Electrochemical and Solid-State Letters, 9 共2兲 G27-G30 共2006兲
G27
1099-0062/2005/9共2兲/G27/4/$20.00 © The Electrochemical Society
CCl4-Based RIE Pattern Transfer into Facets of Mesas Formed
by Wet Etching in InP(100)
P. Eliáš,z Š. Haščík,a J. Martaus,a I. Kostič,b J. Šoltýs,a and I. Hotovýc
a
Institute of Electrical Engineering, Slovak Academy of Sciences, 841 04 Bratislava, Slovak Republic
Institute of Informatics, Slovak Academy of Sciences, 845 07 Bratislava, Slovak Republic
c
Department of Microelectronics, Slovak University of Technology, 812 19 Bratislava, Slovak Republic
b
We performed reactive ion etching 共RIE兲 in CCl4 /He plasma to transfer patterns into 共100兲, 共110兲, and 共211兲A wet-etch exposed
in InP共100兲 wafers. At a power, pressure, self-bias, and substrate temperature of 100 W, 0.8 Pa, −150 V, and 25°C, respectively,
etching rates were: r共100兲, r共110兲, r共211兲A = 53.8 ± 1.7; 56.0 ± 1.5; 57.7 ± 2.7 nm min−1, respectively. Original and dry-etched 共100兲
surfaces exhibited root-mean-square roughness ␴ = 0.59 and 1.65 nm, respectively. r共100兲:r共211兲A:r共110兲 ⬇ 1:1.07:1.04 suggests that
the etching removed surface chlorides more efficiently from the slanted 共211兲A and 共110兲 facets than from the normal-incidence
共100兲 surface.
© 2005 The Electrochemical Society. 关DOI: 10.1149/1.2139978兴 All rights reserved.
Manuscript submitted June 22, 2005; revised manuscript received September 20, 2005.
Available electronically December 12, 2005.
Nonplanar processing of semiconductor electronic devices extends the use of planar device concepts as well as introduces functionalities unattainable in planar devices, exemplified, e.g., in microelectro mechanical systems 共MEMS兲,1 microsensors, and
microprobes.2,3 In the processing of such devices, traditionally
two-dimensional4 or genuinely three-dimensional approaches are
used.5-7
We report on the use of reactive ion etching 共RIE兲 in CCl4 /He
plasma for the nonplanar transfer of test patterns onto tops and sides
of test mesa objects micromachined in 共100兲 semi-insulating InP
substrates. The test objects were 9-70-␮m-high mesa ridges formed
by wet etching. The ridges were confined at the top by 共100兲-related
surfaces and at the sides by 共110兲- and 共11̄0兲-related positively
sloped facets 共further denoted as 兵110其兲 and by 共211兲A- and
共21̄1̄兲A-related positively sloped facets 共further denoted as 兵211其A兲
共Fig. 1兲, into which patterns were etched via resist under various
RIE conditions. The process was evaluated by atomic force microscopy 共AFM兲 and scanning electron microscopy 共SEM兲.
This experiment is part of our work aimed at a technology that is
used to define electronic device topologies in 共100兲-, 兵110其-, and
兵211其A-related tops and sides of micromachined InP objects of specific, e.g., pyramidal shape. It is required that the topologies be
transferred into the 共100兲, 兵110其, and 兵211其A surfaces and facets at
identical or similar etching rates with preferably zero etching mask
undercutting. This condition cannot be achieved if wet etching is
used for the transfer. This is primarily because all known solutions,
such as those based on HCl or bromine in methanol, etch InP at
共100兲 much faster than at 共110兲 and 共211兲A. Also, it is difficult to
prepare a sufficiently adherent resist mask on the 共100兲-, 兵110其-, and
兵211其A-related surfaces to secure a small and identical 共or at least
similar兲 degree of resist mask undercutting on all surfaces. As the
result, a wet-etching process is not appropriate for the nonplanar
pattern transfer via resist mask. However, a dry-etching process can
be expected to be appropriate for the above purpose.
