Methods for repair of photomasks

#This invention generally relates to a method of repairing defects in photomasks. More particularly, it relates to the use of a coating on a photomask during steps to repair clean and opaque defects on photomasks. It also relates to the use of short duration laser pulses to repair opaque defects on photomasks.

#Photomasks are extensively used in the fabrication of integrated circuits on semiconductor wafers. Standard photomasks include a patterned absorbing or opaque film on a transparent substrate. A metal, such as chromium, having a thickness on the order of about 1000 Å is often used as the opaque film. Nickel and aluminum are also used. A transparent material, such as quartz, is often used as the substrate, though materials such as glass and sapphire can also be used. Phase shift photomasks may include in addition to patterned chromium on quartz, means to change the phase of light near the chromium edge, such as an etched region in the quartz or a transparent layer on the quartz near a chromium edge. Phase shift photomasks can also include, instead of a totally opaque material, an absorbing material, also called an attenuator, that permits transmission of a small fraction of the light.

#The fabrication of photomasks usually involves a blanket deposition of a thin film of the chromium opaque film on the quartz substrate. The chromium is then coated with photoresist, which is exposed with a high resolution technique, such as an electron beam, and developed to form the desired pattern in the resist. This pattern is then transferred into the chromium by etching, leaving opaque and transparent regions on the mask.

#The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description of the invention, as illustrated in the accompanying drawings, in which:

#FIGS. 1a-1f are top views of six types of mask defects known in the prior art.

#FIGS. 2a-2c are top views of the steps of repairing an opaque defect similar to the defect of FIG. 1b according to an etching embodiment of the present invention.

#The present invention provides several process embodiments for repair of opaque and clear defects. In most embodiments described hereinbelow, a coating is used that protects the mask during subsequent repair steps, such as etching, laser ablating, focused ion beam sputtering, laser deposition, or focused ion beam deposition. The coating may be a polymer, such as photoresist, a metal, such as copper, or a material, such as carbon that can be etched selective to mask materials, including chrome lines, quartz, and attenuator. The coating may be deposited on or under chrome lines of the mask. Alternatively, a short duration laser pulse may be used with or without a protective coating to achieve similar results.

#Prepositions, such as “on,” “over,” and “under” are defined with respect to a planar surface of the mask, regardless of the orientation the mask is actually held. A layer is on another layer even if there are intervening layers.

#Applicants of the present invention have discovered that laser ablation of opaque defects can cause damage in addition to the damage to adjacent lines and to the quartz that was described hereinabove. Applicants discovered a stain in the neighborhood of laser ablated opaque defects and found that the stain significantly reduced transmission in those regions, particularly when the mask is used for exposure at short wavelengths, such as 248 nm and 365 nm. Applicants then characterized the stain and found it to be a redeposition of laser ablated material that falls back onto the mask as fallout.