REVIEW 4 major objections 5 minor 13 references
Atomic Layer Deposition Niobium Nitride Films for High-Q Resonators
T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Atomic-layer-deposited niobium nitride can make microwave resonators with internal quality factors above one million.
desk verdict First RF characterization of ALD NbN resonators, with credible high-Q values but a headline yield statistic that needs more statistical backing before it can carry the paper's weight. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The mechanism that carries the argument is plasma-enhanced atomic layer deposition using the niobium precursor TBTDEN and an argon plasma, in which one ALD cycle grows roughly one atomic monolayer. This gives measured growth rates of $0.51 ± 0.05$ Å/cycle at 250 °C and $0.62 ± 0.05$ Å/cycle at 300 °C, and it is this atomic-scale thickness control that provides the wafer-scale uniformity claimed for 4–30 nm films. The deposited film is patterned into a planar lumped-element resonator with an interdigitated capacitor and inductor, and its response is fitted to a complex transmission model; Mattis-Bardeen relations convert the temperature dependence of frequency shift and internal loss into the kinetic inductance fraction, gap energy, and RF critical temperature.
What would settle it
Take a single ALD NbN wafer, measure its DC $T_c$ and resistivity on an unpatterned piece, pattern identical resonators on the rest, and compare the RF-derived $T_c$ and $Q_i$ after the full etch and clean process: a processing-induced drop in $T_c$, a broadened transition, or a $Q_i$ below $10^6$ would indicate that the reported quality factors do not measure the intrinsic ALD NbN film.
Extended reading notes
Core claim
The central claim is that plasma-enhanced atomic layer deposition of NbN, despite not matching the highest $T_c$ of sputtered films, yields microwave resonators whose internal quality factors are comparable to or better than sputtered NbN. At a stage temperature of 300 mK, 75% of resonators made from films deposited at 250 °C and 43% of those deposited at 300 °C have $Q_i > 10^6$, and almost all measured resonators exceed $10^5$; the highest DC critical temperature observed is 10.87 K, with RF-derived $T_c$ around 11–12 K. The paper also extracts kinetic inductance $L_k = 5.4 ± 2$ pH/square for a 300-cycle 250 °C film and $1.7 ± 0.5$ pH/square for a 300-cycle 300 °C film. These numbers establish ALD NbN as a candidate material for high-$Q$ resonator circuits, with the caveat that films thinner than about 75 cycles were not observed to superconduct.
Load-bearing premise
The load-bearing assumption is that the lithography and etch sequence—photoresist, fluorine plasma etch, oxygen ash, and hot solvent cleaning—does not degrade the film, because $T_c$ is measured on unpatterned films while $Q_i$ is measured on patterned resonators.
Editorial extensions
If this is right
- Detector readout can be multiplexed more aggressively, since a larger fraction of resonators in a given band will sit above $Q_i = 10^6$.
- Wafer-scale thickness uniformity down to a few nanometers makes ALD NbN attractive for on-chip mm-wave band-defining features and nanowire detectors where sputtered thickness control is a concern.
- Deposition temperature becomes a design lever: 250 °C films show higher quality factors, while 300 °C films grow faster and show lower kinetic inductance.
- Even though $T_c$ maxes near 10.9 K, the measured $Q_i$ already rivals sputtered NbN CPW resonators, so the lower transition temperature does not by itself block detector use.
Reading between the lines
- The paper does not isolate where the factor-of-10 $Q_i$ spread comes from; a natural inference is that local thickness or interface variation, not intrinsic film loss, sets the ceiling, which could be tested by measuring the same resonator after successive processing steps.
- The transition-width data connect thinner films to broader transitions and lower $T_c$; the implied granularity limit near 4 nm suggests that grain structure, visible by microscopy, may be the same quantity that caps $Q_i$ in the ultrathin regime.
- The authors' planned variation of substrate and surface preparation implies a testable hypothesis: the same ALD films on sapphire or (111) Si should show higher $T_c$ and possibly higher $Q_i$ if lattice mismatch and interface contamination are the limiting losses.
- The paper lists kinetic-inductance devices as motivation but builds only resonators; a logical next step is an ALD NbN parametric amplifier or nanowire detector, whose noise and saturation behavior would extend the quality-factor result into a full device demonstration.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This manuscript reports atomic layer deposition of niobium nitride (ALD NbN) films and their DC and microwave characterization for use in high-Q superconducting resonators. Films of roughly 4-30 nm thickness were grown at 250 and 300 °C; the authors report growth rates, XRD texture, AFM roughness, DC critical temperatures (up to 10.87 K), and resistivity. Single-layer lumped-element resonators were fabricated and measured at dilution temperatures, yielding internal quality factors Qi above 10^5 for nearly all resonators, with 75% above 10^6 for a 250 °C film and 43% above 10^6 for a 300 °C film. Additional fits of Mattis-Bardeen relations yield kinetic inductance fractions and RF critical temperatures, and comparison with aluminum reference devices gives Lk = 1.7-5.4 pH/square. The paper concludes that ALD NbN is a viable alternative to sputtered NbN for microwave resonators.
