Difference between revisions of "A Review of Tip-based Nanofabrication"

Revision as of 16:31, 20 August 2018

Huan Hu

ZJU-UIUC Joint Institute, International Campus, Zhejiang University, Haining, Zhejiang 314400, China.

College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China

1 Introduction

Nanotechnology is revolutionizing many crucial technologies in our society such as faster and smaller transistors for more powerful circuits, faster and lower-cost DNA sequencing, early disease diagnosis, higher-efficiency energy conversion, and storage, etc. All these fantastic technologies rely upon the capability of fabricating these nanostructures, nano-devices, and nano-systems. For example, semiconductor manufacturing is powered by high-volume processing lithography method using light with smaller and smaller wavelength to produce billions of transistors. This type of lithography technology also demands accompanying technology for flexible low-volume production, mask fabrication and prototyping of the next-generation devices. The dominant mask production is to use the electron-beam lithography. In parallel to the beam-based nanofabrication technology, tip-based nanofabrication (TBN) technology is also receiving renewed interests because of their flexibility of handling novel materials and their capability of reaching atomic scale. Tip-based nanofabrication is a nanofabrication technology that uses a nanoscale tip to interact with a substrate for fabricating nanometer-scale patterns. It can be categorized based on the platform it is implemented: atomic force microscope (AFM) or scanning tunneling microscope (STM).

2 History

Tip-based nanofabrication (TBN) traces back to the invention of the scanning tunneling microscope (STM) by two IBM scientists. In STM, a sharp metal tip with several nanometers dimensions approaches the substrate, while monitoring the tunneling constant between the tip and the substrate. The tunneling current increases when the tip approaches the substrate. By controlling a constant tunneling current by moving the tip up and down when scanning along a surface, the distance between the tip and the substrate can be kept constant. Therefore, the movement of the tip at each spot can represent the topography of the sample. Accidentally, when scanning a sample, contaminations can be observed on the substrate, which leads to the idea of fabricating nanostructures using this nanoscale tip and eventually to this concept of TBN. Later, silicon microcantilever with a tip was used and tip-substrate interaction forces were used to control the distance between the tip and the substrate, the atomic force microscope was invented which applies not only to conductive surfaces but also to non-conductive surfaces, extending the applications to biology and chemistry.

3 Technology

Based on the platform the TBN technology is implemented, it can be categorized into two types：AFM-based and STM-based.

3.1 AFM-based

This type of TBN technology uses the AFM as the implementation platform. The tip is normally mounted on the free end of a microcantilever. A laser beam is incident upon the backside of a microcantilever and reflected back to an optical quadruple for measuring the deflection of the microcantilever. The deflection feedback helps the control circuits to keep the tip in close contact with the substrate with nanometer resolution in the z-axis. According to nanopattern formation mechanisms, it can be categorized into the material transfer, material change, and material removal. Figure 1 shows the major TBN technology using AFM as the implementation platform. In material transfer scheme, the material is delivered to the substrate through the tip. It can be through the water meniscus naturally formed between the tip and the substrate by diffusion [1] or can be through thermally-induced surface tension [2]. In material change type, the nanopatterns come directly from the chemical change of the substrate induced by the tip. It is either through electrochemical reaction taking place within the water meniscus [3]or due to the photons surrounding the tip causing photochemical reaction [4]. In material removal type, the material is removed from the substrate to form the nanostructures. The material removal can be initialized by scratching the tip across the substrate, using mechanical force to cause material removal from the substrate [5, 6]. Or it can be caused by an AFM tip locally heating up the resist to sublimation [7, 8]. Or it can be a heated tip to cause local thermal reflow of the resist [9]. IBM demonstrated a storage technology based on a thermal tip array that reaches the storage density of 200 Gb/inch</math>^{2}</math> in early 2000 [9].