JP2019054048A - Method for manufacturing semiconductor device - Google Patents

Abstract

To provide a technique which is advantageous for manufacturing a semiconductor device.SOLUTION: A method for manufacturing a semiconductor device comprises: a step of forming an organic material film on a substrate, and an inorganic material film on the organic material film; a step of forming a photoresist pattern having an opening on the inorganic material film; a first etching step of etching, by plasma, the inorganic material film through the opening of the photoresist pattern in a dry manner to form an opening in the inorganic material film; and a second etching step of etching, by plasma, the organic material film exposed from the opening of the photoresist pattern and inorganic material film in a dry manner. The first and second etching steps are executed in a chamber arranged so as to be able to supply plasma with power of first frequency of a wavelength of 1 mm or longer and 1 m or shorter, and power of a second frequency lower than the first frequency through different introduction parts. In the second etching step, the plasma is supplied with the power of the first frequency and the power of the second frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing a semiconductor device.

  In manufacturing a semiconductor device, it may be necessary to form an opening in an organic material film. Patent Document 1 discloses a method of forming an opening in an organic material film. Specifically, an inorganic material film is formed on the organic material film that forms the opening, and a photoresist pattern is formed on the inorganic material film using a photoresist. An opening is formed in the organic material film by performing dry etching of the inorganic material film using this photoresist pattern and then performing dry etching of the organic material film.

JP 2000-164836 A

  In the manufacturing method of Patent Document 1, when the inorganic material film is dry-etched, the surface of the photoresist is hardened, and the photoresist is removed in the dry etching of the organic material film and the stripping process using a stripping solution. There are cases where it is not possible. If the photoresist cannot be removed, the yield may decrease due to the remaining photoresist.

  An object of this invention is to provide the technique advantageous to manufacture of a semiconductor device.

  Means for solving the above-described problem is a method for manufacturing a semiconductor device, the step of forming at least one organic material film on a substrate, and the formation of an inorganic material film on the organic material film A step of forming a photoresist pattern having an opening on the inorganic material film, and dry etching the inorganic material film using plasma through the opening of the photoresist pattern to form an inorganic material film A first etching step of forming an opening in the first etching step, and a second etching step of dry-etching the organic material film exposed at the opening of the photoresist pattern and the inorganic material film using plasma. The etching step and the second etching step include that the plasma has a first frequency power having a wavelength of 1 mm or more and 1 m or less and a second circumference having a frequency lower than the first frequency. The first frequency power and the second frequency power are supplied to the plasma in the second etching step in a chamber capable of supplying a plurality of powers from different introduction parts. To do.

  According to the present invention, it is possible to provide a technique advantageous for manufacturing a semiconductor device.

Sectional drawing which shows the manufacturing method of the semiconductor device which concerns on embodiment of this invention. Sectional drawing which shows the manufacturing method of the semiconductor device which concerns on embodiment of this invention. Sectional drawing which shows the manufacturing method of the semiconductor device which concerns on embodiment of this invention. The schematic diagram which shows the structure of the etching chamber used for manufacture of the semiconductor device which concerns on embodiment of this invention.

  Hereinafter, specific embodiments of a method for manufacturing a semiconductor device according to the present invention will be described with reference to the accompanying drawings. Note that, in the following description and drawings, common reference numerals are given to common configurations over a plurality of drawings. Therefore, a common configuration is described with reference to a plurality of drawings, and a description of a configuration with a common reference numeral is omitted as appropriate.

First Embodiment With reference to FIGS. 1A to 1B, a structure and manufacturing method of a semiconductor device according to an embodiment of the present invention will be described. FIGS. 1A to 1B are cross-sectional views showing respective steps of a method for manufacturing a semiconductor device according to the first embodiment of the present invention.

  As shown in FIG. 1A, the semiconductor device according to the present embodiment includes an imaging region A and a pad region B that are formed on a semiconductor substrate 100 such as silicon and include a photoelectric conversion unit 101. The photoelectric conversion unit 101 converts light incident from the outside of the semiconductor device into an electrical signal. For this reason, the semiconductor device of this embodiment can also be called an imaging device.

