215
Original scientific paper
Journal of Microelectronics, 
Electronic Components and Materials
Vol. 54, No. 3(2024), 215 – 223
https://doi.org/10.33180/InfMIDEM2024.305
Preparation of Dielectric Layers for Applications 
in Digital Microfluidic Thermal Switches 
Blaž Velkavrh
1,2,3,*
, Urban Tomc
2
, Matej Šadl
1
, Victor Regis
1,3
, Maja Koblar
1
, Bianka Colarič
2
, 
Andrej Kitanovski
2
 and Hana Uršič
1,3,*
1
Jožef Stefan Institute, Electronic Ceramics Department, Ljubljana, Slovenia
2
University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia
3
Jožef Stefan International Postgraduate School, Ljubljana, Slovenia
Abstract: In this work, we prepared dielectric layers of three different dielectric materials – Al
2
O
3
, polyimide and epoxy-based 
photopolymer SU-8 and investigated their properties. Aerosol deposition method was used to prepare Al
2
O
3
 and polyimide layers, 
while spin-coating method was used for SU-8 layers. Microstructural analysis revealed dense layers with no anomalies. Temperature- 
and frequency-independent dielectric permittivity ε’ was observed for Al
2
O
3
 and SU-8 layers, while there was slight downside trend 
with increasing temperature for polyimide layers. According to Young-Lippmann equation of electrowetting on dielectric (EWOD) 
effect, Al
2
O
3
 is considered to be the best due to highest ε’ (~11) among all three materials, since it requires the lowest voltage to 
achieve certain droplet contact angle with EWOD.
Keywords: dielectric layers, aerosol deposition method, spin-coating method, microfluidics, thermal switch
Priprava dielektričnih plasti za uporabo v 
digitalnih mikrofluidnih toplotnih stikalih
Izvleček: V tem raziskavi smo pripravili dielektrične plasti iz treh dielektričnih materialov – Al
2
O
3
, poliimid in fotopolimer na osnovi 
epoksida SU-8 ter raziskali njihove lastnosti. Za pripravo Al
2
O
3
 in poliimidnih plasti je bila uporabljena metoda nanašanja v aerosolu, 
za plasti SU-8 pa metoda nanašanja z vrtenjem. Mikrostrukturna analiza je pokazala goste plasti. Pri plasteh Al
2
O
3
 in SU-8 smo opazili 
temperaturno in frekvenčno neodvisno dielektričnost ε’, medtem ko je pri poliimidnih plasteh viden rahlo padajoč trend z naraščajočo 
temperaturo. V skladu z Young-Lippmannovo enačbo učinka elektro-omočenja na dielektriku (EWOD) je Al
2
O
3
 zaradi najvišjega ε’ (~11) 
smatran za najbolj ustrezen material za uporabo v EWOD, saj zahteva najnižjo napetost za doseganje določenega kontaktnega kota 
kapljice z EWOD.
Ključne besede: Dielektrične plasti, metoda nanašanja v aerosolu, metoda nanašanja z vrtenjem, mikrofluidika, toplotno stikalo
* Corresponding Author’s e-mail: blaz.velkavrh@ijs.si, hana.ursic@ijs.si
How to cite:
B. Velkavrh et al., “Preparation of Dielectric Layers for Applications in Digital Microfluidic Thermal Switches", Inf. Midem-J. Microelectron. 
Electron. Compon. Mater., Vol. 54, No. 3(2024), pp. 215–223
1 Introduction
The manufacture of electronic, optical, and mechanical 
devices is experiencing a continuous trend of minia-
turization, making devices small and compact, as well 
as increasing their power density and efficiency. One 
of the main techniques for manufacturing miniatur-
ized electronic devices in large volumes is multilayer 
technology, where layered structures are deposited on 
a substrate/board. These structures are prepared with 
additive processes and can consist of several conduc-
tive, semiconductive, or insulating dielectric layers 
with a typical thickness above 1 μm. The layers can 
be manufactured with different methods, for example 
powder-based technologies like screen-printing [1–3] 
and aerosol deposition (AD) [4–6], or solution-based 
like spin-coating method [7–9].

