Below 200 nm lasers such as ArF excimer lasers are the illumination sources of choice for the microlithographic industry. The industry constantly demands more performance from excimer laser sources. As a result, greater demands are constantly placed on excimer laser optical components, for example, the highly reflective mirrors and other optical components that are used in 193 nm wavelength excimer lasers that operate at high repetition rates. The highly reflective mirrors are typically made using at least one high refractive index material and one low refractive index material deposited in multiple layers on a selected substrate.
The deposition of thin optical thin films is known in the art and several different methods or technologies have been used to deposit such films. Among the methods that have been used to deposit tin films, all of which are carried out in vacuum, are (1) Conventional Deposition (“CD”), (2) Ion Assisted Deposition (“IAD”), (3) Ion Beam Sputtering (“or IBS”), and (4) Plasma Ion Assisted Deposition (“PIAD”).
In the Conventional Deposition (CD) method, the material(s) to be deposited are heated to the molten state by either a resistance heating method or by electron bombardment, the heating being done in the presence of a substrate upon which a film is to be deposited. When the material to be deposited is molten, evaporation of the material occurs and a film is condensed on the surface of the substrate. At the molten material temperatures used by this method some disassociation of the evaporant takes place. While this dissociation is not a problem when an elemental material is being deposited (for example, elemental aluminum, silver, nickel, etc.), it does present a problem when the material to be deposited is a compound (for example, SiO2, HfO2). In the case of oxide materials, small amounts of oxygen are bled into the chamber during deposition in an attempt to re-store stoichiometry—a so-called reactive deposition. The films that are deposited by the CD method are generally porous and lack sufficient kinetic energy (surface mobility) upon deposition to overcome surface energy (adhesion). Film growth is typically columnar (K. Guenther, Applied Optics, Vol. 23 (1984), pp. 3806-3816) with growth in the direction to the source and having a porosity that increases with increasing film thickness. In addition to high film porosity, other problems encountered with CD deposited films include index of refraction inhomogeneity, excessive top surface roughness, and weak absorption. Some improvements, though slight, are possible by adjusting the depositions rate and by increasing the substrate temperature during deposition. However, overall considerations of the final product dictate that CD techniques are not suitable for high quality optical components, for example, telecommunications elements, filters, laser components, and sensors.
Ion Assisted Deposition (IAD) is similar to the CD method described above, with the added feature that the film being deposited is bombarded with energetic ions of an inert gas (for example, argon) during the deposition process, plus some ionized oxygen (which in the case of oxide films is generally necessary to improve film stoichiometry). While ion energies are typically in the range 300 eV to 1000 eV, ion current at the substrate is low, typically a few micro-amps/cm2. (IAD is thus a high voltage, low current density process.) The bombardment serves to transfer momentum to the depositing film and to provide sufficient surface mobility so that surface energies are overcome and dense, smooth films are produced. The index inhomogeneity and transparency of the deposited films are also improved and little or no substrate heating is required for the IAD method.
Ion Beam Sputtering (IBS) is a method in which an energetic ion beam (for example, argon ions in the range 500 eV-1500 eV) is directed to a target material, typically an oxide material. The momentum transferred upon impact is sufficient to sputter-off target material to a substrate where it is deposited as a smooth, dense film. Sputtered material arrives at the substrate with high energy, on the order of several hundred electron volts leading to high packing density and smooth surface, but high absorption of the deposited films is a common by-product of the IAB process. As a result, an IBS process might also include an IAD source to both improve stoichiometry and absorption. While the IBS process is an improvement over CD and IAD, there are nonetheless problems with IBS. Some of the problems with the IBS deposition process include: (1) the deposition process is very slow; (2) it is more of a laboratory technique than a production process; (3) there are few IBS installations in existence, typically remnants from the telecom bubble, and these have only one or two machines operated by a small staff; (4) substrate capacity is quite limited; (5) deposition uniformity over the substrate can become a limitation, which in turn affects product quality and results in a high discard rate; (6) as the target is eroded the uniformity of the film being deposited changes, thus resulting in further quality problems and frequent target change-outs with associated down-time and costs; and (7) the bombardment energy is quite high which in turn leads to disassociation of the deposited materials and hence absorption.
Plasma Ion Assisted Deposition (PIAD) is similar to the IAD process above, except momentum is transferred to the depositing film via a low voltage, high current density plasma. Typical bias voltages are in the range 90-160 v and current densities in the range of milli-amps/cm2. While PIAD instruments are common in the precision optics industry and have been used to deposit films, there are some problems with the PIAD method, particularly in regard to the homogeneity of the deposited film. PIAD deposition has been described in U.S. Pat. No. 7,465,681 in the name of G. Hart, R. Maier and Jue Wang as inventors.
