- Email: [email protected]
The determination of the current-voltage characteristics of a closed-field unbalanced magnetron sputtering system
SURIACE
&COAlIN6S
ELSEVIER
Surface and Coatings Technology 98 ( 1998) 1370-1376
IFGHNOJDGY
The determination of the current-voltage characteristics of a closed-field unbalanced magnetron sputtering system P.I. Kelly *, R.D. Arnell Research Institutefor Design. Manufacture and Marketing. University of Salford. Salford. M54WT. UK
Abstract An investigation has been carried out into the current-voltage (I-V) characteristics of a closed-field unbalanced magnetron sputtering system (CFUBMS). CFUBMS is a versatile technique for the high-rate deposition of high quality metal, anoy and ceramic coatings. A key factor in this system is the ability to transport high ion currents to the substrate. This can enhance the formation of funy dense coatings at relatively low values of homologous temperature. A study has been made of the parameters that determine the 1- V characteristics of this system, at both the target and the substrate. From this, empirical relationships have been developed to describe the substrate ion current density and the substrate self-bias potential. Measurements have been taken of the discharge voltage, the substrate self-bias voltage, and substrate current using aluminium, zirconium, tungsten, and copper targets, at target currents of up to 8 A, over the pressure range 0.5-3 mTorr, and at substrate• to-target separations of 80 mm, 110 mm and 150 mm. The discharge voltage, VT , follows the established relationship IT=P(VT- VO)2, where IT is the discharge current and Vo is the minimum voltage required to strike a discharge. The variations of Pand Vo with target material and pressure are discussed. At the substrate, a linear relationship, Is = mIT' was observed between the ion current drawn at the substrate, Is, and the discharge current, IT' where the coefficient of proportionality, m, varies with chamber pressure and substrate-to-target separation. Also, self-bias voltages, in the range -17 to - 30 V, were measured. The value was largely independent of target material and substrate-to-target separation, but depended strongly on chamber pressure. © 1998 Elsevier Science S.A.
Keywords: Current-voltage characteristics; Unbalanced magnetron sputtering; Sputtering
1. Introduction
BNFL, in collaboration with Salford University, are currently undertaking a long-term, fundamental study of a closed-field unbalanced magnetron sputtering (CFUBMS) system (1,2]. One aim of this study is to investigate the current-voltage (l- V) characteristics of this system in order to understand the relationships between deposition parameters, and to determine which parameters are critical in establishing optimum condi• tions, both at the target and at the substrate. It has been shown [3,4] that the I-V characteristics of magnetron sputtering sources can be accurately described by the expression: IT = {3(VT - VO)2,
(l )
where IT is the discharge, or target current, VT is the
• Corresponding author. 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. Pll S0257-8972(97)00260-0
discharge, or target voltage and Vo is the mInImum voltage at which the discharge can be maintained. Both Vo and fi were found to vary with pressure and target material. In this study, the I-V characteristics of the CFUBMS system have been investigated for several target materials over a range of sputtering pressures. The data were in good agreement with Eq. (I). The values obtained for fi and Vo are compared with published values for planar magnetron sources [4]. The ability of unbalanced magnetrons to transport high ion currents to the substrate has long been recog• nized [5-9]. Indeed, it is a key factor in the success of this deposition technique. The development of multiple 'closed-field' magnetron arrangements [10, II] has led to further increases in the levels of ion current density that can be achieved at the substrate. Earlier work in this study [1] has shown that the high levels of ion bombardment achieved in a CFUBMS system can enhance the formation of fully dense 'high temperature'
P.I.
Ke/~r.
R.D. Arnell I Su~face and Coatings Teclmology 98 (1998) 1370-1376
coating structures at relatively low values of homologous temperature, when compared with other sputtering sys• tems [12]. Although many researchers have studied the influence of ion current density on the structure and properties of coatings, few have attempted to express this current in terms of other deposition parameters. Measurements of the ion current at the substrate were, therefore, made over a range of operating conditions, and from this, an empirical expression was derived, describing ion current in terms of chamber pressure, discharge current, and substrate-to-target separation. In addition to high ion currents, high substrate self• bias potentials, V.b' have also been observed in unbal• anced magnetron systems [5,7,9,13]. Indeed, earlier work at Salford has shown that it is possible to produce fully dense coatings by unbalanced magnetron sputter• ing, without the use of external biasing [14]. In this study, the influence of target material, discharge current, substrate-to-target separation and chamber pressure on the self-bias potential is examined, and an empirical relationship is derived.
