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j.surfcoat.2017.10.041

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Surface & Coatings Technology 331 (2017) 85–89
Contents lists available at ScienceDirect
Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat
Comparative surfacing studies of sputtered and thermal annealed ZnO films
wet etched with NH4Cl and acidic etchants
MARK
Liang-Kui Liu, Cheng Shi, Guan-Qing Wang, Yi Wang⁎, Lei Sun⁎
Institute of Microelectronics, Peking University, Beijing 100871, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Wet etching
ZnO films
Annealing
The wet etchings of as-grown and thermal annealed ZnO films were carried out by using NH4Cl and acidic
etchants respectively. The lateral profiles and side faces of the etched ZnO samples were exhibited by profiler
and SEM measurements. NH4Cl etchant can avoid W-like lateral profile and eave-like slope at the side face of the
etched ZnO patterns for both as-grown and post-annealed ZnO samples. The etching rate has been compared
between NH4Cl etchant and acidic etchants for both as-grown and post-annealed ZnO samples. NH4Cl etchant
can maintain slower etching rate with higher NH4Cl concentration. The thermal annealing procedure can be
helpful to achieving uniform etching surface, and the etching rates of the annealed ZnO samples are even slower,
which can be explained by the better crystallinity and lower density of oxygen vacancy.
1. Introduction
As one kind of wide bandgap metal oxide materials, zinc oxide plays
an important role in microelectronic field. Wang et al. reported the
application of ZnO nanowire arrays for powering nano-devices, considering the outstanding piezoelectric properties of ZnO [1]. Hoffman
et al. applied ZnO as transparent channel layer for thin film transistors,
which can be used in flat panel display applications [2]. Minami et al.
used doped ZnO thin film as transparent electrode for optical electronic
application [3]. The growth, patterning and etching of ZnO films always constitute the major steps of ZnO devices fabrication, among
which the film etching technology can directly affect the manufacture
efficiency and device reliability. Comparing with dry etching for ZnO
films [4–6], wet etching has shown the advantages of high selectivity,
high etching rate and lower cost. A large amount of researches studied
on acid etchants or their mixture, such as HCl [7–9], H3PO4 [8,10],
HNO3 [10], H2SO4 [11], formic acid [12], acetic acid [9,11], oxalic acid
[11], citric acid [12] and hydrogen peroxide [13]. Aqueous acidic salts,
such as FeCl3 [11,14] and NH4Cl [15–17], were also proposed. NH4Cl
will be a better choice, because it is free of Fe contamination. Up to
now, most of the researches focused on the etching rate of the individual etchant, and the lateral profile and the side face morphology of
the etched ZnO patterns have been scarcely reported. On the other
hand, thermal annealing procedures have been widely used to improve
crystallinity and film properties of ZnO [2,18,19], and the etching
properties of the annealed ZnO samples, especially by wet etching, still
need further researches.
⁎
In this article, we've compared the surfacing morphology and
etching rate by using NH4Cl and acidic etchants for both as-grown ZnO
films and post-annealed ZnO samples. The etching surfacing profiles
and etching rates have been measured by profiler, Scanning Electron
Microscope (SEM), and Atomic Force Microscope (AFM) measurements.
The crystallinity of the annealed samples were measured by X-ray
Diffraction (XRD). And we've found that the annealing condition will
significantly influence the etching properties of the samples. And the Wshape lateral profiles eave-like side face can be avoid by using NH4Cl
etchant for both as-grown and post-annealed ZnO samples.
2. Experimental details
N-type (100) silicon substrates were used during the experiments.
Before the depositions of ZnO films, SiO2 buffer layer was firstly grown
by thermal oxidation. ZnO films were then deposited by radio frequency magnetron controlled sputtering (Kurt J. Lesker, PVD-75).
During the depositions, ceramic ZnO target (with purity of 99.99%) was
used. The sputtering power was 70 W, and the gas pressure was kept at
1 Pa. After the growth of ZnO film, about 2 μm-thick positive photoresist (Suzhou Ruihong Electronic Chemicals Co., Ltd., RZJ 304) was
spun on ZnO film surface. Lithography and development were then
performed (Suzhou Ruihong Electronic Chemicals Co., Ltd., RZX 3038).
The common baking steps after spin, exposure and development lasted
50 s, 60 s and 65 s respectively, and the baking temperatures were kept
at 120 °C. After the patterning, the prepared samples were etched with
NH4Cl aqueous solution etchant (with concentration ranged from
Corresponding authors.
E-mail addresses: wangyi@ime.pku.edu.cn (Y. Wang), sunl@pku.edu.cn (L. Sun).
http://dx.doi.org/10.1016/j.surfcoat.2017.10.041
Received 7 April 2017; Received in revised form 29 September 2017; Accepted 12 October 2017
Available online 12 October 2017
0257-8972/ © 2017 Published by Elsevier B.V.
Surface & Coatings Technology 331 (2017) 85–89
L.-K. Liu et al.
