Interhalogen plasma chemistries for dry etch patterning of Ni, Fe

Applied Surface Science 140 Ž1999. 215–222
Interhalogen plasma chemistries for dry etch patterning of Ni, Fe,
NiFe and NiFeCo thin films
H. Cho
a
a,)
, K.B. Jung a , D.C. Hays a , Y.B. Hahn a,1, E.S. Lambers a , T. Feng a ,
Y.D. Park a , J.R. Childress b, S.J. Pearton a
Department of Materials Science and Engineering, UniÕersity of Florida, 132 Rhines Hall, PO Box 116400, GainesÕille, FL 32611, USA
b
IBM Almaden Research Center, San Jose, CA 95120, USA
Received 28 August 1998; accepted 1 October 1998
Abstract
IClrAr and IBrrAr plasmas operated in an inductively coupled plasma ŽICP. source have been examined for dry etching
of Ni, Fe, NiFe and NiFeCo. The removal of the Fe etch products limits the etch rates under most conditions, but rates of
˚ miny1 are obtained for both NiFe and NiFeCo in both chemistries. The etched surfaces are smooth Žatomic force
; 500 A
microscopy root-mean-square roughness - 1 nm. over a broad range of plasma conditions, with small residual halogen
concentrations ŽF 2 at.%.. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Interhalogen; Dry etch; Thin film
1. Introduction
The dry etching of magnetic multilayer structures
represents a challenge because of the relative involatility of the etch products w1–8x. For this reason,
most of the patterning for magnetic sensors, nonvolatile memory elements, and readrwrite heads are
performed using ion beam milling to physically sputter the material w2–7x. High ion energies Ž; 1 kV.
during this process have been found to lower the
coercivity of magnetic elements by up to a factor of
eight, probably due to creation of magnetic dead)
Corresponding author. Fax: q1-352-846-1182
Present address. Department of Chemical Engineering and
Technology, Chonbuk National University, Chonju 561-756, South
Korea.
1
layers on the exposed sidewalls w9x. Etch processes
with a chemical component, in addition to purely
physical sputtering, should have a number of advantages, including higher etch rates, better selectivity to
mask materials, lower ion energies and reduced redeposition on feature sidewalls w10x. One method of
enhancing the etch product volatility in plasma etching is to heat the sample during the process Žgenerally 200–3008C would be required. w1x, but in giant
magneto-resistive ŽGMR. multilayers, the component
˚ thick and there is only a
layers may only be 15–20 A
very limited thermal budget available before interdiffusion occurs w2x.
Another method for removing the etch products is
by providing a high ion flux incident simultaneously
with the reactive neutral flux. This provides impetus
for ion-assisted desorption of the etch products. Ex-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 9 8 - 4
216
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
perimentally, we have found that ion-neutral ratios
G 0.02 are necessary for achieving high etch rates of
NiFe and NiFeCo in Cl 2rAr plasma chemistries
w11–13x. These flux ratios are only available in high
density plasma sources, such as inductively coupled
plasmas ŽICP. or electron cyclotron resonance ŽECR.
microwave plasmas. In conventional reactive ion
etch tools, the ion-neutral flux ratio is typically in the
10y5 –10y6 range, and the absence of a strong ionassisted desorption contribution leads to the build-up
of a selvedge or reaction layer of chlorinated etch
products on the sample surface.
While the Cl 2rAr plasma chemistry operated under high density conditions produces effective etching of magnetic materials, there are other mixtures of
interest. In particular, the interhalogens, ICl and IBr,
have been found to dissociate readily in high density
Fig. 2. Etch rates of Ni, Fe, NiFe and NiFeCo in 250 W rf chuck
power, 5 mTorr discharges of 2 IClr13 Ar Žtop. or 2 IBrr13 Ar
Žbottom., as a function of source power.
plasma sources, producing high concentrations of
reactive species w14x. In this paper, we report a
parametric investigation of the effect of plasma conditions on etch rates, surface morphology and surface
composition of Ni, Fe, Ni 0.8 Fe 0.2 and Ni 0.8 Fe 0.13Co 0.07 in ICP discharges of IClrAr and IBrrAr.
2. Experimental
Fig. 1. Etch rates of Ni, Fe, NiFe and NiFeCo in 750 W source
power, 250 W rf chuck power, 5 mTorr discharges of IClrAr
Žtop. or IBrrAr Žbottom., as a function of plasma composition.
