Fabrication and Characterization of Silicon Micro-Funnels and Tapered Micro-Channels for Stochastic Sensing Applications

Abstract

We present a simplified, highly reproducible process to fabricate arrays of tapered silicon micro-funnels and micro-channels using a single lithographic step with a silicon oxide (SiO2) hard mask on at a wafer scale. Two approaches were used for the fabrication. The first one involves a single wet anisotropic etch step in concentrated potassium hydroxide (KOH) and the second one is a combined approach comprising Deep Reactive Ion Etch (DRIE) followed by wet anisotropic etching. The etching is performed through a 500 mm thick silicon wafer, and the resulting structures are characterized by sharp tapered ends with a sub-micron cross-sectional area at the tip. We discuss the influence of various parameters involved in the fabrication such as the size and thickness variability of the substrate, dry and wet anisotropic etching conditions, the etchant composition, temperature, diffusion and micro-masking effects, the quality of the hard mask in the uniformity and reproducibility of the structures, and the importance of a complete removal of debris and precipitates. The presence of apertures at the tip of the structures is corroborated through current voltage measurements and by the translocation of DNA through the apertures. The relevance of the results obtained in this report is discussed in terms of the potential use of these structures for stochastic sensing.

Keywords: anisotropic etching.; silicon; stochastic sensing.

Figure 1.

Schematic representation of the micro-funnels and tapered micro-channels.

Figure 2.

Scanning Electron Microscopy images of the structures fabricated with a 600 μm (a,b) and a 700 μm (c,d) square mask after re-etching in the same bath at 40 °C with addition of IPA to saturation. The presence of an agglomeration of hillocks at the bottom of the 600 μm mask opening as well as the circular pits at the bottom of the 700 μm opening are still evident.

Figure 3.

Scanning Electron Microscopy images of the tapered tip from structures fabricated with different mask openings sizes. (a) 400 μm, (b) 500 μm, (c) 600 μm and (d) 700 μm. Etching was performed in 9 M KOH at room temperature with no stirring. The etching was performed to a 500 μm depth.

Figure 4.

Scanning Electron Microscopy images of the tip of a representative set of micro-funnels fabricated with the optimized conditions (9 M KOH, 65 °C with no stirring). The openings were 600 μm width and the etching was performed to a 500 μm depth.

Figure 5.

Scanning Electron Microscopy of the structures fabricated with the 700 μm square openings under the same conditions as the structures presented in figure 3.

Figure 6.

Characteristic current-voltage (I-V) curve obtained from a single micro-funnel that etched through the wafer during the anisotropic etching. The light gray line corresponds to the measurement from a solid silicon piece obtained from the same wafer. The difference in the slope indicates that an ionic current is flowing though the tip of the micro-funnel.

Figure 7.

Representative image of the PCR products from the collected human DNA after translocation through a micro-funnel structure using a -500 mV potential. Lane 1, molecular weight marker, lane 2, PCR negative control; lane 3, PCR product from the remanent DNA collected at the cathode (-); lane 4, PCR product from the DNA that translocated from the cathode and was collected at the anode (+).

Figure 8.

(a) Cross-section Scanning Electron Microscopy image of a structure fabricated using two consecutive deep reactive ion etch steps. The bottom width corresponds to the original mask width (150 μm) but the entrance of the structure has been widened. (b) High contrast scanning electron microscopy image of a micro-channel fabricated by four consecutive deep reactive ion etch steps (Bosch) followed by anisotropic etching in KOH. The lines defining the ribs of the inverted pyramid extend from each corner to the bottom of the taper suggesting that the four delimiting walls are at 54.7° with respect to the plane.

Figure 9.

Scanning Electron Microscopy images of a collection of tapered micro-channels fabricated using (a-c) two consecutive steps of deep reactive ion etching (Bosch) followed by anisotropic etching to taper the tip. The diffusion constraints produced by the narrow geometry channel produces non-uniformities and residue accumulation at the tip. The use of four consecutive steps (d-f) leads to a wider channel entrance that facilitates diffusion, producing tapers with a more uniform size range and geometry.

Figure 10.

Scanning Electron Microscopy images of a set of structures in which the mask opening was performed using reactive ion etch (a-c) and buffered oxide etch (d-f) after wet anisotropic etching with KOH. The non-uniform etch of the silicon oxide mask using reactive ion etch reduces the sidewall smoothness and the reproducibility of the tapered structures. The etching was performed to a 500 μm depth.

Figure 11.

Scanning Electron Microscopy images of devices after stripping the silicon oxide mask and abundant rinsing with MilliQ water (a-b). Residue is present and accumulates at the bottom of the taper and on the plateau. The same devices after cleaning with 25% HF in ethanol and storage in 50% ethanol (c-d).

Figure 12.

Process flow. The substrate used is a <100> p-type silicon wafer with a silicon oxide layer of 1 or 2 μm. Patterning is performed using standard lithographic steps.