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Nexray

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This project targets the development of novel pocket X-ray sources and X-ray direct detectors that will be combined in a distributed network to solve important tasks, for example in the field of …

This project targets the development of novel pocket X-ray sources and X-ray direct detectors that will be combined in a distributed network to solve important tasks, for example in the field of security, by ensuring reliable and real-time monitoring of failure sensitive parts in large manufacturing plants or in public transportation.

The miniaturized X-ray sources are based on multi-wall carbon nanotube (CNT) cold electron emitters and advanced microsystems technology. The electron field emission properties of CNTs, with their high current densities, make them prime candidates for cold emitter cathodes. Using CNT cold electron emitters will make it possible to miniaturize the whole X-ray source. Additionally, as opposed to classical thermionic emission, field electron emission of the CNT is voltage-controlled which allows for high modulation frequencies up to GHz level. The X-ray direct detectors in turn are based on crystalline germanium absorption layers grown directly on a CMOS sensor chip yielding high resolution and high sensitivity X-ray detectors. Single photon detection will allow for a significant improvement of contrast for applications in security, health care and nondestructive testing.

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  • 1. Nexray A. Dommann A , H. von Känel C , P. Gröning B , N. Blanc A , C. A. Bosshard A , A. D. Brenzikofer A , S. Giudice A , R. Jose James A , R. Kaufmann A , C. Kottler A , C. Lotto A , A. Neels A , P. Niedermann A , P. Seitz A , G. Spinola Durante A , C. Urban A , H.R. Elsener B , O. Gröning B , B. Batlogg C , C.V. Falub C , K. Mattenberger C , E. Müller C , P. Wägli C Bern, 13. 5. 2011 A: CSEM; B: EMPA, C: ETHZ Network of integrated miniaturized X-ray systems operating in complex environments
  • 2. A system approach Source Sample Detector Contrast mechanism Resolution, Size, Efficiency Spectrum, power, Coherence, Size Miniaturized, fast and programmable X-ray sources Phase contrast X-ray imaging Direct X-ray detectors Breakthroughs in all key building blocks of X-ray systems: Sources, Contrast mechanism and Detectors
  • 3. Network of integrated miniaturized X-ray systems operating in complex environments Single-photon solid-state X-ray detection Si-Ge layers for high-energy X-ray detection Phase contrast X-ray imaging Miniaturized, fast and programmable X-ray sources
  • 4. Static Computed Tomography
    • Array of sources replaces moving parts in CT-systems
    • Sequencial operation of sources, also with alternating high voltage
  • 5. Novel Concepts of Applications
    • Large area X-ray sources
    • Pixelated X-ray sources
    • Pulsed operation of X-ray source (and individual source-pixels)
    • Highly efficient sensors, applicable in medical diagnostics
    • Energy resolved X-ray image detection
    High frequency source modulation compatible with ToF-technology Allows for distance measurement to object in reflexion geometry
  • 6. Medicine and Nondestructive Testing
    • Static CT for emergency medicine
    • Miniaturised X-ray systems for monitoring purposes during surgery, e.g. for cardiovascular or brain surgeries
    • Large area sources for radiation therapies
    • Fast static CT for in-line product inspection
    • Imaging of fast phenomena due to high switching frequency of cold electron emitters
    • Depth measurements inside objects due to TOF operation mode
  • 7. A system approach Source Sample Detector Contrast mechanism Resolution, Size, Efficiency Spectrum, power, Coherence, Size Miniaturized, fast and programmable X-ray sources Phase contrast X-ray imaging Direct X-ray detectors
  • 8. X-ray source microfabrication Extraction Anode Emission Cathode Diamond X-ray Window
