解説

先進セラミックス研究センター年報（2022）. Vol. 11, 13‑26

Applications of Hydrogen Silsesquioxane in
Nanomanufacturing and Nanofabrication
Yuping Xu*, Yunzi Xin*, Takashi Shirai *,**
*Advanced Ceramics Research Center, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPAN**Department of Life Science and Applied Chemistry,
Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPAN
Abstract: Hydrogen silsesquioxane (HSQ) is a versatile inorganic material that has garnered significant attention in thesemiconductor industry, such as in micro-/nano-electromechanical systems, photonic devices, and nanoelectronics, owing to its
high silicon content, small molecular size, low dielectric constant, excellent line edge roughness, high etching resistance, andlocal planarization capabilities. This review article discusses the application-related aspects of HSQ in nanomanufacturing forintegrated circuits and nanoscale patterning, and the technical applications of HSQ in the fabrication of semiconductor siliconbased nanomaterials.

Hydrogen silsesquioxane (HSQ), a completely
inorganic silsesquioxane first synthesized by Collins andFrye in 1970[1], has a general chemical composition of(HSiO 1.5) n, including commercial (H 8Si 8O 12) (n = 8)
cubic clusters and laboratory-synthesized (HSiO 1.5) n(n>8) polymers. HSQ has a cage-like structure (Fig. 1-a)
and a random network of partially formed cages (Fig.
1-b). Cage-like silsesquioxanes are typically referred toas polyhedral oligosilsesquioxanes or polyhedraloligomeric silsesquioxanes. This class of well-defined,
highly symmetric molecules usually features ananoscopic size of approximately 1–3 nm in diameter,
which can be regarded as one of the smallest possible
silica particles. Molecules with a T8 cubic inorganic corecomposed of silicon–oxygen (H8Si8O12) linkages are themost prevalent systems studied. As a critical alternative
to commercial cubic clusters, the HSQ polymer(HSiO1.5)n (n>8) can be easily synthesized by mixing
water and trichlorosilane in a short time via hydrolysis
and polycondensation reactions in a one-pot reaction
with a yield as high as ~ 95% [1-3]. In short, HSQ iscommercially available in methyl isobutyl ketones as a
flowable oxide (Fox) or is synthesized with a highdegree of purity, which makes it ideal for use innanofabrication and other high-tech applications where
purity is important. HSQ exhibits a low dielectric
constant, ranging from 2.6 and 3, making it a good
insulator for use in electronic devices [4]. Owing to its
high silicon content, HSQ is resistant to oxygen plasma
and it exhibits a sufficiently high etching resistance[5-7].

HSQ is also appreciated for its excellent line-edge
roughness and local planarization capabilities[8]. In theearly 1990s, HSQ was used as a carbonless protective
coating and interlayer dielectric material in highperformance integrated circuits in the semiconductor
industry[9-13]. In 1998, Namatsu et al. discovered HSQ as
an inorganic negative-tone resistance for electron beamlithography (EBL) [5, 14]. Based on these findings, thesemiconductor grade HSQ has already been an industrial
standard negative-tone resist applicable for highresolution and small linewidth fluctuations electronbeam patterns [15-28] , extreme ultraviolet (EUV)
lithography [29-36], proton beam writing[37, 38], molding
material in nanoimprint lithography[39-47], step and flash
imprint lithography [48], and making masks using dry
etching processes[49-53]. In addition to creating physicalpatterns, HSQ can be engineered to exhibit a wide rangeof functional properties that transcend the notion of

Figure 1. Chemical structures of HSQ: (a) cage form, (b)network form.

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Applications of Hydrogen Silsesquioxane in Nanomanufacturing and Nanofabrication

resists being used singularly as sacrificial components totransfer a pattern into functional materials. HSQ has
been utilized as a popular molecular precursor for Sibased nanomaterials, that is, nanocrystalline Si/ SiO2
composites, SixGe1-x, through high-temperature thermalt r a n s f o r m a t i o n [2, 3, 54-66] o r r o o m - t e m p e r a t u r emechanochemical treatment[67].
HSQ combines mechanical, thermal, and chemical
stability with solution processing, simple thin-film
fabrication processing, and the flexibility of soft
materials. In this review, we discuss the utilization ofthese attributes in a diverse range of nanoscalepatterning applications. In comparison with traditionallithographic resists, HSQ exhibit several advantages interms of the mechanical and physical properties that areimportant for nanoscale patterning media. HSQ can becustomized to showcase a wide range of functional
applications that go beyond creating physical patterns as
resists or sacrificial components for transferring patterns
into functional materials but act as precursors forfunctional silicon-based nanomaterials. In this study, we
focus on the application-related aspects ofnanomanufacturing for integrated circuits and nanoscalepatterning and review the technical applications of HSQ
in the fabrication of functional silicon-based materials.
1. Interlayer dielectric and protective coating
materials