  First, the photoelectric conversion unit 101 and the wiring structure 102 are formed in the imaging region A. The wiring structure 102 includes a transistor and a plurality of wiring layers each insulated by an interlayer insulating film. In the pad region B, a wiring structure 102 including a pad electrode 103 for electrical connection with the outside of the semiconductor device is formed. Here, the wiring structure 102 between the imaging region A and the pad region B can share an interlayer insulating film and a wiring layer.

  Next, a protective film 104 is formed on the wiring structure 102 and the pad electrode 103. After the formation of the protective film 104, the protective film 104 disposed on the pad electrode 103 in the pad region B in the protective film 104 is selectively etched to expose the upper part of the pad electrode 103. A portion 105 is formed. At this time, as shown in FIG. 1A, the protective film 104 on the wiring structure 102 in the imaging region A may also be removed by etching at the same time. Further, the protective film 104 may be left on the wiring structure 102 in the imaging region A. For the protective film 104, for example, silicon oxide, silicon nitride, silicon oxynitride, or the like may be used.

  After the formation of the protective film opening 105, the planarization film 106 is formed so as to cover the imaging region A and the pad region B. The planarization film 106 is an organic material film using an organic material. As the organic material for forming the organic material film, various resin materials can be appropriately used.

  After the planarization film 106 is formed, in the imaging region A, a color filter material of the first color is applied, exposed, and developed on the planarization film 106 so as to correspond to each photoelectric conversion unit 101. A filter 107 is formed. Similarly, the second color filter 108 and the third color filter 109 are formed.

  After the formation of the color filters 107, 108, and 109, the planarization film 110 is formed so as to cover the imaging region A and the pad region B. The planarization film 110 is an organic material film using an organic material. The planarization film 106 and the planarization film 110 may be formed using the same organic material, or may be formed using different organic materials. Thus, in this embodiment, at least one layer of organic material film is formed on the substrate 100.

  After the planarization film 110 is formed, the microlenses 111 are formed on the planarization film 110 in the imaging region A so as to correspond to the respective photoelectric conversion units 101. At this time, the microlens 111 is not formed on the planarizing film 110 in the pad region B. In the present embodiment, the microlens 111 is formed by applying, exposing, and developing a microlens material. However, the method of forming the microlens 111 is not limited to this, and the manufacturing process of the semiconductor device is appropriately performed. A suitable process can be selected.

  After the formation of the micro lens 111, an antireflection film 112 is formed on the planarization film 110 and the micro lens 111 so as to cover the imaging region A and the pad region B. The antireflection film 112 is an inorganic material film using an inorganic material. As the inorganic material for forming the inorganic material film, silicon oxide, silicon nitride, silicon oxynitride, or the like can be used. In other words, the inorganic material film can include silicon and at least one of oxygen and nitrogen. The antireflection film 112 may have a single layer structure of such an inorganic material film, or may have a stacked structure in which a plurality of inorganic material films are stacked.

  After the formation of the antireflection film 112, a photoresist is applied on the antireflection film 112, and a photoresist pattern 113 is formed using a photolithography process in which exposure and development are performed. As shown in FIG. 1A, the photoresist pattern 113 includes an opening 118 above the pad electrode 103 (protective film opening 105).

  Next, etching for exposing the upper surface of the pad electrode 103 is performed through the opening 118 of the photoresist pattern 113. This etching process is performed in a chamber 410 capable of supplying power of two kinds of frequencies to plasma for dry etching from different introduction parts using the etching apparatus 400 shown in FIG. The chamber 410 is connected to power supplies 401 and 402 for supplying power to the plasma. The power supply 401 supplies power with a frequency of a wavelength of 1 mm or more and 1 m or less to the plasma via the matching box 402 and the introduction unit 403. Hereinafter, in this specification, an electromagnetic wave having a wavelength of 1 mm or more and 1 m or less output from the power supply 401 is referred to as a microwave. The microwave output from the power supply 401 has a frequency of 450 MHz (wavelength: 667 mm), 915 MHz (wavelength: 328 mm), 2.45 GHz (wavelength: 122 mm), 8.3 GHz (wavelength: 36 mm), for example. May be. The power source 411 supplies power with a frequency lower than the frequency of the microwave to the plasma through the introduction unit 413 that also serves as a stage on which the matching box 412 and the substrate 100 are placed. Hereinafter, the electromagnetic wave in the frequency band output from the power source 411 is referred to as an RF wave in this specification. The wavelength of the RF wave output from the power source 411 may be greater than 1 m and not greater than 100 km. The RF wave output from the power supply 411 is, for example, 100 kHz (wavelength: 3.0 km), 400 kHz (wavelength: 749 m), 2 MHz (wavelength: 150 m), 4 MHz (wavelength: 75 m), 13.56 MHz (wavelength: 22 m). 27.12 MHz (wavelength: 11 m), 40.68 MHz (wavelength: 7.4 m), 100 MHz (wavelength: 3.0 m), or the like. The chamber 410 is connected to a gas introduction port 404 for introducing a process gas into the chamber 410 and an exhaust port 405 for bringing the inside of the chamber 410 to an appropriate pressure.

