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Method for monitoring focus on an integrated wafer

  • US 9,046,788 B2
  • Filed: 05/19/2008
  • Issued: 06/02/2015
  • Est. Priority Date: 05/19/2008
  • Status: Expired due to Fees
First Claim
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1. A method of monitoring focus during photolithography on a wafer comprising the steps of:

  • loading a wafer and a patterned reticle into a step-and-scan wafer imaging photolithography exposure system;

    exposing the reticle using the step-and-scan system using a same dose but a varying focus for each exposure field of the reticle pattern on the wafer;

    measuring the critical dimensions of two different features in two different regions in each exposure field, the features and regions being the same in each exposure field, the two different regions being positioned in each exposure field so that the step-and-scan wafer imaging photolithography exposure system cannot expose both of them at best focus;

    plotting Bossung curves of critical dimension versus focus for the two different features in the two different regions;

    determining a first best focus for the first of the two different features and a second best focus for the second of the two different features;

    correlating the first and second best focuses with each other to determine their intersection point which comprises a best focus of the exposure field;

    plotting the critical dimension difference between the Bossung curves for the two different features in the two different regions versus focus shift using data from the Bossung curves of the two different features and the best focus of the exposure field to obtain a sloped plot;

    determining if the difference plot has sufficient sensitivity to be used as a focus monitor by determining whether the sloped plot has a desired minimum slope needed for the sloped plot to be used as the focus monitor, and if the sloped plot does not have the desired minimum slope to meet sufficient sensitivity, choosing new features in two different regions and repeating the measuring, best focus determination, correlating and plotting steps until sufficient sensitivity for use as a focus monitor is achieved;

    determining focus instruction values based on the difference plot focus monitor;

    scanning and exposing a new wafer based on the determined focus instruction values.

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