Co-Cr layers for the perpendicular recording mode were deposited by means of RF-sputtering. The most important sputter parameters, i.e. the RF sputter high voltage VRF, the argon pressure Parand the substrate holder temperature Tsh, gave an optimum value for perpendicular orientation of the magnetization. The crystal structure is always hcp within the ranges of varied parameters and no other magnetic phases were observed. If the sputter parameters do not have optimum values an additional hcp compound with in-plane orientation of the c-axis is observed. This orientation causes an increase of the in-plane remanenceS//= (M_{r}/M_{s}). Measurements of the substrate temperature Tsas an function of the various sputter parameters lead to the conclusion that an exclusive perpendicular c-axis orientation is only obtained atT_{s} sim 15thetadegc. At other Tsestablished either directly by changing Tshor indirectly by changing the sputter conditions an additional hcp compound with in-plane c-axis orientation appears. We concluded that Tsis the dominant parameter for sputtering CoCr layers

Lodder, J.C.

Wielinga, T.

English

Co-Cr layers for the perpendicular recording mode were deposited by means of RF-sputtering. The most important sputter parameters, i.e. the RF sputter high voltage VRF, the argon pressure Par and the substrate holder temperature Tsh, gave an optimum value for perpendicular orientation of the magnetization. The crystal structure is always hcp within the range of varied parameters and no other magnetic phases were observed. If the sputter parameters do not have optimum values an additional hcp compound with in-plane orientation of the c-axis is observed. This orientation causes an increase of the in-plane remanence S//= (Mr/Ms). Measurements of the substrate temperature Ts as an function of the various sputter parameters lead to the conclusion that an exclusive perpendicular c-axis orientation is only obtained at Ts~150oC. At other Ts established either directly by changing Tsh or indirectly by changing the sputter conditions an additional hcp compound with in-plane c-axis orientation appears. We concluded that Ts is the dominant parameter for sputtering CoCr layers

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Influence of R.F. sputter parameters on the magnetic orientation of Co-Cr layers

