We are not able to produce silicon oxide membrane windows that are completely flat. We believe the flatness produced will be acceptable to most of our customers. As our customers say "it is better to have something that is less than perfect than not have it at all." We are not aware of anyone making perfectly flat silicon oxide membrane.
At a thickness of 50 nm there is not enough material to contribute to a diffraction pattern. The membrane itself is structureless and featureless (Remember nothing is completely sturctureless or featureless). Hence, anything seen either by diffraction or in terms of the image itself can be attributed solely to the sample and not to any structure in the support film.
We have every reason to believe that our silicon oxide membranes can be used without problem to at least 200 KV if not also higher.
After the wafer is patterned, the next step is to back etch through the silicon wafer to the (SiO2) layer, thereby producing the "membrane window" effect. While the membranes are very thin and fragile, they are sufficiently "robust" to permit one to do their experiments. The product is meant for a single use. We do not recommend trying to clean off old samples for reuse. Discriminating between the two sides with the naked eye is easy (I.E.) the membrane side vs. the etch pit side are easy to tell apart. We have not yet made AFM measurements to determine surface roughness but we expect that it will be comparable to that which we achieve for our silicon nitride membranes since that roughness is a function of the roughness of the starting silicon wafer. What is noticeably different is not so much the roughness but the flatness of the membrane.
The main attraction for SiO2 membranes over Si3N4 membranes is for those working with nitrogen containing samples, and wanting to do EDS studies without potential confusion from nitrogen in the base membrane. The oxide membranes are also preferred by some of those who are nucleating nanofibers across holes and find that the fiber, with their process, nucleate better with the oxide than the nitride composition membrane holes.
For most users, the "right" side for the sample is usually the "membrane side" of the grid as opposed to the "etch pit side". However, we have heard of the etch pit side being used in special situations. There are not intrinsic reasons why the etch pit side could not be used, except for those wanting to do AFM work on their samples, since no cantilever will go down far enough to actually get into the etch pit and "see" the membrane surface.
Anyone studying nanoparticles, especially those with nitrogen content, find these membrane window grids indispensable for their work. Those studying aerogels and/or xerogels because of the smallness in size of the basic particles, find the SPI silicon oxide membrane window grids especially valuable for their work. We are of extrapolating our comments here from our experience with the hugley successful silicon nitride membrane window grids. Because the silicon oxide membranes are new and we don't have that much actual data on silicon oxide membranes.
For high temperature applications, the oxide membranes are not as thermally stable as the nitride counterparts. High Temperature applications, we recommend the nitride membranes.
The windows if treated carefully are surprisingly robust. They can be picked up easily with the sharpest of our tweezers without risk to the membrane and it can be handled just like any other TEM grid.
The grids, as manufactured tend to be hydrophobic, so if samples are being applied from an aqueous suspension, this can cause suspended nanoparticles to appear in a nonuniform distribution across the membrane. The membranes can be made hydrophilic by treatment in a plasma etcher, such as the SPI Plasma Prep II Plasma Etcher, however the effect of the treatment is transitory (I.E. short period of time). For a fee we can treat the membrane window grids in this way but we can not guarantee the longevity of the effect.
Visually one can "See Through" the windows 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 transparent in the visible range of the spectrum. When considering optical properties, keep in mind that the absorption edge is just below 13 nm. Hence at 13 nm the transmission for a 100 nm membrane is about 44%. At 12.4 nm it drops down to about 13%. Always keep in mind where they are relative to the absorption edge when trying to estimate membrane optical properties. Our target for refraction index is approximately 2.15, but we make no gaurantees nor represent this as a specification of the product.
When considering optical properties, always keep in mind where you are relative to the absorption edge of silicon oxide when trying to estimate membrane's optical properties.
The silicon oxide membranes are not as flat as the silicon nitride membrane window grids. Indeed, making membranes free of wrinkles is one of the biggest challenges facing anyone trying to make them. We believe that the flatness now being achieved will be acceptable for most users but not all users. If flatness is an issue, and you can tolerate the presence of nitrogen, we recommend using the silicon nitride version of this kind of product.
The "standard" window size is 0.5 mm square and useful for most TEM workers. Because of the tendency of the membrane to wrinkle during production, we have ended up using a "partitioned" design where the window itself is partitioned off into 25 smaller windows. This approach does result in areas on the membrane that are more flat, but comparable to the flatness we obtain with silicon nitride membrane window grids.
The silicon oxide membrane window grids should not need further cleaning before use. There are occasionally small pieces of oxide or even of silicon from the breaking of the individual grids out of the wafer. The presence of such debris particles can't be prevented. We do not feel they pose any problems or interference to the user.
We suggest using H2 SO4 : H2O2 (1:1) for organic and H2O:HCl: H2O2 (5:3:3) for metals.
Ultrasonic treatments is not recommended as the windows will shatter.
All the commonly used coating methods can be used on the membrane but there can be adhesion problems. Not because they are membranes but because they are normal surfaces with the properties of normal surfaces. One can use vacuum evaporation, sputter coating, or any other kind of evaporative process without any problems. Spin coating is discourage until the issues of better smoothness and flatness can be achieved.
We have successfully made our first perforated silicon oxide membranes using novel micro fabrication techniques. Such grids make possible the depositing of nanofibers or other samples across the holes so data can be taken without any contribution from the underlying support being present in the final data.
Membranes are sold in packs of 10.
A s ubstantial number of membrane grid customer order tweezers (to safely pick them up) and additional grid storage boxes for storing the prepared grids.
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