Related Prior Arts It is generally recognized that optical fibers made of glass, which are most popular as optical communication materials, are very strong when they are coated with resin. However, when a device using optical fibers is fabricated, such resin coating has to be peeled off, and thus there has been a problem that the device with the resin coating peeled off suddenly becomes fragile due to the surface minute cracks caused when the coating was peeled off.

In recent years, demands for a large-sized, flat glass plate have increasingly risen in association with the large-sizing of displays in plasma display panel (PDP) or the like. However, since glass becomes easier to crack even against the slightest force from the exterior as the size thereof becomes larger, techniques for improving fracture strength of glass, that is, methods for improving anti-impact properties thereof, have attracted the attention.

Conventional glass strengthening methods includes a thermally strengthening method using air blast to a heated plate glass, surface crystallization method forming a crystallized layer on the surface thereof and so on. However, the foregoing conventional methods would need heat treatment at high temperature, and thus the plate glasses must have a thickness enough to withstand such treatment. In the thermally strengthening method, for example, it is said that the method is not applicable to a thin plate less than about 3.2mm in thickness.

On the other hand, methods of obtaining a thin strengthened glass include a chemical strengthening method. This technique is also called an ion exchange method, and is the one to obtain strengthened glass through the exchange of alkali ion (normally sodium ion) in the glass with other alkali ions of melted salt (normally potassium ion) at a high temperature so as to form a compression stress layer on the surface of the glass. This method has an advantage that twice or larger strength can be obtained compared with thermally strengthening method and is applicable regardless of the shape of the glass.

According to this method, however, the composition of the glass material is subjected to restraints to perform ion exchange, the surface is sometimes deteriorated due to chemical reaction on the surface, resulting in loss of transparency, and a partial strengthening is difficult.

Glass strengthening methods by irradiating a plate glass with ions include a method of irradiating the plate glass with nitrogen ion, the plate glass having a chemical bond of silicon and oxygen to thereby form a layer having a chemical bond of silicon and nitrogen on the surface of the plate glass, as disclosed in Japanese Un-Examined Patent Publication No. 1-246159.

However, this method is only applicable to a specific kind of glass materials that have a chemical bond of silicon and oxygen, and the kind of ion is limited to nitrogen only. Whilst this method improves the surface hardness of glass by forming the surface of the glass with a strong bond of silicon and nitrogen so as to improve radiation, abrasion and corrosion resistance, it is recognized that there is no correlation between surface hardness and fracture strength. Besides this technique, there is also a technique for improving surface hardness or abrasion resistance by ion implantation, as disclosed in, for example, Japanese Un-Examined Patent Publication No. 5-254890. As discussed above, however, the surface stiffening does not necessarily lead to the improvement of fracture strength.

On the other hand, a glass strengthening method using a femtosecond laser that is subjected to less restraints in thickness and material of glass is proposed, as disclosed in Japanese Un-Examined Patent Publication No.

2003-286048.

This technique is a method of forming a different phase of from one to tens of nm spot level located below the depth of between 10-100 u m by irradiating the glass with focused femtosecond laser beam to thereby suppress the progress of cracks. According to this laser-used glass strengthening method, however, there is a problem that an optical system for focusing the laser beam is very complex and expensive, and the femtosecond laser system itself is also too expensive. Moreover, irradiating a large-area plate glass is difficult because the method allows only tens of nm spot to be processed in a single laser pulse.

Furthermore, in a case that the glass to be processed is of a non-flat type, but for example is cylindrical one such as optical fibers, the control of light focusing point becomes too complex since the point needs to be controlled along the glass's surface shape, and thus it is difficulty to apply this method to the glass having a complex surface.

Summary of Invention As discussed in the foregoing, conventional glass strengthening methods have various drawbacks such as restraints in thickness, surface shape and material of glass, and costs and time being too expensive and too consuming. The present invention has been made to solve these problems, and it is, accordingly, an object of the invention to provide a method of strengthening a glass sample at low cost and in a short time, without being subjected to restraints in thickness, surface shape and material of the glass.

According to an aspect of the invention, there is provided a method of strengthening a glass sample by implanting accelerated ions into a surface of the glass material, wherein the ions are implanted more deeply than minute cracks that exist on the surface of the glass.

