Micro-coaxial Cables Research Articles

Evaluation of Cost-Effective Flexible High-Speed Data Exchange Links for Heterogeneously Integrated Multi-Chip Modules and Boards

With increasing complexity and heterogeneity, miniaturized integrated system on both multi-chip module and printed circuit board levels relies more and more on high-speed point-to-point serial signal transmission links as the backbone of inter-chip data signal exchange network, which are inevitably accompanied by signal integrity problems such as loss, reflection, and crosstalk. The difficulties in the physical design phase of interconnect routing for high-speed link on densely occupied package interposers or printed circuit boards and the associated planar area overhead (keep-out zone for the crosstalk prevention and impedance matching, etc.), as well as the relevant cost in manufacturing, performance analysis and actual measurement or evaluation, are also sharply increasing; furthermore, the interconnects implemented on interposer or printed circuit board substrate are physically inflexible, and the corresponding microsystems are not suitable for emerging applications that generally demand good deformability and good conformality to 3-dimensionally shaped mounting surface. How to simultaneously elevate the cost-effectiveness, signal transmission performance and physical flexibility of the high-speed link, while keeping low profile of the whole module, has become one of the major challenges in the evolution of micro/nano system technology. Accordingly, this paper proposes a cable-based point-to-point link method, which adopts physically flexible and widely available off-the-shell cables that can be assembled onto the surface of the interposers or boards as a potential cost-effective solution to the challenge mentioned above. Universal Serial Bus, micro-coaxial cables, and Twinax cables are explored as candidates. The full links for each cable, from one of the paired connectors to another, are physically modeled and validated by an electromagnetic full-wave structural simulation tool, with key components of each type of link taken into considerations. The simulated results and the measured results from vector network analyzers are compared, which verifies the effectiveness of our proposal.

Co-Fabrication of Microcoaxial Interconnects and Substrate Junctions for Multichip Microelectronic Systems

RF and millimeter-wave designers continue to use wire bonds and flip chip in their designs when bonding chips or multichip systems. Due to electrical and mechanical constraints of wire bonding and flip chip, designers are forced to compensate for mismatched impedances, high loss, or mechanically unreliable interconnects. In this article, we propose microcoaxial cables (MCCs) as an alternative interconnect, specifically, 30–75- $\Omega $ MCCs for signals, and low-inductance MCCs for bias and supply. In addition, each shielded interconnect provides sufficient isolation to prevent electromagnetic interference (EMI) and crosstalk. The in situ fabrication method presented here utilizes only conventional wire bonding and microfabrication techniques, providing a high-feasibility path toward a new interconnect paradigm based on MCCs. Each cable measured consists of a 25.4- $\mu \text{m}$ gold bond wire coated first with a dielectric and then with a 5.0- $\mu \text{m}$ -thick gold shield. MCCs appropriate for signals have a 38- $\mu \text{m}$ -thick Parylene C dielectric and a characteristic impedance of 39– $68~\Omega $ . For the first time, to the best of our knowledge, we introduce low-inductance MCCs, where the dielectrics evaluated are 1.0- $\mu \text{m}$ -thick Parylene C dielectric and 100-nm-thick HfO2. Their inductances are measured to be no more than 140 pH/mm. For 3.5-mm-long wire with a 0.51-mm pitch, the crosstalk is −62 dB at 1 GHz for signal MCCs. Crosstalk increases to −30 dB at 26.5 GHz.

Fabrication and Characterization of a Miniaturized 15-MHz Side-Looking Phased-Array Transducer Catheter.

This paper describes the development of a miniaturized 15-MHz side-looking phased-array transducer catheter. The array features a 2-2 linear composite with 64 piezoelectric elements mechanically diced into a piece of PMN-30%PT single crystal and separated by non-conductive epoxy kerfs at a 50-μm pitch, yielding a total active aperture of 3.2 mm in the azimuth direction and 1.8 mm in the elevation direction, with an elevation natural focal depth of 8.1 mm. The array includes non-conductive epoxy backing and two front matching layers. A custom flexible circuit connects the array piezoelectric elements to a bundle of 64 individual 48-AWG micro-coaxial cables enclosed within a 1.5-m long 10F catheter. Performance characterization was evaluated via finite element analysis simulations and afterwards compared against obtained measurement results, which showed an average center frequency of 17.7 MHz, an average bandwidth of 52.2% at -6 dB, and crosstalk less than -30 dB. Imaging of a tungsten fine-wire phantom resulted in axial and lateral spatial resolutions of approximately 90 μm and 420 ìm, respectively. The imaging capability was further evaluated with colorectal tissue-mimicking phantoms, demonstrating the potential suitability of the proposed phased-array transducer for the intraoperative assessment of surgical margins during minimally invasive colorectal surgery procedures.

