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2.5

IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 5 , SEPTEMBER 1989

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STRESS RELATED ANISOTROPY STUDIES IN DC-MAGNETRON SPUTTERED

TbCo AND TbFe FILMS

S-C. N. Cheng, Mark H. Kryder, and M. C. A. Mathur *

Magnetics Technology Center Carnegie Mellon University

*Visiting professor at CMU from IBM, Tucson during 1988.

ABSTRACT

A series of TbCo films and a series of TbFe films were prepared by dc-magnetron sputtering a t different deposition powers and Ar sputtering pressures. It was found tha t anisotropy decreased with an increase of deposition power. Anisotropy also showed a peak within the range of 2.5 mtorr to 11.5 mtorr of Ar sputtering pressures. T h e perpendicular magnetic anisotropy of films which were still attached t o their substrates and films which had been removed from their substrates were compared. T h e percentage change in Ku , which occurred when the film was removed from its substrate, correlated with the rise and fall of perpendicular anisotropy, although changes were also typically large a t 2.5 mtorr of Ar sputtering pressure. Changes in ICu after removal from the substrate were as large as 4G % in TbFe films deposited a t 2.5 mtorr of Ar sputtering pressure. Larger percentage changes in ICu was found in dc-magnetron sputtered films than were previously reported for rf-sputtered TbFe and TbCo films. T h e films deposited onto thick polycarbonate substrates had the largest anisotropy and also suffered the largest percentage change in anisotropy when they were removed from the substrate.

INTRODUCTION

Perpendicular anisotropy, ICu, of rare earth transition metal (RE-TM) amorphous films is one of the critical factors in determining the size and regularity of domains written during magneto-optical (MO) recording. I t has been extensively studied in rf-diode sputtered films and shown to be controlled by varying Ar sputtering pressure and substrate bias voltage. Because of higher sputtering rates and lower substrate temperature, dc-magnetron sputtering has recently been popular as a preparation method for M - 0 media. However, few reports have been published on the anisotropy of dc-magnetron sputtered films.

Tb27C073 and Tb25,5Fe74.5, respectively. In addition, Tb,,Co,, and Tb2?Fei3, areal composition, targets were used t o deposit films were deposited onto 20 mil, 10 mil, and 5 mil thick glass, 7 mil, 3 mil, and 1 mil thick Mylar and 50 mil thick polycarbonate with 5 mtorr of Ar sputtering pressure a t 40 watts of deposition power. In order t o make it possible t o peel the films from the substrates, the substra,tes were coated with a thin layer of photoresist.

Approximately, 2000 a films were used in these studies. Prior t o deposition, the sputtering system was evacuated t o below 5 x IO7 torr.

Coercivity and magnetization da ta were taken from a vibrating sample magnetometer in magnetic fields up t o 15 KG. T h e perpendicular anisotropy, ICu, of films both before and after peeling from the substrates was measured with a torque magnetometer in a magnetic field of 14 ICG.

RESULTS and DISCUSSION

1. Perpendicular Anisotropy Constant ICu vs. Deposition Power:

T h e his, Hc, ICu of as-deposited and peeled films, and percentage changes in ICu as a result of peeling from their substrates for films prepared by targets with the areal compositions of Tb,,,Coi, and T ~ I ~ ~ , ~ F ~ ~ ~ , ~ are plotted in Figs. 1 and 2 , respectively. All the films are rare earth rich and the compensation temperatures are all above room temperature. The magnetizations of both kinds of films decrease with increasing deposition power, because the compositions shift from the RE-rich side towards the TM-rich side. The percentage of Ar inclusion in films sputtered under similar conditions t o these was analyzed found t o be less than 1%.

K U ( 1 O 6 e rgs icc) Ms(emu/cc) Hc(102 O e ) AKUiK, (%)

In this study, we have examined the perpendicular magnetic anisotropy by depositing the films a t different powers and Ar sputtering pressures. Stress was believed to be a very important cause of anisotropy in these films. Therefore, anisotropy was measured both in as-deposited films and in films removed from their substrates. The effects of thermal stress t o the ansiotropy have also been studied through the use of different thicknesses of plastic and glass substrates since plastic has a greater thermal expansion coefficient than glass. TbFe and TbCo binary films were chosen as compositions for comparison, since i t is believed tha t F e alloys have higher magnetostriction than CO alloys.

