Fabrication Method and Structure for Multilayer Optical Anti-Reflection Coating, and Optical Component and Optical System Using Multilayer Optical Anti-Reflection Coating

This invention relates to optical anti-reflection coatings, and more specifically to multilayer anti-reflection coatings. Furthermore, this invention relates to optical components (lens elements) and optical systems using multilayer optical coatings.

Anti-reflection coatings (ARC) are widely used on optical surfaces (e.g., in lenses, prisms etc.) to suppress undesirable reflections. In general, such anti-reflection coatings are fabricated by a "dry" process, meaning non-liquid coating processes such as vacuum evaporation, sputtering or CVD (Chemical Vapor Deposition). See e.g. Joy George, Preparation of Thin Films (Marcel Dekker, Inc., New York, 1992) and Francois R. Flory, Thin Films for Optical Systems (Marcel Dekker, Inc., New York, 1995). In the case of a multilayer ARC, in order to obtain high performance (wide AR wavelength bandwidth, very low reflectance and wide angular AR bandwidth), it is well known that several kinds of coating materials which have respectively different refractive indices are needed. It is also known that the larger the difference of the refractive indices of the various coating materials, the better the optical performance, and the lower the refractive index of the lowest refractive index coating material used, the better the optical performance. Furthermore, it is possible to reduce the number of coating layers by use of both a large difference between the refractive indices of the various coating materials used and one of the various coating materials used having a very low refractive index.

However, for shorter wavelengths of incident light (e.g. near 200 nm), many kinds of coating materials cannot be used for an ARC which requires high transmittance, because of optical loss due to the light absorption by the coating material. Therefore, the number of coating materials which can be used at such short wavelengths (such as in the 200 nm ultra violet region) is limited, and so it is difficult to obtain a large enough difference between the refractive indices of the coating materials used, and to obtain a very low refractive index of one of the various coating materials used. Thus, it is difficult to design and fabricate high performance anti-refraction coatings with adequate performance for such wavelength regions.

For instance, for a typical ARC material formed by a dry process for the visible light region, a variety of ARC materials can be used. In general, in the visible light region, the highest refractive index material available is TiO.sub.2 (n=2.4 to 2.7 at 500 nm), and MgF.sub.2 (n=1.38 at 500 nm) is the lowest refractive index material available (where n is refractive index). For shorter wavelengths of incident light such as near 200 nm (ultra violet), however few ARC materials are available. In general, LaF.sub.3, NdF.sub.3 and GdF.sub.3 (all having n of about 1.7 at about 200 nm) are the highest refractive index coating materials available and Na.sub.3 A1F.sub.6 (n=1.36 at 200 nm) is the lowest refractive index coating material available. Therefore the difference between refractive indices of coating materials which can be used for the 200 nm wavelength region is much smaller than the difference of those for the visible light region. Those skilled in the art will readily appreciate that one consequence of the limited coating materials for short wavelength incident light is that ARC design and fabrication is much more difficult for the short wavelength region than for the visible light region.

In the case of an ARC for the visible light region, it is possible to obtain a low reflectance ARC having wide bandwidth that suppresses reflections over a large range of the visible light region (e.g. the reflectance is less than 0.5% from 400 to 800 nm wavelength.). However, many ARC layers (i.e. more than eight or nine) may be needed, extending manufacturing time increasing cost.

In the case of wet process anti-reflection coatings, such coatings are typically fabricated by the hydrolysis and the polymerization of a metal alkoxide solution, i.e., using a liquid; this wet process is called the sol-gel process. It is well known that optical coatings of SiO.sub.2, ZrO.sub.2, HfO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, etc. can be fabricated not only by the dry process but also by the sol-gel process. See e.g. Ian M. Thomas, Applied optics Vol. 26, No. 21 (1987) pp. 4688-4691 and Ian M. Thomas, SPIE vol. 2288 Sol-Gel Optics III (1994) pp. 50-55. In the case of a SiO.sub.2 coating formed by the sol-gel process, the colloidal SiO.sub.2 suspensions that are suitable for the preparation of the SiO.sub.2 coating are usually prepared by the hydrolysis of silicon alkoxides in a parent alcohol as a solvent. The hydrolysis of tetraethyl silicate in ethanol, for instance, can be summarized by the following:

This reaction is complex and many variables such as catalyst, solvent, water ratio, and temperature all have an effect on the nature of the product. Furthermore, three types of liquid coating methods: spin, dip and meniscus are normally used to make wet process coatings. Spin and dip coating are common and widely used. Dip coating is preferred for large samples of irregular shape or having a curved surface. Spinning is excellent for small, round, flat or gently curved samples. Meniscus is particularly good for large, flat substrates. See e.g. Brinker and Scherer, Sol-Gel Science (Academic Press, Inc., San Diego, 1990); and Floch, Priotton and et al., Thin Solid Films, Vol. 175 (1989) pp. 173-178.

