Surface Modification of Electroosmotic Silicon Microchannel ... - MDPI

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Surface Modification of Electroosmotic Silicon Microchannel Using Thermal Dry Oxidation Tuan Norjihan Tuan Yaakub 1,2 , Jumril Yunas 1, * ID , Rhonira Latif 1 , Azrul Azlan Hamzah 1 , Mohd Farhanulhakim Mohd Razip Wee 1 and Burhanuddin Yeop Majlis 1 1

2

*

Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia (UKM), 43600 UKM Bangi, Selangor, Malaysia; [email protected] (T.N.T.Y.); [email protected] (R.L.), [email protected] (A.A.H.); [email protected] (M.F.M.R.W.); [email protected] (B.Y.M.) Department of Electronics Engineering, Faculty of Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia Correspondence: [email protected]; Tel.: +60-3-8911-8541

Received: 30 March 2018; Accepted: 4 May 2018; Published: 7 May 2018

Abstract: A simple fabrication method for the surface modification of an electroosmotic silicon microchannel using thermal dry oxidation is presented. The surface modification is done by coating the silicon surface with a silicon dioxide (SiO2 ) layer using a thermal oxidation process. The process aims not only to improve the surface quality of the channel to be suitable for electroosmotic fluid transport but also to reduce the channel width using a simple technique. Initially, the parallel microchannel array with dimensions of 0.5 mm length and a width ranging from 1.8 µm to 2 µm are created using plasma etching on the 2 cm × 2 cm silicon substrate . The oxidation of the silicon channel in a thermal chamber is then conducted to create the SiO2 layer. The layer properties and the quality of the surface are analyzed using scanning electron microscopy (SEM) and a surface profiler, respectively. The results show that the maximum oxidation growth rate occurs in the first 4 h of oxidation time and the rate decreases over time as the oxide layer becomes thicker. It is also found that the surface roughness is reduced with the increase of the process temperature and the oxide thickness. The scallop effect on the vertical wall due to the plasma etching process also improved with the presence of the oxide layer. After oxidation, the channel width is reduced by ~40%. The demonstrated method is suggested for the fabrication of a uniform channel cross section with high aspect ratio in sub-micro and nanometer scale that will be useful for the electroosmotic (EO) ion manipulation of the biomedical fluid sample. Keywords: surface modification; electroosmotic flow; microfluidic; silicon nanochannel; thermal oxidation

1. Introduction The past decade has seen the rapid advancement of microfluidic chip development. The main reason is because of the fluid flow characteristics in the micrometer structure that allows for an extremely slow fluid movement, less sample volume consumption and precision in fluid control. In microfluidic systems, the electroosmotic device (EO) becomes one of the most important parts in which the fluid can be manipulated by the electric potential between inlet and outlet. EO flow mechanism in a microchannel has been used for numerous microfluidic applications, including for fluids transport and manipulation [1], charge separation and ion transport [2] and fluids pumps [3]. As an example, the EO flow mechanism has been applied to transport fluids as in electroosmotic pump (EOP) [4]. The system was capable of generating a maximum flow rate of 15 µL/min, a significant amount of flow rate per device volume for a microfluidic system. The microchannel width in the fabricated pump is much greater than the Micromachines 2018, 9, 222; doi:10.3390/mi9050222

