Cover glass for mobile terminals, manufacturing method of the same and mobile terminal device

Abstract

To provide cover glass for mobile terminals exhibiting high strength in a thin plate thickness state to enable reductions in thickness of apparatuses when inserted in the apparatuses, cover glass (1) for a mobile terminal of the invention is cover glass (1) that is obtained by forming a resist pattern on main surfaces of a plate-shaped glass substrate, then etching the glass substrate with an etchant using the resist pattern as a mask, and thereby cutting the glass substrate into a desired shape and that protects a display screen of the mobile terminal, where an edge face of the cover glass (1) is formed of a molten glass surface, and as surface roughness of the edge face, arithmetic mean roughness Ra is 10 nm or less.

Claims

1. A plate-shaped glass, wherein an edge face of the plate-shaped glass is formed by etching, wherein the plate-shaped glass has a substantially rectangular shape, wherein the plate-shaped glass comprises a hole penetrating a pair of main surfaces, wherein an inner wall surface of the hole is formed by etching, and wherein each of the edge face and the inner wall surface comprises: a maximum protruding portion, and inclined surfaces inclined toward the main surfaces, respectively, from the maximum protruding portion.

2. The plate-shaped glass according to claim 1, wherein each of the main surfaces comprises a compressive stress layer.

3. The plate-shaped glass according to claim 1, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

4. A plate-shaped glass, wherein an edge face of the plate-shaped glass is formed by etching, wherein the plate-shaped glass has a substantially rectangular shape, wherein an outer circumferential portion of the plate-shaped glass includes a concave portion recessed inward, and an inner wall surface of the concave portion is formed by etching, and wherein the outer circumferential portion of the plate-shaped glass includes an outwardly protruding portion, wherein concave-curved inclined surfaces extend from either side of the protruding portion to respective main surfaces of the plate-shaped glass.

5. The plate-shaped glass according to claim 4, wherein each of the main surfaces comprises a compressive stress layer.

6. The plate-shaped glass according to claim 4, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

7. A plate-shaped glass having first and second main surfaces, the plate-shaped glass comprising an etched edge face, wherein the plate-shaped glass has a substantially rectangular shape, wherein the etched edge face of the plate-shaped glass includes a protruding portion, wherein the protruding portion is formed by first and second surfaces that meet along a common edge, the first surface being connected with the first main surface and the second surface being connected with the second main surface, and wherein each of the first surface and the second surface, which form the protruding portion, comprises a curved concave surface.

8. The plate-shaped glass according to claim 7, wherein each of the main surfaces comprises a compressive stress layer.

9. The plate-shaped glass according to claim 7, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

10. The plate-shaped glass according to claim 7, wherein the common edge forms a ridge.

11. The plate-shaped glass according to claim 7, wherein the plate-shaped glass is rounded at a boundary between the first main surface and the first surface and is rounded at a boundary between the second main surface and the second surface.

12. A plate-shaped glass, wherein arithmetic mean roughness Ra of an edge face of the plate-shaped glass is 10 nm or less, and the plate-shaped glass has a substantially rectangular shape, wherein the plate-shaped glass includes a hole penetrating a pair of main surfaces, wherein an inner wall surface of the hole is formed by etching, and wherein each of the edge face and the inner wall surface comprises: a maximum protruding portion, and inclined surfaces inclined toward the main surfaces, respectively, from the maximum protruding portion.

13. The plate-shaped glass according to claim 12, wherein each of the main surfaces comprises a compressive stress layer.

14. The plate-shaped glass according to claim 12, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

15. A plate-shaped glass, wherein arithmetic mean roughness Ra of an edge face of the plate-shaped glass is 10 nm or less, and the plate-shaped glass has a substantially rectangular shape, wherein an outer circumferential portion of the plate-shaped glass has a concave portion recessed inward, wherein an inner wall surface of the concave portion is formed by etching, and wherein the outer circumferential portion of the plate-shaped glass includes an outwardly protruding portion, wherein concave-curved inclined surfaces extend from either side of the protruding portion to respective main surfaces of the plate-shaped glass.

16. The plate-shaped glass according to claim 15, wherein each of the main surfaces comprises a compressive stress layer.

