WO2019172442A1 - Alkali-free glass substrate - Google Patents

Abstract

The present invention relates to an alkali-free glass substrate which has a strain point of 650°C or more and an average thermal expansion coefficient of from 30 × 10^-7/°C to 45 × 10^-7/°C for the range of from 50°C to 350°C, and which contains, in mass% based on oxides, 54-66% of SiO₂, 10-25% of Al₂O₃, 0.1-12% of B₂O₃, and 7-25% in total of one or more oxides that are selected from the group consisting of MgO, CaO, SrO and BaO, while containing 150-2,000 ppm by mass of Na₂O. This alkali-free glass substrate is also configured such that the amount of Na₂O in at least one main surface of the glass substrate is lower than the amount of Na₂O in the inner part of the glass substrate by 20 ppm by mass or more.

Description

Alkali-free glass substrate

The present invention relates to a glass substrate suitable for forming a thin film transistor (TFT) or the like.

A borosilicate glass substrate (so-called alkali-free glass substrate) containing almost no alkali metal component is used as a glass substrate for various displays, particularly when a thin film such as a TFT is formed on a glass substrate. This is because when a glass substrate containing a large amount of an alkali metal component is used, the alkali metal component in the glass deteriorates the film characteristics and causes a decrease in the reliability of the TFT. The glass substrate for display is also required to have a high strain point and high acid resistance.

¡Alkali-free glass with high strain point and excellent acid resistance needs to melt the glass raw material at a high temperature of 1400-1800 ° C, and it is not easy to efficiently produce a high-quality glass substrate. Patent Document 1 describes a method of melting glass by containing alkali metal oxide in an alkali-free glass in an amount of 200 to 2000 ppm and using heating by a burner combustion flame and heating by energizing molten glass. ing.

A float method is known as a method for forming glass into a plate shape. In the float process, molten glass that is continuously supplied onto molten metal (for example, simply “bath”) in a float bath (hereinafter referred to simply as “bath”) is flowed over the molten metal and formed into a plate shape. To do.
The glass ribbon formed on the molten metal is then conveyed using a roller. In that case, it is known that the part which contacts a roller tends to be damaged.

As a method of preventing such scratches, sulfurous acid gas (SO ₂ gas) is sprayed on the back surface of the glass substrate and reacted with an alkali metal (for example, sodium) present in the glass to form sodium sulfate on the back surface of the glass substrate. In addition, a method for making it work as a protective film is known (for example, Patent Document 2). Patent Document 3 describes a method in which sodium tetraborate or the like is sprayed on the surface of the display substrate glass and then sulfurous acid gas is sprayed.

International Publication No. 2013/084832 International Publication No. 2002/051767 International Publication No. 2008/004480

However, the method described in Patent Document 2 cannot be applied to the production of an alkali-free glass substrate in which the amount of alkali metal present in the glass is extremely small. Moreover, although the method of patent document 3 is applicable also to an alkali free glass substrate, since two types of gases are sprayed on the surface of a glass ribbon, a process is complicated and an apparatus also becomes complicated.

Recently, demands for TFT characteristics and glass substrate characteristics have been further increased, and it has become more difficult to maintain both TFT characteristics on the glass substrate and improve the meltability of the glass.

The present invention has been made in view of the above problems, and has an object to provide a high-quality alkali-free glass substrate that is less likely to deteriorate the reliability of the TFT due to an alkali metal component, has few scratches, and is excellent in productivity. To do.

As a result of intensive studies, the present inventors have found that the strain point and the average thermal expansion coefficient at 50 to 350 ° C. are within a predetermined range, have a specific composition, and Na ₂ O on the surface of at least one glass substrate. The present inventors have found that the above problem can be solved by a glass substrate whose amount is 20 mass ppm or more less than the amount of Na ₂ O inside the glass substrate, and completed the present invention.

That is, the present invention is an alkali-free glass having a strain point of 650 ° C. or higher and an average coefficient of thermal expansion of 30 × 10 ⁻⁷ to 45 × 10 ⁻⁷ / ° C. at 50 to 350 ° C. the SiO ₂ 54 ~ 66% by mass percentage of the _{Al 2 O 3 10 ~ 25%} , the _{B 2 O 3 0.1 ~ 12%} , is selected MgO, CaO, from the group consisting of SrO and BaO 1 contain 7-25% in total of the above components, Na ₂ O and containing 150-2000 mass ppm, 20 from Na ₂ O weight Na ₂ O amount is the glass substrate of the glass substrate surface in at least one major surface It is a non-alkali glass substrate having a mass ppm or less.

The alkali-free glass substrate of the present invention has a specific composition, and the amount of Na ₂ O on the surface of at least one glass substrate is 20 mass ppm or less less than the amount of Na ₂ O inside the glass substrate. The damage of the TFT due to the alkali metal component can be suppressed.

