16. High
Resolution Raman Shift
Department of Chemical and Life Sciences, E-Mail Derek.Gardiner@UNN.AC.UK
Abstract This feature article sets out to describe some important materials science applications of Raman imaging. In addition we illustrate the value of using images to reveal spatially significant, spectral changes which might otherwise be overlooked. Stress fields in micro-indented silicon, edge stresses associated with thin film pads and temperature variations in polysilicon heater devices are discussed as examples.
Introduction Raman microscope imaging was first demonstrated by Delhaye and Dhamelincourt [1] in 1974. A double monochromator with non-astigmatic gratings was used as a relatively wide bandpass filter to select the required Raman shift frequencies and the image was captured on a vidicon detector. The technique, though an important milestone, was of limited value and only worked well with strong Raman scatterers and where narrow frequency band image selection was not required. The routine generation of micro-Raman images became achievable with the development of the ccd detector. An instrument employing tunable bandpass filters to select Raman scattered light from a globally illuminated sample and to image it onto a cooled slow scan ccd detector was first described by Batchelder [2] in 1992. Various attempts have also been made to generate Raman images from point by point collection of an array of spectral data [3]. The Microline Focus Spectrometer (MiFS) which we have developed has been described in detail elsewhere [4, 5]. The instrument uses cylindrical optics to produce a line focus of the incident laser beam on the surface of the sample. Light scattered from this line is then imaged through a subtractive filter stage to remove the incident laser frequency and a spectrograph stage to form a spectrally dispersed image of the Raman scatter on a cooled ccd detector. The non-astigmatic imaging optics employed ensure that the spatial integrity of the origin of the scattered light is retained, thus generating Raman data from the length of the focused laser line at a single exposure. Raman images are obtained by collecting data from a series of adjacent lines on the sample surface. The process is summarised in Figure 1. Raman spectra are dispersed on the ccd chip (lambda), with y as the spatial direction. At each line position on the sample (x) the required spectral parameters are extracted and manipulated to remove background, determine integrated intensity (Iy), position (ry) and bandwidth at half height (by). These data are then written as a greyscale image to a monitor screen before moving on to the next line position. In this way the Raman image is generated in near real time whilst at the same time the full dynamic range of the image data is retained for three dimensional plotting or for further analysis. This approach is an objective compromise between the time consuming and high positional stability demands of point by point sampling of the surface and the spectroscopically limited global illumination method. In addition the method does not rely upon moving optical components and thus is inherently stable. Systematic error in the spectral data is also eliminated and the inclusion of a convenient laser plasma line allows high precision Raman shift data to be realised. Stress variations and structural defects in
bulk silicon are of concern to developments in microelectronics and more recently in
microengineering structures. In the former, stress is generated as a consequence of
epitaxial growth and the varying architecture of component device layers, whilst in the
latter, stress variations may arise from high doping levels and from etch removal of bulk
silicon to produce the required microstructure. The response of the silicon Raman spectrum
to stress and temperature has been reviewed [6], along with the use of Raman microscopy to
determine the spatial variation of these parameters [7]. Much of the Raman microscopy work
to date has used single point probing to explore the spatial variations in silicon
structures with the result that only limited Raman data can be obtained and evaluated
within a reasonable time. The development of Raman imaging instruments makes it possible
to collect large Raman data sets over a surface area and to process these into stress and
structure characteristic images, at high spatial and spectral resolution within an
acceptable time-scale. Here we show how the results of applied stress and temperature can
be visualised using Raman shift and bandwidth images. Figure 1. Schematic of the MiFS mapping/imaging process
Pure single crystal silicon is cubic, belongs to the crystal symmetry class Oh and gives rise to a single triply degenerate Raman active T2g phonon mode at around 520 cm-1. Amorphous silicon gives rise to broad features at 140 and 475 cm-1. Polysilicon or microcrystalline silicon formed by CVD methods, gives rise to a broader band around 518 cm-1, often with additional structure on the low frequency side dependent upon the deposition conditions. The application of an in-plane, biaxial compressive stress results in a positive shift of the phonon frequency, while tensile stress produces a negative shift. Extreme stresses such as those developed under the high impact loads experienced adjacent to an indenter tool, or as a result of surface scratching [8], can result in splitting and asymmetry of the phonon band. This response can be variously interpreted as indications of regions retaining a high degree of non-isotropic strain, or the development of metastable silicon phases. Increase in temperature decreases the phonon relaxation time and alters the phonon state population with the result that the phonon band shifts to lower frequencies and broadens with increasing temperature.
The Raman spectra were excited using either the 488nm or the 514.5nm line from an Ar+ laser generating around 5-10 mW at the sample. The incident beam was polarised parallel to the sample x direction and in the work presented here no analyser was used and all the scattered light was detected. Objectives of 40x and 100x magnification were used producing line lengths of 120 µm or 50 µm respectively on the sample. When the 488nm line was employed, a convenient laser plasma line at 529.66 cm-1 shift was used as a reference.
