JPH0648751A - Method for removing impurity of glass for liquid crystal display element - Google Patents

Abstract

PURPOSE:To remove impurities such as alkali metal, etc., contained in a quartz glass substrate useful for an active matrix type liquid crystal display element, etc. CONSTITUTION:A quartz glass substrate 3 is set in a boat 4 and is introduced to a reaction pipe 1 of an upright heat-treating device. A hydrogen chloride- containing process gas is sent through a gas supply pipe 2 to the interior of the reaction pipe 1. Simultaneously an electric current is sent to a heater 7 to heat the quartz glass substrate 3. By these treatments, metal impurities contained in the quartz glass substrate 3 is vaporized to give high-purity glass for liquid crystal display element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing impurities from glass for liquid crystal display devices. More specifically, the present invention relates to a method for removing impurities such as an alkali metal contained in a quartz glass substrate on which thin film transistors, pixel electrodes and the like are integrally formed.

[0002]

2. Description of the Related Art Substrates on which thin film elements such as thin film transistors are integrally formed have been widely used as constituent members for active matrix type liquid crystal display elements, plasma displays, EL displays, line sensors and the like. The thin film transistor is formed in an integrated manner by applying an LSI manufacturing process using a thin film made of polycrystalline silicon or amorphous silicon formed on the surface of the substrate as a semiconductor active layer. The LSI manufacturing process includes a high temperature heat treatment such as an impurity diffusion process, a gate insulating film forming process or an annealing process, and the substrate must have excellent heat resistance. For example, when the thermal oxidation process is used as the method for forming the gate insulating film, the substrate processing temperature is 100.
It reaches about 0 ° C. In this respect, quartz glass is about 1060 ℃
Since it has a strain temperature of 1 and excellent heat resistance, it is generally used as a transistor array substrate. The characteristics of quartz glass greatly differ depending on the manufacturing method, but they can be roughly classified into two types. One is fused silica glass obtained by melting natural quartz at high temperature to form an ingot. The other is S
It is a synthetic quartz glass obtained by a vapor phase method represented by a CVD method using iCl ₄ , H ₂ and O ₂ as raw materials. From the viewpoint of heat resistance, fused silica glass is superior to synthetic silica glass. However, since natural quartz is used as a raw material, it is unavoidable to mix alkali metal impurities such as Na, K and Li and other metal impurities such as Al and Fe. If a trace amount of alkali impurities is eluted during the semiconductor process for forming a thin film transistor, the characteristics of the functional film made of polycrystalline silicon or the like may be deteriorated. Therefore, the operating characteristics of the thin film transistor are adversely affected.

[0003]

The alkali metal ions such as Na and Li contained in the quartz glass substrate are affected by the strong electric field generated near the high electric field region when the thin film transistor formed thereon operates. Then, it moves in the quartz glass substrate. The moving alkali metal ions change the potential energy of the semiconductor active layer.
Further, these impurities diffuse in the quartz glass substrate in the high temperature process or the plasma process, and cause autodoping of the thin film transistor.

Generally, even if the quartz glass substrate is simply washed, the metal impurities cannot be completely removed. Therefore,
Conventionally, as a precoat, a measure has been taken to contain a metal impurity by forming a high-purity SiO ₂ film on the surface of a quartz glass substrate by a CVD method or a sputtering method. However, since alkali metal ions and the like have a relatively large diffusion coefficient in the SiO ₂ film and are transmitted,
There is a problem that it is difficult to achieve complete inactivation.

[0005]

SUMMARY OF THE INVENTION In view of the above problems of the prior art, it is an object of the present invention to provide a method for effectively removing impurities contained in silica glass used for transistor array substrates and the like. . In order to achieve such an object, a means of heat-treating the quartz glass substrate in an atmosphere containing hydrogen chloride was taken. A highly reliable transistor array substrate can be manufactured by forming the thin film transistor group on the heat-treated quartz glass substrate. Furthermore, an active matrix type liquid crystal display device can be manufactured using such a transistor array substrate.

[0006]