In the semiconductor industry, as the feature size ofdevices is scaled down to the deep sub-micrometerregion, the interconnection delay (i.e., resistancecapacitance delay) becomes a performance-limiting
factor for ultra-large-scale integration circuits. Dielectricmaterials with lower dielectric constants are required toreduce wiring capacitance and improve device
performance. The integration of HSQ as an interlayer
dielectric in multilevel interconnects has garneredconsiderable attention in recent years. This class of
inorganic spin-on dielectrics potentially combines thedesired features of a low dielectric constant (< 3),excellent gap fill, planarization performance, non-etchback processes, and the capability of using a standard
spin-on production technique, which has been reported
to lower the capacitance between adjacent metalinterconnects, reducing electrical delay and increasing
the information processing rate of the device[9, 10, 68-78].
Curing is a significant process during the backend of theline fabrication of integrated circuits because it affects
the structure and properties of the spin on the dielectricmaterial. However, both the leakage current and thedielectric constant of HSQ increase rapidly as the film
being treated in temperatures higher than 400 ℃[79-82].
Furthermore, based on an experimental investigation ofthermal curing, the following possible reactions were
proposed[77, 80-81, 83-86]:
2 HSiO3/2 → H2SiO + SiO2 (1)H2SiO + HSiO3/2 → H3SiO1/2 + SiO2 (2)3 H2SiO + 6 HSiO3/2 → (H8Si4O4) + 4 SiO2 + SiH4 (3)H3SiO1/2 + HSiO3/2 → SiO2 + SiH4 (4)SiH4 → Si + 2H2 (5)SiH4 +O2 → SiO2 + 2 H2 (6)At a temperature of less than 200 ℃, the removal of
the residual solvent was performed. In the ranges of
250–350 and 350–450 ℃, the predominant reactions are
expressed in Equations 1–3 and 4, respectively, with
SiH4 and H2 being the main product. At temperatureshigher than 450 ℃, SiH4 evolution decreased, and H2
was formed. This formation could be the result of thethermal decomposition of SiH4 in the gas phase underinert conditions, as expressed in Equation 5. Traces ofoxygen promoted the oxidation of SiH4, as shown in thelast step. The unusual thermal curing behavior of the
HSQ films is reflected in their physical properties.
Bremmer et al. reported temperatures of 400 ℃ or higher
resulted in increased dielectric constant and film
stress [81]. In addition to the curing temperature, thecuring time and atmospheric process parameters also
affect the structure and properties of HSQ films. To
enhance the thermal stability of low-dielectric-constant
HSQ, ion implantation[87], H2 plasma pretreatments [70, 88, 89],
metal composition (Cu, Ti, Ta, TiN, and TaN), barrier
layer[90-92] deposition, and optimized process conditions [93]have been widely studied.
In protective coatings, HSQ acts as an isolated matrix
to protect Si fins for advanced multiple-gate MOSFETs(Fig. 2-a), exhibiting excellent planarization and gap fillcapabilities[94]. Moreover, HSQ has also been investigatedas a passivation layer in thin-film transistor liquid crystal
displays, as shown in Fig. 2-b, which not only lowers
the resistive-capacitive time delay in the device but alsoenhances the brightness of the displays[95, 96].

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Yuping Xu, Yunzi Xin, Takashi Shirai

Figure 2. (a) Picture of HSQ matrix covering the silicon finsfor a multi-gated MOSFET process (reprinted from Appl. Surf.
Sci., 2006, 253, 395-399. Copyright © 2006 Elsevier B.V.). (b)The schematic of BCE a-Si:H TFT passivated by HSQ (low-kfilm) (reprinted from Thin Solid Films, 2006, 498, 70-74.
Copyright © 2006 Elsevier B.V.).

smaller nanoscale feature sizes. Currently, the mostadvanced lithography systems can produce features as
small as a few nanometers[28], allowing the creation ofcomplex integrated circuits with billions of transistors.
Owing to its intrinsic capacity for patterning, HSQ is
widely utilized in lithography and other applications thatincorporate or require nanoscale patterns. This study
investigated the utilization of HSQ as an inorganic
negative-tone resist for various lithographies, including
EBL, EUV lithography, nanoimprint lithography, stepand flash imprint lithography, and masking in dryetching processes.

