxide film on the surface of the Si substrate, and a step of applying the dielectric thin film on the Si substrate. The method is characterized in that it includes a step of forming. The dielectric thin film is made of Ta ₂ O ₅ or the like.

The present invention is different from the prior art in that it uses a sputtering device equipped with a shutter mechanism that covers the entire substrate electrode, thereby reducing the surface area caused by the substrate being exposed to an atmosphere containing plasma during the pre-sputtering process. Suppresses the growth of oxide film,
The difference is that the dielectric thin film is formed after controlling the surface oxide film thickness using the shutter mechanism.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a sputtering apparatus used in carrying out the present invention, and FIG. 2 is a process diagram for forming a Ta ₂ O ₅ thin film for explaining the embodiment. The procedure for forming a thin film will be described below.

(Step 1) Vacuum chamber 4 is evacuated to 10 ^-6 Torr or less.

(Step 2) Next, the vacuum chamber 4 is filled with argon gas mixed with oxygen at a ratio of 0% to 50% as a sputtering atmosphere gas until a vacuum level of 10 ^-2 Torr to 10 ^-3 Torr is reached.

(Step 3) Thereafter, with the shutters A7 and B8 closed, a voltage is applied between the target electrode 5 and the substrate electrode 10 to generate discharge. This step 3 is called pre-sputtering and is for removing impurities such as moisture adsorbed on the surface of the target 6 and cleaning the surface of the target 6.

(Step 4) Then, by opening and closing the shutter B7 with the shutter A7 closed while maintaining the discharge, the time period during which the surface of the Si substrate is exposed to the atmosphere containing plasma is adjusted, and the surface of the Si substrate 9 is Control the thickness of the growing surface oxide film.

Figure 3 shows Ta ₂ O ₅ as the target and oxygen as 10
This figure shows the relationship between the opening time of shutter B and the thickness of the surface oxide film on the Si substrate when a discharge is generated in an argon atmosphere containing 5%. Needless to say, shutter A is in a closed state here. The initial value of the oxide film is the thickness of the natural oxide film existing on the surface of the Si substrate at the time the substrate 9 is loaded into the sputtering apparatus. Unlike conventional devices, the sputtering device used in this example is equipped with a shutter B8 that covers the entire substrate 9, so no surface oxide film grows during the pre-sputtering process (step 3), and as shown in this figure. Like, shutter B8
The thickness of the oxide film on the surface of the substrate 1 can be controlled by the opening time. On the other hand, since conventional sputtering equipment is not equipped with shutter B, in the conventional manufacturing method, the surface oxide film grows due to the substrate surface being exposed to an atmosphere containing plasma during the pre-sputtering process. However, it is impossible to control the film thickness as shown in FIG. Furthermore, since pre-sputtering is usually required for 30 minutes or more, conventional methods cannot avoid the growth of a surface oxide film of about 30 Å on the substrate surface before forming the thin film.

(Step 5) After controlling the thickness of the surface oxide film to a desired value in the pre-step 4, the shutter A7 is opened to form a dielectric thin film.

As mentioned above, in the conventional method, the process is
Step 1, Step 2, Step 3, and Step 5 are followed, and while a dielectric thin film must be formed on the Si substrate on which a surface oxide film of about 30 Å has grown, this method covers the entire Si substrate. The dielectric thin film is formed by controlling the surface oxide film thickness, which is made possible by using a sputtering device with a structural shutter mechanism.

Note that the influence of the thickness of the surface oxide film before forming the dielectric thin film on the process of forming the dielectric thin film will be explained here. FIGS. 1 to 3 sequentially show the process of forming a thin film of Ta ₂ O ₅ as a dielectric thin film on a Si substrate having a surface oxide film of 20 Å or less by sputtering. Here, 1 is a Si substrate, 11 is a surface oxide film, 12 is a primer oxide layer, 13 is a mixed layer of Si ₂ and Ta ₂ O 5, and 14 is a mixed layer of Si 2 and Ta 2 O ₅ .
Showing ₅ layers of Ta ₂ O. The surface oxide film 1 has a major effect on the leakage current and dielectric strength of thin film capacitors.
1, a plasma oxide film 12, and a mixed layer 13 of SiO ₂ and Ta ₂ O ₅ . As described later, in order to obtain a thin film capacitor with low leakage current and high dielectric strength, the interface between the Si substrate and the insulating layer must be formed with a plasma oxide layer.
It is necessary to consist of a mixed layer of SiO ₂ and Ta ₂ O ₅ . The role played by the surface oxide film in the formation of the plasma oxide layer and the mixed layer of SiO ₂ and Ta ₂ O ₅ will be explained below along with the process of forming the dielectric thin film.