University of Twente Research Information

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. 1,  JANUARY 1984 57 I N F L U E N C E  OF R.F. S P U T T E R   P A R A M E T E R S   O N   T H E   M A G N E T I C  ORIENTATION OF Co-Cr  LAYERS. J.C.Lodder and T.Wielinga Abstract - Co-Cr layers  for  the  perpendicular  recording mode  were  deposited  by  means  of  RF-sputtering.  The  most important  sputter  parameters,  i.e.  the RF sputter  high voltage  VRF,  the  argon  pressure  Par  and  the substrate  holder  temperature  Tshl  gave  an  optimum value  for  perpendicular  orientation  of  the magnetization.  The  crystal'structure is always  hcp within  the  ranges  of  varied  parameters  and  no  other magnetic  phases  were  observed.  If the sputter parameters  do  not  have  optimum  values  an  additional  hcp compound  with  in-plane  orientation  of  the  c-axis is observed.  This  Orientation  causes  an  increase  of  the in-plane  remanence S//= (Mrfis).  Measurements  of the  substrate  temperature Ts as  an  function  of  the various  sputter  parameters  lead  to  the  conclusion  that an  exclusive  perpendicular  c-axis  orientation  is  only obtained  at Ts . 15PC. At  other Ts established either  directly  by  changing TA or  indirectly  by changing  the  sputter  conditions  an  additional  hcp compolind  with  in-plane  c-axis  orientation  appears. concluded  that Ts is the  dominant  parameter  for sputtering  CoCr  layers. INTRODUCTION W-sputtered  Co-Cr  layers  show  suitable  properties  for perpendicular  recording  [1,2].  The  layers  only  exhibit the  hexagonal  close-packed  (hcp)  structure  with  perpen- ,dicular  c-axis  orientation,  thereby  establishing  large perpendicular  crystal  anisotropy.  The optimm perpendi- cular  orientation  of  the  magnetization  strongly  depends on  the  sputter  parameters  used:  VRF,  Par  and  Tsh. ?he  aims  of  this  investigation,  were  first  to  determine the  optimal  sputter  parameters  and  then  to  clarify  the causes  of  magnetic  deterioration  outside  this  set  of optimal  parameters.  The  quality  of  the  perpendicular orientation  of  the  magnetization  is,easily  perceived  by means  of  the  hard-axis  (in-plane)  hysteresis  loop.  In the  ideal  case,  i.e.  the  c-axis s perfectly  perpendi- cularly  oriented,  the  anisotropy'axis  has  no  dispersion and  the  hard-axis  loop,  exhibiting  no  in-plane  remanen- ce,  should  be  observed.  In  fact  Co-Cr  films  sputtered with  optimal  parameters  have a very  small S/ of about 5%, as  shown  by  the  hard-axis  loop  in  6ig.la. However, S i /  is considerably  larger  at  other  values of  the  sputter  parameters  as  shown  in  Fig.lb.  We  use S as a measure  of  the  perpendicular  orientation  of tke  magnetization  and  assume  that a non-zero  value  of S / may  have  the  following  causes:  a)  Dispersion  of d e  c-axis  around  the  film  normal.  b)  Orientations  of the  c-axis  other  than  the  film  normal.  c)  Other  magne- tic  phases  with  low  magnetic  anisotropy  like  for  .exam- ple  fcc  Co-Cr or magnetic-  oxides. V R F -  1.60 kV VRF = 0.m hV S,, = 0.14 0 200 400 600 800 0 2C0 400 600 800 - H [kA/m] - H [kA/m] Fig.  1.  The  in-plane (VSM) hysteresis  loops:  a)  under optimal  conditions.  b)  under  other  sputter  conditions, e.g. a different v ~ ;  s i /  is considerably  increased Manuscript  received J u l y  15, 1983. The authors  are  with mente University of TechnoZogy, P. 0. Box 217, 7500 AE Enschede, The  NetherZands. EXPERIMENTAL  PROCEDURE A comercial Leytmld-Heraeus  RF  sputter  unit  (type 2400) was  used  for  the  deposition.  The  19  at%Cr  alloy target,  had a diameter  of 10 cm and a purity  of  99,99%. ?he  target-substrate  distance  was 5 cm. Monocrystalline (100)-oriented  Si  substrates (0 .3 mn thick)  were  used for  magnetic  and  X-ray  measurements  and  freshly  cleaved mica  sheats  covered  with  carbon  for  transmission  elec- tron  microscopy (TEM). The  substrate  holder  was  water cooled,.but  for  experiments  with  varying  Tsh a heated one  was  used.  Diffusion  of  the  Co-Cr  into  the  Si  sub- strate  takes  place  at  temperatures  higher  than  about 100~~. Therefore  only  mica-carbon  substrates  are used  for  sputtering  on  the  heated  substrate  holder.  'Ihe thickness  of  all  layers is approx.  150 nm and  has  been so chosen  because  the  c-axis  dispersion s then  no  lon- ger  critically  dependent  on  the  film  thickness [3] whi- le TEM is still  possible.  After a background  pressure of  2.lW7mbar is reached  by  pumping 10 hours  with a turbomolecular  pump,  the  pressure is further  reduced  to below lW7mbar by means  of a liquid N2 Nissner trap  and  argon  (purity 99.998 %) was  admitted  as  sput- ter  gas.  