We believe that, to our best knowledge, the use of RIE for pattern transfer into side facets of objects micromachined in InP共100兲 is
published for the first time. This study aims to advance the development of microscale nonplanar monolithic vector magnetic field
sensors, whose key sensing areas are defined in epitaxial layers
grown on 共100兲-, 兵110其-, and 兵211其A-related surfaces and facets of
objects micromachined in InP共100兲.8
z
E-mail: [email protected]
Experimental
For the experiment standard 共100兲 semi-insulating InP:Fe wafers
were used. The preparatory steps included patterned substrate formation and nonplanar optical lithography.
The InP共100兲 substrates were patterned by wet etching in
3HCl:1H3PO4 at 共16 ± 0.05兲°C with complete selectivity over narrow convex-cornered strips defined in a lattice-matched InGaAs
layer, deposited by organometallic vapor phase epitaxy 共OMVPE兲.
The strips had long edges oriented in parallel either with 关01̄1兴 or
with a direction 0.5° off 关001兴 toward 关01̄1兴 共Fig. 1, on the left兲. The
different strip orientations resulted in different mesas: 共a兲 the
关01̄1兴-oriented strips led to ordinary mesas confined at the sides by
facets related to 共211兲A and 共21̄1̄兲A, inclined at approximately 35°
to 共100兲 共Fig. 1, at the top right corner兲; and 共b兲 the 0.5°-off-关001兴to-关01̄1兴-oriented strips led to mesas confined at the sides by facets
related to 共110兲 and 共11̄0兲, inclined at approximately 45° to 共100兲
共Fig. 1, at the bottom right corner兲.8
The mesa-patterned substrates were coated conformally with
positive-tone resist 共AZ5214-E兲 using the following nonstandard
technique. The sample to be coated was put on a holder and submerged in distilled water in a temperature-controlled vessel. Using a
Hamilton syringe, a small, precisely controlled drop of resist was
gently deposited on the water surface in the vessel. Upon contact
with water, the drop quickly spread on the surface forming a floating
layer, which was solidified and flexible. The resist layers used for
this experiment were 2 and 4 ␮m thick. The layers were subsequently draped over samples by slowly lowering the water surface
below the level at which the samples were held. The technique is
capable of providing conformal resist layers on patterned
substrates.9 Its details will be described elsewhere. After resist deposition, the samples were dried in an oven at 95°C for 20 min. Standard optical lithography was used to define strips in the resist 共Fig.
1兲. Prior to RIE experiments, the samples were soft- or hard-baked
at various temperatures for variable time intervals.
The samples were etched in RIE mode in CCl4 /He-based plasma
in a Roth & Rau Microsys 350 machine. The plasma was generated
by a radio frequency 共rf兲 field at 13.56 MHz supplied via a stainless
steel electrode 共쏗 200 mm兲 whose temperature was stabilized at
25°C by He blown into the chamber at 4 sccm. Before the introduction of CCl4 /He, the chamber was evacuated to a background pressure ⬍5 · 10−4 Pa. During etching the flow of CCl4 /He was 13.6
sccm. The samples, put on the electrode by a load-lock system, were
etched under conditions summarized in Table I.
To evaluate the pattern transfer process at the 共100兲 tops and
兵110其- and 兵211其A-related sides of the mesas, AFM, SEM, and op-
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Electrochemical and Solid-State Letters, 9 共2兲 G27-G30 共2006兲
G28
Figure 1. Patterned InP共100兲 substrates used for nonplanar CCl4-based RIE
pattern transfer into 共100兲-related surfaces, and 兵211其A- and 兵110其-related
facets of high mesa objects formed in 3 HCl:1 H3PO4 at 共16 ± 0.05兲°C over
InGaAs strips aligned in 关01̄1兴 and in 0.5° off 关001兴 to 关01̄1兴, respectively.
The surfaces were RIE-patterned in a Roth & Rau Microsys 350 machine via
mask strips defined by optical lithography in conformally deposited positivetone AZ 5214-E resist layers.
tical microscopy were used. The etching rates, r共100兲, r共110兲, and
r共211兲A, were measured using a standard AFM. It was necessary to
make sure that all surfaces and facets were scanned with the AFM
tip positioned perpendicular to them. This was realized with a special homemade table. The table was equipped with wedge-shaped
sample holders with the sides tilted at 35° and 45° 共Fig. 2兲. Ridges
with 兵211其A-related facets were scanned on a 35°-tapered wedge
holder. This tilted the facets perpendicular to the tip, as the angle
between 共100兲 and 兵211其A was close to 35.0 ± 0.5°. Ridges with
兵110其-related facets were scanned on a 45°-tapered wedge holder.
