"There are special requirements for micro and nano manufacturing, and metrology is a critical technology to ensure high quality micro and nano product development"

Metrology: Measuring to the Nanoscale

There is no doubt that many developed countries are highly competitive in global R&D endeavours in micro and nano technology, and it is important that the commercialisation of these technologies keeps pace. More and more products rely on materials with structures engineered in the micro- and nano-metre ranges, which offer new or enhanced properties such as increased strength, light emission, and biocompatibility. These structures and particles offer a route to improved performance of products in a diverse range of sectors, from inhalers and skin creams in the pharmaceutical industry, micro electronics and MEMS devices, through to fuel additives and anti-fog windscreens in the automotive industry. The incentive to develop and manufacture safe, reliable products that are smaller, cheaper, harder, stronger, and/or lighter inherently drives the need for advanced, high quality measurement and characterisation at the micro and nano scale. Of course, the successful supply of competitive products and services is critically dependent on manufacturers to meet market requirements in terms of cost and functionality, and the use of metrology is essential here. Testing is often required at various process stages, to ensure micro- and nano-scale components meet their exacting dimensional tolerances, or to verify their effective enhancement of material properties or device performance. Increasingly, therefore, micro and nano scale measurement and characterisation play a significant role in industry by enabling and accelerating the development of new products from initial concept, through evaluation and testing of prototypes, to the manufacture of final devices and products.

Fast Access to Problem-Solving Metrology

An immense range of measurement and characterisation techniques are available for resolving problems in development or manufacture, and until recently, the process of selection and access to the most appropriate and cost-effective technology has presented a significant barrier or time-delay.

However, through the collaboration of four world-leading industrial companies and the Technology Strategy Board, CEMMNT has been set up to enable industry to benefit from a single point of access and advice to the widest range of state-of-the-art metrology techniques. This extensive range is complemented by open access to the knowledge and experience of over 250 industry-leading experts, each committed to providing clients with the highest quality in measurement and characterisation technologies specific to their industry. The following techniques are just some of the metrology options highlighted by CEMMNT for the micro and nano technology sector.

Microscopy

The effectiveness of micro and nano scale materials depends on a range of factors, which include critical dimensions and chemical, mechanical, optical, and thermal properties. With increasingly stringent requirements for product performance, accurate metrology and characterisation down to the atomic scales becomes critical.

Scanning electron microscopy (SEM) provides high-resolution, large depth-of-field imaging of solid materials, allowing visualisation and metrology of 3D features, surfaces, and cross-sections. Secondary- and backscattered- electrons are collected to build up images dominated by surface topography- and composition-related contrast respectively. The incident electrons also interact to produce fluorescent x-rays with element specific energies.

Electron backscatter diffraction (EBSD) determines crystallographic information from samples in a SEM enabling crystallographic texture, preferred orientation, grain structure and micro structure to be mapped.

Transmission electron microscopy (TEM) is a core tool for nano scale measurement, with unparalleled capabilities for the resolution of multilayer thin films, semiconductor hetero-structures, synthesised nano particles, carbon nanotubes and nano composites. TEM requires high-level operator skills and complex, expensive instrumentation, but is often the only method capable of giving a direct visualisation of such materials, capable of giving structural information at the most detailed level (for example lattice imaging).

TEM can be coupled with additional methods, such as energy-dispersive x-ray analysis (EDX) or electron energy loss spectroscopy (EELS). EDX provides local-area (nano scale) elemental analysis, and EELS expands this further to give chemical bonding information and parallel imaging.

Atomic force microscopy (AFM) generates three dimensional images and surface property information by scanning a stylus over areas of 100 micrometres and below with sub-nanometre resolution. This non-destructive technique requires little sample preparation and operates in air, fluid, electrochemical and vacuum environments using laser feedback to minimalise forces between the tip and sample. Sample temperature can be varied and experiments performed under humidity control. Advanced imaging modes enable a wide range of materials properties to be characterised with nanometre resolution, and the probe can also be accurately positioned for local spectroscopic analysis. Applications range from the measurement of surface roughness down to the nano scale, to the identification of phases and domains on polymers, coatings, and composites and the measurement of cell, nano particle, colloid, and biomolecule adhesion.

