JPH10289854A - Exposure device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To contrive to lessen the deterioration of the performance of an expo sure device extending over along term even in any exposure wavelength by a method wherein an active oxygen producing catalyst is spread on the inner surface of one part of a lens barrel, which encircles the optical path of light emitted from a light source and receives the scattered lights of exposure light, or the like to hold the one part of the lens barrel in an oxygen atmosphere. SOLUTION: Ultraviolet rays radiated from a luminous tube 11, which is used as a light source, are adjusted so that a reticle 14 is uniformly illuminated by optical components 12, such as a capacitor lens and a zoom lens, to make the ultraviolet rays fall on a sealing box 102 for sealing a group 15 of reduced exposure lenses housed in a lens barrel 16 encircling the optical path of light emitted from the light source and moreover, the ultraviolet rays transfer a reticle pattern on a wafer 17. At this time, an exposure is performed on an object of exposure by such a method that oxygen gas purified by an oxygen gas bomb 103 and a gas purifier 104 is made to flow through the lens barrel 16 in the box 102 to make one part of exposure light fall on a catalyst 106 spread on the interior of the lens barrel 16 and one part of the oxygen gas in the interior of the lens barrel 16 is made to activate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure apparatus used for manufacturing a semiconductor device, and more particularly to an exposure apparatus using a stepper exposure apparatus including a reduction exposure system.

[0002]

2. Description of the Related Art A stepper exposure apparatus reduces a pattern on a reticle or mask (hereinafter, collectively referred to as a reticle) by 1/10 to 1/5 to form a pattern on an exposure object such as a photoresist film on a semiconductor wafer. Image.

With the increase in the degree of integration and miniaturization of semiconductor devices, the exposure wavelength of a stepper exposure apparatus has been shortened from a g-line of a mercury lamp to an i-line, and further to a KrF laser beam and an ArF laser beam of an excimer laser. We are working to shorten the wavelength.

[0004] The ultraviolet light used for these exposures has high photon energy, and causes a chemical change in the irradiated photoreceptor such as a photoresist to form a reticle pattern on an object to be exposed.

FIG. 6 shows a conventional stepper exposure apparatus. In FIG. 6, a lamp 61 that emits ultraviolet light and a condenser lens 62 for irradiating light emitted from the lamp 61 onto a reticle 64 are arranged in a lens barrel 601 of an illumination system. Emitted from lamp 61, condenser lens 62
The light converted into a parallel light beam at irradiates the reticle 64.

Further, the ultraviolet light passing through the opening of the reticle 64 passes through the projection system lens barrel 602, is focused by the reduction projection system lens group 65, and exposes the photoresist film applied on the semiconductor wafer 67. . The semiconductor wafer 67 is mounted on an XY stage 69 fixed on a base 68.
It is placed on.

After exposing at an appropriate illuminance using a shutter, a diaphragm, and the like, which are omitted in FIG. 6, which shows this conventional exposure apparatus, the XY stage 69 is moved, and exposure is repeatedly performed in the same procedure.

In order to increase the processing speed of the stepper, it is necessary to increase the amount of light transmitted through all the lens groups of the illumination system and the exposure system, and flare and illuminance unevenness on the object to be exposed must be minimized. No.

Usually, an antireflection film for the exposure wavelength is formed on the lens surface in order to increase the transmittance and suppress flare.

[0010]

If the g-line and i-line steppers are used for a long period of time, the lens surfaces of the illumination lens group 62 and the projection lens group 65 in FIG.
Organic matter accumulates. Generally, the surface of these lens groups is coated with an anti-reflection film for the exposure wavelength, and the reflectance changes due to cloudiness and the deposition of organic substances, which reduces the transmittance and reduces the scattering of the exposure light. Had also occurred.

[0011] If the accumulation of fogging and the adhesion of organic substances are uneven over the entire surface of the lens constituting the optical system, the illuminance may be uneven on the object to be exposed. Also, when the deposit adhering to the lens group surface absorbs the exposure light, similarly, the transmittance of the lens decreases and the transmittance in the lens surface becomes uneven. In such a state, the projection illuminance of the stepper decreases, illuminance unevenness occurs, flare and the like increase, and the performance deteriorates as a whole.

