JP2004260043A - Euv optical system and euv exposure device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an EUV optical system wherein a mirror is used to efficiently absorb a light unnecessary for exposure. <P>SOLUTION: The wavelength of a light source includes a continuous spectrum shown in a diagram (a) on the side of a longer wavelength than an EUV light to be used for exposure. It is assumed that the wavelength is allowed to reflect on a mirror 1 and a mirror 2. The spectral reflection factors of the mirrors 1 and 2 are made different as shown in a diagram (b), and in case when the mirror 2 has a low reflection factor in a wavelength of the mirror 1 with a high reflection factor and the mirror 1 has a low reflection factor in a wavelength of the mirror 2 with a high reflection factor, the spectrum intensity of lights reflected on two mirrors becomes low in all wavelenghts as shown in a diagram (d). Therefore, a thermal load on the downstream side of the mirrors can be made small. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Prior art]
In recent years, with the miniaturization of semiconductor integrated circuit elements, in order to improve the resolution of an optical system limited by the diffraction limit of light, projection lithography technology using EUV light having a shorter wavelength instead of conventional ultraviolet light in order to improve the resolution. Is being developed. An EUV (extremely short ultraviolet) exposure apparatus used in this technology mainly includes an EUV light source, an illumination optical system, a mask, a projection optical system, a wafer stage, and the like.
[0002]
As the EUV light source, a radiation light or a laser plasma light source, a Dense Plasma Focus light source, or the like is used. The illumination optical system includes an oblique incidence mirror, a multilayer mirror, and a filter that reflects or transmits only EUV light having a predetermined wavelength, and illuminates the mask with EUV light having a desired wavelength. A reflective mask is used as the mask. The reflective mask is a pattern in which a pattern having a low reflectance is provided in a predetermined shape on a multilayer film that reflects EUV light, for example. The pattern formed on such a mask is imaged on a sensitive substrate such as a wafer coated with a photoresist and transferred to the resist by a projection optical system composed of a plurality of multilayer mirrors. Since the EUV light is absorbed by the atmosphere and attenuated, all the optical paths are maintained at a predetermined degree of vacuum.
[0003]
In the wavelength region of EUV light, no transparent substance exists and the reflectance on the substance surface is very low, so that ordinary optical elements such as lenses and mirrors cannot be used. For this reason, the optical system for EUV light uses an oblique incidence mirror that reflects the EUV light that has entered the reflecting surface from an oblique direction by using total reflection, or an interference that matches the phase of the reflected light at each interface of the multilayer film. It is constituted by an EUV mirror or the like coated with a multilayer film, which obtains a high reflectance by the effect.
[0004]
The reflection mask also uses such an EUV multilayer mirror. With such an EUV multilayer mirror, the highest reflectance is obtained when a multilayer film composed of molybdenum and silicon is used on the long wavelength side of the L absorption edge (12.3 nm) of silicon, but at a wavelength of 13 to 15 nm. It is about 70% regardless of the angle of incidence. On the shorter wavelength side than the L absorption edge of silicon, almost no multilayer film has been developed that can obtain a reflectance of 30% or more at normal incidence. As a substrate material of the multilayer mirror, a glass material such as quartz or low-thermal-expansion glass, which can be processed with high shape accuracy and small surface roughness, is used.
[0005]
The EUV light source simultaneously emits ultraviolet light and visible light other than EUV light. Since these lights affect the exposure accuracy of the exposure apparatus and raise the temperature of the mirror, in a conventional optical system, a transmission type filter is provided in the optical path, and this filter absorbs unnecessary light. Had been removed. Further, in a conventional optical system, an optical element such as a mirror has its side surface and the like held by a holding member such as a metal frame. The heat absorbed by the optical element is transmitted to the holding member by heat conduction, and is released to the outside by cooling the holding member. A pipe is crawled on the holding member, and a cooling medium such as florinate is flowed through the pipe to cool the pipe.
[0006]
[Problems to be solved by the invention]
The transmission filter absorbs EUV light used for exposure in addition to light of an unnecessary wavelength. Since the absorption by the filter reaches several tens of percent, the amount of EUV light used for exposure is reduced, resulting in a problem that the throughput of the exposure apparatus is greatly reduced.
