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Improvements i.o.r.t. cathode sputtering apparatus
VACUUM Classified Abstracts
Abstract No. and References
Article by R. B. Belser & W. H. ]:[icklin t~ev. Sci. Instrum. 27, May 1956 993-296
66/III
L. A. Holland & W. Edwards & Co. (London) Ltd. Brit. Pat. 736,512
67/II1
Article by G. K. Wehner Phys. Bey. Io2, 1.5.1956 690-704
68/III
Letter by J. F. Straehan & N. L. Harris _Nature x78, 15.9. 1956 588-589
69/tii
II I --
Vacuum
Processing Contd.
Techniques
~
II I
on the adhesion of fihns to glass, q u a r t z and glazed tile s u b s t r a t e s are described. The films produced by sputtering are characterised b y high adherence and resistance to s t r u c t u r a l changes induced by heating. R e f r a c t o r y metals m a y be s p u t t e r e d fairly readily whereas their evaporation is difficult or impossible. Uses of the hard, chemically resistant films produced b y s p u t t e r i n g these metals are discussed.
Improvements i.o.r.t. Cathode Sputtering Apparatus United Kingdom. Details are given of a s p u t t e r i n g p l a n t which is designed to facilitate operation at lower t h a n conventional pressures w i t h o u t affecting the distance of cathode to article. Basically the p l a n t consists of a cylindrical c h a m b e r positioned vertically on a baseplate and closed b y a door at the top end. Baseplate and door form the pole-pieces of an electromagnet. The s p u t t e r i n g cathode is suspended from the door on insulating s u p p o r t s and the baseplate carries an a n n u l a r cathode s u r r o u n d i n g the work holder. I n operation the c h a m b e r is p u m p e d down to a pressure of 0.08 mm. H g b y conventional means. Then the magnetic field is supplied b y energising the m a g n e t and the pressure is f u r t h e r reduced to 3.5 × 10 -~ m m . Hg. I n the presence of the magnetic field (200 gauss) the electrons travelling on a longer p a t h t h a n in the conventional a p p a r a t u s are able to produce more effective ionisation of the residual gas and t h u s enable a cold cathode glow discharge to be sustained at the low pressure mentioned. The anode is connected to the earthed positive terminal of a d.c. s u p p l y providing 1,500 volts, the total c u r r e n t being of the order of 150 m A and the c u r r e n t density 0.25 mA sq.cm.
Controlled Sputtering of Metals by Low-Energy Hg Ions United States. S p u t t e r i n g m e a s u r e m e n t s made using the glow discharges as a source of positive ions arc shown to be unreliable. The w o r k reported here was carried out u n d e r well controlled conditions b y placing the target in a high density p l a s m a of m e r c u r y ions sustained by an arc discharge from a pool-type cathode. Measurements of the n u m b e r of metal a t o m s sputtered under the i m p a c t of one positive ion (yield) were made from p l a t i n u m in the polycrystalline state. S p u t t e r i n g from single crystals of metal showed characteristic p a t t e r n s according to the orientation of the crystal. I t was found t h a t a t o m s were preferentially ejected in directions corresponding to closest packing, i.e. along the (110) and (111) crystal planes in the case of face-centred cubic and body-centred cubic crystals respectively. The possibility of ejected a t o m s being t r a p p e d again on the surface is discussed, and micrographs illustrate the re-growth of silver crystallites on a single crystal of silver subjected to ion b o m b a r d ment. The main p a r t of the work concerns the m e a s u r e m e n t of the m i n i m u m ion energy required for sputtering. The threshold energies of t w e n t y - t h r e e different metals are measured u n d e r m e r c u r y b o m b a r d m e n t , at normal incidence. A single relationship was revealed. At threshold energy it was s h o w n t h a t the p r o d u c t of incident ion energy transferred to target a t o m s and the velocity of sound in the t a r g e t i s p r o p o r t i o n a l to the heat of sublimation of the t a r g e t metal. A s s u m i n g this law, values of threshold energy have been calculated for other gas ions. E x p e r i m e n t s were also carried out at oblique incidence, when it was found t h a t the threshold energies were considerably lower. I t is concluded t h a t at an ion energy less t h a n 300 eV the f u n d a m e n t a l process of s p u t t e r i n g is p r o b a b l y a direct m o m e n t u m transfer. At higher energies, the process is possibly different, and could better be described in t e r m s of evaporation from locally disturbed regions of the target surface.
