Michael Bowden

Monitoring the quality of diamond films using Raman spectra excited at 514.5 nm and 633 nm

Abstract A relative Raman scattering cross-section has been experimentally determined for diamond and non-diamond carbon spectra excited by argon ion 514.5 nm radiation. This has been used to arrive at a semi-quantitative evaluation of diamond film quality. The results are compared with those obtained using helium-neon excitation at 633 nm.

Stress and Structural Images of Microindented Silicon by Raman Microscopy

The microline focus spectrometer (MiFS) Raman imaging process is described and is used to investigate stress and structure defect patterns in micro-indented single-crystal silicon. Raman intensity, frequency, and bandwidth images are reported with 0.3-μm pixel resolution, which reveal residual compressive stress distributions around the indentation site and areas of tensile stress at the crack tips. A previously unreported annular structural defect region, remote from the indent site, is observed in images where the indenter tip edges are aligned with the 110 direction of the silicon crystal.

Automated micro-Raman mapping and imaging applied to silicon devices and zirconia ceramic stress and grain boundary morphology

An automated, point-by-point, Raman mapping and imaging system is described that combines a 0.5-μm stepper-motor-driven stage, a Raman microscope, and a filter spectrograph with an intensified diode array detector. High-resolution Raman images of a silicon device structure and a map showing the presence of the monoclinic phase of zirconia (ZrO2) at grain boundaries and in stressed regions of a sintered tetragonally stabilized ceramic are reported. The importance of image processing is demonstrated and emphasised.

Automated Mapping of High-Temperature Corrosion Products on Iron Chromium Alloy Using Raman Microscopy

An automated Raman microscope mapping system is described which uses single point analysis combined with a stepper-motor-driven microscope stage. Methods for one-dimensional full spectra line-scans and repetitive frequency selective line-scans providing a two-dimensional species map are reported. The technique is applied to a surface feature appearing on a sample of Fe-Cr steel oxidized at 800°C for 143 h. The results suggest that Fe3O4, FeCr2O4 and Cr2O3 develop fairly uniformly across much of the surface and that the formation of raised areas of more extensive corrosion is due to the absence of Cr2O3 in these regions. In addition, a sample oxidized at 675°C for two hours was ball-cratered to provide a Raman depth profile. The corrosion scale was complex—the outermost layer comprising Fe2O3 and some Fe3O4, while the inner layer consisted mainly of FeCr2O4, with some evidence of Cr2O3.

Stress and crystallinity in 〈100〉, 〈110〉, and 〈111〉 oriented diamond films studied using Raman microscopy

Stress and crystallinity variations along the growth direction in three diamond films of different preferred orientations have been investigated using Raman microscopy to monitor the change in band center and full width at half maximum (FWHM) of the first‐order diamond phonon with distance along a single diamond crystal. The results showed a consistent trend for the 〈100〉 oriented film, with both the peak position and FWHM being largest close to the silicon substrate and decreasing along the direction of crystal growth. The 〈110〉 and 〈111〉 orientations showed random variation throughout.

Raman and finite-element analysis of a mechanically strained silicon microstructure

Raman microspectroscopy has been used to determine the volumetric micro-strain distribution in mechanically stressed silicon microstructures. Data are presented as strain images with a spatial resolution of around 0.8 µm. A useful correlation is demonstrated between finite-element analysis calculations of volumetric strain and Raman shift. The results demonstrate that silicon beam structures incorporating a 90° bend will experience a non-uniform stress distribution along the bend radius for small radii of curvature.

Patterns of stress-induced phase transformation in MgO-stabilized zirconia ceramic revealed using micro-Raman imaging

An automated Raman microscope system has been used to collect mapped Raman data from a Goodfellow 3% MgO stabilized zirconia ceramic tile. The data have been transformed to produce images which show the relative concentrations and distributions of the monoclinic and tetragonal phases in the mapped areas. The images reveal concentrations of the monoclinic phase at grain boundaries. Regions surrounding indents in the tile created with a Vickers hardness tester, were also mapped to reveal the extent and pattern of stress-induced phase transformation. A Raman map was also generated from an area before and after indentation. Comparison of the Raman images with the optical white light images reveals a relationship between the pattern of grain boundaries on the sample and the distribution of transformed material.