FIG. 1 is a diagram illustrating an example of a signal intensity ratio profile of ²³ Na ⁺ and ³⁰ Si ^{+ in} the thickness direction of a glass substrate. FIG. 2 is a diagram showing an example of a Na ₂ O content profile near the glass substrate surface. FIG. 3 is a conceptual diagram showing a glass manufacturing apparatus using a float process. FIG. 4 shows an example of the relationship between the variation amount of the threshold voltage in the reliability test of the TFT formed on the glass substrate surface and the amount of Na ₂ O on the glass substrate surface. FIG. 5 is a schematic diagram of a TFT element.

Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below.

In the present specification, the glass composition is expressed in terms of mass% based on oxide in principle, and “%” and “ppm” used for the glass composition in the present specification are “mass%”, “ It means “ppm by mass”.

In the present specification, “non-alkali glass” refers to a glass in which the content of alkali metal components such as lithium, sodium, and potassium is 5000 ppm or less in terms of oxide.

In this specification, “glass ribbon” refers to a molten glass formed into a plate shape. The glass ribbon is cooled and cut to become a glass substrate.

The “bottom surface” is the surface of the glass ribbon or glass substrate manufactured by the float process that is in contact with the molten metal in the float bath. The “top surface” is a surface facing the bottom surface.

In the present specification, the Na ₂ O content of the glass substrate is obtained by pulverizing the glass substrate, thermally decomposing the obtained glass powder with sulfuric acid, nitric acid and hydrofluoric acid, and then concentrating until white sulfuric acid smoke is generated. A constant volume solution dissolved in dilute nitric acid is obtained, and the Na concentration in the constant volume solution is determined by ICP mass spectrometry [unit: mass ppm].
The “Na ₂ O content inside the glass substrate” is equal to the Na ₂ O content of the glass substrate determined by the above method.

“Na ₂ O content on the surface of the glass substrate” is a standard value obtained by cutting out a part of the glass substrate to be evaluated and etching it about 10 μm (specifically 8 to 12 μm) from the surface using a 5% hydrogen fluoride aqueous solution. The sample is obtained from a Na content profile obtained by a time-of-flight secondary ion mass spectrometry (TOF-SIMS) method using C ₆₀ sputtering described later.
Specifically, determined from Na content profile of the glass, the average content of Na ₂ O depth in the region of 0.30μm from 0.25μm from the surface and the content of Na ₂ O of the glass substrate surface.

FIG. 1 is a signal intensity ratio profile of ²³ Na ⁺ and ³⁰ Si ^{+ in} the thickness direction of the glass substrate obtained by the following procedure.
That is, five small pieces were cut out from the present glass substrate, and the surface of each of the four pieces was etched using a 5% hydrogen fluoride aqueous solution. By changing the etching time for the five small pieces, 1 μm, 3 μm, 5 μm, and 10 μm were etched from the glass substrate surface, respectively. The etching thickness was measured using a micrometer.
FIG. 1 shows a plot of the signal intensity ratio of ²³ Na ⁺ and ³⁰ Si ⁺ measured for each piece by time-of-flight secondary ion mass spectrometry (TOF-SIMS) using C ₆₀ sputter. . Since Si is the main component of this glass substrate, and its content is substantially uniform in the thickness direction of the glass substrate, the signal intensity ratio between ²³ Na ⁺ and ³⁰ Si ⁺ indicates a profile of Na content. It can be considered.
From FIG. 1, in the region from the glass substrate surface to a depth of about 5 μm, the Na content is smaller than that of the deeper portion, but at a depth of about 10 μm or more, the Na content is It turns out that it does not fluctuate.

As a method for obtaining the Na ₂ O content on the surface of the glass substrate, a part of the glass substrate to be evaluated is cut out and etched from the surface by using a 5% aqueous hydrogen fluoride solution about 10 μm (specifically, 8 to 12 μm). Using Na ₂ O as a standard sample for quantification of Na ₂ O, the Na content profile near the glass substrate surface is measured by measuring the signal intensity ratio of ²³ Na ⁺ to ³⁰ Si ⁺ with respect to the sputtering time for an unetched glass substrate. A method is mentioned.

Since the Na content does not change at a depth of about 10 μm or more, the signal intensity ratio of ²³ Na ⁺ and ³⁰ Si ⁺ obtained for the standard sample is the Na ₂ O content obtained by IPC mass spectrometry. Equivalent to. Therefore, the signal intensity ratio can be converted into the Na ₂ O content using the value. Further, after the TOF-SIMS measurement, a surface shape measuring device (e.g. manufactured by Veeco; Dektak150) Measurement of the shaved depth _{C 60} sputtering using, can convert the sputtering time to depth from the glass surface.

Thereby, a Na ₂ O content profile as shown in FIG. 2 is obtained. In this way the content of Na ₂ O profile obtained, the average Na ₂ O content of the region of 0.30μm depth from the surface is from 0.25μm to Na ₂ O of the glass surface.

In this specification, the “β-OH value” of a glass substrate is obtained by the following method.

That is, the infrared transmittance of the glass substrate was measured using an infrared spectrophotometer, the minimum value of the transmittance at a wave number of 3500 to 3700 cm ⁻¹ was I _a [unit:%], and the transmittance at a wave number of 4000 cm ⁻¹ was I _{If b} [unit:%] and the thickness of the glass substrate is d [unit: mm], the β-OH value is − (log (I _a / I _b )) / d [unit: mm ⁻¹ ]. .

