Time-multiplexed, inductively coupled plasma process with separate
SiCl4 and O2 steps for etching of GaAs with high selectivity
S. Golka,a兲 M. Arens, M. Reetz, and T. Kwapien
SENTECH Instruments GmbH, Carl-Scheele-Straße 16, 12489 Berlin, Germany
S. Bouchoule and G. Patriarche
Laboratoire de Photonique et de Nanostructures (LPN), CNRS, Route de Nozay, 91460 Marcoussis, France
共Received 6 April 2009; accepted 17 August 2009; published 29 September 2009兲
The authors present the results and the optimization procedure for a time-multiplexed dry etching
process to etch GaAs in an inductively coupled plasma reactive ion etching system. The gas feed
chopping sequence employed a SiCl4 etch phase and an O2 passivation phase. Care is taken not to
intermix O2 with SiCl4. The investigated structures consist of pillars, trenches, stripes, and holes, all
with lateral structure size of 1 ␮m or less. This feature size is interesting for diffractive elements
and cavities in integrated mid-IR optoelectronics. They achieve an aspect ratio of 10 for holes, 17
for trenches, and 30 for stripes with a selectivity of 200:1 on open areas. The improvements in the
sidewall morphology are related to the O2 passivation step investigated by optical emission
spectroscopy and energy dispersive x-ray analysis that reveals a Si-rich SiOX sidewall. © 2009
American Vacuum Society. 关DOI: 10.1116/1.3225599兴
I. INTRODUCTION
The achievement of a high aspect ratio 共AR兲 in deep
etched structures in III-V semiconductors is crucial for substrate based photonic crystals 共PhCs兲 and monolithic onedimensional distributed Bragg reflectors 共DBRs兲. Reactive
ion etching 共RIE兲 by a high density plasma,1 such as an
inductively coupled plasma 共ICP兲, is currently the most
widely used technique for their production. PhCs and DBRs
are important building blocks in integrated optoelectronics
that are able to superimpose a passive optic onto an electrooptically active structure. In the midinfrared 共MIR兲 and terahertz regime, GaAs-based multiple quantum well devices using intersubband 共ISB兲 transitions play a decisive role. The
recent achievement of a GaAs–AlGaAs terahertz quantum
cascade laser 共QCL兲 that was grown by the commercially
important gas phase epitaxy2 has demonstrated that there
is an ongoing demand for GaAs-based high quality ISB
devices.
In QCLs, the incorporation of a PhC hole pattern into the
active layer resulted in surface emission which again has
commercial importance due to the reduced numerical aperture in the far field. The PhC hole radii of a laser reported
with surface emission vary from r = 3 ␮m at ␭ = 70 ␮m
emission wavelength, to r = 10.8 ␮m at ␭ = 110 ␮m in terahertz lasers,3,4 to r = 0.85 ␮m at ␭ = 8 ␮m in a MIR laser.5
Radii depend not only on wavelength but also on the waveguide design. Besides QCLs, detectors make use of ISB transitions, namely, quantum well infrared photodetectors
共QWIPs兲. All ISB based devices rely on a grating coupler to
couple from the internal transverse magnetic mode to surface
emission or illumination. This is due to ISB selection rules.
A simple vertical cavity with no PhC is not possible for ISB
devices. In QWIPs a deep PhC 共r = 0.7 ␮m兲 can tailor the
a兲
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J. Vac. Sci. Technol. B 27„5…, Sep/Oct 2009
absorption spectrum in the 7 – 12 ␮m region.6 The achievement of a high quality factor Q in an optical cavity in QWIPs
can enhance the signal to noise ratio 共SNR兲 and/or give rise
to interesting strong coupling phenomena, such as polariton
splitting of ISB transitions.7 In GaAs QWIPs, a low SNR at
high operating temperature is caused by the dark current that
could be decreased by decreasing the doping. To account for
the reduced light absorption, a higher Q is needed. PhC patterns can produce high Q’s. However, the key factor to
achieve high Q’s in PhC based cavities in lasers and detectors is a high AR of the holes.8 Another challenging point
that needs to be addressed in dry etching of active devices is
the simultaneous achievement of narrow and open structures
in a single etch process. This occurs in the processing of
deep diffraction gratings along with rib waveguides, as in
MIR distributed feedback resonators at ␭ = 10.6 ␮m 共Ref. 9兲
or MIR Bragg reflectors 共DBR兲 with rib lasers.10 The respective trench widths are w = 1, 2, and 6 ␮m at ␭ = 10.6, 9, and
4 ␮m, respectively, with depth of approximately 10 ␮m for
both.
The unavoidable slowing down of the etch rate in narrow
structures 共“RIE lag”兲 and limited thickness of practical hard
masks require a huge selectivity of semiconductor etching
over mask etching. Therefore, the chemical etching component must be large compared to physical sputtering, like it is
the case in Cl-based RIE. However, chemical etching is inherently isotropic. Because net anisotropy is required to produce vertical walls, an etch inhibiting layer must cover the
structure edge in order to protect it from lateral etching. To
achieve this, two chemical reaction products are required.
