Unusual behaviour of current–voltage relations in an unbalanced grid-assisted magnetron

sputtering system

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2012 J. Phys. D: Appl. Phys. 45 505204

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 45 (2012) 505204 (5pp) doi:10.1088/0022-3727/45/50/505204

Unusual behaviour of current–voltagerelations in an unbalanced grid-assistedmagnetron sputtering systemJ C Sagas2, D A Duarte1,2 and L C Fontana1

1 Santa Catarina State University, Plasmas Physics Laboratory, 89223-100, Joinville, SC, Brazil2 Technological Institute of Aeronautics, Plasmas & Processes Laboratory, 12228-900, S. J. dos Campos,SP, Brazil

E-mail: juliocesarsagas@gmail.com

Received 2 July 2012, in final form 30 October 2012Published 20 November 2012Online at stacks.iop.org/JPhysD/45/505204

AbstractCurrent–voltage relations were measured in an unbalanced grid-assisted magnetron sputteringsystem at different inter-electrode distances and magnetic field configurations. The intensityand configuration of the magnetic field were adjusted through an electromagnetic coil placedat the magnetron centre. This scheme allows observation of the current–voltage relationsunder conditions of weak and strong plasma confinement. Surprisingly, for someinter-electrode distances, a decreasing discharge current with increased magnetic field wasobserved. This unusual behaviour in grid-assisted magnetron discharges can be useful for abetter understanding of abnormal glow discharges assisted by a magnetic field.

(Some figures may appear in colour only in the online journal)

1. Introduction

The confinement of abnormal glow discharges by aninhomogeneous magnetic field is the key for high-rate filmdeposition by sputtering. The plasma confinement allows theachievement of high electron densities and, as a consequence,high sputtering rates. This strong confinement is reflectedon the current–voltage (I–V ) relations by a high slope,which was called by Thornton ‘magnetron mode of operation’[1]. Other factors also contribute to the behaviour ofI–V relations, such as the secondary electron yield [2],recapture of secondary electrons [3], gas rarefaction [4, 5],loss of high-energy electrons [6, 7] and sheath ionization [2, 5].However, despite all knowledge about magnetron discharges,the I–V relations are not fully understood, specially dueto observations of unusual behaviour, such as the hysteresisin I–V curves taken under non-reactive discharges [8] and‘flattened curves’ measured in weak magnetic fields [1, 6] andHiPIMS discharges [9].

The complexity of the phenomena occurring in magnetronsputter deposition makes the derivation of an analyticalexpression, which describes the I–V relations, a hard task. Themost recent attempt for balanced magnetrons is the equation

proposed by Musschoot et al [5] considering the recapture ofsecondary electrons and sheath ionization. By considering theloss of high-energy electrons this equation was extended forunbalanced magnetrons as [6]

J = (1 + γeff)4

9ε0αW

(2e

M

)1/2V 3/2(

W

γeff− V εeεi

) (1)

with J the target current density, ε0 the vacuum electricalpermittivity, α a constant related to sheath ionization, e

the fundamental charge, εe the fraction of ions producedon average by a secondary electron before being lost, εi

the coefficient of ion collection, W the effective ionizationpotential in volt [10], M the ion mass and γeff the effectivesecondary electron yield by ion bombardment. The latter isrelated to electron recapture, thus, it is a function of the workingpressure, magnetic field and gas rarefaction in front of the target[4, 5]. Equation (1) was obtained from the Child–Langmuirlaw, which, in theory, is not directly applicable to magnetrondischarges, once the effect of the inhomogeneous magneticfield in charge transport is not considered. On the other hand,the magnetic field effect is included into α, εe andγeff ; however,

0022-3727/12/505204+05$33.00 1 © 2012 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 45 (2012) 505204 J C Sagas et al

Figure 1. Grid-assisted magnetron sputtering system.

the experimental determination of these three parameters isnot always an easy task. The radial inhomogeneity of J isnot considered in this derivation. Another problem, relatedto equation (1), is the lack of a minimum cathode voltagenecessary to sustain the discharge.

