Film thickness variation in a cylindrical magnetron deposition device

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Thin Solid Films293 ( 1997) 78-82

Film thickness variation in a cylindrical magnetron deposition device Tihomir Car *, Nikola Radic RlIgjer BQIkavic Institute, Department ofPhysics. DMsinn (IfMaterials Science and Electronics, BijeniiJka cesta 54. P.O.B. 10]6. Zagreb. Croatia

Received 3 Janu:uy 1996; accepted 5 June 1996

Abstract The 2D thickness distributions of Cu and Al planar thin films prepared by a home-made cylindrical magnetron with rectangular exit window were examined. The experimental results were compared with several models of sputtered particle transport. It has been shown that in the azimuthal direction the best agreement with experimental results was achieved with the free flight model. Due 10 an uneven target erosion, theagreement with the model calculations proved to be somewhat less satisfactory in the axial direction. An overall similarity of thickness distributions inthe case of AI deposition at PA1::::0.7 Paand Cu deposition at PAr:::: 2.25 Paworking gas pressure has been found. However. iI hasbeen found that the portion of backscattcred flux was greater inthecase of AI than it was for Cusputtering. Keywords: Aluminium; Copper; Plasma processing noddeposition; Spuuering

1. Introduction D,c. magnetron sputtering systems arc widely employed in the preparation of thin films. Magnetron devices have the advantage of high deposition rate. discharge electron confinement, applicabilityfor refractory metals etc. Codeposition magnetron devices allow in particular the mixing of thermodynamically immiscible metals such as various binary combinations of good electric conductors (AI, Cu, Ag, Au) and refractory metals (Nb, Mo, W. Ta) [1]. Theparticle transport in magnetron sputtering systems and theresultingdepositionprofileshave been thesubjectofsome recent publications. They can be grouped into Montc Carlo simulations [2-5] and analyticalcomputations [6-9] . Some resultsof deposition ratesand particletransport given in Refs. [6.7J will be applied below. In this paper we have calculated the thickness profile of the film deposited upon a planar substrate through a rectangular exit window at the cylindrical magnetron anode. The substrate plane is parallel to the magnetron axis but skewed with respect to the window plane. A cosine law is assumed for the angular distribution of the sputtered particles.Three particle transport regimes have been considered. i.e, free flight, free flight reduced by backscaucring and free flight combined with diffusion. The results of computations arc

• Corresponding author. 0040·6090/97/$17.00 C 1997 Elsevier ScienceS.A. All rights reserved

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compared to the measured thickness distribution of the prepared AI and Cu films. The pcrfonncd analysis is employed to assert the composition unifonnity of the binary alloy samples preparedin a two-magnetron eodepositiondevice.

2. Model calculations for cylindrical magnetron The general relation for the thickness distribution (mass Mr deposited per unit arcaA r ) in films deposited by the sputtering method is given by the cosine law [10.11] dMr(t!>,{J)

dAr

rMc

-"2 cos c/J cos it

(l)

17f

where Me is the total mass of sputtered material, Mc=Kf,IA.dAedt. K is a constant containing the mass and the mean velocity of sputteredatoms and the sputteringyield. dA e is the emitting surfaceclement. c/J is the azimuthalangle relative to the target normal, {J is the angle of sputtered particleincidence upon substrate. dA,. is theclementof'thereceiving surface and r is the distance between
T. Car. N. Radic/ Thill Solid Films 29j (/ 997) 78-82

79

(2) where (PI.cPl,ZI) and (P2' cP2..z:z) arccylindrical coordinates of dA e and dAn respectively. In the actual situation PI is the outerdiameter of thetargetcathodeandisconsideredconstant during thedeposition. The requiredangles cP and {J arc given by: (3)

and Pleos c/J2 - PICOS tfJl ..jpr+ pi- 2ptP1.cos (cP2 - cPl) + (Z2- Zt)2'

cos -{j

Fig. I. The geometry of the magnetron-substrate configumlion.