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height, to ease the fabrication process. Such designs suffer from clogging due to structure collapsing after bonding process for microsystem integration. Therefore, a microchannel structure for EO device requires a sufficient high channel aspect ratio (H/W >> 1) to create a uniform electric field distribution along the channel that induces a stable electroosmotic flow. For ion separation purposes, a small channel cross section is necessary due to stronger ion interaction between the charged system (ionic solution and charged channel surface) [5]. Microfluidic channels are typically fabricated in glass, silicon and polymeric substrate. The fabrication procedures of polymeric material like Polydimethylsiloxane (PDMS) and Poly (methyl methacrylate) (PMMA) offer a significantly lower cost and ease fabrication procedures [6,7]. Despite this, the polymeric based materials have a lower wall zeta potential and heat dissipation rates as compared to glass because glass has superior chemical properties for surface electrostatics reactions. Furthermore, a few methods to modify the PDMS and PMMA based microchannel surface have been previously reported. For instance, the plasma-based technique to tune the wettability of the surface has been demonstrated [8,9], indicating that a surface modification is preferable for improving the quality of the channel surface. Glass also has an excellent dielectric property that is important for electroosmotic flow [10]. However, the glass fabricating technique is a major drawback due to its low etching rate, which must be considered when designing a glass based EO microfluidic device. On the other hand, silicon material has been, for many years, established as the material to employ for a wide range of microelectronics devices and applications. A silicon microchannel has similar electrical properties to a glass substrate and it can be fabricated using the well-established integrated circuit (IC) fabrication technology [11]. Reactive ion etching using SF6 plasma is widely used to create uniform micro-channels with a vertical wall for microfluidic devices but the wall quality is poor due to the scallop effect, resulting from the repetitive alternating phase of passivation gas (C4 F8 ) deposition and the etching gas (SF6 ) on the targeted wall. Some work on reducing the scallop effects by optimizing the deep reactive ion etching (DRIE) parameter has been reported. However, this complicated studies, due to the additional gas composition [12], an optimum etch and passivation cycle time [13] as well as the controlled flow rates of the etching/passivation ratio [14]. In this paper, we report the implementation of the dry oxidation process to grow an oxide layer on a prefabricated silicon based microfluidic channel in order to improve its surface quality [15] and at the same time to increase the channel aspect ratio by reducing the channel dimension down to the nanoscale for ion transport purposes. This method is also preferable because it is simple, compatible with the complementary metal-oxide-semiconductor (CMOS) process and eliminates the complex nanolithography process. 2. Microchannel Electroosmotic System Design The principle of electroosmotic ion transport is based on the spontaneous reaction of a solid surface in contact with an ionic solution. For a negative surface charge, an accumulation of positive charge forms a stern layer in an electrical double layer (EDL) on the capillary wall. Therefore, the EO flow mechanism is produced by way of the induction principle of cations in the diffuse layer in the electric double layer (EDL), which move with the presence of the electric field. The cations migration will drag the bulk fluid in the capillary towards the negative potential in the flat flow profile. The microfluidic EO system consists of several surface–modified microfluidic channels in parallel array with an SiO2 coating with a length of 500 µm that is fabricated on a 390 µm thick silicon wafer . The channel shape is rectangular with a width smaller than 1 µm and a depth of about 2 µm. A microchannel input (Inlet A—Outlet 1), having a channel geometry of 200 µm width and 20 mm length are integrated with the SiO2 coated silicon microchannel array. On the other end of the surface modified channel, the outlet channel (outlet 2 with similar dimensions to the inlet channel is integrated. The inlet port contains buffer and ionic solution that will be transported through the microchannel array towards the outlet reservoir by electroosmotic flow. For that purpose, a voltage potential V is applied across the microchannel array to create an electric field as shown in Figure 1.

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Figure1.1. Themicrochannel microchannelelectroosmotic electroosmotic system. Figure Figure 1.The The microchannel electroosmoticsystem. system.

