Photomask metrology using a 193nm scatterfield microscope

Abstract

The current photomask linewidth Standard Reference Material (SRM) supplied by the National Institute of Standards and Technology (NIST), SRM 2059, is the fifth generation of such standards for mask metrology. The calibration of this mask has been usually done using an in house NIST ultra-violet transmission microscope and an Atomic Force Microscope (AFM). Recently, a new optical reflection scatterfield microscope has been developed at NIST for wafer inspection, Critical Dimension (CD) and overlay metrology purposes. Scatterfield microscopy relies on illumination engineering at a sufficiently large Conjugate Back Focal Plane (CBFP) of the microscope. 1 Our new scatterfield reflection microscope uses 193 nm excimer laser light as well as sophisticated configurations to allow measurement of both the image plane and the Fourier plane using full-field and angle-resolved illumination. By reducing the wavelength compared to many current metrology tools that work in the visible light and near ultra-violet range, we have made substantial improvements in image resolution 2 and commensurate gains in sensitivity to geometrical parameters. We present a preliminary study on the use of this new microscope to calibrate and measure features of this SRM photomask. The 193 nm scatterfield microscope is used in full-field mode with a NA range from 0.12 to 0.74 using our scatterfield imaging method. Experimental results obtained on isolated lines for different polarization states of the illumination are presented and discussed. Pitch measurements are compared to the measurements done on our NIST Ultra-Violet (UV) transmission microscope.

Appraising the Extensibility of Optics-Based Metrology for Emerging Materials

“…Prior to the introduction of µDBO, our group proposed and realized what we termed "scatterfield microscopy" (3,4,(24)(25)(26), an approach to microscopy that combines sophisticated illumination engineering with the optimized collection of information from the full 3-D electromagnetic scattered field about targets of interest. Imaging these targets permits spatial localization of a region of interest (ROI) smaller than the scatterometric spot size, and several targets can be imaged within the field of view of the microscope without degrading the dimensional measurements (27). Our group has also yielded imagebased measurements of overlay using finite sets of arrayed lines (3) and has also concentrated on the measurement of line width, often referred to in nanoelectronics as the "critical dimension" (CD) (28).…”