[0007]
Light other than the exposure wavelength emitted from the EUV light source such as the radiation light or the plasma light source is also absorbed by the multilayer mirror. If a transmission filter is not used, a wavelength that is easily reflected by the multilayer film reaches a downstream mirror, is absorbed by a mirror of the projection optical system, and leads to an increase in temperature. Since the mirror of the projection optical system requires particularly high shape accuracy, there is a problem of thermal deformation due to temperature rise.
[0008]
The present invention has been made in view of such circumstances, and an EUV optical system that can efficiently absorb light unnecessary for exposure by a mirror, an EUV optical system that can efficiently cool a mirror, Further, it is an object to provide an EUV exposure apparatus using these EUV optical systems.
[0009]
[Means for Solving the Problems]
A first means for solving the above-mentioned problem is that, among a plurality of multilayer mirrors constituting an optical system, at least one of the mirrors has a spectral reflectance different from that of the other mirrors. An EUV optical system (1) having a low spectral reflectance with respect to at least a part of wavelengths other than wavelengths used for exposure among wavelengths having high spectral reflectance. ).
[0010]
The operation and effect of the present means will be described with reference to FIG. For example, it is assumed that the light source has a continuous spectrum as shown in (a) on the wavelength side longer than the EUV light used for exposure. Consider that this is reflected by two mirrors, mirror 1 and mirror 2. As shown in (b), the mirrors 1 and 2 have different spectral reflectances, and at a wavelength at which the mirror 1 has a high reflectance, the mirror 2 has a low reflectance and the mirror 2 has a high reflectance. If the mirror 1 is set to have a low reflectance at the wavelength, the spectral intensity of the light reflected by the two mirrors becomes as shown in (d) and becomes smaller for all the wavelengths. Therefore, the heat load on the mirrors downstream of these mirrors can be reduced.
[0011]
On the other hand, if the mirrors 1 and 2 have the same spectral reflectance as shown in (c), the spectral intensities of the light reflected by these two mirrors are as shown in (e). As a result, a wavelength having a high spectral intensity remains. Light of this wavelength is absorbed by the downstream mirror and induces thermal deformation of the downstream mirror.
[0012]
In this means, by using a combination of mirrors as shown in (b), light in a wavelength range that cannot be absorbed by other mirrors is absorbed, the heat load on the downstream mirror is reduced, and thermal deformation is reduced. It has the effect of doing.
[0013]
In the case of the spectral reflectance as shown in (b), light absorption occurs intensively at the mirror 1 and the mirror 2, so that the temperature of the mirror 1 and the mirror 2 rises. Therefore, it is preferable to reduce the thermal deformation by sufficiently cooling the mirror having a large heat load.
[0014]
The spectral reflectance of the multilayer film can be changed depending on the optical characteristics of the multilayer film. For example, the constituent materials of the multilayer film may be changed. For EUV light having a wavelength of about 13 nm, a multilayer film in which molybdenum and silicon are alternately deposited is generally used. To change the spectral reflectance, for example, ruthenium or rhodium may be used instead of molybdenum. This makes it possible to change the spectral reflectance of other wavelengths while having the same EUV light reflectance as molybdenum. Further, a silicon compound (eg, silicon carbide, silicon boride, or the like) may be used instead of silicon.
[0015]
Further, the constituent material, film thickness or density of the thin film layer on the outermost surface of the multilayer film may be changed from that of another mirror. In the case of a multilayer film composed of molybdenum and silicon, the uppermost layer is often made of silicon. However, it is preferable to increase the thickness of the silicon or reduce the density because the spectral reflectance at wavelengths other than EUV light changes. Further, a material other than silicon may be formed on the uppermost layer.
[0016]
A second means for solving the above-mentioned problem is the first means, wherein the mirror having a different spectral reflectance from another mirror is a mirror of an illumination optical system. Item 2).
[0017]
Since a mirror having a spectral reflectance different from that of the other mirrors has a relatively large amount of absorbed heat, it is preferable that the mirror is adapted to a mirror which has a relatively large shape accuracy error. Since the mirror of the illumination optical system has a higher degree of thermal deformation tolerance than the mirror of the projection optical system, it is preferable to use a mirror having a different spectral reflectance from the other mirrors as the mirror of the illumination optical system. In addition, by absorbing light in a wide wavelength range in the illumination optical system, it is possible to reduce the thermal deformation of the mirror of the projection optical system that is located downstream of the illumination optical system and has a strict tolerance for thermal deformation.
[0018]
A third means for solving the above problem is the first means or the second means, wherein the mirror having a different spectral reflectance from other mirrors has a large thermal deformation tolerance. (Chart 3).
