22 August 2001 CD measurement of re-entry (overhang) obtained by liftoff techniques in 6-in. GaAs IC process
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In a 6 inch GaAs IC fabrication process, a resist liftoff technique was employed to provide definition of features in evaporated metal. The re-entry (overhang) of the resist profile for liftoff, defined by the differences between the top and bottom of the openings, is very critical; in order to protect the defined areas from residual edge metal deposition it is important that the re-entrant profile maintains sufficient overhang to shadow the metal deposition below the edges of the top opening. It is therefore desirable to get the critical dimension (CD) differences of the re-entry measured so that one can monitor the performance of the process form wafer to wafer/lot to lot instead of inspecting irregular wafer samples using the destructive cross section SEM. A methodology was established for this purpose, using a fully automated IVS120 optical CD system, with a study on the application of the focus, measurement algorithms provided by the CD tool. In this work, we resist the profiles with about 0.1 ~ 0.25 micron re-entry at each side were studied and a calibration performed on the optical CD system using cross section SEM as a reference tool. Gauge repeatability and reproducibility was studied before release the method to production line.
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Ying Liu, Iain Black, "CD measurement of re-entry (overhang) obtained by liftoff techniques in 6-in. GaAs IC process", Proc. SPIE 4344, Metrology, Inspection, and Process Control for Microlithography XV, (22 August 2001); doi: 10.1117/12.436804; https://doi.org/10.1117/12.436804

Semiconducting wafers

Critical dimension metrology

Scanning electron microscopy


Photoresist processing

Gallium arsenide

Image processing

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