U.S. patent application number 09/879156 was filed with the patent office on 2001-10-18 for selective growth of ferromagnetic films.
Invention is credited to Bojarczuk, Nestor A. JR., Duncombe, Peter R., Guha, Supratik, Gupta, Arunava, Karasinski, Joseph M., Li, Xinwei.
Application Number | 20010031384 09/879156 |
Document ID | / |
Family ID | 22628646 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031384 |
Kind Code |
A1 |
Bojarczuk, Nestor A. JR. ;
et al. |
October 18, 2001 |
Selective growth of Ferromagnetic films
Abstract
A device and a method of forming the device, includes selective
area deposition of a ferromagnetic material on a substrate,. The
substrate surface is partially covered with material having a
crystal structure having at least one symmetry relation with the
crystal structure of the ferromagnetic material
Inventors: |
Bojarczuk, Nestor A. JR.;
(Poughkeepsie, NY) ; Duncombe, Peter R.;
(Peekskill, NY) ; Guha, Supratik; (Yorktown
Heights, NY) ; Gupta, Arunava; (Valley Cottage,
NY) ; Karasinski, Joseph M.; (Yorktown Heights,
NY) ; Li, Xinwei; (Mohegan Lake, NY) |
Correspondence
Address: |
MCGINN & GIBB, PLLC
8321 OLD COURTHOUSE ROAD
SUITE 200
VIENNA
VA
22182-3817
US
|
Family ID: |
22628646 |
Appl. No.: |
09/879156 |
Filed: |
June 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09879156 |
Jun 13, 2001 |
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09172659 |
Oct 15, 1998 |
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Current U.S.
Class: |
428/831 ;
G9B/5.288; G9B/5.306 |
Current CPC
Class: |
G11B 5/73915 20190501;
Y10T 428/24802 20150115; Y10T 428/24752 20150115; G11B 5/73919
20190501; G11B 5/855 20130101; H01L 43/12 20130101 |
Class at
Publication: |
428/694.0TS |
International
Class: |
G11B 005/64 |
Claims
Having thus described our invention, what we claim as new and
desire to secure by Letters Patent is as follows:
1. A method of forming a magnetic device, comprising: selective
area deposition of a ferromagnetic material on a substrate, said
substrate surface partially covered with material having a crystal
structure having at least one symmetry relation with the crystal
structure of said ferromagnetic material.
2. The method according to claim 1, wherein said ferromagnetic
material is CrO.sub.2.
3. The method according to claim 2, wherein at a predetermined
temperature of said substrate, said CrO.sub.2 grows only on certain
surfaces of said substrate and does not form on other surfaces.
4. The method according to claim 3, wherein said substrate
comprises one of a single crystal sapphire (Al.sub.2O.sub.3) and
titanium oxide (TiO.sub.2).
5. The method according to claim 1, wherein at a predetermined
temperature of said substrate, said ferromagnetic material grows
only on certain surfaces of said substrate and does not form on
other surfaces.
6. The method according to claim 2, wherein said substrate includes
a crystal structure having a shape one of trigonal, hexagonal,
monoclinic, orthorhombic, tetragonal, and cubic.
7. The method according to claim 1, wherein said substrate
comprises one of a single crystal sapphire (Al.sub.2O.sub.3) and
titanium oxide (TiO.sub.2).
8. The method according to claim 1, wherein a portion of said
substrate includes a material that is amorphous.
9. The method according to claim 1, wherein a portion of said
substrate comprises one of amorphous SiO.sub.2, Si.sub.3N.sub.4,
and a compound of amorphous SiO.sub.2 and Si.sub.3N.sub.4.
10. A method of forming a magnetic device, comprising: selective
area deposition of a ferromagnetic material on a substrate, wherein
a portion of said substrate contains material that bears a symmetry
relation with the ferromagnetic material, and wherein said
substrate comprises an amorphous material upon which the
ferromagnetic material does not grow and which does not share a
symmetry relation with the ferromagnetic material.
11. The method according to claim 10, wherein said amorphous
material comprises one of SiO.sub.2 and Si.sub.3N.sub.4.
12. A method of forming a magnetic device, comprising: coating a
substrate with a ferromagnetic layer; depositing an insulating
barrier layer on said ferromagnetic layer; subsequent to
lithographic patterning of said insulating barrier layer,
depositing a SiO.sub.2 layer; opening contact holes in said
SiO.sub.2 layer to said insulating barrier layer; selectively
growing a ferromagnetic layer in the contact holes; and depositing
a metallization layer on the selectively grown ferromagnetic
layer.
13. The method according to claim 12, wherein said ferromagnetic
material comprises CrO.sub.2.