2D and 3D Dimensional Surface Metrology

Properties of a surface can strongly influence its functionality, and these properties must be taken into account when designing or manufacturing MNT-based devices. New industrial applications are requiring manufacturing surfaces that are "structured" by deterministically shaping the surface topography.

Stylus profilometry can measure form, dimension, texture, and step height. A diamond stylus is traversed across the component or surface under test. Undulations in the surface and the overall shape are traced by the stylus and digitised to provide a 2D or 3D surface map. Stylus profilometry can scan long areas with sub-nanometre vertical resolution. Advances in tip technology and staging now allow steep sided surfaces to be accurately profiled.

Roundness measurement enables quality control of nominally circular and cylindrical products by testing the accuracy of their form. The component is rotated on a highly accurate spindle that provides a circular datum. During rotation, a transducer with ball tipped stylus measures radial variations of the component with respect to the spindle axis. Adding a vertical column datum and a radial arm datum, allows circular traces, which can be combined to give a measure of cylindricity, flatness, and squareness.

Optical profilometry delivers rapid non-contact 3D surface analysis. Using an interferomic lens the recombinant reflected light results in interference maxima at best focus providing sub-nanometre level vertical resolution and lateral resolution down to 0.5 micrometres. Automated stages allow large area images to be obtained whilst maintaining high lateral resolution. The thickness of transparent films can be measured and interface structures characterised.

Confocal and laser scanning profilometry provides a flexible approach to non-contact 3D mapping of a wide variety of products in many different materials. The choices of gauges, scanning speeds, and data spacing enables the optimum solution to best fit the metrology challenge.

Co-ordinate measuring machines (CMM) are now commonplace in most advanced manufacturing industries and their use has greatly reduced the complexity, down-time and operator skill required for critical measurements. The measurement of micro and nano manufactured parts - for example, diesel or ink-jet injector nozzles, micro-optics, small medical devices, and micro fluidic components - present some unique challenges. Despite their small size, many of the parts that need measuring are very complex, with high aspect ratio structures that may be constructed from materials that are difficult to contact with a mechanical probe (e.g. polymers or bio-materials). Micro CMM systems are intended to meet these special measurement tasks. Usually, these instruments have relatively large probe tips for measuring micro and nano scale structures, whereas what is actually required, are truly micrometre-scale probes that measure in 3D - a design and manufacturing challenge in its own right.

Dynamic MEMS profilometry (DMEMS) strobes and synchronises the light source of an interferometric optical profiler to acquire 3D images of MEMS devices at each stage in their motion cycle. In addition to generating movies of device motion, both in and out of plane performance can be determined. Laser Doppler Vibrometry (LDV) is a complementary non-invasive method which can be used to characterise in-situ performance of MEMS structures and devices. The Doppler shift of a laser reflected from a vibrating area is detected, which gives the velocity of that region along the axis of the laser light. The laser can also be scanned laterally to map vibration and displacement modes. These two methods in combination are key tools for characterising MEMS devices at each fabrication stage. Hence, device failures can be identified at early process stages saving time and money. DMEMS can be used with temperature stages and vacuum chambers to characterise MEMS properties as a function of temperature and pressure.

Spectroscopy

Fourier transform infra-red (FTIR) spectroscopy provides information on molecular vibrations, and allows chemical fingerprinting of sample species. It is widely used for analysis of both organic and inorganic materials including contaminants. Spectra can be obtained in either transmission or reflectance, and small-area analysis is possible. Thin films can be quantitatively analysed or compared.

UV-Vis Spectrophotometry measures the intensity of transmitted, reflected and/or absorbed light from samples across the UK and visible regions. It enables characterisation of the optical properties of a wide range of materials from optics through to thin films, pharmaceuticals, textiles, and inks, and provides film thickness measurements, analysis of structured anti-reflection coatings and refractive index determination.

