Packaging and integration of the entire radio including the RF front end is one of the most expensive portions of wireless hardware. The ultimate goal could be the so-called system-on-the-chip in which all the constituent elements of wireless communication hardware are in a single chip including the baseband DSP, RF-front end, and even antennas. In practice, however, the approach is to combine several different chips or functional elements and connect them together to form a functional block. The so-called multi-chip module (MCM) of this type. In many attempts, the techniques developed at lower frequencies can be modified for high frequency applications. Flip-chip mounting of the MMIC on a motherboard is one such example (see Fig. 5.15).
Fig. 5.15. A typical flip-chip MMIC structure by NEC.
Matsushita Research Institute Tokyo has been engaged in flip chip technology with micro-bump for quite some time with very successful results. More recently, it has demonstrated a 3D hybrid IC as a future PC-card-type radio terminal for millimeter-wave frequencies. This structure consists of a dual-mode filter in a dry-etching micromachined cavity, multilayered thin films, and flip-chip bonding of GaAs devices (see Fig. 5.16).
Fig. 5.16. Matsushita mm-wave system integration on a chip.
On the other hand, NTT has for some time proposed 3D MMIC with 6 metal layers and polyamide for insulation (see Fig. 5.17). This has resulted in a comparable microstrip-line-based MMIC 1/3 to 1/20 in size, for applications up to 65 GHz. NTT has built a U-band single-chip down converter based on this technology with a conversion gain of 0 dB ±1.5 dB and an image rejection ratio greater than 15 dB in a chip size of 1.78 mm × 1.28 mm (see Fig. 5.18). The K-band Si 3D MMIC has an 0.70 mm × 0.46 mm amplifier together with an 0.46 mm × 0.42 mm mixer (see Fig. 5.19).
Fig. 5.17. NTT's concept on 3D MMIC
Fig. 5.18. NTT U-band single-chip down converter.
Fig. 5.19. NTT K-band Si 3D MMIC examples.
The millimeter wave band is expected to be a frequency resource for the next generation's mobile communication system and is aimed at achieving a high-frequency PC-card-type radio terminal. To realize all the wireless functions including the antenna and the filter on a chip, a 3D hybrid IC structure is one of the most effective solutions. Matsushita has recently developed a 3D millimeter-wave IC that uses silicon micromachining. For the development of this IC, several technologies were used, including a dual mode resonant filter, multi-layer thin-films on silicon, and flip-chip bonding for the GaAs devices. This circuit has been utilized in a 25 GHz receiver down-converter and has an area of 11 mm2 including the built-in micromachined filter.
With the progress of technology and the invention of the transistor, it became apparent that very small transmission lines compatible with the planar technology of the newly discovered two- and three-terminal devices are needed to effectively couple the power of microscopic devices and macroscopic systems. Circuit miniaturization can be achieved by use of 3D integration where circuits are laid in all dimensions of the space. This approach has been effectively used in designing microprocessors and has resulted in a dramatic reduction of size and an unbelievable increase of speed. The next leap beyond the current state of the art multichip modules (MCMs) is the development of a technology that can integrate high-frequency Si-based active circuits, advanced micro-electromechanical (MEMS) devices, and micromachined components (e.g., filter/multiplexers) into one wafer. By employing 3D integration, significant reduction in mass (by a factor of 10) in physical volume and in cost can be achieved easily.
New concepts in integrated conformal packaging have been introduced, leading the way for micromachining to impact planar microwave circuits beyond the component level and into the system integration area. Micromachining has the potential to revolutionize microwave devices by offering new techniques that can be used to integrate entire systems onto a single IC. One scenario for total system integration calls for the use of multiple layers to accomplish various system functions such as amplification, signal reception, down conversion, and filtering. Micromachining offers the possibility of connecting these multiple layers together vertically to achieve new levels of high density integration. This concept has already been applied to the development of high frequency transmit modules and has demonstrated very high density integration along with excellent RF circuit performance.