WO2018056329A1 - Method for producing reinforced glass sheet, glass sheet for reinforcing, and reinforced glass sheet - Google Patents

Abstract

This method for producing a reinforced glass sheet includes reinforcing a glass sheet by means of an ion exchange method. The method for producing a reinforced glass sheet is characterized by the following: including a film formation step of forming an ion permeation prevention film that prevents ion exchange on a part of a surface of a glass sheet, and a selective ion exchange step of bringing the film-bearing glass sheet into contact with a molten salt bath so as to selectively subject regions other than the film-bearing region to ion exchange; and in that the ion permeation prevention film is an inorganic film containing 50-100 mass% of ZrO₂.

Description

Method for producing tempered glass plate, tempered glass plate, and tempered glass plate

The present invention relates to a method for producing a tempered glass plate, a tempered glass plate, and a tempered glass plate, and more specifically to a method for producing a tempered glass plate that chemically strengthens a glass plate by an ion exchange method.

Conventionally, a tempered glass plate that has been chemically strengthened is used as a cover glass plate for touch panel displays mounted on electronic devices such as smartphones and tablet PCs.

Such a tempered glass plate is generally produced by chemically treating a glass plate containing an alkali metal as a composition with a tempering solution to form a compressive stress layer on the surface. Since such a tempered glass plate has a compressive stress layer on the surface, impact resistance and the like are improved. However, even such a tempered glass plate has a lower impact resistance at the edge portion and the peripheral portion than the impact resistance on the main surface, which causes damage to the tempered glass plate. When the compressive stress layer on the surface of the tempered glass sheet is deepened to prevent such damage, the tensile stress formed inside the glass sheet becomes excessive, and the damage caused by the tensile stress (so-called self-destruction) There is a problem that tends to occur.

In order to solve the above problems, a technique for selectively forming a deep compressive stress layer only on a part of the tempered glass plate surface has been developed. For example, in the method disclosed in Patent Document 1, only the central portion of the main surface is shielded with a mask material, whereby only the peripheral portion that is not shielded can be ion-exchanged for strengthening treatment. After that, the shielding is removed, and the strengthening process is performed again, so that a deep compressive stress layer can be formed on the edge portion that has been strengthened in advance, and a shallow compressive stress layer can be formed on the shielded main surface.

US Patent Application Publication No. 2012/0236477

Patent Document 1 exemplifies materials such as metal foil and polyimide as a mask material (ion exchange prevention film) constituting the shield. However, ion exchange prevention membranes made of these materials are easily eroded, decomposed or peeled off by chemical reaction with the molten salt during the ion exchange treatment, and ion exchange cannot be sufficiently suppressed, and strengthening having desired strengthening characteristics. In some cases, glass could not be obtained stably. In addition, such wear and tear of the ion exchange preventing film may become more prominent as the ion exchange treatment temperature is higher.

Therefore, in the conventional ion exchange prevention film of the material, it is necessary to increase the film thickness to such an extent that it can withstand wear, or to lower the processing temperature and increase the processing time, which may reduce the productivity.

The present invention has been made in view of such circumstances, and a method for producing a tempered glass plate capable of stably producing a tempered glass plate having high strength with high productivity, and a tempered glass plate. An object is to provide a tempered glass sheet.

The method for producing a tempered glass plate of the present invention is a method for producing a tempered glass plate for strengthening a glass plate by using an ion exchange method, and an ion permeation preventive film for preventing ion exchange on a part of the surface of the glass plate. And a selective ion exchange step of selectively ion-exchanging a region other than the film formation region by bringing the formed glass plate into contact with the molten salt, It is an inorganic film containing ZrO ₂ .

According to the method for producing a tempered glass plate of the present invention, since the ion permeation preventive film contains ZrO ₂ , the film is not easily worn even when contacted with a molten salt such as high-temperature potassium nitrate, and a relatively thin film. Even if it is thick, ion transmission can be sufficiently blocked. Therefore, a tempered glass sheet having high strength can be manufactured stably with high productivity.

In the method for producing a tempered glass sheet of the present invention, the ion permeation preventive film is preferably an inorganic film containing 50 to 100% by mass of ZrO ₂ .

In the method for producing a tempered glass sheet of the present invention, it is preferable to immerse the glass sheet in a molten salt bath having a temperature of more than 400 ° C. and less than 500 ° C. in the selective ion exchange step.

