Continuous Exposure Over Active Area Topography
Below we show the importance of the independently-controlled force and current feedback loops in hybrid AFM / STM lithography. The top graph shows polysilicon topography across the field-to-active transition. Also shown is the topography of the resist after it was spun on top of the polysilicon. The resist thickness variation over the transistor topography is shown below. The resist--which is nominally 65-nm thick when spun on a flat sample--changes abruptly from less than 30 nm to more than 90 nm. The voltage and field-emitted current during exposure are plotted in the bottom graph. The voltage required to maintain 0.05 nA (50 pA) ranges from 22 V to 81 V. The current feedback maintains a constant exposure dose of electrons independent of the resist thickness.