Study of a rf planar magnetron sputtering discharge: Discharge characteristics and plasma diagnostics

Vacuum/volume Printed in Great

45/number Britain

1 /pages

89 to 95/l

0042-207X/94$6.00+.00

994

@ 1993

Pergamon

Press Ltd

Study of a rf planar magnetron sputtering discharge : discharge characteristics and plasma diagnostics P-Y Jouan 7 lo-CNRS, received

and G Lemperiere, Laboratoire des Plasmas et des Couches 2 rue de la Houssinitire-44072 Nantes Cedex 03, France

23 October

Minces,

lnstitut

des Mate’riaux-UMR

1992

Electrical and Langmuir probe measurements and energy analysis of the ions impinging on the substrate electrode were used to characterize a rf planar magnetron sputtering discharge. The pressure range examined was 0.26-5.33 Pa in both argon and argon-nitrogen mixtures. The electrical measurements allowed us to determine the rf target voltage V,, and the self-bias target voltage V, as a function of rf input power, process pressure and dc substrate bias voltage. The electron density n,, the electron temperature T, and the mean plasma potential 0, were measured using Langmuir probe diagnostics. These time-averaged plasma parameters were investigated as a function of rf input power, process pressure and dc substrate bias voltage. An energy electrostatic analyser was used to study the energy of ions impinging on the negatively biased substrate electrode. The energy distributions depended on the voltage drop across the substrate sheath and on the collisions in the sheath. They were broadened by the rf modulation.

1. Introduction

The rfmagnetron sputtering is currently used today to obtain insulating thin films especially supraconducting films’.‘. However few papers deal with the characteristics and the plasma diagnostics of the rf planar magnetron discharge’ ‘. The modelling of this discharge is only beginning” and much work remains to be done. The purpose of this work is to determine the electrical characteristics of a rf planar magnetron sputtering discharge (13.56 MHz) in argon and argon-nitrogen mixtures, to measure the plasma parameters with Langmuir probes and to investigate the energy distributions of ions arriving at the dc biased substrate holder electrode.

I

2. Experimental

-PUMP

-/

The experimental system is shown in Figures I and 2. There are three chambers separately evacuated by turbomolecular pumps and arranged as follows : (I) a stainless steel magnetic discharge chamber, 320 mm in diameter and 330 mm in height is fitted with a 2 in. US GUN IIrM magnetron cathode cooled by water and with a substrate table electrode, 104 mm in diameter, negatively dc biased between 0 and - 200 V. The anode is formed by the walls of the discharge vessel maintained at ground potential. The residual pressure is 5x 10 ’ Pa. The cathode is capacitively coupled via a matching network (Advanced Energy ATX 600 model) in series with a 13.56 MHz rf power supply (Advanced Energy, RFX 600 model ; maximum power 600 W, output impedance 50 0). The cathode

-PUMP

2 POWER

Figure 1. Experimental

-

PUMP

OLTAGE ‘PLY

arrangement 89

P-Y Jouan

and G Lemperiere:

rf magnetron (his clcctrode is self-biased negatively as in a t-f diode discharge. The voltage of the rfclcctrodc may bc wriltcn :

r.f.voltage (13,56 MHz)

I ‘,)( /) = C’,,,+ I’,, sin (0t.

c

sightglass

substratetable

Figure

2.

The I’,, voltage wus measured with a I’6172 Tektronix probe (500 V, 100 MH/) connected to ;I Tektronix ozcilloscopc (50 MHz. 2225 model). The self-bias voltage I’~,,,was determined vi;1 a low pass tiltcr in sorics with a vollmctcr. WC have tried to measure the discharge current I,,,, sith a wide band currcnl transformer (Pearson. 2100 model). Lnfortunatel! the obtained values arc very high and probably wrong.

Electrical circuit.

is fitted with two samarium cobalt annular- mapnels applying a 0.045 T magnetic field near the target; (3) an intermediate chamber where the pressure is always less than lxl0 ‘, Pa. The ions extracted from the discharge pass through an orifice of 200 ilrn in diameter and depth 200 ltrn drilled in the centre of the substrate table electrode and enter the analysis chamber without acceleration. The substrate table electrode and the entrance slit of the analyser arc maintained at the same potential ; (3) an analysis chamber where the pressure is always I x IO ” Pa and which can be isolated from Ihe intcrmediatc chamber by a gate valve.