EXPERIMENTAL PROCEDURES

TbCo and TbFe films were dc-magnetron sputtered for 4 mins from 3 " dia,meter mosaic targets onto 1 " squere glass substrates. T h e mosaic target areal compositions used were

Cons tan t Ar Sputtering Pressure = 5 rntorr

50 55 60 65 70

Deposit ion Power ( watts )

Figure 1: Hc, Ms, ICu, &Cu, and AICu/ICU of non-peeled and peeled Tb27C023 films deposited on photoresist coated substrates a t different powers with constant Ar sputtering pressure of 5 mtorr.

001 8-9464/89/0900-4018$01 .WO 1989 IEEE

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HC(1O2 Oe) AKuIKu(%) Ku(106 ergs/cc) Ms(emu/cc)

I I I I

Perpendicular anisotropy, KU, of both kinds of films decreases with an increase of deposition power even though the composition is moving closer t o the compensation composition. This is contrary to Tsunashima and Van Dover's resultslP2 which showed an increase in the anisotropy when the composition was close to the compensation composition. Our findings indicate the anisotropy is not strongly composition dependent, but rather depends more on the deposition conditions. The decrease in anisotropy with an increase of deposition power correlates with an increase of peak substrate temperature as determined using temperature sensitive tapes, from 80' C to 100' C as the deposition power increases from 55 watts to 70 watts. It is suggested that the reason increased deposition power leads to lower K U may be due to reduced stress which occurs as a result of higher energy and therefore higher surface mobility caused by the higher power deposition.

The perpendicular anisotropy, KU, of non-peeled T b C o films

falls in the narrow range of 1.3 to 2 x lo6 ergs/cc for the deposition power range from 55 t o 70 watts. However, the ICu of as-deposited

Constant Ar Sputtering Pressure = 5 mtorr

250 25

K.: (Non-Peeled Films) 20

150 10

0.5 loo

I I I I I 50 55 60 65 70

Deposition Power ( watts )

Figure 2: He, Ms, KU, AKU, and AKU/KU of non-peeled and peeled Tb25.5Fe,4,5 films deposited on photoresist coated substrates a t different powers with constant Ar sputtering pressure of 5 mtorr.

TbFe films falls in the wider range of 0.8 to 1.9 x lo6 ergs/cc for the same range of applied deposition power. The peeled TbFe films also show a larger variation in K U with deposition power than the TbCo films. The stronger dependence of K U on deposition power and therefore stress is believed to be caused by a larger magnetostriction of Fe alloys than of CO alloys. This can be seen clearly from the curves of percentage K U changes of peeled and non-peeled films. The percentage K U changes of T b C o films fall in the range of 13 to 27 percent, which is smaller than the 35 to 44 percent changes of TbFe films.

These percentage changes of K U as a result of peeling them from the substrates are larger than those previously shown for rf- sputtered films'. This may be explained by the lower substrate temperature in dc-magnetron sputtering and therefore that less annealing occurred during deposition.

2. Anisotropy variation vs. Ar Sputtering Pressures:

Ms, Hc, KU, and AKU/KU of both peeled and non-peeled Tb25,5Fe74.5films are plotted in Figs. 3 against different Ar sputtering pressures at a constant deposition power of 65 watts.

Constant Deposition Power = 65 watts 30 -1 50

A %/Ku M Ks (Non-Peeled Fi KZ (Peeled Films)

HC U U

80

I I I I I I I I I 1 0 2.5 5 7.5 10 12.5

Ar Sputtering Pressure ( mtorr )

Figure 3: Hc, Ms, KU, AKU, and AKU/KU of non-peeled and peeled Tb25,5Fe74.5 films deposited at different Ar sputtering pressures with constant deposition power of 65 watts.

The drop in anisotropy a t high Ar sputtering pressure is believed to be due to the less dense film microstructure3 which is caused by less energetic bombardment during sputtering and resulting from the effect of gas scattering4. The less dense microstructure is believed to allow stress relaxation and has been shown to occur in films deposited at high Ar pressures.

The curve for the percentage change of K U after peeling, which varies from 28% to 46%, is also plotted in Fig. 3. The peak in AKU/ICu above 5 mtorr of Ar sputtering pressure follows the KU trend of both peeled and non-peeled films. The anisotropy differences between peeled and non-peeled films are caused by the elimination of the film-to-substrate stress. I t is also seen tha t both AKU and AKU/KU increase when the Ar sputtering pressure is decreased from 7.5 mtorr t o 2.5 mtorr. This leads us to believe that the film-to-substrate stress increases as the Ar sputtering pressure is decreased. The increase of the stress, when the Ar sputtering pressure decreases, is probably due to an increase of bombardment energy of sputtered atoms.

