JPS63941B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for improving dielectric breakdown of an insulating film subjected to ion irradiation when ion implantation is used in the manufacture of semiconductor devices.

In recent years, ion implantation has been widely used in the manufacture of semiconductor devices. In particular, MOSIC
In manufacturing, when forming a source and drain using the gate electrode as a mask, or when doping a channel region to adjust the threshold voltage, the features of this method, such as the depth of impurity distribution and It is widely used because its concentration can be freely controlled and its lateral spread is small.

In such cases, the surface of the semiconductor wafer is usually partially provided with a shielding layer to prevent ion implantation, and the area where ion implantation is to be performed may also be covered with a thin insulating film.

As the shielding layer, photoresist, silicon dioxide (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), etc. are mainly used, but when such an insulator is irradiated with ions, charges are accumulated on its surface. , when the thickness is small, consideration must be given to dielectric breakdown.

In particular, when implanting ions into the channel region through the gate insulating film of a MOS transistor,
Since the gate insulating film is thin with a thickness of 1000 Å or less, dielectric breakdown is likely to occur due to charge accumulation. Furthermore, when a polycrystalline silicon layer (hereinafter referred to as poly-Si layer) is formed as a gate electrode and ion implantation is performed using this as a mask, the poly-Si layer may be electrically floating until the wiring layer is formed. Therefore, a large amount of charge will be accumulated here.
This also applies to floating gate structures used as non-volatile memories, and considering the situation in which leakage current tends to concentrate, it should be seen that the conditions are even more severe.

The reason why such charge accumulation has recently come to be considered a major problem is due to the increase in ion implantation current.

In other words, if the ion current is 1 mA or less as in the conventional case, the charge accumulated in the insulating film such as SiO ₂ will not flow on the surface activated by the injection, or it will flow at a level that does not cause permanent destruction. This did not pose any particular problem because the current was discharged by flowing through the insulating film to the substrate.

However, as the size of silicon ware used in recent years has increased, the speed of ion implantation has also increased, and as the ion current value has also increased, it has become necessary to promote discharge by some means. I'm getting old.

In view of the above circumstances, the present invention proposes a charge discharge method based on new knowledge obtained by the inventors of the present invention.

The inventors have found that by heat-treating a SiO ₂ film in a gas with a molecular structure containing nitrogen to convert its surface into silicon nitride, the leakage current that can flow without permanent destruction can be significantly increased. I found out what to do.

FIGS. 1 and 2 explain this phenomenon and a first embodiment that utilizes it.

500 Å on its surface, as shown in Figure 1a.
The semiconductor substrate 1 on which the SiO ₂ film 2 is formed is subjected to heat treatment at 1000° C. in a gas containing ammonia (NH ₃ ) to convert a part of the SiO ₂ film into a nitride layer 3,
A product in the state shown in FIG. 1b was obtained.

After forming a MOS diode by attaching an electrode with a diameter of 400 μm on the nitrided insulating film, the first
The results of measuring the leakage current as shown in Figure c are shown in Figure 2.

In FIG. 2, the time during which the above-described nitriding treatment was performed is plotted on the horizontal axis, and the treatment time O means that such heat treatment was not performed. The vertical axis is the leakage current value that caused permanent dielectric breakdown. As is clear from the figure, when nitriding treatment is performed at 1000℃ for 2 hours or more, it is possible to flow nearly two orders of magnitude larger current without causing dielectric breakdown than when no such treatment is performed at all. be.

In other words, even during ion implantation, a SiO ₂ film that has undergone this type of treatment can release charge at a rate nearly two orders of magnitude faster than one that is not subjected to such treatment, making it possible to perform ion implantation with a correspondingly large current. Make it.

As a nitriding method for silicon oxide film,
A method such as that described in Publication No. 50-147877, that is, a method of heat treatment in an atmosphere containing ammonia or hydrazine (N ₂ H ₄ ), can be used. , the surface can be nitrided.