450

This chamber is fitted with a 90 deflection cylindrical electrostatic energy analyser and field correctors. This analyscr is the same as that used by Grolleau”’ and Turban ct lr/ ’ ’ in the same laboratory. The analyser mean radius of curvature is I’ = 50 mm. The entrance and cxil slits .r’ and .s” have the same width 0.2 mm. The analyser constant. which links the particle cncrgy E to Ihc voltage U applied to the sectors. is k’ = 9.09. The apparatus resolving power R = (E/A,!?) is constant and equal to 200. At the exit of the analyscr, ions arc detected by an cleclron multiplier (RTC channeltron X919AL model) conncctcd to ;I preamplifier. a counter and a DEC MINC 23 microcomputer 10 acquire mcasuremcnls. The target is made of titanium (purity 99.93%). The discharge gases arc argon (purity 99.9995%). They arc introduced \,ia calibrated Alcatcl leak valves and their flow controlled by Brooks Ilowmeters. The discharge pressure is mcasurcd by an MKS capacitance manometer. The discharge conditions arc as follows : rf power applied to the target : I-f power density on the target: argon pressure : nitrogen partial pressure : dc substrate bias voltage : distance between the electrodes :

1O-200 w 0.5-10 W cm 0.26~ 5.33 Pa

’

o--o.2 Pa O-200 v 50 mm.

3. Results 3. I. Electrical characteristics of the rf planar magnetron discharge. Since the target is capacitively coupled to the rf input generator. 90

I ‘I

I/,

Figure 3. (a) Values 01’1,, asa l‘unctlon 01‘the rf power- for 0ircc dilt‘creni pressures in lhc argon discharge : (0 : 0.65Pa: n : I .3i Pa : A : 2.7 Pa) (h) Values of c’,,as a function of the rl’pvucr for three diffcrcnt pl-cssurc, in the argon nitrogen discharge : (0 : 0.65 Pa : n : I .3?Pa; A : 3.7 Pa). (C) Values of I’,, as il function of the (rf power)’ ’ for three dill’erent pressures in the argon discharge. (0 : 0.65Pa : W : I .13Pa : A : 2.7 Pa).

P-Y Jouan

and G Lemperiere:

rf magnetron

We think this system is probably not adapted to this kind of measurement. Figure 3(a)-(c) shows the Vrf changes as a function of the rf input power in an argon discharge and in argon-nitrogen mixtures. It may be observed that : (1) The Vrf voltage is independent of the operating pressure in the studied range and increases with the rf input power P,. (2) The V,,- voltage is a little lower in argon discharge than in the argon-nitrogen discharge. because the change in the secondary emission coefficient of the nitrided target ’ *. (3) The Vrf voltage shows no significant dependence on the negative substrate bias voltage V,, when V,, is varied from 0 to - 200 v. (4) The V,f voltage exhibits a linear dependence on P)‘, a result also found in the rf diode discharges ’ 3. Figure 4(a) and (b) shows the self-bias voltage Vdc as a function of the rf input power P, in the argon and argon-nitrogen discharges. In ail cases Vdc < Vrrand Vdc increases with an increasing rf target power P, ; V,, is independent of the operating pressure ; V,, shows no dependence on the bias voltage Vb if V, is varied from 0 to -200 V. Figure 5(a)-(c) illustrates the change in the substrate electrode current I,, with the bias voltage V,, and with the rf input power P,. I,, increases rapidly for V,, in the range 0 to -20 V and very slowly for more negative bias voltages, both in the argon discharge and in the argon-nitrogen discharge.

100

80

2

60

.5 2

40

20

I

0

50

I

I

100

150

203

,

I

250

Vb ro 100

I

I Argon-Nimgen

(b)

o t

0

I

I

I

/

50

100

150

200

250

- Vb W) 70

I

/ Argon

I

60

(c)

10 1 0

PI

20

40

60

80

100

r.f. Power(w) (a) 0

20

40

60

80

loo

i/

I

1.f. Power ON)

260 r 240

I

I

I

Argon-Nitrogen

c

220

Figure 5. (a) Values of the substrate

current lh as a function of the bias voltage Vh for P = 0.67 Pa and for two different powers in the argon discharge: (0 : 50 W; n : 100 W). (b) Values of the substrate current Ih as a function of the bias voltage Vb for P = 0.67 Pa and for two different powers in the argon-nitrogen discharge: (0 : 50 W: n : 100 W). (c) Values of the substrate current &, as a function of the rf power for three different pressures in the argon discharge : (0 : 0.65 Pa ; n : I .33 Pa ; A : 2.7 Pa).

