EDS Detectors

Considerations for your New Year’s Resolutions from Dr. Pat

Dr. Patrick Camus, Director of Research and Innovation, EDAX

The beginning of the new calendar year is a time to reflect and evaluate important items in your life. At work, it might also be a time to evaluate the age and capabilities of the technical equipment in your lab. If you are a lucky employee, you may work in a newly refurbished lab where most of your equipment is less than 3 years old. If you are such a fortunate worker, the other colleagues in the field will be envious. They usually have equipment that is much more than 5 years old, some of it possibly dating from the last century!

Old Jalopy circa 1970 EDAX windowless Si(Li) detector circa early 70’s

In my case, at home my phone is 3 years old and my 3 vehicles are 18, 16, and 3 years old. We are definitely evaluating the household budget this year to upgrade the oldest automobile. We need to decide what are the highest priority items and which are not so important for our usage. It’s often important to sort through the different features offered and decide what’s most relevant … whether that’s at home or in the lab.

Octane Elite Silicon Drift Detector 2017 Dr. Pat’s Possible New Vehicle 2017

If your lab equipment is older than your vehicles, you need to determine whether the latest generation of equipment will improve either your throughput or the quality of your work. The latest generations of EDAX equipment can enormously speed up throughput and the improve quality of your analysis over that of previous generations – it’s just a matter of convincing your boss that this has value for the company. There is no time like the present for you to gather your arguments into a proposal to get the budget for the new generation of equipment that will benefit both you and the company.
Best of luck in the new year!

Adding a New Dimension to Analysis

Dr. Oleg Lourie, Regional Manager A/P, EDAX

With every dimension, we add to the volume of data, we believe that we add a new perspective in our understanding and interpretation of the data. In microanalysis adding space or time dimensionality has led to the development of 3D compositional tomography and dynamic or in situ compositional experiments. 3D compositional tomography or 3D EDS is developing rapidly and getting wider acceptance, although it still presents challenges such as the photon absorption, associated with sample thickness and time consuming acquisition process, which requires a high level of stability, especially for TEM microscopes. After setting up a multi hour experiment in a TEM to gain a 3D compositional EDS map, one may wonder Is there any shortcut to getting a ‘quick’ glimpse into 3-dimensional elemental distribution? The good news is that there is one and compared to tilt series tomography, it can be a ‘snapshot’ type of the 3D EDS map.

3D distribution of Nd in steel.

3D distribution of Nd in steel.

To enable such 3D EDS mapping on the conceptual level we would need at least two identical 2D TEM EDS maps acquired with photons having different energy – so you can slide along the energy axis (adding a new dimension?) and use photon absorption as a natural yardstick to probe the element distribution along the X-ray path. Since the characteristic X-rays have discrete energies (K, L, M lines), it might work if you subtract the K line map from the L line or M line map to see an element distribution based on different absorption between K and L or M line maps. Ideally, one of EDS maps should be acquired with high energy X-rays, such as K lines for high atomic number elements, and another with low energy X-rays where the absorption has a significant effect, such as for example M lines. Indeed, in the case of elements with a high atomic number, the energies for K lines area ranged in tens of keV having virtually 0 absorption even in a thick TEM sample.

So, it all looks quite promising except for one important detail – current SDDs have the absorption efficiency for high energy photons close to actual 0. Even if you made your SDD sensor as large 150 mm2 it would still be 0. Increasing it to 200 mm2 would keep it steady close to 0. So, having a large silicon sensor for EDS does not seem to matter, what matters is the absorption properties of the sensor material. Here we add a material selection dimension to generate a new perspective for 3D EDS. And indeed, when we selected a CdTe EDS sensor we would able to acquire X-rays with the energies up to 100 keV or more.

To summarize, using a CdTe sensor will open an opportunity for a ‘snapshot’ 3D EDS technique, which can add more insight about elemental volume distribution, sample topography and will not be limited by a sample thickness. It would clearly be more practical for elements with high atomic numbers. Although it might be utilized for a wide yet selected range of samples, this concept could be a complementary and fast (!) alternative to 3D EDS tomography.

