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The preparation of tantalum nitride targets by reactive sputtering
NUCLEAR
INSTRUMENTS
AND
METHODS
233-235; © N O R T H - H O L L A N D P U B L I S H I N G
IO 7 (t973)
CO.
T H E P R E P A R A T I O N OF T A N T A L U M N I T R I D E T A R G E T S BY REACTIVE S P U T T E R I N G M. R. WORMALD*, B. Y. U N D E R W O O D and K. W. ALLEN
Nuclear Physics Laboratory, Oxford, England Received 13 October 1972
A method of making thin nitride films for nuclear targets by reactive sputtering is discussed. The process is found to be simple
and reliable and is capable of producing high purity films ort a widerange of backings.
1. Introduction
chemically with the gas so that the nitride of the cathode metal is deposited on the anode. The choice of cathode material thus determines the metal nitride formed. Possible metals are titanium, vanadium, tantalum, hafnium, and tungsten, all of which form very stable high melting point nitrides and can readily be sputtered. Tantalum was chosen for this work as it has a high atomic number and thus gives only relatively weak, low energy gamma-rays (from Coulomb excitation) when bombarded with alpha particles up to E~ = 8 MeV. Read 3) has shown that the use of pure nitrogen rather than the more usual mixture of 90% argon, 10% nitrogen gives a higher nitride resembling TaN2 rather than TaN. The use of the higher nitride is clearly desirable for maximum reaction yield. Since the process is basically a ' c o l d ' one, the
The reactive sputtering technique for the deposition of thin layers of metallic compounds is well known and frequently employed in industrial processes, but seems to have been little used in the preparation of targets for nuclear physics experiments. In the course of a study in this laboratory of the reaction lSN(~, )')19F, we had to produce nitride targets of high purity on suitable backings capable of withstanding large ( ~ 25 #A) currents of 6 MeV a-particles. Initially we used the well known technique of heating an evaporated layer of a suitable metal (e.g. titanium) on a tantalum backing in a ~SN atmosphere. However, at the bombarding energies used in our experiments, convenient metals such as titanium and zirconium gave an undesirable nuclear reaction yield and with tantalum backings it was difficult to avoid light element contamination, especially carbon, which gave a large yield of high energy ),-rays from the reaction 13C(~, n)160.
We therefore turned to the preparation of nitride layers on gold backings by reactive sputtering, and this paper describes the technique we used to prepare tantalum nitride films. The method is very convenient and simple, and permits excellent control of the thickness of the deposited film. We have also made oxide targets in this laboratory by the sputtering method.
L iclu~iid N2 cooled s c r ~ n
High purity Ta disc Shutter
Su bstrat~
2. Reactive sputtering
,
!
The reactive sputtering process has been described elsewhere 1) and sputtered nitride films have been studied in detail in relation to their chemical composition, crystal structure, and resistive properties2). The process involves striking a discharge in nitrogen gas between a pair of parallel electrodes by applying a dc potential. Bombardment of the cathode by nitrogen ions knocks out atoms of the electrode, which combine * Present address: Department of Physics, University of Guelph, Guelph, Ontario, Canada.
!
ono~
I
0
I
1
I
2
I
3 INCHES
Fig. 1. The electrode assembly was contained in a stainless steel chamber, the pressure in which was less than 10-6 torr prior to admitting N2 gas. The shutter has a radiation shield to protect the substrate. The substrate can be out-gassed at 900°C to reduce its carbon content. 233
234
M.R.
WORMALD
possibility of cracking hydrocarbons on to the target is minimized although the presence of methane or carbon dioxide can lead to the deposition of tantalum carbide.
3 kV. The area of the cathode disc was 45 cm z. The thickness was determined during sputtering by observing the colour of the film on the gold backing. The colour changes from natural gold to a deep gold at 16 ~ 20/~g/cm 2, then to red, purple and finally silver at 40/~g/cm 2 or more. Exact thicknesses were found by weighing and from the observed widths of known alpha-capture resonances• The components of the vacuum system and the sputtering module itself were fabricated from stainless steel. A high speed mercury diffusion pump (Edwards, type EM2), followed by a booster (Edwards, type 2M4) backed by sorption pumps, was used to evacuate the system. To prevent oil vapour contamination, no rotary p u m p of any sort was ever used. A quadrupole mass spectrometer was used to analyse the 15N z sputtering gas and to search for contaminants such as water-vapour and hydro-carbons. It was shown that the purity of the 15N 2 gas could be maintained during the sputtering process.
