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Energy considerations for preparing thin films by ion beam deposition
266
Thin Solid k)'hns, 219 (1992) 266 269
Energy considerations for preparing thin films by ion beam deposition Arvind Jain Nuclear Ph3sics Diz:ision. Bhabha Atomic Research Centre. Bom& O' 400085 (India)
(Received November 15, 1991: revised March 11, 1992; accepted May 27, 1992)
Abstract A three-parameter fit has been made to experimental sell-sputtering data at low energies using an empirically modified form of the Sigmund expression. With the parameters obtained from this global fit, the energies have been calculated at which the sputtering yield becomes 0.25, 0.5, 0.75 and 1.0 for all the non-gaseous elements in the periodic table for many of which experimental data are not available in the literature. This information is useful in selecting the beam energy while preparing thin films using ion beam deposition.
i. Introduction The formation of thin films by the direct deposition of a low energy ion beam is a well-known technique described in an earlier paper by A m a n o [1]. The various parameters such as energy, dose rate and purity were discussed in that work and it was pointed out that an important advantage of this method is the precise controllability of the deposition parameters which can be achieved during the deposition. However, self-sputtering during the deposition is one of the limiting mechanisms in the formation of the films. The film will build up only if the self-sputtering coefficient Y, or the number of atoms knocked out per incoming ion, is less than unity. In the literature, no convenient self-sputtering data are available at very low energies over the whole periodic table from which one can choose directly the beam energy at which the deposition for a given element should be carried out. Such information is also needed for making thin targets in some isotope separators where the ion deposition is carried out at low energy using a retarding beam technique after mass selection. In the present work we perform a leastsquares fit to the experimental self-sputtering data available at low energies to an empirically modified form of the Sigmund sputtering yield expression [2]. With the optimum parameters obtained in such a global fit, a table is presented for all the non-gaseous elements in the periodic table for selecting the beam energy while preparing thin films using ion beam deposition.
2. Sputtering yield Using the linear cascade theory, the sputtering yield Y at normal incidence has been derived by Sigmund [2]:
0040-6090/92/$5.00
(1)
Y = 0.42~S,(E)/U,
where Sn (eV cm 2 ( 10 ~5 atoms) ~) is the elastic stopping cross-section and U~ (eV) is the sublimation (or binding) energy. The parameter ~ is a quantity depending on the target-to-projectile mass ratio [2] /x = M 2 / M ~. At very low energies, the assumptions of the Sigmund theory are not valid [3]. Equation (1) has been empirically modified by several researchers to describe sputtering at low energies near the "threshold". Using a systematic scaling, Bohdansky et al. [3] have given an empirical energy dependence of the sputtering at low energies for the case of a low projectile-to-target mass ratio: M ~ / M 2 < I. The explicit energy dependence of the sputtering yield near threshold has been considered empirically by other researchers also using the power law cross-section. Matsunami et al. [4] have modified the Sigmund equation (1) by multiplying by a factor 1 - ( E t h / E ) t/2, i.e. Y = 0.42~Sn(E)[ 1 -- (E,h/E)'/2]/U~
(2)
Recent simulations have shown that, although sputtering can commence even at the binding energy, the yields are quite small (10 6). The assumption of a "threshold" energy Eth in eqn. (2) at which sputtering is assumed to commence makes the energy term in eqn. (2) dimensionless. This is convenient when making a global fit since all the elements follow the same form of" a sharply rising sputtering yield curve near thc threshold. Bohdansky has given another expression for the energy dependence of the sputtering yield at the threshold
[5]: Y = 0.42~(Rp/R)Sn (E)[ I -- (E¢h/E) 2/s][ l -- ( E,h/E)] 2/U, (3)
:~ 1992
Elsevier Sequoia. All rights reserved
A. .lain / Energy considerations in ion beam deposition
In Bohdansky's fit [5], the value c~* = ~ R p / R is considered a fitting parameter for each element and the fit has been carried out for each element separately. In the present work, the available self-sputtering data for the elements have been fitted simultaneously. We note that the three variables c~, Eth and the exponent of the ratio Eth/E appear to be common factors in all the above representations.
