Micro and nanoscale measurement and characterisation accelerate development of new products from initial conceptualisation through to evaluation and testing of prototypes and manufacture of final devices. The Centre of Excellence in Metrology for Micro and Nano Technologies (CEMMNT) provides access to equipment and expertise to accelerate product commercialisation as illustrated by the following examples.
Atomic Force Microscopy (AFM) of Nanotube-Based Gas Sensors
Small knowledge-intensive companies often reach a barrier in the development of their intellectual property, due to lack of resources and technical facilities. Applied Nanodetectors (www.applied-nanodetectors.com) is a SME with significant intellectual property in the field of nanotube-based gas sensors for environmental and medical applications. Its Managing Director, Dr Victor Higgs, approached CEMMNT partner, the National Physical Laboratory (NPL), and received consultancy funded by the DTI’s Measurement for Innovators (MFI) scheme. Atomic Force Microscopy (AFM) was used to determine 3-dimensional nanotube morphology and is being extended to characterise the electrical (electron transfer) properties of carbon nanotubes using metal coated probes to carry out conductivity and surface potential measurements. Figure 1 shows how the highly conductive carbon nanotubes can be identified using conductive AFM. Transmission Electron Microscopy (TEM) images provided a larger area overview of nanotube morphology and valuable data on tube structure and crystallinity.
Figure 1. Conductive AFM image of carbon nanotubes on a silicon substrate (Copyright NPL).
Characterisation of the Sensitivity of Gas Sensors
The sensitivity of the gas sensors to trace element pollutants was characterised. Most conventional sensors can currently detect 50 – 100 ppb of the pollutant nitrogen dioxide (NO2). The automotive industry is seeking a new generation of higher-sensitivity, low-cost miniature sensors to measure pollution levels inside cars. The NPL team characterised the response of a number of sensors over a large range of NO2 aerosol concentrations. A number of sensors responded to exceptionally low levels of NO2 – down to 4ppb in one sensor which is far beyond the sensitivity of any sensor currently on the market. This example illustrates the benefits that the CEMMNT partnership and its open access philosophy can bring to SME’s operating in the field of micro and nano technology.
Transmission Electron Microscopy (TEM) of Carbon Nanotubes
Transmission Electron Microscopy (TEM) is a core nano technology characterisation tool, ideally suited to the analysis of multilayer thin films, semiconductor hetero-structures, synthesised nanoparticles, carbon nanotubes and nano-composites. High-resolution TEM (HRTEM) imaging gives structural information at the most detailed (e.g. lattice imaging) level, and can be coupled with additional methods, such as energy-dispersive X-ray fluorescence detection (EDX) or electron energy loss spectroscopy (EELS), to allow nanoscale elemental analysis. Figure 2 shows an HRTEM image from a segment of a filled multi-walled carbon nanotube. TEM provides key information relating to nanotube properties, including the number of walls (single, double or multi-wall), tube diameter, wall roughness (which relates to graphitic crystallinity in the case of multi-walled tubes) and identification of co-located species such as fillers or catalyst particles.
Figure 2. HRTEM image from a segment of a filled multi-walled carbon nanotube (Copyright QinetiQ).
Optical Profilometry of MEMS Devices
The MEMS industry is projected to grow at 16% annually and reach over $25 billion by 2009 (source: NEXUS). It is vital that SME’s with new devices can accurately design and test prototype performance. CEMMNT offers a full range of metrology services for static and dynamic characterisation of MEMS devices. For static dimensional measurements, a range of micro-interferometry (optical profilers) and stylus-probe instruments are available. The 3D geometry of a MEMS pressure sensor is determined by optical profilometry in Figure 3. Dynamic measurements offer the ability to determine displacement voltage characteristics and device resonant responses under operational conditions. For out-of-plane displacements optical profilometry under stroboscopic illumination and micro-imaging vibrometry (at NPL) are used. The techniques are applicable over the 50Hz – 1MHz frequency range with sub-nanometre vertical sensitivities. In-plane motions are tracked using micro-interferometry and video-microscopy techniques, and lateral sensitivities down to sub-50 nanometres are achievable.
Figure 3. Optical Profiler image of a MEMS PZT pressure sensor (Copyright Taylor Hobson).
Optical Profilometry of Implants
Orthopaedic implants have seen a dramatic increase in demand over recent decades as the global population ages. Both enhanced performance and longevity are required, which drive component quality control. In total hip replacement systems (THR), the head of the femur is replaced with a ball and stem. Characterisation of the primary components and subsequent component wear require a range of metrology techniques. The roundness of the femoral cup is crucial to its load bearing capability and can be determined using Taylor Hobson Talyrond stylus inspection systems. Wear and volume of lost material are automatically determined by software. In modular implants, accurate control of the taper on the head and stem is required to optimise the rigidity of the implant. Vertical scanning enables accurate and repeatable measurement of cone angle on modular stems. Often a non-contact approach is beneficial to characterise wear on femoral heads. The nanoscale polished finish on a ceramic cup surface is shown in Figure 4. Optical profilometry also allows non-destructive measurement of coating thickness, which is widely used in both the stents industry and for determining passivation layer thickness in microelectronic devices.
Figure 4. Optical Profiler image of a ceramic hip cup surface.
These application examples highlight how CEMMNT’s partnership can combine its expertise and facilities to provide either standard off-the-shelf or bespoke customised solutions at each stage of product and process development for all industry sectors.