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Glow discharge and sputtering characteristics of copper alloys
Vacuum/volume
47/number g/pages 1043 to 1046/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207X/96
$15.00+.00
PII: SOO42-207X(96)00135-2
Glow discharge and sputtering characteristics of copper alloys S K Habib, I A Mousa and A Rizk, Department Heliopolis, Cairo, Egypt received
19 January
of Physics, faculty of Education, Ain-Shams
University,
7996
A study has been made of the glow discharge and sputtering characteristics of the two binary alloy targets Cu-6 wt% Ag and Cu-70 wt% Sn. The results were compared with those of a pure copper target in order to investigate the different effects of alloying both elements individually with Cu. It was found that Ag in the Cu/Ag alloy activated its oxidizing and nitriding processes while Sn in the Cu/Sn target hindered the two processes. On the other hand, alloying the two elements individually with copper enhanced its glow discharge characteristics, where higher current values and lower breakdown voltages were obtained. However, Ag was found to be more effective in improving the above mentioned characteristics than when Sn was used instead. Moreover, the presence of Ag increased the total sputtering yield of the Cu/Ag target while Sn on the other hand decreased that of the Cu/Sn target at all values of input power chosen. Copyright 0 1996 Elsevier Science Ltd.
Introduction
The sputtering of compound and alloy targets is of considerable practical interest arising from the need to understand processes such as: wall erosion of nuclear reactors, different plasma material processings, sputter deposition of alloys and compound thin films, and surface analysis techniques using sputtering. During ion bombardment of binary alloy systems, as an example, the different components are sputtered at different rates resulting in surface segregation of one component. This effect is called differential or preferential sputtering, and within the last few years there has been extensive experimental’-’ and theoretical*-” studies relating to this effect. It is therefore the object of this paper to continue previous studies and investigate the influence of alloying Ag and Sn individually with Cu as well as their preferential sputtering on the glow discharge and sputtering characteristics of Cu in two prepared binary alloy systems.
into discs of 54 mm diameter and then cleaned by ethanol followed by an ultrasonic cleaning. Sputtering system
The DC planar magnetron (PM) sputtering system used in this study has been described in detail elsewhere.12 Three targets of Cu, Cu/Ag and Cu/Sn of 54 mm diameter and 0.5 mm thickness were used separately to form the outer face of the water-cooled cathode. The system was pumped down to an ultimate pressure of 10-l mbar before gas admission using a two stage rotary pump. Argon, nitrogen and oxygen gases of 99.999% purity were employed and different mixtures were premixed using a gas reservoir. All gases were passed at a constant flow rate through a copper coil immersed in liquid nitrogen before admission to the vessel to remove water vapour and Pz05 was used as a drying agent inside the vessel. Experimental results and discussion
Alloys preparation
wt% Ag and Cu-10 wt% Sn alloys were prepared from the high purity (99.999%) Cu, Sn And Ag metals. The appropriate proportions of both alloys were preheated separately in a vacuum furnace for 2h at 1100” followed by water quenching. The obtained alloys were then annealed for 3 days at different annealing temperatures (800’ for Cu/Ag and 600” for Cu/Sn) to form solid solutions for both, followed again by water quenching. The formed alloys were cold rolled at a 10% reduction per pass to a total thickness of 0.5 mm. The sheets obtained were cut Cu-6
characteristics (Z/V). The discharge current (I) as a function of applied voltage (V) for the three targets Cu, Cu/Sn and Cu/Ag critically depends on the ion bombardment current density and energy, material of the target, as well as gas pressure and gas type. This is shown by the I/ I/ curves in Figure 1 and interpreted according to the following plasma processings. Current-voltage
Physical sputtering in Ar glow discharge. Ion bombardment, in the Ar glow discharge of the three targets separately at a constant gas pressure of 0.05 mbar showed that as the applied voltage (V) 1043
S K Habib et al: Glow discharge and sputtering characteristics of copper alloys
I40 120
SO%Ar + SO’%02 PI = 0.05 mb
m Cu-Ag
I40
0 Cu-Sn . r-t,
120
SO%Ar + 507&N? PI = 0.0s mb c
:;I&:;:I.f,“;I, ($__#/f , 200 240 280 320 360 400 440 480 520
v (volt)
200
240
280
320
360 400
v
440
480
520
(volt)
Figure 1. I-V characteristics of sputtering Cu-Ag, Cu-Sn and Cu targets: (a) In dry Ar atmosphere, (b) the effect of changing gas pressure, (c) in dry (50% Arf atmosphere.