Using the etching apparatus described above, first, the antireflection film 112 is dry-etched using the plasma through the opening 118 of the photoresist pattern 113 to form an antireflection film pattern 115 having the opening 114 in the antireflection film 112. . The etching process of the antireflection film 112 may be performed by supplying power of 500 W at a frequency of 13.56 MHz as an RF wave from a power source 411 and supplying 200 sccm of CF ₄ as a process gas. Further, the power of the RF wave supplied to the plasma may be, for example, 100 W or more and 1500 W or less. In addition to CF ₄ , a gas containing carbon and fluorine such as C ₄ F ₈ or C ₅ F ₈ may be used as the process gas. Further, a rare gas such as Ar or O ₂ may be added to the process gas. In the etching process of the antireflection film 112, the pressure in the chamber 410 may be, for example, 20 Pa, but is not limited thereto, and a pressure in the range of 4 Pa to 100 Pa can be used, for example. In the present embodiment, in the etching process of the antireflection film 112, the microwave power may not be supplied from the power source 401 to the plasma.

  In the step of etching the antireflection film 112, a hardened layer 116 is generated on the surface of the photoresist pattern 113 as shown in FIG. The hardened layer 116 can function as a mask when the antireflection film 112 is etched, but it is difficult to remove due to the insolubility of the hardened layer 116 with an organic stripping solution for stripping the photoresist pattern 113 in a later step. There is. If the photoresist pattern 113 cannot be removed, there is a possibility that the manufacturing yield of the semiconductor device is reduced due to the remaining photoresist pattern 113.

  After the etching process of the antireflection film 112, the planarization films 110 and 106, which are organic material films exposed in the opening 118 of the photoresist pattern 113 and the opening 114 of the antireflection film 112, are dry-etched using plasma. The etching process of the planarizing films 110 and 106 is performed by supplying microwave power from the power source 401 and RF power from the power source 411 to the plasma. Thereby, not only the etching of the planarizing films 110 and 106 but also the hardened layer 116 generated on the surface of the photoresist pattern 113 in the etching process of the antireflection film 112 is removed as shown in FIG. . This suppresses a decrease in manufacturing yield of the semiconductor device due to the remaining photoresist pattern 113. Further, in this embodiment, in the etching process of the planarization films 110 and 106, the entire portions of the planarization films 110 and 106 exposed to the opening 118 of the photoresist pattern 113 and the opening 114 of the antireflection film 112 are etched. Is done. As a result, the upper surface of the pad electrode 103 is exposed and a pad opening 117 is formed.

  In the etching process of the planarization films 110 and 106, 800 W of power is supplied to the plasma as a microwave from the power supply 401 at a frequency of 2.45 GHz, and 400 W of power is supplied from the power supply 411 as a RF wave at a frequency of 13.56 MHz. Also good. As described above, in the etching process of the planarization films 110 and 106, the power of the microwave supplied to the plasma may be larger than the power of the RF wave supplied to the plasma. However, the power supplied to the plasma is not limited to this. For example, power of 400 W or more and 2000 W or less may be supplied from the power supply 401 to the plasma. Further, power of 100 W or more and 500 W or less may be supplied from the power source 411 to the plasma. Further, the power of the RF wave supplied to the plasma in the etching process of the planarization films 110 and 106 may be smaller than the power of the RF wave supplied to the plasma in the etching process of the antireflection film 112 described above. This is because, in the etching process of the antireflection film 112, the microwave power is not supplied to the plasma only by the RF wave power. However, in the etching process of the planarization films 110 and 106, the microwave and the RF wave power are applied to the plasma. This is because it is supplied.

