UC Berkeley

Photoresist lithography is a critical step in producing components for high-density data storage and high-speed information processing, as well as in the fabrication of many novel micro and nanoscale devices. With potential applications in next generation nanolithography, the chemistry of a high resolution photoresist material, hydrogen silsesquioxane (HSQ), is studied with two different state-of-the-art, chemically selective microscope systems. Broadband coherent anti-Stokes Raman scattering (CARS) micro-spectroscopy and scanning transmission X­ray microscopy (STXM) reveal the rate of the photoinduced HSQ cross-linking, providing insight into the reaction order, possible mechanisms and species involved in the reactions.

Near infrared (NIR) multiphoton absorption polymerization (MAP) is a relatively new technique for producing sub-diffraction limited structures in photoresists, and in this work it is utilized in HSQ for the first time. By monitoring changes in the characteristic Raman active modes over time with ~500 ms time resolution, broadband CARS micro-spectroscopy provides real-time, in situ measurements of the reaction rate as the HSQ thin films transform to a glass-like network (cross-linked) structure under the focused, pulsed NIR irradiation. The effect of laser power and temporal dispersion (chirp) on the cross-linking rate are studied in detail, revealing that the process is highly non-linear in the peak power of the laser pulses, requiring ~6 photons (on average) to induce each cross-linking event at high laser power, which opens the possibility for high resolution MAP lithography of HSQ. Reducing the peak power of the laser pulses, by reducing average laser power or increasing the chirp, allows fine control of the HSQ cross-linking rate and effective halting of the cross-linking reaction when desired, such that broadband CARS spectra can also be obtained without altering the material.

Direct-write X­ray lithography of HSQ and subsequent high resolution STXM imaging of line patterns reveals a dose and thickness dependent spread in the cross-linking reaction of greater than 70 nm from the exposed regions for 300 nm to 500 nm thick HSQ films. This spread leads to proximity effects such as area dependent exposure sensitivity. Possible mechanisms responsible for the reaction spread are presented in the context of previously reported results. X­ray lithography and imaging is also used to assess the X­ray induced cross-linking rate, and similarities are observed between NIR MAP and X­ray induced network formation of HSQ.