50% 0,) atmosphere
and (d) in dry (50% Ar+ 50% NJ
increased the external current (I) increased for all targets used, Figure l(a). The Z/V characteristics obtained can be divided into two regions of linear current-voltage dependence; the first voltage portion (250-350 V) was characterized by a current increment of 20 mA for Cu, 22 mA for Cu/Sn and 40mA for Cu/Ag, while the increment in current along the second voltage portion (350-450 V) was 55 mA for Cu, 65mA for Cu/Sn and 50 mA for Cu/Ag. The appearance of these two distinguished regions in the I/P’ characteristics showed clearly the effect of ion energy on the discharge current during the sputtering process. The effect of alloying Ag and Sn individually with Cu on the discharge current of pure copper was also evident in the figure where higher current values were obtained. This increase in current can be attributed to the high coefficients” of secondary electron emission of both Ag and Sn with respect to that of Cu. On the other hand, raising the Ar gas pressure from 0.05 to 0.1 m bar increased the discharge current at all values of the applied voltage. This increase in current was due to the enhanced number of ions presented in the Ar glow discharge as the gas pressure increased. This effect is shown in Figure l(b) when the Cu/Ag target (arbitrarily chosen) was used as the cathode material. Reactive sputtering in Ar/O, glow discharge. The Z/V characteristics given in Figure l(c) for Cu, Cu/Sn and Cu/Ag targets sputtered separately in (50% Ar+50% 0,) glow discharge showed that the highest current values were obtained for the CuAg target, where a small increase in the applied voltage (70 V) caused a rapid increase in the discharge current (e 100 mA). However, high current values are expected to be obtained since the formed oxides may have enhancedI the emission of secondary electrons and therefore the total sputtering current. On the other hand, both of the Cu/Sn and Cu targets showed 1044
different transitions (a small transition for Cu/Sn while a large one for Cu) as their external currents begin to increase towards maximum. For instance, a transition of 100 V was needed for the copper target, whereas a transition of 30 V only was needed for the Cu/Sn target. In the I/V characteristics of the Cu/Ag target, no transition was observed. The different transitions obtained are attributed to the different types of oxides formed on the targets and the different binding forces between the molecules of each. Therefore, different energy values were generally needed to remove those types of oxides formed on the different targets, and the more energy consumption the more cohesive the deposit was, and vice versa. The consumption of ion energy was necessarily associated with a current reduction. This explaination is supported by our data given by the i/V characteristics in which the greatest current value was obtained for Cu/Ag target (at 310 V) followed by that of both Cu/Sn (at 380 V) and Cu (at 500 V) targets, respectively. Arising from the foregoing, it can be concluded that alloying Ag with Cu highly activated the oxidizing process of the cathode material and caused a noticeable increase in the sputtering current where as alloying Sn with Cu hindered the process to some extent. Reactive sputtering in Ar/N, glow discharge. Sputtering the three targets separately in a (50% Ar+SO%NJ DC glow discharge resulted in the t/V characteristics shown in Figure l(d). These curves showed that, lower current values were generally obtained for the three targets treated separately in the Ar/N, glow discharge than those obtained earlier when the same targets were treated in the Ar/O, glow discharge at the same gas pressure. Typical maximum current values obtained for the different targets treated in the two glow discharge were as follows. l
l
In Ar/Nz glow discharge: 55m A for Cu/Ag, 42 m A for Cu/Sn and 35 m A for Cu. In Ar/OZ glow discharge: 110 m A for Cu/Ag, 65 m A for Cu/Sn and 60 m A for Cu.
