- Email: [email protected]
A refractory metal thin film evaporator for backscattering studies
NUCLEAR
INSTRUMENTS
AND
METHODS
I2 3
(I975) 465-47o;
©
NORTH-HOLLAND
PUBLISHING
CO.
A REFRACTORY METAL THIN FILM EVAPORATOR FOR BACKSCATTERING STUDIES* D. F. R O L L I N a n d J. E. R O B I N S O N
Department of Engineering Physics and Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada Received 30 September 1974 T h e design a n d the thin film characteristics o f a refractory metal thin film evaporator-target c h a m b e r for keV ion backscattering studies are reported. High temperature evaporation is accomplished using a 3 k W electron b e a m at a b a c k g r o u n d pressure o f
1 0 - 9 - 1 0 - 1 0 t o r r . V a n a d i u m films (300-1800/~) are analysed using a variety o f s t a n d a r d characterization techniques. T h e films are f o u n d to be relatively flat with a consistently reproducible polycrystalline B.C.C. microstructure.
1. Introduction
held in a rotatable assembly and can be aligned with the axis of a beam of keV light ions for the scattering studies. The evaporation and target alignment can be done without exposing the films to oxygen or other potential surface impurities. A photograph of the target chamber (upper chamber) and evaporator (lower chamber) is shown in fig. 1. The target and evaporation chambers are interconnected by a 4½ conflat flange. This interconnection also acts a a limiting aperture for the evaporated vapour beam. The system is completely stainless steel with copper gaskets and can be baked to 250 °C. Using two ion pumps (1501/s), the system can reach a base pressure of less than 2 x 10-9 torr. A titanium sublimation pump (2000 l/s) located in the target chamber can supplement this ion pumping to further reduce the pressure to approximately 10-1° torr. Fig. 2 shows a schematic of the electron beam evaporator and its metal vapour shield. The filament F, acting as the cathode, is at a potential of - 4000 V. The filament is directly heated to produce an electron beam of up to 0.75 A. This beam is focussed and deflected 270 ° onto the crucible. The shield shown in fig. 2 is used to prevent voltage breakdown between the high voltage electric wire leads, A, and any part of the chamber at ground potential. Breakdown can be caused by the high pressure evaporant gas (10 -6 torr) or by the release of small flakes of deposited material from the evaporation chamber walls. This thickness of the film is measured using a quartz crystal oscillator situated beside the film substrate. The assembly consists of an oscillator head and a digital monitoring unit. Thickness is computed directly from the frequency shift of the oscillator and a density input. This computed thickness is an average over the crystal
The thin film evaporator described here is part of a larger accelerator-target system used for studying the scattering of light KeV ions from solid surfaces. Recent interest has been shown in the scattering and radiation damaging properties of light keV ions where both nuclear and electronic stopping are important 1-4. This energy range is also of special importance in fusion research where light ion bombardment may cause serious first wall erosion problemsS'6). As is well known, the scattering of light ions by solid surfaces can be complicated by the presence of a surface oxide layer or by other surface impurities7-9). Here we present the design and film characteristics of an evaporator system capable of producing thin impurity free target films (300-2000 A~) in situ under ultra high vacuum conditions (10-1°torr). The production of vanadium films is described. Since crystal structure can also affect low energy light ion scattering, the microstructure of the deposited films is examined in detail3). The design of the evaporator is presented in section 2, while section 3 contains a description of the substrate preparation. Film characterization is presented in section 4 and a summary of the film characteristics given in section 5.
2. Evaporator-target system In order to produce oxide-free high purity reactive metal films, an electron beam evaporator is used to evaporate the target at high temperature and under ultra high vacuum conditions. The evaporated vapour is deposited on carefully prepared glass substrates to form films of specific thicknesses. The glass substrate is
* Research supported by the National Research Council o f C a n a d a .
465
466
D. F. R O L L 1 N A N D J. E. R O B I N S O N
Fig. 1. The e v a p o r a t o r - t a r g e t c h a m b e r system.
area (I cm 2) and contains an error due to the variable microscopic structure of the film.
