Inductively coupled Cl2/Ar/O2 plasma etch of

Journal of the Korean Physical Society, Vol. 37, No. 6, December 2000, pp. 842∼845
Inductively Coupled Cl2/Ar/O2 Plasma Etching of GaN, InGaN, and AlGaN
Ji-Myon Lee, Ki-Myung Chang and Seong-Ju Park ∗
Department of Materials Science and Engineering and Center for Optoelectronic Materials Research,
Kwangju Institute of Science and Technology, Kwangju 500-712
Hong-Kyu Jang
Atomic-scale Surface Science Research Center, Yonsei University, Seoul 120-749
(Received 12 April 2000)
The etch selectivities of GaN and In0.12Ga0.88N over Al0.1Ga0.9N were investigated using an
inductively coupled Cl2/Ar/O2 plasma and the results were as high as 24 and 32, respectively. An
X-ray photoelectron spectroscopic (XPS) analysis of the etched surface showed that an Al-O bond
was formed on the AlGaN surface during the Cl2/Ar/O2 plasma etching, so the high selectivity
thus obtained could be attributed to the etch-resistant oxide layer. This oxide layer could be
easily etched off by using an HF-based solution during the mask removal process. The atomic
force microscopic image of the surface morphology showed the presence of an Al/Ga droplet-like
structure on the nitride surfaces, that had been etched by oxygen-free plasma while those that had
been etched using an oxygen-containing plasma showed a droplet-free smooth surface. A gallium
oxynitride layer, which prevented the preferential sputtering of nitrogen on the nitride surface, was
also observed by XPS.
I. INTRODUCTION
III-nitride materials have great potential, both present
and future, for use as light emitters and detectors. Currently,
attention is now being directed to the development
of GaN-based electronic devices for high power
switching and transmission applications [1,2]. The dry
etching process represents a crucial step in the fabrication
of these devices, and it is very important to achieve
controlled degrees of anisotropy and high etch rates with
low damage and to have techniques for controlling etch
stop [3]. For the fabrication of an etch stop layer, high
etch selectivity is desirable. Furthermore, with these
highly selective etch schemes, care must be taken to retain
smooth surface morphologies in order to obtain reliable
contact characteristics. Four major dry techniques
have been employed for etching nitride materials. These
include reactive ion etching, electron cyclotron resonance
etching, chemically assisted ion beam etching, and inductively
coupled plasma (ICP) etching [3–6]. The high
density and low pressure plasmas have been shown to
perform very well for group-III nitride patterning [3,7]
because the high plasma flux generated in these systems
enhances the bond-breaking efficiency of the nitrides,
∗ E-mail: sjpark@kjist.ac.kr, Fax: +82-62-970-2304, Tel:
+82-62-970-2309
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as well as the sputter desorption of the etch products.
Improved etch results for these materials have been reported
at a relatively high dc-bias, which is required to
break the strong bonds of group III-N. With a high dcbias,
however, the etch selectivity is reduced and induced
plasma damage, such as a nonstoichiometric surface due
to preferential removal of nitrogen, can be generated [8].
Therefore, it is very important to understand the effects
which are induced by the plasma etching because a metal
contact is formed on the etched surface.
In this paper, we wish to report on an investigation
of highly selective ICP etching of III-nitrides using a
Cl2/Ar/O2 gas mixture, as well as the effect of plasma
exposure to the nitride surfaces. Very high selectivities
of GaN and InGaN with respect to Al0.10Ga0.90N were
achieved by controlling the O2 flow rate in the gas mixture.
O2 was found to play a critical role in both etch
selectivity and surface morphology by enhancing the formation
of an etch-resistive oxide layer on the AlGaN surface.
II. EXPERIMENT
Undoped-GaN, Al0.05Ga0.95N, Al0.1Ga0.9N, and
In0.12Ga0.88N epilayers of 1.0 µm were grown on a cplane
sapphire by using metalorganic chemical vapor deposition
with trimethylgallium, trimethylaluminum, and

Inductively Coupled Cl2/Ar/O2 Plasma Etching of · · · – Ji-Myon Lee et al. -843-
Fig. 1. (a) Etch rate of GaN, In0.12Ga0.88N, Al0.05
Ga0.95N, and Al0.1Ga0.9N as functions of O2 flow rate in a
Cl2/Ar/O2 gas mixture under a 1000 W ICP power and a 100
W RF table power, and (b) corresponding selectivity values
for GaN and In0.12Ga0.88N over AlxGa1−xN.
trimethylindium as sources of Ga, Al, and In, respectively,
and NH3 as the nitrogen source. The ICP reactor,
which was equipped with a 3 kW ICP power supply,
was connected to a load-lock chamber. The dc-bias for
the ion energy was provided by superimposing an RF table
bias (13.56 MHz) on the sample. All samples were
mounted on an anodized Al carrier clamped to a cathode
and were backside cooled with He gas. The base pressure
was less than 1×10 −6 Torr prior to the etching experiments.
