JPH0758788B2 - Method for manufacturing field effect transistor - Google Patents

Description

The present invention relates to a method for manufacturing a field effect transistor, and more particularly to a method for manufacturing a gate oxide film having improved radiation resistance.

[Conventional technology]

The formation of the gate oxide film of the conventional field effect transistor is
A silicon substrate is placed in a quartz tube, dry oxygen or a mixed gas of oxygen and hydrogen is supplied as an oxidizing atmosphere, and heating is performed at 900 to 1200 ° C. using a heating wire. Then, in order to improve the radiation resistance of the gate oxide film, after the oxide film is grown, a heat treatment is performed in an atmosphere of oxygen, argon, nitrogen or the like to reduce the hole trap density in the oxide film,
It reduces the damage of oxide film (accumulation of fixed positive charge and generation of interface state) due to radiation irradiation.

Recently, it was reported that thermal nitridation by lamp heating was performed in a nitrogen atmosphere after forming a gate oxide film, and the amount of interface states generated during irradiation was reduced.
aresan) et al. IEEE Transactions on Nieuclear Science (IEEE Transactions
on Nuchlear Science) NS-33, December 1223 (1986)
Has been reported to.

[Problems to be Solved by the Invention]

When the gate oxide film of a field effect transistor is exposed to ionizing radiation such as electron beams, X-rays, and gamma rays, an interface level is generated at the interface between the semiconductor substrate and the gate oxide film. Further, positive charges are captured by the hole traps inside the oxide film, and fixed positive charges are accumulated. Generation of interface states and accumulation of fixed positive charges inside the gate oxide film changes the threshold voltage of the field effect transistor. When the variation exceeds the allowable amount of the semiconductor device, the function of the device is impaired.

Further, if hydrogen is contained in the gate oxide film, the amount of generated interface states and hole traps is significantly increased and the radiation resistance is deteriorated. Adoption of process is necessary. Therefore, it is said that dry oxygen containing no hydrogen is more preferable than the mixed gas of oxygen and hydrogen as the oxidizing atmosphere. However, dry oxygen is inevitably mixed with hydrogen when the oxide film is formed, and hydrogen is taken into the oxide film.

In this way, the gate oxide film formed by the conventional manufacturing method is
In the growth process, since hydrogen is taken into the inside, a large amount of interface states are generated and fixed positive charges are accumulated at the time of irradiation with radiation, and the radiation resistance of the gate oxide film is deteriorated. Several models have been proposed for the role of hydrogen in the generation of interface states and hole traps, but the outline is as follows.

Hydrogen mixed in the film during the formation of the oxide film exists in the form of Si-H or Si-OH. Electron-hole pairs are generated in the oxide film by the incidence of ionizing radiation, and drift movement is caused by the electric field in the film. In this case the holes were cut Si-H, a Si-OH bond, to release hydrogen in the form of H ₂ or H ⁺ in the film. When this hydrogen reaches the interface between the substrate silicon and the oxide film by diffusion or drift, the interface level is formed by the interaction with the interface, and the hole trap that causes the fixed charge accumulation is formed.

There is also a theory that holes or hydrogen (H ₂ or H ⁺ ) cuts Si—H and Si—OH bonds at the silicon / oxide film interface to directly form an interface level. In any case, it is known that hydrogen contained in the oxide film contributes to the generation of the interface state.

As described above, since the gate oxide film formed by the conventional method for manufacturing a field effect transistor takes in hydrogen during the growth process, a large amount of interface states and fixed charges are generated when exposed to radiation, so that the radiation resistance is high. It has the drawback of being reduced.

It is an object of the present invention to provide a method for manufacturing a field effect transistor having a gate oxide film with improved radiation resistance by eliminating the above drawbacks.

[Means for Solving the Problems]

A method of manufacturing a field effect transistor according to the present invention is a method of manufacturing a field effect transistor in which a gate oxide film is formed on a silicon substrate and then lamp heating is performed. The lamp heating of the silicon substrate is performed in a fluorine compound gas atmosphere. It is done in.

The gate oxide film formed by this method contains fluorine. Fluorine is known to exist mainly in the form of Si-F bonds, and few -O-F bonds are known. The Si-F bond is in an extremely stable state, and even if it is decomposed by irradiation with radiation, fluorine is easily chemically recombined in a highly active state and radiation damage does not occur. Further, even if hydrogen is present in the oxide film, decomposition of protons (H ⁺ ) and migration to the SiO ₂ / Si interface can be suppressed if fluorine having a high electrocathode degree is closely bound to the oxide film. For the same reason, if Si—F is formed at the SiO ₂ / Si interface, the interface state, that is, the generation of Si dangling bonds (Si ·) can be suppressed.

In this way, the gate oxide film is subjected to a lamp heat treatment in a gas atmosphere of a fluorine compound, and fluorine is incorporated into the film, so that the generation of interface states during radiation irradiation and the fixed positive charge accumulation are suppressed. The property is improved. However, if the atmospheric gas at the time of heating contains carbon, carbon enters the oxide film and the withstand voltage of the oxide film deteriorates. Therefore, the use of a fluorine compound containing carbon is not preferable.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of a lamp heater used in the first embodiment of the present invention.