In the method for producing a tempered glass plate of the present invention, in the film forming step, as an ion permeation preventive film, ZrO ₂ is 70 to 99% by mass and at least of Y ₂ O ₃ , CaO, Al ₂ O ₃ , CeO, and HfO ₂ . It is preferable to form an inorganic film containing any one of 1 to 30%.

In the method for producing a tempered glass sheet of the present invention, it is preferable to form an ion permeation preventive film so that the thickness is 400 nm or less in the film forming step.

In the method for producing a tempered glass plate of the present invention, a removal step of removing the ion permeation prevention film from the glass plate after the selective ion exchange step, and an overall ion exchange step of performing ion exchange treatment on the entire surface of the glass plate after the removal step. It is preferable to further provide.

In the method for producing a tempered glass plate of the present invention, the surface roughness Ra of the surface of the glass plate in the film formation region is preferably 0.4 nm or less.

The glass sheet for strengthening of the present invention is a glass sheet for strengthening that is subjected to a strengthening treatment using an ion exchange method, and has a surface coated with an inorganic film containing 50 to 100% by mass of ZrO _2. It has a film part and the exposed part which is not coat | covered with the inorganic film in at least one part of an end surface, It is characterized by the above-mentioned.

The tempered glass plate of the present invention is a tempered glass plate that is partially tempered using an ion exchange method, and has a film-forming part coated on the surface with an inorganic film containing 50 to 100% by mass of ZrO ₂ And an exposed portion that is not coated with an inorganic film on at least a part of the end face, and the depth of the compressive stress layer on the glass surface in the film forming portion is smaller than the compressive stress depth in the exposed portion, To do.

It is a figure which shows the manufacturing method of the tempered glass board which concerns on 1st embodiment of this invention. It is a figure which shows the manufacturing method of the tempered glass board which concerns on 1st embodiment of this invention. It is a figure which shows the manufacturing method of the tempered glass board which concerns on 1st embodiment of this invention. It is a figure which shows the manufacturing method of the tempered glass board which concerns on 1st embodiment of this invention. It is a figure which shows the manufacturing method of the tempered glass board which concerns on 1st embodiment of this invention. It is a figure which shows an example of the film-forming area | region in the manufacturing method of the tempered glass board of this invention.

Hereinafter, the manufacturing method of the tempered glass board of the embodiment of the present invention is explained. 1A to 1E are diagrams showing an example of a method for producing a tempered glass sheet of the present invention.

First, the preparatory process shown in FIG. 1A is performed. The preparation step is a step of preparing the original glass plate G1. The original glass plate G1 is a plate-like glass plate that can be strengthened using an ion exchange method.

The original glass plate G1 contains, as a glass plate, mass% and contains SiO ₂ 45 to 75%, Al ₂ O ₃ 1 to 30%, Na ₂ O 0 to 20%, K ₂ O 0 to 20%. preferable. If the glass plate composition range is regulated as described above, it is easy to achieve both ion exchange performance and devitrification resistance at a high level.

The original glass plate G1 has a thickness of, for example, 1.5 mm or less, preferably 1.3 mm or less, 1.1 mm or less, 1.0 mm or less, 0.8 mm or less, 0.7 mm or less, 0.6 mm or less. 0.5 mm or less, 0.4 mm or less, 0.3 mm or less, 0.2 mm or less, especially 0 mm. 1 mm or less. As the plate thickness of the tempered glass plate substrate is smaller, the tempered glass plate substrate can be reduced in weight, and as a result, the device can be reduced in thickness and weight. In consideration of productivity and the like, the thickness of the original glass plate G1 is preferably 0.01 mm or more.

The size of the main surface of the original glass plate G1 can be arbitrarily set, and is, for example, 480 × 320 mm to 3350 × 3950 mm.

The original glass plate G1 is formed by using, for example, an overflow down draw method. In addition, you may select arbitrarily the shaping | molding method and processed state of the original glass plate G1. For example, the original glass plate G1 may be formed using a float process, and the main surface S and the end surface E may be polished.