[Glass substrate manufacturing method]
First, in order to help understanding of the present invention, a method for producing a glass substrate by a float method will be described as one embodiment of a method for producing an alkali-free glass substrate of the present invention (hereinafter also referred to as “the glass substrate of the present invention”). The manufacturing method of the glass substrate of this invention is not limited to this.

Glass substrate production by float method
(I) “melting process” of melting raw materials to produce molten glass;
(II) "Molding process" for introducing a molten glass into a float bath and forming a glass ribbon;
(III) “Slow cooling step” of gradually cooling the glass ribbon in a slow cooling furnace.

Hereafter, each process is demonstrated using FIG. 3 which is a conceptual diagram which shows the example of the glass manufacturing apparatus by a float glass process.

<Dissolution process>
In the melting step, molten glass 4 is obtained by putting the glass raw material prepared and mixed in accordance with a desired glass composition into melting furnace 3. The temperature of the melting furnace 3 may be appropriately adjusted depending on the glass raw material used, and is, for example, about 1400 ° C. to 1600 ° C.

<Molding process>
In the forming step, a glass ribbon is formed by continuously flowing molten glass 4 from melting furnace 3 onto the molten tin surface of float bath 2 filled with molten tin 1.

The glass ribbon formed on the molten metal is transported to the slow cooling furnace 6 by the transport roller 5 and gradually cooled. However, the glass ribbon is in a period from when the glass ribbon leaves the float bath 2 until it contacts the transport roller 5. It is preferable to blow the SO ₂ gas against the bottom surface of the.

By spraying the SO ₂ gas, Na ₂ O present on the glass surface reacts with the SO ₂ gas to form sodium sulfate on the glass surface. The sodium sulfate has an effect of preventing the glass ribbon from being damaged by the contact between the glass ribbon and the transport roller 5 and can be easily removed by washing with water, so that the quality of the glass substrate is not affected. .

Here, in the production process of the glass substrate of the present invention which is an alkali-free glass substrate having a low Na ₂ O content, in order to form a sufficient amount of sodium sulfate on the glass surface to prevent scratches on the glass substrate, for example, By appropriately adjusting the water vapor concentration in the float bath, the upstream and downstream temperatures of the float bath 2, the residence time of the molten glass 4, the dissolved oxygen concentration in the molten tin 1, etc., the Na ₂ O inside the glass is adjusted. By moving to the vicinity of the bottom surface and unevenly distributed, it becomes possible to generate sodium sulfate sufficiently on the glass surface by blowing SO ₂ gas, and a glass substrate with few scratches can be obtained.

Further, by eccentrically moving the Na 2 _O in the glass in the vicinity of the bottom surface, it is possible to increase the amount of Na 2 _O to diffuse from the glass surface to the molten tin, Na ₂ contained in the entire glass substrate The amount of O can be reduced.

Specific examples of the conditions of the upstream and downstream temperatures of the float bath 2, the residence time of the molten glass 4, and the dissolved oxygen concentration in the molten tin 1 include the following (1) to (4).
(1) The temperature on the upstream side of the float bath 2 is preferably 1400 ° C. to 900 ° C., more preferably 1300 ° C. to 1000 ° C., and further preferably 1250 ° C. to 1100 ° C. The temperature on the upstream side of the float bath 2 indicates the temperature of the upstream glass ribbon and can be measured with a radiation thermometer.
(2) The temperature on the downstream side of the float bath 2 is preferably 600 ° C. to 850 ° C., more preferably 650 ° C. to 850 ° C., and further preferably 700 ° C. to 800 ° C. The temperature on the downstream side of the float bath 2 indicates the temperature of the downstream glass ribbon and can be measured with a radiation thermometer.
(3) The residence time of the molten glass 4 is preferably 5 minutes to 60 minutes, more preferably 10 minutes to 40 minutes, and even more preferably 15 minutes to 30 minutes.
(4) The dissolved oxygen concentration in the molten tin 1 is preferably 10 ppm or less, more preferably 5 ppm or less, still more preferably 3 ppm or less, and most preferably 0 ppm. The dissolved oxygen concentration in the molten tin 1 can be measured with a tin oxygen concentration meter (Redox).

Na ₂ O collected near the bottom surface of the glass substrate by the blowing of SO ₂ gas becomes sodium sulfate and goes out of the glass substrate. Therefore, in the glass substrate obtained by the above method, the amount of Na ₂ O on the glass surface on the bottom surface side is smaller than the amount of Na ₂ O inside the glass, specifically, 20 ppm by mass or more, 40 ppm by mass. The content is preferably less, more preferably 90 mass ppm or less.

In general, the temperature of the heat treatment at the time of manufacturing the TFT is 400 ° C. to 500 ° C., but the temperature of the glass ribbon when blowing the SO ₂ gas is about 600 ° C. to 750 ° C., which is higher than the temperature of the heat treatment at the time of manufacturing the TFT. . Therefore, by blowing SO ₂ gas, Na ions existing at a position deeper than the depth at which Na ions in the glass substrate diffuse during the heat treatment during TFT manufacturing can be reacted with SO ₂ and removed. .

