SPI Supplies^® Silicon Nitride Membrane Windows^® for X-ray Microscopy

X-ray microscopy represents the newest emerging branch of microscopy for which SPI Supplies is now a worldwide global supplier. The SPI X-ray Windows provide the very highest transmission possible for soft x-rays (e.g. vacuum ultra violet). In some laboratories, in some parts of the world, instead of "soft x-ray", the terms "extreme UV" or even "vacuum UV" are used. So far as we know, these terms are used interchangeably. Synchrotron radiation is sometimes the source but laser generated x-ray plasmas (where there is a high powered laser beam focussed on a target material which leads to the generation of characteristic x-rays), pinched sources could also be involved. The softer the x-rays, the less the penetrating layer, therefore requiring a thinner and thinner window, which unfortunately also means a higher and higher price. Thinner windows are also demanded when working "off axis", that is, where the membrane is at an angle to the beam, so the x-rays now see a greater effective thickness.

The windows are very similar in construction to the highly successful SPI silicon nitride membrane window grids for transmission electron microscopy. For applications in x-ray microscopy the windows have the following specifications:

Outside frame dimensions (4 standard choices):

• 5 mm x 5 mm (Window size: 1.0 mm and 1.5 mm square)
• 7.5 mm x 7.5 mm (Window sizes: 2.0 mm and 2.5 mm)
• 10 mm x 10 mm (Window size: 3.0 mm and 5 mm square)
• 12.5 mm x 12.5 mm (Window size: 5.0 mm square)

Frame thickness: 200 µm standard, 381 µm on special order, 30% pricing premium, option not available in packs of 10

Window thickness Si₃ N₄: 100, 150 and 200 nm available

The SPI X-ray Windows for x-ray microscopy are designed specifically for use with x-ray microscopes and other soft x-ray applications. If you are getting started in this field, then you might want to become familiar with previously published papers in this field. The typical user of the SPI Supplies x-ray windows would be someone (probably) making use of an x-ray port associated with a synchrotron or possibly also a laser generated x-ray plasma source.

The membrane itself is "low stress" or LS silicon nitride, as opposed to the earlier technology window that resulted in "stoichiometric" (ST) nitride layers but which were under considerable stress. The low stress products are far more robust than the earlier ST nitride layers and in general, are the preferred nitride if one is given a choice.

Optical transparency:

We are often times asked about the optical transparency of the membrane windows. We have not yet ourselves made such measurements, although we hope to do that soon. But visually, one can indeed "see through" the windows, for example, if you view them by (transmission) light microscopy. There has to be some thickness beyond which the optical transparency starts to decline, but even at 200 nm thickness, they are quite transparent in the visible range of the spectrum. In any case, for the x-ray microscopy application, it is sometimes necessary to make "environmental chambers" which consist of membrane "sandwiches" and it is often times useful to see what is actually happening inside.

Vacuum compatibility:

SPI Supplies has been careful not to be offering these products as vacuum windows. However, for lack of any other good alternatives, we do have awareness that many existing and prospective customers are using them this way. Although one of the features of the SPI Supplies windows is that they are more robust than windows made by others, there obviously has to be a combination of membrane thickness and window area, beyond which, the window will not support that differential and fail. We don't have information about vacuum compatibility quite to the extent some of our customers would like to see, we can report the following data. If the window has the following or larger membrane thicknesses or less than or equal to the following areas, then the window will indeed survive a 1 atm pressure differential:

Membrane thickness	Window dimensions	Pressure differential
50 nm	1.0 x 1.0 mm	1 atm
100 nm	1.5 x 1.5 mm	1 atm
200 nm	2.5 x 2.5 mm	1 atm

We can not guarantee any lifetime under vacuum nor can we ever guarantee that they will survive, but for the most part the windows if the above window thicknesses or greater, or above areas or smaller, should survive such vacuum exposure conditions. If one wanted to use even thinner nitride windows they would have to use an even smaller window area, but just how much smaller we don't ourselves know at this point.

Some comments about installing your SPI Supplies Brand x-ray windows:

Other than the fact that great care must be taken to not damage with window when handling, always remember that the pressure must be applied such that the membrane is pushed against (e.g. being pulled against) the substrate silicon and not away from it.

Some of our x-ray window customers find that having some Vacseal^® Vacuum Leak Sealant near by for reducing leakage can have great value in a UVH system.

Availability of silicon nitride coated wafers:

We are now able to offer to our customers the same silicon nitride coated wafers used by SPI Supplies for the production of the membrane window product line.

Silicon oxide membranes coming!

We have under development and will be introducing in the near future, a parallel product line of products composed of silicon oxide membrane windows. While the oxide, relative to the nitride, is far more susceptible to chemical attack, and in general, is less robust, for x-ray microscopy, not having the N present is more desirable in terms of the N absorption edge.

Some other applications:
Substrates for filters such as Ti, V, Mo. Also used in x-ray optics, for example, as beam splitters.

X-ray Windows, Si₃N₄:

5 mm x 5 mm frames
Membrane window size: 0.5 mm x 0.5 mm
Membrane thickness: 100 nm