[0019]
The tolerance of thermal deformation is determined for each mirror by design, and the tolerance differs depending on the mirror. Mirrors with different spectral reflectances from other mirrors have a relatively large amount of absorbed heat, so if they are applied to a mirror with a large thermal deformation tolerance, even if the thermal deformation is relatively large, the effect on optical characteristics will be small. The design of the cooling mechanism and the like can be facilitated.
[0020]
A fourth means for solving the above problem is any one of the first means to the third means, wherein the mirror having a different spectral reflectance from the other mirror has a higher thermal expansion coefficient than the other mirror. Are small mirrors (claim 4).
[0021]
A mirror having a spectral reflectance different from that of another mirror has a relatively large amount of absorbed heat, but by applying this to a mirror having a small coefficient of thermal expansion, while suppressing the amount of thermal deformation of the mirror, other mirrors can be added to the mirror. Light in a wavelength range not absorbed by the mirror can be absorbed.
[0022]
A fifth means for solving the above problem is that at least two of the plurality of mirrors constituting the optical system are provided with a temperature adjustment mechanism for controlling the temperature, and at least one of the temperature adjustment mechanisms is provided. One is an EUV projection optical system (claim 5), wherein the temperature adjustment mode of the mirror is different from that of the other temperature adjustment mechanisms.
[0023]
It is preferable that each of the plurality of mirrors constituting the optical system be cooled as necessary. There are several methods for cooling the mirror, each of which has advantages and disadvantages. For example, when a water cooling mechanism is directly joined to the mirror, there is a problem in that attaching the cooling mechanism applies a force to the mirror and deforms the mirror. Such a method is difficult to adapt to a mirror that requires high shape accuracy. On the other hand, as a method of cooling the mirror so as not to be deformed, there are cooling means using radiation, a thermo tunneling method, a method of wiping a rare gas to the mirror, and the like. Is low.
[0024]
In this means, at least two mirrors are provided with a temperature adjustment mechanism for controlling the temperature, and one of the temperature adjustment mechanisms is configured such that the temperature adjustment mode of the mirror is different from those of the other temperature adjustment mechanisms. I have. A temperature adjustment mechanism that does not involve mirror deformation is used for mirrors that do not allow relative shape errors. It is preferable to employ a mechanism. For example, the former temperature adjustment mechanism may be applied to the mirror of the projection optical system, and the latter cooling mechanism may be applied to the mirror of the illumination optical system. Further, it is preferable that the mirror adopting the former cooling mechanism has a spectral reflectance different from that of other mirrors, and absorbs a wavelength that is hardly absorbed by other mirrors.
[0025]
As a method of changing the form of the temperature adjustment mechanism, the heat exchange positions of the mirror and the temperature adjustment mechanism may be different. Further, the heat exchange method may be different. As a replacement method, a method using radiation, a method in which gas contacts a mirror, a method in which a liquid contacts a mirror, a method in which a solid contacts a mirror, a thermotunneling method, or the like can be adopted. In the thermotunneling method, a member for temperature adjustment is arranged on a member to be temperature-adjusted with a small interval, and the interval is particularly preferably 30 nm or less from the viewpoint of heat transfer.
[0026]
As a method of changing the form of the temperature adjustment mechanism, a heat transfer method may be changed. As the heat transfer method, a heat conduction method, an electronic cooling method, a heat pipe method, a method of flowing gas or liquid, or the like can be adopted.
[0027]
Further, it is preferable to select a material having a low elastic modulus as the heat pipe type pipe material. In this way, even when the coefficient of thermal expansion of the mirror is different from that of the pipe material, it is possible to reduce the occurrence of stress accompanying the difference.
[0028]
As described above, in the present means, thermal deformation of the mirror can be efficiently suppressed by appropriately selecting and using these according to the performance required of the mirror.
[0029]
A sixth means for solving the above problem is any of the first means to the fifth means, wherein no EUV projection optical system is provided. It is.
[0030]
In this means, since there is no transmission filter, EUV light used for exposure is not absorbed, and the amount of irradiation can be increased.
[0031]
According to a seventh aspect of the present invention, there is provided an EUV exposure apparatus comprising: an EUV optical system according to any one of the first to sixth aspects.
[0032]
Since this means includes the EUV optical system according to any one of the first to sixth means, the functions and effects described in the respective description sections can be obtained.