FIG. 4 1 shows the state before the dielectric thin film is formed, and only the surface oxide film 11 exists on the surface of the Si substrate 1. FIG. 4 2 shows the state immediately after plasma is generated and the formation of Ta ₂ O ₅ is started on the substrate 1 in the above-mentioned state 1. Since the plasma contains ionized oxygen particles and ionized TaO ₅ particles, the ionized particles diffuse into the surface oxide film 11 during the initial formation process. And surface oxide film 1
Oxygen that has reached the interface between Si substrate 1 and Si substrate 1 reacts with Si substrate 1 to form plasma oxide layer 12 .
On the other hand, ionized Ta ₂ O ₅ has a slower diffusion rate than oxygen, so it does not reach the interface and is mixed with SiO ₂ .
A mixed layer 13 with Ta ₂ O ₅ is formed. Figure 4 3
shows the case where the formation continues. As the formation of the thin film progresses on the Si substrate 1, the concentration of oxygen arriving at the interface decreases due to diffusion, and the growth of the plasma oxide layer 12 finally stops. Also,
In the mixed layer 13 of SiO ₂ and Ta ₂ O ₅ , the ratio of Ta ₂ O ₅ increases as the formation progresses, and finally becomes uniform.
A state is reached in which a Ta ₂ O ₅ layer 14 is formed.

In this way, when forming Ta ₂ O ₅ on a Si substrate, the insulating layer consists of the plasma oxide layer 12, SiO ₂ and
It is composed of a mixed layer 13 with Ta ₂ O ₅ and a uniform Ta ₂ O ₅ layer 14.

If the surface oxide film on the substrate immediately before forming Ta ₂ O ₅ is sufficiently thin, a thin film will be formed according to the process described above, but if the surface oxide film is thick, changes will occur in the formation process. . FIGS. 1 to 3 show the process of forming a thin film when the surface oxide film is thick. In FIG. 5, only the surface oxide film 11 exists on the surface of the Si substrate 1 before formation. FIG. 5 shows the state immediately after plasma is generated and the formation of Ta ₂ O ₅ is started on the substrate in the above-mentioned state 1. In this case as well, in the initial process of formation, ionized particles diffuse in the surface oxide film 11, but because the surface oxide film 1 is thick, the concentration of oxygen reaching the interface between the surface oxide film 11 and the Si substrate 1 is low. The plasma oxide layer 12 is not formed. Therefore, the formation of the thin film proceeds with the mixed layer 13 of SiO ₂ and Ta ₂ O ₅ formed on the surface oxide film 11. FIG. 5 3 shows a case where the formation has further progressed, and the mixture layer 6 of SiO ₂ and Ta ₂ O ₅ is
It shows the process in which the ratio of Ta ₂ O ₅ increases and finally a uniform Ta ₂ O ₅ layer is formed. Therefore, the thin film has a structure in which a mixed layer 13 of SiO ₂ and Ta ₂ O ₅ and a uniform Ta ₂ O ₅ layer 14 are formed on a surface oxide film with many defects.

As shown in FIG. 4 and FIG. 5, the composition of the interface between the insulating layer and the Si substrate changes greatly depending on the thickness of the surface oxide film. Therefore, the effects of these compositional differences on the characteristics of thin film capacitors will be explained using quantitative data, and the usefulness of the manufacturing method of the present invention will be demonstrated.

Figure 6 shows that the surface oxide film has a thickness of 7 Å (sample 1).
on Si substrates of 20 Å (sample 2) and 26 Å (sample 3).
This is a comparison of the current-voltage characteristics of thin film capacitors formed using Ta ₂ O ₅ thin film as a dielectric thin film under the same manufacturing conditions. Surface oxide film thickness is 20Å
In Samples 1 and 2 below, the leakage current is less than 10 ^-8 A/cm ² when an electric field with a strength of 1 MV/cm is applied, whereas in Sample 3 with a surface oxide film thickness of 26 Å.
The leakage current increases rapidly to 10 ^-1 A/cm ² or more. In this way, the composition near the interface between the insulating layer and the Si substrate changes depending on the thickness of the surface oxide film, and controlling the surface oxide film to around 20 Å or less can reduce the leakage current of thin film capacitors. It turns out that it has a big effect.

