Defining the emitter window is one of the most critical lithographic steps in a BiCMOS process. Step-and-scan exposure
tools are typically the largest component of fixed capital expense so the industry is constantly trying to push the
resolution limit without purchasing new equipment. As the industry extends KrF scanner to 0.13μm BiCMOS process,
the lithography process window of shallow trenches patterns which are typical for emitter windows will be challenged.
Generally speaking, printing these patterns with conventional single exposure methods will suffer from lack of sufficient
process window and severe line-end shortening. In this paper, we present a method that uses KrF scanners and several
resolution enhancement techniques including attenuated phase shift masks (att-PSM), model-based optical proximity
correction (MbOPC), and a multi-focal exposure technique, to improve the process window of 0.13μm emitter window
features.
Characterization results are shown for the process window, side lobe printability margin, and line-end shortening.
Comparisons are made to a traditional exposure method. The results demonstrate a significant increase in depth of focus
as well as improvements in line-end shortening and intra-wafer CD uniformity.
Monte Carlo simulations were performed to delineate the role of local fluence rates and absorption in histologic success and tensile strength analysis of laser welding of ocular corneal tissue using an erbium fiber laser system operating at 1455nm wavelength. Porcine cornea was used for in vitro welding, while varying power, scan time, and irradiance. Immediate histologic analysis was performed, as well as tensile strength studies. Simulations were performed using MCML code, with a total of 109 photons started. CONV code was used to convolve the output from MCML for a flat photon beam of 80-800 μ focal spot size and power specified by the experiment. The absorption coefficient, μa, was assumed to reflect that of water, 28.6 cm-1. The scattering coefficient, μs, and anisotropy factor, g, were both neglected due to the poor scattering capabilities of water in the wavelength of the laser beam. Fluence rates were determined and were within 0.3%-4% of surface dose calculations for a beam diameter of 80 μ. Interactive Data Language (IDL) was used to sum the dose for one convolved beam to an experiment with multiple scans across the porcine cornea. Achieving optimal usage of the laser system requires maximal use of the variables (power, scan patterns, scan time, irradiance) available to use, and the correlation between Monte Carlo-aided dosimetry and the histopathological and tensile strength studies was performed. Optimal parameters for use in this 1455 nm laser system can be studied, and will allow users the ability to predict histology scores of welding success and tissue injury based on absorption values. These results can refine our experience with laser tissue welding of porcine cornea and aid in determining optimal delivered dose for successful tissue apposition and minimal adverse thermal heating.
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