Effect of ion bombardment on the surface morphology of Zn-films sputtered in an unbalanced magnetron

Vacuum/volume

46/number

Pergamon

2fpages 203 to 210/1995 Elsevier Science Ltd Printed in Great Britain

0042-207x/95

0042-207X(94)E0041-V

$9.50+.00

Effect of ion bombardment on the surface morphology of Zn-films sputtered in an unbalanced magnetron J Musil, J MatouS and V Valvoda Czech Republic received

institute

of Physics, Academy

of Sciences,

Na Slovance

2, 180 40 Prague 8,

9 April 1994

It is well known that magnetron sputtered films of low melting point T, materials have (due to their crystallisation at low substrate temperatures, T < 100°C) rough and diffusely reflecting surfaces, even when thin, for instance about 20 nm for In films. Only extremely thin films have a smooth and specular reflecting surface. This paper reports on the possibility of sputtering thick films of low T, materials with a smooth, optically specular reflecting surface using an unbalanced magnetron. To demonstrate this possibility, Zn films were studied and it was shown that a surface roughness of the film can be effectively controlled by ion bombardment of the film during growth. The smoothing of the Zn film does not depend on film thickness but on ion bombardment of the growing film.

1. Introduction It is well known that the surface morphology of thin films depends not only on the substrate surface roughness’ and deposition conditions under which they are prepared, but also on the melting point T,,, of the film materia124. Experiments show that the surface morphology of metallic films with low T,,,, such as InSn(90/10) (T,,, = 150°C), In (T,,, = 156.6”C) or Sn (T,,, = 231.8”C) and that with high T,,,, for instance Cr (T, = 1875”C), strongly differ when they are deposited at the same substrate temperature T. This result arises from the large variation in the normalised substrate temperature T/T,,, for deposition under similar conditions. Even for T, close to the room temperature T,, films made of low T,,, have a high ratio T/T, > 0.5 while that made of high T, have a low ratio T/T,,, CC0.5. It means that low T,,, films easily crystallise already on unheated substrates ; this is the main reason why low T,,, films have a rough and diffusely reflecting surface and exhibit a milky colour and matt appearance5,6. The surface roughness strongly increases with increasing thickness of the film. Similar behaviour, as the low T, films have, is exhibited by sputtered Zn (T,,, = 419.5”C) films. A diffusely reflecting surface of low m.p. metal films can be a disadvantage of their utilization in applications where thick films with smooth, specular reflecting surfaces are required. We do not know at present of thick low r,,, films with smooth surfaces being deposited and this implies that there is a principal question if it is possible to produce thick low T,,, films with a smooth and specular surface. This paper tries to provide an answer and reports on the possibility of preparing several microns thick Zn films with a smooth surface, using a magnetron bias sputtering. Our experimental concept was to bias the substrate with a

negative voltage Us considerably higher than the floating potential U, of the substrate and thereby gently resputter the surface of the growing film to produce a smooth thick low T, film ; i.e. smooth thick Zn films in this study. The procedure is based on ion bombardment of the film during its deposition, as in ref 7 and the method of surface smoothening has been demonstrated in many experimentsa.

2. Experimental details The Zn films were sputtered using a planar disc unbalanced magnetron with a Zn target of 100 mm dia. in a stainless steel cylindrical deposition chamber (dia. 152 mm), as schematically shown in Figure 1. The chamber was evacuated by an oil diffusion pump (250 1 SK’) to a base pressure p0 = 5. 10m4 Pa. The unbalanced magnetron consists of a conventional magnetron equipped with an internal coil C, (2000 turns, 4I = 32 mm) and an external coil C, (1200 turns, 4, = 126 mm). The magnetic field of the conventional magnetron was produced by the internal electromagnet coil and the unbalanced magnetic field by the external electromagnet coil. Both coils are placed behind the Zn target. The currents I, and Iz control the magnetic field distribution above the Zn target and thus the unbalanced magnetron discharge. For more detail see ref 8. The substrates were optically polished 12% Cr steel circular plates 18 mm dia. and 5 mm thick. Prior to entering the deposition chamber, the substrates were ultrasonically cleaned in a mixture of distilled water and the detergent Sinalod (5 wt%), rinsed in flowing water, showered in redistilled water and dried with a blast of hot air. Then the cleaned substrates were immediately inserted into the chamber. The substrate was mounted in a special 203

J. Musil et al: Bombardment

of Zn-films

Figure 1. Schematic illustration of sputtering device equipped with planar disc magnetron and the substrate holder which makes it possible to measure the energy delivered to the sample (substrate) by incident particles.

