X-ray Fluorescence Analysis for Process Control

Interview conducted by Mychealla RiceJan 21 2019

X-ray Fluorescence (XRF) is a technique that probes the elemental composition of a material. In this interview, Bruce Scruggs, from EDAX talks to AZoM about the XLNCE SMX product line and its capabilities for performing XRF measurements in a process control environment. 

Please give our readers an overview of EDAX’s products for Process Metrology.

EDAX supplies a variety of XRF process metrology tools, known as the XLNCE SMX series, which can be configured around various material handling systems. Specifically, EDAX supplies an in-line tool (SMX-ILH), which can be built around a panel conveyor or spooled substrate. The SMX-ILH, which operates at atmospheric conditions can also be configured as an at-line tool loaded either by hand or by an isolated conveyor. There is also an in-situ tool (SMX-ISI) which can be built into a vacuum chamber. The in-situ tool measures the product through one or more X-ray transparent windows which EDAX installs in the vacuum chamber. Finally, EDAX can supply a benchtop version of the SMX product (SMX-BEN) which uses the same measuring head as the process metrology tools. The SMX-BEN can be used to provide laboratory support for the process metrology tools or it can be used for process measurements of more irregularly-shaped products, as the benchtop system uses the same software and measurement reporting methods as the SMX in-line and in-situ products.

What role does the XLNCE SMX product line play in Process Metrology?

The flexible design of the XLNCE SMX product line allows for complete integration into a process, enabling real-time analysis of composition and thickness. This is essential to control production. SMX measurements can be used for process start-up guidance, yield management, quality, and homogeneity of coating over the entire product area.

The in-line version of the SMX has two additional features that expand the role of these systems in-process metrology. The SMX-ILH employs a patented thermal shield allowing real-time XRF data to be taken off hot substrates, up to 300°C, reducing panel dwell times and improving process throughput.

There is also a devoted measuring height adjustment feature that enhances measurement quality by accommodating bow, warp, and planarity deviations common to large panels, especially at higher temperatures.

What kind of materials and layer stacks can be measured?

The SMX products measure coating thickness and composition using X-ray Fluorescence. Hence, SMX systems can measure layers and layer stacks comprised of elements in the range of Na and heavier, including compounds that contain these elements. By compounds, I mean metal oxides, nitrides, carbides or other compounds as long as there is an element detectable by XRF.

Of course, X-ray physics dictate the minimum and maximum thickness that can be measured based on the energy of the observed signal. A typical emission signal scenario is shown in the figure below:

In this plot, we are looking at the signal intensity from a single layer film, I (f, layer), ratioed to the signal intensity from “bulk” material comprised of the same material as the layer, I (∞, layer). As the single layer approaches “infinite” thickness, the intensity ratio asymptotes at a value of 1. The layer looks like the bulk material with respect to the X-ray emission signal. More energetic X-ray lines are capable of traversing a thicker layer while weaker X-ray lines are only able to traverse thinner layers.

XRF can be used to measure layers in the nanometer range to greater than 100 micrometers. Just remember that the specifics of any situation are dictated by the elemental signals measured and the X-ray physics involved.

The SMX product line measures single layers and multiple layer stacks as well, based on the same X-ray physics described previously. You just need to define the overall layer structure, i.e. which elements are in the top layer and which elements are in the second layer, and so on, all the way down to the substrate. We use this basic structural information to model the x-ray physics involved in the layer stack.

The SMX XRF systems are ideal for measuring metal and metal oxide layer stacks such as electrical contacts, photovoltaic devices, and another protective, functional or decorative coatings. Where the stoichiometry is known, we have even used the SMX system to measure the composition of complex metal brazing formulations.

The material to be measured can be in web, continuous sheet form or individual samples such as solar panels.

The XLNCE SMX-BEN is an excellent choice for non-destructive applications. Why?

The XLNCE SMX product line uses a low power 50 W X-ray tube to excite the X-ray fluorescence signal. The low power excitation energy is non-destructive, and no contact with the sample is necessary. Samples can be measured “as is” without destructive sample preparation.

How does EDAX handle integration into the factory?

The level of integration and the integration interface is typically specified by the customer and EDAX works with the customer to achieve the level of integration desired. Integration involves both the physical integration of the measuring equipment and communications integrated into the factory network.

The atmospheric in-line system is built around the material conveyance system for either rigid or flexible product substrates, e.g. glass panels or flexible, spooling sheets of aluminum or polymer. The in-line system can also be set up off-line with a manual loading system or isolated conveyor.  The vacuum in-situ system can be installed horizontally or vertically into the deposition chamber.

The XLNCE SMX product line uses both Programmable Logic Controllers (PLC) and/or Open Platform Communication (OPC) server to communicate with the factory’s conveyance system.  Measurement data itself is archived locally in an SQL database format. However, for real-time process control, that data is uploaded into the factory’s MES (Manufacturing Execution System) and the MES and production supervisors make decisions about any necessary process adjustments based on the XRF data.

What specialized application areas can the XLNCE SMX-ILH be a huge benefit for?

The strength of XLNCE SMX-ILH measurements is in measuring the composition of bulk materials or the composition and thickness of metal layers or compound layers involving an inorganic elements such as metal oxides, carbides, and nitrides. The SMX-ILH can have huge benefits for any product utilizing these types of materials.

SMX systems have been used in measuring CIGS and Se-doped photovoltaics on solar panels and flexible web substrates; anti-corrosion coatings on biomedical alloys; electrical contacts; Nickel-Phosphorous (aka electroless Nickel) coatings; functional coatings on Aluminum; anti-wear and anti-corrosion coatings on steel alloys; thermal barrier coatings and thermally conductive oxide coatings.

Are there any options for using the XLNCE SMX product line for measuring organic layers?

There are some possibilities for this. Recently, EDAX held a webinar discussing measurements of Aluminum metal coatings on Silicon wafers and Nickel substrates. Instead of measuring the Aluminum signal directly, because the signal is weak in atmospheric measurement systems, I measured the stronger substrate signal as it is absorbed in the Aluminum layer.

The thicker the Aluminum layer, the more the substrate signal is absorbed. The situation is similar to the discussion above for the emission signal, but the absorption signal ratio is approximately 1 for very thin coatings and asymptotes at a value of 0 for thick coatings. See the plot below:

The same absorptive measurement principle can be applied to measure the thickness of organic layers given the overall composition of the organic layer. The measurement range will depend on the substrate material and the X-ray energy of the measured substrate signal.

What does the XLNCE-SMX product line mean for the future of Process Control?

EDAX continues to innovate and develop the SMX product line. In the Process Control area, we are always looking to make measurements faster because customers in the Process Control area always have an interest in this feature. Shorter measurement times mean higher product throughput and better measurement precision which translates into better product yields.

Where can our readers go to find out more?

More information is available at: https://www.edax.com/

Go to the Products section under X-ray Metrology.  Also, check out our product webinars under the News / Events section.  You can always send us an e-mail as well.

About Bruce Scruggs

Bruce is the XRF Product Manager for EDAX. He has a bachelor’s degree in Chemistry from Valparaiso University and master’s and Ph.D. degree in Chemical Engineering from the Massachusetts Institute of Technology.

Bruce started at EDAX in 1997 working on laboratory-based small spot XRF systems. Within EDAX, Bruce has been involved in many roles including product management; product design and development; SW GUI design; applications; customer, technical and service support.