Thus, due to the employment of ions, any light-focusing optical system for use in laser beam is not necessary. Further, since ions can be implanted into a larger area at a time, it is possible to strengthen the glass of a large area in a short time. Moreover, due to the depth to which ions are implanted from the surface of glass is constant and not affected by the surface shape of the glass, it is not necessary to adjust the focusing point to the surface shape of the glass, unlike the case using laser beam. Nor is this method subjected to the restrains in thickness and material of the glass.

Moreover, whilst glass is subjected to certain change in property, such as densification or crystallization in a position where ions are implanted, the method of the invention allows ions to be implanted from minute cracks present on the surface of the glass into a deeper part, and thus such transformed areas are formed not on the surface of glass, but in a portion of the glass that is deeper than the minute cracks. Thus, the glass is strengthened, presumably due to the further progress of the cracks on the surface of glass being suppressed owing to these transformed areas.

In a preferred form of the foregoing glass strengthening method, the ions may be implanted from the surface of the glass to the depth of 5 u m or more.

According to this method, due to the employment of ions, any light-focusing optical system for use in laser beam is not necessary. Further, since ions can be implanted into a larger area at a time, it is possible to strengthen the glass over a large area in a short time. Moreover, due to the depth to which ions are implanted from the surface of glass is constant and not affected by the surface shape of the glass, it is not necessary to adjust the focusing point to the surface shape of the glass, unlike the case using laser beam. Nor is this method subjected to the restrains in thickness and material of the glass.

Moreover, whilst glass is transformed where ions are implanted, the method of the invention allows ions to be implanted from the surface of the glass to the depth of 5 u m or more, and thus such transformed areas are formed not on the surface of glass, but in a portion of the glass that is located 5 u m or more deep. Thus, even if minute cracks are formed on surface of the glass, the glass is strengthened, presumably due to the further progress of the cracks on the surface of glass being suppressed owing to these transformed areas.

In a more detailed aspect of the invention, the aforesaid ion is hydrogen ion or helium ion. Thus, due to the small mass of the ions, it is possible to easily accelerate the ions to high speed, and to allow them to advance deep into the glass after being implanted thereinto, and thus the ions can be easily implanted more deeply than the cracks on the surface of the glass or to the depth of 5 It m or more from the surface.

In a further detailed aspect of the invention, the ions are accelerated by accelerating energy in the order of MeV or below. Accordingly, high-speed ions are capable of being generated easily through the acceleration by practicable high energy, enabling the implantation of the ions deeply, and thus the ions can be easily implanted more deeply than the cracks on the surface of the glass or to the depth of 5 u m or more from the surface.

In a still further detailed aspect of the invention, a conductive material is applied to or deposited on the surface of the glass prior to the implantation of the ion. Accordingly, although ions are electrically charged particles and glass is generally insulating material, the glass is earthed via the conductive material applied or deposited on the surface of the glass, thus enabling the glass to be prevented from being electrically charged.

In an even more specific aspect of the invention, the surface of the glass is covered with a conductive mesh or two or more wires, so that the ions are irradiated from thereabove. Accordingly, although ions are electrically charged particles and glass is generally insulating material, the glass is earthed via the conductive mesh or two or more wires covering the surface of the glass, thus enabling the glass to be prevented from being electrically charged.

In an alternative aspect of the invention, there is provided a strengthened glass that is strengthened by implanting accelerated ions into a surface of the glass material, wherein the accelerated ions are implanted more deeply than minute cracks that exist on the surface of the glass.

Thus, whilst glass is transformed where ions are implanted, the strengthened glass of the invention includes ions implanted from minute cracks present on the surface of the glass into a deeper part, and thus such transformed areas are formed not on the surface of glass, but in a portion of the glass that is deeper than the minute cracks. Thus, the glass is strengthened, presumably due to the further progress of the cracks on the surface of glass being suppressed owing to these transformed areas.

In a further specific aspect of the invention, the strengthened glass may include the ions implanted from the surface of the glass to the depth of 5 g m or more.

