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The mechanism of reactive sputtering
Vacuum Congress
A four element sputtering system E (" M u l y and A J Aronson, Equipment Division, National Research Corporathm, Sub.s'idiarv o f Norton Company, Newton, Mass, U S A S u b s t a n t i a l i m p r o v e m e n t s Call be o b t a i n e d ill the stability and o p e r a t i n g characteristics, particularly at low pressure, of the c o n v e n t i o n a l three element electron assisted discharge by the a d d i t i o n of a fourth electrode. Typical volt-ampere characteristics o f a three element system are presented a n d c o m p a r e d with the e n h a n c e d characteristics of a four element system. The a d v a n t a g e s gained ill sputtering applications is b r o u g h t a b o u t by the increase of an o r d e r of m a g n i t u d e in pressure o p e r a t i n g range a n d the ability to m a i n t a i n the low pressure a n o d e voltage below the sputtering threshold, even at high target voltages. I m p r o v e m e n t s gained in the striking characteristics of the plasma p e r m i t the e l i m i n a t i o n of special igniting elements. T h e increased stability is s h o w n to occur in b o t h the high and m e d i u m v a c u u m ranges of operation.
Tetrode sputtering system and its application for the deposition of tantalum, silica, and alumina thin films T Asamaki, Z Oda, H M a l l , T Mizonobe, Nippon Electric Co, Kawasaki, Japan T h e basic c o n s t r u c t i o n , discharge characteristics a n d the sputtering rate of a " t e t r o d e sputtering s y s t e m " are described in detail. Also, experimental results are r e p o r t e d on the electrical a n d physicochemical properties of t a n t a l u m , silica, a n d a l u m i n a films deposited in this system. T h e electrodes of this sputtering system consist o f a hot filament cathode, a n anode, a target to be sputtered a n d a keep-allying electrode, which enables the discharge starting even below 1 ~ l0 ~ t o r r of a r g o n pressure. By use of a magnetic field which confines the p l a s m a in the space between the target a n d the o p p o s i n g substrate, the sputtering rate can be increased with low energy ions even at low pressures below I :~ 10 :' torr. T a n t a l u m films are sputtered o n glass substrates as functions of a r g o n pressure PA a n d target voltage V T. A b o v e PA : I - 10 :' torr a n d VT=: 600 V, the films show a high specific resistivity (250 / ~ - c m ) a n d a low t e m p e r a t u r e coefficient o f resistance T C R (-100 p p m / C ) : these results are consistent with X-ray diffraction analyses. Silica films a n d a l u m i n a films are deposited by rf sputtering of a fused q u a r t z plate a n d a ceramic a l u m i n a plate, respectively. [nfrared a b s o r p t i o n spectra a n d dielectric c o n s t a n t are comp a r e d with those of the bulk, a n d also the frequency dependence of silica film c a p a c i t a n c e is measured.
Capacitance and impedance of the glow discharge in RF sputtering Robert T C TS'lli, I B M Watson Research ( i , nter, Heights, Neu, York, U S A
Yorktown
F r o m the theoretical calculation of ion energy d i s t r i b u t i o n , ' it is possible to o b t a i n at least a p p r o x i m a t e l y , values o f the c a p a c i t a n c e a n d i m p e d a n c e of the a b n o r m a l glow discharge in typical radio-frequency sputtering conditions. The a p p r o x i m a tion arises mainly because of averaged, instead of distributed ion a n d electron velocities are used in the capacitance calcula-
150
lion. T h e positive space charge density in thc c a t h o d e fall region is d e t e r m i n e d from tile experimental values of the plasma density a n d t e m p e r a t u r e in the rf discharge by L a n g m u i r p r o b e measurements. The load ilnpedance of the discharge, which can be deduced fl'om the capacitance calculation are c o m p a r e d with experimentally measured wihles.
References 1 R T (" Tsui, ,4PS (,~l.veou~ l:'/~'¢tloni~ (o#tl~,leli~,', ,Sd#l l-r~l#tll~lo, (¥diflornia, October 1967, (to he publMhed).
