JPH0371792B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a gate insulating film in a thin film transistor.

In recent years, research on forming thin film transistors on insulating substrates has been actively conducted. This technology is used to create active matrix panels that use inexpensive insulating substrates to create thin displays, three-dimensional integrated circuits that form active elements such as transistors on a regular semiconductor circuit, and inexpensive, high-performance images. It is expected to have many applications, including sensors and high-density memory.

When thin film transistors are applied to these fields, the required characteristics are generally classified into the following two types.

(1) A sufficiently large current can flow between the source and drain when the thin film transistor is turned on.

(2) When the thin film transistor is turned off, as little current as possible should flow between the source and drain.

(1) relates to the characteristics required when the thin film transistor is in the ON state. When applying thin film transistors to any field,
The current flowing between the source and drain during the ON state (hereinafter referred to as ON current) is required to be sufficiently large. The ON current determines the upper operating frequency of the entire system. This is because the operating speed of the thin film transistor is determined by the time constants of charging and discharging various capacitances. Therefore, in order to increase the upper limit frequency of operation, it is necessary to sufficiently increase the ON current and decrease the time constant for charging and discharging the capacitor. In the saturation characteristic region of a thin film transistor, the ON current ION is given by the following equation.

ION=COXμW/L(VGS−Vth) ^2where COX is the capacitance of the gate insulating film per unit area, μ is the carrier mobility, L and W are the channel length and channel width of the thin film transistor, respectively, and V _G is the gate voltage. , Vth is the threshold voltage. COX is determined by the thickness of the gate insulating film, and L and W are determined by the required degree of integration and patterning technology. Further, V _G is determined by the system drive voltage. Therefore, in order to increase the ON current, it is clear from the above equation that the mobility μ must be increased and the threshold voltage Vth must be decreased. In general, thin film transistors have a frequency of several MHz.
In order to satisfy this requirement, the mobility μ must be 10cm ² /
It is necessary that the voltage is Vsec or more. To achieve this, polycrystalline silicon and other materials are considered to be promising semiconductor thin film materials. Furthermore, in order to reduce Vth, it is necessary to reduce the density of interface states existing at the interface between the semiconductor thin film and the gate insulating film. The interface state density is closely related to the method of forming the gate insulating film. Generally, the gate insulating film is formed by chemical vapor deposition (hereinafter referred to as CVD) or physical vapor deposition (hereinafter referred to as PVD). It is known that when silicon dioxide is formed, the interface state density increases, and when silicon dioxide formed by thermal oxidation of silicon is used as a gate insulating film, the interface state density decreases. Therefore, in order to reduce Vth and achieve good transistor characteristics, it is desirable to thermally oxidize a silicon thin film to form silicon dioxide and use this as a gate insulating film.

(2) is a characteristic required when the thin film transistor is in the OFF state. Thin film transistor
The current flowing between the source and drain in the OFF state (hereinafter referred to as OFF current) is required to be as small as possible. The OFF current determines data retention characteristics, quiescent current, lower operating frequency, etc.

Therefore, it is essential to sufficiently reduce the OFF current of the thin film transistor. The OFF current IOFF of the thin film transistor is determined by the resistance value of the semiconductor thin film, and is expressed by the following equation.

IOFF=dW/PLVD where d is the thickness of the semiconductor thin film, P is the specific resistance of the semiconductor thin film, and VD is the drain voltage. Also L,
W is as described above. As can be seen from this equation, it can be seen that in order to reduce the OFF current, the thickness d of the semiconductor thin film can be reduced. Further, the film quality of the semiconductor thin film varies greatly depending on the thickness of the formed film, and therefore the specific resistance P of the semiconductor thin film changes with the film thickness d. In other words, the smaller d is, the more P
becomes large, and according to the results of experiments conducted by the present applicant, the relationship IOFF∝d ⁿ (n≈2.5) has been confirmed. Therefore, a decrease in d greatly contributes to a reduction in IOFF. Furthermore, reducing d also contributes to improving the ON characteristics. That is, when the film thickness d decreases, the absolute amount of space charge in the semiconductor thin film decreases, so the threshold voltage Vth decreases, and the ON characteristics improve significantly.

To summarize the above, in order to realize a thin film transistor with excellent ON and OFF characteristics, it is necessary to thermally oxidize a silicon thin film formed as thin as possible to form silicon dioxide, and use this as a gate insulating film. It can be said that there is.

However, with such conventional methods, the manufacturing process margin is small, making it difficult to stably manufacture transistors using sufficiently thin silicon thin films. Hereinafter, with reference to FIG. 1, the drawbacks of such a conventional thin film transistor manufacturing method will be described.

The final thickness t ₁ of the silicon thin film is t ₁ = t ₀ =
Since it is given by 0.45tox, in this case t ₁ ≒ 800
It becomes Å.