[0033]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 shows an EUV optical system according to an embodiment of the present invention. A target gas is introduced into the light source 1, and a plasma 3 is formed by laser light from a laser (not shown), and EUV light is emitted from the plasma 3. The EUV light is reflected by the mirror C <b> 1, collected, and emitted to the outside of the light source 1. The EUV light is sequentially reflected by mirrors C2, C3, C4, and C5 constituting an illumination optical system, and illuminates the reticle L through a field stop (shape opening) 4. (In this specification, the mirror C1 is also used as the illumination optical system. Is considered as part of).
[0034]
The EUV light reflected by the pattern portion formed on the reticle L passes through the field stop 4 and is sequentially reflected by mirrors M1, M2, M3, M4, M5, and M6 constituting the projection optical system, and the pattern of the reticle L is formed. An image is formed on the wafer W. An aperture stop 5 is provided between the mirrors M2 and M3 of the projection optical system to determine the NA of the projection optical system. Each of the mirrors is a multilayer mirror.
[0035]
Since the mirrors C1 and C2, which are the most upstream in the illumination optical system, are located closest to the light source 1, the amount of light incident on the mirror is also the largest. Therefore, these mirrors are provided with a cooling mechanism having a high cooling capacity. That is, the cooling pipes 6 and 7 are fixed to the back surfaces of the mirrors C1 and C2, respectively, and a coolant such as Fluorinert flows in the cooling pipes 6 and 7. The cooling pipes 6 and 7 have a coefficient of thermal expansion almost equal to that of the mirror.
[0036]
For example, when the mirrors C1 and C2 are made of Invar or Super Invar having a high thermal conductivity and a small coefficient of thermal expansion, the cooling pipes 6 and 7 may be made of Invar or Super Invar. Further, the coolant may directly contact the back surfaces of the mirrors C1 and C2. For example, by pressing the plate-like member having the grooves against the back surfaces of the mirrors C1 and C2 and flowing the refrigerant through the grooves, the heat of the mirrors C1 and C2 can be directly transferred to the refrigerant, and high cooling efficiency can be obtained. .
[0037]
Further, another cooling mechanism is further added to the mirror C1. That is, a rare gas such as helium is wiped from the gas nozzle 2 to the surface of the mirror. In order to prevent the vacuum degree around the mirror from being deteriorated by wiping the gas, an exhaust system may be disposed near the mirror to collect the gas. The gas may be recycled after collection. When the light source uses a target such as a gas, the cooling gas may be collected together with the gas.
[0038]
Further, a radiation cooling mechanism is provided as a cooling mechanism for the mirror C1. That is, by disposing the plate-shaped member 16 around the light beam traveling from the mirror C1 to the mirror C2, the radiant heat from the mirror C1 can be absorbed by the plate-shaped member 16. The plate member 16 may be cooled by water cooling or the like.
[0039]
The cooling method of the mirror C1 is preferably a combination of the above methods. By combining a plurality of techniques, the cooling capacity of the mirror is improved.
[0040]
On the mirror C1 and the mirror C2, a multilayer film having a different spectral reflectance from other mirrors is formed. That is, a multilayer film made of rhodium and silicon carbide is formed on C1 and a multilayer film made of rhodium and silicon is formed on C2. A cooling mechanism having a cooling pipe 7 similar to the mirror C1 is mounted on the mirror C2.
[0041]
The mirror C3 and the mirror C4 play a role as an integrator. Since the shape accuracy of the mirror is stricter than those of C1 and C2, it is not preferable to make the cooling member directly contact the back surface. Therefore, the other end of the spring is cooled by the Peltier elements 8 and 9 by bringing the leaf spring into contact with the back surface. Although the Peltier elements 8 and 9 themselves need to be cooled, a heat pipe (not shown) is used for this cooling.
[0042]
When the heat pipe is fixed to the metal frame of the illumination optical system, a force is applied to the Peltier elements 8, 9 and the force deforms a leaf spring connecting the Peltier elements 8, 9 with the mirrors C3, C4. The mirrors C3 and C4 may be deformed. In order to suppress the transmission of such deformation, it is preferable that the heat pipe be made of a material having a low elastic modulus. For example, the heat pipe may be made of resin such as fluororesin instead of metal.
[0043]
Mirrors constituting a projection optical system are required to have orders of magnitude higher precision than mirrors of an illumination optical system. On the other hand, since most of the light emitted from the light source is absorbed by the mirror of the illumination optical system, the heat energy input to the mirror is relatively small. Therefore, it is preferable to select a different cooling method for the mirror of the projection optical system than for the mirror of the illumination optical system.