In this way, empirical equations are the most used to fit theI–V relations in magnetron sputtering discharges. Westwoodet al [11] suggested the following one:

I = β(V − V0)2, (2)

where V is the cathode voltage, V0 is the minimum cathodevoltage necessary to sustain the discharge and β is a functionof the plasma-forming gas, material target, working pressure,system geometry and magnetic field, being a measure of thesteepness of the I–V curve, which is related to the efficiencyin trapping free electrons. For this work, in particular,equation (2) was used due to the better agreement withexperimental data in comparison with the fits obtained fromthe classical Thornton equation (I = kV n) [1].

To study the fundamental aspects of magnetrondischarges, a system that allows modifications in dischargeconfinement (εe) without changes in the magnetic fieldis necessary. One option is the grid-assisted magnetronsputtering (GAMS) system, also called triode magnetronsputtering in previous papers [6, 12–21]. In GAMS, agrid is inserted between the target and the substrate [12].When the grid is kept grounded it becomes the dischargeanode and the inter-electrode distance can be changed,leading to a modification of plasma volume and conductivity.This geometry allows a better confinement compared withconventional magnetron systems [18]. Then, in this work,the GAMS system was used to study the I–V relations ofmagnetron sputtering discharges for different magnetic fieldconfigurations and grid-to-target distances (dg–t). The maingoal of this paper is to investigate the behaviour of I–V

relations for plasmas confined between the grid and the target.

2. Experimental setup

The deposition system used in this work is schematicallyshown in figure 1. The circular planar magnetron has adiameter of 125.0 mm. The central magnetic field wasproduced by an association of SmCo permanent magnets anda coil (with approximately 310 turns), in which it was possibleto input a dc current of up to 20 A. Thus, modificationsin the magnetic field were possible by adjusting the coilcurrent (Ic). The outer magnetic field was generated by aset of 16 SmCo magnets. An aluminium target (99.5%)was used as the sputtering source. The anode (groundedaustenitic stainless steel grid) does not affect the magneticfield distribution. The grid-to-target distance (dg–t) could bechanged from 10.0 to 50.0 mm. The vacuum chamber wasevacuated to a base pressure of 4 × 10−3 Pa (3 × 10−5 Torr),and afterwards, Ar (99.99%) was injected until a workingpressure of 0.4 Pa (3 mTorr). The target-to-substrate distancewas fixed at 60.0 mm. Four magnetic field configurations(corresponding to coil currents of 0.0, 5.0, 10.0 and 15.0 A)were mapped using a Hall probe at a height of 6.0 mm abovethe target surface. From the mapping of the parallel and normalcomponents of the magnetic field it was possible to determinethe absolute value of the magnetic field vector (magnetic fieldintensity).

Figure 2 illustrates three possible configurations of themagnetic field that can be obtained by adjusting the coil current.In figure 2(a), for Ic = 0, the case where field lines are driventowards the chamber walls is shown, and a few lines are driventowards the grid in this case. Figure 2(b) shows the case wherethe coil current has an intermediate value and contributes todirect field lines towards the substrate surface. In figure 2(c),the coil current is increased and the magnetic field strength, inthe central coil, contributes to close most of the field lines.

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J. Phys. D: Appl. Phys. 45 (2012) 505204 J C Sagas et al

Figure 2. Magnetic field configurations for (a) coil current equal to zero, (b) low coil current (up to 10 A) and (c) high coil current (higherthan 10 A).

Figure 3. I–V relations of the discharge with coil current equal to (a) 0 A, (b) 5 A, (c) 10 A and (d) 15 A for three different grid-to-targetdistances. The respective lines are the fits obtained from equation (2).

3. Results and discussion

3.1. General features

I–V curves taken at different coil currents and grid-to-targetdistances are summarized in figure 3. Most of the curves can

be fitted by equation (2). For grid-to-target distances of 20and 25 mm, some deviations can be observed for Ic = 0. Thedischarge current for Ic = 15 A anddg–t = 15 mm saturates forcathode voltages greater than −500 V, while for other grid-to-target distances the target current still increases with increasingvoltage.