In Inthis this work, work,the theSiO SiO222 coated coated silicon silicon microchannel microchannel reduces reduces not not only only the the channel channel cross-section cross-section In this work, the SiO coated silicon microchannel reduces not only the channel cross-section dimension but also improves the surface quality. To create micron size rectangular microchannels, dimension but also improves the surface quality. To create micron size rectangular microchannels, dimension but also improves the surface quality. To create micron size rectangular microchannels, we weutilized utilizedstandard standardphotolithography photolithographyand andthe thereactive reactiveion ionetching etchingtechnique. technique.The The thermal oxidation we utilized standard photolithography and the reactive ion etching technique. Thethermal thermaloxidation oxidation was performed under atmospheric pressure with a constant O 2 stream flow of 2000 mL/min at was performed under atmospheric pressure with a constant O 2 stream flow of 2000 mL/min at various was performed under atmospheric pressure with a constant O2 stream flow of 2000 mL/min atvarious various process temperatures. Through the conducted procedures, a uniform oxide growth on both vertical process temperatures. temperatures. Through conducted procedures, uniform oxide oxide growth growth on on both both vertical vertical process Through the the conducted procedures, aa uniform and andhorizontal horizontalsurfaces surfacesand andSiO SiO222rectangular rectangular channels of uniform size and shape in parallelarray array and horizontal surfaces and SiO rectangularchannels channelsof ofaaauniform uniformsize sizeand andshape shapein inparallel parallel array are produced. are produced. are produced. As Asshown shownin inFigure Figure2, 2,the theoxidation oxidationprocess processshould shouldproduce producean anapproximately approximatelyuniform uniformoxide oxide As shown in Figure 2, the oxidation process should produce an approximately uniform oxide growth rate on both the horizontal and vertical silicon walls. Hence, accurate control of the depth to growth rate on both the horizontal and vertical silicon walls. Hence, accurate control of the depth to growth rate on both the horizontal and vertical silicon walls. Hence, accurate control of the depth width ratio (H/W) of the microchannels can be achieved after the oxidation process. The final width ratio (H/W) of the microchannels can be achieved after the oxidation process. The final to width ratio (H/W) of the microchannels can be achieved after the oxidation process. The final dimension dimensionwidth widthof ofthe theelectroosmotic electroosmoticchannel channelisis isin inthe therange rangebelow below111µm µm withaaahigher higheraspect aspect ratio. dimension width of the electroosmotic channel in the range below µmwith with higher aspectratio. ratio.

HH W W

Figure Figure2.2.Mechanism Mechanismof ofdry dryoxidation oxidationon onsilicon siliconmicrochannel. microchannel. Figure 2. Mechanism of dry oxidation on silicon microchannel.

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Channel System System 3. Fabrication Process of Electroosmotic Device (EO) Channel 3.1. Fabrication of Silicon Microchannel Using Plasma Etching Etching Procedure Procedure Figure Initially, a set of electroosmotic Figure 3 shows the process flow to create the Si-microchannel. Si-microchannel. Initially, microchannel fabricated using the standard photolithography and deep ion etching microchannelarrays arrayswas was fabricated using the standard photolithography andreactive deep reactive ion (DRIE) on the silicon wafer . The parallel microchannels were designed to have a length of 0.5 mm etching (DRIE) on the silicon wafer . The parallel microchannels were designed to have a length and a variation widths from 1.8 µmfrom to 2 1.8 µm.µm Thetomicrochannel was formed was in a formed straight in line and the of 0.5 mm and aofvariation of widths 2 µm. The microchannel a straight distance eachbetween channel was to 5 µm. line and between the distance eachset channel was set to 5 µm. The microchannel was transferred ontoonto a cleaned silicon silicon surface surface by exposing the substrate microchanneldesign design was transferred a cleaned by exposing the coated withcoated a positive under UV-light under exposure. To pattern a vertical on the substrate withphotoresist a positive photoresist UV-light exposure. Tomicrochannel pattern a vertical silicon substrate, the high-density reactive SF6 etching, with passivation gas Cwith microchannel on we the employed silicon substrate, we employed the ion high-density reactive ion SF6 etching, 4 H8 and O2 at cryogenic with an approximate 1 µm per minute. After DRIE passivation gas C4Htemperature 8 and O2 at cryogenic temperatureetch withrate an of approximate etch rate of 1the µm per process, mask wasprocess, removedthe andmask the geometries of the microchannels were observed minute. the After theresist DRIE resist was removed and thearray geometries of the using a scanning electron (SEM). microchannels array weremicroscope observed using a scanning electron microscope (SEM).

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(c)

(b)

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Figure 3. 3. The diagram of of silicon silicon microchannel microchannel fabrication fabrication process (a) Photoresist Photoresist as as etching etching mask mask Figure The diagram process (a) coating; (b) Transfer pattern using photolithography; (c) Pattern development using resist developer; coating; (b) Transfer pattern using photolithography; (c) Pattern development using resist developer; (d) Si Si DRIE DRIE along along the the unprotected unprotected microchannel microchannellines lines& &photoresist photoresistremoval. removal. (d)

