Electron microscopy, along with many other surface science and analytical techniques, offers an array of complementary
sub-techniques that provide additional information to enhance the primary analysis or imaging mode. Most electron
microscopes are built with several additional ports for the installation of complementary analysis modules. One type of
analysis which is particular useful in geology and semiconductor analysis is cathodoluminescence (CL).
A new technique has been developed to allow complementary optical measurements using the electron beam from the
SEM, compatible with most standard commercial SEM systems. Among the optical measurements accessible using the
Cathodoluminescence Universal Extension (CLUE) module are CL, Raman, PL and EDX spectroscopy and imaging.
This paper shows the advantages of using these complementary techniques, and how they can be applied to analysis of
geological and semiconductor materials.
In spite of the fact that the original Raman microscope was designed in the early 1970's for Raman imaging,
wide-spread practical use of the technology did not appear until the last 5 years. The instruments are smaller,
faster, easier-to-use, promoting reports of a variety of interesting applications in fields as diverse as
nanomaterials, pharmaceuticals, composites, semiconductors, bio-clinical studies, polymers, ceramics and
glasses. While the information content in Raman analysis is quite high, the time to acquire an image has been
a deterrent to its application. Recent innovations including Swift and DUO Scan have addressed and are
addressing these issues. SWIFT (Scanning with Incredibly Fast Times) is a rapid CCD read-out technique
that is based on the synchronization between the XY motion of the motorized or piezo stage and the CCD
readout. DUO scanning uses a set of scanning mirrors above the microscope objective to raster rapidly the
laser beam across a sample area. This can be used to create a "giant pixel" in the map without compromising
the NA of the light collection, or to create a map with step sizes as small as 10nm. Swift, in combination with
DUO scan, as been used to produce full spectral maps of pharmaceutical tablets in times as short as 10
minutes, something that was previously believed to be near impossible. Off-line analysis of such a map using
multivariate techniques produces Raman images indicating the quality of component mixing, and also the
presence of minor, difficult-to-detect components (such as Mgstearate in pharmaceutical tablets).
Implementation of higher spectral and spatial resolution in dispersive Raman microscopes, including access to a variety of excitation wavelengths, has proven beneficial in the semiconductor industry. UV adaptations accommodate measurement of smaller defects, higher sensitivity to thin films (to the exclusion of the substrate) and access to enhancement conditions for materials such as GaN-based photodiodes and lasers, and diamond. The availability of a high dispersion spectrograph, especially for UV wavelengths, avoids compromising spectral resolution. Examples of successful analysis requiring longer focal length, mirror-based spectrographs are shown; these include stress in silicon-based devices, Raman and PL of InGaN (which provides information on composition) and carbon nanotube studies.
The structural elucidation of complex systems may be simplified with multi-dimensional spectroscopic techniques with some combination of spatial and spectral resolution. Raman spectroscopy permits the addition of another variable to this scenario -- excitation wavelength. Data obtained using excitation wavelengths from the UV (244 nm) to near-IR (785 nm) regions will be presented showing the qualitative and quantitative study of diamond-like carbon (DLC), silicon, and other systems of an industrial or biomedical nature. The choice of appropriate wavelength provides an additional advantage over other spectroscopic techniques for elucidating specific structural information from these systems. The advantages of UV-Raman for materials science and thin film studies will be considered. The design of instruments and probes for the application of Raman spectroscopy to industrial process control and the development of Raman spectroscopic libraries for contaminant analysis will be discussed.
The utility of Raman microscopy and imaging for the characterization of a variety of chemical and biological systems is discussed. Measurements have been carried out with an optical microscope coupled to a Raman spectrometer that contains light paths for both single point and imaging measurements. Laser irradiation and signal collection are implemented using epi-illumination through a single microscope objective. For point Raman microspectroscopy. In our arrangement, the laser is defocused to provide wide- field illumination. The Raman signal from within the irradiated sample area is directed through a narrow-band liquid crystal tunable filter (LCTF) and imaged onto the CCD. Spectroscopic information is obtained by recording Raman images through the LCTF over successively tuned frequencies. Raman spectra for various point within the sample thus are obtained in parallel by each pixel in the detector array. Microspectra were recorded within various sample, including bacteria. Spectroscopic features of interest were then investigated in greater spatial detail using the LCTF imaging methodology.
Imaging methodologies present some of the most exciting new frontiers in the biological and medical sciences. Raman spectroscopic imaging combines the power of chemical imaging with the spatial resolution for translating microscopic spectroscopic information into statements relevant to biological and medical function. Imaging results will be presented using mapping, dielectric filters, and liquid- crystalline tunable filters at different excitation wavelengths for selectively determining the spatial distribution of biomaterials in a variety of biological systems.
Recent improvements in filters, multi-element detectors and instrument design have transformed Raman spectroscopy from a difficult to use specialist technique into a widely used multi-dimensional spectroscopic method. Raman spectroscopy is non destructive and offers a spatial resolution of one micro or better. A Raman spectrum gives specific information regarding the chemical bonding of molecules and can therefore be used to identify different molecules in a system. Through the use of xyz mapping techniques, specific types of material can be imaged in living cells, drug formulations and polymer mixtures to give but a few examples. Raman technologies allow areas as large as 500 microns to be imaged directly using filters tuned specifically to look for a particular chemical species. The Raman technique uses visible or close to visible light which is ideal for coupling into optical fibers. It is therefore very easy to build ruggedized spectrometers using fiber optic probes for remote sensing in extremely difficult and/or hazardous environments; for example process monitoring and recently endoscopic diagnostic work in living subjects. This paper will describe the methodology used in direct Raman imaging, Raman mapping experiments and remote sensing with reference to specific examples of biological, pharmaceutical, mineral and crystal studies.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.