Direct current ŽDC. magnetron sputtering was
˚ thick layers of each of
used to deposit 3000–5000 A
the materials on Si substrates. For etch rate experiment, samples were masked with Apiezon wax,
which was removed after the plasma exposure and
the etch step measured by profilometry. Etching was
performed in a Plasma Therm 790 system, involving
an ICP source with a three-turn coil antenna operat-
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
217
3. Results and discussion
The influence of plasma composition on Ni, Fe,
NiFe and NiFeCo etch rates in ICP IClrAr Žtop. and
IBrrAr Žbottom. discharges at fixed source power
Ž750 W., rf chuck power Ž250 W. and pressure Ž5
mTorr. is shown in Fig. 1. For the IClrAr plasma
chemistry, the rates for NiFe, NiFeCo and Ni initially increase as ICl is added, but then decrease
beyond particular discharge compositions. This is
consistent with a mechanism in which the adsorbed
reactive neutral flux must be balanced with the ionassisted removal of the resultant etch products w11–
13x. Beyond the optimum discharge compositions,
we believe there is blocking of the surface to ion
bombardment by the high chlorine and iodine concentrations. The rate-limiting step appears to be re-
Fig. 3. Etch rates of Ni, Fe, NiFe and NiFeCo in 750 W source
power, 5 mTorr discharges of 2 IClr13 Ar Žtop. or 2 IBrr13 Ar
Žbottom., as a function of rf chuck power.
ing at 2 MHz and powers up to 1000 W. The
samples are thermally bonded to a radio frequency
Žrf.-powered Ž13.56 MHz, 0–350 W., He backsidecooled chuck. Process pressure was varied from 5 to
20 mTorr, with a gas load of 15 standard cubic
centimeters per minute Žsccm.. ICl and IBr are crystalline solids with melting temperatures of 27 and
458C, respectively, and were contained in a stainless
steel vacuum vessel heated to ; 508C to enhance the
vapor pressure w14x. The resulting gases were injected directly into the ICP source through electronic
mass controllers. Electronic grade Ar was always
added to provide a strong physical component to the
etching. The etched surfaces were examined by
atomic force microscopy ŽAFM. and Auger electron
spectroscopy ŽAES. to look at morphology and
near-surface composition, respectively.
Fig. 4. Etch rates Žtop. and etch yields Žbottom. of Ni, Fe, NiFe
and NiFeCo in 2 IClr13 Ar, 750 W source power, 250 W rf
chuck power discharges, as a function of process pressure.
218
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
moval of the FeCl x and FeI x etch products, based on
the low etch rate of the Fe layers. Indeed, for very
high ICl percentages, there is net deposition on the
Fe due to the inability to effectively remove the
chloride etch products. Note also that as the ICl
percentage in the discharge increases, the chuck
Fig. 5. The AFM scans of NiFe after etching in 750 W source power, 250 W rf chuck power, 5 mTorr discharges, as a function of plasma
composition.
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
self-bias also increases. This indicates that the positive ion density in the plasma is decreasing w15x, as
expected since both chlorine and iodine are electronegative gases.
219
The results for IBrrAr discharges are shown at
the bottom of Fig. 1. The etch rate behavior for Ni is
similar to that with IClrAr, but the Fe etches much
more rapidly in IBrrAr discharges, especially at
Fig. 6. The AFM scans of NiFeCo after etching in 750 W source power, 250 W rf chuck power, 5 mTorr discharges, as a function of plasma
composition.
220
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
high halogen concentrations. This is due to the higher
volatility of the FeBr x etch products relative to FeCl x
w12x, and is not a strong function of bias. This effect
leads to a small increase in NiFe and NiFeCo etch
rate at IBr percentages beyond ; 60%.
The effect of ICP source power on the material
etch rates is shown for IClrAr Žtop. and IBrrAr
Žbottom. in Fig. 2. For the IClrAr, the etch rates for
Ni, NiFeCo and NiFe increase monotonically with
increasing ion flux, even though the self-bias decreases because of the larger conductivity of the
plasma. For Fe, there is essentially no etching until
source powers ) 600 W, which illustrates the point
that balancing the ion and reactive neutral fluxes can
lead to a positive etch rate w11x. The behavior of
NiFe and NiFeCo in IBrrAr discharges is basically
similar to that with IClrAr. The Ni and Fe etch rates
go in different directions at high flux, due to the ion
energy falling below that needed to efficiently desorb NiBr x and NiI x . The low rates may also be
attributed to the chemical kinetics of the reaction.
Under high flux conditions, the reactive species may
sputter off the surface prior to reaction.