17. The plate-shaped glass according to claim 15, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

18. A plate-shaped glass having first and second main surfaces, wherein the plate-shaped glass has substantially rectangular shape, wherein an edge face of the plate-shaped glass includes first and second concave surfaces, the first concave surface being connected with the first main surface and the second concave surface being connected with the second main surface, and wherein the plate-shaped glass is rounded at a boundary between the first concave surface and the first main surface and is rounded at a boundary between the second concave surface and the second main surface.

19. The plate-shaped glass according to claim 18, wherein each of the main surfaces comprises a compressive stress layer.

20. The plate-shaped glass according to claim 18, wherein the thickness of the plate-shaped glass is 0.5 mm or less.

21. The plate-shaped glass according to claim 19, wherein a protruding portion is formed on the edge face by connecting the first and second concave surfaces with each other.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view showing part of a mobile terminal equipped with cover glass for mobile terminals according to an Embodiment of the invention;

(2) FIG. 2(a) is a diagram to explain the external form of the cover glass as shown in FIG. 1; FIG. 2(b) is a diagram to explain the negative curvature; FIG. 2(c) is a diagram to explain the negative curvature and positive curvature;

(3) FIG. 3 is a diagram showing a shape of an edge face of the cover glass as shown in FIG. 2;

(4) FIGS. 4(a) to 4(c) are diagrams to explain shape processing by etching of a glass substrate;

(5) FIG. 5 is a diagram to explain a shape of cover glass of Example 3; and

(6) FIGS. 6(a) to 6(c) are diagrams to explain a manufacturing process to prepare the cover glass as shown in FIG. 2(a) by a manufacturing method of a Comparative Example.

BEST MODE FOR CARRYING OUT THE INVENTION

(7) An Embodiment of the invention will specifically be described below with reference to accompanying drawings.

(8) FIG. 1 is a cross-sectional view showing part of a mobile terminal device equipped with cover glass for mobile terminals according to an Embodiment of the invention. In the mobile terminal device as shown in FIG. 1, cover glass 1 is disposed at a distance D above a liquid crystal display panel 2. The liquid crystal display panel 2 is configured such that a pair of glass substrates 21 and 22 sandwich a liquid crystal layer 23. In addition, in FIG. 1, the other members are omitted which are generally used in a liquid crystal display panel.

(9) The cover glass in this Embodiment of the invention is obtained by forming a resist pattern on a main surface of a plate-shaped glass substrate, then etching the glass substrate with an etchant using the resist pattern as a mask, and thereby cutting the glass substrate into a desired shape, while an edge face 14 of the cover glass 1 as shown in FIG. 3 is formed of a molten glass surface, and surface roughness (arithmetic mean roughness Ra) in the edge face 14 is 10 nm or less. Thus, in the cover glass according to the invention, since the external form is formed by etching, the edge face 14 formed by etching has extremely high smoothness, is formed of the molten glass surface, and therefore, is in a state without micro-cracks that certainly exist on an edge face formed by mechanical processing. In the cover glass with such a configuration, even when the shape of the external form of the cover glass for mobile terminals is a complicated shape, it is possible to process the external form into a desired shape with ease, and it is possible obtain high mechanical strength required for cover glass for mobile terminals.

(10) Further, for example, even when the plate thickness is 0.5 mm or less and thus thin, it is possible to maintain high mechanical strength. When such cover glass with a thin thickness is inserted in an apparatus, since the cover glass is hard to bend by external force due to the high mechanical strength, it is possible to reduce the distance between the cover glass and display. As a result, it is possible to intend to thin the apparatus.

(11) In addition, as an etching method to etch the glass substrate, either of wet etching and dry etching is available. From the viewpoint of reducing the processing cost, wet etching is preferable. Any etchant capable of etching a glass substrate can be used as an etchant used in wet etching. For example, it is possible to use an acidic solution containing hydrofluoric acid as the main ingredient, mixed acid containing hydrofluoric acid and at least one kind of acid among sulfuric acid, nitric acid, hydrochloric acid, and hydrofluorosilicic acid, etc. Further, as an etchant used in dry etching, any etchant capable of etching a glass substrate can be used, and for example, it is possible to use fluorine gas.