14. The method according to claim 12, wherein said metallization is
formed of Au and wherein said substrate comprises a semiconductor
substrate.
15. The method according to claim 12, wherein a bottom magnetic
electrode of said device includes a CrO.sub.2 layer and an
insulating barrier layer, said CrO.sub.2 layer and said insulating
barrier layer being photolithographically patterned and etched
prior to the deposition of SiO.sub.2 layer and subsequent selective
growth of the CrO.sub.2 layer and the metallization layer.
16. The method according to claim 12, wherein said insulating
barrier layer includes at least one of TiO.sub.2 and
Al.sub.2O.sub.3
17. The method according to claim 12, wherein said substrate
includes an amorphous material upon which the ferromagnetic
material does not grow and which does not share a symmetry relation
with the ferromagnetic material.
18. The method according to claim 17, wherein said amorphous
material comprises one of SiO.sub.2, Si.sub.3N.sub.4, and a
compound of SiO.sub.2 and Si.sub.3N.sub.4.
19. A method of forming a film, comprising: coating a single
crystal substrate with a SiO.sub.2 layer, said single crystal
substrate comprising one of a TiO.sub.2 substrate and a sapphire
substrate; forming a window in the SiO.sub.2 layer; and selectively
growing ferromagnetic material in said window.
20. The method according to claim 19, wherein lateral overgrowth
from sides into the SiO.sub.2-coated single crystal substrate
occurs due to growth of side facets of the ferromagnetic material
layer having been deposited.
21. The method according to claim 20, wherein said lateral
overgrowth is substantially defect-free.
22. The method according to claim 20, wherein said substrate
includes an amorphous material upon which the ferromagnetic
material does not grow and which does not share a symmetry relation
with the ferromagnetic material.
23. The method according to claim 22, wherein said amorphous
material comprises one of SiO.sub.2, Si.sub.3N.sub.4, and a
compound of SiO.sub.2 and Si.sub.3N.sub.4.
24. A storage device, comprising: a substrate having a
ferromagnetic material selectively deposited thereon, said
substrate surface partially covered with material having a crystal
structure having at least one symmetry relation with the crystal
structure of said ferromagnetic material.
25. The storage device according to claim 24, wherein said
ferromagnetic material comprises CrO.sub.2.
26. The storage device according to claim 25, wherein at a
predetermined temperature of said substrate, said CrO.sub.2 grows
only on certain surfaces of said substrate and does not form on
other surfaces.
27. The storage device according to claim 26, wherein said
substrate comprises one of a single crystal sapphire
(Al.sub.2O.sub.3) and titanium oxide (TiO.sub.2).
28. The storage device according to claim 25, wherein said
substrate includes a crystal structure having a shape one of
trigonal, hexagonal, monoclinic, orthorhombic, tetragonal, and
cubic.
29. The storage device according to claim 25, wherein said
substrate comprises one of a single crystal sapphire
(Al.sub.2O.sub.3) and titanium oxide (TiO.sub.2).
30. The storage device according to claim 25, wherein a portion of
said substrate includes a material that is amorphous.
31. The storage device according to claim 25, wherein a portion of
said substrate comprises one of amorphous SiO.sub.2,
Si.sub.3N.sub.4, and a compound of amorphous SiO.sub.2, and
Si.sub.3N.sub.4.
32. A device, comprising: a substrate having thereon selective
area-deposited ferromagnetic material, wherein a portion of said
substrate contains material that bears a symmetry relation with the
ferromagnetic material, and wherein said substrate comprises one of
an amorphous material upon which the ferromagnetic material does
not grow and which does not share a symmetry relation with the
ferromagnetic material.
32. The device according to claim 31, wherein said amorphous
material comprises one of SiO.sub.2, Si.sub.3N.sub.4, and a
compound of SiO.sub.2 and Si.sub.3N.sub.4.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is related to U.S. patent
application Ser. No. 09/______, filed on ______, to Supratik Guha
et al., entitled "HIGH DENSITY MAGNETIC RECORDING MEDIUM UTILIZING
SELECTIVE GROWTH OF FERROMAGNETIC MATERIAL" having IBM Docket No.
YO998-269, assigned to the present assignee, and incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to a method for
depositing a ferromagnetic compound, such as chromium dioxide
(CrO.sub.2), in thin film form in a selective fashion over a
substrate, such that the growth occurs only above specific regions
of the substrate that have been appropriately modified.
[0004] More specifically, the invention relates to a method for
depositing such a ferromagnetic compound in a selective growth
process for a plurality of applications including magnetic memory
and storage-based devices, as well as other devices.
