Indirect Measurement Methods for Quality and Process Control in Nanomanufacturing

Rapid advancement over the past decades in nanomanufacturing has led to the realization of a broad range of nanostructures such as nanoparticles, nanotubes, and nanowires. The unique mechanical, chemical, and electrical properties of these nanostructures have made them increasingly desired as key components in industrial and commercial applications. As the geometric dimension of nano-manufactured products is on the sub-micron to nanometer scale, different mechanisms and effects are involved in the nanomanufacturing process as compared to those for macro-scale manufacturing. Although direct measurement methods using atomic force microscopy and electron beam microscopy can determine the dimensions of the nano structure with high accuracy, these methods are not suited for online process control and quality assurance. In comparison, indirect measurement methods analyze in-process parameters as the basis for inferring the dimensional variations in the nano products, thereby enabling online feedback for process control and quality assurance. This paper provides a comprehensive review of relevant indirect measurement methods, starting with their respective working principles, and subsequently discussing their characteristics and applications in terms of two different approaches: data-based and physics-based methods. Relevant mathematical and physics models for each of the methods are summarized, together with the associated effect of key process parameters on the quality of the final product. Based on the comprehensive literature conducted, it was found that: (1) indirect measurement, especially the data-based method, plays a critical role when it comes to online process control and quality assurance in nanomanufacturing, because of the short processing time compared to the direct method, and (2) physics-based method is providing a way to optimize the process set up for desired geometrical dimensions.