Ellipsometry measures a change in polarisation as light reflects or transmits from a material structure and is frequently used to determine the optical constants and thickness of surface films and multilayers. The method - traditionally used at single laser wavelengths - is now increasingly applied spectroscopically over the UV-Vis and even IR ranges to allow a more complete characterisation of the material or its optical properties.

Micro-Raman spectroscopy is a powerful tool for local chemical fingerprinting or assessment of crystallinity, which exploits inelastic light scattering (the Raman effect). Raman is amenable to liquid and solid samples, including some intractable by FTIR such as aqueous dispersions and carbons. In micro-Raman microscopy, the analysis area is determined by the focused laser spot size (typically ~1 micrometre diameter). The spatial distributions of selected species, the complete chemical fingerprint, or local strain or temperature, can then be mapped over a sample if required.

X-ray photoelectron spectroscopy gives elemental and compositional information from the top few surface atomic layers of a surface. Information on bonding, oxidation states and chemical environments is derived from small "chemical" shifts in the characteristic peak energies. Sampling depth is characterised by an exponential decay length of typically a few monolayers. Surface sensitivity can be further enhanced by use of shallow take-off angles, and depth profiling can be carried out using ion sputtering.

Secondary ion mass spectrometry (SIMS) enables sensitive depth profiling over sample depths from a few nanometres up to tens of micrometres below the surface. Determination of elemental composition as a function of depth, identification of trace-level contaminants and dopants (with typical detection limits in the ppm - ppb range), and isotope ratio measurements, are key applications. The technique can be employed in depth profiling mode, in imaging mode to map spatial variations, or in microprobe mode for spot analysis of features or particles.

Crystallinity and Defect Measurements

X-ray diffraction is a commonly used non-destructive technique to determine crystallite phase of micro crystalline powders and thin films, for texture analysis (polycrystallite orientation) and to determine residual stress/strain. High resolution x-ray diffraction (HRXRD) is applicable to single crystal materials, for high precision lattice parameter measurement, and to determine the composition, thickness, and quality of epitaxial layers, including multilayers. A wide range of XRD instrumental configurations and data analysis approaches are available to suit the application, for example pole figure plots to analyse polycrystal orientation, and double-crystal rocking curve measurement in conjunction with full dynamical simulations to quantify epitaxial layers.

X-ray topography (XRT) is a non-destructive characterisation technique for imaging, by means of x-ray diffraction, the micrometre to centimetre defect microstructure of crystals. Contrast features are seen where there are imperfections in the crystal that cause variations in the long-range atomic order. XRT is a powerful technique for characterising the growth of crystal substrates and thin (e.g. epitaxial) films.

Particle Sizing

Different particle sizing techniques enable size and volume distributions to be made in both fluid and airborne environments. These approaches complement microscopy techniques such as SEM and TEM which provide high resolution shape and morphology data. Dynamic light scattering (DLS) illuminates particles (down to 10 nm-20 nm) in solution with a laser and the resultant interference of scattered light causes a fluctuation in the intensity of light that reflects the particles' size. Airborne particle size distributions can be measured using a scanning mobility particle analyser. Particles entering the system are charged using a radioactive source and are classified according to electrical mobility, with only particles of a narrow range of mobility exiting through the output slit. This mono-dispersed distribution then goes to a condensation particle counter which determines the particle concentration at that size and subsequently detecting size via light scattering.

When presented with a process or material challenge, it is rarely sufficient to load a sample, press a button and be presented with a sensible answer from the analytical 'black box'. It is invariably the expertise of dedicated technologists that recognise and understand the relevant sample artefacts under examination, coupled with the correlation of data from multiple metrology techniques that provide the complete solution.

This article, written by Paul Newbatt (Business Development Director of CEMMNT) appears in the latest edition of the Commercial Micro Manufacturing Magazine. Click here to view website.

Contact CEMMNT today to discover how your business can benefit from the latest in state-of-the-art metrology: 

   Call us on 01509 635279                   enquiry@cemmnt.co.uk