One of the products must be volatile for bulk etching of the
semiconductor 共e.g., GaCl3兲, whereas the sidewall deposit
must be nonvolatile under the local conditions present on the
structure edge. There is no requirement to treat both products
separately, as is nicely demonstrated in Ar/ Cl2 etching,
where sticky etch products such as InCl3 act as both.10,11 At
1071-1023/2009/27„5…/2270/10/$25.00
©2009 American Vacuum Society
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Golka et al.: Time-multiplexed, inductively coupled plasma
selectivities approaching 100, only a very small ion bombardment energy is allowed. That makes such approaches
such as Ar/ Cl2 RIE increasingly difficult. A much more versatile approach is to use a gas or mixture that etches on the
floor under ion bombardment and polymerizes on the sidewall with no or too little bombardment, where instead it reacts to form a compound that is completely different from
the volatile etch product. The feed gases CH4, BCl3, HBr,
CCl4, and SiCl4 are examples. SiCl4 is particularly well
suited for GaAs–AlGaAs etching.9,12–15 A third approach
uses the simultaneous etching of a cover plate in the plasma
chamber to produce the polymerizing gas mixture in the
plasma. This is, for example, a Si plate in a H2 / Cl2 plasma16
to etch InP, producing a Si–Cl polymer formed as a byproduct. Yet another approach is a cyclic depositing and
etching to preserve the high selectivity of a chemical, isotropic etch, but still produce a vertical sidewall. This method is
commonly used for Si etching with SF6 / C4F8, resulting in a
C–F polymer on the walls.17
Recently, a cyclic SiCl4 – O2 – N2 etching process for MIR
Bragg gratings in GaAs was demonstrated.18 The process
consisted of an alternating SiCl4 – O2 – N2 deposition phase
and a SiCl4 – N2 etch phase. In the following, we will discuss
how to improve that concept with respect to high selectivity
and simultaneous achievement of narrow and open patterns.
First, it has to be pointed out that the SiCl4/SiCl4 – O2 does
not strictly represent a cyclic deposition and etching. Without
bias 共as on sidewalls兲, SiCl4 by itself can deposit a layer.
This has been observed for both reactions with traces of O
共Ref. 15兲 or an intentional O2 addition.13,14 It is an open
question whether it is helpful to distinguish sharply between
deposition and etching if the etch step already induces some
anisotropy on its own. The use of a slightly anisotropic etch
phase in a cyclic process has been shown to result in
smoother sidewalls at the expense of selectivity in Si
etching.19 However, in chlorine-based GaAs etching processes, the problem is not simply anisotropy. The achievable
AR is limited by underetching caused by poor sidewall quality, involving strain18 and possibly porosity.16 Hence, we
might as well think of periodic sidewall improvement by
means of a dedicated step. Real devices have AlGaAs layers
with 30%–45% Al in the MIR range and 10%–15% Al in the
terahertz range. In this work, we show only the etching of
GaAs for the sake of simplicity and due to the easier availability of bulk GaAs wafers. Preliminary tests with SiCl4 – O2
processes on AlGaAs-QWIPs have displayed no notable difference in the profiles. No dependence on Al content has
been observed as well for no O2 共Ref. 9兲 and O2 addition.14
In SiCl4 RIE an etch rate slightly higher for AlGaAs than for
GaAs has been reported.12
II. EXPERIMENTAL SETUP
Our experiments have been performed in an ICP etching
system 共Sentech SI-500兲 that has been modified 共Fig. 1兲 for
this experiment. All reactor walls are aluminum. The planar
ICP coil consists of three parallel windings with one turn
each and couples to the plasma via a Faraday shield and an
JVST B - Microelectronics and Nanometer Structures
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planar
ICP source
sapphire
viewport
~ 13,56 MHz
"ICP"
Faraday shield
AlOx
SiCl4
N2 / O2
10 cm
Si
to
spectrometer
sapphire window
1 mm cappillaries
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sample
~ 13,56 MHz
"CCP"
FIG. 1. Schematic of the SI500 etching system used.
AlOX ceramic plate that seals the vacuum. Both electrodes
are driven by radio frequency 共rf兲 generators at 13.56 MHz.
rf power levels are referred to as ICP and capacitively
coupled plasma 共CCP兲 power, PICP and PCCP. The electrodes
are electrically driven with a fixed phase to each other. The
reactor volume is 38 l and is pumped to a base pressure of
10−5 Pa. The gas flows are 20 SCCM 共SCCM denotes cubic
centimeters per minute at STP兲 for SiCl4 共at 0.4 Pa relating
to an average residence time of ⬃0.6 s in the reactor兲 and 70
SCCM for all other gases. Two gas rings with holes that are
integrated into the chamber wall have been used: one for
SiCl4 and one for both N2 and O2. The reactor pressure is
maintained by an automated throttle valve that is sufficiently
fast to maintain the chamber pressure for a limited time at
the set point after all gases have been switched off. With
gradually closing throttle valve, the residual pressure in the
ring suffices for more than one second. The pressure in the
SiCl4 ring is about 100 Pa, while the other ring is about 1000
Pa. With a very short 共⬃1 s兲 intermediate state where all the
gases are off, the ring pressure of the preceding gas in the
chopping sequence has already dropped by the time the next
gas is switched on. Off times were kept sufficiently short so
that at least 10% of the pumping aperture was left open by
the throttle valve. By the described procedure, gas intermixing was minimized. All valves/flow controllers were controlled via a script language, allowing the change 共“ramp”兲 in
etch parameters during the process. The sample is loaded
with a load lock pumped by a scroll pump to a transfer pressure of 3 Pa. This way, a small amount of air might enter the
reactor when the sample is transferred. A 6 in. n-doped Si
wafer is used as a carrier. A He back side pressure of 1200 Pa
connects the carrier thermally to the chuck that is set to
10 ° C by a chiller. The temperature, as measured with a
fiber-optic probe at the Si-wafer back side rises slowly to
about 20 ° C during etching at the given chiller temperature.
The 3 ⫻ 3 mm2 GaAs sample is glued with vacuum oil to the
Si carrier wafer in a way that no oil is exposed to the plasma.
