Energy Dispersive Spectroscopy (EDS) instrument manufacturers and research labs are continually pressing for x-ray components that enable higher performance at a lower cost. High performing x-ray detectors are critical to building quality instruments. Moxtek^® has improved the sensitivity of our Si-PIN detectors by using thicker diodes and thinner beryllium windows. The increased performance enables faster sample times, improved sensitivity, and expanded range of photon detection.

Detector Diode Thickness

Improved Energy Resolution At short peaking times, where the shot noise has very little impact on the resolution, the two dominant noise sources—thermal and 1/f—depend very strongly on the total capacitance between the anode and any grounded or DC-biased conductors (see Figure 1). Therefore, reducing that capacitance has an enormous impact on the detector resolution.

One way to reduce the capacitance of a PIN diode detector is by making the diode thicker. For example, a 400-μm diode has an anode-to-backside capacitance of 1.6 pF, whereas a comparable 625-μm fully-depleted diode has a capacitance of only 1.0 pF (see Figure 2). Two challenges involved in making Si-PIN diodes thicker: 1. Full depletion requires either a higher voltage bias or silicon material with a higher resistivity. 2. Diodes with thicker depletion regions have larger volumes and therefore higher leakage currents, so using a diode manufacturing process that results in very low leakage currents is essential. Moxtek has developed Si-PIN detectors with 625μm and 875μm diodes (875μm device is currently in research and development stages) which have lower capacitance resulting in better energy resolutions than the 400μm devices, especially at short peaking times which make higher count rates possible. The thicker devices offer performance advantages that begin to approach SDDs but at a much lower price (See Figure 3).

Higher Energy Absorption

Another advantage of thicker Si-PIN diodes is greater energy absorption at the higher energies which is important for heavy elemental analysis. Figure 4 shows the absorption curves of three different diodes thickness. Note that at energies above 25 keV, the 625μm device has about 50% greater absorption efficiency than the 400μm device.

New! XPIN™ Detector Platform

The XPIN™ is Moxtek’s next generation Energy Dispersive X-ray Fluorescence detector (EDXRF). The XPIN takes advantage of thicker Si-PIN diode material as well as thin beryllium windows providing optimal energy resolutions and x-ray photon sensitivity. The XPIN comes in multiple configurations for design flexibility for a variety of industrial EDXRF applications.

DuraBeryllium^® Plus windows are Moxtek’s most robust coated beryllium x-ray window. This window has an additional thin coating of polymer “Plus” applied to the already robust DuraBeryllium window. The Plus layer is more resistant to water and other harsh chemicals. DuraBeryllium Plus windows successfully withstand continuous exposure to heated water as well as to other chemicals. A series of tests were completed on DuraBeryllium windows as seen in the test summary below. Part 1 of this application note details exposure of DuraBeryllium Plus to water, KOH, and HCl. Part 2 will be released later in 2012 and will include details of exposure of this window to other harsh chemicals.

X-ray Transmission

DuraBeryllium protective layers provide corrosive shielding as well as a hermetic barrier to the beryllium foil. These protective layers are very thin and the resulting x-ray attenuation is equivalent to about 1μm of beryllium, having little impact on x-ray transmission. See chart below.

Pressure and Thermal Cycling

To demonstrate strength of the DuraBeryllium window, Moxtek pressure and thermal cycled thirty mounted windows. All thirty mounted windows were exposed to 10,000 cycles (15psi to atmosphere). Ten of these endcaps went through an additional 10,000 cycles. All windows withstood the pressure and thermal cycling without failure. The flowchart below details the testing procedure used.

Corrosion Resistance

8μm DuraBeryllium windows are very robust when continuously exposed to water at both room temperature and at 65º C. Conversely uncoated 8 μm beryllium windows will corrode when exposed to water within days. The flowchart below details the testing procedure used.

DuraBeryllium When Exposed to Water

The table below shows the robustness of DuraBeryllium windows when exposed to water.

