Monday, July 16, 2018

20 nm Nanoparticles

In this image gold nano-particles were imaged at 2MX using the through lens detector (TLD) in field immersion using secondary electrons. The sample was drop-cast directly onto an aluminum stub and then subsequently heated at over 100 C to drive off any physisorbed water and organic material. Sub 20 nm particles are clearly imaged and differentiated from each other. This is remarkable as TEM or AFM is generally required to image particles of this size.

There are several concerns in taking images at this high magnification. One is that there needs to be sufficient secondary electron contrast to image the objects. This is Au on Al so there is sufficient secondary electron contrast to differentiate the nanoparticles from the stub. Note that the stub looks fairly featureless while an AFM image of the same stub would show structure. There is just no secondary electron contrast from Al on Al at these feature scales. Thus Al particles drop cast on the aluminum SEM stub would impossible to image.

Another concern is sample preparation and chamber vacuum. At very high magnifications the current density is exceedingly large. Organic material on the sample surface and in the SEM chamber will darken the image where the SEM beam interacts with the sample. At low magnifications this effect can be minimal. At the very worst leaving behind dark rectangles of reduced secondary electron contrast when the magnification is subsequently reduced. However at 2MX the image will just turn dark and featureless in seconds.

Another concern is stability. This includes acoustic noise in the SEM lab, mechanical stage drift, thermal drift of the sample as well as of the SEM column, and electromagnetic interference in the SEM lab. Mitigating the effects of electromagnetic interference may require active field compensation or shielding in the SEM lab. The effects from mechanical vibration and stage drift may require changing scan speeds and applying the stage clamp which helps lock the stage down. The objective lens is not a constant-current lens, and there is always some image drift until it comes to thermal equilibrium. Another significant instability is magnetic hysteresis in the objective lens. Remnant magnetic field in the lens can lead to drift. This is mitigated using the lens reset function.

The point of this application note is that 2MX magnification may seem very exciting-- but results are entirely a matter of sample preparation and operator skill!

Sunday, July 15, 2018

Sample Preparation: Dilution, Dispersion and Surface Charge

Successful SEM imaging requires not only competent microscope operation but also expert sample preparation. In the case of imaging nanostructures, sample preparation is generally the most difficult task. In this example cellulose nanofibers were imaged at low energy due to their low density. In the first image we see an undifferentiated mass as the undiluted but dried fibers were imaged after being dried on a stub and coated with 4 nm of iridium to suppress charging.

The individual fibers are largely unobserved. This is due to the surface charge of the fibers causing them to aggregate to not only each other but also the stub itself. At this point successful imaging comes down to successful sample preparation.

The first plan of attack in any dispersion problem is dilution. In the second image the fibers were imaged after significant dilution. Dilution helps disperse nano-structures by simply keeping them from each other so that they can not aggregate. Dilutions of 1:100 to 1:1000 or more can be necessary.

It was found that dilution alone did not solve the aggregation problem. Dilution provided small patches of agglomerated fibers that resembled the first image. This was because cellulose fibers are generally tangled together and can not be separated upon dilution-- only individual clumps can be separated.

To mitigate surface charge the nanocellulose fibers were subject to washing in DI water and drying with a polar solvent to screen surface charge. Then the material was placed in a non-polar solvent to maintain space between the fibers. This mixture was then quickly dried so that free space between fibers was preserved.

In practice the surface potential of nanoparticles may need to be measured using an instrument that measures zeta-potential, and the surface potential neutralized using a solvent of an appropriate pH. this is often the case with such nanoparticle systems as aluminum oxide where surface potential is highly dependent upon synthesis and post synthesis handling.

The moral of the application note is to not give up when faced with useless images-- success is often a matter of a little experimentation with sample preparation.

Natural Nanopillars: Cicada Wing

Sometimes it is useful to have a source of regular nano-structures for a variety of imaging and metrology tests. Dispersing structures can be difficult due to aggregation due to surface change or substrate charge. Luckily nature provides arrays of these on cicada wings. The natural nanopillars on cicada wings have a natural anti-microbial affect as micro-organisms are torn apart by these pillars.

In this image taken at lower beam energy (3 kV) using the through lens detector in field immersion a large field of regular nanopillars is seen at 30 kX magnification. The pillars have local order, falling into regular domains without long term order.

The second image taken at 300 kX shows the faceted structure of the nanopillars and confirms their diameter as less than 100 nm on average.

The cicada wing was pulled off a dead insect and coated with 4 nm of iridium to suppress charging without any additional sample preparation.

Wednesday, July 11, 2018

Subsurface Imaging with SE2 Secondary Electrons

The range of electrons in materials is dependent upon their energy. This is a physical property that can be exploited to image sub-surface structures. This is a thermoplastic coated with 4 nm of Ir to suppress charging. In the first image the sample is imaged at 50kX at 15 kV using the through-lens (TLD) detector and field-immersion using secondary electrons.

