- Email: [email protected]
The magnetic properties of r.f.-sputtered permalloy and Mumetal films
Thin Sohd Fdms, 86 (1981) 165-174
PREPARATIONANDCHARACTERIZATION
165
THE MAGNETIC PROPERTIES OF R.F.-SPUTTERED PERMALLOY AND MUMETAL FILMS* A. J. COLLINS, C. J.
PRIORANDR.
C. J. HICKS
Wolfson Centre, 30 The Parade, Roath, Cardtff CF2 3AD (Gt. Brltam)
Results arepresented for the effect of the deposition conditions on the magnetic properties of r.f.-sputtered permalloy and Mumetal films in the thickness range 0.45 Ixm.In particular, the effects of the substrate bias potential, both d.c. and a.c., on the coercivity, the dispersion, the anisotropy field, the permeability and the saturation magnetization were investigated. The changes which occur in these properties as the bias potential is varied were related to changes in the purity, the structure and, in the case of the saturation magnetizaUon, the composition of the films.
1. INTRODUCTION Interest in sputtered ferromagnetic films of thickness exceeding 0.4 ktm has been stimulated by the rapid development of (a) magnetic bubble domain devices 1 requiring ferromagnetic overlays for bubble propagation and detection elements, (b) inductive thin film magnetic recording heads 2 and (c) magnetoreslstive read-only heads and their associated ferromagnetic shields 3. However, most ferromagnetic films produced by sputtering have been less than 0.5 lam thick 4' s, 6 and few detailed investigations have been made of thicker sputtered films 7 In the present paper results are presented on the effect of the sputtering deposition conditions on the magnetic properties of r.f.-diode-sputtered high permeability ferromagnetic films in the thickness range 0.1-6 lam. In particular, the effects of the substrate bias voltage and the temperature on the coercivity, the magnetization dispersion, the anisotropy field, the hard axis permeability and the saturation magnetization were investigated. R.f. and d.c. substrate biasing were both investigated; however, the variations in the magnetic properties with the bias potential are qualitatively similar for both types of biasing. The variations in the magnetic properties of Ni-Fe and Mumetal films with deposition conditions are in the majority of cases also qualitatively similar. Therefore, except where specific differences were observed, only representative results are presented. *Paper presented at the International Seminar on Film PreparaUon and Etching by Plasma Technology,Brighton,Gt. Britain,March25-27, 1981 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedm The Netherlands
166
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
2. EXPERIMENTAL DEPOSITION TECHNIQUES Ferromagnetic films were produced from targets of Mumetat and permalloy of diameter 3 - 4 in by r.f. sputtering with r f or d.c. substrate bias The targets were commercial Mumetal and permalloy, the compositions of which were 77wt ~,N114wt ~oFe-5wt.~oCu-4wt ~oMo and 82wt.~Ni-18wt.~oFe respecnvely. The sputtering system was lmtially evacuated to about 10 -6 Torr: then high purity bottled argon, which had been further purified using a titanium chemadsorption purifier, was introduced into the sputtering system to raise the chamber pressure to the sputtering pressure of 10 r e t o r t , Before deposation of the films the targets were sputter etched for approximately 30 mln The films were deposited m a uniform ahgning field of 60 Oe applied parallel to the film plane to induce a unlaxlal magnetic amsotropy m a known direction. An anlsotropy was reduced in the films because the lnvestiganon was mainly concerned with the production of magnetic films for recording head applicanons 2. The deposinon rate of the films was between 0.5 and 3 0 pm h - 1 depending on the deposition conditions which were as follows: target voltage, 0.5-3.0 kV; substrate bias voltage, 0 to - 4 0 0 V; substrate soleplate temperature, 0-250 °C s The substrates were C o m i n g 7059 boroslhcate glass, this material was selectcd because of its smoothness 9 and suitabdlty for mechanical machining, which is necessary for the ultimate producnon of thin film magnetxc recording heads The substrates were heated by means of a soleplate heater. In the case of d.c. biasing the bins potential was applied to the film via stainless steel chps which were pressed an intimate contact with the glass substrate, and hence the apphed bias was not effective untd a conducting layer had been deposited. However, no difference in magnetic properties could be determined between the front and back surfaces of the films, indicating that this initial layer thickness was small. 3. MEASUREMENT TECHNIQUES The coerclvlty H c, the anlsotropy field H K and the magnetization dispersion ~50 were measured using a transverse Kerr magneto-optic loop plotter The measurements were made on small areas at the centres of the films to minimize the demagnetizing effects. Measurements of the bulk coercivity using an inductive c011 technique revealed that the coercivlty of the surface determined using the Kerr magneto-optic system was within the experimental errors the same as that of the bulk of the film. The dispersion eso was measured by Crowther's first method l° and the anlsotropy field H K was determined by the improved Kobelev method 11. The hard axis initial permeabdlty was measured using a coil and a Q meter ~2"13 or by determining the effective anisotropy field HK' from extrapolation of the hard axis minor loop followed by calculation of the permeability/~ from the relationship = B~/H K'
