JP3485421B2 - Method for etching silicon nitride film - Google Patents

Description

BACKGROUND OF THE INVENTION [0001] [Technical Field of the Invention The present invention is an etching method, particularly regarding <br/> wet etching how the silicon nitride film. 2. Description of the Related Art Generally, an LOCO is used for separating a semiconductor element.
The S step is used, and this step will be briefly described with reference to FIG. It should be noted that hatching (oblique lines) representing cross sections in the drawings is partially omitted for easy understanding of the description. First, for example, an SiO ₂ film 43 is formed on a Si substrate 41, and a silicon nitride film (also referred to as a Si ₃ N ₄ film) 45 which is an oxidation-resistant film is deposited thereon (FIG. 6).
(A)). Next, the Si of the region corresponding to the element isolation region 47 is
_The 3N ₄ film 45 and the SiO ₂ film 43 are removed (FIG. 6).
(B)). Next, the Si substrate 41 corresponding to the element isolation region 47 which is not covered with the silicon nitride film 45 is selectively oxidized thickly to obtain a structure as shown in FIG. After forming the element isolation region 47, this S
Since the region covered with the i ₃ N ₄ film 45 becomes the transistor formation region 49, a silicon nitride film (Si ₃ N ₄ film)
45 is removed (etched) (FIG. 6D). In general, as the etching method, for example, dry etching using SF ₆ -based gas, and Reference 1: “Mechanism of silicon nitride film etching by hot phosphoric acid, 11th VLSI
Ultra Clean Technology Workshop, pp9
7-107 (1991) ", wet etching with a high-temperature phosphoric acid solution has been used. [0003] However, in the above dry etching, a silicon nitride film and an underlayer (generally, Si
The etching selectivity with respect to the substrate or the SiO ₂ film is small, and the over-etching may damage the base. In wet etching with hot phosphoric acid, a means for shutting off the atmosphere is required to prevent environmental pollution due to phosphoric acid mist generated during etching. Further, in the process using phosphoric acid, the wafer is usually placed in a jig (carrier or the like) and processed.
i in the phosphoric acid solution
The wafer is lifted up, thereby changing the position of the wafer in the carrier, which may cause a transport trouble. Accordingly, a silicon nitride film (S
i ₃ N ₄ film) In the removal of a large etching selectivity with the base, in the wet etching can be processed at the same degree of etch rate as when using hot phosphoric acid, and easily and accurately Si ₃ from wafer
N ₄ is an etching method and apparatus as removed has been desired. Therefore, the inventor of the present application has found that although the etching rate of Reference 1 is smaller than that of hot phosphoric acid, the silicon nitride film can be etched with pure water. After conducting various research and experiments, focusing on the description that the pure water is confined in a closed tank and adjusting the atmospheric pressure appropriately, the temperature of the pure water can be reduced by the conventional etching rate. Some 30Å (3nm) / mi
It has been discovered that it is possible to achieve temperatures that allow for a range of etch rates around n degrees. Therefore, according to the wet etching method of the present invention , the silicon nitride film is formed on the surface of the semiconductor wafer.
Pure water in an etching tank that seals the structure provided with
And the pure water is in a temperature range of 130-170 ° C.
To a temperature of, and wet etch the silicon nitride film.
After the etching of the silicon nitride film is completed,
Dry wafer continuously in high temperature steam in etching tank
It is characterized by making it. When the temperature of pure water is 130 ° C. or higher, the etching process can be performed at a preferable etching rate. In order to perform the etching process within the withstand voltage limit of the etching apparatus, the processing is preferably performed at 170 ° C. or lower. Therefore, etching can be performed in high-temperature pure water at 130 to 170 ° C. at a conventional etching rate of about 30 ° (3 nm) / min using hot phosphoric acid. Further, since pure water hardly etches the underlying Si substrate or SiO ₂ film, the etching selectivity is improved. Further, since the wafer does not float in pure water, the silicon nitride film can be accurately removed by etching, and there is no possibility of causing a transport trouble. further,
Since etching is performed using pure water only, there is no need to worry about environmental pollution. In addition, etching and drying steps
Can be continuously processed in the same device, and the drying device
Becomes unnecessary, and the process can be simplified. An embodiment of the present invention will be described below with reference to the drawings. It should be noted that the drawings are only schematically shown to the extent that the invention can be understood . Particular materials and conditions cited in or in the following description are merely preferred examples, thus the invention is not intended to be limited thereto. Further, the pure water described in this specification refers to ultrapure water usually used in a semiconductor device manufacturing process. <First Reference Example > Normally, a wet etching process using hot phosphoric acid used for removing a silicon nitride film (Si ₃ N ₄ film) formed on the surface of a semiconductor wafer is 15 steps.