2. Application in nanoscale lithography

Lithography is a process used in the semiconductor
industry to create a desired pattern in a resist layer andthe subsequent transfer of that pattern into or onto theunderlying substrate. The basic steps of the lithographic
process are schematically illustrated in Fig. 3, including
coating, exposure, development, and pattern transfer,
such as lift-off or etching[24]. The lithography process iscritical in the production of semiconductor devices
because it determines the size and shape of the circuits
printed onto the substrate. As circuit designs havebecome increasingly complex and the demand forsmaller and more powerful chips increased, lithography
technology had to keep pace with the need for ever-

2.1 EBL resist

In 1998, a 20-nm wide negative HSQ resist with arectangular cross-sectional shape for EBL was firstreported by Namatsu et al[5]. Fig. 4-a shows a SEM
image of the HSQ patterns. The HSQ resist films werespin-coated onto silicon wafers using flowable oxide, acommercially available HSQ solution manufactured by
the Dow Corning Corporation (Midland, MI, USA), andsubsequently baked and exposed to an electron beam to

Figure 4. (a) SEM image of HSQ patterns (Reprinted fromMicroelectron. Eng., 1998, 41/42, 331-334. Copyright ©1998
Elsevier Science B.V). (b) SEM image of 4.5-nm-half-pitch
nested-L structures patterned in 10-nm-thick HSQ at 10 kV
acceleration voltage (Reprinted from J. Vac. Sci. Technol. B,
2009, 27, 2622. Copyright © 2009 American Vacuum Society).
(c) Left: schematic flow chart of suggested patterning
procedure of Ge2Sb 2Te 5 film. Right: HSQ line arrays withwidths of a-40, b-60, c-80, and d-100 nm on an a-Si/Ge2Sb2Te5/oxide substrate (reprinted from J. Electrochem. Soc., 2007, 154,
Figure 3. Schematic representation of the basic steps of aH844-H847. Copyright © 2007 The Electrochemical Society).

lithographic process including coating, exposure, development(d) Schematic of the fabrication procedure of ITO photonic

and pattern transfer such as lift-off or etching. (Reprinted fromcrystal using e-beam patterned HSQ resist (reprinted from J.

Nanotechnology. 2009, 20, 292001. Copyright © 2009 IOP

Vac. Sci. Technol. B. 2020, 38, 022802. Copyright © 2020

Publishing).

American Vacuum Society).
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Applications of Hydrogen Silsesquioxane in Nanomanufacturing and Nanofabrication

withstand subsequent processing, typically developed in
an aqueous alkali solution such as tetramethyl
ammonium hydroxide. They suggested that the reaction
mechanism of the HSQ resist was based on two steps:(1) Si–H bonds were broken during electron beamirradiation and converted to silanol groups in thepresence of absorbed moisture, and (2) unstable silanolgroups were crosslinked to form a linear network. HSQ
was employed as a single layer, allowing dense lines andspaces with a resolution as small as 20 nm and single
lines with widths as small as 15 nm were established by
Falco et al[16]. The limiting factors of the EBL resolution
include spot size, electron scattering, secondary electron
range, resist development, and mechanical stability of
the resist[97]. In general, the patterning resolution can be
improved by reducing the width of the electron-beam
exposure point-spread function and increasing the resist
contrast, which can also enhance the density ofnanopatterning[98-105]. By refining the exposure strategies,
development procedures, and reducing the film
thickness, high-resolution isolated lines measuring 6 nm
in width and gratings with periodicities as small as 27
nm were reported [17] . Namatsu et al. [18] accurately
measured the diameter of a well-configured electron
beam as 3.6 nm. Using this fine beam, they successfully
formed patterns that were 5–6 nm wide in the HSQ resistat the center and corners of the main deflection field.
However, the minimum electron beam point spreadfunction is not always easily modified. For instance,
smaller point-spread functions can be achieved using
high electron-beam acceleration voltages. Manfrinato et
al. reported that a 200 keV EBL with an aberrationcorrected STEM can be used to pattern 2 nm isolated
features and 5 nm half-pitch structures in HSQ[28]. The
dependence of the contrast enhancement on theconcentration and type of cations and anions in the
aqueous developer solution was observed. Contrast
values as high as 10 in a 115-nm-thick resist wereachieved by developing HSQ in an aqueous mixture of
NaOH alkali and NaCl salt[21]. The improved contrast of
HSQ enabled the fabrication of 7 nm half-pitch nested“ L ” structures in a 35 nm-thick resist using a 30 kVelectron-beam acceleration voltage. Subsequently,
Berggren et al. [23] demonstrated the patterning of 4.5 nm
half-pitch structures (Fig. 4-b) using EBL exposure
combined with a high-contrast salty solution
development process.
It is easy to understand that EBL provides excellent
patterning resolution with the resist of HSQ in