3. The threshold energy The parameter Eth in eqn. (2) for a given element has been found to be related to the binding energy Us. Stuart and Wehner [6] found in their experiments that, for a given target, the constant Eth is proportional to the surface binding energy EB (or Us): Eth =4/-/5
(4)
Matsunami et al. [4] have fitted a large amount of sputtering data for various projectile-target combinations and observed that, when the ratio E t h / U S is plotted as a function of the target-to-projectile mass ratio t~ = M 2 / M ~ , it follows an "average" curve. For the Case of self-sputtering, # = 1, and from their average curve one can obtain [4], for this case, Eth = 14.85Us
(5)
Bohdansky et al. [3] however suggest the expression Eth = 8 U s ( M , / M 2 ) 2/5
M , / M 2 > 0.3
(6)
for Eth from which, for M I / M 2 = 1, Eth = 8Us. From eqns. (4)-(6), the constant Eth is expected to lie approximately in the range 4U S< Eth < 14.85Us.
4. Optimization of the parameters
267
parameters obtained represent the entire mass range. The data used in the fit lie in the range 0 < Y < 1.5 corresponding to an energy range 62 eV < E < 2017 eV. While the data were fitted, the elastic stopping crosssection S , ( E ) required in eqn. (7a) was calculated at each energy using the "average" interatomic potential given by Wilson et al. [11]. This potential has been found to give a good fit to the low energy range energy data [ 11]. The screening function of Wilson et al. used in their interatomic potential is given by ~b(x) -- 0.006 905 exp( -0.131 825x) + 0.166 929 exp( - 0 . 3 0 7 856x) + 0.826 165 exp( - 0.916760x)
(8)
where x = r/a is the reduced length. For the screening function given by eqn. (8), we have parametrized the reduced elastic cross-section sn as ln(l + 1.4835~) Sn = 2(e + 0.009 758 2~ °26333 + 0.342 84e °5)
(9)
which follows a form given by Ziegler [12]. Here e is the reduced energy given by (10)
e = aE/2e2Z 2
Z is the atomic number of the projectile, e the elementary charge and a the screening length: a=O.5577aoZ
1/3
(11)
ao = 0.5292 A is the Bohr radius. The elastic cross-section Sn (eV cm 2 (10 ~5 atoms) ~) required in eqn. (7a) is then given by Sn = 2.6654ZS/3s,/M
(12)
5. Results and discussion
We have carried out a least-squares fit to the experimental self-sputtering data at low energies available in the literature. The following empirically modified form of the Sigmund expression has been used: Y = 0.42c~Sn(E)[ 1 - (Eth/E)n]/U S
(7a)
E m = k Us
(7b)
The three constants c~, k and the exponent n in eqn. (7) have been kept as free parameters in the fit and all the data have been fitted simultaneously. We note that, near the threshold, eqn. (7a) is an empirical expression. However, when E >> Eth, it reduces to the Sigmund expression (eqn. (1)). Self-sputtering data have been considered for the elements aluminium [7], chromium [7], cobalt [8], nickel [8, 9], copper [7, 8], silver [7], tin [8], tungsten [10] and gold [7] so that the best-fit
The least-squares fit to the data of nine elements simultaneously results in the following values for the optimum parameters in eqn. (7): = 0.142 05 Eth = 11.485 Us n = 1.5118 In this fit, the average deviations of the fitted values are found to be within 31.6% of the experimental values for all the data which have been used. This may partly be due to the experimental errors in the data, the neglect of shell effects at these low energies, and perhaps the inadequacy of eqn. (7a) itself to describe sputtering at low energies. Using the form of the energy dependence given by Bohdansky (eqn. (3)) in the least-squares fit,
A. ,lain / Energy considerations in ion beam deposition
268
we did not find a better fit to the global experimental data. Using the optimum parameters obtained in the fit, we have used eqn. (7) to generate tables for the self-sputtering yields for all the non-gaseous elements in the periodic table. For many of these elements measurements are not available in the literature. The energies at which the sputtering yield Y becomes 0.25, 0.5, 0.75 and 1 are summarized for all the non-gaseous elements in Table 1. The calculated values of the energies for Y = 1 using the fitted parameters are compared with the corresponding experimental values given by Almen and
Bruce [ 13] below.
