There are two main approaches used in the fabrication of nanomaterials. These are top-down and bottom-up. Bottom-up involves the deposition of atoms to build up a molecular structure from scratch. On the other hand, top-down fabrication methods take bulk materials and ‘chip away’ at the material to form a desired nanopattern on the material. Both method classes have their own merits and their own uses. In this article, we’re focusing on the many top-down methods that are available to nanoscientists for when they want to create a nanomaterial with a desired pattern and/or introduce a certain topography at the nanoscale.
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Etching is a way of removing a material through either chemical or physical means. There are many ways etching of the surface a material can be performed, but in the world of nanoscale etching, reactive ion etching (RIE) and deep reactive ion etching (DRIE) are two of the most commonly used methods.
Reactive Ion Etching (RIE)
Reactive ion etching is a form of dry etching, i.e., a method that uses a bombardment of ions in the form a plasma to etch the nanomaterial, rather than a liquid etchant. RIE is performed in a vacuum environment, where the combination of low pressure, an applied electronic field, and radiofrequency waves (RF) highly ionizes gas molecules so that they form a plasma. The removed electrons from the molecules within the plasma then move up and down the etching chamber and many eventually end up on the surface of the nanomaterial. It should be noted that the platform that the nanomaterial sits on is not affected by the applied electric field, leading to a DC isolation within the system. This leads to a highly positively charged plasma and a highly negatively charged substrate within the vacuum chamber. This causes some of the lighter ions within the plasma to accelerate towards the negatively charged nanomaterial, and the collision causes the nanomaterial to be etched through kinetic and chemical interactions.
Deep Reactive Ion Etching (DRIE)
Despite the similar name, the differences between deep reactive ion etching (DRIE) and RIE are very clear. Whilst DRIE is essentially a more penetrative form of RIE, it does employ some slight modes of operation and is a much faster etching process than conventional RIE.
In DRIE, a gaseous plasma is still formed, but this time it is a high-density plasma which is generated. The plasma is generated in the same way—under low pressure, RF waves, etc.—however, a higher density plasma is generated because the RF antenna and the plasma inductively couple to each other. This also leads to the generation of an alternating RF magnetic field around the plasma, which in turn induces RF electric fields. This highly ionizes the plasma. However, in conventional RIE, many of the heavier ions are lost to the wall of the vacuum, but the coupled nature of the plasma in DRIE means that hardly any heavy ions are lost from the plasma—hence the higher density. In DRIE, the plate which the nanomaterial sits on is this time connected to a separate RF power source which induces a DC bias between the plasma and the nanomaterial. Because there is a higher density of ions within the plasma to collide with the nanomaterial, deeper etches can be created.
Lithography is a popular technique to remove portions of a nanomaterial and create a pre-determined pattern the surface of a nanomaterial. When nanomaterials are specifically involved, the techniques are often referred to as nanolithography methods.
Regardless of the nomenclature used, an energy-intensive process of some descript—which varies from technique to technique—is used to pattern the material into the desired form. However, because the plan is to form a specific geometry or pattern, it is often the case that some of the material needs to be protected—although this is not the case for all lithography methods. Without this protection, all the material is susceptible to being removed. If a material needs to be protected, a mask is used, and the mask contains certain properties (properties relevant to protect the material for that specific technique) that prevents the material under the mask from being removed.
There are in fact many different types of lithography methods which we will look at below. However, the focus is on the three main types of lithography, namely photolithography, electron beam lithography (EBL), and Nanoimprint Lithography (NIL). Other types of nanolithography methods which are less common include multiphoton lithography and scanning probe lithography (SPL).
Photolithography, otherwise known as optical lithography, is one of the oldest lithographic methods but is still used today. In photolithography, a combination of light, etching, a photomask, and a resist material are used. Photolithography works by depositing a light-sensitive material on top of the nanomaterial, called a photoresist, followed by etching. The mask is usually patterned with holes so that when the light beam hits the mask, the areas which are not covered will be ablated and the areas which are covered will be protected; thus, the material underneath the mask adopts the same geometric pattern as the mask.
The process of patterning a nanomaterial involves many stages, which starts off by cleaning the surface of the nanomaterial and depositing the photoresist. This is followed up with heating it so that any moisture is removed from the photoresist layer. Light is then focused on the surface, and the areas below the ‘solid’ parts of the mask are protected, but the exposed photoresist is removed. The nanomaterial below the ablated photoresist is then chemically etched to form the desired pattern. The remaining photoresist is then removed to leave the patterned nanomaterial.
Electron Beam Lithography (EBL)
Electron beam lithography (EBL) works like photolithography in many ways but involves the patterning of a surface with metal contacts. In EBL, the focused beam is a beam of electrons, and the resist material is an electron-sensitive material. EBL is, in fact, a direct write technique, so a mask is not required. The beam can be focused to a more localized area than light can, and only the parts of the resist which encounter the electron beam will have their chemical make-up changed. The reaction between the electron beam and the resist causes those areas to be dissolvable in certain solvents. These areas can then be removed using specific chemicals, which exposes certain regions of the nanomaterial, and leaves the untouched areas of the resist on the surface of the nanomaterial. A metal is deposited across the top of the surface of the nanomaterial/resist, and as the remaining resist is removed via aggressive chemical reactions, the metal attached to the resist is also lifted off. The areas directly attached to the nanomaterial are left alone during this process, and the surface of the nanomaterial becomes patterned with metal contacts.
Nanoimprint Lithography (NIL)
Whilst nanoimprint lithography (NIL) is a subset of lithography, there are in fact many types of NIL. Overall, NIL is a technique that patterns nanomaterials using mechanical deformation of the resist. Given that the resist must be susceptible to mechanical deformation, polymers are often cured or molded to the surface of the nanomaterial. The two main types of NIL are UV-NIL and thermal NIL, and in each of these methods, the resist material is cured by UV radiation or is subject to thermal treatments, respectively.
The most common type of NIL is thermal NIL, so we’re going to focus on that. Once a layer of resist material has been coated onto the surface of a nanomaterial, it is heated above its glass transition temperature, so that it becomes malleable. A mold with a predetermined pattern is then brought into contact with the polymer, and the soft polymer fills the mold and adopts the same shape. Once the polymer is cooled down, it stays in the predetermined pattern, and the mold is removed, leaving the patterned polymer on the surface of the nanomaterial.
Sources and Further Reading:
- Melbourne Centre for Nanofabrication: http://nanomelbourne.com/services/etching/
- London Centre for Nanotechnology: https://www.london-nano.com/research-and-facilities/themes/techniques/nanolithography
- Lithoguru: http://www.lithoguru.com/scientist/lithobasics.html
- Minnesota Nano Center: https://www.mnc.umn.edu/sites/g/files/pua586/f/photolithography_graphical_material_v4.pdf
- Sheffield University: https://www.sheffield.ac.uk/ebl/patterning
- Friedrich-Alexander-Universität Erlangen-Nürnberg: http://www.optik.uni-erlangen.de/odem/download/v43/v43_electron_beam_lithography.pdf
- NIL Technology: https://www.nilt.com/523/nanoimprint-lithography
- Corial: https://www.corial.com/en/technologies/reactive-ion-etching-rie/ https://www.corial.com/en/technologies/drie-deep-reactive-ion-etching/
- Oxford Instruments: https://plasma.oxinst.com/campaigns/technology/reactive-ion-etching