Tip-directed Field-emission Assisted Nanofabrication (TFAN)|
Precise and fast manipulation of surfaces using scanning tunneling microscopy (STM) principles.
Goal: To fabricate nano-scale features from conductor, semi-conductor or insulator materials on a conductive or semi-conductive surface, with high precision. An STM is utilized to enable precise fabrication using its probe tip and controlling the delivered energy. STM is used both to image the surface and to interact with it.
Approach: An STM is used to image the surface of interest in high resolution. Short electrical pulses delivered from STM end-effector are used to modify surfaces and fabricate user-defined patterns on the surface. This writing principle is straight-forward and is proven to work on conductive and semi-conductive surfaces prior to this study.
In order to achieve high precision required for the fabrication, we will be building our custom control electronics and software to automate the fabrication procedure that is done by a commercially available ultra high vacuum (UHV) STM.
The displacements that will be needed from the positioning system are on the order of a few nanometers; therefore, positioning sensors and feedback control methods cannot be used for reliable positioning. Another problem with precision positioning comes from the nonlinear nature of actuators used for nanometer scale positioning, which makes it harder to achieve repeatable high precision positioning. In addition to these problems, STM based manipulation is a serial process, which generates speed issues.
To solve the precision problem, we will model the positioning system accurately and use nonlinear open-loop or feed-forward control techniques to achieve repeatable, high precision positioning. Also, we will automate the positioning system; hence keep the operator out of the loop. This will enable us to do fabrication in a faster and more reliable manner, without human error. Another advantage of the custom control system will be the flexibility, which will enable our research group to investigate the surface conditions and nano-scale fabrication with unlikely scenarios. We will also be utilizing AFM cantilevers as STM end-effectors in order to enable the possibility of parallel writing using a cantilever array, in the future to solve the speed issues.
We have performed several experiments to show the feasibility of the writing procedure and seen that fabrication using the proposed principles is viable.
We are currently building a custom control system for our STM to achieve flexibility.
We have also modeled the tip-sample interaction, cantilever dynamics and STM operation extensively to gather an understanding of the physics and the interaction of the tip and the sample at these small scales.
Benefits: STM based fabrication has comparable (if not finer) size resolution compared to cutting-edge fabrication methods that are more expensive than an STM. However STM suffers from lack of speed and precision hence cannot replace its more expensive alternatives. This work aims to tackle these issues which would enable STM based fabrication a viable alternative for nanoscale fabrication.
This work is supported primarily by DARPA Tip-Based Nanofabrication Program and is done in collaboration with Profs. Ricketts (ECE), Bain (ECE), Carley(ECE), Davis (Material Sci.), Fedder (ECE), and Islam (Material Sci.) at CMU. For TFAN webpage: link
|[Schematic of the proposed fabrication process of nanostructures]
|[Schematic of the parallel-manipulator-type scan head used in Scanning Probe Microscope]