Anti-corrosion layer on objective lens for liquid immersion lithography applications

Disclosed is an objective lens adapted for use in liquid immersion photolithography and a method for making such a lens. In one example, the objective lens has multiple lens elements, one of which includes a transparent substrate and a layer of anti-corrosion coating (ACC). The ACC is formed proximate to the transparent substrate and is positioned between a liquid used during the liquid immersion photolithography and the transparent substrate to protect the transparent substrate from the liquid.

BACKGROUND

The present disclosure relates generally to lenses for photolithography systems and, more particularly, to an objective lens for use in a liquid immersion lithography process.

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing have been needed.

The growth of the semiconductor IC industry is driven by Moore's law: “The complexity of minimum component cost has increased at a rate of roughly a factor of two per year”. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while feature size (i.e., the smallest component or line that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs.

One of the factors enabling the continued minimization of feature size is improvement in photolithography and its ability to print smaller features. The minimum feature size in an optical lithography system may be determined in part by diffraction, which is related to the wavelength of light and the medium through which the light shines. Accordingly, one approach to reducing feature size and improving resolution is to use light with a shorter wavelength. Another approach is to use a medium between the lens and the substrate other than air. Since the index of refraction of a medium (referred to as ‘n’) is larger than one, the wavelength in the medium is reduced by the factor of n. This may improve resolution.

One such method of enhancing resolution using a non-air medium is referred to as immersion lithography. Depending on the inserted medium, immersion lithography may be classified as solid immersion lithography or liquid immersion lithography. However, in liquid immersion lithography, there may be chemical reaction between an optical lens used during lithography and the liquid used for immersion. Such a chemical reaction may damage the optical system and degrade the system's resolution.

Accordingly, what is needed is an improved optical lens and method of manufacturing such a lens.

DETAILED DESCRIPTION

The present disclosure relates generally to lenses for photolithography systems and, more particularly, to an objective lens for use in a liquid immersion lithography process. It is understood, however, that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring toFIG. 1, in one embodiment, a liquid immersion lithography system100includes a light source110, an illumination system (e.g., a condenser)120, a photomask130, and an objective lens140. An immersion liquid150may be used with the system100to aid in the formation of a pattern on a semiconductor substrate160.

The light source110may be any suitable light source. For example, the light source110may be a mercury lamp having a wavelength of 436 nm (G-line) or 365 nm (I-line); a Krypton Fluoride (KrF) excimer laser with wavelength of 248 nm; an Argon Fluoride (ArF) excimer laser with a wavelength of 193 nm; a Fluoride (F2) excimer laser with a wavelength of 157 nm; or other light sources having a wavelength below approximately 100 nm.

In lithography systems with wavelengths below approximately 157 nm, various system components may demonstrate a high level of absorption. This may be solved by some extent with the use of reflective optics, but these optics may introduce other challenges. Immersion lithography may be used to reduce feature sizes in lithography while addressing some of the concerns with wavelengths below 157 nm. It is understood that in the above description of light sources, each light source may have a certain wavelength distribution, or line width, rather than an exact single wavelength. For example, the I-line (e.g., 365 nm) wavelength of the mercury lamp may not be exactly 365 nm, but may be centered at approximately 365 nm with a range of varying wavelengths extending above and below 365 nm. This range may be used to determine a minimum possible line width during photolithography, with less variation from the desired 365 nm wavelength resulting in a thinner line width.

The illumination system (e.g., the condenser)120may comprise a single lens or may comprise a lens system having multiple lenses and/or other lens components. For example, the illumination system may comprise microlens arrays, shadow masks, or other structures designed to aid in directing light from the light source110onto the photomask130.

The photomask130may comprise a transparent substrate and an absorption layer. The transparent substrate may use fused silica (SiO2) relatively free of defects, such as borosilicate glass and soda-lime glass. The absorption layer may be formed using a number of different processes and materials, such as depositing a metal film made with chromium (Cr) oxide and iron oxide, or an inorganic film made with MoSi, ZrSiO, SiN, and/or TiN. The absorption layer may be patterned to have one or more openings through which light may travel without being absorbed by the absorption layer.

The objective lens140may comprise a single lens or a plurality of lens elements illustrated as170,172,174. As will be described later in greater detail, each lens element may include a transparent substrate and may have an anti-reflection coating (ARC) on at least one surface of each lens element. The transparent substrate may be a conventional objective lens, and may be made of fused silica (SiO2), calcium-fluoride (CaF2), lithium fluoride (LiF), or barium fluoride (BaF2). The materials used for each lens element may be chosen based on the wavelength of light used in the lithography process to lessen absorption and scattering.