Element
E~ac.( Y = I) (eV)
E~xpt ( Y = I ) (eV)
Ni Cu Ag Sn
1100 590 270 280
1300 400 200 450
In a typical measurement on our isotope separator, ?6Mg was collected on a tantalum foil between the
TABLE I. The energies at which the sputtering yield Y = 0.25, 0.5, 0.75 and 1.0 for all the non-gaseous elements in the periodic table calculated
using the optimum parameters metioned in the text Element
Z
E (eV)
Us
Element
Z
E (eV)
b',
(eV)
Li Be B C Na Mg A1 Si P S K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Tc Ru Pd Ag
3 4 5 6 11 12 13 14 15 16 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 37 38 39 40 41 42 44 45 46 47
Y = 0.25
Y = 0.5
Y = 0.75
Y = 1.0
> > > >
> > > >
> > > >
> > > >
10000 10000 10000 10000 25 37 120 190 99 79 17 39 96 150 170 120 66 120 120 120 81 24 59 90 22 41 270 30 96 170 210 180 170 140 81 57
U~ is the sublimation energy.
10000 10000 10000 10000 50 81 370 720 280 210 27 74 230 410 460 280 140 280 280 270 180 38 120 190 33 70 720 48 200 380 520 420 390 300 160 99
10000 10000 10000 10000 97 170 1100 2400 690 460 43 140 480 955 II00 600 270 580 580 560 340 59 210 370 50 120 1600 75 370 770 1100 850 780 580 280 170
10000 10000 10000 10000 180 330 2600 10000 1500 955 66 240 890 1900 2200 1200 470 I100 ll00 1100 590 90 350 640 75 190 3100 120 620 1400 2000 1600 1400 985 450 270
1.67 3.38 5.73 7.4l 1.12 1.54 3.36 4.70 3.27 2.88 0.93 1.83 3.49 4.89 5.33 4.12 2.92 4.34 4.43 4.46 3.52 1.35 2.82 3.88 1.26 2.14 8.63 1.70 4.24 6.33 7.59 6.83 6.69 5.78 3.91 2.97
(eV)
Cd In Sn Sb Te Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu HI" Ta W Re Os [r Pt Au Hg TI Pb Bi Po Th U Pu
48 49 50 51 52 55 56 57 58 59 60 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 90 92 94
Y = 0.25
Y = 0.5
Y = 0.75
Y = 1.0
19 46 60 50 35 13 31 90 85 72 61 37 31 67 74 52 55 55 44 29 83 140 190 210 190 190 150 120 70 140 31 33 36 24 120 110 71
27 75 110 84 54 17 47 170 160 130 I10 56 45 120 130 83 89 88 67 41 150 260 380 430 380 380 290 220 120 250 43 48 52 32 210 190 120
37 130 180 140 84 22 70 290 270 210 170 85 67 190 220 130 140 140 110 58 240 260 710 800 700 700 520 390 190 44(1 62 69 75 44 360 310 180
52 190 280 220 130 29 110 470 430 330 270 130 96 290 340 200 220 220 160 82 380 750 200 1400 1200 1200 840 620 280 700 86 96 I10 60 560 470 270
1.16 2.49 3.12 2.72 2.02 0.82 1.84 4.42 4.23 3.71 3.28 2.16 1.85 3.57 3.89 2.89 3.05 3.05 ,". 5 , 1.74 4.29 6.31 8.1(I 8.68 8.09 8.13 6.90 5.86 3.80 6.36 1.88 2.03 2.17 1.50 5.93 5.42 3.98
A. Jain / Energy considerations in ion beam deposition
energies 42 and 119 eV. The quantity of magnesium expected on the foil for the incident integrated beam current using Table 1 was 30.9 ~tg. The quantity measured on the foil using the atomic absorption technique was 21.9 lag. The deviation between the measured and calculated value is 29.1%, in good agreement with the average deviation of 31.6°/,, in the fit mentioned earlier. In the absence of any experimental data, Table 1 can be used to select the beam energy while making thin films by ion beam deposition. While selfsputtering is at a minimum at the lowest energy, the beam current delivered to a given area of the substrate increases almost linearly with the energy [14], and Fair [14] has suggested an optimum energy for deposition ½E~ where E 1 is the energy at which Y = 1. We have found Table 1 to be a useful guide while depositing thin film targets in our isotope separator.
