ADVANCED MATERIALS AND ON WAFER CHIP EVALUATION FOR …

ADVANCED MATERIALS AND ON WAFER CHIP EVALUATION FOR SECOND GENERATION ALMA SUPERCONDUCTING MIXERS 06-15

University of Virginia Microfabrication Laboratories (UVML)

Electrical & Computer Engineering Arthur Lichtenberger: Professor & Director UVML / Principal Investigator Michael Cyberey: Scientist / Engineering Lead University of Virginia Functional Thin Film Laboratory

Materials Science Jiwei Liu: Associate Professor Saliporn Kittiwatanakul: Scientist

ADVANCED MATERIALS AND ON WAFER CHIP EVALUATION FOR SECOND GENERATION ALMA SUPERCONDUCTING MIXERS


North America ALMA Strategic Context

•  ALMA’s continued & long-term success is of critical importance to North American (NA) & international radio astronomy

•  8 of the 10 ALMA bands currently use superconducting (SIS) mixers

•  Maintaining the NA investment in Bands 3&6 and creating significant new NA opportunities requires efficient & productive access to a robust SIS foundry

•  Realizing next generation ALMA SIS mixers that operate near their quantum limit with improved performance will require new materials & designs, and similar access to such an advanced SIS foundry

There exists only one North American SIS foundry capable of making and

developing such radio astronomy mixers - the University of Virginia Microfabrication Laboratories (UVML).

In addition to the technical thrusts, the proposed funding is important for continuity of ALMA's essential infrastructure by helping to maintain the UVML.


University of Virginia Microfabrication Laboratories (UVML)

•  Long standing international program of excellence in THz materials, devices, circuits and metrology

•  1970s pioneering development, in collaboration with NRAO, of semiconductor Schottky barrier diode devices & receivers

•  Collaborated with the NRAO & other groups > 30 years to develop state of the art superconducting receivers for radio astronomy

•  Demonstrated SIS mixers that exceed design specs for ALMA Band3, 6, 7 & 8

•  Is the SIS foundry for all Band3 and Band6 SIS mixer chips

•  Maintain sophisticated fabrication process for superconducting devices and circuits

•  Excel at fast prototyping of new processes and the development of superconducting circuits in close collaboration with our radio astronomer partners

•  Excellently positioned for the investigation and development of new SIS materials and chip architecture for ALMA


Study Proposal - Big Picture

The UVML ALMA study proposal has two main thrusts: The first thrust is a study of the suitability of an alternative materials deposition technology – Reactive Bias Target Ion Beam Deposition (RBTIBD) – that offers unique capabilities to tailor materials and interfaces, to realize all NbTiN, higher energy gap (hence higher frequency) SIS devices needed for optimal second-generation band9 and band10 receivers and to provide a technical path forward for a future band11 receiver. The second thrust is the study of an all-wafer SIS device screening approach. Current SIS device evaluation is realized by time consuming hand mounting of a few individual device chips onto a carrier chip for dip testing in a liquid helium dewar. Realizing on-wafer cryogenic dc probing of SIS wafers would be an enormous benefit to all ALMA superconducting receiver programs.


Current ALMA SIS Mixers:

• Use of an Al overlayer onto the Nb base electrode • Magnetron sputtering of the SIS Trilayer • Al oxide ~ 1nm critical tunnel barrier grown from part of the Al overlayer • Nb/Al-AlOx/Nb • SIS Nb energy gap, nominal ~700 GHz

First Thrust: All NbTiN SIS junctions (NbTiN/AlN/NbTiN)

Gurvitch Al overlayer process [2]


•  Ideal switch-like I-V at 2.9 mV (energy gap) ~700 GHz

•  SIS low loss operation limited in f by energy gap

•  Actual I-V has turn-on below 2.9 mV ~ 650 GHz

•  Mixer performance will suffer at 650 GHz & above

•  Conductor loss & Josephson noise

•  ALMA band 9 (602-720 GHz) band 10 (787-950 GHz)

•  B9 & B10 suffer from all Nb junction material choice

•  Higher gap SIS electrodes will push the optimal frequency performance limit to higher frequencies

Current ALMA Nb/Al-AlOx/Nb SIS Mixers:

Typical SIS I-V curve for a high quality UVA Nb/Al-oxide/Nb junction [1].