(12) Moreover, the cover glass 1 of the invention can be formed in a shape including a portion having the negative curvature in part of a contour constituting the cover glass as shown in FIG. 2(a). Herein, the positive curvature and negative curvature are defined. It is assumed that the contour constituting the cover glass is represented by curve C1 and curve C2 as shown in FIG. 2(b), and that the cover glass is represented by area A. At this point, when the contour constituting the cover glass is traced while always seeing the inside of the area A on the left side, a portion of the contour turning to the left as proceeds is defined as the positive curvature, another portion of the contour turning to the right as proceeds is defined as the negative curvature, and the curvature of a straight portion that turns to neither the left nor right is defined as 0. Accordingly, the curve C1 as shown in FIG. 2(b) is a curve having the positive curvature, and the curve C2 as shown in FIG. 2(b) is a curve having the negative curvature. Further, also when the contour constituting the cover glass is complicated and becomes a shape as shown in FIG. 2(c), according to the aforementioned definition, a segment C+ is the positive curvature, a segment C is the negative curvature, and the curvature of a segment C0 is 0.

(13) When the negative curvature is defined as described above, the portion having the negative curvature in the cover glass 1 as shown in FIG. 2(a) means corner portions 121 of a hole portion 12, corner portions 111 of a concave portion 11, etc. which are formed within the main surface of the cover glass 1.

(14) Such a shape including portions having the negative curvature is a shape hard to process by mechanical processing that is the external form processing in the ordinary manufacturing process of the glass substrate. It is possible to actualize such a shape with ease by using photolithography and etching as described later. In addition, the corner portion 121 of the hole portion 12 and the corner portion 111 of the concave portion 11 described herein do not include portions caused by chipping of the glass or surface roughness and surface swell comprised of minute convex portion and/or concave portion formed on the cover glass surface. In other words, such portions do not include a convex portion caused by chipping of the glass in the portion having the positive curvature, concave portion in surface roughness or surface swell, or the like.

(15) The cover glass 1 can be fabricated using a plate-shaped glass substrate (sheet glass) formed by a down-draw method. Among glass enabling glass plate formation by a down-draw method is aluminosilicate glass containing SiO.sub.2, Al.sub.2O.sub.3, LI.sub.2O and/or Na.sub.2O. In particular, the aluminosilicate glass preferably contains 62 percent to 75 percent by weight of SiO.sub.2, 5 percent to 15 percent by weight of Al.sub.2O.sub.3, 4 percent to 10 percent by weight of Li.sub.2O, 4 percent to 12 percent by weight of Na.sub.2O, and 5.5 percent to 15 percent by weight of ZrO.sub.2. Further, such a composition is preferable that the ratio by weight of Na.sub.2O/ZrO.sub.2 ranges from 0.5 to 2.0, and that the ratio by weight of Al.sub.2O.sub.3/ZrO.sub.2 ranges from 0.4 to 2.5.

(16) SiO.sub.2 is the prime component forming a glass skeleton. The cover glass for mobile terminal devices, particularly, for cellular phones is used in significantly severe environments such that the cover glass comes into contact with human skin, water, rain, etc. and even in such environments, is required to exert sufficient chemical durability. With consideration given to the chemical durability and melting temperature, the content of SiO.sub.2 preferably ranges from 62 percent by weight to 75 percent by weight.

(17) Al.sub.2O.sub.3 is contained to enhance ion exchange performance of the glass surface. Further, the cover glass for mobile terminals needs to have viewability i.e. transparency. With consideration given to chemical durability and transparent durability, the content of Al.sub.2O.sub.3 preferably ranges from 5 percent by weight to 15 percent by weight.

(18) Li.sub.2O is ion-exchanged with Na ion mainly in an ion exchange treatment bath in the glass surface portion, and is thus an essential component in chemically strengthening the glass. With consideration given to ion exchange performance, transparent resistance and chemical durability, the content of LiO.sub.2 preferably ranges from 4 percent by weight to 10 percent by weight.

(19) Na.sub.2O is ion-exchanged with K ion in the ion exchange treatment bath in the glass surface portion, and is thus an essential component in chemically strengthening the cover glass. Further, the mobile terminal device is used under circumstances where shock and/or external force is applied to the display screen by dropping the apparatus, pressing the display screen repeatedly, or by opening and closing in the case of open/close type mobile terminal devices, and even in such use environments, is required to need sufficient mechanical strength. With consideration given to the mechanical strength, transparent resistance and chemical durability, the content of Na.sub.2O preferably ranges from 4 percent by weight to 12 percent by weight.