SiO2 was deposited on the sample as an etch
mask by means of plasma-enhanced chemical vapor deposition,
and the sample was then coated with a carbonbased
photoresist. After photolithography via conventional
flood exposure, the SiO2 mask was patterned
with a buffered oxide etchant and stripped away after
the plasma etch process. Undoped-GaN, AlxGa1−xN,
and In0.12Ga0.88N were simultaneously loaded into the
plasma process chamber for purposes of ICP etching.
The etch conditions used in this study were 30 sccm Cl2,
10 sccm Ar, 10 mTorr chamber pressure, 1000 W ICP
power, 20 ◦ C table temperature, 100∼250 W RF power,
and 0∼8 sccm O2. O2 was added to the Cl2/Ar gas mixture
in order to examine its effect on etch selectivity and
surface morphology.
Etch rates were measured with a surface profilometer
after removing the SiO2 mask. X-ray photoelectron spectroscopy
(XPS) was used to examine the surface bond
configuration, and the surface morphology was investi-
gated by using atomic force microscopy (AFM) and scanning
electron microscopy (SEM). Ar ion sputtering was
done with a 3 keV accelerating voltage in conjunction
with XPS measurements.
III. RESULTS AND DISCUSSION
Figure 1(a) shows the etch rates of GaN,
In0.12Ga0.88N, Al0.1Ga0.9N, and Al0.05Ga0.95N as a function
of the O2 flow rate. The etch rates of all samples
monotonically decreased with increasing O2 flow rate.
This indicates that a film which is more etch-resistive
than the nitride is formed on the sample surface in an
oxygen ambient, and this conclusion is supported by
the XPS results in this study. Figure 1(b) shows relevant
selectivity values for GaN and In0.12Ga0.88N over
AlxGa1−xN, where the selectivity is defined as the ratio
of etch rates. The selectivities first increase rapidly with
O2 flow rate in the range of 0-2 standard cubic centimeter
per minute (sccm), reaching maximum selectivities of 24
for GaN over Al0.1Ga0.9N and 32 for In0.12Ga0.88N over
Al0.1Ga0.9N and then decrease with further increases in
flow rate. These are the highest values ever reported
for a ternary AlxGa1−xN film with a low Al composition
(x=0.1). In contrast to Al0.1Ga0.9N, the selectivity
of GaN over Al0.05Ga0.95N does not appear to be influenced
by added O2, suggesting that these high selectivity
values of GaN over AlGaN are obtained only when the
Al composition is larger than 10 %. It is well known that
the bond strength is one of the most important factors
which affect etch rate [9]. The higher selectivity for the
case of an AlGaN film with higher Al levels can be attributed
to the fact that the bond strength of Al-N (11.5
eV/atom) is larger than that of Ga-N (8.9 eV/atom) and
that the AlGaN film with higher Al levels gives rise to
a more etch-resistant surface. In addition, it was found
that the O2 added to the plasma played a critical role in
the enhancement of etch selectivity. When O2 is added to
the Cl2/Ar plasma, the selectivity value increases drastically
up to 24 for GaN over Al0.1Ga0.9N. This result
clearly shows that the added O2 is more effective in enhancing
etch selectivity with increasing Al composition
in a GaN/AlGaN or InGaN/AlGaN system.
In an effort to clarify the origin of the enhanced etch selectivity
and the effect of plasma exposure on the surface
as a function of added O2, the chemical bonding states
of the Al0.1Ga0.9N surface etched by the Cl2/Ar/O2
(30/10/2 sccm) plasma were investigated by using XPS.
As shown in Fig. 2, the as-grown sample surface in spectrum
(a) exhibits a peak at 74.6 eV, which corresponds
to the Al-N binding energy, while the ICP etched Al-
GaN surface in spectrum (b) reveals a peak at 75.5 eV,
corresponding to the Al-O bonds [10]. This constitutes
convincing evidence that oxide layers are formed on the
AlGaN surface after the ICP etching in the oxygen ambient.
The XPS result indicates that the origin of the

-844- Journal of the Korean Physical Society, Vol. 37, No. 6, December 2000
Fig. 2. Al 2p core level spectra taken from (a) as-grown,
(b) ICP-etched, (c) ICP-etched and subsequently 2 s sputtered,
(d) ICP-etched and subsequently 30 s sputtered, and
(e) ICP-etched and subsequently HF etched Al0.1Ga0.9N. The
ICP etching was done under a 1000 W ICP power and a 100
W RF table power.
enhanced selectivity is derived from the formation of surface
oxide which leads to a more etch-resistive surface.