ALMA’s all Nb SIS (Nb/Al-AlOx/Nb) junction Receiver Results

SSB receiver noise temperature vs frequency for typical ALMA SIS receivers. Bands 3-8 have sideband-separating mixers and Bands 9 and 10 have double-sideband mixers [1]. The performance of the upper half of band9 and the entire band10 could be improved substantially with high energy gap NbTiN/AlN/NbTiN SSB mixers and Bands 6 and above improved with AlN barriers.


Candidate materials for higher energy gap SIS junctions

NbN and NbTiN (1.6 - 1.7 times larger energy gap than Nb)

•  NbN has been studied for decades for SIS junctions [3-10] •  Yet due to performance issues, not selected for ALMA’s higher bands

•  Mixer performance limited due to surface resistance in NbN films [11-12]

•  NbN/AlN/NbN SIS mixers have performed poorer than anticipated- primary problems with superconductive properties, including penetration depth and loss, resulting in “drastically deteriorated mixing properties” [14]

l  However, NbTiN may be a suitable candidate •  NbTiN exhibited significantly smaller surface resistance than NbN in studies of RF cavities [13].

•  UVA’s Nb/Al-AlN/NbTiN junctions are of extremely high quality. à Unexplored, all NbTiN (NbTiN/AlN/NbTiN) SIS mixers clearly the optimal path to pursue for low noise operation up to ~1.1 THz


Reactive Bias Target Ion Beam Deposition (RBTIBD)

RBTIBD is a hybrid between ion beam and conventional magnetron sputter deposition that combines the best of each technique.

Left: schematic of Reactive Bias Target Ion Beam Deposition system; Right: Photograph of the UVA 4-Wave RBTIBD tool that can take 50 & 100mm wafers, automatic or manual operation.

Study the use of RBTIBD in a radical departure from the standard Al overlayer approach with the direct sputtering of AlN tunnel barrier (NbTiN/AlN/NbTiN).


Reactive Bias Target Ion Beam Deposition (RBTIBD)

•  Ion source: uniform low-energy (5-50eV) inert ions _no re-sputtering •  Flux & dynamics of ions to targets is manipulated by independent pulsed dc bias •  Target voltage can be set •  Target current is fine-tuned through the pulse width and frequency •  Ion flux can also be directed at the wafer •  Rotatable wafer stage •  Wafer stage temperature control to 650C (control material crystallinity) •  Taylor/match lattice constants of NbTiN and AlN films

→ epitaxial growth of AlN on NbTiN

RBTIBD: unique ability to precisely and separately control incoming sputtered atom energies to tailor composition, phase formation, surface roughness and interfaces [22-27] – properties critical to achieving such a direct sputtered AlN tunnel barrier.


Experience • Lichtenberger+Cyberey group’s experience in optimizing Tc, penetration depth, resistivity & stress for magnetron films

• Lu+Kittiwatanakul materials science group’s extensive experience with the RBTIBD tool and associated materials/interfaces/surface engineering and analysis

• Both groups currently collaborating on C3 iARPA Nb superconducting memory (JMRAM) project for SFS type trilayer structure using RBTIBD approach

Primary Characterization Tools • Characterize microstructures and physical properties as a function of growth conditions, the substrate type, and the film thickness. • X-ray diffraction (XRD) characterize the microstructure as well as the film strain • Atomic force microscopy (AFM) characterize film roughness and the grain size • X-ray photoemission spectroscopy (XPS) characterize the electronic structure


How to screen a finished SIS mixer wafer: • One could post processing (lap/thin) individual mixer chips, mount in a mixer block, cool and dc evaluate the SIS junctions … painstaking and time-consuming

• Or, one could mount a few mixer chips on a carrier and dip into liquid helium for SIS I-V testing. Time consuming. Care not to damage the mixers. Not practical for screening an entire wafer of thousands of chips

Second Thrust: Whole SIS-Wafer Cryogenic Screening

Finished Band8 UVML-NRAO 5um Si mixer chip with 2um Au beam leads

SOI chip mounted with silver paint to a quartz test carrier wafer

Ideally a mixer wafer could also be dc evaluated in a simpler screening process and the selected chips, with now known electrical characteristics, used in the receivers.


