DEVICE FABRICATION AND EXPERIMENTAL TECHNIQUES
3.2 Microwave packaging of quantum devices
The fabricated devices are wire-bonded to a printed circuit board (PCB) for electrical connection to external control wiring and packaged inside a metal enclosure for electromagnetic shielding and good thermalization. For high-fidelity control of multiple qubits, it is crucial for microwave packaging to have low crosstalk and to accommodate high wiring density without spurious modes [152]. Also, the packaging must be mechanically robust and thermally stable to be able to undergo multiple rounds of experiments with reconfiguration and thermal cycling. In this section, I will introduce the microwave packages used for experiments described in the thesis.
16-port packaging for 1 cm×1 cm chips
The 16-port device packaging system illustrated in Fig. 3.1a was developed by former undergraduate student Paul Dieterle in 2017 and has enabled a multitude of successful experiments in Refs. [56, 108, 109, 153].
The core of the PCB in this packaging is Arlon AD1000 (dielectric constant𝜖𝑟 =10.2 and loss tangent tan𝛿=0.0023 at 10 GHz) with the thickness of 20 mils (508𝜇m), 0.5 oz (18𝜇m) copper electrodeposited on the top surface, 2 oz (72𝜇m) copper electrodeposited on the bottom surface. The top and bottom copper films of the PCB are connected by a large number of copper vias of diameter 10 mil (254𝜇m) which are periodically arranged in a two-dimensional grid spaced by 1 mm, in order to minimize stray resonances. Planar microwave transmission lines in the form of conductor-backed coplanar waveguide (CBCPW) [154] are patterned on the PCB to interface the chip’s input and output ports to end-launch surface mount MMPX connectors (Huber+Suhner 92_MMPX-S50-0-1/111_NM) with operating frequencies up to 65 GHz. The PCB is electroplated with 50𝜇in (1.27𝜇m) of wire- bondable soft gold without nickel (i.e., non-magnetic) to prevent oxidation and to improve thermal contact with the enclosure. The designed PCB was manufactured by Hughes Circuits, Inc. The MMPX connectors are soldered to the PCB by using low-temperature Sn42/Bi57.6/Ag0.4 solder paste5and a heat gun.
The enclosure (commonly referred to as “box”) for the chip and the PCB is manu- factured by machining either oxygen-free high-conductivity (OFHC) copper (C101) for best thermalization or aluminum alloy (6061-T651) for shielding against ex- ternal magnetic field6. It consists of a top part (cover) and a bottom part (mount plate) joined with UNC #2-56 brass screws. The cover has a 11.4 mm×11.4 mm
× 1.5 mm rectangular opening above the chip surface. The mount plate was ini- tially designed without opening but a pocket of 8 mm×8 mm×3 mm was milled out in later revisions to suppress cavity resonances formed by the mount plate and the ground plane of the chip due to high dielectric constant 𝜖𝑟 = 11.65 of the Si substrate.
While many experiments were conducted in this packaging, there has been several issues found over time. First, there is no mechanism to tightly mount the PCB to the enclosure. This made the wirebonding of the chip to PCB challenging and is also expected to make the thermalization of PCB inefficient. Second, there were many cases when the MMPX connectors soldered to PCB “pop out” due to mechanical force unintendedly acting on the connectors when closing the lid of the box, making the assembly process very challenging. This is due to the over-constrained design of
5Chip Quik SMDLTLFP, melting point 138◦C.
6Aluminum becomes a superconductor at temperatures below∼1 K and can repel magnetic field due to the Meissner effect.
the assembly associated with mating of connectors, PCB, and the enclosure. Finally, MMPX connectors are costly relative to other kinds of RF connectors to be used as consumables. These issues are addressed in the new packaging design which is described below.
26-port and 16-port packaging for 2 cm×1 cm chips
In 2021, we have developed a new packaging standard illustrated in Fig. 3.1b to enable larger-scale experiments and to accommodate larger number of control lines.