First, a silicon substrate is heated in a dry oxygen atmosphere at 950 ° C. to form a gate oxide film having a thickness of 250 Å by the same operation as the conventional manufacturing method. Next, this silicon substrate 3 is first
Insert into the quartz tube 1 of the lamp heater shown in the figure using the quartz tray 5, and supply sulfur hexafluoride (SF ₆ ) 4 which decomposes at 500 ° C as an atmospheric gas, and at 1100 ° C by the halogen lamp 2. Heat for 30 seconds. By this heating, fluorine is incorporated into the gate oxide film.

The depth distributions of fluorine, silicon, and oxygen concentrations in the gate oxide film after lamp heating were obtained by Auger spectroscopy. The results are shown in FIG.

It can be seen from FIG. 2 that fluorine is distributed at a high concentration especially at the interface between the substrate silicon and the gate oxide film.

Fluorine in the oxide film mainly exists in the form of Si-F. Si-F
The bond is stable, and fluorine has high electronegativity and suppresses proton (H ⁺ ) transfer. Therefore, it prevents hydrogen from participating in the interface state and the accumulation of positive charges.

Thus, in the gate oxide film containing fluorine, the generation of interface states and the accumulation of fixed positive charge are suppressed. Therefore, the field-effect transistor using such a gate oxide film has a small amount of threshold variation when irradiated with radiation and has high radiation resistance.

FIG. 3 shows the fluctuation amount ΔV _T of the threshold voltage and the absorbed gamma dose when an N-channel field effect transistor having a gate oxide film formed by the manufacturing method of the first embodiment of the present invention is irradiated with gamma rays. Shows the relationship. However, the gate bias during irradiation is 5V. For comparison, the case of an N-channel transistor manufactured by the conventional method is shown in FIG.

The variation amount of the threshold voltage is the sum of the component A caused by the generation of the interface state and the component B caused by the fixed charge accumulation. The component B based on the fixed charge shifts in the negative direction as the absorbed dose increases. On the other hand, in the N-channel transistor, the acceptor type of the negatively charged interface state increases as the dose increases, so that the component A based on the interface state shifts in the positive direction.
This sum ΔV _T shifts in the negative direction in the apparently low dose region, and shifts in the positive direction as the dose increases.

As shown in FIGS. 3 and 4, the threshold voltage fluctuation amount of the transistor manufactured in the first embodiment has a higher interface level than the threshold voltage fluctuation amount of the transistor manufactured by the conventional manufacturing method. Both the component A based on the fixed charge and the component B based on the fixed charge have small values, and it can be seen that the effect of the present embodiment is particularly large in the component A based on the interface state.

In this way, by introducing fluorine into the gate oxide film, generation of interface states and fixed positive charge accumulation due to radiation irradiation,
In particular, the generation of interface states can be effectively suppressed and radiation resistance can be improved.

In addition, heating with a lamp annealer can heat only the substrate in a short time, so there is less generation of hydrogen fluoride (HF) due to the reaction between sulfur hexafluoride and hydrogen mixed in the quartz furnace, and less furnace erosion. Has the advantage. Further, since sulfur hexafluoride is an extremely stable and harmless substance at room temperature, it is easy to store.

Next, a second embodiment of the present invention will be described.

First, in the same manner as in the first embodiment, 950 ° C. in a dry oxygen atmosphere.
Then, a 250 Å thick gate oxide film is formed on the silicon substrate. Then, using a lamp heater, a fluorine compound that decomposes at 1200 ° C. or lower, for example, nitrogen trifluoride gas that decomposes at 500 ° C. is supplied as an atmospheric gas, and heated at 1000 to 1100 ° C. for 50 seconds. As a result, fluorine is mainly incorporated into the gate oxide film in the form of Si-F, as in the first embodiment.

The Si-F bond is a stable bond and easily recombined even if decomposed by irradiation with radiation. Furthermore, the fluorine atom, which has a high electronegativity, also functions to suppress the movement of protons (H ⁺ ) generated in the vicinity. Therefore, the accumulation of fixed charges and the generation of interface states are greatly reduced.

In this way, by heating the lamp in a nitrogen trifluoride atmosphere to introduce fluorine into the gate oxide film, generation of interface states and fixed positive charge accumulation during irradiation of radiation are suppressed, and the resistance of the film is improved. Radioactivity can be improved. Nitrogen trifluoride is also very stable in the greenhouse and therefore easy to store.

〔The invention's effect〕

As described above, according to the present invention, after forming a gate oxide film on a silicon substrate and performing lamp heating in a gas atmosphere of a fluorine compound, a field effect transistor having improved radiation resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a lamp heater used in the first embodiment of the present invention, and FIG. 2 is a diagram showing distributions of fluorine, silicon and oxygen in a gate oxide film formed in the first embodiment, FIG. 3 and FIG. 4 are diagrams showing the relationship between the absorbed gamma dose and the threshold voltage fluctuation amount of the field effect transistor manufactured according to the first embodiment and the conventional example. 1 ... Quartz tube, 2 ... Halogen lamp, 3 ... Silicon substrate, 4 ... Sulfur hexafluoride, 5 ... Quartz tray, 6 ...
Container, 7 ... Shutter.

Claims (1)

[Claims] 1. A method of manufacturing a field effect transistor in which a gate oxide film is formed on a silicon substrate and then lamp heating is performed. The lamp heating of the silicon substrate is performed in a fluorine compound gas atmosphere. Effect transistor manufacturing method.