3. Substrate preparation The main requirement of the substrate is that it does not introduce effects which could interface with the scattering studies. For our backscattering studies, the requirement is that the atomic mass of the film material be much larger than that of the substrate. This requirement is fulfilled with the use of glass slides as the substrate material for the study of relatively heavy refractory metals such as vanadium or niobium. Glass slides have a number of advantages as a substrate material. That is, they have very fiat surfaces ( + 100/~), they do not have strong structural features to prevent film growth, and they can be easily handled during examination of the deposited film. The selected substrate was soda-lime glass in the form of commercially
available microscope slides (75 mm x 25 mm x 1 mm). The chemical composition of such slides is essentially as follows: 72.68% SiO2, 12.95% C20 and 13.17% NaO. The glass slides did not require polishing due to the high quality of their surface finish. However, intensive surface cleaning was required to remove all dust particles and traces of organic compounds. The complete cleaning and substrate selection procedure involves a series of cleaning and rinsing stages, that is: 1) a preliminary cleaning with cotton wool and acetone, 2) a cleaning in a boiling solution of detergent (alkylsulphate) for 20 min, 3) a rinsing in boiling demineralized water for 20 min, 4) a cleaning in boiling ethyl alcohol for 20 min,
REFRACTORY
METAL THIN
5) a cleaning in an ultrasonic agitator for 10 min in hot ethyl alcohol and 6) a cleaning in an ethyl alcohol evaporator. The slides are then allowed to cool in the evaporator. Failure to remove all grease spots or dust particles results in the appearance of holes in the deposited films. 4. Characterization of the target films Although it can be assumed that the deposited films have oxide free surfaces due to the good vacuum conditions, other properties such as the grain size, surface roughness, density and the lattice constant are important. To characterize the bulk properties of the films, the films were removed from the target chamber and analysed using a variety of standard characterization techniques: electron and optical microscopy, electron diffraction, ellipsometry, electrical resistivity, characteristic X-rays, electron microprobing, and multiple beam interferometryl°). It should be noted
FILM EVAPORATOR
467
that the surface features could not be analysed in detail due to the formation of an oxide layer on the films. An oxide layer formed immediately upon the removal of a film from the vacuum system. Scanning electron and optical microscopes were used to study the gross features of the substrate covered with the vanadium film. In order to prevent evaporation, the electron microscope magnification was limited to a maximum of 3000 x . At this magnification, the details of the film structure cannot be seen. However, electrons
C
TOP VIEW
A
Fig. 3a. T r a n s m i s s i o n m i c r o g r a p h for a 335 A free standing v a n a d i u m film, 93000 ×.
r
I;
',I -4
.,2,1 ,, :: ,~-
] L .... J ~ D
D
SIDE VIEW
SIDE VIEW
A' B' C;
ELECTRICAL LEADS ELECTRON-GUN EVAPORATOR COOLING LINES
D:
VAPOUR
E : F"
FILAMENT EVAPORATIONCRUCIBLE
SHIELD
Fig. 2. Schematic o f the electron b e a m evaporator.
Fig. 3b. T r a n s m i s s i o n diffraction pattern for a 335 A free standing v a n a d i u m film.
468
D. F. ROLLIN AND J. E. ROBINSON
accumulate on non-conducting surfaces revealing discontinuities and holes in the films. Despite the precaution taken in the substrate preparation, approximately 0.1% of the deposited film areas consisted of small holes. These holes were of the order of a micron or less in diameter. A transmission microscope was also used to obtain micrographs and diffraction patterns. F o r these cases, the samples had to be prepared using a special technique. That is, the films had to be peeled off of the substrate and placed on a mesh to permit transmission of
the analysing electron beam. In order to accomplish this, a very thin layer of hard wax was first deposited on the glass slides. After deposition of the vanadium, the wax could be dissolved and small pieces of the film could be placed on a wire mesh for analysis. Although the pieces were small, they were larger than the field of view of the transmission microscope. Some typical transmission micrographs are shown in figs. 3a and 4a. For both cases the sizes of the grain are less than 1000 fi,. For the thinner film (335 ,~) the grain sizes range from 200 to 500,~, while for the thicker film (1592 A) they range from 700 to 1000 A. For both cases the grains are interconnected. It is possible that many neighbour grains have a c o m m o n lattice orientation since few sharp boundaries are seen. Diffraction patterns of the areas examined in the micrographs are shown in figs. 3b and 4b. Spotty or giainy rings are normally associated with crystallites which have a diameter a fraction of the electron beam spot size. F r o m this assumption, the grain sizes determined from the micrographs are consistent with the observed diffraction pattern. The lattice parameter can also be calculated from the diffraction patterns. The spacing between planes of atoms in a direction (h, k, l) is given by:
dh.k,t
= a0(h 2 + k 2 +/2)-'~,
(1)
Fig. 4a. Transmission micrograph for a 1592 A free standing vanadium film, 64000 x.