X - R a y Hazard in Sputtering Apparatus United Kingdom. The a u t h o r s d r a w a t t e n t i o n to the x-ray hazard arising from s p u t t e r i n g experiments with lowpressure gas discharge t u b e s or industrial processes involving ion b o m b a r d m e n t at ca. 10 kV and above. The need for exercising the precautions of testing and shielding, as for magnetrons, gas-filled and v a c u u m valves, is stressed. These conclusions were d r a w n from e x p e r i m e n t s in which targets of materials, including iron, carbon and u r a n i u m , were exposed to the discharge p l a s m a of argon or m e r c u r y v a p o u r discharge tubes. Negative potentials of up to 20 kV were applied to the targets, and t a r g e t c u r r e n t s ranged from 0.1 to 10 mA. Positive ion b o m b a r d m e n t at the higher voltages gave rise to copious secondary electron emission. Acceleration of these electrons across the positive ion s h e a t h s u r r o u n d i n g the t a r g e t and collision with the walls of the discharge tube gave rise to the observed x-rays. Fogging of x - r a y film at 8 kV and above indicated radiation intensities of ca. 10 -s r./sec, six inches from the tube.
The Deposition of Thin Films of Gold on Cylindrical Specimens by Sputtering United Kingdom. The sputtering c h a m b e r consisted of a cylinder of glass t u b i n g on the inside of which a gold foil cathode was held b y washers and bolts. E a c h end of the glass t u b e was closed b y disc-shaped a l u m i n i u m or steel anodes. The ultimate object was to deposit thin films of gold on c a d m i u m single crystals b u t in the preliminary experiments 1 m m . diameter polycrystalline c a d m i u m wire was used. This was s u p p o r t e d along the axis of the discharge t u b e and was insulated from the anodes b y small glass insulators. Thus, the actual discharge took place between the anodes and cathode only and excessive heating of the wire was avoided. Before s p u t t e r ing, the v a c u u m s y s t e m was outgassed at 0.1 ~ for an hour. Sputtering was carried out in dry air a d m i t t e d via a needle valve after passing t h r o u g h a solid carbon dioxide t r a p to remove w a t e r vapour. The pressure was kept as low as possible (10 V) consistent with the m a i n t e n a n c e of the discharge, in order to obtain the m a x i m u m s p u t t e r i n g rate and the s p u t t e r i n g voltage was kept to the lowest possible value (350 volts) to minimise heating of the wire. I t was found t h a t for times of s p u t t e r i n g between two and t h i r t y minutes, the gold films showed interference colours. Their appearance indicated t h a t the circular s y m m e t r y was excellent and the longitudinal
272
Vac~¢um Vc~l. 17
October, 1956
Abstract No. and References
Article by R. B. Belser & W. H. ]:[icklin t~ev. Sci. Instrum. 27, May 1956 993-296
66/III
L. A. Holland & W. Edwards & Co. (London) Ltd. Brit. Pat. 736,512
67/II1
Article by G. K. Wehner Phys. Bey. Io2, 1.5.1956 690-704
68/III
Letter by J. F. Straehan & N. L. Harris _Nature x78, 15.9. 1956 588-589
69/tii
II I --
Vacuum
Processing Contd.
Techniques
~
II I
on the adhesion of fihns to glass, q u a r t z and glazed tile s u b s t r a t e s are described. The films produced by sputtering are characterised b y high adherence and resistance to s t r u c t u r a l changes induced by heating. R e f r a c t o r y metals m a y be s p u t t e r e d fairly readily whereas their evaporation is difficult or impossible. Uses of the hard, chemically resistant films produced b y s p u t t e r i n g these metals are discussed.
Improvements i.o.r.t. Cathode Sputtering Apparatus United Kingdom. Details are given of a s p u t t e r i n g p l a n t which is designed to facilitate operation at lower t h a n conventional pressures w i t h o u t affecting the distance of cathode to article. Basically the p l a n t consists of a cylindrical c h a m b e r positioned vertically on a baseplate and closed b y a door at the top end. Baseplate and door form the pole-pieces of an electromagnet. The s p u t t e r i n g cathode is suspended from the door on insulating s u p p o r t s and the baseplate carries an a n n u l a r cathode s u r r o u n d i n g the work holder. I n operation the c h a m b e r is p u m p e d down to a pressure of 0.08 mm. H g b y conventional means. Then the magnetic field is supplied b y energising the m a g n e t and the pressure is f u r t h e r reduced to 3.5 × 10 -~ m m . Hg. I n the presence of the magnetic field (200 gauss) the electrons travelling on a longer p a t h t h a n in the conventional a p p a r a t u s are able to produce more effective ionisation of the residual gas and t h u s enable a cold cathode glow discharge to be sustained at the low pressure mentioned. The anode is connected to the earthed positive terminal of a d.c. s u p p l y providing 1,500 volts, the total c u r r e n t being of the order of 150 m A and the c u r r e n t density 0.25 mA sq.cm.