200 $ Y >v

180

120

(b)

,oo

t

L

0

40

60

80

100

1.f. Power (WI

Figure 4. (a) Values of Vdcas a function of the rf power for three different pressures in the argon discharge : (0 : 0.65 Pa; w : I .33 Pa ; A : 2.7 Pa). (b) Values of Vd,as a function of the rf power for three different pressures in the argons nitrogen discharge : (0 : 0.65 Pa; n : 1.33Pa ; A : 2.7 Pa).

I,, is strongly affected by the discharge pressure ; a decrease in the working pressure results in an increase of the substra:c electrode current. That seems to demonstrate an enhancement of the gas ionization when the working pressure is reduced. Ih also increases when the target rf power is increased, indicating that the gas ionization is also increased. In conclusion we can say that changes in the substrate parameters V, and I, do not affect the target parameters. 3.2. Plasma diagnostics with Langmuir probes. Few Langmuir probe measurements have been published for planar rf mag91

P-Y Jouan

and G Lemperiere.

rf magnetron

dccrcascs when

the prcswre

the simultaneous

dccrcase

The electron potcnti;ll

C)(a)

density

II, incrc;Iscs

V

then

All

and

the electrons

rctlcctcd X-Y Recorder

cleclrons

accclcratcd

towards

b> rapid discharge clTcct

the

h. In

plasma is obtained probe

consists

discharge parallel

with

axis at 25 mm

above

\urlitcc.

may

on

table

elcctrodc.

i\ out

near the target (Figure

probably

bc compared

of

ol‘thc

irapid

the

F,, dccrcases dischxy

slightly but

\vhcn

In this study thun

the

a~ailahlc”.

dischayc.

the

the

6). In this Ircgion.

tcmpcraturc

of tlic semi-lognrithmic ion

using_

potential

the clcctron line\

The

two

01‘ ;I 1-1‘

electron

The

when

i;hows

discharge.

electron

but dccrwscs

92

ol‘thc

h!

region. the

The

flwting

heating

to

~-cd

from

dischurgcs.

the I-1‘ power

whcrcas

in

;I saturation

value

I’oI-

due

to the change

nitridcd

in the

tai-get. in the argon

nitrogen

arpon

discharge

50 to 700 W. P,, measured

and cyual

r,> on the

7; ;lnd

nits-oyzn

0 to 4.6 cV in the

potential

constant

with rcachcs

TC is constant

incrcascs

Iniuturc

Figure

~)l‘

01‘ cxtrapolatcd

01‘ ,I~

argon

is perhaps

fI_om

plasma

remains

and

II,

coeficicnt

tempcraturc

yound

l‘unctmn

argon

II, incrwscs

decreases

mean

in the argon

plot

:

“’

the dcpcndcncc

saturation

the rl’powcr

The

mwn

The

I‘rom

M;IS clcancd

wire

discharpc

emission

and

the

prohc

the xamc wlucs.

I’,, in the

100 W. This

mlxturc

‘.

and rctwd;ltion

the c‘hcn relation

nitrogen

sccondury

apainst l‘rom

the semi-lofarithnii~

bc calculntcd

arc

bombardment.

7(a) -(c)

argon

P,, =

also

ctirrcnt

point

much

the slope

from

calcul:ltions’

l‘rom

saturation

give

r(‘ power

In the argon

;irc thcoriez

II, is detcrmincd

the intcrscction

ninq

r,,

methods

Figure

the

winy

tip of the titanium

input

01‘ the clcctron

Lafl-amboisc

electron

I ., using

The using

the

potential

potcnti;ll

the

radius

‘1;. is dcri\cd

density

I’,, is dcriked

current

I‘rom

plasma

plot

The clcctron

current

plxsnia

Larmor

so that the usual probe

radius

The clcctron

probe \oltagc.

and ion

clcclron

the probe

to

I8 V

with

respect

in the argon

to

nitropcn

27 to 24 V when the rl‘powcrincrcascs

from

dischzirgc.

