JPH07247132A - Production of quartz glass - Google Patents

Abstract

PURPOSE:To obtain a quartz glass having high ultraviolet transmission and high uni formity of refractive index by swinging the holding temperature plural times within the range from the distortion temperature to the sublimation temperature of the quartz glass and performing the heat-treatment of the quartz glass taking advantage of the generated thermal stress. CONSTITUTION:This process for the production of a quartz glass comprises the heating of a quartz glass preform containing 2>=500ppm of OH group to a holding temperature, the holding of the preform at the temperature for a prescribed period and the slow cooling of the preform to complete the heat-treatment of the preform. In the above process, the holding temperature is reciprocated plural times within the range between the distortion temperature and the sublimation temperature of the quartz glass. The heat-treatment of quartz glass is supposed to be impossible in the devitrification temperature range, i.e., from 1200 deg.C to 1700 deg.C owing to the crystallization (devitrification) phenomenon. In this process, the temperature is reciprocated within the range between the upper limit and the lower limit without using the holding of the quartz glass at a definite upper or lower limit temperature, the holding after the temperature increase and the holding after the temperature decrease. The devitrification can be prevented by this process to enable the heat-treatment even in the devitrification temperature range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing quartz glass, and more particularly to a field requiring a synthetic quartz glass member requiring a high homogeneity of refractive index, for example, photolithography and high precision spectroscope. The present invention relates to a highly homogeneous synthetic quartz glass for optics which is useful for precision optical instruments such as lasers.

[0002]

2. Description of the Related Art With the recent high integration of LSI, fine patterning is being advanced in an optical lithography exposure apparatus for exposing and transferring an integrated circuit on a silicon wafer. Therefore, the wavelength of the light source is becoming shorter, and quartz glass is used as an optical element for ultraviolet lithography that satisfies the high transmittance of ultraviolet light and the high homogeneity of the refractive index. In order to realize a high transmittance of ultraviolet light, it is necessary to suppress the impurity concentration in quartz glass. Therefore, as a method for producing such a quartz glass, a Si compound gas as a raw material and a carrier gas (for example, H ₂ ,
A flame hydrolysis method in which a combustion gas for heating (O ₂ gas or the like) and a combustion gas for heating is ejected from a burner to deposit quartz glass in the flame is generally used.

It is known that this method makes it possible to obtain high-purity quartz glass because impurities in raw materials and combustion gas can be easily suppressed. However, quartz glass as an optical element for ultraviolet lithography is highly homogeneous. It is also essential to have a large refractive index distribution. The main causes of making the refractive index distribution of quartz glass inhomogeneous are fluctuations of various conditions that occur during the synthesis of quartz glass, such as changes in the temperature distribution on the synthesis surface due to flames, flame hydrolysis reactions or thermal decomposition. These include thermal oxidation reaction and changes in the diffusion state of impurities into glass. It is known that fluctuations in these conditions result in distribution of growth fringes called striae and refractive index in the radial direction in the quartz glass.

[0004]

It is generally known that the structure of the quartz glass changes in the temperature range higher than the strain point of 1025 ° C., which is the strain point of quartz glass. Examples of changes in the physical properties of quartz glass due to structural changes include refractive index, strain, transmittance, UV resistance (solarization, fluorescence), and the like. Therefore, in controlling various physical properties, heat treatment in a temperature range above the strain point and changes in the physical properties due to the heat treatment are drawing attention.

Among these changes in physical properties, changes in transmittance and ultraviolet resistance are caused by microscopic structural changes. Therefore, at the same temperature as de-straining (usually 1100 to 1200 ° C.) or lower temperature. Heat treatment is common. For example, JP-A-63-297
No. 234, "Method for producing quartz glass" and JP-A-63-295452
In the article "Method of preventing fluorescence phenomenon in the ultraviolet region of quartz glass", at least quartz glass synthesized by the sol-gel method,
By applying a thermal shock of raising and lowering the temperature near the de-straining temperature,
It has been shown that fluorescence generation by 254 nm irradiation is suppressed. As for thermal shock, it has been shown to repeat heating, holding, cooling, and holding between 1000 ° C and 1300 ° C.