216
B. Velkavrh et al.; Informacije Midem, Vol. 54, No. 3(2024), 215 – 223
Advances in miniaturization have opened new prob-
lems of thermal management in small devices. With 
high power densities of compact devices, conventional 
heat sinks in combination with fans, heat pipes, or wa-
ter cooling are insufficient to dissipate large amounts 
of heat to the ambient on a small scale. Potential so-
lutions to improve thermal management on a smaller 
scale include thermal control devices, one of which 
is a digital microfluidic thermal switch based on elec-
trowetting on dielectric (EWOD) effect [10–12]. Such a 
thermal switch requires a multilayer structure, consist-
ing of a dielectric layer sandwiched between two elec-
trode layers. The fabrication process of the dielectric 
layer has strong implications on its dielectric and ther-
mal properties, which are a crucial factor in the perfor-
mance of the thermal switch based on EWOD effect.
In this work, we investigated three different dielectric 
materials for EWOD applications. These three materials 
are alumina (Al
2
O
3
), polyimide, and epoxy-based pho-
topolymer. Al
2
O
3
 was chosen due to its high electrical 
insulation, chemical inertness, and good mechanical 
properties [13–16]. On the other hand, dielectric poly-
mers are low-cost materials with high electrical insula-
tion [17]. The dielectrics were prepared in layer forms 
using AD (Al
2
O
3
 and polyimide) or spin-coating method 
(epoxy-based photopolymer) and their impacts on the 
voltage-dependent droplet contact angle were esti-
mated by theoretical calculations.
2 Materials and methods
For preparation of dielectric layers, three different 
precursors were used, namely Al
2
O
3
 powder (A 16 SG, 
Almatis, Germany), polyimide powder (P84 NT, Evon-
ik, Germany) and epoxy-based photopolymer SU-8 
(GM1070, Gersteltec, Switzerland). Al
2
O
3
 and polyimide 
layers were prepared with the AD method, while epoxy-
based photopolymer SU-8 layers were prepared with 
the spin-coating method. For preparation of polyim-
ide and epoxy-based photopolymer SU-8 layers, both 
precursors were used as received, while Al
2
O
3
 powder 
needed a pre-treatment to achieve a high deposition 
rate and homogeneous microstructure without large 
pores as reported in [14, 18].
Raw Al
2
O
3
 powder was first thermally pre-treated in 
a chamber furnace (Custom-made, Terna, Slovenia) 
at 1150 °C for 1 h with 5 K min
-1
 heating and cooling 
rates, as suggested in [6, 14]. After thermal treatment, 
the powder was milled to obtain an appropriate parti-
cle size for AD, which is reported to be between 0.2 μm 
and 2 μm for the ceramic powders [4]. In our case, the 
d
50
 of the Al
2
O
3
 powder was 0.6 μm, as shown in Sup-
plementary material: Figure S1. The milling was per-
formed in a planetary mill (PM400, Retsch, Germany) at 
200 min
-1
 for 4 h, using yttria-stabilized zirconia milling 
balls with isopropanol as a liquid medium.
For the preparation of dielectric layers, different sub-
strates were chosen to optimize the deposition rate. 
Commercially available stainless-steel substrates (SS; 
no. 304, American Iron and Steel Institute) with a pol-
ished surface (A480: no. 8, American Society for Testing 
and Materials) were used for the ceramic Al
2
O
3
 dielec-
tric layers, as it had previously been shown that a high 
deposition rate of the ceramic powder can be achieved 
on these substrates [19, 20]. For polyimide and epoxy-
based photopolymer SU-8 layers, glass was used as a 
substrate. Cr/Au bottom electrodes with a thickness of 
∼100 nm were sputtered on the glass substrates by a 
magneton sputtering (Cinquepascal SRL, Italy). 
The AD equipment was provided by Invertech, Ger-
many. The process parameters during the AD for both 
Al
2
O
3
 and polyimide powders are gathered in Table 1.
For the spin-coating process, a spin-coater (WS-650MZ-
23NPPB, Laurell, USA) was used to prepare epoxy-
based photopolymer SU-8 layers. For better adhesion 
of epoxy-based photopolymer SU-8 on a gold-sput-
tered glass substrate, an adhesion promoter Omni-
Coat (G112850, Kayaku Advanced Materials, USA) was 
used. During the preparation process, samples were 
thermally treated with an electric heater (C-MAG HS 7, 
IKA, Germany), according to instructions in the techni-
cal datasheet of epoxy-based photopolymer SU-8 [21]. 
The deposition was performed once without any rep-
etition. The whole process is schematically presented 
in Figure 1.
Table 1: Aerosol deposition process parameters.
Process parameters Value
Pressure in aerosol chamber [mbar] <10
Nozzle slit size [mm
2
] (0.5 × 10) 
Carrier gas N
2
Gas flow rate [L min
-1
] 2 – 4
Nozzle-substrate 
distance [mm]
5
Sweep speed [mm s
-1
] 10
The thickness and root-mean-square roughness (R
q
) 
of the prepared layers were evaluated from line pro-
files, measured with a contact profilometer (DektakXT, 
Bruker, USA). Thickness was determined from the step 
height of the layer, while R
q
 was evaluated with filter-
ing the total profile using Gaussian regression with a 
cut-off 0.08 mm.