By heat-treating the quartz glass substrate in an atmosphere containing hydrogen chloride, impurities such as alkali metals become chlorides and volatilize, and are extremely effectively removed from the surface of the quartz glass substrate. Hereinafter, this process will be referred to as hydrogen chloride purging in this specification. Incidentally, when a MOSIC or CCD device is produced by a normal semiconductor process using a bulk silicon wafer, a so-called halogen oxidation technique is used. This halogen oxidation is to add halogens such as hydrogen chloride, chlorine, and bromine to the process gas at the time of performing oxidation treatment of silicon to remove contaminants in the silicon oxide film and suppress structural defects. . The present invention intends to apply this halogen oxidation technique as a hydrogen chloride purge to a thin film transistor manufacturing process.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The impurity removing method according to the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an example of a vertical heat treatment apparatus used for carrying out hydrogen chloride purging according to the present invention. A cylindrical quartz reaction tube with a bottom and an opening at the lower end and extending upward
A gas supply pipe 2 is provided in the. The individual quartz glass substrates 3 are horizontally held on the boat 4 in a state of being separated from each other at a predetermined pitch, and are conveyed through the heat insulating cylinder 5. In addition, boat 4
Is made of, for example, quartz glass having high heat resistance and high purity. The opening of the reaction tube 1 is adapted to be sealed by a cap 6 fixed to the heat retaining cylinder 5 when the boat 4 is completely inserted therein. A heater 7 is provided around the reaction tube 1 to maintain the ambient temperature at a high temperature. The vertical heat treatment apparatus having such a configuration is excellent in a small space and uniformity of heat treatment, and has an ability to heat a large amount of quartz glass substrates in high quality in a large amount. When carrying out the hydrogen chloride purging according to the present invention, it goes without saying that a horizontal heat treatment apparatus may be used instead of the vertical heat treatment apparatus. The horizontal type has a structure in which a plurality of quartz glass substrates to be heat-treated are placed in a furnace while leaning against each other in a boat, and is characterized in that there is little backflow of air during substrate transportation.

The method of purging hydrogen chloride will be described in detail with reference to FIG. Before starting the thin film transistor manufacturing process, a quartz glass substrate as a raw material is set in the boat 4 and housed inside the reaction tube 1. In some cases, hydrogen chloride purge may be performed during the thin film transistor manufacturing process. Next, the process gas is introduced through the gas supply pipe 2. When performing hydrogen chloride purging, a process gas in which an inert gas is mixed with hydrogen chloride of several%, preferably about 3% is used. As the inert gas species, nitrogen, argon, helium or the like can be used. In addition, oxygen gas may be used instead of the inert gas in some cases. Simultaneously with the introduction of the process gas, the heater 7 is energized to heat the quartz glass substrate to about 1000 ° C. By performing annealing for tens of minutes to several hours in this state, impurities such as alkali metals contained in the quartz glass substrate become chlorides and volatilize.

Next, in order to examine the effect of the hydrogen chloride purging according to the present invention, an experiment was conducted on a treated product and an untreated product. A quartz glass substrate was used as a sample. The treated product was purged with hydrogen chloride using nitrogen gas containing 3% hydrogen chloride, and annealed at 1000 ° C. for 30 minutes. The flow rate of nitrogen gas was set to 15 clm. Only uncleaned products were washed normally.

As an experimental method, P was used as an impurity sensor.
Type high-resistance silicon wafer (about 1000 Ωcm) is used, and a set is created by sandwiching this from above and below with a pair of treated quartz glass substrates and a pair of untreated quartz glass substrates.
A heat treatment was applied to measure the resistance value of the high resistance silicon wafer. The effect of hydrogen chloride purging is investigated by utilizing the phenomenon that impurities contained in a quartz glass substrate are adsorbed on the silicon wafer side and the resistance value of the silicon wafer changes depending on the amount thereof. Specifically, the set was put in the vertical heat treatment apparatus shown in FIG. 1 and heat treatment was applied at 1000 ° C. for 1 hour. Then, the silicon wafer was taken out and the resistance value per unit area of each silicon wafer was measured. The results are shown in the graph of FIG. When sandwiched between untreated quartz glass substrates, the resistance of the silicon wafer decreased to about 10 to 20 kΩ / □, while when sandwiched between hydrogen chloride purge treated quartz glass substrates, it was higher than 100 kΩ / □. I kept my resistance. The decrease in resistance in the untreated product set is presumed to be due to the metal impurities such as Na, Li, P, and B contained in the quartz glass substrate thermally adsorbed in the silicon wafer. On the other hand, in the processed product, a large amount of such metal impurities are removed in advance, and the thermal adsorption of impurities into the silicon wafer does not occur.

Finally, with reference to FIG. 3, an example of a liquid crystal display device produced using a substrate subjected to hydrogen chloride purge is shown. As shown in the figure, an active matrix type liquid crystal display device is assembled on a quartz glass substrate 11. As described above, the substrate 11 has the metal impurities removed in advance by hydrogen chloride purging, and has excellent reliability. Note that some impurities may remain after the hydrogen chloride purge. Therefore, the surface of the quartz glass substrate 11 is precoated with the high-purity SiO ₂ film 12. This precoat is simultaneously applied to the quartz glass substrate 1
1 also has a function of flattening the surface. The thin film transistor 13 is formed on this precoat. The thin film transistor 13 is formed on the semiconductor thin film 14 patterned in an island shape. The semiconductor thin film 14 has a drain region D, a source region S, and a channel region between them. A gate electrode G is formed on the channel region via a gate insulating film. As described above, since impurities such as alkali metals are almost completely removed from the quartz glass substrate 11, there is no fear that the operating characteristics of the thin film transistor 13 will be adversely affected. On the quartz glass substrate 11, a transparent pixel electrode 16, an auxiliary capacitor 17, a metal wiring 18 and the like are further formed via an interlayer insulating film 15. On the other hand, on the inner surface of the counter substrate 19, the counter electrode 20 and the light shielding film 2 are sequentially arranged.
1, a color filter film 22 and the like are laminated. The opening 23 of the light shielding film 21 is aligned with the pixel electrode 16. The counter substrate 19 is bonded to the transistor array substrate 11 with a predetermined gap. A liquid crystal layer 24 is enclosed and filled in the gap between the two substrates.