The current values obtained when the two targets were sputtered separately in the Ar/N, glow discharge show that, alloying Ag with Cu enhanced the nitriding process of the cathode material and increased its electric current values, while alloying Sn with Cu hindered the nitriding process and did not show any improvement in the electric current values unless higher voltages were applied to the target. Breakdown voltage (V,,). The breakdown voltage, i.e. the voltage of the discharge onset, depends on many factors such as the material of the cathode, the magnetic field used in the sputtering system and first of all gas pressure and gas type. In the present work we ignited the glow discharge of Ar, 50% Ar+ 50% 02, and 50% Ar+ 50% N,, respectively using the different targets Cu, Cu/Sn, and Cu/Ag separately. The breakdown voltages (V,) were recorded for each gastarget combination as a function of gas pressure and are plotted in Figure 2. As can be seen from the figure, that V, decreased rapidly with increasing gas pressure until a certain pressure was reached, depended on both types of gas and target material, and then increased slowly. When the Ar glow discharge was used, the breakdown voltages obtained in case of using Cu/Ag target were found to be lower than those obtained for both Cu/Sn and Cu targets, respectively, Figure 2(a). This was due to the high coefficient of secondary
S K Habib
eta/: Glow discharge
and sputtering
characteristics
of copper
(a) Ar
n
Cu-Ag
o Cu-Sn 6 cu
alloys
oxides (or nitrides) formed on the target which consumed low ion energies in its removal as well as the high current values obtained. Table 1 gives the minimum breakdown voltages ( I/,,,,) of glow discharges for the different gas-target combinations. The targets are arranged in order according to the value of I’,,,,. From this table the following remarks can be made: l
0.2
0
0.6
0.4
0.8 l
(b) 380 a 360 I- _
50% Ar t 50% O2
c l l
0
l
l
0.4
0.2
0
0.6
0.8
Ar, Ar/O* and Ar/N, glow discharges were ignited at relatively lower voltages in case of alloying either Ag or Sn with Cu than when using the pure Cu target. The different glow discharges were ignited at relatively lower voltages in case of alloying Ag with Cu than when alloying Sn with Cu. Oxides and nitrides formed separately on both Cu/Ag and Cu/Sn targets required higher voltages to cause glow ignition compared to the breakdown voltages in case of Ar for the two targets, respectively. Oxides required less breakdown voltages for glow ignition than nitrides. This is due to their high coefficient of secondary electron emission.
The data represented here are however of a considerable importance specially when one thinks of a material of low operating breakdown voltage.
a
50% Ar + 50% O2 6 g .
%j
.
, 0
0.2
Q
n
, 0.4
,+:y 016
0.8
P (m bar) Figure 2. Breakdown
voltage as a function of gas pressure of sputtering CupAg, Cu-Sn and Cu targets: (a) in dry Ar atmosphere, (b) in dry (50% Ar + 50% Oz) atmosphere and (c) in dry (50% Arf 50% N2) atmosphere.