Two optical ports are used: one in the top dielectric plate
vertically above the sample and one looking horizontally
above the chuck. Prior to the experiments, the chamber was
cleaned with a 5:1 SF6 : O2 mixture at 1100 W ICP power.
Golka et al.: Time-multiplexed, inductively coupled plasma
III. EXPERIMENTAL RESULTS
First, the optimized continuous process is presented in
which no parameter is changed while the discharge is burning. Then, the chopped process is based on these results. The
optimization relies on scanning electron microscopy 共SEM兲
images. If not otherwise noted, the SEM points onto the
cleaved facet 共always after etching兲 with ⬃10° tilt in order to
partly see the sample’s front side.
A. Planar inductive SiCl4 discharge
A pure inductive SiCl4 plasma has a net deposition rate.
This includes all parts of the chamber that if opened, heavily
outgases HCl due to the reaction between air and Si–Cl–O.
Deposition on the Si carrier wafer depends on the bombardment energy controlled by the chuck bias. At PCCP ⬇ 10 W,
the net deposition rate is zero and with PCCP further increased a net sputter removal will take place. Unfortunately,
the exact value is badly reproducible. At the time of the
experiments, the reactor was used for various hydrogen containing processes 共H2, CHF3, and NH3兲 as well. We found
that the deposition rate was much higher 共⬃250 nm/ min at
PICP = 400 W兲 if the reactor had only seen 1 h SiCl4 conditioning after a hydrogen containing process. On the other
hand, introducing an additional O2 plasma before SiCl4 conditioning resulted in a deposition rate for an inductive SiCl4
plasma that was near zero. This is in accordance with Ref.
21, where H is believed to assist Si–Cl–O growth. An O2
plasma at 1 Pa and PICP = 1100 W reduces the H␣ emission
line at 656 nm to the noise level in 30–60 min. Although the
exact threshold PCCP for transition to deposition remains
badly reproducible even after 60 min O2 plasma, it makes the
process results well reproducible, which we attribute to the
removed hydrogen. The decision to fix PICP = 150 W in further experiments was based on the small undercut for GaAs
samples at that power. Equally important was the fact that
J. Vac. Sci. Technol. B, Vol. 27, No. 5, Sep/Oct 2009
8
7
6
200
400
5
600
Wavelength (nm)
800
1000
397 nm
SiCl3
150 W
Further cleaning involved 1 h O2 plasma at 1100 W ICP and
finally conditioning for 1 h with the respective gas chopping
recipe.
The etch mask used with all samples was a dense SiNX
deposited by plasma enhanced chemical vapor deposition
共PECVD兲. There were only minor differences between
samples in the fabrication of that mask. The samples with
trenches and stripes were masked by 400 nm SiNX structured
by a CHF3 plasma, resulting in smooth 90° SiNX sidewalls.
This SiNX had an index of n = 2.0 at 632 nm, determined by
ellipsometry. The planar PhCs had a similar mask that was
300 nm thick. The mask of the samples for scanning transmission electron microscopy 共STEM兲 consisted of pillars
with diameter of 400–700 nm. The patterns were defined by
e-beam lithography and have been prepared as described in
Refs. 20 and 16. Then 50 nm Cr were deposited by the liftoff technique on a 500 nm thick PECVD SiNX layer that
subsequently was etched in a SF6 / CHF3 plasma using Cr as
a mask. After deep etching, the etched pillars were cut from
the substrate and dispersed on a carbon membrane for STEM
analysis.
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Optical emission (a.u.)
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4
3
2
725 nm
Cl
1
0
100
200
325 nm
SiCl2
300
251 nm
Si
400
500
ICP power (W)
FIG. 2. Optical emission intensity from four different transitions as a function of ICP power. The inset shows a sample emission spectrum at 150 W.
The four arrows correspond to the four line intensities plotted below.
the upper sapphire window 共Fig. 1兲 remains transparent for
many hours of plasma. At higher power, a deposit darkens
the window quickly. On the other hand, PICP should exceed
the power20 of the capacitive-inductive 共E-H兲 transition of
the plasma source.
Figure 2 shows the emission intensities of Cl, SiCl2,
SiCl3, and Si as a function of PICP. Obviously, by adjusting
PICP the ratio of Cl to SiCl2 concentration can be set. Since
Cl can etch GaAs isotropically, more undercut would be expected for an increased Cl emission at higher PICP. While the
Cl intensity rises, the SiCl2 line saturates, indicating an increased dissociation of all products at higher PICP. SiCl2 radicals, in particular, have been considered for the sidewall passivation formation.22 Finally, it is interesting to note that
despite the distinctly different plasma source geometries 共cylindrical兲 in another reactor,9 the results obtained for continuous N2 / SiCl4 etching of GaAs were optimum at the lower
end of stable powers PICP for that source as well.
