Preparation of dielectric layers for applications in digital microfluidic thermal switches

: In this work, we prepared dielectric layers of three different dielectric materials – Al 2 O 3 , polyimide and epoxy-based pho-topolymer 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.


Introduction
The manufacture of electronic, optical, and mechanical devices is experiencing a continuous trend of miniaturization, making devices small and compact, as well as increasing their power density and efficiency.One of the main techniques for manufacturing miniaturized 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 conductive, 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][2][3] and aerosol deposition (AD) [4][5][6], or solution-based, like spincoating method [7][8][9].
Advances in miniaturization have opened new problems of thermal management in small devices.With high power densities of compact devices, conventional heat sinks in combination with fans, heat pipes, or water cooling are insufficient to dissipate large amounts of heat to the ambient on a small scale.Potential solutions to improve thermal management on a smaller scale include thermal control devices, one of which is a digital microfluidic thermal switch based on electrowetting on dielectric (EWOD) effect [10][11][12].Such a thermal switch requires a multilayer structure, consisting of a dielectric layer sandwiched between two electrode layers.The fabrication process of the dielectric layer has strong implications on its dielectric and thermal properties, which are a crucial factor in the performance 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 photopolymer.Al 2 O 3 was chosen due to its high electrical insulation, chemical inertness, and good mechanical properties [13][14][15][16].On the other hand, dielectric polymers are low-cost materials with high electrical insulation [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 estimated by theoretical calculations.

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 (P84NT, Evonik, Germany) and epoxy-based photopolymer SU-8 (GM1070, Gersteltec, Switzerland).Al 2 O 3 and polyimide layers were prepared with the AD method, while epoxybased photopolymer SU-8 layers were prepared with the spin-coating method.For preparation of polyimide 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 particle 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 Supplementary material: Figure S1.The milling was performed 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 substrates were chosen to optimize the deposition rate.Commercially available stainless-steel substrates (SS; no.304, American Iron and Steel Institute) with a polished surface (A480: no. 8, American Society for Testing and Materials) were used for the ceramic Al 2 O 3 dielectric 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, Germany.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 epoxybased photopolymer SU-8 layers.For better adhesion of epoxy-based photopolymer SU-8 on a gold-sputtered glass substrate, an adhesion promoter OmniCoat (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 technical datasheet of epoxy-based photopolymer SU-8 [21].The deposition was performed once without any repetition.The whole process is schematically presented in Figure 1.
The thickness and root-mean-square roughness (R q ) of the prepared layers were evaluated from line profiles, measured with a contact profilometer (DektakXT, Bruker, USA).Thickness was determined from the step height of the layer, while R q was evaluated with filtering the total profile using Gaussian regression with a cut-off 0.08 mm.
The topography images of the prepared dielectric layers were determined with the atomic force microscopes (AFM; Jupiter XR and MFP 3D, Asylum Research AFM, Oxford Instruments, USA).Images were scanned in AC air topography mode using tetrahedral platinumcoated 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 crosssection, 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].

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.

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 commonly found in aerosol-deposited layers can be found in the AFM height image (Figure 2b).These surface characteristics are formed by collision of powder particles with the surface layer during the AD process, as discussed previously in [23].SEM layer-surface images (Figure 2d

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 tapping amplitude images and SEM images in Figure 4b-Figure 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 Figure 4e), particles with a size of several tens to hundreds of nanometres can be seen.In the SEM images of the polyimide powder before AD (Supplementary material: Figure S2), a similar particle size was observed, 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

Epoxy-based photopolymer SU-8 layers prepared by spin-coating method
Figure 6a shows a photograph of epoxy-based photopolymer SU-8 layer on a gold-sputtered glass substrate.AFM height and tapping amplitude images as well as SEM surface images in Figure 6b-Figure 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 epoxybased photopolymer SU-8 with a size in nanometre 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-dependent ε' measurements are shown in Figure 7.The ε' remains constant at ∼6.5, independent of both temperature and frequency.The tan(δ) slightly decreases with increasing temperature, similar as in the case of polyimide layers (Figure 5).However, it remains below 0.05 over the entire measurement range.

Water droplet contact angles on dielectric layers
The roughness R q and dielectric permittivity ε' of prepared 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 comparing Al 2 O 3 and polyimide layers, both prepared with AD method, huge difference in roughness can be observed.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 permittivity 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.

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 epoxybased photopolymer SU-8 layers.The voltage-dependent droplet contact angles of the water droplet were calculated according to Young-Lippmann equation [11]: where ε' was taken from Table 2.The  eq is the initial droplet contact angle,  ew a contact angle when electric field is applied,  0 dielectric permittivity of a vacuum,  voltage, d thickness of the dielectric layer and  lv surface tension of a liquid droplet.For a water droplet in air atmosphere at room temperature,  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   eq .Therefore,  eq = 120° was used according to technical datasheet of fluoropolymer FluoroPel 1601V (Cytonix, USA), commonly used for EWOD applications [25].Graphs in Figure 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 obtain certain contact angle at chosen layer thickness d.However, Young-Lippmann equation assumes smooth and ideally flat surfaces, but the roughness of the dielectric layers also needs to be considered, since it influences the surface wettabilitydroplet 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 effect.Therefore, SU-8 might also be appropriate due to lowest roughness, which would positively effect heat transfer capabilities of multilayer structure.

Conclusions
The microstructural and electrical properties of different dielectric layers were investigated and their influence on 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 breaking 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-independent dielectric permittivity ε' for Al 2 O 3 and epoxy-based photopolymer SU-8, while slight temperature dependency 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.

Figure 2 :
Figure2ashows a photograph of an Al 2 O 3 layer on a stainless-steel substrate.AFM height and tapping amplitude images and SEM images in Figure2b-2e revealed a layer surface with the root-mean-square roughness R q ≈ 40 nm.The concave depressions commonly found in aerosol-deposited layers can be found in the AFM height image (Figure2b).These surface characteristics are formed by collision of powder particles with the surface layer during the AD process, as discussed previously in[23].SEM layer-surface images (Figure2d 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

Figure 4 :
Figure4ashows a photograph of polyimide layer on a gold-sputtered glass substrate.AFM height and tapping amplitude images and SEM images in Figure4b-Figure4erevealed a rough layer surface with R q ≈ 1.4 μm.The concave depressions commonly found in AD layers are also visible in this case (Figure4b), but they are deeper than in Al 2 O 3 , resulting in a higher surface roughness.In SEM images of the polyimide layer surface (Figure4dand Figure4e), particles with a size of several tens to hundreds of nanometres can be seen.In the SEM images of the polyimide powder before AD (Supplementary material: FigureS2), a similar particle size was observed, 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 agglomerates of polyimide particles break apart, while polyimide particles deform and stick together, resulting 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 Figure4f.No large anomalies or pores are visible through the

Figure 3 :
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.

Figure 5 :
Figure 5: Temperature-dependent ε' and tan(δ) of polyimide layer at different frequencies.The vertical black dashed indicates increase in frequency.

Figure 6 :
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 epoxybased photopolymer SU-8 layer.

Figure 7 :
Figure 7: Temperature-dependent ε' and tan(δ) of epoxy-based photopolymer SU-8 layer at different frequencies.The vertical black dashed arrow indicates increase in frequency.

Figure 8 :
Figure 8: Theoretically calculated voltage-dependent contact angles for water droplets on (a) Al 2 O 3 , (b) polyimide and (c) epoxy-based photopolymer SU-8 layers at different dielectric layer thicknesses.