electrons of Ag compared to that of both Sn and Cu. The high electron emission of Ag increased the total current value I (I = I+ +?I+) and consequently led to the observed decrease in the breakdown voltages at all gas pressures used. The sharp decrease in V, (zone ab) is similar to the lower branch of the well known Paschen breakdown curve and reflected the rapid decrease in target voltage required to maintain a given sputtering current, while the observed increase in V, (zone bc) is believed necessary to reorient the major fraction of ions which were scattered in the Ar glow discharge at the high pressures used. On the other hand, Figures 2 (b) and (c) show the V,-P characteristics for the three given targets in (50% Ar + 50% 0,) and (50% Ar+ 50% NJ glow discharges, respectively. The characteristics obtained were generally of the same shape as that obtained for the Ar glow discharge except that higher breakdown voltages were obtained (along the zone bc) than those obtained earlier using the Ar glow. The high breakdown voltages observed are believed to be necessary for oxide (or nitride) removal as well as glow ignition. The formed deposits were continuously accumulating on the surface of each target separately by time. Again as in the case of Ar, the lowest breakdown voltages in both glow discharges were obtained when the Cu/Ag target was used. This was due to the low cohesive nature of the mixed
Voltage-pressure characteristics (V/P). It is interesting to notice that increasing the external current obtained at the breakdown voltage to an arbitrary chosen value (15 mA) resulted in the voltage-pressure characteristics shown in Figure 3. The characteristics were of an opposite shape to those obtained earlier for the breakdown characteristics shown in Figure 2. The V-P characteristics show that as the gas pressure (P) increased, the applied voltage (V) increased (zone ab) until a point was reached where the further increase in pressure was accompanied by a pressure independence region (zone bc) followed by a decrease in the target voltage (zone cd). The observed increase and decrease in target voltages (zones ab and cd) with Ar gas pressure shown in Figure 3(a), were to maintain the required current value. On the other hand when O2 or Nz was used in the glow discharge, oxides and/or nitrides were formed on the targets, where the higher the gas pressure used the thicker was the target’s deposit formed. Therefore along the zone (ab) Figures 3 (b) and (c) one expects that the increase in target voltage was necessarily to remove the accumulated deposits and then maintaining the required current value, while the steady target voltage region observed along the zone (bc) is believed to be sufficient for deposit removal and maintaining the current. On the other hand, the decrease in target voltage along the zone (cd) is believed to be due to the increasing number of ions and emitted secondary electrons which led to increasing current values and therefore the continuous decrease in the voltage is observed.
Table 1. Target’s alloy and minimum Gas
voltage
(V,,,.)
Ar 50%Ar + 50%02
Cu/Ag Cu/Ag
230 V 260 V
50%Ar + 50%NZ Cu/Ag 330 V
breakdown
CuiSn 236 V
Cu 250 V
Cu/Sn 286 V Cu/Sn 330 V
cu 212 v Cu 338 V
1045
S K Habib et a/: Glow discharge and sputtering characteristics
of copper alloys
(8)
It is to be noted that more voltage drop was recorded along the zone (cd) of Figure 3(b) than that obtained for the same zone in Figures 3 (a) and (c). This is due to the enhanced emission of secondary electrons from the oxides formed.
340 320 h 5
300 280
’
260
S’
240
Conclusions l
2ooOo.2
0.8
@I 340r
50% Ar + 50% 0,
320 2
l
(I = 15 mA) n
Cu-Ag
300
l
s;+=&=;
200 0
I 0.2
I 0.4
I 0.6
References
. 0.8
(c) 340 320c
0
50% Ar + 50% N2 b C
0.2
0.4
(I = 15 mA)
0.6
0.8
P (m bar) Figure 3. Voltage-pressure characteristics of sputtering Cu-Ag, Cu-Sn and Cu targets at a constant current value (15 mA): (a) in dry Ar atmosphere, (b) in dry (50% Arf 50% 0,) atmosphere and (c) in dry (50% Ar + 50% N:) atmosphere.
1046
Alloying silver and tin individually with copper generally enhanced the glow discharge characteristics of copper in the two formed binary alloys. Alloying Ag with Cu activated both oxidizing and nitriding processes of the Cu/Ag target where as Sn in the Cu/Sn target hindered the two processes. Low operating breakdown voltages were obtained in the different plasma atmospheres when either Ag or Sn was alloyed with Cu, but Ag was found to be more effective than Sn.
