U.S. patent application number 13/549500 was filed with the patent office on 2014-01-16 for method for making a perpendicular thermally-assisted recording (tar) magnetic recording disk having a carbon segregant.
The applicant listed for this patent is Oleksandr Mosendz, Simone Pisana, James William Reiner, Franck Dreyfus Rose. Invention is credited to Oleksandr Mosendz, Simone Pisana, James William Reiner, Franck Dreyfus Rose.
Application Number | 20140014616 13/549500 |
Document ID | / |
Family ID | 49840836 |
Filed Date | 2014-01-16 |
United States Patent
Application |
20140014616 |
Kind Code |
A1 |
Mosendz; Oleksandr ; et
al. |
January 16, 2014 |
METHOD FOR MAKING A PERPENDICULAR THERMALLY-ASSISTED RECORDING
(TAR) MAGNETIC RECORDING DISK HAVING A CARBON SEGREGANT
Abstract
A method of making a thermally-assisted recording (TAR) disk
includes etching an initial layer of generally spherically shaped
FePt grains encapsulated by shells of graphitic carbon layers. The
etching partially or completely removes the carbon layers on the
tops of the shells, exposing the FePt grains while leaving carbon
segregant material between the FePt grains. Additional Fe, Pt and C
are then simultaneously deposited. The additional Fe and Pt grow on
the exposed FePt grains and increase the vertical height of the
grains, resulting in growth of columnar FePt grains. The additional
C forms on top of the grains that together with the intergranular
carbon form larger carbon shells. The resulting FePt grains thus
have a generally columnar shape with perpendicular magnetic
anisotropy, rather than a generally spherical shape. Lateral grain
isolation is maintained by the carbon segregant remaining between
the grains.
Inventors: |
Mosendz; Oleksandr; (San
Jose, CA) ; Pisana; Simone; (San Jose, CA) ;
Reiner; James William; (Palo Alto, CA) ; Rose; Franck
Dreyfus; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mosendz; Oleksandr
Pisana; Simone
Reiner; James William
Rose; Franck Dreyfus |
San Jose
San Jose
Palo Alto
San Jose |
CA
CA
CA
CA |
US
US
US
US |
|
|
Family ID: |
49840836 |
Appl. No.: |
13/549500 |
Filed: |
July 15, 2012 |
Current U.S.
Class: |
216/22 |
Current CPC
Class: |
G11B 5/84 20130101; G11B
5/65 20130101 |
Class at
Publication: |
216/22 |
International
Class: |
B44C 1/22 20060101
B44C001/22 |
Claims
1. A method for making a perpendicular magnetic recording disk
comprising: providing a substrate; forming on the substrate a
plurality of grains of a substantially chemically-ordered alloy
comprising Pt and an element selected from Fe and Co, the grains
being covered by a plurality of layers of graphitic carbon; etching
the carbon layers to expose said grains; and depositing Pt, said
selected element and carbon simultaneously onto the exposed said
grains to increase the thickness of said grains, the simultaneously
deposited carbon forming a plurality of layers of graphitic carbon
over the increased-thickness grains.
2. (canceled)
3. The method of claim 1 wherein, after exposing said grains,
depositing Pt, said selected element and carbon simultaneously
comprises depositing Pt, said selected element and carbon
simultaneously onto a substrate maintained at a temperature between
about 500 to 700.degree. C.
4. The method of claim 1 wherein, after exposing said grains,
depositing Pt, said selected element and carbon simultaneously
comprises depositing Pt, said selected element and carbon
simultaneously onto a substrate maintained at a temperature between
about 350 to 500.degree. C. and thereafter annealing the deposited
Pt, said selected element and carbon to a temperature between about
500 to 700.degree. C. for between about 10 to 120 minutes.
5. The method of claim 1 wherein etching comprises etching with a
chemically reactive plasma of Ar and hydrogen (H.sub.2).
6. The method of claim 1 wherein forming said grains covered by
said carbon layers comprises depositing Pt, said selected element
and carbon simultaneously onto a substrate maintained at a
temperature between about 500 to 700.degree. C.
7. The method of claim 1 wherein forming said grains covered by
said carbon layers comprises depositing Pt, said selected element
and carbon simultaneously onto a substrate maintained at a
temperature between about 350 to 500.degree. C. and thereafter
annealing the deposited Pt, said selected element and carbon to a
temperature between about 500 to 700.degree. C. for between about
10 to 120 minutes.
8. The method of claim 1 wherein forming on the substrate a
plurality of grains of a substantially chemically-ordered alloy
comprises forming a substantially chemically-ordered pseudo-binary
FePtX alloy in the L1.sub.0 phase, where X is one or more of Ni,
Au, Cu, Pd, Mn and Ag.