[0005] 2. Description of the Related Art
[0006] Conventional systems utilize chromium dioxide (CrO.sub.2) as
an important ferromagnetic material which is used, for example, as
a particulate magnetic recording media. Chromium dioxide has the
rutile crystal structure which is tetragonal with lattice
parameters of a=4.423 .ANG. and c=2.917 .ANG.. The chromium ions
are in the Cr.sup.+4 state with the electronic configuration
[Ar]3d.sup.2 with a magnetic moment of 2.mu..sub.B per ion.
[0007] Because of its half-metallic nature, where the majority spin
electrons have a metallic character and the minority spin electrons
have a semiconducting character (e.g., see K. Schwarz, J. Phys. F
16, 211 (1986)), chromium dioxide also is ideally suited for use in
magnetic tunnel junction (MTJ) devices. Suitable applications for
CrO.sub.2 include, as a media for magnetic storage, and as a
component in magnetic tunnel junction devices.
[0008] Recently, there have been attempts to deposit CrO.sub.2 in
the form of thin films (though not in a selective area fashion), as
described, for example, in R. C. DeVries, "Epitaxial Growth of
CrO.sub.2", Mat. Res. Bull. 1, 83 (1966); S. Ishibashi, T.
Namikawa, and M. Satou, "Epitaxial Growth of CrO.sub.2 in Air",
Japan J. Appl. Phys. 17, 249 (1978); S. Ishibashi, T. Namikawa, and
M. Satou, "Epitaxial Growth of Ferromagnetic CrO.sub.2 Films in
Air", Mat. Res. Bull. 14, 51 (1979); and K. P. Kamper, W. Schmitt,
G. Guntherodt, R. J. Gambino, and R. Ruff, "CrO.sub.2--A New
Half-Metallic Ferromagnet?", Phys. Rev. Lett. 59, 2788 (1987).
[0009] However, these studies have been confined to the uniform
deposition of CrO.sub.2 that covered the entire substrate (e.g., a
substantially continuous deposition). Further, selective area
growth of CrO.sub.2 has not been attempted on predetermined
locations (e.g., those that could be patterned) on the substrate
surface with arbitrary size dimensions. Thus, control in the
deposition of such films has been limited, and thus the
applications of such films have been limited.
[0010] Selective area growth, which consists of depositing a
material over a substrate in such a fashion that material is
deposited only on specific regions of substrate t have been
appropriately modified, is an established technique in the field of
compound semiconductors and metal deposition.
[0011] In the area of semiconductor deposition, selective growth
has been used for the production of higher quality thin film
optoelectronic devices for making better light emitters, and in the
case of metals, selective growth has been used for different
applications such as plugs for hole filling in Si technology. The
particular technical details for selective area growth of various
systems are quite diverse due to the different chemistries
involved.
[0012] Thus, as noted above, selective area growth has not been
used for applications in ferromagnetic thin films or in forming any
substrates used in magnetic memory applications, for example.
[0013] The conventional method of patterning blanket thin films
using photolithography and etching is difficult in the case of
CrO.sub.2 since it is not readily etched in a reactive plasma or
with wet chemicals. Hence, selective area growth would be a
preferred approach since it requires no subsequent patterning
steps.
SUMMARY OF THE INVENTION
[0014] In view of the foregoing and other problems of the
conventional structures and methods, it is an object of the present
invention to provide a structure and method for allowing selective
area deposition of a ferromagnetic material such as CrO.sub.2.
[0015] In a first aspect of the invention, a method of forming a
magnetic device, includes selective area deposition of a
ferromagnetic material on a substrate, part of the substrate
surface being covered with material with a crystalline structure
that shares one or more symmetry relations with the crystal
structure of the ferromagnetic material.
[0016] These symmetry relations do not have to be the
characteristic symmetry element that defines membership into the
particular crystal system. As an example, a substrate is considered
that is hexagonal, so that its characteristic symmetry element is a
6-fold rotation axis. Further considered is a ferromagnetic thin
film that is tetragonal so that its characteristic symmetry element
is a single 4-fold rotation axis. In this case, the symmetry
element that these two structures share is that of 2-fold rotation
symmetry about these axes since it is obvious that both the 6-fold
axis (of the hexagonal structure) and the 4-fold axis (of the
tetragonal structure) possess 2-fold symmetry.
[0017] Generally, the technique utilizes the idea of deposition of
CrO.sub.2 on a surface by the reaction
CrO.sub.3=CrO.sub.2+1/2O.sub.2.