Characterization of Low-Inductance Microcoaxial Cables for Power Distribution

Abstract Low-impedance microcoaxial cables have been developed to supply power to microchips. These uniquely low-inductance cables are enabled by a very thin dielectric compared with a conventional 50-Ω cable. These cables will be used in a novel packaging platform in which traditional interconnects are replaced by microscale coaxial cables. This method saves time and cost for small production volumes and custom electronics, compared with high density interconnects and silicon interposer technologies. These microcoaxial cables are designed to have minimal impedance to meet the stringent power supply requirements of today's electronics. As a concrete example, we consider a Kintex 7 Field-Programmable Gate Array (FPGA). To power this chip with interconnect lengths of 25 mm and a voltage ripple less than 30 mV, a resistance of 3.20–6.40 mΩ/mm and an inductance of 12–15 pH/mm is needed. The tight voltage ripple constraint is what makes this device challenging to design power distribution for. One cable fabricated by Draper, to achieve these power requirements, is the focus of this article. The Draper cable consists of a 127-μm Copper core, 12-μm polyesterimide dielectric layer, and 55-μm gold shield. The measured resistance per unit length at DC, inductance per unit length, capacitance per unit length, and characteristic impedance of the Draper cable are 2.0 mΩ/mm, 40 pH/mm, 118 pF/mm, and 6.56 Ω, respectively.

Forward-looking 30-MHz phased-array transducer for peripheral intravascular imaging

This paper describes the design, fabrication, and testing of a forward-looking 30-MHz phased-array transducer for peripheral intravascular imaging. The array features 32 piezoelectric elements arranged in a linear 2-2 composite configuration dry etched into a piece of PMN-30%PT single crystal and separated by non-conductive epoxy kerfs at a 25-μm pitch, yielding a total active aperture of 0.8 mm in the azimuth direction and 1.0 mm in the elevation direction, with an elevation natural focal depth of 5.0 mm. The array includes non-conductive epoxy backing and two front matching layers. A custom flexible circuit connects the array piezoelectric elements to a bundle of 32 individual 48-AWG micro-coaxial cables enclosed within a 0.6-m long 8 F catheter. Performance characterization was evaluated via finite element analysis simulations and afterwards compared to measured results, which showed an average center frequency of 28.9 MHz, an average bandwidth of 36.4% at -6 dB, and crosstalk less than -26.5 dB. Imaging of a tungsten fine-wire phantom resulted in axial and lateral spatial resolutions of approximately 65 μm and 215 μm, respectively. The imaging capability was further evaluated with a porcine carotid artery phantom, demonstrating the suitability of the proposed phased-array transducer for peripheral intravascular imaging.

A Front-End ASIC With High-Voltage Transmit Switching and Receive Digitization for 3-D Forward-Looking Intravascular Ultrasound Imaging

This paper presents an area- and power-efficient application-specified integrated circuit (ASIC) for 3-D forward-looking intravascular ultrasound imaging. The ASIC is intended to be mounted at the tip of a catheter, and has a circular active area with a diameter of 1.5 mm on the top of which a 2-D array of piezoelectric transducer elements is integrated. It requires only four micro-coaxial cables to interface 64 receive (RX) elements and 16 transmit (TX) elements with an imaging system. To do so, it routes high-voltage (HV) pulses generated by the system to selected TX elements using compact HV switch circuits, digitizes the resulting echo signal received by a selected RX element locally, and employs an energy-efficient load-modulation datalink to return the digitized echo signal to the system in a robust manner. A multi-functional command line provides the required sampling clock, configuration data, and supply voltage for the HV switches. The ASIC has been realized in a 0.18- $\mu \text{m}$ HV CMOS technology and consumes only 9.1 mW. Electrical measurements show 28-V HV switching and RX digitization with a 16-MHz bandwidth and 53-dB dynamic range. Acoustical measurements demonstrate successful pulse transmission and reception. Finally, a 3-D ultrasound image of a three-needle phantom is generated to demonstrate the imaging capability.

