Development and Analysis of a Tip-based Mechanical Nano-Manufacturing Process: Nanomilling

As an alternative to well-established nano-lithographic methods, mechanical manufacturing methods which are widely applied in macro and micro-scales, have also been adapted to nano-scale. Recently, tip-based mechanical nanomanufacturing have been realized through the use of Atomic Force Microscopes (AFM) where the ultra-sharp AFM probe is used to induce sufficiently high contact stresses to remove material from variety of surfaces. Even though the basic applicability of the AFM-based mechanical nanomanufacturing have been demonstrated, its application as a viable controlled nanomanufacturing method is hindered by a number of issues including high dimensional uncertainty, limited shape capability, rapid tip wear, and inefficient material removal.

In this Ph.D. research, to address the issues with the current application of AFM-based methods, a new tip-based mechanical nanomanufacturing process, referred to as nanomilling, is proposed. The overarching objective of this Ph.D. research is to develop, implement and analyze a novel nanomanufacturing process, nanomilling, and the associated equipment. The nanomilling process imposes high-frequency rotational motions to the ultra-sharp probe tip (nanotool) using a piezoelectric actuator, achieving a configuration similar to that of the conventional milling process. By imposing controlled highfrequency motions and implementing a high-stiffness nanotool configuration, nanomilling has the potential to exhibit high-dimensional accuracy and repeatability, achieve high material removal rates and effectively reduce tip wear and forces. Development, implementation and analysis of the nanomilling process constitute the foundation for realization of high-volume, controlled tip-based mechanical nanomanufacturing.

To achieve the overarching research objective, first, the nanomilling method and the associated system is developed. Two fundamental nanomilling configurations are identified: in-plane and out-of-plane configurations where the nanotool is rotated within a plane parallel and perpendicular to the sample surface, respectively. The nanomilling system mainly includes a three-axis piezo-stack actuator hosting the nanotool and imposing the highfrequency rotations, a high-stiffness nanotool assembly and a high-precision nanopositioning stage that controls the feeding motions to create desired feature shapes during nanomilling.

Successful implementation of the nanomilling process requires precision characterization and control of the high-frequency nanotool rotations and feeding motions. To characterize the motions of the piezo-stack actuator and the nanopositioning stage, a three-dimensional motion measurement setup is constructed. To generate accurate nanotool rotations, a comprehensive method is developed for characterization and mathematical representation of non-linear dynamics of piezo-stack actuators. The obtained dynamic representations are then utilized to devise a frequency-domain open-loop method to control the piezo-stack actuator motions. The capability of this method to generate desired nanotool rotations with high accuracy is demonstrated.

Having developed the nanomilling method, the associated system and established the methodology for precision control of nanomilling motions, the dimensional accuracy of the process is evaluated. It was shown that the removal depth and width can be controlled with 5 nm accuracy using the nanomilling process. The preliminary observations on material removal mechanism showed that nanomilling process is able to remove material in the form of long and curled chips.

Finally, the preliminary studies are conducted on the nanotool wear characteristics of the process. A method for experimentation and quantitative analysis of nanotool wear is devised.

Specific contributions of this thesis research include: (1) A novel rotating-tip-based mechanical nanomanufacturing method and associated system that is capable of creating nanoscale features with 5 nm dimensional accuracy, (2) A new method for accurate characterization and representation of three-dimensional dynamic motions of piezo-stack actuators, including those used in the nanomilling system, (3) An experimental understanding on the characteristics of the non-linear dynamic motions of piezo-stack actuators, (4) A frequency-domain-based open-loop method for controlling the non-linear, three-dimensional motions of multi-axis piezo-stack actuators, (5) An approach for quantitative analysis of wear in nanometrically-sharp probe-tips.