4
2
o~
,'.o DIAMETER
Fig. 4b. Transmission diffraction pattern for a 1592 A free standing vanadium film.
,'.5 OF
i.o
2'.5
RINGS (era)
Fig. 5. Diameters of transmission diffraction pattern rings vs (h2+k2+12)½ for a 1592A free standing vanadium film.
REFRACTORY
where a0 is the lattice constant11). The scaling of the rings due to the configuration of the camera is given by the well known expression }~
dh,k, z
-- Dh, k,l
--, 2L
(2)
where Dh.k,t is the ring diameter and L is the distance from the sample to the photographic plate. The values of ( h 2 + k 2 + I 2 ) k for B.C.C. crystals can be plotted versus the ring diameters or shown in fig. 5. ao can then be determined from the slope using a gold reference with eqs. (1) and (2). The lattice parameter determined with this technique equals that for B.C.C. vanadium. Reflection diffraction patterns for grazing incidence electrons were also obtained for films deposited directly onto the glass substrate. A typical diffraction pattern is shown in fig. 6. From the lattice constant obtained from these measurements, the presence of oxide on the vanadium surface was indicated. Such measurements could not be used to yield information on the bulk properties. The oxide layer observed in the reflection diffraction studies was confirmed by ellipsometry measurementsX2). Subsequent to removal from the vacuum chamber, a 1727 A (quartz crystal thickness) film was analysed by ellipsometric techniques. It was then analysed again after 12 h, and a third time after 113 h. The measurements indicated the presence of a 39 ,~ film 2 h after removal from the evaporator. The film grew to 50 ,~ after the 112 h interval. Electrical resistivity measurements were undertaken to investigate the continuity and bulk properties of the film. The instrument used was a four-point microprobe with a low current source. The resistivity is assumed to be given by p = 4.53x 10_ 8 __AVt, AI
469
METAL THIN FILM EVAPORATOR
where t is the film thickness in angstromsla). Fig. 7 shows measured resistivity values as a function of film thickness. The resistivity approaches that of bulk vanadium for a film thickness greater than 2000 A. Characteristic X-rays were used with rolatively thick films (1500 A) to detect impurities. The samples and a reference substrate were irradiated with X-rays from a tungsten target. Trace amounts of nickel were detected (less than 100 ppm). It could not be determined if this iron was due to contamination from the steel crucible or from the glass substrates. Surface roughness was measured using an electron microprobe normally incident on the film and substrate. Also, the film was etched along five lines and multiple beam interferometry (Fizeau interferometry) was used to measure the film thickness and roughness. For a 1700/~ film, the quartz crystal thickness and the interferometric measurements were in agreement within the limits of experimental accuracy. The roughness for both measurement techniques was approximately 100 A root-mean-square. 5. Summary and conclusions An ultra-high vacuum integrated evaporator-target chamber has been constructed for backscattering studies. Vanadium films were analysed using a variety of standard characterization techniques. Polycrystalline B.C.C. vanadium films were produced at ambient
12 xlO"'~
IO
(3) ¢,1 I
>I'--
>
I-03 hi IZ
BULK
0
I
I
400
Fig. 6. Reflection diffraction pattern for grazing incidence on a 1725/~ v a n a d i u m film.
800 FILM
I
1200
RESISTIVITY
I
1600
I
2000
THICKNESS ( ~ )
Fig. 7. Electrical resistivity vs v a n a d i u m film thickness.