At least one lens element may also have an anti-corrosion coating (ACC) to minimize or prevent corrosion of the objective lens140by the immersion liquid150. If the immersion liquid150is corrosive, the lens element170may have an asymmetric coating. For example, the lens element170may have an ARC on one surface and an anti-corrosion coating on the surface that is proximate to or in contact with the immersion liquid150. In the present example, the lens element170of the objective lens140may be in direct contact with the immersion liquid150. The immersion liquid150may be selected both for optical qualities and for compatibility with the semiconductor substrate160. For example, the immersion liquid150may be selected to minimize chemical reactions, scattering, and light absorption, as well as to be relatively bubble-free and thermally stable during lithography. Water may be used, as may other liquids.

The semiconductor substrate160may be a wafer comprising an elementary semiconductor such as crystal silicon, polycrystalline silicon, amorphous silicon, germanium, and diamond, a compound semiconductor such as silicon carbide and gallium arsenic, an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs, and GaInP, or any combination thereof.

Referring now toFIG. 2, one embodiment of the lens element170ofFIG. 1is illustrated with a transparent substrate210and an ACC220. The transparent substrate210may be a conventional lens, and may be include of fused silica (SiO2), calcium-fluoride (CaF2), lithium fluoride (LiF), barium fluoride (BaF2), or another material. In some embodiments, the material may be chosen based on the wavelength of light used in the lithography process.

The ACC220may be formed either directly on or proximate to (if there are intervening layers) the transparent substrate210. The material forming the ACC220may be selected to adhere relatively well to the transparent substrate210, to be transparent to the light used during lithography, and to be chemically stable with the immersion liquid150. It is understood that the material may change depending on the particular immersion liquid used during lithography. In the present example, chemically stable indicates that the chemical reaction rate between the ACC220and the immersion liquid is less than 10−5per hour, where the chemical reaction rate is defined as the decrease of the relative thickness of the ACC220per hour. The chemical stability of the ACC220in the immersion liquid150enables the ACC to protect the lens element170of the objective lens140from damage by the immersion liquid. Accordingly, the resolution of lithography may be maintained while prolonging the lifetime of the objective lens140.

As one example, SiO2may be used for the ACC220when the transparent substrate210is formed of CaF2, LiF, or BaF2. SiO2is compatible with water used as the immersion liquid150. Other materials that may be used for the ACC220include indium tin oxide (ITO), polymer, and the non-photo-active resin of a photoresist.

In the present example, the thickness of the ACC220may be less than 1000 μm, and the surface flatness of the ACC220may be controlled so that the variation of the surface is less than the half of the wavelength of the light used in lithography. In some embodiments, the thickness of the ACC220may be increased within a range of up to approximately 1 mm, enabling the ACC220to perform both anti-corrosion and anti-reflection functions. The deposition of the ACC220may be implemented by spin-on coating, chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD) such as sputtering and evaporation, or epitaxy growth.

Referring now toFIG. 3, another embodiment of the lens element170is illustrated with the transparent substrate210, the ACC220, and an ARC310. In the present example, the ARC310is positioned proximate to the transparent substrate210on the side opposite the ACC220. The ARC310may have a multilayer structure or a graded structure where the refractive index of the ARC changes gradually to match the refractive indexes of neighboring materials. The ARC310may use an organic material containing at least one of hydrogen, carbon, and oxygen; compound materials including MgF2, SiO, Cr2O3, ITO, SiO2, SiN, TaO5, Al2O3, TiN, and ZrO; metal materials such as Al, Ag, Au, and In; or a combination thereof.

Referring now toFIG. 4, still another embodiment of the lens element170is illustrated with the transparent substrate210, the ACC220, and an ARC410. In the present example, the ARC410is positioned between the transparent substrate210and the ACC220. The ARC410may have a multilayer structure or a graded structure where the refractive index of the ARC changes gradually to match the refractive indexes of neighboring materials. The ARC410may use an organic material containing at least one of hydrogen, carbon, and oxygen; compound materials including MgF2, SiO, Cr2O3, ITO, SiO2, SiN, TaO5, Al2O3, TiN, and ZrO; metal materials such as Al, Ag, Au, and In; or a combination thereof.

Referring now toFIG. 5, yet another embodiment of the lens element170is illustrated with the transparent substrate210, the ACC220, and the two ARCs310,410. In the present example, the ARC310is positioned proximate to the transparent substrate210on the side opposite the ACC220, and the ARC410is positioned between the transparent substrate210and the ACC220. One or both ARCs310,410may have a multilayer structure or a graded structure where the refractive index of the ARC changes gradually to match the refractive indexes of neighboring materials. In addition, one or both ARCs310,410may use an organic material containing at least one of hydrogen, carbon, and oxygen; compound materials including MgF2, SiO, Cr2O3, ITO, SiO2, SiN, TaO5, Al2O3, TiN, and ZrO; metal materials such as Al, Ag, Au, and In; or a combination thereof.

