LDCN is involved in a fundamental research project being performed in collaboration with Zyvex Labs (Richardson, TX) and funded by DARPA as part of their Atoms to Product (A2P) Program. One of the aims of this project is to develop the ability to manufacture nanoscale elements with atomic-scale engineering tolerance. Toward this end, a Scanning Tunneling Microscope (STM) is used in the nanolithography process. A tunneling current causes a chemical reaction with designated atoms on a hydrogen-passivated silicon surface, which removes the hydrogen and leaves a silicon-patterned surface. This pattern is then reproduced by the deposition of additional layers of silicon. The final outcome of the process will be a nanoscale structure which is built atom-by-atom, and thus, is atomically precise.

The STM is a member of the broader family of Scanning Probe Microscopes (SPMs), which uses a probing tip to collect nanoscale topography data through a physical phenomenon that arises between the tip and sample. In STM this phenomenon is a quantum mechanics effect known as tunneling current, which refers to the electrical current established between a conducting tip and a sample under a DC bias voltage when their relative distance drops below one nanometer. The tunneling current is exponentially dependent on the tip-sample gap, thus, it maps the surface topography of the sample. In addition to generating STM topography images, the tunneling current can be used to trigger chemical reactions at the surface during the nanolithography process, which makes the STM an ideal tool to be used to achieve the objectives of the DARPA A2P program.

The STM requires a control system that measures the tunneling current and adjusts the tip-sample distance to keep the current constant. Conventionally, a Proportional-Integral (PI) controller is used for this purpose. However, due to the complexities of the tunneling physics and many other uncertainties in the system, in certain conditions the controller can lose performance and cause the tip and sample to touch, causing irreversible damage to both. A tip-sample crash is therefore one of the main problems in STM operation. This problem is more severe in the nanolithography application, which needs an unchanging tip shape. The role of LDCN in this project is to design an ultra-high-precision control system for the STM that facilitates the stable operation of the STM in nanolithography and prevents tip-sample crashes. To this end, system identification techniques along with advanced control synthesis methods are being customized and designed to address the STM control challenges. 

This concept of nanolithography for atomically precise manufacturing is demonstrated via this STM image of a hydrogen-passivated silicon sample with vertical lines showing dimer rows. STM-based nanolithography is performed to remove hydrogen atoms and leave a pattern. Photo credit Zyvex Labs.