PR Pattern
200
Etched after:
(a)
0
10s
20s
Profile (angstrom)
-200
-400
ZnO
-600
Oxide
-800
100nm
-1000
-1200
(a)
-1400
0
50
100
150
200
Lateral Position (um)
PR Pattern
ZnO
500
Etched after:
(b)
10s
20s
Profile (angstrom)
0
Oxide
-500
100nm
-1000
(b)
-1500
-2000
-2500
0
50
100
150
200
Lateral Position (um)
ZnO
PR Pattern
Oxide
200nm
Profile (angstrom)
0
Etched after:
(c)
40s
80s
(c)
Fig. 2. SEM photographs of as-grown ZnO film's cross sections: (a) etched with HCl
etchant, (b) etched with H3PO4 etchant, (c) etched with NH4Cl etchant.
-500
10 kV to 15 kV) and AFM (SII NanoTechnology Inc., SPI3800N/SPA400, at contact mode with Si3N4 tip). The etchings with acidic etchants
(HCl and H3PO4 are selected as control groups) have also been carried
out. The acid concentration is set as 6.7 mmol/L, 10.0 mmol/L,
12.5 mmol/L, 15.0 mmol/L and 16.7 mmol/L for both HCl etchant and
H3PO4 etchant. The etching morphology and etching rate with NH4Cl
etchant were then compared with the control groups. To study the influence of anneal on the etching results, some ZnO samples were annealed in O2 and N2 ambience in tube-type anneal furnace
(Nabertherm, C250) respectively. All annealing temperature was controlled at 400 °C, and annealing time was kept for 1 h. Same as the
treatments above, the annealed ZnO films were patterned and etched
with different etchants. Measurements of the annealed ZnO films after
the etchings were also the same as the case for as-grown samples.
-1000
-1500
-2000
0
50
100
150
200
Lateral Position (um)
Fig. 1. Etching profiles of as-grown ZnO film: (a) etched with HCl etchant (12.5 mmol/L),
(b) etched with H3PO4 etchant (12.5 mmol/L), (c) etched with NH4Cl etchant (3.0 mol/
L).
1.0 mol/L to 4.0 mol/L) at room temperature. The morphologies of the
lateral and vertical etching surfaces were detected by Profiler (AMBiOS,
XP-1), SEM (FEI, Quanta 600, with operating voltage ranging from
86
Surface & Coatings Technology 331 (2017) 85–89
L.-K. Liu et al.
Fig. 3. SEM photographs of ZnO film's cross sections: (a)
annealed in N2 and etched with HCl etchant (12.5 mmol/
L), (b) annealed in N2 and etched with NH4Cl etchant
(1 mol/L), (c) annealed in O2 and etched with HCl etchant
(12.5 mmol/L), (d) annealed in O2 and etched with NH4Cl
etchant (1 mol/L). All samples were over-etched for measurements.
ZnO
Oxide
ZnO
Oxide
Substrate
Substrate
1um
1um
(b)
(a)
ZnO
ZnO
Oxide
Oxide
Substrate
Substrate
1um
1um
(c)
(d)
etched with HCl, H3PO4 and NH4Cl etchants respectively. The samples
were over etched until the SiO2 buffer layers were revealed in the unprotected regions. SEM measurements have shown the presence of
eave-like vertical side faces in the samples etched with HCl and H3PO4
etchants, which would bring difficulty to patterning control (see Fig. 2a
and b). In the contrary, the vertical side face of the sample etched with
NH4Cl etchant is nearly perpendicular to the substrate. We've not found
significant eave-like profile even after longer over etching time. The
uniform etching profile in Fig. 2c are consistent with the absence of Wlike etching profile in Fig. 1c. It's because there are little porous
structures (holes) or other structural defects formed during the etching
with NH4Cl etchant, however, such structural defects might appear
when acid etchant is used [7,12].
The relationship between surface roughness of etched ZnO film and
etching time (with NH4Cl etchant) has been studied by AFM measurements. The root mean squares (RMS) of the surface roughness are
4.492 nm (etched after 20 s), 17.62 nm (etched after 40 s), 27.46 nm
(etched after 80 s) and 31.48 nm (etched after 100 s) respectively. The
RMS of surface roughness increases when etching time becomes longer.
Such difference might be induced by ZnO film itself rather than by the
etching procedure. ZnO films were grown by sputtering, and the growth
procedure follows the island-growth principle (Volmer-Weber growth
model) [20–22]. The islands expand and connect together, finally fill in
the whole surface of the substrate and construct the continuous films.
The etching of ZnO likes a converse procedure of growing. When ZnO
film is etched, the island structure would appear again, therefore ZnO
film surface becomes rougher.