470
D. F. ROLLIN AND J. E. ROBINSON
t e m p e r a t u r e . T h e films were relatively flat a n d h a d a c o n s i s t e n t l y r e p r o d u c i b l e m i c r o s t r u c t u r e . G r a i n sizes were a p p r o x i m a t e l y 300-1000 ~ d e p e n d i n g o n the film thickness. T h e films are sufficiently flat a n d r e p r o d u c i b l e for b a c k s c a t t e r i n g studies. T h e a u t h o r s wish to t h a n k D r J. P. M a r t o n ( W e l w y n C a n a d a Ltd.) for p e r f o r m i n g t h e e l l i p s o m e t r i c m e a s u r e m e n t s a n d M i k e M a r c h a n d ( M c M a s t e r U n i v e r s i t y ) for the c h a r a c t e r i s t i c X - r a y tests. T h e a u t h o r s also a c k n o w l e d g e A. Singh, F. S m i t h a n d L. G o o d r i d g e for their t e c h n i c a l s u p p o r t . T h e a u t h o r s also t h a n k D r D. A. T h o m p s o n ( M c M a s t e r U n i v e r s i t y ) for r e v i e w i n g the m a n u s c r i p t .
References 1) G. M. McCracken and N. J. Freeman, J. Phys. B2 (1969) 661.
2) R. Behrisch and R. Weissmann, Phys. Letters 30A (1969) 506. 3) E. S. Mashkova and V. A. Molchanov, Rad. Effects 16 (1972) 143. 4) j. Bottiger and K. B. Winterbon, Rad. Effects 20 (1973) 65. .5) M. Kaminsky and S. K. Das, Appl. Phys. Letters 21 (1972) 443. ~;) R. Behrisch, Nucl. Fusion 12 (1972) 6. 7) j. A. Phillips, Phys. Rev. 97 (1955) 404. s) K. Morita, H. Akimune and T. Suita, J. J. Phys. 7 (1968) 916. .9) T. M. Buck, L. C. Feldman and G. H. Wheatley, Atomic collisons in solids (Ed. S. Datz; Plenum Press, New York, 1974). lo) j. L. McCall and W. M. Mueller, Microstructure analysis (Plenum Press, New York, 1973). 11) C. S. Barrett and T. B. Massalski, Structure of metals (McGraw-Hill, New York, 1966). 12) j. A. Allen, C. C. Evan and J. W. Mitchell, Structure and properties of thin films (eds. Neugebauer, Newkirk and Vermibyea; J. Wiley, New York, 1959). 13) S. M. Sze, Physics of semiconductor devices (J. Wiley, New York, 1969).
INSTRUMENTS
AND
METHODS
I2 3
(I975) 465-47o;
©
NORTH-HOLLAND
PUBLISHING
CO.
A REFRACTORY METAL THIN FILM EVAPORATOR FOR BACKSCATTERING STUDIES* D. F. R O L L I N a n d J. E. R O B I N S O N
Department of Engineering Physics and Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada Received 30 September 1974 T h e design a n d the thin film characteristics o f a refractory metal thin film evaporator-target c h a m b e r for keV ion backscattering studies are reported. High temperature evaporation is accomplished using a 3 k W electron b e a m at a b a c k g r o u n d pressure o f
1 0 - 9 - 1 0 - 1 0 t o r r . V a n a d i u m films (300-1800/~) are analysed using a variety o f s t a n d a r d characterization techniques. T h e films are f o u n d to be relatively flat with a consistently reproducible polycrystalline B.C.C. microstructure.