[0018] The present inventors have found that, at an appropriate
substrate temperature, the above reaction will proceed only on
certain surfaces, but the reaction will not occur on other
surfaces. This discovery by the present inventors is the basis for
a selective area growth process. If a substrate surface is
patterned in the form of features consisting of two materials, A
and B such that CrO.sub.2 grows on A but not on B, then the spatial
growth of CrO.sub.2 can be controlled selectively by appropriately
pre-patterning the substrate surface with features of A and B. The
relationship between the surface symmetry of the two constituents
is important. Thus, surfaces on which growth can occur will be from
a crystalline system that is trigonal, monoclinic, or a system with
a symmetry higher than monoclinic such as orthorhombic, tetragonal,
or cubic; or a system with a symmetry higher than trigonal such as
hexagonal--as long as one can define a symmetry relationship
between CrO.sub.2 and the substrate. Substrates on which growth
will not occur will be amorphous such as SiO.sub.2.
[0019] Thus, with the unique and unobvious features of the present
invention, selective area growth can be used for applications of
ferromagnetic thin films including magnetic memory
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of
preferred embodiments of the invention with reference to the
drawings, in which:
[0021] FIGS. 1(a)-1(f) are schematic diagrams of a process of the
present invention for magnetic tunnel junction device processing;
and
[0022] FIGS. 2(a)-2(g) are schematic diagrams of another process of
the present invention for magnetic tunnel junction device
processing;
[0023] FIGS. 3(a)-3(c) are schematics showing a process for growth
of high quality CrO.sub.2 layers according to the present
invention;
[0024] FIG. 4 is a schematic diagram of an atmospheric pressure
chemical vapor deposition apparatus;
[0025] FIGS. 5(a)-5(f) are schematic diagrams showing a process
according to the present invention using the apparatus of FIG.
4,
[0026] FIGS. 6(a)-6(c) are scanning electron micrographs (SEMs)
showing features of different sizes produced according to the
process of the present invention;
[0027] FIG. 6(d) is an SEM showing the morphology of an epitaxial
film which has been blanket-deposited on a sapphire substrate and
is for comparison with the SEMs of FIGS. 6(a)-6(c) for structures
produced according to the present invention;
[0028] FIG. 7 is an SEM illustrating some nonselectivity in growth
due to incomplete cleaning of a surface;
[0029] FIGS. 8(a)-8(e) are schematic diagrams showing a method
using a sapphire substrate according to the present invention;
[0030] FIGS. 9(a)-9(d) are SEMs showing the results of the growth
on the sapphire substrate shown in FIGS. 8(a)-8(e); and
[0031] FIGS. 10(a) and 10(b) show the result of selective area
growth on a TiO.sub.2 substrate that has a patterned SiO.sub.2
layer on it, and specifically FIG. 10(a) shows a smooth, high
quality microstructure of the selectively grown CrO.sub.2 and FIG.
10(b) shows the lateral overgrowth that has occurred on the
SiO.sub.2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0032] Referring now to the drawings, and more particularly to
FIGS. 1(a)-10(b), preferred embodiments of the present invention
will be described.
[0033] Generally and as mentioned above, the present invention
allows selective area deposition of a ferromagnetic material such
as CrO.sub.2. The technique utilizes the idea of deposition of
CrO.sub.2 on a surface by the reaction
CrO.sub.3=CrO.sub.2+1/2O.sub.2.
[0034] Specifically, at an appropriate substrate temperature the
reaction will proceed only on certain surfaces, while it will not
occur on other surfaces. This discovery is the basis for a
selective area growth strategy.
[0035] As mentioned above, if a substrate surface is patterned in
the form of features consisting of two materials, A and B such that
CrO.sub.2 grows on A but not on B, then upon growth the spatial
growth of CrO.sub.2 can be controlled selectively by appropriately
prepatterning the substrate surface with features of A and B.
[0036] For example, the present inventors have found that CrO.sub.2
will deposit on a surface such as single crystal sapphire
(Al.sub.2O.sub.3), or titanium oxide (TiO.sub.2), but will not
deposit upon a SiO.sub.2 surface. In terms of crystal structure,
sapphire is hexagonal, TiO.sub.2 is tetragonal, and CrO.sub.2
itself is tetragonal. SiO.sub.2 is amorphous in nature. For
CrO.sub.2 deposition to occur on a surface, the crystal structure
of that surface preferably should bear a symmetry relation with the
crystal structure of CrO.sub.2.
[0037] Specifically, the reason is that the CrO.sub.2 phase itself
is a metastable tetragonal phase. For the formation of a stable
nuclei of CrO.sub.2 on a surface, the interfacial energy at the
CrO.sub.2 nucleus/substrate surface should be low, since otherwise
the energy cost for nucleus formation will be high. The existence
of a symmetry relation between the substrate surface and the
metastable CrO.sub.2 phase will result in an epitaxial
stabilization of the CrO.sub.2 phase and the formation of stable
nuclei as a consequence of a favorable interfacial energy.