Therefore, when a TFT is manufactured using the glass substrate obtained by the above method, since the diffusion of Na ions in the glass during the heat treatment into the TFT is small and fluctuations in threshold voltage can be suppressed, a TFT having excellent reliability is manufactured. can do.

<Slow cooling process>
In the slow cooling step, the glass ribbon carried by the transport roller 5 is slowly cooled in the slow cooling furnace 6 to obtain the glass substrate of the present invention. Although the temperature of the slow cooling furnace 6 is not particularly limited, for example, it can be set to 550 to 750 ° C. on the upstream side of the slow cooling furnace 6 and 200 to 300 ° C. on the downstream side, as in the conditions of a general float process.

[Glass composition]
Next, the glass composition of the glass substrate of the present invention will be described.

The glass substrate of the present invention has a SiO ₂ content of 54 to 66%, an Al ₂ O ₃ content of 10 to 25%, a B ₂ O ₃ content of 0.1 to 12%, MgO, CaO, and SrO in terms of mass% based on oxide. and one or more components selected from the group consisting of BaO contain 7-25% in total, Na ₂ O and containing 150-2000 mass ppm, Na ₂ O amount of at least one glass surface of the inner glass Na less than 20 ppm by weight from ₂ O amount.

Hereinafter, each component will be described in detail.

<SiO ₂ >
SiO ₂ is an essential component of alkali-free glass.
When the content of SiO ₂ is reduced in the glass substrate of the present invention, the strain point is low, the thermal expansion coefficient is large, and the density is increased. Therefore, the content of SiO _{2 in} the glass substrate of the present invention is 54% or more, preferably 57% or more, and more preferably 58% or more.

On the other hand, the viscosity of the glass becomes high when the content of SiO ₂ becomes large, the temperature _{T 4} which is a temperature _{T 2} and ¹⁰ 4 dPa · s glass viscosity becomes ¹⁰ 2 dPa · s is increased, the devitrification temperature rises To do. Therefore, the content of SiO _{2 in} the glass substrate of the present invention is 66% or less, preferably 63% or less, more preferably 62% or less.

<Al ₂ O ₃ >
When the content of Al ₂ O ₃ is reduced in the glass substrate of the present invention, glass phase separation occurs, and the strain point also decreases. Therefore, the content of Al ₂ O _{3 in} the glass substrate of the present invention is 10% or more, preferably 14% or more, and more preferably 15% or more.

On the other hand, _Al 2 when the content of _{O 3} is increased, the possibility that the glass viscosity of ¹⁰ 2 temperature _{T 2} and the temperature _{T 4} which is a ¹⁰ 4 dPa · s as a dPa · s is increased, or the liquidus temperature rises is there. Therefore, the content of Al ₂ O _{3 in} the glass substrate of the present invention is 25% or less, preferably 21% or less, and more preferably 18% or less.

<B ₂ O ₃ >
When the content of B ₂ O ₃ decreases in the glass substrate of the present invention, the viscosity of the glass increases and the devitrification temperature increases. Therefore, the content of B ₂ O _{3 in} the glass substrate of the present invention is 0.1% or more, preferably 0.5% or more, more preferably 1% or more, and further preferably 2% or more. In particular, 3% or more is preferable and 5% or more is more preferable from the viewpoint of preventing haze from being generated by etching using buffered hydrofluoric acid.

On the other hand, when the content of B ₂ O ₃ increases, the strain point decreases. Therefore, the content of B ₂ O _{3 in} the glass substrate of the present invention is 12% or less, preferably 11% or less, more preferably 9% or less. Further, when it is desired to increase the strain point, it is preferably 7% or less, more preferably 5% or less, and further preferably 3% or less.

<MgO, CaO, SrO and BaO>
In the glass substrate of the present invention, MgO, CaO, SrO and BaO are not essential, but these components have the effect of lowering the viscosity of the glass and maintaining chemical durability. Therefore, the total content of these components in the glass substrate of the present invention is 7% or more, preferably 9% or more, and more preferably 12% or more.

On the other hand, if these components are contained excessively, the thermal expansion coefficient of the glass becomes excessive. Therefore, the total content of these components in the glass substrate of the present invention is 25% or less, preferably 21% or less, and more preferably 18% or less.

MgO is a component having a relatively small effect of increasing the thermal expansion coefficient of glass among alkaline earth oxides. It is also a component that can increase the Young's modulus while keeping the glass density low. The content of MgO is preferably 1% or more, more preferably 2% or more, and further preferably 3% or more. On the other hand, since the devitrification temperature can be lowered by reducing the MgO content, the MgO content is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less.

CaO is a component that can increase the Young's modulus without increasing the thermal expansion coefficient and density too much. The content of CaO is preferably 2% or more, and more preferably 3% or more. On the other hand, when the CaO content is reduced, the glass becomes difficult to devitrify. Therefore, the CaO content is preferably 15% or less, more preferably 10% or less, and even more preferably 6% or less.

SrO has an effect of decreasing the viscosity without increasing the devitrification temperature of the glass, and it is preferable to contain 6% or more. On the other hand, since the thermal expansion coefficient can be reduced by lowering the SrO content, the SrO content is preferably 15% or less, more preferably 10% or less, and even more preferably 9% or less.