Before  deposition  the  target is sputter  clea- ned for 30 min.  and  the  substrates  cleaned  by  means  of glow  discharge.  Both  the  film  composition  and  thickness of  the  deposited  films  were  determined  by  means  of  X- ray  fluorescence [4]. The  film  composition  appeared  to be  independent  of  the  sputter  parameters  and  close  to the  target  composition  of  19(5,5)  at%Cr.  The  in-plane hysteresis  loop  of  the  Co-Cr  layers  was  measured  with a vibrating  sample  magnetometer (VSM). The Si/ could  be de ermined  with a relative  accuracy  of  about 5%. In  or- der  to  analyse  the  crystallographic  phases  and  texture, we used a 200kV  Jeol  microscope  and  Philips  X-ray diffraction  apparatus,  using  Cu-Ka  radiation.  Electron diffraction  was  primarily  applied  to  investigate  which alien  crystal  structures  were  present  in  the  hcp  Co-Cr layers,  because  its  sensitivity  to  small  amounts  of  mi- xed  phases is higher  than  that  of  X-ray  diffraction. Furthermore  electron  diffraction  also  reveals a possi- ble  in-plane  orientation  of  the  c-axis,  in  which  case the  (00.2)  and {lO.l] planes  are  parallel  to  the  elec- tron  beam  and  cause  diffraction.  The  diffraction  pat- t rns  were  optically  read-out  by  means  of a microdensi- tometer  and  recorded. We then  only  needed  information on  out-of-plane  orientations  of  the  c-axis  which  could readily  be  measured  by  means  of  the  X-ray  rocking  curve method [3]. The  (00.2)  rocking  curve  enables  registra- tion  of  almost  all  c-axis  orientations  between  the  nor- mal  nd  the  plane  of  the  layer.  Only  an  in-plane  orien- tation  of  the  c-axis  cannot be detected,  but  this is obtained  from  the  diffraction  pattern.  The  dispersion of  the  c-axis is denoted  byA850the  half-width  angle  of the  (00.2)  rocking  curve  peak  around  the  film  normal. The  Tsh  was  measured  by  means  of a chromel-alumel thermocouple,  glued  in a piece  of  aluminum  oxide.  This and  the  substrates  were  laid  on  the  substrate  holder without  further  measures  to  improve  the  heat  conducti- vity.  The  measured  temperature  was  stationary  only  af- ter  about  one  hour.  In  principle  the  optimum  value  of one.sputter  parameter,  VRF  for  example,  may  depend  on the  values  of  other  parameters  such  as  Par-  It is not known  in  advance  how  much  the  parameters  are  mutually dependent.  This  complicates  the  optimalization  of  the sputter  process  and  forces  us  to  vary  each  parameter  at various  values  of  the  others.  In  order  to  reduce  the number  of  runs  needed for a systematic  variation  of  all parameters; w first  fixed Vw'at 1.60 kV  when  vary- ing  the  Par  and  used  the  water-cooled  substrate  hol- 0018-9464/84/0100-0057$01.00 0 1984 IEEE 58 der. A t  t h i s  Vw a reasonable deposition rate R is obtained ( R=0.2 run/s) and furthermore other authors have found th is  vo l tage  to  be optimal [5]. We then va- ried the VRF a t  the optimum Par. Finally the depen- dence of the magnetic orientation on TSh was investi- gated using the optimized Vm and Rar. EXPERIMENTAL RESULTS Fig. 2 presents S i /  andA050 a s  a function of Par. The lower limit of Par is determined by the require- ments for  s table  plasma operation, which is about 2.10-3nbar for our equipment. The S i /  exhibits a f a i n t  minimum a t  Par=4.10-har  and increases s t rongly at  higher  Par. Surprisingly t h e A 0 5 0  shows the opposite behaviour, viz. an improving normal c-axis orientation at  higher Par. Obviously S// originates from causes  (b) and (c ) ,  mentioned above. The (00.2) rocking curve however exhibits no other orientations of the c-axis than the perpendicular one, but electron diffraction reveals the presence of the {lO.l] and the (00.2) ref lect ions in  layers ,  sput tered at  2.10-3 and 1 . 5 ~ 1 0 - ~  mbar. These reflections are not observed in the layer, sputtered a t  4 . 1 0 - h r  ( s e e  Fig.3a.). No reflections of other magnetic phases are observed, l i k e  fcc  Co-Cr a s  was concluded by the absence of the (200) fcc reflection. Furthermore, the diffraction rings of the layer sputtered at Par=l.5~10-2mbar, are mre blurred, which indicates a defect-rich crystal  structu- re. This is not accompanied by an increase ofA050 , which improves a t  higher Par. The increase of S// for argon pressures, which are  different  by 4.10-3 mbar, is apparently caused by the additional presence of an hcp compound with in-plane orientation of the c- VnF = 1.50 kV P2 10-3 10-2 - Pa, [mbar] Fig. 2.  The half width angleA0.50 and the in-plane re- manence S / of 150 run thick Co-Cr layers as a func- t ion of t l e  argon pressure Par. i5  P3 25 15 23 25 - s:~tmr#ro E W l L  B i-8,) IrclIIcring O"*,e 9 :mi": c3 0 Fig. 3. Optical read-outs of electron diffraction pat-  terns using a microdensitometer: a )  a t  var ious  argon pressures b) at increasing substrate holder temperature axis. In the following runs the Par was therefore kept constant a t  4 . l O - h a r .  