Topographic evolution of heat-treated Nb upon electropolishing for superconducting rf applications

Surface finish plays an essential role in the performance of superconducting radio frequency cavities. Several surface treatments have been developed to reduce surface resistance at a moderate accelerating gradient. We investigated the effects of sequential electropolishing on samples vacuum heat-treated at 300 and $600°\mathrm{C}$ and N-doped Nb samples using atomic force microscopy. The N-doping process precipitates niobium nitrides within grains and, most notably, continuously and deeply along some grain boundaries. Upon electropolishing, the nitrides are preferentially removed leaving behind a topographically imperfect surface marked by relatively deep holes and grooves with low radius of curvature edges. The progression of magnetic field enhancement and superheating field suppression factors upon electropolishing were investigated using atomic force micrographs. While minor changes in magnetic field enhancement and superheating field suppression factors are observed for the 300 and $600°\mathrm{C}$ heat-treated Nb, substantial improvements are observed for N-doped Nb. In this system, the most severe topographic defects are the grain boundary grooves which substantially suppress the superheating field. We find that the severity of topographic defects is related to the N-doping process.

Figure 1

(a) Optical microscope images showing the effect of ${\mathrm{H}}_2{\mathrm{O}}_2$ on mechanically polished Nb without and (b) with the addition of ${\mathrm{H}}_2{\mathrm{O}}_2$, the yellow scale bar indicates 1 mm. The grain-textured surface returns if polished without ${\mathrm{H}}_2{\mathrm{O}}_2$. (c–e) EBSD OIM of samples subjected to (c) no electropolishing, (d) $6\text{−}\mathrm{\mu }\mathrm{m}$ electropolishing, and (d) $20\text{−}\mathrm{\mu }\mathrm{m}$ electropolishing. The white and black scale bars indicate 350 and $16\mathrm{\mu }\mathrm{m}$, respectively.

Figure 2

Representative tapping-mode AFM topographies of the heat-treated surfaces. The topography labeled EP is representative of a sample heat treated at $600°\mathrm{C}$ for 10 h then subjected to a $20\text{−}\mathrm{\mu }\mathrm{m}$ electropolish, the sample labeled $600°\mathrm{C}$ is representative of a sample heat treated at $600°\mathrm{C}$ for 10 h without postanneal electropolish. The topography labeled $300°\mathrm{C}$ is representative of a sample heat treated at $300°\mathrm{C}$ for 3 h and the topographies labeled 2N0, 2N6, and 3N60 are N doped as described in the text, all without post-heat-treatment electropolishing. Scale bars are $10\mathrm{\mu }\mathrm{m}$.

Figure 3

Representative tapping-mode AFM topographies of the heat-treated surfaces after electropolishing within grains. Note the difference in scales between the vacuum-annealed and N-doped samples. Scale bars are $10\mathrm{\mu }\mathrm{m}$.

Figure 4

Evolution of the average surface roughness, $S_a$, and rms roughness, $S_q$, from the tapping-mode AFM topographies of the as heat-treated surfaces within a grain. Pre-EP, $S_a\le 5\mathrm{nm}$ for the 600 and $300°\mathrm{C}$ heat-treated samples, while $S_a\le 10\mathrm{nm}$ for the N-doped samples. The increase in $S_a$ for the N-doped samples is due to the formation of nitrides.

Figure 5

(a) Phase map and (b) orientation map of a 2N0 N-doped sample showing the formation of Nb nitrides within grains and along grain boundaries. (c) Scanning transmission electron micrograph of a 3N60 N-doped Nb sample showing the formation of sharp Nb nitrides that intrude into the surface. All N-doped recipes investigated in this work produce qualitatively similar surfaces with nitrides forming within grains and along grain boundaries.

Figure 6

Tapping-mode AFM topographies of the native surface of an as-treated 2N0 sample (left) and after $1\mathrm{\mu }\mathrm{m}$ surface removal via electropolishing (right). After electropolishing grooves develop along grain boundaries. The scale bar is $5\mathrm{\mu }\mathrm{m}$.

Figure 7

Representative tapping-mode AFM topographies of the heat-treated surfaces within a grain. Note the difference in color scales between the vacuum-annealed and N-doped samples. Scale bars are $10\mathrm{\mu }\mathrm{m}$.

Figure 8

(a) Representative AFM topography, (b) local slope angle, $\theta$, (c) slope angle histogram, and (d) $\eta (\mathbf{r})$ for a 2N6 sample after $3\text{−}\mathrm{\mu }\mathrm{m}$ surface removal via electropolishing. Scale bars are $10\mathrm{\mu }\mathrm{m}$.

Figure 9

(a) Tapping-mode AFM topography of a 2N6 sample after $3\mathrm{\mu }\mathrm{m}$ of electropolishing used to construct the model mesh. (b) Mesh used for the solution of the magnetic scalar potential. (c) Local magnetic field enhancement factor at the surface. (d) Histogram of magnetic field enhancement factors. Scale bars are $10\mathrm{\mu }\mathrm{m}$.

Figure 10

(a) Representative triple junction $\eta$ maps upon electropolishing. Scale bars are $10\mathrm{\mu }\mathrm{m}$. (b) Evolution of intragrain $\eta (\mathbf{r})$ histograms upon electropolishing. (c) Evolution of triple junction $\eta (\mathbf{r})$ histograms upon electropolishing.

Figure 11

(a) Representative evolution of triple junction $\beta (\mathbf{r})$ maps upon electropolishing. Scale bars are $10\mathrm{\mu }\mathrm{m}$. (b) Evolution of intragrain $\beta (\mathbf{r})$ histograms upon electropolishing. (c) Evolution of triple junction $\beta (\mathbf{r})$ histograms upon electropolishing.

Figure 12

SIMS depth profile of N impurities in 2N0, 2N6, and 3N60 samples.

Figure 13

Comparison of $\eta$ between the 2N0 and 2N6 processes.