‘H Bubert and R P H Garten, Appl Surf&i, 81,203-214 (1994). ‘W Fischer, A Naoumidis and H Nickel, J Analy Arom Specfro, 9, 375380 (1994). ‘D Popov, P Mashkov and D Tzaneva, Vacuum, 45,967-971 (1994). 4A A Hussain et al., Vacuum, 45, 121-125 (1994). ‘A A Hussain and M Sayer, Vacuum, 45, 115-l 19 (1994). 6N Bibic. I H Wilson and T Nenadovic, J Appl Phys, 53, 52X-5257 (1982). ‘5 W Coburn, Thin Solid Films, 64,371&382 (1979). “C Fan and D Yang, Vacuum, 43,231-234 (1992). ‘J J Jimenez-Rodrfguez et al., Rad Effects, 41, 165-173 (1979). “‘G Carter, R Webb, R Collins and D A Thompson, Rad Effects, 40, 119-121 (1979). ” R Collins, Rad Efjcfs, 37, 13-l 9 (I 978). “A Rizk, S B Youssef and S K Habib, Vacuum, 38,93%95 (1988). Ii Hondbook of chemistry andphysics, 62nd Edilion, 198 I-1982, E-313. 14S Maniv and W D Westwood, J Vat Sci Technol, 17,743-751 (1980). 15G Carter and J S Colhgon, Ion bombardmmf c$solids. American Elsevier, New York (1968).
47/number g/pages 1043 to 1046/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207X/96
$15.00+.00
PII: SOO42-207X(96)00135-2
Glow discharge and sputtering characteristics of copper alloys S K Habib, I A Mousa and A Rizk, Department Heliopolis, Cairo, Egypt received
19 January
of Physics, faculty of Education, Ain-Shams
University,
7996
A study has been made of the glow discharge and sputtering characteristics of the two binary alloy targets Cu-6 wt% Ag and Cu-70 wt% Sn. The results were compared with those of a pure copper target in order to investigate the different effects of alloying both elements individually with Cu. It was found that Ag in the Cu/Ag alloy activated its oxidizing and nitriding processes while Sn in the Cu/Sn target hindered the two processes. On the other hand, alloying the two elements individually with copper enhanced its glow discharge characteristics, where higher current values and lower breakdown voltages were obtained. However, Ag was found to be more effective in improving the above mentioned characteristics than when Sn was used instead. Moreover, the presence of Ag increased the total sputtering yield of the Cu/Ag target while Sn on the other hand decreased that of the Cu/Sn target at all values of input power chosen. Copyright 0 1996 Elsevier Science Ltd.
Introduction
The sputtering of compound and alloy targets is of considerable practical interest arising from the need to understand processes such as: wall erosion of nuclear reactors, different plasma material processings, sputter deposition of alloys and compound thin films, and surface analysis techniques using sputtering. During ion bombardment of binary alloy systems, as an example, the different components are sputtered at different rates resulting in surface segregation of one component. This effect is called differential or preferential sputtering, and within the last few years there has been extensive experimental’-’ and theoretical*-” studies relating to this effect. It is therefore the object of this paper to continue previous studies and investigate the influence of alloying Ag and Sn individually with Cu as well as their preferential sputtering on the glow discharge and sputtering characteristics of Cu in two prepared binary alloy systems.
into discs of 54 mm diameter and then cleaned by ethanol followed by an ultrasonic cleaning. Sputtering system
The DC planar magnetron (PM) sputtering system used in this study has been described in detail elsewhere.12 Three targets of Cu, Cu/Ag and Cu/Sn of 54 mm diameter and 0.5 mm thickness were used separately to form the outer face of the water-cooled cathode. The system was pumped down to an ultimate pressure of 10-l mbar before gas admission using a two stage rotary pump. Argon, nitrogen and oxygen gases of 99.999% purity were employed and different mixtures were premixed using a gas reservoir. All gases were passed at a constant flow rate through a copper coil immersed in liquid nitrogen before admission to the vessel to remove water vapour and Pz05 was used as a drying agent inside the vessel. Experimental results and discussion
Alloys preparation
wt% Ag and Cu-10 wt% Sn alloys were prepared from the high purity (99.999%) Cu, Sn And Ag metals. The appropriate proportions of both alloys were preheated separately in a vacuum furnace for 2h at 1100” followed by water quenching. The obtained alloys were then annealed for 3 days at different annealing temperatures (800’ for Cu/Ag and 600” for Cu/Sn) to form solid solutions for both, followed again by water quenching. The formed alloys were cold rolled at a 10% reduction per pass to a total thickness of 0.5 mm. The sheets obtained were cut Cu-6
characteristics (Z/V). The discharge current (I) as a function of applied voltage (V) for the three targets Cu, Cu/Sn and Cu/Ag critically depends on the ion bombardment current density and energy, material of the target, as well as gas pressure and gas type. This is shown by the I/ I/ curves in Figure 1 and interpreted according to the following plasma processings. Current-voltage
Physical sputtering in Ar glow discharge. Ion bombardment, in the Ar glow discharge of the three targets separately at a constant gas pressure of 0.05 mbar showed that as the applied voltage (V) 1043
S K Habib et al: Glow discharge and sputtering characteristics of copper alloys
I40 120
SO%Ar + SO’%02 PI = 0.05 mb
m Cu-Ag
I40
0 Cu-Sn . r-t,
120
SO%Ar + 507&N? PI = 0.0s mb c
:;I&:;:I.f,“;I, ($__#/f , 200 240 280 320 360 400 440 480 520
v (volt)
200
240
280
320
360 400
v
440
480
520
(volt)
Figure 1. I-V characteristics of sputtering Cu-Ag, Cu-Sn and Cu targets: (a) In dry Ar atmosphere, (b) the effect of changing gas pressure, (c) in dry (50% Arf atmosphere.