(20) ZrO.sub.2 has the effect of increasing the mechanical strength. With consideration given to chemical durability and stable manufacturing of homogenous glass, the content of ZrO.sub.2 preferably ranges from 5.5 percent by weight to 15 percent by weight.

(21) Further, in the aforementioned aluminosilicate glass, by performing chemical strengthening by ion exchange treatment, and thereby forming a compressive stress layer in the glass surface, it is possible to further increase the mechanical strength. In addition, as a substitute for the aluminosilicate glass, other multi-component glass may be used. Further, when required transparency is guaranteed as the cover glass for mobile terminals, crystallized glass may be used.

(22) In the invention, the effect is particularly exhibited when the thickness of the cover glass 1 is 0.5 mm or less. Further, as glass constituting the cover glass 1, it is preferable to use the glass chemically strengthened by ion exchange treatment as described above. The chemically strengthened glass is glass strengthened by replacing alkali metal ions constituting the glass with alkali metal ions larger in size than the alkali metal ions constituting the glass. A compressive stress layer is formed in the surface of thus chemically strengthened glass.

(23) Described next is a method of manufacturing cover glass for mobile terminals of the invention.

(24) In an external form processing step in manufacturing of the cover glass, the external form processing is performed by performing photolithography and etching on a glass substrate. In an etching step, as shown in FIG. 4(a), both main surfaces of the glass substrate 1 are coated with resist materials 31. Next, the resist materials are exposed through a photo mask having a pattern with a desired external form shape (for example, external form including portions having the negative curvature). Then, as shown in FIG. 4(b), the exposed resist materials are developed, a resist pattern is formed (an opening portion 31a is formed) in an area except areas to be etched of the glass substrate, and the areas to be etched of the glass substrate are etched. At this point, in the case of using a wet etchant as an etchant, as shown in FIG. 4(b), the glass is isotropically etched, and by this means, the edge face 14 becomes the shape as shown in FIG. 4(c). In other words, in the edge face 14, a center portion 14a protrudes outward the most, and inclined surfaces 14b, 14b are formed which are gently curved respectively toward both main surface 13 sides from the center portion 14a. In addition, it is preferable that the boundary between the inclined surface 14b and the main surface 13 and the boundary (center portion 14a) between the inclined surfaces 14b are in a rounder shape with a radius of several tens of micrometers. By forming such an edge-face shape, in inserting the cover glass in a frame, etc. of a mobile terminal device, it is possible to perform insertion with ease without galling and/or chipping occurring.

(25) As a resist material used in the etching step, any material is available which has resistance to the etchant used in etching the glass using a resist pattern as a mask. The glass is normally etched by wet etching of an aqueous solution containing hydrofluoric acid, or dry etching of fluorine gas, and therefore, for example, it is possible to use resist materials excellent in resistance to hydrofluoric acid.

(26) As an etchant used in the etching step, it is possible to use a mixed acid containing hydrofluoric acid and at least one kind of acid among sulfuric acid, nitric acid, hydrochloric acid and hydrofluorosilicic acid, etc. By using the aforementioned mixed acid aqueous solution as an etchant, the cover glass having an extremely high surface state is obtained such that the edge face of the cover glass cut into the desired shape has surface roughness of 10 nm or less and thus has high smoothness of the order of nanometers, and that micro-cracks do not exist which are certainly formed in forming the external form by mechanical processing. Further, since photolithography is adopted in forming the external form, dimensional accuracy of the cut cover glass is also good. Accordingly, even when the shape of the external form of the cover glass for mobile terminals is a complicated shape, the cover glass with good dimensional accuracy is obtained, and it is possible to obtain high mechanical strength required for the cover glass for mobile terminals. Further, by this external form processing by photolithography and etching, it is also possible to enhance productivity and reduce the processing cost. Furthermore, as a remover solution to remove the resist material from the glass substrate, it is preferable to use an alkali solution of KOH, NaOH, etc. In addition, kinds of the resist material, etchant and remover solution are capable of being selected as appropriate corresponding to the material of the glass substrate that is a material to be etched.

