1. Field of the Invention
The present invention pertains to optics, and in particular, to optics in microlithography.
2. Related Art
Photolithography (also called microlithography) is a semiconductor fabrication technology. Photolithography uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as, diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to carry out photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system can include a light source, reticle, optical reduction system, and a wafer alignment stage. An image of a semiconductor pattern is printed or fabricated on the reticle (also called a mask). A light source illuminates the reticle to generate an image of the particular reticle pattern. An optical reduction system is used to pass a high-quality image of the reticle pattern to a wafer. See, Nonogaki et al., Microlithography Fundamentals in Semiconductor Devices and Fabrication Technology (Marcel Dekker, Inc.: New York, N.Y. 1998), incorporated in its entirety herein by reference.
Integrated circuit designs are becoming increasingly complex. The number of components and integration density of components in layouts is increasing. Demand for an ever-decreasing minimum feature size is high. The minimum feature size (also called line width) refers to the smallest dimension of a semiconductor feature that can be fabricated within acceptable tolerances. As result, it is increasingly important that photolithographic systems and techniques provide a higher resolution.
One approach to improve resolution is to shorten the wavelength of light used in fabrication. Increasing the numerical aperture (NA) of the optical reduction system also improves resolution. Indeed, commercial exposure systems have been developed with decreasing wavelengths of light and increasing NA. For example, Silicon Valley Group Lithography (SVG), formerly Perkins-Elmer, has introduced a number of ultra-violet exposure systems with catadioptric types of optical reduction systems and step-and-scan reticle/wafer stage systems. See, e.g., Nonogaki, at section 2.5.5, pp. 20-24. These UV exposure systems available from SVG have light sources operating at a wavelength of 248 nanometer (nm) with an associated NA of 0.5 or 0.6, and at a wavelength of 193 nm with an associated NA of 0.5 or 0.6. However, light at wavelengths equal to or less than 170 nm, and for example at 157 nm, has not been made available in photolithographic applications for semiconductor fabrication. A numeric aperture greater than 0.6, and for example at 0.75, is also not available.
Catadioptric optical reduction systems include a mirror that reflects the imaging light after it passes through the reticle onto a wafer. A beam splitter cube is used in the optical path of the system. A conventional beam splitter cube, however, transmits 50% of input light and reflects 50% of the input light. Thus, depending upon the particular configuration of optical paths, significant light loss can occur at the beam splitter.
In UV photolithography, however, it is important to maintain a high light transmissivity through an optical reduction system with little or no loss. Exposure time and the overall semiconductor fabrication time depends upon the intensity or magnitude of light output onto the wafer. To reduce light loss at the beam splitter, a polarizing beam splitter and quarter-wave plates are used.
FIGS. 1A and 1B illustrate an example conventional polarizing beam splitter cube 100 used in a conventional catadioptric optical reduction system. Polarizing beam splitter cuber 100 includes two prisms 110, 150, and a coating interface 120. Prisms 120, 150 are made of fused silica and are transmissive at wavelengths of 248 nm and 193 nm. Coating interface 120 is a multi-layer stack. The multi-layer stack includes alternating thin film layers. The alternating thin film layers are made of thin films having relatively high and low indices of refraction (n1 and n2). The alternating thin film layers and their respective indices of refraction are selected such that the MacNeille condition (also called Brewster condition) is satisfied. In one example, the high index of refraction thin film material is an aluminum oxide. The low index of refraction material is aluminum fluoride. A protective layer may be added during the fabrication of the stack. Cement or glue is included to attach one of the alternating layers to a prism 150 at face 152 or to attach the protective layer to prism 110 at face 112.
As shown in FIG. 2A, the MacNeille condition (as described in U.S. Pat. No. 2,403,731) is a condition at which light 200 incident upon the multi-layer stack is separated into two beams 260, 280 having different polarization states. For example, output beam 260 is an S-polarized beam, and output beam 280 is a P-polarized beam. FIG. 2B shows the advantage of using a polarizing beam splitter in a catadioptric optical reduction system to minimize light loss. Incident light 200 (usually having S and P polarization states) passes through a quarter-wave plate 210. Quarter wave plate 210 converts all of incident light 200 to a linearly polarized beam in an S polarization state. Beam splitter cube 100 reflects all or nearly all of the S polarization to quarter wave plate 220 and mirror 225. Quarter wave plate 220 when doubled passed acts like ahalfwaveplate. See, e.g., xe2x80x9cWaveplates,xe2x80x9d  less than http://www.casix.com/new/waveplate.htm, two pages. Quarter wave plate 220 converts the S polarization light to circular polarization, and after reflection from mirror 225, converts light into P-polarized light. The P-polarized light is transmitted by beam splitter cube 100 and output as a P-polarized beam 290 toward the wafer. In this way, the polarizing beam splitter 100 and quarter wave plates 210, 220 avoid light loss in a catadioptric optical reduction system that includes a mirror 225. Note, as an alternative, mirror 225 and quarter wave plate 220 can be positioned at face B of cube 100 rather than at face A and still achieve the same complete or nearly complete light transmission over a compact optical path length.
