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SPI Supplies® Brand Silicon Nitride Membrane Window Grids for TEM

A study to determine the surface texture (e.g. smoothness) of the silicon nitride membrane windows



For some number of years, ever since the inception of the concept of the silicon nitride membrane window grid, our main focus has been on electron (and also x-ray) transparency and high vacuum compatibility so that the membranes could be used as windows and not just as support films for samples being examined by TEM. For such applications, one does not worry about the a) the flatness or b) the "smoothness" of the membrane. But recently a growing number of researchers have been asking us about the "smoothness" or some would say "roughness" of the surface of the silicon nitride membrane. The one thing these researchers all had in common was that they were working on self-assembled arrays.

SPI asked a leading independent laboratory conducting AFM studies to give us surface roughness information for both the SPI Supplies brand of silicon nitride membrane window grids vs. grids from another source. We present here the summary of the results from the independent laboratory's testing.

Roughness results:
An independent lab used Atomic Force Microscopy (AFM) to measure surface roughness on two samples each of our membranes and a competitor's. The roughness measurements were made near the center of the window using 5 µm images captured in TappingMode™ with 512X512 pixels* and produced the following results:

Sample RMS (Rq) Mean Roughness (Ra) Max. Height (Rmax) Surface Area Diff Box dimensions
SPI
B8 0.74 nm 0.48 nm 24.11 nm 0.21% 5000 nm
A7 0.67 0.47 12.89 0.19 5000
Competitor
A 0.88 0.54 17.85 nm 0.25 5000
B 7.65 1.30 192.16 0.60 5000
B (ex) 0.78 0.50 16.02 0.61 5000


Note: Sample B (of competitor) contained a very large bump. We present data for competitive sample B both inclusive and exclusive of the data corresponding to the bump.

*Images captured at different sizes and resolutions, or using different scanning techniques, may produce different results. TappingMode™ is a trademark of Veeco Instruments. We would prefer to not now make further interpretations of the results and permit the viewer to draw their own conclusions. We know that two samples are not necessarily representative of our production, but we were mainly interested in getting a general picture of just what would actually be the roughness values. We believe that we did accomplish our goals for having funded this comparative study.

The original AFM images are shown below. Instead of offering further interpretation of the results, we let our readers draw their own conclusions. We know that two samples are not necessarily representative of our production, but we were mainly interested in getting a general picture of just what would actually be the roughness values. We believe that we did accomplish our goals for having funded this comparative study.

We would also point out that SPI Supplies customers would never receive samples like the competitor window with the large bump. Our quality control procedures include rigorous inspection and such defects are rejected. Our philosophy is for us to be the quality control lab, not our customers.

Original independent laboratory AFM results:


Figure 1 is a dual height and phase image of an SPI grid. Here the sample surface is viewed from the top. In the height image, bright yellow and white indicates high spots and dark red or brown indicates low spots. The surface contains many tiny bumps which contribute to the roughness tabulated above. In the phase image, bright and dark contrast is related to edges and slopes and serves here to highlight the tiny bumps.



Figure 2 is a perspective view of the height data for the SPI grid. Note that the magnification of the Z axis is much higher than the XY magnification, so that you can perceive the fine structure of what is really an extremely smooth surface. Such dual magnification is customary in the AFM field.



Figure 3 is the same type of image as figure 1, here showing a competitor grid.




Figure 4 is a perspective view of the height data for the competitor grid.


Remember that SPI Supplies has been the pioneer in introducing silicon nitride membrane windows to the worldwide scientific community. We have had many years of experience in the perfecting not only of the smoothness of the membranes but for a given membrane thickness, their UHV compatibility. Nothing beats in importance the value of this kind of experience. Many of our customers push their experiments to the envelop of what is possible and/or what has been done before, which sometimes requires us to carry our unique technology one step further. Let us know your requirements. If you can use one of our standard products, this will always be the most economical solution for you. But if you need something custom, let us know what you require, remembering also to tell us any particular specification requirements you might have (e.g. specifications on the outside frame dimensions).

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Wednesday February 08, 2012
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