9. The method of claim 1 wherein forming on the substrate a
plurality of grains of a substantially chemically-ordered alloy
comprises forming a substantially chemically-ordered pseudo-binary
CoPtX alloy in the L1.sub.0 phase, where X is one or more of Ni,
Au, Cu, Pd, Mn and Ag.
10. The method of claim 1 wherein said grains are also separated
from one another on the substrate by a plurality of layers of
graphitic carbon, and wherein etching the carbon layers comprises
etching both the carbon layers covering the grains and the carbon
layers separating the grains.
11. The method of claim 1 further comprising, after exposing said
grains, depositing Pt, said selected element, carbon and a
segregant selected from one or more of SiO.sub.2, TiO.sub.2,
TaO.sub.x, SiC, SiN, TiC, and TiN.
12. (canceled)
13. A method for making a perpendicular magnetic recording disk
comprising: providing a substrate; forming on the substrate a
plurality of grains of a substantially chemically-ordered alloy
comprising Fe and Pt and a plurality of layers of graphitic carbon,
the carbon layers generally encapsulating the FePt grains; etching
the carbon layers in Ar and hydrogen (H.sub.2) to expose the tops
of said FePt grains; and depositing Fe, Pt and carbon
simultaneously onto the exposed FePt grains to increase the
thickness of said grains, the simultaneously deposited carbon
forming a plurality of layers of graphitic carbon over the
increased-thickness grains.
14. The method of claim 13 wherein forming said grains encapsulated
by said carbon layers comprises depositing Fe, Pt and carbon
simultaneously while maintaining the substrate at a temperature
between 500 to 700.degree. C.
15. The method of claim 13 wherein forming said grains encapsulated
by said carbon layers comprises depositing Fe, Pt and carbon
simultaneously while maintaining the substrate at a temperature
between 350 to 500.degree. C. and thereafter annealing the
deposited Fe, Pt, and carbon to a temperature between about 500 to
700.degree. C. for between about 10 to 120 minutes.
16. The method of claim 13 wherein depositing Fe, Pt and carbon
simultaneously onto the exposed FePt grains to increase the
thickness of said grains comprises depositing Fe, Pt and carbon
simultaneously while maintaining the substrate at a temperature
between 500 to 700.degree. C.
17. The method of claim 13 wherein depositing Fe, Pt and carbon
simultaneously onto the exposed FePt grains to increase the
thickness of said grains comprises depositing Fe, Pt and carbon
simultaneously while maintaining the substrate at a temperature
between 350 to 500.degree. C. and thereafter annealing the
deposited Fe, Pt, and carbon to a temperature between about 500 to
700.degree. C. for between about 10 to 120 minutes.
18. The method of claim 13 wherein forming a plurality of grains of
a substantially chemically-ordered alloy comprising Fe and Pt
comprises forming a substantially chemically-ordered pseudo-binary
FePtX alloy in the L1.sub.0 phase, where X is one or more of Ni,
Au, Cu, Pd, Mn and Ag.
19. The method of claim 13 wherein etching the carbon layers
further comprises etching the carbon between the grains to remove
substantially all of the carbon.
20. The method of claim 19 wherein depositing Fe, Pt and carbon
simultaneously onto the exposed FePt grains comprises depositing
one or more segregant materials selected from SiO.sub.2, TiO.sub.2,
TaO.sub.x, SiC, SiN, TiC, and TiN.
21. A method for making a perpendicular magnetic recording disk
comprising: providing a substrate; forming on the substrate a
plurality of grains of a substantially chemically-ordered alloy
comprising Pt and an element selected from Fe and Co, the grains
being covered by a plurality of layers of graphitic carbon; etching
the carbon layers to expose said grains; and after exposing said
grains, depositing Pt, said selected element, carbon and a
segregant selected from one or more of SiO.sub.2, TiO.sub.2,
TaO.sub.x, SiC, SiN, TiC, and TiN onto said exposed grains.
22. A method for making a perpendicular magnetic recording disk
comprising: providing a substrate; forming on the substrate a first
layer of a plurality of grains of a substantially
chemically-ordered alloy comprising Fe and Pt and a plurality of
layers of graphitic carbon, the carbon layers generally
encapsulating the FePt grains; etching the carbon layers to expose
the tops of said FePt grains in said first layer; forming a second
layer of FePt on said FePt grains in said first layer by depositing
Fe and Pt simultaneously onto the exposed FePt grains in said first
layer to thereby form increased-thickness FePt grains; and
depositing a layer of ferromagnetic material directly on and in
contact with said increased-thickness FePt grains, said
ferromagnetic layer being exchange coupled with said
increased-thickness FePt grains.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to perpendicular magnetic
recording media for use as thermally-assisted recording (TAR)
media, and more particularly to a method for making a TAR disk
wherein the magnetic recording layer includes a carbon
segregant.