[0044]
In the present embodiment, the mirrors of the projection optical system are all cooled by a non-contact cooling method. That is, in the mirrors M1, M2, M4, and M6, the cooling plates 10, 11, 13, and 15 are respectively brought close to the back surfaces of the mirrors, and the distance between the back surfaces of the mirrors and the cooling plates is maintained to be 30 nm or less. I have. By providing such close proximity at extremely small intervals, high cooling efficiency can be obtained due to the thermotunneling effect.
[0045]
In order to keep the distance between the back surface of the mirror and the cooling plate at a very small value, the cooling plate is arranged on a piezo stage so that the distance between the cooling plate and the back surface of the mirror can be adjusted with high precision (not shown). . Further, a feedback sensor may be provided by providing a distance sensor such as an electrostatic sensor for measuring a distance from the back surface of the mirror to the cooling plate.
[0046]
It is difficult to arrange a cooling mechanism on the back surface of the mirrors M3 and M5 because another mirror or wafer is close to the back surface of the mirrors. Therefore, cooling plates 12 and 14 are brought close to the side surfaces of the mirror, respectively, to cool the mirrors by a radiation method. These cooling plates 12 and 14 are cooled by a Peltier element, and the heat generated by the Peltier element is released to the outside by a heat pipe.
[0047]
The field stop 4 has a ring-shaped aperture similar to the aberration-free field of the projection optical system. The field stop 4 is replaceable, so that the effective field of view can be adjusted. The aperture stop 5 is disposed between the mirrors M2 and M3 at a position where the light beam intersects the rotational symmetry axis of the optical system. The aperture stop 5 is also provided with a mechanism capable of replacing a plurality of stops having different aperture sizes so that the effective numerical aperture of the optical system can be adjusted. These throttles are provided with a cooling mechanism to control the temperature so that it does not rise above a desired value.
[0048]
As described above, since the mirror C1 and the mirror C2 absorb useless light in the wavelength range that is not absorbed by the other mirrors, a transmission-type filter is not used in the present embodiment. Therefore, the absorption of the EUV light by the filter is eliminated, and the irradiation amount can be increased accordingly, so that the throughput of the exposure apparatus can be increased.
[0049]
【The invention's effect】
As described above, according to the present invention, an EUV optical system capable of efficiently absorbing light not required for exposure by a mirror, an EUV optical system capable of efficiently cooling a mirror, and an EUV of these An EUV exposure apparatus using an optical system can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an effect of changing a spectral reflectance of a mirror.
FIG. 2 is a diagram illustrating an EUV optical system that is an example of an embodiment of the present invention.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 light source 2 gas nozzle 3 plasma 4 field stop 5 aperture stop 6 cooling pipe 7 cooling pipe 8 peltier element 9 peltier element 10 cooling plate 15 cooling plate , 16: plate member, C1 to C5: mirror, M1 to M6: mirror, L: reticle, W: wafer

Claims (7)

The spectral reflectance of at least one of the multilayer mirrors constituting the optical system is different from the spectral reflectance of the other mirror, and the other mirror is used for exposure at a wavelength having a high spectral reflectance. An EUV projection optical system characterized in that it has a low spectral reflectance for at least some wavelengths other than the wavelength to be irradiated. 2. The EUV projection optical system according to claim 1, wherein the mirror having a different spectral reflectance from another mirror is a mirror of an illumination optical system. The EUV projection optical system according to claim 1 or 2, wherein the mirror having a different spectral reflectance from the other mirrors has a large thermal deformation tolerance. The EUV projection optical system according to any one of claims 1 to 3, wherein the mirror having a different spectral reflectance from the other mirror has a smaller coefficient of thermal expansion than the other mirror. . At least two of the plurality of mirrors constituting the optical system are provided with a temperature adjustment mechanism for controlling the temperature, and at least one of the temperature adjustment mechanisms has a mirror whose temperature adjustment mode is different from that of another mirror. An EUV projection optical system characterized in that the EUV projection optical system is different from that of the temperature adjustment mechanism. The EUV projection optical system according to any one of claims 1 to 5, wherein the EUV projection optical system does not include a transmission filter. An EUV exposure apparatus comprising the EUV optical system according to any one of claims 1 to 6.

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