(27) Further, since the processing for external form is performed by etching step, it is possible to form the concave portion 11 and hole portion 12 including portions of the negative curvature in the cover glass with ease. Furthermore, similarly, since the etching step is used, by adjusting the mask pattern, it is possible to add a design (various shapes of the external form) such as the logo to the cover glass. By this means, it is possible to easily actualize a complicated design which will be required for the cover glass in the future but cannot be implemented by mechanical processing.

(28) Furthermore, as the plate-shaped glass substrate, it is possible to use glass substrates that are directly formed in the shape of a sheet from molten glass, or glass substrates obtained by cutting a glass material formed in some thickness into a predetermined thickness, polishing the main surface, and finishing in a predetermined thickness. It is preferable to use glass substrates that are directly formed in the shape of a sheet from molten glass. This is because the main surface of the glass substrate that is directly formed in the shape of a sheet from molten glass is a surface formed by hot forming has extremely high smoothness, and further has a surface state without micro-cracks. Methods for directly forming the molten glass in the shape of a sheet include a down-draw method and float method. The down-draw method is preferable among the methods. In addition to the above-mentioned effects of high smoothness, etc. in the case of performing the external form processing by etching step, since it is possible to perform etching uniformly from both main surfaces in etching the both main surfaces of the glass substrate using resist patterns formed on the both main surfaces of the glass substrate as a mask, dimensional accuracy is good, and the shape in cross section of the edge face of the cover glass is excellent, thus being preferable.

(29) Moreover, for the same reason as described above, the glass in the method of manufacturing cover glass of the invention is preferably aluminosilicate glass containing SiO.sub.2, Al.sub.2O.sub.3, Li.sub.2O and/or Na.sub.2O.

(30) Further, by performing chemical strengthening by ion exchange treatment on the cover glass after the step of the external form processing by etching, a compressive stress layer is formed in the surface of the cover glass, and it is thereby possible to further enhance mechanical strength of the cover glass. In the case of using the above-mentioned plate-shaped glass substrate obtained by the method such as the down-draw method of directly forming molten glass in the shape of a sheet, since both main surfaces of the glass substrate are surfaces formed by hot forming, and thereby have surface states with extremely high smoothness without micro-cracks that are surely formed when the external from is formed by mechanical processing, the compressive stress layer formed by chemical strengthening is required to be 5 pm or more. The thickness of the compressive stress layer is preferably 50 pm or more, and more preferably, 100 pm. Since the cover glass for protecting a display screen of a mobile terminal is provided to cover the display screen, flaws are tend to occur in handling, and further, in consideration of the respect that shock and/or external force is applied by pressing the display screen repeatedly, or by opening and closing in the case of open/close type mobile terminal devices, the compressive stress layer is desired to be formed deeply.

(31) Furthermore, the cover glass 1 fabricated by the above-mentioned manufacturing method of cover glass has the main surface 13 and the edge face 14 as shown in FIG. 3, and the cross-sectional shape of the edge face 14 is almost the same in the entire edge face 14 in the external form as shown in FIG. 2. In the cover glass according to the invention, since the external form is formed by etching as described above, all of the obtained edge face 14 of the external form is formed under the same conditions. Therefore, in the case of forming the external form by wet etching, since the glass is etched isotropically, the edge face 14 is in the shape as shown in FIG. 3 over the whole of the external form.

(32) Described next are Examples performed to clarify the effects of the invention.

(33) Descriptions are given below using cover glass for a cellular phone to protect a display screen of the cellular phone as an example.

Example 1

(34) First, aluminosilicate glass containing 63.5 percent by weight of SiO.sub.2, 8.2 percent by weight of Al.sub.2O.sub.3, 8.0 percent by weight of Li.sub.2O, 10.4 percent by weight of Na.sub.2O and 11.9 percent by weight of ZrO.sub.2 was formed into plate-shaped glass substrates (sheet-shaped glass) with a sheet thickness of 0.5 mm by a down-draw method. Surface roughness (arithmetic means roughness Ra) of the main surface of the sheet-shaped glass formed by the down-draw method was measured using an atomic force microscope, and was 0.2 nm.