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J. Phys. D: Appl. Phys. 45 (2012) 505204 J C Sagas et al

Table 1. Values of β, for different magnetic field configurations andgrid-to-target distances, according to the fit using equation (2).

β (10−5 A V−2)

dg–t (mm) Ic = 0 A Ic = 5 A Ic = 10 A Ic = 15 A

15 1.8a 2.3 4.0a 3.7a

20 2.8a 5.3 8.3 10.425 4.3a 8.1 10.5 13.5

a Equation (2) does not fit the experimental data accurately(see figure 3).

Figure 4. Discharge current as a function of coil current fordifferent grid-to-target distances at V = −500 V.

In figure 3, an increase in the I–V curve steepness anda shift towards lower voltages are observed with increasedgrid-to-target distance. This behavior can be understood fromequation (1). For a small grid-to-target distance, high-energyelectrons are collected by the grid, thus reducing the coefficientεe and resulting in a lower current for a given voltage.Increasing the grid-to-target distance, a higher number ofelectron–ion pairs are created per electron emitted from thetarget before they are collected by the grid. Because of this,the coefficient εe and the target current increase for a givencathode voltage [6].

In table 1, an increase in the magnetron efficiency (β)is observed for dg–t = 20 and 25 mm, as the coil currentincreases. This effect can be explained by considering thatthe increase in the coil current leads to an increase in themagnetic field intensity, which in other words means anincrease in the plasma confinement. In the case for weakmagnetic fields (Ic = 0) and/or small inter-electrode distances(dg–t = 15 mm), equation (2) does not fit the experimental dataaccurately; see table 1 and figure 3. Under these conditions,more energetic electrons (secondary electrons) are lost justafter a few ionizing collisions with neutral gas atoms (low εe).

3.2. Decreasing discharge current with increased magneticfield

The discharge current as a function of coil current at differentgrid-to-target distances is shown in figure 4. It is noted that,

Figure 5. I–V relations for different coil currents. Grid-to-targetdistance was fixed at 15 mm.

Figure 6. Mapping of the magnetic field intensity at a height of6.0 mm above the target, for different coil currents.

for large grid-to-target distances (dg–t = 20 and 25 mm), thedischarge current increases as a function of coil current. On theother hand, for small grid-to-target distances (dg–t = 12 and15 mm), the discharge current decreases with increased coilcurrent and for dg–t = 18 mm, the discharge current initiallyincreases and then decreases. In fact, one can observe that thetarget current for Ic = 15 A is lower than that for Ic = 10 Aat V > −500 V and dg–t = 15 mm; see figure 5. The increasein the coil current leads to an increase in the magnetic fieldintensity in front of the target (figure 6), as a consequence,an increase in the plasma confinement and target current for agiven voltage is expected. In this way, a decrease in the targetcurrent, for increased coil current and small inter-electrodedistances, is a very interesting and surprising result.

It must be pointed out that this behaviour is observedonly for small grid-to-target distances, i.e. in situations

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J. Phys. D: Appl. Phys. 45 (2012) 505204 J C Sagas et al

Figure 7. Interception of magnetic field lines by the grid.

where the loss of high-energy electrons is high. Thus, areasonable explanation for this issue can be found from theloss mechanism of high-energy electrons. For increased coilcurrent, not only does the field intensity change, but also theshape of the field, as depicted in figure 6. On increasing thecentral magnetic field, more field lines tend to close betweenthe outer and central magnets; however, if the grid is nearthe target, it intercepts some field lines, thus intercepting theelectron trajectories; see figure 7. These interceptions leadto an increased loss of high-energy electrons, thus decreasingthe discharge current. When the grid is far from the target,most of the field lines close without being intercepted by thegrid so that the increased magnetic field (and improved plasmaconfinement) leads to a reduced loss of high-energy electrons,therefore increasing the discharge current.

