WO2024019122A1

WO2024019122A1 - Dry etching method for carbon atom-containing film

Info

Publication number: WO2024019122A1
Application number: PCT/JP2023/026644
Authority: WO
Inventors: 勝堀; 健治石川; ティトゥイガーグエン; 優太青木; 修司津野
Original assignee: 住友精化株式会社
Priority date: 2022-07-22
Filing date: 2023-07-20
Publication date: 2024-01-25

Abstract

This dry etching method for a carbon atom-containing film involves using an etching gas to etch a carbon atom-containing film which contains carbon atoms. This method includes: a mixed gas introduction step in which a mixed gas containing at least oxygen and sulfur dioxide is introduced to an etching chamber in which disposed is a structure comprising a carbon atom-containing film, and a mask having a first opening portion; and an etching step in which the mixed gas is turned into a plasma inside the etching chamber to produce a plasma gas, and using this plasma gas as an etching gas, the carbon atom-containing film of the structure is etched to form a second opening portion. In the etching step, the second opening portion is formed so that an aspect ratio of 1-40 is achieved.

Description

Dry etching method for carbon atom-containing film

The present disclosure relates to a method of dry etching a film containing carbon atoms.

In semiconductor integrated circuits, the miniaturization and lamination of elements are progressing, and in the manufacture of semiconductor integrated circuits, a mask such as a photoresist is used to pattern the film to be etched, and openings are created in the film to be etched. There is a need for technology that can process small, deep holes and trenches with large aspect ratios with high precision and high speed.
As such a technique, for example, the method described in Patent Document 1 is known. The publication reports that processing accuracy is improved by using a plasma gas, which is a mixed gas of oxygen and carbonyl sulfide, as an etching gas for a carbonaceous layer containing carbon.

Japanese Patent Application Publication No. 2009-200459

However, the method described in Patent Document 1 has room for improvement in terms of improving the etching rate of the carbonaceous layer.

The present disclosure has been made in view of the above problems, and aims to provide a method for dry etching a carbon atom-containing film that can improve the etching rate of the carbon atom-containing film.

The inventors of the present disclosure have determined that while controlling the anisotropy (aspect ratio) of the opening of the carbon atom-containing film after etching, the gas added to oxygen during etching of the carbon atom-containing film is changed from carbonyl sulfide to carbonyl dioxide. It was unexpectedly discovered that the above problems could be solved by using sulfur, leading to the present disclosure.

That is, one aspect of the present disclosure is a dry etching method for a carbon atom-containing film, the method comprising etching a carbon atom-containing film with an etching gas, the method comprising using a mixed gas containing at least oxygen and sulfur dioxide as a carbon atom-containing film. a step of introducing a mixed gas into an etching chamber in which a structure including a containing film and a mask having a first opening is arranged, and generating plasma gas by converting the mixed gas into plasma in the etching chamber, and an etching step of etching the carbon atom-containing film of the structure using the plasma gas to form a second opening, and in the etching step, the aspect ratio defined by the following formula (1) is A method of dry etching a carbon atom-containing film is provided, in which the second opening is formed to have a diameter of 1 to 40.
Aspect ratio = L2/L1...(1)
(In the formula (1), L1 represents the design width of the first opening, and L2 represents the depth of the second opening.)

According to the dry etching method for a carbon atom-containing film, the carbon atom-containing film is etched with a plasma gas of a mixed gas containing oxygen and sulfur dioxide while controlling the aspect ratio during etching within the above range. The etching rate for a carbon atom-containing film can be improved compared to the case where a carbon atom-containing film is etched with a plasma gas of a mixed gas containing carbonyl sulfide.

The aspect ratio may be 4 to 40.

The shape of the first opening may be a trench or a hole.

The carbon atom-containing film may include amorphous carbon.

The mask may include an oxygen-containing material.

The oxygen-containing material may be silicon dioxide.
The thickness of the carbon atom-containing film may be 10.0 μm or less.
The thickness of the mask may be 0.01 times or more the thickness of the carbon atom-containing film.
The thickness of the mask may be 0.5 times or less the thickness of the carbon atom-containing film.

In the mixed gas, the content of sulfur dioxide in the total volume of the sulfur dioxide and oxygen may be 20 to 40% by volume.