The below 200 nm lasers, also known as deep ultraviolet or “DUV” lasers, have been extensively used in advanced optical lithography to mass-produce patterned silicon wafers for use in semiconductor manufacturing. As the semiconductor process progresses from the 65 nm node to the 45 nm node and beyond, “at wavelength” optical inspection is required for increased resolution. The “at wavelength” optical metrology demands more performance from optical components used in connection with the inspection systems, for example, wide-angle high reflective mirrors for both p-polarization and s-polarization with angles of incidence ranging from 40° to 50° at the wavelength of 193 nm. The wide-angle highly reflective mirrors may also be required in the other areas where an ArF excimer laser is being used, for example as in medical surgery, ultra-precision machining and measurement, large-scale integrated electronic devices, and components for communications.
Generally, at least one high refractive index and one low refractive index material are required for making highly reflective mirrors. A wide-angle highly reflective mirror corresponds to a broad bandwidth in wavelength. The bandwidth is dominated by the refractive index ratio of the coating materials. At a high angle of incidence, the bandwidth of p-polarization narrows and the reflectance decreases; and this makes the preparation of a wide-angle highly reflective mirror a technical challenge when both s- and p-polarizations need to be considered. Highly reflective mirrors can be fabricated by the multilayering of metal oxides, fluorides and fluoride-oxide hybrids. For the oxides the material selection is very limited at 193 nm. A combination of Al2O3 and SiO2 is frequently used as the high and low refractive index coating materials, respectively. For the Al2O3 and SiO2 system the refractive index ratio (high index÷low index) is relative small (˜1.16) at 193 nm when compared to a ratio of 1.56 for an HfO2—SiO2 combination at 248 nm and 2.07 for TiO2—SiO2 combination at 550 nm. For fluoride materials, GdF3 and LaF3 are considered as high refractive index materials, whereas MgF2 and AlF3 are the low refractive index materials. [See D. Ristau et al, “Ultraviolet optical and microstructural properties of MgF2 and LaF3 coating deposited by ion-beam sputtering and boat and electron-beam evaporation”, Applied Optics 41, 3196-3204 (2002); C. C. Lee et al, “Characterization of AlF3 thin films at 193 nm by thermal evaporation”, Applied Optics 44, 7333-7338 (2005); and Jue Wang et al., “Nanoporous structure of a GdF3 thin film evaluated by variable angle spectroscopic ellipsometry,” Applied Optics 46(16), 3221-3226 (2007).] A combination of GdF3—AlF3 gives a refractive index ratio of 1.23 at 193 nm, which is higher than that of Al2O3—SiO2 combination that has a ratio of approximately 1.16. Thermal resistance evaporation of fluorides has been proved to be a good way to evaporate fluoride without introducing fluorine depletion. However, surface/interface roughness and inhomogeneity of fluoride multilayer increase resulting in high scatter loss as the number of fluoride layers and their thickness increase. As a result, the mean refractive index ratio decreases as number of fluoride layers increases and this restricts the achievable reflectance and bandwidth.
U.S. application Ser. No. 12/156,429, filed May 29, 2008 (Publication No. US 20080204862A1, assigned to Corning Incorporated) describes the use of an oxide-fluoride hybrid approach to eliminate scatter loss and increase environmental stability. By using the approach described in US 20080204862A1, a reflectance of approximately 98.5% at 193 nm at normal angle of incidence has been achieved. However, the bandwidth of the fluoride-oxide hybrid high reflective mirror is limited due to the refractive index ratio of Al2O3—SiO2 which is approximately 1.16 at 193 nm. The bandwidth of the highly reflective mirror can be improved by changing coating materials to increase the ratio. One technical approach is to use a sol-gel process in which nano-porous structures are introduced to further reduce film refractive indices. The nano-porous films can be deposited by a dip coating or a spin coating. The refractive index of nano-porous silica film can be as low as 1.20. [See Jue Wang et al., “Scratch-resistant improvement of sol-gel derived nano-porous silica films,” J. Sol-Gel Sci. and Technol. 18, 219-224 (2000).] The advantages of the sol-gel derived ultralow refractive index has been demonstrated on wide-angle anti-reflection coating at 193 nm, in which 1 layer of the ultralow refractive index material is spin-coated on top of physically evaporated films. [See T. Murata et al., “Preparation of high-performance optical coatings with fluoride nanoparticles films made from autoclaved sols,” Applied Optics 45 1465-1468 (2006).] However, this process is not suitable for highly reflective mirrors. Thus, while considerable effort has been put forth to find a suitable high/low refractive index coating system to improve the performance of highly reflective optical elements, no satisfactory coating system currently exists in the art.