8(a)

(c) illustrates

01‘ the disch;lrse

the ch:inges

tot;11 pressure.

in II,. The

r,

electron

and

1’,, as ;I

density

11~

llian

nc~

ICI ,_I arc

ionization\.

clc‘ctrodc

;il-i: also

to ioniz:Ltion muinl~

in ;I rl_ magnetron

is I erh

I’,, IS changed

;lrc‘ const;lnt

diode ctischargc. larger

tLli_&Jet:

electrons

‘tunnel

to the plasma

le\s

~~ 50

I’,, \oltagc\.

;lrc producctl

clcctrons

I’roni

in diamctct

is located

of the dischayc

probe

sccond;~ry

coming

1‘roni 0 to

induw

suhstratc

I:,, on the

The clcctron

and c;in contrihut~:

The cq lindrical

the substrate

In this region

;It the

discharge

the

0.5 mm

probe

( < 50 <;) and the

located

plasma

the plasma

prohc. Mire.

The

of the clcctr~~n

II, [or

12angmuir

tube.

to the target

density

tip of titanium

an alumina

mapnct~c field iy IOU ol‘densc

the evaluation

work

21sin@

(>(‘a 5 mm

with

shicldcd

this

r, and 01‘ the clcctron

cniittcd

number

7; and

ncgati\c

an cncl-g) can

I,,.

plasma


and

thins

can bc ncplcctcci.

7;. and

tcnipcraturc

more

thcq

the plasma

: but thcsc slcctrons

V in the argon 11c1r011 discharges'

I\ hcrc

mcxn

clectrodc.

WIICII I -,, i5 clxrn& I’or

uith

current

mluture.

ol‘~~.

txblc

haLin 9 acquired

Secondary

enhanccmcnt

substrate

constant

111the plasma

The

nitrogen

the depcndencc

slightly

Ircmains

is in ayxmcnt clcctrodc

const;mt.

in the argon

(c) shons I’,, 01‘ the

\o,ltqc

This

wbstratc

r, remains

f,, decrcascs

I~l~urc bi:is

tcmpcraturc

incrcascs. of the

(a)

t‘1roni 0 to

in the arpon-

700

nitrogen

P-Y Jouan

and G Lemperiere:

rf magnetron

In conclusion, the influence of the negative bias voltage Vh applied to the substrate table electrode is small in this discharge. The plasma parameters are mainly determined by the discharge voltage and the discharge current and consequently by the rf input power and also by the gas pressure. 3.3. Energy analysis of the ions arriving at the substrate table electrode. In front of the substrate table electrode, in contact with the plasma, an ionic sheath develops. The sheath thickness depends on the potential difference between the sheath boundary and the substrate electrode, on the ionic current density and on the plasma parameters. 3.3.1. Mean sheath thickness. The mean sheath thickness 4 may be estimated from different relations” ” which give similar results. For our experimental conditions, the d$ values are in the range 0.3-3 mm. The energy of the ions arriving at the substrate electrode depends on the potential difference between the plasma and the

.

l

.

l

.

.

l1 .

(4

Figure 9. (a) Values of the electron density II, as a function of the bias voltage V,, for a total pressure P = 1.33 Pa and 2 rf power P,, = 50 W in the argon and argon-nitrogen (85% Ar and 15% N2) discharges. (b) Values of the electron temperature T, as a function of the bias voltage Ifh, for a total pressure P = 1.33 Pa and a rf power P,, = 50 W in the argon and argon nitrogen (SS”/, Ar and 15% N2) discharges. (c) Values of the mean plasma potential v,_ as a function of the bias voltage I’,, for a total pressure P = 1.33 Pa and a rf power P,, = 50 W in the argon and argon-nitrogen (SS”/, Ar and 15% N?) discharges.

(b) substrate electrode and on the number of inelastic collisions the ions suffer in the sheath. The more important collisions are the symmetric charge transfer collisions for Ar + ions in which these ions lose all their energy. The Ar’ ion energy is transferred to a neutral Ar atom. To determine the Art mean free path j.,(Ar+) we have used the collision cross-sections measured by Cramer’“. The mean number of collisions in the sheath will be given by the ratio 4/j.,.