The enhanced selectivity of GaN over AlGaN in an O2
ambient is attributed to the fact that the bond strength
difference between Al-O (21.2 eV/atom) and Ga-O (14.7
eV/atom) is much larger than the difference between Al-
N (11.5 eV/atom) and Ga-N (8.9 eV/atom) [11]. Once
the oxide layers are formed on the surface, the etch rate is
limited by the surface oxide because the bond strengths
of the oxides are larger than those of the nitrides. This
also accounts for the observed decrease in etch rate with
increasing O2 flow rate, as shown in Fig. 1(a).
As shown in Figs. 2(c) and 2(d), the Al-O peak from
the sputtered surface is gradually shifted to a lower binding
energy with increasing sputter time and is finally positioned
at the binding energy of Al-N. This indicates
that the aluminum oxide layer is located within a few
surface layers and is completely removed after a 30 s
sputtering period. The chemical shift of the aluminum
peak to a lower energy by 0.3 eV from the peak at 74.5 eV
observed on the 30 s sputtered sample can be attributed
to the removal of aluminum oxide and the preferential
sputtering of nitrogen from the AlGaN surface (see Fig.
2(d)). The surface oxides, which are formed after etching,
may be the source of serious problems in subsequent
processing of device fabrication [12] and should be removed.
In spectrum (e) in Fig. 2, only an aluminum
nitride peak can be seen on the ICP-etch + HF-treated
sample surface, indicating that the surface Al-O layer
and the nitrogen deficient aluminum species were completely
removed during the mask removal process. In
addition, the XPS core level spectra of Ga is very similar.
In Fig. 3(a), the as-grown sample surface exhibits
the broad Ga 2p3 spectrum, which peaks at 1118.2 eV,
Fig. 3. Ga 2p3 core level spectra taken from (a) as-grown,
(b) ICP-etched, and (c) ICP-etched and subsequently HFetched
Al0.1Ga0.9N. The ICP etching was done under a 1000
W ICP power and a 100 W RF table power.
representing the chemical bond between Ga and N [13].
After dry etching using an O2-containing plasma, only a
0.7 eV shift in the peak positions from 1118.2 eV is observed,
as shown in Fig. 3(b). In contrast to a previous
study [13], which reported the core level peak position
at 1119.5 eV for the Ga-O bond, the origin of the Ga
2p3 peak position at 1118.9 eV reported in this study
may not be derived exclusively from the Ga-O bond. Instead,
the small peak shift of 0.7 eV can be attributed
to an oxynitride layer, which was not fully oxidized by
the Cl2/Ar/O2 plasma. A similar oxynitride layer was
observed in a previous oxidation study [14]. In spectrum
(c) in Fig. 3, only the GaN peak can be seen on
the ICP-etch + HF-treated sample, indicating that the
oxynitride layers were also removed from the surface by
the HF solutions.
In addition to the surface bonding state, the morphology
of the etched surface is of interest since a smooth
surface would be expected to enhance electrical properties,
such as metal contacts, thus improving the reliability,
as well as the quality, of the device features
fabricated on this surface [15]. To investigate the effect
of Cl2/Ar/O2 plasma exposure, we examined the
surface morphology by means of AFM. Figure 4 shows
the microscopic images of Al0.1Ga0.9N as a function of
O2 flow rate under a 1000 W ICP power. The as-grown
Al0.1Ga0.9N surface shows a featureless surface morphology,
as shown Fig. 4(a). When the Al0.1Ga0.9N is etched
under an O2-free plasma, the Al0.1Ga0.9N surface shows
an Al/Ga droplet-like structure (Fig. 4(b)), which can
be attributed to a preferential removal of nitrogen by energetic
ion bombardment [3,9]. The corresponding rootmean-square
(RMS) roughness shows its highest value
on the Cl2/Ar plasma etched-surface, as shown in Fig
4(d). This preferential sputtering effect under a Cl2/Ar
plasma condition dominated the nitride etch process due
to the bombardment by chlorine and argon ions which

Inductively Coupled Cl2/Ar/O2 Plasma Etching of · · · – Ji-Myon Lee et al. -845-
Fig. 4. AFM images of the surfaces of (a) as-grown, (b)
Cl2/Ar-etched, and (c) Cl2/Ar/O2-etched Al0.1Ga0.9N and
(d) RMS roughness.
have large molecular masses. For this reason, the highest
etch rates of GaN and AlGaN are observed under
O2-free plasma conditions. Therefore, the etch selectivity
of GaN over AlGaN is lower for the Cl2/Ar plasma
than for the oxygen-containing plasma since the bondbreaking
efficiencies for Al-N and Ga-N are higher under
ion bombardment conditions, as mentioned earlier.
