• Poposal 539: A Second-Generation Wide-Band Low-Noise SIS Mixer for ALMA Band 10 Based on Advanced Materials and Circuits

  • Reviewer 2

Review of proposal "A Second-Generation Wide-Band Low-Noise SIS Mixer for ALMA Band 10 Based on Advanced Materials and Circuits"

Proposal index number: 539

Principal Investigator: Omid Noroozian

Cost: $399,458

In-kind contribution: $210,775 (University of Virginia)

Review date: 8 July 2017

Overall score: 1.5 (0.1 best to 9.9 worst)

Proposal Summary The proposed project is to develop and test a new SIS mixer for ALMA Band 10. The main innovation will be the use of a new material stack for the SIS junctions, with NbTiN for both electrodes and AlN for the barrier. The higher energy gap in this superconductor is expected to result in substantially better noise performance at the Band 10 frequencies (787-950 GHz) compared with the present Nb/Al-oxide/Nb material stack. Additional innovations in the new mixer include fabrication of the mixer circuit on a thin Si membrane (rather than quartz), and use of a balanced IF amplifier fabricated with a superconducting hybrid and MMIC chip amplfiers from Low Noise Factory. Each of the innovations in the new mixer follows from earlier work at UVa, much of which was supported by ALMA Development grants. There is already considerable experience with NbTiN as a top electrode, but not yet as both electrodes in a tri-layer. The proposal shows how the new mixer technology, if used to create a sideband-separating (2SB) mixer, could reduce the system temperature of the telescope relative to the current DSB receivers by a factor of 2 during the 10% of the time when the water vapor content of the atmosphere is reasonably low (PWV < 0.3 mm). However, the additional work to create a 2SB mixer is not included here; it is expected to be in a future ALMA Development proposal. The new mixer is a technology demonstration and not a deliverable. No attempt will be made to integrate it into the current Band 10 receiver.

Strengths and Weaknesses The investigators are well positioned to carry out this work, having already demonstrated portions of the required technologies. It is closely aligned with the goal of improved sensitivity in the ALMA 2030 Roadmap. The fact that conditions at the ALMA site are favorable for near-THz observing only a small fraction of the time makes sensitivity improvements in Band 10 especially valuable. Section 4, Study Scope, spends a great deal of its text and figures digressing into benefits and background and future extensions. It would be better to move those things to separate sections and to focus here on exactly what this study will do. The poor organization makes the proposal difficult to evaluate. After reading all of it several times, I still can't tell how much of the described technology will be used in the mixer that will actually be built and tested during this study. Ideally it will include all of the things described in section 4.6, New Receiver Technology for Band 10 — not only the new material stack, but also the described mixer circuit design (4.6.1), the thin Si membrane fabrication (4.6.2), a micromachined input horn (4.6.3), and a compact balanced IF amplifier (4.6.4). If so, then the prototype mixer would be nearly ready for integration into ALMA's Band 10 receiver, even though it is still DSB. But the proposers do not claim such readiness. In Section 5, Study Deliverables, the only innovation mentioned is the new material stack, and it is stated (for the third time) that in case that arrangement fails to produce high-quality junctions they will fall back to Nb/Al-oxide/NbTiN junctions (new material on the top layer only, still an improvement).

Comments on Details of Proposal Contents Sec 3, Science Case. An extensive argument is given for the scientific importance of observations in Band 10, but no connection is made to the proposed project. The main benefit of the project is improved sensitivity, as shown much later in section 4.5 (Figure 11). Sec 8, Period of Performance. The investigators expect the 24 months to begin 2018 Jan 1 rather than 2017 Oct 2. It is not clear to me whether this is acceptable. Sec 9, Staffing. The PI at NRAO will devote 30% of his time to this project over its 24 month duration. Others at NRAO will devote smaller fractions of time for 12 months. The time commitments of the two CoIs at UVa are 35% and 10%. It appears that no person will devote a majority of his time to this project, so there will be no one whose main responsibility is this project. That creates a risk that it will get insufficient attention and not be completed on schedule. The total effort at NRAO adds up to 1.43 person-years (PY), and the total effort at UVa adds up to 0.9 PY. Sec 10, Cost Breakdown. From Table 4.0, 190.4k$ is allocated to the 1.43 PY of labor at NRAO. This seems too low. From Table 5.0, 210.8k$ is allocated to the 0.90 PY of labor at UVa. Sec 12.5, Risk Management. It is estimated that there is a probability of 1/3 that the desired material stack will not produce acceptable junctions, but that this has no cost impact. The mitigation is to use an alternative material stack, but it is hard to see how this can be done at no cost. Additional tri-layer fabrication runs and mixer circuit patterning runs would be needed.