According to the present disclosure, a method for dry etching a carbon atom-containing film is provided that can improve the etching rate of the carbon atom-containing film.

FIG. 2 is a cross-sectional view illustrating an example of a structure before an etching step in the dry etching method for a carbon atom-containing film according to the present disclosure. FIG. 2 is a schematic diagram showing an etching chamber in which the structure of FIG. 1 is placed. FIG. 2 is a cross-sectional view illustrating an example of a structure after an etching step of the dry etching method for a carbon atom-containing film according to the present disclosure. 4 is a partially enlarged view of the mask and carbon atom-containing film in FIG. 3. FIG. 2 is a graph showing the results of plotting the etching rate against the aspect ratio of the second opening of the carbon atom-containing film in Examples 1 to 3 and Comparative Examples 1 to 3.

Hereinafter, embodiments of the method of dry etching a carbon atom-containing film of the present disclosure will be described in detail with reference to FIGS. 1 to 4. However, the present disclosure is not limited to the following embodiments.

FIG. 1 is a cross-sectional view showing an example of a structure before the etching process of the dry etching method for a carbon atom-containing film of the present disclosure, and FIG. 2 is a schematic diagram showing an etching chamber in which the structure shown in FIG. 1 is arranged. 3 is a cross-sectional view showing an example of a structure after the etching process of the dry etching method for a carbon atom-containing film of the present disclosure, and FIG. 4 is a partially enlarged view of the carbon atom-containing film in FIG.
The method of dry etching a carbon atom-containing film according to the present disclosure is a method of etching a carbon atom-containing film 20 containing carbon atoms with an etching gas, in which a mixed gas G containing at least oxygen and sulfur dioxide is applied to the carbon atom-containing film 20. , and a step of introducing a mixed gas into the etching chamber 1 in which the structure 100 including the mask 30 having the first opening 31 is arranged, and converting the mixed gas G into plasma in the etching chamber 1 to generate plasma gas. , and an etching step of etching the carbon atom-containing film 20 of the structure 100 using this plasma gas to form the second opening 21 (see FIGS. 1 to 3). Then, in the etching process, the second opening 21 is formed so that the aspect ratio defined by the following formula (1) is 1 to 40.
Aspect ratio = L2/L1...(1)
(In the above formula (1), L1 represents the design width of the first opening 31, and L2 represents the depth of the second opening 21.)

According to the dry etching method for a carbon atom-containing film, the carbon atom-containing film 20 is etched with a plasma gas of a mixed gas G containing oxygen and sulfur dioxide while controlling the aspect ratio during etching within the above range. The etching rate for the carbon atom-containing film 20 can be improved compared to the case where the carbon atom-containing film 20 is etched with a plasma gas of a mixed gas G containing oxygen and carbonyl sulfide.

Hereinafter, the mixed gas introduction step and etching step will be described in detail.

<Mixed gas introduction process>
The structure 100 includes a carbon atom-containing film 20 containing carbon atoms, and a mask 30 having a first opening 31. The structure 100 may further include a support 10 that supports the carbon atom-containing film 20, as shown in FIG. In this case, the carbon atom-containing film 20 is placed between the mask 30 and the support 10. Further, the structure 100 may further include an intermediate film (not shown) between the support body 10 and the carbon atom-containing film 20.

(Support)
Although the support body 10 is not particularly limited as long as it is a member that supports the carbon atom-containing film 20, examples of the material constituting the support body 10 include silicon, germanium, and the like. Among them, silicon is preferred. In this case, since the bandgap is wide, durability under high pressure is further improved.

The thickness of the support 10 is not particularly limited, but may be 254 μm or more, or 520 μm or more. When the thickness of the support 10 is 254 μm or more, the mechanical strength is further improved. Moreover, the thickness of the support body 10 may be 795 μm or less, or may be 725 μm or less. When the thickness of the support 10 is 795 μm or less, the structure 100 can be easily cut into wafers of a predetermined size.
(intermediate film)
Examples of the intermediate film include silica (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), amorphous silicon (a:Si), and polycrystalline silicon (poly:Si).