3.2. Surface Modification of Fabricated Silicon Microchannel Using Thermal Oxidation 3.2. Surface Modification of Fabricated Silicon Microchannel Using Thermal Oxidation The surface modification of the prefabricated silicon microchannel was done using an oxidation The surface modification of the prefabricated silicon microchannel was done using an oxidation process in a high thermal ambience. Basically, the oxide was grown on the heated silicon surface at a process in a high thermal ambience. Basically, the oxide was grown on the heated silicon surface at very high temperature between 800–1200 °C with the presence of oxygen gas flow or water vapor, a very high temperature between 800–1200 ◦ C with the presence of oxygen gas flow or water vapor, referring to dry and wet oxidation respectively. In this work, we created the SiO2 microchannel based referring to dry and wet oxidation respectively. In this work, we created the SiO2 microchannel based on the oxidation of silicon in dry flowing oxygen at atmospheric pressure in three different process on the oxidation of silicon in dry flowing oxygen at atmospheric pressure in three different process temperatures, 990 °C, 1020 °C and 1040 °C. The formation of silicon dioxide layer on silicon channels temperatures, 990 ◦ C, 1020 ◦ C and 1040 ◦ C. The formation of silicon dioxide layer on silicon channels was described by the chemical reaction below: was described by the chemical reaction below: Si + O2 = SiO2 (1) Si + O2 = SiO2 (1) In dry thermal oxidation, O2 molecules diffuse through the surface oxide layer and react with the silicon atom at oxidation, the Si-SiOO 2 interface. The resulting SiO2 surface layer is not coplanar with the In dry thermal 2 molecules diffuse through the surface oxide layer and react with the original silicon surface, which means 0.44d of silicon consumed fornot a thickness of d the oxide layer silicon atom at the Si-SiO2 interface. The resulting SiO2 is surface layer is coplanar with original growthsurface, in the thermal dry oxidation silicon which means 0.44 d of process. silicon is consumed for a thickness of d oxide layer growth in the Before starting the oxidation procedure, the etched sample was cleaned in acetone and a thermal dry oxidation process. methanol ultrasonic bath for five minutes each followed by was rinsing the in substrate DIa water for Before starting the oxidation procedure, the etched sample cleaned acetone in and methanol 1 min. Thebath sample was then soaked in 10%by HFrinsing solution 60 s toineliminate the1undesired native ultrasonic for five minutes each followed the for substrate DI water for min. The sample oxide layer before it was rinsed again in DI water and dried with nitrogen. Then, the samples were placed in the ceramic boat and were put at the mouth of the oxidation glass tube furnace that had previously been filled up with nitrogen (N2) gas at a 2000 mL/min stream


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was then soaked in 10% HF solution for 60 s to eliminate the undesired native oxide layer before it was Micromachines x 5 of 9 rinsed again2018, in DI9, water and dried with nitrogen. Micromachines 2018, 9, x 5 of 9 Then, the samples were placed in the ceramic boat and were put at the mouth of the oxidation rate. The resistance heater on the tube furnace was then heated up according to the targeted process glass tube furnace that had previously been filled up with nitrogen (N2 ) gas at a 2000 mL/min stream rate. The resistance heater on the tube furnace was then uptemperature, according tothe thesample targeted process temperature. After the furnace temperature reached the heated required boat was rate. The resistance heater on the tube furnace was then heated up according to the targeted process temperature. thezone, furnace temperature reached temperature, the sample boat was pushed to theAfter heated which is at the center of the required furnace tube. The oxidation process began temperature. After the furnace temperature reached the required temperature, the sample boat was pushed the heated zone, which is at thestarted centertoofflow the furnace Thegas oxidation began after we to stopped the supplied N2 gas and the driedtube. oxygen into theprocess tube furnace pushed to the heated zone, which is at the center of the furnace tube. The oxidation process began after after we stopped flow the supplied N2 gas and started to flow the driedwas oxygen gas intounder the tube furnace under a constant rate of 2000 mL/min. The oxidation process maintained a constant we stopped the supplied N2 gas and started to flow the dried oxygen gas into the tube furnace under under a flow constant rate of 2000 pressure mL/min. from The oxidation process maintained under a constant oxygen rate flow at atmospheric 4 to 12 h. Figurewas 4 shows the oxidation furnace a constant flow rate of 2000 mL/min. The oxidation process was maintained under a constant oxygen oxygen flow rate at atmospheric pressure from 4 toscheme 12 h. Figure shows abnormality the oxidation furnace apparatus for thermal dry oxidation. A uniform color with no4 surface appeared flow rate at atmospheric pressure from 4 to 12 h. Figure 4 shows the oxidation furnace apparatus for apparatus for thermal dry oxidation. color conditions. scheme withThe no surface abnormality on the oxidized sample’s surface at Aalluniform experiment morphology of SiOappeared 2 growth thermal dry oxidation. A uniform color scheme with no surface abnormality appeared on the oxidized on the oxidized sample’s all experiment conditions. morphology of SiOwhile 2 growth surface on the sample wassurface scannedatusing F50 Thin Film MetricsThe at 20 different points the sample’s surface at all experiment conditions. The morphology of SiO2 growth surface on the sample surface on the scanned using F50 Thin Film at 20 different points while the cross-section of sample the SiO2was microchannel was verified using theMetrics scanning electron microscope. was scanned using F50 Thin Film Metrics at 20 different points while the cross-section of the SiO2 cross-section of the SiO2 microchannel was verified using the scanning electron microscope. microchannel was verified using the scanning electron microscope.

