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Method for the measurement of reticle plane power using an external calibration sensor

IP.com Disclosure Number: IPCOM000131868D
Publication Date: 2005-Nov-21
Document File: 3 page(s) / 46K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for the measurement of reticle plane power using an external calibration sensor. Benefits include improved functionality.

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Method for the measurement of reticle plane power using an external calibration sensor

Disclosed is a method for the measurement of reticle plane power using an external calibration sensor. Benefits include improved functionality.

Background

      The optics used in deep ultraviolet (DUV) lithography tools -become damaged with use due in part to the photo-deposition of contaminant films on top of the optical antireflective coatings. This contamination build-up can be particularly damaging to the primary lens between the reticle and wafer planes. The primary lens optics are typically not designed to be field-replaceable and are exceedingly costly. As a result, the level of contamination must be tracked so imaging issues can be predicted before they occur and cause manufacturing yield loss.

              Conventionally, optical power can be measured at the wafer plane but not at the reticle plane. By having a tool to measure reticle plane power, the power loss can be measured directly through the primary lens system of a lithography tool. This power transmission loss can be used to determine the contamination level compared to the uncontaminated level of the primary lens system.

      The level of contamination is inferred through other methods, such as changes in the wafer plane power distribution across the exposure field and primary lens flare measurements.

      The cross-field power distribution changes as a function of the contamination level due to the mechanism at work. The photo-induced deposition is proportional to the number of photons passing through a given portion of the lens. With more photons passing through the center of the lens than the edges, the deposition is highest and transmission the lowest in the center. Over time, the power at the center of the lens (field) degrades at a higher rate than at the edge of the exposure field.

      Contamination degrades the performance of the antireflective coatings on the primary lens optics. As a result, a higher proportion of internal reflections occur in the center of the lens than at the edges. Because flare is a measurement of the internal reflections, the flare maximum and cross-field range values increase as the contamination level increases.

      Wafer plane power measurements are influenced by changes in the entire optical system. Because all optics (not just the primary lenses) on a lithography tool change with usage, the contributions of the secondary optics to the wafer plane power measurements must be removed. A technique for removing flare includes replacing all of the secondary optics with new optics. However, this technique is prohibitively expensive. Another technique is to measure the power at a second location within the optical system, such as between the primary lenses. The power loss can be used to indicate the damage to the antireflective coatings.

      Conventional data collection tools tha...