In recognition of the importance of graphene, the new wonder material that is ultra-light and flexible yet 200 times stronger than steel, Nikon Metrology focuses on providing inspection solutions for research of these new industrial materials and its application areas. The purpose of this document is to advise on excellent, proven methods for optically examining and measuring features on graphene in its raw and processed states.
Bill Clement, Business Development Director at Nikon Metrology, has 28 years of experience in microscopy and coordinates the activities of a team of experienced staff and engineers across Europe. Their remit includes supporting research and development into the inspection of innovative materials and assisting in the realisation of components and products utilising these new materials.
Graphene is currently one of the most prominent of these. The disruptive technology is extremely important, as it has the potential to revolutionise a host of applications from water filtration and medical engineering through electronics and optics to energy generation and lighting. A visit to the UK’s University of Manchester website www.graphene.manchester.ac.uk, where the crystalline material was first extracted from graphite, provides a comprehensive background to the material’s discovery and uses. It also gives an insight into example graphene applications. Inventions include its use in long-lasting light bulbs that are about to be marketed, mobile phones that can theoretically be bent double, and specialised infill material to realise performance-enhancing carbon fibre tennis racquets and golf club shafts.
Producers of the raw material, developers of products containing graphene, and researchers in industry and academe will all benefit from the coherent strategy that Nikon Metrology is putting forward. It brings together the company’s established and recently developed products that are suitable for non-contact inspection of the material and is coupled with a new advisory service to help optimise their utilisation.
Mr Clement said, “The graphene community is amazingly open, with knowledge shared readily between experts and practitioners in the field.
We are modelling our graphene advisory service on the same spirit of openness and if a Nikon Metrology product is not the best solution for a particular task, we will say so.
However, we have a wealth of different equipment for optical inspection, selection and quality control of various forms of chemically, epitaxially and micromechanically synthesised graphene sheet, layers and printed structures, from the Nano metric range up to typically one hundred microns thick. I should like to outline in this article what we are able to provide.”
The first pillar of Nikon Metrology’s offering is its SMZ series of stereomicroscopes equipped with a digital camera such as the Nikon DS-Ri2 or DS-fi2 with L3 or U3 controllers for capturing and archiving images. Several are in use at the University of Manchester for initial selection of suitable graphite flakes from which to isolate graphene. The British Government’s Chancellor of the Exchequer, George Osborne, visited the university examining samples through one of the instruments at the official opening of the National Graphene Institute on 20th March 2015. The research centre enables academics and industry to work side-by-side on graphene applications of the future.
The SMZ stereo microscope range, with its industry-leading 25:1 zoom, is ideal for many tasks including initial grading and sorting of graphite samples owing to the instrument’s long working distance and wide field of view. Combined with the stereoscopic image, they enable the sample’s texture and shape to be understood and then manipulated effectively.
The next stage in the inspection process is provided by a Nikon Metrology LV Eclipse series microscope, which has a range of interchangeable nosepieces, allowing higher magnification and a full range of contrast techniques. Featuring episcopic and diascopic illumination of the sample, the model family including the LV100ND allows the user to see and judge how many layers thick the graphene is, according to the colours observed and the edge contrast. Such detailed study is very fast indeed and provides useful information on the suitability of the flake for a next process as well as continually verifying the method of production.
A major advantage of the instrument over most other similar microscopes on the market is that the image is the right way round and the right way up. It is a longstanding feature of LV Eclipse microscopes, which were developed for use in the semiconductor industry, where correct orientation of the image of the wafer or integrated circuit is essential. A majority of microscopes are designed primarily for medical applications in which it may be less critical to see which way round a cell is viewed, so manufacturers may omit the required corrective optics.
In Mr Clement’s opinion, the process of graphene production and component development requires the use of both SMZ and LV Eclipse microscopes as a minimum. The third stage would be to investigate the material structure in significantly closer detail, for which three alternatives exist.
First is the benchtop scanning electron microscope (SEM), such as the JCM 6000 NeoScope, which is presented to customers as the result of a collaboration between Nikon and Tokyo-based JEOL, who are experts in many areas including electron microscopy.
The simple and affordable, touch-panel-controlled imaging tool combines the high resolution and depth of field of a powerful SEM with the familiarity of a digital camera. Its magnification of up to x60,000 allows the quality of raw graphene to be inspected in very high detail when compared with optical microscopy.
An enhancement to the SEM, which can be retrofitted, is energy dispersive spectroscopy (EDS) employing X-ray emissions from the scan area. Backscattered electron images display compositional information resulting from different chemical elements within a sample and their distribution. EDS allows areas of interest to be analysed, elements identified and their relative proportions to be measured. Notably, this equipment is positioned safely and out of sight under the covers of the JCM 6000 NeoScope, rather than being an external and vulnerable addition mounted on the side.
The second instrument offered for detailed sample inspection is an enhanced version of the LV Eclipse microscope, designated BW range, equipped for white light interferometry (WLI). It includes interferometry objectives manufactured by a special team in a world-class factory in Japan, together with a high speed digital camera and a Nikon-developed, dedicated software algorithm combining all the product elements for providing outstanding Z-axis data.
A BW microscope is a very high speed and repeatable solution that provides information of the graphene sample primarily in the Z-axis. As thickness and surface quality of the layers are often the parameters that are needed most, the BW is very well suited to graphene inspection. With measuring performance far into the sub-nanometre range, WLI provides an efficient and economical method for evaluating the suitability of a graphene flake and the capability of the process that produced it. Naturally the image is upright and the right way round, so printed or etched features are presented correctly to the operator.
Thirdly, atomic force microscopy, for which a raster-scanning, high resolution probe microscope is used, provides a large amount of dimensional information of the sample in the X, Y and Z axes, as well as indicating its magnetic and chemical properties. This type of equipment provides highly accurate and detailed data, with each inspection generally taking many hours to complete, unlike a Nikon BW interferometer which can return results within 20 seconds.
In addition to these front-line instruments, Nikon Metrology has other products in its range that are proving useful and relevant for graphene inspection and quality control. One is the iNEXIV series video coordinate measuring systems, whose large field of view, long working distance and wide XYZ stroke enable easy, automatic measuring of 3D components. They are therefore ideal for checking the dimensions of a graphene sample. Some models, such as the confocal NEXIV VMR-K3040C (Z120X), employ an Eclipse LV optical lens with a top-of-the-range confocal imaging option for increasing the optical Z-axis resolution and contrast of the image, enabling highest quality for making coordinate measurements with great speed over extended X/Y areas. It is well suited, for example, to examining printed graphene structures extending beyond a microscope’s field of view.
Underpinning the use of all Nikon’s measuring and inspection microscopes is an advanced range of manually driven and motorised, high precision stages with software for automated large area recording functionality. They derive from the manufacturer’s long experience in semiconductor wafer handling and inspection. The NWL200 series of wafer loaders are capable of transferring graphene samples on a wafer substrate automatically to a microscope stage, with improved wafer sensing functions and edge-chipping detection to help prevent damage. 3-axis micromanipulators for high precision control of samples under a microscope can be provided.