BaO is a component that lowers the viscosity. However, the BaO content is preferably 5% or less, more preferably 1% or less because the coefficient of thermal expansion can be reduced by lowering the BaO content. In particular, when it is desired to reduce the density for the purpose of reducing the weight of the glass substrate, the content is preferably 0.5% or less, and more preferably substantially not contained.

<Na ₂ O>
Usually, when a TFT is formed on a glass substrate, a heat treatment is performed thereafter. However, if Na ₂ O is contained in the glass in a large amount, Na ions in the glass diffuse into the TFT during the heat treatment, and the threshold value of the TFT. Since the voltage fluctuates, the reliability of the TFT decreases. Therefore, in the glass substrate of the present invention, the Na ₂ O content is 2000 mass ppm or less, preferably 1000 mass ppm or less, more preferably 800 mass ppm or less.

On the other hand, if the Na ₂ O content of the glass is too small, the melting characteristics of the glass deteriorate. Therefore, in the glass substrate of the present invention, the Na ₂ O content is 150 mass ppm or more, preferably 300 mass ppm or more, and more preferably 500 mass ppm or more.

As described above, the glass substrate of the present invention, by decreasing least 20 mass ppm from Na 2 _O content of the glass substrate to Na 2 _O of the glass substrate surface in at least one major surface, a wound surface of the glass substrate Can be reduced.

Therefore, in the glass substrate of the present invention, the amount of Na ₂ O on the glass substrate surface in at least one main surface is 20 mass ppm or less, preferably 40 mass ppm or more, more preferably 90 mass, less than the amount of Na ₂ O inside the glass substrate. It shall be less than ppm.

In the glass substrate of the present invention, by setting the Na ₂ O content in the above-described range, a decrease in the reliability of the TFT when the TFT is formed on the glass substrate is prevented, but this particularly affects the reliability of the TFT. It is Na ₂ O present on the surface of the glass substrate. Therefore, in the glass substrate of the present invention, at least in the one principal surface, a Na 2 _O of the glass substrate surface by less than Na 2 _O content of the glass substrate, further suppressing a decrease in reliability of the TFT be able to.

The reliability of the TFT can be evaluated by, for example, a BT test (bias temperature stress test). FIG. 4 shows the amount of variation in threshold voltage and Na on the surface of the glass substrate when a low-temperature polysilicon thin film transistor (LTPS-TFT) is formed on the surface of the bottom surface of the glass substrate and a positive bias is applied. It is the figure which showed the relationship of ₂ O content.

Here, this LTPS-TFT structure is a standard top gate coplanar structure, and the source drain region is formed by ion implantation of boron to form a p-channel TFT. The polysilicon film is crystallized by irradiating an amorphous silicon film having a thickness of 50 nm obtained by a plasma CVD method with a XeCl excimer laser (wavelength 308 nm).

In addition, a silicon oxide film having a thickness of 100 nm is formed as a barrier film between the glass substrate and the silicon film. LTPS-TFTs are characterized by being more strongly affected by the Na ₂ O content on the glass substrate surface because they are processed at a higher temperature than amorphous silicon TFTs, which are currently the mainstream.

From FIG. 4, the smaller the Na ₂ O content on the glass substrate surface, the smaller the threshold voltage variation, that is, the higher the reliability of the TFT. Also, this feature is found to be significantly smaller the content of Na 2 _O of the glass substrate surface.

Therefore, in the glass substrate of the present invention, the amount of Na ₂ O on the glass substrate surface on at least one main surface is preferably 500 ppm by mass or less, more preferably 300 ppm by mass or less, and further preferably 250 ppm by mass or less.

<Other ingredients>
Although the glass substrate of the present invention is alkali-free glass, it is known that alkali metal oxides are inevitably mixed as impurities in the glass raw material. Usually, Na ₂ O occupies most of the alkali metal oxide mixed as an impurity from the raw material, but it may contain Li ₂ O and K ₂ O. When these are contained, the total amount of alkali metal oxides including Na ₂ O is 5000 ppm by mass or less, preferably 2000 ppm by mass or less, more preferably 1000 ppm by mass or less, and even more preferably 800 ppm by mass or less. .

Further, the glass substrate of the present invention, to the extent that the effect of the present invention _F, Cl, may contain SO _3, SnO _2, ZrO 2 or the like.

[Physical properties of glass substrate]
Then, the physical property of the glass substrate of this invention is demonstrated.
The glass substrate of the present invention has a strain point of 650 ° C. or higher, and an average coefficient of thermal expansion at 50 to 350 ° C. is 30 × 10 ⁻⁷ to 45 × 10 ⁻⁷ / ° C.

<Strain point>
In the glass substrate of the present invention, when the strain point is low, when the glass substrate is exposed to a high temperature in a thin film forming process such as a display, shrinkage (thermal shrinkage) accompanying the deformation of the glass substrate and the stabilization of the glass structure is likely to occur. Become. Therefore, the strain point of the glass substrate of the present invention is 650 ° C. or higher, preferably 660 ° C. or higher, and more preferably 670 ° C. or higher.