1371
over the pressure range 0.5-3 mTorr (0.07-0.4 Pal, which corresponds to the coating pressure range typi• cally used in this rig [1,2]. For each set of experiments, the chamber was evacuated to a pressure of better than 10- 5 mbar (10- 3 Pal, before being backfilled with high purity argon to the required operating pressure, as measured using an Edwards 655 Barocell capacitance manometer gauge. In all, four target materials were used, namely aluminium, zirconium, tungsten and copper. The AI, Zr and eu targets were directly cooled, whereas, the tungsten targets were bonded to copper backing plates. To prevent de-bonding, the maximum discharge current setting for the W targets was fixed at 4 A. Substrate-to-target separation, d.- t , was also varied, with characteristics being taken at d.- t values of 80 mm, 110 mm and 150 mm. The magnetron drivers were operated in current regu• lation mode, whereas the bias supply was operated in voltage regulation mode. At each level of target current, the pressure was initially set to 0.5 mTorr. The target voltages for each magnetron were then recorded from the MDX drivers, as was the substrate self-bias voltage. The substrate bias was then increased incrementally up to - 300 V. The current drawn at the substrate at each increment of bias voltage was recorded from the bias supply. The voltage was then decreased, and repeat readings were taken at each increment. This process was repeated for each target material, at each pressure setting, target current and d.- t value. The substrate current was found to saturate at bias voltages of - 100 V, and above. At bias voltages of this magnitude, it was assumed that all the electrons approaching the substrate would be repelled. Therefore, in accordance with probe theory [15], the saturation current was taken as being representative of the flux of
2. Experimental The rig selected for characterization was a Teer Coatings Ltd UDP 450. The rig is equipped with two 300 mm x 100 mm vertically opposed unbalanced mag• netrons installed in a closed-field configuration. Each magnetron is connected to a 5-kW Advanced Energy MDX magnetron driver. An additional MDX driver is used as the bias supply. This rig has been described in detail elsewhere [1,2]. The 1- V characteristics of the system were investigated
I-V characteristics Variation with pressure
-
O.5mTorr
500
O.75mTorr
>
oj
1.0mTorr
g
15mTorr
OJ III
~ 400
-........
...-+....
...
2.0mTorr
U
3.0mTorr
Q)
OJ III J:
.21 300 0
200 '--_ _
o
----'
' - - ~
0.5
1
1.5
2
-
---J
2.5
3
Square Root of Discharge Current. N'O.5 Fig. I. Variation with pressure of the relationship between VT and
(/T)I/2
for the Zr target at d, _t = 110 mm.
P.J. Kelly, R.D. Arnell I Surface and Coatings Technology 98 ( 1998) 1370-1376
1372
ions arriving at the substrate during deposition. This was converted to an ion current density using the surface area of the substrate holder. Although a number of assumptions are made in this approach, it was consid• ered adequate for comparative purposes.
3. Results
For all conditions tested, the 1- V characteristics at the target were in good agreement with the expression:
(2) where VT is the discharge voltage with the substrate at floating potential (a small decrease in VT was observed as the substrate bias was increased). Examples of these results are shown in Fig. I. The curves were extrapolated to determine values of Vo for each set of conditions. The data were then re-plotted in the form of Eq. (I). Fig. 2 shows the results obtained for Zr and Al targets, whereas, Fig. 3 shows the results for Wand eu targets. As can be seen in Figs. 2 and 3, the data are in very good agreement with Eq. (I). Values obtained for Vo and f3 are listed in Table I. For each target material, very little variation in the values of Vo and f3 were observed with changes in d.- t • Therefore, only one set of data is included for each metal in Table I. The variation of f3 with pressure, for each target metal, is shown in Fig. 4.
... , . •.. • . ..
<
.: c:
8
Q) ~ ~
::::l 0
6
Q)
~ ro 4 .c: 0 en i5 2
Material
Pressure (mTorr)
Va (V)
(J (A/kV 2 )
Zirconium d._t=llOmm
0.5 I 1.5 2 3 0.5 I 1.5 2 3 0.75 1.75 2.25 0.75 1.5 3
253 209 213 226 212 365 318 313 324 326 204 226 227 436 320 317
122 297 339 370 760 158 104 110 145 204 27 38 39 241 95 101
Aluminium d._ t =80mm
3.1. I-Vcharacteristics at the target
10
Table I Values obtained for Va and f3 for Zr. AI, Wand Cu targets, over the pressure range 0.5-3 mTorr. where IT = {3(VT- Va)2
Tungsten d._ t =150mm Copper d.- t = 150 mm
3.2. Substrate ion current
Fig. 5 shows how the relationship between bias voltage and the current drawn at the substrate varies with pressure for an aluminium target, at a discharge current of 8 A and d.- t = 110 mm. These are typical examples of the results obtained from this investigation. At each pressure, the substrate current saturates at bias voltages above -100 V. However, the magnitude of the satura• tion current, I., decreases with increasing pressure. The saturation current, I .. was considered to be the substrate ion current. Over the range of conditions investigated, the substrate ion current was found to vary
I-V Characteristics Variation with pressure ZrO 5mTorr
..... ....
Zr 1.5 mTorr Zr 3 mTorr
.
•-
Al1 mTorr A l2 mTorr
..
A l3 mTorr -;.0.-
0"""-------'------'-----..1-------' 2E+4 4E+4 8E+4 OE+O 6E+4
(VT - Vo)"2. V"2 Fig. 2. Variation with pressure of the relationship between IT and (VT- Va)2 for Zr and AI targets.
1373
P.I. Kelly, R.D. Arnell / Surface and Coatings Technology 98 ( 1998) 1370-/376
I-V Characteristics Variation with pressure 10
«
WO.75mTorr Wl,75mTorr
8
•.•J •.
..•..
.:
W225mTorr
c
...... Q) ~
•
6
Cu 0.75 mTorr
(J
...
Q) C)
CU .t=
Cu 1.5 mTorr
4
.,/*',.
(J
VJ
is
2
o
;';~., •• ,.,., ' •. '.,
~"
-+•
.
Cu 3 mTorr ~
..• l·: !,::,': ~'., .. 1<.::-_.
