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Advances in mm-wave and THz Region M aterials Measurements

Advances in mm-wave and THz Region M aterials Measurements. Jon Martens Jeffrey Hesler Anritsu Company Virginia Diodes Inc. Agenda. Background Measurement systems Quasi-Optical method Lower mm-wave free-space method and waveguide methods

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Advances in mm-wave and THz Region M aterials Measurements

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  1. Advances in mm-wave and THz Region Materials Measurements Jon Martens Jeffrey Hesler Anritsu Company Virginia Diodes Inc. EuMW Seminars 2013

  2. Agenda • Background • Measurement systems • Quasi-Optical method • Lower mm-wave free-space method and waveguide methods • Mm-wave open coax method for liquids • Summary EuMW Seminars 2013

  3. Background • Millimeter-wave materials measurements are increasingly needed for electronic applications, better chemical and biological models, for materials development, etc. • Many lower frequency methods extend logically with reduced sample size requirements but some things change • Free-space and quasi-optical methods are more practical and can be very useful • S-parameter uncertainties change and this can affect extraction • The materials parameters change which also affect uncertainty distributions EuMW Seminars 2013

  4. Anritsu measurement systems The VectorStar VNA (20, 40, 50 and 70 GHz versions) forms the basis for many mm-wave measurements. The base VNA can be combined with the 3743A mm-wave modules for broadband coverage of 70 kHz-125 GHz… … or used with VDI modules for much higher frequency measurements. EuMW Seminars 2013

  5. Virginia Diodes – VDI Company Overview • Company founded to respond to the needs of the scientific community for THz sources and receivers • Founded in 1996, focused on diodes • Reorganized in 2001, added components and systems • Originally focused primarily on astronomy, spectroscopy and plasma diagnostics • Field now expanding • Imaging, radar, EPR/NMR, communications and general THz test and measurement • Developed a full range of broadband electrically tunable solid state sources and detectors • Components from 50 GHz to 3 THz • Ambient, no mechanical tuning • Applying this technology to THz systems • ESR/NMR Systems • THz VNA Extenders EuMW Seminars 2013

  6. VDI - Core TechnologyUse nonlinear diodes to extend the frequency range of traditional microwave electronics Microwave Technology VDI Technology X8 16.7 GHz 40 GHz 40 GHz 320 GHz X3 20mW 1.5W Schottky Diodes Planar Advanced fabrication technology • CAD Design • First-time design • Broadband & Tunerless • High Efficiency EuMW Seminars 2013

  7. VDI VNA Extenders • VDI Extenders available from WM-2540 (WR-10, 75-110 GHz) thru WM-250 (WR-1.0, 750-1050GHz) • State-of-the-art Dynamic range • 120 dB (typ.) at WM-2540 (WR-10, 70-110 GHz) • 120 dB (typ.) at WM-1295 (WR-5.1, 40-220 GHz) • 100 dB (typ.) at WM-380 (WR-1.5, 500-750 GHz) • 70 dB (typ.) at WM-310 (WR-1.2, 600-900 GHz) • 60 dB (typ.) at WM-250 (WR-1.0, 750-1050 GHz) • Excellent amplitude and phase stability EuMW Seminars 2013

  8. THz Frequency Extension of a VNA 50 GHz VNA RF, LO & IF Signal Cables THz Extenders Waveguide Test Ports EuMW Seminars 2013

  9. WM-250 (WR-1.0) VNA Extender • Dynamic Range: 60 dB typical at 10Hz BW • Dynamic Range: 40 dB minimum at 10Hz BW • Magnitude Stability: ±1 dB • Phase Stability: ±15° • Test Port Power: -35 dBm • Test Port Input Limit (dBm, saturation/damage): -20/13 • Directivity: 30 dB • Typical Dimensions: 8 x 5 x 3 inches EuMW Seminars 2013

  10. THz Waveguide Calibration • mm-Wave waveguide calibration methods • Short-Open-Load-Thru (SOLT, TOSM) • Open typically uses ¼-wave delayed short • Thru-Reflect-Line (TRL, LRL) • Line is typically a ¼-wave thru shim • Many others possibilities as well… • Sub-mm wave introduces a new set of challenges • Thru-Reflect-Line (TRL, LRL) • ¼-wave shims difficult to fabricate and fragile • Common to instead use two lines with ¼-wave difference in length • However, this means more connections and interfaces  less accuracy • Short-Open-Load-Thru (SOLT, TOSM) • Challenging to achieve a high return loss load Machined Quarter-wave Delay Short EuMW Seminars 2013