Fabrication and Preparation of Micro-Coaxial Wires: Electrodeposition of Conductive Shields

We describe the fabrication of micro-coaxial cables with diameters of 25 to 300 micrometers. Draper Laboratory is developing a new microelectronics packaging platform that relies on such microcoax, in place of copper interconnects, for both power and signal distribution among the packaged electronic components. This platform could save design time and fabrication labor over conventional packaging techniques for custom electronics with small production volumes and offers a way to rapid prototype and/or edit circuits. In this work, we focus on the fabrication of microcoax for power distribution. The fabrication process of the metal-dielectric-metal cross-section of the microcoax begins with a commercial metal wire of 18 to 150 micrometer diameter, which is either purchased with a polymer insulation of 1 to 12 micrometer thickness, or is insulated with SiO2 or Al2O3 of 0.1 to 2 micrometer thickness by PECVD or ALD processes. Next, a thin Ti/Au bilayer is applied to the insulated wire by electron beam evaporation and acts both as an adhesive layer and as electrical contact to plate the outer metal shield to its full desired thickness. The metal shield is electroplated using a cyanide-free gold sulfite chemistry that produces a soft satin to bright electrodeposit, meeting Type IIIA of Mil-G-45204 C requirements (i.e., ≥ 99.9 % gold purity and Knoop hardness 90 maximum). Straight DC, temperature of 60 °C, current density of 2 mA/cm2and mild agitation were chosen as parameters for Au electroplating conditions. Gold electrodeposits exhibit very low roughness, good corrosion resistance, excellent solderability, high shine and with evaporated seed metal, good adhesion to the polymer dielectrics. The shield thickness is chosen to give a resistance equal to or less than the resistance of the metal core, e.g., 34 micrometers of plated Au for a 127 micrometer diameter Cu core. The use of the microcoax for packaging requires lengths of microcoax of < 15 mm with the wire tips prepared for electrical connection, i.e., tips where the metal shield ends abruptly leaving the center metal wire exposed for bonding, and so we introduce at periodic intervals along the wire length regions where the outer layers are more easily stripped to expose the metal core. We have accomplished this either by ad hoc placement of a plating-resistant (nail polish or soft-baked photoresist) bead on the seed metal to pattern regions where no shield metal is electrodeposited; or by an evaporation mask during seed metal deposition to pattern regions where neither the metal seed nor the metal shield are deposited. The core metal is then exposed in the beaded or masked regions by wet etching the seed metal, if necessary, and UV laser ablation of the polymer dielectric. In addition to the fabrication processes, we will also discuss the fixturing and handling protocols required for these fine diameter wires. Fixturing is designed to accomplish four criteria: to minimize handling among the various process steps (insulating the wire, e-beam evaporation, electroplating); to pattern the microcoax lengths and abrupt edges to the electrodeposited metal; to make electrical contact to the seed metal during electroplating; and for the necessary volume and yield. We will describe our scale-up from an initial proof-of-concept process that produced 0.1 meters of microcoax of arbitrary segment lengths to a simpler process that produces 2 meters of microcoax of well-defined segment length.

Radiation hard micro-coaxial cables for the ATLAS liquid argon calorimeters

The ATLAS collaboration has chosen for the electromagnetic barrel calorimeter and for all the end-cap calorimeters a sampling technique, with liquid argon as the active medium. The read-out electronics and the calibration pulsers are located in boxes outside the cryostats housing the detectors. Signals are transmitted between the detectors and the electronic boxes through custom-designed micro-coaxial cables, which are the subject of this paper. These cables have to satisfy very stringent tolerances in terms of signal transmission, dimensions and radiation hardness. Following a successful pre-series production, these cables have been selected for equipping the ATLAS calorimeter.

Design criterion for coaxial-cable p.c.m. telephone systems

A method is presented for optimum design of a p.c.m. transmitter and receiver for a noisy channel, free of timing jitter. Results are given for 8.192 Mbit/s p.c.m. telephone systems, transmitted via microcoaxial cables in the presence of thermal noise.