B. Continuous SiCl4 etching
The aim of this section is first the development of a suitable anisotropic etch phase for the chopped process and then
understanding of the limiting factors on performance of continuous etching. We start with the variation in chamber pressure. The magnitude of the negative wall angle 共Fig. 3兲 in
GaAs profiles increases slightly with higher pressure. This
should be simply due to the broader angular distribution of
ions impinging onto the sample. Consequently, we keep the
pressure to 0.4 Pa. That is, close to the limit given by the
pumping efficiency of the setup. However, the effect of pressure was small in the investigated range of 0.4–0.8 Pa. The
smoothness of the profiles did not depend on pressure. The
next etching parameter, the sample temperature, was kept at
20 ° C owed to the qualitative observation that elevated temperature did decrease the Si–Cl–O deposition rate. On the
other hand, the etch rate of GaAs does not significantly increase with temperature. Next, the use of N2 and O2 as gas
additives has been investigated. Addition of N2, as in Ref. 9,
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Golka et al.: Time-multiplexed, inductively coupled plasma
a)
b)
10 µm
5 µm
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FIG. 3. SEM images of sample etched under continuous conditions, PCCP
= 0.8 Pa. 共a兲 Mesa. 共b兲 Trench, cleaved after etching.
resulted in an increased roughness in this work, while no
direct benefit could be seen from the N2 addition so we decided not to dilute with N2 in the etch phase. The addition of
⬎10% of O2 under low bias conditions led to roughness as
well. This can be attributed to deposition and etching competing with each other even on bombarded areas. A typical
example for an O2 flow equal to 10% is shown in Fig. 4.
There, we can compare the sidewall 共I兲 that was shielded
from bombardment by the high AR in the trench to the sidewall 共II兲 in the open center area that was more exposed to the
plasma. In the latter 共II兲, the sidewall is intact, while in the
first 共I兲 it is not. This is again not advantageous to our aim of
optimizing for small holes and simultaneously for open areas
and can be attributed to an increased Cl production rate that
has been observed for O2 addition.23,24 When going to even
smaller percentages of O2 flow, it gets increasingly difficult
to distinguish between feed gas and background concentration. Finally, for the addition of 6% and 5% of O2 共1 and 3
SCCM兲, also no apparent increase in the sidewall thickness
was observed.
Next PCCP is adjusted. The transition from etching to
deposition on the Si carrier can be seen by a decrease in the
dc bias caused by covering the electrode with a nonmetallic
film. The measured dc bias returns to the full value once
PCCP is increased again. PCCP = 30 W 共−120 Vdc兲 has been
found to have a long term stable bias that is robust to slight
changes in reactor conditioning. Smaller values are possible,
a)
b)
FIG. 5. In situ measurements all referring to the same time axis 共see bottom兲.
Two processes are shown that are exactly identical except for the choice of
the gas X in the chopped sequence. 共a兲 Gas chopping sequence. 共b兲 Laser
interferometer 共reflection兲 signal with ␭ = 632 nm. 共c兲 Optical emission line
intensity for various species.
but are too close to the transition to the deposition regime. If
PCCP comes too close to the threshold for deposition, etching
is preferred at sites where a mask edge is present, likely due
to ions reflected from the wall. As a consequence, a rough
but very deep trench around masked areas forms. No sidewall angle near to 90° could be obtained in trenches with a
pure SiCl4 plasma. The O2 addition improved the verticality
in some cases but had the disadvantages described above. A
typical non-90° trench shape is that shown in Fig. 3共b兲. However, for the cyclic process no complete anisotropy is needed
in the etch phase for the net process to be completely
anisotropic.19
SiNx
top view:
C. Time-multiplexed etching
1 µm
SiNx
I. II.
SiNx
I.
II.
cleave
SiNx
5 µm
FIG. 4. SEM image of sample etched with a flow ratio SiCl4 : O2 of 10:1. 共a兲
Schematic top view of the mask pattern. The dashed line depicts the position
of the cleave. 共b兲 SEM view onto the cleaved facet as illustrated by the eye
in 共a兲. In closed 共I兲 areas the sidewall is damaged. No damage is seen in
more open 共II兲 areas where the plasma has more access to the sidewall.
Note: the left hand area has not cleaved perfectly.
JVST B - Microelectronics and Nanometer Structures
The gas chopping sequence used is shown in Fig. 5共a兲. A
cycle comprises of four phases SiCl4, N2, O2, and N2. The
discharge is burning at any time. Besides the little breaks
with all the gases shut off, the SiCl4 and O2 phases are separated by two N2 phases. These intermediate N2 phases further prevent direct gas transfer between the SiCl4 and O2
phase. For now, all other process parameters are held constant throughout all phases to the values found in Sec. III B.
Just gases are changed. As a cross-check, the same process is
repeated but with N2 instead of O2 in the respective phase
共X兲. The duration of the N2 phase has been chosen to be
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Golka et al.: Time-multiplexed, inductively coupled plasma
about three times of the gas ring empty time, the O2 phase
duration has been optimized to result in 90° sidewalls in the
trenches, given the SiCl4 duration of 40 s, which was chosen
beforehand. One 40 s long SiCl4 phase etches about 600 nm
of GaAs. The etch rate can be seen in real time using the
interferometer signal 关Fig. 5共b兲兴. Each oscillation is produced
from an additional 316 nm etch depth. The delayed onset of
oscillations in the case X = O2 is indicated by “delay” in Fig.
5共b兲. Because it is not seen for X = N2, the delay must be
attributed to a different chamber/sample surface termination.
Besides 共inefficient兲 sputter removal of an oxide on the
sample floor, the delay could also be due to re-establishment
of an equilibrium on the chamber walls that in the case X
= O2 have been passivated as well. This equilibrium is characterized by a Si–Cl共–O兲 layer on the chamber wall that has
at least partly reacted with O 共Ref. 25兲 for X = O2. Residual
gaseous O2 cannot be responsible for this effect since in the
intermediate N2 phase it was removed from the system.
Small amounts of O2 would increase the Cl concentration
that again would increase the etch rate instead of stopping it.
Besides the interferometric signal, some artifacts and drift
from stray light and absorption in the window can be seen in
the curve, but these cannot explain a delay of oscillations
seen in all SiCl4 phases for X = O2. The observed delay is of
the order of 7 s and in this particular case would represent
the lower limit for the SiCl4 phase duration.
