Researchers at the University of Texas at Dallas have solved a longstanding problem that has been plaguing the scanning tunneling microscope for more than 35 years: How to prevent the tip of a scanning tunneling microscope from crashing into the surface of a material during imaging or lithography. Scanning tunneling microscopes (STMs) operate in an ultra-high vacuum, bringing a fine-tipped probe with a single atom at its apex very close to the surface of a sample. When voltage is applied to the surface, electrons can jump, or tunnel, across the gap between the tip and sample.
The tip can be thought of as a needle that is atomically sharp. The microscope is like a robotic arm, able to reach atoms on the sample surface and manipulate them. The problem is, sometimes the tungsten tip crashes into the sample. If it physically touches the surface, it may inadvertently rearrange the atoms, or create a “crater,” which could damage the sample. Such a “tip crash” often forces operators to replace the tip many times, forfeiting valuable time.
Presently, a feedback controller in the STM measures the current between the microscope and the sample and moves the needle up and down, positioning the microscope from one atom to another across an uneven surface. Because of that, the distance between the sample and tip changes along with the current. While the controller tries to move the tip up and down to maintain a constant current, it does not always respond well, nor does it always regulate the tip correctly. The resulting movement of the tip is therefore often unstable. The problem is that the feedback controller fails to protect the tip from crashing into the surface. When the electronic properties vary across the sample surface, the tip is more prone to crash under conventional control systems. When the tip crashes into the sample, it breaks, curls backward, and flattens.
The solution developed by the researchers is based on the physics of the tunneling between the tip and the surface — there is a small electronic barrier that controls the rate of tunneling. They figured out a way of measuring that local barrier height and adjusting the gain on the control system accordingly. The new system adjusts the control parameters on the fly to compensate for the erratic behavior of the tip.
The goal of this project is to pave the way for atomically precise manufacturing, which is the way forward for nanotechnology. Building structures atom by atom, will enable the creation of innovative new materials.