Thus, whilst glass is transformed where ions are implanted, the strengthened glass of the invention includes ions implanted from the surface of the glass to the depth of 5 u m or more from the surface, and thus such transformed areas are formed not on the surface of glass, but in a portion of the glass that is 5 It m or more deep from the surface. Thus, even if minute cracks are formed on the surface of the glass, the glass is strengthened, presumably due to the progress of the cracks on the surface of glass being suppressed owing to these transformed areas.

In a further specific aspect of the invention, the aforesaid ion is hydrogen ion or helium ion. Thus, due to the small mass of the ions, it is possible to easily accelerate the ions to high speed, and to allow them to advance deep into the glass after being implanted thereinto, and thus the ions can be easily implanted more deeply than the cracks on the surface of the glass or to the depth of 5 A m or more from the surface.

It is to be noted that according to the invention, there can be provided a method of strengthening a glass sample at low cost and in a short time, without being subjected to restraints in thickness, surface shape and material of the glass.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of an ion implantation device used for an embodiment of the invention; Fig. 2 is a partially cross-sectional schematic diagram of a glass sample with hydrogen ion implanted thereinto according to the embodiment; Fig. 3A is a diagram illustrating the ions implanted into the glass sample where a conductive film of an appropriate thickness is applied, while Fig. 3B shows that of too large a thickness is applied.

Fig. 4 is a schematic diagram illustrating a structure for applying electric or magnetic field to a path of an ion beam by arranging an electrode or a magnet in the path; Fig. 5 is a schematic diagram showing an outer peripheral portion of the glass that is strengthened by the ions implanted into the neighborhood thereof; Fig. 6 shows a gradation example of the implanting amount of ions; Fig. 7A is a cross-sectional schematic diagram illustrating a manner in which glass is moved relative to the ion beam, while Fig. 7B a perspective view thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS Hereinafter are described preferred embodiment of the invention.

Fig. 1 is a schematic diagram of an ion implantation device 1 used for carrying out the ion implantation method in accordance with an embodiment of the invention. As shown in Fig. 1, the ion implantation device 1 includes: an ion source 2 to ionize any atom or molecule; a mass spectrograph 3 for selecting any desirable ions from among the ions from the ion source 2; an acceleration tube 4 for accelerating the thus selected ions to a desired velocity; a quadrupole lens 5 for forming an ion beam composed of the accelerated ions; and an implantation chamber 6 in which the ion bean thus formed is irradiated to the sample. The implantation chamber 6 is provided with a retaining base 7 for retaining the glass sample 8 thereon.

Since glass is normally an insulator and an ion is an electrically charged particle, electrical charges are accumulated in the glass if one keeps on the irradiation of ions. As the accumulated charges and the ions have the same polarities, they repulse against each other. If the glass sample is small, this repulsion does not cause so serious a problem as the amount of electrical charges is also small. If the glass sample is large, however, the amount of electrical charges increases, the path of the ions to be implanted is distorted by the repulsive force from the accumulated charges of the same polarity, thus making it difficult to implant ions. Moreover, electrical spark discharge takes place from the glass when the charge is accumulated in the glass to some degree, and thus the surface of the glass is likely to be damaged. For this reason, there may be formed a conductive film 9 to let this charge go away to thereby earth the glass. This conductive film 9 may cover the glass over an area enough to let the charge go away. In a preferred form of the invention, the conductive film 9 may be formed so as to cover the whole surface of the glass.

Accelerated ions 11 are implanted into the glass sample 8 after penetrating through this film 9. Fig. 3 illustrates a manner such ion implantation is carried out. Numeral 12 in the drawing denotes the implanted ions. If this film is too thick, ions 11 with low acceleration energy cannot penetrate through this film, and it does not reach the glass sample 8, as shown in Fig. 3 (b). Further, very high acceleration energy is required for the ion 11 to penetrate through this film 9, and to reach the glass sample 8, and thus the ion implantation device for performing this process becomes expensive. Accordingly, about 20 m is considered to be an upper limit in the thickness of this film 9. On the other hand, if the thickness of the film is about 1 to 2 nm or less, then it is too thin to obtain a good electrical conductivity. Accordingly, the thickness of this film may be desirably 2 nm to 20 u m. However, in the case that the glass to be processed is small or the amount of ion to be implanted does not need to be large, then the electrical charge itself is small, and thus the conductive film is not necessary.