where B, is the saturation induction. The saturation magnetizanon M; was measured on a vibrating sample magnetometer The film thickness was determined by a stylus method and the film composition was determined by X-ray fluorescence
MAGNETICPROPERTIESOF PERMALLOYAND MUMETALFILMS
167
4. RESULTS 4.1. Coercivity and dispersion Ctso The coercivity as a function of the substrate bias potential for films of both N i Fe and Mumetal is shown in Fig. 1. Similar results have been reported for thinner films 5. F o r substrate bias potentials less negative than about - 5 0 V (Fig. 1) the coercivities increase rapidly and the films tend to be isotropic. These large values of the coercive force at low substrate bias potentials are usually attributed to the formation of a void-type columnar microstructure, i.e. a columnar microstructure with voids formed between the columns, as reported by Cargill et al. 14 Such a film structure would be expected to have a large micromagnetic structure constant S 15, resulting in the large observed values of the coercivity. Cargill et al. also reported that the network of crack-like voids vanished at substrate bias potentials of between - 60 and - 120 V. At these bias potentials a much more homogeneous film structure due to a reduction in the impurity concentration within the films was observed. Therefore it is probable that the minimum in the coercivity observed for substrate bias potentials between - 50 and - 100 V can be accounted for by an improvement in the microstructure of the film and a reduction in the structure constant. The increase in coercivity for bias potentials more negative than about - 150 V can be attributed to the implantation of argon ions as they b o m b a r d the growing film 16. This increase in argon concentration would also be expected to produce increases in the structure constants and hence in the coercivity. Although compositional changes occur as the substrate bias potential is
8 _~4uJ
8
/
I' I I
-1oo -21oo -3ool SUBSTRATI¢ BIAS POTENTIAL VOLTS
-2oo
Fig 1 Dependenceofthefilmcoerovltyonthesubstrateblaspotentlal: II,permalloyfilmsl 31xmthlck prepared at a soleplate temperature of 100 °C and under a d c bias; ©, Mumetal films 0 4 ~tm thick prepared at ambient soleplatetemperature and under an r f bias
168
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
increased, they cannot in general account for the large change observed in the coerclvlty The variation in the coerclvlty with sputtered film thickness is shown in Fig. 2. The coerclvity of Mumetal films for a given film thickness and substrate temperature is less than that of N i - F e films. This lower value ofcoerclvlty is probably associated with the fact that Mumetal has a lower local anlsotropy constant K s and hence a lower structure constant S than N i - F e does iv. The variation in coercivlty with film thickness t follows a t - " law where n varies between 0.3 and 1.2 depending on the deposition conditions This compares with the theoretical thickness dependence derived by N6elI 8 of t-4/3. Similar relations have been reported for evaporated films but over a much smaller thickness range a9
2-5
~0
0
1-0-
4\
110
2' 0 3'0 FILM THICKNESS pm
4'0
5'0
Fig 2 Coerclvlty as a function of film thickness O, Mumetal films prepared at amNent soleplate temperature and under an r f bias of - 60 V, II, permalloy films prepared at a soleplate temperature of 200 "C and under a d c bias of - 150 V
The varlatlorL in the coerclvlty of sputtered films with increasing substrate temperature is in general similar to that for evaporated films 19, the coerclvity remaining approximately constant or decreasing slightly with Increasing substrate temperature between ambient and approximately 270 °C At substrate temperatures of 270-300 °C recrystalhzatlon and grain growth occur in the films, producing a rapid increase in the structure constant S and a corresponding increase in the coerclvity. The coerclvlty for N1-Fe films as a function of the substrate soleplate heater temperature is shown in Fig 3. However, it should be appreciated that the soleplate heater temperature is not necessarily equal to that of the front surface of the substrate. The effect of the substrate bias potential on the magnetization dispersion is shown in Fig 4 The variation is similar to that of the coerclvity and can be attributed to the same mechanisms. In general the dispersion shows a similar dependence on the substrate temperature as the coercivity However, while the coercivlty decreases with Increasing film thickness (Fig. 2) the dispersion was observed to increase rapidly in NI Fe films for film thicknesses In excess of about 1 gm. This can be accounted for by an increase in the demagnetizing field of the samples 2° and/or an increase in the grain size 21 and hence in the structure constant.
169
MAGNETIC PROPERTIES OF PERMALLOY AND MUMETAL FILMS
6"0
B'O
4O
40
2"0
2"0
4-4 1OO 200 SOLEPLATE TEMPERATURE CC
t,t 300
- 100
-200
, -300
, - 400
SUBSTRATE BIAS POTENTIALVOLTS
Fig 3 Coerclvlty as a funct]on ofsoleplate temperature for permalloy films 1 3 Jam thick prepared under a d c bias o f - 1 5 0 V F]g 4 Dependence of the magnetization dispersion ~5o on the substrate bias potential C), Mumetal films 0 3 - 0 4 ktm thick prepared at ambient soleplate temperature and under an r.f bias of - 6 0 V; II, permalloy films 1 3 pm thick prepared at a soleplate temperature of 100 °C and under a d c bias of -150 V
This increase in dispers]on is also accompanied by a decrease in the hard ax]s initial permeabil]ty (Section 4.2).