The optical plasma emission intensities versus time
curves in Fig. 5共c兲 are all recorded simultaneously with the
interferometer signal by means of a detector array in the
grating spectrometer. The wavelengths of the tracked emission lines have been chosen to not coincide with any other
line present in some other phases of the chopped process.
From the nitrogen intensities that exactly overlap for the two
respective N2 phases in the two processes X = O2 and X = N2,
we can first conclude that the differences observed for the
other lines are real and not due to a measurement artifact
such as different window deposits, etc. The absolute intensity
is difficult to interpret because, especially for O, small
amounts of added gases can significantly alter the emission
intensity of the other gas. Nevertheless, we can get a qualitative indication that no or little O2 should be directly transferred to the SiCl4 phase because the O line quickly decays
关marked “tran.” in Fig. 5共c兲兴 after O2 shut off. The Si line
resembles the SiCl2 line, except that it has a peak at the time
when SiCl4 is shut off. This peak is also seen in another
decomposition product, Cl, and can be attributed to the
longer chamber residence time during gas shut off. This peak
is missing in the SiCl2 trace probably because SiCl2 decomposes relatively easily. A longer residence time should be
equivalent to a larger PICP 共Fig. 2兲 when thinking of the
decomposition. The 397 nm 共SiCl3兲 curve 共not shown in Fig.
5兲 is simply proportional to the SiCl2 curve. The Cl line
shows two remarkable differences between the two processes. First, for X = O2 the Cl intensity in the SiCl4 phase is
larger. This is in line with the undercut that is observed in
SEM images 共Fig. 4兲 for O2 addition to a continuous process
and the fact that O2 addition indeed does not automatically
J. Vac. Sci. Technol. B, Vol. 27, No. 5, Sep/Oct 2009
2274
FIG. 6. SEM images of cleaved facets of trenches 共top兲 and nearby freestanding pillars 共bottom兲 from the two processes displayed in Fig. 5. 关共a兲 and
共b兲兴 X = O2. 关共c兲 and 共d兲兴 X = N2. Etching time was 35 min total.
improve the sidewall protection. Even more interesting is the
increased Cl intensity 关denoted “inc.” in Fig. 5共c兲兴 during the
O2 phase, which is highest during shut-off gas, as might be
expected since at that point the pumping efficiency is smallest. However, the increase in the Cl peak from before to after
the O2 phase indicates that the O2 plasma releases Cl. Cl
must come from the sample, the chamber walls, and the carrier wafer which in combination has a large surface. No
SiCl4 gas is present and the remaining gaseous Cl species
were rinsed in the intermediate N2 phase. The Cl release
further supports the assumption that O is not simply oxidizing the sidewall but actually exchanges O for Cl.26
The samples etched in the two processes discussed above
are shown in Figs. 6共a兲 and 6共b兲 for X = O2 and Figs. 6共c兲 and
6共d兲 for X = N2. The top portions of narrow trenches are compared in the top row of the figure. For trenches from X = O2,
scallops form, but the average wall is vertical and a visible
sheet of sidewall deposit formed. For X = N2, the trench wall
straightens in deeper regions but it is retracted from the position defined by the mask. Almost no scallops can be seen in
the trench. This is different for sidewalls on open areas 关Figs.
6共b兲 and 6共d兲兴. For X = O2, a smooth wall is formed, while for
X = N2, periodic damage/trenching into the sidewall is seen.
From this it can be concluded not only that the passivation
with O2 works, but furthermore that in this process the passivation relies on the O2 phase. This is distinctly different
from reports where background O was used.
The main requirement for this process stated in the beginning was that the top portion just below the mask resists the
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w = 900 nm
d
Aspect ratio (w/d)
Golka et al.: Time-multiplexed, inductively coupled plasma
Etch rate (nm/min)
2275
Cycle Nr.
FIG. 7. SEM images of cleaved facet after etching of stripes. 共a兲 Different
stripes with increasing widths from left to right. The leftmost, thinnest stripe
broke ex situ and the sidewall deposit peeled off. 共b兲 Close-up 共BSE image兲
of the stripe indicated by an arrow in 共a兲.
FIG. 9. Etch rate and AR of a trench as a function of time 共cycle number兲.
The data have been obtained by measuring the scallop lengths. The data are
obtained from a trench with width 900 nm 共inset兲. Etch rate refers to the 40
s SiCl4 step of the individual phase not the entire cycle.
process for a long time. Figure 7 shows the results for the
same process as shown above with X = O2, but for longer
time 共75 min, 50 cycles兲. The AR of the smallest 共intact兲 rib
is 30. From the remaining mask thickness of 150 nm, originally 400 nm, and the 50 ␮m depth on open areas follows a
selectivity of 200:1. The sidewall has detached 关cf. Fig. 7共b兲兴
from the GaAs stripe that is intact otherwise. Clearly, it can
be seen that the sidewall shrank. From the fact that there is
no undercut, it can be concluded that it shrunk after the process, probably during handling at atmosphere 共air兲. The thinnest rib in the series that decreases in thickness from right to
left has been broken and the sidewall detached. This must
have happened after etching as well since the thin GaAs
stripe would have been completely etched within 1 min with
the sidewall removed. The shape of the sidewall further hints
on a tensile strain in the sidewall that has been released here.