217
The topography images of the prepared dielectric 
layers were determined with the atomic force micro-
scopes (AFM; Jupiter XR and MFP 3D, Asylum Research 
AFM, Oxford Instruments, USA). Images were scanned 
in AC air topography mode using tetrahedral plati-
num-coated silicon tips (OMCL-AC240TM-R3, Olympus, 
Japan). Prepared sample surfaces and their polished 
cross-sections were further investigated with scanning 
electron microscope (SEM; Verios G4 HP , Thermo Fisher 
Scientific, USA). To analyse the layers in cross-section, 
the samples were prepared by cutting, mounting in 
epoxy resin (EpoFixKit, Struers, Denmark), grinding, 
and fine polishing using a colloidal SiO
2
 suspension 
(OP-S, Struers, Denmark).
For dielectric measurements, Au electrodes with a 
0.5 mm diameter were sputtered on the top surface of 
prepared dielectric layers by a magneton sputtering 
(Cinquepascal SRL, Italy). The temperature-dependent 
dielectric permittivity ε’ and dielectric losses tan( δ) 
were measured with Aixacct TF Analyzer 2000 (Aixacct 
Systems GMbH, Germany) and a HP 4284 A Precision 
LCR impedance meter (Hewlett-Packard, USA), using 
AC amplitude of 1 V at different frequencies during 
cooling in the temperature range from 100 °C to -30 °C. 
Theoretical voltage-dependent contact angles for a 
water droplet were calculated with a Young-Lippmann 
equation [11].
3 Results
Dielectric layers were prepared from ceramic Al
2
O
3
, 
polyimide and epoxy-based photopolymer SU-8. The 
microstructural and electrical properties are shown 
first. Later, to determine the influence of the dielectric 
layers on EWOD effect, the voltage-dependent contact 
angles for a water droplet were calculated.
3.1 Al
2
O
3
 layers prepared by the aerosol deposition 
method
Figure 2a shows a photograph of an Al
2
O
3
 layer on a 
stainless-steel substrate. AFM height and tapping 
amplitude images and SEM images in Figure 2b–2e 
revealed a layer surface with the root-mean-square 
roughness R
q
 ≈ 40 nm. The concave depressions com-
monly found in aerosol-deposited layers can be found 
in the AFM height image (Figure 2b). These surface 
characteristics are formed by collision of powder par-
ticles with the surface layer during the AD process, as 
discussed previously in [23]. SEM layer-surface images 
(Figure 2d and 2e) revealed small powder particles 
with a size in the range of nanometres as part of the 
Al
2
O
3
 layer surface. A comparison of the particle size of 
the Al
2
O
3
 powder before AD (Supplementary material: 
Figure S1) with the particles in the layers indicates that 
Al
2
O
3
 particles break during the AD process, as previ-
ously discussed in [6]. The cross-section SEM image in 
Figure 2f revealed a dense 4 μm-thick Al
2
O
3
 layer on a 
stainless-steel substrate. No large defects or pores are 
observed, similar to Al
2
O
3
 layers, previously prepared 
by the same procedure and deposited on gadolinium 
substrates, as reported in [6]. Temperature-dependent 
ε’ measurements are shown in Figure 3. The ε’ remains 
constant at ∼11, independent of both temperature and 
frequency. The tan( δ) slightly increases with increasing 
temperature but remains below 0.02 over the entire 
measurement range.
3.2 Polyimide layers prepared by the aerosol 
deposition method
Figure 4a shows a photograph of polyimide layer on a 
gold-sputtered glass substrate. AFM height and tap-
ping amplitude images and SEM images in Figure 4b–
4e revealed a rough layer surface with R
q
 ≈ 1.4 μm. The 
concave depressions commonly found in AD layers are 
also visible in this case (Figure 4b), but they are deeper 
than in Al
2
O
3
, resulting in a higher surface roughness. 
In SEM images of the polyimide layer surface (Figure 
4d and 4e), particles with a size of several tens to hun-
dreds of nanometres can be seen. In the SEM images of 
the polyimide powder before AD (Supplementary ma-
terial: Figure S2), a similar particle size was observed, 
Figure 1: Schematic presentation of epoxy-based pho-
topolymer SU-8 preparation process (after [21, 22]).
B. Velkavrh et al.; Informacije Midem, Vol. 54, No. 3(2024), 215 – 223