[0003] 2. Description of the Related Art
[0004] In conventional continuous magnetic recording media, the
magnetic recording layer is a continuous layer over the entire
surface of the disk. In magnetic recording disk drives the magnetic
material (or media) for the recording layer on the disk is chosen
to have sufficient coercivity such that the magnetized data regions
that define the data "bits" are written precisely and retain their
magnetization state until written over by new data bits. As the
areal data density (the number of bits that can be recorded on a
unit surface area of the disk) increases, the magnetic grains that
make up the data bits can be so small that they can be demagnetized
simply from thermal instability or agitation within the magnetized
bit (the so-called "superparamagnetic" effect). To avoid thermal
instabilities of the stored magnetization, media with high
magneto-crystalline anisotropy (K.sub.u) are required. The thermal
stability of a magnetic grain is to a large extent determined by
K.sub.uV, where V is the volume of the magnetic grain. Thus a
recording layer with a high K.sub.u is important for thermal
stability. However, increasing K.sub.u also increases the
coercivity of the media, which can exceed the write field
capability of the write head.
[0005] Since it is known that the coercivity of the magnetic
material of the recording layer is temperature dependent, one
proposed solution to the thermal stability problem is
thermally-assisted recording (TAR), also called heat-assisted
magnetic recording (HAMR), wherein the magnetic recording material
is heated locally during writing to lower the coercivity enough for
writing to occur, but where the coercivity/anisotropy is high
enough for thermal stability of the recorded bits at the ambient
temperature of the disk drive (i.e., the normal operating
temperature range of approximately 15-60.degree. C.). In some
proposed TAR systems, the magnetic recording material is heated to
near or above its Curie temperature. The recorded data is then read
back at ambient temperature by a conventional magnetoresistive read
head.
[0006] One type of proposed TAR disk drive uses a "small-area"
heater to directly heat just the area of the data track where data
is to be written by the write head. The most common type of
small-area TAR disk drive uses a laser source and an optical
waveguide with a near-field transducer (NFT). A "near-field"
transducer refers to "near-field optics", wherein the passage of
light is through an element with subwavelength features and the
light is coupled to a second element, such as a substrate like a
magnetic recording medium, located a subwavelength distance from
the first element. The NFT is typically located at the air-bearing
surface (ABS) of the air-bearing slider that also supports the
read/write head and rides or "files" above the disk surface.
[0007] One type of proposed high-K.sub.u TAR media with
perpendicular magnetic anisotropy is an alloy of FePt (or CoPt)
alloy chemically-ordered in the L1.sub.0 phase. The
chemically-ordered FePt alloy, in its bulk form, is known as a
face-centered tetragonal (FCT) L1.sub.0-ordered phase material
(also called a CuAu material). The c-axis of the L1.sub.0 phase is
the easy axis of magnetization and is oriented perpendicular to the
disk substrate. The FePt alloy requires deposition at high
temperature or subsequent high-temperature annealing to achieve the
desired chemical ordering to the L1.sub.0 phase, and typically
include a segregant like SiO.sub.2, B, BN or SiNx that forms
between the FePt grains and reduces the grain size.
[0008] The use of carbon (C) has been proposed as a segregant for
the FePt grains in TAR media. To obtain the required microstructure
and magnetic properties, the FePt needs to be deposited with the
substrate maintained at high temperatures (e.g., about 500 to
700.degree. C.). In pending application Ser. No. 13/290,940 filed
Nov. 7, 2011 and titled "FePt--C BASED MAGNETIC RECORDING MEDIA
WITH ONION-LIKE CARBON PROTECTION LAYER" assigned to the same
assignee as this application, the C segregant is described as
shells of multiple graphitic carbon layers that encapsulate the
FePt grains, which then have a generally spherical shape.
[0009] What is needed is a method for making a FePt TAR disk with a
carbon segregant wherein the FePt grains can be made thicker and
thus have a more columnar and less spherical shape.
SUMMARY OF THE INVENTION
[0010] In the method of making the TAR disk according to this
invention, after forming an initial layer of generally spherically
shaped FePt grains encapsulated by shells of graphitic carbon
layers, an etching step is performed to partially or completely
remove the carbon layers on the tops of the shells. The etching may
be by inductively coupled plasma (ICP) etching in a chemically
reactive plasma of Ar and H.sub.2 and may be performed so as to
remove just the tops of the carbon shells, without removing the
segregant carbon between the FePt grains. Additional Fe, Pt and C
are then simultaneously deposited. The additional Fe and Pt grow on
the exposed FePt grains and increase the vertical height of the
grains, resulting in growth of columnar FePt grains. The additional
carbon forms on top of the grains that together with the
intergranular carbon form larger carbon shells. The resulting FePt
grains thus have a generally columnar shape with perpendicular
magnetic anisotropy, rather than a generally spherical shape.
Lateral grain isolation is maintained by the carbon segregant
remaining between the grains.
