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Research Article

Rational Strategy for Space-Confined Atomic Layer Deposition
Ryoma Kamei, Takuro Hosomi, Masaki Kanai, Eisuke Kanao, Jiangyang Liu, Tsunaki Takahashi,
Wenjun Li, Wataru Tanaka, Kazuki Nagashima, Katsuya Nakano, Koji Otsuka, Takuya Kubo,
and Takeshi Yanagida*
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ABSTRACT: Atomic layer deposition (ALD) offers excellent controllability of spatial
uniformity, film thickness at the Angstrom level, and film composition even for high-aspectratio nanostructured surfaces, which are rarely attainable by other conventional deposition
methodologies. Although ALD has been successfully applied to various substrates under opentop circumstances, the applicability of ALD to confined spaces has been limited because of theinherent difficulty of supplying precursors into confined spaces. Here, we propose a rational
methodology to apply ALD growths to confined spaces (meter-long microtubes with an aspectratio of up to 10 000). The ALD system, which can generate differential pressures to confined
spaces, was newly developed. By using this ALD system, it is possible to deposit TiOx layers onto
the inner surface of capillary tubes with a length of 1000 mm and an inner diameter of 100 μm
with spatial deposition uniformity. Furthermore, we show the superior thermal and chemicalrobustness of TiOx-coated capillary microtubes for molecular separations when compared toconventional molecule-coated capillary microtubes. Thus, the present rational strategy of spaceconfined ALD offers a useful approach to design the chemical and physical properties of the inner surfaces of various confined
spaces.
KEYWORDS: atomic layer deposition, confined space, metal oxides, capillary tubes, high aspect ratio■

INTRODUCTION
Decorating artificially inner surface properties of confined
spaces offers an interesting approach to design novel chemical
events within spaces,1−7 since an interaction between
molecules and the surrounding inner surfaces dominates
phenomena within confined spaces.8−13 Although there arevarious surface decoration methods for inner surfaces ofconfined spaces,14−16 inorganic solid decorations are particularly interesting approaches due to their thermal and chemicalrobustness and the elemental variation, which are rarelyattainable with conventional organic surface modifications.8,17,18 Among various inorganic solid surface decoration
methods, atomic layer deposition (ALD) is one of powerfuldeposition techniques because of the excellent spatial
deposition uniformity and atomic layer level thickness
controllability even for complex nanostructured surfaces
including needlelike high-aspect-ratio nanostructures, which
are rarely depositable by other conventional methods.19−35 In
conventional ALD processes, the objects and/or targets to bedeposited have been mostly various shaped substrates underopen-top circumstances,19,20 rather than the inner surface ofconfined spaces. Therefore, commercially available various
ALD systems have been designed to operate under such opentop environments, where introducing precursors and/or waters
into ALD chambers is easily performed without adding any
special structures due to the relatively high flow conductance.22© 2023 American Chemical Society

In principle of ALD, supplying and/or removing precursors (orwaters) on surfaces are critical processes to appropriately
perform depositions.26 Inherently, such supply and removal of
precursors tends to be more difficult for confined spaces
especially at high aspect ratios.23 This is because the flow
conductance for exchanging precursors tends to decrease as the
aspect ratio of confined spaces increases.36 Although theapplication of unique ALD techniques to confined spaces isclearly fascinating for designing chemical events within spaces,
above inherent limitation of ALD hinders conventional ALD
methodologies from performing depositions for inner surfacesof confined spaces. Here we propose a rational methodology toovercome above difficulty of ALD for confined spaces. We
design our own ALD system, which can generate differential
pressures to confined spaces. This ALD system enables us tosuccessfully deposit a TiOx layer onto the inner surface ofcapillary tubes with a length of 1000 mm and an innerdiameter of 100 μm, where conventional ALD methods are notapplicable. Furthermore, we show the superior thermal andReceived: February 14, 2023
Accepted: April 21, 2023
Published: May 8, 2023

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Research Article

Figure 1. (a) Schematic of typical ALD system employed. (b) Spatial uniformity data of TiOx depositions by conventional ALD system onto theinner surface of microtubes when varying inner diameters; the relative EDS intensity ratio of Ti/Si (ITi/ISi) and the deposition thickness are shown.
(c) Comparison between the present data and previous data [point 1 is from ref 1, point 2 is from ref 37, point 3 is from ref 38, and point 4 is fromref 39] on the depositable geometry of microtubes by conventional ALD.