Next, after the preparation step, the selective ion exchange step shown in FIGS. 1B and 1C is performed. The selective ion exchange step is a step of partially chemically strengthening the surface of the original glass plate G1. Specifically, in the selected ion exchange step, the selected region (peripheral portion S2 and end surface E) set on a part of the surface of the original glass plate G1 is more than the non-selected region (center portion S1) other than the selected region. This is a step of performing a process of forming a deep compressive stress layer. The selective ion exchange process includes a film forming process, a selective ion exchange process, and a removal process.

In the selective ion exchange process, first, the film forming process shown in FIG. 1B is performed. The film forming step is a step of obtaining the glass plate with film G2 by forming the ion permeation preventive film M in the non-selection region set on at least a part of the surface of the original glass plate G1. In the present embodiment, as shown in FIG. 2, a case where the central portion S1 of the front and back main surfaces S of the original glass plate G1 is set as a non-selection region will be described as an example. 1B corresponds to a cross-sectional view taken along arrow AA in FIG. Of the surface of the original glass plate G1, the region other than the central portion S1, that is, the peripheral edge S2 and the end surface E are selected regions and are exposed. The peripheral edge S2 is an area surrounding the central portion S1 of the main surface S. The ion permeation preventive membrane M is a membrane layer that suppresses or blocks the permeation of ions when performing ion exchange on the surface layer of the original glass plate G1 in the selective ion exchange step described later.

The ion permeation preventive film M is an inorganic film containing ZrO ₂ . Preferably, the ion permeation preventive film M contains 50 to 100% by mass of ZrO ₂ . By using such a material as the ion permeation preventive film M, it is possible to suppress wear and the like in the selective ion exchange process described later. Specifically, even when immersed in a molten salt (strengthening liquid) at a temperature higher than 450 ° C., peeling of the ion permeation preventive film M from the film-attached glass plate G2 is extremely unlikely to occur.

When the ion permeation preventive film M mainly composed of ZrO ₂ is formed, the surface roughness Ra in at least a non-selection region (film formation region) on the surface of the original glass plate G1 is preferably 0.4 nm or less. This is because when Ra is large (for example, in the case of a general polishing surface), the film quality of the ion permeation preventive film M mainly composed of ZrO ₂ is adversely affected, and the function as the ion permeation preventive film is remarkably deteriorated. Because there is. Therefore, the surface of the original glass plate G1 is preferably an overflow surface (unpolished fired surface) formed by the overflow downdraw method, but may be a polished surface if it is very flat. In addition, it is preferable that surface roughness Ra of the original glass plate G1 is 0.05 nm or more from viewpoints of manufacturing cost.

Further, when the ion permeation preventive film M mainly composed of ZrO ₂ is exposed to a high-temperature molten salt, the crystallinity of the film changes. Specifically, it changes from amorphous to crystalline. Crystallization does not change the function of the ion permeation prevention film, but the volume of the film decreases with crystallization. As a result, a crack is generated on the surface of the ion permeation preventive film M, and ion exchange may be performed from the crack. Since such crystallization was confirmed when the ion exchange temperature was 500 ° C, the ion exchange temperature is preferably less than 500 ° C and 490 ° C or less.

The ion permeation preventive film M contains 70 to 99% of ZrO ₂ as a composition by mass and 1 to 30% in total of at least one of Y ₂ O ₃ , CaO, Al ₂ O ₃ , CeO, and HfO _2. It is more preferable. In particular, it is preferable to include 0.5% by mass or more of Y ₂ O ₃ because the phase transition of the ion permeation preventive film M at high temperatures can be suppressed.

The thickness of the ion permeation preventive film M may be any thickness as long as ion permeation can be blocked and suppressed. However, if the ion permeation preventive film M is excessively thick, the film formation time, material cost, and the like increase, and therefore it is preferable to form the ion permeation preventive film M as thin as possible so that ion permeation can be blocked and suppressed. The film thickness of the ion permeation prevention film M is preferably 5000 nm or less, 400 nm or less, 350 nm or less, 100 nm or less, 50 nm or less, or 25 nm or less. The film thickness of the ion permeation preventive film M is preferably 1 nm or more, 5 nm or more, and 10 nm or more. Specifically, the film thickness of the ion permeation prevention film M is preferably, for example, 1 to 5000 nm, more preferably 5 to 350 nm, and still more preferably 10 to 100 nm.