4.2. The anisotropyfield and hard axis permeabdity Typical results for the variation in the anisotropy field HK (determined using the improved Kobelev method) with the substrate bias potential are shown in Fig. 5. For substrate bias potentials less negatiye than - 50 V, the films tend to be ]sotropic, as indicated by the large values of the dispersion (Fig. 4). The easy axis is therefore not well defined, resulting in large errors in the measured anisotropy field. Similarly, for films deposited at substrate bins potentials more negative than - 250 V the films also tend to become isotropic. The variation in the anisotropy field with the substrate bins is similar to those of the coercivity and the dispersion. However, for the N i - F e films deposited at substrate potentials more negative than - 300 V, some of the ]ncrease in the anisotropy field could be expected to be due to changes in the composition of the film (Section 4.3) as well as to structural changes (Sect]on 4.1). The present investigation has also revealed that within the experimental errors of the measurements the anisotropy field for Ni-Fe films, as measured by the improved Kobelev method, is independent of the thickness in the range 0.1-6 pro. However, the errors involved in the measurements are large in films of th]ckness greater than approximately 2 pm, owing to the high dispersion in thick films. Typically H K is 6 + 1 0 e at a soleplate temperature of 100 °C and a substrate bias of - 100 V. The anisotropy also decreases as the substrate temperature increases; th~s
170
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
observation is quahtatively consistent with the N6el-Taniguchi theory 22. However, the errors in the measurements are again large for films deposited at substrate temperatures exceeding 250 °C because of the high dispersion exhibited by these thick films. The variation in the hard axis permeability with the substrate bias potential is shown in Fig. 6. The results determined using a coil and a Q meter are lower than those calculated from/~ = BJHK'. These discrepancies can be accounted for by the facts that firstly the relationship /~--Bs/H K' is only a first approximation and secondly there are large errors involved in determining H K' The maximum in the permeability occurs at a substrate bias potential at which the coerclvlty, the dispersion and the anlsotropy are minima
3e
60O
d 1200
FILMS TENDTOBE ISOTROPIC 0 •t
[
~k~I ~
ti/
10
{{/ t
I 1 -100 -2~) 300 SUBSTRATEBIASPOTENTIALVOLTS
-4/0
100 200 300 SUBSTRATEBIAS POTENTIALVOLTS
400
F]g 5 The amsotropy field H K as a function of the substrate bias potential II, permalloy films 1 3 gm thxck prepared at a soleplate temperature of 100 °C and under a d c Nas, O, Mumetal films 0 4 gm thick prepared at ambient substrate temperature and under an r f bins Fig 6 The dependence of the hard axis lmtlal permeabdlty on the substrate bias potential for permalloy films 1 3 lain thick prepared at a soleplate temperature of 100 °C and under a d c bxas I , calculated from Bs/Ha'. IS], measured value using a cod and a Q meter
The hard axis permeability, measured using the coil and Q meter, as a function of the film thickness is shown in Fig. 7. The results are for films deposited at substrate temperatures below 250°C. Films deposited at higher substrate temperatures tended to be lsotropic and to have high values of the magnetization dispersion, resulting in low values of the hard axis permeability. The decrease in permeabdity with increasing thickness, as shown in Fig. 7, can also be attributed to the fact that the films become highly dispersed. In such films the magnetization changes would be expected to take place by domain wall motion, resulting in low values of the
MAGNETICPROPERTIESOF PERMALLOYAND MUMETALFILMS
171
2000
1800
,.i m
|
400
i
t
2"0 4-0 FILM THICKNESS p m
t
8"0
Fig 7 Hard axis lmtml permeabdltyas a functzonof film thickness for permalloyfilms prepared at a soleplate temperatureof 100-200°C and under a d c bias of - 150 V permeability 23. The permeability of about 850 for films 2.0-2.5 lam thick is similar to the values reported by Philipp and Tiemann 23.
4.3. Saturatton magnetization The variation in the composition of an Ni-Fe film with the substrate bias potential is shown in Fig. 8. At bias potentials less than - 150 V, the preferential removal of nickel from the target surface, due to the normally incident positively charged gas ions 24, is probably dominant in comparison with the resputtering of the film material, resulting in films with a small nickel enrichment. The reduction in nickel concentration as the bias potential is made more negative than - 1 5 0 V occurs because the preferential resputtering of nickel from the deposited film becomes dominant. This would be expected as the sputtering yield of nickel is higher than that of iron for all values of bombarding argon ion energies from near the threshold up to 1 keV 25,26. The saturation magnetization as a function of the substrate bias potential is shown in Fig. 9. The approximate bulk values of the saturation magnetization of N i - F e alloys are also shown in Fig. 9 as functions of the alloy composition. These values were obtained from the variation in the bulk value with the alloy composition reported by Bozorth 2T, using the results for the composition as a function of the substrate bias potential shown in Fig. 9. The linear increase in the saturation magnetization with the substrate bias potential can primarily be attributed to compositional changes produced by the preferential removal of nickel as the substrate bias potential is made more negative. Investigations revealed that the measured saturation magnetization of Mumetal films is far more sensitive than that of permalloy films to changes m
172
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
FILM 83 '
COMPOSITION 82 I
wt°/oNICKEL
80 I
75 J
70
100
10OC O
TARGET
~ae
COMPOSITION
4---/
8oc
/
BULK
VALUE
/
60 6oc
0
I --100 SUBSTRATE
I I --200 --300 BIAS POTENTIAL VOLTS
I --400
0
I --100 SUBSTRATE
r I --200 --300 B I A S P O T E N T I A L VOLTS
I --400
Fig 8 The dependenceofNl-Fe filmcompositionon the substrate bias potential (soleplate temperature. 100"C, d c bias) Fig 9 Saturationmagnetization of N1-Fe films as a function of the substrate bias potential and the film composmon
deposition conditions. The variation in the saturation magnetization of Mumetal films with the target voltage is shown in Fig 10 with substrate bias voltage as a parameter The low value of the saturation magnetization exhibited by films produced with low target voltages or a high substrate bias voltage could be increased to the bulk value of the saturation magnetization by post-deposition annealing in a vacuum at 300°C for approximately 0.5 h. No increase in the saturation magnetization was observed for annealing at temperatures below 300 '~C. even after annealing for 10 h The annealing temperature of 300 °C corresponds to the recrystallization temperature generally observed in thin films. X-ray diffraction investigations of the films annealed at 300°C showed that the grain size had Increased from the as-deposited value of 160+20 A to more than 1000 A. As preliminary investigations of the composition of the films revealed only small changes m the composition w~th deposition c o n d i t i o n s - - t h o u g h in general for substrate biases less negative than - 1 5 0 V the films tended to be rich in nickel. similar to N1-Fe films the low values of the saturation magnetization were tentatively attributed to a highly disordered structure which also gives rise to the high internal stresses in the films, as observed by X-ray diffraction. The variation in the saturation magnetization with the deposition conditions is closely related to the deposition rate, as decreasing the target voltage or Increasing the substrate bias voltage caused a decrease in the deposition rate This was further confirmed by the observation that decreasing the deposition rate by increasing the substrate temperature also produced a decrease in the measured saturanon magnetization.
