Fabrication and magnetic characterization of embedded permalloy structures
T.Tezuka, T.Yamamoto, K. Machida, T. Ishibashi, Y. Morishita, A. Koukitu and K.Sato
Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
Abstract
Regularly aligned magnetic patterns of square (1 m1 m), rectangular (300 nm100 nm) and circular (100 nm in diameter) dots with thickness of 150 nm and pattern-separation of 300 nm embedded in silicon wafers have been successfully
fabricated by the damascene technique using an electron beam (EB) lithography and chemical mechanical polishing (CMP): Mask patterns were exposed using an EB pattern generator on the resist films spin-coated on Si(100) wafer. Using the mask pattern the Si
wafer was dry-etched using CF4 gas. Permalloy films were deposited using an electron beam evaporator on to the pit arrays and the magnetic materials outside the pit was polished out by CMP. The magnetic structures of these nano dot arrays were observed using a
magnetic force microscope (MFM:SPA-300/SPI3800N with high moment-tip coated CoPtCr:500A (SII)). Details have been published elsewhere.
In the present study, we investigated cross-shaped array structures of permalloy (Ni80Fe20), in order to elucidate spin structures of magnetic dots with more complicated shape. Fabrication of the cross-shaped patterns was performed using the same damascene
technique as mentioned above.
Two kinds of structures named CROSS1 [200 nm in width, 3 m in length with separation of 3 m] and CROSS2 [100 nm in width, 1.5 m in length with separation of 1.5m] were fabricated. All the MFM images (with high-moment tips) of these cross-shaped patterns
show magnetic poles at the end of the cross bars. The left edge of horizontal wing (tip scan direction) is always bright regardless of magnetization direction of the sample, suggesting that the MFM image is subjected to the influence of the stray field from the tip.
Therefore, we tried to use low-moment probe tips (MFM:SPI-4000/SPA300HV with low-moment probe tip coated CoPtCr:240A(SII)). A preliminary theoretical work on the spin structure of the pattern using LLG equation does not show the presence of magnetic poles at
the end of the cross-bar, suggesting that the importance of the static magnetic interaction between patterns in the array structure.
2 models of cross-patterned dot array
Fabrication by Damascean technique
Spin coating
of resist
Electron beam
deposition
1500nm
3000nm
EB exposure
Development
Etching
Resist removal
flatting
Buried Permalloy
Silicon wafer
Si substrate
〔1〕 EB-resist （ZEP-520） thickness： 300 nm
〔1〕Etching gas:
〔2〕 Rotation : 1000rpm/5sec→5000 rpm/90sec
〔2〕Vacuum:
〔3〕 Baking : 160℃/20min （ after development 120℃/20min ）
〔3〕Gas pressure 9.2Pa
〔4〕 Accelerating voltage ： 50kV
〔5〕 Beam current： 20pA
3.0×10-3Pa
Comparison between AFM image and MFM images
MFM image
AFM image
〔2〕Polishing chemicals: Si wafer
GLANZOX SP-15 ( FUJIMI ) pH:11
〔3〕 Accelerating voltage： 4kV
400W
700nm
100nm
〔3〕Weight: 320g
〔4〕 Deposition rate： 0.4～0.8Å/sec
1500nm
〔4〕Rotation: 150rpm
90sec
3000nm
MFM observation with a high - moment probe tip
Comparison between high-moment-tip
and low-moment-tip observations [Ⅰ]
MFM image
CROSS1
Past Data
Past Data
〔1〕 Slurry: Al2O3 /Grain-size: 20nm
〔2〕 Vacuum： 3.0×10-6Torr
〔5〕Etching rate: 1.6nm/sec
〔6〕Etching time:
AFM image
〔1〕 material： permalloy（Ni80Fe20）
CF4
〔4〕RF power:
〔6〕 Patterned area ： 3mm×3mm
Embedding of permalloy
1400nm
Dry etching process
200nm
EB-patterning process
CROSS2 (small)
CROSS1 (large)
Chemical mechanical
polishing
Square dots
1m ×1m square dots ( 300nm space )
100nm ×300nm rectangular dots ( 300nm space )
Magnetization
High-moment tip
(CoPtCr/500Å in Air)
Sample
AFM image
Pattern variations with different magnetization direction of the sample
MFM image
AFM image
Probe tip
MFM image
CROSS2
200nm ×3000nm cross dots ( 3000nm space )
Low-moment tip
(CoPtCr/240Å in HV)
Rectangular dots
Zoom up at center of a cross dot
Comparison between high-moment-tip
and low-moment-tip observations [Ⅱ]
Comparison between high-moment-tip
and low-moment-tip observations [Ⅲ]
CROSS1
CROSS2
Comparison of MFM observations
for different directions of a tip-magnetization
Magnetization
High-moment tip
(CoPtCr/500Å in Air)
Low-moment tip
(CoPtCr/240Å in HV)
High-moment tip
(CoPtCr/500Å in Air)
Demagnetized sample
Sample
Low-moment tip
(CoPtCr/240Å in HV)
Magnetization direction of the tip was changed
Probe tip
MFM observation with
demagnetized tip
Theoretical cross-dot spin structure
calculated using LLG equation
VSM measurements of CROSS1
y
To parallel
To perpendicular
2000
M(emu/cm3)
2000
M(emu/cm3)
L
0
-2000
0
z (E.A.)
-2000
-10
0
H(kOe)
10
x
W
-10
0
H(kOe)
10
VSM measurements of CROSS2
Hz = 20 kOe → 0 Oe
To perpendicular
Saturation magnetization (Ms)
800 emu/cm3
Exchange field (A)
1×10-6 erg/cm3
divMz
2000
2000
Anisotropic constant (Ku)
1000 erg/cm3
1000
1000
Gyro magnetic constant（γ)
-1.76×107 rad/(s・Oe)
Damping constant（α）
0.2
Easy axis
Z direction
-1000
Cross-pattern size
500 (L) nm×100 (w) nm ×50 (T)nm
-2000
Number of cross-pattern
1
Mesh size
10 nm×10 nm×10 nm
M(emu/cm3)
M(emu/cm3)
To parallel
divM
0
-1000
-2000
-10
0
H(kOe)
10
0
-10
0
H(kOe)
10
Summary
•Regularly aligned permalloy dots with sub-micron cross-shaped pattern have
been successfully fabricated using Damascene technique on the silicon
substrates.
•All the MFM images clearly show magnetic poles at the end of the cross bars
and complicated structures at the crossing region.
•The upper edge of the vertical wing shows bright or dark spots depending on
the magnetization direction of the sample.
•All the MFM patterns of the cross-array structure are aligned in the same
direction, even though it is demagnetized. Since the such phenomenon occurred
with a demagnetized tip, the magnetic alignment cannot be attributed to the stray
field from the probe tip.
•A vortex-like image is observed at the crossing region using a high-moment
probe tip, while asymmetrical domain-like pattern is observed with a lowmoment tip.
•An LLG analysis on a single cross-shaped pattern does not show poles at the
end of the cross bar. Non negligible effect of magneto-static interaction between
patterns is suggested.
•Further theoretical studies are underway including magneto-static interactions.
Acknowledgements
We are very grateful to Dr. Yamaoka of Seiko Insturuments Inc. for his help in MFM
measurements with a low-moment probe tip.
This work has been carried out under the 21st-century COE program of TUAT on “Future
Nano Materials.”