During a recent "Intensive SEM" coarse, that took place in Johannesburg South Africa , the students came up with a superb cleaning technique that they wished to report upon.  The results are or should be interesting to everyone who has to clean a cathode assembly!  

From my side the course contains a number of carefully structured practicals that are designed to present the operating variables to the students in the clearest possible fashion.  From time to time one finds oneself developing practicals "on the run" to suit the situation.  It was such a case that is reported below; the filament failed!  

Introduction

The most time consuming operation during the routine use of a SEM or TEM is often the cleaning of a cathode assembly.  The procedure outlined requires little operator intervention, is free from possible cathode contamination by the cleaning media and takes comparatively very little time.  

History

There is a vast array of cleaning media used by laboratories to clean the cathode assemblies of electron microscopes.  With many of these the biggest failing is the difficulty of removing completely the cleaning media leading to excessive contamination within the system.  This problem is further complicated by the human hazards associated with some of the solvents being used.  In some countries acetone and ether are not permitted in the laboratory!

Steve Chapman has been using and teaching an ultrasonic cleaning technique he developed in 1964.  The procedure took advantage of tungsten being soluble in an ammonia solution (NH4OH) and combined this media with a metal polish that was also soluble in ammonia.  The technique used an ammonia solution that had been diluted from a stock solution down to 10 to 15 parts water to 1 part ammonia.  A range of metal polishing media had been used dictated by their availability in various countries of the world.  This cleaning procedure relies more upon the abrasive effect of the metal polishing media rather than the chemical attack from the ammonia.  Subsequent to the metal polish ultrasonic cleaning period of about 30 minutes, the polishing media was removed by way of two further 5 minute ultrasonic cleaning periods in the dilute ammonia alone, to ensure complete removal of the metal polishing media.  The components were then washed in alcohol and dried with a hot air blower.  With severely contaminated cathodes, as would be typical of a SEM used at high emission currents, a degree of manual cleaning was often required in the "burnt on" tungsten areas around the cathode aperture.  That could be prior to or after the initial ultrasonic cleaning procedure.

The New Procedure

The cathode assembly was placed, aperture face upwards, in a beaker of stock ammonia solution diluted 3 parts ammonia to one part water.  The stock solution was thought to be about 40% ammonia.  After 15 minutes in the ultrasonic cleaner the beaker was placed under running water and thoroughly flushed through.  Care was taken to ensure that none of the clamping or alignment screws had fallen out of the cathode assembly and could be flushed away!  The cathode was then washed with alcohol before being dried with a hair drier.  A new filament was fitted and centred.  The assembly was checked for cleanliness by observing with a 20X lens prior to re installation in the microscope.  Total time for this procedure should be less than 25 minutes break to pump down.

Safety

Great care was taken not to allow the ammonia solution to make contact with the skin or eyes of the operator.  When flushing the solution through with water its flow was set so as not to splash the solution over the operator prior to placing the beaker under the flow.

Conclusions

The procedure was used on a severely contaminated SEM cathode and a cathode assembly from an electron probe.  Contamination rate observations in the SEM before and after cleaning indicated little or no increase in contamination levels.

We were all amazed at the results, a totally wet cleaning method, a perfectly clean cathode assembly with apparently no instrument problems?  I do not know how ammonia reacts with tantalum (the Philips cathode aperture) but I would guess this technique would be applicable to any SEM, TEM, or probe?