3. Experimental The apparatus used is shown in fig. 1. The discharge is restricted to the region between the electrodes by screening the cathode with a third earthed electrode in the usual manner. No discharge and therefore no sputtering occurs in the narrow gap between the screen and the cathode as electrons traversing the gap make too few collisions to maintain a discharge. It is essential that the nitrogen gas in the system is kept free from water vapour and hydro-carbons which would otherwise form oxides and carbides4). The ion bombardment of the walls of the sputtering vessel can result in outgassing which contaminates the nitrogen gas. To avoid this contamination, a shutter system was introduced to cover the substrate during a preliminary sputter, thus enabling the system to be cleaned up without depositing an impure film on the target. The contaminated gas was then pumped away, fresh nitrogen was admitted, and the shutter removed prior to sputtering. After the initial clean-up, the cathode screen was cooled by liquid nitrogen to keep the system free of water vapour and hydro-carbons. Heat generated in the anode plate by electron bombardment was removed by water cooling. Typically, the time taken to deposit a 40 ~tg/cm2 film of TaN2 is thirty minutes with a gas pressure of 50 pm and with a current of 30 mA at TiN
8o
40
O
I
O
Targets made in the manner described having thicknesses varying from 20--, 80 #g/cm 2 have proved to be superior to targets made in the conventional manner in several ways. Fig. 2 shows a comparison of ~,-ray spectra produced by a-particle bombardment of a sputtered T a N 2 film on gold and a TiN film on tantalum prepared by evaporation and heating. These spectra have been normalized to the yield at a strong
TANTALUM
AT
Ed.-
5.30 MeV
~
1000
2000
14N(~,p6 )17O
10OO
3000 ToN2 ON
8708 k~V
o
4. Discussion
Xl
H-
Z 40
ON
e t al.
GOLD
4000 AT E ~ -
539
1
,
2000
5000
6000
MeV BC(~,n ~ )u60 6129.4keV
,
30OOENERGY ( keY ) 4 0 0 0
5000
6000
Fig. 2. Ge(Li) gamma-ray spectra from (a) 5.30 MeV ~-particles incident on a tantalum-backed TitSN film, (b) 5.39 MeV ~-particles incident on a gold-backed Ta15N2 sputtered film of the same thickness.
TANTALUM NITRIDE TARGETS
I"~N(ct, y)19F resonance. At equivalent energies, the yield of 6.129 MeV gamma rays from 13C(0~,n~)160 is about four times less from the TaN 2 on gold than from the TiN on tantalum. The average background below 4 MeV is also less by at least a factor of three and the spectrum below 1.5 MeV is strikingly cleaner due to the absence of y-rays from reactions on titanium. It is gratifying that the deposition of the TaN 2 film by sputtering does not significantly increase the carbon content of the targets as determined from the intensity of the 13C(0qn3))160 line at 6.129 MeV. An upper limit on the carbon content of a 10 #g/cm 2 film of 0.1% by weight has been deduced. Thus the two most important advantages of the sputtering process in our work are the ability to choose the backing material which gives the lowest gamma-ray background, and the cleanliness of the process. The process also makes the preparation of nitride films on thin fragile backings feasible6). By observation of interference fringes, the sputtered films are found to be of very uniform thickness across the 3.2 cm diameter backings used. The adhesion of the films to the backings is particularly good, possibly due
235
to the high thermal energy of the sputtered material. The films are thus able to withstand for long periods the high beam currents needed in particle-capture studies. We would like to thank A. R. MacGill, V. Saxton and R. Griffiths for assistance and helpful discussions. B. Y. Underwood wishes to thank S.R.C. for the award of an S.R.C. Research Studentship and M. R. Wormmald for an S.R.C. Research Fellowship during the tenures of which this work was carried out. References 1) L. Holland, Vacuum deposition of thin films (Chapman and Hall, London, 1963). 2) H. J. Coyne and R. N. Tauber, J. Appl. Phys. 39, no. 12 (1968) 5585. 3) M. H. Read, private communication, cited in ref. 2. 4) D. Gerstenberg and C. J. Calbick, J. Appl. Phys. 35, no. 2 (1964) 4O52. 5) D. M. Mattox and J. E. McDonald, J. Appl. Phys. 34, no. 8 (1963) 2493. 6) H. W. Fullbright, J. A. Robins, M. Blann and D. G. Fleming, Phys. Rev. 184 (1969) 1068.
