WO2009106134A1

WO2009106134A1 - Method for producing an optical component of quartz glass

Info

Publication number: WO2009106134A1
Application number: PCT/EP2008/052370
Authority: WO
Inventors: Martin Trommer; Stefan Ochs; Steffen Zwarg; Bodo Kuehn
Original assignee: Heraeus Quarzglas GmbH and Co KG
Current assignee: Heraeus Quarzglas GmbH and Co KG
Priority date: 2008-02-27
Filing date: 2008-02-27
Publication date: 2009-09-03
Anticipated expiration: 2010-08-27
Also published as: DE112008003728B4; DE112008003728T5

Abstract

To provide a method for the reproducible and reliable production of synthetic, UV- radiation resistant quartz glass which has a predetermined hydroxyl group content and containshardly any chlorine and oxygen defect centers, if possible, the present invention suggests a method which comprises the following steps: (a) producing an SiO₂soot body, (b) drying the soot body such that a mean hydroxyl group content of less than 60 wtppm is obtained, and (c) vitrifying the soot body with formation of a cylindrical quartz glass blank which is further processed into the optical component, wherein prior to vitrification according to method step (c) the soot body is subjected to a conditioning treatment which comprises a treatment with a nitrogen containing gas, preferably a nitrogen oxide,or a sequence of treatments with ammonia and an oxidizing agent.

Description

Method for producing an optical component of quartz glass

Description

The present invention relates to a method for producing an optical component of quartz glass for use in an objective for microlithography at a working wavelength below 250 nm, the method comprising the following steps:

(a) producing an SiO2 soot body,

(b) drying the soot body such that a mean hydroxyl group content of less than 60 wtppm is obtained,

(c) vitrifying the soot body with formation of a cylindrical quartz glass blank which is further processed into the optical component.

Technical Background

The soot body is a hollow cylinder or a solid cylinder of porous SiO2 soot which is obtained according to the known VAD (vapor axial deposition) method or according to the OVD (outside vapor deposition) method. As a rule, such soot bodies have a high content of hydroxyl groups (OH groups) due to the manufacturing process. These have an impact on the resistance of quartz glass to shortwave UV radiation, for instance on the damage patterns known as "compaction" and "decompaction".

Many methods are known that are used for minimizing the hydroxyl group content or for setting it to a predetermined value. The soot body is often subjected to a dehydration treatment in which it is exposed to a chlorine-containing atmosphere at a high temperature around 1000⁰C. In this process OH groups are substituted by chlorine.

As a rule, the removal of chlorine from a soot body is based on diffusion processes and chemical reactions. These processes depend on time and temperature. A complete removal of chlorine from a soot body is therefore tedious and expensive, especially in the case of large soot wall thicknesses, and there is the risk that chlorine residues remain in the soot body, especially since quantitative analysis and checking are difficult to perform at low chlorine contents.

It is however known that chlorine may deteriorate the UV radiation resistance of quartz glass, particularly its compaction behavior, and chlorine contributes to induced birefringence under UV laser radiation. The damage pattern known as "compaction" occurs during or after laser irradiation and manifests itself in a local increase in density of the glass in the volume penetrated by radiation, which in turn leads to a locally inhomogeneous change in the refractive index and thus to a deterioration of the imaging properties of the optical component.

US 2005/0187092 deals with synthetic quartz glass of high UV radiation resistance for lenses, prisms and other optical components for a lithographic device. For the removal of chlorine from the porous SiO2 soot body the publication suggests that the body should be treated in an atmosphere consisting of helium with 3% oxygen and should be slowly heated in this process to a temperature of 1490⁰C over a period of several hours. At the same time the soot body is sintered into a transparent quartz glass body having a hydroxyl group content of about 10 wtppm. The oxygen amount of the vitrification atmosphere simultaneously effects a reduction of the number of oxygen defect centers in the network structure of the quartz glass.

US2006/0137399 A1 describes a similar method for producing synthetic quartz glass of low polarization-induced birefringence. An SiO2 soot body is here dried by using a reactive drying agent, with chlorine, boron or halogen-containing compounds, carbon monoxide and carbon dioxide being indicated as drying agents. The dried soot body has a hydroxyl group concentration of less than 50 wtppm and is subsequently vitrified in a water-containing atmosphere. To remove oxygen deficiency defects, oxygen is additionally added to the water-containing atmosphere.

It has been found that an efficient elimination of oxygen defect centers by means of the known methods is only possible at high temperatures in the range of the vitrification temperature or higher, i.e. at temperatures clearly above 1200⁰C. The use of oxygen at such elevated temperatures, however, requires a complicated corrosion protection, particularly a furnace construction with materials which withstand both high temperatures and an oxygen attack, and which in addition must not contaminate the SiO2 body to be treated. The choice of materials optimally satisfying said conditions is very limited.

Summary of the Invention

It is therefore the object of the present invention to provide an alternative method which permits a reproducible and reliable production of synthetic, UV-radiation resistant quartz glass having a predetermined hydroxyl group content, which contains hardly any chlorine and oxygen defect centers, if possible, whereby the quart glass is suitable for use in a projection objective or in an illumination objective of a microlithography apparatus.