(carbon atom-containing film)
The carbon atom-containing film 20 is not particularly limited as long as it contains carbon atoms. The carbon atom-containing film 20 may be an inorganic carbon film such as amorphous carbon, or may be an organic polymer film such as a resist film or a polyimide film. When the carbon atom-containing film 20 is amorphous carbon, the etching selectivity (i.e., the ratio of the etching speed Vc of the carbon atom-containing film to the etching speed Vm of the mask) is increased when transferring the pattern to the carbon atom-containing film 20. Can be done.

The thickness of the carbon atom-containing film 20 is not particularly limited, but may be 0.1 μm or more, or 0.5 μm or more. When the thickness of the carbon atom-containing film 20 is 0.1 μm or more, if an intermediate film is laminated as a layer to be etched under the carbon atom-containing film 20, the carbon atom-containing film 20 can act as a mask for the layer to be etched. It becomes possible to demonstrate the functions of Further, the thickness of the carbon atom-containing film 20 may be 10.0 μm or less, or may be 5.0 μm or less. When the thickness of the carbon atom-containing film 20 is 10.0 μm or less, the carbon atom-containing film 20 becomes difficult to collapse after etching.

(mask)
The mask 30 has a first opening 31 that allows the etching gas to pass through and guide it to the carbon atom-containing film 20 . The first opening 31 may be a trench or a hole. It is preferable that the mask 30 has a lower etching rate with an etching gas than the carbon atom-containing film 20, and such a mask 30 preferably contains an oxygen-containing material. In this case, the etching rate by the etching gas becomes low. Examples of the oxygen-containing material include silicon dioxide and silicon oxynitride. Among them, silicon dioxide is preferred from the viewpoint of economy.

The thickness of the mask 30 is not particularly limited, but may be 0.01 times or more, or 0.05 times or more, the thickness of the carbon atom-containing film 20. When the thickness of the mask 30 is 0.01 times or more the thickness of the carbon atom-containing film 20, anisotropic etching of the carbon atom-containing film 20 becomes possible. Further, the thickness of the mask 30 may be 0.5 times or less, or 0.2 times or less, the thickness of the carbon atom-containing film 20. When the thickness of the mask 30 is 0.5 times or less than the thickness of the carbon atom-containing film 20, the carbon atom-containing film 20 becomes difficult to collapse after etching.

(etching chamber)
The etching chamber 1 is a container in which the carbon atom-containing film 20 is etched by a plasma gas in which a mixed gas G containing oxygen and sulfur dioxide is turned into plasma, and constitutes a part of an etching apparatus.
Examples of etching equipment include microwave ECR plasma etching equipment, capacitively coupled plasma (CCP) etching equipment, and inductively coupled plasma (ICP) etching equipment, but etching equipment is not limited to these. It's not something you can do.

(mixed gas)
Mixed gas G contains oxygen and sulfur dioxide. The content of sulfur dioxide in the total volume of oxygen and sulfur dioxide is not particularly limited as long as it is larger than 0% by volume, but may be 20 to 40% by volume, or may be 25 to 35% by volume. .
When the content of sulfur dioxide in the total volume of oxygen and sulfur dioxide is within the range of 20 to 40% by volume, the etching rate for the carbon atom-containing film 20 can be more effectively improved.

The flow rate of the mixed gas G when introduced into the etching chamber 1 may be 0.1 mL/min or more, 1 mL/min or more, or 10 mL/min or more. When the flow rate of the mixed gas G is 1 mL/min or more, it becomes possible to efficiently generate ions and radicals necessary for etching the carbon atom-containing film 20.
The flow rate of the mixed gas G when introduced into the etching chamber 1 may be 10000 mL/min or less, 1000 mL/min or less, or 100 mL/min or less. When the flow rate of the mixed gas G is 10,000 mL/min or less, the degree of vacuum in the etching apparatus can be easily maintained at a low pressure.

<Etching process>
In the etching process, the mixed gas G is turned into plasma in the etching chamber 1 to generate plasma gas, and the carbon atom-containing film 20 of the structure 100 is etched using this plasma gas to form the second opening 21. It is a process. The structure 100 becomes a structure 200 through the etching process.

(Pressure inside the etching chamber)
The pressure inside the etching chamber 1 during dry etching may be between 0.1 mTorr and 100 mTorr, and may also be between 0.1 mTorr and 100 mTorr. When the pressure inside the etching chamber 1 is between 0.1 mTorr and 100 mTorr, the pressure is low, so it becomes possible to perform excellent shape control on the second opening 21.