Pass (P) = He Leak Tight (<1 x 10-10 mbar * L/sec)

Leak Testing

~60°C H2O

Room Temperature H2O

DuraBeryllium Plus

DuraBeryllium

DuraBeryllium Plus

DuraBeryllium

Approx. Time Exposure

A0177

A0156

V311

misc

A0159

A0174

A0168

A0166

A0153

3D

4D

V3112

3 hours

P

P

P

P

P

4 hours

P

P

P

5 hours

P

P

P

10 hours

P

P

P

P

P

P

15 hours

P

P

20 hours

P

P

P

P

P

24 hours

P

P

P

2 days

P

P

P

P

P

P

P

P

P

3 days

P

P

P

4 days

P

P

P

P

P

P

P

P

P

6 days

P

P

P

P

P

7 days

P

P

P

8 days

P

P

11 days

P

P

P

P

P

P

P

P

14 days

P

P

P

P

P

18 days

P

P

P

P

P

P

P

21 days

P

P

P

P

P

1 month

P

P

P

P

P

P

P

P

P

P

P

P

2 month

P

P

P

P

P

P

P

P

P

P

P

P

3 month

P

P

P

P

P

P

P

P

P

P

P

P

4 month

P

P

P

P

P

P

P

P

P

X

Fail

P

5 month

P

P

P

P

P

P

P

P

P

P

6 month

P

P

P

P

P

P

P

P

P

P

Blank box= no data collected. X= damaged.

Visual Observation

The images below were taken four months after exposure to H2O at room temperature. As seen in the images, there was some discoloration on the window coating but all windows were leak tight.

KOH Exposure Test (Exposure = 1 week)

Under extreme KOH exposure for one week, DuraBeryllium Plus did not develop any leaks. Some visual polymer delamination was observed after 6 days. Prolonged exposure to KOH will eventually dissolve the polymer “Plus” layer.

	Leak Testing Results
	10% KOH
Time Exposure	DuraBeryllium Plus		DuraBeryllium
	A0172	A0154	no number
3 hours	P	P	P
4 hours	P	P	-
5 hours	-	-	-
6 hours	P	P	P
10 hours	P	P	-
15 hours	P	P	-
20 hours	-	-	-
24 hours	-	-	P
2 days	P	P	-
3 days	-	-	P
4 days	-	-	P
6 days	P	P
7 days	P	P

10% KOH Exposure at room temp

HCl Exposure Text (Exposure = 1 week)

HCl reacts quickly with bare beryllium and corrodes 8μm Be windows within minutes. Both DuraBeryllium Plus and DuraBeryllium add some resistance to this harsh chemical but only for a few hours, as seen in the table below.

	Leak Testing Results
	10% HCl
Time Exposure	DuraBeryllium Plus		DuraBeryllium
	A0160	A0179	misc
3 hours	F	P	F
3 hours		F

DuraBeryllium Plus and DuraBeryllium Visual image above of DuraBeryllium Plus and DuraBeryllium after exposure to HCl for ~3 hours. HCl speedily reacts under the surface of DuraBeryllium.

Conclusion

Corrosion Resistance: DuraBeryllium Plus windows demonstrate remarkable resistance to corrosion after prolonged exposure to heated water. Corrosion on DuraBeryllium Plus windows is minimal and delimitation is non-existent after continuous exposure times of over 6 months without any windows developing a leak. DuraBeryllium Plus windows are affected by exposure of HCl. This chemical will slowly dissolve the outer polymer layer but the beryllium window will still be protected by the inner DuraCoat layer. More testing will be completed at Moxtek to determine resistivity of the DuraCoat to this chemical and will be reported in a later application note. HCl reacts quickly with bare beryllium and corrodes through a 8μm Be foil within minutes. Both DuraBeryllium Plus and DuraBeryllium add some resistance to this harsh chemical but only for a few hours. X-ray Transmission: DuraCoat Plus layers are applied to all Moxtek 8μm thick DuraBeryllium Plus window. The DuraCoat Plus layers consist of very thin layers of DuraCoat and polymer. These protective layers are very thin, and have little impact on x-ray transmission.

