Developing and Analysing sub-10 µm Fluidic Systems with Integrated Electrodes for Pumping and Sensing in Nanotechnology Applications

Abstract

In this thesis, sub-10 µm fluidic systems with integrated electrodes for pumping and sensing in nanotechnology applications were developed and analyzed. This work contributes to the development of the scanning ion pipette (SIP), a tool to investigate surface changes on the nanometer scale induced by locally administering chemically or bio-active solutions. For this purpose, the already existing technology of a micropipette integrated into a scanning force microscope (SFM)-chip was enhanced by the use of electrodes for on-chip electrochemical sensing and electroosmotic (EO) pumping. The integration of an EO pump offered the possibility of storing, selecting and dispensing multiple different liquids with the SIP chip. For a high density and convenient electronic integration, an EO pump with a small footprint (less than 100 µm × 100 µm) and low actuation voltage (less than 10 V) had to be developed. The thesis starts with a detailed analysis of the microfabrication process to build the SIP’s network of capillaries, the freestanding cantilever and the tip. The fabrication was based on standard micromachining, from well established MEMS processes. The main innovation, previously developed by Hug et al., was to outline the capillaries of the fluidic system, the cantilever and the tip in one wafer, and fabricating the fluidic through wafer connections and the SIP chip outline in another wafer. By bonding the two wafers together, the former trenches were capped and by a subsequent high temperature oxidation, the hydrophilic silicon dioxide (SiO2) capillaries were formed. Afterwards, the buried fluidic system and the cantilever were released, providing optical access to the capillaries. Finally, the outlet hole was drilled next to the tip apex with a focussed ion beam. This SIP fabrication process was highly versatile with regards to the capillary geometry, allowing the design of a complex capillary geometry for a multifunctional microfluidic system. To obtain a first experience with these small capillary dimensions, the integration of an evaporation based micropump into the SIP was investigated. Its actuation did not require any electrodes and hence, it could be directly implemented into the SIP fabrication process, without any additional fabrications steps. The working principle of an evaporation based micropump is as follows: The hydrophilic capillaries of the SIP were spontaneously filled with a water based solution. Once the fluid reached the capillary outlet inside, it started evaporating. The capillary pressure kept the outlet of the capillaries wetted, and thereby, automatically replaced the evaporation loss by drawing additional water through the capillaries. This resulted in a unidirectional pump, which could be controlled by the temperature at the evaporation area. The evaporation induced flowrate was experimentally determined to range from 7 pl·s-1 at 23° C up to 53 pl·s-1 at 65° C depending exponentially on the temperature. A more advanced bidirectional EO pump with platinum (Pt) electrodes, based on a simplified fabrication process, resulting in comparable SIP capillary dimensions, was experimentally analyzed and modelled. The current-coupling between the Pt electrodes and the solution required a SIP specific on-chip design to ventilate the emerging electrolyzed gases. This was achieved by integrating the electrodes into a novel liquid-gas (LG) separator. The LG-separator separated the gas bubbles from the liquid and guided them away from the EO pump. Its operation principle is solely based on the LG-separator’s geometry of tapered sidewalls, taking advantage of the high capillary pressure occurring at the bubble’s liquid gas interfaces at this small scale. The LG-separator was experimentally analyzed and modelled. In the experimental analysis, the maximum backpressure of the LG-separator was determined to be 0.6 kPa. It was able to reliably separate and ventilate an emerging gas flow of 2 pl·s-1. For a deeper understanding, the development and the propagation of the bubble within the LG-separator was analytically described in three dimensions. The model and the derived design guidelines show that Pt electrodes, combined with the LG-separator, open an interesting new field for complex high density electrohydrodynamic and electrochemical microfluidic applications. A microfluidic system, containing two LG-separators sandwiching an EO pump, was also analyzed and modelled. The EO pump achieved a flow rate of 50 pl·s-1 at a low actuation voltage of 5 V. The developed corresponding model of the flows within the fluidic system was in good agreement with the measured values. According to the model, an EO pump with a high backpressure (3.6 kPa·V-1) enabling a high dispensing flowrate of 1.5 pl·V-1·s-1 (corresponding to a SIP immersed in water, outlet hole radius of 100 nm) can be built. The performance and integration of a second type of electrodes, based on silver/silver chloride (Ag/AgCl), into the SIP was investigated. These electrodes had the outstanding advantage that during electrode actuation the electrochemical reaction continued to transform Ag into AgCl and vice versa, rather than electrolyzing the liquid. Moreover, these electrodes could be integrated in a post SIP capillary fabrication step, circumventing electrode instability caused by the high temperature oxidation step to form the SiO2 of the capillary sidewalls. The general processflow to integrate the Ag/AgCl electrodes into the fabricated SIP capillary fabrication step was: The adhesion of the Ag electrode to the SiO2 capillary sidewall was improved by using an intermediate polymeric layer consisting of 3-mercaptopropylmethyldimethoxy silane (MPS). This silanization step turned out to be essential for reliably stable Ag electrodes in a capillary dimension of less than 10 µm, since the strong capillary force tended to delaminate the