Mathematics – Logic
Scientific paper
Apr 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt........10p&link_type=abstract
Inaugural-Dissertation, University of Koln, 23 April 2007, 153 pages
Mathematics
Logic
Scientific paper
Many interesting astronomical objects, such as galaxies, molecular clouds, star-forming regions, protostars, evolved stars, planets, and comets, have rich submillimeter spectra. In order to avoid line blending and to resolve line shapes, most of this information can only be obtained using high resolution spectroscopy. The incoming signals are very weak and the receiver must be therefore extremely low-noise. High resolution spectroscopy of weak sources is carried out primarily using heterodyne receivers. Direct detection is intrinsically more sensitive than heterodyne but is constrained by the spectrometers which are needed to achieve the required spectral resolution (~107, see chapter 1). In a heterodyne receiver the astronomical signal is downconverted to much lower frequencies and amplified prior to spectral analysis. Therefore, much lower relative spectral resolution is needed to achieve the same absolute spectral resolution (easily in exceed of 107). In the heterodyne receiver the astronomical signal is mixed with a stronger monochromatic source (Local Oscillator, LO) to the difference frequency (Intermediate Frequency, IF). A basic part of the receiver is the mixer, which is a device with a non-linear response to the signal intensity amplitude and downconverts the received signal to the IF (~1-10 GHz), which is then amplified. Since the mixer is the first active component in the receiver, it has the greatest influence on the total receiver noise (see chapter 1). Best mixer performance is currently achieved with Superconductor-Insulator-Superconductor (SIS) junctions, but their highest operation frequency is intrinsically limited by the energy gap of the superconductors used. Above ~1 THz superconducting phonon-cooled Hot Electron Bolometers (HEB) are nowadays the mixer of choice for heterodyne receivers. Superconducting phonon-cooled HEBs consist of an ultrathin (<5 nm) superconducting NbTiN (or NbN) layer between two normal conducting electrodes (heat sinks) and embedded in a THz coupling environment. The HEB is operated at 4.2 K. The RF power is coupled into the superconducting film. The power dissipation in the film brings it to its critical temperature (~8-9 K) in which a small variation in the dissipated power results in a large variation in the device resistance. The resistance change is translated into an IF voltage variation by the bias current. Since the dissipated power is proportional to the square of the sum of the signal intensities, the voltage drop over the HEB resistance is modulated at the intermediate frequency of the receiver, which can be amplified and analyzed by the following spectrometer. The cooling performance of the HEB limits the response time in which the HEB can follow the variations of the dissipated power and therefore its IF bandwidth. The main cooling mechanism of the superconducting layer is the phonon transport to the substrate (phonon-cooled HEB). Therefore, to enhance the IF bandwidth the superconducting NbTiN or NbN layer has to be made extremely thin (<5 nm). An IF bandwidth of at least several GHz is desired to register spectral lines with large Doppler width. This corresponds to a HEB response time t of just some tens picoseconds. The few groups worldwide developing HEB-based THz-receivers have concentrated on a lens-substrate mixer mount. It is easy in handling due to the thick device substrates but the coupling efficiency of a planar antenna/dielectric lens system to a Gaussian beam is often not as good as that of a high performance waveguide horn. The mixers fabricated during this thesis use waveguide mounts. Waveguide manufacturing is technically demanding at THz frequencies due to the small dimensions. Scaling the dielectric substrate to frequencies beyond 1 THz becomes increasingly difficult. In order to avoid RF loss due to waveguide modes in the substrate channel, a quartz substrate at 1.9 THz would have to be 10 mm thick or less. Considerably thinner substrates have been realized in this thesis by fabricating the mixer device on a thin (i.e. 2 micron) membrane layer deposited on a bulk silicon carrier wafer. The membrane is released by backside etching of the wafer after device fabrication. A large supporting frame is needed around the device for handling and contacting. A possible approach is to fabricate the frame and the mixer simultaneously on one wafer. In the process presented here the membrane-HEB devices and the supporting frames are fabricated separately avoiding the loss of wafer "real estate" for the support frames and thus allowing for up to 690 devices to be simultaneously produced on a single 30 mm square wafer. Fabricating many identical devices for arrays is possible even if many devices are damaged during fabrication. Further it is possible to test many different parameter-variations on one single wafer. The price to pay is a slightly more involved assembly procedure, which has been realized for the first time for 1.4 and 1.9 THz in this thesis. The fabrication of the phonon-cooled HEBs was fully realized at the KOSMA clean room facilities. The sputter deposition of ultra-thin NbTiN films with critical temperatures as high as 8.5 K is one of the technological highlights, already described at [1]. This thesis focused on the optimization of the contact interfaces between the electrodes and the ultrathin superconducting layer. Etching the interface previous to deposition of the electrodes leads to a better control over the interface. The effect of the etching parameters on the HEBs has been investigated. To restore the superconducting film properties that might be affected by the cleaning process, a NbTiN layer (20 nm) is deposited on top of the contact area. The additional superconducting layer between the bolometer thin film and the heat sinks has a positive effect on the mixer performance but introduced complications in the fabrication process which have been solved in this thesis. The fabricated waveguide HEB mixers on thin silicon nitride membranes for GREAT on SOFIA and CONDOR on APEX have been demonstrated at LO frequencies of 1.4 THz and 1.9 THz with good noise performance and excellent stability (Allan Stability time over 30 s) as compared to results of the few other groups. The noise temperature shows a flat response over the measured IF-bandwidth (1.2-1.75 GHz). The 1.4 THz mixer has already been used at the APEX telescope.
No associations
LandOfFree
Waveguide Heterodyne Mixers at THz-Frequencies - Superconducting Hot Electron Bolometers on 2-micron Si3N4 Membranes for GREAT and CONDOR does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Waveguide Heterodyne Mixers at THz-Frequencies - Superconducting Hot Electron Bolometers on 2-micron Si3N4 Membranes for GREAT and CONDOR, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Waveguide Heterodyne Mixers at THz-Frequencies - Superconducting Hot Electron Bolometers on 2-micron Si3N4 Membranes for GREAT and CONDOR will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1360408