nanotechnology research and development of nanoscale
semiconductor devices[19, 106-116] and biodevices[117, 118].
Baek et al. developed HSQ resist trimming and hybrid
lithography techniques to scale down gate-line patterns
to sub-10 nm with high accuracy and reproducibility[19].
Using this novel process, triple-gate MOSFETs with agate length of 6 nm were successfully fabricated.
Although the length of this device is extremely small, its
operational characteristics exhibit excellent performance,such as, a good subthreshold slope and large on/off
current ratio. A fabrication process based on HSQ resist
has the potential to produce high-performance siliconon-insulator-based photonic wire device structures
owing to the high resolution and etch resistance of HSQ,
which allows direct masking of the silicon core[108].
Moreover, HSQ acts as a sacrificial part; in other words,
a mask for transferring patterns onto functional materials
has been reported[106, 107, 116]. Nam et al. demonstrated thesuccessful electron-beam patterning of Ge 2 Sb 2 Te 5nanostructures in the form of lines and dots using an
HSQ resist[107]. The fabrication process is shown in Fig.
4-c, where the patterns were transferred using a two-stepetching process, in which Cl2 and Ar were used in eachstep. HSQ was used as a high-resolution EBL resist to
pattern Ge2Sb2Te5 nanostructures with feature sizes as
small as 40 nm. To improve the light extraction andquality factor cavity applications of organic lightemitting diodes, EBL was used for ITO patterning. A
technological process for fabricating photonic crystals in
the ITO layer in the visible range was developed using
an electron-beam-patterned HSQ resist, as illustrated inFig. 4-d[116]. In this study, owing to its high resistanceand mechanical strength, a high-resolution patterned
HSQ resist was used as a mask to transfer photonic
crystal nanostructures to a 150 nm-thick ITO layer. For
biodevices, the use of electron-beam-patterned HSQ
resists in the fabrication of triangular nanochannels fordeoxyribonucleic acid (DNA) analysis, drug delivery[117],and solid supports for light-directed in situ ssDNA
synthesis[118] have been introduced.
2.2 EUV lithography resist

EUV lithography at a wavelength of 13.5 nm has
proven to be a promising solution to meet the resolution
requirements of the microelectronics industry,
particularly for the production of high-density featureswith critical dimensions of 45 nm and below. Nealey etal. [29] reported that HSQ has a sensitivity of 11.5 mJ/cm2
with a contrast of 1.64 in EUV exposure and is able to― 16 ―

Yuping Xu, Yunzi Xin, Takashi Shirai

resolve 26 nm dense lines in ~70 nm thick film with alow line edge roughness of 5.1 nm, indicating that HSQ
exhibit distinct advantages over traditional chemically
amplified resists for application in EUV lithography.
Stefan et al. presented EUV lithography on an HSQ
resist at resolutions of 16 and 22 nm by adding silanol or
carbinol (R-CH3OH) groups to partially cross-linked
HSQ cages to increase its tetramethylammoniumhydroxide developer and EUV sensitivity [30]. They
reported that to achieve high-resolution, high-volume
manufacturing via EUV lithography, additional
refinements to the HSQ-based system are required tosurmount the developer solubility and dose trade-off,which is still under investigation. HSQ patterns with ahalf-pitch as small as 20 nm and linewidths as small as
10 nm using EUV interference lithography have beenreported[31]. This resolution was achieved through thedevelopment of high- concentration developers for longdevelopment times, which is in line with previous results
obtained with EBL. Sub-10 nm half-pitch resolution
using an HSQ resist has been reported[32].
EUV interference lithography uses diffracted, coherent
light beams to create high-resolution patterns. However,creating the necessary diffraction grating masks throughEBL is limited by the electron proximity effect andpattern transfer issues. To overcome these limitations, ateam of researchers patterned HSQ lines at a relaxed
pitch and used conformal iridium deposition via atomic
layer deposition to create diffraction gratings, resulting
in a 6 nm half-pitch pattern[119]. Winter et al. reported
that EUV could be used to create periodic patterns on
1-nm thick carbon nanomembranes and single-layer
graphene with sizes ranging from 20 to 500 nm, as
illustrated in Fig. 5-a[120]. The resulting nanosheets andnanoribbons can be made into supported films or
freestanding objects on holey supports and grids (Fig.
5-b). In this study, HSQ was employed as a standard
negative photoresist in high-resolution lithography tofabricate nanoribbon patterns with widths as small as ~20 nm in carbon nanomembranes. HSQ has also beenused as an EUV resist in etching-mask applications forthe synthesis of metal nanoparticle arrays [121, 122] ,nanowires, and nanodots[123] packed over large areas.
2.3 Proton beam writing resist

Although HSQ is primarily used as a high-resolution
negative-tone electron beam resist, it is also expected to
exhibit negative-tone behavior for photon beam
exposures. HSQ is a superior resist for p-beam writing,Figure 5. (a) Schematic of the nanopatterning process basedon EUV-IL. (a:a) The 2D material (black) on a substrate(yellow, Au) is spin-coated with a photoresist layer. (a:b) The
photoresist is irradiated with the EUV interference pattern andsubsequently developed (a:c). Thereafter, the structure is
induced into the underlying material via RIE (a:d). Finally, thephotoresist is dissolved (a:e). The structure can subsequently betransferred onto a holey substrate (a:f ) to prepare a selfsupporting, perforated sheet or an array of nanoribbons. (b)Freestanding graphene monolayers on holey lacey carbon
support films: graphene nano-mesh with 175 ± 5 nm wide
openings and 90 ± 5 nm wide graphene features on an area of
50 μm × 50 μm (reprinted from 2D Mater., 2019, 6, 021002.
Copyright © 2019 IOP Publishing Ltd).