[0011] Alternatively, the etching may be performed so as to
completely remove the carbon shells, including the carbon segregant
between the FePt grains, leaving just the FePt grains. Then
additional Fe and Pt grows on top of the FePt grains to form taller
grains with a columnar shape and the additional carbon,
co-deposited with the additional Fe and Pt, forms shells over the
FePt grains and also forms as intergranular segregant material
between the columnar grains.
[0012] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWING
[0013] FIG. 1 is a top view of a thermally-assisted recording (TAR)
disk drive according to the invention.
[0014] FIG. 2 depicts a sectional view, not drawn to scale because
of the difficulty in showing the very small features, of an
air-bearing slider for use in TAR disk drive and a portion of a TAR
disk according to the invention.
[0015] FIG. 3 is a sectional view showing a TAR disk with a FePt
continuous recording layer (RL) according to the prior art.
[0016] FIG. 4A is a transmission electron microscopy (TEM) image
plan view of layers of FePt grains surrounded by shells of
graphitic carbon layers that function as intergranular
segregant.
[0017] FIG. 4B is a TEM image in sectional view showing multiple of
layers of FePt grains surrounded by shells of graphitic carbon
layers.
[0018] FIGS. 5A-5D are side sectional views of a graphical
representation of the growth of the FePt--C shells on a TAR disk
during simultaneous sputter deposition of Fe, Pt and C atoms on a
heated substrate.
[0019] FIG. 6A is a side sectional view of the structure of FIG. 5C
after the etching step according to the method of this
invention.
[0020] FIG. 6B is a side sectional view of the structure of FIG. 6A
after additional co-sputtering of Fe, Pt and C.
[0021] FIGS. 7A-7C are side sectional views of a TAR disk
illustrating the method of this invention to completely remove the
carbon shells, including the carbon segregant between the FePt
grains.
[0022] FIG. 8 is x-ray diffraction (XRD) spectra for a control
sample of FePt media made without the etching steps of this
invention and a test sample made with the etching steps of this
invention.
[0023] FIG. 9 is a side sectional view of a TAR disk with a first
layer of FePt, a second layer formed on the first layer after the
etching step to form the columnar FePt grains, and an upper
exchange-coupled ferromagnetic layer formed directly on top of the
columnar FePt grains.
DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 is a top view of a thermally-assisted recording (TAR)
disk drive 100 according to the invention. In FIG. 1, the TAR disk
drive 100 is depicted with a disk 200 with magnetic the recording
layer 31 patterned into discrete data islands 30 of magnetizable
material arranged in radially-spaced circular tracks 118. Only a
few representative islands 30 and representative tracks 118 near
the inner and outer diameters of disk 200 are shown. However,
instead of the bit-patterned-media (BPM) shown with discrete data
islands 30 in FIG. 1, the TAR disk drive may instead use disks in
which the recording layer 31 is a conventional continuous magnetic
recording layer of magnetizable material.
[0025] The drive 100 has a housing or base 112 that supports an
actuator 130 and a drive motor for rotating the magnetic recording
disk 200. The actuator 130 may be a voice coil motor (VCM) rotary
actuator that has a rigid arm 131 and rotates about pivot 132 as
shown by arrow 133. A head-suspension assembly includes a
suspension 135 that has one end attached to the end of actuator arm
131 and a head carrier, such as an air-bearing slider 120, attached
to the other end of suspension 135. The suspension 135 permits the
slider 120 to be maintained very close to the surface of disk 200
and enables it to "pitch" and "roll" on the air-bearing generated
by the disk 200 as it rotates in the direction of arrow 20. The
slider 120 supports the TAR head (not shown), which includes a
magnetoresistive read head, an inductive write head, the near-field
transducer (NFT) and optical waveguide. A semiconductor laser 90
with a wavelength of 780 to 980 nm may used as the TAR light source
and is depicted as being supported on the top of slider 120.
Alternatively the laser may be located on suspension 135 and
coupled to slider 120 by an optical channel. As the disk 200
rotates in the direction of arrow 20, the movement of actuator 130
allows the TAR head on the slider 120 to access different data
tracks 118 on disk 200. The slider 120 is typically formed of a
composite material, such as a composite of alumina/titanium-carbide
(Al.sub.2O.sub.3/TiC). Only one disk surface with associated slider
and read/write head is shown in FIG. 1, but there are typically
multiple disks stacked on a hub that is rotated by a spindle motor,
with a separate slider and TAR head associated with each surface of
each disk.
