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Bristol University - Wednesday 7th January 1998
Introduction & Welcome
by Prof. GC Allen, Dep.Dir.
Interface Analysis Centre
Professor Allen opened the meeting by reminding all the
delegates of the previous UK ESCA Users meeting held at the IAC, at the Victoria Rooms
conference facilities. The theme of that earlier meeting was also on high spatial
resolution surface analysis. His main memory of that particular meeting was how cold the
conference rooms were, and that there were two choices:
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Hear the talks and freeze, or |
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Have the jet heaters on and ignore the
speakers! |
He was pleased to note that the current meeting was in a smaller warmer
(only slightly!), conference room (in the Russian Language department) and that
like the last meeting held in Bristol, it was very well attended.
"High Spatial Resolution Analysis using
Vintage Instrumentation!"
Dr. RK Wild, Interface Analysis
Centre, Bristol.
r.k.wild@bris.ac.uk
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Figure 1. 1984
Vintage PHI Auger Analyser at the IRC |
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Bob Wild described how due to their limited budget, the IAC had been
forced to look for alternative ways to get their hands on "state of the art"
instrumentation. The policy that had adopted was the use of system upgrades, plus the use
of considerable in-house expertise in the development of new instrumentation and software.
In particular he showed the recent modifications to their 1984 PHI 595 Auger spectrometer
(figure 1), and their "new" LMI-MS system, which was based on an old
UHV chamber or two, a magnetic sector mass spectrometer removed from 20 year old GC/MS and
a brand new FEI liquid metal ion gun. Both were now up and running, and producing
excellent results.
The Auger spectrometer used the York Electron Optics Schottky Field
Emission gun, which gave the system 100nm spatial resolution (for spectroscopy). The limit
on the spatial resolution was set not by the electron gun, which would ultimately give
20nm spatial resolution, but by the stage vibration. This may be more difficult to resolve
given the design of the instrument.
The mass spectrometer, also theoretically also had 20nm resolution, but
again this was badly affected by machine vibration. However, the sensitivity of the
instrument was exceptional, with polished sections of Inconel 600 showing boron
segregation at grain boundaries. The maps were acquired in 1 second without O2 dosing.
Image of Dopant effects using High Energy Resolution
AES
Dr J Wolstenholme , VG Scientific
Wolstenholme@vgscientific.com
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Figure 2. VG Microlab 310F |
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Figure 3. SEM of AU Particles on
Carbon
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Dr. Wolstenholme showed results obtained from the VG Microlab
310F (figure 2), using an optimum spatial resolution of ~20nm (Au on C, figure
3), using a beam voltage of 10kV, and a spectral resolution of 0.05%. The project
involved the careful examination of doped and un-doped Si wafer specimens, noting possible
1) Beam Energy effects and 2) "Z" Stage Positions on the Si peak position. After
completing a number of carefully controlled sets of experiments, it was concluded that
there was no effect on peak position by these parameters!
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Figure
4. Si KLL Auger Peaks Shifts in High Resolution Mode (Note, Example is difference
between n and p type doped Si)
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Further experiments were then performed on in-situ fractured phosphorus
doped samples, where it was clearly shown that there was a 0.7 eV peak shift (Si KLL)
between the doped and un-doped material (figure 4). This difference could be used
to generate AES maps of both regions and hence determine a diffusion profile of the
region. The samples shift was also shown not to be sensitive to ion sputtering (Ar+) and
could therefore be used on real samples.
XPS and SIMS for surface Imaging Phase Separation in
Polymer Blends
Dr. R Short, Sheffield University
r.short@sheffield.ac.uk
Dr. Short gave an excellent presentation on the detailed
examination of phase separation using a multi-technique/multi-instrument approach. Over a
period of approximately 7 years his research group at Sheffield University had examined
possible surface segregation phenomena in a PMMA/PVC blend. The work was part sponsored by
ICI (Wilton) who had also contributed instrument time on their ESCASCOPE, Scienta ESCA 300
and PHI 7200 ToF-SIMS. Surface measurements were also made on the VG CLAM 200 (XPS) and VG
Ionex IXL23LS ToF-SIMS at Sheffield University!