However, in the heat treatment in this destraining temperature range, a gentle refractive index distribution in the radial direction in the bulk (the difference between the maximum and minimum values of the refractive index distribution is regarded as homogeneity, and hereinafter, the homogeneity of the refractive index is indicated by Δn). However, it was impossible to control the local waviness of the refractive index distribution. In order to remove the refractive index distribution caused by the fluctuation of the conditions during the synthesis of the quartz glass by the heat treatment after the synthesis, it was necessary to perform the heat treatment at a temperature higher than 1800 ° C.

According to Japanese Examined Patent Publication No. 3-17775, "Method for homogenizing quartz glass", high-purity quartz glass should be at least 2 atm, preferably 5 atm.
A method of heating to 1800 ° C or higher, preferably 2200 to 2400 ° C in an Ar gas atmosphere of -25 atm has been proposed.
In particular, the result of optimizing the homogenization conditions that are industrially feasible and have high production efficiency is that the temperature between 2200 and 2400 ° C
It is a combination of atmospheric pressure for 2 hours to 5 minutes, and it has been confirmed that this has the effect of eliminating all granular structures and striae. And, in the example, it takes a long time to remove striae at a pressure of 2100 ° C. or less, and a high pressure (10 kg / cm ² or more) to suppress sublimation / evaporation
It says that you must do it.

However, heat treatment at high temperature and high pressure promotes diffusion of impurities into the quartz glass, so that absorption occurs in the ultraviolet region. Therefore, for example, in an optical system where absorption near 200 nm is a problem, such as silica glass used for ArF excimer laser, silica glass that has been heat-treated at high temperature and high pressure has a high transmittance and refraction of ultraviolet light. The requirements of high homogeneity of the rates could not be met at the same time.

For these problems, it is most effective to lower the heat treatment temperature, but it is attempted to suppress the homogeneity of the refractive index of quartz glass after heat treatment to about Δn = 1 × 10 ^-6 or less. In some cases, the homogeneity of the refractive index depends not only on the temperature condition of the heat treatment but also on the homogeneity of the refractive index of the quartz glass preform before the heat treatment. Therefore, in order to obtain reproducibility in the homogeneity after heat treatment in the conventional manufacturing method, the thermal conditions and chemical reactions received during the synthesis of the quartz glass base material, the diffusion of impurities, etc. should be suppressed within a certain range and the refractive index It was necessary to use quartz glass having a homogeneity of distribution and having a locally non-homogeneous refractive index distribution.

Therefore, it is an object of the present invention to solve these problems and to provide a method for producing a quartz glass having a high transmittance of ultraviolet light and a high homogeneity of a refractive index.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted refraction due to thermal conditions, chemical reactions, diffusion of impurities, etc., received during the synthesis of quartz glass in a temperature range as low as possible so that absorption does not occur in the ultraviolet range. We have conducted extensive studies on the conditions for eliminating the inhomogeneous distribution of the rates. Then, specifically, in the temperature holding process during the heat treatment, the holding temperature is raised and lowered a plurality of times to forcibly create a temperature distribution in the quartz glass, and the thermal stress generated by the temperature distribution is utilized to produce the quartz. The inventors have found that the refractive index distribution of glass is homogenized, and have completed the present invention.

Therefore, in the present invention, in the method for producing a quartz glass, the "heat-treated quartz glass preform which is optically inhomogeneous is heated to a holding temperature, held for a certain period of time, and then gradually cooled. The method for producing quartz glass is characterized in that the holding temperature is raised and lowered a plurality of times within a range from the strain point of the quartz glass to the sublimation point thereof.

[0013]

It is believed that there is a correlation between the holding temperature and the homogenization time, but details have not yet been elucidated in controlling the refractive index profile at a precise level, which is a problem in the present invention. In addition, it is generally known that if the time is infinite at any temperature, it will be a stable quartz glass with a uniform refractive index, but it is practically impossible if industrial productivity is taken into consideration. is there.

The present invention will be described in detail below. In the quartz glass containing a large amount of OH groups, the present inventors have paid attention to the fact that there is a portion terminated by a large amount of OH groups in the network structure of quartz glass, and conducted diligent research. As a result, the heat treatment using the quartz glass base material containing a large amount of OH groups as compared with the quartz glass base material containing no OH groups, sufficiently relaxes the structure in the medium distance,
It was found that the phenomenon is remarkable in the quartz glass base material containing 0 ppm or more, relaxation is likely to occur, and the local improvement of the refractive index distribution occurs.

