Month: July 2016

“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.

How To Get the Maximum Benefit from Visiting the Show Floor at a Microanalysis Conference.

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

Control 2016

This is the time of year when many analysts are scrambling to finalize details for the Microscopy & Microanalysis Conference – to be held this year in Columbus, OH. We too are striving to present our products in the best light for attendees to evaluate.

As conference attendees, you may well be coming with the task of evaluating and comparing software and equipment from a variety of vendors. Many will also be booking demonstrations, provided by the very capable application specialists of the representative companies. Their job, as well as mine, is to sell you the best product available, which obviously is from EDAX (wink, wink).

But what is your task for the week, and how should you prepare? I have a few universal topics that you might like to consider before you even hit the show floor.

Your primary task is to get enough information to make an educated decision about the best system at the fairest price to benefit the customers of your lab. That system may be the BEST IN THE WORLD system or it may have the absolute lowest price, but knowing the criteria before seeing the competing systems will help in balancing the cost and the benefits and select the best system for your lab.

Below I will present some criteria for system selection. I will use x-ray microanalysis systems as examples because that is the equipment that EDAX sells, but the approach is universal for all equipment purchases.

  • Understand and appreciate all the system specifications because they are the best indicators of the system quality and performance, but emphasize those that that you currently employ or realistically could implement. For instance, if you have a low-level SEM, do you or will you operate at the maximum beam current of the system? How often do you really operate under the conditions necessary to obtain resolutions specifications? Make sure you understand how the system operates AWAY from the conditions used for specifications. These deviations may be more indicative of how your users operate and how the system will be useful for them.
  • Appreciate aesthetics, but look beneath the system “skin” to actual technical performance substance. Do your current operators work that way or can they be retrained to work that way? Is the technology truly new or just re-skinned? The workflow may demo well, but do your operators work in that manner?
  • Ask about your projected local service engineers. Ask for an interview with them before the sale. Over the lifetime of the system, you will probably work with them more than anyone else at the company.
  • During a demo, perform tests under your typical or expected operating conditions to get a feeling for real-world performance in your lab. But also ask for suggested optimized conditions for better performance for future analyses. How much training is included or can be upgraded? Would training at your site or at the vendor site be more effective for those involved?

These are just a few of the topics that you should consider. This is a lot of preparation work to do before you even hit the show floor, but the answers to these topics will make your system selection that much more satisfying in the long run. And job satisfaction for you and your users goes a long way!