1. Introduction
held in a rotatable assembly and can be aligned with the axis of a beam of keV light ions for the scattering studies. The evaporation and target alignment can be done without exposing the films to oxygen or other potential surface impurities. A photograph of the target chamber (upper chamber) and evaporator (lower chamber) is shown in fig. 1. The target and evaporation chambers are interconnected by a 4½ conflat flange. This interconnection also acts a a limiting aperture for the evaporated vapour beam. The system is completely stainless steel with copper gaskets and can be baked to 250 °C. Using two ion pumps (1501/s), the system can reach a base pressure of less than 2 x 10-9 torr. A titanium sublimation pump (2000 l/s) located in the target chamber can supplement this ion pumping to further reduce the pressure to approximately 10-1° torr. Fig. 2 shows a schematic of the electron beam evaporator and its metal vapour shield. The filament F, acting as the cathode, is at a potential of - 4000 V. The filament is directly heated to produce an electron beam of up to 0.75 A. This beam is focussed and deflected 270 ° onto the crucible. The shield shown in fig. 2 is used to prevent voltage breakdown between the high voltage electric wire leads, A, and any part of the chamber at ground potential. Breakdown can be caused by the high pressure evaporant gas (10 -6 torr) or by the release of small flakes of deposited material from the evaporation chamber walls. This thickness of the film is measured using a quartz crystal oscillator situated beside the film substrate. The assembly consists of an oscillator head and a digital monitoring unit. Thickness is computed directly from the frequency shift of the oscillator and a density input. This computed thickness is an average over the crystal
The thin film evaporator described here is part of a larger accelerator-target system used for studying the scattering of light KeV ions from solid surfaces. Recent interest has been shown in the scattering and radiation damaging properties of light keV ions where both nuclear and electronic stopping are important 1-4. This energy range is also of special importance in fusion research where light ion bombardment may cause serious first wall erosion problemsS'6). As is well known, the scattering of light ions by solid surfaces can be complicated by the presence of a surface oxide layer or by other surface impurities7-9). Here we present the design and film characteristics of an evaporator system capable of producing thin impurity free target films (300-2000 A~) in situ under ultra high vacuum conditions (10-1°torr). The production of vanadium films is described. Since crystal structure can also affect low energy light ion scattering, the microstructure of the deposited films is examined in detail3). The design of the evaporator is presented in section 2, while section 3 contains a description of the substrate preparation. Film characterization is presented in section 4 and a summary of the film characteristics given in section 5.
2. Evaporator-target system In order to produce oxide-free high purity reactive metal films, an electron beam evaporator is used to evaporate the target at high temperature and under ultra high vacuum conditions. The evaporated vapour is deposited on carefully prepared glass substrates to form films of specific thicknesses. The glass substrate is
* Research supported by the National Research Council o f C a n a d a .
465
466
D. F. R O L L 1 N A N D J. E. R O B I N S O N
Fig. 1. The e v a p o r a t o r - t a r g e t c h a m b e r system.
area (I cm 2) and contains an error due to the variable microscopic structure of the film.
3. Substrate preparation The main requirement of the substrate is that it does not introduce effects which could interface with the scattering studies. For our backscattering studies, the requirement is that the atomic mass of the film material be much larger than that of the substrate. This requirement is fulfilled with the use of glass slides as the substrate material for the study of relatively heavy refractory metals such as vanadium or niobium. Glass slides have a number of advantages as a substrate material. That is, they have very fiat surfaces ( + 100/~), they do not have strong structural features to prevent film growth, and they can be easily handled during examination of the deposited film. The selected substrate was soda-lime glass in the form of commercially
available microscope slides (75 mm x 25 mm x 1 mm). The chemical composition of such slides is essentially as follows: 72.68% SiO2, 12.95% C20 and 13.17% NaO. The glass slides did not require polishing due to the high quality of their surface finish. However, intensive surface cleaning was required to remove all dust particles and traces of organic compounds. The complete cleaning and substrate selection procedure involves a series of cleaning and rinsing stages, that is: 1) a preliminary cleaning with cotton wool and acetone, 2) a cleaning in a boiling solution of detergent (alkylsulphate) for 20 min, 3) a rinsing in boiling demineralized water for 20 min, 4) a cleaning in boiling ethyl alcohol for 20 min,
REFRACTORY
METAL THIN
5) a cleaning in an ultrasonic agitator for 10 min in hot ethyl alcohol and 6) a cleaning in an ethyl alcohol evaporator. The slides are then allowed to cool in the evaporator. Failure to remove all grease spots or dust particles results in the appearance of holes in the deposited films. 4. Characterization of the target films Although it can be assumed that the deposited films have oxide free surfaces due to the good vacuum conditions, other properties such as the grain size, surface roughness, density and the lattice constant are important. To characterize the bulk properties of the films, the films were removed from the target chamber and analysed using a variety of standard characterization techniques: electron and optical microscopy, electron diffraction, ellipsometry, electrical resistivity, characteristic X-rays, electron microprobing, and multiple beam interferometryl°). It should be noted
FILM EVAPORATOR
467
that the surface features could not be analysed in detail due to the formation of an oxide layer on the films. An oxide layer formed immediately upon the removal of a film from the vacuum system. Scanning electron and optical microscopes were used to study the gross features of the substrate covered with the vanadium film. In order to prevent evaporation, the electron microscope magnification was limited to a maximum of 3000 x . At this magnification, the details of the film structure cannot be seen. However, electrons
C
TOP VIEW
A
Fig. 3a. T r a n s m i s s i o n m i c r o g r a p h for a 335 A free standing v a n a d i u m film, 93000 ×.
r
I;
',I -4
.,2,1 ,, :: ,~-
] L .... J ~ D
D
SIDE VIEW
SIDE VIEW
A' B' C;
ELECTRICAL LEADS ELECTRON-GUN EVAPORATOR COOLING LINES
D:
VAPOUR
E : F"
FILAMENT EVAPORATIONCRUCIBLE
SHIELD
Fig. 2. Schematic o f the electron b e a m evaporator.