-----'
4E+4
OE+O
-'-
1.2E+5
8E+4
----'
1.6E+5
(VT - Vo)"2, V"2 Fig. 3. Variation with pressure of the relationship between IT and (VT- Vo)2 for Wand Cu targets. 800
r - - - - - - - - - - - - - - - - - - - - - - , ,------,
Zr AI .......
600
>
N
Cu -+-
~ 400
CU
W ..........
...
Q)
CD
200
.................................................................... O'---~_...l....._"---..L_~_L-~
o
0.5
1
1.5
~---l..._~~_~--.l
2
2.5
3
3.5
Pressure, mTorr Fig. 4. Relationship between p and pressure for Zr. AI. Wand Cu targets. where p=/T/(VT- Vo)2.
from 0.16 to 2.52 A, which equates to ion current densities in the range of 0.5-8.2 mA(cm 2 • Fig. 6 shows how the relationship between I. and IT varies with pressure, target material and d.- t • As can be seen, I. is directly proportional to IT' with the constant of proportionality decreasing with increasing pressure. Also, for any combination of target material and pres• sure, I. decreases with increasing d.- t , i.e. (3)
where m varies with pressure, d.- t , and target material. To investigate further the relationship between target current and substrate ion current, the values obtained for m were plotted against chamber pressure, PCh' for different values of d.- t . In all cases, a power law
relationship was observed of the form: m=a(Pch)b,
(4)
where the constants a and b vary with target material and d.- t • The values obtained for a and b are listed in Table 2. Although the values of a decrease with increas• ing d.-to no other systematic trends were observed in these data. Fig. 7 shows how m varies with discharge current and d.- t . 3.3. Substrate self-bias potential
The substrate self-bias potential was found to vary from - 17 V to - 30 V. Fig. 8 shows the variation of Yab with target material, discharge current, d.- t , and
P.J. Kelly. R. D. Arnell! Surface and Coatings Technology 98 ( /998) /370-/376
1374
Substrate current Variation with bias voltage and pressure
2.51 2 .
< .;
;
;
0.5 mTorr
--+-075 mTorr
---.-
1.0 mTorr
c::
---+--
~ 1.5
15mTorr
::l (J
2
2.0 mTorr
~
iii .0
3.0mTorr
::l
en
0.5 OL--------:.--------.....;....------'------1
o
50
100
150
200
250
300
Substrate bias voltage. V (-ve) Fig. 5. Variation with pressure of relationship between substrate current and substrate bias voltage for Al target at d, - I = 80 mm.
3.----------------------,,------, Zrds-t=80mm 05 mTorr
--• -.-
2.5
Zrds-t=80mm 10mTorr
2
Zr ds·t=8Omm 20mTorr
c:: .2 1.5
AI ds·t=80mm 10mTorr
<:
'E
e L.
--.-
::J
.....
U
-
AI ds-I= 11 Omm
.$
10 mTorr
nl
L.
.(/)c ::J
en
AI ds-t=150mm 1.0mTorr
0.5
Cu ds-t-15Omm 1.5mTorr
·····t·.·····
o..-::..--------L-----'------'-----' 02468
W ds-t= 150mm 15mTorr
- +-.
Discharge current, A Fig. 6. Variation with target material. d'_I' and pressure of the relationship between Is and IT'
pressure. Very similar trends were observed for all conditions tested. As can be seen in Fig. 8, V.b appears to be largely independent of target material. discharge current, and d.- t • However, it shows a strong relation• ship with chamber pressure, PCh' of the form: V.b= C(Pch)d,
(5)
where C is in the range 22-24 and d is in the range -0.2 to -0.3, for PCh' measured in mTorr. 4. Discussion Over the pressure range 1-3 mTorr, clear trends in all the characteristics were observed. However, at pressures
below this, conditions at the target and at the substrate changed rapidly, and trends were harder to determine. As stated earlier, the 1- V characteristics at the target were in very good agreement with Eq. (1). Other studies have shown that Vo decreases with increasing pressure, whereas f3 increases with pressure [4]. In this study, f3 was found to follow this trend, over the pressure range stated above. However. Vo for each target material remained almost constant over this range. Average values of Vo over this range for Zr, AI, eu and W were 215 V, 320 V, 319 V and 227 V, respectively. This may imply that Vo is a material property that varies with magnetron design and target condition. (No account was made in this study of changes in the 1- V characteris• tics as the target becomes eroded. In practice, changes
1375
P.J. Kelly, R.D. Arnell / Surface and Coatings Technology 98 (1998) 1370-1376
Table 2 Values obtained for a and b for Zr, AL, Wand Cu targets, at d.- t values of 80 mm, II0mm and 150 mm, where Is =mI, and m=a(Pchl Material Zirconium d._ t =80mm d._t=IIOmm d._ t =150mm Aluminium d._.=80mm d._ t =llOmm d._.=150mm Tungsten d._ t =80 mm d._.=IIOmm d._.=150mm Copper d._ t =150mm
a
b
0.264 0.235 0.218
-0.117 -0.118 -0.081
0.276 0.249 0.243
-0.153 -0.183 -0.126
0.263 0.25 0.217
-0.117 -0.309 -0.244
0.214
-0.198
in the characteristics have been observed, and a longer• term study is being carried out to quantify these changes.) The values obtained for Vo and f3 for Al were com• pared to published data for planar magnetrons [4]. At 3 mTorr, Vo was found to be slightly higher than pub• lished values (280 V), whereas, the published value of f3 was noticeably lower (75 A/ky 2) than that obtained from this study (204 A/ky 2). It would appear that f3 depends not only on target material and pressure, but also on the magnetron and system design. This study has shown that ion current density in a CFUBMS system is directly proportional to discharge current, but decreases with increasing pressure and d. - I ' in the manner described in Eqs. (3) and (4). There also may be a weak dependence on target material, but this is unclear at this stage. Studies of other planar unbalanced magnetrons have also shown the dependence
0.3 [ ' T " " " - - - - - - - - - - - - - - - - , , - - - - - - ,
--
ZJ Os-! = 80mm
0.28
Zr OS-I = 110mm
0.26
E
-+•
W Os-I = 110mm
-.-
0.24
W OS-I = 150mm
0.22
-3• Cu
OS-I = 150mm
0.2
AI Os-I = 110mm
0.18 0.16
... -+....
