On-Chip Refractive Index Sensors Using Plasmonic TiN Nanohole Arrays

Introduction

Integrated biosensors have a wide range of potential applications, from precision agriculture to personalized medicine. Plasmonic-based refractive index (RI) sensors offer the advantage of miniaturization, making them attractive for on-chip bio-sensing. However, commercial plasmonic biosensing is currently limited in size by the need for external instrumentation such as microscopes or spectrometers. An integrated biosensor that can directly convert optical signals from plasmonic structures into electrical signals for on-chip processing represents a cost-effective and highly portable approach.

In this context, the combination of plasmonic nanohole arrays (NHAs) with germanium (Ge) photodetectors is a promising device concept. The optical properties of the NHA, and hence the photocurrent generated in the Ge detector, depend strongly on the refractive index of the surrounding medium. This allows for highly sensitive detection of bulk refractive index changes.

However, realizing such devices on a large scale requires transferring the concept to a complementary metal-oxide-semiconductor (CMOS) compatible production line, which brings challenges in fabrication and device design. This tutorial discusses the successful demonstration of TiN NHAs combined with Ge photodetectors, fabricated in an 8" CMOS production line, for the detection of refractive index changes in liquids.

TiN NHAs for Refractive Index Sensing

A key requirement for CMOS compatibility is avoiding commonly used plasmonic metals like gold. Titanium nitride (TiN) is a CMOS-compatible, biocompatible metal that is stable at high temperatures, as shown in the scanning electron microscope image in Figure 1.

Fig. 1 Scanning electron microscope image of TiN NHA.

However, TiN has higher optical losses compared to metals like silver, which can broaden plasmonic resonances. Therefore, extensive simulations and experiments were carried out to optimize the TiN NHA design.

TiN NHAs can be fabricated with excellent structural homogeneity using industrial processes like sputtering deposition, deep ultraviolet photolithography, and reactive ion etching. When using the measured TiN permittivity in finite-difference time-domain simulations, good agreement is obtained between simulated and measured reflectance spectra for a 150 nm thick TiN NHA with 700 nm pitch and 455 nm nanohole diameter under different angles of incidence for p-polarized light (Figure 2).

Fig. 2 Measured and simulated reflectance spectra of a 150 nm thick TiN NHA with a pitch of 700 nm and a nanohole diameter of 455 nm under different angles of incidence and p-polarization

Both simulations and measurements show dips in the reflection spectra resulting from extraordinary optical transmission through the NHAs. The asymmetric line shape is characteristic of a Fano resonance, which is usually associated with high refractive index sensing performance. Although the resonances are broader in TiN compared to silver due to higher losses, the resonance width can be tuned by varying the angle of incidence. This opens up the possibility of optimizing TiN NHA geometries for good sensing performance despite the higher losses.

On-Chip RI Sensor Characterization

The integrated RI sensors consist of TiN NHAs placed on top of Ge photodetectors, fabricated in a CMOS-compatible process. They were characterized using the setup shown in Figure 3, with a supercontinuum light source, acoustic filter, and collimated output from a photonic crystal fiber.

Fig. 3 Photocurrent measurement setup for the measurement of bulk RI changes in liquids.

Optoelectronic characterization was performed by submerging the devices in different liquids and measuring the photocurrent spectra from 1200 nm to 1650 nm under vertical illumination. The results in Figure 4 show a pronounced resonance at 1250 nm for devices submerged in ethanol (n = 1.353) or isopropanol (n = 1.375), but not in air.

Fig. 4 Photocurrent measurement results of the on-chip sensor under perpendicular incidence of p-polarized light in air, ethanol and isopropanol obtained from a Ge photodetector at external bias of -1 V.

The resonance redshift from ethanol to isopropanol is consistent with the increase in superstrate refractive index. The broad nature of this resonance can be attributed to the high losses of TiN under vertical incidence. However, the influence of oblique incidence and design choices on device performance and reproducibility are also discussed.

Conclusion

In summary, the combination of TiN NHAs with Ge photodetectors, realized through a CMOS-compatible fabrication process, has been demonstrated as a promising concept for on-chip refractive index sensing. Despite the higher losses in TiN compared to conventional plasmonic metals, narrow Fano resonances can be achieved by optimizing the NHA geometry and angle of incidence.

Optoelectronic characterization of the fabricated devices has confirmed their capability for refractive index sensing, with clear changes in the photocurrent spectra observed when the devices are submerged in different liquids. These results pave the way for cost-effective, large-scale realization of on-chip refractive index sensors on the silicon platform, with potential applications in areas such as biosensing and environmental monitoring.

Reference

[1] S. Reiter, A. Sengül, C. Mai, D. Spirito, C. Wenger, I. A. Fischer, "On-Chip Refractive Index Sensors Based on Plasmonic TiN Nanohole Arrays," Experimentalphysik und funktionale Materialien, BTU Cottbus-Senftenberg, Cottbus, Germany; IHP – Leibniz-Institut für innovative Mikroelektronik, Frankfurt (Oder), Germany, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.