Optical spectroscopy for analysis in rf sputtering of CoCr

•1• ,~

ELSEVIER

Journal of magnetism and magnetic mMerlals

Journal of Magnetism and Magnetic Materials 151 (1995) 87-94

Optical spectroscopy for analysis in rf sputtering of CoCr Shigeo Honda * Physical Electronics, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739, Japan Received 1 February 1995; in revised form 17 April 1995

Abstract The optical emission spectra in plasma have been studied during sputtering of Co, Cr and CoCr. The emission intensities of Co and Cr increase with rf power (W). However, the intensities relative to the sputtering amounts (I c° and I cr) are independent of W. The value of I C° increases linearly with Ar gas pressure (PAr), while lrcr is proportional to P~. If a negative bias voltage is applied to the substrate or a magnetic field is applied perpendicular to the electrodes, the emission intensities are remarkably enhanced by multiplication of secondary electrons or cyclotron motion of the electrons.

1. Introduction

inelastic electron collision. According to Green et al. [7], the emission intensity is given by

Magnetic and crystallographic properties of CoCr sputtered films vary considerably depending on sputtering conditions: substrate temperature [1], input rf power [1], pressure of sputtering gas [2], magnetic field applied normal to substrates [3,4] and negative substrate bias voltage [4-6]. The variation in the properties is related to the kinetic energy or the velocity of sputtered atoms arriving at the substrate surface, and to the sputtering rate or deposition rate. For a sputtered atom (or ion) 'a', the velocity, v ~, which is a function of the distance from the target surface, can be evaluated from the optical emission i n t e n s i t y , i a , radiated from a t o m / i o n ' a ' excited by

i a = N a . p(a). T a ) .

* Email: 227195.

[email protected]; fax:

+ 81-824-

(a = Co, Cr)

(1)

where K a is a constant depending on the measuring system and optical wavelength, p(a) the probability of exciting a t o m / i o n ' a ' to the excited state i, being proportional to both the cross section for the excitation collision and the spatial electron density, and T~j) the radiative transition probability from the excited state i to the lower state j related to the measuring optical wavelength. N a is the number density of a t o m / i o n 'a', and it is given by Na=ga.fa/(6.ua).

(2)

Here, fa is a function of the spatial distribution of a t o m / i o n 'a', 6 the observed cross-sectional area of the discharge slice, and S a the total amount of a t o m s / i o n s sputtered per hour. Thus N a, and s o /a, are proportional to the total amount of sputtered

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved

SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 3 9 9 - 1

K a

S. Honda/Journal of Magnetism and Magnetic Materials 151 (1995) 87-94

88

Electrode(Substrate)

• lWater

~

A.rlOOm~'~

VB I l l _ ! ! 0~i

Target

OOom

Water

Fig. 1. Systemfor sputteringand opticalmeasurements. generated by a coil placed under the target was applied normal to the electrodes for focusing the plasma. Optical emission radiated near the center (about 2.5 cm apart from the target surface) between both electrodes was guided to a monochromator placed outside of the chamber (Fig. 1) and analyzed in wavelength range of 360-660 nm. The sputtering rate was determined by the decrease in the weight of Co and Cr tips put on the disk target. The composition of CoCr film deposited from the composite target was measured by Particle Induced X-ray Emission (PIXE).

atoms/ions 'a', and to the reciprocal of the atomic velocity, namely

I a= S a .fa.p(a). T(~j)"Ka/( 3. ua).

(3)

In this paper, we discuss the optical emission in rf sputtering of Co and Cr with various conditions.

2. Experiments Optical spectroscopy was performed using an rf diode sputtering system (Fig. 1), whose electrodes of 100 mm in diameter were about 41 mm apart. Three types of targets were used: Co and Cr disks, and also a composite CoCr target in which Cr tips were put on a Co disk with an area of 13%. Sputtering was done in Ar gas with PAr = 5--50 mTorr with rf power, W, ranging from 50 to 250 W. Negative bias voltage ( - V B = 0 - b 1 2 0 0 V) was applied to the substrate, and a magnetic field ( H - 0 - 1 1 6 Oe) ¢

~o

'

3.1. Emissionspectra Typical emission spectra are shown in Fig. 2, where the CoCr composite target was sputtered with ICr

CoszCr13 ~o~,

'

3. Results and discussion

]1

~;o

'

560

'

5;o

'

6;0

'

6;0

Wavelength (.,,0 Fig. 2. Emissionspectrafor sputteringthe compositetargetof Co87Cr13

at

W= 100 W, PAr = 30 mTorrand Va = H = 0.