0

holder on its axis and electrically and thermally isolated from it by glass fabric, see Figure 1. This treatment allowed the substrate to be biased and also enabled the measurement of the energy flux arriving at the growing film by incident particles using a thermocouple. Independent measurements of the total thermal power P, and electric power P, delivered to the substrate permit estimation of the contribution of neutral particles to the heating of the substrate during the deposition, i.e. Pt - P,. Also, it should be noted that the substrate holder was at the same potential as the isolated substrate. The substrate holder is movable along the deposition chamber axis and the substrate-to-target distance d,_, can be varied from 40 mm to 120 mm. To reduce contamination of the deposits, the magnetron Zn target was presputtered in a pure argon atmosphere at a pressure of 0.3 Pa for 10 min. During sputter-cleaning of the magnetron target the substrate was shielded by a shutter. After this operation the shutter was opened and the Zn film was deposited onto a steel sample. The Zn films were deposited on unheated substrates in pure argon (99.99%) as follows: discharge current Id = 0.4 A, discharge voltage U, = 530-550 V, current in internal electromagnet coil of the unbalanced magnetron I, = 2 A, current in external electromagnet coil of the unbalanced magnetron Zz = 1.8 A, target-to-substrate distance d,_, = 60 mm and at two argon pressures p = 0.3 Pa and p = 0.04 Pa. The film thickness h was controlled by the deposition time fd and the substrate bias Us. The substrate was held either at a floating potential UR or negatively biased up to Us = -800 V. Films deposited on unheated substrates had their temperature T raised during growth from energy released by incident particles. In all experiments the final thermocouple temperature of the substrate Tdid not exceed 12O’C. The highest T was reached at a high negative bias Us = -400 V and a long deposition time td = 8 minutes, see Figure 2. Substrate heating by radiation from the magnetron target was negligible. Film thickness was measured either with an Alphastep 100 tester or determined from the weight difference (assuming bulk density) between the coated and uncoated substrate. The surface morphology and cross-section structure were studied using a JEOL scanning electron microscope (SEM). The argon content of the ion plated Zn films was determined by electron probe microanalysis. The preferred orientation of Zn films was investigated by X-ray diffraction using Cu Kcc radiation. 204

500

250 -

td Ls 1

Figure 2. Time development of the substrate temperature T raised from energy by incident particles for different values of negative substrate bias Us at I,, = 0.4 A, I, = 2 A, I, = 1.8 A, d,, = 60 mm and pAr = 0.3 Pa.

3. Results

The surface roughness of the film increases with the Zn film thickness when the film is sputtered on either an insulator (glass) or conductor (steel) substrate held at floating potential U, in the glow discharge, see Figure 3. Figure 3 shows the development of surface morphology of Zn films sputtered in an unbalanced magnetron onto Cr steel samples at a constant deposition rate of 550 nm min-’ and argon pressurep = 0.3 Pa under the discharge conditions given above. The number of grains decreases and their size increases with film growth. The Zn films of about 0.43 pm thickness exhibit well developed grains whose size reaches 0.5 pm, i.e. the grain size is comparable with the film thickness. The effect of varying the Ar pressure on the Zn morphology was studied. The surface morphology of two Zn films prepared under two different argon pressures p = 0.3 Pa and p = 0.04 Pa, but with the same deposition conditions given above, is given in Figure 4. In spite of a great difference in the substrate floating potential, fJ, = -33 V atp = 0.3 Pa and U, = -90 V atp = 0.04 Pa, no change in surface morphology of Zn films was observed. All the Zn films deposited under the conditions given above have a rough diffusely reflecting surface due to their strong crystallisation during its deposition. Therefore, the next experiments were devoted to an investigation of Zn surface smoothing induced by ion bombardment of the film during its growth. Results of these experiments are summarised in Figures 610. Figure 5 shows the deposition rate a, of sputtered Zn films as a function of negative bias Us. As expected, the deposition rate of an decreases with increasing negative bias Us and is almost independent of the deposition time t,. Thus the film thickness linearly increases with sputtering time, see Figure 6. Also in Figure 6 the transition from milky Zn films with rough, diffusely reflecting surfaces to shiny Zn films with smooth, specular reflecting surfaces is marked and the transition region, between Us = -200 V and -300 V, is almost independent of the film thickness h. Zinc deposited at low negative biases / U,I < 200 V is light