allowing the production of high-aspect-ratio structures
as small as 22 nm [37]. HSQ shows great potential for
proton-beam writing in three-dimensional (3D)
nanolithography [38]. Using HSQ as a resist, it has beenpossible to directly write details with a width as small as
20 nm and a height of 850 nm. Proton beam writing with
HSQ provides a new method for creating high-density
3D nanostructures with reduced proximity effects
compared with the widely adopted e-beam writing
technique.
2.4 Molding material in nanoimprint lithography

Nanoimprint lithography is an attractive method forachieving high-resolution pattern replication at a low
cost and high throughput. This process involves thephysical deformation of a heated polymer coated on asilicon substrate using a mold deposited on the substrate.
The resulting resist pattern is thereafter subjected toreactive ion etching to create a useful profile for thesubsequent hard material pattern transfer. HSQ has beensuccessfully used as a mold owing to its strong adhesion
to silicon substrates and structural similarity to SiO2
after thermal or electron-beam curing [39, 40]. Roomtemperature nanoimprint lithography without a resist
thermal cycle or UV exposure using HSQ was also
reported by Matsui et al[42-47].
Gold nanoparticle (Au NP) patterns fabricated using― 17 ―

Applications of Hydrogen Silsesquioxane in Nanomanufacturing and Nanofabrication

imprinted HSQ patterns for surface-enhanced Raman
scattering (SERS) was first reported by Matsui et al[124].
The successful assembly of the negatively charged Au
NPs followed the geometry of the negatively charged
HSQ pattern. Moreover, a comparison between substrates
fabricated using an inorganic polymer (HSQ) and anorganic polymer demonstrated that the SERS substratefabricated from HSQ was well suited for organic
chemical analysis.

utilization of HSQ as a resist in the nanopatterning
process and as a mask in the dry etching process has also
been developed for the fabrication of MOSFET gateswith an even 6 nm gate length[19, 125], low-loss nanowires
in silicon-on-insulator (Fig. 7)[108], sub-micron GaAs[49],sub-10 nm features into metal [126] , and metallic
nanostructures for nanophotonic and plasmonic[127].
3. Molecular precursors for silicon-based
nanomaterials

2.5 Step and flash imprint lithography resist

Step and flash imprint lithography is a variation of
nanoimprint lithography that uses a UV-transparent
template with a relief image to replicate patterns, and is
performed at room temperature and low pressure. This
involves coating the wafers with an organic polymertransfer layer, pressing the template into the layer, curing
the resist, removing the template, performing afluorocarbon reactive ion etching step to remove thepolymer layer residues, and performing oxygen etching
to transfer the pattern to the substrate. In their study,
Mancini et al. explored the use of HSQ for directelectron-beam patterning of step and flash imprint
lithography templates[48]. They coated quartz photomask
substrates with an ITO charge-dissipation layer andpatterned them with an electron beam using a 100 nm
thick HSQ resist layer. The highest resolutions of 20 and
30 nm were achieved on the templates and imprinted
wafers, respectively (Fig. 6).
2.6 Mask in dry etching processes

As previously discussed in the EBL resist section,
HSQ is a promising material for negative resist in
nanopatterning and hard mask during dry etching owing
to the nanoscale device fabrication because of its highresolution, high etching resistance, and compatibility
with various substrate materials[106, 107, 116]. Moreover, theSemiconductor materials composed of group IV
elements have been intensely studied for a broad rangeof potential applications, from photovoltaics andfluorescent contrast agents to color displays, such as therecently commercialized quantum dot light-emitting
diode displays. In this section, HSQ is introduced as the
precursor of oxide-embedded and freestanding Sicontaining group IV semiconductor nanomaterials withwell-defined sizes and optical properties, such as
photoluminescence (PL) and quantum yield (QY).
3.1 Oxide-embedded and freestanding silicon
nanocrystals (Si NCs)

In 2006, the first preparation of nanocrystalline Si–
SiO2 composites via thermal pyrolysis of a well-definedmolecular HSQ precursor was reported by Veinot etal.[54], as illustrated in Fig. 8-a. Si NCs-embedded SiO2
started to be observed after one-hour thermal pyrolysis
of HSQ at temperatures higher than 800 ℃ under amixed inert atmosphere of 4% H2 and 96% N2. The sizeof the Si NCs can be tuned by controlling the annealing
temperature and/or time. Si NCs with a size of 3.3 nm
can be obtained at temperatures close to 1100 ℃, which