In the above-mentioned Japanese Patent Application _Laid -open No. 147877,
Although a technique for forming a Si ₃ N ₄ layer has been disclosed,
There is no mention of performing ion implantation after forming the Si ₃ N ₄ layer, nor does it hint at the possibility.

Furthermore, while the invention described in the above-mentioned publication relates to a MNOS type nonvolatile memory and is aimed at a device for storing electric charge, the present invention is different from both in that its purpose is to discharge electric charge. It should be noted that there is a large difference between

Second and third embodiments of the invention will now be described. In the case of the structure shown in FIG. 3a, there is a SiO ₂ film 2 on a semiconductor substrate 1, and a conductor layer 5 made of metal or poly-Si is partially formed on the SiO ₂ film. If ion implantation is performed when the SiO 2 film is ion-implanted, the charges injected into the conductor layer can move freely within the conductor layer 5, causing electric field concentration at the thinnest part of the SiO ₂ film, resulting in permanent dielectric breakdown. is likely to occur.

Even in such a case, as shown in Figure 3b,
By forming the nitride layer 3 on the surface of the SiO ₂ film 2 and forming the conductor layer 5 thereon, the dielectric breakdown strength is improved.

Furthermore, in the case of the structure shown in FIG. 4a, that is, there is a SiO ₂ film 2 on a semiconductor substrate 1, and a conductor layer 5 is partially formed on it, the conductor layer is made of SiO ₂ When covered by layer 6, dielectric breakdown is more likely to occur than in the case of FIG. 3, since the accumulated charges cannot escape through the surface of activated SiO ₂ layer 6.

In such a case, as shown in Figure 4b,
By forming the nitride layer 3 on top of the SiO ₂ layer 2, the breakdown strength is improved.

As described above, according to the present invention, it is possible to perform ion implantation with a large current into a semiconductor substrate partially or completely covered with an oxide layer.

[Brief explanation of the drawing]

1 to 4 are diagrams for explaining the present invention, in which 1 is a semiconductor substrate, 2 and 6 are SiO ₂ layers, 3 is a silicon nitride layer, 4 is an Al electrode, and 5 is a conductor layer.

Claims (1)

[Scope of Claims] 1. A semiconductor substrate having a silicon dioxide layer formed on its surface is heat-treated in an atmosphere containing nitrogen atoms to nitride the surface of the silicon dioxide layer, and then a silicon dioxide layer with the surface nitrided. A method of manufacturing a semiconductor device, comprising the step of implanting ions into the semiconductor substrate through the semiconductor substrate. 2. A semiconductor substrate with a silicon dioxide layer formed on its surface is heat-treated in an atmosphere containing nitrogen atoms to nitride the surface of the silicon dioxide layer, and then a conductive layer is selectively formed on the silicon dioxide layer whose surface has been nitrided. 2. The method of manufacturing a semiconductor device according to claim 1, further comprising the step of forming a silicon dioxide layer having a nitrided surface and then implanting ions into the semiconductor substrate through the silicon dioxide layer whose surface is nitrided. 3. A semiconductor substrate with a silicon dioxide layer formed on its surface is heat-treated in an atmosphere containing nitrogen atoms to nitride the surface of the silicon dioxide layer, and then a conductive layer is selectively formed on the silicon dioxide layer whose surface has been nitrided. and then forming an insulating layer covering the conductive layer, and then implanting ions into the semiconductor substrate through the surface-nitrided silicon dioxide layer. 2. A method for manufacturing a semiconductor device according to item 1. 4 Heat treatment performed on a semiconductor substrate with a silicon dioxide layer formed on the surface in an atmosphere containing nitrogen atoms is performed by heating the semiconductor substrate with a silicon dioxide layer formed on the surface in a plasma atmosphere containing excited nitrogen atoms. 2. The method of manufacturing a semiconductor device according to claim 1, wherein the method is carried out by exposing.