Figure 2. (a) Schematic of molecular behaviors into microtubes in typical ALD systems and/or in the forced-flow system proposed in this study,(b) Estimated molecular supply time tsup of a microtube for various inner diameters (ID = 0.1, 1, or 10 mm) plotted against the differential
pressures (Phigh − Plow) of the inside and outside of the tube. The molecular supply time tsup is defined as the time to transport the number ofprecursor molecules required to form a monolayer on the inner wall of the microtubes.

chemical robustness of such TiOx-coated capillary tubes formolecular separations when compared to conventional
molecule-coated capillary tubes.

■

it is difficult to directly measure the TiOx thin film thickness on
the inner wall of microtubes with a length of 100 mm, we
performed EDS measurements on the inner wall to measure
the deposited Ti amount. A glass microtube was cut at several
points, as illustrated in Figure 1b, and the EDS spectra at crosssectional samples were measured. The EDS intensity ratio (ITi/
ISi) of Si (a component of the microtube body) and Ti wasused as the index of spatial deposition uniformity, as shown in
Figure 1b. In addition, the correlation between the ITi/Isi andthe film thickness was extracted as shown in Figure S2. The
correlation was obtained for TiOx films on a flat SiOx substrate.
Typically, an EDS intensity ratio (ITi/ISi) of 0.02 corresponds
to a TiOx thickness of 4.0 nm. The estimated film thickness
data are also shown in Figure 1b. As clearly seen in Figure 1b,

RESULTS AND DISCUSSION

Figure 1a illustrates the schematic of experimental systems
employed for conventional ALD system in the present
experiments for microtubes. Microtubes with various innerdiameters ID (ranging from 0.5 mm to 2 mm) and a length of
100 mm were placed into the ALD chamber. A Ti precursor(TDMAT) was used for these depositions. The deposition
temperature was 150 °C. Figure 1b shows the spatial
uniformity of TiOx depositions onto the inner surfaces of
microtubes when varying the size of the inner diameter. Since
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Research Article

Figure 3. (a) Schematic of the developed ALD system, which can generate differential pressures for a forced flow. (b) Schematic of proposed ALD
cycles and 6 steps to perform material supply and material removal for confined microtubes.

portation. Based on Arnold’s formula,37 a diffusion is inversely
proportional to pressure. Thus, within the framework ofdiffusion-rate-determining processes, depositing at lower
pressures is required to enhance the deposition rate via
increased diffusion constant. However, for microtubes, aprecursor supply only via diffusion requires an extremely
long time (e.g., estimated to be around 7000 s for a length of
100 mm and an inner diameter of 100 μm; see the calculation
details in Figure S3), which is clearly not a realistic time range
for cycled ALD experiments. One possible strategy toovercome this issue is to utilize differential pressures acrossmicrotubes, generating a forced flow. As shown in Figure 2b,
we calculate a time to transport the number of precursor
molecules required to form a monolayer on the inner wall of
the microtubes (tsup) as a function of differential pressureswhen varying the inner diameter of microtubes. For example,
in the microtubes with ID = 0.1 mm, the differential pressures
are required to be above around 500 Pa to generate a forced
flow in the realistic deposition time range. Thus, it is possible
to supply precursors into microtubes and remove them frommicrotubes by applying the appropriate differential pressuresacross microtubes. However, such experimental procedures are
not feasible for conventional ALD systems. For this reason, we
have developed an ALD system that can perform depositions
for microtubes with the low flow conductance by generating
forced flows.
Figure 3a shows the schematic of newly developed ALD
system. This system is designed to be capable of applying therequired differential pressures across microtubes during bothprocesses including supplying precursors into microtubes andthere is a significant size effect of inner diameter (ID) on thespatial uniformity of TiOx depositions. In the case of
microtube with an inner diameter of 2.0 mm, the TiOx
deposition was spatially uniform over the microtube lengthof 100 mm. However, such deposition uniformity tends todeteriorate as the inner diameter decreases. For the innerdiameter of 1.0 mm, the TiOx layer could be deposited only
near both edges of microtubes, but not for the center part.
When the inner diameter was smaller than 0.5 mm, the TiOx
could not be deposited at all for entire microtubes. Figure 1cshows the comparison between previous reports and thepresent data on ALD growth data within microtubes whenvarying the inner diameter and the length.1,37−39 As seen in the
figure, the ALD depositable geometry data of microtubes inprevious works are consistent with the present data. The
reason why there is difficulty in performing ALD growth within
microtubes is clearly related to the low flow conductance ofcapillary microtubes, i.e., the supplied precursors cannot reachthe center part of microtubes. Thus, there is an inherent
limitation of conventional ALD methodology to deposit the
inner surfaces of microtubes, where the space confinement
effect dominates.
Conventional ALD processes for film depositions are
performed in the presence of flows to deliver precursors
and/or waters onto the target surfaces. However, as illustrated
in Figure 2a, the necessity of such a flow process inherently
limits the precursor supply into confined spaces like microtubes. This is because the flow conductance of microtubes
tends to decrease as the inner diameter decreases, and adiffusion of precursors dominates the total precursor trans23933

