Ph.D. Thesis - Physics

in trap depth, however, comes decompensation of the trap. This fact is demonstrated by
plotting cross sections of the pseudopotential for two different top plate voltages in Fig. 6-3.
With Vtop = -25.4 V, we predict that the trap should be compensated, with a trap depth of
1.0 eV. By contrast, Vtop = 15 V leads to a decompensated trap with a depth of 5.4 eV. A
typical strategy, at least in the early period of surface-electrode ion trapping in our group,
was to apply a voltage to the top plate to enhance the depth for initial trapping, and then
gradually tune the compensation voltages and laser positions while reducing Vtop to the
compensated value. In Fig. 6-4, we plot both the trap depth and the displacement of the
trap center from the rf null as a function of the top plate voltage Vtop.
We have seen the principle illustrated in Fig. 6-4 used in Ch. 5 already, in the macroion
experiment, when a high trap depth was useful, but compensation was not critical to the
measurements. In the atomic ion experiment of that chapter, fortunately, applying a voltage
to the top plate was not required. The main caveat regarding this theoretical work is that the
real compensated values may be quite different from the prediction due to stray fields. In the
next section we will actually measure the stray fields and find quite different compensation
voltages. However, it is still useful to have an idea of how the trap will behave if there are
no stray fields; hopefully, they are but a perturbation on the controlled fields.
6.2.2 Constructing and mounting the trap
The trap was manufactured by Hughes Circuits in San Diego, CA. The electrodes are
copper deposited on a fiberglass epoxy substrate, Rogers 4350B, which features a small
rf loss tangent and UHV-compatibility. The thickness of the copper is about 25 µm. The
minimum feature size is about 75 µm, which obviously limits the extension of the technology
to microfabricated traps. In addition, the slot size was set at 850 µm since this was the
smallest standard slot size. To reduce the accumulation of stray charges, the dielectric
material in between rf electrodes was milled out by Hughes, and the sides plated with
copper.
The electrodes were polished using a multi-step diamond grid process. This process
involves putting diamond grit paste containing diamond pieces of a given average size,
together with a lubricating oil, onto a paper disk that is mechanically rotated. Polishing is
done by moving the rotating disk across a trap that is mounted securely on a countertop.
Following each polishing step, the trap is thoroughly cleaned with acetone and isopropanol
and then a smaller-caliber grit paste is applied to a new disk for the next step. Diamond sizes
ranging from 15 µm to 1 µm were used. Following this process, the trap was cleaned using
the multi-step vacuum cleaning process described in Sec. 5.3.1. Fig. 6-5 is a photograph of
the trap after the polishing and cleaning processes.
Following the cleaning, the trap was mounted in the vacuum chamber using a breadboard-
style component from Kimball Physics that is secured to the “grabber grooves” that are a
part of the spherical octagon vacuum chamber. The top plate, consisting for this experi-
134