L-
~_.L.--~_"--~_.L._~ _
0.5
1.5 2 2.5 Pressure, mTorr
_'__...:.:::l
3
AI
Ds-I = 150mm ·····Ei·····
Fig. 7. Variation with target material and d.- t of the relationship between m and pressure, where m=Is/IT •
32 30
>
28
.,.
Zr,4A ds-I=80mm
\~ '~..'
Zr.8A ds-I= 110mm
~:1.
--•
-+•
.::.,
"!~
If! 26
.•
AI,8A ds-I=80mm
'~~:.
.....:.
-AI,8A
,'Y:.
c
ell
.,~.~.~~.::.:%f',.-.~~.-.~.-....
i5 24 c.
Ul
~ 22
,.!.
~ 20
>o."
dS-I= 110mm -[} Cu,4A ds-I= 150mm
eU,8A ds-I= 150mm
..........-
18
•
W.2A Ds-I = 110mm
16 0
0.5
1.5
2
Pressure. mTorr Fig. 8. Variation with target material,
2.5
3
W,3A ds-I = 150mm
d._. and IT of the relationship between pressure and substrate self-bias potential.
1376
P.l. Kelly. R.D. Arnell ( Surface and Coatings Teclm%gy 98 (/998) /370-/376
of ion current on discharge current [6,14], but no attempts were made to describe the relationships. Further studies [16, 17] have investigated the relationship between ion current and d.- t • As would be expected, for single magnetron systems, losses to the chamber walls result in a more rapid decrease in ion current with increasing d.- t , when compared to a dual 'closed-field' system. Substrate self-bias potential showed a strong depen• dence on chamber pressure, but was largely independent of target material, discharge current, or d.- t • The inde• pendence of V.b from IT has been reported elsewhere [6] for single magnetron systems. However, such systems also showed a strong decrease in V.b with increasing d.-I, [14], which was not observed in the CFUBMS system. Again, this is attributed to the strong confine• ment of the plasma between the targets and the sub• strate, preventing losses to the chamber walls.
5. Conclusions The 1- V characteristics of a CFUBMS system have been investigated. At the target, a good agreement with established relationships for other magnetron systems was observed. At the substrate, empirical relationships have been developed to describe both the ion current drawn and the self-bias potential, in terms of operating parameters, such as discharge current, chamber pressure and substrate-to-target separation. In both cases, the CFUBMS system showed less dependence on substrate• to-target separation, when compared to single magnet• ron systems. This study was purely empirical. In order to character• ize the CFUBMS system in more detail, Langmuir probe and mass energy analysis studies are being carried out
that will allow the observed relationships to be described in terms of plasma properties. Acknowledgement The authors would like to acknowledge the financial support given to this project by BNFL.
References [I] P.J. Kelly, R.O. Amell, Presented at ICMCTF'96. Accepted for
publication. [2] P.J. Kelly, O.A. Abu-Zeid, R.O. Amell, J. Tong, Presented at ICMCTF'96. Accepted for publication. [3] S. Maniv, W.O. Westwood, P.J. Scanlon, J. Appl. Phys. 53 (2) (1982) 856-860. [4] W.O. Westwood, S. Maniv, P.J. Scanlon, J. Appl. Phys. 54 (2) (1983) 6841-6846. [5] B. Window, N. Savvides, J. Vac. Sci. Techno\. A4 (2) (1986) 196-201. [6] B. Window, N. Savvides, J. Vac. Sci. Techno\. A4 (2) (1986) 453-456. [7] N. Savvides, B. Window, J. Vac. Sci. Technol. A4 (2) (1986) 504-508. [8] D.G. Teer, Surf. Coat. Techno\. 39(40 (1989) 565-572. [9) R.P. Howson, H.A. J'afer, A.G. Spencer, Thin Solid Films 193( 194 (1990) 127-137. (10) D.G. Teer, UK Patent GB 2 258 343 B, Magnetron Sputter Ion Plating, 1991. [II) W.O. Sproul, Surf. Coat. Techno\. 49 (1991) 284-289. (12) J.A. Thornton, J. Vac. Sci. Technol. II (1974) 666-670. [13] S.L. Rohde, W.O. Sproul, J.R. Rohde, J. Vac. Sci. Technol. A9 ( 1991) 1178-1183. (14) D. Monaghan, R.D. Amell, Surf. Coat. Techno\. 49 (1991) 298-303. [15) M. Konuma, Film Deposition by Plasma Techniques, Springer, Berlin, 1992. [16) J. Musil, S. Kadlec, Vacuum 40 (1990) 435-444. (17) J. Musil, S. Kadlec, W.-D. Munz, J. Vac. Sci. Techno\. A9 (1991) 1171-1177.