S. H o n d a / J o u r n a l of Magnetism and Magnetic Materials 151 (1995) 87-94

C°87Cr13

,.Co OAr

/ / /

/ i

89

C°87Cr13

"

/

/ 150

.//?

I

C

,ool

/// s°

./

50

Cr 50

Io0

150

200

250

W (w)

0

i

G05

~

QO

i

015

i

020

i

025

SCo , SC, (~a,o,,,)

W (,0

Fig. 3. Rf power dependence of emission intensities (a) and sputtering rate (b), and the relation between the emission intensities and the total amounts of sputtered atoms per one hour (c).

100 W rf power at 30 mTorr under VB = 0 and H = 0. The peaks at 412, 420, 423 and 473 nm arise from Co atoms [8], those at 425, 427, 429 and 521 nm from Cr atoms [8], and others from Ar. In this figure, any emission spectra from Co or Cr ions are not detected. The shape of the spectra was independent of sputtering conditions, and then we evaluated the emission intensities here using the peak heights of Co (412 nm), Cr (521 nm) and Ar (556 nm).

3.2. Rf power dependence Emission intensities of Co, Cr and Ar (/Co, icr and iar), and also the sputtering rate per unit target surface area (erCo and o-cr) are plotted in Figs. 3(a) and (b), respectively, as a function of input rf power, W, where Co, Cr and CoCr targets were sputtered at 10 mTorr under VB = 0 and H = 0. The emission intensities and the sputtering rate increase linearly with W. The linear relation between the sputtering rate er a and W is consistent with a discussion reported by Shinoki [9], who derived theoretically the relations; eracI W 0'8 for lower pressure, and o'act W for higher pressure. Thus, as indicated in Fig. 3(c), the intensities/Co and i f r are proportional to the total amounts of sputtered Co and Cr, S c° = Aco. erCo and S Cr = A Cr - o "Cr, where A c° and Acr are the surface areas of Co and Cr targets, respectively. The linear relation between I a and S a means that the value of f~ "P(~i)"Teaj)~ va is constant, independent of W in Eq. (3). In general, the electron and

Ar ÷ ion densities increase with W, resulting in an increase in sputtering rate as shown in Fig. 3(b) and also an increase in the number of collisions between sputtered atoms and electrons, which brings about the enhancement of P(~). Therefore, the constant value of f"'P(a)'T(~)/va means that the atomic velocity, v a, increases proportionally to P(]) with W, because f a . T~j) is considered to be nearly constant. This is consistent with more detailed data [10] on Ag atoms sputtered with Hg + ions. The film morphology and the crystallographic structure might be affected by the slight increase in the velocity or the kinetic energy of the sputtered atom, a n d / o r by the increase in the deposition rate. In particular, Coughlin et al. [1] pointed out that the grain size and the (002) XRD peak intensity become larger with the deposition rate. , 60C

(a)

,

, , ,,

Co87Cr13

2/40O

| W=lOOw ~/

W=I OOw 3oc

. . . .

6°° Cotarget ar~ t 500~I Cr

50C ~40C

'(b) . 400F

ir2 ::l

/

~" 20C

1600 1200

ool

,3

2OOO

800

10C

oo

5

I0

30 50

PAr (reTort)

0 5

10

30 50

PAr (mTorr)

Fig. 4. PAr dependence of emission intensities for sputtering the C087Cr13 composite target (a) or Co and Cr targets (b) at W = 100 W a n d VB = H = 0 .

S. Honda/Journal of Magnetism and Magnetic Materials 151 (1995) 87-94

90

i

3.3. Ar gas pressure dependence The pressure dependence of I a is plotted in Fig. 4 for W = 100 W and VB = H = 0 as a function of Ar pressure, PAr" The emission intensity for At, I Ar, changes slightly with Par by the relation of IArat p~58. On the other hand, /Co and I cr increase steeply with PAr" In particular, the increment of I ° is very large for PAr higher than 20 mTorr. As suggested from Eq. (3), the increment of I a is probably related to the change in S a (or o"a) and v a, which have been discussed by Shinoki [9]. Therefore, we measured the Par dependence of o-a, and show the results in Fig. 5. The values of o-C° and o-Cr become larger with Par up to 20 mTorr, and then saturate or decrease. The behavior of o-a can be explained by the variation in the cathode dark space with Par [9]. The emission intensities relative to the total amounts of sputtered atoms, Irc° = I c ° / S c° and I ° = I C r / s cr, can be replotted against PAr as shown in Fig. 6. In logarithmic scale, the relation between I a and Par is linear, and given by Irc°

cI PAr

and Ircr ~ P2 r .