Starting from the above-indicated method this object is achieved according to the invention in that prior to vitrification according to method step (c) the soot body is subjected to a conditioning treatment which comprises a treatment with nitrogen containing gas , preferably nitrogen oxide, or a sequence of treatments with ammonia and an oxidizing agent.

The gist of the method according to the invention resides in the elimination of oxygen defect centers at a temperature that is as low as possible by treating the soot body in a thermal-oxidative conditioning treatment with treatment reagents that are capable of reacting with oxygen deficiency centers at a low temperature. It has been found that suitable treatment reagents comprise nitrogen oxides and ammonia for this purpose, the treatment with ammonia obligatorily including an after-treatment with an oxidizing agent. Nitrogen oxides particularly encompass nitrogen dioxide (NO2), nitrogen monoxide (NO), and nitrous oxide (N₂O); ..ammonia" (NH₃) also covers ..deuterized ammonia" (ND₃) .

In the method according to the invention the soot body is subjected at a comparatively low temperature (below 1200⁰C) to the thermal conditioning treatment in an atmosphere containing one or more of the above-mentioned treatment reagents. This conditioning treatment may be carried out during drying of the soot body according to method step (b) and/or it may be realized as a separate pre-treatment prior to vitrification of the soot body. Optionally, drying of the soot body and pre-treatment may be carried out in separate furnaces or in one furnace and in one operation.

The conditioning treatment comprises one or several method steps. As a consequence of the thermal decomposition of the nitrogen oxide, reactive nitrogen or reactive oxygen are formed, respectively reactive nitrogen containing components by reaction with or decomposition of ammonia. These reactive components particularly react with reactive locations of the quartz glass network with formation of Si-N and other nitrogen compounds. Groups of Si-H, Si-Cl, Si- OH, Si (free Si bond) or Si-Si should here be mentioned by way of example as reactive locations of the quartz glass network. Defects that have been produced during the drying treatment are for example also saturated with nitrogen by this conditioning treatment. Si nitrogen compounds produced in this process can react with oxygen at a temperature below the vitrification temperature. Stable Si-O or Si- ON bonds are thereby formed.

The soot body is a synthetic body made of porous Siθ2 which may be obtained by oxidizing (or hydrolyzing) a silicon containing raw material thereby producing fine SiO2 particles which are deposited on a substrate.

Synthetic quartz glass with an advantageous damage behavior vis-a-vis shortwave UV laser radiation is obtained by subsequent vitrification from the soot body treated in this way. Due to the manufacturing process, the synthetic quartz glass according to the invention may contain nitrogen, which is chemically bound in the glass network.

The method according to the invention permits the manufacture of low-defect, UV- radiation resistant quartz glass without the need for a technically complicated high- temperature treatment of the Siθ2 soot body in an oxygen atmosphere.

..Optical component" stands here for the ready-for-use component of transparent synthetic quartz glass, such as an enclosed optical lens, but also an intermediate product (blank) which for the manufacture of the component still requires some after-treatment, such as machining by drilling, sawing, grinding or polishing. The quartz glass shows minimum absorption, if possible, preferably less than 0,001/cm, for UV-radiation of a wavelength of 193 nm.

The synthetic quartz glass is produced with the help of the known methods. Flame hydrolysis of SiCI₄ or other hydrolyzable silicon compounds or the plasma- supported deposition of SiO₂ particles should here be mentioned by way of example.

Preferred variants of the method for the treatment of the soot body with nitrogen oxides on the one hand and for the treatment with ammonia (deutehzed ammonia) on the other hand differ from one another in part. First of all suitable modifications of the method for the treatment with nitrogen oxides shall be explained in more detail in the following.

Preferably, nitrogen dioxide (NO₂) is used as the nitrogen oxide.

Particularly reactive nitrogen and oxygen atoms that react already at low temperatures (< 1 ,200⁰C) with the said "reactive centers" of the quartz glass network structure with formation of Si-N, Si-ON, or the like, are formed during the thermal decomposition of nitrogen dioxide. The low treatment temperature permits the use of constructionally relatively simple furnaces, which may e.g. also include furnace parts of graphite.

It has also turned out to be useful when dinitrogen oxide (nitrous oxide, N₂O) is used as the nitrogen oxide.

Dinitrogen oxide has the particular advantage that it is obtainable on a large scale in high purity and that it is relatively nontoxic and harmless. Also in the thermal decomposition of dinitrogen oxide, particularly reactive nitrogen and oxygen atoms are formed that react already at low temperatures (< 1 ,200⁰C) with "reactive centers" of the quartz glass network structure with formation of Si-N, Si-ON, or the like. The low treatment temperature permits the use of constructionally relatively simple furnaces, which may for example also include furnace parts of graphite. With regard to this the treatment with nitrogen oxide is preferably carried out at a treatment temperature below 1100⁰C, preferably below 900⁰C.