On the other hand, the exposure wavelength is excimer KrF laser (2
48nm) and ArF laser (193nm), and when the photon energy and energy density increase, organic substances and the like attached in the clean room and in the lens barrel are removed, and the illuminance on the exposed surface can be reduced even in a short period. There were problems such as changes.

To solve the above problems, Japanese Patent Laid-Open No.
The -201702 discloses improvements are disclosed for filling the barrel with gas such as N ₂ is a non-oxidizing gas. Further, Japanese Patent Application Laid-Open No. 7-273016 discloses an improvement measure for preventing performance deterioration by preventing sticking to a lens or the like with a seal glass and making the seal glass replaceable.

However, in the lens barrel, there are adhesives for fixing the lens and the lens barrel, and contaminants such as organic substances attached to the inner surface of the lens barrel. These contaminants are decomposed or decomposed by the energy of ultraviolet rays. Upon reaction, decomposition products or reaction products adhere to the lens surface. Further, the organic matter adheres to the lens surface without irradiating ultraviolet rays. Therefore, the above measures are not enough to prevent the deterioration of the optical performance due to the adhesion of the organic substance.

Further, there is a problem that conventional measures cannot cope with the problem, for example, it is very difficult to replace the reduction exposure system lens group 65.

An object of the present invention is to provide an exposure apparatus in which the performance does not deteriorate for a long time at any exposure wavelength.

[0017]

The above objects can be attained by the present invention described below. That is, the present invention relates to an exposure apparatus that passes a light beam from a light source through an optical system including a lens and exposes an object to be exposed through a reticle, such as a lens barrel that surrounds an optical path and receives scattered light of exposure light. An exposure apparatus is disclosed in which at least a part of the inner surface is coated with an active oxygen generation catalyst and is maintained in an oxygen atmosphere.

According to another aspect of the present invention, there is provided an exposure apparatus for transmitting a light beam from a light source through an optical system including a lens and exposing the object on an exposure object through a reticle, wherein at least a part of the optical system is shielded from outside air. The present invention also discloses an exposure apparatus characterized in that the exposure apparatus is covered with a container that can be filled with an inert gas that is not contaminated with a metal compound or an organic compound, and that the inner surface of the container is coated with a high filling rate.

Further, according to the present invention, there is provided an exposure apparatus for passing a light beam from a light source through an optical system including a lens and exposing the object on an exposure object through a reticle, wherein at least a part of the optical system is shielded from outside air. The present invention also discloses an exposure apparatus, which is covered with a container which can be filled with an inert gas which is not contaminated with a metal compound and an organic compound, and a trap having a high organic gas adsorption rate is provided on the inner surface of the container. is there.

An exposure apparatus according to the present invention is an exposure apparatus that passes light emitted from a light source on an optical path including a lens, and exposes an object to be exposed through a mask by oxidizing a lens atmosphere existing on the optical path by oxidizing the lens. While maintaining such a state that an impurity substance that forms an oxide film on the surface is removed, a coating serving as an active oxygen generation catalyst is applied to a position where a part of the exposure light is irradiated under a lens holding environment. Alternatively, an inner surface of the container that covers the lens is coated with a coating having a high filling rate that does not allow the organic gas released from the container to pass through, and a trap that adsorbs the organic gas floating in the lens atmosphere is provided.

The active oxygen generating catalyst generates active oxygen atoms and radicals from oxygen in the environment by ultraviolet light irradiation. Since this active oxygen removes the organic thin film adhered to the lens surface, stabilization of optical performance is realized.

Further, by blocking or adsorbing and separating the supply of the organic gas from the container covering the lens, which is the organic gas supply source in the lens holding environment, the organic deposits are adsorbed or synthesized on the lens surface. And optical performance is stabilized.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described.

[0024]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 The lens of the deteriorated i-ray exposure was analyzed by a Fourier transform infrared spectrometer (hereinafter, referred to as FTIR), and various kinds of organic compounds such as hydrocarbons, carboxylic compounds and acrylamide were obtained. It was found to be deposited on the lens surface. When the same unused lens system was analyzed, various hydrocarbons were detected.