[0038] In contrast, if there is no such symmetry relation, the
interfacial energy cost will be high enough so that the reaction
CrO.sub.3=CrO.sub.2+1/2O.sub.2 cannot proceed (e.g., to the right
of the equality above), and stable CrO.sub.2 nuclei will not form.
Hence, the relationship between the surface symmetry of the two
constituents is important. Thus, substrates on which growth can
occur will be from a crystalline system that is trigonal,
monoclinic, or a system with a higher symmetry than monoclinic such
as orthorhombic, tetragonal, or cubic; or a system with a higher
symmetry than trigonal such as hexagonal as long as one can define
a symmetry relationship between CrO.sub.2 and the substrate.
Substrates on which growth will not occur will be amorphous such as
SiO.sub.2 or the like (amorphous Si.sub.3N.sub.4, amorphous carbon,
glass).
[0039] The process above is powerful, since it allows growing
CrO.sub.2 only at a specific desired location. There are many
potential applications which would find great benefit with the
concept of the invention. Two exemplary applications (e.g.,
magnetic tunnel junction device processing and growth of high
quality CrO.sub.2 layers) are described below.
Magnetic Tunnel Junction Device Processing
[0040] Tunnel junction devices, where the current flow is between
the top and bottom magnetic electrode through a thin insulating
barrier layer, must be patterned into small structures for testing.
Such patterning is typically performed by using a multiple step
patterning and etching process after growth of the bottom
electrode, insulating barrier and the top electrode.
[0041] Normally, ion beam milling is used for etching the magnetic
layers since no preferred reactive ion etching process exists for
magnetic materials such as Co, NiFe, CoFe, etc. Redeposition during
ion milling can lead to microshorts in the tunnel junction
structure which will degrade the performance of the device.
Moreover, by using ion milling, it is difficult to obtain a sharp
etch profile and have an etch stop right below the barrier
level.
[0042] In contrast, by using selective area growth, the processing
can be made easier as shown in the schematics of FIGS. 1(a)-1(f)
and FIGS. 2(a)-2(f).
[0043] In a simplified process as shown in FIGS. 1(a)-1(f), a
substrate 1, preferably formed of Al.sub.2O.sub.3, TiO.sub.2,
silicon or the like is provided, as shown in FIG. 1(a). A bottom
magnetic CrO.sub.2 layer 2 and a thin insulating barrier layer 3
(e.g., preferably TiO.sub.2 and/or Al.sub.2O.sub.3, or the like
(SnO.sub.2) are deposited, e.g., by chemical vapor deposition in
turn on the substrate 1. Preferably, the CrO.sub.2 layer 2 has a
thickness substantially within a range of approximately 200 .ANG.
to approximately 2000 .ANG., and the insulating barrier layer 3 has
a thickness substantially within a range of approximately 10 .ANG.
to approximately 50 .ANG..
[0044] Next, as shown in FIGS. 1(b)-1(c), after lithographically
patterning the surface with a photoresist 4 or the like, a
SiO.sub.2 layer 5 is deposited. Preferably, the SiO.sub.2 layer 5
has a thickness substantially within a range of approximately 500
.ANG. to approximately 2000 .ANG..
[0045] Then, as shown in FIG. 1(d), the photoresist 4 remaining is
used as a lift-off stencil to open contact holes (e.g., vias) 6. A
CrO.sub.2 layer 7 is grown selectively in the opened vias 6 on
layer 3 (e.g., the TiO.sub.2 or Al.sub.2O.sub.3 surface 3).
Preferably, the vias 6 are opened to a depth substantially within a
range of approximately 0.3 .mu.m to approximately 10 .mu.m, and
most preferably 2 .mu.m. Preferably, the CrO.sub.2 layer 7 has a
thickness substantially within a range of approximately 100 .ANG.
to approximately 2000 .ANG., and most preferably 500 .ANG..
[0046] Finally, a metallization layer 8 (e.g. Au, Ag, Pt) is
deposited on the CrO.sub.2 layer 7 by sputtering or evaporation and
portions of the upper surface of the SiO.sub.2 layer 5, and
subsequently patterned to make contact to the top electrode.
Preferably, the metallization layer 8 has a thickness within a
range of approximately 1000 .ANG. to approximately 3000 .ANG., and
most preferably 1500 .ANG.. The selective growth of CrO.sub.2 is
preferably carried out by chemical vapor deposition. The
above-described process is especially useful for magnetic tunnel
junction device processing.