The low treatment temperature enables the saturation of defects of the quartz glass network by using a comparatively low amount of energy and furnaces that are constructionally not very complicated. The minimum temperature for the treatment with nitrogen oxide depends on the specific decomposition temperature in contact with the SiO2 surface of the soot body and is normally above 150°C.

It has turned out to be particularly advantageous when the treatment with nitrogen oxide comprises a low-temperature treatment phase in which the treatment temperature is set to be less than 500⁰C, preferably less than 450°C. During the low-temperature treatment phase the soot body is substantially infiltrated, in which infiltration process the nitrogen oxide is distributed substantially uniformly within the porous soot body. The nitrogen oxide is here preferably introduced by means of an inert carrier gas stream into the soot body.

In this connection the treatment with nitrogen oxide preferably comprises also a high-temperature treatment phase in which the treatment temperature is set to be higher than 500°C, preferably higher than 550⁰C.

During the high-temperature treatment phase the nitrogen oxide is thermally decomposed, so that the nitrogen oxide uniformly distributed in the soot body now decomposes and homogeneously reacts with the oxygen deficiency centers or other defects in the quartz glass structure. Ideally, further introduction of nitrogen oxide into the treatment furnace can here be dispensed with, which further reduces the corrosive attack of the nitrogen oxide decomposition products on the furnace components also at the elevated treatment temperature. Therefore, the high-temperature treatment phase is preferably preceded by a low-temperature treatment phase.

It has turned out to be advantageous in the treatment with the nitrogen containing gas the nitrogen oxide content is between 0.1 vol.-% and 10 vol.-%, preferably between 0.5 vol.-% and 5 vol.-% With nitrogen oxide contents below 0.1 vol.-% there is a low oxidative action, and with nitrogen oxide contents above 5 vol.-% there may be an overloading with nitrogen and formation of bubbles during subsequent treatment at high temperature , e.g. during vitrification.

Since the defect centers are saturated and eliminated by means of the method according to the invention at a low temperature, the thermal conditioning treatment in a nitrogen oxide-containing atmosphere can be repeated. At temperatures below about 1200⁰C the porous structure of the soot body is maintained, so that it is ensured that the gaseous treatment reagents can penetrate the soot body by diffusion and uniformly react with the quartz glass network. Therefore, in a preferred variant of the method it is intended that the conditioning treatment comprises repeated running through the low-temperature phase and/or the high- temperature treatment phase.

In the other particularly preferred variant of the method according to the invention with a sequence of treatments with ammonia and an oxidizing agent it is intended that the treatment with ammonia is carried out at a treatment temperature in the range between 100⁰C and 800°C.

Normally, the soot body is dried in a chlorine-containing atmosphere. However, it has been found that chlorine introduced in this process can hardly be removed quantitatively at a later time. Remaining chlorine, however, has a detrimental effect on compaction.

Therefore, in the preferred variant of the method the soot body is first treated with ammonia (or with deuterized ammonia) at a relatively low temperature. Ammonia is a strong nucleophile and, similar to chlorine, has a substitution-reactive effect on hydroxyl groups. Hence, ammonia acts at the same time as a drying agent, so that the treatment of the soot body with ammonia already effects a certain OH reduction. "Ammonia drying" serves in general as a supportive measure during soot body drying, in the case of which the otherwise intended drying measure need not be so intensive. For instance, in the case of a supplementary thermal drying process, a lower treatment temperature, a shorter treatment period or less vacuum is appropriate, or in a dehydration treatment in chlorine also a lower chlorine content, for accomplishing the predetermined drying action.

However, it is also essential that with the ammonia treatment the majority of the defect centers in the soot body should be replaced by amine groups (NH₂) with formation of Si-NH₂ bonds or any other nitrogen containing group as for example imide group (Si-NH-Si). The basic idea is that these defects should be masked such that they can be eliminated in the subsequent oxidation treatment at a temperature that is as low as possible. The following preconditions must here be satisfied:

(1 ) First of all, the soot body has to be subjected to a chemical pre-treatment with ammonia (NH₃ or ND₃) or an ammonia-forming start substance. This is suited to react with reactive locations of the quartz glass network structure, namely at a reaction temperature that is lower than that of the reaction of the corresponding defect center with oxygen or another oxygen-containing oxidizing agent. Structural defects of the quartz glass network and also reactive

Si compounds, for instance Si-Cl-, Si-H- or Si-OH groups, are suited as ..reactive centers" of the quartz glass network structure. A selection of such centers and their reaction with ammonia shall now be explained in more detail with reference to chemical equations: SSii--HH ++ NNHH₃₃ ^ SSii--NNHH₂₂ ++ HH₂₂ (1 )

Si-Cl + NH₃ Si-NH₂ + HCI (2)

Si-OH + NH₃ Si-NH₂ + H₂O (3)

Si + NH₃ Si-NH₂ + 1/2H₂ (4)

Si-Si + 2NH₃ 2 Si-NH₂ + H₂ (5) (2) The amine compound (Si-NH₂) obtained during pre-treatment is in turn of such a kind that in a later oxidative treatment of the soot body it reacts with the oxidizing agent at a lower temperature than the non-masked reactive center of the quartz glass network structure.