Therefore, it can be determined that the carboxylic acid compound, acrylamide and the like are formed by the reaction of the components in the atmosphere and the outgas components from the lens assembly.

It is known that various kinds of hydrocarbons adhere to a sample which is usually left in the atmosphere. For hydrocarbons, it is possible to distinguish between those adhered in the air and those formed by a photochemical reaction. Although not shown, it has been clarified that it adheres to the lens surface during the normal lens making process and after cleaning.

FIG. 1 is a schematic view showing a main part of an i-line exposure apparatus according to the present invention. In the figure, reference numeral 101 denotes an illumination lens sealing chamber; 102, a projection lens sealing chamber;
Is an oxygen gas cylinder, 104 is a gas purifier, 105 is a gas exhaust hole, and 106 is an active oxygen generation catalyst coating.

The details of the present invention will be described with reference to FIG.
Reference numeral 11 denotes a light emitting tube as a light source, which has a high-luminance light emitting portion that emits ultraviolet light, far ultraviolet light, and the like. Ultraviolet rays radiated from 11 are adjusted to uniformly illuminate the reticle by 12 condenser lenses and optical components such as an i-line filter (not shown), a mirror, an optical integrator, and a zoom lens.

The ultraviolet light that has passed through the reticle 14 is
The light enters the sealing box 102 for sealing the reduction exposure lens group 15 housed therein, and further enters the reduction exposure lens group 1.
Through the step 5, the reticle pattern is transferred onto the wafer 17.

The lens barrel 16 includes an adhesive for bonding the reduction exposure lens group 15 and the lens barrel 16, a lens fixing jig, and the like, but is omitted.

Oxygen gas cylinder 103, gas purifier 104
As a result, purified oxygen gas is introduced into the sealing box 102. The oxygen gas used here removes, from the inside of the sealing box 102, a gas that generates a solid by a photochemical reaction, in addition to an organic solvent and an organic metal-based gas mixed in the clean room, and keeps a state where deposition does not occur. I have.

The purified oxygen gas is supplied to the lens barrel 16 in the sealed box 102 by the reduction exposure system lens group 15.
The through-hole 105 is provided so that the heat can be efficiently transmitted to the inside. This oxygen gas is flowed into the sealing box 102 for a while to sufficiently replace the impurity gas in the lens barrel. Thereafter, exposure is performed while flowing oxygen gas.

During exposure, the exposure light is diffracted by the reticle 14 and partially enters the catalyst 106 coated inside the lens barrel. This catalyst activates a part of oxygen inside the lens barrel with the energy of ultraviolet rays.

The activated oxygen reacts with the organic impurities attached to the lens surface inside the lens barrel to form C having a high vapor pressure.
Generates O ₂ , CO, etc. H ₂ O is also generated at the same time. These generated gases are discharged from the exhaust hole 105 to the outside of the lens barrel together with oxygen gas circulating in the lens barrel.

In this way, organic deposits on the lens surface that change the optical performance of the exposure apparatus can be constantly removed, and the performance can be stabilized for a long period of time.

It is desirable to use a material having high resistance to active oxygen, such as an adhesive for bonding the lens barrel to the lens and the inner surface of the barrel, or to coat a protective film so as not to be exposed to active oxygen. .

In this embodiment, an i-line exposure apparatus has been exemplified.
Even in an exposure apparatus such as g-line, excimer KrF, ArF, etc., the optical performance is stabilized by the cleaning effect of the organic substances adhering to the lens surface.

Although oxygen gas is used in this embodiment, N ₂ , Ar, etc. may be mixed into the oxygen gas, and the same effect can be obtained as long as the active oxygen generating catalyst generates active oxygen. Is obtained.

In this embodiment, the embodiment in which only the reduction exposure system lens is cleaned has been described. However, the same processing can be applied to the illumination system lens if a gas introduction mechanism, a circulation hole, a discharge hole, and a catalyst are provided. Needless to say, this is possible.

As the active oxygen catalyst coating, Ti
O ₂ , ZrO ₂ , HfO ₂ and a mixed film of these materials are preferably used.