Another requirement on the process was simultaneous
achievement of open and narrow areas. Figure 8共a兲 shows a
trench from the same sample that is shown in Fig. 7. The
trench profile 关Fig. 8共b兲兴 is very smooth up to about 1 ␮m
depth below the mask. The first scallop seems to be missing
but the backscattered e− 共BSE兲 picture 关Fig. 8共c兲兴 reveals a
region that has been filled completely with a deposit. The
sidewall position is retracted approximately 100 nm from the
line defined by the SiNX mask edge, but this “growth” in the
trench width remains constant from top to the bottom of the
trench. Figure 8共c兲 further shows something like a void between SiNX and the Si–O–Cl sidewall that cannot be seen in
the secondary e− picture. The most probable explanation is a
region filled with very low density material, possibly porous
material that nevertheless was able to protect the underlying
GaAs.
The total etch depth of 17 ␮m obtained for high AR
trenches 共Fig. 8兲 converts into 50 ␮m total depth on open
structures 关Fig. 7共a兲, left兴 with AR⬇ 0. This aspect ratio dependent etching is inevitable for reactive ion etching, in general. The exact dependency of depth and AR is shown in Fig.
9. It is valid for a definite type of pattern that in this case is
a trench of 1 ␮m width. At AR= 9 the etch rate is almost a
third of its value for open areas 共AR⬇ 0兲. At increasing
depth the scallops’ amplitude decreases and at AR⬎ 9 the
scallops become too shallow to be evaluated in SEM pictures. The top scallop’s period/amplitude is roughly 1000/
120 nm 共stripes兲 and 750/150 nm 共trenches兲. This “roughness” should not negatively influence the optical attenuation
of real devices. First the amplitude is less than typical MIR
or terahertz wavelengths and second, the corrugation is parallel to the plane of possible light propagation.
b)
a)
1 µm
c)
D. Optimization for holes
1 µm
10 µm
FIG. 8. SEM images 共a兲 of the cleaved facet perpendicular to a 1 ␮m wide
trench. 共b兲 Close-up of entrance area of the trench, where the scalloping is
more pronounced than deeper in the hole. 共c兲 BSE image of the same spot as
in 共b兲.
JVST B - Microelectronics and Nanometer Structures
Holes have again one dimension less than trenches in
which ions and radicals could have access to the etch floor.
As a consequence the etch rate in holes decays even quicker
with increased AR than it does in trenches. If one simply
increases the total number of cycles, the selectivity starts to
be an issue. One strategy to solve this dilemma is to speed up
the etch rate toward the end of the process by increasing the
Cl concentration and going into an even more chemical regime. In other processes, this would easily destroy the fragile
equilibrium between deposition and etching that protects the
2276
Golka et al.: Time-multiplexed, inductively coupled plasma
2276
a)
a)
b)
300 nm
2 µm
b)
1 µm
top region of sidewall that is exposed most to the plasma.
However, in our case this region is already covered with a
100 nm thick wall at the time we would like to change the
process conditions. In Fig. 10 holes are shown that have been
etched with a cyclic process. After reaching AR⬇ 5 with a
chopped process at PICP = 150 W, PICP is increased to 300 W
and SiCl4 remains on continuously for 200 s. At this power
共cf. Fig. 2兲 the Cl production is considerably higher, leading
to a higher GaAs etch rate while the mask sputtering rate
changes little. Due to the missing O2 phases in the 200 s of
uninterrupted etching, the hole broadens at the bottom. The
sidewall is damaged in the region up to 1 ␮m above the
switching point to the 300 W phase. This region comprises of
about five SiCl4 phases where the time for sidewall growth
was insufficient. The top part of the hole where the sidewall
was growing since the start of the process the sidewall is
intact. The scallop resulting from the 200 s etch phase is
about 1 ␮m high, relating to an etch rate of 300 nm/min.
Therefore, it seems feasible to increase PICP later in the process if the sidewall passivation of the preceding scallops is
sufficient. This is likely to be achieved with a longer O2
phase. Making the SiCl4 phases longer at large depth should
have a positive effect on the selectivity because less N2 and
O2 phases exist that remove 共sputter兲 the mask without etching GaAs. Also generally scalloping is not as pronounced at
large depth; therefore, we should start with short SiCl4
phases and then increase their length.
These measures have been implemented in the process
shown in Fig. 11. The O2 phase remains the same but is
extended by an additional 10 s at PICP = 250 W toward the
end of the process 关inset in Fig. 11共c兲兴. All SiCl4 phases start
and end at PICP = 150 W but later on in the process they are
ramped up to higher power. The ramping avoids jumps in
power to the adjacent N2 phases that might cause transitional
instability in the rf matching network. In Fig. 11共a兲 the transitions between cycle types 1, 2, and 3 can be clearly distinguished. The first scallop of each type is pronounced. Then
amplitude and height slowly decay until the next cycle type
is started. In the plot of rate versus cycle number 关Fig. 11共c兲兴
only data points from scallops that clearly appear in SEM
pictures are plotted. In the data points for an open structure
J. Vac. Sci. Technol. B, Vol. 27, No. 5, Sep/Oct 2009
3 µm
c)
Etch rate (nm/min)
FIG. 10. Holes etched with cyclic process. Three types of cycles have been
used for this sample sequentially. First 10⫻ 20 s 共PICP共SiCl4兲 = 150 W兲,
then 15⫻ 40 s 共150 W兲, and last 1 ⫻ 200 s 共300 W兲. These expressions
denote the number of cycles and their respective durations of the SiCl4
phase with these phases’ respective PICP. The O2 and N2 phases remain
exactly the same in all cycles.
ture
truc
ns
ope
Ph
C
13 × 20 s (150 W)
13 × 40 s (150 W)
5 × 55 s (200 W) + add. 10 s O2
18 × 65 s (300 W) + add. 10 s O2
Cycle Nr.