A ..masking" of the reactive centers thus permits their elimination or substitution at a comparatively low temperature. This is guaranteed due to the fact that, by analogy with the strongly exothermic reaction of ammonia and oxygen, even Si-NH₂ compounds in the porous soot body can be oxidized relatively easily. The amine groups are therefore comparatively sensitive to oxidation. The reactive centers of the network structure masked in this way can be eliminated (oxidized or substituted) in the subsequent oxidation treatment much more efficiently than without such a masking in a purely thermally dried soot body or, for example, in a soot body containing chlorine compounds.

Hence, the subsequent oxidation treatment of the reactive centers masked in this way effects a reaction taking place in a substantially quantitative way, namely at a temperature lower than would otherwise be the case with a chlorinated soot body or with an untreated soot body. Apart from oxygen, appropriate oxidizing agents for the oxidation treatment are e.g. also nitrogen oxides and ozone.

The oxidation treatment is preferably carried out in the still porous state of the soot body, i.e. before completion of the vitrification process. The thermal-oxidative treatment with a gaseous oxidizing agent thus starts before the vitrification of the soot body or at least during a first vitrification phase in which the soot body is still in a porous state.

The amine groups are incorporated at least in part into the network structure of the quartz glass due to the oxidation reaction as triple-coordinated nitrogen. Hence, oxinithde compounds are formed that enhance the mechanical strength of the network structure and can thus improve the compaction behavior of the quartz glass with respect to UV radiation.

Preferably, the treatment with ammonia is carried out in a treatment atmosphere containing less than 5 vol.-%, preferably less than 1 vol.-% and more than 0.05 vol.-% of ammonia.

At ammonia concentration of more than 5 vol.-% a large number of amine groups are formed that in later treatment steps may lead to foaming and to the formation of bubbles. At ammonia concentrations below 0.1 vol.-% the reactive centers are less masked and there is consequently a less positive effect. Treatment of the soot body with ammonia is preferably carried out by using a soot body that is first dried in a purely thermal or additionally reactive way by using a halogen-containing atmosphere.

In a first variant of the method with purely thermal drying (in vacuum or in an inert gas) the drying of the soot body according to step (b) comprises a treatment of the soot body at a drying temperature is in the range between 1100⁰C and 1350⁰C, preferably not more than 1300°C.

In a purely thermal drying process halogen-containing drying agents are dispensed with, so that the introduction of halogens into the soot body is prevented, and these need not be removed from the soot body at a later time. On the other hand, the long-winded thermal treatment under reducing conditions creates oxygen defects because an appropriate substituent is not immediately available for the removed OH groups. The oxygen defects impair the UV radiation resistance of the quartz glass. It is for this reason that the above-described masking of the oxygen defects and their subsequent oxidative after-treatment are provided. Ammonia may be used as a supportive drying agent.

By contrast, in a second equally suited variant of the method according to the invention, the drying of the soot body according to step (b) comprises a treatment of the soot body in a halogen-containing atmosphere.

With this drying in a halogen-containing atmosphere, particularly in a chlorine- containing atmosphere, the hydroxyl group content of the soot body can also be reduced at a low temperature to a value below 60 wtppm, preferably to a value below 30 wtppm.

It has been found that ammonia is capable of expelling chlorine, which has for instance been introduced into the soot body due to a dehydration treatment.

Chemically bound chlorine atoms are here evidently replaced by nitrogen atoms. Moreover, reactive nitrogen shows an adequate reactivity vis-a-vis hydroxyl groups at high temperatures and is adapted to displace hydroxyl groups out of the SiO2 soot body. Hence, this variant of the method permits, on the one hand, an efficient drying of the soot body by using chlorine and, on the other hand, the removal of chlorine introduced thereby with simultaneous introduction of chemically bound nitrogen.

The partial incorporation of nitrogen strengthens the quartz glass network structure, which contributes to a stabilization of the structure and an improvement of the UV radiation resistance of the quartz glass.

The hydroxyl group content (OH content) follows from a measurement of the IR absorption according to the method of D. M. Dodd et al. (..Optical Determinations of OH in Fused Silica", (1966), p. 3911 ).

The known substances, such as oxygen and ozone, are suited as oxidizing agents. However, it has turned out to be particularly advantageous when a nitrogen oxide is used as the oxidizing agent during the oxidation treatment.

The nitrogen oxide used as the oxidizing agent or the used nitrogen oxides additionally effect an incorporation of nitrogen into the quartz glass network. It has been found that such a nitrogen loading of the quartz glass contributes to an improved compaction behavior with respect to high-energy UV radiation.

In a preferred development of the above-explained variant of the method using ammonia as the treatment reagent, it is intended that after its treatment with ammonia the soot body is subjected to a silylation treatment using a silylation reagent at a silylation temperature.

It has been found that amine groups in a nitrided soot body can be cross-linked more strongly in the SiO2 network structure by thermal reaction with a volatile silylation agent. By said slylation treatment, additional oxygen atoms or silicon atoms are distributed over the quartz glass network structure so that nitrogen atoms which are adsorbed at the surface of the network structure or nitrogen atoms being chemically bonded are physically neighbored to such silicon or oxygen which may result in an easier and stronger bonding of the nitrogen in subsequent treatment step at higher temperatures.