J. Musil

et a/: Bombardment

of Zn-films Effect

h h-4

of Argon pressure h = 0.5pm 0.3Pa

270

1080

lw

2160

of surface morphology of two Zn films sputtered Figure 4. Comparison on Cr steel substrates held on floating potential C.Jflat pAr = 0.3 Pa and /I,,~ = 0.04 Pa. Deposition conditions: I, = 0.4 A. U, = CT+ I, = 2 A, I? = 1.8 A, d,_, = 60 mm and t, = 60 s.

id-0.4A.

I,=

d,_t=60mm,

2A. 12=1.8A PAr’0.3 Pa

9 4320

[ nm/min]

I

600

Figure 3. Development of surface morphology of sputtered Zn films on Cr steel substrates held on floating potential U, with the film thickness h, i.e. with the deposition time t,. Deposition conditions : I,, = 0.4 A, I, = 2 A, I, = 1.8A, d, , = 60 mm and pAr = 0.3 Pa.

scattering but that deposited at high negative biases lU,l > 300 V is specular. This finding is illustrated in Figure 7, where a development of the Zn film surface morphology with increasing negative bias Us and the film thickness h, i.e. the deposition time t,, is displayed. For r/, < 200 V = const the grain size increases with increasing h or t,. For td = const the density of grains decreases with increasing negative bias U, and for 1Us\ > 300 V the film surface is smooth and free of grains. These experiments clearly show that energetic ion bombardment of growing Zn films smooths the film surface. A comparison of the surface morphology of specular reflecting Zn films created at US = -300 V and -400 V and diffusely reflecting Zn films created at floating potential U, = - 35 V and

td[mifl] *

. . . 000

+++

2

4 8

--u, [ vI Figure 5. The deposition rate a,, of sputtered Zn films as a function of negative substrate bias U, measured for three values of deposition time td.Deposition conditions : I,, = 0.4 A, I, = 2 A, I? = 1.8 A, d, f = 60 mm and pizr = 0.3 Pa. 205

J. Musil eta/: Bombardment

of Zn-films Zn film

appearence

5 h

240 120

--U,[Vl Figure 6. The thickness II of Zn films sputtered for three deposition times, td = 120, 240 and 480 s, as a function of negative substrate bias U,. Deposition conditions: I, = 0.4 A, I, = 2 A, Iz = 1.8 A, d-, = 60 mm and pAI = 0.3 Pa.

at low negative bias U, = - 100 V is also given in Figure 8. This figure again shows that diffusely reflecting Zn films contain well developed grains and their size increases with increasing t,, i.e. the film thickness k. The specular reflecting films have smooth surfaces but their surface morphology also indicates that the

