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Resolution of the atomic structure of a metal surface by the field ion microscope
VACUUM Classified Abstracts
Abstract No. and References
56/II Article by J. A. Becket & R. G. Brandes J. Appl. Phys. 27, March 1956 221-223
57/II
Article by W. P. Dyke & J. P. Barbour J. Appl. Phys. 27, April 1956 356-366
58/II
Article by E. W. Muller J. Appl. Phys. 27, May 1956 474-476
59/II
II
--
Vacuum
Apparatus and Contd.
Auxiliaries
--
II
A Favourable Condition for Seeing Simple Molecules in a Field Emission Microscope United States. I t has been s h o w n theoretically t h a t irregularities on a metal tip cause increased local resolution in a field emission system. The p a p e r p r e s e n t s experimental evidence s u p p o r t i n g the theory. Baking the s y s t e m at a b o u t 350°C is believed to h a v e caused irregularities in the t u n g s t e n tip b y the etching action of w a t e r v a p o u r and spots were observed on the screen. T h a t irregularities were p r e s e n t was deduced from t h e voltage-current characteristics of the system. I t has also been s h o w n t h a t adsorbed molecules give b r i g h t spots on the screen.
Pulsed T-F Emission Electron Projection Microscopy United States. H i g h - t e m p e r a t u r e high-field emission microscopy suffers from the drawbacks of cathode instability and the effects of electrostatic forces on the mechanisms being observed. By pulsing the s y s t e m at low d u t y cycle (1 ix sec. pulses at 30 pps.) these effects are made negligible. The a p p a r a t u s has been described elsewhere and enables observations to be made of a n u m b e r of p h e n o m e n a and u n d e r various conditions which are n o t possible using a steady field. E x a m p l e s are given of volume diffusion followed b y surface migration of impurities in the cathode, condensation and surface migration of barium, and the relative m o v e m e n t of the cathode surface and twin b o u n d a r y in the cathode material. Cathode distortion and the high steady fields in the n o r m a l m e t h o d would not allow the observation of these effects.
Resolution of the Atomic Structure of a Metal Surface by the Field Ion Microscope United States. W o r k is described on the field ion microscope, developed from the field emission nlicroscope in which electrons emitted from a p o i n t electrode at a high potential are received on a fluorescent screen. I n the a p p a r a t u s described, the point is positive and a gas filling at a few microns pressure (He is preferred) gives rise to ionisation of some of the gas molecules which a p p r o a c h the point, and the fluorescent image is formed b y the ions. Theoretical discussion of the factors affecting the resolution shows the i m p o r t a n c e of a gas with a low polarisability. I n the a p p a r a t u s described, the point is of tungsten, p r o t r u d i n g f r o m the inner wall of w h a t is essentially a D e w a r vessel. The point can t h u s be cooled b y filling the vessel with liquid N 2 or H 2 and f u r t h e r cooling of the gas is obtained b y s u r r o u n d i n g the inner vessel b y an a n n u l a r space which can also be filled w i t h refrigerant. This drastic cooling increases the a c c o m m o d a t i o n coefficient of the gas, so t h a t the ions formed from the rebounding molecules will h a v e s u b s t a n t i a l l y the t e m p e r a t u r e of the point and a correspondingly small transverse kinetic energy. I t is estimated t h a t using a point of 1,000 A radius and He gas the theoretical diameter of the scattering disc is reduced from 6 to 1.5 ~ b y cooling. P h o t o g r a p h s t a k e n u n d e r these conditions show a large increase in resolution c o m p a r e d to a similar image formed b y electrons. I n m a n y p a r t s of the pictures the individual a t o m s constituting the concentric edges o f t h e (0II) net planes are resolved. Field evaporation (at liquid H 2 t e m p e r a t u r e and a field of a b o u t 5.7 × 10 s V/cm) is found to produce a m u c h more perfect surface t h a n annealing. W o r k is in progress observing the damage caused b y ion b o m b a r d m e n t , etc., and various kinds of dislocations have been found. Their extension into the depth can be analysed b y controlled evaporation of the surface layers.