The mechanism of reactive sputtering E J Hollands aml D S ('ampbell, Alh'n ('lark Rexearch (k'ntrc, The Plessey Company Limited, (aswell, Towcc.rter, Norlhaltl.~, England In the f o r m a t i o n of oxide films by reactive sputtering it is possible to envisage the oxidation occurring at three possible sites- at the target, in flight to the substrate and at the substrate. The work reported here was designed to resolve tile controvers} concellfing the d o m i n a n t reaction site by studying the sputtering yield, film g r o w t h rate and oxygen c o n t e n t as functions of the partial oxygen pressure and substrate temperature. The p r o b l e m was s o m e w h a t simplified by sputtering at 10 4 tort, after a residual pressure of 2.10 ~ torr had been attained, so that the possibility of oxidation in flight was reduced to a negligible level. U n d e r the experimental c o n d i t i o n s used for this work it has been found that the location of the site of reaction is strongly d e p e n d e n t on tile oxygen partial pressure. Up to all oxygen pressure of 6.0 • l0 :' torr (the balance was a r g o n ) oxidation occurred at the substrate a n d the rate of lihn growth increased with increase in oxygen pressure. As the oxygen presstire was raised from 6 . 0 . 10 ' torr to the total working pressnre, tile growth rate of the sputtered tilm fell sharply. This is attributed to the shift o f the d o m i n a n t oxidation site from the substrate to the target surface. U p to a s u b s t r a t e t e m p e r a t u r e of 350 ( tile rate of f]tm growth a p p e a r s to be i n d e p e n d e n t of t e m p e r a t u r e at any given oxygen pressure. Variations in film thickness witil t e m p e r a t u r e which have been observed have been shown to be due to density variations. The oxygen content of the lilms exhibited a substrate temperature dependence. At all oxygen partial prcsstlre of 4.0 I() '~ torr it was f o u n d that an increase in tile substrate t e m p e r a t u r e from 50 to 350 C resulted in a decrease of resistivity of a b o u t seven orders of magnitude.
A comparative study of open-ended and closed-end continuous sputtering systems 4 M Hanl)nann and co-authors, Western Electric ( o , Ira, 555 Union Boulevard, ,411entown, Pa, US, I In the closed-end design of a c o n t i n u o u s sputtering system, tile substrates enter a n d leave the v a c u u m c h a n l b e r (or c h a m b e r s ) t h r o u g h loading and u n l o a d i n g valves or locks (closures). In the open-ended design, there are no loading or u n l o a d i n g valves or locks, but a series of individually exhausted c h a m b e r s is necessary to achieve a transition from a t m o s p h e r i c pressure to vacuum. The open-ended raachine has Eeen a mass prociuclion tool
A four element sputtering system E (" M u l y and A J Aronson, Equipment Division, National Research Corporathm, Sub.s'idiarv o f Norton Company, Newton, Mass, U S A S u b s t a n t i a l i m p r o v e m e n t s Call be o b t a i n e d ill the stability and o p e r a t i n g characteristics, particularly at low pressure, of the c o n v e n t i o n a l three element electron assisted discharge by the a d d i t i o n of a fourth electrode. Typical volt-ampere characteristics o f a three element system are presented a n d c o m p a r e d with the e n h a n c e d characteristics of a four element system. The a d v a n t a g e s gained ill sputtering applications is b r o u g h t a b o u t by the increase of an o r d e r of m a g n i t u d e in pressure o p e r a t i n g range a n d the ability to m a i n t a i n the low pressure a n o d e voltage below the sputtering threshold, even at high target voltages. I m p r o v e m e n t s gained in the striking characteristics of the plasma p e r m i t the e l i m i n a t i o n of special igniting elements. T h e increased stability is s h o w n to occur in b o t h the high and m e d i u m v a c u u m ranges of operation.
Tetrode sputtering system and its application for the deposition of tantalum, silica, and alumina thin films T Asamaki, Z Oda, H M a l l , T Mizonobe, Nippon Electric Co, Kawasaki, Japan T h e basic c o n s t r u c t i o n , discharge characteristics a n d the sputtering rate of a " t e t r o d e sputtering s y s t e m " are described in detail. Also, experimental results are r e p o r t e d on the electrical a n d physicochemical properties of t a n t a l u m , silica, a n d a l u m i n a films deposited in this system. T h e electrodes of this sputtering system consist o f a hot filament cathode, a n anode, a target to be sputtered a n d a keep-allying electrode, which enables the discharge starting even below 1 ~ l0 ~ t o r r of a r g o n pressure. By use of a magnetic field which confines the p l a s m a in the space between the target a n d the o p p o s i n g substrate, the sputtering rate can be increased with low energy ions even at low pressures below I :~ 10 :' torr. T a n t a l u m films are sputtered o n glass substrates as functions of a r g o n pressure PA a n d target voltage V T. A b o v e PA : I - 10 :' torr a n d VT=: 600 V, the films show a high specific resistivity (250 / ~ - c m ) a n d a low t e m p e r a t u r e coefficient o f resistance T C R (-100 p p m / C ) : these results are consistent with X-ray diffraction analyses. Silica films a n d a l u m i n a films are deposited by rf sputtering of a fused q u a r t z plate a n d a ceramic a l u m i n a plate, respectively. [nfrared a b s o r p t i o n spectra a n d dielectric c o n s t a n t are comp a r e d with those of the bulk, a n d also the frequency dependence of silica film c a p a c i t a n c e is measured.