The film formation method of the ion permeation preventive film M is a PVD method (physical vapor deposition method) such as a sputtering method or a vacuum vapor deposition method, a CVD method (chemical vapor deposition method) such as a thermal CVD method or a plasma CVD method, or dip coating. A wet coating method such as a method or a slit coating method can be used. In particular, a sputtering method and a dip coating method are preferable. When the sputtering method is used, the ion permeation preventive film M can be easily and uniformly formed. The region where the ion permeation preventive film M is formed may be set by any method. For example, film formation can be performed in a state where the selected region (peripheral portion S2, end surface E) is masked. Alternatively, the ion permeation preventive film M previously formed into a sheet shape may be bonded to the main surface of the original glass plate G1 to form a film.

Next, after the film formation step, the selective ion exchange step shown in FIG. 1C is performed. The selective ion exchange step is a step of obtaining the film-reinforced glass plate G3 by chemically strengthening the film-coated glass plate G2 by an ion exchange method. Specifically, ion exchange is performed by immersing the film-coated glass plate G2 in a molten salt T1 containing alkali metal ions. The molten salt T1 in the present embodiment is, for example, a potassium nitrate molten salt.

The temperature of the molten salt T1 in the selective ion exchange step may be arbitrarily determined. The temperature of the molten salt T1 is preferably 350 ° C. or higher, 370 ° C. or higher, more than 400 ° C., or 420 ° C. or higher. Moreover, it is preferable that the temperature of molten salt T1 is 500 degrees C or less, less than 500 degrees C, and 480 degrees C or less. Specifically, for example, it is preferably 350 to 500 ° C., more than 400 ° C. and less than 500 ° C., and 370 to 480 ° C. The time for immersing the film-coated glass plate G2 in the molten salt T1 may be arbitrarily determined. For example, it is 0.1 to 150 hours, preferably 0.3 to 100 hours, more preferably 0.5 to 50. It's time.

Next, after the selective ion exchange step, the removal step shown in FIG. 1D is performed.

The removal step is a step of removing the ion permeation preventive film M from the tempered glass plate G3 with film. As a method for removing the ion permeation preventive film M, for example, a method such as polishing or etching can be used.

As a polishing apparatus used for polishing, a well-known double-side polishing machine or single-side polishing machine can be used. When the ion permeation preventive film M is removed by polishing, only the ion permeation preventive film M may be polished, or the glass plate portion may be polished together with the ion permeation preventive film M.

Etching methods such as dry etching and wet etching can be used.

When dry etching is used, it is particularly preferable to use plasma of Ar, O ₂ , CH ₄ , BC ₁₃ , C ₁₂ , SF ₆ or the like.

As an etchant used for wet etching, for example, a solution containing fluorine, TMAH, EDP, KOH, NaOH, or the like can be used as the etchant, and a hydrofluoric acid solution is particularly preferably used as the etchant. In the case where only the ion permeation preventive film M is removed using a hydrofluoric acid solution without changing the dimensions of the glass, the concentration of HF in the hydrofluoric acid solution is preferably 10% or less.

The tempered glass sheet G4 obtained as described above has a deep compressive stress layer C at the peripheral edge S2 and the end face E. That is, the tempered glass sheet G4 is a glass that has high impact resistance at the edge and can reduce internal tensile stress and is less likely to break due to the tensile stress.

Since the ion permeation preventive film M blocks the permeation of ions in the selective ion exchange step, the compressive stress layer C is not formed in the central portion S1 of the tempered glass plate G4. Therefore, in order to improve the strength of the central portion S1, it is preferable to perform the following overall ion exchange step after the removal step and form the compressive stress layer C also in the central portion S1.

The whole ion exchange step is a step of exchanging ions on the surface layer by bringing the molten salt into contact with the entire surface of the tempered glass sheet G4 as shown in FIG. 1E. Specifically, the tempered glass plate G4 is immersed in a molten salt T2 containing alkali metal ions and ion exchange is performed to obtain a tempered glass plate G5 having a compressive stress layer C shallower than the peripheral edge S2 and the end face E in the central part S1. . The molten salt T2 is, for example, a potassium nitrate molten salt. It is preferable that DOL of a non-film-forming area | region (end surface E and peripheral part S2) is 1/4 or less of plate | board thickness of the tempered glass board G5. The DOL in the film formation region (central portion S1) is preferably 1/8 or less of the thickness of the tempered glass plate G5.

