Elimination of Wire Bonding for Cryogenic Applications

Much of what is known of our universe on cosmological scales has been enabled by quantum sensors in the form of transition edge sensors (TES). Many of the most promising dark matter searches employ TES, superconducting quantum interference devices (SQUIDs), and other quantum-based devices such as DM Radio, CDMS. As traditional, silicon-based circuitry approaches the limit of the atomic scale, quantum computing offers the promise of a revolutionary approach to solving certain classes of problems. As the field of quantum information science matures, device interconnections are becoming an increasing problem due to the growing complexity of quantum sensing and computing systems. Cosmic Microwave Background (CMB) telescopes have experienced a Moore-like increase in channel count over time. The BICEP (Background Imaging of Cosmic Extragalactic Polarization) series of experiments have had arrays sizes of: 49, 250, and 1280 pixels with CMB-Stage 4 planned with 100,000 pixels. Each pixel needs several electrical connections. Historically within CMB, the standard method of forming electrical contacts has been wire bonding. However, wire bonding is a serial process where each connection is formed one at a time. Additionally, wire bonds provide only a 1-dimensional, linear arrangement. The fragile nature and production cost of superconducting bolometer focal plane arrays has caused the use of manual wire bonding by experts. Wire bonds are quite delicate and prone to mechanical damage. Wire bonds also tend have lengths on the millimeter scale, which can cause unwanted inductance, capacitance, and act as radiating antennas. Wire bonding will represent a constantly increasing cost for projects, a significant assembly risk, and a drag on scientific progress. Solder and indium bumps address some of the above issues, but introduce additional problems associated with fabrication and assembly, requiring heat, pressure, and extensive handling while forming the bumps on delicate substrates. Solder also present radiopurity concerns. To avoid retarding future QIS progress a new technique is needed. Transmission of radio frequency signals from components at the mK level to warmer components at temperatures between 1 K and 4 K is also a limiting factor in current and future QIS projects. A novel anisotropic conductive epoxy (ACE) developed by Sunray Scientific offers a new paradigm for high density interconnections at milliKelvin temperatures. The proposed method is simpler to implement, two-dimensional, and can form thousands of parallel connections simultaneously at high areal density. In contrast to wire bonding, no expensive machine is needed. Sunray’s ACE requires no pressure, modest heat, and avoids sophisticated flip-chip equipment. It involves application of a uniquely formulated epoxy paste that has sub-micron, ferromagnetic particles with a high-conductivity coating randomly suspended. Upon application of a magnetic field, the ferromagnetic particles self-assemble into vertical columns, forming a set of spatially separated, low-resistivity paths between the substrates. When cured, the adhesive sets and conductive columns become fixed. Quartz chips with Sunray’s ACE have been constructed with DOE-HEP funding and have been cycled to below a Kelvin with no adverse effects. Sunray’s ACE has been shown to be radiation hard, can achieve fine pitches at room temperature, and form connections to rigid and flexible substrates. It may also be possible to simultaneously form connections between multiple substrates in a vertical stack such as in a “wedding cake” geometry. Successful completion of the proposed Phase I program will support the execution of the Phase II program that will focus on improving ZTACH anisotropic conductive epoxy formulation to reduce defects, redesign of the package input/output to reduce high density interconnect cable assembly challenges, and developing a fast cure formulation to improve throughput and yield. Collectively, these efforts will support commercialization of ACE for use in constructing quantum computing systems. Commercialized ACE material will revolutionize the electronics industry by enabling the continued miniaturization of high-density interconnect electronics and the assembly of flexible and three-dimensional electronics that cannot handle pressure and high temperature assembly. Furthermore, to the proposed material enables high throughput manufacturing and reduced costs.