Polarizing beam splitter cube 100, however, is not transmissive at wavelengths less than 170 nm. Prisms 120, 150 are made of fused silica which is opaque at wavelengths less than 170 nm. Similarly, coating interface 120 is also based on the MacNeille condition which is only explicitly described for infra-red wavelengths. Such coatings 120 are not effective at ultraviolet wavelengths less than 170 nm. Cement or glue used in bonding coating interface 120 to fused silica prisms 110, 150 can degrade when exposed to light at 170 nm or less.
What is needed is a polarizing beam splitter that supports an even higher resolution. A polarizing beam splitter is needed that is transmissive to light at ultraviolet wavelengths equal to or less than 170 nm, and for example at 157 nm. A polarizing beam splitter is needed that can image at high quality light incident over a wide range of reflectance and transmittance angles. A polarizing beam splitter is needed that can accommodate divergent light in an optical reduction system having a numeric aperture at a wafer plane greater than 0.6, and for example at 0.75.
The present invention provides a ultraviolet (UV) polarizing beam splitter. The UV polarizing beam splitter is transmissive to light at wavelengths equal to or less than 170 nm, and for example at 157 nm. The UV polarizing beam splitter can image at high quality light incident over a wide range of reflectance and transmittance angles. The UV polarizing beam splitter can accommodate divergent light in an optical reduction system having a numeric aperture at a wafer plane greater than 0.6, and for example at 0.75. In different embodiments, the UV polarizing beam splitter has a cubic, rectangular cubic, or truncated cubic shape, or approximates a cubic, rectangular cubic, or truncated cubic shape.
In one embodiment, a UV polarizing beam splitter cube comprises a pair of prisms and a coating interface. The prisms are made of at least a fluoride material, such as, calcium fluoride (CaF2) or barium fluoride (BaF2). The coating interface has at least one layer of a thin film fluoride material. In one example implementation, the coating interface includes a multi-layer stack of alternating layers of quarter wavelength thick thin film fluoride materials. The alternating layers of thin film fluoride materials comprise first and second fluoride materials. The first and second fluoride materials have respective first and second refractive indices. The first refractive index is greater than (or higher than) the second refractive index. In one feature of the present invention, the first and second refractive indices form a stack of fluoride materials having relatively high and low refractive indices of refraction such that the coating interface separates UV light (including light at wavelengths equal to or less than 170 nm, and for example at 157 nm) depending on two polarized states.
In one example, the coating interface comprises a multi-layer design of the form (H L)n H, where H indicates a layer of a quarter wavelength optical thickness of a first fluoride material having a relatively high refractive index. The first fluoride material can include, but is not limited to, gadolinium tri-fluoride (GdF3), lanthanum tri-fluoride (LaF3), samarium fluoride (SmF3), europium fluoride (EuF3), terbium fluoride (TbF3), dysprosium fluoride (DyF3), holmium fluoride (HoF3), erbium fluoride (ErF3), thulium fluoride (TmF3), ytterbium fluoride (YbF3), lutetium fluoride (LuF3), zirconium fluoride (ZrF4), hafnium fluoride (HfF4), yttrium fluoride (YF3), neodymium fluoride (NdF3), any of the other lanthanide series tri-fluorides, metallic fluorides, or other high index, ultraviolet transparent material. L indicates a layer of a quarter wavelength optical thickness of a second fluoride material having a relatively low refractive index. The second fluoride material can include, but is not limited to, magnesium fluoride (MgF2), aluminum tri-fluoride (AlF3), barium fluoride (BaF2), strontium fluoride (SrF2), calcium fluoride (CaF2), lithium fluoride (LiF), and sodium fluoride (NaF), or other low index, ultraviolet transparent material. The value xe2x80x9cnxe2x80x9d indicates the basic (H L) group is repeated n times in a multi-layer stack, where n is a whole number equal to one or more. In one example, n is between 3 and 15. In another example, n is between 5 and 10.
According to a further feature, the prisms and coating interface are joined by optical contact. No cement is needed.
Further multi-layer designs can be generated by computer iterated design. Layers in a multi-layer stack can also be graded across the hypotenuse face of a prism to adjust layer thicknesses at any point so as to compensate for changes in the incidence angle of the light.
In another embodiment, a UV polarizing beam splitter (cube or truncated cube) can be used in a high resolution catadioptric optical reduction system.
The present invention provides a method for splitting an incident light beam based on polarization state. The method includes the step of orienting a coating interface having at least one layer ofa fluoride material at an angle relative to the incident light such that the coating interface transmits incident light in a first polarization state and reflects incident light in a second polarization state. In one example, the method further includes the step of selecting thicknesses of alternating thin film layers and their respective indices of refraction such that the coating interface transmits incident light at a wavelength equal to or less than 170 nm in a first polarization state and reflects incident light at a wavelength equal to or less than 170 nm in a second polarization state.
Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.