Figure 8. (a) Values of the electron density 11, as a function of the total pressure P, for a bias voltage Vh = ~ 50 V and a rf power P,, = 50 W in the argon and argon-nitrogen (85% Ar and 15% N2) discharges. (b) Values of the electron temperature T, as a function of the total pressure P, for a bias voltage L”, = ~ 50 V and a rf power P,, = 50 W in the argon and argon-nitrogen (85% Ar and 15% NZ) discharges. (c) Values of the mean plasma potential v,, as a function of the total pressure P. for a bias voltage C;,= -- 50 V and a rf power P,, = 50 W in the argon and argon nitrogen (85% Ar and 15% N?) discharges.

3.3.2. Modulation of the sheath thickness and of the ion energy. In this type of discharge. the plasma potential is modulated by the rf field and may be written :

VP(t) = Vpdc+ VP,,-sin tot, with Vpdc = v,, and V,,r, = k Vpdc. For a capacitive sheath k = I, for a partially resistive sheath k -c 1, as is the case here. This modulation of the plasma potential induces a modu93

P-Y Jouan

and

G Lemperiere

rf

magnetron

Iation ol‘the sheath thickness” and simultaneously of the cncrgy of ions crossing the sheath. The ion energy modulation results in a large broadening in the energy distributions with a spread AE given by the relation” :

In this discharge, mcnk the C:,,values from 0 to -200 V. relation are between 3.3.X

according to the Langmuir probe measureare in the range IX 2X V when v,, is \ariccl The AE values calculated from Ihc abo\c IO and 20 CV Ihr the AI-. ions.

Energy distributions of the ions arriving at the substrate

electrode.

(a) Influence of the bias voltage Ih. The ion cncrgy is dircctlq linked to the potential diffcrcncc (1’;,,- V,). Figure IO(a) shows the cncrpy distributions obtained at I/‘,,= 0 . ~ 100 and ~ 200 V ; LII If’, = 0 V the distribution is narrow und exhibits one pcuh ccntrcd around the energy cl’,,. When c’, incrcascs. a broadcnlng ofthc peak is observed due to the incrcasc ofthc collision numbci

in the sheath because the t/,/i., ratio increases [Figure IO(b)]. The distribution modulation appears for Vi, = 200 V [Figure IO(a)]. In the argon-nitrogen discharge, the distributions are widct hccausc the rf modulation of the nitrogen ion cncrgy is more important. indeed, I‘or the same I ‘,,value, the (I, ,i., ratio is highci than for argon ions. The obtained distributions ;lrc ~cry complex hccausc Ihc analyscr takes all ions (Ar +. Ar + ’ . N ‘. Nf ,) having the same energy. Besides. according to Grcenc C/ ~1” this complexity is also due to the diamctcr (200 q) or the cxtraction orifice which influences the distribution shape. The mean ion cncrgy is near c( rp - C’,,). (b) lnflucncc o1‘thc rl’input power. When the rl‘power applied 10 the target increases the energq spectrum broudcns and tv,o peaks are observed in the energy distribution [Figure I I (a)]. Thi, etrect is due only to the sheath modulation which incrcascs with the rl’ power while the t/,:2, ratio slightly decreases. This modulation is n&c apparent in the argon nitrogen discharge [Figure

1I (h)l. (c) Plasma potential

and ion cncrgy distributions. To dctcrmint the plasma potential C:,, from the ion encrpy distribution. WC LISC~the melhod proposed by Kiihlcr (‘r t/l” l’or lhc distributions in which a modulation is obscrvcd. The agrccmcnt bctuccn the values 1’1rom the energy discributions and the values obtained li-orn the Langmuir probe measurements is good (Table I ). It is ;IISO possible to dctcrminc r,, Yrom the thcorctical model ol‘thc rf diode discharge: indeed. if the cathode sheath IS pul-cl> capacitive then” :

(a)

P-Y Jouan

and G Lemperiere:

rf magnetron

Table 1. Comparison of the vp values obtained from the ion energy distributions obtained from the Langmuir probe measurements