(antenna power)
When an inductively coupled plasma (ICP) etching device is used as the etching device, the antenna power is not particularly limited, but may be 50 to 1000 W, or 100 to 800 W. It may be 200 to 600W. By setting the antenna power to 50 to 1000 W, the carbon atom-containing film 20 can be etched at high speed and anisotropically.

(bias power)
When an inductively coupled plasma (ICP) etching device is used as the etching device, the bias power is not particularly limited, but may be 10 W or more, 25 W or more, 50 W or more. It may be. By setting the bias power to 10 W or more, it becomes easier to increase the aspect ratio.
Further, the bias power may be 500W or less, 300W or less, or 200W or less. By setting the bias power to 500 W or less, it becomes easier to appropriately control dry etching.

(Second opening)
The shape of the second opening 21 of the carbon atom-containing film 20 after etching is the same as the shape of the first opening 31. That is, when the first opening 31 is a trench, the second opening 21 is also a trench, and when the first opening 31 is a hole, the second opening 21 is also a hole.

The aspect ratio after etching is not particularly limited as long as it is 1 to 40, but may be 4 to 40, 5 to 40, or 5 to 25.
When the aspect ratio is 40 or less, the etching rate for the carbon atom-containing film 20 can be improved compared to the case where the carbon atom-containing film 20 is etched with a plasma gas of a mixed gas containing oxygen and carbonyl sulfide. Note that when a layer (lower layer) is provided on the opposite side of the mask 30 to the carbon atom-containing film 20, the aspect ratio is 1 or more, so that the carbon atom-containing film 20 can be used as a mask during etching of the lower layer, for example. It becomes easier to show the effect. Examples of the lower layer include silica (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), amorphous silicon (a:Si), and polycrystalline silicon (poly:Si).

Here, the aspect ratio is expressed by the above formula (1). That is, the aspect ratio refers to the ratio (L2/L1) of the depth (L2) of the second opening 21 to the designed width (L1) of the first opening 31 (see FIG. 4). The design width of the first opening 31 refers to the length of the first opening 31 along the interface between the carbon atom-containing film 20 and the mask 30 in the cross section of the mask 30. Here, when the first opening 31 of the mask 30 is a trench pattern, the cross section of the mask 30 refers to a cross section along a surface perpendicular to the longitudinal direction of the trench and along the thickness direction of the mask 30. . The depth of the second opening 21 is the length from the interface between the carbon atom-containing film 20 and the mask 30 to the bottom surface of the second opening 21 in the cross section of the carbon atom-containing film 20. 20 along the thickness direction.

Analytical instruments for confirming etching performance include SEM (scanning electron microscope) and TEM (transmission electron microscope), but analytical instruments are particularly limited as long as they are capable of confirming etching speed and occurrence of bowing. It's not a thing.

The gist of the present disclosure is as follows.
[1] A method for dry etching a carbon atom-containing film, which comprises etching a carbon atom-containing film with an etching gas, wherein a mixed gas containing at least oxygen and sulfur dioxide is etched into the carbon atom-containing film and the carbon atom-containing film. a step of introducing a mixed gas into an etching chamber in which a structure including a mask having one opening is placed; a step of introducing a mixed gas into an etching chamber in which a structure including a mask having one opening; a plasma gas is generated by converting the mixed gas into plasma in the etching chamber; and the plasma gas is used in the etching process. an etching step of etching the carbon atom-containing film of the structure using a gas to form a second opening; A method of dry etching a carbon atom-containing film to form a part.
[2] The method for dry etching a carbon atom-containing film according to [1], wherein the aspect ratio is 4 to 40.
[3] The method of dry etching a carbon atom-containing film according to [1] or [2], wherein the first opening has a shape of a trench or a hole.
[4] The method of dry etching a carbon atom-containing film according to any one of [1] to [3], wherein the carbon atom-containing film contains amorphous carbon.
[5] The method for dry etching a carbon atom-containing film according to any one of [1] to [4], wherein the mask contains an oxygen-containing material.
[6] The method for dry etching a carbon atom-containing film according to [5], wherein the oxygen-containing material is silicon dioxide.
[7] The method for dry etching a carbon atom-containing film according to any one of [1] to [6], wherein the thickness of the carbon atom-containing film is 10.0 μm or less.
[8] The method for dry etching a carbon atom-containing film according to any one of [1] to [7], wherein the thickness of the mask is 0.01 times or more the thickness of the carbon atom-containing film.
[9] The method for dry etching a carbon atom-containing film according to any one of [1] to [8], wherein the thickness of the mask is 0.5 times or less the thickness of the carbon atom-containing film.
[10] The carbon atom according to any one of [1] to [9], wherein in the mixed gas, the content of the sulfur dioxide in the total volume of the sulfur dioxide and the oxygen is 20 to 40% by volume. Dry etching method for containing film.