Fig. 3b. T r a n s m i s s i o n diffraction pattern for a 335 A free standing v a n a d i u m film.
468
D. F. ROLLIN AND J. E. ROBINSON
accumulate on non-conducting surfaces revealing discontinuities and holes in the films. Despite the precaution taken in the substrate preparation, approximately 0.1% of the deposited film areas consisted of small holes. These holes were of the order of a micron or less in diameter. A transmission microscope was also used to obtain micrographs and diffraction patterns. F o r these cases, the samples had to be prepared using a special technique. That is, the films had to be peeled off of the substrate and placed on a mesh to permit transmission of
the analysing electron beam. In order to accomplish this, a very thin layer of hard wax was first deposited on the glass slides. After deposition of the vanadium, the wax could be dissolved and small pieces of the film could be placed on a wire mesh for analysis. Although the pieces were small, they were larger than the field of view of the transmission microscope. Some typical transmission micrographs are shown in figs. 3a and 4a. For both cases the sizes of the grain are less than 1000 fi,. For the thinner film (335 ,~) the grain sizes range from 200 to 500,~, while for the thicker film (1592 A) they range from 700 to 1000 A. For both cases the grains are interconnected. It is possible that many neighbour grains have a c o m m o n lattice orientation since few sharp boundaries are seen. Diffraction patterns of the areas examined in the micrographs are shown in figs. 3b and 4b. Spotty or giainy rings are normally associated with crystallites which have a diameter a fraction of the electron beam spot size. F r o m this assumption, the grain sizes determined from the micrographs are consistent with the observed diffraction pattern. The lattice parameter can also be calculated from the diffraction patterns. The spacing between planes of atoms in a direction (h, k, l) is given by:
dh.k,t
= a0(h 2 + k 2 +/2)-'~,
(1)
Fig. 4a. Transmission micrograph for a 1592 A free standing vanadium film, 64000 x.
4
2
o~
,'.o DIAMETER
Fig. 4b. Transmission diffraction pattern for a 1592 A free standing vanadium film.
,'.5 OF
i.o
2'.5
RINGS (era)
Fig. 5. Diameters of transmission diffraction pattern rings vs (h2+k2+12)½ for a 1592A free standing vanadium film.
REFRACTORY
where a0 is the lattice constant11). The scaling of the rings due to the configuration of the camera is given by the well known expression }~
dh,k, z
-- Dh, k,l
--, 2L
(2)
where Dh.k,t is the ring diameter and L is the distance from the sample to the photographic plate. The values of ( h 2 + k 2 + I 2 ) k for B.C.C. crystals can be plotted versus the ring diameters or shown in fig. 5. ao can then be determined from the slope using a gold reference with eqs. (1) and (2). The lattice parameter determined with this technique equals that for B.C.C. vanadium. Reflection diffraction patterns for grazing incidence electrons were also obtained for films deposited directly onto the glass substrate. A typical diffraction pattern is shown in fig. 6. From the lattice constant obtained from these measurements, the presence of oxide on the vanadium surface was indicated. Such measurements could not be used to yield information on the bulk properties. The oxide layer observed in the reflection diffraction studies was confirmed by ellipsometry measurementsX2). Subsequent to removal from the vacuum chamber, a 1727 A (quartz crystal thickness) film was analysed by ellipsometric techniques. It was then analysed again after 12 h, and a third time after 113 h. The measurements indicated the presence of a 39 ,~ film 2 h after removal from the evaporator. The film grew to 50 ,~ after the 112 h interval. Electrical resistivity measurements were undertaken to investigate the continuity and bulk properties of the film. The instrument used was a four-point microprobe with a low current source. The resistivity is assumed to be given by p = 4.53x 10_ 8 __AVt, AI