Figure 7. SEM image of a photonic wire Bragg grating fully
Figure 6. Top down SEM of lines down at 25 and 30 nm pitchetched into silicon waveguide core layer with HSQ as an EBL

resolutions written with SFIL on (a) templates and (b)resist and mask (reprinted from Electron. Lett., 2008, 44, 115-

imprinted wafers (reprinted from J. Vac. Sci. Technol. B, 2002,

116. Copyright © 2008 The Institution of Engineering and20, 2896. Copyright © 2002 American Vacuum Society).

Technology).
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Yuping Xu, Yunzi Xin, Takashi Shirai

increased to 9 nm as the temperature increased to1400 ℃. An increase in size was also observed with anincrease in the annealing time [56]. The formation ofoxide-embedded Si NCs through the thermal pyrolysis
of HSQ in a reducing atmosphere (i.e., 96% N2 or Ar
and 4% H2) has been identified in three stages[2, 54, 56]:
1. HSQ structural changes, including trace solvent
loss (< 200 ℃), cage network rearrangements withassociated loss of SiH4 (250–350 ℃), Si–H thermal
dissociation with the formation of SiH4 and H 2(350–450 ℃), and HSQ cross-linking.
2. With the temperature increasing to 450 ℃, rapid
thermal decomposition of SiH 4 results in theformation of oxide-embedded Si nanodomains. The
specific role of hydrogen in thermal processing wasinvestigated. It is possible that low H2 concentrations
limit or even prevent the dehydrogenation of HSQ,
thus increasing the amount of SiH4 available forthermal decomposition.
3. Nanodomain crystallization and growth. At
temperatures higher than 900 ℃, processing
temperature drives the growth and increases
crystallinity of Si nanodomains, whereas prolonged
heating at 1100 ℃ results in an annealing processthat passivates volume defects.
The luminescent nanocrystalline Si/SiO2 thin films
and patterns formed after thermal pyrolysis (1100 ℃)
from solution processable HSQ have been reported[128].
Free-standing Si NCs with hydrogen-terminated surfaces(H–Si NCs) can be liberated from the matrix composite

through the chemical etching of SiO2 using hydrogen
fluoride (HF), as shown in Fig. 8-b. The etchingprocedures have been reported in previous studies[129].
The most significant procedure is the use of HF/
hydrogen chloride, which provides faster etching. The
emission of these freestanding H–Si NCs in the range of
520–750 nm depended on the etching time (Fig. 8-d).
The observed blue shift in the PL maximum upon
extended exposure to HF can be attributed to thequantum confinement effects arising from the decreased
size. The PL and QYs of the H–Si NCs was approximately
4%. However, H–Si NCs were significantly unstable andeasily oxidized because of the low disassociation bondenergy of Si–H. Various surface passivation methods
have been outlined in previous studies[130-137]. Typically,
the surface passivation of alkyl groups is induced
through thermal-[130, 131], photo-[132], radical-initiated-[133],
and Lewis-acid-initiated [135] reactions between H–Si
NCs and alkenes, resulting in the formation of robust
Si–C covalent bonds instead of Si–H on the surface (Fig.
8-c). Dasog et al. reported the preparation of surfacepassivated Si NCs with PL across the entire visible
spectral region via surface engineering with alkyl,
amine, phosphine, and acetal functional groups without
altering the size to 3–4 nm (Fig. 9-a)[134]. The surfacestate-dependent emission exhibited short-lived excitedstates and higher relative PL QYs than Si–NCs ofequivalent size, which exhibited emission originatingfrom the bandgap transition. This study highlights theimportance of surface states and their crucial role in the

Figure 8. (a) Synthesis of oxide-embedded Si NCs through thermal pyrolysis. (b) Liberation offreestanding H–Si NCs through HF chemical etching. (c) Surface passivation of H–Si NCs by alkene via a)thermal, b) photo, c) radical initiated, and d) Lewisʼs acid reaction (reprinted from Chem. Mater., 2017, 29,
80-89. Copyright © 2016 American Chemical Society). (d) PL of liberating H–Si NCs by varying etchingtime: 50 (red), 85 (orange), 115 (yellow), and 135 minutes (green), resulting in decreased size. Inset:
photographs of PL observed upon exposure to a standard handheld UV light (reprinted from Chem. Mater.,
2006, 18, 6139-6146. Copyright © 2006 American Chemical Society).
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Applications of Hydrogen Silsesquioxane in Nanomanufacturing and Nanofabrication