(35) Next, both main surfaces of the sheet-shaped glass were coated with negative type hydrofluoric-acid resistant resist in a thickness of 30 (m, and the hydrofluoric-acid resistant resist underwent baking processing at 150 C. for 30 minutes. Then, the hydrofluoric-acid resistant resist was exposed from the both surfaces through a photo-mask having a pattern of the external form including portions having the negative curvature as shown in FIG. 2(a), the exposed hydrofluoric-acid resistant resist was developed using a developer (Na.sub.2CO.sub.3 solution), and a resist pattern was formed where the hydrofluoric-acid resistant resist was left in areas except etching-target areas on the sheet-shaped glass.

(36) Next, using a mixed acid aqueous solution of hydrofluoric acid and hydrochloric acid as an etchant, the etching-target areas of the sheet-shaped glass were etched from the both main surface sides using the resist pattern as a mask, and the glass was cut into the external form including the portions having the negative curvature as shown in FIG. 2(a). Then, the hydrofluoric-acid resistant resist remaining on the glass was swollen using an NaOH solution, and was removed from the glass, and rinsing treatment was performed. In this way, obtained was cover glass for a cellular phone of the Example having the external form as shown in FIG. 2(a).

(37) When the cross-sectional shape of the edge face was examined along the external form of thus obtained cover glass (Example) using a light microscope, it was confirmed that the cross-sectional shape was the shape as shown in FIG. 4(c) and almost the same over the entire external form. Further, surface roughness (arithmetic means roughness Ra) of the main surface of the obtained cover glass was measured using an atomic force microscope, was 0.2 nm, did not change from the surface state immediately after forming by the down-draw method, and had high smoothness. Furthermore, surface roughness (arithmetic means roughness Ra) of the edge face of the cover glass was measured using the atomic force microscope, and was 1.2 to 1.3 nm over the entire external form. As the reason why surface roughness of the edge face was thus low, since the etching step was adopted without performing mechanical processing in processing the external form where the glass substrate was etched with an etchant using a resist pattern formed on the sheet glass as a mask and thereby cut into a desired shape, it is considered that brush marks and/or micro-cracks did not exist that occur in mechanical processing by polishing grain, grinding grain or the like.

(38) Further, when the presence or absence of micro-crack of the edge face of the cover glass was checked using a scanning electron microscope, any micro-crack was not found.

Example 2

(39) The glass cut by etching from which the resist was removed in aforementioned Example 1 was immersed in a treatment bath of mixed acid of 60 percent of potassium nitrate (KNO.sub.3) and 40 percent of sodium nitrate (NaNO.sub.3) kept at 385 to 405 C. for 4 hours to undergo ion exchange treatment, and chemically strengthened cover glass for a cellular phone was prepared where a compressive stress layer of 150 m was formed in the glass surface.

(40) When surface roughness of the main surface and edge face was measured as in Example 1, surface roughness of the main surface was 0.3 nm and surface roughness of the edge face was 1.4 to 1.5 nm. Further, when checked whether micro-cracks were present in the edge face, it was confirmed that any micro-crack did not exist.

Example 3

(41) Cover glass for a cellular phone was prepared as in above-mentioned Example 1 except that the shape of the external form of the cover glass was a rectangular shape (size: 50 mm40 mm, thickness: 0.5 mm) as shown in FIG. 5. Surface roughness of the main surface and edge face of the cover glass and micro-cracks were checked as in Example 1, and it was confirmed that surface roughness was the same as in Example 1, and that any micro-crack did not exist.

Example 4

(42) As in above-mentioned Example 1 except that a mixed acid aqueous solution of hydrofluoric acid and nitric acid was used as an etchant, the etching-target areas of the sheet-shaped glass were etched from the both main surface sides using the resist pattern as a mask, the glass was cut into the external form including the portions having the negative curvature as shown in FIG. 2(a), and the cover glass for a mobile terminal was prepared. When surface roughness of the main surface and edge face of the cover glass and micro-cracks were checked as in Example 1, surface roughness of the main surface was 0.2 nm, surface roughness of the edge face was 10 nm, and it was confirmed that any micro-crack did not exist.

Comparative Example 1

(43) The same aluminosilicate glass as in above-mentioned Example 1 was formed into plate-shaped glass substrates (sheet-shaped glass) by the down-draw method. Next, the formed sheet glass was cut into a rectangle slightly larger than finished measurements using a scriber, processed into a desired shape by grinding the outer edge using a rotary grinder with diamond grain embedded therein, and processed into the shape as shown in FIG. 6(a).