  11. WM-380 (WR-1.5) Precision Loads EuMW Seminars 2013

  12. WM-250 (WR-1) Calibrated Measurements • Calibrated waveguide measurements were performed using the Extender • Short-Open-Load-Thru calibration method • ¼-wave delay was used as the Open standard • 1 kHz IF Bandwidth for calibrated measurements WR-1 Waveguide Milled Delay Quarter-wave Delay Precision Load EuMW Seminars 2013

  13. Quasi-Optical Measurements Anritsu VectorStar VDI WM-250 (WR-1) Extender Test Port with Feedhorn Reflecting Plate Off-Axis Parabolic Focusing Mirrors EuMW Seminars 2013

  14. Two-Port Quasi-Optical Measurements 2-Port Measurement in Collimated Beam • Requires large sample • Sample in collimated beam • Can use small sample • Good for devices (grid arrays, focused optics, …) • Difficult Alignment Feedhorn Feedhorn Lens Lens DUT 2-Port Measurement in Focused Beam EuMW Seminars 2013

  15. One-Port Quasi-Optical Measurements 1-Port Measurement in Collimated Beam • Requires large sample • Sample in collimated beam • Can use small sample • Good for devices (grid arrays, focused optics, …) Reflecting Plate Feedhorn Lens DUT 1-Port Measurement in Focused Beam EuMW Seminars 2013

  16. Quasi-Optical Measurement Calibration Measurement of Sample in Focused Beam • Slide lens and sample as a unit • Translation occurs in collimated section • Improved accuracy, minimal effect on focusing • e.g. see Arsenovic 2013 MTT • Measure multiple delay lengths EuMW Seminars 2013

  17. One-Port Quasi-Optical Measurement Setup Reflecting Plate Test Port with Diagonal Feedhorn Device Under Test taped to plate Collimated Beam EuMW Seminars 2013

  18. Quasi-Optical Measurement Setup Focusing Mirror and Sample move as a unit EuMW Seminars 2013

  19. Over-Determined Least Squares Calibration • For calibration, use an un-weighted least squares method • e.g. see Wong 2004 ARFTG • Calibration standards • Series of delayed reflections with known delay distances • Metal plate at focal point of QO system • Metal plate and focusing mirror are both mounted on the same moving stage • Matched load • Used absorber at 45 degree angle as rough termination • Measurement bandwidth 300 Hz EuMW Seminars 2013

  20. Over-determined Least Squares Calibration EuMW Seminars 2013

  21. Evaluation of Calibration Quality • Look at S-parameters of the calibration standards • Over-determined calibration  a measure of the calibration quality • Gives an indication of the measurement quality that can be achieved EuMW Seminars 2013

  22. S-Parameters of Quasi-Optical System • A measurement of the S-parameters of the Quasi-Optical system • Measured using a two-tiered extraction method • Uses a waveguide calibration followed by a separate QO calibration • See Arsenovic 2013 MTT • Vector Star has onboard two-tiered calibration available • Network Extraction Water Line EuMW Seminars 2013

  23. Measurements of Dielectric Samples 0.014” Aluminum NItride • Samples were taped to the metal plate at the focal point • Repeated measurements were made of each sample • Sample removed from plate and re-mounted for each measurement • Calibration using the QO method described earlier • Not a two-tiered calibration 1 mm Quartz (Fused Silica) EuMW Seminars 2013

  24. 0.014” Aluminum Nitride EuMW Seminars 2013

  25. 0.014” Aluminum Nitride EuMW Seminars 2013

  26. 0.014” Aluminum Nitride EuMW Seminars 2013

  27. 0.014” Aluminum Nitride EuMW Seminars 2013

  28. 1 mm Quartz (Fused Silica) EuMW Seminars 2013

  29. 1 mm Quartz (Fused Silica) EuMW Seminars 2013

  30. 1 mm Quartz (Fused Silica) EuMW Seminars 2013

  31. 1 mm Quartz (Fused Silica) EuMW Seminars 2013

  32. Lower mm-wave free-space • TRL calibrations are another option; particularly in the 100 GHz range • The position for the Line is not critical as long as under a half-wavelength (1.5mm at 100 GHz). A stop on the translation stage allows a relatively precise return to the main reference plane. • A shorting plate at the reference plane can be the reflect. The sensitivities to non-planarities here are surprisingly low. • Multipath reflections are still a larger concern. Main reference plane Translation stage to position L Absorber To VNA port 1 To VNA port 2 EuMW Seminars 2013

  33. mm-wave free-space • Fairly large reflect asymmetries have relatively small impact. Of course, it could be tilted enough to make it into a load standard…. Multipath can be an issue without absorbers or another strategy. To port 2 To port 1 EuMW Seminars 2013

  34. mm-wave free-space: cellulose example • In the first measurement, multipath control and focusing were only partially adequate. Uncertainties increased… • Gating does help again (gate width here ~1 cm) . EuMW Seminars 2013

