Abstract
Surface Channel Technology is known as the fabrication platform to make free-hanging microchannels for various microfluidic sensors and actuators. In this technology, thin film metal electrodes, such as platinum or gold, are often used for electrical sensing and actuation purposes. As a result that they are located at the top surface of the microfluidic channels, only topside sensing and actuation is possible. Moreover, in microreactor applications, high temperature degradation of thin film metal layers limits their performance as robust microheaters. In this paper, we report on an innovative idea to make microfluidic devices with integrated silicon sidewall electrodes, and we demonstrate their use as microheaters. This is achieved by modifying the original Surface Channel Technology with optimized mask designs. The modified technology allows to embed heavily-doped bulk silicon electrodes in between the sidewalls of two adjacent free-hanging microfluidic channels. The bulk silicon electrodes have the same electrical properties as the extrinsic silicon substrate. Their cross-sectional geometry and overall dimensions can be designed by optimizing the mask design, hence the resulting resistance of each silicon electrode can be customized. Furthermore, each silicon electrode can be electrically insulated from the silicon substrate. They can be designed with large cross-sectional areas and allow for high power dissipation when used as microheater. A demonstrator device is presented which reached at a power of , limited by thermal conduction through the surrounding air. Other potential applications are sensors using the silicon sidewall electrodes as resistive or capacitive readout.
Highlights
It was previously reported that silicon microheaters can be embedded in the channel sidewalls by the Trench-Assisted Surface Channel Technology (TASCT) process (Figure 1c) and temperatures up to 400 ◦C were reached by Joule heating, limited by the absence of flexure structures to allow for thermal expansion [26]
The fabricated silicon electrodes have cross-sectional areas ranging from approximately 35 μm2 to 720 μm2 and a resistance per unit length ranging from approximately 1 · Ω m−1 to 3 · Ω m−1
A demonstrator chip employing the silicon electrode as a microheater embedded between the adjacent free-hanging microchannels is designed and fabricated
Summary
Silicon-based microfluidic channels (i.e., microchannels) have been developed for various applications such as mass flow sensors [1], biosensors [2], and chemical reactors [3]. Microchannels with large cross-sectional areas can be realized by bonding two wafers with cavities together and thin film metal or poly-crystalline silicon electrodes can be integrated [4,5,6,7,8,9,10]. Free-hanging microchannels with intermediate cross-sectional areas can be made by bulk-micromachining of the silicon substrate, which allows integration of both thin film and bulk silicon electrodes [14,15,16,17,18,19]. Surface channel technology allows the fabrication of free-hanging microfluidic channels with hydraulic diameters ranging from approximately 20 μm to 100 μm [16] These microchannels have very thin channel walls in the range of 1 μm to 1.5 μm thickness and are made using low pressure chemical vapor deposition (LPCVD) of silicon-rich silicon nitride (SiRN). We report an extension to the standard SCT process to integrate silicon sidewall electrodes between adjacent free-hanging microchannels in a silicon wafer
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