Figure 4. Schematic diagram of oxidation furnace. Figure4.4.Schematic Schematicdiagram diagramof ofoxidation oxidationfurnace. furnace. Figure

4. Results and Discussion 4. Results Results and and Discussion Discussion 4. 4.1. SiO2 Layer Thickness and Oxidation Growth Rate Thickness and and Oxidation Oxidation Growth Growth Rate Rate 4.1. SiO SiO22 Layer Thickness 4.1. The average thickness of the oxide layer on the silicon microchannel was highly controlled by Theaverage average thickness theoxide oxide layer silicon microchannel was highly controlled by The thickness ofofthe layer onon thethe microchannel wasthickness highly controlled by the the process temperature and the oxidation time. Asilicon plot of average oxide against time at the process temperature and the oxidation time. of time average oxide against time at process temperature and the oxidation time. A plot A of plot average oxide thickness time various various process temperatures is shown in Figure 5. In the range of 4 thickness toagainst 12 h for all at oxidation varioustemperatures processthe temperatures is Figure shown 5.found In theto time of all 4 to h fortemperatures, all oxidation process is shown layer in 5.inInFigure thewas time range of 4increase torange 12 h for oxidation temperatures, oxidation thickness with the12oxidation time. In temperatures, the thickness oxidation layer thickness was found to the increase with the oxidation time. In the oxidation was found to increase with the oxidation time. In addition, higher the addition, thelayer higher the process temperature, the thicker oxide layer will bethe grown. This addition, theofhigher thethicker process temperature, thebethicker oxide will be grown. This process temperature, the oxide grown.the This relationship of with temperature and relationship temperature andthe time forlayer the will oxide thickness was welllayer agreed the finding relationship of temperature and time for with the perceived oxide thickness wasinwell agreed[16]. with the finding time for the thickness well agreed the finding reported Reference From the plot, reported in oxide Reference [16].was From the plot, we the maximum thickness of 520 nm oxide reported in Reference From the of plot, theatmaximum thickness of 520 nm oxide we perceived the 520 we nm perceived oxide grown 1040 ◦ C for 12 h of oxidation time. grown at 1040 °Cmaximum for 12[16]. h of thickness oxidation time. grown at 1040 °C for 12 h of oxidation time.

Figure 5. 5. Oxide Oxide thickness thickness against against oxidation oxidation time time of of thermal thermaldry dryoxidation oxidationfor forsilicon silicon. . Figure Figure 5. Oxide thickness against oxidation time of thermal dry oxidation for silicon .

The oxide thickness was measured at intervals after 4, 6, 8, 10 and 12 h of the oxidation process. The oxide thickness wasobserved measured atthe intervals after 4, 6, 8, 10 time and 12 of process the oxidation process. The highest growth rate was for first 4 h of oxidation forhall temperatures. The growth highest rate growth rate was observed fortemperatures the first 4 h ofisoxidation for all process temperatures. The at all conducted process presentedtime in Figure 6. It is shown that for Thefirst growth rate at all conducted process temperatures presented in Figure It is that for the 4 h of oxidation, the maximum growth rate wasisup to 72 nm/h at 10406.°C. Asshown the oxidation the first 4 h of oxidation, the maximum growth rate was up to 72 nm/h at 1040 °C. As the oxidation


