Room temperature thermopile THz sensor

In this paper, we present the design, fabrication, and characterization of a thermopile-based Terahertz (THz) sensor operating at room temperature The sensor is designed to detect electromagnetic fields at 3 THz and consists of a metallic grid absorber that is also one of the thermocouple materials We base the absorber design on a multilayer theoretical model incorporating equivalent resistivity calculations while considering small diffraction effects Since future applications may require sub-wavelength resolution we also explore how reducing the grid size affects equivalent resistivity The sensor is constructed on a circular silicon dioxide (SiO₂) membrane with a radius of 1.5 mm We evaluate its time constant using both THz and optical sources finding consistency with finite element simulations Sensitivity and detection limits are also analyzed However initial measurements at 0.3 THz instead of the designed 3 THz frequency—due to testing equipment limitations—show a sensitivity of 35 nV/(W/m²) and an electric field detection limit of 23 V/m The high noise levels are identified as a significant challenge prompting further developments to improve sensitivity.

The THz frequency range spanning 100 GHz to 10 THz has historically been difficult to exploit due to challenges in both the generation and detection of such waves In recent years however advancements in THz technology have made it increasingly useful for material characterization biomedical spectroscopy robotic vision and security applications Room temperature THz sensors with moderate sensitivity in the 100–3000 GHz range are particularly valuable in these applications as they avoid the complexities of cryogenic cooling Our project aims to develop a THz sensor that provides good spatial resolution while remaining easy to handle without bulky cooling systems The thermopile approach was chosen due to its compatibility with existing microfabrication techniques and the ability to miniaturize it at a reasonable cost. The sensor relies on a structured metallic absorber instead of a continuous thin film because thin metal films must be extremely thin—often on the nanometer scale—to achieve strong absorption which is difficult to control in microfabrication Instead the structured absorber consists of a metallic grid which allows for a more practical fabrication process while still maintaining high absorption The theoretical model used to design the grid accounts for the equivalent resistivity of the structure optimizing its absorption by considering different geometries and the presence of a dielectric layer The absorber is deposited on a SiO₂ membrane to enhance thermal isolation The thermopile is constructed from thin-film thermocouples chosen for their Seebeck coefficients Among the materials tested Ti/Al is selected for the first prototype due to its relatively high Seebeck coefficient and ease of fabrication A later version is planned with Ti/doped Si thermocouples which exhibit even higher Seebeck coefficients potentially improving sensitivity. To evaluate the absorption properties of the metallic grid we conduct measurements at 0.3 THz using titanium grids of various geometries deposited on glass substrates These measurements determine absorption through transmission and reflection analysis The experimental setup involves a THz electronic source focused onto the sample using parabolic mirrors while a liquid helium-cooled bolometer measures transmitted or reflected signals Comparison between experimental and theoretical absorption values reveals good agreement though some discrepancies arise due to factors such as thin film resistivity variations substrate thickness irregularities and lift-off process defects that introduce unwanted reflections To address this issue an improved fabrication method using wet etching instead of lift-off is implemented.

To investigate sub-wavelength sensor performance we study how grid dimensions smaller than the THz wavelength affect absorption Instead of directly testing at THz frequencies we perform these experiments at RF frequencies between 900 MHz and 10 GHz where the same electromagnetic behaviors apply without requiring prohibitively small structures We use a TEM transmission line to analyze how grid size affects resistivity finding that as the grid size decreases below the wavelength the effective resistivity increases due to current density limitations This insight informs the design of future sub-wavelength THz sensors.

The fabrication process involves several steps to create a thermally isolated structure The membrane is patterned using aluminum masks and deep reactive ion etching (DRIE) followed by thermal oxidation to create the SiO₂ membrane The metallic grid and first thermocouple track are deposited using evaporation and lift-off techniques followed by deposition of the second thermocouple track in aluminum To enhance absorption at 3 THz and improve mechanical robustness a dielectric layer of SU-8 is added with a thickness corresponding to a quarter-wavelength of the target frequency The final step involves individual membrane release to prevent non-uniform etching The completed sensor is characterized using scanning electron microscopy to verify structural integrity. Characterization of the sensor involves measuring its thermal time constant using a modulated THz source The response exhibits a low-pass filter characteristic with a cut-off frequency of approximately 0.8 Hz corresponding to a time constant of 200 ms Finite element simulations using COMSOL Multiphysics confirm this value showing that convective heat transfer plays a crucial role in determining response time.

Sensitivity and detection limits are evaluated by placing the sensor 5 mm behind a horn antenna emitting a 0.3 THz signal rather than the intended 3 THz frequency due to equipment limitations The sensor detects a signal of 550 nV for an incident power of 118 µW Simulations predict a temperature increase of 41 mK at steady-state and 25 mK under modulated conditions The measured sensor impedance is 5.7 kΩ and the estimated responsivity is 35 nV/(W/m²) The noise-equivalent power (NEP) is approximately 8 × 10⁻⁵ W/Hz¹/² which is relatively high indicating the need for further improvements The limit of detection is estimated at 23 V/m which is suitable for many applications but could be improved by reducing noise levels. To demonstrate the sensor’s potential we use it to map the H-plane of a 0.3 THz horn antenna A micrometric XY translation stage is used to scan the sensor across the beam profile revealing the expected radiation pattern However the signal-to-noise ratio remains a limiting factor for detailed imaging applications Future improvements include replacing Ti/Al thermocouples with Ti/doped Si thermocouples which have a Seebeck coefficient 25 times higher than Ti/Al This change is expected to significantly improve sensitivity and signal-to-noise ratio Additionally removing the SU-8 layer which does not contribute to absorption at 0.3 THz will further enhance performance These modifications will allow for reduced sensor dimensions improving spatial resolution for near-field applications.

In conclusion we develop and characterize a room-temperature THz sensor based on a thermopile The design incorporates a structured metallic absorber on a SiO₂ membrane to optimize thermal isolation while maintaining a practical fabrication process Measurements confirm that absorption follows theoretical predictions while revealing challenges related to noise and sensitivity Future iterations will incorporate higher performance thermocouples and optimized absorber structures to enhance responsivity and resolution paving the way for compact efficient THz sensing in various applications.

Bibtex citation : 

@article{mbarek2013room,
  title={Room temperature thermopile THz sensor},
  author={Mbarek, Sofiane Ben and Euphrasie, S{\'e}bastien and Baron, Thomas and Thiery, Laurent and Vairac, Pascal and Cretin, Bernard and Guillet, Jean-Paul and Chusseau, Laurent},
  journal={Sensors and Actuators A: Physical},
  volume={193},
  pages={155--160},
  year={2013},
  publisher={Elsevier}
}