“It’s not the size of the dog in the fight, it’s the size of the fight in the dog.” (Mark Twain)

Dr. Oleg Lourie, Senior Product Manager, EDAX

San Javier, Spain, October 18, 2015: Airbus A400M airlifter escorted by Sains Patulla Aguila squad on their 30th anniversary celebration event.

Many of us like to travel and some people are fascinated by the view of gigantic A380’ planes slowly navigating on tarmac with projected gracious and powerful determination. I too could not overcome a feel of fascination every time I observed these magnificent planes, they are really – literally big..  The airline industry however seems to have a more practical perspective on this matter – the volume of the A380s purchase is on decline and according to the recent reports Airbus is considering reducing their production based on growing preference towards smaller and faster airplanes. Although the connection may seem slightly tenuous,  in my mind I see a fairly close analogy to this situation in EDS market, when the discussion comes to the size of EDS sensors.

In modern microanalysis where the studies of a compositional structure rapidly become dependent on a time scale, the use of the large sensors can no longer be a single solution to optimize the signal. The energy resolution of an EDS spectrometer can be related to its signal detection capability, which determines the signal to noise ratio and as a result the energy resolution of the detector. Fundamentally, to increase signal to noise ratio one may choose to increase signal, or number of counts, or as alternative to reduce the noise of the detector electronics and improve its sensitivity. The first methodology, based on larger number of counts, is directly related to the amount of input X-rays determined by a solid angle of the detector, and/or the acquisition time. A good example for this approach would be a large SDD sensor operating at long shaping times. A conceptually alternative methodology, would be to employ a sensor with a) reduced electronics noise; and b) having higher efficiency in X-ray transmission, which implies less X-ray losses in transit from sample to the recorded signal in the spectra.

Using this methodology signal to noise ratio can be increased with a smaller sensor having higher transmissivity and operating at higher count rates vs larger sensor operating at lower count rates.

To understand the advantage of using a small sensor at higher count rates we can review a simple operation model for SDD.  A time for a drift of the charge generated by X-ray in Si body of the sensor can be modeled either based on a simple linear trajectory or a random walk model. In both cases, we would arrive to approximate l~√t dependence, where l is the distance traveled by charge from cathode to anode and t is the drift time. In regard to the sensor size this means that a time to collect charge from a single X-ray event is proportional to the sensor area. As an example, a simple calculation with assumed electron mobility of 1500 cm2/V-1s and bias 200 V results in 1 µs drift time estimate for 100 mm2 and 100 ns drift time for 10 mm2 sensors. This implies that in order to collect a full charge in a large sensor the rise time for preamplifier needs to be in the range of 1 µs vs 100 ns rise time that can be used with 10 mm2 sensor.  With 10 times higher readout frequency for 10 mm2 sensor it will collect equivalent signal to a 100 mm2 sensor.

What will happen if we run a large sensor at the high count rates? Let’s assume that a 100mm2 sensor in this example can utilize the 100 ns rise time. In this case, since the rise time is much shorter than the charge drift time (~1 µs), not all electrons, produced by an X-ray event, will be collected. This shortage will result in an incomplete charge collection effect (ICC), which will be introducing artifacts and deteriorating the energy resolution. A single characteristic X-ray for Cu (L) and Cu Kα will generate around 245 and 2115 electrons respectively in Si, which will drift to anode, forced by applied bias, in quite large electron packets.  Such large electron packets are rapidly expanding during the drift with ultimately linear expansion rate vs drift time. If the rise time used to collect the electron packet is too short, some of the electrons in the packet will be ‘left out’ which will result in less accurate charge counting and consequently less accurate readout of the X-ray energy. This artifact, called a ‘ballistic deficit’ (BD), will be negatively affecting the energy resolution at high count rates. It is important to note that both ICC and BD effects for the large sensors are getting more enhanced with increasing energy of the characteristic X-rays, which means the resolution stability will deteriorate even more rapidly for higher Z elements compare to the low energy/light elements range.

Figure 1: Comparative Resolution at MnKa (eV).

