Materials Science

Silicon Windows

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Science  27 Aug 2004:
Vol. 305, Issue 5688, pp. 1214
DOI: 10.1126/science.305.5688.1214c

Although the sample chambers in most electron microscopes are under vacuum, environmental scanning electron microscopes are making it feasible to analyze biological samples at ambient pressures. For these microscopes to work, the electron column, where the beam is formed, has to stay under high vacuum, and so a cascade of pressure stages (like a series of locks in a canal) is used to maintain a pressure gradient. Similarly, if x-ray detectors are used, they need to be protected from contamination with a window made either of beryllium, which cuts off x-rays below 1 keV, or of a polymer, which can be fragile.

Schilling et al. have fabricated a macroporous silicon membrane using photoelectrochemical etching to generate the pores, followed by oxidation and chemical etching to smooth them out. The resulting structure features 50-μm-long pores that are capped with dome-shaped silicon dioxide shells, only 60 nm thick. The nonporous regions of the membrane allow it to withstand a pressure differential of ambient on one side and vacuum on the other. Transmission levels for an electron beam with an accelerating voltage of 25 keV were as high as 22%, albeit with significant variation from pore to pore. Further tests showed that these membranes would also transmit x-rays and infrared radiation. — MSL

Appl. Phys. Lett. 85, 1152 (2004).

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