Significance. If the yield claim is reproducible, this is a useful advance for the detector community: ALD provides wafer-scale thickness control in the ultrathin regime where sputtered NbN uniformity is a known concern, and the reported quality factors are competitive with some sputtered NbN devices. The paper's strengths include the multi-technique characterization (ellipsometry, XRD, AFM, DC transport, resonator S21 fits), the explicit reporting of wafer-to-wafer and device-to-device variation, and the use of an external aluminum reference for the kinetic inductance extraction. However, the headline statistic that 43-75% of resonators exceed Qi = 10^6 is not yet supported by the data as presented, because no resonator counts, confidence intervals, or acceptance criteria are given. The paper also does not establish that the full lithography and etching process preserves the as-deposited film properties. These issues must be addressed before the quantitative claims can be accepted.
major comments (4)
- [§3.3, Fig. 5] The central yield claim in the abstract ('with ≥43% of resonators above Qi = 10^6') is not supported by the reported data. Section 3.3 reports 75% above 10^6 for a 250C film and 43% for a 300C film, but gives no number of resonators measured, no per-chip or per-wafer breakdown, and no confidence intervals. The Fig. 5 caption states that the Qi values were 'obtained with a formal fit for resonators with Qc values between 10^4 and 10^6,' which means resonators outside this coupling window were excluded; this selection must be justified and its effect on the yield quantified. The same paragraph notes that average quality factors from the same 4-inch wafer differ by a factor of 10, so without the full per-resonator data the reported fraction cannot be distinguished from position-dependent or fitting-selection effects. Please provide per-resonator data (or a table/histogram with N) for each deposition condition and a sensitivity analysis of the yield to the Qc acceptance window.
- [§3.2 and §3.3] The paper does not reconcile the RF-derived critical temperatures with the DC measurements. Section 3.3 reports T_c^(RF) ~ 11K for 250C films and ~12K for 300C films, while Section 3.2 reports a maximum DC Tc of 10.87K, with 12% sample-to-sample variation. Because T_c^(RF) is obtained by fitting Mattis-Bardeen expressions for the gap Δ0 and then converting with the BCS relation Δ0 = 1.76 k Tc, fit degeneracies or the local-limit assumption could bias the converted temperature. Please report the uncertainties in the fitted Δ0 and αk, show the quality of the fits, and quantify whether the apparent 1-2 K excess over DC Tc is statistically significant. If the discrepancy is real, it should be discussed; if it is a fitting artifact, the T_c^(RF) values should be presented only with appropriate caveats.
- [§2.2 vs §3.2] The DC Tc and resistivity are measured on unpatterned as-deposited films, while Qi is measured on patterned resonators that have undergone photoresist coating, fluorine-based ICP etching, oxygen ash, and heated NMP cleaning. The manuscript does not test whether these steps degrade the superconducting or microwave properties of the film. If the fabrication process damages the film (e.g., by etching or oxidizing the surface), the reported Qi values would not represent the intrinsic quality of ALD NbN. Please add a control measurement, such as DC Tc and sheet resistance measured before and after the full patterning process, or a comparison of patterned versus unpatterned films from the same deposition.
- [Eq. (3)] Equation (3) is typeset as a single relation but is meant to contain two distinct equations for δf0/f0 and δ(1/Qi). As printed, the expression is ungrammatical and the second relation appears to have σ2 in the numerator, whereas the standard Mattis-Bardeen result for δ(1/Qi) has the conductivity change divided by σ2 (or otherwise normalized). Because this equation is the basis for the αk and Δ0 fits that produce T_c^(RF), the fitting procedure cannot be independently reproduced as written. Please correct the equation, define every symbol (including the meaning of 'd' versus 'δ'), and provide the explicit functional forms used in the fit.
minor comments (5)
- [Abstract and §3.3] The abstract states only '≥43% of resonators above Qi = 10^6' while Section 3.3 reports 75% for the 250C film and 43% for the 300C film; please state both numbers or explicitly say that 43% is the lower bound obtained from the less favorable deposition condition.
