Plasma treatment of bulk niobium surface for superconducting rf cavities: Optimization of the experimental conditions on flat samples

Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate nonsuperconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk niobium (Nb) presents an alternative surface preparation method to the commonly used buffered chemical polishing and electropolishing methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on flat samples. We have achieved an etching rate of $1.7\mu \mathrm{m}/\min $ using only 3% chlorine in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the sample surface to the environment. We intend to apply the optimized experimental conditions to the preparation of single cell cavities, pursuing the improvement of their rf performance.

Figure 1

Etching rate and surface RMS dependence on ${\mathrm{Cl}}_2$ concentration. Experimental conditions: total gas flow 196 sccm, pressure in reaction chamber 340 mTorr, and input power density $1.40\mathrm{W}/{\mathrm{cm}}^3$.

Figure 2

Etching rate and surface RMS dependence on input power density. Experimental conditions: 3% of ${\mathrm{Cl}}_2$ in reactive gas mixture and pressure in reaction chamber 340 mTorr.

Figure 3

Etching rate and surface RMS dependence on total gas pressure. Experimental conditions: 3% of ${\mathrm{Cl}}_2$ in reactive gas mixture, total gas flow 196 sccm, and input power density of $1.40\mathrm{W}/{\mathrm{cm}}^3$.

Figure 4

Excitation temperature in the absence of Nb sample as a function of power density. Error bars reflect the statistical error of the individual sets of data and the uncertainty in local power density. Sets A, B, and C correspond to Table . Experimental conditions: pressure 1 Torr and the gas mixture $\mathrm{Ar}/{\mathrm{Cl}}_2$ ratio $97:3$.

Figure 5

Excitation temperature as a function of time. Data are taken during the etching process. Set B corresponds to Table .

Figure 6

Intensity of chlorine and niobium lines as a function of time. Data are taken during the etching process. The behavior is typical for all lines. Statistical errors are indicated.

Figure 7

Normalized XPS spectrum of oxygen $1s$ for an untreated Nb sample.

Figure 8

Normalized XPS spectrum of oxygen $1s$ for the Nb sample after $\mathrm{Ar}/{\mathrm{Cl}}_2$ plasma treatment. The same sample is used in Fig. 7 (before treatment) and Fig. 8 (after treatment).

Figure 9

AFM scan ($50\mu \mathrm{m}\times 50\mu \mathrm{m}$) of BCP prepared surface.

Figure 10

AFM scan ($50\mu \mathrm{m}\times 50\mu \mathrm{m}$) of surface prepared by plasma etching.