The images of Figure 2 were obtained from the
(001) surface of crystalline silicon indented with a Vickers micro-indenter using a square
pyramidal diamond tip with a load of 0.1N applied for 15 seconds. The diagonals of the
indenter tip were aligned with the <100> direction. The white light image of the
damage site showed only a small ~3 µm wide square indentation. The Raman shift image
(Figure 2a) in which white represents a positive and black a negative shift, reveals
extensive regions due to compressive modification adjacent to the edges of the indent and
extending symmetrically for 2-3 µm. Between these regions and radiating from the corners
of the indent are darker areas arising from silicon under tension. No evidence of cracking
is apparent. Figure 2b confirms that the phonon bandwidth is most affected close to the
indent edges where it is assumed some structural modification has occurred.
Figure 2. Raman images of a 12µm square indent The three dimensional plot of Figure 3 is an alternative presentation of
the same data and shows the compressive and tensile regions. The scale of the phonon
frequency changes which were observed are made clear in Figure 4 which is a plot of the
cross section (xx) indicated in Figure 2a. Here the relative scale of the tensile and
compressive frequency responses can be seen, which cover a range of around 4 cm-1. Figure 3. Three dimensional plot of the data of
Figure 2a Figure 4. Section xx from Figure 2a The Raman shift images (a,b,c) and the
corresonding halfbandwidth images (d,e,f) shown in Figure 5 arise from higher load indents
and reveal varying amounts of cracking. Raman shift images show compressive regions as
before, but at the crack tips, small, dark, tensile regions can be seen, consistent with
crack propagation. The halfbandwidth images appear to indicate incipient lateral crack or
flake formation. Figure 5d in which the indent diagonals are aligned with the <110>
direction (the most favourable cleave direction) identifies a ring of structural damage at
the crack radius. A similar result is seen in Figure 5f. The image of Figure 5e results
from an indent where the diagonals were aligned with the <100> direction. Here the
cracking is more complex, tending to follow the <110> direction and a well developed
lateral crack, which is a precursor to flaking can be observed in the lower right hand
quadrant. The indent of Figure 6 has resulted in flake formation indicated by the region
of low strain on the right hand side (Figure 6a) with structural damage indicated by the
increased bandwidth (Figure 6b) around the perimeter of the flake. Figure 5. Raman images of 50 µm square regions Figure 6. Raman images of a 12 x 20 µm region Silicon Nitride Pad Edge Effects in
Silicon It is well established that thin films, deposited on
silicon, will exhibit varying degrees of stress. When the thin films are deposited as
pads, then the silicon substrate will experience stresses at the pad edge [9]. We have
investigated this effect by studying a checkerboard of 10mm square silicon nitride pads
deposited on silicon (100) under a range of conditions. Initial experiments involved using
single exposures, with the line focus at a small angle to the pad edge in order to
increase the pixel resolution in the spatial direction. A typical result is shown in
Figure 7. In the clear silicon region a constant phonon frequency of 520.7 cm-1
is observed until at around Figure 7. Plot of phonon frequency The three dimensional plot displayed in Figure 8 was obtained from the
same sample by mapping over the corners of two proximate pads as indicated. The Raman
shift data has been adjusted to show tensile stresses as positive in the vertical
direction and compressive stresses as negative, in line with convention. The stress trough
around the edges of the pad is clearly discernible. Other checkerboard samples prepared
with pad stresses from tensile to compressive were studied and revealed complementary
results. A sample produced with no pad stress showed no edge effects. Figure 8. Silicon strain image of a 15 x 18 µm region
As described in the introduction, the polysilicon phonon band changes dramatically with temperature. The effect over 600° C is illustrated in Figure 9.
As the temperature rises from room temperature (red trace) to 600° C
(grey trace) the frequency decreases as the bandwidth increases. The intensity variations
arise from the refocusing required at each temperature and changes in the density of
states distribution as the temperature increases. The frequency change over this range is
linear as shown in the plot of Figure 10 and can be used to monitor temperature. Figure 10. Plot of Raman shift data from As an example of this application, which is discussed in more detail
elsewhere [10], the MiFS mapping procedure was used to produce a Raman shift image of a
working polysilicon micro-heater composed of a free standing polysilicon meander anchored
at each end to a contact pad. The data was then transformed using the results of
Figure 10 to produce the temperature image shown in Figure 11. The temperature can be seen
to increase from ambient to around 350° C at the centre of the meander. Figure 11. Temperature
image of a Conclusions The Raman imaging applications described here demonstrate the value of using image data to detect subtle, spatially significant, spectral changes. It is the patterns revealed in a complete image which lend significance to small spectral variations The bandwidth halos in Figures 5d and 5f, the flake in Figure 6 and the stress trough in Figure 8, are strikingly revealed but might only be discernible from individual point measurements following extensive repeat measurements and data analysis.
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