A method of manufacturing the active matrix type liquid crystal display device according to the present invention will be described with reference to FIG. First, the quartz glass substrate 11 as a raw material is heat-treated in an atmosphere containing hydrogen chloride to remove impurities such as alkali metals. Typical processing conditions for this hydrogen chloride purge are a substrate heating temperature of 800 ° C. to 1100 ° C., a heating time of several tens of minutes to several hours, and a hydrogen chloride concentration of several% with respect to an inert carrier gas. It is a degree. It should be noted that it is preferable to wash the substrate before purging with hydrogen chloride. As the cleaning liquid, for example, a surfactant, a weak alkaline solvent, a weak acidic solvent or the like is used, and the surface of the quartz glass substrate is cleaned by repeating ultrasonic cleaning, bubble cleaning, steam cleaning or the like a plurality of times. However, this cleaning cannot remove the metal impurities, and as described above, the hydrogen chloride purge can perform almost complete impurity removal. Then, the SiO ₂ film 12
To a thickness of about 500 nm. By this SiO ₂ film deposition process, impurities that may still remain in the quartz glass substrate 11 can be contained and the substrate surface can be flattened. In this embodiment, SiO 2 is formed by using the low pressure chemical vapor deposition method.
_Two films were formed. The SiO ₂ film formed by the low pressure chemical vapor deposition method is excellent as a precoat because it has a dense composition and good coverage. Next, the thin film transistor manufacturing process is performed. The thin film transistors 13 are integrated and formed on the quartz glass substrate 11 by making full use of the usual thin film manufacturing technology and semiconductor manufacturing technology. Since the surface impurity concentration is suppressed, it does not adversely affect the electrical characteristics of the formed thin film transistor 13. Subsequently, after patterning a transparent conductive film such as ITO to form the pixel electrode 16,
An active matrix type liquid crystal display device is assembled by stacking the quartz glass substrate 11 and the counter substrate 19 and filling and sealing the liquid crystal layer 24 in the gap between them.

Although the quartz glass material is used as the substrate in the above-described embodiments, the present invention is not limited to this, and various kinds of glass plate materials can be used according to the application. Further, although the thin film transistor element of polycrystalline silicon or amorphous silicon is formed in the above-mentioned specific example, the present invention is not limited to this, and a semiconductor thin film element such as CdSe, a MIM element, a plasma display or an EL display is formed. It is also useful when forming a thin film element used for a line sensor or the like.

[0014]

As described above, according to the present invention,
By subjecting the quartz glass substrate to a heat treatment in an atmosphere containing hydrogen chloride, impurities such as alkali metals can be removed in advance. Therefore, when thin film transistors are integratedly formed on a highly purified quartz glass substrate, there is an effect that the operation characteristics are stabilized and the reliability is improved. Along with this, there is an effect that the manufacturing yield of an active matrix type liquid crystal display device or the like can be improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a vertical heat treatment apparatus used for carrying out a method for removing impurities from glass for liquid crystal display elements according to the present invention.

FIG. 2 is a graph showing the effect of the impurity removing method according to the present invention.

FIG. 3 is a schematic partial cross-sectional view showing a liquid crystal display element assembled using a quartz glass substrate treated by the method for removing impurities according to the present invention.

[Explanation of symbols]

1 Reaction Tube 2 Gas Supply Tube 3 Quartz Glass Substrate 4 Boat 7 Heater 11 Quartz Glass Substrate 12 SiO ₂ Film 13 Thin Film Transistor 16 Pixel Electrode 19 Counter Substrate 20 Counter Electrode 24 Liquid Crystal Layer

Claims (3)

[Claims] 1. A method for removing impurities from a glass for a liquid crystal display device, which comprises heat-treating a quartz glass substrate in an atmosphere containing hydrogen chloride. 2. A method of manufacturing a transistor array substrate, which comprises forming a thin film transistor group on a quartz glass substrate which is heat-treated in an atmosphere containing hydrogen chloride. 3. A method of manufacturing a liquid crystal display device in which a liquid crystal is sandwiched between a pixel electrode connected to a pixel transistor and a counter electrode facing the pixel electrode, the quartz glass substrate being heat-treated in an atmosphere containing hydrogen chloride. A method of manufacturing a liquid crystal display device, which comprises forming a plurality of thin film pixel transistors on the top.