Increase in RF surface resistance of niobium as a result of acid treatment

The authors have systematically studied the effect on R/sub s/ (surface resistance) of several different acid mixtures and the conditions of their use. Reverse electropolishing, which generates large quantities of hydrogen at the niobium surface, can drastically increase R/sub s/. R/sub s/ can be decreased by subsequent heat treatment for 2 h at 200-300 degrees C, indicating that the hydrogen is concentrated in a thin surface layer. This was confirmed by RRR (residual resistance ratio) measurements on thin samples. Etching niobium with 1:1:1 buffered chemical polish under open conditions, i.e., copious amounts of room temperature acid with hydrogen, does not seriously load a sample with hydrogen. Using the same acid under closed conditions, i.e. restricted flow of acid which leads to a substantial increase in temperature, can increase R/sub s/ significantly. A 1:1:4 LCP acid mixture, in which phosphoric acid is replaced by lactic acid, does not introduce hydrogen under closed conditions. This mixture can even remove the hydrogen-enriched layer in a previously contaminated sample, restoring R/sub s/ to its theoretical value. Vacuum heat treatment for 2 h at 900 degrees C removes all dissolved hydrogen and restores R/sub s/.<>