On the other hand, as will be described later, the degree of the inhomogeneous distribution that can be removed in a given time in each holding temperature range during the heat treatment depends on the inhomogeneous distribution of the refractive index existing in the base material to be heat treated. From this, it is understood that the heat treatment conditions such as the holding temperature and the homogenization time should be set according to the inhomogeneous distribution of the refractive index of the quartz glass base material, but the effect of the OH group contained in the quartz glass base material is It is not limited to this.

The inhomogeneous distribution of the refractive index is formed by the dispersion of the medium-range network structure of glass and the distribution of trace impurities. The impurities will be defined here. Impurities (due to synthesis conditions) contained in the quartz glass base material during synthesis are primary impurities, and impurities mixed in by secondary treatment such as heat treatment are secondary impurities. Secondary impurities are a problem in the present invention.

In the conventional heat treatment, the homogenization of the refractive index is carried out by physical / chemical equilibrium. In this case, the quartz glass is kept at a high temperature, which not only promotes the diffusion of primary impurities in the quartz glass but also mixes and diffuses new secondary impurities during the heat treatment, and further Can cause convection to. That is, the conventional homogenization method which mainly depends on physical / chemical equilibrium has various adverse effects.

The inventors of the present invention have examined a heat treatment method for eliminating the inhomogeneous distribution of the refractive index, in particular, a method for highly homogenizing quartz glass. As a result, not only physical / chemical equilibrium is used, but temporary stress is generated in the quartz glass, and by utilizing the force, synthetic quartz glass for optics with high refractive index is obtained. I understood it.

In the present invention, the temporary stress generated in the quartz glass during the heat treatment is obtained by raising and lowering the holding temperature a plurality of times. By raising and lowering this holding temperature a plurality of times, it became possible to set the holding temperature to a lower temperature than before, and a highly homogeneous quartz glass was obtained by heat treatment at a low temperature. The range of the holding temperature is from the strain point to the sublimation point (a temperature at which sublimation does not matter) or less.

Viscous flow occurs near the strain point where the strain is said to disappear, and the physical properties are relaxed. From this fact, it is possible to remove the inhomogeneous distribution of the refractive index by heat treatment near the strain point, but in the conventional heat treatment method, the thermal history at the time of synthesis, especially the refractive index of Δn = 1 × 10 ^-6 order The inhomogeneity could not be removed at the set temperature near the strain point. Therefore, the present inventors considered that the inhomogeneous distribution of the refractive index is mainly composed of inhomogeneity due to strain and other inhomogeneities. Then, the conventional heat treatment removes the local non-homogeneous distribution in which the strain is not involved, and the highly homogeneous quartz glass can be obtained only under the high temperature and high pressure treatment conditions.

Reference will be made to the heat treatment temperature of the present invention. Although the pressure dependence of the strain point of quartz glass has not been clarified so far, it is usually around 1025 ° C. In contrast,
The sublimation point of quartz glass is largely changed by pressure, and is, for example, 1900 ° C. to 2100 ° C. in vacuum or at atmospheric pressure. When the pressure is 25 atm, the sublimation point is around 2500 ° C. The present invention can be carried out under such high temperature and high pressure conditions, but the heat treatment at high temperature and high pressure has a large demerit in terms of not only the mixing of secondary impurities but also the cost of the apparatus. Or atmospheric pressure.

Therefore, considering these, the upper limit temperature is
It is desirable that the temperature is 1750 ° C or lower and the lower limit temperature is 1100 ° C or higher. In the present invention, the lower limit temperature when raising and lowering the holding temperature is set to a temperature at which the inhomogeneity due to strain can be eliminated, and the upper limit temperature is set at a temperature at which other inhomogeneities can be eliminated. Thereby, homogenization is performed more efficiently.

It is said that heat treatment of quartz glass cannot be carried out in the devitrification temperature range of 1200 to 1700 ° C. because of crystallization (devitrification). However, in the present invention, the holding temperature is set to the upper limit and the lower limit without holding at a constant temperature at the upper limit temperature or the lower limit temperature, holding after raising the temperature, and holding after lowering the temperature, and this temperature is repeated.
This makes it possible to prevent devitrification, and heat treatment can be performed even in the devitrification temperature range.