MAGNETIC PROPERTIES OF PERMALLOY AND MUMETAL FILMS
173
700 BULK VALUE BOO O
_N ,oc :!
200.
0"5
10
15
20
25
TARGET VOLTAGE K VOLTS
Fig 10 The variation in the saturation magnetization of Mumetal films as a function of the target voltage w]th the r f bias potential as a parameter III, _ 60 V bias, O, - 150 V bias 5. CONCLUSIONS
While the magnetic properties and composition of r.f.-sputtered ferromagnetic films are dependent on the deposition conditions, in general these can be controlled sufficiently accurately to enable the production of reproducible films. Ferromagnetic films can be produced by sputtering, making them ideally suited for Incorporation in magnetic recording heads and bubble devices. The advantage of r.f. sputtering over other techniques (such as electroplating) of producing thick (greater than 0.5 Ixm) magnetic films is that it can be employed to deposit all the materials required in integrated magnetic devices, such as conductors, insulators and adhesion layers. In some devices this allows multiple-layer depositions to be undertaken. Furthermore, by sputtering it is possible to produce conformal coatings of magnetic material over the edges of previously deposited layers, such as conductors. This is important in, for example, the production of high efficiency magnetic recording heads. ACKNOWLEDGMENTS
The authors wish to thank Dr. G. M. Steed and Mr. P. C. Fisher of the Department of Mineral Exploitation, University College, Cardiff, for the compositional measurements of the Ni-Fe films and Mr. R. D. Garwood of the Department of Materials Science, University College, Cardiff, for compositional and structural investigations of the Mumetal films. Dr. C. J. Prior and Mr. R. C. J. Hicks are indebted to the Science Research Council for financial support during this investigation. REFERENCES 1
T H O'Dell, Magnetw Bubbles, Macmdlan, London, 1974
174 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
A.J. COLLINS, C. J. PRIOR, R. C. J. HICKS
A J Colhns and I Preece, Radto Electron Eng, 34 (3) (1974) 153 R. I Potter, IEEE Trans Magn., 10 (1974) 502. W N Mayer, IEEETrans Magn,4(1966) 166 A.J G n e s t a n d B L Flur, J Appl. Phys,38(1967) 1431 B L F l u r a n d A J Gnest, J AppLPhys,38(1967) 1478. H. Shlbaya and I Fukuda, IEEE Trans Magn, 13 (1977) 1029 J L Vossen and J J O'Nedl, RCA Rev, 29 (1968) 149 L I Malssel and R Glang (eds), Handbook of Thin Ftlm Technology, McGraw-Hdk New York, 1970 T S Crowther, J Appl Phys ,34(1963)580 K Kempter and H Hoffmann, Z. Angew Phy3,28 (1970) 251. K M Pohvanov and A L Frumkm, Trans Sctenufic and Te~hmcal Conf on Methods and Equtpment for Testmg Magnettc Matertals, 1961, p 278 J P Lazzanandl. Melnxck, IEEETrans Magn,7(1971) 146 G S Cargill, S R. Herd, W. E Krull and K Y Ahn, IEEE Trans. Magn, 15 (1979) 1821 H. Hoffmann, Phys. Status Sohdl, 33 (1969) 175 H F Winters and E Kay, J Appl Phys, 38 (1967) 3928 W D. DoyleandT F Fmnegan, J Appl Phys,39(1968) 3355 L. N6el, J Phys Radmm, 17 (1956) 250. M. Prutton, Thm Ferromagnetzc Fdms, Butterworths, London, 1964 S. Mlddlehoek, m J Smlt (ed), Thm Fdms m Magnetw Propertws of Materials, McGraw-Hill, New York, 1971 K L Chopra, Thin Fdm Phenomena, McGraw-Hall, New York, 1969 D O Sooho, Magnettc Thtn Films, Harper and Row, New York, 1965 H. R Phthpp and J J. Tlemann, J Appl Phy~, 43 (1972) 3542. R.R. Olson and G. K Wehner, J Vac Sct Technol, 14 (1977) 319 C H Weljsenfeld, A Hoogendoom and M Koedam, Phystca, 27 (1961) 763. N Laegned and G K Wehner, J Appl. Phys., 32 (1961) 365. R M. Bozorth, Ferromagnettsm, Van Nostrand, Princeton, N J, 1951
PREPARATIONANDCHARACTERIZATION
165
THE MAGNETIC PROPERTIES OF R.F.-SPUTTERED PERMALLOY AND MUMETAL FILMS* A. J. COLLINS, C. J.