  35. mm-wave free-space: thin conductor example • A conductivity estimate can be extracted from the complex permittivity assuming a simple transport model. For disordered materials, this can cause deviations from DC expectations. Proper reference plane positioning does help (~20mm accuracy in the ‘better’ case). |S11| of order unity Kernel from Nicolson-Ross-Weir EuMW Seminars 2013

  36. Mm-wave waveguide/fixtured methods • Unlike free-space, waveguide setups offer fewer modal/multipath issues but sample size is more constrained. • In transmission line-like methods, the S-parameters are usually related to the materials parameters (G=G(e,m), g=g(e,m)) via: L1 L2 d er, mr • Depending on how well-known the reference plane positions and sample thickness are, simplifications are possible. EuMW Seminars 2013

  37. Background: S-parameter uncertainties At mm-wave frequencies, noise floors degrade, repeatability becomes more challenging and calibration material knowledge may decrease. Net uncertainties tend to degrade, particularly for highly lossy materials. EuMW Seminars 2013

  38. Waveguide/fixtured methods: uncertainty • Uncertainty propagation: • Sample length dependency tends to scale roughly with l/L ; decreasing wavelength helps for fixed size Transmission dominant measurement of low-loss material; permittivity decreasing from 5 to 2; fixed length uncertainty of 5mm • While absolute S-parameter uncertainties increase with frequency, permittivities tend to decrease…these can sometimes balance out EuMW Seminars 2013

  39. Waveguide/fixtured example: slushy sea water in W-band • Precise temperature control can be critical for some measurements • The use of thin walled ‘thermal break’ waveguide sections and radiative blocks can help get << 1C control • Use of membrane-cal fixtures helps get reference planes correct Rest of thermal wrap not shown here… Slush sample in the middle waveguide section Waveguide calibration at the break interfaces. EuMW Seminars 2013

  40. Waveguide/fixtured methods: example • With decent temperature control and proper calibration planes, one can get close to expected results… Basic S-parameter component of the uncertainty here is <0.05. EuMW Seminars 2013

  41. Waveguide/fixtured methods: incomplete fill • When sample size is not ideal, aperture effects can be de-embedded. Often these effects are model-able with good accuracy. Unfilled WG aperture sample EuMW Seminars 2013

  42. Waveguide conducting film measurement example • A small gap can have a significant impact, particularly for highly lossy/conductive samples. • S11 phase is the dominant measurement. Expected value range based on other measurements… EuMW Seminars 2013

  43. Mm-wave open-ended coax • Also used for years at lower frequencies, this method has become of increasing interest for studies of liquids at higher frequencies • Multiple time constants for different mechanisms • Better model fitting 1mm connector; 110 GHz (125 GHz) 1.85mm connector; 70 GHz EuMW Seminars 2013

  44. Open-ended coax • Coax diameter (b) tends to determine upper frequency limit while probe size (c) tends to set the lower frequency limit (together with side structure) • A sealed glass bead at the interface keeps porosity and contamination possibilities low. • A variety of plating and/or base metal choices for the probe body can be chosen for more challenging environments. Default is gold-plated aluminum. c b b< 1mm for the 125 GHz probe, >1mm for the 70 GHz probe EuMW Seminars 2013

  45. Open-ended coax • In the simplest analysis, one can integrate over the aperture to get an expression for the admittance*. • We need at least 3 admittance standards at the aperture to calibrate. Three media of known e will do. • Air, water and a shorting medium are commonly used. • High order modes (particularly at mm-wave) complicate things but, through modeling, can be corrected on a probe-type basis. b a *J. Hunger, Max Planck Institute for Polymer Research EuMW Seminars 2013

  46. Open-ended coax • Measurement of water one day after initial calibration. Since this was a calibration standard, this is more of a calibration stability test. Values returned to <<1%. EuMW Seminars 2013

  47. Open-ended coax • Broadband isopropanol example (20 mL sample) in four measurement trials EuMW Seminars 2013

  48. Open-ended coax: repeatability • Sample-to-sample repeatability can be <<1%. • Cal-to-cal repeatability can also be good but may be limited by DI water purity EuMW Seminars 2013

  49. Open-ended coax: repeatability and probe position • In a small volume container, results can still be reasonably consistent staying at least 5 coax radii from the walls (~1mm here) Open top b c a d e f g Position of aperture relative to container EuMW Seminars 2013

  50. Open-ended coax: short repeatability • A fine-pore metallic foam, very soft foil or silver paint can make a good short standard. A harder foil can reduce repeatability, particularly at low frequencies where capacitive coupling is weak. mm-wave range repeatability is, oddly, easier to achieve EuMW Seminars 2013

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