The technique for letting the charge go away is not limited to the foregoing technique for forming the conductive film 9. Alternatively, the electrical charge may be let go by covering the surface of the glass with an electrically conductive mesh or wire, or otherwise, by forming mesh-shaped or grid-shaped conductive layer on the surface of the glass so as to earth these conductive material.

Next is a description of a glass strengthening method using ion implantation in accordance with the invention. It is considered that glass is strengthened due to two underlying mechanisms as hereinbelow described: One mechanism is as follows: When the glass is irradiated with accelerated ions, the glass changes in property in the vicinity of a position where <BR> <BR> the ions are stopped (i. e. , in the vicinity of a position where the ions were implanted). For silica-based glass, for instance, the glass is transformed into high-density glass in the vicinity of the position where the ions are stopped. This is a phenomenon due to the ion's mainly colliding with the atoms in the glass. It is believed that such densified area suppresses the progress of cracks on the surface of the glass so that the glass is strengthened. Accordingly, if the ions are implanted into a position deeper than the minute cracks that are present originally in the surface of the glass, then it is possible to control the progress of the minute cracks, thereby enabling the glass to be made less likely to be broken.

It is, however, sometimes difficult to determine the depth of the minute cracks that exist in the surface of the glass. Nevertheless, the ions may be implanted from the surface of the glass to the depth of 5 u m or above, preferably 10 u m or above, more preferably 20 u m or above, if the depth of the minute cracks are difficult to determine, since minute cracks (polishing scratches) present on the surface of glass after optical polishing are generally about 5 u m deep, and the cracks of the glass that is unpolished after the manufacture thereof are generally more minute.

If the ions are implanted into the uppermost surface to allow the glass to be transformed in the uppermost surface of the glass in a case where minute cracks are absent on the surface or the depths of the cracks are in the order of nanometers, yet the implantation of ions into the uppermost surface is still effective for preventing the cracks from developing on the surface of the glass. If the cracks develop thereafter, however, it is presumed that the improvement of strength against the external force can not be so much expected. Accordingly, for improving the strength of glass against the external force as well, it is desirable to strengthen the glass by implanting ions from the surface to the depth of at least 5 u m, preferably 10 u m or above, more preferably 20 u m or above.

Nevertheless, no improvement of strength was recognized for 20% of the glass samples into which the ions were implanted to the depth of about 23 ß m.

For 5 to 10 % of the glass samples into which the ions were implanted to the depth of about 45 u m as well, no improvement of strength was recognized, as described in hereinafter described working examples 1 and 2. This is presumably due to the fact that those samples include some cracks, although not many, which are deeper than the above-mentioned depths, and thus the samples were broken at these cracks Accordingly, in order to improve strength most efficiently, a strengthening process should be carried out prior to a large crack being developed on the surface of the glass. Further, the ions should desirably be implanted deeper than the minute cracks present on the surface of the glass to be strengthened after grasping the depth of the minute cracks beforehand.

However, polishing and/or cutting, etc. is/are sometimes necessary prior to the strengthening process in the manufacture process of glass, and there is the high possibility that a big crack is developed on the surface of the glass during such polishing or cutting process. In that case, it is necessary to statistically figure out the depth of the crack developed by these processes so as to determine how deeply to implant the ions.

Ions of small mass such as hydrogen ion or helium ion are desirable, though not limited to them, in order to implant the ions to the above-described depth, in view of a hereinafter described accelerating energy in the practical order of MeV If ions of large mass are accelerated, resultant speed is small even if it is accelerated by the same acceleration energy. Further, ions of large mass are difficult to implant deep into the inside from the glass surface due to the ions being subjected to large resistance after implantation. That is why ions of small mass are preferable. For acceleration energy, the larger the energy is, the better it is in general since higher energy enables ions to be implanted even deeper.

However, a device for generating energy higher than lOMeV becomes a large scale one and expensive, it is practicable in general to employ acceleration energy in the order of MeV or less. For example, when hydrogen ions are implanted into synthetic silica glass to the depth of 5 u m or more, acceleration energy of 450keV or more may be applied.