(4)

In Eq. (3), the quantities of fa'T(aij ) ' K a and 6 depend only little on Par, and then we can write I a cx P(])/v a. Thus, Eq. (4) can be rewritten as

I

I

I

0003-o Cr (CoB/Cr13) ACr •

i

I

i

Co(C%C%)

,

4

,

i

,~ ~/ 't ~

30( i "Co(C%TC%)

,," y

10 o / ° 5 ¢ W=lOOw I

I

5 10

I

I

I

I

30 50

PAr (mTorr) Fig. 6. Relative emission intensities per sputtered amounts versus PAr for sputtering at W = 100 W and Va = H = 0.

Since the excitation probability p(a) depends on the spatial electron density and the kinetic energy of electron in the plasma, the values of P(iC)° and P(iC)r will show the same pressure dependence. Therefore, Eq. (5) means that the velocity or the kinetic energy of Cr changes with Par much more than that of Co. The kinetic energy of the sputtered atom decreases during travelling in sputtering Ar gas because of the collision with Ar, and the value at the position x from the target surface given by [9]

E"( x) = ( E~) - kaTAr) exp( --~aPnr~ax/kaTAr )

(5)

P(CC/vC° o(PAr and P(C~/v°c~Pdr"

,

ocr (coj%) 50( . ~ cr

+ ksTAr,

(6)

where ksTgr is the thermal energy of Ar gas, E~ the initial energy of the atom 'a' at x = 0, ~a the cross section of the collision between the atom 'a' and Ar, and ~ a the rate of the energy loss per one collision. The value of ~ a is given by [9] C a = ln(gia//E~)

~_ 0.002

= 1 - {(M a - 1 ) Z / 2 M a} In I ( M a + 1)

/(g b

0.0 01

W= l OOw

o

;

;o

'

3'o '50

PAr (mTorr) Fig. 5. Relation between sputtering rates and A r gas pressure for sputtering at W = 100 W and Vn = H = 0.

a - 1)l-

(7)

Here, E a and E~ are the atomic energies before and after collision, respectively, and M a is the mass ratio of the sputtering Ar gas to the sputtered atom 'a'. The values of ~Co and ~ Cr are calculated as 0.877 and 0.914, respectively. The larger value of ~Cr means that the PAr dependence of the kinetic energy at the position x in Eq. (6) is larger for Cr than for Co, being consistent with the data of Fig. 6.

91

S. Honda//Journal of Magnetism and Magnetic Materials 151 (1995) 87-94

81 ~ 261 2

5 ~;,

~ / R C r

O.i

~

,.~ ~

L~ 22

1

"~

~.o

o

, ~.:~%. °,% %) i 89 Oe)

C~."~

a~o'/

Co (v,=o)

Co (.=o;

,

,3

t~ " O.OE

~ 0.05l

Cr (H=89o,)

Cr (vo=-lOOv)

0.5 ~ 2O

Cr (H=o)

I

-6 -%0 -~o -2~o

0

L

,

i

i

L

i

20

40

60

BO

100

120

H (Oe)

v~ (v) 16~

t

5

~

10

~

30

L

i

50

0

Fig. 8. Total amounts of sputtered Co and Cr versus VB (a) and H (b) for sputtering at 100 W and 10 mTorr.

PAr (mTorr)

Fig. 7. PAr dependencies of the relative sputtering rate of Cr (R cr ) and the Cr content in the deposited films (C ° ) for sputtering the composite target Co87Cr~3. The ratio c c r / R c~ is also shown here.

When PAr becomes larger, the Cr energy decreases steeply. This causes the increase in the number of collisions between Cr and Ar with higher rate than that for Co, and brings about the frequent scattering of Cr more than Co. In other words, the amount of Cr reaching the substrate decreases with PAr more than Co. Namely, the Cr concentration, C cr, decreases with PAr in the film deposited from the composite target. In Fig. 7, however, the value of C cr increases with PAr higher than 10 mTorr. This arises from the increase in the sputtering rate of Cr, as plotted also in Fig. 7; the relative sputtering rate of Cr, R c~ = s c r / ( S c° + sC0, increases with PAr higher than 10 mTorr. However, as inferred above, the ratio of c c r / R cr decreases monotonically with increasing PA,.

3.4. Ve- and H-dependencies The total amounts of sputtered Co and Cr per hour, S c° and S c', are shown in Fig. 8 as a function of the negative bias voltage, --VB or the focussing field, H, for sputtering at 100 W and 10 mTorr. With increasing - V B a n d / o r H, the quantities S c° and S c' become larger, except for a slight decrease at H = 61 Oe for the S a - H curves of VB = 0 in Fig.

