An Approach to Ring Resonator Biosensing Assisted by Dielectrophoresis: Design, Simulation and Fabrication

Abstract

The combination of extreme miniaturization with a high sensitivity and the potential to be integrated in an array form on a chip has made silicon-based photonic microring resonators a very attractive research topic. As biosensors are approaching the nanoscale, analyte mass transfer and bonding kinetics have been ascribed as crucial factors that limit their performance. One solution may be a system that applies dielectrophoretic forces, in addition to microfluidics, to overcome the diffusion limits of conventional biosensors. Dielectrophoresis, which involves the migration of polarized dielectric particles in a non-uniform alternating electric field, has previously been successfully applied to achieve a 1000-fold improved detection efficiency in nanopore sensing and may significantly increase the sensitivity in microring resonator biosensing. In the current work, we designed microring resonators with integrated electrodes next to the sensor surface that may be used to explore the effect of dielectrophoresis. The chip design, including two different electrode configurations, electric field gradient simulations, and the fabrication process flow of a dielectrohoresis-enhanced microring resonator-based sensor, is presented in this paper. Finite element method (FEM) simulations calculated for both electrode configurations revealed ∇E² values above 10¹⁷ V²m^-3 around the sensing areas. This is comparable to electric field gradients previously reported for successful interactions with larger molecules, such as proteins and antibodies.

Keywords: biosensor; dielectrophoresis; mass transfer; micro fabrication; microring resonator; photonic sensor.

Figure 9

(a) A typical experimental transmission spectrum of the ring resonators. The red peaks are attributed to the reference ring and the blue to the sensor ring. (b) The resonance peak shift of the ring resonators exposed to MeOH (n = 1.3118), [51] H₂O (n = 1.3164), EtOH (n = 1.3503), and isopropanol (n = 1.3661) normalized to the peak position in air (n = 1.003). [52] (c) Nyquist and Bode plot representing the impedance measurement of NaCl aqueous solutions, as performed with the coplanar electrode configuration.

Figure 1

Schematic drawing of the principle of ring resonators and their generated transmission spectrum [8].

Figure 2

(a) Cross-section of the nano rib waveguide structure used in this work. (b) Schematic figure of the general IHP photonic BICMOS architecture. In this work, the wafers were only processed up to the first metallization layer.

Figure 3

(a) Contacting the silicon waveguide: Cross-section of the silicon waveguide (xy plane) used for the 3D electrode geometry. The red areas represent highly-doped silicon that is contacted via a channel from the metal 1 layers. Most of the waveguide is covered by an SiO₂ layer to protect it from the outside and prevent losses. (b) In the sensing areas, the SiO₂ is etched off, to allow exposure of the waveguide to the analyte medium. The doped area on the waveguide functions as a dielectrophoresis (DEP) electrode, whereas the counter electrode is placed on top of a microfluidic channel with the distance h in the y-direction to the waveguide. (c) 3D view of the sensor region.

Figure 4

Finite element method (FEM) simulations, as described in Section 2.2, of the ∇E² distribution in the 3D geometry with a 10 V peak to peak potential applied, given in V²m⁻³. The figure shows the cross-section (xy plane) at different heights h in relation to the counter electrode. The waveguide is placed in center at the bottom, extending in the z-direction. The analytes are expected to be transported from the bulk solution to the area with the strongest ∇E², next to the core waveguide.

Figure 5

(a) Cross-section of the sensing region with the coplanar electrode pair geometry. (b) Top view of the coplanar electrode pair geometry. (c) 3D view of the sensing region.

Figure 6

(a) Cross-section (xy plane) of the coplanar electrode configuration with different distances (d) between the electrodes. The waveguide is in the center at the bottom, extending in the z-direction (b) Cross-section (yz plane) at the center of the waveguide with the coplanar electrode configuration. Simulated ∇E² distribution at different thorn-to-thorn distances (t).

Figure 7

Ring resonator layout. Each sensor consists of two ring resonators (dark blue) coupled to a common bus waveguide (dark blue) using directional couplers. One ring is completely covered with SiO2 (light blue) and acts as a reference to control for ambient temperature. The two variants differ in terms of using doped waveguides (left) and coplanar electrodes (right) for dielectrophoresis.

Figure 8

SEM image of the fabricated sensors. (a) The sensor region in the doped waveguide, corresponding to Figure 3c. The core waveguide can be observed at the center as a bright line next to a thin dark gray area representing the undoped rib. The brighter gray area is attributed to the doped rib, while the whiter region is attributed to the salcide segment. (b) The coplanar electrodes: A 4 × 90 µm large window that is etched with reactive ion etching (RIE) that exposes the core waveguide can be observed at the center of the opening. (c) An overview of the chip layout, showing the ring resonators, sensing areas, and electrode structures.