Brighter regions are visible, indicating regions of higher secondary electron emission. A close examination seems to suggest that these particles are not resting on the top of the surface. In the second image the beam energy is dropped down to 2 kV and these bright regions disappear showing a fairly smooth surface. What is going on?

These images show the difference between SE1 and SE2 secondary electrons. In the first image taken at 15 kV there are secondary electrons coming from the sample surface, but also back-scattered electrons scattering from denser objects just under the sample surface. These elastically scattered electrons have considerable energy as they leave the surface, nearly 15 kV, and can generate secondary electrons themselves. These secondaries are called SE2 and appear in the vicinity of the dense sub-surface particles.

By dropping down to 2 kV these SE2 are no longer generated because the cross section for elastic scattering is greatly reduced, as is the cross section for generating SE2's from lower energy electrons. Thus the lower energy image fails to show these subsurface structures.

The point of this application note is to show that SE2's can reveal subsurface structure. This can be exploited to see objects under the surface of a sample. It can also be a source of confusion-- and imaging at several beam energies may be required to resolve this confusion.

The final images shows a back-scattered electron image (BSED) confirming the presence of higher-density inclusions in this material.

Thursday, July 5, 2018

Stigmators: Adjusting and Optimizing Astigmatism

The correction of astigmatism is essential to high resolution imaging, particularly at higher magnifications. Astigmatism is an aberration where the electron column has different focal lengths in two orthogonal directions. This means the image will be focused in one direction at one focal length, and focused in an orthogonal direction at a different focal length. In pragmatic terms this can also be understood as the objective lens (OL) being able to focus to a "blob" but not a point at the optimal focus.

Stigmators are octopole lenses (sometimes 12-pole lenses) that compensate for astigmatism at the level of the objective lens.  This series of images shows the effect of stigmator adjustment. The top image shows the best possible focus of a small particle at 40kX. The working distance, reflecting the OL focal length, is 19.4 mm.

The next image the OL focal length is reduced to 19.2 mm and the image shows streaks going from bottom right towards top left. Another way of looking at that is the image is focused along a roughly 45 degree diagonal from bottom left to top right.

The third image shows the OL focal length increased to 19.5 mm and the image shows streaks going from the bottom left to the top right. Another way of looking at that is the image is focused sharply along a roughly 135 degree diagonal from bottom right to top left.

This streaking in one direction followed by streaking in an orthogonal direction at a different focus is a classic sign of a significant amount of astigmatism.

The final image shows the same particle with the stigmators properly adjusted. Without astigmatism correction this would have looked like a particle on a smooth background, not a background of nanoparticles.

The best approach in astigmatism correction is to find the OL focus settings that provide the second and third images, and then focus in between. This should provide the first image. Then adjust the X- and Y-stigmators, followed by fine focus, iteratively, until one arrives at the last image.

Thursday, June 15, 2017

FEI Nova 400 nanoSEM resolution!

The FEI Nova 400 nanoSEM  has a combination of features which allows for superior imaging quality at nanoscales. In addition to the FEG source which provides a smaller and more stable emitter than a tungsten thermionic emitter, there are special detector configurations that are ideal for high resolution work at small working distances.

The ET-detector works optimally for larger working distances because of the larger solid angle captured by the biased cage on the end the detector.  For small working distances, a through lens detector (TLD) is better, as a larger solid angle can be captured by the opening of the objective lens and directed into the secondary electron detector (SED)-- the scintillator and photo-multiplier shared with the ET-detector, or in FEI jargon, ETD. To increase secondary electron detection efficiency, field immersion can be applied. In this case the field of the objective lens is allowed to penetrate into the space around the sample. Electrons generated within this region are directed into the objective lens (OL) and into the TLD.

The result is extraordinary resolution. In this image of Au clusters on HOPG taken at 15 kV with TLD in field immersion, the facets on and grain boundaries between individual Au-clusters are visible, as are clusters as small as 10 nm.

Much higher resolutions are also possible. The bottom image is of NIST SRM 8012-- 30 nm Au particles. This SRM was diluted and drop cast right onto a standard Al stub and imaged directly. n-mers of spherical particles are readily discernible, remarkable as TEM is generally required for structures of this size.

Monday, August 15, 2016

SEM Upgrade

The Condensed Matter and Materials Physics (CMMP) group at the FSU Department of Physics and the Biological Sciences Imaging Resource (BSIR) have combined their efforts to bring more advanced scanning electron microscopy (SEM) to the FSU research community.

CMMP has decommissioned its JEOL 5900 to help BSIR support and manage its FEI Nova nanoSEM 400. This system includes a FEG source, low vacuum capability, and an Oxford INCA x-sight EDS detector. A Raith ELPHY e-beam lithography (EBL) package is available for e-beam lithography.