On the other hand, if the strain point is low, the temperature of the molding apparatus can be lowered, so that there is an advantage that the life of the molding apparatus can be improved, and the strain point is preferably not too high. Therefore, the strain point of the glass substrate of the present invention is preferably 800 ° C. or lower, more preferably 750 ° C. or lower, and further preferably 730 ° C. or lower.

The strain point can be measured using a fiber drawing method according to the method defined in JIS R3103-2 (2001).

<Average thermal expansion coefficient at 50 to 350 ° C>
In order to obtain a glass substrate having excellent thermal shock resistance and excellent productivity when manufacturing a TFT panel, the average thermal expansion coefficient of the glass substrate of the present invention at 50 to 350 ° C. is 30 × 10 ⁻⁷ to 45 × 10 ⁻⁷ / ℃. The average thermal expansion coefficient at 50 to 350 ° C. of the glass substrate of the present invention is preferably 33 × 10 ⁻⁷ / ° C. or more, and more preferably 35 × 10 ⁻⁷ / ° C. or more. Further, it is preferably 42 × 10 ⁻⁷ / ° C. or less, and more preferably 40 × 10 ⁻⁷ / ° C. or less.

The average coefficient of thermal expansion can be measured using a thermal dilatometer according to the method specified in ASTM E831.

<Density>
Although the density of the glass substrate of this invention is not specifically limited, 3.0 g / cm < ³ > or less is preferable from a viewpoint of implement | achieving weight reduction of a product and raising a specific elastic modulus. More preferably, it is 2.8 g / cm ³ or less, and still more preferably 2.6 g / cm ³ or less.

<Temperature T _{2 at which the} viscosity becomes 10 ² poise (dPa · s)>
Further, the glass substrate of the present invention is easy to dissolve because the temperature T _{2 at} which the viscosity η is 10 ² poise (dPa · s) is relatively low. The temperature T ₂ is preferably 1800 ° C. or less, more preferably 1750 ° C. or less, further preferably 1700 ° C. or less, and particularly preferably 1680 ° C. or less from the viewpoint of solubility.

<Temperature T _{4 at which} viscosity η is 10 ⁴ poise (dPa · s)>
The glass substrate of the present invention is suitable for float forming because the temperature T _{4 at} which the viscosity η is 10 ⁴ poise (dPa · s) is relatively low. The temperature T ₄ is preferably 1350 ° C. or less, more preferably 1325 ° C. or less, still more preferably 1300 ° C. or less, and particularly preferably 1290 ° C. or less from the viewpoint of float moldability.

The temperature _{T 2} and the temperature _{T 4} in accordance with the method specified in ASTM C965-96, can be measured using a rotational viscometer.

<Young's modulus>
The Young's modulus of the glass substrate of the present invention is preferably 70 GPa or more, and more preferably 75 GPa or more. The Young's modulus can be measured by an ultrasonic pulse method according to the method defined in JIS Z2280 (1993).

<Photoelastic constant>
The photoelastic constant of the glass substrate of the present invention is preferably 33 nm / MPa / cm or less.
Due to the birefringence of the glass substrate due to stress generated during the manufacturing process of the liquid crystal display panel and the liquid crystal display device, a phenomenon in which the black display becomes gray and the contrast of the liquid crystal display decreases may be observed.

It is preferable to set the photoelastic constant to 33 nm / MPa / cm or less because this phenomenon can be suppressed to a small level. The photoelastic constant is more preferably 32 nm / MPa / cm or less, and still more preferably 30 nm / MPa / cm or less.

The photoelastic constant of the glass substrate of the present invention is preferably 21 nm / MPa / cm or more, and more preferably 23 nm / MPa / cm or more, considering the ease of securing other physical properties. The photoelastic constant is measured by a disk compression method at a measurement wavelength of 546 nm.

<Relative permittivity>
When the glass substrate of the present invention is applied to an in-cell type touch panel (a liquid crystal display panel with a built-in touch sensor), from the viewpoint of improving the sensing sensitivity of the touch sensor, lowering the driving voltage, and reducing power consumption. It is better that the relative dielectric constant of is higher.

Therefore, the relative dielectric constant of the glass substrate of the present invention is preferably 5.0 or more, more preferably 5.5 or more, and even more preferably 5.7 or more. The relative dielectric constant can be measured by the method described in JIS C-2141 (1992).

<Β-OH value>
The β-OH value of the glass substrate of the present invention can be appropriately selected according to the required characteristics of the glass substrate. From the viewpoint of increasing the strain point of the glass substrate, the β-OH value is preferably low. Specifically, beta-OH value is preferably 0.50 mm ^-1 or less, more preferably 0.45 mm ^-1 or less, more preferably 0.40 mm ^-1 or less.
The β-OH value can be adjusted by various conditions during melting of the raw material, for example, the amount of water in the glass raw material, the water vapor concentration in the melting kiln, the residence time of the molten glass in the melting kiln, and the like.