•  Numerous reports of cryogenic probing of devices in the literature [21-26], especially for low noise amplifiers (LNAs)

•  But “cryogenic” is used to describe probed sample temperatures > 15K

•  There is no ~ 4.2K DUT wafer probing solution

•  Enormously important for ALMA: large number of receivers, increasingly sophisticated mixer architecture of balanced and or sideband separating mixers that require multiple matched mixer chips, and future array receivers.

•  Of particular benefit to all ALMA superconducting receiver programs

•  Allow quick verification of an SIS mixer wafer

•  Rejection of inferior devices

•  Important matching of similar device characteristics in a mixer

•  Feasible path toward populating receiver arrays with suitable mixer chips

•  SOI wafers- allow for frontside screening of the wafer before proceeding to the two week long, labor and tool intensive backside wafer processing.

Whole wafer cryogenic screening


Whole wafer cryogenic screening In this study

• Investigate and optimize the thermal design, connections and heat management of our existing Lake Shore cryogenic probe station,

• Design and realize improved dc cryogenic wafer probes

• Design and realize a prototype mixer chip layout to facilitate probing.

(left) top level thermal schematic (right) picture with four dc probes


DC probing of individual (previously verified) SIS chips

• Superconducting properties (e.g. Nb gap) depend on temperature (90% at 5.5K) • Platter temperature measured, reaches 4.2K • SIS I-V curve at platter temperature of 4.2K gives actual I-V of ~ 7.5-8K • I-V at platter temperature of 7.9K is resistive (not superconductive) • Clearly the SIS chip temperature is significantly higher than the platter temperature • As is, not usable for screening SIS devices • Additional Limitations:

•  Currently requires 4 large DC probes •  Commercial probes not well heat-sunk •  Use of 20 mil DC wires to outside world

UVA’s Lake Shore Cryogenic Probe Station

I-V plotted versus measured platter temperature.


We will design our own improved dc wafer probe

• UVML has extensive experience in the engineering of wafer probes •  Developed 1st THz wafer probe, split-block design, 15um SOI chip [28-31]

• In collaboration with Lake Shore to develop RF WR5.1 cryogenic probe

• Development of a dc cryogenic probe - synergistic UVML activity

• Our wafer probe - compatible with 4.2K environment

• Our Au plated split-block probe - excellent thermal connection to Au plated straps

• Si probe chip with Au mounting pads, mechanically clamped between the splits blocks, will provide an improved thermal circuit

• UVML probe: realize arbitrary probe tip configurations in the Si chip •  Realize two isolated lines •  Allow a 4-point measurement with only two probes (or future four isolated

lines, allowing use of only one probe) • Design a thermal stage: recess for our 50 mm wafers with ring fixture to better thermally clamp the wafer around the entire perimeter.

• Au metalizing the backside of our wafers for improved thermal connection


•  Study is not aimed at benefiting or realizing just one particular ALMA project or receiver.

•  This work will technically benefit all ALMA superconducting research efforts and the resulting astronomical science.

•  This includes ongoing plans for greater sensitivity, more bandwidth, more beams on sky, and more flexibility in observing modes including multi-band observations.

à  A successful high-energy gap NbTiN/AlN/NbTiN study would point all ALMA

partners toward optimal next generation higher frequency receiver materials & designs.

à  Realizing on wafer cryogenic dc screening of SIS wafers would benefit all ALMA superconducting receiver programs, including the ongoing ALMA Study for a 2nd generation Band6 receiver, and multi-beam receivers.