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Figure 5. The surface of a spin
cast PMMA/PVC blend (THF Solvent)
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The work examined the possible segregation of a wide
number of PVC/PMMA blends (0-80%), which were spin cast from THF. Quantitative XPS
analysis of the surface (CLAM 200) indicated a slight surface excess of PMMA although, the
results were always difficult to reproduce. Optical images of the polymers, however,
clearly showed phase separation. Using the ToF-SIMS (IXL23LS) to generate elemental ion
maps it was possible to show that the different phases seen in the optical micrographs
(Cl- for PVC & O/OH- for PMMA). However, high spatial resolution XPS line scans made
on the Scienta ESCA 300 indicated that there was a surface layer of PMMA on the PVC phase.
This was shown by the O1s linescans across a sample section. ToF-SIMS molecular ion images
(PHI 7200) and XPS images (ESCASCOPE) clearly confirmed these earlier results. The
conclusions from the work were as follows:
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In blends cast from THF, phase separation "extends to
the surface", however, |
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There is still a PMMA overlayer on the polymer blend. |
The model of the surface is shown in figure 5. However, some questions
remain unresolved, such as what is the thickness of the surface PMMA layer and is it
uniform? Rob Short and his group will let us know in a few years time.
Dr. M El-Gomati, University of York
mmg@ohm.york.ac.uk
Dr El-Gomati described recent research at the University of York
on the development of a combined Back-Scattered Electron Detector (BSED)/Cylindrical
Mirror Analyser (CMA) fitted to an electron microscope which operates at low beam voltage
(>20V) but retains spatial resolution! The system was shown to be highly effective in
determining surface information and very surface sensitive (<10nm). The developments
were driven by the semiconductor industries requirements for a rapid, high spatial
resolution, low radiation analytical technique. Already the developments are very
promising and the instrument may be commercial available in a few years.
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Figure 6. Dr. Robert Wild
Presenting Dr. Steve Evans with the 1998 Rivieré Prize. |
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Dr Dave Briggs addressed the meeting and thanked the delegates
for the 1997 Rivieré prize presented to him at QSA 7 at Surrey University. The prize had
been completely unexpected and he was grateful for the opportunity to thank the members of
the Surface Analysis Forum for the honour. In an attempt to continue this tradition, Dr. Bob
Wild then surprised Dr Steve Evans with the 1998 prize (figure 6). Steve was so
surprised that he did not know what to say, but he has promised to make his acceptance
speech at the July meeting at NPL!
The committee is indebted to the new Surface Analysis Forum official
photographer, Dr Alan Carrick (Acolyte Consultants), for capturing the event for
posterity.
"Probing the surface on the nano-scale
spatially and as a function of depth"
Dr. C Blomfield, Kratos Analytical
Blomfield@kratos.co.uk
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Figure
7a. The Kratos Ultra |
Dr Blomfield gave a presentation based on data obtained from the
Kratos ULTRA (figure 7a), which offers both high spatial resolution spectroscopy
(Images ~2 microns) with high spectral resolution (figures 7b, 7c) using the
monochromator/charge neutraliser developed at Kratos analytical. The first example was
ARXPS measurements on PMMA, using the high sensitivity of the spectrometer to
differentiate between the various tacticities of PMMA, (figure 8).
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Figure 7b. Spectroscopy Mode |
Figure 7c. Imaging Mode |
The system has sufficient sensitivity that good quality
valance band spectra (suitable for chemical state determination) can be obtained in <20
minutes, (figure 9).
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| Figure 9. High resolution
valence band spectra of two PMMA isomers, (isotactic in red, syndiotactic in green)
overlayed and normalised at 8eV. 15 degree take of angle (close to grazing surface)
spectra show a number of subtle differences. |
Dr. Blomfield then showed several high spatial
resolution examples on the oxidation of TiN films on steels. Using the new imaging
machine, Chris was able to show the formation of ~100m m Fe/Cr Oxide islands in the
surface of the TiN film. Instrument was then reset to take spectra from nominally a 27m m
area, in order to give a detailed chemical description of these features. The ultimate
performance of the instrument at present appears to be around ~2m m, whilst retaining good
spectrometer resolution. The examples shown by Dr. Blomfield were in general acquired in
2-20 minutes, which means the system can be used to tackle real problems within a
realistic timescale.