optical properties of Si NCs. Subsequent studies revealed
that by altering the surface ligands, the PL range could
be extended to encompass the full color spectrum within
the 400–800 nm wavelengths[61-63, 138-140]. For instance,
the surface passivation of Br, I, and Cl ligands, insteadof hydrogen in the H–Si NCs, generates red, orange, andblue PL, respectively (Fig. 9-b)[61]. Recent research hasreported QYs reaching up to 80% in the orange-red
wavelength region [139] . Several studies have been
conducted on the thermal pyrolysis of a well-definedcommercial HSQ, namely, the H8Si8O12 cubic cluster. Si
NCs with a size of 1–10 nm have been prepared throughthermal pyrolysis (1100 ℃) of lab synthesized (HSiO1.5)n(n>8) polymer with different ratios of network-to-cagestructures (as shown in Fig. 9-c) [2, 3, 141]. During thepolymer synthesis process, the network-to-cagestructures of the obtained molecules can be controlled
by the experimental conditions, such as the species and/or amounts of chemical reagents and the reaction speed.
The higher network-to-cage degree and cross-linking
density exhibited by the (HSiO1.5)n polymer increased
the energy required for Si to diffuse across the oxide
matrix, resulting in the formation of smaller Si NCs.
Furthermore, the freestanding Si NCs exhibited sizedependent PL in accordance with the principles ofquantum confinement, with tunable PL across the visible
and near-infrared (NIR) spectra and QYs ranging from 3

to 10%.
In brief, to control the size of Si NCs in the
aforementioned process, two alternative methods wereadopted by precisely manipulating the temperature/time
during thermal pyrolysis and modifying the chemical
structure of the (HSiO 1.5)n polymer. The use of costeffective (HSiO 1.5) n polymers as precursors for the
production of Si–NCs is advantageous. However, therequirement for extremely high temperatures during
thermal pyrolysis and a continuous supply of H 2
throughout the pyrolysis and cooling processes are
significant drawbacks. Xu et al. demonstrated asustainable and green alternative to conventional thermaltransformations from (HSiO1.5)n polymers to functional
Si NCs via a room-temperature mechanochemical processwith zero H2 supply[67]. The size of the Si NCs (<10 nm)
embedded in the SiO2 matrix could be easily controlled
by varying the milling ball size (ZrO2, 3–15 mm) andmechanochemical duration (400 rpm, 1–3 hours). The
mechanism of the reaction from (HSiO1.5)n to Si NCs via
a mechanochemical route distinct from that ofconventional thermal pyrolysis is as follows (Fig. 10-a)[67]:1. Network structure breakage under mechanicalstress, Si–H bond cleavage accompanied by formation

Figure 9. (a) Photograph of emissive Si NCs passivated bydifferent functional groups (reprinted from ACS Nano, 2014,

Figure 10. (a)Transformations from HSQ polymer to emissive

8(9), 9636-9648. Copyright: © 2014 American Chemical

Si NCs via a room- temperature mechanochemical process

Society). (b) Photograph of emissive Si NCs passivated byreported by Xu et al. (b) PL of free-standing alkyl-Si NCs

different Halide groups (reprinted from Chem. Mater. 2015, 27,

synthesized from the mechanochemical process with milling4, 1153-1156. Copyright © 2015 American Chemical Society).

ball in diameter of 10 mm. (c) Photographs of alkyl-Si NCs(c) Size-controlled Si NCs synthesized using the structure ofdispersion under ambient light (left) and UV light with aselectively synthesized HSQ polymers (reprinted from Chem.

wavelength of 350 nm (right) (reprinted from J. Mater. Chem. C,

Lett. 2017, 46, 5, 699-702. Copyright: © 2017 The Chemical

2022, 10, 12588. Copyright: © The Royal Society of ChemistrySociety of Japan).

2022).
― 20 ―

Yuping Xu, Yunzi Xin, Takashi Shiraiof radicals and redistribution of Si–O–Si bonds.
2. H2 and Si nuclei generation.
3. Si nuclei diffusion and formation of Si nanodomains.
4. Particle growth and crystallization of Si nanodomains
by continuously induced mechanical energy.
5. Further particle growth. With the increased
reactivity of the additional polymers, the quantity
and size of the Si NCs increased because of theintensified formation of Si radicals and nuclei. The
amount of Si NCs was significantly increased bysubsequent mechanochemical treatments.
The author stated that to achieve an efficient reactionof the (HSiO1.5)n polymer, it is necessary to not only
induce sufficient mechanical impact energy (determinedby the size of the milling balls) but also frequentcollisions (dependent on the number of milling balls)
between the milling balls and the (HSiO1.5)n precursor.
The freestanding alkyl-passivated Si NCs exhibitedbright emission in visible light region with QY of 9.8%,
whose value is comparable to that synthesized by theconventional thermal pyrolysis of (HSiO1.5)n precursor(Figs. 10-b and c). Moreover, when processing sameamounts of (HSiO 1.5 ) n polymer, mechanochemicaltreatment consumed only one-fifteenth of the energy anda negligible amount of gas compared to hightemperature thermal pyrolysis (1100 ℃). This novel andfacile mechanochemical route opens new avenues forthe smart synthesis of functional nanomaterials.
3.2 Oxide-embedded and freestanding Si xGe 1-x
alloy NCs (64 < x < 100)