The other glass strengthening mechanism is presumed as follows: when accelerated ions are implanted into a substance, the energy of the ions is transferred to the substance, so that the substance is turned into a high temperature state instantaneously. If ions are implanted into Si02 glass, for instance, there is a report that a portion of the glass passed by the ions reaches several thousand degrees Celsius or above. However, after the ions have passed, the portion of the glass is rapidly cooled. As a result, a rapid cooling effect like the conventional thermally strengthening method is achieved by this ion irradiation, thus strengthening the glass.

Naturally, if the amount of ions to be implanted is too little, glass strengthening effect is hardly expected. On the other hand, if too much ions are implanted, then the surface of the glass cracks. Accordingly, a dose of ions to be implanted may be preferably in a range from 1x1013/cm2 to 5x1015/cm2, more preferably from 1 x 1014/cm2 to 1X10l5/cm2.

When ion implantation is carried out, a stress is generated in the vicinity of the surface from which the ions are implanted, due to the glass being subjected to change in property or being cooled rapidly, as discussed above. Accordingly, the glass is sometimes curved due to this stress. If the glass to be processed is plate glass, then it is preferable to apply ion implantation equivalently to both sides thereof in order to prevent such curve.

Alternatively, there may be formed two or more ion-implanted layers, although there is formed a single ion-implanted layer in the foregoing example.

For silica glass, for instance, when hydrogen ions accelerated by 1. 7MeV are implanted after implantation of the hydrogen ions accelerated by 2MeV, then an ion-implanted layer is formed in positions located about 45 u m deep and about 35 , u m deep from the surface of the glass, respectively. By forming two or more ion-implanted layers thus way, the glass strengthening effect is further enhanced.

It should be noted, herein, that it is not always necessary to use the same kind of ion to form each layer when forming the two or more ion-implanted layers. For silica glass, for instance, when hydrogen ions and helium ions accelerated by 2MeV are implanted, an ion-implanted layer can be formed in positions located about 45 t m deep and about 7.5 u m deep from the surface of the glass, respectively.

Moreover, acceleration energy may be slightly changed so as to thicken a region where the ions are implanted, which also can contribute to enhancing the glass strengthening efficiency. For instance, when the ions are implanted into the silica glass by 2. 0MeV only, most of the ions are implanted within the range of about 45il u m in depth. On the other hand, when implanted by 2. 00. lMeV, then the ions can be implanted within the range of about 454 It m in depth. By thickening the ion-implanting region in the depth direction thus way as well, the glass can be strengthened further.

According to the embodiment of the invention, any type of glass member can be strengthened without being subjected to restraints in thickness, surface shape and material of glass. The method of the invention can be applied not only to a large-sized plate glass used for plasma display panel (PDP), but also to optical fibers in order to make them less likely to crack, such optical fibers being recognized as becoming extremely fragile after peeling off their coatings.

Moreover, the strengthening of glass can be achieved at a far higher speed than conventional methods by increasing the amount of ions to be irradiated (i. e. , the amount of ion current).

For amount of the current, a comparatively high amount of the current is required particularly for a large-sized plate glass used for a display or the like.

For instance, if the ions are implanted into a single plate glass of 10cmxlOcm over an entire surface thereof in a dose amount of 2x10l4/cm2, the amount of the ion current of about 5 u A is needed to finish processing the single plate glass in about ten minutes. If the ions are implanted into a single plate glass of 50cmx50cm over an entire surface thereof in a dose amount of 3xlOWcm, then the amount of the ion current of about 200 u A is needed to finish processing the single plate glass in about ten minutes. As is apparent from the foregoing, the amount of the ion current is desirably at least 5 u A, more desirably 20 u A, even more desirably 100 u A in order to finish the plate glass strengthening process in a time limited to some extent (i. e. , in an industrially practical time).

On the other hand, however, if the ions with the amount of the ion current of from several 10 u A to several 100 usa that are accelerated by the accelerating energy of from several 100keV to several MeV are implanted into a minute region, the region reaches an extremely high temperature momentarily.

At that time, there are no serious problems with high-purity silica glass, but glass members for use as window glass, display glass or the like that have impurities are sometimes transformed and stained. Accordingly, the density of the ion current is desirably 10 u A/cm2 or less. Particularly for the glass that is stained easily, the ion current density of about 1 u A/cm2 or less is more preferable.