The Vw was then var- ied between 0.70 and 2.0 kV a t  Par = 4 .10-ha r ,   a s  depicted in Fig.4. The S// has a c lear  minimum a t  V e 1 . 6 0  kV but again there is no correlation between S i /  and the perpendicular c-axis dispersion. TheA050 is almost constant in the entire voltage range, showing only a s l ight  increase in  the lower part, while no other orientations of the c-axis are revealed by the (00.2) rocking curve. Electron diffraction reveals the same behaviour of the crystal  s t ructure ,  as  was found for Par variation. Co-Cr layers sputtered a t  an RF voltage, &ich is di f fe ren t  from the optimal value of 1.60 kV, show the presence of a cer ta in  amount of a hcp 0-0 ' 0 0.5 10 15 20 - V q F  [kV] Fig. 4. The relation between the in-place remanence SI/, the half width angleA05oand the RF-sputter vol- tage Vw. P,, = 4 . W 3  mbar 0 2 t /  i l  14 0 I I 0 0 100 200 300 400 --j T s ~  PC1 angleA05o a t  elevated  substrate  iolder  temperature Fig. 5.  The in-plane remanence S / and the half width Tsh. phase with in-plane orientation of the c-axis, as indi- cated by the {lO.l} and (00.2) diffraction rings.  Also the diffraction rings of the high voltage layer are mo- re blurred, like those of the layer sputtered at high Par. A t  an elevated Tsh the magnetic orientation is strongly deteriorated (see Fig.5). The increase of is in  fac t  so steep, that the lowest possible and Unfortunately no substrate cooling, e.g.  by  means of liquid nitrogen, was possible in our equipment. Never- theless it appears that the perpendicular magnetic orientation depends strongly on Tsh. A t  elevated tem- perature,'A050 hardly changes (see Fig.5), but again the {lO.l] and (00.2)diffraction rings become appacent and mre blurred at increasing temperature (see Fig. 3b,). The very strong relation between the in-plane c- axis  or ientat ion and the temperature m y  imply, that  the dependence of the in-plane c-axis orientation on the other parameters is indirectly caused by Ts vari- ations. An increase of Vw for example is accompanied by an increase of the heat f lux to the fi lm surface,  not only by secondary electrons from the target, but also by the increased kinetic energy of the condensing atoms. In order to obtain an indication of the heating by the sputter process, Ts was measured with the same thermocouple used for measuring the heated substrate holder. %is measured "bulk" temperature d i f f e r s  from the surface temperature where the actual layer grows [3]. Nevertheless it will correlate with the surface temperature and can therefore be used t o  examine the heating from the plasma. Fig.6. shows the measured Ts when Par is varied. Surprisingly Ts exhibits a m i -  nimum a t  Par=4.10-3 rnbar, as  was also found for S//  (see  Fig.2).  This  minimum  of TS is  probably caused  by  the  competitive  heating  by  the  kinetic  energy of  the  condensating  atoms  and  the  secondary  electrons from  the  target.  At  lower  Par,  the  heating  by  the  se- condary  electrons is reduced,  but  the  mean  free  path  in the  argon  gas i simultaneously  enlarged. As a result the  sputtered  atoms  will  arrive  at  the  film  surface with  higher  kinetic  energy,  less  decelerated  by  colli- sions  with  argon  atoms. As an  indication  we  can  calcu- late  the  mean  free  path  Faroof  the  argon  gas  at 4.10-3nbar.  At 1 bar  and 20 C the  mean  free  path Far is [6]. With Far: :l/Par we see that Far=2.5 cm  at Par=4.10-br, half  the  target  to substrate  holder  distance of 5 cm,and  at  Par=2.10-3 mbar  Far  even  equals  this  distance  (corrections  for the  different  collision  diameters  of  the  Co,  Cr  and  Ar atom, are  disregarded).  The  minimum  of Ts is not caused  by  an  anomalous minim in  the  sputter  power, because R continuously  increases  in  the  entire  argon range,  as  shown  in  Fig.6.  Figure 7 shows Ts, as  mea- sured  in  the V w  range  from 0.7 to  2.0  kV.  There  is acontinuous  increase  of Ts as  there is of R, with  the rate  of  increase  of  Ts  largest  in  the  lower  voltage range.  This  corresponds  qualitatively  with  the  rate  of decrease  of SI/, (see Fig.4). rC] 1 VRF - 1.60 kV I Fig. 6. The  substrate  temperaure Ts, established  at various  argon  pressures  by  heating  from  the  sputter process  and  the  deposition  rate R. [ U s 1  4 3 R : r  0 - "RF r k v l  Fig. 7. The  substrate  temperature Ts and  the  deposi- tion  rate R as  measured  for  different  sputter  voltages VRF. DISCUSSION AND CONCLUSIONS ?he  most  important  sputter  parameters,  i.e. vm,par and  Tsh,  bere  found  to  exhibit  an  optimum  value  for perpendicular  orientation of the  magnetization.  