Hydrogen-free silanes, for instance SiCI₄ or SiF₄, are primarily suited as silylation agents. Therefore, following nitration with formation of amine groups, thermal silylation is carried out according to the invention, using a silylation agent and a temperature below the decomposition temperature of the silane in question.

With this treatment the majority of the rather volatile amine groups are converted into imide groups (Si-NH-Si) and into nitride groups (Si-N). As a consequence, nitrogen is incorporated into the SiO2 matrix to a much stronger degree, which in turn results in consolidation and stabilization, respectively, of the SiO2 network. Moreover, this measure reduces the hydrogen amount of the amine groups. This is desired because hydrogen can form Si-H groups in the subsequent treatment steps at a high temperature, said groups forming so-called "precursor defects" and being easily converted into oxygen deficiency centers upon UV radiation.

A quartz glass treated in this way is distinguished by improved compaction behavior.

In a particularly preferred variant of the method, ammonia is used in the form of deuterized ammonia. The special advantage of deuterized ammonia is that said substance is a very efficient deuteration agent for quartz glass. Deuterium is here incorporated into the quartz glass structure instead of hydrogen. It has been found that the radiation resistance of quartz glass can thereby be improved.

Modifications of the method according to the invention shall now be explained in more detail. These modifications have turned out to be useful in a conditioning treatment either with nitrogen oxides or with ammonia with an oxidative-thermal after-treatment.

It has also turned out to be particularly useful when the conditioning treatment is preceded by a pre-treatment in reducing atmosphere.

The treatment of the soot body in reducing atmosphere may be accompanied by a drying of the soot body. It is e.g. carried out in the presence of hydrogen, carbon monoxide, hydrocarbons or ammonia. On account of the reducing action, oxygen deficiency centers are generated in the soot body in a uniform way, said centers being subsequently saturated by treatment with nitrogen-containing treatment reagents, with chemically bound nitrogen being incorporated into the quartz glass network. It has here turned out to be particularly advantageous when after its pre-treatment in an atmosphere with a reducing action the soot body exhibits a concentration of oxygen deficiency centers of at least 2 x 10¹⁵ cm^"3.

As a result of the great number of oxygen deficiency centers, the quartz glass is loaded with nitrogen to a particularly high degree, which has an advantageous effect on UV radiation resistance. Moreover, the use of ammonia during pre- treatment effects a certain drying of the soot body, as has already been explained above, so that the use of halogens during drying can be dispensed with and the disadvantages entailed by halogen incorporation are avoided.

To avoid a loading of the quartz glass with halogens as much as possible, the drying operation is preferably carried out in a purely thermal way (also by application of a vacuum). The drying of the soot body according to step (b) comprises a treatment at a temperature preferably in the range between 1100⁰C and 1350⁰C, preferably not more than 1300°C. Drying temperature means the temperature measured on the surface of the soot body by means of a pyrometer.

Preferably, during drying of the soot body a mean hydroxyl group content of the quartz glass of less than 60 wtppm is obtained, preferably in the range between 10 wtppm and 30 wtppm.

A low hydroxyl group content effects enhanced viscosity of the quartz glass and improves the behavior vis-a-vis a local isotropic density change at the same time.

Moreover, it has turned out to be advantageous when the mean hydrogen content of the quartz glass is set to a value between 5 x 10¹⁵ and 3 x 10¹⁷ molecules/cm³.

The higher the hydrogen content, the greater is its defect-healing effect in case of UV irradiation. On the other hand, a high hydrogen content can promote the formation of SiH groups, so that the mean hydrogen content of the quartz glass is preferably less than 3 x 10¹⁷ molecules/cm³. This upper limit is thus due to the fact that at a high hydrogen content the risk of SiH formation through the reaction

H₂ + Si-O-Si -> SiH + SiOH (6) increases. The hydrogen content is set after vitrification by heating in a hydrogen- containing atmosphere, preferably at a temperature below 500⁰C, to reduce the formation of SiH groups. SiH groups in quartz glass are undesired because upon irradiation with high-energy UV light a so-called E' center and atomic hydrogen are formed therefrom.

The hydrogen content (H₂ content) is determined with the help of a Raman measurement, which was suggested by Khotimchenko et al. for the first time (..Determining the Content of Hydrogen Dissolved in Quartz Glass Using the Methods of Raman Scattering and Mass Spectrometry" Zhurnal Prikladnoi Spektroskopii, Vol. 46, No. 6 (June 1987), pp. 987-991 ).

It has turned out to be advantageous when the nitrogen content of the quartz glass is set to a value in the range between 1 wtppm and 150 wtppm.

The nitrogen oxide used as the treatment and oxidizing agent, or the used nitrogen oxides additionally effect the incorporation of nitrogen into the quartz glass network. Especially the procedure of treating the soot body with ammonia (deutehzed ammonia) and the subsequent ,,oxidation-thermal healing" of the oxygen deficiency defects caused by this treatment result in the stable incorporation of nitrogen into the quartz glass.