Recent Developments in Specimen Preparation Techniques for Electron Microscopy
Article by V. E. Cosslett & R. W. Horne Vacuum 5, Oct. 1955 109-130
United Kingdom. Present-day m e t h o d s employed in the p r e p a r a t i o n of specimens for examination in the electron microscope are reviewed. After discussing the properties required of the specimen s u p p o r t s and mentioning the s t a n d a r d s available for a critical assessment of the m i c r o g r a p h the a u t h o r s describe microtomes available for ultra-thin sectioning, staining techniques, freeze drying techniques, and various replication m e t h o d s employed in the p r e p a r a t i o n of specimens of biological materials. Special m e n t i o n is m a d e of the use of the recently developed techniques for the deposition of carbon films. I n the second p a r t of the p a p e r the a u t h o r s investigate the present techniques employed for the specimen p r e p a r a t i o n of inorganic materials, in particular the crystalline s t r u c t u r e and surface conditions of metals. Silicon monoxide replication m e t h o d s are mentioned in connection w i t h the p r e p a r a t i o n of specimens for chemical studies a n d certain aspects of the specimen prep a r a t i o n of fibrous materials are mentioned.
6O/II
A Simple Adaptation of the Carbon Replica Technique for the Examination of Selected Areas in the Electron
Microscope United Kingdom. The p a p e r describes an i m p r o v e d m e t h o d of preparing carbon replicas which makes it possible to examine areas or features which have been selected previously in the optical microscope. The m e t h o d is simpler t h a n those previously devised and requires only an a t t a c h m e n t to a 16 ram. optical microscope objective. This consists of a fiat plate, drilled so t h a t it can be clamped between the objective cap and the m a i n b o d y of the microscope, which s u p p o r t s a second plate b y a hinge at one end and a hinged bolt at the other. The r o t a t i o n of a screw on the bolt enables the separation of the plates to be varied. Three small pins are fixed undern e a t h the b o t t o m plate r o u n d a small hole which lies on the optical axis of the objectives; these s u p p o r t the specimen grid which is a t t a c h e d w i t h a suitable adhesive. Provision is m a d e for placing a reference scratch on the specimen b y m e a n s of a printed d i a m o n d or sapphire m o u n t e d on a lever p i v o t t e d from the fixed portion of the microscope stage. After m a r k i n g the specimen, it is removed from the optical microscope and coated with a thin film of F o r m v a r . I t is t h e n r e t u r n e d to the optical microscope and the selected area is located again using the o r d i n a r y objective. This is n e x t replaced b y the objective modified as above and with a specimen grid
246
Vacuum Vol. VI
October, 1956
Abstract No. and References
56/II Article by J. A. Becket & R. G. Brandes J. Appl. Phys. 27, March 1956 221-223
57/II
Article by W. P. Dyke & J. P. Barbour J. Appl. Phys. 27, April 1956 356-366
58/II
Article by E. W. Muller J. Appl. Phys. 27, May 1956 474-476
59/II
II
--
Vacuum
Apparatus and Contd.
Auxiliaries
--
II
A Favourable Condition for Seeing Simple Molecules in a Field Emission Microscope United States. I t has been s h o w n theoretically t h a t irregularities on a metal tip cause increased local resolution in a field emission system. The p a p e r p r e s e n t s experimental evidence s u p p o r t i n g the theory. Baking the s y s t e m at a b o u t 350°C is believed to h a v e caused irregularities in the t u n g s t e n tip b y the etching action of w a t e r v a p o u r and spots were observed on the screen. T h a t irregularities were p r e s e n t was deduced from t h e voltage-current characteristics of the system. I t has also been s h o w n t h a t adsorbed molecules give b r i g h t spots on the screen.
Pulsed T-F Emission Electron Projection Microscopy United States. H i g h - t e m p e r a t u r e high-field emission microscopy suffers from the drawbacks of cathode instability and the effects of electrostatic forces on the mechanisms being observed. By pulsing the s y s t e m at low d u t y cycle (1 ix sec. pulses at 30 pps.) these effects are made negligible. The a p p a r a t u s has been described elsewhere and enables observations to be made of a n u m b e r of p h e n o m e n a and u n d e r various conditions which are n o t possible using a steady field. E x a m p l e s are given of volume diffusion followed b y surface migration of impurities in the cathode, condensation and surface migration of barium, and the relative m o v e m e n t of the cathode surface and twin b o u n d a r y in the cathode material. Cathode distortion and the high steady fields in the n o r m a l m e t h o d would not allow the observation of these effects.