In order to obtain the above-mentioned high ion current and small ion current density, it is necessary to broaden an ion beam area. A method of broadening an ion beam area, for example, is one in which ion beam is broadened, using a quadrupole lens 5 that forms the ion beam. However, the use of the quadrupole lens 5 not only has a limit in broadening the beam area but it makes it difficult to broaden the same uniformly.

Consequently, the present inventor proposes a beam broadening method as illustrated in Fig. 4, where ion beam B accelerated in an acceleration tube is formed by the quadrupole lens 5 into ion beam C, and then an electrode or a magnet 12 is disposed in a beam path to apply an electric field or a magnetic field to it, and then to switch the direction of the electric or magnetic field at high speed, thereby imparting a minute deflection or swing to the traveling direction of the beam to thereby expand an apparent beam area. At that moment, the electrode or the magnet 12 may make the beam area even larger, by preparing two pairs respectively, so as to swing the beam in two directions, such as in the vertical direction and the horizontal direction in Fig. 4 (horizontal direction not shown).

For a large glass sample, there naturally occurs a case that the glass sample is larger than the beam area of ion. In that case, the glass sample may be moved relative to the ion beam to thereby irradiate the sample therewith entirely.

Whilst the amount of the ion current is discussed in the foregoing, if the glass to be processed is even larger, there is a limit in increasing the amount of the ion current further. Besides, the ion implantation device becomes too expensive and thus it is not industrially practical. Nonetheless, it is not industrially applicable if the ion implantation time becomes too long. Thus, it is practicable, in such case, to carry out ion implantation only into a part that is particularly likely to crack, instead of implanting the ions to the entire surface of glass. In general, a large plate glass is cut out from a larger plate glass and formed into a shape for practical use. At that time, a part of the glass to be cut, i. e. , an outer peripheral part thereof becomes likely to develop cracks and susceptible to fracture. Accordingly, ion implantation may be carried out only to the neighborhood of the periphery of the glass. For example, it will be sufficiently effective to strengthen a region 14 located within about a few centimeters of an edge of the plate glass by implanting ions only thereto, without implanting the ions into an interior 13 of the plate glass, as illustrated in Fig. 5. Moreover, a higher effect is obtained if this strengthening process is implemented to a certain part such as the center of the glass which is subjected to the greatest force when an external force is applied to the glass.

If the glass strengthening process is performed by implementing the above-mentioned partial implantation of ions, however, the glass is likely to be strained due to the stress developed in a boundary portion between the ion implanted region 14 and the ion-non-implanted region 13, depending on the presence or absence of change caused by the ion implantation. Accordingly, the implantation of ions may desirably be implemented according to a gradual gradation zone ranging from 1mm to 10cm, preferably from 5mm to 5cm where the amount of ions to be implanted is gradually changed, without clearly demarcating the region 14 from the region 13. An example of gradated ion implantation is illustrated in Fig. 6.

As for a method for forming such gradation, the speed of the glass when the glass is moved relative to the ion beam at the time of ion implantation may be varied. Alternatively, the electrode or magnet 12 of Fig. 4 may be controlled so as to give the desirable gradation to the amount of the current in the ion beam. Fig.

7A and Fig. 7B are schematic diagrams illustrating an operating manner how the glass sample 8 is moved relative to the ion beam 18. The ion beam 18 provided lengthwise along the same direction as the end edge of the glass sample 8 is formed by using the electrode or the magnet 12 of Fig. 4, and then the glass sample 8 is moved in the vertical direction 19 to the end edge of the glass sample 8. At this moment, the glass may be moved slowly at one side where the amount of the ion implantation is to be high, while it may be moved gradually faster as it approaches another side where the amount of the ion implantation should become zero, thus enabling the ion implantation with ion distribution as shown in Fig. 6.

Next is a description of specific working examples of the invention.

Working Example 1 In this example, synthetic silica glass of 0.8mm thickness and 4x4cm square was used as the glass sample 8. Ni film 9 of about 500nm was vapor deposited on the front and rear surfaces thereof, and the film 9 was earthed. The purpose for forming such Ni film 9 is to prevent the glass sample 8 from being electrically charged up. In this example, hydrogen ion was used as an ion species.