  • 9. Plasma Enhanced-CVD growth of CNTs Utilization of a Plasma during deposition allows the growth of vertically oriented CNTs Ni dot of Da = 70 nm -> catalyst for growth of straight CNTs TiN for homogenisation of CNTs electron emission
  • 10. X-ray source packaging aspects
    • Research on multilayer UBM stable at high temperature for CNT deposition (600°C)
    •  Annealing, bonding and hermeticity tests with different combinations of evaporated thin films
    • Tests based on AuSn for high vacuum packaging
    •  10 -5 mbar required for functioning of CNTs
    •  Tests with AuSn bonding processes allowing getter integration and activation
    Pt UBM Au UBM showing good hermeticity ζ phase Eutectic gold tin Au UBM
  • 11. High vacuum sealing of test vehicle
      • Goal :
      • - Tests with all the developed elements together and characterization
      • Increase in melting point of solder and getter activation during life time
      • Vacuum level measurement and finer hermeticity test with µPirani
    CNT substrate Thin film getter µPirani AuSn solder ring
  • 12. X-ray source experimental platform: The concept
  • 13. Silicon chips for cathodes
    • For development of high vacuum hermetic packaging
    • Different variants of Pt and Au based UBM metal stacks
    • 2 wafer runs
  • 14. Microfabricated grids 2 x 2 mm grid 10 µm grid lines Diced wafer
  • 15. Emission characteristics: longtime-stability Applied elec. field 20, 100, 500 µA Longtime measurement: 13 h Distance: 20 µm Emission current: 50 µA (constant) I-V measurement after longtime test
  • 16. A system approach Source Sample Detector Contrast mechanism Resolution, Size, Efficiency Spectrum, power, Coherence, Size Miniaturized, fast and programmable X-ray sources Phase contrast X-ray imaging Direct X-ray detectors
  • 17. Low-Energy Plasma-Enhanced CVD (LEPECVD) • Electrons emitted by a hot filament sustain a DC plasma • Low (~10eV) ion energy – no ion damage • Discharge confined by a magnetic field (~1 mT) • Deposition rates 0.01-10nm/s depending on gas flow and plasma density • Gas phase precursors: SiH 4 , GeH 4
  • 18. CHALLENGES: Mismatched Epitaxy, e.g. Si-Ge Si Ge cracks Si Ge TD MD Ge Si • Lattice mismatch ( strain = 4.2 %). • Mismatch of thermal expansion coefficients.
    • High density of misfit (MD) and threading dislocations (TD), wafer bowing and cracks, which can significantly degrade the performance of a device.
  • 19. Problems related to Si:Ge Epitaxy LATTICE MISMATCH (a Si = 0.543095 nm, a Ge = 5.564613 nm   a/a = 4.2 % compressive ) Ge % Si Ge
    • Only 4 monolayers of Ge (~ 2.2. nm) can be grown epitaxially on Si !
    • Plastic Deformation (i.e. relaxation) by misfit ( M ) and threading ( T ) dislocations: bad quality
    Threading dislocations Strained Ge on Si substrate strained Ge bulk Si a ┴ > a Si a ║ = a Si relaxed Ge bulk Si Misfit Relaxed SiGe on Si substrate a 0 a 0 Misfit Threading
  • 20. Monolithic Integration on CMOS Wafers Demonstrated
    • Monolithic integration of Ge photodetectors on CMOS demonstrated for infrared applications (2 µm layer thickness)
    • 64 x 64 pixel NIR image sensor exists
    • Optimisation of process is going on
  • 21. INNOVATION: Self-aligned epitaxial Ge crystals Micromachined Si pillars Epitaxial Ge pillars on Si Ge Si 5  m Ge ~30  m No limitation for layer thickness!
  • 22. INNOVATION: Selective Epitaxy on pre-patterned Si Ge fully relaxed Ge partially strained (0.14%) Perfect crystal structure despite lattice strain! Perfect basic understanding of the growth morphology Simulations Experiment
  • 23. Defect free Pillars
  • 24. Nexray detector technology & chip schematic concept
  • 25. SiGe Pillars RSMs on Ge/Si(004) and Ge/Si(115) – measured on patterned part of the wafer Relaxed Ge (115) (004) Si-Substrate Patterned: Very small mosaicity. No tilt compared to #56558.
  • 26. Photon Counting Circuits
    • X-ray quantum counting: Every single X-ray photon is counted
    • Test-chip exists
    • Low noise circuit with band-pass filtering
    • Measured noise limit of 12 e - RMS at 1 µs pulse length
    • X-ray energy resolution possible with pulse-height measurements
  • 27. THANK YOU FOR YOUR ATTENTION

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