Controlled Sputtering of Metals by Low-Energy Hg Ions United States. S p u t t e r i n g m e a s u r e m e n t s made using the glow discharges as a source of positive ions arc shown to be unreliable. The w o r k reported here was carried out u n d e r well controlled conditions b y placing the target in a high density p l a s m a of m e r c u r y ions sustained by an arc discharge from a pool-type cathode. Measurements of the n u m b e r of metal a t o m s sputtered under the i m p a c t of one positive ion (yield) were made from p l a t i n u m in the polycrystalline state. S p u t t e r i n g from single crystals of metal showed characteristic p a t t e r n s according to the orientation of the crystal. I t was found t h a t a t o m s were preferentially ejected in directions corresponding to closest packing, i.e. along the (110) and (111) crystal planes in the case of face-centred cubic and body-centred cubic crystals respectively. The possibility of ejected a t o m s being t r a p p e d again on the surface is discussed, and micrographs illustrate the re-growth of silver crystallites on a single crystal of silver subjected to ion b o m b a r d ment. The main p a r t of the work concerns the m e a s u r e m e n t of the m i n i m u m ion energy required for sputtering. The threshold energies of t w e n t y - t h r e e different metals are measured u n d e r m e r c u r y b o m b a r d m e n t , at normal incidence. A single relationship was revealed. At threshold energy it was s h o w n t h a t the p r o d u c t of incident ion energy transferred to target a t o m s and the velocity of sound in the t a r g e t i s p r o p o r t i o n a l to the heat of sublimation of the t a r g e t metal. A s s u m i n g this law, values of threshold energy have been calculated for other gas ions. E x p e r i m e n t s were also carried out at oblique incidence, when it was found t h a t the threshold energies were considerably lower. I t is concluded t h a t at an ion energy less t h a n 300 eV the f u n d a m e n t a l process of s p u t t e r i n g is p r o b a b l y a direct m o m e n t u m transfer. At higher energies, the process is possibly different, and could better be described in t e r m s of evaporation from locally disturbed regions of the target surface.
X - R a y Hazard in Sputtering Apparatus United Kingdom. The a u t h o r s d r a w a t t e n t i o n to the x-ray hazard arising from s p u t t e r i n g experiments with lowpressure gas discharge t u b e s or industrial processes involving ion b o m b a r d m e n t at ca. 10 kV and above. The need for exercising the precautions of testing and shielding, as for magnetrons, gas-filled and v a c u u m valves, is stressed. These conclusions were d r a w n from e x p e r i m e n t s in which targets of materials, including iron, carbon and u r a n i u m , were exposed to the discharge p l a s m a of argon or m e r c u r y v a p o u r discharge tubes. Negative potentials of up to 20 kV were applied to the targets, and t a r g e t c u r r e n t s ranged from 0.1 to 10 mA. Positive ion b o m b a r d m e n t at the higher voltages gave rise to copious secondary electron emission. Acceleration of these electrons across the positive ion s h e a t h s u r r o u n d i n g the t a r g e t and collision with the walls of the discharge tube gave rise to the observed x-rays. Fogging of x - r a y film at 8 kV and above indicated radiation intensities of ca. 10 -s r./sec, six inches from the tube.
The Deposition of Thin Films of Gold on Cylindrical Specimens by Sputtering United Kingdom. The sputtering c h a m b e r consisted of a cylinder of glass t u b i n g on the inside of which a gold foil cathode was held b y washers and bolts. E a c h end of the glass t u b e was closed b y disc-shaped a l u m i n i u m or steel anodes. The ultimate object was to deposit thin films of gold on c a d m i u m single crystals b u t in the preliminary experiments 1 m m . diameter polycrystalline c a d m i u m wire was used. This was s u p p o r t e d along the axis of the discharge t u b e and was insulated from the anodes b y small glass insulators. Thus, the actual discharge took place between the anodes and cathode only and excessive heating of the wire was avoided. Before s p u t t e r ing, the v a c u u m s y s t e m was outgassed at 0.1 ~ for an hour. Sputtering was carried out in dry air a d m i t t e d via a needle valve after passing t h r o u g h a solid carbon dioxide t r a p to remove w a t e r vapour. The pressure was kept as low as possible (10 V) consistent with the m a i n t e n a n c e of the discharge, in order to obtain the m a x i m u m s p u t t e r i n g rate and the s p u t t e r i n g voltage was kept to the lowest possible value (350 volts) to minimise heating of the wire. I t was found t h a t for times of s p u t t e r i n g between two and t h i r t y minutes, the gold films showed interference colours. Their appearance indicated t h a t the circular s y m m e t r y was excellent and the longitudinal
272
Vac~¢um Vc~l. 17
October, 1956