INSTRUMENTS
AND
METHODS
233-235; © N O R T H - H O L L A N D P U B L I S H I N G
IO 7 (t973)
CO.
T H E P R E P A R A T I O N OF T A N T A L U M N I T R I D E T A R G E T S BY REACTIVE S P U T T E R I N G M. R. WORMALD*, B. Y. U N D E R W O O D and K. W. ALLEN
Nuclear Physics Laboratory, Oxford, England Received 13 October 1972
A method of making thin nitride films for nuclear targets by reactive sputtering is discussed. The process is found to be simple
and reliable and is capable of producing high purity films ort a widerange of backings.
1. Introduction
chemically with the gas so that the nitride of the cathode metal is deposited on the anode. The choice of cathode material thus determines the metal nitride formed. Possible metals are titanium, vanadium, tantalum, hafnium, and tungsten, all of which form very stable high melting point nitrides and can readily be sputtered. Tantalum was chosen for this work as it has a high atomic number and thus gives only relatively weak, low energy gamma-rays (from Coulomb excitation) when bombarded with alpha particles up to E~ = 8 MeV. Read 3) has shown that the use of pure nitrogen rather than the more usual mixture of 90% argon, 10% nitrogen gives a higher nitride resembling TaN2 rather than TaN. The use of the higher nitride is clearly desirable for maximum reaction yield. Since the process is basically a ' c o l d ' one, the
The reactive sputtering technique for the deposition of thin layers of metallic compounds is well known and frequently employed in industrial processes, but seems to have been little used in the preparation of targets for nuclear physics experiments. In the course of a study in this laboratory of the reaction lSN(~, )')19F, we had to produce nitride targets of high purity on suitable backings capable of withstanding large ( ~ 25 #A) currents of 6 MeV a-particles. Initially we used the well known technique of heating an evaporated layer of a suitable metal (e.g. titanium) on a tantalum backing in a ~SN atmosphere. However, at the bombarding energies used in our experiments, convenient metals such as titanium and zirconium gave an undesirable nuclear reaction yield and with tantalum backings it was difficult to avoid light element contamination, especially carbon, which gave a large yield of high energy ),-rays from the reaction 13C(~, n)160.
We therefore turned to the preparation of nitride layers on gold backings by reactive sputtering, and this paper describes the technique we used to prepare tantalum nitride films. The method is very convenient and simple, and permits excellent control of the thickness of the deposited film. We have also made oxide targets in this laboratory by the sputtering method.
L iclu~iid N2 cooled s c r ~ n
High purity Ta disc Shutter
Su bstrat~
2. Reactive sputtering
,
!
The reactive sputtering process has been described elsewhere 1) and sputtered nitride films have been studied in detail in relation to their chemical composition, crystal structure, and resistive properties2). The process involves striking a discharge in nitrogen gas between a pair of parallel electrodes by applying a dc potential. Bombardment of the cathode by nitrogen ions knocks out atoms of the electrode, which combine * Present address: Department of Physics, University of Guelph, Guelph, Ontario, Canada.
!
ono~
I
0
I
1
I
2
I
3 INCHES
Fig. 1. The electrode assembly was contained in a stainless steel chamber, the pressure in which was less than 10-6 torr prior to admitting N2 gas. The shutter has a radiation shield to protect the substrate. The substrate can be out-gassed at 900°C to reduce its carbon content. 233
234
M.R.
WORMALD
possibility of cracking hydrocarbons on to the target is minimized although the presence of methane or carbon dioxide can lead to the deposition of tantalum carbide.
3 kV. The area of the cathode disc was 45 cm z. The thickness was determined during sputtering by observing the colour of the film on the gold backing. The colour changes from natural gold to a deep gold at 16 ~ 20/~g/cm 2, then to red, purple and finally silver at 40/~g/cm 2 or more. Exact thicknesses were found by weighing and from the observed widths of known alpha-capture resonances• The components of the vacuum system and the sputtering module itself were fabricated from stainless steel. A high speed mercury diffusion pump (Edwards, type EM2), followed by a booster (Edwards, type 2M4) backed by sorption pumps, was used to evacuate the system. To prevent oil vapour contamination, no rotary p u m p of any sort was ever used. A quadrupole mass spectrometer was used to analyse the 15N z sputtering gas and to search for contaminants such as water-vapour and hydro-carbons. It was shown that the purity of the 15N 2 gas could be maintained during the sputtering process.