According to the method for producing quartz glass of the present invention, the inhomogeneity of the refractive index whose homogeneity of the refractive index is about Δn = 1 × 10 ⁻⁴ cannot be removed. Refractive index homogeneity is Δn = 1
× 10 ^- whereas heterogeneous refractive index is about ⁴ takes a long homogenisation time, since a longer homogenization time in a temperature range abruptly crystallization of the present invention proceeds, [Delta] n as quartz glass preform It is preferable to use quartz glass of about 1 × 10 ⁻⁵ . As a result, the refractive index variation Δn = 1 × 10 ^-6
It is possible to obtain silica glass that is optically homogeneous to a degree or less.

Number of times the holding temperature is raised and lowered (here, one cycle is the lower limit temperature → the upper limit temperature → the lower limit temperature)
Is not particularly limited, but unnecessarily increasing and decreasing the number of times prolongs the homogenization time, and it is not preferable because impurities are mixed and absorption occurs. By the way, when silica glass is used in an optical system of precision optical equipment such as photolithography, high-precision spectroscope, and laser, the ideal refractive index distribution is that there is no difference in refractive index. However,
It is very difficult to produce quartz glass having no refractive index distribution even if the above heat treatment is performed. However,
Depending on the shape of the refractive index distribution, it is possible to reduce the refractive difference by correcting the lens shape and the optical axis when assembling the optical system. This correctable refractive index distribution is only a centrally symmetric refractive index distribution having one extreme value with respect to the optical axis of incident light when quartz glass is used as an optical element.

Therefore, during the heat treatment of the present invention, the quartz glass base material is rotated at 100 rpm or less. Thereby, points equidistant from the center of rotation receive the same thermal history, and a centrally symmetric refractive index distribution can be obtained. The number of rotations can be set arbitrarily, but considering the structure of the device and the effects obtained thereby, 100 rpm or less is realistic.

Furthermore, as described above, not only the physical / chemical equilibrium is utilized, but a temporary stress is generated in the quartz glass, and the force is utilized to cause the quartz glass base material to "incident optical axis". Even in the case of a silica glass base material having a refractive index distribution with three or more extreme values in a cross section including “,” the heat treatment of the present invention “provides a single central extremal refractive index distribution with one extreme value in the cross section including the incident optical axis. Quartz glass with "can be modified.

Further, the inventors of the present invention described in JP-A-5-116969, "Manufacturing Method of Quartz Glass," Dispersion of Refractive Index Δn =
Optically inhomogeneous quartz glass of about 1 × 10 ^-5 ", SiO ₂
Of “optically homogeneous quartz glass with Δn = 1 × 10 ⁻⁶ or less”, characterized by heat treatment under a pressure of 0 to 10 kg / cm ² in a matrix made of powder or lumps of Proposing a method. Among them, it is desirable that the temperature of the entire quartz glass be decreased uniformly after the heat treatment.However, even if the temperature decrease rate is sufficiently slow, the temperature decrease rate is different between the outside and the inside of the quartz glass, so the temperature distribution can be made after the temperature decrease. , That it appears as a refractive index profile. Especially when viewed from the thickness direction (optical axis direction when used as an optical element), it was confirmed that the number of isotherms increases in the peripheral part of the silica glass, and an altered layer with large variations in refractive index is formed in this part. did.

Therefore, in the present invention, in order to eliminate the formation of this altered layer, that is, the inhomogeneous distribution of the refractive index, the temperature is fixed at the lower limit temperature and the upper limit temperature until the temperature distribution in the quartz glass becomes uniform (diameter). 200mm, about 70mm thick quartz glass, about 10min.), The cooling rate is 100
It is preferable to set the temperature to not more than ° C / H. In addition, Japanese Patent Laid-Open No. 5-1
High-purity SiO ₂ powder or S, as proposed in 16969
By performing the heat treatment of the present invention using a master block made of a lump of iO ₂ powder melted, the refractive index can be further homogenized. This is because the use of the matrix reduces the temperature distribution in the quartz glass when the temperature of the heat treatment is decreased, and eliminates the inhomogeneous distribution of the refractive index due to the temperature distribution. Furthermore, since heat treatment is performed without direct contact with the outer mold or atmosphere, homogenization, gas release from the interior of the quartz glass, gas diffusion from the atmosphere to the interior of the silica glass,
It is possible to prevent diffusion of impurities from the outer mold into the quartz glass.