PRIORANDR.
C. J. HICKS
Wolfson Centre, 30 The Parade, Roath, Cardtff CF2 3AD (Gt. Brltam)
Results arepresented for the effect of the deposition conditions on the magnetic properties of r.f.-sputtered permalloy and Mumetal films in the thickness range 0.45 Ixm.In particular, the effects of the substrate bias potential, both d.c. and a.c., on the coercivity, the dispersion, the anisotropy field, the permeability and the saturation magnetization were investigated. The changes which occur in these properties as the bias potential is varied were related to changes in the purity, the structure and, in the case of the saturation magnetizaUon, the composition of the films.
1. INTRODUCTION Interest in sputtered ferromagnetic films of thickness exceeding 0.4 ktm has been stimulated by the rapid development of (a) magnetic bubble domain devices 1 requiring ferromagnetic overlays for bubble propagation and detection elements, (b) inductive thin film magnetic recording heads 2 and (c) magnetoreslstive read-only heads and their associated ferromagnetic shields 3. However, most ferromagnetic films produced by sputtering have been less than 0.5 lam thick 4' s, 6 and few detailed investigations have been made of thicker sputtered films 7 In the present paper results are presented on the effect of the sputtering deposition conditions on the magnetic properties of r.f.-diode-sputtered high permeability ferromagnetic films in the thickness range 0.1-6 lam. In particular, the effects of the substrate bias voltage and the temperature on the coercivity, the magnetization dispersion, the anisotropy field, the hard axis permeability and the saturation magnetization were investigated. R.f. and d.c. substrate biasing were both investigated; however, the variations in the magnetic properties with the bias potential are qualitatively similar for both types of biasing. The variations in the magnetic properties of Ni-Fe and Mumetal films with deposition conditions are in the majority of cases also qualitatively similar. Therefore, except where specific differences were observed, only representative results are presented. *Paper presented at the International Seminar on Film PreparaUon and Etching by Plasma Technology,Brighton,Gt. Britain,March25-27, 1981 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedm The Netherlands
166
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
2. EXPERIMENTAL DEPOSITION TECHNIQUES Ferromagnetic films were produced from targets of Mumetat and permalloy of diameter 3 - 4 in by r.f. sputtering with r f or d.c. substrate bias The targets were commercial Mumetal and permalloy, the compositions of which were 77wt ~,N114wt ~oFe-5wt.~oCu-4wt ~oMo and 82wt.~Ni-18wt.~oFe respecnvely. The sputtering system was lmtially evacuated to about 10 -6 Torr: then high purity bottled argon, which had been further purified using a titanium chemadsorption purifier, was introduced into the sputtering system to raise the chamber pressure to the sputtering pressure of 10 r e t o r t , Before deposation of the films the targets were sputter etched for approximately 30 mln The films were deposited m a uniform ahgning field of 60 Oe applied parallel to the film plane to induce a unlaxlal magnetic amsotropy m a known direction. An anlsotropy was reduced in the films because the lnvestiganon was mainly concerned with the production of magnetic films for recording head applicanons 2. The deposinon rate of the films was between 0.5 and 3 0 pm h - 1 depending on the deposition conditions which were as follows: target voltage, 0.5-3.0 kV; substrate bias voltage, 0 to - 4 0 0 V; substrate soleplate temperature, 0-250 °C s The substrates were C o m i n g 7059 boroslhcate glass, this material was selectcd because of its smoothness 9 and suitabdlty for mechanical machining, which is necessary for the ultimate producnon of thin film magnetxc recording heads The substrates were heated by means of a soleplate heater. In the case of d.c. biasing the bins potential was applied to the film via stainless steel chps which were pressed an intimate contact with the glass substrate, and hence the apphed bias was not effective untd a conducting layer had been deposited. However, no difference in magnetic properties could be determined between the front and back surfaces of the films, indicating that this initial layer thickness was small. 3. MEASUREMENT TECHNIQUES The coerclvlty H c, the anlsotropy field H K and the magnetization dispersion ~50 were measured using a transverse Kerr magneto-optic loop plotter The measurements were made on small areas at the centres of the films to minimize the demagnetizing effects. Measurements of the bulk coercivity using an inductive c011 technique revealed that the coercivlty of the surface determined using the Kerr magneto-optic system was within the experimental errors the same as that of the bulk of the film. The dispersion eso was measured by Crowther's first method l° and the anlsotropy field H K was determined by the improved Kobelev method 11. The hard axis initial permeabdlty was measured using a coil and a Q meter ~2"13 or by determining the effective anisotropy field HK' from extrapolation of the hard axis minor loop followed by calculation of the permeability/~ from the relationship = B~/H K'
where B, is the saturation induction. The saturation magnetizanon M; was measured on a vibrating sample magnetometer The film thickness was determined by a stylus method and the film composition was determined by X-ray fluorescence
