Low cost fabrication of solid state nanopores in robust silicon wafer

M. VEGA; P. GRANELL; F. GOLMAR ; M.J. DIEGUEZ; C. LASORSA; B .LERNER; M. S. PEREZ

The Hague

Conferencia; 41 st micro and nano engineering MNE 2015; 2015

In this research an easy, reproducible and inexpensive technique for the production of solid state nanopores, using silicon wafer substrate is proposed. The method uses an automatic braking pore formation, without the intervention of an external agent, when this has dimensions of around 20 nm to 200 µm. The manufacturing method is based on the chemical braking of etching in silicon wafers for produce solid state nanopores. Nanopores were manufactured using a Potassium hydroxide (KOH) etching and subsequently a chemical braking was performed with Hydrochloric acid (HCl), in order to automatically control the nanopore opening. The technique is based on pore control formation, by neutralization etchant (KOH) with a strong acid. Thus, a local neutralization is produced around the nanopore, which stops the silicon etching. The etching process was performed with 7M KOH at 80°C. The control of the pore formation with the braking acid method was done using 12M HCl. A schematic picture of the pattern forming process is shown in figure 1. The substrates used were 700 µm thick double-side polished oriented silicon wafers (Virginia Semiconductor In). Silicon nitride layer of 50 nm thickness were deposited on a wafer using Plasma Enhanced chemical vapor deposition (PECVD) at 600°C. First, the etching was performed simultaneously on both sides of the silicon wafer. The reaction was stopped before the inverted pyramids formed joined. The pore size was adjusted by controlling the reaction temperature and / or applying an electrical potential. 15 samples were studied i) at 25°C, ii) at 80°C and iii) at 80°C applying a potential. These temperatures were chosen according to previous studies [1]. The chemical braking was performed by introducing the silicon wafer previously attacked with KOH at 80 °C (first etching), in a PMDS device between two compartments isolated for performing a second etching with KOH 4 M in cis face, while the trans side was exposed to HCl 12M at 25 °C. Thus, the attack continued only on the cis side of the substrate shows the etching rate in the early hours with a speed of 0.083 µm/min, and when the depth of the window reaches 512 µm, the etching is completely stopped, even after 50 h etching. Thus, it was found that the nanopores are produced after about 8 h of chemical braking, and the process is able to control its formation, up to 50 h of etching later. To evaluate the effect of high temperature, the same process carried out at 80°C. In the first minute, the chemical braking is carried out at a rate of 1.23 µm/min, similar to reported rate in literature for these conditions [2] However, to reach a depth of 510 ± 2 microns, attack speed changed due to the contact of the two openings in the silicon wafer and the resulting mixture of HCl with KOH. However, due to temperature, KOH neutralization process is not completely generated and therefore the reaction of silicon attack is not stopped at time to forming a nanopore. Consequently, the result was the production of micropores. The same behavior was observed at lower temperatures up to 60 °C. Thus, the etching continued, but only in areas where the KOH was not neutralized. Since at the extreme condition temperature of 80 °C, the HCl do not have enough time to stop the etching with KOH in order to produce nanopores, the same etching temperature was evaluated, but applying a potential. Therefore, the potential acting as a driving force of the passage of protons to the trans side and thus accelerates the chemical braking to control the nanopores formation. An electric potential of 12 V was applied with platinum electrode between the two compartments. Etching continued until the two inverted pyramids found each other, and then KOH was neutralized by HCl, automatically stopping the etching forming the nanopore. The application of electric potential also had the advantage of acting as a cleaning process of the wafer surface. Figure 2 shows a compilation the results of the etch depth versus time for the three processes studied and Figure 3 shows solid state nanopores using this technique. The manufacture of nanopores with braking chemical method has shown several advantages over other manufacturing methods. First, the method allows to automatically stopping the nanopore manufacturing process, without any feedback system or the intervention of an external agent and second, it allows locating the nanopores in specific locations of the substrate.