Selection of Correct Material of Construction

SPI Supplies offers apertures made from three distinctly different materials of construction, each offering its own pros and cons. Proper aperture selection requires knowing the trade-offs !

The actual composition is not pure platinum but is an alloy of platinum with 5% Iridium

Advantages: Can be easily cleaned with an aperture flamer over a bunsen burner flame in air. Easily handled, both when cold as well as when hot. No need to invest, use and/or purchase a costly vacuum evaporator.

Disadvantages: Apertures will eventually "fail" due to grain growth and a saw tooth appearance around the edge resulting from repeated heating cycles and cooling. The rate of grain growth with Pt is noticeably faster than with Mo, which is a refractory metal.

Advantages:Molybdenum is a refractory metal; and grains grow almost imperceptibly slow. A molybdenum aperture can be cleaned many more times than a platinum aperture and will "outlive" a Pt aperture roughly 5 to 10 times. It will contaminate just as quickly, but it can be cleaned and recleaned many more times than Pt.

Disadvantages: Cannot be cleaned in air; must be cleaned under high vacuum, in a vacuum evaporator, using a tungsten boat. Discoloration will occur over time if molybdenum is exposed to air when hot. This is not a contamination problem and for most users at least need not be a concern. However, what is formed is most likely a nitride which of course is not going to be conductive, and while we might not be able to prove it, we just believe that starting out with a bit of nonconductive coating on a clean aperture is just not the best practice of electron microscopy. If you too share our concerns, we suggest you use a platinum aperture. Or alternatively, you might want to try one of our own SPI gold foil thin film apertures, made to a standard that is unsurpassed.

Advantages: Highest performance; longest life. Gold is deposited to form the disc, then acid etched to form the aperture, producing the roundest possible holes of any aperture made today. The contamination rate is the very slowest of all aperture choices because it operates at such a high temperature and that is why it is described as being "self-cleaning".

Disadvantages: Very fragile; can only be cleaned by heating in situ. Easily damaged by electron beam if not concentrated on "hole" of aperture. Smallest hole size (e.g. 1 um) requires relatively clean instruments in order to get anything approaching "reasonable" aperture lifetimes. Useful lifetimes have been reported as being on the order of 60 minutes in diffusion pumped instruments that are thought to be reasonably clean. However, customers who have used them seem to be very happy since they report results just not obtainable via any other means. There have also been reports of apertures being recleaned for reuse with an SPI Plasma Prep II but we here at SPI Supplies have not confirmed such rejuvenation can in fact be done.

Information about Tolerances

           Pt and Mo                         Gold Foil
            Drilled                          Thin Film
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        Hole Dia.       Tolerance       Hole Dia.       Tolerance
                                         1µm            +/- 0.01µm
         5-10µm         +/- 1 µm         2-5µm          +/- 0.5µm
         10-20µm        +/- 1.0µm        20-500µm       +/- 10% of size
         30-70µm        +/- 2.0µm        90-150µm       +/- 5.0µm
         300-1000µm     +/- 1% of size
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