Organic substances also adhere to the atmosphere side of the exposure light irradiating portions of the sealing chambers 101 and 102. However, anti-adhesion glass or the like is provided in this portion, and the glass is easily replaced and replaced.

By using the exposure apparatus shown in FIG. 1 according to the present invention, it is possible to change the reflectance by adhering to the lens surface or to constantly remove organic substances that absorb i-rays. In addition, it is possible to perform stable exposure for a long period without changing the illuminance unevenness. In addition, the apparatus can be used efficiently without the need for disassembling and cleaning the lens group.

[Embodiment 2] FIG. 2 is a schematic view showing a main part of an excimer KrF exposure apparatus according to a second embodiment of the present invention. In the figure, 201 is an active oxygen generation catalyst, 202
Is an ultraviolet lamp, 203 is a mixed gas of nitrogen and oxygen, 204
Denotes a purifier, 205 denotes an excimer laser, and 206 denotes a mirror.

The details of the present invention will be described with reference to FIG.
The laser light emitted from the laser 205 is adjusted to uniformly illuminate the reticle 14 by optical components such as a mirror 206, a condenser lens 12 and an optical integrator and a zoom lens which are omitted in this schematic diagram.

The laser beam that has passed through the reticle 14 is
The reticle pattern enters the sealing box 102 that seals the reduction exposure system lens group 15 housed in 6 and passes through the reduction exposure lens group 15 to transfer the reticle pattern onto the wafer 17.

The lens barrel 16 includes an adhesive for bonding the reduction exposure lens group 15 and the lens barrel 16, a lens fixing jig, and the like, but is omitted.

Before the exposure, the inside of the sealed box 102 is sufficiently purged with the nitrogen / oxygen mixed gas of 203 purified by the purifier 204. The nitrogen / oxygen mixed gas 203 used here is one from which impurities causing deposition such as organic metal and NH ₄ have been removed.

After purging with a mixed gas of nitrogen and oxygen,
The second ultraviolet lamp is turned on to irradiate the active oxygen generating catalyst 201 coated in the sealing box 102 with ultraviolet light. At this time, the sealing box 102 is made of a material having a high transmittance of ultraviolet light so that the catalyst is effectively irradiated with ultraviolet light,
For example, quartz or the like is preferably used.

Active oxygen is generated in the sealed box by a catalyst and ultraviolet rays, and the reduced exposure system lens group 1
5 is introduced. Here, the organic matter adhering to the lens surface is removed and discharged from the ventilation hole.

In this embodiment, even when exposure is not performed,
Active oxygen can be generated to remove organic substances adhering to the lens surface, and prevent the optical performance from being changed by the organic substances adhering when not exposed. Further, the lens surface can be cleaned without disassembling the reduction exposure system lens group.

In this embodiment, a mixed gas of nitrogen and oxygen is used. However, a mixed gas of oxygen gas and a mixed gas of oxygen and an inert gas such as Ar and Ne may be used. Air may be used, but in any case, it is preferable that the gas is inexpensive, high-purity gas is available, and has a refractive index similar to that of the atmosphere.

[Embodiment 3] FIG. 3 is a schematic view showing a main part of an i-line exposure apparatus according to a third embodiment of the present invention. In the figure, 301 and 302 are Si ₃ N ₄ coatings.

Reference numeral 11 denotes an arc tube as a light source, which has a high-luminance light-emitting portion that emits ultraviolet rays, far ultraviolet rays, and the like. Ultraviolet rays radiated from 11 are adjusted so as to uniformly illuminate the reticle by 12 condenser lenses and optical components such as an i-line filter, a mirror, an optical integrator, and a zoom lens which are omitted in this schematic diagram.

The ultraviolet light that has passed through the reticle 14 is
The light enters the sealing box 102 for sealing the reduction exposure lens group 15 housed therein, and further enters the reduction exposure lens group 1.
Through the step 5, the reticle pattern is transferred onto the wafer 17.

The lens barrel 16 includes an adhesive for bonding the reduction exposure lens group 15 and the lens barrel 16, a lens fixing jig, and the like, but is omitted.

