| Autor: |
Michal E. Pawlowski, Ryan Munden, Corey Loke, Steve Roux, Aage Bendiksen |
| Rok vydání: |
2021 |
| Předmět: |
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| Zdroj: |
Emerging Digital Micromirror Device Based Systems and Applications XIII. |
| DOI: |
10.1117/12.2576450 |
| Popis: |
A frequency modulation technique is described to distinguish between particle signals and stray light in a prototype of a photolithographic mask inspection system. Strict cleanliness requirements apply to the reticles used in photolithography, and to guarantee correct alignment and pattern reproduction both sides of a reticle (pellicle on front side and reticle back side) need to be inspected for contamination. Reticles constitute difficult inspection targets for imaging systems, because their diffractive patterns can redirect light in arbitrary directions which may potentially result in false positive detections. In this paper we introduce a frequency encoded structural illumination system which enables automatic delineation between stray light and particle signal. In our approach, a sinusoidal pattern whose temporal frequency changes with spatial coordinates is used to obliquely illuminate the surface under test. Incoherent illumination is used, and thus the signal recorded by the imaging optics positioned normal to the inspected surface is a superposition of light scattered by particles and stray light redirected by the photolithographic pattern. Because the inspected surface and reticle pattern are from the perspective of an observation system axially separated, the frequency response of each individual detector point is z position dependent. Therefore, Fourier analysis can be used to analyze the frequency content of each pixel individually and by comparing measured with expected frequencies, one can distinguish between light scattered by particles and stray light. The frequency encoding technique can be used in both imaging and scatterometry based systems. Example experimental results are presented. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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