In 2001, Veinot et al. reported the preparation of
SixGe1-x alloy NCs using HSQ and soluble germanium
diiodide (GeI2) complexes[64]. Trialkylphosphine adducts
facilitate the formation of a homogenous precursor foroxide-embedded SixGe1-x NCs via thermal pyrolysis at
1100 ℃ in 1–9 hours. The freestanding passivated
SixGe1-x NCs (< 20 nm) exhibit bright NIR PL and QYs
of 16 ± 3%. The GeI2 precursor facilitated the nucleationof larger SixGe1-x NCs. A separate population of Si-rich
NCs is formed through matrix diffusion, and may also
incorporate low amounts of Ge through matrix diffusion
processes.
3.3 Boron (B)- and/or phosphorus (P)- doped SiNCs

For electronic applications, organic-functionalized Si
NC surfaces deteriorate the carrier transport of films
because of the increased NC-to-NC distance. To

overcome this problem, B-and P-doped all-inorganic Si
NCs were developed [65, 66]. Si NCs doped with B and/or
P were synthesized in glass matrices using annealing
solutions containing HSQ and dopant acids as illustrated
in Fig. 11. Subsequently, the NCs were extracted fromthe matrices via HF etching. The resulting free-standing
NCs exhibited dispersibility in methanol without
requiring any additional surface functionalization
processes because the hydrophilic shells were heavily
doped with B and P on the surface of the Si NCs. The
NCs exhibited size-tunable PL with high efficiency,
covering a broad range from the NIR to the visible
region.

Figure 11. (a) Preparation procedure of all-inorganic B- and Pdoped colloidal Si NCs. (Scale bars: 1 cm) (b) Schematic of thestructure of B-and P-doped crystalline Si core (reprinted from
Nanoscale, 2014, 6, 12354-12359. Copyright: © The Royal
Society of Chemistry 2014).

In summary, HSQ is a promising dielectric material

for use in integrated circuits owing to its low dielectric
constant, excellent gap fill and planarization properties,
and the capability to use standard spin-on production
techniques. However, its thermal stability is a significant
issue, as the leakage current and dielectric constantincrease rapidly at temperatures higher than 400 ℃.
Various methods have been proposed to enhance thethermal stability of HSQ, including ion implantation, H2
plasma pretreatment, and metal-composite barrier layer
deposition. Additionally, HSQ has also been used as aprotective coating and passivation layer in advanced
multiple-gate MOSFETs and thin-film transistor liquid
crystal displays, respectively. In lithography, HSQ has
been used as a standard industrial negative-tone resist
applicable for high-resolution and small linewidth
fluctuations in EBL, EUV lithography, nanoimprint
lithography, and step and flash imprint lithography. Sub10-nm isolated features are now frequently obtained in
HSQ and even 4.5 nm-half-pitch structures have beensuccessfully obtained. HSQ is used as a resist in several
applications, including the fabrication of MOSFET gateswith a 6 nm gate length, etching masks in bilayer― 21 ―

Applications of Hydrogen Silsesquioxane in Nanomanufacturing and Nanofabrication

lithographic processes, and imprinting templates forselective metal deposition, and step and flash imprint
lithography. In addition to nanomanufacturing forphotonic and electronic devices, HSQ has also been
extensively reported as a precursor for fabrication ofsemiconductor nanomaterials, such as Si NCs andSi xGe 1-x, at extremely high temperatures (≥1100 ℃)
under continuous supply of H2 and inert atmosphere.
Moreover, it is difficult to control the particle size by
precisely manipulating the temperature during thermal
pyrolysis and modifying the chemical structure of HSQ.
A sustainable and green alternative to conventional
thermal transformations from HSQ to functional Si NCs
via a room-temperature mechanochemical process withzero H2 supply was also demonstrated. Si NCs weresynthesized using different methods and their sizes andoptical properties were extensively studied. We
demonstrated that Si NCs with a quantum-confinementeffect-induced unique electronic structure and opticalproperties hold great promise for applications in nextg e n e r a t i o n o p t o e l e c t r o n i c d e v i c e s [57, 140, 142-144],
photovoltaic [145-147] , and in vivo and in vitro bioimaging[148-150]. Overall, HSQ is a versatile and valuable
material with a wide range of applications in
nanomanufacturing and nanofabrication. As technology
continues to advance, the use of HSQ is expected to
become increasingly important in the development of
new and innovative nanoscale devices and functional
semiconductor materials.

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