1. Field of the Invention
The present invention relates to a method for forming a reticle for use in photolithography and, more particularly, to a method for cleaning a reticle.
2. Description of the Related Art
Optical systems are widely used in the microelectronics industry to manufacture semiconductor wafers by a process known as photolithography. Typically, a photolithography system comprises a light source configured to project light rays to a condenser lens. The condenser lens collimates the light rays towards a pellicle placed before (or after) a reticle, sometimes referred to as a photomask. Typically, the reticle includes an optically transparent substrate underlying an optically opaque layer in a pattern used to project an image onto a portion of a photoresist-coated wafer. The pellicle is a substantially thin, optically transparent film which seals off the reticle surface from airborne particulates and other forms of contamination. Typically, a projection lens is placed after the reticle to receive and focus the pattern of light rays onto an exposure field on a photoresist (“resist”) layer overlying a wafer. Exposed or unexposed portions of the photoresist are then developed, replicating the reticle pattern in the photoresist layer.
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC's with hundreds of millions of transistors at feature sizes of 0.25 micrometers (“micron”), 0.18 micron, 0.10 micron, and less are becoming routine. Improvement in overlay tolerances in optical photolithography and the introduction of new light sources with progressively shorter wavelengths have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far below one micron.
There are various types of reticles available in the art. Binary masks comprise a pattern (e.g., an integrated circuit (IC) pattern) formed in a layer of an opaque material (e.g., chrome or chrome oxide) overlying a transparent substrate (e.g., quartz). The pattern may be formed using any lithographic technique known in the art, such as optical photolithography or electron-beam lithography.
Attenuated phase shift masks (APSMs) have provided improved image contrast and lithographic resolution over standard binary masks. An APSM comprises a layer of an optically transparent material (e.g., quartz) below a layer of an optically low-transmission material, such as molybdenum doped silicon oxynitride (MoSixOyNz, wherein “x”, “y” and “z” are numbers greater than zero; also referred to as “MoSi” herein) or molybdenum doped silicon oxide (MoSixOy, wherein “x” and “y” are numbers greater than zero; also referred to as “MoSiO” herein). An APSM forms shift patterns through adjacent areas of the transparent and low-transmission materials. Unlike an optically opaque material, such as chrome, MoSi allows a small percentage of light to pass through. However, the amount that passes through is “weak” and does not expose the resist on the wafer. The light that does pass through is 180° out of phase compared to the light passing through neighboring quartz areas. Therefore, where the material and the quartz meet, light interferes in such a way as to sharpen the edges of the design, producing a faithful replica of the intended pattern in the resist. Approximately 6% of light that is incident on the layer of an optically low transmission material of an APSM is transmitted through the reticle. High transmission attenuated reticles (HTARs) are similar to APSMs, with the exception that the thickness of low-transmission material layer is reduced relative to that of an APSM. Depending on the thickness of the layer of the optically low transmission material, about 18% or 30% of light that is incident on the layer of the optically low transmission material of an HTAR is transmitted through the reticle.
Manufacturing imperfections can render the quality of images formed using various reticles, such as APSMs, undesirable. For example, reticle defects, which may arise from impurities in and/or on the MoSi layer, can cause intolerable or unacceptable variations in CD's in the exposure field. To solve these problems, some manufacturers simply replace a defective reticle with a new one, which ultimately increases production costs. Other manufacturers resort to methods for cleaning reticles.
A reticle is typically cleaned by a conventional method based on RCA cleaning, typically using a mixture of an acid, such as sulfuric acid, and hydrogen peroxide (or a mixture of an alkaline agent, such as aqueous ammonia, and hydrogen peroxide), which is also widely used for cleaning wafers. In a first step, a hot mixture of sulfuric acid and hydrogen peroxide is introduced to the reticle to decompose organic objects, such as resist present on the surface of a reticle. This step improves the wettability of the surface of the reticle, thereby enhancing the efficiency of the subsequent cleaning steps. In a second step, the reticle is rinsed with hot pure water to remove residual agents, such as sulfuric acid, from the surface of the reticle. In a third step, the reticle is dipped in and cleaned with a heated or ambient temperature mixture of ammonia hydroxide and hydrogen peroxide (APM clean) for the purpose of removing foreign objects attached to the reticle. During this step, an ultrasonic wave may be applied to the dipping tank to remove foreign objects more effectively. This step, too, is typically followed by rinsing with pure water. In a final step, the reticle is dried.
The conventional cleaning process has several disadvantages. For example, the treatment with sulfuric acid/hydrogen peroxide is followed by rinsing with a large amount of pure water or hot pure water to discharge any remaining sulfuric acid. This rinsing step consumes a large amount of pure water and considerable electric energy for heating pure water, thus adding to increased processing costs. As another example, if the cleaning efficiency (yield in cleaning) is poor, the number of cleaning times required per reticle increases, which also increases processing costs.
Several alternatives to the conventional cleaning method are available. For example, U.S. Pat. No. 6,071,376, filed Jul. 27, 1998, the disclosure of which is entirely incorporated herein by reference, teaches a reticle cleaning method comprising a first step of cleaning the surface of a reticle with a hot mixture of sulfuric acid and hydrogen peroxide to decompose organic and metallic objects present thereon, a second step of removing residual sulfuric acid from the surface of the reticle using anodic water, a third step of removing foreign objects attached to the surface of said reticle with cathodic water, and a fourth step of drying the reticle. As another example, U.S. Pat. No. 7,001,470, filed Apr. 10, 2002, the disclosure of which is entirely incorporated herein by reference, teaches using ozone (O3) gas solved water to eliminate organic substances adhered on a surface of a reticle. The reticle is subsequently cleaned using an alkaline chemical, such as alkaline ionized water or hydrogenated water. The reticle is dried after completion of the cleaning steps. As yet another example, a UVO cleaning device by the Jelight Company, Inc, removes organic molecules from reticles upon excitation and/or dissociation of contaminants by absorption of short-wavelength ultraviolet (UV) radiation. Dissociation of molecular oxygen with 184.9 nanometer (nm) wavelength light produces atomic oxygen that reacts with contaminant molecules to form volatile products that desorb from the reticle.