3. Experimental The apparatus used is shown in fig. 1. The discharge is restricted to the region between the electrodes by screening the cathode with a third earthed electrode in the usual manner. No discharge and therefore no sputtering occurs in the narrow gap between the screen and the cathode as electrons traversing the gap make too few collisions to maintain a discharge. It is essential that the nitrogen gas in the system is kept free from water vapour and hydro-carbons which would otherwise form oxides and carbides4). The ion bombardment of the walls of the sputtering vessel can result in outgassing which contaminates the nitrogen gas. To avoid this contamination, a shutter system was introduced to cover the substrate during a preliminary sputter, thus enabling the system to be cleaned up without depositing an impure film on the target. The contaminated gas was then pumped away, fresh nitrogen was admitted, and the shutter removed prior to sputtering. After the initial clean-up, the cathode screen was cooled by liquid nitrogen to keep the system free of water vapour and hydro-carbons. Heat generated in the anode plate by electron bombardment was removed by water cooling. Typically, the time taken to deposit a 40 ~tg/cm2 film of TaN2 is thirty minutes with a gas pressure of 50 pm and with a current of 30 mA at TiN
8o
40
O
I
O
Targets made in the manner described having thicknesses varying from 20--, 80 #g/cm 2 have proved to be superior to targets made in the conventional manner in several ways. Fig. 2 shows a comparison of ~,-ray spectra produced by a-particle bombardment of a sputtered T a N 2 film on gold and a TiN film on tantalum prepared by evaporation and heating. These spectra have been normalized to the yield at a strong
TANTALUM
AT
Ed.-
5.30 MeV
~
1000
2000
14N(~,p6 )17O
10OO
3000 ToN2 ON
8708 k~V
o
4. Discussion
Xl
H-
Z 40
ON
e t al.
GOLD
4000 AT E ~ -
539
1
,
2000
5000
6000
MeV BC(~,n ~ )u60 6129.4keV
,
30OOENERGY ( keY ) 4 0 0 0
5000
6000
Fig. 2. Ge(Li) gamma-ray spectra from (a) 5.30 MeV ~-particles incident on a tantalum-backed TitSN film, (b) 5.39 MeV ~-particles incident on a gold-backed Ta15N2 sputtered film of the same thickness.
TANTALUM NITRIDE TARGETS
I"~N(ct, y)19F resonance. At equivalent energies, the yield of 6.129 MeV gamma rays from 13C(0~,n~)160 is about four times less from the TaN 2 on gold than from the TiN on tantalum. The average background below 4 MeV is also less by at least a factor of three and the spectrum below 1.5 MeV is strikingly cleaner due to the absence of y-rays from reactions on titanium. It is gratifying that the deposition of the TaN 2 film by sputtering does not significantly increase the carbon content of the targets as determined from the intensity of the 13C(0qn3))160 line at 6.129 MeV. An upper limit on the carbon content of a 10 #g/cm 2 film of 0.1% by weight has been deduced. Thus the two most important advantages of the sputtering process in our work are the ability to choose the backing material which gives the lowest gamma-ray background, and the cleanliness of the process. The process also makes the preparation of nitride films on thin fragile backings feasible6). By observation of interference fringes, the sputtered films are found to be of very uniform thickness across the 3.2 cm diameter backings used. The adhesion of the films to the backings is particularly good, possibly due
235
to the high thermal energy of the sputtered material. The films are thus able to withstand for long periods the high beam currents needed in particle-capture studies. We would like to thank A. R. MacGill, V. Saxton and R. Griffiths for assistance and helpful discussions. B. Y. Underwood wishes to thank S.R.C. for the award of an S.R.C. Research Studentship and M. R. Wormmald for an S.R.C. Research Fellowship during the tenures of which this work was carried out. References 1) L. Holland, Vacuum deposition of thin films (Chapman and Hall, London, 1963). 2) H. J. Coyne and R. N. Tauber, J. Appl. Phys. 39, no. 12 (1968) 5585. 3) M. H. Read, private communication, cited in ref. 2. 4) D. Gerstenberg and C. J. Calbick, J. Appl. Phys. 35, no. 2 (1964) 4O52. 5) D. M. Mattox and J. E. McDonald, J. Appl. Phys. 34, no. 8 (1963) 2493. 6) H. W. Fullbright, J. A. Robins, M. Blann and D. G. Fleming, Phys. Rev. 184 (1969) 1068.