RESOLUTION IMPROVEMENT OF PHOTOLITHOGRAPHIC TECHNIQUES BASED ON VISIBLE LIGHT
Fourkas, John T
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The semiconductor industry is planning to use Extreme Ultraviolet lithography as its next-generation patterning technique. However, this technique has run into many roadblocks due to its cost and complexity. An alternative approach employs light in the near-UV. A 2-color photolithographic technique based on combination of two colors on the near-UV or visible light has shown promising results in creating structures with sizes at a fraction of the excitation light wavelength. One color of light excites photoinitiator molecules to a chemically active state that leads initiation of polymerization. A second color of light deactivates photoinitiator molecules before they form radicals, inhibiting polymerization. In this thesis we show how extending 2-color lithography to include a third color (3CL) can achieve super-resolution for applications requiring fabrication of closely packed structures. The advantage of the 3CL process is in its separation of polymerization initiation and deactivation steps by involving different chemical states that allow for more efficient deactivation and for increased resolution. Some of the crucial elements needed to achieve an optimized scheme for 3CL are the determination of the intramolecular transitions that participate in the process, the lifetimes of the photoinitiators, and the exposure parameters. Several photoinitiators were studied to determine the optimal exposure conditions. Polymerization action spectra and deactivation action spectra were used to determine the combinations of excitation and deactivation parameters resulting in the most efficient deactivation. The 2-beam initiation threshold (2-BIT) method was introduced for in situ measurement of the order of eective nonlinearity of photoresists. The order of the effective nonlinearity was determined for a series of photoinitiators under various excitation wavelengths and fabrication velocities. Additionally, a photoinitiator with a proportional velocity (PROVE) dependence, in which feature size increases with the velocity, was found to undergo efficient self-deactivation at increased temperatures. This dependence was demonstrated by gradually heating the sample and analyzing the fabricated feature sizes. Spot heating with a laser beam was also used to locally prevent polymerization. The correlation between polymerization rate and temperature opens opportunities for high speed fabrication that uses temperature gradients to create finer structures.