[TFT manufacturing method]
Next, in order to help understanding of the present invention, a manufacturing method of the TFT element 10 using a glass substrate will be described by taking a top gate coplanar type LTPS-TFT manufacturing method shown in FIG. 5 as an example. The use of the glass substrate is not limited to this.

If necessary, first, a barrier film 12 is formed on one main surface of the glass substrate 11. The barrier film 12 is made of, for example, silicon oxide, silicon oxynitride, silicon nitride, or alumina, but may be omitted. Next, an amorphous silicon layer is formed as a semiconductor on the barrier film 12 (or the glass substrate 11).

Next, if the hydrogen concentration in the amorphous silicon layer is reduced by performing heat treatment, film peeling or the like in the subsequent process can be prevented. This heat treatment is performed in the range of 450 to 600 ° C., for example. Next, amorphous silicon is crystallized by laser annealing to obtain a polysilicon layer 13. Laser annealing is performed, for example, by a method of irradiating an excimer laser with a wavelength of 308 nm. Thereafter, the polysilicon layer 13 is patterned into a predetermined shape. The patterning is performed by, for example, photolithography and an etching method. Subsequently, an insulating film and a conductive film are formed. This insulating film is made of, for example, silicon oxide, silicon oxynitride, silicon nitride, or alumina, and later becomes the gate insulating film 14a. The thickness of the insulating film is, for example, 30 to 600 nm. The conductive film is made of, for example, a metal such as chromium, molybdenum, aluminum, copper, silver, or an alloy containing them, and is later patterned to form the gate electrode 15. The thickness of the conductive film is, for example, 30 to 600 nm.

After the gate electrode 15 is formed, a process of reducing the electrical resistance value (low resistance process) is performed on the portion of the polysilicon layer 13 protruding from the gate electrode 15. The resistance reduction process is performed by a method such as ion implantation of B (boron) ions into the polysilicon layer 13, for example. Further, the implanted ions are activated by the heat treatment. This heat treatment may be performed at 450 to 600 ° C. for 10 to 60 minutes, for example.
Next, an interlayer insulating film 14b is formed. The interlayer insulating film 14b is made of, for example, silicon oxide, silicon oxynitride, silicon nitride, alumina, or the like. The interlayer insulating film 14 b is patterned so that part of the protruding portion of the polysilicon layer 13 is exposed on both sides of the gate electrode 15.

Next, the source electrode 16 and the drain electrode 17 are formed. These electrodes are formed, for example, by forming a conductive film made of a metal such as chromium, molybdenum, aluminum, copper, silver, or an alloy containing them, and then patterning the conductive film.

Next, a passivation film 18 is formed so as to cover them. The passivation film 18 is made of, for example, silicon oxide, silicon oxynitride, silicon nitride, or alumina. The thickness is, for example, 30 to 600 nm.
The TFT element 10 can be manufactured as described above.

[Application of glass substrate]
Although the use of the glass substrate of this invention is not specifically limited, It is useful as a glass substrate for displays, such as a liquid crystal display device.

Hereinafter, the effects of the present invention will be described more specifically with examples of the present invention, but the present invention is not limited to these.

The glass raw material was melted and molded by the float process to obtain the glass substrates of Examples 1 to 5. In any of the examples, the SO ₂ gas was sprayed onto the bottom surface of the glass ribbon between the time when the glass ribbon left the float bath and the time when the glass ribbon contacted the transport roller in the float process.
The composition of the obtained glass substrate is shown in Table 1 in terms of mass% based on oxide. In addition, Example 1 is manufactured under the conventional manufacturing conditions, Examples 2 and 3 are manufactured so that the water vapor concentration in the float bath is slightly high, and Examples 4 and 5 are in the float bath. It was manufactured with a higher water vapor concentration.

In addition, since the composition of SiO ₂ , Al ₂ O ₃ , B ₂ O ₃ , MgO, CaO, SrO and BaO in the glass substrate obtained by the float process does not substantially change during the glass melting process, the content of these components is It was determined from the blending amount of the glass raw material.

As for the amount of Na ₂ O inside the glass substrate, the obtained glass substrate is pulverized, the obtained glass powder is thermally decomposed with sulfuric acid, nitric acid and hydrofluoric acid, and then concentrated until white smoke of sulfuric acid is produced. A constant volume solution dissolved in nitric acid was obtained, and the Na concentration in the constant volume solution was determined by ICP mass spectrometry [unit: mass ppm].

The Na ₂ O of the glass substrate surface, the Na ₂ O content of the surface of the bottom side of the glass substrate by using a _{C 60} sputtering TOF-SIMS method, was measured by the method described above. The measurement conditions were as follows.
Measuring device: TOF manufactured by ION-TOF. SIMS5
Primary ion species: Bi ⁺
Primary ion acceleration voltage: 25 kV
Primary ion current value: 1 pA (at 10 kHz)
Raster size of primary ions: 20 × 20 μm ²
Primary ion bunching: Yes Sputtering ion species: C ₆₀ ⁺⁺
Sputtering ion acceleration voltage: 10 kV
Sputtered ion current value: 1.1 nA (at 10 kHz)
Raster size of sputter ions: 100 × 100 μm ²
Sputtering mode: non-interlaced mode
Degree of vacuum: 5.0 × 10 ⁻⁶ mbar

For the glass substrates of Examples 1 to 5, the β-OH value, density, Young's modulus, average thermal expansion coefficient at 50 to 350 ° C., T ₂ , T ₄ , glass transition point, strain point, photoelastic constant, relative dielectric constant The measured results are shown in Table 1.