Oxygen gas cylinder 103, gas purifier 104
As a result, purified oxygen gas is introduced into the sealing box 102. As the oxygen gas used here, in addition to the organic solvent and the organic metal-based gas mixed in the clean room, a gas that generates a solid by a photochemical reaction is removed from the inside of the sealing box 102, and the oxygen gas is kept in a state where deposition does not occur. I have.

The purified oxygen gas is supplied to the lens barrel 16 in the sealing box 102 by the reduction exposure system lens group 15.
The through-hole 105 is provided so that the heat can be efficiently transmitted to the inside. This oxygen gas is flowed into the sealing box 102 for a while to sufficiently replace the impurity gas in the lens barrel. Thereafter, exposure is performed while flowing oxygen gas.

The inner surfaces of the sealing box 102 and the lens barrel 16 are coated with high density Si ₃ N ₄ coatings 301 and 302. The Si3N4 coating has a high density and prevents outgassing from Al used as a lens barrel material or a sealing box material. This coating also has good surface flatness and can reduce the amount of organic substances adsorbed on the surface.

After assembling the exposure apparatus, it is more preferable that an oxidizing gas such as ozone is flowed into the sealing box and the lens barrel to remove organic substances adhering to the coating surface.

When exposure is performed with the above configuration, the amount of organic substances adhered to the lens can be reduced, and the optical performance of the exposure apparatus can be stabilized.

[Embodiment 4] FIG. 4 is a schematic view showing a main part of an i-line exposure apparatus according to a fourth embodiment of the present invention. In the figure, reference numerals 401 and 402 denote organic gas traps.

Reference numeral 11 denotes a light emitting tube as a light source, which has a high-luminance light emitting portion that emits ultraviolet light, far ultraviolet light, and the like. Ultraviolet rays radiated from 11 are adjusted so as to uniformly illuminate the reticle by 12 condenser lenses and optical components such as an i-line filter, a mirror, an optical integrator, and a zoom lens which are omitted in this schematic diagram.

The ultraviolet light that has passed through the reticle 14 is
The light enters the sealing box 102 for sealing the reduction exposure lens group 15 housed therein, and further enters the reduction exposure lens group 1.
Through the step 5, the reticle pattern is transferred onto the wafer 17. The lens barrel 16 includes an adhesive for bonding the reduction exposure lens group 15 and the lens barrel 16, a lens fixing jig, and the like, but is omitted.

The sealing box 102 containing the lens barrel 16 is shielded from the outside air, and a carbon trap 402 for an organic gas is provided inside. Trap 401 inside the lens barrel
Are arranged. The organic gas released from the lens barrel and the sealed box is captured by this trap. Although not shown in the drawing, the trap 402 can be replaced without exposing the inside of the sealed box 102 to the outside air, so that the effect can be maintained for a long time.

With the above configuration, the partial pressure of the organic gas inside the lens barrel can be reduced, and the adhesion of organic substances can be suppressed.

Therefore, it is possible to stabilize the transmittance of the reduction exposure system lens group 15 disposed inside the lens barrel, and to stabilize the performance of the exposure apparatus.

[Embodiment 5] FIG. 5 is a schematic view showing a main part of an i-line exposure apparatus according to a fifth embodiment of the present invention. In the figure, 501 is an exhaust system, 502 and 503 are organic gas traps, and 504 is a heater.

The sealed box 102 containing the lens barrel 16 is shielded from the outside air, and a carbon trap 402 for organic gas is provided inside. Trap 401 inside the lens barrel
Are arranged. The organic gas released from the lens barrel and the sealed box is captured by this trap. Although not shown in the drawing, the trap 402 can be replaced without exposing the inside of the sealed box to the outside air, so that the effect can be maintained for a long time.

With the above configuration, the partial pressure of the organic gas in the lens barrel can be reduced, and the adhesion of organic substances can be suppressed.

[0071] In this apparatus, reduction exposure system metal oxide easily removed can not be deposits metals and metal compounds from a cylinder of 203 so as not to precipitate such, N ₂ / O ₂ removing the organic impurities or the like By performing exposure while purging with a mixed gas, it is possible to prevent decomposition of an organic gas and lens contamination due to a synthesis reaction, and to stabilize the transmittance of the reduction exposure lens group 15 disposed inside the lens barrel. As a result, the performance of the exposure apparatus can be stabilized.