Resolution of the Atomic Structure of a Metal Surface by the Field Ion Microscope United States. W o r k is described on the field ion microscope, developed from the field emission nlicroscope in which electrons emitted from a p o i n t electrode at a high potential are received on a fluorescent screen. I n the a p p a r a t u s described, the point is positive and a gas filling at a few microns pressure (He is preferred) gives rise to ionisation of some of the gas molecules which a p p r o a c h the point, and the fluorescent image is formed b y the ions. Theoretical discussion of the factors affecting the resolution shows the i m p o r t a n c e of a gas with a low polarisability. I n the a p p a r a t u s described, the point is of tungsten, p r o t r u d i n g f r o m the inner wall of w h a t is essentially a D e w a r vessel. The point can t h u s be cooled b y filling the vessel with liquid N 2 or H 2 and f u r t h e r cooling of the gas is obtained b y s u r r o u n d i n g the inner vessel b y an a n n u l a r space which can also be filled w i t h refrigerant. This drastic cooling increases the a c c o m m o d a t i o n coefficient of the gas, so t h a t the ions formed from the rebounding molecules will h a v e s u b s t a n t i a l l y the t e m p e r a t u r e of the point and a correspondingly small transverse kinetic energy. I t is estimated t h a t using a point of 1,000 A radius and He gas the theoretical diameter of the scattering disc is reduced from 6 to 1.5 ~ b y cooling. P h o t o g r a p h s t a k e n u n d e r these conditions show a large increase in resolution c o m p a r e d to a similar image formed b y electrons. I n m a n y p a r t s of the pictures the individual a t o m s constituting the concentric edges o f t h e (0II) net planes are resolved. Field evaporation (at liquid H 2 t e m p e r a t u r e and a field of a b o u t 5.7 × 10 s V/cm) is found to produce a m u c h more perfect surface t h a n annealing. W o r k is in progress observing the damage caused b y ion b o m b a r d m e n t , etc., and various kinds of dislocations have been found. Their extension into the depth can be analysed b y controlled evaporation of the surface layers.
Recent Developments in Specimen Preparation Techniques for Electron Microscopy
Article by V. E. Cosslett & R. W. Horne Vacuum 5, Oct. 1955 109-130
United Kingdom. Present-day m e t h o d s employed in the p r e p a r a t i o n of specimens for examination in the electron microscope are reviewed. After discussing the properties required of the specimen s u p p o r t s and mentioning the s t a n d a r d s available for a critical assessment of the m i c r o g r a p h the a u t h o r s describe microtomes available for ultra-thin sectioning, staining techniques, freeze drying techniques, and various replication m e t h o d s employed in the p r e p a r a t i o n of specimens of biological materials. Special m e n t i o n is m a d e of the use of the recently developed techniques for the deposition of carbon films. I n the second p a r t of the p a p e r the a u t h o r s investigate the present techniques employed for the specimen p r e p a r a t i o n of inorganic materials, in particular the crystalline s t r u c t u r e and surface conditions of metals. Silicon monoxide replication m e t h o d s are mentioned in connection w i t h the p r e p a r a t i o n of specimens for chemical studies a n d certain aspects of the specimen prep a r a t i o n of fibrous materials are mentioned.
6O/II
A Simple Adaptation of the Carbon Replica Technique for the Examination of Selected Areas in the Electron
Microscope United Kingdom. The p a p e r describes an i m p r o v e d m e t h o d of preparing carbon replicas which makes it possible to examine areas or features which have been selected previously in the optical microscope. The m e t h o d is simpler t h a n those previously devised and requires only an a t t a c h m e n t to a 16 ram. optical microscope objective. This consists of a fiat plate, drilled so t h a t it can be clamped between the objective cap and the m a i n b o d y of the microscope, which s u p p o r t s a second plate b y a hinge at one end and a hinged bolt at the other. The r o t a t i o n of a screw on the bolt enables the separation of the plates to be varied. Three small pins are fixed undern e a t h the b o t t o m plate r o u n d a small hole which lies on the optical axis of the objectives; these s u p p o r t the specimen grid which is a t t a c h e d w i t h a suitable adhesive. Provision is m a d e for placing a reference scratch on the specimen b y m e a n s of a printed d i a m o n d or sapphire m o u n t e d on a lever p i v o t t e d from the fixed portion of the microscope stage. After m a r k i n g the specimen, it is removed from the optical microscope and coated with a thin film of F o r m v a r . I t is t h e n r e t u r n e d to the optical microscope and the selected area is located again using the o r d i n a r y objective. This is n e x t replaced b y the objective modified as above and with a specimen grid
246
Vacuum Vol. VI
October, 1956