Scoring The proposal deserves high marks for its highly advanced work in materials and fabrication methods, building on earlier work. The investigators are very competent to carry out the proposed work, made possible by existing facilities at UVa and NRAO. If the work proposed here is successful and it is eventually deployed in improved Band 10 receivers for ALMA, it will have substantial impact by reducing system noise temperature by a factor ~1.4 (or ~2 if 2SB mixers are later developed), and thus reducing observing time to achieve a given detection by a factor ~2 (or ~4). This is in close alignment with ALMA 2030 goals.

For these reasons, I am inclined to give this proposal a score of 0.5, nearly as favorable as possible. However, my final score is degraded to 1.5 to reflect uncertainty about how much of the improved technology discussed in the proposal will actually be incorporated into the tested mixer (due to the proposal's lack of clarity about this) and to reflect concerns about the budget and staffing and risk management, as discussed above.

  • Reviewer 5

Grade: 2

A SECOND-GENERATION WIDE-BAND LOW-NOISE SIS MIXER FOR ALMA BAND 10 BASED ON ADVANCED MATERIALS AND CIRCUITS

1. Alignment with NA ALMA Partnership strategic goals;

Excellent. Improvements in sensitivity as well and work towards developing double sideband receivers for bands 9 & 10 are specifically called for in the strategic goals.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ALMA 2030 development documents.

Increasing the sensitivity of these receivers will enhance all science in Band 10. In addition, imaging studies of star formation regions will be new science that cannot be currently conducted because of the long integrations required by the current receivers.

3. Quality of the upgrade conceptual design;

The approach is well founded. Significant work has already taken place to demonstrate a high critical current SIS junction fabricated with a novel material deposition technique, which is a critical aspect of the technology.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

Moderately ready. This work will demonstrate a band 10 DSB mixer that would directly lead to a development proposal for sideband-separating band 10 receivers.

5. Strength of the consortium organization (if applicable);

Excellent. NRAO and UVa have a long-standing history of productive collaboration.

6. Qualifications of the key personnel of the Study;

Excellent. Professor Lichtenberger of UVa has been at the forefront of SIS mixer technology for decades.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

See #6. UVa has unique deposition equipment that is enabling higher critical current SIS junctions.

8. Assessment of the level of risk inherent in the design;

Low to Medium. Although the high critical current has been demonstrated on a lower-frequency design, challenges could arise when the anode side is reduced to operate at a higher frequency. Having an alternate SIS stackup as well as multiple fabrication runs over the course of the study mitigates this risk.

9. Strength of the Scientific Team supporting the Study;

N/A.

10. Level of support guaranteed by the Institutes;

Excellent. Much of the labor costs are being supported by in-kind contributions of both UVa and NRAO.

11. Budgeted cost of the Study;

Acceptable.

  • Reviewer 7

Title of ALMADev Proposal: "A Second-Generation wideband low-noise SIS Mixer for ALMA Band 10 based on advanced materials and circuits" by Noroozian et al.

Review Comments

The proposal seeks to develop new low-noise SIS mixers and receivers for ALMA Band 10. The proposal will leverage ongoing advances at UVML in the production of Nb/AlN/NbTiN devices. These hybrid NbTiN have a higher bandgap frequency and will lower the noise temperature of new devices compared to the current Band 10 devices which are based on Nb devices. The proposal will also develop all NbTiN devices and couple with balanced IF amplifiers to considerably enhance the sensitivity of ALMA receivers for Band 10.

The science case for the proposal is very strong. Even though the ALMA site has 350 um weather 10% of the time, Band 10 is rich in chemical species (like CI and CO) and has a window into CII and NII species (which are crucial coolants of the ISM) for moderate redshift galaxies. Due to the limitations of operating Nb SIS devices above its bandgap frequency, Band 10 receivers are significantly higher noise temperature compared to the trend established by all other lower frequency receivers. The development of NbTiN and AlN insulator devices will be crucial to gain the full benefit of ALMA's angular resolution and sensitivity in the 350 um atmospheric window.

The assembled team is a strong one. UVML has had good success in the production of NbTiN hybrid devices. The 2016 ALMA development grant has resulted in impressive all-NbTiN devices with 5mV of energy gap which is an impressive accomplishment. The proposal also leverages some of the strengths of the CDL and UVML groups - the parallel development of balanced IF amplifiers, beam lead devices on Si substrates, Si membrane based LO couplers and new micromachined spline horns.