FIG. 11. Ramped cyclic process for planar PhC pattern. 共a兲 SEM image of
cleaved facet. Holes are approximately 900 nm in diameter. 共b兲 Close-up of
the first scallops. Four types of cycles 共numbered 1–4兲 have been used for
this sample sequentially. 共c兲 Etch rate during the SiCl4 phase vs the amount
of completed cycles. The legend shows the different parameters in the four
cycle types.
that has been etched simultaneously we see the increase in
etch rate for higher PICP. In the PhC this can ease the slow
down at high AR but not stop it. However, at the last cycle
共number 50, depth 10 ␮m兲 the etch rate is still 100 nm/min.
The selectivity is ⬃33 and the top portion of the hole 关Fig.
11共b兲兴 is well covered with a sidewall, no gap between sidewall and SiNX mask is visible, and the first scallop in the
GaAs is smooth and unaltered by subsequent etch cycles.
Hence, a thicker mask and more cycles might further increase the AR of 10 shown here.
Large structures etched simultaneously turn out to be
50 ␮m deep with intact sidewalls 关Figs. 12共a兲 and 12共b兲兴.
Trenches 共w = 900 nm兲 are 15 ␮m deep and slightly growing in width with increasing AR. This effect is counterintuitive since holes are straight and open walls 关Fig. 12共b兲兴 are
even slightly overcut. One would have expected that generally overcut increases in the series hole, trench, open area.
Trenches obviously had a larger ratio of chemical attack over
sidewall deposition than both holes and open areas. In Fig.
12共a兲 we can furthermore see the changing scallop geometry
with respect to the etch phase duration. By making the etch
phase longer 共20–40 s兲, the corrugation period 共70–150 nm兲
and amplitude 共150–830 nm兲 can be increased intentionally.
Making the etch duration very short is a common way to
remove corrugation/roughness in Si based devices.17
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Golka et al.: Time-multiplexed, inductively coupled plasma
2277
A
FIG. 12. Sample etched simultaneously with the PhC sample in Fig. 11. 共a兲
Rib. Clearly the first 13⫻ 20 s and the first four scallops with 40 s etch time
共SiCl4 phase兲 can be seen. 共b兲 Cut through a large mesa edge. 共c兲 Trench.
B
FIG. 14. Composition of the passivation layer in terms of atomic percentage
measured with EDX. The EDX signal is spatially resolved along the line
from A to B as indicated in the dark field STEM image 共inset兲.
E. Analysis of the sidewall
The nature of the passivation layer was analyzed ex situ
by energy dispersive x-ray spectroscopy 共EDX兲 coupled to a
STEM 共Jeol 2200FS 200 kV field emission STEM/TEM with
a CEOS GmbH hexapole Cs corrector for the probe-forming
lens, and equipped with Jeol EDX system兲. The presence of
any passivation layer on the pillar edge can be detected with
a spatial resolution better than 1 nm, and the composition of
this layer can be estimated with a typical spatial resolution of
⬃5 nm using the EDX spectroscopy system installed in the
microscope with the transmitted electron beam as the excitation source. A typical STEM image is shown in Fig. 13. The
freestanding pillars used for EDX analysis were about 6 ␮m
high etched by 25 cycles with a 20 s SiCl4 phase. Assuming
that all cycles deposited the same amount of sidewall in the
top region 共⬃200 nm thick兲 and further assuming that sputtering removal was negligible, then the deposition rate was
⬃8 nm/ cycle. With exception of the first scallop, the sidewall thickness in Fig. 13共a兲 decreases linearly with the cycle
number. The bright glow just below the mask is attributed to
high STEM signal from a reduced density of the sidewall
a)
1 µm
b)
200 nm
FIG. 13. Bright field STEM image of a pillar that has been etched with a
cyclic process with 25⫻ 20 s 共150 W兲 SiCl4 phases.
JVST B - Microelectronics and Nanometer Structures
density or a crack that developed ex situ. This effect can be
seen in STEM but not in secondary e− SEM and is likely to
be related to the dark region in BSE SEM images 关Fig. 8共c兲兴.
The composition of the passivation layer in terms of
atomic percentage is reported on in Fig. 14. A horizontal
EDX profile is performed across the passivation layer. The
atomic percentages were deduced from the intensities of the
K lines of the EDX spectrum for all elements. A Si-rich
silicon oxide layer with Si/O ⬃1 / 1 is found. Chlorine is
detected as a weak trace 共Cl% ⬍ 1%兲. Ex situ analysis cannot
be used to accurately determine high Cl content because water vapor from the ambient air would quickly reduce the Cl
content through the formation of HCl. However, other
works15,18 have shown that at least few percent of Cl can
remain in similar air-exposed sidewall layers of comparable
thickness 共⬎100 nm兲. Ga and As are also only weakly
present 共Ga% ⬃ 5% and As% ⬃ 2.6%兲 with an increase in the
As and Ga signal when the transmission electron spot starts
to penetrate into the GaAs pillar core. No nitrogen 共N%
= 0%兲 could be detected in this layer in spite of the two
plasma N2 phases used in one process cycle, indicating that
nitridation is not a passivating mechanism. This lack of N is
consistent with previous sidewall passivation analysis performed on InP samples etched in N2 / Cl2-based plasmas.16
The exact values of atomic percentages may depend on the K
factor value used to deduce the atomic percentage from the
line intensities. In order to confirm the Si/O ratio of ⬃1 / 1
found in the passivation layer, a powder obtained from pure
fused silica 共Si/ O = 1 / 2兲 was used as a standard to precisely
adjust the K factor for O. It was observed that the default
K-factor value led to a slight overestimation of the O percentage by ⬃5%. The data in Fig. 14 have been obtained
using the K factors deduced from the SiO2 standard.