In order to maintain the performance of the trap, while heating with a heater 504 from the outside, N ₂ /
By purging with an O ₂ mixed gas, the organic gas released from the trap is removed from the sealed box. Depending on the situation, the release of organic gas from the trap can be promoted by reducing the pressure in the exhaust system 501.

[0073]

As described above, according to the present invention, there is provided an exposure apparatus which can operate stably for a long period of time using ultraviolet rays.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to a first embodiment of the present invention.

FIG. 2 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to a second embodiment of the present invention.

FIG. 3 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to a third embodiment of the present invention.

FIG. 4 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to Embodiment 4 of the present invention.

FIG. 5 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to Embodiment 5 of the present invention.

FIG. 6 is a schematic sectional view showing an outline of a lens system of an exposure apparatus according to a conventional technique.

[Explanation of symbols]

11,61 arc tube, light source (lamp) 12,62 condenser lens, illumination system lens group 14,64 reticle 15,65 reduction exposure / projection system lens group 16 lens barrel 17,67 silicon wafer, semiconductor wafer 68 base 69 XY stage DESCRIPTION OF SYMBOLS 101 Illumination lens sealing chamber 102 Projection lens sealing chamber (sealing box) 103 Oxygen gas cylinder 104, 204 Gas purifier, 105 Gas exhaust hole (through hole) 106 Active oxygen generation catalyst coating 201 Active oxygen generation catalyst 202 Ultraviolet rays lamp 203 N ₂ / O ₂ mixed gas 205 excimer laser 206 mirrors 301, 302 Si ₃ N ₄ coating 401,402,502,503 organic gas carbon trap 501 exhaust system 504 heater 601 illumination system lens barrel 602 projection system lens barrel

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Ryuji Bilo 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Hidehiro Kanazawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Non Corporation

Claims (8)

[Claims] 1. An exposure apparatus which passes a light beam from a light source through an optical system including a lens and exposes an object to be exposed through a reticle, such as a lens barrel surrounding an optical path and receiving scattered light of exposure light. An exposure apparatus characterized in that at least a part of the inner surface is coated with an active oxygen generation catalyst and is kept in an oxygen atmosphere. 2. The method according to claim 1, wherein the active oxygen generating catalyst is Ti, TiO.
_2. The exposure apparatus according to claim 1, wherein the exposure apparatus is ₂ or a compound thereof. 3. The method according to claim 2, wherein the active oxygen generating catalyst is Zr, ZrO.
_2. The exposure apparatus according to claim 1, wherein the exposure apparatus is ₂ or a compound thereof. 4. The method according to claim 1, wherein the active oxygen generating catalyst is Hf, HfO.
_2. The exposure apparatus according to claim 1, wherein the exposure apparatus is ₂ or a compound thereof. 5. An exposure apparatus for passing a light beam from a light source through an optical system including a lens and exposing the object on an exposure object through a reticle, wherein at least a part of the optical system is blocked from outside air, An exposure apparatus comprising: a container which can be filled with an inert gas which is not contaminated with a compound or an organic compound; and a coating having a high filling rate is applied to an inner surface of the container. 6. The exposure apparatus according to claim 5, wherein the substance used for the coating having a high filling factor is selected from the group consisting of silicon nitride, aluminum oxide, titanium nitride, and a compound thereof. 7. An exposure apparatus for passing a light beam from a light source through an optical system including a lens and exposing the object on an exposure object through a reticle, wherein at least a part of the optical system is shielded from outside air, An exposure apparatus, comprising: a container which can be filled with an inert gas which is not contaminated with a compound or an organic compound; and a trap having a high organic gas adsorption rate is provided on an inner surface of the container. 8. The substance used for the trap having a high organic gas adsorption rate is carbon, Al ₂ O ₃ , SiO ₂ , ZrO ₂ ,
The exposure apparatus according to claim 7, wherein the exposure apparatus is selected from the group consisting of graphite, Ti, and a compound thereof.