269
References 1 J. Amano, Thin Solid Films, 92(1982) 115. 2 P. Sigmund, Phys. Rev., 184 (1969) 383. 3 J. Bohdansky, J. Roth and H. L. Bay, J. Appl. Phys., 51 (5) (1980) 2861. 4 N. Matsunami, Y. Yamamura, Y. Itikawa, N. ltoh, Y. Zazumata, S. Miyagawa, K. Morita and R. Shimizu, Rep. 1PPJ-AM-14, June 1980 (Institute of Plasma Physics, Nagoya University); Radiat. Effl Lett., 57 (1980) 15. 5 J. Bohdansky, Nucl. lnstrum. Methods B, 2 (1984) 587. 6 R. V, Stuart and K. Wehner, Phys. Rev. Lett., 4 (1960) 409. 7 W. H. Hayward and A. R. Wolter, J. Appl. Phys., 40(1969) 2911. 8 A. Fontell and E. Arminen, Can. J. Phys., 47 (1969) 2405. 9 E. Hechtl, H. L. Bay and J. Bohdansky, J. Appl. Phys., 16 (1978) 147. 10 M. Saidoh and K. Sone, Jpn. J. Appl. Phys., 22(1983) 1361. 11 W. D. Wilson, L. G. Haggmark and J. B. Biersack, Phys. Rev. B, 15 (1977) 2458. 12 J. F. Ziegler, in J. F. Ziegler (ed.), Ion Implantation Science and Technology, Academic Press, New York 1984, p. 74. 13 O. Almen and G. Bruce, Nucl. lnstrum. Methods, II (1961) 287. 14 R. B. Fair, J. Appl. Phys., 42 (1971) 3176.
Thin Solid k)'hns, 219 (1992) 266 269
Energy considerations for preparing thin films by ion beam deposition Arvind Jain Nuclear Ph3sics Diz:ision. Bhabha Atomic Research Centre. Bom& O' 400085 (India)
(Received November 15, 1991: revised March 11, 1992; accepted May 27, 1992)
Abstract A three-parameter fit has been made to experimental sell-sputtering data at low energies using an empirically modified form of the Sigmund expression. With the parameters obtained from this global fit, the energies have been calculated at which the sputtering yield becomes 0.25, 0.5, 0.75 and 1.0 for all the non-gaseous elements in the periodic table for many of which experimental data are not available in the literature. This information is useful in selecting the beam energy while preparing thin films using ion beam deposition.
i. Introduction The formation of thin films by the direct deposition of a low energy ion beam is a well-known technique described in an earlier paper by A m a n o [1]. The various parameters such as energy, dose rate and purity were discussed in that work and it was pointed out that an important advantage of this method is the precise controllability of the deposition parameters which can be achieved during the deposition. However, self-sputtering during the deposition is one of the limiting mechanisms in the formation of the films. The film will build up only if the self-sputtering coefficient Y, or the number of atoms knocked out per incoming ion, is less than unity. In the literature, no convenient self-sputtering data are available at very low energies over the whole periodic table from which one can choose directly the beam energy at which the deposition for a given element should be carried out. Such information is also needed for making thin targets in some isotope separators where the ion deposition is carried out at low energy using a retarding beam technique after mass selection. In the present work we perform a leastsquares fit to the experimental self-sputtering data available at low energies to an empirically modified form of the Sigmund sputtering yield expression [2]. With the optimum parameters obtained in such a global fit, a table is presented for all the non-gaseous elements in the periodic table for selecting the beam energy while preparing thin films using ion beam deposition.
