JPH11297607A - Pattern forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent resist pattern configuration. SOLUTION: This pattern forming method comprises a process forming a first photosensitive resin film 12 on a substrate 11 to be worked, a process forming a latent image 13 by exposing a desired region of the first resin film 12 to a light, a process forming a second resin film 14 soluble in developer for the first resin film on the first resin film 12 on which the latent image is formed, a process developing the first resin film 12 by exposing the second resin film 14 to developer 15 of the first resin film 12, and a process cleaning the substrate surface exposed to the developer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a pattern forming method used in a semiconductor device manufacturing process and the like.

[0002]

2. Description of the Related Art In a resist process, conditions close to the resolution limit, that is, conditions having dimensions close to the limit resolution determined by diffraction phenomena, conditions in which an optical image is degraded due to a positional shift of a focal point, and conditions around a pattern to be formed. Under a condition where the light irradiation area is large, for example, the boundary of a desired pattern or the pattern itself is exposed to exposure light. When such a pattern is developed, there arises a problem that the thickness of the pattern is reduced or the pattern itself is missing.

In order to solve such a problem, a method has been proposed in which a resin contained in a resist film is previously mixed in a developing solution (Japanese Patent No. 2582901). When development is performed by this method, the pattern can be prevented from being lost, and the profile of the pattern can be improved. However, since the solubility of the resin in the developer is limited, dissolution of several percent or more is impossible, and although a slight improvement effect can be obtained, further improvement is desired in producing a device. .

[0004] On the other hand, a developer (resin or the like) of a resist exists in a developing solution on a substrate to be processed. In the conventional developing method, in order to prevent such precipitation of the dissolved substance, the pattern surface of the substrate to be processed is once rinsed with a fresh developing solution and then washed with pure water. However, even if the pattern surface of the substrate to be processed is rinsed with a fresh developing solution, if a very small amount of the resist dissolved material remains, the resist dissolved material precipitates in the washing step with pure water, which causes foreign matters. Would.

[0005]

As described above, the conventional developing process has a problem that a pattern is missing or a pattern profile is deteriorated. Further, in the conventional developing process, there is also a problem that a dissolved substance in a developing solution precipitates in a washing step and causes foreign matters.

The present invention has been made to solve the above-mentioned conventional problems, and has as its first object to provide a method capable of obtaining a good pattern shape. A second object is to provide a method that can be prevented.

[0007]

A pattern forming method according to the present invention comprises the steps of forming a photosensitive first resin film on a substrate to be processed, and exposing a desired area of the first resin film to light. Forming a latent image on the first resin film on which the latent image is formed, forming a second resin film soluble in a developer of the first resin film on the first resin film on which the latent image is formed, First where the resin film has a latent image
Exposing the substrate formed on the first resin film to a developing solution of the first resin film to develop the first resin film; and cleaning the surface of the substrate exposed to the developing solution. Features (Claim 1).

According to the present invention, a supersaturated state is always created by dissolving the resin from the second resin film in the developer in the interface region to be developed. Therefore, development is always performed in an ideal state, and a good pattern shape can be obtained even when the optical image is deteriorated.

The step of exposing the first resin film to form a latent image includes irradiating the first resin film with ultraviolet light or X-rays through a mask on which a predetermined pattern is formed to form a latent image. Alternatively, a latent image may be formed by directly irradiating a desired region of the first resin film with ultraviolet light, X-rays, or an electron beam.

As the developing solution, it is preferable to use a developing solution containing a basic compound widely used for the current resist as a main component. As the resin constituting the second resin film, a resin containing styrene or its derivative as a skeleton (for example, polyhydroxystyrene resin) can be used (claim 2). At present, it is widely used as a raw material for DUV resist, and can suppress a decrease in film thickness during resist development, can make a side wall shape vertical, and can prevent a resist pattern from deteriorating even in a portion where image intensity is weak. it can.

As the resin constituting the second resin film, a novolak resin or a derivative thereof can be used. This is currently widely used as a raw material for I-line resists. In this case as well, it is possible to suppress a decrease in film thickness during development of the resist and to make the side wall shape vertical, so that the resist pattern can be formed even in a portion where image intensity is weak. Deterioration can be prevented.