- [§3.1, Fig. 2] The XRD peak labels in Fig. 2 are '111' and '200', but the text refers to the ⟨100⟩ orientation; please reconcile the crystallographic notation (e.g., by explaining that the 200 reflection corresponds to the ⟨100⟩ texture).
- [§3.1] Growth rates are written as '.51 ± .05 Å/cycle' and '.62 ± .05 Å/cycle' without leading zeros; please use '0.51' and '0.62' for consistency with the rest of the text.
- [Fig. 5] The right panel of Fig. 5 is labeled 'Resonator d f/f vs T behavior' and is said to correspond to a 75-cycle film, but the caption does not identify which resonator or measurement power is shown; please add a legend or the missing axis details.
- [§4] The comparison with sputtered NbN resonators (refs. 12 and 13) is qualitative and does not state the film thickness, measurement temperature, or readout power for the compared devices; a short table with these conditions would make the comparison meaningful.
Circularity Check
No circularity: all derived quantities are fits or cross-calibrated against external references, and no load-bearing self-citation chain appears.
full rationale
The paper reports direct measurements of ALD NbN resonator quality factors, DC critical temperatures, and kinetic inductance. Internal quality factors are extracted by fitting S21 data to the standard resonator equation (Eq. 2); Qi is not defined in terms of any target claim and there is no fitted parameter later renamed as a prediction. DC Tc is obtained from an explicitly ad hoc fit function (Eq. 1) and is a measurement convention, not a derived prediction. The radio-frequency Tc is obtained by fitting Mattis-Bardeen relations to df/f0 and 1/Qi, then converting the fitted gap via the BCS relation Δ0 = 1.76 k Tc; this is a standard model assumption and is presented as a fit ('we fitted for Tc(RF)'), not as an independent prediction. Kinetic inductance is extracted by comparing with previously tested Al devices of the same circuit design (Eq. 4), using an external reference to solve for geometric inductance and capacitance; this is a cross-calibration, not circular. The yield statistic ('≥43% of resonators above Qi = 10^6') lacks explicit resonator counts and depends on the Qc selection window described only in the Fig. 5 caption, but incomplete statistical reporting is not circular reasoning. The paper contains no self-citations, no imported uniqueness theorems, and no ansatz smuggled in via citation. Every load-bearing quantitative result is either a direct measurement, an explicitly labeled fit, or a cross-calibration against an external reference. Therefore no significant circularity is present.
Assumptions & free parameters
free parameters (4)
- Tc (fit in Eq. 1) =
7.5-10.9 K depending on sample
- kappa (transition width) =
not quoted
- alpha_k (kinetic inductance fraction) =
not quoted
- Delta0 (superconducting gap) =
not quoted
assumptions (5)
- domain assumption The local limit with gamma = 1/2 applies to the ALD NbN films.
- standard math The BCS relation Delta0 = 1.76 kTc holds for these films.
- domain assumption The geometric inductance and capacitance of NbN resonators are identical to previously tested Al devices with the same design.
- ad hoc to paper The ad hoc R(T) function of Eq. (1) provides an unbiased Tc estimate.
- domain assumption The device fabrication process (Section 2.2) does not degrade the film's superconducting or microwave properties.
Cite this review
Pith. "Pith review of Atomic Layer Deposition Niobium Nitride Films for High-Q Resonators." pith.science (2026). https://pith.science/paper/Y3AXPJS4
@misc{pith2026190807146,
author = {Pith},
title = {Pith review of: Atomic Layer Deposition Niobium Nitride Films for High-Q Resonators},
year = {2026},
howpublished = {\url{https://pith.science/paper/Y3AXPJS4}},
note = {Machine review of arXiv:1908.07146}
}
read the original abstract
Niobium nitride (NbN) is a useful material for fabricating detectors because of its high critical temperature and relatively high kinetic inductance. In particular, NbN can be used to fabricate nanowire detectors and mm-wave transmission lines. When deposited, NbN is usually sputtered, leaving room for concern about uniformity at small thicknesses. We present atomic layer deposition niobium nitride (ALD NbN) as an alternative technique that allows for precision control of deposition parameters such as film thickness, stage temperature, and nitrogen flow. Atomic-scale control over film thickness admits wafer-scale uniformity for films 4-30 nm thick; control over deposition temperature gives rise to growth rate changes, which can be used to optimize film thickness and critical temperature. In order to characterize ALD NbN in the radio-frequency regime, we construct single-layer microwave resonators and test their performance as a function of stage temperature and input power. ALD processes can admit high resonator quality factors, which in turn increase detector multiplexing capabilities. We present measurements of the critical temperature and internal quality factor of ALD NbN resonators under the variation of ALD parameters.
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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