-u, tvl/td[sl

60

120

material of the film crystallises during its deposition. A confirmation of this fact is also given from XRD patterns taken from specular reflecting and diffusely reflecting films, see Figure 13. A more detailed study of the surface and cross-section of diffusely reflecting and specular reflecting Zn films is shown in Figs 9 and 10. Here, it is clearly seen that diffusely reflecting Zn film sputtered on a Si substrate held at the floating potential U,, = -35 V is thicker (h = 2.7 pm) than the specular reflecting Zn film sputtered at negative bias U, = -300 V (h = 1.5 pm). There is practically no visible difference in the microstructure of diffusely and specular reflecting Zn film. As can be seen from cross-sectional micrographs, the microstructure of both films corresponds well to zone 3 of the Thornton structural zone model. The difference in the surface roughness of diffusely and specular reflecting Zn films is evident. Figure 11 shows the content of Ar incorporated into Zn film during its growth. It increases with increasing negative substrate bias to a maximum 8 at% at I/, = -400 V. For higher negative biases of IU,l > 400 V it decreases to about one third of its maximum value, i.e. approximately 2.7% at U, = -600 V, and keeps this value up to U,= - 800V. The reason for the decrease of Ar content and its saturation at 2.7at% in Zn films deposited at high negative biases 1U,la 400 V is not clear at present. We cannot exclude that the decrease of Ar content in created films is connected with its outdiffusion from growing film, due to an increase of energy loading of the growing film and so to an increase of Tin deposition at high negative biases U,. Therefore, we also investigated the development of the substrate temperature 240

460

Figure 7. Development of surface morphology of sputtered Zn films with the deposition time td and the negative substrate bias CI,. Zn films sputtered at 1(/,I > 200 V have the specular reflecting surface. Deposition conditions : Id = 0.4 A, I, = 2 A. Iz = 1.8 A, rl,_, = 60 mm and P.,? = 0.3 Pa. 206

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T with the deposition time td and the magnitude of energy delivered to the growing film by incident particles. Figure 2 shows the increase of temperature of the Cr steel substrate during sputtering of Zn film under the deposition conditions given above. The substrate temperature T increases linearly with deposition time td and the increase is higher when higher negative bias U, is used. For deposition times up to 5 min, which are used in our experiments, a final substrate temperature r, reached the value of 6O’C for substrates held at floating potential U, and about 12O’C for substrates biased at US = -400 V. This result means that in sputter deposition of Zn films with thickness of about 1 pm, or greater, at high negative biases of 1r/,1 > 400 V, the heating of the substrate during deposition is significant and plays a dominant role in the mechanism of the film growth because the melting point of Zn is 7’,,,= 419.C. For T, = 100°C the ratio T/Tm reaches the already high value of 0.54. A question of particular interest is what kind of particles contribute to heating of the substrate. Therefore, we measured the total thermal power P, delivered to the growing film by incident particles and the electric power P, delivered to the growing film by ions, i.e. P, = I, Us. Results of our measurements, carried out at argon pressure p = 0.3 Pa, are given in Figure 12. The total power P, is greater than the electric power P,. It means that neutral particles also contribute to the substrate heating. Their contribution is given approximately by the difference P, - P,. It is necessary to note that the magnitude of substrate heating by neutral particles will depend on operating gas pressure and the substrate heating by neutral particles will decrease with the increase of argon pressure due to particle collisions, i.e. by a plasma thermalisation. The specular and diffusely reflecting films were also characterised by X-ray diffraction. For this analysis, two pairs of specular/diffusely reflecting films were sputtered at two different deposition times, as shown in Table 1. The XRD patterns of matt (sample No. 347) and shiny (sample No. 344) Zn films are given in Figure 13. Both the matt and shiny Zn films exhibit a strong (002) reflection. However, the intensity of the (002) reflection is much higher for the shiny Zn film, by about 10 times, than that of the matt Zn film. Also. as shown in Table 2, while the (002) reflection of the shiny Zn film strongly increases with the film thickness (from h = 1.2 pm to 2.6 pm, i.e. about 4 times), the (002) reflection of the matt Zn film only slightly increases with increasing thickness (from h = 2.5 pm to 4.0 pm, i.e. about I .25 times). The extremely high (002) preferred orientation of the Zn films sputtered at large negative bias U, = - 300 V is unexpected and

Surface

Shiny

Lsl

ld

-u, Is

344 341 348 352 * Determined

films

480

480

[VI

300

300

400

P-W

2,1

2,1

2,1

Matt films

td is] -u, I5

[VI IW

30

120

240

34

36

100

0

0

2,1

Figure 8. Comparison of surface morphology of specular reflecting (shiny) and diffuse reflecting (matt) sputtered Zn films prepared at different values of t,, U_ and I,. Constant deposition conditions: I, = 0.4 A, I, = 2 A, Iz = 1.8 A, c/, , = 60 mm and psr = 0.3 Pa.

analysis.