In the etching process of the planarization films 110 and 106, the process gas may contain at least one of oxygen and nitrogen. For example, the etching process of the planarization films 110 and 106 may be performed by supplying N ₂ at 90 sccm and O ₂ at 750 sccm as process gases. Further, as a process gas, a rare gas such as Ar or a CF-based gas such as CF ₄ may be added. Further, in the etching process of the planarization films 110 and 106, the pressure inside the chamber 410 may be, for example, 40 Pa, but is not limited thereto, and the pressure may be, for example, 4 Pa or more and 100 Pa or less. .

  The etching process of the antireflection film 112 and the etching process of the planarization films 110 and 106 may be performed using chambers 410 of different etching apparatuses 400, respectively. Further, the etching process of the antireflection film 112 and the etching process of the planarization films 110 and 106 may be continuously performed without taking out the substrate 100 from the chamber 410 of the same etching apparatus 400. In this case, for example, the end point of the etching of the antireflection film 112 is detected by monitoring the light emission of the plasma, and the process proceeds from the etching process of the antireflection film 112 to the etching process of the planarization films 110 and 106 accordingly. May be. After the pad opening 117 is formed, as shown in FIG. 2B, the photoresist pattern 113 from which the hardened layer 116 has been removed is removed using an organic stripping solution.

  In this embodiment, the planarization films 110 and 106 are etched by supplying microwave power and RF wave power to the plasma. Thus, the hardened layer 116 generated on the surface of the photoresist pattern 113 in the etching process of the antireflection film 112 as well as the planarization films 110 and 106 is removed. As a result, as shown in FIG. 2B, in the step of removing the photoresist pattern 113, the photoresist pattern 113 is suppressed from remaining, and the semiconductor device is manufactured due to the remaining photoresist pattern 113. The decrease in the yield is suppressed.

Second Embodiment With reference to FIGS. 3A to 3C, a structure and manufacturing method of a semiconductor device according to an embodiment of the present invention will be described. FIG. 3A to FIG. 3C are cross-sectional views showing respective steps of the method for manufacturing the imaging device according to the second embodiment of the present invention. In the present embodiment, the process up to forming the antireflection film pattern 115 having the opening 114 in the antireflection film 112 may be the same as the process shown in FIGS. 1A and 1B described above. Therefore, description of the process shown in FIGS. 1A and 1B is omitted here, and a process after the etching process of the antireflection film 112 for forming the opening 114 will be described.

  After the etching process of the antireflection film 112, the planarization films 110 and 106, which are organic material films exposed in the opening 118 of the photoresist pattern 113 and the opening 114 of the antireflection film 112, are dry-etched using plasma. In the present embodiment, the etching process of the planarizing films 110 and 106 is performed in two stages unlike the first embodiment described above. First, in the first step of the etching process of the planarization films 110 and 106, a part of the planarization films 110 and 106 exposed to the opening 118 of the photoresist pattern 113 and the opening 114 of the antireflection film 112 is etched. The For example, as shown in FIG. 3A, the etching may be controlled to stop in the planarizing film 110, or the etching may be controlled to stop in the planarizing film 106. The first stage of the etching process of the planarization films 110 and 106 is performed by supplying microwave power from the power supply 401 and RF power from the power supply 411 to the plasma, as in the first embodiment. Thereby, not only the etching of the planarization films 110 and 106 but also the hardened layer 116 generated on the surface of the photoresist pattern 113 in the etching process of the antireflection film 112 is removed as shown in FIG. .

  In the first stage of the etching process of the planarizing films 110 and 106, 800 W of power is supplied to the plasma from the power source 401 as a microwave at a frequency of 2.45 GHz, and 400 W is supplied from the power source 411 as an RF wave at a frequency of 13.56 MHz. You may supply. Thus, in the first stage of the etching process of the planarization films 110 and 106, the power of the microwave supplied to the plasma may be larger than the power of the RF wave supplied to the plasma. However, the power supplied to the plasma is not limited to this. For example, power of 400 W or more and 2000 W or less may be supplied from the power supply 401 to the plasma. Further, power of 100 W or more and 500 W or less may be supplied from the power source 411 to the plasma.