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removing them from microtubes. As illustrated in Figure 3a,
the pressure at both ends of the microtube can be
independently controlled by valve operations, and relatively
large differential pressures around 104 Pa can be generated forthe microtube. The processes for one ALD cycle consist of 6
steps, as illustrated in Figure 3b. The first process (Step 1) is awhole evacuation of all ALD chamber components. The
background pressure values are typically around 0.5 Pa. In Step2, N2 gas mixed with precursors is filled into pipes surrounding
microtubes. In Step 3, by opening two values (V2 and V3), N2
gas mixed with precursors is introduced from both ends ofmicrotubes via the generated differential pressure. After thisprecursor deposition onto the inner surface of microtube (Step3), residual precursors must be removed before the next
precursor supply. In Step 4, by opening three valves (V2, V3,
and V4), residual N2 gas mixed with precursors is evacuatedfrom all pipes surrounding microtube. However, due to the low
flow conductance of the microtube, it is impossible tocompletely remove the residual precursor gas from themicrotube, as illustrated in Figure 3b. To fully evacuate theresidual precursors from microtube, in Step 5, pure N2 gas is
introduced into surrounding pipes by opening two valves (V1and V2). Note that one side pipe of microtube near valve (V3)
is intentionally closed to keep low pressure level (0.5 Pa). And
then in Step 6, by opening two valves (V3 and V4), theresidual precursors can be completely moved from microtubes
by the N2 forced flow via the pregenerated differential pressure.
Thus, by utilizing the differential pressures across microtubes,
supplying and/or removing precursors into/from microtube
can be performed for ALD processes. Note that theseexperimental procedures utilizing differential pressures are
essential to perform ALD growths for any confined spaces.
Figure 4 shows the application of developed ALD
procedures to deposit TiOx (Ti precursor: TDMAT) onto