Hydrogen gas was ionized in the ion source 2, and the hydrogen ions electrically charged to the desirable value were selected in the mass spectrograph 3, and then the selected hydrogen ions were accelerated at about 2MeV in the acceleration tube 4.

The ions thus accelerated were formed into a beam, introducing it into the implantation chamber 6, so that the glass sample 8 was irradiated therewith.

As a comparative example, other glass sample with the hydrogen ions accelerated at 380keV implanted thereinto was also prepared.

In either case, hydrogen ions were irradiated to the entire surface of the glass in a dose amount of about 3X10l4/cm2. At that moment, when the ions were implanted into the glass sample 8, the electric charge was not accumulated on the surface of the glass sample 8 because it flew to the earth through the Ni film.

After implanting the ions into the surface of the glass sample 8 in this way, the rear surface of each glass sample was turned to upside and a similar ion implantation was applied to the rear surface as well. The glass sample could be strengthened even further by implanting the ions to both sides of the glass sample, preventing the curving of the glass.

Fig. 2 is a partly cross-sectional schematic diagram showing the glass sample 8 into which the hydrogen ions accelerated by 2MeV was implanted.

Implanted hydrogen 10 was distributed in the vicinity of about 45 u m from the front surface and the back surface. Concentric circle load bending test was carried out relative to the glass sample 8 into which the hydrogen ions accelerated by 2MeV were implanted, and thus the glass strength was measured.

As a result, it was acknowledged that strength of the glass increased about 1.6 times on an average by the ion irradiation as compared with before the ions were irradiated However, strength of the glass did not increase in the above-mentioned comparative sample irradiated with the hydrogen ions accelerated by 380keV. It is presumably due to the fact that the acceleration energy by 380keV was not enough to allow the hydrogen ions to reach deeper than the minute cracks that had formed on the surface of the glass. The depth of implantation of the hydrogen ions accelerated by 380keV into a synthetic silica glass is less than 4 u m, while the glass prior to the ion implantation normally undergoes surface polishing to optically smoothen the surface, so that minute cracks of about 5 m depth are considered to have already been present on the surface. For these underlying reasons, there could presumably not be recognized any rise in glass strength under the above-mentioned ion implantation condition.

However, not all the samples with the hydrogen ions accelerated by the above-mentioned 2MeV implanted showed the rise in glass strength, and about 8% samples showed no change in strength. This is presumably because those samples had cracks deeper than 45 IL m due to the surface polishing, which prevented the ion implantation effect from being seen.

Working Example 2 In this example, doped fused silica glass of 1. 0mm thickness and 4x4cm square was used as the glass sample 8. Ni film 9 of about 50nm was vapor deposited on the front and rear surfaces thereof, and the film 9 was earthed. In this example also, hydrogen ion was used as an ion species. Ion acceleration energy was set at 1. 3MeV so that the depth of ion implantation was about 23 u m.

The amount of ions to be implanted (i. e. , a dose amount of ion) was set at about 3x1014/cm2, and the ions were implanted into both sides of the glass sample.

In this sample, the strength of the glass increased about 1.5 times on an average by the ion irradiation as compared with before the ions were irradiated.

However, not all the samples examined showed the rise in glass strength in this example, and about 20% samples showed no change in strength. This is also presumably because those samples had cracks deeper than 23 m due to the surface polishing, which prevented the ion implantation effect from being seen.

Working Example 3 In this example, synthetic silica glass of 1. 0mm thickness and 4x4cm square was used as the glass sample 8. Ni film 9 of about 30nm was vapor deposited on the front and rear surfaces thereof, and the film 9 was earthed. In this example also, hydrogen ion was used as an ion species. Ion acceleration energy was set at two levels: 1. 3MeV and 2. 0MeV so that ion implantation was carried out twice. The amount of ions to be implanted (i. e. , a dose amount of ion) was set at about 3X10l4/cm2 in each ion implantation, and the ions were implanted into both sides of the glass sample.

In this sample as well, the strength about 2.1 times the original was obtained. Even higher strengthening effect could be obtained by carrying out ion implantation under two or more different conditions.