Nanofabrication involves processes and methods for creating structures and devices having minimum dimensions lower than 100 nm. In spite of the large variety of the processes and methods used, they all can be categorised into two main approaches, bottom-up and top-down. In the bottom-up approach, the structures and devices are created from small to large, i.e. assembled from their subcomponents (atoms, molecules or even cells) in an additive fashion. On the other hand, in the top-down approach the fabrication goes from large to small using sculpting or etching to carve nanostructures and devices from a larger piece of material, usually in a subtractive fashion.
The top-down approach is still the prevailing one, being the main workhorse in the micro- and nanoelectronics industry. It relies on two main steps: (i) nanolithography, where a stencil with the required pattern is created in a sacrificial layer called “resist”, deposited on the main working material, and (ii) transfer of the pattern through the resist stencil into the base material.
There are number of nanolithography methods, e.g. photolithography, electron beam lithography (EBL), soft lithography, nanoimprint lithography (NIL), X-ray lithography, ion-beam lithography, scanning probe lithography, directed self-assembly (DSA) of block co-polymers (BCP), etc. Although the photolithography, in particular the deep ultraviolet (DUV) lithography is still the main techniques used for mass production in the semiconductor industry, the EBL is becoming increasingly widespread in research and development (R&D) as well as in small volume production. The main reasons for this are its flexibility and mask-less nature, very high (sub-10 nm) resolution as well as maturity and affordable price of equipment. This is also the main nanolithography method that we at MCAG extensively use.
As the name suggests, an EBL system uses a finely focussed beam of electrons (down to a spot size of about 1-2 nm and even below) to directly irradiate the resist and create the desired pattern. Therefore, it is a mask-less lithography. EBL resists are especially formulated to be sensitive to electrons, which locally alternate their chemistry by either forming bonds between the resist molecules (negative resist) or cleaving the bonds (positive resists). In this way, the exposed areas of a negative resist become insoluble, while those of a positive resist become soluble in the appropriate developer as illustrated in the figure below.
Schematic illustration of development of positive and negative resists after electron beam exposure
In MCAG, we have extensive experience in ultrahigh-resolution patterning of different substrate materials such as silicon (Si), silicon-on-insulator (SOI), germanium (Ge) and germanium-on-insulator (GeOI). We operate the two EBL systems available at the Tyndall National Institute: JEOL JBX 6000FS and Raith e_LiNE plus, and use three positive resists: polymethyl methacrylate (PMMA), ZEP520A, and SML as well as one negative resists: hydrogen silsesquioxane (HSQ). The figures below show scanning electron microscope (SEM) images of some ultrahigh-resolution EBL structures fabricated by MCAG researchers.
Georgiev, Y. M.; Petkov, N. P.; McCarthy, B.; Yu, R.; Djara, V.; O’Connell, D.; Lotty, O.; Nightingale, A. M.; Thamsumet, N.; DeMello, J.; Blake, A.; Das, S.; Holmes, J. D. ‘Fully CMOS-compatible top-down fabrication of sub-50 nm silicon nanowire sensing devices’ Microelectron. Eng. 2014, 118, 47-53.
Gangnaik, A.; Georgiev, Y.; McCarthy, B.; Petkov, Nikolay; Djara, Vladimir; Holmes, J. D. ‘Characterisation of a novel electron beam lithography resist, SML and its comparison to PMMA and ZEP resists’ Microelectron. Eng. 2014, 123, 126-130.3.
Duffy, R.; Shayesteh, M.; Thomas, K.; Pelucchi, E.; Yu, R.; Gangnaik, A.; Georgiev, Y. M.; Carolan, P.; Petkov, N.; Long, B. Holmes, J. D. ‘Access resistance reduction in Ge nanowires and substrates based on non-destructive gas-source dopant in-diffusion’ J. Mater. Chem. C 2014, 2 (43), 9248-9257.
Georgiev, Y. M.; Yu, R.; Petkov, N.; Lotty, O.; Nightingale, A.; de Mello, J. C.; Duffy, R.; Holmes, J. D. ‘Silicon and germanium junctionless nanowire Transistors for Sensing and Digital Electronics Applications’ in Functional Nanomaterials and Devices for Electronics, Sensors and Energy Harvesting Engineering Materials; Springer, Switzerland. 2014, Part IV NanoSensors and MEMS/NEMS, Pg. 367-388