&COAlIN6S
ELSEVIER
Surface and Coatings Technology 98 ( 1998) 1370-1376
IFGHNOJDGY
The determination of the current-voltage characteristics of a closed-field unbalanced magnetron sputtering system P.I. Kelly *, R.D. Arnell Research Institutefor Design. Manufacture and Marketing. University of Salford. Salford. M54WT. UK
Abstract An investigation has been carried out into the current-voltage (I-V) characteristics of a closed-field unbalanced magnetron sputtering system (CFUBMS). CFUBMS is a versatile technique for the high-rate deposition of high quality metal, anoy and ceramic coatings. A key factor in this system is the ability to transport high ion currents to the substrate. This can enhance the formation of funy dense coatings at relatively low values of homologous temperature. A study has been made of the parameters that determine the 1- V characteristics of this system, at both the target and the substrate. From this, empirical relationships have been developed to describe the substrate ion current density and the substrate self-bias potential. Measurements have been taken of the discharge voltage, the substrate self-bias voltage, and substrate current using aluminium, zirconium, tungsten, and copper targets, at target currents of up to 8 A, over the pressure range 0.5-3 mTorr, and at substrate• to-target separations of 80 mm, 110 mm and 150 mm. The discharge voltage, VT , follows the established relationship IT=P(VT- VO)2, where IT is the discharge current and Vo is the minimum voltage required to strike a discharge. The variations of Pand Vo with target material and pressure are discussed. At the substrate, a linear relationship, Is = mIT' was observed between the ion current drawn at the substrate, Is, and the discharge current, IT' where the coefficient of proportionality, m, varies with chamber pressure and substrate-to-target separation. Also, self-bias voltages, in the range -17 to - 30 V, were measured. The value was largely independent of target material and substrate-to-target separation, but depended strongly on chamber pressure. © 1998 Elsevier Science S.A.
Keywords: Current-voltage characteristics; Unbalanced magnetron sputtering; Sputtering
1. Introduction
BNFL, in collaboration with Salford University, are currently undertaking a long-term, fundamental study of a closed-field unbalanced magnetron sputtering (CFUBMS) system (1,2]. One aim of this study is to investigate the current-voltage (l- V) characteristics of this system in order to understand the relationships between deposition parameters, and to determine which parameters are critical in establishing optimum condi• tions, both at the target and at the substrate. It has been shown [3,4] that the I-V characteristics of magnetron sputtering sources can be accurately described by the expression: IT = {3(VT - VO)2,
(l )
where IT is the discharge, or target current, VT is the
• Corresponding author. 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. Pll S0257-8972(97)00260-0
discharge, or target voltage and Vo is the mInImum voltage at which the discharge can be maintained. Both Vo and fi were found to vary with pressure and target material. In this study, the I-V characteristics of the CFUBMS system have been investigated for several target materials over a range of sputtering pressures. The data were in good agreement with Eq. (I). The values obtained for fi and Vo are compared with published values for planar magnetron sources [4]. The ability of unbalanced magnetrons to transport high ion currents to the substrate has long been recog• nized [5-9]. Indeed, it is a key factor in the success of this deposition technique. The development of multiple 'closed-field' magnetron arrangements [10, II] has led to further increases in the levels of ion current density that can be achieved at the substrate. Earlier work in this study [1] has shown that the high levels of ion bombardment achieved in a CFUBMS system can enhance the formation of fully dense 'high temperature'
P.I.
Ke/~r.
R.D. Arnell I Su~face and Coatings Teclmology 98 (1998) 1370-1376
coating structures at relatively low values of homologous temperature, when compared with other sputtering sys• tems [12]. Although many researchers have studied the influence of ion current density on the structure and properties of coatings, few have attempted to express this current in terms of other deposition parameters. Measurements of the ion current at the substrate were, therefore, made over a range of operating conditions, and from this, an empirical expression was derived, describing ion current in terms of chamber pressure, discharge current, and substrate-to-target separation. In addition to high ion currents, high substrate self• bias potentials, V.b' have also been observed in unbal• anced magnetron systems [5,7,9,13]. Indeed, earlier work at Salford has shown that it is possible to produce fully dense coatings by unbalanced magnetron sputter• ing, without the use of external biasing [14]. In this study, the influence of target material, discharge current, substrate-to-target separation and chamber pressure on the self-bias potential is examined, and an empirical relationship is derived.