Referring toFIG. 6, in another embodiment, a method600may be used to fabricate the objective lens140ofFIG. 1or a lens element, such as the lens element170ofFIGS. 2-5. In the present example, the method600is illustrated with respect to lens element170ofFIG. 3. In step610, a surface of the lens element170is prepared. The preparation may include a sequence of polishing and cleaning, and may use mechanical, chemical, and/or other surface preparation methods.

In step620, a material for an ACC (e.g., the ACC220ofFIG. 3) is selected based on one or both of the immersion liquid with which the lens element170is to be used and the material of the lens element170. For example, if the lens element170comprises CaF2, LiF, or BrF2and water is to be used as the immersion liquid, then SiO2may be selected as the ACC material. Other materials that may be used for the ACC220include indium tin oxide (ITO), polymer, and the non-photo-active resin of a photoresist.

In step630, the ACC is formed on one surface of the lens element170. The ACC may be formed either directly on or proximate to (if there are intervening layers) the transparent substrate. The material forming the ACC may be selected to adhere relatively well to the transparent substrate, to be transparent to the light used during lithography, and to be chemically stable with the immersion liquid150. It is understood that the material may change depending on the particular immersion liquid used during lithography. In the present example, chemically stable indicates that the chemical reaction rate between the ACC and the immersion liquid is less than 10−5per hour, where the chemical reaction rate is defined as the decrease of the relative thickness of the ACC220per hour.

In the present example, the thickness of the ACC may be less than 1000 μm, and the surface flatness of the ACC may be controlled so that the variation of the surface is less than the half of the wavelength of the light used in lithography. In some embodiments, the thickness of the ACC may be increased within a range of up to approximately 1 mm, enabling the ACC to perform both anti-corrosion and anti-reflection functions. The formed ACC may be planarized through processing such as polishing.

In step640, an ARC (e.g., the ARC310ofFIG. 3) may be formed on the opposite surface relative to the ACC of the lens element170of the objective lens. The ARC may have multilayer structure, or may have a graded structure where the refractive index of the ARC may change gradually to match the refractive indexes of neighboring materials. The ARC may comprise organic material containing at least one of hydrogen, carbon, and oxygen; compound materials including Cr2O3, ITO, SiO2, SiN, TaO5, Al2O3, TiN, and ZrO; metal materials such as Al, Ag, Au, and In; or combination of thereof. It is understood that the ARC may not be used on the lens element170, or may be formed on the surface prior to forming the ACC on the same surface (e.g.,FIG. 4).

In step650, the lens element170with its ACC and ARC may be assembled with other lens elements into an objective lens. It is understood that such assembly may not be required. For example, the lens element may be used alone or may already be part of an objective lens when one or both of the ACC and ARC are formed. Other lens elements used with the lens element170may or may not have an ACC or an ARC.

Referring toFIG. 7, a method700illustrates an exemplary use of the objective lens170ofFIGS. 1-5during liquid immersion lithography. In step710, an objective lens having a suitable ACC is selected based at least partly on a liquid to be used during the immersion lithography. As the ACC is to protect or minimize corrosion of the lens by the immersion liquid, the material used to form the ACC should be a factor in selecting an objective lens. An exemplary immersion liquid may have an index of refraction larger than one, relatively low optical absorption at a predetermined wavelength such as 193 nm, and may be compatible with a photoresist applied to the semiconductor substrate. In addition, the immersion liquid may be relatively chemically stable, uniform, non-contaminating, bubble-free, and thermally stable. As an example, pure water may be used as an immersion liquid. Further, the temperature of the immersion liquid may be controlled due to its effect on shifting the index of refraction.

At step720, the immersion liquid is dispensed between the substrate to be patterned and the objective lens, with the objective lens positioned with the ACC close to or in contact with the immersion liquid. In some embodiments, the objective lens may be at least partly held in position by the surface tension of the immersion liquid.

At step730, the objective lens is illuminated to perform lithography on the substrate. The illuminating light may be from a mercury lamp with a wavelength of 436 nm (G-line) or 365 nm (I-line); a Krypton Fluoride (KrF) excimer laser with a wavelength of 248 nm; an Argon Fluoride (ArF) excimer laser with a wavelength of 193 nm; or a Fluoride (F2) excimer laser with a wavelength of 157 nm. For example, a 193 nm ArF excimer laser may be used in liquid immersion lithography to enhance the resolution of the pattern.

The present disclosure has been described relative to a preferred embodiment. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the application. It is understood that several modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.