[Paper Review] Micro-thermocouple on nano-membrane: thermometer for nanoscale measurements

Armandas Balčytis, Meguya Ryu+2 more|arXiv (Cornell University)|Apr 8, 2018

Thermal properties of materials11 references3 citations

TL;DR

This paper presents a micro-thermocouple fabricated on a 30-nm-thick Si₃N₄ nano-membrane using low-resolution electron beam lithography and lift-off, enabling nanoscale thermal sensing with high spatial and temporal resolution. The device achieves a temperature response rate of up to ~2×10⁵ K/s at 10 kHz, demonstrating sub-micron spatial resolution and real-time detection of rapid thermal changes from laser and electron beam excitation.

ABSTRACT

A thermocouple of Au-Ni with only 2.5-micrometers-wide electrodes on a 30-nm-thick Si3N4 membrane was fabricated by a simple low-resolution electron beam lithography and lift off procedure. The thermocouple is shown to be sensitive to heat generated by laser as well as an electron beam. Nano-thin membrane was used to reach a high spatial resolution of energy deposition and to realise a heat source of sub-1 micrometer diameter. This was achieved due to a limited generation of secondary electrons, which increase a lateral energy deposition. A low thermal capacitance of the fabricated devices is useful for the real time monitoring of small and fast temperature changes, e.g., due to convection, and can be detected through an optical and mechanical barrier of the nano-thin membrane. Temperature changes up to ~2x10^5 K/s can be measured at 10 kHz rate. A simultaneous down-sizing of both, the heat detector and heat source strongly required for creation of thermal microscopy is demonstrated. Peculiarities of Seebeck constant (thermopower) dependence on electron injection into thermocouple are discussed. Modeling of thermal flows on a nano-membrane with presence of a micro-thermocouple was carried out to compare with experimentally measured temporal response.

Motivation & Objective

• To develop a miniaturized thermocouple for direct, real-time measurement of nanoscale thermal phenomena with high spatial and temporal resolution.
• To address the limitations of conventional micro-thermocouples, which suffer from high thermal capacitance and poor spatial resolution due to large heat capacity and secondary electron effects.
• To enable simultaneous miniaturization of both heat source and detector for thermal microscopy applications.
• To investigate the influence of electron injection on thermopower (Seebeck coefficient) in ultra-small junctions under electron beam exposure.
• To provide a calibration method for absolute temperature measurements in nanoscale devices using reference junctions on the same substrate.

Proposed method

• Fabrication of Au-Ni micro-thermocouples with 2.5-µm-wide electrodes on 30-nm-thick Si₃N₄ membranes using low-resolution electron beam lithography and lift-off processes.
• Use of a laser-scribed optical mask to define secondary contact pads, enabling electrical interfacing with minimal thermal gradient.
• Employment of a Cr adhesion layer (5 nm) followed by 50 nm Au and 50 nm Ni deposition via thermal evaporation, with lift-off to define the thermocouple structure.
• Utilization of an Au-Au junction as a reference thermocouple on the same substrate to enable relative temperature measurements via optical modulation and lock-in detection.
• Application of a 10.1 µV/K sensitivity calibration constant derived from similar Au-Ni thermocouples for temperature estimation under laser and electron beam excitation.
• Thermal modeling of heat dissipation in the nano-membrane system to compare with experimental temporal response and identify dominant heat transfer pathways.

Experimental results

Research questions

• RQ1 Can a micro-thermocouple on a 30-nm Si₃N₄ membrane achieve sub-micron spatial resolution in thermal sensing while maintaining high temporal response?
• RQ2 How does the thermal response of the nano-membrane device compare to bulk substrates in terms of heat dissipation and secondary electron generation?
• RQ3 What is the impact of electron beam-induced electron injection on the Seebeck coefficient and thermopower in ultra-small Au-Ni junctions?
• RQ4 To what extent does the low thermal capacitance of the nano-membrane enable real-time monitoring of fast temperature changes (e.g., >10⁵ K/s)?
• RQ5 Can the thermocouple be effectively calibrated for absolute temperature measurements in nanoscale devices with varying film thicknesses?

Key findings

• The micro-thermocouple on a 30-nm Si₃N₄ membrane demonstrated a temporal response rate of up to ~2×10⁵ K/s at a 10 kHz measurement rate, enabling real-time monitoring of rapid thermal transients.
• Laser heating at 1 mW power induced a measurable temperature rise of ~0.1 K, confirming sensitivity to low-energy inputs.
• The nano-membrane reduced secondary electron generation, limiting lateral energy deposition to ~1–2 µm cross-section, which enhances spatial resolution compared to bulk substrates.
• Thermal modeling showed that heat dissipation is dominated by the Si₃N₄ membrane, but experimental data revealed faster dissipation than predicted, indicating a significant contribution from electron injection and metallic leads.
• The Seebeck coefficient of the Au-Ni junction was found to be sensitive to electron injection, with dynamic changes in thermopower observed under electron beam exposure, requiring separation from pure thermal effects.
• The device enabled direct, non-contact thermal monitoring of samples on the opposite side of the membrane, such as biological cells in buffer solution, with potential for in situ applications in synchrotron radiation and optical microscopy.