prepared by conventional ALD system did not show any TiOx
growth in the microtube. On the other hands, spatially uniform
TiOx deposition can be observed for sample prepared by newly
developed ALD system. These results highlight that ourdeveloped ALD system enables us to perform spatially uniform
depositions on the inner surface of a long capillary microtube
with a high aspect ratio of 10 000. Note that any previous ALD
systems are not capable of depositing for such capillary
microtubes.
Finally, we examine the applicability of fabricated capillary
microtubes with TiOx on the inner surface for thermally andchemically robust gas chromatography, as illustrated in Figure
5a. Figure 5b shows the comparison between the present
capillary microtubes with TiOx and a conventional molecularly
decorated InertCap FFAP capillary column on the retention
data using carboxylic acids (nonanoic acid, hexanoic acid). To
examine the thermal robustness, we measured the thermal
durability of both capillary columns. Figure 5c summarizes thethermal durability of the retention time when varying thethermal cycling time from room temperature to 350 °C. As canseen in the figure, the present capillary microtubes with TiOx
on the inner surface exhibited superior thermal stability whencompared with the conventional FFAP capillary column. As
the cycle number of thermal endurance increased, theretention time of FFAP significantly decreased, whereas theretention time of the present capillary microtubes with TiOx
could be maintained. Thus, the present capillary column
coated by TiOx via ALD can separate highly polar moleculeswith thermal robustness, which had been difficult using
conventional molecule-coated capillary columns. Furthermore,
our ALD method could be applied to the nanostructures
formed within microtubes. We have previously constructed
ZnO nanowires (ZnO-NW) on the inner wall of microtube
capillary and implemented the capillary as a liquid phaseseparation column; the ZnO-NWs could adsorb phosphonicacids via electrostatic interaction.40,41 However, the corrosion
morphology of ZnO-NWs in the capillary column has been
confirmed after exposures to acetate buffer in 4 h as seen in
Figure 5d, e, and this corrosion has led to a loss of adsorption
capacity for phosphonic acid (see the details in Figures S4 andS5). In contrast, ZnO-NWs coated by TiOx via ALD could
keep their specific adsorption capacity for phosphonic acids
without corrosion of their inner-nanowire structures (Figure5e). Briefly, the TiOx coating via ALD improved the chemicalrobustness as a separation media of ZnO-NWs. Since thiscapillary column application is solely one example of various
applications of the proposed ALD system to decorate the innersurface of confined spaces, the present rational strategy ofspace-confined ALD offers an approach to design the chemicaland physical properties of various confined spaces.

Figure 4. (a) Photograph of a capillary microtube. the spatial
locations measured for EDS were highlighted as A−J. (b) Spatial
uniformity of TiOx deposition on the inner surfaces at various
positions of the microtube.

■

Research Article

CONCLUSION
We demonstrate a rational methodology to apply ALD
depositions to confined spaces (meter-long microtubes of
100 μm inner diameter with an aspect ratio of up to 10 000).
ALD system, which can generate differential pressure toconfined spaces, was newly developed. This ALD systemenables us to successfully deposit TiOx layer onto the innersurface of capillary tubes with the length of 1000 mm and theinner diameter of 100 μm. Furthermore, we show the superior
thermal robustness of such TiOx-coated capillary tubes formolecular separations when compared to conventional
molecule-coated capillary tubes. The present rational strategy