While the effort is for DSB receivers in this proposal, members of the proposing team have shown success with 2SB systems which will be needed for application with ALMA to make maximum use of the 350 um window.

If the all NbTiN devices do not have enough critical current and as the proposal suggests they may have to settle on hybrid Nb/AlN/NbTiN devices, what is the noise temperature penalty? This was not explained in the proposal.

The study is well aligned with ALMA's long term goals.

Review Score: 5.0

  • Reviewer 8

No review received.

  • Reviewer 10

539 A Second-Generation Wide-Band Low-Noise SIS Mixer for ALMA Band 10 

Based on Advanced Materials and Circuits Omid Noroozian

Score 3

The main goal of this proposal is to improve the performance of the band-10 system by developing mixers that work well in this frequency band (787-950 GHz). The current mixers employ Nb junctions. Unfortunately, an SIS mixer stops working at frequencies where photons can break Cooper pairs, which is below 750 GHz for Nb. As the energy gap is a monotonically decreasing function of temperature, a material with an gap energy well beyond 1 THz is required for band 10 mixers to be operated at a physical temperature of 4 K. NbTiN has a higher gap energy and is thus a more appropriate material for use in ALMA band 10 mixers. The team includes one of the world leaders in SIS fabrication and they have already worked with NbTiN in the past. While developing the junctions may require solving several technical challenges, the team is well poised to make the devices work. The resulting mixers would greatly improve the performance of the band ten system.

It appears that the bulk of the mixer development work will be led by CO-I Lichtenberger, who is a recognized expert in the field of SIS mixer fabrication. As such, it is a weakness that he is not the lead on this proposal. Nonetheless, the breakdown in funds does reflect the fact that his group would be responsible for a significant portion of the work. Another weak point in this proposal was that it seemed to assume that another proposal submitted to this call would get funded and the resulting IF amplifiers would be available.

  • Reviewer 11

Grade: 1

Title A SECOND-GENERATION WIDE-BAND LOW-NOISE SIS MIXER FOR ALMA BAND 10 BASED ON ADVANCED MATERIALS AND CIRCUITS

1. Alignment with NA ALMA Partnership strategic goals; This study aims to increase sensitivity at Band 10, thereby aligning with the strategic goal of improving throughput and mapping efficiency (1.1). Also 4.2

2. Strength of the scientific case for the proposed ALMA upgrade concept; The strong science case for band 10 is well described, but it is less clear how significantly this proposal directly affects those science goals, other than improving efficiency, allowing more such studies to be performed in a given number of telescope hours. In particular, it seems that all of the science goals of the proposal can potentially be accomplished with the existing band 10 mixers at reduced observing efficiency compared to the proposed upgrade. There is no mention of how much subscription pressure there is at band 10 which could be reduced by improved sensitivity. Comment on the relevance to the ‘ALMA 2030’ development documents. This proposal aligns with development path 2, by increasing usable bandwidth and improving sensitivity for Band 10.

3. Quality of the upgrade conceptual design; The concept of the upgrade seems very well thought out and a large amount of prior work has been done to demonstrate the feasibility of making NbTiN/AlN/NbTiN mixers and the surety of the Nb/AlAlN/NbTiN fallback option. The proposal presents substantial forethought next steps after this particular proposal as well, looking towards the full mixer modules beyond the SIS junctions themselves.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development); Initial demonstration of the NbTiN/AlN/NbTiN junction technology is presented and looks promising. The proposers have clearly thought through the path to production. There is significant development work required beyond this proposal, which is suggested to be the subject of future development proposals. This puts the technology as viable in a medium-term timescale.

5. Strength of the consortium organization (if applicable); The UVa lab is an established world leader for SIS technology.

6. Qualifications of the key personnel of the Study; See next question

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study; Technical facilities and expertise are world leading in the field. It would be difficult to find a more qualified team.

8. Assessment of the level of risk inherent in the design; The risk appears to be quite low given the demonstration data presented in the proposal. The proposed fall-back option also appears to be extremely low risk. However, it’s not clear to me that the fall-back option would be scientifically compelling enough to justify the development.

9. Strength of the Scientific Team supporting the Study; No concerns

10. Level of support guaranteed by the Institutes; Very high, including substantial in-kind contribution from UVa.

11. Budgeted cost of the Study; No concerns

-- AlWootten - 2017-07-18

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Topic revision: 2017-07-18, AlWootten
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