Calibration of Microscope-coupled Fourier Transform Infrared Spectrometers for CW and Modulated Light Emission Measurements

TL;DR

This work tackles the calibration challenge of microscope-coupled FTIR for measuring weak infrared emission from microstructures by introducing an emissivity-based calibration that leverages a heated doped-silicon radiator. A responsivity function $m(\tilde{\nu})$ is extracted using a differential two-temperature approach that cancels background, relating raw counts to spectral power via $S_{\mathrm{th}}(\tilde{\nu},T)=\theta\epsilon(\tilde{\nu},T)B(\tilde{\nu},T)$. The method is demonstrated on a Bruker Invenio R/Hyperion II system with MCT and InSb detectors in both CW and step-scan modes, showing that $m(\tilde{\nu})$ is largely throughput-independent and that the measured instrument noise floor agrees with theoretical expectations. Results indicate lower noise in step-scan due to background rejection and confirm the practical viability for quantitative, throughput-aware emission spectroscopy at micro/nano scales. This calibration enables more accurate MEMS and NFTPV-related infrared emission studies using microscope-coupled FTIR.

Abstract

Measurement of low power infrared light emission spectra from microstructures can be challenging, but is of key importance in several research fields. Fourier transform infrared spectrometers (FTIR) can be used for characterizing such weak light emitters, but this requires additional custom user calibration compared to traditional FTIR measurements of, e.g., transmission or reflection. These calibration techniques are well documented for standalone FTIR instruments but not for microscope coupled-FTIRs, even though such an architecture greatly simplifies collection of light from micro and nano scale structures. We propose and demonstrate a calibration method for microsope-FTIRs based on the well-known emissivity of doped silicon at high temperature. With this method, we measure responsivity and noise floor of a recently installed microscope-FTIR instrument (Bruker\textsuperscript{\textcopyright} Invenio\textsuperscript{\textregistered} R coupled with a Hyperion II microscope), which is found to be within theoretically predicted values. The method is demonstrated for two different detectors (Mercury Cadmium Telluride and Indium Antimonide), in both continuous wave (CW) and modulated (step-scan) emission measurements mode.

Figures (11)

• Figure 1: a) Schematic of the microscope-coupled-FTIR, reproduced from bruker2015invenioR, with a zoomed in view of the controlled thermal emitter. b) Temperature of the thermal emitter measured from the resistance change in the metal ceramic heater and from a resistance temperature detector. c) Calculated spectral emissivity of the emitter at various temperatures.
• Figure 2: a) Measured raw emission signal $S_{\mathrm{Si,meas}}$ collected with the MCT detector in CW mode. b) Response function $m(\tilde{\nu})$ of the MCT detector in CW mode. c) Measured raw emission signal $S_{\mathrm{Si,meas}}$ collected with the MCT detector in step-scan mode. d) Response function $m(\tilde{\nu})$ of the MCT detector in step-scan mode. All measurements are taken at various emitter temperatures ($T_\mathrm{RTD}$).
• Figure 3: a) Measured raw emission signal $S_{\mathrm{Si,meas}}$ collected with the InSb detector in CW mode. b) Response function $m(\tilde{\nu})$ of the InSb detector in CW mode. c) Measured raw emission signal $S_{\mathrm{Si,meas}}$ collected with the InSb detector in step-scan mode. d) Response function $m(\tilde{\nu})$ of the InSb detector in step-scan mode. All measurements are taken at various emitter temperatures ($T_\mathrm{RTD}$).
• Figure 4: a) Theoretical and measured noise of the FTIR equipped with the MCT detector, in CW mode, including the detector noise and the thermal background. The resolution is $\Delta\tilde{\nu}=4$ cm-1 b) Theoretical and measured noise of the MCT detector, in step-scan for multiple resolutions. In step-scan we only include the detector noise since there is no thermal background.
• Figure 5: a) Theoretical and measured noise of the FTIR equipped with the InSb detector, in CW mode, including the detector noise and the thermal background. The resolution is $\Delta\tilde{\nu}=4$ cm-1 b) Theoretical and measured noise of the InSb detector, in step-scan for multiple resolutions. In step-scan we only include the detector noise since there is no thermal background.