Never- theless,  the  crystal  structure  is  always  hcp  within  the ranges  of  the  varied  parameters  and  no  other  magnetic phases  were  observed.  We  only  found  that  if  sputtering is not carried  out  at  the  optimum  values  of  the  parame- ters,  an  additional  hcp  compound  with  in-plane  orienta- tion  of  the  c-axis observed.  The {lO.l} and  (00.2) diffraction  rings  then  become  apparent  in  the  electron diffraction  pattern.  This  hcp  compound  with  an  in-plane orientation  of  the  c-axis  also  causes  some  in-plane  an- isotropy  and  consequently  an  increase of Si/. Measu- rements  of  Ts  as  function  of  the  different  sputter parameters  lead  to  the  conclusion,  that  an  exclusive perpengicular  c-axis  orientation is only  obtained  at Ts-150 C.  At  other  temperatures,  established  ei- 59 ther  directly  by a change  of  Tsh  or  indirectly  by chan ing a sputter  parameter,  an  additional  hcp  com- pound  with  in-plane  c-axis  orientation is formed.  The optimum  value  Ts-150°C is more  or  less  in  agree- me t  with  the  temperature  found  by  Ouchi  and  Iwasaki [7] for  minimum  dispersion  of  the  perpendicular  c-axis orientation.  At a constant V m  of  1.60  kV  and  varying Par, a minimum  of S i /  was  found  at  Par=4.10-3 mbar,  because  the  optimum  Ts-1500C  was  reached  at this  argon  pressure.  This  minimum  can  be  considered  as a coincidence  because a different  value  of Vm, for example  l.OkV,  would  not  have  given  such a minimum.  In order  to  verify  our  conclusions, we sputtered  Co-Cr  un- der  modified  conditions.  First a Co-Cr  layer was sput- tered  at Vm= 0.70 kV  and  Par=4.10-%bar  but  at  elevated  Tsh=1300C  (see  Fig.4).  Actually Si/ shows a significant  decrease.  The  same  is  obser- ved  if  Co-Cr is sputtered  at  Par=l.5x10-2mbar  on the  water  cooled  substrate  holder  but  at a lower Vm'1.35 kV  (see  Fig.2).  Both  results  make it plau- sible  that  Tsh is the  dominant  parameter,  no  matter whether it is determined  by  direct  or  indirect  heating of the  substrate.  In  literature  [2,5,7]  the  deteriora- ti n of the  perpendicular  magnetic  orientation  is  com- mnly marked  by a strong  increase  ofA850.  This  was  not observed  in  the  actual  Co-Cr  layers,  which  always  exhi- bited  an  excellent  orientation  of  the  perpendicular  c- axis.  Probably  this  discrepancy is caused  by  the  diffe- rent  film  thicknesses  used.  In  literature,  data  are presented  of  about 1 micron  thick  Co-Cr  layers,  while ours  were  0.15  micron  thick.  This  paper  emphasizes  the perpendicular  orientation  of  the  magnetization  but other  magnetic  parameters,  like  the  coercivity,  are  no less  important  for  the  recording  performance.  Fortuna- tely,  the  coercivity of the  Co-Cr  layers,  sputtered with  optimum  parameters,  has a reasonable  magnitude  of abut 50 kA/m depending  on  the  thickness [8]. Sputte- ring  at a higher  Tsh  for  example,  yields  coercivities in  the  order  of 100 kA/m  [2,7],  which  are  too  high  for c o m n  recording  head  materials.  Finally  we  must  con- clude,  that the,set of  optimal  parameters,  as  found  for our  sputter  equipment,  is  not  directly  applicable  for other  systems.  The  heating  of  the  substrate  from  the plasma  plays  an  important  role  for  the  determination  of the  optimal  value  of  these  parameters.  This  heating strongly  depends  on  the  geometry  of  the  sputter  system used,  e.g.  the  target  to  substrate  distance  and  the target  diameter  are  important  dimensions  in  connection with  the  substrate  heating  by  secondary  electrons. An unsatisfactory  conclusion is therefore  that  the  optimal parameters  have  to  be  independently  determined  for  each sputter  system. REFERENCES 1. S. Iwasaki  and K. Ouchi,  IEEE  Trans.Magn.,  Vol. MAG- 2.  T.  Wielinga  and  J.C.  Ladder,  IEEE  Trans.Magn., 3.  J.C.  Ladder,  T.  Wielinga  and  J.  WOrst,  Thin  Solid Films, 101, 1983, pp. 61-73. 4. J.H.H.G.  van  Willigen,  H.T.  Weber,  M. Bos and  W.E. van  der  Linden, An. Chem.  Acta,  136  1982,  pp.379-384. 5. Y. Uesaka  and  J.H.  Judy,  1980  Intermag  Conference, Boston,  paper  34-1. 6.  Handbook  of  chemistry  and  physics,  Florida:  CRC ress, 60th  edition,  1980. 7. K. Ouchi  and S. Iwasaki,  Proceedings  of  1982  Sendai symposium  on  perpendicular  magnetic  recording,  pp.131- 141.8. T. Wielinga,  Thesis %ne Universty  of  Technology, 1983,  Enschede,  The  Netherlands. 14,  N0.5,  1978,  pp.849-851. Vol.MAG-17,  No.6,  1981,  pp.3178-3180. 

Influence of R.F. sputter parameters on the magnetic orientation of Co-Cr layers

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