50% 0,) atmosphere
and (d) in dry (50% Ar+ 50% NJ
increased the external current (I) increased for all targets used, Figure l(a). The Z/V characteristics obtained can be divided into two regions of linear current-voltage dependence; the first voltage portion (250-350 V) was characterized by a current increment of 20 mA for Cu, 22 mA for Cu/Sn and 40mA for Cu/Ag, while the increment in current along the second voltage portion (350-450 V) was 55 mA for Cu, 65mA for Cu/Sn and 50 mA for Cu/Ag. The appearance of these two distinguished regions in the I/P’ characteristics showed clearly the effect of ion energy on the discharge current during the sputtering process. The effect of alloying Ag and Sn individually with Cu on the discharge current of pure copper was also evident in the figure where higher current values were obtained. This increase in current can be attributed to the high coefficients” of secondary electron emission of both Ag and Sn with respect to that of Cu. On the other hand, raising the Ar gas pressure from 0.05 to 0.1 m bar increased the discharge current at all values of the applied voltage. This increase in current was due to the enhanced number of ions presented in the Ar glow discharge as the gas pressure increased. This effect is shown in Figure l(b) when the Cu/Ag target (arbitrarily chosen) was used as the cathode material. Reactive sputtering in Ar/O, glow discharge. The Z/V characteristics given in Figure l(c) for Cu, Cu/Sn and Cu/Ag targets sputtered separately in (50% Ar+50% 0,) glow discharge showed that the highest current values were obtained for the CuAg target, where a small increase in the applied voltage (70 V) caused a rapid increase in the discharge current (e 100 mA). However, high current values are expected to be obtained since the formed oxides may have enhancedI the emission of secondary electrons and therefore the total sputtering current. On the other hand, both of the Cu/Sn and Cu targets showed 1044
different transitions (a small transition for Cu/Sn while a large one for Cu) as their external currents begin to increase towards maximum. For instance, a transition of 100 V was needed for the copper target, whereas a transition of 30 V only was needed for the Cu/Sn target. In the I/V characteristics of the Cu/Ag target, no transition was observed. The different transitions obtained are attributed to the different types of oxides formed on the targets and the different binding forces between the molecules of each. Therefore, different energy values were generally needed to remove those types of oxides formed on the different targets, and the more energy consumption the more cohesive the deposit was, and vice versa. The consumption of ion energy was necessarily associated with a current reduction. This explaination is supported by our data given by the i/V characteristics in which the greatest current value was obtained for Cu/Ag target (at 310 V) followed by that of both Cu/Sn (at 380 V) and Cu (at 500 V) targets, respectively. Arising from the foregoing, it can be concluded that alloying Ag with Cu highly activated the oxidizing process of the cathode material and caused a noticeable increase in the sputtering current where as alloying Sn with Cu hindered the process to some extent. Reactive sputtering in Ar/N, glow discharge. Sputtering the three targets separately in a (50% Ar+SO%NJ DC glow discharge resulted in the t/V characteristics shown in Figure l(d). These curves showed that, lower current values were generally obtained for the three targets treated separately in the Ar/N, glow discharge than those obtained earlier when the same targets were treated in the Ar/O, glow discharge at the same gas pressure. Typical maximum current values obtained for the different targets treated in the two glow discharge were as follows. l
l
In Ar/Nz glow discharge: 55m A for Cu/Ag, 42 m A for Cu/Sn and 35 m A for Cu. In Ar/OZ glow discharge: 110 m A for Cu/Ag, 65 m A for Cu/Sn and 60 m A for Cu.
