IRAM Progress Report FP6 - RadioNet - AMSTAR (May-October 2004)

W.P. HEB-Mixer film fabrication and optimization

This work package concerns the development of phonon-cooled Hot Electron Bolometers (HEB) Mixers for observations above 1 THz (SOFIA) with the goal of fundamental study and optimization of ultrathin NbN film parameters.

The heterodyne detection of weak THz signals in radioastronomy with ultrathin superconducting microbridge bolometers necessitates optimized NbN film qualities to allow mixing with IF bandwidths up to 10 GHz. In this first step the following activities has been started:

• Production of 100 nm thick NbN films by RF sputtering of niobium in a nitrogen/methane/argon atmosphere to control the process parameters and the superconducting transport parameters of the deposited films ($Tc \approx 15$ K, resistance ratio $\approx 0.96$, specific resistance $\approx 130\; \mu\Omega$cm).
• Production of ultrathin (3 - 5 nm) NbN films on fused quartz with a 15 nm thick MgO seed layer ( $T_c \approx 12$ K).

  Figure 4:

  
• Determination of the optical constants of the deposited NbN films by means of ellipsometry (complex refractive index ${\bf n} = 1.89 - i \times 4.36$). The large value of the adsorption coefficient corresponds to a light penetration depth of about d = 5 nm. Therefore, Raman spectroscopy is a useful method to investigate very thin NbN films and their acoustic properties for the phonon cooling mechanism.
• Measurement of acoustic phonon density of states of thick and ultrathin (3 - 5 nm thick) NbN films by Raman spectroscopy. The measurements will help to derive the acoustic phonon matching for the realization of high IF bandwidths in the future HEB devices. A first result (phonon wavenumber $\approx 150$ cm$^{-1}$) is shown in Figure 5.

  Figure 5:

  
• The evaluation of the crystalline film growth and the corresponding superconducting transport properties depending on the substrate temperature (range: room temperature to $700^\circ$C) during the deposition process will be carried out in the near future. Raman spectra for different deposition temperatures are already measured in the range of room temperature until $300^\circ$C (Fig. 6). In this low temperature range the spectra show similar behaviour. For higher temperatures a calibration procedure is necessary and will be done in the next fabrication step.

  Figure 6:

  
• The design and manufacturing of a 230 and 650 GHz photolithographic mask set for HEBs (3 chromium masks) with 0.5 and 1 micron bridge length for rapid IF bandwidth measurement up to 10 GHz is completed.

• Fabrication of a first batch on fused quartz / 15 nm MgO / 5nm NbN. The DC I/V-characteristics of the HEB test device is drawn in Figure 7.

  Figure 7:

  
• Preliminary mixer measurements at LO frequency of 230 GHz in the LHe-cryostat shows an IF bandwidth of about $\Delta f = 2$ GHz. The NbN film deposition temperature in this first test device was room temperature. We used the bandwidth measurement setup described in Fig. 8.

  Figure 8:

  

Publications

• T. A. Scherer, M. Frommberger, M. Schicke, K. Schuster, ``Development of Phonon Cooled Hot-Electron-Bolometers (HEB) for Applications in THz Radioastronomy'' , Proc. Of the 29th Int. Conf. On Infrared and Millimeter Waves, 12th Int. Conf. On Terahertz Electronics, Sept. 27 - Oct. 1 2004, Karlsruhe, Germany.

• T. A. Scherer, M. Frommberger, M. Schicke, K. Schuster, ``Entwicklung Phonon-gekuehlter Hot-Electron-Bolometer (HEB) fuer Anwendungen in der THz-Radioastronomie'' , Kryoelektronische Bauelemente 2004, Goslar, Germany, 12.- 14. Sept. 2004.

T. A. Scherer, K. Schuster, M. Schicke