In addition, acrylic resins, methacrylic resins, carboxylated alicyclic methacrylic resins, and the like are also applicable to the process for resists having these resin skeletons.

[0013] Further, the pattern forming method according to the present invention comprises:
Forming a photosensitive resin film on the substrate to be processed, and forming a latent image by exposing a desired area of the resin film,
Exposing the substrate, on which the resin film having the latent image is formed, to a developing solution to develop the resin film, and applying a first chemical solution containing a basic compound as a main component to the substrate surface exposed to the developing solution. And a step of increasing the mixing ratio of the second chemical solution to the first chemical solution with the lapse of time using a second chemical solution composed of pure water and cleaning.

[0014] As in the prior art, when rinsing with pure water or rinsing with a fresh developer and then washing with pure water, the resin component is insoluble in pure water. Precipitates. Therefore, in the present invention, the cleaning is performed by gradually decreasing the ratio of the first chemical solution containing a basic compound as a main component and gradually increasing the ratio of the second chemical solution made of pure water. Then, the supply of the first chemical solution is stopped, and the cleaning is performed with substantially only pure water. As a result, the resin remaining on the resist film surface and the resin dissolved in the developing solution are discharged while being dissolved in the chemical solution, and the basicity gradually weakens as the concentration of the dissolved material decreases, so The substrate to be processed can be washed without depositing an object, and a pattern free of foreign matter can be formed on the substrate to be processed.

Further, the pattern forming method according to the present invention comprises:
Forming a photosensitive resin film on the substrate to be processed, and forming a latent image by exposing a desired area of the resin film,
Exposing the substrate on which the resin film having the latent image is formed to a developing solution to develop the resin film; and exposing the substrate surface exposed to the developing solution to a photosensitive resin containing a basic compound as a main component. Cleaning with a first chemical solution adjusted to a concentration at which development of the film does not proceed, and then washing with a second chemical solution made of pure water (claim 4).

According to the present invention, by processing using a first chemical solution containing a basic compound as a main component and then cleaning using a second chemical solution consisting of pure water, the workpiece can be processed without depositing a dissolved substance. The substrate can be cleaned, and a pattern free of foreign matter can be formed on the substrate to be processed.
In addition, since the first chemical solution is adjusted to a concentration at which the development of the photosensitive resin film does not proceed, the processing dimensions of the photosensitive resin film processed by the development are maintained even after the cleaning process, so that the pattern Processing accuracy can be improved.

The invention according to claims 3 and 4 can be applied to a case where a resin film is further formed on the photosensitive resin film as in the invention according to claim 1, and the resin film is formed on the photosensitive resin film. Can be applied to the case where is not formed.

[0018]

(Embodiment 1) Hereinafter, a first embodiment of the present invention will be described with reference to FIG. First, a resist film 12 having a polystyrene skeleton was formed with a thickness of 0.3 μm on a substrate 11 to be processed by a spin coating method (FIG. 1A). Subsequently, reduction projection exposure was performed using light of 248 nm (KrF laser) through an exposure mask to form a latent image 13 on the resist film (FIG. 1B).
Next, a solution containing a polystyrene resin was poured on the resist film 13, and a resin thin film 14 having a thickness of 0.05 μm was formed by a spin coating method (FIG. 1C). Subsequently, a developing solution 15 of TMAH (tetramethylammonium hydroxide) having a concentration of 0.21 N (normal) is applied on the substrate on which the resin thin film 14 is formed so that the liquid thickness becomes about 2 mm.
This was left for 60 seconds for development (FIG. 1 (d)).

Next, as a first rinsing step, a process of pouring a first rinsing liquid while rotating the substrate to be processed was performed for 5 seconds. A 0.21N concentration solution of TMAH (chemical solution 1) was used as the first rinse solution. This step is performed for the purpose of completely dissolving and removing the resin present on the resist film and in the developing solution, and the processing time is determined by whether or not the shape of the resist pattern formed by the developing process is the presence or absence of the rinsing process. It was set within a range that hardly changed. Then, the second
As in the first rinsing step, the second rinsing liquid was poured while rotating the substrate to be processed as in the first rinsing step. As the second rinsing solution, a solution prepared by adjusting the mixing ratio between the chemical solution 1 having a 0.21N concentration of TMAH and pure water (chemical solution 2) was used. FIG. 2 schematically shows the configuration of the apparatus at this time, in which the substrate 21 to be processed is placed on the rotating part 22 and the tube 24 a
And 24b are supplied to the mixing valve 25, respectively, and the chemical mixed by the mixing valve 25 is supplied from the nozzle 23 to the substrate 21 to be processed.