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The oxide thickness was measured at intervals after 4, 6, 8, 10 and 12 h of the oxidation process. The highest growth rate was observed for the first 4 h of oxidation time for all process temperatures. Micromachines 2018, 9, x 6 of 9 The growth rate Micromachines 2018,at 9, xall conducted process temperatures is presented in Figure 6. It is shown that for6the of 9 ◦ first 4 h of oxidation, the maximum growth rate was up to 72 nm/h at 1040 C. As the oxidation time time increased, we observed a consistent decrease of growth rate. The same trend was observed for time increased, we observed a consistent decrease of growth rate.same The trend same was trendobserved was observed for increased, we observed a consistent decrease of growth rate. The for other other process temperatures◦ of 1020 °C◦ and 990 °C. The decreasing growth rate by time can be other process temperatures 1020990 °C C. and 990 °C. The growth decreasing growth ratebebyexplained time candue be process temperatures of 1020 of C and The decreasing rate by time can explained due to a lower supply of oxygen atoms at the silicon interface as the thickness of the oxide explained due to aoflower supply atoms at the silicon of increases. the oxide to a lower supply oxygen atomsofatoxygen the silicon interface as the interface thicknessas of the the thickness oxide layer layer increases. We also found that the maximum oxide growth rate was 33% higher with an increase ◦ C in layer increases. We also found that oxide the maximum oxidewas growth waswith 33%an higher withof an50 increase We also found that the maximum growth rate 33% rate higher increase of 50 °C in the process temperature for 12 h of oxidation time. This finding proved the strong relation of 50 °C in the process temperature 12 h of time. oxidation This finding proved strongbetween relation the process temperature for 12 h of for oxidation This time. finding proved the strongthe relation between the temperature and the oxide growth rate as the oxidant diffusivity increased with the between the temperature and the oxide growth as the oxidant diffusivity increased with the the temperature and the oxide growth rate as therate oxidant diffusivity increased with the increasing increasing temperature [11,16,17]. increasing temperature temperature [11,16,17]. [11,16,17].

Figure 6. Oxide growth rate of thermal dry oxidation for silicon . Figure Figure6.6.Oxide Oxidegrowth growthrate rateof ofthermal thermaldry dryoxidation oxidationfor forsilicon silicon. .

4.2. Surface Uniformity 4.2. Surface Surface Uniformity Uniformity 4.2. The thickness of the oxide layer on the sample surface at all oxidation durations was found to Thethickness thickness oxide onsample the sample at all oxidation to The of of thethe oxide layerlayer on the surfacesurface at all oxidation durationsdurations was foundwas to befound uniform be uniform across a 2 cm × 2 cm sample area. The thickness measurements were characterized using be uniform across a 2 cm × 2 cm sample area. The thickness measurements were characterized using across a 2 cm × 2 cm sample area. The thickness measurements were characterized using F50 Filmetrics at F50 Filmetrics at 20 random scanned points across the samples. The measurement principle was F50 Filmetrics at 20 random points the samples. The measurement principle was 20 random scanned points acrossscanned the samples. Theacross measurement principle was based on the light scattering based on the light scattering effect of the incident light normal to the thin film surface. basedofon light scattering effect ofthin the incident light normal to the thin film surface. effect thethe incident light normal to the film surface. Figure 7 shows the surface roughness, indicating the uniformity of the oxide layer thickness that Figure 7 shows the uniformity of the oxide layer thickness that Figure shows the thesurface surfaceroughness, roughness,indicating indicating the uniformity of the oxide layer thickness is calculated as the difference between the maximum and minimum thickness of the grown oxide is calculated as the between the maximum and minimum thickness of theofgrown oxide that is calculated asdifference the difference between the maximum and minimum thickness the grown layer divided by the average thickness at every oxidation temperature. In general, the oxide thickness layer divided by the by average thickness at everyatoxidation temperature. In general, oxidethe thickness oxide layer divided the average thickness every oxidation temperature. In the general, oxide measurement showed that the oxide layer was grown with the lowest roughness as the oxidation measurement showed that the oxide layer was layer grown with the lowest roughness as the oxidation thickness measurement showed that the oxide was grown with the lowest roughness as the temperature increased. The surface roughness of 2.03% was observed for the oxidation temperature temperature increased. increased. The surfaceThe roughness 2.03% was forobserved the oxidation temperature oxidation temperature surface of roughness of observed 2.03% was for the oxidation of 1040 °C. The oxide surface roughness also improved when we prolong the oxidation time to 12 h. of 1040 °C. The oxide◦ C. surface roughness improved when we prolong time to 12 h. temperature of 1040 The oxide surfacealso roughness also improved when the we oxidation prolong the oxidation The longer oxidation time allows for a thicker oxide layer growth with better uniformity of the The to longer allowstime for allows a thicker layer growth with better of the time 12 h. oxidation The longertime oxidation for aoxide thicker oxide layer growth withuniformity better uniformity microchannel surface. microchannel surface. of the microchannel surface.

