For those who are doing either spin coating or further processing of the membrane windows,
such as vacuum deposition, purchasing their membranes as a multi-frame array
(MFA) offers the convenience of time savings as well as uniformity of processing of quantities
of membranes at one time.
The outside frame size is the same for all membrane sizes, namely 23.5 x 23.5 mm using our
standard stock 200 µm thick silicon wafers. Other
silicon thicknesses can be used, but at higher costs which can be as much as 2X higher in cost and possibly larger minimum
quantities.
Remember that the nitride layer is always "low stress" silicon nitride (as opposed to stoichiometric silicon nitride).
For most users, the "low stress" membrane is the desired option. We can make the membrane in
the multiframe arrays in just about whatever thickness might be required subject
only to the limitations in thickness imposed by the window size. Generally speaking,
as the membrane thickness drops below 100 nm, the cost starts to escalate.
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Typical multi-frame array of membrane windows
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This approach is also very useful for anyone wanting some kind of "special" window size but
does not have the budget for an entire wafer's worth of production. The cost per membrane
using the MFA approach is higher, but the minimum order is significantly lower than the
production from an entire wafer. Hence, the researcher can get something special without
having to take the production from an entire wafer, since a single wafer can have a number
of
different multi-frame arrays!
We have received many compliments from customers who have taken advantage of the MFA approach
since it enables them to do their experiments without using up all of their budget on the
production of vast quantities of membranes that they will never use.
Just remember that this approach is for special order membranes, not for standard membranes,
unless further processing is going to be done. On a per-membrane basis, the cost will
always be higher when ordered as part of a MFA than if as individual membranes because our
yields for an MFA are much lower; if just one out of 49 membranes for example, breaks, we
can not offer it as a MFA and the individual membrane chips have to be snapped out and packaged
for sale as separate membranes.
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Typical detail of the membrane window:
Example is 0.5x0.5 mm square
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We can produce whatever geometry one might want so long as it can be handled
within the confines of the 23.5 mm square frame. Most applications require
square frames. For TEM applications, we can produce 2.65 mm square frames with
"clipped back corners" so that the diagonal will be 3.05 mm which of course
will fit comfortably into anyone's standard 3.05 mm TEM grid holder. We
should correct one misconception that has prevailed, perhaps in part caused
inadvertently by our own use of the term "clipped back corners". We are
not talking about using some kind of a mechanical "clipping" device like
nail clippers (!) but the corners are being
chemically etched back to
produce the final desired product.
The dimensions given below are for the individual units (outside frame size)
and not the over all dimensions of the total array. For example, if one
wanted to order SPI#
4138SN-AB which 3x3 mm outside frame size, since this
corresponds to a 6x6 array, the actual array dimensions would be 18x18 mm
but when the outside "rim" is added, the overall dimensions for the outside
frame would be 23.5 x 23.5 mm. Since costs are determined primarily by how
many arrays we can get on to a single wafer, the larger the array's
overall size, the higher the cost.
This is the only MFA product we recommend for direct TEM applications,
namely the 2.65 x 2.65 outside frame size, and which is designated as SPI #4134SN-AB.
The 2.65 mm squares, with their corners clipped, will fit comfortably into any standard
TEM grid holder. 2.65 mm is the repeating distance of the squares in the array.
We recommend the
Model KW-4A spin coater because of
its ease of use and also it budget level price. Note that one should not be using vacuum to hold the
MFA to the "chuck" since that vacuum will cause the membrane to bow outward and therefore reduce its
resistance to fracturing. This also means that this deformation will lead to an uneven coating.
Some researchers have developed special "chucks" that use clips rather than vacuum and this is
another benefit of the MFA substrate.
The array is made with the scribe lines down, that is, they can not be seen from above. Then press down gently with
tweezers at the end of the scribe lines and you will see a line start to appear as the wafer bends. A little more
pressure and it starts to crack. At this point, the array should be turned over and the same thing done from the other
side. Once one edge is off, take the other edges off in turn and then break out the columns of individual frames.
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