FIG. 3 shows the change over time of the mixing ratio of the liquid medicine 1 and the liquid medicine 2. That is, the mixing ratio of the chemical solution is such that the ratio of the chemical solution 1 is high (almost 1) in the initial stage of rinsing, and the relative amount of the chemical solution 1 decreases as the rinsing progresses, and
, The ratio of the second chemical solution in the second rinsing solution is gradually increased. Then, after the second rinsing liquid is almost pure water (chemical liquid 2), it is rinsed for a while, and then
The supply of the second rinsing liquid was stopped, and the rotation of the substrate to be processed was performed at a high speed, followed by shaking off and drying. Through such steps, a resist pattern 16 was formed on the substrate 11 to be processed (FIG. 1E).

The pattern formed by the above steps has a dimension of 0.15 μm in design, and the remaining (line) and blank (space) patterns are alternately positioned (0.15 μm).
(L & S) pattern and a 0.15 μm line and a 0.6 μm space around it (a 0.15 μm isolated L) pattern. As shown in FIG. The pattern could be formed with an error of 7%, and the pattern shape could be made rectangular. Also,
When the base 17 was processed using these resist patterns 16 as a mask, a desired pattern shape could be obtained as shown in FIG.

In the conventional developing method (when no resin is mixed in the developing solution), by adjusting the exposure amount, as shown in FIG.
Even if either one of the 15 μm isolated L patterns could be formed, one of the patterns was missing. Further, in the system in which the resin is mixed in the phenomenon liquid in advance (Japanese Patent No. 2582901), although the pattern is not missing, the amount of the resin mixed is insufficient, so that as shown in FIG. When the 15 µmL & S pattern is processed to the desired size, the isolated L is 0.15 µm.
The pattern formation size was 0.08 μm and the film thickness was about half. Further, when the underlayer 17 was processed using the resist pattern 16 as a mask, as shown in FIG. 8, a drop occurred during the etching, and a desired pattern could not be obtained after the etching.

As described above, in the present embodiment, a favorable pattern shape could be obtained by forming a resin film on the resist after exposure and performing development. In addition,
In the present embodiment, the effect was also confirmed with respect to the reduction of defects (foreign matter) caused by the dissolved resist. FIG. 9 shows the result. FIG. 9 shows a cleaning step after development.
(1) When washing with pure water only, (2) TMAH 0.2
After washing with 1N chemical solution for 5 seconds, and then washing with pure water,
(3) After cleaning with a chemical solution of TMAH 0.21N for 5 seconds, and then cleaning with a chemical solution in which the mixing ratio with pure water is changed with respect to the cleaning time, 0.3 μm or less of the surface of an 8-inch wafer. The amount of foreign matter caused by the resist was measured.

In the case of washing with pure water only (1), the amount of foreign matter was too large to measure. In the case of washing with pure water after washing for 5 seconds with a chemical solution of TMHA 0.21N (2), it was confirmed that about 100 particles were present, although the amount of foreign substances was greatly reduced. On the other hand, in the case of cleaning with a chemical solution of TMAH 0.21N for 5 seconds and then cleaning with a chemical solution in which the mixing ratio with pure water was changed with respect to the cleaning time (3), no foreign matter was confirmed. (As a result of examining many wafers, 1-3 wafers / 10
Only about one sheet of foreign matter was confirmed).

As described above, by using the method of the present embodiment, it was possible to perform development without generating resist-induced foreign matter. The chemical solution 1 and the chemical solution 2
The change of the mixing ratio with respect to the time is not limited to that shown in FIG. The composition of the supply liquid is almost chemical liquid 1 at the beginning of development, and almost chemical liquid 2 at the end of development.
Further, the present invention is applicable as long as the composition in the middle changes monotonously. Further, the composition of the chemical solution may be such as to show a change as shown in FIG. 10, for example, as long as the composition is adjusted so that the substance contained in the processing solution on the substrate does not precipitate. Further, in consideration of the fact that a large amount of resist is eluted in the developing solution at the end of development, for example, as shown in FIG. 11, the ratio of the chemical solution 1 in the first rinsing solution is supplied in a slightly smaller amount. Is also good. In this case, it is desirable that the hydrogen ion concentration (pH) of the supply liquid is higher than the pH of the developer on the substrate to be processed.