The formation of a Si-rich silicon oxide passivation layer
on the sidewall of InP patterns has previously been reported
in Cl2 – H2 and HBr inductively coupled plasmas for both
GaAs 共Ref. 20兲 and InP.16 However, as soon as small
2278
Golka et al.: Time-multiplexed, inductively coupled plasma
amounts of O2 are added to these process, the sidewall stoichiometry changed from SiO to SiO2. Observation of SiO2
with EDX is reported for SiCl4 – O2 etching13 and for
SiCl4 / Cl2 with O traces15 as well. Deposit formed in the
SiCl4 – O2 – N2 deposition phase of a chopped process18 was
SiO2 as well. The sidewall deposition rate in this work is
higher or at least comparable to that in the works cited
above, while the Si/O ratio is not. Hence the sidewall formation mechanism must have been different to some extent.
Si–O–Cl sidewalls also form in Si etching for transistor
gates. Here O2 – Cl2 – 共HBr兲 are used as etch gases. SiCl2 that
is formed from etched Si is reported to be the dominant
radical,22 governing the wall deposition/etching equilibrium
that can lead to process drift27 due to changing chamber wall
coverage. A SiOX sidewall deposit is found on the transistor
gate after the etch process. The process has an etch stop on
the underlying SiO2 layer. Desvoivres et al.26 used x-ray
photoelectron spectroscopy to compare the sidewalls of
samples etched with and without overetch time after the end
point. During overetch only Br/Cl is available but unlike in
the main etch no more Si. In the overetch O substitutes Cl or
Br in the sidewall. Based on the MIR spectra25 recorded by a
total internal reflection probe installed on the chamber wall
during Cl2 / O2 Si etching, it is proposed that the observed
Si–Cl–O film is deposited through oxidation of SiClX radicals. These radicals that are responsible for the film formation are adsorbed on the sample and chamber walls. No gas
phase reaction plays a considerable role for deposition. Tiller
and Sameith24 proposed chamber wall contamination by
共SiCl2兲N in a pure SiCl4 plasma. With the in situ measurements used in our work we have no evidence for a specific
stoichiometric Si–Cl ratio deposited in SiCl4 phase. However, the above confirms that a Si–Cl layer with little O is
formed during the SiCl4 phase, and that in the O2 phase the
plasma chemistry exchanges Cl for O. Since almost no Cl is
found in the sidewall, this exchange can be assumed complete. Hence we can assume that the Si content is defined by
cross-linking of SiCl2 or similar SiClX. This takes place before O introduction. In the subsequent O2 phase O cannot
further oxidize the sidewall, it can just exchange the Cl in the
predefined sites. The absence of O in the geometric formation of the sidewall could further explain the smoothness of
the scallops 共cf. Fig. 13兲. On GaAs surfaces no micromasking by SiO can take place since O plays a minor role until the
O2 phase starts.
IV. CONCLUSION
We have presented a cyclic etch process for the etching of
GaAs in a planar ICP-RIE. The development of this process
was motivated by the drawbacks presented for continuous
SiCl4 etching and the difficulty to improve the process by
continuous O2 addition. The main concern was the ability to
etch structures with very high AR simultaneously to low AR
structures with high selectivity to a standard SiNX mask. The
improvement in verticality and sidewall stability was
achieved by a chopping cycle in which an O2 passivation
J. Vac. Sci. Technol. B, Vol. 27, No. 5, Sep/Oct 2009
2278
phase is separate from a SiCl4 etch/deposition phase. A chopping sequence SiCl4 – N2 – O2 – N2 – SiCl4 – ¯ is used to ensure this. In situ laser interferometry reveals that oxygen passivation delays the onset of fast etching in the subsequent
etch phase. Optical emission spectroscopy shows that O introduction is freeing Cl. We proposed that O is substituting
Cl in a Si–Cl layer deposited previously on all chamber and
sample walls. This is further substantiated by EDX analysis
showing a Si:O ratio of 1:1 which was in contrast to results
published for continuous O2 addition where Si:O was 1:2.
Furthermore, the layer had low density but contained no Cl.
The sidewall deposit was stable upon a change in process
parameters and/or if no more O2 was flowing. All sidewalls
showed scallops that relate to gas cycling. These have a typical amplitude of 100 nm, which should not generate too high
scattering losses at MIR wavelengths. The inside of the scallops was smooth, from which we conclude that no micromasking due to Si–O particles was present. Trenches with
width of 1 ␮m were achieved that had a depth of 17 ␮m. To
transfer the process that worked well for trenches to the fabrication of holes with ⬃1 ␮m diameter, it was helpful to
increase the ICP power toward the end of the process. This
enhances the etching aspect of SiCl4, as shown by optical
emission spectroscopy. Increasing ICP power reduced the
slow down of the etch rate at increased AR. With the
chopped process, an AR of 30 is obtained for isolated stripes
with a very high selectivity against the SiNX mask reaching
200:1 in open areas. The achieved AR of 10 in ⬃1 ␮m
diameter holes and the dimensional accuracy make this process a good candidate for high Q PhC cavities for GaAs MIR
to terahertz optoelectronics.
ACKNOWLEDGMENTS
Jens Pfeiffer and Ralf Behmel are gratefully acknowledged for their technical help. Luc le Gratiet of LPN is acknowledged for precious assistance in e-beam lithography.
This work was supported by the BMWi 共German Federal
Ministry of Economics and Technology through Grant No.
KA0218102FK7 in the project Pro INNO II.
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