Hereinafter, the present disclosure will be described in more detail with reference to Examples and Comparative Examples. However, the present disclosure is not limited to the following examples.

[Example 1]
First, a laminate consisting of a Si substrate as a support and an amorphous carbon film (thickness: about 700 nm) as a carbon atom-containing film was prepared. Then, on the amorphous carbon film of this laminate, a mask pattern as a first opening is formed by lithography, and a silicon dioxide film having a silicon film as an underlying layer (total thickness of silicon film and silicon dioxide film: approx. 50 nm) was placed as a mask, and a 20 mm square structure was prepared (see FIG. 1). At this time, the mask pattern of the mask is a trench pattern, the trench design width (design width of the first opening) L1 is 80 nm, and the mask design width (design width of the actual part of the mask other than the first opening (trench pattern) is 80 nm. The width (width between the two)) was 80 nm (see FIG. 1). Then, the structure obtained as described above was attached to a wafer having a diameter of 150 mm, and the wafer was placed on a processing stage in an etching chamber of an etching apparatus. At this time, an inductively coupled plasma (ICP) etching apparatus (product name "NLD6000", manufactured by ULVAC) was used as the etching apparatus.
Then, dry etching of the amorphous carbon film was performed as follows. That is, the vacuum pressure in the etching chamber was set to 3.8 mTorr, the antenna power was set to 400 W, and the bias power was set to 100 W, and a mixed gas was introduced into the etching chamber at a flow rate of 50 mL/min to generate plasma gas as an etching gas. The amorphous carbon film was dry etched using this plasma gas to form a trench pattern as a second opening in the amorphous carbon film. In this way, dry etching of the carbon atom-containing film was completed.
At this time, the mixed gas is composed of a mixed gas of oxygen and sulfur dioxide, the etching time is 2 minutes and 30 seconds, and the content of sulfur dioxide in the total volume of oxygen and sulfur dioxide is 30% by volume (oxygen content The ratio was 70% by volume). Further, the second opening was formed so that the etching depth L2 was 600 nm, that is, the aspect ratio was 7.5. Note that the aspect ratio was calculated using the following formula (1).

Aspect ratio = Etching depth L2 (nm) ÷ Trench design width L1 (nm)...(1)

After the etching was completed, the cross section of the amorphous carbon film was observed using a SEM (product name "SU8230", manufactured by Hitachi High-Tech Corporation), and the etching depth (L2) of the trench pattern formed in the amorphous carbon film was actually confirmed. The actual value of the etching depth L2 was 599 nm, and the etching rate was 240 nm/min as shown in Table 1. Note that the aspect ratio based on the actually measured value of the etching depth L2 was 7.5.
In addition, when observing the cross section of the amorphous carbon film with SEM, we confirmed that the inner wall surface of the second opening was not etched much, and the occurrence of bowing was suppressed, making it anisotropic. It was found that highly sensitive etching was progressing.

[Comparative example 1]
Dry etching of the amorphous carbon film was performed in the same manner as in Example 1 except that the mixed gas, etching time, and aspect ratio were as shown in Table 1. Then, the etching rate was calculated. The results are shown in Table 1. As shown in Table 1, the etching rate was 170 nm/min.

[Example 2]
The thickness of the amorphous carbon film of the laminate is approximately 2400 nm, the thickness of the silicon dioxide film having a silicon film as a lower layer (the total thickness of the silicon film and the silicon dioxide film) is approximately 350 nm, and the trench design width (the Using a mask with a mask design width (design width of one opening) L1 of 80 nm and a mask design width (design width of the actual part of the mask other than the first opening) W of 320 nm, the mixed gas, etching time, and aspect ratio are shown in Table 2. Dry etching of the amorphous carbon film was carried out in the same manner as in Example 1 except that the procedure was as shown in . Then, the etching rate was calculated. The results are shown in Table 2. As shown in Table 2, the etching rate was 154 nm/min.