Determination of bandshifts as a function of strain in carbon fibres using Raman microline focus spectrometry (MiFS)

Raman Microline Focus Spectrometry has been shown to have major advantages over conventional point focus Raman microscopy for the study of stress-induced shifts in carbon fibres. Averaged, representative spectra have been collected from a sample length of 140 μm, reducing possible errors in band positions incurred from localised stress and surface variations. In addition, it has been shown that data can be collected quickly at low laser powers, which avoid the localised heating effects experienced with point focus illumination. Unsized, pitch-based, carbon fibres, 6 μm in diameter, were studied during a strain-release cycle. Within the scatter of the data, the E2g band returns to the original position when the tension is released. However, the Alg band remains downshifted by ca. −5.6 cm−1 upon release.

Characterization of a micro-engineered selective strain-coupling structure using Raman spectroscopy

This paper reports the physical characterization of a novel micro-electro mechanical system (MEMS) packaging structure using micro Raman spectroscopy. The structure is designed to reduce the effects of unwanted residual packaging strain on a resonant strain gauge device. It does this by maintaining the resonator alignment and selectively coupling the resonator to strain in one degree of freedom only. Previous work has demonstrated the ability of the structure to maintain resonator alignment. In this work, the ability of the structure to provide selective coupling is determined experimentally using micro Raman spectroscopy. The experimental results are shown to agree well with the predicted performance of the structure.

Raman spectroscopy as a mapping tool for localized strain in microengineered structures

The behaviour of microengineered devices is routinely modelled using finite element analysis (FEA). Whilst this is becoming increasingly sophisticated, the values of the material data used in FEA packages have considerable associated uncertainty. In particular, there is no general technique to measure localized strain: the FEA is done using an average value taken from, for instance, a displacement measurement followed by calculations on bending stresses. The aim of this paper is to show how Raman spectroscopy can be used to assess localized strain, by first using the technique to calibrate doping profiles in silicon: doping itself being a major contributor to the strain in a device. Localized results are important because many applications require heavily doped silicon structures: in high concentrations the dopant will distort the silicon lattice considerably. In addition, dopants are known to aggregate at certain crystal defects such as dislocations. Thus the distribution of the dopant, and the associated strain, will depend on the original crystal quality and its processing history prior to dopant incorporation. Results have previously been reported on Raman spectroscopy of boron doped silicon [1], using a destructive etch technique to obtain a through-wafer doping profile. To our knowledge no results have been published on localized measurement of doping with the spatial resolution (,1 μm) proposed here, and using a nondestructive technique. There is an additional benefit from the work. Some microengineered devices, in particular those which require resonant motion, may have long term failure mechanisms which are caused by localized strain maxima. The ability to map the distribution of strain within a device, and then correlate this data with points of failure, should lead to more reliable design and manufacturing. The principles of this technique have been reported elsewhere, e.g. [2], as have details of the microline focus spectrometer (MiFS) used in our experiments [3]. Fig. 1 is a schematic of the instrument. Both the band shift and band half-width can be plotted as a function of position to give a direct two-dimensional picture. The established strain induced shift in orientated single crystal silicon and other materials points to the use of shifts in the doped silicon phonon frequency to determine relative strain distributions in the fabricated structures. As a result, strain peaks in the device, including overlays, can be observed. The instrumentation is capable of obtaining spectra with sub-micron resolution and can generate profiles and images representing intensity (species concentration), frequency (stress) and bandwidth (crystallinity). Applied mechanical stresses will result in phonon frequency changes, and structural alterations will change the phonon density of states population and thus the band shape. In the case of silicon these spectral responses to stress have been published [4], although the manifestation of all the theoretically predicted spectral changes has not been observed. A (1 0 0) n-type silicon wafer, of 10 U cm resistivity, was used as the starting material. After cleaning and etching to remove the top 3 μm of the wafer surface, the substrate was loaded into a diffusion furnace alongside a solid source of boron. Diffusion was carried out at 1100 8C for two hours. This is enough to produce a heavily doped p-type surface layer, with a concentration in excess of 3 3 10 ions cmy3 at a depth of 2 μm; confirmation is provided by this process being used to produce an etch stop layer in an ethylenediamine pyrocatechol (EDP)-based process. Substantial changes to the phonon spectrum of silicon in boron-diffused material have been ob-