This tilted the facets perpendicular to the tip, as the angle between
共100兲 and 兵110其 was close to 45.1 ± 0.3°. The AFM was able to
compensate for a ±1° angular misalignment of the facet positioning.
Each facet was scanned with the tip being moved along lines in
parallel with the top edges of the mesa objects. The tip with a typical
tip curvature radius of 10 nm probed the surfaces in the noncontact
mode.
Results and Discussion
At first, the initial RIE runs were performed to specify conditions
leading to a smooth and reasonably fast etching of InP共100兲 surfaces
through the resist patterns. Table I and Fig. 3 summarize initial
results. Eight different sets of RIE conditions were used to determine r共100兲 vs negative self-bias at the sample-holding electrode with
the chamber pressure P as the parameter. At a constant P, the selfbias was adjusted by setting the rf field power.
Runs 1, 2, 3, and 4 were performed at 0.8 Pa. Run 1 yielded a
low etching rate and moderate trenching: r共100兲 ⬇ 8.83 nm min−1
Figure 2. The pattern transfer was evaluated with an AFM equipped with a
homemade positioning system. Ridges with 兵211其A-related facets were
scanned on a 35°-tapered wedge holder, which tilted them perpendicular to
the AFM tip, as the angle between 共100兲 and 兵211其A was 35.0 ± 0.5°. Ridges
with 兵110其-related facets were scanned on a 45°-tapered wedge holder, which
tilted them perpendicular to the tip, as the angle between 共100兲 and 兵110其 was
45.1 ± 0.3°. The AFM was able to compensate for a ±1° misalignment of
facet positioning. Each facet was scanned along parallel lines with top edges
of the mesas.
and ⬇10 nm min−1 at plateau areas between trenches, and inside the
trenches, respectively 共see Fig. 3a and the profile in the inset in the
top left corner兲. At runs 2 and 3, r共100兲 slightly increased and the
trenching was suppressed. Run 4 gave a reasonably high etching rate
and no trenching: r共100兲 ⬇ 53.85 nm min−1 共see Fig. 3a and the profile in the inset in the top right corner兲. Runs 5 to 8 performed at 1.5
Pa yielded heavy trenching and low top-to-plateau r共100兲 etching
rates 共⬍4 nm min−1兲 in all cases 共Fig. 3b; the inset exemplifies a
profile formed at the run 8 conditions兲.
RIE at the run 4 conditions via a 2-␮m-thick AZ5214-E resist
layer was studied for surface roughness using AFM at 10
⫻ 10 ␮m-sized areas 共Fig. 4a and b兲. The 共100兲 surface 共after wet
etching and before RIE兲 exhibited average roughness
Ra = 0.48 ± 0.08 nm and root-mean-square roughness 共rms兲
␴ = 0.59 ± 0.10 nm. The 10 min long RIE at the run 4 conditions
left the surface with Ra = 1.31 ± 0.08 nm and ␴ = 1.65 ± 0.1 nm.
RIE at the run 3 and 4 conditions was used for pattern transfer
into the tops and side facets of the mesa ridges 共Fig. 1兲.
Figure 5 shows depths etched into the 共100兲, 兵110其, and 兵211其A
surfaces vs tetch under the run 4 conditions. The results suggest
r共100兲 艋 r共110兲 艋 r共211兲A. Average rates were determined from data at
tetch = 10 and 20 min as follows: r共100兲 = 53.8 ± 1.7nm min−1;
r共110兲 = 56.0 ± 1.5 nm min−1; and r共211兲A = 57.7 ± 2.7 nm min−1,
which gave a r共100兲:r共211兲A:r共110兲 ratio of ⬇1:1.07:1.04.
Figure 6 shows SEM images of RIE pattern transfer at the run 3
conditions via a 4 ␮m thick AZ5214-E resist layer into 共100兲, 兵110其related, and 兵211其A-related facets of 9-␮m-high mesa ridges. Figure
6 a depicts a SEM image of a Greek cross topology transferred into
Table I. Etched depth dtop-plateau and dtop-trench bottom „nm… into (100) semi-insulating InP at CCl4-based RIE vs self-bias at the sample-holding
electrode with the chamber pressure P as the parameter. At a constant P, the self-bias was adjusted by setting the rf field power.