This highlights CMMP's committed effort to work across organizational boundaries to support research at FSU.

Wednesday, January 7, 2015

The Choice of Beam Energy: The Letter "a"

Any SEM image is simply the output of some detector as a function of the scanned electron beam position. As such the X and Y axes relate directly to lateral position, and this position can be calibrated using a lithographed length standard. The image brightness and contrast relates to some probe-sample interaction that is being monitored using any number of detectors available to the microscope. The most common are secondary electron detectors (SEI), such as the ET-detector (Everhardt-Thornley detector), which detect low energy electrons generated through a large number of inelastic scattering events, and back-scattered electron detectors (BEI), which detect elastically scattered electrons that undergo a small number of elastic scattering events. The key to interpreting SEM images is an understanding of these probe-sample interactions, but the choice of operational parameters is also dependent upon the basic physics of electron-sample interactions.

In this example a 7-point Garamond letter "a" was printed on a laser printer and coated with a few 10's nm of graphite to suppress charging. In the first image this letter "a" was imaged at 20 kV using secondary electrons (SEI mode) where the presumption would be that the column performance is quite good. The performance of the JEOL 5900 is specified as a resolution of ~ 10 nm as measured imaging Au clusters on HOPG at 30 kV-- so this image should be near the optimal column performance. The fibrous structure of the paper is very clear, as are brighter non-fibrous structures that were shown to be surface treatment material through EDS. There is very little evidence of our little letter "a" even though we know it is there by visual inspection. The laser printer has clearly fused enough toner to locally change the optical properties of the paper surface, but those regions don't produce significant secondary electron contrast to see the "a" when imaging in SEI mode.

In the second image, taken at 10 kV, small amorphous blobs of fused toner start to become visible, especially on some of the larger flat fibers. Nevertheless, it's largely impossible to make out the "a" letter.

The third image is taken at 2.5 kV and the soft low-density toner produces enough secondary electron contrast to be imaged. An outline of the letter "a" is very clearly visible, and it is obvious that it is constructed of fused blobs of amorphous polymer bonded to the paper's cellulose fibers.

The point of this application note is that the beam energy must be selected depending upon the system to be imaged-- not just in consideration of electron-optical performance of the column.  In this case the actual part of the sample to be imaged does not require optimal column performance in terms of probe diameter, but sufficient secondary electron emission is required-- and that is possible only at lower beam energies.

The last image is a back-scattered compositional image (BEI COMPO) which reflects the local density or average atomic number of the printed "a" letter. This image was taken at the same beam energy as the first image where the "a" was invisible. The region of interest, the pattern provided by the fused toner, is very slightly less dense than the surrounding cellulose matrix and is easily visible in this imaging mode. The paper additives are also more obvious and appear as the very bright domains.

That is the other point of this application note. Not only does the beam energy need to be selected according to sample, but also the imaging detector. In many cases a sample is best suited imaged at several beam energies and using several detectors to fully understand the sample under investigation.

Tuesday, January 6, 2015

Pectinariidae Tubes

In the following images the magnification and length scales are incorrect due to an instrument failure! There is a lesson in this: always check measurements to standards!

Pectinariidae are a family of marine polychaete or segmented worms that build conical trumpet or ice cream cone shaped homes. The conical homes are upside down in the marine sea bottom, with the small opening at the surface and the cone opening up wider and wider in the sediment. The pectinariidae or "ice cream cone worms" use their robust setae to dig into the sediment, while using tentacle like structures to bring food to their mouth. Grains of sand are passed through their guts or between the gap between the worm and its conical home. Much of the material it digs and sifts through is passed through its alimentary canal and expelled from the cone.

The conical homes are assembled by the worm using its mouth parts to select and locate grains according to shape. Since the cones are exactly one grain thick, the grains must be well fit for structural strength. EDS spectroscopy verifies the grains to be simple sand, quartz, and the presence of NaCl. The samples were found, dried, and flash coated with graphite without additional processing to remove salt or other debris such as the occasion diatoms that can be seen hitch-hiking.

Grains of sand are cemented together using a protein-based cement from worm's mouth. EDS spectroscopy shows that the interstitial regions contain NaCl and light-element material: carbon, oxygen, etc., which is consistent with an organic material. In this image some diatoms can be see near the top of the image. The cement appears to be porous, but upon further inspection it is shown to consist of bubbles. The formation of bubbles may reflect the production of the protein at the ice cream cone worm's mouth, and may be useful in the structural project of cementing these grains together as it allows the worm to fill as much gap-volume as possible with the smallest amount of actual cement material.

The final micrograph shows the edge of the tube at high tilt. The large depth of field at large working distance allows one to see much of the length of the tube in addition to the edge of the tube. The length scale of individual grains are clear, and the tube is clearly a single sand grain thick.

