Summary of Antimony Work

Transcription

1 Summary of Antimony Work Evan Shockley July 21, 2014 Contents 1 Introduction 1 2 Methods for Cleaning Antimony Pulsed laser cleaning C bake Cesiation Determining Cleanliness Resistivity XPS OSEE Conclusions 4 1 Introduction The development of a procedure for in situ photocathode production is important for lowering cost and optimizing the manufacturing process. Since we can synthesize an air-stable K 2 Cs compound, we plan to deposit Sb on the window, make window-base seal, incorporate the bialkali vapor to form photoacthode, and then vacuum seal the tile. This process is very similar to the standard approach of photocathode formation in PMTs, but, while the geometry of PMTs allows for the inclusion of an antimony bead for complete in situ photocathode production, LAPPDs simply don t have the space. Therefore, Sb must be applied to the window prior to sealing. This means that 1) the antimony will be exposed to the environment, or 2) vacuum transfer is necessary. If we avoid the latter and thus allow the antimony to be exposed to the (even controlled) environment, we risk oxidation and adsorption on the antimony layer. The goal of this work was to determine a method for cleaning antimony as well as one for evaluating the cleanliness of the metal film. 1

2 2 Methods for Cleaning Antimony The main concern for exposing antimony to the atmosphere is that the metal could oxidize and have a thin film of antimony oxide rather than pure metal. Three different methods were considered for cleaning this oxide. 2.1 Pulsed laser cleaning A paper [1] was found by Petit and Caudano suggesting that pulsed laser annealing (PLA) is a good method for removing oxide from antimony. At sufficient energy density (>130 mj/cm 2 )ofuv, oxidesprogessivelyleavethesurface as well as organic contaminants. The authors also performed simulations using a thermal model as the mechanism for chemical reduction C bake Yunpeng Ji found a paper [2] by Asryan, et. al. that claimed in the temperature range K ( C), Sb 2 O 3 should leave the antimony surface through the sublimation reaction 2Sb 2 O 3 (s)! Sb 4 O 6 (g). Since we want to bake the tile at high temperatures during production, this was a very exciting find. However, we ran into some other problems with this method (see Section 3.2). 2.3 Cesiation We did not find any papers on this method of cleaning, but since cesium is highly reactive it was suggested that maybe it would clean oxide from antimony. Since cesium will be incorporated for photocathode formation, this seemed like areasonablewaytochemicallycleanthesurface. Toseeifitwouldworkin theory, a free energy calculation was done. The desired reaction is Sb 2 O 3 (s)+6cs(g)!2sb(s)+3cs 2 O(s). However, the thermodynamic data on Sb 2 O 3 was unclear. I was able to find a H formation value of kj/mol for Sb 2 O 3 and H formation and S values of kj/mol and J/mol K, respectively, for Sb 4 O 6. It seems strange that the heats of formation would be so different for such similar compounds. If the oxide on a layer of antimony is Sb 2 O 3,thereactionaboveshouldcertainly take place. If it is Sb 4 O 6, the calculation yields (at STP) H rxn and S rxn values of kj/mol and J/mol K. Therefore, G is very negative and the reaction is favorable. Further calculations are needed to verify this for the tile conditions, but if it happens at room temperature it should happen at high temperature and low pressure. Jeff Elam has a program that might help solve this, and UC might pay for access as well. 2

3 Figure 1: The third generation resistance probe. Known mass weights could easily rest on top of the probe to increase pressure on metal surfaces. 3 Determining Cleanliness 3.1 Resistivity The first idea for determining cleanliness was to measure the resistivity of the metal. Antimony has a much lower resistivity than antimony oxide, and so a precise measurement of the sheet resistance should determine the cleanliness. Three generations of a simple four-point probe were built to try to accurately measure the resistance (Figure 1). To check if the device worked properly, known amounts of mass were added to the probe to increase the pressure of the test leads on the surface. If the device worked, as weight increased we would expect a resistance curve with the appearance of a decaying exponential that approaches a constant value. Upon cleaning, we would expect the curve to reach it s constant point more quickly. Bar Keeper s Friend was used as a cleaning agent, and copper was the metal being tested. As seen in Figure 2, the results are as expected. Next, the device was used to measure the resistances of three different thin metal films: copper, gold, and antimony (Figure 3). Copper oxidizes and gold does not, so it was a good test to see any differences in the plots. Since gold does not oxidize, the decaying behavior was not expected. However, the test leads, or fingers, are flexible and thus the contact area increases as weight is added. This could account for the plots in Figures 2 and 3. Finally, some copper and antimony samples (not tested with this device) were brought to Argonne and tested using Anil s several-thousand dollar sheet 3

4 Figure 2: A comparison between dirty (oxidized in air) and clean copper resistances. Current is held constant so the voltage drop is proportional to resistance. resistance probe. He measured resistivity of many oxidized samples and remeasured one after cleaning with Micro90. The resistance increased after cleaning. This was very unexpected and led us to the conclusion that the cleaning must remove layers of the metal, and thus increase the resistance. After these tests and talking to many experts, including Jeff Elam and Anil, it was concluded that resistance is not a great way to determine whether the metal is clean. 3.2 XPS Alex Zinovev at Argonne was kind enough to perform XPS on Sb samples while heating to 400 ºC. Surprisingly, he did not see any oxides on the sample to the extent XPS could measure. Furthermore, all of the antimony metal evaporated from the glass at 400 degrees! Suprised by the lack of oxide on the antimony film, more research/calculations were done and it appears that antiony does not in fact oxidize at room temperature. 3.3 OSEE Optically Stimulated Electron Emission (OSEE) is another possible cleanliness analysis tool, but it is expensive ($18k) and might not be necessary anyway. 4 Conclusions Many questions remain after this work, but some were answered as well. seems reasonable to conclude that It 1. Resistance is not a good method for measuring cleanliness in this application; 2. Heating a thin film of pure antimony to 400 ºC evaporatesthemetalfrom the glass; and 4

5 (a) Cu (b) Au (c) Sb Figure 3: The device was used to measure the resistances for three different metals. The copper sample was a piece of copper shim, and the gold and antimony samples were deposited on glass. The dashed lines give a range expected from rough calculations, and should not be taken too seriously as many estimations were made. 5

6 3. Antimony does not oxidize as previously assumed. The most pressing question that remains also concerns #3 above. If antimony does not oxidize at room temperature, then why are there papers that discuss cleaning antimony of oxide? This certainly requires further work. In addition, we still do not know the best way to clean antimony of other contaminants as well. The pulsed laser cleaning deserves another look. References [1] Roland Caudano Etienne J. Petit. Pulsed laser cleaning of antimony single crystal <111> surface. MRS Proceedings, 129,1988. [2] A.S. Alikhanyan N.A. Asryan and G.D. Nipan. p-t-x phase diagram of the sb-o system. Inorganic Materials, 40(6): ,