The temperature of the molten salt T2 in the entire ion exchange step may be arbitrarily determined, and is, for example, 350 to 500 ° C, preferably 370 to 480 ° C. Further, the time for immersing the tempered glass plate G4 in the molten salt T2 may be arbitrarily determined. For example, it is 0.1 to 72 hours, preferably 0.3 to 50 hours, more preferably 0.5 to 24 hours. It is.

The molten salt T2 may be the same as the molten salt T1 described above. That is, the tempered glass plate G4 may be immersed again in the salt bath used in the selective ion exchange step. In this case, since it is possible to perform a plurality of processes in a single salt bath, the manufacturing cost can be suppressed.

Further, the molten salt T2 may be different from the molten salt T1, and the processing temperature and processing time in the entire ion exchange step may be different from the processing temperature and processing time in the selective ion exchange step. For example, the ion exchange treatment time in the entire ion exchange step is preferably shorter than the treatment time in the selective ion exchange step. According to such a process, the depth of the compressive stress layer C in the central portion S2 is not excessive, and an increase in tensile stress can be suppressed.

In addition, after the whole ion exchange process, you may implement the process of a finishing process (not shown). In the finishing process, the surface of the tempered glass sheet G5, for example, at least one of the main surface S and the end surface E is polished. When the dimensions and surface state of the tempered glass sheet G5 are not in a desired state such as a product standard due to the processing in the entire ion exchange step, it can be brought into a desired state by performing the processing in such a finishing step.

As described above, according to the method for manufacturing a tempered glass sheet according to the embodiment of the present invention, the tempered glass sheets G4 and G5 with little breakage from the end face can be stably and efficiently manufactured.

In addition, before and after the arbitrary process shown above, a machining process for performing any one of cutting, end face machining, and drilling may be provided. In addition, before and after the arbitrary steps described above, the glass plate may be appropriately washed and dried.

Moreover, in the said embodiment, although the case where molten salt T1, T2 was potassium nitrate molten salt was demonstrated as an example, it is not restricted to this, It replaces with well-known molten salt used for the ion exchange of a glass plate, or combines. May be used. For example, the molten salts T1 and T2 may be a mixed salt of a potassium nitrate molten salt and a sodium nitrate molten salt.

Moreover, although the case where chemical strengthening was performed by exchanging Na ions and K ions in the above embodiment, chemical strengthening may be performed by exchanging arbitrary ions. For example, chemical strengthening may be performed by exchanging Li ions and Na ions, or exchanging Li ions and K ions. In this case, the original glass plate preferably contains 0.5 to 7.5% by weight of Li ₂ O as a glass composition, for example, 3.0% or 4.5%.

In addition, the process of the selective ion exchange step is not limited to the above-described method. For example, a deep compressive stress layer C is partially formed by immersing only a selected region in a molten salt for ion exchange or applying a molten salt. You may do it.

Here, the stress characteristics of the tempered glass can be measured using, for example, FSM-6000 manufactured by Orihara Seisakusho. When the depth of the compressive stress layer of the aluminosilicate glass exceeds 100 μm, or when ion exchange of Li ions is performed, the stress characteristics of the tempered glass can be measured using, for example, SLP-1000 manufactured by Orihara Seisakusho. it can. When a cross-sectional sample can be prepared by cutting tempered glass, it is desirable to observe the internal stress distribution and confirm the stress depth using, for example, WPA-micro manufactured by Photonic Lattice or Abrio manufactured by Tokyo Instruments. .

Hereinafter, the present invention will be described in detail based on examples.

In Table 1, Nos. 1 to 3 show examples of the present invention, and Nos. 4 to 7 show comparative examples.

Each sample of Table 1 was produced as follows. First, the glass composition contains, in mass%, SiO ₂ 61.6%, Al ₂ O ₃ 19.6%, B ₂ O ₃ 0.8%, Na ₂ O 16%, K ₂ O 2%. Glass raw materials were prepared and melted, and formed into a plate shape using an overflow downdraw method to obtain a tempered glass having a thickness of 0.7 mm. Next, ion-exchange suppression films having the respective compositions and film thicknesses shown in Table 1 were formed on both main surfaces of the tempering glass using a sputtering method, and then cut and the end surfaces were not formed (glass (Exposed) was obtained. Subsequently, the obtained glass with film was immersed in a molten potassium nitrate salt having a molten salt temperature shown in Table 1 for the immersion time shown in Table 1, washed with pure water and naturally dried, and then the ion exchange suppression film was removed by polishing. No. in Table 1 1-7 tempered glass plate samples were obtained.