(Evaluation of vulnerability)
It is considered that sulfate reacts with Na ₂ O on the surface of the glass substrate and SO _x in the atmosphere, and the sulfate acts as a buffer lubricant to prevent scratches. The amount of sulfate on the surface of the glass substrate is measured, and the ease of scratching is evaluated. The higher the amount of sulfate, the higher the effect of suppressing scratches. The amount of sulfate on the surface of the glass substrate can be measured by measuring the amount of S using fluorescent X-rays.
The amount of S was measured using a fluorescent X-ray analyzer (manufacturer: Rigaku, model: ZSX-Primus II). The measurement conditions were Target Rh, tube voltage was 50 KV, and tube current was 60 mV. The optical conditions were an attenuator 1/1 and a slit S4, a spectral crystal Ge, and a detector PC.
A calibration curve was prepared using several standard samples, and the sulfate amount of each sample was measured using the calibration curve. The results of S amount measurement for each sample are shown in Table 2 below.

Next, the ease of scratching was evaluated for each sample using a friction and wear tester (manufacturer: Shinto Kagaku Co., Ltd., model: TYPE 40 (high temperature specification)).
Measurement temperature: 600 ° C
Scratch indenter: Pin made of silicon nitride (tip radius 25 microns)
Load condition: 10g
Indenter moving speed: 30 mm / min

As a result of evaluating the ease of scratching of each sample, strong scratches occurred in Example 1, and no scratches occurred in Examples 2 to 5. In Example 1, since the amount of sulfate produced was small, the result was easily damaged.
That is, the glass substrate of Example 1 the difference between the Na 2 _O content and the glass substrate inside the Na 2 _O content of the glass substrate surface is less than 20, scratched for production of sodium sulphate in the bottom surface the surface is insufficient It was easy and there was a quality control problem.
The glass substrates of Examples 2 and 3 were hardly scratched because sodium sulfate was formed on the bottom surface. The glass substrates of Examples 4 and 5 were more difficult to be damaged because a large amount of sodium sulfate was formed on the bottom surface.

In addition, since the glass substrate in any of Examples 2 to 5 has a sufficiently small amount of surface Na ₂ O, it is considered that there is little deterioration in TFT characteristics when a TFT is formed on the bottom surface. Since the substrate has a particularly small amount of surface Na ₂ O, it is considered that the degradation of TFT characteristics is particularly small.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on March 9, 2018 (Japanese Patent Application No. 2018-043493), the contents of which are incorporated herein by reference.

DESCRIPTION OF SYMBOLS 100 ... Float glass manufacturing apparatus 1 ... Molten tin 2 ... Float bath 3 ... Melting kiln 4 ... Molten glass 5 ... Conveying roller 6 ... Slow cooling furnace 10 ... TFT element 11 ... Glass substrate 12 ... Barrier film 13 ... Polysilicon layer 14 ... Insulating layer 14a ... Gate insulating film 14b ... Interlayer insulating film 15 ... Gate electrode 16 ... Source electrode 17 ... Drain electrode 18 ... Passivation film

Claims (8)

1. An alkali-free glass substrate having a strain point of 650 ° C. or higher and an average coefficient of thermal expansion at 50 to 350 ° C. of 30 × 10 ⁻⁷ to 45 × 10 ⁻⁷ / ° C.
   SiO ₂ is 54 to 66% by mass% based on oxide,
   Al ₂ O ₃ 10-25%,
   0.1 to 12% of B ₂ O ₃
   7 to 25% in total containing at least one component selected from the group consisting of MgO, CaO, SrO and BaO,
   Containing 150 to 2000 ppm by mass of Na ₂ O,
   An alkali-free glass substrate in which the amount of Na ₂ O on the glass substrate surface on at least one main surface is 20 mass ppm or less less than the amount of Na ₂ O inside the glass substrate.
2. The alkali-free glass substrate according to claim 1, wherein the amount of Na ₂ O on the glass surface on at least one main surface is 500 ppm by mass or less.
3. The alkali-free glass substrate according to claim 1 or 2, wherein the amount of Na ₂ O inside the glass substrate is 300 mass ppm or more.
4. Temperature _{T 4} which glass viscosity of ¹⁰ 4 dPa · s is 1350 ° C. or less, alkali-free glass substrate according to any one of claims 1 to 3.
5. The alkali-free glass substrate according to any one of claims 1 to 4, wherein a temperature T _{2 at} which the glass viscosity becomes 10 ² dPa · s is 1800 ° C or lower.
6. The alkali-free glass substrate according to any one of claims 1 to 5, which has a β-OH value of 0.50 mm ^-1 or less.
7. The alkali-free glass substrate according to any one of claims 1 to 6 obtained by a float process.
8. The alkali-free glass substrate according to claim 7, wherein the one main surface is a bottom surface.

Images