The dependence of material etch rates on rf chuck
power is shown in Fig. 3 Žtop. for 2 IClr13 Ar and
Fig. 3 Žbottom. for 2 IBrr13 Ar discharges at fixed
source power Ž750 W. and pressure Ž5 mTorr.. For
both chemistries, the etch rates Žexcept those for Ni.
are basically linearly dependent on chuck power,
indicative of a desorption-limited process. For Ni in
both chemistries and Fe in IBrrAr, the rates initially
increase as the rf chuck power Žand hence DC
self-bias is increased., but then decrease beyond
particular maxima. This is often observed in highdensity plasma etching of materials, and is usually
ascribed to desorption of the adsorbed chlorine neutrals before they can react with the surface of the
metal w10x. The reaction rate is presumably different
on the alloys, where this trend is not observed up to
our maximum of chuck power.
Fig. 4 shows the pressure dependence of material
etch rates in 2 IClr13 Ar discharges Ž750 W source
power, 250 W rf chuck power.. We were not able to
produce stable IBrrAr discharges at pressures above
5 mTorr. Even though DC self-bias increases with
pressure, the etch rates of all of the materials decrease with increasing pressure. We suspect that the
ionrneutral ratio falls below that necessary for effec-
tive balance of the product formation and desorption.
Once again, the rate-limiting step is removal of the
Fe. The etch yields and ion fluxes calculated from
the etch rate and DC self-bias on the chuck are
shown at the bottom of the figure w16x. The low etch
yields show why high density plasma conditions are
needed to produce practical etch rates for the magnetic materials.
The surfaces of the NiFe and NiFeCo were smooth
over a broad range of plasma conditions. Fig. 5
shows AFM scans from NiFe samples after etching
˚ of material in IBrrAr discharges Ž750
of ; 2000 A
Fig. 7. The AES surface scans of NiFe after etching in either
IClrAr Žtop. or IBrrAr Žcenter and bottom. discharges Ž750 W
source power, 250 W rf chuck power, 5 mTorr., as a function of
plasma composition.
H. Cho et al.r Applied Surface Science 140 (1999) 215–222
W source power, 250 W rf chuck power, 5 mTorr. at
different gas compositions. The as-grown rootmean-square ŽRMS. roughness is ; 0.55 nm. At low
IBr compositions, the surface is significantly rougher
Ž1.8 nm RMS., but as the chemical component of the
etching increases, the surfaces are as good or slightly
better than the control value. A similar trend was
observed with NiFeCo layers, as shown in Fig. 6.
The main difference is that even for the low IBr
concentration, the RMS roughness is still as good as
the control value. These data show that there is a
wide process window for maintaining high quality
surfaces with the interhalogen plasma chemistries.
The AES data showed that the samples retained their
initial stoichiometry under these conditions.
The surfaces were also relatively clean after etching. Fig. 7 shows AES surface scans of NiFe after
either IClrAr Žtop. or IBrrAr etching Žcenter and
bottom. at different plasma compositions. We observe adventitious carbon and a native oxide originating from exposure to ambient during transfer
from etch chamber to analysis chamber. There is
only a slightly amount of residual chlorine detected
on the ICl etched material ŽF 1 at.%., which is
consistent with the mechanism involving efficient
desorption of the etch products by the attendant
ion flux. There was no Br Žmain Auger transition at
1396 eV. detected on any of the samples, while any I
signal would be swamped by that due to oxygen
Žiodine transition at 511 eV..
Similar data were obtained for etched NiFeCo
samples. Once again, the surfaces were relatively
clean, with residual bromine concentrations below
the detection limit of AES w11–13x. We have previously reported that use of in-situ H 2 plasma cleaning
is effective in volatilizing halogen residues, producing clean surfaces on the etched field w13x. More
work needs to be done to establish the chemical state
of the sidewalls of etched features, since this is what
will determine the extent of long-term corrosion on
patterned magnetic multilayers.
4. Summary and conclusions
The interhalogen compounds, ICl and IBr, are
effective dry etchants for Ni, Fe, NiFe and NiFeCo
221
under high ion density conditions. The maximum
etch rates are similar to those we have achieved with
pure Cl 2 under the same conditions in the same
reactor, but the surfaces are smoother over a broader
range of conditions than with Cl 2 Žwhich typically
produced RMS values a factor of 2–3 higher.. This
appears to be related to the lower amount of residual
halogen on the etched surfaces Žour past results with
Cl 2 typically show 1–3 at.% chlorine residues.. The
etch rates are strongly dependent on plasma composition, source power, rf chuck power and pressure.
All of these trends with plasma parameters are consistent with the etching being limited by the removal
of the halogenated reaction products, and the need to
balance the formation and removal of these species.
Acknowledgements
The work at UF is partially supported by a DOD
MURI monitored by AFOSR ŽH.C. DeLong.,
contract F49620-96-1-0026, and by ONR contract
N00014-96-C-2114 through the Honeywell MRAM
program. The work of H.C. is partially supported by
KOSEF. YBH gratefully acknowledges the support
of Korea Research Foundation for Faculty Research
Abroad.
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