469
METAL THIN FILM EVAPORATOR
where t is the film thickness in angstromsla). Fig. 7 shows measured resistivity values as a function of film thickness. The resistivity approaches that of bulk vanadium for a film thickness greater than 2000 A. Characteristic X-rays were used with rolatively thick films (1500 A) to detect impurities. The samples and a reference substrate were irradiated with X-rays from a tungsten target. Trace amounts of nickel were detected (less than 100 ppm). It could not be determined if this iron was due to contamination from the steel crucible or from the glass substrates. Surface roughness was measured using an electron microprobe normally incident on the film and substrate. Also, the film was etched along five lines and multiple beam interferometry (Fizeau interferometry) was used to measure the film thickness and roughness. For a 1700/~ film, the quartz crystal thickness and the interferometric measurements were in agreement within the limits of experimental accuracy. The roughness for both measurement techniques was approximately 100 A root-mean-square. 5. Summary and conclusions An ultra-high vacuum integrated evaporator-target chamber has been constructed for backscattering studies. Vanadium films were analysed using a variety of standard characterization techniques. Polycrystalline B.C.C. vanadium films were produced at ambient
12 xlO"'~
IO
(3) ¢,1 I
>I'--
>
I-03 hi IZ
BULK
0
I
I
400
Fig. 6. Reflection diffraction pattern for grazing incidence on a 1725/~ v a n a d i u m film.
800 FILM
I
1200
RESISTIVITY
I
1600
I
2000
THICKNESS ( ~ )
Fig. 7. Electrical resistivity vs v a n a d i u m film thickness.
470
D. F. ROLLIN AND J. E. ROBINSON
t e m p e r a t u r e . T h e films were relatively flat a n d h a d a c o n s i s t e n t l y r e p r o d u c i b l e m i c r o s t r u c t u r e . G r a i n sizes were a p p r o x i m a t e l y 300-1000 ~ d e p e n d i n g o n the film thickness. T h e films are sufficiently flat a n d r e p r o d u c i b l e for b a c k s c a t t e r i n g studies. T h e a u t h o r s wish to t h a n k D r J. P. M a r t o n ( W e l w y n C a n a d a Ltd.) for p e r f o r m i n g t h e e l l i p s o m e t r i c m e a s u r e m e n t s a n d M i k e M a r c h a n d ( M c M a s t e r U n i v e r s i t y ) for the c h a r a c t e r i s t i c X - r a y tests. T h e a u t h o r s also a c k n o w l e d g e A. Singh, F. S m i t h a n d L. G o o d r i d g e for their t e c h n i c a l s u p p o r t . T h e a u t h o r s also t h a n k D r D. A. T h o m p s o n ( M c M a s t e r U n i v e r s i t y ) for r e v i e w i n g the m a n u s c r i p t .
References 1) G. M. McCracken and N. J. Freeman, J. Phys. B2 (1969) 661.
2) R. Behrisch and R. Weissmann, Phys. Letters 30A (1969) 506. 3) E. S. Mashkova and V. A. Molchanov, Rad. Effects 16 (1972) 143. 4) j. Bottiger and K. B. Winterbon, Rad. Effects 20 (1973) 65. .5) M. Kaminsky and S. K. Das, Appl. Phys. Letters 21 (1972) 443. ~;) R. Behrisch, Nucl. Fusion 12 (1972) 6. 7) j. A. Phillips, Phys. Rev. 97 (1955) 404. s) K. Morita, H. Akimune and T. Suita, J. J. Phys. 7 (1968) 916. .9) T. M. Buck, L. C. Feldman and G. H. Wheatley, Atomic collisons in solids (Ed. S. Datz; Plenum Press, New York, 1974). lo) j. L. McCall and W. M. Mueller, Microstructure analysis (Plenum Press, New York, 1973). 11) C. S. Barrett and T. B. Massalski, Structure of metals (McGraw-Hill, New York, 1966). 12) j. A. Allen, C. C. Evan and J. W. Mitchell, Structure and properties of thin films (eds. Neugebauer, Newkirk and Vermibyea; J. Wiley, New York, 1959). 13) S. M. Sze, Physics of semiconductor devices (J. Wiley, New York, 1969).