[0047] An alternative process, used only for MTJ, where the bottom
magnetic electrode and barrier (e.g., layers 2 and 3) are
photolithographically patterned (e.g., see FIG. 2(b) illustrating
the deposition of the photoresist 4 and FIG. 2(c) which illustrates
the structure after ion mill etching or the like) prior to the
deposition of SiO.sub.2 layer 5 (there is an additional
photolithography step after 2(c) to define the lift-off vias for
growth of top, CrO.sub.2 before the deposition of SiO.sub.2 as
depicted in 2(d)) and subsequent selective growth of the CrO.sub.2
layer 7 and metallization layer 8, is shown in FIGS. 2(a)-2(g)
which use the same reference numerals as FIGS. 1(a)-1(f) for like
elements. The remaining steps (e.g., shown in FIGS. 2(d)-2(g)) are
substantially the same as in FIGS. 1(c)-1(f).
[0048] Thus, with the first embodiment, selective area deposition
of a ferromagnetic material such as CrO.sub.2 is optimized for
magnetic memory applications and the like.
Growth of High Quality CrO.sub.2, Layers
[0049] CrO.sub.2 films preferably should have high crystalline
quality for improved tunneling device performance. However, when
CrO.sub.2 is grown on sapphire (Al.sub.2O.sub.3) or TiO.sub.2, the
thin film contains many defects that result from lattice mismatch.
The presence of planar defects may influence the magnetic and
magnetotransport properties of the films.
[0050] Using selective area growth, and utilizing the property of
lateral overgrowth that arises from selective area growth,
high-quality, defect-free CrO.sub.2 layers may be obtained, as
shown in FIGS. 3(a)-3(c).
[0051] Essentially, a single crystal TiO.sub.2 or sapphire
substrate 31 (or a thin film of TiO.sub.2 or sapphire on a silicon
substrate 30 as shown) is coated with a layer of SiO.sub.2 32.
Preferably, there is no layer between 31 and 30. One may either
start with a single crystal TiO.sub.2 or sapphire substrate, or
deposit a TiO.sub.2 film on a Si substrate. It is important to
provide a TiO.sub.2 or sapphire surface for CrO.sub.2 growth.
Preferably, layer 31, if it is a thin film and not a substrate
itself, has a thickness substantially within a range of
approximately 1000 .ANG. to approximately a few microns, and most
preferably 1 micron, and layer 32 has a thickness substantially
within a range of approximately 200 .ANG. to approximately 1
micron, and most preferably 2000 .ANG..
[0052] Windows 34 (e.g., preferably having lateral dimensions of a
few hundred angstroms to a few hundred microns) are opened up in
the SiO.sub.2 32, as shown in FIG. 3(a).
[0053] Thereafter, selective area growth is performed with
deposited CrO.sub.2 35, as shown in FIG. 3(b). Preferably, the
CrO.sub.2 deposition 35 has a thickness substantially within a
range of approximately few hundred angstroms to approximately a few
microns. Lateral overgrowth from the sides into the
SiO.sub.2-covered surface occurs due to the growth of the side
facets 36 of the deposited CrO.sub.2 35. This lateral overgrowth 37
is substantially defect-free (or at least has a decreased defect
density), since the surface below is SiO.sub.2 32, and offers no
lattice mismatch since it is amorphous.
[0054] Further, since defects propagate upwards on the CrO.sub.2 35
growth in the defective region, no lateral migration of the defects
occurs on the overgrown region. FIG. 3(c) illustrates poor quality
directly overgrown CrO.sub.2 on TiO.sub.2 at reference numeral 38,
whereas reference numeral 39 illustrates high quality laterally
overgrown CrO.sub.2 on SiO.sub.2. This is a general technique, and
has been successfully employed for the growth of compound
semiconductors such as GaAs (e.g., see B. Y. Tsaur et al., Appl.
Phys. Lett., V41, 347 (1982); U.S. Pat. Nos. 4,670,088, 4,826,784
and 4,868,633) and GaN. However, such has not been applied for
ferromagnetic materials and the present invention clearly is novel
in at least this respect.
[0055] Thus, using selective area growth, and utilizing the
property of lateral overgrowth that arises from selective area
growth, high-quality, defect-free CrO.sub.2 layers are
produced.
[0056] FIG. 4 shows a schematic of an atmospheric pressure chemical
vapor deposition apparatus 40 for practicing the selective area
growth according to the present invention. The setup is similar to
what has been described by Isibashi et al., mentioned above and
incorporated herein by reference.