8(b). The increase in the sputtering rate might be caused by the multiplication of Ar + ions; When - V B is applied, a lot of secondary electrons are emitted from the substrate surface bombarded by Ar + ions accelerated with - V B , and promote the ionization of Ar. And the focussing field H brings about a cyclotron motion of the electrons, which causes also the multiplication of Ar + ions due to the frequent collision with Ar atoms. The increase in the number of collisions can be confirmed by the change in the Ar emission intensity, IAr. As shown in Fig. 9, IAr becomes intense with - V B a n d / o r H owing to the frequent collision.

150

'

'

"

H=89 oe

•

¢ /

•

~00

&

. .....

"'" . . - "

- -

H=6]

f

5C i" r"" -0- - -0- - - 6 - - -C

H=O 5~0

-,oo ~ -~so ~ -2oo VB (v)

Fig. 9. I m versus VB with H as a parameter for sputtering at 100 W and 10 mTorr. The broken lines indicate the calculations for Eq. (9).

S. Honda/Journal of Magnetisra and Magnetic Materials 151 (1995) 87-94

92

,

Cuomo et al. [11] reported that the spatial density of Ar ÷ ions (n ÷) increases with - VB by the relation: n + = n~- (1 + m I I VB I q),

(8)

where n~ is the Ar + density at VB = 0, and m 1 and q are constants. The electron density in the plasma increases also with the same relation as Eq. (8). If H is additionally applied, the number of collisions will increase further. Therefore, the Ar emission intensity will be enhanced by - V B and H with the relation:

(a) VB=O 40C

where I ~ r is the Ar emission intensity at VB = 0 and H = 0. The term in round brackets indicates the change due to - V B , and that in square brackets is due to the additional field H. The calculated results for q = 0.3, m 1 = 0.071 and m 2 = 0.032 are shown by the broken lines in Fig. 9. These results fit well with the experimental results. The sputtering rates S c° and S c~ and the number of collisions between Co or Cr and electrons will increase with - V a and H, because of the increments in Ar + and electron densities. Thus, the emission intensities per sputtering rate, I c° and I ° , will be enhanced with a similar relation to Eq. (9):

Ira=lr~(l +ml

VB

Iq)[l +m2(l +mllVa I q ) n ] . (10)

i

i

,

'

0

Cr/ ,'6

p"

I

I

I

20

40

eo

20C

/

I

I

~

0

.

2

~0'

6'0 80'

' ]oo

H (Oe)

The calculated results for Co with q = 0.3, m I = 0.07 and m 2 = 0.012, and for Cr with q = 0.3, m 1 = 0.21 and m 2 = 0.04 are shown by the broken lines in Figs. 10 and 11, which indicate a good agreement with experiments. Thus, the emission intensity changes simultaneously with the variation of - V B or H, and so we can use the emission intensity as a monitor for observing the plasma state as reported previously [12].

3.5. Effects of oxygen Crystalline and magnetic properties of CoCr sputtered films are affected by the residual oxygen in the Co87Cr13 PAr+Po2=lOmrorr W= IOOw P02=O mTorr CO I

~.~

Cr

i

.(

i

--(

100(

Co

I

80 ,oo ,2o

co Cr

/

Fig. 11. Relative emission intensities (lrc° and icr) versus H for sputtering at 100 W and 10 mTorr under VB = 0 (a) or - 100 V (b). The broken lines indicate the calculations for Eq. (10).

" " Cr

,100

/t

i" 0

50(

9e'o.-'" Co o

//

/

//

20O

8

0

,

Cr ~"

,

(b) H=89 Oe

(a)H=O

,

Iooo /

H (Oe) (9)

,

(b) VB=-IOOv

L. L)

IA~=IAr(I+mtIVB I q) × [1 + m2(1 + m I IV B I q ) H ] ,

,

~

50

/

/o P02= l.O m T o r r

'l I I

I i

Co

..%- - - . r - - - " - -i

,' ) i

i i

o

-s

I

i

-,oo -,5o -2oo

vB (v)

I

-so

i

i

i

-,oo -,5o -2oo

v~ (v)

Fig. 10. Relative emission intensities (/,co and I c ' ) versus VB for sputtering at 100 W and 10 reTort under H = 0 (a) or 89 Oe (b). The broken lines indicate the calculations for Eq. (10).

I

400

I

I

I

450

I

I

500

Wavelength (nm) Fig. 12. Emission spectra during sputtering the composite target Co87Cq3 in mixed gas of Ar and 0 2 .