Figure Figure7.7.Surface Surfaceroughness roughnessof ofoxide oxidelayer layerfor forvarious variousprocess processtemperature. temperature. Figure 7. Surface roughness of oxide layer for various process temperature.

Figure 8 shows the SEM observations on the EO channel before and after oxidation process. The Figure 8 shows the SEM observations on the EO channel before and after oxidation process. The scallop effect becomes a major challenge for high aspect ratio microchannel fabrication using DRIE scallop effect becomes a major challenge for high aspect ratio microchannel fabrication using DRIE [18]. The scallop roughness, as shown in Figure 8a, was not desirable especially for an electroosmotic [18]. The scallop roughness, as shown in Figure 8a, was not desirable especially for an electroosmotic microfluidic system because it can create unnecessary flow characteristics and induce unwanted microfluidic system because it can create unnecessary flow characteristics and induce unwanted


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Figure 8 shows the SEM observations on the EO channel before and after oxidation process. The scallop effect becomes a major challenge for high aspect ratio microchannel fabrication using DRIE [18]. The scallop roughness, as shown Micromachines 2018, 9, x in Figure 8a, was not desirable especially for an electroosmotic microfluidic system 7 of 9 Micromachines 9, xunnecessary flow characteristics and induce unwanted pressure inside the microchannel. 7 of 9 because it can2018, create On top of that, thethe surface roughness can thethat, EDL the properties near the surfacecan wall, hence reduce the pressure inside microchannel. Onchange top of surface roughness change the EDL pressure inside the microchannel. On top of that, the surface roughness can change the EDL electroosmotic flow the microchannel 8b shows the effect oxidation the side wall properties near theinside surface wall, hence [19,20]. reduce Figure the electroosmotic flowofinside the on microchannel properties near the surface wall, hence reduce the electroosmotic flow inside the microchannel smoothness. It can seen the thateffect the scallop roughness vertical of the microchannel resulted [19,20]. Figure 8b be shows of oxidation onon thethe side wallwall smoothness. It can be seen thatfrom the [19,20]. Figure 8b shows the effect of oxidation on the side wall smoothness. It can be seen that the the plasma etching on process was improved oxide layer growth the silicon vertical wall after 12 h scallop roughness the vertical wall of by thethe microchannel resultedonfrom the plasma etching process scallop roughness on the vertical wall of the microchannel resulted from the plasma etching process of oxidation. was improved by the oxide layer growth on the silicon vertical wall after 12 h of oxidation. was improved by the oxide layer growth on the silicon vertical wall after 12 h of oxidation.

(a) (a)

(b) (b)

Figure after oxidation. Figure 8. 8. Scallop Scallop effect effect on on the the silicon silicon vertical vertical wall wall (a) (a) before before and and (b) (b) Figure 8. Scallop effect on the silicon vertical wall (a) before and (b) after after oxidation. oxidation.