1371
over the pressure range 0.5-3 mTorr (0.07-0.4 Pal, which corresponds to the coating pressure range typi• cally used in this rig [1,2]. For each set of experiments, the chamber was evacuated to a pressure of better than 10- 5 mbar (10- 3 Pal, before being backfilled with high purity argon to the required operating pressure, as measured using an Edwards 655 Barocell capacitance manometer gauge. In all, four target materials were used, namely aluminium, zirconium, tungsten and copper. The AI, Zr and eu targets were directly cooled, whereas, the tungsten targets were bonded to copper backing plates. To prevent de-bonding, the maximum discharge current setting for the W targets was fixed at 4 A. Substrate-to-target separation, d.- t , was also varied, with characteristics being taken at d.- t values of 80 mm, 110 mm and 150 mm. The magnetron drivers were operated in current regu• lation mode, whereas the bias supply was operated in voltage regulation mode. At each level of target current, the pressure was initially set to 0.5 mTorr. The target voltages for each magnetron were then recorded from the MDX drivers, as was the substrate self-bias voltage. The substrate bias was then increased incrementally up to - 300 V. The current drawn at the substrate at each increment of bias voltage was recorded from the bias supply. The voltage was then decreased, and repeat readings were taken at each increment. This process was repeated for each target material, at each pressure setting, target current and d.- t value. The substrate current was found to saturate at bias voltages of - 100 V, and above. At bias voltages of this magnitude, it was assumed that all the electrons approaching the substrate would be repelled. Therefore, in accordance with probe theory [15], the saturation current was taken as being representative of the flux of
2. Experimental The rig selected for characterization was a Teer Coatings Ltd UDP 450. The rig is equipped with two 300 mm x 100 mm vertically opposed unbalanced mag• netrons installed in a closed-field configuration. Each magnetron is connected to a 5-kW Advanced Energy MDX magnetron driver. An additional MDX driver is used as the bias supply. This rig has been described in detail elsewhere [1,2]. The 1- V characteristics of the system were investigated
I-V characteristics Variation with pressure
-
O.5mTorr
500
O.75mTorr
>
oj
1.0mTorr
g
15mTorr
OJ III
~ 400
-........
...-+....
...
2.0mTorr
U
3.0mTorr
Q)
OJ III J:
.21 300 0
200 '--_ _
o
----'
' - - ~
0.5
1
1.5
2
-
---J
2.5
3
Square Root of Discharge Current. N'O.5 Fig. I. Variation with pressure of the relationship between VT and
(/T)I/2
for the Zr target at d, _t = 110 mm.
P.J. Kelly, R.D. Arnell I Surface and Coatings Technology 98 ( 1998) 1370-1376
1372
ions arriving at the substrate during deposition. This was converted to an ion current density using the surface area of the substrate holder. Although a number of assumptions are made in this approach, it was consid• ered adequate for comparative purposes.
3. Results
For all conditions tested, the 1- V characteristics at the target were in good agreement with the expression:
(2) where VT is the discharge voltage with the substrate at floating potential (a small decrease in VT was observed as the substrate bias was increased). Examples of these results are shown in Fig. I. The curves were extrapolated to determine values of Vo for each set of conditions. The data were then re-plotted in the form of Eq. (I). Fig. 2 shows the results obtained for Zr and Al targets, whereas, Fig. 3 shows the results for Wand eu targets. As can be seen in Figs. 2 and 3, the data are in very good agreement with Eq. (I). Values obtained for Vo and f3 are listed in Table I. For each target material, very little variation in the values of Vo and f3 were observed with changes in d.- t • Therefore, only one set of data is included for each metal in Table I. The variation of f3 with pressure, for each target metal, is shown in Fig. 4.
... , . •.. • . ..
<
.: c:
8
Q) ~ ~
::::l 0
6
Q)
~ ro 4 .c: 0 en i5 2
Material
Pressure (mTorr)
Va (V)
(J (A/kV 2 )
Zirconium d._t=llOmm
0.5 I 1.5 2 3 0.5 I 1.5 2 3 0.75 1.75 2.25 0.75 1.5 3
253 209 213 226 212 365 318 313 324 326 204 226 227 436 320 317
122 297 339 370 760 158 104 110 145 204 27 38 39 241 95 101
Aluminium d._ t =80mm
3.1. I-Vcharacteristics at the target
10
Table I Values obtained for Va and f3 for Zr. AI, Wand Cu targets, over the pressure range 0.5-3 mTorr. where IT = {3(VT- Va)2
Tungsten d._ t =150mm Copper d.- t = 150 mm
3.2. Substrate ion current
Fig. 5 shows how the relationship between bias voltage and the current drawn at the substrate varies with pressure for an aluminium target, at a discharge current of 8 A and d.- t = 110 mm. These are typical examples of the results obtained from this investigation. At each pressure, the substrate current saturates at bias voltages above -100 V. However, the magnitude of the satura• tion current, I., decreases with increasing pressure. The saturation current, I .. was considered to be the substrate ion current. Over the range of conditions investigated, the substrate ion current was found to vary
I-V Characteristics Variation with pressure ZrO 5mTorr
..... ....
Zr 1.5 mTorr Zr 3 mTorr
.
•-
Al1 mTorr A l2 mTorr
..
A l3 mTorr -;.0.-
0"""-------'------'-----..1-------' 2E+4 4E+4 8E+4 OE+O 6E+4
(VT - Vo)"2. V"2 Fig. 2. Variation with pressure of the relationship between IT and (VT- Va)2 for Zr and AI targets.
1373
P.I. Kelly, R.D. Arnell / Surface and Coatings Technology 98 ( 1998) 1370-/376
I-V Characteristics Variation with pressure 10
«
WO.75mTorr Wl,75mTorr
8
•.•J •.
..•..
.:
W225mTorr
c
...... Q) ~
•
6
Cu 0.75 mTorr
(J
...
Q) C)
CU .t=
Cu 1.5 mTorr
4
.,/*',.
(J
VJ
is
2
o
;';~., •• ,.,., ' •. '.,
~"
-+•
.
Cu 3 mTorr ~
..• l·: !,::,': ~'., .. 1<.::-_.
-----'
4E+4
OE+O
-'-
1.2E+5
8E+4
----'
1.6E+5
(VT - Vo)"2, V"2 Fig. 3. Variation with pressure of the relationship between IT and (VT- Vo)2 for Wand Cu targets. 800
r - - - - - - - - - - - - - - - - - - - - - - , ,------,
Zr AI .......