Figure 7 shows a comparison of the frequency dispersion of the normalized capacitance, where the capacitance of the insulating layer is normalized to C _1M Hz at 1MHz, for the three types of thin film capacitors described in Figure 6. . In sample 3, which was formed on a Si substrate with a 26 Å surface oxide film, significant frequency dispersion was observed in the capacitance, indicating that many defects were present in the thin film. On the other hand, in samples 1 and 2, which were formed on Si substrates with a surface oxide film of 20 Å or less, no frequency dispersion was observed in the capacitance, and high-quality insulating layers with few defects were obtained. I understand that. In addition, the dielectric loss of sample 3 is 6%, while the dielectric loss of samples 1 and 2 is very small, less than 0.1%, which also shows that high quality insulating layers are obtained. . Furthermore, the dielectric constants of the insulating layers of Samples 1 and 2 are 16.5 to 17.0, and the dielectric constant of the two-layer insulating layer is 10.
A high dielectric constant can be achieved compared to the following.

As is clear from the results below, by controlling the thickness of the surface oxide film on the Si substrate before forming the dielectric thin film to about 20 Å or less using the manufacturing method of the present invention,
It becomes possible to control the composition of the interface between the insulating layer and the Si substrate,
A high-quality insulating layer with a high dielectric constant, low leakage current, high dielectric strength voltage, and low dielectric loss can be formed on a Si substrate. In addition, in the above example, the case where Ta ₂ O ₅ was used as the dielectric thin film was explained.
It goes without saying that the same effect can be obtained by controlling the surface oxide film thickness when Nb ₂ O ₅ is made of other materials such as BaTiO.

〔Effect of the invention〕

As explained above, according to the present invention, by controlling the film thickness of the surface oxide film on the Si substrate, it is possible to form a high quality plasma oxide film at the interface between the insulating layer and the Si substrate. A high-quality insulating layer with a high dielectric constant, low leakage current, high withstand voltage, and low dielectric loss can be formed on the surface.

[Brief explanation of drawings]

Figure 1 is a block diagram of the sputtering apparatus used in the present invention, Figure 2 is a process diagram explaining the manufacturing method of the present invention, and Figure 3 is the relationship between the opening time of shutter B and the surface oxide film thickness. FIG. 4 is an explanatory diagram of the thin film formation process when Ta ₂ O ₅ is formed on a Si substrate with a thin surface oxide film, and FIG. Figure 6 is an explanatory diagram of the thin film formation process when ₂ O ₅ is formed, and the surface oxide film thickness is 7 Å (sample 1), 20 Å (sample 2),
Figure 7 is a diagram comparing the current-voltage characteristics of a thin film capacitor with a Ta ₂ O ₅ dielectric thin film formed on a 26 Å (sample 3) Si substrate under the same sputtering conditions. , 2 and 3, and FIG. 8 is a diagram showing the cross-sectional structure of a general thin film capacitor. DESCRIPTION OF SYMBOLS 1...Si substrate, 2...Insulating layer, 3...Metal electrode, 4...Vacuum chamber, 5...Target electrode, 6...
...Target, 7...Shutter A, 8...Shutter B, 9...Substrate, 10...Substrate electrode, 11
...Surface oxide film, 12...Plasma oxide layer, 13
...Mixed layer of SiO ₂ and Ta ₂ O ₅ , 14...Ta ₂ O ₅
layer.

Claims (1)

[Claims] 1. In a method for manufacturing a thin film capacitor in which an insulating layer is a dielectric thin film formed by sputtering on a Si substrate, at least a shutter that covers the entire Si substrate is used, and the shutter is closed. A method for manufacturing a thin film capacitor, comprising the steps of sputtering, controlling the thickness of an ultra-thin surface oxide film on the surface of a Si substrate, and forming the dielectric thin film on the Si substrate. 2. The method for manufacturing a thin film capacitor according to claim 1, wherein the dielectric thin film is a Ta ₂ O ₅ thin film.