the inner surface of microtube with the inner diameter of 100
μm and the length of 1000 mm (the aspect ratio: 10 000).
Figure 4a shows the photograph of employed whole capillary
microtube, and the measurements for deposition uniformity
were performed at various positions A−J. Figure 4b shows thecomparison between the developed ALD system and conventional ALD system for the TiOx deposition onto the innersurface of microtubes. As clearly seen in Figure 4b, the sample
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Figure 5. (a) Schematic of molecular separation experiments with ALD-deposited microtubes used as GC column. (b) Change in carboxylic acids
peak retention time after five measurements on a PEG-coated column (left) and a TiOx-deposited column (right). (c) Comparison between thepresent TiOx-deposited column and conventional PEG-coated column on cycled retention time data. (d) Schematic of liquid-phase leaching
resistance tests, and 10 mM acetate buffer (pH 4.5) was used as the liquid phase. (e) SEM images of a ZnO nanowire-arrayed capillary microtube(bare-NW) and a TiOx-deposited ZnO nanowire-arrayed capillary microtube (TiOx-deposited) after soaking with acetate buffer solution for 4 h.
microtube (1 min). Step 7: Evacuate the entire flow path for 30 s toexhaust the gas from the microtubes. The system temperature wasmaintained at 250 °C throughout the cycle. It takes approximately 20
min to complete one cycle. The deposition rate of TiOx at the centerof the tube was calculated to be 0.04 nm/cycle in the case of thecapillary tube (Molex 1068150023).
Inner-tube Deposition with a Typical ALD System. The
microtube was placed in the chamber of a commercially available
ALD system (Veeco Instruments Inc. Savannah S100) as shown in
Figure 1a. The following five steps are performed to form one ALD
cycle. Step 1: Evacuate the chamber (about 20 Pa) with 20 sccm N2
flow. Step 2: Supply TDMAT by opening the precursor-supplying
valve for 0.1 s. Step 3: Wait for 5 s with continuous N2 flow. Step 4:
Supply H2O by opening the precursor-supplying valve for 0.015 s.
Step 5: Wait for 5 s with continuous N2 flow. Deposition rate of TiOx
on flat SiO2 substrates is 0.04 nm/cycle.
Analysis of the Deposited Layer within the Microtube. The
morphology of the TiOx within microtubes was characterized by fieldemission scanning electron microscopy with energy dispersive X-ray
spectroscopy (FESEM-EDS, JEOL JSM-7610F 30 kV) to quantify theatomic existing ratios. The microtubes were cut at 10 cm intervals toallow EDS measurements at each point. To evaluate the thickness of
TiOx films using EDS, calibration was performed using a standard
sample. The ratio of Ti(Kα) to Si(Kα) characteristic X-ray intensities(ITi/ISi) of a 500 μm thick TiOx film deposited on a SiO2 substrate by
ALD was measured to obtain the relationship between TiOx film
thickness and ITi/ISi value.
Gas Chromatography Analysis. The deposited microtubes were
installed in a gas chromatograph system with a flame ionization
detector (Shimadzu corp. GC-2030). A mixture of nonanoic acid,hexanoic acid, nonanal, hexanal, 1-nonanol, and 1-hexanol in ethanol(0.1 mM) was used as the standard sample mixture. The linearof space-confined ALD offers a useful approach to design thechemical and physical properties of inner surfaces of various
confined spaces.

■

METHODS

Materials. For glass microtubes, commercially available SiO2 glass
tubes (inner diameter/outer diameter = 2.0 mm/6.0 mm, 1.0 mm/4.0
mm, and 0.5 mm/3.0 mm) were used. For capillary microtubes, apolyimide-coated fused silica capillary tube (Molex 1068150023,
inner diameter = 100 μm, outer diameter = 375 μm) was used. For
the ALD TiOx precursor, tetrakis(dimethylamino)titanium(IV)(TDMAT, purchased from Tri Chemical Laboratories Inc.) wasused. N2 gas (purity: 99.99%) was purchased from Taiyo Nippon
Sanso Gas Co., Ltd. All chemicals in the GC measurements (nonanoic
acid, hexanoic acid, nonanal, hexanal, 1-nonanol, 1-hexanol, andethanol) were purchased from Tokyo Chemical Industry Co., Ltd.,and used without further purifications. Adenosine monophosphate(AMP) was purchased from Sigma-Aldrich, and used as is.
Inner-tube Deposition with a Forced-Flow ALD System. The
microtube was installed in the high differential pressure ALD shownin the Figure 3 with carbon ferrule joints. Prior to the deposition, thecapillary tube was filled with N2 at 8000 Pa and kept at 250 °C for 5
min. The following seven steps are performed sequentially for
TDMAT and H2O as the precursors to form one ALD cycle. Step 1:
Evacuate the entire flow path for 1 min to reach 0.5 Pa. Step 2: Fillthe outer flow path with 10 000 Pa precursor-mixed N2. Step 3:
Introduce precursor-mixed N2 into the microtubes, and wait until thepressure stabilizes (within 30 s). Step 4: Evacuate the entire flow pathfor 1 min to roughly exhaust the precursor-mixed N2 from the system.
Step 5: Fill the path with clean N2 at 10 000 Pa. Step 6: A vacuum isdrawn from the downstream side to expel residual gas in the23935