Power (W)

Pressure (Pa)

Bias voltage (V)

V, (probe V)

VP(spectrum V)

50

1.33 1.33 1.33 1.33 1.33 1.33 5.32

0 -50 -100 -150 -200 -50 -50

33.8 26.9 23 26.7 27 24 28.6

29.1 21.7 25.8 22.5 22.5 25.8 20

50 50 50 50 200 50

If the cathode

with the values

sheath is purely resistive then14 :

References ‘N Homma. H Takahashi. S Okayama, T Morishita and S Tanaka, .I

In this rf magnetron discharge, the vp values determined from the energy distributions agree with the assumption of a cathode resistive sheath whereas with the assumption of a capacitive sheath before the target, the v,, values calculated are too high.

4. Conclusion Electrical and Langmuir probe measurements and energy analysis of the ions arriving at the substrate biased electrode have allowed us to characterize a planar rf magnetron sputtering discharge in the low pressure range. The electrical measurements show that the parameters of the rf electrode (target), rf voltage, rf current do not depend on the changes in the parameters of the substrate biased electrode. The electron density ncr the electron temperature T, and the mean plasma potential vp have been studied using Langmuir probes as a function of the input rf power, of the discharge pressure and of the bias voltage applied to the substrate electrode. The electron density n, increases with increasing rf power but tends towards a saturation value, and n, decreases when the working pressure increases. r, and V, remain constant in the argon-nitrogen discharge but decrease in the argon discharge with increasing rf input power. The substrate bias voltage Vb has little influence on the plasma parameters. The energy distributions of ions arriving at the substrate table electrode are broadened by the rf modulation. This rf modulation increases with increasing rf power and substrate bias voltage.

Mater Res, 7, 813 (1992). ‘Q X Jia, Z Q Shi and W A Anderson, Thin Solid Films, 2, 230 (1992). ’ i Holland, Thin Solid Films, 86,227 (I 98 I). “G Samuel and L Holland, 8th Int Vacuum Cong September 1980 Cannes, Le Vide-Le.7 Couches Minces, Suppl201, 34 (1980). ’ S Maniv and W D Weswood, J Vat Sci Tech&, 17,743 (1980). ‘R K Waits. J Vat Sci Tech&. lS(21, 179 (19781. ‘J G Cook, S R Das and T A QuaAce. J Aipl PI&s, 68, 1635 (1990). ‘H D Lowe, H H Goto and T Ohmi, J Vat Sci Technol, A9, 3090 (1991). ’ M A Libermann, A J Lichtenberg and S E Savas, IEEE Tram Plasma sci, 19, 189 (1991). “‘B Grolleau, Thise de Doctorat d’Etat, Nantes (1973). ” G Turban, B Grolleau, P Launay and P Briaud, Rrc Phq’s Appl, 20, 609 (1985). “H D Hagstrum. Ph?;.s Rec. 104,317. 672, 1516 (1956). lZA J Roosmalen, W G M Van den Hock and H Kalter, J Appl Phys, 58,653 (1985). lJB Lipschultz, I Hutchinson. B Labombard and A Wan, J Vat Sci Technol, A4, 1810 (1986). “J G Laframboise, Rapport No 100, Universitk de Toronto (1966). “F Chen Plasma Diagnostics Techniques. Chap 4. Academic Press, New York (19i5). “J Langmuir, Phys Rev, 26, 585 (1925) : 33, 954 (1929). Ix S G Ingram and N St J Braithwaite, J Appl. Phys, 57, 59 (1984). ” E Badareu and I Popescu, Ga: lonisk. Dunod, Paris (I 968). “‘W H Cramer, J Chem Phys, 30, 641 (I 959). “K KGhler, J W Coburn, D E Horne, E Kay and J H Keller, J Appl Phys, 57, 59 (1984). 22J W Coburn and E Kay, J Appl Phys, 43( I2), 4965 (1972). “W M Greene, M A Hartney, W G Oldham and D W Hess, J Appl Phys. 63, 1367 (1988). 24K K(ihler, D E Horne and J W Coburn, J Appl Phys, 58,335O (1985). *’ B N Chapman Glo,r Discharge Procrs.w.s, p 41, Wiley-Interscience. New York (1980;.

95