In the first stage of the etching process of the planarization films 110 and 106, the process gas may contain at least one of oxygen and nitrogen. For example, the etching process of the planarization films 110 and 106 may be performed by supplying N ₂ at 90 sccm and O ₂ at 750 sccm as process gases. Further, a rare gas such as Ar or a CF-based gas may be added as a process gas. Further, in the first stage of the etching process of the planarization films 110 and 106, the pressure inside the chamber 410 may be, for example, 40 Pa, but is not limited thereto, and the pressure is, for example, 4 Pa or more and 100 Pa or less. There may be.

  After the first stage of the etching process of the planarization films 110 and 106, the photoresist pattern 113 is removed using an organic stripping solution. In the first stage of the etching process of the planarization films 110 and 106, the hardened layer 116 generated on the surface of the photoresist pattern 113 in the etching process of the antireflection film 112 is removed. Therefore, as shown in FIG. 3B, the photoresist pattern 113 is suppressed from remaining in the step of removing the photoresist pattern 113. Further, unlike the above-described first embodiment, the photoresist pattern 113 is removed before the pad opening 117 is opened. For this reason, the charge accumulated in the photoresist pattern 113 in the first stage of the etching process of the planarizing films 110 and 106 is suppressed from flowing into the pad electrode 103, so that each element including a transistor and the like formed in the semiconductor device is transferred. The electrical damage of can be suppressed.

  After removing the photoresist pattern 113, the second stage of the etching process of the planarization films 110 and 106 is performed. Specifically, a portion of the exposed portion of the antireflection film 112 that has not been etched by the first etching process of the planarization films 110 and 106 is etched through the opening 114 of the antireflection film 112. . By this step, as shown in FIG. 3C, the upper surface of the pad electrode 103 is exposed and a pad opening 117 is formed.

In the second stage of the etching process of the planarization films 110 and 106, since the photoresist pattern 113 has already been removed from the substrate 100, there are less restrictions on the etching conditions. Therefore, for example, in the second stage of the etching process of the planarizing films 110 and 106, the etching may be performed within the range of the above-described conditions for performing the first stage etching. Further, for example, in order to improve the continuity of the shape of the side surface of the pad opening 117 formed in the planarizing films 110 and 106, the first and second stages of the etching process of the planarizing films 110 and 106 are etched. The conditions may be the same. Further, for example, in the second stage of the etching process of the planarizing films 110 and 106, RF wave power is supplied to the plasma in order to improve the verticality of the side surface of the pad opening 117 formed in the planarizing films 110 and 106. Alternatively, it may be performed without supplying microwave power to the plasma. Further, for example, in order to increase the verticality of the side surface to the deep part of the pad opening 117 formed in the planarization films 110 and 106, the second stage of the etching process of the planarization films 110 and 106 is performed in the chamber 410 rather than the first stage. It may be carried out under the condition that the pressure inside is low. Further, for example, in order to suppress physical and electrical damage to each element disposed in the pad electrode and the semiconductor device connected to the pad electrode, the second stage of the etching process of the planarization films 110 and 106 is one stage. It may be performed under the condition that the power of the RF wave supplied to the plasma is lower than that of the eyes. For example, in order to increase the etching rate of the planarizing films 110 and 106 which are organic material films, the second stage of the etching process of the planarizing films 110 and 106 is oxygen of process gas supplied into the chamber 410. The flow rate ratio may be larger than in the first stage. For example, the second stage etching step of the planarization film 110,106 is a _{N 2} as a process gas 40 sccm, the _{O 2} may be performed in 800sccm supply. Furthermore, the second stage of the etching process of the planarizing films 110 and 106 may be performed by wet etching.

  In this embodiment, the planarization films 110 and 106 are etched by supplying microwave power and RF wave power to the plasma. Accordingly, the hardened layer 116 generated on the surface of the photoresist pattern 113 in the etching process of the antireflection film 112 as well as the planarization films 110 and 106 is removed. As a result, in the step of removing the photoresist pattern 113, it is suppressed that the photoresist pattern 113 remains, and a decrease in manufacturing yield of the semiconductor device due to the remaining photoresist pattern 113 is suppressed. In this embodiment, the planarization films 110 and 106 are etched in two stages, and the photoresist pattern 113 is removed before the pad opening 117 is opened. Therefore, electrical damage to each element of the semiconductor device due to charges accumulated in the photoresist pattern 113 in the first stage of the etching process of the planarizing films 110 and 106 can be suppressed.