For each glass sample obtained as described above, the stress depth DOL of each of the film formation region (main surface) and the non-film formation region (end surface) was measured with a stress meter (FSM-6000LE and FsmXP manufactured by Orihara Seisakusho). . When the DOL value of the film formation region (main surface) is zero, it is indicated that the compressive stress layer is not formed under the film formation region, and the ion exchange is sufficiently blocked by the ion exchange suppression film.

As shown in Table 1, the sample Nos. 1 to 3 as the examples contained an appropriate amount of ZrO ₂ in the ion exchange suppressing membrane, so that no wear occurred and ion exchange could be suitably blocked. In particular, excessive wear did not occur even at a relatively high molten salt temperature and a relatively thin film thickness, and ion exchange could be suitably interrupted.

On the other hand, sample Nos. 4, _5, and 7 use ion exchange suppression membranes made of SiO ₂ and Nb ₂ O ₅ , so that the membranes are worn out at a molten salt temperature of 430 ° C. or higher, and ion exchange is performed. Could not be blocked. Sample No. No. 6 was able to block ion exchange because the temperature of the molten salt was relatively low, but the film thickness of the ion exchange suppression film had to be thicker than in the examples. That is, as compared with the examples, a long film formation time was required and the productivity was low.

The tempered glass plate and the method for producing the same of the present invention are useful as a tempered glass plate used for a touch panel display and the like, and a method for producing the same.

G1 Original glass plate G2 Glass plate with film G3, Tempered glass plate with film G4, G5 Tempered glass plate M Ion permeation prevention film T1 First molten salt T2 Second molten salt

Claims (9)

1. A method for producing a tempered glass plate for strengthening a glass plate using an ion exchange method,
   A film forming step of forming an ion permeation preventive film for preventing the ion exchange on a part of the surface of the glass plate;
   A selective ion exchange step of selectively ion-exchanging a region other than the film-forming region by bringing the formed glass plate into contact with a molten salt,
   The method for producing a tempered glass sheet, wherein the ion permeation preventive film is an inorganic film containing ZrO ₂ .
2. The method for producing a tempered glass sheet according to claim 1, wherein the ion permeation preventive film is an inorganic film containing 50 to 100% by mass of ZrO ₂ .
3. The method for producing a tempered glass sheet according to claim 1 or 2, wherein, in the selective ion exchange step, the glass sheet is immersed in a molten salt bath having a temperature of more than 400 ° C and less than 500 ° C.
4. In the film formation step, the ion permeation preventive film is composed of 70 to 99% by mass of ZrO ₂ and at least one of Y ₂ O ₃ , CaO, Al ₂ O ₃ , CeO, and HfO _{2 in} a total amount of 1 to 30. The method for producing a tempered glass sheet according to any one of claims 1 to 3, wherein an inorganic film containing 1% is formed.
5. The method for producing a tempered glass sheet according to any one of claims 1 to 4, wherein in the film forming step, the ion permeation preventive film is formed to have a thickness of 400 nm or less.
6. A removal step of removing the ion permeation prevention film from the glass plate after the selective ion exchange step;
   The method for producing a tempered glass sheet according to any one of claims 1 to 5, further comprising a whole ion exchange step of performing ion exchange treatment on the entire surface of the glass plate after the removing step.
7. The method for producing a tempered glass sheet according to any one of claims 1 to 6, wherein the surface roughness Ra of the surface of the glass sheet in the film formation region is 0.4 nm or less.
8. A tempered glass plate subjected to a tempering treatment using an ion exchange method,
   On the surface,
   A film forming portion coated with an inorganic film containing 50 to 100% by mass of ZrO ₂ ;
   An tempered glass plate comprising: an exposed portion not covered with the inorganic film on at least a part of an end surface.
9. A tempered glass plate partially tempered using an ion exchange method,
   On the surface,
   A film forming portion coated with an inorganic film containing 50 to 100% by mass of ZrO ₂ ;
   An exposed portion that is not covered with the inorganic film on at least a part of the end surface;
   A tempered glass sheet, wherein the compressive stress layer on the glass surface in the film forming portion has a depth smaller than the compressive stress depth in the exposed portion.

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