MAGNETICPROPERTIESOF PERMALLOYAND MUMETALFILMS
167
4. RESULTS 4.1. Coercivity and dispersion Ctso The coercivity as a function of the substrate bias potential for films of both N i Fe and Mumetal is shown in Fig. 1. Similar results have been reported for thinner films 5. F o r substrate bias potentials less negative than about - 5 0 V (Fig. 1) the coercivities increase rapidly and the films tend to be isotropic. These large values of the coercive force at low substrate bias potentials are usually attributed to the formation of a void-type columnar microstructure, i.e. a columnar microstructure with voids formed between the columns, as reported by Cargill et al. 14 Such a film structure would be expected to have a large micromagnetic structure constant S 15, resulting in the large observed values of the coercivity. Cargill et al. also reported that the network of crack-like voids vanished at substrate bias potentials of between - 60 and - 120 V. At these bias potentials a much more homogeneous film structure due to a reduction in the impurity concentration within the films was observed. Therefore it is probable that the minimum in the coercivity observed for substrate bias potentials between - 50 and - 100 V can be accounted for by an improvement in the microstructure of the film and a reduction in the structure constant. The increase in coercivity for bias potentials more negative than about - 150 V can be attributed to the implantation of argon ions as they b o m b a r d the growing film 16. This increase in argon concentration would also be expected to produce increases in the structure constants and hence in the coercivity. Although compositional changes occur as the substrate bias potential is
8 _~4uJ
8
/
I' I I
-1oo -21oo -3ool SUBSTRATI¢ BIAS POTENTIAL VOLTS
-2oo
Fig 1 Dependenceofthefilmcoerovltyonthesubstrateblaspotentlal: II,permalloyfilmsl 31xmthlck prepared at a soleplate temperature of 100 °C and under a d c bias; ©, Mumetal films 0 4 ~tm thick prepared at ambient soleplatetemperature and under an r f bias
168
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
increased, they cannot in general account for the large change observed in the coerclvlty The variation in the coerclvlty with sputtered film thickness is shown in Fig. 2. The coerclvity of Mumetal films for a given film thickness and substrate temperature is less than that of N i - F e films. This lower value ofcoerclvlty is probably associated with the fact that Mumetal has a lower local anlsotropy constant K s and hence a lower structure constant S than N i - F e does iv. The variation in coercivlty with film thickness t follows a t - " law where n varies between 0.3 and 1.2 depending on the deposition conditions This compares with the theoretical thickness dependence derived by N6elI 8 of t-4/3. Similar relations have been reported for evaporated films but over a much smaller thickness range a9
2-5
~0
0
1-0-
4\
110
2' 0 3'0 FILM THICKNESS pm
4'0
5'0
Fig 2 Coerclvlty as a function of film thickness O, Mumetal films prepared at amNent soleplate temperature and under an r f bias of - 60 V, II, permalloy films prepared at a soleplate temperature of 200 "C and under a d c bias of - 150 V
The varlatlorL in the coerclvlty of sputtered films with increasing substrate temperature is in general similar to that for evaporated films 19, the coerclvity remaining approximately constant or decreasing slightly with Increasing substrate temperature between ambient and approximately 270 °C At substrate temperatures of 270-300 °C recrystalhzatlon and grain growth occur in the films, producing a rapid increase in the structure constant S and a corresponding increase in the coerclvity. The coerclvlty for N1-Fe films as a function of the substrate soleplate heater temperature is shown in Fig 3. However, it should be appreciated that the soleplate heater temperature is not necessarily equal to that of the front surface of the substrate. The effect of the substrate bias potential on the magnetization dispersion is shown in Fig 4 The variation is similar to that of the coerclvity and can be attributed to the same mechanisms. In general the dispersion shows a similar dependence on the substrate temperature as the coercivity However, while the coercivlty decreases with Increasing film thickness (Fig. 2) the dispersion was observed to increase rapidly in NI Fe films for film thicknesses In excess of about 1 gm. This can be accounted for by an increase in the demagnetizing field of the samples 2° and/or an increase in the grain size 21 and hence in the structure constant.
169
MAGNETIC PROPERTIES OF PERMALLOY AND MUMETAL FILMS
6"0
B'O
4O
40
2"0
2"0
4-4 1OO 200 SOLEPLATE TEMPERATURE CC
t,t 300
- 100
-200
, -300
, - 400
SUBSTRATE BIAS POTENTIALVOLTS
Fig 3 Coerclvlty as a funct]on ofsoleplate temperature for permalloy films 1 3 Jam thick prepared under a d c bias o f - 1 5 0 V F]g 4 Dependence of the magnetization dispersion ~5o on the substrate bias potential C), Mumetal films 0 3 - 0 4 ktm thick prepared at ambient soleplate temperature and under an r.f bias of - 6 0 V; II, permalloy films 1 3 pm thick prepared at a soleplate temperature of 100 °C and under a d c bias of -150 V
This increase in dispers]on is also accompanied by a decrease in the hard ax]s initial permeabil]ty (Section 4.2).