The etching is performed at a high temperature of 0 ° C. or more, and the etching rate is 30 ° (3 nm) / min (Å / min represents a film thickness etched per minute). Therefore, in order to etch the Si ₃ N ₄ film with pure water, first,
It is necessary to know the relationship between the temperature of pure water and the etching rate. Si ₃ by pure water described in the above-mentioned reference 1
FIG. 2 is a characteristic diagram showing extrapolated values of measured data at a temperature of 100 ° C. or more based on the pure water temperature dependence of the etching rate in the N ₄ film etching. This figure 2
In the graph, the horizontal axis indicates temperature (1000 / T), and the vertical axis indicates etching rate (Å / min), and shows the pure water temperature dependence of the etching rate of the Si ₃ N ₄ film. This figure 2
According to the above, in order to increase the etching rate of the Si ₃ N ₄ film by pure water to 30 ° (3 nm) / min or more, the temperature must be 16
It can be understood that the temperature must be about 0 ° C. or higher.
In order to further increase the temperature of pure water to about 160 ° C. or more,
It is necessary to raise the boiling point. Therefore, the boiling point of pure water and the external pressure (atmospheric pressure)
As a result of an experiment, the relationship as shown in FIG. 3 was obtained. In FIG. 3, the horizontal axis indicates the atmospheric pressure (outside air pressure) (hP
Taking a), the boiling point (° C.) of pure water is plotted on the vertical axis to show the dependency of the boiling point of pure water on the outside pressure. According to this, 160 ° C
It can be understood that in order to raise the temperature of pure water to about or more, the external pressure should be about 5900 hPa or more. The temperature range actually suitable for etching is about 130 ° C. to 170 ° C., and the optimum value is 160 ° C. The atmospheric pressure range corresponding to this temperature range is from 2600 hPa to 6
800 hPa. In addition, this 130 to 170 degreeC
The etching rate corresponding to FIG.
(0.8 nm) / min to 100 ° (10 nm) / min. Such a temperature range, an atmospheric pressure range, and an etching rate range are correlated with each other, and within these ranges, it is necessary to shorten the etching processing time to increase the etching processing capability of the apparatus and to reduce the atmospheric pressure. This is because consideration was given to keeping the pressure within the withstand pressure limit of the device. The etching rate of the Si ₃ N ₄ film with pure water depends only on the temperature of pure water. [0012] Referring now to FIG. 1, a description will be given of a first reference example of the etching apparatus and etching steps. The etching apparatus used in the present invention includes an etching tank 11 for performing an etching process, and a water pipe provided above the etching tank 11 for introducing pure water as an etching solution into the tank 11. 13 and
A water valve 15 attached to the water pipe 13 for controlling the amount of water from the water pipe 13, a gas pipe 17 provided above the etching tank 11 and introducing an inert gas into the tank 11, Gas valve 19 attached to gas pipe 17 to control the amount of gas from gas pipe 17
And a heater 21 mounted in the etching bath 11 for heating the pure water. Next, the etching process will be described. An oxide film (SiO ₂ ) is formed on the surface of a silicon (Si) wafer, and a structure 23 in which a Si ₃ N ₄ film is formed on the SiO ₂ film (for example, as shown in FIG. 6C) An example of removing the Si ₃ N ₄ film by etching will be described. First, the water valve 15 is opened, and pure water 25 is introduced into the etching tank 11 through the water pipe 13. The above-mentioned structure 23 is immersed in pure water 25 of this etching tank 11. Next, the etching tank 11 is sealed, and for example, an N ₂ gas 27 as an inert gas is opened through the gas valve 19 and introduced through the gas pipe 17. When the atmospheric pressure in the etching tank 11 becomes 5900 hPa or more, the gas valve 17 is closed. At this point, electricity is supplied to the heater 21 to heat the pure water 25. The temperature of pure water rises to 10
When the temperature reaches 0 ° C. or higher, etching of the Si ₃ N ₄ film starts. Under the atmospheric pressure of about 5900 hPa (it is considered that the atmospheric pressure is slightly increased by water vapor volatilized by heating), the temperature of pure water 25 rises to about 160 ° C. (boiling point). If the boiling point is maintained while heating by the heater 21, the Si ₃ N ₄ film can be removed at an etching rate of about 30 ° (3 nm) / min. However, considering the pressure resistance of the apparatus, the atmospheric pressure cannot be increased so much, and the maximum pressure is preferably about 6800 hPa. Therefore, it is considered that the maximum temperature of the pure water is about 170 ° C. As a result, pure water is used as a base (Si substrate or S substrate).