Etch run
tetch 共min兲
RF 共W兲
P 共Pa兲
Bias 共V兲
Telectrode 共°C兲
dtop-plateau 共nm兲
dtop-trench bottom 共nm兲
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
43
67
79
100
50
67
83
100
0.8
0.8
0.8
0.8
1.5
1.5
1.5
1.5
−100
−125
−135
−150
−75
−100
−125
−147
25
25
25
25
25
25
25
25
88.3 ± 3.7
173.9 ± 6.0
197.3 ± 12.2
538.5 ± 35.6
7.0 ± 1.0
15.0 ± 10.0
4.7 ± 2.4
44.0 ± 27.4
99.8 ± 3.2
Less than in run 1
Less than in run 1
None
Immeasurable
102.4 ± 8.3
164.4 ± 39.5
239.8 ± 12.3
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Electrochemical and Solid-State Letters, 9 共2兲 G27-G30 共2006兲
G29
Figure 4. 共a兲 AFM image of a InP共100兲 sample etched during 10 min under
the run 4 conditions via a 2-␮m-thick AZ5214-E resist layer 共Table I兲; 共b兲
Detail of the dry-etched surface. Before the RIE, the 共100兲 surface exhibited
a standard roughness Ra and a rms roughness ␴ of 0.48 ± 0.08 and
0.59 ± 0.10 nm, respectively. After the 10-min-long RIE, it had Ra and ␴
increased to 1.31 ± 0.08 and 1.65 ± 0.10 nm, respectively. The figures were
obtained from 10 ⫻ 10-␮m-sized areas.
Figure 3. r共100兲 at planar InP共100兲 semi-insulating substrates vs self-bias
with the chamber pressure P as the parameter 共Table I兲. 共a兲 r共100兲 at runs 1 to
4. The insets exemplify etch profiles formed during run 1 and 4; 共b兲 r共100兲 at
runs 5 to 8, which all resulted in excessive trenching. The inset shows a
profile revealed during run 8.
the 共100兲 top surface and a 共211兲A-related side facet into a depth of
approximately 200 nm. Figure 6b shows a SEM image of the same
transfer into 共100兲 and 共110兲 surfaces. Figure 6c exemplifies a detail
of the pattern definition at a 共100兲-共211兲A top edge. Figure 6d shows
a SEM image of the pattern transfer at 共100兲 and 共211兲A into a depth
of about 650 nm at the run 3 conditions for 30 min.
In general, the RIE of InP in chlorine-based chemistries
proceeds through surface chloride formation succeeded
by the removal of the chlorides via ion bombardment.10 The
r共100兲:r共211兲A:r共110兲 ⬇ 1:1.07:1.04 ratio, observed at the run 4 conditions, gives evidence that the etching process at 共100兲, 兵110其, and
兵211其A was quite isotropic. The results cannot be accounted for
considering only 共i兲 the density of bombarding ions and 共ii兲 the
likelihood of surface chloride formation at the respective surfaces,
because they would theoretically give r共100兲 higher than r共110兲 and
r共211兲A.
The density of bombarding ions should be at a maximum at the
共100兲 surfaces, and it should decrease by a factor of
cos共35°兲 ⬇ 0.82 at the 35°-inclined 兵211其A facets, and by a factor
of cos共45°兲 ⬇ 0.71 at the 45°-inclined 兵110其 facets, because ions in
the plasma travel through the sheath region perpendicular to the
sample-holding electrode.
Figure 5. Depth etched into the 共100兲-, 兵110其-, and 兵211其A-related surfaces
vs tetch at the run 4 conditions 共Table I兲. The results suggest that
r共100兲 艋 r共110兲 艋 r共211兲A.