By using such a wet process, one generally obtains coatings which can have either high packing density and low packing density. In order to obtain wet process coatings with a high packing density equal to that of coatings formed by a dry process, it is necessary to heat the coating to high temperature (e.g. more than 450 degree C.), in the fabricating process. Not only does this result in a long manufacturing time and high cost, but also may result in damage or degradation to the substrate. However, it is easy to obtain low packing density coatings using the wet process, because an additional process such as heating is not required (since the process can be carried out at room temperature or less than 150 degrees C.).

Since the structure of coatings can be defined as micro-pores spacing apart deposited solid material, the relation between packing density and refractive index of optical coating is:

Where n.sub.p is the refractive index of the material (e.g. air, water) which fills up the micro-pores, n.sub.f and n.sub.o are respectively the actual refractive index (dependent on packing density) and the refractive index of the deposited solid material, and P is the packing density of the coating. Furthermore packing density P is defined as:

P=(Volume of the solid part of the coating)/(Total volume of the coating (solid+pores)).

Thus, high and low packing density mean high and low refractive index respectively. In the case of a SiO.sub.2 coating fabricated by a wet process, the packing density can vary from 1 to approximately 0.5, and thereby the refractive index can vary from 1.45 to 1.22 in the visible light region. Thus, one obtains a nearly 0% reflectance single layer ARC on optical glass by using a wet process low packing density SiO.sub.2. This SiO.sub.2 ARC is also well known to have a high laser light damage threshold value. Therefore, this ARC is used for high power lasers, e.g. for nuclear fusion. See Ian M. Thomas, Applied Optics Vol. 31, No. 18 (1992) pp. 6145-6149.

In accordance with this invention, both dry process and wet process coating materials are used in combination to fabricate a multilayer anti-reflection coating having a wide bandwidth, thereby to obtain a high performance anti-reflection coating.

This allows fabrication of multilayer anti-reflection coatings including a wet process coating layer having a refractive index which cannot be attained by a conventional dry process. This results in an ARC having superior performance compared to an all dry process ARC. Thus, using a wet process layer one can fabricate an anti-reflection coating having characteristics not available using only a dry process (e.g. an ARC having wide bandwidth for the ultra violet region or a wide range of incident angles for the light of an ArF laser). It is also possible to reduce the number of coating layers, in comparison with the number of layers using only the conventional dry process, while maintaining or improving the anti-reflection performance.

There are several advantages of an ARC in accordance with the present invention. It is possible to fabricate a high performance (wide bandwidth, wide range of incidence angles, etc.) low reflectance anti-reflection coating not possible to form using only a conventional dry process ARC. For the same anti-reflection performance as achieved by a conventional dry process, it is possible to reduce the number of ARC layers by using this invention. It is possible to reduce the manufacturing time and cost due to the reduced number of ARC layers. ARCs using this invention are adapted for shorter wavelengths due to the small number of layers, because the thinner total thickness of the multilayer coating causes smaller absorption by the coating. It is possible to obtain an ARC with a higher laser damage threshold value than that formed by a conventional dry process.

FIG. 1 shows a configuration of a two layer ARC in accordance with the invention.

FIG. 2 shows performance of the FIG. 1 ARC.

FIG. 3 shows a configuration of a three layer ARC in accordance with the invention.

FIG. 4 shows performance of the FIG. 3 ARC.

FIG. 5 shows a configuration of a six layer ARC in accordance with the invention.

FIG. 6 shows performance of the FIG. 5 ARC.

FIG. 7 shows a configuration of a seven layer ARC in accordance with the invention.

FIG. 8 shows performance of the FIG. 7 ARC.

The following discloses four examples of ARCs in accordance with the invention: these are illustrative and not limiting. Example 1 is a wideband ARC intended for the visible light region, e.g. for lenses and prisms of a camera or other optical systems, for the entire visible light spectrum. The visible light spectrum is light having a wavelength from approximately 400 to 800 nm. This ARC is shown in cross section (and not to scale) in FIG. 1, and includes two ARC layers formed on substrate 12 (e.g. optical glass). The first layer 16 is a SiO.sub.2 coating having a refractive index of 1.45 at 500 nm and a optical thickness of 132 nm. This is equal to 0.264.times.500 nm, the nominal center wavelength .lambda..sub.0 for calculation of optical characteristics of the coating. The first (lower) layer 16 can be fabricated by either a dry or wet process, but it is easier to use a dry process. The dry process is preferred as it reduces the manufacturing time and cost.

