Nanofabrication involves processes and methods for creating structures and devices having minimum dimensions lower than 100 nm. In spite of the large variety of these processes and methods, 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 in a subtractive fashion.
The bottom-up technique is the more recent one of the two approaches and relies to a great extent on self-organisation processes, which still have to be fully understood and properly steered and applied. Bottom-up techniques such as Directed Self-Assembly (DSA) of Block Co-Polymers (BCP), DNA origami, nanosphere lithography, etc. attract lately large interest in research and development (R&D) as a potential application for scaling down semiconductor device patterning.
In contrast to its bottom-up “competitor”, the top-down approach is based on two long-established and well known processes:
(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) Pattern Transfer through the resist stencil into the base material. Therefore, the top-down approach is still the prevailing one, being the main workhorse in the micro- and nanoelectronics industry. Nevertheless, the bottom-up approach is rapidly gaining momentum and there should be no doubt that the future belongs to the smart and inventive combination of the two “competing” approaches.
Since at the Nanofabrication Facility in Rossendorf (NanoFaRo) we mainly use the top-down approach, below we will briefly describe the two basic processes of this technique.
There are a number of nanolithography methods, e.g. deep ultraviolet (DUV) lithography, extreme ultraviolet (EUV) lithography, electron beam lithography (EBL), soft lithography, nanoimprint lithography (NIL), X-ray lithography, ion-beam lithography, scanning probe lithography (SPL), directed self-assembly (DSA) of block co-polymers (BCP), etc. Although the DUV lithography employing the wavelength of 193 nm is still the main technique 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 resolution (down to 5 nm and even below) as well as maturity and affordable price of equipment. This is also the main nanolithography method that is being extensively used at NanoFaRo.
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. Since the pattern is exposed step by step by a usually very small electron beam, EBL is a highly precise but relatively slow technique, which is its main drawback.
EBL resists are especially formulated to be sensitive to electrons, which locally alternate their chemistry by either creating new bonds between the resist molecules forming in this way larger molecules (negative resists) or cleaving the already existing bonds and forming smaller molecules (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.
At NanoFaRo we are accumulating extensive experience in high-resolution patterning (down to 6-7 nm) of the main semiconductor substrate materials such as silicon (Si) and silicon-on-insulator (SOI) for nanoelectronics and photonics applications as well as of a wide range of magnetic materials for spintronics, magnonics, etc. applications. We operate two EBL systems, Raith 150 Two and Raith e_LiNE plus (Raith GmbH), and mainly use three positive resists: polymethyl methacrylate (PMMA) of different molecular weights, ZEP520A, and the copolymer resist (MMA (8.5) MAA) as well as two negative resists: hydrogen silsesquioxane (HSQ) and ma-N 240x. The figure below shows scanning electron microscope (SEM) images of some high-resolution EBL structures fabricated at NanoFaRo.
To transfer the lithographic pattern created in the resist into the base material (the substrate), two main types of processes are usually used: (i) additive processes: doping (ion implantation or other doping techniques), metallisation or other kind of thin film deposition combined with lift-off, etc. and (ii) subtractive processes: wet chemical etching or dry etching.
For thin film deposition we use mainly two evaporation facilities: UHV evaporation tool BETty (BESTEC GmbH) and LAB 500 evaporation tool (Leybold Optics GmbH), both equipped with electron beam and thermal evaporators, as well as a sputter facility NORDIKO 2000 (NORDIKO Ltd.) equipped with 4 magnetrons: 2 operating in DC-mode and 2 operating in DC- or RF-mode. The figures below show sample SEM images of structures fabricated at NanoFaRo using EBL, metal deposition and lift-off.
For dry etching we use two facilities: inductively-coupled plasma (ICP) etching tool SI 591 (SENTECH Instruments GmbH) operating with several gases such as SF6, CF4, C4F8, O2, and Ar as well as reactive ion beam etcher (RIBE) IonSys 500 (Roth & Rau) operating with Ar and CF4 gases. The figures below show sample SEM images of structures fabricated at NanoFaRo using ICP dry etching.
At NanoFaRo we achieved very good results in high-precision contacting of bottom-up synthesized nanostructures (e.g. nanowires, DNA origami, 2D material flakes, etc.) that are randomly distributed on a wafer surface. This technology is very important for the integration of such structures into devices and systems. The figures below show SEM micrographs of randomly distributed nanostructures contacted with a high precision by EBL with a positive resist, metal deposition and lift-off.