It has been found that such a nitrogen loading of the quartz glass leads not only to an increase in the viscosity of quartz glass, but also contributes to improved compaction behavior vis-a-vis high-energy UV radiation.

The optical component produced according to the method of the invention is distinguished by low sensitivity to a local anisotropic and isotropic density change upon irradiation with shortwave UV radiation. Therefore, it is preferably used as an optical component in a projection system of an automatic exposure apparatus for microlithography for the purpose of transmitting ultraviolet, pulsed or linearly polarized UV laser radiation of a wavelength between 190 nm and 250 nm. Preferred Embodiments

The invention shall now be explained with reference to embodiments and a drawing in more detail, in which drawing

Figure 1 shows a diagram with transmission spectra of a sample produced according to the invention and a comparative example.

Example 1

A soot body is produced by flame hydrolysis of SiCI₄ with the help of the known OVD method. The soot body is dehydrated at a temperature of 1200⁰C in a heating furnace with a heating element of graphite in vacuum. The graphite existing in the heating furnace effects the setting of reducing conditions. Upon completion of the dehydration treatment after 50 hours the hydroxyl group content of the soot body is about 25 wtppm. The quartz glass of the soot body contains oxygen defects in the order of 1.7 x 10¹⁶ cm^"3. This defect concentration was indirectly determined through the transmission loss. The transmission loss is here based on the division of the oxygen defects under laser irradiation into two so- called E' centers, said centers showing a typical absorption at a wavelength of 210 mm.

The thermally dried soot body is subsequently treated at a temperature in the range of 500⁰C to 800°C in a nitrous oxide atmosphere (N₂O, also known as "laughing gas"). The soot body is here first thermally treated with nitrous oxide in a carrier gas stream consisting of nitrogen at temperatures of less than 500⁰C for four hours and the temperature is raised from 500°C to 800°C while maintaining the N₂O/N₂ gas stream. The nitrous oxide concentration is set to 1.5 vol.-% during this pre-treatment phase.

Above 500 ⁰C N₂O decomposes into oxygen and reactive nitrogen atoms or compounds that are capable of reacting with the defects of the quartz glass network structure and of saturating and thereby eliminating the same.

The dried and after-treated soot body is then vitrified in a sintering furnace at a temperature of about 1400 ⁰C in vacuum (10^~2 mbar) into a transparent quartz glass blank. Said blank is subsequently homogenized by thermal mechanical homogenization (twisting) and formation of a quartz glass cylinder.

For reducing mechanical stresses and for reducing birefringence and for producing a compaction-resistant glass structure the quartz glass cylinder is subjected to an annealing treatment in which the quartz glass cylinder is heated to 1130⁰C for a holding period of 8 hours in air and at atmospheric pressure and is subsequently cooled at a cooling rate of 4°C/h to a temperature of 1050⁰C and held at this temperature for 4 hours. Thereupon, the quartz glass cylinder is cooled at a higher cooling rate of 50°C/h to a temperature of 300⁰C, whereupon the furnace is switched off and the quartz glass cylinder is allowed to cool freely in the furnace.

The quartz glass cylinder treated in this way has an outer diameter of 350 mm and a thickness of 60 mm. Since the cylinder in its edge regions exhibits relatively strong stress birefringence, part of the overdimension with respect to the component contour is removed from the faces, namely a thickness of 3 mm.

Thereupon the quartz glass cylinder is held in an atmosphere of 80 vol.-% nitrogen and 20 vol.-% hydrogen at 400°C at an absolute pressure of 1 bar for a period of 80 days.

The quartz glass cylinder obtained thereafter is substantially free from chlorine, oxygen defects and SiH groups (below the detection limit of 5 x 10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of 3 x 10¹⁶ molecules/cm³ and a hydroxyl group content of 25 wtppm.

The above-described pre-treatment and N₂O atmosphere can be repeated several times. For improving the reactivity between the quartz glass of the soot body and the nitrogen oxide the pre-treatment of the soot body can also be carried out in a reducing atmosphere. Instead of an inert nitrogen carrier gas stream, an NH₃- containing nitrogen carrier gas stream is for instance used. Example 2

Another quartz glass cylinder is produced as has been described above with reference to Example 1 , with the difference that, instead of nitrous oxide, nitrogen dioxide (NO2) is used as the oxidizing agent.

To this end the soot body is treated in a nitrogen dioxide atmosphere after the dehydration treatment. In this process the soot body is first treated in a mixture of NO and O2 which gases are transported in a carrier gas stream of nitrogen into a treating furnace at a temperature of 100⁰C, whereby in situ-generating NO2. After a treatment for four hours the temperature is raised from 400⁰C to 700°C and at that temperature the sample is treated for 20 h, with the nitrogen dioxide concentration being set to 1.5 vol.-%

Above 200⁰C, NO2 decomposes into oxygen and reactive nitrogen atoms or nitrogen compounds that are capable of reacting with the defects of the quartz glass network structure and of saturating and thereby eliminating the same.