4.2. The anisotropyfield and hard axis permeabdity Typical results for the variation in the anisotropy field HK (determined using the improved Kobelev method) with the substrate bias potential are shown in Fig. 5. For substrate bias potentials less negatiye than - 50 V, the films tend to be ]sotropic, as indicated by the large values of the dispersion (Fig. 4). The easy axis is therefore not well defined, resulting in large errors in the measured anisotropy field. Similarly, for films deposited at substrate bins potentials more negative than - 250 V the films also tend to become isotropic. The variation in the anisotropy field with the substrate bins is similar to those of the coercivity and the dispersion. However, for the N i - F e films deposited at substrate potentials more negative than - 300 V, some of the ]ncrease in the anisotropy field could be expected to be due to changes in the composition of the film (Section 4.3) as well as to structural changes (Sect]on 4.1). The present investigation has also revealed that within the experimental errors of the measurements the anisotropy field for Ni-Fe films, as measured by the improved Kobelev method, is independent of the thickness in the range 0.1-6 pro. However, the errors involved in the measurements are large in films of th]ckness greater than approximately 2 pm, owing to the high dispersion in thick films. Typically H K is 6 + 1 0 e at a soleplate temperature of 100 °C and a substrate bias of - 100 V. The anisotropy also decreases as the substrate temperature increases; th~s
170
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
observation is quahtatively consistent with the N6el-Taniguchi theory 22. However, the errors in the measurements are again large for films deposited at substrate temperatures exceeding 250 °C because of the high dispersion exhibited by these thick films. The variation in the hard axis permeability with the substrate bias potential is shown in Fig. 6. The results determined using a coil and a Q meter are lower than those calculated from/~ = BJHK'. These discrepancies can be accounted for by the facts that firstly the relationship /~--Bs/H K' is only a first approximation and secondly there are large errors involved in determining H K' The maximum in the permeability occurs at a substrate bias potential at which the coerclvlty, the dispersion and the anlsotropy are minima
3e
60O
d 1200
FILMS TENDTOBE ISOTROPIC 0 •t
[
~k~I ~
ti/
10
{{/ t
I 1 -100 -2~) 300 SUBSTRATEBIASPOTENTIALVOLTS
-4/0
100 200 300 SUBSTRATEBIAS POTENTIALVOLTS
400
F]g 5 The amsotropy field H K as a function of the substrate bias potential II, permalloy films 1 3 gm thxck prepared at a soleplate temperature of 100 °C and under a d c Nas, O, Mumetal films 0 4 gm thick prepared at ambient substrate temperature and under an r f bins Fig 6 The dependence of the hard axis lmtlal permeabdlty on the substrate bias potential for permalloy films 1 3 lain thick prepared at a soleplate temperature of 100 °C and under a d c bxas I , calculated from Bs/Ha'. IS], measured value using a cod and a Q meter
The hard axis permeability, measured using the coil and Q meter, as a function of the film thickness is shown in Fig. 7. The results are for films deposited at substrate temperatures below 250°C. Films deposited at higher substrate temperatures tended to be lsotropic and to have high values of the magnetization dispersion, resulting in low values of the hard axis permeability. The decrease in permeabdity with increasing thickness, as shown in Fig. 7, can also be attributed to the fact that the films become highly dispersed. In such films the magnetization changes would be expected to take place by domain wall motion, resulting in low values of the
MAGNETICPROPERTIESOF PERMALLOYAND MUMETALFILMS
171
2000
1800
,.i m
|
400
i
t
2"0 4-0 FILM THICKNESS p m
t
8"0
Fig 7 Hard axis lmtml permeabdltyas a functzonof film thickness for permalloyfilms prepared at a soleplate temperatureof 100-200°C and under a d c bias of - 150 V permeability 23. The permeability of about 850 for films 2.0-2.5 lam thick is similar to the values reported by Philipp and Tiemann 23.
4.3. Saturatton magnetization The variation in the composition of an Ni-Fe film with the substrate bias potential is shown in Fig. 8. At bias potentials less than - 150 V, the preferential removal of nickel from the target surface, due to the normally incident positively charged gas ions 24, is probably dominant in comparison with the resputtering of the film material, resulting in films with a small nickel enrichment. The reduction in nickel concentration as the bias potential is made more negative than - 1 5 0 V occurs because the preferential resputtering of nickel from the deposited film becomes dominant. This would be expected as the sputtering yield of nickel is higher than that of iron for all values of bombarding argon ion energies from near the threshold up to 1 keV 25,26. The saturation magnetization as a function of the substrate bias potential is shown in Fig. 9. The approximate bulk values of the saturation magnetization of N i - F e alloys are also shown in Fig. 9 as functions of the alloy composition. These values were obtained from the variation in the bulk value with the alloy composition reported by Bozorth 2T, using the results for the composition as a function of the substrate bias potential shown in Fig. 9. The linear increase in the saturation magnetization with the substrate bias potential can primarily be attributed to compositional changes produced by the preferential removal of nickel as the substrate bias potential is made more negative. Investigations revealed that the measured saturation magnetization of Mumetal films is far more sensitive than that of permalloy films to changes m
172
A . J . COLLINS, C. J. PRIOR, R. C. J. HICKS
FILM 83 '
COMPOSITION 82 I
wt°/oNICKEL
80 I
75 J
70
100
10OC O
TARGET
~ae
COMPOSITION
4---/
8oc
/
BULK
VALUE
/
60 6oc
0
I --100 SUBSTRATE
I I --200 --300 BIAS POTENTIAL VOLTS
I --400
0
I --100 SUBSTRATE
r I --200 --300 B I A S P O T E N T I A L VOLTS
I --400
Fig 8 The dependenceofNl-Fe filmcompositionon the substrate bias potential (soleplate temperature. 100"C, d c bias) Fig 9 Saturationmagnetization of N1-Fe films as a function of the substrate bias potential and the film composmon
deposition conditions. The variation in the saturation magnetization of Mumetal films with the target voltage is shown in Fig 10 with substrate bias voltage as a parameter The low value of the saturation magnetization exhibited by films produced with low target voltages or a high substrate bias voltage could be increased to the bulk value of the saturation magnetization by post-deposition annealing in a vacuum at 300°C for approximately 0.5 h. No increase in the saturation magnetization was observed for annealing at temperatures below 300 '~C. even after annealing for 10 h The annealing temperature of 300 °C corresponds to the recrystallization temperature generally observed in thin films. X-ray diffraction investigations of the films annealed at 300°C showed that the grain size had Increased from the as-deposited value of 160+20 A to more than 1000 A. As preliminary investigations of the composition of the films revealed only small changes m the composition w~th deposition c o n d i t i o n s - - t h o u g h in general for substrate biases less negative than - 1 5 0 V the films tended to be rich in nickel. similar to N1-Fe films the low values of the saturation magnetization were tentatively attributed to a highly disordered structure which also gives rise to the high internal stresses in the films, as observed by X-ray diffraction. The variation in the saturation magnetization with the deposition conditions is closely related to the deposition rate, as decreasing the target voltage or Increasing the substrate bias voltage caused a decrease in the deposition rate This was further confirmed by the observation that decreasing the deposition rate by increasing the substrate temperature also produced a decrease in the measured saturanon magnetization.