Thursday, August 1, 2013

A Biological Example: A Centric Diatom

This image taken at 20 kV with secondary electrons shows the centric diatom cyclotella choctawhatcheeana Prasad in a presentation that resembles an exploded view. The topmost structure is the external surface of the valve. Behind it are the circumferential "girdles" and and the very bottom the internal surface of the valve. Identification of diatoms from their unique morphological signatures gives insight into the water chemistry and ecology of the water column through its annual cycle.

Thanks to Dr. A. K. S. K. Prasad of the FSU Department of Biology who discovered this particular diatom species.

Tuesday, July 2, 2013

Light Element EDS

The ability to resolve very light elements is entirely depending upon the pulse shaping time constants used. The top images shows the lower energy portion of an EDS spectrum of magnetite (Fe2O3) taken at 30 kV using a 16 µs shaping time constant. The Fe Lα peak is clearly visible to the right of the O Kα peak. There is a small shoulder for the C Kα peak since the sample was coated with graphite to suppress charging. The peak just above the Si Kα line is actually the Si K absorption edge. Using data of this quality one could, with the use of standards, quantitate oxygen using O Kα. The Cr Lα peak is shown near the O Kα peak to illustrate a common overlap with O Kα. Chromium is also a common trace contaminant in mineralogical magnetite.

The detection efficiency of the light elements is less than the higher X-ray energy elements, and thus one might be tempted to decrease the shaping time constant, TC, to increase the throughput of the EDS detector allowing one to perform EDS with more beam current and higher acquisition rates.  The second image shows the same energy region with a time constant of 0.5 µs.  The energy range is attenuated around 2 keV and all information regarding light elements is lost. This is an artifact of pulse processing.

The point of this brief example is very simple: use the larger TC's for light element studies! A TC as small as 8 µs should allow for the resolution of C Kα, and for O Kα one can go as low as 1 µs.

Galena: An Illustration of EDS Spectral Overlaps

With an energy resolution of ~ 143 eV at the Mn Kα line (5.893 keV), the problem of spectral overlaps is a common problem with EDS applications. In the top image a spectrum of the mineral galena, PbS, is taken at 30 kV using a 0.5 µs pulse shaping time constant. The S Kα line is at 2.306 keV and the S Kβ is at 2.464 keV, with the Pb Mα and Mβ in the general vicinity-- at 2.342 keV and 2.442 keV respectively. Because of the high Z of Pb, there are other lines just above and below: the Pb Mz at 1.839 keV, the Mγ at 2.652 keV and the M2N4 at 3.124 keV. The difference between the S Kα, S Kβ, Pb Mα, Pb Mβ and Pb Mγ are all within the resolution of the EDS detector which is ~ 263 eV at this time constant.

Having an accurate estimate of the EDS detector resolution (the FWHM of the peaks) as a function of energy, together with the energies and relative intensities of the Kα/Kβ and Mα/Mβ doublets allows one to attempt to deconvolute the Pb and S contributions to this slightly asymmetric peak.  The result?  Pb:S is determined, without standards using ZAF matrix corrections, to be 73:27.

If one is to use EDS, the only possibility is to increase the resolution by increasing the pulse-shaping time constant as shown in a former application on this blog.  Increasing the TC from 0.5 µs  to 32 µs  drops the peak FWHM for Mn Kα from ~ 263 eV to ~ 142 eV. That data is shown in the second image. The increased resolution is immediately apparent. The Pb Mz is very clearly resolved as a separate peak on the left of the main peak, as is the Pb Mγ on the right. The S Kβ and Pb Mβ is apparent as a slight shoulder on the right of the main peak.  The S Kα and Pb Mα are still left unresolved.

Again, knowing the detector resolution as well as the energies and relative intensities of the Kα/Kβ and Mα/Mβ doublets one can attempt to deconvolute the Pb and S contributions in this higher resolution peak. The result: Pb:S is 61:39.  Better, but still far from being a convincing estimate of the 1:1 stoichiometry of galena. This is not a failure of not using standards or a limitation of the matrix corrections. The EDS spectrometer simply can not resolve the peak shape with sufficient resolution and statistical certainty to allow the deconvolution of S Kα and Pb Mα which are 36 eV apart.

How to do better?-- wavelength dispersive spectroscopy (WDS) where the X-rays are diffracted from crystals or multi-layers.

EDS Pulse Processing Time Constants

The fundamental principle of EDS is energy dispersion. By that it is meant that X-ray energies are measured by directly measuring their energy. In EDS detectors, X-rays generate an electrical cascade in a solid state detector where the resultant pulse height is proportional to the X-ray energy.

