JPH0114698B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas plasma etching method, and a method for controlling excessive etching or etching of a substrate by changing the etching rate near the end of desired etching.

Gas plasma etching is often used as a means of patterning thin films of silicon compounds or films of metals such as chromium. In particular, when forming patterns on silicon compounds during the manufacturing process of semiconductor integrated circuits, the adhesion between the photoresist as an etching mask and the material to be etched has a greater influence on pattern formation accuracy than etching with conventionally known chemicals. It has become indispensable as a manufacturing technology for semiconductor integrated circuits because of its low degree of etching and the simple etching of polycrystalline silicon films or silicon nitride films (Si ₃ N ₄ ). However, in highly integrated circuit elements, 3~
It is necessary to form patterns of silicon compounds, etc., with a width of 4 μm or less, and the pattern accuracy is within the standard deviation value when etching a thin film of about 0.5 μm.
A thickness of about 0.1 μm is required, which is extremely difficult even in normal gas plasma etching.

For example, Fig. 1 shows that Freon (CF ₄ ) gas contains 5
Using a gas mixed with % oxygen (O ₂ ), the gas pressure was 0.4 Torr, the plasma discharge output was 200 Watts,
A thermal oxide film (SiO ₂ ) was grown on the silicon wafer at a temperature of 130°C inside the reaction tube, and a 0.5 μm thick polycrystalline silicon film was grown using this as a base, and the negative photoresist was etched. FIG. 4 is a correlation diagram between the etching time and the amount of change in pattern dimensions of a polycrystalline silicon film when etching is performed using the mask. In Figure 1, 1a is the pattern dimension of the photoresist, 1b is the point at which etching of the 0.5 μm thick polycrystalline silicon film has been completed, and 1c is the point at which etching has proceeded for an additional 10 seconds beyond the etching completion point 1b. . FIG. 2 is a sectional view showing the progress of etching in correspondence with points 1a, 1b, and 1c in FIG. The state of 1c is shown.

In FIG. 2, 2 is an etching mask made of photoresist, 3 is a polycrystalline silicon film, 4 is an underlying thermal oxide film, and 5 is a silicon substrate. 1 in Figure 1
From a to 1b shows the etching time and dimensional change depending on the film thickness, and the dimensional change is 0.9 μm with an etching time of 58 seconds, but from 1b to 1c
shows a dimensional change of 0.8 μm after only 10 seconds of excessive etching.

Based on these characteristics, when determining the etching end point, it is necessary to take into consideration the non-uniformity of the film thickness of the polycrystalline silicon film 3 or the non-uniformity between cycles.
It is very difficult to achieve the above-mentioned required precision in forming pattern dimensions. The reason why the dimensional change in Figure 1 has a large inflection point near the end of etching is thought to be due to the phenomenon that when the area of the sample to be etched in the reaction system decreases, the etching rate increases approximately in proportion to the area of the sample. It will be done. In other words, since the exposed area of the material to be etched is limited to the cross section of the pattern from the end point of etching, excessive etching appears as a phenomenon of rapid side etching.

As a characteristic that supports this, FIG. 3 shows the etching rate depending on the area dependence of various silicon compounds. In FIG. 3, line 6 is a polycrystalline silicon film, 7 is a single crystal silicon film, 8 is a silicon nitride film (Si ₃ N ₄ ),
9 is a silicon thermal oxide film (SiO ₂ ). The greater the silicon compound has a higher reaction rate in Freon gas plasma, the more the area dependence of the etching rate appears even when the area to be etched is small.

This invention focuses on the above-mentioned proportional relationship between the area of the sample to be etched and the etching rate, and reduces the etching rate of the entire reaction system near the end of etching the film to be etched, thereby reducing the dimensional change near the end of etching shown in FIG. The purpose is to relax the angle of the inflection point 1b of the quantity. If the etching rate of the entire reaction system is reduced near the end of etching,
The etching amount of the material to be etched per unit time decreases, and relatively reaches the inflection point 1 shown in FIG.
The angle of the dimensional change curve b becomes larger, and a large margin is obtained in terms of etching time control of the amount of dimensional change due to side etching after the etching end point. Furthermore, it is possible to alleviate the influence of variations in etching dimensions caused by non-uniform etching progress within a silicon wafer or non-uniform film thickness of a material to be etched. In the continuous etching process, the etching rate of the entire reaction system can be changed by the following method depending on the characteristics of gas plasma etching. (1) changes the discharge output. (2)
The amount or type of gas introduced as an etchant is changed. (3) Change the degree of vacuum in the reaction system. Further, detection near the end of etching can be easily confirmed by the following method. (1) Determine the progress of etching on the wafer surface by visual observation. (2)
A photoelectric element detects light of a specific wavelength generated by a plasma chemical reaction, and judgments are made based on the amount of light.

Next, a specific embodiment of this invention will be shown. This method is an example in which the etching rate of the entire reaction system is reduced near the end of etching by reducing the discharge output. Same conditions as shown in Figure 1, i.e. the introduced gas is Freon (CF ₄ ).
A polycrystalline silicon film with a thickness of 0.5 μm is etched using a mixed gas containing 5% oxygen at a gas pressure of 0.4 Torr and a temperature inside the reaction tube of 130° C. The initial discharge output was 200 watts _, and near the end of etching was detected by a photoelectric element _, the plasma discharge light (wavelength
When the discharge output (nearly 9000 Å) decreases to 70% of the maximum value, the discharge output is reduced to 80 watts. FIG. 4 is a correlation diagram between the etching time and the amount of change in pattern dimensions of the polycrystalline silicon film in this method. In FIG. 4, 4a is the pattern size of the photoresist used as an etching mask, 4b is the point at which the discharge output was reduced, and 4c is the etching end point. The amount of dimensional change after the etching end point 4c is reduced to 1/4 per unit time compared to FIG. 1.

In the above embodiments, a polycrystalline silicon film was used as the material to be etched, but as can be seen from FIG. 3, the same effect can be obtained with a single crystal silicon film or a silicon nitride film.

In this way, when this method is used, the progress of partial over-etching due to non-uniform etching completion time of the material to be etched in the reaction system is alleviated,
It is possible to further reduce the amount of dimensional change in the material to be etched due to etching from the photoresist pattern dimension, and it is useful as a pattern processing technique for semiconductor integrated circuits using ultra-fine patterns.

It goes without saying that the present invention can also be applied to etching using plasma of Freon gas or a mixed gas of Freon gas and other inert gases such as helium and argon.

[Brief explanation of drawings]

Figure 1 is a correlation diagram between the amount of change in pattern dimension of a polycrystalline silicon film and etching time in conventional gas plasma etching, Figure 2 is a cross-sectional view showing the progress of gas plasma etching, and Figure 3 is a graph of freon etching of various silicon compounds. FIG. 4 is a correlation diagram between etching rate and area dependence of a material to be etched in gas plasma. FIG. 4 is a correlation diagram between pattern size and etching time in etching a polycrystalline silicon film according to an embodiment of the present invention. In the figure, 2 is an etching mask, 3 is a polycrystalline silicon film, 4 is a thermal oxide film, and 5 is a silicon substrate. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. In a method of etching a material to be etched with gas plasma, the material to be etched is a polycrystalline silicon film, a single crystal silicon film, a silicon nitride film, etc. whose etching rate has a relatively large dependence on area; A gas plasma etching method characterized in that the energy state within the etching system or the amount of etching species is reduced in order to keep the change in etching rate approximately constant after the end of etching.