MAGNETIC PROPERTIES OF PERMALLOY AND MUMETAL FILMS
173
700 BULK VALUE BOO O
_N ,oc :!
200.
0"5
10
15
20
25
TARGET VOLTAGE K VOLTS
Fig 10 The variation in the saturation magnetization of Mumetal films as a function of the target voltage w]th the r f bias potential as a parameter III, _ 60 V bias, O, - 150 V bias 5. CONCLUSIONS
While the magnetic properties and composition of r.f.-sputtered ferromagnetic films are dependent on the deposition conditions, in general these can be controlled sufficiently accurately to enable the production of reproducible films. Ferromagnetic films can be produced by sputtering, making them ideally suited for Incorporation in magnetic recording heads and bubble devices. The advantage of r.f. sputtering over other techniques (such as electroplating) of producing thick (greater than 0.5 Ixm) magnetic films is that it can be employed to deposit all the materials required in integrated magnetic devices, such as conductors, insulators and adhesion layers. In some devices this allows multiple-layer depositions to be undertaken. Furthermore, by sputtering it is possible to produce conformal coatings of magnetic material over the edges of previously deposited layers, such as conductors. This is important in, for example, the production of high efficiency magnetic recording heads. ACKNOWLEDGMENTS
The authors wish to thank Dr. G. M. Steed and Mr. P. C. Fisher of the Department of Mineral Exploitation, University College, Cardiff, for the compositional measurements of the Ni-Fe films and Mr. R. D. Garwood of the Department of Materials Science, University College, Cardiff, for compositional and structural investigations of the Mumetal films. Dr. C. J. Prior and Mr. R. C. J. Hicks are indebted to the Science Research Council for financial support during this investigation. REFERENCES 1
T H O'Dell, Magnetw Bubbles, Macmdlan, London, 1974
174 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
A.J. COLLINS, C. J. PRIOR, R. C. J. HICKS
A J Colhns and I Preece, Radto Electron Eng, 34 (3) (1974) 153 R. I Potter, IEEE Trans Magn., 10 (1974) 502. W N Mayer, IEEETrans Magn,4(1966) 166 A.J G n e s t a n d B L Flur, J Appl. Phys,38(1967) 1431 B L F l u r a n d A J Gnest, J AppLPhys,38(1967) 1478. H. Shlbaya and I Fukuda, IEEE Trans Magn, 13 (1977) 1029 J L Vossen and J J O'Nedl, RCA Rev, 29 (1968) 149 L I Malssel and R Glang (eds), Handbook of Thin Ftlm Technology, McGraw-Hdk New York, 1970 T S Crowther, J Appl Phys ,34(1963)580 K Kempter and H Hoffmann, Z. Angew Phy3,28 (1970) 251. K M Pohvanov and A L Frumkm, Trans Sctenufic and Te~hmcal Conf on Methods and Equtpment for Testmg Magnettc Matertals, 1961, p 278 J P Lazzanandl. Melnxck, IEEETrans Magn,7(1971) 146 G S Cargill, S R. Herd, W. E Krull and K Y Ahn, IEEE Trans. Magn, 15 (1979) 1821 H. Hoffmann, Phys. Status Sohdl, 33 (1969) 175 H F Winters and E Kay, J Appl Phys, 38 (1967) 3928 W D. DoyleandT F Fmnegan, J Appl Phys,39(1968) 3355 L. N6el, J Phys Radmm, 17 (1956) 250. M. Prutton, Thm Ferromagnetzc Fdms, Butterworths, London, 1964 S. Mlddlehoek, m J Smlt (ed), Thm Fdms m Magnetw Propertws of Materials, McGraw-Hill, New York, 1971 K L Chopra, Thin Fdm Phenomena, McGraw-Hall, New York, 1969 D O Sooho, Magnettc Thtn Films, Harper and Row, New York, 1965 H. R Phthpp and J J. Tlemann, J Appl Phy~, 43 (1972) 3542. R.R. Olson and G. K Wehner, J Vac Sct Technol, 14 (1977) 319 C H Weljsenfeld, A Hoogendoom and M Koedam, Phystca, 27 (1961) 763. N Laegned and G K Wehner, J Appl. Phys., 32 (1961) 365. R M. Bozorth, Ferromagnettsm, Van Nostrand, Princeton, N J, 1951