[0026] FIG. 2 is a schematic cross-sectional view illustrating a
configuration example of a TAR head according to the present
invention. The X direction denotes a direction perpendicular to the
air-bearing surface (ABS) of the slider, the Y direction denotes a
track width or cross-track direction, and the Z direction denotes
an along-the-track direction. In FIG. 2, the disk 200 is depicted
with the recording layer 31 being a conventional continuous
magnetic recording layer of magnetizable material with magnetized
regions or "bits" 34. The air-bearing slider 120 is supported by
suspension 135 and has an ABS that faces the disk 200 and supports
the magnetic write head 50, read head 60, and magnetically
permeable read head shields S1 and S2. A recording magnetic field
is generated by the write head 50 made up of a coil 56, a magnetic
pole 53 for transmitting flux generated by the coil 56, a main pole
52, and a return pole 54. A magnetic field generated by the coil 56
is transmitted through the magnetic pole 53 to the main pole 52
arranged in a vicinity of an optical near-field transducer (NFT)
74. At the moment of recording, the recording layer 31 of disk 200
is heated by an optical near-field generated by the NFT 74 and, at
the same time, a region or "bit" 34 is magnetized and thus written
onto the recording layer 31 by applying a recording magnetic field
generated by the main pole 52.
[0027] A semiconductor laser 90 is mounted to the top surface of
slider 120. An optical waveguide 73 for guiding light from laser 90
to the NFT 74 is formed inside the slider 120. Materials that
ensure a refractive index of the waveguide 73 core material to be
greater than a refractive index of the cladding material may be
used for the waveguide 73. For example, Al.sub.2O.sub.3 may be used
as the cladding material and TiO.sub.2, T.sub.2O.sub.5 and
SiO.sub.xN.sub.y as the core material. Alternatively, SiO.sub.2 may
be used as the cladding material and Ta.sub.2O.sub.5, TiO.sub.2,
SiO.sub.xN.sub.y, or Ge-doped SiO.sub.2 as the core material.
[0028] FIG. 3 is a sectional view showing TAR disk 200 with a
continuous recording layer (RL) 31 of a substantially
chemically-ordered FePt alloy (or CoPt alloy) as proposed in the
prior art. The disk 200 is a substrate 201 having a generally
planar surface on which the representative layers are sequentially
deposited, typically by sputtering. The hard disk substrate 201 may
be any commercially available glass substrate, but may also be a
conventional aluminum alloy with a NiP surface coating, or an
alternative substrate, such as silicon or silicon-carbide.
[0029] The perpendicular media that forms the RL 31 is a
high-H.sub.k substantially chemically-ordered FePt alloy (or CoPt
alloy) with perpendicular magnetic anisotropy. Substantially
chemically-ordered means that the FePt alloy has a composition of
the form Fe.sub.(y)Pt.sub.(100-y) where y is between about 45 and
55 atomic percent. Such alloys of FePt (and CoPt) ordered in
L1.sub.0 are known for their high magneto-crystalline anisotropy
and magnetization, properties that are desirable for high-density
magnetic recording materials. The substantially chemically-ordered
FePt alloy, in its bulk form, is known as a face-centered
tetragonal (FCT) L1.sub.0-ordered phase material (also called a
CuAu material). The c-axis of the L1.sub.0 phase is the easy axis
of magnetization and is oriented perpendicular to the disk
substrate. The substantially chemically-ordered FePt alloy may also
be a pseudo-binary alloy based on the FePt L1.sub.0 phase, e.g.,
(Fe.sub.(y)Pt.sub.(100-y))--X, where y is between about 45 and 55
atomic percent and the element X may be one or more of Ni, Au, Cu,
Pd, Mn and Ag and present in the range of between about 0% to about
20% atomic percent. While the pseudo-binary alloy in general has
similarly high anisotropy as the binary alloy FePt, it allows
additional control over the magnetic and other properties of the
RL. For example, Ag improves the formation of the L1.sub.0 phase
and Cu reduces the Curie temperature. While the method will be
described for media with a FePt RL, the method is also fully
applicable to media with a CoPt (or a pseudo-binary CoPt--X alloy
based on the CoPt L.sub.10 phase) RL.
[0030] The FePt RL is sputter deposited to a thickness of between
about 4 to 15 nm while the disk substrate 201 is maintained at an
elevated temperature, for example between about 300 and 700.degree.
C. The FePt RL may be sputter deposited from a single composite
target having generally equal atomic amounts of Fe and Pt and with
the desired amounts of X-additives and segregant, or co-sputtered
from separate targets. As an alternative method for forming the
FePt RL, sequential alternating layers of Fe and Pt can be
deposited by sputter depositing from separate Fe and Pt targets,
using a shutter to alternately cover the Fe and Pt targets, with
each Fe and Pt layer having a thickness in the range of about 0.15
nm to 0.25 nm to the desired total thickness.