Constant

deposition

/I* (nm)

Film appearance Shiny Matt Shiny Matt

of Zn films

120

Table 1. Basic process parameters and properties of Zn films used for X-ray diffraction conditions : Id = 0.4 A, I, = 2 A, Iz = 1.8A, d,_, = 60 mm and .uAr = 0.3 Pa

Sample no.

morphology

-300 un = -34 - 300 U,, = -36

2.15 0 2.10 0

514 518 508 515

240 240 480 480

1224 2020 2568 4056

from sample weight 207

J. Musil

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le of

Of

ation

Film

0

ion

-Surface

lm

Film

0

-Interface substrate

Interface substrate

Film surface

Film surface t Film

5

-Interface substrate

lm

Film surface J Film

20

-Interface substrate

of surface and cross-section Figure 9. Morphology (matt) Zn film sputtered on Si substrate. Deposition A. I,=2 A, I:= 1.8 A. d,,=60 mm. PAr=0.3

of diffuse reflecting conditions: I, = 0.4 Pa, r,=240 s and

Cl, = C’“.

is now undergoing a detailed will be published later.

Figure 10. Morphology of surface and cross-section of specular reflecting (shiny) Zn film sputtered on Si substrate. Deposition conditions : f, = 0.4 A. I, = 2 A, I, = 1.8 A. (1, t = 60 mm. I’,,~ = 0.3 Pa. fd = 240 s and C’, = -300 v.

XRD study. Results of this study 10

4. Discussion The main results of experiments marised as follows :

Rr content

described

above can be sum-

that Zn films deposited by magnetron 1. It was demonstrated sputter ion plating with a negative substrate bias (U,I > 200 V permitted the production of smooth, specular reflecting Zn films, whereas Zn films prepared at 1UJ < 200 V had a rough, diffusely reflecting surface of matt and milky appearance. 2. Smooth specular reflecting Zn films with thickness up to 2500 nm were prepared. 3. Argon ion buried in sputtered Zn films rises with the negative substrate bias U, and reaches a maximum value of about 8 at% at U, = -450 V. At ICI,] > 450 V the argon content decreases to about 3.5 at%, at U, = -600 V, and remains at this value up to U, = - 800 V. 208

[at Xl

1

-I

era\

8

II

200

I

1

,

400

600

800

--U,[Vl Figure 11. Atomic content of Ar incorporated into sputtered Zn films as a function of negative substrate bias CT,.Deposition conditions : I, = 0.4 A, U,=500 V, I,=2 A, Iz= 1.8 A, I,=2 to 2.2 mA, r,,=240 s. Iz(L/,) = 2160-548 nm, 4, = 60 mm and phr = 0.3 Pa.

J. Musil eta/: Bombardment

of Zn-films In film

lni~nsil~

I,shiny

[EPSI 0.6,

t

NQ.344

10000-

[ W/cm21 0.5 1

1

3000 lntensily

[USI

Oi....2;0....1 -

1

z II

500 -us

[ v1

Figure 12. The power P delivered to the substrate during Zn film deposition by all incident particles (P,) and by ions (PC = I, U,), as a function of negative substrate bias V,. Deposition conditions : I, = 0.4 A, I, = 2 A, I, = 1.8 A, &, = 60 mm and pAr = 0.3 Pa. The substrate surface S = 2.24 cm2.

Figure 13. XRD patterns

Table 2. Integral intensity of (002) reflection from Zn samples of Table 1 determined by XRD method

Sample no.

h (nm)

I(OO2)

344 348 347 352

1224 2568 2020 4056

3064.8 12,659.5 340.8 402.7

4.

Film appearance - 300 - 300 ufl ufl

Shiny Shiny Matt Matt

XRD

patterns measured from diffusely and specular reflecting Zn films exhibit a strong (002) reflection. The intensity of the (002) reflection is about 10 times greater for specular reflecting Zn films. This result is unexpected and is now under study.