Angle Resolved XPS: –
"The development of a practical spreadsheet package for the
average analyst"
Dr. P Cumpson, NPL.
pjc2@npl.co.uk
Angle-Resolved X-ray photoelectron spectroscopy (ARXPS) can be
used non-destructively to define the depth-distribution of chemical species. Often one
needs a simple qualitative result, such as that "layer A" is above (or below)
"layer B". In these simple cases one examines how the ratio of the two peak
intensities changes as one tilts the specimen close to grazing emission. Sometimes one
needs to use ARXPS quantitatively. This is often the case if chemical state information
from the first 5nm or so is needed, since this is very rapidly destroyed by alternative
depth-profiling methods involving sputtering. By acquiring spectra at a few different
angles of emission we are typically trying to answer one of four types of question;
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Thickness Measurement (TM) - e.g. "What is the
thickness of this aluminium oxide?"
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Depth Measurement (DM) - e.g. "What is the average
depth of this low energy implant?"
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Layer Modelling (LM) - e.g. "What is the order of the
layers in a complex oxide, and how thick are they?"
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Depth Profile (DP) - e.g. "How does
the concentration of different oxidation states of carbon vary with depth over the first
few nanometres in this bio-compatible material?"
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Figure 10. THe NPL ARXPS
Spreadsheet |
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At NPL
we have reviewed the twenty or so different quantification algorithms proposed since 1974
for solving problems like these. One can classify them into families. Most recently we
have developed a computer spreadsheet which allows one to apply the simplest and most
reliable approaches to answer questions of this kind, and incorporate ones prior knowledge
about the specimen. The use of some graphics and the standard "windows" user
interface controls make the spreadsheet reasonably user-friendly. Prior knowledge
turns-out to be extremely important, especially for the more challenging methods, which
give a depth profile.
Comments from Surface Analysis Forum members at the Bristol meeting in
January will help us refine the spreadsheet, to make it easy to apply to the widest
possible range of typical samples. The July '98 Surface Analysis Forum meeting at NPL will
include a "hands-on" computer-based workshop, focussing on how to apply the
quantification methods available within the spreadsheet to typical sets of measured
spectra.
"Recent Advances in Scanning Probe Microscopy"
Dr. J Leckenby, TopoMetrix UK.
topometrix.uk@dial.pipex.com
Due to Dr. Readings unavailability, Dr. Leckenby gave a
presentation on both the general use of AFM and the use of the CASM technique.
He reviewed the various modes of analysis such as:
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Contact |
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Oscillating Cantilever |
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Profile Mode |
Dr. Leckanby then described in detail several examples of AFM studies
on in-situ and ex-situ experiments such as the growth of oxide films on aluminium alloys,
the corrosion of Cu and Fe, and the solvent etching of polymers. Further details of these
particular examples can be found on the TopoMetrix website http://www.topometrix.com/
Two of examples, which Dr. Leckanby gave, were of the most recent
advances in the use of AFM. One was the Layered Imaging experiment, which map surface
compliance and the other was Scanning Thermal Microscopy.
1) Layered Imaging
In Layered Imaging, the user records the cantilever deflection through
a partial or entire force distance curve for each pixel in an image. The actions are
described in Figure 11 below.
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AB-Probe not in contact but approaching the sample, no
attractive or repulsive force, |
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BE-"Jump to Contact", due to attractive pull on
cantilever |
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CD-Upward motion due to sample deforming in response to
cantilever, slope of curve indicates hardness. |
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DE-Reverse of CD, and if CD¹ DE then indicates difference
in elastic or plastic deformation. |
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EF-motion of cantilever from surface to neutral deflection
point, F=total adhesive force between probe and sample. |
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Figure 11. Force Distance
Curve for SPM in Pulse Force Mode |
An example of this type of experiment is shown below (figure
12), where recessed gum on a surface was examined using the SPM. The initial
cantilever force was 0.14nN, then the tip was "pulled" away approximately 265nm
from the surface.
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Figure 12 SPM Compression
Experiment |
2) Scanning Thermal Microscopy (SThM)
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Figure 13. Polymer Blend at 40°C
in the SPM |
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The image shows a polymer blend run at 40C in the SPM. Showing
the two immiscible phases (PVC & PB). When the sample temperature is raised the two
phases interact and merge. This experiment can be monitored in the SPM and the changes in
dimension monitored as the process takes place. It is also possible to use the system to
perform local DMA measurements on the specimens with a spatial resolution of around 200nm.
This shows that SPM now yields chemical information at a very high spatial resolution,
something only surface specific techniques have only really been good at until recently
(and possibly TEM at a push).
The Surface Analysis Forum committee would like to thank Kratos, TopoMetrix
and VG, for allowing data from their Websites to
be included in the meeting report. All three Websites give many more examples of the
latest instrumentation and there application. |
