Moxtek is a manufacturer of X-ray windows for low energy X-ray detection. Moxtek is a manufacturer of ultra-thin polymer X-ray windows that are attached to metal mounts which house energy dispersive X-ray detectors such as silicon drift detectors (SDDs). These windows enable transmission of low energy X-rays at the same time maintaining a hermetic seal critical for the X-ray detector’s performance.
While Moxtek ensures all windows meet published limits for leak rates, no material is the right gas barrier and gases will diffuse through window assemblies over time. In order to ensure constant improvement, Moxtek studied the impact of plasma cleaning the metal mounts before attaching to a window, and achieved around a 40% reduction in the diffusion of helium through the epoxy-to-mount interface.
Overview of X-Ray Windows
One of the key functions of an X-ray window is to act as a gas barrier. It is a known fact that no material is a perfect gas barrier. Gasses diffuse at different rates through materials depending on many factors. Diffusion was determined at different locations on the window assembly and the epoxy-to-mount interface was identified as a significant source of diffusion.
Efforts were made to reduce diffusion at the epoxy-to-mount interface. Moxtek studied the leak rate of helium in a metal mount assembly and a window blank. The window blank was used for eliminating the diffusion signal from the thin polymer film, hence concentrating on the diffusion through the metal-to-window frame seal. Helium is a small inert molecule, which is an industrial standard in vacuum leak detection. Reduction in helium diffusion through Moxtek window assemblies is a good indicator that should be reduced as well. Surface conditions on the metal mount such as surface energy and contamination play a key role in adhesion of the epoxy to the mount which effects gas permeability. The impact of different plasma cleaning process on the metal mount surface were also evaluated by Moxtek in order to optimize adhesion of the epoxy to the mount.
Helium diffusion through Moxtek AP3 window assemblies was measured by using a fixture designed to eliminate the effects of helium diffusing through anything other than the area of interest, and by using a window blank to isolate diffusion through the metal-window frame seal.
Window assemblies were exposed to helium for long periods of time until a steady-state diffusion rate was achieved. The epoxy-mount interface was one area identified as a source of diffusion as shown in Figure 1.
For reducing gas diffusion through the epoxy-mount interface, plasma cleaning the mounts was investigated as a way to reduce surface contamination on metal mounts and to increase wettability of the epoxy. SEM/EDS analysis was done to check possible contamination on the metal mount before and after plasma cleaning. XPS was done for comparing plasma cleaning processes and a chemical cleaning in their effectiveness in removing contamination. Epoxy wettability on the metal mount was evaluated by checking the surface tension of the metal surface using dyne solutions.
Figure 1. Epoxy-mount diffusion path in AP3 Window
Finally, solid metal disks were epoxied into window mounts so that all interfaces with the epoxy would be epoxy-to-metal. Certain mounts were plasma cleaned before attaching while others were not. All parts were then evacuated on the vacuum side and then exposed on the other side to 1atm of helium for 10h allowing a steady state diffusion to occur. Diffused helium was detected using a helium leak detector.
The results from the SEM/EDS analysis on metal mounts showed areas of carbon based contamination that could affect how well the epoxy bonds to the metal mount as shown in Figure 2.
Figure 2. Typical contamination found on metal mounts
The XPS data in Figure 3 compares the effectiveness of different plasma processes in the removal of carbon-based contamination. In all cases, plasma cleaning was more effective in removing carbon-based contamination than only using Moxtek’s standard chemical cleaning.
Figure 3. Relative percent carbon measured by XPS on metal window mounts that were cleaned by different processes
Increasing the surface energy or wettability of the metal mount is another important factor in improving adhesion.
Figure 4 shows the impact of plasma exposure times on surface energy. The highest dyne solution was 72mN/m and was reached within 1s of exposure to plasma. It is possible that the surface energy continued to rise after one second, however, the rapid increase in surface energy over time measured in this experiment was sufficient for this application.
The final test as shown in Figure 5, was to measure the diffusion rate of helium through the epoxy-to-metal interface of assemblies where metal mounts had been plasma cleaned and metal mounts that were not plasma cleaned. The red bars are the helium steady-state diffusion rates of window assemblies whose mounts were only chemically cleaned. The blue bars show the steady state diffusion rates of assemblies whose metal mounts were plasma cleaned by Process 6 from Figure 3 before attaching the solid metal disks. A 41% average decrease in helium diffusion rate occurred in parts that received a plasma clean from the ones that did not.
Figure 4. Surface energy as a function of exposure time of plasma on a metal surface
Figure 5. Difference in steady state diffusion of helium from mounts that were treated with plasma prior to attaching metal blank and those that were not
Plasma cleaning of metal mounts brings down carbon-based contamination and increases the surface energy leading to better adhesion of the epoxy to the metal mount. Understanding the impact of different plasma processes on surface conditions is critical to achieving a better bonding surface that reduces the diffusion of gasses, including helium, at the epoxy-metal interfaces. While no material is a perfect gas barrier, Moxtek is continuously working to improve the hermetic properties of X-ray windows.
This information has been sourced, reviewed and adapted from materials provided by Moxtek, Inc.
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