The atmosphere during the heat treatment is He, N ₂ , which is inert to the carbon member used in the furnace at a high temperature,
It is necessary to use Ar or its mixed gas,
Especially, when it is necessary to increase the hydrogen concentration in the quartz glass, H ₂ or a mixed gas of H ₂ and He, N ₂ , Ar is used. In addition, the above atmosphere is effective because the mother die is not completely sealed even in the heat treatment using the mother die.

As a secondary effect of the present invention, striae of the grade 2 (class B) of Japan Optical Glass Industry Standard (JOGIS-11-1975) can be eliminated.

[0032]

EXAMPLE FIG. 2 is a schematic sectional view of an apparatus for producing quartz glass used in this example. Quartz glass base material 1
Is placed in a master block ₂ of SiO ₂ powder or a melted mass of SiO ₂ powder, and heated while rotating at 5 rpm while being set in an outer mold 3 made of carbon graphite. A carbon fiber felt 6 was provided on the inner surface of the outer die in order to prevent the mother die from being taken out of the outer die after the heat treatment. The processing furnace has heaters 4 in the upper and lower parts and side parts, and the entire heating furnace 5 is covered with a heat insulating layer.

The sample shape used was a cylinder of φ150 × t50. FIG. 1 is a diagram schematically showing temperature conditions during heat treatment in each of the examples and comparative examples. Table 1 shows the qualities of the samples used in each of the examples and comparative examples before the heat treatment (on the left side of the arrow) and the qualities obtained by heat-treating the samples under the temperature conditions shown in Table 2 (the arrow indicates (Right side). The temperature conditions A to L in Table 2 correspond to the temperature conditions shown in FIG. The homogeneity of the refractive index (dispersion of the refractive index within the measurement region) is indicated by the difference Δn (PV value) between the maximum value and the minimum value of the refractive index within the measurement region, and the optical property when used as a lens. The RMS value (after power component correction) of the wavefront aberration that directly affects the performance is shown.

[0034]

[Table 1]

[0035]

[Table 2]

Example 1 The sample was heat-treated under the temperature conditions shown in Table 2. When the refractive index distribution was measured by an interferometer, Δn (PV value) was changed from an initial value of 6.3 × 10 ^-6 to 0.7 × 10 ^-6 , which is 1 × 10 ^-6 or less. Moreover, the extreme value of the refractive index distribution generated during the synthesis of the base material was three, but when the rotary heat treatment was carried out under the above conditions, it was reduced to only one in the central part, and further the rotation during the synthesis of the quartz glass ingot was performed. The centrally asymmetric refractive index profile due to being cut off from the axis was corrected to a centrally symmetrical refractive index profile. The RMS value (after correction of the power component) is improved because the effect of power correction is large in the centrally symmetric refractive index distribution with one extreme value. The transmittance was measured using a precisely adjusted spectrophotometer, and the 193 nm internal transmittance of a 10 mm-thick test piece that was highly accurately polished was measured. As a result, 99.9% was maintained even after the heat treatment.

Comparative Example 1 The sample was heat-treated under the temperature conditions shown in Table 2. When the refractive index distribution was measured by an interferometer, Δn (PV value) was not sufficiently improved. In addition, the extreme values of the refractive index distribution generated at the time of synthesis remained three, and the refractive index distribution was also asymmetric in the center. Therefore, the effect of power correction was small even in the RMS value (after power component correction). Since the internal transmittance was kept at a high temperature of 2000 ° C. for a long time, the internal transmittance of 10 mm at 193 nm decreased to 99.6%.

Example 2 A sample was heat-treated under the temperature conditions shown in Table 2. Although most of the heat treatment temperature was in the crystallization temperature range, no crystallization was observed because the temperature was not maintained at the upper and lower limit temperatures. When the refractive index distribution was measured by an interferometer, Δn (PV value) was 1 × 10 ⁻⁶ or less. In addition, the extreme value of the refractive index distribution generated during synthesis is 3
The number was reduced from one to only one in the central portion, and the refractive index distribution was corrected to have central symmetry. The RMS value (after power component correction) also improved and the 10 mm internal transmittance at 193 nm also maintained 99.9%.