FIG. 1 shows a conventional method of manufacturing a thin film transistor. As shown in FIG. 1a, a silicon thin film 102 is deposited on an insulating substrate 101. As shown in FIG. At this time, for the reasons mentioned above, the thickness of the silicon thin film 102 (referred to as t ₀ ) is formed to be small (for example, t ₀ =1500 Å). Next, as shown in FIG. 1b, the silicon thin film is oxidized in a high temperature oxygen atmosphere, and silicon dioxide 1
Form 03. The film thickness tox of this silicon dioxide 103 is, for example, tox=1500 Å. Next, as shown in FIG. 1c, the formed silicon dioxide is used as a gate insulating film, and a gate electrode 104 is formed thereon.

Thereafter, impurities are introduced into the silicon thin film by ion implantation to form a source region 105 and a drain region 106. Next, as shown in FIG. 1d, an interlayer insulating film 107 is deposited, contact holes are opened, and a source electrode 108 and a drain electrode 109 are formed.

FIG. 2 shows typical characteristics of the thin film transistor manufactured in this manner. The horizontal axis of the graph is the gate voltage VG with respect to the source, and the vertical axis is the common logarithm value of the drain current ID. Drain voltage is VD = 4V, transistor size is L = 20μm,
W=10 μm. It can be seen that the OFF current is still large and the threshold voltage Vth is also high. In order to further improve this characteristic, it would be sufficient to reduce the final silicon thin film thickness t ₁ , but this is actually difficult to achieve for the following reasons. In other words, when forming a gate oxide film by thermal oxidation, the silicon thin film is exposed to high temperatures (e.g. 1100°C).
As the material undergoes heat treatment, polycrystalization progresses and crystal grains grow larger. As a result, asperities on the surface of the silicon thin film become significant, and preferential oxidation of grain boundaries is likely to occur particularly when the silicon thin film is thin. Therefore, the vicinity of the crystal grain boundaries of the silicon thin film becomes silicon dioxide, carrier mobility is significantly reduced, and characteristics as a transistor cannot be obtained. According to the results of experiments conducted by the present applicant, it has been found that the final thickness of the silicon thin film is required to be 500 Å or more. In addition, the thickness of silicon dioxide formed by thermal oxidation of a silicon thin film has large variations during manufacturing. For example, when tox = 1500 Å is set, ±200 Å
A margin of approximately 100 Å must be provided. Furthermore, considering the variation in the thickness of the silicon thin film during deposition, the final silicon thin film thickness t ₁ is approximately
700Å or more is required.

Therefore, the characteristics shown in FIG. 2 are almost at the limit, and it is difficult to improve the characteristics further with the current process.

The present invention aims to eliminate these drawbacks, and its purpose is to further reduce the thickness of the silicon thin film without deteriorating the interface between the silicon thin film and the gate insulating film, thereby significantly improving transistor characteristics. The purpose is to improve. In order to achieve this, in the present invention, in a method for manufacturing a thin film transistor including a silicon thin film and a silicon dioxide film serving as a gate insulating film, a silicon dioxide film is deposited from the outside on the silicon thin film, and then thermally oxidized. Provided is a method for manufacturing a thin film transistor, characterized in that a gate insulating film is formed by performing the following steps. Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 3 shows a method for manufacturing a thin film transistor according to the present invention. As shown in Figure 3a,
A silicon thin film 302 is deposited on an insulating substrate 301 . The film thickness is, for example, t ₀ =500 Å.

Next, as shown in FIG. 3b, a silicon dioxide film 30
3 is deposited on the entire surface. The film thickness is, for example, toxo
=1300Å. The silicon dioxide film at this time may be formed by any method as long as it is not formed by oxidizing a silicon thin film but is directly deposited from the outside. For example, reduced pressure
CVD methods such as CVD, normal pressure CVD, plasma CVD,
Alternatively, PVD methods such as spatsuta may be used. Next, as shown in Figure 3c, thermal oxidation treatment is applied to form a silicon dioxide film with a thickness of 30 mm
Grow 4. The increase in the thickness of the silicon dioxide film due to this is, for example, 200 Å, and the final thickness of the gate insulating film is tox = 1500 Å. Due to this thermal oxidation, the final thickness of the silicon thin film is t ₁ =
It becomes 400Å. The subsequent manufacturing process is exactly the same as the conventional method, and as shown in FIG.
6 and a drain region 307 are formed. Next, after depositing an interlayer insulating film 308, contact holes are opened, and a source electrode 309 and a drain electrode 310 are formed.

FIG. 4 is a graph showing the characteristics of the thin film transistor manufactured by the above manufacturing method in comparison with the characteristics of the conventional thin film transistor shown in FIG. The horizontal axis of the graph is the gate voltage to the source
V _G and the vertical axis is the common logarithm value of the drain current _ID . The drain voltage is V _D =4V, and the transistor size is L=20 μm and W=10 μm. The solid line graph shows the characteristics of the transistor according to the present invention, and the broken line graph shows the characteristics of the conventional transistor.