3. Anisotropy variation vs. substrate materials and thicknesses:

The effective anisotropy (KU-2rM?), before and after peeling, and percentage changes in anisotropy of Tb30Co,0 and Tb27Fe73 films are tabulated in Tables 1 and 2, respectively.

Changes in substrate materials and thicknesses do not cause significant differences in the perpendicular anisotropy among glass and mylar substrates. However, the films deposited on thick polycarbonate substrates show greater perpendicular anisotropy than those on glass substrates. The percentage change of anisotropy in films when they were peeled from Mylar and polycarbonate substrates is greater than that of films tha t were peeled from glass substrates. We believe that the larger percentage change in K U for the films peeled from the plastic substrates is due to the larger film-to-substrate stress caused by the larger coefficient of thermal expansion of plastic and the fact that the plastic is not as thermally conductive as glass resulting in a higher temperature during film deposition. The films deposited on the polycarbonate substrates show the greatest change in anisotropy when peeled because the polycarbonate is the thickest, causing the growth temperature to be the highest and also being less likely to partially relax the film stress.

Anisotropies show a peak as the Ar sputtering pressure is varied. This trend is seen in both the peeled and non-peeled films.

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photoresist coated Mylar, 1 mil

photoresist coated glass, lomil

photoresist coated glass, 5 mil

photoresist coated polycarbon., 50 mil

Tb,,Co,, sputtered on a variety of substrates

0.55 0.38 30.9

0.55 0.46 15.6

0.51 0.44 12.9

1.11 0.26 77

photoresist coated Mylar, 3 mil I 0.68 I 0.46 I 32.4 I

photoresist coated Mylar, 1 mil

photoresist coated glass, 10 mil

photoresist coated glass, 5 mil

photoresist coated polycarbon., 50 mil

0.76 0.43 43.4

0.8 0.53 33.3

0.7 0.51 27.3

1.67 0.5 70.4

Tb2,Fe7, sputtered on a variety of substrates

photoresist coated Mylar, 7 mil 1 0.7 1 0.38 I 45.3

photoresist coated Mylar, 3 mil 1 0.7 I 0.48 I 31.7

Table 2: KU-2?rM: and percentage change in anisotropy of Tb,,Fe,, films deposited on a variety of thicknesses and materials of substrates. * indicates the da t a was taken from films peeled from their substrates.

SUMMARY

Stress has been shown to play a significant role in creating perpendicular anisotropy in TbCo and TbFe films. In TbCo films film-to-substrate stress was found responsible for 13 to 27 percent of the perpendicular anisotropy, which in TbFe films film-to- substrate stress was found to account for as much as 35 to 44 percent of the percent of the perpendicular anisotropy.

pressure until a maximum is reached and, then decreases. The KU of the peeled films follows a similar trend.

I t was also found that the substrates onto which the films were deposited affected the anisotropy. Although films deposited onto glass or thin Mylar were similar, films deposited onto thick polycarbonate substrates exhibited larger anisotropy when still on the substrate, but essentially the same anisotropy after removed from the substrate. This is presumably because of a larger film-to- substrate stress on polycarbonate, produced by both a higher substrate temperature caused by the poorer thermal conductivity of polycarbonate compared to glass and by the larger mismatch between the film and polycarbonate than between the film and glass.

ACKNOWLEDGEMENT

This work is supported by the Magnetic Materials Research Group, through the Division of Materials Research, National Science Foundation, under Grant No. DMR-8613386.

REFERENCES

1. S. Tsunashima, H. Takagi, K . Kamegaki, T . Fujii and S. Uchimaya, IEEE Trans. Mag., MAG-14, 844 (1978).

2. R. B. Van Dover, M. Hong, E. M. Gyorgy, J. F. Dillon, Jr., and S. D. Albiston, J. Appl. Phys., Vol. 57, 3897 (1985).

3. M. Hong, E. M. Gyorgy, R. B. Van Dover, S. Nakahara, D. D. Bacon, and P . K . Gallagher, J. Appl. Phys. Vol. 59, 551 (1986).

4. J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., Vol. 14, 164 (1977)

The contributions to anisotropy from stress depend upon the deposition conditions, the perpendicular anisotropy of dc- magnetron sputtered films decreases with increasing deposition power regardless of whether the film is on a substrate or removed. On the other hand, K U increases with increasing Ar sputtering