The pulse processing electronics of the EDS detector can be modified to change the performance of the detector. The most important parameter is the pulse shaping time constant, or as it is indicated in the software-- TC. The top graph shows the Mn Kα peak width as estimated by the FWHM as a function of the time constant, TC.  The smaller TC's result in wider peaks and thus lower spectral resolution.  As the TC is reduced the Mn Kα width reaches a floor of ~ 142 eV which represents the resolution of the EDS detector.  It should be noted that it is convention to measure the resolution of an EDS detector at the Mn Kα line: 5.893 keV.

An obvious question is why would anyone then want to increase the TC and thus reduce the resolution of the EDS detector given the potential problem of overlaps?

The second graph shows the maximum detected rate-- defined as the maximum rate at ~ 28% dead time-- on a Mn sample with a 30 kV beam. Note that empirically there is a power-law that relates TC to maximum rate through a negative exponent close to -1.  As such the maximum rate is nearly inversely  proportional (but not exactly) to TC.  At at 0.3 µs time constant the maximum rate is about 90 kc/s, while at the 32 µs time constant the maximum rate is 1.5 kc/s.

As such the smaller time constants are best for high throughput applications. These would include EDS mapping and the detection of trace elements in the absence of potential overlaps. The larger time constants are best suited to higher resolution applications, such as quantitation that requires deconvolution of overlapping peaks. The larger time constants are also useful for very light elements as will be demonstrated in a future application note.

When are Standards Necessary?

The EDS spectrum shown was taken at 30 kV on a piece of GaAs wafer with a 2 µs shaping time-constant. The Ga and As Lα peaks are visible just above 1 keV while the Ga and As Kα and Kβ peaks are from 9.2 to 11.7 keV. These two sets of peaks have overlaps that can be resolved by deconvolution, and the peak sets differ by roughly 10 keV.

This is a very good test example to illustrate when standards are absolutely necessary for EDS quantitation. Ga and As were quantitated using standard ZAF corrections which account for such effects as the excitation volume, back-scattered current, absorption and fluorescence in the sample matrix. Obviously Ga:As should be 1:1 for a GaAs wafer-- and in this example we will look at Ga:As when quantitating with different combinations of these lines.

Common practice would be to quant using the Ga Kα and Kβ lines as there is less overlap with the K-lines than the L-lines. Ga Kα:As Kα yields 49.7:50.3-- which is quite close to the 1:1 expected from a GaAs wafer. If one is concerned about an accurate estimation of the As Kα intensity given the need to deconvolute the Ga Kβ peak, one can also quant Ga Kα:As Kβ-- which yields 49.3:50.7.  Both approaches are very close to the expected 1:1 value.  The error in any component is < 1%.

For the sake of exploration, consider quantitating with the low energy Lα lines. Ga Lα:As Lα is 48.6:51.4. The quant is now more than 1%, but just marginally so.

Let's consider quantitating with one high energy Kα line and one low energy Lα line. Ga Lα:As Kα is 44.7:55.3 and Ga Kα:As Lα is 53.0:47.0. In the first case the lighter element, Ga, is underestimated by 5.3% and in the later case the heavier element is underestimated by 3.0%-- but in each case the lower-energy component is the component underestimated.  We are now unable to argue convincingly that the GaAs wafer is stoichiometric 1:1.

What we find is that we can adequately quantitate without standards if the X-ray lines are close in energy.  When the lines differ by close to 10 keV-- then there are problems. Why?  Without resorting to standards we are not accounting for detector-specific losses such as absorption in the EDS detector window and the material that is cryo-pumped on its surface, as well as absorption in the body of the silicon EDS detector itself.  Such absorption is more significant at lower energies and thus we tend to underestimate those  components without resorting to standards. Another important effect is the energy dependent quantum efficiency of the EDS detector itself.

Thursday, June 27, 2013

Au coating

Unless one wishes to image at very low beam energies-- 0.5-2.0 kV for most materials-- insulating materials must be coated with a conducting material before imaging. This conducting over-layer allows electrons to be sourced from the grounded sample stage to neutralize positive surface charge due to secondary and back-scattered electron current leaving the sample. In the case of an environmental SEM or low-vacuum SEM this charging can also be dissipated using a quench gas such as water vapor or environmental gasses.  The CMMP JEOL 5900 does not allow this.

Coating with a conducting material also improves image resolution as it enhances secondary electron contrast in SEI imaging modes.  Since SEI images are essentially maps of secondary electron current versus lateral surface position, metal coating can greatly improve spatial resolution by increasing the secondary electron contrast between small structural features.  Even if a sample is conducting or semi-conducting enough to allow SEM imaging, it is often beneficial to perform metal coating.

Generally AuPd is used to coat samples as the metal grains are on the order of 20-25 nm. Given that the JEOL 5900 has a resolution on the order of ~ 20 nm at 30 kV and a 6 mm working distance, such grains would never been seen, though they would certainly be seen in a field emission SEM.

