A simple condition for maximum bandwidth of a chip packaged in an mm-wave waveguide

Abstract

Abstract- INTRODUCTIONComplex millimetre wave systems are difficult to construct due to packaging problems. Various monolithic active chips need to be interconnected with each other, but standard techniques, such as bond wires cannot be used due to large unknown parasitics. One of the possible solutions, still under development [1], is to use waveguides both for packaging and as interconnection media, with electromagnetic coupling between the chips and the waveguide. The simplest approach is to place the chip with suitable input and output slotline tapers in the E-plane of the waveguide (see Fig. 1). However, the effect of high permittivity (12.9 for GaAs) of a semiconductor chip on propagation modes in a waveguide needs to be investigated. Traditionally, finline structures consisting of a dielectric with a slotline placed in the E-plane were used to reduce the lower cut-off frequency of the fundamental mode, enabling usage of relatively small dimension waveguides at low frequencies. Unfortunately, the cut-off frequencies of the higher order modes are also reduced. When a waveguide is used as both interconnection medium and a package, then a useful bandwidth of the whole system is between the cut-off frequency of the fundamental mode of the empty waveguide and the first higher order mode of the waveguide with the chip. The electric loading from the semiconductor substrate of a chip and the slotline circuitry on it can dramatically reduce the cut-off frequencies of higher order modes and consequently cause severe reduction in a useful system bandwidth. These effects have been already investigated in [2]. For example for WR6 waveguide, when a finline with a relatively thick substrate of 200µm is used, the cut-off frequency of the first higher order mode is only 112 GHz, which is below the lower frequency of the practical bandwidth of a hollow WR6 waveguide. Waveguide packaging cannot be used in this case, as there is no useful bandwidth in the system. One way to increase the bandwidth is to use a very thin chip substrate. However, thinning GaAs or InP semiconductor substrates is not only quite expensive, but also produces very brittle chips that are difficult to handle. An alternative solution was proposed in [3], in which the slotline was mounted at an offset in a plane with zero electric field for TE 20 mode, rather than in the centre of the waveguide. It was shown that the frequency of the first higher order mode of the WR6 waveguide with a 200µm chip has increased by 14 GHz to 126 GHz, representing a substantial improvement. In this paper we use transverse resonance analysis to derive a simple implicit condition for an optimum offset of the chip. We compare the results obtained by the solution of this condition with those obtained from rigorous 3-D electromagnetic simulations of WR6 and WR10 waveguide packages for typical GaAs and InP chip dimensions, showing excellent agreement.

Design study of a thermocouple power sensor as a monolithic fin-line

“…WR-6 waveguide has internal dimensions small enough to allow a microchip to be fitted directly within the guide cavity. Work has been done toward packaging ICs as finline components directly in waveguide but the idea is not commonplace [7]. The use of an IC as an integrated finline component is a new approach made possible by the small dimensions of waveguide at this frequency range.…”