Rough masks naturally create speckle at the wafer plane of an EUV optical tool. The speckle can be used to estimate the transfer function of the imaging system (optical aberrations) if the aerial image can be recorded on a camera. This is not directly feasible in a micro exposure tool since the aerial image is captured by the photoresist instead. We propose using the latent image in the resist to estimate the aerial image intensity of speckle and consequently recover tool aberrations.
The latent image is measured on both PMMA and CAR resists to capture the speckle created by the mask roughness. The dose calibration tool at the Lawrence Berkeley National Lab is used to expose the resist, and an atomic force microscope (AFM) measures its surface before the bake. In the the linear regime of the resist, the shape of the resist surface (or the latent resist image) before bake or develop is proportional to aerial image intensity, for low values of the incoming dose. For larger values, the acid that shrinks the resist polymer will saturate, and hence the resist is no longer linear in the image intensity. However since at EUV wavelengths most materials are weakly scattering, the speckle created by the mask roughness at atomic scales has low variance and the latent image is linear in the speckle intensity. Additionally, since the aerial image is sampled by the resist as non-uniform grains instead of the periodic pixels of a CCD camera, there is a natural antialiasing in the captured spectral on resist when compared with similar measurements on a camera in an aerial imaging tool.
Since the speckle scatters into the whole pupil of the imaging system, it encodes the optical aberrations in its power spectrum. The aberrations are consequently recovered in a sparse Zernike basis using a contrast transfer formulation for weakly scattering object . Aberration characterization typically requires special test masks to measure the point spread function of the imaging system, which is time consuming and expensive. Our method promises to be an in situ aberration measurement technique for EUV exposure tools, since speckle from any blank area of the mask can be used to characterize field of view dependent aberrations of the imaging optics in a single shot.
Aamod Shanker, Patrick Naulleau, and Laura Waller, "EUV exposure tool aberration estimation using latent resist image of mask roughness (Conference Presentation)," Proc. SPIE 10583, Extreme Ultraviolet (EUV) Lithography IX, 105830T (Presented at SPIE Advanced Lithography: February 28, 2018; Published: 19 March 2018); https://doi.org/10.1117/12.2299605.5754439008001.
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