The second (upper) layer 20 is a SiO.sub.2 coating having a refractive index of 1.22 at 500 nm and an optical thickness of 134 nm (0.268 times the nominal center wavelength .lambda..sub.0). The second layer 20 is fabricated by a wet process to obtain the desired refractive index. The optical reflection performance of this coating (when the entrance medium is air) at 0 degree incidence onto optical glass BK7 manufactured by Schott Glass Technologies, Inc. (having n=1.52 at 500 nm) and optical glass F2 manufactured by Schott Glass Technologies, Inc. (having n=1.62 at 500 nm) substrates, which are the best known conventional glasses for cameras or other visible light optical systems, is shown in FIG. 2. The solid line in FIG. 2 is the optical reflection performance of this coating on BK7 glass and the dotted line is that on F2 glass; both reflectances are less than 0.5% over the entire visible light spectrum. This coating can be favorably applied to the lenses and prisms for cameras and the other optical systems.

Example 2 is a wideband ARC intended for the ultra violet light region (wavelength of 180 to 350 nm), shown in cross section (and not to scale) in FIG. 3 formed on substrate 40. This coating has three layers; the first (lower) layer 44 is LaF.sub.3, or NdF.sub.3, or GdF.sub.3, having a refractive index of approximately 1.70 at 200 nm and an optical thickness of 12 nm (0.06 times the nominal center wavelength .lambda..sub.0 of 200 nm); the second layer 48 is SiO.sub.2 having a refractive index of 1.52 at 200 nm and an optical thickness of 82 nm (0.41 times the nominal center wavelength), and the third layer 50 is SiO.sub.2 having a refractive index of 1.25 at 200 nm and a optical thickness of 60 nm (0.3 times the nominal center wavelength). The first layer 44 is fabricated using a dry process and the second layer 48 can be fabricated using either a wet or dry process, but it is easier to use a dry process. Thus, the dry process is preferred to reduce the manufacturing time and the cost. The third layer 50 is fabricated using a wet process. The optical reflection performance for this coating (when the entrance medium is air) at 0 degree incidence when coated onto a fused silica glass substrate 40 (which has high transmission in the ultra violet region, has a refractive index of 1.54 at 200 nm is shown in FIG. 4; the reflectance is less than 0.5% over the very wide ultra violet region (i.e. approximately 180 to 350 nm).

Example 3 is a wide range of incident angles (e.g. 0 to 55 degrees) ARC intended for KrF laser light (having a wavelength of 248.4 nm). This ARC may be utilized for optical components and systems for use in optical lithography equipment, such as, for example, lenses and prisms for the projection and illumination systems of a lithography tool. This ARC is shown in cross section (and not to scale) in FIG. 5, and includes six ARC layers formed on substrate 60. All of the layers are dry process layers except for the topmost SiO.sub.2 layer, and having the center wavelength of 248 nm. The coating materials, the refractive indices and the optical thicknesses of various layers, from the substrate up are shown in Table 1.

The optical reflection performance for KrF laser light (248.4 nm) for various incident angles for this coating when coated onto a fused silica glass substrate 60 is shown in FIG. 6. In FIG. 6, the entrance medium is air and the substrate 60, which has high transmission in the ultra violet region, has a refractive index of 1.54 at 200 nm. The solid line in FIG. 6 is the optical reflection performance at 248.4 nm to incident angles of s polarization light and the dotted line is that of p polarization light; both reflectances are favorably less than 0.5% from 0 to 55 degrees angle of incidence. As mentioned previously, this coating can be applied to the optical surfaces of lenses and prisms used in projection and illumination components of a lithography tool.

Example 4 is a wide range of incidence angles (e.g. 0 to 55 degrees) ARC intended for ArF laser light (having a wavelength of 193.4 nm). This ARC may be utilized for optical components of optical lithography equipment, such as, for example lenses and prisms for the projection and illumination systems of a step and repeator step and scan system. This ARC is shown in cross section (and not to scale) in FIG. 7 and includes seven ARC layers on substrate 70. All of the layers are dry process layers except for the topmost SiO.sub.2 layer, and have a center wavelength of 193 nm. The coating materials, the refractive indices and the optical thicknesses of various layers, from the substrate up are shown in Table 2.

The optical reflection performance for ArF laser light (193.4 nm wavelength) to various incidence angles for this coating when coated onto a fused silica glass substrate 70 is shown in FIG. 8. In FIG. 8, the entrance medium is air and the substrate 70, which has a high transmission in the ultra violet regiun, has a refractive index of 1.54 at 200 nm. The solid line in FIG. 8 is the optical reflection performance at 193.4 nm to incidence angles of s polarization light and the dotted line is that of p polarization light; both reflectances are favorably less than 0.5% from 0 to approximately 55 degrees angle of incidence. As mentioned previously, this coating can be applied to the optical surfaces for lenses and prisms used in projection and illumination components of a lithography system.

In order to obtain high adhesion between a low packing density wet process coating and a high packing density coating, one can locate the layer of high packing density oxide coatings (which can be fabricated by either a dry or wet process) under the low packing density SiO.sub.2 layer. Chemical bonding is believed to occur between the low packing density SiO.sub.2 layer and the high packing density oxide layer. All four of the above discussed examples use this configuration. However, the present invention is in no way limited to such a configuration.

This disclosure is illustrative and not limiting; further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.