Recently the AuPd sputter target failed, and having several Au targets in stock we have migrated to Au coating for the time being. Au coating is a little less desirable as the metal grains are on the order of ~ 50 nm, potentially just visible at the highest magnifications in the JEOL 5900. For routine work at several 10kX and below, it's a non-issue. Like AuPd, Au is also a very good secondary electron contrast coating.

One important difference between the AuPd and Au sputter targets is the sputtering yield. Au is softer than the AuPd allow and produces more sputtered material per unit time per unit of plasma current. While the AuPd target with a 25 mm target distance and 30 mA plasma current produced a ~ 15 nm film, the Au target produces a ~ 45 nm film under the same conditions.

The figure above shows the fit of Au film thickness versus sputtering time for a 25 mm target distance and a 30 mA plasma current.  The fit is: thickness [nm] = 0.629 [nm/s] * sputter time [s] + 1.389 [nm].

The target thickness is really dependent upon the surface morphology and the method of coating.  A very smooth surface with very little morphological relief can be coated with as little as 5 nm of a small-grain metal coating like AuPd or Ir-- just thick enough to produce a continuous film.  Samples with very large surface relief require thicker coatings or coating with the planetary motion attachment to assist coating the peaks and valleys in the sample topography.  Given that the resolution of the JEOL 5900 is ~ 20 nm, we've been aiming at ~ 15 nm coatings.  Very rough topographies might warrant 30 nm coatings-- even thicker or coating using the planetary motion attachment.

For a 25 mm target distance and a 30 mA plasma current the following guidelines can be used:
  • 10 nm: 14 seconds
  • 15 nm: 22 seconds
  • 20 nm: 30 seconds
  • 25 nm: 38 seconds
  • 30 nm: 46 seconds
The above data was taken by coating microscope cover slips, scratching the surface and profiling the scratch with a stylus profilometer.

Saturday, June 1, 2013

Which Beam Energy Should I Use?

A common question is which primary beam energy is most desirable for imaging a particular sample. The question can only be answered by identifying imaging objectives and considering the physics of electron-beam sample interactions. While SEM column specifications are often optimal at higher energies, typically 30-35 kV, it is generally not primary beam diameter that determines image quality unless one is looking at nano-structures that are comparable in size to the probe size.

In this study activated charcoal was imaged at different beam energies at the same magnification, probe current and working distance. The same porous area of the charcoal surface was imaged, though some image rotation is noticeable due to the Lorentz force exerted on the rastered e-bean in the objective lens. Being graphitic, the sample was uncoated and was known to contain ash mineral contamination. To avoid operator "aesthetic bias" all images were ACB'd-- automatic contrast and brightness adjusted-- to highlight systematic changes with beam energy.

The top image was taken at 30keV.  It clearly shows the pore structures in the activated charcoal-- the horizontal ridges on the sample surface and the pores that go into the sample volume. The pores appear quite clearly, and there is some evidence of edge effects-- the edge brightness enhancement on the horizontal margins of some of the larger pores. All in all, the surface of the porous tubes looks fairly smooth, and there is some evidence of little particles on their surface. This isn't very shocking as graphitic materials have a low efficiency for generating secondary electrons. The sample looks "soft" and largely featureless, much like the carbon tape that holds it down.  It would look more so without the particulate material coating the pores.

Dropping the beam energy 10 kV is shown in the second image. It's clearly the same structure-- but it is slightly nuanced. The electron range is significantly less at 10 kV, even in a low density matrix like activated charcoal. This results in the enhancement of edge effects.  In the second image all of the pore edges are brighter due to edge enhancement. Edge enhancement occurs in secondary electron imaging--SEI-- because secondary electrons can be collected not only from the sample surface, but also the edge of the feature and perhaps even hidden surfaces of a feature. Much more surface topography of the pores is also visible at 10 kV. There are clearly ribs and ridges on the pore surface, and smaller secondary pores that emerge from the larger pores.

Most striking is the enhancement of the particulate surface trash. It is very prominent at 10 kV. It could simply be due to an increase in secondary electron emission efficiency at 10 kV. In general, we have to differentiate SE1, secondary electrons produced by the primary beam, and SE2, secondaries produced by back-scattered electrons. The lower electron range at 10 kV in the graphitic matrix increases the probability of generating SE2, and this perhaps is the cause of increased contrast of the surface trash.

In the final image the beam energy is dropped to 1 kV. The efficiency for generating secondary electrons is very low at these low beam energies, and I like to characterize these low-energy images as "chalky". We aren't inventing or losing any morphological features as we change beam energies. The same features-- pores, pore ridges, small side pores, particulate matter-- are all visible, as the such image artifacts as edge enhancement, but the image quality is largely driven by detection efficiency. We get a very clear sense of the relief of the pores actually penetrating the sample.

So, which is correct? They all are. The SEM isn't lying. Which is best? It depends upon the story one wishes to tell with the SEM images. We finish the example with three images taken at much lower magnification, taken at 1 kV, 10 kV and 30 kV. Same general morphology-- different aesthetic quality and different features naturally enhanced.