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G30
Electrochemical and Solid-State Letters, 9 共2兲 G27-G30 共2006兲
and smooth etching of the 共100兲-, 兵211其A-, and 兵110其-related surfaces of the mesa ridges in InP共100兲. This is very promising for
technologies of microscale nonplanar monolithic vector magnetic
field sensors, for which reliable nonplanar pattern transfer is needed
to define key sensing areas in epitaxial layers grown on 共100兲-,
兵110其-, and 兵211其A-related surfaces and side facets of objects micromachined in 共100兲 InP.8,17,18
Conclusions
Figure 6. Pattern transfer at the run 3 conditions via a 4-␮m-thick
AZ5214-E resist layer into 共100兲, 兵110其-, and 兵211其A-related surfaces of
9-␮m-high mesa ridges. 共a兲 SEM image of a Greek cross topology transferred into 共100兲 and 共211兲A into a depth of ⬇200 nm; 共b兲 SEM image of the
same transfer into 共100兲 and 共110兲; 共c兲 and 共d兲 details of the pattern definition
at 共100兲 and 共211兲A into a depth at 共100兲 of ⬇200 and ⬇637 nm during 10
and 30 min, respectively.
At the same time, the surface chloride formation should be more
likely to occur at 共100兲 than at 兵211其A and 兵110其, because 共100兲
possesses a higher atomic packing density and a higher density of
dangling bonds than 兵211其A and 兵110其.11
Hence, to explain the r共100兲:r共211兲A:r共110兲 ⬇ 1:1.07:1.04 observation, one should also consider the efficiency of chloride removal vs
angle of incidence of incoming ions. The ratio indicates that the
chlorides were more easily removed at the slanted surfaces: as the
angle of incidence increased from 0° through 35° to 45° for 共100兲,
兵211其A, and 兵110其 surfaces, respectively, chloride removal was the
slowest at 共100兲, reached a maximum at 兵211其A, and slowed down at
兵110其. The increase in the chloride removal efficiency at the slanted
surfaces made 兵110其 and 兵211其A etch away as fast 共or even faster兲 as
the 共100兲 despite a lower ion bombardment intensity and a slower
chloride formation.
The phenomenon of increased efficiency of ion sputtering with
an angle of incidence has been generally observed in dry etching
experiments. The efficiency increases because the energy of incoming ions is transferred better to surface atoms if they hit the surface
at oblique angles.12,13 The phenomenon has been observed in highenergy ion-beam-etching experiments 共acceleration between hundreds and about 2000 V兲 regardless the type of material 共crystalline,
polycrystalline, or amorphous兲,14,15 as well as in lower energy RIE
experiments 共acceleration between tens and hundreds volts兲.16
RIE under run 3 and 4 conditions provided for a nearly isotropic
Reactive ion etching 共RIE兲 in CCl4 /He plasma was used for the
nonplanar transfer of test patterns via AZ 5214-E resist into tops and
sides of 9-70-␮m-high test mesa ridges micromachined in 共100兲
semi–insulating InP substrates. The ridges were confined by the top
共100兲-related surfaces and by 兵110其- and 兵211其A-related positively
sloped facets. Atomic force microscopy was used to evaluate r共100兲,
r共110兲, and r共211兲A etching rates under various RIE conditions.
between
8.8 ± 0.4
and
The
RIE
yielded
r共100兲
53.8 ± 1.7 nm min−1, very little or no trenching, and good surface
finish at a chamber pressure P of 0.8 Pa, and self-bias between −100
and −150 V, and the sample-holding electrode kept at 25°C. After
10-min-long RIE the 共100兲 surface had rms roughness
␴ = 1.65 ± 0.1 nm 共before RIE ␴ = 0.59 ± 0.10 nm兲.
At P = 0.8 Pa and a self-bias of −150 V, the
共100兲-, 兵110其-, 兵211其A-related surfaces were etched at
almost identical rates: r共100兲, r共110兲, r共211兲A were 53.8 ± 1.7;
respectively,
with
56.0 ± 1.5;
and
57.7 ± 2.7 nm min−1,
r共100兲:r共211兲A:r共110兲 ⬇ 1:1.07:1.04. The etching process thus proceeded slightly faster at the 兵110其- and 兵211其A-related surfaces than
at the 共100兲 surface, because of a higher efficiency of surface chloride removal from the slanted surfaces.
In conclusion, we demonstrated that RIE in CCl4 /He plasma is
appropriate for the transfer of device topologies at nearly identical
etching rates into 共100兲-, 兵110其- and 兵211其A-related surfaces and
facets of objects micromachined 共100兲 InP.
Acknowledgments
This research was sponsored under projects APVT-26-020902,
APVT-51-050602, VEGA 2/3187/23, and VEGA 2/3114/23.
Slovak Academy of Sciences assisted in meeting the publication costs of
this article.
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