Flip Chip Process Achieves Conversion Loss Under 8 dB in EBand Diode Harmonic Mixers
A low-cost process in which discrete semiconductor chips are flip-attached by thermocompression bonding to 30-micron-diameter gold bumps on fine-featured passive integrated circuits was employed in the fabrication of diode harmonic mixers for use over the 71-76 GHz and 81-86 GHz ranges. Incorporated in the substrate were highprecision grounded heat sinks, semi-lumped filters, and matching networks in low-loss microstrip format. Measurements demonstrate an upconversion loss minimum of 6 dB and downconversion loss minimum of 8 dB respectively. The performance is attributable to the high cutoff frequency of the discrete diodes and the low loss of the passive circuitry. Index Terms — Millimeter-wave mixers, multichip modules, frequency conversion, millimeter-wave integrated circuits, integrated circuit design, flip-chip devices.
Since mixers generally have a large ratio of passive circuit area to semiconductor device area, it is attractive to consider a technology in which a small and inexpensive semiconductor chip containing the diodes is flip-attached onto a low-cost passive integrated circuit. Diode chips with exceptionally high cutoff frequencies are unavailable on microwave monolithic integrated circuits (MMICs) due to limitations on process complexity and surface topology, but speciallyprocessed discrete diodes with cutoff frequencies approaching 2 THz can be obtained in chip form at low cost by virtue of their small size. The present work shows the results that can be achieved when these diodes are flip-attached onto low-loss passive integrated circuits to form E-band harmonic mixers. The flip-chip-on-passive-integrated-circuit technology used in this study is termed MLMS™ (MultiLithic MicroSystem™). In MLMS™ technology small semiconductor dice containing the active or nonlinear elements are flip attached by way of an automated assembly process onto an inexpensive substrate that contains all of the passive elements of the subsystem. This passive integrated circuit substrate features 5-micron minimum trace and space widths, two metal layers, and integrated metal-insulator-metal capacitors and thin-film resistors. The die attachment is achieved through thermocompression welding of the pads on the semiconductor dice to 30-micron gold bumps on the substrate—the same metallurgy as in wire bonding. The technology lends itself to the integration of complete subsystems on a single substrate thereby avoiding the use of bond wires and other variables in the millimeter-wave path. These MLMS™ assemblies withstand repeated cycling between liquid nitrogen and a 300º C hot plate and repeated cycling between freezing and boiling while under water. They have been tested under mechanical shock at 3000 Gs, and, based on measured die pull forces, are expected to withstand 100,000 Gs. The MLMS™ substrate, a cross-section of which is shown conceptually in Fig. 1, has built into it high-thermalconductivity pedestals that are precisely shaped to thermally and electrically ground a bump while maintaining electrical isolation to other bumps spaced as little as 65 microns away center-to-center. The dielectric, with a thickness of 100 microns, is thin enough to allow microstrip circuitry to operate with low loss and good isolation up to 100 GHz and to allow low-inductance grounding through the pedestals. Its dielectric constant is less than half that of a typical MMIC substrate. The harmonic mixers, one of which is shown in Fig. 2, reside on 2.6 mm by 1.5 mm substrates. A 0.36 mm by 0.32 mm custom-designed GaAs dual-diode chip is flip-attached onto the substrate. The Schottky diodes on this chip have a cutoff frequency of 2 THz.