Spurious mode suppression using micromachined pillars in superconducting quantum devices

As quantum circuits scale in size, the enclosures used to house them will contain spurious resonant electromagnetic modes detrimental to the circuits unless preventative steps are taken. A standard solution in microwave circuits is the use of through-chip vias. Here we present an alternative that moves the through-chip electrical connection off the substrate and to the enclosure, which suppresses substrate and enclosure modes simultaneously. We achieve this by placing a substrate in a rectangular cavity incorporating an array of micromachined pillars, such that the minimum mode frequency is set by the pillar spacing and not the enclosure dimensions. To accommodate the pillars the substrate is machined. We investigate the compatibility of both CNC and laser machining of holes in silicon with superconducting qubit fabrication. We present proof of principle experiments on enclosures incorporating pillars, and produce simulations with more complex arrangements of pillars in larger-scale devices.