EM - Aperture Materials and Cleaning

APERTURE MATERIALS

Whether an aperture can be cleaned and reused depends partially on the material.

Platinum Apertures

Platinum apertures are easily cleaned by either an aperture flamer or over a bunsen burner in the open air. The platinum aperture should be handled with a pair of tweezers reserved solely for this purpose.

Platinum apertures can also be cleaned chemically, however neither method will reliably remove all contaminants. Even an aperture that appears clean may produce astigmatism when reused in the microscope. The platinum aperture therefore has a limited work life. 

Molybdenum Apertures

Molybdenum apertures can be cleaned 5 to 10 times more than a platinum aperture, however they must be cleaned under high vacuum, using a vacuum evaporator. 

Gold Foil Disc Apertures 

Conventional mechanical cleaning involving costly down-time is no longer necessary with gold foil “self-cleaning” apertures. 

Advanced techniques of thin metal film application permits the apertures to retain a clean hole edge during prolonged beam exposure. Ultra-thin, half-micron design discourages contaminant accumulation and stabilises astigmatic conditions to provide longer life. Because the foil is ultra thin, care must be taken when handling the gold foil apertures.

CLEANING

Aperture Cleaning using Flamer

The following method is for cleaning platinum apertures; clean tungsten and molybdenum apertures by heating in vacuum.

Pre-clean apertures by storing them for a while in a strong solvent such as chloroform.

1. Place aperture disc in cup or hang strip from hook on cup.
2. Turn Bunsen burner to low (small blue flame).
3. Hold apertures on flamer 12mm - 20mm above aperture.
4. Adjust heat if required to turn aperture bright red. No more than 5 seconds.  Greater heat and long (more than a few seconds) flaming times increase crystalinity of the disk or strip and eventually ruin the apertures.
5. Inspect aperture on a microslide with a microscope. Repeat heating if required.

Note: the force of the heat, rather than the flame itself, cleans the aperture.

Cleaning MAP Apertures

When mounting Pt or Mo apertures, please note:

Very small and loose particles may contaminate new and well made apertures. Inspect aperture, after placing it flat side up on a clean watch glass. Use a light microscope with a 10 to 40 x objective.

If required, ultra-sonicate aperture in absolute alcohol and blot, flat side up on filter paper. Work with aperture container and electron microscope holder above lint free material, such as aluminium foil or a large filter paper.

Use tweezers, exerting little pressure, and lift aperture straight into aperture holder.