Figure 1: Comparative Resolution at MnKα (eV) *

As the factual illustration to this topic, the actual SDD performance for sensors with different areas is shown in the Fig. 1. It displays the effect of the acquisition rates on the energy resolution for the EDS detectors having different sensors size and electronics design. Two clear trends can be observed – a rapid energy resolution deterioration with increase of the sensor size for the traditional electronics design; and much more stable resolution performance at high count rates for the sensor with new CMOS based electronics. In particular, the data for Elite Plus with 30 mm2 sensor shows stable resolution below 0.96 µs shaping time, which corresponds to >200 kcps OCR.

In conclusion, conceptually, employing a smaller sensor with optimized signal collection efficiency at higher count rates does offer an attractive alternative to acquiring the X-ray signal matching the one from large area sensors, yet combined with high throughput and improved energy resolution. Ultimately, the ideal solution for low flux applications will be a combination of several smaller sensors arranged in an array, which will combine all the benefits of smaller geometry, higher count rates, higher transmissivity and maximized solid angle.

* SDD performance data courtesy of the EDAX Applications Team.

Return Ticket from the East Coast to East Asia

Dr. Jens Rafaelsen, Applications Engineer, EDAX

Figure 1

As I write this I am on my way back to the US after having spent the past week in Singapore with my schedule filled with meetings and training sessions with both local microscope vendors and for customers, and discussions with the EDAX sales and applications people from China, India and Singapore. A good amount of time was spent discussing detector specifics and how to really make the advantages of our silicon nitride window and Elite detectors shine, but there was also general knowledge transfer and comparison between the challenges that we see in the different regions.

Singapore is definitely a change from the east coast of the United States, with the tropical climate and architecture including a sky-rise hotel with a ship parked on top, buildings with the exterior designed to look like the shell of the Durian fruit, or giant steel tree structures in the middle of the city park. But it is also a central hub where we have one of our regional offices and a state that invests heavily in the knowledge industry.
Figure 2
While the primary reason for my trip was the training of our local team and introduction of new and up-coming projects and software features, I also wanted to gather input and knowledge to bring back to our main office in Mahwah. Often we get so used to what we see every day that we forget that there’s a whole world out there. What we in the US think should be the major focus can be of less interest in other regions and vice versa. One of the things I learned was that the Asia/Pacific region sees a larger proportion of operators being technicians with limited insight into the advantages and limitations of the technique, than we usually do in the US and Europe. At the same time the microscope vendors were impressed with the level of analysis and how powerful the TEAM™ software is. These are things that we will have to take into consideration for future development, making it easier for novice users to apply the flexibility and power of the software but still allowing our advanced users access to all the bells and whistles that we have to offer.

Although we have conference systems, phone meetings and e-mail, there’s definitely something to be said for meeting face to face. The discussions and interactions flow much more easily when we can actually point to the same thing on the screen, draw on a piece of paper or just chat over coffee. Of course it can be a little overwhelming to come back to the hotel after a long day and find an overflowing inbox when you open the computer (not to mention getting calls at 3 AM from people who aren’t aware that you are travelling), but this is easily compensated by the experience of the culture, local food, and the chance to catch up with colleagues. Who knew that fried fish skin with salted egg goes so well with a cold beer?

With my Singapore trip over, I am making my way through the 24-hour travel back to the US and I have time to contemplate the experiences and discussions that I have had during the past week. There’s plenty of data to analyze, ideas for new software features, and input from microscope vendors to consider, but all that will have to wait. For now, it’s time to catch some sleep, try to get back on east coast time and maybe not worry about the line at immigration and New York traffic till I actually have to deal with it!

The origin of ideas

Dr. Patrick Camus, Director of Research and Innovation, EDAX

iStock_000035437584XLarge__teamwork

Stimulation for new research approaches and topics can come from odd origins and at the most unexpected times.

We recently held a Sales Meeting at the factory in Mahwah. During a presentation by Dr. Jens Rafaelsen, an Applications Scientist, he mentioned an unexpected EDS result. He found that a brand new EDS Elite detector was collecting more x-rays than a larger older Octane detector for the same geometry and SEM conditions. This result is quite unexpected and seems to violate physics and our typical ideas about x-ray detection. If confirmed, this result has far reaching implications for Sales and Marketing and would be exploited in the coming months. But the science behind the result is unknown at the time.