The dried and treated soot body is sintered, homogenized and loaded with hydrogen, as described with reference to Example 1. The quartz glass cylinder obtained thereafter is substantially free from chlorine, oxygen defects and SiH groups (below the detection limit of 5x10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of 3 x 10¹⁶ molecules/cm³ and a hydroxyl group content of 25 wtppm.

Example 3

Another quartz glass cylinder is produced as has been described above with reference to Example 1 , but with the following differences:

The soot body is dehydrated at a temperature of 1200⁰C in a heating furnace with a heating element of graphite in vacuum, the dehydration treatment being only completed after 100 hours. The hydroxyl group content of the soot body is about 15 wtppm after said treatment. The quartz glass of the soot body contains oxygen defects in the order of 3 x 10¹⁶ molecules/cm³. The dried soot body is then treated at a temperature of 750⁰C for a period of 20 h in an atmosphere consisting of ammonia and nitrogen. The content of ammonia in the treatment atmosphere is here 0.3 vol.-%. It is ensured through the ammonia treatment that most of the defect centers that have e.g. been generated during thermal drying are replaced by amine groups.

The amine groups are sensitive to oxidation and can be replaced in a subsequent oxidation treatment by oxygen. Therefore, the ammonia treatment is followed by an oxygen treatment. For that purpose the soot body is treated in a nitrogen gas stream containing 1 vol.-% oxygen whereby the temperature is raised from 750 ⁰C to 1200⁰C within 10 hours and subsequently held at that temperature for 20 h, whereby the soot body is partially densified. After that the partially densified soot body is vitrified at a temperature of 1400 °C.

During that treatment the amine groups or any other nitrogen containing groups of the silica network are replaced in part by oxygen, another part is incorporated as triple-coordinated nitrogen in the SiO2 network, which enhances the strength of the network. The stable incorporation of nitrogen in the quartz glass network has the effect that it does not degas in subsequent hot treatment processes and that the quartz glass does therefore not foam.

The soot body is thereafter vitrified and further treated, as has been described with reference to Example 1. The resulting quartz glass cylinder is substantially free from chlorine, oxygen defects and SiH groups (below the detection limit of 5 x10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of about 3 x 10¹⁶ molecules/cm³ and by a hydroxyl group content of 20 wtppm.

Example 4

In a modification of the production method, as has been described with reference to Example 2, the soot body is treated after the ammonia treatment with a silylation agent in the form of vaporous SiCI₄ (1 Vol.-% SiCI₄ in a nitrogen gas stream as a carrier gas). This treatment is carried out at a temperature of 1000⁰C and for a period of 10 h. Most of the amine groups are thereby converted into imide groups (Si-NH-Si) or into nitrides. This results in a stable incorporation of nitrogen into the quartz glass network. As for the subsequent oxidation treatment in an oxygen-containing atmosphere (1.5 vol.-% O2) at a temperature of 1000⁰C, a treatment period of 10 hours is sufficient.

The soot body is thereafter vitrified and further treated, as has been described above with reference to Example 1. The resulting quartz glass cylinder is substantially free from chlorine, oxygen defects and SiH groups (below the detection limit of 5 x10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of about 3 x 10¹⁶ molecules/cm³ and by a hydroxyl group content of 30 wtppm.

Example 5

In a modification of the production method, as has been described above with reference to Example 3, an oxidation treatment is carried out - after the ammonia treatment and after treatment with a silylation agent in the form of vaporous SiCI₄ - in an atmosphere consisting of nitrogen and N₂O. For that purpose the soot body is treated for 5 hours in a nitrogen gas stream containing 2 vol.-% of N₂O which is fed to the treating furnace that is hold at a temperature of 500⁰C. Subsequently the temperature of the furnace is increased to 900 °C followed by treatment period of 20 hours.

The soot body is thereafter vitrified and further treated, as has been described above with reference to Example 1. The resulting quartz glass cylinder is substantially free from chlorine, oxygen defects and SiH groups (below the detection limit of 5 x10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of about 3 x 10¹⁶ molecules/cm³ and a hydroxyl group content of 25 wtppm.

Comparative Example

A soot body is produced and thermally dried, as has been described with reference to Example 1. The soot body is treated without treatment in a nitrogen dioxide atmosphere (N₂O) in a sintering furnace at a temperature of 1200 ⁰C, first in an oxygen atmosphere (100%), and it is subsequently vitrified in an He atmosphere at 1450⁰C to obtain a transparent quartz glass blank.

The further processing and after-treatment of the quartz glass blank corresponds to that of Example 1. The resulting quartz glass cylinder is substantially free from chlorine and SiH groups (below the detection limit of 5 x 10¹⁶ molecules/cm³), and it is distinguished within a diameter of 280 mm (CA area) by a mean hydrogen content of 3 x 10¹⁶ molecules/cm³ and a hydroxyl group content of 48 wtppm.