4.3. Microchannel Structure Improvement after Thermal Oxidation 4.3. Microchannel Microchannel Structure Structure Improvement 4.3. Improvement after after Thermal Thermal Oxidation Oxidation A high aspect ratio SiO2 microchannel was successfully created by using dry thermal oxidation A high microchannel was was successfully successfully created created by by using using dry dry thermal thermal oxidation oxidation A high aspect aspect ratio ratio SiO SiO22 microchannel that is suitable for an electroosmotic flow application. The shape and cross section of the that is suitable for an electroosmotic flow application. The shape and cross section of the microchannel that is suitable for an electroosmotic flow application. The shape and cross section of the microchannel was uniform, as demonstrated by Figure 9a,b. The width of the microchannel was was uniform, as demonstrated Figure 9a,b.by The width9a,b. of the microchannel reduced to 42% microchannel was uniform, as by demonstrated Figure The width of thewas microchannel was reduced to 42% with an oxide layer of 520 nm on the channel wall. The color scheme on the top with an oxide layer of 520 nm on the channel wall. The color scheme on the top surface of the substrate reduced to 42% with an oxide layer of 520 nm on the channel wall. The color scheme on the top surface of the substrate was seen consistently without any abnormal spots. was seen without anyconsistently abnormal spots. surface ofconsistently the substrate was seen without any abnormal spots.

(a) (a)

(b) (b)

Figure 9. (a) Silicon microchannel before oxidation process; (b) SiO2 microchannel after 12 h thermal Figure 9. (a) microchannel before before oxidation oxidation process; process; (b) (b) SiO SiO2 microchannel after 12 h thermal Figure 9. (a) Silicon Silicon dry oxidation at 1040microchannel °C. 2 microchannel after 12 h thermal dry oxidation at 1040 °C. ◦ dry oxidation at 1040 C.

5. Conclusions 5. Conclusions In conclusion, we presented a simple technique for fabricating a silicon electroosmotic (EO) In conclusion, we presented a simple technique for fabricating a silicon electroosmotic (EO) channel with surface modification. The microchannel arrays were initially realized by reactive ion channel with surface modification. The microchannel arrays were initially realized by reactive ion etching coupled with post processed thermal dry oxidation to produce high aspect ratio etching coupled with post processed thermal dry oxidation to produce high aspect ratio microchannels with an improved cross section and surface morphology. The strong relation between microchannels with an improved cross section and surface morphology. The strong relation between oxide thickness and the temperature was presented by performing the thermal dry oxidation at oxide thickness and the temperature was presented by performing the thermal dry oxidation at 990 °C, 1020 °C and 1040 °C. The oxide growth rate was found to increase at higher process 990 °C, 1020 °C and 1040 °C. The oxide growth rate was found to increase at higher process temperatures. By using this method, the microchannel width was reduced by ~40% with the


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5. Conclusions In conclusion, we presented a simple technique for fabricating a silicon electroosmotic (EO) channel with surface modification. The microchannel arrays were initially realized by reactive ion etching coupled with post processed thermal dry oxidation to produce high aspect ratio microchannels with an improved cross section and surface morphology. The strong relation between oxide thickness and the temperature was presented by performing the thermal dry oxidation at 990 ◦ C, 1020 ◦ C and 1040 ◦ C. The oxide growth rate was found to increase at higher process temperatures. By using this method, the microchannel width was reduced by ~40% with the minimum aspect ratio (H/W) of 2. The surface quality was also improved as the scallop effect from the plasma etching process was reduced by adding an adequate amount of oxide layer. A uniform cross section with a good quality of surface roughness was essential for demonstrating a steady electroosmotic flow inside the microchannel. The final structure of the SiO2 microchannels array will be integrated with a PDMS structure to work as a complete electroosmotic microfluidic device. This process was foreseen to be able to produce a high aspect ratio sub-microchannel and nanochannels without implementing the nanolithography procedure that will be beneficiary for ion transport purposes. Author Contributions: J.Y., T.N.T.Y. and B.Y.M. conceived and designed the experiments; J.Y., and T.N.T.Y. performed the experiments and analyzed the data; A.A.H. and M.F.M.R.W. contributed reagents/materials, T.N.T.Y., J.Y. and R.L. wrote the paper. Acknowledgments: This work is supported by Ministry of Higher Education under LRGS Project Grant No. LRGS/2015/UKM-UKM/NANOMITE/04/01 and FRGS Project Grant No. FRGS/1/2016/TK05/UKM/02/2 and partly supported by the French RENATECH Network and its FEMTO-ST Technology facilities. Conflicts of Interest: The authors declare no conflict of interest.

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