METHODS AND APPARATUS FOR DETERMINING FOCUS
First Claim
1. A method of determining optimal focus for a photolithography system, the method comprising:
- acquiring a plurality of optical signals from a particular target located in a plurality of fields on a semiconductor wafer, wherein the fields were formed using different process parameters, including different focus values;
extracting a feature from the optical signals related to changes in focus;
fitting a symmetric curve to the extracted feature of the optical signals as a function of focus; and
determining and reporting an extreme point in the symmetric curve as an optimal focus for use in the photolithography system.
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Abstract
Disclosed are apparatus and methods for determining optimal focus for a photolithography system. A plurality of optical signals are acquired from a particular target located in a plurality of fields on a semiconductor wafer, and the fields were formed using different process parameters, including different focus values. A feature is extracted from the optical signals related to changes in focus. A symmetric curve is fitted to the extracted feature of the optical signals as a function of focus. An extreme point in the symmetric curve is determined and reported as an optimal focus for use in the photolithography system.
60 Citations
23 Claims
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1. A method of determining optimal focus for a photolithography system, the method comprising:
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acquiring a plurality of optical signals from a particular target located in a plurality of fields on a semiconductor wafer, wherein the fields were formed using different process parameters, including different focus values; extracting a feature from the optical signals related to changes in focus; fitting a symmetric curve to the extracted feature of the optical signals as a function of focus; and determining and reporting an extreme point in the symmetric curve as an optimal focus for use in the photolithography system. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A system for inspecting or measuring a specimen, comprising:
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an illuminator for generating illumination; illumination optics for directing the illumination towards a particular target located in a plurality of fields on a semiconductor wafer, wherein the fields were formed using different process parameters, including different focus values; collection optics for directing a plurality of optical signals from the particular target located in the plurality of fields in response to the illumination to a detector system; the detector system for acquiring the plurality of optical signals from the particular target from the plurality of fields in response to the illumination; and a processor and memory configured for performing the following operations; extracting a feature from the optical signals related to changes in focus; fitting a symmetric curve to the extracted feature of the optical signals as a function of focus; and determining and reporting an extreme point in the symmetric curve as an optimal focus for use in the photolithography system. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
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Specification