2. Sputtering yield Using the linear cascade theory, the sputtering yield Y at normal incidence has been derived by Sigmund [2]:
0040-6090/92/$5.00
(1)
Y = 0.42~S,(E)/U,
where Sn (eV cm 2 ( 10 ~5 atoms) ~) is the elastic stopping cross-section and U~ (eV) is the sublimation (or binding) energy. The parameter ~ is a quantity depending on the target-to-projectile mass ratio [2] /x = M 2 / M ~. At very low energies, the assumptions of the Sigmund theory are not valid [3]. Equation (1) has been empirically modified by several researchers to describe sputtering at low energies near the "threshold". Using a systematic scaling, Bohdansky et al. [3] have given an empirical energy dependence of the sputtering at low energies for the case of a low projectile-to-target mass ratio: M ~ / M 2 < I. The explicit energy dependence of the sputtering yield near threshold has been considered empirically by other researchers also using the power law cross-section. Matsunami et al. [4] have modified the Sigmund equation (1) by multiplying by a factor 1 - ( E t h / E ) t/2, i.e. Y = 0.42~Sn(E)[ 1 -- (E,h/E)'/2]/U~
(2)
Recent simulations have shown that, although sputtering can commence even at the binding energy, the yields are quite small (10 6). The assumption of a "threshold" energy Eth in eqn. (2) at which sputtering is assumed to commence makes the energy term in eqn. (2) dimensionless. This is convenient when making a global fit since all the elements follow the same form of" a sharply rising sputtering yield curve near thc threshold. Bohdansky has given another expression for the energy dependence of the sputtering yield at the threshold
[5]: Y = 0.42~(Rp/R)Sn (E)[ I -- (E¢h/E) 2/s][ l -- ( E,h/E)] 2/U, (3)
:~ 1992
Elsevier Sequoia. All rights reserved
A. .lain / Energy considerations in ion beam deposition
In Bohdansky's fit [5], the value c~* = ~ R p / R is considered a fitting parameter for each element and the fit has been carried out for each element separately. In the present work, the available self-sputtering data for the elements have been fitted simultaneously. We note that the three variables c~, Eth and the exponent of the ratio Eth/E appear to be common factors in all the above representations.
3. The threshold energy The parameter Eth in eqn. (2) for a given element has been found to be related to the binding energy Us. Stuart and Wehner [6] found in their experiments that, for a given target, the constant Eth is proportional to the surface binding energy EB (or Us): Eth =4/-/5
(4)
Matsunami et al. [4] have fitted a large amount of sputtering data for various projectile-target combinations and observed that, when the ratio E t h / U S is plotted as a function of the target-to-projectile mass ratio t~ = M 2 / M ~ , it follows an "average" curve. For the Case of self-sputtering, # = 1, and from their average curve one can obtain [4], for this case, Eth = 14.85Us
(5)
Bohdansky et al. [3] however suggest the expression Eth = 8 U s ( M , / M 2 ) 2/5
M , / M 2 > 0.3
(6)
for Eth from which, for M I / M 2 = 1, Eth = 8Us. From eqns. (4)-(6), the constant Eth is expected to lie approximately in the range 4U S< Eth < 14.85Us.