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EDS spectrum and modelling of Mg-Calcite.

A further discussion with Jens after his presentation inspired me to draft some notes on the scrap of paper that I had on hand. From these notes, I drafted an approach to an x-ray detection modelling experiment that would require input from Jens and another Scientist within the company. The experiment is to go beyond the simple description of associating detector detection performance with simply solid angle. That method may work when much of the sub-assemblies of the detection system are similar. However, for the latest generation of EDS detection systems, the use of modern materials requires a more complete system analysis.

Together, we will refine the model, compare the results to empirical results, and hope to publish both internal and external publications.

All of this work was sparked by a subtle but original observation by a coworker. Inspiration can come from unexpected sources and at unexpected times. Where have your inspirations come from?

Click here to watch Global Applications Manager, Tara Nylese presenting an overview of the Octane Elite at M&M 2015.

BLOG UPDATE FROM PAT – March 23, 2016
A new result has been found while modelling different detector configurations. The thickness of the Silicon support grid for the windows is significantly different for the traditional polymer (>300 um) and the new Si-N (<50 um) windows. This creates a different absorption of x-rays as a function of x-ray energy. This is illustrated in the following figure.

The predicted increase of the transparency of the Si-N window grid at intermediate x-ray energies has the potential to increase the total count rates of the detection system by a significant amount. More details to follow.

It’s All About Speed!

Dr. Oleg Lourie, Senior Product Manager EDS, EDAX

Different perceptions of speed can be measured differently, and yet in my opinion speed is one of those few fascinating concepts, which you are always aware of regardless of your activity. The world of speed is enriched with various emotional flavors which generate a multitude of reactions:  curiosity, when I observed the 690m/h cruising speed during my recent flight with KLM (‘are we getting close to 1Mach and when?’), or a contemplative focus when you accelerate to 170m/h on the German Autobahn near Düsseldorf.

In all circumstances speed inevitably arrests your attention, just as blazing fast EDS mapping did for me recently, when I saw a literally staggering acquisition speed below 200 us/pixel, which translated into a 512×400 pixel, fully quantifiable elemental map, which was collected in less than 1 min.

The ‘Octane’ EDS power, that ‘fueled’ this racing performance is equally remarkable – holding above 2Mcps in X-ray input counts without a single complaint  and exploding with 860Kcps for a single channel at about 50% dead time. I should admit I simply enjoyed it. It is inspiring to push the ‘limits’. The new electronics for this system will move things even further by leveling the throughput up to 1.8Mcps for a single channel – literally doubling the processing speed of the system.

1. Phase map of mineral clearly showing separation of zirconium silicate and calcium phosphate phases. 2. Spectrum of zirconium silicate

While astounded at the extreme throughput, a casual observer may wonder where this power can be applied in a ‘daily commute’ for elemental information. The answer is everywhere! It affects all your materials analysis when there are no boundaries imposed by your spectrometer on the scope of your experiment. It is indispensable for setting automated runs where sudden changes in sample composition, geometry or topography can impact acquisition. It aids in the formulation of statistics, where you need the fastest screening to acquire reliable statistical data. It is essential in ‘in situ’ studies where you rapidly change the sample compositional structure during the observation. It is useful in observing live Direct Phase Mapping and showing various phase distributions immediately after the scanned image is acquired. With more than 860 kcps ‘under the hood’, low noise CUBE electronics design and pulse processing times geared from 7.8 us to 120 ns, you can focus on driving your experiment at any speed you can imagine to achieve superior results in less time.

3. Spectrum of calcium phosphate 4. Superimposed spectra of 2 and 3 showing an complete overlap of the P and Zr peaks, which makes them undistinguishable in the RGB elemental map.

With all this ‘Octane’ power to keep your acquisition limits tunable on demand, there are many more exciting experiments further ‘down the road’. And yes, the roads can be icy and slippery in December. It is more fun to race with your fast EDS, collecting powerful, streamlined data and aiming towards the holidays with new observations, and possibly new discoveries.