Recommendation

DuraBeryllium and DuraBerylllium Plus windows are recommended for all applications that require 8μm thick beryllium windows.  

Moxtek^® is a leading manufacturer of X-ray windows for low energy X-ray detection. Moxtek manufactures 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 allow for transmission of low energy X-rays while maintaining a hermetic seal critical for the X-ray detector’s performance. While Moxtek ensures that all windows meet published limits for leak rates, no material is a perfect gas barrier and gasses will diffuse through window assemblies over time. As a continuous improvement effort, Moxtek investigated the effects of plasma cleaning the metal mounts before attaching a window, and achieved around a 40% reduction in the diffusion of helium through the epoxy-to-mount interface.  

Introduction

One of the primary functions of an X-ray window is to act as a gas barrier. It is well known that no material is a perfect gas barrier. Gasses diffuse at different rates through materials depending on many factors. Diffusion was measured 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 evaluated the leak rate of helium in a window blank and metal mount assembly. The window blank was used to eliminate the diffusion signal from the thin polymer film, thereby concentrating on the diffusion through the metal-to-window frame seal. Helium is an industrial standard in vacuum leak detection. Helium is a small, inert molecule; a reduction in the diffusion of helium through Moxtek window assemblies is a good indicator that the diffusion of other gasses should be reduced as well. Surface conditions on the metal mount, such as contamination and surface energy, play a key role in adhesion of the epoxy to the mount which effects gas permeability. The effects of different plasma cleaning processes on the surface of the metal mounts were also evaluated by Moxtek in order to optimize adhesion of the epoxy to the mount.

Methods

Diffusion of helium 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 (Figure 1). In order to reduce diffusion of gasses 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 for possible contamination on the metal mount before and after plasma cleaning. XPS was done to compare plasma cleaning processes and a chemical cleaning in their effectiveness in removing contamination. Wettability of the epoxy on the metal mount was evaluated by checking the surface tension of the metal surface using dyne solutions. Finally, solid metal disks were epoxied into window mounts so that all interfaces with the epoxy would be epoxy-to-metal. Some mounts were plasma cleaned prior to attaching while others were not. All parts were then evacuated on the vacuum side and then exposed on the other side to 1 atm of helium for 10 hours allowing a steady state diffusion to occur. Diffused helium was detected using a helium leak detector.

Results

Analysis/ Conclusion

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 (Figure 2). 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. Increasing the wettability or surface energy of the metal mount is another key factor in improving adhesion. Figure 4 shows the effect of plasma exposure times on surface energy. The highest dyne solution was 72 mN/m and was reached within 1 second 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, 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 bars in red 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 prior to 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. Plasma cleaning metal mounts reduces carbon based contamination and increases the surface energy leading to better adhesion of the epoxy to the metal mount. Understanding the effects 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.

Moxtek^® X-ray Windows are very thin and fragile, great care should be taken while handling these windows. The guidelines below must be strictly followed in order to prevent damage to the window. (more…)

AP3 ultra-thin polymer windows manufactured at Moxtek^® receive two quality inspections before being shipped to customers, once before installation into various mounts (more…)

This preliminary setup will lead to a prototype simultaneous XRD/XRF instrument. (more…)

Moxtek® delivers each AP window in a mount appropriate for each customer. The windows are bonded to a mount using vacuum compatible adhesive. This Technical Note provides general guidelines to use when designing mounts for Moxtek AP windows. (more…)

A first prototype XRD/XRF instrument was developed at NASA Ames Research Center and is currently in use at Los Alamos National Laboratory. The intended applications for this instrument are for planetary exploration and as a portable instrument for terrestrial use. (more…)

Moxtek ISO 9001 Certificate

The MX-40 has a very small gate capacitance (0.26 pF). This is accomplished by making the diameter (“width”) of the gate very small. (more…)