Bryan K. Choo

Improving stigmation control of the CD-SEM

Production fads currently rely on CD-SEM metrology for linewidth control in lithography and etch. The quality of the measurement data is therefore directly tied to the quality of the SEM imaging and that of the electron beam. Experience has shown that even within a 12 hour period, the beam alignment can drift sufficiently to cause a shift of several nanometers in the measured CD. Furthermore, the alignment of the electron beam has traditionally been performed manually, with the quality of the SEM image then being dependent on the judgement of the operator. While this has been adequate in the past, the drive towards ever smaller geometries means that even slight changes in the beam spot can lead to unacceptable variation in the CD measurements. In this paper, we will describe how a more consistent electron beam can be achieved by obtaining quantitative feedback on its sharpness and eccentricity. Using this quantitative information removes the subjective nature of the manual beam alignment and requires less training on part of the users. Furthermore, this procedure results in beam conditions that are at least as good as the alignment performed by an experienced operator, and in fact generally improves the alignment when compared to that obtained by subjective judgement of the image quality. Once a baseline is established, SPC charts for the sharpness and eccentricity values can be used to track tool performance. If either value falls out of a specified range, the system can be flagged as having a problem and efforts to restore the image resolution can begin immediately, reducing the risk of erroneous measurements and thus preventing lots from being misprocessed or mistakenly reworked.

Determination of lithography process control metrics by spectroscopic scatterometry

Spectroscopic Scatterometry was employed for definition of control metrics for lithography process tool optimization. Normal incidence unpolarized reflectance data was acquired in active device regions on resist gratings that defined the first gate level for the core array of a flash memory circuit. Calculations of the interactions of incident boradband light with the scattering structures were perforemd utilizing both a database of pre-computed spectra as well as a real-time method. Results from both inversion techniques were highly correlated. Scatterometry-based CDs were also highly correlated with dimensions determiend by CD-SEM and the modeled profiles clsoely matched cross section SEM data. Test wafer sets were patterned both at uniform exposure and as focus-exposure matrices, and measured to determine resist critical dimensions and thicknesses. Process variations were tracked across fields, across wafers and over time. Analytical models were applied to the profile data to determine rpocess windows and to define optimal scanner settings.