When O2 was added to the Cl2/Ar plasma, the etched
surface showed a droplet-free structure, as shown in Fig.
4(c). The integrated intensity ratio of Ga 2p3 to N 1s,
as determined by XPS, were 1.12 and 0.85 after the Al-
GaN samples were etched using a Cl2/Ar plasma and a
Cl2/Ar/O2 plasma, respectively. The RMS roughness of
the sample etched using the oxygen-containing plasma
was even smaller than that of the as-grown sample, indicating
that nitrogen is not preferentially removed. These
results suggest that an etch-resistant layer, which contains
nitrogen, is formed on the surface and hinders nitrogen
from being preferentially removed, which is consistent
with our interpretation of the data in Fig. 3(b).
This result is also of importance in developing GaNbased
electronic devices since the Schottky contact is
formed on the etched AlGaN surface.
IV. CONCLUSIONS
The selective etch characteristics of GaN and InGaN
over AlGaN, along with its effect on the surface characteristics
of AlGaN have been studied using an inductively
coupled Cl2/Ar/O2 plasma. The maximum selec-
tivities of GaN and In0.12Ga0.88N over Al0.1Ga0.9N were
24 and 32, respectively. An XPS study indicated that the
high selectivity could be attributed to the formation of
aluminum oxide on the AlGaN surface, which was etchresistant,
as a result of the bond strength of Al-O, which
is higher than those of Al-N and Ga-N. A gallium oxynitride
layer, which prevented the preferential sputtering
of nitrogen, was also observed by using XPS. These oxide
and oxynitride layers could be easily removed during
the mask removal process. The Cl2/Ar/O2 plasma
chemistry appears to be well suited for electronic device
applications.
ACKNOWLEDGMENTS
This work was supported by grants from the Institute
of Information Technology Assessment and Brain Korea
21 program.
REFERENCES
[1] M. A. Khan, M. S. Shur, J. N. Kuznia, O. Chen, J. Burm
and W. Schaff. Appl. Phys. Lett. 66, 1083 (1996).
[2] O. Aktas, Z. Fan, S. N. Mohammad, A. Botchkarev and
H. Morkoc, Appl. Phys. Lett. 69, 3872 (1996).
[3] S. A. Smith, C. A. Wolden, M. D. Bremser, A. D. Hanser,
R. F. Davis and W. V. Lampert, Appl. Phys. Lett. 71,
3631 (1997).
[4] R. J. Shul, S. P. Kilcoyne, M. Hagerott Crawford, J.
E. Parmeter, C. B. Vartuli, C. R. Abernathy and S. J.
Pearton, Appl. Phys. Lett. 66, 1761 (1995).
[5] T. Ping, I. Adesida and M. Asif Khan, Appl. Phys. Lett.
67, 1250 (1995).
[6] J. M. Lee, K. M. Chang, I. H. Lee and S. J. Park, J.
Electrochem. Soc. 147, 1859 (2000).
[7] C. B. Vartuli, S. J. Pearton, J. W. Lee, J. Hong, J. D.
MacKenzie, C. R. Abernathy and R. J. Shul, Appl. Phys.
Lett. 69, 1426 (1996).
[8] R. J. Shul, C. G. Willison, M. M. Bridges, J. Han, J. W.
Lee, S. J. Pearton, C. R. Abernathy, J. D. Mackenzie and
S. M. Donovan, Solid State Electron. 42, 2269 (1998).
[9] C. B. Vartuli, S. J. Pearton, J. W. Lee, A. Y. Polyakov,
M. Shin, D. W. Grave, M. Skronowski and R. J. Shul,
Electrochem. Soc. 144, 2146 (1997).
[10] E. A. Moon, J. L. Lee and H. M. Yoo, J. Appl. Phys. 84,
3933 (1998).
[11] CRC Handbook of Chemistry and Physics, edited by D.
R. Lide, (CRC, Boca Raton, FL, 1996), pp. (9-51) - (9-
56).
[12] B. T. Lee, D. K. Kim and J. H. Ahn, Semicond. Sci.
Technol. 11, 1456 (1996).
[13] S. D. Wolter, B. P. Luther, D. L. Waltemyer, C. Onneby,
S. E. Mohney and R. J. Molnar, Appl. Phys. Lett. 70,
2156 (1997).
[14] A. D. Katnami and K. I. Papathomas, J. Vac. Sci. Technol.
A5, 1335 (1987).
[15] M. W. Cole, R. Ren and S. J. Pearton, Appl. Phys. Lett.
71, 3004 (1997).