Fig. 3 shows the cross-section of the vertical etching profiles of the
annealing samples. The samples were annealed in N2 and O2 ambience,
and etched respectively. In the case with HCl etchant, eave-like vertical
side face has been suppressed (see Fig. 3a), and the abrupt vertical side
face is kept for the one etched with NH4Cl etchant (see Fig. 3b). Annealing in N2 ambience also conducts smoother side face. The etching
morphology with both HCl and NH4Cl etchants becomes acceptable
after annealing. Such phenomenon can be explained by the enhancement of ZnO film crystallinity. In Fig. 4, the influence of annealing
Intensity (arb. units)
ZnO annealed in N2
ZnO annealed in O2
as-grown ZnO
20
25
30
35
40
45
50
2Theta (Degree)
Fig. 4. XRD measurement profiles of as-grown ZnO film (black line), annealed in O2 at
400 °C (blue line), and annealed one in N2 at 400 °C (red line). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
3. Results and discussions
Fig. 1 shows the etching profiles of as-grown ZnO films etched with
the three different etchants. For HCl and H3PO4 etchants, and we can
observe obvious over-etching near the dual boundary side of the photoresist pattern region, which turns out to form a “W”-like etching
trench profile before the whole ZnO film is completely removed. In the
case with NH4Cl etchant, etching rate is 1.5 nm/s, which is much
slower than those with acidic etchants, and therefore W-like profile
doesn't appear even during a longer etch time (see Fig. 1c).
Fig. 2 shows the cross-section morphology of as-grown ZnO films
87
Surface & Coatings Technology 331 (2017) 85–89
L.-K. Liu et al.
Etching depth (nm)
absorbed by ZnO during the annealing, remove extra hanging bonds
which are induced by the oxygen vacancy, and construct a more stable
state for ZnO [23–25].
In Fig. 5, we can see the etching depth owns a linear relationship
with etching time, while the slope value represents the etching rate.
Clearly displayed in Fig. 5, etching rate of the as-grown ZnO film is
faster than that of the annealed ZnO film, because the ZnO film becomes denser and more stable bind connection appeared in crystal after
annealing. As shown in Fig. 6, we also obtained much slower etching
rate with NH4Cl etchant than with acidic etchants, while the concentration of NH4Cl is about one order magnitude larger than that of
acidic etchants. All of them own a positive correspondence relationship
between the etchant concentration and the etching rate. The following
equations describe the etching mechanism of NH4Cl:
(a)
200
Conc. of 1mol/L
Conc. of 4mol/L
150
100
50
Etching of as-grown ZnO
0
20
40
60
80
100
120
140
160
H2 O ↔ H+ + OH−
NH 4+ + OH− ↔ NH 4 OH
2H+ + ZnO(s) → Zn2 + + H2 O(l)
Etching time (s)
(b)
160
Etching depth (nm)
NH4Cl will partially ionize in water which makes its etching rate
lower than that of acidic ones which is totally ionized. Meanwhile,
partial ionization of NH4Cl is also helpful for keeping the pH value
stable during the etching procedure. In our experiments, we observed
that the fluctuation of NH4Cl solution's pH value was slightly changed
while acid solution's pH value had an obvious raise when etching time
extended. And the gradient of etching rate with NH4Cl etchant is also
smoother than the cases with HCl and H3PO4 etchants. With relative
higher applicable concentration and smoother etching rate gradient, it
would bring us a wider design window and lower precision requirement
to control the etchant concentration.
Circle: annealed in O2
Triangle: annealed in N2
Red: Conc. of 1mol/L
Blue: Conc. of 4mol/L
120
80
40
4. Conclusions
Etching of annealed ZnO
0
0
50
100
150
200
250
The etching profiles with NH4Cl and acidic etchants were compared,
and NH4Cl etchant has shown better etching controllability for the asgrown ZnO samples. Both W-like shape on horizontal etching surface
and eave-like slope at vertical etching side face can be avoided with
NH4Cl etchant. After the thermal annealing of ZnO samples, the crystallinity has been improved and the eave-like slope side face has also
been suppressed with acidic etchant. NH4Cl etchant shows a better
applicable design window for industrial applications, because of the
optimized size control and stable etching rate. And the thermal annealing procedure is also recommended to obtain uniform etching
profiles for ZnO films.
300
Etching time (s)
Fig. 5. Relationship between etching depth and etching time with NH4Cl etchant for: (a)
as-grown ZnO films, (b) ZnO films annealed in O2 and N2 ambient respectively.
Acid Concentration (mmol/L)
8
10
12
14
16
800
NH4Cl etchant
HCl etchant
H3PO4 etchant
100
90
700
600
80
500
70
400
60
300
50
200
40
30
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Acidic etchant etching rate (nm/min)
NH4Cl etchant etching rate (nm/min)
110
6
Acknowledgments
The authors would like to thank the financial support from Natural
Science Foundation of China (No. 61474005), Importation and
Development of High-Caliber Talents Project of Beijing Municipal
Institutions (Great Wall Scholar, No.·CIT & TCD20150320) and Beijing
Nova Program Interdisciplinary Studies Cooperative projects (No.
Z161100004916036).
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