Our experiments demonstrated that by using a magnetron sputter ion plating process, it is possible to produce low T, films with a smooth specular reflecting surface. This result can be obtained in spite of the fact that low T, materials easily crystallise at low substrate temperatures T close to T, because the normalised temperature T/T,,, > 0.5. A smoothing of the film surface can be explained, either by an ion bombardment which erodes the peaks of rough surface and fills its valleys with a material produced by ‘forward sputtering”, or by an incorporation of sputtering Ar gas into Zn film. The Ar embedded in sputter deposited Zn films suppresses grain growth, see for example ref 9. It results in a decrease of grain size and so in the smoothing of Zn film surface. Obtained results indicate that the second phenomenon induced by ion bombardment during growth could be the main mechanism responsible for surface smoothening of low m.p. metal films. The necessary condition for preparation of a film with a

from specular reflecting (shiny) (sample No. 344) and diffuse reflecting (matt) (sample No.347) sputtered Zn films. Deposition conditions: I, = 0.4 A, I, = 2 A, Z2 = 1.8 A, d,, = 60 mm, pAr = 0.3 Pa, U, = U, = -34,6 V for sample No.347 and U, = -300 V for sample No.344.

smooth specular reflecting surface is sufficient ion bombardment of the film during its growth. In our case, high ( Us\ > 250 V is needed to deposit thick, smooth, specular reflecting Zn films at deposition rate of 550 nm mini. For higher deposition rates un > 550 nm min’ it is reasonable to expect that higher ion fluxes and/or higher negative biases Us should be used. If we take into consideration that a, decreases with increasing negative bias Us there is probably a certain physical limit for ub, because for every material there is a given value of negative Us when all deposited material is resputtered, i.e. there is no net film growth. Also, it should be noted that the thickness of low T, films with a smooth surface will be at a given value of negative bias Us limited by an increase of the substrate temperature T from particle bombardment of the film during growth. 5. Conclusion It is well established, from many experiments, that low m.p. metal films crystallise at low substrate temperatures, close to room temperature, and develop a rough, diffused reflecting surface of milky colour and matt appearance, even when still only a few nanometres thick. Experiments described in this paper clearly demonstrated that low m.p. metal films with a smooth and specular reflecting surface can be produced by the sputter ion plating process, i.e. at high negative biases, 1UsI > 250 V, when the surface of the growing film is bombarded by accelerated ions. Under such conditions, the low m.p. Zn films with thickness of 2.5 urn were prepared. Smooth specular reflecting Zn films of greater thickness can be 209

J. Musil et al: Bombardment

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prepared if a rise of the substrate bardment is prevented.

temperature

T from ion bom-

Acknowledgements The authors would like to thank Dr R Kuiel Jr of Charles University in Prague for XRD measurements. Also, we wish to thank Prof L Holland for his kind, stimulating comments and suggestions which strongly improved this paper. This work was supported in part by the Grant Agency of the Czech Republic under Grant No. 202/93/0508. References ‘P Bai, J F McDonald and T -M Lu, J Vuc Sci Technol, A9(4), 2113 (1991).

210

‘J A Thornton, Ann Reu Mater Sci, 7,239 (1977). ‘S Graig and G L Harding, J Vat Sci Technol, 19(2), 2113 (1981). 4Z W Kowalski, J Mafer Sci Lett, 7, 845 (1988). 5T Minami, H Sato, S Takata, N Ogawa and T Mouri, Jpn J Appl Ph,vs, 31, Part 2, No.8A, L1106 (1992). 6J Musil, J MatouS, J VlEek. L Koydl and K Mailer, Czech J Ph,vs, 44(6). 565 (1994). ‘R D Bland, G J Kominiak and D M Mattox, J Vat Sci Technol, 11(4), 671 (1974). ‘S Kadlec, J Musil, W-D Munz, G Hakansson and J E Sundgren, Swface Coat Technol., 39/40,487 (1989). 9T Kiyota, S Toyoda, K Tamagawa and H Yamanaka, Proc 2nd Int .S~jmpon Sputtering and Plasma Processes, ISSP-93. May 27-28. 1993, Tokyo, Japan, pp 1355140.