  As mentioned above, although embodiment which concerns on this invention was shown, it cannot be overemphasized that this invention is not limited to these embodiment, In the range which does not deviate from the summary of this invention, embodiment mentioned above can be changed and combined suitably. Is possible.

100: substrate, 113: photoresist pattern, 116: hardened layer, 410: chamber

Claims (20)

A method for manufacturing a semiconductor device, comprising:
Forming at least one organic material film on the substrate;
Forming an inorganic material film on the organic material film;
Forming a photoresist pattern having an opening on the inorganic material film;
A first etching step of dry-etching the inorganic material film through the opening of the photoresist pattern using plasma to form an opening in the inorganic material film;
A second etching step of dry-etching the organic material film exposed at the opening of the photoresist pattern and the inorganic material film using plasma,
The first etching step and the second etching step include a first frequency power having a wavelength of 1 mm or more and 1 m or less in plasma, and a second frequency power lower than the first frequency. Is performed in a chamber that can be supplied from different introduction parts,
In the second etching step, the power having the first frequency and the power having the second frequency are supplied to the plasma.   The manufacturing method according to claim 1, wherein a process gas used in the second etching step includes at least one of oxygen and nitrogen.   3. The manufacturing method according to claim 1, wherein a pressure inside the chamber in the second etching step is 4 Pa or more and 100 Pa or less.   The power of the first frequency supplied to the plasma in the second etching step is higher than the power of the second frequency supplied to the plasma. 2. The production method according to item 1.   5. The method according to claim 1, wherein, in the second etching step, the entire portion of the organic material film exposed at the opening of the photoresist pattern and the inorganic material film is etched. 2. The production method according to item 1.   The manufacturing method according to claim 1, wherein the manufacturing method further includes a step of removing the photoresist pattern using a stripping solution after the second etching step. . In the second etching step, a part of the organic material film exposed at the opening of the photoresist pattern and the inorganic material film is etched,
The manufacturing method is
After the second etching step, a stripping step of removing the photoresist pattern using a stripping solution;
And a third etching step of etching a portion of the exposed portion that has not been etched by the second etching step through the opening of the inorganic material film after the peeling step. The manufacturing method according to any one of claims 1 to 4. A process gas used in the second etching step and the third etching step contains oxygen;
The manufacturing method according to claim 7, wherein the third etching step has a larger flow rate ratio of oxygen contained in the process gas than the second etching step.   In the third etching step, the pressure of the second frequency supplied to the plasma is lower than that in the second etching step, and the pressure inside the chamber is lower than that in the second etching step. The production method according to claim 7 or 8, wherein the production is performed under at least one of the conditions.   10. The method according to claim 7, wherein the third etching step is performed without supplying power of the second frequency to plasma and supplying power of the first frequency to plasma. 2. The production method according to item 1.   The manufacturing method according to claim 7, wherein the third etching step is performed by wet etching.   The manufacturing method according to claim 7, wherein the conditions for performing the etching in the second etching step and the third etching step are the same.   The manufacturing method according to any one of claims 1 to 12, wherein the process gas of the first etching step includes a gas containing carbon and fluorine.   14. The method according to claim 1, wherein the first etching step is performed without supplying power of the second frequency to the plasma and without supplying power of the first frequency to the plasma. 2. The production method according to item 1.   The power of the second frequency supplied to the plasma in the second etching step is smaller than the power of the second frequency supplied to the plasma in the first etching step. 15. The manufacturing method according to any one of 1 to 14.   The manufacturing method according to claim 1, wherein the inorganic material film includes silicon and at least one of oxygen and nitrogen.   The manufacturing method according to any one of claims 1 to 16, wherein the wavelength of the second frequency is greater than 1 m and equal to or less than 100 km.   18. The manufacturing method according to claim 1, wherein the first etching step and the second etching step are continuously performed without removing the substrate from the chamber. . The semiconductor device includes an imaging region including a photoelectric conversion unit, and a pad region including a pad electrode for connecting to the outside of the semiconductor device,
The manufacturing method according to claim 1, wherein the opening of the photoresist pattern is disposed on the pad electrode. In the first etching step, a cured layer is generated on the surface of the photoresist pattern,
The manufacturing method according to claim 1, wherein the hardened layer is removed in the second etching step.

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