IMPROVED RESOLUTION IN FREEZE-ETCHING

Resolution in freeze-etching is primarily limited by the need for shadowing the replica. In addition to giving high resolution, any technique suitable for freeze-etching must allow short shadowing times, a low thermal load for the specimen, and must provide a final film capable of surviving the drastic chemical procedures used for cleaning the replica. Simultaneous evaporation of platinum and carbon is at present the standard shadowing method in freeze etching. Replicas of higher resolution can in principle be obtained by using very high melting metals such as tungsten or tantalum. Extensive specimen damage caused by long evaporation times and excessive thermal load have however prevented the successful application of such ultrashadowing methods to freeze etching. Since neither long shadowing times nor a high thermal load are properties intrinsic to ultrashadowing, a suitable electron beam evaporator for high melting metals was built and thus ultrashadowing successfully applied to freeze-etching. All parts of the gun are water cooled and can be outgased by electron bombardment. The source can thus be operated reproducibly at high rates and without affecting the vacuum. The actual source (3 mm Ø) is the only hot part of the gun not shielded from the specimen. The ions which are generated during evaporation, and can cause considerable specimen damage, are deflected from the specimen by an electric field. The evaporation is done in 7-10 seconds, the source-specimen distance is 200 mm. For the shadowing material we use a tantalum-tungsten alloy. The resultant films are stable in 70% sulfure acid. At lower magnification freeze-etched specimen which are shadowed with Ta/W look just like Pt/C replicas which indicates that no additional artification took place. High magnification micrographs of ultrashadowed objects show a resolution considerably higher than those of platinum-carbon replicas published in the literature. Since heat damage is a crucial problem in freeze-etching the thermal load for tantalum-tungsten-and platinum-carbon-shadowing was calculated. For both methods a theoretical value of approximately 16 mW·cm-2 was obtained provided the above shadowing conditions are observed and the ions are deflected from the specimen. Furthermore, the calculations show that the load caused by thermal radiation can be reduced drastically, without increasing the shadowing time, if the gun is operated at a higher rate thus allowing the use of either a smaller source or a longer source-specimen distance. Measurements of the thermal load agreed basically with the calculations. The values measured using the tantalum-tungsten gun were about half the ones obtained during platinum-carbon shadowing with a commercial evaporator. © 1969 Springer-Verlag.