(4)

2.1. Model OJ

Foreasy visual presentation of the film thickness distribution. wehave also introduced (local) cartesian coordinates (x, z') of the da, element at the planar substrate (see Fig. 1). The transformations between the (local) cartesian and thecylindrical coordinates are given by d P2=--

x=dtan c/J,. z'=z

cos cP2

(5)

whered is the distance betweenthecenterof the cathodeand the substrate. Finally. the differential surface element of the source (cathode) is dA e = PI dcP dz

(6)

After inserting Eqs. (2)-(6) into Eq, (I) and withsome modifications, we arrive at

Fig. 2 shows the 3D results of the integration of Eq. (7) for the straight passing of the sputtered particles. In this as well as in subsequent figures. the film thickness distribution is normalized to its maximum value. The free flight model approximates the situation when the number of deflecting collisions isnegligible along thesputteredatomstrajectorythus, the ratio between themean free path Aandthe travelling distance r should be AIr» 1. However,intheactualsituation thesputtered particles move firstacross the dischargeplasma and then through the working gas beyond the exit window. Theyexhibit collisions mostly with the neutral gas. The collisions deflect some ofthe particlesand backscatter the others. To account for these collisions, we have recalculated the Eq. (7) in two ways.

I

dMr(d,Zz.eP2) dA

=

KJ I olol

I"'''.#.

:1.l

U',..l

d

Zl

r

X

p~COS2

7TtP

(d - Picas q,l) [ (COS .pI+ tan
cP2 2

PI

+

tP cos

A.

•

q,J )d -

A.

PI] 2

2 A... - 2prd( cos cPr + tan'f'Zsm 't'l) + (Z2- 41) 'f'2

(7) which allows one to calculate the film thicknessdistribution for the actual magnetron-substrate geometry. Herein. the integration over t/Jl and 41 variables is performed within the limitsdetermined by the Jines ofa viewbetween thedA c and dAr through the rectangular exit window at the anode. The actual integration of Eq, (7) is performed numerically. We have considered three model cases: (i) free flight of the sputtered particles (l"= I); (ii) clastic collisions between the sputtered particles and the workinggas atcms-e-these give risetothe backscattercd fraction which does not arrive at the substrate (r= r( r) < 1); (iii) a diffuslon-controlled transport of the sputtered particles.

2.2. Model (ii) By reducing theincoming flux in Eq. (7) withthe assumption that all backscattered particles are lost. e.g. multiplying the subintcgral expression in Eq. (7) by cxp( - rl A). where r=!(p,c/J,?) (Eq, (2) andAis the mean freepath calculated according to the kinetic gas theory [6]:

A-, = Vzwn,a; +'I1fl,(0~+0,)'

::x:

I,

+ m.

(8)

In!:

[10- 3 m)

Fig. 2. :m pIal or calculated film thickness distribution in the C&C of free fllghl.

T. Cor, N. Rudie I TJlin Solid Films 293 (1997) 78-82

where ns and ng are the numbers of sputtered and gas atoms per unit volumes, as and ag arc the atom diameters. With the assumption n; «n g , we can neglect the first term and thus, we find: "\AI==0.0191 m (P=O.7 Pa) and Acu =s 0.OO54 m (P=2.25 Pa). 2.3. Model (iii)

According to the Keller and Simmons formula [7 J, the total particleflux at distance r inonedimension in adiffusioncontrolled regime is given by

(2r)J

3roA [ l-exp - 3,.\ ~=~

(9)

where Fa is the particle nux leaving the target and A= vI v,,, in which v is the average particle velocity, and Vc is the collision frequency of the streaming particles. This formula has been inserted into Eq. (7), assuming diffusion only in theradial direction. Ifone sets AfromEq. (8) and assuming energies of sputtered atoms about 1eV [4,12] (incidention energies is estimated to be 300 eV [13J), the collision frequency: vc(A\) =:: 1.4 X lOs s -I and l1c(CII) =:: 3.2 X lOs s- t is found. The results of the above calculations (models (ii) and (iii» are given in Fig. 3(a) and (b).