In Conclusion


References [1] A. R. Kerr, S.-K. Pan, and W. G. Lyons, "The Genesis of SIS Mixers – The Legacy of John Tucker in Radio Astronomy,” IEEE MTT-S International Microwave Symposium Digest, 2015. [2] M. Gurvitch, M.A. Washington, and H.A. Huggins, “High Quality Refractory Josephson Tunnel Junctions Utilizing Thin Aluminum Layers,” Applied Physics Letters, 42, 1983. [3] B.D. Hunt, H.G. LeDuc, S.R. Cypher, and J.A. Stern, "NbN/MgO/NbN Edge-Geometry Tunnel Junctions", Appl. Phys. Lett. 55,81 (1989). [4] W.R. McGrath, H.H.S. Javadi, S.R. Cypher, B. Bumber, B.D Hunt, H.G. Leduc, “Low-Noise 205 GHz SIS Mixers Using High Current Density Nb and NbN Junctions,” Second International Symposium on Space Terahertz Technology, 1991 423-438 [5] H.G. Leduc, A. Juda, S.R. Cypher, B. Bumble, B.D. Hunt, and J.A. Stern, “Submicron Area NbN/MgO/NbN Tunnel Junctions for SIS Mixer Applications,” IEEE Trans Magn. Vol 27, No-2, March 1991 [6] Z. Wang, A. Kawakami, Y. Uzawa, and B. Komiyama, "NbN/AlN/NbN tunnel junctions fabricated at ambient substrate temperature," IEEE Trans. on Applied Superconductivity, Vol. 5, No.2, pp2322-2325, June 1995. [7] A. Karpov, B. Plathner, J. Blondel, M. Schicke, K. H. Gundlach, M Aoyagi and S. Takada,"Noise and Gain in Frequency Mixers with NbN SIS Junctions," IEEE Trans. on Superconductivity, Vol. 7, No. 2, pp1077-1080, June 1997 [8] A. Kawakami and Z. Wang, “Fabrication and characterization of epitaxial NbN/MgO/NbN Josephson tunnel junctions,” J. Appl. Phys., vol. 90, no. 9, pp. 4796–4799, 2001 [9] A. Kawakami, Y. Uzawa and Z. Wang, " Development of epitaxial NbN/MgO/NbN superconductor-insulator -superconductor mixers for operations over the Nb gap frequency," Applied Physics Letters, Vol. 83, No. 19, pp3954-3956, Nov. 2003. [10] Z. Wang, A. Kawakami, Y. Uzawa, and B. Komiyama,"NbN/AlN/NbN tunnel junctions fabricated at ambient substrate temperature," IEEE Trans. on Applied Superconductivity, Vol. 5, No.2, pp. 2322-2325, June 1995. [11] Wang, A. Kawakami, Y. Uzawa, and B. Komiyama, “High critical current density NbN/AIN/NbN tunnel junctions fabricated on ambient temperature MgO substrates,” Appl. Phys. Lett. 64, pp. 2034-2036, 1994 [12] A. Karpov, B. Plather, and J. Blondel, “Noise and gain in high frequency mixers with NbN SIS junctions,” IEEE Trans. Applied Supercondtivity 7, pp. 1077-1080, 1997. [13] R. Di Leo, A. Nigro, G. Nobile, and R.Vaglio, “Niobium- titanium nitride thin films for superconducting rf accelerator cavities,” J. Low Temp. Phys. 78, pp. 4 1-50, 1990. [14] M. Takeda, W. Shan, T. Kojima, S. Saito, M. Kroug, Y. Uzawa and Z. Wang, “Low-noise waveguide SIS mixer with NbN/AlN/NbN tunnel junctions tuned by an NbN/MgO/NbTiN microstrip circuit,” Superconductor. Sci. Technol. 22, June, 200


