Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
Experiments with Supersonic Beams as a Source of Cold Atoms
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PTFE or PFA. The procedure takes place under the fume hood, and the working<br />
surface is covered by a layer <strong>of</strong> clean room wipes.<br />
The initial cleaning is done by sonicating the crystal in isopropanol for 10<br />
minutes, followed by another 10 minutes in acetone. The crystals are then rinsed <strong>with</strong><br />
water to remove these organic solvents before continuing the cleaning process. The<br />
next step is to place the wafers in an RCA I solution which removes organic residues<br />
and grows a uniform oxide layer. This solution consists <strong>of</strong> 1 part 30% ammonium<br />
hydroxide, 1 part 30 % hydrogen peroxide, and 5 parts water. The water is heated<br />
in a water bath on a hot plate to 80 ◦ C and the ammonium hydroxide and hydrogen<br />
peroxide are added just before the wafers. The solution is stirred gently <strong>with</strong> a<br />
magnetic stirrer on the hot plate. The crystal is left in the RCA I solution for 5<br />
minutes, at which point it is removed from the solution and rinsed <strong>with</strong> water. Each<br />
rinsing step described here involves a significant quantity <strong>of</strong> water, typically at le<strong>as</strong>t<br />
half a liter is poured over each crystal during the rinsing ph<strong>as</strong>es.<br />
The next step is the wet etching, which is done in a 40 % solution <strong>of</strong> ammonium<br />
fluoride. Before the crystals are etched, the disolved oxygen in the ammonium fluoride<br />
must first be removed by bubbling ultra pure argon through the solution for at le<strong>as</strong>t<br />
40 minutes prior to inserting the crystals. For the crystals used here, etch times<br />
between 10 and 15 minutes are optimal. Shorter times do not completely remove the<br />
surface oxide layer, while longer times result in deep etch pits in the surface. The<br />
crystals used in the experiments described below are etched for 12 minutes. Unlike<br />
in the previous steps, the ammonium fluoride solution is not stirred or disturbed in<br />
any way while etching the crystals. Even continuing to gently bubble argon in the<br />
solution negatively affects the resulting surface.<br />
After the etching is complete, the crystal is rinsed a final time. The p<strong>as</strong>sivated<br />
surface is hydrophobic, and the water does not wet the surface if the etching is<br />
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