As the resist and the resin formed thereon, it is preferable to use a resin containing styrene or a derivative thereof as a skeleton. However, the present invention is not limited to this, and the resin formed on the resist film may be formed of a resist. What is necessary is just to have the skeleton contained in at least one part.

Further, the film thickness of the resist is not limited to the value of the above embodiment, but may be appropriately set to a thickness required for processing in a subsequent step. When the film thickness is small, a decrease in film thickness after development can be prevented, and when the film thickness is large, the line width can be prevented from becoming thin. The thickness of the resin film is desirably about 10 to 20% with respect to the thickness of the resist film, but the thickness of the resin film may be 10% or less or 20% or more depending on the type of the resist and the size of the pattern to be processed. It may be desirable to set it. Further, an antireflection film made of an organic or inorganic film may be provided between the substrate to be processed and the resist film.

Further, regarding exposure, Kr of 248 nm
It is not limited to the one using the F laser,
The present invention is also applicable to exposure using a 93 nm ArF laser, a fluorine laser, or other narrow band oscillation lines of a mercury lamp.

(Embodiment 2) Hereinafter, a second embodiment of the present invention will be described with reference to FIG. First, a resist film 12 having a polystyrene skeleton was formed to a thickness of 0.6 μm on a substrate 11 to be processed by a spin coating method (FIG. 12A). Subsequently, reduction projection exposure is performed by using light of 248 nm (KrF laser) through an exposure mask.
A latent image 13 was formed on the resist film (FIG. 12B). Next, a TMAH 0.2
A developing solution 15 having a concentration of 1N (prescribed) was applied so that the solution thickness became about 2 mm, and the solution was left standing for 60 seconds to perform development (FIG. 12C).

Next, as in the first embodiment, a rinsing step was performed using an apparatus as shown in FIG. That is, first, as a first rinsing step, a process of pouring the first rinsing liquid while rotating the substrate to be processed was performed for 5 seconds. A 0.21N concentration solution of TMAH (chemical solution 1) was used as the first rinse solution. This step is performed for the purpose of completely dissolving and removing the resin present in the developing solution on the substrate, and the processing time is almost the same as that of the resist pattern formed by the developing process depending on the presence or absence of the rinsing process. Set within the range that does not change. Subsequently, as a second rinsing step, a second rinsing liquid was poured while rotating the substrate to be processed. Second
As the rinsing liquid, the one prepared by adjusting the mixing ratio between the chemical liquid 1 having a TMAH concentration of 0.21N and pure water (chemical liquid 2) was used.

As in the first embodiment (see FIG. 3), the mixing ratio of the chemical 1 is high at the initial stage of rinsing (approximately 1). By increasing 2, the ratio of the second chemical solution in the second rinse solution is gradually increased. Then, after the second rinsing liquid is almost pure water (chemical liquid 2) only, it is rinsed for a while, and then the supply of the second rinsing liquid is stopped. went. Through these steps, a resist pattern 16 was formed on the substrate 11 to be processed (FIG. 12D).

The pattern formed by the above process has a dimension of 0.18 μm in design, and a left (line) and a blank (space) pattern are alternately positioned (0.18 μm).
(L & S) pattern and a 0.18 μm line and a 0.6 μm space around it (a 0.18 μm isolated L) pattern. Both patterns are formed with an error of ± 7% with respect to design dimensions. And the pattern shape could be made rectangular. Further, when the base was processed using these resist patterns as masks, a desired pattern shape could be obtained.