3. Experimental results and discussion

The general outlook of the actual cylindrical magnetron sputtering devices is givenin Refs. [14,15] and the detailed geometry of the magnetron-substrate configuration is presented in Fig. 1. The minimal distance between the cathode surface and the substrate is D = 33 mm, the height of the cathode is h = 20 mm, the radiusof thecathode is P« =8 mm and the radius of the anode is PIl =22 mm, The sputtered atoms can leave the discharge comportment only through the 20 mm X20 mm rectangular window at the anode. Oneedge of the window is adjusted to the line of minimal cathodesubstrate distance. TItus. the window covers approximately 60° of azimuthal angle with respect to the minimal distance line. The experimental conditions for the Cu and AI sputtering are given in Table 1.The magnetic field has been increased in the case of aluminum by the serial arrangement of the magnetic circuit. Asn consequence theworkinggas pressure is lowered intheoptimal operating regime forAImagnetrons. Filmthickness has been measured by two methods: optically by the MlOO Angstrommeter andmechanically by the Alphastep profllometer. The experimental error ofboth methods is estimated to be within 10%. Under the given conditions, the maximum thickness of the Cu film after 1 h deposition is 3.2 ....m and the maximum thickness of Al film after 3 h deposition is := 2.4 J.Lm. Maximums inthickness distributions are positionedat ep = 7~ and z == 0 in bothcases, respectively.

=

z·..xI I110'" ml Fig. 3. Comparison between experimental thickness distribution of Cu and AlliUgets. respectively, and calculmed distributions. (0.) In the azimuthal direction (for :=0) and (b) in the axial direction (for 0 ..... Cu experimental results: ~ • AI experimental results; O. cosine law; 0, eosine taw reduced by backseattering; O. cosinelawsuperimposed by diffusiom •• cosinelaw induding erosion profile in axinl direction. 127°)

The 3D plots of the measured thickness variation of Cu and Al arc given in Fig.4(a) and 4(b), respectively. The observed difference in the maximum of the local deposition rate for the case of Cu and Al calls for some explanation. After adjustments of respective atomic volumes of Cu and AI, it appears that the deposition rate for copperis about 4-5 times higher than the one for aluminium. Such a difference can be partially ascribed to the sputtering yield of copper which istwiceas large asthatofaluminium at 300cVincident Ar+ energy [16]. However, in the almostfree flight regime, the remaining difference in growth rate has to be attributed to thedifference inthe largely unknown stickingcoefficients of thesputtered CuandAI atomson the growing film surface. Toole I Experiment.11 conditions forCu and AI magnetron sputtering {Y denotes the spullcring yield lit 300eVincident Ion energies) Cu

AJ

P",-2.2S Pa

PAr-D.? Pli U=380 V

U=400V 1-100mA ,-60 min H=0.0251 Y-1.20±O.30

1-IOOmA ,-180mln

H=O.040T r-O.6S±O.lS

T. Car, N. Radic I ThinSolid Films 293 (J 997) 78-82

S1

..

o.~

. c., ..

C.t ;:0

:;: .~ . =... -.

- 0 .7

C.O

...;:tJ' ~

:r

0 .' 0 .'

r;'

~

::l III III

II>

:z:

[10:- 3 mJ

integration area hasno significant influence on thethickness distribution in azimuthal direction. From the relative position of experimental curves in Fig. 3( a) and3(b), it seemsthat thefraction of backscattered particles is greater in the case of AI than in the case of Cu sputtering, Probably, it is a consequence of the smaller AI atomic mass. As well it can becaused by deviations from the cosine law for theemission from the target surface [ 16].

Fig. 5. Experimental erosion profile of Ihe Cu target after np"roltimalcly 10h cf cxploltatlon.