(44) Next, only the outer edge portion was ground using a diamond grinder, and underwent predetermined chamfering processing. Then, a hundred sheets of plate-shaped glass in the shape as shown in FIG. 6(a) were stacked, and the portion (concave portion 11) having the negative curvature was formed using a diamond grinder by mechanical processing (FIG. 6(b)).

(45) Subsequently, five sheets of plate-shaped glass in the shape as shown in FIG. 6(b) were stacked, the portion (hole portion 12) having the negative curvature was formed using a diamond grinder by mechanical processing, and the glass was formed into the shape as shown in FIG. 6(c). Then, finally both main surfaces were polished into mirror surfaces using eerie oxide, and the sheet thickness was adjusted to 0.5 mm. In this way, obtained was cover glass for a cellular phone of Comparative Example 1 having the shape as shown in FIG. 2(a).

(46) Surface roughness Ra of the main surface and edge face of thus obtained cover glass (Comparative Example 1) was measured as in the Examples. Surface roughness of the main surface was 0.3 nm, and thus was not different from that in Example 1 so much, but surface roughness of the edge face was 0.2 m and was a significantly large value. Further, when the presence or absence of micro-crack in the edge face of the cover glass was examined, it was confirmed there were many micro-cracks with depths ranging from dozens to hundreds of micrometers. As the reason why surface roughness Ra was thus large or many micro-cracks existed, it is considered that mechanical processing was adopted in shape processing.

Comparative Example 2

(47) The cover glass of aforementioned Comparative Example 1 was subjected to ion exchange treatment under the same conditions as in Example 2, and chemically strengthened cover glass for a mobile terminal was prepared where a compressive stress layer was formed in the glass surface. When surface roughness of the main surface and edge face of the cover glass and micro-cracks were checked as in Comparative Example 1, it was confirmed that surface roughness was the same as in Comparative Example 1, and many micro-cracks existed.

Comparative Example 3

(48) Cover glass for a cellular phone was prepared as in Comparative Example 2 except that in above-mentioned Comparative Example 1, the shape of the external form of the cover glass was made the same shape as shown in Example 3. When surface roughness of the main surface and edge face of the cover glass and micro-cracks were checked as in Comparative Example 1, it was confirmed that surface roughness was the same as in Comparative Example 1, and many micro-cracks existed.

(49) (Evaluation Test of Mechanical Strength of the Cover Glass of Examples 1 to 4 and Comparative Examples 1 to 3)

(50) The cover glass was set on a support mount coming into contact with the outer circumferential edge portion in 3 mm in the main surface of the cover glass, and static pressure strength tests were performed while pressing the center portion of the cover glass from the main surface side opposite to the side in contact with the support mount using a pressurizing member. Used as the pressurizing material was a material made of a stainless alloy having a front end with (I) of 5 mm.

(51) As a result, in the cover glass of Examples 1 to 4, the breaking load when the glass was broken exceeded 50 kgf, and the glass had the extremely high mechanical strength. Meanwhile, the cover glass of Comparative Examples 1, 2 and 3 was of 5 kgf, 14 kgf, and 17 kgf, and thus had the significantly low strength. Particularly, the cover glass of Comparative Example 1 was extremely weak in mechanical strength. When the state of cracking of the cover glass of Comparative Example 1 was checked, it was confirmed that cracking proceeded from micro-cracks existing in areas including the portions having the negative curvature.

(52) The above-mentioned cover glass for mobile terminals of the invention is applicable to mobile terminal devices such as cellular phones by being provided above a display screen of the apparatus body having the display screen. Each of the cover glass of Examples 1 to 4 was provided on a display screen of a cellular phone, the cellular phone was subjected to a repetitive drop test, and each cover glass had high mechanical strength without any cracking being found.

(53) The present invention is not limited to the above-mentioned Embodiment, and is capable of being carried into practice with modifications thereof as appropriate. For example, the shape of the external form, the numbers of members, sizes, processing procedures and the like in the above-mentioned Embodiment are examples, and are capable of being carried into practice with various modifications thereof within the scope of exhibiting the effects of the invention. Further, the invention is capable of being carried into practice with modifications thereof as appropriate without departing from the scope of the object of the invention.