Capacitance and impedance of the glow discharge in RF sputtering Robert T C TS'lli, I B M Watson Research ( i , nter, Heights, Neu, York, U S A
Yorktown
F r o m the theoretical calculation of ion energy d i s t r i b u t i o n , ' it is possible to o b t a i n at least a p p r o x i m a t e l y , values o f the c a p a c i t a n c e a n d i m p e d a n c e of the a b n o r m a l glow discharge in typical radio-frequency sputtering conditions. The a p p r o x i m a tion arises mainly because of averaged, instead of distributed ion a n d electron velocities are used in the capacitance calcula-
150
lion. T h e positive space charge density in thc c a t h o d e fall region is d e t e r m i n e d from tile experimental values of the plasma density a n d t e m p e r a t u r e in the rf discharge by L a n g m u i r p r o b e measurements. The load ilnpedance of the discharge, which can be deduced fl'om the capacitance calculation are c o m p a r e d with experimentally measured wihles.
References 1 R T (" Tsui, ,4PS (,~l.veou~ l:'/~'¢tloni~ (o#tl~,leli~,', ,Sd#l l-r~l#tll~lo, (¥diflornia, October 1967, (to he publMhed).
The mechanism of reactive sputtering E J Hollands aml D S ('ampbell, Alh'n ('lark Rexearch (k'ntrc, The Plessey Company Limited, (aswell, Towcc.rter, Norlhaltl.~, England In the f o r m a t i o n of oxide films by reactive sputtering it is possible to envisage the oxidation occurring at three possible sites- at the target, in flight to the substrate and at the substrate. The work reported here was designed to resolve tile controvers} concellfing the d o m i n a n t reaction site by studying the sputtering yield, film g r o w t h rate and oxygen c o n t e n t as functions of the partial oxygen pressure and substrate temperature. The p r o b l e m was s o m e w h a t simplified by sputtering at 10 4 tort, after a residual pressure of 2.10 ~ torr had been attained, so that the possibility of oxidation in flight was reduced to a negligible level. U n d e r the experimental c o n d i t i o n s used for this work it has been found that the location of the site of reaction is strongly d e p e n d e n t on tile oxygen partial pressure. Up to all oxygen pressure of 6.0 • l0 :' torr (the balance was a r g o n ) oxidation occurred at the substrate a n d the rate of lihn growth increased with increase in oxygen pressure. As the oxygen presstire was raised from 6 . 0 . 10 ' torr to the total working pressnre, tile growth rate of the sputtered tilm fell sharply. This is attributed to the shift o f the d o m i n a n t oxidation site from the substrate to the target surface. U p to a s u b s t r a t e t e m p e r a t u r e of 350 ( tile rate of f]tm growth a p p e a r s to be i n d e p e n d e n t of t e m p e r a t u r e at any given oxygen pressure. Variations in film thickness witil t e m p e r a t u r e which have been observed have been shown to be due to density variations. The oxygen content of the lilms exhibited a substrate temperature dependence. At all oxygen partial prcsstlre of 4.0 I() '~ torr it was f o u n d that an increase in tile substrate t e m p e r a t u r e from 50 to 350 C resulted in a decrease of resistivity of a b o u t seven orders of magnitude.
A comparative study of open-ended and closed-end continuous sputtering systems 4 M Hanl)nann and co-authors, Western Electric ( o , Ira, 555 Union Boulevard, ,411entown, Pa, US, I In the closed-end design of a c o n t i n u o u s sputtering system, tile substrates enter a n d leave the v a c u u m c h a n l b e r (or c h a m b e r s ) t h r o u g h loading and u n l o a d i n g valves or locks (closures). In the open-ended design, there are no loading or u n l o a d i n g valves or locks, but a series of individually exhausted c h a m b e r s is necessary to achieve a transition from a t m o s p h e r i c pressure to vacuum. The open-ended raachine has Eeen a mass prociuclion tool