[0031] A set of underlayers are located between the substrate 201
and the FePt RL 31. An optional soft underlayer (SUL) 210 of
magnetically permeable material that serves as a flux return path
for the magnetic flux from the write head may be formed on
substrate 201. The SUL 210 may be formed of magnetically permeable
materials such as alloys of CoNiFe, FeCoB, CoCuFe, NiFe, FeAlSi,
FeTaN, FeN, FeTaC, CoTaZr, CoFeTaZr, CoFeB, and CoZrNb. The SUL 210
may also be a laminated or multilayered SUL formed of multiple soft
magnetic films separated by nonmagnetic films, such as electrically
conductive films of Al or CoCr. The SUL 210 may also be a laminated
or multilayered SUL formed of multiple soft magnetic films
separated by interlayer films that mediate an antiferromagnetic
coupling, such as Ru, Ir, or Cr or alloys thereof. The SUL 210 may
have a thickness in the range of about 5 to 50 nm.
[0032] An optional heat sink layer 220 may be located on substrate
201 (or on optional SUL 210) and formed of a material that is a
good thermal conductor, like Cr, Cu, Au, Ag or other suitable
metals or metal alloys. Heat sink layer 220 may be necessary to
facilitate the transfer of heat away from the RL to prevent
spreading of heat to regions of the RL adjacent to where data is
desired to be written, thus preventing overwriting of data in
adjacent data tracks.
[0033] An insulating layer 240, typically MgO, but also TiN or TiC,
with a thickness between about 2-20 nm, is located below the FePt
RL 31 to define a texture for the subsequently deposited FePt RL
31. An optional seed layer 230 for the insulating layer 240 may be
used to enhance the crystalline growth of the insulating layer 240.
If the insulating layer is MgO, the preferred seed layer 230 is a
NiTa alloy with a thickness in the range of about 5-100 nm.
[0034] A protective overcoat (OC) 260 is deposited on the RL 31,
preferably to a thickness between about 1-5 nm. OC 260 is
preferably a layer of amorphous carbon, like amorphous diamond-like
carbon (DLC). The amorphous carbon or DLC may also be hydrogenated
and/or nitrogenated, as is well-known in the art. On the completed
disk, a liquid lubricant, like a perfluorpolyether (PFPE), is
coated on OC 260.
[0035] FePt L1.sub.0 phase based thin films exhibit strong
perpendicular anisotropy, which potentially leads to small (e.g.,
3-9 nm in diameter) thermally stable grains for ultrahigh density
magnetic recording. To fabricate small grain FePt L1.sub.0 media
some form of segregant to separate grains can be used as an
integral part of the magnetic recording layer. Thus in the TAR disk
200, the RL also typically includes a segregant, such as one or
more of SiO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5, C, and BN that forms
between the FePt grains and reduces the grain size. The use of
carbon (C) atoms provides segregation of FePt grains that are well
isolated and magnetically de-coupled for TAR media. To obtain the
required microstructure and magnetic properties, the FePt needs to
be deposited with the substrate maintained at high temperatures
(e.g., about 500 to 700.degree. C.). At this high temperature, the
carbon segregant forms shells of multiple graphitic carbon layers
that encapsulate the FePt grains. The graphitic carbon layers are
sheets or partial sheets of hexagonal crystalline carbon, often of
a single atomic layer like graphene, that overlap, much like an
onion-skin, to form the carbon shells. This is described in pending
application Ser. No. 13/290,940 filed Nov. 7, 2011 and titled
"FePt--C BASED MAGNETIC RECORDING MEDIA WITH ONION-LIKE CARBON
PROTECTION LAYER" assigned to the same assignee as this
application. In the '940 application, the carbon that encapsulates
the FePt grains also serves as the protective overcoat for the RL,
eliminating the need for a separate sputter deposition step.
[0036] As part of this invention it has been discovered that while
the formation of these carbon shells enhances grain isolation, it
impairs the columnar growth of the FePt grains. The presence of the
carbon shells drives the FePt grains toward a spherical shape. At
the first stages of deposition, a shell is not fully formed and is,
as a result, permeable to Fe and Pt atoms. A spherical FePt grain
grows inside the carbon shell. When the multilayered shell has
attained a critical thickness or structural integrity it becomes
impermeable to Fe and Pt atoms, and the growth of the spherical
FePt grain is stopped. From this point in media deposition onward,
FePt forms new grains above the first layer. These FePt grains in
the second layer will, in general, not be oriented so that the
magnetic easy axis of the FePt crystal is out of the plane, and as
a result will impair recording performance. For optimal recording
performance in TAR media, separate control of the lateral and
vertical size of the first layer of FePt grains is required.
Decreasing lateral grain size is required to improve areal
recording density while increasing vertical grain size is required
to maintain thermal stability and magnetic signal amplitude.
[0037] In the method of this invention, after the layer of FePt--C
shells is formed, an inductively coupled plasma (ICP) etching step
is performed to partially or completely remove the carbon layers on
the tops of the shells. The tops of the shells encapsulating the
FePt grains are thus removed, allowing additional Fe and Pt atoms
to be subsequently deposited to form columnar grains. Lateral grain
isolation is maintained by the carbon segregant remaining between
the grains.