The current values obtained when the two targets were sputtered separately in the Ar/N, glow discharge show that, alloying Ag with Cu enhanced the nitriding process of the cathode material and increased its electric current values, while alloying Sn with Cu hindered the nitriding process and did not show any improvement in the electric current values unless higher voltages were applied to the target. Breakdown voltage (V,,). The breakdown voltage, i.e. the voltage of the discharge onset, depends on many factors such as the material of the cathode, the magnetic field used in the sputtering system and first of all gas pressure and gas type. In the present work we ignited the glow discharge of Ar, 50% Ar+ 50% 02, and 50% Ar+ 50% N,, respectively using the different targets Cu, Cu/Sn, and Cu/Ag separately. The breakdown voltages (V,) were recorded for each gastarget combination as a function of gas pressure and are plotted in Figure 2. As can be seen from the figure, that V, decreased rapidly with increasing gas pressure until a certain pressure was reached, depended on both types of gas and target material, and then increased slowly. When the Ar glow discharge was used, the breakdown voltages obtained in case of using Cu/Ag target were found to be lower than those obtained for both Cu/Sn and Cu targets, respectively, Figure 2(a). This was due to the high coefficient of secondary
S K Habib
eta/: Glow discharge
and sputtering
characteristics
of copper
(a) Ar
n
Cu-Ag
o Cu-Sn 6 cu
alloys
oxides (or nitrides) formed on the target which consumed low ion energies in its removal as well as the high current values obtained. Table 1 gives the minimum breakdown voltages ( I/,,,,) of glow discharges for the different gas-target combinations. The targets are arranged in order according to the value of I’,,,,. From this table the following remarks can be made: l
0.2
0
0.6
0.4
0.8 l
(b) 380 a 360 I- _
50% Ar t 50% O2
c l l
0
l
l
0.4
0.2
0
0.6
0.8
Ar, Ar/O* and Ar/N, glow discharges were ignited at relatively lower voltages in case of alloying either Ag or Sn with Cu than when using the pure Cu target. The different glow discharges were ignited at relatively lower voltages in case of alloying Ag with Cu than when alloying Sn with Cu. Oxides and nitrides formed separately on both Cu/Ag and Cu/Sn targets required higher voltages to cause glow ignition compared to the breakdown voltages in case of Ar for the two targets, respectively. Oxides required less breakdown voltages for glow ignition than nitrides. This is due to their high coefficient of secondary electron emission.
The data represented here are however of a considerable importance specially when one thinks of a material of low operating breakdown voltage.
a
50% Ar + 50% O2 6 g .
%j
.
, 0
0.2
Q
n
, 0.4
,+:y 016
0.8
P (m bar) Figure 2. Breakdown
voltage as a function of gas pressure of sputtering CupAg, Cu-Sn and Cu targets: (a) in dry Ar atmosphere, (b) in dry (50% Ar + 50% Oz) atmosphere and (c) in dry (50% Arf 50% N2) atmosphere.