Measurement samples were taken for determining the resistance of the quartz glass to irradiation with UV excimer laser radiation. The measurement samples were taken from the quartz glass cylinders prior to their hydrogen loading because hydrogen doping would otherwise distort the measurement effect. The measurement samples with a thickness of 5 mm were exposed to UV radiation of a wavelength of 193 nm, a pulse width of 25 ns and an energy density of 80 mJ/cm² and a frequency of 200 Hz. After a pulse number of 20,000 pulses the transmission was measured over the wavelength range of 180 nm to 300 m.

Figure 1 shows corresponding transmission spectra of measurement samples that have been produced according to Example 1 (curve 1 ) and according to Comparative Example 1 (curve 2). The y-axis has plotted thereon a transmission value "T" normalized to the respective initial transmission (before treatment with UV excimer laser radiation). Wavelength ,,λ" is plotted in mm on the x-axis.

It can be seen in the transmission spectrum of curve 1 that the transmission of the measurement sample has not significantly changed after treatment with UV excimer laser radiation; the relative transmission is about 1 over the whole wavelength range.

By contrast, transmission in the comparative sample has noticeably deteriorated after treatment with UV excimer laser radiation. In the case of curve 2 the relative transmission is below 1 , and a further deterioration occurs at a wavelength around 210 nm. This comparison demonstrates that the optical component produced according to the invention has a lower content of oxygen deficit centers and is particularly suited for use in a projection system of an automatic exposure device for microlithography for the purpose of transmitting ultraviolet, pulsed and linearly polarized UV laser radiation of a wavelength between 190 nm and 250 nm.

Claims

Patent Claims

1. A method for producing an optical component of quartz glass for use in an objective for microlithography at a working wavelength below 250 nm, the method comprising the following steps:

(a) producing an SiO₂ soot body,

(c) vitrifying the soot body with formation of a cylindrical quartz glass blank which is further processed into the optical component, characterized in that prior to vitrification according to method step (c) the soot body is subjected to a conditioning treatment which comprises a treatment with a nitrogen containing gas, preferably with nitrogen oxide, or a sequence of treatments with ammonia and an oxidizing agent.

2. The method according to claim 1 , characterized in that nitrogen dioxide (NO₂) is used as the nitrogen oxide.

3. The method according to claim 1 or 2, characterized in that nitrous oxide (N₂O) is used as the nitrogen oxide.

4. The method according to any one of the preceding claims, characterized in that the treatment with nitrogen containing gas (nitrogen oxide) takes place at a treatment temperature below 1000⁰C, preferably below 900⁰C.

5. The method according to any one of the preceding claims, characterized in that the treatment with nitrogen containing gas (nitrogen oxide) comprises a low- temperature treatment phase in which the treatment temperature is set to be less than 500°C, preferably less than 450⁰C.

6. The method according to claims 4 and 5, characterized in that the treatment with nitrogen containing gas (nitrogen oxide) comprises a high-temperature treatment phase in which the treatment temperature is set to be higher than 500⁰C, preferably higher than 550⁰C.

7. The method according to any one of the preceding claims, characterized in that in the treatment with the nitrogen containing gas the nitrogen oxide content is between 0.1 vol.-% and 10 vol.-%, preferably between 0.5 vol.-% and 5 vol.-%

8. The method according to any one of claims 4 to 7, characterized in that the conditioning treatment comprises repeated running through the low- temperature treatment phase and/or the high-temperature treatment phase.

9. The method according to claim 1 , characterized in that the sequence of treatments with ammonia and an oxidizing agent comprises a treatment with ammonia at a treatment temperature in the range between 100°C and 800⁰C.

10. The method according to claim 9, characterized in that the treatment with ammonia is carried out in a treatment atmosphere containing less than 5 vol.-

%, preferably less than 1 vol.-% and more than 0.05 vol.-%, of ammonia.

11.The method of according to claim 9 or 10, characterized in that the drying of the soot body according to step (b) comprises a treatment of the soot body at a drying temperature is in the range between 1100°C and 1350°C, preferably not more than 1300⁰C.

12. The method according to claim 9 or 10, characterized in that the drying of the soot body according to step (b) comprises a treatment in a halogen-containing atmosphere.

13. The method according to any one of claims 9 to 12, characterized in that a nitrogen oxide is used as an oxidizing agent during the oxidation treatment.

14. The method according to any one of claims 9 to 13, characterized in that after treatment with ammonia according to method step (c) the soot body is subjected to a silylation treatment using a silylation reagent at a silylation temperature.

15. The method according to claim 14, characterized in that a silane is used as the silylation reagent, and that the silylation temperature is set to a temperature below the decomposition temperature of the silane.

16. The method according to any one of the preceding claims, characterized in that during drying of the soot body a mean hydroxyl group content of the quartz glass is set in the range between 10 wtppm and 30 wtppm.

17. The method according to any one of the preceding claims, characterized in that a mean hydrogen content of the quartz glass is set in the range between 5 x 10¹⁵ and 3 x 10¹⁷ molecules/cm³ in the quartz glass.

18. The method according to any one of the preceding claims, characterized in that the nitrogen content of the quartz glass is set to a value in the range between

1 wtppm and 150 wtppm.