In the conventional developing method (when no resin is mixed in the developing solution), by adjusting the exposure amount,
Even when one of the 0.18 μm L & S pattern and the 0.18 μm isolated L pattern can be formed, one of the patterns is missing. Also,
A method in which a resin is mixed in a phenomenon liquid in advance (Japanese Patent No. 25
No. 82901), a resist pattern having a shape similar to that of the present embodiment could be obtained, but a large number of foreign matters caused by resin mixed in the developer were observed. At the time of the base processing, these foreign substances became a mask, and there were many short places. On the other hand, in the present embodiment, there was no foreign matter, and there was no short-circuited portion, and a good device could be obtained.

In this embodiment, as in the first embodiment, the time change of the mixing ratio of the chemical solution 1 and the chemical solution 2 in the rinsing step, the material of the resist, its film thickness, the exposure wavelength and the like can be appropriately changed. In addition, an antireflection film made of an organic or inorganic film may be provided between the substrate to be processed and the resist film.

(Embodiment 3) Hereinafter, a third embodiment of the present invention will be described with reference to FIG. First, a resist film 12 (M20G: JSR) is formed on a substrate 11 to be processed.
Was formed with a thickness of 0.3 μm by a spin coating method (FIG. 12A). Subsequently, reduction projection exposure is performed by using light of 248 nm (KrF laser) through an exposure mask.
A latent image 13 was formed on the resist film (FIG. 12B). Next, a TMAH 0.2
A developing solution 15 having a concentration of 7N (pH 13.4) was applied so that the solution thickness became about 2 mm, and the solution was left standing for 60 seconds to perform development (FIG. 12C).

Next, a rinsing step was performed using an apparatus having a schematic configuration as shown in FIG. That is, the substrate 21 to be processed is placed on the rotating part 22, and first, as a first rinsing step, a process of pouring the first rinsing liquid from the nozzle 26a while rotating the substrate to be processed is performed for 30 seconds. As the first rinsing solution, a solution obtained by diluting the developing solution to 1/10 concentration (pH 12) was used. This step is performed for the purpose of stopping the progress of development, and completely dissolving and removing the resin present in the developer from the substrate.
The processing time and the concentration of the rinsing liquid were set within a range in which the shape of the resist pattern formed in the developing processing hardly changed depending on the presence or absence of the rinsing. After the first rinsing step, as a second rinsing step, the nozzle 26 is rotated while the substrate to be processed is rotated.
Pure water was poured from b to wash with water to remove alkali components on the surface of the substrate to be processed. Lastly, the substrate to be processed was rotated at a high speed and dried by shaking off.

By these steps, the pattern could be formed without deteriorating the pattern dimension, and a remarkable effect could be obtained on the reduction of defects caused by the dissolved resist product. FIG. 14 shows the result.

FIG. 14 shows that the washing step after development is (1) when washing with pure water only, and (2) washing with a rinse solution (pH 7.5) diluted to 1/10 ⁵ for 30 seconds. Thereafter, when washing with pure water, (3) washing with a rinse solution (pH 9) diluted to 1/10 ³ for 30 seconds, and then washing with pure water, (4) developing solution is reduced to 1/2. Rinse solution (pH 13) with almost the same pH as the developer at the end point of development
(5) washing with pure water after washing for 30 seconds with a rinse solution (pH 13.8) using the developing solution as it is,
This is a measurement of the amount of foreign matter due to a resist dissolution product of 0.3 μm or less on the surface of an 8-inch wafer.

When the cleaning was performed only with pure water (1), the amount of foreign matters was about 200. In the case of (2), the amount of foreign matters was about 300. In the case of (3), the amount of foreign matter was reduced to about 50. In the case of (4), only a few foreign matters were observed. In the case of (5), the amount of foreign substances increased to about 30 pieces.

FIG. 14 shows the pattern size (0.2 μmL & 2 μm) in the wafer surface for the above cases (1) to (5).
3) of S) is also shown. In the cases of (1) to (4), 3σ was almost the same, showing a good value of 10 nm. However, in the case of (5), 3σ is 18n
m. The reason for this is that the dimension immediately below the rinse nozzle is extremely thinner than the surrounding area.

From these results, in the above example, it was found that cleaning by the cleaning method (4) was the most effective in reducing foreign substances and suppressing generation of defects. These results may vary depending on the resist, the process, and the like, and the concentrations of the developing solution and the first rinsing solution can be appropriately changed according to the resist and the process to be used. For example, the pH of the first rinsing liquid is preferably 9 to 13. However, the pH of the first rinsing liquid is set lower than that of the developing solution so that the first rinsing solution has a concentration at which development does not proceed.