According to the manufacturing method of the present invention, the thickness of the final silicon thin film is 1/2 that of the conventional method, so it is possible to
The current is reduced to 1/5 to 1/6, and the ON characteristics are also significantly improved due to the lower Vth.

That is, the overall transistor characteristics are significantly improved, and the great effect of the present invention can be understood.

A feature of the present invention is that, as shown in FIGS. 3b and 3c, silicon dioxide is deposited externally and then thermally oxidized. The significance of adopting such a manufacturing method and its effects will be described below.

First of all, the processing time for thermal oxidation can be significantly shortened compared to conventional manufacturing methods. This will be explained with reference to FIG. FIG. 5 is a graph showing the relationship between the oxidation time T and the grown film thickness t of silicon dioxide when a silicon thin film is thermally oxidized at a temperature of 1100°C. In the conventional method, a thermal oxidation time of 90 minutes is required to obtain t=1500 Å, but in the present invention, for example, according to the above example, a silicon dioxide film of 1300 Å is initially deposited from the outside. Therefore, it can be seen that it takes only 27 minutes of thermal oxidation time to grow to 1500 Å. Therefore, asperities on the surface of the silicon thin film are suppressed, and even if the silicon thin film is made even thinner than before, sufficient carrier mobility can be obtained, and good transistor characteristics can be achieved. Furthermore, in the present invention, since the surface of the silicon thin film is coated with silicon dioxide in advance during thermal oxidation, the development of asperities is more likely to be suppressed. Because of these effects, it has been confirmed through experiments by the applicant that the present invention can achieve a final silicon thin film thickness of 300 Å or less.

Second, in connection with the above point, the thickness of the silicon thin film that is oxidized to silicon dioxide is small. In the conventional method, about 700 Å of silicon thin film reacts with oxygen to form silicon dioxide, whereas in the present invention, only about 100 Å of silicon thin film is consumed. Therefore, it becomes very easy to control the thickness of the final silicon thin film. As mentioned above, transistor characteristics are very sensitive to the thickness of the silicon thin film, so the present invention dramatically improves the uniformity and reproducibility of transistor characteristics.

Thirdly, the interface between the silicon thin film and the silicon dioxide film is maintained in the same state as that achieved by conventional thermal oxidation alone. Said first
In order to achieve the second effect, it would be possible to omit the thermal oxidation step in the present invention, but in reality, this would reduce the interface quasi existing at the interface between the silicon thin film and the silicon dioxide film. Since the density of potentials increases significantly, the threshold voltage Vth of the transistor increases significantly, and the transistor characteristics, especially the ON characteristics, are significantly oxidized. In contrast, in the present invention, since the interface between the silicon thin film and the silicon dioxide film is newly formed by thermal oxidation, the interface state density is small, and the interface state is not caused only by conventional thermal oxidation. It can be said that they are completely equivalent. It has been confirmed through experiments by the applicant that the silicon dioxide film thickness required by thermal oxidation to improve the interface is 100 to 200 Å.

Fourthly, variations in the thickness of a silicon dioxide film deposited directly from the outside on a silicon thin film can be corrected by a thermal oxidation process. If there is variation in the thickness of silicon dioxide deposited from the outside, a thick thermal oxide film will grow at a thin film thickness, and a thin thermal oxide film will grow at a thick film thickness, resulting in the final gate insulation The variation in film thickness becomes smaller than at the initial stage. This can be easily understood from the graph in FIG. The main effect of the thermal oxidation process in the present invention is to reduce the interface state density as described in Section 3, but it also has the effect of uniformizing the thickness of the gate insulating film as described here. It also has the important effect of The thickness of the gate insulating film is an important parameter that determines transistor characteristics, and due to this effect, the thickness of the gate insulating film of the thin film transistor according to the present invention can be controlled very accurately.

Note that the various numerical values used in the embodiments of the present invention described above are set according to the purpose and margin, and do not have absolute meaning.

As described above, the present invention has the excellent effect of realizing thin film transistors having excellent characteristics with good reproducibility and good uniformity.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional method for manufacturing a thin film transistor. FIG. 2 is a graph showing typical characteristics of conventional thin film transistors. FIG. 3 is a diagram showing a method for manufacturing a thin film transistor according to the present invention. FIG. 4 is a graph showing the characteristics of the thin film transistor according to the present invention in comparison with the conventional characteristics. FIG. 5 is a graph showing oxidation time and silicon dioxide growth film thickness used to explain the present invention.

Claims (1)

[Claims] 1. A method for manufacturing a thin film transistor, comprising the steps of: forming a non-single crystal silicon thin film on a substrate; depositing an insulating film on the non-single crystal silicon thin film; and then thermally oxidizing the gate oxide film; 1. A method for manufacturing a thin film transistor, comprising the step of forming a thin film transistor.