600
>
N
Cu -+-
~ 400
CU
W ..........
...
Q)
CD
200
.................................................................... O'---~_...l....._"---..L_~_L-~
o
0.5
1
1.5
~---l..._~~_~--.l
2
2.5
3
3.5
Pressure, mTorr Fig. 4. Relationship between p and pressure for Zr. AI. Wand Cu targets. where p=/T/(VT- Vo)2.
from 0.16 to 2.52 A, which equates to ion current densities in the range of 0.5-8.2 mA(cm 2 • Fig. 6 shows how the relationship between I. and IT varies with pressure, target material and d.- t • As can be seen, I. is directly proportional to IT' with the constant of proportionality decreasing with increasing pressure. Also, for any combination of target material and pres• sure, I. decreases with increasing d.- t , i.e. (3)
where m varies with pressure, d.- t , and target material. To investigate further the relationship between target current and substrate ion current, the values obtained for m were plotted against chamber pressure, PCh' for different values of d.- t . In all cases, a power law
relationship was observed of the form: m=a(Pch)b,
(4)
where the constants a and b vary with target material and d.- t • The values obtained for a and b are listed in Table 2. Although the values of a decrease with increas• ing d.-to no other systematic trends were observed in these data. Fig. 7 shows how m varies with discharge current and d.- t . 3.3. Substrate self-bias potential
The substrate self-bias potential was found to vary from - 17 V to - 30 V. Fig. 8 shows the variation of Yab with target material, discharge current, d.- t , and
P.J. Kelly. R. D. Arnell! Surface and Coatings Technology 98 ( /998) /370-/376
1374
Substrate current Variation with bias voltage and pressure
2.51 2 .
< .;
;
;
0.5 mTorr
--+-075 mTorr
---.-
1.0 mTorr
c::
---+--
~ 1.5
15mTorr
::l (J
2
2.0 mTorr
~
iii .0
3.0mTorr
::l
en
0.5 OL--------:.--------.....;....------'------1
o
50
100
150
200
250
300
Substrate bias voltage. V (-ve) Fig. 5. Variation with pressure of relationship between substrate current and substrate bias voltage for Al target at d, - I = 80 mm.
3.----------------------,,------, Zrds-t=80mm 05 mTorr
--• -.-
2.5
Zrds-t=80mm 10mTorr
2
Zr ds·t=8Omm 20mTorr
c:: .2 1.5
AI ds·t=80mm 10mTorr
<:
'E
e L.
--.-
::J
.....
U
-
AI ds-I= 11 Omm
.$
10 mTorr
nl
L.
.(/)c ::J
en
AI ds-t=150mm 1.0mTorr
0.5
Cu ds-t-15Omm 1.5mTorr
·····t·.·····
o..-::..--------L-----'------'-----' 02468
W ds-t= 150mm 15mTorr
- +-.
Discharge current, A Fig. 6. Variation with target material. d'_I' and pressure of the relationship between Is and IT'
pressure. Very similar trends were observed for all conditions tested. As can be seen in Fig. 8, V.b appears to be largely independent of target material. discharge current, and d.- t • However, it shows a strong relation• ship with chamber pressure, PCh' of the form: V.b= C(Pch)d,
(5)
where C is in the range 22-24 and d is in the range -0.2 to -0.3, for PCh' measured in mTorr. 4. Discussion Over the pressure range 1-3 mTorr, clear trends in all the characteristics were observed. However, at pressures
below this, conditions at the target and at the substrate changed rapidly, and trends were harder to determine. As stated earlier, the 1- V characteristics at the target were in very good agreement with Eq. (1). Other studies have shown that Vo decreases with increasing pressure, whereas f3 increases with pressure [4]. In this study, f3 was found to follow this trend, over the pressure range stated above. However. Vo for each target material remained almost constant over this range. Average values of Vo over this range for Zr, AI, eu and W were 215 V, 320 V, 319 V and 227 V, respectively. This may imply that Vo is a material property that varies with magnetron design and target condition. (No account was made in this study of changes in the 1- V characteris• tics as the target becomes eroded. In practice, changes
1375
P.J. Kelly, R.D. Arnell / Surface and Coatings Technology 98 (1998) 1370-1376
Table 2 Values obtained for a and b for Zr, AL, Wand Cu targets, at d.- t values of 80 mm, II0mm and 150 mm, where Is =mI, and m=a(Pchl Material Zirconium d._ t =80mm d._t=IIOmm d._ t =150mm Aluminium d._.=80mm d._ t =llOmm d._.=150mm Tungsten d._ t =80 mm d._.=IIOmm d._.=150mm Copper d._ t =150mm
a
b
0.264 0.235 0.218
-0.117 -0.118 -0.081
0.276 0.249 0.243
-0.153 -0.183 -0.126
0.263 0.25 0.217
-0.117 -0.309 -0.244
0.214
-0.198
in the characteristics have been observed, and a longer• term study is being carried out to quantify these changes.) The values obtained for Vo and f3 for Al were com• pared to published data for planar magnetrons [4]. At 3 mTorr, Vo was found to be slightly higher than pub• lished values (280 V), whereas, the published value of f3 was noticeably lower (75 A/ky 2) than that obtained from this study (204 A/ky 2). It would appear that f3 depends not only on target material and pressure, but also on the magnetron and system design. This study has shown that ion current density in a CFUBMS system is directly proportional to discharge current, but decreases with increasing pressure and d. - I ' in the manner described in Eqs. (3) and (4). There also may be a weak dependence on target material, but this is unclear at this stage. Studies of other planar unbalanced magnetrons have also shown the dependence
0.3 [ ' T " " " - - - - - - - - - - - - - - - - , , - - - - - - ,
--
ZJ Os-! = 80mm
0.28
Zr OS-I = 110mm
0.26
E
-+•
W Os-I = 110mm
-.-
0.24
W OS-I = 150mm
0.22
-3• Cu
OS-I = 150mm
0.2
AI Os-I = 110mm
0.18 0.16
... -+....