218
but these particles were mainly agglomerated. Similar 
particle size before and after AD indicates that particles 
were not heavily fractured during the AD process, in 
contrast to ceramic Al
2
O
3
 particles. During AD, the ag-
glomerates of polyimide particles break apart, while 
polyimide particles deform and stick together, result-
ing in the formation of dense polyimide layers. The 
cross-section SEM image of such a dense polyimide 
layer with a thickness d ≈ 19 μm is shown in Figure 4f. 
No large anomalies or pores are visible through the 
layer thickness. We can observe slight delamination of 
the bottom electrode from the glass substrate, which 
is caused by mechanical forces arising from the curing 
process of the polymer epoxy-resin EpoFixKit during 
the cross-section sample preparation. Temperature-
dependent ε’ measurements are shown in Figure 5. 
The graph shows a slight downward trend of ε’ with 
increasing temperature. No large frequency depend-
ence is observed. On average, ε’ remains at ∼5.5. The 
tan( δ) slightly decreases with the increasing tempera-
ture but remains below 0.03 over the entire measure-
ment range.
3.3 Epoxy-based photopolymer SU-8 layers prepared 
by spin-coating method
Figure 6a shows a photograph of epoxy-based pho-
topolymer SU-8 layer on a gold-sputtered glass sub-
strate. AFM height and tapping amplitude images as 
well as SEM surface images in Figure 6b–6e revealed 
a smooth layer surface with R
q
 ≈ 4 nm. While the SEM 
layer surface image at lower magnification (Figure 
6d) does not reveal any details, SEM image at higher 
magnification (Figure 6e) shows small particles of 
epoxy-based photopolymer SU-8 with a size in nano-
metre range. The SEM cross-section image in Figure 
6f revealed a dense layer with a thickness d ≈ 30 μm 
with no large anomalies or bubbles. Temperature-de-
pendent ε’ measurements are shown in Figure 7. The 
ε’ remains constant at ∼6.5, independent of both tem-
perature and frequency. The tan( δ) slightly decreases 
Figure 2: (a) Photograph of aerosol-deposited Al
2
O
3
 layer on stainless-steel substrate. AFM (b) height and (c) tapping 
mode amplitude images. SEM (d, e) surface and (f) cross-section images of the Al
2
O
3
 layer.
Figure 3: Temperature-dependent ε’ and tan( δ) of 
Al
2
O
3
 layer at different frequencies. The vertical black 
dashed arrow indicates increase in frequency.
B. Velkavrh et al.; Informacije Midem, Vol. 54, No. 3(2024), 215 – 223

219
with increasing temperature, similar as in the case of 
polyimide layers (Figure 5). However, it remains below 
0.05 over the entire measurement range. 
3.4 Water droplet contact angles on dielectric layers
The roughness R
q
 and dielectric permittivity ε’ of pre-
pared layers are collected in Table 2. Polyimide layers 
prepared with AD method have the highest R
q
, while 
epoxy-based photopolymer SU-8 layers, prepared with 
spin-coating method have the lowest R
q
 between all 
three different types of layers. In AD, powder particles 
collide with layer surface and form rougher surface in 
comparison to spin-coating method. When compar-
ing Al
2
O
3
 and polyimide layers, both prepared with 
AD method, huge difference in roughness can be ob-
served. While ceramic Al
2
O
3
 particles break during the 
AD process, forming the surface with lower roughness, 
polyimide particles deform and stick together, forming 
the surface with much higher roughness. The ceramic 
Al
2
O
3
 layers exhibited the highest dielectric permittiv-
ity ε’ compared to both polyimide and epoxy-based 
photopolymer SU-8 layers.
Table 2: The root-mean-square surface roughness R
q
 and 
dielectric permittivity ε’ of prepared dielectric layers.
Aerosol  
deposition
Spin-coating
Material Al
2
O
3
Polyimide SU-8
R
q
 [nm] 40 1400 4
ε’ [/] @ 10 kHz 11 5.5 6.5
Figure 8 shows theoretical voltage-dependent water 
droplet contact angles on Al
2
O
3
, polyimide and epoxy-
based photopolymer SU-8 layers. The voltage-depend-
ent droplet contact angles of the water droplet were 
Figure 4: (a) Photograph of aerosol-deposited polyimide layer on gold-sputtered glass substrate. AFM (b) height and 
(c) tapping mode amplitude images. SEM (d, e) surface and (f ) cross-section images of the polyimide layer. Please note 
that on panel (f), two white doted lines in the corners of the image between the polyimide layer and the epoxy resin 
EpoFixKit are only the guide for the eye.
Figure 5: Temperature-dependent ε’ and tan( δ) of pol-
yimide layer at different frequencies. The vertical black 
dashed arrow indicates increase in frequency. 
B. Velkavrh et al.; Informacije Midem, Vol. 54, No. 3(2024), 215 – 223