In particular, packaging for 2 cm×1 cm chips was conceived to enable fabrication of longer superconducting metamaterial waveguides for constructing 10-qubit system to study quantum many-body physics.
The PCB has a large footprint of 45 mm×45 mm to allow for up to 26 RF connectors along its perimeter. We use the same PCB core (Arlon AD1000 with 2 oz/0.5 oz electrodeposited copper) as the old packaging version and pattern similar CBCPW transmission lines to connect on-chip launchers to RF connectors. We place vias of diameter 10 mil (254𝜇m) as “fences” surrounding each waveguide trace and the footprint of each RF connector to suppress spurious resonances. Also, the vias are placed along the inner edges of the PCB and in a two-dimensional grid spaced by 1 mm automatically positioned according to the PCB fabrication rules. We decided to use the SMPM connector7, a miniature version of the well-known SMP connector considered the availability and the cost. We use full-detent through-hole connectors (Amphenol RF 925-138J-51P) soldered to the PCB rather than surface mount ones to achieve mechanically robust connections. Stub resonances caused by ∼ 2 mm extruded pins of this connector are expected to lie above∼ 38 GHz which will not cause problems in the experiments. The PCB is gold-plated in a similar manner as described above. The SMPM connectors are soldered to the PCB from the bottom side by using eutectic Sn63/Pb37 solder paste8and a heat gun.
The machined OFHC copper enclosure consists of the cover and the mount plate.
The cover has an opening of 21.4 mm×11.4mm×1.6mm above the top surface of the chip. The mount plate has a 20 mm×10 mm wide and 3 mm deep pocket to be placed under the chip to suppress the resonances above 10 GHz. Also, pedestals at the corners of the pocket provide mechanical support of the chip. The top cover and the mount plate are assembled using four UNC #2-56 brass screws.
7SMPM is also known as mini-SMP or Corning Gilbert GPPO, with maximum operating frequency of 65 GHz.
8Chip Quik SMD291AX10, melting point 183◦C
During the test of this packaging, we have observed degradation of mating between the PCB connectors and right-angled cable connectors with thermal cycling. To prevent this, we have later added another copper piece to the assembly for cable connectors to be tightly clamped to the PCB connectors, which solved the problems.
Assembly procedure
The PCB is soldered to RF connectors and then sonicated in acetone and IPA in order to remove residues of solder paste. The enclosure parts made of aluminum (copper) are cleaned with Transene Aluminum Etchant A (1.0M citric acid) followed by rinse in DI water and IPA prior to the assembly.
After cleaning, the PCB and the fabricated chip is placed on top of the mount plate and wire-bonded. In case of the new packaging, the fabricated chip is bonded to the pedestals by using GE Varnish (VGE-7031) to provide strong mechanical support prior to wire-bonding and cryogenic heatsink [155]. Also, the PCB is tightly attached to the mount plate by using four UNC #1-64 brass screws with a thin layer of Apiezon N grease [156] applied at the interface to maintain good thermal conductance at cryogenic temperatures. The box is then closed by joining the cover and the mount plate with brass screws.
The packaging assembly is then installed vertically to the sample mount machined with OFHC copper and mounted to the mixing chamber plate of the dilution refrig- erator. We use non-magnetic semi-flexible coaxial cables (Micro-Coax UT-085C- FORM) to route input/output signals between the PCB connectors and the cryogenic semi-rigid coaxial cables discussed in Sec. 3.3. After this, we enclose the packaging assembly with two 1.5 mm-thick cylindrical Cryophy magnetic shields of outer di- ameters 70 mm and 90 mm and heights 185 mm and 200 mm, respectively. Finally, there exists a large cylindrical mu-metal shield (thickness 1 mm, inner diameter 395 mm, height 750 mm) placed inside the vacuum can of the dilution refrigerator for additional protection from external magnetic field.