Why you should never leave home without your plasma cleaner – at least if you are going to M&M

Dr. Jens Rafaelsen – Applications Engineer, EDAX

One of the things I learned during the 2015 Microscopy and Microanalysis meeting was just how efficient plasma cleaners really are and this is a short story about how it saved the day for us. We had shipped our older Hitachi S3400N microscope from Mahwah to Portland for the show and had tested everything before it went on the truck. The meeting opened Monday August 3 at noon so Sunday was set aside for getting everything set up and calibrated. While our service group had done most of the work, I had a bit of data I wanted to collect for the days to follow. So I sat down at the microscope, turned on the beam, and stared at the current meter showing next to nothing. I checked the usual microscope settings and fidgeted with the apertures but still couldn’t get a decent current down through the column. Since we were a little short on time and the Hitachi booth was close by, we went over and looked sufficiently desperate for the Hitachi service guys to take pity on us and come to help.

I noticed the Hitachi guys going through the same steps I had done and end up with the same problem, so at least it wasn’t just down to my short comings regarding microscope service. As the last step they pulled out the aperture strip and the black gunk covering all three apertures gave us a pretty good indication of the problem: the beam was being severely attenuated simply because the apertures were clogged up with carbon contamination. Of course the Hitachi guys’ immediate question was “Did you bring a new aperture strip?” and my answer was a meek “No…”. But then I remembered that I did bring a plasma cleaner. I didn’t really believe that it would be able to do much with the level of contamination that we had on the apertures but it was still worth a shot. So I put the aperture strip in the cleaner chamber and ran it at a pressure of 2*10-2 mbar with a power of 50 W.

I have to say that I was extremely surprised when the aperture strip looked as good as new after only 10 minutes of plasma exposure. Both the EDAX and Hitachi service guys were equally impressed and after mounting the strip back in the column we were up and running again. So 10 minutes of plasma cleaning saved us from having to either try to have an aperture strip shipped in overnight or run the microscope with no aperture and ensuing risk of sample damage and reduced imaging capability. Unfortunately I didn’t take any pictures before and after cleaning, as I honestly was not expecting it to work, but the picture below shows us busy running demos on the Hitachi during the show.

The EDAX booth at M&M 2015

The EDAX booth at M&M 2015

At this point you might wonder why I had brought a plasma cleaner in the first place. Well, one of the things that we were highlighting with the new Octane Elite detector we launched at the show was the silicon nitride window and its durability. I had run a test on my office desk with a live detector mounted directly on an asher chamber (shown in Figure 1) that I borrowed from Vince Carlino of ibss Group, Inc. When the asher chamber is running, it looks like something out of a science fiction movie so we wanted to do something similar at the M&M meeting as a visual prop.

Figure 1: Silicon nitride window detector mounted on ibss asher chamber.

Figure 1: Silicon nitride window detector mounted on ibss asher chamber.

Since a full detector takes up space we simply put a single detector module directly in the asher chamber and started the cleaning process on Monday when the exhibition began. I took pictures of the controller for the system and the module at the start and end of each day as can be seen in the picture sequence below.

Figure 2: The controller and module at the start and end of each day.

Figure 2: The controller and module at the start and end of each day.

After almost 76 hours of continuous plasma exposure, the silicon nitride window shows no signs of degradation and knowing what plasma cleaning did to the aperture strip, I am pretty certain that was absolutely no carbon contamination on the window. Of course this is more of a show-and-tell kind of experiment and the testing I did before this involved detailed monitoring of the module performance and temperature to detect any pin-holes that would not be visible by eye. That report will be available shortly.

The next step will be to try the same with a polymer window but I am still thinking about exactly how to design the experiment. Of course I could just clean it for an extended period of time and see if the window is still intact but it would be nice to have a metric of how fast the damage occurs (or not). One idea would be to use a bare window and correlate the ratio of the carbon and aluminum signal of the window to the silicon peak from the support grid in order to monitor any changes in thickness, but if anyone has other suggestions, I would be happy to hear them.

Even though the tabletop plasma cleaner has been around for a number of years, its complete usefulness is sometimes is overlooked because it is a small piece of  auxillary equipment. Sometimes, however,  the smallest of equipment can provide the largest benefit!