220
calculated according to Young-Lippmann equation 
[11]:

2 0
ew eq
lv
'
cos cos  ,
2
UU
d




 

  (1)
where ε’ was taken from Table 2. The θ
eq
 is the initial drop-
let contact angle, θ
ew
 a contact angle when electric field is 
applied, ε
0
 dielectric permittivity of a vacuum, 𝑈 voltage, 
d thickness of the dielectric layer and γ
lv
 surface tension 
of a liquid droplet. For a water droplet in air atmosphere 
at room temperature, γ
lv
 = 0.072 N m
-1
 was used [24]. 
An additional hydrophobic layer can be applied on the 
top of the dielectric layer to achieve high θ
eq
. Therefore, 
θ
eq 
= 120
o
 was used according to technical datasheet of 
fluoropolymer FluoroPel 1601V (Cytonix, USA), com-
monly used for EWOD applications [25]. Graphs in Fig-
ure 8 indicate Al
2
O
3
 to be the best choice between all 
three materials for EWOD applications, since it has the 
highest ε’, resulting in lower voltage required to ob-
tain certain contact angle at chosen layer thickness d. 
However, Young-Lippmann equation assumes smooth 
and ideally flat surfaces, but the roughness of the di-
electric layers also needs to be considered, since it in-
fluences the surface wettability – droplet contact angle 
[26]. In addition, the roughness also has the influence 
on the interface thermal resistances in the multilayer 
structure, which effects the heat transfer capabilities of 
digital microfluidic thermal switch based on EWOD ef-
fect. Therefore, SU-8 might also be appropriate due to 
lowest roughness, which would positively effect heat 
transfer capabilities of multilayer structure.
4 Conclusions
The microstructural and electrical properties of different 
dielectric layers were investigated and their influence on 
Figure 6: (a) Photograph of spin-coated epoxy-based photopolymer SU-8 layer on gold-sputtered glass substrate. 
AFM (b) height and (c) tapping mode amplitude images. SEM (d, e) surface and (f ) cross-section images of the epoxy-
based photopolymer SU-8 layer.
Figure 7: Temperature-dependent ε’ and tan(δ) of 
epoxy-based photopolymer SU-8 layer at different fre-
quencies. The vertical black dashed arrow indicates in-
crease in frequency.
B. Velkavrh et al.; Informacije Midem, Vol. 54, No. 3(2024), 215 – 223

221
the EWOD effect was determined. Al
2
O
3
 and polyimide 
layers were prepared with AD method, while epoxy-based 
photopolymer SU-8 was prepared with spin-coating 
method. Microstructural analysis revealed dense layers 
without any anomalies. Particle analysis indicates break-
ing of ceramic Al
2
O
3
 particles during the AD process. In 
the case of polyimide, big agglomerates, observed in raw 
powder, break apart during the AD process, while smaller 
polyimide particles deform and stick together. Dielectric 
measurements revealed temperature- and frequency-in-
dependent dielectric permittivity ε’ for Al
2
O
3
 and epoxy-
based photopolymer SU-8, while slight temperature de-
pendency of ε’ can be observed in polyimide. Highest 
dielectric permittivity ε’ between all three materials was 
measured in Al
2
O
3
 layers (ε’ ∼11), indicating Al
2
O
3
 as the 
optimal choice for EWOD application.
5 Supplementary material
Supplementary material available on the publisher’s 
web page contains:
- S1: Particle size distribution and SEM analysis of 
Al
2
O
3
 powder
- S2: Particle size distribution and SEM analysis of 
polyimide powder
6 Acknowledgments
The authors acknowledge financial support from the 
transnational consortium M-ERA.NET for the project 
Cool BatMan: Battery Thermal Management System 
Based on High Power Density Digital Microfluidic Mag-
netocaloric Cooling (No. 9400, Slovenian part of the 
project is financed by Ministry of Higher Education, Sci-
ence and Innovation). Authors also acknowledge the 
financial support of the Slovenian Research and Inno-
vation Agency for the research core fundings (No. P2-
0422, P2-0223, J2-1738-1 and P2-0105). The assistance 
of Jena Cilenšek, Val Fišinger, Brigita Kmet and Andreja 
Benčan is gratefully acknowledged.
7 Conflict of interest
The authors declare no conflict of interest. The found-
ing sponsors had no role in the design of the study; in 
the collection, analyses, or interpretation of data; in the 
writing of the manuscript, and in the decision to pub-
lish the results.
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