electrode. Crucial for a successful silanization was the use of a gas phase deposition on a dehydrated surface, to avoid the formation of polymeric MPS globuly. Electroless deposition provided a highly flexible and unique tool to deposit the electrodes in the closed SIP capillaries. The general idea was to fill a solution of Ag ions, as well as a reducing agent into the capillary. During the electrochemical reaction, the Ag electrodes started to grow on the capillary sidewall. The deposition of thick electrodes was required since during the electrode actuation, either Ag or AgCl was continuously consumed. This deposition of thick electrodes was especially challenging due to a minute available capillary volume, hence high concentrations within the electroless solution were used. The best electroless deposition process control was achieved with an improved Tollens solution and the reaction speed was controlled with the sodium citrate concentration (decrease) and the sodium hydroxide concentration (increase). Two different methods of electroless deposition were employed: First, a batch-like dip process of multiple electroless depositions, and second, a single electrode flow deposition process providing continuously fresh electroless solution. The electrode structuring was performed by controlling the capillary filling of the electroless solution within the fluidic system by microfluidic stopvalves. The stopvalve functionality was twofold, the solution was reliably stopped during the electroless deposition and afterwards the stopvalve was void free filled to ensure correct fluidic actuation of the final device. This should be done preferably without applying any external pressure. The switching from stopping to transmitting the solution was induced by a change in the solution’s surface tension. The stopvalve performance was modelled by improving the previously two dimensional model to three dimensions including, additionally, the design fabrication specific corner rounding and low capillary cross section’s aspect ratio. After the electroless deposition and structuring, the Ag electrode was further transformed into an Ag/AgCl electrode. Similarly to the electroless Ag deposition, the electrode transformation was performed with a flow of either sodium hypochloride or ferric(III) chloride. The transformation should be limited to 20% of the initial Ag layer thickness, due to stress related electrode delamination or cracking, originating in the density difference of Ag and AgCl. Interestingly, the electrochemical transformation process from Ag into AgCl could be described by the Deal-Grove model for the oxidation of silicon. The growth of AgCl depended nonlinearly on electrochemical reaction time. After an AgCl thickness of about 40 nm, the electrochemical reaction was dominated by the diffusion of the oxidizing species through an increasing layer of AgCl. Electrodes were deposited into three different capillaries: a) into the SIP capillary itself (cross section 2.2 µm × 3.7 µm), b) for larger availability and easier accessibility, into commercially available round capillaries (radius up to 520 µm), and c) a polymeric microfluidic system with rectangular capillaries (cross section 55 µm × 65 µm). Inside the SIP, the functionality of the microfluidic stopvalve was experimentally verified, a binary solution of 20% ethanol in water (contact angle 82°) was stopped and a binary solution of 40% ethanol in water (contact angle 58°) filled the stopvalve void free. In addition, a successful dip electroless deposition and stopvalve structuring of Ag electrodes was shown. Inside round capillaries, multiple electroless depositions revealed that each deposition increased the Ag layer thickness of 51 nm. The deposited Ag layer had a high specific conductivity of 6 × 107 S·m-1, indicating a high purity and density. The further transformation into Ag/AgCl provided electrodes to electrochemically measure different pH values. A linear pH sensitivity of 57.4 mV·pH-1 at 22.7° C with a good agreement of Nernstian behavior was reached. During these experiments, it turned out that the electroless Ag deposition was highly contamination sensitive which was strongly enhanced by the small capillary cross section. Therefore, the deposition into the polymeric fluidic system was performed with a flow of electroless solution instead of previously used multiple depositions. Despite the individual electrode deposition, the flow deposition had the advantage that the concentration could be kept constant throughout the capillary during the complete deposition time. This provided a better reaction control due to a lower concentration and in addition, it reduced the effect of the minute available capillary volume. The EO pump inside the polymeric fluidic system had an experimentally determined pump rate of 0.12 nl·s-1·V-1. First, experiments with the SIP for imaging and dispensing were performed. In order to get a first hand-on experience, a less delicate sample, in less challenging conditions, was chosen, then anticipated for the expected SIP imaging of a living cell. The imaging capabilities were illustrated by imaging in tapping mode a fixed and dried Escherichia Coli bacteria. The obtained images had a reasonable image quality and resolution. Moreover, no special skills in handling the SFM were required, since it did not perform differently with a mounted SIP than with a mounted standard commercially available SFM. In case of dispensing, with an externally applied pressure, the development of a bubble at the outlet hole of the tip was observed. With the currently used method of gluing the SIP to the SFM holder, no satisfying and reliable mounting was achieved. The main reasons of failure were leakage afflicted sealing between the SIP and the SFM holder, contamination of the SIP capillaries and finally breaking of the SIP cantilever during the complex and lengthy mounting procedure. This, again, shows the necessity of improving SIP techniques towards autonomous on-chip fluid handling.