References continued [15] K. G. West, J. W. Lu, J. Yu, D. Kirkwood, W. Chen, Y. H. Pei, J. Claassen, and S. A. Wolf, Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition. Journal of Vacuum Science & Technology A, 26(1), 133-139, 2008. [16] J. W. Lu, K. G. West, and S. A. Wolf, Very large anisotropy in the dc conductivity of epitaxial VO2 thin films grown on (011) rutile TiO2 substrates. Applied Physics Letters, 93(26), 2008. [17] S. Kittiwatanakul, J. W. Lu, and S. A. Wolf, Transport Anisotropy of Epitaxial VO2 Films near the Metal-Semiconductor Transition. Applied Physics Express, 4(9), 2011. [18] S. Kittiwatanakul, J. Laverock, D. Newby, K. E. Smith, S. A. Wolf, and J. Lu, Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure. Journal of Applied Physics, 114(5), 053703, 2013. [19] S. Kittiwatanakul, S. A. Wolf, and J. Lu, Large epitaxial bi-axial strain induces a Mott-like phase transition in VO2. Applied Physics Letters, 105(7), 073112, 2014. [20] Y. Wang, R. B. Comes, S. Kittwantanakul, S. A. Wolf and J. W. Lu, Epitaxial niobium dioxide thin films by reactive-biased target ion beam deposition, Journal of Vacuum Science & Technology A, 33, 021516 (2015) [21] H. Meschede, R. Reuter, J. Albers, J. Kraus, D. Peters, W. Brockerhoff, F. Tegude, M. Bode, J. Schubert, and W. Zander, ”On-wafer microwave measurement setup for investigation on HEMT’s and high Tc superconductors at cryogenic temperatures down to 20 K”, IEEE Trans. Microw. Theory Techn., vol. 40, no. 12, Dec. 1992. [22] V. Hietala, M. Housel, R. Caldwell, ”Network analyzer calibration for cryogenic on-wafer measurements”, ARFTG Conference, vol. 25, San Diego, May 1994. [23] J. Laskar, J. Bautista, M. Nishimoto, M. Hamai, and R. Lai,” Development of accurate on-wafer, cryogenic characterization techniques”, IEEE Trans. Microw. Theory Techn., vol. 44, no. 7, Dec. 1996. [24] T. Vaha-Heikkila, J. Varis, H. Hakojarvi, and J. Tuovinen, ”Wideband cryogenic on-wafer measurements at 20-295 K and 50-110 GHz”, European Microwave Conference, vol 33, Munich, 2003. [25] D. Russell, K. Cleary, and R. Reeves, "Cryogenic probe station for on-wafer characterization of electrical devices," Review of Scientific Instruments 83, 044703, 2012. [26] R. Reeves, K. Cleary, R. Gawande, J. Kooi, , J. Lamb, A. Readhead, , S. Weinreb, T. Gaier, , P. Kangaslahti, D. Russell, L. Samoska, M. Varonen, R. Lai, and S. Sarkozy, "Cryogenic Probing of mm-Wave MMIC LNAs for Large Focal-Plane Arrays in Radio-Astronomy," IEEE Proceedings from the 9th European Microwave Integrated Circuit Conference (EuMIC), Oct 2014. [27] A. R. Kerr and N. Horner, “The Low temperature Thermal Resistance of High Purity Copper and bolted Copper Joints,” NRAO Electronics Division Technical Note No. 163, August 30, 1991.


References continued [28] M. Bauwens, N. Alijabbari, N. S. Barker, A. Lichtenberger, R. M. Weikle, "A 1.1 THz Micromachined On-Wafer Probe," International Microwave Symposium (IMS) Digest, 4, 2014. [29] M. F. Bauwens, L. Chen, C. Zhang, A. I. Arsenovic, N. Alijabbari, A. W. Lichtenberger, N. Scott Barker and R. M. Weikle, “Characterization of Micromachined on-Wafer Probes for the 600–900 GHz Waveguide Band II,” IEEE Transactions on Terahertz Science and Technology, Vol 4, 527-529, July 2014. [30] L. Chen, C. Zhang, T. J. Reck, C. Groppi, A. Arsenovic, A. Lichtenberger, R. M. Weikle, N. S. Barker, “Terahertz Micromachined On-wafer Probes: Repeatability & Robustness,” IEEE Trans. Microwave Theory and Tech., 2012. [31] L. Chen, C. Zhang, T.J. Reck, A. Arsenovic, M. Bauwens, C. Groppi, A.W. Lichtenberger, R.M. Weikle, II, and N. Scott Barker “Terahertz micromachined on-wafer probes: repeatability and reliability,” IEEE Trans. Microwave Theory and Tech., Vol. 60, No. 9, 2894-2902, Sept 2012.

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