[0057] Apparatus 40 includes a two-zone furnace 41 with a quartz
tube. The precursor powder 42 (e.g., CrO.sub.3) is placed in a
quartz boat in the source zone and the substrates 43 placed on a
tilted susceptor placed in the reaction zone. The tilting of the
susceptor helps maintain a relatively uniform deposition on the
substrates by increasing the flow velocity due to constriction in
the channel and thereby thinning the boundary layer. The precursor
powder is sublimed and carried to the substrates using oxygen
(O.sub.2) 44 where it decomposes to form the CrO.sub.2 film by
evolving oxygen. The temperature of the source zone has been varied
between about 250-300.degree. C. and the substrate temperature from
about 385-450.degree. C. for different runs. Preferred source
temperature is substantially within a range of about 260 to about
270.degree. C., and a preferred substrate temperature is
substantially within a range of about 390 to about 420.degree. C.
The flow rate of oxygen has similarly been varied between 50-500
cc/min. Reference numeral 45 illustrates an exhaust. A gas flow
controller 46 also is provided.
[0058] The phase purity and morphology of the films depend on the
substrate and source temperatures and the oxygen flow rate, with
optimum films (e.g., in terms of phase purity and morphology) being
obtained at substrate temperatures of approximately 390 to about
420.degree. C., with source temperature of around 260 to about
270.degree. C. and oxygen flow rate of approximately 100
cc/min.
[0059] The present inventors have found that the deposition of
CrO.sub.2 is highly sensitive to the surface of the substrate on
which it is deposited. CrO.sub.2 will not deposit on clean,
amorphous SiO.sub.2 (or amorphous Si.sub.3N.sub.4 or a compound of
amorphous SiO.sub.2 and Si.sub.3N.sub.4), but will deposit on a
surface of TiO.sub.2 or sapphire (Al.sub.2O.sub.3), as demonstrated
in the following description and as shown in FIGS. 5(a)-5(f).
[0060] In FIG. 5(a), a substrate including a Si (100) wafer 50 had
a SiO.sub.2 layer 51 having a thickness of approximately 5000 .ANG.
deposited thereon. Then, a Ti film 52 having a thickness of
approximately 500 .ANG., was deposited on top of the silicon
dioxide layer 51, as shown in FIG. 5(a). A Sn film and/or Ru film
can be used instead of or in addition to the Ti film.
[0061] As shown in FIG. 5(b), a photoresist 53 was then spun onto
this wafer, and the wafer patterned in the form of squares,
rectangles, etc. and lines with dimensions ranging from about 1
micron to about 300 microns using standard lithographic procedures.
Then, the wafer was placed in a HNO.sub.3:HF:H.sub.2O (1:1:50)
solution to etch the Ti, thereby leaving 1-300 micron features of
Ti on SiO.sub.2 as shown in FIG. 5(c).
[0062] Then, as shown in FIG. 5(d), the photoresist 53 was
dissolved away in a suitable solvent such as acetone or the
like.
[0063] Thereafter, the patterned wafer was given a final acetone
clean bath, and inserted into the growth reactor with the substrate
heated to approximately 395.degree. C. The source temperature was
maintained at approximately 260.degree. C. and the oxygen flow rate
was approximately 100 cc/min. Thus, the Ti 52 was converted to
TiO.sub.2 54, as shown in FIG. 5(e). If Sn and/or Ru are employed,
they also would be converted to the oxides.
[0064] As shown in FIG. 5(f), deposition of the CrO.sub.2 55
resulted in preferential growth only on the TiO.sub.2 54 and not on
the SiO.sub.2 51. There is no excess CrO.sub.2. All the growth
occurs on the TiO.sub.2-covered surfaces.
[0065] After deposition, the wafer was removed from the reactor and
studied by scanning electron microscopy (SEM) to reveal the
selective area growth.
[0066] CrO.sub.2 was observed not to have been deposited on the
SiO.sub.2. However, it grew readily on the TiO.sub.2 formed by
oxidation of the Ti film. This can be seen in the scanning electron
micrographs (SEMs) of FIGS. 6(a)-6(c) showing that no growth of
CrO.sub.2 has occurred on the SiO.sub.2. FIGS. 6(a)-6(c) show
features of different sizes.
[0067] For example FIG. 6(a) shows a 750 .mu.m feature size, FIG.
6(b) shows a 20.0 .mu.m feature size, and FIG. 6(c) shows a 4.3
.mu.m feature size. The bars in the micrograph correspond to the
scale from which the feature sizes can be estimated. They are not
the actual feature sizes. It is noted that the growth habit occurs
only on the patterned areas containing the TiO.sub.2 layer. The
morphology of an epitaxial film which has been blanket deposited on
a sapphire substrate is shown in FIG. 6(d) for comparison.
[0068] A subsequent experiment showed that selective growth of
CrO.sub.2 on TiO.sub.2 could be accomplished even with a Ti layer
as thin as 20 .ANG.. Further, the pressure was reduced in the
reactor tube down to 10 Torr and selective growth occurred using
the same range of temperatures for the source and substrate as for
atmospheric pressure growth.