L-
~_.L.--~_"--~_.L._~ _
0.5
1.5 2 2.5 Pressure, mTorr
_'__...:.:::l
3
AI
Ds-I = 150mm ·····Ei·····
Fig. 7. Variation with target material and d.- t of the relationship between m and pressure, where m=Is/IT •
32 30
>
28
.,.
Zr,4A ds-I=80mm
\~ '~..'
Zr.8A ds-I= 110mm
~:1.
--•
-+•
.::.,
"!~
If! 26
.•
AI,8A ds-I=80mm
'~~:.
.....:.
-AI,8A
,'Y:.
c
ell
.,~.~.~~.::.:%f',.-.~~.-.~.-....
i5 24 c.
Ul
~ 22
,.!.
~ 20
>o."
dS-I= 110mm -[} Cu,4A ds-I= 150mm
eU,8A ds-I= 150mm
..........-
18
•
W.2A Ds-I = 110mm
16 0
0.5
1.5
2
Pressure. mTorr Fig. 8. Variation with target material,
2.5
3
W,3A ds-I = 150mm
d._. and IT of the relationship between pressure and substrate self-bias potential.
1376
P.l. Kelly. R.D. Arnell ( Surface and Coatings Teclm%gy 98 (/998) /370-/376
of ion current on discharge current [6,14], but no attempts were made to describe the relationships. Further studies [16, 17] have investigated the relationship between ion current and d.- t • As would be expected, for single magnetron systems, losses to the chamber walls result in a more rapid decrease in ion current with increasing d.- t , when compared to a dual 'closed-field' system. Substrate self-bias potential showed a strong depen• dence on chamber pressure, but was largely independent of target material, discharge current, or d.- t • The inde• pendence of V.b from IT has been reported elsewhere [6] for single magnetron systems. However, such systems also showed a strong decrease in V.b with increasing d.-I, [14], which was not observed in the CFUBMS system. Again, this is attributed to the strong confine• ment of the plasma between the targets and the sub• strate, preventing losses to the chamber walls.
5. Conclusions The 1- V characteristics of a CFUBMS system have been investigated. At the target, a good agreement with established relationships for other magnetron systems was observed. At the substrate, empirical relationships have been developed to describe both the ion current drawn and the self-bias potential, in terms of operating parameters, such as discharge current, chamber pressure and substrate-to-target separation. In both cases, the CFUBMS system showed less dependence on substrate• to-target separation, when compared to single magnet• ron systems. This study was purely empirical. In order to character• ize the CFUBMS system in more detail, Langmuir probe and mass energy analysis studies are being carried out
that will allow the observed relationships to be described in terms of plasma properties. Acknowledgement The authors would like to acknowledge the financial support given to this project by BNFL.
References [I] P.J. Kelly, R.O. Amell, Presented at ICMCTF'96. Accepted for
publication. [2] P.J. Kelly, O.A. Abu-Zeid, R.O. Amell, J. Tong, Presented at ICMCTF'96. Accepted for publication. [3] S. Maniv, W.O. Westwood, P.J. Scanlon, J. Appl. Phys. 53 (2) (1982) 856-860. [4] W.O. Westwood, S. Maniv, P.J. Scanlon, J. Appl. Phys. 54 (2) (1983) 6841-6846. [5] B. Window, N. Savvides, J. Vac. Sci. Techno\. A4 (2) (1986) 196-201. [6] B. Window, N. Savvides, J. Vac. Sci. Techno\. A4 (2) (1986) 453-456. [7] N. Savvides, B. Window, J. Vac. Sci. Technol. A4 (2) (1986) 504-508. [8] D.G. Teer, Surf. Coat. Techno\. 39(40 (1989) 565-572. [9) R.P. Howson, H.A. J'afer, A.G. Spencer, Thin Solid Films 193( 194 (1990) 127-137. (10) D.G. Teer, UK Patent GB 2 258 343 B, Magnetron Sputter Ion Plating, 1991. [II) W.O. Sproul, Surf. Coat. Techno\. 49 (1991) 284-289. (12) J.A. Thornton, J. Vac. Sci. Technol. II (1974) 666-670. [13] S.L. Rohde, W.O. Sproul, J.R. Rohde, J. Vac. Sci. Technol. A9 ( 1991) 1178-1183. (14) D. Monaghan, R.D. Amell, Surf. Coat. Techno\. 49 (1991) 298-303. [15) M. Konuma, Film Deposition by Plasma Techniques, Springer, Berlin, 1992. [16) J. Musil, S. Kadlec, Vacuum 40 (1990) 435-444. (17) J. Musil, S. Kadlec, W.-D. Munz, J. Vac. Sci. Techno\. A9 (1991) 1171-1177.