4. Optimization of the parameters
267
parameters obtained represent the entire mass range. The data used in the fit lie in the range 0 < Y < 1.5 corresponding to an energy range 62 eV < E < 2017 eV. While the data were fitted, the elastic stopping crosssection S , ( E ) required in eqn. (7a) was calculated at each energy using the "average" interatomic potential given by Wilson et al. [11]. This potential has been found to give a good fit to the low energy range energy data [ 11]. The screening function of Wilson et al. used in their interatomic potential is given by ~b(x) -- 0.006 905 exp( -0.131 825x) + 0.166 929 exp( - 0 . 3 0 7 856x) + 0.826 165 exp( - 0.916760x)
(8)
where x = r/a is the reduced length. For the screening function given by eqn. (8), we have parametrized the reduced elastic cross-section sn as ln(l + 1.4835~) Sn = 2(e + 0.009 758 2~ °26333 + 0.342 84e °5)
(9)
which follows a form given by Ziegler [12]. Here e is the reduced energy given by (10)
e = aE/2e2Z 2
Z is the atomic number of the projectile, e the elementary charge and a the screening length: a=O.5577aoZ
1/3
(11)
ao = 0.5292 A is the Bohr radius. The elastic cross-section Sn (eV cm 2 (10 ~5 atoms) ~) required in eqn. (7a) is then given by Sn = 2.6654ZS/3s,/M
(12)
5. Results and discussion
We have carried out a least-squares fit to the experimental self-sputtering data at low energies available in the literature. The following empirically modified form of the Sigmund expression has been used: Y = 0.42c~Sn(E)[ 1 - (Eth/E)n]/U S
(7a)
E m = k Us
(7b)
The three constants c~, k and the exponent n in eqn. (7) have been kept as free parameters in the fit and all the data have been fitted simultaneously. We note that, near the threshold, eqn. (7a) is an empirical expression. However, when E >> Eth, it reduces to the Sigmund expression (eqn. (1)). Self-sputtering data have been considered for the elements aluminium [7], chromium [7], cobalt [8], nickel [8, 9], copper [7, 8], silver [7], tin [8], tungsten [10] and gold [7] so that the best-fit
The least-squares fit to the data of nine elements simultaneously results in the following values for the optimum parameters in eqn. (7): = 0.142 05 Eth = 11.485 Us n = 1.5118 In this fit, the average deviations of the fitted values are found to be within 31.6% of the experimental values for all the data which have been used. This may partly be due to the experimental errors in the data, the neglect of shell effects at these low energies, and perhaps the inadequacy of eqn. (7a) itself to describe sputtering at low energies. Using the form of the energy dependence given by Bohdansky (eqn. (3)) in the least-squares fit,
A. ,lain / Energy considerations in ion beam deposition
268
we did not find a better fit to the global experimental data. Using the optimum parameters obtained in the fit, we have used eqn. (7) to generate tables for the self-sputtering yields for all the non-gaseous elements in the periodic table. For many of these elements measurements are not available in the literature. The energies at which the sputtering yield Y becomes 0.25, 0.5, 0.75 and 1 are summarized for all the non-gaseous elements in Table 1. The calculated values of the energies for Y = 1 using the fitted parameters are compared with the corresponding experimental values given by Almen and
Bruce [ 13] below.
Element
E~ac.( Y = I) (eV)
E~xpt ( Y = I ) (eV)
Ni Cu Ag Sn
1100 590 270 280
1300 400 200 450
In a typical measurement on our isotope separator, ?6Mg was collected on a tantalum foil between the
TABLE I. The energies at which the sputtering yield Y = 0.25, 0.5, 0.75 and 1.0 for all the non-gaseous elements in the periodic table calculated
using the optimum parameters metioned in the text Element
Z
E (eV)
Us
Element
Z
E (eV)
b',
(eV)
Li Be B C Na Mg A1 Si P S K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Tc Ru Pd Ag
3 4 5 6 11 12 13 14 15 16 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 37 38 39 40 41 42 44 45 46 47
Y = 0.25
Y = 0.5
Y = 0.75
Y = 1.0
> > > >
> > > >
> > > >
> > > >
10000 10000 10000 10000 25 37 120 190 99 79 17 39 96 150 170 120 66 120 120 120 81 24 59 90 22 41 270 30 96 170 210 180 170 140 81 57
U~ is the sublimation energy.