[Comparative example 2]
Dry etching of the amorphous carbon film was performed in the same manner as in Example 2 except that the mixed gas, etching time, and aspect ratio were as shown in Table 2. Then, the etching rate was calculated. The results are shown in Table 2. As shown in Table 2, the etching rate was 107 nm/min.

[Example 3]
The thickness of the amorphous carbon film of the laminate is approximately 2400 nm, the thickness of the silicon dioxide film having a silicon film as a lower layer (the total thickness of the silicon film and the silicon dioxide film) is approximately 350 nm, and the trench design width (the Using a mask in which the design width (design width of one opening) L1 is 80 nm and the mask design width (design width of the actual part of the mask other than the first opening) W is 320 nm, the mixed gas, etching time, and aspect ratio are shown in Table 3. Dry etching of the amorphous carbon film was carried out in the same manner as in Example 1 except that the procedure was as shown in . Then, the etching rate was calculated. The results are shown in Table 3. As shown in Table 3, the etching rate was 123 nm/min.

[Comparative example 3]
Dry etching of the amorphous carbon film was performed in the same manner as in Example 3 except that the mixed gas, etching time, and aspect ratio were as shown in Table 3. Then, the etching rate was calculated. The results are shown in Table 3. As shown in Table 3, the etching rate was 88 nm/min.

Based on the results shown in Tables 1 to 3, the etching rate was plotted against the aspect ratio in Examples 1 to 3 and Comparative Examples 1 to 3, and the results are shown in FIG.
From the results shown in Figure 5, when the aspect ratios are almost the same, when dry etching a carbon atom-containing film, a mixed gas containing sulfur dioxide improves the etching rate than a mixed gas containing carbonyl sulfide. I know what you're doing.

DESCRIPTION OF SYMBOLS 1... Etching chamber, 10... Support, 20... Carbon atom-containing film, 21... Second opening, 30... Mask, 31... First opening, 100... Structure, 200... Structure, L1... First opening L2... Depth of the second opening, W... Mask design width (width of the actual portion of the mask other than the first opening), G... Mixed gas.

Claims

A method for dry etching a carbon atom-containing film, the method comprising etching a carbon atom-containing film with an etching gas, the method comprising:
A mixed gas introducing step of introducing a mixed gas containing at least oxygen and sulfur dioxide into an etching chamber in which a structure including the carbon atom-containing film and a mask having a first opening is disposed;
Etching in which the mixed gas is turned into plasma in the etching chamber to generate a plasma gas, and the carbon atom-containing film of the structure is etched using the plasma gas as the etching gas to form a second opening. including the process,
A dry etching method for a carbon atom-containing film, wherein in the etching step, the second opening is formed so that the aspect ratio defined by the following formula (1) is 1 to 40.
Aspect ratio = L2/L1...(1)
(In the formula (1), L1 represents the design width of the first opening, and L2 represents the depth of the second opening.)
The method for dry etching a carbon atom-containing film according to claim 1, wherein the aspect ratio is 4 to 40.
The method of dry etching a carbon atom-containing film according to claim 1, wherein the first opening has a shape of a trench or a hole.
The method for dry etching a carbon atom-containing film according to claim 1, wherein the carbon atom-containing film contains amorphous carbon.
The method for dry etching a carbon atom-containing film according to claim 1, wherein the mask includes an oxygen-containing material.
The method for dry etching a carbon atom-containing film according to claim 5, wherein the oxygen-containing material is silicon dioxide.
The method of dry etching a carbon atom-containing film according to claim 1, wherein the thickness of the carbon atom-containing film is 10.0 μm or less.
The method for dry etching a carbon atom-containing film according to claim 1, wherein the thickness of the mask is 0.01 times or more that of the carbon atom-containing film.
The method for dry etching a carbon atom-containing film according to claim 1, wherein the thickness of the mask is 0.5 times or less that of the carbon atom-containing film.
Drying a carbon atom-containing film according to any one of claims 1 to 9, wherein in the mixed gas, the content of the sulfur dioxide in the total volume of the sulfur dioxide and the oxygen is 20 to 40% by volume. Etching method.