Friday, March 1, 2013

Tilt, Image Collection and Morphology

One of the keys to obtaining high quality SEM images is an understanding of image formation. A secondary electron image is a map of secondary electron intensity as a function of position. Where this becomes subtle is how are the secondary electrons detected?

In an Everhart-Thornley (ET) detector secondary electrons are collected by a slightly positively biased cage where they hit a scintillator and create light which is then subsequently detected using a photo-multiplier tube.

One trick in obtaining high quality images is learning to exploit the collection characteristics of the detectors one is using. In the case of the standard ET-detector, there is a sampling bias-- electrons emitted in the direction of the ET-detector are detected more efficiently. In the first image we see Au clusters of HOPG. The clusters are well resolved, but there is some hint of ordering of these clusters as indicated by the faint lines from the bottom left towards the top right of the image.

If we tilt the sample towards the ET-detector, then we will increase the secondary electron detection efficiency. In a sample like this there is very little vertical morphological contrast, so there is very little secondary electron collection contrast between higher and lower parts of the sample.

In the second image the sample is tilted + 30 degrees towards the ET detector. Now, even though it has vertical features of only a few 10's nm we can clearly see the structure of the cleaved surface of the HOPG. This level of structure is lost without the tilt of +30 degrees.

In the final image the sample is tilted - 10 degrees away from the ET detector. In this case all structure from the cleavage of the HOPG surface is lost. All we see in the final image is secondary electron contrast from the Au clusters, not from the terraces of the HOPG crystal.

The purpose of this short note is to show how to gain structural resolution by tilting the sample towards the ET-detector. Sample tilting is more than just seeing the undersides of things!

Friday, October 5, 2012

Field Emission SEM

This is an image of the Au cluster on HPOG standard used to check the resolution of the JEOL 5900 SEM. This image was taken using an FEI Nova 400 nanoSEM field-emission FE-SEM in the FSU BSIR. A comparison can be made with an image taken with the JEOL 5900 in a previous post.

While the JEOL 5900 SEM image taken at CMMP was not under optimal conditions-- it was a particularly bad day for building noise-- it should be noted that not all electron microscopes are the same! SEM's and FE-SEM's are different instruments with unique capabilities and limitations.

One of the big differences is the type of emitter. SEM's using tungsten filaments use thermionic emission to generate electrons. Such emitters are spatially extended and subject to thermal drift while producing a fairly low brightness source. Field emitters used in FE-SEM's extract electrons using electric field as well thermal energy, and thus produce very small high brightness sources.  The FE-SEM emitter is always on so these sources are less subject to thermal drift. On the other hand, field emission sources are much more expensive and prone to destruction by poor vacuum.

Electron optical column design aside, there are other important differences.  Because of the high spatial resolutions possible with FE-SEM's, images are often not taken with the traditional ET-detector. Instead, a "through lens" detector or TLD is used, where secondary electrons are detected in the objective lens. In this case, because of the small working distances typically used for high resolution work, a significant solid angle of secondary electrons can pass into the objective lens. The detection efficiency is increased in field immersion mode, where the sample area is embedded the magnetic field of the objective lens, as this increases the number of electrons that make it up the column to the TLD.

This last image at 500,000X shows the capabilities of the Nova 400 FE-SEM on this Au cluster sample.  This image was taken at 15 kV with TLD and field immersion.  Individual nanoparticles are visible in the gaps that are unresolvable in the JEOL 5900.

For more information about the BSIR FE-SEM look here.

Monday, July 30, 2012

AuPd Coating Procedure Part 4: Coating and Venting

In the previous three parts of this post, the various parts of the sputter deposition recipe were configured one by one. First, the sample-target distance was set. Then the sputter chamber was pumped out to its based pressure and back filled with a target partial pressure of Ar. Then given a deposition calibration and a target film thickness a plasma current and deposition time were configured and set.

During this period the target shutter was closed to prevent any material from being deposited on the sample.  Now the shutter must be opened. The top of the sputter attachment is shown. The knob is on the red or CLOSED configuration. Turning it to the green is the OPEN configuration.

Now with the shutter open, the high tension START button "O" is pushed-- see Coater Parts to find this button-- and the film is deposited. It is good practice to check the Ar base pressure and plasma current just before beginning the deposition. Adjust the plasma current during the deposition to maintain the target plasma current and thus the target film thickness.

When the timer times out then the plasma current will natural go to 0 mA. Shut off the gas, gauging, turbo pump and mains power.  Turning off he mains power vents the sputter chamber so that the samples can be removed. 

In summary:
  • Open shutter.
  • Press START to begin sputtering.
  • Wait until timer finishes.
  • Close needle valve and Ar bottle.
  • Turn off gauging.
  • Turn off turbo pump and make sure it's not on half speed setting.
  • Turn off mains power to vent.

