Electron Dose Calibration and PMMA Resist Characterization Using a Transmission Electron Microscope

Understanding the electron-dose response of resist materials is essential for precision control in nanoscale lithography. We studied the behavior of thick PMMA resist from electron exposure using a transmission electron microscope (TEM). A8 950 PMMA was spun at 5000~rpm onto 3.5~$\times$~3.5~mm silicon chips containing 150~$\mu$m windows, yielding $\approx$600~nm thick films optimized for adhesion and uniformity. Ten circular exposures, each 10~$\mu$m in diameter, were written in a line with 30~$\mu$m center-to-center spacing, spanning a calibrated dose range from $\sim$200 to 1000~$\mu$C/cm$^{2}$ based on measured beam current and illumination area. In this run, exposures were limited to beam diameters $\geq$3~$\mu$m due to procedural choices: dwell times were restricted to $\geq$1~s and the screen-current readout resolution was 1~pA. After exposure, the development strength and time were progressively increased, beginning with 15~s in 100\% isopropanol and extending to 90~s in a 1:3 MIBK:IPA solution. This series revealed a clear dose-dependent transition from intact to fully developed resist. Underdeveloped central regions resembling shallow discs were visible within the exposed areas, possibly arising from internal electron scattering in the thick PMMA or from a nonuniform beam intensity profile. Subsequent PVD deposition of 50~\AA~Ti / 300~\AA~Au followed by liftoff verified that the exposed areas were cleanly removed, confirming successful pattern definition and metal transfer.