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velocity of the carrier gas (He) was 2.0 cm/min. The inlet
temperature is 400 °C. The column oven temperature was first held
at 35 °C for 5 min, and then increased to 350 °C at a rate of 20 °C/
min and held at that temperature for 10 min. InertCap FFAP (GLSciences Inc.) was used for the PEG-coated capillary column. The
peaks were attributed to compounds by comparison of retention times
with standard reagents.
Liquid-Phase Leaching Tolerance Tests. A bare-NW column(5.0 cm × 100 μm ID) was prepared by growing ZnO nanowire arrayson the inner wall of a capillary microtube by a method we recently
reported.41 The TiOx-deposited column (5.0 cm × 100 μm ID) was
prepared by depositing TiOx by ALD on the nanowires of the bareNW column. Ten millimolar acetate buffer (pH4.5) was flowed into
the columns for 4 h, and the morphology changes were observed by
SEM. The SEM images before the liquid-phase leaching tolerance
tests are shown in Figure 5e. Furthermore, The adsorption capacity of
the capillary for phosphonic acid was evaluated by nanoliquid
chromatography (nano-LC) after treatment with acetate buffer. A
Nexera Micros (Shimadzu, Kyoto, Japan) series was used for nano-LC
analysis. This system consisted of LC-Mikros (Shimadzu, Kyoto,
Japan) as the pump, MU701 (GL Sciences, Tokyo, Japan) as the UV
detector, a CHEMINERT as the sample injector, and CTO-20AC(Shimadzu, Kyoto, Japan) as the column oven. The detailed
conditions are described in the Supporting Information.

Tsunaki Takahashi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8654, Japan; JST-PRESTO, Kawaguchi, Saitama
332-0012, Japan; orcid.org/0000-0002-2840-8038
Wenjun Li − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138654, Japan
Wataru Tanaka − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8654, Japan
Kazuki Nagashima − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8654, Japan; JST-PRESTO, Kawaguchi, Saitama
332-0012, Japan; orcid.org/0000-0003-0180-816X
Katsuya Nakano − Graduate School of Engineering, Kyoto
University, Kyoto 615-8510, Japan
Koji Otsuka − Graduate School of Engineering, Kyoto
University, Kyoto 615-8510, Japan
Takuya Kubo − Graduate School of Engineering, Kyoto
University, Kyoto 615-8510, Japan; orcid.org/00000002-9274-3295
Complete contact information is available at:https://pubs.acs.org/10.1021/acsami.3c01443■

ASSOCIATED CONTENT

sı Supporting Information
*

Notes

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.3c01443.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
This work was supported by KAKENHI. T.T., T.H., K.N., J.T.,
and T.Y. were supported by JST CREST, Japan. This work was
performed under the Cooperative Research Program of“Network Joint Research Center for Materials and Devices”
and the MEXT Project of “Integrated Research Consortium on
Chemical Sciences”.

Deposition results (Figure S1), correlation between EDS
data and film thickness in TiOx ALD depositions (FigureS2), estimated time for molecules to pass through themicrotube when varying the pressure and the diameter(Figure S3), chemical robustness of TiOx-coatedmicrotubes with ZnO nanowires (Figures S4 and FigureS5) (PDF)■

Research Article■

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Corresponding Author

Takeshi Yanagida − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8654, Japan; Institute for Materials Chemistry
and Engineering, Kyushu University, Fukuoka 816-8580,
Japan; orcid.org/0000-0003-4837-5701;
Email: yanagida@g.ecc.u-tokyo.ac.jp
Authors

Ryoma Kamei − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138654, Japan; Institute for Materials Chemistry andEngineering, Kyushu University, Fukuoka 816-8580, Japan
Takuro Hosomi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8654, Japan; JST-PRESTO, Kawaguchi, Saitama
332-0012, Japan; orcid.org/0000-0002-5649-6696
Masaki Kanai − Institute for Materials Chemistry andEngineering, Kyushu University, Fukuoka 816-8580, Japan
Eisuke Kanao − Graduate School of Pharmaceutical Sciences,
Kyoto University, Kyoto 606-8501, Japan; National
Institutes of Biomedical Innovation, Health and Nutrition,
Ibaraki, Osaka 567-0085, Japan
Jiangyang Liu − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138654, Japan; orcid.org/0000-0001-5456-7705
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Research Article

https://doi.org/10.1021/acsami.3c01443
ACS Appl. Mater. Interfaces 2023, 15, 23931−23937