AuPd Coating Procedure Part 3: Configuring Plasma

For sputtering, a partial pressure of Ar is required in the sputtering chamber. As such, it is counter productive to rapidly pump away the Ar introduced as a sputter agent. To increase the Ar base pressure in the sputtering chamber we first set the pump to "half speed" by pressing the "L" button on the turbo pump controller.  Look here if you can't find the half speed button. The turbo will gradually slow down, but will do so more quickly as the gas load is increased when argon is introduced. As the turbo slows several of the green LED's on the pump controller will dim. Look here to locate the half-speed button and the turbo speed indicating LED's.

First one opens the Ar cylinder using the valve V1. Make sure the regulated pressure is just a few PSI as indicated by the red arrow. One is leaking a very small amount of Ar into the sputter chamber, so a large pressure head is not required as there is very little gas flow. Open V2 to open the gas cylinder to the sputter coater.

Ar can now be leaked into the sputter coater using the leak valve shown to the left. This valve is easily damaged, so do not over tighten. Only finger tighten to close. Again, without a deposition rate monitor, the film thickness can only be determined by recipe, and for the current deposition calibration an Ar partial pressure of 5x10-2 mbar was used. As Ar is leaked into the chamber the turbo pump, now set to half-speed, will start to slow-- which will reduce the pumping efficiency. As a result the Ar partial pressure will be unstable until the turbo pump fully reaches half speed. Once the pressure is stable one can move on to the next step which is setting the high voltage for the desired plasma current.

Make sure the sputter coater is configured for sputtering by setting switch "B" to HTS which stands for "high tension supply". Now knob "C" controls the high voltage supply. On the high tension supply unit set switch "N" to SPUT for sputtering.

At this point we have a predetermined sample-target distance of 25 mm and a Ar partial pressure of 5x10-2 mbar. We now need to set a fixed deposition time. As determined in the previous AuPd coater calibration, for the parameters above a 55 s deposition time yields a 25 nm coating at 30 mA emission current.

The deposition time can be set using the set of push buttons indicated by "Q".

Now with the shutter closed, press the green high voltage START button "O" and turn the knob "C" up to somewhere between 5-6 to read a plasma current of 30 mA on meter "R". When the START button "O" is pushed, the timer will start counting down as indicated by the LCD display "P". If the timer times out then the voltage will turn off and the plasma current will naturally drop to 0 mA. Press "O" again and continue to adjust knob "C" to set the desired plasma emission current. It's important to perform this step with the shutter closed or material will be deposited on the samples while one adjusts the coater parameters.

Since the plasma current is a function of the Ar partial pressure, the plasma current reading at meter "R" will vary over time if the turbo pump is not fully at half-speed and the Ar partial pressure isn't stable.

In summary:
  • Set coater to HTS mode.
  • Set high tension supply to SPUT.
  • Make sure target shutter is closed.
  • Set timer to the desired deposition time.
  • Press high tension supply START button-- timer starts.
  • Adjust knob "C" to desired emission current on meter "R".
  • Proceed to AuPd Coating Procedure Part 4: Coating and Venting

AuPd Coating Procedure Part 2: Pumping

With the sample stage height set, the samples in place, and the O-ring and chamber sealing surfaces carefully cleaned (and if needed lightly lubricated with vacuum grease)-- start pumping.

Turn the mains "A" on.  Look here if you can't find the main power switch.

Turn on the pump power by pressing "K".  Make sure "L", the pump half-speed setting, is NOT pushed in. The green LED's in the indicator "M" will start lighting. To make sure there is a good vacuum seal, trying lifting the sputter head upwards slightly. If it doesn't move-- then the chamber is sealed.  f one can lift it then there is a major leak. Turn the pump off using "K" and check for leaks.

Turn on the gauging using "H". As the system pumps one should see the pressure gauge drop on the scale "I". One is initially reading the Pirani gauge on the black scale. The indicator "J" will be off.

In time the pressure will drop below 10-3 mbar on the black scale and the gauges will switch. The indicator "J" will go on indicating that the scale "I" is reading the cold cathode gauge on the red scale.

Wait until the pressure reaches the desired base pressure. For best results, the base pressure should be < 10-4 mbar. The absolute base pressure is between 10-6 and 5x10-5 mbar. The time to reach the base pressure is determined by many factors including cleanliness while handling the components of the sputter coating (e.g. bare hands vs. gloves), the porosity of the samples (e.g. mesoporous silica), any water or solvents present in samples (e.g. clay minerals, sample mounting (e.g. Ag or C paints). An empty chamber should pump to 5x10-5 mbar in roughly 30-40 minutes.

This last photo shows the vacuum gauging at the highest base pressure recommended for AuPd sputter coating-- just above 10-4 mbar.

In summary: