text: 1. Field of the Invention
  Generally, the present disclosure relates to the manufacture of semiconductor devices, and, more specifically, to various embodiments of a novel blazed grating spectral purity filter and various methods of making such a filter.
  2. Description of the Related Art
  The fabrication of advanced integrated circuits, such as CPU's, storage devices, ASIC's (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout, wherein field effect transistors (NMOS and PMOS transistors) represent one important type of circuit element used in manufacturing such integrated circuit devices. In general, integrated circuit devices are formed by performing a number of process operations in a detailed sequence or process flow. Such process operations typically include deposition, etching, ion implantation, photolithography and heating processes that are performed in a very detailed sequence to produce the final device.
  Device designers are under constant pressure to increase the operating speed and electrical performance of transistors and integrated circuit products that employ such transistors. One technique that continues to be employed to achieve such results is the reduction in size of the various devices, such as the gate length of the transistors. The gate length (the distance between the source and drain regions) on modern transistor devices may be approximately 30-50 nm, and further downward scaling is anticipated in the future. Manufacturing devices that are so small is a very difficult challenge, particularly for some processes, such as photolithography tools and techniques.
  A typical photolithography process generally involves the steps of: (1) applying a layer of photoresist (a light-sensitive material) above a wafer or substrate, typically accomplished by a spin-coating process; (2) pre-baking (or soft-baking) the layer of photoresist at a temperature of approximately 90-120° C. to reduce the level of solvents in the layer of photoresist and to improve the adhesion characteristics of the photoresist; (3) performing an exposure process, wherein a pattern on a reticle or mask is projected onto the layer of photoresist used in a stepper tool to create a latent image in the layer of photoresist; (4) performing a post-exposure bake on the layer of photoresist at a temperature approximately 5-15° C. higher than the pre-bake process; (5) performing a develop process to turn the latent image in the layer of photoresist into the final resist image; and (6) performing a post-bake process (or hard-bake) at a temperature of approximately 125-160° C. to remove residual solids and to improve adhesion of the patterned photoresist mask. These process steps result in a “post-litho” patterned etch mask that may be used for a variety of purposes, e.g., as an etch mask to form trench/hole type features in an underlying layer of insulating material. The above processes are well known to those skilled in the art and, thus, will not be described herein in any greater detail.
  By way of background, photolithography tools and systems typically include a source of radiation at a desired wavelength, an optical system and, as noted above, a mask or reticle that contains a pattern that is desired to be formed on a wafer. Radiation is provided through or reflected off the mask or reticle to form an image on a light-sensitive layer of photoresist material that is formed above a semiconductor wafer. The radiation used in such systems may be light, such as ultraviolet light, deep ultraviolet light (DUV), vacuum ultraviolet light (VUV), extreme ultraviolet light (EUV), etc. The radiation may also be x-ray radiation, e-beam radiation, etc. Currently, most of the photolithography systems employed in semiconductor manufacturing operations are so-called deep ultraviolet (DUV) systems that generate radiation at a wavelength of 248 nm or 193 nm. However, the capabilities and limits of traditional DUV photolithography systems are being tested as device dimensions continue to shrink. This has led to the development of so-called extreme ultraviolet systems, i.e., EUV systems, that use radiation with a much shorter wavelength, e.g., less than 20 nm and in some particular cases about 13.5 nm. One fundamental difference between DUV systems and EUV systems involves the structure of the reticle, and the manner in which light interacts with the reticle. In a DUV system, light (from the light source) passes through the reticle and irradiates a layer of light sensitive material. In contrast, in an EUV system, light (from the light source) is reflected off of a multi-layer mask onto the light sensitive material.
  As device dimensions continue to shrink, it is believed the EUV photolithography systems will be required to make future generation devices. However, there are some practical problems associated with effectively implementing EUV systems into production manufacturing environments. For example, low source power and excessive defect generation are two problems that need to be addressed to allow implementation of cost-effective EUV systems.
  The typical radiation source in an EUV system emits multiple wavelengths of energy, such as, for example, 10.6 μm, 13.5 nm, 193 nm, 248 nm, etc. EUV systems are designed to use radiation at a wavelength of about 13.5 nm. Radiation at a wavelength of 10.6 μm is typically referred to as infra-red (IR) radiation. Radiation at wavelengths other than 13.5 nm and 10.6 μm is typically referred to as out-of-band (OoB) light or radiation. Thus, given the multiple wavelengths in the radiation source for an EUV system, a spectral purity filter (SPF) must be employed to filter out the undesirable wavelengths of light, i.e., to filter out the IR and OoB wavelengths. If the IR and OoB wavelengths are not effectively blocked, the EUV system may not be as effective in forming patterned features because such undesirable wavelengths reduce the contrast during the EUV photolithography process. However, when filtering the undesirable IR and OoB wavelengths, some of the desirable 13.5 nm wavelength light is also filtered out. Accordingly, what is desired is a spectral purity filter that allows a high transmission of EUV light and a low transmission of the IR and OoB wavelengths.
  There are various known techniques and systems that are used in an attempt to filter the IR and OoB wavelengths while allowing transmission of the desirable EUV light. Typically, such known techniques and systems try to block the IR and OoB wavelengths separately. In general, there is more energy in the IR band and, accordingly, absorption of IR radiation tends to generate a lot of heat. Thus, heat management is an important issue when it comes to designing and implementing a spectral purity filter. Moreover, the inventors are not aware of any prior art filtering systems that block both the IR and OoB wavelengths while at the same time providing acceptable levels of heat management.
  One type of spectral purity filter is a gas-based spectral purity filter. As the name implies, a gas (e.g., SF4 or Kr) is used to absorb IR light. The positive attributes of such a gas-based spectral purity filter is that it has a relatively high EUV transmission rate (about 80-90%), it is relatively good in terms of heat management and it is effective in blocking debris. The negative aspects of such a gas-based spectral purity filter are that it requires the use of a bulk processing chamber and gas turbulence may induce aberrations in the EUV light, which may result in reduced imaging quality in the resulting patterned layer of photoresist.
  Another type of spectral purity filter is typically referred to as an ML Bragg grating spectral purity filter. An ML Bragg grating filter is typically comprised of multiple layers of B4C/Si and metal layers to absorb IR light. The positive attributes of such an ML Bragg grating filter is that it is physically small, it is effective at blocking IR radiation (IR transmission less than about 0.5% at 10.6 μm) and it is relatively good in terms of heat management. The negative aspect of such an ML Bragg grating filter is that it has a relatively low EUV transmission rate (about 45% at 13.5 nm).
  Yet another type of spectral purity filter is typically referred to as a grid spectral purity filter. In a grid filter, a grid cell is used to block IR light. The positive attributes of such a grid filter is that it is physically small and it is effective at blocking IR radiation (IR transmission less than about 0.1% at 10.6 μm). The negative aspect of such a grid filter is that it has a relatively low EUV transmission rate (about 74% at 13.5 nm).
  Another type of spectral purity filter is typically referred to as a blazed grating spectral purity filter. As the name implies, a blazed grating filter includes a blazed grating that is used to separate the EUV, IR and OoB wavelengths. The positive attributes of such a blazed grating filter is that it is one of the few types of filters wherein the IR and OoB wavelengths are blocked at the same time, it is physically small, it exhibits a low rate of transmission for both IR and OoB wavelengths, and it is relatively good in terms of heat management. The negative aspects of such a prior art blazed grating filter are that it has a relatively low EUV transmission rate (about 63% at 13.5 nm) and the fabrication of such a prior art blazed grating spectral purity filter is complex.
  Yet another type of spectral purity filter is typically referred to as an OoB spectral purity filter. In an OoB filter, a free-standing foil or membrane is used to block OoB wavelength light. The positive attributes of such an OoB filter are that it is physically small and it exhibits a low rate of transmission for OoB wavelength light. The negative aspects of such an OoB filter are that it is very fragile, exhibits a relatively low EUV transmission rate (about 70% at 13.5 nm) and it is relatively poor in terms of heat management.
  One prior art EUV system (manufactured by ASML) includes both a grid filter and an OoB filter. It is believed that this system has a relatively low EUV transmission rate (about 50% at 13.5 nm), it is very fragile (due to the foil in the OoB filter) and it is very expensive.
  FIG. 1 is a simplistic depiction of a prior art blazed grating filter 10. Such a blazed grating structure 10 may be a stand-alone device, or it may be fabricated into the surface of a mirror within an EUV system. As shown therein, the blazed grating filter 10 is comprised of a specially constructed “saw-toothed” shaped surface 12 that includes a plurality of facet surfaces 14 that have a specifically designed periodicity “d”. Additionally, the facet surfaces 14 may be oriented at a specifically designed blaze angle, which may typically range from about 7-10° depending upon the particular application. The normal directions for the facet surfaces 14 and the blazed grating 10 are different, as noted in FIG. 1. The geometry and shape of surface 12 is designed such that incident radiation may be separated into various orders, with illustrative examples of the first order diffraction and the zeroth order diffraction depicted in FIG. 1.
  Typically, such a prior art blazed grating filter 10 for use in an EUV system was formed by first fabricating the saw-toothed surface 12 into a substrate and thereafter depositing the various films that define the well-known multi-layer structure used in such filters onto the saw-toothed surface 12. In general, such prior art blazed grating filters 10 were fabricated in one of two ways: by etching the substrate or by using a so-called electron-beam lithography tool to cut the saw-toothed surfaces into the substrate. One illustrative example of a method of forming a blazed grating filter 10 using an etching process is described in an article by R. K. Heilmann et al. (Heilmann et al., “Advances in reflection grating technology for Constellation-X,” Proc. of SPIE, 5168:271-82, 2004). The method generally involves the formation of a masking layer above the substrate, performing an etching process that exposes the <111> planes of the substrate, stripping the masking layer from the etched substrate, attaching a transparent template to the etched substrate (which acts as a mold) using a UV curable polymer material, curing the polymer material and removing the etched silicon mold. Such a manufacturing process is very complex and very time consuming. One example of a method of manufacturing a blazed grating filter using an electron-beam lithography tool is described in an article by P. P. Naulleau et al. (Naulleau et al., “Fabrication of high-efficiency multilayer-coated binary blazed gratings in the EUV regime,” Optics Communications, 200:27-34, 2001). Another example of a method of manufacturing a blazed grating filter using an electron-beam lithography tool is described in an article by J. A. Liddle et al. (Liddle et al., “Nanoscale topography control for the fabrication of advanced diffractive optics,” J. Vac. Sci. Technol. B, 21:2980, 2003). Manufacturing blazed grating filters using electron-beam lithography techniques has two primary problems. First, due to the nature of the electron-beam lithography tool, the surfaces are not as smooth as is desired and such surface roughness can cause errors in the diffraction of incident light. Moreover, such a manufacturing technique is time-consuming and expensive.
  The present invention is directed to various embodiments of a novel blazed grating spectral purity filter and various methods of making such a filter that may reduce or eliminate one or more of the problems identified above.