IEDM2024|Mid-Infared On-Chip Spectroscopy and Waveguide Integrated Thermal Radiation Detector Technology

Introduction

Mid-infared spectroscopy offers unique advantages for gas and biochemical substance detection. The mid-infrared wavelength range (2-20 µm) encompasses various molecular vibration and rotation absorption peaks, enabling high sensitivity and specificity in molecular detection [1].

Figure 1: (a) Schematic diagram of an on-chip mid-infrared spectrometer based on a Germanium-on-Insulator (Ge-OI) photonics platform. (b) Design and working principle of the self-supported waveguide integrated thermal radiation detector.

Traditional laser absorption spectroscopy systems perform well but are limited by their bulky size and complex optical alignment requirements. Waveguide-based on-chip laser absorption spectroscopy systems offer a compact solution, seamlessly integrating photonic and electronic devices while reducing power consumption. However, achieving stringent detection limits required for industrial applications remains challenging due to short optical paths and high transmission losses.

Design Principles and Working Mechanism

The self-supported waveguide integrated thermal radiation detector presented in this paper achieves significant progress in addressing these challenges. The device employs an innovative air-slot structure, enhancing thermal isolation and improving overall performance.

Figure 2: Schematic of heat conduction and its impact on thermal radiation detector performance, illustrating how reduced thermal crosstalk and improved thermal efficiency enhance device performance.

The working principle includes several critical steps. Input light enters the self-supported waveguide via a ridge waveguide and is subsequently absorbed by heavily doped p-type germanium through free-carrier absorption (FCA). The absorbed light converts into heat, conducted to the upper thermally sensitive layer composed of TiO₂ and Ti.

Figure 3: Mid-infrared absorption coefficients of intrinsic, p-type, and n-type germanium as functions of wavelength.

The air-slot structure significantly improves the device's thermal efficiency (defined as the average temperature rise per unit input power). This enhancement primarily results from effectively inhibiting heat conduction from the waveguide to the buried oxide layer.

Fabrication Process and Implementation

The fabrication process employs CMOS-compatible materials and techniques, involving multiple precision steps.

Figure 4: (a) Step-by-step fabrication process of the Ge-OI wafer and self-supported film. (b) Transmission electron microscope images detailing the structure. (c) High-resolution X-ray diffraction image demonstrating crystal quality.

Initially, a 4 µm-deep air-slot is fabricated using reactive ion etching. Subsequently, 1.5 µm-thick Y₂O₃ is deposited as a buried oxide layer via RF sputtering. Chemical mechanical polishing ensures the surface flatness required for wafer bonding, achieving a roughness of approximately 0.522 nm.

Figure 5: Optical microscope images showing (a) the completed device, including input waveguides and reference waveguides, and (b) detailed views of the self-supported thermal radiation detector positioned above the air-slot.

Performance Characterization and Testing

The device demonstrates excellent performance across multiple parameters.

Figure 6: Comprehensive characterization of the thermal-sensitive layer, including I-V characteristics, temperature dependency, Arrhenius plots, and TCR measurement results.

Temperature-dependent measurements indicate significant ohmic characteristics of the thermal-sensitive layer. The device exhibits a notable resistance decrease with increasing temperature, achieving a temperature coefficient of resistance (TCR) of -2.49%/K at 293 K.

Figure 7: Optical measurement results showing normalized resistance variation with input power, extracted sensitivity measurements, and stability test outcomes.

At an input optical power of 73 µW, the device achieves a sensitivity of -1.789%/µW, significantly improved over traditional designs. Stability tests at 0.1 Hz confirm stable photocurrent values, validating the excellent stability of the self-supported waveguide integrated thermal radiation detector.

Combining CMOS process compatibility and spectral coverage up to 13 µm, this device offers an effective solution for high-performance mid-infrared on-chip spectroscopy applications, enabling the development of compact, highly sensitive molecular detection systems.

Reference

[1] Kim, J. Shim, J. Lim, J. Jeong, B. H. Kim, and S. Kim, "Highly-Sensitive Free-Standing Waveguide-Integrated Bolometer on Germanium-on-Insulator Platform for Mid-Infrared on-Chip Spectroscopy," in 2024 IEEE International Electron Devices Meeting (IEDM), 2024.