4. Conclusions

Current–voltage relations for a grid-assisted magnetronsputtering system were investigated through modifications inmagnetic field configuration and grid-to-target distance. It wasverified that the variation of grid-to-target distance modifiesthe effect of an increased magnetic field. For small inter-electrode distances, the grid intercepts the field lines, andtherefore the electron trajectories. Thus, if the magneticfield is highly unbalanced, an increase in the central fielddrives free electrons towards the grid, increasing the loss ofhigh-energy electrons and decreasing the discharge current.In these situations of high loss of energetic electrons, theempirical equation (2) cannot be used to fit the experimentaldata. However, for large inter-electrode distances, field lines(electron trajectories) are not intercepted by the grid, leading tothe conventional behaviour of increased current with increasedmagnetic field, and equation (2) fits the experimental dataaccurately. The strong dependence of plasma characteristicson the grid position can also be observed by the increase indischarge current up to almost three times with the increasein the grid-to-target distance from 15 to 25 mm using a coilcurrent of 15 A. Then, the geometry of the GAMS systemallows a greater flexibility regarding the discharge parameters,making this system a feasible tool for basic studies about theprinciples of magnetron sputtering discharges.

Acknowledgments

The authors acknowledge the financial support from theBrazilian Research Agencies CNPq, CAPES and FAPESP.

References

[1] Thornton J A 1978 J. Vac. Sci. Technol. 15 171[2] Depla D, Mahieu S and De Gryse R 2009 Thin Solid Films

517 2825[3] Buyle G, De Bosscher W, Depla D, Eufinger K, Haemers J and

De Gryse R 2003 Vacuum 70 29[4] Rossnagel S M and Kaufman H R 1987 J. Vac. Sci. Technol. A

5 2276[5] Musschoot J, Depla D, Buyle G, Haemers J and De Gryse R

2008 J. Phys. D: Appl. Phys. 41 015209[6] Sagas J C, Fontana L C and Maciel H S 2011 Vacuum 85 705[7] Depla D, Buyle G, Haemers J and De Gryse R 2006 Surf.

Coat. Technol. 200 4329[8] Musschoot J, Depla D, Haemers J and De Gryse R 2008

Plasma Sources Sci. Technol. 17 015010[9] Capek J, Hala M, Zabeida O, Klemberg-Sapieha J E and

Martinu L 2012 J. Appl. Phys. 111 023301[10] Lieberman M A and Lichtenberg A J 1994 Principles of

Plasma Discharges and Materials Processing 1st edn (NewYork: Wiley)

[11] Westwood W D, Maniv S and Scanlon P J 1983 J. Appl. Phys.54 6841

[12] Fontana L C and Muzart J R L 1998 Surf. Coat. Technol.107 24

[13] Fontana L C and Muzart J R L 1999 Surf. Coat. Technol.114 7

[14] Recco A A C, Lopez D, Bevilacqua A F, Silva F andTschiptschin A P 2007 Surf. Coat. Technol. 202 993

[15] Recco A A C, Oliveira I C, Massi M, Maciel H S andTschiptschin A P 2007 Surf. Coat. Technol. 202 1078

[16] Recco A A C, Viafara C C, Sinatora A and Tschiptschin A P2009 Wear 267 1146

[17] Devia D M, Restrepo-Parra E, Arango P J, Tschiptschin A Pand Velez J M 2011 Appl. Surf. Sci. 257 6181

[18] Sagas J C, Duarte D A and Fissmer S F 2011 Vacuum 85 1042[19] Soethe V L, Nohara E L, Fontana L C and Rezende M C 2011

J. Aerospace Technol. Manag. 3 279[20] Sagas J C, Duarte D A, Irala D R, Fontana L C and Rosa T R

2011 Surf. Coat. Technol. 206 1765[21] Duarte D A, Sagas J C, Fontana L C, da Silva Sobrinho A S

and Cinelli M J 2010 Eur. Phys. J. Appl. Phys. 52 31001

5