Translume is introducing a service for fabricating small glass ion traps. This capability is being developed with funding from the Air Force Office of Scientific Research, and in collaboration with the group of Professor Chris Monroe of the Joint Quantum Institute at the University of Maryland.

Our traps are fabricated in fused silica. To date we have specialized in volumetric (3D) traps which offer excellent trapping depth. Fabrication of surface electrode traps is also possible with our microfabrication processes. Electrical and magnetic functionalities are implemented using RF and DC electrodes fabricated using physical vapor deposition processes (sputtering, with electroplating if necessary). To accommodate 3D trap geometry these electrodes can be fabricated on flat surfaces and slanted surfaces.

We are also developing a unique capability to integrate various optical elements within the body of the trap using our proprietary micromachining processes. Interfacing to the external world can be provided via permanently attached and aligned optical fibers. Combining these features we can integrate electrical, magnetic and optical functionalities in a monolithic trap. Multiple traps can be located on a single substrate.

The fabrication of an ion trap is a multistep process.

The fabrication is split in four main phases:

(1) glass shaping,
(2) electrode deposition,
(3) fiber and optics interfacing,
(4) wire bonding.

Additional steps may be required if waveguides are integrated in the platform.

For instructive purposes, these main phases are presented below in an extended sequence of steps.

Step 1A:
The fabrication sequence starts with the selection of a fused silica wafer of the desired thickness. We typically work with 500-µm or 1-mm thick substrates, but our technique is compatible with other thicknesses.

Using our femtoEtch process we define the substrate outside dimensions. Attachment points and alignment features may also be defined at this time.

Step 1B:
The trap boundaries are then defined.

In this case we selected a symmetrical cross-section, but this is not a requirement.


        

Step 1C:
We can integrate optics either by direct-write or through insertion of pre-manufactured small optics.

In this example we choose to insert a high index of refraction ball lens. A well into which optics will later be inserted is defined in Step 1C. Note that a ball lens support is an integral part of the well.

Other optical elements can be fabricated (See below).

Step 1D:
A tunnel is created to connect the optic well with the center of the trap. It is entirely submerged within the body of trap substrate. This tunnel has a conical shape (see cross-section below) to allow for the use of large-NA optics, to minimize disturbing the RF trapping field, and to reduce direct line-of-sight exposure of the trapped ions to nearby bare dielectric surfaces.


        

Step 1E:
A fiber port/fiber positioner assembly is defined. In this example the assembly is designed for a 125-micron outside diameter fiber.

Step 1F:
Electrical vias are formed. In this example, in order to simplify the later wire-bonding process, vias are used to move all the DC and RF electrodes to one side of the chip

Step 2A:
Electrodes (DC and RF) are fabricated.

The electrodes are fabricated using standard physical vapor deposition processes, followed, if desired, with an electroplating step. Note however these are 3D electrodes, created with 3D masks that follow the contour of the trap.

The RF electrode (in the central region, shown in red) is deposited on the vertical walls of the platform enclosing the through slot, and extends approximately 10 microns over the top and bottom surface of the mid-layer platform.

The DC electrodes are separated from the RF electrode by a 25-micron gap in this design, although different spacings can be used.

As noted above we have proprietary techniques to fabricate electrodes that follow three-dimensional geometries. Here the DC electrodes dip down into the trap area from the main surface of the substrates.

Note that the slanted walls have been moved some distance from the RF trap to prevent direct line-of-sight path between any uncoated areas and the trapped ions.

Shown at left are two micrographs of one of our traps. The insert shows a close-up view of the separation between the upper lip of the RF electrode and the end of the DC electrode.

Step 2B:
Large ground planes are added to reduce the unwanted effects associated with the presence of charges on uncoated dielectrics.
A similar process is applied to the back side (not shown here).

Step 3A:
A ruby ball lens is inserted in the optic well. It rests on the support fabricated in step 1C. The lens can be bonded in place using an UHV compatible bonding agent, thermally bonded, or laser welded. Ball lenses of various materials (fused silica, sapphire, high index optical glasses) can be used.

Finally a fiber is inserted in the fiber port (See Step 1E). Its position is optimized (in this case for maximum fluorescence collection). The fiber can be single-mode or multimode, and polarization preserving or not.

A microphotograph of a 3D trap with large NA collection optics and RF and DC electrodes. This three-dimensional trap demonstrator is shown after the DC and RF electrode deposition fabrication step, but prior to ground planes deposition. Each DC electrodes is terminated with an electrical via hole. Wirebounding connections are on the back side. The trap is optically connected to an output fiber using a conical tunnel and a fused silica micro-ball lens. The fiber is inserted in a high precision fiber port. The lens is supported by an integrated precision-machined seat.

In the example shown above, a single high index ball lens is used to collect the fluorescence signal from a trap ion. Other optical elements, such as subsurface waveguides (shown in the micrograph at the far left) and integrated lenses (shown in the micrograph at the near left) can be used in addition to, or in place of, ball lenses.

Professor Christopher Monroe's Research Group at the University of Maryland

The Joint Quantum Institute