S. H o n d a / J o u r n a l of Magnetism and Magnetic Materials 151 (1995) 87-94

( Cr disk PA, +Po2=20mT"o,r 3.=521 n,n (a) ,°02=0.98 mTorr ,--:

I

.'

I

I

93

tering for 120 s, because the target surface is covered by the oxidized layer. Thus, the observation of the emission intensity is a useful tool for monitoring the target surface state, especially for the reactive materials such as rare earth metals.

.'

Po2= l'38mT°rr

4. Conclusions

100

200

3 0

z, 0

500

soo

Sputtering time (sec)

Fig. 13. Time dependence of I ° during sputtering Cr target at 100 W and PAr + Po2 = 20 mTorr with Po2 = 0.98 mTorr (a) or 1.38 mTorr (b).

sputtering chamber [13]. Therefore, observing a relation between the optical emission and the partial oxygen pressure is very important for controlling the film quality. We examined the optical spectroscopy during sputtering in mixed gas of Ar and O 2. We could not detect any spectra for oxygen atoms or ions in the wavelength range of 300-1000 nm. However, we can see a new spectrum at near 485 nm as shown in Fig. 12, arising from carbon oxide, CO [7], which might be produced by oxidation of oil having flowed from the diffusion pump. Fig. 12 shows that the emission intensities for Co and Cr decrease with increasing partial oxygen pressure, Po2. The reduction of the emission intensities for Co and Cr is mainly caused by the decrease in the sputtering rate due to the oxidation of the target surface. Indeed, we have confirmed that the sputtering rate for Co becomes nearly zero at higher oxygen pressure than 2 mTorr in the total pressure of 20 mTorr. Therefore, the state of the target surface can be monitored by observing the emission intensity. Fig. 13 shows the time dependence of iCr in sputtering the Cr target at POE = 0.98 mTorr (a) or 1.38 mTorr (b) under PAr + Po2 = 20 mTorr. In Fig. 13(a) for lower oxygen pressure, the value of iCr is nearly constant, indicating the oxidation to be negligible small. On the other hand, in Fig. 13(b) for higher oxygen pressure, I cr drops to nearly zero after sput-

The optical emission spectra radiated from sputtered Co and Cr were examined as a function of the sputtering condition. The emission intensities of Co and Cr increased with W. However, the relative intensities to the sputtering amounts (I c° and ICr) were independent of W. The value of I c° increased linearly with PAr, while Ircr was proportional to PA2r. If a negative bias voltage was applied to the substrate or a magnetic field was applied perpendicular to the electrodes, the emission intensities increased remarkably, because of an increment in secondary electrons or of a cyclotron motion of the electrons. When oxygen was introduced into the sputtering gas, the target surface was oxidized, and then the sputtering rate and so the emission intensity were decreased.

Acknowledgements The author would like to thank H. Noguchi for experimental assistance, F. Nishiyama for the PIXE measurement, and Prof. T. Kusuda for useful comments.

References [1] T.M. Coughlin, J.H. Judy and E.R. Wuori, IEEE Trans. Magn. 17 (1981) 3169. [2] T. Wielinga and J.C. Lodder, IEEE Trans. Magn. 17 (1981) 3178. [3] H. Maeda, J. Appl. Phys. 54 (1983) 2429. [4] S. Honda, H. Noguchi and T. Kusuda, J. Magn. Soc. Jpn. 13 S-1 (1989) 913. [5] W.G. Haines, J. Appl. Phys. 55 (1984) 2263. [6] D.J. Mapps, N. Mahvan and M.A. Akhter, IEEE Trans. Magn. 23 (1987) 2473. [7] J.E. Green, J. Vac. Sci. Tech. 15 (1978) 1718.

94

S. Honda//Journal of Magnetism and Magnetic Materials 151 (1995) 87-94

[8] M.I.T.-Wavelength Tables, ed. G.R. Harrison (MIT Press, Cambridge, MA, 1969). [9] F. Shinoki, Res. Electrotechnical Lab. No. 806 (1980). [10] R.V. Stuart and G.K. Wehner, J. Appl. Phys. 35 (1964) 1819. [11] J.J. Cuomo, R.J. Gambino and R. Rosenberg, J. Vac. Sci. Tech. 11 (1974) 34.

[12] S. Honda, F. Tsuneda and T. Kusuda, Trans. JIM 29 (1988) 411. [13] S. lwasaki, K. Ouchi, M. Kimura and K. Saiki, J. Magn. Soc. Jpn. 10 (1986) 61.