111e measured and calculated thickness distributions normalized to the respective maximum values arc jointly given in Fig.3(a) and 3(b). It can be seen that there is a good agreement between the experimental results and computationsaccording tothefreeflight model intheazimuthal direction. for both Cu and Al films. In the axial direction (Fig. 3(b») somediscrepancy between the calculations and the experiment appears, however. Since this discrepancy increases towards theouterendsof thecathodes, it isprobably due to the uneven erosion of the target cathode along the z axis (Fig. 5). Therefore, by reducing the integration area on the target along the <: axis by about 10%. the computed distribution in the axial direction can be brought to much better agreement with the experiments (Fig. 3(b»). The reduced

4. Conclusions

The measured andcomputed thickness distributions of'Cu and AI thin films prepared in a home-made cylindrical magnetron sputtering device have been compared. It has been shown that the measured thickness distribution can be described in a satisfactory manner by the variation of sputtered particles angular distribution and free night regime of particle transport to the substrate. Very good agreement between the experimental results nnd the computation is in the azimuthal direction. while in theaxial direction the thickness distribution is remarkably influenced by the uneven target erosion. Because of theestimated portion of backscattered particles and diffusion, we include terms for backscaucring and diffusion. One can see that theponlon ofbackscattcrcd particles is greater in AI than in Cu sputtering,

82

T. Cllr, N.Rudie IThin Salid Films 293 f 1997) 'J8~

Finally, theabove results corroborateasatisfactory (within 10% variation) compositional uniformity of binary alloy samples produced by codeposition upon 1 em diameter CiT· cular substrate positioned symmetrically wilh respect to two adjacentcylindrical magnetrons r15J. Acknowledgements The authors thank H. Zorc and J. Turkovic for film thickness measurements.

References (IJ N. Rndi~ and D. Ornein, Fiziktl A.4 (1995) 233; J. lvkov,T. Car, N. Rudie and E. Babic, SolidState Cammun., 88 ( 1993) 633. [2] J. Staehe, J. Vac. Sci.Technol, A, 12 (5) (1994) 2867. [3] E..Shidoji, M. Nemoto, T. Ncmuraand Y. Yoshik.o.Wll, J. Appl. Pllys., 33 (1994) 4281.

[4] O.M. Turner, I.S. Falconer. B.W.1:lmes and D.R. McKenzie, J. AppJ. Ph:;.:.• 65 (9) (1989) 3671. [SJ Y. Yamamura nnd M. Ishida, J. Vac. Sci. Tee/lllol. A. J3 (1) (1995) 101. [6J W.O. Westwood. J. Vo". Sci. Technol., 15 (1) (1978) J. (7] J.H.Keller lindR.O. Simmons, 10MJ. Res. Develop•• 2J (J) (1979) 24. [8] Q. FtIJI, X. Chen and Y. Zhllllg, Vacuum. 46 (1995) 229. [9] A.Gras-Marti and l.A. Valles-Abraca, J. Appl. Phys.. 54 (2) (1983)

1071. [10] L.1. Malssel and R. Glans, Handbook of ru« Film Technology, McGraw-Hili. NewYork, 1970,Chapt, I, pp.32-34. [II J L. Holland. VacuumDepositionofTllin Films, Wiley, New York, 1960, ChllPt. 5, pp, J41-164. [12] M.W. Thompson. Phllos. Mag., 18 (1968) 377. [131 W.O. Dtlvis and T.A. VlllldersJiec, Pllys. Rev.. 131 (J963) 219. (14J B. Gdcta, N. Radi~. D. Gracin, T. Do~li6 and T. Car. J. Non-Cryst. Solids, 170 (1994) 101. [IS] N. Rodic. B. Gdela, D. Gracln and T. Car, tu« Solid Films. 228 (J993) 225. (16] H.H. Andersen and H.L. Bay. Spullcring Yield Measurements, in R. Berisch (ed .), Sputtering by Particle Bombardment I. SpringerVerlag. Berlin, 1981.