Comparative Example 2 A sample was heat-treated under the temperature conditions shown in Table 2. Optical properties could not be evaluated due to remarkable crystallization.

[0040]

As described above, according to the method for producing quartz glass of the present invention, quartz glass having high transmittance of ultraviolet light and high homogeneity of refractive index can be obtained. Conventionally, if there was a local inhomogeneous distribution of the refractive index of about Δn = 1 × 10 ^{-5 in} the quartz glass base material before the heat treatment, the heat treatment at 2000 ° C or below causes the local inhomogeneity in the refractive index. While the refractive index distribution remained, the method for producing quartz glass of the present invention can achieve homogenization even at 2000 ° C. or lower.

Temporary stress generated in the quartz glass base material to be heat-treated should be maintained at high temperature and high pressure even if the inhomogeneous distribution of the refractive index of the quartz glass base material produced during synthesis is locally inhomogeneous. It is performed by raising and lowering the low holding temperature multiple times. In the present invention, heat treatment at high temperature and high pressure is unnecessary, and control of processing conditions such as temperature and pressure becomes easy. It is possible to suppress the diffusion of impurities into the quartz glass during the heat treatment.

Further, by performing heat treatment in a mother die made of high-purity SiO ₂ powder or agglomerates, the temperature distribution during the heat treatment can be reduced and the reaction with carbon graphite as the outer die can be suppressed. . Furthermore, even if a silica glass base material having a locally inhomogeneous refractive index distribution of about Δn = 1 × 10 ^-5 is used, the dispersion of the refractive index is Δn = 1 × 10 ^-6 It becomes possible to manufacture quartz glass.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing temperature conditions during heat treatment in Examples and Comparative Examples.

FIG. 2 is a schematic cross-sectional view of a quartz glass manufacturing apparatus used in this example.

[Explanation of symbols]

1 Quartz glass base material 2 Base mold (SiO ₂ ) 3 Outer mold 4 Heater 5 Heating furnace 6 Felt

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Hiraiwa 3 2-3 Marunouchi, Chiyoda-ku, Tokyo Inside Nikon Corporation

Claims (8)

[Claims] 1. A method for producing quartz glass, wherein a quartz glass base material containing 500 ppm or more of OH groups is heated to a holding temperature, held for a certain period of time, and then heat-treated gradually to lower the holding temperature. A method for producing quartz glass, which comprises raising and lowering a plurality of times within a range from the strain point of the glass to the sublimation point thereof. 2. The method for producing quartz glass according to claim 1, wherein the quartz glass base material is rotated at 100 rpm or less during the heat treatment. 3. The method for producing quartz glass according to claim 1, wherein the quartz glass base material is made of high-purity SiO ₂ during the heat treatment.
A process for producing quartz glass, characterized in that the powder or SiO ₂ powder is put into a matrix made of a molten mass. 4. The method for producing quartz glass according to claim 1, wherein the quartz glass base material has a “refractive index homogeneity Δn = 1.
× 10 ^-5 about less optically inhomogeneous quartz glass preform "feature that" the refractive index homogeneity Δn = 1 × 10 in that it is
^-Method for producing "optically homogeneous quartz glass of about ⁶ or less". 5. The method for producing quartz glass according to claim 1, wherein the atmosphere during the heat treatment is He, N ₂ , Ar or a mixed gas thereof. 6. The method for producing quartz glass according to claim 1, wherein the atmosphere during the heat treatment is H ₂ or H ₂ and He,
A method for producing quartz glass, which is a mixed gas of N ₂ and Ar. 7. The method for producing silica glass according to claim 1, wherein the silica glass base material is “a silica glass base material having a refractive index distribution with three or more extreme values in the cross section including the incident optical axis”. A method of manufacturing "quartz glass having a centrally symmetric refractive index distribution with a single extreme value in the cross section including the incident optical axis". 8. The method for producing quartz glass according to claim 1, wherein the holding temperature range of the heat treatment is 1100 to 1750 ° C.
A method for producing quartz glass, characterized in that