In this embodiment, similarly to the first and second embodiments, the material of the resist, its film thickness, exposure wavelength, and the like can be appropriately changed, and the reflection between the substrate to be processed and the resist film can be changed. A protective film may be provided.

Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the present invention. .

[0044]

According to the present invention, since the resin formed on the photosensitive resin film is melted in the developing step, the development is always performed in an ideal state, so that a good pattern shape can be obtained. Becomes

In the present invention, a first chemical solution containing a basic compound as a main component and a second chemical solution composed of pure water are used.
By performing the cleaning mainly using the second chemical liquid after the cleaning mainly using the first chemical liquid, the cleaning can be performed without depositing a dissolved substance, and a pattern free of foreign matter is formed on the substrate to be processed. It becomes possible to do.

[Brief description of the drawings]

FIG. 1 is a process sectional view showing an example of a process according to an embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of an example of a chemical solution supply unit according to the embodiment of the present invention.

FIG. 3 is a diagram showing an example of a change over time of a mixing ratio of a chemical solution according to the embodiment of the present invention.

FIG. 4 is a view showing a resist pattern obtained by an embodiment of the present invention.

FIG. 5 is a view showing a case where a base is processed using a resist pattern obtained according to an embodiment of the present invention.

FIG. 6 is a view showing a resist pattern formed using a conventional technique.

FIG. 7 is a view showing a resist pattern formed using a conventional technique.

FIG. 8 is a diagram showing a case where a base is processed using a resist pattern formed using a conventional technique.

FIG. 9 is a diagram showing the effect of reducing the amount of foreign matter by the method of the present invention.

FIG. 10 is a diagram showing another example of the change over time of the mixing ratio of the chemical solution according to the embodiment of the present invention.

FIG. 11 is a diagram showing another example of a temporal change of the mixing ratio of the chemical solution according to the embodiment of the present invention.

FIG. 12 is a process cross-sectional view showing another example of the process according to the embodiment of the present invention.

FIG. 13 is a diagram showing a schematic configuration of another example of the chemical solution supply unit according to the embodiment of the present invention.

FIG. 14 is a view showing the effect of reducing the amount of foreign matter by the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Substrate to be processed 12 ... Resist film 13 ... Latent image 14 ... Resin film 15 ... Developer 16 ... Resist pattern 17 ... Base pattern 21 ... Substrate to be processed 22 ... Rotating part 23, 26a, 26b ... Nozzle 24a, 24b ... Tube 25: Mixing valve

Continued on the front page (72) Inventor Katsuya Okumura 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Pref.

Claims (4)

[Claims] A step of forming a photosensitive first resin film on a substrate to be processed; a step of exposing a desired area of the first resin film to form a latent image; Forming a second resin film soluble in a developer of the first resin film on the formed first resin film; and forming the second resin film on the first resin film having a latent image. A step of exposing the substrate formed in step (b) to a developing solution for the first resin film to develop the first resin film, and a step of cleaning the surface of the substrate exposed to the developing solution. Forming method. 2. The pattern forming method according to claim 1, wherein the resin constituting the second resin film contains styrene or a derivative thereof as a skeleton. 3. A step of forming a photosensitive resin film on a substrate to be processed, a step of exposing a desired area of the resin film to form a latent image, and a step of forming a resin film having the latent image. Exposing the exposed substrate to a developing solution to develop the resin film, and subjecting the substrate surface exposed to the developing solution to a first chemical solution containing a basic compound as a main component and a second chemical solution consisting of pure water. A step of increasing the mixing ratio of the second chemical solution to the first chemical solution over time and performing cleaning. 4. A step of forming a photosensitive resin film on a substrate to be processed, a step of exposing a desired area of the resin film to form a latent image, and a step of forming a resin film having the latent image. Exposing the exposed substrate to a developing solution to develop the resin film, and adjusting the surface of the substrate exposed to the developing solution to a concentration containing a basic compound as a main component and preventing the development of the photosensitive resin film from proceeding. Cleaning using a first chemical solution and then cleaning using a second chemical solution made of pure water.