[0069] The growth of CrO.sub.2 occurs over a narrow window of
temperature and flow parameters. Preferably such growth occurs over
this narrow window/range, although some selectivity can still be
maintained beyond this range. Selective area growth was observed
across this entire window of deposition conditions. The selectivity
is strongly dependent upon the cleanliness of the SiO.sub.2. If
there is contamination on the SiO.sub.2, CrO.sub.2 will deposit
upon the contamination and selectivity will be compromised, as
shown in the SEM of FIG. 7.
[0070] In FIG. 7, the SiO.sub.2 was contaminated due to incomplete
cleaning of the surface. As shown, while there is extensive growth
of CrO.sub.2 on the TiO.sub.2, some deposition also has occurred on
the SiO.sub.2. CrO.sub.2 is a metastable phase, and its successful
nucleation is facilitated by the presence of a crystalline template
that is close to its crystal structure. This is provided by the
TiO.sub.2 surface, and not provided by the amorphous SiO.sub.2
surface, and hence the selectivity. A CrO.sub.3 molecule impinging
upon the SiO.sub.2 surface will have a long migration length and
will subsequently desorb.
[0071] In contrast, if there is contamination present, the
CrO.sub.3 will decompose at the contamination site to form
CrO.sub.2. On a TiO.sub.2 surface, the arriving CrO.sub.3 molecule
has a short migration length and reacts to form CrO.sub.2 due to
stabilization of the substrate which acts as a catalyst for the
reaction. Because of the epitaxial match with A1.sub.2O.sub.3,
CrO.sub.2 can also be selectively grown on this template.
[0072] In the example shown, the TiO.sub.2 template was
polycrystalline. Hence, the ensuing CrO.sub.2 deposit was
polycrystalline as well, and the faceted grains can be easily
observed in the micrographs of FIGS. 6(a)-6(c) and 7. When a single
crystal template is used, the microstructure observed is much
smoother. For example, this relative smoothness is observed in the
case of growth on single crystal non-patterned sapphire substrates
as shown in FIG. 6(d).
[0073] FIGS. 8(a)-8(e) illustrate a process according to the
present invention in which the substrate used is sapphire (e.g.,
single crystal sapphire(Al.sub.2O.sub.3)).
[0074] Specifically, FIGS. 8(a)-8(e) show where a SiO.sub.2 film 81
was deposited on a sapphire substrate, and patterned with a
photoresist 82 to open up windows 83 in the SiO.sub.2 film for
selective area growth using standard photolithographic techniques
and wet chemical etching. Unlike the structure shown in FIG. 5, no
Ti deposition and subsequent oxidation are performed here since the
growth of CrO.sub.2 occurs on the sapphire substrate inside the
windows 83. Growth of CrO.sub.2 84 is now performed in
substantially the same way as was done for the case of growth of
CrO.sub.2 on TiO.sub.2. However, due to the single crystal nature
of the sapphire substrate the growth microstructure is far smoother
as clearly observed in the SEM micrographs of FIGS. 9(a) and 9(b)
which show the results of growth on the patterned sapphire
substrate 80 shown in FIGS. 8(a)-8(e).
[0075] As clearly shown from the SEM micrographs of FIG. 9(a)-9(d),
selective area growth has occurred with growth occurring on the
exposed sapphire surface, but not on the adjoining SiO.sub.2
surface, and the growth is smoother compared to growth on TiO.sub.2
since the sapphire surface is single crystal in nature. It is noted
that the growth at the edges of the features is not very uniform as
compared to the growth away from the edges and this may be related
to anomalous behavior of the surface diffusion of the reactant
species near the feature edges.
[0076] FIG. 10 shows the situation when the growth is performed on
single crystal TiO.sub.2. In this case, a single crystal TiO.sub.2
wafer was coated with 120 nm SiO.sub.2 and patterned so as to open
up 5-micron- and 10-micron-stripe windows in the SiO.sub.2. When
CrO.sub.2 growth was performed, again as expected, selective area
growth was observed as can be seen in the SEM micrograph of FIG.
10(a). The growth in this case is of high quality as evidenced from
the smooth growth microstructure.
[0077] FIG. 10(b) shows the growth cross-section observed at an
angle. Clearly, epitaxial lateral overgrowth, which was described
earlier, is observed and the CrO.sub.2 penetrates laterally over
the SiO.sub.2 and from the edge of the stripe at a velocity that is
roughly 50% of the growth velocity in the vertical (growth)
direction.
[0078] While the invention has been described in terms of several
preferred embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the appended claims.
* * * * *