electrons of Ag compared to that of both Sn and Cu. The high electron emission of Ag increased the total current value I (I = I+ +?I+) and consequently led to the observed decrease in the breakdown voltages at all gas pressures used. The sharp decrease in V, (zone ab) is similar to the lower branch of the well known Paschen breakdown curve and reflected the rapid decrease in target voltage required to maintain a given sputtering current, while the observed increase in V, (zone bc) is believed necessary to reorient the major fraction of ions which were scattered in the Ar glow discharge at the high pressures used. On the other hand, Figures 2 (b) and (c) show the V,-P characteristics for the three given targets in (50% Ar + 50% 0,) and (50% Ar+ 50% NJ glow discharges, respectively. The characteristics obtained were generally of the same shape as that obtained for the Ar glow discharge except that higher breakdown voltages were obtained (along the zone bc) than those obtained earlier using the Ar glow. The high breakdown voltages observed are believed to be necessary for oxide (or nitride) removal as well as glow ignition. The formed deposits were continuously accumulating on the surface of each target separately by time. Again as in the case of Ar, the lowest breakdown voltages in both glow discharges were obtained when the Cu/Ag target was used. This was due to the low cohesive nature of the mixed
Voltage-pressure characteristics (V/P). It is interesting to notice that increasing the external current obtained at the breakdown voltage to an arbitrary chosen value (15 mA) resulted in the voltage-pressure characteristics shown in Figure 3. The characteristics were of an opposite shape to those obtained earlier for the breakdown characteristics shown in Figure 2. The V-P characteristics show that as the gas pressure (P) increased, the applied voltage (V) increased (zone ab) until a point was reached where the further increase in pressure was accompanied by a pressure independence region (zone bc) followed by a decrease in the target voltage (zone cd). The observed increase and decrease in target voltages (zones ab and cd) with Ar gas pressure shown in Figure 3(a), were to maintain the required current value. On the other hand when O2 or Nz was used in the glow discharge, oxides and/or nitrides were formed on the targets, where the higher the gas pressure used the thicker was the target’s deposit formed. Therefore along the zone (ab) Figures 3 (b) and (c) one expects that the increase in target voltage was necessarily to remove the accumulated deposits and then maintaining the required current value, while the steady target voltage region observed along the zone (bc) is believed to be sufficient for deposit removal and maintaining the current. On the other hand, the decrease in target voltage along the zone (cd) is believed to be due to the increasing number of ions and emitted secondary electrons which led to increasing current values and therefore the continuous decrease in the voltage is observed.
Table 1. Target’s alloy and minimum Gas
voltage
(V,,,.)
Ar 50%Ar + 50%02
Cu/Ag Cu/Ag
230 V 260 V
50%Ar + 50%NZ Cu/Ag 330 V
breakdown
CuiSn 236 V
Cu 250 V
Cu/Sn 286 V Cu/Sn 330 V
cu 212 v Cu 338 V
1045
S K Habib et a/: Glow discharge and sputtering characteristics
of copper alloys
(8)
It is to be noted that more voltage drop was recorded along the zone (cd) of Figure 3(b) than that obtained for the same zone in Figures 3 (a) and (c). This is due to the enhanced emission of secondary electrons from the oxides formed.
340 320 h 5
300 280
’
260
S’
240
Conclusions l
2ooOo.2
0.8
@I 340r
50% Ar + 50% 0,
320 2
l
(I = 15 mA) n
Cu-Ag
300
l
s;+=&=;
200 0
I 0.2
I 0.4
I 0.6
References
. 0.8
(c) 340 320c
0
50% Ar + 50% N2 b C
0.2
0.4
(I = 15 mA)
0.6
0.8
P (m bar) Figure 3. Voltage-pressure characteristics of sputtering Cu-Ag, Cu-Sn and Cu targets at a constant current value (15 mA): (a) in dry Ar atmosphere, (b) in dry (50% Arf 50% 0,) atmosphere and (c) in dry (50% Ar + 50% N:) atmosphere.
1046
Alloying silver and tin individually with copper generally enhanced the glow discharge characteristics of copper in the two formed binary alloys. Alloying Ag with Cu activated both oxidizing and nitriding processes of the Cu/Ag target where as Sn in the Cu/Sn target hindered the two processes. Low operating breakdown voltages were obtained in the different plasma atmospheres when either Ag or Sn was alloyed with Cu, but Ag was found to be more effective than Sn.
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