[0038] Experimental evidence for the formation of onion-like
graphitic carbon encapsulating FePt grains is shown by transmission
electron microscopy (TEM) images in the plan view of FIG. 4A and
the side sectional view of FIG. 4B. This particular FePt film was
grown by magnetron sputtering co-deposition (in Ar gas at 3 mTorr)
from a C target and an Fe.sub.55Pt.sub.45 alloy target (where the
subscripts are in atomic percent) while the substrate was
maintained at a temperature of 600.degree. C. The substrate was a
0.8 mm thick glass substrate and an underlayer of MgO was located
below the FePt film. The C added to FePt during deposition at
elevated temperatures forms graphitic carbon layers that
encapsulate FePt grains. They manifest themselves as the
black/white curved lines in the intergranular material as can be
seen in the plan view TEM image in FIG. 4A, resulting in an average
spacing between the grains of about 3.5 .ANG.. The TEM image in the
sectional view of FIG. 4B shows 3-4 layers of FePt--C shells and
illustrates how each layer of generally spherical shells forms on
top of the layer beneath it, preventing the vertical growth of the
FePt grains. Only the bottom or first layer of FePt--C shells
directly in contact with the underlying MgO is useful because only
it will have perpendicular magnetic anisotropy; the other layers
will grow with a magnetic anisotropy pointing in random
directions.
[0039] FIGS. 5A-5D are side sectional views of a graphical
representation of the growth of the FePt--C shells during
simultaneous sputter deposition of Fe, Pt and C atoms on a heated
substrate to between 500 to 700.degree. C. FIG. 5A shows the first
stage of FePt grains 300 surrounded by initial stages of an initial
carbon layer 302. The Fe and Pt atoms can diffuse through the
carbon in layer 302, resulting in growth of the generally
spherically shaped FePt grains 300 and the formation of additional
carbon layers 303, as shown in FIG. 5B. In FIG. 5C, the carbon
layers have thickened to form carbon shells 304, preventing further
diffusion of Fe and Pt atoms, and thus cessation of growth of the
FePt grains 300. Continued co-sputtering of Fe, Pt and C results in
an additional layer of FePt--C shells 305 on top of and possibly
surrounding the shells in the initial layer, as shown in FIG.
5D.
[0040] The simultaneous deposition of Fe, Pt and C on a substrate
maintained at a temperature between 500 to 700.degree. C. causes
the FePt to form as the desired chemically-ordered L1.sub.0-ordered
phase material and also results in the formation of the carbon
shells that encapsulate the FePt grains. However, it has been
discovered that the carbon shells will form at a lower temperature
between 350 to 500.degree. C. Thus it is possible to form the FePt
grains 300 and carbon shells 304 as shown in FIG. 5C by
simultaneous deposition of Fe, Pt and C on a substrate maintained
between 350 to 500.degree. C. and thereafter annealing the
deposited Fe, Pt, and carbon to a temperature between about 500 to
700.degree. C. for between about 10 to 120 minutes.
[0041] FIG. 6A is a side sectional view of the structure of FIG. 5C
after the ICP etching step in the method of this invention. The ICP
etching is performed so as to remove just the tops of the carbon
shells 304, without removing the segregant C between the FePt
grains 300. The ICP etching step parameters may be determined
experimentally. For carbon shells with a thickness of about 1 nm,
ICP etching for 5 sec. with an Ar and H.sub.2 gas mixture (30%
Ar/70% H.sub.2 volumetric flow rate ratio) at a total pressure of
about 20 mT and at a temperature below 100.degree. C. will remove
the tops of the shells while leaving untouched most of the
intergranular carbon. FIG. 6B shows the structure after additional
co-sputtering of Fe, Pt and C with the substrate to between 500 to
700.degree. C. The additional Fe and Pt grow on the FePt grains 300
and increase the vertical height of the grains, resulting in growth
of columnar grains 306. The additional C forms on top of the grains
306 that together with the intergranular C form larger carbon
shells 307. The resulting FePt grains 306 thus have a generally
columnar shape with perpendicular magnetic anisotropy, rather than
the generally spherical shape of grains 300 in FIG. 5D. With the
method of this invention, the thickness of the columnar grains is
between 4-10 nm, the diameter of the columnar grains is between 3-8
nm and the grains are spaced apart by the carbon segregant material
by between 2-4 nm.
[0042] The simultaneous deposition of the additional Fe, Pt and C
after the etching step in FIG. 6A is preferably done with the
substrate maintained at a temperature between about 500 to
700.degree. C. However, this may alternatively be done by
simultaneous deposition of Fe, Pt and C on a substrate maintained
between 350 to 500.degree. C. and thereafter annealing the
deposited Fe, Pt, and carbon to a temperature between about 500 to
700.degree. C. for between about 10 to 120 minutes.