Since almost no etching is performed on the iO ₂ film), the etching selectivity is improved. Further, since only pure water is used as the etching solution, there is no concern about environmental pollution. Further, since there is a difference in specific gravity between Si and pure water, the wafer does not float in pure water. In addition, the etching with pure water can be performed at the same etching rate as the etching with hot phosphoric acid. <Second Reference Example > Next, a second reference example will be described with reference to FIG. The etching apparatus has a configuration in which the gas pipe 17 and the gas valve 19 are removed from the configuration already described in the first reference example . In this example, a structure 23 similar to that described in the first reference example is immersed in pure water 25,
The etching tank 11 is sealed, and pure water 25 is
The temperature is raised. The atmospheric pressure in the etching tank 11 increases due to water vapor generated when the pure water 25 boils,
Accordingly, the boiling point of pure water 25 also exceeds 100 ° C. When the atmospheric pressure in the etching tank becomes 5900 hPa or more, the temperature of the pure water 25 can be raised to 160 ° C. or more corresponding to the pressure. If the temperature of 160 ° C. or more is maintained, the first reference
As described in the example , the Si ₃ N ₄ film can be etched at an etching rate of 30 ° (3 nm) / min or more according to the temperature at which the Si ₃ N ₄ film is maintained. Also in this case, the maximum atmospheric pressure is 6 for the same reason as in the first reference example.
The pressure is preferably about 800 hPa. [0019] After referring to completion of the etching of the <implementation of Embodiment> Next 5, the drying process of the wafers can be continuously performed will be described. As for the apparatus, a drain pipe 29 and a drain valve 31 are newly provided at a lower portion of the etching tank 11 as a means for depressurizing and draining the etching tank 11 in the etching apparatus described in the first and second reference examples . . After completion of the etching, the heater 21 is stopped, the drain valve 31 is opened, and the pure water 25 is gradually drained from the drain pipe 29, whereby the atmospheric pressure in the etching tank 11 is reduced.
The pure water 25 remaining in the tank 11 starts boiling again.
When the drainage is completed, the etching tank 11 is filled with high-temperature steam, and the wafer after the etching can be dried using the high-temperature steam. As described above, it is possible to continuously finish the wafer drying step in the same apparatus after the completion of the etching, so that the time and cost for using the drying apparatus can be reduced. In addition, unless the gas pipe 17 and the gas valve 19 and the drain pipe 29 and the drain valve 31 of FIG.
It is substantially equivalent to the device of FIG. In addition, if the drain pipe 29 and the drain valve 31 are not used in FIG. 5, the apparatus is substantially the same as the apparatus in FIG. As is apparent from the above description, in removing the Si ₃ N ₄ film formed on the surface of the semiconductor wafer, wet etching is performed in high-temperature pure water having a temperature range of 130 to 170 ° C. When this is done, since high-temperature pure water hardly etches the base made of Si or SiO ₂ , the etching selectivity of the Si ₃ N ₄ film is improved, and the Si ₃ N ₄ can be removed from the wafer surface without damaging the base. The film can be removed. Further, since there is a difference in specific gravity between pure water and Si, the wafer does not float in the pure water, so that a transport trouble does not occur. Further, since the etching is performed only with pure water, there is no need to worry about environmental pollution. In addition, the boiling point of pure water rises because the inside of the etching tank of the etching apparatus is sealed and a high-pressure atmosphere is established. As a result, it is possible to perform etching at the same etching rate as wet etching with hot phosphoric acid. became. Further, a drainage means is provided in the etching apparatus. As a result, when the drainage is performed after the etching, the pressure in the etching tank is reduced, and the pure water remaining in the tank starts to boil again. When the drainage is completed, the inside of the etching tank is filled with high-temperature steam, and the high-temperature steam can dry the etched wafer. As a result, a drying device becomes unnecessary, and time and cost can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of an etching apparatus used for explaining a first reference example . FIG. 2 is a characteristic curve diagram showing a relationship between a pure water temperature and an Si ₃ N ₄ etching rate. FIG. 3 is a characteristic curve diagram showing a relationship between an external pressure and a boiling point of pure water. FIG. 4 is a schematic cross-sectional view of an etching apparatus used for explaining a second reference example . 5 is a schematic cross-sectional view of an etching apparatus for explaining implementation of forms. FIG. 6 is a process diagram briefly showing an element isolation technique for explaining a conventional technique. [Description of Signs] 11: Etching tank 13: Water pipe 15: Water valve 17: Gas pipe 19: Gas valve 21: Heater 23: Structure 25: Pure water 27: Gas 29: Drain pipe 31: Drain valve 41: Si substrate 43: SiO ₂ film 45: Si ₃ N ₄ film 47: element isolation region 49: transistor formation region

Claims (1)

(57) Claims 1. A structure provided with a silicon nitride film on a surface of a semiconductor wafer is immersed in pure water in a sealed etching tank, and the pure water is immersed in 130-170. The silicon nitride film is heated to a temperature within a temperature range of 150 ° C. to wet-etch the silicon nitride film.
The wafer is continuously dried in high-temperature steam in a
A method for etching a silicon nitride film.