10000 10000 10000 10000 50 81 370 720 280 210 27 74 230 410 460 280 140 280 280 270 180 38 120 190 33 70 720 48 200 380 520 420 390 300 160 99
10000 10000 10000 10000 97 170 1100 2400 690 460 43 140 480 955 II00 600 270 580 580 560 340 59 210 370 50 120 1600 75 370 770 1100 850 780 580 280 170
10000 10000 10000 10000 180 330 2600 10000 1500 955 66 240 890 1900 2200 1200 470 I100 ll00 1100 590 90 350 640 75 190 3100 120 620 1400 2000 1600 1400 985 450 270
1.67 3.38 5.73 7.4l 1.12 1.54 3.36 4.70 3.27 2.88 0.93 1.83 3.49 4.89 5.33 4.12 2.92 4.34 4.43 4.46 3.52 1.35 2.82 3.88 1.26 2.14 8.63 1.70 4.24 6.33 7.59 6.83 6.69 5.78 3.91 2.97
(eV)
Cd In Sn Sb Te Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu HI" Ta W Re Os [r Pt Au Hg TI Pb Bi Po Th U Pu
48 49 50 51 52 55 56 57 58 59 60 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 90 92 94
Y = 0.25
Y = 0.5
Y = 0.75
Y = 1.0
19 46 60 50 35 13 31 90 85 72 61 37 31 67 74 52 55 55 44 29 83 140 190 210 190 190 150 120 70 140 31 33 36 24 120 110 71
27 75 110 84 54 17 47 170 160 130 I10 56 45 120 130 83 89 88 67 41 150 260 380 430 380 380 290 220 120 250 43 48 52 32 210 190 120
37 130 180 140 84 22 70 290 270 210 170 85 67 190 220 130 140 140 110 58 240 260 710 800 700 700 520 390 190 44(1 62 69 75 44 360 310 180
52 190 280 220 130 29 110 470 430 330 270 130 96 290 340 200 220 220 160 82 380 750 200 1400 1200 1200 840 620 280 700 86 96 I10 60 560 470 270
1.16 2.49 3.12 2.72 2.02 0.82 1.84 4.42 4.23 3.71 3.28 2.16 1.85 3.57 3.89 2.89 3.05 3.05 ,". 5 , 1.74 4.29 6.31 8.1(I 8.68 8.09 8.13 6.90 5.86 3.80 6.36 1.88 2.03 2.17 1.50 5.93 5.42 3.98
A. Jain / Energy considerations in ion beam deposition
energies 42 and 119 eV. The quantity of magnesium expected on the foil for the incident integrated beam current using Table 1 was 30.9 ~tg. The quantity measured on the foil using the atomic absorption technique was 21.9 lag. The deviation between the measured and calculated value is 29.1%, in good agreement with the average deviation of 31.6°/,, in the fit mentioned earlier. In the absence of any experimental data, Table 1 can be used to select the beam energy while making thin films by ion beam deposition. While selfsputtering is at a minimum at the lowest energy, the beam current delivered to a given area of the substrate increases almost linearly with the energy [14], and Fair [14] has suggested an optimum energy for deposition ½E~ where E 1 is the energy at which Y = 1. We have found Table 1 to be a useful guide while depositing thin film targets in our isotope separator.
269
References 1 J. Amano, Thin Solid Films, 92(1982) 115. 2 P. Sigmund, Phys. Rev., 184 (1969) 383. 3 J. Bohdansky, J. Roth and H. L. Bay, J. Appl. Phys., 51 (5) (1980) 2861. 4 N. Matsunami, Y. Yamamura, Y. Itikawa, N. ltoh, Y. Zazumata, S. Miyagawa, K. Morita and R. Shimizu, Rep. 1PPJ-AM-14, June 1980 (Institute of Plasma Physics, Nagoya University); Radiat. Effl Lett., 57 (1980) 15. 5 J. Bohdansky, Nucl. lnstrum. Methods B, 2 (1984) 587. 6 R. V, Stuart and K. Wehner, Phys. Rev. Lett., 4 (1960) 409. 7 W. H. Hayward and A. R. Wolter, J. Appl. Phys., 40(1969) 2911. 8 A. Fontell and E. Arminen, Can. J. Phys., 47 (1969) 2405. 9 E. Hechtl, H. L. Bay and J. Bohdansky, J. Appl. Phys., 16 (1978) 147. 10 M. Saidoh and K. Sone, Jpn. J. Appl. Phys., 22(1983) 1361. 11 W. D. Wilson, L. G. Haggmark and J. B. Biersack, Phys. Rev. B, 15 (1977) 2458. 12 J. F. Ziegler, in J. F. Ziegler (ed.), Ion Implantation Science and Technology, Academic Press, New York 1984, p. 74. 13 O. Almen and G. Bruce, Nucl. lnstrum. Methods, II (1961) 287. 14 R. B. Fair, J. Appl. Phys., 42 (1971) 3176.