[0043] The initial FePt--C layer, i.e., the layer before the ICP
etching step, as well as the subsequent FePt--C material after
etching, may be formed by co-sputtering, as described for the
structure shown with FePt grains in FIGS. 4A-4B. However, the
FePt--C may also be formed by use of a composite FePt--C
target.
[0044] FIGS. 7A-7C are side sectional views illustrating the method
of this invention for completely removing the carbon shells,
including the carbon between the FePt grains. FIG. 7A shows the
FePt grains 300 surrounded by carbon shells 304. In FIG. 7B, the
ICP etching is performed so as to remove all of the carbon shells
304, not just the tops of the carbon shells 304 as in FIG. 6A. The
ICP etching step parameters may be determined experimentally. For
carbon shells with a thickness of about 1 nm, ICP etching for 20
sec. with an Ar and H.sub.2 gas mixture (30% Ar/70% H.sub.2
volumetric flow rate ratio) at a total pressure of about 20 mT and
at a temperature below 100.degree. C. will remove all of the carbon
shells, including the intergranular carbon. While a pure Ar gas
etch process would remove FePt at a rate comparable or higher than
C is removed, the H.sub.2-rich gas mixture removes C far more
quickly than FePt. As a result, all of the C can be removed during
the ICP etch step while little to no FePt is removed. FIG. 7C shows
the structure after additional co-sputtering of Fe, Pt and C. The
additional Fe and Pt has grown on top of the grains 300 in FIG. 7B
to form taller grains 308 and the FePt grains 308 now have a
columnar shape. The subsequent co-deposition of the C has formed
shells 309 over the FePt grains 308 and also formed as
intergranular segregant material between the grains.
[0045] In the embodiment of FIGS. 7A-7C, one or more additional
segregant material may be added to the C during the subsequent
deposition step of FIG. 7C. Materials such as SiO.sub.2, TiO.sub.2,
TaO.sub.x, SiC, SiN, TiC, and TiN can be co-sputtered with the C.
The C should be at least 60% of the total volume of the added
mixture of segregants to assure the formation of the carbon shells.
For small grain size, e.g., 3-9 nm diameter grains, up to 60% by
volume of the whole film is occupied by segregants.
[0046] To demonstrate the practicality and advantage of the method
of this invention, two groups of FePt media were prepared, using
identical deposition steps except for the presence or absence of
the ICP etching step. In the control group, 9 nm thick FePt--C
media was deposited in three steps of 3 nm each on an MgO
underlayer without any ICP etching. In the test group, 9 nm thick
FePt--C media was also deposited on an MgO underlayer in three
steps of 3 nm each, but with an ICP etching step performed after
the first and second FePt--C depositions. The FePt--C depositions
were done with the substrate maintained between 550-600.degree. C.
The ICP etching steps were performed for 5 sec. with an Ar and
H.sub.2 gas mixture (30%/70%) at a total pressure of 20 mT and at a
temperature below 100.degree. C. The control group and the test
group were then both characterized using x-ray diffraction (XRD).
XRD provides an estimate of the total volume of the FePt grains
with favorable crystalline alignment (i.e., L1.sub.0 order with the
c-axis aligned within 10 degrees of perpendicular). The XRD spectra
are shown in FIG. 8. The higher intensity of the FePt diffraction
peaks for the test sample (solid line) made with the ICP etching
steps, as compared to the control sample (dashed line) indicates
that more of the deposited FePt material (about twice as much) is
found in the first layer of grains attached to the MgO. There is a
balancing decrease in the amount of FePt found in grains not
attached to the MgO, which because of their random crystallographic
orientation do not contribute to these spectra. This increase in
the FePt diffraction intensity indicates that the ICP etching steps
were successful in removing the graphitic sheets on the top of the
FePt and opening up the carbon shells, which would otherwise form a
barrier to the continual addition of Fe and Pt atoms to the first
layer of FePt grains.
[0047] It is also possible to use the method of this invention to
form exchange-coupled media. Thus instead of depositing a final
layer of FePt--C to form the columnar grains 301 surrounded by the
carbon shells, as shown in FIG. 6B, a separate layer of different
magnetic material can be deposited on the tops of the FePt grains
from which the tops of the carbon shells have been removed by ICP
etching. FIG. 9 shows a sectional view of a TAR disk with a first
layer 310 of FePt, a second layer 312 formed on the first layer
after the ICP etching step to form the columnar grains 301, and an
upper exchange coupled ferromagnetic layer 320 formed directly on
top of the columnar FePt grains 306. The use of exchange-coupled
media is well-known. The ferromagnetic layer 320 may be formed of a
low anisotropy magnetic material with a crystallization temperature
higher than the Curie temperature of the FePt RL, such as FeCoTaB
and FeCoZrB alloys. Such a layer will assist in the magnetic
recording process through the exchange interaction with the FePt
RL, but only when it is in direct contact with the FePt grains.
[0048] While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *